hotspot/src/share/vm/runtime/synchronizer.cpp
changeset 1 489c9b5090e2
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     1 /*
       
     2  * Copyright 1998-2007 Sun Microsystems, Inc.  All Rights Reserved.
       
     3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
       
     4  *
       
     5  * This code is free software; you can redistribute it and/or modify it
       
     6  * under the terms of the GNU General Public License version 2 only, as
       
     7  * published by the Free Software Foundation.
       
     8  *
       
     9  * This code is distributed in the hope that it will be useful, but WITHOUT
       
    10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
       
    11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
       
    12  * version 2 for more details (a copy is included in the LICENSE file that
       
    13  * accompanied this code).
       
    14  *
       
    15  * You should have received a copy of the GNU General Public License version
       
    16  * 2 along with this work; if not, write to the Free Software Foundation,
       
    17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
       
    18  *
       
    19  * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
       
    20  * CA 95054 USA or visit www.sun.com if you need additional information or
       
    21  * have any questions.
       
    22  *
       
    23  */
       
    24 
       
    25 # include "incls/_precompiled.incl"
       
    26 # include "incls/_synchronizer.cpp.incl"
       
    27 
       
    28 #if defined(__GNUC__) && !defined(IA64)
       
    29   // Need to inhibit inlining for older versions of GCC to avoid build-time failures
       
    30   #define ATTR __attribute__((noinline))
       
    31 #else
       
    32   #define ATTR
       
    33 #endif
       
    34 
       
    35 // Native markword accessors for synchronization and hashCode().
       
    36 //
       
    37 // The "core" versions of monitor enter and exit reside in this file.
       
    38 // The interpreter and compilers contain specialized transliterated
       
    39 // variants of the enter-exit fast-path operations.  See i486.ad fast_lock(),
       
    40 // for instance.  If you make changes here, make sure to modify the
       
    41 // interpreter, and both C1 and C2 fast-path inline locking code emission.
       
    42 //
       
    43 // TODO: merge the objectMonitor and synchronizer classes.
       
    44 //
       
    45 // -----------------------------------------------------------------------------
       
    46 
       
    47 #ifdef DTRACE_ENABLED
       
    48 
       
    49 // Only bother with this argument setup if dtrace is available
       
    50 // TODO-FIXME: probes should not fire when caller is _blocked.  assert() accordingly.
       
    51 
       
    52 HS_DTRACE_PROBE_DECL5(hotspot, monitor__wait,
       
    53   jlong, uintptr_t, char*, int, long);
       
    54 HS_DTRACE_PROBE_DECL4(hotspot, monitor__waited,
       
    55   jlong, uintptr_t, char*, int);
       
    56 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notify,
       
    57   jlong, uintptr_t, char*, int);
       
    58 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notifyAll,
       
    59   jlong, uintptr_t, char*, int);
       
    60 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__enter,
       
    61   jlong, uintptr_t, char*, int);
       
    62 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__entered,
       
    63   jlong, uintptr_t, char*, int);
       
    64 HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__exit,
       
    65   jlong, uintptr_t, char*, int);
       
    66 
       
    67 #define DTRACE_MONITOR_PROBE_COMMON(klassOop, thread)                      \
       
    68   char* bytes = NULL;                                                      \
       
    69   int len = 0;                                                             \
       
    70   jlong jtid = SharedRuntime::get_java_tid(thread);                        \
       
    71   symbolOop klassname = ((oop)(klassOop))->klass()->klass_part()->name();  \
       
    72   if (klassname != NULL) {                                                 \
       
    73     bytes = (char*)klassname->bytes();                                     \
       
    74     len = klassname->utf8_length();                                        \
       
    75   }
       
    76 
       
    77 #define DTRACE_MONITOR_WAIT_PROBE(monitor, klassOop, thread, millis)       \
       
    78   {                                                                        \
       
    79     if (DTraceMonitorProbes) {                                            \
       
    80       DTRACE_MONITOR_PROBE_COMMON(klassOop, thread);                       \
       
    81       HS_DTRACE_PROBE5(hotspot, monitor__wait, jtid,                       \
       
    82                        (monitor), bytes, len, (millis));                   \
       
    83     }                                                                      \
       
    84   }
       
    85 
       
    86 #define DTRACE_MONITOR_PROBE(probe, monitor, klassOop, thread)             \
       
    87   {                                                                        \
       
    88     if (DTraceMonitorProbes) {                                            \
       
    89       DTRACE_MONITOR_PROBE_COMMON(klassOop, thread);                       \
       
    90       HS_DTRACE_PROBE4(hotspot, monitor__##probe, jtid,                    \
       
    91                        (uintptr_t)(monitor), bytes, len);                  \
       
    92     }                                                                      \
       
    93   }
       
    94 
       
    95 #else //  ndef DTRACE_ENABLED
       
    96 
       
    97 #define DTRACE_MONITOR_WAIT_PROBE(klassOop, thread, millis, mon)    {;}
       
    98 #define DTRACE_MONITOR_PROBE(probe, klassOop, thread, mon)          {;}
       
    99 
       
   100 #endif // ndef DTRACE_ENABLED
       
   101 
       
   102 // ObjectWaiter serves as a "proxy" or surrogate thread.
       
   103 // TODO-FIXME: Eliminate ObjectWaiter and use the thread-specific
       
   104 // ParkEvent instead.  Beware, however, that the JVMTI code
       
   105 // knows about ObjectWaiters, so we'll have to reconcile that code.
       
   106 // See next_waiter(), first_waiter(), etc.
       
   107 
       
   108 class ObjectWaiter : public StackObj {
       
   109  public:
       
   110   enum TStates { TS_UNDEF, TS_READY, TS_RUN, TS_WAIT, TS_ENTER, TS_CXQ } ;
       
   111   enum Sorted  { PREPEND, APPEND, SORTED } ;
       
   112   ObjectWaiter * volatile _next;
       
   113   ObjectWaiter * volatile _prev;
       
   114   Thread*       _thread;
       
   115   ParkEvent *   _event;
       
   116   volatile int  _notified ;
       
   117   volatile TStates TState ;
       
   118   Sorted        _Sorted ;           // List placement disposition
       
   119   bool          _active ;           // Contention monitoring is enabled
       
   120  public:
       
   121   ObjectWaiter(Thread* thread) {
       
   122     _next     = NULL;
       
   123     _prev     = NULL;
       
   124     _notified = 0;
       
   125     TState    = TS_RUN ;
       
   126     _thread   = thread;
       
   127     _event    = thread->_ParkEvent ;
       
   128     _active   = false;
       
   129     assert (_event != NULL, "invariant") ;
       
   130   }
       
   131 
       
   132   void wait_reenter_begin(ObjectMonitor *mon) {
       
   133     JavaThread *jt = (JavaThread *)this->_thread;
       
   134     _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
       
   135   }
       
   136 
       
   137   void wait_reenter_end(ObjectMonitor *mon) {
       
   138     JavaThread *jt = (JavaThread *)this->_thread;
       
   139     JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
       
   140   }
       
   141 };
       
   142 
       
   143 enum ManifestConstants {
       
   144     ClearResponsibleAtSTW   = 0,
       
   145     MaximumRecheckInterval  = 1000
       
   146 } ;
       
   147 
       
   148 
       
   149 #undef TEVENT
       
   150 #define TEVENT(nom) {if (SyncVerbose) FEVENT(nom); }
       
   151 
       
   152 #define FEVENT(nom) { static volatile int ctr = 0 ; int v = ++ctr ; if ((v & (v-1)) == 0) { ::printf (#nom " : %d \n", v); ::fflush(stdout); }}
       
   153 
       
   154 #undef  TEVENT
       
   155 #define TEVENT(nom) {;}
       
   156 
       
   157 // Performance concern:
       
   158 // OrderAccess::storestore() calls release() which STs 0 into the global volatile
       
   159 // OrderAccess::Dummy variable.  This store is unnecessary for correctness.
       
   160 // Many threads STing into a common location causes considerable cache migration
       
   161 // or "sloshing" on large SMP system.  As such, I avoid using OrderAccess::storestore()
       
   162 // until it's repaired.  In some cases OrderAccess::fence() -- which incurs local
       
   163 // latency on the executing processor -- is a better choice as it scales on SMP
       
   164 // systems.  See http://blogs.sun.com/dave/entry/biased_locking_in_hotspot for a
       
   165 // discussion of coherency costs.  Note that all our current reference platforms
       
   166 // provide strong ST-ST order, so the issue is moot on IA32, x64, and SPARC.
       
   167 //
       
   168 // As a general policy we use "volatile" to control compiler-based reordering
       
   169 // and explicit fences (barriers) to control for architectural reordering performed
       
   170 // by the CPU(s) or platform.
       
   171 
       
   172 static int  MBFence (int x) { OrderAccess::fence(); return x; }
       
   173 
       
   174 struct SharedGlobals {
       
   175     // These are highly shared mostly-read variables.
       
   176     // To avoid false-sharing they need to be the sole occupants of a $ line.
       
   177     double padPrefix [8];
       
   178     volatile int stwRandom ;
       
   179     volatile int stwCycle ;
       
   180 
       
   181     // Hot RW variables -- Sequester to avoid false-sharing
       
   182     double padSuffix [16];
       
   183     volatile int hcSequence ;
       
   184     double padFinal [8] ;
       
   185 } ;
       
   186 
       
   187 static SharedGlobals GVars ;
       
   188 
       
   189 
       
   190 // Tunables ...
       
   191 // The knob* variables are effectively final.  Once set they should
       
   192 // never be modified hence.  Consider using __read_mostly with GCC.
       
   193 
       
   194 static int Knob_LogSpins           = 0 ;       // enable jvmstat tally for spins
       
   195 static int Knob_HandOff            = 0 ;
       
   196 static int Knob_Verbose            = 0 ;
       
   197 static int Knob_ReportSettings     = 0 ;
       
   198 
       
   199 static int Knob_SpinLimit          = 5000 ;    // derived by an external tool -
       
   200 static int Knob_SpinBase           = 0 ;       // Floor AKA SpinMin
       
   201 static int Knob_SpinBackOff        = 0 ;       // spin-loop backoff
       
   202 static int Knob_CASPenalty         = -1 ;      // Penalty for failed CAS
       
   203 static int Knob_OXPenalty          = -1 ;      // Penalty for observed _owner change
       
   204 static int Knob_SpinSetSucc        = 1 ;       // spinners set the _succ field
       
   205 static int Knob_SpinEarly          = 1 ;
       
   206 static int Knob_SuccEnabled        = 1 ;       // futile wake throttling
       
   207 static int Knob_SuccRestrict       = 0 ;       // Limit successors + spinners to at-most-one
       
   208 static int Knob_MaxSpinners        = -1 ;      // Should be a function of # CPUs
       
   209 static int Knob_Bonus              = 100 ;     // spin success bonus
       
   210 static int Knob_BonusB             = 100 ;     // spin success bonus
       
   211 static int Knob_Penalty            = 200 ;     // spin failure penalty
       
   212 static int Knob_Poverty            = 1000 ;
       
   213 static int Knob_SpinAfterFutile    = 1 ;       // Spin after returning from park()
       
   214 static int Knob_FixedSpin          = 0 ;
       
   215 static int Knob_OState             = 3 ;       // Spinner checks thread state of _owner
       
   216 static int Knob_UsePause           = 1 ;
       
   217 static int Knob_ExitPolicy         = 0 ;
       
   218 static int Knob_PreSpin            = 10 ;      // 20-100 likely better
       
   219 static int Knob_ResetEvent         = 0 ;
       
   220 static int BackOffMask             = 0 ;
       
   221 
       
   222 static int Knob_FastHSSEC          = 0 ;
       
   223 static int Knob_MoveNotifyee       = 2 ;       // notify() - disposition of notifyee
       
   224 static int Knob_QMode              = 0 ;       // EntryList-cxq policy - queue discipline
       
   225 static volatile int InitDone       = 0 ;
       
   226 
       
   227 
       
   228 // hashCode() generation :
       
   229 //
       
   230 // Possibilities:
       
   231 // * MD5Digest of {obj,stwRandom}
       
   232 // * CRC32 of {obj,stwRandom} or any linear-feedback shift register function.
       
   233 // * A DES- or AES-style SBox[] mechanism
       
   234 // * One of the Phi-based schemes, such as:
       
   235 //   2654435761 = 2^32 * Phi (golden ratio)
       
   236 //   HashCodeValue = ((uintptr_t(obj) >> 3) * 2654435761) ^ GVars.stwRandom ;
       
   237 // * A variation of Marsaglia's shift-xor RNG scheme.
       
   238 // * (obj ^ stwRandom) is appealing, but can result
       
   239 //   in undesirable regularity in the hashCode values of adjacent objects
       
   240 //   (objects allocated back-to-back, in particular).  This could potentially
       
   241 //   result in hashtable collisions and reduced hashtable efficiency.
       
   242 //   There are simple ways to "diffuse" the middle address bits over the
       
   243 //   generated hashCode values:
       
   244 //
       
   245 
       
   246 static inline intptr_t get_next_hash(Thread * Self, oop obj) {
       
   247   intptr_t value = 0 ;
       
   248   if (hashCode == 0) {
       
   249      // This form uses an unguarded global Park-Miller RNG,
       
   250      // so it's possible for two threads to race and generate the same RNG.
       
   251      // On MP system we'll have lots of RW access to a global, so the
       
   252      // mechanism induces lots of coherency traffic.
       
   253      value = os::random() ;
       
   254   } else
       
   255   if (hashCode == 1) {
       
   256      // This variation has the property of being stable (idempotent)
       
   257      // between STW operations.  This can be useful in some of the 1-0
       
   258      // synchronization schemes.
       
   259      intptr_t addrBits = intptr_t(obj) >> 3 ;
       
   260      value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom ;
       
   261   } else
       
   262   if (hashCode == 2) {
       
   263      value = 1 ;            // for sensitivity testing
       
   264   } else
       
   265   if (hashCode == 3) {
       
   266      value = ++GVars.hcSequence ;
       
   267   } else
       
   268   if (hashCode == 4) {
       
   269      value = intptr_t(obj) ;
       
   270   } else {
       
   271      // Marsaglia's xor-shift scheme with thread-specific state
       
   272      // This is probably the best overall implementation -- we'll
       
   273      // likely make this the default in future releases.
       
   274      unsigned t = Self->_hashStateX ;
       
   275      t ^= (t << 11) ;
       
   276      Self->_hashStateX = Self->_hashStateY ;
       
   277      Self->_hashStateY = Self->_hashStateZ ;
       
   278      Self->_hashStateZ = Self->_hashStateW ;
       
   279      unsigned v = Self->_hashStateW ;
       
   280      v = (v ^ (v >> 19)) ^ (t ^ (t >> 8)) ;
       
   281      Self->_hashStateW = v ;
       
   282      value = v ;
       
   283   }
       
   284 
       
   285   value &= markOopDesc::hash_mask;
       
   286   if (value == 0) value = 0xBAD ;
       
   287   assert (value != markOopDesc::no_hash, "invariant") ;
       
   288   TEVENT (hashCode: GENERATE) ;
       
   289   return value;
       
   290 }
       
   291 
       
   292 void BasicLock::print_on(outputStream* st) const {
       
   293   st->print("monitor");
       
   294 }
       
   295 
       
   296 void BasicLock::move_to(oop obj, BasicLock* dest) {
       
   297   // Check to see if we need to inflate the lock. This is only needed
       
   298   // if an object is locked using "this" lightweight monitor. In that
       
   299   // case, the displaced_header() is unlocked, because the
       
   300   // displaced_header() contains the header for the originally unlocked
       
   301   // object. However the object could have already been inflated. But it
       
   302   // does not matter, the inflation will just a no-op. For other cases,
       
   303   // the displaced header will be either 0x0 or 0x3, which are location
       
   304   // independent, therefore the BasicLock is free to move.
       
   305   //
       
   306   // During OSR we may need to relocate a BasicLock (which contains a
       
   307   // displaced word) from a location in an interpreter frame to a
       
   308   // new location in a compiled frame.  "this" refers to the source
       
   309   // basiclock in the interpreter frame.  "dest" refers to the destination
       
   310   // basiclock in the new compiled frame.  We *always* inflate in move_to().
       
   311   // The always-Inflate policy works properly, but in 1.5.0 it can sometimes
       
   312   // cause performance problems in code that makes heavy use of a small # of
       
   313   // uncontended locks.   (We'd inflate during OSR, and then sync performance
       
   314   // would subsequently plummet because the thread would be forced thru the slow-path).
       
   315   // This problem has been made largely moot on IA32 by inlining the inflated fast-path
       
   316   // operations in Fast_Lock and Fast_Unlock in i486.ad.
       
   317   //
       
   318   // Note that there is a way to safely swing the object's markword from
       
   319   // one stack location to another.  This avoids inflation.  Obviously,
       
   320   // we need to ensure that both locations refer to the current thread's stack.
       
   321   // There are some subtle concurrency issues, however, and since the benefit is
       
   322   // is small (given the support for inflated fast-path locking in the fast_lock, etc)
       
   323   // we'll leave that optimization for another time.
       
   324 
       
   325   if (displaced_header()->is_neutral()) {
       
   326     ObjectSynchronizer::inflate_helper(obj);
       
   327     // WARNING: We can not put check here, because the inflation
       
   328     // will not update the displaced header. Once BasicLock is inflated,
       
   329     // no one should ever look at its content.
       
   330   } else {
       
   331     // Typically the displaced header will be 0 (recursive stack lock) or
       
   332     // unused_mark.  Naively we'd like to assert that the displaced mark
       
   333     // value is either 0, neutral, or 3.  But with the advent of the
       
   334     // store-before-CAS avoidance in fast_lock/compiler_lock_object
       
   335     // we can find any flavor mark in the displaced mark.
       
   336   }
       
   337 // [RGV] The next line appears to do nothing!
       
   338   intptr_t dh = (intptr_t) displaced_header();
       
   339   dest->set_displaced_header(displaced_header());
       
   340 }
       
   341 
       
   342 // -----------------------------------------------------------------------------
       
   343 
       
   344 // standard constructor, allows locking failures
       
   345 ObjectLocker::ObjectLocker(Handle obj, Thread* thread, bool doLock) {
       
   346   _dolock = doLock;
       
   347   _thread = thread;
       
   348   debug_only(if (StrictSafepointChecks) _thread->check_for_valid_safepoint_state(false);)
       
   349   _obj = obj;
       
   350 
       
   351   if (_dolock) {
       
   352     TEVENT (ObjectLocker) ;
       
   353 
       
   354     ObjectSynchronizer::fast_enter(_obj, &_lock, false, _thread);
       
   355   }
       
   356 }
       
   357 
       
   358 ObjectLocker::~ObjectLocker() {
       
   359   if (_dolock) {
       
   360     ObjectSynchronizer::fast_exit(_obj(), &_lock, _thread);
       
   361   }
       
   362 }
       
   363 
       
   364 // -----------------------------------------------------------------------------
       
   365 
       
   366 
       
   367 PerfCounter * ObjectSynchronizer::_sync_Inflations                  = NULL ;
       
   368 PerfCounter * ObjectSynchronizer::_sync_Deflations                  = NULL ;
       
   369 PerfCounter * ObjectSynchronizer::_sync_ContendedLockAttempts       = NULL ;
       
   370 PerfCounter * ObjectSynchronizer::_sync_FutileWakeups               = NULL ;
       
   371 PerfCounter * ObjectSynchronizer::_sync_Parks                       = NULL ;
       
   372 PerfCounter * ObjectSynchronizer::_sync_EmptyNotifications          = NULL ;
       
   373 PerfCounter * ObjectSynchronizer::_sync_Notifications               = NULL ;
       
   374 PerfCounter * ObjectSynchronizer::_sync_PrivateA                    = NULL ;
       
   375 PerfCounter * ObjectSynchronizer::_sync_PrivateB                    = NULL ;
       
   376 PerfCounter * ObjectSynchronizer::_sync_SlowExit                    = NULL ;
       
   377 PerfCounter * ObjectSynchronizer::_sync_SlowEnter                   = NULL ;
       
   378 PerfCounter * ObjectSynchronizer::_sync_SlowNotify                  = NULL ;
       
   379 PerfCounter * ObjectSynchronizer::_sync_SlowNotifyAll               = NULL ;
       
   380 PerfCounter * ObjectSynchronizer::_sync_FailedSpins                 = NULL ;
       
   381 PerfCounter * ObjectSynchronizer::_sync_SuccessfulSpins             = NULL ;
       
   382 PerfCounter * ObjectSynchronizer::_sync_MonInCirculation            = NULL ;
       
   383 PerfCounter * ObjectSynchronizer::_sync_MonScavenged                = NULL ;
       
   384 PerfLongVariable * ObjectSynchronizer::_sync_MonExtant              = NULL ;
       
   385 
       
   386 // One-shot global initialization for the sync subsystem.
       
   387 // We could also defer initialization and initialize on-demand
       
   388 // the first time we call inflate().  Initialization would
       
   389 // be protected - like so many things - by the MonitorCache_lock.
       
   390 
       
   391 void ObjectSynchronizer::Initialize () {
       
   392   static int InitializationCompleted = 0 ;
       
   393   assert (InitializationCompleted == 0, "invariant") ;
       
   394   InitializationCompleted = 1 ;
       
   395   if (UsePerfData) {
       
   396       EXCEPTION_MARK ;
       
   397       #define NEWPERFCOUNTER(n)   {n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,CHECK); }
       
   398       #define NEWPERFVARIABLE(n)  {n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,CHECK); }
       
   399       NEWPERFCOUNTER(_sync_Inflations) ;
       
   400       NEWPERFCOUNTER(_sync_Deflations) ;
       
   401       NEWPERFCOUNTER(_sync_ContendedLockAttempts) ;
       
   402       NEWPERFCOUNTER(_sync_FutileWakeups) ;
       
   403       NEWPERFCOUNTER(_sync_Parks) ;
       
   404       NEWPERFCOUNTER(_sync_EmptyNotifications) ;
       
   405       NEWPERFCOUNTER(_sync_Notifications) ;
       
   406       NEWPERFCOUNTER(_sync_SlowEnter) ;
       
   407       NEWPERFCOUNTER(_sync_SlowExit) ;
       
   408       NEWPERFCOUNTER(_sync_SlowNotify) ;
       
   409       NEWPERFCOUNTER(_sync_SlowNotifyAll) ;
       
   410       NEWPERFCOUNTER(_sync_FailedSpins) ;
       
   411       NEWPERFCOUNTER(_sync_SuccessfulSpins) ;
       
   412       NEWPERFCOUNTER(_sync_PrivateA) ;
       
   413       NEWPERFCOUNTER(_sync_PrivateB) ;
       
   414       NEWPERFCOUNTER(_sync_MonInCirculation) ;
       
   415       NEWPERFCOUNTER(_sync_MonScavenged) ;
       
   416       NEWPERFVARIABLE(_sync_MonExtant) ;
       
   417       #undef NEWPERFCOUNTER
       
   418   }
       
   419 }
       
   420 
       
   421 // Compile-time asserts
       
   422 // When possible, it's better to catch errors deterministically at
       
   423 // compile-time than at runtime.  The down-side to using compile-time
       
   424 // asserts is that error message -- often something about negative array
       
   425 // indices -- is opaque.
       
   426 
       
   427 #define CTASSERT(x) { int tag[1-(2*!(x))]; printf ("Tag @%X\n", tag); }
       
   428 
       
   429 void ObjectMonitor::ctAsserts() {
       
   430   CTASSERT(offset_of (ObjectMonitor, _header) == 0);
       
   431 }
       
   432 
       
   433 static int Adjust (volatile int * adr, int dx) {
       
   434   int v ;
       
   435   for (v = *adr ; Atomic::cmpxchg (v + dx, adr, v) != v; v = *adr) ;
       
   436   return v ;
       
   437 }
       
   438 
       
   439 // Ad-hoc mutual exclusion primitives: SpinLock and Mux
       
   440 //
       
   441 // We employ SpinLocks _only for low-contention, fixed-length
       
   442 // short-duration critical sections where we're concerned
       
   443 // about native mutex_t or HotSpot Mutex:: latency.
       
   444 // The mux construct provides a spin-then-block mutual exclusion
       
   445 // mechanism.
       
   446 //
       
   447 // Testing has shown that contention on the ListLock guarding gFreeList
       
   448 // is common.  If we implement ListLock as a simple SpinLock it's common
       
   449 // for the JVM to devolve to yielding with little progress.  This is true
       
   450 // despite the fact that the critical sections protected by ListLock are
       
   451 // extremely short.
       
   452 //
       
   453 // TODO-FIXME: ListLock should be of type SpinLock.
       
   454 // We should make this a 1st-class type, integrated into the lock
       
   455 // hierarchy as leaf-locks.  Critically, the SpinLock structure
       
   456 // should have sufficient padding to avoid false-sharing and excessive
       
   457 // cache-coherency traffic.
       
   458 
       
   459 
       
   460 typedef volatile int SpinLockT ;
       
   461 
       
   462 void Thread::SpinAcquire (volatile int * adr, const char * LockName) {
       
   463   if (Atomic::cmpxchg (1, adr, 0) == 0) {
       
   464      return ;   // normal fast-path return
       
   465   }
       
   466 
       
   467   // Slow-path : We've encountered contention -- Spin/Yield/Block strategy.
       
   468   TEVENT (SpinAcquire - ctx) ;
       
   469   int ctr = 0 ;
       
   470   int Yields = 0 ;
       
   471   for (;;) {
       
   472      while (*adr != 0) {
       
   473         ++ctr ;
       
   474         if ((ctr & 0xFFF) == 0 || !os::is_MP()) {
       
   475            if (Yields > 5) {
       
   476              // Consider using a simple NakedSleep() instead.
       
   477              // Then SpinAcquire could be called by non-JVM threads
       
   478              Thread::current()->_ParkEvent->park(1) ;
       
   479            } else {
       
   480              os::NakedYield() ;
       
   481              ++Yields ;
       
   482            }
       
   483         } else {
       
   484            SpinPause() ;
       
   485         }
       
   486      }
       
   487      if (Atomic::cmpxchg (1, adr, 0) == 0) return ;
       
   488   }
       
   489 }
       
   490 
       
   491 void Thread::SpinRelease (volatile int * adr) {
       
   492   assert (*adr != 0, "invariant") ;
       
   493   OrderAccess::fence() ;      // guarantee at least release consistency.
       
   494   // Roach-motel semantics.
       
   495   // It's safe if subsequent LDs and STs float "up" into the critical section,
       
   496   // but prior LDs and STs within the critical section can't be allowed
       
   497   // to reorder or float past the ST that releases the lock.
       
   498   *adr = 0 ;
       
   499 }
       
   500 
       
   501 // muxAcquire and muxRelease:
       
   502 //
       
   503 // *  muxAcquire and muxRelease support a single-word lock-word construct.
       
   504 //    The LSB of the word is set IFF the lock is held.
       
   505 //    The remainder of the word points to the head of a singly-linked list
       
   506 //    of threads blocked on the lock.
       
   507 //
       
   508 // *  The current implementation of muxAcquire-muxRelease uses its own
       
   509 //    dedicated Thread._MuxEvent instance.  If we're interested in
       
   510 //    minimizing the peak number of extant ParkEvent instances then
       
   511 //    we could eliminate _MuxEvent and "borrow" _ParkEvent as long
       
   512 //    as certain invariants were satisfied.  Specifically, care would need
       
   513 //    to be taken with regards to consuming unpark() "permits".
       
   514 //    A safe rule of thumb is that a thread would never call muxAcquire()
       
   515 //    if it's enqueued (cxq, EntryList, WaitList, etc) and will subsequently
       
   516 //    park().  Otherwise the _ParkEvent park() operation in muxAcquire() could
       
   517 //    consume an unpark() permit intended for monitorenter, for instance.
       
   518 //    One way around this would be to widen the restricted-range semaphore
       
   519 //    implemented in park().  Another alternative would be to provide
       
   520 //    multiple instances of the PlatformEvent() for each thread.  One
       
   521 //    instance would be dedicated to muxAcquire-muxRelease, for instance.
       
   522 //
       
   523 // *  Usage:
       
   524 //    -- Only as leaf locks
       
   525 //    -- for short-term locking only as muxAcquire does not perform
       
   526 //       thread state transitions.
       
   527 //
       
   528 // Alternatives:
       
   529 // *  We could implement muxAcquire and muxRelease with MCS or CLH locks
       
   530 //    but with parking or spin-then-park instead of pure spinning.
       
   531 // *  Use Taura-Oyama-Yonenzawa locks.
       
   532 // *  It's possible to construct a 1-0 lock if we encode the lockword as
       
   533 //    (List,LockByte).  Acquire will CAS the full lockword while Release
       
   534 //    will STB 0 into the LockByte.  The 1-0 scheme admits stranding, so
       
   535 //    acquiring threads use timers (ParkTimed) to detect and recover from
       
   536 //    the stranding window.  Thread/Node structures must be aligned on 256-byte
       
   537 //    boundaries by using placement-new.
       
   538 // *  Augment MCS with advisory back-link fields maintained with CAS().
       
   539 //    Pictorially:  LockWord -> T1 <-> T2 <-> T3 <-> ... <-> Tn <-> Owner.
       
   540 //    The validity of the backlinks must be ratified before we trust the value.
       
   541 //    If the backlinks are invalid the exiting thread must back-track through the
       
   542 //    the forward links, which are always trustworthy.
       
   543 // *  Add a successor indication.  The LockWord is currently encoded as
       
   544 //    (List, LOCKBIT:1).  We could also add a SUCCBIT or an explicit _succ variable
       
   545 //    to provide the usual futile-wakeup optimization.
       
   546 //    See RTStt for details.
       
   547 // *  Consider schedctl.sc_nopreempt to cover the critical section.
       
   548 //
       
   549 
       
   550 
       
   551 typedef volatile intptr_t MutexT ;      // Mux Lock-word
       
   552 enum MuxBits { LOCKBIT = 1 } ;
       
   553 
       
   554 void Thread::muxAcquire (volatile intptr_t * Lock, const char * LockName) {
       
   555   intptr_t w = Atomic::cmpxchg_ptr (LOCKBIT, Lock, 0) ;
       
   556   if (w == 0) return ;
       
   557   if ((w & LOCKBIT) == 0 && Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {
       
   558      return ;
       
   559   }
       
   560 
       
   561   TEVENT (muxAcquire - Contention) ;
       
   562   ParkEvent * const Self = Thread::current()->_MuxEvent ;
       
   563   assert ((intptr_t(Self) & LOCKBIT) == 0, "invariant") ;
       
   564   for (;;) {
       
   565      int its = (os::is_MP() ? 100 : 0) + 1 ;
       
   566 
       
   567      // Optional spin phase: spin-then-park strategy
       
   568      while (--its >= 0) {
       
   569        w = *Lock ;
       
   570        if ((w & LOCKBIT) == 0 && Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {
       
   571           return ;
       
   572        }
       
   573      }
       
   574 
       
   575      Self->reset() ;
       
   576      Self->OnList = intptr_t(Lock) ;
       
   577      // The following fence() isn't _strictly necessary as the subsequent
       
   578      // CAS() both serializes execution and ratifies the fetched *Lock value.
       
   579      OrderAccess::fence();
       
   580      for (;;) {
       
   581         w = *Lock ;
       
   582         if ((w & LOCKBIT) == 0) {
       
   583             if (Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {
       
   584                 Self->OnList = 0 ;   // hygiene - allows stronger asserts
       
   585                 return ;
       
   586             }
       
   587             continue ;      // Interference -- *Lock changed -- Just retry
       
   588         }
       
   589         assert (w & LOCKBIT, "invariant") ;
       
   590         Self->ListNext = (ParkEvent *) (w & ~LOCKBIT );
       
   591         if (Atomic::cmpxchg_ptr (intptr_t(Self)|LOCKBIT, Lock, w) == w) break ;
       
   592      }
       
   593 
       
   594      while (Self->OnList != 0) {
       
   595         Self->park() ;
       
   596      }
       
   597   }
       
   598 }
       
   599 
       
   600 void Thread::muxAcquireW (volatile intptr_t * Lock, ParkEvent * ev) {
       
   601   intptr_t w = Atomic::cmpxchg_ptr (LOCKBIT, Lock, 0) ;
       
   602   if (w == 0) return ;
       
   603   if ((w & LOCKBIT) == 0 && Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {
       
   604     return ;
       
   605   }
       
   606 
       
   607   TEVENT (muxAcquire - Contention) ;
       
   608   ParkEvent * ReleaseAfter = NULL ;
       
   609   if (ev == NULL) {
       
   610     ev = ReleaseAfter = ParkEvent::Allocate (NULL) ;
       
   611   }
       
   612   assert ((intptr_t(ev) & LOCKBIT) == 0, "invariant") ;
       
   613   for (;;) {
       
   614     guarantee (ev->OnList == 0, "invariant") ;
       
   615     int its = (os::is_MP() ? 100 : 0) + 1 ;
       
   616 
       
   617     // Optional spin phase: spin-then-park strategy
       
   618     while (--its >= 0) {
       
   619       w = *Lock ;
       
   620       if ((w & LOCKBIT) == 0 && Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {
       
   621         if (ReleaseAfter != NULL) {
       
   622           ParkEvent::Release (ReleaseAfter) ;
       
   623         }
       
   624         return ;
       
   625       }
       
   626     }
       
   627 
       
   628     ev->reset() ;
       
   629     ev->OnList = intptr_t(Lock) ;
       
   630     // The following fence() isn't _strictly necessary as the subsequent
       
   631     // CAS() both serializes execution and ratifies the fetched *Lock value.
       
   632     OrderAccess::fence();
       
   633     for (;;) {
       
   634       w = *Lock ;
       
   635       if ((w & LOCKBIT) == 0) {
       
   636         if (Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {
       
   637           ev->OnList = 0 ;
       
   638           // We call ::Release while holding the outer lock, thus
       
   639           // artificially lengthening the critical section.
       
   640           // Consider deferring the ::Release() until the subsequent unlock(),
       
   641           // after we've dropped the outer lock.
       
   642           if (ReleaseAfter != NULL) {
       
   643             ParkEvent::Release (ReleaseAfter) ;
       
   644           }
       
   645           return ;
       
   646         }
       
   647         continue ;      // Interference -- *Lock changed -- Just retry
       
   648       }
       
   649       assert (w & LOCKBIT, "invariant") ;
       
   650       ev->ListNext = (ParkEvent *) (w & ~LOCKBIT );
       
   651       if (Atomic::cmpxchg_ptr (intptr_t(ev)|LOCKBIT, Lock, w) == w) break ;
       
   652     }
       
   653 
       
   654     while (ev->OnList != 0) {
       
   655       ev->park() ;
       
   656     }
       
   657   }
       
   658 }
       
   659 
       
   660 // Release() must extract a successor from the list and then wake that thread.
       
   661 // It can "pop" the front of the list or use a detach-modify-reattach (DMR) scheme
       
   662 // similar to that used by ParkEvent::Allocate() and ::Release().  DMR-based
       
   663 // Release() would :
       
   664 // (A) CAS() or swap() null to *Lock, releasing the lock and detaching the list.
       
   665 // (B) Extract a successor from the private list "in-hand"
       
   666 // (C) attempt to CAS() the residual back into *Lock over null.
       
   667 //     If there were any newly arrived threads and the CAS() would fail.
       
   668 //     In that case Release() would detach the RATs, re-merge the list in-hand
       
   669 //     with the RATs and repeat as needed.  Alternately, Release() might
       
   670 //     detach and extract a successor, but then pass the residual list to the wakee.
       
   671 //     The wakee would be responsible for reattaching and remerging before it
       
   672 //     competed for the lock.
       
   673 //
       
   674 // Both "pop" and DMR are immune from ABA corruption -- there can be
       
   675 // multiple concurrent pushers, but only one popper or detacher.
       
   676 // This implementation pops from the head of the list.  This is unfair,
       
   677 // but tends to provide excellent throughput as hot threads remain hot.
       
   678 // (We wake recently run threads first).
       
   679 
       
   680 void Thread::muxRelease (volatile intptr_t * Lock)  {
       
   681   for (;;) {
       
   682     const intptr_t w = Atomic::cmpxchg_ptr (0, Lock, LOCKBIT) ;
       
   683     assert (w & LOCKBIT, "invariant") ;
       
   684     if (w == LOCKBIT) return ;
       
   685     ParkEvent * List = (ParkEvent *) (w & ~LOCKBIT) ;
       
   686     assert (List != NULL, "invariant") ;
       
   687     assert (List->OnList == intptr_t(Lock), "invariant") ;
       
   688     ParkEvent * nxt = List->ListNext ;
       
   689 
       
   690     // The following CAS() releases the lock and pops the head element.
       
   691     if (Atomic::cmpxchg_ptr (intptr_t(nxt), Lock, w) != w) {
       
   692       continue ;
       
   693     }
       
   694     List->OnList = 0 ;
       
   695     OrderAccess::fence() ;
       
   696     List->unpark () ;
       
   697     return ;
       
   698   }
       
   699 }
       
   700 
       
   701 // ObjectMonitor Lifecycle
       
   702 // -----------------------
       
   703 // Inflation unlinks monitors from the global gFreeList and
       
   704 // associates them with objects.  Deflation -- which occurs at
       
   705 // STW-time -- disassociates idle monitors from objects.  Such
       
   706 // scavenged monitors are returned to the gFreeList.
       
   707 //
       
   708 // The global list is protected by ListLock.  All the critical sections
       
   709 // are short and operate in constant-time.
       
   710 //
       
   711 // ObjectMonitors reside in type-stable memory (TSM) and are immortal.
       
   712 //
       
   713 // Lifecycle:
       
   714 // --   unassigned and on the global free list
       
   715 // --   unassigned and on a thread's private omFreeList
       
   716 // --   assigned to an object.  The object is inflated and the mark refers
       
   717 //      to the objectmonitor.
       
   718 //
       
   719 // TODO-FIXME:
       
   720 //
       
   721 // *  We currently protect the gFreeList with a simple lock.
       
   722 //    An alternate lock-free scheme would be to pop elements from the gFreeList
       
   723 //    with CAS.  This would be safe from ABA corruption as long we only
       
   724 //    recycled previously appearing elements onto the list in deflate_idle_monitors()
       
   725 //    at STW-time.  Completely new elements could always be pushed onto the gFreeList
       
   726 //    with CAS.  Elements that appeared previously on the list could only
       
   727 //    be installed at STW-time.
       
   728 //
       
   729 // *  For efficiency and to help reduce the store-before-CAS penalty
       
   730 //    the objectmonitors on gFreeList or local free lists should be ready to install
       
   731 //    with the exception of _header and _object.  _object can be set after inflation.
       
   732 //    In particular, keep all objectMonitors on a thread's private list in ready-to-install
       
   733 //    state with m.Owner set properly.
       
   734 //
       
   735 // *  We could all diffuse contention by using multiple global (FreeList, Lock)
       
   736 //    pairs -- threads could use trylock() and a cyclic-scan strategy to search for
       
   737 //    an unlocked free list.
       
   738 //
       
   739 // *  Add lifecycle tags and assert()s.
       
   740 //
       
   741 // *  Be more consistent about when we clear an objectmonitor's fields:
       
   742 //    A.  After extracting the objectmonitor from a free list.
       
   743 //    B.  After adding an objectmonitor to a free list.
       
   744 //
       
   745 
       
   746 ObjectMonitor * ObjectSynchronizer::gBlockList = NULL ;
       
   747 ObjectMonitor * volatile ObjectSynchronizer::gFreeList  = NULL ;
       
   748 static volatile intptr_t ListLock = 0 ;      // protects global monitor free-list cache
       
   749 #define CHAINMARKER ((oop)-1)
       
   750 
       
   751 ObjectMonitor * ATTR ObjectSynchronizer::omAlloc (Thread * Self) {
       
   752     // A large MAXPRIVATE value reduces both list lock contention
       
   753     // and list coherency traffic, but also tends to increase the
       
   754     // number of objectMonitors in circulation as well as the STW
       
   755     // scavenge costs.  As usual, we lean toward time in space-time
       
   756     // tradeoffs.
       
   757     const int MAXPRIVATE = 1024 ;
       
   758     for (;;) {
       
   759         ObjectMonitor * m ;
       
   760 
       
   761         // 1: try to allocate from the thread's local omFreeList.
       
   762         // Threads will attempt to allocate first from their local list, then
       
   763         // from the global list, and only after those attempts fail will the thread
       
   764         // attempt to instantiate new monitors.   Thread-local free lists take
       
   765         // heat off the ListLock and improve allocation latency, as well as reducing
       
   766         // coherency traffic on the shared global list.
       
   767         m = Self->omFreeList ;
       
   768         if (m != NULL) {
       
   769            Self->omFreeList = m->FreeNext ;
       
   770            Self->omFreeCount -- ;
       
   771            // CONSIDER: set m->FreeNext = BAD -- diagnostic hygiene
       
   772            guarantee (m->object() == NULL, "invariant") ;
       
   773            return m ;
       
   774         }
       
   775 
       
   776         // 2: try to allocate from the global gFreeList
       
   777         // CONSIDER: use muxTry() instead of muxAcquire().
       
   778         // If the muxTry() fails then drop immediately into case 3.
       
   779         // If we're using thread-local free lists then try
       
   780         // to reprovision the caller's free list.
       
   781         if (gFreeList != NULL) {
       
   782             // Reprovision the thread's omFreeList.
       
   783             // Use bulk transfers to reduce the allocation rate and heat
       
   784             // on various locks.
       
   785             Thread::muxAcquire (&ListLock, "omAlloc") ;
       
   786             for (int i = Self->omFreeProvision; --i >= 0 && gFreeList != NULL; ) {
       
   787                 ObjectMonitor * take = gFreeList ;
       
   788                 gFreeList = take->FreeNext ;
       
   789                 guarantee (take->object() == NULL, "invariant") ;
       
   790                 guarantee (!take->is_busy(), "invariant") ;
       
   791                 take->Recycle() ;
       
   792                 omRelease (Self, take) ;
       
   793             }
       
   794             Thread::muxRelease (&ListLock) ;
       
   795             Self->omFreeProvision += 1 + (Self->omFreeProvision/2) ;
       
   796             if (Self->omFreeProvision > MAXPRIVATE ) Self->omFreeProvision = MAXPRIVATE ;
       
   797             TEVENT (omFirst - reprovision) ;
       
   798             continue ;
       
   799         }
       
   800 
       
   801         // 3: allocate a block of new ObjectMonitors
       
   802         // Both the local and global free lists are empty -- resort to malloc().
       
   803         // In the current implementation objectMonitors are TSM - immortal.
       
   804         assert (_BLOCKSIZE > 1, "invariant") ;
       
   805         ObjectMonitor * temp = new ObjectMonitor[_BLOCKSIZE];
       
   806 
       
   807         // NOTE: (almost) no way to recover if allocation failed.
       
   808         // We might be able to induce a STW safepoint and scavenge enough
       
   809         // objectMonitors to permit progress.
       
   810         if (temp == NULL) {
       
   811             vm_exit_out_of_memory (sizeof (ObjectMonitor[_BLOCKSIZE]), "Allocate ObjectMonitors") ;
       
   812         }
       
   813 
       
   814         // Format the block.
       
   815         // initialize the linked list, each monitor points to its next
       
   816         // forming the single linked free list, the very first monitor
       
   817         // will points to next block, which forms the block list.
       
   818         // The trick of using the 1st element in the block as gBlockList
       
   819         // linkage should be reconsidered.  A better implementation would
       
   820         // look like: class Block { Block * next; int N; ObjectMonitor Body [N] ; }
       
   821 
       
   822         for (int i = 1; i < _BLOCKSIZE ; i++) {
       
   823            temp[i].FreeNext = &temp[i+1];
       
   824         }
       
   825 
       
   826         // terminate the last monitor as the end of list
       
   827         temp[_BLOCKSIZE - 1].FreeNext = NULL ;
       
   828 
       
   829         // Element [0] is reserved for global list linkage
       
   830         temp[0].set_object(CHAINMARKER);
       
   831 
       
   832         // Consider carving out this thread's current request from the
       
   833         // block in hand.  This avoids some lock traffic and redundant
       
   834         // list activity.
       
   835 
       
   836         // Acquire the ListLock to manipulate BlockList and FreeList.
       
   837         // An Oyama-Taura-Yonezawa scheme might be more efficient.
       
   838         Thread::muxAcquire (&ListLock, "omAlloc [2]") ;
       
   839 
       
   840         // Add the new block to the list of extant blocks (gBlockList).
       
   841         // The very first objectMonitor in a block is reserved and dedicated.
       
   842         // It serves as blocklist "next" linkage.
       
   843         temp[0].FreeNext = gBlockList;
       
   844         gBlockList = temp;
       
   845 
       
   846         // Add the new string of objectMonitors to the global free list
       
   847         temp[_BLOCKSIZE - 1].FreeNext = gFreeList ;
       
   848         gFreeList = temp + 1;
       
   849         Thread::muxRelease (&ListLock) ;
       
   850         TEVENT (Allocate block of monitors) ;
       
   851     }
       
   852 }
       
   853 
       
   854 // Place "m" on the caller's private per-thread omFreeList.
       
   855 // In practice there's no need to clamp or limit the number of
       
   856 // monitors on a thread's omFreeList as the only time we'll call
       
   857 // omRelease is to return a monitor to the free list after a CAS
       
   858 // attempt failed.  This doesn't allow unbounded #s of monitors to
       
   859 // accumulate on a thread's free list.
       
   860 //
       
   861 // In the future the usage of omRelease() might change and monitors
       
   862 // could migrate between free lists.  In that case to avoid excessive
       
   863 // accumulation we could  limit omCount to (omProvision*2), otherwise return
       
   864 // the objectMonitor to the global list.  We should drain (return) in reasonable chunks.
       
   865 // That is, *not* one-at-a-time.
       
   866 
       
   867 
       
   868 void ObjectSynchronizer::omRelease (Thread * Self, ObjectMonitor * m) {
       
   869     guarantee (m->object() == NULL, "invariant") ;
       
   870     m->FreeNext = Self->omFreeList ;
       
   871     Self->omFreeList = m ;
       
   872     Self->omFreeCount ++ ;
       
   873 }
       
   874 
       
   875 // Return the monitors of a moribund thread's local free list to
       
   876 // the global free list.  Typically a thread calls omFlush() when
       
   877 // it's dying.  We could also consider having the VM thread steal
       
   878 // monitors from threads that have not run java code over a few
       
   879 // consecutive STW safepoints.  Relatedly, we might decay
       
   880 // omFreeProvision at STW safepoints.
       
   881 //
       
   882 // We currently call omFlush() from the Thread:: dtor _after the thread
       
   883 // has been excised from the thread list and is no longer a mutator.
       
   884 // That means that omFlush() can run concurrently with a safepoint and
       
   885 // the scavenge operator.  Calling omFlush() from JavaThread::exit() might
       
   886 // be a better choice as we could safely reason that that the JVM is
       
   887 // not at a safepoint at the time of the call, and thus there could
       
   888 // be not inopportune interleavings between omFlush() and the scavenge
       
   889 // operator.
       
   890 
       
   891 void ObjectSynchronizer::omFlush (Thread * Self) {
       
   892     ObjectMonitor * List = Self->omFreeList ;  // Null-terminated SLL
       
   893     Self->omFreeList = NULL ;
       
   894     if (List == NULL) return ;
       
   895     ObjectMonitor * Tail = NULL ;
       
   896     ObjectMonitor * s ;
       
   897     for (s = List ; s != NULL ; s = s->FreeNext) {
       
   898         Tail = s ;
       
   899         guarantee (s->object() == NULL, "invariant") ;
       
   900         guarantee (!s->is_busy(), "invariant") ;
       
   901         s->set_owner (NULL) ;   // redundant but good hygiene
       
   902         TEVENT (omFlush - Move one) ;
       
   903     }
       
   904 
       
   905     guarantee (Tail != NULL && List != NULL, "invariant") ;
       
   906     Thread::muxAcquire (&ListLock, "omFlush") ;
       
   907     Tail->FreeNext = gFreeList ;
       
   908     gFreeList = List ;
       
   909     Thread::muxRelease (&ListLock) ;
       
   910     TEVENT (omFlush) ;
       
   911 }
       
   912 
       
   913 
       
   914 // Get the next block in the block list.
       
   915 static inline ObjectMonitor* next(ObjectMonitor* block) {
       
   916   assert(block->object() == CHAINMARKER, "must be a block header");
       
   917   block = block->FreeNext ;
       
   918   assert(block == NULL || block->object() == CHAINMARKER, "must be a block header");
       
   919   return block;
       
   920 }
       
   921 
       
   922 // Fast path code shared by multiple functions
       
   923 ObjectMonitor* ObjectSynchronizer::inflate_helper(oop obj) {
       
   924   markOop mark = obj->mark();
       
   925   if (mark->has_monitor()) {
       
   926     assert(ObjectSynchronizer::verify_objmon_isinpool(mark->monitor()), "monitor is invalid");
       
   927     assert(mark->monitor()->header()->is_neutral(), "monitor must record a good object header");
       
   928     return mark->monitor();
       
   929   }
       
   930   return ObjectSynchronizer::inflate(Thread::current(), obj);
       
   931 }
       
   932 
       
   933 // Note that we could encounter some performance loss through false-sharing as
       
   934 // multiple locks occupy the same $ line.  Padding might be appropriate.
       
   935 
       
   936 #define NINFLATIONLOCKS 256
       
   937 static volatile intptr_t InflationLocks [NINFLATIONLOCKS] ;
       
   938 
       
   939 static markOop ReadStableMark (oop obj) {
       
   940   markOop mark = obj->mark() ;
       
   941   if (!mark->is_being_inflated()) {
       
   942     return mark ;       // normal fast-path return
       
   943   }
       
   944 
       
   945   int its = 0 ;
       
   946   for (;;) {
       
   947     markOop mark = obj->mark() ;
       
   948     if (!mark->is_being_inflated()) {
       
   949       return mark ;    // normal fast-path return
       
   950     }
       
   951 
       
   952     // The object is being inflated by some other thread.
       
   953     // The caller of ReadStableMark() must wait for inflation to complete.
       
   954     // Avoid live-lock
       
   955     // TODO: consider calling SafepointSynchronize::do_call_back() while
       
   956     // spinning to see if there's a safepoint pending.  If so, immediately
       
   957     // yielding or blocking would be appropriate.  Avoid spinning while
       
   958     // there is a safepoint pending.
       
   959     // TODO: add inflation contention performance counters.
       
   960     // TODO: restrict the aggregate number of spinners.
       
   961 
       
   962     ++its ;
       
   963     if (its > 10000 || !os::is_MP()) {
       
   964        if (its & 1) {
       
   965          os::NakedYield() ;
       
   966          TEVENT (Inflate: INFLATING - yield) ;
       
   967        } else {
       
   968          // Note that the following code attenuates the livelock problem but is not
       
   969          // a complete remedy.  A more complete solution would require that the inflating
       
   970          // thread hold the associated inflation lock.  The following code simply restricts
       
   971          // the number of spinners to at most one.  We'll have N-2 threads blocked
       
   972          // on the inflationlock, 1 thread holding the inflation lock and using
       
   973          // a yield/park strategy, and 1 thread in the midst of inflation.
       
   974          // A more refined approach would be to change the encoding of INFLATING
       
   975          // to allow encapsulation of a native thread pointer.  Threads waiting for
       
   976          // inflation to complete would use CAS to push themselves onto a singly linked
       
   977          // list rooted at the markword.  Once enqueued, they'd loop, checking a per-thread flag
       
   978          // and calling park().  When inflation was complete the thread that accomplished inflation
       
   979          // would detach the list and set the markword to inflated with a single CAS and
       
   980          // then for each thread on the list, set the flag and unpark() the thread.
       
   981          // This is conceptually similar to muxAcquire-muxRelease, except that muxRelease
       
   982          // wakes at most one thread whereas we need to wake the entire list.
       
   983          int ix = (intptr_t(obj) >> 5) & (NINFLATIONLOCKS-1) ;
       
   984          int YieldThenBlock = 0 ;
       
   985          assert (ix >= 0 && ix < NINFLATIONLOCKS, "invariant") ;
       
   986          assert ((NINFLATIONLOCKS & (NINFLATIONLOCKS-1)) == 0, "invariant") ;
       
   987          Thread::muxAcquire (InflationLocks + ix, "InflationLock") ;
       
   988          while (obj->mark() == markOopDesc::INFLATING()) {
       
   989            // Beware: NakedYield() is advisory and has almost no effect on some platforms
       
   990            // so we periodically call Self->_ParkEvent->park(1).
       
   991            // We use a mixed spin/yield/block mechanism.
       
   992            if ((YieldThenBlock++) >= 16) {
       
   993               Thread::current()->_ParkEvent->park(1) ;
       
   994            } else {
       
   995               os::NakedYield() ;
       
   996            }
       
   997          }
       
   998          Thread::muxRelease (InflationLocks + ix ) ;
       
   999          TEVENT (Inflate: INFLATING - yield/park) ;
       
  1000        }
       
  1001     } else {
       
  1002        SpinPause() ;       // SMP-polite spinning
       
  1003     }
       
  1004   }
       
  1005 }
       
  1006 
       
  1007 ObjectMonitor * ATTR ObjectSynchronizer::inflate (Thread * Self, oop object) {
       
  1008   // Inflate mutates the heap ...
       
  1009   // Relaxing assertion for bug 6320749.
       
  1010   assert (Universe::verify_in_progress() ||
       
  1011           !SafepointSynchronize::is_at_safepoint(), "invariant") ;
       
  1012 
       
  1013   for (;;) {
       
  1014       const markOop mark = object->mark() ;
       
  1015       assert (!mark->has_bias_pattern(), "invariant") ;
       
  1016 
       
  1017       // The mark can be in one of the following states:
       
  1018       // *  Inflated     - just return
       
  1019       // *  Stack-locked - coerce it to inflated
       
  1020       // *  INFLATING    - busy wait for conversion to complete
       
  1021       // *  Neutral      - aggressively inflate the object.
       
  1022       // *  BIASED       - Illegal.  We should never see this
       
  1023 
       
  1024       // CASE: inflated
       
  1025       if (mark->has_monitor()) {
       
  1026           ObjectMonitor * inf = mark->monitor() ;
       
  1027           assert (inf->header()->is_neutral(), "invariant");
       
  1028           assert (inf->object() == object, "invariant") ;
       
  1029           assert (ObjectSynchronizer::verify_objmon_isinpool(inf), "monitor is invalid");
       
  1030           return inf ;
       
  1031       }
       
  1032 
       
  1033       // CASE: inflation in progress - inflating over a stack-lock.
       
  1034       // Some other thread is converting from stack-locked to inflated.
       
  1035       // Only that thread can complete inflation -- other threads must wait.
       
  1036       // The INFLATING value is transient.
       
  1037       // Currently, we spin/yield/park and poll the markword, waiting for inflation to finish.
       
  1038       // We could always eliminate polling by parking the thread on some auxiliary list.
       
  1039       if (mark == markOopDesc::INFLATING()) {
       
  1040          TEVENT (Inflate: spin while INFLATING) ;
       
  1041          ReadStableMark(object) ;
       
  1042          continue ;
       
  1043       }
       
  1044 
       
  1045       // CASE: stack-locked
       
  1046       // Could be stack-locked either by this thread or by some other thread.
       
  1047       //
       
  1048       // Note that we allocate the objectmonitor speculatively, _before_ attempting
       
  1049       // to install INFLATING into the mark word.  We originally installed INFLATING,
       
  1050       // allocated the objectmonitor, and then finally STed the address of the
       
  1051       // objectmonitor into the mark.  This was correct, but artificially lengthened
       
  1052       // the interval in which INFLATED appeared in the mark, thus increasing
       
  1053       // the odds of inflation contention.
       
  1054       //
       
  1055       // We now use per-thread private objectmonitor free lists.
       
  1056       // These list are reprovisioned from the global free list outside the
       
  1057       // critical INFLATING...ST interval.  A thread can transfer
       
  1058       // multiple objectmonitors en-mass from the global free list to its local free list.
       
  1059       // This reduces coherency traffic and lock contention on the global free list.
       
  1060       // Using such local free lists, it doesn't matter if the omAlloc() call appears
       
  1061       // before or after the CAS(INFLATING) operation.
       
  1062       // See the comments in omAlloc().
       
  1063 
       
  1064       if (mark->has_locker()) {
       
  1065           ObjectMonitor * m = omAlloc (Self) ;
       
  1066           // Optimistically prepare the objectmonitor - anticipate successful CAS
       
  1067           // We do this before the CAS in order to minimize the length of time
       
  1068           // in which INFLATING appears in the mark.
       
  1069           m->Recycle();
       
  1070           m->FreeNext      = NULL ;
       
  1071           m->_Responsible  = NULL ;
       
  1072           m->OwnerIsThread = 0 ;
       
  1073           m->_recursions   = 0 ;
       
  1074           m->_SpinDuration = Knob_SpinLimit ;   // Consider: maintain by type/class
       
  1075 
       
  1076           markOop cmp = (markOop) Atomic::cmpxchg_ptr (markOopDesc::INFLATING(), object->mark_addr(), mark) ;
       
  1077           if (cmp != mark) {
       
  1078              omRelease (Self, m) ;
       
  1079              continue ;       // Interference -- just retry
       
  1080           }
       
  1081 
       
  1082           // We've successfully installed INFLATING (0) into the mark-word.
       
  1083           // This is the only case where 0 will appear in a mark-work.
       
  1084           // Only the singular thread that successfully swings the mark-word
       
  1085           // to 0 can perform (or more precisely, complete) inflation.
       
  1086           //
       
  1087           // Why do we CAS a 0 into the mark-word instead of just CASing the
       
  1088           // mark-word from the stack-locked value directly to the new inflated state?
       
  1089           // Consider what happens when a thread unlocks a stack-locked object.
       
  1090           // It attempts to use CAS to swing the displaced header value from the
       
  1091           // on-stack basiclock back into the object header.  Recall also that the
       
  1092           // header value (hashcode, etc) can reside in (a) the object header, or
       
  1093           // (b) a displaced header associated with the stack-lock, or (c) a displaced
       
  1094           // header in an objectMonitor.  The inflate() routine must copy the header
       
  1095           // value from the basiclock on the owner's stack to the objectMonitor, all
       
  1096           // the while preserving the hashCode stability invariants.  If the owner
       
  1097           // decides to release the lock while the value is 0, the unlock will fail
       
  1098           // and control will eventually pass from slow_exit() to inflate.  The owner
       
  1099           // will then spin, waiting for the 0 value to disappear.   Put another way,
       
  1100           // the 0 causes the owner to stall if the owner happens to try to
       
  1101           // drop the lock (restoring the header from the basiclock to the object)
       
  1102           // while inflation is in-progress.  This protocol avoids races that might
       
  1103           // would otherwise permit hashCode values to change or "flicker" for an object.
       
  1104           // Critically, while object->mark is 0 mark->displaced_mark_helper() is stable.
       
  1105           // 0 serves as a "BUSY" inflate-in-progress indicator.
       
  1106 
       
  1107 
       
  1108           // fetch the displaced mark from the owner's stack.
       
  1109           // The owner can't die or unwind past the lock while our INFLATING
       
  1110           // object is in the mark.  Furthermore the owner can't complete
       
  1111           // an unlock on the object, either.
       
  1112           markOop dmw = mark->displaced_mark_helper() ;
       
  1113           assert (dmw->is_neutral(), "invariant") ;
       
  1114 
       
  1115           // Setup monitor fields to proper values -- prepare the monitor
       
  1116           m->set_header(dmw) ;
       
  1117 
       
  1118           // Optimization: if the mark->locker stack address is associated
       
  1119           // with this thread we could simply set m->_owner = Self and
       
  1120           // m->OwnerIsThread = 1.  Note that a thread can inflate an object
       
  1121           // that it has stack-locked -- as might happen in wait() -- directly
       
  1122           // with CAS.  That is, we can avoid the xchg-NULL .... ST idiom.
       
  1123           m->set_owner (mark->locker());
       
  1124           m->set_object(object);
       
  1125           // TODO-FIXME: assert BasicLock->dhw != 0.
       
  1126 
       
  1127           // Must preserve store ordering. The monitor state must
       
  1128           // be stable at the time of publishing the monitor address.
       
  1129           guarantee (object->mark() == markOopDesc::INFLATING(), "invariant") ;
       
  1130           object->release_set_mark(markOopDesc::encode(m));
       
  1131 
       
  1132           // Hopefully the performance counters are allocated on distinct cache lines
       
  1133           // to avoid false sharing on MP systems ...
       
  1134           if (_sync_Inflations != NULL) _sync_Inflations->inc() ;
       
  1135           TEVENT(Inflate: overwrite stacklock) ;
       
  1136           if (TraceMonitorInflation) {
       
  1137             if (object->is_instance()) {
       
  1138               ResourceMark rm;
       
  1139               tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
       
  1140                 (intptr_t) object, (intptr_t) object->mark(),
       
  1141                 Klass::cast(object->klass())->external_name());
       
  1142             }
       
  1143           }
       
  1144           return m ;
       
  1145       }
       
  1146 
       
  1147       // CASE: neutral
       
  1148       // TODO-FIXME: for entry we currently inflate and then try to CAS _owner.
       
  1149       // If we know we're inflating for entry it's better to inflate by swinging a
       
  1150       // pre-locked objectMonitor pointer into the object header.   A successful
       
  1151       // CAS inflates the object *and* confers ownership to the inflating thread.
       
  1152       // In the current implementation we use a 2-step mechanism where we CAS()
       
  1153       // to inflate and then CAS() again to try to swing _owner from NULL to Self.
       
  1154       // An inflateTry() method that we could call from fast_enter() and slow_enter()
       
  1155       // would be useful.
       
  1156 
       
  1157       assert (mark->is_neutral(), "invariant");
       
  1158       ObjectMonitor * m = omAlloc (Self) ;
       
  1159       // prepare m for installation - set monitor to initial state
       
  1160       m->Recycle();
       
  1161       m->set_header(mark);
       
  1162       m->set_owner(NULL);
       
  1163       m->set_object(object);
       
  1164       m->OwnerIsThread = 1 ;
       
  1165       m->_recursions   = 0 ;
       
  1166       m->FreeNext      = NULL ;
       
  1167       m->_Responsible  = NULL ;
       
  1168       m->_SpinDuration = Knob_SpinLimit ;       // consider: keep metastats by type/class
       
  1169 
       
  1170       if (Atomic::cmpxchg_ptr (markOopDesc::encode(m), object->mark_addr(), mark) != mark) {
       
  1171           m->set_object (NULL) ;
       
  1172           m->set_owner  (NULL) ;
       
  1173           m->OwnerIsThread = 0 ;
       
  1174           m->Recycle() ;
       
  1175           omRelease (Self, m) ;
       
  1176           m = NULL ;
       
  1177           continue ;
       
  1178           // interference - the markword changed - just retry.
       
  1179           // The state-transitions are one-way, so there's no chance of
       
  1180           // live-lock -- "Inflated" is an absorbing state.
       
  1181       }
       
  1182 
       
  1183       // Hopefully the performance counters are allocated on distinct
       
  1184       // cache lines to avoid false sharing on MP systems ...
       
  1185       if (_sync_Inflations != NULL) _sync_Inflations->inc() ;
       
  1186       TEVENT(Inflate: overwrite neutral) ;
       
  1187       if (TraceMonitorInflation) {
       
  1188         if (object->is_instance()) {
       
  1189           ResourceMark rm;
       
  1190           tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
       
  1191             (intptr_t) object, (intptr_t) object->mark(),
       
  1192             Klass::cast(object->klass())->external_name());
       
  1193         }
       
  1194       }
       
  1195       return m ;
       
  1196   }
       
  1197 }
       
  1198 
       
  1199 
       
  1200 // This the fast monitor enter. The interpreter and compiler use
       
  1201 // some assembly copies of this code. Make sure update those code
       
  1202 // if the following function is changed. The implementation is
       
  1203 // extremely sensitive to race condition. Be careful.
       
  1204 
       
  1205 void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock, bool attempt_rebias, TRAPS) {
       
  1206  if (UseBiasedLocking) {
       
  1207     if (!SafepointSynchronize::is_at_safepoint()) {
       
  1208       BiasedLocking::Condition cond = BiasedLocking::revoke_and_rebias(obj, attempt_rebias, THREAD);
       
  1209       if (cond == BiasedLocking::BIAS_REVOKED_AND_REBIASED) {
       
  1210         return;
       
  1211       }
       
  1212     } else {
       
  1213       assert(!attempt_rebias, "can not rebias toward VM thread");
       
  1214       BiasedLocking::revoke_at_safepoint(obj);
       
  1215     }
       
  1216     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
       
  1217   }
       
  1218 
       
  1219   THREAD->update_highest_lock((address)lock);
       
  1220   slow_enter (obj, lock, THREAD) ;
       
  1221 }
       
  1222 
       
  1223 void ObjectSynchronizer::fast_exit(oop object, BasicLock* lock, TRAPS) {
       
  1224   assert(!object->mark()->has_bias_pattern(), "should not see bias pattern here");
       
  1225   // if displaced header is null, the previous enter is recursive enter, no-op
       
  1226   markOop dhw = lock->displaced_header();
       
  1227   markOop mark ;
       
  1228   if (dhw == NULL) {
       
  1229      // Recursive stack-lock.
       
  1230      // Diagnostics -- Could be: stack-locked, inflating, inflated.
       
  1231      mark = object->mark() ;
       
  1232      assert (!mark->is_neutral(), "invariant") ;
       
  1233      if (mark->has_locker() && mark != markOopDesc::INFLATING()) {
       
  1234         assert(THREAD->is_lock_owned((address)mark->locker()), "invariant") ;
       
  1235      }
       
  1236      if (mark->has_monitor()) {
       
  1237         ObjectMonitor * m = mark->monitor() ;
       
  1238         assert(((oop)(m->object()))->mark() == mark, "invariant") ;
       
  1239         assert(m->is_entered(THREAD), "invariant") ;
       
  1240      }
       
  1241      return ;
       
  1242   }
       
  1243 
       
  1244   mark = object->mark() ;
       
  1245 
       
  1246   // If the object is stack-locked by the current thread, try to
       
  1247   // swing the displaced header from the box back to the mark.
       
  1248   if (mark == (markOop) lock) {
       
  1249      assert (dhw->is_neutral(), "invariant") ;
       
  1250      if ((markOop) Atomic::cmpxchg_ptr (dhw, object->mark_addr(), mark) == mark) {
       
  1251         TEVENT (fast_exit: release stacklock) ;
       
  1252         return;
       
  1253      }
       
  1254   }
       
  1255 
       
  1256   ObjectSynchronizer::inflate(THREAD, object)->exit (THREAD) ;
       
  1257 }
       
  1258 
       
  1259 // This routine is used to handle interpreter/compiler slow case
       
  1260 // We don't need to use fast path here, because it must have been
       
  1261 // failed in the interpreter/compiler code.
       
  1262 void ObjectSynchronizer::slow_enter(Handle obj, BasicLock* lock, TRAPS) {
       
  1263   markOop mark = obj->mark();
       
  1264   assert(!mark->has_bias_pattern(), "should not see bias pattern here");
       
  1265 
       
  1266   if (mark->is_neutral()) {
       
  1267     // Anticipate successful CAS -- the ST of the displaced mark must
       
  1268     // be visible <= the ST performed by the CAS.
       
  1269     lock->set_displaced_header(mark);
       
  1270     if (mark == (markOop) Atomic::cmpxchg_ptr(lock, obj()->mark_addr(), mark)) {
       
  1271       TEVENT (slow_enter: release stacklock) ;
       
  1272       return ;
       
  1273     }
       
  1274     // Fall through to inflate() ...
       
  1275   } else
       
  1276   if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
       
  1277     assert(lock != mark->locker(), "must not re-lock the same lock");
       
  1278     assert(lock != (BasicLock*)obj->mark(), "don't relock with same BasicLock");
       
  1279     lock->set_displaced_header(NULL);
       
  1280     return;
       
  1281   }
       
  1282 
       
  1283 #if 0
       
  1284   // The following optimization isn't particularly useful.
       
  1285   if (mark->has_monitor() && mark->monitor()->is_entered(THREAD)) {
       
  1286     lock->set_displaced_header (NULL) ;
       
  1287     return ;
       
  1288   }
       
  1289 #endif
       
  1290 
       
  1291   // The object header will never be displaced to this lock,
       
  1292   // so it does not matter what the value is, except that it
       
  1293   // must be non-zero to avoid looking like a re-entrant lock,
       
  1294   // and must not look locked either.
       
  1295   lock->set_displaced_header(markOopDesc::unused_mark());
       
  1296   ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);
       
  1297 }
       
  1298 
       
  1299 // This routine is used to handle interpreter/compiler slow case
       
  1300 // We don't need to use fast path here, because it must have
       
  1301 // failed in the interpreter/compiler code. Simply use the heavy
       
  1302 // weight monitor should be ok, unless someone find otherwise.
       
  1303 void ObjectSynchronizer::slow_exit(oop object, BasicLock* lock, TRAPS) {
       
  1304   fast_exit (object, lock, THREAD) ;
       
  1305 }
       
  1306 
       
  1307 // NOTE: must use heavy weight monitor to handle jni monitor enter
       
  1308 void ObjectSynchronizer::jni_enter(Handle obj, TRAPS) { // possible entry from jni enter
       
  1309   // the current locking is from JNI instead of Java code
       
  1310   TEVENT (jni_enter) ;
       
  1311   if (UseBiasedLocking) {
       
  1312     BiasedLocking::revoke_and_rebias(obj, false, THREAD);
       
  1313     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
       
  1314   }
       
  1315   THREAD->set_current_pending_monitor_is_from_java(false);
       
  1316   ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);
       
  1317   THREAD->set_current_pending_monitor_is_from_java(true);
       
  1318 }
       
  1319 
       
  1320 // NOTE: must use heavy weight monitor to handle jni monitor enter
       
  1321 bool ObjectSynchronizer::jni_try_enter(Handle obj, Thread* THREAD) {
       
  1322   if (UseBiasedLocking) {
       
  1323     BiasedLocking::revoke_and_rebias(obj, false, THREAD);
       
  1324     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
       
  1325   }
       
  1326 
       
  1327   ObjectMonitor* monitor = ObjectSynchronizer::inflate_helper(obj());
       
  1328   return monitor->try_enter(THREAD);
       
  1329 }
       
  1330 
       
  1331 
       
  1332 // NOTE: must use heavy weight monitor to handle jni monitor exit
       
  1333 void ObjectSynchronizer::jni_exit(oop obj, Thread* THREAD) {
       
  1334   TEVENT (jni_exit) ;
       
  1335   if (UseBiasedLocking) {
       
  1336     BiasedLocking::revoke_and_rebias(obj, false, THREAD);
       
  1337   }
       
  1338   assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
       
  1339 
       
  1340   ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj);
       
  1341   // If this thread has locked the object, exit the monitor.  Note:  can't use
       
  1342   // monitor->check(CHECK); must exit even if an exception is pending.
       
  1343   if (monitor->check(THREAD)) {
       
  1344      monitor->exit(THREAD);
       
  1345   }
       
  1346 }
       
  1347 
       
  1348 // complete_exit()/reenter() are used to wait on a nested lock
       
  1349 // i.e. to give up an outer lock completely and then re-enter
       
  1350 // Used when holding nested locks - lock acquisition order: lock1 then lock2
       
  1351 //  1) complete_exit lock1 - saving recursion count
       
  1352 //  2) wait on lock2
       
  1353 //  3) when notified on lock2, unlock lock2
       
  1354 //  4) reenter lock1 with original recursion count
       
  1355 //  5) lock lock2
       
  1356 // NOTE: must use heavy weight monitor to handle complete_exit/reenter()
       
  1357 intptr_t ObjectSynchronizer::complete_exit(Handle obj, TRAPS) {
       
  1358   TEVENT (complete_exit) ;
       
  1359   if (UseBiasedLocking) {
       
  1360     BiasedLocking::revoke_and_rebias(obj, false, THREAD);
       
  1361     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
       
  1362   }
       
  1363 
       
  1364   ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
       
  1365 
       
  1366   return monitor->complete_exit(THREAD);
       
  1367 }
       
  1368 
       
  1369 // NOTE: must use heavy weight monitor to handle complete_exit/reenter()
       
  1370 void ObjectSynchronizer::reenter(Handle obj, intptr_t recursion, TRAPS) {
       
  1371   TEVENT (reenter) ;
       
  1372   if (UseBiasedLocking) {
       
  1373     BiasedLocking::revoke_and_rebias(obj, false, THREAD);
       
  1374     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
       
  1375   }
       
  1376 
       
  1377   ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
       
  1378 
       
  1379   monitor->reenter(recursion, THREAD);
       
  1380 }
       
  1381 
       
  1382 // This exists only as a workaround of dtrace bug 6254741
       
  1383 int dtrace_waited_probe(ObjectMonitor* monitor, Handle obj, Thread* thr) {
       
  1384   DTRACE_MONITOR_PROBE(waited, monitor, obj(), thr);
       
  1385   return 0;
       
  1386 }
       
  1387 
       
  1388 // NOTE: must use heavy weight monitor to handle wait()
       
  1389 void ObjectSynchronizer::wait(Handle obj, jlong millis, TRAPS) {
       
  1390   if (UseBiasedLocking) {
       
  1391     BiasedLocking::revoke_and_rebias(obj, false, THREAD);
       
  1392     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
       
  1393   }
       
  1394   if (millis < 0) {
       
  1395     TEVENT (wait - throw IAX) ;
       
  1396     THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
       
  1397   }
       
  1398   ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
       
  1399   DTRACE_MONITOR_WAIT_PROBE(monitor, obj(), THREAD, millis);
       
  1400   monitor->wait(millis, true, THREAD);
       
  1401 
       
  1402   /* This dummy call is in place to get around dtrace bug 6254741.  Once
       
  1403      that's fixed we can uncomment the following line and remove the call */
       
  1404   // DTRACE_MONITOR_PROBE(waited, monitor, obj(), THREAD);
       
  1405   dtrace_waited_probe(monitor, obj, THREAD);
       
  1406 }
       
  1407 
       
  1408 void ObjectSynchronizer::waitUninterruptibly (Handle obj, jlong millis, TRAPS) {
       
  1409   if (UseBiasedLocking) {
       
  1410     BiasedLocking::revoke_and_rebias(obj, false, THREAD);
       
  1411     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
       
  1412   }
       
  1413   if (millis < 0) {
       
  1414     TEVENT (wait - throw IAX) ;
       
  1415     THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
       
  1416   }
       
  1417   ObjectSynchronizer::inflate(THREAD, obj()) -> wait(millis, false, THREAD) ;
       
  1418 }
       
  1419 
       
  1420 void ObjectSynchronizer::notify(Handle obj, TRAPS) {
       
  1421  if (UseBiasedLocking) {
       
  1422     BiasedLocking::revoke_and_rebias(obj, false, THREAD);
       
  1423     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
       
  1424   }
       
  1425 
       
  1426   markOop mark = obj->mark();
       
  1427   if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
       
  1428     return;
       
  1429   }
       
  1430   ObjectSynchronizer::inflate(THREAD, obj())->notify(THREAD);
       
  1431 }
       
  1432 
       
  1433 // NOTE: see comment of notify()
       
  1434 void ObjectSynchronizer::notifyall(Handle obj, TRAPS) {
       
  1435   if (UseBiasedLocking) {
       
  1436     BiasedLocking::revoke_and_rebias(obj, false, THREAD);
       
  1437     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
       
  1438   }
       
  1439 
       
  1440   markOop mark = obj->mark();
       
  1441   if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
       
  1442     return;
       
  1443   }
       
  1444   ObjectSynchronizer::inflate(THREAD, obj())->notifyAll(THREAD);
       
  1445 }
       
  1446 
       
  1447 intptr_t ObjectSynchronizer::FastHashCode (Thread * Self, oop obj) {
       
  1448   if (UseBiasedLocking) {
       
  1449     // NOTE: many places throughout the JVM do not expect a safepoint
       
  1450     // to be taken here, in particular most operations on perm gen
       
  1451     // objects. However, we only ever bias Java instances and all of
       
  1452     // the call sites of identity_hash that might revoke biases have
       
  1453     // been checked to make sure they can handle a safepoint. The
       
  1454     // added check of the bias pattern is to avoid useless calls to
       
  1455     // thread-local storage.
       
  1456     if (obj->mark()->has_bias_pattern()) {
       
  1457       // Box and unbox the raw reference just in case we cause a STW safepoint.
       
  1458       Handle hobj (Self, obj) ;
       
  1459       // Relaxing assertion for bug 6320749.
       
  1460       assert (Universe::verify_in_progress() ||
       
  1461               !SafepointSynchronize::is_at_safepoint(),
       
  1462              "biases should not be seen by VM thread here");
       
  1463       BiasedLocking::revoke_and_rebias(hobj, false, JavaThread::current());
       
  1464       obj = hobj() ;
       
  1465       assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
       
  1466     }
       
  1467   }
       
  1468 
       
  1469   // hashCode() is a heap mutator ...
       
  1470   // Relaxing assertion for bug 6320749.
       
  1471   assert (Universe::verify_in_progress() ||
       
  1472           !SafepointSynchronize::is_at_safepoint(), "invariant") ;
       
  1473   assert (Universe::verify_in_progress() ||
       
  1474           Self->is_Java_thread() , "invariant") ;
       
  1475   assert (Universe::verify_in_progress() ||
       
  1476          ((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;
       
  1477 
       
  1478   ObjectMonitor* monitor = NULL;
       
  1479   markOop temp, test;
       
  1480   intptr_t hash;
       
  1481   markOop mark = ReadStableMark (obj);
       
  1482 
       
  1483   // object should remain ineligible for biased locking
       
  1484   assert (!mark->has_bias_pattern(), "invariant") ;
       
  1485 
       
  1486   if (mark->is_neutral()) {
       
  1487     hash = mark->hash();              // this is a normal header
       
  1488     if (hash) {                       // if it has hash, just return it
       
  1489       return hash;
       
  1490     }
       
  1491     hash = get_next_hash(Self, obj);  // allocate a new hash code
       
  1492     temp = mark->copy_set_hash(hash); // merge the hash code into header
       
  1493     // use (machine word version) atomic operation to install the hash
       
  1494     test = (markOop) Atomic::cmpxchg_ptr(temp, obj->mark_addr(), mark);
       
  1495     if (test == mark) {
       
  1496       return hash;
       
  1497     }
       
  1498     // If atomic operation failed, we must inflate the header
       
  1499     // into heavy weight monitor. We could add more code here
       
  1500     // for fast path, but it does not worth the complexity.
       
  1501   } else if (mark->has_monitor()) {
       
  1502     monitor = mark->monitor();
       
  1503     temp = monitor->header();
       
  1504     assert (temp->is_neutral(), "invariant") ;
       
  1505     hash = temp->hash();
       
  1506     if (hash) {
       
  1507       return hash;
       
  1508     }
       
  1509     // Skip to the following code to reduce code size
       
  1510   } else if (Self->is_lock_owned((address)mark->locker())) {
       
  1511     temp = mark->displaced_mark_helper(); // this is a lightweight monitor owned
       
  1512     assert (temp->is_neutral(), "invariant") ;
       
  1513     hash = temp->hash();              // by current thread, check if the displaced
       
  1514     if (hash) {                       // header contains hash code
       
  1515       return hash;
       
  1516     }
       
  1517     // WARNING:
       
  1518     //   The displaced header is strictly immutable.
       
  1519     // It can NOT be changed in ANY cases. So we have
       
  1520     // to inflate the header into heavyweight monitor
       
  1521     // even the current thread owns the lock. The reason
       
  1522     // is the BasicLock (stack slot) will be asynchronously
       
  1523     // read by other threads during the inflate() function.
       
  1524     // Any change to stack may not propagate to other threads
       
  1525     // correctly.
       
  1526   }
       
  1527 
       
  1528   // Inflate the monitor to set hash code
       
  1529   monitor = ObjectSynchronizer::inflate(Self, obj);
       
  1530   // Load displaced header and check it has hash code
       
  1531   mark = monitor->header();
       
  1532   assert (mark->is_neutral(), "invariant") ;
       
  1533   hash = mark->hash();
       
  1534   if (hash == 0) {
       
  1535     hash = get_next_hash(Self, obj);
       
  1536     temp = mark->copy_set_hash(hash); // merge hash code into header
       
  1537     assert (temp->is_neutral(), "invariant") ;
       
  1538     test = (markOop) Atomic::cmpxchg_ptr(temp, monitor, mark);
       
  1539     if (test != mark) {
       
  1540       // The only update to the header in the monitor (outside GC)
       
  1541       // is install the hash code. If someone add new usage of
       
  1542       // displaced header, please update this code
       
  1543       hash = test->hash();
       
  1544       assert (test->is_neutral(), "invariant") ;
       
  1545       assert (hash != 0, "Trivial unexpected object/monitor header usage.");
       
  1546     }
       
  1547   }
       
  1548   // We finally get the hash
       
  1549   return hash;
       
  1550 }
       
  1551 
       
  1552 // Deprecated -- use FastHashCode() instead.
       
  1553 
       
  1554 intptr_t ObjectSynchronizer::identity_hash_value_for(Handle obj) {
       
  1555   return FastHashCode (Thread::current(), obj()) ;
       
  1556 }
       
  1557 
       
  1558 bool ObjectSynchronizer::current_thread_holds_lock(JavaThread* thread,
       
  1559                                                    Handle h_obj) {
       
  1560   if (UseBiasedLocking) {
       
  1561     BiasedLocking::revoke_and_rebias(h_obj, false, thread);
       
  1562     assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
       
  1563   }
       
  1564 
       
  1565   assert(thread == JavaThread::current(), "Can only be called on current thread");
       
  1566   oop obj = h_obj();
       
  1567 
       
  1568   markOop mark = ReadStableMark (obj) ;
       
  1569 
       
  1570   // Uncontended case, header points to stack
       
  1571   if (mark->has_locker()) {
       
  1572     return thread->is_lock_owned((address)mark->locker());
       
  1573   }
       
  1574   // Contended case, header points to ObjectMonitor (tagged pointer)
       
  1575   if (mark->has_monitor()) {
       
  1576     ObjectMonitor* monitor = mark->monitor();
       
  1577     return monitor->is_entered(thread) != 0 ;
       
  1578   }
       
  1579   // Unlocked case, header in place
       
  1580   assert(mark->is_neutral(), "sanity check");
       
  1581   return false;
       
  1582 }
       
  1583 
       
  1584 // Be aware of this method could revoke bias of the lock object.
       
  1585 // This method querys the ownership of the lock handle specified by 'h_obj'.
       
  1586 // If the current thread owns the lock, it returns owner_self. If no
       
  1587 // thread owns the lock, it returns owner_none. Otherwise, it will return
       
  1588 // ower_other.
       
  1589 ObjectSynchronizer::LockOwnership ObjectSynchronizer::query_lock_ownership
       
  1590 (JavaThread *self, Handle h_obj) {
       
  1591   // The caller must beware this method can revoke bias, and
       
  1592   // revocation can result in a safepoint.
       
  1593   assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
       
  1594   assert (self->thread_state() != _thread_blocked , "invariant") ;
       
  1595 
       
  1596   // Possible mark states: neutral, biased, stack-locked, inflated
       
  1597 
       
  1598   if (UseBiasedLocking && h_obj()->mark()->has_bias_pattern()) {
       
  1599     // CASE: biased
       
  1600     BiasedLocking::revoke_and_rebias(h_obj, false, self);
       
  1601     assert(!h_obj->mark()->has_bias_pattern(),
       
  1602            "biases should be revoked by now");
       
  1603   }
       
  1604 
       
  1605   assert(self == JavaThread::current(), "Can only be called on current thread");
       
  1606   oop obj = h_obj();
       
  1607   markOop mark = ReadStableMark (obj) ;
       
  1608 
       
  1609   // CASE: stack-locked.  Mark points to a BasicLock on the owner's stack.
       
  1610   if (mark->has_locker()) {
       
  1611     return self->is_lock_owned((address)mark->locker()) ?
       
  1612       owner_self : owner_other;
       
  1613   }
       
  1614 
       
  1615   // CASE: inflated. Mark (tagged pointer) points to an objectMonitor.
       
  1616   // The Object:ObjectMonitor relationship is stable as long as we're
       
  1617   // not at a safepoint.
       
  1618   if (mark->has_monitor()) {
       
  1619     void * owner = mark->monitor()->_owner ;
       
  1620     if (owner == NULL) return owner_none ;
       
  1621     return (owner == self ||
       
  1622             self->is_lock_owned((address)owner)) ? owner_self : owner_other;
       
  1623   }
       
  1624 
       
  1625   // CASE: neutral
       
  1626   assert(mark->is_neutral(), "sanity check");
       
  1627   return owner_none ;           // it's unlocked
       
  1628 }
       
  1629 
       
  1630 // FIXME: jvmti should call this
       
  1631 JavaThread* ObjectSynchronizer::get_lock_owner(Handle h_obj, bool doLock) {
       
  1632   if (UseBiasedLocking) {
       
  1633     if (SafepointSynchronize::is_at_safepoint()) {
       
  1634       BiasedLocking::revoke_at_safepoint(h_obj);
       
  1635     } else {
       
  1636       BiasedLocking::revoke_and_rebias(h_obj, false, JavaThread::current());
       
  1637     }
       
  1638     assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
       
  1639   }
       
  1640 
       
  1641   oop obj = h_obj();
       
  1642   address owner = NULL;
       
  1643 
       
  1644   markOop mark = ReadStableMark (obj) ;
       
  1645 
       
  1646   // Uncontended case, header points to stack
       
  1647   if (mark->has_locker()) {
       
  1648     owner = (address) mark->locker();
       
  1649   }
       
  1650 
       
  1651   // Contended case, header points to ObjectMonitor (tagged pointer)
       
  1652   if (mark->has_monitor()) {
       
  1653     ObjectMonitor* monitor = mark->monitor();
       
  1654     assert(monitor != NULL, "monitor should be non-null");
       
  1655     owner = (address) monitor->owner();
       
  1656   }
       
  1657 
       
  1658   if (owner != NULL) {
       
  1659     return Threads::owning_thread_from_monitor_owner(owner, doLock);
       
  1660   }
       
  1661 
       
  1662   // Unlocked case, header in place
       
  1663   // Cannot have assertion since this object may have been
       
  1664   // locked by another thread when reaching here.
       
  1665   // assert(mark->is_neutral(), "sanity check");
       
  1666 
       
  1667   return NULL;
       
  1668 }
       
  1669 
       
  1670 // Iterate through monitor cache and attempt to release thread's monitors
       
  1671 // Gives up on a particular monitor if an exception occurs, but continues
       
  1672 // the overall iteration, swallowing the exception.
       
  1673 class ReleaseJavaMonitorsClosure: public MonitorClosure {
       
  1674 private:
       
  1675   TRAPS;
       
  1676 
       
  1677 public:
       
  1678   ReleaseJavaMonitorsClosure(Thread* thread) : THREAD(thread) {}
       
  1679   void do_monitor(ObjectMonitor* mid) {
       
  1680     if (mid->owner() == THREAD) {
       
  1681       (void)mid->complete_exit(CHECK);
       
  1682     }
       
  1683   }
       
  1684 };
       
  1685 
       
  1686 // Release all inflated monitors owned by THREAD.  Lightweight monitors are
       
  1687 // ignored.  This is meant to be called during JNI thread detach which assumes
       
  1688 // all remaining monitors are heavyweight.  All exceptions are swallowed.
       
  1689 // Scanning the extant monitor list can be time consuming.
       
  1690 // A simple optimization is to add a per-thread flag that indicates a thread
       
  1691 // called jni_monitorenter() during its lifetime.
       
  1692 //
       
  1693 // Instead of No_Savepoint_Verifier it might be cheaper to
       
  1694 // use an idiom of the form:
       
  1695 //   auto int tmp = SafepointSynchronize::_safepoint_counter ;
       
  1696 //   <code that must not run at safepoint>
       
  1697 //   guarantee (((tmp ^ _safepoint_counter) | (tmp & 1)) == 0) ;
       
  1698 // Since the tests are extremely cheap we could leave them enabled
       
  1699 // for normal product builds.
       
  1700 
       
  1701 void ObjectSynchronizer::release_monitors_owned_by_thread(TRAPS) {
       
  1702   assert(THREAD == JavaThread::current(), "must be current Java thread");
       
  1703   No_Safepoint_Verifier nsv ;
       
  1704   ReleaseJavaMonitorsClosure rjmc(THREAD);
       
  1705   Thread::muxAcquire(&ListLock, "release_monitors_owned_by_thread");
       
  1706   ObjectSynchronizer::monitors_iterate(&rjmc);
       
  1707   Thread::muxRelease(&ListLock);
       
  1708   THREAD->clear_pending_exception();
       
  1709 }
       
  1710 
       
  1711 // Visitors ...
       
  1712 
       
  1713 void ObjectSynchronizer::monitors_iterate(MonitorClosure* closure) {
       
  1714   ObjectMonitor* block = gBlockList;
       
  1715   ObjectMonitor* mid;
       
  1716   while (block) {
       
  1717     assert(block->object() == CHAINMARKER, "must be a block header");
       
  1718     for (int i = _BLOCKSIZE - 1; i > 0; i--) {
       
  1719       mid = block + i;
       
  1720       oop object = (oop) mid->object();
       
  1721       if (object != NULL) {
       
  1722         closure->do_monitor(mid);
       
  1723       }
       
  1724     }
       
  1725     block = (ObjectMonitor*) block->FreeNext;
       
  1726   }
       
  1727 }
       
  1728 
       
  1729 void ObjectSynchronizer::oops_do(OopClosure* f) {
       
  1730   assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
       
  1731   for (ObjectMonitor* block = gBlockList; block != NULL; block = next(block)) {
       
  1732     assert(block->object() == CHAINMARKER, "must be a block header");
       
  1733     for (int i = 1; i < _BLOCKSIZE; i++) {
       
  1734       ObjectMonitor* mid = &block[i];
       
  1735       if (mid->object() != NULL) {
       
  1736         f->do_oop((oop*)mid->object_addr());
       
  1737       }
       
  1738     }
       
  1739   }
       
  1740 }
       
  1741 
       
  1742 // Deflate_idle_monitors() is called at all safepoints, immediately
       
  1743 // after all mutators are stopped, but before any objects have moved.
       
  1744 // It traverses the list of known monitors, deflating where possible.
       
  1745 // The scavenged monitor are returned to the monitor free list.
       
  1746 //
       
  1747 // Beware that we scavenge at *every* stop-the-world point.
       
  1748 // Having a large number of monitors in-circulation negatively
       
  1749 // impacts the performance of some applications (e.g., PointBase).
       
  1750 // Broadly, we want to minimize the # of monitors in circulation.
       
  1751 // Alternately, we could partition the active monitors into sub-lists
       
  1752 // of those that need scanning and those that do not.
       
  1753 // Specifically, we would add a new sub-list of objectmonitors
       
  1754 // that are in-circulation and potentially active.  deflate_idle_monitors()
       
  1755 // would scan only that list.  Other monitors could reside on a quiescent
       
  1756 // list.  Such sequestered monitors wouldn't need to be scanned by
       
  1757 // deflate_idle_monitors().  omAlloc() would first check the global free list,
       
  1758 // then the quiescent list, and, failing those, would allocate a new block.
       
  1759 // Deflate_idle_monitors() would scavenge and move monitors to the
       
  1760 // quiescent list.
       
  1761 //
       
  1762 // Perversely, the heap size -- and thus the STW safepoint rate --
       
  1763 // typically drives the scavenge rate.  Large heaps can mean infrequent GC,
       
  1764 // which in turn can mean large(r) numbers of objectmonitors in circulation.
       
  1765 // This is an unfortunate aspect of this design.
       
  1766 //
       
  1767 // Another refinement would be to refrain from calling deflate_idle_monitors()
       
  1768 // except at stop-the-world points associated with garbage collections.
       
  1769 //
       
  1770 // An even better solution would be to deflate on-the-fly, aggressively,
       
  1771 // at monitorexit-time as is done in EVM's metalock or Relaxed Locks.
       
  1772 
       
  1773 void ObjectSynchronizer::deflate_idle_monitors() {
       
  1774   assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
       
  1775   int nInuse = 0 ;              // currently associated with objects
       
  1776   int nInCirculation = 0 ;      // extant
       
  1777   int nScavenged = 0 ;          // reclaimed
       
  1778 
       
  1779   ObjectMonitor * FreeHead = NULL ;  // Local SLL of scavenged monitors
       
  1780   ObjectMonitor * FreeTail = NULL ;
       
  1781 
       
  1782   // Iterate over all extant monitors - Scavenge all idle monitors.
       
  1783   TEVENT (deflate_idle_monitors) ;
       
  1784   for (ObjectMonitor* block = gBlockList; block != NULL; block = next(block)) {
       
  1785     assert(block->object() == CHAINMARKER, "must be a block header");
       
  1786     nInCirculation += _BLOCKSIZE ;
       
  1787     for (int i = 1 ; i < _BLOCKSIZE; i++) {
       
  1788       ObjectMonitor* mid = &block[i];
       
  1789       oop obj = (oop) mid->object();
       
  1790 
       
  1791       if (obj == NULL) {
       
  1792         // The monitor is not associated with an object.
       
  1793         // The monitor should either be a thread-specific private
       
  1794         // free list or the global free list.
       
  1795         // obj == NULL IMPLIES mid->is_busy() == 0
       
  1796         guarantee (!mid->is_busy(), "invariant") ;
       
  1797         continue ;
       
  1798       }
       
  1799 
       
  1800       // Normal case ... The monitor is associated with obj.
       
  1801       guarantee (obj->mark() == markOopDesc::encode(mid), "invariant") ;
       
  1802       guarantee (mid == obj->mark()->monitor(), "invariant");
       
  1803       guarantee (mid->header()->is_neutral(), "invariant");
       
  1804 
       
  1805       if (mid->is_busy()) {
       
  1806          if (ClearResponsibleAtSTW) mid->_Responsible = NULL ;
       
  1807          nInuse ++ ;
       
  1808       } else {
       
  1809          // Deflate the monitor if it is no longer being used
       
  1810          // It's idle - scavenge and return to the global free list
       
  1811          // plain old deflation ...
       
  1812          TEVENT (deflate_idle_monitors - scavenge1) ;
       
  1813          if (TraceMonitorInflation) {
       
  1814            if (obj->is_instance()) {
       
  1815              ResourceMark rm;
       
  1816                tty->print_cr("Deflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
       
  1817                     (intptr_t) obj, (intptr_t) obj->mark(), Klass::cast(obj->klass())->external_name());
       
  1818            }
       
  1819          }
       
  1820 
       
  1821          // Restore the header back to obj
       
  1822          obj->release_set_mark(mid->header());
       
  1823          mid->clear();
       
  1824 
       
  1825          assert (mid->object() == NULL, "invariant") ;
       
  1826 
       
  1827          // Move the object to the working free list defined by FreeHead,FreeTail.
       
  1828          mid->FreeNext = NULL ;
       
  1829          if (FreeHead == NULL) FreeHead = mid ;
       
  1830          if (FreeTail != NULL) FreeTail->FreeNext = mid ;
       
  1831          FreeTail = mid ;
       
  1832          nScavenged ++ ;
       
  1833       }
       
  1834     }
       
  1835   }
       
  1836 
       
  1837   // Move the scavenged monitors back to the global free list.
       
  1838   // In theory we don't need the freelist lock as we're at a STW safepoint.
       
  1839   // omAlloc() and omFree() can only be called while a thread is _not in safepoint state.
       
  1840   // But it's remotely possible that omFlush() or release_monitors_owned_by_thread()
       
  1841   // might be called while not at a global STW safepoint.  In the interest of
       
  1842   // safety we protect the following access with ListLock.
       
  1843   // An even more conservative and prudent approach would be to guard
       
  1844   // the main loop in scavenge_idle_monitors() with ListLock.
       
  1845   if (FreeHead != NULL) {
       
  1846      guarantee (FreeTail != NULL && nScavenged > 0, "invariant") ;
       
  1847      assert (FreeTail->FreeNext == NULL, "invariant") ;
       
  1848      // constant-time list splice - prepend scavenged segment to gFreeList
       
  1849      Thread::muxAcquire (&ListLock, "scavenge - return") ;
       
  1850      FreeTail->FreeNext = gFreeList ;
       
  1851      gFreeList = FreeHead ;
       
  1852      Thread::muxRelease (&ListLock) ;
       
  1853   }
       
  1854 
       
  1855   if (_sync_Deflations != NULL) _sync_Deflations->inc(nScavenged) ;
       
  1856   if (_sync_MonExtant  != NULL) _sync_MonExtant ->set_value(nInCirculation);
       
  1857 
       
  1858   // TODO: Add objectMonitor leak detection.
       
  1859   // Audit/inventory the objectMonitors -- make sure they're all accounted for.
       
  1860   GVars.stwRandom = os::random() ;
       
  1861   GVars.stwCycle ++ ;
       
  1862 }
       
  1863 
       
  1864 // A macro is used below because there may already be a pending
       
  1865 // exception which should not abort the execution of the routines
       
  1866 // which use this (which is why we don't put this into check_slow and
       
  1867 // call it with a CHECK argument).
       
  1868 
       
  1869 #define CHECK_OWNER()                                                             \
       
  1870   do {                                                                            \
       
  1871     if (THREAD != _owner) {                                                       \
       
  1872       if (THREAD->is_lock_owned((address) _owner)) {                              \
       
  1873         _owner = THREAD ;  /* Convert from basiclock addr to Thread addr */       \
       
  1874         _recursions = 0;                                                          \
       
  1875         OwnerIsThread = 1 ;                                                       \
       
  1876       } else {                                                                    \
       
  1877         TEVENT (Throw IMSX) ;                                                     \
       
  1878         THROW(vmSymbols::java_lang_IllegalMonitorStateException());               \
       
  1879       }                                                                           \
       
  1880     }                                                                             \
       
  1881   } while (false)
       
  1882 
       
  1883 // TODO-FIXME: eliminate ObjectWaiters.  Replace this visitor/enumerator
       
  1884 // interface with a simple FirstWaitingThread(), NextWaitingThread() interface.
       
  1885 
       
  1886 ObjectWaiter* ObjectMonitor::first_waiter() {
       
  1887   return _WaitSet;
       
  1888 }
       
  1889 
       
  1890 ObjectWaiter* ObjectMonitor::next_waiter(ObjectWaiter* o) {
       
  1891   return o->_next;
       
  1892 }
       
  1893 
       
  1894 Thread* ObjectMonitor::thread_of_waiter(ObjectWaiter* o) {
       
  1895   return o->_thread;
       
  1896 }
       
  1897 
       
  1898 // initialize the monitor, exception the semaphore, all other fields
       
  1899 // are simple integers or pointers
       
  1900 ObjectMonitor::ObjectMonitor() {
       
  1901   _header       = NULL;
       
  1902   _count        = 0;
       
  1903   _waiters      = 0,
       
  1904   _recursions   = 0;
       
  1905   _object       = NULL;
       
  1906   _owner        = NULL;
       
  1907   _WaitSet      = NULL;
       
  1908   _WaitSetLock  = 0 ;
       
  1909   _Responsible  = NULL ;
       
  1910   _succ         = NULL ;
       
  1911   _cxq          = NULL ;
       
  1912   FreeNext      = NULL ;
       
  1913   _EntryList    = NULL ;
       
  1914   _SpinFreq     = 0 ;
       
  1915   _SpinClock    = 0 ;
       
  1916   OwnerIsThread = 0 ;
       
  1917 }
       
  1918 
       
  1919 ObjectMonitor::~ObjectMonitor() {
       
  1920    // TODO: Add asserts ...
       
  1921    // _cxq == 0 _succ == NULL _owner == NULL _waiters == 0
       
  1922    // _count == 0 _EntryList  == NULL etc
       
  1923 }
       
  1924 
       
  1925 intptr_t ObjectMonitor::is_busy() const {
       
  1926   // TODO-FIXME: merge _count and _waiters.
       
  1927   // TODO-FIXME: assert _owner == null implies _recursions = 0
       
  1928   // TODO-FIXME: assert _WaitSet != null implies _count > 0
       
  1929   return _count|_waiters|intptr_t(_owner)|intptr_t(_cxq)|intptr_t(_EntryList ) ;
       
  1930 }
       
  1931 
       
  1932 void ObjectMonitor::Recycle () {
       
  1933   // TODO: add stronger asserts ...
       
  1934   // _cxq == 0 _succ == NULL _owner == NULL _waiters == 0
       
  1935   // _count == 0 EntryList  == NULL
       
  1936   // _recursions == 0 _WaitSet == NULL
       
  1937   // TODO: assert (is_busy()|_recursions) == 0
       
  1938   _succ          = NULL ;
       
  1939   _EntryList     = NULL ;
       
  1940   _cxq           = NULL ;
       
  1941   _WaitSet       = NULL ;
       
  1942   _recursions    = 0 ;
       
  1943   _SpinFreq      = 0 ;
       
  1944   _SpinClock     = 0 ;
       
  1945   OwnerIsThread  = 0 ;
       
  1946 }
       
  1947 
       
  1948 // WaitSet management ...
       
  1949 
       
  1950 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
       
  1951   assert(node != NULL, "should not dequeue NULL node");
       
  1952   assert(node->_prev == NULL, "node already in list");
       
  1953   assert(node->_next == NULL, "node already in list");
       
  1954   // put node at end of queue (circular doubly linked list)
       
  1955   if (_WaitSet == NULL) {
       
  1956     _WaitSet = node;
       
  1957     node->_prev = node;
       
  1958     node->_next = node;
       
  1959   } else {
       
  1960     ObjectWaiter* head = _WaitSet ;
       
  1961     ObjectWaiter* tail = head->_prev;
       
  1962     assert(tail->_next == head, "invariant check");
       
  1963     tail->_next = node;
       
  1964     head->_prev = node;
       
  1965     node->_next = head;
       
  1966     node->_prev = tail;
       
  1967   }
       
  1968 }
       
  1969 
       
  1970 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
       
  1971   // dequeue the very first waiter
       
  1972   ObjectWaiter* waiter = _WaitSet;
       
  1973   if (waiter) {
       
  1974     DequeueSpecificWaiter(waiter);
       
  1975   }
       
  1976   return waiter;
       
  1977 }
       
  1978 
       
  1979 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
       
  1980   assert(node != NULL, "should not dequeue NULL node");
       
  1981   assert(node->_prev != NULL, "node already removed from list");
       
  1982   assert(node->_next != NULL, "node already removed from list");
       
  1983   // when the waiter has woken up because of interrupt,
       
  1984   // timeout or other spurious wake-up, dequeue the
       
  1985   // waiter from waiting list
       
  1986   ObjectWaiter* next = node->_next;
       
  1987   if (next == node) {
       
  1988     assert(node->_prev == node, "invariant check");
       
  1989     _WaitSet = NULL;
       
  1990   } else {
       
  1991     ObjectWaiter* prev = node->_prev;
       
  1992     assert(prev->_next == node, "invariant check");
       
  1993     assert(next->_prev == node, "invariant check");
       
  1994     next->_prev = prev;
       
  1995     prev->_next = next;
       
  1996     if (_WaitSet == node) {
       
  1997       _WaitSet = next;
       
  1998     }
       
  1999   }
       
  2000   node->_next = NULL;
       
  2001   node->_prev = NULL;
       
  2002 }
       
  2003 
       
  2004 static char * kvGet (char * kvList, const char * Key) {
       
  2005     if (kvList == NULL) return NULL ;
       
  2006     size_t n = strlen (Key) ;
       
  2007     char * Search ;
       
  2008     for (Search = kvList ; *Search ; Search += strlen(Search) + 1) {
       
  2009         if (strncmp (Search, Key, n) == 0) {
       
  2010             if (Search[n] == '=') return Search + n + 1 ;
       
  2011             if (Search[n] == 0)   return (char *) "1" ;
       
  2012         }
       
  2013     }
       
  2014     return NULL ;
       
  2015 }
       
  2016 
       
  2017 static int kvGetInt (char * kvList, const char * Key, int Default) {
       
  2018     char * v = kvGet (kvList, Key) ;
       
  2019     int rslt = v ? ::strtol (v, NULL, 0) : Default ;
       
  2020     if (Knob_ReportSettings && v != NULL) {
       
  2021         ::printf ("  SyncKnob: %s %d(%d)\n", Key, rslt, Default) ;
       
  2022         ::fflush (stdout) ;
       
  2023     }
       
  2024     return rslt ;
       
  2025 }
       
  2026 
       
  2027 // By convention we unlink a contending thread from EntryList|cxq immediately
       
  2028 // after the thread acquires the lock in ::enter().  Equally, we could defer
       
  2029 // unlinking the thread until ::exit()-time.
       
  2030 
       
  2031 void ObjectMonitor::UnlinkAfterAcquire (Thread * Self, ObjectWaiter * SelfNode)
       
  2032 {
       
  2033     assert (_owner == Self, "invariant") ;
       
  2034     assert (SelfNode->_thread == Self, "invariant") ;
       
  2035 
       
  2036     if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
       
  2037         // Normal case: remove Self from the DLL EntryList .
       
  2038         // This is a constant-time operation.
       
  2039         ObjectWaiter * nxt = SelfNode->_next ;
       
  2040         ObjectWaiter * prv = SelfNode->_prev ;
       
  2041         if (nxt != NULL) nxt->_prev = prv ;
       
  2042         if (prv != NULL) prv->_next = nxt ;
       
  2043         if (SelfNode == _EntryList ) _EntryList = nxt ;
       
  2044         assert (nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant") ;
       
  2045         assert (prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant") ;
       
  2046         TEVENT (Unlink from EntryList) ;
       
  2047     } else {
       
  2048         guarantee (SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant") ;
       
  2049         // Inopportune interleaving -- Self is still on the cxq.
       
  2050         // This usually means the enqueue of self raced an exiting thread.
       
  2051         // Normally we'll find Self near the front of the cxq, so
       
  2052         // dequeueing is typically fast.  If needbe we can accelerate
       
  2053         // this with some MCS/CHL-like bidirectional list hints and advisory
       
  2054         // back-links so dequeueing from the interior will normally operate
       
  2055         // in constant-time.
       
  2056         // Dequeue Self from either the head (with CAS) or from the interior
       
  2057         // with a linear-time scan and normal non-atomic memory operations.
       
  2058         // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
       
  2059         // and then unlink Self from EntryList.  We have to drain eventually,
       
  2060         // so it might as well be now.
       
  2061 
       
  2062         ObjectWaiter * v = _cxq ;
       
  2063         assert (v != NULL, "invariant") ;
       
  2064         if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) {
       
  2065             // The CAS above can fail from interference IFF a "RAT" arrived.
       
  2066             // In that case Self must be in the interior and can no longer be
       
  2067             // at the head of cxq.
       
  2068             if (v == SelfNode) {
       
  2069                 assert (_cxq != v, "invariant") ;
       
  2070                 v = _cxq ;          // CAS above failed - start scan at head of list
       
  2071             }
       
  2072             ObjectWaiter * p ;
       
  2073             ObjectWaiter * q = NULL ;
       
  2074             for (p = v ; p != NULL && p != SelfNode; p = p->_next) {
       
  2075                 q = p ;
       
  2076                 assert (p->TState == ObjectWaiter::TS_CXQ, "invariant") ;
       
  2077             }
       
  2078             assert (v != SelfNode,  "invariant") ;
       
  2079             assert (p == SelfNode,  "Node not found on cxq") ;
       
  2080             assert (p != _cxq,      "invariant") ;
       
  2081             assert (q != NULL,      "invariant") ;
       
  2082             assert (q->_next == p,  "invariant") ;
       
  2083             q->_next = p->_next ;
       
  2084         }
       
  2085         TEVENT (Unlink from cxq) ;
       
  2086     }
       
  2087 
       
  2088     // Diagnostic hygiene ...
       
  2089     SelfNode->_prev  = (ObjectWaiter *) 0xBAD ;
       
  2090     SelfNode->_next  = (ObjectWaiter *) 0xBAD ;
       
  2091     SelfNode->TState = ObjectWaiter::TS_RUN ;
       
  2092 }
       
  2093 
       
  2094 // Caveat: TryLock() is not necessarily serializing if it returns failure.
       
  2095 // Callers must compensate as needed.
       
  2096 
       
  2097 int ObjectMonitor::TryLock (Thread * Self) {
       
  2098    for (;;) {
       
  2099       void * own = _owner ;
       
  2100       if (own != NULL) return 0 ;
       
  2101       if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
       
  2102          // Either guarantee _recursions == 0 or set _recursions = 0.
       
  2103          assert (_recursions == 0, "invariant") ;
       
  2104          assert (_owner == Self, "invariant") ;
       
  2105          // CONSIDER: set or assert that OwnerIsThread == 1
       
  2106          return 1 ;
       
  2107       }
       
  2108       // The lock had been free momentarily, but we lost the race to the lock.
       
  2109       // Interference -- the CAS failed.
       
  2110       // We can either return -1 or retry.
       
  2111       // Retry doesn't make as much sense because the lock was just acquired.
       
  2112       if (true) return -1 ;
       
  2113    }
       
  2114 }
       
  2115 
       
  2116 // NotRunnable() -- informed spinning
       
  2117 //
       
  2118 // Don't bother spinning if the owner is not eligible to drop the lock.
       
  2119 // Peek at the owner's schedctl.sc_state and Thread._thread_values and
       
  2120 // spin only if the owner thread is _thread_in_Java or _thread_in_vm.
       
  2121 // The thread must be runnable in order to drop the lock in timely fashion.
       
  2122 // If the _owner is not runnable then spinning will not likely be
       
  2123 // successful (profitable).
       
  2124 //
       
  2125 // Beware -- the thread referenced by _owner could have died
       
  2126 // so a simply fetch from _owner->_thread_state might trap.
       
  2127 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
       
  2128 // Because of the lifecycle issues the schedctl and _thread_state values
       
  2129 // observed by NotRunnable() might be garbage.  NotRunnable must
       
  2130 // tolerate this and consider the observed _thread_state value
       
  2131 // as advisory.
       
  2132 //
       
  2133 // Beware too, that _owner is sometimes a BasicLock address and sometimes
       
  2134 // a thread pointer.  We differentiate the two cases with OwnerIsThread.
       
  2135 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
       
  2136 // with the LSB of _owner.  Another option would be to probablistically probe
       
  2137 // the putative _owner->TypeTag value.
       
  2138 //
       
  2139 // Checking _thread_state isn't perfect.  Even if the thread is
       
  2140 // in_java it might be blocked on a page-fault or have been preempted
       
  2141 // and sitting on a ready/dispatch queue.  _thread state in conjunction
       
  2142 // with schedctl.sc_state gives us a good picture of what the
       
  2143 // thread is doing, however.
       
  2144 //
       
  2145 // TODO: check schedctl.sc_state.
       
  2146 // We'll need to use SafeFetch32() to read from the schedctl block.
       
  2147 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/
       
  2148 //
       
  2149 // The return value from NotRunnable() is *advisory* -- the
       
  2150 // result is based on sampling and is not necessarily coherent.
       
  2151 // The caller must tolerate false-negative and false-positive errors.
       
  2152 // Spinning, in general, is probabilistic anyway.
       
  2153 
       
  2154 
       
  2155 int ObjectMonitor::NotRunnable (Thread * Self, Thread * ox) {
       
  2156     // Check either OwnerIsThread or ox->TypeTag == 2BAD.
       
  2157     if (!OwnerIsThread) return 0 ;
       
  2158 
       
  2159     if (ox == NULL) return 0 ;
       
  2160 
       
  2161     // Avoid transitive spinning ...
       
  2162     // Say T1 spins or blocks trying to acquire L.  T1._Stalled is set to L.
       
  2163     // Immediately after T1 acquires L it's possible that T2, also
       
  2164     // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
       
  2165     // This occurs transiently after T1 acquired L but before
       
  2166     // T1 managed to clear T1.Stalled.  T2 does not need to abort
       
  2167     // its spin in this circumstance.
       
  2168     intptr_t BlockedOn = SafeFetchN ((intptr_t *) &ox->_Stalled, intptr_t(1)) ;
       
  2169 
       
  2170     if (BlockedOn == 1) return 1 ;
       
  2171     if (BlockedOn != 0) {
       
  2172       return BlockedOn != intptr_t(this) && _owner == ox ;
       
  2173     }
       
  2174 
       
  2175     assert (sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant") ;
       
  2176     int jst = SafeFetch32 ((int *) &((JavaThread *) ox)->_thread_state, -1) ; ;
       
  2177     // consider also: jst != _thread_in_Java -- but that's overspecific.
       
  2178     return jst == _thread_blocked || jst == _thread_in_native ;
       
  2179 }
       
  2180 
       
  2181 
       
  2182 // Adaptive spin-then-block - rational spinning
       
  2183 //
       
  2184 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
       
  2185 // algorithm.  On high order SMP systems it would be better to start with
       
  2186 // a brief global spin and then revert to spinning locally.  In the spirit of MCS/CLH,
       
  2187 // a contending thread could enqueue itself on the cxq and then spin locally
       
  2188 // on a thread-specific variable such as its ParkEvent._Event flag.
       
  2189 // That's left as an exercise for the reader.  Note that global spinning is
       
  2190 // not problematic on Niagara, as the L2$ serves the interconnect and has both
       
  2191 // low latency and massive bandwidth.
       
  2192 //
       
  2193 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
       
  2194 // acquisition attempts where we opt to spin --  at 100% and vary the spin count
       
  2195 // (duration) or we can fix the count at approximately the duration of
       
  2196 // a context switch and vary the frequency.   Of course we could also
       
  2197 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
       
  2198 // See http://j2se.east/~dice/PERSIST/040824-AdaptiveSpinning.html.
       
  2199 //
       
  2200 // This implementation varies the duration "D", where D varies with
       
  2201 // the success rate of recent spin attempts. (D is capped at approximately
       
  2202 // length of a round-trip context switch).  The success rate for recent
       
  2203 // spin attempts is a good predictor of the success rate of future spin
       
  2204 // attempts.  The mechanism adapts automatically to varying critical
       
  2205 // section length (lock modality), system load and degree of parallelism.
       
  2206 // D is maintained per-monitor in _SpinDuration and is initialized
       
  2207 // optimistically.  Spin frequency is fixed at 100%.
       
  2208 //
       
  2209 // Note that _SpinDuration is volatile, but we update it without locks
       
  2210 // or atomics.  The code is designed so that _SpinDuration stays within
       
  2211 // a reasonable range even in the presence of races.  The arithmetic
       
  2212 // operations on _SpinDuration are closed over the domain of legal values,
       
  2213 // so at worst a race will install and older but still legal value.
       
  2214 // At the very worst this introduces some apparent non-determinism.
       
  2215 // We might spin when we shouldn't or vice-versa, but since the spin
       
  2216 // count are relatively short, even in the worst case, the effect is harmless.
       
  2217 //
       
  2218 // Care must be taken that a low "D" value does not become an
       
  2219 // an absorbing state.  Transient spinning failures -- when spinning
       
  2220 // is overall profitable -- should not cause the system to converge
       
  2221 // on low "D" values.  We want spinning to be stable and predictable
       
  2222 // and fairly responsive to change and at the same time we don't want
       
  2223 // it to oscillate, become metastable, be "too" non-deterministic,
       
  2224 // or converge on or enter undesirable stable absorbing states.
       
  2225 //
       
  2226 // We implement a feedback-based control system -- using past behavior
       
  2227 // to predict future behavior.  We face two issues: (a) if the
       
  2228 // input signal is random then the spin predictor won't provide optimal
       
  2229 // results, and (b) if the signal frequency is too high then the control
       
  2230 // system, which has some natural response lag, will "chase" the signal.
       
  2231 // (b) can arise from multimodal lock hold times.  Transient preemption
       
  2232 // can also result in apparent bimodal lock hold times.
       
  2233 // Although sub-optimal, neither condition is particularly harmful, as
       
  2234 // in the worst-case we'll spin when we shouldn't or vice-versa.
       
  2235 // The maximum spin duration is rather short so the failure modes aren't bad.
       
  2236 // To be conservative, I've tuned the gain in system to bias toward
       
  2237 // _not spinning.  Relatedly, the system can sometimes enter a mode where it
       
  2238 // "rings" or oscillates between spinning and not spinning.  This happens
       
  2239 // when spinning is just on the cusp of profitability, however, so the
       
  2240 // situation is not dire.  The state is benign -- there's no need to add
       
  2241 // hysteresis control to damp the transition rate between spinning and
       
  2242 // not spinning.
       
  2243 //
       
  2244 // - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
       
  2245 //
       
  2246 // Spin-then-block strategies ...
       
  2247 //
       
  2248 // Thoughts on ways to improve spinning :
       
  2249 //
       
  2250 // *  Periodically call {psr_}getloadavg() while spinning, and
       
  2251 //    permit unbounded spinning if the load average is <
       
  2252 //    the number of processors.  Beware, however, that getloadavg()
       
  2253 //    is exceptionally fast on solaris (about 1/10 the cost of a full
       
  2254 //    spin cycle, but quite expensive on linux.  Beware also, that
       
  2255 //    multiple JVMs could "ring" or oscillate in a feedback loop.
       
  2256 //    Sufficient damping would solve that problem.
       
  2257 //
       
  2258 // *  We currently use spin loops with iteration counters to approximate
       
  2259 //    spinning for some interval.  Given the availability of high-precision
       
  2260 //    time sources such as gethrtime(), %TICK, %STICK, RDTSC, etc., we should
       
  2261 //    someday reimplement the spin loops to duration-based instead of iteration-based.
       
  2262 //
       
  2263 // *  Don't spin if there are more than N = (CPUs/2) threads
       
  2264 //        currently spinning on the monitor (or globally).
       
  2265 //    That is, limit the number of concurrent spinners.
       
  2266 //    We might also limit the # of spinners in the JVM, globally.
       
  2267 //
       
  2268 // *  If a spinning thread observes _owner change hands it should
       
  2269 //    abort the spin (and park immediately) or at least debit
       
  2270 //    the spin counter by a large "penalty".
       
  2271 //
       
  2272 // *  Classically, the spin count is either K*(CPUs-1) or is a
       
  2273 //        simple constant that approximates the length of a context switch.
       
  2274 //    We currently use a value -- computed by a special utility -- that
       
  2275 //    approximates round-trip context switch times.
       
  2276 //
       
  2277 // *  Normally schedctl_start()/_stop() is used to advise the kernel
       
  2278 //    to avoid preempting threads that are running in short, bounded
       
  2279 //    critical sections.  We could use the schedctl hooks in an inverted
       
  2280 //    sense -- spinners would set the nopreempt flag, but poll the preempt
       
  2281 //    pending flag.  If a spinner observed a pending preemption it'd immediately
       
  2282 //    abort the spin and park.   As such, the schedctl service acts as
       
  2283 //    a preemption warning mechanism.
       
  2284 //
       
  2285 // *  In lieu of spinning, if the system is running below saturation
       
  2286 //    (that is, loadavg() << #cpus), we can instead suppress futile
       
  2287 //    wakeup throttling, or even wake more than one successor at exit-time.
       
  2288 //    The net effect is largely equivalent to spinning.  In both cases,
       
  2289 //    contending threads go ONPROC and opportunistically attempt to acquire
       
  2290 //    the lock, decreasing lock handover latency at the expense of wasted
       
  2291 //    cycles and context switching.
       
  2292 //
       
  2293 // *  We might to spin less after we've parked as the thread will
       
  2294 //    have less $ and TLB affinity with the processor.
       
  2295 //    Likewise, we might spin less if we come ONPROC on a different
       
  2296 //    processor or after a long period (>> rechose_interval).
       
  2297 //
       
  2298 // *  A table-driven state machine similar to Solaris' dispadmin scheduling
       
  2299 //    tables might be a better design.  Instead of encoding information in
       
  2300 //    _SpinDuration, _SpinFreq and _SpinClock we'd just use explicit,
       
  2301 //    discrete states.   Success or failure during a spin would drive
       
  2302 //    state transitions, and each state node would contain a spin count.
       
  2303 //
       
  2304 // *  If the processor is operating in a mode intended to conserve power
       
  2305 //    (such as Intel's SpeedStep) or to reduce thermal output (thermal
       
  2306 //    step-down mode) then the Java synchronization subsystem should
       
  2307 //    forgo spinning.
       
  2308 //
       
  2309 // *  The minimum spin duration should be approximately the worst-case
       
  2310 //    store propagation latency on the platform.  That is, the time
       
  2311 //    it takes a store on CPU A to become visible on CPU B, where A and
       
  2312 //    B are "distant".
       
  2313 //
       
  2314 // *  We might want to factor a thread's priority in the spin policy.
       
  2315 //    Threads with a higher priority might spin for slightly longer.
       
  2316 //    Similarly, if we use back-off in the TATAS loop, lower priority
       
  2317 //    threads might back-off longer.  We don't currently use a
       
  2318 //    thread's priority when placing it on the entry queue.  We may
       
  2319 //    want to consider doing so in future releases.
       
  2320 //
       
  2321 // *  We might transiently drop a thread's scheduling priority while it spins.
       
  2322 //    SCHED_BATCH on linux and FX scheduling class at priority=0 on Solaris
       
  2323 //    would suffice.  We could even consider letting the thread spin indefinitely at
       
  2324 //    a depressed or "idle" priority.  This brings up fairness issues, however --
       
  2325 //    in a saturated system a thread would with a reduced priority could languish
       
  2326 //    for extended periods on the ready queue.
       
  2327 //
       
  2328 // *  While spinning try to use the otherwise wasted time to help the VM make
       
  2329 //    progress:
       
  2330 //
       
  2331 //    -- YieldTo() the owner, if the owner is OFFPROC but ready
       
  2332 //       Done our remaining quantum directly to the ready thread.
       
  2333 //       This helps "push" the lock owner through the critical section.
       
  2334 //       It also tends to improve affinity/locality as the lock
       
  2335 //       "migrates" less frequently between CPUs.
       
  2336 //    -- Walk our own stack in anticipation of blocking.  Memoize the roots.
       
  2337 //    -- Perform strand checking for other thread.  Unpark potential strandees.
       
  2338 //    -- Help GC: trace or mark -- this would need to be a bounded unit of work.
       
  2339 //       Unfortunately this will pollute our $ and TLBs.  Recall that we
       
  2340 //       spin to avoid context switching -- context switching has an
       
  2341 //       immediate cost in latency, a disruptive cost to other strands on a CMT
       
  2342 //       processor, and an amortized cost because of the D$ and TLB cache
       
  2343 //       reload transient when the thread comes back ONPROC and repopulates
       
  2344 //       $s and TLBs.
       
  2345 //    -- call getloadavg() to see if the system is saturated.  It'd probably
       
  2346 //       make sense to call getloadavg() half way through the spin.
       
  2347 //       If the system isn't at full capacity the we'd simply reset
       
  2348 //       the spin counter to and extend the spin attempt.
       
  2349 //    -- Doug points out that we should use the same "helping" policy
       
  2350 //       in thread.yield().
       
  2351 //
       
  2352 // *  Try MONITOR-MWAIT on systems that support those instructions.
       
  2353 //
       
  2354 // *  The spin statistics that drive spin decisions & frequency are
       
  2355 //    maintained in the objectmonitor structure so if we deflate and reinflate
       
  2356 //    we lose spin state.  In practice this is not usually a concern
       
  2357 //    as the default spin state after inflation is aggressive (optimistic)
       
  2358 //    and tends toward spinning.  So in the worst case for a lock where
       
  2359 //    spinning is not profitable we may spin unnecessarily for a brief
       
  2360 //    period.  But then again, if a lock is contended it'll tend not to deflate
       
  2361 //    in the first place.
       
  2362 
       
  2363 
       
  2364 intptr_t ObjectMonitor::SpinCallbackArgument = 0 ;
       
  2365 int (*ObjectMonitor::SpinCallbackFunction)(intptr_t, int) = NULL ;
       
  2366 
       
  2367 // Spinning: Fixed frequency (100%), vary duration
       
  2368 
       
  2369 int ObjectMonitor::TrySpin_VaryDuration (Thread * Self) {
       
  2370 
       
  2371     // Dumb, brutal spin.  Good for comparative measurements against adaptive spinning.
       
  2372     int ctr = Knob_FixedSpin ;
       
  2373     if (ctr != 0) {
       
  2374         while (--ctr >= 0) {
       
  2375             if (TryLock (Self) > 0) return 1 ;
       
  2376             SpinPause () ;
       
  2377         }
       
  2378         return 0 ;
       
  2379     }
       
  2380 
       
  2381     for (ctr = Knob_PreSpin + 1; --ctr >= 0 ; ) {
       
  2382       if (TryLock(Self) > 0) {
       
  2383         // Increase _SpinDuration ...
       
  2384         // Note that we don't clamp SpinDuration precisely at SpinLimit.
       
  2385         // Raising _SpurDuration to the poverty line is key.
       
  2386         int x = _SpinDuration ;
       
  2387         if (x < Knob_SpinLimit) {
       
  2388            if (x < Knob_Poverty) x = Knob_Poverty ;
       
  2389            _SpinDuration = x + Knob_BonusB ;
       
  2390         }
       
  2391         return 1 ;
       
  2392       }
       
  2393       SpinPause () ;
       
  2394     }
       
  2395 
       
  2396     // Admission control - verify preconditions for spinning
       
  2397     //
       
  2398     // We always spin a little bit, just to prevent _SpinDuration == 0 from
       
  2399     // becoming an absorbing state.  Put another way, we spin briefly to
       
  2400     // sample, just in case the system load, parallelism, contention, or lock
       
  2401     // modality changed.
       
  2402     //
       
  2403     // Consider the following alternative:
       
  2404     // Periodically set _SpinDuration = _SpinLimit and try a long/full
       
  2405     // spin attempt.  "Periodically" might mean after a tally of
       
  2406     // the # of failed spin attempts (or iterations) reaches some threshold.
       
  2407     // This takes us into the realm of 1-out-of-N spinning, where we
       
  2408     // hold the duration constant but vary the frequency.
       
  2409 
       
  2410     ctr = _SpinDuration  ;
       
  2411     if (ctr < Knob_SpinBase) ctr = Knob_SpinBase ;
       
  2412     if (ctr <= 0) return 0 ;
       
  2413 
       
  2414     if (Knob_SuccRestrict && _succ != NULL) return 0 ;
       
  2415     if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
       
  2416        TEVENT (Spin abort - notrunnable [TOP]);
       
  2417        return 0 ;
       
  2418     }
       
  2419 
       
  2420     int MaxSpin = Knob_MaxSpinners ;
       
  2421     if (MaxSpin >= 0) {
       
  2422        if (_Spinner > MaxSpin) {
       
  2423           TEVENT (Spin abort -- too many spinners) ;
       
  2424           return 0 ;
       
  2425        }
       
  2426        // Slighty racy, but benign ...
       
  2427        Adjust (&_Spinner, 1) ;
       
  2428     }
       
  2429 
       
  2430     // We're good to spin ... spin ingress.
       
  2431     // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
       
  2432     // when preparing to LD...CAS _owner, etc and the CAS is likely
       
  2433     // to succeed.
       
  2434     int hits    = 0 ;
       
  2435     int msk     = 0 ;
       
  2436     int caspty  = Knob_CASPenalty ;
       
  2437     int oxpty   = Knob_OXPenalty ;
       
  2438     int sss     = Knob_SpinSetSucc ;
       
  2439     if (sss && _succ == NULL ) _succ = Self ;
       
  2440     Thread * prv = NULL ;
       
  2441 
       
  2442     // There are three ways to exit the following loop:
       
  2443     // 1.  A successful spin where this thread has acquired the lock.
       
  2444     // 2.  Spin failure with prejudice
       
  2445     // 3.  Spin failure without prejudice
       
  2446 
       
  2447     while (--ctr >= 0) {
       
  2448 
       
  2449       // Periodic polling -- Check for pending GC
       
  2450       // Threads may spin while they're unsafe.
       
  2451       // We don't want spinning threads to delay the JVM from reaching
       
  2452       // a stop-the-world safepoint or to steal cycles from GC.
       
  2453       // If we detect a pending safepoint we abort in order that
       
  2454       // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
       
  2455       // this thread, if safe, doesn't steal cycles from GC.
       
  2456       // This is in keeping with the "no loitering in runtime" rule.
       
  2457       // We periodically check to see if there's a safepoint pending.
       
  2458       if ((ctr & 0xFF) == 0) {
       
  2459          if (SafepointSynchronize::do_call_back()) {
       
  2460             TEVENT (Spin: safepoint) ;
       
  2461             goto Abort ;           // abrupt spin egress
       
  2462          }
       
  2463          if (Knob_UsePause & 1) SpinPause () ;
       
  2464 
       
  2465          int (*scb)(intptr_t,int) = SpinCallbackFunction ;
       
  2466          if (hits > 50 && scb != NULL) {
       
  2467             int abend = (*scb)(SpinCallbackArgument, 0) ;
       
  2468          }
       
  2469       }
       
  2470 
       
  2471       if (Knob_UsePause & 2) SpinPause() ;
       
  2472 
       
  2473       // Exponential back-off ...  Stay off the bus to reduce coherency traffic.
       
  2474       // This is useful on classic SMP systems, but is of less utility on
       
  2475       // N1-style CMT platforms.
       
  2476       //
       
  2477       // Trade-off: lock acquisition latency vs coherency bandwidth.
       
  2478       // Lock hold times are typically short.  A histogram
       
  2479       // of successful spin attempts shows that we usually acquire
       
  2480       // the lock early in the spin.  That suggests we want to
       
  2481       // sample _owner frequently in the early phase of the spin,
       
  2482       // but then back-off and sample less frequently as the spin
       
  2483       // progresses.  The back-off makes a good citizen on SMP big
       
  2484       // SMP systems.  Oversampling _owner can consume excessive
       
  2485       // coherency bandwidth.  Relatedly, if we _oversample _owner we
       
  2486       // can inadvertently interfere with the the ST m->owner=null.
       
  2487       // executed by the lock owner.
       
  2488       if (ctr & msk) continue ;
       
  2489       ++hits ;
       
  2490       if ((hits & 0xF) == 0) {
       
  2491         // The 0xF, above, corresponds to the exponent.
       
  2492         // Consider: (msk+1)|msk
       
  2493         msk = ((msk << 2)|3) & BackOffMask ;
       
  2494       }
       
  2495 
       
  2496       // Probe _owner with TATAS
       
  2497       // If this thread observes the monitor transition or flicker
       
  2498       // from locked to unlocked to locked, then the odds that this
       
  2499       // thread will acquire the lock in this spin attempt go down
       
  2500       // considerably.  The same argument applies if the CAS fails
       
  2501       // or if we observe _owner change from one non-null value to
       
  2502       // another non-null value.   In such cases we might abort
       
  2503       // the spin without prejudice or apply a "penalty" to the
       
  2504       // spin count-down variable "ctr", reducing it by 100, say.
       
  2505 
       
  2506       Thread * ox = (Thread *) _owner ;
       
  2507       if (ox == NULL) {
       
  2508          ox = (Thread *) Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
       
  2509          if (ox == NULL) {
       
  2510             // The CAS succeeded -- this thread acquired ownership
       
  2511             // Take care of some bookkeeping to exit spin state.
       
  2512             if (sss && _succ == Self) {
       
  2513                _succ = NULL ;
       
  2514             }
       
  2515             if (MaxSpin > 0) Adjust (&_Spinner, -1) ;
       
  2516 
       
  2517             // Increase _SpinDuration :
       
  2518             // The spin was successful (profitable) so we tend toward
       
  2519             // longer spin attempts in the future.
       
  2520             // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
       
  2521             // If we acquired the lock early in the spin cycle it
       
  2522             // makes sense to increase _SpinDuration proportionally.
       
  2523             // Note that we don't clamp SpinDuration precisely at SpinLimit.
       
  2524             int x = _SpinDuration ;
       
  2525             if (x < Knob_SpinLimit) {
       
  2526                 if (x < Knob_Poverty) x = Knob_Poverty ;
       
  2527                 _SpinDuration = x + Knob_Bonus ;
       
  2528             }
       
  2529             return 1 ;
       
  2530          }
       
  2531 
       
  2532          // The CAS failed ... we can take any of the following actions:
       
  2533          // * penalize: ctr -= Knob_CASPenalty
       
  2534          // * exit spin with prejudice -- goto Abort;
       
  2535          // * exit spin without prejudice.
       
  2536          // * Since CAS is high-latency, retry again immediately.
       
  2537          prv = ox ;
       
  2538          TEVENT (Spin: cas failed) ;
       
  2539          if (caspty == -2) break ;
       
  2540          if (caspty == -1) goto Abort ;
       
  2541          ctr -= caspty ;
       
  2542          continue ;
       
  2543       }
       
  2544 
       
  2545       // Did lock ownership change hands ?
       
  2546       if (ox != prv && prv != NULL ) {
       
  2547           TEVENT (spin: Owner changed)
       
  2548           if (oxpty == -2) break ;
       
  2549           if (oxpty == -1) goto Abort ;
       
  2550           ctr -= oxpty ;
       
  2551       }
       
  2552       prv = ox ;
       
  2553 
       
  2554       // Abort the spin if the owner is not executing.
       
  2555       // The owner must be executing in order to drop the lock.
       
  2556       // Spinning while the owner is OFFPROC is idiocy.
       
  2557       // Consider: ctr -= RunnablePenalty ;
       
  2558       if (Knob_OState && NotRunnable (Self, ox)) {
       
  2559          TEVENT (Spin abort - notrunnable);
       
  2560          goto Abort ;
       
  2561       }
       
  2562       if (sss && _succ == NULL ) _succ = Self ;
       
  2563    }
       
  2564 
       
  2565    // Spin failed with prejudice -- reduce _SpinDuration.
       
  2566    // TODO: Use an AIMD-like policy to adjust _SpinDuration.
       
  2567    // AIMD is globally stable.
       
  2568    TEVENT (Spin failure) ;
       
  2569    {
       
  2570      int x = _SpinDuration ;
       
  2571      if (x > 0) {
       
  2572         // Consider an AIMD scheme like: x -= (x >> 3) + 100
       
  2573         // This is globally sample and tends to damp the response.
       
  2574         x -= Knob_Penalty ;
       
  2575         if (x < 0) x = 0 ;
       
  2576         _SpinDuration = x ;
       
  2577      }
       
  2578    }
       
  2579 
       
  2580  Abort:
       
  2581    if (MaxSpin >= 0) Adjust (&_Spinner, -1) ;
       
  2582    if (sss && _succ == Self) {
       
  2583       _succ = NULL ;
       
  2584       // Invariant: after setting succ=null a contending thread
       
  2585       // must recheck-retry _owner before parking.  This usually happens
       
  2586       // in the normal usage of TrySpin(), but it's safest
       
  2587       // to make TrySpin() as foolproof as possible.
       
  2588       OrderAccess::fence() ;
       
  2589       if (TryLock(Self) > 0) return 1 ;
       
  2590    }
       
  2591    return 0 ;
       
  2592 }
       
  2593 
       
  2594 #define TrySpin TrySpin_VaryDuration
       
  2595 
       
  2596 static void DeferredInitialize () {
       
  2597   if (InitDone > 0) return ;
       
  2598   if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
       
  2599       while (InitDone != 1) ;
       
  2600       return ;
       
  2601   }
       
  2602 
       
  2603   // One-shot global initialization ...
       
  2604   // The initialization is idempotent, so we don't need locks.
       
  2605   // In the future consider doing this via os::init_2().
       
  2606   // SyncKnobs consist of <Key>=<Value> pairs in the style
       
  2607   // of environment variables.  Start by converting ':' to NUL.
       
  2608 
       
  2609   if (SyncKnobs == NULL) SyncKnobs = "" ;
       
  2610 
       
  2611   size_t sz = strlen (SyncKnobs) ;
       
  2612   char * knobs = (char *) malloc (sz + 2) ;
       
  2613   if (knobs == NULL) {
       
  2614      vm_exit_out_of_memory (sz + 2, "Parse SyncKnobs") ;
       
  2615      guarantee (0, "invariant") ;
       
  2616   }
       
  2617   strcpy (knobs, SyncKnobs) ;
       
  2618   knobs[sz+1] = 0 ;
       
  2619   for (char * p = knobs ; *p ; p++) {
       
  2620      if (*p == ':') *p = 0 ;
       
  2621   }
       
  2622 
       
  2623   #define SETKNOB(x) { Knob_##x = kvGetInt (knobs, #x, Knob_##x); }
       
  2624   SETKNOB(ReportSettings) ;
       
  2625   SETKNOB(Verbose) ;
       
  2626   SETKNOB(FixedSpin) ;
       
  2627   SETKNOB(SpinLimit) ;
       
  2628   SETKNOB(SpinBase) ;
       
  2629   SETKNOB(SpinBackOff);
       
  2630   SETKNOB(CASPenalty) ;
       
  2631   SETKNOB(OXPenalty) ;
       
  2632   SETKNOB(LogSpins) ;
       
  2633   SETKNOB(SpinSetSucc) ;
       
  2634   SETKNOB(SuccEnabled) ;
       
  2635   SETKNOB(SuccRestrict) ;
       
  2636   SETKNOB(Penalty) ;
       
  2637   SETKNOB(Bonus) ;
       
  2638   SETKNOB(BonusB) ;
       
  2639   SETKNOB(Poverty) ;
       
  2640   SETKNOB(SpinAfterFutile) ;
       
  2641   SETKNOB(UsePause) ;
       
  2642   SETKNOB(SpinEarly) ;
       
  2643   SETKNOB(OState) ;
       
  2644   SETKNOB(MaxSpinners) ;
       
  2645   SETKNOB(PreSpin) ;
       
  2646   SETKNOB(ExitPolicy) ;
       
  2647   SETKNOB(QMode);
       
  2648   SETKNOB(ResetEvent) ;
       
  2649   SETKNOB(MoveNotifyee) ;
       
  2650   SETKNOB(FastHSSEC) ;
       
  2651   #undef SETKNOB
       
  2652 
       
  2653   if (os::is_MP()) {
       
  2654      BackOffMask = (1 << Knob_SpinBackOff) - 1 ;
       
  2655      if (Knob_ReportSettings) ::printf ("BackOffMask=%X\n", BackOffMask) ;
       
  2656      // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)
       
  2657   } else {
       
  2658      Knob_SpinLimit = 0 ;
       
  2659      Knob_SpinBase  = 0 ;
       
  2660      Knob_PreSpin   = 0 ;
       
  2661      Knob_FixedSpin = -1 ;
       
  2662   }
       
  2663 
       
  2664   if (Knob_LogSpins == 0) {
       
  2665      ObjectSynchronizer::_sync_FailedSpins = NULL ;
       
  2666   }
       
  2667 
       
  2668   free (knobs) ;
       
  2669   OrderAccess::fence() ;
       
  2670   InitDone = 1 ;
       
  2671 }
       
  2672 
       
  2673 // Theory of operations -- Monitors lists, thread residency, etc:
       
  2674 //
       
  2675 // * A thread acquires ownership of a monitor by successfully
       
  2676 //   CAS()ing the _owner field from null to non-null.
       
  2677 //
       
  2678 // * Invariant: A thread appears on at most one monitor list --
       
  2679 //   cxq, EntryList or WaitSet -- at any one time.
       
  2680 //
       
  2681 // * Contending threads "push" themselves onto the cxq with CAS
       
  2682 //   and then spin/park.
       
  2683 //
       
  2684 // * After a contending thread eventually acquires the lock it must
       
  2685 //   dequeue itself from either the EntryList or the cxq.
       
  2686 //
       
  2687 // * The exiting thread identifies and unparks an "heir presumptive"
       
  2688 //   tentative successor thread on the EntryList.  Critically, the
       
  2689 //   exiting thread doesn't unlink the successor thread from the EntryList.
       
  2690 //   After having been unparked, the wakee will recontend for ownership of
       
  2691 //   the monitor.   The successor (wakee) will either acquire the lock or
       
  2692 //   re-park itself.
       
  2693 //
       
  2694 //   Succession is provided for by a policy of competitive handoff.
       
  2695 //   The exiting thread does _not_ grant or pass ownership to the
       
  2696 //   successor thread.  (This is also referred to as "handoff" succession").
       
  2697 //   Instead the exiting thread releases ownership and possibly wakes
       
  2698 //   a successor, so the successor can (re)compete for ownership of the lock.
       
  2699 //   If the EntryList is empty but the cxq is populated the exiting
       
  2700 //   thread will drain the cxq into the EntryList.  It does so by
       
  2701 //   by detaching the cxq (installing null with CAS) and folding
       
  2702 //   the threads from the cxq into the EntryList.  The EntryList is
       
  2703 //   doubly linked, while the cxq is singly linked because of the
       
  2704 //   CAS-based "push" used to enqueue recently arrived threads (RATs).
       
  2705 //
       
  2706 // * Concurrency invariants:
       
  2707 //
       
  2708 //   -- only the monitor owner may access or mutate the EntryList.
       
  2709 //      The mutex property of the monitor itself protects the EntryList
       
  2710 //      from concurrent interference.
       
  2711 //   -- Only the monitor owner may detach the cxq.
       
  2712 //
       
  2713 // * The monitor entry list operations avoid locks, but strictly speaking
       
  2714 //   they're not lock-free.  Enter is lock-free, exit is not.
       
  2715 //   See http://j2se.east/~dice/PERSIST/040825-LockFreeQueues.html
       
  2716 //
       
  2717 // * The cxq can have multiple concurrent "pushers" but only one concurrent
       
  2718 //   detaching thread.  This mechanism is immune from the ABA corruption.
       
  2719 //   More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
       
  2720 //
       
  2721 // * Taken together, the cxq and the EntryList constitute or form a
       
  2722 //   single logical queue of threads stalled trying to acquire the lock.
       
  2723 //   We use two distinct lists to improve the odds of a constant-time
       
  2724 //   dequeue operation after acquisition (in the ::enter() epilog) and
       
  2725 //   to reduce heat on the list ends.  (c.f. Michael Scott's "2Q" algorithm).
       
  2726 //   A key desideratum is to minimize queue & monitor metadata manipulation
       
  2727 //   that occurs while holding the monitor lock -- that is, we want to
       
  2728 //   minimize monitor lock holds times.  Note that even a small amount of
       
  2729 //   fixed spinning will greatly reduce the # of enqueue-dequeue operations
       
  2730 //   on EntryList|cxq.  That is, spinning relieves contention on the "inner"
       
  2731 //   locks and monitor metadata.
       
  2732 //
       
  2733 //   Cxq points to the the set of Recently Arrived Threads attempting entry.
       
  2734 //   Because we push threads onto _cxq with CAS, the RATs must take the form of
       
  2735 //   a singly-linked LIFO.  We drain _cxq into EntryList  at unlock-time when
       
  2736 //   the unlocking thread notices that EntryList is null but _cxq is != null.
       
  2737 //
       
  2738 //   The EntryList is ordered by the prevailing queue discipline and
       
  2739 //   can be organized in any convenient fashion, such as a doubly-linked list or
       
  2740 //   a circular doubly-linked list.  Critically, we want insert and delete operations
       
  2741 //   to operate in constant-time.  If we need a priority queue then something akin
       
  2742 //   to Solaris' sleepq would work nicely.  Viz.,
       
  2743 //   http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
       
  2744 //   Queue discipline is enforced at ::exit() time, when the unlocking thread
       
  2745 //   drains the cxq into the EntryList, and orders or reorders the threads on the
       
  2746 //   EntryList accordingly.
       
  2747 //
       
  2748 //   Barring "lock barging", this mechanism provides fair cyclic ordering,
       
  2749 //   somewhat similar to an elevator-scan.
       
  2750 //
       
  2751 // * The monitor synchronization subsystem avoids the use of native
       
  2752 //   synchronization primitives except for the narrow platform-specific
       
  2753 //   park-unpark abstraction.  See the comments in os_solaris.cpp regarding
       
  2754 //   the semantics of park-unpark.  Put another way, this monitor implementation
       
  2755 //   depends only on atomic operations and park-unpark.  The monitor subsystem
       
  2756 //   manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
       
  2757 //   underlying OS manages the READY<->RUN transitions.
       
  2758 //
       
  2759 // * Waiting threads reside on the WaitSet list -- wait() puts
       
  2760 //   the caller onto the WaitSet.
       
  2761 //
       
  2762 // * notify() or notifyAll() simply transfers threads from the WaitSet to
       
  2763 //   either the EntryList or cxq.  Subsequent exit() operations will
       
  2764 //   unpark the notifyee.  Unparking a notifee in notify() is inefficient -
       
  2765 //   it's likely the notifyee would simply impale itself on the lock held
       
  2766 //   by the notifier.
       
  2767 //
       
  2768 // * An interesting alternative is to encode cxq as (List,LockByte) where
       
  2769 //   the LockByte is 0 iff the monitor is owned.  _owner is simply an auxiliary
       
  2770 //   variable, like _recursions, in the scheme.  The threads or Events that form
       
  2771 //   the list would have to be aligned in 256-byte addresses.  A thread would
       
  2772 //   try to acquire the lock or enqueue itself with CAS, but exiting threads
       
  2773 //   could use a 1-0 protocol and simply STB to set the LockByte to 0.
       
  2774 //   Note that is is *not* word-tearing, but it does presume that full-word
       
  2775 //   CAS operations are coherent with intermix with STB operations.  That's true
       
  2776 //   on most common processors.
       
  2777 //
       
  2778 // * See also http://blogs.sun.com/dave
       
  2779 
       
  2780 
       
  2781 void ATTR ObjectMonitor::EnterI (TRAPS) {
       
  2782     Thread * Self = THREAD ;
       
  2783     assert (Self->is_Java_thread(), "invariant") ;
       
  2784     assert (((JavaThread *) Self)->thread_state() == _thread_blocked   , "invariant") ;
       
  2785 
       
  2786     // Try the lock - TATAS
       
  2787     if (TryLock (Self) > 0) {
       
  2788         assert (_succ != Self              , "invariant") ;
       
  2789         assert (_owner == Self             , "invariant") ;
       
  2790         assert (_Responsible != Self       , "invariant") ;
       
  2791         return ;
       
  2792     }
       
  2793 
       
  2794     DeferredInitialize () ;
       
  2795 
       
  2796     // We try one round of spinning *before* enqueueing Self.
       
  2797     //
       
  2798     // If the _owner is ready but OFFPROC we could use a YieldTo()
       
  2799     // operation to donate the remainder of this thread's quantum
       
  2800     // to the owner.  This has subtle but beneficial affinity
       
  2801     // effects.
       
  2802 
       
  2803     if (TrySpin (Self) > 0) {
       
  2804         assert (_owner == Self        , "invariant") ;
       
  2805         assert (_succ != Self         , "invariant") ;
       
  2806         assert (_Responsible != Self  , "invariant") ;
       
  2807         return ;
       
  2808     }
       
  2809 
       
  2810     // The Spin failed -- Enqueue and park the thread ...
       
  2811     assert (_succ  != Self            , "invariant") ;
       
  2812     assert (_owner != Self            , "invariant") ;
       
  2813     assert (_Responsible != Self      , "invariant") ;
       
  2814 
       
  2815     // Enqueue "Self" on ObjectMonitor's _cxq.
       
  2816     //
       
  2817     // Node acts as a proxy for Self.
       
  2818     // As an aside, if were to ever rewrite the synchronization code mostly
       
  2819     // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
       
  2820     // Java objects.  This would avoid awkward lifecycle and liveness issues,
       
  2821     // as well as eliminate a subset of ABA issues.
       
  2822     // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
       
  2823     //
       
  2824 
       
  2825     ObjectWaiter node(Self) ;
       
  2826     Self->_ParkEvent->reset() ;
       
  2827     node._prev   = (ObjectWaiter *) 0xBAD ;
       
  2828     node.TState  = ObjectWaiter::TS_CXQ ;
       
  2829 
       
  2830     // Push "Self" onto the front of the _cxq.
       
  2831     // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
       
  2832     // Note that spinning tends to reduce the rate at which threads
       
  2833     // enqueue and dequeue on EntryList|cxq.
       
  2834     ObjectWaiter * nxt ;
       
  2835     for (;;) {
       
  2836         node._next = nxt = _cxq ;
       
  2837         if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ;
       
  2838 
       
  2839         // Interference - the CAS failed because _cxq changed.  Just retry.
       
  2840         // As an optional optimization we retry the lock.
       
  2841         if (TryLock (Self) > 0) {
       
  2842             assert (_succ != Self         , "invariant") ;
       
  2843             assert (_owner == Self        , "invariant") ;
       
  2844             assert (_Responsible != Self  , "invariant") ;
       
  2845             return ;
       
  2846         }
       
  2847     }
       
  2848 
       
  2849     // Check for cxq|EntryList edge transition to non-null.  This indicates
       
  2850     // the onset of contention.  While contention persists exiting threads
       
  2851     // will use a ST:MEMBAR:LD 1-1 exit protocol.  When contention abates exit
       
  2852     // operations revert to the faster 1-0 mode.  This enter operation may interleave
       
  2853     // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
       
  2854     // arrange for one of the contending thread to use a timed park() operations
       
  2855     // to detect and recover from the race.  (Stranding is form of progress failure
       
  2856     // where the monitor is unlocked but all the contending threads remain parked).
       
  2857     // That is, at least one of the contended threads will periodically poll _owner.
       
  2858     // One of the contending threads will become the designated "Responsible" thread.
       
  2859     // The Responsible thread uses a timed park instead of a normal indefinite park
       
  2860     // operation -- it periodically wakes and checks for and recovers from potential
       
  2861     // strandings admitted by 1-0 exit operations.   We need at most one Responsible
       
  2862     // thread per-monitor at any given moment.  Only threads on cxq|EntryList may
       
  2863     // be responsible for a monitor.
       
  2864     //
       
  2865     // Currently, one of the contended threads takes on the added role of "Responsible".
       
  2866     // A viable alternative would be to use a dedicated "stranding checker" thread
       
  2867     // that periodically iterated over all the threads (or active monitors) and unparked
       
  2868     // successors where there was risk of stranding.  This would help eliminate the
       
  2869     // timer scalability issues we see on some platforms as we'd only have one thread
       
  2870     // -- the checker -- parked on a timer.
       
  2871 
       
  2872     if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
       
  2873         // Try to assume the role of responsible thread for the monitor.
       
  2874         // CONSIDER:  ST vs CAS vs { if (Responsible==null) Responsible=Self }
       
  2875         Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
       
  2876     }
       
  2877 
       
  2878     // The lock have been released while this thread was occupied queueing
       
  2879     // itself onto _cxq.  To close the race and avoid "stranding" and
       
  2880     // progress-liveness failure we must resample-retry _owner before parking.
       
  2881     // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
       
  2882     // In this case the ST-MEMBAR is accomplished with CAS().
       
  2883     //
       
  2884     // TODO: Defer all thread state transitions until park-time.
       
  2885     // Since state transitions are heavy and inefficient we'd like
       
  2886     // to defer the state transitions until absolutely necessary,
       
  2887     // and in doing so avoid some transitions ...
       
  2888 
       
  2889     TEVENT (Inflated enter - Contention) ;
       
  2890     int nWakeups = 0 ;
       
  2891     int RecheckInterval = 1 ;
       
  2892 
       
  2893     for (;;) {
       
  2894 
       
  2895         if (TryLock (Self) > 0) break ;
       
  2896         assert (_owner != Self, "invariant") ;
       
  2897 
       
  2898         if ((SyncFlags & 2) && _Responsible == NULL) {
       
  2899            Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
       
  2900         }
       
  2901 
       
  2902         // park self
       
  2903         if (_Responsible == Self || (SyncFlags & 1)) {
       
  2904             TEVENT (Inflated enter - park TIMED) ;
       
  2905             Self->_ParkEvent->park ((jlong) RecheckInterval) ;
       
  2906             // Increase the RecheckInterval, but clamp the value.
       
  2907             RecheckInterval *= 8 ;
       
  2908             if (RecheckInterval > 1000) RecheckInterval = 1000 ;
       
  2909         } else {
       
  2910             TEVENT (Inflated enter - park UNTIMED) ;
       
  2911             Self->_ParkEvent->park() ;
       
  2912         }
       
  2913 
       
  2914         if (TryLock(Self) > 0) break ;
       
  2915 
       
  2916         // The lock is still contested.
       
  2917         // Keep a tally of the # of futile wakeups.
       
  2918         // Note that the counter is not protected by a lock or updated by atomics.
       
  2919         // That is by design - we trade "lossy" counters which are exposed to
       
  2920         // races during updates for a lower probe effect.
       
  2921         TEVENT (Inflated enter - Futile wakeup) ;
       
  2922         if (ObjectSynchronizer::_sync_FutileWakeups != NULL) {
       
  2923            ObjectSynchronizer::_sync_FutileWakeups->inc() ;
       
  2924         }
       
  2925         ++ nWakeups ;
       
  2926 
       
  2927         // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
       
  2928         // We can defer clearing _succ until after the spin completes
       
  2929         // TrySpin() must tolerate being called with _succ == Self.
       
  2930         // Try yet another round of adaptive spinning.
       
  2931         if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ;
       
  2932 
       
  2933         // We can find that we were unpark()ed and redesignated _succ while
       
  2934         // we were spinning.  That's harmless.  If we iterate and call park(),
       
  2935         // park() will consume the event and return immediately and we'll
       
  2936         // just spin again.  This pattern can repeat, leaving _succ to simply
       
  2937         // spin on a CPU.  Enable Knob_ResetEvent to clear pending unparks().
       
  2938         // Alternately, we can sample fired() here, and if set, forgo spinning
       
  2939         // in the next iteration.
       
  2940 
       
  2941         if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
       
  2942            Self->_ParkEvent->reset() ;
       
  2943            OrderAccess::fence() ;
       
  2944         }
       
  2945         if (_succ == Self) _succ = NULL ;
       
  2946 
       
  2947         // Invariant: after clearing _succ a thread *must* retry _owner before parking.
       
  2948         OrderAccess::fence() ;
       
  2949     }
       
  2950 
       
  2951     // Egress :
       
  2952     // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
       
  2953     // Normally we'll find Self on the EntryList .
       
  2954     // From the perspective of the lock owner (this thread), the
       
  2955     // EntryList is stable and cxq is prepend-only.
       
  2956     // The head of cxq is volatile but the interior is stable.
       
  2957     // In addition, Self.TState is stable.
       
  2958 
       
  2959     assert (_owner == Self      , "invariant") ;
       
  2960     assert (object() != NULL    , "invariant") ;
       
  2961     // I'd like to write:
       
  2962     //   guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
       
  2963     // but as we're at a safepoint that's not safe.
       
  2964 
       
  2965     UnlinkAfterAcquire (Self, &node) ;
       
  2966     if (_succ == Self) _succ = NULL ;
       
  2967 
       
  2968     assert (_succ != Self, "invariant") ;
       
  2969     if (_Responsible == Self) {
       
  2970         _Responsible = NULL ;
       
  2971         // Dekker pivot-point.
       
  2972         // Consider OrderAccess::storeload() here
       
  2973 
       
  2974         // We may leave threads on cxq|EntryList without a designated
       
  2975         // "Responsible" thread.  This is benign.  When this thread subsequently
       
  2976         // exits the monitor it can "see" such preexisting "old" threads --
       
  2977         // threads that arrived on the cxq|EntryList before the fence, above --
       
  2978         // by LDing cxq|EntryList.  Newly arrived threads -- that is, threads
       
  2979         // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
       
  2980         // non-null and elect a new "Responsible" timer thread.
       
  2981         //
       
  2982         // This thread executes:
       
  2983         //    ST Responsible=null; MEMBAR    (in enter epilog - here)
       
  2984         //    LD cxq|EntryList               (in subsequent exit)
       
  2985         //
       
  2986         // Entering threads in the slow/contended path execute:
       
  2987         //    ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
       
  2988         //    The (ST cxq; MEMBAR) is accomplished with CAS().
       
  2989         //
       
  2990         // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
       
  2991         // exit operation from floating above the ST Responsible=null.
       
  2992         //
       
  2993         // In *practice* however, EnterI() is always followed by some atomic
       
  2994         // operation such as the decrement of _count in ::enter().  Those atomics
       
  2995         // obviate the need for the explicit MEMBAR, above.
       
  2996     }
       
  2997 
       
  2998     // We've acquired ownership with CAS().
       
  2999     // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
       
  3000     // But since the CAS() this thread may have also stored into _succ,
       
  3001     // EntryList, cxq or Responsible.  These meta-data updates must be
       
  3002     // visible __before this thread subsequently drops the lock.
       
  3003     // Consider what could occur if we didn't enforce this constraint --
       
  3004     // STs to monitor meta-data and user-data could reorder with (become
       
  3005     // visible after) the ST in exit that drops ownership of the lock.
       
  3006     // Some other thread could then acquire the lock, but observe inconsistent
       
  3007     // or old monitor meta-data and heap data.  That violates the JMM.
       
  3008     // To that end, the 1-0 exit() operation must have at least STST|LDST
       
  3009     // "release" barrier semantics.  Specifically, there must be at least a
       
  3010     // STST|LDST barrier in exit() before the ST of null into _owner that drops
       
  3011     // the lock.   The barrier ensures that changes to monitor meta-data and data
       
  3012     // protected by the lock will be visible before we release the lock, and
       
  3013     // therefore before some other thread (CPU) has a chance to acquire the lock.
       
  3014     // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
       
  3015     //
       
  3016     // Critically, any prior STs to _succ or EntryList must be visible before
       
  3017     // the ST of null into _owner in the *subsequent* (following) corresponding
       
  3018     // monitorexit.  Recall too, that in 1-0 mode monitorexit does not necessarily
       
  3019     // execute a serializing instruction.
       
  3020 
       
  3021     if (SyncFlags & 8) {
       
  3022        OrderAccess::fence() ;
       
  3023     }
       
  3024     return ;
       
  3025 }
       
  3026 
       
  3027 // ExitSuspendEquivalent:
       
  3028 // A faster alternate to handle_special_suspend_equivalent_condition()
       
  3029 //
       
  3030 // handle_special_suspend_equivalent_condition() unconditionally
       
  3031 // acquires the SR_lock.  On some platforms uncontended MutexLocker()
       
  3032 // operations have high latency.  Note that in ::enter() we call HSSEC
       
  3033 // while holding the monitor, so we effectively lengthen the critical sections.
       
  3034 //
       
  3035 // There are a number of possible solutions:
       
  3036 //
       
  3037 // A.  To ameliorate the problem we might also defer state transitions
       
  3038 //     to as late as possible -- just prior to parking.
       
  3039 //     Given that, we'd call HSSEC after having returned from park(),
       
  3040 //     but before attempting to acquire the monitor.  This is only a
       
  3041 //     partial solution.  It avoids calling HSSEC while holding the
       
  3042 //     monitor (good), but it still increases successor reacquisition latency --
       
  3043 //     the interval between unparking a successor and the time the successor
       
  3044 //     resumes and retries the lock.  See ReenterI(), which defers state transitions.
       
  3045 //     If we use this technique we can also avoid EnterI()-exit() loop
       
  3046 //     in ::enter() where we iteratively drop the lock and then attempt
       
  3047 //     to reacquire it after suspending.
       
  3048 //
       
  3049 // B.  In the future we might fold all the suspend bits into a
       
  3050 //     composite per-thread suspend flag and then update it with CAS().
       
  3051 //     Alternately, a Dekker-like mechanism with multiple variables
       
  3052 //     would suffice:
       
  3053 //       ST Self->_suspend_equivalent = false
       
  3054 //       MEMBAR
       
  3055 //       LD Self_>_suspend_flags
       
  3056 //
       
  3057 
       
  3058 
       
  3059 bool ObjectMonitor::ExitSuspendEquivalent (JavaThread * jSelf) {
       
  3060    int Mode = Knob_FastHSSEC ;
       
  3061    if (Mode && !jSelf->is_external_suspend()) {
       
  3062       assert (jSelf->is_suspend_equivalent(), "invariant") ;
       
  3063       jSelf->clear_suspend_equivalent() ;
       
  3064       if (2 == Mode) OrderAccess::storeload() ;
       
  3065       if (!jSelf->is_external_suspend()) return false ;
       
  3066       // We raced a suspension -- fall thru into the slow path
       
  3067       TEVENT (ExitSuspendEquivalent - raced) ;
       
  3068       jSelf->set_suspend_equivalent() ;
       
  3069    }
       
  3070    return jSelf->handle_special_suspend_equivalent_condition() ;
       
  3071 }
       
  3072 
       
  3073 
       
  3074 // ReenterI() is a specialized inline form of the latter half of the
       
  3075 // contended slow-path from EnterI().  We use ReenterI() only for
       
  3076 // monitor reentry in wait().
       
  3077 //
       
  3078 // In the future we should reconcile EnterI() and ReenterI(), adding
       
  3079 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
       
  3080 // loop accordingly.
       
  3081 
       
  3082 void ATTR ObjectMonitor::ReenterI (Thread * Self, ObjectWaiter * SelfNode) {
       
  3083     assert (Self != NULL                , "invariant") ;
       
  3084     assert (SelfNode != NULL            , "invariant") ;
       
  3085     assert (SelfNode->_thread == Self   , "invariant") ;
       
  3086     assert (_waiters > 0                , "invariant") ;
       
  3087     assert (((oop)(object()))->mark() == markOopDesc::encode(this) , "invariant") ;
       
  3088     assert (((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;
       
  3089     JavaThread * jt = (JavaThread *) Self ;
       
  3090 
       
  3091     int nWakeups = 0 ;
       
  3092     for (;;) {
       
  3093         ObjectWaiter::TStates v = SelfNode->TState ;
       
  3094         guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
       
  3095         assert    (_owner != Self, "invariant") ;
       
  3096 
       
  3097         if (TryLock (Self) > 0) break ;
       
  3098         if (TrySpin (Self) > 0) break ;
       
  3099 
       
  3100         TEVENT (Wait Reentry - parking) ;
       
  3101 
       
  3102         // State transition wrappers around park() ...
       
  3103         // ReenterI() wisely defers state transitions until
       
  3104         // it's clear we must park the thread.
       
  3105         {
       
  3106            OSThreadContendState osts(Self->osthread());
       
  3107            ThreadBlockInVM tbivm(jt);
       
  3108 
       
  3109            // cleared by handle_special_suspend_equivalent_condition()
       
  3110            // or java_suspend_self()
       
  3111            jt->set_suspend_equivalent();
       
  3112            if (SyncFlags & 1) {
       
  3113               Self->_ParkEvent->park ((jlong)1000) ;
       
  3114            } else {
       
  3115               Self->_ParkEvent->park () ;
       
  3116            }
       
  3117 
       
  3118            // were we externally suspended while we were waiting?
       
  3119            for (;;) {
       
  3120               if (!ExitSuspendEquivalent (jt)) break ;
       
  3121               if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
       
  3122               jt->java_suspend_self();
       
  3123               jt->set_suspend_equivalent();
       
  3124            }
       
  3125         }
       
  3126 
       
  3127         // Try again, but just so we distinguish between futile wakeups and
       
  3128         // successful wakeups.  The following test isn't algorithmically
       
  3129         // necessary, but it helps us maintain sensible statistics.
       
  3130         if (TryLock(Self) > 0) break ;
       
  3131 
       
  3132         // The lock is still contested.
       
  3133         // Keep a tally of the # of futile wakeups.
       
  3134         // Note that the counter is not protected by a lock or updated by atomics.
       
  3135         // That is by design - we trade "lossy" counters which are exposed to
       
  3136         // races during updates for a lower probe effect.
       
  3137         TEVENT (Wait Reentry - futile wakeup) ;
       
  3138         ++ nWakeups ;
       
  3139 
       
  3140         // Assuming this is not a spurious wakeup we'll normally
       
  3141         // find that _succ == Self.
       
  3142         if (_succ == Self) _succ = NULL ;
       
  3143 
       
  3144         // Invariant: after clearing _succ a contending thread
       
  3145         // *must* retry  _owner before parking.
       
  3146         OrderAccess::fence() ;
       
  3147 
       
  3148         if (ObjectSynchronizer::_sync_FutileWakeups != NULL) {
       
  3149           ObjectSynchronizer::_sync_FutileWakeups->inc() ;
       
  3150         }
       
  3151     }
       
  3152 
       
  3153     // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
       
  3154     // Normally we'll find Self on the EntryList.
       
  3155     // Unlinking from the EntryList is constant-time and atomic-free.
       
  3156     // From the perspective of the lock owner (this thread), the
       
  3157     // EntryList is stable and cxq is prepend-only.
       
  3158     // The head of cxq is volatile but the interior is stable.
       
  3159     // In addition, Self.TState is stable.
       
  3160 
       
  3161     assert (_owner == Self, "invariant") ;
       
  3162     assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
       
  3163     UnlinkAfterAcquire (Self, SelfNode) ;
       
  3164     if (_succ == Self) _succ = NULL ;
       
  3165     assert (_succ != Self, "invariant") ;
       
  3166     SelfNode->TState = ObjectWaiter::TS_RUN ;
       
  3167     OrderAccess::fence() ;      // see comments at the end of EnterI()
       
  3168 }
       
  3169 
       
  3170 bool ObjectMonitor::try_enter(Thread* THREAD) {
       
  3171   if (THREAD != _owner) {
       
  3172     if (THREAD->is_lock_owned ((address)_owner)) {
       
  3173        assert(_recursions == 0, "internal state error");
       
  3174        _owner = THREAD ;
       
  3175        _recursions = 1 ;
       
  3176        OwnerIsThread = 1 ;
       
  3177        return true;
       
  3178     }
       
  3179     if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
       
  3180       return false;
       
  3181     }
       
  3182     return true;
       
  3183   } else {
       
  3184     _recursions++;
       
  3185     return true;
       
  3186   }
       
  3187 }
       
  3188 
       
  3189 void ATTR ObjectMonitor::enter(TRAPS) {
       
  3190   // The following code is ordered to check the most common cases first
       
  3191   // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
       
  3192   Thread * const Self = THREAD ;
       
  3193   void * cur ;
       
  3194 
       
  3195   cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
       
  3196   if (cur == NULL) {
       
  3197      // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
       
  3198      assert (_recursions == 0   , "invariant") ;
       
  3199      assert (_owner      == Self, "invariant") ;
       
  3200      // CONSIDER: set or assert OwnerIsThread == 1
       
  3201      return ;
       
  3202   }
       
  3203 
       
  3204   if (cur == Self) {
       
  3205      // TODO-FIXME: check for integer overflow!  BUGID 6557169.
       
  3206      _recursions ++ ;
       
  3207      return ;
       
  3208   }
       
  3209 
       
  3210   if (Self->is_lock_owned ((address)cur)) {
       
  3211     assert (_recursions == 0, "internal state error");
       
  3212     _recursions = 1 ;
       
  3213     // Commute owner from a thread-specific on-stack BasicLockObject address to
       
  3214     // a full-fledged "Thread *".
       
  3215     _owner = Self ;
       
  3216     OwnerIsThread = 1 ;
       
  3217     return ;
       
  3218   }
       
  3219 
       
  3220   // We've encountered genuine contention.
       
  3221   assert (Self->_Stalled == 0, "invariant") ;
       
  3222   Self->_Stalled = intptr_t(this) ;
       
  3223 
       
  3224   // Try one round of spinning *before* enqueueing Self
       
  3225   // and before going through the awkward and expensive state
       
  3226   // transitions.  The following spin is strictly optional ...
       
  3227   // Note that if we acquire the monitor from an initial spin
       
  3228   // we forgo posting JVMTI events and firing DTRACE probes.
       
  3229   if (Knob_SpinEarly && TrySpin (Self) > 0) {
       
  3230      assert (_owner == Self      , "invariant") ;
       
  3231      assert (_recursions == 0    , "invariant") ;
       
  3232      assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
       
  3233      Self->_Stalled = 0 ;
       
  3234      return ;
       
  3235   }
       
  3236 
       
  3237   assert (_owner != Self          , "invariant") ;
       
  3238   assert (_succ  != Self          , "invariant") ;
       
  3239   assert (Self->is_Java_thread()  , "invariant") ;
       
  3240   JavaThread * jt = (JavaThread *) Self ;
       
  3241   assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
       
  3242   assert (jt->thread_state() != _thread_blocked   , "invariant") ;
       
  3243   assert (this->object() != NULL  , "invariant") ;
       
  3244   assert (_count >= 0, "invariant") ;
       
  3245 
       
  3246   // Prevent deflation at STW-time.  See deflate_idle_monitors() and is_busy().
       
  3247   // Ensure the object-monitor relationship remains stable while there's contention.
       
  3248   Atomic::inc_ptr(&_count);
       
  3249 
       
  3250   { // Change java thread status to indicate blocked on monitor enter.
       
  3251     JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
       
  3252 
       
  3253     DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
       
  3254     if (JvmtiExport::should_post_monitor_contended_enter()) {
       
  3255       JvmtiExport::post_monitor_contended_enter(jt, this);
       
  3256     }
       
  3257 
       
  3258     OSThreadContendState osts(Self->osthread());
       
  3259     ThreadBlockInVM tbivm(jt);
       
  3260 
       
  3261     Self->set_current_pending_monitor(this);
       
  3262 
       
  3263     // TODO-FIXME: change the following for(;;) loop to straight-line code.
       
  3264     for (;;) {
       
  3265       jt->set_suspend_equivalent();
       
  3266       // cleared by handle_special_suspend_equivalent_condition()
       
  3267       // or java_suspend_self()
       
  3268 
       
  3269       EnterI (THREAD) ;
       
  3270 
       
  3271       if (!ExitSuspendEquivalent(jt)) break ;
       
  3272 
       
  3273       //
       
  3274       // We have acquired the contended monitor, but while we were
       
  3275       // waiting another thread suspended us. We don't want to enter
       
  3276       // the monitor while suspended because that would surprise the
       
  3277       // thread that suspended us.
       
  3278       //
       
  3279           _recursions = 0 ;
       
  3280       _succ = NULL ;
       
  3281       exit (Self) ;
       
  3282 
       
  3283       jt->java_suspend_self();
       
  3284     }
       
  3285     Self->set_current_pending_monitor(NULL);
       
  3286   }
       
  3287 
       
  3288   Atomic::dec_ptr(&_count);
       
  3289   assert (_count >= 0, "invariant") ;
       
  3290   Self->_Stalled = 0 ;
       
  3291 
       
  3292   // Must either set _recursions = 0 or ASSERT _recursions == 0.
       
  3293   assert (_recursions == 0     , "invariant") ;
       
  3294   assert (_owner == Self       , "invariant") ;
       
  3295   assert (_succ  != Self       , "invariant") ;
       
  3296   assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
       
  3297 
       
  3298   // The thread -- now the owner -- is back in vm mode.
       
  3299   // Report the glorious news via TI,DTrace and jvmstat.
       
  3300   // The probe effect is non-trivial.  All the reportage occurs
       
  3301   // while we hold the monitor, increasing the length of the critical
       
  3302   // section.  Amdahl's parallel speedup law comes vividly into play.
       
  3303   //
       
  3304   // Another option might be to aggregate the events (thread local or
       
  3305   // per-monitor aggregation) and defer reporting until a more opportune
       
  3306   // time -- such as next time some thread encounters contention but has
       
  3307   // yet to acquire the lock.  While spinning that thread could
       
  3308   // spinning we could increment JVMStat counters, etc.
       
  3309 
       
  3310   DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
       
  3311   if (JvmtiExport::should_post_monitor_contended_entered()) {
       
  3312     JvmtiExport::post_monitor_contended_entered(jt, this);
       
  3313   }
       
  3314   if (ObjectSynchronizer::_sync_ContendedLockAttempts != NULL) {
       
  3315      ObjectSynchronizer::_sync_ContendedLockAttempts->inc() ;
       
  3316   }
       
  3317 }
       
  3318 
       
  3319 void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) {
       
  3320    assert (_owner == Self, "invariant") ;
       
  3321 
       
  3322    // Exit protocol:
       
  3323    // 1. ST _succ = wakee
       
  3324    // 2. membar #loadstore|#storestore;
       
  3325    // 2. ST _owner = NULL
       
  3326    // 3. unpark(wakee)
       
  3327 
       
  3328    _succ = Knob_SuccEnabled ? Wakee->_thread : NULL ;
       
  3329    ParkEvent * Trigger = Wakee->_event ;
       
  3330 
       
  3331    // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
       
  3332    // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
       
  3333    // out-of-scope (non-extant).
       
  3334    Wakee  = NULL ;
       
  3335 
       
  3336    // Drop the lock
       
  3337    OrderAccess::release_store_ptr (&_owner, NULL) ;
       
  3338    OrderAccess::fence() ;                               // ST _owner vs LD in unpark()
       
  3339 
       
  3340    // TODO-FIXME:
       
  3341    // If there's a safepoint pending the best policy would be to
       
  3342    // get _this thread to a safepoint and only wake the successor
       
  3343    // after the safepoint completed.  monitorexit uses a "leaf"
       
  3344    // state transition, however, so this thread can't become
       
  3345    // safe at this point in time.  (Its stack isn't walkable).
       
  3346    // The next best thing is to defer waking the successor by
       
  3347    // adding to a list of thread to be unparked after at the
       
  3348    // end of the forthcoming STW).
       
  3349    if (SafepointSynchronize::do_call_back()) {
       
  3350       TEVENT (unpark before SAFEPOINT) ;
       
  3351    }
       
  3352 
       
  3353    // Possible optimizations ...
       
  3354    //
       
  3355    // * Consider: set Wakee->UnparkTime = timeNow()
       
  3356    //   When the thread wakes up it'll compute (timeNow() - Self->UnparkTime()).
       
  3357    //   By measuring recent ONPROC latency we can approximate the
       
  3358    //   system load.  In turn, we can feed that information back
       
  3359    //   into the spinning & succession policies.
       
  3360    //   (ONPROC latency correlates strongly with load).
       
  3361    //
       
  3362    // * Pull affinity:
       
  3363    //   If the wakee is cold then transiently setting it's affinity
       
  3364    //   to the current CPU is a good idea.
       
  3365    //   See http://j2se.east/~dice/PERSIST/050624-PullAffinity.txt
       
  3366    Trigger->unpark() ;
       
  3367 
       
  3368    // Maintain stats and report events to JVMTI
       
  3369    if (ObjectSynchronizer::_sync_Parks != NULL) {
       
  3370       ObjectSynchronizer::_sync_Parks->inc() ;
       
  3371    }
       
  3372    DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
       
  3373 }
       
  3374 
       
  3375 
       
  3376 // exit()
       
  3377 // ~~~~~~
       
  3378 // Note that the collector can't reclaim the objectMonitor or deflate
       
  3379 // the object out from underneath the thread calling ::exit() as the
       
  3380 // thread calling ::exit() never transitions to a stable state.
       
  3381 // This inhibits GC, which in turn inhibits asynchronous (and
       
  3382 // inopportune) reclamation of "this".
       
  3383 //
       
  3384 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
       
  3385 // There's one exception to the claim above, however.  EnterI() can call
       
  3386 // exit() to drop a lock if the acquirer has been externally suspended.
       
  3387 // In that case exit() is called with _thread_state as _thread_blocked,
       
  3388 // but the monitor's _count field is > 0, which inhibits reclamation.
       
  3389 //
       
  3390 // 1-0 exit
       
  3391 // ~~~~~~~~
       
  3392 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
       
  3393 // the fast-path operators have been optimized so the common ::exit()
       
  3394 // operation is 1-0.  See i486.ad fast_unlock(), for instance.
       
  3395 // The code emitted by fast_unlock() elides the usual MEMBAR.  This
       
  3396 // greatly improves latency -- MEMBAR and CAS having considerable local
       
  3397 // latency on modern processors -- but at the cost of "stranding".  Absent the
       
  3398 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
       
  3399 // ::enter() path, resulting in the entering thread being stranding
       
  3400 // and a progress-liveness failure.   Stranding is extremely rare.
       
  3401 // We use timers (timed park operations) & periodic polling to detect
       
  3402 // and recover from stranding.  Potentially stranded threads periodically
       
  3403 // wake up and poll the lock.  See the usage of the _Responsible variable.
       
  3404 //
       
  3405 // The CAS() in enter provides for safety and exclusion, while the CAS or
       
  3406 // MEMBAR in exit provides for progress and avoids stranding.  1-0 locking
       
  3407 // eliminates the CAS/MEMBAR from the exist path, but it admits stranding.
       
  3408 // We detect and recover from stranding with timers.
       
  3409 //
       
  3410 // If a thread transiently strands it'll park until (a) another
       
  3411 // thread acquires the lock and then drops the lock, at which time the
       
  3412 // exiting thread will notice and unpark the stranded thread, or, (b)
       
  3413 // the timer expires.  If the lock is high traffic then the stranding latency
       
  3414 // will be low due to (a).  If the lock is low traffic then the odds of
       
  3415 // stranding are lower, although the worst-case stranding latency
       
  3416 // is longer.  Critically, we don't want to put excessive load in the
       
  3417 // platform's timer subsystem.  We want to minimize both the timer injection
       
  3418 // rate (timers created/sec) as well as the number of timers active at
       
  3419 // any one time.  (more precisely, we want to minimize timer-seconds, which is
       
  3420 // the integral of the # of active timers at any instant over time).
       
  3421 // Both impinge on OS scalability.  Given that, at most one thread parked on
       
  3422 // a monitor will use a timer.
       
  3423 
       
  3424 void ATTR ObjectMonitor::exit(TRAPS) {
       
  3425    Thread * Self = THREAD ;
       
  3426    if (THREAD != _owner) {
       
  3427      if (THREAD->is_lock_owned((address) _owner)) {
       
  3428        // Transmute _owner from a BasicLock pointer to a Thread address.
       
  3429        // We don't need to hold _mutex for this transition.
       
  3430        // Non-null to Non-null is safe as long as all readers can
       
  3431        // tolerate either flavor.
       
  3432        assert (_recursions == 0, "invariant") ;
       
  3433        _owner = THREAD ;
       
  3434        _recursions = 0 ;
       
  3435        OwnerIsThread = 1 ;
       
  3436      } else {
       
  3437        // NOTE: we need to handle unbalanced monitor enter/exit
       
  3438        // in native code by throwing an exception.
       
  3439        // TODO: Throw an IllegalMonitorStateException ?
       
  3440        TEVENT (Exit - Throw IMSX) ;
       
  3441        assert(false, "Non-balanced monitor enter/exit!");
       
  3442        if (false) {
       
  3443           THROW(vmSymbols::java_lang_IllegalMonitorStateException());
       
  3444        }
       
  3445        return;
       
  3446      }
       
  3447    }
       
  3448 
       
  3449    if (_recursions != 0) {
       
  3450      _recursions--;        // this is simple recursive enter
       
  3451      TEVENT (Inflated exit - recursive) ;
       
  3452      return ;
       
  3453    }
       
  3454 
       
  3455    // Invariant: after setting Responsible=null an thread must execute
       
  3456    // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
       
  3457    if ((SyncFlags & 4) == 0) {
       
  3458       _Responsible = NULL ;
       
  3459    }
       
  3460 
       
  3461    for (;;) {
       
  3462       assert (THREAD == _owner, "invariant") ;
       
  3463 
       
  3464       // Fast-path monitor exit:
       
  3465       //
       
  3466       // Observe the Dekker/Lamport duality:
       
  3467       // A thread in ::exit() executes:
       
  3468       //   ST Owner=null; MEMBAR; LD EntryList|cxq.
       
  3469       // A thread in the contended ::enter() path executes the complementary:
       
  3470       //   ST EntryList|cxq = nonnull; MEMBAR; LD Owner.
       
  3471       //
       
  3472       // Note that there's a benign race in the exit path.  We can drop the
       
  3473       // lock, another thread can reacquire the lock immediately, and we can
       
  3474       // then wake a thread unnecessarily (yet another flavor of futile wakeup).
       
  3475       // This is benign, and we've structured the code so the windows are short
       
  3476       // and the frequency of such futile wakeups is low.
       
  3477       //
       
  3478       // We could eliminate the race by encoding both the "LOCKED" state and
       
  3479       // the queue head in a single word.  Exit would then use either CAS to
       
  3480       // clear the LOCKED bit/byte.  This precludes the desirable 1-0 optimization,
       
  3481       // however.
       
  3482       //
       
  3483       // Possible fast-path ::exit() optimization:
       
  3484       // The current fast-path exit implementation fetches both cxq and EntryList.
       
  3485       // See also i486.ad fast_unlock().  Testing has shown that two LDs
       
  3486       // isn't measurably slower than a single LD on any platforms.
       
  3487       // Still, we could reduce the 2 LDs to one or zero by one of the following:
       
  3488       //
       
  3489       // - Use _count instead of cxq|EntryList
       
  3490       //   We intend to eliminate _count, however, when we switch
       
  3491       //   to on-the-fly deflation in ::exit() as is used in
       
  3492       //   Metalocks and RelaxedLocks.
       
  3493       //
       
  3494       // - Establish the invariant that cxq == null implies EntryList == null.
       
  3495       //   set cxq == EMPTY (1) to encode the state where cxq is empty
       
  3496       //   by EntryList != null.  EMPTY is a distinguished value.
       
  3497       //   The fast-path exit() would fetch cxq but not EntryList.
       
  3498       //
       
  3499       // - Encode succ as follows:
       
  3500       //   succ = t :  Thread t is the successor -- t is ready or is spinning.
       
  3501       //               Exiting thread does not need to wake a successor.
       
  3502       //   succ = 0 :  No successor required -> (EntryList|cxq) == null
       
  3503       //               Exiting thread does not need to wake a successor
       
  3504       //   succ = 1 :  Successor required    -> (EntryList|cxq) != null and
       
  3505       //               logically succ == null.
       
  3506       //               Exiting thread must wake a successor.
       
  3507       //
       
  3508       //   The 1-1 fast-exit path would appear as :
       
  3509       //     _owner = null ; membar ;
       
  3510       //     if (_succ == 1 && CAS (&_owner, null, Self) == null) goto SlowPath
       
  3511       //     goto FastPathDone ;
       
  3512       //
       
  3513       //   and the 1-0 fast-exit path would appear as:
       
  3514       //      if (_succ == 1) goto SlowPath
       
  3515       //      Owner = null ;
       
  3516       //      goto FastPathDone
       
  3517       //
       
  3518       // - Encode the LSB of _owner as 1 to indicate that exit()
       
  3519       //   must use the slow-path and make a successor ready.
       
  3520       //   (_owner & 1) == 0 IFF succ != null || (EntryList|cxq) == null
       
  3521       //   (_owner & 1) == 0 IFF succ == null && (EntryList|cxq) != null (obviously)
       
  3522       //   The 1-0 fast exit path would read:
       
  3523       //      if (_owner != Self) goto SlowPath
       
  3524       //      _owner = null
       
  3525       //      goto FastPathDone
       
  3526 
       
  3527       if (Knob_ExitPolicy == 0) {
       
  3528          // release semantics: prior loads and stores from within the critical section
       
  3529          // must not float (reorder) past the following store that drops the lock.
       
  3530          // On SPARC that requires MEMBAR #loadstore|#storestore.
       
  3531          // But of course in TSO #loadstore|#storestore is not required.
       
  3532          // I'd like to write one of the following:
       
  3533          // A.  OrderAccess::release() ; _owner = NULL
       
  3534          // B.  OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
       
  3535          // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
       
  3536          // store into a _dummy variable.  That store is not needed, but can result
       
  3537          // in massive wasteful coherency traffic on classic SMP systems.
       
  3538          // Instead, I use release_store(), which is implemented as just a simple
       
  3539          // ST on x64, x86 and SPARC.
       
  3540          OrderAccess::release_store_ptr (&_owner, NULL) ;   // drop the lock
       
  3541          OrderAccess::storeload() ;                         // See if we need to wake a successor
       
  3542          if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
       
  3543             TEVENT (Inflated exit - simple egress) ;
       
  3544             return ;
       
  3545          }
       
  3546          TEVENT (Inflated exit - complex egress) ;
       
  3547 
       
  3548          // Normally the exiting thread is responsible for ensuring succession,
       
  3549          // but if other successors are ready or other entering threads are spinning
       
  3550          // then this thread can simply store NULL into _owner and exit without
       
  3551          // waking a successor.  The existence of spinners or ready successors
       
  3552          // guarantees proper succession (liveness).  Responsibility passes to the
       
  3553          // ready or running successors.  The exiting thread delegates the duty.
       
  3554          // More precisely, if a successor already exists this thread is absolved
       
  3555          // of the responsibility of waking (unparking) one.
       
  3556          //
       
  3557          // The _succ variable is critical to reducing futile wakeup frequency.
       
  3558          // _succ identifies the "heir presumptive" thread that has been made
       
  3559          // ready (unparked) but that has not yet run.  We need only one such
       
  3560          // successor thread to guarantee progress.
       
  3561          // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
       
  3562          // section 3.3 "Futile Wakeup Throttling" for details.
       
  3563          //
       
  3564          // Note that spinners in Enter() also set _succ non-null.
       
  3565          // In the current implementation spinners opportunistically set
       
  3566          // _succ so that exiting threads might avoid waking a successor.
       
  3567          // Another less appealing alternative would be for the exiting thread
       
  3568          // to drop the lock and then spin briefly to see if a spinner managed
       
  3569          // to acquire the lock.  If so, the exiting thread could exit
       
  3570          // immediately without waking a successor, otherwise the exiting
       
  3571          // thread would need to dequeue and wake a successor.
       
  3572          // (Note that we'd need to make the post-drop spin short, but no
       
  3573          // shorter than the worst-case round-trip cache-line migration time.
       
  3574          // The dropped lock needs to become visible to the spinner, and then
       
  3575          // the acquisition of the lock by the spinner must become visible to
       
  3576          // the exiting thread).
       
  3577          //
       
  3578 
       
  3579          // It appears that an heir-presumptive (successor) must be made ready.
       
  3580          // Only the current lock owner can manipulate the EntryList or
       
  3581          // drain _cxq, so we need to reacquire the lock.  If we fail
       
  3582          // to reacquire the lock the responsibility for ensuring succession
       
  3583          // falls to the new owner.
       
  3584          //
       
  3585          if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
       
  3586             return ;
       
  3587          }
       
  3588          TEVENT (Exit - Reacquired) ;
       
  3589       } else {
       
  3590          if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
       
  3591             OrderAccess::release_store_ptr (&_owner, NULL) ;   // drop the lock
       
  3592             OrderAccess::storeload() ;
       
  3593             // Ratify the previously observed values.
       
  3594             if (_cxq == NULL || _succ != NULL) {
       
  3595                 TEVENT (Inflated exit - simple egress) ;
       
  3596                 return ;
       
  3597             }
       
  3598 
       
  3599             // inopportune interleaving -- the exiting thread (this thread)
       
  3600             // in the fast-exit path raced an entering thread in the slow-enter
       
  3601             // path.
       
  3602             // We have two choices:
       
  3603             // A.  Try to reacquire the lock.
       
  3604             //     If the CAS() fails return immediately, otherwise
       
  3605             //     we either restart/rerun the exit operation, or simply
       
  3606             //     fall-through into the code below which wakes a successor.
       
  3607             // B.  If the elements forming the EntryList|cxq are TSM
       
  3608             //     we could simply unpark() the lead thread and return
       
  3609             //     without having set _succ.
       
  3610             if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
       
  3611                TEVENT (Inflated exit - reacquired succeeded) ;
       
  3612                return ;
       
  3613             }
       
  3614             TEVENT (Inflated exit - reacquired failed) ;
       
  3615          } else {
       
  3616             TEVENT (Inflated exit - complex egress) ;
       
  3617          }
       
  3618       }
       
  3619 
       
  3620       guarantee (_owner == THREAD, "invariant") ;
       
  3621 
       
  3622       // Select an appropriate successor ("heir presumptive") from the EntryList
       
  3623       // and make it ready.  Generally we just wake the head of EntryList .
       
  3624       // There's no algorithmic constraint that we use the head - it's just
       
  3625       // a policy decision.   Note that the thread at head of the EntryList
       
  3626       // remains at the head until it acquires the lock.  This means we'll
       
  3627       // repeatedly wake the same thread until it manages to grab the lock.
       
  3628       // This is generally a good policy - if we're seeing lots of futile wakeups
       
  3629       // at least we're waking/rewaking a thread that's like to be hot or warm
       
  3630       // (have residual D$ and TLB affinity).
       
  3631       //
       
  3632       // "Wakeup locality" optimization:
       
  3633       // http://j2se.east/~dice/PERSIST/040825-WakeLocality.txt
       
  3634       // In the future we'll try to bias the selection mechanism
       
  3635       // to preferentially pick a thread that recently ran on
       
  3636       // a processor element that shares cache with the CPU on which
       
  3637       // the exiting thread is running.   We need access to Solaris'
       
  3638       // schedctl.sc_cpu to make that work.
       
  3639       //
       
  3640       ObjectWaiter * w = NULL ;
       
  3641       int QMode = Knob_QMode ;
       
  3642 
       
  3643       if (QMode == 2 && _cxq != NULL) {
       
  3644           // QMode == 2 : cxq has precedence over EntryList.
       
  3645           // Try to directly wake a successor from the cxq.
       
  3646           // If successful, the successor will need to unlink itself from cxq.
       
  3647           w = _cxq ;
       
  3648           assert (w != NULL, "invariant") ;
       
  3649           assert (w->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
       
  3650           ExitEpilog (Self, w) ;
       
  3651           return ;
       
  3652       }
       
  3653 
       
  3654       if (QMode == 3 && _cxq != NULL) {
       
  3655           // Aggressively drain cxq into EntryList at the first opportunity.
       
  3656           // This policy ensure that recently-run threads live at the head of EntryList.
       
  3657           // Drain _cxq into EntryList - bulk transfer.
       
  3658           // First, detach _cxq.
       
  3659           // The following loop is tantamount to: w = swap (&cxq, NULL)
       
  3660           w = _cxq ;
       
  3661           for (;;) {
       
  3662              assert (w != NULL, "Invariant") ;
       
  3663              ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
       
  3664              if (u == w) break ;
       
  3665              w = u ;
       
  3666           }
       
  3667           assert (w != NULL              , "invariant") ;
       
  3668 
       
  3669           ObjectWaiter * q = NULL ;
       
  3670           ObjectWaiter * p ;
       
  3671           for (p = w ; p != NULL ; p = p->_next) {
       
  3672               guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
       
  3673               p->TState = ObjectWaiter::TS_ENTER ;
       
  3674               p->_prev = q ;
       
  3675               q = p ;
       
  3676           }
       
  3677 
       
  3678           // Append the RATs to the EntryList
       
  3679           // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
       
  3680           ObjectWaiter * Tail ;
       
  3681           for (Tail = _EntryList ; Tail != NULL && Tail->_next != NULL ; Tail = Tail->_next) ;
       
  3682           if (Tail == NULL) {
       
  3683               _EntryList = w ;
       
  3684           } else {
       
  3685               Tail->_next = w ;
       
  3686               w->_prev = Tail ;
       
  3687           }
       
  3688 
       
  3689           // Fall thru into code that tries to wake a successor from EntryList
       
  3690       }
       
  3691 
       
  3692       if (QMode == 4 && _cxq != NULL) {
       
  3693           // Aggressively drain cxq into EntryList at the first opportunity.
       
  3694           // This policy ensure that recently-run threads live at the head of EntryList.
       
  3695 
       
  3696           // Drain _cxq into EntryList - bulk transfer.
       
  3697           // First, detach _cxq.
       
  3698           // The following loop is tantamount to: w = swap (&cxq, NULL)
       
  3699           w = _cxq ;
       
  3700           for (;;) {
       
  3701              assert (w != NULL, "Invariant") ;
       
  3702              ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
       
  3703              if (u == w) break ;
       
  3704              w = u ;
       
  3705           }
       
  3706           assert (w != NULL              , "invariant") ;
       
  3707 
       
  3708           ObjectWaiter * q = NULL ;
       
  3709           ObjectWaiter * p ;
       
  3710           for (p = w ; p != NULL ; p = p->_next) {
       
  3711               guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
       
  3712               p->TState = ObjectWaiter::TS_ENTER ;
       
  3713               p->_prev = q ;
       
  3714               q = p ;
       
  3715           }
       
  3716 
       
  3717           // Prepend the RATs to the EntryList
       
  3718           if (_EntryList != NULL) {
       
  3719               q->_next = _EntryList ;
       
  3720               _EntryList->_prev = q ;
       
  3721           }
       
  3722           _EntryList = w ;
       
  3723 
       
  3724           // Fall thru into code that tries to wake a successor from EntryList
       
  3725       }
       
  3726 
       
  3727       w = _EntryList  ;
       
  3728       if (w != NULL) {
       
  3729           // I'd like to write: guarantee (w->_thread != Self).
       
  3730           // But in practice an exiting thread may find itself on the EntryList.
       
  3731           // Lets say thread T1 calls O.wait().  Wait() enqueues T1 on O's waitset and
       
  3732           // then calls exit().  Exit release the lock by setting O._owner to NULL.
       
  3733           // Lets say T1 then stalls.  T2 acquires O and calls O.notify().  The
       
  3734           // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
       
  3735           // release the lock "O".  T2 resumes immediately after the ST of null into
       
  3736           // _owner, above.  T2 notices that the EntryList is populated, so it
       
  3737           // reacquires the lock and then finds itself on the EntryList.
       
  3738           // Given all that, we have to tolerate the circumstance where "w" is
       
  3739           // associated with Self.
       
  3740           assert (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
       
  3741           ExitEpilog (Self, w) ;
       
  3742           return ;
       
  3743       }
       
  3744 
       
  3745       // If we find that both _cxq and EntryList are null then just
       
  3746       // re-run the exit protocol from the top.
       
  3747       w = _cxq ;
       
  3748       if (w == NULL) continue ;
       
  3749 
       
  3750       // Drain _cxq into EntryList - bulk transfer.
       
  3751       // First, detach _cxq.
       
  3752       // The following loop is tantamount to: w = swap (&cxq, NULL)
       
  3753       for (;;) {
       
  3754           assert (w != NULL, "Invariant") ;
       
  3755           ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
       
  3756           if (u == w) break ;
       
  3757           w = u ;
       
  3758       }
       
  3759       TEVENT (Inflated exit - drain cxq into EntryList) ;
       
  3760 
       
  3761       assert (w != NULL              , "invariant") ;
       
  3762       assert (_EntryList  == NULL    , "invariant") ;
       
  3763 
       
  3764       // Convert the LIFO SLL anchored by _cxq into a DLL.
       
  3765       // The list reorganization step operates in O(LENGTH(w)) time.
       
  3766       // It's critical that this step operate quickly as
       
  3767       // "Self" still holds the outer-lock, restricting parallelism
       
  3768       // and effectively lengthening the critical section.
       
  3769       // Invariant: s chases t chases u.
       
  3770       // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
       
  3771       // we have faster access to the tail.
       
  3772 
       
  3773       if (QMode == 1) {
       
  3774          // QMode == 1 : drain cxq to EntryList, reversing order
       
  3775          // We also reverse the order of the list.
       
  3776          ObjectWaiter * s = NULL ;
       
  3777          ObjectWaiter * t = w ;
       
  3778          ObjectWaiter * u = NULL ;
       
  3779          while (t != NULL) {
       
  3780              guarantee (t->TState == ObjectWaiter::TS_CXQ, "invariant") ;
       
  3781              t->TState = ObjectWaiter::TS_ENTER ;
       
  3782              u = t->_next ;
       
  3783              t->_prev = u ;
       
  3784              t->_next = s ;
       
  3785              s = t;
       
  3786              t = u ;
       
  3787          }
       
  3788          _EntryList  = s ;
       
  3789          assert (s != NULL, "invariant") ;
       
  3790       } else {
       
  3791          // QMode == 0 or QMode == 2
       
  3792          _EntryList = w ;
       
  3793          ObjectWaiter * q = NULL ;
       
  3794          ObjectWaiter * p ;
       
  3795          for (p = w ; p != NULL ; p = p->_next) {
       
  3796              guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
       
  3797              p->TState = ObjectWaiter::TS_ENTER ;
       
  3798              p->_prev = q ;
       
  3799              q = p ;
       
  3800          }
       
  3801       }
       
  3802 
       
  3803       // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
       
  3804       // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
       
  3805 
       
  3806       // See if we can abdicate to a spinner instead of waking a thread.
       
  3807       // A primary goal of the implementation is to reduce the
       
  3808       // context-switch rate.
       
  3809       if (_succ != NULL) continue;
       
  3810 
       
  3811       w = _EntryList  ;
       
  3812       if (w != NULL) {
       
  3813           guarantee (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
       
  3814           ExitEpilog (Self, w) ;
       
  3815           return ;
       
  3816       }
       
  3817    }
       
  3818 }
       
  3819 // complete_exit exits a lock returning recursion count
       
  3820 // complete_exit/reenter operate as a wait without waiting
       
  3821 // complete_exit requires an inflated monitor
       
  3822 // The _owner field is not always the Thread addr even with an
       
  3823 // inflated monitor, e.g. the monitor can be inflated by a non-owning
       
  3824 // thread due to contention.
       
  3825 intptr_t ObjectMonitor::complete_exit(TRAPS) {
       
  3826    Thread * const Self = THREAD;
       
  3827    assert(Self->is_Java_thread(), "Must be Java thread!");
       
  3828    JavaThread *jt = (JavaThread *)THREAD;
       
  3829 
       
  3830    DeferredInitialize();
       
  3831 
       
  3832    if (THREAD != _owner) {
       
  3833     if (THREAD->is_lock_owned ((address)_owner)) {
       
  3834        assert(_recursions == 0, "internal state error");
       
  3835        _owner = THREAD ;   /* Convert from basiclock addr to Thread addr */
       
  3836        _recursions = 0 ;
       
  3837        OwnerIsThread = 1 ;
       
  3838     }
       
  3839    }
       
  3840 
       
  3841    guarantee(Self == _owner, "complete_exit not owner");
       
  3842    intptr_t save = _recursions; // record the old recursion count
       
  3843    _recursions = 0;        // set the recursion level to be 0
       
  3844    exit (Self) ;           // exit the monitor
       
  3845    guarantee (_owner != Self, "invariant");
       
  3846    return save;
       
  3847 }
       
  3848 
       
  3849 // reenter() enters a lock and sets recursion count
       
  3850 // complete_exit/reenter operate as a wait without waiting
       
  3851 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
       
  3852    Thread * const Self = THREAD;
       
  3853    assert(Self->is_Java_thread(), "Must be Java thread!");
       
  3854    JavaThread *jt = (JavaThread *)THREAD;
       
  3855 
       
  3856    guarantee(_owner != Self, "reenter already owner");
       
  3857    enter (THREAD);       // enter the monitor
       
  3858    guarantee (_recursions == 0, "reenter recursion");
       
  3859    _recursions = recursions;
       
  3860    return;
       
  3861 }
       
  3862 
       
  3863 // Note: a subset of changes to ObjectMonitor::wait()
       
  3864 // will need to be replicated in complete_exit above
       
  3865 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
       
  3866    Thread * const Self = THREAD ;
       
  3867    assert(Self->is_Java_thread(), "Must be Java thread!");
       
  3868    JavaThread *jt = (JavaThread *)THREAD;
       
  3869 
       
  3870    DeferredInitialize () ;
       
  3871 
       
  3872    // Throw IMSX or IEX.
       
  3873    CHECK_OWNER();
       
  3874 
       
  3875    // check for a pending interrupt
       
  3876    if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
       
  3877      // post monitor waited event.  Note that this is past-tense, we are done waiting.
       
  3878      if (JvmtiExport::should_post_monitor_waited()) {
       
  3879         // Note: 'false' parameter is passed here because the
       
  3880         // wait was not timed out due to thread interrupt.
       
  3881         JvmtiExport::post_monitor_waited(jt, this, false);
       
  3882      }
       
  3883      TEVENT (Wait - Throw IEX) ;
       
  3884      THROW(vmSymbols::java_lang_InterruptedException());
       
  3885      return ;
       
  3886    }
       
  3887    TEVENT (Wait) ;
       
  3888 
       
  3889    assert (Self->_Stalled == 0, "invariant") ;
       
  3890    Self->_Stalled = intptr_t(this) ;
       
  3891    jt->set_current_waiting_monitor(this);
       
  3892 
       
  3893    // create a node to be put into the queue
       
  3894    // Critically, after we reset() the event but prior to park(), we must check
       
  3895    // for a pending interrupt.
       
  3896    ObjectWaiter node(Self);
       
  3897    node.TState = ObjectWaiter::TS_WAIT ;
       
  3898    Self->_ParkEvent->reset() ;
       
  3899    OrderAccess::fence();          // ST into Event; membar ; LD interrupted-flag
       
  3900 
       
  3901    // Enter the waiting queue, which is a circular doubly linked list in this case
       
  3902    // but it could be a priority queue or any data structure.
       
  3903    // _WaitSetLock protects the wait queue.  Normally the wait queue is accessed only
       
  3904    // by the the owner of the monitor *except* in the case where park()
       
  3905    // returns because of a timeout of interrupt.  Contention is exceptionally rare
       
  3906    // so we use a simple spin-lock instead of a heavier-weight blocking lock.
       
  3907 
       
  3908    Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ;
       
  3909    AddWaiter (&node) ;
       
  3910    Thread::SpinRelease (&_WaitSetLock) ;
       
  3911 
       
  3912    if ((SyncFlags & 4) == 0) {
       
  3913       _Responsible = NULL ;
       
  3914    }
       
  3915    intptr_t save = _recursions; // record the old recursion count
       
  3916    _waiters++;                  // increment the number of waiters
       
  3917    _recursions = 0;             // set the recursion level to be 1
       
  3918    exit (Self) ;                    // exit the monitor
       
  3919    guarantee (_owner != Self, "invariant") ;
       
  3920 
       
  3921    // As soon as the ObjectMonitor's ownership is dropped in the exit()
       
  3922    // call above, another thread can enter() the ObjectMonitor, do the
       
  3923    // notify(), and exit() the ObjectMonitor. If the other thread's
       
  3924    // exit() call chooses this thread as the successor and the unpark()
       
  3925    // call happens to occur while this thread is posting a
       
  3926    // MONITOR_CONTENDED_EXIT event, then we run the risk of the event
       
  3927    // handler using RawMonitors and consuming the unpark().
       
  3928    //
       
  3929    // To avoid the problem, we re-post the event. This does no harm
       
  3930    // even if the original unpark() was not consumed because we are the
       
  3931    // chosen successor for this monitor.
       
  3932    if (node._notified != 0 && _succ == Self) {
       
  3933       node._event->unpark();
       
  3934    }
       
  3935 
       
  3936    // The thread is on the WaitSet list - now park() it.
       
  3937    // On MP systems it's conceivable that a brief spin before we park
       
  3938    // could be profitable.
       
  3939    //
       
  3940    // TODO-FIXME: change the following logic to a loop of the form
       
  3941    //   while (!timeout && !interrupted && _notified == 0) park()
       
  3942 
       
  3943    int ret = OS_OK ;
       
  3944    int WasNotified = 0 ;
       
  3945    { // State transition wrappers
       
  3946      OSThread* osthread = Self->osthread();
       
  3947      OSThreadWaitState osts(osthread, true);
       
  3948      {
       
  3949        ThreadBlockInVM tbivm(jt);
       
  3950        // Thread is in thread_blocked state and oop access is unsafe.
       
  3951        jt->set_suspend_equivalent();
       
  3952 
       
  3953        if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
       
  3954            // Intentionally empty
       
  3955        } else
       
  3956        if (node._notified == 0) {
       
  3957          if (millis <= 0) {
       
  3958             Self->_ParkEvent->park () ;
       
  3959          } else {
       
  3960             ret = Self->_ParkEvent->park (millis) ;
       
  3961          }
       
  3962        }
       
  3963 
       
  3964        // were we externally suspended while we were waiting?
       
  3965        if (ExitSuspendEquivalent (jt)) {
       
  3966           // TODO-FIXME: add -- if succ == Self then succ = null.
       
  3967           jt->java_suspend_self();
       
  3968        }
       
  3969 
       
  3970      } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
       
  3971 
       
  3972 
       
  3973      // Node may be on the WaitSet, the EntryList (or cxq), or in transition
       
  3974      // from the WaitSet to the EntryList.
       
  3975      // See if we need to remove Node from the WaitSet.
       
  3976      // We use double-checked locking to avoid grabbing _WaitSetLock
       
  3977      // if the thread is not on the wait queue.
       
  3978      //
       
  3979      // Note that we don't need a fence before the fetch of TState.
       
  3980      // In the worst case we'll fetch a old-stale value of TS_WAIT previously
       
  3981      // written by the is thread. (perhaps the fetch might even be satisfied
       
  3982      // by a look-aside into the processor's own store buffer, although given
       
  3983      // the length of the code path between the prior ST and this load that's
       
  3984      // highly unlikely).  If the following LD fetches a stale TS_WAIT value
       
  3985      // then we'll acquire the lock and then re-fetch a fresh TState value.
       
  3986      // That is, we fail toward safety.
       
  3987 
       
  3988      if (node.TState == ObjectWaiter::TS_WAIT) {
       
  3989          Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ;
       
  3990          if (node.TState == ObjectWaiter::TS_WAIT) {
       
  3991             DequeueSpecificWaiter (&node) ;       // unlink from WaitSet
       
  3992             assert(node._notified == 0, "invariant");
       
  3993             node.TState = ObjectWaiter::TS_RUN ;
       
  3994          }
       
  3995          Thread::SpinRelease (&_WaitSetLock) ;
       
  3996      }
       
  3997 
       
  3998      // The thread is now either on off-list (TS_RUN),
       
  3999      // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
       
  4000      // The Node's TState variable is stable from the perspective of this thread.
       
  4001      // No other threads will asynchronously modify TState.
       
  4002      guarantee (node.TState != ObjectWaiter::TS_WAIT, "invariant") ;
       
  4003      OrderAccess::loadload() ;
       
  4004      if (_succ == Self) _succ = NULL ;
       
  4005      WasNotified = node._notified ;
       
  4006 
       
  4007      // Reentry phase -- reacquire the monitor.
       
  4008      // re-enter contended monitor after object.wait().
       
  4009      // retain OBJECT_WAIT state until re-enter successfully completes
       
  4010      // Thread state is thread_in_vm and oop access is again safe,
       
  4011      // although the raw address of the object may have changed.
       
  4012      // (Don't cache naked oops over safepoints, of course).
       
  4013 
       
  4014      // post monitor waited event. Note that this is past-tense, we are done waiting.
       
  4015      if (JvmtiExport::should_post_monitor_waited()) {
       
  4016        JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
       
  4017      }
       
  4018      OrderAccess::fence() ;
       
  4019 
       
  4020      assert (Self->_Stalled != 0, "invariant") ;
       
  4021      Self->_Stalled = 0 ;
       
  4022 
       
  4023      assert (_owner != Self, "invariant") ;
       
  4024      ObjectWaiter::TStates v = node.TState ;
       
  4025      if (v == ObjectWaiter::TS_RUN) {
       
  4026          enter (Self) ;
       
  4027      } else {
       
  4028          guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
       
  4029          ReenterI (Self, &node) ;
       
  4030          node.wait_reenter_end(this);
       
  4031      }
       
  4032 
       
  4033      // Self has reacquired the lock.
       
  4034      // Lifecycle - the node representing Self must not appear on any queues.
       
  4035      // Node is about to go out-of-scope, but even if it were immortal we wouldn't
       
  4036      // want residual elements associated with this thread left on any lists.
       
  4037      guarantee (node.TState == ObjectWaiter::TS_RUN, "invariant") ;
       
  4038      assert    (_owner == Self, "invariant") ;
       
  4039      assert    (_succ != Self , "invariant") ;
       
  4040    } // OSThreadWaitState()
       
  4041 
       
  4042    jt->set_current_waiting_monitor(NULL);
       
  4043 
       
  4044    guarantee (_recursions == 0, "invariant") ;
       
  4045    _recursions = save;     // restore the old recursion count
       
  4046    _waiters--;             // decrement the number of waiters
       
  4047 
       
  4048    // Verify a few postconditions
       
  4049    assert (_owner == Self       , "invariant") ;
       
  4050    assert (_succ  != Self       , "invariant") ;
       
  4051    assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
       
  4052 
       
  4053    if (SyncFlags & 32) {
       
  4054       OrderAccess::fence() ;
       
  4055    }
       
  4056 
       
  4057    // check if the notification happened
       
  4058    if (!WasNotified) {
       
  4059      // no, it could be timeout or Thread.interrupt() or both
       
  4060      // check for interrupt event, otherwise it is timeout
       
  4061      if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
       
  4062        TEVENT (Wait - throw IEX from epilog) ;
       
  4063        THROW(vmSymbols::java_lang_InterruptedException());
       
  4064      }
       
  4065    }
       
  4066 
       
  4067    // NOTE: Spurious wake up will be consider as timeout.
       
  4068    // Monitor notify has precedence over thread interrupt.
       
  4069 }
       
  4070 
       
  4071 
       
  4072 // Consider:
       
  4073 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
       
  4074 // then instead of transferring a thread from the WaitSet to the EntryList
       
  4075 // we might just dequeue a thread from the WaitSet and directly unpark() it.
       
  4076 
       
  4077 void ObjectMonitor::notify(TRAPS) {
       
  4078   CHECK_OWNER();
       
  4079   if (_WaitSet == NULL) {
       
  4080      TEVENT (Empty-Notify) ;
       
  4081      return ;
       
  4082   }
       
  4083   DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
       
  4084 
       
  4085   int Policy = Knob_MoveNotifyee ;
       
  4086 
       
  4087   Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ;
       
  4088   ObjectWaiter * iterator = DequeueWaiter() ;
       
  4089   if (iterator != NULL) {
       
  4090      TEVENT (Notify1 - Transfer) ;
       
  4091      guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
       
  4092      guarantee (iterator->_notified == 0, "invariant") ;
       
  4093      // Disposition - what might we do with iterator ?
       
  4094      // a.  add it directly to the EntryList - either tail or head.
       
  4095      // b.  push it onto the front of the _cxq.
       
  4096      // For now we use (a).
       
  4097      if (Policy != 4) {
       
  4098         iterator->TState = ObjectWaiter::TS_ENTER ;
       
  4099      }
       
  4100      iterator->_notified = 1 ;
       
  4101 
       
  4102      ObjectWaiter * List = _EntryList ;
       
  4103      if (List != NULL) {
       
  4104         assert (List->_prev == NULL, "invariant") ;
       
  4105         assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
       
  4106         assert (List != iterator, "invariant") ;
       
  4107      }
       
  4108 
       
  4109      if (Policy == 0) {       // prepend to EntryList
       
  4110          if (List == NULL) {
       
  4111              iterator->_next = iterator->_prev = NULL ;
       
  4112              _EntryList = iterator ;
       
  4113          } else {
       
  4114              List->_prev = iterator ;
       
  4115              iterator->_next = List ;
       
  4116              iterator->_prev = NULL ;
       
  4117              _EntryList = iterator ;
       
  4118         }
       
  4119      } else
       
  4120      if (Policy == 1) {      // append to EntryList
       
  4121          if (List == NULL) {
       
  4122              iterator->_next = iterator->_prev = NULL ;
       
  4123              _EntryList = iterator ;
       
  4124          } else {
       
  4125             // CONSIDER:  finding the tail currently requires a linear-time walk of
       
  4126             // the EntryList.  We can make tail access constant-time by converting to
       
  4127             // a CDLL instead of using our current DLL.
       
  4128             ObjectWaiter * Tail ;
       
  4129             for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
       
  4130             assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
       
  4131             Tail->_next = iterator ;
       
  4132             iterator->_prev = Tail ;
       
  4133             iterator->_next = NULL ;
       
  4134         }
       
  4135      } else
       
  4136      if (Policy == 2) {      // prepend to cxq
       
  4137          // prepend to cxq
       
  4138          if (List == NULL) {
       
  4139              iterator->_next = iterator->_prev = NULL ;
       
  4140              _EntryList = iterator ;
       
  4141          } else {
       
  4142             iterator->TState = ObjectWaiter::TS_CXQ ;
       
  4143             for (;;) {
       
  4144                 ObjectWaiter * Front = _cxq ;
       
  4145                 iterator->_next = Front ;
       
  4146                 if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
       
  4147                     break ;
       
  4148                 }
       
  4149             }
       
  4150          }
       
  4151      } else
       
  4152      if (Policy == 3) {      // append to cxq
       
  4153         iterator->TState = ObjectWaiter::TS_CXQ ;
       
  4154         for (;;) {
       
  4155             ObjectWaiter * Tail ;
       
  4156             Tail = _cxq ;
       
  4157             if (Tail == NULL) {
       
  4158                 iterator->_next = NULL ;
       
  4159                 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
       
  4160                    break ;
       
  4161                 }
       
  4162             } else {
       
  4163                 while (Tail->_next != NULL) Tail = Tail->_next ;
       
  4164                 Tail->_next = iterator ;
       
  4165                 iterator->_prev = Tail ;
       
  4166                 iterator->_next = NULL ;
       
  4167                 break ;
       
  4168             }
       
  4169         }
       
  4170      } else {
       
  4171         ParkEvent * ev = iterator->_event ;
       
  4172         iterator->TState = ObjectWaiter::TS_RUN ;
       
  4173         OrderAccess::fence() ;
       
  4174         ev->unpark() ;
       
  4175      }
       
  4176 
       
  4177      if (Policy < 4) {
       
  4178        iterator->wait_reenter_begin(this);
       
  4179      }
       
  4180 
       
  4181      // _WaitSetLock protects the wait queue, not the EntryList.  We could
       
  4182      // move the add-to-EntryList operation, above, outside the critical section
       
  4183      // protected by _WaitSetLock.  In practice that's not useful.  With the
       
  4184      // exception of  wait() timeouts and interrupts the monitor owner
       
  4185      // is the only thread that grabs _WaitSetLock.  There's almost no contention
       
  4186      // on _WaitSetLock so it's not profitable to reduce the length of the
       
  4187      // critical section.
       
  4188   }
       
  4189 
       
  4190   Thread::SpinRelease (&_WaitSetLock) ;
       
  4191 
       
  4192   if (iterator != NULL && ObjectSynchronizer::_sync_Notifications != NULL) {
       
  4193      ObjectSynchronizer::_sync_Notifications->inc() ;
       
  4194   }
       
  4195 }
       
  4196 
       
  4197 
       
  4198 void ObjectMonitor::notifyAll(TRAPS) {
       
  4199   CHECK_OWNER();
       
  4200   ObjectWaiter* iterator;
       
  4201   if (_WaitSet == NULL) {
       
  4202       TEVENT (Empty-NotifyAll) ;
       
  4203       return ;
       
  4204   }
       
  4205   DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
       
  4206 
       
  4207   int Policy = Knob_MoveNotifyee ;
       
  4208   int Tally = 0 ;
       
  4209   Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notifyall") ;
       
  4210 
       
  4211   for (;;) {
       
  4212      iterator = DequeueWaiter () ;
       
  4213      if (iterator == NULL) break ;
       
  4214      TEVENT (NotifyAll - Transfer1) ;
       
  4215      ++Tally ;
       
  4216 
       
  4217      // Disposition - what might we do with iterator ?
       
  4218      // a.  add it directly to the EntryList - either tail or head.
       
  4219      // b.  push it onto the front of the _cxq.
       
  4220      // For now we use (a).
       
  4221      //
       
  4222      // TODO-FIXME: currently notifyAll() transfers the waiters one-at-a-time from the waitset
       
  4223      // to the EntryList.  This could be done more efficiently with a single bulk transfer,
       
  4224      // but in practice it's not time-critical.  Beware too, that in prepend-mode we invert the
       
  4225      // order of the waiters.  Lets say that the waitset is "ABCD" and the EntryList is "XYZ".
       
  4226      // After a notifyAll() in prepend mode the waitset will be empty and the EntryList will
       
  4227      // be "DCBAXYZ".
       
  4228 
       
  4229      guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
       
  4230      guarantee (iterator->_notified == 0, "invariant") ;
       
  4231      iterator->_notified = 1 ;
       
  4232      if (Policy != 4) {
       
  4233         iterator->TState = ObjectWaiter::TS_ENTER ;
       
  4234      }
       
  4235 
       
  4236      ObjectWaiter * List = _EntryList ;
       
  4237      if (List != NULL) {
       
  4238         assert (List->_prev == NULL, "invariant") ;
       
  4239         assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
       
  4240         assert (List != iterator, "invariant") ;
       
  4241      }
       
  4242 
       
  4243      if (Policy == 0) {       // prepend to EntryList
       
  4244          if (List == NULL) {
       
  4245              iterator->_next = iterator->_prev = NULL ;
       
  4246              _EntryList = iterator ;
       
  4247          } else {
       
  4248              List->_prev = iterator ;
       
  4249              iterator->_next = List ;
       
  4250              iterator->_prev = NULL ;
       
  4251              _EntryList = iterator ;
       
  4252         }
       
  4253      } else
       
  4254      if (Policy == 1) {      // append to EntryList
       
  4255          if (List == NULL) {
       
  4256              iterator->_next = iterator->_prev = NULL ;
       
  4257              _EntryList = iterator ;
       
  4258          } else {
       
  4259             // CONSIDER:  finding the tail currently requires a linear-time walk of
       
  4260             // the EntryList.  We can make tail access constant-time by converting to
       
  4261             // a CDLL instead of using our current DLL.
       
  4262             ObjectWaiter * Tail ;
       
  4263             for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
       
  4264             assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
       
  4265             Tail->_next = iterator ;
       
  4266             iterator->_prev = Tail ;
       
  4267             iterator->_next = NULL ;
       
  4268         }
       
  4269      } else
       
  4270      if (Policy == 2) {      // prepend to cxq
       
  4271          // prepend to cxq
       
  4272          iterator->TState = ObjectWaiter::TS_CXQ ;
       
  4273          for (;;) {
       
  4274              ObjectWaiter * Front = _cxq ;
       
  4275              iterator->_next = Front ;
       
  4276              if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
       
  4277                  break ;
       
  4278              }
       
  4279          }
       
  4280      } else
       
  4281      if (Policy == 3) {      // append to cxq
       
  4282         iterator->TState = ObjectWaiter::TS_CXQ ;
       
  4283         for (;;) {
       
  4284             ObjectWaiter * Tail ;
       
  4285             Tail = _cxq ;
       
  4286             if (Tail == NULL) {
       
  4287                 iterator->_next = NULL ;
       
  4288                 if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
       
  4289                    break ;
       
  4290                 }
       
  4291             } else {
       
  4292                 while (Tail->_next != NULL) Tail = Tail->_next ;
       
  4293                 Tail->_next = iterator ;
       
  4294                 iterator->_prev = Tail ;
       
  4295                 iterator->_next = NULL ;
       
  4296                 break ;
       
  4297             }
       
  4298         }
       
  4299      } else {
       
  4300         ParkEvent * ev = iterator->_event ;
       
  4301         iterator->TState = ObjectWaiter::TS_RUN ;
       
  4302         OrderAccess::fence() ;
       
  4303         ev->unpark() ;
       
  4304      }
       
  4305 
       
  4306      if (Policy < 4) {
       
  4307        iterator->wait_reenter_begin(this);
       
  4308      }
       
  4309 
       
  4310      // _WaitSetLock protects the wait queue, not the EntryList.  We could
       
  4311      // move the add-to-EntryList operation, above, outside the critical section
       
  4312      // protected by _WaitSetLock.  In practice that's not useful.  With the
       
  4313      // exception of  wait() timeouts and interrupts the monitor owner
       
  4314      // is the only thread that grabs _WaitSetLock.  There's almost no contention
       
  4315      // on _WaitSetLock so it's not profitable to reduce the length of the
       
  4316      // critical section.
       
  4317   }
       
  4318 
       
  4319   Thread::SpinRelease (&_WaitSetLock) ;
       
  4320 
       
  4321   if (Tally != 0 && ObjectSynchronizer::_sync_Notifications != NULL) {
       
  4322      ObjectSynchronizer::_sync_Notifications->inc(Tally) ;
       
  4323   }
       
  4324 }
       
  4325 
       
  4326 // check_slow() is a misnomer.  It's called to simply to throw an IMSX exception.
       
  4327 // TODO-FIXME: remove check_slow() -- it's likely dead.
       
  4328 
       
  4329 void ObjectMonitor::check_slow(TRAPS) {
       
  4330   TEVENT (check_slow - throw IMSX) ;
       
  4331   assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
       
  4332   THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
       
  4333 }
       
  4334 
       
  4335 
       
  4336 // -------------------------------------------------------------------------
       
  4337 // The raw monitor subsystem is entirely distinct from normal
       
  4338 // java-synchronization or jni-synchronization.  raw monitors are not
       
  4339 // associated with objects.  They can be implemented in any manner
       
  4340 // that makes sense.  The original implementors decided to piggy-back
       
  4341 // the raw-monitor implementation on the existing Java objectMonitor mechanism.
       
  4342 // This flaw needs to fixed.  We should reimplement raw monitors as sui-generis.
       
  4343 // Specifically, we should not implement raw monitors via java monitors.
       
  4344 // Time permitting, we should disentangle and deconvolve the two implementations
       
  4345 // and move the resulting raw monitor implementation over to the JVMTI directories.
       
  4346 // Ideally, the raw monitor implementation would be built on top of
       
  4347 // park-unpark and nothing else.
       
  4348 //
       
  4349 // raw monitors are used mainly by JVMTI
       
  4350 // The raw monitor implementation borrows the ObjectMonitor structure,
       
  4351 // but the operators are degenerate and extremely simple.
       
  4352 //
       
  4353 // Mixed use of a single objectMonitor instance -- as both a raw monitor
       
  4354 // and a normal java monitor -- is not permissible.
       
  4355 //
       
  4356 // Note that we use the single RawMonitor_lock to protect queue operations for
       
  4357 // _all_ raw monitors.  This is a scalability impediment, but since raw monitor usage
       
  4358 // is deprecated and rare, this is not of concern.  The RawMonitor_lock can not
       
  4359 // be held indefinitely.  The critical sections must be short and bounded.
       
  4360 //
       
  4361 // -------------------------------------------------------------------------
       
  4362 
       
  4363 int ObjectMonitor::SimpleEnter (Thread * Self) {
       
  4364   for (;;) {
       
  4365     if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
       
  4366        return OS_OK ;
       
  4367     }
       
  4368 
       
  4369     ObjectWaiter Node (Self) ;
       
  4370     Self->_ParkEvent->reset() ;     // strictly optional
       
  4371     Node.TState = ObjectWaiter::TS_ENTER ;
       
  4372 
       
  4373     RawMonitor_lock->lock_without_safepoint_check() ;
       
  4374     Node._next  = _EntryList ;
       
  4375     _EntryList  = &Node ;
       
  4376     OrderAccess::fence() ;
       
  4377     if (_owner == NULL && Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
       
  4378         _EntryList = Node._next ;
       
  4379         RawMonitor_lock->unlock() ;
       
  4380         return OS_OK ;
       
  4381     }
       
  4382     RawMonitor_lock->unlock() ;
       
  4383     while (Node.TState == ObjectWaiter::TS_ENTER) {
       
  4384        Self->_ParkEvent->park() ;
       
  4385     }
       
  4386   }
       
  4387 }
       
  4388 
       
  4389 int ObjectMonitor::SimpleExit (Thread * Self) {
       
  4390   guarantee (_owner == Self, "invariant") ;
       
  4391   OrderAccess::release_store_ptr (&_owner, NULL) ;
       
  4392   OrderAccess::fence() ;
       
  4393   if (_EntryList == NULL) return OS_OK ;
       
  4394   ObjectWaiter * w ;
       
  4395 
       
  4396   RawMonitor_lock->lock_without_safepoint_check() ;
       
  4397   w = _EntryList ;
       
  4398   if (w != NULL) {
       
  4399       _EntryList = w->_next ;
       
  4400   }
       
  4401   RawMonitor_lock->unlock() ;
       
  4402   if (w != NULL) {
       
  4403       guarantee (w ->TState == ObjectWaiter::TS_ENTER, "invariant") ;
       
  4404       ParkEvent * ev = w->_event ;
       
  4405       w->TState = ObjectWaiter::TS_RUN ;
       
  4406       OrderAccess::fence() ;
       
  4407       ev->unpark() ;
       
  4408   }
       
  4409   return OS_OK ;
       
  4410 }
       
  4411 
       
  4412 int ObjectMonitor::SimpleWait (Thread * Self, jlong millis) {
       
  4413   guarantee (_owner == Self  , "invariant") ;
       
  4414   guarantee (_recursions == 0, "invariant") ;
       
  4415 
       
  4416   ObjectWaiter Node (Self) ;
       
  4417   Node._notified = 0 ;
       
  4418   Node.TState    = ObjectWaiter::TS_WAIT ;
       
  4419 
       
  4420   RawMonitor_lock->lock_without_safepoint_check() ;
       
  4421   Node._next     = _WaitSet ;
       
  4422   _WaitSet       = &Node ;
       
  4423   RawMonitor_lock->unlock() ;
       
  4424 
       
  4425   SimpleExit (Self) ;
       
  4426   guarantee (_owner != Self, "invariant") ;
       
  4427 
       
  4428   int ret = OS_OK ;
       
  4429   if (millis <= 0) {
       
  4430     Self->_ParkEvent->park();
       
  4431   } else {
       
  4432     ret = Self->_ParkEvent->park(millis);
       
  4433   }
       
  4434 
       
  4435   // If thread still resides on the waitset then unlink it.
       
  4436   // Double-checked locking -- the usage is safe in this context
       
  4437   // as we TState is volatile and the lock-unlock operators are
       
  4438   // serializing (barrier-equivalent).
       
  4439 
       
  4440   if (Node.TState == ObjectWaiter::TS_WAIT) {
       
  4441     RawMonitor_lock->lock_without_safepoint_check() ;
       
  4442     if (Node.TState == ObjectWaiter::TS_WAIT) {
       
  4443       // Simple O(n) unlink, but performance isn't critical here.
       
  4444       ObjectWaiter * p ;
       
  4445       ObjectWaiter * q = NULL ;
       
  4446       for (p = _WaitSet ; p != &Node; p = p->_next) {
       
  4447          q = p ;
       
  4448       }
       
  4449       guarantee (p == &Node, "invariant") ;
       
  4450       if (q == NULL) {
       
  4451         guarantee (p == _WaitSet, "invariant") ;
       
  4452         _WaitSet = p->_next ;
       
  4453       } else {
       
  4454         guarantee (p == q->_next, "invariant") ;
       
  4455         q->_next = p->_next ;
       
  4456       }
       
  4457       Node.TState = ObjectWaiter::TS_RUN ;
       
  4458     }
       
  4459     RawMonitor_lock->unlock() ;
       
  4460   }
       
  4461 
       
  4462   guarantee (Node.TState == ObjectWaiter::TS_RUN, "invariant") ;
       
  4463   SimpleEnter (Self) ;
       
  4464 
       
  4465   guarantee (_owner == Self, "invariant") ;
       
  4466   guarantee (_recursions == 0, "invariant") ;
       
  4467   return ret ;
       
  4468 }
       
  4469 
       
  4470 int ObjectMonitor::SimpleNotify (Thread * Self, bool All) {
       
  4471   guarantee (_owner == Self, "invariant") ;
       
  4472   if (_WaitSet == NULL) return OS_OK ;
       
  4473 
       
  4474   // We have two options:
       
  4475   // A. Transfer the threads from the WaitSet to the EntryList
       
  4476   // B. Remove the thread from the WaitSet and unpark() it.
       
  4477   //
       
  4478   // We use (B), which is crude and results in lots of futile
       
  4479   // context switching.  In particular (B) induces lots of contention.
       
  4480 
       
  4481   ParkEvent * ev = NULL ;       // consider using a small auto array ...
       
  4482   RawMonitor_lock->lock_without_safepoint_check() ;
       
  4483   for (;;) {
       
  4484       ObjectWaiter * w = _WaitSet ;
       
  4485       if (w == NULL) break ;
       
  4486       _WaitSet = w->_next ;
       
  4487       if (ev != NULL) { ev->unpark(); ev = NULL; }
       
  4488       ev = w->_event ;
       
  4489       OrderAccess::loadstore() ;
       
  4490       w->TState = ObjectWaiter::TS_RUN ;
       
  4491       OrderAccess::storeload();
       
  4492       if (!All) break ;
       
  4493   }
       
  4494   RawMonitor_lock->unlock() ;
       
  4495   if (ev != NULL) ev->unpark();
       
  4496   return OS_OK ;
       
  4497 }
       
  4498 
       
  4499 // Any JavaThread will enter here with state _thread_blocked
       
  4500 int ObjectMonitor::raw_enter(TRAPS) {
       
  4501   TEVENT (raw_enter) ;
       
  4502   void * Contended ;
       
  4503 
       
  4504   // don't enter raw monitor if thread is being externally suspended, it will
       
  4505   // surprise the suspender if a "suspended" thread can still enter monitor
       
  4506   JavaThread * jt = (JavaThread *)THREAD;
       
  4507   if (THREAD->is_Java_thread()) {
       
  4508     jt->SR_lock()->lock_without_safepoint_check();
       
  4509     while (jt->is_external_suspend()) {
       
  4510       jt->SR_lock()->unlock();
       
  4511       jt->java_suspend_self();
       
  4512       jt->SR_lock()->lock_without_safepoint_check();
       
  4513     }
       
  4514     // guarded by SR_lock to avoid racing with new external suspend requests.
       
  4515     Contended = Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) ;
       
  4516     jt->SR_lock()->unlock();
       
  4517   } else {
       
  4518     Contended = Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) ;
       
  4519   }
       
  4520 
       
  4521   if (Contended == THREAD) {
       
  4522      _recursions ++ ;
       
  4523      return OM_OK ;
       
  4524   }
       
  4525 
       
  4526   if (Contended == NULL) {
       
  4527      guarantee (_owner == THREAD, "invariant") ;
       
  4528      guarantee (_recursions == 0, "invariant") ;
       
  4529      return OM_OK ;
       
  4530   }
       
  4531 
       
  4532   THREAD->set_current_pending_monitor(this);
       
  4533 
       
  4534   if (!THREAD->is_Java_thread()) {
       
  4535      // No other non-Java threads besides VM thread would acquire
       
  4536      // a raw monitor.
       
  4537      assert(THREAD->is_VM_thread(), "must be VM thread");
       
  4538      SimpleEnter (THREAD) ;
       
  4539    } else {
       
  4540      guarantee (jt->thread_state() == _thread_blocked, "invariant") ;
       
  4541      for (;;) {
       
  4542        jt->set_suspend_equivalent();
       
  4543        // cleared by handle_special_suspend_equivalent_condition() or
       
  4544        // java_suspend_self()
       
  4545        SimpleEnter (THREAD) ;
       
  4546 
       
  4547        // were we externally suspended while we were waiting?
       
  4548        if (!jt->handle_special_suspend_equivalent_condition()) break ;
       
  4549 
       
  4550        // This thread was externally suspended
       
  4551        //
       
  4552        // This logic isn't needed for JVMTI raw monitors,
       
  4553        // but doesn't hurt just in case the suspend rules change. This
       
  4554            // logic is needed for the ObjectMonitor.wait() reentry phase.
       
  4555            // We have reentered the contended monitor, but while we were
       
  4556            // waiting another thread suspended us. We don't want to reenter
       
  4557            // the monitor while suspended because that would surprise the
       
  4558            // thread that suspended us.
       
  4559            //
       
  4560            // Drop the lock -
       
  4561        SimpleExit (THREAD) ;
       
  4562 
       
  4563            jt->java_suspend_self();
       
  4564          }
       
  4565 
       
  4566      assert(_owner == THREAD, "Fatal error with monitor owner!");
       
  4567      assert(_recursions == 0, "Fatal error with monitor recursions!");
       
  4568   }
       
  4569 
       
  4570   THREAD->set_current_pending_monitor(NULL);
       
  4571   guarantee (_recursions == 0, "invariant") ;
       
  4572   return OM_OK;
       
  4573 }
       
  4574 
       
  4575 // Used mainly for JVMTI raw monitor implementation
       
  4576 // Also used for ObjectMonitor::wait().
       
  4577 int ObjectMonitor::raw_exit(TRAPS) {
       
  4578   TEVENT (raw_exit) ;
       
  4579   if (THREAD != _owner) {
       
  4580     return OM_ILLEGAL_MONITOR_STATE;
       
  4581   }
       
  4582   if (_recursions > 0) {
       
  4583     --_recursions ;
       
  4584     return OM_OK ;
       
  4585   }
       
  4586 
       
  4587   void * List = _EntryList ;
       
  4588   SimpleExit (THREAD) ;
       
  4589 
       
  4590   return OM_OK;
       
  4591 }
       
  4592 
       
  4593 // Used for JVMTI raw monitor implementation.
       
  4594 // All JavaThreads will enter here with state _thread_blocked
       
  4595 
       
  4596 int ObjectMonitor::raw_wait(jlong millis, bool interruptible, TRAPS) {
       
  4597   TEVENT (raw_wait) ;
       
  4598   if (THREAD != _owner) {
       
  4599     return OM_ILLEGAL_MONITOR_STATE;
       
  4600   }
       
  4601 
       
  4602   // To avoid spurious wakeups we reset the parkevent -- This is strictly optional.
       
  4603   // The caller must be able to tolerate spurious returns from raw_wait().
       
  4604   THREAD->_ParkEvent->reset() ;
       
  4605   OrderAccess::fence() ;
       
  4606 
       
  4607   // check interrupt event
       
  4608   if (interruptible && Thread::is_interrupted(THREAD, true)) {
       
  4609     return OM_INTERRUPTED;
       
  4610   }
       
  4611 
       
  4612   intptr_t save = _recursions ;
       
  4613   _recursions = 0 ;
       
  4614   _waiters ++ ;
       
  4615   if (THREAD->is_Java_thread()) {
       
  4616     guarantee (((JavaThread *) THREAD)->thread_state() == _thread_blocked, "invariant") ;
       
  4617     ((JavaThread *)THREAD)->set_suspend_equivalent();
       
  4618   }
       
  4619   int rv = SimpleWait (THREAD, millis) ;
       
  4620   _recursions = save ;
       
  4621   _waiters -- ;
       
  4622 
       
  4623   guarantee (THREAD == _owner, "invariant") ;
       
  4624   if (THREAD->is_Java_thread()) {
       
  4625      JavaThread * jSelf = (JavaThread *) THREAD ;
       
  4626      for (;;) {
       
  4627         if (!jSelf->handle_special_suspend_equivalent_condition()) break ;
       
  4628         SimpleExit (THREAD) ;
       
  4629         jSelf->java_suspend_self();
       
  4630         SimpleEnter (THREAD) ;
       
  4631         jSelf->set_suspend_equivalent() ;
       
  4632      }
       
  4633   }
       
  4634   guarantee (THREAD == _owner, "invariant") ;
       
  4635 
       
  4636   if (interruptible && Thread::is_interrupted(THREAD, true)) {
       
  4637     return OM_INTERRUPTED;
       
  4638   }
       
  4639   return OM_OK ;
       
  4640 }
       
  4641 
       
  4642 int ObjectMonitor::raw_notify(TRAPS) {
       
  4643   TEVENT (raw_notify) ;
       
  4644   if (THREAD != _owner) {
       
  4645     return OM_ILLEGAL_MONITOR_STATE;
       
  4646   }
       
  4647   SimpleNotify (THREAD, false) ;
       
  4648   return OM_OK;
       
  4649 }
       
  4650 
       
  4651 int ObjectMonitor::raw_notifyAll(TRAPS) {
       
  4652   TEVENT (raw_notifyAll) ;
       
  4653   if (THREAD != _owner) {
       
  4654     return OM_ILLEGAL_MONITOR_STATE;
       
  4655   }
       
  4656   SimpleNotify (THREAD, true) ;
       
  4657   return OM_OK;
       
  4658 }
       
  4659 
       
  4660 #ifndef PRODUCT
       
  4661 void ObjectMonitor::verify() {
       
  4662 }
       
  4663 
       
  4664 void ObjectMonitor::print() {
       
  4665 }
       
  4666 #endif
       
  4667 
       
  4668 //------------------------------------------------------------------------------
       
  4669 // Non-product code
       
  4670 
       
  4671 #ifndef PRODUCT
       
  4672 
       
  4673 void ObjectSynchronizer::trace_locking(Handle locking_obj, bool is_compiled,
       
  4674                                        bool is_method, bool is_locking) {
       
  4675   // Don't know what to do here
       
  4676 }
       
  4677 
       
  4678 // Verify all monitors in the monitor cache, the verification is weak.
       
  4679 void ObjectSynchronizer::verify() {
       
  4680   ObjectMonitor* block = gBlockList;
       
  4681   ObjectMonitor* mid;
       
  4682   while (block) {
       
  4683     assert(block->object() == CHAINMARKER, "must be a block header");
       
  4684     for (int i = 1; i < _BLOCKSIZE; i++) {
       
  4685       mid = block + i;
       
  4686       oop object = (oop) mid->object();
       
  4687       if (object != NULL) {
       
  4688         mid->verify();
       
  4689       }
       
  4690     }
       
  4691     block = (ObjectMonitor*) block->FreeNext;
       
  4692   }
       
  4693 }
       
  4694 
       
  4695 // Check if monitor belongs to the monitor cache
       
  4696 // The list is grow-only so it's *relatively* safe to traverse
       
  4697 // the list of extant blocks without taking a lock.
       
  4698 
       
  4699 int ObjectSynchronizer::verify_objmon_isinpool(ObjectMonitor *monitor) {
       
  4700   ObjectMonitor* block = gBlockList;
       
  4701 
       
  4702   while (block) {
       
  4703     assert(block->object() == CHAINMARKER, "must be a block header");
       
  4704     if (monitor > &block[0] && monitor < &block[_BLOCKSIZE]) {
       
  4705       address mon = (address) monitor;
       
  4706       address blk = (address) block;
       
  4707       size_t diff = mon - blk;
       
  4708       assert((diff % sizeof(ObjectMonitor)) == 0, "check");
       
  4709       return 1;
       
  4710     }
       
  4711     block = (ObjectMonitor*) block->FreeNext;
       
  4712   }
       
  4713   return 0;
       
  4714 }
       
  4715 
       
  4716 #endif