--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/hotspot/src/share/vm/runtime/synchronizer.cpp Sat Dec 01 00:00:00 2007 +0000
@@ -0,0 +1,4716 @@
+/*
+ * Copyright 1998-2007 Sun Microsystems, Inc. All Rights Reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
+ * CA 95054 USA or visit www.sun.com if you need additional information or
+ * have any questions.
+ *
+ */
+
+# include "incls/_precompiled.incl"
+# include "incls/_synchronizer.cpp.incl"
+
+#if defined(__GNUC__) && !defined(IA64)
+ // Need to inhibit inlining for older versions of GCC to avoid build-time failures
+ #define ATTR __attribute__((noinline))
+#else
+ #define ATTR
+#endif
+
+// Native markword accessors for synchronization and hashCode().
+//
+// The "core" versions of monitor enter and exit reside in this file.
+// The interpreter and compilers contain specialized transliterated
+// variants of the enter-exit fast-path operations. See i486.ad fast_lock(),
+// for instance. If you make changes here, make sure to modify the
+// interpreter, and both C1 and C2 fast-path inline locking code emission.
+//
+// TODO: merge the objectMonitor and synchronizer classes.
+//
+// -----------------------------------------------------------------------------
+
+#ifdef DTRACE_ENABLED
+
+// Only bother with this argument setup if dtrace is available
+// TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
+
+HS_DTRACE_PROBE_DECL5(hotspot, monitor__wait,
+ jlong, uintptr_t, char*, int, long);
+HS_DTRACE_PROBE_DECL4(hotspot, monitor__waited,
+ jlong, uintptr_t, char*, int);
+HS_DTRACE_PROBE_DECL4(hotspot, monitor__notify,
+ jlong, uintptr_t, char*, int);
+HS_DTRACE_PROBE_DECL4(hotspot, monitor__notifyAll,
+ jlong, uintptr_t, char*, int);
+HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__enter,
+ jlong, uintptr_t, char*, int);
+HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__entered,
+ jlong, uintptr_t, char*, int);
+HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__exit,
+ jlong, uintptr_t, char*, int);
+
+#define DTRACE_MONITOR_PROBE_COMMON(klassOop, thread) \
+ char* bytes = NULL; \
+ int len = 0; \
+ jlong jtid = SharedRuntime::get_java_tid(thread); \
+ symbolOop klassname = ((oop)(klassOop))->klass()->klass_part()->name(); \
+ if (klassname != NULL) { \
+ bytes = (char*)klassname->bytes(); \
+ len = klassname->utf8_length(); \
+ }
+
+#define DTRACE_MONITOR_WAIT_PROBE(monitor, klassOop, thread, millis) \
+ { \
+ if (DTraceMonitorProbes) { \
+ DTRACE_MONITOR_PROBE_COMMON(klassOop, thread); \
+ HS_DTRACE_PROBE5(hotspot, monitor__wait, jtid, \
+ (monitor), bytes, len, (millis)); \
+ } \
+ }
+
+#define DTRACE_MONITOR_PROBE(probe, monitor, klassOop, thread) \
+ { \
+ if (DTraceMonitorProbes) { \
+ DTRACE_MONITOR_PROBE_COMMON(klassOop, thread); \
+ HS_DTRACE_PROBE4(hotspot, monitor__##probe, jtid, \
+ (uintptr_t)(monitor), bytes, len); \
+ } \
+ }
+
+#else // ndef DTRACE_ENABLED
+
+#define DTRACE_MONITOR_WAIT_PROBE(klassOop, thread, millis, mon) {;}
+#define DTRACE_MONITOR_PROBE(probe, klassOop, thread, mon) {;}
+
+#endif // ndef DTRACE_ENABLED
+
+// ObjectWaiter serves as a "proxy" or surrogate thread.
+// TODO-FIXME: Eliminate ObjectWaiter and use the thread-specific
+// ParkEvent instead. Beware, however, that the JVMTI code
+// knows about ObjectWaiters, so we'll have to reconcile that code.
+// See next_waiter(), first_waiter(), etc.
+
+class ObjectWaiter : public StackObj {
+ public:
+ enum TStates { TS_UNDEF, TS_READY, TS_RUN, TS_WAIT, TS_ENTER, TS_CXQ } ;
+ enum Sorted { PREPEND, APPEND, SORTED } ;
+ ObjectWaiter * volatile _next;
+ ObjectWaiter * volatile _prev;
+ Thread* _thread;
+ ParkEvent * _event;
+ volatile int _notified ;
+ volatile TStates TState ;
+ Sorted _Sorted ; // List placement disposition
+ bool _active ; // Contention monitoring is enabled
+ public:
+ ObjectWaiter(Thread* thread) {
+ _next = NULL;
+ _prev = NULL;
+ _notified = 0;
+ TState = TS_RUN ;
+ _thread = thread;
+ _event = thread->_ParkEvent ;
+ _active = false;
+ assert (_event != NULL, "invariant") ;
+ }
+
+ void wait_reenter_begin(ObjectMonitor *mon) {
+ JavaThread *jt = (JavaThread *)this->_thread;
+ _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
+ }
+
+ void wait_reenter_end(ObjectMonitor *mon) {
+ JavaThread *jt = (JavaThread *)this->_thread;
+ JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
+ }
+};
+
+enum ManifestConstants {
+ ClearResponsibleAtSTW = 0,
+ MaximumRecheckInterval = 1000
+} ;
+
+
+#undef TEVENT
+#define TEVENT(nom) {if (SyncVerbose) FEVENT(nom); }
+
+#define FEVENT(nom) { static volatile int ctr = 0 ; int v = ++ctr ; if ((v & (v-1)) == 0) { ::printf (#nom " : %d \n", v); ::fflush(stdout); }}
+
+#undef TEVENT
+#define TEVENT(nom) {;}
+
+// Performance concern:
+// OrderAccess::storestore() calls release() which STs 0 into the global volatile
+// OrderAccess::Dummy variable. This store is unnecessary for correctness.
+// Many threads STing into a common location causes considerable cache migration
+// or "sloshing" on large SMP system. As such, I avoid using OrderAccess::storestore()
+// until it's repaired. In some cases OrderAccess::fence() -- which incurs local
+// latency on the executing processor -- is a better choice as it scales on SMP
+// systems. See http://blogs.sun.com/dave/entry/biased_locking_in_hotspot for a
+// discussion of coherency costs. Note that all our current reference platforms
+// provide strong ST-ST order, so the issue is moot on IA32, x64, and SPARC.
+//
+// As a general policy we use "volatile" to control compiler-based reordering
+// and explicit fences (barriers) to control for architectural reordering performed
+// by the CPU(s) or platform.
+
+static int MBFence (int x) { OrderAccess::fence(); return x; }
+
+struct SharedGlobals {
+ // These are highly shared mostly-read variables.
+ // To avoid false-sharing they need to be the sole occupants of a $ line.
+ double padPrefix [8];
+ volatile int stwRandom ;
+ volatile int stwCycle ;
+
+ // Hot RW variables -- Sequester to avoid false-sharing
+ double padSuffix [16];
+ volatile int hcSequence ;
+ double padFinal [8] ;
+} ;
+
+static SharedGlobals GVars ;
+
+
+// Tunables ...
+// The knob* variables are effectively final. Once set they should
+// never be modified hence. Consider using __read_mostly with GCC.
+
+static int Knob_LogSpins = 0 ; // enable jvmstat tally for spins
+static int Knob_HandOff = 0 ;
+static int Knob_Verbose = 0 ;
+static int Knob_ReportSettings = 0 ;
+
+static int Knob_SpinLimit = 5000 ; // derived by an external tool -
+static int Knob_SpinBase = 0 ; // Floor AKA SpinMin
+static int Knob_SpinBackOff = 0 ; // spin-loop backoff
+static int Knob_CASPenalty = -1 ; // Penalty for failed CAS
+static int Knob_OXPenalty = -1 ; // Penalty for observed _owner change
+static int Knob_SpinSetSucc = 1 ; // spinners set the _succ field
+static int Knob_SpinEarly = 1 ;
+static int Knob_SuccEnabled = 1 ; // futile wake throttling
+static int Knob_SuccRestrict = 0 ; // Limit successors + spinners to at-most-one
+static int Knob_MaxSpinners = -1 ; // Should be a function of # CPUs
+static int Knob_Bonus = 100 ; // spin success bonus
+static int Knob_BonusB = 100 ; // spin success bonus
+static int Knob_Penalty = 200 ; // spin failure penalty
+static int Knob_Poverty = 1000 ;
+static int Knob_SpinAfterFutile = 1 ; // Spin after returning from park()
+static int Knob_FixedSpin = 0 ;
+static int Knob_OState = 3 ; // Spinner checks thread state of _owner
+static int Knob_UsePause = 1 ;
+static int Knob_ExitPolicy = 0 ;
+static int Knob_PreSpin = 10 ; // 20-100 likely better
+static int Knob_ResetEvent = 0 ;
+static int BackOffMask = 0 ;
+
+static int Knob_FastHSSEC = 0 ;
+static int Knob_MoveNotifyee = 2 ; // notify() - disposition of notifyee
+static int Knob_QMode = 0 ; // EntryList-cxq policy - queue discipline
+static volatile int InitDone = 0 ;
+
+
+// hashCode() generation :
+//
+// Possibilities:
+// * MD5Digest of {obj,stwRandom}
+// * CRC32 of {obj,stwRandom} or any linear-feedback shift register function.
+// * A DES- or AES-style SBox[] mechanism
+// * One of the Phi-based schemes, such as:
+// 2654435761 = 2^32 * Phi (golden ratio)
+// HashCodeValue = ((uintptr_t(obj) >> 3) * 2654435761) ^ GVars.stwRandom ;
+// * A variation of Marsaglia's shift-xor RNG scheme.
+// * (obj ^ stwRandom) is appealing, but can result
+// in undesirable regularity in the hashCode values of adjacent objects
+// (objects allocated back-to-back, in particular). This could potentially
+// result in hashtable collisions and reduced hashtable efficiency.
+// There are simple ways to "diffuse" the middle address bits over the
+// generated hashCode values:
+//
+
+static inline intptr_t get_next_hash(Thread * Self, oop obj) {
+ intptr_t value = 0 ;
+ if (hashCode == 0) {
+ // This form uses an unguarded global Park-Miller RNG,
+ // so it's possible for two threads to race and generate the same RNG.
+ // On MP system we'll have lots of RW access to a global, so the
+ // mechanism induces lots of coherency traffic.
+ value = os::random() ;
+ } else
+ if (hashCode == 1) {
+ // This variation has the property of being stable (idempotent)
+ // between STW operations. This can be useful in some of the 1-0
+ // synchronization schemes.
+ intptr_t addrBits = intptr_t(obj) >> 3 ;
+ value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom ;
+ } else
+ if (hashCode == 2) {
+ value = 1 ; // for sensitivity testing
+ } else
+ if (hashCode == 3) {
+ value = ++GVars.hcSequence ;
+ } else
+ if (hashCode == 4) {
+ value = intptr_t(obj) ;
+ } else {
+ // Marsaglia's xor-shift scheme with thread-specific state
+ // This is probably the best overall implementation -- we'll
+ // likely make this the default in future releases.
+ unsigned t = Self->_hashStateX ;
+ t ^= (t << 11) ;
+ Self->_hashStateX = Self->_hashStateY ;
+ Self->_hashStateY = Self->_hashStateZ ;
+ Self->_hashStateZ = Self->_hashStateW ;
+ unsigned v = Self->_hashStateW ;
+ v = (v ^ (v >> 19)) ^ (t ^ (t >> 8)) ;
+ Self->_hashStateW = v ;
+ value = v ;
+ }
+
+ value &= markOopDesc::hash_mask;
+ if (value == 0) value = 0xBAD ;
+ assert (value != markOopDesc::no_hash, "invariant") ;
+ TEVENT (hashCode: GENERATE) ;
+ return value;
+}
+
+void BasicLock::print_on(outputStream* st) const {
+ st->print("monitor");
+}
+
+void BasicLock::move_to(oop obj, BasicLock* dest) {
+ // Check to see if we need to inflate the lock. This is only needed
+ // if an object is locked using "this" lightweight monitor. In that
+ // case, the displaced_header() is unlocked, because the
+ // displaced_header() contains the header for the originally unlocked
+ // object. However the object could have already been inflated. But it
+ // does not matter, the inflation will just a no-op. For other cases,
+ // the displaced header will be either 0x0 or 0x3, which are location
+ // independent, therefore the BasicLock is free to move.
+ //
+ // During OSR we may need to relocate a BasicLock (which contains a
+ // displaced word) from a location in an interpreter frame to a
+ // new location in a compiled frame. "this" refers to the source
+ // basiclock in the interpreter frame. "dest" refers to the destination
+ // basiclock in the new compiled frame. We *always* inflate in move_to().
+ // The always-Inflate policy works properly, but in 1.5.0 it can sometimes
+ // cause performance problems in code that makes heavy use of a small # of
+ // uncontended locks. (We'd inflate during OSR, and then sync performance
+ // would subsequently plummet because the thread would be forced thru the slow-path).
+ // This problem has been made largely moot on IA32 by inlining the inflated fast-path
+ // operations in Fast_Lock and Fast_Unlock in i486.ad.
+ //
+ // Note that there is a way to safely swing the object's markword from
+ // one stack location to another. This avoids inflation. Obviously,
+ // we need to ensure that both locations refer to the current thread's stack.
+ // There are some subtle concurrency issues, however, and since the benefit is
+ // is small (given the support for inflated fast-path locking in the fast_lock, etc)
+ // we'll leave that optimization for another time.
+
+ if (displaced_header()->is_neutral()) {
+ ObjectSynchronizer::inflate_helper(obj);
+ // WARNING: We can not put check here, because the inflation
+ // will not update the displaced header. Once BasicLock is inflated,
+ // no one should ever look at its content.
+ } else {
+ // Typically the displaced header will be 0 (recursive stack lock) or
+ // unused_mark. Naively we'd like to assert that the displaced mark
+ // value is either 0, neutral, or 3. But with the advent of the
+ // store-before-CAS avoidance in fast_lock/compiler_lock_object
+ // we can find any flavor mark in the displaced mark.
+ }
+// [RGV] The next line appears to do nothing!
+ intptr_t dh = (intptr_t) displaced_header();
+ dest->set_displaced_header(displaced_header());
+}
+
+// -----------------------------------------------------------------------------
+
+// standard constructor, allows locking failures
+ObjectLocker::ObjectLocker(Handle obj, Thread* thread, bool doLock) {
+ _dolock = doLock;
+ _thread = thread;
+ debug_only(if (StrictSafepointChecks) _thread->check_for_valid_safepoint_state(false);)
+ _obj = obj;
+
+ if (_dolock) {
+ TEVENT (ObjectLocker) ;
+
+ ObjectSynchronizer::fast_enter(_obj, &_lock, false, _thread);
+ }
+}
+
+ObjectLocker::~ObjectLocker() {
+ if (_dolock) {
+ ObjectSynchronizer::fast_exit(_obj(), &_lock, _thread);
+ }
+}
+
+// -----------------------------------------------------------------------------
+
+
+PerfCounter * ObjectSynchronizer::_sync_Inflations = NULL ;
+PerfCounter * ObjectSynchronizer::_sync_Deflations = NULL ;
+PerfCounter * ObjectSynchronizer::_sync_ContendedLockAttempts = NULL ;
+PerfCounter * ObjectSynchronizer::_sync_FutileWakeups = NULL ;
+PerfCounter * ObjectSynchronizer::_sync_Parks = NULL ;
+PerfCounter * ObjectSynchronizer::_sync_EmptyNotifications = NULL ;
+PerfCounter * ObjectSynchronizer::_sync_Notifications = NULL ;
+PerfCounter * ObjectSynchronizer::_sync_PrivateA = NULL ;
+PerfCounter * ObjectSynchronizer::_sync_PrivateB = NULL ;
+PerfCounter * ObjectSynchronizer::_sync_SlowExit = NULL ;
+PerfCounter * ObjectSynchronizer::_sync_SlowEnter = NULL ;
+PerfCounter * ObjectSynchronizer::_sync_SlowNotify = NULL ;
+PerfCounter * ObjectSynchronizer::_sync_SlowNotifyAll = NULL ;
+PerfCounter * ObjectSynchronizer::_sync_FailedSpins = NULL ;
+PerfCounter * ObjectSynchronizer::_sync_SuccessfulSpins = NULL ;
+PerfCounter * ObjectSynchronizer::_sync_MonInCirculation = NULL ;
+PerfCounter * ObjectSynchronizer::_sync_MonScavenged = NULL ;
+PerfLongVariable * ObjectSynchronizer::_sync_MonExtant = NULL ;
+
+// One-shot global initialization for the sync subsystem.
+// We could also defer initialization and initialize on-demand
+// the first time we call inflate(). Initialization would
+// be protected - like so many things - by the MonitorCache_lock.
+
+void ObjectSynchronizer::Initialize () {
+ static int InitializationCompleted = 0 ;
+ assert (InitializationCompleted == 0, "invariant") ;
+ InitializationCompleted = 1 ;
+ if (UsePerfData) {
+ EXCEPTION_MARK ;
+ #define NEWPERFCOUNTER(n) {n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,CHECK); }
+ #define NEWPERFVARIABLE(n) {n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,CHECK); }
+ NEWPERFCOUNTER(_sync_Inflations) ;
+ NEWPERFCOUNTER(_sync_Deflations) ;
+ NEWPERFCOUNTER(_sync_ContendedLockAttempts) ;
+ NEWPERFCOUNTER(_sync_FutileWakeups) ;
+ NEWPERFCOUNTER(_sync_Parks) ;
+ NEWPERFCOUNTER(_sync_EmptyNotifications) ;
+ NEWPERFCOUNTER(_sync_Notifications) ;
+ NEWPERFCOUNTER(_sync_SlowEnter) ;
+ NEWPERFCOUNTER(_sync_SlowExit) ;
+ NEWPERFCOUNTER(_sync_SlowNotify) ;
+ NEWPERFCOUNTER(_sync_SlowNotifyAll) ;
+ NEWPERFCOUNTER(_sync_FailedSpins) ;
+ NEWPERFCOUNTER(_sync_SuccessfulSpins) ;
+ NEWPERFCOUNTER(_sync_PrivateA) ;
+ NEWPERFCOUNTER(_sync_PrivateB) ;
+ NEWPERFCOUNTER(_sync_MonInCirculation) ;
+ NEWPERFCOUNTER(_sync_MonScavenged) ;
+ NEWPERFVARIABLE(_sync_MonExtant) ;
+ #undef NEWPERFCOUNTER
+ }
+}
+
+// Compile-time asserts
+// When possible, it's better to catch errors deterministically at
+// compile-time than at runtime. The down-side to using compile-time
+// asserts is that error message -- often something about negative array
+// indices -- is opaque.
+
+#define CTASSERT(x) { int tag[1-(2*!(x))]; printf ("Tag @%X\n", tag); }
+
+void ObjectMonitor::ctAsserts() {
+ CTASSERT(offset_of (ObjectMonitor, _header) == 0);
+}
+
+static int Adjust (volatile int * adr, int dx) {
+ int v ;
+ for (v = *adr ; Atomic::cmpxchg (v + dx, adr, v) != v; v = *adr) ;
+ return v ;
+}
+
+// Ad-hoc mutual exclusion primitives: SpinLock and Mux
+//
+// We employ SpinLocks _only for low-contention, fixed-length
+// short-duration critical sections where we're concerned
+// about native mutex_t or HotSpot Mutex:: latency.
+// The mux construct provides a spin-then-block mutual exclusion
+// mechanism.
+//
+// Testing has shown that contention on the ListLock guarding gFreeList
+// is common. If we implement ListLock as a simple SpinLock it's common
+// for the JVM to devolve to yielding with little progress. This is true
+// despite the fact that the critical sections protected by ListLock are
+// extremely short.
+//
+// TODO-FIXME: ListLock should be of type SpinLock.
+// We should make this a 1st-class type, integrated into the lock
+// hierarchy as leaf-locks. Critically, the SpinLock structure
+// should have sufficient padding to avoid false-sharing and excessive
+// cache-coherency traffic.
+
+
+typedef volatile int SpinLockT ;
+
+void Thread::SpinAcquire (volatile int * adr, const char * LockName) {
+ if (Atomic::cmpxchg (1, adr, 0) == 0) {
+ return ; // normal fast-path return
+ }
+
+ // Slow-path : We've encountered contention -- Spin/Yield/Block strategy.
+ TEVENT (SpinAcquire - ctx) ;
+ int ctr = 0 ;
+ int Yields = 0 ;
+ for (;;) {
+ while (*adr != 0) {
+ ++ctr ;
+ if ((ctr & 0xFFF) == 0 || !os::is_MP()) {
+ if (Yields > 5) {
+ // Consider using a simple NakedSleep() instead.
+ // Then SpinAcquire could be called by non-JVM threads
+ Thread::current()->_ParkEvent->park(1) ;
+ } else {
+ os::NakedYield() ;
+ ++Yields ;
+ }
+ } else {
+ SpinPause() ;
+ }
+ }
+ if (Atomic::cmpxchg (1, adr, 0) == 0) return ;
+ }
+}
+
+void Thread::SpinRelease (volatile int * adr) {
+ assert (*adr != 0, "invariant") ;
+ OrderAccess::fence() ; // guarantee at least release consistency.
+ // Roach-motel semantics.
+ // It's safe if subsequent LDs and STs float "up" into the critical section,
+ // but prior LDs and STs within the critical section can't be allowed
+ // to reorder or float past the ST that releases the lock.
+ *adr = 0 ;
+}
+
+// muxAcquire and muxRelease:
+//
+// * muxAcquire and muxRelease support a single-word lock-word construct.
+// The LSB of the word is set IFF the lock is held.
+// The remainder of the word points to the head of a singly-linked list
+// of threads blocked on the lock.
+//
+// * The current implementation of muxAcquire-muxRelease uses its own
+// dedicated Thread._MuxEvent instance. If we're interested in
+// minimizing the peak number of extant ParkEvent instances then
+// we could eliminate _MuxEvent and "borrow" _ParkEvent as long
+// as certain invariants were satisfied. Specifically, care would need
+// to be taken with regards to consuming unpark() "permits".
+// A safe rule of thumb is that a thread would never call muxAcquire()
+// if it's enqueued (cxq, EntryList, WaitList, etc) and will subsequently
+// park(). Otherwise the _ParkEvent park() operation in muxAcquire() could
+// consume an unpark() permit intended for monitorenter, for instance.
+// One way around this would be to widen the restricted-range semaphore
+// implemented in park(). Another alternative would be to provide
+// multiple instances of the PlatformEvent() for each thread. One
+// instance would be dedicated to muxAcquire-muxRelease, for instance.
+//
+// * Usage:
+// -- Only as leaf locks
+// -- for short-term locking only as muxAcquire does not perform
+// thread state transitions.
+//
+// Alternatives:
+// * We could implement muxAcquire and muxRelease with MCS or CLH locks
+// but with parking or spin-then-park instead of pure spinning.
+// * Use Taura-Oyama-Yonenzawa locks.
+// * It's possible to construct a 1-0 lock if we encode the lockword as
+// (List,LockByte). Acquire will CAS the full lockword while Release
+// will STB 0 into the LockByte. The 1-0 scheme admits stranding, so
+// acquiring threads use timers (ParkTimed) to detect and recover from
+// the stranding window. Thread/Node structures must be aligned on 256-byte
+// boundaries by using placement-new.
+// * Augment MCS with advisory back-link fields maintained with CAS().
+// Pictorially: LockWord -> T1 <-> T2 <-> T3 <-> ... <-> Tn <-> Owner.
+// The validity of the backlinks must be ratified before we trust the value.
+// If the backlinks are invalid the exiting thread must back-track through the
+// the forward links, which are always trustworthy.
+// * Add a successor indication. The LockWord is currently encoded as
+// (List, LOCKBIT:1). We could also add a SUCCBIT or an explicit _succ variable
+// to provide the usual futile-wakeup optimization.
+// See RTStt for details.
+// * Consider schedctl.sc_nopreempt to cover the critical section.
+//
+
+
+typedef volatile intptr_t MutexT ; // Mux Lock-word
+enum MuxBits { LOCKBIT = 1 } ;
+
+void Thread::muxAcquire (volatile intptr_t * Lock, const char * LockName) {
+ intptr_t w = Atomic::cmpxchg_ptr (LOCKBIT, Lock, 0) ;
+ if (w == 0) return ;
+ if ((w & LOCKBIT) == 0 && Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {
+ return ;
+ }
+
+ TEVENT (muxAcquire - Contention) ;
+ ParkEvent * const Self = Thread::current()->_MuxEvent ;
+ assert ((intptr_t(Self) & LOCKBIT) == 0, "invariant") ;
+ for (;;) {
+ int its = (os::is_MP() ? 100 : 0) + 1 ;
+
+ // Optional spin phase: spin-then-park strategy
+ while (--its >= 0) {
+ w = *Lock ;
+ if ((w & LOCKBIT) == 0 && Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {
+ return ;
+ }
+ }
+
+ Self->reset() ;
+ Self->OnList = intptr_t(Lock) ;
+ // The following fence() isn't _strictly necessary as the subsequent
+ // CAS() both serializes execution and ratifies the fetched *Lock value.
+ OrderAccess::fence();
+ for (;;) {
+ w = *Lock ;
+ if ((w & LOCKBIT) == 0) {
+ if (Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {
+ Self->OnList = 0 ; // hygiene - allows stronger asserts
+ return ;
+ }
+ continue ; // Interference -- *Lock changed -- Just retry
+ }
+ assert (w & LOCKBIT, "invariant") ;
+ Self->ListNext = (ParkEvent *) (w & ~LOCKBIT );
+ if (Atomic::cmpxchg_ptr (intptr_t(Self)|LOCKBIT, Lock, w) == w) break ;
+ }
+
+ while (Self->OnList != 0) {
+ Self->park() ;
+ }
+ }
+}
+
+void Thread::muxAcquireW (volatile intptr_t * Lock, ParkEvent * ev) {
+ intptr_t w = Atomic::cmpxchg_ptr (LOCKBIT, Lock, 0) ;
+ if (w == 0) return ;
+ if ((w & LOCKBIT) == 0 && Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {
+ return ;
+ }
+
+ TEVENT (muxAcquire - Contention) ;
+ ParkEvent * ReleaseAfter = NULL ;
+ if (ev == NULL) {
+ ev = ReleaseAfter = ParkEvent::Allocate (NULL) ;
+ }
+ assert ((intptr_t(ev) & LOCKBIT) == 0, "invariant") ;
+ for (;;) {
+ guarantee (ev->OnList == 0, "invariant") ;
+ int its = (os::is_MP() ? 100 : 0) + 1 ;
+
+ // Optional spin phase: spin-then-park strategy
+ while (--its >= 0) {
+ w = *Lock ;
+ if ((w & LOCKBIT) == 0 && Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {
+ if (ReleaseAfter != NULL) {
+ ParkEvent::Release (ReleaseAfter) ;
+ }
+ return ;
+ }
+ }
+
+ ev->reset() ;
+ ev->OnList = intptr_t(Lock) ;
+ // The following fence() isn't _strictly necessary as the subsequent
+ // CAS() both serializes execution and ratifies the fetched *Lock value.
+ OrderAccess::fence();
+ for (;;) {
+ w = *Lock ;
+ if ((w & LOCKBIT) == 0) {
+ if (Atomic::cmpxchg_ptr (w|LOCKBIT, Lock, w) == w) {
+ ev->OnList = 0 ;
+ // We call ::Release while holding the outer lock, thus
+ // artificially lengthening the critical section.
+ // Consider deferring the ::Release() until the subsequent unlock(),
+ // after we've dropped the outer lock.
+ if (ReleaseAfter != NULL) {
+ ParkEvent::Release (ReleaseAfter) ;
+ }
+ return ;
+ }
+ continue ; // Interference -- *Lock changed -- Just retry
+ }
+ assert (w & LOCKBIT, "invariant") ;
+ ev->ListNext = (ParkEvent *) (w & ~LOCKBIT );
+ if (Atomic::cmpxchg_ptr (intptr_t(ev)|LOCKBIT, Lock, w) == w) break ;
+ }
+
+ while (ev->OnList != 0) {
+ ev->park() ;
+ }
+ }
+}
+
+// Release() must extract a successor from the list and then wake that thread.
+// It can "pop" the front of the list or use a detach-modify-reattach (DMR) scheme
+// similar to that used by ParkEvent::Allocate() and ::Release(). DMR-based
+// Release() would :
+// (A) CAS() or swap() null to *Lock, releasing the lock and detaching the list.
+// (B) Extract a successor from the private list "in-hand"
+// (C) attempt to CAS() the residual back into *Lock over null.
+// If there were any newly arrived threads and the CAS() would fail.
+// In that case Release() would detach the RATs, re-merge the list in-hand
+// with the RATs and repeat as needed. Alternately, Release() might
+// detach and extract a successor, but then pass the residual list to the wakee.
+// The wakee would be responsible for reattaching and remerging before it
+// competed for the lock.
+//
+// Both "pop" and DMR are immune from ABA corruption -- there can be
+// multiple concurrent pushers, but only one popper or detacher.
+// This implementation pops from the head of the list. This is unfair,
+// but tends to provide excellent throughput as hot threads remain hot.
+// (We wake recently run threads first).
+
+void Thread::muxRelease (volatile intptr_t * Lock) {
+ for (;;) {
+ const intptr_t w = Atomic::cmpxchg_ptr (0, Lock, LOCKBIT) ;
+ assert (w & LOCKBIT, "invariant") ;
+ if (w == LOCKBIT) return ;
+ ParkEvent * List = (ParkEvent *) (w & ~LOCKBIT) ;
+ assert (List != NULL, "invariant") ;
+ assert (List->OnList == intptr_t(Lock), "invariant") ;
+ ParkEvent * nxt = List->ListNext ;
+
+ // The following CAS() releases the lock and pops the head element.
+ if (Atomic::cmpxchg_ptr (intptr_t(nxt), Lock, w) != w) {
+ continue ;
+ }
+ List->OnList = 0 ;
+ OrderAccess::fence() ;
+ List->unpark () ;
+ return ;
+ }
+}
+
+// ObjectMonitor Lifecycle
+// -----------------------
+// Inflation unlinks monitors from the global gFreeList and
+// associates them with objects. Deflation -- which occurs at
+// STW-time -- disassociates idle monitors from objects. Such
+// scavenged monitors are returned to the gFreeList.
+//
+// The global list is protected by ListLock. All the critical sections
+// are short and operate in constant-time.
+//
+// ObjectMonitors reside in type-stable memory (TSM) and are immortal.
+//
+// Lifecycle:
+// -- unassigned and on the global free list
+// -- unassigned and on a thread's private omFreeList
+// -- assigned to an object. The object is inflated and the mark refers
+// to the objectmonitor.
+//
+// TODO-FIXME:
+//
+// * We currently protect the gFreeList with a simple lock.
+// An alternate lock-free scheme would be to pop elements from the gFreeList
+// with CAS. This would be safe from ABA corruption as long we only
+// recycled previously appearing elements onto the list in deflate_idle_monitors()
+// at STW-time. Completely new elements could always be pushed onto the gFreeList
+// with CAS. Elements that appeared previously on the list could only
+// be installed at STW-time.
+//
+// * For efficiency and to help reduce the store-before-CAS penalty
+// the objectmonitors on gFreeList or local free lists should be ready to install
+// with the exception of _header and _object. _object can be set after inflation.
+// In particular, keep all objectMonitors on a thread's private list in ready-to-install
+// state with m.Owner set properly.
+//
+// * We could all diffuse contention by using multiple global (FreeList, Lock)
+// pairs -- threads could use trylock() and a cyclic-scan strategy to search for
+// an unlocked free list.
+//
+// * Add lifecycle tags and assert()s.
+//
+// * Be more consistent about when we clear an objectmonitor's fields:
+// A. After extracting the objectmonitor from a free list.
+// B. After adding an objectmonitor to a free list.
+//
+
+ObjectMonitor * ObjectSynchronizer::gBlockList = NULL ;
+ObjectMonitor * volatile ObjectSynchronizer::gFreeList = NULL ;
+static volatile intptr_t ListLock = 0 ; // protects global monitor free-list cache
+#define CHAINMARKER ((oop)-1)
+
+ObjectMonitor * ATTR ObjectSynchronizer::omAlloc (Thread * Self) {
+ // A large MAXPRIVATE value reduces both list lock contention
+ // and list coherency traffic, but also tends to increase the
+ // number of objectMonitors in circulation as well as the STW
+ // scavenge costs. As usual, we lean toward time in space-time
+ // tradeoffs.
+ const int MAXPRIVATE = 1024 ;
+ for (;;) {
+ ObjectMonitor * m ;
+
+ // 1: try to allocate from the thread's local omFreeList.
+ // Threads will attempt to allocate first from their local list, then
+ // from the global list, and only after those attempts fail will the thread
+ // attempt to instantiate new monitors. Thread-local free lists take
+ // heat off the ListLock and improve allocation latency, as well as reducing
+ // coherency traffic on the shared global list.
+ m = Self->omFreeList ;
+ if (m != NULL) {
+ Self->omFreeList = m->FreeNext ;
+ Self->omFreeCount -- ;
+ // CONSIDER: set m->FreeNext = BAD -- diagnostic hygiene
+ guarantee (m->object() == NULL, "invariant") ;
+ return m ;
+ }
+
+ // 2: try to allocate from the global gFreeList
+ // CONSIDER: use muxTry() instead of muxAcquire().
+ // If the muxTry() fails then drop immediately into case 3.
+ // If we're using thread-local free lists then try
+ // to reprovision the caller's free list.
+ if (gFreeList != NULL) {
+ // Reprovision the thread's omFreeList.
+ // Use bulk transfers to reduce the allocation rate and heat
+ // on various locks.
+ Thread::muxAcquire (&ListLock, "omAlloc") ;
+ for (int i = Self->omFreeProvision; --i >= 0 && gFreeList != NULL; ) {
+ ObjectMonitor * take = gFreeList ;
+ gFreeList = take->FreeNext ;
+ guarantee (take->object() == NULL, "invariant") ;
+ guarantee (!take->is_busy(), "invariant") ;
+ take->Recycle() ;
+ omRelease (Self, take) ;
+ }
+ Thread::muxRelease (&ListLock) ;
+ Self->omFreeProvision += 1 + (Self->omFreeProvision/2) ;
+ if (Self->omFreeProvision > MAXPRIVATE ) Self->omFreeProvision = MAXPRIVATE ;
+ TEVENT (omFirst - reprovision) ;
+ continue ;
+ }
+
+ // 3: allocate a block of new ObjectMonitors
+ // Both the local and global free lists are empty -- resort to malloc().
+ // In the current implementation objectMonitors are TSM - immortal.
+ assert (_BLOCKSIZE > 1, "invariant") ;
+ ObjectMonitor * temp = new ObjectMonitor[_BLOCKSIZE];
+
+ // NOTE: (almost) no way to recover if allocation failed.
+ // We might be able to induce a STW safepoint and scavenge enough
+ // objectMonitors to permit progress.
+ if (temp == NULL) {
+ vm_exit_out_of_memory (sizeof (ObjectMonitor[_BLOCKSIZE]), "Allocate ObjectMonitors") ;
+ }
+
+ // Format the block.
+ // initialize the linked list, each monitor points to its next
+ // forming the single linked free list, the very first monitor
+ // will points to next block, which forms the block list.
+ // The trick of using the 1st element in the block as gBlockList
+ // linkage should be reconsidered. A better implementation would
+ // look like: class Block { Block * next; int N; ObjectMonitor Body [N] ; }
+
+ for (int i = 1; i < _BLOCKSIZE ; i++) {
+ temp[i].FreeNext = &temp[i+1];
+ }
+
+ // terminate the last monitor as the end of list
+ temp[_BLOCKSIZE - 1].FreeNext = NULL ;
+
+ // Element [0] is reserved for global list linkage
+ temp[0].set_object(CHAINMARKER);
+
+ // Consider carving out this thread's current request from the
+ // block in hand. This avoids some lock traffic and redundant
+ // list activity.
+
+ // Acquire the ListLock to manipulate BlockList and FreeList.
+ // An Oyama-Taura-Yonezawa scheme might be more efficient.
+ Thread::muxAcquire (&ListLock, "omAlloc [2]") ;
+
+ // Add the new block to the list of extant blocks (gBlockList).
+ // The very first objectMonitor in a block is reserved and dedicated.
+ // It serves as blocklist "next" linkage.
+ temp[0].FreeNext = gBlockList;
+ gBlockList = temp;
+
+ // Add the new string of objectMonitors to the global free list
+ temp[_BLOCKSIZE - 1].FreeNext = gFreeList ;
+ gFreeList = temp + 1;
+ Thread::muxRelease (&ListLock) ;
+ TEVENT (Allocate block of monitors) ;
+ }
+}
+
+// Place "m" on the caller's private per-thread omFreeList.
+// In practice there's no need to clamp or limit the number of
+// monitors on a thread's omFreeList as the only time we'll call
+// omRelease is to return a monitor to the free list after a CAS
+// attempt failed. This doesn't allow unbounded #s of monitors to
+// accumulate on a thread's free list.
+//
+// In the future the usage of omRelease() might change and monitors
+// could migrate between free lists. In that case to avoid excessive
+// accumulation we could limit omCount to (omProvision*2), otherwise return
+// the objectMonitor to the global list. We should drain (return) in reasonable chunks.
+// That is, *not* one-at-a-time.
+
+
+void ObjectSynchronizer::omRelease (Thread * Self, ObjectMonitor * m) {
+ guarantee (m->object() == NULL, "invariant") ;
+ m->FreeNext = Self->omFreeList ;
+ Self->omFreeList = m ;
+ Self->omFreeCount ++ ;
+}
+
+// Return the monitors of a moribund thread's local free list to
+// the global free list. Typically a thread calls omFlush() when
+// it's dying. We could also consider having the VM thread steal
+// monitors from threads that have not run java code over a few
+// consecutive STW safepoints. Relatedly, we might decay
+// omFreeProvision at STW safepoints.
+//
+// We currently call omFlush() from the Thread:: dtor _after the thread
+// has been excised from the thread list and is no longer a mutator.
+// That means that omFlush() can run concurrently with a safepoint and
+// the scavenge operator. Calling omFlush() from JavaThread::exit() might
+// be a better choice as we could safely reason that that the JVM is
+// not at a safepoint at the time of the call, and thus there could
+// be not inopportune interleavings between omFlush() and the scavenge
+// operator.
+
+void ObjectSynchronizer::omFlush (Thread * Self) {
+ ObjectMonitor * List = Self->omFreeList ; // Null-terminated SLL
+ Self->omFreeList = NULL ;
+ if (List == NULL) return ;
+ ObjectMonitor * Tail = NULL ;
+ ObjectMonitor * s ;
+ for (s = List ; s != NULL ; s = s->FreeNext) {
+ Tail = s ;
+ guarantee (s->object() == NULL, "invariant") ;
+ guarantee (!s->is_busy(), "invariant") ;
+ s->set_owner (NULL) ; // redundant but good hygiene
+ TEVENT (omFlush - Move one) ;
+ }
+
+ guarantee (Tail != NULL && List != NULL, "invariant") ;
+ Thread::muxAcquire (&ListLock, "omFlush") ;
+ Tail->FreeNext = gFreeList ;
+ gFreeList = List ;
+ Thread::muxRelease (&ListLock) ;
+ TEVENT (omFlush) ;
+}
+
+
+// Get the next block in the block list.
+static inline ObjectMonitor* next(ObjectMonitor* block) {
+ assert(block->object() == CHAINMARKER, "must be a block header");
+ block = block->FreeNext ;
+ assert(block == NULL || block->object() == CHAINMARKER, "must be a block header");
+ return block;
+}
+
+// Fast path code shared by multiple functions
+ObjectMonitor* ObjectSynchronizer::inflate_helper(oop obj) {
+ markOop mark = obj->mark();
+ if (mark->has_monitor()) {
+ assert(ObjectSynchronizer::verify_objmon_isinpool(mark->monitor()), "monitor is invalid");
+ assert(mark->monitor()->header()->is_neutral(), "monitor must record a good object header");
+ return mark->monitor();
+ }
+ return ObjectSynchronizer::inflate(Thread::current(), obj);
+}
+
+// Note that we could encounter some performance loss through false-sharing as
+// multiple locks occupy the same $ line. Padding might be appropriate.
+
+#define NINFLATIONLOCKS 256
+static volatile intptr_t InflationLocks [NINFLATIONLOCKS] ;
+
+static markOop ReadStableMark (oop obj) {
+ markOop mark = obj->mark() ;
+ if (!mark->is_being_inflated()) {
+ return mark ; // normal fast-path return
+ }
+
+ int its = 0 ;
+ for (;;) {
+ markOop mark = obj->mark() ;
+ if (!mark->is_being_inflated()) {
+ return mark ; // normal fast-path return
+ }
+
+ // The object is being inflated by some other thread.
+ // The caller of ReadStableMark() must wait for inflation to complete.
+ // Avoid live-lock
+ // TODO: consider calling SafepointSynchronize::do_call_back() while
+ // spinning to see if there's a safepoint pending. If so, immediately
+ // yielding or blocking would be appropriate. Avoid spinning while
+ // there is a safepoint pending.
+ // TODO: add inflation contention performance counters.
+ // TODO: restrict the aggregate number of spinners.
+
+ ++its ;
+ if (its > 10000 || !os::is_MP()) {
+ if (its & 1) {
+ os::NakedYield() ;
+ TEVENT (Inflate: INFLATING - yield) ;
+ } else {
+ // Note that the following code attenuates the livelock problem but is not
+ // a complete remedy. A more complete solution would require that the inflating
+ // thread hold the associated inflation lock. The following code simply restricts
+ // the number of spinners to at most one. We'll have N-2 threads blocked
+ // on the inflationlock, 1 thread holding the inflation lock and using
+ // a yield/park strategy, and 1 thread in the midst of inflation.
+ // A more refined approach would be to change the encoding of INFLATING
+ // to allow encapsulation of a native thread pointer. Threads waiting for
+ // inflation to complete would use CAS to push themselves onto a singly linked
+ // list rooted at the markword. Once enqueued, they'd loop, checking a per-thread flag
+ // and calling park(). When inflation was complete the thread that accomplished inflation
+ // would detach the list and set the markword to inflated with a single CAS and
+ // then for each thread on the list, set the flag and unpark() the thread.
+ // This is conceptually similar to muxAcquire-muxRelease, except that muxRelease
+ // wakes at most one thread whereas we need to wake the entire list.
+ int ix = (intptr_t(obj) >> 5) & (NINFLATIONLOCKS-1) ;
+ int YieldThenBlock = 0 ;
+ assert (ix >= 0 && ix < NINFLATIONLOCKS, "invariant") ;
+ assert ((NINFLATIONLOCKS & (NINFLATIONLOCKS-1)) == 0, "invariant") ;
+ Thread::muxAcquire (InflationLocks + ix, "InflationLock") ;
+ while (obj->mark() == markOopDesc::INFLATING()) {
+ // Beware: NakedYield() is advisory and has almost no effect on some platforms
+ // so we periodically call Self->_ParkEvent->park(1).
+ // We use a mixed spin/yield/block mechanism.
+ if ((YieldThenBlock++) >= 16) {
+ Thread::current()->_ParkEvent->park(1) ;
+ } else {
+ os::NakedYield() ;
+ }
+ }
+ Thread::muxRelease (InflationLocks + ix ) ;
+ TEVENT (Inflate: INFLATING - yield/park) ;
+ }
+ } else {
+ SpinPause() ; // SMP-polite spinning
+ }
+ }
+}
+
+ObjectMonitor * ATTR ObjectSynchronizer::inflate (Thread * Self, oop object) {
+ // Inflate mutates the heap ...
+ // Relaxing assertion for bug 6320749.
+ assert (Universe::verify_in_progress() ||
+ !SafepointSynchronize::is_at_safepoint(), "invariant") ;
+
+ for (;;) {
+ const markOop mark = object->mark() ;
+ assert (!mark->has_bias_pattern(), "invariant") ;
+
+ // The mark can be in one of the following states:
+ // * Inflated - just return
+ // * Stack-locked - coerce it to inflated
+ // * INFLATING - busy wait for conversion to complete
+ // * Neutral - aggressively inflate the object.
+ // * BIASED - Illegal. We should never see this
+
+ // CASE: inflated
+ if (mark->has_monitor()) {
+ ObjectMonitor * inf = mark->monitor() ;
+ assert (inf->header()->is_neutral(), "invariant");
+ assert (inf->object() == object, "invariant") ;
+ assert (ObjectSynchronizer::verify_objmon_isinpool(inf), "monitor is invalid");
+ return inf ;
+ }
+
+ // CASE: inflation in progress - inflating over a stack-lock.
+ // Some other thread is converting from stack-locked to inflated.
+ // Only that thread can complete inflation -- other threads must wait.
+ // The INFLATING value is transient.
+ // Currently, we spin/yield/park and poll the markword, waiting for inflation to finish.
+ // We could always eliminate polling by parking the thread on some auxiliary list.
+ if (mark == markOopDesc::INFLATING()) {
+ TEVENT (Inflate: spin while INFLATING) ;
+ ReadStableMark(object) ;
+ continue ;
+ }
+
+ // CASE: stack-locked
+ // Could be stack-locked either by this thread or by some other thread.
+ //
+ // Note that we allocate the objectmonitor speculatively, _before_ attempting
+ // to install INFLATING into the mark word. We originally installed INFLATING,
+ // allocated the objectmonitor, and then finally STed the address of the
+ // objectmonitor into the mark. This was correct, but artificially lengthened
+ // the interval in which INFLATED appeared in the mark, thus increasing
+ // the odds of inflation contention.
+ //
+ // We now use per-thread private objectmonitor free lists.
+ // These list are reprovisioned from the global free list outside the
+ // critical INFLATING...ST interval. A thread can transfer
+ // multiple objectmonitors en-mass from the global free list to its local free list.
+ // This reduces coherency traffic and lock contention on the global free list.
+ // Using such local free lists, it doesn't matter if the omAlloc() call appears
+ // before or after the CAS(INFLATING) operation.
+ // See the comments in omAlloc().
+
+ if (mark->has_locker()) {
+ ObjectMonitor * m = omAlloc (Self) ;
+ // Optimistically prepare the objectmonitor - anticipate successful CAS
+ // We do this before the CAS in order to minimize the length of time
+ // in which INFLATING appears in the mark.
+ m->Recycle();
+ m->FreeNext = NULL ;
+ m->_Responsible = NULL ;
+ m->OwnerIsThread = 0 ;
+ m->_recursions = 0 ;
+ m->_SpinDuration = Knob_SpinLimit ; // Consider: maintain by type/class
+
+ markOop cmp = (markOop) Atomic::cmpxchg_ptr (markOopDesc::INFLATING(), object->mark_addr(), mark) ;
+ if (cmp != mark) {
+ omRelease (Self, m) ;
+ continue ; // Interference -- just retry
+ }
+
+ // We've successfully installed INFLATING (0) into the mark-word.
+ // This is the only case where 0 will appear in a mark-work.
+ // Only the singular thread that successfully swings the mark-word
+ // to 0 can perform (or more precisely, complete) inflation.
+ //
+ // Why do we CAS a 0 into the mark-word instead of just CASing the
+ // mark-word from the stack-locked value directly to the new inflated state?
+ // Consider what happens when a thread unlocks a stack-locked object.
+ // It attempts to use CAS to swing the displaced header value from the
+ // on-stack basiclock back into the object header. Recall also that the
+ // header value (hashcode, etc) can reside in (a) the object header, or
+ // (b) a displaced header associated with the stack-lock, or (c) a displaced
+ // header in an objectMonitor. The inflate() routine must copy the header
+ // value from the basiclock on the owner's stack to the objectMonitor, all
+ // the while preserving the hashCode stability invariants. If the owner
+ // decides to release the lock while the value is 0, the unlock will fail
+ // and control will eventually pass from slow_exit() to inflate. The owner
+ // will then spin, waiting for the 0 value to disappear. Put another way,
+ // the 0 causes the owner to stall if the owner happens to try to
+ // drop the lock (restoring the header from the basiclock to the object)
+ // while inflation is in-progress. This protocol avoids races that might
+ // would otherwise permit hashCode values to change or "flicker" for an object.
+ // Critically, while object->mark is 0 mark->displaced_mark_helper() is stable.
+ // 0 serves as a "BUSY" inflate-in-progress indicator.
+
+
+ // fetch the displaced mark from the owner's stack.
+ // The owner can't die or unwind past the lock while our INFLATING
+ // object is in the mark. Furthermore the owner can't complete
+ // an unlock on the object, either.
+ markOop dmw = mark->displaced_mark_helper() ;
+ assert (dmw->is_neutral(), "invariant") ;
+
+ // Setup monitor fields to proper values -- prepare the monitor
+ m->set_header(dmw) ;
+
+ // Optimization: if the mark->locker stack address is associated
+ // with this thread we could simply set m->_owner = Self and
+ // m->OwnerIsThread = 1. Note that a thread can inflate an object
+ // that it has stack-locked -- as might happen in wait() -- directly
+ // with CAS. That is, we can avoid the xchg-NULL .... ST idiom.
+ m->set_owner (mark->locker());
+ m->set_object(object);
+ // TODO-FIXME: assert BasicLock->dhw != 0.
+
+ // Must preserve store ordering. The monitor state must
+ // be stable at the time of publishing the monitor address.
+ guarantee (object->mark() == markOopDesc::INFLATING(), "invariant") ;
+ object->release_set_mark(markOopDesc::encode(m));
+
+ // Hopefully the performance counters are allocated on distinct cache lines
+ // to avoid false sharing on MP systems ...
+ if (_sync_Inflations != NULL) _sync_Inflations->inc() ;
+ TEVENT(Inflate: overwrite stacklock) ;
+ if (TraceMonitorInflation) {
+ if (object->is_instance()) {
+ ResourceMark rm;
+ tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
+ (intptr_t) object, (intptr_t) object->mark(),
+ Klass::cast(object->klass())->external_name());
+ }
+ }
+ return m ;
+ }
+
+ // CASE: neutral
+ // TODO-FIXME: for entry we currently inflate and then try to CAS _owner.
+ // If we know we're inflating for entry it's better to inflate by swinging a
+ // pre-locked objectMonitor pointer into the object header. A successful
+ // CAS inflates the object *and* confers ownership to the inflating thread.
+ // In the current implementation we use a 2-step mechanism where we CAS()
+ // to inflate and then CAS() again to try to swing _owner from NULL to Self.
+ // An inflateTry() method that we could call from fast_enter() and slow_enter()
+ // would be useful.
+
+ assert (mark->is_neutral(), "invariant");
+ ObjectMonitor * m = omAlloc (Self) ;
+ // prepare m for installation - set monitor to initial state
+ m->Recycle();
+ m->set_header(mark);
+ m->set_owner(NULL);
+ m->set_object(object);
+ m->OwnerIsThread = 1 ;
+ m->_recursions = 0 ;
+ m->FreeNext = NULL ;
+ m->_Responsible = NULL ;
+ m->_SpinDuration = Knob_SpinLimit ; // consider: keep metastats by type/class
+
+ if (Atomic::cmpxchg_ptr (markOopDesc::encode(m), object->mark_addr(), mark) != mark) {
+ m->set_object (NULL) ;
+ m->set_owner (NULL) ;
+ m->OwnerIsThread = 0 ;
+ m->Recycle() ;
+ omRelease (Self, m) ;
+ m = NULL ;
+ continue ;
+ // interference - the markword changed - just retry.
+ // The state-transitions are one-way, so there's no chance of
+ // live-lock -- "Inflated" is an absorbing state.
+ }
+
+ // Hopefully the performance counters are allocated on distinct
+ // cache lines to avoid false sharing on MP systems ...
+ if (_sync_Inflations != NULL) _sync_Inflations->inc() ;
+ TEVENT(Inflate: overwrite neutral) ;
+ if (TraceMonitorInflation) {
+ if (object->is_instance()) {
+ ResourceMark rm;
+ tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
+ (intptr_t) object, (intptr_t) object->mark(),
+ Klass::cast(object->klass())->external_name());
+ }
+ }
+ return m ;
+ }
+}
+
+
+// This the fast monitor enter. The interpreter and compiler use
+// some assembly copies of this code. Make sure update those code
+// if the following function is changed. The implementation is
+// extremely sensitive to race condition. Be careful.
+
+void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock, bool attempt_rebias, TRAPS) {
+ if (UseBiasedLocking) {
+ if (!SafepointSynchronize::is_at_safepoint()) {
+ BiasedLocking::Condition cond = BiasedLocking::revoke_and_rebias(obj, attempt_rebias, THREAD);
+ if (cond == BiasedLocking::BIAS_REVOKED_AND_REBIASED) {
+ return;
+ }
+ } else {
+ assert(!attempt_rebias, "can not rebias toward VM thread");
+ BiasedLocking::revoke_at_safepoint(obj);
+ }
+ assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
+ }
+
+ THREAD->update_highest_lock((address)lock);
+ slow_enter (obj, lock, THREAD) ;
+}
+
+void ObjectSynchronizer::fast_exit(oop object, BasicLock* lock, TRAPS) {
+ assert(!object->mark()->has_bias_pattern(), "should not see bias pattern here");
+ // if displaced header is null, the previous enter is recursive enter, no-op
+ markOop dhw = lock->displaced_header();
+ markOop mark ;
+ if (dhw == NULL) {
+ // Recursive stack-lock.
+ // Diagnostics -- Could be: stack-locked, inflating, inflated.
+ mark = object->mark() ;
+ assert (!mark->is_neutral(), "invariant") ;
+ if (mark->has_locker() && mark != markOopDesc::INFLATING()) {
+ assert(THREAD->is_lock_owned((address)mark->locker()), "invariant") ;
+ }
+ if (mark->has_monitor()) {
+ ObjectMonitor * m = mark->monitor() ;
+ assert(((oop)(m->object()))->mark() == mark, "invariant") ;
+ assert(m->is_entered(THREAD), "invariant") ;
+ }
+ return ;
+ }
+
+ mark = object->mark() ;
+
+ // If the object is stack-locked by the current thread, try to
+ // swing the displaced header from the box back to the mark.
+ if (mark == (markOop) lock) {
+ assert (dhw->is_neutral(), "invariant") ;
+ if ((markOop) Atomic::cmpxchg_ptr (dhw, object->mark_addr(), mark) == mark) {
+ TEVENT (fast_exit: release stacklock) ;
+ return;
+ }
+ }
+
+ ObjectSynchronizer::inflate(THREAD, object)->exit (THREAD) ;
+}
+
+// This routine is used to handle interpreter/compiler slow case
+// We don't need to use fast path here, because it must have been
+// failed in the interpreter/compiler code.
+void ObjectSynchronizer::slow_enter(Handle obj, BasicLock* lock, TRAPS) {
+ markOop mark = obj->mark();
+ assert(!mark->has_bias_pattern(), "should not see bias pattern here");
+
+ if (mark->is_neutral()) {
+ // Anticipate successful CAS -- the ST of the displaced mark must
+ // be visible <= the ST performed by the CAS.
+ lock->set_displaced_header(mark);
+ if (mark == (markOop) Atomic::cmpxchg_ptr(lock, obj()->mark_addr(), mark)) {
+ TEVENT (slow_enter: release stacklock) ;
+ return ;
+ }
+ // Fall through to inflate() ...
+ } else
+ if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
+ assert(lock != mark->locker(), "must not re-lock the same lock");
+ assert(lock != (BasicLock*)obj->mark(), "don't relock with same BasicLock");
+ lock->set_displaced_header(NULL);
+ return;
+ }
+
+#if 0
+ // The following optimization isn't particularly useful.
+ if (mark->has_monitor() && mark->monitor()->is_entered(THREAD)) {
+ lock->set_displaced_header (NULL) ;
+ return ;
+ }
+#endif
+
+ // The object header will never be displaced to this lock,
+ // so it does not matter what the value is, except that it
+ // must be non-zero to avoid looking like a re-entrant lock,
+ // and must not look locked either.
+ lock->set_displaced_header(markOopDesc::unused_mark());
+ ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);
+}
+
+// This routine is used to handle interpreter/compiler slow case
+// We don't need to use fast path here, because it must have
+// failed in the interpreter/compiler code. Simply use the heavy
+// weight monitor should be ok, unless someone find otherwise.
+void ObjectSynchronizer::slow_exit(oop object, BasicLock* lock, TRAPS) {
+ fast_exit (object, lock, THREAD) ;
+}
+
+// NOTE: must use heavy weight monitor to handle jni monitor enter
+void ObjectSynchronizer::jni_enter(Handle obj, TRAPS) { // possible entry from jni enter
+ // the current locking is from JNI instead of Java code
+ TEVENT (jni_enter) ;
+ if (UseBiasedLocking) {
+ BiasedLocking::revoke_and_rebias(obj, false, THREAD);
+ assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
+ }
+ THREAD->set_current_pending_monitor_is_from_java(false);
+ ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);
+ THREAD->set_current_pending_monitor_is_from_java(true);
+}
+
+// NOTE: must use heavy weight monitor to handle jni monitor enter
+bool ObjectSynchronizer::jni_try_enter(Handle obj, Thread* THREAD) {
+ if (UseBiasedLocking) {
+ BiasedLocking::revoke_and_rebias(obj, false, THREAD);
+ assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
+ }
+
+ ObjectMonitor* monitor = ObjectSynchronizer::inflate_helper(obj());
+ return monitor->try_enter(THREAD);
+}
+
+
+// NOTE: must use heavy weight monitor to handle jni monitor exit
+void ObjectSynchronizer::jni_exit(oop obj, Thread* THREAD) {
+ TEVENT (jni_exit) ;
+ if (UseBiasedLocking) {
+ BiasedLocking::revoke_and_rebias(obj, false, THREAD);
+ }
+ assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
+
+ ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj);
+ // If this thread has locked the object, exit the monitor. Note: can't use
+ // monitor->check(CHECK); must exit even if an exception is pending.
+ if (monitor->check(THREAD)) {
+ monitor->exit(THREAD);
+ }
+}
+
+// complete_exit()/reenter() are used to wait on a nested lock
+// i.e. to give up an outer lock completely and then re-enter
+// Used when holding nested locks - lock acquisition order: lock1 then lock2
+// 1) complete_exit lock1 - saving recursion count
+// 2) wait on lock2
+// 3) when notified on lock2, unlock lock2
+// 4) reenter lock1 with original recursion count
+// 5) lock lock2
+// NOTE: must use heavy weight monitor to handle complete_exit/reenter()
+intptr_t ObjectSynchronizer::complete_exit(Handle obj, TRAPS) {
+ TEVENT (complete_exit) ;
+ if (UseBiasedLocking) {
+ BiasedLocking::revoke_and_rebias(obj, false, THREAD);
+ assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
+ }
+
+ ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
+
+ return monitor->complete_exit(THREAD);
+}
+
+// NOTE: must use heavy weight monitor to handle complete_exit/reenter()
+void ObjectSynchronizer::reenter(Handle obj, intptr_t recursion, TRAPS) {
+ TEVENT (reenter) ;
+ if (UseBiasedLocking) {
+ BiasedLocking::revoke_and_rebias(obj, false, THREAD);
+ assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
+ }
+
+ ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
+
+ monitor->reenter(recursion, THREAD);
+}
+
+// This exists only as a workaround of dtrace bug 6254741
+int dtrace_waited_probe(ObjectMonitor* monitor, Handle obj, Thread* thr) {
+ DTRACE_MONITOR_PROBE(waited, monitor, obj(), thr);
+ return 0;
+}
+
+// NOTE: must use heavy weight monitor to handle wait()
+void ObjectSynchronizer::wait(Handle obj, jlong millis, TRAPS) {
+ if (UseBiasedLocking) {
+ BiasedLocking::revoke_and_rebias(obj, false, THREAD);
+ assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
+ }
+ if (millis < 0) {
+ TEVENT (wait - throw IAX) ;
+ THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
+ }
+ ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
+ DTRACE_MONITOR_WAIT_PROBE(monitor, obj(), THREAD, millis);
+ monitor->wait(millis, true, THREAD);
+
+ /* This dummy call is in place to get around dtrace bug 6254741. Once
+ that's fixed we can uncomment the following line and remove the call */
+ // DTRACE_MONITOR_PROBE(waited, monitor, obj(), THREAD);
+ dtrace_waited_probe(monitor, obj, THREAD);
+}
+
+void ObjectSynchronizer::waitUninterruptibly (Handle obj, jlong millis, TRAPS) {
+ if (UseBiasedLocking) {
+ BiasedLocking::revoke_and_rebias(obj, false, THREAD);
+ assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
+ }
+ if (millis < 0) {
+ TEVENT (wait - throw IAX) ;
+ THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
+ }
+ ObjectSynchronizer::inflate(THREAD, obj()) -> wait(millis, false, THREAD) ;
+}
+
+void ObjectSynchronizer::notify(Handle obj, TRAPS) {
+ if (UseBiasedLocking) {
+ BiasedLocking::revoke_and_rebias(obj, false, THREAD);
+ assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
+ }
+
+ markOop mark = obj->mark();
+ if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
+ return;
+ }
+ ObjectSynchronizer::inflate(THREAD, obj())->notify(THREAD);
+}
+
+// NOTE: see comment of notify()
+void ObjectSynchronizer::notifyall(Handle obj, TRAPS) {
+ if (UseBiasedLocking) {
+ BiasedLocking::revoke_and_rebias(obj, false, THREAD);
+ assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
+ }
+
+ markOop mark = obj->mark();
+ if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
+ return;
+ }
+ ObjectSynchronizer::inflate(THREAD, obj())->notifyAll(THREAD);
+}
+
+intptr_t ObjectSynchronizer::FastHashCode (Thread * Self, oop obj) {
+ if (UseBiasedLocking) {
+ // NOTE: many places throughout the JVM do not expect a safepoint
+ // to be taken here, in particular most operations on perm gen
+ // objects. However, we only ever bias Java instances and all of
+ // the call sites of identity_hash that might revoke biases have
+ // been checked to make sure they can handle a safepoint. The
+ // added check of the bias pattern is to avoid useless calls to
+ // thread-local storage.
+ if (obj->mark()->has_bias_pattern()) {
+ // Box and unbox the raw reference just in case we cause a STW safepoint.
+ Handle hobj (Self, obj) ;
+ // Relaxing assertion for bug 6320749.
+ assert (Universe::verify_in_progress() ||
+ !SafepointSynchronize::is_at_safepoint(),
+ "biases should not be seen by VM thread here");
+ BiasedLocking::revoke_and_rebias(hobj, false, JavaThread::current());
+ obj = hobj() ;
+ assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
+ }
+ }
+
+ // hashCode() is a heap mutator ...
+ // Relaxing assertion for bug 6320749.
+ assert (Universe::verify_in_progress() ||
+ !SafepointSynchronize::is_at_safepoint(), "invariant") ;
+ assert (Universe::verify_in_progress() ||
+ Self->is_Java_thread() , "invariant") ;
+ assert (Universe::verify_in_progress() ||
+ ((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;
+
+ ObjectMonitor* monitor = NULL;
+ markOop temp, test;
+ intptr_t hash;
+ markOop mark = ReadStableMark (obj);
+
+ // object should remain ineligible for biased locking
+ assert (!mark->has_bias_pattern(), "invariant") ;
+
+ if (mark->is_neutral()) {
+ hash = mark->hash(); // this is a normal header
+ if (hash) { // if it has hash, just return it
+ return hash;
+ }
+ hash = get_next_hash(Self, obj); // allocate a new hash code
+ temp = mark->copy_set_hash(hash); // merge the hash code into header
+ // use (machine word version) atomic operation to install the hash
+ test = (markOop) Atomic::cmpxchg_ptr(temp, obj->mark_addr(), mark);
+ if (test == mark) {
+ return hash;
+ }
+ // If atomic operation failed, we must inflate the header
+ // into heavy weight monitor. We could add more code here
+ // for fast path, but it does not worth the complexity.
+ } else if (mark->has_monitor()) {
+ monitor = mark->monitor();
+ temp = monitor->header();
+ assert (temp->is_neutral(), "invariant") ;
+ hash = temp->hash();
+ if (hash) {
+ return hash;
+ }
+ // Skip to the following code to reduce code size
+ } else if (Self->is_lock_owned((address)mark->locker())) {
+ temp = mark->displaced_mark_helper(); // this is a lightweight monitor owned
+ assert (temp->is_neutral(), "invariant") ;
+ hash = temp->hash(); // by current thread, check if the displaced
+ if (hash) { // header contains hash code
+ return hash;
+ }
+ // WARNING:
+ // The displaced header is strictly immutable.
+ // It can NOT be changed in ANY cases. So we have
+ // to inflate the header into heavyweight monitor
+ // even the current thread owns the lock. The reason
+ // is the BasicLock (stack slot) will be asynchronously
+ // read by other threads during the inflate() function.
+ // Any change to stack may not propagate to other threads
+ // correctly.
+ }
+
+ // Inflate the monitor to set hash code
+ monitor = ObjectSynchronizer::inflate(Self, obj);
+ // Load displaced header and check it has hash code
+ mark = monitor->header();
+ assert (mark->is_neutral(), "invariant") ;
+ hash = mark->hash();
+ if (hash == 0) {
+ hash = get_next_hash(Self, obj);
+ temp = mark->copy_set_hash(hash); // merge hash code into header
+ assert (temp->is_neutral(), "invariant") ;
+ test = (markOop) Atomic::cmpxchg_ptr(temp, monitor, mark);
+ if (test != mark) {
+ // The only update to the header in the monitor (outside GC)
+ // is install the hash code. If someone add new usage of
+ // displaced header, please update this code
+ hash = test->hash();
+ assert (test->is_neutral(), "invariant") ;
+ assert (hash != 0, "Trivial unexpected object/monitor header usage.");
+ }
+ }
+ // We finally get the hash
+ return hash;
+}
+
+// Deprecated -- use FastHashCode() instead.
+
+intptr_t ObjectSynchronizer::identity_hash_value_for(Handle obj) {
+ return FastHashCode (Thread::current(), obj()) ;
+}
+
+bool ObjectSynchronizer::current_thread_holds_lock(JavaThread* thread,
+ Handle h_obj) {
+ if (UseBiasedLocking) {
+ BiasedLocking::revoke_and_rebias(h_obj, false, thread);
+ assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
+ }
+
+ assert(thread == JavaThread::current(), "Can only be called on current thread");
+ oop obj = h_obj();
+
+ markOop mark = ReadStableMark (obj) ;
+
+ // Uncontended case, header points to stack
+ if (mark->has_locker()) {
+ return thread->is_lock_owned((address)mark->locker());
+ }
+ // Contended case, header points to ObjectMonitor (tagged pointer)
+ if (mark->has_monitor()) {
+ ObjectMonitor* monitor = mark->monitor();
+ return monitor->is_entered(thread) != 0 ;
+ }
+ // Unlocked case, header in place
+ assert(mark->is_neutral(), "sanity check");
+ return false;
+}
+
+// Be aware of this method could revoke bias of the lock object.
+// This method querys the ownership of the lock handle specified by 'h_obj'.
+// If the current thread owns the lock, it returns owner_self. If no
+// thread owns the lock, it returns owner_none. Otherwise, it will return
+// ower_other.
+ObjectSynchronizer::LockOwnership ObjectSynchronizer::query_lock_ownership
+(JavaThread *self, Handle h_obj) {
+ // The caller must beware this method can revoke bias, and
+ // revocation can result in a safepoint.
+ assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
+ assert (self->thread_state() != _thread_blocked , "invariant") ;
+
+ // Possible mark states: neutral, biased, stack-locked, inflated
+
+ if (UseBiasedLocking && h_obj()->mark()->has_bias_pattern()) {
+ // CASE: biased
+ BiasedLocking::revoke_and_rebias(h_obj, false, self);
+ assert(!h_obj->mark()->has_bias_pattern(),
+ "biases should be revoked by now");
+ }
+
+ assert(self == JavaThread::current(), "Can only be called on current thread");
+ oop obj = h_obj();
+ markOop mark = ReadStableMark (obj) ;
+
+ // CASE: stack-locked. Mark points to a BasicLock on the owner's stack.
+ if (mark->has_locker()) {
+ return self->is_lock_owned((address)mark->locker()) ?
+ owner_self : owner_other;
+ }
+
+ // CASE: inflated. Mark (tagged pointer) points to an objectMonitor.
+ // The Object:ObjectMonitor relationship is stable as long as we're
+ // not at a safepoint.
+ if (mark->has_monitor()) {
+ void * owner = mark->monitor()->_owner ;
+ if (owner == NULL) return owner_none ;
+ return (owner == self ||
+ self->is_lock_owned((address)owner)) ? owner_self : owner_other;
+ }
+
+ // CASE: neutral
+ assert(mark->is_neutral(), "sanity check");
+ return owner_none ; // it's unlocked
+}
+
+// FIXME: jvmti should call this
+JavaThread* ObjectSynchronizer::get_lock_owner(Handle h_obj, bool doLock) {
+ if (UseBiasedLocking) {
+ if (SafepointSynchronize::is_at_safepoint()) {
+ BiasedLocking::revoke_at_safepoint(h_obj);
+ } else {
+ BiasedLocking::revoke_and_rebias(h_obj, false, JavaThread::current());
+ }
+ assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
+ }
+
+ oop obj = h_obj();
+ address owner = NULL;
+
+ markOop mark = ReadStableMark (obj) ;
+
+ // Uncontended case, header points to stack
+ if (mark->has_locker()) {
+ owner = (address) mark->locker();
+ }
+
+ // Contended case, header points to ObjectMonitor (tagged pointer)
+ if (mark->has_monitor()) {
+ ObjectMonitor* monitor = mark->monitor();
+ assert(monitor != NULL, "monitor should be non-null");
+ owner = (address) monitor->owner();
+ }
+
+ if (owner != NULL) {
+ return Threads::owning_thread_from_monitor_owner(owner, doLock);
+ }
+
+ // Unlocked case, header in place
+ // Cannot have assertion since this object may have been
+ // locked by another thread when reaching here.
+ // assert(mark->is_neutral(), "sanity check");
+
+ return NULL;
+}
+
+// Iterate through monitor cache and attempt to release thread's monitors
+// Gives up on a particular monitor if an exception occurs, but continues
+// the overall iteration, swallowing the exception.
+class ReleaseJavaMonitorsClosure: public MonitorClosure {
+private:
+ TRAPS;
+
+public:
+ ReleaseJavaMonitorsClosure(Thread* thread) : THREAD(thread) {}
+ void do_monitor(ObjectMonitor* mid) {
+ if (mid->owner() == THREAD) {
+ (void)mid->complete_exit(CHECK);
+ }
+ }
+};
+
+// Release all inflated monitors owned by THREAD. Lightweight monitors are
+// ignored. This is meant to be called during JNI thread detach which assumes
+// all remaining monitors are heavyweight. All exceptions are swallowed.
+// Scanning the extant monitor list can be time consuming.
+// A simple optimization is to add a per-thread flag that indicates a thread
+// called jni_monitorenter() during its lifetime.
+//
+// Instead of No_Savepoint_Verifier it might be cheaper to
+// use an idiom of the form:
+// auto int tmp = SafepointSynchronize::_safepoint_counter ;
+// <code that must not run at safepoint>
+// guarantee (((tmp ^ _safepoint_counter) | (tmp & 1)) == 0) ;
+// Since the tests are extremely cheap we could leave them enabled
+// for normal product builds.
+
+void ObjectSynchronizer::release_monitors_owned_by_thread(TRAPS) {
+ assert(THREAD == JavaThread::current(), "must be current Java thread");
+ No_Safepoint_Verifier nsv ;
+ ReleaseJavaMonitorsClosure rjmc(THREAD);
+ Thread::muxAcquire(&ListLock, "release_monitors_owned_by_thread");
+ ObjectSynchronizer::monitors_iterate(&rjmc);
+ Thread::muxRelease(&ListLock);
+ THREAD->clear_pending_exception();
+}
+
+// Visitors ...
+
+void ObjectSynchronizer::monitors_iterate(MonitorClosure* closure) {
+ ObjectMonitor* block = gBlockList;
+ ObjectMonitor* mid;
+ while (block) {
+ assert(block->object() == CHAINMARKER, "must be a block header");
+ for (int i = _BLOCKSIZE - 1; i > 0; i--) {
+ mid = block + i;
+ oop object = (oop) mid->object();
+ if (object != NULL) {
+ closure->do_monitor(mid);
+ }
+ }
+ block = (ObjectMonitor*) block->FreeNext;
+ }
+}
+
+void ObjectSynchronizer::oops_do(OopClosure* f) {
+ assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
+ for (ObjectMonitor* block = gBlockList; block != NULL; block = next(block)) {
+ assert(block->object() == CHAINMARKER, "must be a block header");
+ for (int i = 1; i < _BLOCKSIZE; i++) {
+ ObjectMonitor* mid = &block[i];
+ if (mid->object() != NULL) {
+ f->do_oop((oop*)mid->object_addr());
+ }
+ }
+ }
+}
+
+// Deflate_idle_monitors() is called at all safepoints, immediately
+// after all mutators are stopped, but before any objects have moved.
+// It traverses the list of known monitors, deflating where possible.
+// The scavenged monitor are returned to the monitor free list.
+//
+// Beware that we scavenge at *every* stop-the-world point.
+// Having a large number of monitors in-circulation negatively
+// impacts the performance of some applications (e.g., PointBase).
+// Broadly, we want to minimize the # of monitors in circulation.
+// Alternately, we could partition the active monitors into sub-lists
+// of those that need scanning and those that do not.
+// Specifically, we would add a new sub-list of objectmonitors
+// that are in-circulation and potentially active. deflate_idle_monitors()
+// would scan only that list. Other monitors could reside on a quiescent
+// list. Such sequestered monitors wouldn't need to be scanned by
+// deflate_idle_monitors(). omAlloc() would first check the global free list,
+// then the quiescent list, and, failing those, would allocate a new block.
+// Deflate_idle_monitors() would scavenge and move monitors to the
+// quiescent list.
+//
+// Perversely, the heap size -- and thus the STW safepoint rate --
+// typically drives the scavenge rate. Large heaps can mean infrequent GC,
+// which in turn can mean large(r) numbers of objectmonitors in circulation.
+// This is an unfortunate aspect of this design.
+//
+// Another refinement would be to refrain from calling deflate_idle_monitors()
+// except at stop-the-world points associated with garbage collections.
+//
+// An even better solution would be to deflate on-the-fly, aggressively,
+// at monitorexit-time as is done in EVM's metalock or Relaxed Locks.
+
+void ObjectSynchronizer::deflate_idle_monitors() {
+ assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
+ int nInuse = 0 ; // currently associated with objects
+ int nInCirculation = 0 ; // extant
+ int nScavenged = 0 ; // reclaimed
+
+ ObjectMonitor * FreeHead = NULL ; // Local SLL of scavenged monitors
+ ObjectMonitor * FreeTail = NULL ;
+
+ // Iterate over all extant monitors - Scavenge all idle monitors.
+ TEVENT (deflate_idle_monitors) ;
+ for (ObjectMonitor* block = gBlockList; block != NULL; block = next(block)) {
+ assert(block->object() == CHAINMARKER, "must be a block header");
+ nInCirculation += _BLOCKSIZE ;
+ for (int i = 1 ; i < _BLOCKSIZE; i++) {
+ ObjectMonitor* mid = &block[i];
+ oop obj = (oop) mid->object();
+
+ if (obj == NULL) {
+ // The monitor is not associated with an object.
+ // The monitor should either be a thread-specific private
+ // free list or the global free list.
+ // obj == NULL IMPLIES mid->is_busy() == 0
+ guarantee (!mid->is_busy(), "invariant") ;
+ continue ;
+ }
+
+ // Normal case ... The monitor is associated with obj.
+ guarantee (obj->mark() == markOopDesc::encode(mid), "invariant") ;
+ guarantee (mid == obj->mark()->monitor(), "invariant");
+ guarantee (mid->header()->is_neutral(), "invariant");
+
+ if (mid->is_busy()) {
+ if (ClearResponsibleAtSTW) mid->_Responsible = NULL ;
+ nInuse ++ ;
+ } else {
+ // Deflate the monitor if it is no longer being used
+ // It's idle - scavenge and return to the global free list
+ // plain old deflation ...
+ TEVENT (deflate_idle_monitors - scavenge1) ;
+ if (TraceMonitorInflation) {
+ if (obj->is_instance()) {
+ ResourceMark rm;
+ tty->print_cr("Deflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
+ (intptr_t) obj, (intptr_t) obj->mark(), Klass::cast(obj->klass())->external_name());
+ }
+ }
+
+ // Restore the header back to obj
+ obj->release_set_mark(mid->header());
+ mid->clear();
+
+ assert (mid->object() == NULL, "invariant") ;
+
+ // Move the object to the working free list defined by FreeHead,FreeTail.
+ mid->FreeNext = NULL ;
+ if (FreeHead == NULL) FreeHead = mid ;
+ if (FreeTail != NULL) FreeTail->FreeNext = mid ;
+ FreeTail = mid ;
+ nScavenged ++ ;
+ }
+ }
+ }
+
+ // Move the scavenged monitors back to the global free list.
+ // In theory we don't need the freelist lock as we're at a STW safepoint.
+ // omAlloc() and omFree() can only be called while a thread is _not in safepoint state.
+ // But it's remotely possible that omFlush() or release_monitors_owned_by_thread()
+ // might be called while not at a global STW safepoint. In the interest of
+ // safety we protect the following access with ListLock.
+ // An even more conservative and prudent approach would be to guard
+ // the main loop in scavenge_idle_monitors() with ListLock.
+ if (FreeHead != NULL) {
+ guarantee (FreeTail != NULL && nScavenged > 0, "invariant") ;
+ assert (FreeTail->FreeNext == NULL, "invariant") ;
+ // constant-time list splice - prepend scavenged segment to gFreeList
+ Thread::muxAcquire (&ListLock, "scavenge - return") ;
+ FreeTail->FreeNext = gFreeList ;
+ gFreeList = FreeHead ;
+ Thread::muxRelease (&ListLock) ;
+ }
+
+ if (_sync_Deflations != NULL) _sync_Deflations->inc(nScavenged) ;
+ if (_sync_MonExtant != NULL) _sync_MonExtant ->set_value(nInCirculation);
+
+ // TODO: Add objectMonitor leak detection.
+ // Audit/inventory the objectMonitors -- make sure they're all accounted for.
+ GVars.stwRandom = os::random() ;
+ GVars.stwCycle ++ ;
+}
+
+// A macro is used below because there may already be a pending
+// exception which should not abort the execution of the routines
+// which use this (which is why we don't put this into check_slow and
+// call it with a CHECK argument).
+
+#define CHECK_OWNER() \
+ do { \
+ if (THREAD != _owner) { \
+ if (THREAD->is_lock_owned((address) _owner)) { \
+ _owner = THREAD ; /* Convert from basiclock addr to Thread addr */ \
+ _recursions = 0; \
+ OwnerIsThread = 1 ; \
+ } else { \
+ TEVENT (Throw IMSX) ; \
+ THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \
+ } \
+ } \
+ } while (false)
+
+// TODO-FIXME: eliminate ObjectWaiters. Replace this visitor/enumerator
+// interface with a simple FirstWaitingThread(), NextWaitingThread() interface.
+
+ObjectWaiter* ObjectMonitor::first_waiter() {
+ return _WaitSet;
+}
+
+ObjectWaiter* ObjectMonitor::next_waiter(ObjectWaiter* o) {
+ return o->_next;
+}
+
+Thread* ObjectMonitor::thread_of_waiter(ObjectWaiter* o) {
+ return o->_thread;
+}
+
+// initialize the monitor, exception the semaphore, all other fields
+// are simple integers or pointers
+ObjectMonitor::ObjectMonitor() {
+ _header = NULL;
+ _count = 0;
+ _waiters = 0,
+ _recursions = 0;
+ _object = NULL;
+ _owner = NULL;
+ _WaitSet = NULL;
+ _WaitSetLock = 0 ;
+ _Responsible = NULL ;
+ _succ = NULL ;
+ _cxq = NULL ;
+ FreeNext = NULL ;
+ _EntryList = NULL ;
+ _SpinFreq = 0 ;
+ _SpinClock = 0 ;
+ OwnerIsThread = 0 ;
+}
+
+ObjectMonitor::~ObjectMonitor() {
+ // TODO: Add asserts ...
+ // _cxq == 0 _succ == NULL _owner == NULL _waiters == 0
+ // _count == 0 _EntryList == NULL etc
+}
+
+intptr_t ObjectMonitor::is_busy() const {
+ // TODO-FIXME: merge _count and _waiters.
+ // TODO-FIXME: assert _owner == null implies _recursions = 0
+ // TODO-FIXME: assert _WaitSet != null implies _count > 0
+ return _count|_waiters|intptr_t(_owner)|intptr_t(_cxq)|intptr_t(_EntryList ) ;
+}
+
+void ObjectMonitor::Recycle () {
+ // TODO: add stronger asserts ...
+ // _cxq == 0 _succ == NULL _owner == NULL _waiters == 0
+ // _count == 0 EntryList == NULL
+ // _recursions == 0 _WaitSet == NULL
+ // TODO: assert (is_busy()|_recursions) == 0
+ _succ = NULL ;
+ _EntryList = NULL ;
+ _cxq = NULL ;
+ _WaitSet = NULL ;
+ _recursions = 0 ;
+ _SpinFreq = 0 ;
+ _SpinClock = 0 ;
+ OwnerIsThread = 0 ;
+}
+
+// WaitSet management ...
+
+inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
+ assert(node != NULL, "should not dequeue NULL node");
+ assert(node->_prev == NULL, "node already in list");
+ assert(node->_next == NULL, "node already in list");
+ // put node at end of queue (circular doubly linked list)
+ if (_WaitSet == NULL) {
+ _WaitSet = node;
+ node->_prev = node;
+ node->_next = node;
+ } else {
+ ObjectWaiter* head = _WaitSet ;
+ ObjectWaiter* tail = head->_prev;
+ assert(tail->_next == head, "invariant check");
+ tail->_next = node;
+ head->_prev = node;
+ node->_next = head;
+ node->_prev = tail;
+ }
+}
+
+inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
+ // dequeue the very first waiter
+ ObjectWaiter* waiter = _WaitSet;
+ if (waiter) {
+ DequeueSpecificWaiter(waiter);
+ }
+ return waiter;
+}
+
+inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
+ assert(node != NULL, "should not dequeue NULL node");
+ assert(node->_prev != NULL, "node already removed from list");
+ assert(node->_next != NULL, "node already removed from list");
+ // when the waiter has woken up because of interrupt,
+ // timeout or other spurious wake-up, dequeue the
+ // waiter from waiting list
+ ObjectWaiter* next = node->_next;
+ if (next == node) {
+ assert(node->_prev == node, "invariant check");
+ _WaitSet = NULL;
+ } else {
+ ObjectWaiter* prev = node->_prev;
+ assert(prev->_next == node, "invariant check");
+ assert(next->_prev == node, "invariant check");
+ next->_prev = prev;
+ prev->_next = next;
+ if (_WaitSet == node) {
+ _WaitSet = next;
+ }
+ }
+ node->_next = NULL;
+ node->_prev = NULL;
+}
+
+static char * kvGet (char * kvList, const char * Key) {
+ if (kvList == NULL) return NULL ;
+ size_t n = strlen (Key) ;
+ char * Search ;
+ for (Search = kvList ; *Search ; Search += strlen(Search) + 1) {
+ if (strncmp (Search, Key, n) == 0) {
+ if (Search[n] == '=') return Search + n + 1 ;
+ if (Search[n] == 0) return (char *) "1" ;
+ }
+ }
+ return NULL ;
+}
+
+static int kvGetInt (char * kvList, const char * Key, int Default) {
+ char * v = kvGet (kvList, Key) ;
+ int rslt = v ? ::strtol (v, NULL, 0) : Default ;
+ if (Knob_ReportSettings && v != NULL) {
+ ::printf (" SyncKnob: %s %d(%d)\n", Key, rslt, Default) ;
+ ::fflush (stdout) ;
+ }
+ return rslt ;
+}
+
+// By convention we unlink a contending thread from EntryList|cxq immediately
+// after the thread acquires the lock in ::enter(). Equally, we could defer
+// unlinking the thread until ::exit()-time.
+
+void ObjectMonitor::UnlinkAfterAcquire (Thread * Self, ObjectWaiter * SelfNode)
+{
+ assert (_owner == Self, "invariant") ;
+ assert (SelfNode->_thread == Self, "invariant") ;
+
+ if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
+ // Normal case: remove Self from the DLL EntryList .
+ // This is a constant-time operation.
+ ObjectWaiter * nxt = SelfNode->_next ;
+ ObjectWaiter * prv = SelfNode->_prev ;
+ if (nxt != NULL) nxt->_prev = prv ;
+ if (prv != NULL) prv->_next = nxt ;
+ if (SelfNode == _EntryList ) _EntryList = nxt ;
+ assert (nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant") ;
+ assert (prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant") ;
+ TEVENT (Unlink from EntryList) ;
+ } else {
+ guarantee (SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant") ;
+ // Inopportune interleaving -- Self is still on the cxq.
+ // This usually means the enqueue of self raced an exiting thread.
+ // Normally we'll find Self near the front of the cxq, so
+ // dequeueing is typically fast. If needbe we can accelerate
+ // this with some MCS/CHL-like bidirectional list hints and advisory
+ // back-links so dequeueing from the interior will normally operate
+ // in constant-time.
+ // Dequeue Self from either the head (with CAS) or from the interior
+ // with a linear-time scan and normal non-atomic memory operations.
+ // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
+ // and then unlink Self from EntryList. We have to drain eventually,
+ // so it might as well be now.
+
+ ObjectWaiter * v = _cxq ;
+ assert (v != NULL, "invariant") ;
+ if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) {
+ // The CAS above can fail from interference IFF a "RAT" arrived.
+ // In that case Self must be in the interior and can no longer be
+ // at the head of cxq.
+ if (v == SelfNode) {
+ assert (_cxq != v, "invariant") ;
+ v = _cxq ; // CAS above failed - start scan at head of list
+ }
+ ObjectWaiter * p ;
+ ObjectWaiter * q = NULL ;
+ for (p = v ; p != NULL && p != SelfNode; p = p->_next) {
+ q = p ;
+ assert (p->TState == ObjectWaiter::TS_CXQ, "invariant") ;
+ }
+ assert (v != SelfNode, "invariant") ;
+ assert (p == SelfNode, "Node not found on cxq") ;
+ assert (p != _cxq, "invariant") ;
+ assert (q != NULL, "invariant") ;
+ assert (q->_next == p, "invariant") ;
+ q->_next = p->_next ;
+ }
+ TEVENT (Unlink from cxq) ;
+ }
+
+ // Diagnostic hygiene ...
+ SelfNode->_prev = (ObjectWaiter *) 0xBAD ;
+ SelfNode->_next = (ObjectWaiter *) 0xBAD ;
+ SelfNode->TState = ObjectWaiter::TS_RUN ;
+}
+
+// Caveat: TryLock() is not necessarily serializing if it returns failure.
+// Callers must compensate as needed.
+
+int ObjectMonitor::TryLock (Thread * Self) {
+ for (;;) {
+ void * own = _owner ;
+ if (own != NULL) return 0 ;
+ if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
+ // Either guarantee _recursions == 0 or set _recursions = 0.
+ assert (_recursions == 0, "invariant") ;
+ assert (_owner == Self, "invariant") ;
+ // CONSIDER: set or assert that OwnerIsThread == 1
+ return 1 ;
+ }
+ // The lock had been free momentarily, but we lost the race to the lock.
+ // Interference -- the CAS failed.
+ // We can either return -1 or retry.
+ // Retry doesn't make as much sense because the lock was just acquired.
+ if (true) return -1 ;
+ }
+}
+
+// NotRunnable() -- informed spinning
+//
+// Don't bother spinning if the owner is not eligible to drop the lock.
+// Peek at the owner's schedctl.sc_state and Thread._thread_values and
+// spin only if the owner thread is _thread_in_Java or _thread_in_vm.
+// The thread must be runnable in order to drop the lock in timely fashion.
+// If the _owner is not runnable then spinning will not likely be
+// successful (profitable).
+//
+// Beware -- the thread referenced by _owner could have died
+// so a simply fetch from _owner->_thread_state might trap.
+// Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
+// Because of the lifecycle issues the schedctl and _thread_state values
+// observed by NotRunnable() might be garbage. NotRunnable must
+// tolerate this and consider the observed _thread_state value
+// as advisory.
+//
+// Beware too, that _owner is sometimes a BasicLock address and sometimes
+// a thread pointer. We differentiate the two cases with OwnerIsThread.
+// Alternately, we might tag the type (thread pointer vs basiclock pointer)
+// with the LSB of _owner. Another option would be to probablistically probe
+// the putative _owner->TypeTag value.
+//
+// Checking _thread_state isn't perfect. Even if the thread is
+// in_java it might be blocked on a page-fault or have been preempted
+// and sitting on a ready/dispatch queue. _thread state in conjunction
+// with schedctl.sc_state gives us a good picture of what the
+// thread is doing, however.
+//
+// TODO: check schedctl.sc_state.
+// We'll need to use SafeFetch32() to read from the schedctl block.
+// See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/
+//
+// The return value from NotRunnable() is *advisory* -- the
+// result is based on sampling and is not necessarily coherent.
+// The caller must tolerate false-negative and false-positive errors.
+// Spinning, in general, is probabilistic anyway.
+
+
+int ObjectMonitor::NotRunnable (Thread * Self, Thread * ox) {
+ // Check either OwnerIsThread or ox->TypeTag == 2BAD.
+ if (!OwnerIsThread) return 0 ;
+
+ if (ox == NULL) return 0 ;
+
+ // Avoid transitive spinning ...
+ // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
+ // Immediately after T1 acquires L it's possible that T2, also
+ // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
+ // This occurs transiently after T1 acquired L but before
+ // T1 managed to clear T1.Stalled. T2 does not need to abort
+ // its spin in this circumstance.
+ intptr_t BlockedOn = SafeFetchN ((intptr_t *) &ox->_Stalled, intptr_t(1)) ;
+
+ if (BlockedOn == 1) return 1 ;
+ if (BlockedOn != 0) {
+ return BlockedOn != intptr_t(this) && _owner == ox ;
+ }
+
+ assert (sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant") ;
+ int jst = SafeFetch32 ((int *) &((JavaThread *) ox)->_thread_state, -1) ; ;
+ // consider also: jst != _thread_in_Java -- but that's overspecific.
+ return jst == _thread_blocked || jst == _thread_in_native ;
+}
+
+
+// Adaptive spin-then-block - rational spinning
+//
+// Note that we spin "globally" on _owner with a classic SMP-polite TATAS
+// algorithm. On high order SMP systems it would be better to start with
+// a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
+// a contending thread could enqueue itself on the cxq and then spin locally
+// on a thread-specific variable such as its ParkEvent._Event flag.
+// That's left as an exercise for the reader. Note that global spinning is
+// not problematic on Niagara, as the L2$ serves the interconnect and has both
+// low latency and massive bandwidth.
+//
+// Broadly, we can fix the spin frequency -- that is, the % of contended lock
+// acquisition attempts where we opt to spin -- at 100% and vary the spin count
+// (duration) or we can fix the count at approximately the duration of
+// a context switch and vary the frequency. Of course we could also
+// vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
+// See http://j2se.east/~dice/PERSIST/040824-AdaptiveSpinning.html.
+//
+// This implementation varies the duration "D", where D varies with
+// the success rate of recent spin attempts. (D is capped at approximately
+// length of a round-trip context switch). The success rate for recent
+// spin attempts is a good predictor of the success rate of future spin
+// attempts. The mechanism adapts automatically to varying critical
+// section length (lock modality), system load and degree of parallelism.
+// D is maintained per-monitor in _SpinDuration and is initialized
+// optimistically. Spin frequency is fixed at 100%.
+//
+// Note that _SpinDuration is volatile, but we update it without locks
+// or atomics. The code is designed so that _SpinDuration stays within
+// a reasonable range even in the presence of races. The arithmetic
+// operations on _SpinDuration are closed over the domain of legal values,
+// so at worst a race will install and older but still legal value.
+// At the very worst this introduces some apparent non-determinism.
+// We might spin when we shouldn't or vice-versa, but since the spin
+// count are relatively short, even in the worst case, the effect is harmless.
+//
+// Care must be taken that a low "D" value does not become an
+// an absorbing state. Transient spinning failures -- when spinning
+// is overall profitable -- should not cause the system to converge
+// on low "D" values. We want spinning to be stable and predictable
+// and fairly responsive to change and at the same time we don't want
+// it to oscillate, become metastable, be "too" non-deterministic,
+// or converge on or enter undesirable stable absorbing states.
+//
+// We implement a feedback-based control system -- using past behavior
+// to predict future behavior. We face two issues: (a) if the
+// input signal is random then the spin predictor won't provide optimal
+// results, and (b) if the signal frequency is too high then the control
+// system, which has some natural response lag, will "chase" the signal.
+// (b) can arise from multimodal lock hold times. Transient preemption
+// can also result in apparent bimodal lock hold times.
+// Although sub-optimal, neither condition is particularly harmful, as
+// in the worst-case we'll spin when we shouldn't or vice-versa.
+// The maximum spin duration is rather short so the failure modes aren't bad.
+// To be conservative, I've tuned the gain in system to bias toward
+// _not spinning. Relatedly, the system can sometimes enter a mode where it
+// "rings" or oscillates between spinning and not spinning. This happens
+// when spinning is just on the cusp of profitability, however, so the
+// situation is not dire. The state is benign -- there's no need to add
+// hysteresis control to damp the transition rate between spinning and
+// not spinning.
+//
+// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+//
+// Spin-then-block strategies ...
+//
+// Thoughts on ways to improve spinning :
+//
+// * Periodically call {psr_}getloadavg() while spinning, and
+// permit unbounded spinning if the load average is <
+// the number of processors. Beware, however, that getloadavg()
+// is exceptionally fast on solaris (about 1/10 the cost of a full
+// spin cycle, but quite expensive on linux. Beware also, that
+// multiple JVMs could "ring" or oscillate in a feedback loop.
+// Sufficient damping would solve that problem.
+//
+// * We currently use spin loops with iteration counters to approximate
+// spinning for some interval. Given the availability of high-precision
+// time sources such as gethrtime(), %TICK, %STICK, RDTSC, etc., we should
+// someday reimplement the spin loops to duration-based instead of iteration-based.
+//
+// * Don't spin if there are more than N = (CPUs/2) threads
+// currently spinning on the monitor (or globally).
+// That is, limit the number of concurrent spinners.
+// We might also limit the # of spinners in the JVM, globally.
+//
+// * If a spinning thread observes _owner change hands it should
+// abort the spin (and park immediately) or at least debit
+// the spin counter by a large "penalty".
+//
+// * Classically, the spin count is either K*(CPUs-1) or is a
+// simple constant that approximates the length of a context switch.
+// We currently use a value -- computed by a special utility -- that
+// approximates round-trip context switch times.
+//
+// * Normally schedctl_start()/_stop() is used to advise the kernel
+// to avoid preempting threads that are running in short, bounded
+// critical sections. We could use the schedctl hooks in an inverted
+// sense -- spinners would set the nopreempt flag, but poll the preempt
+// pending flag. If a spinner observed a pending preemption it'd immediately
+// abort the spin and park. As such, the schedctl service acts as
+// a preemption warning mechanism.
+//
+// * In lieu of spinning, if the system is running below saturation
+// (that is, loadavg() << #cpus), we can instead suppress futile
+// wakeup throttling, or even wake more than one successor at exit-time.
+// The net effect is largely equivalent to spinning. In both cases,
+// contending threads go ONPROC and opportunistically attempt to acquire
+// the lock, decreasing lock handover latency at the expense of wasted
+// cycles and context switching.
+//
+// * We might to spin less after we've parked as the thread will
+// have less $ and TLB affinity with the processor.
+// Likewise, we might spin less if we come ONPROC on a different
+// processor or after a long period (>> rechose_interval).
+//
+// * A table-driven state machine similar to Solaris' dispadmin scheduling
+// tables might be a better design. Instead of encoding information in
+// _SpinDuration, _SpinFreq and _SpinClock we'd just use explicit,
+// discrete states. Success or failure during a spin would drive
+// state transitions, and each state node would contain a spin count.
+//
+// * If the processor is operating in a mode intended to conserve power
+// (such as Intel's SpeedStep) or to reduce thermal output (thermal
+// step-down mode) then the Java synchronization subsystem should
+// forgo spinning.
+//
+// * The minimum spin duration should be approximately the worst-case
+// store propagation latency on the platform. That is, the time
+// it takes a store on CPU A to become visible on CPU B, where A and
+// B are "distant".
+//
+// * We might want to factor a thread's priority in the spin policy.
+// Threads with a higher priority might spin for slightly longer.
+// Similarly, if we use back-off in the TATAS loop, lower priority
+// threads might back-off longer. We don't currently use a
+// thread's priority when placing it on the entry queue. We may
+// want to consider doing so in future releases.
+//
+// * We might transiently drop a thread's scheduling priority while it spins.
+// SCHED_BATCH on linux and FX scheduling class at priority=0 on Solaris
+// would suffice. We could even consider letting the thread spin indefinitely at
+// a depressed or "idle" priority. This brings up fairness issues, however --
+// in a saturated system a thread would with a reduced priority could languish
+// for extended periods on the ready queue.
+//
+// * While spinning try to use the otherwise wasted time to help the VM make
+// progress:
+//
+// -- YieldTo() the owner, if the owner is OFFPROC but ready
+// Done our remaining quantum directly to the ready thread.
+// This helps "push" the lock owner through the critical section.
+// It also tends to improve affinity/locality as the lock
+// "migrates" less frequently between CPUs.
+// -- Walk our own stack in anticipation of blocking. Memoize the roots.
+// -- Perform strand checking for other thread. Unpark potential strandees.
+// -- Help GC: trace or mark -- this would need to be a bounded unit of work.
+// Unfortunately this will pollute our $ and TLBs. Recall that we
+// spin to avoid context switching -- context switching has an
+// immediate cost in latency, a disruptive cost to other strands on a CMT
+// processor, and an amortized cost because of the D$ and TLB cache
+// reload transient when the thread comes back ONPROC and repopulates
+// $s and TLBs.
+// -- call getloadavg() to see if the system is saturated. It'd probably
+// make sense to call getloadavg() half way through the spin.
+// If the system isn't at full capacity the we'd simply reset
+// the spin counter to and extend the spin attempt.
+// -- Doug points out that we should use the same "helping" policy
+// in thread.yield().
+//
+// * Try MONITOR-MWAIT on systems that support those instructions.
+//
+// * The spin statistics that drive spin decisions & frequency are
+// maintained in the objectmonitor structure so if we deflate and reinflate
+// we lose spin state. In practice this is not usually a concern
+// as the default spin state after inflation is aggressive (optimistic)
+// and tends toward spinning. So in the worst case for a lock where
+// spinning is not profitable we may spin unnecessarily for a brief
+// period. But then again, if a lock is contended it'll tend not to deflate
+// in the first place.
+
+
+intptr_t ObjectMonitor::SpinCallbackArgument = 0 ;
+int (*ObjectMonitor::SpinCallbackFunction)(intptr_t, int) = NULL ;
+
+// Spinning: Fixed frequency (100%), vary duration
+
+int ObjectMonitor::TrySpin_VaryDuration (Thread * Self) {
+
+ // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
+ int ctr = Knob_FixedSpin ;
+ if (ctr != 0) {
+ while (--ctr >= 0) {
+ if (TryLock (Self) > 0) return 1 ;
+ SpinPause () ;
+ }
+ return 0 ;
+ }
+
+ for (ctr = Knob_PreSpin + 1; --ctr >= 0 ; ) {
+ if (TryLock(Self) > 0) {
+ // Increase _SpinDuration ...
+ // Note that we don't clamp SpinDuration precisely at SpinLimit.
+ // Raising _SpurDuration to the poverty line is key.
+ int x = _SpinDuration ;
+ if (x < Knob_SpinLimit) {
+ if (x < Knob_Poverty) x = Knob_Poverty ;
+ _SpinDuration = x + Knob_BonusB ;
+ }
+ return 1 ;
+ }
+ SpinPause () ;
+ }
+
+ // Admission control - verify preconditions for spinning
+ //
+ // We always spin a little bit, just to prevent _SpinDuration == 0 from
+ // becoming an absorbing state. Put another way, we spin briefly to
+ // sample, just in case the system load, parallelism, contention, or lock
+ // modality changed.
+ //
+ // Consider the following alternative:
+ // Periodically set _SpinDuration = _SpinLimit and try a long/full
+ // spin attempt. "Periodically" might mean after a tally of
+ // the # of failed spin attempts (or iterations) reaches some threshold.
+ // This takes us into the realm of 1-out-of-N spinning, where we
+ // hold the duration constant but vary the frequency.
+
+ ctr = _SpinDuration ;
+ if (ctr < Knob_SpinBase) ctr = Knob_SpinBase ;
+ if (ctr <= 0) return 0 ;
+
+ if (Knob_SuccRestrict && _succ != NULL) return 0 ;
+ if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
+ TEVENT (Spin abort - notrunnable [TOP]);
+ return 0 ;
+ }
+
+ int MaxSpin = Knob_MaxSpinners ;
+ if (MaxSpin >= 0) {
+ if (_Spinner > MaxSpin) {
+ TEVENT (Spin abort -- too many spinners) ;
+ return 0 ;
+ }
+ // Slighty racy, but benign ...
+ Adjust (&_Spinner, 1) ;
+ }
+
+ // We're good to spin ... spin ingress.
+ // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
+ // when preparing to LD...CAS _owner, etc and the CAS is likely
+ // to succeed.
+ int hits = 0 ;
+ int msk = 0 ;
+ int caspty = Knob_CASPenalty ;
+ int oxpty = Knob_OXPenalty ;
+ int sss = Knob_SpinSetSucc ;
+ if (sss && _succ == NULL ) _succ = Self ;
+ Thread * prv = NULL ;
+
+ // There are three ways to exit the following loop:
+ // 1. A successful spin where this thread has acquired the lock.
+ // 2. Spin failure with prejudice
+ // 3. Spin failure without prejudice
+
+ while (--ctr >= 0) {
+
+ // Periodic polling -- Check for pending GC
+ // Threads may spin while they're unsafe.
+ // We don't want spinning threads to delay the JVM from reaching
+ // a stop-the-world safepoint or to steal cycles from GC.
+ // If we detect a pending safepoint we abort in order that
+ // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
+ // this thread, if safe, doesn't steal cycles from GC.
+ // This is in keeping with the "no loitering in runtime" rule.
+ // We periodically check to see if there's a safepoint pending.
+ if ((ctr & 0xFF) == 0) {
+ if (SafepointSynchronize::do_call_back()) {
+ TEVENT (Spin: safepoint) ;
+ goto Abort ; // abrupt spin egress
+ }
+ if (Knob_UsePause & 1) SpinPause () ;
+
+ int (*scb)(intptr_t,int) = SpinCallbackFunction ;
+ if (hits > 50 && scb != NULL) {
+ int abend = (*scb)(SpinCallbackArgument, 0) ;
+ }
+ }
+
+ if (Knob_UsePause & 2) SpinPause() ;
+
+ // Exponential back-off ... Stay off the bus to reduce coherency traffic.
+ // This is useful on classic SMP systems, but is of less utility on
+ // N1-style CMT platforms.
+ //
+ // Trade-off: lock acquisition latency vs coherency bandwidth.
+ // Lock hold times are typically short. A histogram
+ // of successful spin attempts shows that we usually acquire
+ // the lock early in the spin. That suggests we want to
+ // sample _owner frequently in the early phase of the spin,
+ // but then back-off and sample less frequently as the spin
+ // progresses. The back-off makes a good citizen on SMP big
+ // SMP systems. Oversampling _owner can consume excessive
+ // coherency bandwidth. Relatedly, if we _oversample _owner we
+ // can inadvertently interfere with the the ST m->owner=null.
+ // executed by the lock owner.
+ if (ctr & msk) continue ;
+ ++hits ;
+ if ((hits & 0xF) == 0) {
+ // The 0xF, above, corresponds to the exponent.
+ // Consider: (msk+1)|msk
+ msk = ((msk << 2)|3) & BackOffMask ;
+ }
+
+ // Probe _owner with TATAS
+ // If this thread observes the monitor transition or flicker
+ // from locked to unlocked to locked, then the odds that this
+ // thread will acquire the lock in this spin attempt go down
+ // considerably. The same argument applies if the CAS fails
+ // or if we observe _owner change from one non-null value to
+ // another non-null value. In such cases we might abort
+ // the spin without prejudice or apply a "penalty" to the
+ // spin count-down variable "ctr", reducing it by 100, say.
+
+ Thread * ox = (Thread *) _owner ;
+ if (ox == NULL) {
+ ox = (Thread *) Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
+ if (ox == NULL) {
+ // The CAS succeeded -- this thread acquired ownership
+ // Take care of some bookkeeping to exit spin state.
+ if (sss && _succ == Self) {
+ _succ = NULL ;
+ }
+ if (MaxSpin > 0) Adjust (&_Spinner, -1) ;
+
+ // Increase _SpinDuration :
+ // The spin was successful (profitable) so we tend toward
+ // longer spin attempts in the future.
+ // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
+ // If we acquired the lock early in the spin cycle it
+ // makes sense to increase _SpinDuration proportionally.
+ // Note that we don't clamp SpinDuration precisely at SpinLimit.
+ int x = _SpinDuration ;
+ if (x < Knob_SpinLimit) {
+ if (x < Knob_Poverty) x = Knob_Poverty ;
+ _SpinDuration = x + Knob_Bonus ;
+ }
+ return 1 ;
+ }
+
+ // The CAS failed ... we can take any of the following actions:
+ // * penalize: ctr -= Knob_CASPenalty
+ // * exit spin with prejudice -- goto Abort;
+ // * exit spin without prejudice.
+ // * Since CAS is high-latency, retry again immediately.
+ prv = ox ;
+ TEVENT (Spin: cas failed) ;
+ if (caspty == -2) break ;
+ if (caspty == -1) goto Abort ;
+ ctr -= caspty ;
+ continue ;
+ }
+
+ // Did lock ownership change hands ?
+ if (ox != prv && prv != NULL ) {
+ TEVENT (spin: Owner changed)
+ if (oxpty == -2) break ;
+ if (oxpty == -1) goto Abort ;
+ ctr -= oxpty ;
+ }
+ prv = ox ;
+
+ // Abort the spin if the owner is not executing.
+ // The owner must be executing in order to drop the lock.
+ // Spinning while the owner is OFFPROC is idiocy.
+ // Consider: ctr -= RunnablePenalty ;
+ if (Knob_OState && NotRunnable (Self, ox)) {
+ TEVENT (Spin abort - notrunnable);
+ goto Abort ;
+ }
+ if (sss && _succ == NULL ) _succ = Self ;
+ }
+
+ // Spin failed with prejudice -- reduce _SpinDuration.
+ // TODO: Use an AIMD-like policy to adjust _SpinDuration.
+ // AIMD is globally stable.
+ TEVENT (Spin failure) ;
+ {
+ int x = _SpinDuration ;
+ if (x > 0) {
+ // Consider an AIMD scheme like: x -= (x >> 3) + 100
+ // This is globally sample and tends to damp the response.
+ x -= Knob_Penalty ;
+ if (x < 0) x = 0 ;
+ _SpinDuration = x ;
+ }
+ }
+
+ Abort:
+ if (MaxSpin >= 0) Adjust (&_Spinner, -1) ;
+ if (sss && _succ == Self) {
+ _succ = NULL ;
+ // Invariant: after setting succ=null a contending thread
+ // must recheck-retry _owner before parking. This usually happens
+ // in the normal usage of TrySpin(), but it's safest
+ // to make TrySpin() as foolproof as possible.
+ OrderAccess::fence() ;
+ if (TryLock(Self) > 0) return 1 ;
+ }
+ return 0 ;
+}
+
+#define TrySpin TrySpin_VaryDuration
+
+static void DeferredInitialize () {
+ if (InitDone > 0) return ;
+ if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
+ while (InitDone != 1) ;
+ return ;
+ }
+
+ // One-shot global initialization ...
+ // The initialization is idempotent, so we don't need locks.
+ // In the future consider doing this via os::init_2().
+ // SyncKnobs consist of <Key>=<Value> pairs in the style
+ // of environment variables. Start by converting ':' to NUL.
+
+ if (SyncKnobs == NULL) SyncKnobs = "" ;
+
+ size_t sz = strlen (SyncKnobs) ;
+ char * knobs = (char *) malloc (sz + 2) ;
+ if (knobs == NULL) {
+ vm_exit_out_of_memory (sz + 2, "Parse SyncKnobs") ;
+ guarantee (0, "invariant") ;
+ }
+ strcpy (knobs, SyncKnobs) ;
+ knobs[sz+1] = 0 ;
+ for (char * p = knobs ; *p ; p++) {
+ if (*p == ':') *p = 0 ;
+ }
+
+ #define SETKNOB(x) { Knob_##x = kvGetInt (knobs, #x, Knob_##x); }
+ SETKNOB(ReportSettings) ;
+ SETKNOB(Verbose) ;
+ SETKNOB(FixedSpin) ;
+ SETKNOB(SpinLimit) ;
+ SETKNOB(SpinBase) ;
+ SETKNOB(SpinBackOff);
+ SETKNOB(CASPenalty) ;
+ SETKNOB(OXPenalty) ;
+ SETKNOB(LogSpins) ;
+ SETKNOB(SpinSetSucc) ;
+ SETKNOB(SuccEnabled) ;
+ SETKNOB(SuccRestrict) ;
+ SETKNOB(Penalty) ;
+ SETKNOB(Bonus) ;
+ SETKNOB(BonusB) ;
+ SETKNOB(Poverty) ;
+ SETKNOB(SpinAfterFutile) ;
+ SETKNOB(UsePause) ;
+ SETKNOB(SpinEarly) ;
+ SETKNOB(OState) ;
+ SETKNOB(MaxSpinners) ;
+ SETKNOB(PreSpin) ;
+ SETKNOB(ExitPolicy) ;
+ SETKNOB(QMode);
+ SETKNOB(ResetEvent) ;
+ SETKNOB(MoveNotifyee) ;
+ SETKNOB(FastHSSEC) ;
+ #undef SETKNOB
+
+ if (os::is_MP()) {
+ BackOffMask = (1 << Knob_SpinBackOff) - 1 ;
+ if (Knob_ReportSettings) ::printf ("BackOffMask=%X\n", BackOffMask) ;
+ // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1)
+ } else {
+ Knob_SpinLimit = 0 ;
+ Knob_SpinBase = 0 ;
+ Knob_PreSpin = 0 ;
+ Knob_FixedSpin = -1 ;
+ }
+
+ if (Knob_LogSpins == 0) {
+ ObjectSynchronizer::_sync_FailedSpins = NULL ;
+ }
+
+ free (knobs) ;
+ OrderAccess::fence() ;
+ InitDone = 1 ;
+}
+
+// Theory of operations -- Monitors lists, thread residency, etc:
+//
+// * A thread acquires ownership of a monitor by successfully
+// CAS()ing the _owner field from null to non-null.
+//
+// * Invariant: A thread appears on at most one monitor list --
+// cxq, EntryList or WaitSet -- at any one time.
+//
+// * Contending threads "push" themselves onto the cxq with CAS
+// and then spin/park.
+//
+// * After a contending thread eventually acquires the lock it must
+// dequeue itself from either the EntryList or the cxq.
+//
+// * The exiting thread identifies and unparks an "heir presumptive"
+// tentative successor thread on the EntryList. Critically, the
+// exiting thread doesn't unlink the successor thread from the EntryList.
+// After having been unparked, the wakee will recontend for ownership of
+// the monitor. The successor (wakee) will either acquire the lock or
+// re-park itself.
+//
+// Succession is provided for by a policy of competitive handoff.
+// The exiting thread does _not_ grant or pass ownership to the
+// successor thread. (This is also referred to as "handoff" succession").
+// Instead the exiting thread releases ownership and possibly wakes
+// a successor, so the successor can (re)compete for ownership of the lock.
+// If the EntryList is empty but the cxq is populated the exiting
+// thread will drain the cxq into the EntryList. It does so by
+// by detaching the cxq (installing null with CAS) and folding
+// the threads from the cxq into the EntryList. The EntryList is
+// doubly linked, while the cxq is singly linked because of the
+// CAS-based "push" used to enqueue recently arrived threads (RATs).
+//
+// * Concurrency invariants:
+//
+// -- only the monitor owner may access or mutate the EntryList.
+// The mutex property of the monitor itself protects the EntryList
+// from concurrent interference.
+// -- Only the monitor owner may detach the cxq.
+//
+// * The monitor entry list operations avoid locks, but strictly speaking
+// they're not lock-free. Enter is lock-free, exit is not.
+// See http://j2se.east/~dice/PERSIST/040825-LockFreeQueues.html
+//
+// * The cxq can have multiple concurrent "pushers" but only one concurrent
+// detaching thread. This mechanism is immune from the ABA corruption.
+// More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
+//
+// * Taken together, the cxq and the EntryList constitute or form a
+// single logical queue of threads stalled trying to acquire the lock.
+// We use two distinct lists to improve the odds of a constant-time
+// dequeue operation after acquisition (in the ::enter() epilog) and
+// to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
+// A key desideratum is to minimize queue & monitor metadata manipulation
+// that occurs while holding the monitor lock -- that is, we want to
+// minimize monitor lock holds times. Note that even a small amount of
+// fixed spinning will greatly reduce the # of enqueue-dequeue operations
+// on EntryList|cxq. That is, spinning relieves contention on the "inner"
+// locks and monitor metadata.
+//
+// Cxq points to the the set of Recently Arrived Threads attempting entry.
+// Because we push threads onto _cxq with CAS, the RATs must take the form of
+// a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
+// the unlocking thread notices that EntryList is null but _cxq is != null.
+//
+// The EntryList is ordered by the prevailing queue discipline and
+// can be organized in any convenient fashion, such as a doubly-linked list or
+// a circular doubly-linked list. Critically, we want insert and delete operations
+// to operate in constant-time. If we need a priority queue then something akin
+// to Solaris' sleepq would work nicely. Viz.,
+// http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
+// Queue discipline is enforced at ::exit() time, when the unlocking thread
+// drains the cxq into the EntryList, and orders or reorders the threads on the
+// EntryList accordingly.
+//
+// Barring "lock barging", this mechanism provides fair cyclic ordering,
+// somewhat similar to an elevator-scan.
+//
+// * The monitor synchronization subsystem avoids the use of native
+// synchronization primitives except for the narrow platform-specific
+// park-unpark abstraction. See the comments in os_solaris.cpp regarding
+// the semantics of park-unpark. Put another way, this monitor implementation
+// depends only on atomic operations and park-unpark. The monitor subsystem
+// manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
+// underlying OS manages the READY<->RUN transitions.
+//
+// * Waiting threads reside on the WaitSet list -- wait() puts
+// the caller onto the WaitSet.
+//
+// * notify() or notifyAll() simply transfers threads from the WaitSet to
+// either the EntryList or cxq. Subsequent exit() operations will
+// unpark the notifyee. Unparking a notifee in notify() is inefficient -
+// it's likely the notifyee would simply impale itself on the lock held
+// by the notifier.
+//
+// * An interesting alternative is to encode cxq as (List,LockByte) where
+// the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
+// variable, like _recursions, in the scheme. The threads or Events that form
+// the list would have to be aligned in 256-byte addresses. A thread would
+// try to acquire the lock or enqueue itself with CAS, but exiting threads
+// could use a 1-0 protocol and simply STB to set the LockByte to 0.
+// Note that is is *not* word-tearing, but it does presume that full-word
+// CAS operations are coherent with intermix with STB operations. That's true
+// on most common processors.
+//
+// * See also http://blogs.sun.com/dave
+
+
+void ATTR ObjectMonitor::EnterI (TRAPS) {
+ Thread * Self = THREAD ;
+ assert (Self->is_Java_thread(), "invariant") ;
+ assert (((JavaThread *) Self)->thread_state() == _thread_blocked , "invariant") ;
+
+ // Try the lock - TATAS
+ if (TryLock (Self) > 0) {
+ assert (_succ != Self , "invariant") ;
+ assert (_owner == Self , "invariant") ;
+ assert (_Responsible != Self , "invariant") ;
+ return ;
+ }
+
+ DeferredInitialize () ;
+
+ // We try one round of spinning *before* enqueueing Self.
+ //
+ // If the _owner is ready but OFFPROC we could use a YieldTo()
+ // operation to donate the remainder of this thread's quantum
+ // to the owner. This has subtle but beneficial affinity
+ // effects.
+
+ if (TrySpin (Self) > 0) {
+ assert (_owner == Self , "invariant") ;
+ assert (_succ != Self , "invariant") ;
+ assert (_Responsible != Self , "invariant") ;
+ return ;
+ }
+
+ // The Spin failed -- Enqueue and park the thread ...
+ assert (_succ != Self , "invariant") ;
+ assert (_owner != Self , "invariant") ;
+ assert (_Responsible != Self , "invariant") ;
+
+ // Enqueue "Self" on ObjectMonitor's _cxq.
+ //
+ // Node acts as a proxy for Self.
+ // As an aside, if were to ever rewrite the synchronization code mostly
+ // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
+ // Java objects. This would avoid awkward lifecycle and liveness issues,
+ // as well as eliminate a subset of ABA issues.
+ // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
+ //
+
+ ObjectWaiter node(Self) ;
+ Self->_ParkEvent->reset() ;
+ node._prev = (ObjectWaiter *) 0xBAD ;
+ node.TState = ObjectWaiter::TS_CXQ ;
+
+ // Push "Self" onto the front of the _cxq.
+ // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
+ // Note that spinning tends to reduce the rate at which threads
+ // enqueue and dequeue on EntryList|cxq.
+ ObjectWaiter * nxt ;
+ for (;;) {
+ node._next = nxt = _cxq ;
+ if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ;
+
+ // Interference - the CAS failed because _cxq changed. Just retry.
+ // As an optional optimization we retry the lock.
+ if (TryLock (Self) > 0) {
+ assert (_succ != Self , "invariant") ;
+ assert (_owner == Self , "invariant") ;
+ assert (_Responsible != Self , "invariant") ;
+ return ;
+ }
+ }
+
+ // Check for cxq|EntryList edge transition to non-null. This indicates
+ // the onset of contention. While contention persists exiting threads
+ // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
+ // operations revert to the faster 1-0 mode. This enter operation may interleave
+ // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
+ // arrange for one of the contending thread to use a timed park() operations
+ // to detect and recover from the race. (Stranding is form of progress failure
+ // where the monitor is unlocked but all the contending threads remain parked).
+ // That is, at least one of the contended threads will periodically poll _owner.
+ // One of the contending threads will become the designated "Responsible" thread.
+ // The Responsible thread uses a timed park instead of a normal indefinite park
+ // operation -- it periodically wakes and checks for and recovers from potential
+ // strandings admitted by 1-0 exit operations. We need at most one Responsible
+ // thread per-monitor at any given moment. Only threads on cxq|EntryList may
+ // be responsible for a monitor.
+ //
+ // Currently, one of the contended threads takes on the added role of "Responsible".
+ // A viable alternative would be to use a dedicated "stranding checker" thread
+ // that periodically iterated over all the threads (or active monitors) and unparked
+ // successors where there was risk of stranding. This would help eliminate the
+ // timer scalability issues we see on some platforms as we'd only have one thread
+ // -- the checker -- parked on a timer.
+
+ if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) {
+ // Try to assume the role of responsible thread for the monitor.
+ // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
+ Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
+ }
+
+ // The lock have been released while this thread was occupied queueing
+ // itself onto _cxq. To close the race and avoid "stranding" and
+ // progress-liveness failure we must resample-retry _owner before parking.
+ // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
+ // In this case the ST-MEMBAR is accomplished with CAS().
+ //
+ // TODO: Defer all thread state transitions until park-time.
+ // Since state transitions are heavy and inefficient we'd like
+ // to defer the state transitions until absolutely necessary,
+ // and in doing so avoid some transitions ...
+
+ TEVENT (Inflated enter - Contention) ;
+ int nWakeups = 0 ;
+ int RecheckInterval = 1 ;
+
+ for (;;) {
+
+ if (TryLock (Self) > 0) break ;
+ assert (_owner != Self, "invariant") ;
+
+ if ((SyncFlags & 2) && _Responsible == NULL) {
+ Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ;
+ }
+
+ // park self
+ if (_Responsible == Self || (SyncFlags & 1)) {
+ TEVENT (Inflated enter - park TIMED) ;
+ Self->_ParkEvent->park ((jlong) RecheckInterval) ;
+ // Increase the RecheckInterval, but clamp the value.
+ RecheckInterval *= 8 ;
+ if (RecheckInterval > 1000) RecheckInterval = 1000 ;
+ } else {
+ TEVENT (Inflated enter - park UNTIMED) ;
+ Self->_ParkEvent->park() ;
+ }
+
+ if (TryLock(Self) > 0) break ;
+
+ // The lock is still contested.
+ // Keep a tally of the # of futile wakeups.
+ // Note that the counter is not protected by a lock or updated by atomics.
+ // That is by design - we trade "lossy" counters which are exposed to
+ // races during updates for a lower probe effect.
+ TEVENT (Inflated enter - Futile wakeup) ;
+ if (ObjectSynchronizer::_sync_FutileWakeups != NULL) {
+ ObjectSynchronizer::_sync_FutileWakeups->inc() ;
+ }
+ ++ nWakeups ;
+
+ // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
+ // We can defer clearing _succ until after the spin completes
+ // TrySpin() must tolerate being called with _succ == Self.
+ // Try yet another round of adaptive spinning.
+ if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ;
+
+ // We can find that we were unpark()ed and redesignated _succ while
+ // we were spinning. That's harmless. If we iterate and call park(),
+ // park() will consume the event and return immediately and we'll
+ // just spin again. This pattern can repeat, leaving _succ to simply
+ // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks().
+ // Alternately, we can sample fired() here, and if set, forgo spinning
+ // in the next iteration.
+
+ if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
+ Self->_ParkEvent->reset() ;
+ OrderAccess::fence() ;
+ }
+ if (_succ == Self) _succ = NULL ;
+
+ // Invariant: after clearing _succ a thread *must* retry _owner before parking.
+ OrderAccess::fence() ;
+ }
+
+ // Egress :
+ // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
+ // Normally we'll find Self on the EntryList .
+ // From the perspective of the lock owner (this thread), the
+ // EntryList is stable and cxq is prepend-only.
+ // The head of cxq is volatile but the interior is stable.
+ // In addition, Self.TState is stable.
+
+ assert (_owner == Self , "invariant") ;
+ assert (object() != NULL , "invariant") ;
+ // I'd like to write:
+ // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
+ // but as we're at a safepoint that's not safe.
+
+ UnlinkAfterAcquire (Self, &node) ;
+ if (_succ == Self) _succ = NULL ;
+
+ assert (_succ != Self, "invariant") ;
+ if (_Responsible == Self) {
+ _Responsible = NULL ;
+ // Dekker pivot-point.
+ // Consider OrderAccess::storeload() here
+
+ // We may leave threads on cxq|EntryList without a designated
+ // "Responsible" thread. This is benign. When this thread subsequently
+ // exits the monitor it can "see" such preexisting "old" threads --
+ // threads that arrived on the cxq|EntryList before the fence, above --
+ // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
+ // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
+ // non-null and elect a new "Responsible" timer thread.
+ //
+ // This thread executes:
+ // ST Responsible=null; MEMBAR (in enter epilog - here)
+ // LD cxq|EntryList (in subsequent exit)
+ //
+ // Entering threads in the slow/contended path execute:
+ // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
+ // The (ST cxq; MEMBAR) is accomplished with CAS().
+ //
+ // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
+ // exit operation from floating above the ST Responsible=null.
+ //
+ // In *practice* however, EnterI() is always followed by some atomic
+ // operation such as the decrement of _count in ::enter(). Those atomics
+ // obviate the need for the explicit MEMBAR, above.
+ }
+
+ // We've acquired ownership with CAS().
+ // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
+ // But since the CAS() this thread may have also stored into _succ,
+ // EntryList, cxq or Responsible. These meta-data updates must be
+ // visible __before this thread subsequently drops the lock.
+ // Consider what could occur if we didn't enforce this constraint --
+ // STs to monitor meta-data and user-data could reorder with (become
+ // visible after) the ST in exit that drops ownership of the lock.
+ // Some other thread could then acquire the lock, but observe inconsistent
+ // or old monitor meta-data and heap data. That violates the JMM.
+ // To that end, the 1-0 exit() operation must have at least STST|LDST
+ // "release" barrier semantics. Specifically, there must be at least a
+ // STST|LDST barrier in exit() before the ST of null into _owner that drops
+ // the lock. The barrier ensures that changes to monitor meta-data and data
+ // protected by the lock will be visible before we release the lock, and
+ // therefore before some other thread (CPU) has a chance to acquire the lock.
+ // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
+ //
+ // Critically, any prior STs to _succ or EntryList must be visible before
+ // the ST of null into _owner in the *subsequent* (following) corresponding
+ // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
+ // execute a serializing instruction.
+
+ if (SyncFlags & 8) {
+ OrderAccess::fence() ;
+ }
+ return ;
+}
+
+// ExitSuspendEquivalent:
+// A faster alternate to handle_special_suspend_equivalent_condition()
+//
+// handle_special_suspend_equivalent_condition() unconditionally
+// acquires the SR_lock. On some platforms uncontended MutexLocker()
+// operations have high latency. Note that in ::enter() we call HSSEC
+// while holding the monitor, so we effectively lengthen the critical sections.
+//
+// There are a number of possible solutions:
+//
+// A. To ameliorate the problem we might also defer state transitions
+// to as late as possible -- just prior to parking.
+// Given that, we'd call HSSEC after having returned from park(),
+// but before attempting to acquire the monitor. This is only a
+// partial solution. It avoids calling HSSEC while holding the
+// monitor (good), but it still increases successor reacquisition latency --
+// the interval between unparking a successor and the time the successor
+// resumes and retries the lock. See ReenterI(), which defers state transitions.
+// If we use this technique we can also avoid EnterI()-exit() loop
+// in ::enter() where we iteratively drop the lock and then attempt
+// to reacquire it after suspending.
+//
+// B. In the future we might fold all the suspend bits into a
+// composite per-thread suspend flag and then update it with CAS().
+// Alternately, a Dekker-like mechanism with multiple variables
+// would suffice:
+// ST Self->_suspend_equivalent = false
+// MEMBAR
+// LD Self_>_suspend_flags
+//
+
+
+bool ObjectMonitor::ExitSuspendEquivalent (JavaThread * jSelf) {
+ int Mode = Knob_FastHSSEC ;
+ if (Mode && !jSelf->is_external_suspend()) {
+ assert (jSelf->is_suspend_equivalent(), "invariant") ;
+ jSelf->clear_suspend_equivalent() ;
+ if (2 == Mode) OrderAccess::storeload() ;
+ if (!jSelf->is_external_suspend()) return false ;
+ // We raced a suspension -- fall thru into the slow path
+ TEVENT (ExitSuspendEquivalent - raced) ;
+ jSelf->set_suspend_equivalent() ;
+ }
+ return jSelf->handle_special_suspend_equivalent_condition() ;
+}
+
+
+// ReenterI() is a specialized inline form of the latter half of the
+// contended slow-path from EnterI(). We use ReenterI() only for
+// monitor reentry in wait().
+//
+// In the future we should reconcile EnterI() and ReenterI(), adding
+// Knob_Reset and Knob_SpinAfterFutile support and restructuring the
+// loop accordingly.
+
+void ATTR ObjectMonitor::ReenterI (Thread * Self, ObjectWaiter * SelfNode) {
+ assert (Self != NULL , "invariant") ;
+ assert (SelfNode != NULL , "invariant") ;
+ assert (SelfNode->_thread == Self , "invariant") ;
+ assert (_waiters > 0 , "invariant") ;
+ assert (((oop)(object()))->mark() == markOopDesc::encode(this) , "invariant") ;
+ assert (((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;
+ JavaThread * jt = (JavaThread *) Self ;
+
+ int nWakeups = 0 ;
+ for (;;) {
+ ObjectWaiter::TStates v = SelfNode->TState ;
+ guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
+ assert (_owner != Self, "invariant") ;
+
+ if (TryLock (Self) > 0) break ;
+ if (TrySpin (Self) > 0) break ;
+
+ TEVENT (Wait Reentry - parking) ;
+
+ // State transition wrappers around park() ...
+ // ReenterI() wisely defers state transitions until
+ // it's clear we must park the thread.
+ {
+ OSThreadContendState osts(Self->osthread());
+ ThreadBlockInVM tbivm(jt);
+
+ // cleared by handle_special_suspend_equivalent_condition()
+ // or java_suspend_self()
+ jt->set_suspend_equivalent();
+ if (SyncFlags & 1) {
+ Self->_ParkEvent->park ((jlong)1000) ;
+ } else {
+ Self->_ParkEvent->park () ;
+ }
+
+ // were we externally suspended while we were waiting?
+ for (;;) {
+ if (!ExitSuspendEquivalent (jt)) break ;
+ if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
+ jt->java_suspend_self();
+ jt->set_suspend_equivalent();
+ }
+ }
+
+ // Try again, but just so we distinguish between futile wakeups and
+ // successful wakeups. The following test isn't algorithmically
+ // necessary, but it helps us maintain sensible statistics.
+ if (TryLock(Self) > 0) break ;
+
+ // The lock is still contested.
+ // Keep a tally of the # of futile wakeups.
+ // Note that the counter is not protected by a lock or updated by atomics.
+ // That is by design - we trade "lossy" counters which are exposed to
+ // races during updates for a lower probe effect.
+ TEVENT (Wait Reentry - futile wakeup) ;
+ ++ nWakeups ;
+
+ // Assuming this is not a spurious wakeup we'll normally
+ // find that _succ == Self.
+ if (_succ == Self) _succ = NULL ;
+
+ // Invariant: after clearing _succ a contending thread
+ // *must* retry _owner before parking.
+ OrderAccess::fence() ;
+
+ if (ObjectSynchronizer::_sync_FutileWakeups != NULL) {
+ ObjectSynchronizer::_sync_FutileWakeups->inc() ;
+ }
+ }
+
+ // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
+ // Normally we'll find Self on the EntryList.
+ // Unlinking from the EntryList is constant-time and atomic-free.
+ // From the perspective of the lock owner (this thread), the
+ // EntryList is stable and cxq is prepend-only.
+ // The head of cxq is volatile but the interior is stable.
+ // In addition, Self.TState is stable.
+
+ assert (_owner == Self, "invariant") ;
+ assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
+ UnlinkAfterAcquire (Self, SelfNode) ;
+ if (_succ == Self) _succ = NULL ;
+ assert (_succ != Self, "invariant") ;
+ SelfNode->TState = ObjectWaiter::TS_RUN ;
+ OrderAccess::fence() ; // see comments at the end of EnterI()
+}
+
+bool ObjectMonitor::try_enter(Thread* THREAD) {
+ if (THREAD != _owner) {
+ if (THREAD->is_lock_owned ((address)_owner)) {
+ assert(_recursions == 0, "internal state error");
+ _owner = THREAD ;
+ _recursions = 1 ;
+ OwnerIsThread = 1 ;
+ return true;
+ }
+ if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
+ return false;
+ }
+ return true;
+ } else {
+ _recursions++;
+ return true;
+ }
+}
+
+void ATTR ObjectMonitor::enter(TRAPS) {
+ // The following code is ordered to check the most common cases first
+ // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
+ Thread * const Self = THREAD ;
+ void * cur ;
+
+ cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
+ if (cur == NULL) {
+ // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
+ assert (_recursions == 0 , "invariant") ;
+ assert (_owner == Self, "invariant") ;
+ // CONSIDER: set or assert OwnerIsThread == 1
+ return ;
+ }
+
+ if (cur == Self) {
+ // TODO-FIXME: check for integer overflow! BUGID 6557169.
+ _recursions ++ ;
+ return ;
+ }
+
+ if (Self->is_lock_owned ((address)cur)) {
+ assert (_recursions == 0, "internal state error");
+ _recursions = 1 ;
+ // Commute owner from a thread-specific on-stack BasicLockObject address to
+ // a full-fledged "Thread *".
+ _owner = Self ;
+ OwnerIsThread = 1 ;
+ return ;
+ }
+
+ // We've encountered genuine contention.
+ assert (Self->_Stalled == 0, "invariant") ;
+ Self->_Stalled = intptr_t(this) ;
+
+ // Try one round of spinning *before* enqueueing Self
+ // and before going through the awkward and expensive state
+ // transitions. The following spin is strictly optional ...
+ // Note that if we acquire the monitor from an initial spin
+ // we forgo posting JVMTI events and firing DTRACE probes.
+ if (Knob_SpinEarly && TrySpin (Self) > 0) {
+ assert (_owner == Self , "invariant") ;
+ assert (_recursions == 0 , "invariant") ;
+ assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
+ Self->_Stalled = 0 ;
+ return ;
+ }
+
+ assert (_owner != Self , "invariant") ;
+ assert (_succ != Self , "invariant") ;
+ assert (Self->is_Java_thread() , "invariant") ;
+ JavaThread * jt = (JavaThread *) Self ;
+ assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
+ assert (jt->thread_state() != _thread_blocked , "invariant") ;
+ assert (this->object() != NULL , "invariant") ;
+ assert (_count >= 0, "invariant") ;
+
+ // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
+ // Ensure the object-monitor relationship remains stable while there's contention.
+ Atomic::inc_ptr(&_count);
+
+ { // Change java thread status to indicate blocked on monitor enter.
+ JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
+
+ DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
+ if (JvmtiExport::should_post_monitor_contended_enter()) {
+ JvmtiExport::post_monitor_contended_enter(jt, this);
+ }
+
+ OSThreadContendState osts(Self->osthread());
+ ThreadBlockInVM tbivm(jt);
+
+ Self->set_current_pending_monitor(this);
+
+ // TODO-FIXME: change the following for(;;) loop to straight-line code.
+ for (;;) {
+ jt->set_suspend_equivalent();
+ // cleared by handle_special_suspend_equivalent_condition()
+ // or java_suspend_self()
+
+ EnterI (THREAD) ;
+
+ if (!ExitSuspendEquivalent(jt)) break ;
+
+ //
+ // We have acquired the contended monitor, but while we were
+ // waiting another thread suspended us. We don't want to enter
+ // the monitor while suspended because that would surprise the
+ // thread that suspended us.
+ //
+ _recursions = 0 ;
+ _succ = NULL ;
+ exit (Self) ;
+
+ jt->java_suspend_self();
+ }
+ Self->set_current_pending_monitor(NULL);
+ }
+
+ Atomic::dec_ptr(&_count);
+ assert (_count >= 0, "invariant") ;
+ Self->_Stalled = 0 ;
+
+ // Must either set _recursions = 0 or ASSERT _recursions == 0.
+ assert (_recursions == 0 , "invariant") ;
+ assert (_owner == Self , "invariant") ;
+ assert (_succ != Self , "invariant") ;
+ assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
+
+ // The thread -- now the owner -- is back in vm mode.
+ // Report the glorious news via TI,DTrace and jvmstat.
+ // The probe effect is non-trivial. All the reportage occurs
+ // while we hold the monitor, increasing the length of the critical
+ // section. Amdahl's parallel speedup law comes vividly into play.
+ //
+ // Another option might be to aggregate the events (thread local or
+ // per-monitor aggregation) and defer reporting until a more opportune
+ // time -- such as next time some thread encounters contention but has
+ // yet to acquire the lock. While spinning that thread could
+ // spinning we could increment JVMStat counters, etc.
+
+ DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
+ if (JvmtiExport::should_post_monitor_contended_entered()) {
+ JvmtiExport::post_monitor_contended_entered(jt, this);
+ }
+ if (ObjectSynchronizer::_sync_ContendedLockAttempts != NULL) {
+ ObjectSynchronizer::_sync_ContendedLockAttempts->inc() ;
+ }
+}
+
+void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) {
+ assert (_owner == Self, "invariant") ;
+
+ // Exit protocol:
+ // 1. ST _succ = wakee
+ // 2. membar #loadstore|#storestore;
+ // 2. ST _owner = NULL
+ // 3. unpark(wakee)
+
+ _succ = Knob_SuccEnabled ? Wakee->_thread : NULL ;
+ ParkEvent * Trigger = Wakee->_event ;
+
+ // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
+ // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
+ // out-of-scope (non-extant).
+ Wakee = NULL ;
+
+ // Drop the lock
+ OrderAccess::release_store_ptr (&_owner, NULL) ;
+ OrderAccess::fence() ; // ST _owner vs LD in unpark()
+
+ // TODO-FIXME:
+ // If there's a safepoint pending the best policy would be to
+ // get _this thread to a safepoint and only wake the successor
+ // after the safepoint completed. monitorexit uses a "leaf"
+ // state transition, however, so this thread can't become
+ // safe at this point in time. (Its stack isn't walkable).
+ // The next best thing is to defer waking the successor by
+ // adding to a list of thread to be unparked after at the
+ // end of the forthcoming STW).
+ if (SafepointSynchronize::do_call_back()) {
+ TEVENT (unpark before SAFEPOINT) ;
+ }
+
+ // Possible optimizations ...
+ //
+ // * Consider: set Wakee->UnparkTime = timeNow()
+ // When the thread wakes up it'll compute (timeNow() - Self->UnparkTime()).
+ // By measuring recent ONPROC latency we can approximate the
+ // system load. In turn, we can feed that information back
+ // into the spinning & succession policies.
+ // (ONPROC latency correlates strongly with load).
+ //
+ // * Pull affinity:
+ // If the wakee is cold then transiently setting it's affinity
+ // to the current CPU is a good idea.
+ // See http://j2se.east/~dice/PERSIST/050624-PullAffinity.txt
+ Trigger->unpark() ;
+
+ // Maintain stats and report events to JVMTI
+ if (ObjectSynchronizer::_sync_Parks != NULL) {
+ ObjectSynchronizer::_sync_Parks->inc() ;
+ }
+ DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
+}
+
+
+// exit()
+// ~~~~~~
+// Note that the collector can't reclaim the objectMonitor or deflate
+// the object out from underneath the thread calling ::exit() as the
+// thread calling ::exit() never transitions to a stable state.
+// This inhibits GC, which in turn inhibits asynchronous (and
+// inopportune) reclamation of "this".
+//
+// We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
+// There's one exception to the claim above, however. EnterI() can call
+// exit() to drop a lock if the acquirer has been externally suspended.
+// In that case exit() is called with _thread_state as _thread_blocked,
+// but the monitor's _count field is > 0, which inhibits reclamation.
+//
+// 1-0 exit
+// ~~~~~~~~
+// ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
+// the fast-path operators have been optimized so the common ::exit()
+// operation is 1-0. See i486.ad fast_unlock(), for instance.
+// The code emitted by fast_unlock() elides the usual MEMBAR. This
+// greatly improves latency -- MEMBAR and CAS having considerable local
+// latency on modern processors -- but at the cost of "stranding". Absent the
+// MEMBAR, a thread in fast_unlock() can race a thread in the slow
+// ::enter() path, resulting in the entering thread being stranding
+// and a progress-liveness failure. Stranding is extremely rare.
+// We use timers (timed park operations) & periodic polling to detect
+// and recover from stranding. Potentially stranded threads periodically
+// wake up and poll the lock. See the usage of the _Responsible variable.
+//
+// The CAS() in enter provides for safety and exclusion, while the CAS or
+// MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
+// eliminates the CAS/MEMBAR from the exist path, but it admits stranding.
+// We detect and recover from stranding with timers.
+//
+// If a thread transiently strands it'll park until (a) another
+// thread acquires the lock and then drops the lock, at which time the
+// exiting thread will notice and unpark the stranded thread, or, (b)
+// the timer expires. If the lock is high traffic then the stranding latency
+// will be low due to (a). If the lock is low traffic then the odds of
+// stranding are lower, although the worst-case stranding latency
+// is longer. Critically, we don't want to put excessive load in the
+// platform's timer subsystem. We want to minimize both the timer injection
+// rate (timers created/sec) as well as the number of timers active at
+// any one time. (more precisely, we want to minimize timer-seconds, which is
+// the integral of the # of active timers at any instant over time).
+// Both impinge on OS scalability. Given that, at most one thread parked on
+// a monitor will use a timer.
+
+void ATTR ObjectMonitor::exit(TRAPS) {
+ Thread * Self = THREAD ;
+ if (THREAD != _owner) {
+ if (THREAD->is_lock_owned((address) _owner)) {
+ // Transmute _owner from a BasicLock pointer to a Thread address.
+ // We don't need to hold _mutex for this transition.
+ // Non-null to Non-null is safe as long as all readers can
+ // tolerate either flavor.
+ assert (_recursions == 0, "invariant") ;
+ _owner = THREAD ;
+ _recursions = 0 ;
+ OwnerIsThread = 1 ;
+ } else {
+ // NOTE: we need to handle unbalanced monitor enter/exit
+ // in native code by throwing an exception.
+ // TODO: Throw an IllegalMonitorStateException ?
+ TEVENT (Exit - Throw IMSX) ;
+ assert(false, "Non-balanced monitor enter/exit!");
+ if (false) {
+ THROW(vmSymbols::java_lang_IllegalMonitorStateException());
+ }
+ return;
+ }
+ }
+
+ if (_recursions != 0) {
+ _recursions--; // this is simple recursive enter
+ TEVENT (Inflated exit - recursive) ;
+ return ;
+ }
+
+ // Invariant: after setting Responsible=null an thread must execute
+ // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
+ if ((SyncFlags & 4) == 0) {
+ _Responsible = NULL ;
+ }
+
+ for (;;) {
+ assert (THREAD == _owner, "invariant") ;
+
+ // Fast-path monitor exit:
+ //
+ // Observe the Dekker/Lamport duality:
+ // A thread in ::exit() executes:
+ // ST Owner=null; MEMBAR; LD EntryList|cxq.
+ // A thread in the contended ::enter() path executes the complementary:
+ // ST EntryList|cxq = nonnull; MEMBAR; LD Owner.
+ //
+ // Note that there's a benign race in the exit path. We can drop the
+ // lock, another thread can reacquire the lock immediately, and we can
+ // then wake a thread unnecessarily (yet another flavor of futile wakeup).
+ // This is benign, and we've structured the code so the windows are short
+ // and the frequency of such futile wakeups is low.
+ //
+ // We could eliminate the race by encoding both the "LOCKED" state and
+ // the queue head in a single word. Exit would then use either CAS to
+ // clear the LOCKED bit/byte. This precludes the desirable 1-0 optimization,
+ // however.
+ //
+ // Possible fast-path ::exit() optimization:
+ // The current fast-path exit implementation fetches both cxq and EntryList.
+ // See also i486.ad fast_unlock(). Testing has shown that two LDs
+ // isn't measurably slower than a single LD on any platforms.
+ // Still, we could reduce the 2 LDs to one or zero by one of the following:
+ //
+ // - Use _count instead of cxq|EntryList
+ // We intend to eliminate _count, however, when we switch
+ // to on-the-fly deflation in ::exit() as is used in
+ // Metalocks and RelaxedLocks.
+ //
+ // - Establish the invariant that cxq == null implies EntryList == null.
+ // set cxq == EMPTY (1) to encode the state where cxq is empty
+ // by EntryList != null. EMPTY is a distinguished value.
+ // The fast-path exit() would fetch cxq but not EntryList.
+ //
+ // - Encode succ as follows:
+ // succ = t : Thread t is the successor -- t is ready or is spinning.
+ // Exiting thread does not need to wake a successor.
+ // succ = 0 : No successor required -> (EntryList|cxq) == null
+ // Exiting thread does not need to wake a successor
+ // succ = 1 : Successor required -> (EntryList|cxq) != null and
+ // logically succ == null.
+ // Exiting thread must wake a successor.
+ //
+ // The 1-1 fast-exit path would appear as :
+ // _owner = null ; membar ;
+ // if (_succ == 1 && CAS (&_owner, null, Self) == null) goto SlowPath
+ // goto FastPathDone ;
+ //
+ // and the 1-0 fast-exit path would appear as:
+ // if (_succ == 1) goto SlowPath
+ // Owner = null ;
+ // goto FastPathDone
+ //
+ // - Encode the LSB of _owner as 1 to indicate that exit()
+ // must use the slow-path and make a successor ready.
+ // (_owner & 1) == 0 IFF succ != null || (EntryList|cxq) == null
+ // (_owner & 1) == 0 IFF succ == null && (EntryList|cxq) != null (obviously)
+ // The 1-0 fast exit path would read:
+ // if (_owner != Self) goto SlowPath
+ // _owner = null
+ // goto FastPathDone
+
+ if (Knob_ExitPolicy == 0) {
+ // release semantics: prior loads and stores from within the critical section
+ // must not float (reorder) past the following store that drops the lock.
+ // On SPARC that requires MEMBAR #loadstore|#storestore.
+ // But of course in TSO #loadstore|#storestore is not required.
+ // I'd like to write one of the following:
+ // A. OrderAccess::release() ; _owner = NULL
+ // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
+ // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
+ // store into a _dummy variable. That store is not needed, but can result
+ // in massive wasteful coherency traffic on classic SMP systems.
+ // Instead, I use release_store(), which is implemented as just a simple
+ // ST on x64, x86 and SPARC.
+ OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
+ OrderAccess::storeload() ; // See if we need to wake a successor
+ if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
+ TEVENT (Inflated exit - simple egress) ;
+ return ;
+ }
+ TEVENT (Inflated exit - complex egress) ;
+
+ // Normally the exiting thread is responsible for ensuring succession,
+ // but if other successors are ready or other entering threads are spinning
+ // then this thread can simply store NULL into _owner and exit without
+ // waking a successor. The existence of spinners or ready successors
+ // guarantees proper succession (liveness). Responsibility passes to the
+ // ready or running successors. The exiting thread delegates the duty.
+ // More precisely, if a successor already exists this thread is absolved
+ // of the responsibility of waking (unparking) one.
+ //
+ // The _succ variable is critical to reducing futile wakeup frequency.
+ // _succ identifies the "heir presumptive" thread that has been made
+ // ready (unparked) but that has not yet run. We need only one such
+ // successor thread to guarantee progress.
+ // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
+ // section 3.3 "Futile Wakeup Throttling" for details.
+ //
+ // Note that spinners in Enter() also set _succ non-null.
+ // In the current implementation spinners opportunistically set
+ // _succ so that exiting threads might avoid waking a successor.
+ // Another less appealing alternative would be for the exiting thread
+ // to drop the lock and then spin briefly to see if a spinner managed
+ // to acquire the lock. If so, the exiting thread could exit
+ // immediately without waking a successor, otherwise the exiting
+ // thread would need to dequeue and wake a successor.
+ // (Note that we'd need to make the post-drop spin short, but no
+ // shorter than the worst-case round-trip cache-line migration time.
+ // The dropped lock needs to become visible to the spinner, and then
+ // the acquisition of the lock by the spinner must become visible to
+ // the exiting thread).
+ //
+
+ // It appears that an heir-presumptive (successor) must be made ready.
+ // Only the current lock owner can manipulate the EntryList or
+ // drain _cxq, so we need to reacquire the lock. If we fail
+ // to reacquire the lock the responsibility for ensuring succession
+ // falls to the new owner.
+ //
+ if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
+ return ;
+ }
+ TEVENT (Exit - Reacquired) ;
+ } else {
+ if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
+ OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock
+ OrderAccess::storeload() ;
+ // Ratify the previously observed values.
+ if (_cxq == NULL || _succ != NULL) {
+ TEVENT (Inflated exit - simple egress) ;
+ return ;
+ }
+
+ // inopportune interleaving -- the exiting thread (this thread)
+ // in the fast-exit path raced an entering thread in the slow-enter
+ // path.
+ // We have two choices:
+ // A. Try to reacquire the lock.
+ // If the CAS() fails return immediately, otherwise
+ // we either restart/rerun the exit operation, or simply
+ // fall-through into the code below which wakes a successor.
+ // B. If the elements forming the EntryList|cxq are TSM
+ // we could simply unpark() the lead thread and return
+ // without having set _succ.
+ if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) {
+ TEVENT (Inflated exit - reacquired succeeded) ;
+ return ;
+ }
+ TEVENT (Inflated exit - reacquired failed) ;
+ } else {
+ TEVENT (Inflated exit - complex egress) ;
+ }
+ }
+
+ guarantee (_owner == THREAD, "invariant") ;
+
+ // Select an appropriate successor ("heir presumptive") from the EntryList
+ // and make it ready. Generally we just wake the head of EntryList .
+ // There's no algorithmic constraint that we use the head - it's just
+ // a policy decision. Note that the thread at head of the EntryList
+ // remains at the head until it acquires the lock. This means we'll
+ // repeatedly wake the same thread until it manages to grab the lock.
+ // This is generally a good policy - if we're seeing lots of futile wakeups
+ // at least we're waking/rewaking a thread that's like to be hot or warm
+ // (have residual D$ and TLB affinity).
+ //
+ // "Wakeup locality" optimization:
+ // http://j2se.east/~dice/PERSIST/040825-WakeLocality.txt
+ // In the future we'll try to bias the selection mechanism
+ // to preferentially pick a thread that recently ran on
+ // a processor element that shares cache with the CPU on which
+ // the exiting thread is running. We need access to Solaris'
+ // schedctl.sc_cpu to make that work.
+ //
+ ObjectWaiter * w = NULL ;
+ int QMode = Knob_QMode ;
+
+ if (QMode == 2 && _cxq != NULL) {
+ // QMode == 2 : cxq has precedence over EntryList.
+ // Try to directly wake a successor from the cxq.
+ // If successful, the successor will need to unlink itself from cxq.
+ w = _cxq ;
+ assert (w != NULL, "invariant") ;
+ assert (w->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
+ ExitEpilog (Self, w) ;
+ return ;
+ }
+
+ if (QMode == 3 && _cxq != NULL) {
+ // Aggressively drain cxq into EntryList at the first opportunity.
+ // This policy ensure that recently-run threads live at the head of EntryList.
+ // Drain _cxq into EntryList - bulk transfer.
+ // First, detach _cxq.
+ // The following loop is tantamount to: w = swap (&cxq, NULL)
+ w = _cxq ;
+ for (;;) {
+ assert (w != NULL, "Invariant") ;
+ ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
+ if (u == w) break ;
+ w = u ;
+ }
+ assert (w != NULL , "invariant") ;
+
+ ObjectWaiter * q = NULL ;
+ ObjectWaiter * p ;
+ for (p = w ; p != NULL ; p = p->_next) {
+ guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
+ p->TState = ObjectWaiter::TS_ENTER ;
+ p->_prev = q ;
+ q = p ;
+ }
+
+ // Append the RATs to the EntryList
+ // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
+ ObjectWaiter * Tail ;
+ for (Tail = _EntryList ; Tail != NULL && Tail->_next != NULL ; Tail = Tail->_next) ;
+ if (Tail == NULL) {
+ _EntryList = w ;
+ } else {
+ Tail->_next = w ;
+ w->_prev = Tail ;
+ }
+
+ // Fall thru into code that tries to wake a successor from EntryList
+ }
+
+ if (QMode == 4 && _cxq != NULL) {
+ // Aggressively drain cxq into EntryList at the first opportunity.
+ // This policy ensure that recently-run threads live at the head of EntryList.
+
+ // Drain _cxq into EntryList - bulk transfer.
+ // First, detach _cxq.
+ // The following loop is tantamount to: w = swap (&cxq, NULL)
+ w = _cxq ;
+ for (;;) {
+ assert (w != NULL, "Invariant") ;
+ ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
+ if (u == w) break ;
+ w = u ;
+ }
+ assert (w != NULL , "invariant") ;
+
+ ObjectWaiter * q = NULL ;
+ ObjectWaiter * p ;
+ for (p = w ; p != NULL ; p = p->_next) {
+ guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
+ p->TState = ObjectWaiter::TS_ENTER ;
+ p->_prev = q ;
+ q = p ;
+ }
+
+ // Prepend the RATs to the EntryList
+ if (_EntryList != NULL) {
+ q->_next = _EntryList ;
+ _EntryList->_prev = q ;
+ }
+ _EntryList = w ;
+
+ // Fall thru into code that tries to wake a successor from EntryList
+ }
+
+ w = _EntryList ;
+ if (w != NULL) {
+ // I'd like to write: guarantee (w->_thread != Self).
+ // But in practice an exiting thread may find itself on the EntryList.
+ // Lets say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
+ // then calls exit(). Exit release the lock by setting O._owner to NULL.
+ // Lets say T1 then stalls. T2 acquires O and calls O.notify(). The
+ // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
+ // release the lock "O". T2 resumes immediately after the ST of null into
+ // _owner, above. T2 notices that the EntryList is populated, so it
+ // reacquires the lock and then finds itself on the EntryList.
+ // Given all that, we have to tolerate the circumstance where "w" is
+ // associated with Self.
+ assert (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
+ ExitEpilog (Self, w) ;
+ return ;
+ }
+
+ // If we find that both _cxq and EntryList are null then just
+ // re-run the exit protocol from the top.
+ w = _cxq ;
+ if (w == NULL) continue ;
+
+ // Drain _cxq into EntryList - bulk transfer.
+ // First, detach _cxq.
+ // The following loop is tantamount to: w = swap (&cxq, NULL)
+ for (;;) {
+ assert (w != NULL, "Invariant") ;
+ ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ;
+ if (u == w) break ;
+ w = u ;
+ }
+ TEVENT (Inflated exit - drain cxq into EntryList) ;
+
+ assert (w != NULL , "invariant") ;
+ assert (_EntryList == NULL , "invariant") ;
+
+ // Convert the LIFO SLL anchored by _cxq into a DLL.
+ // The list reorganization step operates in O(LENGTH(w)) time.
+ // It's critical that this step operate quickly as
+ // "Self" still holds the outer-lock, restricting parallelism
+ // and effectively lengthening the critical section.
+ // Invariant: s chases t chases u.
+ // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
+ // we have faster access to the tail.
+
+ if (QMode == 1) {
+ // QMode == 1 : drain cxq to EntryList, reversing order
+ // We also reverse the order of the list.
+ ObjectWaiter * s = NULL ;
+ ObjectWaiter * t = w ;
+ ObjectWaiter * u = NULL ;
+ while (t != NULL) {
+ guarantee (t->TState == ObjectWaiter::TS_CXQ, "invariant") ;
+ t->TState = ObjectWaiter::TS_ENTER ;
+ u = t->_next ;
+ t->_prev = u ;
+ t->_next = s ;
+ s = t;
+ t = u ;
+ }
+ _EntryList = s ;
+ assert (s != NULL, "invariant") ;
+ } else {
+ // QMode == 0 or QMode == 2
+ _EntryList = w ;
+ ObjectWaiter * q = NULL ;
+ ObjectWaiter * p ;
+ for (p = w ; p != NULL ; p = p->_next) {
+ guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ;
+ p->TState = ObjectWaiter::TS_ENTER ;
+ p->_prev = q ;
+ q = p ;
+ }
+ }
+
+ // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
+ // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
+
+ // See if we can abdicate to a spinner instead of waking a thread.
+ // A primary goal of the implementation is to reduce the
+ // context-switch rate.
+ if (_succ != NULL) continue;
+
+ w = _EntryList ;
+ if (w != NULL) {
+ guarantee (w->TState == ObjectWaiter::TS_ENTER, "invariant") ;
+ ExitEpilog (Self, w) ;
+ return ;
+ }
+ }
+}
+// complete_exit exits a lock returning recursion count
+// complete_exit/reenter operate as a wait without waiting
+// complete_exit requires an inflated monitor
+// The _owner field is not always the Thread addr even with an
+// inflated monitor, e.g. the monitor can be inflated by a non-owning
+// thread due to contention.
+intptr_t ObjectMonitor::complete_exit(TRAPS) {
+ Thread * const Self = THREAD;
+ assert(Self->is_Java_thread(), "Must be Java thread!");
+ JavaThread *jt = (JavaThread *)THREAD;
+
+ DeferredInitialize();
+
+ if (THREAD != _owner) {
+ if (THREAD->is_lock_owned ((address)_owner)) {
+ assert(_recursions == 0, "internal state error");
+ _owner = THREAD ; /* Convert from basiclock addr to Thread addr */
+ _recursions = 0 ;
+ OwnerIsThread = 1 ;
+ }
+ }
+
+ guarantee(Self == _owner, "complete_exit not owner");
+ intptr_t save = _recursions; // record the old recursion count
+ _recursions = 0; // set the recursion level to be 0
+ exit (Self) ; // exit the monitor
+ guarantee (_owner != Self, "invariant");
+ return save;
+}
+
+// reenter() enters a lock and sets recursion count
+// complete_exit/reenter operate as a wait without waiting
+void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
+ Thread * const Self = THREAD;
+ assert(Self->is_Java_thread(), "Must be Java thread!");
+ JavaThread *jt = (JavaThread *)THREAD;
+
+ guarantee(_owner != Self, "reenter already owner");
+ enter (THREAD); // enter the monitor
+ guarantee (_recursions == 0, "reenter recursion");
+ _recursions = recursions;
+ return;
+}
+
+// Note: a subset of changes to ObjectMonitor::wait()
+// will need to be replicated in complete_exit above
+void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
+ Thread * const Self = THREAD ;
+ assert(Self->is_Java_thread(), "Must be Java thread!");
+ JavaThread *jt = (JavaThread *)THREAD;
+
+ DeferredInitialize () ;
+
+ // Throw IMSX or IEX.
+ CHECK_OWNER();
+
+ // check for a pending interrupt
+ if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
+ // post monitor waited event. Note that this is past-tense, we are done waiting.
+ if (JvmtiExport::should_post_monitor_waited()) {
+ // Note: 'false' parameter is passed here because the
+ // wait was not timed out due to thread interrupt.
+ JvmtiExport::post_monitor_waited(jt, this, false);
+ }
+ TEVENT (Wait - Throw IEX) ;
+ THROW(vmSymbols::java_lang_InterruptedException());
+ return ;
+ }
+ TEVENT (Wait) ;
+
+ assert (Self->_Stalled == 0, "invariant") ;
+ Self->_Stalled = intptr_t(this) ;
+ jt->set_current_waiting_monitor(this);
+
+ // create a node to be put into the queue
+ // Critically, after we reset() the event but prior to park(), we must check
+ // for a pending interrupt.
+ ObjectWaiter node(Self);
+ node.TState = ObjectWaiter::TS_WAIT ;
+ Self->_ParkEvent->reset() ;
+ OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
+
+ // Enter the waiting queue, which is a circular doubly linked list in this case
+ // but it could be a priority queue or any data structure.
+ // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
+ // by the the owner of the monitor *except* in the case where park()
+ // returns because of a timeout of interrupt. Contention is exceptionally rare
+ // so we use a simple spin-lock instead of a heavier-weight blocking lock.
+
+ Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ;
+ AddWaiter (&node) ;
+ Thread::SpinRelease (&_WaitSetLock) ;
+
+ if ((SyncFlags & 4) == 0) {
+ _Responsible = NULL ;
+ }
+ intptr_t save = _recursions; // record the old recursion count
+ _waiters++; // increment the number of waiters
+ _recursions = 0; // set the recursion level to be 1
+ exit (Self) ; // exit the monitor
+ guarantee (_owner != Self, "invariant") ;
+
+ // As soon as the ObjectMonitor's ownership is dropped in the exit()
+ // call above, another thread can enter() the ObjectMonitor, do the
+ // notify(), and exit() the ObjectMonitor. If the other thread's
+ // exit() call chooses this thread as the successor and the unpark()
+ // call happens to occur while this thread is posting a
+ // MONITOR_CONTENDED_EXIT event, then we run the risk of the event
+ // handler using RawMonitors and consuming the unpark().
+ //
+ // To avoid the problem, we re-post the event. This does no harm
+ // even if the original unpark() was not consumed because we are the
+ // chosen successor for this monitor.
+ if (node._notified != 0 && _succ == Self) {
+ node._event->unpark();
+ }
+
+ // The thread is on the WaitSet list - now park() it.
+ // On MP systems it's conceivable that a brief spin before we park
+ // could be profitable.
+ //
+ // TODO-FIXME: change the following logic to a loop of the form
+ // while (!timeout && !interrupted && _notified == 0) park()
+
+ int ret = OS_OK ;
+ int WasNotified = 0 ;
+ { // State transition wrappers
+ OSThread* osthread = Self->osthread();
+ OSThreadWaitState osts(osthread, true);
+ {
+ ThreadBlockInVM tbivm(jt);
+ // Thread is in thread_blocked state and oop access is unsafe.
+ jt->set_suspend_equivalent();
+
+ if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
+ // Intentionally empty
+ } else
+ if (node._notified == 0) {
+ if (millis <= 0) {
+ Self->_ParkEvent->park () ;
+ } else {
+ ret = Self->_ParkEvent->park (millis) ;
+ }
+ }
+
+ // were we externally suspended while we were waiting?
+ if (ExitSuspendEquivalent (jt)) {
+ // TODO-FIXME: add -- if succ == Self then succ = null.
+ jt->java_suspend_self();
+ }
+
+ } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
+
+
+ // Node may be on the WaitSet, the EntryList (or cxq), or in transition
+ // from the WaitSet to the EntryList.
+ // See if we need to remove Node from the WaitSet.
+ // We use double-checked locking to avoid grabbing _WaitSetLock
+ // if the thread is not on the wait queue.
+ //
+ // Note that we don't need a fence before the fetch of TState.
+ // In the worst case we'll fetch a old-stale value of TS_WAIT previously
+ // written by the is thread. (perhaps the fetch might even be satisfied
+ // by a look-aside into the processor's own store buffer, although given
+ // the length of the code path between the prior ST and this load that's
+ // highly unlikely). If the following LD fetches a stale TS_WAIT value
+ // then we'll acquire the lock and then re-fetch a fresh TState value.
+ // That is, we fail toward safety.
+
+ if (node.TState == ObjectWaiter::TS_WAIT) {
+ Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ;
+ if (node.TState == ObjectWaiter::TS_WAIT) {
+ DequeueSpecificWaiter (&node) ; // unlink from WaitSet
+ assert(node._notified == 0, "invariant");
+ node.TState = ObjectWaiter::TS_RUN ;
+ }
+ Thread::SpinRelease (&_WaitSetLock) ;
+ }
+
+ // The thread is now either on off-list (TS_RUN),
+ // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
+ // The Node's TState variable is stable from the perspective of this thread.
+ // No other threads will asynchronously modify TState.
+ guarantee (node.TState != ObjectWaiter::TS_WAIT, "invariant") ;
+ OrderAccess::loadload() ;
+ if (_succ == Self) _succ = NULL ;
+ WasNotified = node._notified ;
+
+ // Reentry phase -- reacquire the monitor.
+ // re-enter contended monitor after object.wait().
+ // retain OBJECT_WAIT state until re-enter successfully completes
+ // Thread state is thread_in_vm and oop access is again safe,
+ // although the raw address of the object may have changed.
+ // (Don't cache naked oops over safepoints, of course).
+
+ // post monitor waited event. Note that this is past-tense, we are done waiting.
+ if (JvmtiExport::should_post_monitor_waited()) {
+ JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
+ }
+ OrderAccess::fence() ;
+
+ assert (Self->_Stalled != 0, "invariant") ;
+ Self->_Stalled = 0 ;
+
+ assert (_owner != Self, "invariant") ;
+ ObjectWaiter::TStates v = node.TState ;
+ if (v == ObjectWaiter::TS_RUN) {
+ enter (Self) ;
+ } else {
+ guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
+ ReenterI (Self, &node) ;
+ node.wait_reenter_end(this);
+ }
+
+ // Self has reacquired the lock.
+ // Lifecycle - the node representing Self must not appear on any queues.
+ // Node is about to go out-of-scope, but even if it were immortal we wouldn't
+ // want residual elements associated with this thread left on any lists.
+ guarantee (node.TState == ObjectWaiter::TS_RUN, "invariant") ;
+ assert (_owner == Self, "invariant") ;
+ assert (_succ != Self , "invariant") ;
+ } // OSThreadWaitState()
+
+ jt->set_current_waiting_monitor(NULL);
+
+ guarantee (_recursions == 0, "invariant") ;
+ _recursions = save; // restore the old recursion count
+ _waiters--; // decrement the number of waiters
+
+ // Verify a few postconditions
+ assert (_owner == Self , "invariant") ;
+ assert (_succ != Self , "invariant") ;
+ assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
+
+ if (SyncFlags & 32) {
+ OrderAccess::fence() ;
+ }
+
+ // check if the notification happened
+ if (!WasNotified) {
+ // no, it could be timeout or Thread.interrupt() or both
+ // check for interrupt event, otherwise it is timeout
+ if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
+ TEVENT (Wait - throw IEX from epilog) ;
+ THROW(vmSymbols::java_lang_InterruptedException());
+ }
+ }
+
+ // NOTE: Spurious wake up will be consider as timeout.
+ // Monitor notify has precedence over thread interrupt.
+}
+
+
+// Consider:
+// If the lock is cool (cxq == null && succ == null) and we're on an MP system
+// then instead of transferring a thread from the WaitSet to the EntryList
+// we might just dequeue a thread from the WaitSet and directly unpark() it.
+
+void ObjectMonitor::notify(TRAPS) {
+ CHECK_OWNER();
+ if (_WaitSet == NULL) {
+ TEVENT (Empty-Notify) ;
+ return ;
+ }
+ DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
+
+ int Policy = Knob_MoveNotifyee ;
+
+ Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ;
+ ObjectWaiter * iterator = DequeueWaiter() ;
+ if (iterator != NULL) {
+ TEVENT (Notify1 - Transfer) ;
+ guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
+ guarantee (iterator->_notified == 0, "invariant") ;
+ // Disposition - what might we do with iterator ?
+ // a. add it directly to the EntryList - either tail or head.
+ // b. push it onto the front of the _cxq.
+ // For now we use (a).
+ if (Policy != 4) {
+ iterator->TState = ObjectWaiter::TS_ENTER ;
+ }
+ iterator->_notified = 1 ;
+
+ ObjectWaiter * List = _EntryList ;
+ if (List != NULL) {
+ assert (List->_prev == NULL, "invariant") ;
+ assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
+ assert (List != iterator, "invariant") ;
+ }
+
+ if (Policy == 0) { // prepend to EntryList
+ if (List == NULL) {
+ iterator->_next = iterator->_prev = NULL ;
+ _EntryList = iterator ;
+ } else {
+ List->_prev = iterator ;
+ iterator->_next = List ;
+ iterator->_prev = NULL ;
+ _EntryList = iterator ;
+ }
+ } else
+ if (Policy == 1) { // append to EntryList
+ if (List == NULL) {
+ iterator->_next = iterator->_prev = NULL ;
+ _EntryList = iterator ;
+ } else {
+ // CONSIDER: finding the tail currently requires a linear-time walk of
+ // the EntryList. We can make tail access constant-time by converting to
+ // a CDLL instead of using our current DLL.
+ ObjectWaiter * Tail ;
+ for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
+ assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
+ Tail->_next = iterator ;
+ iterator->_prev = Tail ;
+ iterator->_next = NULL ;
+ }
+ } else
+ if (Policy == 2) { // prepend to cxq
+ // prepend to cxq
+ if (List == NULL) {
+ iterator->_next = iterator->_prev = NULL ;
+ _EntryList = iterator ;
+ } else {
+ iterator->TState = ObjectWaiter::TS_CXQ ;
+ for (;;) {
+ ObjectWaiter * Front = _cxq ;
+ iterator->_next = Front ;
+ if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
+ break ;
+ }
+ }
+ }
+ } else
+ if (Policy == 3) { // append to cxq
+ iterator->TState = ObjectWaiter::TS_CXQ ;
+ for (;;) {
+ ObjectWaiter * Tail ;
+ Tail = _cxq ;
+ if (Tail == NULL) {
+ iterator->_next = NULL ;
+ if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
+ break ;
+ }
+ } else {
+ while (Tail->_next != NULL) Tail = Tail->_next ;
+ Tail->_next = iterator ;
+ iterator->_prev = Tail ;
+ iterator->_next = NULL ;
+ break ;
+ }
+ }
+ } else {
+ ParkEvent * ev = iterator->_event ;
+ iterator->TState = ObjectWaiter::TS_RUN ;
+ OrderAccess::fence() ;
+ ev->unpark() ;
+ }
+
+ if (Policy < 4) {
+ iterator->wait_reenter_begin(this);
+ }
+
+ // _WaitSetLock protects the wait queue, not the EntryList. We could
+ // move the add-to-EntryList operation, above, outside the critical section
+ // protected by _WaitSetLock. In practice that's not useful. With the
+ // exception of wait() timeouts and interrupts the monitor owner
+ // is the only thread that grabs _WaitSetLock. There's almost no contention
+ // on _WaitSetLock so it's not profitable to reduce the length of the
+ // critical section.
+ }
+
+ Thread::SpinRelease (&_WaitSetLock) ;
+
+ if (iterator != NULL && ObjectSynchronizer::_sync_Notifications != NULL) {
+ ObjectSynchronizer::_sync_Notifications->inc() ;
+ }
+}
+
+
+void ObjectMonitor::notifyAll(TRAPS) {
+ CHECK_OWNER();
+ ObjectWaiter* iterator;
+ if (_WaitSet == NULL) {
+ TEVENT (Empty-NotifyAll) ;
+ return ;
+ }
+ DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
+
+ int Policy = Knob_MoveNotifyee ;
+ int Tally = 0 ;
+ Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notifyall") ;
+
+ for (;;) {
+ iterator = DequeueWaiter () ;
+ if (iterator == NULL) break ;
+ TEVENT (NotifyAll - Transfer1) ;
+ ++Tally ;
+
+ // Disposition - what might we do with iterator ?
+ // a. add it directly to the EntryList - either tail or head.
+ // b. push it onto the front of the _cxq.
+ // For now we use (a).
+ //
+ // TODO-FIXME: currently notifyAll() transfers the waiters one-at-a-time from the waitset
+ // to the EntryList. This could be done more efficiently with a single bulk transfer,
+ // but in practice it's not time-critical. Beware too, that in prepend-mode we invert the
+ // order of the waiters. Lets say that the waitset is "ABCD" and the EntryList is "XYZ".
+ // After a notifyAll() in prepend mode the waitset will be empty and the EntryList will
+ // be "DCBAXYZ".
+
+ guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
+ guarantee (iterator->_notified == 0, "invariant") ;
+ iterator->_notified = 1 ;
+ if (Policy != 4) {
+ iterator->TState = ObjectWaiter::TS_ENTER ;
+ }
+
+ ObjectWaiter * List = _EntryList ;
+ if (List != NULL) {
+ assert (List->_prev == NULL, "invariant") ;
+ assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
+ assert (List != iterator, "invariant") ;
+ }
+
+ if (Policy == 0) { // prepend to EntryList
+ if (List == NULL) {
+ iterator->_next = iterator->_prev = NULL ;
+ _EntryList = iterator ;
+ } else {
+ List->_prev = iterator ;
+ iterator->_next = List ;
+ iterator->_prev = NULL ;
+ _EntryList = iterator ;
+ }
+ } else
+ if (Policy == 1) { // append to EntryList
+ if (List == NULL) {
+ iterator->_next = iterator->_prev = NULL ;
+ _EntryList = iterator ;
+ } else {
+ // CONSIDER: finding the tail currently requires a linear-time walk of
+ // the EntryList. We can make tail access constant-time by converting to
+ // a CDLL instead of using our current DLL.
+ ObjectWaiter * Tail ;
+ for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
+ assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
+ Tail->_next = iterator ;
+ iterator->_prev = Tail ;
+ iterator->_next = NULL ;
+ }
+ } else
+ if (Policy == 2) { // prepend to cxq
+ // prepend to cxq
+ iterator->TState = ObjectWaiter::TS_CXQ ;
+ for (;;) {
+ ObjectWaiter * Front = _cxq ;
+ iterator->_next = Front ;
+ if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
+ break ;
+ }
+ }
+ } else
+ if (Policy == 3) { // append to cxq
+ iterator->TState = ObjectWaiter::TS_CXQ ;
+ for (;;) {
+ ObjectWaiter * Tail ;
+ Tail = _cxq ;
+ if (Tail == NULL) {
+ iterator->_next = NULL ;
+ if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
+ break ;
+ }
+ } else {
+ while (Tail->_next != NULL) Tail = Tail->_next ;
+ Tail->_next = iterator ;
+ iterator->_prev = Tail ;
+ iterator->_next = NULL ;
+ break ;
+ }
+ }
+ } else {
+ ParkEvent * ev = iterator->_event ;
+ iterator->TState = ObjectWaiter::TS_RUN ;
+ OrderAccess::fence() ;
+ ev->unpark() ;
+ }
+
+ if (Policy < 4) {
+ iterator->wait_reenter_begin(this);
+ }
+
+ // _WaitSetLock protects the wait queue, not the EntryList. We could
+ // move the add-to-EntryList operation, above, outside the critical section
+ // protected by _WaitSetLock. In practice that's not useful. With the
+ // exception of wait() timeouts and interrupts the monitor owner
+ // is the only thread that grabs _WaitSetLock. There's almost no contention
+ // on _WaitSetLock so it's not profitable to reduce the length of the
+ // critical section.
+ }
+
+ Thread::SpinRelease (&_WaitSetLock) ;
+
+ if (Tally != 0 && ObjectSynchronizer::_sync_Notifications != NULL) {
+ ObjectSynchronizer::_sync_Notifications->inc(Tally) ;
+ }
+}
+
+// check_slow() is a misnomer. It's called to simply to throw an IMSX exception.
+// TODO-FIXME: remove check_slow() -- it's likely dead.
+
+void ObjectMonitor::check_slow(TRAPS) {
+ TEVENT (check_slow - throw IMSX) ;
+ assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
+ THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
+}
+
+
+// -------------------------------------------------------------------------
+// The raw monitor subsystem is entirely distinct from normal
+// java-synchronization or jni-synchronization. raw monitors are not
+// associated with objects. They can be implemented in any manner
+// that makes sense. The original implementors decided to piggy-back
+// the raw-monitor implementation on the existing Java objectMonitor mechanism.
+// This flaw needs to fixed. We should reimplement raw monitors as sui-generis.
+// Specifically, we should not implement raw monitors via java monitors.
+// Time permitting, we should disentangle and deconvolve the two implementations
+// and move the resulting raw monitor implementation over to the JVMTI directories.
+// Ideally, the raw monitor implementation would be built on top of
+// park-unpark and nothing else.
+//
+// raw monitors are used mainly by JVMTI
+// The raw monitor implementation borrows the ObjectMonitor structure,
+// but the operators are degenerate and extremely simple.
+//
+// Mixed use of a single objectMonitor instance -- as both a raw monitor
+// and a normal java monitor -- is not permissible.
+//
+// Note that we use the single RawMonitor_lock to protect queue operations for
+// _all_ raw monitors. This is a scalability impediment, but since raw monitor usage
+// is deprecated and rare, this is not of concern. The RawMonitor_lock can not
+// be held indefinitely. The critical sections must be short and bounded.
+//
+// -------------------------------------------------------------------------
+
+int ObjectMonitor::SimpleEnter (Thread * Self) {
+ for (;;) {
+ if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
+ return OS_OK ;
+ }
+
+ ObjectWaiter Node (Self) ;
+ Self->_ParkEvent->reset() ; // strictly optional
+ Node.TState = ObjectWaiter::TS_ENTER ;
+
+ RawMonitor_lock->lock_without_safepoint_check() ;
+ Node._next = _EntryList ;
+ _EntryList = &Node ;
+ OrderAccess::fence() ;
+ if (_owner == NULL && Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) {
+ _EntryList = Node._next ;
+ RawMonitor_lock->unlock() ;
+ return OS_OK ;
+ }
+ RawMonitor_lock->unlock() ;
+ while (Node.TState == ObjectWaiter::TS_ENTER) {
+ Self->_ParkEvent->park() ;
+ }
+ }
+}
+
+int ObjectMonitor::SimpleExit (Thread * Self) {
+ guarantee (_owner == Self, "invariant") ;
+ OrderAccess::release_store_ptr (&_owner, NULL) ;
+ OrderAccess::fence() ;
+ if (_EntryList == NULL) return OS_OK ;
+ ObjectWaiter * w ;
+
+ RawMonitor_lock->lock_without_safepoint_check() ;
+ w = _EntryList ;
+ if (w != NULL) {
+ _EntryList = w->_next ;
+ }
+ RawMonitor_lock->unlock() ;
+ if (w != NULL) {
+ guarantee (w ->TState == ObjectWaiter::TS_ENTER, "invariant") ;
+ ParkEvent * ev = w->_event ;
+ w->TState = ObjectWaiter::TS_RUN ;
+ OrderAccess::fence() ;
+ ev->unpark() ;
+ }
+ return OS_OK ;
+}
+
+int ObjectMonitor::SimpleWait (Thread * Self, jlong millis) {
+ guarantee (_owner == Self , "invariant") ;
+ guarantee (_recursions == 0, "invariant") ;
+
+ ObjectWaiter Node (Self) ;
+ Node._notified = 0 ;
+ Node.TState = ObjectWaiter::TS_WAIT ;
+
+ RawMonitor_lock->lock_without_safepoint_check() ;
+ Node._next = _WaitSet ;
+ _WaitSet = &Node ;
+ RawMonitor_lock->unlock() ;
+
+ SimpleExit (Self) ;
+ guarantee (_owner != Self, "invariant") ;
+
+ int ret = OS_OK ;
+ if (millis <= 0) {
+ Self->_ParkEvent->park();
+ } else {
+ ret = Self->_ParkEvent->park(millis);
+ }
+
+ // If thread still resides on the waitset then unlink it.
+ // Double-checked locking -- the usage is safe in this context
+ // as we TState is volatile and the lock-unlock operators are
+ // serializing (barrier-equivalent).
+
+ if (Node.TState == ObjectWaiter::TS_WAIT) {
+ RawMonitor_lock->lock_without_safepoint_check() ;
+ if (Node.TState == ObjectWaiter::TS_WAIT) {
+ // Simple O(n) unlink, but performance isn't critical here.
+ ObjectWaiter * p ;
+ ObjectWaiter * q = NULL ;
+ for (p = _WaitSet ; p != &Node; p = p->_next) {
+ q = p ;
+ }
+ guarantee (p == &Node, "invariant") ;
+ if (q == NULL) {
+ guarantee (p == _WaitSet, "invariant") ;
+ _WaitSet = p->_next ;
+ } else {
+ guarantee (p == q->_next, "invariant") ;
+ q->_next = p->_next ;
+ }
+ Node.TState = ObjectWaiter::TS_RUN ;
+ }
+ RawMonitor_lock->unlock() ;
+ }
+
+ guarantee (Node.TState == ObjectWaiter::TS_RUN, "invariant") ;
+ SimpleEnter (Self) ;
+
+ guarantee (_owner == Self, "invariant") ;
+ guarantee (_recursions == 0, "invariant") ;
+ return ret ;
+}
+
+int ObjectMonitor::SimpleNotify (Thread * Self, bool All) {
+ guarantee (_owner == Self, "invariant") ;
+ if (_WaitSet == NULL) return OS_OK ;
+
+ // We have two options:
+ // A. Transfer the threads from the WaitSet to the EntryList
+ // B. Remove the thread from the WaitSet and unpark() it.
+ //
+ // We use (B), which is crude and results in lots of futile
+ // context switching. In particular (B) induces lots of contention.
+
+ ParkEvent * ev = NULL ; // consider using a small auto array ...
+ RawMonitor_lock->lock_without_safepoint_check() ;
+ for (;;) {
+ ObjectWaiter * w = _WaitSet ;
+ if (w == NULL) break ;
+ _WaitSet = w->_next ;
+ if (ev != NULL) { ev->unpark(); ev = NULL; }
+ ev = w->_event ;
+ OrderAccess::loadstore() ;
+ w->TState = ObjectWaiter::TS_RUN ;
+ OrderAccess::storeload();
+ if (!All) break ;
+ }
+ RawMonitor_lock->unlock() ;
+ if (ev != NULL) ev->unpark();
+ return OS_OK ;
+}
+
+// Any JavaThread will enter here with state _thread_blocked
+int ObjectMonitor::raw_enter(TRAPS) {
+ TEVENT (raw_enter) ;
+ void * Contended ;
+
+ // don't enter raw monitor if thread is being externally suspended, it will
+ // surprise the suspender if a "suspended" thread can still enter monitor
+ JavaThread * jt = (JavaThread *)THREAD;
+ if (THREAD->is_Java_thread()) {
+ jt->SR_lock()->lock_without_safepoint_check();
+ while (jt->is_external_suspend()) {
+ jt->SR_lock()->unlock();
+ jt->java_suspend_self();
+ jt->SR_lock()->lock_without_safepoint_check();
+ }
+ // guarded by SR_lock to avoid racing with new external suspend requests.
+ Contended = Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) ;
+ jt->SR_lock()->unlock();
+ } else {
+ Contended = Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) ;
+ }
+
+ if (Contended == THREAD) {
+ _recursions ++ ;
+ return OM_OK ;
+ }
+
+ if (Contended == NULL) {
+ guarantee (_owner == THREAD, "invariant") ;
+ guarantee (_recursions == 0, "invariant") ;
+ return OM_OK ;
+ }
+
+ THREAD->set_current_pending_monitor(this);
+
+ if (!THREAD->is_Java_thread()) {
+ // No other non-Java threads besides VM thread would acquire
+ // a raw monitor.
+ assert(THREAD->is_VM_thread(), "must be VM thread");
+ SimpleEnter (THREAD) ;
+ } else {
+ guarantee (jt->thread_state() == _thread_blocked, "invariant") ;
+ for (;;) {
+ jt->set_suspend_equivalent();
+ // cleared by handle_special_suspend_equivalent_condition() or
+ // java_suspend_self()
+ SimpleEnter (THREAD) ;
+
+ // were we externally suspended while we were waiting?
+ if (!jt->handle_special_suspend_equivalent_condition()) break ;
+
+ // This thread was externally suspended
+ //
+ // This logic isn't needed for JVMTI raw monitors,
+ // but doesn't hurt just in case the suspend rules change. This
+ // logic is needed for the ObjectMonitor.wait() reentry phase.
+ // We have reentered the contended monitor, but while we were
+ // waiting another thread suspended us. We don't want to reenter
+ // the monitor while suspended because that would surprise the
+ // thread that suspended us.
+ //
+ // Drop the lock -
+ SimpleExit (THREAD) ;
+
+ jt->java_suspend_self();
+ }
+
+ assert(_owner == THREAD, "Fatal error with monitor owner!");
+ assert(_recursions == 0, "Fatal error with monitor recursions!");
+ }
+
+ THREAD->set_current_pending_monitor(NULL);
+ guarantee (_recursions == 0, "invariant") ;
+ return OM_OK;
+}
+
+// Used mainly for JVMTI raw monitor implementation
+// Also used for ObjectMonitor::wait().
+int ObjectMonitor::raw_exit(TRAPS) {
+ TEVENT (raw_exit) ;
+ if (THREAD != _owner) {
+ return OM_ILLEGAL_MONITOR_STATE;
+ }
+ if (_recursions > 0) {
+ --_recursions ;
+ return OM_OK ;
+ }
+
+ void * List = _EntryList ;
+ SimpleExit (THREAD) ;
+
+ return OM_OK;
+}
+
+// Used for JVMTI raw monitor implementation.
+// All JavaThreads will enter here with state _thread_blocked
+
+int ObjectMonitor::raw_wait(jlong millis, bool interruptible, TRAPS) {
+ TEVENT (raw_wait) ;
+ if (THREAD != _owner) {
+ return OM_ILLEGAL_MONITOR_STATE;
+ }
+
+ // To avoid spurious wakeups we reset the parkevent -- This is strictly optional.
+ // The caller must be able to tolerate spurious returns from raw_wait().
+ THREAD->_ParkEvent->reset() ;
+ OrderAccess::fence() ;
+
+ // check interrupt event
+ if (interruptible && Thread::is_interrupted(THREAD, true)) {
+ return OM_INTERRUPTED;
+ }
+
+ intptr_t save = _recursions ;
+ _recursions = 0 ;
+ _waiters ++ ;
+ if (THREAD->is_Java_thread()) {
+ guarantee (((JavaThread *) THREAD)->thread_state() == _thread_blocked, "invariant") ;
+ ((JavaThread *)THREAD)->set_suspend_equivalent();
+ }
+ int rv = SimpleWait (THREAD, millis) ;
+ _recursions = save ;
+ _waiters -- ;
+
+ guarantee (THREAD == _owner, "invariant") ;
+ if (THREAD->is_Java_thread()) {
+ JavaThread * jSelf = (JavaThread *) THREAD ;
+ for (;;) {
+ if (!jSelf->handle_special_suspend_equivalent_condition()) break ;
+ SimpleExit (THREAD) ;
+ jSelf->java_suspend_self();
+ SimpleEnter (THREAD) ;
+ jSelf->set_suspend_equivalent() ;
+ }
+ }
+ guarantee (THREAD == _owner, "invariant") ;
+
+ if (interruptible && Thread::is_interrupted(THREAD, true)) {
+ return OM_INTERRUPTED;
+ }
+ return OM_OK ;
+}
+
+int ObjectMonitor::raw_notify(TRAPS) {
+ TEVENT (raw_notify) ;
+ if (THREAD != _owner) {
+ return OM_ILLEGAL_MONITOR_STATE;
+ }
+ SimpleNotify (THREAD, false) ;
+ return OM_OK;
+}
+
+int ObjectMonitor::raw_notifyAll(TRAPS) {
+ TEVENT (raw_notifyAll) ;
+ if (THREAD != _owner) {
+ return OM_ILLEGAL_MONITOR_STATE;
+ }
+ SimpleNotify (THREAD, true) ;
+ return OM_OK;
+}
+
+#ifndef PRODUCT
+void ObjectMonitor::verify() {
+}
+
+void ObjectMonitor::print() {
+}
+#endif
+
+//------------------------------------------------------------------------------
+// Non-product code
+
+#ifndef PRODUCT
+
+void ObjectSynchronizer::trace_locking(Handle locking_obj, bool is_compiled,
+ bool is_method, bool is_locking) {
+ // Don't know what to do here
+}
+
+// Verify all monitors in the monitor cache, the verification is weak.
+void ObjectSynchronizer::verify() {
+ ObjectMonitor* block = gBlockList;
+ ObjectMonitor* mid;
+ while (block) {
+ assert(block->object() == CHAINMARKER, "must be a block header");
+ for (int i = 1; i < _BLOCKSIZE; i++) {
+ mid = block + i;
+ oop object = (oop) mid->object();
+ if (object != NULL) {
+ mid->verify();
+ }
+ }
+ block = (ObjectMonitor*) block->FreeNext;
+ }
+}
+
+// Check if monitor belongs to the monitor cache
+// The list is grow-only so it's *relatively* safe to traverse
+// the list of extant blocks without taking a lock.
+
+int ObjectSynchronizer::verify_objmon_isinpool(ObjectMonitor *monitor) {
+ ObjectMonitor* block = gBlockList;
+
+ while (block) {
+ assert(block->object() == CHAINMARKER, "must be a block header");
+ if (monitor > &block[0] && monitor < &block[_BLOCKSIZE]) {
+ address mon = (address) monitor;
+ address blk = (address) block;
+ size_t diff = mon - blk;
+ assert((diff % sizeof(ObjectMonitor)) == 0, "check");
+ return 1;
+ }
+ block = (ObjectMonitor*) block->FreeNext;
+ }
+ return 0;
+}
+
+#endif