diff -r fd16c54261b3 -r 489c9b5090e2 hotspot/src/share/vm/runtime/synchronizer.cpp --- /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 ; +// +// 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 = 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