/* * Copyright (c) 1998, 2014, Oracle and/or its affiliates. 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 Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. * */ #include "precompiled.hpp" #include "classfile/vmSymbols.hpp" #include "memory/resourceArea.hpp" #include "oops/markOop.hpp" #include "oops/oop.inline.hpp" #include "runtime/biasedLocking.hpp" #include "runtime/handles.inline.hpp" #include "runtime/interfaceSupport.hpp" #include "runtime/mutexLocker.hpp" #include "runtime/objectMonitor.hpp" #include "runtime/objectMonitor.inline.hpp" #include "runtime/osThread.hpp" #include "runtime/stubRoutines.hpp" #include "runtime/synchronizer.hpp" #include "runtime/thread.inline.hpp" #include "utilities/dtrace.hpp" #include "utilities/events.hpp" #include "utilities/preserveException.hpp" #ifdef TARGET_OS_FAMILY_linux # include "os_linux.inline.hpp" #endif #ifdef TARGET_OS_FAMILY_solaris # include "os_solaris.inline.hpp" #endif #ifdef TARGET_OS_FAMILY_windows # include "os_windows.inline.hpp" #endif #ifdef TARGET_OS_FAMILY_bsd # include "os_bsd.inline.hpp" #endif #if defined(__GNUC__) // Need to inhibit inlining for older versions of GCC to avoid build-time failures #define ATTR __attribute__((noinline)) #else #define ATTR #endif // 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. // // // ----------------------------------------------------------------------------- #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. #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \ char* bytes = NULL; \ int len = 0; \ jlong jtid = SharedRuntime::get_java_tid(thread); \ Symbol* klassname = ((oop)(obj))->klass()->name(); \ if (klassname != NULL) { \ bytes = (char*)klassname->bytes(); \ len = klassname->utf8_length(); \ } #ifndef USDT2 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); #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \ { \ if (DTraceMonitorProbes) { \ DTRACE_MONITOR_PROBE_COMMON(obj, thread); \ HS_DTRACE_PROBE5(hotspot, monitor__wait, jtid, \ (monitor), bytes, len, (millis)); \ } \ } #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \ { \ if (DTraceMonitorProbes) { \ DTRACE_MONITOR_PROBE_COMMON(obj, thread); \ HS_DTRACE_PROBE4(hotspot, monitor__##probe, jtid, \ (uintptr_t)(monitor), bytes, len); \ } \ } #else /* USDT2 */ #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \ { \ if (DTraceMonitorProbes) { \ DTRACE_MONITOR_PROBE_COMMON(obj, thread); \ HOTSPOT_MONITOR_WAIT(jtid, \ (uintptr_t)(monitor), bytes, len, (millis)); \ } \ } #define HOTSPOT_MONITOR_PROBE_waited HOTSPOT_MONITOR_WAITED #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \ { \ if (DTraceMonitorProbes) { \ DTRACE_MONITOR_PROBE_COMMON(obj, thread); \ HOTSPOT_MONITOR_PROBE_##probe(jtid, /* probe = waited */ \ (uintptr_t)(monitor), bytes, len); \ } \ } #endif /* USDT2 */ #else // ndef DTRACE_ENABLED #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;} #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;} #endif // ndef DTRACE_ENABLED // 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; } #define NINFLATIONLOCKS 256 static volatile intptr_t InflationLocks [NINFLATIONLOCKS] ; ObjectMonitor * ObjectSynchronizer::gBlockList = NULL ; ObjectMonitor * volatile ObjectSynchronizer::gFreeList = NULL ; ObjectMonitor * volatile ObjectSynchronizer::gOmInUseList = NULL ; int ObjectSynchronizer::gOmInUseCount = 0; static volatile intptr_t ListLock = 0 ; // protects global monitor free-list cache static volatile int MonitorFreeCount = 0 ; // # on gFreeList static volatile int MonitorPopulation = 0 ; // # Extant -- in circulation #define CHAINMARKER (cast_to_oop(-1)) // ----------------------------------------------------------------------------- // Fast Monitor Enter/Exit // 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"); } 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 (true, THREAD) ; } // ----------------------------------------------------------------------------- // Interpreter/Compiler Slow Case // 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) ; } // ----------------------------------------------------------------------------- // Class Loader support to workaround deadlocks on the class loader lock objects // Also used by GC // 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); } // ----------------------------------------------------------------------------- // JNI locks on java objects // 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) { Handle h_obj(THREAD, obj); BiasedLocking::revoke_and_rebias(h_obj, false, THREAD); obj = h_obj(); } 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(true, THREAD); } } // ----------------------------------------------------------------------------- // Internal VM locks on java objects // 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); } } // ----------------------------------------------------------------------------- // Wait/Notify/NotifyAll // NOTE: must use heavy weight monitor to handle wait() int 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_0(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); return 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); } // ----------------------------------------------------------------------------- // Hash Code handling // // 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. 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 ; static int MonitorScavengeThreshold = 1000000 ; static volatile int ForceMonitorScavenge = 0 ; // Scavenge required and pending 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 = (cast_from_oop(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 } } } // 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 = cast_from_oop(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 = cast_from_oop(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; } // 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) { // owning_thread_from_monitor_owner() may also return NULL here 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; } // 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; } } // 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; } 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()); } } } } // ----------------------------------------------------------------------------- // 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. // // Constraining monitor pool growth via MonitorBound ... // // The monitor pool is grow-only. We scavenge at STW safepoint-time, but the // the rate of scavenging is driven primarily by GC. As such, we can find // an inordinate number of monitors in circulation. // To avoid that scenario we can artificially induce a STW safepoint // if the pool appears to be growing past some reasonable bound. // Generally we favor time in space-time tradeoffs, but as there's no // natural back-pressure on the # of extant monitors we need to impose some // type of limit. Beware that if MonitorBound is set to too low a value // we could just loop. In addition, if MonitorBound is set to a low value // we'll incur more safepoints, which are harmful to performance. // See also: GuaranteedSafepointInterval // // The current implementation uses asynchronous VM operations. // static void InduceScavenge (Thread * Self, const char * Whence) { // Induce STW safepoint to trim monitors // Ultimately, this results in a call to deflate_idle_monitors() in the near future. // More precisely, trigger an asynchronous STW safepoint as the number // of active monitors passes the specified threshold. // TODO: assert thread state is reasonable if (ForceMonitorScavenge == 0 && Atomic::xchg (1, &ForceMonitorScavenge) == 0) { if (ObjectMonitor::Knob_Verbose) { ::printf ("Monitor scavenge - Induced STW @%s (%d)\n", Whence, ForceMonitorScavenge) ; ::fflush(stdout) ; } // Induce a 'null' safepoint to scavenge monitors // Must VM_Operation instance be heap allocated as the op will be enqueue and posted // to the VMthread and have a lifespan longer than that of this activation record. // The VMThread will delete the op when completed. VMThread::execute (new VM_ForceAsyncSafepoint()) ; if (ObjectMonitor::Knob_Verbose) { ::printf ("Monitor scavenge - STW posted @%s (%d)\n", Whence, ForceMonitorScavenge) ; ::fflush(stdout) ; } } } /* Too slow for general assert or debug void ObjectSynchronizer::verifyInUse (Thread *Self) { ObjectMonitor* mid; int inusetally = 0; for (mid = Self->omInUseList; mid != NULL; mid = mid->FreeNext) { inusetally ++; } assert(inusetally == Self->omInUseCount, "inuse count off"); int freetally = 0; for (mid = Self->omFreeList; mid != NULL; mid = mid->FreeNext) { freetally ++; } assert(freetally == Self->omFreeCount, "free count off"); } */ 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") ; if (MonitorInUseLists) { m->FreeNext = Self->omInUseList; Self->omInUseList = m; Self->omInUseCount ++; // verifyInUse(Self); } else { m->FreeNext = NULL; } 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; ) { MonitorFreeCount --; ObjectMonitor * take = gFreeList ; gFreeList = take->FreeNext ; guarantee (take->object() == NULL, "invariant") ; guarantee (!take->is_busy(), "invariant") ; take->Recycle() ; omRelease (Self, take, false) ; } Thread::muxRelease (&ListLock) ; Self->omFreeProvision += 1 + (Self->omFreeProvision/2) ; if (Self->omFreeProvision > MAXPRIVATE ) Self->omFreeProvision = MAXPRIVATE ; TEVENT (omFirst - reprovision) ; const int mx = MonitorBound ; if (mx > 0 && (MonitorPopulation-MonitorFreeCount) > mx) { // We can't safely induce a STW safepoint from omAlloc() as our thread // state may not be appropriate for such activities and callers may hold // naked oops, so instead we defer the action. InduceScavenge (Self, "omAlloc") ; } 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]), OOM_MALLOC_ERROR, "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]") ; MonitorPopulation += _BLOCKSIZE-1; MonitorFreeCount += _BLOCKSIZE-1; // 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. // void ObjectSynchronizer::omRelease (Thread * Self, ObjectMonitor * m, bool fromPerThreadAlloc) { guarantee (m->object() == NULL, "invariant") ; // Remove from omInUseList if (MonitorInUseLists && fromPerThreadAlloc) { ObjectMonitor* curmidinuse = NULL; for (ObjectMonitor* mid = Self->omInUseList; mid != NULL; ) { if (m == mid) { // extract from per-thread in-use-list if (mid == Self->omInUseList) { Self->omInUseList = mid->FreeNext; } else if (curmidinuse != NULL) { curmidinuse->FreeNext = mid->FreeNext; // maintain the current thread inuselist } Self->omInUseCount --; // verifyInUse(Self); break; } else { curmidinuse = mid; mid = mid->FreeNext; } } } // FreeNext is used for both onInUseList and omFreeList, so clear old before setting new 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. // // Also return the monitors of a moribund thread"s omInUseList to // a global gOmInUseList under the global list lock so these // will continue to be scanned. // // 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 ; ObjectMonitor * Tail = NULL ; int Tally = 0; if (List != NULL) { ObjectMonitor * s ; for (s = List ; s != NULL ; s = s->FreeNext) { Tally ++ ; 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") ; } ObjectMonitor * InUseList = Self->omInUseList; ObjectMonitor * InUseTail = NULL ; int InUseTally = 0; if (InUseList != NULL) { Self->omInUseList = NULL; ObjectMonitor *curom; for (curom = InUseList; curom != NULL; curom = curom->FreeNext) { InUseTail = curom; InUseTally++; } // TODO debug assert(Self->omInUseCount == InUseTally, "inuse count off"); Self->omInUseCount = 0; guarantee (InUseTail != NULL && InUseList != NULL, "invariant"); } Thread::muxAcquire (&ListLock, "omFlush") ; if (Tail != NULL) { Tail->FreeNext = gFreeList ; gFreeList = List ; MonitorFreeCount += Tally; } if (InUseTail != NULL) { InUseTail->FreeNext = gOmInUseList; gOmInUseList = InUseList; gOmInUseCount += InUseTally; } Thread::muxRelease (&ListLock) ; TEVENT (omFlush) ; } // 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. 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->_Responsible = NULL ; m->OwnerIsThread = 0 ; m->_recursions = 0 ; m->_SpinDuration = ObjectMonitor::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, true) ; 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 (ObjectMonitor::_sync_Inflations != NULL) ObjectMonitor::_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", (void *) object, (intptr_t) object->mark(), 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->_Responsible = NULL ; m->_SpinDuration = ObjectMonitor::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, true) ; 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 (ObjectMonitor::_sync_Inflations != NULL) ObjectMonitor::_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", (void *) object, (intptr_t) object->mark(), object->klass()->external_name()); } } return m ; } } // Note that we could encounter some performance loss through false-sharing as // multiple locks occupy the same $ line. Padding might be appropriate. // 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. // // We have added a flag, MonitorInUseLists, which creates a list // of active monitors for each thread. deflate_idle_monitors() // only scans the per-thread inuse lists. omAlloc() puts all // assigned monitors on the per-thread list. deflate_idle_monitors() // returns the non-busy monitors to the global free list. // When a thread dies, omFlush() adds the list of active monitors for // that thread to a global gOmInUseList acquiring the // global list lock. deflate_idle_monitors() acquires the global // list lock to scan for non-busy monitors to the global free list. // An alternative could have used a single global inuse list. The // downside would have been the additional cost of acquiring the global list lock // for every omAlloc(). // // 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. // enum ManifestConstants { ClearResponsibleAtSTW = 0, MaximumRecheckInterval = 1000 } ; // Deflate a single monitor if not in use // Return true if deflated, false if in use bool ObjectSynchronizer::deflate_monitor(ObjectMonitor* mid, oop obj, ObjectMonitor** FreeHeadp, ObjectMonitor** FreeTailp) { bool deflated; // 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 ; deflated = false; } 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", (void *) obj, (intptr_t) obj->mark(), 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. if (*FreeHeadp == NULL) *FreeHeadp = mid; if (*FreeTailp != NULL) { ObjectMonitor * prevtail = *FreeTailp; assert(prevtail->FreeNext == NULL, "cleaned up deflated?"); // TODO KK prevtail->FreeNext = mid; } *FreeTailp = mid; deflated = true; } return deflated; } // Caller acquires ListLock int ObjectSynchronizer::walk_monitor_list(ObjectMonitor** listheadp, ObjectMonitor** FreeHeadp, ObjectMonitor** FreeTailp) { ObjectMonitor* mid; ObjectMonitor* next; ObjectMonitor* curmidinuse = NULL; int deflatedcount = 0; for (mid = *listheadp; mid != NULL; ) { oop obj = (oop) mid->object(); bool deflated = false; if (obj != NULL) { deflated = deflate_monitor(mid, obj, FreeHeadp, FreeTailp); } if (deflated) { // extract from per-thread in-use-list if (mid == *listheadp) { *listheadp = mid->FreeNext; } else if (curmidinuse != NULL) { curmidinuse->FreeNext = mid->FreeNext; // maintain the current thread inuselist } next = mid->FreeNext; mid->FreeNext = NULL; // This mid is current tail in the FreeHead list mid = next; deflatedcount++; } else { curmidinuse = mid; mid = mid->FreeNext; } } return deflatedcount; } 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 bool deflated = false; ObjectMonitor * FreeHead = NULL ; // Local SLL of scavenged monitors ObjectMonitor * FreeTail = NULL ; TEVENT (deflate_idle_monitors) ; // Prevent omFlush from changing mids in Thread dtor's during deflation // And in case the vm thread is acquiring a lock during a safepoint // See e.g. 6320749 Thread::muxAcquire (&ListLock, "scavenge - return") ; if (MonitorInUseLists) { int inUse = 0; for (JavaThread* cur = Threads::first(); cur != NULL; cur = cur->next()) { nInCirculation+= cur->omInUseCount; int deflatedcount = walk_monitor_list(cur->omInUseList_addr(), &FreeHead, &FreeTail); cur->omInUseCount-= deflatedcount; // verifyInUse(cur); nScavenged += deflatedcount; nInuse += cur->omInUseCount; } // For moribund threads, scan gOmInUseList if (gOmInUseList) { nInCirculation += gOmInUseCount; int deflatedcount = walk_monitor_list((ObjectMonitor **)&gOmInUseList, &FreeHead, &FreeTail); gOmInUseCount-= deflatedcount; nScavenged += deflatedcount; nInuse += gOmInUseCount; } } else for (ObjectMonitor* block = gBlockList; block != NULL; block = next(block)) { // Iterate over all extant monitors - Scavenge all idle monitors. 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 ; } deflated = deflate_monitor(mid, obj, &FreeHead, &FreeTail); if (deflated) { mid->FreeNext = NULL ; nScavenged ++ ; } else { nInuse ++; } } } MonitorFreeCount += nScavenged; // Consider: audit gFreeList to ensure that MonitorFreeCount and list agree. if (ObjectMonitor::Knob_Verbose) { ::printf ("Deflate: InCirc=%d InUse=%d Scavenged=%d ForceMonitorScavenge=%d : pop=%d free=%d\n", nInCirculation, nInuse, nScavenged, ForceMonitorScavenge, MonitorPopulation, MonitorFreeCount) ; ::fflush(stdout) ; } ForceMonitorScavenge = 0; // Reset // Move the scavenged monitors back to the global free list. if (FreeHead != NULL) { guarantee (FreeTail != NULL && nScavenged > 0, "invariant") ; assert (FreeTail->FreeNext == NULL, "invariant") ; // constant-time list splice - prepend scavenged segment to gFreeList FreeTail->FreeNext = gFreeList ; gFreeList = FreeHead ; } Thread::muxRelease (&ListLock) ; if (ObjectMonitor::_sync_Deflations != NULL) ObjectMonitor::_sync_Deflations->inc(nScavenged) ; if (ObjectMonitor::_sync_MonExtant != NULL) ObjectMonitor::_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 ++ ; } // Monitor cleanup on JavaThread::exit // 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(); } //------------------------------------------------------------------------------ // Non-product code #ifndef PRODUCT // 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