/* * Copyright (c) 1998, 2017, 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 "logging/log.hpp" #include "memory/metaspaceShared.hpp" #include "memory/padded.hpp" #include "memory/resourceArea.hpp" #include "oops/markOop.hpp" #include "oops/oop.inline.hpp" #include "runtime/atomic.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 "runtime/vframe.hpp" #include "trace/traceMacros.hpp" #include "trace/tracing.hpp" #include "utilities/dtrace.hpp" #include "utilities/events.hpp" #include "utilities/preserveException.hpp" // 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(); \ } #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_notify HOTSPOT_MONITOR_NOTIFY #define HOTSPOT_MONITOR_PROBE_notifyAll HOTSPOT_MONITOR_NOTIFYALL #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); \ } \ } #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 gInflationLocks[NINFLATIONLOCKS]; // global list of blocks of monitors // gBlockList is really PaddedEnd *, but we don't // want to expose the PaddedEnd template more than necessary. ObjectMonitor * volatile ObjectSynchronizer::gBlockList = NULL; // global monitor free list ObjectMonitor * volatile ObjectSynchronizer::gFreeList = NULL; // global monitor in-use list, for moribund threads, // monitors they inflated need to be scanned for deflation ObjectMonitor * volatile ObjectSynchronizer::gOmInUseList = NULL; // count of entries in gOmInUseList int ObjectSynchronizer::gOmInUseCount = 0; static volatile intptr_t gListLock = 0; // protects global monitor lists static volatile int gMonitorFreeCount = 0; // # on gFreeList static volatile int gMonitorPopulation = 0; // # Extant -- in circulation static void post_monitor_inflate_event(EventJavaMonitorInflate&, const oop, const ObjectSynchronizer::InflateCause); #define CHAINMARKER (cast_to_oop(-1)) // =====================> Quick functions // The quick_* forms are special fast-path variants used to improve // performance. In the simplest case, a "quick_*" implementation could // simply return false, in which case the caller will perform the necessary // state transitions and call the slow-path form. // The fast-path is designed to handle frequently arising cases in an efficient // manner and is just a degenerate "optimistic" variant of the slow-path. // returns true -- to indicate the call was satisfied. // returns false -- to indicate the call needs the services of the slow-path. // A no-loitering ordinance is in effect for code in the quick_* family // operators: safepoints or indefinite blocking (blocking that might span a // safepoint) are forbidden. Generally the thread_state() is _in_Java upon // entry. // // Consider: An interesting optimization is to have the JIT recognize the // following common idiom: // synchronized (someobj) { .... ; notify(); } // That is, we find a notify() or notifyAll() call that immediately precedes // the monitorexit operation. In that case the JIT could fuse the operations // into a single notifyAndExit() runtime primitive. bool ObjectSynchronizer::quick_notify(oopDesc * obj, Thread * self, bool all) { assert(!SafepointSynchronize::is_at_safepoint(), "invariant"); assert(self->is_Java_thread(), "invariant"); assert(((JavaThread *) self)->thread_state() == _thread_in_Java, "invariant"); NoSafepointVerifier nsv; if (obj == NULL) return false; // slow-path for invalid obj const markOop mark = obj->mark(); if (mark->has_locker() && self->is_lock_owned((address)mark->locker())) { // Degenerate notify // stack-locked by caller so by definition the implied waitset is empty. return true; } if (mark->has_monitor()) { ObjectMonitor * const mon = mark->monitor(); assert(mon->object() == obj, "invariant"); if (mon->owner() != self) return false; // slow-path for IMS exception if (mon->first_waiter() != NULL) { // We have one or more waiters. Since this is an inflated monitor // that we own, we can transfer one or more threads from the waitset // to the entrylist here and now, avoiding the slow-path. if (all) { DTRACE_MONITOR_PROBE(notifyAll, mon, obj, self); } else { DTRACE_MONITOR_PROBE(notify, mon, obj, self); } int tally = 0; do { mon->INotify(self); ++tally; } while (mon->first_waiter() != NULL && all); OM_PERFDATA_OP(Notifications, inc(tally)); } return true; } // biased locking and any other IMS exception states take the slow-path return false; } // The LockNode emitted directly at the synchronization site would have // been too big if it were to have included support for the cases of inflated // recursive enter and exit, so they go here instead. // Note that we can't safely call AsyncPrintJavaStack() from within // quick_enter() as our thread state remains _in_Java. bool ObjectSynchronizer::quick_enter(oop obj, Thread * Self, BasicLock * lock) { assert(!SafepointSynchronize::is_at_safepoint(), "invariant"); assert(Self->is_Java_thread(), "invariant"); assert(((JavaThread *) Self)->thread_state() == _thread_in_Java, "invariant"); NoSafepointVerifier nsv; if (obj == NULL) return false; // Need to throw NPE const markOop mark = obj->mark(); if (mark->has_monitor()) { ObjectMonitor * const m = mark->monitor(); assert(m->object() == obj, "invariant"); Thread * const owner = (Thread *) m->_owner; // Lock contention and Transactional Lock Elision (TLE) diagnostics // and observability // Case: light contention possibly amenable to TLE // Case: TLE inimical operations such as nested/recursive synchronization if (owner == Self) { m->_recursions++; return true; } // This Java Monitor is inflated so obj's header will never be // displaced to this thread's BasicLock. Make the displaced header // non-NULL so this BasicLock is not seen as recursive nor as // being locked. We do this unconditionally so that this thread's // BasicLock cannot be mis-interpreted by any stack walkers. For // performance reasons, stack walkers generally first check for // Biased Locking in the object's header, the second check is for // stack-locking in the object's header, the third check is for // recursive stack-locking in the displaced header in the BasicLock, // and last are the inflated Java Monitor (ObjectMonitor) checks. lock->set_displaced_header(markOopDesc::unused_mark()); if (owner == NULL && Atomic::cmpxchg_ptr(Self, &(m->_owner), NULL) == NULL) { assert(m->_recursions == 0, "invariant"); assert(m->_owner == Self, "invariant"); return true; } } // Note that we could inflate in quick_enter. // This is likely a useful optimization // Critically, in quick_enter() we must not: // -- perform bias revocation, or // -- block indefinitely, or // -- reach a safepoint return false; // revert to slow-path } // ----------------------------------------------------------------------------- // 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) { markOop mark = object->mark(); // We cannot check for Biased Locking if we are racing an inflation. assert(mark == markOopDesc::INFLATING() || !mark->has_bias_pattern(), "should not see bias pattern here"); markOop dhw = lock->displaced_header(); if (dhw == NULL) { // If the displaced header is NULL, then this exit matches up with // a recursive enter. No real work to do here except for diagnostics. #ifndef PRODUCT if (mark != markOopDesc::INFLATING()) { // Only do diagnostics if we are not racing an inflation. Simply // exiting a recursive enter of a Java Monitor that is being // inflated is safe; see the has_monitor() comment below. assert(!mark->is_neutral(), "invariant"); assert(!mark->has_locker() || THREAD->is_lock_owned((address)mark->locker()), "invariant"); if (mark->has_monitor()) { // The BasicLock's displaced_header is marked as a recursive // enter and we have an inflated Java Monitor (ObjectMonitor). // This is a special case where the Java Monitor was inflated // after this thread entered the stack-lock recursively. When a // Java Monitor is inflated, we cannot safely walk the Java // Monitor owner's stack and update the BasicLocks because a // Java Monitor can be asynchronously inflated by a thread that // does not own the Java Monitor. ObjectMonitor * m = mark->monitor(); assert(((oop)(m->object()))->mark() == mark, "invariant"); assert(m->is_entered(THREAD), "invariant"); } } #endif return; } if (mark == (markOop) lock) { // If the object is stack-locked by the current thread, try to // swing the displaced header from the BasicLock back to the mark. assert(dhw->is_neutral(), "invariant"); if ((markOop) Atomic::cmpxchg_ptr(dhw, object->mark_addr(), mark) == mark) { TEVENT(fast_exit: release stack-lock); return; } } // We have to take the slow-path of possible inflation and then exit. ObjectSynchronizer::inflate(THREAD, object, inflate_cause_vm_internal)->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; } // 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(), inflate_cause_monitor_enter)->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(), inflate_cause_vm_internal); 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(), inflate_cause_vm_internal); 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) { // 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(), inflate_cause_jni_enter)->enter(THREAD); THREAD->set_current_pending_monitor_is_from_java(true); } // 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, inflate_cause_jni_exit); // 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(), inflate_cause_wait); 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, remove the call // and change this function back into a "void" func. // 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(), inflate_cause_wait)->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(), inflate_cause_notify)->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(), inflate_cause_notify)->notifyAll(THREAD); } // ----------------------------------------------------------------------------- // Hash Code handling // // Performance concern: // OrderAccess::storestore() calls release() which at one time stored 0 // into the global volatile OrderAccess::dummy variable. This store was // unnecessary for correctness. Many threads storing into a common location // causes considerable cache migration or "sloshing" on large SMP systems. // As such, I avoided using OrderAccess::storestore(). 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.oracle.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 { char _pad_prefix[DEFAULT_CACHE_LINE_SIZE]; // These are highly shared mostly-read variables. // To avoid false-sharing they need to be the sole occupants of a cache line. volatile int stwRandom; volatile int stwCycle; DEFINE_PAD_MINUS_SIZE(1, DEFAULT_CACHE_LINE_SIZE, sizeof(volatile int) * 2); // Hot RW variable -- Sequester to avoid false-sharing volatile int hcSequence; DEFINE_PAD_MINUS_SIZE(2, DEFAULT_CACHE_LINE_SIZE, sizeof(volatile int)); }; 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::naked_yield(); 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(gInflationLocks + ix, "gInflationLock"); 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::naked_yield(); } } Thread::muxRelease(gInflationLocks + 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()) { // Handle for oop obj in case of 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() || DumpSharedSpaces || !SafepointSynchronize::is_at_safepoint(), "invariant"); assert(Universe::verify_in_progress() || DumpSharedSpaces || Self->is_Java_thread() , "invariant"); assert(Universe::verify_in_progress() || DumpSharedSpaces || ((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, inflate_cause_hash_code); // 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 queries 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 // owner_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) { PaddedEnd * block = (PaddedEnd *)OrderAccess::load_ptr_acquire(&gBlockList); while (block != NULL) { assert(block->object() == CHAINMARKER, "must be a block header"); for (int i = _BLOCKSIZE - 1; i > 0; i--) { ObjectMonitor* mid = (ObjectMonitor *)(block + i); oop object = (oop)mid->object(); if (object != NULL) { closure->do_monitor(mid); } } block = (PaddedEnd *)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) { if (MonitorInUseLists) { // When using thread local monitor lists, we only scan the // global used list here (for moribund threads), and // the thread-local monitors in Thread::oops_do(). global_used_oops_do(f); } else { global_oops_do(f); } } void ObjectSynchronizer::global_oops_do(OopClosure* f) { assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint"); PaddedEnd * block = (PaddedEnd *)OrderAccess::load_ptr_acquire(&gBlockList); for (; block != NULL; block = (PaddedEnd *)next(block)) { assert(block->object() == CHAINMARKER, "must be a block header"); for (int i = 1; i < _BLOCKSIZE; i++) { ObjectMonitor* mid = (ObjectMonitor *)&block[i]; if (mid->object() != NULL) { f->do_oop((oop*)mid->object_addr()); } } } } void ObjectSynchronizer::global_used_oops_do(OopClosure* f) { assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint"); list_oops_do(gOmInUseList, f); } void ObjectSynchronizer::thread_local_used_oops_do(Thread* thread, OopClosure* f) { assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint"); list_oops_do(thread->omInUseList, f); } void ObjectSynchronizer::list_oops_do(ObjectMonitor* list, OopClosure* f) { assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint"); ObjectMonitor* mid; for (mid = list; mid != NULL; mid = mid->FreeNext) { 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 gListLock. 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) { tty->print_cr("INFO: Monitor scavenge - Induced STW @%s (%d)", Whence, ForceMonitorScavenge) ; tty->flush(); } // 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) { tty->print_cr("INFO: Monitor scavenge - STW posted @%s (%d)", Whence, ForceMonitorScavenge) ; tty->flush(); } } } void ObjectSynchronizer::verifyInUse(Thread *Self) { ObjectMonitor* mid; int in_use_tally = 0; for (mid = Self->omInUseList; mid != NULL; mid = mid->FreeNext) { in_use_tally++; } assert(in_use_tally == Self->omInUseCount, "in-use count off"); int free_tally = 0; for (mid = Self->omFreeList; mid != NULL; mid = mid->FreeNext) { free_tally++; } assert(free_tally == Self->omFreeCount, "free count off"); } ObjectMonitor* 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 gListLock 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++; if (ObjectMonitor::Knob_VerifyInUse) { 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(&gListLock, "omAlloc"); for (int i = Self->omFreeProvision; --i >= 0 && gFreeList != NULL;) { gMonitorFreeCount--; ObjectMonitor * take = gFreeList; gFreeList = take->FreeNext; guarantee(take->object() == NULL, "invariant"); guarantee(!take->is_busy(), "invariant"); take->Recycle(); omRelease(Self, take, false); } Thread::muxRelease(&gListLock); Self->omFreeProvision += 1 + (Self->omFreeProvision/2); if (Self->omFreeProvision > MAXPRIVATE) Self->omFreeProvision = MAXPRIVATE; TEVENT(omFirst - reprovision); const int mx = MonitorBound; if (mx > 0 && (gMonitorPopulation-gMonitorFreeCount) > 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. // Ideally, we'd write "new ObjectMonitor[_BLOCKSIZE], but we want // each ObjectMonitor to start at the beginning of a cache line, // so we use align_size_up(). // A better solution would be to use C++ placement-new. // BEWARE: As it stands currently, we don't run the ctors! assert(_BLOCKSIZE > 1, "invariant"); size_t neededsize = sizeof(PaddedEnd) * _BLOCKSIZE; PaddedEnd * temp; size_t aligned_size = neededsize + (DEFAULT_CACHE_LINE_SIZE - 1); void* real_malloc_addr = (void *)NEW_C_HEAP_ARRAY(char, aligned_size, mtInternal); temp = (PaddedEnd *) align_size_up((intptr_t)real_malloc_addr, DEFAULT_CACHE_LINE_SIZE); // 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(neededsize, OOM_MALLOC_ERROR, "Allocate ObjectMonitors"); } (void)memset((void *) temp, 0, neededsize); // 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 = (ObjectMonitor *)&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 gListLock to manipulate gBlockList and gFreeList. // An Oyama-Taura-Yonezawa scheme might be more efficient. Thread::muxAcquire(&gListLock, "omAlloc [2]"); gMonitorPopulation += _BLOCKSIZE-1; gMonitorFreeCount += _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; // There are lock-free uses of gBlockList so make sure that // the previous stores happen before we update gBlockList. OrderAccess::release_store_ptr(&gBlockList, temp); // Add the new string of objectMonitors to the global free list temp[_BLOCKSIZE - 1].FreeNext = gFreeList; gFreeList = temp + 1; Thread::muxRelease(&gListLock); 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. // // Key constraint: all ObjectMonitors on a thread's free list and the global // free list must have their object field set to null. This prevents the // scavenger -- deflate_idle_monitors -- from reclaiming them. void ObjectSynchronizer::omRelease(Thread * Self, ObjectMonitor * m, bool fromPerThreadAlloc) { guarantee(m->object() == NULL, "invariant"); guarantee(((m->is_busy()|m->_recursions) == 0), "freeing in-use monitor"); // Remove from omInUseList if (MonitorInUseLists && fromPerThreadAlloc) { ObjectMonitor* cur_mid_in_use = NULL; bool extracted = false; for (ObjectMonitor* mid = Self->omInUseList; mid != NULL; cur_mid_in_use = mid, mid = mid->FreeNext) { if (m == mid) { // extract from per-thread in-use list if (mid == Self->omInUseList) { Self->omInUseList = mid->FreeNext; } else if (cur_mid_in_use != NULL) { cur_mid_in_use->FreeNext = mid->FreeNext; // maintain the current thread in-use list } extracted = true; Self->omInUseCount--; if (ObjectMonitor::Knob_VerifyInUse) { verifyInUse(Self); } break; } } assert(extracted, "Should have extracted from in-use list"); } // FreeNext is used for both omInUseList 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; // The thread is going away, the per-thread free monitors // are freed via set_owner(NULL) // Link them to tail, which will be linked into the global free list // gFreeList below, under the gListLock 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 *cur_om; // The thread is going away, however the omInUseList inflated // monitors may still be in-use by other threads. // Link them to inUseTail, which will be linked into the global in-use list // gOmInUseList below, under the gListLock for (cur_om = inUseList; cur_om != NULL; cur_om = cur_om->FreeNext) { inUseTail = cur_om; inUseTally++; } assert(Self->omInUseCount == inUseTally, "in-use count off"); Self->omInUseCount = 0; guarantee(inUseTail != NULL && inUseList != NULL, "invariant"); } Thread::muxAcquire(&gListLock, "omFlush"); if (tail != NULL) { tail->FreeNext = gFreeList; gFreeList = list; gMonitorFreeCount += tally; } if (inUseTail != NULL) { inUseTail->FreeNext = gOmInUseList; gOmInUseList = inUseList; gOmInUseCount += inUseTally; } Thread::muxRelease(&gListLock); 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, inflate_cause_vm_internal); } ObjectMonitor* ObjectSynchronizer::inflate(Thread * Self, oop object, const InflateCause cause) { // Inflate mutates the heap ... // Relaxing assertion for bug 6320749. assert(Universe::verify_in_progress() || !SafepointSynchronize::is_at_safepoint(), "invariant"); EventJavaMonitorInflate event; 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"); event.cancel(); // let's not post an inflation event, unless we did the deed ourselves 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->_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-word. // 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. // 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 ... OM_PERFDATA_OP(Inflations, inc()); TEVENT(Inflate: overwrite stacklock); if (log_is_enabled(Debug, monitorinflation)) { if (object->is_instance()) { ResourceMark rm; log_debug(monitorinflation)("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s", p2i(object), p2i(object->mark()), object->klass()->external_name()); } } if (event.should_commit()) { post_monitor_inflate_event(event, object, cause); } 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->_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->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 ... OM_PERFDATA_OP(Inflations, inc()); TEVENT(Inflate: overwrite neutral); if (log_is_enabled(Debug, monitorinflation)) { if (object->is_instance()) { ResourceMark rm; log_debug(monitorinflation)("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s", p2i(object), p2i(object->mark()), object->klass()->external_name()); } } if (event.should_commit()) { post_monitor_inflate_event(event, object, cause); } return m; } } // 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 in-use 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 in-use 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 }; // 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 (log_is_enabled(Debug, monitorinflation)) { if (obj->is_instance()) { ResourceMark rm; log_debug(monitorinflation)("Deflating object " INTPTR_FORMAT " , " "mark " INTPTR_FORMAT " , type %s", p2i(obj), p2i(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 freeHeadp, freeTailp if (*freeHeadp == NULL) *freeHeadp = mid; if (*freeTailp != NULL) { ObjectMonitor * prevtail = *freeTailp; assert(prevtail->FreeNext == NULL, "cleaned up deflated?"); prevtail->FreeNext = mid; } *freeTailp = mid; deflated = true; } return deflated; } // Walk a given monitor list, and deflate idle monitors // The given list could be a per-thread list or a global list // Caller acquires gListLock int ObjectSynchronizer::deflate_monitor_list(ObjectMonitor** listHeadp, ObjectMonitor** freeHeadp, ObjectMonitor** freeTailp) { ObjectMonitor* mid; ObjectMonitor* next; ObjectMonitor* cur_mid_in_use = NULL; int deflated_count = 0; for (mid = *listHeadp; mid != NULL;) { oop obj = (oop) mid->object(); if (obj != NULL && deflate_monitor(mid, obj, freeHeadp, freeTailp)) { // if deflate_monitor succeeded, // extract from per-thread in-use list if (mid == *listHeadp) { *listHeadp = mid->FreeNext; } else if (cur_mid_in_use != NULL) { cur_mid_in_use->FreeNext = mid->FreeNext; // maintain the current thread in-use list } next = mid->FreeNext; mid->FreeNext = NULL; // This mid is current tail in the freeHeadp list mid = next; deflated_count++; } else { cur_mid_in_use = mid; mid = mid->FreeNext; } } return deflated_count; } 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 * freeHeadp = NULL; // Local SLL of scavenged monitors ObjectMonitor * freeTailp = 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(&gListLock, "scavenge - return"); if (MonitorInUseLists) { int inUse = 0; for (JavaThread* cur = Threads::first(); cur != NULL; cur = cur->next()) { nInCirculation+= cur->omInUseCount; int deflated_count = deflate_monitor_list(cur->omInUseList_addr(), &freeHeadp, &freeTailp); cur->omInUseCount-= deflated_count; if (ObjectMonitor::Knob_VerifyInUse) { verifyInUse(cur); } nScavenged += deflated_count; nInuse += cur->omInUseCount; } // For moribund threads, scan gOmInUseList if (gOmInUseList) { nInCirculation += gOmInUseCount; int deflated_count = deflate_monitor_list((ObjectMonitor **)&gOmInUseList, &freeHeadp, &freeTailp); gOmInUseCount-= deflated_count; nScavenged += deflated_count; nInuse += gOmInUseCount; } } else { PaddedEnd * block = (PaddedEnd *)OrderAccess::load_ptr_acquire(&gBlockList); for (; block != NULL; block = (PaddedEnd *)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 = (ObjectMonitor*)&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, &freeHeadp, &freeTailp); if (deflated) { mid->FreeNext = NULL; nScavenged++; } else { nInuse++; } } } } gMonitorFreeCount += nScavenged; // Consider: audit gFreeList to ensure that gMonitorFreeCount and list agree. if (ObjectMonitor::Knob_Verbose) { tty->print_cr("INFO: Deflate: InCirc=%d InUse=%d Scavenged=%d " "ForceMonitorScavenge=%d : pop=%d free=%d", nInCirculation, nInuse, nScavenged, ForceMonitorScavenge, gMonitorPopulation, gMonitorFreeCount); tty->flush(); } ForceMonitorScavenge = 0; // Reset // Move the scavenged monitors back to the global free list. if (freeHeadp != NULL) { guarantee(freeTailp != NULL && nScavenged > 0, "invariant"); assert(freeTailp->FreeNext == NULL, "invariant"); // constant-time list splice - prepend scavenged segment to gFreeList freeTailp->FreeNext = gFreeList; gFreeList = freeHeadp; } Thread::muxRelease(&gListLock); OM_PERFDATA_OP(Deflations, inc(nScavenged)); OM_PERFDATA_OP(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) { if (ObjectMonitor::Knob_VerifyMatch != 0) { ResourceMark rm; Handle obj(THREAD, (oop) mid->object()); tty->print("INFO: unexpected locked object:"); javaVFrame::print_locked_object_class_name(tty, obj, "locked"); fatal("exiting JavaThread=" INTPTR_FORMAT " unexpectedly owns ObjectMonitor=" INTPTR_FORMAT, p2i(THREAD), p2i(mid)); } (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"); NoSafepointVerifier nsv; ReleaseJavaMonitorsClosure rjmc(THREAD); Thread::muxAcquire(&gListLock, "release_monitors_owned_by_thread"); ObjectSynchronizer::monitors_iterate(&rjmc); Thread::muxRelease(&gListLock); THREAD->clear_pending_exception(); } const char* ObjectSynchronizer::inflate_cause_name(const InflateCause cause) { switch (cause) { case inflate_cause_vm_internal: return "VM Internal"; case inflate_cause_monitor_enter: return "Monitor Enter"; case inflate_cause_wait: return "Monitor Wait"; case inflate_cause_notify: return "Monitor Notify"; case inflate_cause_hash_code: return "Monitor Hash Code"; case inflate_cause_jni_enter: return "JNI Monitor Enter"; case inflate_cause_jni_exit: return "JNI Monitor Exit"; default: ShouldNotReachHere(); } return "Unknown"; } static void post_monitor_inflate_event(EventJavaMonitorInflate& event, const oop obj, const ObjectSynchronizer::InflateCause cause) { #if INCLUDE_TRACE assert(event.should_commit(), "check outside"); event.set_monitorClass(obj->klass()); event.set_address((TYPE_ADDRESS)(uintptr_t)(void*)obj); event.set_cause((u1)cause); event.commit(); #endif } //------------------------------------------------------------------------------ // Debugging code void ObjectSynchronizer::sanity_checks(const bool verbose, const uint cache_line_size, int *error_cnt_ptr, int *warning_cnt_ptr) { u_char *addr_begin = (u_char*)&GVars; u_char *addr_stwRandom = (u_char*)&GVars.stwRandom; u_char *addr_hcSequence = (u_char*)&GVars.hcSequence; if (verbose) { tty->print_cr("INFO: sizeof(SharedGlobals)=" SIZE_FORMAT, sizeof(SharedGlobals)); } uint offset_stwRandom = (uint)(addr_stwRandom - addr_begin); if (verbose) tty->print_cr("INFO: offset(stwRandom)=%u", offset_stwRandom); uint offset_hcSequence = (uint)(addr_hcSequence - addr_begin); if (verbose) { tty->print_cr("INFO: offset(_hcSequence)=%u", offset_hcSequence); } if (cache_line_size != 0) { // We were able to determine the L1 data cache line size so // do some cache line specific sanity checks if (offset_stwRandom < cache_line_size) { tty->print_cr("WARNING: the SharedGlobals.stwRandom field is closer " "to the struct beginning than a cache line which permits " "false sharing."); (*warning_cnt_ptr)++; } if ((offset_hcSequence - offset_stwRandom) < cache_line_size) { tty->print_cr("WARNING: the SharedGlobals.stwRandom and " "SharedGlobals.hcSequence fields are closer than a cache " "line which permits false sharing."); (*warning_cnt_ptr)++; } if ((sizeof(SharedGlobals) - offset_hcSequence) < cache_line_size) { tty->print_cr("WARNING: the SharedGlobals.hcSequence field is closer " "to the struct end than a cache line which permits false " "sharing."); (*warning_cnt_ptr)++; } } } #ifndef PRODUCT // 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) { PaddedEnd * block = (PaddedEnd *)OrderAccess::load_ptr_acquire(&gBlockList); while (block != NULL) { assert(block->object() == CHAINMARKER, "must be a block header"); if (monitor > (ObjectMonitor *)&block[0] && monitor < (ObjectMonitor *)&block[_BLOCKSIZE]) { address mon = (address)monitor; address blk = (address)block; size_t diff = mon - blk; assert((diff % sizeof(PaddedEnd)) == 0, "must be aligned"); return 1; } block = (PaddedEnd *)block->FreeNext; } return 0; } #endif