jdk8-mips64-public/hotspot: src/share/vm/runtime/synchronizer.cpp@fa83ab460c54 (annotated)

src/share/vm/runtime/synchronizer.cpp@fa83ab460c54 (annotated)

src/share/vm/runtime/synchronizer.cpp

Fri, 22 Oct 2010 15:59:34 -0400

author: acorn
date: Fri, 22 Oct 2010 15:59:34 -0400
changeset 2233: fa83ab460c54
parent 1995: bfc89697cccb
child 2314: f95d63e2154a
permissions: -rw-r--r--

6988353: refactor contended sync subsystem
Summary: reduce complexity by factoring synchronizer.cpp
Reviewed-by: dholmes, never, coleenp

 /*
  * Copyright (c) 1998, 2009, 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 "incls/_precompiled.incl"
 # include "incls/_synchronizer.cpp.incl"
 #if defined(__GNUC__) && !defined(IA64)
   // Need to inhibit inlining for older versions of GCC to avoid build-time failures
   #define ATTR __attribute__((noinline))
 #else
   #define ATTR
 #endif
 // 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.
 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_PROBE_COMMON(klassOop, thread)                      \
   char* bytes = NULL;                                                      \
   int len = 0;                                                             \
   jlong jtid = SharedRuntime::get_java_tid(thread);                        \
   symbolOop klassname = ((oop)(klassOop))->klass()->klass_part()->name();  \
   if (klassname != NULL) {                                                 \
     bytes = (char*)klassname->bytes();                                     \
     len = klassname->utf8_length();                                        \
   }
 #define DTRACE_MONITOR_WAIT_PROBE(monitor, klassOop, thread, millis)       \
   {                                                                        \
     if (DTraceMonitorProbes) {                                            \
       DTRACE_MONITOR_PROBE_COMMON(klassOop, thread);                       \
       HS_DTRACE_PROBE5(hotspot, monitor__wait, jtid,                       \
                        (monitor), bytes, len, (millis));                   \
     }                                                                      \
   }
 #define DTRACE_MONITOR_PROBE(probe, monitor, klassOop, thread)             \
   {                                                                        \
     if (DTraceMonitorProbes) {                                            \
       DTRACE_MONITOR_PROBE_COMMON(klassOop, thread);                       \
       HS_DTRACE_PROBE4(hotspot, monitor__##probe, jtid,                    \
                        (uintptr_t)(monitor), bytes, len);                  \
     }                                                                      \
   }
 #else //  ndef DTRACE_ENABLED
 #define DTRACE_MONITOR_WAIT_PROBE(klassOop, thread, millis, mon)    {;}
 #define DTRACE_MONITOR_PROBE(probe, klassOop, thread, mon)          {;}
 #endif // ndef DTRACE_ENABLED
 // 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 ((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 (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) {
     BiasedLocking::revoke_and_rebias(obj, false, THREAD);
   }
   assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
   ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj);
   // If this thread has locked the object, exit the monitor.  Note:  can't use
   // monitor->check(CHECK); must exit even if an exception is pending.
   if (monitor->check(THREAD)) {
      monitor->exit(THREAD);
   }
 }
 // -----------------------------------------------------------------------------
 // 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()
 void ObjectSynchronizer::wait(Handle obj, jlong millis, TRAPS) {
   if (UseBiasedLocking) {
     BiasedLocking::revoke_and_rebias(obj, false, THREAD);
     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
   }
   if (millis < 0) {
     TEVENT (wait - throw IAX) ;
     THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
   }
   ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
   DTRACE_MONITOR_WAIT_PROBE(monitor, obj(), THREAD, millis);
   monitor->wait(millis, true, THREAD);
   /* This dummy call is in place to get around dtrace bug 6254741.  Once
      that's fixed we can uncomment the following line and remove the call */
   // DTRACE_MONITOR_PROBE(waited, monitor, obj(), THREAD);
   dtrace_waited_probe(monitor, obj, THREAD);
 }
 void ObjectSynchronizer::waitUninterruptibly (Handle obj, jlong millis, TRAPS) {
   if (UseBiasedLocking) {
     BiasedLocking::revoke_and_rebias(obj, false, THREAD);
     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
   }
   if (millis < 0) {
     TEVENT (wait - throw IAX) ;
     THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
   }
   ObjectSynchronizer::inflate(THREAD, obj()) -> wait(millis, false, THREAD) ;
 }
 void ObjectSynchronizer::notify(Handle obj, TRAPS) {
  if (UseBiasedLocking) {
     BiasedLocking::revoke_and_rebias(obj, false, THREAD);
     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
   }
   markOop mark = obj->mark();
   if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
     return;
   }
   ObjectSynchronizer::inflate(THREAD, obj())->notify(THREAD);
 }
 // NOTE: see comment of notify()
 void ObjectSynchronizer::notifyall(Handle obj, TRAPS) {
   if (UseBiasedLocking) {
     BiasedLocking::revoke_and_rebias(obj, false, THREAD);
     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
   }
   markOop mark = obj->mark();
   if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
     return;
   }
   ObjectSynchronizer::inflate(THREAD, obj())->notifyAll(THREAD);
 }
 // -----------------------------------------------------------------------------
 // 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.
 static int  MBFence (int x) { OrderAccess::fence(); return x; }
 struct SharedGlobals {
     // These are highly shared mostly-read variables.
     // To avoid false-sharing they need to be the sole occupants of a $ line.
     double padPrefix [8];
     volatile int stwRandom ;
     volatile int stwCycle ;
     // Hot RW variables -- Sequester to avoid false-sharing
     double padSuffix [16];
     volatile int hcSequence ;
     double padFinal [8] ;
 } ;
 static SharedGlobals GVars ;
 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 = (intptr_t(obj) >> 5) & (NINFLATIONLOCKS-1) ;
          int YieldThenBlock = 0 ;
          assert (ix >= 0 && ix < NINFLATIONLOCKS, "invariant") ;
          assert ((NINFLATIONLOCKS & (NINFLATIONLOCKS-1)) == 0, "invariant") ;
          Thread::muxAcquire (InflationLocks + ix, "InflationLock") ;
          while (obj->mark() == markOopDesc::INFLATING()) {
            // Beware: NakedYield() is advisory and has almost no effect on some platforms
            // so we periodically call Self->_ParkEvent->park(1).
            // We use a mixed spin/yield/block mechanism.
            if ((YieldThenBlock++) >= 16) {
               Thread::current()->_ParkEvent->park(1) ;
            } else {
               os::NakedYield() ;
            }
          }
          Thread::muxRelease (InflationLocks + ix ) ;
          TEVENT (Inflate: INFLATING - yield/park) ;
        }
     } else {
        SpinPause() ;       // SMP-polite spinning
     }
   }
 }
 // hashCode() generation :
 //
 // Possibilities:
 // * MD5Digest of {obj,stwRandom}
 // * CRC32 of {obj,stwRandom} or any linear-feedback shift register function.
 // * A DES- or AES-style SBox[] mechanism
 // * One of the Phi-based schemes, such as:
 //   2654435761 = 2^32 * Phi (golden ratio)
 //   HashCodeValue = ((uintptr_t(obj) >> 3) * 2654435761) ^ GVars.stwRandom ;
 // * A variation of Marsaglia's shift-xor RNG scheme.
 // * (obj ^ stwRandom) is appealing, but can result
 //   in undesirable regularity in the hashCode values of adjacent objects
 //   (objects allocated back-to-back, in particular).  This could potentially
 //   result in hashtable collisions and reduced hashtable efficiency.
 //   There are simple ways to "diffuse" the middle address bits over the
 //   generated hashCode values:
 //
 static inline intptr_t get_next_hash(Thread * Self, oop obj) {
   intptr_t value = 0 ;
   if (hashCode == 0) {
      // This form uses an unguarded global Park-Miller RNG,
      // so it's possible for two threads to race and generate the same RNG.
      // On MP system we'll have lots of RW access to a global, so the
      // mechanism induces lots of coherency traffic.
      value = os::random() ;
   } else
   if (hashCode == 1) {
      // This variation has the property of being stable (idempotent)
      // between STW operations.  This can be useful in some of the 1-0
      // synchronization schemes.
      intptr_t addrBits = intptr_t(obj) >> 3 ;
      value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom ;
   } else
   if (hashCode == 2) {
      value = 1 ;            // for sensitivity testing
   } else
   if (hashCode == 3) {
      value = ++GVars.hcSequence ;
   } else
   if (hashCode == 4) {
      value = intptr_t(obj) ;
   } else {
      // Marsaglia's xor-shift scheme with thread-specific state
      // This is probably the best overall implementation -- we'll
      // likely make this the default in future releases.
      unsigned t = Self->_hashStateX ;
      t ^= (t << 11) ;
      Self->_hashStateX = Self->_hashStateY ;
      Self->_hashStateY = Self->_hashStateZ ;
      Self->_hashStateZ = Self->_hashStateW ;
      unsigned v = Self->_hashStateW ;
      v = (v ^ (v >> 19)) ^ (t ^ (t >> 8)) ;
      Self->_hashStateW = v ;
      value = v ;
   }
   value &= markOopDesc::hash_mask;
   if (value == 0) value = 0xBAD ;
   assert (value != markOopDesc::no_hash, "invariant") ;
   TEVENT (hashCode: GENERATE) ;
   return value;
 }
 //
 intptr_t ObjectSynchronizer::FastHashCode (Thread * Self, oop obj) {
   if (UseBiasedLocking) {
     // NOTE: many places throughout the JVM do not expect a safepoint
     // to be taken here, in particular most operations on perm gen
     // objects. However, we only ever bias Java instances and all of
     // the call sites of identity_hash that might revoke biases have
     // been checked to make sure they can handle a safepoint. The
     // added check of the bias pattern is to avoid useless calls to
     // thread-local storage.
     if (obj->mark()->has_bias_pattern()) {
       // Box and unbox the raw reference just in case we cause a STW safepoint.
       Handle hobj (Self, obj) ;
       // Relaxing assertion for bug 6320749.
       assert (Universe::verify_in_progress() ||
               !SafepointSynchronize::is_at_safepoint(),
              "biases should not be seen by VM thread here");
       BiasedLocking::revoke_and_rebias(hobj, false, JavaThread::current());
       obj = hobj() ;
       assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
     }
   }
   // hashCode() is a heap mutator ...
   // Relaxing assertion for bug 6320749.
   assert (Universe::verify_in_progress() ||
           !SafepointSynchronize::is_at_safepoint(), "invariant") ;
   assert (Universe::verify_in_progress() ||
           Self->is_Java_thread() , "invariant") ;
   assert (Universe::verify_in_progress() ||
          ((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;
   ObjectMonitor* monitor = NULL;
   markOop temp, test;
   intptr_t hash;
   markOop mark = ReadStableMark (obj);
   // object should remain ineligible for biased locking
   assert (!mark->has_bias_pattern(), "invariant") ;
   if (mark->is_neutral()) {
     hash = mark->hash();              // this is a normal header
     if (hash) {                       // if it has hash, just return it
       return hash;
     }
     hash = get_next_hash(Self, obj);  // allocate a new hash code
     temp = mark->copy_set_hash(hash); // merge the hash code into header
     // use (machine word version) atomic operation to install the hash
     test = (markOop) Atomic::cmpxchg_ptr(temp, obj->mark_addr(), mark);
     if (test == mark) {
       return hash;
     }
     // If atomic operation failed, we must inflate the header
     // into heavy weight monitor. We could add more code here
     // for fast path, but it does not worth the complexity.
   } else if (mark->has_monitor()) {
     monitor = mark->monitor();
     temp = monitor->header();
     assert (temp->is_neutral(), "invariant") ;
     hash = temp->hash();
     if (hash) {
       return hash;
     }
     // Skip to the following code to reduce code size
   } else if (Self->is_lock_owned((address)mark->locker())) {
     temp = mark->displaced_mark_helper(); // this is a lightweight monitor owned
     assert (temp->is_neutral(), "invariant") ;
     hash = temp->hash();              // by current thread, check if the displaced
     if (hash) {                       // header contains hash code
       return hash;
     }
     // WARNING:
     //   The displaced header is strictly immutable.
     // It can NOT be changed in ANY cases. So we have
     // to inflate the header into heavyweight monitor
     // even the current thread owns the lock. The reason
     // is the BasicLock (stack slot) will be asynchronously
     // read by other threads during the inflate() function.
     // Any change to stack may not propagate to other threads
     // correctly.
   }
   // Inflate the monitor to set hash code
   monitor = ObjectSynchronizer::inflate(Self, obj);
   // Load displaced header and check it has hash code
   mark = monitor->header();
   assert (mark->is_neutral(), "invariant") ;
   hash = mark->hash();
   if (hash == 0) {
     hash = get_next_hash(Self, obj);
     temp = mark->copy_set_hash(hash); // merge hash code into header
     assert (temp->is_neutral(), "invariant") ;
     test = (markOop) Atomic::cmpxchg_ptr(temp, monitor, mark);
     if (test != mark) {
       // The only update to the header in the monitor (outside GC)
       // is install the hash code. If someone add new usage of
       // displaced header, please update this code
       hash = test->hash();
       assert (test->is_neutral(), "invariant") ;
       assert (hash != 0, "Trivial unexpected object/monitor header usage.");
     }
   }
   // We finally get the hash
   return hash;
 }
 // Deprecated -- use FastHashCode() instead.
 intptr_t ObjectSynchronizer::identity_hash_value_for(Handle obj) {
   return FastHashCode (Thread::current(), obj()) ;
 }
 bool ObjectSynchronizer::current_thread_holds_lock(JavaThread* thread,
                                                    Handle h_obj) {
   if (UseBiasedLocking) {
     BiasedLocking::revoke_and_rebias(h_obj, false, thread);
     assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
   }
   assert(thread == JavaThread::current(), "Can only be called on current thread");
   oop obj = h_obj();
   markOop mark = ReadStableMark (obj) ;
   // Uncontended case, header points to stack
   if (mark->has_locker()) {
     return thread->is_lock_owned((address)mark->locker());
   }
   // Contended case, header points to ObjectMonitor (tagged pointer)
   if (mark->has_monitor()) {
     ObjectMonitor* monitor = mark->monitor();
     return monitor->is_entered(thread) != 0 ;
   }
   // Unlocked case, header in place
   assert(mark->is_neutral(), "sanity check");
   return false;
 }
 // Be aware of this method could revoke bias of the lock object.
 // This method querys the ownership of the lock handle specified by 'h_obj'.
 // If the current thread owns the lock, it returns owner_self. If no
 // thread owns the lock, it returns owner_none. Otherwise, it will return
 // ower_other.
 ObjectSynchronizer::LockOwnership ObjectSynchronizer::query_lock_ownership
 (JavaThread *self, Handle h_obj) {
   // The caller must beware this method can revoke bias, and
   // revocation can result in a safepoint.
   assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
   assert (self->thread_state() != _thread_blocked , "invariant") ;
   // Possible mark states: neutral, biased, stack-locked, inflated
   if (UseBiasedLocking && h_obj()->mark()->has_bias_pattern()) {
     // CASE: biased
     BiasedLocking::revoke_and_rebias(h_obj, false, self);
     assert(!h_obj->mark()->has_bias_pattern(),
            "biases should be revoked by now");
   }
   assert(self == JavaThread::current(), "Can only be called on current thread");
   oop obj = h_obj();
   markOop mark = ReadStableMark (obj) ;
   // CASE: stack-locked.  Mark points to a BasicLock on the owner's stack.
   if (mark->has_locker()) {
     return self->is_lock_owned((address)mark->locker()) ?
       owner_self : owner_other;
   }
   // CASE: inflated. Mark (tagged pointer) points to an objectMonitor.
   // The Object:ObjectMonitor relationship is stable as long as we're
   // not at a safepoint.
   if (mark->has_monitor()) {
     void * owner = mark->monitor()->_owner ;
     if (owner == NULL) return owner_none ;
     return (owner == self ||
             self->is_lock_owned((address)owner)) ? owner_self : owner_other;
   }
   // CASE: neutral
   assert(mark->is_neutral(), "sanity check");
   return owner_none ;           // it's unlocked
 }
 // FIXME: jvmti should call this
 JavaThread* ObjectSynchronizer::get_lock_owner(Handle h_obj, bool doLock) {
   if (UseBiasedLocking) {
     if (SafepointSynchronize::is_at_safepoint()) {
       BiasedLocking::revoke_at_safepoint(h_obj);
     } else {
       BiasedLocking::revoke_and_rebias(h_obj, false, JavaThread::current());
     }
     assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
   }
   oop obj = h_obj();
   address owner = NULL;
   markOop mark = ReadStableMark (obj) ;
   // Uncontended case, header points to stack
   if (mark->has_locker()) {
     owner = (address) mark->locker();
   }
   // Contended case, header points to ObjectMonitor (tagged pointer)
   if (mark->has_monitor()) {
     ObjectMonitor* monitor = mark->monitor();
     assert(monitor != NULL, "monitor should be non-null");
     owner = (address) monitor->owner();
   }
   if (owner != NULL) {
     return Threads::owning_thread_from_monitor_owner(owner, doLock);
   }
   // Unlocked case, header in place
   // Cannot have assertion since this object may have been
   // locked by another thread when reaching here.
   // assert(mark->is_neutral(), "sanity check");
   return NULL;
 }
 // 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]), "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",
                 (intptr_t) object, (intptr_t) object->mark(),
                 Klass::cast(object->klass())->external_name());
             }
           }
           return m ;
       }
       // CASE: neutral
       // TODO-FIXME: for entry we currently inflate and then try to CAS _owner.
       // If we know we're inflating for entry it's better to inflate by swinging a
       // pre-locked objectMonitor pointer into the object header.   A successful
       // CAS inflates the object *and* confers ownership to the inflating thread.
       // In the current implementation we use a 2-step mechanism where we CAS()
       // to inflate and then CAS() again to try to swing _owner from NULL to Self.
       // An inflateTry() method that we could call from fast_enter() and slow_enter()
       // would be useful.
       assert (mark->is_neutral(), "invariant");
       ObjectMonitor * m = omAlloc (Self) ;
       // prepare m for installation - set monitor to initial state
       m->Recycle();
       m->set_header(mark);
       m->set_owner(NULL);
       m->set_object(object);
       m->OwnerIsThread = 1 ;
       m->_recursions   = 0 ;
       m->_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",
             (intptr_t) object, (intptr_t) object->mark(),
             Klass::cast(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",
                 (intptr_t) obj, (intptr_t) obj->mark(), Klass::cast(obj->klass())->external_name());
        }
      }
      // Restore the header back to obj
      obj->release_set_mark(mid->header());
      mid->clear();
      assert (mid->object() == NULL, "invariant") ;
      // Move the object to the working free list defined by FreeHead,FreeTail.
      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 ;
 //   <code that must not run at safepoint>
 //   guarantee (((tmp ^ _safepoint_counter) | (tmp & 1)) == 0) ;
 // Since the tests are extremely cheap we could leave them enabled
 // for normal product builds.
 void ObjectSynchronizer::release_monitors_owned_by_thread(TRAPS) {
   assert(THREAD == JavaThread::current(), "must be current Java thread");
   No_Safepoint_Verifier nsv ;
   ReleaseJavaMonitorsClosure rjmc(THREAD);
   Thread::muxAcquire(&ListLock, "release_monitors_owned_by_thread");
   ObjectSynchronizer::monitors_iterate(&rjmc);
   Thread::muxRelease(&ListLock);
   THREAD->clear_pending_exception();
 }
 //------------------------------------------------------------------------------
 // Non-product code
 #ifndef PRODUCT
 void ObjectSynchronizer::trace_locking(Handle locking_obj, bool is_compiled,
                                        bool is_method, bool is_locking) {
   // Don't know what to do here
 }
 // Verify all monitors in the monitor cache, the verification is weak.
 void ObjectSynchronizer::verify() {
   ObjectMonitor* block = gBlockList;
   ObjectMonitor* mid;
   while (block) {
     assert(block->object() == CHAINMARKER, "must be a block header");
     for (int i = 1; i < _BLOCKSIZE; i++) {
       mid = block + i;
       oop object = (oop) mid->object();
       if (object != NULL) {
         mid->verify();
       }
     }
     block = (ObjectMonitor*) block->FreeNext;
   }
 }
 // Check if monitor belongs to the monitor cache
 // The list is grow-only so it's *relatively* safe to traverse
 // the list of extant blocks without taking a lock.
 int ObjectSynchronizer::verify_objmon_isinpool(ObjectMonitor *monitor) {
   ObjectMonitor* block = gBlockList;
   while (block) {
     assert(block->object() == CHAINMARKER, "must be a block header");
     if (monitor > &block[0] && monitor < &block[_BLOCKSIZE]) {
       address mon = (address) monitor;
       address blk = (address) block;
       size_t diff = mon - blk;
       assert((diff % sizeof(ObjectMonitor)) == 0, "check");
       return 1;
     }
     block = (ObjectMonitor*) block->FreeNext;
   }
   return 0;
 }
 #endif

Mercurial > jdk8-mips64-public > hotspot / annotate

src/share/vm/runtime/synchronizer.cpp@fa83ab460c54 (annotated)

src/share/vm/runtime/synchronizer.cpp