1 /*
   2  * Copyright (c) 1998, 2018, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.
   8  *
   9  * This code is distributed in the hope that it will be useful, but WITHOUT
  10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  12  * version 2 for more details (a copy is included in the LICENSE file that
  13  * accompanied this code).
  14  *
  15  * You should have received a copy of the GNU General Public License version
  16  * 2 along with this work; if not, write to the Free Software Foundation,
  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  18  *
  19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  20  * or visit www.oracle.com if you need additional information or have any
  21  * questions.
  22  *
  23  */
  24 
  25 #include "precompiled.hpp"
  26 #include "classfile/vmSymbols.hpp"
  27 #include "logging/log.hpp"
  28 #include "jfr/jfrEvents.hpp"
  29 #include "memory/allocation.inline.hpp"
  30 #include "memory/metaspaceShared.hpp"
  31 #include "memory/padded.hpp"
  32 #include "memory/resourceArea.hpp"
  33 #include "oops/markOop.hpp"
  34 #include "oops/oop.inline.hpp"
  35 #include "runtime/atomic.hpp"
  36 #include "runtime/biasedLocking.hpp"
  37 #include "runtime/handles.inline.hpp"
  38 #include "runtime/interfaceSupport.inline.hpp"
  39 #include "runtime/mutexLocker.hpp"
  40 #include "runtime/objectMonitor.hpp"
  41 #include "runtime/objectMonitor.inline.hpp"
  42 #include "runtime/osThread.hpp"
  43 #include "runtime/safepointVerifiers.hpp"
  44 #include "runtime/sharedRuntime.hpp"
  45 #include "runtime/stubRoutines.hpp"
  46 #include "runtime/synchronizer.hpp"
  47 #include "runtime/thread.inline.hpp"
  48 #include "runtime/vframe.hpp"
  49 #include "runtime/vmThread.hpp"
  50 #include "utilities/align.hpp"
  51 #include "utilities/dtrace.hpp"
  52 #include "utilities/events.hpp"
  53 #include "utilities/preserveException.hpp"
  54 
  55 // The "core" versions of monitor enter and exit reside in this file.
  56 // The interpreter and compilers contain specialized transliterated
  57 // variants of the enter-exit fast-path operations.  See i486.ad fast_lock(),
  58 // for instance.  If you make changes here, make sure to modify the
  59 // interpreter, and both C1 and C2 fast-path inline locking code emission.
  60 //
  61 // -----------------------------------------------------------------------------
  62 
  63 #ifdef DTRACE_ENABLED
  64 
  65 // Only bother with this argument setup if dtrace is available
  66 // TODO-FIXME: probes should not fire when caller is _blocked.  assert() accordingly.
  67 
  68 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread)                           \
  69   char* bytes = NULL;                                                      \
  70   int len = 0;                                                             \
  71   jlong jtid = SharedRuntime::get_java_tid(thread);                        \
  72   Symbol* klassname = ((oop)(obj))->klass()->name();                       \
  73   if (klassname != NULL) {                                                 \
  74     bytes = (char*)klassname->bytes();                                     \
  75     len = klassname->utf8_length();                                        \
  76   }
  77 
  78 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis)            \
  79   {                                                                        \
  80     if (DTraceMonitorProbes) {                                             \
  81       DTRACE_MONITOR_PROBE_COMMON(obj, thread);                            \
  82       HOTSPOT_MONITOR_WAIT(jtid,                                           \
  83                            (uintptr_t)(monitor), bytes, len, (millis));    \
  84     }                                                                      \
  85   }
  86 
  87 #define HOTSPOT_MONITOR_PROBE_notify HOTSPOT_MONITOR_NOTIFY
  88 #define HOTSPOT_MONITOR_PROBE_notifyAll HOTSPOT_MONITOR_NOTIFYALL
  89 #define HOTSPOT_MONITOR_PROBE_waited HOTSPOT_MONITOR_WAITED
  90 
  91 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread)                  \
  92   {                                                                        \
  93     if (DTraceMonitorProbes) {                                             \
  94       DTRACE_MONITOR_PROBE_COMMON(obj, thread);                            \
  95       HOTSPOT_MONITOR_PROBE_##probe(jtid, /* probe = waited */             \
  96                                     (uintptr_t)(monitor), bytes, len);     \
  97     }                                                                      \
  98   }
  99 
 100 #else //  ndef DTRACE_ENABLED
 101 
 102 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon)    {;}
 103 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon)          {;}
 104 
 105 #endif // ndef DTRACE_ENABLED
 106 
 107 // This exists only as a workaround of dtrace bug 6254741
 108 int dtrace_waited_probe(ObjectMonitor* monitor, Handle obj, Thread* thr) {
 109   DTRACE_MONITOR_PROBE(waited, monitor, obj(), thr);
 110   return 0;
 111 }
 112 
 113 #define NINFLATIONLOCKS 256
 114 static volatile intptr_t gInflationLocks[NINFLATIONLOCKS];
 115 
 116 // global list of blocks of monitors
 117 PaddedEnd<ObjectMonitor> * volatile ObjectSynchronizer::gBlockList = NULL;
 118 // global monitor free list
 119 ObjectMonitor * volatile ObjectSynchronizer::gFreeList  = NULL;
 120 // global monitor in-use list, for moribund threads,
 121 // monitors they inflated need to be scanned for deflation
 122 ObjectMonitor * volatile ObjectSynchronizer::gOmInUseList  = NULL;
 123 // count of entries in gOmInUseList
 124 int ObjectSynchronizer::gOmInUseCount = 0;
 125 
 126 static volatile intptr_t gListLock = 0;      // protects global monitor lists
 127 static volatile int gMonitorFreeCount  = 0;  // # on gFreeList
 128 static volatile int gMonitorPopulation = 0;  // # Extant -- in circulation
 129 
 130 #define CHAINMARKER (cast_to_oop<intptr_t>(-1))
 131 
 132 
 133 // =====================> Quick functions
 134 
 135 // The quick_* forms are special fast-path variants used to improve
 136 // performance.  In the simplest case, a "quick_*" implementation could
 137 // simply return false, in which case the caller will perform the necessary
 138 // state transitions and call the slow-path form.
 139 // The fast-path is designed to handle frequently arising cases in an efficient
 140 // manner and is just a degenerate "optimistic" variant of the slow-path.
 141 // returns true  -- to indicate the call was satisfied.
 142 // returns false -- to indicate the call needs the services of the slow-path.
 143 // A no-loitering ordinance is in effect for code in the quick_* family
 144 // operators: safepoints or indefinite blocking (blocking that might span a
 145 // safepoint) are forbidden. Generally the thread_state() is _in_Java upon
 146 // entry.
 147 //
 148 // Consider: An interesting optimization is to have the JIT recognize the
 149 // following common idiom:
 150 //   synchronized (someobj) { .... ; notify(); }
 151 // That is, we find a notify() or notifyAll() call that immediately precedes
 152 // the monitorexit operation.  In that case the JIT could fuse the operations
 153 // into a single notifyAndExit() runtime primitive.
 154 
 155 bool ObjectSynchronizer::quick_notify(oopDesc * obj, Thread * self, bool all) {
 156   assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
 157   assert(self->is_Java_thread(), "invariant");
 158   assert(((JavaThread *) self)->thread_state() == _thread_in_Java, "invariant");
 159   NoSafepointVerifier nsv;
 160   if (obj == NULL) return false;  // slow-path for invalid obj
 161   const markOop mark = obj->mark();
 162 
 163   if (mark->has_locker() && self->is_lock_owned((address)mark->locker())) {
 164     // Degenerate notify
 165     // stack-locked by caller so by definition the implied waitset is empty.
 166     return true;
 167   }
 168 
 169   if (mark->has_monitor()) {
 170     ObjectMonitor * const mon = mark->monitor();
 171     assert(oopDesc::equals((oop) mon->object(), obj), "invariant");
 172     if (mon->owner() != self) return false;  // slow-path for IMS exception
 173 
 174     if (mon->first_waiter() != NULL) {
 175       // We have one or more waiters. Since this is an inflated monitor
 176       // that we own, we can transfer one or more threads from the waitset
 177       // to the entrylist here and now, avoiding the slow-path.
 178       if (all) {
 179         DTRACE_MONITOR_PROBE(notifyAll, mon, obj, self);
 180       } else {
 181         DTRACE_MONITOR_PROBE(notify, mon, obj, self);
 182       }
 183       int tally = 0;
 184       do {
 185         mon->INotify(self);
 186         ++tally;
 187       } while (mon->first_waiter() != NULL && all);
 188       OM_PERFDATA_OP(Notifications, inc(tally));
 189     }
 190     return true;
 191   }
 192 
 193   // biased locking and any other IMS exception states take the slow-path
 194   return false;
 195 }
 196 
 197 
 198 // The LockNode emitted directly at the synchronization site would have
 199 // been too big if it were to have included support for the cases of inflated
 200 // recursive enter and exit, so they go here instead.
 201 // Note that we can't safely call AsyncPrintJavaStack() from within
 202 // quick_enter() as our thread state remains _in_Java.
 203 
 204 bool ObjectSynchronizer::quick_enter(oop obj, Thread * Self,
 205                                      BasicLock * lock) {
 206   assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
 207   assert(Self->is_Java_thread(), "invariant");
 208   assert(((JavaThread *) Self)->thread_state() == _thread_in_Java, "invariant");
 209   NoSafepointVerifier nsv;
 210   if (obj == NULL) return false;       // Need to throw NPE
 211   const markOop mark = obj->mark();
 212 
 213   if (mark->has_monitor()) {
 214     ObjectMonitor * const m = mark->monitor();
 215     assert(oopDesc::equals((oop) m->object(), obj), "invariant");
 216     Thread * const owner = (Thread *) m->_owner;
 217 
 218     // Lock contention and Transactional Lock Elision (TLE) diagnostics
 219     // and observability
 220     // Case: light contention possibly amenable to TLE
 221     // Case: TLE inimical operations such as nested/recursive synchronization
 222 
 223     if (owner == Self) {
 224       m->_recursions++;
 225       return true;
 226     }
 227 
 228     // This Java Monitor is inflated so obj's header will never be
 229     // displaced to this thread's BasicLock. Make the displaced header
 230     // non-NULL so this BasicLock is not seen as recursive nor as
 231     // being locked. We do this unconditionally so that this thread's
 232     // BasicLock cannot be mis-interpreted by any stack walkers. For
 233     // performance reasons, stack walkers generally first check for
 234     // Biased Locking in the object's header, the second check is for
 235     // stack-locking in the object's header, the third check is for
 236     // recursive stack-locking in the displaced header in the BasicLock,
 237     // and last are the inflated Java Monitor (ObjectMonitor) checks.
 238     lock->set_displaced_header(markOopDesc::unused_mark());
 239 
 240     if (owner == NULL && Atomic::replace_if_null(Self, &(m->_owner))) {
 241       assert(m->_recursions == 0, "invariant");
 242       assert(m->_owner == Self, "invariant");
 243       return true;
 244     }
 245   }
 246 
 247   // Note that we could inflate in quick_enter.
 248   // This is likely a useful optimization
 249   // Critically, in quick_enter() we must not:
 250   // -- perform bias revocation, or
 251   // -- block indefinitely, or
 252   // -- reach a safepoint
 253 
 254   return false;        // revert to slow-path
 255 }
 256 
 257 // -----------------------------------------------------------------------------
 258 //  Fast Monitor Enter/Exit
 259 // This the fast monitor enter. The interpreter and compiler use
 260 // some assembly copies of this code. Make sure update those code
 261 // if the following function is changed. The implementation is
 262 // extremely sensitive to race condition. Be careful.
 263 
 264 void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock,
 265                                     bool attempt_rebias, TRAPS) {
 266   if (UseBiasedLocking) {
 267     if (!SafepointSynchronize::is_at_safepoint()) {
 268       BiasedLocking::Condition cond = BiasedLocking::revoke_and_rebias(obj, attempt_rebias, THREAD);
 269       if (cond == BiasedLocking::BIAS_REVOKED_AND_REBIASED) {
 270         return;
 271       }
 272     } else {
 273       assert(!attempt_rebias, "can not rebias toward VM thread");
 274       BiasedLocking::revoke_at_safepoint(obj);
 275     }
 276     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
 277   }
 278 
 279   slow_enter(obj, lock, THREAD);
 280 }
 281 
 282 void ObjectSynchronizer::fast_exit(oop object, BasicLock* lock, TRAPS) {
 283   markOop mark = object->mark();
 284   // We cannot check for Biased Locking if we are racing an inflation.
 285   assert(mark == markOopDesc::INFLATING() ||
 286          !mark->has_bias_pattern(), "should not see bias pattern here");
 287 
 288   markOop dhw = lock->displaced_header();
 289   if (dhw == NULL) {
 290     // If the displaced header is NULL, then this exit matches up with
 291     // a recursive enter. No real work to do here except for diagnostics.
 292 #ifndef PRODUCT
 293     if (mark != markOopDesc::INFLATING()) {
 294       // Only do diagnostics if we are not racing an inflation. Simply
 295       // exiting a recursive enter of a Java Monitor that is being
 296       // inflated is safe; see the has_monitor() comment below.
 297       assert(!mark->is_neutral(), "invariant");
 298       assert(!mark->has_locker() ||
 299              THREAD->is_lock_owned((address)mark->locker()), "invariant");
 300       if (mark->has_monitor()) {
 301         // The BasicLock's displaced_header is marked as a recursive
 302         // enter and we have an inflated Java Monitor (ObjectMonitor).
 303         // This is a special case where the Java Monitor was inflated
 304         // after this thread entered the stack-lock recursively. When a
 305         // Java Monitor is inflated, we cannot safely walk the Java
 306         // Monitor owner's stack and update the BasicLocks because a
 307         // Java Monitor can be asynchronously inflated by a thread that
 308         // does not own the Java Monitor.
 309         ObjectMonitor * m = mark->monitor();
 310         assert(((oop)(m->object()))->mark() == mark, "invariant");
 311         assert(m->is_entered(THREAD), "invariant");
 312       }
 313     }
 314 #endif
 315     return;
 316   }
 317 
 318   if (mark == (markOop) lock) {
 319     // If the object is stack-locked by the current thread, try to
 320     // swing the displaced header from the BasicLock back to the mark.
 321     assert(dhw->is_neutral(), "invariant");
 322     if (object->cas_set_mark(dhw, mark) == mark) {
 323       return;
 324     }
 325   }
 326 
 327   // We have to take the slow-path of possible inflation and then exit.
 328   ObjectSynchronizer::inflate(THREAD,
 329                               object,
 330                               inflate_cause_vm_internal)->exit(true, THREAD);
 331 }
 332 
 333 // -----------------------------------------------------------------------------
 334 // Interpreter/Compiler Slow Case
 335 // This routine is used to handle interpreter/compiler slow case
 336 // We don't need to use fast path here, because it must have been
 337 // failed in the interpreter/compiler code.
 338 void ObjectSynchronizer::slow_enter(Handle obj, BasicLock* lock, TRAPS) {
 339   markOop mark = obj->mark();
 340   assert(!mark->has_bias_pattern(), "should not see bias pattern here");
 341 
 342   if (mark->is_neutral()) {
 343     // Anticipate successful CAS -- the ST of the displaced mark must
 344     // be visible <= the ST performed by the CAS.
 345     lock->set_displaced_header(mark);
 346     if (mark == obj()->cas_set_mark((markOop) lock, mark)) {
 347       return;
 348     }
 349     // Fall through to inflate() ...
 350   } else if (mark->has_locker() &&
 351              THREAD->is_lock_owned((address)mark->locker())) {
 352     assert(lock != mark->locker(), "must not re-lock the same lock");
 353     assert(lock != (BasicLock*)obj->mark(), "don't relock with same BasicLock");
 354     lock->set_displaced_header(NULL);
 355     return;
 356   }
 357 
 358   // The object header will never be displaced to this lock,
 359   // so it does not matter what the value is, except that it
 360   // must be non-zero to avoid looking like a re-entrant lock,
 361   // and must not look locked either.
 362   lock->set_displaced_header(markOopDesc::unused_mark());
 363   ObjectSynchronizer::inflate(THREAD,
 364                               obj(),
 365                               inflate_cause_monitor_enter)->enter(THREAD);
 366 }
 367 
 368 // This routine is used to handle interpreter/compiler slow case
 369 // We don't need to use fast path here, because it must have
 370 // failed in the interpreter/compiler code. Simply use the heavy
 371 // weight monitor should be ok, unless someone find otherwise.
 372 void ObjectSynchronizer::slow_exit(oop object, BasicLock* lock, TRAPS) {
 373   fast_exit(object, lock, THREAD);
 374 }
 375 
 376 // -----------------------------------------------------------------------------
 377 // Class Loader  support to workaround deadlocks on the class loader lock objects
 378 // Also used by GC
 379 // complete_exit()/reenter() are used to wait on a nested lock
 380 // i.e. to give up an outer lock completely and then re-enter
 381 // Used when holding nested locks - lock acquisition order: lock1 then lock2
 382 //  1) complete_exit lock1 - saving recursion count
 383 //  2) wait on lock2
 384 //  3) when notified on lock2, unlock lock2
 385 //  4) reenter lock1 with original recursion count
 386 //  5) lock lock2
 387 // NOTE: must use heavy weight monitor to handle complete_exit/reenter()
 388 intptr_t ObjectSynchronizer::complete_exit(Handle obj, TRAPS) {
 389   if (UseBiasedLocking) {
 390     BiasedLocking::revoke_and_rebias(obj, false, THREAD);
 391     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
 392   }
 393 
 394   ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD,
 395                                                        obj(),
 396                                                        inflate_cause_vm_internal);
 397 
 398   return monitor->complete_exit(THREAD);
 399 }
 400 
 401 // NOTE: must use heavy weight monitor to handle complete_exit/reenter()
 402 void ObjectSynchronizer::reenter(Handle obj, intptr_t recursion, TRAPS) {
 403   if (UseBiasedLocking) {
 404     BiasedLocking::revoke_and_rebias(obj, false, THREAD);
 405     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
 406   }
 407 
 408   ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD,
 409                                                        obj(),
 410                                                        inflate_cause_vm_internal);
 411 
 412   monitor->reenter(recursion, THREAD);
 413 }
 414 // -----------------------------------------------------------------------------
 415 // JNI locks on java objects
 416 // NOTE: must use heavy weight monitor to handle jni monitor enter
 417 void ObjectSynchronizer::jni_enter(Handle obj, TRAPS) {
 418   // the current locking is from JNI instead of Java code
 419   if (UseBiasedLocking) {
 420     BiasedLocking::revoke_and_rebias(obj, false, THREAD);
 421     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
 422   }
 423   THREAD->set_current_pending_monitor_is_from_java(false);
 424   ObjectSynchronizer::inflate(THREAD, obj(), inflate_cause_jni_enter)->enter(THREAD);
 425   THREAD->set_current_pending_monitor_is_from_java(true);
 426 }
 427 
 428 // NOTE: must use heavy weight monitor to handle jni monitor exit
 429 void ObjectSynchronizer::jni_exit(oop obj, Thread* THREAD) {
 430   if (UseBiasedLocking) {
 431     Handle h_obj(THREAD, obj);
 432     BiasedLocking::revoke_and_rebias(h_obj, false, THREAD);
 433     obj = h_obj();
 434   }
 435   assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
 436 
 437   ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD,
 438                                                        obj,
 439                                                        inflate_cause_jni_exit);
 440   // If this thread has locked the object, exit the monitor.  Note:  can't use
 441   // monitor->check(CHECK); must exit even if an exception is pending.
 442   if (monitor->check(THREAD)) {
 443     monitor->exit(true, THREAD);
 444   }
 445 }
 446 
 447 // -----------------------------------------------------------------------------
 448 // Internal VM locks on java objects
 449 // standard constructor, allows locking failures
 450 ObjectLocker::ObjectLocker(Handle obj, Thread* thread, bool doLock) {
 451   _dolock = doLock;
 452   _thread = thread;
 453   debug_only(if (StrictSafepointChecks) _thread->check_for_valid_safepoint_state(false);)
 454   _obj = obj;
 455 
 456   if (_dolock) {
 457     ObjectSynchronizer::fast_enter(_obj, &_lock, false, _thread);
 458   }
 459 }
 460 
 461 ObjectLocker::~ObjectLocker() {
 462   if (_dolock) {
 463     ObjectSynchronizer::fast_exit(_obj(), &_lock, _thread);
 464   }
 465 }
 466 
 467 
 468 // -----------------------------------------------------------------------------
 469 //  Wait/Notify/NotifyAll
 470 // NOTE: must use heavy weight monitor to handle wait()
 471 int ObjectSynchronizer::wait(Handle obj, jlong millis, TRAPS) {
 472   if (UseBiasedLocking) {
 473     BiasedLocking::revoke_and_rebias(obj, false, THREAD);
 474     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
 475   }
 476   if (millis < 0) {
 477     THROW_MSG_0(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
 478   }
 479   ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD,
 480                                                        obj(),
 481                                                        inflate_cause_wait);
 482 
 483   DTRACE_MONITOR_WAIT_PROBE(monitor, obj(), THREAD, millis);
 484   monitor->wait(millis, true, THREAD);
 485 
 486   // This dummy call is in place to get around dtrace bug 6254741.  Once
 487   // that's fixed we can uncomment the following line, remove the call
 488   // and change this function back into a "void" func.
 489   // DTRACE_MONITOR_PROBE(waited, monitor, obj(), THREAD);
 490   return dtrace_waited_probe(monitor, obj, THREAD);
 491 }
 492 
 493 void ObjectSynchronizer::waitUninterruptibly(Handle obj, jlong millis, TRAPS) {
 494   if (UseBiasedLocking) {
 495     BiasedLocking::revoke_and_rebias(obj, false, THREAD);
 496     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
 497   }
 498   if (millis < 0) {
 499     THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
 500   }
 501   ObjectSynchronizer::inflate(THREAD,
 502                               obj(),
 503                               inflate_cause_wait)->wait(millis, false, THREAD);
 504 }
 505 
 506 void ObjectSynchronizer::notify(Handle obj, TRAPS) {
 507   if (UseBiasedLocking) {
 508     BiasedLocking::revoke_and_rebias(obj, false, THREAD);
 509     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
 510   }
 511 
 512   markOop mark = obj->mark();
 513   if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
 514     return;
 515   }
 516   ObjectSynchronizer::inflate(THREAD,
 517                               obj(),
 518                               inflate_cause_notify)->notify(THREAD);
 519 }
 520 
 521 // NOTE: see comment of notify()
 522 void ObjectSynchronizer::notifyall(Handle obj, TRAPS) {
 523   if (UseBiasedLocking) {
 524     BiasedLocking::revoke_and_rebias(obj, false, THREAD);
 525     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
 526   }
 527 
 528   markOop mark = obj->mark();
 529   if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
 530     return;
 531   }
 532   ObjectSynchronizer::inflate(THREAD,
 533                               obj(),
 534                               inflate_cause_notify)->notifyAll(THREAD);
 535 }
 536 
 537 // -----------------------------------------------------------------------------
 538 // Hash Code handling
 539 //
 540 // Performance concern:
 541 // OrderAccess::storestore() calls release() which at one time stored 0
 542 // into the global volatile OrderAccess::dummy variable. This store was
 543 // unnecessary for correctness. Many threads storing into a common location
 544 // causes considerable cache migration or "sloshing" on large SMP systems.
 545 // As such, I avoided using OrderAccess::storestore(). In some cases
 546 // OrderAccess::fence() -- which incurs local latency on the executing
 547 // processor -- is a better choice as it scales on SMP systems.
 548 //
 549 // See http://blogs.oracle.com/dave/entry/biased_locking_in_hotspot for
 550 // a discussion of coherency costs. Note that all our current reference
 551 // platforms provide strong ST-ST order, so the issue is moot on IA32,
 552 // x64, and SPARC.
 553 //
 554 // As a general policy we use "volatile" to control compiler-based reordering
 555 // and explicit fences (barriers) to control for architectural reordering
 556 // performed by the CPU(s) or platform.
 557 
 558 struct SharedGlobals {
 559   char         _pad_prefix[DEFAULT_CACHE_LINE_SIZE];
 560   // These are highly shared mostly-read variables.
 561   // To avoid false-sharing they need to be the sole occupants of a cache line.
 562   volatile int stwRandom;
 563   volatile int stwCycle;
 564   DEFINE_PAD_MINUS_SIZE(1, DEFAULT_CACHE_LINE_SIZE, sizeof(volatile int) * 2);
 565   // Hot RW variable -- Sequester to avoid false-sharing
 566   volatile int hcSequence;
 567   DEFINE_PAD_MINUS_SIZE(2, DEFAULT_CACHE_LINE_SIZE, sizeof(volatile int));
 568 };
 569 
 570 static SharedGlobals GVars;
 571 static int MonitorScavengeThreshold = 1000000;
 572 static volatile int ForceMonitorScavenge = 0; // Scavenge required and pending
 573 
 574 static markOop ReadStableMark(oop obj) {
 575   markOop mark = obj->mark();
 576   if (!mark->is_being_inflated()) {
 577     return mark;       // normal fast-path return
 578   }
 579 
 580   int its = 0;
 581   for (;;) {
 582     markOop mark = obj->mark();
 583     if (!mark->is_being_inflated()) {
 584       return mark;    // normal fast-path return
 585     }
 586 
 587     // The object is being inflated by some other thread.
 588     // The caller of ReadStableMark() must wait for inflation to complete.
 589     // Avoid live-lock
 590     // TODO: consider calling SafepointSynchronize::do_call_back() while
 591     // spinning to see if there's a safepoint pending.  If so, immediately
 592     // yielding or blocking would be appropriate.  Avoid spinning while
 593     // there is a safepoint pending.
 594     // TODO: add inflation contention performance counters.
 595     // TODO: restrict the aggregate number of spinners.
 596 
 597     ++its;
 598     if (its > 10000 || !os::is_MP()) {
 599       if (its & 1) {
 600         os::naked_yield();
 601       } else {
 602         // Note that the following code attenuates the livelock problem but is not
 603         // a complete remedy.  A more complete solution would require that the inflating
 604         // thread hold the associated inflation lock.  The following code simply restricts
 605         // the number of spinners to at most one.  We'll have N-2 threads blocked
 606         // on the inflationlock, 1 thread holding the inflation lock and using
 607         // a yield/park strategy, and 1 thread in the midst of inflation.
 608         // A more refined approach would be to change the encoding of INFLATING
 609         // to allow encapsulation of a native thread pointer.  Threads waiting for
 610         // inflation to complete would use CAS to push themselves onto a singly linked
 611         // list rooted at the markword.  Once enqueued, they'd loop, checking a per-thread flag
 612         // and calling park().  When inflation was complete the thread that accomplished inflation
 613         // would detach the list and set the markword to inflated with a single CAS and
 614         // then for each thread on the list, set the flag and unpark() the thread.
 615         // This is conceptually similar to muxAcquire-muxRelease, except that muxRelease
 616         // wakes at most one thread whereas we need to wake the entire list.
 617         int ix = (cast_from_oop<intptr_t>(obj) >> 5) & (NINFLATIONLOCKS-1);
 618         int YieldThenBlock = 0;
 619         assert(ix >= 0 && ix < NINFLATIONLOCKS, "invariant");
 620         assert((NINFLATIONLOCKS & (NINFLATIONLOCKS-1)) == 0, "invariant");
 621         Thread::muxAcquire(gInflationLocks + ix, "gInflationLock");
 622         while (obj->mark() == markOopDesc::INFLATING()) {
 623           // Beware: NakedYield() is advisory and has almost no effect on some platforms
 624           // so we periodically call Self->_ParkEvent->park(1).
 625           // We use a mixed spin/yield/block mechanism.
 626           if ((YieldThenBlock++) >= 16) {
 627             Thread::current()->_ParkEvent->park(1);
 628           } else {
 629             os::naked_yield();
 630           }
 631         }
 632         Thread::muxRelease(gInflationLocks + ix);
 633       }
 634     } else {
 635       SpinPause();       // SMP-polite spinning
 636     }
 637   }
 638 }
 639 
 640 // hashCode() generation :
 641 //
 642 // Possibilities:
 643 // * MD5Digest of {obj,stwRandom}
 644 // * CRC32 of {obj,stwRandom} or any linear-feedback shift register function.
 645 // * A DES- or AES-style SBox[] mechanism
 646 // * One of the Phi-based schemes, such as:
 647 //   2654435761 = 2^32 * Phi (golden ratio)
 648 //   HashCodeValue = ((uintptr_t(obj) >> 3) * 2654435761) ^ GVars.stwRandom ;
 649 // * A variation of Marsaglia's shift-xor RNG scheme.
 650 // * (obj ^ stwRandom) is appealing, but can result
 651 //   in undesirable regularity in the hashCode values of adjacent objects
 652 //   (objects allocated back-to-back, in particular).  This could potentially
 653 //   result in hashtable collisions and reduced hashtable efficiency.
 654 //   There are simple ways to "diffuse" the middle address bits over the
 655 //   generated hashCode values:
 656 
 657 static inline intptr_t get_next_hash(Thread * Self, oop obj) {
 658   intptr_t value = 0;
 659   if (hashCode == 0) {
 660     // This form uses global Park-Miller RNG.
 661     // On MP system we'll have lots of RW access to a global, so the
 662     // mechanism induces lots of coherency traffic.
 663     value = os::random();
 664   } else if (hashCode == 1) {
 665     // This variation has the property of being stable (idempotent)
 666     // between STW operations.  This can be useful in some of the 1-0
 667     // synchronization schemes.
 668     intptr_t addrBits = cast_from_oop<intptr_t>(obj) >> 3;
 669     value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom;
 670   } else if (hashCode == 2) {
 671     value = 1;            // for sensitivity testing
 672   } else if (hashCode == 3) {
 673     value = ++GVars.hcSequence;
 674   } else if (hashCode == 4) {
 675     value = cast_from_oop<intptr_t>(obj);
 676   } else {
 677     // Marsaglia's xor-shift scheme with thread-specific state
 678     // This is probably the best overall implementation -- we'll
 679     // likely make this the default in future releases.
 680     unsigned t = Self->_hashStateX;
 681     t ^= (t << 11);
 682     Self->_hashStateX = Self->_hashStateY;
 683     Self->_hashStateY = Self->_hashStateZ;
 684     Self->_hashStateZ = Self->_hashStateW;
 685     unsigned v = Self->_hashStateW;
 686     v = (v ^ (v >> 19)) ^ (t ^ (t >> 8));
 687     Self->_hashStateW = v;
 688     value = v;
 689   }
 690 
 691   value &= markOopDesc::hash_mask;
 692   if (value == 0) value = 0xBAD;
 693   assert(value != markOopDesc::no_hash, "invariant");
 694   return value;
 695 }
 696 
 697 intptr_t ObjectSynchronizer::FastHashCode(Thread * Self, oop obj) {
 698   if (UseBiasedLocking) {
 699     // NOTE: many places throughout the JVM do not expect a safepoint
 700     // to be taken here, in particular most operations on perm gen
 701     // objects. However, we only ever bias Java instances and all of
 702     // the call sites of identity_hash that might revoke biases have
 703     // been checked to make sure they can handle a safepoint. The
 704     // added check of the bias pattern is to avoid useless calls to
 705     // thread-local storage.
 706     if (obj->mark()->has_bias_pattern()) {
 707       // Handle for oop obj in case of STW safepoint
 708       Handle hobj(Self, obj);
 709       // Relaxing assertion for bug 6320749.
 710       assert(Universe::verify_in_progress() ||
 711              !SafepointSynchronize::is_at_safepoint(),
 712              "biases should not be seen by VM thread here");
 713       BiasedLocking::revoke_and_rebias(hobj, false, JavaThread::current());
 714       obj = hobj();
 715       assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
 716     }
 717   }
 718 
 719   // hashCode() is a heap mutator ...
 720   // Relaxing assertion for bug 6320749.
 721   assert(Universe::verify_in_progress() || DumpSharedSpaces ||
 722          !SafepointSynchronize::is_at_safepoint(), "invariant");
 723   assert(Universe::verify_in_progress() || DumpSharedSpaces ||
 724          Self->is_Java_thread() , "invariant");
 725   assert(Universe::verify_in_progress() || DumpSharedSpaces ||
 726          ((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
 727 
 728   ObjectMonitor* monitor = NULL;
 729   markOop temp, test;
 730   intptr_t hash;
 731   markOop mark = ReadStableMark(obj);
 732 
 733   // object should remain ineligible for biased locking
 734   assert(!mark->has_bias_pattern(), "invariant");
 735 
 736   if (mark->is_neutral()) {
 737     hash = mark->hash();              // this is a normal header
 738     if (hash) {                       // if it has hash, just return it
 739       return hash;
 740     }
 741     hash = get_next_hash(Self, obj);  // allocate a new hash code
 742     temp = mark->copy_set_hash(hash); // merge the hash code into header
 743     // use (machine word version) atomic operation to install the hash
 744     test = obj->cas_set_mark(temp, mark);
 745     if (test == mark) {
 746       return hash;
 747     }
 748     // If atomic operation failed, we must inflate the header
 749     // into heavy weight monitor. We could add more code here
 750     // for fast path, but it does not worth the complexity.
 751   } else if (mark->has_monitor()) {
 752     monitor = mark->monitor();
 753     temp = monitor->header();
 754     assert(temp->is_neutral(), "invariant");
 755     hash = temp->hash();
 756     if (hash) {
 757       return hash;
 758     }
 759     // Skip to the following code to reduce code size
 760   } else if (Self->is_lock_owned((address)mark->locker())) {
 761     temp = mark->displaced_mark_helper(); // this is a lightweight monitor owned
 762     assert(temp->is_neutral(), "invariant");
 763     hash = temp->hash();              // by current thread, check if the displaced
 764     if (hash) {                       // header contains hash code
 765       return hash;
 766     }
 767     // WARNING:
 768     //   The displaced header is strictly immutable.
 769     // It can NOT be changed in ANY cases. So we have
 770     // to inflate the header into heavyweight monitor
 771     // even the current thread owns the lock. The reason
 772     // is the BasicLock (stack slot) will be asynchronously
 773     // read by other threads during the inflate() function.
 774     // Any change to stack may not propagate to other threads
 775     // correctly.
 776   }
 777 
 778   // Inflate the monitor to set hash code
 779   monitor = ObjectSynchronizer::inflate(Self, obj, inflate_cause_hash_code);
 780   // Load displaced header and check it has hash code
 781   mark = monitor->header();
 782   assert(mark->is_neutral(), "invariant");
 783   hash = mark->hash();
 784   if (hash == 0) {
 785     hash = get_next_hash(Self, obj);
 786     temp = mark->copy_set_hash(hash); // merge hash code into header
 787     assert(temp->is_neutral(), "invariant");
 788     test = Atomic::cmpxchg(temp, monitor->header_addr(), mark);
 789     if (test != mark) {
 790       // The only update to the header in the monitor (outside GC)
 791       // is install the hash code. If someone add new usage of
 792       // displaced header, please update this code
 793       hash = test->hash();
 794       assert(test->is_neutral(), "invariant");
 795       assert(hash != 0, "Trivial unexpected object/monitor header usage.");
 796     }
 797   }
 798   // We finally get the hash
 799   return hash;
 800 }
 801 
 802 // Deprecated -- use FastHashCode() instead.
 803 
 804 intptr_t ObjectSynchronizer::identity_hash_value_for(Handle obj) {
 805   return FastHashCode(Thread::current(), obj());
 806 }
 807 
 808 
 809 bool ObjectSynchronizer::current_thread_holds_lock(JavaThread* thread,
 810                                                    Handle h_obj) {
 811   if (UseBiasedLocking) {
 812     BiasedLocking::revoke_and_rebias(h_obj, false, thread);
 813     assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
 814   }
 815 
 816   assert(thread == JavaThread::current(), "Can only be called on current thread");
 817   oop obj = h_obj();
 818 
 819   markOop mark = ReadStableMark(obj);
 820 
 821   // Uncontended case, header points to stack
 822   if (mark->has_locker()) {
 823     return thread->is_lock_owned((address)mark->locker());
 824   }
 825   // Contended case, header points to ObjectMonitor (tagged pointer)
 826   if (mark->has_monitor()) {
 827     ObjectMonitor* monitor = mark->monitor();
 828     return monitor->is_entered(thread) != 0;
 829   }
 830   // Unlocked case, header in place
 831   assert(mark->is_neutral(), "sanity check");
 832   return false;
 833 }
 834 
 835 // Be aware of this method could revoke bias of the lock object.
 836 // This method queries the ownership of the lock handle specified by 'h_obj'.
 837 // If the current thread owns the lock, it returns owner_self. If no
 838 // thread owns the lock, it returns owner_none. Otherwise, it will return
 839 // owner_other.
 840 ObjectSynchronizer::LockOwnership ObjectSynchronizer::query_lock_ownership
 841 (JavaThread *self, Handle h_obj) {
 842   // The caller must beware this method can revoke bias, and
 843   // revocation can result in a safepoint.
 844   assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
 845   assert(self->thread_state() != _thread_blocked, "invariant");
 846 
 847   // Possible mark states: neutral, biased, stack-locked, inflated
 848 
 849   if (UseBiasedLocking && h_obj()->mark()->has_bias_pattern()) {
 850     // CASE: biased
 851     BiasedLocking::revoke_and_rebias(h_obj, false, self);
 852     assert(!h_obj->mark()->has_bias_pattern(),
 853            "biases should be revoked by now");
 854   }
 855 
 856   assert(self == JavaThread::current(), "Can only be called on current thread");
 857   oop obj = h_obj();
 858   markOop mark = ReadStableMark(obj);
 859 
 860   // CASE: stack-locked.  Mark points to a BasicLock on the owner's stack.
 861   if (mark->has_locker()) {
 862     return self->is_lock_owned((address)mark->locker()) ?
 863       owner_self : owner_other;
 864   }
 865 
 866   // CASE: inflated. Mark (tagged pointer) points to an objectMonitor.
 867   // The Object:ObjectMonitor relationship is stable as long as we're
 868   // not at a safepoint.
 869   if (mark->has_monitor()) {
 870     void * owner = mark->monitor()->_owner;
 871     if (owner == NULL) return owner_none;
 872     return (owner == self ||
 873             self->is_lock_owned((address)owner)) ? owner_self : owner_other;
 874   }
 875 
 876   // CASE: neutral
 877   assert(mark->is_neutral(), "sanity check");
 878   return owner_none;           // it's unlocked
 879 }
 880 
 881 // FIXME: jvmti should call this
 882 JavaThread* ObjectSynchronizer::get_lock_owner(ThreadsList * t_list, Handle h_obj) {
 883   if (UseBiasedLocking) {
 884     if (SafepointSynchronize::is_at_safepoint()) {
 885       BiasedLocking::revoke_at_safepoint(h_obj);
 886     } else {
 887       BiasedLocking::revoke_and_rebias(h_obj, false, JavaThread::current());
 888     }
 889     assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
 890   }
 891 
 892   oop obj = h_obj();
 893   address owner = NULL;
 894 
 895   markOop mark = ReadStableMark(obj);
 896 
 897   // Uncontended case, header points to stack
 898   if (mark->has_locker()) {
 899     owner = (address) mark->locker();
 900   }
 901 
 902   // Contended case, header points to ObjectMonitor (tagged pointer)
 903   if (mark->has_monitor()) {
 904     ObjectMonitor* monitor = mark->monitor();
 905     assert(monitor != NULL, "monitor should be non-null");
 906     owner = (address) monitor->owner();
 907   }
 908 
 909   if (owner != NULL) {
 910     // owning_thread_from_monitor_owner() may also return NULL here
 911     return Threads::owning_thread_from_monitor_owner(t_list, owner);
 912   }
 913 
 914   // Unlocked case, header in place
 915   // Cannot have assertion since this object may have been
 916   // locked by another thread when reaching here.
 917   // assert(mark->is_neutral(), "sanity check");
 918 
 919   return NULL;
 920 }
 921 
 922 // Visitors ...
 923 
 924 void ObjectSynchronizer::monitors_iterate(MonitorClosure* closure) {
 925   PaddedEnd<ObjectMonitor> * block = OrderAccess::load_acquire(&gBlockList);
 926   while (block != NULL) {
 927     assert(block->object() == CHAINMARKER, "must be a block header");
 928     for (int i = _BLOCKSIZE - 1; i > 0; i--) {
 929       ObjectMonitor* mid = (ObjectMonitor *)(block + i);
 930       oop object = (oop)mid->object();
 931       if (object != NULL) {
 932         closure->do_monitor(mid);
 933       }
 934     }
 935     block = (PaddedEnd<ObjectMonitor> *)block->FreeNext;
 936   }
 937 }
 938 
 939 // Get the next block in the block list.
 940 static inline PaddedEnd<ObjectMonitor>* next(PaddedEnd<ObjectMonitor>* block) {
 941   assert(block->object() == CHAINMARKER, "must be a block header");
 942   block = (PaddedEnd<ObjectMonitor>*) block->FreeNext;
 943   assert(block == NULL || block->object() == CHAINMARKER, "must be a block header");
 944   return block;
 945 }
 946 
 947 static bool monitors_used_above_threshold() {
 948   if (gMonitorPopulation == 0) {
 949     return false;
 950   }
 951   int monitors_used = gMonitorPopulation - gMonitorFreeCount;
 952   int monitor_usage = (monitors_used * 100LL) / gMonitorPopulation;
 953   return monitor_usage > MonitorUsedDeflationThreshold;
 954 }
 955 
 956 bool ObjectSynchronizer::is_cleanup_needed() {
 957   if (MonitorUsedDeflationThreshold > 0) {
 958     return monitors_used_above_threshold();
 959   }
 960   return false;
 961 }
 962 
 963 void ObjectSynchronizer::oops_do(OopClosure* f) {
 964   if (MonitorInUseLists) {
 965     // When using thread local monitor lists, we only scan the
 966     // global used list here (for moribund threads), and
 967     // the thread-local monitors in Thread::oops_do().
 968     global_used_oops_do(f);
 969   } else {
 970     global_oops_do(f);
 971   }
 972 }
 973 
 974 void ObjectSynchronizer::global_oops_do(OopClosure* f) {
 975   assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
 976   PaddedEnd<ObjectMonitor> * block = OrderAccess::load_acquire(&gBlockList);
 977   for (; block != NULL; block = next(block)) {
 978     assert(block->object() == CHAINMARKER, "must be a block header");
 979     for (int i = 1; i < _BLOCKSIZE; i++) {
 980       ObjectMonitor* mid = (ObjectMonitor *)&block[i];
 981       if (mid->object() != NULL) {
 982         f->do_oop((oop*)mid->object_addr());
 983       }
 984     }
 985   }
 986 }
 987 
 988 void ObjectSynchronizer::global_used_oops_do(OopClosure* f) {
 989   assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
 990   list_oops_do(gOmInUseList, f);
 991 }
 992 
 993 void ObjectSynchronizer::thread_local_used_oops_do(Thread* thread, OopClosure* f) {
 994   assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
 995   list_oops_do(thread->omInUseList, f);
 996 }
 997 
 998 void ObjectSynchronizer::list_oops_do(ObjectMonitor* list, OopClosure* f) {
 999   assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
1000   ObjectMonitor* mid;
1001   for (mid = list; mid != NULL; mid = mid->FreeNext) {
1002     if (mid->object() != NULL) {
1003       f->do_oop((oop*)mid->object_addr());
1004     }
1005   }
1006 }
1007 
1008 
1009 // -----------------------------------------------------------------------------
1010 // ObjectMonitor Lifecycle
1011 // -----------------------
1012 // Inflation unlinks monitors from the global gFreeList and
1013 // associates them with objects.  Deflation -- which occurs at
1014 // STW-time -- disassociates idle monitors from objects.  Such
1015 // scavenged monitors are returned to the gFreeList.
1016 //
1017 // The global list is protected by gListLock.  All the critical sections
1018 // are short and operate in constant-time.
1019 //
1020 // ObjectMonitors reside in type-stable memory (TSM) and are immortal.
1021 //
1022 // Lifecycle:
1023 // --   unassigned and on the global free list
1024 // --   unassigned and on a thread's private omFreeList
1025 // --   assigned to an object.  The object is inflated and the mark refers
1026 //      to the objectmonitor.
1027 
1028 
1029 // Constraining monitor pool growth via MonitorBound ...
1030 //
1031 // The monitor pool is grow-only.  We scavenge at STW safepoint-time, but the
1032 // the rate of scavenging is driven primarily by GC.  As such,  we can find
1033 // an inordinate number of monitors in circulation.
1034 // To avoid that scenario we can artificially induce a STW safepoint
1035 // if the pool appears to be growing past some reasonable bound.
1036 // Generally we favor time in space-time tradeoffs, but as there's no
1037 // natural back-pressure on the # of extant monitors we need to impose some
1038 // type of limit.  Beware that if MonitorBound is set to too low a value
1039 // we could just loop. In addition, if MonitorBound is set to a low value
1040 // we'll incur more safepoints, which are harmful to performance.
1041 // See also: GuaranteedSafepointInterval
1042 //
1043 // The current implementation uses asynchronous VM operations.
1044 
1045 static void InduceScavenge(Thread * Self, const char * Whence) {
1046   // Induce STW safepoint to trim monitors
1047   // Ultimately, this results in a call to deflate_idle_monitors() in the near future.
1048   // More precisely, trigger an asynchronous STW safepoint as the number
1049   // of active monitors passes the specified threshold.
1050   // TODO: assert thread state is reasonable
1051 
1052   if (ForceMonitorScavenge == 0 && Atomic::xchg (1, &ForceMonitorScavenge) == 0) {
1053     // Induce a 'null' safepoint to scavenge monitors
1054     // Must VM_Operation instance be heap allocated as the op will be enqueue and posted
1055     // to the VMthread and have a lifespan longer than that of this activation record.
1056     // The VMThread will delete the op when completed.
1057     VMThread::execute(new VM_ScavengeMonitors());
1058   }
1059 }
1060 
1061 void ObjectSynchronizer::verifyInUse(Thread *Self) {
1062   ObjectMonitor* mid;
1063   int in_use_tally = 0;
1064   for (mid = Self->omInUseList; mid != NULL; mid = mid->FreeNext) {
1065     in_use_tally++;
1066   }
1067   assert(in_use_tally == Self->omInUseCount, "in-use count off");
1068 
1069   int free_tally = 0;
1070   for (mid = Self->omFreeList; mid != NULL; mid = mid->FreeNext) {
1071     free_tally++;
1072   }
1073   assert(free_tally == Self->omFreeCount, "free count off");
1074 }
1075 
1076 ObjectMonitor* ObjectSynchronizer::omAlloc(Thread * Self) {
1077   // A large MAXPRIVATE value reduces both list lock contention
1078   // and list coherency traffic, but also tends to increase the
1079   // number of objectMonitors in circulation as well as the STW
1080   // scavenge costs.  As usual, we lean toward time in space-time
1081   // tradeoffs.
1082   const int MAXPRIVATE = 1024;
1083   for (;;) {
1084     ObjectMonitor * m;
1085 
1086     // 1: try to allocate from the thread's local omFreeList.
1087     // Threads will attempt to allocate first from their local list, then
1088     // from the global list, and only after those attempts fail will the thread
1089     // attempt to instantiate new monitors.   Thread-local free lists take
1090     // heat off the gListLock and improve allocation latency, as well as reducing
1091     // coherency traffic on the shared global list.
1092     m = Self->omFreeList;
1093     if (m != NULL) {
1094       Self->omFreeList = m->FreeNext;
1095       Self->omFreeCount--;
1096       // CONSIDER: set m->FreeNext = BAD -- diagnostic hygiene
1097       guarantee(m->object() == NULL, "invariant");
1098       if (MonitorInUseLists) {
1099         m->FreeNext = Self->omInUseList;
1100         Self->omInUseList = m;
1101         Self->omInUseCount++;
1102       } else {
1103         m->FreeNext = NULL;
1104       }
1105       return m;
1106     }
1107 
1108     // 2: try to allocate from the global gFreeList
1109     // CONSIDER: use muxTry() instead of muxAcquire().
1110     // If the muxTry() fails then drop immediately into case 3.
1111     // If we're using thread-local free lists then try
1112     // to reprovision the caller's free list.
1113     if (gFreeList != NULL) {
1114       // Reprovision the thread's omFreeList.
1115       // Use bulk transfers to reduce the allocation rate and heat
1116       // on various locks.
1117       Thread::muxAcquire(&gListLock, "omAlloc");
1118       for (int i = Self->omFreeProvision; --i >= 0 && gFreeList != NULL;) {
1119         gMonitorFreeCount--;
1120         ObjectMonitor * take = gFreeList;
1121         gFreeList = take->FreeNext;
1122         guarantee(take->object() == NULL, "invariant");
1123         guarantee(!take->is_busy(), "invariant");
1124         take->Recycle();
1125         omRelease(Self, take, false);
1126       }
1127       Thread::muxRelease(&gListLock);
1128       Self->omFreeProvision += 1 + (Self->omFreeProvision/2);
1129       if (Self->omFreeProvision > MAXPRIVATE) Self->omFreeProvision = MAXPRIVATE;
1130 
1131       const int mx = MonitorBound;
1132       if (mx > 0 && (gMonitorPopulation-gMonitorFreeCount) > mx) {
1133         // We can't safely induce a STW safepoint from omAlloc() as our thread
1134         // state may not be appropriate for such activities and callers may hold
1135         // naked oops, so instead we defer the action.
1136         InduceScavenge(Self, "omAlloc");
1137       }
1138       continue;
1139     }
1140 
1141     // 3: allocate a block of new ObjectMonitors
1142     // Both the local and global free lists are empty -- resort to malloc().
1143     // In the current implementation objectMonitors are TSM - immortal.
1144     // Ideally, we'd write "new ObjectMonitor[_BLOCKSIZE], but we want
1145     // each ObjectMonitor to start at the beginning of a cache line,
1146     // so we use align_up().
1147     // A better solution would be to use C++ placement-new.
1148     // BEWARE: As it stands currently, we don't run the ctors!
1149     assert(_BLOCKSIZE > 1, "invariant");
1150     size_t neededsize = sizeof(PaddedEnd<ObjectMonitor>) * _BLOCKSIZE;
1151     PaddedEnd<ObjectMonitor> * temp;
1152     size_t aligned_size = neededsize + (DEFAULT_CACHE_LINE_SIZE - 1);
1153     void* real_malloc_addr = (void *)NEW_C_HEAP_ARRAY(char, aligned_size,
1154                                                       mtInternal);
1155     temp = (PaddedEnd<ObjectMonitor> *)
1156              align_up(real_malloc_addr, DEFAULT_CACHE_LINE_SIZE);
1157 
1158     // NOTE: (almost) no way to recover if allocation failed.
1159     // We might be able to induce a STW safepoint and scavenge enough
1160     // objectMonitors to permit progress.
1161     if (temp == NULL) {
1162       vm_exit_out_of_memory(neededsize, OOM_MALLOC_ERROR,
1163                             "Allocate ObjectMonitors");
1164     }
1165     (void)memset((void *) temp, 0, neededsize);
1166 
1167     // Format the block.
1168     // initialize the linked list, each monitor points to its next
1169     // forming the single linked free list, the very first monitor
1170     // will points to next block, which forms the block list.
1171     // The trick of using the 1st element in the block as gBlockList
1172     // linkage should be reconsidered.  A better implementation would
1173     // look like: class Block { Block * next; int N; ObjectMonitor Body [N] ; }
1174 
1175     for (int i = 1; i < _BLOCKSIZE; i++) {
1176       temp[i].FreeNext = (ObjectMonitor *)&temp[i+1];
1177     }
1178 
1179     // terminate the last monitor as the end of list
1180     temp[_BLOCKSIZE - 1].FreeNext = NULL;
1181 
1182     // Element [0] is reserved for global list linkage
1183     temp[0].set_object(CHAINMARKER);
1184 
1185     // Consider carving out this thread's current request from the
1186     // block in hand.  This avoids some lock traffic and redundant
1187     // list activity.
1188 
1189     // Acquire the gListLock to manipulate gBlockList and gFreeList.
1190     // An Oyama-Taura-Yonezawa scheme might be more efficient.
1191     Thread::muxAcquire(&gListLock, "omAlloc [2]");
1192     gMonitorPopulation += _BLOCKSIZE-1;
1193     gMonitorFreeCount += _BLOCKSIZE-1;
1194 
1195     // Add the new block to the list of extant blocks (gBlockList).
1196     // The very first objectMonitor in a block is reserved and dedicated.
1197     // It serves as blocklist "next" linkage.
1198     temp[0].FreeNext = gBlockList;
1199     // There are lock-free uses of gBlockList so make sure that
1200     // the previous stores happen before we update gBlockList.
1201     OrderAccess::release_store(&gBlockList, temp);
1202 
1203     // Add the new string of objectMonitors to the global free list
1204     temp[_BLOCKSIZE - 1].FreeNext = gFreeList;
1205     gFreeList = temp + 1;
1206     Thread::muxRelease(&gListLock);
1207   }
1208 }
1209 
1210 // Place "m" on the caller's private per-thread omFreeList.
1211 // In practice there's no need to clamp or limit the number of
1212 // monitors on a thread's omFreeList as the only time we'll call
1213 // omRelease is to return a monitor to the free list after a CAS
1214 // attempt failed.  This doesn't allow unbounded #s of monitors to
1215 // accumulate on a thread's free list.
1216 //
1217 // Key constraint: all ObjectMonitors on a thread's free list and the global
1218 // free list must have their object field set to null. This prevents the
1219 // scavenger -- deflate_idle_monitors -- from reclaiming them.
1220 
1221 void ObjectSynchronizer::omRelease(Thread * Self, ObjectMonitor * m,
1222                                    bool fromPerThreadAlloc) {
1223   guarantee(m->object() == NULL, "invariant");
1224   guarantee(((m->is_busy()|m->_recursions) == 0), "freeing in-use monitor");
1225   // Remove from omInUseList
1226   if (MonitorInUseLists && fromPerThreadAlloc) {
1227     ObjectMonitor* cur_mid_in_use = NULL;
1228     bool extracted = false;
1229     for (ObjectMonitor* mid = Self->omInUseList; mid != NULL; cur_mid_in_use = mid, mid = mid->FreeNext) {
1230       if (m == mid) {
1231         // extract from per-thread in-use list
1232         if (mid == Self->omInUseList) {
1233           Self->omInUseList = mid->FreeNext;
1234         } else if (cur_mid_in_use != NULL) {
1235           cur_mid_in_use->FreeNext = mid->FreeNext; // maintain the current thread in-use list
1236         }
1237         extracted = true;
1238         Self->omInUseCount--;
1239         break;
1240       }
1241     }
1242     assert(extracted, "Should have extracted from in-use list");
1243   }
1244 
1245   // FreeNext is used for both omInUseList and omFreeList, so clear old before setting new
1246   m->FreeNext = Self->omFreeList;
1247   Self->omFreeList = m;
1248   Self->omFreeCount++;
1249 }
1250 
1251 // Return the monitors of a moribund thread's local free list to
1252 // the global free list.  Typically a thread calls omFlush() when
1253 // it's dying.  We could also consider having the VM thread steal
1254 // monitors from threads that have not run java code over a few
1255 // consecutive STW safepoints.  Relatedly, we might decay
1256 // omFreeProvision at STW safepoints.
1257 //
1258 // Also return the monitors of a moribund thread's omInUseList to
1259 // a global gOmInUseList under the global list lock so these
1260 // will continue to be scanned.
1261 //
1262 // We currently call omFlush() from Threads::remove() _before the thread
1263 // has been excised from the thread list and is no longer a mutator.
1264 // This means that omFlush() can not run concurrently with a safepoint and
1265 // interleave with the scavenge operator. In particular, this ensures that
1266 // the thread's monitors are scanned by a GC safepoint, either via
1267 // Thread::oops_do() (if safepoint happens before omFlush()) or via
1268 // ObjectSynchronizer::oops_do() (if it happens after omFlush() and the thread's
1269 // monitors have been transferred to the global in-use list).
1270 
1271 void ObjectSynchronizer::omFlush(Thread * Self) {
1272   ObjectMonitor * list = Self->omFreeList;  // Null-terminated SLL
1273   Self->omFreeList = NULL;
1274   ObjectMonitor * tail = NULL;
1275   int tally = 0;
1276   if (list != NULL) {
1277     ObjectMonitor * s;
1278     // The thread is going away, the per-thread free monitors
1279     // are freed via set_owner(NULL)
1280     // Link them to tail, which will be linked into the global free list
1281     // gFreeList below, under the gListLock
1282     for (s = list; s != NULL; s = s->FreeNext) {
1283       tally++;
1284       tail = s;
1285       guarantee(s->object() == NULL, "invariant");
1286       guarantee(!s->is_busy(), "invariant");
1287       s->set_owner(NULL);   // redundant but good hygiene
1288     }
1289     guarantee(tail != NULL && list != NULL, "invariant");
1290   }
1291 
1292   ObjectMonitor * inUseList = Self->omInUseList;
1293   ObjectMonitor * inUseTail = NULL;
1294   int inUseTally = 0;
1295   if (inUseList != NULL) {
1296     Self->omInUseList = NULL;
1297     ObjectMonitor *cur_om;
1298     // The thread is going away, however the omInUseList inflated
1299     // monitors may still be in-use by other threads.
1300     // Link them to inUseTail, which will be linked into the global in-use list
1301     // gOmInUseList below, under the gListLock
1302     for (cur_om = inUseList; cur_om != NULL; cur_om = cur_om->FreeNext) {
1303       inUseTail = cur_om;
1304       inUseTally++;
1305     }
1306     assert(Self->omInUseCount == inUseTally, "in-use count off");
1307     Self->omInUseCount = 0;
1308     guarantee(inUseTail != NULL && inUseList != NULL, "invariant");
1309   }
1310 
1311   Thread::muxAcquire(&gListLock, "omFlush");
1312   if (tail != NULL) {
1313     tail->FreeNext = gFreeList;
1314     gFreeList = list;
1315     gMonitorFreeCount += tally;
1316     assert(Self->omFreeCount == tally, "free-count off");
1317     Self->omFreeCount = 0;
1318   }
1319 
1320   if (inUseTail != NULL) {
1321     inUseTail->FreeNext = gOmInUseList;
1322     gOmInUseList = inUseList;
1323     gOmInUseCount += inUseTally;
1324   }
1325 
1326   Thread::muxRelease(&gListLock);
1327 }
1328 
1329 static void post_monitor_inflate_event(EventJavaMonitorInflate* event,
1330                                        const oop obj,
1331                                        ObjectSynchronizer::InflateCause cause) {
1332   assert(event != NULL, "invariant");
1333   assert(event->should_commit(), "invariant");
1334   event->set_monitorClass(obj->klass());
1335   event->set_address((uintptr_t)(void*)obj);
1336   event->set_cause((u1)cause);
1337   event->commit();
1338 }
1339 
1340 // Fast path code shared by multiple functions
1341 ObjectMonitor* ObjectSynchronizer::inflate_helper(oop obj) {
1342   markOop mark = obj->mark();
1343   if (mark->has_monitor()) {
1344     assert(ObjectSynchronizer::verify_objmon_isinpool(mark->monitor()), "monitor is invalid");
1345     assert(mark->monitor()->header()->is_neutral(), "monitor must record a good object header");
1346     return mark->monitor();
1347   }
1348   return ObjectSynchronizer::inflate(Thread::current(),
1349                                      obj,
1350                                      inflate_cause_vm_internal);
1351 }
1352 
1353 ObjectMonitor* ObjectSynchronizer::inflate(Thread * Self,
1354                                                      oop object,
1355                                                      const InflateCause cause) {
1356 
1357   // Inflate mutates the heap ...
1358   // Relaxing assertion for bug 6320749.
1359   assert(Universe::verify_in_progress() ||
1360          !SafepointSynchronize::is_at_safepoint(), "invariant");
1361 
1362   EventJavaMonitorInflate event;
1363 
1364   for (;;) {
1365     const markOop mark = object->mark();
1366     assert(!mark->has_bias_pattern(), "invariant");
1367 
1368     // The mark can be in one of the following states:
1369     // *  Inflated     - just return
1370     // *  Stack-locked - coerce it to inflated
1371     // *  INFLATING    - busy wait for conversion to complete
1372     // *  Neutral      - aggressively inflate the object.
1373     // *  BIASED       - Illegal.  We should never see this
1374 
1375     // CASE: inflated
1376     if (mark->has_monitor()) {
1377       ObjectMonitor * inf = mark->monitor();
1378       assert(inf->header()->is_neutral(), "invariant");
1379       assert(oopDesc::equals((oop) inf->object(), object), "invariant");
1380       assert(ObjectSynchronizer::verify_objmon_isinpool(inf), "monitor is invalid");
1381       return inf;
1382     }
1383 
1384     // CASE: inflation in progress - inflating over a stack-lock.
1385     // Some other thread is converting from stack-locked to inflated.
1386     // Only that thread can complete inflation -- other threads must wait.
1387     // The INFLATING value is transient.
1388     // Currently, we spin/yield/park and poll the markword, waiting for inflation to finish.
1389     // We could always eliminate polling by parking the thread on some auxiliary list.
1390     if (mark == markOopDesc::INFLATING()) {
1391       ReadStableMark(object);
1392       continue;
1393     }
1394 
1395     // CASE: stack-locked
1396     // Could be stack-locked either by this thread or by some other thread.
1397     //
1398     // Note that we allocate the objectmonitor speculatively, _before_ attempting
1399     // to install INFLATING into the mark word.  We originally installed INFLATING,
1400     // allocated the objectmonitor, and then finally STed the address of the
1401     // objectmonitor into the mark.  This was correct, but artificially lengthened
1402     // the interval in which INFLATED appeared in the mark, thus increasing
1403     // the odds of inflation contention.
1404     //
1405     // We now use per-thread private objectmonitor free lists.
1406     // These list are reprovisioned from the global free list outside the
1407     // critical INFLATING...ST interval.  A thread can transfer
1408     // multiple objectmonitors en-mass from the global free list to its local free list.
1409     // This reduces coherency traffic and lock contention on the global free list.
1410     // Using such local free lists, it doesn't matter if the omAlloc() call appears
1411     // before or after the CAS(INFLATING) operation.
1412     // See the comments in omAlloc().
1413 
1414     if (mark->has_locker()) {
1415       ObjectMonitor * m = omAlloc(Self);
1416       // Optimistically prepare the objectmonitor - anticipate successful CAS
1417       // We do this before the CAS in order to minimize the length of time
1418       // in which INFLATING appears in the mark.
1419       m->Recycle();
1420       m->_Responsible  = NULL;
1421       m->_recursions   = 0;
1422       m->_SpinDuration = ObjectMonitor::Knob_SpinLimit;   // Consider: maintain by type/class
1423 
1424       markOop cmp = object->cas_set_mark(markOopDesc::INFLATING(), mark);
1425       if (cmp != mark) {
1426         omRelease(Self, m, true);
1427         continue;       // Interference -- just retry
1428       }
1429 
1430       // We've successfully installed INFLATING (0) into the mark-word.
1431       // This is the only case where 0 will appear in a mark-word.
1432       // Only the singular thread that successfully swings the mark-word
1433       // to 0 can perform (or more precisely, complete) inflation.
1434       //
1435       // Why do we CAS a 0 into the mark-word instead of just CASing the
1436       // mark-word from the stack-locked value directly to the new inflated state?
1437       // Consider what happens when a thread unlocks a stack-locked object.
1438       // It attempts to use CAS to swing the displaced header value from the
1439       // on-stack basiclock back into the object header.  Recall also that the
1440       // header value (hashcode, etc) can reside in (a) the object header, or
1441       // (b) a displaced header associated with the stack-lock, or (c) a displaced
1442       // header in an objectMonitor.  The inflate() routine must copy the header
1443       // value from the basiclock on the owner's stack to the objectMonitor, all
1444       // the while preserving the hashCode stability invariants.  If the owner
1445       // decides to release the lock while the value is 0, the unlock will fail
1446       // and control will eventually pass from slow_exit() to inflate.  The owner
1447       // will then spin, waiting for the 0 value to disappear.   Put another way,
1448       // the 0 causes the owner to stall if the owner happens to try to
1449       // drop the lock (restoring the header from the basiclock to the object)
1450       // while inflation is in-progress.  This protocol avoids races that might
1451       // would otherwise permit hashCode values to change or "flicker" for an object.
1452       // Critically, while object->mark is 0 mark->displaced_mark_helper() is stable.
1453       // 0 serves as a "BUSY" inflate-in-progress indicator.
1454 
1455 
1456       // fetch the displaced mark from the owner's stack.
1457       // The owner can't die or unwind past the lock while our INFLATING
1458       // object is in the mark.  Furthermore the owner can't complete
1459       // an unlock on the object, either.
1460       markOop dmw = mark->displaced_mark_helper();
1461       assert(dmw->is_neutral(), "invariant");
1462 
1463       // Setup monitor fields to proper values -- prepare the monitor
1464       m->set_header(dmw);
1465 
1466       // Optimization: if the mark->locker stack address is associated
1467       // with this thread we could simply set m->_owner = Self.
1468       // Note that a thread can inflate an object
1469       // that it has stack-locked -- as might happen in wait() -- directly
1470       // with CAS.  That is, we can avoid the xchg-NULL .... ST idiom.
1471       m->set_owner(mark->locker());
1472       m->set_object(object);
1473       // TODO-FIXME: assert BasicLock->dhw != 0.
1474 
1475       // Must preserve store ordering. The monitor state must
1476       // be stable at the time of publishing the monitor address.
1477       guarantee(object->mark() == markOopDesc::INFLATING(), "invariant");
1478       object->release_set_mark(markOopDesc::encode(m));
1479 
1480       // Hopefully the performance counters are allocated on distinct cache lines
1481       // to avoid false sharing on MP systems ...
1482       OM_PERFDATA_OP(Inflations, inc());
1483       if (log_is_enabled(Debug, monitorinflation)) {
1484         if (object->is_instance()) {
1485           ResourceMark rm;
1486           log_debug(monitorinflation)("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1487                                       p2i(object), p2i(object->mark()),
1488                                       object->klass()->external_name());
1489         }
1490       }
1491       if (event.should_commit()) {
1492         post_monitor_inflate_event(&event, object, cause);
1493       }
1494       return m;
1495     }
1496 
1497     // CASE: neutral
1498     // TODO-FIXME: for entry we currently inflate and then try to CAS _owner.
1499     // If we know we're inflating for entry it's better to inflate by swinging a
1500     // pre-locked objectMonitor pointer into the object header.   A successful
1501     // CAS inflates the object *and* confers ownership to the inflating thread.
1502     // In the current implementation we use a 2-step mechanism where we CAS()
1503     // to inflate and then CAS() again to try to swing _owner from NULL to Self.
1504     // An inflateTry() method that we could call from fast_enter() and slow_enter()
1505     // would be useful.
1506 
1507     assert(mark->is_neutral(), "invariant");
1508     ObjectMonitor * m = omAlloc(Self);
1509     // prepare m for installation - set monitor to initial state
1510     m->Recycle();
1511     m->set_header(mark);
1512     m->set_owner(NULL);
1513     m->set_object(object);
1514     m->_recursions   = 0;
1515     m->_Responsible  = NULL;
1516     m->_SpinDuration = ObjectMonitor::Knob_SpinLimit;       // consider: keep metastats by type/class
1517 
1518     if (object->cas_set_mark(markOopDesc::encode(m), mark) != mark) {
1519       m->set_object(NULL);
1520       m->set_owner(NULL);
1521       m->Recycle();
1522       omRelease(Self, m, true);
1523       m = NULL;
1524       continue;
1525       // interference - the markword changed - just retry.
1526       // The state-transitions are one-way, so there's no chance of
1527       // live-lock -- "Inflated" is an absorbing state.
1528     }
1529 
1530     // Hopefully the performance counters are allocated on distinct
1531     // cache lines to avoid false sharing on MP systems ...
1532     OM_PERFDATA_OP(Inflations, inc());
1533     if (log_is_enabled(Debug, monitorinflation)) {
1534       if (object->is_instance()) {
1535         ResourceMark rm;
1536         log_debug(monitorinflation)("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1537                                     p2i(object), p2i(object->mark()),
1538                                     object->klass()->external_name());
1539       }
1540     }
1541     if (event.should_commit()) {
1542       post_monitor_inflate_event(&event, object, cause);
1543     }
1544     return m;
1545   }
1546 }
1547 
1548 
1549 // Deflate_idle_monitors() is called at all safepoints, immediately
1550 // after all mutators are stopped, but before any objects have moved.
1551 // It traverses the list of known monitors, deflating where possible.
1552 // The scavenged monitor are returned to the monitor free list.
1553 //
1554 // Beware that we scavenge at *every* stop-the-world point.
1555 // Having a large number of monitors in-circulation negatively
1556 // impacts the performance of some applications (e.g., PointBase).
1557 // Broadly, we want to minimize the # of monitors in circulation.
1558 //
1559 // We have added a flag, MonitorInUseLists, which creates a list
1560 // of active monitors for each thread. deflate_idle_monitors()
1561 // only scans the per-thread in-use lists. omAlloc() puts all
1562 // assigned monitors on the per-thread list. deflate_idle_monitors()
1563 // returns the non-busy monitors to the global free list.
1564 // When a thread dies, omFlush() adds the list of active monitors for
1565 // that thread to a global gOmInUseList acquiring the
1566 // global list lock. deflate_idle_monitors() acquires the global
1567 // list lock to scan for non-busy monitors to the global free list.
1568 // An alternative could have used a single global in-use list. The
1569 // downside would have been the additional cost of acquiring the global list lock
1570 // for every omAlloc().
1571 //
1572 // Perversely, the heap size -- and thus the STW safepoint rate --
1573 // typically drives the scavenge rate.  Large heaps can mean infrequent GC,
1574 // which in turn can mean large(r) numbers of objectmonitors in circulation.
1575 // This is an unfortunate aspect of this design.
1576 
1577 enum ManifestConstants {
1578   ClearResponsibleAtSTW = 0
1579 };
1580 
1581 // Deflate a single monitor if not in-use
1582 // Return true if deflated, false if in-use
1583 bool ObjectSynchronizer::deflate_monitor(ObjectMonitor* mid, oop obj,
1584                                          ObjectMonitor** freeHeadp,
1585                                          ObjectMonitor** freeTailp) {
1586   bool deflated;
1587   // Normal case ... The monitor is associated with obj.
1588   guarantee(obj->mark() == markOopDesc::encode(mid), "invariant");
1589   guarantee(mid == obj->mark()->monitor(), "invariant");
1590   guarantee(mid->header()->is_neutral(), "invariant");
1591 
1592   if (mid->is_busy()) {
1593     if (ClearResponsibleAtSTW) mid->_Responsible = NULL;
1594     deflated = false;
1595   } else {
1596     // Deflate the monitor if it is no longer being used
1597     // It's idle - scavenge and return to the global free list
1598     // plain old deflation ...
1599     if (log_is_enabled(Debug, monitorinflation)) {
1600       if (obj->is_instance()) {
1601         ResourceMark rm;
1602         log_debug(monitorinflation)("Deflating object " INTPTR_FORMAT " , "
1603                                     "mark " INTPTR_FORMAT " , type %s",
1604                                     p2i(obj), p2i(obj->mark()),
1605                                     obj->klass()->external_name());
1606       }
1607     }
1608 
1609     // Restore the header back to obj
1610     obj->release_set_mark(mid->header());
1611     mid->clear();
1612 
1613     assert(mid->object() == NULL, "invariant");
1614 
1615     // Move the object to the working free list defined by freeHeadp, freeTailp
1616     if (*freeHeadp == NULL) *freeHeadp = mid;
1617     if (*freeTailp != NULL) {
1618       ObjectMonitor * prevtail = *freeTailp;
1619       assert(prevtail->FreeNext == NULL, "cleaned up deflated?");
1620       prevtail->FreeNext = mid;
1621     }
1622     *freeTailp = mid;
1623     deflated = true;
1624   }
1625   return deflated;
1626 }
1627 
1628 // Walk a given monitor list, and deflate idle monitors
1629 // The given list could be a per-thread list or a global list
1630 // Caller acquires gListLock.
1631 //
1632 // In the case of parallel processing of thread local monitor lists,
1633 // work is done by Threads::parallel_threads_do() which ensures that
1634 // each Java thread is processed by exactly one worker thread, and
1635 // thus avoid conflicts that would arise when worker threads would
1636 // process the same monitor lists concurrently.
1637 //
1638 // See also ParallelSPCleanupTask and
1639 // SafepointSynchronize::do_cleanup_tasks() in safepoint.cpp and
1640 // Threads::parallel_java_threads_do() in thread.cpp.
1641 int ObjectSynchronizer::deflate_monitor_list(ObjectMonitor** listHeadp,
1642                                              ObjectMonitor** freeHeadp,
1643                                              ObjectMonitor** freeTailp) {
1644   ObjectMonitor* mid;
1645   ObjectMonitor* next;
1646   ObjectMonitor* cur_mid_in_use = NULL;
1647   int deflated_count = 0;
1648 
1649   for (mid = *listHeadp; mid != NULL;) {
1650     oop obj = (oop) mid->object();
1651     if (obj != NULL && deflate_monitor(mid, obj, freeHeadp, freeTailp)) {
1652       // if deflate_monitor succeeded,
1653       // extract from per-thread in-use list
1654       if (mid == *listHeadp) {
1655         *listHeadp = mid->FreeNext;
1656       } else if (cur_mid_in_use != NULL) {
1657         cur_mid_in_use->FreeNext = mid->FreeNext; // maintain the current thread in-use list
1658       }
1659       next = mid->FreeNext;
1660       mid->FreeNext = NULL;  // This mid is current tail in the freeHeadp list
1661       mid = next;
1662       deflated_count++;
1663     } else {
1664       cur_mid_in_use = mid;
1665       mid = mid->FreeNext;
1666     }
1667   }
1668   return deflated_count;
1669 }
1670 
1671 void ObjectSynchronizer::prepare_deflate_idle_monitors(DeflateMonitorCounters* counters) {
1672   counters->nInuse = 0;          // currently associated with objects
1673   counters->nInCirculation = 0;  // extant
1674   counters->nScavenged = 0;      // reclaimed
1675 }
1676 
1677 void ObjectSynchronizer::deflate_idle_monitors(DeflateMonitorCounters* counters) {
1678   assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
1679   bool deflated = false;
1680 
1681   ObjectMonitor * freeHeadp = NULL;  // Local SLL of scavenged monitors
1682   ObjectMonitor * freeTailp = NULL;
1683 
1684   // Prevent omFlush from changing mids in Thread dtor's during deflation
1685   // And in case the vm thread is acquiring a lock during a safepoint
1686   // See e.g. 6320749
1687   Thread::muxAcquire(&gListLock, "scavenge - return");
1688 
1689   if (MonitorInUseLists) {
1690     // Note: the thread-local monitors lists get deflated in
1691     // a separate pass. See deflate_thread_local_monitors().
1692 
1693     // For moribund threads, scan gOmInUseList
1694     if (gOmInUseList) {
1695       counters->nInCirculation += gOmInUseCount;
1696       int deflated_count = deflate_monitor_list((ObjectMonitor **)&gOmInUseList, &freeHeadp, &freeTailp);
1697       gOmInUseCount -= deflated_count;
1698       counters->nScavenged += deflated_count;
1699       counters->nInuse += gOmInUseCount;
1700     }
1701 
1702   } else {
1703     PaddedEnd<ObjectMonitor> * block = OrderAccess::load_acquire(&gBlockList);
1704     for (; block != NULL; block = next(block)) {
1705       // Iterate over all extant monitors - Scavenge all idle monitors.
1706       assert(block->object() == CHAINMARKER, "must be a block header");
1707       counters->nInCirculation += _BLOCKSIZE;
1708       for (int i = 1; i < _BLOCKSIZE; i++) {
1709         ObjectMonitor* mid = (ObjectMonitor*)&block[i];
1710         oop obj = (oop)mid->object();
1711 
1712         if (obj == NULL) {
1713           // The monitor is not associated with an object.
1714           // The monitor should either be a thread-specific private
1715           // free list or the global free list.
1716           // obj == NULL IMPLIES mid->is_busy() == 0
1717           guarantee(!mid->is_busy(), "invariant");
1718           continue;
1719         }
1720         deflated = deflate_monitor(mid, obj, &freeHeadp, &freeTailp);
1721 
1722         if (deflated) {
1723           mid->FreeNext = NULL;
1724           counters->nScavenged++;
1725         } else {
1726           counters->nInuse++;
1727         }
1728       }
1729     }
1730   }
1731 
1732   // Move the scavenged monitors back to the global free list.
1733   if (freeHeadp != NULL) {
1734     guarantee(freeTailp != NULL && counters->nScavenged > 0, "invariant");
1735     assert(freeTailp->FreeNext == NULL, "invariant");
1736     // constant-time list splice - prepend scavenged segment to gFreeList
1737     freeTailp->FreeNext = gFreeList;
1738     gFreeList = freeHeadp;
1739   }
1740   Thread::muxRelease(&gListLock);
1741 
1742 }
1743 
1744 void ObjectSynchronizer::finish_deflate_idle_monitors(DeflateMonitorCounters* counters) {
1745   gMonitorFreeCount += counters->nScavenged;
1746 
1747   // Consider: audit gFreeList to ensure that gMonitorFreeCount and list agree.
1748 
1749   ForceMonitorScavenge = 0;    // Reset
1750 
1751   OM_PERFDATA_OP(Deflations, inc(counters->nScavenged));
1752   OM_PERFDATA_OP(MonExtant, set_value(counters->nInCirculation));
1753 
1754   // TODO: Add objectMonitor leak detection.
1755   // Audit/inventory the objectMonitors -- make sure they're all accounted for.
1756   GVars.stwRandom = os::random();
1757   GVars.stwCycle++;
1758 }
1759 
1760 void ObjectSynchronizer::deflate_thread_local_monitors(Thread* thread, DeflateMonitorCounters* counters) {
1761   assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
1762   if (!MonitorInUseLists) return;
1763 
1764   ObjectMonitor * freeHeadp = NULL;  // Local SLL of scavenged monitors
1765   ObjectMonitor * freeTailp = NULL;
1766 
1767   int deflated_count = deflate_monitor_list(thread->omInUseList_addr(), &freeHeadp, &freeTailp);
1768 
1769   Thread::muxAcquire(&gListLock, "scavenge - return");
1770 
1771   // Adjust counters
1772   counters->nInCirculation += thread->omInUseCount;
1773   thread->omInUseCount -= deflated_count;
1774   counters->nScavenged += deflated_count;
1775   counters->nInuse += thread->omInUseCount;
1776 
1777   // Move the scavenged monitors back to the global free list.
1778   if (freeHeadp != NULL) {
1779     guarantee(freeTailp != NULL && deflated_count > 0, "invariant");
1780     assert(freeTailp->FreeNext == NULL, "invariant");
1781 
1782     // constant-time list splice - prepend scavenged segment to gFreeList
1783     freeTailp->FreeNext = gFreeList;
1784     gFreeList = freeHeadp;
1785   }
1786   Thread::muxRelease(&gListLock);
1787 }
1788 
1789 // Monitor cleanup on JavaThread::exit
1790 
1791 // Iterate through monitor cache and attempt to release thread's monitors
1792 // Gives up on a particular monitor if an exception occurs, but continues
1793 // the overall iteration, swallowing the exception.
1794 class ReleaseJavaMonitorsClosure: public MonitorClosure {
1795  private:
1796   TRAPS;
1797 
1798  public:
1799   ReleaseJavaMonitorsClosure(Thread* thread) : THREAD(thread) {}
1800   void do_monitor(ObjectMonitor* mid) {
1801     if (mid->owner() == THREAD) {
1802       if (ObjectMonitor::Knob_VerifyMatch != 0) {
1803         ResourceMark rm;
1804         Handle obj(THREAD, (oop) mid->object());
1805         tty->print("INFO: unexpected locked object:");
1806         javaVFrame::print_locked_object_class_name(tty, obj, "locked");
1807         fatal("exiting JavaThread=" INTPTR_FORMAT
1808               " unexpectedly owns ObjectMonitor=" INTPTR_FORMAT,
1809               p2i(THREAD), p2i(mid));
1810       }
1811       (void)mid->complete_exit(CHECK);
1812     }
1813   }
1814 };
1815 
1816 // Release all inflated monitors owned by THREAD.  Lightweight monitors are
1817 // ignored.  This is meant to be called during JNI thread detach which assumes
1818 // all remaining monitors are heavyweight.  All exceptions are swallowed.
1819 // Scanning the extant monitor list can be time consuming.
1820 // A simple optimization is to add a per-thread flag that indicates a thread
1821 // called jni_monitorenter() during its lifetime.
1822 //
1823 // Instead of No_Savepoint_Verifier it might be cheaper to
1824 // use an idiom of the form:
1825 //   auto int tmp = SafepointSynchronize::_safepoint_counter ;
1826 //   <code that must not run at safepoint>
1827 //   guarantee (((tmp ^ _safepoint_counter) | (tmp & 1)) == 0) ;
1828 // Since the tests are extremely cheap we could leave them enabled
1829 // for normal product builds.
1830 
1831 void ObjectSynchronizer::release_monitors_owned_by_thread(TRAPS) {
1832   assert(THREAD == JavaThread::current(), "must be current Java thread");
1833   NoSafepointVerifier nsv;
1834   ReleaseJavaMonitorsClosure rjmc(THREAD);
1835   Thread::muxAcquire(&gListLock, "release_monitors_owned_by_thread");
1836   ObjectSynchronizer::monitors_iterate(&rjmc);
1837   Thread::muxRelease(&gListLock);
1838   THREAD->clear_pending_exception();
1839 }
1840 
1841 const char* ObjectSynchronizer::inflate_cause_name(const InflateCause cause) {
1842   switch (cause) {
1843     case inflate_cause_vm_internal:    return "VM Internal";
1844     case inflate_cause_monitor_enter:  return "Monitor Enter";
1845     case inflate_cause_wait:           return "Monitor Wait";
1846     case inflate_cause_notify:         return "Monitor Notify";
1847     case inflate_cause_hash_code:      return "Monitor Hash Code";
1848     case inflate_cause_jni_enter:      return "JNI Monitor Enter";
1849     case inflate_cause_jni_exit:       return "JNI Monitor Exit";
1850     default:
1851       ShouldNotReachHere();
1852   }
1853   return "Unknown";
1854 }
1855 
1856 //------------------------------------------------------------------------------
1857 // Debugging code
1858 
1859 u_char* ObjectSynchronizer::get_gvars_addr() {
1860   return (u_char*)&GVars;
1861 }
1862 
1863 u_char* ObjectSynchronizer::get_gvars_hcSequence_addr() {
1864   return (u_char*)&GVars.hcSequence;
1865 }
1866 
1867 size_t ObjectSynchronizer::get_gvars_size() {
1868   return sizeof(SharedGlobals);
1869 }
1870 
1871 u_char* ObjectSynchronizer::get_gvars_stwRandom_addr() {
1872   return (u_char*)&GVars.stwRandom;
1873 }
1874 
1875 #ifndef PRODUCT
1876 
1877 // Check if monitor belongs to the monitor cache
1878 // The list is grow-only so it's *relatively* safe to traverse
1879 // the list of extant blocks without taking a lock.
1880 
1881 int ObjectSynchronizer::verify_objmon_isinpool(ObjectMonitor *monitor) {
1882   PaddedEnd<ObjectMonitor> * block = OrderAccess::load_acquire(&gBlockList);
1883   while (block != NULL) {
1884     assert(block->object() == CHAINMARKER, "must be a block header");
1885     if (monitor > &block[0] && monitor < &block[_BLOCKSIZE]) {
1886       address mon = (address)monitor;
1887       address blk = (address)block;
1888       size_t diff = mon - blk;
1889       assert((diff % sizeof(PaddedEnd<ObjectMonitor>)) == 0, "must be aligned");
1890       return 1;
1891     }
1892     block = (PaddedEnd<ObjectMonitor> *)block->FreeNext;
1893   }
1894   return 0;
1895 }
1896 
1897 #endif