1 /*
   2  * Copyright (c) 1998, 2012, 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 "memory/resourceArea.hpp"
  28 #include "oops/markOop.hpp"
  29 #include "oops/oop.inline.hpp"
  30 #include "runtime/biasedLocking.hpp"
  31 #include "runtime/handles.inline.hpp"
  32 #include "runtime/interfaceSupport.hpp"
  33 #include "runtime/mutexLocker.hpp"
  34 #include "runtime/objectMonitor.hpp"
  35 #include "runtime/objectMonitor.inline.hpp"
  36 #include "runtime/osThread.hpp"
  37 #include "runtime/stubRoutines.hpp"
  38 #include "runtime/synchronizer.hpp"
  39 #include "utilities/dtrace.hpp"
  40 #include "utilities/events.hpp"
  41 #include "utilities/preserveException.hpp"
  42 #ifdef TARGET_OS_FAMILY_linux
  43 # include "os_linux.inline.hpp"
  44 # include "thread_linux.inline.hpp"
  45 #endif
  46 #ifdef TARGET_OS_FAMILY_solaris
  47 # include "os_solaris.inline.hpp"
  48 # include "thread_solaris.inline.hpp"
  49 #endif
  50 #ifdef TARGET_OS_FAMILY_windows
  51 # include "os_windows.inline.hpp"
  52 # include "thread_windows.inline.hpp"
  53 #endif
  54 #ifdef TARGET_OS_FAMILY_bsd
  55 # include "os_bsd.inline.hpp"
  56 # include "thread_bsd.inline.hpp"
  57 #endif
  58 
  59 #if defined(__GNUC__) && !defined(IA64)
  60   // Need to inhibit inlining for older versions of GCC to avoid build-time failures
  61   #define ATTR __attribute__((noinline))
  62 #else
  63   #define ATTR
  64 #endif
  65 
  66 // The "core" versions of monitor enter and exit reside in this file.
  67 // The interpreter and compilers contain specialized transliterated
  68 // variants of the enter-exit fast-path operations.  See i486.ad fast_lock(),
  69 // for instance.  If you make changes here, make sure to modify the
  70 // interpreter, and both C1 and C2 fast-path inline locking code emission.
  71 //
  72 //
  73 // -----------------------------------------------------------------------------
  74 
  75 #ifdef DTRACE_ENABLED
  76 
  77 // Only bother with this argument setup if dtrace is available
  78 // TODO-FIXME: probes should not fire when caller is _blocked.  assert() accordingly.
  79 
  80 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread)                           \
  81   char* bytes = NULL;                                                      \
  82   int len = 0;                                                             \
  83   jlong jtid = SharedRuntime::get_java_tid(thread);                        \
  84   Symbol* klassname = ((oop)(obj))->klass()->name();                       \
  85   if (klassname != NULL) {                                                 \
  86     bytes = (char*)klassname->bytes();                                     \
  87     len = klassname->utf8_length();                                        \
  88   }
  89 
  90 #ifndef USDT2
  91 HS_DTRACE_PROBE_DECL5(hotspot, monitor__wait,
  92   jlong, uintptr_t, char*, int, long);
  93 HS_DTRACE_PROBE_DECL4(hotspot, monitor__waited,
  94   jlong, uintptr_t, char*, int);
  95 
  96 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis)            \
  97   {                                                                        \
  98     if (DTraceMonitorProbes) {                                            \
  99       DTRACE_MONITOR_PROBE_COMMON(obj, thread);                            \
 100       HS_DTRACE_PROBE5(hotspot, monitor__wait, jtid,                       \
 101                        (monitor), bytes, len, (millis));                   \
 102     }                                                                      \
 103   }
 104 
 105 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread)                  \
 106   {                                                                        \
 107     if (DTraceMonitorProbes) {                                            \
 108       DTRACE_MONITOR_PROBE_COMMON(obj, thread);                            \
 109       HS_DTRACE_PROBE4(hotspot, monitor__##probe, jtid,                    \
 110                        (uintptr_t)(monitor), bytes, len);                  \
 111     }                                                                      \
 112   }
 113 
 114 #else /* USDT2 */
 115 
 116 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis)            \
 117   {                                                                        \
 118     if (DTraceMonitorProbes) {                                            \
 119       DTRACE_MONITOR_PROBE_COMMON(obj, thread);                            \
 120       HOTSPOT_MONITOR_WAIT(jtid,                                           \
 121                            (uintptr_t)(monitor), bytes, len, (millis));  \
 122     }                                                                      \
 123   }
 124 
 125 #define HOTSPOT_MONITOR_PROBE_waited HOTSPOT_MONITOR_PROBE_WAITED
 126 
 127 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread)                  \
 128   {                                                                        \
 129     if (DTraceMonitorProbes) {                                            \
 130       DTRACE_MONITOR_PROBE_COMMON(obj, thread);                            \
 131       HOTSPOT_MONITOR_PROBE_##probe(jtid, /* probe = waited */             \
 132                        (uintptr_t)(monitor), bytes, len);                  \
 133     }                                                                      \
 134   }
 135 
 136 #endif /* USDT2 */
 137 #else //  ndef DTRACE_ENABLED
 138 
 139 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon)    {;}
 140 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon)          {;}
 141 
 142 #endif // ndef DTRACE_ENABLED
 143 
 144 // This exists only as a workaround of dtrace bug 6254741
 145 int dtrace_waited_probe(ObjectMonitor* monitor, Handle obj, Thread* thr) {
 146   DTRACE_MONITOR_PROBE(waited, monitor, obj(), thr);
 147   return 0;
 148 }
 149 
 150 #define NINFLATIONLOCKS 256
 151 static volatile intptr_t InflationLocks [NINFLATIONLOCKS] ;
 152 
 153 ObjectMonitor * ObjectSynchronizer::gBlockList = NULL ;
 154 ObjectMonitor * volatile ObjectSynchronizer::gFreeList  = NULL ;
 155 ObjectMonitor * volatile ObjectSynchronizer::gOmInUseList  = NULL ;
 156 int ObjectSynchronizer::gOmInUseCount = 0;
 157 static volatile intptr_t ListLock = 0 ;      // protects global monitor free-list cache
 158 static volatile int MonitorFreeCount  = 0 ;      // # on gFreeList
 159 static volatile int MonitorPopulation = 0 ;      // # Extant -- in circulation
 160 #define CHAINMARKER ((oop)-1)
 161 
 162 // -----------------------------------------------------------------------------
 163 //  Fast Monitor Enter/Exit
 164 // This the fast monitor enter. The interpreter and compiler use
 165 // some assembly copies of this code. Make sure update those code
 166 // if the following function is changed. The implementation is
 167 // extremely sensitive to race condition. Be careful.
 168 
 169 void ObjectSynchronizer::fast_enter(Handle obj, BasicLock* lock, bool attempt_rebias, TRAPS) {
 170  if (UseBiasedLocking) {
 171     if (!SafepointSynchronize::is_at_safepoint()) {
 172       BiasedLocking::Condition cond = BiasedLocking::revoke_and_rebias(obj, attempt_rebias, THREAD);
 173       if (cond == BiasedLocking::BIAS_REVOKED_AND_REBIASED) {
 174         return;
 175       }
 176     } else {
 177       assert(!attempt_rebias, "can not rebias toward VM thread");
 178       BiasedLocking::revoke_at_safepoint(obj);
 179     }
 180     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
 181  }
 182 
 183  slow_enter (obj, lock, THREAD) ;
 184 }
 185 
 186 void ObjectSynchronizer::fast_exit(oop object, BasicLock* lock, TRAPS) {
 187   assert(!object->mark()->has_bias_pattern(), "should not see bias pattern here");
 188   // if displaced header is null, the previous enter is recursive enter, no-op
 189   markOop dhw = lock->displaced_header();
 190   markOop mark ;
 191   if (dhw == NULL) {
 192      // Recursive stack-lock.
 193      // Diagnostics -- Could be: stack-locked, inflating, inflated.
 194      mark = object->mark() ;
 195      assert (!mark->is_neutral(), "invariant") ;
 196      if (mark->has_locker() && mark != markOopDesc::INFLATING()) {
 197         assert(THREAD->is_lock_owned((address)mark->locker()), "invariant") ;
 198      }
 199      if (mark->has_monitor()) {
 200         ObjectMonitor * m = mark->monitor() ;
 201         assert(((oop)(m->object()))->mark() == mark, "invariant") ;
 202         assert(m->is_entered(THREAD), "invariant") ;
 203      }
 204      return ;
 205   }
 206 
 207   mark = object->mark() ;
 208 
 209   // If the object is stack-locked by the current thread, try to
 210   // swing the displaced header from the box back to the mark.
 211   if (mark == (markOop) lock) {
 212      assert (dhw->is_neutral(), "invariant") ;
 213      if ((markOop) Atomic::cmpxchg_ptr (dhw, object->mark_addr(), mark) == mark) {
 214         TEVENT (fast_exit: release stacklock) ;
 215         return;
 216      }
 217   }
 218 
 219   ObjectSynchronizer::inflate(THREAD, object)->exit (THREAD) ;
 220 }
 221 
 222 // -----------------------------------------------------------------------------
 223 // Interpreter/Compiler Slow Case
 224 // This routine is used to handle interpreter/compiler slow case
 225 // We don't need to use fast path here, because it must have been
 226 // failed in the interpreter/compiler code.
 227 void ObjectSynchronizer::slow_enter(Handle obj, BasicLock* lock, TRAPS) {
 228   markOop mark = obj->mark();
 229   assert(!mark->has_bias_pattern(), "should not see bias pattern here");
 230 
 231   if (mark->is_neutral()) {
 232     // Anticipate successful CAS -- the ST of the displaced mark must
 233     // be visible <= the ST performed by the CAS.
 234     lock->set_displaced_header(mark);
 235     if (mark == (markOop) Atomic::cmpxchg_ptr(lock, obj()->mark_addr(), mark)) {
 236       TEVENT (slow_enter: release stacklock) ;
 237       return ;
 238     }
 239     // Fall through to inflate() ...
 240   } else
 241   if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
 242     assert(lock != mark->locker(), "must not re-lock the same lock");
 243     assert(lock != (BasicLock*)obj->mark(), "don't relock with same BasicLock");
 244     lock->set_displaced_header(NULL);
 245     return;
 246   }
 247 
 248 #if 0
 249   // The following optimization isn't particularly useful.
 250   if (mark->has_monitor() && mark->monitor()->is_entered(THREAD)) {
 251     lock->set_displaced_header (NULL) ;
 252     return ;
 253   }
 254 #endif
 255 
 256   // The object header will never be displaced to this lock,
 257   // so it does not matter what the value is, except that it
 258   // must be non-zero to avoid looking like a re-entrant lock,
 259   // and must not look locked either.
 260   lock->set_displaced_header(markOopDesc::unused_mark());
 261   ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);
 262 }
 263 
 264 // This routine is used to handle interpreter/compiler slow case
 265 // We don't need to use fast path here, because it must have
 266 // failed in the interpreter/compiler code. Simply use the heavy
 267 // weight monitor should be ok, unless someone find otherwise.
 268 void ObjectSynchronizer::slow_exit(oop object, BasicLock* lock, TRAPS) {
 269   fast_exit (object, lock, THREAD) ;
 270 }
 271 
 272 // -----------------------------------------------------------------------------
 273 // Class Loader  support to workaround deadlocks on the class loader lock objects
 274 // Also used by GC
 275 // complete_exit()/reenter() are used to wait on a nested lock
 276 // i.e. to give up an outer lock completely and then re-enter
 277 // Used when holding nested locks - lock acquisition order: lock1 then lock2
 278 //  1) complete_exit lock1 - saving recursion count
 279 //  2) wait on lock2
 280 //  3) when notified on lock2, unlock lock2
 281 //  4) reenter lock1 with original recursion count
 282 //  5) lock lock2
 283 // NOTE: must use heavy weight monitor to handle complete_exit/reenter()
 284 intptr_t ObjectSynchronizer::complete_exit(Handle obj, TRAPS) {
 285   TEVENT (complete_exit) ;
 286   if (UseBiasedLocking) {
 287     BiasedLocking::revoke_and_rebias(obj, false, THREAD);
 288     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
 289   }
 290 
 291   ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
 292 
 293   return monitor->complete_exit(THREAD);
 294 }
 295 
 296 // NOTE: must use heavy weight monitor to handle complete_exit/reenter()
 297 void ObjectSynchronizer::reenter(Handle obj, intptr_t recursion, TRAPS) {
 298   TEVENT (reenter) ;
 299   if (UseBiasedLocking) {
 300     BiasedLocking::revoke_and_rebias(obj, false, THREAD);
 301     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
 302   }
 303 
 304   ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
 305 
 306   monitor->reenter(recursion, THREAD);
 307 }
 308 // -----------------------------------------------------------------------------
 309 // JNI locks on java objects
 310 // NOTE: must use heavy weight monitor to handle jni monitor enter
 311 void ObjectSynchronizer::jni_enter(Handle obj, TRAPS) { // possible entry from jni enter
 312   // the current locking is from JNI instead of Java code
 313   TEVENT (jni_enter) ;
 314   if (UseBiasedLocking) {
 315     BiasedLocking::revoke_and_rebias(obj, false, THREAD);
 316     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
 317   }
 318   THREAD->set_current_pending_monitor_is_from_java(false);
 319   ObjectSynchronizer::inflate(THREAD, obj())->enter(THREAD);
 320   THREAD->set_current_pending_monitor_is_from_java(true);
 321 }
 322 
 323 // NOTE: must use heavy weight monitor to handle jni monitor enter
 324 bool ObjectSynchronizer::jni_try_enter(Handle obj, Thread* THREAD) {
 325   if (UseBiasedLocking) {
 326     BiasedLocking::revoke_and_rebias(obj, false, THREAD);
 327     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
 328   }
 329 
 330   ObjectMonitor* monitor = ObjectSynchronizer::inflate_helper(obj());
 331   return monitor->try_enter(THREAD);
 332 }
 333 
 334 
 335 // NOTE: must use heavy weight monitor to handle jni monitor exit
 336 void ObjectSynchronizer::jni_exit(oop obj, Thread* THREAD) {
 337   TEVENT (jni_exit) ;
 338   if (UseBiasedLocking) {
 339     BiasedLocking::revoke_and_rebias(obj, false, THREAD);
 340   }
 341   assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
 342 
 343   ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj);
 344   // If this thread has locked the object, exit the monitor.  Note:  can't use
 345   // monitor->check(CHECK); must exit even if an exception is pending.
 346   if (monitor->check(THREAD)) {
 347      monitor->exit(THREAD);
 348   }
 349 }
 350 
 351 // -----------------------------------------------------------------------------
 352 // Internal VM locks on java objects
 353 // standard constructor, allows locking failures
 354 ObjectLocker::ObjectLocker(Handle obj, Thread* thread, bool doLock) {
 355   _dolock = doLock;
 356   _thread = thread;
 357   debug_only(if (StrictSafepointChecks) _thread->check_for_valid_safepoint_state(false);)
 358   _obj = obj;
 359 
 360   if (_dolock) {
 361     TEVENT (ObjectLocker) ;
 362 
 363     ObjectSynchronizer::fast_enter(_obj, &_lock, false, _thread);
 364   }
 365 }
 366 
 367 ObjectLocker::~ObjectLocker() {
 368   if (_dolock) {
 369     ObjectSynchronizer::fast_exit(_obj(), &_lock, _thread);
 370   }
 371 }
 372 
 373 
 374 // -----------------------------------------------------------------------------
 375 //  Wait/Notify/NotifyAll
 376 // NOTE: must use heavy weight monitor to handle wait()
 377 void ObjectSynchronizer::wait(Handle obj, jlong millis, TRAPS) {
 378   if (UseBiasedLocking) {
 379     BiasedLocking::revoke_and_rebias(obj, false, THREAD);
 380     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
 381   }
 382   if (millis < 0) {
 383     TEVENT (wait - throw IAX) ;
 384     THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
 385   }
 386   ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD, obj());
 387   DTRACE_MONITOR_WAIT_PROBE(monitor, obj(), THREAD, millis);
 388   monitor->wait(millis, true, THREAD);
 389 
 390   /* This dummy call is in place to get around dtrace bug 6254741.  Once
 391      that's fixed we can uncomment the following line and remove the call */
 392   // DTRACE_MONITOR_PROBE(waited, monitor, obj(), THREAD);
 393   dtrace_waited_probe(monitor, obj, THREAD);
 394 }
 395 
 396 void ObjectSynchronizer::waitUninterruptibly (Handle obj, jlong millis, TRAPS) {
 397   if (UseBiasedLocking) {
 398     BiasedLocking::revoke_and_rebias(obj, false, THREAD);
 399     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
 400   }
 401   if (millis < 0) {
 402     TEVENT (wait - throw IAX) ;
 403     THROW_MSG(vmSymbols::java_lang_IllegalArgumentException(), "timeout value is negative");
 404   }
 405   ObjectSynchronizer::inflate(THREAD, obj()) -> wait(millis, false, THREAD) ;
 406 }
 407 
 408 void ObjectSynchronizer::notify(Handle obj, TRAPS) {
 409  if (UseBiasedLocking) {
 410     BiasedLocking::revoke_and_rebias(obj, false, THREAD);
 411     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
 412   }
 413 
 414   markOop mark = obj->mark();
 415   if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
 416     return;
 417   }
 418   ObjectSynchronizer::inflate(THREAD, obj())->notify(THREAD);
 419 }
 420 
 421 // NOTE: see comment of notify()
 422 void ObjectSynchronizer::notifyall(Handle obj, TRAPS) {
 423   if (UseBiasedLocking) {
 424     BiasedLocking::revoke_and_rebias(obj, false, THREAD);
 425     assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
 426   }
 427 
 428   markOop mark = obj->mark();
 429   if (mark->has_locker() && THREAD->is_lock_owned((address)mark->locker())) {
 430     return;
 431   }
 432   ObjectSynchronizer::inflate(THREAD, obj())->notifyAll(THREAD);
 433 }
 434 
 435 // -----------------------------------------------------------------------------
 436 // Hash Code handling
 437 //
 438 // Performance concern:
 439 // OrderAccess::storestore() calls release() which STs 0 into the global volatile
 440 // OrderAccess::Dummy variable.  This store is unnecessary for correctness.
 441 // Many threads STing into a common location causes considerable cache migration
 442 // or "sloshing" on large SMP system.  As such, I avoid using OrderAccess::storestore()
 443 // until it's repaired.  In some cases OrderAccess::fence() -- which incurs local
 444 // latency on the executing processor -- is a better choice as it scales on SMP
 445 // systems.  See http://blogs.sun.com/dave/entry/biased_locking_in_hotspot for a
 446 // discussion of coherency costs.  Note that all our current reference platforms
 447 // provide strong ST-ST order, so the issue is moot on IA32, x64, and SPARC.
 448 //
 449 // As a general policy we use "volatile" to control compiler-based reordering
 450 // and explicit fences (barriers) to control for architectural reordering performed
 451 // by the CPU(s) or platform.
 452 
 453 static int  MBFence (int x) { OrderAccess::fence(); return x; }
 454 
 455 struct SharedGlobals {
 456     // These are highly shared mostly-read variables.
 457     // To avoid false-sharing they need to be the sole occupants of a $ line.
 458     double padPrefix [8];
 459     volatile int stwRandom ;
 460     volatile int stwCycle ;
 461 
 462     // Hot RW variables -- Sequester to avoid false-sharing
 463     double padSuffix [16];
 464     volatile int hcSequence ;
 465     double padFinal [8] ;
 466 } ;
 467 
 468 static SharedGlobals GVars ;
 469 static int MonitorScavengeThreshold = 1000000 ;
 470 static volatile int ForceMonitorScavenge = 0 ; // Scavenge required and pending
 471 
 472 static markOop ReadStableMark (oop obj) {
 473   markOop mark = obj->mark() ;
 474   if (!mark->is_being_inflated()) {
 475     return mark ;       // normal fast-path return
 476   }
 477 
 478   int its = 0 ;
 479   for (;;) {
 480     markOop mark = obj->mark() ;
 481     if (!mark->is_being_inflated()) {
 482       return mark ;    // normal fast-path return
 483     }
 484 
 485     // The object is being inflated by some other thread.
 486     // The caller of ReadStableMark() must wait for inflation to complete.
 487     // Avoid live-lock
 488     // TODO: consider calling SafepointSynchronize::do_call_back() while
 489     // spinning to see if there's a safepoint pending.  If so, immediately
 490     // yielding or blocking would be appropriate.  Avoid spinning while
 491     // there is a safepoint pending.
 492     // TODO: add inflation contention performance counters.
 493     // TODO: restrict the aggregate number of spinners.
 494 
 495     ++its ;
 496     if (its > 10000 || !os::is_MP()) {
 497        if (its & 1) {
 498          os::NakedYield() ;
 499          TEVENT (Inflate: INFLATING - yield) ;
 500        } else {
 501          // Note that the following code attenuates the livelock problem but is not
 502          // a complete remedy.  A more complete solution would require that the inflating
 503          // thread hold the associated inflation lock.  The following code simply restricts
 504          // the number of spinners to at most one.  We'll have N-2 threads blocked
 505          // on the inflationlock, 1 thread holding the inflation lock and using
 506          // a yield/park strategy, and 1 thread in the midst of inflation.
 507          // A more refined approach would be to change the encoding of INFLATING
 508          // to allow encapsulation of a native thread pointer.  Threads waiting for
 509          // inflation to complete would use CAS to push themselves onto a singly linked
 510          // list rooted at the markword.  Once enqueued, they'd loop, checking a per-thread flag
 511          // and calling park().  When inflation was complete the thread that accomplished inflation
 512          // would detach the list and set the markword to inflated with a single CAS and
 513          // then for each thread on the list, set the flag and unpark() the thread.
 514          // This is conceptually similar to muxAcquire-muxRelease, except that muxRelease
 515          // wakes at most one thread whereas we need to wake the entire list.
 516          int ix = (intptr_t(obj) >> 5) & (NINFLATIONLOCKS-1) ;
 517          int YieldThenBlock = 0 ;
 518          assert (ix >= 0 && ix < NINFLATIONLOCKS, "invariant") ;
 519          assert ((NINFLATIONLOCKS & (NINFLATIONLOCKS-1)) == 0, "invariant") ;
 520          Thread::muxAcquire (InflationLocks + ix, "InflationLock") ;
 521          while (obj->mark() == markOopDesc::INFLATING()) {
 522            // Beware: NakedYield() is advisory and has almost no effect on some platforms
 523            // so we periodically call Self->_ParkEvent->park(1).
 524            // We use a mixed spin/yield/block mechanism.
 525            if ((YieldThenBlock++) >= 16) {
 526               Thread::current()->_ParkEvent->park(1) ;
 527            } else {
 528               os::NakedYield() ;
 529            }
 530          }
 531          Thread::muxRelease (InflationLocks + ix ) ;
 532          TEVENT (Inflate: INFLATING - yield/park) ;
 533        }
 534     } else {
 535        SpinPause() ;       // SMP-polite spinning
 536     }
 537   }
 538 }
 539 
 540 // hashCode() generation :
 541 //
 542 // Possibilities:
 543 // * MD5Digest of {obj,stwRandom}
 544 // * CRC32 of {obj,stwRandom} or any linear-feedback shift register function.
 545 // * A DES- or AES-style SBox[] mechanism
 546 // * One of the Phi-based schemes, such as:
 547 //   2654435761 = 2^32 * Phi (golden ratio)
 548 //   HashCodeValue = ((uintptr_t(obj) >> 3) * 2654435761) ^ GVars.stwRandom ;
 549 // * A variation of Marsaglia's shift-xor RNG scheme.
 550 // * (obj ^ stwRandom) is appealing, but can result
 551 //   in undesirable regularity in the hashCode values of adjacent objects
 552 //   (objects allocated back-to-back, in particular).  This could potentially
 553 //   result in hashtable collisions and reduced hashtable efficiency.
 554 //   There are simple ways to "diffuse" the middle address bits over the
 555 //   generated hashCode values:
 556 //
 557 
 558 static inline intptr_t get_next_hash(Thread * Self, oop obj) {
 559   intptr_t value = 0 ;
 560   if (hashCode == 0) {
 561      // This form uses an unguarded global Park-Miller RNG,
 562      // so it's possible for two threads to race and generate the same RNG.
 563      // On MP system we'll have lots of RW access to a global, so the
 564      // mechanism induces lots of coherency traffic.
 565      value = os::random() ;
 566   } else
 567   if (hashCode == 1) {
 568      // This variation has the property of being stable (idempotent)
 569      // between STW operations.  This can be useful in some of the 1-0
 570      // synchronization schemes.
 571      intptr_t addrBits = intptr_t(obj) >> 3 ;
 572      value = addrBits ^ (addrBits >> 5) ^ GVars.stwRandom ;
 573   } else
 574   if (hashCode == 2) {
 575      value = 1 ;            // for sensitivity testing
 576   } else
 577   if (hashCode == 3) {
 578      value = ++GVars.hcSequence ;
 579   } else
 580   if (hashCode == 4) {
 581      value = intptr_t(obj) ;
 582   } else {
 583      // Marsaglia's xor-shift scheme with thread-specific state
 584      // This is probably the best overall implementation -- we'll
 585      // likely make this the default in future releases.
 586      unsigned t = Self->_hashStateX ;
 587      t ^= (t << 11) ;
 588      Self->_hashStateX = Self->_hashStateY ;
 589      Self->_hashStateY = Self->_hashStateZ ;
 590      Self->_hashStateZ = Self->_hashStateW ;
 591      unsigned v = Self->_hashStateW ;
 592      v = (v ^ (v >> 19)) ^ (t ^ (t >> 8)) ;
 593      Self->_hashStateW = v ;
 594      value = v ;
 595   }
 596 
 597   value &= markOopDesc::hash_mask;
 598   if (value == 0) value = 0xBAD ;
 599   assert (value != markOopDesc::no_hash, "invariant") ;
 600   TEVENT (hashCode: GENERATE) ;
 601   return value;
 602 }
 603 //
 604 intptr_t ObjectSynchronizer::FastHashCode (Thread * Self, oop obj) {
 605   if (UseBiasedLocking) {
 606     // NOTE: many places throughout the JVM do not expect a safepoint
 607     // to be taken here, in particular most operations on perm gen
 608     // objects. However, we only ever bias Java instances and all of
 609     // the call sites of identity_hash that might revoke biases have
 610     // been checked to make sure they can handle a safepoint. The
 611     // added check of the bias pattern is to avoid useless calls to
 612     // thread-local storage.
 613     if (obj->mark()->has_bias_pattern()) {
 614       // Box and unbox the raw reference just in case we cause a STW safepoint.
 615       Handle hobj (Self, obj) ;
 616       // Relaxing assertion for bug 6320749.
 617       assert (Universe::verify_in_progress() ||
 618               !SafepointSynchronize::is_at_safepoint(),
 619              "biases should not be seen by VM thread here");
 620       BiasedLocking::revoke_and_rebias(hobj, false, JavaThread::current());
 621       obj = hobj() ;
 622       assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
 623     }
 624   }
 625 
 626   // hashCode() is a heap mutator ...
 627   // Relaxing assertion for bug 6320749.
 628   assert (Universe::verify_in_progress() ||
 629           !SafepointSynchronize::is_at_safepoint(), "invariant") ;
 630   assert (Universe::verify_in_progress() ||
 631           Self->is_Java_thread() , "invariant") ;
 632   assert (Universe::verify_in_progress() ||
 633          ((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ;
 634 
 635   ObjectMonitor* monitor = NULL;
 636   markOop temp, test;
 637   intptr_t hash;
 638   markOop mark = ReadStableMark (obj);
 639 
 640   // object should remain ineligible for biased locking
 641   assert (!mark->has_bias_pattern(), "invariant") ;
 642 
 643   if (mark->is_neutral()) {
 644     hash = mark->hash();              // this is a normal header
 645     if (hash) {                       // if it has hash, just return it
 646       return hash;
 647     }
 648     hash = get_next_hash(Self, obj);  // allocate a new hash code
 649     temp = mark->copy_set_hash(hash); // merge the hash code into header
 650     // use (machine word version) atomic operation to install the hash
 651     test = (markOop) Atomic::cmpxchg_ptr(temp, obj->mark_addr(), mark);
 652     if (test == mark) {
 653       return hash;
 654     }
 655     // If atomic operation failed, we must inflate the header
 656     // into heavy weight monitor. We could add more code here
 657     // for fast path, but it does not worth the complexity.
 658   } else if (mark->has_monitor()) {
 659     monitor = mark->monitor();
 660     temp = monitor->header();
 661     assert (temp->is_neutral(), "invariant") ;
 662     hash = temp->hash();
 663     if (hash) {
 664       return hash;
 665     }
 666     // Skip to the following code to reduce code size
 667   } else if (Self->is_lock_owned((address)mark->locker())) {
 668     temp = mark->displaced_mark_helper(); // this is a lightweight monitor owned
 669     assert (temp->is_neutral(), "invariant") ;
 670     hash = temp->hash();              // by current thread, check if the displaced
 671     if (hash) {                       // header contains hash code
 672       return hash;
 673     }
 674     // WARNING:
 675     //   The displaced header is strictly immutable.
 676     // It can NOT be changed in ANY cases. So we have
 677     // to inflate the header into heavyweight monitor
 678     // even the current thread owns the lock. The reason
 679     // is the BasicLock (stack slot) will be asynchronously
 680     // read by other threads during the inflate() function.
 681     // Any change to stack may not propagate to other threads
 682     // correctly.
 683   }
 684 
 685   // Inflate the monitor to set hash code
 686   monitor = ObjectSynchronizer::inflate(Self, obj);
 687   // Load displaced header and check it has hash code
 688   mark = monitor->header();
 689   assert (mark->is_neutral(), "invariant") ;
 690   hash = mark->hash();
 691   if (hash == 0) {
 692     hash = get_next_hash(Self, obj);
 693     temp = mark->copy_set_hash(hash); // merge hash code into header
 694     assert (temp->is_neutral(), "invariant") ;
 695     test = (markOop) Atomic::cmpxchg_ptr(temp, monitor, mark);
 696     if (test != mark) {
 697       // The only update to the header in the monitor (outside GC)
 698       // is install the hash code. If someone add new usage of
 699       // displaced header, please update this code
 700       hash = test->hash();
 701       assert (test->is_neutral(), "invariant") ;
 702       assert (hash != 0, "Trivial unexpected object/monitor header usage.");
 703     }
 704   }
 705   // We finally get the hash
 706   return hash;
 707 }
 708 
 709 // Deprecated -- use FastHashCode() instead.
 710 
 711 intptr_t ObjectSynchronizer::identity_hash_value_for(Handle obj) {
 712   return FastHashCode (Thread::current(), obj()) ;
 713 }
 714 
 715 
 716 bool ObjectSynchronizer::current_thread_holds_lock(JavaThread* thread,
 717                                                    Handle h_obj) {
 718   if (UseBiasedLocking) {
 719     BiasedLocking::revoke_and_rebias(h_obj, false, thread);
 720     assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
 721   }
 722 
 723   assert(thread == JavaThread::current(), "Can only be called on current thread");
 724   oop obj = h_obj();
 725 
 726   markOop mark = ReadStableMark (obj) ;
 727 
 728   // Uncontended case, header points to stack
 729   if (mark->has_locker()) {
 730     return thread->is_lock_owned((address)mark->locker());
 731   }
 732   // Contended case, header points to ObjectMonitor (tagged pointer)
 733   if (mark->has_monitor()) {
 734     ObjectMonitor* monitor = mark->monitor();
 735     return monitor->is_entered(thread) != 0 ;
 736   }
 737   // Unlocked case, header in place
 738   assert(mark->is_neutral(), "sanity check");
 739   return false;
 740 }
 741 
 742 // Be aware of this method could revoke bias of the lock object.
 743 // This method querys the ownership of the lock handle specified by 'h_obj'.
 744 // If the current thread owns the lock, it returns owner_self. If no
 745 // thread owns the lock, it returns owner_none. Otherwise, it will return
 746 // ower_other.
 747 ObjectSynchronizer::LockOwnership ObjectSynchronizer::query_lock_ownership
 748 (JavaThread *self, Handle h_obj) {
 749   // The caller must beware this method can revoke bias, and
 750   // revocation can result in a safepoint.
 751   assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
 752   assert (self->thread_state() != _thread_blocked , "invariant") ;
 753 
 754   // Possible mark states: neutral, biased, stack-locked, inflated
 755 
 756   if (UseBiasedLocking && h_obj()->mark()->has_bias_pattern()) {
 757     // CASE: biased
 758     BiasedLocking::revoke_and_rebias(h_obj, false, self);
 759     assert(!h_obj->mark()->has_bias_pattern(),
 760            "biases should be revoked by now");
 761   }
 762 
 763   assert(self == JavaThread::current(), "Can only be called on current thread");
 764   oop obj = h_obj();
 765   markOop mark = ReadStableMark (obj) ;
 766 
 767   // CASE: stack-locked.  Mark points to a BasicLock on the owner's stack.
 768   if (mark->has_locker()) {
 769     return self->is_lock_owned((address)mark->locker()) ?
 770       owner_self : owner_other;
 771   }
 772 
 773   // CASE: inflated. Mark (tagged pointer) points to an objectMonitor.
 774   // The Object:ObjectMonitor relationship is stable as long as we're
 775   // not at a safepoint.
 776   if (mark->has_monitor()) {
 777     void * owner = mark->monitor()->_owner ;
 778     if (owner == NULL) return owner_none ;
 779     return (owner == self ||
 780             self->is_lock_owned((address)owner)) ? owner_self : owner_other;
 781   }
 782 
 783   // CASE: neutral
 784   assert(mark->is_neutral(), "sanity check");
 785   return owner_none ;           // it's unlocked
 786 }
 787 
 788 // FIXME: jvmti should call this
 789 JavaThread* ObjectSynchronizer::get_lock_owner(Handle h_obj, bool doLock) {
 790   if (UseBiasedLocking) {
 791     if (SafepointSynchronize::is_at_safepoint()) {
 792       BiasedLocking::revoke_at_safepoint(h_obj);
 793     } else {
 794       BiasedLocking::revoke_and_rebias(h_obj, false, JavaThread::current());
 795     }
 796     assert(!h_obj->mark()->has_bias_pattern(), "biases should be revoked by now");
 797   }
 798 
 799   oop obj = h_obj();
 800   address owner = NULL;
 801 
 802   markOop mark = ReadStableMark (obj) ;
 803 
 804   // Uncontended case, header points to stack
 805   if (mark->has_locker()) {
 806     owner = (address) mark->locker();
 807   }
 808 
 809   // Contended case, header points to ObjectMonitor (tagged pointer)
 810   if (mark->has_monitor()) {
 811     ObjectMonitor* monitor = mark->monitor();
 812     assert(monitor != NULL, "monitor should be non-null");
 813     owner = (address) monitor->owner();
 814   }
 815 
 816   if (owner != NULL) {
 817     return Threads::owning_thread_from_monitor_owner(owner, doLock);
 818   }
 819 
 820   // Unlocked case, header in place
 821   // Cannot have assertion since this object may have been
 822   // locked by another thread when reaching here.
 823   // assert(mark->is_neutral(), "sanity check");
 824 
 825   return NULL;
 826 }
 827 // Visitors ...
 828 
 829 void ObjectSynchronizer::monitors_iterate(MonitorClosure* closure) {
 830   ObjectMonitor* block = gBlockList;
 831   ObjectMonitor* mid;
 832   while (block) {
 833     assert(block->object() == CHAINMARKER, "must be a block header");
 834     for (int i = _BLOCKSIZE - 1; i > 0; i--) {
 835       mid = block + i;
 836       oop object = (oop) mid->object();
 837       if (object != NULL) {
 838         closure->do_monitor(mid);
 839       }
 840     }
 841     block = (ObjectMonitor*) block->FreeNext;
 842   }
 843 }
 844 
 845 // Get the next block in the block list.
 846 static inline ObjectMonitor* next(ObjectMonitor* block) {
 847   assert(block->object() == CHAINMARKER, "must be a block header");
 848   block = block->FreeNext ;
 849   assert(block == NULL || block->object() == CHAINMARKER, "must be a block header");
 850   return block;
 851 }
 852 
 853 
 854 void ObjectSynchronizer::oops_do(OopClosure* f) {
 855   assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
 856   for (ObjectMonitor* block = gBlockList; block != NULL; block = next(block)) {
 857     assert(block->object() == CHAINMARKER, "must be a block header");
 858     for (int i = 1; i < _BLOCKSIZE; i++) {
 859       ObjectMonitor* mid = &block[i];
 860       if (mid->object() != NULL) {
 861         f->do_oop((oop*)mid->object_addr());
 862       }
 863     }
 864   }
 865 }
 866 
 867 
 868 // -----------------------------------------------------------------------------
 869 // ObjectMonitor Lifecycle
 870 // -----------------------
 871 // Inflation unlinks monitors from the global gFreeList and
 872 // associates them with objects.  Deflation -- which occurs at
 873 // STW-time -- disassociates idle monitors from objects.  Such
 874 // scavenged monitors are returned to the gFreeList.
 875 //
 876 // The global list is protected by ListLock.  All the critical sections
 877 // are short and operate in constant-time.
 878 //
 879 // ObjectMonitors reside in type-stable memory (TSM) and are immortal.
 880 //
 881 // Lifecycle:
 882 // --   unassigned and on the global free list
 883 // --   unassigned and on a thread's private omFreeList
 884 // --   assigned to an object.  The object is inflated and the mark refers
 885 //      to the objectmonitor.
 886 //
 887 
 888 
 889 // Constraining monitor pool growth via MonitorBound ...
 890 //
 891 // The monitor pool is grow-only.  We scavenge at STW safepoint-time, but the
 892 // the rate of scavenging is driven primarily by GC.  As such,  we can find
 893 // an inordinate number of monitors in circulation.
 894 // To avoid that scenario we can artificially induce a STW safepoint
 895 // if the pool appears to be growing past some reasonable bound.
 896 // Generally we favor time in space-time tradeoffs, but as there's no
 897 // natural back-pressure on the # of extant monitors we need to impose some
 898 // type of limit.  Beware that if MonitorBound is set to too low a value
 899 // we could just loop. In addition, if MonitorBound is set to a low value
 900 // we'll incur more safepoints, which are harmful to performance.
 901 // See also: GuaranteedSafepointInterval
 902 //
 903 // The current implementation uses asynchronous VM operations.
 904 //
 905 
 906 static void InduceScavenge (Thread * Self, const char * Whence) {
 907   // Induce STW safepoint to trim monitors
 908   // Ultimately, this results in a call to deflate_idle_monitors() in the near future.
 909   // More precisely, trigger an asynchronous STW safepoint as the number
 910   // of active monitors passes the specified threshold.
 911   // TODO: assert thread state is reasonable
 912 
 913   if (ForceMonitorScavenge == 0 && Atomic::xchg (1, &ForceMonitorScavenge) == 0) {
 914     if (ObjectMonitor::Knob_Verbose) {
 915       ::printf ("Monitor scavenge - Induced STW @%s (%d)\n", Whence, ForceMonitorScavenge) ;
 916       ::fflush(stdout) ;
 917     }
 918     // Induce a 'null' safepoint to scavenge monitors
 919     // Must VM_Operation instance be heap allocated as the op will be enqueue and posted
 920     // to the VMthread and have a lifespan longer than that of this activation record.
 921     // The VMThread will delete the op when completed.
 922     VMThread::execute (new VM_ForceAsyncSafepoint()) ;
 923 
 924     if (ObjectMonitor::Knob_Verbose) {
 925       ::printf ("Monitor scavenge - STW posted @%s (%d)\n", Whence, ForceMonitorScavenge) ;
 926       ::fflush(stdout) ;
 927     }
 928   }
 929 }
 930 /* Too slow for general assert or debug
 931 void ObjectSynchronizer::verifyInUse (Thread *Self) {
 932    ObjectMonitor* mid;
 933    int inusetally = 0;
 934    for (mid = Self->omInUseList; mid != NULL; mid = mid->FreeNext) {
 935      inusetally ++;
 936    }
 937    assert(inusetally == Self->omInUseCount, "inuse count off");
 938 
 939    int freetally = 0;
 940    for (mid = Self->omFreeList; mid != NULL; mid = mid->FreeNext) {
 941      freetally ++;
 942    }
 943    assert(freetally == Self->omFreeCount, "free count off");
 944 }
 945 */
 946 ObjectMonitor * ATTR ObjectSynchronizer::omAlloc (Thread * Self) {
 947     // A large MAXPRIVATE value reduces both list lock contention
 948     // and list coherency traffic, but also tends to increase the
 949     // number of objectMonitors in circulation as well as the STW
 950     // scavenge costs.  As usual, we lean toward time in space-time
 951     // tradeoffs.
 952     const int MAXPRIVATE = 1024 ;
 953     for (;;) {
 954         ObjectMonitor * m ;
 955 
 956         // 1: try to allocate from the thread's local omFreeList.
 957         // Threads will attempt to allocate first from their local list, then
 958         // from the global list, and only after those attempts fail will the thread
 959         // attempt to instantiate new monitors.   Thread-local free lists take
 960         // heat off the ListLock and improve allocation latency, as well as reducing
 961         // coherency traffic on the shared global list.
 962         m = Self->omFreeList ;
 963         if (m != NULL) {
 964            Self->omFreeList = m->FreeNext ;
 965            Self->omFreeCount -- ;
 966            // CONSIDER: set m->FreeNext = BAD -- diagnostic hygiene
 967            guarantee (m->object() == NULL, "invariant") ;
 968            if (MonitorInUseLists) {
 969              m->FreeNext = Self->omInUseList;
 970              Self->omInUseList = m;
 971              Self->omInUseCount ++;
 972              // verifyInUse(Self);
 973            } else {
 974              m->FreeNext = NULL;
 975            }
 976            return m ;
 977         }
 978 
 979         // 2: try to allocate from the global gFreeList
 980         // CONSIDER: use muxTry() instead of muxAcquire().
 981         // If the muxTry() fails then drop immediately into case 3.
 982         // If we're using thread-local free lists then try
 983         // to reprovision the caller's free list.
 984         if (gFreeList != NULL) {
 985             // Reprovision the thread's omFreeList.
 986             // Use bulk transfers to reduce the allocation rate and heat
 987             // on various locks.
 988             Thread::muxAcquire (&ListLock, "omAlloc") ;
 989             for (int i = Self->omFreeProvision; --i >= 0 && gFreeList != NULL; ) {
 990                 MonitorFreeCount --;
 991                 ObjectMonitor * take = gFreeList ;
 992                 gFreeList = take->FreeNext ;
 993                 guarantee (take->object() == NULL, "invariant") ;
 994                 guarantee (!take->is_busy(), "invariant") ;
 995                 take->Recycle() ;
 996                 omRelease (Self, take, false) ;
 997             }
 998             Thread::muxRelease (&ListLock) ;
 999             Self->omFreeProvision += 1 + (Self->omFreeProvision/2) ;
1000             if (Self->omFreeProvision > MAXPRIVATE ) Self->omFreeProvision = MAXPRIVATE ;
1001             TEVENT (omFirst - reprovision) ;
1002 
1003             const int mx = MonitorBound ;
1004             if (mx > 0 && (MonitorPopulation-MonitorFreeCount) > mx) {
1005               // We can't safely induce a STW safepoint from omAlloc() as our thread
1006               // state may not be appropriate for such activities and callers may hold
1007               // naked oops, so instead we defer the action.
1008               InduceScavenge (Self, "omAlloc") ;
1009             }
1010             continue;
1011         }
1012 
1013         // 3: allocate a block of new ObjectMonitors
1014         // Both the local and global free lists are empty -- resort to malloc().
1015         // In the current implementation objectMonitors are TSM - immortal.
1016         assert (_BLOCKSIZE > 1, "invariant") ;
1017         ObjectMonitor * temp = new ObjectMonitor[_BLOCKSIZE];
1018 
1019         // NOTE: (almost) no way to recover if allocation failed.
1020         // We might be able to induce a STW safepoint and scavenge enough
1021         // objectMonitors to permit progress.
1022         if (temp == NULL) {
1023             vm_exit_out_of_memory (sizeof (ObjectMonitor[_BLOCKSIZE]), "Allocate ObjectMonitors") ;
1024         }
1025 
1026         // Format the block.
1027         // initialize the linked list, each monitor points to its next
1028         // forming the single linked free list, the very first monitor
1029         // will points to next block, which forms the block list.
1030         // The trick of using the 1st element in the block as gBlockList
1031         // linkage should be reconsidered.  A better implementation would
1032         // look like: class Block { Block * next; int N; ObjectMonitor Body [N] ; }
1033 
1034         for (int i = 1; i < _BLOCKSIZE ; i++) {
1035            temp[i].FreeNext = &temp[i+1];
1036         }
1037 
1038         // terminate the last monitor as the end of list
1039         temp[_BLOCKSIZE - 1].FreeNext = NULL ;
1040 
1041         // Element [0] is reserved for global list linkage
1042         temp[0].set_object(CHAINMARKER);
1043 
1044         // Consider carving out this thread's current request from the
1045         // block in hand.  This avoids some lock traffic and redundant
1046         // list activity.
1047 
1048         // Acquire the ListLock to manipulate BlockList and FreeList.
1049         // An Oyama-Taura-Yonezawa scheme might be more efficient.
1050         Thread::muxAcquire (&ListLock, "omAlloc [2]") ;
1051         MonitorPopulation += _BLOCKSIZE-1;
1052         MonitorFreeCount += _BLOCKSIZE-1;
1053 
1054         // Add the new block to the list of extant blocks (gBlockList).
1055         // The very first objectMonitor in a block is reserved and dedicated.
1056         // It serves as blocklist "next" linkage.
1057         temp[0].FreeNext = gBlockList;
1058         gBlockList = temp;
1059 
1060         // Add the new string of objectMonitors to the global free list
1061         temp[_BLOCKSIZE - 1].FreeNext = gFreeList ;
1062         gFreeList = temp + 1;
1063         Thread::muxRelease (&ListLock) ;
1064         TEVENT (Allocate block of monitors) ;
1065     }
1066 }
1067 
1068 // Place "m" on the caller's private per-thread omFreeList.
1069 // In practice there's no need to clamp or limit the number of
1070 // monitors on a thread's omFreeList as the only time we'll call
1071 // omRelease is to return a monitor to the free list after a CAS
1072 // attempt failed.  This doesn't allow unbounded #s of monitors to
1073 // accumulate on a thread's free list.
1074 //
1075 
1076 void ObjectSynchronizer::omRelease (Thread * Self, ObjectMonitor * m, bool fromPerThreadAlloc) {
1077     guarantee (m->object() == NULL, "invariant") ;
1078 
1079     // Remove from omInUseList
1080     if (MonitorInUseLists && fromPerThreadAlloc) {
1081       ObjectMonitor* curmidinuse = NULL;
1082       for (ObjectMonitor* mid = Self->omInUseList; mid != NULL; ) {
1083        if (m == mid) {
1084          // extract from per-thread in-use-list
1085          if (mid == Self->omInUseList) {
1086            Self->omInUseList = mid->FreeNext;
1087          } else if (curmidinuse != NULL) {
1088            curmidinuse->FreeNext = mid->FreeNext; // maintain the current thread inuselist
1089          }
1090          Self->omInUseCount --;
1091          // verifyInUse(Self);
1092          break;
1093        } else {
1094          curmidinuse = mid;
1095          mid = mid->FreeNext;
1096       }
1097     }
1098   }
1099 
1100   // FreeNext is used for both onInUseList and omFreeList, so clear old before setting new
1101   m->FreeNext = Self->omFreeList ;
1102   Self->omFreeList = m ;
1103   Self->omFreeCount ++ ;
1104 }
1105 
1106 // Return the monitors of a moribund thread's local free list to
1107 // the global free list.  Typically a thread calls omFlush() when
1108 // it's dying.  We could also consider having the VM thread steal
1109 // monitors from threads that have not run java code over a few
1110 // consecutive STW safepoints.  Relatedly, we might decay
1111 // omFreeProvision at STW safepoints.
1112 //
1113 // Also return the monitors of a moribund thread"s omInUseList to
1114 // a global gOmInUseList under the global list lock so these
1115 // will continue to be scanned.
1116 //
1117 // We currently call omFlush() from the Thread:: dtor _after the thread
1118 // has been excised from the thread list and is no longer a mutator.
1119 // That means that omFlush() can run concurrently with a safepoint and
1120 // the scavenge operator.  Calling omFlush() from JavaThread::exit() might
1121 // be a better choice as we could safely reason that that the JVM is
1122 // not at a safepoint at the time of the call, and thus there could
1123 // be not inopportune interleavings between omFlush() and the scavenge
1124 // operator.
1125 
1126 void ObjectSynchronizer::omFlush (Thread * Self) {
1127     ObjectMonitor * List = Self->omFreeList ;  // Null-terminated SLL
1128     Self->omFreeList = NULL ;
1129     ObjectMonitor * Tail = NULL ;
1130     int Tally = 0;
1131     if (List != NULL) {
1132       ObjectMonitor * s ;
1133       for (s = List ; s != NULL ; s = s->FreeNext) {
1134           Tally ++ ;
1135           Tail = s ;
1136           guarantee (s->object() == NULL, "invariant") ;
1137           guarantee (!s->is_busy(), "invariant") ;
1138           s->set_owner (NULL) ;   // redundant but good hygiene
1139           TEVENT (omFlush - Move one) ;
1140       }
1141       guarantee (Tail != NULL && List != NULL, "invariant") ;
1142     }
1143 
1144     ObjectMonitor * InUseList = Self->omInUseList;
1145     ObjectMonitor * InUseTail = NULL ;
1146     int InUseTally = 0;
1147     if (InUseList != NULL) {
1148       Self->omInUseList = NULL;
1149       ObjectMonitor *curom;
1150       for (curom = InUseList; curom != NULL; curom = curom->FreeNext) {
1151         InUseTail = curom;
1152         InUseTally++;
1153       }
1154 // TODO debug
1155       assert(Self->omInUseCount == InUseTally, "inuse count off");
1156       Self->omInUseCount = 0;
1157       guarantee (InUseTail != NULL && InUseList != NULL, "invariant");
1158     }
1159 
1160     Thread::muxAcquire (&ListLock, "omFlush") ;
1161     if (Tail != NULL) {
1162       Tail->FreeNext = gFreeList ;
1163       gFreeList = List ;
1164       MonitorFreeCount += Tally;
1165     }
1166 
1167     if (InUseTail != NULL) {
1168       InUseTail->FreeNext = gOmInUseList;
1169       gOmInUseList = InUseList;
1170       gOmInUseCount += InUseTally;
1171     }
1172 
1173     Thread::muxRelease (&ListLock) ;
1174     TEVENT (omFlush) ;
1175 }
1176 
1177 // Fast path code shared by multiple functions
1178 ObjectMonitor* ObjectSynchronizer::inflate_helper(oop obj) {
1179   markOop mark = obj->mark();
1180   if (mark->has_monitor()) {
1181     assert(ObjectSynchronizer::verify_objmon_isinpool(mark->monitor()), "monitor is invalid");
1182     assert(mark->monitor()->header()->is_neutral(), "monitor must record a good object header");
1183     return mark->monitor();
1184   }
1185   return ObjectSynchronizer::inflate(Thread::current(), obj);
1186 }
1187 
1188 
1189 // Note that we could encounter some performance loss through false-sharing as
1190 // multiple locks occupy the same $ line.  Padding might be appropriate.
1191 
1192 
1193 ObjectMonitor * ATTR ObjectSynchronizer::inflate (Thread * Self, oop object) {
1194   // Inflate mutates the heap ...
1195   // Relaxing assertion for bug 6320749.
1196   assert (Universe::verify_in_progress() ||
1197           !SafepointSynchronize::is_at_safepoint(), "invariant") ;
1198 
1199   for (;;) {
1200       const markOop mark = object->mark() ;
1201       assert (!mark->has_bias_pattern(), "invariant") ;
1202 
1203       // The mark can be in one of the following states:
1204       // *  Inflated     - just return
1205       // *  Stack-locked - coerce it to inflated
1206       // *  INFLATING    - busy wait for conversion to complete
1207       // *  Neutral      - aggressively inflate the object.
1208       // *  BIASED       - Illegal.  We should never see this
1209 
1210       // CASE: inflated
1211       if (mark->has_monitor()) {
1212           ObjectMonitor * inf = mark->monitor() ;
1213           assert (inf->header()->is_neutral(), "invariant");
1214           assert (inf->object() == object, "invariant") ;
1215           assert (ObjectSynchronizer::verify_objmon_isinpool(inf), "monitor is invalid");
1216           return inf ;
1217       }
1218 
1219       // CASE: inflation in progress - inflating over a stack-lock.
1220       // Some other thread is converting from stack-locked to inflated.
1221       // Only that thread can complete inflation -- other threads must wait.
1222       // The INFLATING value is transient.
1223       // Currently, we spin/yield/park and poll the markword, waiting for inflation to finish.
1224       // We could always eliminate polling by parking the thread on some auxiliary list.
1225       if (mark == markOopDesc::INFLATING()) {
1226          TEVENT (Inflate: spin while INFLATING) ;
1227          ReadStableMark(object) ;
1228          continue ;
1229       }
1230 
1231       // CASE: stack-locked
1232       // Could be stack-locked either by this thread or by some other thread.
1233       //
1234       // Note that we allocate the objectmonitor speculatively, _before_ attempting
1235       // to install INFLATING into the mark word.  We originally installed INFLATING,
1236       // allocated the objectmonitor, and then finally STed the address of the
1237       // objectmonitor into the mark.  This was correct, but artificially lengthened
1238       // the interval in which INFLATED appeared in the mark, thus increasing
1239       // the odds of inflation contention.
1240       //
1241       // We now use per-thread private objectmonitor free lists.
1242       // These list are reprovisioned from the global free list outside the
1243       // critical INFLATING...ST interval.  A thread can transfer
1244       // multiple objectmonitors en-mass from the global free list to its local free list.
1245       // This reduces coherency traffic and lock contention on the global free list.
1246       // Using such local free lists, it doesn't matter if the omAlloc() call appears
1247       // before or after the CAS(INFLATING) operation.
1248       // See the comments in omAlloc().
1249 
1250       if (mark->has_locker()) {
1251           ObjectMonitor * m = omAlloc (Self) ;
1252           // Optimistically prepare the objectmonitor - anticipate successful CAS
1253           // We do this before the CAS in order to minimize the length of time
1254           // in which INFLATING appears in the mark.
1255           m->Recycle();
1256           m->_Responsible  = NULL ;
1257           m->OwnerIsThread = 0 ;
1258           m->_recursions   = 0 ;
1259           m->_SpinDuration = ObjectMonitor::Knob_SpinLimit ;   // Consider: maintain by type/class
1260 
1261           markOop cmp = (markOop) Atomic::cmpxchg_ptr (markOopDesc::INFLATING(), object->mark_addr(), mark) ;
1262           if (cmp != mark) {
1263              omRelease (Self, m, true) ;
1264              continue ;       // Interference -- just retry
1265           }
1266 
1267           // We've successfully installed INFLATING (0) into the mark-word.
1268           // This is the only case where 0 will appear in a mark-work.
1269           // Only the singular thread that successfully swings the mark-word
1270           // to 0 can perform (or more precisely, complete) inflation.
1271           //
1272           // Why do we CAS a 0 into the mark-word instead of just CASing the
1273           // mark-word from the stack-locked value directly to the new inflated state?
1274           // Consider what happens when a thread unlocks a stack-locked object.
1275           // It attempts to use CAS to swing the displaced header value from the
1276           // on-stack basiclock back into the object header.  Recall also that the
1277           // header value (hashcode, etc) can reside in (a) the object header, or
1278           // (b) a displaced header associated with the stack-lock, or (c) a displaced
1279           // header in an objectMonitor.  The inflate() routine must copy the header
1280           // value from the basiclock on the owner's stack to the objectMonitor, all
1281           // the while preserving the hashCode stability invariants.  If the owner
1282           // decides to release the lock while the value is 0, the unlock will fail
1283           // and control will eventually pass from slow_exit() to inflate.  The owner
1284           // will then spin, waiting for the 0 value to disappear.   Put another way,
1285           // the 0 causes the owner to stall if the owner happens to try to
1286           // drop the lock (restoring the header from the basiclock to the object)
1287           // while inflation is in-progress.  This protocol avoids races that might
1288           // would otherwise permit hashCode values to change or "flicker" for an object.
1289           // Critically, while object->mark is 0 mark->displaced_mark_helper() is stable.
1290           // 0 serves as a "BUSY" inflate-in-progress indicator.
1291 
1292 
1293           // fetch the displaced mark from the owner's stack.
1294           // The owner can't die or unwind past the lock while our INFLATING
1295           // object is in the mark.  Furthermore the owner can't complete
1296           // an unlock on the object, either.
1297           markOop dmw = mark->displaced_mark_helper() ;
1298           assert (dmw->is_neutral(), "invariant") ;
1299 
1300           // Setup monitor fields to proper values -- prepare the monitor
1301           m->set_header(dmw) ;
1302 
1303           // Optimization: if the mark->locker stack address is associated
1304           // with this thread we could simply set m->_owner = Self and
1305           // m->OwnerIsThread = 1. Note that a thread can inflate an object
1306           // that it has stack-locked -- as might happen in wait() -- directly
1307           // with CAS.  That is, we can avoid the xchg-NULL .... ST idiom.
1308           m->set_owner(mark->locker());
1309           m->set_object(object);
1310           // TODO-FIXME: assert BasicLock->dhw != 0.
1311 
1312           // Must preserve store ordering. The monitor state must
1313           // be stable at the time of publishing the monitor address.
1314           guarantee (object->mark() == markOopDesc::INFLATING(), "invariant") ;
1315           object->release_set_mark(markOopDesc::encode(m));
1316 
1317           // Hopefully the performance counters are allocated on distinct cache lines
1318           // to avoid false sharing on MP systems ...
1319           if (ObjectMonitor::_sync_Inflations != NULL) ObjectMonitor::_sync_Inflations->inc() ;
1320           TEVENT(Inflate: overwrite stacklock) ;
1321           if (TraceMonitorInflation) {
1322             if (object->is_instance()) {
1323               ResourceMark rm;
1324               tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1325                 (intptr_t) object, (intptr_t) object->mark(),
1326                 object->klass()->external_name());
1327             }
1328           }
1329           return m ;
1330       }
1331 
1332       // CASE: neutral
1333       // TODO-FIXME: for entry we currently inflate and then try to CAS _owner.
1334       // If we know we're inflating for entry it's better to inflate by swinging a
1335       // pre-locked objectMonitor pointer into the object header.   A successful
1336       // CAS inflates the object *and* confers ownership to the inflating thread.
1337       // In the current implementation we use a 2-step mechanism where we CAS()
1338       // to inflate and then CAS() again to try to swing _owner from NULL to Self.
1339       // An inflateTry() method that we could call from fast_enter() and slow_enter()
1340       // would be useful.
1341 
1342       assert (mark->is_neutral(), "invariant");
1343       ObjectMonitor * m = omAlloc (Self) ;
1344       // prepare m for installation - set monitor to initial state
1345       m->Recycle();
1346       m->set_header(mark);
1347       m->set_owner(NULL);
1348       m->set_object(object);
1349       m->OwnerIsThread = 1 ;
1350       m->_recursions   = 0 ;
1351       m->_Responsible  = NULL ;
1352       m->_SpinDuration = ObjectMonitor::Knob_SpinLimit ;       // consider: keep metastats by type/class
1353 
1354       if (Atomic::cmpxchg_ptr (markOopDesc::encode(m), object->mark_addr(), mark) != mark) {
1355           m->set_object (NULL) ;
1356           m->set_owner  (NULL) ;
1357           m->OwnerIsThread = 0 ;
1358           m->Recycle() ;
1359           omRelease (Self, m, true) ;
1360           m = NULL ;
1361           continue ;
1362           // interference - the markword changed - just retry.
1363           // The state-transitions are one-way, so there's no chance of
1364           // live-lock -- "Inflated" is an absorbing state.
1365       }
1366 
1367       // Hopefully the performance counters are allocated on distinct
1368       // cache lines to avoid false sharing on MP systems ...
1369       if (ObjectMonitor::_sync_Inflations != NULL) ObjectMonitor::_sync_Inflations->inc() ;
1370       TEVENT(Inflate: overwrite neutral) ;
1371       if (TraceMonitorInflation) {
1372         if (object->is_instance()) {
1373           ResourceMark rm;
1374           tty->print_cr("Inflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1375             (intptr_t) object, (intptr_t) object->mark(),
1376             object->klass()->external_name());
1377         }
1378       }
1379       return m ;
1380   }
1381 }
1382 
1383 // Note that we could encounter some performance loss through false-sharing as
1384 // multiple locks occupy the same $ line.  Padding might be appropriate.
1385 
1386 
1387 // Deflate_idle_monitors() is called at all safepoints, immediately
1388 // after all mutators are stopped, but before any objects have moved.
1389 // It traverses the list of known monitors, deflating where possible.
1390 // The scavenged monitor are returned to the monitor free list.
1391 //
1392 // Beware that we scavenge at *every* stop-the-world point.
1393 // Having a large number of monitors in-circulation negatively
1394 // impacts the performance of some applications (e.g., PointBase).
1395 // Broadly, we want to minimize the # of monitors in circulation.
1396 //
1397 // We have added a flag, MonitorInUseLists, which creates a list
1398 // of active monitors for each thread. deflate_idle_monitors()
1399 // only scans the per-thread inuse lists. omAlloc() puts all
1400 // assigned monitors on the per-thread list. deflate_idle_monitors()
1401 // returns the non-busy monitors to the global free list.
1402 // When a thread dies, omFlush() adds the list of active monitors for
1403 // that thread to a global gOmInUseList acquiring the
1404 // global list lock. deflate_idle_monitors() acquires the global
1405 // list lock to scan for non-busy monitors to the global free list.
1406 // An alternative could have used a single global inuse list. The
1407 // downside would have been the additional cost of acquiring the global list lock
1408 // for every omAlloc().
1409 //
1410 // Perversely, the heap size -- and thus the STW safepoint rate --
1411 // typically drives the scavenge rate.  Large heaps can mean infrequent GC,
1412 // which in turn can mean large(r) numbers of objectmonitors in circulation.
1413 // This is an unfortunate aspect of this design.
1414 //
1415 
1416 enum ManifestConstants {
1417     ClearResponsibleAtSTW   = 0,
1418     MaximumRecheckInterval  = 1000
1419 } ;
1420 
1421 // Deflate a single monitor if not in use
1422 // Return true if deflated, false if in use
1423 bool ObjectSynchronizer::deflate_monitor(ObjectMonitor* mid, oop obj,
1424                                          ObjectMonitor** FreeHeadp, ObjectMonitor** FreeTailp) {
1425   bool deflated;
1426   // Normal case ... The monitor is associated with obj.
1427   guarantee (obj->mark() == markOopDesc::encode(mid), "invariant") ;
1428   guarantee (mid == obj->mark()->monitor(), "invariant");
1429   guarantee (mid->header()->is_neutral(), "invariant");
1430 
1431   if (mid->is_busy()) {
1432      if (ClearResponsibleAtSTW) mid->_Responsible = NULL ;
1433      deflated = false;
1434   } else {
1435      // Deflate the monitor if it is no longer being used
1436      // It's idle - scavenge and return to the global free list
1437      // plain old deflation ...
1438      TEVENT (deflate_idle_monitors - scavenge1) ;
1439      if (TraceMonitorInflation) {
1440        if (obj->is_instance()) {
1441          ResourceMark rm;
1442            tty->print_cr("Deflating object " INTPTR_FORMAT " , mark " INTPTR_FORMAT " , type %s",
1443                 (intptr_t) obj, (intptr_t) obj->mark(), obj->klass()->external_name());
1444        }
1445      }
1446 
1447      // Restore the header back to obj
1448      obj->release_set_mark(mid->header());
1449      mid->clear();
1450 
1451      assert (mid->object() == NULL, "invariant") ;
1452 
1453      // Move the object to the working free list defined by FreeHead,FreeTail.
1454      if (*FreeHeadp == NULL) *FreeHeadp = mid;
1455      if (*FreeTailp != NULL) {
1456        ObjectMonitor * prevtail = *FreeTailp;
1457        assert(prevtail->FreeNext == NULL, "cleaned up deflated?"); // TODO KK
1458        prevtail->FreeNext = mid;
1459       }
1460      *FreeTailp = mid;
1461      deflated = true;
1462   }
1463   return deflated;
1464 }
1465 
1466 // Caller acquires ListLock
1467 int ObjectSynchronizer::walk_monitor_list(ObjectMonitor** listheadp,
1468                                           ObjectMonitor** FreeHeadp, ObjectMonitor** FreeTailp) {
1469   ObjectMonitor* mid;
1470   ObjectMonitor* next;
1471   ObjectMonitor* curmidinuse = NULL;
1472   int deflatedcount = 0;
1473 
1474   for (mid = *listheadp; mid != NULL; ) {
1475      oop obj = (oop) mid->object();
1476      bool deflated = false;
1477      if (obj != NULL) {
1478        deflated = deflate_monitor(mid, obj, FreeHeadp, FreeTailp);
1479      }
1480      if (deflated) {
1481        // extract from per-thread in-use-list
1482        if (mid == *listheadp) {
1483          *listheadp = mid->FreeNext;
1484        } else if (curmidinuse != NULL) {
1485          curmidinuse->FreeNext = mid->FreeNext; // maintain the current thread inuselist
1486        }
1487        next = mid->FreeNext;
1488        mid->FreeNext = NULL;  // This mid is current tail in the FreeHead list
1489        mid = next;
1490        deflatedcount++;
1491      } else {
1492        curmidinuse = mid;
1493        mid = mid->FreeNext;
1494     }
1495   }
1496   return deflatedcount;
1497 }
1498 
1499 void ObjectSynchronizer::deflate_idle_monitors() {
1500   assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
1501   int nInuse = 0 ;              // currently associated with objects
1502   int nInCirculation = 0 ;      // extant
1503   int nScavenged = 0 ;          // reclaimed
1504   bool deflated = false;
1505 
1506   ObjectMonitor * FreeHead = NULL ;  // Local SLL of scavenged monitors
1507   ObjectMonitor * FreeTail = NULL ;
1508 
1509   TEVENT (deflate_idle_monitors) ;
1510   // Prevent omFlush from changing mids in Thread dtor's during deflation
1511   // And in case the vm thread is acquiring a lock during a safepoint
1512   // See e.g. 6320749
1513   Thread::muxAcquire (&ListLock, "scavenge - return") ;
1514 
1515   if (MonitorInUseLists) {
1516     int inUse = 0;
1517     for (JavaThread* cur = Threads::first(); cur != NULL; cur = cur->next()) {
1518       nInCirculation+= cur->omInUseCount;
1519       int deflatedcount = walk_monitor_list(cur->omInUseList_addr(), &FreeHead, &FreeTail);
1520       cur->omInUseCount-= deflatedcount;
1521       // verifyInUse(cur);
1522       nScavenged += deflatedcount;
1523       nInuse += cur->omInUseCount;
1524      }
1525 
1526    // For moribund threads, scan gOmInUseList
1527    if (gOmInUseList) {
1528      nInCirculation += gOmInUseCount;
1529      int deflatedcount = walk_monitor_list((ObjectMonitor **)&gOmInUseList, &FreeHead, &FreeTail);
1530      gOmInUseCount-= deflatedcount;
1531      nScavenged += deflatedcount;
1532      nInuse += gOmInUseCount;
1533     }
1534 
1535   } else for (ObjectMonitor* block = gBlockList; block != NULL; block = next(block)) {
1536   // Iterate over all extant monitors - Scavenge all idle monitors.
1537     assert(block->object() == CHAINMARKER, "must be a block header");
1538     nInCirculation += _BLOCKSIZE ;
1539     for (int i = 1 ; i < _BLOCKSIZE; i++) {
1540       ObjectMonitor* mid = &block[i];
1541       oop obj = (oop) mid->object();
1542 
1543       if (obj == NULL) {
1544         // The monitor is not associated with an object.
1545         // The monitor should either be a thread-specific private
1546         // free list or the global free list.
1547         // obj == NULL IMPLIES mid->is_busy() == 0
1548         guarantee (!mid->is_busy(), "invariant") ;
1549         continue ;
1550       }
1551       deflated = deflate_monitor(mid, obj, &FreeHead, &FreeTail);
1552 
1553       if (deflated) {
1554         mid->FreeNext = NULL ;
1555         nScavenged ++ ;
1556       } else {
1557         nInuse ++;
1558       }
1559     }
1560   }
1561 
1562   MonitorFreeCount += nScavenged;
1563 
1564   // Consider: audit gFreeList to ensure that MonitorFreeCount and list agree.
1565 
1566   if (ObjectMonitor::Knob_Verbose) {
1567     ::printf ("Deflate: InCirc=%d InUse=%d Scavenged=%d ForceMonitorScavenge=%d : pop=%d free=%d\n",
1568         nInCirculation, nInuse, nScavenged, ForceMonitorScavenge,
1569         MonitorPopulation, MonitorFreeCount) ;
1570     ::fflush(stdout) ;
1571   }
1572 
1573   ForceMonitorScavenge = 0;    // Reset
1574 
1575   // Move the scavenged monitors back to the global free list.
1576   if (FreeHead != NULL) {
1577      guarantee (FreeTail != NULL && nScavenged > 0, "invariant") ;
1578      assert (FreeTail->FreeNext == NULL, "invariant") ;
1579      // constant-time list splice - prepend scavenged segment to gFreeList
1580      FreeTail->FreeNext = gFreeList ;
1581      gFreeList = FreeHead ;
1582   }
1583   Thread::muxRelease (&ListLock) ;
1584 
1585   if (ObjectMonitor::_sync_Deflations != NULL) ObjectMonitor::_sync_Deflations->inc(nScavenged) ;
1586   if (ObjectMonitor::_sync_MonExtant  != NULL) ObjectMonitor::_sync_MonExtant ->set_value(nInCirculation);
1587 
1588   // TODO: Add objectMonitor leak detection.
1589   // Audit/inventory the objectMonitors -- make sure they're all accounted for.
1590   GVars.stwRandom = os::random() ;
1591   GVars.stwCycle ++ ;
1592 }
1593 
1594 // Monitor cleanup on JavaThread::exit
1595 
1596 // Iterate through monitor cache and attempt to release thread's monitors
1597 // Gives up on a particular monitor if an exception occurs, but continues
1598 // the overall iteration, swallowing the exception.
1599 class ReleaseJavaMonitorsClosure: public MonitorClosure {
1600 private:
1601   TRAPS;
1602 
1603 public:
1604   ReleaseJavaMonitorsClosure(Thread* thread) : THREAD(thread) {}
1605   void do_monitor(ObjectMonitor* mid) {
1606     if (mid->owner() == THREAD) {
1607       (void)mid->complete_exit(CHECK);
1608     }
1609   }
1610 };
1611 
1612 // Release all inflated monitors owned by THREAD.  Lightweight monitors are
1613 // ignored.  This is meant to be called during JNI thread detach which assumes
1614 // all remaining monitors are heavyweight.  All exceptions are swallowed.
1615 // Scanning the extant monitor list can be time consuming.
1616 // A simple optimization is to add a per-thread flag that indicates a thread
1617 // called jni_monitorenter() during its lifetime.
1618 //
1619 // Instead of No_Savepoint_Verifier it might be cheaper to
1620 // use an idiom of the form:
1621 //   auto int tmp = SafepointSynchronize::_safepoint_counter ;
1622 //   <code that must not run at safepoint>
1623 //   guarantee (((tmp ^ _safepoint_counter) | (tmp & 1)) == 0) ;
1624 // Since the tests are extremely cheap we could leave them enabled
1625 // for normal product builds.
1626 
1627 void ObjectSynchronizer::release_monitors_owned_by_thread(TRAPS) {
1628   assert(THREAD == JavaThread::current(), "must be current Java thread");
1629   No_Safepoint_Verifier nsv ;
1630   ReleaseJavaMonitorsClosure rjmc(THREAD);
1631   Thread::muxAcquire(&ListLock, "release_monitors_owned_by_thread");
1632   ObjectSynchronizer::monitors_iterate(&rjmc);
1633   Thread::muxRelease(&ListLock);
1634   THREAD->clear_pending_exception();
1635 }
1636 
1637 //------------------------------------------------------------------------------
1638 // Non-product code
1639 
1640 #ifndef PRODUCT
1641 
1642 void ObjectSynchronizer::trace_locking(Handle locking_obj, bool is_compiled,
1643                                        bool is_method, bool is_locking) {
1644   // Don't know what to do here
1645 }
1646 
1647 // Verify all monitors in the monitor cache, the verification is weak.
1648 void ObjectSynchronizer::verify() {
1649   ObjectMonitor* block = gBlockList;
1650   ObjectMonitor* mid;
1651   while (block) {
1652     assert(block->object() == CHAINMARKER, "must be a block header");
1653     for (int i = 1; i < _BLOCKSIZE; i++) {
1654       mid = block + i;
1655       oop object = (oop) mid->object();
1656       if (object != NULL) {
1657         mid->verify();
1658       }
1659     }
1660     block = (ObjectMonitor*) block->FreeNext;
1661   }
1662 }
1663 
1664 // Check if monitor belongs to the monitor cache
1665 // The list is grow-only so it's *relatively* safe to traverse
1666 // the list of extant blocks without taking a lock.
1667 
1668 int ObjectSynchronizer::verify_objmon_isinpool(ObjectMonitor *monitor) {
1669   ObjectMonitor* block = gBlockList;
1670 
1671   while (block) {
1672     assert(block->object() == CHAINMARKER, "must be a block header");
1673     if (monitor > &block[0] && monitor < &block[_BLOCKSIZE]) {
1674       address mon = (address) monitor;
1675       address blk = (address) block;
1676       size_t diff = mon - blk;
1677       assert((diff % sizeof(ObjectMonitor)) == 0, "check");
1678       return 1;
1679     }
1680     block = (ObjectMonitor*) block->FreeNext;
1681   }
1682   return 0;
1683 }
1684 
1685 #endif