rev 51780 : imported patch syncknobs-00-base
rev 51781 : imported patch syncknobs-01-Knob_ReportSettings
rev 51782 : imported patch syncknobs-02-Knob_SpinBackOff
rev 51783 : imported patch syncknobs-03-BackOffMask
rev 51784 : imported patch syncknobs-04-Knob_ExitRelease
rev 51785 : imported patch syncknobs-05-Knob_InlineNotify
rev 51786 : imported patch syncknobs-06-Knob_Verbose
rev 51787 : imported patch syncknobs-07-Knob_VerifyInUse
rev 51788 : imported patch syncknobs-08-Knob_VerifyMatch
rev 51789 : imported patch syncknobs-09-Knob_SpinBase
rev 51790 : imported patch syncknobs-10-Knob_CASPenalty
rev 51791 : imported patch syncknobs-11-Knob_OXPenalty
rev 51792 : imported patch syncknobs-12-Knob_SpinSetSucc

   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 "jfr/jfrEvents.hpp"
  28 #include "jfr/support/jfrThreadId.hpp"
  29 #include "memory/allocation.inline.hpp"
  30 #include "memory/resourceArea.hpp"
  31 #include "oops/markOop.hpp"
  32 #include "oops/oop.inline.hpp"
  33 #include "runtime/atomic.hpp"
  34 #include "runtime/handles.inline.hpp"
  35 #include "runtime/interfaceSupport.inline.hpp"
  36 #include "runtime/mutexLocker.hpp"
  37 #include "runtime/objectMonitor.hpp"
  38 #include "runtime/objectMonitor.inline.hpp"
  39 #include "runtime/orderAccess.hpp"
  40 #include "runtime/osThread.hpp"
  41 #include "runtime/safepointMechanism.inline.hpp"
  42 #include "runtime/sharedRuntime.hpp"
  43 #include "runtime/stubRoutines.hpp"
  44 #include "runtime/thread.inline.hpp"
  45 #include "services/threadService.hpp"
  46 #include "utilities/dtrace.hpp"
  47 #include "utilities/macros.hpp"
  48 #include "utilities/preserveException.hpp"
  49 #if INCLUDE_JFR
  50 #include "jfr/support/jfrFlush.hpp"
  51 #endif
  52 
  53 #ifdef DTRACE_ENABLED
  54 
  55 // Only bother with this argument setup if dtrace is available
  56 // TODO-FIXME: probes should not fire when caller is _blocked.  assert() accordingly.
  57 
  58 
  59 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread)                           \
  60   char* bytes = NULL;                                                      \
  61   int len = 0;                                                             \
  62   jlong jtid = SharedRuntime::get_java_tid(thread);                        \
  63   Symbol* klassname = ((oop)obj)->klass()->name();                         \
  64   if (klassname != NULL) {                                                 \
  65     bytes = (char*)klassname->bytes();                                     \
  66     len = klassname->utf8_length();                                        \
  67   }
  68 
  69 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis)            \
  70   {                                                                        \
  71     if (DTraceMonitorProbes) {                                             \
  72       DTRACE_MONITOR_PROBE_COMMON(obj, thread);                            \
  73       HOTSPOT_MONITOR_WAIT(jtid,                                           \
  74                            (monitor), bytes, len, (millis));               \
  75     }                                                                      \
  76   }
  77 
  78 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
  79 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
  80 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
  81 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
  82 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
  83 
  84 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread)                  \
  85   {                                                                        \
  86     if (DTraceMonitorProbes) {                                             \
  87       DTRACE_MONITOR_PROBE_COMMON(obj, thread);                            \
  88       HOTSPOT_MONITOR_##probe(jtid,                                        \
  89                               (uintptr_t)(monitor), bytes, len);           \
  90     }                                                                      \
  91   }
  92 
  93 #else //  ndef DTRACE_ENABLED
  94 
  95 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon)    {;}
  96 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon)          {;}
  97 
  98 #endif // ndef DTRACE_ENABLED
  99 
 100 // Tunables ...
 101 // The knob* variables are effectively final.  Once set they should
 102 // never be modified hence.  Consider using __read_mostly with GCC.
 103 
 104 int ObjectMonitor::Knob_SpinLimit    = 5000;    // derived by an external tool -
 105 
 106 static int Knob_SpinSetSucc         = 1;       // spinners set the _succ field
 107 static int Knob_SpinEarly           = 1;
 108 static int Knob_SuccEnabled         = 1;       // futile wake throttling
 109 static int Knob_SuccRestrict        = 0;       // Limit successors + spinners to at-most-one
 110 static int Knob_MaxSpinners         = -1;      // Should be a function of # CPUs
 111 static int Knob_Bonus               = 100;     // spin success bonus
 112 static int Knob_BonusB              = 100;     // spin success bonus
 113 static int Knob_Penalty             = 200;     // spin failure penalty
 114 static int Knob_Poverty             = 1000;
 115 static int Knob_SpinAfterFutile     = 1;       // Spin after returning from park()
 116 static int Knob_FixedSpin           = 0;
 117 static int Knob_OState              = 3;       // Spinner checks thread state of _owner
 118 static int Knob_UsePause            = 1;
 119 static int Knob_ExitPolicy          = 0;
 120 static int Knob_PreSpin             = 10;      // 20-100 likely better
 121 static int Knob_ResetEvent          = 0;
 122 
 123 static int Knob_FastHSSEC           = 0;
 124 static int Knob_MoveNotifyee        = 2;       // notify() - disposition of notifyee
 125 static int Knob_QMode               = 0;       // EntryList-cxq policy - queue discipline
 126 static volatile int InitDone        = 0;
 127 
 128 // -----------------------------------------------------------------------------
 129 // Theory of operations -- Monitors lists, thread residency, etc:
 130 //
 131 // * A thread acquires ownership of a monitor by successfully
 132 //   CAS()ing the _owner field from null to non-null.
 133 //
 134 // * Invariant: A thread appears on at most one monitor list --
 135 //   cxq, EntryList or WaitSet -- at any one time.
 136 //
 137 // * Contending threads "push" themselves onto the cxq with CAS
 138 //   and then spin/park.
 139 //
 140 // * After a contending thread eventually acquires the lock it must
 141 //   dequeue itself from either the EntryList or the cxq.
 142 //
 143 // * The exiting thread identifies and unparks an "heir presumptive"
 144 //   tentative successor thread on the EntryList.  Critically, the
 145 //   exiting thread doesn't unlink the successor thread from the EntryList.
 146 //   After having been unparked, the wakee will recontend for ownership of
 147 //   the monitor.   The successor (wakee) will either acquire the lock or
 148 //   re-park itself.
 149 //
 150 //   Succession is provided for by a policy of competitive handoff.
 151 //   The exiting thread does _not_ grant or pass ownership to the
 152 //   successor thread.  (This is also referred to as "handoff" succession").
 153 //   Instead the exiting thread releases ownership and possibly wakes
 154 //   a successor, so the successor can (re)compete for ownership of the lock.
 155 //   If the EntryList is empty but the cxq is populated the exiting
 156 //   thread will drain the cxq into the EntryList.  It does so by
 157 //   by detaching the cxq (installing null with CAS) and folding
 158 //   the threads from the cxq into the EntryList.  The EntryList is
 159 //   doubly linked, while the cxq is singly linked because of the
 160 //   CAS-based "push" used to enqueue recently arrived threads (RATs).
 161 //
 162 // * Concurrency invariants:
 163 //
 164 //   -- only the monitor owner may access or mutate the EntryList.
 165 //      The mutex property of the monitor itself protects the EntryList
 166 //      from concurrent interference.
 167 //   -- Only the monitor owner may detach the cxq.
 168 //
 169 // * The monitor entry list operations avoid locks, but strictly speaking
 170 //   they're not lock-free.  Enter is lock-free, exit is not.
 171 //   For a description of 'Methods and apparatus providing non-blocking access
 172 //   to a resource,' see U.S. Pat. No. 7844973.
 173 //
 174 // * The cxq can have multiple concurrent "pushers" but only one concurrent
 175 //   detaching thread.  This mechanism is immune from the ABA corruption.
 176 //   More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
 177 //
 178 // * Taken together, the cxq and the EntryList constitute or form a
 179 //   single logical queue of threads stalled trying to acquire the lock.
 180 //   We use two distinct lists to improve the odds of a constant-time
 181 //   dequeue operation after acquisition (in the ::enter() epilogue) and
 182 //   to reduce heat on the list ends.  (c.f. Michael Scott's "2Q" algorithm).
 183 //   A key desideratum is to minimize queue & monitor metadata manipulation
 184 //   that occurs while holding the monitor lock -- that is, we want to
 185 //   minimize monitor lock holds times.  Note that even a small amount of
 186 //   fixed spinning will greatly reduce the # of enqueue-dequeue operations
 187 //   on EntryList|cxq.  That is, spinning relieves contention on the "inner"
 188 //   locks and monitor metadata.
 189 //
 190 //   Cxq points to the set of Recently Arrived Threads attempting entry.
 191 //   Because we push threads onto _cxq with CAS, the RATs must take the form of
 192 //   a singly-linked LIFO.  We drain _cxq into EntryList  at unlock-time when
 193 //   the unlocking thread notices that EntryList is null but _cxq is != null.
 194 //
 195 //   The EntryList is ordered by the prevailing queue discipline and
 196 //   can be organized in any convenient fashion, such as a doubly-linked list or
 197 //   a circular doubly-linked list.  Critically, we want insert and delete operations
 198 //   to operate in constant-time.  If we need a priority queue then something akin
 199 //   to Solaris' sleepq would work nicely.  Viz.,
 200 //   http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
 201 //   Queue discipline is enforced at ::exit() time, when the unlocking thread
 202 //   drains the cxq into the EntryList, and orders or reorders the threads on the
 203 //   EntryList accordingly.
 204 //
 205 //   Barring "lock barging", this mechanism provides fair cyclic ordering,
 206 //   somewhat similar to an elevator-scan.
 207 //
 208 // * The monitor synchronization subsystem avoids the use of native
 209 //   synchronization primitives except for the narrow platform-specific
 210 //   park-unpark abstraction.  See the comments in os_solaris.cpp regarding
 211 //   the semantics of park-unpark.  Put another way, this monitor implementation
 212 //   depends only on atomic operations and park-unpark.  The monitor subsystem
 213 //   manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
 214 //   underlying OS manages the READY<->RUN transitions.
 215 //
 216 // * Waiting threads reside on the WaitSet list -- wait() puts
 217 //   the caller onto the WaitSet.
 218 //
 219 // * notify() or notifyAll() simply transfers threads from the WaitSet to
 220 //   either the EntryList or cxq.  Subsequent exit() operations will
 221 //   unpark the notifyee.  Unparking a notifee in notify() is inefficient -
 222 //   it's likely the notifyee would simply impale itself on the lock held
 223 //   by the notifier.
 224 //
 225 // * An interesting alternative is to encode cxq as (List,LockByte) where
 226 //   the LockByte is 0 iff the monitor is owned.  _owner is simply an auxiliary
 227 //   variable, like _recursions, in the scheme.  The threads or Events that form
 228 //   the list would have to be aligned in 256-byte addresses.  A thread would
 229 //   try to acquire the lock or enqueue itself with CAS, but exiting threads
 230 //   could use a 1-0 protocol and simply STB to set the LockByte to 0.
 231 //   Note that is is *not* word-tearing, but it does presume that full-word
 232 //   CAS operations are coherent with intermix with STB operations.  That's true
 233 //   on most common processors.
 234 //
 235 // * See also http://blogs.sun.com/dave
 236 
 237 
 238 void* ObjectMonitor::operator new (size_t size) throw() {
 239   return AllocateHeap(size, mtInternal);
 240 }
 241 void* ObjectMonitor::operator new[] (size_t size) throw() {
 242   return operator new (size);
 243 }
 244 void ObjectMonitor::operator delete(void* p) {
 245   FreeHeap(p);
 246 }
 247 void ObjectMonitor::operator delete[] (void *p) {
 248   operator delete(p);
 249 }
 250 
 251 // -----------------------------------------------------------------------------
 252 // Enter support
 253 
 254 void ObjectMonitor::enter(TRAPS) {
 255   // The following code is ordered to check the most common cases first
 256   // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
 257   Thread * const Self = THREAD;
 258 
 259   void * cur = Atomic::cmpxchg(Self, &_owner, (void*)NULL);
 260   if (cur == NULL) {
 261     // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
 262     assert(_recursions == 0, "invariant");
 263     assert(_owner == Self, "invariant");
 264     return;
 265   }
 266 
 267   if (cur == Self) {
 268     // TODO-FIXME: check for integer overflow!  BUGID 6557169.
 269     _recursions++;
 270     return;
 271   }
 272 
 273   if (Self->is_lock_owned ((address)cur)) {
 274     assert(_recursions == 0, "internal state error");
 275     _recursions = 1;
 276     // Commute owner from a thread-specific on-stack BasicLockObject address to
 277     // a full-fledged "Thread *".
 278     _owner = Self;
 279     return;
 280   }
 281 
 282   // We've encountered genuine contention.
 283   assert(Self->_Stalled == 0, "invariant");
 284   Self->_Stalled = intptr_t(this);
 285 
 286   // Try one round of spinning *before* enqueueing Self
 287   // and before going through the awkward and expensive state
 288   // transitions.  The following spin is strictly optional ...
 289   // Note that if we acquire the monitor from an initial spin
 290   // we forgo posting JVMTI events and firing DTRACE probes.
 291   if (Knob_SpinEarly && TrySpin (Self) > 0) {
 292     assert(_owner == Self, "invariant");
 293     assert(_recursions == 0, "invariant");
 294     assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
 295     Self->_Stalled = 0;
 296     return;
 297   }
 298 
 299   assert(_owner != Self, "invariant");
 300   assert(_succ != Self, "invariant");
 301   assert(Self->is_Java_thread(), "invariant");
 302   JavaThread * jt = (JavaThread *) Self;
 303   assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
 304   assert(jt->thread_state() != _thread_blocked, "invariant");
 305   assert(this->object() != NULL, "invariant");
 306   assert(_count >= 0, "invariant");
 307 
 308   // Prevent deflation at STW-time.  See deflate_idle_monitors() and is_busy().
 309   // Ensure the object-monitor relationship remains stable while there's contention.
 310   Atomic::inc(&_count);
 311 
 312   JFR_ONLY(JfrConditionalFlushWithStacktrace<EventJavaMonitorEnter> flush(jt);)
 313   EventJavaMonitorEnter event;
 314   if (event.should_commit()) {
 315     event.set_monitorClass(((oop)this->object())->klass());
 316     event.set_address((uintptr_t)(this->object_addr()));
 317   }
 318 
 319   { // Change java thread status to indicate blocked on monitor enter.
 320     JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
 321 
 322     Self->set_current_pending_monitor(this);
 323 
 324     DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
 325     if (JvmtiExport::should_post_monitor_contended_enter()) {
 326       JvmtiExport::post_monitor_contended_enter(jt, this);
 327 
 328       // The current thread does not yet own the monitor and does not
 329       // yet appear on any queues that would get it made the successor.
 330       // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
 331       // handler cannot accidentally consume an unpark() meant for the
 332       // ParkEvent associated with this ObjectMonitor.
 333     }
 334 
 335     OSThreadContendState osts(Self->osthread());
 336     ThreadBlockInVM tbivm(jt);
 337 
 338     // TODO-FIXME: change the following for(;;) loop to straight-line code.
 339     for (;;) {
 340       jt->set_suspend_equivalent();
 341       // cleared by handle_special_suspend_equivalent_condition()
 342       // or java_suspend_self()
 343 
 344       EnterI(THREAD);
 345 
 346       if (!ExitSuspendEquivalent(jt)) break;
 347 
 348       // We have acquired the contended monitor, but while we were
 349       // waiting another thread suspended us. We don't want to enter
 350       // the monitor while suspended because that would surprise the
 351       // thread that suspended us.
 352       //
 353       _recursions = 0;
 354       _succ = NULL;
 355       exit(false, Self);
 356 
 357       jt->java_suspend_self();
 358     }
 359     Self->set_current_pending_monitor(NULL);
 360 
 361     // We cleared the pending monitor info since we've just gotten past
 362     // the enter-check-for-suspend dance and we now own the monitor free
 363     // and clear, i.e., it is no longer pending. The ThreadBlockInVM
 364     // destructor can go to a safepoint at the end of this block. If we
 365     // do a thread dump during that safepoint, then this thread will show
 366     // as having "-locked" the monitor, but the OS and java.lang.Thread
 367     // states will still report that the thread is blocked trying to
 368     // acquire it.
 369   }
 370 
 371   Atomic::dec(&_count);
 372   assert(_count >= 0, "invariant");
 373   Self->_Stalled = 0;
 374 
 375   // Must either set _recursions = 0 or ASSERT _recursions == 0.
 376   assert(_recursions == 0, "invariant");
 377   assert(_owner == Self, "invariant");
 378   assert(_succ != Self, "invariant");
 379   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
 380 
 381   // The thread -- now the owner -- is back in vm mode.
 382   // Report the glorious news via TI,DTrace and jvmstat.
 383   // The probe effect is non-trivial.  All the reportage occurs
 384   // while we hold the monitor, increasing the length of the critical
 385   // section.  Amdahl's parallel speedup law comes vividly into play.
 386   //
 387   // Another option might be to aggregate the events (thread local or
 388   // per-monitor aggregation) and defer reporting until a more opportune
 389   // time -- such as next time some thread encounters contention but has
 390   // yet to acquire the lock.  While spinning that thread could
 391   // spinning we could increment JVMStat counters, etc.
 392 
 393   DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
 394   if (JvmtiExport::should_post_monitor_contended_entered()) {
 395     JvmtiExport::post_monitor_contended_entered(jt, this);
 396 
 397     // The current thread already owns the monitor and is not going to
 398     // call park() for the remainder of the monitor enter protocol. So
 399     // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
 400     // event handler consumed an unpark() issued by the thread that
 401     // just exited the monitor.
 402   }
 403   if (event.should_commit()) {
 404     event.set_previousOwner((uintptr_t)_previous_owner_tid);
 405     event.commit();
 406   }
 407   OM_PERFDATA_OP(ContendedLockAttempts, inc());
 408 }
 409 
 410 // Caveat: TryLock() is not necessarily serializing if it returns failure.
 411 // Callers must compensate as needed.
 412 
 413 int ObjectMonitor::TryLock(Thread * Self) {
 414   void * own = _owner;
 415   if (own != NULL) return 0;
 416   if (Atomic::replace_if_null(Self, &_owner)) {
 417     // Either guarantee _recursions == 0 or set _recursions = 0.
 418     assert(_recursions == 0, "invariant");
 419     assert(_owner == Self, "invariant");
 420     return 1;
 421   }
 422   // The lock had been free momentarily, but we lost the race to the lock.
 423   // Interference -- the CAS failed.
 424   // We can either return -1 or retry.
 425   // Retry doesn't make as much sense because the lock was just acquired.
 426   return -1;
 427 }
 428 
 429 #define MAX_RECHECK_INTERVAL 1000
 430 
 431 void ObjectMonitor::EnterI(TRAPS) {
 432   Thread * const Self = THREAD;
 433   assert(Self->is_Java_thread(), "invariant");
 434   assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant");
 435 
 436   // Try the lock - TATAS
 437   if (TryLock (Self) > 0) {
 438     assert(_succ != Self, "invariant");
 439     assert(_owner == Self, "invariant");
 440     assert(_Responsible != Self, "invariant");
 441     return;
 442   }
 443 
 444   DeferredInitialize();
 445 
 446   // We try one round of spinning *before* enqueueing Self.
 447   //
 448   // If the _owner is ready but OFFPROC we could use a YieldTo()
 449   // operation to donate the remainder of this thread's quantum
 450   // to the owner.  This has subtle but beneficial affinity
 451   // effects.
 452 
 453   if (TrySpin (Self) > 0) {
 454     assert(_owner == Self, "invariant");
 455     assert(_succ != Self, "invariant");
 456     assert(_Responsible != Self, "invariant");
 457     return;
 458   }
 459 
 460   // The Spin failed -- Enqueue and park the thread ...
 461   assert(_succ != Self, "invariant");
 462   assert(_owner != Self, "invariant");
 463   assert(_Responsible != Self, "invariant");
 464 
 465   // Enqueue "Self" on ObjectMonitor's _cxq.
 466   //
 467   // Node acts as a proxy for Self.
 468   // As an aside, if were to ever rewrite the synchronization code mostly
 469   // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
 470   // Java objects.  This would avoid awkward lifecycle and liveness issues,
 471   // as well as eliminate a subset of ABA issues.
 472   // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
 473 
 474   ObjectWaiter node(Self);
 475   Self->_ParkEvent->reset();
 476   node._prev   = (ObjectWaiter *) 0xBAD;
 477   node.TState  = ObjectWaiter::TS_CXQ;
 478 
 479   // Push "Self" onto the front of the _cxq.
 480   // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
 481   // Note that spinning tends to reduce the rate at which threads
 482   // enqueue and dequeue on EntryList|cxq.
 483   ObjectWaiter * nxt;
 484   for (;;) {
 485     node._next = nxt = _cxq;
 486     if (Atomic::cmpxchg(&node, &_cxq, nxt) == nxt) break;
 487 
 488     // Interference - the CAS failed because _cxq changed.  Just retry.
 489     // As an optional optimization we retry the lock.
 490     if (TryLock (Self) > 0) {
 491       assert(_succ != Self, "invariant");
 492       assert(_owner == Self, "invariant");
 493       assert(_Responsible != Self, "invariant");
 494       return;
 495     }
 496   }
 497 
 498   // Check for cxq|EntryList edge transition to non-null.  This indicates
 499   // the onset of contention.  While contention persists exiting threads
 500   // will use a ST:MEMBAR:LD 1-1 exit protocol.  When contention abates exit
 501   // operations revert to the faster 1-0 mode.  This enter operation may interleave
 502   // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
 503   // arrange for one of the contending thread to use a timed park() operations
 504   // to detect and recover from the race.  (Stranding is form of progress failure
 505   // where the monitor is unlocked but all the contending threads remain parked).
 506   // That is, at least one of the contended threads will periodically poll _owner.
 507   // One of the contending threads will become the designated "Responsible" thread.
 508   // The Responsible thread uses a timed park instead of a normal indefinite park
 509   // operation -- it periodically wakes and checks for and recovers from potential
 510   // strandings admitted by 1-0 exit operations.   We need at most one Responsible
 511   // thread per-monitor at any given moment.  Only threads on cxq|EntryList may
 512   // be responsible for a monitor.
 513   //
 514   // Currently, one of the contended threads takes on the added role of "Responsible".
 515   // A viable alternative would be to use a dedicated "stranding checker" thread
 516   // that periodically iterated over all the threads (or active monitors) and unparked
 517   // successors where there was risk of stranding.  This would help eliminate the
 518   // timer scalability issues we see on some platforms as we'd only have one thread
 519   // -- the checker -- parked on a timer.
 520 
 521   if (nxt == NULL && _EntryList == NULL) {
 522     // Try to assume the role of responsible thread for the monitor.
 523     // CONSIDER:  ST vs CAS vs { if (Responsible==null) Responsible=Self }
 524     Atomic::replace_if_null(Self, &_Responsible);
 525   }
 526 
 527   // The lock might have been released while this thread was occupied queueing
 528   // itself onto _cxq.  To close the race and avoid "stranding" and
 529   // progress-liveness failure we must resample-retry _owner before parking.
 530   // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
 531   // In this case the ST-MEMBAR is accomplished with CAS().
 532   //
 533   // TODO: Defer all thread state transitions until park-time.
 534   // Since state transitions are heavy and inefficient we'd like
 535   // to defer the state transitions until absolutely necessary,
 536   // and in doing so avoid some transitions ...
 537 
 538   int nWakeups = 0;
 539   int recheckInterval = 1;
 540 
 541   for (;;) {
 542 
 543     if (TryLock(Self) > 0) break;
 544     assert(_owner != Self, "invariant");
 545 
 546     // park self
 547     if (_Responsible == Self) {
 548       Self->_ParkEvent->park((jlong) recheckInterval);
 549       // Increase the recheckInterval, but clamp the value.
 550       recheckInterval *= 8;
 551       if (recheckInterval > MAX_RECHECK_INTERVAL) {
 552         recheckInterval = MAX_RECHECK_INTERVAL;
 553       }
 554     } else {
 555       Self->_ParkEvent->park();
 556     }
 557 
 558     if (TryLock(Self) > 0) break;
 559 
 560     // The lock is still contested.
 561     // Keep a tally of the # of futile wakeups.
 562     // Note that the counter is not protected by a lock or updated by atomics.
 563     // That is by design - we trade "lossy" counters which are exposed to
 564     // races during updates for a lower probe effect.
 565 
 566     // This PerfData object can be used in parallel with a safepoint.
 567     // See the work around in PerfDataManager::destroy().
 568     OM_PERFDATA_OP(FutileWakeups, inc());
 569     ++nWakeups;
 570 
 571     // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
 572     // We can defer clearing _succ until after the spin completes
 573     // TrySpin() must tolerate being called with _succ == Self.
 574     // Try yet another round of adaptive spinning.
 575     if ((Knob_SpinAfterFutile & 1) && TrySpin(Self) > 0) break;
 576 
 577     // We can find that we were unpark()ed and redesignated _succ while
 578     // we were spinning.  That's harmless.  If we iterate and call park(),
 579     // park() will consume the event and return immediately and we'll
 580     // just spin again.  This pattern can repeat, leaving _succ to simply
 581     // spin on a CPU.  Enable Knob_ResetEvent to clear pending unparks().
 582     // Alternately, we can sample fired() here, and if set, forgo spinning
 583     // in the next iteration.
 584 
 585     if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
 586       Self->_ParkEvent->reset();
 587       OrderAccess::fence();
 588     }
 589     if (_succ == Self) _succ = NULL;
 590 
 591     // Invariant: after clearing _succ a thread *must* retry _owner before parking.
 592     OrderAccess::fence();
 593   }
 594 
 595   // Egress :
 596   // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
 597   // Normally we'll find Self on the EntryList .
 598   // From the perspective of the lock owner (this thread), the
 599   // EntryList is stable and cxq is prepend-only.
 600   // The head of cxq is volatile but the interior is stable.
 601   // In addition, Self.TState is stable.
 602 
 603   assert(_owner == Self, "invariant");
 604   assert(object() != NULL, "invariant");
 605   // I'd like to write:
 606   //   guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
 607   // but as we're at a safepoint that's not safe.
 608 
 609   UnlinkAfterAcquire(Self, &node);
 610   if (_succ == Self) _succ = NULL;
 611 
 612   assert(_succ != Self, "invariant");
 613   if (_Responsible == Self) {
 614     _Responsible = NULL;
 615     OrderAccess::fence(); // Dekker pivot-point
 616 
 617     // We may leave threads on cxq|EntryList without a designated
 618     // "Responsible" thread.  This is benign.  When this thread subsequently
 619     // exits the monitor it can "see" such preexisting "old" threads --
 620     // threads that arrived on the cxq|EntryList before the fence, above --
 621     // by LDing cxq|EntryList.  Newly arrived threads -- that is, threads
 622     // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
 623     // non-null and elect a new "Responsible" timer thread.
 624     //
 625     // This thread executes:
 626     //    ST Responsible=null; MEMBAR    (in enter epilogue - here)
 627     //    LD cxq|EntryList               (in subsequent exit)
 628     //
 629     // Entering threads in the slow/contended path execute:
 630     //    ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
 631     //    The (ST cxq; MEMBAR) is accomplished with CAS().
 632     //
 633     // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
 634     // exit operation from floating above the ST Responsible=null.
 635   }
 636 
 637   // We've acquired ownership with CAS().
 638   // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
 639   // But since the CAS() this thread may have also stored into _succ,
 640   // EntryList, cxq or Responsible.  These meta-data updates must be
 641   // visible __before this thread subsequently drops the lock.
 642   // Consider what could occur if we didn't enforce this constraint --
 643   // STs to monitor meta-data and user-data could reorder with (become
 644   // visible after) the ST in exit that drops ownership of the lock.
 645   // Some other thread could then acquire the lock, but observe inconsistent
 646   // or old monitor meta-data and heap data.  That violates the JMM.
 647   // To that end, the 1-0 exit() operation must have at least STST|LDST
 648   // "release" barrier semantics.  Specifically, there must be at least a
 649   // STST|LDST barrier in exit() before the ST of null into _owner that drops
 650   // the lock.   The barrier ensures that changes to monitor meta-data and data
 651   // protected by the lock will be visible before we release the lock, and
 652   // therefore before some other thread (CPU) has a chance to acquire the lock.
 653   // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
 654   //
 655   // Critically, any prior STs to _succ or EntryList must be visible before
 656   // the ST of null into _owner in the *subsequent* (following) corresponding
 657   // monitorexit.  Recall too, that in 1-0 mode monitorexit does not necessarily
 658   // execute a serializing instruction.
 659 
 660   return;
 661 }
 662 
 663 // ReenterI() is a specialized inline form of the latter half of the
 664 // contended slow-path from EnterI().  We use ReenterI() only for
 665 // monitor reentry in wait().
 666 //
 667 // In the future we should reconcile EnterI() and ReenterI(), adding
 668 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
 669 // loop accordingly.
 670 
 671 void ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) {
 672   assert(Self != NULL, "invariant");
 673   assert(SelfNode != NULL, "invariant");
 674   assert(SelfNode->_thread == Self, "invariant");
 675   assert(_waiters > 0, "invariant");
 676   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
 677   assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
 678   JavaThread * jt = (JavaThread *) Self;
 679 
 680   int nWakeups = 0;
 681   for (;;) {
 682     ObjectWaiter::TStates v = SelfNode->TState;
 683     guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
 684     assert(_owner != Self, "invariant");
 685 
 686     if (TryLock(Self) > 0) break;
 687     if (TrySpin(Self) > 0) break;
 688 
 689     // State transition wrappers around park() ...
 690     // ReenterI() wisely defers state transitions until
 691     // it's clear we must park the thread.
 692     {
 693       OSThreadContendState osts(Self->osthread());
 694       ThreadBlockInVM tbivm(jt);
 695 
 696       // cleared by handle_special_suspend_equivalent_condition()
 697       // or java_suspend_self()
 698       jt->set_suspend_equivalent();
 699       Self->_ParkEvent->park();
 700 
 701       // were we externally suspended while we were waiting?
 702       for (;;) {
 703         if (!ExitSuspendEquivalent(jt)) break;
 704         if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
 705         jt->java_suspend_self();
 706         jt->set_suspend_equivalent();
 707       }
 708     }
 709 
 710     // Try again, but just so we distinguish between futile wakeups and
 711     // successful wakeups.  The following test isn't algorithmically
 712     // necessary, but it helps us maintain sensible statistics.
 713     if (TryLock(Self) > 0) break;
 714 
 715     // The lock is still contested.
 716     // Keep a tally of the # of futile wakeups.
 717     // Note that the counter is not protected by a lock or updated by atomics.
 718     // That is by design - we trade "lossy" counters which are exposed to
 719     // races during updates for a lower probe effect.
 720     ++nWakeups;
 721 
 722     // Assuming this is not a spurious wakeup we'll normally
 723     // find that _succ == Self.
 724     if (_succ == Self) _succ = NULL;
 725 
 726     // Invariant: after clearing _succ a contending thread
 727     // *must* retry  _owner before parking.
 728     OrderAccess::fence();
 729 
 730     // This PerfData object can be used in parallel with a safepoint.
 731     // See the work around in PerfDataManager::destroy().
 732     OM_PERFDATA_OP(FutileWakeups, inc());
 733   }
 734 
 735   // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
 736   // Normally we'll find Self on the EntryList.
 737   // Unlinking from the EntryList is constant-time and atomic-free.
 738   // From the perspective of the lock owner (this thread), the
 739   // EntryList is stable and cxq is prepend-only.
 740   // The head of cxq is volatile but the interior is stable.
 741   // In addition, Self.TState is stable.
 742 
 743   assert(_owner == Self, "invariant");
 744   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
 745   UnlinkAfterAcquire(Self, SelfNode);
 746   if (_succ == Self) _succ = NULL;
 747   assert(_succ != Self, "invariant");
 748   SelfNode->TState = ObjectWaiter::TS_RUN;
 749   OrderAccess::fence();      // see comments at the end of EnterI()
 750 }
 751 
 752 // By convention we unlink a contending thread from EntryList|cxq immediately
 753 // after the thread acquires the lock in ::enter().  Equally, we could defer
 754 // unlinking the thread until ::exit()-time.
 755 
 756 void ObjectMonitor::UnlinkAfterAcquire(Thread *Self, ObjectWaiter *SelfNode) {
 757   assert(_owner == Self, "invariant");
 758   assert(SelfNode->_thread == Self, "invariant");
 759 
 760   if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
 761     // Normal case: remove Self from the DLL EntryList .
 762     // This is a constant-time operation.
 763     ObjectWaiter * nxt = SelfNode->_next;
 764     ObjectWaiter * prv = SelfNode->_prev;
 765     if (nxt != NULL) nxt->_prev = prv;
 766     if (prv != NULL) prv->_next = nxt;
 767     if (SelfNode == _EntryList) _EntryList = nxt;
 768     assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
 769     assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
 770   } else {
 771     assert(SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant");
 772     // Inopportune interleaving -- Self is still on the cxq.
 773     // This usually means the enqueue of self raced an exiting thread.
 774     // Normally we'll find Self near the front of the cxq, so
 775     // dequeueing is typically fast.  If needbe we can accelerate
 776     // this with some MCS/CHL-like bidirectional list hints and advisory
 777     // back-links so dequeueing from the interior will normally operate
 778     // in constant-time.
 779     // Dequeue Self from either the head (with CAS) or from the interior
 780     // with a linear-time scan and normal non-atomic memory operations.
 781     // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
 782     // and then unlink Self from EntryList.  We have to drain eventually,
 783     // so it might as well be now.
 784 
 785     ObjectWaiter * v = _cxq;
 786     assert(v != NULL, "invariant");
 787     if (v != SelfNode || Atomic::cmpxchg(SelfNode->_next, &_cxq, v) != v) {
 788       // The CAS above can fail from interference IFF a "RAT" arrived.
 789       // In that case Self must be in the interior and can no longer be
 790       // at the head of cxq.
 791       if (v == SelfNode) {
 792         assert(_cxq != v, "invariant");
 793         v = _cxq;          // CAS above failed - start scan at head of list
 794       }
 795       ObjectWaiter * p;
 796       ObjectWaiter * q = NULL;
 797       for (p = v; p != NULL && p != SelfNode; p = p->_next) {
 798         q = p;
 799         assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
 800       }
 801       assert(v != SelfNode, "invariant");
 802       assert(p == SelfNode, "Node not found on cxq");
 803       assert(p != _cxq, "invariant");
 804       assert(q != NULL, "invariant");
 805       assert(q->_next == p, "invariant");
 806       q->_next = p->_next;
 807     }
 808   }
 809 
 810 #ifdef ASSERT
 811   // Diagnostic hygiene ...
 812   SelfNode->_prev  = (ObjectWaiter *) 0xBAD;
 813   SelfNode->_next  = (ObjectWaiter *) 0xBAD;
 814   SelfNode->TState = ObjectWaiter::TS_RUN;
 815 #endif
 816 }
 817 
 818 // -----------------------------------------------------------------------------
 819 // Exit support
 820 //
 821 // exit()
 822 // ~~~~~~
 823 // Note that the collector can't reclaim the objectMonitor or deflate
 824 // the object out from underneath the thread calling ::exit() as the
 825 // thread calling ::exit() never transitions to a stable state.
 826 // This inhibits GC, which in turn inhibits asynchronous (and
 827 // inopportune) reclamation of "this".
 828 //
 829 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
 830 // There's one exception to the claim above, however.  EnterI() can call
 831 // exit() to drop a lock if the acquirer has been externally suspended.
 832 // In that case exit() is called with _thread_state as _thread_blocked,
 833 // but the monitor's _count field is > 0, which inhibits reclamation.
 834 //
 835 // 1-0 exit
 836 // ~~~~~~~~
 837 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
 838 // the fast-path operators have been optimized so the common ::exit()
 839 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
 840 // The code emitted by fast_unlock() elides the usual MEMBAR.  This
 841 // greatly improves latency -- MEMBAR and CAS having considerable local
 842 // latency on modern processors -- but at the cost of "stranding".  Absent the
 843 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
 844 // ::enter() path, resulting in the entering thread being stranding
 845 // and a progress-liveness failure.   Stranding is extremely rare.
 846 // We use timers (timed park operations) & periodic polling to detect
 847 // and recover from stranding.  Potentially stranded threads periodically
 848 // wake up and poll the lock.  See the usage of the _Responsible variable.
 849 //
 850 // The CAS() in enter provides for safety and exclusion, while the CAS or
 851 // MEMBAR in exit provides for progress and avoids stranding.  1-0 locking
 852 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
 853 // We detect and recover from stranding with timers.
 854 //
 855 // If a thread transiently strands it'll park until (a) another
 856 // thread acquires the lock and then drops the lock, at which time the
 857 // exiting thread will notice and unpark the stranded thread, or, (b)
 858 // the timer expires.  If the lock is high traffic then the stranding latency
 859 // will be low due to (a).  If the lock is low traffic then the odds of
 860 // stranding are lower, although the worst-case stranding latency
 861 // is longer.  Critically, we don't want to put excessive load in the
 862 // platform's timer subsystem.  We want to minimize both the timer injection
 863 // rate (timers created/sec) as well as the number of timers active at
 864 // any one time.  (more precisely, we want to minimize timer-seconds, which is
 865 // the integral of the # of active timers at any instant over time).
 866 // Both impinge on OS scalability.  Given that, at most one thread parked on
 867 // a monitor will use a timer.
 868 //
 869 // There is also the risk of a futile wake-up. If we drop the lock
 870 // another thread can reacquire the lock immediately, and we can
 871 // then wake a thread unnecessarily. This is benign, and we've
 872 // structured the code so the windows are short and the frequency
 873 // of such futile wakups is low.
 874 
 875 void ObjectMonitor::exit(bool not_suspended, TRAPS) {
 876   Thread * const Self = THREAD;
 877   if (THREAD != _owner) {
 878     if (THREAD->is_lock_owned((address) _owner)) {
 879       // Transmute _owner from a BasicLock pointer to a Thread address.
 880       // We don't need to hold _mutex for this transition.
 881       // Non-null to Non-null is safe as long as all readers can
 882       // tolerate either flavor.
 883       assert(_recursions == 0, "invariant");
 884       _owner = THREAD;
 885       _recursions = 0;
 886     } else {
 887       // Apparent unbalanced locking ...
 888       // Naively we'd like to throw IllegalMonitorStateException.
 889       // As a practical matter we can neither allocate nor throw an
 890       // exception as ::exit() can be called from leaf routines.
 891       // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
 892       // Upon deeper reflection, however, in a properly run JVM the only
 893       // way we should encounter this situation is in the presence of
 894       // unbalanced JNI locking. TODO: CheckJNICalls.
 895       // See also: CR4414101
 896       assert(false, "Non-balanced monitor enter/exit! Likely JNI locking");
 897       return;
 898     }
 899   }
 900 
 901   if (_recursions != 0) {
 902     _recursions--;        // this is simple recursive enter
 903     return;
 904   }
 905 
 906   // Invariant: after setting Responsible=null an thread must execute
 907   // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
 908   _Responsible = NULL;
 909 
 910 #if INCLUDE_JFR
 911   // get the owner's thread id for the MonitorEnter event
 912   // if it is enabled and the thread isn't suspended
 913   if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
 914     _previous_owner_tid = JFR_THREAD_ID(Self);
 915   }
 916 #endif
 917 
 918   for (;;) {
 919     assert(THREAD == _owner, "invariant");
 920 
 921     if (Knob_ExitPolicy == 0) {
 922       // release semantics: prior loads and stores from within the critical section
 923       // must not float (reorder) past the following store that drops the lock.
 924       // On SPARC that requires MEMBAR #loadstore|#storestore.
 925       // But of course in TSO #loadstore|#storestore is not required.
 926       // I'd like to write one of the following:
 927       // A.  OrderAccess::release() ; _owner = NULL
 928       // B.  OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
 929       // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
 930       // store into a _dummy variable.  That store is not needed, but can result
 931       // in massive wasteful coherency traffic on classic SMP systems.
 932       // Instead, I use release_store(), which is implemented as just a simple
 933       // ST on x64, x86 and SPARC.
 934       OrderAccess::release_store(&_owner, (void*)NULL);   // drop the lock
 935       OrderAccess::storeload();                        // See if we need to wake a successor
 936       if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
 937         return;
 938       }
 939       // Other threads are blocked trying to acquire the lock.
 940 
 941       // Normally the exiting thread is responsible for ensuring succession,
 942       // but if other successors are ready or other entering threads are spinning
 943       // then this thread can simply store NULL into _owner and exit without
 944       // waking a successor.  The existence of spinners or ready successors
 945       // guarantees proper succession (liveness).  Responsibility passes to the
 946       // ready or running successors.  The exiting thread delegates the duty.
 947       // More precisely, if a successor already exists this thread is absolved
 948       // of the responsibility of waking (unparking) one.
 949       //
 950       // The _succ variable is critical to reducing futile wakeup frequency.
 951       // _succ identifies the "heir presumptive" thread that has been made
 952       // ready (unparked) but that has not yet run.  We need only one such
 953       // successor thread to guarantee progress.
 954       // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
 955       // section 3.3 "Futile Wakeup Throttling" for details.
 956       //
 957       // Note that spinners in Enter() also set _succ non-null.
 958       // In the current implementation spinners opportunistically set
 959       // _succ so that exiting threads might avoid waking a successor.
 960       // Another less appealing alternative would be for the exiting thread
 961       // to drop the lock and then spin briefly to see if a spinner managed
 962       // to acquire the lock.  If so, the exiting thread could exit
 963       // immediately without waking a successor, otherwise the exiting
 964       // thread would need to dequeue and wake a successor.
 965       // (Note that we'd need to make the post-drop spin short, but no
 966       // shorter than the worst-case round-trip cache-line migration time.
 967       // The dropped lock needs to become visible to the spinner, and then
 968       // the acquisition of the lock by the spinner must become visible to
 969       // the exiting thread).
 970 
 971       // It appears that an heir-presumptive (successor) must be made ready.
 972       // Only the current lock owner can manipulate the EntryList or
 973       // drain _cxq, so we need to reacquire the lock.  If we fail
 974       // to reacquire the lock the responsibility for ensuring succession
 975       // falls to the new owner.
 976       //
 977       if (!Atomic::replace_if_null(THREAD, &_owner)) {
 978         return;
 979       }
 980     } else {
 981       if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
 982         OrderAccess::release_store(&_owner, (void*)NULL);   // drop the lock
 983         OrderAccess::storeload();
 984         // Ratify the previously observed values.
 985         if (_cxq == NULL || _succ != NULL) {
 986           return;
 987         }
 988 
 989         // inopportune interleaving -- the exiting thread (this thread)
 990         // in the fast-exit path raced an entering thread in the slow-enter
 991         // path.
 992         // We have two choices:
 993         // A.  Try to reacquire the lock.
 994         //     If the CAS() fails return immediately, otherwise
 995         //     we either restart/rerun the exit operation, or simply
 996         //     fall-through into the code below which wakes a successor.
 997         // B.  If the elements forming the EntryList|cxq are TSM
 998         //     we could simply unpark() the lead thread and return
 999         //     without having set _succ.
1000         if (!Atomic::replace_if_null(THREAD, &_owner)) {
1001           return;
1002         }
1003       }
1004     }
1005 
1006     guarantee(_owner == THREAD, "invariant");
1007 
1008     ObjectWaiter * w = NULL;
1009     int QMode = Knob_QMode;
1010 
1011     if (QMode == 2 && _cxq != NULL) {
1012       // QMode == 2 : cxq has precedence over EntryList.
1013       // Try to directly wake a successor from the cxq.
1014       // If successful, the successor will need to unlink itself from cxq.
1015       w = _cxq;
1016       assert(w != NULL, "invariant");
1017       assert(w->TState == ObjectWaiter::TS_CXQ, "Invariant");
1018       ExitEpilog(Self, w);
1019       return;
1020     }
1021 
1022     if (QMode == 3 && _cxq != NULL) {
1023       // Aggressively drain cxq into EntryList at the first opportunity.
1024       // This policy ensure that recently-run threads live at the head of EntryList.
1025       // Drain _cxq into EntryList - bulk transfer.
1026       // First, detach _cxq.
1027       // The following loop is tantamount to: w = swap(&cxq, NULL)
1028       w = _cxq;
1029       for (;;) {
1030         assert(w != NULL, "Invariant");
1031         ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1032         if (u == w) break;
1033         w = u;
1034       }
1035       assert(w != NULL, "invariant");
1036 
1037       ObjectWaiter * q = NULL;
1038       ObjectWaiter * p;
1039       for (p = w; p != NULL; p = p->_next) {
1040         guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1041         p->TState = ObjectWaiter::TS_ENTER;
1042         p->_prev = q;
1043         q = p;
1044       }
1045 
1046       // Append the RATs to the EntryList
1047       // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
1048       ObjectWaiter * Tail;
1049       for (Tail = _EntryList; Tail != NULL && Tail->_next != NULL;
1050            Tail = Tail->_next)
1051         /* empty */;
1052       if (Tail == NULL) {
1053         _EntryList = w;
1054       } else {
1055         Tail->_next = w;
1056         w->_prev = Tail;
1057       }
1058 
1059       // Fall thru into code that tries to wake a successor from EntryList
1060     }
1061 
1062     if (QMode == 4 && _cxq != NULL) {
1063       // Aggressively drain cxq into EntryList at the first opportunity.
1064       // This policy ensure that recently-run threads live at the head of EntryList.
1065 
1066       // Drain _cxq into EntryList - bulk transfer.
1067       // First, detach _cxq.
1068       // The following loop is tantamount to: w = swap(&cxq, NULL)
1069       w = _cxq;
1070       for (;;) {
1071         assert(w != NULL, "Invariant");
1072         ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1073         if (u == w) break;
1074         w = u;
1075       }
1076       assert(w != NULL, "invariant");
1077 
1078       ObjectWaiter * q = NULL;
1079       ObjectWaiter * p;
1080       for (p = w; p != NULL; p = p->_next) {
1081         guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1082         p->TState = ObjectWaiter::TS_ENTER;
1083         p->_prev = q;
1084         q = p;
1085       }
1086 
1087       // Prepend the RATs to the EntryList
1088       if (_EntryList != NULL) {
1089         q->_next = _EntryList;
1090         _EntryList->_prev = q;
1091       }
1092       _EntryList = w;
1093 
1094       // Fall thru into code that tries to wake a successor from EntryList
1095     }
1096 
1097     w = _EntryList;
1098     if (w != NULL) {
1099       // I'd like to write: guarantee (w->_thread != Self).
1100       // But in practice an exiting thread may find itself on the EntryList.
1101       // Let's say thread T1 calls O.wait().  Wait() enqueues T1 on O's waitset and
1102       // then calls exit().  Exit release the lock by setting O._owner to NULL.
1103       // Let's say T1 then stalls.  T2 acquires O and calls O.notify().  The
1104       // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1105       // release the lock "O".  T2 resumes immediately after the ST of null into
1106       // _owner, above.  T2 notices that the EntryList is populated, so it
1107       // reacquires the lock and then finds itself on the EntryList.
1108       // Given all that, we have to tolerate the circumstance where "w" is
1109       // associated with Self.
1110       assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1111       ExitEpilog(Self, w);
1112       return;
1113     }
1114 
1115     // If we find that both _cxq and EntryList are null then just
1116     // re-run the exit protocol from the top.
1117     w = _cxq;
1118     if (w == NULL) continue;
1119 
1120     // Drain _cxq into EntryList - bulk transfer.
1121     // First, detach _cxq.
1122     // The following loop is tantamount to: w = swap(&cxq, NULL)
1123     for (;;) {
1124       assert(w != NULL, "Invariant");
1125       ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1126       if (u == w) break;
1127       w = u;
1128     }
1129 
1130     assert(w != NULL, "invariant");
1131     assert(_EntryList == NULL, "invariant");
1132 
1133     // Convert the LIFO SLL anchored by _cxq into a DLL.
1134     // The list reorganization step operates in O(LENGTH(w)) time.
1135     // It's critical that this step operate quickly as
1136     // "Self" still holds the outer-lock, restricting parallelism
1137     // and effectively lengthening the critical section.
1138     // Invariant: s chases t chases u.
1139     // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1140     // we have faster access to the tail.
1141 
1142     if (QMode == 1) {
1143       // QMode == 1 : drain cxq to EntryList, reversing order
1144       // We also reverse the order of the list.
1145       ObjectWaiter * s = NULL;
1146       ObjectWaiter * t = w;
1147       ObjectWaiter * u = NULL;
1148       while (t != NULL) {
1149         guarantee(t->TState == ObjectWaiter::TS_CXQ, "invariant");
1150         t->TState = ObjectWaiter::TS_ENTER;
1151         u = t->_next;
1152         t->_prev = u;
1153         t->_next = s;
1154         s = t;
1155         t = u;
1156       }
1157       _EntryList  = s;
1158       assert(s != NULL, "invariant");
1159     } else {
1160       // QMode == 0 or QMode == 2
1161       _EntryList = w;
1162       ObjectWaiter * q = NULL;
1163       ObjectWaiter * p;
1164       for (p = w; p != NULL; p = p->_next) {
1165         guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1166         p->TState = ObjectWaiter::TS_ENTER;
1167         p->_prev = q;
1168         q = p;
1169       }
1170     }
1171 
1172     // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1173     // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1174 
1175     // See if we can abdicate to a spinner instead of waking a thread.
1176     // A primary goal of the implementation is to reduce the
1177     // context-switch rate.
1178     if (_succ != NULL) continue;
1179 
1180     w = _EntryList;
1181     if (w != NULL) {
1182       guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1183       ExitEpilog(Self, w);
1184       return;
1185     }
1186   }
1187 }
1188 
1189 // ExitSuspendEquivalent:
1190 // A faster alternate to handle_special_suspend_equivalent_condition()
1191 //
1192 // handle_special_suspend_equivalent_condition() unconditionally
1193 // acquires the SR_lock.  On some platforms uncontended MutexLocker()
1194 // operations have high latency.  Note that in ::enter() we call HSSEC
1195 // while holding the monitor, so we effectively lengthen the critical sections.
1196 //
1197 // There are a number of possible solutions:
1198 //
1199 // A.  To ameliorate the problem we might also defer state transitions
1200 //     to as late as possible -- just prior to parking.
1201 //     Given that, we'd call HSSEC after having returned from park(),
1202 //     but before attempting to acquire the monitor.  This is only a
1203 //     partial solution.  It avoids calling HSSEC while holding the
1204 //     monitor (good), but it still increases successor reacquisition latency --
1205 //     the interval between unparking a successor and the time the successor
1206 //     resumes and retries the lock.  See ReenterI(), which defers state transitions.
1207 //     If we use this technique we can also avoid EnterI()-exit() loop
1208 //     in ::enter() where we iteratively drop the lock and then attempt
1209 //     to reacquire it after suspending.
1210 //
1211 // B.  In the future we might fold all the suspend bits into a
1212 //     composite per-thread suspend flag and then update it with CAS().
1213 //     Alternately, a Dekker-like mechanism with multiple variables
1214 //     would suffice:
1215 //       ST Self->_suspend_equivalent = false
1216 //       MEMBAR
1217 //       LD Self_>_suspend_flags
1218 //
1219 // UPDATE 2007-10-6: since I've replaced the native Mutex/Monitor subsystem
1220 // with a more efficient implementation, the need to use "FastHSSEC" has
1221 // decreased. - Dave
1222 
1223 
1224 bool ObjectMonitor::ExitSuspendEquivalent(JavaThread * jSelf) {
1225   const int Mode = Knob_FastHSSEC;
1226   if (Mode && !jSelf->is_external_suspend()) {
1227     assert(jSelf->is_suspend_equivalent(), "invariant");
1228     jSelf->clear_suspend_equivalent();
1229     if (2 == Mode) OrderAccess::storeload();
1230     if (!jSelf->is_external_suspend()) return false;
1231     // We raced a suspension -- fall thru into the slow path
1232     jSelf->set_suspend_equivalent();
1233   }
1234   return jSelf->handle_special_suspend_equivalent_condition();
1235 }
1236 
1237 
1238 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) {
1239   assert(_owner == Self, "invariant");
1240 
1241   // Exit protocol:
1242   // 1. ST _succ = wakee
1243   // 2. membar #loadstore|#storestore;
1244   // 2. ST _owner = NULL
1245   // 3. unpark(wakee)
1246 
1247   _succ = Knob_SuccEnabled ? Wakee->_thread : NULL;
1248   ParkEvent * Trigger = Wakee->_event;
1249 
1250   // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1251   // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1252   // out-of-scope (non-extant).
1253   Wakee  = NULL;
1254 
1255   // Drop the lock
1256   OrderAccess::release_store(&_owner, (void*)NULL);
1257   OrderAccess::fence();                               // ST _owner vs LD in unpark()
1258 
1259   DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1260   Trigger->unpark();
1261 
1262   // Maintain stats and report events to JVMTI
1263   OM_PERFDATA_OP(Parks, inc());
1264 }
1265 
1266 
1267 // -----------------------------------------------------------------------------
1268 // Class Loader deadlock handling.
1269 //
1270 // complete_exit exits a lock returning recursion count
1271 // complete_exit/reenter operate as a wait without waiting
1272 // complete_exit requires an inflated monitor
1273 // The _owner field is not always the Thread addr even with an
1274 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1275 // thread due to contention.
1276 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1277   Thread * const Self = THREAD;
1278   assert(Self->is_Java_thread(), "Must be Java thread!");
1279   JavaThread *jt = (JavaThread *)THREAD;
1280 
1281   DeferredInitialize();
1282 
1283   if (THREAD != _owner) {
1284     if (THREAD->is_lock_owned ((address)_owner)) {
1285       assert(_recursions == 0, "internal state error");
1286       _owner = THREAD;   // Convert from basiclock addr to Thread addr
1287       _recursions = 0;
1288     }
1289   }
1290 
1291   guarantee(Self == _owner, "complete_exit not owner");
1292   intptr_t save = _recursions; // record the old recursion count
1293   _recursions = 0;        // set the recursion level to be 0
1294   exit(true, Self);           // exit the monitor
1295   guarantee(_owner != Self, "invariant");
1296   return save;
1297 }
1298 
1299 // reenter() enters a lock and sets recursion count
1300 // complete_exit/reenter operate as a wait without waiting
1301 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1302   Thread * const Self = THREAD;
1303   assert(Self->is_Java_thread(), "Must be Java thread!");
1304   JavaThread *jt = (JavaThread *)THREAD;
1305 
1306   guarantee(_owner != Self, "reenter already owner");
1307   enter(THREAD);       // enter the monitor
1308   guarantee(_recursions == 0, "reenter recursion");
1309   _recursions = recursions;
1310   return;
1311 }
1312 
1313 
1314 // -----------------------------------------------------------------------------
1315 // A macro is used below because there may already be a pending
1316 // exception which should not abort the execution of the routines
1317 // which use this (which is why we don't put this into check_slow and
1318 // call it with a CHECK argument).
1319 
1320 #define CHECK_OWNER()                                                       \
1321   do {                                                                      \
1322     if (THREAD != _owner) {                                                 \
1323       if (THREAD->is_lock_owned((address) _owner)) {                        \
1324         _owner = THREAD;  /* Convert from basiclock addr to Thread addr */  \
1325         _recursions = 0;                                                    \
1326       } else {                                                              \
1327         THROW(vmSymbols::java_lang_IllegalMonitorStateException());         \
1328       }                                                                     \
1329     }                                                                       \
1330   } while (false)
1331 
1332 // check_slow() is a misnomer.  It's called to simply to throw an IMSX exception.
1333 // TODO-FIXME: remove check_slow() -- it's likely dead.
1334 
1335 void ObjectMonitor::check_slow(TRAPS) {
1336   assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1337   THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1338 }
1339 
1340 static int Adjust(volatile int * adr, int dx) {
1341   int v;
1342   for (v = *adr; Atomic::cmpxchg(v + dx, adr, v) != v; v = *adr) /* empty */;
1343   return v;
1344 }
1345 
1346 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1347                                     ObjectMonitor* monitor,
1348                                     jlong notifier_tid,
1349                                     jlong timeout,
1350                                     bool timedout) {
1351   assert(event != NULL, "invariant");
1352   assert(monitor != NULL, "invariant");
1353   event->set_monitorClass(((oop)monitor->object())->klass());
1354   event->set_timeout(timeout);
1355   event->set_address((uintptr_t)monitor->object_addr());
1356   event->set_notifier(notifier_tid);
1357   event->set_timedOut(timedout);
1358   event->commit();
1359 }
1360 
1361 // -----------------------------------------------------------------------------
1362 // Wait/Notify/NotifyAll
1363 //
1364 // Note: a subset of changes to ObjectMonitor::wait()
1365 // will need to be replicated in complete_exit
1366 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1367   Thread * const Self = THREAD;
1368   assert(Self->is_Java_thread(), "Must be Java thread!");
1369   JavaThread *jt = (JavaThread *)THREAD;
1370 
1371   DeferredInitialize();
1372 
1373   // Throw IMSX or IEX.
1374   CHECK_OWNER();
1375 
1376   EventJavaMonitorWait event;
1377 
1378   // check for a pending interrupt
1379   if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1380     // post monitor waited event.  Note that this is past-tense, we are done waiting.
1381     if (JvmtiExport::should_post_monitor_waited()) {
1382       // Note: 'false' parameter is passed here because the
1383       // wait was not timed out due to thread interrupt.
1384       JvmtiExport::post_monitor_waited(jt, this, false);
1385 
1386       // In this short circuit of the monitor wait protocol, the
1387       // current thread never drops ownership of the monitor and
1388       // never gets added to the wait queue so the current thread
1389       // cannot be made the successor. This means that the
1390       // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1391       // consume an unpark() meant for the ParkEvent associated with
1392       // this ObjectMonitor.
1393     }
1394     if (event.should_commit()) {
1395       post_monitor_wait_event(&event, this, 0, millis, false);
1396     }
1397     THROW(vmSymbols::java_lang_InterruptedException());
1398     return;
1399   }
1400 
1401   assert(Self->_Stalled == 0, "invariant");
1402   Self->_Stalled = intptr_t(this);
1403   jt->set_current_waiting_monitor(this);
1404 
1405   // create a node to be put into the queue
1406   // Critically, after we reset() the event but prior to park(), we must check
1407   // for a pending interrupt.
1408   ObjectWaiter node(Self);
1409   node.TState = ObjectWaiter::TS_WAIT;
1410   Self->_ParkEvent->reset();
1411   OrderAccess::fence();          // ST into Event; membar ; LD interrupted-flag
1412 
1413   // Enter the waiting queue, which is a circular doubly linked list in this case
1414   // but it could be a priority queue or any data structure.
1415   // _WaitSetLock protects the wait queue.  Normally the wait queue is accessed only
1416   // by the the owner of the monitor *except* in the case where park()
1417   // returns because of a timeout of interrupt.  Contention is exceptionally rare
1418   // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1419 
1420   Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1421   AddWaiter(&node);
1422   Thread::SpinRelease(&_WaitSetLock);
1423 
1424   _Responsible = NULL;
1425 
1426   intptr_t save = _recursions; // record the old recursion count
1427   _waiters++;                  // increment the number of waiters
1428   _recursions = 0;             // set the recursion level to be 1
1429   exit(true, Self);                    // exit the monitor
1430   guarantee(_owner != Self, "invariant");
1431 
1432   // The thread is on the WaitSet list - now park() it.
1433   // On MP systems it's conceivable that a brief spin before we park
1434   // could be profitable.
1435   //
1436   // TODO-FIXME: change the following logic to a loop of the form
1437   //   while (!timeout && !interrupted && _notified == 0) park()
1438 
1439   int ret = OS_OK;
1440   int WasNotified = 0;
1441   { // State transition wrappers
1442     OSThread* osthread = Self->osthread();
1443     OSThreadWaitState osts(osthread, true);
1444     {
1445       ThreadBlockInVM tbivm(jt);
1446       // Thread is in thread_blocked state and oop access is unsafe.
1447       jt->set_suspend_equivalent();
1448 
1449       if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1450         // Intentionally empty
1451       } else if (node._notified == 0) {
1452         if (millis <= 0) {
1453           Self->_ParkEvent->park();
1454         } else {
1455           ret = Self->_ParkEvent->park(millis);
1456         }
1457       }
1458 
1459       // were we externally suspended while we were waiting?
1460       if (ExitSuspendEquivalent (jt)) {
1461         // TODO-FIXME: add -- if succ == Self then succ = null.
1462         jt->java_suspend_self();
1463       }
1464 
1465     } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1466 
1467     // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1468     // from the WaitSet to the EntryList.
1469     // See if we need to remove Node from the WaitSet.
1470     // We use double-checked locking to avoid grabbing _WaitSetLock
1471     // if the thread is not on the wait queue.
1472     //
1473     // Note that we don't need a fence before the fetch of TState.
1474     // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1475     // written by the is thread. (perhaps the fetch might even be satisfied
1476     // by a look-aside into the processor's own store buffer, although given
1477     // the length of the code path between the prior ST and this load that's
1478     // highly unlikely).  If the following LD fetches a stale TS_WAIT value
1479     // then we'll acquire the lock and then re-fetch a fresh TState value.
1480     // That is, we fail toward safety.
1481 
1482     if (node.TState == ObjectWaiter::TS_WAIT) {
1483       Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1484       if (node.TState == ObjectWaiter::TS_WAIT) {
1485         DequeueSpecificWaiter(&node);       // unlink from WaitSet
1486         assert(node._notified == 0, "invariant");
1487         node.TState = ObjectWaiter::TS_RUN;
1488       }
1489       Thread::SpinRelease(&_WaitSetLock);
1490     }
1491 
1492     // The thread is now either on off-list (TS_RUN),
1493     // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1494     // The Node's TState variable is stable from the perspective of this thread.
1495     // No other threads will asynchronously modify TState.
1496     guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1497     OrderAccess::loadload();
1498     if (_succ == Self) _succ = NULL;
1499     WasNotified = node._notified;
1500 
1501     // Reentry phase -- reacquire the monitor.
1502     // re-enter contended monitor after object.wait().
1503     // retain OBJECT_WAIT state until re-enter successfully completes
1504     // Thread state is thread_in_vm and oop access is again safe,
1505     // although the raw address of the object may have changed.
1506     // (Don't cache naked oops over safepoints, of course).
1507 
1508     // post monitor waited event. Note that this is past-tense, we are done waiting.
1509     if (JvmtiExport::should_post_monitor_waited()) {
1510       JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1511 
1512       if (node._notified != 0 && _succ == Self) {
1513         // In this part of the monitor wait-notify-reenter protocol it
1514         // is possible (and normal) for another thread to do a fastpath
1515         // monitor enter-exit while this thread is still trying to get
1516         // to the reenter portion of the protocol.
1517         //
1518         // The ObjectMonitor was notified and the current thread is
1519         // the successor which also means that an unpark() has already
1520         // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1521         // consume the unpark() that was done when the successor was
1522         // set because the same ParkEvent is shared between Java
1523         // monitors and JVM/TI RawMonitors (for now).
1524         //
1525         // We redo the unpark() to ensure forward progress, i.e., we
1526         // don't want all pending threads hanging (parked) with none
1527         // entering the unlocked monitor.
1528         node._event->unpark();
1529       }
1530     }
1531 
1532     if (event.should_commit()) {
1533       post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1534     }
1535 
1536     OrderAccess::fence();
1537 
1538     assert(Self->_Stalled != 0, "invariant");
1539     Self->_Stalled = 0;
1540 
1541     assert(_owner != Self, "invariant");
1542     ObjectWaiter::TStates v = node.TState;
1543     if (v == ObjectWaiter::TS_RUN) {
1544       enter(Self);
1545     } else {
1546       guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1547       ReenterI(Self, &node);
1548       node.wait_reenter_end(this);
1549     }
1550 
1551     // Self has reacquired the lock.
1552     // Lifecycle - the node representing Self must not appear on any queues.
1553     // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1554     // want residual elements associated with this thread left on any lists.
1555     guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1556     assert(_owner == Self, "invariant");
1557     assert(_succ != Self, "invariant");
1558   } // OSThreadWaitState()
1559 
1560   jt->set_current_waiting_monitor(NULL);
1561 
1562   guarantee(_recursions == 0, "invariant");
1563   _recursions = save;     // restore the old recursion count
1564   _waiters--;             // decrement the number of waiters
1565 
1566   // Verify a few postconditions
1567   assert(_owner == Self, "invariant");
1568   assert(_succ != Self, "invariant");
1569   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
1570 
1571   // check if the notification happened
1572   if (!WasNotified) {
1573     // no, it could be timeout or Thread.interrupt() or both
1574     // check for interrupt event, otherwise it is timeout
1575     if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1576       THROW(vmSymbols::java_lang_InterruptedException());
1577     }
1578   }
1579 
1580   // NOTE: Spurious wake up will be consider as timeout.
1581   // Monitor notify has precedence over thread interrupt.
1582 }
1583 
1584 
1585 // Consider:
1586 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1587 // then instead of transferring a thread from the WaitSet to the EntryList
1588 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1589 
1590 void ObjectMonitor::INotify(Thread * Self) {
1591   const int policy = Knob_MoveNotifyee;
1592 
1593   Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1594   ObjectWaiter * iterator = DequeueWaiter();
1595   if (iterator != NULL) {
1596     guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1597     guarantee(iterator->_notified == 0, "invariant");
1598     // Disposition - what might we do with iterator ?
1599     // a.  add it directly to the EntryList - either tail (policy == 1)
1600     //     or head (policy == 0).
1601     // b.  push it onto the front of the _cxq (policy == 2).
1602     // For now we use (b).
1603     if (policy != 4) {
1604       iterator->TState = ObjectWaiter::TS_ENTER;
1605     }
1606     iterator->_notified = 1;
1607     iterator->_notifier_tid = JFR_THREAD_ID(Self);
1608 
1609     ObjectWaiter * list = _EntryList;
1610     if (list != NULL) {
1611       assert(list->_prev == NULL, "invariant");
1612       assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1613       assert(list != iterator, "invariant");
1614     }
1615 
1616     if (policy == 0) {       // prepend to EntryList
1617       if (list == NULL) {
1618         iterator->_next = iterator->_prev = NULL;
1619         _EntryList = iterator;
1620       } else {
1621         list->_prev = iterator;
1622         iterator->_next = list;
1623         iterator->_prev = NULL;
1624         _EntryList = iterator;
1625       }
1626     } else if (policy == 1) {      // append to EntryList
1627       if (list == NULL) {
1628         iterator->_next = iterator->_prev = NULL;
1629         _EntryList = iterator;
1630       } else {
1631         // CONSIDER:  finding the tail currently requires a linear-time walk of
1632         // the EntryList.  We can make tail access constant-time by converting to
1633         // a CDLL instead of using our current DLL.
1634         ObjectWaiter * tail;
1635         for (tail = list; tail->_next != NULL; tail = tail->_next) {}
1636         assert(tail != NULL && tail->_next == NULL, "invariant");
1637         tail->_next = iterator;
1638         iterator->_prev = tail;
1639         iterator->_next = NULL;
1640       }
1641     } else if (policy == 2) {      // prepend to cxq
1642       if (list == NULL) {
1643         iterator->_next = iterator->_prev = NULL;
1644         _EntryList = iterator;
1645       } else {
1646         iterator->TState = ObjectWaiter::TS_CXQ;
1647         for (;;) {
1648           ObjectWaiter * front = _cxq;
1649           iterator->_next = front;
1650           if (Atomic::cmpxchg(iterator, &_cxq, front) == front) {
1651             break;
1652           }
1653         }
1654       }
1655     } else if (policy == 3) {      // append to cxq
1656       iterator->TState = ObjectWaiter::TS_CXQ;
1657       for (;;) {
1658         ObjectWaiter * tail = _cxq;
1659         if (tail == NULL) {
1660           iterator->_next = NULL;
1661           if (Atomic::replace_if_null(iterator, &_cxq)) {
1662             break;
1663           }
1664         } else {
1665           while (tail->_next != NULL) tail = tail->_next;
1666           tail->_next = iterator;
1667           iterator->_prev = tail;
1668           iterator->_next = NULL;
1669           break;
1670         }
1671       }
1672     } else {
1673       ParkEvent * ev = iterator->_event;
1674       iterator->TState = ObjectWaiter::TS_RUN;
1675       OrderAccess::fence();
1676       ev->unpark();
1677     }
1678 
1679     // _WaitSetLock protects the wait queue, not the EntryList.  We could
1680     // move the add-to-EntryList operation, above, outside the critical section
1681     // protected by _WaitSetLock.  In practice that's not useful.  With the
1682     // exception of  wait() timeouts and interrupts the monitor owner
1683     // is the only thread that grabs _WaitSetLock.  There's almost no contention
1684     // on _WaitSetLock so it's not profitable to reduce the length of the
1685     // critical section.
1686 
1687     if (policy < 4) {
1688       iterator->wait_reenter_begin(this);
1689     }
1690   }
1691   Thread::SpinRelease(&_WaitSetLock);
1692 }
1693 
1694 // Consider: a not-uncommon synchronization bug is to use notify() when
1695 // notifyAll() is more appropriate, potentially resulting in stranded
1696 // threads; this is one example of a lost wakeup. A useful diagnostic
1697 // option is to force all notify() operations to behave as notifyAll().
1698 //
1699 // Note: We can also detect many such problems with a "minimum wait".
1700 // When the "minimum wait" is set to a small non-zero timeout value
1701 // and the program does not hang whereas it did absent "minimum wait",
1702 // that suggests a lost wakeup bug.
1703 
1704 void ObjectMonitor::notify(TRAPS) {
1705   CHECK_OWNER();
1706   if (_WaitSet == NULL) {
1707     return;
1708   }
1709   DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1710   INotify(THREAD);
1711   OM_PERFDATA_OP(Notifications, inc(1));
1712 }
1713 
1714 
1715 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1716 // from the waitset to the EntryList. This could be done more efficiently with a
1717 // single bulk transfer but in practice it's not time-critical. Beware too,
1718 // that in prepend-mode we invert the order of the waiters. Let's say that the
1719 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1720 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1721 
1722 void ObjectMonitor::notifyAll(TRAPS) {
1723   CHECK_OWNER();
1724   if (_WaitSet == NULL) {
1725     return;
1726   }
1727 
1728   DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1729   int tally = 0;
1730   while (_WaitSet != NULL) {
1731     tally++;
1732     INotify(THREAD);
1733   }
1734 
1735   OM_PERFDATA_OP(Notifications, inc(tally));
1736 }
1737 
1738 // -----------------------------------------------------------------------------
1739 // Adaptive Spinning Support
1740 //
1741 // Adaptive spin-then-block - rational spinning
1742 //
1743 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1744 // algorithm.  On high order SMP systems it would be better to start with
1745 // a brief global spin and then revert to spinning locally.  In the spirit of MCS/CLH,
1746 // a contending thread could enqueue itself on the cxq and then spin locally
1747 // on a thread-specific variable such as its ParkEvent._Event flag.
1748 // That's left as an exercise for the reader.  Note that global spinning is
1749 // not problematic on Niagara, as the L2 cache serves the interconnect and
1750 // has both low latency and massive bandwidth.
1751 //
1752 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1753 // acquisition attempts where we opt to spin --  at 100% and vary the spin count
1754 // (duration) or we can fix the count at approximately the duration of
1755 // a context switch and vary the frequency.   Of course we could also
1756 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1757 // For a description of 'Adaptive spin-then-block mutual exclusion in
1758 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1759 //
1760 // This implementation varies the duration "D", where D varies with
1761 // the success rate of recent spin attempts. (D is capped at approximately
1762 // length of a round-trip context switch).  The success rate for recent
1763 // spin attempts is a good predictor of the success rate of future spin
1764 // attempts.  The mechanism adapts automatically to varying critical
1765 // section length (lock modality), system load and degree of parallelism.
1766 // D is maintained per-monitor in _SpinDuration and is initialized
1767 // optimistically.  Spin frequency is fixed at 100%.
1768 //
1769 // Note that _SpinDuration is volatile, but we update it without locks
1770 // or atomics.  The code is designed so that _SpinDuration stays within
1771 // a reasonable range even in the presence of races.  The arithmetic
1772 // operations on _SpinDuration are closed over the domain of legal values,
1773 // so at worst a race will install and older but still legal value.
1774 // At the very worst this introduces some apparent non-determinism.
1775 // We might spin when we shouldn't or vice-versa, but since the spin
1776 // count are relatively short, even in the worst case, the effect is harmless.
1777 //
1778 // Care must be taken that a low "D" value does not become an
1779 // an absorbing state.  Transient spinning failures -- when spinning
1780 // is overall profitable -- should not cause the system to converge
1781 // on low "D" values.  We want spinning to be stable and predictable
1782 // and fairly responsive to change and at the same time we don't want
1783 // it to oscillate, become metastable, be "too" non-deterministic,
1784 // or converge on or enter undesirable stable absorbing states.
1785 //
1786 // We implement a feedback-based control system -- using past behavior
1787 // to predict future behavior.  We face two issues: (a) if the
1788 // input signal is random then the spin predictor won't provide optimal
1789 // results, and (b) if the signal frequency is too high then the control
1790 // system, which has some natural response lag, will "chase" the signal.
1791 // (b) can arise from multimodal lock hold times.  Transient preemption
1792 // can also result in apparent bimodal lock hold times.
1793 // Although sub-optimal, neither condition is particularly harmful, as
1794 // in the worst-case we'll spin when we shouldn't or vice-versa.
1795 // The maximum spin duration is rather short so the failure modes aren't bad.
1796 // To be conservative, I've tuned the gain in system to bias toward
1797 // _not spinning.  Relatedly, the system can sometimes enter a mode where it
1798 // "rings" or oscillates between spinning and not spinning.  This happens
1799 // when spinning is just on the cusp of profitability, however, so the
1800 // situation is not dire.  The state is benign -- there's no need to add
1801 // hysteresis control to damp the transition rate between spinning and
1802 // not spinning.
1803 
1804 // Spinning: Fixed frequency (100%), vary duration
1805 int ObjectMonitor::TrySpin(Thread * Self) {
1806   // Dumb, brutal spin.  Good for comparative measurements against adaptive spinning.
1807   int ctr = Knob_FixedSpin;
1808   if (ctr != 0) {
1809     while (--ctr >= 0) {
1810       if (TryLock(Self) > 0) return 1;
1811       SpinPause();
1812     }
1813     return 0;
1814   }
1815 
1816   for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1817     if (TryLock(Self) > 0) {
1818       // Increase _SpinDuration ...
1819       // Note that we don't clamp SpinDuration precisely at SpinLimit.
1820       // Raising _SpurDuration to the poverty line is key.
1821       int x = _SpinDuration;
1822       if (x < Knob_SpinLimit) {
1823         if (x < Knob_Poverty) x = Knob_Poverty;
1824         _SpinDuration = x + Knob_BonusB;
1825       }
1826       return 1;
1827     }
1828     SpinPause();
1829   }
1830 
1831   // Admission control - verify preconditions for spinning
1832   //
1833   // We always spin a little bit, just to prevent _SpinDuration == 0 from
1834   // becoming an absorbing state.  Put another way, we spin briefly to
1835   // sample, just in case the system load, parallelism, contention, or lock
1836   // modality changed.
1837   //
1838   // Consider the following alternative:
1839   // Periodically set _SpinDuration = _SpinLimit and try a long/full
1840   // spin attempt.  "Periodically" might mean after a tally of
1841   // the # of failed spin attempts (or iterations) reaches some threshold.
1842   // This takes us into the realm of 1-out-of-N spinning, where we
1843   // hold the duration constant but vary the frequency.
1844 
1845   ctr = _SpinDuration;
1846   if (ctr <= 0) return 0;
1847 
1848   if (Knob_SuccRestrict && _succ != NULL) return 0;
1849   if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
1850     return 0;
1851   }
1852 
1853   int MaxSpin = Knob_MaxSpinners;
1854   if (MaxSpin >= 0) {
1855     if (_Spinner > MaxSpin) {
1856       return 0;
1857     }
1858     // Slightly racy, but benign ...
1859     Adjust(&_Spinner, 1);
1860   }
1861 
1862   // We're good to spin ... spin ingress.
1863   // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1864   // when preparing to LD...CAS _owner, etc and the CAS is likely
1865   // to succeed.
1866   int sss     = Knob_SpinSetSucc;
1867   if (sss && _succ == NULL) _succ = Self;

1868   Thread * prv = NULL;
1869 
1870   // There are three ways to exit the following loop:
1871   // 1.  A successful spin where this thread has acquired the lock.
1872   // 2.  Spin failure with prejudice
1873   // 3.  Spin failure without prejudice
1874 
1875   while (--ctr >= 0) {
1876 
1877     // Periodic polling -- Check for pending GC
1878     // Threads may spin while they're unsafe.
1879     // We don't want spinning threads to delay the JVM from reaching
1880     // a stop-the-world safepoint or to steal cycles from GC.
1881     // If we detect a pending safepoint we abort in order that
1882     // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1883     // this thread, if safe, doesn't steal cycles from GC.
1884     // This is in keeping with the "no loitering in runtime" rule.
1885     // We periodically check to see if there's a safepoint pending.
1886     if ((ctr & 0xFF) == 0) {
1887       if (SafepointMechanism::poll(Self)) {
1888         goto Abort;           // abrupt spin egress
1889       }
1890       if (Knob_UsePause & 1) SpinPause();
1891     }
1892 
1893     if (Knob_UsePause & 2) SpinPause();
1894 
1895     // Probe _owner with TATAS
1896     // If this thread observes the monitor transition or flicker
1897     // from locked to unlocked to locked, then the odds that this
1898     // thread will acquire the lock in this spin attempt go down
1899     // considerably.  The same argument applies if the CAS fails
1900     // or if we observe _owner change from one non-null value to
1901     // another non-null value.   In such cases we might abort
1902     // the spin without prejudice or apply a "penalty" to the
1903     // spin count-down variable "ctr", reducing it by 100, say.
1904 
1905     Thread * ox = (Thread *) _owner;
1906     if (ox == NULL) {
1907       ox = (Thread*)Atomic::cmpxchg(Self, &_owner, (void*)NULL);
1908       if (ox == NULL) {
1909         // The CAS succeeded -- this thread acquired ownership
1910         // Take care of some bookkeeping to exit spin state.
1911         if (sss && _succ == Self) {
1912           _succ = NULL;
1913         }
1914         if (MaxSpin > 0) Adjust(&_Spinner, -1);
1915 
1916         // Increase _SpinDuration :
1917         // The spin was successful (profitable) so we tend toward
1918         // longer spin attempts in the future.
1919         // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
1920         // If we acquired the lock early in the spin cycle it
1921         // makes sense to increase _SpinDuration proportionally.
1922         // Note that we don't clamp SpinDuration precisely at SpinLimit.
1923         int x = _SpinDuration;
1924         if (x < Knob_SpinLimit) {
1925           if (x < Knob_Poverty) x = Knob_Poverty;
1926           _SpinDuration = x + Knob_Bonus;
1927         }
1928         return 1;
1929       }
1930 
1931       // The CAS failed ... we can take any of the following actions:
1932       // * penalize: ctr -= CASPenalty
1933       // * exit spin with prejudice -- goto Abort;
1934       // * exit spin without prejudice.
1935       // * Since CAS is high-latency, retry again immediately.
1936       prv = ox;
1937       goto Abort;
1938     }
1939 
1940     // Did lock ownership change hands ?
1941     if (ox != prv && prv != NULL) {
1942       goto Abort;
1943     }
1944     prv = ox;
1945 
1946     // Abort the spin if the owner is not executing.
1947     // The owner must be executing in order to drop the lock.
1948     // Spinning while the owner is OFFPROC is idiocy.
1949     // Consider: ctr -= RunnablePenalty ;
1950     if (Knob_OState && NotRunnable (Self, ox)) {
1951       goto Abort;
1952     }
1953     if (sss && _succ == NULL) _succ = Self;


1954   }
1955 
1956   // Spin failed with prejudice -- reduce _SpinDuration.
1957   // TODO: Use an AIMD-like policy to adjust _SpinDuration.
1958   // AIMD is globally stable.
1959   {
1960     int x = _SpinDuration;
1961     if (x > 0) {
1962       // Consider an AIMD scheme like: x -= (x >> 3) + 100
1963       // This is globally sample and tends to damp the response.
1964       x -= Knob_Penalty;
1965       if (x < 0) x = 0;
1966       _SpinDuration = x;
1967     }
1968   }
1969 
1970  Abort:
1971   if (MaxSpin >= 0) Adjust(&_Spinner, -1);
1972   if (sss && _succ == Self) {
1973     _succ = NULL;
1974     // Invariant: after setting succ=null a contending thread
1975     // must recheck-retry _owner before parking.  This usually happens
1976     // in the normal usage of TrySpin(), but it's safest
1977     // to make TrySpin() as foolproof as possible.
1978     OrderAccess::fence();
1979     if (TryLock(Self) > 0) return 1;
1980   }
1981   return 0;
1982 }
1983 
1984 // NotRunnable() -- informed spinning
1985 //
1986 // Don't bother spinning if the owner is not eligible to drop the lock.
1987 // Spin only if the owner thread is _thread_in_Java or _thread_in_vm.
1988 // The thread must be runnable in order to drop the lock in timely fashion.
1989 // If the _owner is not runnable then spinning will not likely be
1990 // successful (profitable).
1991 //
1992 // Beware -- the thread referenced by _owner could have died
1993 // so a simply fetch from _owner->_thread_state might trap.
1994 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
1995 // Because of the lifecycle issues, the _thread_state values
1996 // observed by NotRunnable() might be garbage.  NotRunnable must
1997 // tolerate this and consider the observed _thread_state value
1998 // as advisory.
1999 //
2000 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2001 // a thread pointer.
2002 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2003 // with the LSB of _owner.  Another option would be to probabilistically probe
2004 // the putative _owner->TypeTag value.
2005 //
2006 // Checking _thread_state isn't perfect.  Even if the thread is
2007 // in_java it might be blocked on a page-fault or have been preempted
2008 // and sitting on a ready/dispatch queue.
2009 //
2010 // The return value from NotRunnable() is *advisory* -- the
2011 // result is based on sampling and is not necessarily coherent.
2012 // The caller must tolerate false-negative and false-positive errors.
2013 // Spinning, in general, is probabilistic anyway.
2014 
2015 
2016 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) {
2017   // Check ox->TypeTag == 2BAD.
2018   if (ox == NULL) return 0;
2019 
2020   // Avoid transitive spinning ...
2021   // Say T1 spins or blocks trying to acquire L.  T1._Stalled is set to L.
2022   // Immediately after T1 acquires L it's possible that T2, also
2023   // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2024   // This occurs transiently after T1 acquired L but before
2025   // T1 managed to clear T1.Stalled.  T2 does not need to abort
2026   // its spin in this circumstance.
2027   intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
2028 
2029   if (BlockedOn == 1) return 1;
2030   if (BlockedOn != 0) {
2031     return BlockedOn != intptr_t(this) && _owner == ox;
2032   }
2033 
2034   assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant");
2035   int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);;
2036   // consider also: jst != _thread_in_Java -- but that's overspecific.
2037   return jst == _thread_blocked || jst == _thread_in_native;
2038 }
2039 
2040 
2041 // -----------------------------------------------------------------------------
2042 // WaitSet management ...
2043 
2044 ObjectWaiter::ObjectWaiter(Thread* thread) {
2045   _next     = NULL;
2046   _prev     = NULL;
2047   _notified = 0;
2048   _notifier_tid = 0;
2049   TState    = TS_RUN;
2050   _thread   = thread;
2051   _event    = thread->_ParkEvent;
2052   _active   = false;
2053   assert(_event != NULL, "invariant");
2054 }
2055 
2056 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2057   JavaThread *jt = (JavaThread *)this->_thread;
2058   _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2059 }
2060 
2061 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2062   JavaThread *jt = (JavaThread *)this->_thread;
2063   JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2064 }
2065 
2066 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2067   assert(node != NULL, "should not add NULL node");
2068   assert(node->_prev == NULL, "node already in list");
2069   assert(node->_next == NULL, "node already in list");
2070   // put node at end of queue (circular doubly linked list)
2071   if (_WaitSet == NULL) {
2072     _WaitSet = node;
2073     node->_prev = node;
2074     node->_next = node;
2075   } else {
2076     ObjectWaiter* head = _WaitSet;
2077     ObjectWaiter* tail = head->_prev;
2078     assert(tail->_next == head, "invariant check");
2079     tail->_next = node;
2080     head->_prev = node;
2081     node->_next = head;
2082     node->_prev = tail;
2083   }
2084 }
2085 
2086 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2087   // dequeue the very first waiter
2088   ObjectWaiter* waiter = _WaitSet;
2089   if (waiter) {
2090     DequeueSpecificWaiter(waiter);
2091   }
2092   return waiter;
2093 }
2094 
2095 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2096   assert(node != NULL, "should not dequeue NULL node");
2097   assert(node->_prev != NULL, "node already removed from list");
2098   assert(node->_next != NULL, "node already removed from list");
2099   // when the waiter has woken up because of interrupt,
2100   // timeout or other spurious wake-up, dequeue the
2101   // waiter from waiting list
2102   ObjectWaiter* next = node->_next;
2103   if (next == node) {
2104     assert(node->_prev == node, "invariant check");
2105     _WaitSet = NULL;
2106   } else {
2107     ObjectWaiter* prev = node->_prev;
2108     assert(prev->_next == node, "invariant check");
2109     assert(next->_prev == node, "invariant check");
2110     next->_prev = prev;
2111     prev->_next = next;
2112     if (_WaitSet == node) {
2113       _WaitSet = next;
2114     }
2115   }
2116   node->_next = NULL;
2117   node->_prev = NULL;
2118 }
2119 
2120 // -----------------------------------------------------------------------------
2121 // PerfData support
2122 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts       = NULL;
2123 PerfCounter * ObjectMonitor::_sync_FutileWakeups               = NULL;
2124 PerfCounter * ObjectMonitor::_sync_Parks                       = NULL;
2125 PerfCounter * ObjectMonitor::_sync_Notifications               = NULL;
2126 PerfCounter * ObjectMonitor::_sync_Inflations                  = NULL;
2127 PerfCounter * ObjectMonitor::_sync_Deflations                  = NULL;
2128 PerfLongVariable * ObjectMonitor::_sync_MonExtant              = NULL;
2129 
2130 // One-shot global initialization for the sync subsystem.
2131 // We could also defer initialization and initialize on-demand
2132 // the first time we call inflate().  Initialization would
2133 // be protected - like so many things - by the MonitorCache_lock.
2134 
2135 void ObjectMonitor::Initialize() {
2136   static int InitializationCompleted = 0;
2137   assert(InitializationCompleted == 0, "invariant");
2138   InitializationCompleted = 1;
2139   if (UsePerfData) {
2140     EXCEPTION_MARK;
2141 #define NEWPERFCOUNTER(n)                                                \
2142   {                                                                      \
2143     n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,  \
2144                                         CHECK);                          \
2145   }
2146 #define NEWPERFVARIABLE(n)                                                \
2147   {                                                                       \
2148     n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,  \
2149                                          CHECK);                          \
2150   }
2151     NEWPERFCOUNTER(_sync_Inflations);
2152     NEWPERFCOUNTER(_sync_Deflations);
2153     NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2154     NEWPERFCOUNTER(_sync_FutileWakeups);
2155     NEWPERFCOUNTER(_sync_Parks);
2156     NEWPERFCOUNTER(_sync_Notifications);
2157     NEWPERFVARIABLE(_sync_MonExtant);
2158 #undef NEWPERFCOUNTER
2159 #undef NEWPERFVARIABLE
2160   }
2161 }
2162 
2163 void ObjectMonitor::DeferredInitialize() {
2164   if (InitDone > 0) return;
2165   if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2166     while (InitDone != 1) /* empty */;
2167     return;
2168   }
2169 
2170   // One-shot global initialization ...
2171   // The initialization is idempotent, so we don't need locks.
2172   // In the future consider doing this via os::init_2().
2173 
2174   if (!os::is_MP()) {
2175     Knob_SpinLimit = 0;
2176     Knob_PreSpin   = 0;
2177     Knob_FixedSpin = -1;
2178   }
2179 
2180   OrderAccess::fence();
2181   InitDone = 1;
2182 }
2183 
--- EOF ---