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

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

1848   if (ctr <= 0) return 0;
1849 
1850   if (Knob_SuccRestrict && _succ != NULL) return 0;
1851   if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
1852     return 0;
1853   }
1854 
1855   int MaxSpin = Knob_MaxSpinners;
1856   if (MaxSpin >= 0) {
1857     if (_Spinner > MaxSpin) {
1858       return 0;
1859     }
1860     // Slightly racy, but benign ...
1861     Adjust(&_Spinner, 1);
1862   }
1863 
1864   // We're good to spin ... spin ingress.
1865   // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1866   // when preparing to LD...CAS _owner, etc and the CAS is likely
1867   // to succeed.
1868   int caspty  = Knob_CASPenalty;
1869   int oxpty   = Knob_OXPenalty;
1870   int sss     = Knob_SpinSetSucc;
1871   if (sss && _succ == NULL) _succ = Self;
1872   Thread * prv = NULL;
1873 
1874   // There are three ways to exit the following loop:
1875   // 1.  A successful spin where this thread has acquired the lock.
1876   // 2.  Spin failure with prejudice
1877   // 3.  Spin failure without prejudice
1878 
1879   while (--ctr >= 0) {
1880 
1881     // Periodic polling -- Check for pending GC
1882     // Threads may spin while they're unsafe.
1883     // We don't want spinning threads to delay the JVM from reaching
1884     // a stop-the-world safepoint or to steal cycles from GC.
1885     // If we detect a pending safepoint we abort in order that
1886     // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1887     // this thread, if safe, doesn't steal cycles from GC.
1888     // This is in keeping with the "no loitering in runtime" rule.
1889     // We periodically check to see if there's a safepoint pending.
1890     if ((ctr & 0xFF) == 0) {
1891       if (SafepointMechanism::poll(Self)) {
1892         goto Abort;           // abrupt spin egress
1893       }
1894       if (Knob_UsePause & 1) SpinPause();
1895     }
1896 
1897     if (Knob_UsePause & 2) SpinPause();
1898 
1899     // Probe _owner with TATAS
1900     // If this thread observes the monitor transition or flicker
1901     // from locked to unlocked to locked, then the odds that this
1902     // thread will acquire the lock in this spin attempt go down
1903     // considerably.  The same argument applies if the CAS fails
1904     // or if we observe _owner change from one non-null value to
1905     // another non-null value.   In such cases we might abort
1906     // the spin without prejudice or apply a "penalty" to the
1907     // spin count-down variable "ctr", reducing it by 100, say.
1908 
1909     Thread * ox = (Thread *) _owner;
1910     if (ox == NULL) {
1911       ox = (Thread*)Atomic::cmpxchg(Self, &_owner, (void*)NULL);
1912       if (ox == NULL) {
1913         // The CAS succeeded -- this thread acquired ownership
1914         // Take care of some bookkeeping to exit spin state.
1915         if (sss && _succ == Self) {
1916           _succ = NULL;
1917         }
1918         if (MaxSpin > 0) Adjust(&_Spinner, -1);
1919 
1920         // Increase _SpinDuration :
1921         // The spin was successful (profitable) so we tend toward
1922         // longer spin attempts in the future.
1923         // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
1924         // If we acquired the lock early in the spin cycle it
1925         // makes sense to increase _SpinDuration proportionally.
1926         // Note that we don't clamp SpinDuration precisely at SpinLimit.
1927         int x = _SpinDuration;
1928         if (x < Knob_SpinLimit) {
1929           if (x < Knob_Poverty) x = Knob_Poverty;
1930           _SpinDuration = x + Knob_Bonus;
1931         }
1932         return 1;
1933       }
1934 
1935       // The CAS failed ... we can take any of the following actions:
1936       // * penalize: ctr -= Knob_CASPenalty
1937       // * exit spin with prejudice -- goto Abort;
1938       // * exit spin without prejudice.
1939       // * Since CAS is high-latency, retry again immediately.
1940       prv = ox;
1941       if (caspty == -2) break;
1942       if (caspty == -1) goto Abort;
1943       ctr -= caspty;
1944       continue;
1945     }
1946 
1947     // Did lock ownership change hands ?
1948     if (ox != prv && prv != NULL) {
1949       if (oxpty == -2) break;
1950       if (oxpty == -1) goto Abort;
1951       ctr -= oxpty;
1952     }
1953     prv = ox;
1954 
1955     // Abort the spin if the owner is not executing.
1956     // The owner must be executing in order to drop the lock.
1957     // Spinning while the owner is OFFPROC is idiocy.
1958     // Consider: ctr -= RunnablePenalty ;
1959     if (Knob_OState && NotRunnable (Self, ox)) {
1960       goto Abort;
1961     }
1962     if (sss && _succ == NULL) _succ = Self;
1963   }
1964 
1965   // Spin failed with prejudice -- reduce _SpinDuration.
1966   // TODO: Use an AIMD-like policy to adjust _SpinDuration.
1967   // AIMD is globally stable.
1968   {
1969     int x = _SpinDuration;
1970     if (x > 0) {
1971       // Consider an AIMD scheme like: x -= (x >> 3) + 100
1972       // This is globally sample and tends to damp the response.
1973       x -= Knob_Penalty;
1974       if (x < 0) x = 0;
1975       _SpinDuration = x;
1976     }
1977   }
1978 
1979  Abort:
1980   if (MaxSpin >= 0) Adjust(&_Spinner, -1);
1981   if (sss && _succ == Self) {
1982     _succ = NULL;
1983     // Invariant: after setting succ=null a contending thread
1984     // must recheck-retry _owner before parking.  This usually happens
1985     // in the normal usage of TrySpin(), but it's safest
1986     // to make TrySpin() as foolproof as possible.
1987     OrderAccess::fence();
1988     if (TryLock(Self) > 0) return 1;
1989   }
1990   return 0;
1991 }
1992 
1993 // NotRunnable() -- informed spinning
1994 //
1995 // Don't bother spinning if the owner is not eligible to drop the lock.
1996 // Spin only if the owner thread is _thread_in_Java or _thread_in_vm.
1997 // The thread must be runnable in order to drop the lock in timely fashion.
1998 // If the _owner is not runnable then spinning will not likely be
1999 // successful (profitable).
2000 //
2001 // Beware -- the thread referenced by _owner could have died
2002 // so a simply fetch from _owner->_thread_state might trap.
2003 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2004 // Because of the lifecycle issues, the _thread_state values
2005 // observed by NotRunnable() might be garbage.  NotRunnable must
2006 // tolerate this and consider the observed _thread_state value
2007 // as advisory.
2008 //
2009 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2010 // a thread pointer.
2011 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2012 // with the LSB of _owner.  Another option would be to probabilistically probe
2013 // the putative _owner->TypeTag value.
2014 //
2015 // Checking _thread_state isn't perfect.  Even if the thread is
2016 // in_java it might be blocked on a page-fault or have been preempted
2017 // and sitting on a ready/dispatch queue.
2018 //
2019 // The return value from NotRunnable() is *advisory* -- the
2020 // result is based on sampling and is not necessarily coherent.
2021 // The caller must tolerate false-negative and false-positive errors.
2022 // Spinning, in general, is probabilistic anyway.
2023 
2024 
2025 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) {
2026   // Check ox->TypeTag == 2BAD.
2027   if (ox == NULL) return 0;
2028 
2029   // Avoid transitive spinning ...
2030   // Say T1 spins or blocks trying to acquire L.  T1._Stalled is set to L.
2031   // Immediately after T1 acquires L it's possible that T2, also
2032   // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2033   // This occurs transiently after T1 acquired L but before
2034   // T1 managed to clear T1.Stalled.  T2 does not need to abort
2035   // its spin in this circumstance.
2036   intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
2037 
2038   if (BlockedOn == 1) return 1;
2039   if (BlockedOn != 0) {
2040     return BlockedOn != intptr_t(this) && _owner == ox;
2041   }
2042 
2043   assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant");
2044   int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);;
2045   // consider also: jst != _thread_in_Java -- but that's overspecific.
2046   return jst == _thread_blocked || jst == _thread_in_native;
2047 }
2048 
2049 
2050 // -----------------------------------------------------------------------------
2051 // WaitSet management ...
2052 
2053 ObjectWaiter::ObjectWaiter(Thread* thread) {
2054   _next     = NULL;
2055   _prev     = NULL;
2056   _notified = 0;
2057   _notifier_tid = 0;
2058   TState    = TS_RUN;
2059   _thread   = thread;
2060   _event    = thread->_ParkEvent;
2061   _active   = false;
2062   assert(_event != NULL, "invariant");
2063 }
2064 
2065 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2066   JavaThread *jt = (JavaThread *)this->_thread;
2067   _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2068 }
2069 
2070 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2071   JavaThread *jt = (JavaThread *)this->_thread;
2072   JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2073 }
2074 
2075 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2076   assert(node != NULL, "should not add NULL node");
2077   assert(node->_prev == NULL, "node already in list");
2078   assert(node->_next == NULL, "node already in list");
2079   // put node at end of queue (circular doubly linked list)
2080   if (_WaitSet == NULL) {
2081     _WaitSet = node;
2082     node->_prev = node;
2083     node->_next = node;
2084   } else {
2085     ObjectWaiter* head = _WaitSet;
2086     ObjectWaiter* tail = head->_prev;
2087     assert(tail->_next == head, "invariant check");
2088     tail->_next = node;
2089     head->_prev = node;
2090     node->_next = head;
2091     node->_prev = tail;
2092   }
2093 }
2094 
2095 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2096   // dequeue the very first waiter
2097   ObjectWaiter* waiter = _WaitSet;
2098   if (waiter) {
2099     DequeueSpecificWaiter(waiter);
2100   }
2101   return waiter;
2102 }
2103 
2104 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2105   assert(node != NULL, "should not dequeue NULL node");
2106   assert(node->_prev != NULL, "node already removed from list");
2107   assert(node->_next != NULL, "node already removed from list");
2108   // when the waiter has woken up because of interrupt,
2109   // timeout or other spurious wake-up, dequeue the
2110   // waiter from waiting list
2111   ObjectWaiter* next = node->_next;
2112   if (next == node) {
2113     assert(node->_prev == node, "invariant check");
2114     _WaitSet = NULL;
2115   } else {
2116     ObjectWaiter* prev = node->_prev;
2117     assert(prev->_next == node, "invariant check");
2118     assert(next->_prev == node, "invariant check");
2119     next->_prev = prev;
2120     prev->_next = next;
2121     if (_WaitSet == node) {
2122       _WaitSet = next;
2123     }
2124   }
2125   node->_next = NULL;
2126   node->_prev = NULL;
2127 }
2128 
2129 // -----------------------------------------------------------------------------
2130 // PerfData support
2131 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts       = NULL;
2132 PerfCounter * ObjectMonitor::_sync_FutileWakeups               = NULL;
2133 PerfCounter * ObjectMonitor::_sync_Parks                       = NULL;
2134 PerfCounter * ObjectMonitor::_sync_Notifications               = NULL;
2135 PerfCounter * ObjectMonitor::_sync_Inflations                  = NULL;
2136 PerfCounter * ObjectMonitor::_sync_Deflations                  = NULL;
2137 PerfLongVariable * ObjectMonitor::_sync_MonExtant              = NULL;
2138 
2139 // One-shot global initialization for the sync subsystem.
2140 // We could also defer initialization and initialize on-demand
2141 // the first time we call inflate().  Initialization would
2142 // be protected - like so many things - by the MonitorCache_lock.
2143 
2144 void ObjectMonitor::Initialize() {
2145   static int InitializationCompleted = 0;
2146   assert(InitializationCompleted == 0, "invariant");
2147   InitializationCompleted = 1;
2148   if (UsePerfData) {
2149     EXCEPTION_MARK;
2150 #define NEWPERFCOUNTER(n)                                                \
2151   {                                                                      \
2152     n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,  \
2153                                         CHECK);                          \
2154   }
2155 #define NEWPERFVARIABLE(n)                                                \
2156   {                                                                       \
2157     n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,  \
2158                                          CHECK);                          \
2159   }
2160     NEWPERFCOUNTER(_sync_Inflations);
2161     NEWPERFCOUNTER(_sync_Deflations);
2162     NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2163     NEWPERFCOUNTER(_sync_FutileWakeups);
2164     NEWPERFCOUNTER(_sync_Parks);
2165     NEWPERFCOUNTER(_sync_Notifications);
2166     NEWPERFVARIABLE(_sync_MonExtant);
2167 #undef NEWPERFCOUNTER
2168 #undef NEWPERFVARIABLE
2169   }
2170 }
2171 
2172 void ObjectMonitor::DeferredInitialize() {
2173   if (InitDone > 0) return;
2174   if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2175     while (InitDone != 1) /* empty */;
2176     return;
2177   }
2178 
2179   // One-shot global initialization ...
2180   // The initialization is idempotent, so we don't need locks.
2181   // In the future consider doing this via os::init_2().
2182 
2183   if (!os::is_MP()) {
2184     Knob_SpinLimit = 0;

2185     Knob_PreSpin   = 0;
2186     Knob_FixedSpin = -1;
2187   }
2188 
2189   OrderAccess::fence();
2190   InitDone = 1;
2191 }
2192 
--- EOF ---