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

   1 /*
   2  * Copyright (c) 1998, 2018, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.
   8  *
   9  * This code is distributed in the hope that it will be useful, but WITHOUT
  10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  12  * version 2 for more details (a copy is included in the LICENSE file that
  13  * accompanied this code).
  14  *
  15  * You should have received a copy of the GNU General Public License version
  16  * 2 along with this work; if not, write to the Free Software Foundation,
  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  18  *
  19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  20  * or visit www.oracle.com if you need additional information or have any
  21  * questions.
  22  *
  23  */
  24 
  25 #include "precompiled.hpp"
  26 #include "classfile/vmSymbols.hpp"
  27 #include "jfr/jfrEvents.hpp"
  28 #include "jfr/support/jfrThreadId.hpp"
  29 #include "memory/allocation.inline.hpp"
  30 #include "memory/resourceArea.hpp"
  31 #include "oops/markOop.hpp"
  32 #include "oops/oop.inline.hpp"
  33 #include "runtime/atomic.hpp"
  34 #include "runtime/handles.inline.hpp"
  35 #include "runtime/interfaceSupport.inline.hpp"
  36 #include "runtime/mutexLocker.hpp"
  37 #include "runtime/objectMonitor.hpp"
  38 #include "runtime/objectMonitor.inline.hpp"
  39 #include "runtime/orderAccess.hpp"
  40 #include "runtime/osThread.hpp"
  41 #include "runtime/safepointMechanism.inline.hpp"
  42 #include "runtime/sharedRuntime.hpp"
  43 #include "runtime/stubRoutines.hpp"
  44 #include "runtime/thread.inline.hpp"
  45 #include "services/threadService.hpp"
  46 #include "utilities/dtrace.hpp"
  47 #include "utilities/macros.hpp"
  48 #include "utilities/preserveException.hpp"
  49 #if INCLUDE_JFR
  50 #include "jfr/support/jfrFlush.hpp"
  51 #endif
  52 
  53 #ifdef DTRACE_ENABLED
  54 
  55 // Only bother with this argument setup if dtrace is available
  56 // TODO-FIXME: probes should not fire when caller is _blocked.  assert() accordingly.
  57 
  58 
  59 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread)                           \
  60   char* bytes = NULL;                                                      \
  61   int len = 0;                                                             \
  62   jlong jtid = SharedRuntime::get_java_tid(thread);                        \
  63   Symbol* klassname = ((oop)obj)->klass()->name();                         \
  64   if (klassname != NULL) {                                                 \
  65     bytes = (char*)klassname->bytes();                                     \
  66     len = klassname->utf8_length();                                        \
  67   }
  68 
  69 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis)            \
  70   {                                                                        \
  71     if (DTraceMonitorProbes) {                                             \
  72       DTRACE_MONITOR_PROBE_COMMON(obj, thread);                            \
  73       HOTSPOT_MONITOR_WAIT(jtid,                                           \
  74                            (monitor), bytes, len, (millis));               \
  75     }                                                                      \
  76   }
  77 
  78 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
  79 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
  80 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
  81 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
  82 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
  83 
  84 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread)                  \
  85   {                                                                        \
  86     if (DTraceMonitorProbes) {                                             \
  87       DTRACE_MONITOR_PROBE_COMMON(obj, thread);                            \
  88       HOTSPOT_MONITOR_##probe(jtid,                                        \
  89                               (uintptr_t)(monitor), bytes, len);           \
  90     }                                                                      \
  91   }
  92 
  93 #else //  ndef DTRACE_ENABLED
  94 
  95 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon)    {;}
  96 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon)          {;}
  97 
  98 #endif // ndef DTRACE_ENABLED
  99 
 100 // Tunables ...
 101 // The knob* variables are effectively final.  Once set they should
 102 // never be modified hence.  Consider using __read_mostly with GCC.
 103 
 104 int ObjectMonitor::Knob_SpinLimit    = 5000;    // derived by an external tool -
 105 

 106 static int Knob_SpinEarly           = 1;
 107 static int Knob_SuccEnabled         = 1;       // futile wake throttling
 108 static int Knob_SuccRestrict        = 0;       // Limit successors + spinners to at-most-one
 109 static int Knob_MaxSpinners         = -1;      // Should be a function of # CPUs
 110 static int Knob_Bonus               = 100;     // spin success bonus
 111 static int Knob_BonusB              = 100;     // spin success bonus
 112 static int Knob_Penalty             = 200;     // spin failure penalty
 113 static int Knob_Poverty             = 1000;
 114 static int Knob_SpinAfterFutile     = 1;       // Spin after returning from park()
 115 static int Knob_FixedSpin           = 0;
 116 static int Knob_OState              = 3;       // Spinner checks thread state of _owner
 117 static int Knob_UsePause            = 1;
 118 static int Knob_ExitPolicy          = 0;
 119 static int Knob_PreSpin             = 10;      // 20-100 likely better
 120 static int Knob_ResetEvent          = 0;
 121 
 122 static int Knob_FastHSSEC           = 0;
 123 static int Knob_MoveNotifyee        = 2;       // notify() - disposition of notifyee
 124 static int Knob_QMode               = 0;       // EntryList-cxq policy - queue discipline
 125 static volatile int InitDone        = 0;
 126 
 127 // -----------------------------------------------------------------------------
 128 // Theory of operations -- Monitors lists, thread residency, etc:
 129 //
 130 // * A thread acquires ownership of a monitor by successfully
 131 //   CAS()ing the _owner field from null to non-null.
 132 //
 133 // * Invariant: A thread appears on at most one monitor list --
 134 //   cxq, EntryList or WaitSet -- at any one time.
 135 //
 136 // * Contending threads "push" themselves onto the cxq with CAS
 137 //   and then spin/park.
 138 //
 139 // * After a contending thread eventually acquires the lock it must
 140 //   dequeue itself from either the EntryList or the cxq.
 141 //
 142 // * The exiting thread identifies and unparks an "heir presumptive"
 143 //   tentative successor thread on the EntryList.  Critically, the
 144 //   exiting thread doesn't unlink the successor thread from the EntryList.
 145 //   After having been unparked, the wakee will recontend for ownership of
 146 //   the monitor.   The successor (wakee) will either acquire the lock or
 147 //   re-park itself.
 148 //
 149 //   Succession is provided for by a policy of competitive handoff.
 150 //   The exiting thread does _not_ grant or pass ownership to the
 151 //   successor thread.  (This is also referred to as "handoff" succession").
 152 //   Instead the exiting thread releases ownership and possibly wakes
 153 //   a successor, so the successor can (re)compete for ownership of the lock.
 154 //   If the EntryList is empty but the cxq is populated the exiting
 155 //   thread will drain the cxq into the EntryList.  It does so by
 156 //   by detaching the cxq (installing null with CAS) and folding
 157 //   the threads from the cxq into the EntryList.  The EntryList is
 158 //   doubly linked, while the cxq is singly linked because of the
 159 //   CAS-based "push" used to enqueue recently arrived threads (RATs).
 160 //
 161 // * Concurrency invariants:
 162 //
 163 //   -- only the monitor owner may access or mutate the EntryList.
 164 //      The mutex property of the monitor itself protects the EntryList
 165 //      from concurrent interference.
 166 //   -- Only the monitor owner may detach the cxq.
 167 //
 168 // * The monitor entry list operations avoid locks, but strictly speaking
 169 //   they're not lock-free.  Enter is lock-free, exit is not.
 170 //   For a description of 'Methods and apparatus providing non-blocking access
 171 //   to a resource,' see U.S. Pat. No. 7844973.
 172 //
 173 // * The cxq can have multiple concurrent "pushers" but only one concurrent
 174 //   detaching thread.  This mechanism is immune from the ABA corruption.
 175 //   More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
 176 //
 177 // * Taken together, the cxq and the EntryList constitute or form a
 178 //   single logical queue of threads stalled trying to acquire the lock.
 179 //   We use two distinct lists to improve the odds of a constant-time
 180 //   dequeue operation after acquisition (in the ::enter() epilogue) and
 181 //   to reduce heat on the list ends.  (c.f. Michael Scott's "2Q" algorithm).
 182 //   A key desideratum is to minimize queue & monitor metadata manipulation
 183 //   that occurs while holding the monitor lock -- that is, we want to
 184 //   minimize monitor lock holds times.  Note that even a small amount of
 185 //   fixed spinning will greatly reduce the # of enqueue-dequeue operations
 186 //   on EntryList|cxq.  That is, spinning relieves contention on the "inner"
 187 //   locks and monitor metadata.
 188 //
 189 //   Cxq points to the set of Recently Arrived Threads attempting entry.
 190 //   Because we push threads onto _cxq with CAS, the RATs must take the form of
 191 //   a singly-linked LIFO.  We drain _cxq into EntryList  at unlock-time when
 192 //   the unlocking thread notices that EntryList is null but _cxq is != null.
 193 //
 194 //   The EntryList is ordered by the prevailing queue discipline and
 195 //   can be organized in any convenient fashion, such as a doubly-linked list or
 196 //   a circular doubly-linked list.  Critically, we want insert and delete operations
 197 //   to operate in constant-time.  If we need a priority queue then something akin
 198 //   to Solaris' sleepq would work nicely.  Viz.,
 199 //   http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
 200 //   Queue discipline is enforced at ::exit() time, when the unlocking thread
 201 //   drains the cxq into the EntryList, and orders or reorders the threads on the
 202 //   EntryList accordingly.
 203 //
 204 //   Barring "lock barging", this mechanism provides fair cyclic ordering,
 205 //   somewhat similar to an elevator-scan.
 206 //
 207 // * The monitor synchronization subsystem avoids the use of native
 208 //   synchronization primitives except for the narrow platform-specific
 209 //   park-unpark abstraction.  See the comments in os_solaris.cpp regarding
 210 //   the semantics of park-unpark.  Put another way, this monitor implementation
 211 //   depends only on atomic operations and park-unpark.  The monitor subsystem
 212 //   manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
 213 //   underlying OS manages the READY<->RUN transitions.
 214 //
 215 // * Waiting threads reside on the WaitSet list -- wait() puts
 216 //   the caller onto the WaitSet.
 217 //
 218 // * notify() or notifyAll() simply transfers threads from the WaitSet to
 219 //   either the EntryList or cxq.  Subsequent exit() operations will
 220 //   unpark the notifyee.  Unparking a notifee in notify() is inefficient -
 221 //   it's likely the notifyee would simply impale itself on the lock held
 222 //   by the notifier.
 223 //
 224 // * An interesting alternative is to encode cxq as (List,LockByte) where
 225 //   the LockByte is 0 iff the monitor is owned.  _owner is simply an auxiliary
 226 //   variable, like _recursions, in the scheme.  The threads or Events that form
 227 //   the list would have to be aligned in 256-byte addresses.  A thread would
 228 //   try to acquire the lock or enqueue itself with CAS, but exiting threads
 229 //   could use a 1-0 protocol and simply STB to set the LockByte to 0.
 230 //   Note that is is *not* word-tearing, but it does presume that full-word
 231 //   CAS operations are coherent with intermix with STB operations.  That's true
 232 //   on most common processors.
 233 //
 234 // * See also http://blogs.sun.com/dave
 235 
 236 
 237 void* ObjectMonitor::operator new (size_t size) throw() {
 238   return AllocateHeap(size, mtInternal);
 239 }
 240 void* ObjectMonitor::operator new[] (size_t size) throw() {
 241   return operator new (size);
 242 }
 243 void ObjectMonitor::operator delete(void* p) {
 244   FreeHeap(p);
 245 }
 246 void ObjectMonitor::operator delete[] (void *p) {
 247   operator delete(p);
 248 }
 249 
 250 // -----------------------------------------------------------------------------
 251 // Enter support
 252 
 253 void ObjectMonitor::enter(TRAPS) {
 254   // The following code is ordered to check the most common cases first
 255   // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
 256   Thread * const Self = THREAD;
 257 
 258   void * cur = Atomic::cmpxchg(Self, &_owner, (void*)NULL);
 259   if (cur == NULL) {
 260     // Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
 261     assert(_recursions == 0, "invariant");
 262     assert(_owner == Self, "invariant");
 263     return;
 264   }
 265 
 266   if (cur == Self) {
 267     // TODO-FIXME: check for integer overflow!  BUGID 6557169.
 268     _recursions++;
 269     return;
 270   }
 271 
 272   if (Self->is_lock_owned ((address)cur)) {
 273     assert(_recursions == 0, "internal state error");
 274     _recursions = 1;
 275     // Commute owner from a thread-specific on-stack BasicLockObject address to
 276     // a full-fledged "Thread *".
 277     _owner = Self;
 278     return;
 279   }
 280 
 281   // We've encountered genuine contention.
 282   assert(Self->_Stalled == 0, "invariant");
 283   Self->_Stalled = intptr_t(this);
 284 
 285   // Try one round of spinning *before* enqueueing Self
 286   // and before going through the awkward and expensive state
 287   // transitions.  The following spin is strictly optional ...
 288   // Note that if we acquire the monitor from an initial spin
 289   // we forgo posting JVMTI events and firing DTRACE probes.
 290   if (Knob_SpinEarly && TrySpin (Self) > 0) {
 291     assert(_owner == Self, "invariant");
 292     assert(_recursions == 0, "invariant");
 293     assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
 294     Self->_Stalled = 0;
 295     return;
 296   }
 297 
 298   assert(_owner != Self, "invariant");
 299   assert(_succ != Self, "invariant");
 300   assert(Self->is_Java_thread(), "invariant");
 301   JavaThread * jt = (JavaThread *) Self;
 302   assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
 303   assert(jt->thread_state() != _thread_blocked, "invariant");
 304   assert(this->object() != NULL, "invariant");
 305   assert(_count >= 0, "invariant");
 306 
 307   // Prevent deflation at STW-time.  See deflate_idle_monitors() and is_busy().
 308   // Ensure the object-monitor relationship remains stable while there's contention.
 309   Atomic::inc(&_count);
 310 
 311   JFR_ONLY(JfrConditionalFlushWithStacktrace<EventJavaMonitorEnter> flush(jt);)
 312   EventJavaMonitorEnter event;
 313   if (event.should_commit()) {
 314     event.set_monitorClass(((oop)this->object())->klass());
 315     event.set_address((uintptr_t)(this->object_addr()));
 316   }
 317 
 318   { // Change java thread status to indicate blocked on monitor enter.
 319     JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
 320 
 321     Self->set_current_pending_monitor(this);
 322 
 323     DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
 324     if (JvmtiExport::should_post_monitor_contended_enter()) {
 325       JvmtiExport::post_monitor_contended_enter(jt, this);
 326 
 327       // The current thread does not yet own the monitor and does not
 328       // yet appear on any queues that would get it made the successor.
 329       // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
 330       // handler cannot accidentally consume an unpark() meant for the
 331       // ParkEvent associated with this ObjectMonitor.
 332     }
 333 
 334     OSThreadContendState osts(Self->osthread());
 335     ThreadBlockInVM tbivm(jt);
 336 
 337     // TODO-FIXME: change the following for(;;) loop to straight-line code.
 338     for (;;) {
 339       jt->set_suspend_equivalent();
 340       // cleared by handle_special_suspend_equivalent_condition()
 341       // or java_suspend_self()
 342 
 343       EnterI(THREAD);
 344 
 345       if (!ExitSuspendEquivalent(jt)) break;
 346 
 347       // We have acquired the contended monitor, but while we were
 348       // waiting another thread suspended us. We don't want to enter
 349       // the monitor while suspended because that would surprise the
 350       // thread that suspended us.
 351       //
 352       _recursions = 0;
 353       _succ = NULL;
 354       exit(false, Self);
 355 
 356       jt->java_suspend_self();
 357     }
 358     Self->set_current_pending_monitor(NULL);
 359 
 360     // We cleared the pending monitor info since we've just gotten past
 361     // the enter-check-for-suspend dance and we now own the monitor free
 362     // and clear, i.e., it is no longer pending. The ThreadBlockInVM
 363     // destructor can go to a safepoint at the end of this block. If we
 364     // do a thread dump during that safepoint, then this thread will show
 365     // as having "-locked" the monitor, but the OS and java.lang.Thread
 366     // states will still report that the thread is blocked trying to
 367     // acquire it.
 368   }
 369 
 370   Atomic::dec(&_count);
 371   assert(_count >= 0, "invariant");
 372   Self->_Stalled = 0;
 373 
 374   // Must either set _recursions = 0 or ASSERT _recursions == 0.
 375   assert(_recursions == 0, "invariant");
 376   assert(_owner == Self, "invariant");
 377   assert(_succ != Self, "invariant");
 378   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
 379 
 380   // The thread -- now the owner -- is back in vm mode.
 381   // Report the glorious news via TI,DTrace and jvmstat.
 382   // The probe effect is non-trivial.  All the reportage occurs
 383   // while we hold the monitor, increasing the length of the critical
 384   // section.  Amdahl's parallel speedup law comes vividly into play.
 385   //
 386   // Another option might be to aggregate the events (thread local or
 387   // per-monitor aggregation) and defer reporting until a more opportune
 388   // time -- such as next time some thread encounters contention but has
 389   // yet to acquire the lock.  While spinning that thread could
 390   // spinning we could increment JVMStat counters, etc.
 391 
 392   DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
 393   if (JvmtiExport::should_post_monitor_contended_entered()) {
 394     JvmtiExport::post_monitor_contended_entered(jt, this);
 395 
 396     // The current thread already owns the monitor and is not going to
 397     // call park() for the remainder of the monitor enter protocol. So
 398     // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
 399     // event handler consumed an unpark() issued by the thread that
 400     // just exited the monitor.
 401   }
 402   if (event.should_commit()) {
 403     event.set_previousOwner((uintptr_t)_previous_owner_tid);
 404     event.commit();
 405   }
 406   OM_PERFDATA_OP(ContendedLockAttempts, inc());
 407 }
 408 
 409 // Caveat: TryLock() is not necessarily serializing if it returns failure.
 410 // Callers must compensate as needed.
 411 
 412 int ObjectMonitor::TryLock(Thread * Self) {
 413   void * own = _owner;
 414   if (own != NULL) return 0;
 415   if (Atomic::replace_if_null(Self, &_owner)) {
 416     // Either guarantee _recursions == 0 or set _recursions = 0.
 417     assert(_recursions == 0, "invariant");
 418     assert(_owner == Self, "invariant");
 419     return 1;
 420   }
 421   // The lock had been free momentarily, but we lost the race to the lock.
 422   // Interference -- the CAS failed.
 423   // We can either return -1 or retry.
 424   // Retry doesn't make as much sense because the lock was just acquired.
 425   return -1;
 426 }
 427 
 428 #define MAX_RECHECK_INTERVAL 1000
 429 
 430 void ObjectMonitor::EnterI(TRAPS) {
 431   Thread * const Self = THREAD;
 432   assert(Self->is_Java_thread(), "invariant");
 433   assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant");
 434 
 435   // Try the lock - TATAS
 436   if (TryLock (Self) > 0) {
 437     assert(_succ != Self, "invariant");
 438     assert(_owner == Self, "invariant");
 439     assert(_Responsible != Self, "invariant");
 440     return;
 441   }
 442 
 443   DeferredInitialize();
 444 
 445   // We try one round of spinning *before* enqueueing Self.
 446   //
 447   // If the _owner is ready but OFFPROC we could use a YieldTo()
 448   // operation to donate the remainder of this thread's quantum
 449   // to the owner.  This has subtle but beneficial affinity
 450   // effects.
 451 
 452   if (TrySpin (Self) > 0) {
 453     assert(_owner == Self, "invariant");
 454     assert(_succ != Self, "invariant");
 455     assert(_Responsible != Self, "invariant");
 456     return;
 457   }
 458 
 459   // The Spin failed -- Enqueue and park the thread ...
 460   assert(_succ != Self, "invariant");
 461   assert(_owner != Self, "invariant");
 462   assert(_Responsible != Self, "invariant");
 463 
 464   // Enqueue "Self" on ObjectMonitor's _cxq.
 465   //
 466   // Node acts as a proxy for Self.
 467   // As an aside, if were to ever rewrite the synchronization code mostly
 468   // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
 469   // Java objects.  This would avoid awkward lifecycle and liveness issues,
 470   // as well as eliminate a subset of ABA issues.
 471   // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
 472 
 473   ObjectWaiter node(Self);
 474   Self->_ParkEvent->reset();
 475   node._prev   = (ObjectWaiter *) 0xBAD;
 476   node.TState  = ObjectWaiter::TS_CXQ;
 477 
 478   // Push "Self" onto the front of the _cxq.
 479   // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
 480   // Note that spinning tends to reduce the rate at which threads
 481   // enqueue and dequeue on EntryList|cxq.
 482   ObjectWaiter * nxt;
 483   for (;;) {
 484     node._next = nxt = _cxq;
 485     if (Atomic::cmpxchg(&node, &_cxq, nxt) == nxt) break;
 486 
 487     // Interference - the CAS failed because _cxq changed.  Just retry.
 488     // As an optional optimization we retry the lock.
 489     if (TryLock (Self) > 0) {
 490       assert(_succ != Self, "invariant");
 491       assert(_owner == Self, "invariant");
 492       assert(_Responsible != Self, "invariant");
 493       return;
 494     }
 495   }
 496 
 497   // Check for cxq|EntryList edge transition to non-null.  This indicates
 498   // the onset of contention.  While contention persists exiting threads
 499   // will use a ST:MEMBAR:LD 1-1 exit protocol.  When contention abates exit
 500   // operations revert to the faster 1-0 mode.  This enter operation may interleave
 501   // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
 502   // arrange for one of the contending thread to use a timed park() operations
 503   // to detect and recover from the race.  (Stranding is form of progress failure
 504   // where the monitor is unlocked but all the contending threads remain parked).
 505   // That is, at least one of the contended threads will periodically poll _owner.
 506   // One of the contending threads will become the designated "Responsible" thread.
 507   // The Responsible thread uses a timed park instead of a normal indefinite park
 508   // operation -- it periodically wakes and checks for and recovers from potential
 509   // strandings admitted by 1-0 exit operations.   We need at most one Responsible
 510   // thread per-monitor at any given moment.  Only threads on cxq|EntryList may
 511   // be responsible for a monitor.
 512   //
 513   // Currently, one of the contended threads takes on the added role of "Responsible".
 514   // A viable alternative would be to use a dedicated "stranding checker" thread
 515   // that periodically iterated over all the threads (or active monitors) and unparked
 516   // successors where there was risk of stranding.  This would help eliminate the
 517   // timer scalability issues we see on some platforms as we'd only have one thread
 518   // -- the checker -- parked on a timer.
 519 
 520   if (nxt == NULL && _EntryList == NULL) {
 521     // Try to assume the role of responsible thread for the monitor.
 522     // CONSIDER:  ST vs CAS vs { if (Responsible==null) Responsible=Self }
 523     Atomic::replace_if_null(Self, &_Responsible);
 524   }
 525 
 526   // The lock might have been released while this thread was occupied queueing
 527   // itself onto _cxq.  To close the race and avoid "stranding" and
 528   // progress-liveness failure we must resample-retry _owner before parking.
 529   // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
 530   // In this case the ST-MEMBAR is accomplished with CAS().
 531   //
 532   // TODO: Defer all thread state transitions until park-time.
 533   // Since state transitions are heavy and inefficient we'd like
 534   // to defer the state transitions until absolutely necessary,
 535   // and in doing so avoid some transitions ...
 536 
 537   int nWakeups = 0;
 538   int recheckInterval = 1;
 539 
 540   for (;;) {
 541 
 542     if (TryLock(Self) > 0) break;
 543     assert(_owner != Self, "invariant");
 544 
 545     // park self
 546     if (_Responsible == Self) {
 547       Self->_ParkEvent->park((jlong) recheckInterval);
 548       // Increase the recheckInterval, but clamp the value.
 549       recheckInterval *= 8;
 550       if (recheckInterval > MAX_RECHECK_INTERVAL) {
 551         recheckInterval = MAX_RECHECK_INTERVAL;
 552       }
 553     } else {
 554       Self->_ParkEvent->park();
 555     }
 556 
 557     if (TryLock(Self) > 0) break;
 558 
 559     // The lock is still contested.
 560     // Keep a tally of the # of futile wakeups.
 561     // Note that the counter is not protected by a lock or updated by atomics.
 562     // That is by design - we trade "lossy" counters which are exposed to
 563     // races during updates for a lower probe effect.
 564 
 565     // This PerfData object can be used in parallel with a safepoint.
 566     // See the work around in PerfDataManager::destroy().
 567     OM_PERFDATA_OP(FutileWakeups, inc());
 568     ++nWakeups;
 569 
 570     // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
 571     // We can defer clearing _succ until after the spin completes
 572     // TrySpin() must tolerate being called with _succ == Self.
 573     // Try yet another round of adaptive spinning.
 574     if ((Knob_SpinAfterFutile & 1) && TrySpin(Self) > 0) break;
 575 
 576     // We can find that we were unpark()ed and redesignated _succ while
 577     // we were spinning.  That's harmless.  If we iterate and call park(),
 578     // park() will consume the event and return immediately and we'll
 579     // just spin again.  This pattern can repeat, leaving _succ to simply
 580     // spin on a CPU.  Enable Knob_ResetEvent to clear pending unparks().
 581     // Alternately, we can sample fired() here, and if set, forgo spinning
 582     // in the next iteration.
 583 
 584     if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) {
 585       Self->_ParkEvent->reset();
 586       OrderAccess::fence();
 587     }
 588     if (_succ == Self) _succ = NULL;
 589 
 590     // Invariant: after clearing _succ a thread *must* retry _owner before parking.
 591     OrderAccess::fence();
 592   }
 593 
 594   // Egress :
 595   // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
 596   // Normally we'll find Self on the EntryList .
 597   // From the perspective of the lock owner (this thread), the
 598   // EntryList is stable and cxq is prepend-only.
 599   // The head of cxq is volatile but the interior is stable.
 600   // In addition, Self.TState is stable.
 601 
 602   assert(_owner == Self, "invariant");
 603   assert(object() != NULL, "invariant");
 604   // I'd like to write:
 605   //   guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
 606   // but as we're at a safepoint that's not safe.
 607 
 608   UnlinkAfterAcquire(Self, &node);
 609   if (_succ == Self) _succ = NULL;
 610 
 611   assert(_succ != Self, "invariant");
 612   if (_Responsible == Self) {
 613     _Responsible = NULL;
 614     OrderAccess::fence(); // Dekker pivot-point
 615 
 616     // We may leave threads on cxq|EntryList without a designated
 617     // "Responsible" thread.  This is benign.  When this thread subsequently
 618     // exits the monitor it can "see" such preexisting "old" threads --
 619     // threads that arrived on the cxq|EntryList before the fence, above --
 620     // by LDing cxq|EntryList.  Newly arrived threads -- that is, threads
 621     // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
 622     // non-null and elect a new "Responsible" timer thread.
 623     //
 624     // This thread executes:
 625     //    ST Responsible=null; MEMBAR    (in enter epilogue - here)
 626     //    LD cxq|EntryList               (in subsequent exit)
 627     //
 628     // Entering threads in the slow/contended path execute:
 629     //    ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
 630     //    The (ST cxq; MEMBAR) is accomplished with CAS().
 631     //
 632     // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
 633     // exit operation from floating above the ST Responsible=null.
 634   }
 635 
 636   // We've acquired ownership with CAS().
 637   // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
 638   // But since the CAS() this thread may have also stored into _succ,
 639   // EntryList, cxq or Responsible.  These meta-data updates must be
 640   // visible __before this thread subsequently drops the lock.
 641   // Consider what could occur if we didn't enforce this constraint --
 642   // STs to monitor meta-data and user-data could reorder with (become
 643   // visible after) the ST in exit that drops ownership of the lock.
 644   // Some other thread could then acquire the lock, but observe inconsistent
 645   // or old monitor meta-data and heap data.  That violates the JMM.
 646   // To that end, the 1-0 exit() operation must have at least STST|LDST
 647   // "release" barrier semantics.  Specifically, there must be at least a
 648   // STST|LDST barrier in exit() before the ST of null into _owner that drops
 649   // the lock.   The barrier ensures that changes to monitor meta-data and data
 650   // protected by the lock will be visible before we release the lock, and
 651   // therefore before some other thread (CPU) has a chance to acquire the lock.
 652   // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
 653   //
 654   // Critically, any prior STs to _succ or EntryList must be visible before
 655   // the ST of null into _owner in the *subsequent* (following) corresponding
 656   // monitorexit.  Recall too, that in 1-0 mode monitorexit does not necessarily
 657   // execute a serializing instruction.
 658 
 659   return;
 660 }
 661 
 662 // ReenterI() is a specialized inline form of the latter half of the
 663 // contended slow-path from EnterI().  We use ReenterI() only for
 664 // monitor reentry in wait().
 665 //
 666 // In the future we should reconcile EnterI() and ReenterI(), adding
 667 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the
 668 // loop accordingly.
 669 
 670 void ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) {
 671   assert(Self != NULL, "invariant");
 672   assert(SelfNode != NULL, "invariant");
 673   assert(SelfNode->_thread == Self, "invariant");
 674   assert(_waiters > 0, "invariant");
 675   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
 676   assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
 677   JavaThread * jt = (JavaThread *) Self;
 678 
 679   int nWakeups = 0;
 680   for (;;) {
 681     ObjectWaiter::TStates v = SelfNode->TState;
 682     guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
 683     assert(_owner != Self, "invariant");
 684 
 685     if (TryLock(Self) > 0) break;
 686     if (TrySpin(Self) > 0) break;
 687 
 688     // State transition wrappers around park() ...
 689     // ReenterI() wisely defers state transitions until
 690     // it's clear we must park the thread.
 691     {
 692       OSThreadContendState osts(Self->osthread());
 693       ThreadBlockInVM tbivm(jt);
 694 
 695       // cleared by handle_special_suspend_equivalent_condition()
 696       // or java_suspend_self()
 697       jt->set_suspend_equivalent();
 698       Self->_ParkEvent->park();
 699 
 700       // were we externally suspended while we were waiting?
 701       for (;;) {
 702         if (!ExitSuspendEquivalent(jt)) break;
 703         if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
 704         jt->java_suspend_self();
 705         jt->set_suspend_equivalent();
 706       }
 707     }
 708 
 709     // Try again, but just so we distinguish between futile wakeups and
 710     // successful wakeups.  The following test isn't algorithmically
 711     // necessary, but it helps us maintain sensible statistics.
 712     if (TryLock(Self) > 0) break;
 713 
 714     // The lock is still contested.
 715     // Keep a tally of the # of futile wakeups.
 716     // Note that the counter is not protected by a lock or updated by atomics.
 717     // That is by design - we trade "lossy" counters which are exposed to
 718     // races during updates for a lower probe effect.
 719     ++nWakeups;
 720 
 721     // Assuming this is not a spurious wakeup we'll normally
 722     // find that _succ == Self.
 723     if (_succ == Self) _succ = NULL;
 724 
 725     // Invariant: after clearing _succ a contending thread
 726     // *must* retry  _owner before parking.
 727     OrderAccess::fence();
 728 
 729     // This PerfData object can be used in parallel with a safepoint.
 730     // See the work around in PerfDataManager::destroy().
 731     OM_PERFDATA_OP(FutileWakeups, inc());
 732   }
 733 
 734   // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
 735   // Normally we'll find Self on the EntryList.
 736   // Unlinking from the EntryList is constant-time and atomic-free.
 737   // From the perspective of the lock owner (this thread), the
 738   // EntryList is stable and cxq is prepend-only.
 739   // The head of cxq is volatile but the interior is stable.
 740   // In addition, Self.TState is stable.
 741 
 742   assert(_owner == Self, "invariant");
 743   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
 744   UnlinkAfterAcquire(Self, SelfNode);
 745   if (_succ == Self) _succ = NULL;
 746   assert(_succ != Self, "invariant");
 747   SelfNode->TState = ObjectWaiter::TS_RUN;
 748   OrderAccess::fence();      // see comments at the end of EnterI()
 749 }
 750 
 751 // By convention we unlink a contending thread from EntryList|cxq immediately
 752 // after the thread acquires the lock in ::enter().  Equally, we could defer
 753 // unlinking the thread until ::exit()-time.
 754 
 755 void ObjectMonitor::UnlinkAfterAcquire(Thread *Self, ObjectWaiter *SelfNode) {
 756   assert(_owner == Self, "invariant");
 757   assert(SelfNode->_thread == Self, "invariant");
 758 
 759   if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
 760     // Normal case: remove Self from the DLL EntryList .
 761     // This is a constant-time operation.
 762     ObjectWaiter * nxt = SelfNode->_next;
 763     ObjectWaiter * prv = SelfNode->_prev;
 764     if (nxt != NULL) nxt->_prev = prv;
 765     if (prv != NULL) prv->_next = nxt;
 766     if (SelfNode == _EntryList) _EntryList = nxt;
 767     assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
 768     assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
 769   } else {
 770     assert(SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant");
 771     // Inopportune interleaving -- Self is still on the cxq.
 772     // This usually means the enqueue of self raced an exiting thread.
 773     // Normally we'll find Self near the front of the cxq, so
 774     // dequeueing is typically fast.  If needbe we can accelerate
 775     // this with some MCS/CHL-like bidirectional list hints and advisory
 776     // back-links so dequeueing from the interior will normally operate
 777     // in constant-time.
 778     // Dequeue Self from either the head (with CAS) or from the interior
 779     // with a linear-time scan and normal non-atomic memory operations.
 780     // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
 781     // and then unlink Self from EntryList.  We have to drain eventually,
 782     // so it might as well be now.
 783 
 784     ObjectWaiter * v = _cxq;
 785     assert(v != NULL, "invariant");
 786     if (v != SelfNode || Atomic::cmpxchg(SelfNode->_next, &_cxq, v) != v) {
 787       // The CAS above can fail from interference IFF a "RAT" arrived.
 788       // In that case Self must be in the interior and can no longer be
 789       // at the head of cxq.
 790       if (v == SelfNode) {
 791         assert(_cxq != v, "invariant");
 792         v = _cxq;          // CAS above failed - start scan at head of list
 793       }
 794       ObjectWaiter * p;
 795       ObjectWaiter * q = NULL;
 796       for (p = v; p != NULL && p != SelfNode; p = p->_next) {
 797         q = p;
 798         assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
 799       }
 800       assert(v != SelfNode, "invariant");
 801       assert(p == SelfNode, "Node not found on cxq");
 802       assert(p != _cxq, "invariant");
 803       assert(q != NULL, "invariant");
 804       assert(q->_next == p, "invariant");
 805       q->_next = p->_next;
 806     }
 807   }
 808 
 809 #ifdef ASSERT
 810   // Diagnostic hygiene ...
 811   SelfNode->_prev  = (ObjectWaiter *) 0xBAD;
 812   SelfNode->_next  = (ObjectWaiter *) 0xBAD;
 813   SelfNode->TState = ObjectWaiter::TS_RUN;
 814 #endif
 815 }
 816 
 817 // -----------------------------------------------------------------------------
 818 // Exit support
 819 //
 820 // exit()
 821 // ~~~~~~
 822 // Note that the collector can't reclaim the objectMonitor or deflate
 823 // the object out from underneath the thread calling ::exit() as the
 824 // thread calling ::exit() never transitions to a stable state.
 825 // This inhibits GC, which in turn inhibits asynchronous (and
 826 // inopportune) reclamation of "this".
 827 //
 828 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
 829 // There's one exception to the claim above, however.  EnterI() can call
 830 // exit() to drop a lock if the acquirer has been externally suspended.
 831 // In that case exit() is called with _thread_state as _thread_blocked,
 832 // but the monitor's _count field is > 0, which inhibits reclamation.
 833 //
 834 // 1-0 exit
 835 // ~~~~~~~~
 836 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
 837 // the fast-path operators have been optimized so the common ::exit()
 838 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
 839 // The code emitted by fast_unlock() elides the usual MEMBAR.  This
 840 // greatly improves latency -- MEMBAR and CAS having considerable local
 841 // latency on modern processors -- but at the cost of "stranding".  Absent the
 842 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
 843 // ::enter() path, resulting in the entering thread being stranding
 844 // and a progress-liveness failure.   Stranding is extremely rare.
 845 // We use timers (timed park operations) & periodic polling to detect
 846 // and recover from stranding.  Potentially stranded threads periodically
 847 // wake up and poll the lock.  See the usage of the _Responsible variable.
 848 //
 849 // The CAS() in enter provides for safety and exclusion, while the CAS or
 850 // MEMBAR in exit provides for progress and avoids stranding.  1-0 locking
 851 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
 852 // We detect and recover from stranding with timers.
 853 //
 854 // If a thread transiently strands it'll park until (a) another
 855 // thread acquires the lock and then drops the lock, at which time the
 856 // exiting thread will notice and unpark the stranded thread, or, (b)
 857 // the timer expires.  If the lock is high traffic then the stranding latency
 858 // will be low due to (a).  If the lock is low traffic then the odds of
 859 // stranding are lower, although the worst-case stranding latency
 860 // is longer.  Critically, we don't want to put excessive load in the
 861 // platform's timer subsystem.  We want to minimize both the timer injection
 862 // rate (timers created/sec) as well as the number of timers active at
 863 // any one time.  (more precisely, we want to minimize timer-seconds, which is
 864 // the integral of the # of active timers at any instant over time).
 865 // Both impinge on OS scalability.  Given that, at most one thread parked on
 866 // a monitor will use a timer.
 867 //
 868 // There is also the risk of a futile wake-up. If we drop the lock
 869 // another thread can reacquire the lock immediately, and we can
 870 // then wake a thread unnecessarily. This is benign, and we've
 871 // structured the code so the windows are short and the frequency
 872 // of such futile wakups is low.
 873 
 874 void ObjectMonitor::exit(bool not_suspended, TRAPS) {
 875   Thread * const Self = THREAD;
 876   if (THREAD != _owner) {
 877     if (THREAD->is_lock_owned((address) _owner)) {
 878       // Transmute _owner from a BasicLock pointer to a Thread address.
 879       // We don't need to hold _mutex for this transition.
 880       // Non-null to Non-null is safe as long as all readers can
 881       // tolerate either flavor.
 882       assert(_recursions == 0, "invariant");
 883       _owner = THREAD;
 884       _recursions = 0;
 885     } else {
 886       // Apparent unbalanced locking ...
 887       // Naively we'd like to throw IllegalMonitorStateException.
 888       // As a practical matter we can neither allocate nor throw an
 889       // exception as ::exit() can be called from leaf routines.
 890       // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
 891       // Upon deeper reflection, however, in a properly run JVM the only
 892       // way we should encounter this situation is in the presence of
 893       // unbalanced JNI locking. TODO: CheckJNICalls.
 894       // See also: CR4414101
 895       assert(false, "Non-balanced monitor enter/exit! Likely JNI locking");
 896       return;
 897     }
 898   }
 899 
 900   if (_recursions != 0) {
 901     _recursions--;        // this is simple recursive enter
 902     return;
 903   }
 904 
 905   // Invariant: after setting Responsible=null an thread must execute
 906   // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
 907   _Responsible = NULL;
 908 
 909 #if INCLUDE_JFR
 910   // get the owner's thread id for the MonitorEnter event
 911   // if it is enabled and the thread isn't suspended
 912   if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
 913     _previous_owner_tid = JFR_THREAD_ID(Self);
 914   }
 915 #endif
 916 
 917   for (;;) {
 918     assert(THREAD == _owner, "invariant");
 919 
 920     if (Knob_ExitPolicy == 0) {
 921       // release semantics: prior loads and stores from within the critical section
 922       // must not float (reorder) past the following store that drops the lock.
 923       // On SPARC that requires MEMBAR #loadstore|#storestore.
 924       // But of course in TSO #loadstore|#storestore is not required.
 925       // I'd like to write one of the following:
 926       // A.  OrderAccess::release() ; _owner = NULL
 927       // B.  OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL;
 928       // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both
 929       // store into a _dummy variable.  That store is not needed, but can result
 930       // in massive wasteful coherency traffic on classic SMP systems.
 931       // Instead, I use release_store(), which is implemented as just a simple
 932       // ST on x64, x86 and SPARC.
 933       OrderAccess::release_store(&_owner, (void*)NULL);   // drop the lock
 934       OrderAccess::storeload();                        // See if we need to wake a successor
 935       if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
 936         return;
 937       }
 938       // Other threads are blocked trying to acquire the lock.
 939 
 940       // Normally the exiting thread is responsible for ensuring succession,
 941       // but if other successors are ready or other entering threads are spinning
 942       // then this thread can simply store NULL into _owner and exit without
 943       // waking a successor.  The existence of spinners or ready successors
 944       // guarantees proper succession (liveness).  Responsibility passes to the
 945       // ready or running successors.  The exiting thread delegates the duty.
 946       // More precisely, if a successor already exists this thread is absolved
 947       // of the responsibility of waking (unparking) one.
 948       //
 949       // The _succ variable is critical to reducing futile wakeup frequency.
 950       // _succ identifies the "heir presumptive" thread that has been made
 951       // ready (unparked) but that has not yet run.  We need only one such
 952       // successor thread to guarantee progress.
 953       // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
 954       // section 3.3 "Futile Wakeup Throttling" for details.
 955       //
 956       // Note that spinners in Enter() also set _succ non-null.
 957       // In the current implementation spinners opportunistically set
 958       // _succ so that exiting threads might avoid waking a successor.
 959       // Another less appealing alternative would be for the exiting thread
 960       // to drop the lock and then spin briefly to see if a spinner managed
 961       // to acquire the lock.  If so, the exiting thread could exit
 962       // immediately without waking a successor, otherwise the exiting
 963       // thread would need to dequeue and wake a successor.
 964       // (Note that we'd need to make the post-drop spin short, but no
 965       // shorter than the worst-case round-trip cache-line migration time.
 966       // The dropped lock needs to become visible to the spinner, and then
 967       // the acquisition of the lock by the spinner must become visible to
 968       // the exiting thread).
 969 
 970       // It appears that an heir-presumptive (successor) must be made ready.
 971       // Only the current lock owner can manipulate the EntryList or
 972       // drain _cxq, so we need to reacquire the lock.  If we fail
 973       // to reacquire the lock the responsibility for ensuring succession
 974       // falls to the new owner.
 975       //
 976       if (!Atomic::replace_if_null(THREAD, &_owner)) {
 977         return;
 978       }
 979     } else {
 980       if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
 981         OrderAccess::release_store(&_owner, (void*)NULL);   // drop the lock
 982         OrderAccess::storeload();
 983         // Ratify the previously observed values.
 984         if (_cxq == NULL || _succ != NULL) {
 985           return;
 986         }
 987 
 988         // inopportune interleaving -- the exiting thread (this thread)
 989         // in the fast-exit path raced an entering thread in the slow-enter
 990         // path.
 991         // We have two choices:
 992         // A.  Try to reacquire the lock.
 993         //     If the CAS() fails return immediately, otherwise
 994         //     we either restart/rerun the exit operation, or simply
 995         //     fall-through into the code below which wakes a successor.
 996         // B.  If the elements forming the EntryList|cxq are TSM
 997         //     we could simply unpark() the lead thread and return
 998         //     without having set _succ.
 999         if (!Atomic::replace_if_null(THREAD, &_owner)) {
1000           return;
1001         }
1002       }
1003     }
1004 
1005     guarantee(_owner == THREAD, "invariant");
1006 
1007     ObjectWaiter * w = NULL;
1008     int QMode = Knob_QMode;
1009 
1010     if (QMode == 2 && _cxq != NULL) {
1011       // QMode == 2 : cxq has precedence over EntryList.
1012       // Try to directly wake a successor from the cxq.
1013       // If successful, the successor will need to unlink itself from cxq.
1014       w = _cxq;
1015       assert(w != NULL, "invariant");
1016       assert(w->TState == ObjectWaiter::TS_CXQ, "Invariant");
1017       ExitEpilog(Self, w);
1018       return;
1019     }
1020 
1021     if (QMode == 3 && _cxq != NULL) {
1022       // Aggressively drain cxq into EntryList at the first opportunity.
1023       // This policy ensure that recently-run threads live at the head of EntryList.
1024       // Drain _cxq into EntryList - bulk transfer.
1025       // First, detach _cxq.
1026       // The following loop is tantamount to: w = swap(&cxq, NULL)
1027       w = _cxq;
1028       for (;;) {
1029         assert(w != NULL, "Invariant");
1030         ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1031         if (u == w) break;
1032         w = u;
1033       }
1034       assert(w != NULL, "invariant");
1035 
1036       ObjectWaiter * q = NULL;
1037       ObjectWaiter * p;
1038       for (p = w; p != NULL; p = p->_next) {
1039         guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1040         p->TState = ObjectWaiter::TS_ENTER;
1041         p->_prev = q;
1042         q = p;
1043       }
1044 
1045       // Append the RATs to the EntryList
1046       // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time.
1047       ObjectWaiter * Tail;
1048       for (Tail = _EntryList; Tail != NULL && Tail->_next != NULL;
1049            Tail = Tail->_next)
1050         /* empty */;
1051       if (Tail == NULL) {
1052         _EntryList = w;
1053       } else {
1054         Tail->_next = w;
1055         w->_prev = Tail;
1056       }
1057 
1058       // Fall thru into code that tries to wake a successor from EntryList
1059     }
1060 
1061     if (QMode == 4 && _cxq != NULL) {
1062       // Aggressively drain cxq into EntryList at the first opportunity.
1063       // This policy ensure that recently-run threads live at the head of EntryList.
1064 
1065       // Drain _cxq into EntryList - bulk transfer.
1066       // First, detach _cxq.
1067       // The following loop is tantamount to: w = swap(&cxq, NULL)
1068       w = _cxq;
1069       for (;;) {
1070         assert(w != NULL, "Invariant");
1071         ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1072         if (u == w) break;
1073         w = u;
1074       }
1075       assert(w != NULL, "invariant");
1076 
1077       ObjectWaiter * q = NULL;
1078       ObjectWaiter * p;
1079       for (p = w; p != NULL; p = p->_next) {
1080         guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1081         p->TState = ObjectWaiter::TS_ENTER;
1082         p->_prev = q;
1083         q = p;
1084       }
1085 
1086       // Prepend the RATs to the EntryList
1087       if (_EntryList != NULL) {
1088         q->_next = _EntryList;
1089         _EntryList->_prev = q;
1090       }
1091       _EntryList = w;
1092 
1093       // Fall thru into code that tries to wake a successor from EntryList
1094     }
1095 
1096     w = _EntryList;
1097     if (w != NULL) {
1098       // I'd like to write: guarantee (w->_thread != Self).
1099       // But in practice an exiting thread may find itself on the EntryList.
1100       // Let's say thread T1 calls O.wait().  Wait() enqueues T1 on O's waitset and
1101       // then calls exit().  Exit release the lock by setting O._owner to NULL.
1102       // Let's say T1 then stalls.  T2 acquires O and calls O.notify().  The
1103       // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1104       // release the lock "O".  T2 resumes immediately after the ST of null into
1105       // _owner, above.  T2 notices that the EntryList is populated, so it
1106       // reacquires the lock and then finds itself on the EntryList.
1107       // Given all that, we have to tolerate the circumstance where "w" is
1108       // associated with Self.
1109       assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1110       ExitEpilog(Self, w);
1111       return;
1112     }
1113 
1114     // If we find that both _cxq and EntryList are null then just
1115     // re-run the exit protocol from the top.
1116     w = _cxq;
1117     if (w == NULL) continue;
1118 
1119     // Drain _cxq into EntryList - bulk transfer.
1120     // First, detach _cxq.
1121     // The following loop is tantamount to: w = swap(&cxq, NULL)
1122     for (;;) {
1123       assert(w != NULL, "Invariant");
1124       ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1125       if (u == w) break;
1126       w = u;
1127     }
1128 
1129     assert(w != NULL, "invariant");
1130     assert(_EntryList == NULL, "invariant");
1131 
1132     // Convert the LIFO SLL anchored by _cxq into a DLL.
1133     // The list reorganization step operates in O(LENGTH(w)) time.
1134     // It's critical that this step operate quickly as
1135     // "Self" still holds the outer-lock, restricting parallelism
1136     // and effectively lengthening the critical section.
1137     // Invariant: s chases t chases u.
1138     // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1139     // we have faster access to the tail.
1140 
1141     if (QMode == 1) {
1142       // QMode == 1 : drain cxq to EntryList, reversing order
1143       // We also reverse the order of the list.
1144       ObjectWaiter * s = NULL;
1145       ObjectWaiter * t = w;
1146       ObjectWaiter * u = NULL;
1147       while (t != NULL) {
1148         guarantee(t->TState == ObjectWaiter::TS_CXQ, "invariant");
1149         t->TState = ObjectWaiter::TS_ENTER;
1150         u = t->_next;
1151         t->_prev = u;
1152         t->_next = s;
1153         s = t;
1154         t = u;
1155       }
1156       _EntryList  = s;
1157       assert(s != NULL, "invariant");
1158     } else {
1159       // QMode == 0 or QMode == 2
1160       _EntryList = w;
1161       ObjectWaiter * q = NULL;
1162       ObjectWaiter * p;
1163       for (p = w; p != NULL; p = p->_next) {
1164         guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1165         p->TState = ObjectWaiter::TS_ENTER;
1166         p->_prev = q;
1167         q = p;
1168       }
1169     }
1170 
1171     // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1172     // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1173 
1174     // See if we can abdicate to a spinner instead of waking a thread.
1175     // A primary goal of the implementation is to reduce the
1176     // context-switch rate.
1177     if (_succ != NULL) continue;
1178 
1179     w = _EntryList;
1180     if (w != NULL) {
1181       guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1182       ExitEpilog(Self, w);
1183       return;
1184     }
1185   }
1186 }
1187 
1188 // ExitSuspendEquivalent:
1189 // A faster alternate to handle_special_suspend_equivalent_condition()
1190 //
1191 // handle_special_suspend_equivalent_condition() unconditionally
1192 // acquires the SR_lock.  On some platforms uncontended MutexLocker()
1193 // operations have high latency.  Note that in ::enter() we call HSSEC
1194 // while holding the monitor, so we effectively lengthen the critical sections.
1195 //
1196 // There are a number of possible solutions:
1197 //
1198 // A.  To ameliorate the problem we might also defer state transitions
1199 //     to as late as possible -- just prior to parking.
1200 //     Given that, we'd call HSSEC after having returned from park(),
1201 //     but before attempting to acquire the monitor.  This is only a
1202 //     partial solution.  It avoids calling HSSEC while holding the
1203 //     monitor (good), but it still increases successor reacquisition latency --
1204 //     the interval between unparking a successor and the time the successor
1205 //     resumes and retries the lock.  See ReenterI(), which defers state transitions.
1206 //     If we use this technique we can also avoid EnterI()-exit() loop
1207 //     in ::enter() where we iteratively drop the lock and then attempt
1208 //     to reacquire it after suspending.
1209 //
1210 // B.  In the future we might fold all the suspend bits into a
1211 //     composite per-thread suspend flag and then update it with CAS().
1212 //     Alternately, a Dekker-like mechanism with multiple variables
1213 //     would suffice:
1214 //       ST Self->_suspend_equivalent = false
1215 //       MEMBAR
1216 //       LD Self_>_suspend_flags
1217 //
1218 // UPDATE 2007-10-6: since I've replaced the native Mutex/Monitor subsystem
1219 // with a more efficient implementation, the need to use "FastHSSEC" has
1220 // decreased. - Dave
1221 
1222 
1223 bool ObjectMonitor::ExitSuspendEquivalent(JavaThread * jSelf) {
1224   const int Mode = Knob_FastHSSEC;
1225   if (Mode && !jSelf->is_external_suspend()) {
1226     assert(jSelf->is_suspend_equivalent(), "invariant");
1227     jSelf->clear_suspend_equivalent();
1228     if (2 == Mode) OrderAccess::storeload();
1229     if (!jSelf->is_external_suspend()) return false;
1230     // We raced a suspension -- fall thru into the slow path
1231     jSelf->set_suspend_equivalent();
1232   }
1233   return jSelf->handle_special_suspend_equivalent_condition();
1234 }
1235 
1236 
1237 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) {
1238   assert(_owner == Self, "invariant");
1239 
1240   // Exit protocol:
1241   // 1. ST _succ = wakee
1242   // 2. membar #loadstore|#storestore;
1243   // 2. ST _owner = NULL
1244   // 3. unpark(wakee)
1245 
1246   _succ = Knob_SuccEnabled ? Wakee->_thread : NULL;
1247   ParkEvent * Trigger = Wakee->_event;
1248 
1249   // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1250   // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1251   // out-of-scope (non-extant).
1252   Wakee  = NULL;
1253 
1254   // Drop the lock
1255   OrderAccess::release_store(&_owner, (void*)NULL);
1256   OrderAccess::fence();                               // ST _owner vs LD in unpark()
1257 
1258   DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1259   Trigger->unpark();
1260 
1261   // Maintain stats and report events to JVMTI
1262   OM_PERFDATA_OP(Parks, inc());
1263 }
1264 
1265 
1266 // -----------------------------------------------------------------------------
1267 // Class Loader deadlock handling.
1268 //
1269 // complete_exit exits a lock returning recursion count
1270 // complete_exit/reenter operate as a wait without waiting
1271 // complete_exit requires an inflated monitor
1272 // The _owner field is not always the Thread addr even with an
1273 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1274 // thread due to contention.
1275 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1276   Thread * const Self = THREAD;
1277   assert(Self->is_Java_thread(), "Must be Java thread!");
1278   JavaThread *jt = (JavaThread *)THREAD;
1279 
1280   DeferredInitialize();
1281 
1282   if (THREAD != _owner) {
1283     if (THREAD->is_lock_owned ((address)_owner)) {
1284       assert(_recursions == 0, "internal state error");
1285       _owner = THREAD;   // Convert from basiclock addr to Thread addr
1286       _recursions = 0;
1287     }
1288   }
1289 
1290   guarantee(Self == _owner, "complete_exit not owner");
1291   intptr_t save = _recursions; // record the old recursion count
1292   _recursions = 0;        // set the recursion level to be 0
1293   exit(true, Self);           // exit the monitor
1294   guarantee(_owner != Self, "invariant");
1295   return save;
1296 }
1297 
1298 // reenter() enters a lock and sets recursion count
1299 // complete_exit/reenter operate as a wait without waiting
1300 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1301   Thread * const Self = THREAD;
1302   assert(Self->is_Java_thread(), "Must be Java thread!");
1303   JavaThread *jt = (JavaThread *)THREAD;
1304 
1305   guarantee(_owner != Self, "reenter already owner");
1306   enter(THREAD);       // enter the monitor
1307   guarantee(_recursions == 0, "reenter recursion");
1308   _recursions = recursions;
1309   return;
1310 }
1311 
1312 
1313 // -----------------------------------------------------------------------------
1314 // A macro is used below because there may already be a pending
1315 // exception which should not abort the execution of the routines
1316 // which use this (which is why we don't put this into check_slow and
1317 // call it with a CHECK argument).
1318 
1319 #define CHECK_OWNER()                                                       \
1320   do {                                                                      \
1321     if (THREAD != _owner) {                                                 \
1322       if (THREAD->is_lock_owned((address) _owner)) {                        \
1323         _owner = THREAD;  /* Convert from basiclock addr to Thread addr */  \
1324         _recursions = 0;                                                    \
1325       } else {                                                              \
1326         THROW(vmSymbols::java_lang_IllegalMonitorStateException());         \
1327       }                                                                     \
1328     }                                                                       \
1329   } while (false)
1330 
1331 // check_slow() is a misnomer.  It's called to simply to throw an IMSX exception.
1332 // TODO-FIXME: remove check_slow() -- it's likely dead.
1333 
1334 void ObjectMonitor::check_slow(TRAPS) {
1335   assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner");
1336   THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner");
1337 }
1338 
1339 static int Adjust(volatile int * adr, int dx) {
1340   int v;
1341   for (v = *adr; Atomic::cmpxchg(v + dx, adr, v) != v; v = *adr) /* empty */;
1342   return v;
1343 }
1344 
1345 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1346                                     ObjectMonitor* monitor,
1347                                     jlong notifier_tid,
1348                                     jlong timeout,
1349                                     bool timedout) {
1350   assert(event != NULL, "invariant");
1351   assert(monitor != NULL, "invariant");
1352   event->set_monitorClass(((oop)monitor->object())->klass());
1353   event->set_timeout(timeout);
1354   event->set_address((uintptr_t)monitor->object_addr());
1355   event->set_notifier(notifier_tid);
1356   event->set_timedOut(timedout);
1357   event->commit();
1358 }
1359 
1360 // -----------------------------------------------------------------------------
1361 // Wait/Notify/NotifyAll
1362 //
1363 // Note: a subset of changes to ObjectMonitor::wait()
1364 // will need to be replicated in complete_exit
1365 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1366   Thread * const Self = THREAD;
1367   assert(Self->is_Java_thread(), "Must be Java thread!");
1368   JavaThread *jt = (JavaThread *)THREAD;
1369 
1370   DeferredInitialize();
1371 
1372   // Throw IMSX or IEX.
1373   CHECK_OWNER();
1374 
1375   EventJavaMonitorWait event;
1376 
1377   // check for a pending interrupt
1378   if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1379     // post monitor waited event.  Note that this is past-tense, we are done waiting.
1380     if (JvmtiExport::should_post_monitor_waited()) {
1381       // Note: 'false' parameter is passed here because the
1382       // wait was not timed out due to thread interrupt.
1383       JvmtiExport::post_monitor_waited(jt, this, false);
1384 
1385       // In this short circuit of the monitor wait protocol, the
1386       // current thread never drops ownership of the monitor and
1387       // never gets added to the wait queue so the current thread
1388       // cannot be made the successor. This means that the
1389       // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1390       // consume an unpark() meant for the ParkEvent associated with
1391       // this ObjectMonitor.
1392     }
1393     if (event.should_commit()) {
1394       post_monitor_wait_event(&event, this, 0, millis, false);
1395     }
1396     THROW(vmSymbols::java_lang_InterruptedException());
1397     return;
1398   }
1399 
1400   assert(Self->_Stalled == 0, "invariant");
1401   Self->_Stalled = intptr_t(this);
1402   jt->set_current_waiting_monitor(this);
1403 
1404   // create a node to be put into the queue
1405   // Critically, after we reset() the event but prior to park(), we must check
1406   // for a pending interrupt.
1407   ObjectWaiter node(Self);
1408   node.TState = ObjectWaiter::TS_WAIT;
1409   Self->_ParkEvent->reset();
1410   OrderAccess::fence();          // ST into Event; membar ; LD interrupted-flag
1411 
1412   // Enter the waiting queue, which is a circular doubly linked list in this case
1413   // but it could be a priority queue or any data structure.
1414   // _WaitSetLock protects the wait queue.  Normally the wait queue is accessed only
1415   // by the the owner of the monitor *except* in the case where park()
1416   // returns because of a timeout of interrupt.  Contention is exceptionally rare
1417   // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1418 
1419   Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1420   AddWaiter(&node);
1421   Thread::SpinRelease(&_WaitSetLock);
1422 
1423   _Responsible = NULL;
1424 
1425   intptr_t save = _recursions; // record the old recursion count
1426   _waiters++;                  // increment the number of waiters
1427   _recursions = 0;             // set the recursion level to be 1
1428   exit(true, Self);                    // exit the monitor
1429   guarantee(_owner != Self, "invariant");
1430 
1431   // The thread is on the WaitSet list - now park() it.
1432   // On MP systems it's conceivable that a brief spin before we park
1433   // could be profitable.
1434   //
1435   // TODO-FIXME: change the following logic to a loop of the form
1436   //   while (!timeout && !interrupted && _notified == 0) park()
1437 
1438   int ret = OS_OK;
1439   int WasNotified = 0;
1440   { // State transition wrappers
1441     OSThread* osthread = Self->osthread();
1442     OSThreadWaitState osts(osthread, true);
1443     {
1444       ThreadBlockInVM tbivm(jt);
1445       // Thread is in thread_blocked state and oop access is unsafe.
1446       jt->set_suspend_equivalent();
1447 
1448       if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1449         // Intentionally empty
1450       } else if (node._notified == 0) {
1451         if (millis <= 0) {
1452           Self->_ParkEvent->park();
1453         } else {
1454           ret = Self->_ParkEvent->park(millis);
1455         }
1456       }
1457 
1458       // were we externally suspended while we were waiting?
1459       if (ExitSuspendEquivalent (jt)) {
1460         // TODO-FIXME: add -- if succ == Self then succ = null.
1461         jt->java_suspend_self();
1462       }
1463 
1464     } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1465 
1466     // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1467     // from the WaitSet to the EntryList.
1468     // See if we need to remove Node from the WaitSet.
1469     // We use double-checked locking to avoid grabbing _WaitSetLock
1470     // if the thread is not on the wait queue.
1471     //
1472     // Note that we don't need a fence before the fetch of TState.
1473     // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1474     // written by the is thread. (perhaps the fetch might even be satisfied
1475     // by a look-aside into the processor's own store buffer, although given
1476     // the length of the code path between the prior ST and this load that's
1477     // highly unlikely).  If the following LD fetches a stale TS_WAIT value
1478     // then we'll acquire the lock and then re-fetch a fresh TState value.
1479     // That is, we fail toward safety.
1480 
1481     if (node.TState == ObjectWaiter::TS_WAIT) {
1482       Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1483       if (node.TState == ObjectWaiter::TS_WAIT) {
1484         DequeueSpecificWaiter(&node);       // unlink from WaitSet
1485         assert(node._notified == 0, "invariant");
1486         node.TState = ObjectWaiter::TS_RUN;
1487       }
1488       Thread::SpinRelease(&_WaitSetLock);
1489     }
1490 
1491     // The thread is now either on off-list (TS_RUN),
1492     // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1493     // The Node's TState variable is stable from the perspective of this thread.
1494     // No other threads will asynchronously modify TState.
1495     guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1496     OrderAccess::loadload();
1497     if (_succ == Self) _succ = NULL;
1498     WasNotified = node._notified;
1499 
1500     // Reentry phase -- reacquire the monitor.
1501     // re-enter contended monitor after object.wait().
1502     // retain OBJECT_WAIT state until re-enter successfully completes
1503     // Thread state is thread_in_vm and oop access is again safe,
1504     // although the raw address of the object may have changed.
1505     // (Don't cache naked oops over safepoints, of course).
1506 
1507     // post monitor waited event. Note that this is past-tense, we are done waiting.
1508     if (JvmtiExport::should_post_monitor_waited()) {
1509       JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1510 
1511       if (node._notified != 0 && _succ == Self) {
1512         // In this part of the monitor wait-notify-reenter protocol it
1513         // is possible (and normal) for another thread to do a fastpath
1514         // monitor enter-exit while this thread is still trying to get
1515         // to the reenter portion of the protocol.
1516         //
1517         // The ObjectMonitor was notified and the current thread is
1518         // the successor which also means that an unpark() has already
1519         // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1520         // consume the unpark() that was done when the successor was
1521         // set because the same ParkEvent is shared between Java
1522         // monitors and JVM/TI RawMonitors (for now).
1523         //
1524         // We redo the unpark() to ensure forward progress, i.e., we
1525         // don't want all pending threads hanging (parked) with none
1526         // entering the unlocked monitor.
1527         node._event->unpark();
1528       }
1529     }
1530 
1531     if (event.should_commit()) {
1532       post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1533     }
1534 
1535     OrderAccess::fence();
1536 
1537     assert(Self->_Stalled != 0, "invariant");
1538     Self->_Stalled = 0;
1539 
1540     assert(_owner != Self, "invariant");
1541     ObjectWaiter::TStates v = node.TState;
1542     if (v == ObjectWaiter::TS_RUN) {
1543       enter(Self);
1544     } else {
1545       guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1546       ReenterI(Self, &node);
1547       node.wait_reenter_end(this);
1548     }
1549 
1550     // Self has reacquired the lock.
1551     // Lifecycle - the node representing Self must not appear on any queues.
1552     // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1553     // want residual elements associated with this thread left on any lists.
1554     guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1555     assert(_owner == Self, "invariant");
1556     assert(_succ != Self, "invariant");
1557   } // OSThreadWaitState()
1558 
1559   jt->set_current_waiting_monitor(NULL);
1560 
1561   guarantee(_recursions == 0, "invariant");
1562   _recursions = save;     // restore the old recursion count
1563   _waiters--;             // decrement the number of waiters
1564 
1565   // Verify a few postconditions
1566   assert(_owner == Self, "invariant");
1567   assert(_succ != Self, "invariant");
1568   assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant");
1569 
1570   // check if the notification happened
1571   if (!WasNotified) {
1572     // no, it could be timeout or Thread.interrupt() or both
1573     // check for interrupt event, otherwise it is timeout
1574     if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1575       THROW(vmSymbols::java_lang_InterruptedException());
1576     }
1577   }
1578 
1579   // NOTE: Spurious wake up will be consider as timeout.
1580   // Monitor notify has precedence over thread interrupt.
1581 }
1582 
1583 
1584 // Consider:
1585 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1586 // then instead of transferring a thread from the WaitSet to the EntryList
1587 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1588 
1589 void ObjectMonitor::INotify(Thread * Self) {
1590   const int policy = Knob_MoveNotifyee;
1591 
1592   Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1593   ObjectWaiter * iterator = DequeueWaiter();
1594   if (iterator != NULL) {
1595     guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1596     guarantee(iterator->_notified == 0, "invariant");
1597     // Disposition - what might we do with iterator ?
1598     // a.  add it directly to the EntryList - either tail (policy == 1)
1599     //     or head (policy == 0).
1600     // b.  push it onto the front of the _cxq (policy == 2).
1601     // For now we use (b).
1602     if (policy != 4) {
1603       iterator->TState = ObjectWaiter::TS_ENTER;
1604     }
1605     iterator->_notified = 1;
1606     iterator->_notifier_tid = JFR_THREAD_ID(Self);
1607 
1608     ObjectWaiter * list = _EntryList;
1609     if (list != NULL) {
1610       assert(list->_prev == NULL, "invariant");
1611       assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1612       assert(list != iterator, "invariant");
1613     }
1614 
1615     if (policy == 0) {       // prepend to EntryList
1616       if (list == NULL) {
1617         iterator->_next = iterator->_prev = NULL;
1618         _EntryList = iterator;
1619       } else {
1620         list->_prev = iterator;
1621         iterator->_next = list;
1622         iterator->_prev = NULL;
1623         _EntryList = iterator;
1624       }
1625     } else if (policy == 1) {      // append to EntryList
1626       if (list == NULL) {
1627         iterator->_next = iterator->_prev = NULL;
1628         _EntryList = iterator;
1629       } else {
1630         // CONSIDER:  finding the tail currently requires a linear-time walk of
1631         // the EntryList.  We can make tail access constant-time by converting to
1632         // a CDLL instead of using our current DLL.
1633         ObjectWaiter * tail;
1634         for (tail = list; tail->_next != NULL; tail = tail->_next) {}
1635         assert(tail != NULL && tail->_next == NULL, "invariant");
1636         tail->_next = iterator;
1637         iterator->_prev = tail;
1638         iterator->_next = NULL;
1639       }
1640     } else if (policy == 2) {      // prepend to cxq
1641       if (list == NULL) {
1642         iterator->_next = iterator->_prev = NULL;
1643         _EntryList = iterator;
1644       } else {
1645         iterator->TState = ObjectWaiter::TS_CXQ;
1646         for (;;) {
1647           ObjectWaiter * front = _cxq;
1648           iterator->_next = front;
1649           if (Atomic::cmpxchg(iterator, &_cxq, front) == front) {
1650             break;
1651           }
1652         }
1653       }
1654     } else if (policy == 3) {      // append to cxq
1655       iterator->TState = ObjectWaiter::TS_CXQ;
1656       for (;;) {
1657         ObjectWaiter * tail = _cxq;
1658         if (tail == NULL) {
1659           iterator->_next = NULL;
1660           if (Atomic::replace_if_null(iterator, &_cxq)) {
1661             break;
1662           }
1663         } else {
1664           while (tail->_next != NULL) tail = tail->_next;
1665           tail->_next = iterator;
1666           iterator->_prev = tail;
1667           iterator->_next = NULL;
1668           break;
1669         }
1670       }
1671     } else {
1672       ParkEvent * ev = iterator->_event;
1673       iterator->TState = ObjectWaiter::TS_RUN;
1674       OrderAccess::fence();
1675       ev->unpark();
1676     }
1677 
1678     // _WaitSetLock protects the wait queue, not the EntryList.  We could
1679     // move the add-to-EntryList operation, above, outside the critical section
1680     // protected by _WaitSetLock.  In practice that's not useful.  With the
1681     // exception of  wait() timeouts and interrupts the monitor owner
1682     // is the only thread that grabs _WaitSetLock.  There's almost no contention
1683     // on _WaitSetLock so it's not profitable to reduce the length of the
1684     // critical section.
1685 
1686     if (policy < 4) {
1687       iterator->wait_reenter_begin(this);
1688     }
1689   }
1690   Thread::SpinRelease(&_WaitSetLock);
1691 }
1692 
1693 // Consider: a not-uncommon synchronization bug is to use notify() when
1694 // notifyAll() is more appropriate, potentially resulting in stranded
1695 // threads; this is one example of a lost wakeup. A useful diagnostic
1696 // option is to force all notify() operations to behave as notifyAll().
1697 //
1698 // Note: We can also detect many such problems with a "minimum wait".
1699 // When the "minimum wait" is set to a small non-zero timeout value
1700 // and the program does not hang whereas it did absent "minimum wait",
1701 // that suggests a lost wakeup bug.
1702 
1703 void ObjectMonitor::notify(TRAPS) {
1704   CHECK_OWNER();
1705   if (_WaitSet == NULL) {
1706     return;
1707   }
1708   DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1709   INotify(THREAD);
1710   OM_PERFDATA_OP(Notifications, inc(1));
1711 }
1712 
1713 
1714 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1715 // from the waitset to the EntryList. This could be done more efficiently with a
1716 // single bulk transfer but in practice it's not time-critical. Beware too,
1717 // that in prepend-mode we invert the order of the waiters. Let's say that the
1718 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1719 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1720 
1721 void ObjectMonitor::notifyAll(TRAPS) {
1722   CHECK_OWNER();
1723   if (_WaitSet == NULL) {
1724     return;
1725   }
1726 
1727   DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1728   int tally = 0;
1729   while (_WaitSet != NULL) {
1730     tally++;
1731     INotify(THREAD);
1732   }
1733 
1734   OM_PERFDATA_OP(Notifications, inc(tally));
1735 }
1736 
1737 // -----------------------------------------------------------------------------
1738 // Adaptive Spinning Support
1739 //
1740 // Adaptive spin-then-block - rational spinning
1741 //
1742 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1743 // algorithm.  On high order SMP systems it would be better to start with
1744 // a brief global spin and then revert to spinning locally.  In the spirit of MCS/CLH,
1745 // a contending thread could enqueue itself on the cxq and then spin locally
1746 // on a thread-specific variable such as its ParkEvent._Event flag.
1747 // That's left as an exercise for the reader.  Note that global spinning is
1748 // not problematic on Niagara, as the L2 cache serves the interconnect and
1749 // has both low latency and massive bandwidth.
1750 //
1751 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1752 // acquisition attempts where we opt to spin --  at 100% and vary the spin count
1753 // (duration) or we can fix the count at approximately the duration of
1754 // a context switch and vary the frequency.   Of course we could also
1755 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1756 // For a description of 'Adaptive spin-then-block mutual exclusion in
1757 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1758 //
1759 // This implementation varies the duration "D", where D varies with
1760 // the success rate of recent spin attempts. (D is capped at approximately
1761 // length of a round-trip context switch).  The success rate for recent
1762 // spin attempts is a good predictor of the success rate of future spin
1763 // attempts.  The mechanism adapts automatically to varying critical
1764 // section length (lock modality), system load and degree of parallelism.
1765 // D is maintained per-monitor in _SpinDuration and is initialized
1766 // optimistically.  Spin frequency is fixed at 100%.
1767 //
1768 // Note that _SpinDuration is volatile, but we update it without locks
1769 // or atomics.  The code is designed so that _SpinDuration stays within
1770 // a reasonable range even in the presence of races.  The arithmetic
1771 // operations on _SpinDuration are closed over the domain of legal values,
1772 // so at worst a race will install and older but still legal value.
1773 // At the very worst this introduces some apparent non-determinism.
1774 // We might spin when we shouldn't or vice-versa, but since the spin
1775 // count are relatively short, even in the worst case, the effect is harmless.
1776 //
1777 // Care must be taken that a low "D" value does not become an
1778 // an absorbing state.  Transient spinning failures -- when spinning
1779 // is overall profitable -- should not cause the system to converge
1780 // on low "D" values.  We want spinning to be stable and predictable
1781 // and fairly responsive to change and at the same time we don't want
1782 // it to oscillate, become metastable, be "too" non-deterministic,
1783 // or converge on or enter undesirable stable absorbing states.
1784 //
1785 // We implement a feedback-based control system -- using past behavior
1786 // to predict future behavior.  We face two issues: (a) if the
1787 // input signal is random then the spin predictor won't provide optimal
1788 // results, and (b) if the signal frequency is too high then the control
1789 // system, which has some natural response lag, will "chase" the signal.
1790 // (b) can arise from multimodal lock hold times.  Transient preemption
1791 // can also result in apparent bimodal lock hold times.
1792 // Although sub-optimal, neither condition is particularly harmful, as
1793 // in the worst-case we'll spin when we shouldn't or vice-versa.
1794 // The maximum spin duration is rather short so the failure modes aren't bad.
1795 // To be conservative, I've tuned the gain in system to bias toward
1796 // _not spinning.  Relatedly, the system can sometimes enter a mode where it
1797 // "rings" or oscillates between spinning and not spinning.  This happens
1798 // when spinning is just on the cusp of profitability, however, so the
1799 // situation is not dire.  The state is benign -- there's no need to add
1800 // hysteresis control to damp the transition rate between spinning and
1801 // not spinning.
1802 
1803 // Spinning: Fixed frequency (100%), vary duration
1804 int ObjectMonitor::TrySpin(Thread * Self) {
1805   // Dumb, brutal spin.  Good for comparative measurements against adaptive spinning.
1806   int ctr = Knob_FixedSpin;
1807   if (ctr != 0) {
1808     while (--ctr >= 0) {
1809       if (TryLock(Self) > 0) return 1;
1810       SpinPause();
1811     }
1812     return 0;
1813   }
1814 
1815   for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1816     if (TryLock(Self) > 0) {
1817       // Increase _SpinDuration ...
1818       // Note that we don't clamp SpinDuration precisely at SpinLimit.
1819       // Raising _SpurDuration to the poverty line is key.
1820       int x = _SpinDuration;
1821       if (x < Knob_SpinLimit) {
1822         if (x < Knob_Poverty) x = Knob_Poverty;
1823         _SpinDuration = x + Knob_BonusB;
1824       }
1825       return 1;
1826     }
1827     SpinPause();
1828   }
1829 
1830   // Admission control - verify preconditions for spinning
1831   //
1832   // We always spin a little bit, just to prevent _SpinDuration == 0 from
1833   // becoming an absorbing state.  Put another way, we spin briefly to
1834   // sample, just in case the system load, parallelism, contention, or lock
1835   // modality changed.
1836   //
1837   // Consider the following alternative:
1838   // Periodically set _SpinDuration = _SpinLimit and try a long/full
1839   // spin attempt.  "Periodically" might mean after a tally of
1840   // the # of failed spin attempts (or iterations) reaches some threshold.
1841   // This takes us into the realm of 1-out-of-N spinning, where we
1842   // hold the duration constant but vary the frequency.
1843 
1844   ctr = _SpinDuration;
1845   if (ctr <= 0) return 0;
1846 
1847   if (Knob_SuccRestrict && _succ != NULL) return 0;
1848   if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) {
1849     return 0;
1850   }
1851 
1852   int MaxSpin = Knob_MaxSpinners;
1853   if (MaxSpin >= 0) {
1854     if (_Spinner > MaxSpin) {
1855       return 0;
1856     }
1857     // Slightly racy, but benign ...
1858     Adjust(&_Spinner, 1);
1859   }
1860 
1861   // We're good to spin ... spin ingress.
1862   // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1863   // when preparing to LD...CAS _owner, etc and the CAS is likely
1864   // to succeed.
1865   if (_succ == NULL) {
1866     _succ = Self;
1867   }
1868   Thread * prv = NULL;
1869 
1870   // There are three ways to exit the following loop:
1871   // 1.  A successful spin where this thread has acquired the lock.
1872   // 2.  Spin failure with prejudice
1873   // 3.  Spin failure without prejudice
1874 
1875   while (--ctr >= 0) {
1876 
1877     // Periodic polling -- Check for pending GC
1878     // Threads may spin while they're unsafe.
1879     // We don't want spinning threads to delay the JVM from reaching
1880     // a stop-the-world safepoint or to steal cycles from GC.
1881     // If we detect a pending safepoint we abort in order that
1882     // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1883     // this thread, if safe, doesn't steal cycles from GC.
1884     // This is in keeping with the "no loitering in runtime" rule.
1885     // We periodically check to see if there's a safepoint pending.
1886     if ((ctr & 0xFF) == 0) {
1887       if (SafepointMechanism::poll(Self)) {
1888         goto Abort;           // abrupt spin egress
1889       }
1890       if (Knob_UsePause & 1) SpinPause();
1891     }
1892 
1893     if (Knob_UsePause & 2) SpinPause();
1894 
1895     // Probe _owner with TATAS
1896     // If this thread observes the monitor transition or flicker
1897     // from locked to unlocked to locked, then the odds that this
1898     // thread will acquire the lock in this spin attempt go down
1899     // considerably.  The same argument applies if the CAS fails
1900     // or if we observe _owner change from one non-null value to
1901     // another non-null value.   In such cases we might abort
1902     // the spin without prejudice or apply a "penalty" to the
1903     // spin count-down variable "ctr", reducing it by 100, say.
1904 
1905     Thread * ox = (Thread *) _owner;
1906     if (ox == NULL) {
1907       ox = (Thread*)Atomic::cmpxchg(Self, &_owner, (void*)NULL);
1908       if (ox == NULL) {
1909         // The CAS succeeded -- this thread acquired ownership
1910         // Take care of some bookkeeping to exit spin state.
1911         if (_succ == Self) {
1912           _succ = NULL;
1913         }
1914         if (MaxSpin > 0) Adjust(&_Spinner, -1);
1915 
1916         // Increase _SpinDuration :
1917         // The spin was successful (profitable) so we tend toward
1918         // longer spin attempts in the future.
1919         // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
1920         // If we acquired the lock early in the spin cycle it
1921         // makes sense to increase _SpinDuration proportionally.
1922         // Note that we don't clamp SpinDuration precisely at SpinLimit.
1923         int x = _SpinDuration;
1924         if (x < Knob_SpinLimit) {
1925           if (x < Knob_Poverty) x = Knob_Poverty;
1926           _SpinDuration = x + Knob_Bonus;
1927         }
1928         return 1;
1929       }
1930 
1931       // The CAS failed ... we can take any of the following actions:
1932       // * penalize: ctr -= CASPenalty
1933       // * exit spin with prejudice -- goto Abort;
1934       // * exit spin without prejudice.
1935       // * Since CAS is high-latency, retry again immediately.
1936       prv = ox;
1937       goto Abort;
1938     }
1939 
1940     // Did lock ownership change hands ?
1941     if (ox != prv && prv != NULL) {
1942       goto Abort;
1943     }
1944     prv = ox;
1945 
1946     // Abort the spin if the owner is not executing.
1947     // The owner must be executing in order to drop the lock.
1948     // Spinning while the owner is OFFPROC is idiocy.
1949     // Consider: ctr -= RunnablePenalty ;
1950     if (Knob_OState && NotRunnable (Self, ox)) {
1951       goto Abort;
1952     }
1953     if (_succ == NULL) {
1954       _succ = Self;
1955     }
1956   }
1957 
1958   // Spin failed with prejudice -- reduce _SpinDuration.
1959   // TODO: Use an AIMD-like policy to adjust _SpinDuration.
1960   // AIMD is globally stable.
1961   {
1962     int x = _SpinDuration;
1963     if (x > 0) {
1964       // Consider an AIMD scheme like: x -= (x >> 3) + 100
1965       // This is globally sample and tends to damp the response.
1966       x -= Knob_Penalty;
1967       if (x < 0) x = 0;
1968       _SpinDuration = x;
1969     }
1970   }
1971 
1972  Abort:
1973   if (MaxSpin >= 0) Adjust(&_Spinner, -1);
1974   if (_succ == Self) {
1975     _succ = NULL;
1976     // Invariant: after setting succ=null a contending thread
1977     // must recheck-retry _owner before parking.  This usually happens
1978     // in the normal usage of TrySpin(), but it's safest
1979     // to make TrySpin() as foolproof as possible.
1980     OrderAccess::fence();
1981     if (TryLock(Self) > 0) return 1;
1982   }
1983   return 0;
1984 }
1985 
1986 // NotRunnable() -- informed spinning
1987 //
1988 // Don't bother spinning if the owner is not eligible to drop the lock.
1989 // Spin only if the owner thread is _thread_in_Java or _thread_in_vm.
1990 // The thread must be runnable in order to drop the lock in timely fashion.
1991 // If the _owner is not runnable then spinning will not likely be
1992 // successful (profitable).
1993 //
1994 // Beware -- the thread referenced by _owner could have died
1995 // so a simply fetch from _owner->_thread_state might trap.
1996 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
1997 // Because of the lifecycle issues, the _thread_state values
1998 // observed by NotRunnable() might be garbage.  NotRunnable must
1999 // tolerate this and consider the observed _thread_state value
2000 // as advisory.
2001 //
2002 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2003 // a thread pointer.
2004 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2005 // with the LSB of _owner.  Another option would be to probabilistically probe
2006 // the putative _owner->TypeTag value.
2007 //
2008 // Checking _thread_state isn't perfect.  Even if the thread is
2009 // in_java it might be blocked on a page-fault or have been preempted
2010 // and sitting on a ready/dispatch queue.
2011 //
2012 // The return value from NotRunnable() is *advisory* -- the
2013 // result is based on sampling and is not necessarily coherent.
2014 // The caller must tolerate false-negative and false-positive errors.
2015 // Spinning, in general, is probabilistic anyway.
2016 
2017 
2018 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) {
2019   // Check ox->TypeTag == 2BAD.
2020   if (ox == NULL) return 0;
2021 
2022   // Avoid transitive spinning ...
2023   // Say T1 spins or blocks trying to acquire L.  T1._Stalled is set to L.
2024   // Immediately after T1 acquires L it's possible that T2, also
2025   // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2026   // This occurs transiently after T1 acquired L but before
2027   // T1 managed to clear T1.Stalled.  T2 does not need to abort
2028   // its spin in this circumstance.
2029   intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
2030 
2031   if (BlockedOn == 1) return 1;
2032   if (BlockedOn != 0) {
2033     return BlockedOn != intptr_t(this) && _owner == ox;
2034   }
2035 
2036   assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant");
2037   int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);;
2038   // consider also: jst != _thread_in_Java -- but that's overspecific.
2039   return jst == _thread_blocked || jst == _thread_in_native;
2040 }
2041 
2042 
2043 // -----------------------------------------------------------------------------
2044 // WaitSet management ...
2045 
2046 ObjectWaiter::ObjectWaiter(Thread* thread) {
2047   _next     = NULL;
2048   _prev     = NULL;
2049   _notified = 0;
2050   _notifier_tid = 0;
2051   TState    = TS_RUN;
2052   _thread   = thread;
2053   _event    = thread->_ParkEvent;
2054   _active   = false;
2055   assert(_event != NULL, "invariant");
2056 }
2057 
2058 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2059   JavaThread *jt = (JavaThread *)this->_thread;
2060   _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
2061 }
2062 
2063 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2064   JavaThread *jt = (JavaThread *)this->_thread;
2065   JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
2066 }
2067 
2068 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2069   assert(node != NULL, "should not add NULL node");
2070   assert(node->_prev == NULL, "node already in list");
2071   assert(node->_next == NULL, "node already in list");
2072   // put node at end of queue (circular doubly linked list)
2073   if (_WaitSet == NULL) {
2074     _WaitSet = node;
2075     node->_prev = node;
2076     node->_next = node;
2077   } else {
2078     ObjectWaiter* head = _WaitSet;
2079     ObjectWaiter* tail = head->_prev;
2080     assert(tail->_next == head, "invariant check");
2081     tail->_next = node;
2082     head->_prev = node;
2083     node->_next = head;
2084     node->_prev = tail;
2085   }
2086 }
2087 
2088 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2089   // dequeue the very first waiter
2090   ObjectWaiter* waiter = _WaitSet;
2091   if (waiter) {
2092     DequeueSpecificWaiter(waiter);
2093   }
2094   return waiter;
2095 }
2096 
2097 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2098   assert(node != NULL, "should not dequeue NULL node");
2099   assert(node->_prev != NULL, "node already removed from list");
2100   assert(node->_next != NULL, "node already removed from list");
2101   // when the waiter has woken up because of interrupt,
2102   // timeout or other spurious wake-up, dequeue the
2103   // waiter from waiting list
2104   ObjectWaiter* next = node->_next;
2105   if (next == node) {
2106     assert(node->_prev == node, "invariant check");
2107     _WaitSet = NULL;
2108   } else {
2109     ObjectWaiter* prev = node->_prev;
2110     assert(prev->_next == node, "invariant check");
2111     assert(next->_prev == node, "invariant check");
2112     next->_prev = prev;
2113     prev->_next = next;
2114     if (_WaitSet == node) {
2115       _WaitSet = next;
2116     }
2117   }
2118   node->_next = NULL;
2119   node->_prev = NULL;
2120 }
2121 
2122 // -----------------------------------------------------------------------------
2123 // PerfData support
2124 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts       = NULL;
2125 PerfCounter * ObjectMonitor::_sync_FutileWakeups               = NULL;
2126 PerfCounter * ObjectMonitor::_sync_Parks                       = NULL;
2127 PerfCounter * ObjectMonitor::_sync_Notifications               = NULL;
2128 PerfCounter * ObjectMonitor::_sync_Inflations                  = NULL;
2129 PerfCounter * ObjectMonitor::_sync_Deflations                  = NULL;
2130 PerfLongVariable * ObjectMonitor::_sync_MonExtant              = NULL;
2131 
2132 // One-shot global initialization for the sync subsystem.
2133 // We could also defer initialization and initialize on-demand
2134 // the first time we call inflate().  Initialization would
2135 // be protected - like so many things - by the MonitorCache_lock.
2136 
2137 void ObjectMonitor::Initialize() {
2138   static int InitializationCompleted = 0;
2139   assert(InitializationCompleted == 0, "invariant");
2140   InitializationCompleted = 1;
2141   if (UsePerfData) {
2142     EXCEPTION_MARK;
2143 #define NEWPERFCOUNTER(n)                                                \
2144   {                                                                      \
2145     n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,  \
2146                                         CHECK);                          \
2147   }
2148 #define NEWPERFVARIABLE(n)                                                \
2149   {                                                                       \
2150     n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,  \
2151                                          CHECK);                          \
2152   }
2153     NEWPERFCOUNTER(_sync_Inflations);
2154     NEWPERFCOUNTER(_sync_Deflations);
2155     NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2156     NEWPERFCOUNTER(_sync_FutileWakeups);
2157     NEWPERFCOUNTER(_sync_Parks);
2158     NEWPERFCOUNTER(_sync_Notifications);
2159     NEWPERFVARIABLE(_sync_MonExtant);
2160 #undef NEWPERFCOUNTER
2161 #undef NEWPERFVARIABLE
2162   }
2163 }
2164 
2165 void ObjectMonitor::DeferredInitialize() {
2166   if (InitDone > 0) return;
2167   if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2168     while (InitDone != 1) /* empty */;
2169     return;
2170   }
2171 
2172   // One-shot global initialization ...
2173   // The initialization is idempotent, so we don't need locks.
2174   // In the future consider doing this via os::init_2().
2175 
2176   if (!os::is_MP()) {
2177     Knob_SpinLimit = 0;
2178     Knob_PreSpin   = 0;
2179     Knob_FixedSpin = -1;
2180   }
2181 
2182   OrderAccess::fence();
2183   InitDone = 1;
2184 }
2185 
--- EOF ---