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
rev 51793 : imported patch syncknobs-13-Knob_SpinEarly
rev 51794 : imported patch syncknobs-14-Knob_SuccEnabled
rev 51795 : imported patch syncknobs-15-Knob_SuccRestrict
rev 51796 : imported patch syncknobs-16-Knob_MaxSpinners
rev 51797 : imported patch syncknobs-17-Knob_SpinAfterFutile
rev 51798 : imported patch syncknobs-18-Knob_OState
rev 51799 : imported patch syncknobs-19-Knob_UsePause
rev 51800 : imported patch syncknobs-20-Knob_ExitPolicy
rev 51801 : imported patch syncknobs-21-Knob_ResetEvent
rev 51802 : imported patch syncknobs-22-Knob_FastHSSEC
rev 51803 : imported patch syncknobs-23-Knob_MoveNotifyee

   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_Bonus               = 100;     // spin success bonus
 107 static int Knob_BonusB              = 100;     // spin success bonus
 108 static int Knob_Penalty             = 200;     // spin failure penalty
 109 static int Knob_Poverty             = 1000;
 110 static int Knob_FixedSpin           = 0;
 111 static int Knob_PreSpin             = 10;      // 20-100 likely better
 112 

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


1525   Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1526   ObjectWaiter * iterator = DequeueWaiter();
1527   if (iterator != NULL) {
1528     guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1529     guarantee(iterator->_notified == 0, "invariant");
1530     // Disposition - what might we do with iterator ?
1531     // a.  add it directly to the EntryList - either tail (policy == 1)
1532     //     or head (policy == 0).
1533     // b.  push it onto the front of the _cxq (policy == 2).
1534     // For now we use (b).
1535 
1536     iterator->TState = ObjectWaiter::TS_ENTER;
1537 
1538     iterator->_notified = 1;
1539     iterator->_notifier_tid = JFR_THREAD_ID(Self);
1540 
1541     ObjectWaiter * list = _EntryList;
1542     if (list != NULL) {
1543       assert(list->_prev == NULL, "invariant");
1544       assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1545       assert(list != iterator, "invariant");
1546     }
1547 
1548     // prepend to cxq

























1549     if (list == NULL) {
1550       iterator->_next = iterator->_prev = NULL;
1551       _EntryList = iterator;
1552     } else {
1553       iterator->TState = ObjectWaiter::TS_CXQ;
1554       for (;;) {
1555         ObjectWaiter * front = _cxq;
1556         iterator->_next = front;
1557         if (Atomic::cmpxchg(iterator, &_cxq, front) == front) {
1558           break;
1559         }
1560       }
1561     }























1562 
1563     // _WaitSetLock protects the wait queue, not the EntryList.  We could
1564     // move the add-to-EntryList operation, above, outside the critical section
1565     // protected by _WaitSetLock.  In practice that's not useful.  With the
1566     // exception of  wait() timeouts and interrupts the monitor owner
1567     // is the only thread that grabs _WaitSetLock.  There's almost no contention
1568     // on _WaitSetLock so it's not profitable to reduce the length of the
1569     // critical section.
1570 

1571     iterator->wait_reenter_begin(this);

1572   }
1573   Thread::SpinRelease(&_WaitSetLock);
1574 }
1575 
1576 // Consider: a not-uncommon synchronization bug is to use notify() when
1577 // notifyAll() is more appropriate, potentially resulting in stranded
1578 // threads; this is one example of a lost wakeup. A useful diagnostic
1579 // option is to force all notify() operations to behave as notifyAll().
1580 //
1581 // Note: We can also detect many such problems with a "minimum wait".
1582 // When the "minimum wait" is set to a small non-zero timeout value
1583 // and the program does not hang whereas it did absent "minimum wait",
1584 // that suggests a lost wakeup bug.
1585 
1586 void ObjectMonitor::notify(TRAPS) {
1587   CHECK_OWNER();
1588   if (_WaitSet == NULL) {
1589     return;
1590   }
1591   DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1592   INotify(THREAD);
1593   OM_PERFDATA_OP(Notifications, inc(1));
1594 }
1595 
1596 
1597 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1598 // from the waitset to the EntryList. This could be done more efficiently with a
1599 // single bulk transfer but in practice it's not time-critical. Beware too,
1600 // that in prepend-mode we invert the order of the waiters. Let's say that the
1601 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1602 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1603 
1604 void ObjectMonitor::notifyAll(TRAPS) {
1605   CHECK_OWNER();
1606   if (_WaitSet == NULL) {
1607     return;
1608   }
1609 
1610   DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1611   int tally = 0;
1612   while (_WaitSet != NULL) {
1613     tally++;
1614     INotify(THREAD);
1615   }
1616 
1617   OM_PERFDATA_OP(Notifications, inc(tally));
1618 }
1619 
1620 // -----------------------------------------------------------------------------
1621 // Adaptive Spinning Support
1622 //
1623 // Adaptive spin-then-block - rational spinning
1624 //
1625 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1626 // algorithm.  On high order SMP systems it would be better to start with
1627 // a brief global spin and then revert to spinning locally.  In the spirit of MCS/CLH,
1628 // a contending thread could enqueue itself on the cxq and then spin locally
1629 // on a thread-specific variable such as its ParkEvent._Event flag.
1630 // That's left as an exercise for the reader.  Note that global spinning is
1631 // not problematic on Niagara, as the L2 cache serves the interconnect and
1632 // has both low latency and massive bandwidth.
1633 //
1634 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1635 // acquisition attempts where we opt to spin --  at 100% and vary the spin count
1636 // (duration) or we can fix the count at approximately the duration of
1637 // a context switch and vary the frequency.   Of course we could also
1638 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1639 // For a description of 'Adaptive spin-then-block mutual exclusion in
1640 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1641 //
1642 // This implementation varies the duration "D", where D varies with
1643 // the success rate of recent spin attempts. (D is capped at approximately
1644 // length of a round-trip context switch).  The success rate for recent
1645 // spin attempts is a good predictor of the success rate of future spin
1646 // attempts.  The mechanism adapts automatically to varying critical
1647 // section length (lock modality), system load and degree of parallelism.
1648 // D is maintained per-monitor in _SpinDuration and is initialized
1649 // optimistically.  Spin frequency is fixed at 100%.
1650 //
1651 // Note that _SpinDuration is volatile, but we update it without locks
1652 // or atomics.  The code is designed so that _SpinDuration stays within
1653 // a reasonable range even in the presence of races.  The arithmetic
1654 // operations on _SpinDuration are closed over the domain of legal values,
1655 // so at worst a race will install and older but still legal value.
1656 // At the very worst this introduces some apparent non-determinism.
1657 // We might spin when we shouldn't or vice-versa, but since the spin
1658 // count are relatively short, even in the worst case, the effect is harmless.
1659 //
1660 // Care must be taken that a low "D" value does not become an
1661 // an absorbing state.  Transient spinning failures -- when spinning
1662 // is overall profitable -- should not cause the system to converge
1663 // on low "D" values.  We want spinning to be stable and predictable
1664 // and fairly responsive to change and at the same time we don't want
1665 // it to oscillate, become metastable, be "too" non-deterministic,
1666 // or converge on or enter undesirable stable absorbing states.
1667 //
1668 // We implement a feedback-based control system -- using past behavior
1669 // to predict future behavior.  We face two issues: (a) if the
1670 // input signal is random then the spin predictor won't provide optimal
1671 // results, and (b) if the signal frequency is too high then the control
1672 // system, which has some natural response lag, will "chase" the signal.
1673 // (b) can arise from multimodal lock hold times.  Transient preemption
1674 // can also result in apparent bimodal lock hold times.
1675 // Although sub-optimal, neither condition is particularly harmful, as
1676 // in the worst-case we'll spin when we shouldn't or vice-versa.
1677 // The maximum spin duration is rather short so the failure modes aren't bad.
1678 // To be conservative, I've tuned the gain in system to bias toward
1679 // _not spinning.  Relatedly, the system can sometimes enter a mode where it
1680 // "rings" or oscillates between spinning and not spinning.  This happens
1681 // when spinning is just on the cusp of profitability, however, so the
1682 // situation is not dire.  The state is benign -- there's no need to add
1683 // hysteresis control to damp the transition rate between spinning and
1684 // not spinning.
1685 
1686 // Spinning: Fixed frequency (100%), vary duration
1687 int ObjectMonitor::TrySpin(Thread * Self) {
1688   // Dumb, brutal spin.  Good for comparative measurements against adaptive spinning.
1689   int ctr = Knob_FixedSpin;
1690   if (ctr != 0) {
1691     while (--ctr >= 0) {
1692       if (TryLock(Self) > 0) return 1;
1693       SpinPause();
1694     }
1695     return 0;
1696   }
1697 
1698   for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1699     if (TryLock(Self) > 0) {
1700       // Increase _SpinDuration ...
1701       // Note that we don't clamp SpinDuration precisely at SpinLimit.
1702       // Raising _SpurDuration to the poverty line is key.
1703       int x = _SpinDuration;
1704       if (x < Knob_SpinLimit) {
1705         if (x < Knob_Poverty) x = Knob_Poverty;
1706         _SpinDuration = x + Knob_BonusB;
1707       }
1708       return 1;
1709     }
1710     SpinPause();
1711   }
1712 
1713   // Admission control - verify preconditions for spinning
1714   //
1715   // We always spin a little bit, just to prevent _SpinDuration == 0 from
1716   // becoming an absorbing state.  Put another way, we spin briefly to
1717   // sample, just in case the system load, parallelism, contention, or lock
1718   // modality changed.
1719   //
1720   // Consider the following alternative:
1721   // Periodically set _SpinDuration = _SpinLimit and try a long/full
1722   // spin attempt.  "Periodically" might mean after a tally of
1723   // the # of failed spin attempts (or iterations) reaches some threshold.
1724   // This takes us into the realm of 1-out-of-N spinning, where we
1725   // hold the duration constant but vary the frequency.
1726 
1727   ctr = _SpinDuration;
1728   if (ctr <= 0) return 0;
1729 
1730   if (NotRunnable(Self, (Thread *) _owner)) {
1731     return 0;
1732   }
1733 
1734   // We're good to spin ... spin ingress.
1735   // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1736   // when preparing to LD...CAS _owner, etc and the CAS is likely
1737   // to succeed.
1738   if (_succ == NULL) {
1739     _succ = Self;
1740   }
1741   Thread * prv = NULL;
1742 
1743   // There are three ways to exit the following loop:
1744   // 1.  A successful spin where this thread has acquired the lock.
1745   // 2.  Spin failure with prejudice
1746   // 3.  Spin failure without prejudice
1747 
1748   while (--ctr >= 0) {
1749 
1750     // Periodic polling -- Check for pending GC
1751     // Threads may spin while they're unsafe.
1752     // We don't want spinning threads to delay the JVM from reaching
1753     // a stop-the-world safepoint or to steal cycles from GC.
1754     // If we detect a pending safepoint we abort in order that
1755     // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1756     // this thread, if safe, doesn't steal cycles from GC.
1757     // This is in keeping with the "no loitering in runtime" rule.
1758     // We periodically check to see if there's a safepoint pending.
1759     if ((ctr & 0xFF) == 0) {
1760       if (SafepointMechanism::poll(Self)) {
1761         goto Abort;           // abrupt spin egress
1762       }
1763       SpinPause();
1764     }
1765 
1766     // Probe _owner with TATAS
1767     // If this thread observes the monitor transition or flicker
1768     // from locked to unlocked to locked, then the odds that this
1769     // thread will acquire the lock in this spin attempt go down
1770     // considerably.  The same argument applies if the CAS fails
1771     // or if we observe _owner change from one non-null value to
1772     // another non-null value.   In such cases we might abort
1773     // the spin without prejudice or apply a "penalty" to the
1774     // spin count-down variable "ctr", reducing it by 100, say.
1775 
1776     Thread * ox = (Thread *) _owner;
1777     if (ox == NULL) {
1778       ox = (Thread*)Atomic::cmpxchg(Self, &_owner, (void*)NULL);
1779       if (ox == NULL) {
1780         // The CAS succeeded -- this thread acquired ownership
1781         // Take care of some bookkeeping to exit spin state.
1782         if (_succ == Self) {
1783           _succ = NULL;
1784         }
1785 
1786         // Increase _SpinDuration :
1787         // The spin was successful (profitable) so we tend toward
1788         // longer spin attempts in the future.
1789         // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
1790         // If we acquired the lock early in the spin cycle it
1791         // makes sense to increase _SpinDuration proportionally.
1792         // Note that we don't clamp SpinDuration precisely at SpinLimit.
1793         int x = _SpinDuration;
1794         if (x < Knob_SpinLimit) {
1795           if (x < Knob_Poverty) x = Knob_Poverty;
1796           _SpinDuration = x + Knob_Bonus;
1797         }
1798         return 1;
1799       }
1800 
1801       // The CAS failed ... we can take any of the following actions:
1802       // * penalize: ctr -= CASPenalty
1803       // * exit spin with prejudice -- goto Abort;
1804       // * exit spin without prejudice.
1805       // * Since CAS is high-latency, retry again immediately.
1806       prv = ox;
1807       goto Abort;
1808     }
1809 
1810     // Did lock ownership change hands ?
1811     if (ox != prv && prv != NULL) {
1812       goto Abort;
1813     }
1814     prv = ox;
1815 
1816     // Abort the spin if the owner is not executing.
1817     // The owner must be executing in order to drop the lock.
1818     // Spinning while the owner is OFFPROC is idiocy.
1819     // Consider: ctr -= RunnablePenalty ;
1820     if (NotRunnable(Self, ox)) {
1821       goto Abort;
1822     }
1823     if (_succ == NULL) {
1824       _succ = Self;
1825     }
1826   }
1827 
1828   // Spin failed with prejudice -- reduce _SpinDuration.
1829   // TODO: Use an AIMD-like policy to adjust _SpinDuration.
1830   // AIMD is globally stable.
1831   {
1832     int x = _SpinDuration;
1833     if (x > 0) {
1834       // Consider an AIMD scheme like: x -= (x >> 3) + 100
1835       // This is globally sample and tends to damp the response.
1836       x -= Knob_Penalty;
1837       if (x < 0) x = 0;
1838       _SpinDuration = x;
1839     }
1840   }
1841 
1842  Abort:
1843   if (_succ == Self) {
1844     _succ = NULL;
1845     // Invariant: after setting succ=null a contending thread
1846     // must recheck-retry _owner before parking.  This usually happens
1847     // in the normal usage of TrySpin(), but it's safest
1848     // to make TrySpin() as foolproof as possible.
1849     OrderAccess::fence();
1850     if (TryLock(Self) > 0) return 1;
1851   }
1852   return 0;
1853 }
1854 
1855 // NotRunnable() -- informed spinning
1856 //
1857 // Don't bother spinning if the owner is not eligible to drop the lock.
1858 // Spin only if the owner thread is _thread_in_Java or _thread_in_vm.
1859 // The thread must be runnable in order to drop the lock in timely fashion.
1860 // If the _owner is not runnable then spinning will not likely be
1861 // successful (profitable).
1862 //
1863 // Beware -- the thread referenced by _owner could have died
1864 // so a simply fetch from _owner->_thread_state might trap.
1865 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
1866 // Because of the lifecycle issues, the _thread_state values
1867 // observed by NotRunnable() might be garbage.  NotRunnable must
1868 // tolerate this and consider the observed _thread_state value
1869 // as advisory.
1870 //
1871 // Beware too, that _owner is sometimes a BasicLock address and sometimes
1872 // a thread pointer.
1873 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
1874 // with the LSB of _owner.  Another option would be to probabilistically probe
1875 // the putative _owner->TypeTag value.
1876 //
1877 // Checking _thread_state isn't perfect.  Even if the thread is
1878 // in_java it might be blocked on a page-fault or have been preempted
1879 // and sitting on a ready/dispatch queue.
1880 //
1881 // The return value from NotRunnable() is *advisory* -- the
1882 // result is based on sampling and is not necessarily coherent.
1883 // The caller must tolerate false-negative and false-positive errors.
1884 // Spinning, in general, is probabilistic anyway.
1885 
1886 
1887 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) {
1888   // Check ox->TypeTag == 2BAD.
1889   if (ox == NULL) return 0;
1890 
1891   // Avoid transitive spinning ...
1892   // Say T1 spins or blocks trying to acquire L.  T1._Stalled is set to L.
1893   // Immediately after T1 acquires L it's possible that T2, also
1894   // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
1895   // This occurs transiently after T1 acquired L but before
1896   // T1 managed to clear T1.Stalled.  T2 does not need to abort
1897   // its spin in this circumstance.
1898   intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
1899 
1900   if (BlockedOn == 1) return 1;
1901   if (BlockedOn != 0) {
1902     return BlockedOn != intptr_t(this) && _owner == ox;
1903   }
1904 
1905   assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant");
1906   int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);;
1907   // consider also: jst != _thread_in_Java -- but that's overspecific.
1908   return jst == _thread_blocked || jst == _thread_in_native;
1909 }
1910 
1911 
1912 // -----------------------------------------------------------------------------
1913 // WaitSet management ...
1914 
1915 ObjectWaiter::ObjectWaiter(Thread* thread) {
1916   _next     = NULL;
1917   _prev     = NULL;
1918   _notified = 0;
1919   _notifier_tid = 0;
1920   TState    = TS_RUN;
1921   _thread   = thread;
1922   _event    = thread->_ParkEvent;
1923   _active   = false;
1924   assert(_event != NULL, "invariant");
1925 }
1926 
1927 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
1928   JavaThread *jt = (JavaThread *)this->_thread;
1929   _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
1930 }
1931 
1932 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
1933   JavaThread *jt = (JavaThread *)this->_thread;
1934   JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
1935 }
1936 
1937 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
1938   assert(node != NULL, "should not add NULL node");
1939   assert(node->_prev == NULL, "node already in list");
1940   assert(node->_next == NULL, "node already in list");
1941   // put node at end of queue (circular doubly linked list)
1942   if (_WaitSet == NULL) {
1943     _WaitSet = node;
1944     node->_prev = node;
1945     node->_next = node;
1946   } else {
1947     ObjectWaiter* head = _WaitSet;
1948     ObjectWaiter* tail = head->_prev;
1949     assert(tail->_next == head, "invariant check");
1950     tail->_next = node;
1951     head->_prev = node;
1952     node->_next = head;
1953     node->_prev = tail;
1954   }
1955 }
1956 
1957 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
1958   // dequeue the very first waiter
1959   ObjectWaiter* waiter = _WaitSet;
1960   if (waiter) {
1961     DequeueSpecificWaiter(waiter);
1962   }
1963   return waiter;
1964 }
1965 
1966 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
1967   assert(node != NULL, "should not dequeue NULL node");
1968   assert(node->_prev != NULL, "node already removed from list");
1969   assert(node->_next != NULL, "node already removed from list");
1970   // when the waiter has woken up because of interrupt,
1971   // timeout or other spurious wake-up, dequeue the
1972   // waiter from waiting list
1973   ObjectWaiter* next = node->_next;
1974   if (next == node) {
1975     assert(node->_prev == node, "invariant check");
1976     _WaitSet = NULL;
1977   } else {
1978     ObjectWaiter* prev = node->_prev;
1979     assert(prev->_next == node, "invariant check");
1980     assert(next->_prev == node, "invariant check");
1981     next->_prev = prev;
1982     prev->_next = next;
1983     if (_WaitSet == node) {
1984       _WaitSet = next;
1985     }
1986   }
1987   node->_next = NULL;
1988   node->_prev = NULL;
1989 }
1990 
1991 // -----------------------------------------------------------------------------
1992 // PerfData support
1993 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts       = NULL;
1994 PerfCounter * ObjectMonitor::_sync_FutileWakeups               = NULL;
1995 PerfCounter * ObjectMonitor::_sync_Parks                       = NULL;
1996 PerfCounter * ObjectMonitor::_sync_Notifications               = NULL;
1997 PerfCounter * ObjectMonitor::_sync_Inflations                  = NULL;
1998 PerfCounter * ObjectMonitor::_sync_Deflations                  = NULL;
1999 PerfLongVariable * ObjectMonitor::_sync_MonExtant              = NULL;
2000 
2001 // One-shot global initialization for the sync subsystem.
2002 // We could also defer initialization and initialize on-demand
2003 // the first time we call inflate().  Initialization would
2004 // be protected - like so many things - by the MonitorCache_lock.
2005 
2006 void ObjectMonitor::Initialize() {
2007   static int InitializationCompleted = 0;
2008   assert(InitializationCompleted == 0, "invariant");
2009   InitializationCompleted = 1;
2010   if (UsePerfData) {
2011     EXCEPTION_MARK;
2012 #define NEWPERFCOUNTER(n)                                                \
2013   {                                                                      \
2014     n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,  \
2015                                         CHECK);                          \
2016   }
2017 #define NEWPERFVARIABLE(n)                                                \
2018   {                                                                       \
2019     n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,  \
2020                                          CHECK);                          \
2021   }
2022     NEWPERFCOUNTER(_sync_Inflations);
2023     NEWPERFCOUNTER(_sync_Deflations);
2024     NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2025     NEWPERFCOUNTER(_sync_FutileWakeups);
2026     NEWPERFCOUNTER(_sync_Parks);
2027     NEWPERFCOUNTER(_sync_Notifications);
2028     NEWPERFVARIABLE(_sync_MonExtant);
2029 #undef NEWPERFCOUNTER
2030 #undef NEWPERFVARIABLE
2031   }
2032 }
2033 
2034 void ObjectMonitor::DeferredInitialize() {
2035   if (InitDone > 0) return;
2036   if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) {
2037     while (InitDone != 1) /* empty */;
2038     return;
2039   }
2040 
2041   // One-shot global initialization ...
2042   // The initialization is idempotent, so we don't need locks.
2043   // In the future consider doing this via os::init_2().
2044 
2045   if (!os::is_MP()) {
2046     Knob_SpinLimit = 0;
2047     Knob_PreSpin   = 0;
2048     Knob_FixedSpin = -1;
2049   }
2050 
2051   OrderAccess::fence();
2052   InitDone = 1;
2053 }
2054 
--- EOF ---