1 /*
   2  * Copyright (c) 1998, 2021, 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 "gc/shared/oopStorage.hpp"
  28 #include "gc/shared/oopStorageSet.hpp"
  29 #include "jfr/jfrEvents.hpp"
  30 #include "jfr/support/jfrThreadId.hpp"
  31 #include "logging/log.hpp"
  32 #include "logging/logStream.hpp"
  33 #include "memory/allocation.inline.hpp"
  34 #include "memory/resourceArea.hpp"
  35 #include "oops/markWord.hpp"
  36 #include "oops/oop.inline.hpp"
  37 #include "oops/oopHandle.inline.hpp"
  38 #include "oops/weakHandle.inline.hpp"
  39 #include "prims/jvmtiDeferredUpdates.hpp"
  40 #include "prims/jvmtiExport.hpp"
  41 #include "runtime/atomic.hpp"
  42 #include "runtime/handles.inline.hpp"
  43 #include "runtime/interfaceSupport.inline.hpp"
  44 #include "runtime/mutexLocker.hpp"
  45 #include "runtime/objectMonitor.hpp"
  46 #include "runtime/objectMonitor.inline.hpp"
  47 #include "runtime/orderAccess.hpp"
  48 #include "runtime/osThread.hpp"
  49 #include "runtime/perfData.hpp"
  50 #include "runtime/safefetch.inline.hpp"
  51 #include "runtime/safepointMechanism.inline.hpp"
  52 #include "runtime/sharedRuntime.hpp"
  53 #include "runtime/thread.inline.hpp"
  54 #include "services/threadService.hpp"
  55 #include "utilities/dtrace.hpp"
  56 #include "utilities/macros.hpp"
  57 #include "utilities/preserveException.hpp"
  58 #if INCLUDE_JFR
  59 #include "jfr/support/jfrFlush.hpp"
  60 #endif
  61 
  62 #ifdef DTRACE_ENABLED
  63 
  64 // Only bother with this argument setup if dtrace is available
  65 // TODO-FIXME: probes should not fire when caller is _blocked.  assert() accordingly.
  66 
  67 
  68 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread)                           \
  69   char* bytes = NULL;                                                      \
  70   int len = 0;                                                             \
  71   jlong jtid = SharedRuntime::get_java_tid(thread);                        \
  72   Symbol* klassname = obj->klass()->name();                                \
  73   if (klassname != NULL) {                                                 \
  74     bytes = (char*)klassname->bytes();                                     \
  75     len = klassname->utf8_length();                                        \
  76   }
  77 
  78 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis)            \
  79   {                                                                        \
  80     if (DTraceMonitorProbes) {                                             \
  81       DTRACE_MONITOR_PROBE_COMMON(obj, thread);                            \
  82       HOTSPOT_MONITOR_WAIT(jtid,                                           \
  83                            (monitor), bytes, len, (millis));               \
  84     }                                                                      \
  85   }
  86 
  87 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
  88 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
  89 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
  90 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
  91 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
  92 
  93 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread)                  \
  94   {                                                                        \
  95     if (DTraceMonitorProbes) {                                             \
  96       DTRACE_MONITOR_PROBE_COMMON(obj, thread);                            \
  97       HOTSPOT_MONITOR_##probe(jtid,                                        \
  98                               (uintptr_t)(monitor), bytes, len);           \
  99     }                                                                      \
 100   }
 101 
 102 #else //  ndef DTRACE_ENABLED
 103 
 104 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon)    {;}
 105 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon)          {;}
 106 
 107 #endif // ndef DTRACE_ENABLED
 108 
 109 // Tunables ...
 110 // The knob* variables are effectively final.  Once set they should
 111 // never be modified hence.  Consider using __read_mostly with GCC.
 112 
 113 int ObjectMonitor::Knob_SpinLimit    = 5000;    // derived by an external tool -
 114 
 115 static int Knob_Bonus               = 100;     // spin success bonus
 116 static int Knob_BonusB              = 100;     // spin success bonus
 117 static int Knob_Penalty             = 200;     // spin failure penalty
 118 static int Knob_Poverty             = 1000;
 119 static int Knob_FixedSpin           = 0;
 120 static int Knob_PreSpin             = 10;      // 20-100 likely better
 121 
 122 DEBUG_ONLY(static volatile bool InitDone = false;)
 123 
 124 OopStorage* ObjectMonitor::_oop_storage = NULL;
 125 
 126 // -----------------------------------------------------------------------------
 127 // Theory of operations -- Monitors lists, thread residency, etc:
 128 //
 129 // * A thread acquires ownership of a monitor by successfully
 130 //   CAS()ing the _owner field from null to non-null.
 131 //
 132 // * Invariant: A thread appears on at most one monitor list --
 133 //   cxq, EntryList or WaitSet -- at any one time.
 134 //
 135 // * Contending threads "push" themselves onto the cxq with CAS
 136 //   and then spin/park.
 137 //
 138 // * After a contending thread eventually acquires the lock it must
 139 //   dequeue itself from either the EntryList or the cxq.
 140 //
 141 // * The exiting thread identifies and unparks an "heir presumptive"
 142 //   tentative successor thread on the EntryList.  Critically, the
 143 //   exiting thread doesn't unlink the successor thread from the EntryList.
 144 //   After having been unparked, the wakee will recontend for ownership of
 145 //   the monitor.   The successor (wakee) will either acquire the lock or
 146 //   re-park itself.
 147 //
 148 //   Succession is provided for by a policy of competitive handoff.
 149 //   The exiting thread does _not_ grant or pass ownership to the
 150 //   successor thread.  (This is also referred to as "handoff" succession").
 151 //   Instead the exiting thread releases ownership and possibly wakes
 152 //   a successor, so the successor can (re)compete for ownership of the lock.
 153 //   If the EntryList is empty but the cxq is populated the exiting
 154 //   thread will drain the cxq into the EntryList.  It does so by
 155 //   by detaching the cxq (installing null with CAS) and folding
 156 //   the threads from the cxq into the EntryList.  The EntryList is
 157 //   doubly linked, while the cxq is singly linked because of the
 158 //   CAS-based "push" used to enqueue recently arrived threads (RATs).
 159 //
 160 // * Concurrency invariants:
 161 //
 162 //   -- only the monitor owner may access or mutate the EntryList.
 163 //      The mutex property of the monitor itself protects the EntryList
 164 //      from concurrent interference.
 165 //   -- Only the monitor owner may detach the cxq.
 166 //
 167 // * The monitor entry list operations avoid locks, but strictly speaking
 168 //   they're not lock-free.  Enter is lock-free, exit is not.
 169 //   For a description of 'Methods and apparatus providing non-blocking access
 170 //   to a resource,' see U.S. Pat. No. 7844973.
 171 //
 172 // * The cxq can have multiple concurrent "pushers" but only one concurrent
 173 //   detaching thread.  This mechanism is immune from the ABA corruption.
 174 //   More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
 175 //
 176 // * Taken together, the cxq and the EntryList constitute or form a
 177 //   single logical queue of threads stalled trying to acquire the lock.
 178 //   We use two distinct lists to improve the odds of a constant-time
 179 //   dequeue operation after acquisition (in the ::enter() epilogue) and
 180 //   to reduce heat on the list ends.  (c.f. Michael Scott's "2Q" algorithm).
 181 //   A key desideratum is to minimize queue & monitor metadata manipulation
 182 //   that occurs while holding the monitor lock -- that is, we want to
 183 //   minimize monitor lock holds times.  Note that even a small amount of
 184 //   fixed spinning will greatly reduce the # of enqueue-dequeue operations
 185 //   on EntryList|cxq.  That is, spinning relieves contention on the "inner"
 186 //   locks and monitor metadata.
 187 //
 188 //   Cxq points to the set of Recently Arrived Threads attempting entry.
 189 //   Because we push threads onto _cxq with CAS, the RATs must take the form of
 190 //   a singly-linked LIFO.  We drain _cxq into EntryList  at unlock-time when
 191 //   the unlocking thread notices that EntryList is null but _cxq is != null.
 192 //
 193 //   The EntryList is ordered by the prevailing queue discipline and
 194 //   can be organized in any convenient fashion, such as a doubly-linked list or
 195 //   a circular doubly-linked list.  Critically, we want insert and delete operations
 196 //   to operate in constant-time.  If we need a priority queue then something akin
 197 //   to Solaris' sleepq would work nicely.  Viz.,
 198 //   http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
 199 //   Queue discipline is enforced at ::exit() time, when the unlocking thread
 200 //   drains the cxq into the EntryList, and orders or reorders the threads on the
 201 //   EntryList accordingly.
 202 //
 203 //   Barring "lock barging", this mechanism provides fair cyclic ordering,
 204 //   somewhat similar to an elevator-scan.
 205 //
 206 // * The monitor synchronization subsystem avoids the use of native
 207 //   synchronization primitives except for the narrow platform-specific
 208 //   park-unpark abstraction.  See the comments in os_solaris.cpp regarding
 209 //   the semantics of park-unpark.  Put another way, this monitor implementation
 210 //   depends only on atomic operations and park-unpark.  The monitor subsystem
 211 //   manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
 212 //   underlying OS manages the READY<->RUN transitions.
 213 //
 214 // * Waiting threads reside on the WaitSet list -- wait() puts
 215 //   the caller onto the WaitSet.
 216 //
 217 // * notify() or notifyAll() simply transfers threads from the WaitSet to
 218 //   either the EntryList or cxq.  Subsequent exit() operations will
 219 //   unpark the notifyee.  Unparking a notifee in notify() is inefficient -
 220 //   it's likely the notifyee would simply impale itself on the lock held
 221 //   by the notifier.
 222 //
 223 // * An interesting alternative is to encode cxq as (List,LockByte) where
 224 //   the LockByte is 0 iff the monitor is owned.  _owner is simply an auxiliary
 225 //   variable, like _recursions, in the scheme.  The threads or Events that form
 226 //   the list would have to be aligned in 256-byte addresses.  A thread would
 227 //   try to acquire the lock or enqueue itself with CAS, but exiting threads
 228 //   could use a 1-0 protocol and simply STB to set the LockByte to 0.
 229 //   Note that is is *not* word-tearing, but it does presume that full-word
 230 //   CAS operations are coherent with intermix with STB operations.  That's true
 231 //   on most common processors.
 232 //
 233 // * See also http://blogs.sun.com/dave
 234 
 235 
 236 void* ObjectMonitor::operator new (size_t size) throw() {
 237   return AllocateHeap(size, mtInternal);
 238 }
 239 void* ObjectMonitor::operator new[] (size_t size) throw() {
 240   return operator new (size);
 241 }
 242 void ObjectMonitor::operator delete(void* p) {
 243   FreeHeap(p);
 244 }
 245 void ObjectMonitor::operator delete[] (void *p) {
 246   operator delete(p);
 247 }
 248 
 249 // Check that object() and set_object() are called from the right context:
 250 static void check_object_context() {
 251 #ifdef ASSERT
 252   Thread* self = Thread::current();
 253   if (self->is_Java_thread()) {
 254     // Mostly called from JavaThreads so sanity check the thread state.
 255     JavaThread* jt = JavaThread::cast(self);
 256     switch (jt->thread_state()) {
 257     case _thread_in_vm:    // the usual case
 258     case _thread_in_Java:  // during deopt
 259       break;
 260     default:
 261       fatal("called from an unsafe thread state");
 262     }
 263     assert(jt->is_active_Java_thread(), "must be active JavaThread");
 264   } else {
 265     // However, ThreadService::get_current_contended_monitor()
 266     // can call here via the VMThread so sanity check it.
 267     assert(self->is_VM_thread(), "must be");
 268   }
 269 #endif // ASSERT
 270 }
 271 
 272 ObjectMonitor::ObjectMonitor(oop object) :
 273   _header(markWord::zero()),
 274   _object(_oop_storage, object),
 275   _owner(NULL),
 276   _previous_owner_tid(0),
 277   _next_om(NULL),
 278   _recursions(0),
 279   _EntryList(NULL),
 280   _cxq(NULL),
 281   _succ(NULL),
 282   _Responsible(NULL),
 283   _Spinner(0),
 284   _SpinDuration(ObjectMonitor::Knob_SpinLimit),
 285   _contentions(0),
 286   _WaitSet(NULL),
 287   _waiters(0),
 288   _WaitSetLock(0)
 289 { }
 290 
 291 ObjectMonitor::~ObjectMonitor() {
 292   _object.release(_oop_storage);
 293 }
 294 
 295 oop ObjectMonitor::object() const {
 296   check_object_context();
 297   if (_object.is_null()) {
 298     return NULL;
 299   }
 300   return _object.resolve();
 301 }
 302 
 303 oop ObjectMonitor::object_peek() const {
 304   if (_object.is_null()) {
 305     return NULL;
 306   }
 307   return _object.peek();
 308 }
 309 
 310 void ObjectMonitor::ExitOnSuspend::operator()(JavaThread* current) {
 311   if (current->is_suspended()) {
 312     _om->_recursions = 0;
 313     _om->_succ = NULL;
 314     // Don't need a full fence after clearing successor here because of the call to exit().
 315     _om->exit(current, false /* not_suspended */);
 316     _om_exited = true;
 317 
 318     current->set_current_pending_monitor(_om);
 319   }
 320 }
 321 
 322 void ObjectMonitor::ClearSuccOnSuspend::operator()(JavaThread* current) {
 323   if (current->is_suspended()) {
 324     if (_om->_succ == current) {
 325       _om->_succ = NULL;
 326       OrderAccess::fence(); // always do a full fence when successor is cleared
 327     }
 328   }
 329 }
 330 
 331 // -----------------------------------------------------------------------------
 332 // Enter support
 333 
 334 bool ObjectMonitor::enter(JavaThread* current) {
 335   // The following code is ordered to check the most common cases first
 336   // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
 337 
 338   void* cur = try_set_owner_from(NULL, current);
 339   if (cur == NULL) {
 340     assert(_recursions == 0, "invariant");
 341     return true;
 342   }
 343 
 344   if (cur == current) {
 345     // TODO-FIXME: check for integer overflow!  BUGID 6557169.
 346     _recursions++;
 347     return true;
 348   }
 349 
 350   if (current->is_lock_owned((address)cur)) {
 351     assert(_recursions == 0, "internal state error");
 352     _recursions = 1;
 353     set_owner_from_BasicLock(cur, current);  // Convert from BasicLock* to Thread*.
 354     return true;
 355   }
 356 
 357   // We've encountered genuine contention.
 358   assert(current->_Stalled == 0, "invariant");
 359   current->_Stalled = intptr_t(this);
 360 
 361   // Try one round of spinning *before* enqueueing current
 362   // and before going through the awkward and expensive state
 363   // transitions.  The following spin is strictly optional ...
 364   // Note that if we acquire the monitor from an initial spin
 365   // we forgo posting JVMTI events and firing DTRACE probes.
 366   if (TrySpin(current) > 0) {
 367     assert(owner_raw() == current, "must be current: owner=" INTPTR_FORMAT, p2i(owner_raw()));
 368     assert(_recursions == 0, "must be 0: recursions=" INTX_FORMAT, _recursions);
 369     assert(object()->mark() == markWord::encode(this),
 370            "object mark must match encoded this: mark=" INTPTR_FORMAT
 371            ", encoded this=" INTPTR_FORMAT, object()->mark().value(),
 372            markWord::encode(this).value());
 373     current->_Stalled = 0;
 374     return true;
 375   }
 376 
 377   assert(owner_raw() != current, "invariant");
 378   assert(_succ != current, "invariant");
 379   assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
 380   assert(current->thread_state() != _thread_blocked, "invariant");
 381 
 382   // Keep track of contention for JVM/TI and M&M queries.
 383   add_to_contentions(1);
 384   if (is_being_async_deflated()) {
 385     // Async deflation is in progress and our contentions increment
 386     // above lost the race to async deflation. Undo the work and
 387     // force the caller to retry.
 388     const oop l_object = object();
 389     if (l_object != NULL) {
 390       // Attempt to restore the header/dmw to the object's header so that
 391       // we only retry once if the deflater thread happens to be slow.
 392       install_displaced_markword_in_object(l_object);
 393     }
 394     current->_Stalled = 0;
 395     add_to_contentions(-1);
 396     return false;
 397   }
 398 
 399   JFR_ONLY(JfrConditionalFlushWithStacktrace<EventJavaMonitorEnter> flush(current);)
 400   EventJavaMonitorEnter event;
 401   if (event.is_started()) {
 402     event.set_monitorClass(object()->klass());
 403     // Set an address that is 'unique enough', such that events close in
 404     // time and with the same address are likely (but not guaranteed) to
 405     // belong to the same object.
 406     event.set_address((uintptr_t)this);
 407   }
 408 
 409   { // Change java thread status to indicate blocked on monitor enter.
 410     JavaThreadBlockedOnMonitorEnterState jtbmes(current, this);
 411 
 412     assert(current->current_pending_monitor() == NULL, "invariant");
 413     current->set_current_pending_monitor(this);
 414 
 415     DTRACE_MONITOR_PROBE(contended__enter, this, object(), current);
 416     if (JvmtiExport::should_post_monitor_contended_enter()) {
 417       JvmtiExport::post_monitor_contended_enter(current, this);
 418 
 419       // The current thread does not yet own the monitor and does not
 420       // yet appear on any queues that would get it made the successor.
 421       // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
 422       // handler cannot accidentally consume an unpark() meant for the
 423       // ParkEvent associated with this ObjectMonitor.
 424     }
 425 
 426     OSThreadContendState osts(current->osthread());
 427 
 428     assert(current->thread_state() == _thread_in_vm, "invariant");
 429 
 430     for (;;) {
 431       ExitOnSuspend eos(this);
 432       {
 433         ThreadBlockInVMPreprocess<ExitOnSuspend> tbivs(current, eos, true /* allow_suspend */);
 434         EnterI(current);
 435         current->set_current_pending_monitor(NULL);
 436         // We can go to a safepoint at the end of this block. If we
 437         // do a thread dump during that safepoint, then this thread will show
 438         // as having "-locked" the monitor, but the OS and java.lang.Thread
 439         // states will still report that the thread is blocked trying to
 440         // acquire it.
 441         // If there is a suspend request, ExitOnSuspend will exit the OM
 442         // and set the OM as pending.
 443       }
 444       if (!eos.exited()) {
 445         // ExitOnSuspend did not exit the OM
 446         assert(owner_raw() == current, "invariant");
 447         break;
 448       }
 449     }
 450 
 451     // We've just gotten past the enter-check-for-suspend dance and we now own
 452     // the monitor free and clear.
 453   }
 454 
 455   add_to_contentions(-1);
 456   assert(contentions() >= 0, "must not be negative: contentions=%d", contentions());
 457   current->_Stalled = 0;
 458 
 459   // Must either set _recursions = 0 or ASSERT _recursions == 0.
 460   assert(_recursions == 0, "invariant");
 461   assert(owner_raw() == current, "invariant");
 462   assert(_succ != current, "invariant");
 463   assert(object()->mark() == markWord::encode(this), "invariant");
 464 
 465   // The thread -- now the owner -- is back in vm mode.
 466   // Report the glorious news via TI,DTrace and jvmstat.
 467   // The probe effect is non-trivial.  All the reportage occurs
 468   // while we hold the monitor, increasing the length of the critical
 469   // section.  Amdahl's parallel speedup law comes vividly into play.
 470   //
 471   // Another option might be to aggregate the events (thread local or
 472   // per-monitor aggregation) and defer reporting until a more opportune
 473   // time -- such as next time some thread encounters contention but has
 474   // yet to acquire the lock.  While spinning that thread could
 475   // spinning we could increment JVMStat counters, etc.
 476 
 477   DTRACE_MONITOR_PROBE(contended__entered, this, object(), current);
 478   if (JvmtiExport::should_post_monitor_contended_entered()) {
 479     JvmtiExport::post_monitor_contended_entered(current, this);
 480 
 481     // The current thread already owns the monitor and is not going to
 482     // call park() for the remainder of the monitor enter protocol. So
 483     // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
 484     // event handler consumed an unpark() issued by the thread that
 485     // just exited the monitor.
 486   }
 487   if (event.should_commit()) {
 488     event.set_previousOwner(_previous_owner_tid);
 489     event.commit();
 490   }
 491   OM_PERFDATA_OP(ContendedLockAttempts, inc());
 492   return true;
 493 }
 494 
 495 // Caveat: TryLock() is not necessarily serializing if it returns failure.
 496 // Callers must compensate as needed.
 497 
 498 int ObjectMonitor::TryLock(JavaThread* current) {
 499   void* own = owner_raw();
 500   if (own != NULL) return 0;
 501   if (try_set_owner_from(NULL, current) == NULL) {
 502     assert(_recursions == 0, "invariant");
 503     return 1;
 504   }
 505   // The lock had been free momentarily, but we lost the race to the lock.
 506   // Interference -- the CAS failed.
 507   // We can either return -1 or retry.
 508   // Retry doesn't make as much sense because the lock was just acquired.
 509   return -1;
 510 }
 511 
 512 // Deflate the specified ObjectMonitor if not in-use. Returns true if it
 513 // was deflated and false otherwise.
 514 //
 515 // The async deflation protocol sets owner to DEFLATER_MARKER and
 516 // makes contentions negative as signals to contending threads that
 517 // an async deflation is in progress. There are a number of checks
 518 // as part of the protocol to make sure that the calling thread has
 519 // not lost the race to a contending thread.
 520 //
 521 // The ObjectMonitor has been successfully async deflated when:
 522 //   (contentions < 0)
 523 // Contending threads that see that condition know to retry their operation.
 524 //
 525 bool ObjectMonitor::deflate_monitor() {
 526   if (is_busy()) {
 527     // Easy checks are first - the ObjectMonitor is busy so no deflation.
 528     return false;
 529   }
 530 
 531   if (ObjectSynchronizer::is_final_audit() && owner_is_DEFLATER_MARKER()) {
 532     // The final audit can see an already deflated ObjectMonitor on the
 533     // in-use list because MonitorList::unlink_deflated() might have
 534     // blocked for the final safepoint before unlinking all the deflated
 535     // monitors.
 536     assert(contentions() < 0, "must be negative: contentions=%d", contentions());
 537     // Already returned 'true' when it was originally deflated.
 538     return false;
 539   }
 540 
 541   const oop obj = object_peek();
 542 
 543   if (obj == NULL) {
 544     // If the object died, we can recycle the monitor without racing with
 545     // Java threads. The GC already broke the association with the object.
 546     set_owner_from(NULL, DEFLATER_MARKER);
 547     assert(contentions() >= 0, "must be non-negative: contentions=%d", contentions());
 548     _contentions = INT_MIN; // minimum negative int
 549   } else {
 550     // Attempt async deflation protocol.
 551 
 552     // Set a NULL owner to DEFLATER_MARKER to force any contending thread
 553     // through the slow path. This is just the first part of the async
 554     // deflation dance.
 555     if (try_set_owner_from(NULL, DEFLATER_MARKER) != NULL) {
 556       // The owner field is no longer NULL so we lost the race since the
 557       // ObjectMonitor is now busy.
 558       return false;
 559     }
 560 
 561     if (contentions() > 0 || _waiters != 0) {
 562       // Another thread has raced to enter the ObjectMonitor after
 563       // is_busy() above or has already entered and waited on
 564       // it which makes it busy so no deflation. Restore owner to
 565       // NULL if it is still DEFLATER_MARKER.
 566       if (try_set_owner_from(DEFLATER_MARKER, NULL) != DEFLATER_MARKER) {
 567         // Deferred decrement for the JT EnterI() that cancelled the async deflation.
 568         add_to_contentions(-1);
 569       }
 570       return false;
 571     }
 572 
 573     // Make a zero contentions field negative to force any contending threads
 574     // to retry. This is the second part of the async deflation dance.
 575     if (Atomic::cmpxchg(&_contentions, 0, INT_MIN) != 0) {
 576       // Contentions was no longer 0 so we lost the race since the
 577       // ObjectMonitor is now busy. Restore owner to NULL if it is
 578       // still DEFLATER_MARKER:
 579       if (try_set_owner_from(DEFLATER_MARKER, NULL) != DEFLATER_MARKER) {
 580         // Deferred decrement for the JT EnterI() that cancelled the async deflation.
 581         add_to_contentions(-1);
 582       }
 583       return false;
 584     }
 585   }
 586 
 587   // Sanity checks for the races:
 588   guarantee(owner_is_DEFLATER_MARKER(), "must be deflater marker");
 589   guarantee(contentions() < 0, "must be negative: contentions=%d",
 590             contentions());
 591   guarantee(_waiters == 0, "must be 0: waiters=%d", _waiters);
 592   guarantee(_cxq == NULL, "must be no contending threads: cxq="
 593             INTPTR_FORMAT, p2i(_cxq));
 594   guarantee(_EntryList == NULL,
 595             "must be no entering threads: EntryList=" INTPTR_FORMAT,
 596             p2i(_EntryList));
 597 
 598   if (obj != NULL) {
 599     if (log_is_enabled(Trace, monitorinflation)) {
 600       ResourceMark rm;
 601       log_trace(monitorinflation)("deflate_monitor: object=" INTPTR_FORMAT
 602                                   ", mark=" INTPTR_FORMAT ", type='%s'",
 603                                   p2i(obj), obj->mark().value(),
 604                                   obj->klass()->external_name());
 605     }
 606 
 607     // Install the old mark word if nobody else has already done it.
 608     install_displaced_markword_in_object(obj);
 609   }
 610 
 611   // We leave owner == DEFLATER_MARKER and contentions < 0
 612   // to force any racing threads to retry.
 613   return true;  // Success, ObjectMonitor has been deflated.
 614 }
 615 
 616 // We might access the dead object headers for parsable heap walk, make sure
 617 // headers are in correct shape, e.g. monitors deflated.
 618 void ObjectMonitor::maybe_deflate_dead(oop* p) {
 619   oop obj = *p;
 620   assert(obj != NULL, "must not yet been cleared");
 621   markWord mark = obj->mark();
 622   if (mark.has_monitor()) {
 623     ObjectMonitor* monitor = mark.monitor();
 624     if (p == monitor->_object.ptr_raw()) {
 625       assert(monitor->object_peek() == obj, "lock object must match");
 626       markWord dmw = monitor->header();
 627       obj->set_mark(dmw);
 628     }
 629   }
 630 }
 631 
 632 // Install the displaced mark word (dmw) of a deflating ObjectMonitor
 633 // into the header of the object associated with the monitor. This
 634 // idempotent method is called by a thread that is deflating a
 635 // monitor and by other threads that have detected a race with the
 636 // deflation process.
 637 void ObjectMonitor::install_displaced_markword_in_object(const oop obj) {
 638   // This function must only be called when (owner == DEFLATER_MARKER
 639   // && contentions <= 0), but we can't guarantee that here because
 640   // those values could change when the ObjectMonitor gets moved from
 641   // the global free list to a per-thread free list.
 642 
 643   guarantee(obj != NULL, "must be non-NULL");
 644 
 645   // Separate loads in is_being_async_deflated(), which is almost always
 646   // called before this function, from the load of dmw/header below.
 647 
 648   // _contentions and dmw/header may get written by different threads.
 649   // Make sure to observe them in the same order when having several observers.
 650   OrderAccess::loadload_for_IRIW();
 651 
 652   const oop l_object = object_peek();
 653   if (l_object == NULL) {
 654     // ObjectMonitor's object ref has already been cleared by async
 655     // deflation or GC so we're done here.
 656     return;
 657   }
 658   assert(l_object == obj, "object=" INTPTR_FORMAT " must equal obj="
 659          INTPTR_FORMAT, p2i(l_object), p2i(obj));
 660 
 661   markWord dmw = header();
 662   // The dmw has to be neutral (not NULL, not locked and not marked).
 663   assert(dmw.is_neutral(), "must be neutral: dmw=" INTPTR_FORMAT, dmw.value());
 664 
 665   // Install displaced mark word if the object's header still points
 666   // to this ObjectMonitor. More than one racing caller to this function
 667   // can rarely reach this point, but only one can win.
 668   markWord res = obj->cas_set_mark(dmw, markWord::encode(this));
 669   if (res != markWord::encode(this)) {
 670     // This should be rare so log at the Info level when it happens.
 671     log_info(monitorinflation)("install_displaced_markword_in_object: "
 672                                "failed cas_set_mark: new_mark=" INTPTR_FORMAT
 673                                ", old_mark=" INTPTR_FORMAT ", res=" INTPTR_FORMAT,
 674                                dmw.value(), markWord::encode(this).value(),
 675                                res.value());
 676   }
 677 
 678   // Note: It does not matter which thread restored the header/dmw
 679   // into the object's header. The thread deflating the monitor just
 680   // wanted the object's header restored and it is. The threads that
 681   // detected a race with the deflation process also wanted the
 682   // object's header restored before they retry their operation and
 683   // because it is restored they will only retry once.
 684 }
 685 
 686 // Convert the fields used by is_busy() to a string that can be
 687 // used for diagnostic output.
 688 const char* ObjectMonitor::is_busy_to_string(stringStream* ss) {
 689   ss->print("is_busy: waiters=%d, ", _waiters);
 690   if (contentions() > 0) {
 691     ss->print("contentions=%d, ", contentions());
 692   } else {
 693     ss->print("contentions=0");
 694   }
 695   if (!owner_is_DEFLATER_MARKER()) {
 696     ss->print("owner=" INTPTR_FORMAT, p2i(owner_raw()));
 697   } else {
 698     // We report NULL instead of DEFLATER_MARKER here because is_busy()
 699     // ignores DEFLATER_MARKER values.
 700     ss->print("owner=" INTPTR_FORMAT, NULL);
 701   }
 702   ss->print(", cxq=" INTPTR_FORMAT ", EntryList=" INTPTR_FORMAT, p2i(_cxq),
 703             p2i(_EntryList));
 704   return ss->base();
 705 }
 706 
 707 #define MAX_RECHECK_INTERVAL 1000
 708 
 709 void ObjectMonitor::EnterI(JavaThread* current) {
 710   assert(current->thread_state() == _thread_blocked, "invariant");
 711 
 712   // Try the lock - TATAS
 713   if (TryLock (current) > 0) {
 714     assert(_succ != current, "invariant");
 715     assert(owner_raw() == current, "invariant");
 716     assert(_Responsible != current, "invariant");
 717     return;
 718   }
 719 
 720   if (try_set_owner_from(DEFLATER_MARKER, current) == DEFLATER_MARKER) {
 721     // Cancelled the in-progress async deflation by changing owner from
 722     // DEFLATER_MARKER to current. As part of the contended enter protocol,
 723     // contentions was incremented to a positive value before EnterI()
 724     // was called and that prevents the deflater thread from winning the
 725     // last part of the 2-part async deflation protocol. After EnterI()
 726     // returns to enter(), contentions is decremented because the caller
 727     // now owns the monitor. We bump contentions an extra time here to
 728     // prevent the deflater thread from winning the last part of the
 729     // 2-part async deflation protocol after the regular decrement
 730     // occurs in enter(). The deflater thread will decrement contentions
 731     // after it recognizes that the async deflation was cancelled.
 732     add_to_contentions(1);
 733     assert(_succ != current, "invariant");
 734     assert(_Responsible != current, "invariant");
 735     return;
 736   }
 737 
 738   assert(InitDone, "Unexpectedly not initialized");
 739 
 740   // We try one round of spinning *before* enqueueing current.
 741   //
 742   // If the _owner is ready but OFFPROC we could use a YieldTo()
 743   // operation to donate the remainder of this thread's quantum
 744   // to the owner.  This has subtle but beneficial affinity
 745   // effects.
 746 
 747   if (TrySpin(current) > 0) {
 748     assert(owner_raw() == current, "invariant");
 749     assert(_succ != current, "invariant");
 750     assert(_Responsible != current, "invariant");
 751     return;
 752   }
 753 
 754   // The Spin failed -- Enqueue and park the thread ...
 755   assert(_succ != current, "invariant");
 756   assert(owner_raw() != current, "invariant");
 757   assert(_Responsible != current, "invariant");
 758 
 759   // Enqueue "current" on ObjectMonitor's _cxq.
 760   //
 761   // Node acts as a proxy for current.
 762   // As an aside, if were to ever rewrite the synchronization code mostly
 763   // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
 764   // Java objects.  This would avoid awkward lifecycle and liveness issues,
 765   // as well as eliminate a subset of ABA issues.
 766   // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
 767 
 768   ObjectWaiter node(current);
 769   current->_ParkEvent->reset();
 770   node._prev   = (ObjectWaiter*) 0xBAD;
 771   node.TState  = ObjectWaiter::TS_CXQ;
 772 
 773   // Push "current" onto the front of the _cxq.
 774   // Once on cxq/EntryList, current stays on-queue until it acquires the lock.
 775   // Note that spinning tends to reduce the rate at which threads
 776   // enqueue and dequeue on EntryList|cxq.
 777   ObjectWaiter* nxt;
 778   for (;;) {
 779     node._next = nxt = _cxq;
 780     if (Atomic::cmpxchg(&_cxq, nxt, &node) == nxt) break;
 781 
 782     // Interference - the CAS failed because _cxq changed.  Just retry.
 783     // As an optional optimization we retry the lock.
 784     if (TryLock (current) > 0) {
 785       assert(_succ != current, "invariant");
 786       assert(owner_raw() == current, "invariant");
 787       assert(_Responsible != current, "invariant");
 788       return;
 789     }
 790   }
 791 
 792   // Check for cxq|EntryList edge transition to non-null.  This indicates
 793   // the onset of contention.  While contention persists exiting threads
 794   // will use a ST:MEMBAR:LD 1-1 exit protocol.  When contention abates exit
 795   // operations revert to the faster 1-0 mode.  This enter operation may interleave
 796   // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
 797   // arrange for one of the contending thread to use a timed park() operations
 798   // to detect and recover from the race.  (Stranding is form of progress failure
 799   // where the monitor is unlocked but all the contending threads remain parked).
 800   // That is, at least one of the contended threads will periodically poll _owner.
 801   // One of the contending threads will become the designated "Responsible" thread.
 802   // The Responsible thread uses a timed park instead of a normal indefinite park
 803   // operation -- it periodically wakes and checks for and recovers from potential
 804   // strandings admitted by 1-0 exit operations.   We need at most one Responsible
 805   // thread per-monitor at any given moment.  Only threads on cxq|EntryList may
 806   // be responsible for a monitor.
 807   //
 808   // Currently, one of the contended threads takes on the added role of "Responsible".
 809   // A viable alternative would be to use a dedicated "stranding checker" thread
 810   // that periodically iterated over all the threads (or active monitors) and unparked
 811   // successors where there was risk of stranding.  This would help eliminate the
 812   // timer scalability issues we see on some platforms as we'd only have one thread
 813   // -- the checker -- parked on a timer.
 814 
 815   if (nxt == NULL && _EntryList == NULL) {
 816     // Try to assume the role of responsible thread for the monitor.
 817     // CONSIDER:  ST vs CAS vs { if (Responsible==null) Responsible=current }
 818     Atomic::replace_if_null(&_Responsible, current);
 819   }
 820 
 821   // The lock might have been released while this thread was occupied queueing
 822   // itself onto _cxq.  To close the race and avoid "stranding" and
 823   // progress-liveness failure we must resample-retry _owner before parking.
 824   // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
 825   // In this case the ST-MEMBAR is accomplished with CAS().
 826   //
 827   // TODO: Defer all thread state transitions until park-time.
 828   // Since state transitions are heavy and inefficient we'd like
 829   // to defer the state transitions until absolutely necessary,
 830   // and in doing so avoid some transitions ...
 831 
 832   int nWakeups = 0;
 833   int recheckInterval = 1;
 834 
 835   for (;;) {
 836 
 837     if (TryLock(current) > 0) break;
 838     assert(owner_raw() != current, "invariant");
 839 
 840     // park self
 841     if (_Responsible == current) {
 842       current->_ParkEvent->park((jlong) recheckInterval);
 843       // Increase the recheckInterval, but clamp the value.
 844       recheckInterval *= 8;
 845       if (recheckInterval > MAX_RECHECK_INTERVAL) {
 846         recheckInterval = MAX_RECHECK_INTERVAL;
 847       }
 848     } else {
 849       current->_ParkEvent->park();
 850     }
 851 
 852     if (TryLock(current) > 0) break;
 853 
 854     if (try_set_owner_from(DEFLATER_MARKER, current) == DEFLATER_MARKER) {
 855       // Cancelled the in-progress async deflation by changing owner from
 856       // DEFLATER_MARKER to current. As part of the contended enter protocol,
 857       // contentions was incremented to a positive value before EnterI()
 858       // was called and that prevents the deflater thread from winning the
 859       // last part of the 2-part async deflation protocol. After EnterI()
 860       // returns to enter(), contentions is decremented because the caller
 861       // now owns the monitor. We bump contentions an extra time here to
 862       // prevent the deflater thread from winning the last part of the
 863       // 2-part async deflation protocol after the regular decrement
 864       // occurs in enter(). The deflater thread will decrement contentions
 865       // after it recognizes that the async deflation was cancelled.
 866       add_to_contentions(1);
 867       break;
 868     }
 869 
 870     // The lock is still contested.
 871     // Keep a tally of the # of futile wakeups.
 872     // Note that the counter is not protected by a lock or updated by atomics.
 873     // That is by design - we trade "lossy" counters which are exposed to
 874     // races during updates for a lower probe effect.
 875 
 876     // This PerfData object can be used in parallel with a safepoint.
 877     // See the work around in PerfDataManager::destroy().
 878     OM_PERFDATA_OP(FutileWakeups, inc());
 879     ++nWakeups;
 880 
 881     // Assuming this is not a spurious wakeup we'll normally find _succ == current.
 882     // We can defer clearing _succ until after the spin completes
 883     // TrySpin() must tolerate being called with _succ == current.
 884     // Try yet another round of adaptive spinning.
 885     if (TrySpin(current) > 0) break;
 886 
 887     // We can find that we were unpark()ed and redesignated _succ while
 888     // we were spinning.  That's harmless.  If we iterate and call park(),
 889     // park() will consume the event and return immediately and we'll
 890     // just spin again.  This pattern can repeat, leaving _succ to simply
 891     // spin on a CPU.
 892 
 893     if (_succ == current) _succ = NULL;
 894 
 895     // Invariant: after clearing _succ a thread *must* retry _owner before parking.
 896     OrderAccess::fence();
 897   }
 898 
 899   // Egress :
 900   // current has acquired the lock -- Unlink current from the cxq or EntryList.
 901   // Normally we'll find current on the EntryList .
 902   // From the perspective of the lock owner (this thread), the
 903   // EntryList is stable and cxq is prepend-only.
 904   // The head of cxq is volatile but the interior is stable.
 905   // In addition, current.TState is stable.
 906 
 907   assert(owner_raw() == current, "invariant");
 908 
 909   UnlinkAfterAcquire(current, &node);
 910   if (_succ == current) _succ = NULL;
 911 
 912   assert(_succ != current, "invariant");
 913   if (_Responsible == current) {
 914     _Responsible = NULL;
 915     OrderAccess::fence(); // Dekker pivot-point
 916 
 917     // We may leave threads on cxq|EntryList without a designated
 918     // "Responsible" thread.  This is benign.  When this thread subsequently
 919     // exits the monitor it can "see" such preexisting "old" threads --
 920     // threads that arrived on the cxq|EntryList before the fence, above --
 921     // by LDing cxq|EntryList.  Newly arrived threads -- that is, threads
 922     // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
 923     // non-null and elect a new "Responsible" timer thread.
 924     //
 925     // This thread executes:
 926     //    ST Responsible=null; MEMBAR    (in enter epilogue - here)
 927     //    LD cxq|EntryList               (in subsequent exit)
 928     //
 929     // Entering threads in the slow/contended path execute:
 930     //    ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
 931     //    The (ST cxq; MEMBAR) is accomplished with CAS().
 932     //
 933     // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
 934     // exit operation from floating above the ST Responsible=null.
 935   }
 936 
 937   // We've acquired ownership with CAS().
 938   // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
 939   // But since the CAS() this thread may have also stored into _succ,
 940   // EntryList, cxq or Responsible.  These meta-data updates must be
 941   // visible __before this thread subsequently drops the lock.
 942   // Consider what could occur if we didn't enforce this constraint --
 943   // STs to monitor meta-data and user-data could reorder with (become
 944   // visible after) the ST in exit that drops ownership of the lock.
 945   // Some other thread could then acquire the lock, but observe inconsistent
 946   // or old monitor meta-data and heap data.  That violates the JMM.
 947   // To that end, the 1-0 exit() operation must have at least STST|LDST
 948   // "release" barrier semantics.  Specifically, there must be at least a
 949   // STST|LDST barrier in exit() before the ST of null into _owner that drops
 950   // the lock.   The barrier ensures that changes to monitor meta-data and data
 951   // protected by the lock will be visible before we release the lock, and
 952   // therefore before some other thread (CPU) has a chance to acquire the lock.
 953   // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
 954   //
 955   // Critically, any prior STs to _succ or EntryList must be visible before
 956   // the ST of null into _owner in the *subsequent* (following) corresponding
 957   // monitorexit.  Recall too, that in 1-0 mode monitorexit does not necessarily
 958   // execute a serializing instruction.
 959 
 960   return;
 961 }
 962 
 963 // ReenterI() is a specialized inline form of the latter half of the
 964 // contended slow-path from EnterI().  We use ReenterI() only for
 965 // monitor reentry in wait().
 966 //
 967 // In the future we should reconcile EnterI() and ReenterI().
 968 
 969 void ObjectMonitor::ReenterI(JavaThread* current, ObjectWaiter* currentNode) {
 970   assert(current != NULL, "invariant");
 971   assert(currentNode != NULL, "invariant");
 972   assert(currentNode->_thread == current, "invariant");
 973   assert(_waiters > 0, "invariant");
 974   assert(object()->mark() == markWord::encode(this), "invariant");
 975 
 976   assert(current->thread_state() != _thread_blocked, "invariant");
 977 
 978   int nWakeups = 0;
 979   for (;;) {
 980     ObjectWaiter::TStates v = currentNode->TState;
 981     guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
 982     assert(owner_raw() != current, "invariant");
 983 
 984     if (TryLock(current) > 0) break;
 985     if (TrySpin(current) > 0) break;
 986 
 987     {
 988       OSThreadContendState osts(current->osthread());
 989 
 990       assert(current->thread_state() == _thread_in_vm, "invariant");
 991 
 992       {
 993         ClearSuccOnSuspend csos(this);
 994         ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */);
 995         current->_ParkEvent->park();
 996       }
 997     }
 998 
 999     // Try again, but just so we distinguish between futile wakeups and
1000     // successful wakeups.  The following test isn't algorithmically
1001     // necessary, but it helps us maintain sensible statistics.
1002     if (TryLock(current) > 0) break;
1003 
1004     // The lock is still contested.
1005     // Keep a tally of the # of futile wakeups.
1006     // Note that the counter is not protected by a lock or updated by atomics.
1007     // That is by design - we trade "lossy" counters which are exposed to
1008     // races during updates for a lower probe effect.
1009     ++nWakeups;
1010 
1011     // Assuming this is not a spurious wakeup we'll normally
1012     // find that _succ == current.
1013     if (_succ == current) _succ = NULL;
1014 
1015     // Invariant: after clearing _succ a contending thread
1016     // *must* retry  _owner before parking.
1017     OrderAccess::fence();
1018 
1019     // This PerfData object can be used in parallel with a safepoint.
1020     // See the work around in PerfDataManager::destroy().
1021     OM_PERFDATA_OP(FutileWakeups, inc());
1022   }
1023 
1024   // current has acquired the lock -- Unlink current from the cxq or EntryList .
1025   // Normally we'll find current on the EntryList.
1026   // Unlinking from the EntryList is constant-time and atomic-free.
1027   // From the perspective of the lock owner (this thread), the
1028   // EntryList is stable and cxq is prepend-only.
1029   // The head of cxq is volatile but the interior is stable.
1030   // In addition, current.TState is stable.
1031 
1032   assert(owner_raw() == current, "invariant");
1033   assert(object()->mark() == markWord::encode(this), "invariant");
1034   UnlinkAfterAcquire(current, currentNode);
1035   if (_succ == current) _succ = NULL;
1036   assert(_succ != current, "invariant");
1037   currentNode->TState = ObjectWaiter::TS_RUN;
1038   OrderAccess::fence();      // see comments at the end of EnterI()
1039 }
1040 
1041 // By convention we unlink a contending thread from EntryList|cxq immediately
1042 // after the thread acquires the lock in ::enter().  Equally, we could defer
1043 // unlinking the thread until ::exit()-time.
1044 
1045 void ObjectMonitor::UnlinkAfterAcquire(JavaThread* current, ObjectWaiter* currentNode) {
1046   assert(owner_raw() == current, "invariant");
1047   assert(currentNode->_thread == current, "invariant");
1048 
1049   if (currentNode->TState == ObjectWaiter::TS_ENTER) {
1050     // Normal case: remove current from the DLL EntryList .
1051     // This is a constant-time operation.
1052     ObjectWaiter* nxt = currentNode->_next;
1053     ObjectWaiter* prv = currentNode->_prev;
1054     if (nxt != NULL) nxt->_prev = prv;
1055     if (prv != NULL) prv->_next = nxt;
1056     if (currentNode == _EntryList) _EntryList = nxt;
1057     assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
1058     assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
1059   } else {
1060     assert(currentNode->TState == ObjectWaiter::TS_CXQ, "invariant");
1061     // Inopportune interleaving -- current is still on the cxq.
1062     // This usually means the enqueue of self raced an exiting thread.
1063     // Normally we'll find current near the front of the cxq, so
1064     // dequeueing is typically fast.  If needbe we can accelerate
1065     // this with some MCS/CHL-like bidirectional list hints and advisory
1066     // back-links so dequeueing from the interior will normally operate
1067     // in constant-time.
1068     // Dequeue current from either the head (with CAS) or from the interior
1069     // with a linear-time scan and normal non-atomic memory operations.
1070     // CONSIDER: if current is on the cxq then simply drain cxq into EntryList
1071     // and then unlink current from EntryList.  We have to drain eventually,
1072     // so it might as well be now.
1073 
1074     ObjectWaiter* v = _cxq;
1075     assert(v != NULL, "invariant");
1076     if (v != currentNode || Atomic::cmpxchg(&_cxq, v, currentNode->_next) != v) {
1077       // The CAS above can fail from interference IFF a "RAT" arrived.
1078       // In that case current must be in the interior and can no longer be
1079       // at the head of cxq.
1080       if (v == currentNode) {
1081         assert(_cxq != v, "invariant");
1082         v = _cxq;          // CAS above failed - start scan at head of list
1083       }
1084       ObjectWaiter* p;
1085       ObjectWaiter* q = NULL;
1086       for (p = v; p != NULL && p != currentNode; p = p->_next) {
1087         q = p;
1088         assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
1089       }
1090       assert(v != currentNode, "invariant");
1091       assert(p == currentNode, "Node not found on cxq");
1092       assert(p != _cxq, "invariant");
1093       assert(q != NULL, "invariant");
1094       assert(q->_next == p, "invariant");
1095       q->_next = p->_next;
1096     }
1097   }
1098 
1099 #ifdef ASSERT
1100   // Diagnostic hygiene ...
1101   currentNode->_prev  = (ObjectWaiter*) 0xBAD;
1102   currentNode->_next  = (ObjectWaiter*) 0xBAD;
1103   currentNode->TState = ObjectWaiter::TS_RUN;
1104 #endif
1105 }
1106 
1107 // -----------------------------------------------------------------------------
1108 // Exit support
1109 //
1110 // exit()
1111 // ~~~~~~
1112 // Note that the collector can't reclaim the objectMonitor or deflate
1113 // the object out from underneath the thread calling ::exit() as the
1114 // thread calling ::exit() never transitions to a stable state.
1115 // This inhibits GC, which in turn inhibits asynchronous (and
1116 // inopportune) reclamation of "this".
1117 //
1118 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
1119 // There's one exception to the claim above, however.  EnterI() can call
1120 // exit() to drop a lock if the acquirer has been externally suspended.
1121 // In that case exit() is called with _thread_state == _thread_blocked,
1122 // but the monitor's _contentions field is > 0, which inhibits reclamation.
1123 //
1124 // 1-0 exit
1125 // ~~~~~~~~
1126 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
1127 // the fast-path operators have been optimized so the common ::exit()
1128 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
1129 // The code emitted by fast_unlock() elides the usual MEMBAR.  This
1130 // greatly improves latency -- MEMBAR and CAS having considerable local
1131 // latency on modern processors -- but at the cost of "stranding".  Absent the
1132 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
1133 // ::enter() path, resulting in the entering thread being stranding
1134 // and a progress-liveness failure.   Stranding is extremely rare.
1135 // We use timers (timed park operations) & periodic polling to detect
1136 // and recover from stranding.  Potentially stranded threads periodically
1137 // wake up and poll the lock.  See the usage of the _Responsible variable.
1138 //
1139 // The CAS() in enter provides for safety and exclusion, while the CAS or
1140 // MEMBAR in exit provides for progress and avoids stranding.  1-0 locking
1141 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
1142 // We detect and recover from stranding with timers.
1143 //
1144 // If a thread transiently strands it'll park until (a) another
1145 // thread acquires the lock and then drops the lock, at which time the
1146 // exiting thread will notice and unpark the stranded thread, or, (b)
1147 // the timer expires.  If the lock is high traffic then the stranding latency
1148 // will be low due to (a).  If the lock is low traffic then the odds of
1149 // stranding are lower, although the worst-case stranding latency
1150 // is longer.  Critically, we don't want to put excessive load in the
1151 // platform's timer subsystem.  We want to minimize both the timer injection
1152 // rate (timers created/sec) as well as the number of timers active at
1153 // any one time.  (more precisely, we want to minimize timer-seconds, which is
1154 // the integral of the # of active timers at any instant over time).
1155 // Both impinge on OS scalability.  Given that, at most one thread parked on
1156 // a monitor will use a timer.
1157 //
1158 // There is also the risk of a futile wake-up. If we drop the lock
1159 // another thread can reacquire the lock immediately, and we can
1160 // then wake a thread unnecessarily. This is benign, and we've
1161 // structured the code so the windows are short and the frequency
1162 // of such futile wakups is low.
1163 
1164 void ObjectMonitor::exit(JavaThread* current, bool not_suspended) {
1165   void* cur = owner_raw();
1166   if (current != cur) {
1167     if (current->is_lock_owned((address)cur)) {
1168       assert(_recursions == 0, "invariant");
1169       set_owner_from_BasicLock(cur, current);  // Convert from BasicLock* to Thread*.
1170       _recursions = 0;
1171     } else {
1172       // Apparent unbalanced locking ...
1173       // Naively we'd like to throw IllegalMonitorStateException.
1174       // As a practical matter we can neither allocate nor throw an
1175       // exception as ::exit() can be called from leaf routines.
1176       // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
1177       // Upon deeper reflection, however, in a properly run JVM the only
1178       // way we should encounter this situation is in the presence of
1179       // unbalanced JNI locking. TODO: CheckJNICalls.
1180       // See also: CR4414101
1181 #ifdef ASSERT
1182       LogStreamHandle(Error, monitorinflation) lsh;
1183       lsh.print_cr("ERROR: ObjectMonitor::exit(): thread=" INTPTR_FORMAT
1184                     " is exiting an ObjectMonitor it does not own.", p2i(current));
1185       lsh.print_cr("The imbalance is possibly caused by JNI locking.");
1186       print_debug_style_on(&lsh);
1187       assert(false, "Non-balanced monitor enter/exit!");
1188 #endif
1189       return;
1190     }
1191   }
1192 
1193   if (_recursions != 0) {
1194     _recursions--;        // this is simple recursive enter
1195     return;
1196   }
1197 
1198   // Invariant: after setting Responsible=null an thread must execute
1199   // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
1200   _Responsible = NULL;
1201 
1202 #if INCLUDE_JFR
1203   // get the owner's thread id for the MonitorEnter event
1204   // if it is enabled and the thread isn't suspended
1205   if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
1206     _previous_owner_tid = JFR_THREAD_ID(current);
1207   }
1208 #endif
1209 
1210   for (;;) {
1211     assert(current == owner_raw(), "invariant");
1212 
1213     // Drop the lock.
1214     // release semantics: prior loads and stores from within the critical section
1215     // must not float (reorder) past the following store that drops the lock.
1216     // Uses a storeload to separate release_store(owner) from the
1217     // successor check. The try_set_owner() below uses cmpxchg() so
1218     // we get the fence down there.
1219     release_clear_owner(current);
1220     OrderAccess::storeload();
1221 
1222     if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1223       return;
1224     }
1225     // Other threads are blocked trying to acquire the lock.
1226 
1227     // Normally the exiting thread is responsible for ensuring succession,
1228     // but if other successors are ready or other entering threads are spinning
1229     // then this thread can simply store NULL into _owner and exit without
1230     // waking a successor.  The existence of spinners or ready successors
1231     // guarantees proper succession (liveness).  Responsibility passes to the
1232     // ready or running successors.  The exiting thread delegates the duty.
1233     // More precisely, if a successor already exists this thread is absolved
1234     // of the responsibility of waking (unparking) one.
1235     //
1236     // The _succ variable is critical to reducing futile wakeup frequency.
1237     // _succ identifies the "heir presumptive" thread that has been made
1238     // ready (unparked) but that has not yet run.  We need only one such
1239     // successor thread to guarantee progress.
1240     // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1241     // section 3.3 "Futile Wakeup Throttling" for details.
1242     //
1243     // Note that spinners in Enter() also set _succ non-null.
1244     // In the current implementation spinners opportunistically set
1245     // _succ so that exiting threads might avoid waking a successor.
1246     // Another less appealing alternative would be for the exiting thread
1247     // to drop the lock and then spin briefly to see if a spinner managed
1248     // to acquire the lock.  If so, the exiting thread could exit
1249     // immediately without waking a successor, otherwise the exiting
1250     // thread would need to dequeue and wake a successor.
1251     // (Note that we'd need to make the post-drop spin short, but no
1252     // shorter than the worst-case round-trip cache-line migration time.
1253     // The dropped lock needs to become visible to the spinner, and then
1254     // the acquisition of the lock by the spinner must become visible to
1255     // the exiting thread).
1256 
1257     // It appears that an heir-presumptive (successor) must be made ready.
1258     // Only the current lock owner can manipulate the EntryList or
1259     // drain _cxq, so we need to reacquire the lock.  If we fail
1260     // to reacquire the lock the responsibility for ensuring succession
1261     // falls to the new owner.
1262     //
1263     if (try_set_owner_from(NULL, current) != NULL) {
1264       return;
1265     }
1266 
1267     guarantee(owner_raw() == current, "invariant");
1268 
1269     ObjectWaiter* w = NULL;
1270 
1271     w = _EntryList;
1272     if (w != NULL) {
1273       // I'd like to write: guarantee (w->_thread != current).
1274       // But in practice an exiting thread may find itself on the EntryList.
1275       // Let's say thread T1 calls O.wait().  Wait() enqueues T1 on O's waitset and
1276       // then calls exit().  Exit release the lock by setting O._owner to NULL.
1277       // Let's say T1 then stalls.  T2 acquires O and calls O.notify().  The
1278       // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1279       // release the lock "O".  T2 resumes immediately after the ST of null into
1280       // _owner, above.  T2 notices that the EntryList is populated, so it
1281       // reacquires the lock and then finds itself on the EntryList.
1282       // Given all that, we have to tolerate the circumstance where "w" is
1283       // associated with current.
1284       assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1285       ExitEpilog(current, w);
1286       return;
1287     }
1288 
1289     // If we find that both _cxq and EntryList are null then just
1290     // re-run the exit protocol from the top.
1291     w = _cxq;
1292     if (w == NULL) continue;
1293 
1294     // Drain _cxq into EntryList - bulk transfer.
1295     // First, detach _cxq.
1296     // The following loop is tantamount to: w = swap(&cxq, NULL)
1297     for (;;) {
1298       assert(w != NULL, "Invariant");
1299       ObjectWaiter* u = Atomic::cmpxchg(&_cxq, w, (ObjectWaiter*)NULL);
1300       if (u == w) break;
1301       w = u;
1302     }
1303 
1304     assert(w != NULL, "invariant");
1305     assert(_EntryList == NULL, "invariant");
1306 
1307     // Convert the LIFO SLL anchored by _cxq into a DLL.
1308     // The list reorganization step operates in O(LENGTH(w)) time.
1309     // It's critical that this step operate quickly as
1310     // "current" still holds the outer-lock, restricting parallelism
1311     // and effectively lengthening the critical section.
1312     // Invariant: s chases t chases u.
1313     // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1314     // we have faster access to the tail.
1315 
1316     _EntryList = w;
1317     ObjectWaiter* q = NULL;
1318     ObjectWaiter* p;
1319     for (p = w; p != NULL; p = p->_next) {
1320       guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1321       p->TState = ObjectWaiter::TS_ENTER;
1322       p->_prev = q;
1323       q = p;
1324     }
1325 
1326     // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1327     // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1328 
1329     // See if we can abdicate to a spinner instead of waking a thread.
1330     // A primary goal of the implementation is to reduce the
1331     // context-switch rate.
1332     if (_succ != NULL) continue;
1333 
1334     w = _EntryList;
1335     if (w != NULL) {
1336       guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1337       ExitEpilog(current, w);
1338       return;
1339     }
1340   }
1341 }
1342 
1343 void ObjectMonitor::ExitEpilog(JavaThread* current, ObjectWaiter* Wakee) {
1344   assert(owner_raw() == current, "invariant");
1345 
1346   // Exit protocol:
1347   // 1. ST _succ = wakee
1348   // 2. membar #loadstore|#storestore;
1349   // 2. ST _owner = NULL
1350   // 3. unpark(wakee)
1351 
1352   _succ = Wakee->_thread;
1353   ParkEvent * Trigger = Wakee->_event;
1354 
1355   // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1356   // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1357   // out-of-scope (non-extant).
1358   Wakee  = NULL;
1359 
1360   // Drop the lock.
1361   // Uses a fence to separate release_store(owner) from the LD in unpark().
1362   release_clear_owner(current);
1363   OrderAccess::fence();
1364 
1365   DTRACE_MONITOR_PROBE(contended__exit, this, object(), current);
1366   Trigger->unpark();
1367 
1368   // Maintain stats and report events to JVMTI
1369   OM_PERFDATA_OP(Parks, inc());
1370 }
1371 
1372 
1373 // -----------------------------------------------------------------------------
1374 // Class Loader deadlock handling.
1375 //
1376 // complete_exit exits a lock returning recursion count
1377 // complete_exit/reenter operate as a wait without waiting
1378 // complete_exit requires an inflated monitor
1379 // The _owner field is not always the Thread addr even with an
1380 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1381 // thread due to contention.
1382 intx ObjectMonitor::complete_exit(JavaThread* current) {
1383   assert(InitDone, "Unexpectedly not initialized");
1384 
1385   void* cur = owner_raw();
1386   if (current != cur) {
1387     if (current->is_lock_owned((address)cur)) {
1388       assert(_recursions == 0, "internal state error");
1389       set_owner_from_BasicLock(cur, current);  // Convert from BasicLock* to Thread*.
1390       _recursions = 0;
1391     }
1392   }
1393 
1394   guarantee(current == owner_raw(), "complete_exit not owner");
1395   intx save = _recursions; // record the old recursion count
1396   _recursions = 0;         // set the recursion level to be 0
1397   exit(current);           // exit the monitor
1398   guarantee(owner_raw() != current, "invariant");
1399   return save;
1400 }
1401 
1402 // reenter() enters a lock and sets recursion count
1403 // complete_exit/reenter operate as a wait without waiting
1404 bool ObjectMonitor::reenter(intx recursions, JavaThread* current) {
1405 
1406   guarantee(owner_raw() != current, "reenter already owner");
1407   if (!enter(current)) {
1408     return false;
1409   }
1410   // Entered the monitor.
1411   guarantee(_recursions == 0, "reenter recursion");
1412   _recursions = recursions;
1413   return true;
1414 }
1415 
1416 // Checks that the current THREAD owns this monitor and causes an
1417 // immediate return if it doesn't. We don't use the CHECK macro
1418 // because we want the IMSE to be the only exception that is thrown
1419 // from the call site when false is returned. Any other pending
1420 // exception is ignored.
1421 #define CHECK_OWNER()                                                  \
1422   do {                                                                 \
1423     if (!check_owner(THREAD)) {                                        \
1424        assert(HAS_PENDING_EXCEPTION, "expected a pending IMSE here."); \
1425        return;                                                         \
1426      }                                                                 \
1427   } while (false)
1428 
1429 // Returns true if the specified thread owns the ObjectMonitor.
1430 // Otherwise returns false and throws IllegalMonitorStateException
1431 // (IMSE). If there is a pending exception and the specified thread
1432 // is not the owner, that exception will be replaced by the IMSE.
1433 bool ObjectMonitor::check_owner(TRAPS) {
1434   JavaThread* current = THREAD;
1435   void* cur = owner_raw();
1436   if (cur == current) {
1437     return true;
1438   }
1439   if (current->is_lock_owned((address)cur)) {
1440     set_owner_from_BasicLock(cur, current);  // Convert from BasicLock* to Thread*.
1441     _recursions = 0;
1442     return true;
1443   }
1444   THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(),
1445              "current thread is not owner", false);
1446 }
1447 
1448 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1449                                     ObjectMonitor* monitor,
1450                                     uint64_t notifier_tid,
1451                                     jlong timeout,
1452                                     bool timedout) {
1453   assert(event != NULL, "invariant");
1454   assert(monitor != NULL, "invariant");
1455   event->set_monitorClass(monitor->object()->klass());
1456   event->set_timeout(timeout);
1457   // Set an address that is 'unique enough', such that events close in
1458   // time and with the same address are likely (but not guaranteed) to
1459   // belong to the same object.
1460   event->set_address((uintptr_t)monitor);
1461   event->set_notifier(notifier_tid);
1462   event->set_timedOut(timedout);
1463   event->commit();
1464 }
1465 
1466 // -----------------------------------------------------------------------------
1467 // Wait/Notify/NotifyAll
1468 //
1469 // Note: a subset of changes to ObjectMonitor::wait()
1470 // will need to be replicated in complete_exit
1471 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1472   JavaThread* current = THREAD;
1473 
1474   assert(InitDone, "Unexpectedly not initialized");
1475 
1476   CHECK_OWNER();  // Throws IMSE if not owner.
1477 
1478   EventJavaMonitorWait event;
1479 
1480   // check for a pending interrupt
1481   if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1482     // post monitor waited event.  Note that this is past-tense, we are done waiting.
1483     if (JvmtiExport::should_post_monitor_waited()) {
1484       // Note: 'false' parameter is passed here because the
1485       // wait was not timed out due to thread interrupt.
1486       JvmtiExport::post_monitor_waited(current, this, false);
1487 
1488       // In this short circuit of the monitor wait protocol, the
1489       // current thread never drops ownership of the monitor and
1490       // never gets added to the wait queue so the current thread
1491       // cannot be made the successor. This means that the
1492       // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1493       // consume an unpark() meant for the ParkEvent associated with
1494       // this ObjectMonitor.
1495     }
1496     if (event.should_commit()) {
1497       post_monitor_wait_event(&event, this, 0, millis, false);
1498     }
1499     THROW(vmSymbols::java_lang_InterruptedException());
1500     return;
1501   }
1502 
1503   assert(current->_Stalled == 0, "invariant");
1504   current->_Stalled = intptr_t(this);
1505   current->set_current_waiting_monitor(this);
1506 
1507   // create a node to be put into the queue
1508   // Critically, after we reset() the event but prior to park(), we must check
1509   // for a pending interrupt.
1510   ObjectWaiter node(current);
1511   node.TState = ObjectWaiter::TS_WAIT;
1512   current->_ParkEvent->reset();
1513   OrderAccess::fence();          // ST into Event; membar ; LD interrupted-flag
1514 
1515   // Enter the waiting queue, which is a circular doubly linked list in this case
1516   // but it could be a priority queue or any data structure.
1517   // _WaitSetLock protects the wait queue.  Normally the wait queue is accessed only
1518   // by the the owner of the monitor *except* in the case where park()
1519   // returns because of a timeout of interrupt.  Contention is exceptionally rare
1520   // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1521 
1522   Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1523   AddWaiter(&node);
1524   Thread::SpinRelease(&_WaitSetLock);
1525 
1526   _Responsible = NULL;
1527 
1528   intx save = _recursions;     // record the old recursion count
1529   _waiters++;                  // increment the number of waiters
1530   _recursions = 0;             // set the recursion level to be 1
1531   exit(current);               // exit the monitor
1532   guarantee(owner_raw() != current, "invariant");
1533 
1534   // The thread is on the WaitSet list - now park() it.
1535   // On MP systems it's conceivable that a brief spin before we park
1536   // could be profitable.
1537   //
1538   // TODO-FIXME: change the following logic to a loop of the form
1539   //   while (!timeout && !interrupted && _notified == 0) park()
1540 
1541   int ret = OS_OK;
1542   int WasNotified = 0;
1543 
1544   // Need to check interrupt state whilst still _thread_in_vm
1545   bool interrupted = interruptible && current->is_interrupted(false);
1546 
1547   { // State transition wrappers
1548     OSThread* osthread = current->osthread();
1549     OSThreadWaitState osts(osthread, true);
1550 
1551     assert(current->thread_state() == _thread_in_vm, "invariant");
1552 
1553     {
1554       ClearSuccOnSuspend csos(this);
1555       ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */);
1556       if (interrupted || HAS_PENDING_EXCEPTION) {
1557         // Intentionally empty
1558       } else if (node._notified == 0) {
1559         if (millis <= 0) {
1560           current->_ParkEvent->park();
1561         } else {
1562           ret = current->_ParkEvent->park(millis);
1563         }
1564       }
1565     }
1566 
1567     // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1568     // from the WaitSet to the EntryList.
1569     // See if we need to remove Node from the WaitSet.
1570     // We use double-checked locking to avoid grabbing _WaitSetLock
1571     // if the thread is not on the wait queue.
1572     //
1573     // Note that we don't need a fence before the fetch of TState.
1574     // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1575     // written by the is thread. (perhaps the fetch might even be satisfied
1576     // by a look-aside into the processor's own store buffer, although given
1577     // the length of the code path between the prior ST and this load that's
1578     // highly unlikely).  If the following LD fetches a stale TS_WAIT value
1579     // then we'll acquire the lock and then re-fetch a fresh TState value.
1580     // That is, we fail toward safety.
1581 
1582     if (node.TState == ObjectWaiter::TS_WAIT) {
1583       Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1584       if (node.TState == ObjectWaiter::TS_WAIT) {
1585         DequeueSpecificWaiter(&node);       // unlink from WaitSet
1586         assert(node._notified == 0, "invariant");
1587         node.TState = ObjectWaiter::TS_RUN;
1588       }
1589       Thread::SpinRelease(&_WaitSetLock);
1590     }
1591 
1592     // The thread is now either on off-list (TS_RUN),
1593     // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1594     // The Node's TState variable is stable from the perspective of this thread.
1595     // No other threads will asynchronously modify TState.
1596     guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1597     OrderAccess::loadload();
1598     if (_succ == current) _succ = NULL;
1599     WasNotified = node._notified;
1600 
1601     // Reentry phase -- reacquire the monitor.
1602     // re-enter contended monitor after object.wait().
1603     // retain OBJECT_WAIT state until re-enter successfully completes
1604     // Thread state is thread_in_vm and oop access is again safe,
1605     // although the raw address of the object may have changed.
1606     // (Don't cache naked oops over safepoints, of course).
1607 
1608     // post monitor waited event. Note that this is past-tense, we are done waiting.
1609     if (JvmtiExport::should_post_monitor_waited()) {
1610       JvmtiExport::post_monitor_waited(current, this, ret == OS_TIMEOUT);
1611 
1612       if (node._notified != 0 && _succ == current) {
1613         // In this part of the monitor wait-notify-reenter protocol it
1614         // is possible (and normal) for another thread to do a fastpath
1615         // monitor enter-exit while this thread is still trying to get
1616         // to the reenter portion of the protocol.
1617         //
1618         // The ObjectMonitor was notified and the current thread is
1619         // the successor which also means that an unpark() has already
1620         // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1621         // consume the unpark() that was done when the successor was
1622         // set because the same ParkEvent is shared between Java
1623         // monitors and JVM/TI RawMonitors (for now).
1624         //
1625         // We redo the unpark() to ensure forward progress, i.e., we
1626         // don't want all pending threads hanging (parked) with none
1627         // entering the unlocked monitor.
1628         node._event->unpark();
1629       }
1630     }
1631 
1632     if (event.should_commit()) {
1633       post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1634     }
1635 
1636     OrderAccess::fence();
1637 
1638     assert(current->_Stalled != 0, "invariant");
1639     current->_Stalled = 0;
1640 
1641     assert(owner_raw() != current, "invariant");
1642     ObjectWaiter::TStates v = node.TState;
1643     if (v == ObjectWaiter::TS_RUN) {
1644       enter(current);
1645     } else {
1646       guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1647       ReenterI(current, &node);
1648       node.wait_reenter_end(this);
1649     }
1650 
1651     // current has reacquired the lock.
1652     // Lifecycle - the node representing current must not appear on any queues.
1653     // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1654     // want residual elements associated with this thread left on any lists.
1655     guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1656     assert(owner_raw() == current, "invariant");
1657     assert(_succ != current, "invariant");
1658   } // OSThreadWaitState()
1659 
1660   current->set_current_waiting_monitor(NULL);
1661 
1662   guarantee(_recursions == 0, "invariant");
1663   _recursions = save      // restore the old recursion count
1664                 + JvmtiDeferredUpdates::get_and_reset_relock_count_after_wait(current); //  increased by the deferred relock count
1665   _waiters--;             // decrement the number of waiters
1666 
1667   // Verify a few postconditions
1668   assert(owner_raw() == current, "invariant");
1669   assert(_succ != current, "invariant");
1670   assert(object()->mark() == markWord::encode(this), "invariant");
1671 
1672   // check if the notification happened
1673   if (!WasNotified) {
1674     // no, it could be timeout or Thread.interrupt() or both
1675     // check for interrupt event, otherwise it is timeout
1676     if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1677       THROW(vmSymbols::java_lang_InterruptedException());
1678     }
1679   }
1680 
1681   // NOTE: Spurious wake up will be consider as timeout.
1682   // Monitor notify has precedence over thread interrupt.
1683 }
1684 
1685 
1686 // Consider:
1687 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1688 // then instead of transferring a thread from the WaitSet to the EntryList
1689 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1690 
1691 void ObjectMonitor::INotify(JavaThread* current) {
1692   Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1693   ObjectWaiter* iterator = DequeueWaiter();
1694   if (iterator != NULL) {
1695     guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1696     guarantee(iterator->_notified == 0, "invariant");
1697     // Disposition - what might we do with iterator ?
1698     // a.  add it directly to the EntryList - either tail (policy == 1)
1699     //     or head (policy == 0).
1700     // b.  push it onto the front of the _cxq (policy == 2).
1701     // For now we use (b).
1702 
1703     iterator->TState = ObjectWaiter::TS_ENTER;
1704 
1705     iterator->_notified = 1;
1706     iterator->_notifier_tid = JFR_THREAD_ID(current);
1707 
1708     ObjectWaiter* list = _EntryList;
1709     if (list != NULL) {
1710       assert(list->_prev == NULL, "invariant");
1711       assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1712       assert(list != iterator, "invariant");
1713     }
1714 
1715     // prepend to cxq
1716     if (list == NULL) {
1717       iterator->_next = iterator->_prev = NULL;
1718       _EntryList = iterator;
1719     } else {
1720       iterator->TState = ObjectWaiter::TS_CXQ;
1721       for (;;) {
1722         ObjectWaiter* front = _cxq;
1723         iterator->_next = front;
1724         if (Atomic::cmpxchg(&_cxq, front, iterator) == front) {
1725           break;
1726         }
1727       }
1728     }
1729 
1730     // _WaitSetLock protects the wait queue, not the EntryList.  We could
1731     // move the add-to-EntryList operation, above, outside the critical section
1732     // protected by _WaitSetLock.  In practice that's not useful.  With the
1733     // exception of  wait() timeouts and interrupts the monitor owner
1734     // is the only thread that grabs _WaitSetLock.  There's almost no contention
1735     // on _WaitSetLock so it's not profitable to reduce the length of the
1736     // critical section.
1737 
1738     iterator->wait_reenter_begin(this);
1739   }
1740   Thread::SpinRelease(&_WaitSetLock);
1741 }
1742 
1743 // Consider: a not-uncommon synchronization bug is to use notify() when
1744 // notifyAll() is more appropriate, potentially resulting in stranded
1745 // threads; this is one example of a lost wakeup. A useful diagnostic
1746 // option is to force all notify() operations to behave as notifyAll().
1747 //
1748 // Note: We can also detect many such problems with a "minimum wait".
1749 // When the "minimum wait" is set to a small non-zero timeout value
1750 // and the program does not hang whereas it did absent "minimum wait",
1751 // that suggests a lost wakeup bug.
1752 
1753 void ObjectMonitor::notify(TRAPS) {
1754   JavaThread* current = THREAD;
1755   CHECK_OWNER();  // Throws IMSE if not owner.
1756   if (_WaitSet == NULL) {
1757     return;
1758   }
1759   DTRACE_MONITOR_PROBE(notify, this, object(), current);
1760   INotify(current);
1761   OM_PERFDATA_OP(Notifications, inc(1));
1762 }
1763 
1764 
1765 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1766 // from the waitset to the EntryList. This could be done more efficiently with a
1767 // single bulk transfer but in practice it's not time-critical. Beware too,
1768 // that in prepend-mode we invert the order of the waiters. Let's say that the
1769 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1770 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1771 
1772 void ObjectMonitor::notifyAll(TRAPS) {
1773   JavaThread* current = THREAD;
1774   CHECK_OWNER();  // Throws IMSE if not owner.
1775   if (_WaitSet == NULL) {
1776     return;
1777   }
1778 
1779   DTRACE_MONITOR_PROBE(notifyAll, this, object(), current);
1780   int tally = 0;
1781   while (_WaitSet != NULL) {
1782     tally++;
1783     INotify(current);
1784   }
1785 
1786   OM_PERFDATA_OP(Notifications, inc(tally));
1787 }
1788 
1789 // -----------------------------------------------------------------------------
1790 // Adaptive Spinning Support
1791 //
1792 // Adaptive spin-then-block - rational spinning
1793 //
1794 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1795 // algorithm.  On high order SMP systems it would be better to start with
1796 // a brief global spin and then revert to spinning locally.  In the spirit of MCS/CLH,
1797 // a contending thread could enqueue itself on the cxq and then spin locally
1798 // on a thread-specific variable such as its ParkEvent._Event flag.
1799 // That's left as an exercise for the reader.  Note that global spinning is
1800 // not problematic on Niagara, as the L2 cache serves the interconnect and
1801 // has both low latency and massive bandwidth.
1802 //
1803 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1804 // acquisition attempts where we opt to spin --  at 100% and vary the spin count
1805 // (duration) or we can fix the count at approximately the duration of
1806 // a context switch and vary the frequency.   Of course we could also
1807 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1808 // For a description of 'Adaptive spin-then-block mutual exclusion in
1809 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1810 //
1811 // This implementation varies the duration "D", where D varies with
1812 // the success rate of recent spin attempts. (D is capped at approximately
1813 // length of a round-trip context switch).  The success rate for recent
1814 // spin attempts is a good predictor of the success rate of future spin
1815 // attempts.  The mechanism adapts automatically to varying critical
1816 // section length (lock modality), system load and degree of parallelism.
1817 // D is maintained per-monitor in _SpinDuration and is initialized
1818 // optimistically.  Spin frequency is fixed at 100%.
1819 //
1820 // Note that _SpinDuration is volatile, but we update it without locks
1821 // or atomics.  The code is designed so that _SpinDuration stays within
1822 // a reasonable range even in the presence of races.  The arithmetic
1823 // operations on _SpinDuration are closed over the domain of legal values,
1824 // so at worst a race will install and older but still legal value.
1825 // At the very worst this introduces some apparent non-determinism.
1826 // We might spin when we shouldn't or vice-versa, but since the spin
1827 // count are relatively short, even in the worst case, the effect is harmless.
1828 //
1829 // Care must be taken that a low "D" value does not become an
1830 // an absorbing state.  Transient spinning failures -- when spinning
1831 // is overall profitable -- should not cause the system to converge
1832 // on low "D" values.  We want spinning to be stable and predictable
1833 // and fairly responsive to change and at the same time we don't want
1834 // it to oscillate, become metastable, be "too" non-deterministic,
1835 // or converge on or enter undesirable stable absorbing states.
1836 //
1837 // We implement a feedback-based control system -- using past behavior
1838 // to predict future behavior.  We face two issues: (a) if the
1839 // input signal is random then the spin predictor won't provide optimal
1840 // results, and (b) if the signal frequency is too high then the control
1841 // system, which has some natural response lag, will "chase" the signal.
1842 // (b) can arise from multimodal lock hold times.  Transient preemption
1843 // can also result in apparent bimodal lock hold times.
1844 // Although sub-optimal, neither condition is particularly harmful, as
1845 // in the worst-case we'll spin when we shouldn't or vice-versa.
1846 // The maximum spin duration is rather short so the failure modes aren't bad.
1847 // To be conservative, I've tuned the gain in system to bias toward
1848 // _not spinning.  Relatedly, the system can sometimes enter a mode where it
1849 // "rings" or oscillates between spinning and not spinning.  This happens
1850 // when spinning is just on the cusp of profitability, however, so the
1851 // situation is not dire.  The state is benign -- there's no need to add
1852 // hysteresis control to damp the transition rate between spinning and
1853 // not spinning.
1854 
1855 // Spinning: Fixed frequency (100%), vary duration
1856 int ObjectMonitor::TrySpin(JavaThread* current) {
1857   // Dumb, brutal spin.  Good for comparative measurements against adaptive spinning.
1858   int ctr = Knob_FixedSpin;
1859   if (ctr != 0) {
1860     while (--ctr >= 0) {
1861       if (TryLock(current) > 0) return 1;
1862       SpinPause();
1863     }
1864     return 0;
1865   }
1866 
1867   for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1868     if (TryLock(current) > 0) {
1869       // Increase _SpinDuration ...
1870       // Note that we don't clamp SpinDuration precisely at SpinLimit.
1871       // Raising _SpurDuration to the poverty line is key.
1872       int x = _SpinDuration;
1873       if (x < Knob_SpinLimit) {
1874         if (x < Knob_Poverty) x = Knob_Poverty;
1875         _SpinDuration = x + Knob_BonusB;
1876       }
1877       return 1;
1878     }
1879     SpinPause();
1880   }
1881 
1882   // Admission control - verify preconditions for spinning
1883   //
1884   // We always spin a little bit, just to prevent _SpinDuration == 0 from
1885   // becoming an absorbing state.  Put another way, we spin briefly to
1886   // sample, just in case the system load, parallelism, contention, or lock
1887   // modality changed.
1888   //
1889   // Consider the following alternative:
1890   // Periodically set _SpinDuration = _SpinLimit and try a long/full
1891   // spin attempt.  "Periodically" might mean after a tally of
1892   // the # of failed spin attempts (or iterations) reaches some threshold.
1893   // This takes us into the realm of 1-out-of-N spinning, where we
1894   // hold the duration constant but vary the frequency.
1895 
1896   ctr = _SpinDuration;
1897   if (ctr <= 0) return 0;
1898 
1899   if (NotRunnable(current, (JavaThread*) owner_raw())) {
1900     return 0;
1901   }
1902 
1903   // We're good to spin ... spin ingress.
1904   // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1905   // when preparing to LD...CAS _owner, etc and the CAS is likely
1906   // to succeed.
1907   if (_succ == NULL) {
1908     _succ = current;
1909   }
1910   Thread* prv = NULL;
1911 
1912   // There are three ways to exit the following loop:
1913   // 1.  A successful spin where this thread has acquired the lock.
1914   // 2.  Spin failure with prejudice
1915   // 3.  Spin failure without prejudice
1916 
1917   while (--ctr >= 0) {
1918 
1919     // Periodic polling -- Check for pending GC
1920     // Threads may spin while they're unsafe.
1921     // We don't want spinning threads to delay the JVM from reaching
1922     // a stop-the-world safepoint or to steal cycles from GC.
1923     // If we detect a pending safepoint we abort in order that
1924     // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1925     // this thread, if safe, doesn't steal cycles from GC.
1926     // This is in keeping with the "no loitering in runtime" rule.
1927     // We periodically check to see if there's a safepoint pending.
1928     if ((ctr & 0xFF) == 0) {
1929       if (SafepointMechanism::should_process(current)) {
1930         goto Abort;           // abrupt spin egress
1931       }
1932       SpinPause();
1933     }
1934 
1935     // Probe _owner with TATAS
1936     // If this thread observes the monitor transition or flicker
1937     // from locked to unlocked to locked, then the odds that this
1938     // thread will acquire the lock in this spin attempt go down
1939     // considerably.  The same argument applies if the CAS fails
1940     // or if we observe _owner change from one non-null value to
1941     // another non-null value.   In such cases we might abort
1942     // the spin without prejudice or apply a "penalty" to the
1943     // spin count-down variable "ctr", reducing it by 100, say.
1944 
1945     JavaThread* ox = (JavaThread*) owner_raw();
1946     if (ox == NULL) {
1947       ox = (JavaThread*)try_set_owner_from(NULL, current);
1948       if (ox == NULL) {
1949         // The CAS succeeded -- this thread acquired ownership
1950         // Take care of some bookkeeping to exit spin state.
1951         if (_succ == current) {
1952           _succ = NULL;
1953         }
1954 
1955         // Increase _SpinDuration :
1956         // The spin was successful (profitable) so we tend toward
1957         // longer spin attempts in the future.
1958         // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
1959         // If we acquired the lock early in the spin cycle it
1960         // makes sense to increase _SpinDuration proportionally.
1961         // Note that we don't clamp SpinDuration precisely at SpinLimit.
1962         int x = _SpinDuration;
1963         if (x < Knob_SpinLimit) {
1964           if (x < Knob_Poverty) x = Knob_Poverty;
1965           _SpinDuration = x + Knob_Bonus;
1966         }
1967         return 1;
1968       }
1969 
1970       // The CAS failed ... we can take any of the following actions:
1971       // * penalize: ctr -= CASPenalty
1972       // * exit spin with prejudice -- goto Abort;
1973       // * exit spin without prejudice.
1974       // * Since CAS is high-latency, retry again immediately.
1975       prv = ox;
1976       goto Abort;
1977     }
1978 
1979     // Did lock ownership change hands ?
1980     if (ox != prv && prv != NULL) {
1981       goto Abort;
1982     }
1983     prv = ox;
1984 
1985     // Abort the spin if the owner is not executing.
1986     // The owner must be executing in order to drop the lock.
1987     // Spinning while the owner is OFFPROC is idiocy.
1988     // Consider: ctr -= RunnablePenalty ;
1989     if (NotRunnable(current, ox)) {
1990       goto Abort;
1991     }
1992     if (_succ == NULL) {
1993       _succ = current;
1994     }
1995   }
1996 
1997   // Spin failed with prejudice -- reduce _SpinDuration.
1998   // TODO: Use an AIMD-like policy to adjust _SpinDuration.
1999   // AIMD is globally stable.
2000   {
2001     int x = _SpinDuration;
2002     if (x > 0) {
2003       // Consider an AIMD scheme like: x -= (x >> 3) + 100
2004       // This is globally sample and tends to damp the response.
2005       x -= Knob_Penalty;
2006       if (x < 0) x = 0;
2007       _SpinDuration = x;
2008     }
2009   }
2010 
2011  Abort:
2012   if (_succ == current) {
2013     _succ = NULL;
2014     // Invariant: after setting succ=null a contending thread
2015     // must recheck-retry _owner before parking.  This usually happens
2016     // in the normal usage of TrySpin(), but it's safest
2017     // to make TrySpin() as foolproof as possible.
2018     OrderAccess::fence();
2019     if (TryLock(current) > 0) return 1;
2020   }
2021   return 0;
2022 }
2023 
2024 // NotRunnable() -- informed spinning
2025 //
2026 // Don't bother spinning if the owner is not eligible to drop the lock.
2027 // Spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2028 // The thread must be runnable in order to drop the lock in timely fashion.
2029 // If the _owner is not runnable then spinning will not likely be
2030 // successful (profitable).
2031 //
2032 // Beware -- the thread referenced by _owner could have died
2033 // so a simply fetch from _owner->_thread_state might trap.
2034 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2035 // Because of the lifecycle issues, the _thread_state values
2036 // observed by NotRunnable() might be garbage.  NotRunnable must
2037 // tolerate this and consider the observed _thread_state value
2038 // as advisory.
2039 //
2040 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2041 // a thread pointer.
2042 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2043 // with the LSB of _owner.  Another option would be to probabilistically probe
2044 // the putative _owner->TypeTag value.
2045 //
2046 // Checking _thread_state isn't perfect.  Even if the thread is
2047 // in_java it might be blocked on a page-fault or have been preempted
2048 // and sitting on a ready/dispatch queue.
2049 //
2050 // The return value from NotRunnable() is *advisory* -- the
2051 // result is based on sampling and is not necessarily coherent.
2052 // The caller must tolerate false-negative and false-positive errors.
2053 // Spinning, in general, is probabilistic anyway.
2054 
2055 
2056 int ObjectMonitor::NotRunnable(JavaThread* current, JavaThread* ox) {
2057   // Check ox->TypeTag == 2BAD.
2058   if (ox == NULL) return 0;
2059 
2060   // Avoid transitive spinning ...
2061   // Say T1 spins or blocks trying to acquire L.  T1._Stalled is set to L.
2062   // Immediately after T1 acquires L it's possible that T2, also
2063   // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2064   // This occurs transiently after T1 acquired L but before
2065   // T1 managed to clear T1.Stalled.  T2 does not need to abort
2066   // its spin in this circumstance.
2067   intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
2068 
2069   if (BlockedOn == 1) return 1;
2070   if (BlockedOn != 0) {
2071     return BlockedOn != intptr_t(this) && owner_raw() == ox;
2072   }
2073 
2074   assert(sizeof(ox->_thread_state == sizeof(int)), "invariant");
2075   int jst = SafeFetch32((int *) &ox->_thread_state, -1);;
2076   // consider also: jst != _thread_in_Java -- but that's overspecific.
2077   return jst == _thread_blocked || jst == _thread_in_native;
2078 }
2079 
2080 
2081 // -----------------------------------------------------------------------------
2082 // WaitSet management ...
2083 
2084 ObjectWaiter::ObjectWaiter(JavaThread* current) {
2085   _next     = NULL;
2086   _prev     = NULL;
2087   _notified = 0;
2088   _notifier_tid = 0;
2089   TState    = TS_RUN;
2090   _thread   = current;
2091   _event    = _thread->_ParkEvent;
2092   _active   = false;
2093   assert(_event != NULL, "invariant");
2094 }
2095 
2096 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2097   _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(_thread, mon);
2098 }
2099 
2100 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2101   JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(_thread, _active);
2102 }
2103 
2104 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2105   assert(node != NULL, "should not add NULL node");
2106   assert(node->_prev == NULL, "node already in list");
2107   assert(node->_next == NULL, "node already in list");
2108   // put node at end of queue (circular doubly linked list)
2109   if (_WaitSet == NULL) {
2110     _WaitSet = node;
2111     node->_prev = node;
2112     node->_next = node;
2113   } else {
2114     ObjectWaiter* head = _WaitSet;
2115     ObjectWaiter* tail = head->_prev;
2116     assert(tail->_next == head, "invariant check");
2117     tail->_next = node;
2118     head->_prev = node;
2119     node->_next = head;
2120     node->_prev = tail;
2121   }
2122 }
2123 
2124 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2125   // dequeue the very first waiter
2126   ObjectWaiter* waiter = _WaitSet;
2127   if (waiter) {
2128     DequeueSpecificWaiter(waiter);
2129   }
2130   return waiter;
2131 }
2132 
2133 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2134   assert(node != NULL, "should not dequeue NULL node");
2135   assert(node->_prev != NULL, "node already removed from list");
2136   assert(node->_next != NULL, "node already removed from list");
2137   // when the waiter has woken up because of interrupt,
2138   // timeout or other spurious wake-up, dequeue the
2139   // waiter from waiting list
2140   ObjectWaiter* next = node->_next;
2141   if (next == node) {
2142     assert(node->_prev == node, "invariant check");
2143     _WaitSet = NULL;
2144   } else {
2145     ObjectWaiter* prev = node->_prev;
2146     assert(prev->_next == node, "invariant check");
2147     assert(next->_prev == node, "invariant check");
2148     next->_prev = prev;
2149     prev->_next = next;
2150     if (_WaitSet == node) {
2151       _WaitSet = next;
2152     }
2153   }
2154   node->_next = NULL;
2155   node->_prev = NULL;
2156 }
2157 
2158 // -----------------------------------------------------------------------------
2159 // PerfData support
2160 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts       = NULL;
2161 PerfCounter * ObjectMonitor::_sync_FutileWakeups               = NULL;
2162 PerfCounter * ObjectMonitor::_sync_Parks                       = NULL;
2163 PerfCounter * ObjectMonitor::_sync_Notifications               = NULL;
2164 PerfCounter * ObjectMonitor::_sync_Inflations                  = NULL;
2165 PerfCounter * ObjectMonitor::_sync_Deflations                  = NULL;
2166 PerfLongVariable * ObjectMonitor::_sync_MonExtant              = NULL;
2167 
2168 // One-shot global initialization for the sync subsystem.
2169 // We could also defer initialization and initialize on-demand
2170 // the first time we call ObjectSynchronizer::inflate().
2171 // Initialization would be protected - like so many things - by
2172 // the MonitorCache_lock.
2173 
2174 void ObjectMonitor::Initialize() {
2175   assert(!InitDone, "invariant");
2176 
2177   if (!os::is_MP()) {
2178     Knob_SpinLimit = 0;
2179     Knob_PreSpin   = 0;
2180     Knob_FixedSpin = -1;
2181   }
2182 
2183   if (UsePerfData) {
2184     EXCEPTION_MARK;
2185 #define NEWPERFCOUNTER(n)                                                \
2186   {                                                                      \
2187     n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,  \
2188                                         CHECK);                          \
2189   }
2190 #define NEWPERFVARIABLE(n)                                                \
2191   {                                                                       \
2192     n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,  \
2193                                          CHECK);                          \
2194   }
2195     NEWPERFCOUNTER(_sync_Inflations);
2196     NEWPERFCOUNTER(_sync_Deflations);
2197     NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2198     NEWPERFCOUNTER(_sync_FutileWakeups);
2199     NEWPERFCOUNTER(_sync_Parks);
2200     NEWPERFCOUNTER(_sync_Notifications);
2201     NEWPERFVARIABLE(_sync_MonExtant);
2202 #undef NEWPERFCOUNTER
2203 #undef NEWPERFVARIABLE
2204   }
2205 
2206   _oop_storage = OopStorageSet::create_weak("ObjectSynchronizer Weak", mtSynchronizer);
2207 
2208   DEBUG_ONLY(InitDone = true;)
2209 }
2210 
2211 void ObjectMonitor::print_on(outputStream* st) const {
2212   // The minimal things to print for markWord printing, more can be added for debugging and logging.
2213   st->print("{contentions=0x%08x,waiters=0x%08x"
2214             ",recursions=" INTX_FORMAT ",owner=" INTPTR_FORMAT "}",
2215             contentions(), waiters(), recursions(),
2216             p2i(owner()));
2217 }
2218 void ObjectMonitor::print() const { print_on(tty); }
2219 
2220 #ifdef ASSERT
2221 // Print the ObjectMonitor like a debugger would:
2222 //
2223 // (ObjectMonitor) 0x00007fdfb6012e40 = {
2224 //   _header = 0x0000000000000001
2225 //   _object = 0x000000070ff45fd0
2226 //   _pad_buf0 = {
2227 //     [0] = '\0'
2228 //     ...
2229 //     [43] = '\0'
2230 //   }
2231 //   _owner = 0x0000000000000000
2232 //   _previous_owner_tid = 0
2233 //   _pad_buf1 = {
2234 //     [0] = '\0'
2235 //     ...
2236 //     [47] = '\0'
2237 //   }
2238 //   _next_om = 0x0000000000000000
2239 //   _recursions = 0
2240 //   _EntryList = 0x0000000000000000
2241 //   _cxq = 0x0000000000000000
2242 //   _succ = 0x0000000000000000
2243 //   _Responsible = 0x0000000000000000
2244 //   _Spinner = 0
2245 //   _SpinDuration = 5000
2246 //   _contentions = 0
2247 //   _WaitSet = 0x0000700009756248
2248 //   _waiters = 1
2249 //   _WaitSetLock = 0
2250 // }
2251 //
2252 void ObjectMonitor::print_debug_style_on(outputStream* st) const {
2253   st->print_cr("(ObjectMonitor*) " INTPTR_FORMAT " = {", p2i(this));
2254   st->print_cr("  _header = " INTPTR_FORMAT, header().value());
2255   st->print_cr("  _object = " INTPTR_FORMAT, p2i(object_peek()));
2256   st->print_cr("  _pad_buf0 = {");
2257   st->print_cr("    [0] = '\\0'");
2258   st->print_cr("    ...");
2259   st->print_cr("    [%d] = '\\0'", (int)sizeof(_pad_buf0) - 1);
2260   st->print_cr("  }");
2261   st->print_cr("  _owner = " INTPTR_FORMAT, p2i(owner_raw()));
2262   st->print_cr("  _previous_owner_tid = " UINT64_FORMAT, _previous_owner_tid);
2263   st->print_cr("  _pad_buf1 = {");
2264   st->print_cr("    [0] = '\\0'");
2265   st->print_cr("    ...");
2266   st->print_cr("    [%d] = '\\0'", (int)sizeof(_pad_buf1) - 1);
2267   st->print_cr("  }");
2268   st->print_cr("  _next_om = " INTPTR_FORMAT, p2i(next_om()));
2269   st->print_cr("  _recursions = " INTX_FORMAT, _recursions);
2270   st->print_cr("  _EntryList = " INTPTR_FORMAT, p2i(_EntryList));
2271   st->print_cr("  _cxq = " INTPTR_FORMAT, p2i(_cxq));
2272   st->print_cr("  _succ = " INTPTR_FORMAT, p2i(_succ));
2273   st->print_cr("  _Responsible = " INTPTR_FORMAT, p2i(_Responsible));
2274   st->print_cr("  _Spinner = %d", _Spinner);
2275   st->print_cr("  _SpinDuration = %d", _SpinDuration);
2276   st->print_cr("  _contentions = %d", contentions());
2277   st->print_cr("  _WaitSet = " INTPTR_FORMAT, p2i(_WaitSet));
2278   st->print_cr("  _waiters = %d", _waiters);
2279   st->print_cr("  _WaitSetLock = %d", _WaitSetLock);
2280   st->print_cr("}");
2281 }
2282 #endif