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src/hotspot/share/runtime/objectMonitor.cpp

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  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/globals.hpp"
  43 #include "runtime/handles.inline.hpp"
  44 #include "runtime/interfaceSupport.inline.hpp"
  45 #include "runtime/javaThread.inline.hpp"
  46 #include "runtime/mutexLocker.hpp"
  47 #include "runtime/objectMonitor.hpp"
  48 #include "runtime/objectMonitor.inline.hpp"
  49 #include "runtime/orderAccess.hpp"
  50 #include "runtime/osThread.hpp"
  51 #include "runtime/perfData.hpp"
  52 #include "runtime/safefetch.hpp"
  53 #include "runtime/safepointMechanism.inline.hpp"
  54 #include "runtime/sharedRuntime.hpp"

  55 #include "services/threadService.hpp"
  56 #include "utilities/dtrace.hpp"
  57 #include "utilities/globalDefinitions.hpp"
  58 #include "utilities/macros.hpp"
  59 #include "utilities/preserveException.hpp"
  60 #if INCLUDE_JFR
  61 #include "jfr/support/jfrFlush.hpp"
  62 #endif
  63 
  64 #ifdef DTRACE_ENABLED
  65 
  66 // Only bother with this argument setup if dtrace is available
  67 // TODO-FIXME: probes should not fire when caller is _blocked.  assert() accordingly.
  68 
  69 
  70 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread)                           \
  71   char* bytes = nullptr;                                                   \
  72   int len = 0;                                                             \
  73   jlong jtid = SharedRuntime::get_java_tid(thread);                        \
  74   Symbol* klassname = obj->klass()->name();                                \

 108 
 109 #endif // ndef DTRACE_ENABLED
 110 
 111 // Tunables ...
 112 // The knob* variables are effectively final.  Once set they should
 113 // never be modified hence.  Consider using __read_mostly with GCC.
 114 
 115 int ObjectMonitor::Knob_SpinLimit    = 5000;    // derived by an external tool -
 116 
 117 static int Knob_Bonus               = 100;     // spin success bonus
 118 static int Knob_BonusB              = 100;     // spin success bonus
 119 static int Knob_Penalty             = 200;     // spin failure penalty
 120 static int Knob_Poverty             = 1000;
 121 static int Knob_FixedSpin           = 0;
 122 static int Knob_PreSpin             = 10;      // 20-100 likely better
 123 
 124 DEBUG_ONLY(static volatile bool InitDone = false;)
 125 
 126 OopStorage* ObjectMonitor::_oop_storage = nullptr;
 127 



 128 // -----------------------------------------------------------------------------
 129 // Theory of operations -- Monitors lists, thread residency, etc:
 130 //
 131 // * A thread acquires ownership of a monitor by successfully
 132 //   CAS()ing the _owner field from null to non-null.
 133 //
 134 // * Invariant: A thread appears on at most one monitor list --
 135 //   cxq, EntryList or WaitSet -- at any one time.
 136 //
 137 // * Contending threads "push" themselves onto the cxq with CAS
 138 //   and then spin/park.
 139 //
 140 // * After a contending thread eventually acquires the lock it must
 141 //   dequeue itself from either the EntryList or the cxq.
 142 //
 143 // * The exiting thread identifies and unparks an "heir presumptive"
 144 //   tentative successor thread on the EntryList.  Critically, the
 145 //   exiting thread doesn't unlink the successor thread from the EntryList.
 146 //   After having been unparked, the wakee will recontend for ownership of
 147 //   the monitor.   The successor (wakee) will either acquire the lock or

 245     switch (jt->thread_state()) {
 246     case _thread_in_vm:    // the usual case
 247     case _thread_in_Java:  // during deopt
 248       break;
 249     default:
 250       fatal("called from an unsafe thread state");
 251     }
 252     assert(jt->is_active_Java_thread(), "must be active JavaThread");
 253   } else {
 254     // However, ThreadService::get_current_contended_monitor()
 255     // can call here via the VMThread so sanity check it.
 256     assert(self->is_VM_thread(), "must be");
 257   }
 258 #endif // ASSERT
 259 }
 260 
 261 ObjectMonitor::ObjectMonitor(oop object) :
 262   _header(markWord::zero()),
 263   _object(_oop_storage, object),
 264   _owner(nullptr),

 265   _previous_owner_tid(0),
 266   _next_om(nullptr),
 267   _recursions(0),
 268   _EntryList(nullptr),
 269   _cxq(nullptr),
 270   _succ(nullptr),
 271   _Responsible(nullptr),
 272   _Spinner(0),
 273   _SpinDuration(ObjectMonitor::Knob_SpinLimit),
 274   _contentions(0),
 275   _WaitSet(nullptr),
 276   _waiters(0),
 277   _WaitSetLock(0)
 278 { }
 279 
 280 ObjectMonitor::~ObjectMonitor() {
 281   _object.release(_oop_storage);
 282 }
 283 
 284 oop ObjectMonitor::object() const {

 313 
 314 // -----------------------------------------------------------------------------
 315 // Enter support
 316 
 317 bool ObjectMonitor::enter_for(JavaThread* locking_thread) {
 318   // Used by ObjectSynchronizer::enter_for to enter for another thread.
 319   // The monitor is private to or already owned by locking_thread which must be suspended.
 320   // So this code may only contend with deflation.
 321   assert(locking_thread == Thread::current() || locking_thread->is_obj_deopt_suspend(), "must be");
 322 
 323   // Block out deflation as soon as possible.
 324   add_to_contentions(1);
 325 
 326   bool success = false;
 327   if (!is_being_async_deflated()) {
 328     void* prev_owner = try_set_owner_from(nullptr, locking_thread);
 329 
 330     if (prev_owner == nullptr) {
 331       assert(_recursions == 0, "invariant");
 332       success = true;
 333     } else if (prev_owner == locking_thread) {
 334       _recursions++;
 335       success = true;
 336     } else if (prev_owner == DEFLATER_MARKER) {
 337       // Racing with deflation.
 338       prev_owner = try_set_owner_from(DEFLATER_MARKER, locking_thread);
 339       if (prev_owner == DEFLATER_MARKER) {
 340         // Cancelled deflation. Increment contentions as part of the deflation protocol.
 341         add_to_contentions(1);
 342         success = true;
 343       } else if (prev_owner == nullptr) {
 344         // At this point we cannot race with deflation as we have both incremented
 345         // contentions, seen contention > 0 and seen a DEFLATER_MARKER.
 346         // success will only be false if this races with something other than
 347         // deflation.
 348         prev_owner = try_set_owner_from(nullptr, locking_thread);
 349         success = prev_owner == nullptr;
 350       }
 351     } else if (LockingMode == LM_LEGACY && locking_thread->is_lock_owned((address)prev_owner)) {
 352       assert(_recursions == 0, "must be");
 353       _recursions = 1;
 354       set_owner_from_BasicLock(prev_owner, locking_thread);
 355       success = true;
 356     }
 357     assert(success, "Failed to enter_for: locking_thread=" INTPTR_FORMAT
 358            ", this=" INTPTR_FORMAT "{owner=" INTPTR_FORMAT "}, observed owner: " INTPTR_FORMAT,
 359            p2i(locking_thread), p2i(this), p2i(owner_raw()), p2i(prev_owner));
 360   } else {
 361     // Async deflation is in progress and our contentions increment
 362     // above lost the race to async deflation. Undo the work and
 363     // force the caller to retry.
 364     const oop l_object = object();
 365     if (l_object != nullptr) {
 366       // Attempt to restore the header/dmw to the object's header so that
 367       // we only retry once if the deflater thread happens to be slow.
 368       install_displaced_markword_in_object(l_object);
 369     }
 370   }
 371 
 372   add_to_contentions(-1);
 373 
 374   assert(!success || owner_raw() == locking_thread, "must be");
 375 
 376   return success;
 377 }
 378 
 379 bool ObjectMonitor::enter(JavaThread* current) {
 380   assert(current == JavaThread::current(), "must be");
 381   // The following code is ordered to check the most common cases first
 382   // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
 383 
 384   void* cur = try_set_owner_from(nullptr, current);
 385   if (cur == nullptr) {
 386     assert(_recursions == 0, "invariant");
 387     return true;
 388   }
 389 
 390   if (cur == current) {
 391     // TODO-FIXME: check for integer overflow!  BUGID 6557169.
 392     _recursions++;
 393     return true;
 394   }
 395 
 396   if (LockingMode != LM_LIGHTWEIGHT && current->is_lock_owned((address)cur)) {
 397     assert(_recursions == 0, "internal state error");
 398     _recursions = 1;
 399     set_owner_from_BasicLock(cur, current);  // Convert from BasicLock* to Thread*.
 400     return true;
 401   }
 402 
 403   // We've encountered genuine contention.
 404   assert(current->_Stalled == 0, "invariant");
 405   current->_Stalled = intptr_t(this);
 406 
 407   // Try one round of spinning *before* enqueueing current
 408   // and before going through the awkward and expensive state
 409   // transitions.  The following spin is strictly optional ...
 410   // Note that if we acquire the monitor from an initial spin
 411   // we forgo posting JVMTI events and firing DTRACE probes.
 412   if (TrySpin(current) > 0) {
 413     assert(owner_raw() == current, "must be current: owner=" INTPTR_FORMAT, p2i(owner_raw()));
 414     assert(_recursions == 0, "must be 0: recursions=" INTX_FORMAT, _recursions);
 415     assert(object()->mark() == markWord::encode(this),
 416            "object mark must match encoded this: mark=" INTPTR_FORMAT
 417            ", encoded this=" INTPTR_FORMAT, object()->mark().value(),
 418            markWord::encode(this).value());
 419     current->_Stalled = 0;
 420     return true;
 421   }
 422 
 423   assert(owner_raw() != current, "invariant");
 424   assert(_succ != current, "invariant");
 425   assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
 426   assert(current->thread_state() != _thread_blocked, "invariant");
 427 
 428   // Keep track of contention for JVM/TI and M&M queries.
 429   add_to_contentions(1);
 430   if (is_being_async_deflated()) {
 431     // Async deflation is in progress and our contentions increment
 432     // above lost the race to async deflation. Undo the work and
 433     // force the caller to retry.
 434     const oop l_object = object();
 435     if (l_object != nullptr) {
 436       // Attempt to restore the header/dmw to the object's header so that
 437       // we only retry once if the deflater thread happens to be slow.
 438       install_displaced_markword_in_object(l_object);
 439     }
 440     current->_Stalled = 0;
 441     add_to_contentions(-1);
 442     return false;
 443   }

 452     event.set_address((uintptr_t)this);
 453   }
 454 
 455   { // Change java thread status to indicate blocked on monitor enter.
 456     JavaThreadBlockedOnMonitorEnterState jtbmes(current, this);
 457 
 458     assert(current->current_pending_monitor() == nullptr, "invariant");
 459     current->set_current_pending_monitor(this);
 460 
 461     DTRACE_MONITOR_PROBE(contended__enter, this, object(), current);
 462     if (JvmtiExport::should_post_monitor_contended_enter()) {
 463       JvmtiExport::post_monitor_contended_enter(current, this);
 464 
 465       // The current thread does not yet own the monitor and does not
 466       // yet appear on any queues that would get it made the successor.
 467       // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
 468       // handler cannot accidentally consume an unpark() meant for the
 469       // ParkEvent associated with this ObjectMonitor.
 470     }
 471 




















 472     OSThreadContendState osts(current->osthread());
 473 
 474     assert(current->thread_state() == _thread_in_vm, "invariant");
 475 
 476     for (;;) {
 477       ExitOnSuspend eos(this);
 478       {
 479         ThreadBlockInVMPreprocess<ExitOnSuspend> tbivs(current, eos, true /* allow_suspend */);
 480         EnterI(current);
 481         current->set_current_pending_monitor(nullptr);
 482         // We can go to a safepoint at the end of this block. If we
 483         // do a thread dump during that safepoint, then this thread will show
 484         // as having "-locked" the monitor, but the OS and java.lang.Thread
 485         // states will still report that the thread is blocked trying to
 486         // acquire it.
 487         // If there is a suspend request, ExitOnSuspend will exit the OM
 488         // and set the OM as pending.
 489       }
 490       if (!eos.exited()) {
 491         // ExitOnSuspend did not exit the OM
 492         assert(owner_raw() == current, "invariant");
 493         break;
 494       }
 495     }
 496 
 497     // We've just gotten past the enter-check-for-suspend dance and we now own
 498     // the monitor free and clear.
 499   }
 500 
 501   add_to_contentions(-1);
 502   assert(contentions() >= 0, "must not be negative: contentions=%d", contentions());
 503   current->_Stalled = 0;
 504 
 505   // Must either set _recursions = 0 or ASSERT _recursions == 0.
 506   assert(_recursions == 0, "invariant");
 507   assert(owner_raw() == current, "invariant");
 508   assert(_succ != current, "invariant");
 509   assert(object()->mark() == markWord::encode(this), "invariant");
 510 
 511   // The thread -- now the owner -- is back in vm mode.
 512   // Report the glorious news via TI,DTrace and jvmstat.
 513   // The probe effect is non-trivial.  All the reportage occurs
 514   // while we hold the monitor, increasing the length of the critical
 515   // section.  Amdahl's parallel speedup law comes vividly into play.
 516   //
 517   // Another option might be to aggregate the events (thread local or
 518   // per-monitor aggregation) and defer reporting until a more opportune
 519   // time -- such as next time some thread encounters contention but has
 520   // yet to acquire the lock.  While spinning that thread could
 521   // spinning we could increment JVMStat counters, etc.
 522 
 523   DTRACE_MONITOR_PROBE(contended__entered, this, object(), current);
 524   if (JvmtiExport::should_post_monitor_contended_entered()) {
 525     JvmtiExport::post_monitor_contended_entered(current, this);
 526 
 527     // The current thread already owns the monitor and is not going to

 562 // makes contentions negative as signals to contending threads that
 563 // an async deflation is in progress. There are a number of checks
 564 // as part of the protocol to make sure that the calling thread has
 565 // not lost the race to a contending thread.
 566 //
 567 // The ObjectMonitor has been successfully async deflated when:
 568 //   (contentions < 0)
 569 // Contending threads that see that condition know to retry their operation.
 570 //
 571 bool ObjectMonitor::deflate_monitor() {
 572   if (is_busy()) {
 573     // Easy checks are first - the ObjectMonitor is busy so no deflation.
 574     return false;
 575   }
 576 
 577   const oop obj = object_peek();
 578 
 579   if (obj == nullptr) {
 580     // If the object died, we can recycle the monitor without racing with
 581     // Java threads. The GC already broke the association with the object.
 582     set_owner_from(nullptr, DEFLATER_MARKER);
 583     assert(contentions() >= 0, "must be non-negative: contentions=%d", contentions());
 584     _contentions = INT_MIN; // minimum negative int
 585   } else {
 586     // Attempt async deflation protocol.
 587 
 588     // Set a null owner to DEFLATER_MARKER to force any contending thread
 589     // through the slow path. This is just the first part of the async
 590     // deflation dance.
 591     if (try_set_owner_from(nullptr, DEFLATER_MARKER) != nullptr) {
 592       // The owner field is no longer null so we lost the race since the
 593       // ObjectMonitor is now busy.
 594       return false;
 595     }
 596 
 597     if (contentions() > 0 || _waiters != 0) {
 598       // Another thread has raced to enter the ObjectMonitor after
 599       // is_busy() above or has already entered and waited on
 600       // it which makes it busy so no deflation. Restore owner to
 601       // null if it is still DEFLATER_MARKER.
 602       if (try_set_owner_from(DEFLATER_MARKER, nullptr) != DEFLATER_MARKER) {
 603         // Deferred decrement for the JT EnterI() that cancelled the async deflation.
 604         add_to_contentions(-1);
 605       }
 606       return false;
 607     }
 608 
 609     // Make a zero contentions field negative to force any contending threads
 610     // to retry. This is the second part of the async deflation dance.
 611     if (Atomic::cmpxchg(&_contentions, 0, INT_MIN) != 0) {
 612       // Contentions was no longer 0 so we lost the race since the
 613       // ObjectMonitor is now busy. Restore owner to null if it is
 614       // still DEFLATER_MARKER:
 615       if (try_set_owner_from(DEFLATER_MARKER, nullptr) != DEFLATER_MARKER) {
 616         // Deferred decrement for the JT EnterI() that cancelled the async deflation.
 617         add_to_contentions(-1);
 618       }
 619       return false;
 620     }
 621   }
 622 
 623   // Sanity checks for the races:
 624   guarantee(owner_is_DEFLATER_MARKER(), "must be deflater marker");
 625   guarantee(contentions() < 0, "must be negative: contentions=%d",
 626             contentions());
 627   guarantee(_waiters == 0, "must be 0: waiters=%d", _waiters);
 628   guarantee(_cxq == nullptr, "must be no contending threads: cxq="
 629             INTPTR_FORMAT, p2i(_cxq));
 630   guarantee(_EntryList == nullptr,
 631             "must be no entering threads: EntryList=" INTPTR_FORMAT,
 632             p2i(_EntryList));
 633 
 634   if (obj != nullptr) {
 635     if (log_is_enabled(Trace, monitorinflation)) {

 691     log_info(monitorinflation)("install_displaced_markword_in_object: "
 692                                "failed cas_set_mark: new_mark=" INTPTR_FORMAT
 693                                ", old_mark=" INTPTR_FORMAT ", res=" INTPTR_FORMAT,
 694                                dmw.value(), markWord::encode(this).value(),
 695                                res.value());
 696   }
 697 
 698   // Note: It does not matter which thread restored the header/dmw
 699   // into the object's header. The thread deflating the monitor just
 700   // wanted the object's header restored and it is. The threads that
 701   // detected a race with the deflation process also wanted the
 702   // object's header restored before they retry their operation and
 703   // because it is restored they will only retry once.
 704 }
 705 
 706 // Convert the fields used by is_busy() to a string that can be
 707 // used for diagnostic output.
 708 const char* ObjectMonitor::is_busy_to_string(stringStream* ss) {
 709   ss->print("is_busy: waiters=%d"
 710             ", contentions=%d"
 711             ", owner=" PTR_FORMAT
 712             ", cxq=" PTR_FORMAT
 713             ", EntryList=" PTR_FORMAT,
 714             _waiters,
 715             (contentions() > 0 ? contentions() : 0),
 716             owner_is_DEFLATER_MARKER()
 717                 // We report null instead of DEFLATER_MARKER here because is_busy()
 718                 // ignores DEFLATER_MARKER values.
 719                 ? p2i(nullptr)
 720                 : p2i(owner_raw()),
 721             p2i(_cxq),
 722             p2i(_EntryList));
 723   return ss->base();
 724 }
 725 
 726 #define MAX_RECHECK_INTERVAL 1000
 727 
 728 void ObjectMonitor::EnterI(JavaThread* current) {
 729   assert(current->thread_state() == _thread_blocked, "invariant");
 730 
 731   // Try the lock - TATAS
 732   if (TryLock (current) > 0) {
 733     assert(_succ != current, "invariant");
 734     assert(owner_raw() == current, "invariant");
 735     assert(_Responsible != current, "invariant");
 736     return;
 737   }
 738 
 739   if (try_set_owner_from(DEFLATER_MARKER, current) == DEFLATER_MARKER) {
 740     // Cancelled the in-progress async deflation by changing owner from
 741     // DEFLATER_MARKER to current. As part of the contended enter protocol,
 742     // contentions was incremented to a positive value before EnterI()
 743     // was called and that prevents the deflater thread from winning the
 744     // last part of the 2-part async deflation protocol. After EnterI()
 745     // returns to enter(), contentions is decremented because the caller
 746     // now owns the monitor. We bump contentions an extra time here to
 747     // prevent the deflater thread from winning the last part of the
 748     // 2-part async deflation protocol after the regular decrement
 749     // occurs in enter(). The deflater thread will decrement contentions
 750     // after it recognizes that the async deflation was cancelled.
 751     add_to_contentions(1);
 752     assert(_succ != current, "invariant");
 753     assert(_Responsible != current, "invariant");
 754     return;
 755   }
 756 
 757   assert(InitDone, "Unexpectedly not initialized");
 758 
 759   // We try one round of spinning *before* enqueueing current.
 760   //
 761   // If the _owner is ready but OFFPROC we could use a YieldTo()
 762   // operation to donate the remainder of this thread's quantum
 763   // to the owner.  This has subtle but beneficial affinity
 764   // effects.
 765 
 766   if (TrySpin(current) > 0) {
 767     assert(owner_raw() == current, "invariant");
 768     assert(_succ != current, "invariant");
 769     assert(_Responsible != current, "invariant");
 770     return;
 771   }
 772 
 773   // The Spin failed -- Enqueue and park the thread ...
 774   assert(_succ != current, "invariant");
 775   assert(owner_raw() != current, "invariant");
 776   assert(_Responsible != current, "invariant");
 777 
 778   // Enqueue "current" on ObjectMonitor's _cxq.
 779   //
 780   // Node acts as a proxy for current.
 781   // As an aside, if were to ever rewrite the synchronization code mostly
 782   // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
 783   // Java objects.  This would avoid awkward lifecycle and liveness issues,
 784   // as well as eliminate a subset of ABA issues.
 785   // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
 786 
 787   ObjectWaiter node(current);
 788   current->_ParkEvent->reset();
 789   node._prev   = (ObjectWaiter*) 0xBAD;
 790   node.TState  = ObjectWaiter::TS_CXQ;
 791 
 792   // Push "current" onto the front of the _cxq.
 793   // Once on cxq/EntryList, current stays on-queue until it acquires the lock.
 794   // Note that spinning tends to reduce the rate at which threads
 795   // enqueue and dequeue on EntryList|cxq.
 796   ObjectWaiter* nxt;
 797   for (;;) {
 798     node._next = nxt = _cxq;
 799     if (Atomic::cmpxchg(&_cxq, nxt, &node) == nxt) break;
 800 
 801     // Interference - the CAS failed because _cxq changed.  Just retry.
 802     // As an optional optimization we retry the lock.
 803     if (TryLock (current) > 0) {
 804       assert(_succ != current, "invariant");
 805       assert(owner_raw() == current, "invariant");
 806       assert(_Responsible != current, "invariant");
 807       return;
 808     }
 809   }
 810 
 811   // Check for cxq|EntryList edge transition to non-null.  This indicates
 812   // the onset of contention.  While contention persists exiting threads
 813   // will use a ST:MEMBAR:LD 1-1 exit protocol.  When contention abates exit
 814   // operations revert to the faster 1-0 mode.  This enter operation may interleave
 815   // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
 816   // arrange for one of the contending thread to use a timed park() operations
 817   // to detect and recover from the race.  (Stranding is form of progress failure
 818   // where the monitor is unlocked but all the contending threads remain parked).
 819   // That is, at least one of the contended threads will periodically poll _owner.
 820   // One of the contending threads will become the designated "Responsible" thread.
 821   // The Responsible thread uses a timed park instead of a normal indefinite park
 822   // operation -- it periodically wakes and checks for and recovers from potential
 823   // strandings admitted by 1-0 exit operations.   We need at most one Responsible
 824   // thread per-monitor at any given moment.  Only threads on cxq|EntryList may
 825   // be responsible for a monitor.

 833 
 834   if (nxt == nullptr && _EntryList == nullptr) {
 835     // Try to assume the role of responsible thread for the monitor.
 836     // CONSIDER:  ST vs CAS vs { if (Responsible==null) Responsible=current }
 837     Atomic::replace_if_null(&_Responsible, current);
 838   }
 839 
 840   // The lock might have been released while this thread was occupied queueing
 841   // itself onto _cxq.  To close the race and avoid "stranding" and
 842   // progress-liveness failure we must resample-retry _owner before parking.
 843   // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
 844   // In this case the ST-MEMBAR is accomplished with CAS().
 845   //
 846   // TODO: Defer all thread state transitions until park-time.
 847   // Since state transitions are heavy and inefficient we'd like
 848   // to defer the state transitions until absolutely necessary,
 849   // and in doing so avoid some transitions ...
 850 
 851   int nWakeups = 0;
 852   int recheckInterval = 1;






 853 
 854   for (;;) {
 855 
 856     if (TryLock(current) > 0) break;
 857     assert(owner_raw() != current, "invariant");
 858 
 859     // park self
 860     if (_Responsible == current) {
 861       current->_ParkEvent->park((jlong) recheckInterval);
 862       // Increase the recheckInterval, but clamp the value.
 863       recheckInterval *= 8;
 864       if (recheckInterval > MAX_RECHECK_INTERVAL) {
 865         recheckInterval = MAX_RECHECK_INTERVAL;
 866       }
 867     } else {
 868       current->_ParkEvent->park();
 869     }
 870 
 871     if (TryLock(current) > 0) break;
 872 
 873     if (try_set_owner_from(DEFLATER_MARKER, current) == DEFLATER_MARKER) {
 874       // Cancelled the in-progress async deflation by changing owner from
 875       // DEFLATER_MARKER to current. As part of the contended enter protocol,
 876       // contentions was incremented to a positive value before EnterI()
 877       // was called and that prevents the deflater thread from winning the
 878       // last part of the 2-part async deflation protocol. After EnterI()
 879       // returns to enter(), contentions is decremented because the caller
 880       // now owns the monitor. We bump contentions an extra time here to

 906     // We can find that we were unpark()ed and redesignated _succ while
 907     // we were spinning.  That's harmless.  If we iterate and call park(),
 908     // park() will consume the event and return immediately and we'll
 909     // just spin again.  This pattern can repeat, leaving _succ to simply
 910     // spin on a CPU.
 911 
 912     if (_succ == current) _succ = nullptr;
 913 
 914     // Invariant: after clearing _succ a thread *must* retry _owner before parking.
 915     OrderAccess::fence();
 916   }
 917 
 918   // Egress :
 919   // current has acquired the lock -- Unlink current from the cxq or EntryList.
 920   // Normally we'll find current on the EntryList .
 921   // From the perspective of the lock owner (this thread), the
 922   // EntryList is stable and cxq is prepend-only.
 923   // The head of cxq is volatile but the interior is stable.
 924   // In addition, current.TState is stable.
 925 
 926   assert(owner_raw() == current, "invariant");
 927 
 928   UnlinkAfterAcquire(current, &node);
 929   if (_succ == current) _succ = nullptr;
 930 
 931   assert(_succ != current, "invariant");
 932   if (_Responsible == current) {
 933     _Responsible = nullptr;
 934     OrderAccess::fence(); // Dekker pivot-point
 935 
 936     // We may leave threads on cxq|EntryList without a designated
 937     // "Responsible" thread.  This is benign.  When this thread subsequently
 938     // exits the monitor it can "see" such preexisting "old" threads --
 939     // threads that arrived on the cxq|EntryList before the fence, above --
 940     // by LDing cxq|EntryList.  Newly arrived threads -- that is, threads
 941     // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
 942     // non-null and elect a new "Responsible" timer thread.
 943     //
 944     // This thread executes:
 945     //    ST Responsible=null; MEMBAR    (in enter epilogue - here)
 946     //    LD cxq|EntryList               (in subsequent exit)

 962   // STs to monitor meta-data and user-data could reorder with (become
 963   // visible after) the ST in exit that drops ownership of the lock.
 964   // Some other thread could then acquire the lock, but observe inconsistent
 965   // or old monitor meta-data and heap data.  That violates the JMM.
 966   // To that end, the 1-0 exit() operation must have at least STST|LDST
 967   // "release" barrier semantics.  Specifically, there must be at least a
 968   // STST|LDST barrier in exit() before the ST of null into _owner that drops
 969   // the lock.   The barrier ensures that changes to monitor meta-data and data
 970   // protected by the lock will be visible before we release the lock, and
 971   // therefore before some other thread (CPU) has a chance to acquire the lock.
 972   // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
 973   //
 974   // Critically, any prior STs to _succ or EntryList must be visible before
 975   // the ST of null into _owner in the *subsequent* (following) corresponding
 976   // monitorexit.  Recall too, that in 1-0 mode monitorexit does not necessarily
 977   // execute a serializing instruction.
 978 
 979   return;
 980 }
 981 






















































































 982 // ReenterI() is a specialized inline form of the latter half of the
 983 // contended slow-path from EnterI().  We use ReenterI() only for
 984 // monitor reentry in wait().
 985 //
 986 // In the future we should reconcile EnterI() and ReenterI().
 987 
 988 void ObjectMonitor::ReenterI(JavaThread* current, ObjectWaiter* currentNode) {
 989   assert(current != nullptr, "invariant");
 990   assert(currentNode != nullptr, "invariant");
 991   assert(currentNode->_thread == current, "invariant");
 992   assert(_waiters > 0, "invariant");
 993   assert(object()->mark() == markWord::encode(this), "invariant");
 994 
 995   assert(current->thread_state() != _thread_blocked, "invariant");
 996 
 997   int nWakeups = 0;
 998   for (;;) {
 999     ObjectWaiter::TStates v = currentNode->TState;
1000     guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1001     assert(owner_raw() != current, "invariant");
1002 
1003     if (TryLock(current) > 0) break;
1004     if (TrySpin(current) > 0) break;
1005 
1006     {
1007       OSThreadContendState osts(current->osthread());
1008 
1009       assert(current->thread_state() == _thread_in_vm, "invariant");
1010 
1011       {
1012         ClearSuccOnSuspend csos(this);
1013         ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */);
1014         current->_ParkEvent->park();
1015       }
1016     }
1017 
1018     // Try again, but just so we distinguish between futile wakeups and
1019     // successful wakeups.  The following test isn't algorithmically
1020     // necessary, but it helps us maintain sensible statistics.
1021     if (TryLock(current) > 0) break;

1031     // find that _succ == current.
1032     if (_succ == current) _succ = nullptr;
1033 
1034     // Invariant: after clearing _succ a contending thread
1035     // *must* retry  _owner before parking.
1036     OrderAccess::fence();
1037 
1038     // This PerfData object can be used in parallel with a safepoint.
1039     // See the work around in PerfDataManager::destroy().
1040     OM_PERFDATA_OP(FutileWakeups, inc());
1041   }
1042 
1043   // current has acquired the lock -- Unlink current from the cxq or EntryList .
1044   // Normally we'll find current on the EntryList.
1045   // Unlinking from the EntryList is constant-time and atomic-free.
1046   // From the perspective of the lock owner (this thread), the
1047   // EntryList is stable and cxq is prepend-only.
1048   // The head of cxq is volatile but the interior is stable.
1049   // In addition, current.TState is stable.
1050 
1051   assert(owner_raw() == current, "invariant");
1052   assert(object()->mark() == markWord::encode(this), "invariant");
1053   UnlinkAfterAcquire(current, currentNode);
1054   if (_succ == current) _succ = nullptr;
1055   assert(_succ != current, "invariant");
1056   currentNode->TState = ObjectWaiter::TS_RUN;
1057   OrderAccess::fence();      // see comments at the end of EnterI()
1058 }
1059 



























































1060 // By convention we unlink a contending thread from EntryList|cxq immediately
1061 // after the thread acquires the lock in ::enter().  Equally, we could defer
1062 // unlinking the thread until ::exit()-time.
1063 
1064 void ObjectMonitor::UnlinkAfterAcquire(JavaThread* current, ObjectWaiter* currentNode) {
1065   assert(owner_raw() == current, "invariant");
1066   assert(currentNode->_thread == current, "invariant");
1067 
1068   if (currentNode->TState == ObjectWaiter::TS_ENTER) {
1069     // Normal case: remove current from the DLL EntryList .
1070     // This is a constant-time operation.
1071     ObjectWaiter* nxt = currentNode->_next;
1072     ObjectWaiter* prv = currentNode->_prev;
1073     if (nxt != nullptr) nxt->_prev = prv;
1074     if (prv != nullptr) prv->_next = nxt;
1075     if (currentNode == _EntryList) _EntryList = nxt;
1076     assert(nxt == nullptr || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
1077     assert(prv == nullptr || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
1078   } else {
1079     assert(currentNode->TState == ObjectWaiter::TS_CXQ, "invariant");
1080     // Inopportune interleaving -- current is still on the cxq.
1081     // This usually means the enqueue of self raced an exiting thread.
1082     // Normally we'll find current near the front of the cxq, so
1083     // dequeueing is typically fast.  If needbe we can accelerate
1084     // this with some MCS/CHL-like bidirectional list hints and advisory
1085     // back-links so dequeueing from the interior will normally operate
1086     // in constant-time.

1106         q = p;
1107         assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
1108       }
1109       assert(v != currentNode, "invariant");
1110       assert(p == currentNode, "Node not found on cxq");
1111       assert(p != _cxq, "invariant");
1112       assert(q != nullptr, "invariant");
1113       assert(q->_next == p, "invariant");
1114       q->_next = p->_next;
1115     }
1116   }
1117 
1118 #ifdef ASSERT
1119   // Diagnostic hygiene ...
1120   currentNode->_prev  = (ObjectWaiter*) 0xBAD;
1121   currentNode->_next  = (ObjectWaiter*) 0xBAD;
1122   currentNode->TState = ObjectWaiter::TS_RUN;
1123 #endif
1124 }
1125 










1126 // -----------------------------------------------------------------------------
1127 // Exit support
1128 //
1129 // exit()
1130 // ~~~~~~
1131 // Note that the collector can't reclaim the objectMonitor or deflate
1132 // the object out from underneath the thread calling ::exit() as the
1133 // thread calling ::exit() never transitions to a stable state.
1134 // This inhibits GC, which in turn inhibits asynchronous (and
1135 // inopportune) reclamation of "this".
1136 //
1137 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
1138 // There's one exception to the claim above, however.  EnterI() can call
1139 // exit() to drop a lock if the acquirer has been externally suspended.
1140 // In that case exit() is called with _thread_state == _thread_blocked,
1141 // but the monitor's _contentions field is > 0, which inhibits reclamation.
1142 //
1143 // 1-0 exit
1144 // ~~~~~~~~
1145 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of

1165 // exiting thread will notice and unpark the stranded thread, or, (b)
1166 // the timer expires.  If the lock is high traffic then the stranding latency
1167 // will be low due to (a).  If the lock is low traffic then the odds of
1168 // stranding are lower, although the worst-case stranding latency
1169 // is longer.  Critically, we don't want to put excessive load in the
1170 // platform's timer subsystem.  We want to minimize both the timer injection
1171 // rate (timers created/sec) as well as the number of timers active at
1172 // any one time.  (more precisely, we want to minimize timer-seconds, which is
1173 // the integral of the # of active timers at any instant over time).
1174 // Both impinge on OS scalability.  Given that, at most one thread parked on
1175 // a monitor will use a timer.
1176 //
1177 // There is also the risk of a futile wake-up. If we drop the lock
1178 // another thread can reacquire the lock immediately, and we can
1179 // then wake a thread unnecessarily. This is benign, and we've
1180 // structured the code so the windows are short and the frequency
1181 // of such futile wakups is low.
1182 
1183 void ObjectMonitor::exit(JavaThread* current, bool not_suspended) {
1184   void* cur = owner_raw();
1185   if (current != cur) {
1186     if (LockingMode != LM_LIGHTWEIGHT && current->is_lock_owned((address)cur)) {
1187       assert(_recursions == 0, "invariant");
1188       set_owner_from_BasicLock(cur, current);  // Convert from BasicLock* to Thread*.
1189       _recursions = 0;
1190     } else {
1191       // Apparent unbalanced locking ...
1192       // Naively we'd like to throw IllegalMonitorStateException.
1193       // As a practical matter we can neither allocate nor throw an
1194       // exception as ::exit() can be called from leaf routines.
1195       // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
1196       // Upon deeper reflection, however, in a properly run JVM the only
1197       // way we should encounter this situation is in the presence of
1198       // unbalanced JNI locking. TODO: CheckJNICalls.
1199       // See also: CR4414101
1200 #ifdef ASSERT
1201       LogStreamHandle(Error, monitorinflation) lsh;
1202       lsh.print_cr("ERROR: ObjectMonitor::exit(): thread=" INTPTR_FORMAT
1203                     " is exiting an ObjectMonitor it does not own.", p2i(current));
1204       lsh.print_cr("The imbalance is possibly caused by JNI locking.");
1205       print_debug_style_on(&lsh);
1206       assert(false, "Non-balanced monitor enter/exit!");
1207 #endif
1208       return;
1209     }
1210   }
1211 
1212   if (_recursions != 0) {
1213     _recursions--;        // this is simple recursive enter
1214     return;
1215   }
1216 
1217   // Invariant: after setting Responsible=null an thread must execute
1218   // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
1219   _Responsible = nullptr;
1220 
1221 #if INCLUDE_JFR
1222   // get the owner's thread id for the MonitorEnter event
1223   // if it is enabled and the thread isn't suspended
1224   if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
1225     _previous_owner_tid = JFR_THREAD_ID(current);
1226   }
1227 #endif
1228 
1229   for (;;) {
1230     assert(current == owner_raw(), "invariant");
1231 
1232     // Drop the lock.
1233     // release semantics: prior loads and stores from within the critical section
1234     // must not float (reorder) past the following store that drops the lock.
1235     // Uses a storeload to separate release_store(owner) from the
1236     // successor check. The try_set_owner() below uses cmpxchg() so
1237     // we get the fence down there.
1238     release_clear_owner(current);
1239     OrderAccess::storeload();
1240 
1241     if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != nullptr) {
1242       return;
1243     }
1244     // Other threads are blocked trying to acquire the lock.
1245 
1246     // Normally the exiting thread is responsible for ensuring succession,
1247     // but if other successors are ready or other entering threads are spinning
1248     // then this thread can simply store null into _owner and exit without
1249     // waking a successor.  The existence of spinners or ready successors
1250     // guarantees proper succession (liveness).  Responsibility passes to the
1251     // ready or running successors.  The exiting thread delegates the duty.
1252     // More precisely, if a successor already exists this thread is absolved
1253     // of the responsibility of waking (unparking) one.
1254     //
1255     // The _succ variable is critical to reducing futile wakeup frequency.
1256     // _succ identifies the "heir presumptive" thread that has been made

1266     // to drop the lock and then spin briefly to see if a spinner managed
1267     // to acquire the lock.  If so, the exiting thread could exit
1268     // immediately without waking a successor, otherwise the exiting
1269     // thread would need to dequeue and wake a successor.
1270     // (Note that we'd need to make the post-drop spin short, but no
1271     // shorter than the worst-case round-trip cache-line migration time.
1272     // The dropped lock needs to become visible to the spinner, and then
1273     // the acquisition of the lock by the spinner must become visible to
1274     // the exiting thread).
1275 
1276     // It appears that an heir-presumptive (successor) must be made ready.
1277     // Only the current lock owner can manipulate the EntryList or
1278     // drain _cxq, so we need to reacquire the lock.  If we fail
1279     // to reacquire the lock the responsibility for ensuring succession
1280     // falls to the new owner.
1281     //
1282     if (try_set_owner_from(nullptr, current) != nullptr) {
1283       return;
1284     }
1285 
1286     guarantee(owner_raw() == current, "invariant");
1287 
1288     ObjectWaiter* w = nullptr;
1289 
1290     w = _EntryList;
1291     if (w != nullptr) {
1292       // I'd like to write: guarantee (w->_thread != current).
1293       // But in practice an exiting thread may find itself on the EntryList.
1294       // Let's say thread T1 calls O.wait().  Wait() enqueues T1 on O's waitset and
1295       // then calls exit().  Exit release the lock by setting O._owner to null.
1296       // Let's say T1 then stalls.  T2 acquires O and calls O.notify().  The
1297       // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1298       // release the lock "O".  T2 resumes immediately after the ST of null into
1299       // _owner, above.  T2 notices that the EntryList is populated, so it
1300       // reacquires the lock and then finds itself on the EntryList.
1301       // Given all that, we have to tolerate the circumstance where "w" is
1302       // associated with current.
1303       assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1304       ExitEpilog(current, w);
1305       return;
1306     }

1343     }
1344 
1345     // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = nullptr
1346     // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1347 
1348     // See if we can abdicate to a spinner instead of waking a thread.
1349     // A primary goal of the implementation is to reduce the
1350     // context-switch rate.
1351     if (_succ != nullptr) continue;
1352 
1353     w = _EntryList;
1354     if (w != nullptr) {
1355       guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1356       ExitEpilog(current, w);
1357       return;
1358     }
1359   }
1360 }
1361 
1362 void ObjectMonitor::ExitEpilog(JavaThread* current, ObjectWaiter* Wakee) {
1363   assert(owner_raw() == current, "invariant");
1364 
1365   // Exit protocol:
1366   // 1. ST _succ = wakee
1367   // 2. membar #loadstore|#storestore;
1368   // 2. ST _owner = nullptr
1369   // 3. unpark(wakee)
1370 
1371   _succ = Wakee->_thread;








1372   ParkEvent * Trigger = Wakee->_event;
1373 
1374   // Hygiene -- once we've set _owner = nullptr we can't safely dereference Wakee again.
1375   // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1376   // out-of-scope (non-extant).
1377   Wakee  = nullptr;
1378 
1379   // Drop the lock.
1380   // Uses a fence to separate release_store(owner) from the LD in unpark().
1381   release_clear_owner(current);
1382   OrderAccess::fence();
1383 
1384   DTRACE_MONITOR_PROBE(contended__exit, this, object(), current);
1385   Trigger->unpark();






1386 
1387   // Maintain stats and report events to JVMTI
1388   OM_PERFDATA_OP(Parks, inc());
1389 }
1390 
1391 // complete_exit exits a lock returning recursion count
1392 // complete_exit requires an inflated monitor
1393 // The _owner field is not always the Thread addr even with an
1394 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1395 // thread due to contention.
1396 intx ObjectMonitor::complete_exit(JavaThread* current) {
1397   assert(InitDone, "Unexpectedly not initialized");
1398 
1399   void* cur = owner_raw();
1400   if (current != cur) {
1401     if (LockingMode != LM_LIGHTWEIGHT && current->is_lock_owned((address)cur)) {
1402       assert(_recursions == 0, "internal state error");
1403       set_owner_from_BasicLock(cur, current);  // Convert from BasicLock* to Thread*.
1404       _recursions = 0;
1405     }
1406   }
1407 
1408   guarantee(current == owner_raw(), "complete_exit not owner");
1409   intx save = _recursions; // record the old recursion count
1410   _recursions = 0;         // set the recursion level to be 0
1411   exit(current);           // exit the monitor
1412   guarantee(owner_raw() != current, "invariant");
1413   return save;
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   assert(cur != anon_owner_ptr(), "no anon owner here");
1437   if (cur == current) {
1438     return true;
1439   }
1440   if (LockingMode != LM_LIGHTWEIGHT && current->is_lock_owned((address)cur)) {
1441     set_owner_from_BasicLock(cur, current);  // Convert from BasicLock* to Thread*.
1442     _recursions = 0;
1443     return true;
1444   }
1445   THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(),
1446              "current thread is not owner", false);
1447 }
1448 
1449 static inline bool is_excluded(const Klass* monitor_klass) {
1450   assert(monitor_klass != nullptr, "invariant");
1451   NOT_JFR_RETURN_(false);
1452   JFR_ONLY(return vmSymbols::jfr_chunk_rotation_monitor() == monitor_klass->name();)
1453 }
1454 
1455 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1456                                     ObjectMonitor* monitor,
1457                                     uint64_t notifier_tid,
1458                                     jlong timeout,
1459                                     bool timedout) {
1460   assert(event != nullptr, "invariant");
1461   assert(monitor != nullptr, "invariant");
1462   const Klass* monitor_klass = monitor->object()->klass();

1469   // time and with the same address are likely (but not guaranteed) to
1470   // belong to the same object.
1471   event->set_address((uintptr_t)monitor);
1472   event->set_notifier(notifier_tid);
1473   event->set_timedOut(timedout);
1474   event->commit();
1475 }
1476 
1477 // -----------------------------------------------------------------------------
1478 // Wait/Notify/NotifyAll
1479 //
1480 // Note: a subset of changes to ObjectMonitor::wait()
1481 // will need to be replicated in complete_exit
1482 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1483   JavaThread* current = THREAD;
1484 
1485   assert(InitDone, "Unexpectedly not initialized");
1486 
1487   CHECK_OWNER();  // Throws IMSE if not owner.
1488 








1489   EventJavaMonitorWait event;
1490 
1491   // check for a pending interrupt
1492   if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1493     // post monitor waited event.  Note that this is past-tense, we are done waiting.
1494     if (JvmtiExport::should_post_monitor_waited()) {
1495       // Note: 'false' parameter is passed here because the
1496       // wait was not timed out due to thread interrupt.
1497       JvmtiExport::post_monitor_waited(current, this, false);
1498 
1499       // In this short circuit of the monitor wait protocol, the
1500       // current thread never drops ownership of the monitor and
1501       // never gets added to the wait queue so the current thread
1502       // cannot be made the successor. This means that the
1503       // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1504       // consume an unpark() meant for the ParkEvent associated with
1505       // this ObjectMonitor.
1506     }
1507     if (event.should_commit()) {
1508       post_monitor_wait_event(&event, this, 0, millis, false);

1523   current->_ParkEvent->reset();
1524   OrderAccess::fence();          // ST into Event; membar ; LD interrupted-flag
1525 
1526   // Enter the waiting queue, which is a circular doubly linked list in this case
1527   // but it could be a priority queue or any data structure.
1528   // _WaitSetLock protects the wait queue.  Normally the wait queue is accessed only
1529   // by the owner of the monitor *except* in the case where park()
1530   // returns because of a timeout of interrupt.  Contention is exceptionally rare
1531   // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1532 
1533   Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1534   AddWaiter(&node);
1535   Thread::SpinRelease(&_WaitSetLock);
1536 
1537   _Responsible = nullptr;
1538 
1539   intx save = _recursions;     // record the old recursion count
1540   _waiters++;                  // increment the number of waiters
1541   _recursions = 0;             // set the recursion level to be 1
1542   exit(current);               // exit the monitor
1543   guarantee(owner_raw() != current, "invariant");
1544 
1545   // The thread is on the WaitSet list - now park() it.
1546   // On MP systems it's conceivable that a brief spin before we park
1547   // could be profitable.
1548   //
1549   // TODO-FIXME: change the following logic to a loop of the form
1550   //   while (!timeout && !interrupted && _notified == 0) park()
1551 
1552   int ret = OS_OK;
1553   int WasNotified = 0;
1554 
1555   // Need to check interrupt state whilst still _thread_in_vm
1556   bool interrupted = interruptible && current->is_interrupted(false);
1557 
1558   { // State transition wrappers
1559     OSThread* osthread = current->osthread();
1560     OSThreadWaitState osts(osthread, true);
1561 
1562     assert(current->thread_state() == _thread_in_vm, "invariant");
1563 

1632         // consume the unpark() that was done when the successor was
1633         // set because the same ParkEvent is shared between Java
1634         // monitors and JVM/TI RawMonitors (for now).
1635         //
1636         // We redo the unpark() to ensure forward progress, i.e., we
1637         // don't want all pending threads hanging (parked) with none
1638         // entering the unlocked monitor.
1639         node._event->unpark();
1640       }
1641     }
1642 
1643     if (event.should_commit()) {
1644       post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1645     }
1646 
1647     OrderAccess::fence();
1648 
1649     assert(current->_Stalled != 0, "invariant");
1650     current->_Stalled = 0;
1651 
1652     assert(owner_raw() != current, "invariant");
1653     ObjectWaiter::TStates v = node.TState;
1654     if (v == ObjectWaiter::TS_RUN) {
1655       enter(current);
1656     } else {
1657       guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1658       ReenterI(current, &node);
1659       node.wait_reenter_end(this);
1660     }
1661 
1662     // current has reacquired the lock.
1663     // Lifecycle - the node representing current must not appear on any queues.
1664     // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1665     // want residual elements associated with this thread left on any lists.
1666     guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1667     assert(owner_raw() == current, "invariant");
1668     assert(_succ != current, "invariant");
1669   } // OSThreadWaitState()
1670 
1671   current->set_current_waiting_monitor(nullptr);
1672 
1673   guarantee(_recursions == 0, "invariant");
1674   int relock_count = JvmtiDeferredUpdates::get_and_reset_relock_count_after_wait(current);
1675   _recursions =   save          // restore the old recursion count
1676                 + relock_count; //  increased by the deferred relock count
1677   current->inc_held_monitor_count(relock_count); // Deopt never entered these counts.
1678   _waiters--;             // decrement the number of waiters
1679 
1680   // Verify a few postconditions
1681   assert(owner_raw() == current, "invariant");
1682   assert(_succ != current, "invariant");
1683   assert(object()->mark() == markWord::encode(this), "invariant");
1684 
1685   // check if the notification happened
1686   if (!WasNotified) {
1687     // no, it could be timeout or Thread.interrupt() or both
1688     // check for interrupt event, otherwise it is timeout
1689     if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1690       THROW(vmSymbols::java_lang_InterruptedException());
1691     }
1692   }
1693 
1694   // NOTE: Spurious wake up will be consider as timeout.
1695   // Monitor notify has precedence over thread interrupt.
1696 }
1697 
1698 
1699 // Consider:
1700 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1701 // then instead of transferring a thread from the WaitSet to the EntryList

1899   // sample, just in case the system load, parallelism, contention, or lock
1900   // modality changed.
1901   //
1902   // Consider the following alternative:
1903   // Periodically set _SpinDuration = _SpinLimit and try a long/full
1904   // spin attempt.  "Periodically" might mean after a tally of
1905   // the # of failed spin attempts (or iterations) reaches some threshold.
1906   // This takes us into the realm of 1-out-of-N spinning, where we
1907   // hold the duration constant but vary the frequency.
1908 
1909   ctr = _SpinDuration;
1910   if (ctr <= 0) return 0;
1911 
1912   // We're good to spin ... spin ingress.
1913   // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1914   // when preparing to LD...CAS _owner, etc and the CAS is likely
1915   // to succeed.
1916   if (_succ == nullptr) {
1917     _succ = current;
1918   }
1919   Thread* prv = nullptr;
1920 
1921   // There are three ways to exit the following loop:
1922   // 1.  A successful spin where this thread has acquired the lock.
1923   // 2.  Spin failure with prejudice
1924   // 3.  Spin failure without prejudice
1925 
1926   while (--ctr >= 0) {
1927 
1928     // Periodic polling -- Check for pending GC
1929     // Threads may spin while they're unsafe.
1930     // We don't want spinning threads to delay the JVM from reaching
1931     // a stop-the-world safepoint or to steal cycles from GC.
1932     // If we detect a pending safepoint we abort in order that
1933     // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1934     // this thread, if safe, doesn't steal cycles from GC.
1935     // This is in keeping with the "no loitering in runtime" rule.
1936     // We periodically check to see if there's a safepoint pending.
1937     if ((ctr & 0xFF) == 0) {
1938       // Can't call SafepointMechanism::should_process() since that
1939       // might update the poll values and we could be in a thread_blocked
1940       // state here which is not allowed so just check the poll.
1941       if (SafepointMechanism::local_poll_armed(current)) {
1942         goto Abort;           // abrupt spin egress
1943       }
1944       SpinPause();
1945     }
1946 
1947     // Probe _owner with TATAS
1948     // If this thread observes the monitor transition or flicker
1949     // from locked to unlocked to locked, then the odds that this
1950     // thread will acquire the lock in this spin attempt go down
1951     // considerably.  The same argument applies if the CAS fails
1952     // or if we observe _owner change from one non-null value to
1953     // another non-null value.   In such cases we might abort
1954     // the spin without prejudice or apply a "penalty" to the
1955     // spin count-down variable "ctr", reducing it by 100, say.
1956 
1957     JavaThread* ox = static_cast<JavaThread*>(owner_raw());
1958     if (ox == nullptr) {
1959       ox = static_cast<JavaThread*>(try_set_owner_from(nullptr, current));
1960       if (ox == nullptr) {
1961         // The CAS succeeded -- this thread acquired ownership
1962         // Take care of some bookkeeping to exit spin state.
1963         if (_succ == current) {
1964           _succ = nullptr;
1965         }
1966 
1967         // Increase _SpinDuration :
1968         // The spin was successful (profitable) so we tend toward
1969         // longer spin attempts in the future.
1970         // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
1971         // If we acquired the lock early in the spin cycle it
1972         // makes sense to increase _SpinDuration proportionally.
1973         // Note that we don't clamp SpinDuration precisely at SpinLimit.
1974         int x = _SpinDuration;
1975         if (x < Knob_SpinLimit) {
1976           if (x < Knob_Poverty) x = Knob_Poverty;
1977           _SpinDuration = x + Knob_Bonus;
1978         }
1979         return 1;

2020     // must recheck-retry _owner before parking.  This usually happens
2021     // in the normal usage of TrySpin(), but it's safest
2022     // to make TrySpin() as foolproof as possible.
2023     OrderAccess::fence();
2024     if (TryLock(current) > 0) return 1;
2025   }
2026   return 0;
2027 }
2028 
2029 
2030 // -----------------------------------------------------------------------------
2031 // WaitSet management ...
2032 
2033 ObjectWaiter::ObjectWaiter(JavaThread* current) {
2034   _next     = nullptr;
2035   _prev     = nullptr;
2036   _notified = 0;
2037   _notifier_tid = 0;
2038   TState    = TS_RUN;
2039   _thread   = current;
2040   _event    = _thread->_ParkEvent;
2041   _active   = false;
2042   assert(_event != nullptr, "invariant");
2043 }
2044 




2045 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2046   _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(_thread, mon);
2047 }
2048 
2049 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2050   JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(_thread, _active);
2051 }
2052 
2053 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2054   assert(node != nullptr, "should not add null node");
2055   assert(node->_prev == nullptr, "node already in list");
2056   assert(node->_next == nullptr, "node already in list");
2057   // put node at end of queue (circular doubly linked list)
2058   if (_WaitSet == nullptr) {
2059     _WaitSet = node;
2060     node->_prev = node;
2061     node->_next = node;
2062   } else {
2063     ObjectWaiter* head = _WaitSet;
2064     ObjectWaiter* tail = head->_prev;

2140   {                                                                       \
2141     n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,  \
2142                                          CHECK);                          \
2143   }
2144     NEWPERFCOUNTER(_sync_Inflations);
2145     NEWPERFCOUNTER(_sync_Deflations);
2146     NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2147     NEWPERFCOUNTER(_sync_FutileWakeups);
2148     NEWPERFCOUNTER(_sync_Parks);
2149     NEWPERFCOUNTER(_sync_Notifications);
2150     NEWPERFVARIABLE(_sync_MonExtant);
2151 #undef NEWPERFCOUNTER
2152 #undef NEWPERFVARIABLE
2153   }
2154 
2155   _oop_storage = OopStorageSet::create_weak("ObjectSynchronizer Weak", mtSynchronizer);
2156 
2157   DEBUG_ONLY(InitDone = true;)
2158 }
2159 





2160 void ObjectMonitor::print_on(outputStream* st) const {
2161   // The minimal things to print for markWord printing, more can be added for debugging and logging.
2162   st->print("{contentions=0x%08x,waiters=0x%08x"
2163             ",recursions=" INTX_FORMAT ",owner=" INTPTR_FORMAT "}",
2164             contentions(), waiters(), recursions(),
2165             p2i(owner()));
2166 }
2167 void ObjectMonitor::print() const { print_on(tty); }
2168 
2169 #ifdef ASSERT
2170 // Print the ObjectMonitor like a debugger would:
2171 //
2172 // (ObjectMonitor) 0x00007fdfb6012e40 = {
2173 //   _header = 0x0000000000000001
2174 //   _object = 0x000000070ff45fd0
2175 //   _pad_buf0 = {
2176 //     [0] = '\0'
2177 //     ...
2178 //     [43] = '\0'
2179 //   }

  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/globals.hpp"
  43 #include "runtime/handles.inline.hpp"
  44 #include "runtime/interfaceSupport.inline.hpp"
  45 #include "runtime/javaThread.inline.hpp"
  46 #include "runtime/mutexLocker.hpp"
  47 #include "runtime/objectMonitor.hpp"
  48 #include "runtime/objectMonitor.inline.hpp"
  49 #include "runtime/orderAccess.hpp"
  50 #include "runtime/osThread.hpp"
  51 #include "runtime/perfData.hpp"
  52 #include "runtime/safefetch.hpp"
  53 #include "runtime/safepointMechanism.inline.hpp"
  54 #include "runtime/sharedRuntime.hpp"
  55 #include "runtime/threads.hpp"
  56 #include "services/threadService.hpp"
  57 #include "utilities/dtrace.hpp"
  58 #include "utilities/globalDefinitions.hpp"
  59 #include "utilities/macros.hpp"
  60 #include "utilities/preserveException.hpp"
  61 #if INCLUDE_JFR
  62 #include "jfr/support/jfrFlush.hpp"
  63 #endif
  64 
  65 #ifdef DTRACE_ENABLED
  66 
  67 // Only bother with this argument setup if dtrace is available
  68 // TODO-FIXME: probes should not fire when caller is _blocked.  assert() accordingly.
  69 
  70 
  71 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread)                           \
  72   char* bytes = nullptr;                                                   \
  73   int len = 0;                                                             \
  74   jlong jtid = SharedRuntime::get_java_tid(thread);                        \
  75   Symbol* klassname = obj->klass()->name();                                \

 109 
 110 #endif // ndef DTRACE_ENABLED
 111 
 112 // Tunables ...
 113 // The knob* variables are effectively final.  Once set they should
 114 // never be modified hence.  Consider using __read_mostly with GCC.
 115 
 116 int ObjectMonitor::Knob_SpinLimit    = 5000;    // derived by an external tool -
 117 
 118 static int Knob_Bonus               = 100;     // spin success bonus
 119 static int Knob_BonusB              = 100;     // spin success bonus
 120 static int Knob_Penalty             = 200;     // spin failure penalty
 121 static int Knob_Poverty             = 1000;
 122 static int Knob_FixedSpin           = 0;
 123 static int Knob_PreSpin             = 10;      // 20-100 likely better
 124 
 125 DEBUG_ONLY(static volatile bool InitDone = false;)
 126 
 127 OopStorage* ObjectMonitor::_oop_storage = nullptr;
 128 
 129 OopHandle ObjectMonitor::_vthread_cxq_head;
 130 ParkEvent* ObjectMonitor::_vthread_unparker_ParkEvent = nullptr;
 131 
 132 // -----------------------------------------------------------------------------
 133 // Theory of operations -- Monitors lists, thread residency, etc:
 134 //
 135 // * A thread acquires ownership of a monitor by successfully
 136 //   CAS()ing the _owner field from null to non-null.
 137 //
 138 // * Invariant: A thread appears on at most one monitor list --
 139 //   cxq, EntryList or WaitSet -- at any one time.
 140 //
 141 // * Contending threads "push" themselves onto the cxq with CAS
 142 //   and then spin/park.
 143 //
 144 // * After a contending thread eventually acquires the lock it must
 145 //   dequeue itself from either the EntryList or the cxq.
 146 //
 147 // * The exiting thread identifies and unparks an "heir presumptive"
 148 //   tentative successor thread on the EntryList.  Critically, the
 149 //   exiting thread doesn't unlink the successor thread from the EntryList.
 150 //   After having been unparked, the wakee will recontend for ownership of
 151 //   the monitor.   The successor (wakee) will either acquire the lock or

 249     switch (jt->thread_state()) {
 250     case _thread_in_vm:    // the usual case
 251     case _thread_in_Java:  // during deopt
 252       break;
 253     default:
 254       fatal("called from an unsafe thread state");
 255     }
 256     assert(jt->is_active_Java_thread(), "must be active JavaThread");
 257   } else {
 258     // However, ThreadService::get_current_contended_monitor()
 259     // can call here via the VMThread so sanity check it.
 260     assert(self->is_VM_thread(), "must be");
 261   }
 262 #endif // ASSERT
 263 }
 264 
 265 ObjectMonitor::ObjectMonitor(oop object) :
 266   _header(markWord::zero()),
 267   _object(_oop_storage, object),
 268   _owner(nullptr),
 269   _stack_locker(nullptr),
 270   _previous_owner_tid(0),
 271   _next_om(nullptr),
 272   _recursions(0),
 273   _EntryList(nullptr),
 274   _cxq(nullptr),
 275   _succ(nullptr),
 276   _Responsible(nullptr),
 277   _Spinner(0),
 278   _SpinDuration(ObjectMonitor::Knob_SpinLimit),
 279   _contentions(0),
 280   _WaitSet(nullptr),
 281   _waiters(0),
 282   _WaitSetLock(0)
 283 { }
 284 
 285 ObjectMonitor::~ObjectMonitor() {
 286   _object.release(_oop_storage);
 287 }
 288 
 289 oop ObjectMonitor::object() const {

 318 
 319 // -----------------------------------------------------------------------------
 320 // Enter support
 321 
 322 bool ObjectMonitor::enter_for(JavaThread* locking_thread) {
 323   // Used by ObjectSynchronizer::enter_for to enter for another thread.
 324   // The monitor is private to or already owned by locking_thread which must be suspended.
 325   // So this code may only contend with deflation.
 326   assert(locking_thread == Thread::current() || locking_thread->is_obj_deopt_suspend(), "must be");
 327 
 328   // Block out deflation as soon as possible.
 329   add_to_contentions(1);
 330 
 331   bool success = false;
 332   if (!is_being_async_deflated()) {
 333     void* prev_owner = try_set_owner_from(nullptr, locking_thread);
 334 
 335     if (prev_owner == nullptr) {
 336       assert(_recursions == 0, "invariant");
 337       success = true;
 338     } else if (prev_owner == owner_for(locking_thread)) {
 339       _recursions++;
 340       success = true;
 341     } else if (prev_owner == DEFLATER_MARKER) {
 342       // Racing with deflation.
 343       prev_owner = try_set_owner_from(DEFLATER_MARKER, locking_thread);
 344       if (prev_owner == DEFLATER_MARKER) {
 345         // Cancelled deflation. Increment contentions as part of the deflation protocol.
 346         add_to_contentions(1);
 347         success = true;
 348       } else if (prev_owner == nullptr) {
 349         // At this point we cannot race with deflation as we have both incremented
 350         // contentions, seen contention > 0 and seen a DEFLATER_MARKER.
 351         // success will only be false if this races with something other than
 352         // deflation.
 353         prev_owner = try_set_owner_from(nullptr, locking_thread);
 354         success = prev_owner == nullptr;
 355       }





 356     }
 357     assert(success, "Failed to enter_for: locking_thread=" INTPTR_FORMAT
 358            ", this=" INTPTR_FORMAT "{owner=" INTPTR_FORMAT "}, observed owner: " INTPTR_FORMAT,
 359            p2i(locking_thread), p2i(this), p2i(owner_raw()), p2i(prev_owner));
 360   } else {
 361     // Async deflation is in progress and our contentions increment
 362     // above lost the race to async deflation. Undo the work and
 363     // force the caller to retry.
 364     const oop l_object = object();
 365     if (l_object != nullptr) {
 366       // Attempt to restore the header/dmw to the object's header so that
 367       // we only retry once if the deflater thread happens to be slow.
 368       install_displaced_markword_in_object(l_object);
 369     }
 370   }
 371 
 372   add_to_contentions(-1);
 373 
 374   assert(!success || is_owner(locking_thread), "must be");
 375 
 376   return success;
 377 }
 378 
 379 bool ObjectMonitor::enter(JavaThread* current) {
 380   assert(current == JavaThread::current(), "must be");
 381   // The following code is ordered to check the most common cases first
 382   // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
 383 
 384   void* cur = try_set_owner_from(nullptr, current);
 385   if (cur == nullptr) {
 386     assert(_recursions == 0, "invariant");
 387     return true;
 388   }
 389 
 390   if (cur == owner_for(current)) {
 391     // TODO-FIXME: check for integer overflow!  BUGID 6557169.
 392     _recursions++;
 393     return true;
 394   }
 395 







 396   // We've encountered genuine contention.
 397   assert(current->_Stalled == 0, "invariant");
 398   current->_Stalled = intptr_t(this);
 399 
 400   // Try one round of spinning *before* enqueueing current
 401   // and before going through the awkward and expensive state
 402   // transitions.  The following spin is strictly optional ...
 403   // Note that if we acquire the monitor from an initial spin
 404   // we forgo posting JVMTI events and firing DTRACE probes.
 405   if (TrySpin(current) > 0) {
 406     assert(owner_raw() == owner_for(current), "must be current: owner=" INTPTR_FORMAT, p2i(owner_raw()));
 407     assert(_recursions == 0, "must be 0: recursions=" INTX_FORMAT, _recursions);
 408     assert(object()->mark() == markWord::encode(this),
 409            "object mark must match encoded this: mark=" INTPTR_FORMAT
 410            ", encoded this=" INTPTR_FORMAT, object()->mark().value(),
 411            markWord::encode(this).value());
 412     current->_Stalled = 0;
 413     return true;
 414   }
 415 
 416   assert(owner_raw() != owner_for(current), "invariant");
 417   assert(_succ != current, "invariant");
 418   assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
 419   assert(current->thread_state() != _thread_blocked, "invariant");
 420 
 421   // Keep track of contention for JVM/TI and M&M queries.
 422   add_to_contentions(1);
 423   if (is_being_async_deflated()) {
 424     // Async deflation is in progress and our contentions increment
 425     // above lost the race to async deflation. Undo the work and
 426     // force the caller to retry.
 427     const oop l_object = object();
 428     if (l_object != nullptr) {
 429       // Attempt to restore the header/dmw to the object's header so that
 430       // we only retry once if the deflater thread happens to be slow.
 431       install_displaced_markword_in_object(l_object);
 432     }
 433     current->_Stalled = 0;
 434     add_to_contentions(-1);
 435     return false;
 436   }

 445     event.set_address((uintptr_t)this);
 446   }
 447 
 448   { // Change java thread status to indicate blocked on monitor enter.
 449     JavaThreadBlockedOnMonitorEnterState jtbmes(current, this);
 450 
 451     assert(current->current_pending_monitor() == nullptr, "invariant");
 452     current->set_current_pending_monitor(this);
 453 
 454     DTRACE_MONITOR_PROBE(contended__enter, this, object(), current);
 455     if (JvmtiExport::should_post_monitor_contended_enter()) {
 456       JvmtiExport::post_monitor_contended_enter(current, this);
 457 
 458       // The current thread does not yet own the monitor and does not
 459       // yet appear on any queues that would get it made the successor.
 460       // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
 461       // handler cannot accidentally consume an unpark() meant for the
 462       // ParkEvent associated with this ObjectMonitor.
 463     }
 464 
 465 #ifdef LOOM_MONITOR_SUPPORT
 466     ContinuationEntry* ce = current->last_continuation();
 467     if (ce != nullptr && ce->is_virtual_thread() && current->is_on_monitorenter()) {
 468       int result = Continuation::try_preempt(current, ce->cont_oop(current));
 469       if (result == freeze_ok) {
 470         bool acquired = HandlePreemptedVThread(current);
 471         DEBUG_ONLY(int state = java_lang_VirtualThread::state(current->vthread()));
 472         assert((acquired && current->preemption_cancelled() && state == java_lang_VirtualThread::RUNNING) ||
 473                (!acquired && !current->preemption_cancelled() && state == java_lang_VirtualThread::BLOCKING), "invariant");
 474         return true;
 475       }
 476       if (result == freeze_pinned_native || result == freeze_pinned_cs) {
 477         EventVirtualThreadPinned e;
 478         if (e.should_commit()) {
 479           e.commit();
 480         }
 481       }
 482     }
 483 #endif
 484 
 485     OSThreadContendState osts(current->osthread());
 486 
 487     assert(current->thread_state() == _thread_in_vm, "invariant");
 488 
 489     for (;;) {
 490       ExitOnSuspend eos(this);
 491       {
 492         ThreadBlockInVMPreprocess<ExitOnSuspend> tbivs(current, eos, true /* allow_suspend */);
 493         EnterI(current);
 494         current->set_current_pending_monitor(nullptr);
 495         // We can go to a safepoint at the end of this block. If we
 496         // do a thread dump during that safepoint, then this thread will show
 497         // as having "-locked" the monitor, but the OS and java.lang.Thread
 498         // states will still report that the thread is blocked trying to
 499         // acquire it.
 500         // If there is a suspend request, ExitOnSuspend will exit the OM
 501         // and set the OM as pending.
 502       }
 503       if (!eos.exited()) {
 504         // ExitOnSuspend did not exit the OM
 505         assert(owner_raw() == owner_for(current), "invariant");
 506         break;
 507       }
 508     }
 509 
 510     // We've just gotten past the enter-check-for-suspend dance and we now own
 511     // the monitor free and clear.
 512   }
 513 
 514   add_to_contentions(-1);
 515   assert(contentions() >= 0, "must not be negative: contentions=%d", contentions());
 516   current->_Stalled = 0;
 517 
 518   // Must either set _recursions = 0 or ASSERT _recursions == 0.
 519   assert(_recursions == 0, "invariant");
 520   assert(owner_raw() == owner_for(current), "invariant");
 521   assert(_succ != current, "invariant");
 522   assert(object()->mark() == markWord::encode(this), "invariant");
 523 
 524   // The thread -- now the owner -- is back in vm mode.
 525   // Report the glorious news via TI,DTrace and jvmstat.
 526   // The probe effect is non-trivial.  All the reportage occurs
 527   // while we hold the monitor, increasing the length of the critical
 528   // section.  Amdahl's parallel speedup law comes vividly into play.
 529   //
 530   // Another option might be to aggregate the events (thread local or
 531   // per-monitor aggregation) and defer reporting until a more opportune
 532   // time -- such as next time some thread encounters contention but has
 533   // yet to acquire the lock.  While spinning that thread could
 534   // spinning we could increment JVMStat counters, etc.
 535 
 536   DTRACE_MONITOR_PROBE(contended__entered, this, object(), current);
 537   if (JvmtiExport::should_post_monitor_contended_entered()) {
 538     JvmtiExport::post_monitor_contended_entered(current, this);
 539 
 540     // The current thread already owns the monitor and is not going to

 575 // makes contentions negative as signals to contending threads that
 576 // an async deflation is in progress. There are a number of checks
 577 // as part of the protocol to make sure that the calling thread has
 578 // not lost the race to a contending thread.
 579 //
 580 // The ObjectMonitor has been successfully async deflated when:
 581 //   (contentions < 0)
 582 // Contending threads that see that condition know to retry their operation.
 583 //
 584 bool ObjectMonitor::deflate_monitor() {
 585   if (is_busy()) {
 586     // Easy checks are first - the ObjectMonitor is busy so no deflation.
 587     return false;
 588   }
 589 
 590   const oop obj = object_peek();
 591 
 592   if (obj == nullptr) {
 593     // If the object died, we can recycle the monitor without racing with
 594     // Java threads. The GC already broke the association with the object.
 595     set_owner_from_raw(nullptr, DEFLATER_MARKER);
 596     assert(contentions() >= 0, "must be non-negative: contentions=%d", contentions());
 597     _contentions = INT_MIN; // minimum negative int
 598   } else {
 599     // Attempt async deflation protocol.
 600 
 601     // Set a null owner to DEFLATER_MARKER to force any contending thread
 602     // through the slow path. This is just the first part of the async
 603     // deflation dance.
 604     if (try_set_owner_from_raw(nullptr, DEFLATER_MARKER) != nullptr) {
 605       // The owner field is no longer null so we lost the race since the
 606       // ObjectMonitor is now busy.
 607       return false;
 608     }
 609 
 610     if (contentions() > 0 || _waiters != 0) {
 611       // Another thread has raced to enter the ObjectMonitor after
 612       // is_busy() above or has already entered and waited on
 613       // it which makes it busy so no deflation. Restore owner to
 614       // null if it is still DEFLATER_MARKER.
 615       if (try_set_owner_from_raw(DEFLATER_MARKER, nullptr) != DEFLATER_MARKER) {
 616         // Deferred decrement for the JT EnterI() that cancelled the async deflation.
 617         add_to_contentions(-1);
 618       }
 619       return false;
 620     }
 621 
 622     // Make a zero contentions field negative to force any contending threads
 623     // to retry. This is the second part of the async deflation dance.
 624     if (Atomic::cmpxchg(&_contentions, 0, INT_MIN) != 0) {
 625       // Contentions was no longer 0 so we lost the race since the
 626       // ObjectMonitor is now busy. Restore owner to null if it is
 627       // still DEFLATER_MARKER:
 628       if (try_set_owner_from_raw(DEFLATER_MARKER, nullptr) != DEFLATER_MARKER) {
 629         // Deferred decrement for the JT EnterI() that cancelled the async deflation.
 630         add_to_contentions(-1);
 631       }
 632       return false;
 633     }
 634   }
 635 
 636   // Sanity checks for the races:
 637   guarantee(owner_is_DEFLATER_MARKER(), "must be deflater marker");
 638   guarantee(contentions() < 0, "must be negative: contentions=%d",
 639             contentions());
 640   guarantee(_waiters == 0, "must be 0: waiters=%d", _waiters);
 641   guarantee(_cxq == nullptr, "must be no contending threads: cxq="
 642             INTPTR_FORMAT, p2i(_cxq));
 643   guarantee(_EntryList == nullptr,
 644             "must be no entering threads: EntryList=" INTPTR_FORMAT,
 645             p2i(_EntryList));
 646 
 647   if (obj != nullptr) {
 648     if (log_is_enabled(Trace, monitorinflation)) {

 704     log_info(monitorinflation)("install_displaced_markword_in_object: "
 705                                "failed cas_set_mark: new_mark=" INTPTR_FORMAT
 706                                ", old_mark=" INTPTR_FORMAT ", res=" INTPTR_FORMAT,
 707                                dmw.value(), markWord::encode(this).value(),
 708                                res.value());
 709   }
 710 
 711   // Note: It does not matter which thread restored the header/dmw
 712   // into the object's header. The thread deflating the monitor just
 713   // wanted the object's header restored and it is. The threads that
 714   // detected a race with the deflation process also wanted the
 715   // object's header restored before they retry their operation and
 716   // because it is restored they will only retry once.
 717 }
 718 
 719 // Convert the fields used by is_busy() to a string that can be
 720 // used for diagnostic output.
 721 const char* ObjectMonitor::is_busy_to_string(stringStream* ss) {
 722   ss->print("is_busy: waiters=%d"
 723             ", contentions=%d"
 724             ", owner=" INTPTR_FORMAT
 725             ", cxq=" PTR_FORMAT
 726             ", EntryList=" PTR_FORMAT,
 727             _waiters,
 728             (contentions() > 0 ? contentions() : 0),
 729             owner_is_DEFLATER_MARKER()
 730                 // We report null instead of DEFLATER_MARKER here because is_busy()
 731                 // ignores DEFLATER_MARKER values.
 732                 ? p2i(nullptr)
 733                 : p2i(owner_raw()),
 734             p2i(_cxq),
 735             p2i(_EntryList));
 736   return ss->base();
 737 }
 738 
 739 #define MAX_RECHECK_INTERVAL 1000
 740 
 741 void ObjectMonitor::EnterI(JavaThread* current) {
 742   assert(current->thread_state() == _thread_blocked, "invariant");
 743 
 744   // Try the lock - TATAS
 745   if (TryLock (current) > 0) {
 746     assert(_succ != current, "invariant");
 747     assert(owner_raw() == owner_for(current), "invariant");
 748     assert(_Responsible != current, "invariant");
 749     return;
 750   }
 751 
 752   if (try_set_owner_from(DEFLATER_MARKER, current) == DEFLATER_MARKER) {
 753     // Cancelled the in-progress async deflation by changing owner from
 754     // DEFLATER_MARKER to current. As part of the contended enter protocol,
 755     // contentions was incremented to a positive value before EnterI()
 756     // was called and that prevents the deflater thread from winning the
 757     // last part of the 2-part async deflation protocol. After EnterI()
 758     // returns to enter(), contentions is decremented because the caller
 759     // now owns the monitor. We bump contentions an extra time here to
 760     // prevent the deflater thread from winning the last part of the
 761     // 2-part async deflation protocol after the regular decrement
 762     // occurs in enter(). The deflater thread will decrement contentions
 763     // after it recognizes that the async deflation was cancelled.
 764     add_to_contentions(1);
 765     assert(_succ != current, "invariant");
 766     assert(_Responsible != current, "invariant");
 767     return;
 768   }
 769 
 770   assert(InitDone, "Unexpectedly not initialized");
 771 
 772   // We try one round of spinning *before* enqueueing current.
 773   //
 774   // If the _owner is ready but OFFPROC we could use a YieldTo()
 775   // operation to donate the remainder of this thread's quantum
 776   // to the owner.  This has subtle but beneficial affinity
 777   // effects.
 778 
 779   if (TrySpin(current) > 0) {
 780     assert(owner_raw() == owner_for(current), "invariant");
 781     assert(_succ != current, "invariant");
 782     assert(_Responsible != current, "invariant");
 783     return;
 784   }
 785 
 786   // The Spin failed -- Enqueue and park the thread ...
 787   assert(_succ != current, "invariant");
 788   assert(owner_raw() != owner_for(current), "invariant");
 789   assert(_Responsible != current, "invariant");
 790 
 791   // Enqueue "current" on ObjectMonitor's _cxq.
 792   //
 793   // Node acts as a proxy for current.
 794   // As an aside, if were to ever rewrite the synchronization code mostly
 795   // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
 796   // Java objects.  This would avoid awkward lifecycle and liveness issues,
 797   // as well as eliminate a subset of ABA issues.
 798   // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
 799 
 800   ObjectWaiter node(current);
 801   current->_ParkEvent->reset();
 802   node._prev   = (ObjectWaiter*) 0xBAD;
 803   node.TState  = ObjectWaiter::TS_CXQ;
 804 
 805   // Push "current" onto the front of the _cxq.
 806   // Once on cxq/EntryList, current stays on-queue until it acquires the lock.
 807   // Note that spinning tends to reduce the rate at which threads
 808   // enqueue and dequeue on EntryList|cxq.
 809   ObjectWaiter* nxt;
 810   for (;;) {
 811     node._next = nxt = _cxq;
 812     if (Atomic::cmpxchg(&_cxq, nxt, &node) == nxt) break;
 813 
 814     // Interference - the CAS failed because _cxq changed.  Just retry.
 815     // As an optional optimization we retry the lock.
 816     if (TryLock (current) > 0) {
 817       assert(_succ != current, "invariant");
 818       assert(owner_raw() == owner_for(current), "invariant");
 819       assert(_Responsible != current, "invariant");
 820       return;
 821     }
 822   }
 823 
 824   // Check for cxq|EntryList edge transition to non-null.  This indicates
 825   // the onset of contention.  While contention persists exiting threads
 826   // will use a ST:MEMBAR:LD 1-1 exit protocol.  When contention abates exit
 827   // operations revert to the faster 1-0 mode.  This enter operation may interleave
 828   // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
 829   // arrange for one of the contending thread to use a timed park() operations
 830   // to detect and recover from the race.  (Stranding is form of progress failure
 831   // where the monitor is unlocked but all the contending threads remain parked).
 832   // That is, at least one of the contended threads will periodically poll _owner.
 833   // One of the contending threads will become the designated "Responsible" thread.
 834   // The Responsible thread uses a timed park instead of a normal indefinite park
 835   // operation -- it periodically wakes and checks for and recovers from potential
 836   // strandings admitted by 1-0 exit operations.   We need at most one Responsible
 837   // thread per-monitor at any given moment.  Only threads on cxq|EntryList may
 838   // be responsible for a monitor.

 846 
 847   if (nxt == nullptr && _EntryList == nullptr) {
 848     // Try to assume the role of responsible thread for the monitor.
 849     // CONSIDER:  ST vs CAS vs { if (Responsible==null) Responsible=current }
 850     Atomic::replace_if_null(&_Responsible, current);
 851   }
 852 
 853   // The lock might have been released while this thread was occupied queueing
 854   // itself onto _cxq.  To close the race and avoid "stranding" and
 855   // progress-liveness failure we must resample-retry _owner before parking.
 856   // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
 857   // In this case the ST-MEMBAR is accomplished with CAS().
 858   //
 859   // TODO: Defer all thread state transitions until park-time.
 860   // Since state transitions are heavy and inefficient we'd like
 861   // to defer the state transitions until absolutely necessary,
 862   // and in doing so avoid some transitions ...
 863 
 864   int nWakeups = 0;
 865   int recheckInterval = 1;
 866   bool do_timed_parked = false;
 867 
 868   ContinuationEntry* ce = current->last_continuation();
 869   if (ce != nullptr && ce->is_virtual_thread()) {
 870     do_timed_parked = true;
 871   }
 872 
 873   for (;;) {
 874 
 875     if (TryLock(current) > 0) break;
 876     assert(owner_raw() != owner_for(current), "invariant");
 877 
 878     // park self
 879     if (_Responsible == current || do_timed_parked) {
 880       current->_ParkEvent->park((jlong) recheckInterval);
 881       // Increase the recheckInterval, but clamp the value.
 882       recheckInterval *= 8;
 883       if (recheckInterval > MAX_RECHECK_INTERVAL) {
 884         recheckInterval = MAX_RECHECK_INTERVAL;
 885       }
 886     } else {
 887       current->_ParkEvent->park();
 888     }
 889 
 890     if (TryLock(current) > 0) break;
 891 
 892     if (try_set_owner_from(DEFLATER_MARKER, current) == DEFLATER_MARKER) {
 893       // Cancelled the in-progress async deflation by changing owner from
 894       // DEFLATER_MARKER to current. As part of the contended enter protocol,
 895       // contentions was incremented to a positive value before EnterI()
 896       // was called and that prevents the deflater thread from winning the
 897       // last part of the 2-part async deflation protocol. After EnterI()
 898       // returns to enter(), contentions is decremented because the caller
 899       // now owns the monitor. We bump contentions an extra time here to

 925     // We can find that we were unpark()ed and redesignated _succ while
 926     // we were spinning.  That's harmless.  If we iterate and call park(),
 927     // park() will consume the event and return immediately and we'll
 928     // just spin again.  This pattern can repeat, leaving _succ to simply
 929     // spin on a CPU.
 930 
 931     if (_succ == current) _succ = nullptr;
 932 
 933     // Invariant: after clearing _succ a thread *must* retry _owner before parking.
 934     OrderAccess::fence();
 935   }
 936 
 937   // Egress :
 938   // current has acquired the lock -- Unlink current from the cxq or EntryList.
 939   // Normally we'll find current on the EntryList .
 940   // From the perspective of the lock owner (this thread), the
 941   // EntryList is stable and cxq is prepend-only.
 942   // The head of cxq is volatile but the interior is stable.
 943   // In addition, current.TState is stable.
 944 
 945   assert(owner_raw() == owner_for(current), "invariant");
 946 
 947   UnlinkAfterAcquire(current, &node);
 948   if (_succ == current) _succ = nullptr;
 949 
 950   assert(_succ != current, "invariant");
 951   if (_Responsible == current) {
 952     _Responsible = nullptr;
 953     OrderAccess::fence(); // Dekker pivot-point
 954 
 955     // We may leave threads on cxq|EntryList without a designated
 956     // "Responsible" thread.  This is benign.  When this thread subsequently
 957     // exits the monitor it can "see" such preexisting "old" threads --
 958     // threads that arrived on the cxq|EntryList before the fence, above --
 959     // by LDing cxq|EntryList.  Newly arrived threads -- that is, threads
 960     // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
 961     // non-null and elect a new "Responsible" timer thread.
 962     //
 963     // This thread executes:
 964     //    ST Responsible=null; MEMBAR    (in enter epilogue - here)
 965     //    LD cxq|EntryList               (in subsequent exit)

 981   // STs to monitor meta-data and user-data could reorder with (become
 982   // visible after) the ST in exit that drops ownership of the lock.
 983   // Some other thread could then acquire the lock, but observe inconsistent
 984   // or old monitor meta-data and heap data.  That violates the JMM.
 985   // To that end, the 1-0 exit() operation must have at least STST|LDST
 986   // "release" barrier semantics.  Specifically, there must be at least a
 987   // STST|LDST barrier in exit() before the ST of null into _owner that drops
 988   // the lock.   The barrier ensures that changes to monitor meta-data and data
 989   // protected by the lock will be visible before we release the lock, and
 990   // therefore before some other thread (CPU) has a chance to acquire the lock.
 991   // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
 992   //
 993   // Critically, any prior STs to _succ or EntryList must be visible before
 994   // the ST of null into _owner in the *subsequent* (following) corresponding
 995   // monitorexit.  Recall too, that in 1-0 mode monitorexit does not necessarily
 996   // execute a serializing instruction.
 997 
 998   return;
 999 }
1000 
1001 bool ObjectMonitor::HandlePreemptedVThread(JavaThread* current) {
1002   // Either because we acquire the lock below or because we will preempt the
1003   // vthread clear the _Stalled/_current_pending_monitor field from the current JavaThread.
1004   current->_Stalled = 0;
1005   current->set_current_pending_monitor(nullptr);
1006 
1007   // Try once more after freezing the continuation.
1008   if (TryLock (current) > 0) {
1009     assert(owner_raw() == owner_for(current), "invariant");
1010     assert(_succ != current, "invariant");
1011     assert(_Responsible != current, "invariant");
1012     current->set_preemption_cancelled(true);
1013     add_to_contentions(-1);
1014     return true;
1015   }
1016 
1017   if (try_set_owner_from(DEFLATER_MARKER, current) == DEFLATER_MARKER) {
1018     // Cancelled the in-progress async deflation by changing owner from
1019     // DEFLATER_MARKER to current. As part of the contended enter protocol,
1020     // contentions was incremented to a positive value before this call to
1021     // HandlePreemptedVThread(). We avoid decrementing contentions to
1022     // prevent the deflater thread from winning the last part of the
1023     // 2-part async deflation protocol. The deflater thread will decrement
1024     // contentions after it recognizes that the async deflation was cancelled.
1025     assert(_succ != current, "invariant");
1026     assert(_Responsible != current, "invariant");
1027     current->set_preemption_cancelled(true);
1028     return true;
1029   }
1030 
1031   oop vthread = current->vthread();
1032   assert(java_lang_VirtualThread::state(vthread) == java_lang_VirtualThread::RUNNING, "wrong state for vthread");
1033   java_lang_VirtualThread::set_state(vthread, java_lang_VirtualThread::BLOCKING);
1034 
1035   ObjectWaiter* node = new ObjectWaiter(vthread);
1036   node->_prev   = (ObjectWaiter*) 0xBAD;
1037   node->TState  = ObjectWaiter::TS_CXQ;
1038 
1039   // Push node associated with vthread onto the front of the _cxq.
1040   ObjectWaiter* nxt;
1041   for (;;) {
1042     node->_next = nxt = _cxq;
1043     if (Atomic::cmpxchg(&_cxq, nxt, node) == nxt) break;
1044 
1045     // Interference - the CAS failed because _cxq changed.  Just retry.
1046     // As an optional optimization we retry the lock.
1047     if (TryLock (current) > 0) {
1048       assert(owner_raw() == owner_for(current), "invariant");
1049       assert(_succ != current, "invariant");
1050       assert(_Responsible != current, "invariant");
1051       current->set_preemption_cancelled(true);
1052       java_lang_VirtualThread::set_state(vthread, java_lang_VirtualThread::RUNNING);
1053       add_to_contentions(-1);
1054       delete node;
1055       return true;
1056     }
1057   }
1058 
1059   // We have to try once more since owner could have exited monitor and checked
1060   // _cxq before we added the node to the queue.
1061   if (TryLock(current) > 0) {
1062     assert(owner_raw() == owner_for(current), "invariant");
1063     assert(_Responsible != current, "invariant");
1064     current->set_preemption_cancelled(true);
1065     java_lang_VirtualThread::set_state(vthread, java_lang_VirtualThread::RUNNING);
1066     UnlinkAfterAcquire(current, node, vthread);
1067     delete node;
1068     if (_succ == (JavaThread*)java_lang_Thread::thread_id(vthread)) _succ = nullptr;
1069     add_to_contentions(-1);
1070     return true;
1071   }
1072 
1073   if (nxt == nullptr && _EntryList == nullptr) {
1074     // The C2 unlock() fast path first checks if _cxq and _EntryList are empty and
1075     // if they are it just clears the _owner field. Since we always run the risk of
1076     // having that check happening before we added the node to _cxq and the release
1077     // of the monitor happening after the last TryLock attempt we need to do something
1078     // to avoid stranding. We set the _Responsible field which results in a timed-wait.
1079     if (Atomic::replace_if_null(&_Responsible, (JavaThread*)java_lang_Thread::thread_id(vthread))) {
1080       java_lang_VirtualThread::set_recheckInterval(vthread, 1);
1081     }
1082   }
1083 
1084   return false;
1085 }
1086 
1087 // ReenterI() is a specialized inline form of the latter half of the
1088 // contended slow-path from EnterI().  We use ReenterI() only for
1089 // monitor reentry in wait().
1090 //
1091 // In the future we should reconcile EnterI() and ReenterI().
1092 
1093 void ObjectMonitor::ReenterI(JavaThread* current, ObjectWaiter* currentNode) {
1094   assert(current != nullptr, "invariant");
1095   assert(currentNode != nullptr, "invariant");
1096   assert(currentNode->_thread == current, "invariant");
1097   assert(_waiters > 0, "invariant");
1098   assert(object()->mark() == markWord::encode(this), "invariant");
1099 
1100   assert(current->thread_state() != _thread_blocked, "invariant");
1101 
1102   int nWakeups = 0;
1103   for (;;) {
1104     ObjectWaiter::TStates v = currentNode->TState;
1105     guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1106     assert(owner_raw() != owner_for(current), "invariant");
1107 
1108     if (TryLock(current) > 0) break;
1109     if (TrySpin(current) > 0) break;
1110 
1111     {
1112       OSThreadContendState osts(current->osthread());
1113 
1114       assert(current->thread_state() == _thread_in_vm, "invariant");
1115 
1116       {
1117         ClearSuccOnSuspend csos(this);
1118         ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */);
1119         current->_ParkEvent->park();
1120       }
1121     }
1122 
1123     // Try again, but just so we distinguish between futile wakeups and
1124     // successful wakeups.  The following test isn't algorithmically
1125     // necessary, but it helps us maintain sensible statistics.
1126     if (TryLock(current) > 0) break;

1136     // find that _succ == current.
1137     if (_succ == current) _succ = nullptr;
1138 
1139     // Invariant: after clearing _succ a contending thread
1140     // *must* retry  _owner before parking.
1141     OrderAccess::fence();
1142 
1143     // This PerfData object can be used in parallel with a safepoint.
1144     // See the work around in PerfDataManager::destroy().
1145     OM_PERFDATA_OP(FutileWakeups, inc());
1146   }
1147 
1148   // current has acquired the lock -- Unlink current from the cxq or EntryList .
1149   // Normally we'll find current on the EntryList.
1150   // Unlinking from the EntryList is constant-time and atomic-free.
1151   // From the perspective of the lock owner (this thread), the
1152   // EntryList is stable and cxq is prepend-only.
1153   // The head of cxq is volatile but the interior is stable.
1154   // In addition, current.TState is stable.
1155 
1156   assert(owner_raw() == owner_for(current), "invariant");
1157   assert(object()->mark() == markWord::encode(this), "invariant");
1158   UnlinkAfterAcquire(current, currentNode);
1159   if (_succ == current) _succ = nullptr;
1160   assert(_succ != current, "invariant");
1161   currentNode->TState = ObjectWaiter::TS_RUN;
1162   OrderAccess::fence();      // see comments at the end of EnterI()
1163 }
1164 
1165 void ObjectMonitor::redo_enter(JavaThread* current) {
1166   assert(java_lang_VirtualThread::state(current->vthread()) == java_lang_VirtualThread::RUNNING, "wrong state for vthread");
1167   assert(current->is_in_VTMS_transition(), "must be");
1168 
1169   if (TryLock (current) > 0) {
1170     VThreadEpilog(current);
1171     return;
1172   }
1173 
1174   oop vthread = current->vthread();
1175   if (_succ == (JavaThread*)java_lang_Thread::thread_id(vthread)) _succ = nullptr;
1176 
1177   // Invariant: after clearing _succ a thread *must* retry _owner before parking.
1178   OrderAccess::fence();
1179 
1180   if (TryLock (current) > 0) {
1181     assert(owner_raw() == owner_for(current), "invariant");
1182     VThreadEpilog(current);
1183     return;
1184   }
1185 
1186   // Fast preemption. The JT will read this variable on return to the
1187   // monitorenter_redo stub and will just remove enterSpecial frame
1188   // from the stack and return to Continuation.run()
1189   current->set_preempting(true);
1190 
1191   java_lang_VirtualThread::set_state(vthread, java_lang_VirtualThread::BLOCKING);
1192   if (_Responsible == (JavaThread*)java_lang_Thread::thread_id(vthread)) {
1193     int recheckInterval = java_lang_VirtualThread::recheckInterval(vthread);
1194     assert(recheckInterval >= 1 && recheckInterval <= 6, "invariant");
1195     if (recheckInterval < 6) {
1196       recheckInterval++;
1197       java_lang_VirtualThread::set_recheckInterval(vthread, recheckInterval);
1198     }
1199   } else if (java_lang_VirtualThread::recheckInterval(vthread) > 0) {
1200     // No need to do timed park anymore
1201     java_lang_VirtualThread::set_recheckInterval(vthread, 0);
1202   }
1203 }
1204 
1205 void ObjectMonitor::VThreadEpilog(JavaThread* current) {
1206   assert(owner_raw() == owner_for(current), "invariant");
1207   add_to_contentions(-1);
1208 
1209   oop vthread = current->vthread();
1210   if (java_lang_VirtualThread::recheckInterval(vthread) > 0) {
1211     java_lang_VirtualThread::set_recheckInterval(vthread, 0);
1212   }
1213   int64_t threadid = java_lang_Thread::thread_id(vthread);
1214   if (_succ == (JavaThread*)threadid) _succ = nullptr;
1215   if (_Responsible == (JavaThread*)threadid) {
1216     _Responsible = nullptr;
1217     OrderAccess::fence(); // Dekker pivot-point
1218   }
1219   ObjectWaiter* node = LookupWaiter(threadid);
1220   UnlinkAfterAcquire(current, node, vthread);
1221   delete node;
1222 }
1223 
1224 // By convention we unlink a contending thread from EntryList|cxq immediately
1225 // after the thread acquires the lock in ::enter().  Equally, we could defer
1226 // unlinking the thread until ::exit()-time.
1227 
1228 void ObjectMonitor::UnlinkAfterAcquire(JavaThread* current, ObjectWaiter* currentNode, oop vthread) {
1229   assert(owner_raw() == owner_for(current), "invariant");
1230   assert((currentNode->_thread == current) || (currentNode->_thread == nullptr && currentNode->vthread() == vthread), "invariant");
1231 
1232   if (currentNode->TState == ObjectWaiter::TS_ENTER) {
1233     // Normal case: remove current from the DLL EntryList .
1234     // This is a constant-time operation.
1235     ObjectWaiter* nxt = currentNode->_next;
1236     ObjectWaiter* prv = currentNode->_prev;
1237     if (nxt != nullptr) nxt->_prev = prv;
1238     if (prv != nullptr) prv->_next = nxt;
1239     if (currentNode == _EntryList) _EntryList = nxt;
1240     assert(nxt == nullptr || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
1241     assert(prv == nullptr || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
1242   } else {
1243     assert(currentNode->TState == ObjectWaiter::TS_CXQ, "invariant");
1244     // Inopportune interleaving -- current is still on the cxq.
1245     // This usually means the enqueue of self raced an exiting thread.
1246     // Normally we'll find current near the front of the cxq, so
1247     // dequeueing is typically fast.  If needbe we can accelerate
1248     // this with some MCS/CHL-like bidirectional list hints and advisory
1249     // back-links so dequeueing from the interior will normally operate
1250     // in constant-time.

1270         q = p;
1271         assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
1272       }
1273       assert(v != currentNode, "invariant");
1274       assert(p == currentNode, "Node not found on cxq");
1275       assert(p != _cxq, "invariant");
1276       assert(q != nullptr, "invariant");
1277       assert(q->_next == p, "invariant");
1278       q->_next = p->_next;
1279     }
1280   }
1281 
1282 #ifdef ASSERT
1283   // Diagnostic hygiene ...
1284   currentNode->_prev  = (ObjectWaiter*) 0xBAD;
1285   currentNode->_next  = (ObjectWaiter*) 0xBAD;
1286   currentNode->TState = ObjectWaiter::TS_RUN;
1287 #endif
1288 }
1289 
1290 // Fix this. Save ObjectWaiter* when freezing. Or use hashtable.
1291 ObjectWaiter* ObjectMonitor::LookupWaiter(int64_t threadid) {
1292   ObjectWaiter* p;
1293   for (p = _EntryList; p != nullptr && (!p->is_vthread() || java_lang_Thread::thread_id(p->vthread()) != threadid); p = p->_next) {}
1294   if (p != nullptr) return p;
1295   for (p = _cxq; p != nullptr && (!p->is_vthread() || java_lang_Thread::thread_id(p->vthread()) != threadid); p = p->_next) {}
1296   assert(p != nullptr, "should be on either _cxq or _EntryList");
1297   return p;
1298 }
1299 
1300 // -----------------------------------------------------------------------------
1301 // Exit support
1302 //
1303 // exit()
1304 // ~~~~~~
1305 // Note that the collector can't reclaim the objectMonitor or deflate
1306 // the object out from underneath the thread calling ::exit() as the
1307 // thread calling ::exit() never transitions to a stable state.
1308 // This inhibits GC, which in turn inhibits asynchronous (and
1309 // inopportune) reclamation of "this".
1310 //
1311 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
1312 // There's one exception to the claim above, however.  EnterI() can call
1313 // exit() to drop a lock if the acquirer has been externally suspended.
1314 // In that case exit() is called with _thread_state == _thread_blocked,
1315 // but the monitor's _contentions field is > 0, which inhibits reclamation.
1316 //
1317 // 1-0 exit
1318 // ~~~~~~~~
1319 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of

1339 // exiting thread will notice and unpark the stranded thread, or, (b)
1340 // the timer expires.  If the lock is high traffic then the stranding latency
1341 // will be low due to (a).  If the lock is low traffic then the odds of
1342 // stranding are lower, although the worst-case stranding latency
1343 // is longer.  Critically, we don't want to put excessive load in the
1344 // platform's timer subsystem.  We want to minimize both the timer injection
1345 // rate (timers created/sec) as well as the number of timers active at
1346 // any one time.  (more precisely, we want to minimize timer-seconds, which is
1347 // the integral of the # of active timers at any instant over time).
1348 // Both impinge on OS scalability.  Given that, at most one thread parked on
1349 // a monitor will use a timer.
1350 //
1351 // There is also the risk of a futile wake-up. If we drop the lock
1352 // another thread can reacquire the lock immediately, and we can
1353 // then wake a thread unnecessarily. This is benign, and we've
1354 // structured the code so the windows are short and the frequency
1355 // of such futile wakups is low.
1356 
1357 void ObjectMonitor::exit(JavaThread* current, bool not_suspended) {
1358   void* cur = owner_raw();
1359   if (owner_for(current) != cur) {
1360     // Apparent unbalanced locking ...
1361     // Naively we'd like to throw IllegalMonitorStateException.
1362     // As a practical matter we can neither allocate nor throw an
1363     // exception as ::exit() can be called from leaf routines.
1364     // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
1365     // Upon deeper reflection, however, in a properly run JVM the only
1366     // way we should encounter this situation is in the presence of
1367     // unbalanced JNI locking. TODO: CheckJNICalls.
1368     // See also: CR4414101





1369 #ifdef ASSERT
1370     LogStreamHandle(Error, monitorinflation) lsh;
1371     lsh.print_cr("ERROR: ObjectMonitor::exit(): thread=" INTPTR_FORMAT
1372                   " is exiting an ObjectMonitor it does not own.", p2i(current));
1373     lsh.print_cr("The imbalance is possibly caused by JNI locking.");
1374     print_debug_style_on(&lsh);
1375     assert(false, "Non-balanced monitor enter/exit!");
1376 #endif
1377     return;

1378   }
1379 
1380   if (_recursions != 0) {
1381     _recursions--;        // this is simple recursive enter
1382     return;
1383   }
1384 
1385   // Invariant: after setting Responsible=null an thread must execute
1386   // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
1387   _Responsible = nullptr;
1388 
1389 #if INCLUDE_JFR
1390   // get the owner's thread id for the MonitorEnter event
1391   // if it is enabled and the thread isn't suspended
1392   if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
1393     _previous_owner_tid = JFR_THREAD_ID(current);
1394   }
1395 #endif
1396 
1397   for (;;) {
1398     assert(owner_for(current) == owner_raw(), "invariant");
1399 
1400     // Drop the lock.
1401     // release semantics: prior loads and stores from within the critical section
1402     // must not float (reorder) past the following store that drops the lock.
1403     // Uses a storeload to separate release_store(owner) from the
1404     // successor check. The try_set_owner_from() below uses cmpxchg() so
1405     // we get the fence down there.
1406     release_clear_owner(current);
1407     OrderAccess::storeload();
1408 
1409     if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != nullptr) {
1410       return;
1411     }
1412     // Other threads are blocked trying to acquire the lock.
1413 
1414     // Normally the exiting thread is responsible for ensuring succession,
1415     // but if other successors are ready or other entering threads are spinning
1416     // then this thread can simply store null into _owner and exit without
1417     // waking a successor.  The existence of spinners or ready successors
1418     // guarantees proper succession (liveness).  Responsibility passes to the
1419     // ready or running successors.  The exiting thread delegates the duty.
1420     // More precisely, if a successor already exists this thread is absolved
1421     // of the responsibility of waking (unparking) one.
1422     //
1423     // The _succ variable is critical to reducing futile wakeup frequency.
1424     // _succ identifies the "heir presumptive" thread that has been made

1434     // to drop the lock and then spin briefly to see if a spinner managed
1435     // to acquire the lock.  If so, the exiting thread could exit
1436     // immediately without waking a successor, otherwise the exiting
1437     // thread would need to dequeue and wake a successor.
1438     // (Note that we'd need to make the post-drop spin short, but no
1439     // shorter than the worst-case round-trip cache-line migration time.
1440     // The dropped lock needs to become visible to the spinner, and then
1441     // the acquisition of the lock by the spinner must become visible to
1442     // the exiting thread).
1443 
1444     // It appears that an heir-presumptive (successor) must be made ready.
1445     // Only the current lock owner can manipulate the EntryList or
1446     // drain _cxq, so we need to reacquire the lock.  If we fail
1447     // to reacquire the lock the responsibility for ensuring succession
1448     // falls to the new owner.
1449     //
1450     if (try_set_owner_from(nullptr, current) != nullptr) {
1451       return;
1452     }
1453 
1454     guarantee(owner_raw() == owner_for(current), "invariant");
1455 
1456     ObjectWaiter* w = nullptr;
1457 
1458     w = _EntryList;
1459     if (w != nullptr) {
1460       // I'd like to write: guarantee (w->_thread != current).
1461       // But in practice an exiting thread may find itself on the EntryList.
1462       // Let's say thread T1 calls O.wait().  Wait() enqueues T1 on O's waitset and
1463       // then calls exit().  Exit release the lock by setting O._owner to null.
1464       // Let's say T1 then stalls.  T2 acquires O and calls O.notify().  The
1465       // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1466       // release the lock "O".  T2 resumes immediately after the ST of null into
1467       // _owner, above.  T2 notices that the EntryList is populated, so it
1468       // reacquires the lock and then finds itself on the EntryList.
1469       // Given all that, we have to tolerate the circumstance where "w" is
1470       // associated with current.
1471       assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1472       ExitEpilog(current, w);
1473       return;
1474     }

1511     }
1512 
1513     // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = nullptr
1514     // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1515 
1516     // See if we can abdicate to a spinner instead of waking a thread.
1517     // A primary goal of the implementation is to reduce the
1518     // context-switch rate.
1519     if (_succ != nullptr) continue;
1520 
1521     w = _EntryList;
1522     if (w != nullptr) {
1523       guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1524       ExitEpilog(current, w);
1525       return;
1526     }
1527   }
1528 }
1529 
1530 void ObjectMonitor::ExitEpilog(JavaThread* current, ObjectWaiter* Wakee) {
1531   assert(owner_raw() == owner_for(current), "invariant");
1532 
1533   // Exit protocol:
1534   // 1. ST _succ = wakee
1535   // 2. membar #loadstore|#storestore;
1536   // 2. ST _owner = nullptr
1537   // 3. unpark(wakee)
1538 
1539   oop vthread = nullptr;
1540   if (Wakee->_thread != nullptr) {
1541     // Platform thread case
1542     _succ = Wakee->_thread;
1543   } else {
1544     assert(Wakee->vthread() != nullptr, "invariant");
1545     vthread = Wakee->vthread();
1546     _succ = (JavaThread*)java_lang_Thread::thread_id(vthread);
1547   }
1548   ParkEvent * Trigger = Wakee->_event;
1549 
1550   // Hygiene -- once we've set _owner = nullptr we can't safely dereference Wakee again.
1551   // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1552   // out-of-scope (non-extant).
1553   Wakee  = nullptr;
1554 
1555   // Drop the lock.
1556   // Uses a fence to separate release_store(owner) from the LD in unpark().
1557   release_clear_owner(current);
1558   OrderAccess::fence();
1559 
1560   DTRACE_MONITOR_PROBE(contended__exit, this, object(), current);
1561 
1562   if (vthread == nullptr) {
1563     // Platform thread case
1564     Trigger->unpark();
1565   } else if (java_lang_VirtualThread::set_onWaitingList(vthread, _vthread_cxq_head)) {
1566     Trigger->unpark();
1567   }
1568 
1569   // Maintain stats and report events to JVMTI
1570   OM_PERFDATA_OP(Parks, inc());
1571 }
1572 
1573 // complete_exit exits a lock returning recursion count
1574 // complete_exit requires an inflated monitor
1575 // The _owner field is not always the Thread addr even with an
1576 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1577 // thread due to contention.
1578 intx ObjectMonitor::complete_exit(JavaThread* current) {
1579   assert(InitDone, "Unexpectedly not initialized");
1580 
1581   void* cur = owner_raw();
1582   if (owner_for(current) != cur) {
1583     if (LockingMode == LM_LEGACY && is_stack_locker(current)) {
1584       assert(_recursions == 0, "internal state error");
1585       set_owner_from_BasicLock(current);  // Convert from BasicLock* to Thread*.
1586       _recursions = 0;
1587     }
1588   }
1589 
1590   guarantee(owner_for(current) == owner_raw(), "complete_exit not owner");
1591   intx save = _recursions; // record the old recursion count
1592   _recursions = 0;         // set the recursion level to be 0
1593   exit(current);           // exit the monitor
1594   guarantee(owner_raw() != owner_for(current), "invariant");
1595   return save;
1596 }
1597 
1598 // Checks that the current THREAD owns this monitor and causes an
1599 // immediate return if it doesn't. We don't use the CHECK macro
1600 // because we want the IMSE to be the only exception that is thrown
1601 // from the call site when false is returned. Any other pending
1602 // exception is ignored.
1603 #define CHECK_OWNER()                                                  \
1604   do {                                                                 \
1605     if (!check_owner(THREAD)) {                                        \
1606        assert(HAS_PENDING_EXCEPTION, "expected a pending IMSE here."); \
1607        return;                                                         \
1608      }                                                                 \
1609   } while (false)
1610 
1611 // Returns true if the specified thread owns the ObjectMonitor.
1612 // Otherwise returns false and throws IllegalMonitorStateException
1613 // (IMSE). If there is a pending exception and the specified thread
1614 // is not the owner, that exception will be replaced by the IMSE.
1615 bool ObjectMonitor::check_owner(TRAPS) {
1616   JavaThread* current = THREAD;
1617   void* cur = owner_raw();
1618   if (cur == owner_for(current)) {






1619     return true;
1620   }
1621   THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(),
1622              "current thread is not owner", false);
1623 }
1624 
1625 static inline bool is_excluded(const Klass* monitor_klass) {
1626   assert(monitor_klass != nullptr, "invariant");
1627   NOT_JFR_RETURN_(false);
1628   JFR_ONLY(return vmSymbols::jfr_chunk_rotation_monitor() == monitor_klass->name();)
1629 }
1630 
1631 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1632                                     ObjectMonitor* monitor,
1633                                     uint64_t notifier_tid,
1634                                     jlong timeout,
1635                                     bool timedout) {
1636   assert(event != nullptr, "invariant");
1637   assert(monitor != nullptr, "invariant");
1638   const Klass* monitor_klass = monitor->object()->klass();

1645   // time and with the same address are likely (but not guaranteed) to
1646   // belong to the same object.
1647   event->set_address((uintptr_t)monitor);
1648   event->set_notifier(notifier_tid);
1649   event->set_timedOut(timedout);
1650   event->commit();
1651 }
1652 
1653 // -----------------------------------------------------------------------------
1654 // Wait/Notify/NotifyAll
1655 //
1656 // Note: a subset of changes to ObjectMonitor::wait()
1657 // will need to be replicated in complete_exit
1658 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1659   JavaThread* current = THREAD;
1660 
1661   assert(InitDone, "Unexpectedly not initialized");
1662 
1663   CHECK_OWNER();  // Throws IMSE if not owner.
1664 
1665   ContinuationEntry* ce = current->last_continuation();
1666   if (ce != nullptr && ce->is_virtual_thread()) {
1667     EventVirtualThreadPinned e;
1668     if (e.should_commit()) {
1669       e.commit();
1670     }
1671   }
1672 
1673   EventJavaMonitorWait event;
1674 
1675   // check for a pending interrupt
1676   if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1677     // post monitor waited event.  Note that this is past-tense, we are done waiting.
1678     if (JvmtiExport::should_post_monitor_waited()) {
1679       // Note: 'false' parameter is passed here because the
1680       // wait was not timed out due to thread interrupt.
1681       JvmtiExport::post_monitor_waited(current, this, false);
1682 
1683       // In this short circuit of the monitor wait protocol, the
1684       // current thread never drops ownership of the monitor and
1685       // never gets added to the wait queue so the current thread
1686       // cannot be made the successor. This means that the
1687       // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1688       // consume an unpark() meant for the ParkEvent associated with
1689       // this ObjectMonitor.
1690     }
1691     if (event.should_commit()) {
1692       post_monitor_wait_event(&event, this, 0, millis, false);

1707   current->_ParkEvent->reset();
1708   OrderAccess::fence();          // ST into Event; membar ; LD interrupted-flag
1709 
1710   // Enter the waiting queue, which is a circular doubly linked list in this case
1711   // but it could be a priority queue or any data structure.
1712   // _WaitSetLock protects the wait queue.  Normally the wait queue is accessed only
1713   // by the owner of the monitor *except* in the case where park()
1714   // returns because of a timeout of interrupt.  Contention is exceptionally rare
1715   // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1716 
1717   Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1718   AddWaiter(&node);
1719   Thread::SpinRelease(&_WaitSetLock);
1720 
1721   _Responsible = nullptr;
1722 
1723   intx save = _recursions;     // record the old recursion count
1724   _waiters++;                  // increment the number of waiters
1725   _recursions = 0;             // set the recursion level to be 1
1726   exit(current);               // exit the monitor
1727   guarantee(owner_raw() != owner_for(current), "invariant");
1728 
1729   // The thread is on the WaitSet list - now park() it.
1730   // On MP systems it's conceivable that a brief spin before we park
1731   // could be profitable.
1732   //
1733   // TODO-FIXME: change the following logic to a loop of the form
1734   //   while (!timeout && !interrupted && _notified == 0) park()
1735 
1736   int ret = OS_OK;
1737   int WasNotified = 0;
1738 
1739   // Need to check interrupt state whilst still _thread_in_vm
1740   bool interrupted = interruptible && current->is_interrupted(false);
1741 
1742   { // State transition wrappers
1743     OSThread* osthread = current->osthread();
1744     OSThreadWaitState osts(osthread, true);
1745 
1746     assert(current->thread_state() == _thread_in_vm, "invariant");
1747 

1816         // consume the unpark() that was done when the successor was
1817         // set because the same ParkEvent is shared between Java
1818         // monitors and JVM/TI RawMonitors (for now).
1819         //
1820         // We redo the unpark() to ensure forward progress, i.e., we
1821         // don't want all pending threads hanging (parked) with none
1822         // entering the unlocked monitor.
1823         node._event->unpark();
1824       }
1825     }
1826 
1827     if (event.should_commit()) {
1828       post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1829     }
1830 
1831     OrderAccess::fence();
1832 
1833     assert(current->_Stalled != 0, "invariant");
1834     current->_Stalled = 0;
1835 
1836     assert(owner_raw() != owner_for(current), "invariant");
1837     ObjectWaiter::TStates v = node.TState;
1838     if (v == ObjectWaiter::TS_RUN) {
1839       enter(current);
1840     } else {
1841       guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1842       ReenterI(current, &node);
1843       node.wait_reenter_end(this);
1844     }
1845 
1846     // current has reacquired the lock.
1847     // Lifecycle - the node representing current must not appear on any queues.
1848     // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1849     // want residual elements associated with this thread left on any lists.
1850     guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1851     assert(owner_raw() == owner_for(current), "invariant");
1852     assert(_succ != current, "invariant");
1853   } // OSThreadWaitState()
1854 
1855   current->set_current_waiting_monitor(nullptr);
1856 
1857   guarantee(_recursions == 0, "invariant");
1858   int relock_count = JvmtiDeferredUpdates::get_and_reset_relock_count_after_wait(current);
1859   _recursions =   save          // restore the old recursion count
1860                 + relock_count; //  increased by the deferred relock count
1861   NOT_LOOM_MONITOR_SUPPORT(current->inc_held_monitor_count(relock_count);) // Deopt never entered these counts.
1862   _waiters--;             // decrement the number of waiters
1863 
1864   // Verify a few postconditions
1865   assert(owner_raw() == owner_for(current), "invariant");
1866   assert(_succ != current, "invariant");
1867   assert(object()->mark() == markWord::encode(this), "invariant");
1868 
1869   // check if the notification happened
1870   if (!WasNotified) {
1871     // no, it could be timeout or Thread.interrupt() or both
1872     // check for interrupt event, otherwise it is timeout
1873     if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1874       THROW(vmSymbols::java_lang_InterruptedException());
1875     }
1876   }
1877 
1878   // NOTE: Spurious wake up will be consider as timeout.
1879   // Monitor notify has precedence over thread interrupt.
1880 }
1881 
1882 
1883 // Consider:
1884 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1885 // then instead of transferring a thread from the WaitSet to the EntryList

2083   // sample, just in case the system load, parallelism, contention, or lock
2084   // modality changed.
2085   //
2086   // Consider the following alternative:
2087   // Periodically set _SpinDuration = _SpinLimit and try a long/full
2088   // spin attempt.  "Periodically" might mean after a tally of
2089   // the # of failed spin attempts (or iterations) reaches some threshold.
2090   // This takes us into the realm of 1-out-of-N spinning, where we
2091   // hold the duration constant but vary the frequency.
2092 
2093   ctr = _SpinDuration;
2094   if (ctr <= 0) return 0;
2095 
2096   // We're good to spin ... spin ingress.
2097   // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
2098   // when preparing to LD...CAS _owner, etc and the CAS is likely
2099   // to succeed.
2100   if (_succ == nullptr) {
2101     _succ = current;
2102   }
2103   void* prv = nullptr;
2104 
2105   // There are three ways to exit the following loop:
2106   // 1.  A successful spin where this thread has acquired the lock.
2107   // 2.  Spin failure with prejudice
2108   // 3.  Spin failure without prejudice
2109 
2110   while (--ctr >= 0) {
2111 
2112     // Periodic polling -- Check for pending GC
2113     // Threads may spin while they're unsafe.
2114     // We don't want spinning threads to delay the JVM from reaching
2115     // a stop-the-world safepoint or to steal cycles from GC.
2116     // If we detect a pending safepoint we abort in order that
2117     // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
2118     // this thread, if safe, doesn't steal cycles from GC.
2119     // This is in keeping with the "no loitering in runtime" rule.
2120     // We periodically check to see if there's a safepoint pending.
2121     if ((ctr & 0xFF) == 0) {
2122       // Can't call SafepointMechanism::should_process() since that
2123       // might update the poll values and we could be in a thread_blocked
2124       // state here which is not allowed so just check the poll.
2125       if (SafepointMechanism::local_poll_armed(current)) {
2126         goto Abort;           // abrupt spin egress
2127       }
2128       SpinPause();
2129     }
2130 
2131     // Probe _owner with TATAS
2132     // If this thread observes the monitor transition or flicker
2133     // from locked to unlocked to locked, then the odds that this
2134     // thread will acquire the lock in this spin attempt go down
2135     // considerably.  The same argument applies if the CAS fails
2136     // or if we observe _owner change from one non-null value to
2137     // another non-null value.   In such cases we might abort
2138     // the spin without prejudice or apply a "penalty" to the
2139     // spin count-down variable "ctr", reducing it by 100, say.
2140 
2141     void* ox = owner_raw();
2142     if (ox == nullptr) {
2143       ox = try_set_owner_from(nullptr, current);
2144       if (ox == nullptr) {
2145         // The CAS succeeded -- this thread acquired ownership
2146         // Take care of some bookkeeping to exit spin state.
2147         if (_succ == current) {
2148           _succ = nullptr;
2149         }
2150 
2151         // Increase _SpinDuration :
2152         // The spin was successful (profitable) so we tend toward
2153         // longer spin attempts in the future.
2154         // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2155         // If we acquired the lock early in the spin cycle it
2156         // makes sense to increase _SpinDuration proportionally.
2157         // Note that we don't clamp SpinDuration precisely at SpinLimit.
2158         int x = _SpinDuration;
2159         if (x < Knob_SpinLimit) {
2160           if (x < Knob_Poverty) x = Knob_Poverty;
2161           _SpinDuration = x + Knob_Bonus;
2162         }
2163         return 1;

2204     // must recheck-retry _owner before parking.  This usually happens
2205     // in the normal usage of TrySpin(), but it's safest
2206     // to make TrySpin() as foolproof as possible.
2207     OrderAccess::fence();
2208     if (TryLock(current) > 0) return 1;
2209   }
2210   return 0;
2211 }
2212 
2213 
2214 // -----------------------------------------------------------------------------
2215 // WaitSet management ...
2216 
2217 ObjectWaiter::ObjectWaiter(JavaThread* current) {
2218   _next     = nullptr;
2219   _prev     = nullptr;
2220   _notified = 0;
2221   _notifier_tid = 0;
2222   TState    = TS_RUN;
2223   _thread   = current;
2224   _event    = _thread != nullptr ? _thread->_ParkEvent : ObjectMonitor::vthread_unparker_ParkEvent();
2225   _active   = false;
2226   assert(_event != nullptr, "invariant");
2227 }
2228 
2229 ObjectWaiter::ObjectWaiter(oop vthread) : ObjectWaiter((JavaThread*)nullptr) {
2230   _vthread = OopHandle(JavaThread::thread_oop_storage(), vthread);
2231 }
2232 
2233 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2234   _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(_thread, mon);
2235 }
2236 
2237 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2238   JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(_thread, _active);
2239 }
2240 
2241 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2242   assert(node != nullptr, "should not add null node");
2243   assert(node->_prev == nullptr, "node already in list");
2244   assert(node->_next == nullptr, "node already in list");
2245   // put node at end of queue (circular doubly linked list)
2246   if (_WaitSet == nullptr) {
2247     _WaitSet = node;
2248     node->_prev = node;
2249     node->_next = node;
2250   } else {
2251     ObjectWaiter* head = _WaitSet;
2252     ObjectWaiter* tail = head->_prev;

2328   {                                                                       \
2329     n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,  \
2330                                          CHECK);                          \
2331   }
2332     NEWPERFCOUNTER(_sync_Inflations);
2333     NEWPERFCOUNTER(_sync_Deflations);
2334     NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2335     NEWPERFCOUNTER(_sync_FutileWakeups);
2336     NEWPERFCOUNTER(_sync_Parks);
2337     NEWPERFCOUNTER(_sync_Notifications);
2338     NEWPERFVARIABLE(_sync_MonExtant);
2339 #undef NEWPERFCOUNTER
2340 #undef NEWPERFVARIABLE
2341   }
2342 
2343   _oop_storage = OopStorageSet::create_weak("ObjectSynchronizer Weak", mtSynchronizer);
2344 
2345   DEBUG_ONLY(InitDone = true;)
2346 }
2347 
2348 void ObjectMonitor::Initialize2() {
2349   _vthread_cxq_head = OopHandle(JavaThread::thread_oop_storage(), nullptr);
2350   _vthread_unparker_ParkEvent = ParkEvent::Allocate(nullptr);
2351 }
2352 
2353 void ObjectMonitor::print_on(outputStream* st) const {
2354   // The minimal things to print for markWord printing, more can be added for debugging and logging.
2355   st->print("{contentions=0x%08x,waiters=0x%08x"
2356             ",recursions=" INTX_FORMAT ",owner=" INTPTR_FORMAT "}",
2357             contentions(), waiters(), recursions(),
2358             p2i(owner()));
2359 }
2360 void ObjectMonitor::print() const { print_on(tty); }
2361 
2362 #ifdef ASSERT
2363 // Print the ObjectMonitor like a debugger would:
2364 //
2365 // (ObjectMonitor) 0x00007fdfb6012e40 = {
2366 //   _header = 0x0000000000000001
2367 //   _object = 0x000000070ff45fd0
2368 //   _pad_buf0 = {
2369 //     [0] = '\0'
2370 //     ...
2371 //     [43] = '\0'
2372 //   }
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