1 /*
2 * Copyright (c) 1998, 2024, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 #include "precompiled.hpp"
26 #include "classfile/vmSymbols.hpp"
27 #include "gc/shared/oopStorage.hpp"
28 #include "gc/shared/oopStorageSet.hpp"
29 #include "jfr/jfrEvents.hpp"
30 #include "jfr/support/jfrThreadId.hpp"
31 #include "logging/log.hpp"
32 #include "logging/logStream.hpp"
33 #include "memory/allocation.inline.hpp"
34 #include "memory/resourceArea.hpp"
35 #include "oops/markWord.hpp"
36 #include "oops/oop.inline.hpp"
37 #include "oops/oopHandle.inline.hpp"
38 #include "oops/weakHandle.inline.hpp"
39 #include "prims/jvmtiDeferredUpdates.hpp"
40 #include "prims/jvmtiExport.hpp"
41 #include "runtime/atomic.hpp"
42 #include "runtime/globals.hpp"
43 #include "runtime/handles.inline.hpp"
44 #include "runtime/interfaceSupport.inline.hpp"
45 #include "runtime/javaThread.inline.hpp"
46 #include "runtime/lightweightSynchronizer.hpp"
47 #include "runtime/mutexLocker.hpp"
48 #include "runtime/objectMonitor.hpp"
49 #include "runtime/objectMonitor.inline.hpp"
50 #include "runtime/orderAccess.hpp"
51 #include "runtime/osThread.hpp"
52 #include "runtime/perfData.hpp"
53 #include "runtime/safefetch.hpp"
54 #include "runtime/safepointMechanism.inline.hpp"
55 #include "runtime/sharedRuntime.hpp"
56 #include "runtime/synchronizer.hpp"
57 #include "services/threadService.hpp"
58 #include "utilities/debug.hpp"
59 #include "utilities/dtrace.hpp"
60 #include "utilities/globalDefinitions.hpp"
61 #include "utilities/macros.hpp"
62 #include "utilities/preserveException.hpp"
63 #if INCLUDE_JFR
64 #include "jfr/support/jfrFlush.hpp"
65 #endif
66
67 #ifdef DTRACE_ENABLED
68
69 // Only bother with this argument setup if dtrace is available
70 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
71
72
73 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
74 char* bytes = nullptr; \
75 int len = 0; \
76 jlong jtid = SharedRuntime::get_java_tid(thread); \
77 Symbol* klassname = obj->klass()->name(); \
78 if (klassname != nullptr) { \
79 bytes = (char*)klassname->bytes(); \
80 len = klassname->utf8_length(); \
81 }
82
83 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
84 { \
85 if (DTraceMonitorProbes) { \
86 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
87 HOTSPOT_MONITOR_WAIT(jtid, \
88 (monitor), bytes, len, (millis)); \
89 } \
90 }
91
92 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
93 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
94 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
95 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
96 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
97
98 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
99 { \
100 if (DTraceMonitorProbes) { \
101 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
102 HOTSPOT_MONITOR_##probe(jtid, \
103 (uintptr_t)(monitor), bytes, len); \
104 } \
105 }
106
107 #else // ndef DTRACE_ENABLED
108
109 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
110 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
111
112 #endif // ndef DTRACE_ENABLED
113
114 DEBUG_ONLY(static volatile bool InitDone = false;)
115
116 OopStorage* ObjectMonitor::_oop_storage = nullptr;
117
118 // -----------------------------------------------------------------------------
119 // Theory of operations -- Monitors lists, thread residency, etc:
120 //
121 // * A thread acquires ownership of a monitor by successfully
122 // CAS()ing the _owner field from null to non-null.
123 //
124 // * Invariant: A thread appears on at most one monitor list --
125 // cxq, EntryList or WaitSet -- at any one time.
126 //
127 // * Contending threads "push" themselves onto the cxq with CAS
128 // and then spin/park.
129 //
130 // * After a contending thread eventually acquires the lock it must
131 // dequeue itself from either the EntryList or the cxq.
132 //
133 // * The exiting thread identifies and unparks an "heir presumptive"
134 // tentative successor thread on the EntryList. Critically, the
135 // exiting thread doesn't unlink the successor thread from the EntryList.
136 // After having been unparked, the wakee will recontend for ownership of
137 // the monitor. The successor (wakee) will either acquire the lock or
138 // re-park itself.
139 //
140 // Succession is provided for by a policy of competitive handoff.
141 // The exiting thread does _not_ grant or pass ownership to the
142 // successor thread. (This is also referred to as "handoff" succession").
143 // Instead the exiting thread releases ownership and possibly wakes
144 // a successor, so the successor can (re)compete for ownership of the lock.
145 // If the EntryList is empty but the cxq is populated the exiting
146 // thread will drain the cxq into the EntryList. It does so by
147 // by detaching the cxq (installing null with CAS) and folding
148 // the threads from the cxq into the EntryList. The EntryList is
149 // doubly linked, while the cxq is singly linked because of the
150 // CAS-based "push" used to enqueue recently arrived threads (RATs).
151 //
152 // * Concurrency invariants:
153 //
154 // -- only the monitor owner may access or mutate the EntryList.
155 // The mutex property of the monitor itself protects the EntryList
156 // from concurrent interference.
157 // -- Only the monitor owner may detach the cxq.
158 //
159 // * The monitor entry list operations avoid locks, but strictly speaking
160 // they're not lock-free. Enter is lock-free, exit is not.
161 // For a description of 'Methods and apparatus providing non-blocking access
162 // to a resource,' see U.S. Pat. No. 7844973.
163 //
164 // * The cxq can have multiple concurrent "pushers" but only one concurrent
165 // detaching thread. This mechanism is immune from the ABA corruption.
166 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
167 //
168 // * Taken together, the cxq and the EntryList constitute or form a
169 // single logical queue of threads stalled trying to acquire the lock.
170 // We use two distinct lists to improve the odds of a constant-time
171 // dequeue operation after acquisition (in the ::enter() epilogue) and
172 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
173 // A key desideratum is to minimize queue & monitor metadata manipulation
174 // that occurs while holding the monitor lock -- that is, we want to
175 // minimize monitor lock holds times. Note that even a small amount of
176 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
177 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
178 // locks and monitor metadata.
179 //
180 // Cxq points to the set of Recently Arrived Threads attempting entry.
181 // Because we push threads onto _cxq with CAS, the RATs must take the form of
182 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
183 // the unlocking thread notices that EntryList is null but _cxq is != null.
184 //
185 // The EntryList is ordered by the prevailing queue discipline and
186 // can be organized in any convenient fashion, such as a doubly-linked list or
187 // a circular doubly-linked list. Critically, we want insert and delete operations
188 // to operate in constant-time. If we need a priority queue then something akin
189 // to Solaris' sleepq would work nicely. Viz.,
190 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
191 // Queue discipline is enforced at ::exit() time, when the unlocking thread
192 // drains the cxq into the EntryList, and orders or reorders the threads on the
193 // EntryList accordingly.
194 //
195 // Barring "lock barging", this mechanism provides fair cyclic ordering,
196 // somewhat similar to an elevator-scan.
197 //
198 // * The monitor synchronization subsystem avoids the use of native
199 // synchronization primitives except for the narrow platform-specific
200 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
201 // the semantics of park-unpark. Put another way, this monitor implementation
202 // depends only on atomic operations and park-unpark. The monitor subsystem
203 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
204 // underlying OS manages the READY<->RUN transitions.
205 //
206 // * Waiting threads reside on the WaitSet list -- wait() puts
207 // the caller onto the WaitSet.
208 //
209 // * notify() or notifyAll() simply transfers threads from the WaitSet to
210 // either the EntryList or cxq. Subsequent exit() operations will
211 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
212 // it's likely the notifyee would simply impale itself on the lock held
213 // by the notifier.
214 //
215 // * An interesting alternative is to encode cxq as (List,LockByte) where
216 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
217 // variable, like _recursions, in the scheme. The threads or Events that form
218 // the list would have to be aligned in 256-byte addresses. A thread would
219 // try to acquire the lock or enqueue itself with CAS, but exiting threads
220 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
221 // Note that is is *not* word-tearing, but it does presume that full-word
222 // CAS operations are coherent with intermix with STB operations. That's true
223 // on most common processors.
224 //
225 // * See also http://blogs.sun.com/dave
226
227
228 // Check that object() and set_object() are called from the right context:
229 static void check_object_context() {
230 #ifdef ASSERT
231 Thread* self = Thread::current();
232 if (self->is_Java_thread()) {
233 // Mostly called from JavaThreads so sanity check the thread state.
234 JavaThread* jt = JavaThread::cast(self);
235 switch (jt->thread_state()) {
236 case _thread_in_vm: // the usual case
237 case _thread_in_Java: // during deopt
238 break;
239 default:
240 fatal("called from an unsafe thread state");
241 }
242 assert(jt->is_active_Java_thread(), "must be active JavaThread");
243 } else {
244 // However, ThreadService::get_current_contended_monitor()
245 // can call here via the VMThread so sanity check it.
246 assert(self->is_VM_thread(), "must be");
247 }
248 #endif // ASSERT
249 }
250
251 ObjectMonitor::ObjectMonitor(oop object) :
252 _metadata(0),
253 _object(_oop_storage, object),
254 _owner(nullptr),
255 _previous_owner_tid(0),
256 _next_om(nullptr),
257 _recursions(0),
258 _EntryList(nullptr),
259 _cxq(nullptr),
260 _succ(nullptr),
261 _Responsible(nullptr),
262 _SpinDuration(ObjectMonitor::Knob_SpinLimit),
263 _contentions(0),
264 _WaitSet(nullptr),
265 _waiters(0),
266 _WaitSetLock(0)
267 { }
268
269 ObjectMonitor::~ObjectMonitor() {
270 _object.release(_oop_storage);
271 }
272
273 oop ObjectMonitor::object() const {
274 check_object_context();
275 return _object.resolve();
276 }
277
278 void ObjectMonitor::ExitOnSuspend::operator()(JavaThread* current) {
279 if (current->is_suspended()) {
280 _om->_recursions = 0;
281 _om->_succ = nullptr;
282 // Don't need a full fence after clearing successor here because of the call to exit().
283 _om->exit(current, false /* not_suspended */);
284 _om_exited = true;
285
286 current->set_current_pending_monitor(_om);
287 }
288 }
289
290 void ObjectMonitor::ClearSuccOnSuspend::operator()(JavaThread* current) {
291 if (current->is_suspended()) {
292 if (_om->_succ == current) {
293 _om->_succ = nullptr;
294 OrderAccess::fence(); // always do a full fence when successor is cleared
295 }
296 }
297 }
298
299 #define assert_mark_word_consistency() \
300 assert(UseObjectMonitorTable || object()->mark() == markWord::encode(this), \
301 "object mark must match encoded this: mark=" INTPTR_FORMAT \
302 ", encoded this=" INTPTR_FORMAT, object()->mark().value(), \
303 markWord::encode(this).value());
304
305 // -----------------------------------------------------------------------------
306 // Enter support
307
308 bool ObjectMonitor::enter_is_async_deflating() {
309 if (is_being_async_deflated()) {
310 if (!UseObjectMonitorTable) {
311 const oop l_object = object();
312 if (l_object != nullptr) {
313 // Attempt to restore the header/dmw to the object's header so that
314 // we only retry once if the deflater thread happens to be slow.
315 install_displaced_markword_in_object(l_object);
316 }
317 }
318 return true;
319 }
320
321 return false;
322 }
323
324 void ObjectMonitor::enter_for_with_contention_mark(JavaThread* locking_thread, ObjectMonitorContentionMark& contention_mark) {
325 // Used by ObjectSynchronizer::enter_for to enter for another thread.
326 // The monitor is private to or already owned by locking_thread which must be suspended.
327 // So this code may only contend with deflation.
328 assert(locking_thread == Thread::current() || locking_thread->is_obj_deopt_suspend(), "must be");
329 assert(contention_mark._monitor == this, "must be");
330 assert(!is_being_async_deflated(), "must be");
331
332
333 void* prev_owner = try_set_owner_from(nullptr, locking_thread);
334
335 bool success = false;
336
337 if (prev_owner == nullptr) {
338 assert(_recursions == 0, "invariant");
339 success = true;
340 } else if (prev_owner == locking_thread) {
341 _recursions++;
342 success = true;
343 } else if (prev_owner == DEFLATER_MARKER) {
344 // Racing with deflation.
345 prev_owner = try_set_owner_from(DEFLATER_MARKER, locking_thread);
346 if (prev_owner == DEFLATER_MARKER) {
347 // Cancelled deflation. Increment contentions as part of the deflation protocol.
348 add_to_contentions(1);
349 success = true;
350 } else if (prev_owner == nullptr) {
351 // At this point we cannot race with deflation as we have both incremented
352 // contentions, seen contention > 0 and seen a DEFLATER_MARKER.
353 // success will only be false if this races with something other than
354 // deflation.
355 prev_owner = try_set_owner_from(nullptr, locking_thread);
356 success = prev_owner == nullptr;
357 }
358 } else if (LockingMode == LM_LEGACY && locking_thread->is_lock_owned((address)prev_owner)) {
359 assert(_recursions == 0, "must be");
360 _recursions = 1;
361 set_owner_from_BasicLock(prev_owner, locking_thread);
362 success = true;
363 }
364 assert(success, "Failed to enter_for: locking_thread=" INTPTR_FORMAT
365 ", this=" INTPTR_FORMAT "{owner=" INTPTR_FORMAT "}, observed owner: " INTPTR_FORMAT,
366 p2i(locking_thread), p2i(this), p2i(owner_raw()), p2i(prev_owner));
367 }
368
369 bool ObjectMonitor::enter_for(JavaThread* locking_thread) {
370
371 // Block out deflation as soon as possible.
372 ObjectMonitorContentionMark contention_mark(this);
373
374 // Check for deflation.
375 if (enter_is_async_deflating()) {
376 return false;
377 }
378
379 enter_for_with_contention_mark(locking_thread, contention_mark);
380 assert(owner_raw() == locking_thread, "must be");
381 return true;
382 }
383
384 bool ObjectMonitor::try_enter(JavaThread* current) {
385 // TryLock avoids the CAS
386 TryLockResult r = TryLock(current);
387 if (r == TryLockResult::Success) {
388 assert(_recursions == 0, "invariant");
389 return true;
390 }
391
392 if (r == TryLockResult::HasOwner && owner() == current) {
393 _recursions++;
394 return true;
395 }
396
397 void* cur = owner_raw();
398 if (LockingMode == LM_LEGACY && current->is_lock_owned((address)cur)) {
399 assert(_recursions == 0, "internal state error");
400 _recursions = 1;
401 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*.
402 return true;
403 }
404
405 return false;
406 }
407
408 bool ObjectMonitor::spin_enter(JavaThread* current) {
409 assert(current == JavaThread::current(), "must be");
410
411 // Check for recursion.
412 if (try_enter(current)) {
413 return true;
414 }
415
416 // Check for deflation.
417 if (enter_is_async_deflating()) {
418 return false;
419 }
420
421 // We've encountered genuine contention.
422
423 // Do one round of spinning.
424 // Note that if we acquire the monitor from an initial spin
425 // we forgo posting JVMTI events and firing DTRACE probes.
426 if (TrySpin(current)) {
427 assert(owner_raw() == current, "must be current: owner=" INTPTR_FORMAT, p2i(owner_raw()));
428 assert(_recursions == 0, "must be 0: recursions=" INTX_FORMAT, _recursions);
429 assert_mark_word_consistency();
430 return true;
431 }
432
433 return false;
434 }
435
436 bool ObjectMonitor::enter(JavaThread* current) {
437 assert(current == JavaThread::current(), "must be");
438
439 if (spin_enter(current)) {
440 return true;
441 }
442
443 assert(owner_raw() != current, "invariant");
444 assert(_succ != current, "invariant");
445 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
446 assert(current->thread_state() != _thread_blocked, "invariant");
447
448 // Keep is_being_async_deflated stable across the rest of enter
449 ObjectMonitorContentionMark contention_mark(this);
450
451 // Check for deflation.
452 if (enter_is_async_deflating()) {
453 return false;
454 }
455
456 // At this point this ObjectMonitor cannot be deflated, finish contended enter
457 enter_with_contention_mark(current, contention_mark);
458 return true;
459 }
460
461 void ObjectMonitor::enter_with_contention_mark(JavaThread *current, ObjectMonitorContentionMark &cm) {
462 assert(current == JavaThread::current(), "must be");
463 assert(owner_raw() != current, "must be");
464 assert(cm._monitor == this, "must be");
465 assert(!is_being_async_deflated(), "must be");
466
467 JFR_ONLY(JfrConditionalFlush<EventJavaMonitorEnter> flush(current);)
468 EventJavaMonitorEnter event;
469 if (event.is_started()) {
470 event.set_monitorClass(object()->klass());
471 // Set an address that is 'unique enough', such that events close in
472 // time and with the same address are likely (but not guaranteed) to
473 // belong to the same object.
474 event.set_address((uintptr_t)this);
475 }
476
477 { // Change java thread status to indicate blocked on monitor enter.
478 JavaThreadBlockedOnMonitorEnterState jtbmes(current, this);
479
480 assert(current->current_pending_monitor() == nullptr, "invariant");
481 current->set_current_pending_monitor(this);
482
483 DTRACE_MONITOR_PROBE(contended__enter, this, object(), current);
484 if (JvmtiExport::should_post_monitor_contended_enter()) {
485 JvmtiExport::post_monitor_contended_enter(current, this);
486
487 // The current thread does not yet own the monitor and does not
488 // yet appear on any queues that would get it made the successor.
489 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
490 // handler cannot accidentally consume an unpark() meant for the
491 // ParkEvent associated with this ObjectMonitor.
492 }
493
494 OSThreadContendState osts(current->osthread());
495
496 assert(current->thread_state() == _thread_in_vm, "invariant");
497
498 for (;;) {
499 ExitOnSuspend eos(this);
500 {
501 ThreadBlockInVMPreprocess<ExitOnSuspend> tbivs(current, eos, true /* allow_suspend */);
502 EnterI(current);
503 current->set_current_pending_monitor(nullptr);
504 // We can go to a safepoint at the end of this block. If we
505 // do a thread dump during that safepoint, then this thread will show
506 // as having "-locked" the monitor, but the OS and java.lang.Thread
507 // states will still report that the thread is blocked trying to
508 // acquire it.
509 // If there is a suspend request, ExitOnSuspend will exit the OM
510 // and set the OM as pending.
511 }
512 if (!eos.exited()) {
513 // ExitOnSuspend did not exit the OM
514 assert(owner_raw() == current, "invariant");
515 break;
516 }
517 }
518
519 // We've just gotten past the enter-check-for-suspend dance and we now own
520 // the monitor free and clear.
521 }
522
523 assert(contentions() >= 0, "must not be negative: contentions=%d", contentions());
524
525 // Must either set _recursions = 0 or ASSERT _recursions == 0.
526 assert(_recursions == 0, "invariant");
527 assert(owner_raw() == current, "invariant");
528 assert(_succ != current, "invariant");
529 assert_mark_word_consistency();
530
531 // The thread -- now the owner -- is back in vm mode.
532 // Report the glorious news via TI,DTrace and jvmstat.
533 // The probe effect is non-trivial. All the reportage occurs
534 // while we hold the monitor, increasing the length of the critical
535 // section. Amdahl's parallel speedup law comes vividly into play.
536 //
537 // Another option might be to aggregate the events (thread local or
538 // per-monitor aggregation) and defer reporting until a more opportune
539 // time -- such as next time some thread encounters contention but has
540 // yet to acquire the lock. While spinning that thread could
541 // spinning we could increment JVMStat counters, etc.
542
543 DTRACE_MONITOR_PROBE(contended__entered, this, object(), current);
544 if (JvmtiExport::should_post_monitor_contended_entered()) {
545 JvmtiExport::post_monitor_contended_entered(current, this);
546
547 // The current thread already owns the monitor and is not going to
548 // call park() for the remainder of the monitor enter protocol. So
549 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
550 // event handler consumed an unpark() issued by the thread that
551 // just exited the monitor.
552 }
553 if (event.should_commit()) {
554 event.set_previousOwner(_previous_owner_tid);
555 event.commit();
556 }
557 OM_PERFDATA_OP(ContendedLockAttempts, inc());
558 }
559
560 // Caveat: TryLock() is not necessarily serializing if it returns failure.
561 // Callers must compensate as needed.
562
563 ObjectMonitor::TryLockResult ObjectMonitor::TryLock(JavaThread* current) {
564 void* own = owner_raw();
565 if (own != nullptr) return TryLockResult::HasOwner;
566 if (try_set_owner_from(nullptr, current) == nullptr) {
567 assert(_recursions == 0, "invariant");
568 return TryLockResult::Success;
569 }
570 // The lock had been free momentarily, but we lost the race to the lock.
571 // Interference -- the CAS failed.
572 // We can either return -1 or retry.
573 // Retry doesn't make as much sense because the lock was just acquired.
574 return TryLockResult::Interference;
575 }
576
577 // Deflate the specified ObjectMonitor if not in-use. Returns true if it
578 // was deflated and false otherwise.
579 //
580 // The async deflation protocol sets owner to DEFLATER_MARKER and
581 // makes contentions negative as signals to contending threads that
582 // an async deflation is in progress. There are a number of checks
583 // as part of the protocol to make sure that the calling thread has
584 // not lost the race to a contending thread.
585 //
586 // The ObjectMonitor has been successfully async deflated when:
587 // (contentions < 0)
588 // Contending threads that see that condition know to retry their operation.
589 //
590 bool ObjectMonitor::deflate_monitor(Thread* current) {
591 if (is_busy()) {
592 // Easy checks are first - the ObjectMonitor is busy so no deflation.
593 return false;
594 }
595
596 const oop obj = object_peek();
597
598 if (obj == nullptr) {
599 // If the object died, we can recycle the monitor without racing with
600 // Java threads. The GC already broke the association with the object.
601 set_owner_from(nullptr, DEFLATER_MARKER);
602 assert(contentions() >= 0, "must be non-negative: contentions=%d", contentions());
603 _contentions = INT_MIN; // minimum negative int
604 } else {
605 // Attempt async deflation protocol.
606
607 // Set a null owner to DEFLATER_MARKER to force any contending thread
608 // through the slow path. This is just the first part of the async
609 // deflation dance.
610 if (try_set_owner_from(nullptr, DEFLATER_MARKER) != nullptr) {
611 // The owner field is no longer null so we lost the race since the
612 // ObjectMonitor is now busy.
613 return false;
614 }
615
616 if (contentions() > 0 || _waiters != 0) {
617 // Another thread has raced to enter the ObjectMonitor after
618 // is_busy() above or has already entered and waited on
619 // it which makes it busy so no deflation. Restore owner to
620 // null if it is still DEFLATER_MARKER.
621 if (try_set_owner_from(DEFLATER_MARKER, nullptr) != DEFLATER_MARKER) {
622 // Deferred decrement for the JT EnterI() that cancelled the async deflation.
623 add_to_contentions(-1);
624 }
625 return false;
626 }
627
628 // Make a zero contentions field negative to force any contending threads
629 // to retry. This is the second part of the async deflation dance.
630 if (Atomic::cmpxchg(&_contentions, 0, INT_MIN) != 0) {
631 // Contentions was no longer 0 so we lost the race since the
632 // ObjectMonitor is now busy. Restore owner to null if it is
633 // still DEFLATER_MARKER:
634 if (try_set_owner_from(DEFLATER_MARKER, nullptr) != DEFLATER_MARKER) {
635 // Deferred decrement for the JT EnterI() that cancelled the async deflation.
636 add_to_contentions(-1);
637 }
638 return false;
639 }
640 }
641
642 // Sanity checks for the races:
643 guarantee(owner_is_DEFLATER_MARKER(), "must be deflater marker");
644 guarantee(contentions() < 0, "must be negative: contentions=%d",
645 contentions());
646 guarantee(_waiters == 0, "must be 0: waiters=%d", _waiters);
647 guarantee(_cxq == nullptr, "must be no contending threads: cxq="
648 INTPTR_FORMAT, p2i(_cxq));
649 guarantee(_EntryList == nullptr,
650 "must be no entering threads: EntryList=" INTPTR_FORMAT,
651 p2i(_EntryList));
652
653 if (obj != nullptr) {
654 if (log_is_enabled(Trace, monitorinflation)) {
655 ResourceMark rm;
656 log_trace(monitorinflation)("deflate_monitor: object=" INTPTR_FORMAT
657 ", mark=" INTPTR_FORMAT ", type='%s'",
658 p2i(obj), obj->mark().value(),
659 obj->klass()->external_name());
660 }
661 }
662
663 if (UseObjectMonitorTable) {
664 LightweightSynchronizer::deflate_monitor(current, obj, this);
665 } else {
666 if (obj != nullptr) {
667 // Install the old mark word if nobody else has already done it.
668 install_displaced_markword_in_object(obj);
669 }
670 }
671
672 // We leave owner == DEFLATER_MARKER and contentions < 0
673 // to force any racing threads to retry.
674 return true; // Success, ObjectMonitor has been deflated.
675 }
676
677 // Install the displaced mark word (dmw) of a deflating ObjectMonitor
678 // into the header of the object associated with the monitor. This
679 // idempotent method is called by a thread that is deflating a
680 // monitor and by other threads that have detected a race with the
681 // deflation process.
682 void ObjectMonitor::install_displaced_markword_in_object(const oop obj) {
683 assert(!UseObjectMonitorTable, "Lightweight has no dmw");
684 // This function must only be called when (owner == DEFLATER_MARKER
685 // && contentions <= 0), but we can't guarantee that here because
686 // those values could change when the ObjectMonitor gets moved from
687 // the global free list to a per-thread free list.
688
689 guarantee(obj != nullptr, "must be non-null");
690
691 // Separate loads in is_being_async_deflated(), which is almost always
692 // called before this function, from the load of dmw/header below.
693
694 // _contentions and dmw/header may get written by different threads.
695 // Make sure to observe them in the same order when having several observers.
696 OrderAccess::loadload_for_IRIW();
697
698 const oop l_object = object_peek();
699 if (l_object == nullptr) {
700 // ObjectMonitor's object ref has already been cleared by async
701 // deflation or GC so we're done here.
702 return;
703 }
704 assert(l_object == obj, "object=" INTPTR_FORMAT " must equal obj="
705 INTPTR_FORMAT, p2i(l_object), p2i(obj));
706
707 markWord dmw = header();
708 // The dmw has to be neutral (not null, not locked and not marked).
709 assert(dmw.is_neutral(), "must be neutral: dmw=" INTPTR_FORMAT, dmw.value());
710
711 // Install displaced mark word if the object's header still points
712 // to this ObjectMonitor. More than one racing caller to this function
713 // can rarely reach this point, but only one can win.
714 markWord res = obj->cas_set_mark(dmw, markWord::encode(this));
715 if (res != markWord::encode(this)) {
716 // This should be rare so log at the Info level when it happens.
717 log_info(monitorinflation)("install_displaced_markword_in_object: "
718 "failed cas_set_mark: new_mark=" INTPTR_FORMAT
719 ", old_mark=" INTPTR_FORMAT ", res=" INTPTR_FORMAT,
720 dmw.value(), markWord::encode(this).value(),
721 res.value());
722 }
723
724 // Note: It does not matter which thread restored the header/dmw
725 // into the object's header. The thread deflating the monitor just
726 // wanted the object's header restored and it is. The threads that
727 // detected a race with the deflation process also wanted the
728 // object's header restored before they retry their operation and
729 // because it is restored they will only retry once.
730 }
731
732 // Convert the fields used by is_busy() to a string that can be
733 // used for diagnostic output.
734 const char* ObjectMonitor::is_busy_to_string(stringStream* ss) {
735 ss->print("is_busy: waiters=%d"
736 ", contentions=%d"
737 ", owner=" PTR_FORMAT
738 ", cxq=" PTR_FORMAT
739 ", EntryList=" PTR_FORMAT,
740 _waiters,
741 (contentions() > 0 ? contentions() : 0),
742 owner_is_DEFLATER_MARKER()
743 // We report null instead of DEFLATER_MARKER here because is_busy()
744 // ignores DEFLATER_MARKER values.
745 ? p2i(nullptr)
746 : p2i(owner_raw()),
747 p2i(_cxq),
748 p2i(_EntryList));
749 return ss->base();
750 }
751
752 #define MAX_RECHECK_INTERVAL 1000
753
754 void ObjectMonitor::EnterI(JavaThread* current) {
755 assert(current->thread_state() == _thread_blocked, "invariant");
756
757 // Try the lock - TATAS
758 if (TryLock(current) == TryLockResult::Success) {
759 assert(_succ != current, "invariant");
760 assert(owner_raw() == current, "invariant");
761 assert(_Responsible != current, "invariant");
762 return;
763 }
764
765 if (try_set_owner_from(DEFLATER_MARKER, current) == DEFLATER_MARKER) {
766 // Cancelled the in-progress async deflation by changing owner from
767 // DEFLATER_MARKER to current. As part of the contended enter protocol,
768 // contentions was incremented to a positive value before EnterI()
769 // was called and that prevents the deflater thread from winning the
770 // last part of the 2-part async deflation protocol. After EnterI()
771 // returns to enter(), contentions is decremented because the caller
772 // now owns the monitor. We bump contentions an extra time here to
773 // prevent the deflater thread from winning the last part of the
774 // 2-part async deflation protocol after the regular decrement
775 // occurs in enter(). The deflater thread will decrement contentions
776 // after it recognizes that the async deflation was cancelled.
777 add_to_contentions(1);
778 assert(_succ != current, "invariant");
779 assert(_Responsible != current, "invariant");
780 return;
781 }
782
783 assert(InitDone, "Unexpectedly not initialized");
784
785 // We try one round of spinning *before* enqueueing current.
786 //
787 // If the _owner is ready but OFFPROC we could use a YieldTo()
788 // operation to donate the remainder of this thread's quantum
789 // to the owner. This has subtle but beneficial affinity
790 // effects.
791
792 if (TrySpin(current)) {
793 assert(owner_raw() == current, "invariant");
794 assert(_succ != current, "invariant");
795 assert(_Responsible != current, "invariant");
796 return;
797 }
798
799 // The Spin failed -- Enqueue and park the thread ...
800 assert(_succ != current, "invariant");
801 assert(owner_raw() != current, "invariant");
802 assert(_Responsible != current, "invariant");
803
804 // Enqueue "current" on ObjectMonitor's _cxq.
805 //
806 // Node acts as a proxy for current.
807 // As an aside, if were to ever rewrite the synchronization code mostly
808 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
809 // Java objects. This would avoid awkward lifecycle and liveness issues,
810 // as well as eliminate a subset of ABA issues.
811 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
812
813 ObjectWaiter node(current);
814 current->_ParkEvent->reset();
815 node._prev = (ObjectWaiter*) 0xBAD;
816 node.TState = ObjectWaiter::TS_CXQ;
817
818 // Push "current" onto the front of the _cxq.
819 // Once on cxq/EntryList, current stays on-queue until it acquires the lock.
820 // Note that spinning tends to reduce the rate at which threads
821 // enqueue and dequeue on EntryList|cxq.
822 ObjectWaiter* nxt;
823 for (;;) {
824 node._next = nxt = _cxq;
825 if (Atomic::cmpxchg(&_cxq, nxt, &node) == nxt) break;
826
827 // Interference - the CAS failed because _cxq changed. Just retry.
828 // As an optional optimization we retry the lock.
829 if (TryLock(current) == TryLockResult::Success) {
830 assert(_succ != current, "invariant");
831 assert(owner_raw() == current, "invariant");
832 assert(_Responsible != current, "invariant");
833 return;
834 }
835 }
836
837 // Check for cxq|EntryList edge transition to non-null. This indicates
838 // the onset of contention. While contention persists exiting threads
839 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
840 // operations revert to the faster 1-0 mode. This enter operation may interleave
841 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
842 // arrange for one of the contending thread to use a timed park() operations
843 // to detect and recover from the race. (Stranding is form of progress failure
844 // where the monitor is unlocked but all the contending threads remain parked).
845 // That is, at least one of the contended threads will periodically poll _owner.
846 // One of the contending threads will become the designated "Responsible" thread.
847 // The Responsible thread uses a timed park instead of a normal indefinite park
848 // operation -- it periodically wakes and checks for and recovers from potential
849 // strandings admitted by 1-0 exit operations. We need at most one Responsible
850 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
851 // be responsible for a monitor.
852 //
853 // Currently, one of the contended threads takes on the added role of "Responsible".
854 // A viable alternative would be to use a dedicated "stranding checker" thread
855 // that periodically iterated over all the threads (or active monitors) and unparked
856 // successors where there was risk of stranding. This would help eliminate the
857 // timer scalability issues we see on some platforms as we'd only have one thread
858 // -- the checker -- parked on a timer.
859
860 if (nxt == nullptr && _EntryList == nullptr) {
861 // Try to assume the role of responsible thread for the monitor.
862 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=current }
863 Atomic::replace_if_null(&_Responsible, current);
864 }
865
866 // The lock might have been released while this thread was occupied queueing
867 // itself onto _cxq. To close the race and avoid "stranding" and
868 // progress-liveness failure we must resample-retry _owner before parking.
869 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
870 // In this case the ST-MEMBAR is accomplished with CAS().
871 //
872 // TODO: Defer all thread state transitions until park-time.
873 // Since state transitions are heavy and inefficient we'd like
874 // to defer the state transitions until absolutely necessary,
875 // and in doing so avoid some transitions ...
876
877 int recheckInterval = 1;
878
879 for (;;) {
880
881 if (TryLock(current) == TryLockResult::Success) {
882 break;
883 }
884 assert(owner_raw() != current, "invariant");
885
886 // park self
887 if (_Responsible == current) {
888 current->_ParkEvent->park((jlong) recheckInterval);
889 // Increase the recheckInterval, but clamp the value.
890 recheckInterval *= 8;
891 if (recheckInterval > MAX_RECHECK_INTERVAL) {
892 recheckInterval = MAX_RECHECK_INTERVAL;
893 }
894 } else {
895 current->_ParkEvent->park();
896 }
897
898 if (TryLock(current) == TryLockResult::Success) {
899 break;
900 }
901
902 if (try_set_owner_from(DEFLATER_MARKER, current) == DEFLATER_MARKER) {
903 // Cancelled the in-progress async deflation by changing owner from
904 // DEFLATER_MARKER to current. As part of the contended enter protocol,
905 // contentions was incremented to a positive value before EnterI()
906 // was called and that prevents the deflater thread from winning the
907 // last part of the 2-part async deflation protocol. After EnterI()
908 // returns to enter(), contentions is decremented because the caller
909 // now owns the monitor. We bump contentions an extra time here to
910 // prevent the deflater thread from winning the last part of the
911 // 2-part async deflation protocol after the regular decrement
912 // occurs in enter(). The deflater thread will decrement contentions
913 // after it recognizes that the async deflation was cancelled.
914 add_to_contentions(1);
915 break;
916 }
917
918 // The lock is still contested.
919
920 // Keep a tally of the # of futile wakeups.
921 // Note that the counter is not protected by a lock or updated by atomics.
922 // That is by design - we trade "lossy" counters which are exposed to
923 // races during updates for a lower probe effect.
924 // This PerfData object can be used in parallel with a safepoint.
925 // See the work around in PerfDataManager::destroy().
926 OM_PERFDATA_OP(FutileWakeups, inc());
927
928 // Assuming this is not a spurious wakeup we'll normally find _succ == current.
929 // We can defer clearing _succ until after the spin completes
930 // TrySpin() must tolerate being called with _succ == current.
931 // Try yet another round of adaptive spinning.
932 if (TrySpin(current)) {
933 break;
934 }
935
936 // We can find that we were unpark()ed and redesignated _succ while
937 // we were spinning. That's harmless. If we iterate and call park(),
938 // park() will consume the event and return immediately and we'll
939 // just spin again. This pattern can repeat, leaving _succ to simply
940 // spin on a CPU.
941
942 if (_succ == current) _succ = nullptr;
943
944 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
945 OrderAccess::fence();
946 }
947
948 // Egress :
949 // current has acquired the lock -- Unlink current from the cxq or EntryList.
950 // Normally we'll find current on the EntryList .
951 // From the perspective of the lock owner (this thread), the
952 // EntryList is stable and cxq is prepend-only.
953 // The head of cxq is volatile but the interior is stable.
954 // In addition, current.TState is stable.
955
956 assert(owner_raw() == current, "invariant");
957
958 UnlinkAfterAcquire(current, &node);
959 if (_succ == current) _succ = nullptr;
960
961 assert(_succ != current, "invariant");
962 if (_Responsible == current) {
963 _Responsible = nullptr;
964 OrderAccess::fence(); // Dekker pivot-point
965
966 // We may leave threads on cxq|EntryList without a designated
967 // "Responsible" thread. This is benign. When this thread subsequently
968 // exits the monitor it can "see" such preexisting "old" threads --
969 // threads that arrived on the cxq|EntryList before the fence, above --
970 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
971 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
972 // non-null and elect a new "Responsible" timer thread.
973 //
974 // This thread executes:
975 // ST Responsible=null; MEMBAR (in enter epilogue - here)
976 // LD cxq|EntryList (in subsequent exit)
977 //
978 // Entering threads in the slow/contended path execute:
979 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
980 // The (ST cxq; MEMBAR) is accomplished with CAS().
981 //
982 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
983 // exit operation from floating above the ST Responsible=null.
984 }
985
986 // We've acquired ownership with CAS().
987 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
988 // But since the CAS() this thread may have also stored into _succ,
989 // EntryList, cxq or Responsible. These meta-data updates must be
990 // visible __before this thread subsequently drops the lock.
991 // Consider what could occur if we didn't enforce this constraint --
992 // STs to monitor meta-data and user-data could reorder with (become
993 // visible after) the ST in exit that drops ownership of the lock.
994 // Some other thread could then acquire the lock, but observe inconsistent
995 // or old monitor meta-data and heap data. That violates the JMM.
996 // To that end, the 1-0 exit() operation must have at least STST|LDST
997 // "release" barrier semantics. Specifically, there must be at least a
998 // STST|LDST barrier in exit() before the ST of null into _owner that drops
999 // the lock. The barrier ensures that changes to monitor meta-data and data
1000 // protected by the lock will be visible before we release the lock, and
1001 // therefore before some other thread (CPU) has a chance to acquire the lock.
1002 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
1003 //
1004 // Critically, any prior STs to _succ or EntryList must be visible before
1005 // the ST of null into _owner in the *subsequent* (following) corresponding
1006 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
1007 // execute a serializing instruction.
1008
1009 return;
1010 }
1011
1012 // ReenterI() is a specialized inline form of the latter half of the
1013 // contended slow-path from EnterI(). We use ReenterI() only for
1014 // monitor reentry in wait().
1015 //
1016 // In the future we should reconcile EnterI() and ReenterI().
1017
1018 void ObjectMonitor::ReenterI(JavaThread* current, ObjectWaiter* currentNode) {
1019 assert(current != nullptr, "invariant");
1020 assert(current->thread_state() != _thread_blocked, "invariant");
1021 assert(currentNode != nullptr, "invariant");
1022 assert(currentNode->_thread == current, "invariant");
1023 assert(_waiters > 0, "invariant");
1024 assert_mark_word_consistency();
1025
1026 for (;;) {
1027 ObjectWaiter::TStates v = currentNode->TState;
1028 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1029 assert(owner_raw() != current, "invariant");
1030
1031 // This thread has been notified so try to reacquire the lock.
1032 if (TryLock(current) == TryLockResult::Success) {
1033 break;
1034 }
1035
1036 // If that fails, spin again. Note that spin count may be zero so the above TryLock
1037 // is necessary.
1038 if (TrySpin(current)) {
1039 break;
1040 }
1041
1042 {
1043 OSThreadContendState osts(current->osthread());
1044
1045 assert(current->thread_state() == _thread_in_vm, "invariant");
1046
1047 {
1048 ClearSuccOnSuspend csos(this);
1049 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */);
1050 current->_ParkEvent->park();
1051 }
1052 }
1053
1054 // Try again, but just so we distinguish between futile wakeups and
1055 // successful wakeups. The following test isn't algorithmically
1056 // necessary, but it helps us maintain sensible statistics.
1057 if (TryLock(current) == TryLockResult::Success) {
1058 break;
1059 }
1060
1061 // The lock is still contested.
1062
1063 // Assuming this is not a spurious wakeup we'll normally
1064 // find that _succ == current.
1065 if (_succ == current) _succ = nullptr;
1066
1067 // Invariant: after clearing _succ a contending thread
1068 // *must* retry _owner before parking.
1069 OrderAccess::fence();
1070
1071 // Keep a tally of the # of futile wakeups.
1072 // Note that the counter is not protected by a lock or updated by atomics.
1073 // That is by design - we trade "lossy" counters which are exposed to
1074 // races during updates for a lower probe effect.
1075 // This PerfData object can be used in parallel with a safepoint.
1076 // See the work around in PerfDataManager::destroy().
1077 OM_PERFDATA_OP(FutileWakeups, inc());
1078 }
1079
1080 // current has acquired the lock -- Unlink current from the cxq or EntryList .
1081 // Normally we'll find current on the EntryList.
1082 // Unlinking from the EntryList is constant-time and atomic-free.
1083 // From the perspective of the lock owner (this thread), the
1084 // EntryList is stable and cxq is prepend-only.
1085 // The head of cxq is volatile but the interior is stable.
1086 // In addition, current.TState is stable.
1087
1088 assert(owner_raw() == current, "invariant");
1089 assert_mark_word_consistency();
1090 UnlinkAfterAcquire(current, currentNode);
1091 if (_succ == current) _succ = nullptr;
1092 assert(_succ != current, "invariant");
1093 currentNode->TState = ObjectWaiter::TS_RUN;
1094 OrderAccess::fence(); // see comments at the end of EnterI()
1095 }
1096
1097 // By convention we unlink a contending thread from EntryList|cxq immediately
1098 // after the thread acquires the lock in ::enter(). Equally, we could defer
1099 // unlinking the thread until ::exit()-time.
1100
1101 void ObjectMonitor::UnlinkAfterAcquire(JavaThread* current, ObjectWaiter* currentNode) {
1102 assert(owner_raw() == current, "invariant");
1103 assert(currentNode->_thread == current, "invariant");
1104
1105 if (currentNode->TState == ObjectWaiter::TS_ENTER) {
1106 // Normal case: remove current from the DLL EntryList .
1107 // This is a constant-time operation.
1108 ObjectWaiter* nxt = currentNode->_next;
1109 ObjectWaiter* prv = currentNode->_prev;
1110 if (nxt != nullptr) nxt->_prev = prv;
1111 if (prv != nullptr) prv->_next = nxt;
1112 if (currentNode == _EntryList) _EntryList = nxt;
1113 assert(nxt == nullptr || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
1114 assert(prv == nullptr || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
1115 } else {
1116 assert(currentNode->TState == ObjectWaiter::TS_CXQ, "invariant");
1117 // Inopportune interleaving -- current is still on the cxq.
1118 // This usually means the enqueue of self raced an exiting thread.
1119 // Normally we'll find current near the front of the cxq, so
1120 // dequeueing is typically fast. If needbe we can accelerate
1121 // this with some MCS/CHL-like bidirectional list hints and advisory
1122 // back-links so dequeueing from the interior will normally operate
1123 // in constant-time.
1124 // Dequeue current from either the head (with CAS) or from the interior
1125 // with a linear-time scan and normal non-atomic memory operations.
1126 // CONSIDER: if current is on the cxq then simply drain cxq into EntryList
1127 // and then unlink current from EntryList. We have to drain eventually,
1128 // so it might as well be now.
1129
1130 ObjectWaiter* v = _cxq;
1131 assert(v != nullptr, "invariant");
1132 if (v != currentNode || Atomic::cmpxchg(&_cxq, v, currentNode->_next) != v) {
1133 // The CAS above can fail from interference IFF a "RAT" arrived.
1134 // In that case current must be in the interior and can no longer be
1135 // at the head of cxq.
1136 if (v == currentNode) {
1137 assert(_cxq != v, "invariant");
1138 v = _cxq; // CAS above failed - start scan at head of list
1139 }
1140 ObjectWaiter* p;
1141 ObjectWaiter* q = nullptr;
1142 for (p = v; p != nullptr && p != currentNode; p = p->_next) {
1143 q = p;
1144 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
1145 }
1146 assert(v != currentNode, "invariant");
1147 assert(p == currentNode, "Node not found on cxq");
1148 assert(p != _cxq, "invariant");
1149 assert(q != nullptr, "invariant");
1150 assert(q->_next == p, "invariant");
1151 q->_next = p->_next;
1152 }
1153 }
1154
1155 #ifdef ASSERT
1156 // Diagnostic hygiene ...
1157 currentNode->_prev = (ObjectWaiter*) 0xBAD;
1158 currentNode->_next = (ObjectWaiter*) 0xBAD;
1159 currentNode->TState = ObjectWaiter::TS_RUN;
1160 #endif
1161 }
1162
1163 // -----------------------------------------------------------------------------
1164 // Exit support
1165 //
1166 // exit()
1167 // ~~~~~~
1168 // Note that the collector can't reclaim the objectMonitor or deflate
1169 // the object out from underneath the thread calling ::exit() as the
1170 // thread calling ::exit() never transitions to a stable state.
1171 // This inhibits GC, which in turn inhibits asynchronous (and
1172 // inopportune) reclamation of "this".
1173 //
1174 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
1175 // There's one exception to the claim above, however. EnterI() can call
1176 // exit() to drop a lock if the acquirer has been externally suspended.
1177 // In that case exit() is called with _thread_state == _thread_blocked,
1178 // but the monitor's _contentions field is > 0, which inhibits reclamation.
1179 //
1180 // 1-0 exit
1181 // ~~~~~~~~
1182 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
1183 // the fast-path operators have been optimized so the common ::exit()
1184 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
1185 // The code emitted by fast_unlock() elides the usual MEMBAR. This
1186 // greatly improves latency -- MEMBAR and CAS having considerable local
1187 // latency on modern processors -- but at the cost of "stranding". Absent the
1188 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
1189 // ::enter() path, resulting in the entering thread being stranding
1190 // and a progress-liveness failure. Stranding is extremely rare.
1191 // We use timers (timed park operations) & periodic polling to detect
1192 // and recover from stranding. Potentially stranded threads periodically
1193 // wake up and poll the lock. See the usage of the _Responsible variable.
1194 //
1195 // The CAS() in enter provides for safety and exclusion, while the CAS or
1196 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
1197 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
1198 // We detect and recover from stranding with timers.
1199 //
1200 // If a thread transiently strands it'll park until (a) another
1201 // thread acquires the lock and then drops the lock, at which time the
1202 // exiting thread will notice and unpark the stranded thread, or, (b)
1203 // the timer expires. If the lock is high traffic then the stranding latency
1204 // will be low due to (a). If the lock is low traffic then the odds of
1205 // stranding are lower, although the worst-case stranding latency
1206 // is longer. Critically, we don't want to put excessive load in the
1207 // platform's timer subsystem. We want to minimize both the timer injection
1208 // rate (timers created/sec) as well as the number of timers active at
1209 // any one time. (more precisely, we want to minimize timer-seconds, which is
1210 // the integral of the # of active timers at any instant over time).
1211 // Both impinge on OS scalability. Given that, at most one thread parked on
1212 // a monitor will use a timer.
1213 //
1214 // There is also the risk of a futile wake-up. If we drop the lock
1215 // another thread can reacquire the lock immediately, and we can
1216 // then wake a thread unnecessarily. This is benign, and we've
1217 // structured the code so the windows are short and the frequency
1218 // of such futile wakups is low.
1219
1220 void ObjectMonitor::exit(JavaThread* current, bool not_suspended) {
1221 void* cur = owner_raw();
1222 if (current != cur) {
1223 if (LockingMode != LM_LIGHTWEIGHT && current->is_lock_owned((address)cur)) {
1224 assert(_recursions == 0, "invariant");
1225 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*.
1226 _recursions = 0;
1227 } else {
1228 // Apparent unbalanced locking ...
1229 // Naively we'd like to throw IllegalMonitorStateException.
1230 // As a practical matter we can neither allocate nor throw an
1231 // exception as ::exit() can be called from leaf routines.
1232 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
1233 // Upon deeper reflection, however, in a properly run JVM the only
1234 // way we should encounter this situation is in the presence of
1235 // unbalanced JNI locking. TODO: CheckJNICalls.
1236 // See also: CR4414101
1237 #ifdef ASSERT
1238 LogStreamHandle(Error, monitorinflation) lsh;
1239 lsh.print_cr("ERROR: ObjectMonitor::exit(): thread=" INTPTR_FORMAT
1240 " is exiting an ObjectMonitor it does not own.", p2i(current));
1241 lsh.print_cr("The imbalance is possibly caused by JNI locking.");
1242 print_debug_style_on(&lsh);
1243 assert(false, "Non-balanced monitor enter/exit!");
1244 #endif
1245 return;
1246 }
1247 }
1248
1249 if (_recursions != 0) {
1250 _recursions--; // this is simple recursive enter
1251 return;
1252 }
1253
1254 // Invariant: after setting Responsible=null an thread must execute
1255 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
1256 _Responsible = nullptr;
1257
1258 #if INCLUDE_JFR
1259 // get the owner's thread id for the MonitorEnter event
1260 // if it is enabled and the thread isn't suspended
1261 if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
1262 _previous_owner_tid = JFR_THREAD_ID(current);
1263 }
1264 #endif
1265
1266 for (;;) {
1267 assert(current == owner_raw(), "invariant");
1268
1269 // Drop the lock.
1270 // release semantics: prior loads and stores from within the critical section
1271 // must not float (reorder) past the following store that drops the lock.
1272 // Uses a storeload to separate release_store(owner) from the
1273 // successor check. The try_set_owner() below uses cmpxchg() so
1274 // we get the fence down there.
1275 release_clear_owner(current);
1276 OrderAccess::storeload();
1277
1278 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != nullptr) {
1279 return;
1280 }
1281 // Other threads are blocked trying to acquire the lock.
1282
1283 // Normally the exiting thread is responsible for ensuring succession,
1284 // but if other successors are ready or other entering threads are spinning
1285 // then this thread can simply store null into _owner and exit without
1286 // waking a successor. The existence of spinners or ready successors
1287 // guarantees proper succession (liveness). Responsibility passes to the
1288 // ready or running successors. The exiting thread delegates the duty.
1289 // More precisely, if a successor already exists this thread is absolved
1290 // of the responsibility of waking (unparking) one.
1291 //
1292 // The _succ variable is critical to reducing futile wakeup frequency.
1293 // _succ identifies the "heir presumptive" thread that has been made
1294 // ready (unparked) but that has not yet run. We need only one such
1295 // successor thread to guarantee progress.
1296 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1297 // section 3.3 "Futile Wakeup Throttling" for details.
1298 //
1299 // Note that spinners in Enter() also set _succ non-null.
1300 // In the current implementation spinners opportunistically set
1301 // _succ so that exiting threads might avoid waking a successor.
1302 // Another less appealing alternative would be for the exiting thread
1303 // to drop the lock and then spin briefly to see if a spinner managed
1304 // to acquire the lock. If so, the exiting thread could exit
1305 // immediately without waking a successor, otherwise the exiting
1306 // thread would need to dequeue and wake a successor.
1307 // (Note that we'd need to make the post-drop spin short, but no
1308 // shorter than the worst-case round-trip cache-line migration time.
1309 // The dropped lock needs to become visible to the spinner, and then
1310 // the acquisition of the lock by the spinner must become visible to
1311 // the exiting thread).
1312
1313 // It appears that an heir-presumptive (successor) must be made ready.
1314 // Only the current lock owner can manipulate the EntryList or
1315 // drain _cxq, so we need to reacquire the lock. If we fail
1316 // to reacquire the lock the responsibility for ensuring succession
1317 // falls to the new owner.
1318 //
1319 if (try_set_owner_from(nullptr, current) != nullptr) {
1320 return;
1321 }
1322
1323 guarantee(owner_raw() == current, "invariant");
1324
1325 ObjectWaiter* w = nullptr;
1326
1327 w = _EntryList;
1328 if (w != nullptr) {
1329 // I'd like to write: guarantee (w->_thread != current).
1330 // But in practice an exiting thread may find itself on the EntryList.
1331 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1332 // then calls exit(). Exit release the lock by setting O._owner to null.
1333 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1334 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1335 // release the lock "O". T2 resumes immediately after the ST of null into
1336 // _owner, above. T2 notices that the EntryList is populated, so it
1337 // reacquires the lock and then finds itself on the EntryList.
1338 // Given all that, we have to tolerate the circumstance where "w" is
1339 // associated with current.
1340 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1341 ExitEpilog(current, w);
1342 return;
1343 }
1344
1345 // If we find that both _cxq and EntryList are null then just
1346 // re-run the exit protocol from the top.
1347 w = _cxq;
1348 if (w == nullptr) continue;
1349
1350 // Drain _cxq into EntryList - bulk transfer.
1351 // First, detach _cxq.
1352 // The following loop is tantamount to: w = swap(&cxq, nullptr)
1353 for (;;) {
1354 assert(w != nullptr, "Invariant");
1355 ObjectWaiter* u = Atomic::cmpxchg(&_cxq, w, (ObjectWaiter*)nullptr);
1356 if (u == w) break;
1357 w = u;
1358 }
1359
1360 assert(w != nullptr, "invariant");
1361 assert(_EntryList == nullptr, "invariant");
1362
1363 // Convert the LIFO SLL anchored by _cxq into a DLL.
1364 // The list reorganization step operates in O(LENGTH(w)) time.
1365 // It's critical that this step operate quickly as
1366 // "current" still holds the outer-lock, restricting parallelism
1367 // and effectively lengthening the critical section.
1368 // Invariant: s chases t chases u.
1369 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1370 // we have faster access to the tail.
1371
1372 _EntryList = w;
1373 ObjectWaiter* q = nullptr;
1374 ObjectWaiter* p;
1375 for (p = w; p != nullptr; p = p->_next) {
1376 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1377 p->TState = ObjectWaiter::TS_ENTER;
1378 p->_prev = q;
1379 q = p;
1380 }
1381
1382 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = nullptr
1383 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1384
1385 // See if we can abdicate to a spinner instead of waking a thread.
1386 // A primary goal of the implementation is to reduce the
1387 // context-switch rate.
1388 if (_succ != nullptr) continue;
1389
1390 w = _EntryList;
1391 if (w != nullptr) {
1392 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1393 ExitEpilog(current, w);
1394 return;
1395 }
1396 }
1397 }
1398
1399 void ObjectMonitor::ExitEpilog(JavaThread* current, ObjectWaiter* Wakee) {
1400 assert(owner_raw() == current, "invariant");
1401
1402 // Exit protocol:
1403 // 1. ST _succ = wakee
1404 // 2. membar #loadstore|#storestore;
1405 // 2. ST _owner = nullptr
1406 // 3. unpark(wakee)
1407
1408 _succ = Wakee->_thread;
1409 ParkEvent * Trigger = Wakee->_event;
1410
1411 // Hygiene -- once we've set _owner = nullptr we can't safely dereference Wakee again.
1412 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1413 // out-of-scope (non-extant).
1414 Wakee = nullptr;
1415
1416 // Drop the lock.
1417 // Uses a fence to separate release_store(owner) from the LD in unpark().
1418 release_clear_owner(current);
1419 OrderAccess::fence();
1420
1421 DTRACE_MONITOR_PROBE(contended__exit, this, object(), current);
1422 Trigger->unpark();
1423
1424 // Maintain stats and report events to JVMTI
1425 OM_PERFDATA_OP(Parks, inc());
1426 }
1427
1428 // complete_exit exits a lock returning recursion count
1429 // complete_exit requires an inflated monitor
1430 // The _owner field is not always the Thread addr even with an
1431 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1432 // thread due to contention.
1433 intx ObjectMonitor::complete_exit(JavaThread* current) {
1434 assert(InitDone, "Unexpectedly not initialized");
1435
1436 void* cur = owner_raw();
1437 if (current != cur) {
1438 if (LockingMode != LM_LIGHTWEIGHT && current->is_lock_owned((address)cur)) {
1439 assert(_recursions == 0, "internal state error");
1440 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*.
1441 _recursions = 0;
1442 }
1443 }
1444
1445 guarantee(current == owner_raw(), "complete_exit not owner");
1446 intx save = _recursions; // record the old recursion count
1447 _recursions = 0; // set the recursion level to be 0
1448 exit(current); // exit the monitor
1449 guarantee(owner_raw() != current, "invariant");
1450 return save;
1451 }
1452
1453 // Checks that the current THREAD owns this monitor and causes an
1454 // immediate return if it doesn't. We don't use the CHECK macro
1455 // because we want the IMSE to be the only exception that is thrown
1456 // from the call site when false is returned. Any other pending
1457 // exception is ignored.
1458 #define CHECK_OWNER() \
1459 do { \
1460 if (!check_owner(THREAD)) { \
1461 assert(HAS_PENDING_EXCEPTION, "expected a pending IMSE here."); \
1462 return; \
1463 } \
1464 } while (false)
1465
1466 // Returns true if the specified thread owns the ObjectMonitor.
1467 // Otherwise returns false and throws IllegalMonitorStateException
1468 // (IMSE). If there is a pending exception and the specified thread
1469 // is not the owner, that exception will be replaced by the IMSE.
1470 bool ObjectMonitor::check_owner(TRAPS) {
1471 JavaThread* current = THREAD;
1472 void* cur = owner_raw();
1473 assert(cur != anon_owner_ptr(), "no anon owner here");
1474 if (cur == current) {
1475 return true;
1476 }
1477 if (LockingMode != LM_LIGHTWEIGHT && current->is_lock_owned((address)cur)) {
1478 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*.
1479 _recursions = 0;
1480 return true;
1481 }
1482 THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(),
1483 "current thread is not owner", false);
1484 }
1485
1486 static inline bool is_excluded(const Klass* monitor_klass) {
1487 assert(monitor_klass != nullptr, "invariant");
1488 NOT_JFR_RETURN_(false);
1489 JFR_ONLY(return vmSymbols::jdk_jfr_internal_HiddenWait() == monitor_klass->name();)
1490 }
1491
1492 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1493 ObjectMonitor* monitor,
1494 uint64_t notifier_tid,
1495 jlong timeout,
1496 bool timedout) {
1497 assert(event != nullptr, "invariant");
1498 assert(monitor != nullptr, "invariant");
1499 const Klass* monitor_klass = monitor->object()->klass();
1500 if (is_excluded(monitor_klass)) {
1501 return;
1502 }
1503 event->set_monitorClass(monitor_klass);
1504 event->set_timeout(timeout);
1505 // Set an address that is 'unique enough', such that events close in
1506 // time and with the same address are likely (but not guaranteed) to
1507 // belong to the same object.
1508 event->set_address((uintptr_t)monitor);
1509 event->set_notifier(notifier_tid);
1510 event->set_timedOut(timedout);
1511 event->commit();
1512 }
1513
1514 // -----------------------------------------------------------------------------
1515 // Wait/Notify/NotifyAll
1516 //
1517 // Note: a subset of changes to ObjectMonitor::wait()
1518 // will need to be replicated in complete_exit
1519 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1520 JavaThread* current = THREAD;
1521
1522 assert(InitDone, "Unexpectedly not initialized");
1523
1524 CHECK_OWNER(); // Throws IMSE if not owner.
1525
1526 EventJavaMonitorWait event;
1527
1528 // check for a pending interrupt
1529 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1530 // post monitor waited event. Note that this is past-tense, we are done waiting.
1531 if (JvmtiExport::should_post_monitor_waited()) {
1532 // Note: 'false' parameter is passed here because the
1533 // wait was not timed out due to thread interrupt.
1534 JvmtiExport::post_monitor_waited(current, this, false);
1535
1536 // In this short circuit of the monitor wait protocol, the
1537 // current thread never drops ownership of the monitor and
1538 // never gets added to the wait queue so the current thread
1539 // cannot be made the successor. This means that the
1540 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1541 // consume an unpark() meant for the ParkEvent associated with
1542 // this ObjectMonitor.
1543 }
1544 if (event.should_commit()) {
1545 post_monitor_wait_event(&event, this, 0, millis, false);
1546 }
1547 THROW(vmSymbols::java_lang_InterruptedException());
1548 return;
1549 }
1550
1551 current->set_current_waiting_monitor(this);
1552
1553 // create a node to be put into the queue
1554 // Critically, after we reset() the event but prior to park(), we must check
1555 // for a pending interrupt.
1556 ObjectWaiter node(current);
1557 node.TState = ObjectWaiter::TS_WAIT;
1558 current->_ParkEvent->reset();
1559 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1560
1561 // Enter the waiting queue, which is a circular doubly linked list in this case
1562 // but it could be a priority queue or any data structure.
1563 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1564 // by the owner of the monitor *except* in the case where park()
1565 // returns because of a timeout of interrupt. Contention is exceptionally rare
1566 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1567
1568 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1569 AddWaiter(&node);
1570 Thread::SpinRelease(&_WaitSetLock);
1571
1572 _Responsible = nullptr;
1573
1574 intx save = _recursions; // record the old recursion count
1575 _waiters++; // increment the number of waiters
1576 _recursions = 0; // set the recursion level to be 1
1577 exit(current); // exit the monitor
1578 guarantee(owner_raw() != current, "invariant");
1579
1580 // The thread is on the WaitSet list - now park() it.
1581 // On MP systems it's conceivable that a brief spin before we park
1582 // could be profitable.
1583 //
1584 // TODO-FIXME: change the following logic to a loop of the form
1585 // while (!timeout && !interrupted && _notified == 0) park()
1586
1587 int ret = OS_OK;
1588 int WasNotified = 0;
1589
1590 // Need to check interrupt state whilst still _thread_in_vm
1591 bool interrupted = interruptible && current->is_interrupted(false);
1592
1593 { // State transition wrappers
1594 OSThread* osthread = current->osthread();
1595 OSThreadWaitState osts(osthread, true);
1596
1597 assert(current->thread_state() == _thread_in_vm, "invariant");
1598
1599 {
1600 ClearSuccOnSuspend csos(this);
1601 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */);
1602 if (interrupted || HAS_PENDING_EXCEPTION) {
1603 // Intentionally empty
1604 } else if (node._notified == 0) {
1605 if (millis <= 0) {
1606 current->_ParkEvent->park();
1607 } else {
1608 ret = current->_ParkEvent->park(millis);
1609 }
1610 }
1611 }
1612
1613 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1614 // from the WaitSet to the EntryList.
1615 // See if we need to remove Node from the WaitSet.
1616 // We use double-checked locking to avoid grabbing _WaitSetLock
1617 // if the thread is not on the wait queue.
1618 //
1619 // Note that we don't need a fence before the fetch of TState.
1620 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1621 // written by the is thread. (perhaps the fetch might even be satisfied
1622 // by a look-aside into the processor's own store buffer, although given
1623 // the length of the code path between the prior ST and this load that's
1624 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1625 // then we'll acquire the lock and then re-fetch a fresh TState value.
1626 // That is, we fail toward safety.
1627
1628 if (node.TState == ObjectWaiter::TS_WAIT) {
1629 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1630 if (node.TState == ObjectWaiter::TS_WAIT) {
1631 DequeueSpecificWaiter(&node); // unlink from WaitSet
1632 assert(node._notified == 0, "invariant");
1633 node.TState = ObjectWaiter::TS_RUN;
1634 }
1635 Thread::SpinRelease(&_WaitSetLock);
1636 }
1637
1638 // The thread is now either on off-list (TS_RUN),
1639 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1640 // The Node's TState variable is stable from the perspective of this thread.
1641 // No other threads will asynchronously modify TState.
1642 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1643 OrderAccess::loadload();
1644 if (_succ == current) _succ = nullptr;
1645 WasNotified = node._notified;
1646
1647 // Reentry phase -- reacquire the monitor.
1648 // re-enter contended monitor after object.wait().
1649 // retain OBJECT_WAIT state until re-enter successfully completes
1650 // Thread state is thread_in_vm and oop access is again safe,
1651 // although the raw address of the object may have changed.
1652 // (Don't cache naked oops over safepoints, of course).
1653
1654 // post monitor waited event. Note that this is past-tense, we are done waiting.
1655 if (JvmtiExport::should_post_monitor_waited()) {
1656 JvmtiExport::post_monitor_waited(current, this, ret == OS_TIMEOUT);
1657
1658 if (node._notified != 0 && _succ == current) {
1659 // In this part of the monitor wait-notify-reenter protocol it
1660 // is possible (and normal) for another thread to do a fastpath
1661 // monitor enter-exit while this thread is still trying to get
1662 // to the reenter portion of the protocol.
1663 //
1664 // The ObjectMonitor was notified and the current thread is
1665 // the successor which also means that an unpark() has already
1666 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1667 // consume the unpark() that was done when the successor was
1668 // set because the same ParkEvent is shared between Java
1669 // monitors and JVM/TI RawMonitors (for now).
1670 //
1671 // We redo the unpark() to ensure forward progress, i.e., we
1672 // don't want all pending threads hanging (parked) with none
1673 // entering the unlocked monitor.
1674 node._event->unpark();
1675 }
1676 }
1677
1678 if (event.should_commit()) {
1679 post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1680 }
1681
1682 OrderAccess::fence();
1683
1684 assert(owner_raw() != current, "invariant");
1685 ObjectWaiter::TStates v = node.TState;
1686 if (v == ObjectWaiter::TS_RUN) {
1687 enter(current);
1688 } else {
1689 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1690 ReenterI(current, &node);
1691 node.wait_reenter_end(this);
1692 }
1693
1694 // current has reacquired the lock.
1695 // Lifecycle - the node representing current must not appear on any queues.
1696 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1697 // want residual elements associated with this thread left on any lists.
1698 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1699 assert(owner_raw() == current, "invariant");
1700 assert(_succ != current, "invariant");
1701 } // OSThreadWaitState()
1702
1703 current->set_current_waiting_monitor(nullptr);
1704
1705 guarantee(_recursions == 0, "invariant");
1706 int relock_count = JvmtiDeferredUpdates::get_and_reset_relock_count_after_wait(current);
1707 _recursions = save // restore the old recursion count
1708 + relock_count; // increased by the deferred relock count
1709 current->inc_held_monitor_count(relock_count); // Deopt never entered these counts.
1710 _waiters--; // decrement the number of waiters
1711
1712 // Verify a few postconditions
1713 assert(owner_raw() == current, "invariant");
1714 assert(_succ != current, "invariant");
1715 assert_mark_word_consistency();
1716
1717 // check if the notification happened
1718 if (!WasNotified) {
1719 // no, it could be timeout or Thread.interrupt() or both
1720 // check for interrupt event, otherwise it is timeout
1721 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1722 THROW(vmSymbols::java_lang_InterruptedException());
1723 }
1724 }
1725
1726 // NOTE: Spurious wake up will be consider as timeout.
1727 // Monitor notify has precedence over thread interrupt.
1728 }
1729
1730
1731 // Consider:
1732 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1733 // then instead of transferring a thread from the WaitSet to the EntryList
1734 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1735
1736 void ObjectMonitor::INotify(JavaThread* current) {
1737 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1738 ObjectWaiter* iterator = DequeueWaiter();
1739 if (iterator != nullptr) {
1740 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1741 guarantee(iterator->_notified == 0, "invariant");
1742 // Disposition - what might we do with iterator ?
1743 // a. add it directly to the EntryList - either tail (policy == 1)
1744 // or head (policy == 0).
1745 // b. push it onto the front of the _cxq (policy == 2).
1746 // For now we use (b).
1747
1748 iterator->TState = ObjectWaiter::TS_ENTER;
1749
1750 iterator->_notified = 1;
1751 iterator->_notifier_tid = JFR_THREAD_ID(current);
1752
1753 ObjectWaiter* list = _EntryList;
1754 if (list != nullptr) {
1755 assert(list->_prev == nullptr, "invariant");
1756 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1757 assert(list != iterator, "invariant");
1758 }
1759
1760 // prepend to cxq
1761 if (list == nullptr) {
1762 iterator->_next = iterator->_prev = nullptr;
1763 _EntryList = iterator;
1764 } else {
1765 iterator->TState = ObjectWaiter::TS_CXQ;
1766 for (;;) {
1767 ObjectWaiter* front = _cxq;
1768 iterator->_next = front;
1769 if (Atomic::cmpxchg(&_cxq, front, iterator) == front) {
1770 break;
1771 }
1772 }
1773 }
1774
1775 // _WaitSetLock protects the wait queue, not the EntryList. We could
1776 // move the add-to-EntryList operation, above, outside the critical section
1777 // protected by _WaitSetLock. In practice that's not useful. With the
1778 // exception of wait() timeouts and interrupts the monitor owner
1779 // is the only thread that grabs _WaitSetLock. There's almost no contention
1780 // on _WaitSetLock so it's not profitable to reduce the length of the
1781 // critical section.
1782
1783 iterator->wait_reenter_begin(this);
1784 }
1785 Thread::SpinRelease(&_WaitSetLock);
1786 }
1787
1788 // Consider: a not-uncommon synchronization bug is to use notify() when
1789 // notifyAll() is more appropriate, potentially resulting in stranded
1790 // threads; this is one example of a lost wakeup. A useful diagnostic
1791 // option is to force all notify() operations to behave as notifyAll().
1792 //
1793 // Note: We can also detect many such problems with a "minimum wait".
1794 // When the "minimum wait" is set to a small non-zero timeout value
1795 // and the program does not hang whereas it did absent "minimum wait",
1796 // that suggests a lost wakeup bug.
1797
1798 void ObjectMonitor::notify(TRAPS) {
1799 JavaThread* current = THREAD;
1800 CHECK_OWNER(); // Throws IMSE if not owner.
1801 if (_WaitSet == nullptr) {
1802 return;
1803 }
1804 DTRACE_MONITOR_PROBE(notify, this, object(), current);
1805 INotify(current);
1806 OM_PERFDATA_OP(Notifications, inc(1));
1807 }
1808
1809
1810 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1811 // from the waitset to the EntryList. This could be done more efficiently with a
1812 // single bulk transfer but in practice it's not time-critical. Beware too,
1813 // that in prepend-mode we invert the order of the waiters. Let's say that the
1814 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1815 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1816
1817 void ObjectMonitor::notifyAll(TRAPS) {
1818 JavaThread* current = THREAD;
1819 CHECK_OWNER(); // Throws IMSE if not owner.
1820 if (_WaitSet == nullptr) {
1821 return;
1822 }
1823
1824 DTRACE_MONITOR_PROBE(notifyAll, this, object(), current);
1825 int tally = 0;
1826 while (_WaitSet != nullptr) {
1827 tally++;
1828 INotify(current);
1829 }
1830
1831 OM_PERFDATA_OP(Notifications, inc(tally));
1832 }
1833
1834 // -----------------------------------------------------------------------------
1835 // Adaptive Spinning Support
1836 //
1837 // Adaptive spin-then-block - rational spinning
1838 //
1839 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1840 // algorithm. On high order SMP systems it would be better to start with
1841 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1842 // a contending thread could enqueue itself on the cxq and then spin locally
1843 // on a thread-specific variable such as its ParkEvent._Event flag.
1844 // That's left as an exercise for the reader. Note that global spinning is
1845 // not problematic on Niagara, as the L2 cache serves the interconnect and
1846 // has both low latency and massive bandwidth.
1847 //
1848 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1849 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1850 // (duration) or we can fix the count at approximately the duration of
1851 // a context switch and vary the frequency. Of course we could also
1852 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1853 // For a description of 'Adaptive spin-then-block mutual exclusion in
1854 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1855 //
1856 // This implementation varies the duration "D", where D varies with
1857 // the success rate of recent spin attempts. (D is capped at approximately
1858 // length of a round-trip context switch). The success rate for recent
1859 // spin attempts is a good predictor of the success rate of future spin
1860 // attempts. The mechanism adapts automatically to varying critical
1861 // section length (lock modality), system load and degree of parallelism.
1862 // D is maintained per-monitor in _SpinDuration and is initialized
1863 // optimistically. Spin frequency is fixed at 100%.
1864 //
1865 // Note that _SpinDuration is volatile, but we update it without locks
1866 // or atomics. The code is designed so that _SpinDuration stays within
1867 // a reasonable range even in the presence of races. The arithmetic
1868 // operations on _SpinDuration are closed over the domain of legal values,
1869 // so at worst a race will install and older but still legal value.
1870 // At the very worst this introduces some apparent non-determinism.
1871 // We might spin when we shouldn't or vice-versa, but since the spin
1872 // count are relatively short, even in the worst case, the effect is harmless.
1873 //
1874 // Care must be taken that a low "D" value does not become an
1875 // an absorbing state. Transient spinning failures -- when spinning
1876 // is overall profitable -- should not cause the system to converge
1877 // on low "D" values. We want spinning to be stable and predictable
1878 // and fairly responsive to change and at the same time we don't want
1879 // it to oscillate, become metastable, be "too" non-deterministic,
1880 // or converge on or enter undesirable stable absorbing states.
1881 //
1882 // We implement a feedback-based control system -- using past behavior
1883 // to predict future behavior. We face two issues: (a) if the
1884 // input signal is random then the spin predictor won't provide optimal
1885 // results, and (b) if the signal frequency is too high then the control
1886 // system, which has some natural response lag, will "chase" the signal.
1887 // (b) can arise from multimodal lock hold times. Transient preemption
1888 // can also result in apparent bimodal lock hold times.
1889 // Although sub-optimal, neither condition is particularly harmful, as
1890 // in the worst-case we'll spin when we shouldn't or vice-versa.
1891 // The maximum spin duration is rather short so the failure modes aren't bad.
1892 // To be conservative, I've tuned the gain in system to bias toward
1893 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1894 // "rings" or oscillates between spinning and not spinning. This happens
1895 // when spinning is just on the cusp of profitability, however, so the
1896 // situation is not dire. The state is benign -- there's no need to add
1897 // hysteresis control to damp the transition rate between spinning and
1898 // not spinning.
1899
1900 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool
1901
1902 static int Knob_Bonus = 100; // spin success bonus
1903 static int Knob_Penalty = 200; // spin failure penalty
1904 static int Knob_Poverty = 1000;
1905 static int Knob_FixedSpin = 0;
1906 static int Knob_PreSpin = 10; // 20-100 likely better, but it's not better in my testing.
1907
1908 inline static int adjust_up(int spin_duration) {
1909 int x = spin_duration;
1910 if (x < ObjectMonitor::Knob_SpinLimit) {
1911 if (x < Knob_Poverty) {
1912 x = Knob_Poverty;
1913 }
1914 return x + Knob_Bonus;
1915 } else {
1916 return spin_duration;
1917 }
1918 }
1919
1920 inline static int adjust_down(int spin_duration) {
1921 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
1922 // AIMD is globally stable.
1923 int x = spin_duration;
1924 if (x > 0) {
1925 // Consider an AIMD scheme like: x -= (x >> 3) + 100
1926 // This is globally sample and tends to damp the response.
1927 x -= Knob_Penalty;
1928 if (x < 0) { x = 0; }
1929 return x;
1930 } else {
1931 return spin_duration;
1932 }
1933 }
1934
1935 bool ObjectMonitor::short_fixed_spin(JavaThread* current, int spin_count, bool adapt) {
1936 for (int ctr = 0; ctr < spin_count; ctr++) {
1937 TryLockResult status = TryLock(current);
1938 if (status == TryLockResult::Success) {
1939 if (adapt) {
1940 _SpinDuration = adjust_up(_SpinDuration);
1941 }
1942 return true;
1943 } else if (status == TryLockResult::Interference) {
1944 break;
1945 }
1946 SpinPause();
1947 }
1948 return false;
1949 }
1950
1951 // Spinning: Fixed frequency (100%), vary duration
1952 bool ObjectMonitor::TrySpin(JavaThread* current) {
1953
1954 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1955 int knob_fixed_spin = Knob_FixedSpin; // 0 (don't spin: default), 2000 good test
1956 if (knob_fixed_spin > 0) {
1957 return short_fixed_spin(current, knob_fixed_spin, false);
1958 }
1959
1960 // Admission control - verify preconditions for spinning
1961 //
1962 // We always spin a little bit, just to prevent _SpinDuration == 0 from
1963 // becoming an absorbing state. Put another way, we spin briefly to
1964 // sample, just in case the system load, parallelism, contention, or lock
1965 // modality changed.
1966
1967 int knob_pre_spin = Knob_PreSpin; // 10 (default), 100, 1000 or 2000
1968 if (short_fixed_spin(current, knob_pre_spin, true)) {
1969 return true;
1970 }
1971
1972 //
1973 // Consider the following alternative:
1974 // Periodically set _SpinDuration = _SpinLimit and try a long/full
1975 // spin attempt. "Periodically" might mean after a tally of
1976 // the # of failed spin attempts (or iterations) reaches some threshold.
1977 // This takes us into the realm of 1-out-of-N spinning, where we
1978 // hold the duration constant but vary the frequency.
1979
1980 int ctr = _SpinDuration;
1981 if (ctr <= 0) return false;
1982
1983 // We're good to spin ... spin ingress.
1984 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1985 // when preparing to LD...CAS _owner, etc and the CAS is likely
1986 // to succeed.
1987 if (_succ == nullptr) {
1988 _succ = current;
1989 }
1990 Thread* prv = nullptr;
1991
1992 // There are three ways to exit the following loop:
1993 // 1. A successful spin where this thread has acquired the lock.
1994 // 2. Spin failure with prejudice
1995 // 3. Spin failure without prejudice
1996
1997 while (--ctr >= 0) {
1998
1999 // Periodic polling -- Check for pending GC
2000 // Threads may spin while they're unsafe.
2001 // We don't want spinning threads to delay the JVM from reaching
2002 // a stop-the-world safepoint or to steal cycles from GC.
2003 // If we detect a pending safepoint we abort in order that
2004 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
2005 // this thread, if safe, doesn't steal cycles from GC.
2006 // This is in keeping with the "no loitering in runtime" rule.
2007 // We periodically check to see if there's a safepoint pending.
2008 if ((ctr & 0xFF) == 0) {
2009 // Can't call SafepointMechanism::should_process() since that
2010 // might update the poll values and we could be in a thread_blocked
2011 // state here which is not allowed so just check the poll.
2012 if (SafepointMechanism::local_poll_armed(current)) {
2013 break;
2014 }
2015 SpinPause();
2016 }
2017
2018 // Probe _owner with TATAS
2019 // If this thread observes the monitor transition or flicker
2020 // from locked to unlocked to locked, then the odds that this
2021 // thread will acquire the lock in this spin attempt go down
2022 // considerably. The same argument applies if the CAS fails
2023 // or if we observe _owner change from one non-null value to
2024 // another non-null value. In such cases we might abort
2025 // the spin without prejudice or apply a "penalty" to the
2026 // spin count-down variable "ctr", reducing it by 100, say.
2027
2028 JavaThread* ox = static_cast<JavaThread*>(owner_raw());
2029 if (ox == nullptr) {
2030 ox = static_cast<JavaThread*>(try_set_owner_from(nullptr, current));
2031 if (ox == nullptr) {
2032 // The CAS succeeded -- this thread acquired ownership
2033 // Take care of some bookkeeping to exit spin state.
2034 if (_succ == current) {
2035 _succ = nullptr;
2036 }
2037
2038 // Increase _SpinDuration :
2039 // The spin was successful (profitable) so we tend toward
2040 // longer spin attempts in the future.
2041 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2042 // If we acquired the lock early in the spin cycle it
2043 // makes sense to increase _SpinDuration proportionally.
2044 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2045 _SpinDuration = adjust_up(_SpinDuration);
2046 return true;
2047 }
2048
2049 // The CAS failed ... we can take any of the following actions:
2050 // * penalize: ctr -= CASPenalty
2051 // * exit spin with prejudice -- abort without adapting spinner
2052 // * exit spin without prejudice.
2053 // * Since CAS is high-latency, retry again immediately.
2054 break;
2055 }
2056
2057 // Did lock ownership change hands ?
2058 if (ox != prv && prv != nullptr) {
2059 break;
2060 }
2061 prv = ox;
2062
2063 if (_succ == nullptr) {
2064 _succ = current;
2065 }
2066 }
2067
2068 // Spin failed with prejudice -- reduce _SpinDuration.
2069 if (ctr < 0) {
2070 _SpinDuration = adjust_down(_SpinDuration);
2071 }
2072
2073 if (_succ == current) {
2074 _succ = nullptr;
2075 // Invariant: after setting succ=null a contending thread
2076 // must recheck-retry _owner before parking. This usually happens
2077 // in the normal usage of TrySpin(), but it's safest
2078 // to make TrySpin() as foolproof as possible.
2079 OrderAccess::fence();
2080 if (TryLock(current) == TryLockResult::Success) {
2081 return true;
2082 }
2083 }
2084
2085 return false;
2086 }
2087
2088
2089 // -----------------------------------------------------------------------------
2090 // WaitSet management ...
2091
2092 ObjectWaiter::ObjectWaiter(JavaThread* current) {
2093 _next = nullptr;
2094 _prev = nullptr;
2095 _notified = 0;
2096 _notifier_tid = 0;
2097 TState = TS_RUN;
2098 _thread = current;
2099 _event = _thread->_ParkEvent;
2100 _active = false;
2101 assert(_event != nullptr, "invariant");
2102 }
2103
2104 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2105 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(_thread, mon);
2106 }
2107
2108 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2109 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(_thread, _active);
2110 }
2111
2112 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2113 assert(node != nullptr, "should not add null node");
2114 assert(node->_prev == nullptr, "node already in list");
2115 assert(node->_next == nullptr, "node already in list");
2116 // put node at end of queue (circular doubly linked list)
2117 if (_WaitSet == nullptr) {
2118 _WaitSet = node;
2119 node->_prev = node;
2120 node->_next = node;
2121 } else {
2122 ObjectWaiter* head = _WaitSet;
2123 ObjectWaiter* tail = head->_prev;
2124 assert(tail->_next == head, "invariant check");
2125 tail->_next = node;
2126 head->_prev = node;
2127 node->_next = head;
2128 node->_prev = tail;
2129 }
2130 }
2131
2132 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2133 // dequeue the very first waiter
2134 ObjectWaiter* waiter = _WaitSet;
2135 if (waiter) {
2136 DequeueSpecificWaiter(waiter);
2137 }
2138 return waiter;
2139 }
2140
2141 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2142 assert(node != nullptr, "should not dequeue nullptr node");
2143 assert(node->_prev != nullptr, "node already removed from list");
2144 assert(node->_next != nullptr, "node already removed from list");
2145 // when the waiter has woken up because of interrupt,
2146 // timeout or other spurious wake-up, dequeue the
2147 // waiter from waiting list
2148 ObjectWaiter* next = node->_next;
2149 if (next == node) {
2150 assert(node->_prev == node, "invariant check");
2151 _WaitSet = nullptr;
2152 } else {
2153 ObjectWaiter* prev = node->_prev;
2154 assert(prev->_next == node, "invariant check");
2155 assert(next->_prev == node, "invariant check");
2156 next->_prev = prev;
2157 prev->_next = next;
2158 if (_WaitSet == node) {
2159 _WaitSet = next;
2160 }
2161 }
2162 node->_next = nullptr;
2163 node->_prev = nullptr;
2164 }
2165
2166 // -----------------------------------------------------------------------------
2167 // PerfData support
2168 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = nullptr;
2169 PerfCounter * ObjectMonitor::_sync_FutileWakeups = nullptr;
2170 PerfCounter * ObjectMonitor::_sync_Parks = nullptr;
2171 PerfCounter * ObjectMonitor::_sync_Notifications = nullptr;
2172 PerfCounter * ObjectMonitor::_sync_Inflations = nullptr;
2173 PerfCounter * ObjectMonitor::_sync_Deflations = nullptr;
2174 PerfLongVariable * ObjectMonitor::_sync_MonExtant = nullptr;
2175
2176 // One-shot global initialization for the sync subsystem.
2177 // We could also defer initialization and initialize on-demand
2178 // the first time we call ObjectSynchronizer::inflate().
2179 // Initialization would be protected - like so many things - by
2180 // the MonitorCache_lock.
2181
2182 void ObjectMonitor::Initialize() {
2183 assert(!InitDone, "invariant");
2184
2185 if (!os::is_MP()) {
2186 Knob_SpinLimit = 0;
2187 Knob_PreSpin = 0;
2188 Knob_FixedSpin = -1;
2189 }
2190
2191 if (UsePerfData) {
2192 EXCEPTION_MARK;
2193 #define NEWPERFCOUNTER(n) \
2194 { \
2195 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \
2196 CHECK); \
2197 }
2198 #define NEWPERFVARIABLE(n) \
2199 { \
2200 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \
2201 CHECK); \
2202 }
2203 NEWPERFCOUNTER(_sync_Inflations);
2204 NEWPERFCOUNTER(_sync_Deflations);
2205 NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2206 NEWPERFCOUNTER(_sync_FutileWakeups);
2207 NEWPERFCOUNTER(_sync_Parks);
2208 NEWPERFCOUNTER(_sync_Notifications);
2209 NEWPERFVARIABLE(_sync_MonExtant);
2210 #undef NEWPERFCOUNTER
2211 #undef NEWPERFVARIABLE
2212 }
2213
2214 _oop_storage = OopStorageSet::create_weak("ObjectSynchronizer Weak", mtSynchronizer);
2215
2216 DEBUG_ONLY(InitDone = true;)
2217 }
2218
2219 void ObjectMonitor::print_on(outputStream* st) const {
2220 // The minimal things to print for markWord printing, more can be added for debugging and logging.
2221 st->print("{contentions=0x%08x,waiters=0x%08x"
2222 ",recursions=" INTX_FORMAT ",owner=" INTPTR_FORMAT "}",
2223 contentions(), waiters(), recursions(),
2224 p2i(owner()));
2225 }
2226 void ObjectMonitor::print() const { print_on(tty); }
2227
2228 #ifdef ASSERT
2229 // Print the ObjectMonitor like a debugger would:
2230 //
2231 // (ObjectMonitor) 0x00007fdfb6012e40 = {
2232 // _metadata = 0x0000000000000001
2233 // _object = 0x000000070ff45fd0
2234 // _pad_buf0 = {
2235 // [0] = '\0'
2236 // ...
2237 // [43] = '\0'
2238 // }
2239 // _owner = 0x0000000000000000
2240 // _previous_owner_tid = 0
2241 // _pad_buf1 = {
2242 // [0] = '\0'
2243 // ...
2244 // [47] = '\0'
2245 // }
2246 // _next_om = 0x0000000000000000
2247 // _recursions = 0
2248 // _EntryList = 0x0000000000000000
2249 // _cxq = 0x0000000000000000
2250 // _succ = 0x0000000000000000
2251 // _Responsible = 0x0000000000000000
2252 // _SpinDuration = 5000
2253 // _contentions = 0
2254 // _WaitSet = 0x0000700009756248
2255 // _waiters = 1
2256 // _WaitSetLock = 0
2257 // }
2258 //
2259 void ObjectMonitor::print_debug_style_on(outputStream* st) const {
2260 st->print_cr("(ObjectMonitor*) " INTPTR_FORMAT " = {", p2i(this));
2261 st->print_cr(" _metadata = " INTPTR_FORMAT, _metadata);
2262 st->print_cr(" _object = " INTPTR_FORMAT, p2i(object_peek()));
2263 st->print_cr(" _pad_buf0 = {");
2264 st->print_cr(" [0] = '\\0'");
2265 st->print_cr(" ...");
2266 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf0) - 1);
2267 st->print_cr(" }");
2268 st->print_cr(" _owner = " INTPTR_FORMAT, p2i(owner_raw()));
2269 st->print_cr(" _previous_owner_tid = " UINT64_FORMAT, _previous_owner_tid);
2270 st->print_cr(" _pad_buf1 = {");
2271 st->print_cr(" [0] = '\\0'");
2272 st->print_cr(" ...");
2273 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf1) - 1);
2274 st->print_cr(" }");
2275 st->print_cr(" _next_om = " INTPTR_FORMAT, p2i(next_om()));
2276 st->print_cr(" _recursions = " INTX_FORMAT, _recursions);
2277 st->print_cr(" _EntryList = " INTPTR_FORMAT, p2i(_EntryList));
2278 st->print_cr(" _cxq = " INTPTR_FORMAT, p2i(_cxq));
2279 st->print_cr(" _succ = " INTPTR_FORMAT, p2i(_succ));
2280 st->print_cr(" _Responsible = " INTPTR_FORMAT, p2i(_Responsible));
2281 st->print_cr(" _SpinDuration = %d", _SpinDuration);
2282 st->print_cr(" _contentions = %d", contentions());
2283 st->print_cr(" _WaitSet = " INTPTR_FORMAT, p2i(_WaitSet));
2284 st->print_cr(" _waiters = %d", _waiters);
2285 st->print_cr(" _WaitSetLock = %d", _WaitSetLock);
2286 st->print_cr("}");
2287 }
2288 #endif