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/mutexLocker.hpp"
47 #include "runtime/objectMonitor.hpp"
48 #include "runtime/objectMonitor.inline.hpp"
49 #include "runtime/orderAccess.hpp"
50 #include "runtime/osThread.hpp"
51 #include "runtime/perfData.hpp"
52 #include "runtime/safefetch.hpp"
53 #include "runtime/safepointMechanism.inline.hpp"
54 #include "runtime/sharedRuntime.hpp"
55 #include "services/threadService.hpp"
56 #include "utilities/dtrace.hpp"
57 #include "utilities/globalDefinitions.hpp"
58 #include "utilities/macros.hpp"
59 #include "utilities/preserveException.hpp"
60 #if INCLUDE_JFR
61 #include "jfr/support/jfrFlush.hpp"
62 #endif
63
64 #ifdef DTRACE_ENABLED
65
66 // Only bother with this argument setup if dtrace is available
67 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
68
69
70 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
71 char* bytes = nullptr; \
72 int len = 0; \
73 jlong jtid = SharedRuntime::get_java_tid(thread); \
74 Symbol* klassname = obj->klass()->name(); \
75 if (klassname != nullptr) { \
76 bytes = (char*)klassname->bytes(); \
77 len = klassname->utf8_length(); \
78 }
79
80 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
81 { \
82 if (DTraceMonitorProbes) { \
83 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
84 HOTSPOT_MONITOR_WAIT(jtid, \
85 (monitor), bytes, len, (millis)); \
86 } \
87 }
88
89 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
90 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
91 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
92 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
93 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
94
95 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
96 { \
97 if (DTraceMonitorProbes) { \
98 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
99 HOTSPOT_MONITOR_##probe(jtid, \
100 (uintptr_t)(monitor), bytes, len); \
101 } \
102 }
103
104 #else // ndef DTRACE_ENABLED
105
106 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
107 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
108
109 #endif // ndef DTRACE_ENABLED
110
111 // Tunables ...
112 // The knob* variables are effectively final. Once set they should
113 // never be modified hence. Consider using __read_mostly with GCC.
114
115 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool -
116
117 static int Knob_Bonus = 100; // spin success bonus
118 static int Knob_BonusB = 100; // spin success bonus
119 static int Knob_Penalty = 200; // spin failure penalty
120 static int Knob_Poverty = 1000;
121 static int Knob_FixedSpin = 0;
122 static int Knob_PreSpin = 10; // 20-100 likely better
123
124 DEBUG_ONLY(static volatile bool InitDone = false;)
125
126 OopStorage* ObjectMonitor::_oop_storage = nullptr;
127
128 // -----------------------------------------------------------------------------
129 // Theory of operations -- Monitors lists, thread residency, etc:
130 //
131 // * A thread acquires ownership of a monitor by successfully
132 // CAS()ing the _owner field from null to non-null.
133 //
134 // * Invariant: A thread appears on at most one monitor list --
135 // cxq, EntryList or WaitSet -- at any one time.
136 //
137 // * Contending threads "push" themselves onto the cxq with CAS
138 // and then spin/park.
139 //
140 // * After a contending thread eventually acquires the lock it must
141 // dequeue itself from either the EntryList or the cxq.
142 //
143 // * The exiting thread identifies and unparks an "heir presumptive"
144 // tentative successor thread on the EntryList. Critically, the
145 // exiting thread doesn't unlink the successor thread from the EntryList.
146 // After having been unparked, the wakee will recontend for ownership of
147 // the monitor. The successor (wakee) will either acquire the lock or
148 // re-park itself.
149 //
150 // Succession is provided for by a policy of competitive handoff.
151 // The exiting thread does _not_ grant or pass ownership to the
152 // successor thread. (This is also referred to as "handoff" succession").
153 // Instead the exiting thread releases ownership and possibly wakes
154 // a successor, so the successor can (re)compete for ownership of the lock.
155 // If the EntryList is empty but the cxq is populated the exiting
156 // thread will drain the cxq into the EntryList. It does so by
157 // by detaching the cxq (installing null with CAS) and folding
158 // the threads from the cxq into the EntryList. The EntryList is
159 // doubly linked, while the cxq is singly linked because of the
160 // CAS-based "push" used to enqueue recently arrived threads (RATs).
161 //
162 // * Concurrency invariants:
163 //
164 // -- only the monitor owner may access or mutate the EntryList.
165 // The mutex property of the monitor itself protects the EntryList
166 // from concurrent interference.
167 // -- Only the monitor owner may detach the cxq.
168 //
169 // * The monitor entry list operations avoid locks, but strictly speaking
170 // they're not lock-free. Enter is lock-free, exit is not.
171 // For a description of 'Methods and apparatus providing non-blocking access
172 // to a resource,' see U.S. Pat. No. 7844973.
173 //
174 // * The cxq can have multiple concurrent "pushers" but only one concurrent
175 // detaching thread. This mechanism is immune from the ABA corruption.
176 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
177 //
178 // * Taken together, the cxq and the EntryList constitute or form a
179 // single logical queue of threads stalled trying to acquire the lock.
180 // We use two distinct lists to improve the odds of a constant-time
181 // dequeue operation after acquisition (in the ::enter() epilogue) and
182 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
183 // A key desideratum is to minimize queue & monitor metadata manipulation
184 // that occurs while holding the monitor lock -- that is, we want to
185 // minimize monitor lock holds times. Note that even a small amount of
186 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
187 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
188 // locks and monitor metadata.
189 //
190 // Cxq points to the set of Recently Arrived Threads attempting entry.
191 // Because we push threads onto _cxq with CAS, the RATs must take the form of
192 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
193 // the unlocking thread notices that EntryList is null but _cxq is != null.
194 //
195 // The EntryList is ordered by the prevailing queue discipline and
196 // can be organized in any convenient fashion, such as a doubly-linked list or
197 // a circular doubly-linked list. Critically, we want insert and delete operations
198 // to operate in constant-time. If we need a priority queue then something akin
199 // to Solaris' sleepq would work nicely. Viz.,
200 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
201 // Queue discipline is enforced at ::exit() time, when the unlocking thread
202 // drains the cxq into the EntryList, and orders or reorders the threads on the
203 // EntryList accordingly.
204 //
205 // Barring "lock barging", this mechanism provides fair cyclic ordering,
206 // somewhat similar to an elevator-scan.
207 //
208 // * The monitor synchronization subsystem avoids the use of native
209 // synchronization primitives except for the narrow platform-specific
210 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
211 // the semantics of park-unpark. Put another way, this monitor implementation
212 // depends only on atomic operations and park-unpark. The monitor subsystem
213 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
214 // underlying OS manages the READY<->RUN transitions.
215 //
216 // * Waiting threads reside on the WaitSet list -- wait() puts
217 // the caller onto the WaitSet.
218 //
219 // * notify() or notifyAll() simply transfers threads from the WaitSet to
220 // either the EntryList or cxq. Subsequent exit() operations will
221 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
222 // it's likely the notifyee would simply impale itself on the lock held
223 // by the notifier.
224 //
225 // * An interesting alternative is to encode cxq as (List,LockByte) where
226 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
227 // variable, like _recursions, in the scheme. The threads or Events that form
228 // the list would have to be aligned in 256-byte addresses. A thread would
229 // try to acquire the lock or enqueue itself with CAS, but exiting threads
230 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
231 // Note that is is *not* word-tearing, but it does presume that full-word
232 // CAS operations are coherent with intermix with STB operations. That's true
233 // on most common processors.
234 //
235 // * See also http://blogs.sun.com/dave
236
237
238 // Check that object() and set_object() are called from the right context:
239 static void check_object_context() {
240 #ifdef ASSERT
241 Thread* self = Thread::current();
242 if (self->is_Java_thread()) {
243 // Mostly called from JavaThreads so sanity check the thread state.
244 JavaThread* jt = JavaThread::cast(self);
245 switch (jt->thread_state()) {
246 case _thread_in_vm: // the usual case
247 case _thread_in_Java: // during deopt
248 break;
249 default:
250 fatal("called from an unsafe thread state");
251 }
252 assert(jt->is_active_Java_thread(), "must be active JavaThread");
253 } else {
254 // However, ThreadService::get_current_contended_monitor()
255 // can call here via the VMThread so sanity check it.
256 assert(self->is_VM_thread(), "must be");
257 }
258 #endif // ASSERT
259 }
260
261 ObjectMonitor::ObjectMonitor(oop object) :
262 _header(markWord::zero()),
263 _object(_oop_storage, object),
264 _owner(nullptr),
265 _previous_owner_tid(0),
266 _next_om(nullptr),
267 _recursions(0),
268 _EntryList(nullptr),
269 _cxq(nullptr),
270 _succ(nullptr),
271 _Responsible(nullptr),
272 _Spinner(0),
273 _SpinDuration(ObjectMonitor::Knob_SpinLimit),
274 _contentions(0),
275 _WaitSet(nullptr),
276 _waiters(0),
277 _WaitSetLock(0)
278 { }
279
280 ObjectMonitor::~ObjectMonitor() {
281 _object.release(_oop_storage);
282 }
283
284 oop ObjectMonitor::object() const {
285 check_object_context();
286 return _object.resolve();
287 }
288
289 oop ObjectMonitor::object_peek() const {
290 return _object.peek();
291 }
292
293 void ObjectMonitor::ExitOnSuspend::operator()(JavaThread* current) {
294 if (current->is_suspended()) {
295 _om->_recursions = 0;
296 _om->_succ = nullptr;
297 // Don't need a full fence after clearing successor here because of the call to exit().
298 _om->exit(current, false /* not_suspended */);
299 _om_exited = true;
300
301 current->set_current_pending_monitor(_om);
302 }
303 }
304
305 void ObjectMonitor::ClearSuccOnSuspend::operator()(JavaThread* current) {
306 if (current->is_suspended()) {
307 if (_om->_succ == current) {
308 _om->_succ = nullptr;
309 OrderAccess::fence(); // always do a full fence when successor is cleared
310 }
311 }
312 }
313
314 // -----------------------------------------------------------------------------
315 // Enter support
316
317 bool ObjectMonitor::enter_for(JavaThread* locking_thread) {
318 // Used by ObjectSynchronizer::enter_for to enter for another thread.
319 // The monitor is private to or already owned by locking_thread which must be suspended.
320 // So this code may only contend with deflation.
321 assert(locking_thread == Thread::current() || locking_thread->is_obj_deopt_suspend(), "must be");
322
323 // Block out deflation as soon as possible.
324 add_to_contentions(1);
325
326 bool success = false;
327 if (!is_being_async_deflated()) {
328 void* prev_owner = try_set_owner_from(nullptr, locking_thread);
329
330 if (prev_owner == nullptr) {
331 assert(_recursions == 0, "invariant");
332 success = true;
333 } else if (prev_owner == locking_thread) {
334 _recursions++;
335 success = true;
336 } else if (prev_owner == DEFLATER_MARKER) {
337 // Racing with deflation.
338 prev_owner = try_set_owner_from(DEFLATER_MARKER, locking_thread);
339 if (prev_owner == DEFLATER_MARKER) {
340 // Cancelled deflation. Increment contentions as part of the deflation protocol.
341 add_to_contentions(1);
342 success = true;
343 } else if (prev_owner == nullptr) {
344 // At this point we cannot race with deflation as we have both incremented
345 // contentions, seen contention > 0 and seen a DEFLATER_MARKER.
346 // success will only be false if this races with something other than
347 // deflation.
348 prev_owner = try_set_owner_from(nullptr, locking_thread);
349 success = prev_owner == nullptr;
350 }
351 } else if (LockingMode == LM_LEGACY && locking_thread->is_lock_owned((address)prev_owner)) {
352 assert(_recursions == 0, "must be");
353 _recursions = 1;
354 set_owner_from_BasicLock(prev_owner, locking_thread);
355 success = true;
356 }
357 assert(success, "Failed to enter_for: locking_thread=" INTPTR_FORMAT
358 ", this=" INTPTR_FORMAT "{owner=" INTPTR_FORMAT "}, observed owner: " INTPTR_FORMAT,
359 p2i(locking_thread), p2i(this), p2i(owner_raw()), p2i(prev_owner));
360 } else {
361 // Async deflation is in progress and our contentions increment
362 // above lost the race to async deflation. Undo the work and
363 // force the caller to retry.
364 const oop l_object = object();
365 if (l_object != nullptr) {
366 // Attempt to restore the header/dmw to the object's header so that
367 // we only retry once if the deflater thread happens to be slow.
368 install_displaced_markword_in_object(l_object);
369 }
370 }
371
372 add_to_contentions(-1);
373
374 assert(!success || owner_raw() == locking_thread, "must be");
375
376 return success;
377 }
378
379 bool ObjectMonitor::enter(JavaThread* current) {
380 assert(current == JavaThread::current(), "must be");
381 // The following code is ordered to check the most common cases first
382 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
383
384 void* cur = try_set_owner_from(nullptr, current);
385 if (cur == nullptr) {
386 assert(_recursions == 0, "invariant");
387 return true;
388 }
389
390 if (cur == current) {
391 // TODO-FIXME: check for integer overflow! BUGID 6557169.
392 _recursions++;
393 return true;
394 }
395
396 if (LockingMode != LM_LIGHTWEIGHT && current->is_lock_owned((address)cur)) {
397 assert(_recursions == 0, "internal state error");
398 _recursions = 1;
399 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*.
400 return true;
401 }
402
403 // We've encountered genuine contention.
404 assert(current->_Stalled == 0, "invariant");
405 current->_Stalled = intptr_t(this);
406
407 // Try one round of spinning *before* enqueueing current
408 // and before going through the awkward and expensive state
409 // transitions. The following spin is strictly optional ...
410 // Note that if we acquire the monitor from an initial spin
411 // we forgo posting JVMTI events and firing DTRACE probes.
412 if (TrySpin(current) > 0) {
413 assert(owner_raw() == current, "must be current: owner=" INTPTR_FORMAT, p2i(owner_raw()));
414 assert(_recursions == 0, "must be 0: recursions=" INTX_FORMAT, _recursions);
415 assert(object()->mark() == markWord::encode(this),
416 "object mark must match encoded this: mark=" INTPTR_FORMAT
417 ", encoded this=" INTPTR_FORMAT, object()->mark().value(),
418 markWord::encode(this).value());
419 current->_Stalled = 0;
420 return true;
421 }
422
423 assert(owner_raw() != current, "invariant");
424 assert(_succ != current, "invariant");
425 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
426 assert(current->thread_state() != _thread_blocked, "invariant");
427
428 // Keep track of contention for JVM/TI and M&M queries.
429 add_to_contentions(1);
430 if (is_being_async_deflated()) {
431 // Async deflation is in progress and our contentions increment
432 // above lost the race to async deflation. Undo the work and
433 // force the caller to retry.
434 const oop l_object = object();
435 if (l_object != nullptr) {
436 // Attempt to restore the header/dmw to the object's header so that
437 // we only retry once if the deflater thread happens to be slow.
438 install_displaced_markword_in_object(l_object);
439 }
440 current->_Stalled = 0;
441 add_to_contentions(-1);
442 return false;
443 }
444
445 JFR_ONLY(JfrConditionalFlush<EventJavaMonitorEnter> flush(current);)
446 EventJavaMonitorEnter event;
447 if (event.is_started()) {
448 event.set_monitorClass(object()->klass());
449 // Set an address that is 'unique enough', such that events close in
450 // time and with the same address are likely (but not guaranteed) to
451 // belong to the same object.
452 event.set_address((uintptr_t)this);
453 }
454
455 { // Change java thread status to indicate blocked on monitor enter.
456 JavaThreadBlockedOnMonitorEnterState jtbmes(current, this);
457
458 assert(current->current_pending_monitor() == nullptr, "invariant");
459 current->set_current_pending_monitor(this);
460
461 DTRACE_MONITOR_PROBE(contended__enter, this, object(), current);
462 if (JvmtiExport::should_post_monitor_contended_enter()) {
463 JvmtiExport::post_monitor_contended_enter(current, this);
464
465 // The current thread does not yet own the monitor and does not
466 // yet appear on any queues that would get it made the successor.
467 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
468 // handler cannot accidentally consume an unpark() meant for the
469 // ParkEvent associated with this ObjectMonitor.
470 }
471
472 OSThreadContendState osts(current->osthread());
473
474 assert(current->thread_state() == _thread_in_vm, "invariant");
475
476 for (;;) {
477 ExitOnSuspend eos(this);
478 {
479 ThreadBlockInVMPreprocess<ExitOnSuspend> tbivs(current, eos, true /* allow_suspend */);
480 EnterI(current);
481 current->set_current_pending_monitor(nullptr);
482 // We can go to a safepoint at the end of this block. If we
483 // do a thread dump during that safepoint, then this thread will show
484 // as having "-locked" the monitor, but the OS and java.lang.Thread
485 // states will still report that the thread is blocked trying to
486 // acquire it.
487 // If there is a suspend request, ExitOnSuspend will exit the OM
488 // and set the OM as pending.
489 }
490 if (!eos.exited()) {
491 // ExitOnSuspend did not exit the OM
492 assert(owner_raw() == current, "invariant");
493 break;
494 }
495 }
496
497 // We've just gotten past the enter-check-for-suspend dance and we now own
498 // the monitor free and clear.
499 }
500
501 add_to_contentions(-1);
502 assert(contentions() >= 0, "must not be negative: contentions=%d", contentions());
503 current->_Stalled = 0;
504
505 // Must either set _recursions = 0 or ASSERT _recursions == 0.
506 assert(_recursions == 0, "invariant");
507 assert(owner_raw() == current, "invariant");
508 assert(_succ != current, "invariant");
509 assert(object()->mark() == markWord::encode(this), "invariant");
510
511 // The thread -- now the owner -- is back in vm mode.
512 // Report the glorious news via TI,DTrace and jvmstat.
513 // The probe effect is non-trivial. All the reportage occurs
514 // while we hold the monitor, increasing the length of the critical
515 // section. Amdahl's parallel speedup law comes vividly into play.
516 //
517 // Another option might be to aggregate the events (thread local or
518 // per-monitor aggregation) and defer reporting until a more opportune
519 // time -- such as next time some thread encounters contention but has
520 // yet to acquire the lock. While spinning that thread could
521 // spinning we could increment JVMStat counters, etc.
522
523 DTRACE_MONITOR_PROBE(contended__entered, this, object(), current);
524 if (JvmtiExport::should_post_monitor_contended_entered()) {
525 JvmtiExport::post_monitor_contended_entered(current, this);
526
527 // The current thread already owns the monitor and is not going to
528 // call park() for the remainder of the monitor enter protocol. So
529 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
530 // event handler consumed an unpark() issued by the thread that
531 // just exited the monitor.
532 }
533 if (event.should_commit()) {
534 event.set_previousOwner(_previous_owner_tid);
535 event.commit();
536 }
537 OM_PERFDATA_OP(ContendedLockAttempts, inc());
538 return true;
539 }
540
541 // Caveat: TryLock() is not necessarily serializing if it returns failure.
542 // Callers must compensate as needed.
543
544 int ObjectMonitor::TryLock(JavaThread* current) {
545 void* own = owner_raw();
546 if (own != nullptr) return 0;
547 if (try_set_owner_from(nullptr, current) == nullptr) {
548 assert(_recursions == 0, "invariant");
549 return 1;
550 }
551 // The lock had been free momentarily, but we lost the race to the lock.
552 // Interference -- the CAS failed.
553 // We can either return -1 or retry.
554 // Retry doesn't make as much sense because the lock was just acquired.
555 return -1;
556 }
557
558 // Deflate the specified ObjectMonitor if not in-use. Returns true if it
559 // was deflated and false otherwise.
560 //
561 // The async deflation protocol sets owner to DEFLATER_MARKER and
562 // makes contentions negative as signals to contending threads that
563 // an async deflation is in progress. There are a number of checks
564 // as part of the protocol to make sure that the calling thread has
565 // not lost the race to a contending thread.
566 //
567 // The ObjectMonitor has been successfully async deflated when:
568 // (contentions < 0)
569 // Contending threads that see that condition know to retry their operation.
570 //
571 bool ObjectMonitor::deflate_monitor() {
572 if (is_busy()) {
573 // Easy checks are first - the ObjectMonitor is busy so no deflation.
574 return false;
575 }
576
577 if (ObjectSynchronizer::is_final_audit() && owner_is_DEFLATER_MARKER()) {
578 // The final audit can see an already deflated ObjectMonitor on the
579 // in-use list because MonitorList::unlink_deflated() might have
580 // blocked for the final safepoint before unlinking all the deflated
581 // monitors.
582 assert(contentions() < 0, "must be negative: contentions=%d", contentions());
583 // Already returned 'true' when it was originally deflated.
584 return false;
585 }
586
587 const oop obj = object_peek();
588
589 if (obj == nullptr) {
590 // If the object died, we can recycle the monitor without racing with
591 // Java threads. The GC already broke the association with the object.
592 set_owner_from(nullptr, DEFLATER_MARKER);
593 assert(contentions() >= 0, "must be non-negative: contentions=%d", contentions());
594 _contentions = INT_MIN; // minimum negative int
595 } else {
596 // Attempt async deflation protocol.
597
598 // Set a null owner to DEFLATER_MARKER to force any contending thread
599 // through the slow path. This is just the first part of the async
600 // deflation dance.
601 if (try_set_owner_from(nullptr, DEFLATER_MARKER) != nullptr) {
602 // The owner field is no longer null so we lost the race since the
603 // ObjectMonitor is now busy.
604 return false;
605 }
606
607 if (contentions() > 0 || _waiters != 0) {
608 // Another thread has raced to enter the ObjectMonitor after
609 // is_busy() above or has already entered and waited on
610 // it which makes it busy so no deflation. Restore owner to
611 // null if it is still DEFLATER_MARKER.
612 if (try_set_owner_from(DEFLATER_MARKER, nullptr) != DEFLATER_MARKER) {
613 // Deferred decrement for the JT EnterI() that cancelled the async deflation.
614 add_to_contentions(-1);
615 }
616 return false;
617 }
618
619 // Make a zero contentions field negative to force any contending threads
620 // to retry. This is the second part of the async deflation dance.
621 if (Atomic::cmpxchg(&_contentions, 0, INT_MIN) != 0) {
622 // Contentions was no longer 0 so we lost the race since the
623 // ObjectMonitor is now busy. Restore owner to null if it is
624 // still DEFLATER_MARKER:
625 if (try_set_owner_from(DEFLATER_MARKER, nullptr) != DEFLATER_MARKER) {
626 // Deferred decrement for the JT EnterI() that cancelled the async deflation.
627 add_to_contentions(-1);
628 }
629 return false;
630 }
631 }
632
633 // Sanity checks for the races:
634 guarantee(owner_is_DEFLATER_MARKER(), "must be deflater marker");
635 guarantee(contentions() < 0, "must be negative: contentions=%d",
636 contentions());
637 guarantee(_waiters == 0, "must be 0: waiters=%d", _waiters);
638 guarantee(_cxq == nullptr, "must be no contending threads: cxq="
639 INTPTR_FORMAT, p2i(_cxq));
640 guarantee(_EntryList == nullptr,
641 "must be no entering threads: EntryList=" INTPTR_FORMAT,
642 p2i(_EntryList));
643
644 if (obj != nullptr) {
645 if (log_is_enabled(Trace, monitorinflation)) {
646 ResourceMark rm;
647 log_trace(monitorinflation)("deflate_monitor: object=" INTPTR_FORMAT
648 ", mark=" INTPTR_FORMAT ", type='%s'",
649 p2i(obj), obj->mark().value(),
650 obj->klass()->external_name());
651 }
652
653 // Install the old mark word if nobody else has already done it.
654 install_displaced_markword_in_object(obj);
655 }
656
657 // We leave owner == DEFLATER_MARKER and contentions < 0
658 // to force any racing threads to retry.
659 return true; // Success, ObjectMonitor has been deflated.
660 }
661
662 // Install the displaced mark word (dmw) of a deflating ObjectMonitor
663 // into the header of the object associated with the monitor. This
664 // idempotent method is called by a thread that is deflating a
665 // monitor and by other threads that have detected a race with the
666 // deflation process.
667 void ObjectMonitor::install_displaced_markword_in_object(const oop obj) {
668 // This function must only be called when (owner == DEFLATER_MARKER
669 // && contentions <= 0), but we can't guarantee that here because
670 // those values could change when the ObjectMonitor gets moved from
671 // the global free list to a per-thread free list.
672
673 guarantee(obj != nullptr, "must be non-null");
674
675 // Separate loads in is_being_async_deflated(), which is almost always
676 // called before this function, from the load of dmw/header below.
677
678 // _contentions and dmw/header may get written by different threads.
679 // Make sure to observe them in the same order when having several observers.
680 OrderAccess::loadload_for_IRIW();
681
682 const oop l_object = object_peek();
683 if (l_object == nullptr) {
684 // ObjectMonitor's object ref has already been cleared by async
685 // deflation or GC so we're done here.
686 return;
687 }
688 assert(l_object == obj, "object=" INTPTR_FORMAT " must equal obj="
689 INTPTR_FORMAT, p2i(l_object), p2i(obj));
690
691 markWord dmw = header();
692 // The dmw has to be neutral (not null, not locked and not marked).
693 assert(dmw.is_neutral(), "must be neutral: dmw=" INTPTR_FORMAT, dmw.value());
694
695 // Install displaced mark word if the object's header still points
696 // to this ObjectMonitor. More than one racing caller to this function
697 // can rarely reach this point, but only one can win.
698 markWord res = obj->cas_set_mark(dmw, markWord::encode(this));
699 if (res != markWord::encode(this)) {
700 // This should be rare so log at the Info level when it happens.
701 log_info(monitorinflation)("install_displaced_markword_in_object: "
702 "failed cas_set_mark: new_mark=" INTPTR_FORMAT
703 ", old_mark=" INTPTR_FORMAT ", res=" INTPTR_FORMAT,
704 dmw.value(), markWord::encode(this).value(),
705 res.value());
706 }
707
708 // Note: It does not matter which thread restored the header/dmw
709 // into the object's header. The thread deflating the monitor just
710 // wanted the object's header restored and it is. The threads that
711 // detected a race with the deflation process also wanted the
712 // object's header restored before they retry their operation and
713 // because it is restored they will only retry once.
714 }
715
716 // Convert the fields used by is_busy() to a string that can be
717 // used for diagnostic output.
718 const char* ObjectMonitor::is_busy_to_string(stringStream* ss) {
719 ss->print("is_busy: waiters=%d, ", _waiters);
720 if (contentions() > 0) {
721 ss->print("contentions=%d, ", contentions());
722 } else {
723 ss->print("contentions=0");
724 }
725 if (!owner_is_DEFLATER_MARKER()) {
726 ss->print("owner=" INTPTR_FORMAT, p2i(owner_raw()));
727 } else {
728 // We report null instead of DEFLATER_MARKER here because is_busy()
729 // ignores DEFLATER_MARKER values.
730 ss->print("owner=" INTPTR_FORMAT, NULL_WORD);
731 }
732 ss->print(", cxq=" INTPTR_FORMAT ", EntryList=" INTPTR_FORMAT, p2i(_cxq),
733 p2i(_EntryList));
734 return ss->base();
735 }
736
737 #define MAX_RECHECK_INTERVAL 1000
738
739 void ObjectMonitor::EnterI(JavaThread* current) {
740 assert(current->thread_state() == _thread_blocked, "invariant");
741
742 // Try the lock - TATAS
743 if (TryLock (current) > 0) {
744 assert(_succ != current, "invariant");
745 assert(owner_raw() == current, "invariant");
746 assert(_Responsible != current, "invariant");
747 return;
748 }
749
750 if (try_set_owner_from(DEFLATER_MARKER, current) == DEFLATER_MARKER) {
751 // Cancelled the in-progress async deflation by changing owner from
752 // DEFLATER_MARKER to current. As part of the contended enter protocol,
753 // contentions was incremented to a positive value before EnterI()
754 // was called and that prevents the deflater thread from winning the
755 // last part of the 2-part async deflation protocol. After EnterI()
756 // returns to enter(), contentions is decremented because the caller
757 // now owns the monitor. We bump contentions an extra time here to
758 // prevent the deflater thread from winning the last part of the
759 // 2-part async deflation protocol after the regular decrement
760 // occurs in enter(). The deflater thread will decrement contentions
761 // after it recognizes that the async deflation was cancelled.
762 add_to_contentions(1);
763 assert(_succ != current, "invariant");
764 assert(_Responsible != current, "invariant");
765 return;
766 }
767
768 assert(InitDone, "Unexpectedly not initialized");
769
770 // We try one round of spinning *before* enqueueing current.
771 //
772 // If the _owner is ready but OFFPROC we could use a YieldTo()
773 // operation to donate the remainder of this thread's quantum
774 // to the owner. This has subtle but beneficial affinity
775 // effects.
776
777 if (TrySpin(current) > 0) {
778 assert(owner_raw() == current, "invariant");
779 assert(_succ != current, "invariant");
780 assert(_Responsible != current, "invariant");
781 return;
782 }
783
784 // The Spin failed -- Enqueue and park the thread ...
785 assert(_succ != current, "invariant");
786 assert(owner_raw() != current, "invariant");
787 assert(_Responsible != current, "invariant");
788
789 // Enqueue "current" on ObjectMonitor's _cxq.
790 //
791 // Node acts as a proxy for current.
792 // As an aside, if were to ever rewrite the synchronization code mostly
793 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
794 // Java objects. This would avoid awkward lifecycle and liveness issues,
795 // as well as eliminate a subset of ABA issues.
796 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
797
798 ObjectWaiter node(current);
799 current->_ParkEvent->reset();
800 node._prev = (ObjectWaiter*) 0xBAD;
801 node.TState = ObjectWaiter::TS_CXQ;
802
803 // Push "current" onto the front of the _cxq.
804 // Once on cxq/EntryList, current stays on-queue until it acquires the lock.
805 // Note that spinning tends to reduce the rate at which threads
806 // enqueue and dequeue on EntryList|cxq.
807 ObjectWaiter* nxt;
808 for (;;) {
809 node._next = nxt = _cxq;
810 if (Atomic::cmpxchg(&_cxq, nxt, &node) == nxt) break;
811
812 // Interference - the CAS failed because _cxq changed. Just retry.
813 // As an optional optimization we retry the lock.
814 if (TryLock (current) > 0) {
815 assert(_succ != current, "invariant");
816 assert(owner_raw() == current, "invariant");
817 assert(_Responsible != current, "invariant");
818 return;
819 }
820 }
821
822 // Check for cxq|EntryList edge transition to non-null. This indicates
823 // the onset of contention. While contention persists exiting threads
824 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
825 // operations revert to the faster 1-0 mode. This enter operation may interleave
826 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
827 // arrange for one of the contending thread to use a timed park() operations
828 // to detect and recover from the race. (Stranding is form of progress failure
829 // where the monitor is unlocked but all the contending threads remain parked).
830 // That is, at least one of the contended threads will periodically poll _owner.
831 // One of the contending threads will become the designated "Responsible" thread.
832 // The Responsible thread uses a timed park instead of a normal indefinite park
833 // operation -- it periodically wakes and checks for and recovers from potential
834 // strandings admitted by 1-0 exit operations. We need at most one Responsible
835 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
836 // be responsible for a monitor.
837 //
838 // Currently, one of the contended threads takes on the added role of "Responsible".
839 // A viable alternative would be to use a dedicated "stranding checker" thread
840 // that periodically iterated over all the threads (or active monitors) and unparked
841 // successors where there was risk of stranding. This would help eliminate the
842 // timer scalability issues we see on some platforms as we'd only have one thread
843 // -- the checker -- parked on a timer.
844
845 if (nxt == nullptr && _EntryList == nullptr) {
846 // Try to assume the role of responsible thread for the monitor.
847 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=current }
848 Atomic::replace_if_null(&_Responsible, current);
849 }
850
851 // The lock might have been released while this thread was occupied queueing
852 // itself onto _cxq. To close the race and avoid "stranding" and
853 // progress-liveness failure we must resample-retry _owner before parking.
854 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
855 // In this case the ST-MEMBAR is accomplished with CAS().
856 //
857 // TODO: Defer all thread state transitions until park-time.
858 // Since state transitions are heavy and inefficient we'd like
859 // to defer the state transitions until absolutely necessary,
860 // and in doing so avoid some transitions ...
861
862 int nWakeups = 0;
863 int recheckInterval = 1;
864
865 for (;;) {
866
867 if (TryLock(current) > 0) break;
868 assert(owner_raw() != current, "invariant");
869
870 // park self
871 if (_Responsible == current) {
872 current->_ParkEvent->park((jlong) recheckInterval);
873 // Increase the recheckInterval, but clamp the value.
874 recheckInterval *= 8;
875 if (recheckInterval > MAX_RECHECK_INTERVAL) {
876 recheckInterval = MAX_RECHECK_INTERVAL;
877 }
878 } else {
879 current->_ParkEvent->park();
880 }
881
882 if (TryLock(current) > 0) break;
883
884 if (try_set_owner_from(DEFLATER_MARKER, current) == DEFLATER_MARKER) {
885 // Cancelled the in-progress async deflation by changing owner from
886 // DEFLATER_MARKER to current. As part of the contended enter protocol,
887 // contentions was incremented to a positive value before EnterI()
888 // was called and that prevents the deflater thread from winning the
889 // last part of the 2-part async deflation protocol. After EnterI()
890 // returns to enter(), contentions is decremented because the caller
891 // now owns the monitor. We bump contentions an extra time here to
892 // prevent the deflater thread from winning the last part of the
893 // 2-part async deflation protocol after the regular decrement
894 // occurs in enter(). The deflater thread will decrement contentions
895 // after it recognizes that the async deflation was cancelled.
896 add_to_contentions(1);
897 break;
898 }
899
900 // The lock is still contested.
901 // Keep a tally of the # of futile wakeups.
902 // Note that the counter is not protected by a lock or updated by atomics.
903 // That is by design - we trade "lossy" counters which are exposed to
904 // races during updates for a lower probe effect.
905
906 // This PerfData object can be used in parallel with a safepoint.
907 // See the work around in PerfDataManager::destroy().
908 OM_PERFDATA_OP(FutileWakeups, inc());
909 ++nWakeups;
910
911 // Assuming this is not a spurious wakeup we'll normally find _succ == current.
912 // We can defer clearing _succ until after the spin completes
913 // TrySpin() must tolerate being called with _succ == current.
914 // Try yet another round of adaptive spinning.
915 if (TrySpin(current) > 0) break;
916
917 // We can find that we were unpark()ed and redesignated _succ while
918 // we were spinning. That's harmless. If we iterate and call park(),
919 // park() will consume the event and return immediately and we'll
920 // just spin again. This pattern can repeat, leaving _succ to simply
921 // spin on a CPU.
922
923 if (_succ == current) _succ = nullptr;
924
925 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
926 OrderAccess::fence();
927 }
928
929 // Egress :
930 // current has acquired the lock -- Unlink current from the cxq or EntryList.
931 // Normally we'll find current on the EntryList .
932 // From the perspective of the lock owner (this thread), the
933 // EntryList is stable and cxq is prepend-only.
934 // The head of cxq is volatile but the interior is stable.
935 // In addition, current.TState is stable.
936
937 assert(owner_raw() == current, "invariant");
938
939 UnlinkAfterAcquire(current, &node);
940 if (_succ == current) _succ = nullptr;
941
942 assert(_succ != current, "invariant");
943 if (_Responsible == current) {
944 _Responsible = nullptr;
945 OrderAccess::fence(); // Dekker pivot-point
946
947 // We may leave threads on cxq|EntryList without a designated
948 // "Responsible" thread. This is benign. When this thread subsequently
949 // exits the monitor it can "see" such preexisting "old" threads --
950 // threads that arrived on the cxq|EntryList before the fence, above --
951 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
952 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
953 // non-null and elect a new "Responsible" timer thread.
954 //
955 // This thread executes:
956 // ST Responsible=null; MEMBAR (in enter epilogue - here)
957 // LD cxq|EntryList (in subsequent exit)
958 //
959 // Entering threads in the slow/contended path execute:
960 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
961 // The (ST cxq; MEMBAR) is accomplished with CAS().
962 //
963 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
964 // exit operation from floating above the ST Responsible=null.
965 }
966
967 // We've acquired ownership with CAS().
968 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
969 // But since the CAS() this thread may have also stored into _succ,
970 // EntryList, cxq or Responsible. These meta-data updates must be
971 // visible __before this thread subsequently drops the lock.
972 // Consider what could occur if we didn't enforce this constraint --
973 // STs to monitor meta-data and user-data could reorder with (become
974 // visible after) the ST in exit that drops ownership of the lock.
975 // Some other thread could then acquire the lock, but observe inconsistent
976 // or old monitor meta-data and heap data. That violates the JMM.
977 // To that end, the 1-0 exit() operation must have at least STST|LDST
978 // "release" barrier semantics. Specifically, there must be at least a
979 // STST|LDST barrier in exit() before the ST of null into _owner that drops
980 // the lock. The barrier ensures that changes to monitor meta-data and data
981 // protected by the lock will be visible before we release the lock, and
982 // therefore before some other thread (CPU) has a chance to acquire the lock.
983 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
984 //
985 // Critically, any prior STs to _succ or EntryList must be visible before
986 // the ST of null into _owner in the *subsequent* (following) corresponding
987 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
988 // execute a serializing instruction.
989
990 return;
991 }
992
993 // ReenterI() is a specialized inline form of the latter half of the
994 // contended slow-path from EnterI(). We use ReenterI() only for
995 // monitor reentry in wait().
996 //
997 // In the future we should reconcile EnterI() and ReenterI().
998
999 void ObjectMonitor::ReenterI(JavaThread* current, ObjectWaiter* currentNode) {
1000 assert(current != nullptr, "invariant");
1001 assert(currentNode != nullptr, "invariant");
1002 assert(currentNode->_thread == current, "invariant");
1003 assert(_waiters > 0, "invariant");
1004 assert(object()->mark() == markWord::encode(this), "invariant");
1005
1006 assert(current->thread_state() != _thread_blocked, "invariant");
1007
1008 int nWakeups = 0;
1009 for (;;) {
1010 ObjectWaiter::TStates v = currentNode->TState;
1011 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1012 assert(owner_raw() != current, "invariant");
1013
1014 if (TryLock(current) > 0) break;
1015 if (TrySpin(current) > 0) break;
1016
1017 {
1018 OSThreadContendState osts(current->osthread());
1019
1020 assert(current->thread_state() == _thread_in_vm, "invariant");
1021
1022 {
1023 ClearSuccOnSuspend csos(this);
1024 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */);
1025 current->_ParkEvent->park();
1026 }
1027 }
1028
1029 // Try again, but just so we distinguish between futile wakeups and
1030 // successful wakeups. The following test isn't algorithmically
1031 // necessary, but it helps us maintain sensible statistics.
1032 if (TryLock(current) > 0) break;
1033
1034 // The lock is still contested.
1035 // Keep a tally of the # of futile wakeups.
1036 // Note that the counter is not protected by a lock or updated by atomics.
1037 // That is by design - we trade "lossy" counters which are exposed to
1038 // races during updates for a lower probe effect.
1039 ++nWakeups;
1040
1041 // Assuming this is not a spurious wakeup we'll normally
1042 // find that _succ == current.
1043 if (_succ == current) _succ = nullptr;
1044
1045 // Invariant: after clearing _succ a contending thread
1046 // *must* retry _owner before parking.
1047 OrderAccess::fence();
1048
1049 // This PerfData object can be used in parallel with a safepoint.
1050 // See the work around in PerfDataManager::destroy().
1051 OM_PERFDATA_OP(FutileWakeups, inc());
1052 }
1053
1054 // current has acquired the lock -- Unlink current from the cxq or EntryList .
1055 // Normally we'll find current on the EntryList.
1056 // Unlinking from the EntryList is constant-time and atomic-free.
1057 // From the perspective of the lock owner (this thread), the
1058 // EntryList is stable and cxq is prepend-only.
1059 // The head of cxq is volatile but the interior is stable.
1060 // In addition, current.TState is stable.
1061
1062 assert(owner_raw() == current, "invariant");
1063 assert(object()->mark() == markWord::encode(this), "invariant");
1064 UnlinkAfterAcquire(current, currentNode);
1065 if (_succ == current) _succ = nullptr;
1066 assert(_succ != current, "invariant");
1067 currentNode->TState = ObjectWaiter::TS_RUN;
1068 OrderAccess::fence(); // see comments at the end of EnterI()
1069 }
1070
1071 // By convention we unlink a contending thread from EntryList|cxq immediately
1072 // after the thread acquires the lock in ::enter(). Equally, we could defer
1073 // unlinking the thread until ::exit()-time.
1074
1075 void ObjectMonitor::UnlinkAfterAcquire(JavaThread* current, ObjectWaiter* currentNode) {
1076 assert(owner_raw() == current, "invariant");
1077 assert(currentNode->_thread == current, "invariant");
1078
1079 if (currentNode->TState == ObjectWaiter::TS_ENTER) {
1080 // Normal case: remove current from the DLL EntryList .
1081 // This is a constant-time operation.
1082 ObjectWaiter* nxt = currentNode->_next;
1083 ObjectWaiter* prv = currentNode->_prev;
1084 if (nxt != nullptr) nxt->_prev = prv;
1085 if (prv != nullptr) prv->_next = nxt;
1086 if (currentNode == _EntryList) _EntryList = nxt;
1087 assert(nxt == nullptr || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
1088 assert(prv == nullptr || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
1089 } else {
1090 assert(currentNode->TState == ObjectWaiter::TS_CXQ, "invariant");
1091 // Inopportune interleaving -- current is still on the cxq.
1092 // This usually means the enqueue of self raced an exiting thread.
1093 // Normally we'll find current near the front of the cxq, so
1094 // dequeueing is typically fast. If needbe we can accelerate
1095 // this with some MCS/CHL-like bidirectional list hints and advisory
1096 // back-links so dequeueing from the interior will normally operate
1097 // in constant-time.
1098 // Dequeue current from either the head (with CAS) or from the interior
1099 // with a linear-time scan and normal non-atomic memory operations.
1100 // CONSIDER: if current is on the cxq then simply drain cxq into EntryList
1101 // and then unlink current from EntryList. We have to drain eventually,
1102 // so it might as well be now.
1103
1104 ObjectWaiter* v = _cxq;
1105 assert(v != nullptr, "invariant");
1106 if (v != currentNode || Atomic::cmpxchg(&_cxq, v, currentNode->_next) != v) {
1107 // The CAS above can fail from interference IFF a "RAT" arrived.
1108 // In that case current must be in the interior and can no longer be
1109 // at the head of cxq.
1110 if (v == currentNode) {
1111 assert(_cxq != v, "invariant");
1112 v = _cxq; // CAS above failed - start scan at head of list
1113 }
1114 ObjectWaiter* p;
1115 ObjectWaiter* q = nullptr;
1116 for (p = v; p != nullptr && p != currentNode; p = p->_next) {
1117 q = p;
1118 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
1119 }
1120 assert(v != currentNode, "invariant");
1121 assert(p == currentNode, "Node not found on cxq");
1122 assert(p != _cxq, "invariant");
1123 assert(q != nullptr, "invariant");
1124 assert(q->_next == p, "invariant");
1125 q->_next = p->_next;
1126 }
1127 }
1128
1129 #ifdef ASSERT
1130 // Diagnostic hygiene ...
1131 currentNode->_prev = (ObjectWaiter*) 0xBAD;
1132 currentNode->_next = (ObjectWaiter*) 0xBAD;
1133 currentNode->TState = ObjectWaiter::TS_RUN;
1134 #endif
1135 }
1136
1137 // -----------------------------------------------------------------------------
1138 // Exit support
1139 //
1140 // exit()
1141 // ~~~~~~
1142 // Note that the collector can't reclaim the objectMonitor or deflate
1143 // the object out from underneath the thread calling ::exit() as the
1144 // thread calling ::exit() never transitions to a stable state.
1145 // This inhibits GC, which in turn inhibits asynchronous (and
1146 // inopportune) reclamation of "this".
1147 //
1148 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
1149 // There's one exception to the claim above, however. EnterI() can call
1150 // exit() to drop a lock if the acquirer has been externally suspended.
1151 // In that case exit() is called with _thread_state == _thread_blocked,
1152 // but the monitor's _contentions field is > 0, which inhibits reclamation.
1153 //
1154 // 1-0 exit
1155 // ~~~~~~~~
1156 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
1157 // the fast-path operators have been optimized so the common ::exit()
1158 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
1159 // The code emitted by fast_unlock() elides the usual MEMBAR. This
1160 // greatly improves latency -- MEMBAR and CAS having considerable local
1161 // latency on modern processors -- but at the cost of "stranding". Absent the
1162 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
1163 // ::enter() path, resulting in the entering thread being stranding
1164 // and a progress-liveness failure. Stranding is extremely rare.
1165 // We use timers (timed park operations) & periodic polling to detect
1166 // and recover from stranding. Potentially stranded threads periodically
1167 // wake up and poll the lock. See the usage of the _Responsible variable.
1168 //
1169 // The CAS() in enter provides for safety and exclusion, while the CAS or
1170 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
1171 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
1172 // We detect and recover from stranding with timers.
1173 //
1174 // If a thread transiently strands it'll park until (a) another
1175 // thread acquires the lock and then drops the lock, at which time the
1176 // exiting thread will notice and unpark the stranded thread, or, (b)
1177 // the timer expires. If the lock is high traffic then the stranding latency
1178 // will be low due to (a). If the lock is low traffic then the odds of
1179 // stranding are lower, although the worst-case stranding latency
1180 // is longer. Critically, we don't want to put excessive load in the
1181 // platform's timer subsystem. We want to minimize both the timer injection
1182 // rate (timers created/sec) as well as the number of timers active at
1183 // any one time. (more precisely, we want to minimize timer-seconds, which is
1184 // the integral of the # of active timers at any instant over time).
1185 // Both impinge on OS scalability. Given that, at most one thread parked on
1186 // a monitor will use a timer.
1187 //
1188 // There is also the risk of a futile wake-up. If we drop the lock
1189 // another thread can reacquire the lock immediately, and we can
1190 // then wake a thread unnecessarily. This is benign, and we've
1191 // structured the code so the windows are short and the frequency
1192 // of such futile wakups is low.
1193
1194 void ObjectMonitor::exit(JavaThread* current, bool not_suspended) {
1195 void* cur = owner_raw();
1196 if (current != cur) {
1197 if (LockingMode != LM_LIGHTWEIGHT && current->is_lock_owned((address)cur)) {
1198 assert(_recursions == 0, "invariant");
1199 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*.
1200 _recursions = 0;
1201 } else {
1202 // Apparent unbalanced locking ...
1203 // Naively we'd like to throw IllegalMonitorStateException.
1204 // As a practical matter we can neither allocate nor throw an
1205 // exception as ::exit() can be called from leaf routines.
1206 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
1207 // Upon deeper reflection, however, in a properly run JVM the only
1208 // way we should encounter this situation is in the presence of
1209 // unbalanced JNI locking. TODO: CheckJNICalls.
1210 // See also: CR4414101
1211 #ifdef ASSERT
1212 LogStreamHandle(Error, monitorinflation) lsh;
1213 lsh.print_cr("ERROR: ObjectMonitor::exit(): thread=" INTPTR_FORMAT
1214 " is exiting an ObjectMonitor it does not own.", p2i(current));
1215 lsh.print_cr("The imbalance is possibly caused by JNI locking.");
1216 print_debug_style_on(&lsh);
1217 assert(false, "Non-balanced monitor enter/exit!");
1218 #endif
1219 return;
1220 }
1221 }
1222
1223 if (_recursions != 0) {
1224 _recursions--; // this is simple recursive enter
1225 return;
1226 }
1227
1228 // Invariant: after setting Responsible=null an thread must execute
1229 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
1230 _Responsible = nullptr;
1231
1232 #if INCLUDE_JFR
1233 // get the owner's thread id for the MonitorEnter event
1234 // if it is enabled and the thread isn't suspended
1235 if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
1236 _previous_owner_tid = JFR_THREAD_ID(current);
1237 }
1238 #endif
1239
1240 for (;;) {
1241 assert(current == owner_raw(), "invariant");
1242
1243 // Drop the lock.
1244 // release semantics: prior loads and stores from within the critical section
1245 // must not float (reorder) past the following store that drops the lock.
1246 // Uses a storeload to separate release_store(owner) from the
1247 // successor check. The try_set_owner() below uses cmpxchg() so
1248 // we get the fence down there.
1249 release_clear_owner(current);
1250 OrderAccess::storeload();
1251
1252 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != nullptr) {
1253 return;
1254 }
1255 // Other threads are blocked trying to acquire the lock.
1256
1257 // Normally the exiting thread is responsible for ensuring succession,
1258 // but if other successors are ready or other entering threads are spinning
1259 // then this thread can simply store null into _owner and exit without
1260 // waking a successor. The existence of spinners or ready successors
1261 // guarantees proper succession (liveness). Responsibility passes to the
1262 // ready or running successors. The exiting thread delegates the duty.
1263 // More precisely, if a successor already exists this thread is absolved
1264 // of the responsibility of waking (unparking) one.
1265 //
1266 // The _succ variable is critical to reducing futile wakeup frequency.
1267 // _succ identifies the "heir presumptive" thread that has been made
1268 // ready (unparked) but that has not yet run. We need only one such
1269 // successor thread to guarantee progress.
1270 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1271 // section 3.3 "Futile Wakeup Throttling" for details.
1272 //
1273 // Note that spinners in Enter() also set _succ non-null.
1274 // In the current implementation spinners opportunistically set
1275 // _succ so that exiting threads might avoid waking a successor.
1276 // Another less appealing alternative would be for the exiting thread
1277 // to drop the lock and then spin briefly to see if a spinner managed
1278 // to acquire the lock. If so, the exiting thread could exit
1279 // immediately without waking a successor, otherwise the exiting
1280 // thread would need to dequeue and wake a successor.
1281 // (Note that we'd need to make the post-drop spin short, but no
1282 // shorter than the worst-case round-trip cache-line migration time.
1283 // The dropped lock needs to become visible to the spinner, and then
1284 // the acquisition of the lock by the spinner must become visible to
1285 // the exiting thread).
1286
1287 // It appears that an heir-presumptive (successor) must be made ready.
1288 // Only the current lock owner can manipulate the EntryList or
1289 // drain _cxq, so we need to reacquire the lock. If we fail
1290 // to reacquire the lock the responsibility for ensuring succession
1291 // falls to the new owner.
1292 //
1293 if (try_set_owner_from(nullptr, current) != nullptr) {
1294 return;
1295 }
1296
1297 guarantee(owner_raw() == current, "invariant");
1298
1299 ObjectWaiter* w = nullptr;
1300
1301 w = _EntryList;
1302 if (w != nullptr) {
1303 // I'd like to write: guarantee (w->_thread != current).
1304 // But in practice an exiting thread may find itself on the EntryList.
1305 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1306 // then calls exit(). Exit release the lock by setting O._owner to null.
1307 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1308 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1309 // release the lock "O". T2 resumes immediately after the ST of null into
1310 // _owner, above. T2 notices that the EntryList is populated, so it
1311 // reacquires the lock and then finds itself on the EntryList.
1312 // Given all that, we have to tolerate the circumstance where "w" is
1313 // associated with current.
1314 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1315 ExitEpilog(current, w);
1316 return;
1317 }
1318
1319 // If we find that both _cxq and EntryList are null then just
1320 // re-run the exit protocol from the top.
1321 w = _cxq;
1322 if (w == nullptr) continue;
1323
1324 // Drain _cxq into EntryList - bulk transfer.
1325 // First, detach _cxq.
1326 // The following loop is tantamount to: w = swap(&cxq, nullptr)
1327 for (;;) {
1328 assert(w != nullptr, "Invariant");
1329 ObjectWaiter* u = Atomic::cmpxchg(&_cxq, w, (ObjectWaiter*)nullptr);
1330 if (u == w) break;
1331 w = u;
1332 }
1333
1334 assert(w != nullptr, "invariant");
1335 assert(_EntryList == nullptr, "invariant");
1336
1337 // Convert the LIFO SLL anchored by _cxq into a DLL.
1338 // The list reorganization step operates in O(LENGTH(w)) time.
1339 // It's critical that this step operate quickly as
1340 // "current" still holds the outer-lock, restricting parallelism
1341 // and effectively lengthening the critical section.
1342 // Invariant: s chases t chases u.
1343 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1344 // we have faster access to the tail.
1345
1346 _EntryList = w;
1347 ObjectWaiter* q = nullptr;
1348 ObjectWaiter* p;
1349 for (p = w; p != nullptr; p = p->_next) {
1350 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1351 p->TState = ObjectWaiter::TS_ENTER;
1352 p->_prev = q;
1353 q = p;
1354 }
1355
1356 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = nullptr
1357 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1358
1359 // See if we can abdicate to a spinner instead of waking a thread.
1360 // A primary goal of the implementation is to reduce the
1361 // context-switch rate.
1362 if (_succ != nullptr) continue;
1363
1364 w = _EntryList;
1365 if (w != nullptr) {
1366 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1367 ExitEpilog(current, w);
1368 return;
1369 }
1370 }
1371 }
1372
1373 void ObjectMonitor::ExitEpilog(JavaThread* current, ObjectWaiter* Wakee) {
1374 assert(owner_raw() == current, "invariant");
1375
1376 // Exit protocol:
1377 // 1. ST _succ = wakee
1378 // 2. membar #loadstore|#storestore;
1379 // 2. ST _owner = nullptr
1380 // 3. unpark(wakee)
1381
1382 _succ = Wakee->_thread;
1383 ParkEvent * Trigger = Wakee->_event;
1384
1385 // Hygiene -- once we've set _owner = nullptr we can't safely dereference Wakee again.
1386 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1387 // out-of-scope (non-extant).
1388 Wakee = nullptr;
1389
1390 // Drop the lock.
1391 // Uses a fence to separate release_store(owner) from the LD in unpark().
1392 release_clear_owner(current);
1393 OrderAccess::fence();
1394
1395 DTRACE_MONITOR_PROBE(contended__exit, this, object(), current);
1396 Trigger->unpark();
1397
1398 // Maintain stats and report events to JVMTI
1399 OM_PERFDATA_OP(Parks, inc());
1400 }
1401
1402 // complete_exit exits a lock returning recursion count
1403 // complete_exit requires an inflated monitor
1404 // The _owner field is not always the Thread addr even with an
1405 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1406 // thread due to contention.
1407 intx ObjectMonitor::complete_exit(JavaThread* current) {
1408 assert(InitDone, "Unexpectedly not initialized");
1409
1410 void* cur = owner_raw();
1411 if (current != cur) {
1412 if (LockingMode != LM_LIGHTWEIGHT && current->is_lock_owned((address)cur)) {
1413 assert(_recursions == 0, "internal state error");
1414 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*.
1415 _recursions = 0;
1416 }
1417 }
1418
1419 guarantee(current == owner_raw(), "complete_exit not owner");
1420 intx save = _recursions; // record the old recursion count
1421 _recursions = 0; // set the recursion level to be 0
1422 exit(current); // exit the monitor
1423 guarantee(owner_raw() != current, "invariant");
1424 return save;
1425 }
1426
1427 // Checks that the current THREAD owns this monitor and causes an
1428 // immediate return if it doesn't. We don't use the CHECK macro
1429 // because we want the IMSE to be the only exception that is thrown
1430 // from the call site when false is returned. Any other pending
1431 // exception is ignored.
1432 #define CHECK_OWNER() \
1433 do { \
1434 if (!check_owner(THREAD)) { \
1435 assert(HAS_PENDING_EXCEPTION, "expected a pending IMSE here."); \
1436 return; \
1437 } \
1438 } while (false)
1439
1440 // Returns true if the specified thread owns the ObjectMonitor.
1441 // Otherwise returns false and throws IllegalMonitorStateException
1442 // (IMSE). If there is a pending exception and the specified thread
1443 // is not the owner, that exception will be replaced by the IMSE.
1444 bool ObjectMonitor::check_owner(TRAPS) {
1445 JavaThread* current = THREAD;
1446 void* cur = owner_raw();
1447 assert(cur != anon_owner_ptr(), "no anon owner here");
1448 if (cur == current) {
1449 return true;
1450 }
1451 if (LockingMode != LM_LIGHTWEIGHT && current->is_lock_owned((address)cur)) {
1452 set_owner_from_BasicLock(cur, current); // Convert from BasicLock* to Thread*.
1453 _recursions = 0;
1454 return true;
1455 }
1456 THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(),
1457 "current thread is not owner", false);
1458 }
1459
1460 static inline bool is_excluded(const Klass* monitor_klass) {
1461 assert(monitor_klass != nullptr, "invariant");
1462 NOT_JFR_RETURN_(false);
1463 JFR_ONLY(return vmSymbols::jfr_chunk_rotation_monitor() == monitor_klass->name());
1464 }
1465
1466 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1467 ObjectMonitor* monitor,
1468 uint64_t notifier_tid,
1469 jlong timeout,
1470 bool timedout) {
1471 assert(event != nullptr, "invariant");
1472 assert(monitor != nullptr, "invariant");
1473 const Klass* monitor_klass = monitor->object()->klass();
1474 if (is_excluded(monitor_klass)) {
1475 return;
1476 }
1477 event->set_monitorClass(monitor_klass);
1478 event->set_timeout(timeout);
1479 // Set an address that is 'unique enough', such that events close in
1480 // time and with the same address are likely (but not guaranteed) to
1481 // belong to the same object.
1482 event->set_address((uintptr_t)monitor);
1483 event->set_notifier(notifier_tid);
1484 event->set_timedOut(timedout);
1485 event->commit();
1486 }
1487
1488 // -----------------------------------------------------------------------------
1489 // Wait/Notify/NotifyAll
1490 //
1491 // Note: a subset of changes to ObjectMonitor::wait()
1492 // will need to be replicated in complete_exit
1493 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1494 JavaThread* current = THREAD;
1495
1496 assert(InitDone, "Unexpectedly not initialized");
1497
1498 CHECK_OWNER(); // Throws IMSE if not owner.
1499
1500 EventJavaMonitorWait event;
1501
1502 // check for a pending interrupt
1503 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1504 // post monitor waited event. Note that this is past-tense, we are done waiting.
1505 if (JvmtiExport::should_post_monitor_waited()) {
1506 // Note: 'false' parameter is passed here because the
1507 // wait was not timed out due to thread interrupt.
1508 JvmtiExport::post_monitor_waited(current, this, false);
1509
1510 // In this short circuit of the monitor wait protocol, the
1511 // current thread never drops ownership of the monitor and
1512 // never gets added to the wait queue so the current thread
1513 // cannot be made the successor. This means that the
1514 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1515 // consume an unpark() meant for the ParkEvent associated with
1516 // this ObjectMonitor.
1517 }
1518 if (event.should_commit()) {
1519 post_monitor_wait_event(&event, this, 0, millis, false);
1520 }
1521 THROW(vmSymbols::java_lang_InterruptedException());
1522 return;
1523 }
1524
1525 assert(current->_Stalled == 0, "invariant");
1526 current->_Stalled = intptr_t(this);
1527 current->set_current_waiting_monitor(this);
1528
1529 // create a node to be put into the queue
1530 // Critically, after we reset() the event but prior to park(), we must check
1531 // for a pending interrupt.
1532 ObjectWaiter node(current);
1533 node.TState = ObjectWaiter::TS_WAIT;
1534 current->_ParkEvent->reset();
1535 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1536
1537 // Enter the waiting queue, which is a circular doubly linked list in this case
1538 // but it could be a priority queue or any data structure.
1539 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1540 // by the owner of the monitor *except* in the case where park()
1541 // returns because of a timeout of interrupt. Contention is exceptionally rare
1542 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1543
1544 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1545 AddWaiter(&node);
1546 Thread::SpinRelease(&_WaitSetLock);
1547
1548 _Responsible = nullptr;
1549
1550 intx save = _recursions; // record the old recursion count
1551 _waiters++; // increment the number of waiters
1552 _recursions = 0; // set the recursion level to be 1
1553 exit(current); // exit the monitor
1554 guarantee(owner_raw() != current, "invariant");
1555
1556 // The thread is on the WaitSet list - now park() it.
1557 // On MP systems it's conceivable that a brief spin before we park
1558 // could be profitable.
1559 //
1560 // TODO-FIXME: change the following logic to a loop of the form
1561 // while (!timeout && !interrupted && _notified == 0) park()
1562
1563 int ret = OS_OK;
1564 int WasNotified = 0;
1565
1566 // Need to check interrupt state whilst still _thread_in_vm
1567 bool interrupted = interruptible && current->is_interrupted(false);
1568
1569 { // State transition wrappers
1570 OSThread* osthread = current->osthread();
1571 OSThreadWaitState osts(osthread, true);
1572
1573 assert(current->thread_state() == _thread_in_vm, "invariant");
1574
1575 {
1576 ClearSuccOnSuspend csos(this);
1577 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */);
1578 if (interrupted || HAS_PENDING_EXCEPTION) {
1579 // Intentionally empty
1580 } else if (node._notified == 0) {
1581 if (millis <= 0) {
1582 current->_ParkEvent->park();
1583 } else {
1584 ret = current->_ParkEvent->park(millis);
1585 }
1586 }
1587 }
1588
1589 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1590 // from the WaitSet to the EntryList.
1591 // See if we need to remove Node from the WaitSet.
1592 // We use double-checked locking to avoid grabbing _WaitSetLock
1593 // if the thread is not on the wait queue.
1594 //
1595 // Note that we don't need a fence before the fetch of TState.
1596 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1597 // written by the is thread. (perhaps the fetch might even be satisfied
1598 // by a look-aside into the processor's own store buffer, although given
1599 // the length of the code path between the prior ST and this load that's
1600 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1601 // then we'll acquire the lock and then re-fetch a fresh TState value.
1602 // That is, we fail toward safety.
1603
1604 if (node.TState == ObjectWaiter::TS_WAIT) {
1605 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1606 if (node.TState == ObjectWaiter::TS_WAIT) {
1607 DequeueSpecificWaiter(&node); // unlink from WaitSet
1608 assert(node._notified == 0, "invariant");
1609 node.TState = ObjectWaiter::TS_RUN;
1610 }
1611 Thread::SpinRelease(&_WaitSetLock);
1612 }
1613
1614 // The thread is now either on off-list (TS_RUN),
1615 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1616 // The Node's TState variable is stable from the perspective of this thread.
1617 // No other threads will asynchronously modify TState.
1618 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1619 OrderAccess::loadload();
1620 if (_succ == current) _succ = nullptr;
1621 WasNotified = node._notified;
1622
1623 // Reentry phase -- reacquire the monitor.
1624 // re-enter contended monitor after object.wait().
1625 // retain OBJECT_WAIT state until re-enter successfully completes
1626 // Thread state is thread_in_vm and oop access is again safe,
1627 // although the raw address of the object may have changed.
1628 // (Don't cache naked oops over safepoints, of course).
1629
1630 // post monitor waited event. Note that this is past-tense, we are done waiting.
1631 if (JvmtiExport::should_post_monitor_waited()) {
1632 JvmtiExport::post_monitor_waited(current, this, ret == OS_TIMEOUT);
1633
1634 if (node._notified != 0 && _succ == current) {
1635 // In this part of the monitor wait-notify-reenter protocol it
1636 // is possible (and normal) for another thread to do a fastpath
1637 // monitor enter-exit while this thread is still trying to get
1638 // to the reenter portion of the protocol.
1639 //
1640 // The ObjectMonitor was notified and the current thread is
1641 // the successor which also means that an unpark() has already
1642 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1643 // consume the unpark() that was done when the successor was
1644 // set because the same ParkEvent is shared between Java
1645 // monitors and JVM/TI RawMonitors (for now).
1646 //
1647 // We redo the unpark() to ensure forward progress, i.e., we
1648 // don't want all pending threads hanging (parked) with none
1649 // entering the unlocked monitor.
1650 node._event->unpark();
1651 }
1652 }
1653
1654 if (event.should_commit()) {
1655 post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1656 }
1657
1658 OrderAccess::fence();
1659
1660 assert(current->_Stalled != 0, "invariant");
1661 current->_Stalled = 0;
1662
1663 assert(owner_raw() != current, "invariant");
1664 ObjectWaiter::TStates v = node.TState;
1665 if (v == ObjectWaiter::TS_RUN) {
1666 enter(current);
1667 } else {
1668 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1669 ReenterI(current, &node);
1670 node.wait_reenter_end(this);
1671 }
1672
1673 // current has reacquired the lock.
1674 // Lifecycle - the node representing current must not appear on any queues.
1675 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1676 // want residual elements associated with this thread left on any lists.
1677 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1678 assert(owner_raw() == current, "invariant");
1679 assert(_succ != current, "invariant");
1680 } // OSThreadWaitState()
1681
1682 current->set_current_waiting_monitor(nullptr);
1683
1684 guarantee(_recursions == 0, "invariant");
1685 int relock_count = JvmtiDeferredUpdates::get_and_reset_relock_count_after_wait(current);
1686 _recursions = save // restore the old recursion count
1687 + relock_count; // increased by the deferred relock count
1688 current->inc_held_monitor_count(relock_count); // Deopt never entered these counts.
1689 _waiters--; // decrement the number of waiters
1690
1691 // Verify a few postconditions
1692 assert(owner_raw() == current, "invariant");
1693 assert(_succ != current, "invariant");
1694 assert(object()->mark() == markWord::encode(this), "invariant");
1695
1696 // check if the notification happened
1697 if (!WasNotified) {
1698 // no, it could be timeout or Thread.interrupt() or both
1699 // check for interrupt event, otherwise it is timeout
1700 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1701 THROW(vmSymbols::java_lang_InterruptedException());
1702 }
1703 }
1704
1705 // NOTE: Spurious wake up will be consider as timeout.
1706 // Monitor notify has precedence over thread interrupt.
1707 }
1708
1709
1710 // Consider:
1711 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1712 // then instead of transferring a thread from the WaitSet to the EntryList
1713 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1714
1715 void ObjectMonitor::INotify(JavaThread* current) {
1716 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1717 ObjectWaiter* iterator = DequeueWaiter();
1718 if (iterator != nullptr) {
1719 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1720 guarantee(iterator->_notified == 0, "invariant");
1721 // Disposition - what might we do with iterator ?
1722 // a. add it directly to the EntryList - either tail (policy == 1)
1723 // or head (policy == 0).
1724 // b. push it onto the front of the _cxq (policy == 2).
1725 // For now we use (b).
1726
1727 iterator->TState = ObjectWaiter::TS_ENTER;
1728
1729 iterator->_notified = 1;
1730 iterator->_notifier_tid = JFR_THREAD_ID(current);
1731
1732 ObjectWaiter* list = _EntryList;
1733 if (list != nullptr) {
1734 assert(list->_prev == nullptr, "invariant");
1735 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1736 assert(list != iterator, "invariant");
1737 }
1738
1739 // prepend to cxq
1740 if (list == nullptr) {
1741 iterator->_next = iterator->_prev = nullptr;
1742 _EntryList = iterator;
1743 } else {
1744 iterator->TState = ObjectWaiter::TS_CXQ;
1745 for (;;) {
1746 ObjectWaiter* front = _cxq;
1747 iterator->_next = front;
1748 if (Atomic::cmpxchg(&_cxq, front, iterator) == front) {
1749 break;
1750 }
1751 }
1752 }
1753
1754 // _WaitSetLock protects the wait queue, not the EntryList. We could
1755 // move the add-to-EntryList operation, above, outside the critical section
1756 // protected by _WaitSetLock. In practice that's not useful. With the
1757 // exception of wait() timeouts and interrupts the monitor owner
1758 // is the only thread that grabs _WaitSetLock. There's almost no contention
1759 // on _WaitSetLock so it's not profitable to reduce the length of the
1760 // critical section.
1761
1762 iterator->wait_reenter_begin(this);
1763 }
1764 Thread::SpinRelease(&_WaitSetLock);
1765 }
1766
1767 // Consider: a not-uncommon synchronization bug is to use notify() when
1768 // notifyAll() is more appropriate, potentially resulting in stranded
1769 // threads; this is one example of a lost wakeup. A useful diagnostic
1770 // option is to force all notify() operations to behave as notifyAll().
1771 //
1772 // Note: We can also detect many such problems with a "minimum wait".
1773 // When the "minimum wait" is set to a small non-zero timeout value
1774 // and the program does not hang whereas it did absent "minimum wait",
1775 // that suggests a lost wakeup bug.
1776
1777 void ObjectMonitor::notify(TRAPS) {
1778 JavaThread* current = THREAD;
1779 CHECK_OWNER(); // Throws IMSE if not owner.
1780 if (_WaitSet == nullptr) {
1781 return;
1782 }
1783 DTRACE_MONITOR_PROBE(notify, this, object(), current);
1784 INotify(current);
1785 OM_PERFDATA_OP(Notifications, inc(1));
1786 }
1787
1788
1789 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1790 // from the waitset to the EntryList. This could be done more efficiently with a
1791 // single bulk transfer but in practice it's not time-critical. Beware too,
1792 // that in prepend-mode we invert the order of the waiters. Let's say that the
1793 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1794 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1795
1796 void ObjectMonitor::notifyAll(TRAPS) {
1797 JavaThread* current = THREAD;
1798 CHECK_OWNER(); // Throws IMSE if not owner.
1799 if (_WaitSet == nullptr) {
1800 return;
1801 }
1802
1803 DTRACE_MONITOR_PROBE(notifyAll, this, object(), current);
1804 int tally = 0;
1805 while (_WaitSet != nullptr) {
1806 tally++;
1807 INotify(current);
1808 }
1809
1810 OM_PERFDATA_OP(Notifications, inc(tally));
1811 }
1812
1813 // -----------------------------------------------------------------------------
1814 // Adaptive Spinning Support
1815 //
1816 // Adaptive spin-then-block - rational spinning
1817 //
1818 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1819 // algorithm. On high order SMP systems it would be better to start with
1820 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1821 // a contending thread could enqueue itself on the cxq and then spin locally
1822 // on a thread-specific variable such as its ParkEvent._Event flag.
1823 // That's left as an exercise for the reader. Note that global spinning is
1824 // not problematic on Niagara, as the L2 cache serves the interconnect and
1825 // has both low latency and massive bandwidth.
1826 //
1827 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1828 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1829 // (duration) or we can fix the count at approximately the duration of
1830 // a context switch and vary the frequency. Of course we could also
1831 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1832 // For a description of 'Adaptive spin-then-block mutual exclusion in
1833 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1834 //
1835 // This implementation varies the duration "D", where D varies with
1836 // the success rate of recent spin attempts. (D is capped at approximately
1837 // length of a round-trip context switch). The success rate for recent
1838 // spin attempts is a good predictor of the success rate of future spin
1839 // attempts. The mechanism adapts automatically to varying critical
1840 // section length (lock modality), system load and degree of parallelism.
1841 // D is maintained per-monitor in _SpinDuration and is initialized
1842 // optimistically. Spin frequency is fixed at 100%.
1843 //
1844 // Note that _SpinDuration is volatile, but we update it without locks
1845 // or atomics. The code is designed so that _SpinDuration stays within
1846 // a reasonable range even in the presence of races. The arithmetic
1847 // operations on _SpinDuration are closed over the domain of legal values,
1848 // so at worst a race will install and older but still legal value.
1849 // At the very worst this introduces some apparent non-determinism.
1850 // We might spin when we shouldn't or vice-versa, but since the spin
1851 // count are relatively short, even in the worst case, the effect is harmless.
1852 //
1853 // Care must be taken that a low "D" value does not become an
1854 // an absorbing state. Transient spinning failures -- when spinning
1855 // is overall profitable -- should not cause the system to converge
1856 // on low "D" values. We want spinning to be stable and predictable
1857 // and fairly responsive to change and at the same time we don't want
1858 // it to oscillate, become metastable, be "too" non-deterministic,
1859 // or converge on or enter undesirable stable absorbing states.
1860 //
1861 // We implement a feedback-based control system -- using past behavior
1862 // to predict future behavior. We face two issues: (a) if the
1863 // input signal is random then the spin predictor won't provide optimal
1864 // results, and (b) if the signal frequency is too high then the control
1865 // system, which has some natural response lag, will "chase" the signal.
1866 // (b) can arise from multimodal lock hold times. Transient preemption
1867 // can also result in apparent bimodal lock hold times.
1868 // Although sub-optimal, neither condition is particularly harmful, as
1869 // in the worst-case we'll spin when we shouldn't or vice-versa.
1870 // The maximum spin duration is rather short so the failure modes aren't bad.
1871 // To be conservative, I've tuned the gain in system to bias toward
1872 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1873 // "rings" or oscillates between spinning and not spinning. This happens
1874 // when spinning is just on the cusp of profitability, however, so the
1875 // situation is not dire. The state is benign -- there's no need to add
1876 // hysteresis control to damp the transition rate between spinning and
1877 // not spinning.
1878
1879 // Spinning: Fixed frequency (100%), vary duration
1880 int ObjectMonitor::TrySpin(JavaThread* current) {
1881 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1882 int ctr = Knob_FixedSpin;
1883 if (ctr != 0) {
1884 while (--ctr >= 0) {
1885 if (TryLock(current) > 0) return 1;
1886 SpinPause();
1887 }
1888 return 0;
1889 }
1890
1891 for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1892 if (TryLock(current) > 0) {
1893 // Increase _SpinDuration ...
1894 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1895 // Raising _SpurDuration to the poverty line is key.
1896 int x = _SpinDuration;
1897 if (x < Knob_SpinLimit) {
1898 if (x < Knob_Poverty) x = Knob_Poverty;
1899 _SpinDuration = x + Knob_BonusB;
1900 }
1901 return 1;
1902 }
1903 SpinPause();
1904 }
1905
1906 // Admission control - verify preconditions for spinning
1907 //
1908 // We always spin a little bit, just to prevent _SpinDuration == 0 from
1909 // becoming an absorbing state. Put another way, we spin briefly to
1910 // sample, just in case the system load, parallelism, contention, or lock
1911 // modality changed.
1912 //
1913 // Consider the following alternative:
1914 // Periodically set _SpinDuration = _SpinLimit and try a long/full
1915 // spin attempt. "Periodically" might mean after a tally of
1916 // the # of failed spin attempts (or iterations) reaches some threshold.
1917 // This takes us into the realm of 1-out-of-N spinning, where we
1918 // hold the duration constant but vary the frequency.
1919
1920 ctr = _SpinDuration;
1921 if (ctr <= 0) return 0;
1922
1923 if (NotRunnable(current, static_cast<JavaThread*>(owner_raw()))) {
1924 return 0;
1925 }
1926
1927 // We're good to spin ... spin ingress.
1928 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1929 // when preparing to LD...CAS _owner, etc and the CAS is likely
1930 // to succeed.
1931 if (_succ == nullptr) {
1932 _succ = current;
1933 }
1934 Thread* prv = nullptr;
1935
1936 // There are three ways to exit the following loop:
1937 // 1. A successful spin where this thread has acquired the lock.
1938 // 2. Spin failure with prejudice
1939 // 3. Spin failure without prejudice
1940
1941 while (--ctr >= 0) {
1942
1943 // Periodic polling -- Check for pending GC
1944 // Threads may spin while they're unsafe.
1945 // We don't want spinning threads to delay the JVM from reaching
1946 // a stop-the-world safepoint or to steal cycles from GC.
1947 // If we detect a pending safepoint we abort in order that
1948 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1949 // this thread, if safe, doesn't steal cycles from GC.
1950 // This is in keeping with the "no loitering in runtime" rule.
1951 // We periodically check to see if there's a safepoint pending.
1952 if ((ctr & 0xFF) == 0) {
1953 // Can't call SafepointMechanism::should_process() since that
1954 // might update the poll values and we could be in a thread_blocked
1955 // state here which is not allowed so just check the poll.
1956 if (SafepointMechanism::local_poll_armed(current)) {
1957 goto Abort; // abrupt spin egress
1958 }
1959 SpinPause();
1960 }
1961
1962 // Probe _owner with TATAS
1963 // If this thread observes the monitor transition or flicker
1964 // from locked to unlocked to locked, then the odds that this
1965 // thread will acquire the lock in this spin attempt go down
1966 // considerably. The same argument applies if the CAS fails
1967 // or if we observe _owner change from one non-null value to
1968 // another non-null value. In such cases we might abort
1969 // the spin without prejudice or apply a "penalty" to the
1970 // spin count-down variable "ctr", reducing it by 100, say.
1971
1972 JavaThread* ox = static_cast<JavaThread*>(owner_raw());
1973 if (ox == nullptr) {
1974 ox = static_cast<JavaThread*>(try_set_owner_from(nullptr, current));
1975 if (ox == nullptr) {
1976 // The CAS succeeded -- this thread acquired ownership
1977 // Take care of some bookkeeping to exit spin state.
1978 if (_succ == current) {
1979 _succ = nullptr;
1980 }
1981
1982 // Increase _SpinDuration :
1983 // The spin was successful (profitable) so we tend toward
1984 // longer spin attempts in the future.
1985 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
1986 // If we acquired the lock early in the spin cycle it
1987 // makes sense to increase _SpinDuration proportionally.
1988 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1989 int x = _SpinDuration;
1990 if (x < Knob_SpinLimit) {
1991 if (x < Knob_Poverty) x = Knob_Poverty;
1992 _SpinDuration = x + Knob_Bonus;
1993 }
1994 return 1;
1995 }
1996
1997 // The CAS failed ... we can take any of the following actions:
1998 // * penalize: ctr -= CASPenalty
1999 // * exit spin with prejudice -- goto Abort;
2000 // * exit spin without prejudice.
2001 // * Since CAS is high-latency, retry again immediately.
2002 prv = ox;
2003 goto Abort;
2004 }
2005
2006 // Did lock ownership change hands ?
2007 if (ox != prv && prv != nullptr) {
2008 goto Abort;
2009 }
2010 prv = ox;
2011
2012 // Abort the spin if the owner is not executing.
2013 // The owner must be executing in order to drop the lock.
2014 // Spinning while the owner is OFFPROC is idiocy.
2015 // Consider: ctr -= RunnablePenalty ;
2016 if (NotRunnable(current, ox)) {
2017 goto Abort;
2018 }
2019 if (_succ == nullptr) {
2020 _succ = current;
2021 }
2022 }
2023
2024 // Spin failed with prejudice -- reduce _SpinDuration.
2025 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2026 // AIMD is globally stable.
2027 {
2028 int x = _SpinDuration;
2029 if (x > 0) {
2030 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2031 // This is globally sample and tends to damp the response.
2032 x -= Knob_Penalty;
2033 if (x < 0) x = 0;
2034 _SpinDuration = x;
2035 }
2036 }
2037
2038 Abort:
2039 if (_succ == current) {
2040 _succ = nullptr;
2041 // Invariant: after setting succ=null a contending thread
2042 // must recheck-retry _owner before parking. This usually happens
2043 // in the normal usage of TrySpin(), but it's safest
2044 // to make TrySpin() as foolproof as possible.
2045 OrderAccess::fence();
2046 if (TryLock(current) > 0) return 1;
2047 }
2048 return 0;
2049 }
2050
2051 // NotRunnable() -- informed spinning
2052 //
2053 // Don't bother spinning if the owner is not eligible to drop the lock.
2054 // Spin only if the owner thread is _thread_in_Java or _thread_in_vm.
2055 // The thread must be runnable in order to drop the lock in timely fashion.
2056 // If the _owner is not runnable then spinning will not likely be
2057 // successful (profitable).
2058 //
2059 // Beware -- the thread referenced by _owner could have died
2060 // so a simply fetch from _owner->_thread_state might trap.
2061 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
2062 // Because of the lifecycle issues, the _thread_state values
2063 // observed by NotRunnable() might be garbage. NotRunnable must
2064 // tolerate this and consider the observed _thread_state value
2065 // as advisory.
2066 //
2067 // Beware too, that _owner is sometimes a BasicLock address and sometimes
2068 // a thread pointer.
2069 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
2070 // with the LSB of _owner. Another option would be to probabilistically probe
2071 // the putative _owner->TypeTag value.
2072 //
2073 // Checking _thread_state isn't perfect. Even if the thread is
2074 // in_java it might be blocked on a page-fault or have been preempted
2075 // and sitting on a ready/dispatch queue.
2076 //
2077 // The return value from NotRunnable() is *advisory* -- the
2078 // result is based on sampling and is not necessarily coherent.
2079 // The caller must tolerate false-negative and false-positive errors.
2080 // Spinning, in general, is probabilistic anyway.
2081
2082
2083 int ObjectMonitor::NotRunnable(JavaThread* current, JavaThread* ox) {
2084 // Check ox->TypeTag == 2BAD.
2085 if (ox == nullptr) return 0;
2086
2087 // Avoid transitive spinning ...
2088 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
2089 // Immediately after T1 acquires L it's possible that T2, also
2090 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
2091 // This occurs transiently after T1 acquired L but before
2092 // T1 managed to clear T1.Stalled. T2 does not need to abort
2093 // its spin in this circumstance.
2094 intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
2095
2096 if (BlockedOn == 1) return 1;
2097 if (BlockedOn != 0) {
2098 return BlockedOn != intptr_t(this) && owner_raw() == ox;
2099 }
2100
2101 assert(sizeof(ox->_thread_state == sizeof(int)), "invariant");
2102 int jst = SafeFetch32((int *) &ox->_thread_state, -1);;
2103 // consider also: jst != _thread_in_Java -- but that's overspecific.
2104 return jst == _thread_blocked || jst == _thread_in_native;
2105 }
2106
2107
2108 // -----------------------------------------------------------------------------
2109 // WaitSet management ...
2110
2111 ObjectWaiter::ObjectWaiter(JavaThread* current) {
2112 _next = nullptr;
2113 _prev = nullptr;
2114 _notified = 0;
2115 _notifier_tid = 0;
2116 TState = TS_RUN;
2117 _thread = current;
2118 _event = _thread->_ParkEvent;
2119 _active = false;
2120 assert(_event != nullptr, "invariant");
2121 }
2122
2123 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2124 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(_thread, mon);
2125 }
2126
2127 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2128 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(_thread, _active);
2129 }
2130
2131 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2132 assert(node != nullptr, "should not add null node");
2133 assert(node->_prev == nullptr, "node already in list");
2134 assert(node->_next == nullptr, "node already in list");
2135 // put node at end of queue (circular doubly linked list)
2136 if (_WaitSet == nullptr) {
2137 _WaitSet = node;
2138 node->_prev = node;
2139 node->_next = node;
2140 } else {
2141 ObjectWaiter* head = _WaitSet;
2142 ObjectWaiter* tail = head->_prev;
2143 assert(tail->_next == head, "invariant check");
2144 tail->_next = node;
2145 head->_prev = node;
2146 node->_next = head;
2147 node->_prev = tail;
2148 }
2149 }
2150
2151 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2152 // dequeue the very first waiter
2153 ObjectWaiter* waiter = _WaitSet;
2154 if (waiter) {
2155 DequeueSpecificWaiter(waiter);
2156 }
2157 return waiter;
2158 }
2159
2160 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2161 assert(node != nullptr, "should not dequeue nullptr node");
2162 assert(node->_prev != nullptr, "node already removed from list");
2163 assert(node->_next != nullptr, "node already removed from list");
2164 // when the waiter has woken up because of interrupt,
2165 // timeout or other spurious wake-up, dequeue the
2166 // waiter from waiting list
2167 ObjectWaiter* next = node->_next;
2168 if (next == node) {
2169 assert(node->_prev == node, "invariant check");
2170 _WaitSet = nullptr;
2171 } else {
2172 ObjectWaiter* prev = node->_prev;
2173 assert(prev->_next == node, "invariant check");
2174 assert(next->_prev == node, "invariant check");
2175 next->_prev = prev;
2176 prev->_next = next;
2177 if (_WaitSet == node) {
2178 _WaitSet = next;
2179 }
2180 }
2181 node->_next = nullptr;
2182 node->_prev = nullptr;
2183 }
2184
2185 // -----------------------------------------------------------------------------
2186 // PerfData support
2187 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = nullptr;
2188 PerfCounter * ObjectMonitor::_sync_FutileWakeups = nullptr;
2189 PerfCounter * ObjectMonitor::_sync_Parks = nullptr;
2190 PerfCounter * ObjectMonitor::_sync_Notifications = nullptr;
2191 PerfCounter * ObjectMonitor::_sync_Inflations = nullptr;
2192 PerfCounter * ObjectMonitor::_sync_Deflations = nullptr;
2193 PerfLongVariable * ObjectMonitor::_sync_MonExtant = nullptr;
2194
2195 // One-shot global initialization for the sync subsystem.
2196 // We could also defer initialization and initialize on-demand
2197 // the first time we call ObjectSynchronizer::inflate().
2198 // Initialization would be protected - like so many things - by
2199 // the MonitorCache_lock.
2200
2201 void ObjectMonitor::Initialize() {
2202 assert(!InitDone, "invariant");
2203
2204 if (!os::is_MP()) {
2205 Knob_SpinLimit = 0;
2206 Knob_PreSpin = 0;
2207 Knob_FixedSpin = -1;
2208 }
2209
2210 if (UsePerfData) {
2211 EXCEPTION_MARK;
2212 #define NEWPERFCOUNTER(n) \
2213 { \
2214 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \
2215 CHECK); \
2216 }
2217 #define NEWPERFVARIABLE(n) \
2218 { \
2219 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \
2220 CHECK); \
2221 }
2222 NEWPERFCOUNTER(_sync_Inflations);
2223 NEWPERFCOUNTER(_sync_Deflations);
2224 NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2225 NEWPERFCOUNTER(_sync_FutileWakeups);
2226 NEWPERFCOUNTER(_sync_Parks);
2227 NEWPERFCOUNTER(_sync_Notifications);
2228 NEWPERFVARIABLE(_sync_MonExtant);
2229 #undef NEWPERFCOUNTER
2230 #undef NEWPERFVARIABLE
2231 }
2232
2233 _oop_storage = OopStorageSet::create_weak("ObjectSynchronizer Weak", mtSynchronizer);
2234
2235 DEBUG_ONLY(InitDone = true;)
2236 }
2237
2238 void ObjectMonitor::print_on(outputStream* st) const {
2239 // The minimal things to print for markWord printing, more can be added for debugging and logging.
2240 st->print("{contentions=0x%08x,waiters=0x%08x"
2241 ",recursions=" INTX_FORMAT ",owner=" INTPTR_FORMAT "}",
2242 contentions(), waiters(), recursions(),
2243 p2i(owner()));
2244 }
2245 void ObjectMonitor::print() const { print_on(tty); }
2246
2247 #ifdef ASSERT
2248 // Print the ObjectMonitor like a debugger would:
2249 //
2250 // (ObjectMonitor) 0x00007fdfb6012e40 = {
2251 // _header = 0x0000000000000001
2252 // _object = 0x000000070ff45fd0
2253 // _pad_buf0 = {
2254 // [0] = '\0'
2255 // ...
2256 // [43] = '\0'
2257 // }
2258 // _owner = 0x0000000000000000
2259 // _previous_owner_tid = 0
2260 // _pad_buf1 = {
2261 // [0] = '\0'
2262 // ...
2263 // [47] = '\0'
2264 // }
2265 // _next_om = 0x0000000000000000
2266 // _recursions = 0
2267 // _EntryList = 0x0000000000000000
2268 // _cxq = 0x0000000000000000
2269 // _succ = 0x0000000000000000
2270 // _Responsible = 0x0000000000000000
2271 // _Spinner = 0
2272 // _SpinDuration = 5000
2273 // _contentions = 0
2274 // _WaitSet = 0x0000700009756248
2275 // _waiters = 1
2276 // _WaitSetLock = 0
2277 // }
2278 //
2279 void ObjectMonitor::print_debug_style_on(outputStream* st) const {
2280 st->print_cr("(ObjectMonitor*) " INTPTR_FORMAT " = {", p2i(this));
2281 st->print_cr(" _header = " INTPTR_FORMAT, header().value());
2282 st->print_cr(" _object = " INTPTR_FORMAT, p2i(object_peek()));
2283 st->print_cr(" _pad_buf0 = {");
2284 st->print_cr(" [0] = '\\0'");
2285 st->print_cr(" ...");
2286 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf0) - 1);
2287 st->print_cr(" }");
2288 st->print_cr(" _owner = " INTPTR_FORMAT, p2i(owner_raw()));
2289 st->print_cr(" _previous_owner_tid = " UINT64_FORMAT, _previous_owner_tid);
2290 st->print_cr(" _pad_buf1 = {");
2291 st->print_cr(" [0] = '\\0'");
2292 st->print_cr(" ...");
2293 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf1) - 1);
2294 st->print_cr(" }");
2295 st->print_cr(" _next_om = " INTPTR_FORMAT, p2i(next_om()));
2296 st->print_cr(" _recursions = " INTX_FORMAT, _recursions);
2297 st->print_cr(" _EntryList = " INTPTR_FORMAT, p2i(_EntryList));
2298 st->print_cr(" _cxq = " INTPTR_FORMAT, p2i(_cxq));
2299 st->print_cr(" _succ = " INTPTR_FORMAT, p2i(_succ));
2300 st->print_cr(" _Responsible = " INTPTR_FORMAT, p2i(_Responsible));
2301 st->print_cr(" _Spinner = %d", _Spinner);
2302 st->print_cr(" _SpinDuration = %d", _SpinDuration);
2303 st->print_cr(" _contentions = %d", contentions());
2304 st->print_cr(" _WaitSet = " INTPTR_FORMAT, p2i(_WaitSet));
2305 st->print_cr(" _waiters = %d", _waiters);
2306 st->print_cr(" _WaitSetLock = %d", _WaitSetLock);
2307 st->print_cr("}");
2308 }
2309 #endif