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/continuationWrapper.inline.hpp"
43 #include "runtime/globals.hpp"
44 #include "runtime/handles.inline.hpp"
45 #include "runtime/interfaceSupport.inline.hpp"
46 #include "runtime/javaThread.inline.hpp"
47 #include "runtime/lightweightSynchronizer.hpp"
48 #include "runtime/mutexLocker.hpp"
49 #include "runtime/objectMonitor.hpp"
50 #include "runtime/objectMonitor.inline.hpp"
51 #include "runtime/orderAccess.hpp"
52 #include "runtime/osThread.hpp"
53 #include "runtime/perfData.hpp"
54 #include "runtime/safefetch.hpp"
55 #include "runtime/safepointMechanism.inline.hpp"
56 #include "runtime/sharedRuntime.hpp"
57 #include "runtime/threads.hpp"
58 #include "services/threadService.hpp"
59 #include "utilities/debug.hpp"
60 #include "utilities/dtrace.hpp"
61 #include "utilities/globalDefinitions.hpp"
62 #include "utilities/macros.hpp"
63 #include "utilities/preserveException.hpp"
64 #if INCLUDE_JFR
65 #include "jfr/support/jfrFlush.hpp"
66 #endif
67
68 #ifdef DTRACE_ENABLED
69
70 // Only bother with this argument setup if dtrace is available
71 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
72
73
74 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
75 char* bytes = nullptr; \
76 int len = 0; \
77 jlong jtid = SharedRuntime::get_java_tid(thread); \
78 Symbol* klassname = obj->klass()->name(); \
79 if (klassname != nullptr) { \
80 bytes = (char*)klassname->bytes(); \
81 len = klassname->utf8_length(); \
82 }
83
84 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
85 { \
86 if (DTraceMonitorProbes) { \
87 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
88 HOTSPOT_MONITOR_WAIT(jtid, \
89 (monitor), bytes, len, (millis)); \
90 } \
91 }
92
93 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
94 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
95 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
96 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
97 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
98
99 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
100 { \
101 if (DTraceMonitorProbes) { \
102 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
103 HOTSPOT_MONITOR_##probe(jtid, \
104 (uintptr_t)(monitor), bytes, len); \
105 } \
106 }
107
108 #else // ndef DTRACE_ENABLED
109
110 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
111 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
112
113 #endif // ndef DTRACE_ENABLED
114
115 DEBUG_ONLY(static volatile bool InitDone = false;)
116
117 OopStorage* ObjectMonitor::_oop_storage = nullptr;
118
119 OopHandle ObjectMonitor::_vthread_cxq_head;
120 ParkEvent* ObjectMonitor::_vthread_unparker_ParkEvent = nullptr;
121
122 static void post_virtual_thread_pinned_event(JavaThread* current, const char* reason) {
123 EventVirtualThreadPinned e;
124 if (e.should_commit()) {
125 e.set_pinnedReason(reason);
126 e.set_carrierThread(JFR_JVM_THREAD_ID(current));
127 e.commit();
128 }
129 }
130
131 // -----------------------------------------------------------------------------
132 // Theory of operations -- Monitors lists, thread residency, etc:
133 //
134 // * A thread acquires ownership of a monitor by successfully
135 // CAS()ing the _owner field from null to non-null.
136 //
137 // * Invariant: A thread appears on at most one monitor list --
138 // cxq, EntryList or WaitSet -- at any one time.
139 //
140 // * Contending threads "push" themselves onto the cxq with CAS
141 // and then spin/park.
142 //
143 // * After a contending thread eventually acquires the lock it must
144 // dequeue itself from either the EntryList or the cxq.
145 //
146 // * The exiting thread identifies and unparks an "heir presumptive"
147 // tentative successor thread on the EntryList. Critically, the
148 // exiting thread doesn't unlink the successor thread from the EntryList.
149 // After having been unparked, the wakee will recontend for ownership of
150 // the monitor. The successor (wakee) will either acquire the lock or
151 // re-park itself.
152 //
153 // Succession is provided for by a policy of competitive handoff.
154 // The exiting thread does _not_ grant or pass ownership to the
155 // successor thread. (This is also referred to as "handoff" succession").
156 // Instead the exiting thread releases ownership and possibly wakes
157 // a successor, so the successor can (re)compete for ownership of the lock.
158 // If the EntryList is empty but the cxq is populated the exiting
159 // thread will drain the cxq into the EntryList. It does so by
160 // by detaching the cxq (installing null with CAS) and folding
161 // the threads from the cxq into the EntryList. The EntryList is
162 // doubly linked, while the cxq is singly linked because of the
163 // CAS-based "push" used to enqueue recently arrived threads (RATs).
164 //
165 // * Concurrency invariants:
166 //
167 // -- only the monitor owner may access or mutate the EntryList.
168 // The mutex property of the monitor itself protects the EntryList
169 // from concurrent interference.
170 // -- Only the monitor owner may detach the cxq.
171 //
172 // * The monitor entry list operations avoid locks, but strictly speaking
173 // they're not lock-free. Enter is lock-free, exit is not.
174 // For a description of 'Methods and apparatus providing non-blocking access
175 // to a resource,' see U.S. Pat. No. 7844973.
176 //
177 // * The cxq can have multiple concurrent "pushers" but only one concurrent
178 // detaching thread. This mechanism is immune from the ABA corruption.
179 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
180 //
181 // * Taken together, the cxq and the EntryList constitute or form a
182 // single logical queue of threads stalled trying to acquire the lock.
183 // We use two distinct lists to improve the odds of a constant-time
184 // dequeue operation after acquisition (in the ::enter() epilogue) and
185 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
186 // A key desideratum is to minimize queue & monitor metadata manipulation
187 // that occurs while holding the monitor lock -- that is, we want to
188 // minimize monitor lock holds times. Note that even a small amount of
189 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
190 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
191 // locks and monitor metadata.
192 //
193 // Cxq points to the set of Recently Arrived Threads attempting entry.
194 // Because we push threads onto _cxq with CAS, the RATs must take the form of
195 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
196 // the unlocking thread notices that EntryList is null but _cxq is != null.
197 //
198 // The EntryList is ordered by the prevailing queue discipline and
199 // can be organized in any convenient fashion, such as a doubly-linked list or
200 // a circular doubly-linked list. Critically, we want insert and delete operations
201 // to operate in constant-time. If we need a priority queue then something akin
202 // to Solaris' sleepq would work nicely. Viz.,
203 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
204 // Queue discipline is enforced at ::exit() time, when the unlocking thread
205 // drains the cxq into the EntryList, and orders or reorders the threads on the
206 // EntryList accordingly.
207 //
208 // Barring "lock barging", this mechanism provides fair cyclic ordering,
209 // somewhat similar to an elevator-scan.
210 //
211 // * The monitor synchronization subsystem avoids the use of native
212 // synchronization primitives except for the narrow platform-specific
213 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
214 // the semantics of park-unpark. Put another way, this monitor implementation
215 // depends only on atomic operations and park-unpark. The monitor subsystem
216 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
217 // underlying OS manages the READY<->RUN transitions.
218 //
219 // * Waiting threads reside on the WaitSet list -- wait() puts
220 // the caller onto the WaitSet.
221 //
222 // * notify() or notifyAll() simply transfers threads from the WaitSet to
223 // either the EntryList or cxq. Subsequent exit() operations will
224 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
225 // it's likely the notifyee would simply impale itself on the lock held
226 // by the notifier.
227
228 // Check that object() and set_object() are called from the right context:
229 static void check_object_context() {
230 #ifdef ASSERT
231 Thread* self = Thread::current();
232 if (self->is_Java_thread()) {
233 // Mostly called from JavaThreads so sanity check the thread state.
234 JavaThread* jt = JavaThread::cast(self);
235 switch (jt->thread_state()) {
236 case _thread_in_vm: // the usual case
237 case _thread_in_Java: // during deopt
238 break;
239 default:
240 fatal("called from an unsafe thread state");
241 }
242 assert(jt->is_active_Java_thread(), "must be active JavaThread");
243 } else {
244 // However, ThreadService::get_current_contended_monitor()
245 // can call here via the VMThread so sanity check it.
246 assert(self->is_VM_thread(), "must be");
247 }
248 #endif // ASSERT
249 }
250
251 ObjectMonitor::ObjectMonitor(oop object) :
252 _metadata(0),
253 _object(_oop_storage, object),
254 _owner(nullptr),
255 _previous_owner_tid(0),
256 _next_om(nullptr),
257 _recursions(0),
258 _EntryList(nullptr),
259 _cxq(nullptr),
260 _succ(nullptr),
261 _SpinDuration(ObjectMonitor::Knob_SpinLimit),
262 _contentions(0),
263 _WaitSet(nullptr),
264 _waiters(0),
265 _WaitSetLock(0),
266 _stack_locker(nullptr)
267 { }
268
269 ObjectMonitor::~ObjectMonitor() {
270 _object.release(_oop_storage);
271 }
272
273 oop ObjectMonitor::object() const {
274 check_object_context();
275 return _object.resolve();
276 }
277
278 void ObjectMonitor::ExitOnSuspend::operator()(JavaThread* current) {
279 if (current->is_suspended()) {
280 _om->_recursions = 0;
281 _om->_succ = nullptr;
282 // Don't need a full fence after clearing successor here because of the call to exit().
283 _om->exit(current, false /* not_suspended */);
284 _om_exited = true;
285
286 current->set_current_pending_monitor(_om);
287 }
288 }
289
290 void ObjectMonitor::ClearSuccOnSuspend::operator()(JavaThread* current) {
291 if (current->is_suspended()) {
292 if (_om->_succ == current) {
293 _om->_succ = nullptr;
294 OrderAccess::fence(); // always do a full fence when successor is cleared
295 }
296 }
297 }
298
299 #define assert_mark_word_consistency() \
300 assert(UseObjectMonitorTable || object()->mark() == markWord::encode(this), \
301 "object mark must match encoded this: mark=" INTPTR_FORMAT \
302 ", encoded this=" INTPTR_FORMAT, object()->mark().value(), \
303 markWord::encode(this).value());
304
305 // -----------------------------------------------------------------------------
306 // Enter support
307
308 bool ObjectMonitor::enter_is_async_deflating() {
309 if (is_being_async_deflated()) {
310 if (!UseObjectMonitorTable) {
311 const oop l_object = object();
312 if (l_object != nullptr) {
313 // Attempt to restore the header/dmw to the object's header so that
314 // we only retry once if the deflater thread happens to be slow.
315 install_displaced_markword_in_object(l_object);
316 }
317 }
318 return true;
319 }
320
321 return false;
322 }
323
324 bool ObjectMonitor::TryLockWithContentionMark(JavaThread* locking_thread, ObjectMonitorContentionMark& contention_mark) {
325 assert(contention_mark._monitor == this, "must be");
326 assert(!is_being_async_deflated(), "must be");
327
328 void* prev_owner = try_set_owner_from(nullptr, locking_thread);
329 bool success = false;
330
331 if (prev_owner == nullptr) {
332 assert(_recursions == 0, "invariant");
333 success = true;
334 } else if (prev_owner == owner_for(locking_thread)) {
335 _recursions++;
336 success = true;
337 } else if (prev_owner == DEFLATER_MARKER) {
338 // Racing with deflation.
339 prev_owner = try_set_owner_from(DEFLATER_MARKER, locking_thread);
340 if (prev_owner == DEFLATER_MARKER) {
341 // We successfully cancelled the in-progress async deflation by
342 // changing owner from DEFLATER_MARKER to current. We now extend
343 // the lifetime of the contention_mark (e.g. contentions++) here
344 // to prevent the deflater thread from winning the last part of
345 // the 2-part async deflation protocol after the regular
346 // decrement occurs when the contention_mark goes out of
347 // scope. ObjectMonitor::deflate_monitor() which is called by
348 // the deflater thread will decrement contentions after it
349 // recognizes that the async deflation was cancelled.
350 contention_mark.extend();
351 success = true;
352 } else if (prev_owner == nullptr) {
353 // At this point we cannot race with deflation as we have both incremented
354 // contentions, seen contention > 0 and seen a DEFLATER_MARKER.
355 // success will only be false if this races with something other than
356 // deflation.
357 prev_owner = try_set_owner_from(nullptr, locking_thread);
358 success = prev_owner == nullptr;
359 }
360 }
361 assert(!success || owner_raw() == owner_for(locking_thread), "must be");
362
363 return success;
364 }
365
366 void ObjectMonitor::enter_for_with_contention_mark(JavaThread* locking_thread, ObjectMonitorContentionMark& contention_mark) {
367 // Used by LightweightSynchronizer::inflate_and_enter in deoptimization path to enter for another thread.
368 // The monitor is private to or already owned by locking_thread which must be suspended.
369 // So this code may only contend with deflation.
370 assert(locking_thread == Thread::current() || locking_thread->is_obj_deopt_suspend(), "must be");
371 bool success = TryLockWithContentionMark(locking_thread, contention_mark);
372
373 assert(success, "Failed to enter_for: locking_thread=" INTPTR_FORMAT
374 ", this=" INTPTR_FORMAT "{owner=" INTPTR_FORMAT "}",
375 p2i(locking_thread), p2i(this), p2i(owner_raw()));
376 }
377
378 bool ObjectMonitor::enter_for(JavaThread* locking_thread) {
379 // Used by ObjectSynchronizer::enter_for() to enter for another thread.
380 // The monitor is private to or already owned by locking_thread which must be suspended.
381 // So this code may only contend with deflation.
382 assert(locking_thread == Thread::current() || locking_thread->is_obj_deopt_suspend(), "must be");
383
384 // Block out deflation as soon as possible.
385 ObjectMonitorContentionMark contention_mark(this);
386
387 // Check for deflation.
388 if (enter_is_async_deflating()) {
389 return false;
390 }
391
392 bool success = TryLockWithContentionMark(locking_thread, contention_mark);
393
394 assert(success, "Failed to enter_for: locking_thread=" INTPTR_FORMAT
395 ", this=" INTPTR_FORMAT "{owner=" INTPTR_FORMAT "}",
396 p2i(locking_thread), p2i(this), p2i(owner_raw()));
397 assert(owner_raw() == owner_for(locking_thread), "must be");
398 return true;
399 }
400
401 bool ObjectMonitor::try_enter(JavaThread* current, bool check_for_recursion) {
402 // TryLock avoids the CAS and handles deflation.
403 TryLockResult r = TryLock(current);
404 if (r == TryLockResult::Success) {
405 assert(_recursions == 0, "invariant");
406 return true;
407 }
408
409 // If called from SharedRuntime::monitor_exit_helper(), we know that
410 // this thread doesn't already own the lock.
411 if (!check_for_recursion) {
412 return false;
413 }
414
415 if (r == TryLockResult::HasOwner && owner() == owner_for(current)) {
416 _recursions++;
417 return true;
418 }
419
420 return false;
421 }
422
423 bool ObjectMonitor::spin_enter(JavaThread* current) {
424 assert(current == JavaThread::current(), "must be");
425
426 // Check for recursion.
427 if (try_enter(current)) {
428 return true;
429 }
430
431 // Check for deflation.
432 if (enter_is_async_deflating()) {
433 return false;
434 }
435
436 // We've encountered genuine contention.
437
438 // Do one round of spinning.
439 // Note that if we acquire the monitor from an initial spin
440 // we forgo posting JVMTI events and firing DTRACE probes.
441 if (TrySpin(current)) {
442 assert(owner_raw() == owner_for(current), "must be current: owner=" INTPTR_FORMAT, p2i(owner_raw()));
443 assert(_recursions == 0, "must be 0: recursions=" INTX_FORMAT, _recursions);
444 assert_mark_word_consistency();
445 return true;
446 }
447
448 return false;
449 }
450
451 bool ObjectMonitor::enter(JavaThread* current) {
452 assert(current == JavaThread::current(), "must be");
453
454 if (spin_enter(current)) {
455 return true;
456 }
457
458 assert(owner_raw() != owner_for(current), "invariant");
459 assert(_succ != current, "invariant");
460 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
461 assert(current->thread_state() != _thread_blocked, "invariant");
462
463 // Keep is_being_async_deflated stable across the rest of enter
464 ObjectMonitorContentionMark contention_mark(this);
465
466 // Check for deflation.
467 if (enter_is_async_deflating()) {
468 return false;
469 }
470
471 // At this point this ObjectMonitor cannot be deflated, finish contended enter
472 enter_with_contention_mark(current, contention_mark);
473 return true;
474 }
475
476 void ObjectMonitor::enter_with_contention_mark(JavaThread *current, ObjectMonitorContentionMark &cm) {
477 assert(current == JavaThread::current(), "must be");
478 assert(owner_raw() != current, "must be");
479 assert(cm._monitor == this, "must be");
480 assert(!is_being_async_deflated(), "must be");
481
482 JFR_ONLY(JfrConditionalFlush<EventJavaMonitorEnter> flush(current);)
483 EventJavaMonitorEnter event;
484 if (event.is_started()) {
485 event.set_monitorClass(object()->klass());
486 // Set an address that is 'unique enough', such that events close in
487 // time and with the same address are likely (but not guaranteed) to
488 // belong to the same object.
489 event.set_address((uintptr_t)this);
490 }
491
492 { // Change java thread status to indicate blocked on monitor enter.
493 JavaThreadBlockedOnMonitorEnterState jtbmes(current, this);
494
495 assert(current->current_pending_monitor() == nullptr, "invariant");
496 current->set_current_pending_monitor(this);
497
498 DTRACE_MONITOR_PROBE(contended__enter, this, object(), current);
499 if (JvmtiExport::should_post_monitor_contended_enter()) {
500 JvmtiExport::post_monitor_contended_enter(current, this);
501
502 // The current thread does not yet own the monitor and does not
503 // yet appear on any queues that would get it made the successor.
504 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
505 // handler cannot accidentally consume an unpark() meant for the
506 // ParkEvent associated with this ObjectMonitor.
507 }
508
509 ContinuationEntry* ce = current->last_continuation();
510 if (ce != nullptr && ce->is_virtual_thread()) {
511 int result = Continuation::try_preempt(current, ce->cont_oop(current), Continuation::freeze_on_monitorenter);
512 if (result == freeze_ok) {
513 bool acquired = VThreadMonitorEnter(current);
514 if (acquired) {
515 current->set_preemption_cancelled(true);
516 if (JvmtiExport::should_post_monitor_contended_entered()) {
517 // We are going to call thaw again after this and finish the VMTS
518 // transition so no need to do it here. We will post the event there.
519 current->set_contended_entered_monitor(this);
520 }
521 }
522 current->set_current_pending_monitor(nullptr);
523 DEBUG_ONLY(int state = java_lang_VirtualThread::state(current->vthread()));
524 assert((acquired && current->preemption_cancelled() && state == java_lang_VirtualThread::RUNNING) ||
525 (!acquired && !current->preemption_cancelled() && state == java_lang_VirtualThread::BLOCKING), "invariant");
526 return;
527 }
528 if (result == freeze_pinned_native) {
529 post_virtual_thread_pinned_event(current, "Native frame or <clinit> on stack");
530 } else if (result == freeze_unsupported) {
531 post_virtual_thread_pinned_event(current, "Native frame or <clinit> or monitors on stack");
532 }
533 }
534
535 OSThreadContendState osts(current->osthread());
536
537 assert(current->thread_state() == _thread_in_vm, "invariant");
538
539 for (;;) {
540 ExitOnSuspend eos(this);
541 {
542 ThreadBlockInVMPreprocess<ExitOnSuspend> tbivs(current, eos, true /* allow_suspend */);
543 EnterI(current);
544 current->set_current_pending_monitor(nullptr);
545 // We can go to a safepoint at the end of this block. If we
546 // do a thread dump during that safepoint, then this thread will show
547 // as having "-locked" the monitor, but the OS and java.lang.Thread
548 // states will still report that the thread is blocked trying to
549 // acquire it.
550 // If there is a suspend request, ExitOnSuspend will exit the OM
551 // and set the OM as pending.
552 }
553 if (!eos.exited()) {
554 // ExitOnSuspend did not exit the OM
555 assert(owner_raw() == owner_for(current), "invariant");
556 break;
557 }
558 }
559
560 // We've just gotten past the enter-check-for-suspend dance and we now own
561 // the monitor free and clear.
562 }
563
564 assert(contentions() >= 0, "must not be negative: contentions=%d", contentions());
565
566 // Must either set _recursions = 0 or ASSERT _recursions == 0.
567 assert(_recursions == 0, "invariant");
568 assert(owner_raw() == owner_for(current), "invariant");
569 assert(_succ != current, "invariant");
570 assert_mark_word_consistency();
571
572 // The thread -- now the owner -- is back in vm mode.
573 // Report the glorious news via TI,DTrace and jvmstat.
574 // The probe effect is non-trivial. All the reportage occurs
575 // while we hold the monitor, increasing the length of the critical
576 // section. Amdahl's parallel speedup law comes vividly into play.
577 //
578 // Another option might be to aggregate the events (thread local or
579 // per-monitor aggregation) and defer reporting until a more opportune
580 // time -- such as next time some thread encounters contention but has
581 // yet to acquire the lock. While spinning that thread could
582 // spinning we could increment JVMStat counters, etc.
583
584 DTRACE_MONITOR_PROBE(contended__entered, this, object(), current);
585 if (JvmtiExport::should_post_monitor_contended_entered()) {
586 JvmtiExport::post_monitor_contended_entered(current, this);
587
588 // The current thread already owns the monitor and is not going to
589 // call park() for the remainder of the monitor enter protocol. So
590 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
591 // event handler consumed an unpark() issued by the thread that
592 // just exited the monitor.
593 }
594 if (event.should_commit()) {
595 event.set_previousOwner(_previous_owner_tid);
596 event.commit();
597 }
598 OM_PERFDATA_OP(ContendedLockAttempts, inc());
599 }
600
601 // Caveat: TryLock() is not necessarily serializing if it returns failure.
602 // Callers must compensate as needed.
603
604 ObjectMonitor::TryLockResult ObjectMonitor::TryLock(JavaThread* current) {
605 void* own = owner_raw();
606 void* first_own = own;
607
608 for (;;) {
609 if (own == DEFLATER_MARKER) {
610 // Block out deflation as soon as possible.
611 ObjectMonitorContentionMark contention_mark(this);
612
613 // Check for deflation.
614 if (enter_is_async_deflating()) {
615 // Treat deflation as interference.
616 return TryLockResult::Interference;
617 }
618 if (TryLockWithContentionMark(current, contention_mark)) {
619 assert(_recursions == 0, "invariant");
620 return TryLockResult::Success;
621 } else {
622 // Deflation won or change of owner; dont spin
623 break;
624 }
625 } else if (own == nullptr) {
626 void* prev_own = try_set_owner_from(nullptr, current);
627 if (prev_own == nullptr) {
628 assert(_recursions == 0, "invariant");
629 return TryLockResult::Success;
630 } else {
631 // The lock had been free momentarily, but we lost the race to the lock.
632 own = prev_own;
633 }
634 } else {
635 // Retry doesn't make as much sense because the lock was just acquired.
636 break;
637 }
638 }
639 return first_own == own ? TryLockResult::HasOwner : TryLockResult::Interference;
640 }
641
642 // Deflate the specified ObjectMonitor if not in-use. Returns true if it
643 // was deflated and false otherwise.
644 //
645 // The async deflation protocol sets owner to DEFLATER_MARKER and
646 // makes contentions negative as signals to contending threads that
647 // an async deflation is in progress. There are a number of checks
648 // as part of the protocol to make sure that the calling thread has
649 // not lost the race to a contending thread.
650 //
651 // The ObjectMonitor has been successfully async deflated when:
652 // (contentions < 0)
653 // Contending threads that see that condition know to retry their operation.
654 //
655 bool ObjectMonitor::deflate_monitor(Thread* current) {
656 if (is_busy()) {
657 // Easy checks are first - the ObjectMonitor is busy so no deflation.
658 return false;
659 }
660
661 const oop obj = object_peek();
662
663 if (obj == nullptr) {
664 // If the object died, we can recycle the monitor without racing with
665 // Java threads. The GC already broke the association with the object.
666 set_owner_from_raw(nullptr, DEFLATER_MARKER);
667 assert(contentions() >= 0, "must be non-negative: contentions=%d", contentions());
668 _contentions = INT_MIN; // minimum negative int
669 } else {
670 // Attempt async deflation protocol.
671
672 // Set a null owner to DEFLATER_MARKER to force any contending thread
673 // through the slow path. This is just the first part of the async
674 // deflation dance.
675 if (try_set_owner_from_raw(nullptr, DEFLATER_MARKER) != nullptr) {
676 // The owner field is no longer null so we lost the race since the
677 // ObjectMonitor is now busy.
678 return false;
679 }
680
681 if (contentions() > 0 || _waiters != 0) {
682 // Another thread has raced to enter the ObjectMonitor after
683 // is_busy() above or has already entered and waited on
684 // it which makes it busy so no deflation. Restore owner to
685 // null if it is still DEFLATER_MARKER.
686 if (try_set_owner_from_raw(DEFLATER_MARKER, nullptr) != DEFLATER_MARKER) {
687 // Deferred decrement for the JT EnterI() that cancelled the async deflation.
688 add_to_contentions(-1);
689 }
690 return false;
691 }
692
693 // Make a zero contentions field negative to force any contending threads
694 // to retry. This is the second part of the async deflation dance.
695 if (Atomic::cmpxchg(&_contentions, 0, INT_MIN) != 0) {
696 // Contentions was no longer 0 so we lost the race since the
697 // ObjectMonitor is now busy. Restore owner to null if it is
698 // still DEFLATER_MARKER:
699 if (try_set_owner_from_raw(DEFLATER_MARKER, nullptr) != DEFLATER_MARKER) {
700 // Deferred decrement for the JT EnterI() that cancelled the async deflation.
701 add_to_contentions(-1);
702 }
703 return false;
704 }
705 }
706
707 // Sanity checks for the races:
708 guarantee(owner_is_DEFLATER_MARKER(), "must be deflater marker");
709 guarantee(contentions() < 0, "must be negative: contentions=%d",
710 contentions());
711 guarantee(_waiters == 0, "must be 0: waiters=%d", _waiters);
712 guarantee(_cxq == nullptr, "must be no contending threads: cxq="
713 INTPTR_FORMAT, p2i(_cxq));
714 guarantee(_EntryList == nullptr,
715 "must be no entering threads: EntryList=" INTPTR_FORMAT,
716 p2i(_EntryList));
717
718 if (obj != nullptr) {
719 if (log_is_enabled(Trace, monitorinflation)) {
720 ResourceMark rm;
721 log_trace(monitorinflation)("deflate_monitor: object=" INTPTR_FORMAT
722 ", mark=" INTPTR_FORMAT ", type='%s'",
723 p2i(obj), obj->mark().value(),
724 obj->klass()->external_name());
725 }
726 }
727
728 if (UseObjectMonitorTable) {
729 LightweightSynchronizer::deflate_monitor(current, obj, this);
730 } else if (obj != nullptr) {
731 // Install the old mark word if nobody else has already done it.
732 install_displaced_markword_in_object(obj);
733 }
734
735 // We leave owner == DEFLATER_MARKER and contentions < 0
736 // to force any racing threads to retry.
737 return true; // Success, ObjectMonitor has been deflated.
738 }
739
740 // Install the displaced mark word (dmw) of a deflating ObjectMonitor
741 // into the header of the object associated with the monitor. This
742 // idempotent method is called by a thread that is deflating a
743 // monitor and by other threads that have detected a race with the
744 // deflation process.
745 void ObjectMonitor::install_displaced_markword_in_object(const oop obj) {
746 assert(!UseObjectMonitorTable, "ObjectMonitorTable has no dmw");
747 // This function must only be called when (owner == DEFLATER_MARKER
748 // && contentions <= 0), but we can't guarantee that here because
749 // those values could change when the ObjectMonitor gets moved from
750 // the global free list to a per-thread free list.
751
752 guarantee(obj != nullptr, "must be non-null");
753
754 // Separate loads in is_being_async_deflated(), which is almost always
755 // called before this function, from the load of dmw/header below.
756
757 // _contentions and dmw/header may get written by different threads.
758 // Make sure to observe them in the same order when having several observers.
759 OrderAccess::loadload_for_IRIW();
760
761 const oop l_object = object_peek();
762 if (l_object == nullptr) {
763 // ObjectMonitor's object ref has already been cleared by async
764 // deflation or GC so we're done here.
765 return;
766 }
767 assert(l_object == obj, "object=" INTPTR_FORMAT " must equal obj="
768 INTPTR_FORMAT, p2i(l_object), p2i(obj));
769
770 markWord dmw = header();
771 // The dmw has to be neutral (not null, not locked and not marked).
772 assert(dmw.is_neutral(), "must be neutral: dmw=" INTPTR_FORMAT, dmw.value());
773
774 // Install displaced mark word if the object's header still points
775 // to this ObjectMonitor. More than one racing caller to this function
776 // can rarely reach this point, but only one can win.
777 markWord res = obj->cas_set_mark(dmw, markWord::encode(this));
778 if (res != markWord::encode(this)) {
779 // This should be rare so log at the Info level when it happens.
780 log_info(monitorinflation)("install_displaced_markword_in_object: "
781 "failed cas_set_mark: new_mark=" INTPTR_FORMAT
782 ", old_mark=" INTPTR_FORMAT ", res=" INTPTR_FORMAT,
783 dmw.value(), markWord::encode(this).value(),
784 res.value());
785 }
786
787 // Note: It does not matter which thread restored the header/dmw
788 // into the object's header. The thread deflating the monitor just
789 // wanted the object's header restored and it is. The threads that
790 // detected a race with the deflation process also wanted the
791 // object's header restored before they retry their operation and
792 // because it is restored they will only retry once.
793 }
794
795 // Convert the fields used by is_busy() to a string that can be
796 // used for diagnostic output.
797 const char* ObjectMonitor::is_busy_to_string(stringStream* ss) {
798 ss->print("is_busy: waiters=%d"
799 ", contentions=%d"
800 ", owner=" INTPTR_FORMAT
801 ", cxq=" PTR_FORMAT
802 ", EntryList=" PTR_FORMAT,
803 _waiters,
804 (contentions() > 0 ? contentions() : 0),
805 owner_is_DEFLATER_MARKER()
806 // We report null instead of DEFLATER_MARKER here because is_busy()
807 // ignores DEFLATER_MARKER values.
808 ? p2i(nullptr)
809 : p2i(owner_raw()),
810 p2i(_cxq),
811 p2i(_EntryList));
812 return ss->base();
813 }
814
815 void ObjectMonitor::EnterI(JavaThread* current) {
816 assert(current->thread_state() == _thread_blocked, "invariant");
817
818 // Try the lock - TATAS
819 if (TryLock(current) == TryLockResult::Success) {
820 assert(_succ != current, "invariant");
821 assert(owner_raw() == owner_for(current), "invariant");
822 return;
823 }
824
825 assert(InitDone, "Unexpectedly not initialized");
826
827 // We try one round of spinning *before* enqueueing current.
828 //
829 // If the _owner is ready but OFFPROC we could use a YieldTo()
830 // operation to donate the remainder of this thread's quantum
831 // to the owner. This has subtle but beneficial affinity
832 // effects.
833
834 if (TrySpin(current)) {
835 assert(owner_raw() == owner_for(current), "invariant");
836 assert(_succ != current, "invariant");
837 return;
838 }
839
840 // The Spin failed -- Enqueue and park the thread ...
841 assert(_succ != current, "invariant");
842 assert(owner_raw() != owner_for(current), "invariant");
843
844 // Enqueue "current" on ObjectMonitor's _cxq.
845 //
846 // Node acts as a proxy for current.
847 // As an aside, if were to ever rewrite the synchronization code mostly
848 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
849 // Java objects. This would avoid awkward lifecycle and liveness issues,
850 // as well as eliminate a subset of ABA issues.
851 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
852
853 ObjectWaiter node(current);
854 current->_ParkEvent->reset();
855 node._prev = (ObjectWaiter*) 0xBAD;
856 node.TState = ObjectWaiter::TS_CXQ;
857
858 // Push "current" onto the front of the _cxq.
859 // Once on cxq/EntryList, current stays on-queue until it acquires the lock.
860 // Note that spinning tends to reduce the rate at which threads
861 // enqueue and dequeue on EntryList|cxq.
862 ObjectWaiter* nxt;
863 for (;;) {
864 node._next = nxt = _cxq;
865 if (Atomic::cmpxchg(&_cxq, nxt, &node) == nxt) break;
866
867 // Interference - the CAS failed because _cxq changed. Just retry.
868 // As an optional optimization we retry the lock.
869 if (TryLock(current) == TryLockResult::Success) {
870 assert(_succ != current, "invariant");
871 assert(owner_raw() == owner_for(current), "invariant");
872 return;
873 }
874 }
875
876 // The lock might have been released while this thread was occupied queueing
877 // itself onto _cxq. To close the race and avoid "stranding" and
878 // progress-liveness failure we must resample-retry _owner before parking.
879 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
880 // In this case the ST-MEMBAR is accomplished with CAS().
881 //
882 // TODO: Defer all thread state transitions until park-time.
883 // Since state transitions are heavy and inefficient we'd like
884 // to defer the state transitions until absolutely necessary,
885 // and in doing so avoid some transitions ...
886
887 // For virtual threads that are pinned, do a timed-park instead to
888 // alleviate some deadlocks cases where succesor cannot run.
889 static int MAX_RECHECK_INTERVAL = 1000;
890 int recheck_interval = 1;
891 bool do_timed_parked = false;
892 ContinuationEntry* ce = current->last_continuation();
893 if (ce != nullptr && ce->is_virtual_thread()) {
894 do_timed_parked = true;
895 }
896
897 for (;;) {
898
899 if (TryLock(current) == TryLockResult::Success) {
900 break;
901 }
902 assert(owner_raw() != owner_for(current), "invariant");
903
904 // park self
905 if (do_timed_parked) {
906 current->_ParkEvent->park((jlong) recheck_interval);
907 // Increase the recheck_interval, but clamp the value.
908 recheck_interval *= 8;
909 if (recheck_interval > MAX_RECHECK_INTERVAL) {
910 recheck_interval = MAX_RECHECK_INTERVAL;
911 }
912 } else {
913 current->_ParkEvent->park();
914 }
915
916 if (TryLock(current) == TryLockResult::Success) {
917 break;
918 }
919
920 // The lock is still contested.
921
922 // Keep a tally of the # of futile wakeups.
923 // Note that the counter is not protected by a lock or updated by atomics.
924 // That is by design - we trade "lossy" counters which are exposed to
925 // races during updates for a lower probe effect.
926 // This PerfData object can be used in parallel with a safepoint.
927 // See the work around in PerfDataManager::destroy().
928 OM_PERFDATA_OP(FutileWakeups, inc());
929
930 // Assuming this is not a spurious wakeup we'll normally find _succ == current.
931 // We can defer clearing _succ until after the spin completes
932 // TrySpin() must tolerate being called with _succ == current.
933 // Try yet another round of adaptive spinning.
934 if (TrySpin(current)) {
935 break;
936 }
937
938 // We can find that we were unpark()ed and redesignated _succ while
939 // we were spinning. That's harmless. If we iterate and call park(),
940 // park() will consume the event and return immediately and we'll
941 // just spin again. This pattern can repeat, leaving _succ to simply
942 // spin on a CPU.
943
944 if (_succ == current) _succ = nullptr;
945
946 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
947 OrderAccess::fence();
948 }
949
950 // Egress :
951 // current has acquired the lock -- Unlink current from the cxq or EntryList.
952 // Normally we'll find current on the EntryList .
953 // From the perspective of the lock owner (this thread), the
954 // EntryList is stable and cxq is prepend-only.
955 // The head of cxq is volatile but the interior is stable.
956 // In addition, current.TState is stable.
957
958 assert(owner_raw() == owner_for(current), "invariant");
959
960 UnlinkAfterAcquire(current, &node);
961 if (_succ == current) {
962 _succ = nullptr;
963 // Note that we don't need to do OrderAccess::fence() after clearing
964 // _succ here, since we own the lock.
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 or cxq. 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 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.
988
989 return;
990 }
991
992 // ReenterI() is a specialized inline form of the latter half of the
993 // contended slow-path from EnterI(). We use ReenterI() only for
994 // monitor reentry in wait().
995 //
996 // In the future we should reconcile EnterI() and ReenterI().
997
998 void ObjectMonitor::ReenterI(JavaThread* current, ObjectWaiter* currentNode) {
999 assert(current != nullptr, "invariant");
1000 assert(current->thread_state() != _thread_blocked, "invariant");
1001 assert(currentNode != nullptr, "invariant");
1002 assert(currentNode->_thread == current, "invariant");
1003 assert(_waiters > 0, "invariant");
1004 assert_mark_word_consistency();
1005
1006 for (;;) {
1007 uint8_t v = currentNode->TState;
1008 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1009 assert(owner_raw() != owner_for(current), "invariant");
1010
1011 // This thread has been notified so try to reacquire the lock.
1012 if (TryLock(current) == TryLockResult::Success) {
1013 break;
1014 }
1015
1016 // If that fails, spin again. Note that spin count may be zero so the above TryLock
1017 // is necessary.
1018 if (TrySpin(current)) {
1019 break;
1020 }
1021
1022 {
1023 OSThreadContendState osts(current->osthread());
1024
1025 assert(current->thread_state() == _thread_in_vm, "invariant");
1026
1027 {
1028 ClearSuccOnSuspend csos(this);
1029 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */);
1030 current->_ParkEvent->park();
1031 }
1032 }
1033
1034 // Try again, but just so we distinguish between futile wakeups and
1035 // successful wakeups. The following test isn't algorithmically
1036 // necessary, but it helps us maintain sensible statistics.
1037 if (TryLock(current) == TryLockResult::Success) {
1038 break;
1039 }
1040
1041 // The lock is still contested.
1042
1043 // Assuming this is not a spurious wakeup we'll normally
1044 // find that _succ == current.
1045 if (_succ == current) _succ = nullptr;
1046
1047 // Invariant: after clearing _succ a contending thread
1048 // *must* retry _owner before parking.
1049 OrderAccess::fence();
1050
1051 // Keep a tally of the # of futile wakeups.
1052 // Note that the counter is not protected by a lock or updated by atomics.
1053 // That is by design - we trade "lossy" counters which are exposed to
1054 // races during updates for a lower probe effect.
1055 // This PerfData object can be used in parallel with a safepoint.
1056 // See the work around in PerfDataManager::destroy().
1057 OM_PERFDATA_OP(FutileWakeups, inc());
1058 }
1059
1060 // current has acquired the lock -- Unlink current from the cxq or EntryList .
1061 // Normally we'll find current on the EntryList.
1062 // Unlinking from the EntryList is constant-time and atomic-free.
1063 // From the perspective of the lock owner (this thread), the
1064 // EntryList is stable and cxq is prepend-only.
1065 // The head of cxq is volatile but the interior is stable.
1066 // In addition, current.TState is stable.
1067
1068 assert(owner_raw() == owner_for(current), "invariant");
1069 assert_mark_word_consistency();
1070 UnlinkAfterAcquire(current, currentNode);
1071 if (_succ == current) _succ = nullptr;
1072 assert(_succ != current, "invariant");
1073 currentNode->TState = ObjectWaiter::TS_RUN;
1074 OrderAccess::fence(); // see comments at the end of EnterI()
1075 }
1076
1077 // This method is called from two places:
1078 // - On monitorenter contention with a null waiter.
1079 // - After Object.wait() times out or the target is interrupted to reenter the
1080 // monitor, with the existing waiter.
1081 // For the Object.wait() case we do not delete the ObjectWaiter in case we
1082 // succesfully acquire the monitor since we are going to need it on return.
1083 bool ObjectMonitor::VThreadMonitorEnter(JavaThread* current, ObjectWaiter* waiter) {
1084 if (TryLock(current) == TryLockResult::Success) {
1085 assert(owner_raw() == owner_for(current), "invariant");
1086 assert(_succ != current, "invariant");
1087 return true;
1088 }
1089
1090 oop vthread = current->vthread();
1091 ObjectWaiter* node = waiter != nullptr ? waiter : new ObjectWaiter(vthread, this);
1092 node->_prev = (ObjectWaiter*) 0xBAD;
1093 node->TState = ObjectWaiter::TS_CXQ;
1094
1095 // Push node associated with vthread onto the front of the _cxq.
1096 ObjectWaiter* nxt;
1097 for (;;) {
1098 node->_next = nxt = _cxq;
1099 if (Atomic::cmpxchg(&_cxq, nxt, node) == nxt) break;
1100
1101 // Interference - the CAS failed because _cxq changed. Just retry.
1102 // As an optional optimization we retry the lock.
1103 if (TryLock(current) == TryLockResult::Success) {
1104 assert(owner_raw() == owner_for(current), "invariant");
1105 assert(_succ != current, "invariant");
1106 if (waiter == nullptr) delete node; // for Object.wait() don't delete yet
1107 return true;
1108 }
1109 }
1110
1111 // We have to try once more since owner could have exited monitor and checked
1112 // _cxq before we added the node to the queue.
1113 if (TryLock(current) == TryLockResult::Success) {
1114 assert(owner_raw() == owner_for(current), "invariant");
1115 UnlinkAfterAcquire(current, node);
1116 if (_succ == (JavaThread*)java_lang_Thread::thread_id(vthread)) _succ = nullptr;
1117 if (waiter == nullptr) delete node; // for Object.wait() don't delete yet
1118 return true;
1119 }
1120
1121 assert(java_lang_VirtualThread::state(vthread) == java_lang_VirtualThread::RUNNING, "wrong state for vthread");
1122 java_lang_VirtualThread::set_state(vthread, java_lang_VirtualThread::BLOCKING);
1123
1124 // We didn't succeed in acquiring the monitor so increment _contentions and
1125 // save ObjectWaiter* in the chunk since we will need it when resuming execution.
1126 add_to_contentions(1);
1127 oop cont = java_lang_VirtualThread::continuation(vthread);
1128 stackChunkOop chunk = jdk_internal_vm_Continuation::tail(cont);
1129 chunk->set_object_waiter(node);
1130 return false;
1131 }
1132
1133 bool ObjectMonitor::resume_operation(JavaThread* current, ObjectWaiter* node, ContinuationWrapper& cont) {
1134 assert(java_lang_VirtualThread::state(current->vthread()) == java_lang_VirtualThread::RUNNING, "wrong state for vthread");
1135
1136 if (node->is_wait() && !node->at_reenter()) {
1137 bool acquired_monitor = VThreadWaitReenter(current, node, cont);
1138 if (acquired_monitor) return true;
1139 }
1140
1141 // Retry acquiring monitor...
1142
1143 int state = node->TState;
1144 guarantee(state == ObjectWaiter::TS_ENTER || state == ObjectWaiter::TS_CXQ, "invariant");
1145
1146 if (TryLock(current) == TryLockResult::Success) {
1147 VThreadEpilog(current, node);
1148 return true;
1149 }
1150
1151 oop vthread = current->vthread();
1152 if (_succ == (JavaThread*)java_lang_Thread::thread_id(vthread)) _succ = nullptr;
1153
1154 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
1155 OrderAccess::fence();
1156
1157 if (TryLock(current) == TryLockResult::Success) {
1158 VThreadEpilog(current, node);
1159 return true;
1160 }
1161
1162 // The JT will read this variable on return to the resume_monitor_operation stub
1163 // and will unmount (enterSpecial frame removed and return to Continuation.run()).
1164 java_lang_VirtualThread::set_state(vthread, java_lang_VirtualThread::BLOCKING);
1165
1166 return false;
1167 }
1168
1169 void ObjectMonitor::VThreadEpilog(JavaThread* current, ObjectWaiter* node) {
1170 assert(owner_raw() == owner_for(current), "invariant");
1171 add_to_contentions(-1);
1172
1173 oop vthread = current->vthread();
1174 int64_t threadid = java_lang_Thread::thread_id(vthread);
1175 if (_succ == (JavaThread*)threadid) _succ = nullptr;
1176
1177 guarantee(_recursions == 0, "invariant");
1178
1179 if (node->is_wait()) {
1180 _recursions = node->_recursions; // restore the old recursion count
1181 _waiters--; // decrement the number of waiters
1182
1183 if (node->_interrupted) {
1184 // We will throw at thaw end after finishing the mount transition.
1185 current->set_pending_interrupted_exception(true);
1186 }
1187 }
1188
1189 assert(node->TState == ObjectWaiter::TS_ENTER || node->TState == ObjectWaiter::TS_CXQ, "");
1190 UnlinkAfterAcquire(current, node);
1191 delete node;
1192
1193 oop cont = java_lang_VirtualThread::continuation(vthread);
1194 stackChunkOop chunk = jdk_internal_vm_Continuation::tail(cont);
1195 chunk->set_object_waiter(nullptr);
1196
1197 if (JvmtiExport::should_post_monitor_contended_entered()) {
1198 // We are going to call thaw again after this and finish the VMTS
1199 // transition so no need to do it here. We will post the event there.
1200 current->set_contended_entered_monitor(this);
1201 }
1202 }
1203
1204 // By convention we unlink a contending thread from EntryList|cxq immediately
1205 // after the thread acquires the lock in ::enter(). Equally, we could defer
1206 // unlinking the thread until ::exit()-time.
1207
1208 void ObjectMonitor::UnlinkAfterAcquire(JavaThread* current, ObjectWaiter* currentNode) {
1209 assert(owner_raw() == owner_for(current), "invariant");
1210 assert((!currentNode->is_vthread() && currentNode->thread() == current) ||
1211 (currentNode->is_vthread() && currentNode->vthread() == current->vthread()), "invariant");
1212
1213 if (currentNode->TState == ObjectWaiter::TS_ENTER) {
1214 // Normal case: remove current from the DLL EntryList .
1215 // This is a constant-time operation.
1216 ObjectWaiter* nxt = currentNode->_next;
1217 ObjectWaiter* prv = currentNode->_prev;
1218 if (nxt != nullptr) nxt->_prev = prv;
1219 if (prv != nullptr) prv->_next = nxt;
1220 if (currentNode == _EntryList) _EntryList = nxt;
1221 assert(nxt == nullptr || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
1222 assert(prv == nullptr || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
1223 } else {
1224 assert(currentNode->TState == ObjectWaiter::TS_CXQ, "invariant");
1225 // Inopportune interleaving -- current is still on the cxq.
1226 // This usually means the enqueue of self raced an exiting thread.
1227 // Normally we'll find current near the front of the cxq, so
1228 // dequeueing is typically fast. If needbe we can accelerate
1229 // this with some MCS/CHL-like bidirectional list hints and advisory
1230 // back-links so dequeueing from the interior will normally operate
1231 // in constant-time.
1232 // Dequeue current from either the head (with CAS) or from the interior
1233 // with a linear-time scan and normal non-atomic memory operations.
1234 // CONSIDER: if current is on the cxq then simply drain cxq into EntryList
1235 // and then unlink current from EntryList. We have to drain eventually,
1236 // so it might as well be now.
1237
1238 ObjectWaiter* v = _cxq;
1239 assert(v != nullptr, "invariant");
1240 if (v != currentNode || Atomic::cmpxchg(&_cxq, v, currentNode->_next) != v) {
1241 // The CAS above can fail from interference IFF a "RAT" arrived.
1242 // In that case current must be in the interior and can no longer be
1243 // at the head of cxq.
1244 if (v == currentNode) {
1245 assert(_cxq != v, "invariant");
1246 v = _cxq; // CAS above failed - start scan at head of list
1247 }
1248 ObjectWaiter* p;
1249 ObjectWaiter* q = nullptr;
1250 for (p = v; p != nullptr && p != currentNode; p = p->_next) {
1251 q = p;
1252 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
1253 }
1254 assert(v != currentNode, "invariant");
1255 assert(p == currentNode, "Node not found on cxq");
1256 assert(p != _cxq, "invariant");
1257 assert(q != nullptr, "invariant");
1258 assert(q->_next == p, "invariant");
1259 q->_next = p->_next;
1260 }
1261 }
1262
1263 #ifdef ASSERT
1264 // Diagnostic hygiene ...
1265 currentNode->_prev = (ObjectWaiter*) 0xBAD;
1266 currentNode->_next = (ObjectWaiter*) 0xBAD;
1267 currentNode->TState = ObjectWaiter::TS_RUN;
1268 #endif
1269 }
1270
1271 // -----------------------------------------------------------------------------
1272 // Exit support
1273 //
1274 // exit()
1275 // ~~~~~~
1276 // Note that the collector can't reclaim the objectMonitor or deflate
1277 // the object out from underneath the thread calling ::exit() as the
1278 // thread calling ::exit() never transitions to a stable state.
1279 // This inhibits GC, which in turn inhibits asynchronous (and
1280 // inopportune) reclamation of "this".
1281 //
1282 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
1283 // There's one exception to the claim above, however. EnterI() can call
1284 // exit() to drop a lock if the acquirer has been externally suspended.
1285 // In that case exit() is called with _thread_state == _thread_blocked,
1286 // but the monitor's _contentions field is > 0, which inhibits reclamation.
1287 //
1288 // This is the exit part of the locking protocol, often implemented in
1289 // C2_MacroAssembler::fast_unlock()
1290 //
1291 // 1. A release barrier ensures that changes to monitor meta-data
1292 // (_succ, _EntryList, _cxq) and data protected by the lock will be
1293 // visible before we release the lock.
1294 // 2. Release the lock by clearing the owner.
1295 // 3. A storeload MEMBAR is needed between releasing the owner and
1296 // subsequently reading meta-data to safely determine if the lock is
1297 // contended (step 4) without an elected successor (step 5).
1298 // 4. If both _EntryList and _cxq are null, we are done, since there is no
1299 // other thread waiting on the lock to wake up. I.e. there is no
1300 // contention.
1301 // 5. If there is a successor (_succ is non-null), we are done. The
1302 // responsibility for guaranteeing progress-liveness has now implicitly
1303 // been moved from the exiting thread to the successor.
1304 // 6. There are waiters in the entry list (_EntryList and/or cxq are
1305 // non-null), but there is no successor (_succ is null), so we need to
1306 // wake up (unpark) a waiting thread to avoid stranding.
1307 //
1308 // Note that since only the current lock owner can manipulate the _EntryList
1309 // or drain _cxq, we need to reacquire the lock before we can wake up
1310 // (unpark) a waiting thread.
1311 //
1312 // The CAS() in enter provides for safety and exclusion, while the
1313 // MEMBAR in exit provides for progress and avoids stranding.
1314 //
1315 // There is also the risk of a futile wake-up. If we drop the lock
1316 // another thread can reacquire the lock immediately, and we can
1317 // then wake a thread unnecessarily. This is benign, and we've
1318 // structured the code so the windows are short and the frequency
1319 // of such futile wakups is low.
1320
1321 void ObjectMonitor::exit(JavaThread* current, bool not_suspended) {
1322 void* cur = owner_raw();
1323 if (owner_for(current) != cur) {
1324 // Apparent unbalanced locking ...
1325 // Naively we'd like to throw IllegalMonitorStateException.
1326 // As a practical matter we can neither allocate nor throw an
1327 // exception as ::exit() can be called from leaf routines.
1328 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
1329 // Upon deeper reflection, however, in a properly run JVM the only
1330 // way we should encounter this situation is in the presence of
1331 // unbalanced JNI locking. TODO: CheckJNICalls.
1332 // See also: CR4414101
1333 #ifdef ASSERT
1334 LogStreamHandle(Error, monitorinflation) lsh;
1335 lsh.print_cr("ERROR: ObjectMonitor::exit(): thread=" INTPTR_FORMAT
1336 " is exiting an ObjectMonitor it does not own.", p2i(current));
1337 lsh.print_cr("The imbalance is possibly caused by JNI locking.");
1338 print_debug_style_on(&lsh);
1339 assert(false, "Non-balanced monitor enter/exit!");
1340 #endif
1341 return;
1342 }
1343
1344 if (_recursions != 0) {
1345 _recursions--; // this is simple recursive enter
1346 return;
1347 }
1348
1349 #if INCLUDE_JFR
1350 // get the owner's thread id for the MonitorEnter event
1351 // if it is enabled and the thread isn't suspended
1352 if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
1353 _previous_owner_tid = JFR_THREAD_ID(current);
1354 }
1355 #endif
1356
1357 for (;;) {
1358 assert(owner_for(current) == owner_raw(), "invariant");
1359
1360 // Drop the lock.
1361 // release semantics: prior loads and stores from within the critical section
1362 // must not float (reorder) past the following store that drops the lock.
1363 // Uses a storeload to separate release_store(owner) from the
1364 // successor check. The try_set_owner_from() below uses cmpxchg() so
1365 // we get the fence down there.
1366 release_clear_owner(current);
1367 OrderAccess::storeload();
1368
1369 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != nullptr) {
1370 return;
1371 }
1372 // Other threads are blocked trying to acquire the lock.
1373
1374 // Normally the exiting thread is responsible for ensuring succession,
1375 // but if this thread observes other successors are ready or other
1376 // entering threads are spinning after it has stored null into _owner
1377 // then it can exit without waking a successor. The existence of
1378 // spinners or ready successors guarantees proper succession (liveness).
1379 // Responsibility passes to the ready or running successors. The exiting
1380 // thread delegates the duty. More precisely, if a successor already
1381 // exists this thread is absolved of the responsibility of waking
1382 // (unparking) one.
1383
1384 // The _succ variable is critical to reducing futile wakeup frequency.
1385 // _succ identifies the "heir presumptive" thread that has been made
1386 // ready (unparked) but that has not yet run. We need only one such
1387 // successor thread to guarantee progress.
1388 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1389 // section 3.3 "Futile Wakeup Throttling" for details.
1390 //
1391 // Note that spinners in Enter() also set _succ non-null.
1392 // In the current implementation spinners opportunistically set
1393 // _succ so that exiting threads might avoid waking a successor.
1394 // Which means that the exiting thread could exit immediately without
1395 // waking a successor, if it observes a successor after it has dropped
1396 // the lock. Note that the dropped lock needs to become visible to the
1397 // spinner.
1398
1399 // It appears that an heir-presumptive (successor) must be made ready.
1400 // Only the current lock owner can manipulate the EntryList or
1401 // drain _cxq, so we need to reacquire the lock. If we fail
1402 // to reacquire the lock the responsibility for ensuring succession
1403 // falls to the new owner.
1404
1405 if (TryLock(current) != TryLockResult::Success) {
1406 // Some other thread acquired the lock (or the monitor was
1407 // deflated). Either way we are done.
1408 return;
1409 }
1410
1411 guarantee(owner_raw() == owner_for(current), "invariant");
1412
1413 ObjectWaiter* w = nullptr;
1414
1415 w = _EntryList;
1416 if (w != nullptr) {
1417 // I'd like to write: guarantee (w->_thread != current).
1418 // But in practice an exiting thread may find itself on the EntryList.
1419 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1420 // then calls exit(). Exit release the lock by setting O._owner to null.
1421 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1422 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1423 // release the lock "O". T2 resumes immediately after the ST of null into
1424 // _owner, above. T2 notices that the EntryList is populated, so it
1425 // reacquires the lock and then finds itself on the EntryList.
1426 // Given all that, we have to tolerate the circumstance where "w" is
1427 // associated with current.
1428 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1429 ExitEpilog(current, w);
1430 return;
1431 }
1432
1433 // If we find that both _cxq and EntryList are null then just
1434 // re-run the exit protocol from the top.
1435 w = _cxq;
1436 if (w == nullptr) continue;
1437
1438 // Drain _cxq into EntryList - bulk transfer.
1439 // First, detach _cxq.
1440 // The following loop is tantamount to: w = swap(&cxq, nullptr)
1441 for (;;) {
1442 assert(w != nullptr, "Invariant");
1443 ObjectWaiter* u = Atomic::cmpxchg(&_cxq, w, (ObjectWaiter*)nullptr);
1444 if (u == w) break;
1445 w = u;
1446 }
1447
1448 assert(w != nullptr, "invariant");
1449 assert(_EntryList == nullptr, "invariant");
1450
1451 // Convert the LIFO SLL anchored by _cxq into a DLL.
1452 // The list reorganization step operates in O(LENGTH(w)) time.
1453 // It's critical that this step operate quickly as
1454 // "current" still holds the outer-lock, restricting parallelism
1455 // and effectively lengthening the critical section.
1456 // Invariant: s chases t chases u.
1457 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1458 // we have faster access to the tail.
1459
1460 _EntryList = w;
1461 ObjectWaiter* q = nullptr;
1462 ObjectWaiter* p;
1463 for (p = w; p != nullptr; p = p->_next) {
1464 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1465 p->TState = ObjectWaiter::TS_ENTER;
1466 p->_prev = q;
1467 q = p;
1468 }
1469
1470 // We need to: ST EntryList; MEMBAR #storestore; ST _owner = nullptr
1471 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1472
1473 // See if we can abdicate to a spinner instead of waking a thread.
1474 // A primary goal of the implementation is to reduce the
1475 // context-switch rate.
1476 if (_succ != nullptr) continue;
1477
1478 w = _EntryList;
1479 if (w != nullptr) {
1480 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1481 ExitEpilog(current, w);
1482 return;
1483 }
1484 }
1485 }
1486
1487 void ObjectMonitor::ExitEpilog(JavaThread* current, ObjectWaiter* Wakee) {
1488 assert(owner_raw() == owner_for(current), "invariant");
1489
1490 // Exit protocol:
1491 // 1. ST _succ = wakee
1492 // 2. membar #loadstore|#storestore;
1493 // 2. ST _owner = nullptr
1494 // 3. unpark(wakee)
1495
1496 oop vthread = nullptr;
1497 ParkEvent * Trigger;
1498 if (!Wakee->is_vthread()) {
1499 JavaThread* t = Wakee->thread();
1500 assert(t != nullptr, "");
1501 Trigger = t->_ParkEvent;
1502 _succ = t;
1503 } else {
1504 vthread = Wakee->vthread();
1505 assert(vthread != nullptr, "");
1506 Trigger = ObjectMonitor::vthread_unparker_ParkEvent();
1507 _succ = (JavaThread*)java_lang_Thread::thread_id(vthread);
1508 }
1509
1510 // Hygiene -- once we've set _owner = nullptr we can't safely dereference Wakee again.
1511 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1512 // out-of-scope (non-extant).
1513 Wakee = nullptr;
1514
1515 // Drop the lock.
1516 // Uses a fence to separate release_store(owner) from the LD in unpark().
1517 release_clear_owner(current);
1518 OrderAccess::fence();
1519
1520 DTRACE_MONITOR_PROBE(contended__exit, this, object(), current);
1521
1522 if (vthread == nullptr) {
1523 // Platform thread case
1524 Trigger->unpark();
1525 } else if (java_lang_VirtualThread::set_onWaitingList(vthread, _vthread_cxq_head)) {
1526 Trigger->unpark();
1527 }
1528
1529 // Maintain stats and report events to JVMTI
1530 OM_PERFDATA_OP(Parks, inc());
1531 }
1532
1533 // complete_exit exits a lock returning recursion count
1534 // complete_exit requires an inflated monitor
1535 // The _owner field is not always the Thread addr even with an
1536 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1537 // thread due to contention.
1538 intx ObjectMonitor::complete_exit(JavaThread* current) {
1539 assert(InitDone, "Unexpectedly not initialized");
1540
1541 void* cur = owner_raw();
1542 if (owner_for(current) != cur) {
1543 if (LockingMode == LM_LEGACY && is_stack_locker(current)) {
1544 assert(_recursions == 0, "internal state error");
1545 set_owner_from_BasicLock(current); // Convert from BasicLock* to Thread*.
1546 _recursions = 0;
1547 }
1548 }
1549
1550 guarantee(owner_for(current) == owner_raw(), "complete_exit not owner");
1551 intx save = _recursions; // record the old recursion count
1552 _recursions = 0; // set the recursion level to be 0
1553 exit(current); // exit the monitor
1554 guarantee(owner_raw() != owner_for(current), "invariant");
1555 return save;
1556 }
1557
1558 // Checks that the current THREAD owns this monitor and causes an
1559 // immediate return if it doesn't. We don't use the CHECK macro
1560 // because we want the IMSE to be the only exception that is thrown
1561 // from the call site when false is returned. Any other pending
1562 // exception is ignored.
1563 #define CHECK_OWNER() \
1564 do { \
1565 if (!check_owner(THREAD)) { \
1566 assert(HAS_PENDING_EXCEPTION, "expected a pending IMSE here."); \
1567 return; \
1568 } \
1569 } while (false)
1570
1571 // Returns true if the specified thread owns the ObjectMonitor.
1572 // Otherwise returns false and throws IllegalMonitorStateException
1573 // (IMSE). If there is a pending exception and the specified thread
1574 // is not the owner, that exception will be replaced by the IMSE.
1575 bool ObjectMonitor::check_owner(TRAPS) {
1576 JavaThread* current = THREAD;
1577 void* cur = owner_raw();
1578 if (cur == owner_for(current)) {
1579 return true;
1580 }
1581 THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(),
1582 "current thread is not owner", false);
1583 }
1584
1585 static inline bool is_excluded(const Klass* monitor_klass) {
1586 assert(monitor_klass != nullptr, "invariant");
1587 NOT_JFR_RETURN_(false);
1588 JFR_ONLY(return vmSymbols::jdk_jfr_internal_management_HiddenWait() == monitor_klass->name();)
1589 }
1590
1591 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1592 ObjectMonitor* monitor,
1593 uint64_t notifier_tid,
1594 jlong timeout,
1595 bool timedout) {
1596 assert(event != nullptr, "invariant");
1597 assert(monitor != nullptr, "invariant");
1598 const Klass* monitor_klass = monitor->object()->klass();
1599 if (is_excluded(monitor_klass)) {
1600 return;
1601 }
1602 event->set_monitorClass(monitor_klass);
1603 event->set_timeout(timeout);
1604 // Set an address that is 'unique enough', such that events close in
1605 // time and with the same address are likely (but not guaranteed) to
1606 // belong to the same object.
1607 event->set_address((uintptr_t)monitor);
1608 event->set_notifier(notifier_tid);
1609 event->set_timedOut(timedout);
1610 event->commit();
1611 }
1612
1613 static void vthread_monitor_waited_event(JavaThread *current, ObjectWaiter* node, ContinuationWrapper& cont, EventJavaMonitorWait* event, jboolean timed_out) {
1614 // Since we might safepoint set the anchor so that the stack can we walked.
1615 assert(current->last_continuation() != nullptr, "");
1616 JavaFrameAnchor* anchor = current->frame_anchor();
1617 anchor->set_last_Java_sp(current->last_continuation()->entry_sp());
1618 anchor->set_last_Java_pc(current->last_continuation()->entry_pc());
1619
1620 ContinuationWrapper::SafepointOp so(current, cont);
1621
1622 JRT_BLOCK
1623 if (event->should_commit()) {
1624 long timeout = java_lang_VirtualThread::waitTimeout(current->vthread());
1625 post_monitor_wait_event(event, node->_monitor, node->_notifier_tid, timeout, timed_out);
1626 }
1627 if (JvmtiExport::should_post_monitor_waited()) {
1628 ThreadOnMonitorWaitedEvent tmwe(current);
1629 JvmtiExport::vthread_post_monitor_waited(current, node->_monitor, timed_out);
1630 }
1631 JRT_BLOCK_END
1632 current->frame_anchor()->clear();
1633 }
1634
1635 // -----------------------------------------------------------------------------
1636 // Wait/Notify/NotifyAll
1637 //
1638 // Note: a subset of changes to ObjectMonitor::wait()
1639 // will need to be replicated in complete_exit
1640 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1641 JavaThread* current = THREAD;
1642
1643 assert(InitDone, "Unexpectedly not initialized");
1644
1645 CHECK_OWNER(); // Throws IMSE if not owner.
1646
1647 EventJavaMonitorWait event;
1648
1649 // check for a pending interrupt
1650 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1651 // post monitor waited event. Note that this is past-tense, we are done waiting.
1652 if (JvmtiExport::should_post_monitor_waited()) {
1653 // Note: 'false' parameter is passed here because the
1654 // wait was not timed out due to thread interrupt.
1655 JvmtiExport::post_monitor_waited(current, this, false);
1656
1657 // In this short circuit of the monitor wait protocol, the
1658 // current thread never drops ownership of the monitor and
1659 // never gets added to the wait queue so the current thread
1660 // cannot be made the successor. This means that the
1661 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1662 // consume an unpark() meant for the ParkEvent associated with
1663 // this ObjectMonitor.
1664 }
1665 if (event.should_commit()) {
1666 post_monitor_wait_event(&event, this, 0, millis, false);
1667 }
1668 THROW(vmSymbols::java_lang_InterruptedException());
1669 return;
1670 }
1671
1672 current->set_current_waiting_monitor(this);
1673
1674 ContinuationEntry* ce = current->last_continuation();
1675 if (interruptible && ce != nullptr && ce->is_virtual_thread()) {
1676 int result = Continuation::try_preempt(current, ce->cont_oop(current), Continuation::freeze_on_wait);
1677 if (result == freeze_ok) {
1678 VThreadWait(current, millis);
1679 current->set_current_waiting_monitor(nullptr);
1680 return;
1681 }
1682 if (result == freeze_pinned_native || result == freeze_unsupported) {
1683 const Klass* monitor_klass = object()->klass();
1684 if (!is_excluded(monitor_klass)) {
1685 if (result == freeze_pinned_native) {
1686 post_virtual_thread_pinned_event(current,"Native frame or <clinit> on stack");
1687 } else if (result == freeze_unsupported) {
1688 post_virtual_thread_pinned_event(current, "Native frame or <clinit> or monitors on stack");
1689 }
1690 }
1691 }
1692 }
1693
1694 // create a node to be put into the queue
1695 // Critically, after we reset() the event but prior to park(), we must check
1696 // for a pending interrupt.
1697 ObjectWaiter node(current);
1698 node.TState = ObjectWaiter::TS_WAIT;
1699 current->_ParkEvent->reset();
1700 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1701
1702 // Enter the waiting queue, which is a circular doubly linked list in this case
1703 // but it could be a priority queue or any data structure.
1704 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1705 // by the owner of the monitor *except* in the case where park()
1706 // returns because of a timeout of interrupt. Contention is exceptionally rare
1707 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1708
1709 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1710 AddWaiter(&node);
1711 Thread::SpinRelease(&_WaitSetLock);
1712
1713 intx save = _recursions; // record the old recursion count
1714 _waiters++; // increment the number of waiters
1715 _recursions = 0; // set the recursion level to be 1
1716 exit(current); // exit the monitor
1717 guarantee(owner_raw() != owner_for(current), "invariant");
1718
1719 // The thread is on the WaitSet list - now park() it.
1720 // On MP systems it's conceivable that a brief spin before we park
1721 // could be profitable.
1722 //
1723 // TODO-FIXME: change the following logic to a loop of the form
1724 // while (!timeout && !interrupted && _notified == 0) park()
1725
1726 int ret = OS_OK;
1727 int WasNotified = 0;
1728
1729 // Need to check interrupt state whilst still _thread_in_vm
1730 bool interrupted = interruptible && current->is_interrupted(false);
1731
1732 { // State transition wrappers
1733 OSThread* osthread = current->osthread();
1734 OSThreadWaitState osts(osthread, true);
1735
1736 assert(current->thread_state() == _thread_in_vm, "invariant");
1737
1738 {
1739 ClearSuccOnSuspend csos(this);
1740 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */);
1741 if (interrupted || HAS_PENDING_EXCEPTION) {
1742 // Intentionally empty
1743 } else if (!node._notified) {
1744 if (millis <= 0) {
1745 current->_ParkEvent->park();
1746 } else {
1747 ret = current->_ParkEvent->park(millis);
1748 }
1749 }
1750 }
1751
1752 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1753 // from the WaitSet to the EntryList.
1754 // See if we need to remove Node from the WaitSet.
1755 // We use double-checked locking to avoid grabbing _WaitSetLock
1756 // if the thread is not on the wait queue.
1757 //
1758 // Note that we don't need a fence before the fetch of TState.
1759 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1760 // written by the is thread. (perhaps the fetch might even be satisfied
1761 // by a look-aside into the processor's own store buffer, although given
1762 // the length of the code path between the prior ST and this load that's
1763 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1764 // then we'll acquire the lock and then re-fetch a fresh TState value.
1765 // That is, we fail toward safety.
1766
1767 if (node.TState == ObjectWaiter::TS_WAIT) {
1768 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1769 if (node.TState == ObjectWaiter::TS_WAIT) {
1770 DequeueSpecificWaiter(&node); // unlink from WaitSet
1771 assert(!node._notified, "invariant");
1772 node.TState = ObjectWaiter::TS_RUN;
1773 }
1774 Thread::SpinRelease(&_WaitSetLock);
1775 }
1776
1777 // The thread is now either on off-list (TS_RUN),
1778 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1779 // The Node's TState variable is stable from the perspective of this thread.
1780 // No other threads will asynchronously modify TState.
1781 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1782 OrderAccess::loadload();
1783 if (_succ == current) _succ = nullptr;
1784 WasNotified = node._notified;
1785
1786 // Reentry phase -- reacquire the monitor.
1787 // re-enter contended monitor after object.wait().
1788 // retain OBJECT_WAIT state until re-enter successfully completes
1789 // Thread state is thread_in_vm and oop access is again safe,
1790 // although the raw address of the object may have changed.
1791 // (Don't cache naked oops over safepoints, of course).
1792
1793 // post monitor waited event. Note that this is past-tense, we are done waiting.
1794 if (JvmtiExport::should_post_monitor_waited()) {
1795 JvmtiExport::post_monitor_waited(current, this, ret == OS_TIMEOUT);
1796
1797 if (node._notified && _succ == current) {
1798 // In this part of the monitor wait-notify-reenter protocol it
1799 // is possible (and normal) for another thread to do a fastpath
1800 // monitor enter-exit while this thread is still trying to get
1801 // to the reenter portion of the protocol.
1802 //
1803 // The ObjectMonitor was notified and the current thread is
1804 // the successor which also means that an unpark() has already
1805 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1806 // consume the unpark() that was done when the successor was
1807 // set because the same ParkEvent is shared between Java
1808 // monitors and JVM/TI RawMonitors (for now).
1809 //
1810 // We redo the unpark() to ensure forward progress, i.e., we
1811 // don't want all pending threads hanging (parked) with none
1812 // entering the unlocked monitor.
1813 current->_ParkEvent->unpark();
1814 }
1815 }
1816
1817 if (event.should_commit()) {
1818 post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1819 }
1820
1821 OrderAccess::fence();
1822
1823 assert(owner_raw() != owner_for(current), "invariant");
1824 ObjectWaiter::TStates v = node.TState;
1825 if (v == ObjectWaiter::TS_RUN) {
1826 enter(current);
1827 } else {
1828 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1829 ReenterI(current, &node);
1830 node.wait_reenter_end(this);
1831 }
1832
1833 // current has reacquired the lock.
1834 // Lifecycle - the node representing current must not appear on any queues.
1835 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1836 // want residual elements associated with this thread left on any lists.
1837 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1838 assert(owner_raw() == owner_for(current), "invariant");
1839 assert(_succ != current, "invariant");
1840 } // OSThreadWaitState()
1841
1842 current->set_current_waiting_monitor(nullptr);
1843
1844 guarantee(_recursions == 0, "invariant");
1845 int relock_count = JvmtiDeferredUpdates::get_and_reset_relock_count_after_wait(current);
1846 _recursions = save // restore the old recursion count
1847 + relock_count; // increased by the deferred relock count
1848 current->inc_held_monitor_count(relock_count); // Deopt never entered these counts.
1849 _waiters--; // decrement the number of waiters
1850
1851 // Verify a few postconditions
1852 assert(owner_raw() == owner_for(current), "invariant");
1853 assert(_succ != current, "invariant");
1854 assert_mark_word_consistency();
1855
1856 // check if the notification happened
1857 if (!WasNotified) {
1858 // no, it could be timeout or Thread.interrupt() or both
1859 // check for interrupt event, otherwise it is timeout
1860 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1861 THROW(vmSymbols::java_lang_InterruptedException());
1862 }
1863 }
1864
1865 // NOTE: Spurious wake up will be consider as timeout.
1866 // Monitor notify has precedence over thread interrupt.
1867 }
1868
1869 // Consider:
1870 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1871 // then instead of transferring a thread from the WaitSet to the EntryList
1872 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1873
1874 void ObjectMonitor::INotify(JavaThread* current) {
1875 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1876 ObjectWaiter* iterator = DequeueWaiter();
1877 if (iterator != nullptr) {
1878 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1879 guarantee(!iterator->_notified, "invariant");
1880 // Disposition - what might we do with iterator ?
1881 // a. add it directly to the EntryList - either tail (policy == 1)
1882 // or head (policy == 0).
1883 // b. push it onto the front of the _cxq (policy == 2).
1884 // For now we use (b).
1885
1886 if (iterator->is_vthread()) {
1887 oop vthread = iterator->vthread();
1888 java_lang_VirtualThread::set_notified(vthread, true);
1889 int old_state = java_lang_VirtualThread::state(vthread);
1890 // If state is not WAIT/TIMED_WAIT then target could still be on
1891 // unmount transition, or wait could have already timed-out or target
1892 // could have been interrupted. In the first case, the target itself
1893 // will set the state to BLOCKED at the end of the unmount transition.
1894 // In the other cases the target would have been already unblocked so
1895 // there is nothing to do.
1896 if (old_state == java_lang_VirtualThread::WAIT ||
1897 old_state == java_lang_VirtualThread::TIMED_WAIT) {
1898 java_lang_VirtualThread::cmpxchg_state(vthread, old_state, java_lang_VirtualThread::BLOCKED);
1899 }
1900 }
1901
1902 iterator->TState = ObjectWaiter::TS_ENTER;
1903
1904 iterator->_notified = true;
1905 iterator->_notifier_tid = JFR_THREAD_ID(current);
1906
1907 ObjectWaiter* list = _EntryList;
1908 if (list != nullptr) {
1909 assert(list->_prev == nullptr, "invariant");
1910 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1911 assert(list != iterator, "invariant");
1912 }
1913
1914 // prepend to cxq
1915 if (list == nullptr) {
1916 iterator->_next = iterator->_prev = nullptr;
1917 _EntryList = iterator;
1918 } else {
1919 iterator->TState = ObjectWaiter::TS_CXQ;
1920 for (;;) {
1921 ObjectWaiter* front = _cxq;
1922 iterator->_next = front;
1923 if (Atomic::cmpxchg(&_cxq, front, iterator) == front) {
1924 break;
1925 }
1926 }
1927 }
1928
1929 // _WaitSetLock protects the wait queue, not the EntryList. We could
1930 // move the add-to-EntryList operation, above, outside the critical section
1931 // protected by _WaitSetLock. In practice that's not useful. With the
1932 // exception of wait() timeouts and interrupts the monitor owner
1933 // is the only thread that grabs _WaitSetLock. There's almost no contention
1934 // on _WaitSetLock so it's not profitable to reduce the length of the
1935 // critical section.
1936 if (!iterator->is_vthread()) {
1937 iterator->wait_reenter_begin(this);
1938 }
1939 }
1940 Thread::SpinRelease(&_WaitSetLock);
1941 }
1942
1943 // Consider: a not-uncommon synchronization bug is to use notify() when
1944 // notifyAll() is more appropriate, potentially resulting in stranded
1945 // threads; this is one example of a lost wakeup. A useful diagnostic
1946 // option is to force all notify() operations to behave as notifyAll().
1947 //
1948 // Note: We can also detect many such problems with a "minimum wait".
1949 // When the "minimum wait" is set to a small non-zero timeout value
1950 // and the program does not hang whereas it did absent "minimum wait",
1951 // that suggests a lost wakeup bug.
1952
1953 void ObjectMonitor::notify(TRAPS) {
1954 JavaThread* current = THREAD;
1955 CHECK_OWNER(); // Throws IMSE if not owner.
1956 if (_WaitSet == nullptr) {
1957 return;
1958 }
1959 DTRACE_MONITOR_PROBE(notify, this, object(), current);
1960 INotify(current);
1961 OM_PERFDATA_OP(Notifications, inc(1));
1962 }
1963
1964
1965 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1966 // from the waitset to the EntryList. This could be done more efficiently with a
1967 // single bulk transfer but in practice it's not time-critical. Beware too,
1968 // that in prepend-mode we invert the order of the waiters. Let's say that the
1969 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1970 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1971
1972 void ObjectMonitor::notifyAll(TRAPS) {
1973 JavaThread* current = THREAD;
1974 CHECK_OWNER(); // Throws IMSE if not owner.
1975 if (_WaitSet == nullptr) {
1976 return;
1977 }
1978
1979 DTRACE_MONITOR_PROBE(notifyAll, this, object(), current);
1980 int tally = 0;
1981 while (_WaitSet != nullptr) {
1982 tally++;
1983 INotify(current);
1984 }
1985
1986 OM_PERFDATA_OP(Notifications, inc(tally));
1987 }
1988
1989 void ObjectMonitor::VThreadWait(JavaThread* current, jlong millis) {
1990 oop vthread = current->vthread();
1991 ObjectWaiter* node = new ObjectWaiter(vthread, this);
1992 node->_is_wait = true;
1993 node->TState = ObjectWaiter::TS_WAIT;
1994 java_lang_VirtualThread::set_notified(vthread, false); // Reset notified flag
1995
1996 // Enter the waiting queue, which is a circular doubly linked list in this case
1997 // but it could be a priority queue or any data structure.
1998 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1999 // by the owner of the monitor *except* in the case where park()
2000 // returns because of a timeout or interrupt. Contention is exceptionally rare
2001 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
2002
2003 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
2004 AddWaiter(node);
2005 Thread::SpinRelease(&_WaitSetLock);
2006
2007 node->_recursions = _recursions; // record the old recursion count
2008 _recursions = 0; // set the recursion level to be 1
2009 _waiters++; // increment the number of waiters
2010 exit(current); // exit the monitor
2011 guarantee(owner_raw() != owner_for(current), "invariant");
2012
2013 assert(java_lang_VirtualThread::state(vthread) == java_lang_VirtualThread::RUNNING, "wrong state for vthread");
2014 java_lang_VirtualThread::set_state(vthread, millis == 0 ? java_lang_VirtualThread::WAITING : java_lang_VirtualThread::TIMED_WAITING);
2015 java_lang_VirtualThread::set_waitTimeout(vthread, millis);
2016
2017 // Save the ObjectWaiter* in the chunk since we will need it
2018 // when resuming execution.
2019 oop cont = java_lang_VirtualThread::continuation(vthread);
2020 stackChunkOop chunk = jdk_internal_vm_Continuation::tail(cont);
2021 chunk->set_object_waiter(node);
2022 }
2023
2024 bool ObjectMonitor::VThreadWaitReenter(JavaThread* current, ObjectWaiter* node, ContinuationWrapper& cont) {
2025 // First time we run after being preempted on Object.wait().
2026 // We need to check if we were interrupted or wait() timed-out
2027 // and in that case remove ourselves from the _WaitSet queue.
2028 if (node->TState == ObjectWaiter::TS_WAIT) {
2029 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
2030 if (node->TState == ObjectWaiter::TS_WAIT) {
2031 DequeueSpecificWaiter(node); // unlink from WaitSet
2032 assert(!node->_notified, "invariant");
2033 node->TState = ObjectWaiter::TS_RUN;
2034 }
2035 Thread::SpinRelease(&_WaitSetLock);
2036 }
2037
2038 ObjectWaiter::TStates state = node->TState;
2039 bool was_notified = state == ObjectWaiter::TS_ENTER || state == ObjectWaiter::TS_CXQ;
2040 assert(was_notified || state == ObjectWaiter::TS_RUN, "");
2041
2042 // save it so that once we re-acquire the monitor we know if we need to throw IE.
2043 node->_interrupted = !was_notified && current->is_interrupted(false);
2044
2045 EventJavaMonitorWait event;
2046 if (event.should_commit() || JvmtiExport::should_post_monitor_waited()) {
2047 vthread_monitor_waited_event(current, node, cont, &event, !was_notified && !node->_interrupted);
2048 }
2049
2050 node->_at_reenter = true;
2051 assert(owner_raw() != owner_for(current), "invariant");
2052
2053 if (!was_notified) {
2054 bool acquired = VThreadMonitorEnter(current, node);
2055 if (acquired) {
2056 guarantee(_recursions == 0, "invariant");
2057 _recursions = node->_recursions; // restore the old recursion count
2058 _waiters--; // decrement the number of waiters
2059
2060 if (node->_interrupted) {
2061 // We will throw at thaw end after finishing the mount transition.
2062 current->set_pending_interrupted_exception(true);
2063 }
2064
2065 delete node;
2066 stackChunkOop chunk = cont.tail();
2067 chunk->set_object_waiter(nullptr);
2068 return true;
2069 }
2070 } else {
2071 // Already moved to _cxq or _EntryList by notifier, so just add to contentions.
2072 add_to_contentions(1);
2073 }
2074 return false;
2075 }
2076
2077 // -----------------------------------------------------------------------------
2078 // Adaptive Spinning Support
2079 //
2080 // Adaptive spin-then-block - rational spinning
2081 //
2082 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
2083 // algorithm. On high order SMP systems it would be better to start with
2084 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
2085 // a contending thread could enqueue itself on the cxq and then spin locally
2086 // on a thread-specific variable such as its ParkEvent._Event flag.
2087 // That's left as an exercise for the reader. Note that global spinning is
2088 // not problematic on Niagara, as the L2 cache serves the interconnect and
2089 // has both low latency and massive bandwidth.
2090 //
2091 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
2092 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
2093 // (duration) or we can fix the count at approximately the duration of
2094 // a context switch and vary the frequency. Of course we could also
2095 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
2096 // For a description of 'Adaptive spin-then-block mutual exclusion in
2097 // multi-threaded processing,' see U.S. Pat. No. 8046758.
2098 //
2099 // This implementation varies the duration "D", where D varies with
2100 // the success rate of recent spin attempts. (D is capped at approximately
2101 // length of a round-trip context switch). The success rate for recent
2102 // spin attempts is a good predictor of the success rate of future spin
2103 // attempts. The mechanism adapts automatically to varying critical
2104 // section length (lock modality), system load and degree of parallelism.
2105 // D is maintained per-monitor in _SpinDuration and is initialized
2106 // optimistically. Spin frequency is fixed at 100%.
2107 //
2108 // Note that _SpinDuration is volatile, but we update it without locks
2109 // or atomics. The code is designed so that _SpinDuration stays within
2110 // a reasonable range even in the presence of races. The arithmetic
2111 // operations on _SpinDuration are closed over the domain of legal values,
2112 // so at worst a race will install and older but still legal value.
2113 // At the very worst this introduces some apparent non-determinism.
2114 // We might spin when we shouldn't or vice-versa, but since the spin
2115 // count are relatively short, even in the worst case, the effect is harmless.
2116 //
2117 // Care must be taken that a low "D" value does not become an
2118 // an absorbing state. Transient spinning failures -- when spinning
2119 // is overall profitable -- should not cause the system to converge
2120 // on low "D" values. We want spinning to be stable and predictable
2121 // and fairly responsive to change and at the same time we don't want
2122 // it to oscillate, become metastable, be "too" non-deterministic,
2123 // or converge on or enter undesirable stable absorbing states.
2124 //
2125 // We implement a feedback-based control system -- using past behavior
2126 // to predict future behavior. We face two issues: (a) if the
2127 // input signal is random then the spin predictor won't provide optimal
2128 // results, and (b) if the signal frequency is too high then the control
2129 // system, which has some natural response lag, will "chase" the signal.
2130 // (b) can arise from multimodal lock hold times. Transient preemption
2131 // can also result in apparent bimodal lock hold times.
2132 // Although sub-optimal, neither condition is particularly harmful, as
2133 // in the worst-case we'll spin when we shouldn't or vice-versa.
2134 // The maximum spin duration is rather short so the failure modes aren't bad.
2135 // To be conservative, I've tuned the gain in system to bias toward
2136 // _not spinning. Relatedly, the system can sometimes enter a mode where it
2137 // "rings" or oscillates between spinning and not spinning. This happens
2138 // when spinning is just on the cusp of profitability, however, so the
2139 // situation is not dire. The state is benign -- there's no need to add
2140 // hysteresis control to damp the transition rate between spinning and
2141 // not spinning.
2142
2143 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool
2144
2145 static int Knob_Bonus = 100; // spin success bonus
2146 static int Knob_Penalty = 200; // spin failure penalty
2147 static int Knob_Poverty = 1000;
2148 static int Knob_FixedSpin = 0;
2149 static int Knob_PreSpin = 10; // 20-100 likely better, but it's not better in my testing.
2150
2151 inline static int adjust_up(int spin_duration) {
2152 int x = spin_duration;
2153 if (x < ObjectMonitor::Knob_SpinLimit) {
2154 if (x < Knob_Poverty) {
2155 x = Knob_Poverty;
2156 }
2157 return x + Knob_Bonus;
2158 } else {
2159 return spin_duration;
2160 }
2161 }
2162
2163 inline static int adjust_down(int spin_duration) {
2164 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2165 // AIMD is globally stable.
2166 int x = spin_duration;
2167 if (x > 0) {
2168 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2169 // This is globally sample and tends to damp the response.
2170 x -= Knob_Penalty;
2171 if (x < 0) { x = 0; }
2172 return x;
2173 } else {
2174 return spin_duration;
2175 }
2176 }
2177
2178 bool ObjectMonitor::short_fixed_spin(JavaThread* current, int spin_count, bool adapt) {
2179 for (int ctr = 0; ctr < spin_count; ctr++) {
2180 TryLockResult status = TryLock(current);
2181 if (status == TryLockResult::Success) {
2182 if (adapt) {
2183 _SpinDuration = adjust_up(_SpinDuration);
2184 }
2185 return true;
2186 } else if (status == TryLockResult::Interference) {
2187 break;
2188 }
2189 SpinPause();
2190 }
2191 return false;
2192 }
2193
2194 // Spinning: Fixed frequency (100%), vary duration
2195 bool ObjectMonitor::TrySpin(JavaThread* current) {
2196
2197 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
2198 int knob_fixed_spin = Knob_FixedSpin; // 0 (don't spin: default), 2000 good test
2199 if (knob_fixed_spin > 0) {
2200 return short_fixed_spin(current, knob_fixed_spin, false);
2201 }
2202
2203 // Admission control - verify preconditions for spinning
2204 //
2205 // We always spin a little bit, just to prevent _SpinDuration == 0 from
2206 // becoming an absorbing state. Put another way, we spin briefly to
2207 // sample, just in case the system load, parallelism, contention, or lock
2208 // modality changed.
2209
2210 int knob_pre_spin = Knob_PreSpin; // 10 (default), 100, 1000 or 2000
2211 if (short_fixed_spin(current, knob_pre_spin, true)) {
2212 return true;
2213 }
2214
2215 //
2216 // Consider the following alternative:
2217 // Periodically set _SpinDuration = _SpinLimit and try a long/full
2218 // spin attempt. "Periodically" might mean after a tally of
2219 // the # of failed spin attempts (or iterations) reaches some threshold.
2220 // This takes us into the realm of 1-out-of-N spinning, where we
2221 // hold the duration constant but vary the frequency.
2222
2223 int ctr = _SpinDuration;
2224 if (ctr <= 0) return false;
2225
2226 // We're good to spin ... spin ingress.
2227 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
2228 // when preparing to LD...CAS _owner, etc and the CAS is likely
2229 // to succeed.
2230 if (_succ == nullptr) {
2231 _succ = current;
2232 }
2233 void* prv = nullptr;
2234
2235 // There are three ways to exit the following loop:
2236 // 1. A successful spin where this thread has acquired the lock.
2237 // 2. Spin failure with prejudice
2238 // 3. Spin failure without prejudice
2239
2240 while (--ctr >= 0) {
2241
2242 // Periodic polling -- Check for pending GC
2243 // Threads may spin while they're unsafe.
2244 // We don't want spinning threads to delay the JVM from reaching
2245 // a stop-the-world safepoint or to steal cycles from GC.
2246 // If we detect a pending safepoint we abort in order that
2247 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
2248 // this thread, if safe, doesn't steal cycles from GC.
2249 // This is in keeping with the "no loitering in runtime" rule.
2250 // We periodically check to see if there's a safepoint pending.
2251 if ((ctr & 0xFF) == 0) {
2252 // Can't call SafepointMechanism::should_process() since that
2253 // might update the poll values and we could be in a thread_blocked
2254 // state here which is not allowed so just check the poll.
2255 if (SafepointMechanism::local_poll_armed(current)) {
2256 break;
2257 }
2258 SpinPause();
2259 }
2260
2261 // Probe _owner with TATAS
2262 // If this thread observes the monitor transition or flicker
2263 // from locked to unlocked to locked, then the odds that this
2264 // thread will acquire the lock in this spin attempt go down
2265 // considerably. The same argument applies if the CAS fails
2266 // or if we observe _owner change from one non-null value to
2267 // another non-null value. In such cases we might abort
2268 // the spin without prejudice or apply a "penalty" to the
2269 // spin count-down variable "ctr", reducing it by 100, say.
2270
2271 void* ox = owner_raw();
2272 if (ox == nullptr) {
2273 ox = try_set_owner_from(nullptr, current);
2274 if (ox == nullptr) {
2275 // The CAS succeeded -- this thread acquired ownership
2276 // Take care of some bookkeeping to exit spin state.
2277 if (_succ == current) {
2278 _succ = nullptr;
2279 }
2280
2281 // Increase _SpinDuration :
2282 // The spin was successful (profitable) so we tend toward
2283 // longer spin attempts in the future.
2284 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2285 // If we acquired the lock early in the spin cycle it
2286 // makes sense to increase _SpinDuration proportionally.
2287 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2288 _SpinDuration = adjust_up(_SpinDuration);
2289 return true;
2290 }
2291
2292 // The CAS failed ... we can take any of the following actions:
2293 // * penalize: ctr -= CASPenalty
2294 // * exit spin with prejudice -- abort without adapting spinner
2295 // * exit spin without prejudice.
2296 // * Since CAS is high-latency, retry again immediately.
2297 break;
2298 }
2299
2300 // Did lock ownership change hands ?
2301 if (ox != prv && prv != nullptr) {
2302 break;
2303 }
2304 prv = ox;
2305
2306 if (_succ == nullptr) {
2307 _succ = current;
2308 }
2309 }
2310
2311 // Spin failed with prejudice -- reduce _SpinDuration.
2312 if (ctr < 0) {
2313 _SpinDuration = adjust_down(_SpinDuration);
2314 }
2315
2316 if (_succ == current) {
2317 _succ = nullptr;
2318 // Invariant: after setting succ=null a contending thread
2319 // must recheck-retry _owner before parking. This usually happens
2320 // in the normal usage of TrySpin(), but it's safest
2321 // to make TrySpin() as foolproof as possible.
2322 OrderAccess::fence();
2323 if (TryLock(current) == TryLockResult::Success) {
2324 return true;
2325 }
2326 }
2327
2328 return false;
2329 }
2330
2331
2332 // -----------------------------------------------------------------------------
2333 // WaitSet management ...
2334
2335 ObjectWaiter::ObjectWaiter(JavaThread* current) {
2336 _next = nullptr;
2337 _prev = nullptr;
2338 _thread = current;
2339 _monitor = nullptr;
2340 _notifier_tid = 0;
2341 _recursions = 0;
2342 TState = TS_RUN;
2343 _notified = false;
2344 _is_wait = false;
2345 _at_reenter = false;
2346 _interrupted = false;
2347 _active = false;
2348 }
2349
2350 ObjectWaiter::ObjectWaiter(oop vthread, ObjectMonitor* mon) : ObjectWaiter(nullptr) {
2351 assert(oopDesc::is_oop(vthread), "");
2352 _vthread = OopHandle(JavaThread::thread_oop_storage(), vthread);
2353 _monitor = mon;
2354 }
2355
2356 ObjectWaiter::~ObjectWaiter() {
2357 if (is_vthread()) {
2358 assert(vthread() != nullptr, "");
2359 _vthread.release(JavaThread::thread_oop_storage());
2360 }
2361 }
2362
2363 oop ObjectWaiter::vthread() {
2364 return _vthread.resolve();
2365 }
2366
2367 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2368 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(_thread, mon);
2369 }
2370
2371 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2372 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(_thread, _active);
2373 }
2374
2375 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2376 assert(node != nullptr, "should not add null node");
2377 assert(node->_prev == nullptr, "node already in list");
2378 assert(node->_next == nullptr, "node already in list");
2379 // put node at end of queue (circular doubly linked list)
2380 if (_WaitSet == nullptr) {
2381 _WaitSet = node;
2382 node->_prev = node;
2383 node->_next = node;
2384 } else {
2385 ObjectWaiter* head = _WaitSet;
2386 ObjectWaiter* tail = head->_prev;
2387 assert(tail->_next == head, "invariant check");
2388 tail->_next = node;
2389 head->_prev = node;
2390 node->_next = head;
2391 node->_prev = tail;
2392 }
2393 }
2394
2395 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2396 // dequeue the very first waiter
2397 ObjectWaiter* waiter = _WaitSet;
2398 if (waiter) {
2399 DequeueSpecificWaiter(waiter);
2400 }
2401 return waiter;
2402 }
2403
2404 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2405 assert(node != nullptr, "should not dequeue nullptr node");
2406 assert(node->_prev != nullptr, "node already removed from list");
2407 assert(node->_next != nullptr, "node already removed from list");
2408 // when the waiter has woken up because of interrupt,
2409 // timeout or other spurious wake-up, dequeue the
2410 // waiter from waiting list
2411 ObjectWaiter* next = node->_next;
2412 if (next == node) {
2413 assert(node->_prev == node, "invariant check");
2414 _WaitSet = nullptr;
2415 } else {
2416 ObjectWaiter* prev = node->_prev;
2417 assert(prev->_next == node, "invariant check");
2418 assert(next->_prev == node, "invariant check");
2419 next->_prev = prev;
2420 prev->_next = next;
2421 if (_WaitSet == node) {
2422 _WaitSet = next;
2423 }
2424 }
2425 node->_next = nullptr;
2426 node->_prev = nullptr;
2427 }
2428
2429 // -----------------------------------------------------------------------------
2430 // PerfData support
2431 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = nullptr;
2432 PerfCounter * ObjectMonitor::_sync_FutileWakeups = nullptr;
2433 PerfCounter * ObjectMonitor::_sync_Parks = nullptr;
2434 PerfCounter * ObjectMonitor::_sync_Notifications = nullptr;
2435 PerfCounter * ObjectMonitor::_sync_Inflations = nullptr;
2436 PerfCounter * ObjectMonitor::_sync_Deflations = nullptr;
2437 PerfLongVariable * ObjectMonitor::_sync_MonExtant = nullptr;
2438
2439 // One-shot global initialization for the sync subsystem.
2440 // We could also defer initialization and initialize on-demand
2441 // the first time we call ObjectSynchronizer::inflate().
2442 // Initialization would be protected - like so many things - by
2443 // the MonitorCache_lock.
2444
2445 void ObjectMonitor::Initialize() {
2446 assert(!InitDone, "invariant");
2447
2448 if (!os::is_MP()) {
2449 Knob_SpinLimit = 0;
2450 Knob_PreSpin = 0;
2451 Knob_FixedSpin = -1;
2452 }
2453
2454 if (UsePerfData) {
2455 EXCEPTION_MARK;
2456 #define NEWPERFCOUNTER(n) \
2457 { \
2458 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \
2459 CHECK); \
2460 }
2461 #define NEWPERFVARIABLE(n) \
2462 { \
2463 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \
2464 CHECK); \
2465 }
2466 NEWPERFCOUNTER(_sync_Inflations);
2467 NEWPERFCOUNTER(_sync_Deflations);
2468 NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2469 NEWPERFCOUNTER(_sync_FutileWakeups);
2470 NEWPERFCOUNTER(_sync_Parks);
2471 NEWPERFCOUNTER(_sync_Notifications);
2472 NEWPERFVARIABLE(_sync_MonExtant);
2473 #undef NEWPERFCOUNTER
2474 #undef NEWPERFVARIABLE
2475 }
2476
2477 _oop_storage = OopStorageSet::create_weak("ObjectSynchronizer Weak", mtSynchronizer);
2478
2479 DEBUG_ONLY(InitDone = true;)
2480 }
2481
2482 void ObjectMonitor::Initialize2() {
2483 _vthread_cxq_head = OopHandle(JavaThread::thread_oop_storage(), nullptr);
2484 _vthread_unparker_ParkEvent = ParkEvent::Allocate(nullptr);
2485 }
2486
2487 void ObjectMonitor::print_on(outputStream* st) const {
2488 // The minimal things to print for markWord printing, more can be added for debugging and logging.
2489 st->print("{contentions=0x%08x,waiters=0x%08x"
2490 ",recursions=" INTX_FORMAT ",owner=" INTPTR_FORMAT "}",
2491 contentions(), waiters(), recursions(),
2492 p2i(owner()));
2493 }
2494 void ObjectMonitor::print() const { print_on(tty); }
2495
2496 #ifdef ASSERT
2497 // Print the ObjectMonitor like a debugger would:
2498 //
2499 // (ObjectMonitor) 0x00007fdfb6012e40 = {
2500 // _metadata = 0x0000000000000001
2501 // _object = 0x000000070ff45fd0
2502 // _pad_buf0 = {
2503 // [0] = '\0'
2504 // ...
2505 // [43] = '\0'
2506 // }
2507 // _owner = 0x0000000000000000
2508 // _previous_owner_tid = 0
2509 // _pad_buf1 = {
2510 // [0] = '\0'
2511 // ...
2512 // [47] = '\0'
2513 // }
2514 // _next_om = 0x0000000000000000
2515 // _recursions = 0
2516 // _EntryList = 0x0000000000000000
2517 // _cxq = 0x0000000000000000
2518 // _succ = 0x0000000000000000
2519 // _SpinDuration = 5000
2520 // _contentions = 0
2521 // _WaitSet = 0x0000700009756248
2522 // _waiters = 1
2523 // _WaitSetLock = 0
2524 // }
2525 //
2526 void ObjectMonitor::print_debug_style_on(outputStream* st) const {
2527 st->print_cr("(ObjectMonitor*) " INTPTR_FORMAT " = {", p2i(this));
2528 st->print_cr(" _metadata = " INTPTR_FORMAT, _metadata);
2529 st->print_cr(" _object = " INTPTR_FORMAT, p2i(object_peek()));
2530 st->print_cr(" _pad_buf0 = {");
2531 st->print_cr(" [0] = '\\0'");
2532 st->print_cr(" ...");
2533 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf0) - 1);
2534 st->print_cr(" }");
2535 st->print_cr(" _owner = " INTPTR_FORMAT, p2i(owner_raw()));
2536 st->print_cr(" _previous_owner_tid = " UINT64_FORMAT, _previous_owner_tid);
2537 st->print_cr(" _pad_buf1 = {");
2538 st->print_cr(" [0] = '\\0'");
2539 st->print_cr(" ...");
2540 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf1) - 1);
2541 st->print_cr(" }");
2542 st->print_cr(" _next_om = " INTPTR_FORMAT, p2i(next_om()));
2543 st->print_cr(" _recursions = " INTX_FORMAT, _recursions);
2544 st->print_cr(" _EntryList = " INTPTR_FORMAT, p2i(_EntryList));
2545 st->print_cr(" _cxq = " INTPTR_FORMAT, p2i(_cxq));
2546 st->print_cr(" _succ = " INTPTR_FORMAT, p2i(_succ));
2547 st->print_cr(" _SpinDuration = %d", _SpinDuration);
2548 st->print_cr(" _contentions = %d", contentions());
2549 st->print_cr(" _WaitSet = " INTPTR_FORMAT, p2i(_WaitSet));
2550 st->print_cr(" _waiters = %d", _waiters);
2551 st->print_cr(" _WaitSetLock = %d", _WaitSetLock);
2552 st->print_cr("}");
2553 }
2554 #endif