1 /*
2 * Copyright (c) 1998, 2025, 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 "classfile/vmSymbols.hpp"
26 #include "gc/shared/oopStorage.hpp"
27 #include "gc/shared/oopStorageSet.hpp"
28 #include "jfr/jfrEvents.hpp"
29 #include "jfr/support/jfrThreadId.hpp"
30 #include "logging/log.hpp"
31 #include "logging/logStream.hpp"
32 #include "memory/allocation.inline.hpp"
33 #include "memory/resourceArea.hpp"
34 #include "oops/markWord.hpp"
35 #include "oops/oop.inline.hpp"
36 #include "oops/oopHandle.inline.hpp"
37 #include "oops/weakHandle.inline.hpp"
38 #include "prims/jvmtiDeferredUpdates.hpp"
39 #include "prims/jvmtiExport.hpp"
40 #include "runtime/atomic.hpp"
41 #include "runtime/continuationWrapper.inline.hpp"
42 #include "runtime/globals.hpp"
43 #include "runtime/handles.inline.hpp"
44 #include "runtime/interfaceSupport.inline.hpp"
45 #include "runtime/javaThread.inline.hpp"
46 #include "runtime/lightweightSynchronizer.hpp"
47 #include "runtime/mutexLocker.hpp"
48 #include "runtime/objectMonitor.hpp"
49 #include "runtime/objectMonitor.inline.hpp"
50 #include "runtime/orderAccess.hpp"
51 #include "runtime/osThread.hpp"
52 #include "runtime/safefetch.hpp"
53 #include "runtime/safepointMechanism.inline.hpp"
54 #include "runtime/sharedRuntime.hpp"
55 #include "runtime/threads.hpp"
56 #include "services/threadService.hpp"
57 #include "utilities/debug.hpp"
58 #include "utilities/dtrace.hpp"
59 #include "utilities/globalCounter.inline.hpp"
60 #include "utilities/globalDefinitions.hpp"
61 #include "utilities/macros.hpp"
62 #include "utilities/preserveException.hpp"
63 #if INCLUDE_JFR
64 #include "jfr/support/jfrFlush.hpp"
65 #endif
66
67 #ifdef DTRACE_ENABLED
68
69 // Only bother with this argument setup if dtrace is available
70 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
71
72
73 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
74 char* bytes = nullptr; \
75 int len = 0; \
76 jlong jtid = SharedRuntime::get_java_tid(thread); \
77 Symbol* klassname = obj->klass()->name(); \
78 if (klassname != nullptr) { \
79 bytes = (char*)klassname->bytes(); \
80 len = klassname->utf8_length(); \
81 }
82
83 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
84 { \
85 if (DTraceMonitorProbes) { \
86 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
87 HOTSPOT_MONITOR_WAIT(jtid, \
88 (monitor), bytes, len, (millis)); \
89 } \
90 }
91
92 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
93 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
94 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
95 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
96 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
97
98 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
99 { \
100 if (DTraceMonitorProbes) { \
101 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
102 HOTSPOT_MONITOR_##probe(jtid, \
103 (uintptr_t)(monitor), bytes, len); \
104 } \
105 }
106
107 #else // ndef DTRACE_ENABLED
108
109 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
110 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
111
112 #endif // ndef DTRACE_ENABLED
113
114 DEBUG_ONLY(static volatile bool InitDone = false;)
115
116 OopStorage* ObjectMonitor::_oop_storage = nullptr;
117
118 OopHandle ObjectMonitor::_vthread_list_head;
119 ParkEvent* ObjectMonitor::_vthread_unparker_ParkEvent = nullptr;
120
121 // -----------------------------------------------------------------------------
122 // Theory of operations -- Monitors lists, thread residency, etc:
123 //
124 // * A thread acquires ownership of a monitor by successfully
125 // CAS()ing the _owner field from NO_OWNER/DEFLATER_MARKER to
126 // its owner_id (return value from owner_id_from()).
127 //
128 // * Invariant: A thread appears on at most one monitor list --
129 // entry_list or wait_set -- at any one time.
130 //
131 // * Contending threads "push" themselves onto the entry_list with CAS
132 // and then spin/park.
133 // If the thread is a virtual thread it will first attempt to
134 // unmount itself. The virtual thread will first try to freeze
135 // all frames in the heap. If the operation fails it will just
136 // follow the regular path for platform threads. If the operation
137 // succeeds, it will push itself onto the entry_list with CAS and then
138 // return back to Java to continue the unmount logic.
139 //
140 // * After a contending thread eventually acquires the lock it must
141 // dequeue itself from the entry_list.
142 //
143 // * The exiting thread identifies and unparks an "heir presumptive"
144 // tentative successor thread on the entry_list. In case the successor
145 // is an unmounted virtual thread, the exiting thread will first try
146 // to add it to the list of vthreads waiting to be unblocked, and on
147 // success it will unpark the special unblocker thread instead, which
148 // will be in charge of submitting the vthread back to the scheduler
149 // queue. Critically, the exiting thread doesn't unlink the successor
150 // thread from the entry_list. After having been unparked/re-scheduled,
151 // the wakee will recontend for ownership of the monitor. The successor
152 // (wakee) will either acquire the lock or re-park/unmount itself.
153 //
154 // Succession is provided for by a policy of competitive handoff.
155 // The exiting thread does _not_ grant or pass ownership to the
156 // successor thread. (This is also referred to as "handoff succession").
157 // Instead the exiting thread releases ownership and possibly wakes
158 // a successor, so the successor can (re)compete for ownership of the lock.
159 //
160 // * The entry_list forms a queue of threads stalled trying to acquire
161 // the lock. Within the entry_list the next pointers always form a
162 // consistent singly linked list. At unlock-time when the unlocking
163 // thread notices that the tail of the entry_list is not known, we
164 // convert the singly linked entry_list into a doubly linked list by
165 // assigning the prev pointers and the entry_list_tail pointer.
166 //
167 // Example:
168 //
169 // The first contending thread that "pushed" itself onto entry_list,
170 // will be the last thread in the list. Each newly pushed thread in
171 // entry_list will be linked through its next pointer, and have its
172 // prev pointer set to null. Thus pushing six threads A-F (in that
173 // order) onto entry_list, will form a singly linked list, see 1)
174 // below.
175 //
176 // 1) entry_list ->F->E->D->C->B->A->null
177 // entry_list_tail ->null
178 //
179 // Since the successor is chosen in FIFO order, the exiting thread
180 // needs to find the tail of the entry_list. This is done by walking
181 // from the entry_list head. While walking the list we also assign
182 // the prev pointers of each thread, essentially forming a doubly
183 // linked list, see 2) below.
184 //
185 // 2) entry_list ->F<=>E<=>D<=>C<=>B<=>A->null
186 // entry_list_tail ----------------------^
187 //
188 // Once we have formed a doubly linked list it's easy to find the
189 // successor (A), wake it up, have it remove itself, and update the
190 // tail pointer, as seen in and 3) below.
191 //
192 // 3) entry_list ->F<=>E<=>D<=>C<=>B->null
193 // entry_list_tail ------------------^
194 //
195 // At any time new threads can add themselves to the entry_list, see
196 // 4) below.
197 //
198 // 4) entry_list ->I->H->G->F<=>E<=>D->null
199 // entry_list_tail -------------------^
200 //
201 // At some point in time the thread (F) that wants to remove itself
202 // from the end of the list, will not have any prev pointer, see 5)
203 // below.
204 //
205 // 5) entry_list ->I->H->G->F->null
206 // entry_list_tail -----------^
207 //
208 // To resolve this we just start walking from the entry_list head
209 // again, forming a new doubly linked list, before removing the
210 // thread (F), see 6) and 7) below.
211 //
212 // 6) entry_list ->I<=>H<=>G<=>F->null
213 // entry_list_tail --------------^
214 //
215 // 7) entry_list ->I<=>H<=>G->null
216 // entry_list_tail ----------^
217 //
218 // * The monitor itself protects all of the operations on the
219 // entry_list except for the CAS of a new arrival to the head. Only
220 // the monitor owner can read or write the prev links (e.g. to
221 // remove itself) or update the tail.
222 //
223 // * The monitor entry list operations avoid locks, but strictly speaking
224 // they're not lock-free. Enter is lock-free, exit is not.
225 // For a description of 'Methods and apparatus providing non-blocking access
226 // to a resource,' see U.S. Pat. No. 7844973.
227 //
228 // * The entry_list can have multiple concurrent "pushers" but only
229 // one concurrent detaching thread. There is no ABA-problem with
230 // this usage of CAS.
231 //
232 // * As long as the entry_list_tail is known the odds are good that we
233 // should be able to dequeue after acquisition (in the ::enter()
234 // epilogue) in constant-time. This is good since a key desideratum
235 // is to minimize queue & monitor metadata manipulation that occurs
236 // while holding the monitor lock -- that is, we want to minimize
237 // monitor lock holds times. Note that even a small amount of fixed
238 // spinning will greatly reduce the # of enqueue-dequeue operations
239 // on entry_list. That is, spinning relieves contention on the
240 // "inner" locks and monitor metadata.
241 //
242 // Insert and delete operations may not operate in constant-time if
243 // we have interference because some other thread is adding or
244 // removing the head element of entry_list or if we need to convert
245 // the singly linked entry_list into a doubly linked list to find the
246 // tail.
247 //
248 // * The monitor synchronization subsystem avoids the use of native
249 // synchronization primitives except for the narrow platform-specific
250 // park-unpark abstraction. See the comments in os_posix.cpp regarding
251 // the semantics of park-unpark. Put another way, this monitor implementation
252 // depends only on atomic operations and park-unpark.
253 //
254 // * Waiting threads reside on the wait_set list -- wait() puts
255 // the caller onto the wait_set.
256 //
257 // * notify() or notifyAll() simply transfers threads from the wait_set
258 // to the entry_list. Subsequent exit() operations will
259 // unpark/re-schedule the notifyee. Unparking/re-scheduling a
260 // notifyee in notify() is inefficient - it's likely the notifyee
261 // would simply impale itself on the lock held by the notifier.
262
263 // Check that object() and set_object() are called from the right context:
264 static void check_object_context() {
265 #ifdef ASSERT
266 Thread* self = Thread::current();
267 if (self->is_Java_thread()) {
268 // Mostly called from JavaThreads so sanity check the thread state.
269 JavaThread* jt = JavaThread::cast(self);
270 switch (jt->thread_state()) {
271 case _thread_in_vm: // the usual case
272 case _thread_in_Java: // during deopt
273 break;
274 default:
275 fatal("called from an unsafe thread state");
276 }
277 assert(jt->is_active_Java_thread(), "must be active JavaThread");
278 } else {
279 // However, ThreadService::get_current_contended_monitor()
280 // can call here via the VMThread so sanity check it.
281 assert(self->is_VM_thread(), "must be");
282 }
283 #endif // ASSERT
284 }
285
286 ObjectMonitor::ObjectMonitor(oop object) :
287 _metadata(0),
288 _object(_oop_storage, object),
289 _owner(NO_OWNER),
290 _previous_owner_tid(0),
291 _next_om(nullptr),
292 _recursions(0),
293 _entry_list(nullptr),
294 _entry_list_tail(nullptr),
295 _succ(NO_OWNER),
296 _SpinDuration(ObjectMonitor::Knob_SpinLimit),
297 _contentions(0),
298 _wait_set(nullptr),
299 _waiters(0),
300 _wait_set_lock(0),
301 _stack_locker(nullptr)
302 { }
303
304 ObjectMonitor::~ObjectMonitor() {
305 _object.release(_oop_storage);
306 }
307
308 oop ObjectMonitor::object() const {
309 check_object_context();
310 return _object.resolve();
311 }
312
313 void ObjectMonitor::ExitOnSuspend::operator()(JavaThread* current) {
314 if (current->is_suspended()) {
315 _om->_recursions = 0;
316 _om->clear_successor();
317 // Don't need a full fence after clearing successor here because of the call to exit().
318 _om->exit(current, false /* not_suspended */);
319 _om_exited = true;
320
321 current->set_current_pending_monitor(_om);
322 }
323 }
324
325 void ObjectMonitor::ClearSuccOnSuspend::operator()(JavaThread* current) {
326 if (current->is_suspended()) {
327 if (_om->has_successor(current)) {
328 _om->clear_successor();
329 OrderAccess::fence(); // always do a full fence when successor is cleared
330 }
331 }
332 }
333
334 #define assert_mark_word_consistency() \
335 assert(UseObjectMonitorTable || object()->mark() == markWord::encode(this), \
336 "object mark must match encoded this: mark=" INTPTR_FORMAT \
337 ", encoded this=" INTPTR_FORMAT, object()->mark().value(), \
338 markWord::encode(this).value());
339
340 // -----------------------------------------------------------------------------
341 // Enter support
342
343 bool ObjectMonitor::enter_is_async_deflating() {
344 if (is_being_async_deflated()) {
345 if (!UseObjectMonitorTable) {
346 const oop l_object = object();
347 if (l_object != nullptr) {
348 // Attempt to restore the header/dmw to the object's header so that
349 // we only retry once if the deflater thread happens to be slow.
350 install_displaced_markword_in_object(l_object);
351 }
352 }
353 return true;
354 }
355
356 return false;
357 }
358
359 bool ObjectMonitor::try_lock_with_contention_mark(JavaThread* locking_thread, ObjectMonitorContentionMark& contention_mark) {
360 assert(contention_mark._monitor == this, "must be");
361 assert(!is_being_async_deflated(), "must be");
362
363 int64_t prev_owner = try_set_owner_from(NO_OWNER, locking_thread);
364 bool success = false;
365
366 if (prev_owner == NO_OWNER) {
367 assert(_recursions == 0, "invariant");
368 success = true;
369 } else if (prev_owner == owner_id_from(locking_thread)) {
370 _recursions++;
371 success = true;
372 } else if (prev_owner == DEFLATER_MARKER) {
373 // Racing with deflation.
374 prev_owner = try_set_owner_from(DEFLATER_MARKER, locking_thread);
375 if (prev_owner == DEFLATER_MARKER) {
376 // We successfully cancelled the in-progress async deflation by
377 // changing owner from DEFLATER_MARKER to current. We now extend
378 // the lifetime of the contention_mark (e.g. contentions++) here
379 // to prevent the deflater thread from winning the last part of
380 // the 2-part async deflation protocol after the regular
381 // decrement occurs when the contention_mark goes out of
382 // scope. ObjectMonitor::deflate_monitor() which is called by
383 // the deflater thread will decrement contentions after it
384 // recognizes that the async deflation was cancelled.
385 contention_mark.extend();
386 success = true;
387 } else if (prev_owner == NO_OWNER) {
388 // At this point we cannot race with deflation as we have both incremented
389 // contentions, seen contention > 0 and seen a DEFLATER_MARKER.
390 // success will only be false if this races with something other than
391 // deflation.
392 prev_owner = try_set_owner_from(NO_OWNER, locking_thread);
393 success = prev_owner == NO_OWNER;
394 }
395 }
396 assert(!success || has_owner(locking_thread), "must be");
397
398 return success;
399 }
400
401 void ObjectMonitor::enter_for_with_contention_mark(JavaThread* locking_thread, ObjectMonitorContentionMark& contention_mark) {
402 // Used by LightweightSynchronizer::inflate_and_enter in deoptimization path to enter for another thread.
403 // The monitor is private to or already owned by locking_thread which must be suspended.
404 // So this code may only contend with deflation.
405 assert(locking_thread == Thread::current() || locking_thread->is_obj_deopt_suspend(), "must be");
406 bool success = try_lock_with_contention_mark(locking_thread, contention_mark);
407
408 assert(success, "Failed to enter_for: locking_thread=" INTPTR_FORMAT
409 ", this=" INTPTR_FORMAT "{owner=" INT64_FORMAT "}",
410 p2i(locking_thread), p2i(this), owner_raw());
411 }
412
413 bool ObjectMonitor::enter_for(JavaThread* locking_thread) {
414 // Used by ObjectSynchronizer::enter_for() to enter for another thread.
415 // The monitor is private to or already owned by locking_thread which must be suspended.
416 // So this code may only contend with deflation.
417 assert(locking_thread == Thread::current() || locking_thread->is_obj_deopt_suspend(), "must be");
418
419 // Block out deflation as soon as possible.
420 ObjectMonitorContentionMark contention_mark(this);
421
422 // Check for deflation.
423 if (enter_is_async_deflating()) {
424 return false;
425 }
426
427 bool success = try_lock_with_contention_mark(locking_thread, contention_mark);
428
429 assert(success, "Failed to enter_for: locking_thread=" INTPTR_FORMAT
430 ", this=" INTPTR_FORMAT "{owner=" INT64_FORMAT "}",
431 p2i(locking_thread), p2i(this), owner_raw());
432 assert(has_owner(locking_thread), "must be");
433 return true;
434 }
435
436 bool ObjectMonitor::try_enter(JavaThread* current, bool check_for_recursion) {
437 // TryLock avoids the CAS and handles deflation.
438 TryLockResult r = try_lock(current);
439 if (r == TryLockResult::Success) {
440 assert(_recursions == 0, "invariant");
441 return true;
442 }
443
444 // If called from SharedRuntime::monitor_exit_helper(), we know that
445 // this thread doesn't already own the lock.
446 if (!check_for_recursion) {
447 return false;
448 }
449
450 if (r == TryLockResult::HasOwner && has_owner(current)) {
451 _recursions++;
452 return true;
453 }
454
455 return false;
456 }
457
458 bool ObjectMonitor::spin_enter(JavaThread* current) {
459 assert(current == JavaThread::current(), "must be");
460
461 // Check for recursion.
462 if (try_enter(current)) {
463 return true;
464 }
465
466 // Check for deflation.
467 if (enter_is_async_deflating()) {
468 return false;
469 }
470
471 // We've encountered genuine contention.
472
473 // Do one round of spinning.
474 // Note that if we acquire the monitor from an initial spin
475 // we forgo posting JVMTI events and firing DTRACE probes.
476 if (try_spin(current)) {
477 assert(has_owner(current), "must be current: owner=" INT64_FORMAT, owner_raw());
478 assert(_recursions == 0, "must be 0: recursions=%zd", _recursions);
479 assert_mark_word_consistency();
480 return true;
481 }
482
483 return false;
484 }
485
486 bool ObjectMonitor::enter(JavaThread* current) {
487 assert(current == JavaThread::current(), "must be");
488
489 if (spin_enter(current)) {
490 return true;
491 }
492
493 assert(!has_owner(current), "invariant");
494 assert(!has_successor(current), "invariant");
495 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
496 assert(current->thread_state() != _thread_blocked, "invariant");
497
498 // Keep is_being_async_deflated stable across the rest of enter
499 ObjectMonitorContentionMark contention_mark(this);
500
501 // Check for deflation.
502 if (enter_is_async_deflating()) {
503 return false;
504 }
505
506 // At this point this ObjectMonitor cannot be deflated, finish contended enter
507 enter_with_contention_mark(current, contention_mark);
508 return true;
509 }
510
511 void ObjectMonitor::notify_contended_enter(JavaThread* current) {
512 current->set_current_pending_monitor(this);
513
514 DTRACE_MONITOR_PROBE(contended__enter, this, object(), current);
515 if (JvmtiExport::should_post_monitor_contended_enter()) {
516 JvmtiExport::post_monitor_contended_enter(current, this);
517
518 // The current thread does not yet own the monitor and does not
519 // yet appear on any queues that would get it made the successor.
520 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
521 // handler cannot accidentally consume an unpark() meant for the
522 // ParkEvent associated with this ObjectMonitor.
523 }
524 }
525
526 void ObjectMonitor::enter_with_contention_mark(JavaThread* current, ObjectMonitorContentionMark &cm) {
527 assert(current == JavaThread::current(), "must be");
528 assert(!has_owner(current), "must be");
529 assert(cm._monitor == this, "must be");
530 assert(!is_being_async_deflated(), "must be");
531
532 JFR_ONLY(JfrConditionalFlush<EventJavaMonitorEnter> flush(current);)
533 EventJavaMonitorEnter enter_event;
534 if (enter_event.is_started()) {
535 enter_event.set_monitorClass(object()->klass());
536 // Set an address that is 'unique enough', such that events close in
537 // time and with the same address are likely (but not guaranteed) to
538 // belong to the same object.
539 enter_event.set_address((uintptr_t)this);
540 }
541 EventVirtualThreadPinned vthread_pinned_event;
542
543 freeze_result result;
544
545 assert(current->current_pending_monitor() == nullptr, "invariant");
546
547 ContinuationEntry* ce = current->last_continuation();
548 bool is_virtual = ce != nullptr && ce->is_virtual_thread();
549 if (is_virtual) {
550 notify_contended_enter(current);
551 result = Continuation::try_preempt(current, ce->cont_oop(current));
552 if (result == freeze_ok) {
553 bool acquired = vthread_monitor_enter(current);
554 if (acquired) {
555 // We actually acquired the monitor while trying to add the vthread to the
556 // _entry_list so cancel preemption. We will still go through the preempt stub
557 // but instead of unmounting we will call thaw to continue execution.
558 current->set_preemption_cancelled(true);
559 if (JvmtiExport::should_post_monitor_contended_entered()) {
560 // We are going to call thaw again after this and finish the VMTS
561 // transition so no need to do it here. We will post the event there.
562 current->set_contended_entered_monitor(this);
563 }
564 }
565 current->set_current_pending_monitor(nullptr);
566 DEBUG_ONLY(int state = java_lang_VirtualThread::state(current->vthread()));
567 assert((acquired && current->preemption_cancelled() && state == java_lang_VirtualThread::RUNNING) ||
568 (!acquired && !current->preemption_cancelled() && state == java_lang_VirtualThread::BLOCKING), "invariant");
569 return;
570 }
571 }
572
573 {
574 // Change java thread status to indicate blocked on monitor enter.
575 JavaThreadBlockedOnMonitorEnterState jtbmes(current, this);
576
577 if (!is_virtual) { // already notified contended_enter for virtual
578 notify_contended_enter(current);
579 }
580 OSThreadContendState osts(current->osthread());
581
582 assert(current->thread_state() == _thread_in_vm, "invariant");
583
584 for (;;) {
585 ExitOnSuspend eos(this);
586 {
587 ThreadBlockInVMPreprocess<ExitOnSuspend> tbivs(current, eos, true /* allow_suspend */);
588 enter_internal(current);
589 current->set_current_pending_monitor(nullptr);
590 // We can go to a safepoint at the end of this block. If we
591 // do a thread dump during that safepoint, then this thread will show
592 // as having "-locked" the monitor, but the OS and java.lang.Thread
593 // states will still report that the thread is blocked trying to
594 // acquire it.
595 // If there is a suspend request, ExitOnSuspend will exit the OM
596 // and set the OM as pending.
597 }
598 if (!eos.exited()) {
599 // ExitOnSuspend did not exit the OM
600 assert(has_owner(current), "invariant");
601 break;
602 }
603 }
604
605 // We've just gotten past the enter-check-for-suspend dance and we now own
606 // the monitor free and clear.
607 }
608
609 assert(contentions() >= 0, "must not be negative: contentions=%d", contentions());
610
611 // Must either set _recursions = 0 or ASSERT _recursions == 0.
612 assert(_recursions == 0, "invariant");
613 assert(has_owner(current), "invariant");
614 assert(!has_successor(current), "invariant");
615 assert_mark_word_consistency();
616
617 // The thread -- now the owner -- is back in vm mode.
618 // Report the glorious news via TI,DTrace and jvmstat.
619 // The probe effect is non-trivial. All the reportage occurs
620 // while we hold the monitor, increasing the length of the critical
621 // section. Amdahl's parallel speedup law comes vividly into play.
622 //
623 // Another option might be to aggregate the events (thread local or
624 // per-monitor aggregation) and defer reporting until a more opportune
625 // time -- such as next time some thread encounters contention but has
626 // yet to acquire the lock. While spinning that thread could
627 // spinning we could increment JVMStat counters, etc.
628
629 DTRACE_MONITOR_PROBE(contended__entered, this, object(), current);
630 if (JvmtiExport::should_post_monitor_contended_entered()) {
631 JvmtiExport::post_monitor_contended_entered(current, this);
632
633 // The current thread already owns the monitor and is not going to
634 // call park() for the remainder of the monitor enter protocol. So
635 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
636 // event handler consumed an unpark() issued by the thread that
637 // just exited the monitor.
638 }
639 if (enter_event.should_commit()) {
640 enter_event.set_previousOwner(_previous_owner_tid);
641 enter_event.commit();
642 }
643
644 if (current->current_waiting_monitor() == nullptr) {
645 ContinuationEntry* ce = current->last_continuation();
646 if (ce != nullptr && ce->is_virtual_thread()) {
647 current->post_vthread_pinned_event(&vthread_pinned_event, "Contended monitor enter", result);
648 }
649 }
650 }
651
652 // Caveat: try_lock() is not necessarily serializing if it returns failure.
653 // Callers must compensate as needed.
654
655 ObjectMonitor::TryLockResult ObjectMonitor::try_lock(JavaThread* current) {
656 int64_t own = owner_raw();
657 int64_t first_own = own;
658
659 for (;;) {
660 if (own == DEFLATER_MARKER) {
661 // Block out deflation as soon as possible.
662 ObjectMonitorContentionMark contention_mark(this);
663
664 // Check for deflation.
665 if (enter_is_async_deflating()) {
666 // Treat deflation as interference.
667 return TryLockResult::Interference;
668 }
669 if (try_lock_with_contention_mark(current, contention_mark)) {
670 assert(_recursions == 0, "invariant");
671 return TryLockResult::Success;
672 } else {
673 // Deflation won or change of owner; dont spin
674 break;
675 }
676 } else if (own == NO_OWNER) {
677 int64_t prev_own = try_set_owner_from(NO_OWNER, current);
678 if (prev_own == NO_OWNER) {
679 assert(_recursions == 0, "invariant");
680 return TryLockResult::Success;
681 } else {
682 // The lock had been free momentarily, but we lost the race to the lock.
683 own = prev_own;
684 }
685 } else {
686 // Retry doesn't make as much sense because the lock was just acquired.
687 break;
688 }
689 }
690 return first_own == own ? TryLockResult::HasOwner : TryLockResult::Interference;
691 }
692
693 // Push "current" onto the head of the _entry_list. Once on _entry_list,
694 // current stays on-queue until it acquires the lock.
695 void ObjectMonitor::add_to_entry_list(JavaThread* current, ObjectWaiter* node) {
696 node->_prev = nullptr;
697 node->TState = ObjectWaiter::TS_ENTER;
698
699 for (;;) {
700 ObjectWaiter* head = Atomic::load(&_entry_list);
701 node->_next = head;
702 if (Atomic::cmpxchg(&_entry_list, head, node) == head) {
703 return;
704 }
705 }
706 }
707
708 // Push "current" onto the head of the entry_list.
709 // If the _entry_list was changed during our push operation, we try to
710 // lock the monitor. Returns true if we locked the monitor, and false
711 // if we added current to _entry_list. Once on _entry_list, current
712 // stays on-queue until it acquires the lock.
713 bool ObjectMonitor::try_lock_or_add_to_entry_list(JavaThread* current, ObjectWaiter* node) {
714 node->_prev = nullptr;
715 node->TState = ObjectWaiter::TS_ENTER;
716
717 for (;;) {
718 ObjectWaiter* head = Atomic::load(&_entry_list);
719 node->_next = head;
720 if (Atomic::cmpxchg(&_entry_list, head, node) == head) {
721 return false;
722 }
723
724 // Interference - the CAS failed because _entry_list changed. Before
725 // retrying the CAS retry taking the lock as it may now be free.
726 if (try_lock(current) == TryLockResult::Success) {
727 assert(!has_successor(current), "invariant");
728 assert(has_owner(current), "invariant");
729 return true;
730 }
731 }
732 }
733
734 static void post_monitor_deflate_event(EventJavaMonitorDeflate* event,
735 const oop obj) {
736 assert(event != nullptr, "invariant");
737 if (obj == nullptr) {
738 // Accept the case when obj was already garbage-collected.
739 // Emit the event anyway, but without details.
740 event->set_monitorClass(nullptr);
741 event->set_address(0);
742 } else {
743 const Klass* monitor_klass = obj->klass();
744 if (ObjectMonitor::is_jfr_excluded(monitor_klass)) {
745 return;
746 }
747 event->set_monitorClass(monitor_klass);
748 event->set_address((uintptr_t)(void*)obj);
749 }
750 event->commit();
751 }
752
753 // Deflate the specified ObjectMonitor if not in-use. Returns true if it
754 // was deflated and false otherwise.
755 //
756 // The async deflation protocol sets owner to DEFLATER_MARKER and
757 // makes contentions negative as signals to contending threads that
758 // an async deflation is in progress. There are a number of checks
759 // as part of the protocol to make sure that the calling thread has
760 // not lost the race to a contending thread.
761 //
762 // The ObjectMonitor has been successfully async deflated when:
763 // (contentions < 0)
764 // Contending threads that see that condition know to retry their operation.
765 //
766 bool ObjectMonitor::deflate_monitor(Thread* current) {
767 if (is_busy()) {
768 // Easy checks are first - the ObjectMonitor is busy so no deflation.
769 return false;
770 }
771
772 EventJavaMonitorDeflate event;
773
774 const oop obj = object_peek();
775
776 if (obj == nullptr) {
777 // If the object died, we can recycle the monitor without racing with
778 // Java threads. The GC already broke the association with the object.
779 set_owner_from_raw(NO_OWNER, DEFLATER_MARKER);
780 assert(contentions() >= 0, "must be non-negative: contentions=%d", contentions());
781 _contentions = INT_MIN; // minimum negative int
782 } else {
783 // Attempt async deflation protocol.
784
785 // Set a null owner to DEFLATER_MARKER to force any contending thread
786 // through the slow path. This is just the first part of the async
787 // deflation dance.
788 if (try_set_owner_from_raw(NO_OWNER, DEFLATER_MARKER) != NO_OWNER) {
789 // The owner field is no longer null so we lost the race since the
790 // ObjectMonitor is now busy.
791 return false;
792 }
793
794 if (contentions() > 0 || _waiters != 0) {
795 // Another thread has raced to enter the ObjectMonitor after
796 // is_busy() above or has already entered and waited on
797 // it which makes it busy so no deflation. Restore owner to
798 // null if it is still DEFLATER_MARKER.
799 if (try_set_owner_from_raw(DEFLATER_MARKER, NO_OWNER) != DEFLATER_MARKER) {
800 // Deferred decrement for the JT enter_internal() that cancelled the async deflation.
801 add_to_contentions(-1);
802 }
803 return false;
804 }
805
806 // Make a zero contentions field negative to force any contending threads
807 // to retry. This is the second part of the async deflation dance.
808 if (Atomic::cmpxchg(&_contentions, 0, INT_MIN) != 0) {
809 // Contentions was no longer 0 so we lost the race since the
810 // ObjectMonitor is now busy. Restore owner to null if it is
811 // still DEFLATER_MARKER:
812 if (try_set_owner_from_raw(DEFLATER_MARKER, NO_OWNER) != DEFLATER_MARKER) {
813 // Deferred decrement for the JT enter_internal() that cancelled the async deflation.
814 add_to_contentions(-1);
815 }
816 return false;
817 }
818 }
819
820 // Sanity checks for the races:
821 guarantee(owner_is_DEFLATER_MARKER(), "must be deflater marker");
822 guarantee(contentions() < 0, "must be negative: contentions=%d",
823 contentions());
824 guarantee(_waiters == 0, "must be 0: waiters=%d", _waiters);
825 ObjectWaiter* w = Atomic::load(&_entry_list);
826 guarantee(w == nullptr,
827 "must be no entering threads: entry_list=" INTPTR_FORMAT,
828 p2i(w));
829
830 if (obj != nullptr) {
831 if (log_is_enabled(Trace, monitorinflation)) {
832 ResourceMark rm;
833 log_trace(monitorinflation)("deflate_monitor: object=" INTPTR_FORMAT
834 ", mark=" INTPTR_FORMAT ", type='%s'",
835 p2i(obj), obj->mark().value(),
836 obj->klass()->external_name());
837 }
838 }
839
840 if (UseObjectMonitorTable) {
841 LightweightSynchronizer::deflate_monitor(current, obj, this);
842 } else if (obj != nullptr) {
843 // Install the old mark word if nobody else has already done it.
844 install_displaced_markword_in_object(obj);
845 }
846
847 if (event.should_commit()) {
848 post_monitor_deflate_event(&event, obj);
849 }
850
851 // We leave owner == DEFLATER_MARKER and contentions < 0
852 // to force any racing threads to retry.
853 return true; // Success, ObjectMonitor has been deflated.
854 }
855
856 // Install the displaced mark word (dmw) of a deflating ObjectMonitor
857 // into the header of the object associated with the monitor. This
858 // idempotent method is called by a thread that is deflating a
859 // monitor and by other threads that have detected a race with the
860 // deflation process.
861 void ObjectMonitor::install_displaced_markword_in_object(const oop obj) {
862 assert(!UseObjectMonitorTable, "ObjectMonitorTable has no dmw");
863 // This function must only be called when (owner == DEFLATER_MARKER
864 // && contentions <= 0), but we can't guarantee that here because
865 // those values could change when the ObjectMonitor gets moved from
866 // the global free list to a per-thread free list.
867
868 guarantee(obj != nullptr, "must be non-null");
869
870 // Separate loads in is_being_async_deflated(), which is almost always
871 // called before this function, from the load of dmw/header below.
872
873 // _contentions and dmw/header may get written by different threads.
874 // Make sure to observe them in the same order when having several observers.
875 OrderAccess::loadload_for_IRIW();
876
877 const oop l_object = object_peek();
878 if (l_object == nullptr) {
879 // ObjectMonitor's object ref has already been cleared by async
880 // deflation or GC so we're done here.
881 return;
882 }
883 assert(l_object == obj, "object=" INTPTR_FORMAT " must equal obj="
884 INTPTR_FORMAT, p2i(l_object), p2i(obj));
885
886 markWord dmw = header();
887 // The dmw has to be neutral (not null, not locked and not marked).
888 assert(dmw.is_neutral(), "must be neutral: dmw=" INTPTR_FORMAT, dmw.value());
889
890 // Install displaced mark word if the object's header still points
891 // to this ObjectMonitor. More than one racing caller to this function
892 // can rarely reach this point, but only one can win.
893 markWord res = obj->cas_set_mark(dmw, markWord::encode(this));
894 if (res != markWord::encode(this)) {
895 // This should be rare so log at the Info level when it happens.
896 log_info(monitorinflation)("install_displaced_markword_in_object: "
897 "failed cas_set_mark: new_mark=" INTPTR_FORMAT
898 ", old_mark=" INTPTR_FORMAT ", res=" INTPTR_FORMAT,
899 dmw.value(), markWord::encode(this).value(),
900 res.value());
901 }
902
903 // Note: It does not matter which thread restored the header/dmw
904 // into the object's header. The thread deflating the monitor just
905 // wanted the object's header restored and it is. The threads that
906 // detected a race with the deflation process also wanted the
907 // object's header restored before they retry their operation and
908 // because it is restored they will only retry once.
909 }
910
911 // Convert the fields used by is_busy() to a string that can be
912 // used for diagnostic output.
913 const char* ObjectMonitor::is_busy_to_string(stringStream* ss) {
914 ss->print("is_busy: waiters=%d"
915 ", contentions=%d"
916 ", owner=" INT64_FORMAT
917 ", entry_list=" PTR_FORMAT,
918 _waiters,
919 (contentions() > 0 ? contentions() : 0),
920 owner_is_DEFLATER_MARKER()
921 // We report null instead of DEFLATER_MARKER here because is_busy()
922 // ignores DEFLATER_MARKER values.
923 ? NO_OWNER
924 : owner_raw(),
925 p2i(_entry_list));
926 return ss->base();
927 }
928
929 void ObjectMonitor::enter_internal(JavaThread* current) {
930 assert(current->thread_state() == _thread_blocked, "invariant");
931
932 // Try the lock - TATAS
933 if (try_lock(current) == TryLockResult::Success) {
934 assert(!has_successor(current), "invariant");
935 assert(has_owner(current), "invariant");
936 return;
937 }
938
939 assert(InitDone, "Unexpectedly not initialized");
940
941 // We try one round of spinning *before* enqueueing current.
942 //
943 // If the _owner is ready but OFFPROC we could use a YieldTo()
944 // operation to donate the remainder of this thread's quantum
945 // to the owner. This has subtle but beneficial affinity
946 // effects.
947
948 if (try_spin(current)) {
949 assert(has_owner(current), "invariant");
950 assert(!has_successor(current), "invariant");
951 return;
952 }
953
954 // The Spin failed -- Enqueue and park the thread ...
955 assert(!has_successor(current), "invariant");
956 assert(!has_owner(current), "invariant");
957
958 // Enqueue "current" on ObjectMonitor's _entry_list.
959 //
960 // Node acts as a proxy for current.
961 // As an aside, if were to ever rewrite the synchronization code mostly
962 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
963 // Java objects. This would avoid awkward lifecycle and liveness issues,
964 // as well as eliminate a subset of ABA issues.
965 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
966
967 ObjectWaiter node(current);
968 current->_ParkEvent->reset();
969
970 if (try_lock_or_add_to_entry_list(current, &node)) {
971 return; // We got the lock.
972 }
973 // This thread is now added to the _entry_list.
974
975 // The lock might have been released while this thread was occupied queueing
976 // itself onto _entry_list. To close the race and avoid "stranding" and
977 // progress-liveness failure we must resample-retry _owner before parking.
978 // Note the Dekker/Lamport duality: ST _entry_list; MEMBAR; LD Owner.
979 // In this case the ST-MEMBAR is accomplished with CAS().
980 //
981 // TODO: Defer all thread state transitions until park-time.
982 // Since state transitions are heavy and inefficient we'd like
983 // to defer the state transitions until absolutely necessary,
984 // and in doing so avoid some transitions ...
985
986 // For virtual threads that are pinned, do a timed-park instead to
987 // alleviate some deadlocks cases where the succesor is an unmounted
988 // virtual thread that cannot run. This can happen in particular when
989 // this virtual thread is currently loading/initializing a class, and
990 // all other carriers have a vthread pinned to it waiting for said class
991 // to be loaded/initialized.
992 static int MAX_RECHECK_INTERVAL = 1000;
993 int recheck_interval = 1;
994 bool do_timed_parked = false;
995 ContinuationEntry* ce = current->last_continuation();
996 if (ce != nullptr && ce->is_virtual_thread()) {
997 do_timed_parked = true;
998 }
999
1000 for (;;) {
1001
1002 if (try_lock(current) == TryLockResult::Success) {
1003 break;
1004 }
1005 assert(!has_owner(current), "invariant");
1006
1007 // park self
1008 if (do_timed_parked) {
1009 current->_ParkEvent->park((jlong) recheck_interval);
1010 // Increase the recheck_interval, but clamp the value.
1011 recheck_interval *= 8;
1012 if (recheck_interval > MAX_RECHECK_INTERVAL) {
1013 recheck_interval = MAX_RECHECK_INTERVAL;
1014 }
1015 } else {
1016 current->_ParkEvent->park();
1017 }
1018
1019 if (try_lock(current) == TryLockResult::Success) {
1020 break;
1021 }
1022
1023 // The lock is still contested.
1024
1025 // Assuming this is not a spurious wakeup we'll normally find _succ == current.
1026 // We can defer clearing _succ until after the spin completes
1027 // try_spin() must tolerate being called with _succ == current.
1028 // Try yet another round of adaptive spinning.
1029 if (try_spin(current)) {
1030 break;
1031 }
1032
1033 // We can find that we were unpark()ed and redesignated _succ while
1034 // we were spinning. That's harmless. If we iterate and call park(),
1035 // park() will consume the event and return immediately and we'll
1036 // just spin again. This pattern can repeat, leaving _succ to simply
1037 // spin on a CPU.
1038
1039 if (has_successor(current)) clear_successor();
1040
1041 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
1042 OrderAccess::fence();
1043 }
1044
1045 // Egress :
1046 // Current has acquired the lock -- Unlink current from the _entry_list.
1047 unlink_after_acquire(current, &node);
1048 if (has_successor(current)) {
1049 clear_successor();
1050 // Note that we don't need to do OrderAccess::fence() after clearing
1051 // _succ here, since we own the lock.
1052 }
1053
1054 // We've acquired ownership with CAS().
1055 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
1056 // But since the CAS() this thread may have also stored into _succ
1057 // or entry_list. These meta-data updates must be visible __before
1058 // this thread subsequently drops the lock.
1059 // Consider what could occur if we didn't enforce this constraint --
1060 // STs to monitor meta-data and user-data could reorder with (become
1061 // visible after) the ST in exit that drops ownership of the lock.
1062 // Some other thread could then acquire the lock, but observe inconsistent
1063 // or old monitor meta-data and heap data. That violates the JMM.
1064 // To that end, the exit() operation must have at least STST|LDST
1065 // "release" barrier semantics. Specifically, there must be at least a
1066 // STST|LDST barrier in exit() before the ST of null into _owner that drops
1067 // the lock. The barrier ensures that changes to monitor meta-data and data
1068 // protected by the lock will be visible before we release the lock, and
1069 // therefore before some other thread (CPU) has a chance to acquire the lock.
1070 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
1071 //
1072 // Critically, any prior STs to _succ or entry_list must be visible before
1073 // the ST of null into _owner in the *subsequent* (following) corresponding
1074 // monitorexit.
1075
1076 return;
1077 }
1078
1079 // reenter_internal() is a specialized inline form of the latter half of the
1080 // contended slow-path from enter_internal(). We use reenter_internal() only for
1081 // monitor reentry in wait().
1082 //
1083 // In the future we should reconcile enter_internal() and reenter_internal().
1084
1085 void ObjectMonitor::reenter_internal(JavaThread* current, ObjectWaiter* currentNode) {
1086 assert(current != nullptr, "invariant");
1087 assert(current->thread_state() != _thread_blocked, "invariant");
1088 assert(currentNode != nullptr, "invariant");
1089 assert(currentNode->_thread == current, "invariant");
1090 assert(_waiters > 0, "invariant");
1091 assert_mark_word_consistency();
1092
1093 for (;;) {
1094 ObjectWaiter::TStates v = currentNode->TState;
1095 guarantee(v == ObjectWaiter::TS_ENTER, "invariant");
1096 assert(!has_owner(current), "invariant");
1097
1098 // This thread has been notified so try to reacquire the lock.
1099 if (try_lock(current) == TryLockResult::Success) {
1100 break;
1101 }
1102
1103 // If that fails, spin again. Note that spin count may be zero so the above TryLock
1104 // is necessary.
1105 if (try_spin(current)) {
1106 break;
1107 }
1108
1109 {
1110 OSThreadContendState osts(current->osthread());
1111
1112 assert(current->thread_state() == _thread_in_vm, "invariant");
1113
1114 {
1115 ClearSuccOnSuspend csos(this);
1116 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */);
1117 current->_ParkEvent->park();
1118 }
1119 }
1120
1121 // Try again, but just so we distinguish between futile wakeups and
1122 // successful wakeups. The following test isn't algorithmically
1123 // necessary, but it helps us maintain sensible statistics.
1124 if (try_lock(current) == TryLockResult::Success) {
1125 break;
1126 }
1127
1128 // The lock is still contested.
1129
1130 // Assuming this is not a spurious wakeup we'll normally
1131 // find that _succ == current.
1132 if (has_successor(current)) clear_successor();
1133
1134 // Invariant: after clearing _succ a contending thread
1135 // *must* retry _owner before parking.
1136 OrderAccess::fence();
1137 }
1138
1139 // Current has acquired the lock -- Unlink current from the _entry_list.
1140 assert(has_owner(current), "invariant");
1141 assert_mark_word_consistency();
1142 unlink_after_acquire(current, currentNode);
1143 if (has_successor(current)) clear_successor();
1144 assert(!has_successor(current), "invariant");
1145 currentNode->TState = ObjectWaiter::TS_RUN;
1146 OrderAccess::fence(); // see comments at the end of enter_internal()
1147 }
1148
1149 // This method is called from two places:
1150 // - On monitorenter contention with a null waiter.
1151 // - After Object.wait() times out or the target is interrupted to reenter the
1152 // monitor, with the existing waiter.
1153 // For the Object.wait() case we do not delete the ObjectWaiter in case we
1154 // succesfully acquire the monitor since we are going to need it on return.
1155 bool ObjectMonitor::vthread_monitor_enter(JavaThread* current, ObjectWaiter* waiter) {
1156 if (try_lock(current) == TryLockResult::Success) {
1157 assert(has_owner(current), "invariant");
1158 assert(!has_successor(current), "invariant");
1159 return true;
1160 }
1161
1162 oop vthread = current->vthread();
1163 ObjectWaiter* node = waiter != nullptr ? waiter : new ObjectWaiter(vthread, this);
1164 if (try_lock_or_add_to_entry_list(current, node)) {
1165 // We got the lock.
1166 if (waiter == nullptr) delete node; // for Object.wait() don't delete yet
1167 return true;
1168 }
1169 // This thread is now added to the entry_list.
1170
1171 // We have to try once more since owner could have exited monitor and checked
1172 // _entry_list before we added the node to the queue.
1173 if (try_lock(current) == TryLockResult::Success) {
1174 assert(has_owner(current), "invariant");
1175 unlink_after_acquire(current, node);
1176 if (has_successor(current)) clear_successor();
1177 if (waiter == nullptr) delete node; // for Object.wait() don't delete yet
1178 return true;
1179 }
1180
1181 assert(java_lang_VirtualThread::state(vthread) == java_lang_VirtualThread::RUNNING, "wrong state for vthread");
1182 java_lang_VirtualThread::set_state(vthread, java_lang_VirtualThread::BLOCKING);
1183
1184 // We didn't succeed in acquiring the monitor so increment _contentions and
1185 // save ObjectWaiter* in the vthread since we will need it when resuming execution.
1186 add_to_contentions(1);
1187 java_lang_VirtualThread::set_objectWaiter(vthread, node);
1188 return false;
1189 }
1190
1191 // Called from thaw code to resume the monitor operation that caused the vthread
1192 // to be unmounted. Method returns true if the monitor is successfully acquired,
1193 // which marks the end of the monitor operation, otherwise it returns false.
1194 bool ObjectMonitor::resume_operation(JavaThread* current, ObjectWaiter* node, ContinuationWrapper& cont) {
1195 assert(java_lang_VirtualThread::state(current->vthread()) == java_lang_VirtualThread::RUNNING, "wrong state for vthread");
1196 assert(!has_owner(current), "");
1197
1198 if (node->is_wait() && !node->at_reenter()) {
1199 bool acquired_monitor = vthread_wait_reenter(current, node, cont);
1200 if (acquired_monitor) return true;
1201 }
1202
1203 // Retry acquiring monitor...
1204
1205 int state = node->TState;
1206 guarantee(state == ObjectWaiter::TS_ENTER, "invariant");
1207
1208 if (try_lock(current) == TryLockResult::Success) {
1209 vthread_epilog(current, node);
1210 return true;
1211 }
1212
1213 oop vthread = current->vthread();
1214 if (has_successor(current)) clear_successor();
1215
1216 // Invariant: after clearing _succ a thread *must* retry acquiring the monitor.
1217 OrderAccess::fence();
1218
1219 if (try_lock(current) == TryLockResult::Success) {
1220 vthread_epilog(current, node);
1221 return true;
1222 }
1223
1224 // We will return to Continuation.run() and unmount so set the right state.
1225 java_lang_VirtualThread::set_state(vthread, java_lang_VirtualThread::BLOCKING);
1226
1227 return false;
1228 }
1229
1230 void ObjectMonitor::vthread_epilog(JavaThread* current, ObjectWaiter* node) {
1231 assert(has_owner(current), "invariant");
1232 add_to_contentions(-1);
1233
1234 if (has_successor(current)) clear_successor();
1235
1236 guarantee(_recursions == 0, "invariant");
1237
1238 if (node->is_wait()) {
1239 _recursions = node->_recursions; // restore the old recursion count
1240 _waiters--; // decrement the number of waiters
1241
1242 if (node->_interrupted) {
1243 // We will throw at thaw end after finishing the mount transition.
1244 current->set_pending_interrupted_exception(true);
1245 }
1246 }
1247
1248 unlink_after_acquire(current, node);
1249 delete node;
1250
1251 // Clear the ObjectWaiter* from the vthread.
1252 java_lang_VirtualThread::set_objectWaiter(current->vthread(), nullptr);
1253
1254 if (JvmtiExport::should_post_monitor_contended_entered()) {
1255 // We are going to call thaw again after this and finish the VMTS
1256 // transition so no need to do it here. We will post the event there.
1257 current->set_contended_entered_monitor(this);
1258 }
1259 }
1260
1261 // Convert entry_list into a doubly linked list by assigning the prev
1262 // pointers and the entry_list_tail pointer (if needed). Within the
1263 // entry_list the next pointers always form a consistent singly linked
1264 // list. When this function is called, the entry_list will be either
1265 // singly linked, or starting as singly linked (at the head), but
1266 // ending as doubly linked (at the tail).
1267 void ObjectMonitor::entry_list_build_dll(JavaThread* current) {
1268 assert(has_owner(current), "invariant");
1269 ObjectWaiter* prev = nullptr;
1270 // Need acquire here to match the implicit release of the cmpxchg
1271 // that updated entry_list, so we can access w->prev().
1272 ObjectWaiter* w = Atomic::load_acquire(&_entry_list);
1273 assert(w != nullptr, "should only be called when entry list is not empty");
1274 while (w != nullptr) {
1275 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1276 assert(w->prev() == nullptr || w->prev() == prev, "invariant");
1277 if (w->prev() != nullptr) {
1278 break;
1279 }
1280 w->_prev = prev;
1281 prev = w;
1282 w = w->next();
1283 }
1284 if (w == nullptr) {
1285 // We converted the entire entry_list from a singly linked list
1286 // into a doubly linked list. Now we just need to set the tail
1287 // pointer.
1288 assert(prev != nullptr && prev->next() == nullptr, "invariant");
1289 assert(_entry_list_tail == nullptr || _entry_list_tail == prev, "invariant");
1290 _entry_list_tail = prev;
1291 } else {
1292 #ifdef ASSERT
1293 // We stopped iterating through the _entry_list when we found a
1294 // node that had its prev pointer set. I.e. we converted the first
1295 // part of the entry_list from a singly linked list into a doubly
1296 // linked list. Now we just want to make sure the rest of the list
1297 // is doubly linked. But first we check that we have a tail
1298 // pointer, because if the end of the entry_list is doubly linked
1299 // and we don't have the tail pointer, something is broken.
1300 assert(_entry_list_tail != nullptr, "invariant");
1301 while (w != nullptr) {
1302 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1303 assert(w->prev() == prev, "invariant");
1304 prev = w;
1305 w = w->next();
1306 }
1307 assert(_entry_list_tail == prev, "invariant");
1308 #endif
1309 }
1310 }
1311
1312 // Return the tail of the _entry_list. If the tail is currently not
1313 // known, it can be found by first calling entry_list_build_dll().
1314 ObjectWaiter* ObjectMonitor::entry_list_tail(JavaThread* current) {
1315 assert(has_owner(current), "invariant");
1316 ObjectWaiter* w = _entry_list_tail;
1317 if (w != nullptr) {
1318 return w;
1319 }
1320 entry_list_build_dll(current);
1321 w = _entry_list_tail;
1322 assert(w != nullptr, "invariant");
1323 return w;
1324 }
1325
1326 // By convention we unlink a contending thread from _entry_list
1327 // immediately after the thread acquires the lock in ::enter().
1328 // The head of _entry_list is volatile but the interior is stable.
1329 // In addition, current.TState is stable.
1330
1331 void ObjectMonitor::unlink_after_acquire(JavaThread* current, ObjectWaiter* currentNode) {
1332 assert(has_owner(current), "invariant");
1333 assert((!currentNode->is_vthread() && currentNode->thread() == current) ||
1334 (currentNode->is_vthread() && currentNode->vthread() == current->vthread()), "invariant");
1335
1336 // Check if we are unlinking the last element in the _entry_list.
1337 // This is by far the most common case.
1338 if (currentNode->next() == nullptr) {
1339 assert(_entry_list_tail == nullptr || _entry_list_tail == currentNode, "invariant");
1340
1341 ObjectWaiter* w = Atomic::load(&_entry_list);
1342 if (w == currentNode) {
1343 // The currentNode is the only element in _entry_list.
1344 if (Atomic::cmpxchg(&_entry_list, w, (ObjectWaiter*)nullptr) == w) {
1345 _entry_list_tail = nullptr;
1346 currentNode->set_bad_pointers();
1347 return;
1348 }
1349 // The CAS above can fail from interference IFF a contending
1350 // thread "pushed" itself onto entry_list. So fall-through to
1351 // building the doubly linked list.
1352 assert(currentNode->prev() == nullptr, "invariant");
1353 }
1354 if (currentNode->prev() == nullptr) {
1355 // Build the doubly linked list to get hold of
1356 // currentNode->prev().
1357 entry_list_build_dll(current);
1358 assert(currentNode->prev() != nullptr, "must be");
1359 assert(_entry_list_tail == currentNode, "must be");
1360 }
1361 // The currentNode is the last element in _entry_list and we know
1362 // which element is the previous one.
1363 assert(_entry_list != currentNode, "invariant");
1364 _entry_list_tail = currentNode->prev();
1365 _entry_list_tail->_next = nullptr;
1366 currentNode->set_bad_pointers();
1367 return;
1368 }
1369
1370 // If we get here it means the current thread enqueued itself on the
1371 // _entry_list but was then able to "steal" the lock before the
1372 // chosen successor was able to. Consequently currentNode must be an
1373 // interior node in the _entry_list, or the head.
1374 assert(currentNode->next() != nullptr, "invariant");
1375 assert(currentNode != _entry_list_tail, "invariant");
1376
1377 // Check if we are in the singly linked portion of the
1378 // _entry_list. If we are the head then we try to remove ourselves,
1379 // else we convert to the doubly linked list.
1380 if (currentNode->prev() == nullptr) {
1381 ObjectWaiter* w = Atomic::load(&_entry_list);
1382
1383 assert(w != nullptr, "invariant");
1384 if (w == currentNode) {
1385 ObjectWaiter* next = currentNode->next();
1386 // currentNode is at the head of _entry_list.
1387 if (Atomic::cmpxchg(&_entry_list, w, next) == w) {
1388 // The CAS above sucsessfully unlinked currentNode from the
1389 // head of the _entry_list.
1390 assert(_entry_list != w, "invariant");
1391 next->_prev = nullptr;
1392 currentNode->set_bad_pointers();
1393 return;
1394 } else {
1395 // The CAS above can fail from interference IFF a contending
1396 // thread "pushed" itself onto _entry_list, in which case
1397 // currentNode must now be in the interior of the
1398 // list. Fall-through to building the doubly linked list.
1399 assert(_entry_list != currentNode, "invariant");
1400 }
1401 }
1402 // Build the doubly linked list to get hold of currentNode->prev().
1403 entry_list_build_dll(current);
1404 assert(currentNode->prev() != nullptr, "must be");
1405 }
1406
1407 // We now know we are unlinking currentNode from the interior of a
1408 // doubly linked list.
1409 assert(currentNode->next() != nullptr, "");
1410 assert(currentNode->prev() != nullptr, "");
1411 assert(currentNode != _entry_list, "");
1412 assert(currentNode != _entry_list_tail, "");
1413
1414 ObjectWaiter* nxt = currentNode->next();
1415 ObjectWaiter* prv = currentNode->prev();
1416 assert(nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
1417 assert(prv->TState == ObjectWaiter::TS_ENTER, "invariant");
1418
1419 nxt->_prev = prv;
1420 prv->_next = nxt;
1421 currentNode->set_bad_pointers();
1422 }
1423
1424 // -----------------------------------------------------------------------------
1425 // Exit support
1426 //
1427 // exit()
1428 // ~~~~~~
1429 // Note that the collector can't reclaim the objectMonitor or deflate
1430 // the object out from underneath the thread calling ::exit() as the
1431 // thread calling ::exit() never transitions to a stable state.
1432 // This inhibits GC, which in turn inhibits asynchronous (and
1433 // inopportune) reclamation of "this".
1434 //
1435 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
1436 // There's one exception to the claim above, however. enter_internal() can call
1437 // exit() to drop a lock if the acquirer has been externally suspended.
1438 // In that case exit() is called with _thread_state == _thread_blocked,
1439 // but the monitor's _contentions field is > 0, which inhibits reclamation.
1440 //
1441 // This is the exit part of the locking protocol, often implemented in
1442 // C2_MacroAssembler::fast_unlock()
1443 //
1444 // 1. A release barrier ensures that changes to monitor meta-data
1445 // (_succ, _entry_list) and data protected by the lock will be
1446 // visible before we release the lock.
1447 // 2. Release the lock by clearing the owner.
1448 // 3. A storeload MEMBAR is needed between releasing the owner and
1449 // subsequently reading meta-data to safely determine if the lock is
1450 // contended (step 4) without an elected successor (step 5).
1451 // 4. If _entry_list is null, we are done, since there is no
1452 // other thread waiting on the lock to wake up. I.e. there is no
1453 // contention.
1454 // 5. If there is a successor (_succ is non-null), we are done. The
1455 // responsibility for guaranteeing progress-liveness has now implicitly
1456 // been moved from the exiting thread to the successor.
1457 // 6. There are waiters in the entry list (_entry_list is non-null),
1458 // but there is no successor (_succ is null), so we need to
1459 // wake up (unpark) a waiting thread to avoid stranding.
1460 //
1461 // Note that since only the current lock owner can manipulate the
1462 // _entry_list (except for pushing new threads to the head), we need to
1463 // reacquire the lock before we can wake up (unpark) a waiting thread.
1464 //
1465 // The CAS() in enter provides for safety and exclusion, while the
1466 // MEMBAR in exit provides for progress and avoids stranding.
1467 //
1468 // There is also the risk of a futile wake-up. If we drop the lock
1469 // another thread can reacquire the lock immediately, and we can
1470 // then wake a thread unnecessarily. This is benign, and we've
1471 // structured the code so the windows are short and the frequency
1472 // of such futile wakups is low.
1473
1474 void ObjectMonitor::exit(JavaThread* current, bool not_suspended) {
1475 if (!has_owner(current)) {
1476 // Apparent unbalanced locking ...
1477 // Naively we'd like to throw IllegalMonitorStateException.
1478 // As a practical matter we can neither allocate nor throw an
1479 // exception as ::exit() can be called from leaf routines.
1480 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
1481 // Upon deeper reflection, however, in a properly run JVM the only
1482 // way we should encounter this situation is in the presence of
1483 // unbalanced JNI locking. TODO: CheckJNICalls.
1484 // See also: CR4414101
1485 #ifdef ASSERT
1486 LogStreamHandle(Error, monitorinflation) lsh;
1487 lsh.print_cr("ERROR: ObjectMonitor::exit(): thread=" INTPTR_FORMAT
1488 " is exiting an ObjectMonitor it does not own.", p2i(current));
1489 lsh.print_cr("The imbalance is possibly caused by JNI locking.");
1490 print_debug_style_on(&lsh);
1491 assert(false, "Non-balanced monitor enter/exit!");
1492 #endif
1493 return;
1494 }
1495
1496 if (_recursions != 0) {
1497 _recursions--; // this is simple recursive enter
1498 return;
1499 }
1500
1501 #if INCLUDE_JFR
1502 // get the owner's thread id for the MonitorEnter event
1503 // if it is enabled and the thread isn't suspended
1504 if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
1505 _previous_owner_tid = JFR_THREAD_ID(current);
1506 }
1507 #endif
1508
1509 for (;;) {
1510 // If there is a successor we should release the lock as soon as
1511 // possible, so that the successor can acquire the lock. If there is
1512 // no successor, we might need to wake up a waiting thread.
1513 if (!has_successor()) {
1514 ObjectWaiter* w = Atomic::load(&_entry_list);
1515 if (w != nullptr) {
1516 // Other threads are blocked trying to acquire the lock and
1517 // there is no successor, so it appears that an heir-
1518 // presumptive (successor) must be made ready. Since threads
1519 // are woken up in FIFO order, we need to find the tail of the
1520 // entry_list.
1521 w = entry_list_tail(current);
1522 // I'd like to write: guarantee (w->_thread != current).
1523 // But in practice an exiting thread may find itself on the entry_list.
1524 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1525 // then calls exit(). Exit release the lock by setting O._owner to null.
1526 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1527 // notify() operation moves T1 from O's waitset to O's entry_list. T2 then
1528 // release the lock "O". T1 resumes immediately after the ST of null into
1529 // _owner, above. T1 notices that the entry_list is populated, so it
1530 // reacquires the lock and then finds itself on the entry_list.
1531 // Given all that, we have to tolerate the circumstance where "w" is
1532 // associated with current.
1533 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1534 exit_epilog(current, w);
1535 return;
1536 }
1537 }
1538
1539 // Drop the lock.
1540 // release semantics: prior loads and stores from within the critical section
1541 // must not float (reorder) past the following store that drops the lock.
1542 // Uses a storeload to separate release_store(owner) from the
1543 // successor check. The try_set_owner_from() below uses cmpxchg() so
1544 // we get the fence down there.
1545 release_clear_owner(current);
1546 OrderAccess::storeload();
1547
1548 // Normally the exiting thread is responsible for ensuring succession,
1549 // but if this thread observes other successors are ready or other
1550 // entering threads are spinning after it has stored null into _owner
1551 // then it can exit without waking a successor. The existence of
1552 // spinners or ready successors guarantees proper succession (liveness).
1553 // Responsibility passes to the ready or running successors. The exiting
1554 // thread delegates the duty. More precisely, if a successor already
1555 // exists this thread is absolved of the responsibility of waking
1556 // (unparking) one.
1557
1558 // The _succ variable is critical to reducing futile wakeup frequency.
1559 // _succ identifies the "heir presumptive" thread that has been made
1560 // ready (unparked) but that has not yet run. We need only one such
1561 // successor thread to guarantee progress.
1562 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1563 // section 3.3 "Futile Wakeup Throttling" for details.
1564 //
1565 // Note that spinners in Enter() also set _succ non-null.
1566 // In the current implementation spinners opportunistically set
1567 // _succ so that exiting threads might avoid waking a successor.
1568 // Which means that the exiting thread could exit immediately without
1569 // waking a successor, if it observes a successor after it has dropped
1570 // the lock. Note that the dropped lock needs to become visible to the
1571 // spinner.
1572
1573 if (_entry_list == nullptr || has_successor()) {
1574 return;
1575 }
1576
1577 // Only the current lock owner can manipulate the entry_list
1578 // (except for pushing new threads to the head), therefore we need
1579 // to reacquire the lock. If we fail to reacquire the lock the
1580 // responsibility for ensuring succession falls to the new owner.
1581
1582 if (try_lock(current) != TryLockResult::Success) {
1583 // Some other thread acquired the lock (or the monitor was
1584 // deflated). Either way we are done.
1585 return;
1586 }
1587
1588 guarantee(has_owner(current), "invariant");
1589 }
1590 }
1591
1592 void ObjectMonitor::exit_epilog(JavaThread* current, ObjectWaiter* Wakee) {
1593 assert(has_owner(current), "invariant");
1594
1595 // Exit protocol:
1596 // 1. ST _succ = wakee
1597 // 2. membar #loadstore|#storestore;
1598 // 2. ST _owner = nullptr
1599 // 3. unpark(wakee)
1600
1601 oop vthread = nullptr;
1602 ParkEvent * Trigger;
1603 if (!Wakee->is_vthread()) {
1604 JavaThread* t = Wakee->thread();
1605 assert(t != nullptr, "");
1606 Trigger = t->_ParkEvent;
1607 set_successor(t);
1608 } else {
1609 vthread = Wakee->vthread();
1610 assert(vthread != nullptr, "");
1611 Trigger = ObjectMonitor::vthread_unparker_ParkEvent();
1612 set_successor(vthread);
1613 }
1614
1615 // Hygiene -- once we've set _owner = nullptr we can't safely dereference Wakee again.
1616 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1617 // out-of-scope (non-extant).
1618 Wakee = nullptr;
1619
1620 // Drop the lock.
1621 // Uses a fence to separate release_store(owner) from the LD in unpark().
1622 release_clear_owner(current);
1623 OrderAccess::fence();
1624
1625 DTRACE_MONITOR_PROBE(contended__exit, this, object(), current);
1626
1627 if (vthread == nullptr) {
1628 // Platform thread case.
1629 Trigger->unpark();
1630 } else if (java_lang_VirtualThread::set_onWaitingList(vthread, vthread_list_head())) {
1631 // Virtual thread case.
1632 Trigger->unpark();
1633 }
1634 }
1635
1636 // Exits the monitor returning recursion count. _owner should
1637 // be set to current's owner_id, i.e. no ANONYMOUS_OWNER allowed.
1638 intx ObjectMonitor::complete_exit(JavaThread* current) {
1639 assert(InitDone, "Unexpectedly not initialized");
1640 guarantee(has_owner(current), "complete_exit not owner");
1641
1642 intx save = _recursions; // record the old recursion count
1643 _recursions = 0; // set the recursion level to be 0
1644 exit(current); // exit the monitor
1645 guarantee(!has_owner(current), "invariant");
1646 return save;
1647 }
1648
1649 // Checks that the current THREAD owns this monitor and causes an
1650 // immediate return if it doesn't. We don't use the CHECK macro
1651 // because we want the IMSE to be the only exception that is thrown
1652 // from the call site when false is returned. Any other pending
1653 // exception is ignored.
1654 #define CHECK_OWNER() \
1655 do { \
1656 if (!check_owner(THREAD)) { \
1657 assert(HAS_PENDING_EXCEPTION, "expected a pending IMSE here."); \
1658 return; \
1659 } \
1660 } while (false)
1661
1662 // Returns true if the specified thread owns the ObjectMonitor.
1663 // Otherwise returns false and throws IllegalMonitorStateException
1664 // (IMSE). If there is a pending exception and the specified thread
1665 // is not the owner, that exception will be replaced by the IMSE.
1666 bool ObjectMonitor::check_owner(TRAPS) {
1667 JavaThread* current = THREAD;
1668 int64_t cur = owner_raw();
1669 if (cur == owner_id_from(current)) {
1670 return true;
1671 }
1672 THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(),
1673 "current thread is not owner", false);
1674 }
1675
1676 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1677 ObjectMonitor* monitor,
1678 uint64_t notifier_tid,
1679 jlong timeout,
1680 bool timedout) {
1681 assert(event != nullptr, "invariant");
1682 assert(monitor != nullptr, "invariant");
1683 const Klass* monitor_klass = monitor->object()->klass();
1684 if (ObjectMonitor::is_jfr_excluded(monitor_klass)) {
1685 return;
1686 }
1687 event->set_monitorClass(monitor_klass);
1688 event->set_timeout(timeout);
1689 // Set an address that is 'unique enough', such that events close in
1690 // time and with the same address are likely (but not guaranteed) to
1691 // belong to the same object.
1692 event->set_address((uintptr_t)monitor);
1693 event->set_notifier(notifier_tid);
1694 event->set_timedOut(timedout);
1695 event->commit();
1696 }
1697
1698 static void vthread_monitor_waited_event(JavaThread* current, ObjectWaiter* node, ContinuationWrapper& cont, EventJavaMonitorWait* event, jboolean timed_out) {
1699 // Since we might safepoint set the anchor so that the stack can we walked.
1700 assert(current->last_continuation() != nullptr, "");
1701 JavaFrameAnchor* anchor = current->frame_anchor();
1702 anchor->set_last_Java_sp(current->last_continuation()->entry_sp());
1703 anchor->set_last_Java_pc(current->last_continuation()->entry_pc());
1704
1705 ContinuationWrapper::SafepointOp so(current, cont);
1706
1707 JRT_BLOCK
1708 if (event->should_commit()) {
1709 long timeout = java_lang_VirtualThread::timeout(current->vthread());
1710 post_monitor_wait_event(event, node->_monitor, node->_notifier_tid, timeout, timed_out);
1711 }
1712 if (JvmtiExport::should_post_monitor_waited()) {
1713 // We mark this call in case of an upcall to Java while posting the event.
1714 // If somebody walks the stack in that case, processing the enterSpecial
1715 // frame should not include processing callee arguments since there is no
1716 // actual callee (see nmethod::preserve_callee_argument_oops()).
1717 ThreadOnMonitorWaitedEvent tmwe(current);
1718 JvmtiExport::vthread_post_monitor_waited(current, node->_monitor, timed_out);
1719 }
1720 JRT_BLOCK_END
1721 current->frame_anchor()->clear();
1722 }
1723
1724 // -----------------------------------------------------------------------------
1725 // Wait/Notify/NotifyAll
1726 //
1727 // Note: a subset of changes to ObjectMonitor::wait()
1728 // will need to be replicated in complete_exit
1729 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1730 JavaThread* current = THREAD;
1731
1732 assert(InitDone, "Unexpectedly not initialized");
1733
1734 CHECK_OWNER(); // Throws IMSE if not owner.
1735
1736 EventJavaMonitorWait wait_event;
1737 EventVirtualThreadPinned vthread_pinned_event;
1738
1739 // check for a pending interrupt
1740 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1741 JavaThreadInObjectWaitState jtiows(current, millis != 0, interruptible);
1742
1743 if (JvmtiExport::should_post_monitor_wait()) {
1744 JvmtiExport::post_monitor_wait(current, object(), millis);
1745 }
1746 // post monitor waited event. Note that this is past-tense, we are done waiting.
1747 if (JvmtiExport::should_post_monitor_waited()) {
1748 // Note: 'false' parameter is passed here because the
1749 // wait was not timed out due to thread interrupt.
1750 JvmtiExport::post_monitor_waited(current, this, false);
1751
1752 // In this short circuit of the monitor wait protocol, the
1753 // current thread never drops ownership of the monitor and
1754 // never gets added to the wait queue so the current thread
1755 // cannot be made the successor. This means that the
1756 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1757 // consume an unpark() meant for the ParkEvent associated with
1758 // this ObjectMonitor.
1759 }
1760 if (wait_event.should_commit()) {
1761 post_monitor_wait_event(&wait_event, this, 0, millis, false);
1762 }
1763 THROW(vmSymbols::java_lang_InterruptedException());
1764 return;
1765 }
1766
1767 freeze_result result;
1768 ContinuationEntry* ce = current->last_continuation();
1769 bool is_virtual = ce != nullptr && ce->is_virtual_thread();
1770 if (is_virtual) {
1771 if (interruptible && JvmtiExport::should_post_monitor_wait()) {
1772 JvmtiExport::post_monitor_wait(current, object(), millis);
1773 }
1774 current->set_current_waiting_monitor(this);
1775 result = Continuation::try_preempt(current, ce->cont_oop(current));
1776 if (result == freeze_ok) {
1777 vthread_wait(current, millis);
1778 current->set_current_waiting_monitor(nullptr);
1779 return;
1780 }
1781 }
1782 // The jtiows does nothing for non-interruptible.
1783 JavaThreadInObjectWaitState jtiows(current, millis != 0, interruptible);
1784
1785 if (!is_virtual) { // it was already set for virtual thread
1786 if (interruptible && JvmtiExport::should_post_monitor_wait()) {
1787 JvmtiExport::post_monitor_wait(current, object(), millis);
1788
1789 // The current thread already owns the monitor and it has not yet
1790 // been added to the wait queue so the current thread cannot be
1791 // made the successor. This means that the JVMTI_EVENT_MONITOR_WAIT
1792 // event handler cannot accidentally consume an unpark() meant for
1793 // the ParkEvent associated with this ObjectMonitor.
1794 }
1795 current->set_current_waiting_monitor(this);
1796 }
1797 // create a node to be put into the queue
1798 // Critically, after we reset() the event but prior to park(), we must check
1799 // for a pending interrupt.
1800 ObjectWaiter node(current);
1801 node.TState = ObjectWaiter::TS_WAIT;
1802 current->_ParkEvent->reset();
1803 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1804
1805 // Enter the waiting queue, which is a circular doubly linked list in this case
1806 // but it could be a priority queue or any data structure.
1807 // _wait_set_lock protects the wait queue. Normally the wait queue is accessed only
1808 // by the owner of the monitor *except* in the case where park()
1809 // returns because of a timeout of interrupt. Contention is exceptionally rare
1810 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1811
1812 Thread::SpinAcquire(&_wait_set_lock);
1813 add_waiter(&node);
1814 Thread::SpinRelease(&_wait_set_lock);
1815
1816 intx save = _recursions; // record the old recursion count
1817 _waiters++; // increment the number of waiters
1818 _recursions = 0; // set the recursion level to be 1
1819 exit(current); // exit the monitor
1820 guarantee(!has_owner(current), "invariant");
1821
1822 // The thread is on the wait_set list - now park() it.
1823 // On MP systems it's conceivable that a brief spin before we park
1824 // could be profitable.
1825 //
1826 // TODO-FIXME: change the following logic to a loop of the form
1827 // while (!timeout && !interrupted && _notified == 0) park()
1828
1829 int ret = OS_OK;
1830 int WasNotified = 0;
1831
1832 // Need to check interrupt state whilst still _thread_in_vm
1833 bool interrupted = interruptible && current->is_interrupted(false);
1834
1835 { // State transition wrappers
1836 OSThread* osthread = current->osthread();
1837 OSThreadWaitState osts(osthread, true);
1838
1839 assert(current->thread_state() == _thread_in_vm, "invariant");
1840
1841 {
1842 ClearSuccOnSuspend csos(this);
1843 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */);
1844 if (interrupted || HAS_PENDING_EXCEPTION) {
1845 // Intentionally empty
1846 } else if (!node._notified) {
1847 if (millis <= 0) {
1848 current->_ParkEvent->park();
1849 } else {
1850 ret = current->_ParkEvent->park(millis);
1851 }
1852 }
1853 }
1854
1855 // Node may be on the wait_set, or on the entry_list, or in transition
1856 // from the wait_set to the entry_list.
1857 // See if we need to remove Node from the wait_set.
1858 // We use double-checked locking to avoid grabbing _wait_set_lock
1859 // if the thread is not on the wait queue.
1860 //
1861 // Note that we don't need a fence before the fetch of TState.
1862 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1863 // written by the is thread. (perhaps the fetch might even be satisfied
1864 // by a look-aside into the processor's own store buffer, although given
1865 // the length of the code path between the prior ST and this load that's
1866 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1867 // then we'll acquire the lock and then re-fetch a fresh TState value.
1868 // That is, we fail toward safety.
1869
1870 if (node.TState == ObjectWaiter::TS_WAIT) {
1871 Thread::SpinAcquire(&_wait_set_lock);
1872 if (node.TState == ObjectWaiter::TS_WAIT) {
1873 dequeue_specific_waiter(&node); // unlink from wait_set
1874 assert(!node._notified, "invariant");
1875 node.TState = ObjectWaiter::TS_RUN;
1876 }
1877 Thread::SpinRelease(&_wait_set_lock);
1878 }
1879
1880 // The thread is now either on off-list (TS_RUN),
1881 // or on the entry_list (TS_ENTER).
1882 // The Node's TState variable is stable from the perspective of this thread.
1883 // No other threads will asynchronously modify TState.
1884 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1885 OrderAccess::loadload();
1886 if (has_successor(current)) clear_successor();
1887 WasNotified = node._notified;
1888
1889 // Reentry phase -- reacquire the monitor.
1890 // re-enter contended monitor after object.wait().
1891 // retain OBJECT_WAIT state until re-enter successfully completes
1892 // Thread state is thread_in_vm and oop access is again safe,
1893 // although the raw address of the object may have changed.
1894 // (Don't cache naked oops over safepoints, of course).
1895
1896 // post monitor waited event. Note that this is past-tense, we are done waiting.
1897 if (JvmtiExport::should_post_monitor_waited()) {
1898 JvmtiExport::post_monitor_waited(current, this, ret == OS_TIMEOUT);
1899
1900 if (node._notified && has_successor(current)) {
1901 // In this part of the monitor wait-notify-reenter protocol it
1902 // is possible (and normal) for another thread to do a fastpath
1903 // monitor enter-exit while this thread is still trying to get
1904 // to the reenter portion of the protocol.
1905 //
1906 // The ObjectMonitor was notified and the current thread is
1907 // the successor which also means that an unpark() has already
1908 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1909 // consume the unpark() that was done when the successor was
1910 // set because the same ParkEvent is shared between Java
1911 // monitors and JVM/TI RawMonitors (for now).
1912 //
1913 // We redo the unpark() to ensure forward progress, i.e., we
1914 // don't want all pending threads hanging (parked) with none
1915 // entering the unlocked monitor.
1916 current->_ParkEvent->unpark();
1917 }
1918 }
1919
1920 if (wait_event.should_commit()) {
1921 post_monitor_wait_event(&wait_event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1922 }
1923
1924 OrderAccess::fence();
1925
1926 assert(!has_owner(current), "invariant");
1927 ObjectWaiter::TStates v = node.TState;
1928 if (v == ObjectWaiter::TS_RUN) {
1929 // We use the NoPreemptMark for the very rare case where the previous
1930 // preempt attempt failed due to OOM. The preempt on monitor contention
1931 // could succeed but we can't unmount now.
1932 NoPreemptMark npm(current);
1933 enter(current);
1934 } else {
1935 guarantee(v == ObjectWaiter::TS_ENTER, "invariant");
1936 reenter_internal(current, &node);
1937 node.wait_reenter_end(this);
1938 }
1939
1940 // current has reacquired the lock.
1941 // Lifecycle - the node representing current must not appear on any queues.
1942 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1943 // want residual elements associated with this thread left on any lists.
1944 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1945 assert(has_owner(current), "invariant");
1946 assert(!has_successor(current), "invariant");
1947 } // OSThreadWaitState()
1948
1949 current->set_current_waiting_monitor(nullptr);
1950
1951 guarantee(_recursions == 0, "invariant");
1952 int relock_count = JvmtiDeferredUpdates::get_and_reset_relock_count_after_wait(current);
1953 _recursions = save // restore the old recursion count
1954 + relock_count; // increased by the deferred relock count
1955 current->inc_held_monitor_count(relock_count); // Deopt never entered these counts.
1956 _waiters--; // decrement the number of waiters
1957
1958 // Verify a few postconditions
1959 assert(has_owner(current), "invariant");
1960 assert(!has_successor(current), "invariant");
1961 assert_mark_word_consistency();
1962
1963 if (ce != nullptr && ce->is_virtual_thread()) {
1964 current->post_vthread_pinned_event(&vthread_pinned_event, "Object.wait", result);
1965 }
1966
1967 // check if the notification happened
1968 if (!WasNotified) {
1969 // no, it could be timeout or Thread.interrupt() or both
1970 // check for interrupt event, otherwise it is timeout
1971 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1972 THROW(vmSymbols::java_lang_InterruptedException());
1973 }
1974 }
1975
1976 // NOTE: Spurious wake up will be consider as timeout.
1977 // Monitor notify has precedence over thread interrupt.
1978 }
1979
1980 // Consider:
1981 // If the lock is cool (entry_list == null && succ == null) and we're on an MP system
1982 // then instead of transferring a thread from the wait_set to the entry_list
1983 // we might just dequeue a thread from the wait_set and directly unpark() it.
1984
1985 bool ObjectMonitor::notify_internal(JavaThread* current) {
1986 bool did_notify = false;
1987 Thread::SpinAcquire(&_wait_set_lock);
1988 ObjectWaiter* iterator = dequeue_waiter();
1989 if (iterator != nullptr) {
1990 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1991 guarantee(!iterator->_notified, "invariant");
1992
1993 if (iterator->is_vthread()) {
1994 oop vthread = iterator->vthread();
1995 java_lang_VirtualThread::set_notified(vthread, true);
1996 int old_state = java_lang_VirtualThread::state(vthread);
1997 // If state is not WAIT/TIMED_WAIT then target could still be on
1998 // unmount transition, or wait could have already timed-out or target
1999 // could have been interrupted. In the first case, the target itself
2000 // will set the state to BLOCKED at the end of the unmount transition.
2001 // In the other cases the target would have been already unblocked so
2002 // there is nothing to do.
2003 if (old_state == java_lang_VirtualThread::WAIT ||
2004 old_state == java_lang_VirtualThread::TIMED_WAIT) {
2005 java_lang_VirtualThread::cmpxchg_state(vthread, old_state, java_lang_VirtualThread::BLOCKED);
2006 }
2007 }
2008
2009 iterator->_notified = true;
2010 iterator->_notifier_tid = JFR_THREAD_ID(current);
2011 did_notify = true;
2012 add_to_entry_list(current, iterator);
2013
2014 // _wait_set_lock protects the wait queue, not the entry_list. We could
2015 // move the add-to-entry_list operation, above, outside the critical section
2016 // protected by _wait_set_lock. In practice that's not useful. With the
2017 // exception of wait() timeouts and interrupts the monitor owner
2018 // is the only thread that grabs _wait_set_lock. There's almost no contention
2019 // on _wait_set_lock so it's not profitable to reduce the length of the
2020 // critical section.
2021
2022 if (!iterator->is_vthread()) {
2023 iterator->wait_reenter_begin(this);
2024 }
2025 }
2026 Thread::SpinRelease(&_wait_set_lock);
2027 return did_notify;
2028 }
2029
2030 static void post_monitor_notify_event(EventJavaMonitorNotify* event,
2031 ObjectMonitor* monitor,
2032 int notified_count) {
2033 assert(event != nullptr, "invariant");
2034 assert(monitor != nullptr, "invariant");
2035 const Klass* monitor_klass = monitor->object()->klass();
2036 if (ObjectMonitor::is_jfr_excluded(monitor_klass)) {
2037 return;
2038 }
2039 event->set_monitorClass(monitor_klass);
2040 // Set an address that is 'unique enough', such that events close in
2041 // time and with the same address are likely (but not guaranteed) to
2042 // belong to the same object.
2043 event->set_address((uintptr_t)monitor);
2044 event->set_notifiedCount(notified_count);
2045 event->commit();
2046 }
2047
2048 // Consider: a not-uncommon synchronization bug is to use notify() when
2049 // notifyAll() is more appropriate, potentially resulting in stranded
2050 // threads; this is one example of a lost wakeup. A useful diagnostic
2051 // option is to force all notify() operations to behave as notifyAll().
2052 //
2053 // Note: We can also detect many such problems with a "minimum wait".
2054 // When the "minimum wait" is set to a small non-zero timeout value
2055 // and the program does not hang whereas it did absent "minimum wait",
2056 // that suggests a lost wakeup bug.
2057
2058 void ObjectMonitor::notify(TRAPS) {
2059 JavaThread* current = THREAD;
2060 CHECK_OWNER(); // Throws IMSE if not owner.
2061 if (_wait_set == nullptr) {
2062 return;
2063 }
2064
2065 quick_notify(current);
2066 }
2067
2068 void ObjectMonitor::quick_notify(JavaThread* current) {
2069 assert(has_owner(current), "Precondition");
2070
2071 EventJavaMonitorNotify event;
2072 DTRACE_MONITOR_PROBE(notify, this, object(), current);
2073 int tally = notify_internal(current) ? 1 : 0;
2074
2075 if ((tally > 0) && event.should_commit()) {
2076 post_monitor_notify_event(&event, this, /* notified_count = */ tally);
2077 }
2078 }
2079
2080 // notifyAll() transfers the waiters one-at-a-time from the waitset to
2081 // the entry_list. If the waitset is "ABCD" (where A was added first
2082 // and D last) and the entry_list is ->X->Y->Z. After a notifyAll()
2083 // the waitset will be empty and the entry_list will be
2084 // ->D->C->B->A->X->Y->Z, and the next choosen successor will be Z.
2085
2086 void ObjectMonitor::notifyAll(TRAPS) {
2087 JavaThread* current = THREAD;
2088 CHECK_OWNER(); // Throws IMSE if not owner.
2089 if (_wait_set == nullptr) {
2090 return;
2091 }
2092
2093 quick_notifyAll(current);
2094 }
2095
2096 void ObjectMonitor::quick_notifyAll(JavaThread* current) {
2097 assert(has_owner(current), "Precondition");
2098
2099 EventJavaMonitorNotify event;
2100 DTRACE_MONITOR_PROBE(notifyAll, this, object(), current);
2101 int tally = 0;
2102 while (_wait_set != nullptr) {
2103 if (notify_internal(current)) {
2104 tally++;
2105 }
2106 }
2107
2108 if ((tally > 0) && event.should_commit()) {
2109 post_monitor_notify_event(&event, this, /* notified_count = */ tally);
2110 }
2111 }
2112
2113 void ObjectMonitor::vthread_wait(JavaThread* current, jlong millis) {
2114 oop vthread = current->vthread();
2115 ObjectWaiter* node = new ObjectWaiter(vthread, this);
2116 node->_is_wait = true;
2117 node->TState = ObjectWaiter::TS_WAIT;
2118 java_lang_VirtualThread::set_notified(vthread, false); // Reset notified flag
2119
2120 // Enter the waiting queue, which is a circular doubly linked list in this case
2121 // but it could be a priority queue or any data structure.
2122 // _wait_set_lock protects the wait queue. Normally the wait queue is accessed only
2123 // by the owner of the monitor *except* in the case where park()
2124 // returns because of a timeout or interrupt. Contention is exceptionally rare
2125 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
2126
2127 Thread::SpinAcquire(&_wait_set_lock);
2128 add_waiter(node);
2129 Thread::SpinRelease(&_wait_set_lock);
2130
2131 node->_recursions = _recursions; // record the old recursion count
2132 _recursions = 0; // set the recursion level to be 0
2133 _waiters++; // increment the number of waiters
2134 exit(current); // exit the monitor
2135 guarantee(!has_owner(current), "invariant");
2136
2137 assert(java_lang_VirtualThread::state(vthread) == java_lang_VirtualThread::RUNNING, "wrong state for vthread");
2138 java_lang_VirtualThread::set_state(vthread, millis == 0 ? java_lang_VirtualThread::WAITING : java_lang_VirtualThread::TIMED_WAITING);
2139 java_lang_VirtualThread::set_timeout(vthread, millis);
2140
2141 // Save the ObjectWaiter* in the vthread since we will need it when resuming execution.
2142 java_lang_VirtualThread::set_objectWaiter(vthread, node);
2143 }
2144
2145 bool ObjectMonitor::vthread_wait_reenter(JavaThread* current, ObjectWaiter* node, ContinuationWrapper& cont) {
2146 // The first time we run after being preempted on Object.wait() we
2147 // need to check if we were interrupted or the wait timed-out, and
2148 // in that case remove ourselves from the _wait_set queue.
2149 if (node->TState == ObjectWaiter::TS_WAIT) {
2150 Thread::SpinAcquire(&_wait_set_lock);
2151 if (node->TState == ObjectWaiter::TS_WAIT) {
2152 dequeue_specific_waiter(node); // unlink from wait_set
2153 assert(!node->_notified, "invariant");
2154 node->TState = ObjectWaiter::TS_RUN;
2155 }
2156 Thread::SpinRelease(&_wait_set_lock);
2157 }
2158
2159 // If this was an interrupted case, set the _interrupted boolean so that
2160 // once we re-acquire the monitor we know if we need to throw IE or not.
2161 ObjectWaiter::TStates state = node->TState;
2162 bool was_notified = state == ObjectWaiter::TS_ENTER;
2163 assert(was_notified || state == ObjectWaiter::TS_RUN, "");
2164 node->_interrupted = !was_notified && current->is_interrupted(false);
2165
2166 // Post JFR and JVMTI events.
2167 EventJavaMonitorWait wait_event;
2168 if (wait_event.should_commit() || JvmtiExport::should_post_monitor_waited()) {
2169 vthread_monitor_waited_event(current, node, cont, &wait_event, !was_notified && !node->_interrupted);
2170 }
2171
2172 // Mark that we are at reenter so that we don't call this method again.
2173 node->_at_reenter = true;
2174
2175 if (!was_notified) {
2176 bool acquired = vthread_monitor_enter(current, node);
2177 if (acquired) {
2178 guarantee(_recursions == 0, "invariant");
2179 _recursions = node->_recursions; // restore the old recursion count
2180 _waiters--; // decrement the number of waiters
2181
2182 if (node->_interrupted) {
2183 // We will throw at thaw end after finishing the mount transition.
2184 current->set_pending_interrupted_exception(true);
2185 }
2186
2187 delete node;
2188 // Clear the ObjectWaiter* from the vthread.
2189 java_lang_VirtualThread::set_objectWaiter(current->vthread(), nullptr);
2190 return true;
2191 }
2192 } else {
2193 // Already moved to _entry_list by notifier, so just add to contentions.
2194 add_to_contentions(1);
2195 }
2196 return false;
2197 }
2198
2199 // -----------------------------------------------------------------------------
2200 // Adaptive Spinning Support
2201 //
2202 // Adaptive spin-then-block - rational spinning
2203 //
2204 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
2205 // algorithm.
2206 //
2207 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
2208 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
2209 // (duration) or we can fix the count at approximately the duration of
2210 // a context switch and vary the frequency. Of course we could also
2211 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
2212 // For a description of 'Adaptive spin-then-block mutual exclusion in
2213 // multi-threaded processing,' see U.S. Pat. No. 8046758.
2214 //
2215 // This implementation varies the duration "D", where D varies with
2216 // the success rate of recent spin attempts. (D is capped at approximately
2217 // length of a round-trip context switch). The success rate for recent
2218 // spin attempts is a good predictor of the success rate of future spin
2219 // attempts. The mechanism adapts automatically to varying critical
2220 // section length (lock modality), system load and degree of parallelism.
2221 // D is maintained per-monitor in _SpinDuration and is initialized
2222 // optimistically. Spin frequency is fixed at 100%.
2223 //
2224 // Note that _SpinDuration is volatile, but we update it without locks
2225 // or atomics. The code is designed so that _SpinDuration stays within
2226 // a reasonable range even in the presence of races. The arithmetic
2227 // operations on _SpinDuration are closed over the domain of legal values,
2228 // so at worst a race will install and older but still legal value.
2229 // At the very worst this introduces some apparent non-determinism.
2230 // We might spin when we shouldn't or vice-versa, but since the spin
2231 // count are relatively short, even in the worst case, the effect is harmless.
2232 //
2233 // Care must be taken that a low "D" value does not become an
2234 // an absorbing state. Transient spinning failures -- when spinning
2235 // is overall profitable -- should not cause the system to converge
2236 // on low "D" values. We want spinning to be stable and predictable
2237 // and fairly responsive to change and at the same time we don't want
2238 // it to oscillate, become metastable, be "too" non-deterministic,
2239 // or converge on or enter undesirable stable absorbing states.
2240 //
2241 // We implement a feedback-based control system -- using past behavior
2242 // to predict future behavior. We face two issues: (a) if the
2243 // input signal is random then the spin predictor won't provide optimal
2244 // results, and (b) if the signal frequency is too high then the control
2245 // system, which has some natural response lag, will "chase" the signal.
2246 // (b) can arise from multimodal lock hold times. Transient preemption
2247 // can also result in apparent bimodal lock hold times.
2248 // Although sub-optimal, neither condition is particularly harmful, as
2249 // in the worst-case we'll spin when we shouldn't or vice-versa.
2250 // The maximum spin duration is rather short so the failure modes aren't bad.
2251 // To be conservative, I've tuned the gain in system to bias toward
2252 // _not spinning. Relatedly, the system can sometimes enter a mode where it
2253 // "rings" or oscillates between spinning and not spinning. This happens
2254 // when spinning is just on the cusp of profitability, however, so the
2255 // situation is not dire. The state is benign -- there's no need to add
2256 // hysteresis control to damp the transition rate between spinning and
2257 // not spinning.
2258
2259 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool
2260
2261 static int Knob_Bonus = 100; // spin success bonus
2262 static int Knob_Penalty = 200; // spin failure penalty
2263 static int Knob_Poverty = 1000;
2264 static int Knob_FixedSpin = 0;
2265 static int Knob_PreSpin = 10; // 20-100 likely better, but it's not better in my testing.
2266
2267 inline static int adjust_up(int spin_duration) {
2268 int x = spin_duration;
2269 if (x < ObjectMonitor::Knob_SpinLimit) {
2270 if (x < Knob_Poverty) {
2271 x = Knob_Poverty;
2272 }
2273 return x + Knob_Bonus;
2274 } else {
2275 return spin_duration;
2276 }
2277 }
2278
2279 inline static int adjust_down(int spin_duration) {
2280 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2281 // AIMD is globally stable.
2282 int x = spin_duration;
2283 if (x > 0) {
2284 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2285 // This is globally sample and tends to damp the response.
2286 x -= Knob_Penalty;
2287 if (x < 0) { x = 0; }
2288 return x;
2289 } else {
2290 return spin_duration;
2291 }
2292 }
2293
2294 bool ObjectMonitor::short_fixed_spin(JavaThread* current, int spin_count, bool adapt) {
2295 for (int ctr = 0; ctr < spin_count; ctr++) {
2296 TryLockResult status = try_lock(current);
2297 if (status == TryLockResult::Success) {
2298 if (adapt) {
2299 _SpinDuration = adjust_up(_SpinDuration);
2300 }
2301 return true;
2302 } else if (status == TryLockResult::Interference) {
2303 break;
2304 }
2305 SpinPause();
2306 }
2307 return false;
2308 }
2309
2310 // Spinning: Fixed frequency (100%), vary duration
2311 bool ObjectMonitor::try_spin(JavaThread* current) {
2312
2313 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
2314 int knob_fixed_spin = Knob_FixedSpin; // 0 (don't spin: default), 2000 good test
2315 if (knob_fixed_spin > 0) {
2316 return short_fixed_spin(current, knob_fixed_spin, false);
2317 }
2318
2319 // Admission control - verify preconditions for spinning
2320 //
2321 // We always spin a little bit, just to prevent _SpinDuration == 0 from
2322 // becoming an absorbing state. Put another way, we spin briefly to
2323 // sample, just in case the system load, parallelism, contention, or lock
2324 // modality changed.
2325
2326 int knob_pre_spin = Knob_PreSpin; // 10 (default), 100, 1000 or 2000
2327 if (short_fixed_spin(current, knob_pre_spin, true)) {
2328 return true;
2329 }
2330
2331 //
2332 // Consider the following alternative:
2333 // Periodically set _SpinDuration = _SpinLimit and try a long/full
2334 // spin attempt. "Periodically" might mean after a tally of
2335 // the # of failed spin attempts (or iterations) reaches some threshold.
2336 // This takes us into the realm of 1-out-of-N spinning, where we
2337 // hold the duration constant but vary the frequency.
2338
2339 int ctr = _SpinDuration;
2340 if (ctr <= 0) return false;
2341
2342 // We're good to spin ... spin ingress.
2343 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
2344 // when preparing to LD...CAS _owner, etc and the CAS is likely
2345 // to succeed.
2346 if (!has_successor()) {
2347 set_successor(current);
2348 }
2349 int64_t prv = NO_OWNER;
2350
2351 // There are three ways to exit the following loop:
2352 // 1. A successful spin where this thread has acquired the lock.
2353 // 2. Spin failure with prejudice
2354 // 3. Spin failure without prejudice
2355
2356 while (--ctr >= 0) {
2357
2358 // Periodic polling -- Check for pending GC
2359 // Threads may spin while they're unsafe.
2360 // We don't want spinning threads to delay the JVM from reaching
2361 // a stop-the-world safepoint or to steal cycles from GC.
2362 // If we detect a pending safepoint we abort in order that
2363 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
2364 // this thread, if safe, doesn't steal cycles from GC.
2365 // This is in keeping with the "no loitering in runtime" rule.
2366 // We periodically check to see if there's a safepoint pending.
2367 if ((ctr & 0xFF) == 0) {
2368 // Can't call SafepointMechanism::should_process() since that
2369 // might update the poll values and we could be in a thread_blocked
2370 // state here which is not allowed so just check the poll.
2371 if (SafepointMechanism::local_poll_armed(current)) {
2372 break;
2373 }
2374 SpinPause();
2375 }
2376
2377 // Probe _owner with TATAS
2378 // If this thread observes the monitor transition or flicker
2379 // from locked to unlocked to locked, then the odds that this
2380 // thread will acquire the lock in this spin attempt go down
2381 // considerably. The same argument applies if the CAS fails
2382 // or if we observe _owner change from one non-null value to
2383 // another non-null value. In such cases we might abort
2384 // the spin without prejudice or apply a "penalty" to the
2385 // spin count-down variable "ctr", reducing it by 100, say.
2386
2387 int64_t ox = owner_raw();
2388 if (ox == NO_OWNER) {
2389 ox = try_set_owner_from(NO_OWNER, current);
2390 if (ox == NO_OWNER) {
2391 // The CAS succeeded -- this thread acquired ownership
2392 // Take care of some bookkeeping to exit spin state.
2393 if (has_successor(current)) {
2394 clear_successor();
2395 }
2396
2397 // Increase _SpinDuration :
2398 // The spin was successful (profitable) so we tend toward
2399 // longer spin attempts in the future.
2400 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2401 // If we acquired the lock early in the spin cycle it
2402 // makes sense to increase _SpinDuration proportionally.
2403 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2404 _SpinDuration = adjust_up(_SpinDuration);
2405 return true;
2406 }
2407
2408 // The CAS failed ... we can take any of the following actions:
2409 // * penalize: ctr -= CASPenalty
2410 // * exit spin with prejudice -- abort without adapting spinner
2411 // * exit spin without prejudice.
2412 // * Since CAS is high-latency, retry again immediately.
2413 break;
2414 }
2415
2416 // Did lock ownership change hands ?
2417 if (ox != prv && prv != NO_OWNER) {
2418 break;
2419 }
2420 prv = ox;
2421
2422 if (!has_successor()) {
2423 set_successor(current);
2424 }
2425 }
2426
2427 // Spin failed with prejudice -- reduce _SpinDuration.
2428 if (ctr < 0) {
2429 _SpinDuration = adjust_down(_SpinDuration);
2430 }
2431
2432 if (has_successor(current)) {
2433 clear_successor();
2434 // Invariant: after setting succ=null a contending thread
2435 // must recheck-retry _owner before parking. This usually happens
2436 // in the normal usage of try_spin(), but it's safest
2437 // to make try_spin() as foolproof as possible.
2438 OrderAccess::fence();
2439 if (try_lock(current) == TryLockResult::Success) {
2440 return true;
2441 }
2442 }
2443
2444 return false;
2445 }
2446
2447
2448 // -----------------------------------------------------------------------------
2449 // wait_set management ...
2450
2451 ObjectWaiter::ObjectWaiter(JavaThread* current) {
2452 _next = nullptr;
2453 _prev = nullptr;
2454 _thread = current;
2455 _monitor = nullptr;
2456 _notifier_tid = 0;
2457 _recursions = 0;
2458 TState = TS_RUN;
2459 _notified = false;
2460 _is_wait = false;
2461 _at_reenter = false;
2462 _interrupted = false;
2463 _active = false;
2464 }
2465
2466 ObjectWaiter::ObjectWaiter(oop vthread, ObjectMonitor* mon) : ObjectWaiter(nullptr) {
2467 assert(oopDesc::is_oop(vthread), "");
2468 _vthread = OopHandle(JavaThread::thread_oop_storage(), vthread);
2469 _monitor = mon;
2470 }
2471
2472 ObjectWaiter::~ObjectWaiter() {
2473 if (is_vthread()) {
2474 assert(vthread() != nullptr, "");
2475 _vthread.release(JavaThread::thread_oop_storage());
2476 }
2477 }
2478
2479 oop ObjectWaiter::vthread() const {
2480 return _vthread.resolve();
2481 }
2482
2483 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2484 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(_thread, mon);
2485 }
2486
2487 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2488 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(_thread, _active);
2489 }
2490
2491 inline void ObjectMonitor::add_waiter(ObjectWaiter* node) {
2492 assert(node != nullptr, "should not add null node");
2493 assert(node->_prev == nullptr, "node already in list");
2494 assert(node->_next == nullptr, "node already in list");
2495 // put node at end of queue (circular doubly linked list)
2496 if (_wait_set == nullptr) {
2497 _wait_set = node;
2498 node->_prev = node;
2499 node->_next = node;
2500 } else {
2501 ObjectWaiter* head = _wait_set;
2502 ObjectWaiter* tail = head->_prev;
2503 assert(tail->_next == head, "invariant check");
2504 tail->_next = node;
2505 head->_prev = node;
2506 node->_next = head;
2507 node->_prev = tail;
2508 }
2509 }
2510
2511 inline ObjectWaiter* ObjectMonitor::dequeue_waiter() {
2512 // dequeue the very first waiter
2513 ObjectWaiter* waiter = _wait_set;
2514 if (waiter) {
2515 dequeue_specific_waiter(waiter);
2516 }
2517 return waiter;
2518 }
2519
2520 inline void ObjectMonitor::dequeue_specific_waiter(ObjectWaiter* node) {
2521 assert(node != nullptr, "should not dequeue nullptr node");
2522 assert(node->_prev != nullptr, "node already removed from list");
2523 assert(node->_next != nullptr, "node already removed from list");
2524 // when the waiter has woken up because of interrupt,
2525 // timeout or other spurious wake-up, dequeue the
2526 // waiter from waiting list
2527 ObjectWaiter* next = node->_next;
2528 if (next == node) {
2529 assert(node->_prev == node, "invariant check");
2530 _wait_set = nullptr;
2531 } else {
2532 ObjectWaiter* prev = node->_prev;
2533 assert(prev->_next == node, "invariant check");
2534 assert(next->_prev == node, "invariant check");
2535 next->_prev = prev;
2536 prev->_next = next;
2537 if (_wait_set == node) {
2538 _wait_set = next;
2539 }
2540 }
2541 node->_next = nullptr;
2542 node->_prev = nullptr;
2543 }
2544
2545 // -----------------------------------------------------------------------------
2546
2547 // One-shot global initialization for the sync subsystem.
2548 // We could also defer initialization and initialize on-demand
2549 // the first time we call ObjectSynchronizer::inflate().
2550 // Initialization would be protected - like so many things - by
2551 // the MonitorCache_lock.
2552
2553 void ObjectMonitor::Initialize() {
2554 assert(!InitDone, "invariant");
2555
2556 if (!os::is_MP()) {
2557 Knob_SpinLimit = 0;
2558 Knob_PreSpin = 0;
2559 Knob_FixedSpin = -1;
2560 }
2561
2562 _oop_storage = OopStorageSet::create_weak("ObjectSynchronizer Weak", mtSynchronizer);
2563
2564 DEBUG_ONLY(InitDone = true;)
2565 }
2566
2567 // We can't call this during Initialize() because BarrierSet needs to be set.
2568 void ObjectMonitor::Initialize2() {
2569 _vthread_list_head = OopHandle(JavaThread::thread_oop_storage(), nullptr);
2570 _vthread_unparker_ParkEvent = ParkEvent::Allocate(nullptr);
2571 }
2572
2573 void ObjectMonitor::print_on(outputStream* st) const {
2574 // The minimal things to print for markWord printing, more can be added for debugging and logging.
2575 st->print("{contentions=0x%08x,waiters=0x%08x"
2576 ",recursions=%zd,owner=" INT64_FORMAT "}",
2577 contentions(), waiters(), recursions(),
2578 owner_raw());
2579 }
2580 void ObjectMonitor::print() const { print_on(tty); }
2581
2582 #ifdef ASSERT
2583 // Print the ObjectMonitor like a debugger would:
2584 //
2585 // (ObjectMonitor) 0x00007fdfb6012e40 = {
2586 // _metadata = 0x0000000000000001
2587 // _object = 0x000000070ff45fd0
2588 // _pad_buf0 = {
2589 // [0] = '\0'
2590 // ...
2591 // [43] = '\0'
2592 // }
2593 // _owner = 0x0000000000000000
2594 // _previous_owner_tid = 0
2595 // _pad_buf1 = {
2596 // [0] = '\0'
2597 // ...
2598 // [47] = '\0'
2599 // }
2600 // _next_om = 0x0000000000000000
2601 // _recursions = 0
2602 // _entry_list = 0x0000000000000000
2603 // _entry_list_tail = 0x0000000000000000
2604 // _succ = 0x0000000000000000
2605 // _SpinDuration = 5000
2606 // _contentions = 0
2607 // _wait_set = 0x0000700009756248
2608 // _waiters = 1
2609 // _wait_set_lock = 0
2610 // }
2611 //
2612 void ObjectMonitor::print_debug_style_on(outputStream* st) const {
2613 st->print_cr("(ObjectMonitor*) " INTPTR_FORMAT " = {", p2i(this));
2614 st->print_cr(" _metadata = " INTPTR_FORMAT, _metadata);
2615 st->print_cr(" _object = " INTPTR_FORMAT, p2i(object_peek()));
2616 st->print_cr(" _pad_buf0 = {");
2617 st->print_cr(" [0] = '\\0'");
2618 st->print_cr(" ...");
2619 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf0) - 1);
2620 st->print_cr(" }");
2621 st->print_cr(" _owner = " INT64_FORMAT, owner_raw());
2622 st->print_cr(" _previous_owner_tid = " UINT64_FORMAT, _previous_owner_tid);
2623 st->print_cr(" _pad_buf1 = {");
2624 st->print_cr(" [0] = '\\0'");
2625 st->print_cr(" ...");
2626 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf1) - 1);
2627 st->print_cr(" }");
2628 st->print_cr(" _next_om = " INTPTR_FORMAT, p2i(next_om()));
2629 st->print_cr(" _recursions = %zd", _recursions);
2630 st->print_cr(" _entry_list = " INTPTR_FORMAT, p2i(_entry_list));
2631 st->print_cr(" _entry_list_tail = " INTPTR_FORMAT, p2i(_entry_list_tail));
2632 st->print_cr(" _succ = " INT64_FORMAT, successor());
2633 st->print_cr(" _SpinDuration = %d", _SpinDuration);
2634 st->print_cr(" _contentions = %d", contentions());
2635 st->print_cr(" _wait_set = " INTPTR_FORMAT, p2i(_wait_set));
2636 st->print_cr(" _waiters = %d", _waiters);
2637 st->print_cr(" _wait_set_lock = %d", _wait_set_lock);
2638 st->print_cr("}");
2639 }
2640 #endif