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/atomicAccess.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.inline.hpp"
49 #include "runtime/orderAccess.hpp"
50 #include "runtime/osThread.hpp"
51 #include "runtime/safefetch.hpp"
52 #include "runtime/safepointMechanism.inline.hpp"
53 #include "runtime/sharedRuntime.hpp"
54 #include "runtime/threads.hpp"
55 #include "services/threadService.hpp"
56 #include "utilities/debug.hpp"
57 #include "utilities/dtrace.hpp"
58 #include "utilities/globalCounter.inline.hpp"
59 #include "utilities/globalDefinitions.hpp"
60 #include "utilities/macros.hpp"
61 #include "utilities/preserveException.hpp"
62 #if INCLUDE_JFR
63 #include "jfr/support/jfrFlush.hpp"
64 #endif
65
66 #ifdef DTRACE_ENABLED
67
68 // Only bother with this argument setup if dtrace is available
69 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
70
71
72 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
73 char* bytes = nullptr; \
74 int len = 0; \
75 jlong jtid = SharedRuntime::get_java_tid(thread); \
76 Symbol* klassname = obj->klass()->name(); \
77 if (klassname != nullptr) { \
78 bytes = (char*)klassname->bytes(); \
79 len = klassname->utf8_length(); \
80 }
81
82 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
83 { \
84 if (DTraceMonitorProbes) { \
85 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
86 HOTSPOT_MONITOR_WAIT(jtid, \
87 (monitor), bytes, len, (millis)); \
88 } \
89 }
90
91 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
92 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
93 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
94 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
95 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
96
97 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
98 { \
99 if (DTraceMonitorProbes) { \
100 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
101 HOTSPOT_MONITOR_##probe(jtid, \
102 (uintptr_t)(monitor), bytes, len); \
103 } \
104 }
105
106 #else // ndef DTRACE_ENABLED
107
108 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
109 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
110
111 #endif // ndef DTRACE_ENABLED
112
113 DEBUG_ONLY(static volatile bool InitDone = false;)
114
115 OopStorage* ObjectMonitor::_oop_storage = nullptr;
116
117 OopHandle ObjectMonitor::_vthread_list_head;
118 ParkEvent* ObjectMonitor::_vthread_unparker_ParkEvent = nullptr;
119
120 static const jlong MAX_RECHECK_INTERVAL = 1000;
121
122 // -----------------------------------------------------------------------------
123 // Theory of operations -- Monitors lists, thread residency, etc:
124 //
125 // * A thread acquires ownership of a monitor by successfully
126 // CAS()ing the _owner field from NO_OWNER/DEFLATER_MARKER to
127 // its owner_id (return value from owner_id_from()).
128 //
129 // * Invariant: A thread appears on at most one monitor list --
130 // entry_list or wait_set -- at any one time.
131 //
132 // * Contending threads "push" themselves onto the entry_list with CAS
133 // and then spin/park.
134 // If the thread is a virtual thread it will first attempt to
135 // unmount itself. The virtual thread will first try to freeze
136 // all frames in the heap. If the operation fails it will just
137 // follow the regular path for platform threads. If the operation
138 // succeeds, it will push itself onto the entry_list with CAS and then
139 // return back to Java to continue the unmount logic.
140 //
141 // * After a contending thread eventually acquires the lock it must
142 // dequeue itself from the entry_list.
143 //
144 // * The exiting thread identifies and unparks an "heir presumptive"
145 // tentative successor thread on the entry_list. In case the successor
146 // is an unmounted virtual thread, the exiting thread will first try
147 // to add it to the list of vthreads waiting to be unblocked, and on
148 // success it will unpark the special unblocker thread instead, which
149 // will be in charge of submitting the vthread back to the scheduler
150 // queue. Critically, the exiting thread doesn't unlink the successor
151 // thread from the entry_list. After having been unparked/re-scheduled,
152 // the wakee will recontend for ownership of the monitor. The successor
153 // (wakee) will either acquire the lock or re-park/unmount itself.
154 //
155 // Succession is provided for by a policy of competitive handoff.
156 // The exiting thread does _not_ grant or pass ownership to the
157 // successor thread. (This is also referred to as "handoff succession").
158 // Instead the exiting thread releases ownership and possibly wakes
159 // a successor, so the successor can (re)compete for ownership of the lock.
160 //
161 // * The entry_list forms a queue of threads stalled trying to acquire
162 // the lock. Within the entry_list the next pointers always form a
163 // consistent singly linked list. At unlock-time when the unlocking
164 // thread notices that the tail of the entry_list is not known, we
165 // convert the singly linked entry_list into a doubly linked list by
166 // assigning the prev pointers and the entry_list_tail pointer.
167 //
168 // Example:
169 //
170 // The first contending thread that "pushed" itself onto entry_list,
171 // will be the last thread in the list. Each newly pushed thread in
172 // entry_list will be linked through its next pointer, and have its
173 // prev pointer set to null. Thus pushing six threads A-F (in that
174 // order) onto entry_list, will form a singly linked list, see 1)
175 // below.
176 //
177 // 1) entry_list ->F->E->D->C->B->A->null
178 // entry_list_tail ->null
179 //
180 // Since the successor is chosen in FIFO order, the exiting thread
181 // needs to find the tail of the entry_list. This is done by walking
182 // from the entry_list head. While walking the list we also assign
183 // the prev pointers of each thread, essentially forming a doubly
184 // linked list, see 2) below.
185 //
186 // 2) entry_list ->F<=>E<=>D<=>C<=>B<=>A->null
187 // entry_list_tail ----------------------^
188 //
189 // Once we have formed a doubly linked list it's easy to find the
190 // successor (A), wake it up, have it remove itself, and update the
191 // tail pointer, as seen in and 3) below.
192 //
193 // 3) entry_list ->F<=>E<=>D<=>C<=>B->null
194 // entry_list_tail ------------------^
195 //
196 // At any time new threads can add themselves to the entry_list, see
197 // 4) below.
198 //
199 // 4) entry_list ->I->H->G->F<=>E<=>D->null
200 // entry_list_tail -------------------^
201 //
202 // At some point in time the thread (F) that wants to remove itself
203 // from the end of the list, will not have any prev pointer, see 5)
204 // below.
205 //
206 // 5) entry_list ->I->H->G->F->null
207 // entry_list_tail -----------^
208 //
209 // To resolve this we just start walking from the entry_list head
210 // again, forming a new doubly linked list, before removing the
211 // thread (F), see 6) and 7) below.
212 //
213 // 6) entry_list ->I<=>H<=>G<=>F->null
214 // entry_list_tail --------------^
215 //
216 // 7) entry_list ->I<=>H<=>G->null
217 // entry_list_tail ----------^
218 //
219 // * The monitor itself protects all of the operations on the
220 // entry_list except for the CAS of a new arrival to the head. Only
221 // the monitor owner can read or write the prev links (e.g. to
222 // remove itself) or update the tail.
223 //
224 // * The monitor entry list operations avoid locks, but strictly speaking
225 // they're not lock-free. Enter is lock-free, exit is not.
226 // For a description of 'Methods and apparatus providing non-blocking access
227 // to a resource,' see U.S. Pat. No. 7844973.
228 //
229 // * The entry_list can have multiple concurrent "pushers" but only
230 // one concurrent detaching thread. There is no ABA-problem with
231 // this usage of CAS.
232 //
233 // * As long as the entry_list_tail is known the odds are good that we
234 // should be able to dequeue after acquisition (in the ::enter()
235 // epilogue) in constant-time. This is good since a key desideratum
236 // is to minimize queue & monitor metadata manipulation that occurs
237 // while holding the monitor lock -- that is, we want to minimize
238 // monitor lock holds times. Note that even a small amount of fixed
239 // spinning will greatly reduce the # of enqueue-dequeue operations
240 // on entry_list. That is, spinning relieves contention on the
241 // "inner" locks and monitor metadata.
242 //
243 // Insert and delete operations may not operate in constant-time if
244 // we have interference because some other thread is adding or
245 // removing the head element of entry_list or if we need to convert
246 // the singly linked entry_list into a doubly linked list to find the
247 // tail.
248 //
249 // * The monitor synchronization subsystem avoids the use of native
250 // synchronization primitives except for the narrow platform-specific
251 // park-unpark abstraction. See the comments in os_posix.cpp regarding
252 // the semantics of park-unpark. Put another way, this monitor implementation
253 // depends only on atomic operations and park-unpark.
254 //
255 // * Waiting threads reside on the wait_set list -- wait() puts
256 // the caller onto the wait_set.
257 //
258 // * notify() or notifyAll() simply transfers threads from the wait_set
259 // to the entry_list. Subsequent exit() operations will
260 // unpark/re-schedule the notifyee. Unparking/re-scheduling a
261 // notifyee in notify() is inefficient - it's likely the notifyee
262 // would simply impale itself on the lock held by the notifier.
263
264 // Check that object() and set_object() are called from the right context:
265 static void check_object_context() {
266 #ifdef ASSERT
267 Thread* self = Thread::current();
268 if (self->is_Java_thread()) {
269 // Mostly called from JavaThreads so sanity check the thread state.
270 JavaThread* jt = JavaThread::cast(self);
271 switch (jt->thread_state()) {
272 case _thread_in_vm: // the usual case
273 case _thread_in_Java: // during deopt
274 break;
275 default:
276 fatal("called from an unsafe thread state");
277 }
278 assert(jt->is_active_Java_thread(), "must be active JavaThread");
279 } else {
280 // However, ThreadService::get_current_contended_monitor()
281 // can call here via the VMThread so sanity check it.
282 assert(self->is_VM_thread(), "must be");
283 }
284 #endif // ASSERT
285 }
286
287 ObjectMonitor::ObjectMonitor(oop object) :
288 _metadata(0),
289 _object(_oop_storage, object),
290 _owner(NO_OWNER),
291 _previous_owner_tid(0),
292 _next_om(nullptr),
293 _recursions(0),
294 _entry_list(nullptr),
295 _entry_list_tail(nullptr),
296 _succ(NO_OWNER),
297 _SpinDuration(ObjectMonitor::Knob_SpinLimit),
298 _contentions(0),
299 _unmounted_vthreads(0),
300 _wait_set(nullptr),
301 _waiters(0),
302 _wait_set_lock(0)
303 { }
304
305 ObjectMonitor::~ObjectMonitor() {
306 _object.release(_oop_storage);
307 }
308
309 oop ObjectMonitor::object() const {
310 check_object_context();
311 return _object.resolve();
312 }
313
314 void ObjectMonitor::ExitOnSuspend::operator()(JavaThread* current) {
315 if (current->is_suspended()) {
316 _om->_recursions = 0;
317 _om->clear_successor();
318 // Don't need a full fence after clearing successor here because of the call to exit().
319 _om->exit(current, false /* not_suspended */);
320 _om_exited = true;
321
322 current->set_current_pending_monitor(_om);
323 }
324 }
325
326 void ObjectMonitor::ClearSuccOnSuspend::operator()(JavaThread* current) {
327 if (current->is_suspended()) {
328 if (_om->has_successor(current)) {
329 _om->clear_successor();
330 OrderAccess::fence(); // always do a full fence when successor is cleared
331 }
332 }
333 }
334
335 #define assert_mark_word_consistency() \
336 assert(UseObjectMonitorTable || object()->mark() == markWord::encode(this), \
337 "object mark must match encoded this: mark=" INTPTR_FORMAT \
338 ", encoded this=" INTPTR_FORMAT, object()->mark().value(), \
339 markWord::encode(this).value());
340
341 // -----------------------------------------------------------------------------
342 // Enter support
343
344 bool ObjectMonitor::enter_is_async_deflating() {
345 if (is_being_async_deflated()) {
346 if (!UseObjectMonitorTable) {
347 const oop l_object = object();
348 if (l_object != nullptr) {
349 // Attempt to restore the header/dmw to the object's header so that
350 // we only retry once if the deflater thread happens to be slow.
351 install_displaced_markword_in_object(l_object);
352 }
353 }
354 return true;
355 }
356
357 return false;
358 }
359
360 bool ObjectMonitor::try_lock_with_contention_mark(JavaThread* locking_thread, ObjectMonitorContentionMark& contention_mark) {
361 assert(contention_mark._monitor == this, "must be");
362 assert(!is_being_async_deflated(), "must be");
363
364 int64_t prev_owner = try_set_owner_from(NO_OWNER, locking_thread);
365 bool success = false;
366
367 if (prev_owner == NO_OWNER) {
368 assert(_recursions == 0, "invariant");
369 success = true;
370 } else if (prev_owner == owner_id_from(locking_thread)) {
371 _recursions++;
372 success = true;
373 } else if (prev_owner == DEFLATER_MARKER) {
374 // Racing with deflation.
375 prev_owner = try_set_owner_from(DEFLATER_MARKER, locking_thread);
376 if (prev_owner == DEFLATER_MARKER) {
377 // We successfully cancelled the in-progress async deflation by
378 // changing owner from DEFLATER_MARKER to current. We now extend
379 // the lifetime of the contention_mark (e.g. contentions++) here
380 // to prevent the deflater thread from winning the last part of
381 // the 2-part async deflation protocol after the regular
382 // decrement occurs when the contention_mark goes out of
383 // scope. ObjectMonitor::deflate_monitor() which is called by
384 // the deflater thread will decrement contentions after it
385 // recognizes that the async deflation was cancelled.
386 contention_mark.extend();
387 success = true;
388 } else if (prev_owner == NO_OWNER) {
389 // At this point we cannot race with deflation as we have both incremented
390 // contentions, seen contention > 0 and seen a DEFLATER_MARKER.
391 // success will only be false if this races with something other than
392 // deflation.
393 prev_owner = try_set_owner_from(NO_OWNER, locking_thread);
394 success = prev_owner == NO_OWNER;
395 }
396 }
397 assert(!success || has_owner(locking_thread), "must be");
398
399 return success;
400 }
401
402 void ObjectMonitor::enter_for_with_contention_mark(JavaThread* locking_thread, ObjectMonitorContentionMark& contention_mark) {
403 // Used by LightweightSynchronizer::inflate_and_enter in deoptimization path to enter for another thread.
404 // The monitor is private to or already owned by locking_thread which must be suspended.
405 // So this code may only contend with deflation.
406 assert(locking_thread == Thread::current() || locking_thread->is_obj_deopt_suspend(), "must be");
407 bool success = try_lock_with_contention_mark(locking_thread, contention_mark);
408
409 assert(success, "Failed to enter_for: locking_thread=" INTPTR_FORMAT
410 ", this=" INTPTR_FORMAT "{owner=" INT64_FORMAT "}",
411 p2i(locking_thread), p2i(this), owner_raw());
412 }
413
414 bool ObjectMonitor::enter_for(JavaThread* locking_thread) {
415 // Used by ObjectSynchronizer::enter_for() to enter for another thread.
416 // The monitor is private to or already owned by locking_thread which must be suspended.
417 // So this code may only contend with deflation.
418 assert(locking_thread == Thread::current() || locking_thread->is_obj_deopt_suspend(), "must be");
419
420 // Block out deflation as soon as possible.
421 ObjectMonitorContentionMark contention_mark(this);
422
423 // Check for deflation.
424 if (enter_is_async_deflating()) {
425 return false;
426 }
427
428 bool success = try_lock_with_contention_mark(locking_thread, contention_mark);
429
430 assert(success, "Failed to enter_for: locking_thread=" INTPTR_FORMAT
431 ", this=" INTPTR_FORMAT "{owner=" INT64_FORMAT "}",
432 p2i(locking_thread), p2i(this), owner_raw());
433 assert(has_owner(locking_thread), "must be");
434 return true;
435 }
436
437 bool ObjectMonitor::try_enter(JavaThread* current, bool check_for_recursion) {
438 // TryLock avoids the CAS and handles deflation.
439 TryLockResult r = try_lock(current);
440 if (r == TryLockResult::Success) {
441 assert(_recursions == 0, "invariant");
442 return true;
443 }
444
445 // If called from SharedRuntime::monitor_exit_helper(), we know that
446 // this thread doesn't already own the lock.
447 if (!check_for_recursion) {
448 return false;
449 }
450
451 if (r == TryLockResult::HasOwner && has_owner(current)) {
452 _recursions++;
453 return true;
454 }
455
456 return false;
457 }
458
459 bool ObjectMonitor::spin_enter(JavaThread* current) {
460 assert(current == JavaThread::current(), "must be");
461
462 // Check for recursion.
463 if (try_enter(current)) {
464 return true;
465 }
466
467 // Check for deflation.
468 if (enter_is_async_deflating()) {
469 return false;
470 }
471
472 // We've encountered genuine contention.
473
474 // Do one round of spinning.
475 // Note that if we acquire the monitor from an initial spin
476 // we forgo posting JVMTI events and firing DTRACE probes.
477 if (try_spin(current)) {
478 assert(has_owner(current), "must be current: owner=" INT64_FORMAT, owner_raw());
479 assert(_recursions == 0, "must be 0: recursions=%zd", _recursions);
480 assert_mark_word_consistency();
481 return true;
482 }
483
484 return false;
485 }
486
487 bool ObjectMonitor::enter(JavaThread* current) {
488 assert(current == JavaThread::current(), "must be");
489
490 if (spin_enter(current)) {
491 return true;
492 }
493
494 assert(!has_owner(current), "invariant");
495 assert(!has_successor(current), "invariant");
496 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
497 assert(current->thread_state() != _thread_blocked, "invariant");
498
499 // Keep is_being_async_deflated stable across the rest of enter
500 ObjectMonitorContentionMark contention_mark(this);
501
502 // Check for deflation.
503 if (enter_is_async_deflating()) {
504 return false;
505 }
506
507 // At this point this ObjectMonitor cannot be deflated, finish contended enter
508 enter_with_contention_mark(current, contention_mark);
509 return true;
510 }
511
512 void ObjectMonitor::notify_contended_enter(JavaThread* current) {
513 current->set_current_pending_monitor(this);
514
515 DTRACE_MONITOR_PROBE(contended__enter, this, object(), current);
516 if (JvmtiExport::should_post_monitor_contended_enter()) {
517 JvmtiExport::post_monitor_contended_enter(current, this);
518
519 // The current thread does not yet own the monitor and does not
520 // yet appear on any queues that would get it made the successor.
521 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
522 // handler cannot accidentally consume an unpark() meant for the
523 // ParkEvent associated with this ObjectMonitor.
524 }
525 }
526
527 void ObjectMonitor::enter_with_contention_mark(JavaThread* current, ObjectMonitorContentionMark &cm) {
528 assert(current == JavaThread::current(), "must be");
529 assert(!has_owner(current), "must be");
530 assert(cm._monitor == this, "must be");
531 assert(!is_being_async_deflated(), "must be");
532
533 JFR_ONLY(JfrConditionalFlush<EventJavaMonitorEnter> flush(current);)
534 EventJavaMonitorEnter enter_event;
535 if (enter_event.is_started()) {
536 enter_event.set_monitorClass(object()->klass());
537 // Set an address that is 'unique enough', such that events close in
538 // time and with the same address are likely (but not guaranteed) to
539 // belong to the same object.
540 enter_event.set_address((uintptr_t)this);
541 }
542 EventVirtualThreadPinned vthread_pinned_event;
543
544 freeze_result result;
545
546 assert(current->current_pending_monitor() == nullptr, "invariant");
547
548 ContinuationEntry* ce = current->last_continuation();
549 bool is_virtual = ce != nullptr && ce->is_virtual_thread();
550 if (is_virtual) {
551 notify_contended_enter(current);
552 result = Continuation::try_preempt(current, ce->cont_oop(current));
553 if (result == freeze_ok) {
554 bool acquired = vthread_monitor_enter(current);
555 if (acquired) {
556 // We actually acquired the monitor while trying to add the vthread to the
557 // _entry_list so cancel preemption. We will still go through the preempt stub
558 // but instead of unmounting we will call thaw to continue execution.
559 current->set_preemption_cancelled(true);
560 if (JvmtiExport::should_post_monitor_contended_entered()) {
561 // We are going to call thaw again after this and finish the VMTS
562 // transition so no need to do it here. We will post the event there.
563 current->set_contended_entered_monitor(this);
564 }
565 }
566 current->set_current_pending_monitor(nullptr);
567 DEBUG_ONLY(int state = java_lang_VirtualThread::state(current->vthread()));
568 assert((acquired && current->preemption_cancelled() && state == java_lang_VirtualThread::RUNNING) ||
569 (!acquired && !current->preemption_cancelled() && state == java_lang_VirtualThread::BLOCKING), "invariant");
570 return;
571 }
572 }
573
574 {
575 // Change java thread status to indicate blocked on monitor enter.
576 JavaThreadBlockedOnMonitorEnterState jtbmes(current, this);
577
578 if (!is_virtual) { // already notified contended_enter for virtual
579 notify_contended_enter(current);
580 }
581 OSThreadContendState osts(current->osthread());
582
583 assert(current->thread_state() == _thread_in_vm, "invariant");
584
585 for (;;) {
586 ExitOnSuspend eos(this);
587 {
588 ThreadBlockInVMPreprocess<ExitOnSuspend> tbivs(current, eos, true /* allow_suspend */);
589 enter_internal(current);
590 current->set_current_pending_monitor(nullptr);
591 // We can go to a safepoint at the end of this block. If we
592 // do a thread dump during that safepoint, then this thread will show
593 // as having "-locked" the monitor, but the OS and java.lang.Thread
594 // states will still report that the thread is blocked trying to
595 // acquire it.
596 // If there is a suspend request, ExitOnSuspend will exit the OM
597 // and set the OM as pending.
598 }
599 if (!eos.exited()) {
600 // ExitOnSuspend did not exit the OM
601 assert(has_owner(current), "invariant");
602 break;
603 }
604 }
605
606 // We've just gotten past the enter-check-for-suspend dance and we now own
607 // the monitor free and clear.
608 }
609
610 assert(contentions() >= 0, "must not be negative: contentions=%d", contentions());
611
612 // Must either set _recursions = 0 or ASSERT _recursions == 0.
613 assert(_recursions == 0, "invariant");
614 assert(has_owner(current), "invariant");
615 assert(!has_successor(current), "invariant");
616 assert_mark_word_consistency();
617
618 // The thread -- now the owner -- is back in vm mode.
619 // Report the glorious news via TI,DTrace and jvmstat.
620 // The probe effect is non-trivial. All the reportage occurs
621 // while we hold the monitor, increasing the length of the critical
622 // section. Amdahl's parallel speedup law comes vividly into play.
623 //
624 // Another option might be to aggregate the events (thread local or
625 // per-monitor aggregation) and defer reporting until a more opportune
626 // time -- such as next time some thread encounters contention but has
627 // yet to acquire the lock. While spinning that thread could
628 // spinning we could increment JVMStat counters, etc.
629
630 DTRACE_MONITOR_PROBE(contended__entered, this, object(), current);
631 if (JvmtiExport::should_post_monitor_contended_entered()) {
632 JvmtiExport::post_monitor_contended_entered(current, this);
633
634 // The current thread already owns the monitor and is not going to
635 // call park() for the remainder of the monitor enter protocol. So
636 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
637 // event handler consumed an unpark() issued by the thread that
638 // just exited the monitor.
639 }
640 if (enter_event.should_commit()) {
641 enter_event.set_previousOwner(_previous_owner_tid);
642 enter_event.commit();
643 }
644
645 if (current->current_waiting_monitor() == nullptr) {
646 ContinuationEntry* ce = current->last_continuation();
647 if (ce != nullptr && ce->is_virtual_thread()) {
648 current->post_vthread_pinned_event(&vthread_pinned_event, "Contended monitor enter", result);
649 }
650 }
651 }
652
653 // Caveat: try_lock() is not necessarily serializing if it returns failure.
654 // Callers must compensate as needed.
655
656 ObjectMonitor::TryLockResult ObjectMonitor::try_lock(JavaThread* current) {
657 int64_t own = owner_raw();
658 int64_t first_own = own;
659
660 for (;;) {
661 if (own == DEFLATER_MARKER) {
662 // Block out deflation as soon as possible.
663 ObjectMonitorContentionMark contention_mark(this);
664
665 // Check for deflation.
666 if (enter_is_async_deflating()) {
667 // Treat deflation as interference.
668 return TryLockResult::Interference;
669 }
670 if (try_lock_with_contention_mark(current, contention_mark)) {
671 assert(_recursions == 0, "invariant");
672 return TryLockResult::Success;
673 } else {
674 // Deflation won or change of owner; dont spin
675 break;
676 }
677 } else if (own == NO_OWNER) {
678 int64_t prev_own = try_set_owner_from(NO_OWNER, current);
679 if (prev_own == NO_OWNER) {
680 assert(_recursions == 0, "invariant");
681 return TryLockResult::Success;
682 } else {
683 // The lock had been free momentarily, but we lost the race to the lock.
684 own = prev_own;
685 }
686 } else {
687 // Retry doesn't make as much sense because the lock was just acquired.
688 break;
689 }
690 }
691 return first_own == own ? TryLockResult::HasOwner : TryLockResult::Interference;
692 }
693
694 // Push "current" onto the head of the _entry_list. Once on _entry_list,
695 // current stays on-queue until it acquires the lock.
696 void ObjectMonitor::add_to_entry_list(JavaThread* current, ObjectWaiter* node) {
697 node->_prev = nullptr;
698 node->TState = ObjectWaiter::TS_ENTER;
699
700 for (;;) {
701 ObjectWaiter* head = AtomicAccess::load(&_entry_list);
702 node->_next = head;
703 if (AtomicAccess::cmpxchg(&_entry_list, head, node) == head) {
704 return;
705 }
706 }
707 }
708
709 // Push "current" onto the head of the entry_list.
710 // If the _entry_list was changed during our push operation, we try to
711 // lock the monitor. Returns true if we locked the monitor, and false
712 // if we added current to _entry_list. Once on _entry_list, current
713 // stays on-queue until it acquires the lock.
714 bool ObjectMonitor::try_lock_or_add_to_entry_list(JavaThread* current, ObjectWaiter* node) {
715 assert(node->TState == ObjectWaiter::TS_RUN, "");
716 node->_prev = nullptr;
717 node->TState = ObjectWaiter::TS_ENTER;
718
719 for (;;) {
720 ObjectWaiter* head = AtomicAccess::load(&_entry_list);
721 node->_next = head;
722 if (AtomicAccess::cmpxchg(&_entry_list, head, node) == head) {
723 return false;
724 }
725
726 // Interference - the CAS failed because _entry_list changed. Before
727 // retrying the CAS retry taking the lock as it may now be free.
728 if (try_lock(current) == TryLockResult::Success) {
729 assert(!has_successor(current), "invariant");
730 assert(has_owner(current), "invariant");
731 node->TState = ObjectWaiter::TS_RUN;
732 return true;
733 }
734 }
735 }
736
737 static void post_monitor_deflate_event(EventJavaMonitorDeflate* event,
738 const oop obj) {
739 assert(event != nullptr, "invariant");
740 if (obj == nullptr) {
741 // Accept the case when obj was already garbage-collected.
742 // Emit the event anyway, but without details.
743 event->set_monitorClass(nullptr);
744 event->set_address(0);
745 } else {
746 const Klass* monitor_klass = obj->klass();
747 if (ObjectMonitor::is_jfr_excluded(monitor_klass)) {
748 return;
749 }
750 event->set_monitorClass(monitor_klass);
751 event->set_address((uintptr_t)(void*)obj);
752 }
753 event->commit();
754 }
755
756 // Deflate the specified ObjectMonitor if not in-use. Returns true if it
757 // was deflated and false otherwise.
758 //
759 // The async deflation protocol sets owner to DEFLATER_MARKER and
760 // makes contentions negative as signals to contending threads that
761 // an async deflation is in progress. There are a number of checks
762 // as part of the protocol to make sure that the calling thread has
763 // not lost the race to a contending thread.
764 //
765 // The ObjectMonitor has been successfully async deflated when:
766 // (contentions < 0)
767 // Contending threads that see that condition know to retry their operation.
768 //
769 bool ObjectMonitor::deflate_monitor(Thread* current) {
770 if (is_busy()) {
771 // Easy checks are first - the ObjectMonitor is busy so no deflation.
772 return false;
773 }
774
775 EventJavaMonitorDeflate event;
776
777 const oop obj = object_peek();
778
779 if (obj == nullptr) {
780 // If the object died, we can recycle the monitor without racing with
781 // Java threads. The GC already broke the association with the object.
782 set_owner_from_raw(NO_OWNER, DEFLATER_MARKER);
783 assert(contentions() >= 0, "must be non-negative: contentions=%d", contentions());
784 _contentions = INT_MIN; // minimum negative int
785 } else {
786 // Attempt async deflation protocol.
787
788 // Set a null owner to DEFLATER_MARKER to force any contending thread
789 // through the slow path. This is just the first part of the async
790 // deflation dance.
791 if (try_set_owner_from_raw(NO_OWNER, DEFLATER_MARKER) != NO_OWNER) {
792 // The owner field is no longer null so we lost the race since the
793 // ObjectMonitor is now busy.
794 return false;
795 }
796
797 if (contentions() > 0 || _waiters != 0) {
798 // Another thread has raced to enter the ObjectMonitor after
799 // is_busy() above or has already entered and waited on
800 // it which makes it busy so no deflation. Restore owner to
801 // null if it is still DEFLATER_MARKER.
802 if (try_set_owner_from_raw(DEFLATER_MARKER, NO_OWNER) != DEFLATER_MARKER) {
803 // Deferred decrement for the JT enter_internal() that cancelled the async deflation.
804 add_to_contentions(-1);
805 }
806 return false;
807 }
808
809 // Make a zero contentions field negative to force any contending threads
810 // to retry. This is the second part of the async deflation dance.
811 if (AtomicAccess::cmpxchg(&_contentions, 0, INT_MIN) != 0) {
812 // Contentions was no longer 0 so we lost the race since the
813 // ObjectMonitor is now busy. Restore owner to null if it is
814 // still DEFLATER_MARKER:
815 if (try_set_owner_from_raw(DEFLATER_MARKER, NO_OWNER) != DEFLATER_MARKER) {
816 // Deferred decrement for the JT enter_internal() that cancelled the async deflation.
817 add_to_contentions(-1);
818 }
819 return false;
820 }
821 }
822
823 // Sanity checks for the races:
824 guarantee(owner_is_DEFLATER_MARKER(), "must be deflater marker");
825 guarantee(contentions() < 0, "must be negative: contentions=%d",
826 contentions());
827 guarantee(_waiters == 0, "must be 0: waiters=%d", _waiters);
828 ObjectWaiter* w = AtomicAccess::load(&_entry_list);
829 guarantee(w == nullptr,
830 "must be no entering threads: entry_list=" INTPTR_FORMAT,
831 p2i(w));
832
833 if (obj != nullptr) {
834 if (log_is_enabled(Trace, monitorinflation)) {
835 ResourceMark rm;
836 log_trace(monitorinflation)("deflate_monitor: object=" INTPTR_FORMAT
837 ", mark=" INTPTR_FORMAT ", type='%s'",
838 p2i(obj), obj->mark().value(),
839 obj->klass()->external_name());
840 }
841 }
842
843 if (UseObjectMonitorTable) {
844 LightweightSynchronizer::deflate_monitor(current, obj, this);
845 } else if (obj != nullptr) {
846 // Install the old mark word if nobody else has already done it.
847 install_displaced_markword_in_object(obj);
848 }
849
850 if (event.should_commit()) {
851 post_monitor_deflate_event(&event, obj);
852 }
853
854 // We leave owner == DEFLATER_MARKER and contentions < 0
855 // to force any racing threads to retry.
856 return true; // Success, ObjectMonitor has been deflated.
857 }
858
859 // Install the displaced mark word (dmw) of a deflating ObjectMonitor
860 // into the header of the object associated with the monitor. This
861 // idempotent method is called by a thread that is deflating a
862 // monitor and by other threads that have detected a race with the
863 // deflation process.
864 void ObjectMonitor::install_displaced_markword_in_object(const oop obj) {
865 assert(!UseObjectMonitorTable, "ObjectMonitorTable has no dmw");
866 // This function must only be called when (owner == DEFLATER_MARKER
867 // && contentions <= 0), but we can't guarantee that here because
868 // those values could change when the ObjectMonitor gets moved from
869 // the global free list to a per-thread free list.
870
871 guarantee(obj != nullptr, "must be non-null");
872
873 // Separate loads in is_being_async_deflated(), which is almost always
874 // called before this function, from the load of dmw/header below.
875
876 // _contentions and dmw/header may get written by different threads.
877 // Make sure to observe them in the same order when having several observers.
878 OrderAccess::loadload_for_IRIW();
879
880 const oop l_object = object_peek();
881 if (l_object == nullptr) {
882 // ObjectMonitor's object ref has already been cleared by async
883 // deflation or GC so we're done here.
884 return;
885 }
886 assert(l_object == obj, "object=" INTPTR_FORMAT " must equal obj="
887 INTPTR_FORMAT, p2i(l_object), p2i(obj));
888
889 markWord dmw = header();
890 // The dmw has to be neutral (not null, not locked and not marked).
891 assert(dmw.is_neutral(), "must be neutral: dmw=" INTPTR_FORMAT, dmw.value());
892
893 // Install displaced mark word if the object's header still points
894 // to this ObjectMonitor. More than one racing caller to this function
895 // can rarely reach this point, but only one can win.
896 markWord res = obj->cas_set_mark(dmw, markWord::encode(this));
897 if (res != markWord::encode(this)) {
898 // This should be rare so log at the Info level when it happens.
899 log_info(monitorinflation)("install_displaced_markword_in_object: "
900 "failed cas_set_mark: new_mark=" INTPTR_FORMAT
901 ", old_mark=" INTPTR_FORMAT ", res=" INTPTR_FORMAT,
902 dmw.value(), markWord::encode(this).value(),
903 res.value());
904 }
905
906 // Note: It does not matter which thread restored the header/dmw
907 // into the object's header. The thread deflating the monitor just
908 // wanted the object's header restored and it is. The threads that
909 // detected a race with the deflation process also wanted the
910 // object's header restored before they retry their operation and
911 // because it is restored they will only retry once.
912 }
913
914 // Convert the fields used by is_busy() to a string that can be
915 // used for diagnostic output.
916 const char* ObjectMonitor::is_busy_to_string(stringStream* ss) {
917 ss->print("is_busy: waiters=%d"
918 ", contentions=%d"
919 ", owner=" INT64_FORMAT
920 ", entry_list=" PTR_FORMAT,
921 _waiters,
922 (contentions() > 0 ? contentions() : 0),
923 owner_is_DEFLATER_MARKER()
924 // We report null instead of DEFLATER_MARKER here because is_busy()
925 // ignores DEFLATER_MARKER values.
926 ? NO_OWNER
927 : owner_raw(),
928 p2i(_entry_list));
929 return ss->base();
930 }
931
932 void ObjectMonitor::enter_internal(JavaThread* current) {
933 assert(current->thread_state() == _thread_blocked, "invariant");
934
935 // Try the lock - TATAS
936 if (try_lock(current) == TryLockResult::Success) {
937 assert(!has_successor(current), "invariant");
938 assert(has_owner(current), "invariant");
939 return;
940 }
941
942 assert(InitDone, "Unexpectedly not initialized");
943
944 // We try one round of spinning *before* enqueueing current.
945 //
946 // If the _owner is ready but OFFPROC we could use a YieldTo()
947 // operation to donate the remainder of this thread's quantum
948 // to the owner. This has subtle but beneficial affinity
949 // effects.
950
951 if (try_spin(current)) {
952 assert(has_owner(current), "invariant");
953 assert(!has_successor(current), "invariant");
954 return;
955 }
956
957 // The Spin failed -- Enqueue and park the thread ...
958 assert(!has_successor(current), "invariant");
959 assert(!has_owner(current), "invariant");
960
961 // Enqueue "current" on ObjectMonitor's _entry_list.
962 //
963 // Node acts as a proxy for current.
964 // As an aside, if were to ever rewrite the synchronization code mostly
965 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
966 // Java objects. This would avoid awkward lifecycle and liveness issues,
967 // as well as eliminate a subset of ABA issues.
968 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
969
970 ObjectWaiter node(current);
971 current->_ParkEvent->reset();
972
973 if (try_lock_or_add_to_entry_list(current, &node)) {
974 return; // We got the lock.
975 }
976 // This thread is now added to the _entry_list.
977
978 // The lock might have been released while this thread was occupied queueing
979 // itself onto _entry_list. To close the race and avoid "stranding" and
980 // progress-liveness failure we must resample-retry _owner before parking.
981 // Note the Dekker/Lamport duality: ST _entry_list; MEMBAR; LD Owner.
982 // In this case the ST-MEMBAR is accomplished with CAS().
983 //
984 // TODO: Defer all thread state transitions until park-time.
985 // Since state transitions are heavy and inefficient we'd like
986 // to defer the state transitions until absolutely necessary,
987 // and in doing so avoid some transitions ...
988
989 // If there are unmounted virtual threads ahead in the _entry_list we want
990 // to do a timed-park instead to alleviate some deadlock cases where one
991 // of them is picked as the successor but cannot run due to having run out
992 // of carriers. This can happen, for example, if this is a pinned virtual
993 // thread currently loading or initializining a class, and all other carriers
994 // have a pinned vthread waiting for said class to be loaded/initialized.
995 // Read counter *after* adding this thread to the _entry_list. Adding to
996 // _entry_list uses Atomic::cmpxchg() which already provides a fence that
997 // prevents this load from floating up previous store.
998 // Note that we can have false positives where timed-park is not necessary.
999 bool do_timed_parked = has_unmounted_vthreads();
1000 jlong recheck_interval = 1;
1001
1002 for (;;) {
1003
1004 if (try_lock(current) == TryLockResult::Success) {
1005 break;
1006 }
1007 assert(!has_owner(current), "invariant");
1008
1009 // park self
1010 if (do_timed_parked) {
1011 current->_ParkEvent->park(recheck_interval);
1012 // Increase the recheck_interval, but clamp the value.
1013 recheck_interval *= 8;
1014 if (recheck_interval > MAX_RECHECK_INTERVAL) {
1015 recheck_interval = MAX_RECHECK_INTERVAL;
1016 }
1017 } else {
1018 current->_ParkEvent->park();
1019 }
1020
1021 if (try_lock(current) == TryLockResult::Success) {
1022 break;
1023 }
1024
1025 // The lock is still contested.
1026
1027 // Assuming this is not a spurious wakeup we'll normally find _succ == current.
1028 // We can defer clearing _succ until after the spin completes
1029 // try_spin() must tolerate being called with _succ == current.
1030 // Try yet another round of adaptive spinning.
1031 if (try_spin(current)) {
1032 break;
1033 }
1034
1035 // We can find that we were unpark()ed and redesignated _succ while
1036 // we were spinning. That's harmless. If we iterate and call park(),
1037 // park() will consume the event and return immediately and we'll
1038 // just spin again. This pattern can repeat, leaving _succ to simply
1039 // spin on a CPU.
1040
1041 if (has_successor(current)) clear_successor();
1042
1043 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
1044 OrderAccess::fence();
1045 }
1046
1047 // Egress :
1048 // Current has acquired the lock -- Unlink current from the _entry_list.
1049 unlink_after_acquire(current, &node);
1050 if (has_successor(current)) {
1051 clear_successor();
1052 // Note that we don't need to do OrderAccess::fence() after clearing
1053 // _succ here, since we own the lock.
1054 }
1055
1056 // We've acquired ownership with CAS().
1057 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
1058 // But since the CAS() this thread may have also stored into _succ
1059 // or entry_list. These meta-data updates must be visible __before
1060 // this thread subsequently drops the lock.
1061 // Consider what could occur if we didn't enforce this constraint --
1062 // STs to monitor meta-data and user-data could reorder with (become
1063 // visible after) the ST in exit that drops ownership of the lock.
1064 // Some other thread could then acquire the lock, but observe inconsistent
1065 // or old monitor meta-data and heap data. That violates the JMM.
1066 // To that end, the exit() operation must have at least STST|LDST
1067 // "release" barrier semantics. Specifically, there must be at least a
1068 // STST|LDST barrier in exit() before the ST of null into _owner that drops
1069 // the lock. The barrier ensures that changes to monitor meta-data and data
1070 // protected by the lock will be visible before we release the lock, and
1071 // therefore before some other thread (CPU) has a chance to acquire the lock.
1072 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
1073 //
1074 // Critically, any prior STs to _succ or entry_list must be visible before
1075 // the ST of null into _owner in the *subsequent* (following) corresponding
1076 // monitorexit.
1077
1078 return;
1079 }
1080
1081 // reenter_internal() is a specialized inline form of the latter half of the
1082 // contended slow-path from enter_internal(). We use reenter_internal() only for
1083 // monitor reentry in wait().
1084 //
1085 // In the future we should reconcile enter_internal() and reenter_internal().
1086
1087 void ObjectMonitor::reenter_internal(JavaThread* current, ObjectWaiter* currentNode) {
1088 assert(current != nullptr, "invariant");
1089 assert(current->thread_state() != _thread_blocked, "invariant");
1090 assert(currentNode != nullptr, "invariant");
1091 assert(currentNode->_thread == current, "invariant");
1092 assert(_waiters > 0, "invariant");
1093 assert_mark_word_consistency();
1094
1095 // If there are unmounted virtual threads ahead in the _entry_list we want
1096 // to do a timed-park instead to alleviate some deadlock cases where one
1097 // of them is picked as the successor but cannot run due to having run out
1098 // of carriers. This can happen, for example, if a mixed of unmounted and
1099 // pinned vthreads taking up all the carriers are waiting for a class to be
1100 // initialized, and the selected successor is one of the unmounted vthreads.
1101 // Although this method is used for the "notification" case, it could be
1102 // that this thread reached here without been added to the _entry_list yet.
1103 // This can happen if it was interrupted or the wait timed-out at the same
1104 // time. In that case we rely on currentNode->_do_timed_park, which will be
1105 // read on the next loop iteration, after consuming the park permit set by
1106 // the notifier in notify_internal.
1107 // Note that we can have false positives where timed-park is not necessary.
1108 bool do_timed_parked = has_unmounted_vthreads();
1109 jlong recheck_interval = 1;
1110
1111 for (;;) {
1112 ObjectWaiter::TStates v = currentNode->TState;
1113 guarantee(v == ObjectWaiter::TS_ENTER, "invariant");
1114 assert(!has_owner(current), "invariant");
1115
1116 // This thread has been notified so try to reacquire the lock.
1117 if (try_lock(current) == TryLockResult::Success) {
1118 break;
1119 }
1120
1121 // If that fails, spin again. Note that spin count may be zero so the above TryLock
1122 // is necessary.
1123 if (try_spin(current)) {
1124 break;
1125 }
1126
1127 {
1128 OSThreadContendState osts(current->osthread());
1129
1130 assert(current->thread_state() == _thread_in_vm, "invariant");
1131
1132 {
1133 ClearSuccOnSuspend csos(this);
1134 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */);
1135 if (do_timed_parked) {
1136 current->_ParkEvent->park(recheck_interval);
1137 // Increase the recheck_interval, but clamp the value.
1138 recheck_interval *= 8;
1139 if (recheck_interval > MAX_RECHECK_INTERVAL) {
1140 recheck_interval = MAX_RECHECK_INTERVAL;
1141 }
1142 } else {
1143 current->_ParkEvent->park();
1144 }
1145 }
1146 }
1147
1148 // Try again, but just so we distinguish between futile wakeups and
1149 // successful wakeups. The following test isn't algorithmically
1150 // necessary, but it helps us maintain sensible statistics.
1151 if (try_lock(current) == TryLockResult::Success) {
1152 break;
1153 }
1154
1155 // The lock is still contested.
1156
1157 // Assuming this is not a spurious wakeup we'll normally
1158 // find that _succ == current.
1159 if (has_successor(current)) clear_successor();
1160
1161 // Invariant: after clearing _succ a contending thread
1162 // *must* retry _owner before parking.
1163 OrderAccess::fence();
1164
1165 // See comment in notify_internal
1166 do_timed_parked |= currentNode->_do_timed_park;
1167 }
1168
1169 // Current has acquired the lock -- Unlink current from the _entry_list.
1170 assert(has_owner(current), "invariant");
1171 assert_mark_word_consistency();
1172 unlink_after_acquire(current, currentNode);
1173 if (has_successor(current)) clear_successor();
1174 assert(!has_successor(current), "invariant");
1175 currentNode->TState = ObjectWaiter::TS_RUN;
1176 OrderAccess::fence(); // see comments at the end of enter_internal()
1177 }
1178
1179 // This method is called from two places:
1180 // - On monitorenter contention with a null waiter.
1181 // - After Object.wait() times out or the target is interrupted to reenter the
1182 // monitor, with the existing waiter.
1183 // For the Object.wait() case we do not delete the ObjectWaiter in case we
1184 // succesfully acquire the monitor since we are going to need it on return.
1185 bool ObjectMonitor::vthread_monitor_enter(JavaThread* current, ObjectWaiter* waiter) {
1186 if (try_lock(current) == TryLockResult::Success) {
1187 assert(has_owner(current), "invariant");
1188 assert(!has_successor(current), "invariant");
1189 return true;
1190 }
1191
1192 oop vthread = current->vthread();
1193 ObjectWaiter* node = waiter != nullptr ? waiter : new ObjectWaiter(vthread, this);
1194
1195 // Increment counter *before* adding the vthread to the _entry_list.
1196 // Adding to _entry_list uses Atomic::cmpxchg() which already provides
1197 // a fence that prevents reordering of the stores.
1198 inc_unmounted_vthreads();
1199
1200 if (try_lock_or_add_to_entry_list(current, node)) {
1201 // We got the lock.
1202 if (waiter == nullptr) delete node; // for Object.wait() don't delete yet
1203 dec_unmounted_vthreads();
1204 return true;
1205 }
1206 // This thread is now added to the entry_list.
1207
1208 // We have to try once more since owner could have exited monitor and checked
1209 // _entry_list before we added the node to the queue.
1210 if (try_lock(current) == TryLockResult::Success) {
1211 assert(has_owner(current), "invariant");
1212 unlink_after_acquire(current, node);
1213 if (has_successor(current)) clear_successor();
1214 if (waiter == nullptr) delete node; // for Object.wait() don't delete yet
1215 dec_unmounted_vthreads();
1216 return true;
1217 }
1218
1219 assert(java_lang_VirtualThread::state(vthread) == java_lang_VirtualThread::RUNNING, "wrong state for vthread");
1220 java_lang_VirtualThread::set_state(vthread, java_lang_VirtualThread::BLOCKING);
1221
1222 // We didn't succeed in acquiring the monitor so increment _contentions and
1223 // save ObjectWaiter* in the vthread since we will need it when resuming execution.
1224 add_to_contentions(1);
1225 java_lang_VirtualThread::set_objectWaiter(vthread, node);
1226 return false;
1227 }
1228
1229 // Called from thaw code to resume the monitor operation that caused the vthread
1230 // to be unmounted. Method returns true if the monitor is successfully acquired,
1231 // which marks the end of the monitor operation, otherwise it returns false.
1232 bool ObjectMonitor::resume_operation(JavaThread* current, ObjectWaiter* node, ContinuationWrapper& cont) {
1233 assert(java_lang_VirtualThread::state(current->vthread()) == java_lang_VirtualThread::RUNNING, "wrong state for vthread");
1234 assert(!has_owner(current), "");
1235
1236 if (node->is_wait() && !node->at_reenter()) {
1237 bool acquired_monitor = vthread_wait_reenter(current, node, cont);
1238 if (acquired_monitor) return true;
1239 }
1240
1241 // Retry acquiring monitor...
1242
1243 int state = node->TState;
1244 guarantee(state == ObjectWaiter::TS_ENTER, "invariant");
1245
1246 if (try_lock(current) == TryLockResult::Success) {
1247 vthread_epilog(current, node);
1248 return true;
1249 }
1250
1251 oop vthread = current->vthread();
1252 if (has_successor(current)) clear_successor();
1253
1254 // Invariant: after clearing _succ a thread *must* retry acquiring the monitor.
1255 OrderAccess::fence();
1256
1257 if (try_lock(current) == TryLockResult::Success) {
1258 vthread_epilog(current, node);
1259 return true;
1260 }
1261
1262 // We will return to Continuation.run() and unmount so set the right state.
1263 java_lang_VirtualThread::set_state(vthread, java_lang_VirtualThread::BLOCKING);
1264
1265 return false;
1266 }
1267
1268 void ObjectMonitor::vthread_epilog(JavaThread* current, ObjectWaiter* node) {
1269 assert(has_owner(current), "invariant");
1270 add_to_contentions(-1);
1271 dec_unmounted_vthreads();
1272
1273 if (has_successor(current)) clear_successor();
1274
1275 guarantee(_recursions == 0, "invariant");
1276
1277 if (node->is_wait()) {
1278 _recursions = node->_recursions; // restore the old recursion count
1279 _waiters--; // decrement the number of waiters
1280
1281 if (node->_interrupted) {
1282 // We will throw at thaw end after finishing the mount transition.
1283 current->set_pending_interrupted_exception(true);
1284 }
1285 }
1286
1287 unlink_after_acquire(current, node);
1288 delete node;
1289
1290 // Clear the ObjectWaiter* from the vthread.
1291 java_lang_VirtualThread::set_objectWaiter(current->vthread(), nullptr);
1292
1293 if (JvmtiExport::should_post_monitor_contended_entered()) {
1294 // We are going to call thaw again after this and finish the VMTS
1295 // transition so no need to do it here. We will post the event there.
1296 current->set_contended_entered_monitor(this);
1297 }
1298 }
1299
1300 // Convert entry_list into a doubly linked list by assigning the prev
1301 // pointers and the entry_list_tail pointer (if needed). Within the
1302 // entry_list the next pointers always form a consistent singly linked
1303 // list. When this function is called, the entry_list will be either
1304 // singly linked, or starting as singly linked (at the head), but
1305 // ending as doubly linked (at the tail).
1306 void ObjectMonitor::entry_list_build_dll(JavaThread* current) {
1307 assert(has_owner(current), "invariant");
1308 ObjectWaiter* prev = nullptr;
1309 // Need acquire here to match the implicit release of the cmpxchg
1310 // that updated entry_list, so we can access w->prev().
1311 ObjectWaiter* w = AtomicAccess::load_acquire(&_entry_list);
1312 assert(w != nullptr, "should only be called when entry list is not empty");
1313 while (w != nullptr) {
1314 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1315 assert(w->prev() == nullptr || w->prev() == prev, "invariant");
1316 if (w->prev() != nullptr) {
1317 break;
1318 }
1319 w->_prev = prev;
1320 prev = w;
1321 w = w->next();
1322 }
1323 if (w == nullptr) {
1324 // We converted the entire entry_list from a singly linked list
1325 // into a doubly linked list. Now we just need to set the tail
1326 // pointer.
1327 assert(prev != nullptr && prev->next() == nullptr, "invariant");
1328 assert(_entry_list_tail == nullptr || _entry_list_tail == prev, "invariant");
1329 _entry_list_tail = prev;
1330 } else {
1331 #ifdef ASSERT
1332 // We stopped iterating through the _entry_list when we found a
1333 // node that had its prev pointer set. I.e. we converted the first
1334 // part of the entry_list from a singly linked list into a doubly
1335 // linked list. Now we just want to make sure the rest of the list
1336 // is doubly linked. But first we check that we have a tail
1337 // pointer, because if the end of the entry_list is doubly linked
1338 // and we don't have the tail pointer, something is broken.
1339 assert(_entry_list_tail != nullptr, "invariant");
1340 while (w != nullptr) {
1341 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1342 assert(w->prev() == prev, "invariant");
1343 prev = w;
1344 w = w->next();
1345 }
1346 assert(_entry_list_tail == prev, "invariant");
1347 #endif
1348 }
1349 }
1350
1351 // Return the tail of the _entry_list. If the tail is currently not
1352 // known, it can be found by first calling entry_list_build_dll().
1353 ObjectWaiter* ObjectMonitor::entry_list_tail(JavaThread* current) {
1354 assert(has_owner(current), "invariant");
1355 ObjectWaiter* w = _entry_list_tail;
1356 if (w != nullptr) {
1357 return w;
1358 }
1359 entry_list_build_dll(current);
1360 w = _entry_list_tail;
1361 assert(w != nullptr, "invariant");
1362 return w;
1363 }
1364
1365 // By convention we unlink a contending thread from _entry_list
1366 // immediately after the thread acquires the lock in ::enter().
1367 // The head of _entry_list is volatile but the interior is stable.
1368 // In addition, current.TState is stable.
1369
1370 void ObjectMonitor::unlink_after_acquire(JavaThread* current, ObjectWaiter* currentNode) {
1371 assert(has_owner(current), "invariant");
1372 assert((!currentNode->is_vthread() && currentNode->thread() == current) ||
1373 (currentNode->is_vthread() && currentNode->vthread() == current->vthread()), "invariant");
1374
1375 // Check if we are unlinking the last element in the _entry_list.
1376 // This is by far the most common case.
1377 if (currentNode->next() == nullptr) {
1378 assert(_entry_list_tail == nullptr || _entry_list_tail == currentNode, "invariant");
1379
1380 ObjectWaiter* w = AtomicAccess::load(&_entry_list);
1381 if (w == currentNode) {
1382 // The currentNode is the only element in _entry_list.
1383 if (AtomicAccess::cmpxchg(&_entry_list, w, (ObjectWaiter*)nullptr) == w) {
1384 _entry_list_tail = nullptr;
1385 currentNode->set_bad_pointers();
1386 return;
1387 }
1388 // The CAS above can fail from interference IFF a contending
1389 // thread "pushed" itself onto entry_list. So fall-through to
1390 // building the doubly linked list.
1391 assert(currentNode->prev() == nullptr, "invariant");
1392 }
1393 if (currentNode->prev() == nullptr) {
1394 // Build the doubly linked list to get hold of
1395 // currentNode->prev().
1396 entry_list_build_dll(current);
1397 assert(currentNode->prev() != nullptr, "must be");
1398 assert(_entry_list_tail == currentNode, "must be");
1399 }
1400 // The currentNode is the last element in _entry_list and we know
1401 // which element is the previous one.
1402 assert(_entry_list != currentNode, "invariant");
1403 _entry_list_tail = currentNode->prev();
1404 _entry_list_tail->_next = nullptr;
1405 currentNode->set_bad_pointers();
1406 return;
1407 }
1408
1409 // If we get here it means the current thread enqueued itself on the
1410 // _entry_list but was then able to "steal" the lock before the
1411 // chosen successor was able to. Consequently currentNode must be an
1412 // interior node in the _entry_list, or the head.
1413 assert(currentNode->next() != nullptr, "invariant");
1414 assert(currentNode != _entry_list_tail, "invariant");
1415
1416 // Check if we are in the singly linked portion of the
1417 // _entry_list. If we are the head then we try to remove ourselves,
1418 // else we convert to the doubly linked list.
1419 if (currentNode->prev() == nullptr) {
1420 ObjectWaiter* w = AtomicAccess::load(&_entry_list);
1421
1422 assert(w != nullptr, "invariant");
1423 if (w == currentNode) {
1424 ObjectWaiter* next = currentNode->next();
1425 // currentNode is at the head of _entry_list.
1426 if (AtomicAccess::cmpxchg(&_entry_list, w, next) == w) {
1427 // The CAS above sucsessfully unlinked currentNode from the
1428 // head of the _entry_list.
1429 assert(_entry_list != w, "invariant");
1430 next->_prev = nullptr;
1431 currentNode->set_bad_pointers();
1432 return;
1433 } else {
1434 // The CAS above can fail from interference IFF a contending
1435 // thread "pushed" itself onto _entry_list, in which case
1436 // currentNode must now be in the interior of the
1437 // list. Fall-through to building the doubly linked list.
1438 assert(_entry_list != currentNode, "invariant");
1439 }
1440 }
1441 // Build the doubly linked list to get hold of currentNode->prev().
1442 entry_list_build_dll(current);
1443 assert(currentNode->prev() != nullptr, "must be");
1444 }
1445
1446 // We now know we are unlinking currentNode from the interior of a
1447 // doubly linked list.
1448 assert(currentNode->next() != nullptr, "");
1449 assert(currentNode->prev() != nullptr, "");
1450 assert(currentNode != _entry_list, "");
1451 assert(currentNode != _entry_list_tail, "");
1452
1453 ObjectWaiter* nxt = currentNode->next();
1454 ObjectWaiter* prv = currentNode->prev();
1455 assert(nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
1456 assert(prv->TState == ObjectWaiter::TS_ENTER, "invariant");
1457
1458 nxt->_prev = prv;
1459 prv->_next = nxt;
1460 currentNode->set_bad_pointers();
1461 }
1462
1463 // -----------------------------------------------------------------------------
1464 // Exit support
1465 //
1466 // exit()
1467 // ~~~~~~
1468 // Note that the collector can't reclaim the objectMonitor or deflate
1469 // the object out from underneath the thread calling ::exit() as the
1470 // thread calling ::exit() never transitions to a stable state.
1471 // This inhibits GC, which in turn inhibits asynchronous (and
1472 // inopportune) reclamation of "this".
1473 //
1474 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
1475 // There's one exception to the claim above, however. enter_internal() can call
1476 // exit() to drop a lock if the acquirer has been externally suspended.
1477 // In that case exit() is called with _thread_state == _thread_blocked,
1478 // but the monitor's _contentions field is > 0, which inhibits reclamation.
1479 //
1480 // This is the exit part of the locking protocol, often implemented in
1481 // C2_MacroAssembler::fast_unlock()
1482 //
1483 // 1. A release barrier ensures that changes to monitor meta-data
1484 // (_succ, _entry_list) and data protected by the lock will be
1485 // visible before we release the lock.
1486 // 2. Release the lock by clearing the owner.
1487 // 3. A storeload MEMBAR is needed between releasing the owner and
1488 // subsequently reading meta-data to safely determine if the lock is
1489 // contended (step 4) without an elected successor (step 5).
1490 // 4. If _entry_list is null, we are done, since there is no
1491 // other thread waiting on the lock to wake up. I.e. there is no
1492 // contention.
1493 // 5. If there is a successor (_succ is non-null), we are done. The
1494 // responsibility for guaranteeing progress-liveness has now implicitly
1495 // been moved from the exiting thread to the successor.
1496 // 6. There are waiters in the entry list (_entry_list is non-null),
1497 // but there is no successor (_succ is null), so we need to
1498 // wake up (unpark) a waiting thread to avoid stranding.
1499 //
1500 // Note that since only the current lock owner can manipulate the
1501 // _entry_list (except for pushing new threads to the head), we need to
1502 // reacquire the lock before we can wake up (unpark) a waiting thread.
1503 //
1504 // The CAS() in enter provides for safety and exclusion, while the
1505 // MEMBAR in exit provides for progress and avoids stranding.
1506 //
1507 // There is also the risk of a futile wake-up. If we drop the lock
1508 // another thread can reacquire the lock immediately, and we can
1509 // then wake a thread unnecessarily. This is benign, and we've
1510 // structured the code so the windows are short and the frequency
1511 // of such futile wakups is low.
1512
1513 void ObjectMonitor::exit(JavaThread* current, bool not_suspended) {
1514 if (!has_owner(current)) {
1515 // Apparent unbalanced locking ...
1516 // Naively we'd like to throw IllegalMonitorStateException.
1517 // As a practical matter we can neither allocate nor throw an
1518 // exception as ::exit() can be called from leaf routines.
1519 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
1520 // Upon deeper reflection, however, in a properly run JVM the only
1521 // way we should encounter this situation is in the presence of
1522 // unbalanced JNI locking. TODO: CheckJNICalls.
1523 // See also: CR4414101
1524 #ifdef ASSERT
1525 LogStreamHandle(Error, monitorinflation) lsh;
1526 lsh.print_cr("ERROR: ObjectMonitor::exit(): thread=" INTPTR_FORMAT
1527 " is exiting an ObjectMonitor it does not own.", p2i(current));
1528 lsh.print_cr("The imbalance is possibly caused by JNI locking.");
1529 print_debug_style_on(&lsh);
1530 assert(false, "Non-balanced monitor enter/exit!");
1531 #endif
1532 return;
1533 }
1534
1535 if (_recursions != 0) {
1536 _recursions--; // this is simple recursive enter
1537 return;
1538 }
1539
1540 #if INCLUDE_JFR
1541 // get the owner's thread id for the MonitorEnter event
1542 // if it is enabled and the thread isn't suspended
1543 if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
1544 _previous_owner_tid = JFR_THREAD_ID(current);
1545 }
1546 #endif
1547
1548 for (;;) {
1549 // If there is a successor we should release the lock as soon as
1550 // possible, so that the successor can acquire the lock. If there is
1551 // no successor, we might need to wake up a waiting thread.
1552 if (!has_successor()) {
1553 ObjectWaiter* w = AtomicAccess::load(&_entry_list);
1554 if (w != nullptr) {
1555 // Other threads are blocked trying to acquire the lock and
1556 // there is no successor, so it appears that an heir-
1557 // presumptive (successor) must be made ready. Since threads
1558 // are woken up in FIFO order, we need to find the tail of the
1559 // entry_list.
1560 w = entry_list_tail(current);
1561 // I'd like to write: guarantee (w->_thread != current).
1562 // But in practice an exiting thread may find itself on the entry_list.
1563 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1564 // then calls exit(). Exit release the lock by setting O._owner to null.
1565 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1566 // notify() operation moves T1 from O's waitset to O's entry_list. T2 then
1567 // release the lock "O". T1 resumes immediately after the ST of null into
1568 // _owner, above. T1 notices that the entry_list is populated, so it
1569 // reacquires the lock and then finds itself on the entry_list.
1570 // Given all that, we have to tolerate the circumstance where "w" is
1571 // associated with current.
1572 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1573 exit_epilog(current, w);
1574 return;
1575 }
1576 }
1577
1578 // Drop the lock.
1579 // release semantics: prior loads and stores from within the critical section
1580 // must not float (reorder) past the following store that drops the lock.
1581 // Uses a storeload to separate release_store(owner) from the
1582 // successor check. The try_set_owner_from() below uses cmpxchg() so
1583 // we get the fence down there.
1584 release_clear_owner(current);
1585 OrderAccess::storeload();
1586
1587 // Normally the exiting thread is responsible for ensuring succession,
1588 // but if this thread observes other successors are ready or other
1589 // entering threads are spinning after it has stored null into _owner
1590 // then it can exit without waking a successor. The existence of
1591 // spinners or ready successors guarantees proper succession (liveness).
1592 // Responsibility passes to the ready or running successors. The exiting
1593 // thread delegates the duty. More precisely, if a successor already
1594 // exists this thread is absolved of the responsibility of waking
1595 // (unparking) one.
1596
1597 // The _succ variable is critical to reducing futile wakeup frequency.
1598 // _succ identifies the "heir presumptive" thread that has been made
1599 // ready (unparked) but that has not yet run. We need only one such
1600 // successor thread to guarantee progress.
1601 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1602 // section 3.3 "Futile Wakeup Throttling" for details.
1603 //
1604 // Note that spinners in Enter() also set _succ non-null.
1605 // In the current implementation spinners opportunistically set
1606 // _succ so that exiting threads might avoid waking a successor.
1607 // Which means that the exiting thread could exit immediately without
1608 // waking a successor, if it observes a successor after it has dropped
1609 // the lock. Note that the dropped lock needs to become visible to the
1610 // spinner.
1611
1612 if (_entry_list == nullptr || has_successor()) {
1613 return;
1614 }
1615
1616 // Only the current lock owner can manipulate the entry_list
1617 // (except for pushing new threads to the head), therefore we need
1618 // to reacquire the lock. If we fail to reacquire the lock the
1619 // responsibility for ensuring succession falls to the new owner.
1620
1621 if (try_lock(current) != TryLockResult::Success) {
1622 // Some other thread acquired the lock (or the monitor was
1623 // deflated). Either way we are done.
1624 return;
1625 }
1626
1627 guarantee(has_owner(current), "invariant");
1628 }
1629 }
1630
1631 void ObjectMonitor::exit_epilog(JavaThread* current, ObjectWaiter* Wakee) {
1632 assert(has_owner(current), "invariant");
1633
1634 // Exit protocol:
1635 // 1. ST _succ = wakee
1636 // 2. membar #loadstore|#storestore;
1637 // 2. ST _owner = nullptr
1638 // 3. unpark(wakee)
1639
1640 oop vthread = nullptr;
1641 ParkEvent * Trigger;
1642 if (!Wakee->is_vthread()) {
1643 JavaThread* t = Wakee->thread();
1644 assert(t != nullptr, "");
1645 Trigger = t->_ParkEvent;
1646 set_successor(t);
1647 } else {
1648 vthread = Wakee->vthread();
1649 assert(vthread != nullptr, "");
1650 Trigger = ObjectMonitor::vthread_unparker_ParkEvent();
1651 set_successor(vthread);
1652 }
1653
1654 // Hygiene -- once we've set _owner = nullptr we can't safely dereference Wakee again.
1655 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1656 // out-of-scope (non-extant).
1657 Wakee = nullptr;
1658
1659 // Drop the lock.
1660 // Uses a fence to separate release_store(owner) from the LD in unpark().
1661 release_clear_owner(current);
1662 OrderAccess::fence();
1663
1664 DTRACE_MONITOR_PROBE(contended__exit, this, object(), current);
1665
1666 if (vthread == nullptr) {
1667 // Platform thread case.
1668 Trigger->unpark();
1669 } else if (java_lang_VirtualThread::set_onWaitingList(vthread, vthread_list_head())) {
1670 // Virtual thread case.
1671 Trigger->unpark();
1672 }
1673 }
1674
1675 // Exits the monitor returning recursion count. _owner should
1676 // be set to current's owner_id, i.e. no ANONYMOUS_OWNER allowed.
1677 intx ObjectMonitor::complete_exit(JavaThread* current) {
1678 assert(InitDone, "Unexpectedly not initialized");
1679 guarantee(has_owner(current), "complete_exit not owner");
1680
1681 intx save = _recursions; // record the old recursion count
1682 _recursions = 0; // set the recursion level to be 0
1683 exit(current); // exit the monitor
1684 guarantee(!has_owner(current), "invariant");
1685 return save;
1686 }
1687
1688 // Checks that the current THREAD owns this monitor and causes an
1689 // immediate return if it doesn't. We don't use the CHECK macro
1690 // because we want the IMSE to be the only exception that is thrown
1691 // from the call site when false is returned. Any other pending
1692 // exception is ignored.
1693 #define CHECK_OWNER() \
1694 do { \
1695 if (!check_owner(THREAD)) { \
1696 assert(HAS_PENDING_EXCEPTION, "expected a pending IMSE here."); \
1697 return; \
1698 } \
1699 } while (false)
1700
1701 // Returns true if the specified thread owns the ObjectMonitor.
1702 // Otherwise returns false and throws IllegalMonitorStateException
1703 // (IMSE). If there is a pending exception and the specified thread
1704 // is not the owner, that exception will be replaced by the IMSE.
1705 bool ObjectMonitor::check_owner(TRAPS) {
1706 JavaThread* current = THREAD;
1707 int64_t cur = owner_raw();
1708 if (cur == owner_id_from(current)) {
1709 return true;
1710 }
1711 THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(),
1712 "current thread is not owner", false);
1713 }
1714
1715 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1716 ObjectMonitor* monitor,
1717 uint64_t notifier_tid,
1718 jlong timeout,
1719 bool timedout) {
1720 assert(event != nullptr, "invariant");
1721 assert(monitor != nullptr, "invariant");
1722 const Klass* monitor_klass = monitor->object()->klass();
1723 if (ObjectMonitor::is_jfr_excluded(monitor_klass)) {
1724 return;
1725 }
1726 event->set_monitorClass(monitor_klass);
1727 event->set_timeout(timeout);
1728 // Set an address that is 'unique enough', such that events close in
1729 // time and with the same address are likely (but not guaranteed) to
1730 // belong to the same object.
1731 event->set_address((uintptr_t)monitor);
1732 event->set_notifier(notifier_tid);
1733 event->set_timedOut(timedout);
1734 event->commit();
1735 }
1736
1737 static void vthread_monitor_waited_event(JavaThread* current, ObjectWaiter* node, ContinuationWrapper& cont, EventJavaMonitorWait* event, jboolean timed_out) {
1738 // Since we might safepoint set the anchor so that the stack can we walked.
1739 assert(current->last_continuation() != nullptr, "");
1740 JavaFrameAnchor* anchor = current->frame_anchor();
1741 anchor->set_last_Java_sp(current->last_continuation()->entry_sp());
1742 anchor->set_last_Java_pc(current->last_continuation()->entry_pc());
1743
1744 ContinuationWrapper::SafepointOp so(current, cont);
1745
1746 JRT_BLOCK
1747 if (event->should_commit()) {
1748 long timeout = java_lang_VirtualThread::timeout(current->vthread());
1749 post_monitor_wait_event(event, node->_monitor, node->_notifier_tid, timeout, timed_out);
1750 }
1751 if (JvmtiExport::should_post_monitor_waited()) {
1752 // We mark this call in case of an upcall to Java while posting the event.
1753 // If somebody walks the stack in that case, processing the enterSpecial
1754 // frame should not include processing callee arguments since there is no
1755 // actual callee (see nmethod::preserve_callee_argument_oops()).
1756 ThreadOnMonitorWaitedEvent tmwe(current);
1757 JvmtiExport::vthread_post_monitor_waited(current, node->_monitor, timed_out);
1758 }
1759 JRT_BLOCK_END
1760 current->frame_anchor()->clear();
1761 }
1762
1763 // -----------------------------------------------------------------------------
1764 // Wait/Notify/NotifyAll
1765 //
1766 // Note: a subset of changes to ObjectMonitor::wait()
1767 // will need to be replicated in complete_exit
1768 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1769 JavaThread* current = THREAD;
1770
1771 assert(InitDone, "Unexpectedly not initialized");
1772
1773 CHECK_OWNER(); // Throws IMSE if not owner.
1774
1775 EventJavaMonitorWait wait_event;
1776 EventVirtualThreadPinned vthread_pinned_event;
1777
1778 // check for a pending interrupt
1779 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1780 JavaThreadInObjectWaitState jtiows(current, millis != 0, interruptible);
1781
1782 if (JvmtiExport::should_post_monitor_wait()) {
1783 JvmtiExport::post_monitor_wait(current, object(), millis);
1784 }
1785 // post monitor waited event. Note that this is past-tense, we are done waiting.
1786 if (JvmtiExport::should_post_monitor_waited()) {
1787 // Note: 'false' parameter is passed here because the
1788 // wait was not timed out due to thread interrupt.
1789 JvmtiExport::post_monitor_waited(current, this, false);
1790
1791 // In this short circuit of the monitor wait protocol, the
1792 // current thread never drops ownership of the monitor and
1793 // never gets added to the wait queue so the current thread
1794 // cannot be made the successor. This means that the
1795 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1796 // consume an unpark() meant for the ParkEvent associated with
1797 // this ObjectMonitor.
1798 }
1799 if (wait_event.should_commit()) {
1800 post_monitor_wait_event(&wait_event, this, 0, millis, false);
1801 }
1802 THROW(vmSymbols::java_lang_InterruptedException());
1803 return;
1804 }
1805
1806 freeze_result result;
1807 ContinuationEntry* ce = current->last_continuation();
1808 bool is_virtual = ce != nullptr && ce->is_virtual_thread();
1809 if (is_virtual) {
1810 if (interruptible && JvmtiExport::should_post_monitor_wait()) {
1811 JvmtiExport::post_monitor_wait(current, object(), millis);
1812 }
1813 current->set_current_waiting_monitor(this);
1814 result = Continuation::try_preempt(current, ce->cont_oop(current));
1815 if (result == freeze_ok) {
1816 vthread_wait(current, millis);
1817 current->set_current_waiting_monitor(nullptr);
1818 return;
1819 }
1820 }
1821 // The jtiows does nothing for non-interruptible.
1822 JavaThreadInObjectWaitState jtiows(current, millis != 0, interruptible);
1823
1824 if (!is_virtual) { // it was already set for virtual thread
1825 if (interruptible && JvmtiExport::should_post_monitor_wait()) {
1826 JvmtiExport::post_monitor_wait(current, object(), millis);
1827
1828 // The current thread already owns the monitor and it has not yet
1829 // been added to the wait queue so the current thread cannot be
1830 // made the successor. This means that the JVMTI_EVENT_MONITOR_WAIT
1831 // event handler cannot accidentally consume an unpark() meant for
1832 // the ParkEvent associated with this ObjectMonitor.
1833 }
1834 current->set_current_waiting_monitor(this);
1835 }
1836 // create a node to be put into the queue
1837 // Critically, after we reset() the event but prior to park(), we must check
1838 // for a pending interrupt.
1839 ObjectWaiter node(current);
1840 node.TState = ObjectWaiter::TS_WAIT;
1841 current->_ParkEvent->reset();
1842 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1843
1844 // Enter the waiting queue, which is a circular doubly linked list in this case
1845 // but it could be a priority queue or any data structure.
1846 // _wait_set_lock protects the wait queue. Normally the wait queue is accessed only
1847 // by the owner of the monitor *except* in the case where park()
1848 // returns because of a timeout of interrupt. Contention is exceptionally rare
1849 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1850
1851 Thread::SpinAcquire(&_wait_set_lock);
1852 add_waiter(&node);
1853 Thread::SpinRelease(&_wait_set_lock);
1854
1855 intx save = _recursions; // record the old recursion count
1856 _waiters++; // increment the number of waiters
1857 _recursions = 0; // set the recursion level to be 1
1858 exit(current); // exit the monitor
1859 guarantee(!has_owner(current), "invariant");
1860
1861 // The thread is on the wait_set list - now park() it.
1862 // On MP systems it's conceivable that a brief spin before we park
1863 // could be profitable.
1864 //
1865 // TODO-FIXME: change the following logic to a loop of the form
1866 // while (!timeout && !interrupted && node.TState == TS_WAIT) park()
1867
1868 int ret = OS_OK;
1869 bool was_notified = false;
1870
1871 // Need to check interrupt state whilst still _thread_in_vm
1872 bool interrupted = interruptible && current->is_interrupted(false);
1873
1874 { // State transition wrappers
1875 OSThread* osthread = current->osthread();
1876 OSThreadWaitState osts(osthread, true);
1877
1878 assert(current->thread_state() == _thread_in_vm, "invariant");
1879
1880 {
1881 ClearSuccOnSuspend csos(this);
1882 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */);
1883 if (interrupted || HAS_PENDING_EXCEPTION) {
1884 // Intentionally empty
1885 } else if (node.TState == ObjectWaiter::TS_WAIT) {
1886 if (millis <= 0) {
1887 current->_ParkEvent->park();
1888 } else {
1889 ret = current->_ParkEvent->park(millis);
1890 }
1891 }
1892 }
1893
1894 // Node may be on the wait_set, or on the entry_list, or in transition
1895 // from the wait_set to the entry_list.
1896 // See if we need to remove Node from the wait_set.
1897 // We use double-checked locking to avoid grabbing _wait_set_lock
1898 // if the thread is not on the wait queue.
1899 //
1900 // Note that we don't need a fence before the fetch of TState.
1901 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1902 // written by the is thread. (perhaps the fetch might even be satisfied
1903 // by a look-aside into the processor's own store buffer, although given
1904 // the length of the code path between the prior ST and this load that's
1905 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1906 // then we'll acquire the lock and then re-fetch a fresh TState value.
1907 // That is, we fail toward safety.
1908
1909 if (node.TState == ObjectWaiter::TS_WAIT) {
1910 Thread::SpinAcquire(&_wait_set_lock);
1911 if (node.TState == ObjectWaiter::TS_WAIT) {
1912 dequeue_specific_waiter(&node); // unlink from wait_set
1913 node.TState = ObjectWaiter::TS_RUN;
1914 }
1915 Thread::SpinRelease(&_wait_set_lock);
1916 }
1917
1918 // The thread is now either on off-list (TS_RUN),
1919 // or on the entry_list (TS_ENTER).
1920 // The Node's TState variable is stable from the perspective of this thread.
1921 // No other threads will asynchronously modify TState.
1922 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1923 OrderAccess::loadload();
1924 if (has_successor(current)) clear_successor();
1925 was_notified = node.TState == ObjectWaiter::TS_ENTER;
1926
1927 // Reentry phase -- reacquire the monitor.
1928 // re-enter contended monitor after object.wait().
1929 // retain OBJECT_WAIT state until re-enter successfully completes
1930 // Thread state is thread_in_vm and oop access is again safe,
1931 // although the raw address of the object may have changed.
1932 // (Don't cache naked oops over safepoints, of course).
1933
1934 // post monitor waited event. Note that this is past-tense, we are done waiting.
1935 if (JvmtiExport::should_post_monitor_waited()) {
1936 JvmtiExport::post_monitor_waited(current, this, ret == OS_TIMEOUT);
1937
1938 if (was_notified && has_successor(current)) {
1939 // In this part of the monitor wait-notify-reenter protocol it
1940 // is possible (and normal) for another thread to do a fastpath
1941 // monitor enter-exit while this thread is still trying to get
1942 // to the reenter portion of the protocol.
1943 //
1944 // The ObjectMonitor was notified and the current thread is
1945 // the successor which also means that an unpark() has already
1946 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1947 // consume the unpark() that was done when the successor was
1948 // set because the same ParkEvent is shared between Java
1949 // monitors and JVM/TI RawMonitors (for now).
1950 //
1951 // We redo the unpark() to ensure forward progress, i.e., we
1952 // don't want all pending threads hanging (parked) with none
1953 // entering the unlocked monitor.
1954 current->_ParkEvent->unpark();
1955 }
1956 }
1957
1958 if (wait_event.should_commit()) {
1959 post_monitor_wait_event(&wait_event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1960 }
1961
1962 OrderAccess::fence();
1963
1964 assert(!has_owner(current), "invariant");
1965 ObjectWaiter::TStates v = node.TState;
1966 if (v == ObjectWaiter::TS_RUN) {
1967 // We use the NoPreemptMark for the very rare case where the previous
1968 // preempt attempt failed due to OOM. The preempt on monitor contention
1969 // could succeed but we can't unmount now.
1970 NoPreemptMark npm(current);
1971 enter(current);
1972 } else {
1973 guarantee(v == ObjectWaiter::TS_ENTER, "invariant");
1974 reenter_internal(current, &node);
1975 node.wait_reenter_end(this);
1976 }
1977
1978 // current has reacquired the lock.
1979 // Lifecycle - the node representing current must not appear on any queues.
1980 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1981 // want residual elements associated with this thread left on any lists.
1982 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1983 assert(has_owner(current), "invariant");
1984 assert(!has_successor(current), "invariant");
1985 } // OSThreadWaitState()
1986
1987 current->set_current_waiting_monitor(nullptr);
1988
1989 guarantee(_recursions == 0, "invariant");
1990 int relock_count = JvmtiDeferredUpdates::get_and_reset_relock_count_after_wait(current);
1991 _recursions = save // restore the old recursion count
1992 + relock_count; // increased by the deferred relock count
1993 _waiters--; // decrement the number of waiters
1994
1995 // Verify a few postconditions
1996 assert(has_owner(current), "invariant");
1997 assert(!has_successor(current), "invariant");
1998 assert_mark_word_consistency();
1999
2000 if (ce != nullptr && ce->is_virtual_thread()) {
2001 current->post_vthread_pinned_event(&vthread_pinned_event, "Object.wait", result);
2002 }
2003
2004 // check if the notification happened
2005 if (!was_notified) {
2006 // no, it could be timeout or Thread.interrupt() or both
2007 // check for interrupt event, otherwise it is timeout
2008 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
2009 THROW(vmSymbols::java_lang_InterruptedException());
2010 }
2011 }
2012
2013 // NOTE: Spurious wake up will be consider as timeout.
2014 // Monitor notify has precedence over thread interrupt.
2015 }
2016
2017 // Consider:
2018 // If the lock is cool (entry_list == null && succ == null) and we're on an MP system
2019 // then instead of transferring a thread from the wait_set to the entry_list
2020 // we might just dequeue a thread from the wait_set and directly unpark() it.
2021
2022 bool ObjectMonitor::notify_internal(JavaThread* current) {
2023 bool did_notify = false;
2024 Thread::SpinAcquire(&_wait_set_lock);
2025 ObjectWaiter* iterator = dequeue_waiter();
2026 if (iterator != nullptr) {
2027 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
2028
2029 if (iterator->is_vthread()) {
2030 oop vthread = iterator->vthread();
2031 java_lang_VirtualThread::set_notified(vthread, true);
2032 int old_state = java_lang_VirtualThread::state(vthread);
2033 // If state is not WAIT/TIMED_WAIT then target could still be on
2034 // unmount transition, or wait could have already timed-out or target
2035 // could have been interrupted. In the first case, the target itself
2036 // will set the state to BLOCKED at the end of the unmount transition.
2037 // In the other cases the target would have been already unblocked so
2038 // there is nothing to do.
2039 if (old_state == java_lang_VirtualThread::WAIT ||
2040 old_state == java_lang_VirtualThread::TIMED_WAIT) {
2041 java_lang_VirtualThread::cmpxchg_state(vthread, old_state, java_lang_VirtualThread::BLOCKED);
2042 }
2043 // Increment counter *before* adding the vthread to the _entry_list.
2044 // Adding to _entry_list uses Atomic::cmpxchg() which already provides
2045 // a fence that prevents reordering of the stores.
2046 inc_unmounted_vthreads();
2047 }
2048
2049 iterator->_notifier_tid = JFR_THREAD_ID(current);
2050 did_notify = true;
2051 add_to_entry_list(current, iterator);
2052
2053 // _wait_set_lock protects the wait queue, not the entry_list. We could
2054 // move the add-to-entry_list operation, above, outside the critical section
2055 // protected by _wait_set_lock. In practice that's not useful. With the
2056 // exception of wait() timeouts and interrupts the monitor owner
2057 // is the only thread that grabs _wait_set_lock. There's almost no contention
2058 // on _wait_set_lock so it's not profitable to reduce the length of the
2059 // critical section.
2060
2061 if (!iterator->is_vthread()) {
2062 iterator->wait_reenter_begin(this);
2063
2064 // Read counter *after* adding the thread to the _entry_list.
2065 // Adding to _entry_list uses Atomic::cmpxchg() which already provides
2066 // a fence that prevents this load from floating up previous store.
2067 if (has_unmounted_vthreads()) {
2068 // Wake up the thread to alleviate some deadlock cases where the successor
2069 // that will be picked up when this thread releases the monitor is an unmounted
2070 // virtual thread that cannot run due to having run out of carriers. Upon waking
2071 // up, the thread will call reenter_internal() which will use timed-park in case
2072 // there is contention and there are still vthreads in the _entry_list.
2073 // If the target was interrupted or the wait timed-out at the same time, it could
2074 // have reached reenter_internal and read a false value of has_unmounted_vthreads()
2075 // before we added it to the _entry_list above. To deal with that case, we set _do_timed_park
2076 // which will be read by the target on the next loop iteration in reenter_internal.
2077 iterator->_do_timed_park = true;
2078 JavaThread* t = iterator->thread();
2079 t->_ParkEvent->unpark();
2080 }
2081 }
2082 }
2083 Thread::SpinRelease(&_wait_set_lock);
2084 return did_notify;
2085 }
2086
2087 static void post_monitor_notify_event(EventJavaMonitorNotify* event,
2088 ObjectMonitor* monitor,
2089 int notified_count) {
2090 assert(event != nullptr, "invariant");
2091 assert(monitor != nullptr, "invariant");
2092 const Klass* monitor_klass = monitor->object()->klass();
2093 if (ObjectMonitor::is_jfr_excluded(monitor_klass)) {
2094 return;
2095 }
2096 event->set_monitorClass(monitor_klass);
2097 // Set an address that is 'unique enough', such that events close in
2098 // time and with the same address are likely (but not guaranteed) to
2099 // belong to the same object.
2100 event->set_address((uintptr_t)monitor);
2101 event->set_notifiedCount(notified_count);
2102 event->commit();
2103 }
2104
2105 // Consider: a not-uncommon synchronization bug is to use notify() when
2106 // notifyAll() is more appropriate, potentially resulting in stranded
2107 // threads; this is one example of a lost wakeup. A useful diagnostic
2108 // option is to force all notify() operations to behave as notifyAll().
2109 //
2110 // Note: We can also detect many such problems with a "minimum wait".
2111 // When the "minimum wait" is set to a small non-zero timeout value
2112 // and the program does not hang whereas it did absent "minimum wait",
2113 // that suggests a lost wakeup bug.
2114
2115 void ObjectMonitor::notify(TRAPS) {
2116 JavaThread* current = THREAD;
2117 CHECK_OWNER(); // Throws IMSE if not owner.
2118 if (_wait_set == nullptr) {
2119 return;
2120 }
2121
2122 quick_notify(current);
2123 }
2124
2125 void ObjectMonitor::quick_notify(JavaThread* current) {
2126 assert(has_owner(current), "Precondition");
2127
2128 EventJavaMonitorNotify event;
2129 DTRACE_MONITOR_PROBE(notify, this, object(), current);
2130 int tally = notify_internal(current) ? 1 : 0;
2131
2132 if ((tally > 0) && event.should_commit()) {
2133 post_monitor_notify_event(&event, this, /* notified_count = */ tally);
2134 }
2135 }
2136
2137 // notifyAll() transfers the waiters one-at-a-time from the waitset to
2138 // the entry_list. If the waitset is "ABCD" (where A was added first
2139 // and D last) and the entry_list is ->X->Y->Z. After a notifyAll()
2140 // the waitset will be empty and the entry_list will be
2141 // ->D->C->B->A->X->Y->Z, and the next choosen successor will be Z.
2142
2143 void ObjectMonitor::notifyAll(TRAPS) {
2144 JavaThread* current = THREAD;
2145 CHECK_OWNER(); // Throws IMSE if not owner.
2146 if (_wait_set == nullptr) {
2147 return;
2148 }
2149
2150 quick_notifyAll(current);
2151 }
2152
2153 void ObjectMonitor::quick_notifyAll(JavaThread* current) {
2154 assert(has_owner(current), "Precondition");
2155
2156 EventJavaMonitorNotify event;
2157 DTRACE_MONITOR_PROBE(notifyAll, this, object(), current);
2158 int tally = 0;
2159 while (_wait_set != nullptr) {
2160 if (notify_internal(current)) {
2161 tally++;
2162 }
2163 }
2164
2165 if ((tally > 0) && event.should_commit()) {
2166 post_monitor_notify_event(&event, this, /* notified_count = */ tally);
2167 }
2168 }
2169
2170 void ObjectMonitor::vthread_wait(JavaThread* current, jlong millis) {
2171 oop vthread = current->vthread();
2172 ObjectWaiter* node = new ObjectWaiter(vthread, this);
2173 node->_is_wait = true;
2174 node->TState = ObjectWaiter::TS_WAIT;
2175 java_lang_VirtualThread::set_notified(vthread, false); // Reset notified flag
2176
2177 // Enter the waiting queue, which is a circular doubly linked list in this case
2178 // but it could be a priority queue or any data structure.
2179 // _wait_set_lock protects the wait queue. Normally the wait queue is accessed only
2180 // by the owner of the monitor *except* in the case where park()
2181 // returns because of a timeout or interrupt. Contention is exceptionally rare
2182 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
2183
2184 Thread::SpinAcquire(&_wait_set_lock);
2185 add_waiter(node);
2186 Thread::SpinRelease(&_wait_set_lock);
2187
2188 node->_recursions = _recursions; // record the old recursion count
2189 _recursions = 0; // set the recursion level to be 0
2190 _waiters++; // increment the number of waiters
2191 exit(current); // exit the monitor
2192 guarantee(!has_owner(current), "invariant");
2193
2194 assert(java_lang_VirtualThread::state(vthread) == java_lang_VirtualThread::RUNNING, "wrong state for vthread");
2195 java_lang_VirtualThread::set_state(vthread, millis == 0 ? java_lang_VirtualThread::WAITING : java_lang_VirtualThread::TIMED_WAITING);
2196 java_lang_VirtualThread::set_timeout(vthread, millis);
2197
2198 // Save the ObjectWaiter* in the vthread since we will need it when resuming execution.
2199 java_lang_VirtualThread::set_objectWaiter(vthread, node);
2200 }
2201
2202 bool ObjectMonitor::vthread_wait_reenter(JavaThread* current, ObjectWaiter* node, ContinuationWrapper& cont) {
2203 // The first time we run after being preempted on Object.wait() we
2204 // need to check if we were interrupted or the wait timed-out, and
2205 // in that case remove ourselves from the _wait_set queue.
2206 if (node->TState == ObjectWaiter::TS_WAIT) {
2207 Thread::SpinAcquire(&_wait_set_lock);
2208 if (node->TState == ObjectWaiter::TS_WAIT) {
2209 dequeue_specific_waiter(node); // unlink from wait_set
2210 node->TState = ObjectWaiter::TS_RUN;
2211 }
2212 Thread::SpinRelease(&_wait_set_lock);
2213 }
2214
2215 // If this was an interrupted case, set the _interrupted boolean so that
2216 // once we re-acquire the monitor we know if we need to throw IE or not.
2217 ObjectWaiter::TStates state = node->TState;
2218 bool was_notified = state == ObjectWaiter::TS_ENTER;
2219 assert(was_notified || state == ObjectWaiter::TS_RUN, "");
2220 node->_interrupted = !was_notified && current->is_interrupted(false);
2221
2222 // Post JFR and JVMTI events.
2223 EventJavaMonitorWait wait_event;
2224 if (wait_event.should_commit() || JvmtiExport::should_post_monitor_waited()) {
2225 vthread_monitor_waited_event(current, node, cont, &wait_event, !was_notified && !node->_interrupted);
2226 }
2227
2228 // Mark that we are at reenter so that we don't call this method again.
2229 node->_at_reenter = true;
2230
2231 if (!was_notified) {
2232 bool acquired = vthread_monitor_enter(current, node);
2233 if (acquired) {
2234 guarantee(_recursions == 0, "invariant");
2235 _recursions = node->_recursions; // restore the old recursion count
2236 _waiters--; // decrement the number of waiters
2237
2238 if (node->_interrupted) {
2239 // We will throw at thaw end after finishing the mount transition.
2240 current->set_pending_interrupted_exception(true);
2241 }
2242
2243 delete node;
2244 // Clear the ObjectWaiter* from the vthread.
2245 java_lang_VirtualThread::set_objectWaiter(current->vthread(), nullptr);
2246 return true;
2247 }
2248 } else {
2249 // Already moved to _entry_list by notifier, so just add to contentions.
2250 add_to_contentions(1);
2251 }
2252 return false;
2253 }
2254
2255 // -----------------------------------------------------------------------------
2256 // Adaptive Spinning Support
2257 //
2258 // Adaptive spin-then-block - rational spinning
2259 //
2260 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
2261 // algorithm.
2262 //
2263 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
2264 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
2265 // (duration) or we can fix the count at approximately the duration of
2266 // a context switch and vary the frequency. Of course we could also
2267 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
2268 // For a description of 'Adaptive spin-then-block mutual exclusion in
2269 // multi-threaded processing,' see U.S. Pat. No. 8046758.
2270 //
2271 // This implementation varies the duration "D", where D varies with
2272 // the success rate of recent spin attempts. (D is capped at approximately
2273 // length of a round-trip context switch). The success rate for recent
2274 // spin attempts is a good predictor of the success rate of future spin
2275 // attempts. The mechanism adapts automatically to varying critical
2276 // section length (lock modality), system load and degree of parallelism.
2277 // D is maintained per-monitor in _SpinDuration and is initialized
2278 // optimistically. Spin frequency is fixed at 100%.
2279 //
2280 // Note that _SpinDuration is volatile, but we update it without locks
2281 // or atomics. The code is designed so that _SpinDuration stays within
2282 // a reasonable range even in the presence of races. The arithmetic
2283 // operations on _SpinDuration are closed over the domain of legal values,
2284 // so at worst a race will install and older but still legal value.
2285 // At the very worst this introduces some apparent non-determinism.
2286 // We might spin when we shouldn't or vice-versa, but since the spin
2287 // count are relatively short, even in the worst case, the effect is harmless.
2288 //
2289 // Care must be taken that a low "D" value does not become an
2290 // an absorbing state. Transient spinning failures -- when spinning
2291 // is overall profitable -- should not cause the system to converge
2292 // on low "D" values. We want spinning to be stable and predictable
2293 // and fairly responsive to change and at the same time we don't want
2294 // it to oscillate, become metastable, be "too" non-deterministic,
2295 // or converge on or enter undesirable stable absorbing states.
2296 //
2297 // We implement a feedback-based control system -- using past behavior
2298 // to predict future behavior. We face two issues: (a) if the
2299 // input signal is random then the spin predictor won't provide optimal
2300 // results, and (b) if the signal frequency is too high then the control
2301 // system, which has some natural response lag, will "chase" the signal.
2302 // (b) can arise from multimodal lock hold times. Transient preemption
2303 // can also result in apparent bimodal lock hold times.
2304 // Although sub-optimal, neither condition is particularly harmful, as
2305 // in the worst-case we'll spin when we shouldn't or vice-versa.
2306 // The maximum spin duration is rather short so the failure modes aren't bad.
2307 // To be conservative, I've tuned the gain in system to bias toward
2308 // _not spinning. Relatedly, the system can sometimes enter a mode where it
2309 // "rings" or oscillates between spinning and not spinning. This happens
2310 // when spinning is just on the cusp of profitability, however, so the
2311 // situation is not dire. The state is benign -- there's no need to add
2312 // hysteresis control to damp the transition rate between spinning and
2313 // not spinning.
2314
2315 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool
2316
2317 static int Knob_Bonus = 100; // spin success bonus
2318 static int Knob_Penalty = 200; // spin failure penalty
2319 static int Knob_Poverty = 1000;
2320 static int Knob_FixedSpin = 0;
2321 static int Knob_PreSpin = 10; // 20-100 likely better, but it's not better in my testing.
2322
2323 inline static int adjust_up(int spin_duration) {
2324 int x = spin_duration;
2325 if (x < ObjectMonitor::Knob_SpinLimit) {
2326 if (x < Knob_Poverty) {
2327 x = Knob_Poverty;
2328 }
2329 return x + Knob_Bonus;
2330 } else {
2331 return spin_duration;
2332 }
2333 }
2334
2335 inline static int adjust_down(int spin_duration) {
2336 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2337 // AIMD is globally stable.
2338 int x = spin_duration;
2339 if (x > 0) {
2340 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2341 // This is globally sample and tends to damp the response.
2342 x -= Knob_Penalty;
2343 if (x < 0) { x = 0; }
2344 return x;
2345 } else {
2346 return spin_duration;
2347 }
2348 }
2349
2350 bool ObjectMonitor::short_fixed_spin(JavaThread* current, int spin_count, bool adapt) {
2351 for (int ctr = 0; ctr < spin_count; ctr++) {
2352 TryLockResult status = try_lock(current);
2353 if (status == TryLockResult::Success) {
2354 if (adapt) {
2355 _SpinDuration = adjust_up(_SpinDuration);
2356 }
2357 return true;
2358 } else if (status == TryLockResult::Interference) {
2359 break;
2360 }
2361 SpinPause();
2362 }
2363 return false;
2364 }
2365
2366 // Spinning: Fixed frequency (100%), vary duration
2367 bool ObjectMonitor::try_spin(JavaThread* current) {
2368
2369 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
2370 int knob_fixed_spin = Knob_FixedSpin; // 0 (don't spin: default), 2000 good test
2371 if (knob_fixed_spin > 0) {
2372 return short_fixed_spin(current, knob_fixed_spin, false);
2373 }
2374
2375 // Admission control - verify preconditions for spinning
2376 //
2377 // We always spin a little bit, just to prevent _SpinDuration == 0 from
2378 // becoming an absorbing state. Put another way, we spin briefly to
2379 // sample, just in case the system load, parallelism, contention, or lock
2380 // modality changed.
2381
2382 int knob_pre_spin = Knob_PreSpin; // 10 (default), 100, 1000 or 2000
2383 if (short_fixed_spin(current, knob_pre_spin, true)) {
2384 return true;
2385 }
2386
2387 //
2388 // Consider the following alternative:
2389 // Periodically set _SpinDuration = _SpinLimit and try a long/full
2390 // spin attempt. "Periodically" might mean after a tally of
2391 // the # of failed spin attempts (or iterations) reaches some threshold.
2392 // This takes us into the realm of 1-out-of-N spinning, where we
2393 // hold the duration constant but vary the frequency.
2394
2395 int ctr = _SpinDuration;
2396 if (ctr <= 0) return false;
2397
2398 // We're good to spin ... spin ingress.
2399 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
2400 // when preparing to LD...CAS _owner, etc and the CAS is likely
2401 // to succeed.
2402 if (!has_successor()) {
2403 set_successor(current);
2404 }
2405 int64_t prv = NO_OWNER;
2406
2407 // There are three ways to exit the following loop:
2408 // 1. A successful spin where this thread has acquired the lock.
2409 // 2. Spin failure with prejudice
2410 // 3. Spin failure without prejudice
2411
2412 while (--ctr >= 0) {
2413
2414 // Periodic polling -- Check for pending GC
2415 // Threads may spin while they're unsafe.
2416 // We don't want spinning threads to delay the JVM from reaching
2417 // a stop-the-world safepoint or to steal cycles from GC.
2418 // If we detect a pending safepoint we abort in order that
2419 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
2420 // this thread, if safe, doesn't steal cycles from GC.
2421 // This is in keeping with the "no loitering in runtime" rule.
2422 // We periodically check to see if there's a safepoint pending.
2423 if ((ctr & 0xFF) == 0) {
2424 // Can't call SafepointMechanism::should_process() since that
2425 // might update the poll values and we could be in a thread_blocked
2426 // state here which is not allowed so just check the poll.
2427 if (SafepointMechanism::local_poll_armed(current)) {
2428 break;
2429 }
2430 SpinPause();
2431 }
2432
2433 // Probe _owner with TATAS
2434 // If this thread observes the monitor transition or flicker
2435 // from locked to unlocked to locked, then the odds that this
2436 // thread will acquire the lock in this spin attempt go down
2437 // considerably. The same argument applies if the CAS fails
2438 // or if we observe _owner change from one non-null value to
2439 // another non-null value. In such cases we might abort
2440 // the spin without prejudice or apply a "penalty" to the
2441 // spin count-down variable "ctr", reducing it by 100, say.
2442
2443 int64_t ox = owner_raw();
2444 if (ox == NO_OWNER) {
2445 ox = try_set_owner_from(NO_OWNER, current);
2446 if (ox == NO_OWNER) {
2447 // The CAS succeeded -- this thread acquired ownership
2448 // Take care of some bookkeeping to exit spin state.
2449 if (has_successor(current)) {
2450 clear_successor();
2451 }
2452
2453 // Increase _SpinDuration :
2454 // The spin was successful (profitable) so we tend toward
2455 // longer spin attempts in the future.
2456 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2457 // If we acquired the lock early in the spin cycle it
2458 // makes sense to increase _SpinDuration proportionally.
2459 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2460 _SpinDuration = adjust_up(_SpinDuration);
2461 return true;
2462 }
2463
2464 // The CAS failed ... we can take any of the following actions:
2465 // * penalize: ctr -= CASPenalty
2466 // * exit spin with prejudice -- abort without adapting spinner
2467 // * exit spin without prejudice.
2468 // * Since CAS is high-latency, retry again immediately.
2469 break;
2470 }
2471
2472 // Did lock ownership change hands ?
2473 if (ox != prv && prv != NO_OWNER) {
2474 break;
2475 }
2476 prv = ox;
2477
2478 if (!has_successor()) {
2479 set_successor(current);
2480 }
2481 }
2482
2483 // Spin failed with prejudice -- reduce _SpinDuration.
2484 if (ctr < 0) {
2485 _SpinDuration = adjust_down(_SpinDuration);
2486 }
2487
2488 if (has_successor(current)) {
2489 clear_successor();
2490 // Invariant: after setting succ=null a contending thread
2491 // must recheck-retry _owner before parking. This usually happens
2492 // in the normal usage of try_spin(), but it's safest
2493 // to make try_spin() as foolproof as possible.
2494 OrderAccess::fence();
2495 if (try_lock(current) == TryLockResult::Success) {
2496 return true;
2497 }
2498 }
2499
2500 return false;
2501 }
2502
2503
2504 // -----------------------------------------------------------------------------
2505 // wait_set management ...
2506
2507 ObjectWaiter::ObjectWaiter(JavaThread* current) {
2508 _next = nullptr;
2509 _prev = nullptr;
2510 _thread = current;
2511 _monitor = nullptr;
2512 _notifier_tid = 0;
2513 _recursions = 0;
2514 TState = TS_RUN;
2515 _is_wait = false;
2516 _at_reenter = false;
2517 _interrupted = false;
2518 _do_timed_park = false;
2519 _active = false;
2520 }
2521
2522 ObjectWaiter::ObjectWaiter(oop vthread, ObjectMonitor* mon) : ObjectWaiter(nullptr) {
2523 assert(oopDesc::is_oop(vthread), "");
2524 _vthread = OopHandle(JavaThread::thread_oop_storage(), vthread);
2525 _monitor = mon;
2526 }
2527
2528 ObjectWaiter::~ObjectWaiter() {
2529 if (is_vthread()) {
2530 assert(vthread() != nullptr, "");
2531 _vthread.release(JavaThread::thread_oop_storage());
2532 }
2533 }
2534
2535 oop ObjectWaiter::vthread() const {
2536 return _vthread.resolve();
2537 }
2538
2539 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2540 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(_thread, mon);
2541 }
2542
2543 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2544 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(_thread, _active);
2545 }
2546
2547 inline void ObjectMonitor::add_waiter(ObjectWaiter* node) {
2548 assert(node != nullptr, "should not add null node");
2549 assert(node->_prev == nullptr, "node already in list");
2550 assert(node->_next == nullptr, "node already in list");
2551 // put node at end of queue (circular doubly linked list)
2552 if (_wait_set == nullptr) {
2553 _wait_set = node;
2554 node->_prev = node;
2555 node->_next = node;
2556 } else {
2557 ObjectWaiter* head = _wait_set;
2558 ObjectWaiter* tail = head->_prev;
2559 assert(tail->_next == head, "invariant check");
2560 tail->_next = node;
2561 head->_prev = node;
2562 node->_next = head;
2563 node->_prev = tail;
2564 }
2565 }
2566
2567 inline ObjectWaiter* ObjectMonitor::dequeue_waiter() {
2568 // dequeue the very first waiter
2569 ObjectWaiter* waiter = _wait_set;
2570 if (waiter) {
2571 dequeue_specific_waiter(waiter);
2572 }
2573 return waiter;
2574 }
2575
2576 inline void ObjectMonitor::dequeue_specific_waiter(ObjectWaiter* node) {
2577 assert(node != nullptr, "should not dequeue nullptr node");
2578 assert(node->_prev != nullptr, "node already removed from list");
2579 assert(node->_next != nullptr, "node already removed from list");
2580 // when the waiter has woken up because of interrupt,
2581 // timeout or other spurious wake-up, dequeue the
2582 // waiter from waiting list
2583 ObjectWaiter* next = node->_next;
2584 if (next == node) {
2585 assert(node->_prev == node, "invariant check");
2586 _wait_set = nullptr;
2587 } else {
2588 ObjectWaiter* prev = node->_prev;
2589 assert(prev->_next == node, "invariant check");
2590 assert(next->_prev == node, "invariant check");
2591 next->_prev = prev;
2592 prev->_next = next;
2593 if (_wait_set == node) {
2594 _wait_set = next;
2595 }
2596 }
2597 node->_next = nullptr;
2598 node->_prev = nullptr;
2599 }
2600
2601 // -----------------------------------------------------------------------------
2602
2603 // One-shot global initialization for the sync subsystem.
2604 // We could also defer initialization and initialize on-demand
2605 // the first time we call ObjectSynchronizer::inflate().
2606 // Initialization would be protected - like so many things - by
2607 // the MonitorCache_lock.
2608
2609 void ObjectMonitor::Initialize() {
2610 assert(!InitDone, "invariant");
2611
2612 if (!os::is_MP()) {
2613 Knob_SpinLimit = 0;
2614 Knob_PreSpin = 0;
2615 Knob_FixedSpin = -1;
2616 }
2617
2618 _oop_storage = OopStorageSet::create_weak("ObjectSynchronizer Weak", mtSynchronizer);
2619
2620 DEBUG_ONLY(InitDone = true;)
2621 }
2622
2623 // We can't call this during Initialize() because BarrierSet needs to be set.
2624 void ObjectMonitor::Initialize2() {
2625 _vthread_list_head = OopHandle(JavaThread::thread_oop_storage(), nullptr);
2626 _vthread_unparker_ParkEvent = ParkEvent::Allocate(nullptr);
2627 }
2628
2629 void ObjectMonitor::print_on(outputStream* st) const {
2630 // The minimal things to print for markWord printing, more can be added for debugging and logging.
2631 st->print("{contentions=0x%08x,waiters=0x%08x"
2632 ",recursions=%zd,owner=" INT64_FORMAT "}",
2633 contentions(), waiters(), recursions(),
2634 owner_raw());
2635 }
2636 void ObjectMonitor::print() const { print_on(tty); }
2637
2638 #ifdef ASSERT
2639 // Print the ObjectMonitor like a debugger would:
2640 //
2641 // (ObjectMonitor) 0x00007fdfb6012e40 = {
2642 // _metadata = 0x0000000000000001
2643 // _object = 0x000000070ff45fd0
2644 // _pad_buf0 = {
2645 // [0] = '\0'
2646 // ...
2647 // [43] = '\0'
2648 // }
2649 // _owner = 0x0000000000000000
2650 // _previous_owner_tid = 0
2651 // _pad_buf1 = {
2652 // [0] = '\0'
2653 // ...
2654 // [47] = '\0'
2655 // }
2656 // _next_om = 0x0000000000000000
2657 // _recursions = 0
2658 // _entry_list = 0x0000000000000000
2659 // _entry_list_tail = 0x0000000000000000
2660 // _succ = 0x0000000000000000
2661 // _SpinDuration = 5000
2662 // _contentions = 0
2663 // _wait_set = 0x0000700009756248
2664 // _waiters = 1
2665 // _wait_set_lock = 0
2666 // }
2667 //
2668 void ObjectMonitor::print_debug_style_on(outputStream* st) const {
2669 st->print_cr("(ObjectMonitor*) " INTPTR_FORMAT " = {", p2i(this));
2670 st->print_cr(" _metadata = " INTPTR_FORMAT, _metadata);
2671 st->print_cr(" _object = " INTPTR_FORMAT, p2i(object_peek()));
2672 st->print_cr(" _pad_buf0 = {");
2673 st->print_cr(" [0] = '\\0'");
2674 st->print_cr(" ...");
2675 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf0) - 1);
2676 st->print_cr(" }");
2677 st->print_cr(" _owner = " INT64_FORMAT, owner_raw());
2678 st->print_cr(" _previous_owner_tid = " UINT64_FORMAT, _previous_owner_tid);
2679 st->print_cr(" _pad_buf1 = {");
2680 st->print_cr(" [0] = '\\0'");
2681 st->print_cr(" ...");
2682 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf1) - 1);
2683 st->print_cr(" }");
2684 st->print_cr(" _next_om = " INTPTR_FORMAT, p2i(next_om()));
2685 st->print_cr(" _recursions = %zd", _recursions);
2686 st->print_cr(" _entry_list = " INTPTR_FORMAT, p2i(_entry_list));
2687 st->print_cr(" _entry_list_tail = " INTPTR_FORMAT, p2i(_entry_list_tail));
2688 st->print_cr(" _succ = " INT64_FORMAT, successor());
2689 st->print_cr(" _SpinDuration = %d", _SpinDuration);
2690 st->print_cr(" _contentions = %d", contentions());
2691 st->print_cr(" _unmounted_vthreads = " INT64_FORMAT, _unmounted_vthreads);
2692 st->print_cr(" _wait_set = " INTPTR_FORMAT, p2i(_wait_set));
2693 st->print_cr(" _waiters = %d", _waiters);
2694 st->print_cr(" _wait_set_lock = %d", _wait_set_lock);
2695 st->print_cr("}");
2696 }
2697 #endif
--- EOF ---