1 /*
2 * Copyright (c) 1998, 2025, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 #include "classfile/vmSymbols.hpp"
26 #include "gc/shared/oopStorage.hpp"
27 #include "gc/shared/oopStorageSet.hpp"
28 #include "jfr/jfrEvents.hpp"
29 #include "jfr/support/jfrThreadId.hpp"
30 #include "logging/log.hpp"
31 #include "logging/logStream.hpp"
32 #include "memory/allocation.inline.hpp"
33 #include "memory/resourceArea.hpp"
34 #include "oops/markWord.hpp"
35 #include "oops/oop.inline.hpp"
36 #include "oops/oopHandle.inline.hpp"
37 #include "oops/weakHandle.inline.hpp"
38 #include "prims/jvmtiDeferredUpdates.hpp"
39 #include "prims/jvmtiExport.hpp"
40 #include "runtime/atomic.hpp"
41 #include "runtime/continuationWrapper.inline.hpp"
42 #include "runtime/globals.hpp"
43 #include "runtime/handles.inline.hpp"
44 #include "runtime/interfaceSupport.inline.hpp"
45 #include "runtime/javaThread.inline.hpp"
46 #include "runtime/lightweightSynchronizer.hpp"
47 #include "runtime/mutexLocker.hpp"
48 #include "runtime/objectMonitor.hpp"
49 #include "runtime/objectMonitor.inline.hpp"
50 #include "runtime/orderAccess.hpp"
51 #include "runtime/osThread.hpp"
52 #include "runtime/safefetch.hpp"
53 #include "runtime/safepointMechanism.inline.hpp"
54 #include "runtime/sharedRuntime.hpp"
55 #include "runtime/threads.hpp"
56 #include "services/threadService.hpp"
57 #include "utilities/debug.hpp"
58 #include "utilities/dtrace.hpp"
59 #include "utilities/globalCounter.inline.hpp"
60 #include "utilities/globalDefinitions.hpp"
61 #include "utilities/macros.hpp"
62 #include "utilities/preserveException.hpp"
63 #if INCLUDE_JFR
64 #include "jfr/support/jfrFlush.hpp"
65 #endif
66
67 #ifdef DTRACE_ENABLED
68
69 // Only bother with this argument setup if dtrace is available
70 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
71
72
73 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
74 char* bytes = nullptr; \
75 int len = 0; \
76 jlong jtid = SharedRuntime::get_java_tid(thread); \
77 Symbol* klassname = obj->klass()->name(); \
78 if (klassname != nullptr) { \
79 bytes = (char*)klassname->bytes(); \
80 len = klassname->utf8_length(); \
81 }
82
83 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
84 { \
85 if (DTraceMonitorProbes) { \
86 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
87 HOTSPOT_MONITOR_WAIT(jtid, \
88 (monitor), bytes, len, (millis)); \
89 } \
90 }
91
92 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
93 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
94 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
95 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
96 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
97
98 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
99 { \
100 if (DTraceMonitorProbes) { \
101 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
102 HOTSPOT_MONITOR_##probe(jtid, \
103 (uintptr_t)(monitor), bytes, len); \
104 } \
105 }
106
107 #else // ndef DTRACE_ENABLED
108
109 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
110 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
111
112 #endif // ndef DTRACE_ENABLED
113
114 DEBUG_ONLY(static volatile bool InitDone = false;)
115
116 OopStorage* ObjectMonitor::_oop_storage = nullptr;
117
118 OopHandle ObjectMonitor::_vthread_list_head;
119 ParkEvent* ObjectMonitor::_vthread_unparker_ParkEvent = nullptr;
120
121 // -----------------------------------------------------------------------------
122 // Theory of operations -- Monitors lists, thread residency, etc:
123 //
124 // * A thread acquires ownership of a monitor by successfully
125 // CAS()ing the _owner field from NO_OWNER/DEFLATER_MARKER to
126 // its owner_id (return value from owner_id_from()).
127 //
128 // * Invariant: A thread appears on at most one monitor list --
129 // entry_list or wait_set -- at any one time.
130 //
131 // * Contending threads "push" themselves onto the entry_list with CAS
132 // and then spin/park.
133 // If the thread is a virtual thread it will first attempt to
134 // unmount itself. The virtual thread will first try to freeze
135 // all frames in the heap. If the operation fails it will just
136 // follow the regular path for platform threads. If the operation
137 // succeeds, it will push itself onto the entry_list with CAS and then
138 // return back to Java to continue the unmount logic.
139 //
140 // * After a contending thread eventually acquires the lock it must
141 // dequeue itself from the entry_list.
142 //
143 // * The exiting thread identifies and unparks an "heir presumptive"
144 // tentative successor thread on the entry_list. In case the successor
145 // is an unmounted virtual thread, the exiting thread will first try
146 // to add it to the list of vthreads waiting to be unblocked, and on
147 // success it will unpark the special unblocker thread instead, which
148 // will be in charge of submitting the vthread back to the scheduler
149 // queue. Critically, the exiting thread doesn't unlink the successor
150 // thread from the entry_list. After having been unparked/re-scheduled,
151 // the wakee will recontend for ownership of the monitor. The successor
152 // (wakee) will either acquire the lock or re-park/unmount itself.
153 //
154 // Succession is provided for by a policy of competitive handoff.
155 // The exiting thread does _not_ grant or pass ownership to the
156 // successor thread. (This is also referred to as "handoff succession").
157 // Instead the exiting thread releases ownership and possibly wakes
158 // a successor, so the successor can (re)compete for ownership of the lock.
159 //
160 // * The entry_list forms a queue of threads stalled trying to acquire
161 // the lock. Within the entry_list the next pointers always form a
162 // consistent singly linked list. At unlock-time when the unlocking
163 // thread notices that the tail of the entry_list is not known, we
164 // convert the singly linked entry_list into a doubly linked list by
165 // assigning the prev pointers and the entry_list_tail pointer.
166 //
167 // Example:
168 //
169 // The first contending thread that "pushed" itself onto entry_list,
170 // will be the last thread in the list. Each newly pushed thread in
171 // entry_list will be linked through its next pointer, and have its
172 // prev pointer set to null. Thus pushing six threads A-F (in that
173 // order) onto entry_list, will form a singly linked list, see 1)
174 // below.
175 //
176 // 1) entry_list ->F->E->D->C->B->A->null
177 // entry_list_tail ->null
178 //
179 // Since the successor is chosen in FIFO order, the exiting thread
180 // needs to find the tail of the entry_list. This is done by walking
181 // from the entry_list head. While walking the list we also assign
182 // the prev pointers of each thread, essentially forming a doubly
183 // linked list, see 2) below.
184 //
185 // 2) entry_list ->F<=>E<=>D<=>C<=>B<=>A->null
186 // entry_list_tail ----------------------^
187 //
188 // Once we have formed a doubly linked list it's easy to find the
189 // successor (A), wake it up, have it remove itself, and update the
190 // tail pointer, as seen in and 3) below.
191 //
192 // 3) entry_list ->F<=>E<=>D<=>C<=>B->null
193 // entry_list_tail ------------------^
194 //
195 // At any time new threads can add themselves to the entry_list, see
196 // 4) below.
197 //
198 // 4) entry_list ->I->H->G->F<=>E<=>D->null
199 // entry_list_tail -------------------^
200 //
201 // At some point in time the thread (F) that wants to remove itself
202 // from the end of the list, will not have any prev pointer, see 5)
203 // below.
204 //
205 // 5) entry_list ->I->H->G->F->null
206 // entry_list_tail -----------^
207 //
208 // To resolve this we just start walking from the entry_list head
209 // again, forming a new doubly linked list, before removing the
210 // thread (F), see 6) and 7) below.
211 //
212 // 6) entry_list ->I<=>H<=>G<=>F->null
213 // entry_list_tail --------------^
214 //
215 // 7) entry_list ->I<=>H<=>G->null
216 // entry_list_tail ----------^
217 //
218 // * The monitor itself protects all of the operations on the
219 // entry_list except for the CAS of a new arrival to the head. Only
220 // the monitor owner can read or write the prev links (e.g. to
221 // remove itself) or update the tail.
222 //
223 // * The monitor entry list operations avoid locks, but strictly speaking
224 // they're not lock-free. Enter is lock-free, exit is not.
225 // For a description of 'Methods and apparatus providing non-blocking access
226 // to a resource,' see U.S. Pat. No. 7844973.
227 //
228 // * The entry_list can have multiple concurrent "pushers" but only
229 // one concurrent detaching thread. There is no ABA-problem with
230 // this usage of CAS.
231 //
232 // * As long as the entry_list_tail is known the odds are good that we
233 // should be able to dequeue after acquisition (in the ::enter()
234 // epilogue) in constant-time. This is good since a key desideratum
235 // is to minimize queue & monitor metadata manipulation that occurs
236 // while holding the monitor lock -- that is, we want to minimize
237 // monitor lock holds times. Note that even a small amount of fixed
238 // spinning will greatly reduce the # of enqueue-dequeue operations
239 // on entry_list. That is, spinning relieves contention on the
240 // "inner" locks and monitor metadata.
241 //
242 // Insert and delete operations may not operate in constant-time if
243 // we have interference because some other thread is adding or
244 // removing the head element of entry_list or if we need to convert
245 // the singly linked entry_list into a doubly linked list to find the
246 // tail.
247 //
248 // * The monitor synchronization subsystem avoids the use of native
249 // synchronization primitives except for the narrow platform-specific
250 // park-unpark abstraction. See the comments in os_posix.cpp regarding
251 // the semantics of park-unpark. Put another way, this monitor implementation
252 // depends only on atomic operations and park-unpark.
253 //
254 // * Waiting threads reside on the wait_set list -- wait() puts
255 // the caller onto the wait_set.
256 //
257 // * notify() or notifyAll() simply transfers threads from the wait_set
258 // to the entry_list. Subsequent exit() operations will
259 // unpark/re-schedule the notifyee. Unparking/re-scheduling a
260 // notifyee in notify() is inefficient - it's likely the notifyee
261 // would simply impale itself on the lock held by the notifier.
262
263 // Check that object() and set_object() are called from the right context:
264 static void check_object_context() {
265 #ifdef ASSERT
266 Thread* self = Thread::current();
267 if (self->is_Java_thread()) {
268 // Mostly called from JavaThreads so sanity check the thread state.
269 JavaThread* jt = JavaThread::cast(self);
270 switch (jt->thread_state()) {
271 case _thread_in_vm: // the usual case
272 case _thread_in_Java: // during deopt
273 break;
274 default:
275 fatal("called from an unsafe thread state");
276 }
277 assert(jt->is_active_Java_thread(), "must be active JavaThread");
278 } else {
279 // However, ThreadService::get_current_contended_monitor()
280 // can call here via the VMThread so sanity check it.
281 assert(self->is_VM_thread(), "must be");
282 }
283 #endif // ASSERT
284 }
285
286 ObjectMonitor::ObjectMonitor(oop object) :
287 _metadata(0),
288 _object(_oop_storage, object),
289 _owner(NO_OWNER),
290 _previous_owner_tid(0),
291 _next_om(nullptr),
292 _recursions(0),
293 _entry_list(nullptr),
294 _entry_list_tail(nullptr),
295 _succ(NO_OWNER),
296 _SpinDuration(ObjectMonitor::Knob_SpinLimit),
297 _contentions(0),
298 _unmounted_vthreads(0),
299 _wait_set(nullptr),
300 _waiters(0),
301 _wait_set_lock(0),
302 _stack_locker(nullptr)
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 = Atomic::load(&_entry_list);
702 node->_next = head;
703 if (Atomic::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 = Atomic::load(&_entry_list);
721 node->_next = head;
722 if (Atomic::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 (Atomic::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 = Atomic::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 in the _entry_list do a timed-park
990 // instead to alleviate some deadlocks cases where one of them is picked as
991 // the successor but cannot run due to having run out of carriers. This can
992 // happen, for example, if this is a pinned virtual thread currently loading
993 // or initializining a class, and all other carriers have a pinned vthread
994 // waiting for said class to be loaded/initialized.
995 // Read counter *after* adding this thread to the _entry_list.
996 // Adding to _entry_list uses Atomic::cmpxchg() which already provides
997 // a fence that prevents this load from floating up previous store.
998 bool do_timed_parked = has_unmounted_vthreads();
999 static int MAX_RECHECK_INTERVAL = 1000;
1000 int 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((jlong) 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 in the _entry_list do a timed-park
1096 // instead to alleviate some deadlocks cases where one of them is picked as
1097 // the successor but cannot run due to having run out of carriers. This can
1098 // happen, for example, if this is a pinned virtual thread (or plain carrier)
1099 // waiting for a class to be initialized.
1100 // In theory we only get here in the "notification" case where the thread has
1101 // already been added to the _entry_list. But if the thread happened to be interrupted
1102 // at the same time it was being notified, we could have read a state of TS_ENTER
1103 // that led us here but the thread hasn't been added yet to the queue. In that
1104 // case getting a false value from has_unmounted_vthreads() is not a guarantee
1105 // that vthreads weren't added before this thread to the _entry_list. We will live
1106 // with this corner case not only because it would be very rare, but also because
1107 // if there are several carriers blocked in this same situation, this would only
1108 // happen for the first one notified.
1109 bool do_timed_parked = has_unmounted_vthreads();
1110 static int MAX_RECHECK_INTERVAL = 1000;
1111 int recheck_interval = 1;
1112
1113 for (;;) {
1114 ObjectWaiter::TStates v = currentNode->TState;
1115 guarantee(v == ObjectWaiter::TS_ENTER, "invariant");
1116 assert(!has_owner(current), "invariant");
1117
1118 // This thread has been notified so try to reacquire the lock.
1119 if (try_lock(current) == TryLockResult::Success) {
1120 break;
1121 }
1122
1123 // If that fails, spin again. Note that spin count may be zero so the above TryLock
1124 // is necessary.
1125 if (try_spin(current)) {
1126 break;
1127 }
1128
1129 {
1130 OSThreadContendState osts(current->osthread());
1131
1132 assert(current->thread_state() == _thread_in_vm, "invariant");
1133
1134 {
1135 ClearSuccOnSuspend csos(this);
1136 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */);
1137 if (do_timed_parked) {
1138 current->_ParkEvent->park((jlong) recheck_interval);
1139 // Increase the recheck_interval, but clamp the value.
1140 recheck_interval *= 8;
1141 if (recheck_interval > MAX_RECHECK_INTERVAL) {
1142 recheck_interval = MAX_RECHECK_INTERVAL;
1143 }
1144 } else {
1145 current->_ParkEvent->park();
1146 }
1147 }
1148 }
1149
1150 // Try again, but just so we distinguish between futile wakeups and
1151 // successful wakeups. The following test isn't algorithmically
1152 // necessary, but it helps us maintain sensible statistics.
1153 if (try_lock(current) == TryLockResult::Success) {
1154 break;
1155 }
1156
1157 // The lock is still contested.
1158
1159 // Assuming this is not a spurious wakeup we'll normally
1160 // find that _succ == current.
1161 if (has_successor(current)) clear_successor();
1162
1163 // Invariant: after clearing _succ a contending thread
1164 // *must* retry _owner before parking.
1165 OrderAccess::fence();
1166 }
1167
1168 // Current has acquired the lock -- Unlink current from the _entry_list.
1169 assert(has_owner(current), "invariant");
1170 assert_mark_word_consistency();
1171 unlink_after_acquire(current, currentNode);
1172 if (has_successor(current)) clear_successor();
1173 assert(!has_successor(current), "invariant");
1174 currentNode->TState = ObjectWaiter::TS_RUN;
1175 OrderAccess::fence(); // see comments at the end of enter_internal()
1176 }
1177
1178 // This method is called from two places:
1179 // - On monitorenter contention with a null waiter.
1180 // - After Object.wait() times out or the target is interrupted to reenter the
1181 // monitor, with the existing waiter.
1182 // For the Object.wait() case we do not delete the ObjectWaiter in case we
1183 // succesfully acquire the monitor since we are going to need it on return.
1184 bool ObjectMonitor::vthread_monitor_enter(JavaThread* current, ObjectWaiter* waiter) {
1185 if (try_lock(current) == TryLockResult::Success) {
1186 assert(has_owner(current), "invariant");
1187 assert(!has_successor(current), "invariant");
1188 return true;
1189 }
1190
1191 oop vthread = current->vthread();
1192 ObjectWaiter* node = waiter != nullptr ? waiter : new ObjectWaiter(vthread, this);
1193
1194 // Increment counter *before* adding the vthread to the _entry_list.
1195 // Adding to _entry_list uses Atomic::cmpxchg() which already provides
1196 // a fence that prevents reordering of the stores.
1197 inc_unmounted_vthreads();
1198
1199 if (try_lock_or_add_to_entry_list(current, node)) {
1200 // We got the lock.
1201 if (waiter == nullptr) delete node; // for Object.wait() don't delete yet
1202 dec_unmounted_vthreads();
1203 return true;
1204 }
1205 // This thread is now added to the entry_list.
1206
1207 // We have to try once more since owner could have exited monitor and checked
1208 // _entry_list before we added the node to the queue.
1209 if (try_lock(current) == TryLockResult::Success) {
1210 assert(has_owner(current), "invariant");
1211 unlink_after_acquire(current, node);
1212 if (has_successor(current)) clear_successor();
1213 if (waiter == nullptr) delete node; // for Object.wait() don't delete yet
1214 dec_unmounted_vthreads();
1215 return true;
1216 }
1217
1218 assert(java_lang_VirtualThread::state(vthread) == java_lang_VirtualThread::RUNNING, "wrong state for vthread");
1219 java_lang_VirtualThread::set_state(vthread, java_lang_VirtualThread::BLOCKING);
1220
1221 // We didn't succeed in acquiring the monitor so increment _contentions and
1222 // save ObjectWaiter* in the vthread since we will need it when resuming execution.
1223 add_to_contentions(1);
1224 java_lang_VirtualThread::set_objectWaiter(vthread, node);
1225 return false;
1226 }
1227
1228 // Called from thaw code to resume the monitor operation that caused the vthread
1229 // to be unmounted. Method returns true if the monitor is successfully acquired,
1230 // which marks the end of the monitor operation, otherwise it returns false.
1231 bool ObjectMonitor::resume_operation(JavaThread* current, ObjectWaiter* node, ContinuationWrapper& cont) {
1232 assert(java_lang_VirtualThread::state(current->vthread()) == java_lang_VirtualThread::RUNNING, "wrong state for vthread");
1233 assert(!has_owner(current), "");
1234
1235 if (node->is_wait() && !node->at_reenter()) {
1236 bool acquired_monitor = vthread_wait_reenter(current, node, cont);
1237 if (acquired_monitor) return true;
1238 }
1239
1240 // Retry acquiring monitor...
1241
1242 int state = node->TState;
1243 guarantee(state == ObjectWaiter::TS_ENTER, "invariant");
1244
1245 if (try_lock(current) == TryLockResult::Success) {
1246 vthread_epilog(current, node);
1247 return true;
1248 }
1249
1250 oop vthread = current->vthread();
1251 if (has_successor(current)) clear_successor();
1252
1253 // Invariant: after clearing _succ a thread *must* retry acquiring the monitor.
1254 OrderAccess::fence();
1255
1256 if (try_lock(current) == TryLockResult::Success) {
1257 vthread_epilog(current, node);
1258 return true;
1259 }
1260
1261 // We will return to Continuation.run() and unmount so set the right state.
1262 java_lang_VirtualThread::set_state(vthread, java_lang_VirtualThread::BLOCKING);
1263
1264 return false;
1265 }
1266
1267 void ObjectMonitor::vthread_epilog(JavaThread* current, ObjectWaiter* node) {
1268 assert(has_owner(current), "invariant");
1269 add_to_contentions(-1);
1270 dec_unmounted_vthreads();
1271
1272 if (has_successor(current)) clear_successor();
1273
1274 guarantee(_recursions == 0, "invariant");
1275
1276 if (node->is_wait()) {
1277 _recursions = node->_recursions; // restore the old recursion count
1278 _waiters--; // decrement the number of waiters
1279
1280 if (node->_interrupted) {
1281 // We will throw at thaw end after finishing the mount transition.
1282 current->set_pending_interrupted_exception(true);
1283 }
1284 }
1285
1286 unlink_after_acquire(current, node);
1287 delete node;
1288
1289 // Clear the ObjectWaiter* from the vthread.
1290 java_lang_VirtualThread::set_objectWaiter(current->vthread(), nullptr);
1291
1292 if (JvmtiExport::should_post_monitor_contended_entered()) {
1293 // We are going to call thaw again after this and finish the VMTS
1294 // transition so no need to do it here. We will post the event there.
1295 current->set_contended_entered_monitor(this);
1296 }
1297 }
1298
1299 // Convert entry_list into a doubly linked list by assigning the prev
1300 // pointers and the entry_list_tail pointer (if needed). Within the
1301 // entry_list the next pointers always form a consistent singly linked
1302 // list. When this function is called, the entry_list will be either
1303 // singly linked, or starting as singly linked (at the head), but
1304 // ending as doubly linked (at the tail).
1305 void ObjectMonitor::entry_list_build_dll(JavaThread* current) {
1306 assert(has_owner(current), "invariant");
1307 ObjectWaiter* prev = nullptr;
1308 // Need acquire here to match the implicit release of the cmpxchg
1309 // that updated entry_list, so we can access w->prev().
1310 ObjectWaiter* w = Atomic::load_acquire(&_entry_list);
1311 assert(w != nullptr, "should only be called when entry list is not empty");
1312 while (w != nullptr) {
1313 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1314 assert(w->prev() == nullptr || w->prev() == prev, "invariant");
1315 if (w->prev() != nullptr) {
1316 break;
1317 }
1318 w->_prev = prev;
1319 prev = w;
1320 w = w->next();
1321 }
1322 if (w == nullptr) {
1323 // We converted the entire entry_list from a singly linked list
1324 // into a doubly linked list. Now we just need to set the tail
1325 // pointer.
1326 assert(prev != nullptr && prev->next() == nullptr, "invariant");
1327 assert(_entry_list_tail == nullptr || _entry_list_tail == prev, "invariant");
1328 _entry_list_tail = prev;
1329 } else {
1330 #ifdef ASSERT
1331 // We stopped iterating through the _entry_list when we found a
1332 // node that had its prev pointer set. I.e. we converted the first
1333 // part of the entry_list from a singly linked list into a doubly
1334 // linked list. Now we just want to make sure the rest of the list
1335 // is doubly linked. But first we check that we have a tail
1336 // pointer, because if the end of the entry_list is doubly linked
1337 // and we don't have the tail pointer, something is broken.
1338 assert(_entry_list_tail != nullptr, "invariant");
1339 while (w != nullptr) {
1340 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1341 assert(w->prev() == prev, "invariant");
1342 prev = w;
1343 w = w->next();
1344 }
1345 assert(_entry_list_tail == prev, "invariant");
1346 #endif
1347 }
1348 }
1349
1350 // Return the tail of the _entry_list. If the tail is currently not
1351 // known, it can be found by first calling entry_list_build_dll().
1352 ObjectWaiter* ObjectMonitor::entry_list_tail(JavaThread* current) {
1353 assert(has_owner(current), "invariant");
1354 ObjectWaiter* w = _entry_list_tail;
1355 if (w != nullptr) {
1356 return w;
1357 }
1358 entry_list_build_dll(current);
1359 w = _entry_list_tail;
1360 assert(w != nullptr, "invariant");
1361 return w;
1362 }
1363
1364 // By convention we unlink a contending thread from _entry_list
1365 // immediately after the thread acquires the lock in ::enter().
1366 // The head of _entry_list is volatile but the interior is stable.
1367 // In addition, current.TState is stable.
1368
1369 void ObjectMonitor::unlink_after_acquire(JavaThread* current, ObjectWaiter* currentNode) {
1370 assert(has_owner(current), "invariant");
1371 assert((!currentNode->is_vthread() && currentNode->thread() == current) ||
1372 (currentNode->is_vthread() && currentNode->vthread() == current->vthread()), "invariant");
1373
1374 // Check if we are unlinking the last element in the _entry_list.
1375 // This is by far the most common case.
1376 if (currentNode->next() == nullptr) {
1377 assert(_entry_list_tail == nullptr || _entry_list_tail == currentNode, "invariant");
1378
1379 ObjectWaiter* w = Atomic::load(&_entry_list);
1380 if (w == currentNode) {
1381 // The currentNode is the only element in _entry_list.
1382 if (Atomic::cmpxchg(&_entry_list, w, (ObjectWaiter*)nullptr) == w) {
1383 _entry_list_tail = nullptr;
1384 currentNode->set_bad_pointers();
1385 return;
1386 }
1387 // The CAS above can fail from interference IFF a contending
1388 // thread "pushed" itself onto entry_list. So fall-through to
1389 // building the doubly linked list.
1390 assert(currentNode->prev() == nullptr, "invariant");
1391 }
1392 if (currentNode->prev() == nullptr) {
1393 // Build the doubly linked list to get hold of
1394 // currentNode->prev().
1395 entry_list_build_dll(current);
1396 assert(currentNode->prev() != nullptr, "must be");
1397 assert(_entry_list_tail == currentNode, "must be");
1398 }
1399 // The currentNode is the last element in _entry_list and we know
1400 // which element is the previous one.
1401 assert(_entry_list != currentNode, "invariant");
1402 _entry_list_tail = currentNode->prev();
1403 _entry_list_tail->_next = nullptr;
1404 currentNode->set_bad_pointers();
1405 return;
1406 }
1407
1408 // If we get here it means the current thread enqueued itself on the
1409 // _entry_list but was then able to "steal" the lock before the
1410 // chosen successor was able to. Consequently currentNode must be an
1411 // interior node in the _entry_list, or the head.
1412 assert(currentNode->next() != nullptr, "invariant");
1413 assert(currentNode != _entry_list_tail, "invariant");
1414
1415 // Check if we are in the singly linked portion of the
1416 // _entry_list. If we are the head then we try to remove ourselves,
1417 // else we convert to the doubly linked list.
1418 if (currentNode->prev() == nullptr) {
1419 ObjectWaiter* w = Atomic::load(&_entry_list);
1420
1421 assert(w != nullptr, "invariant");
1422 if (w == currentNode) {
1423 ObjectWaiter* next = currentNode->next();
1424 // currentNode is at the head of _entry_list.
1425 if (Atomic::cmpxchg(&_entry_list, w, next) == w) {
1426 // The CAS above sucsessfully unlinked currentNode from the
1427 // head of the _entry_list.
1428 assert(_entry_list != w, "invariant");
1429 next->_prev = nullptr;
1430 currentNode->set_bad_pointers();
1431 return;
1432 } else {
1433 // The CAS above can fail from interference IFF a contending
1434 // thread "pushed" itself onto _entry_list, in which case
1435 // currentNode must now be in the interior of the
1436 // list. Fall-through to building the doubly linked list.
1437 assert(_entry_list != currentNode, "invariant");
1438 }
1439 }
1440 // Build the doubly linked list to get hold of currentNode->prev().
1441 entry_list_build_dll(current);
1442 assert(currentNode->prev() != nullptr, "must be");
1443 }
1444
1445 // We now know we are unlinking currentNode from the interior of a
1446 // doubly linked list.
1447 assert(currentNode->next() != nullptr, "");
1448 assert(currentNode->prev() != nullptr, "");
1449 assert(currentNode != _entry_list, "");
1450 assert(currentNode != _entry_list_tail, "");
1451
1452 ObjectWaiter* nxt = currentNode->next();
1453 ObjectWaiter* prv = currentNode->prev();
1454 assert(nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
1455 assert(prv->TState == ObjectWaiter::TS_ENTER, "invariant");
1456
1457 nxt->_prev = prv;
1458 prv->_next = nxt;
1459 currentNode->set_bad_pointers();
1460 }
1461
1462 // -----------------------------------------------------------------------------
1463 // Exit support
1464 //
1465 // exit()
1466 // ~~~~~~
1467 // Note that the collector can't reclaim the objectMonitor or deflate
1468 // the object out from underneath the thread calling ::exit() as the
1469 // thread calling ::exit() never transitions to a stable state.
1470 // This inhibits GC, which in turn inhibits asynchronous (and
1471 // inopportune) reclamation of "this".
1472 //
1473 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
1474 // There's one exception to the claim above, however. enter_internal() can call
1475 // exit() to drop a lock if the acquirer has been externally suspended.
1476 // In that case exit() is called with _thread_state == _thread_blocked,
1477 // but the monitor's _contentions field is > 0, which inhibits reclamation.
1478 //
1479 // This is the exit part of the locking protocol, often implemented in
1480 // C2_MacroAssembler::fast_unlock()
1481 //
1482 // 1. A release barrier ensures that changes to monitor meta-data
1483 // (_succ, _entry_list) and data protected by the lock will be
1484 // visible before we release the lock.
1485 // 2. Release the lock by clearing the owner.
1486 // 3. A storeload MEMBAR is needed between releasing the owner and
1487 // subsequently reading meta-data to safely determine if the lock is
1488 // contended (step 4) without an elected successor (step 5).
1489 // 4. If _entry_list is null, we are done, since there is no
1490 // other thread waiting on the lock to wake up. I.e. there is no
1491 // contention.
1492 // 5. If there is a successor (_succ is non-null), we are done. The
1493 // responsibility for guaranteeing progress-liveness has now implicitly
1494 // been moved from the exiting thread to the successor.
1495 // 6. There are waiters in the entry list (_entry_list is non-null),
1496 // but there is no successor (_succ is null), so we need to
1497 // wake up (unpark) a waiting thread to avoid stranding.
1498 //
1499 // Note that since only the current lock owner can manipulate the
1500 // _entry_list (except for pushing new threads to the head), we need to
1501 // reacquire the lock before we can wake up (unpark) a waiting thread.
1502 //
1503 // The CAS() in enter provides for safety and exclusion, while the
1504 // MEMBAR in exit provides for progress and avoids stranding.
1505 //
1506 // There is also the risk of a futile wake-up. If we drop the lock
1507 // another thread can reacquire the lock immediately, and we can
1508 // then wake a thread unnecessarily. This is benign, and we've
1509 // structured the code so the windows are short and the frequency
1510 // of such futile wakups is low.
1511
1512 void ObjectMonitor::exit(JavaThread* current, bool not_suspended) {
1513 if (!has_owner(current)) {
1514 // Apparent unbalanced locking ...
1515 // Naively we'd like to throw IllegalMonitorStateException.
1516 // As a practical matter we can neither allocate nor throw an
1517 // exception as ::exit() can be called from leaf routines.
1518 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
1519 // Upon deeper reflection, however, in a properly run JVM the only
1520 // way we should encounter this situation is in the presence of
1521 // unbalanced JNI locking. TODO: CheckJNICalls.
1522 // See also: CR4414101
1523 #ifdef ASSERT
1524 LogStreamHandle(Error, monitorinflation) lsh;
1525 lsh.print_cr("ERROR: ObjectMonitor::exit(): thread=" INTPTR_FORMAT
1526 " is exiting an ObjectMonitor it does not own.", p2i(current));
1527 lsh.print_cr("The imbalance is possibly caused by JNI locking.");
1528 print_debug_style_on(&lsh);
1529 assert(false, "Non-balanced monitor enter/exit!");
1530 #endif
1531 return;
1532 }
1533
1534 if (_recursions != 0) {
1535 _recursions--; // this is simple recursive enter
1536 return;
1537 }
1538
1539 #if INCLUDE_JFR
1540 // get the owner's thread id for the MonitorEnter event
1541 // if it is enabled and the thread isn't suspended
1542 if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
1543 _previous_owner_tid = JFR_THREAD_ID(current);
1544 }
1545 #endif
1546
1547 for (;;) {
1548 // If there is a successor we should release the lock as soon as
1549 // possible, so that the successor can acquire the lock. If there is
1550 // no successor, we might need to wake up a waiting thread.
1551 if (!has_successor()) {
1552 ObjectWaiter* w = Atomic::load(&_entry_list);
1553 if (w != nullptr) {
1554 // Other threads are blocked trying to acquire the lock and
1555 // there is no successor, so it appears that an heir-
1556 // presumptive (successor) must be made ready. Since threads
1557 // are woken up in FIFO order, we need to find the tail of the
1558 // entry_list.
1559 w = entry_list_tail(current);
1560 // I'd like to write: guarantee (w->_thread != current).
1561 // But in practice an exiting thread may find itself on the entry_list.
1562 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1563 // then calls exit(). Exit release the lock by setting O._owner to null.
1564 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1565 // notify() operation moves T1 from O's waitset to O's entry_list. T2 then
1566 // release the lock "O". T1 resumes immediately after the ST of null into
1567 // _owner, above. T1 notices that the entry_list is populated, so it
1568 // reacquires the lock and then finds itself on the entry_list.
1569 // Given all that, we have to tolerate the circumstance where "w" is
1570 // associated with current.
1571 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1572 exit_epilog(current, w);
1573 return;
1574 }
1575 }
1576
1577 // Drop the lock.
1578 // release semantics: prior loads and stores from within the critical section
1579 // must not float (reorder) past the following store that drops the lock.
1580 // Uses a storeload to separate release_store(owner) from the
1581 // successor check. The try_set_owner_from() below uses cmpxchg() so
1582 // we get the fence down there.
1583 release_clear_owner(current);
1584 OrderAccess::storeload();
1585
1586 // Normally the exiting thread is responsible for ensuring succession,
1587 // but if this thread observes other successors are ready or other
1588 // entering threads are spinning after it has stored null into _owner
1589 // then it can exit without waking a successor. The existence of
1590 // spinners or ready successors guarantees proper succession (liveness).
1591 // Responsibility passes to the ready or running successors. The exiting
1592 // thread delegates the duty. More precisely, if a successor already
1593 // exists this thread is absolved of the responsibility of waking
1594 // (unparking) one.
1595
1596 // The _succ variable is critical to reducing futile wakeup frequency.
1597 // _succ identifies the "heir presumptive" thread that has been made
1598 // ready (unparked) but that has not yet run. We need only one such
1599 // successor thread to guarantee progress.
1600 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1601 // section 3.3 "Futile Wakeup Throttling" for details.
1602 //
1603 // Note that spinners in Enter() also set _succ non-null.
1604 // In the current implementation spinners opportunistically set
1605 // _succ so that exiting threads might avoid waking a successor.
1606 // Which means that the exiting thread could exit immediately without
1607 // waking a successor, if it observes a successor after it has dropped
1608 // the lock. Note that the dropped lock needs to become visible to the
1609 // spinner.
1610
1611 if (_entry_list == nullptr || has_successor()) {
1612 return;
1613 }
1614
1615 // Only the current lock owner can manipulate the entry_list
1616 // (except for pushing new threads to the head), therefore we need
1617 // to reacquire the lock. If we fail to reacquire the lock the
1618 // responsibility for ensuring succession falls to the new owner.
1619
1620 if (try_lock(current) != TryLockResult::Success) {
1621 // Some other thread acquired the lock (or the monitor was
1622 // deflated). Either way we are done.
1623 return;
1624 }
1625
1626 guarantee(has_owner(current), "invariant");
1627 }
1628 }
1629
1630 void ObjectMonitor::exit_epilog(JavaThread* current, ObjectWaiter* Wakee) {
1631 assert(has_owner(current), "invariant");
1632
1633 // Exit protocol:
1634 // 1. ST _succ = wakee
1635 // 2. membar #loadstore|#storestore;
1636 // 2. ST _owner = nullptr
1637 // 3. unpark(wakee)
1638
1639 oop vthread = nullptr;
1640 ParkEvent * Trigger;
1641 if (!Wakee->is_vthread()) {
1642 JavaThread* t = Wakee->thread();
1643 assert(t != nullptr, "");
1644 Trigger = t->_ParkEvent;
1645 set_successor(t);
1646 } else {
1647 vthread = Wakee->vthread();
1648 assert(vthread != nullptr, "");
1649 Trigger = ObjectMonitor::vthread_unparker_ParkEvent();
1650 set_successor(vthread);
1651 }
1652
1653 // Hygiene -- once we've set _owner = nullptr we can't safely dereference Wakee again.
1654 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1655 // out-of-scope (non-extant).
1656 Wakee = nullptr;
1657
1658 // Drop the lock.
1659 // Uses a fence to separate release_store(owner) from the LD in unpark().
1660 release_clear_owner(current);
1661 OrderAccess::fence();
1662
1663 DTRACE_MONITOR_PROBE(contended__exit, this, object(), current);
1664
1665 if (vthread == nullptr) {
1666 // Platform thread case.
1667 Trigger->unpark();
1668 } else if (java_lang_VirtualThread::set_onWaitingList(vthread, vthread_list_head())) {
1669 // Virtual thread case.
1670 Trigger->unpark();
1671 }
1672 }
1673
1674 // Exits the monitor returning recursion count. _owner should
1675 // be set to current's owner_id, i.e. no ANONYMOUS_OWNER allowed.
1676 intx ObjectMonitor::complete_exit(JavaThread* current) {
1677 assert(InitDone, "Unexpectedly not initialized");
1678 guarantee(has_owner(current), "complete_exit not owner");
1679
1680 intx save = _recursions; // record the old recursion count
1681 _recursions = 0; // set the recursion level to be 0
1682 exit(current); // exit the monitor
1683 guarantee(!has_owner(current), "invariant");
1684 return save;
1685 }
1686
1687 // Checks that the current THREAD owns this monitor and causes an
1688 // immediate return if it doesn't. We don't use the CHECK macro
1689 // because we want the IMSE to be the only exception that is thrown
1690 // from the call site when false is returned. Any other pending
1691 // exception is ignored.
1692 #define CHECK_OWNER() \
1693 do { \
1694 if (!check_owner(THREAD)) { \
1695 assert(HAS_PENDING_EXCEPTION, "expected a pending IMSE here."); \
1696 return; \
1697 } \
1698 } while (false)
1699
1700 // Returns true if the specified thread owns the ObjectMonitor.
1701 // Otherwise returns false and throws IllegalMonitorStateException
1702 // (IMSE). If there is a pending exception and the specified thread
1703 // is not the owner, that exception will be replaced by the IMSE.
1704 bool ObjectMonitor::check_owner(TRAPS) {
1705 JavaThread* current = THREAD;
1706 int64_t cur = owner_raw();
1707 if (cur == owner_id_from(current)) {
1708 return true;
1709 }
1710 THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(),
1711 "current thread is not owner", false);
1712 }
1713
1714 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1715 ObjectMonitor* monitor,
1716 uint64_t notifier_tid,
1717 jlong timeout,
1718 bool timedout) {
1719 assert(event != nullptr, "invariant");
1720 assert(monitor != nullptr, "invariant");
1721 const Klass* monitor_klass = monitor->object()->klass();
1722 if (ObjectMonitor::is_jfr_excluded(monitor_klass)) {
1723 return;
1724 }
1725 event->set_monitorClass(monitor_klass);
1726 event->set_timeout(timeout);
1727 // Set an address that is 'unique enough', such that events close in
1728 // time and with the same address are likely (but not guaranteed) to
1729 // belong to the same object.
1730 event->set_address((uintptr_t)monitor);
1731 event->set_notifier(notifier_tid);
1732 event->set_timedOut(timedout);
1733 event->commit();
1734 }
1735
1736 static void vthread_monitor_waited_event(JavaThread* current, ObjectWaiter* node, ContinuationWrapper& cont, EventJavaMonitorWait* event, jboolean timed_out) {
1737 // Since we might safepoint set the anchor so that the stack can we walked.
1738 assert(current->last_continuation() != nullptr, "");
1739 JavaFrameAnchor* anchor = current->frame_anchor();
1740 anchor->set_last_Java_sp(current->last_continuation()->entry_sp());
1741 anchor->set_last_Java_pc(current->last_continuation()->entry_pc());
1742
1743 ContinuationWrapper::SafepointOp so(current, cont);
1744
1745 JRT_BLOCK
1746 if (event->should_commit()) {
1747 long timeout = java_lang_VirtualThread::timeout(current->vthread());
1748 post_monitor_wait_event(event, node->_monitor, node->_notifier_tid, timeout, timed_out);
1749 }
1750 if (JvmtiExport::should_post_monitor_waited()) {
1751 // We mark this call in case of an upcall to Java while posting the event.
1752 // If somebody walks the stack in that case, processing the enterSpecial
1753 // frame should not include processing callee arguments since there is no
1754 // actual callee (see nmethod::preserve_callee_argument_oops()).
1755 ThreadOnMonitorWaitedEvent tmwe(current);
1756 JvmtiExport::vthread_post_monitor_waited(current, node->_monitor, timed_out);
1757 }
1758 JRT_BLOCK_END
1759 current->frame_anchor()->clear();
1760 }
1761
1762 // -----------------------------------------------------------------------------
1763 // Wait/Notify/NotifyAll
1764 //
1765 // Note: a subset of changes to ObjectMonitor::wait()
1766 // will need to be replicated in complete_exit
1767 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1768 JavaThread* current = THREAD;
1769
1770 assert(InitDone, "Unexpectedly not initialized");
1771
1772 CHECK_OWNER(); // Throws IMSE if not owner.
1773
1774 EventJavaMonitorWait wait_event;
1775 EventVirtualThreadPinned vthread_pinned_event;
1776
1777 // check for a pending interrupt
1778 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1779 JavaThreadInObjectWaitState jtiows(current, millis != 0, interruptible);
1780
1781 if (JvmtiExport::should_post_monitor_wait()) {
1782 JvmtiExport::post_monitor_wait(current, object(), millis);
1783 }
1784 // post monitor waited event. Note that this is past-tense, we are done waiting.
1785 if (JvmtiExport::should_post_monitor_waited()) {
1786 // Note: 'false' parameter is passed here because the
1787 // wait was not timed out due to thread interrupt.
1788 JvmtiExport::post_monitor_waited(current, this, false);
1789
1790 // In this short circuit of the monitor wait protocol, the
1791 // current thread never drops ownership of the monitor and
1792 // never gets added to the wait queue so the current thread
1793 // cannot be made the successor. This means that the
1794 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1795 // consume an unpark() meant for the ParkEvent associated with
1796 // this ObjectMonitor.
1797 }
1798 if (wait_event.should_commit()) {
1799 post_monitor_wait_event(&wait_event, this, 0, millis, false);
1800 }
1801 THROW(vmSymbols::java_lang_InterruptedException());
1802 return;
1803 }
1804
1805 freeze_result result;
1806 ContinuationEntry* ce = current->last_continuation();
1807 bool is_virtual = ce != nullptr && ce->is_virtual_thread();
1808 if (is_virtual) {
1809 if (interruptible && JvmtiExport::should_post_monitor_wait()) {
1810 JvmtiExport::post_monitor_wait(current, object(), millis);
1811 }
1812 current->set_current_waiting_monitor(this);
1813 result = Continuation::try_preempt(current, ce->cont_oop(current));
1814 if (result == freeze_ok) {
1815 vthread_wait(current, millis, interruptible);
1816 current->set_current_waiting_monitor(nullptr);
1817 return;
1818 }
1819 }
1820 // The jtiows does nothing for non-interruptible.
1821 JavaThreadInObjectWaitState jtiows(current, millis != 0, interruptible);
1822
1823 if (!is_virtual) { // it was already set for virtual thread
1824 if (interruptible && JvmtiExport::should_post_monitor_wait()) {
1825 JvmtiExport::post_monitor_wait(current, object(), millis);
1826
1827 // The current thread already owns the monitor and it has not yet
1828 // been added to the wait queue so the current thread cannot be
1829 // made the successor. This means that the JVMTI_EVENT_MONITOR_WAIT
1830 // event handler cannot accidentally consume an unpark() meant for
1831 // the ParkEvent associated with this ObjectMonitor.
1832 }
1833 current->set_current_waiting_monitor(this);
1834 }
1835 // create a node to be put into the queue
1836 // Critically, after we reset() the event but prior to park(), we must check
1837 // for a pending interrupt.
1838 ObjectWaiter node(current);
1839 node.TState = ObjectWaiter::TS_WAIT;
1840 current->_ParkEvent->reset();
1841 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1842
1843 // Enter the waiting queue, which is a circular doubly linked list in this case
1844 // but it could be a priority queue or any data structure.
1845 // _wait_set_lock protects the wait queue. Normally the wait queue is accessed only
1846 // by the owner of the monitor *except* in the case where park()
1847 // returns because of a timeout of interrupt. Contention is exceptionally rare
1848 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1849
1850 Thread::SpinAcquire(&_wait_set_lock);
1851 add_waiter(&node);
1852 Thread::SpinRelease(&_wait_set_lock);
1853
1854 intx save = _recursions; // record the old recursion count
1855 _waiters++; // increment the number of waiters
1856 _recursions = 0; // set the recursion level to be 1
1857 exit(current); // exit the monitor
1858 guarantee(!has_owner(current), "invariant");
1859
1860 // The thread is on the wait_set list - now park() it.
1861 // On MP systems it's conceivable that a brief spin before we park
1862 // could be profitable.
1863 //
1864 // TODO-FIXME: change the following logic to a loop of the form
1865 // while (!timeout && !interrupted && _notified == 0) park()
1866
1867 int ret = OS_OK;
1868 int WasNotified = 0;
1869
1870 // Need to check interrupt state whilst still _thread_in_vm
1871 bool interrupted = interruptible && current->is_interrupted(false);
1872
1873 { // State transition wrappers
1874 OSThread* osthread = current->osthread();
1875 OSThreadWaitState osts(osthread, true);
1876
1877 assert(current->thread_state() == _thread_in_vm, "invariant");
1878
1879 {
1880 ClearSuccOnSuspend csos(this);
1881 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */);
1882 if (interrupted || HAS_PENDING_EXCEPTION) {
1883 // Intentionally empty
1884 } else if (!node._notified) {
1885 if (millis <= 0) {
1886 current->_ParkEvent->park();
1887 } else {
1888 ret = current->_ParkEvent->park(millis);
1889 }
1890 }
1891 }
1892
1893 // Node may be on the wait_set, or on the entry_list, or in transition
1894 // from the wait_set to the entry_list.
1895 // See if we need to remove Node from the wait_set.
1896 // We use double-checked locking to avoid grabbing _wait_set_lock
1897 // if the thread is not on the wait queue.
1898 //
1899 // Note that we don't need a fence before the fetch of TState.
1900 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1901 // written by the is thread. (perhaps the fetch might even be satisfied
1902 // by a look-aside into the processor's own store buffer, although given
1903 // the length of the code path between the prior ST and this load that's
1904 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1905 // then we'll acquire the lock and then re-fetch a fresh TState value.
1906 // That is, we fail toward safety.
1907
1908 if (node.TState == ObjectWaiter::TS_WAIT) {
1909 Thread::SpinAcquire(&_wait_set_lock);
1910 if (node.TState == ObjectWaiter::TS_WAIT) {
1911 dequeue_specific_waiter(&node); // unlink from wait_set
1912 assert(!node._notified, "invariant");
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 WasNotified = node._notified;
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 (node._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 current->inc_held_monitor_count(relock_count); // Deopt never entered these counts.
1994 _waiters--; // decrement the number of waiters
1995
1996 // Verify a few postconditions
1997 assert(has_owner(current), "invariant");
1998 assert(!has_successor(current), "invariant");
1999 assert_mark_word_consistency();
2000
2001 if (ce != nullptr && ce->is_virtual_thread()) {
2002 current->post_vthread_pinned_event(&vthread_pinned_event, "Object.wait", result);
2003 }
2004
2005 // check if the notification happened
2006 if (!WasNotified) {
2007 // no, it could be timeout or Thread.interrupt() or both
2008 // check for interrupt event, otherwise it is timeout
2009 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
2010 THROW(vmSymbols::java_lang_InterruptedException());
2011 }
2012 }
2013
2014 // NOTE: Spurious wake up will be consider as timeout.
2015 // Monitor notify has precedence over thread interrupt.
2016 }
2017
2018 // Consider:
2019 // If the lock is cool (entry_list == null && succ == null) and we're on an MP system
2020 // then instead of transferring a thread from the wait_set to the entry_list
2021 // we might just dequeue a thread from the wait_set and directly unpark() it.
2022
2023 bool ObjectMonitor::notify_internal(JavaThread* current) {
2024 bool did_notify = false;
2025 Thread::SpinAcquire(&_wait_set_lock);
2026 ObjectWaiter* iterator = dequeue_waiter();
2027 if (iterator != nullptr) {
2028 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
2029 guarantee(!iterator->_notified, "invariant");
2030
2031 if (iterator->is_vthread()) {
2032 oop vthread = iterator->vthread();
2033 java_lang_VirtualThread::set_notified(vthread, true);
2034 int old_state = java_lang_VirtualThread::state(vthread);
2035 // If state is not WAIT/TIMED_WAIT then target could still be on
2036 // unmount transition, or wait could have already timed-out or target
2037 // could have been interrupted. In the first case, the target itself
2038 // will set the state to BLOCKED at the end of the unmount transition.
2039 // In the other cases the target would have been already unblocked so
2040 // there is nothing to do.
2041 if (old_state == java_lang_VirtualThread::WAIT ||
2042 old_state == java_lang_VirtualThread::TIMED_WAIT) {
2043 java_lang_VirtualThread::cmpxchg_state(vthread, old_state, java_lang_VirtualThread::BLOCKED);
2044 }
2045 // Increment counter *before* adding the vthread to the _entry_list.
2046 // Adding to _entry_list uses Atomic::cmpxchg() which already provides
2047 // a fence that prevents reordering of the stores.
2048 inc_unmounted_vthreads();
2049 }
2050
2051 iterator->_notified = true;
2052 iterator->_notifier_tid = JFR_THREAD_ID(current);
2053 did_notify = true;
2054 add_to_entry_list(current, iterator);
2055
2056 // _wait_set_lock protects the wait queue, not the entry_list. We could
2057 // move the add-to-entry_list operation, above, outside the critical section
2058 // protected by _wait_set_lock. In practice that's not useful. With the
2059 // exception of wait() timeouts and interrupts the monitor owner
2060 // is the only thread that grabs _wait_set_lock. There's almost no contention
2061 // on _wait_set_lock so it's not profitable to reduce the length of the
2062 // critical section.
2063
2064 if (!iterator->is_vthread()) {
2065 iterator->wait_reenter_begin(this);
2066
2067 // Read counter *after* adding the thread to the _entry_list.
2068 // Adding to _entry_list uses Atomic::cmpxchg() which already provides
2069 // a fence that prevents this load from floating up previous store.
2070 if (has_unmounted_vthreads()) {
2071 // Wake up the thread to alleviate some deadlocks cases where the successor
2072 // that will be picked up when this thread releases the monitor is an unmounted
2073 // virtual thread that cannot run due to having run out of carriers. Upon waking
2074 // up, the thread will call reenter_internal() which will use time-park in case
2075 // there is contention and there are still vthreads in the _entry_list.
2076 JavaThread* t = iterator->thread();
2077 t->_ParkEvent->unpark();
2078 }
2079 }
2080 }
2081 Thread::SpinRelease(&_wait_set_lock);
2082 return did_notify;
2083 }
2084
2085 static void post_monitor_notify_event(EventJavaMonitorNotify* event,
2086 ObjectMonitor* monitor,
2087 int notified_count) {
2088 assert(event != nullptr, "invariant");
2089 assert(monitor != nullptr, "invariant");
2090 const Klass* monitor_klass = monitor->object()->klass();
2091 if (ObjectMonitor::is_jfr_excluded(monitor_klass)) {
2092 return;
2093 }
2094 event->set_monitorClass(monitor_klass);
2095 // Set an address that is 'unique enough', such that events close in
2096 // time and with the same address are likely (but not guaranteed) to
2097 // belong to the same object.
2098 event->set_address((uintptr_t)monitor);
2099 event->set_notifiedCount(notified_count);
2100 event->commit();
2101 }
2102
2103 // Consider: a not-uncommon synchronization bug is to use notify() when
2104 // notifyAll() is more appropriate, potentially resulting in stranded
2105 // threads; this is one example of a lost wakeup. A useful diagnostic
2106 // option is to force all notify() operations to behave as notifyAll().
2107 //
2108 // Note: We can also detect many such problems with a "minimum wait".
2109 // When the "minimum wait" is set to a small non-zero timeout value
2110 // and the program does not hang whereas it did absent "minimum wait",
2111 // that suggests a lost wakeup bug.
2112
2113 void ObjectMonitor::notify(TRAPS) {
2114 JavaThread* current = THREAD;
2115 CHECK_OWNER(); // Throws IMSE if not owner.
2116 if (_wait_set == nullptr) {
2117 return;
2118 }
2119
2120 quick_notify(current);
2121 }
2122
2123 void ObjectMonitor::quick_notify(JavaThread* current) {
2124 assert(has_owner(current), "Precondition");
2125
2126 EventJavaMonitorNotify event;
2127 DTRACE_MONITOR_PROBE(notify, this, object(), current);
2128 int tally = notify_internal(current) ? 1 : 0;
2129
2130 if ((tally > 0) && event.should_commit()) {
2131 post_monitor_notify_event(&event, this, /* notified_count = */ tally);
2132 }
2133 }
2134
2135 // notifyAll() transfers the waiters one-at-a-time from the waitset to
2136 // the entry_list. If the waitset is "ABCD" (where A was added first
2137 // and D last) and the entry_list is ->X->Y->Z. After a notifyAll()
2138 // the waitset will be empty and the entry_list will be
2139 // ->D->C->B->A->X->Y->Z, and the next choosen successor will be Z.
2140
2141 void ObjectMonitor::notifyAll(TRAPS) {
2142 JavaThread* current = THREAD;
2143 CHECK_OWNER(); // Throws IMSE if not owner.
2144 if (_wait_set == nullptr) {
2145 return;
2146 }
2147
2148 quick_notifyAll(current);
2149 }
2150
2151 void ObjectMonitor::quick_notifyAll(JavaThread* current) {
2152 assert(has_owner(current), "Precondition");
2153
2154 EventJavaMonitorNotify event;
2155 DTRACE_MONITOR_PROBE(notifyAll, this, object(), current);
2156 int tally = 0;
2157 while (_wait_set != nullptr) {
2158 if (notify_internal(current)) {
2159 tally++;
2160 }
2161 }
2162
2163 if ((tally > 0) && event.should_commit()) {
2164 post_monitor_notify_event(&event, this, /* notified_count = */ tally);
2165 }
2166 }
2167
2168 void ObjectMonitor::vthread_wait(JavaThread* current, jlong millis, bool interruptible) {
2169 oop vthread = current->vthread();
2170 ObjectWaiter* node = new ObjectWaiter(vthread, this);
2171 node->_is_wait = true;
2172 node->_interruptible = interruptible;
2173 node->TState = ObjectWaiter::TS_WAIT;
2174 java_lang_VirtualThread::set_notified(vthread, false); // Reset notified flag
2175 java_lang_VirtualThread::set_interruptible_wait(vthread, interruptible);
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 assert(!node->_notified, "invariant");
2211 node->TState = ObjectWaiter::TS_RUN;
2212 }
2213 Thread::SpinRelease(&_wait_set_lock);
2214 }
2215
2216 // If this was an interrupted case, set the _interrupted boolean so that
2217 // once we re-acquire the monitor we know if we need to throw IE or not.
2218 ObjectWaiter::TStates state = node->TState;
2219 bool was_notified = state == ObjectWaiter::TS_ENTER;
2220 assert(was_notified || state == ObjectWaiter::TS_RUN, "");
2221 node->_interrupted = node->_interruptible && !was_notified && current->is_interrupted(false);
2222
2223 // Post JFR and JVMTI events. If non-interruptible we are in
2224 // ObjectLocker case so we don't post anything.
2225 EventJavaMonitorWait wait_event;
2226 if (node->_interruptible && (wait_event.should_commit() || JvmtiExport::should_post_monitor_waited())) {
2227 vthread_monitor_waited_event(current, node, cont, &wait_event, !was_notified && !node->_interrupted);
2228 }
2229
2230 // Mark that we are at reenter so that we don't call this method again.
2231 node->_at_reenter = true;
2232
2233 if (!was_notified) {
2234 bool acquired = vthread_monitor_enter(current, node);
2235 if (acquired) {
2236 guarantee(_recursions == 0, "invariant");
2237 _recursions = node->_recursions; // restore the old recursion count
2238 _waiters--; // decrement the number of waiters
2239
2240 if (node->_interrupted) {
2241 // We will throw at thaw end after finishing the mount transition.
2242 current->set_pending_interrupted_exception(true);
2243 }
2244
2245 delete node;
2246 // Clear the ObjectWaiter* from the vthread.
2247 java_lang_VirtualThread::set_objectWaiter(current->vthread(), nullptr);
2248 return true;
2249 }
2250 } else {
2251 // Already moved to _entry_list by notifier, so just add to contentions.
2252 add_to_contentions(1);
2253 }
2254 return false;
2255 }
2256
2257 // -----------------------------------------------------------------------------
2258 // Adaptive Spinning Support
2259 //
2260 // Adaptive spin-then-block - rational spinning
2261 //
2262 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
2263 // algorithm.
2264 //
2265 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
2266 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
2267 // (duration) or we can fix the count at approximately the duration of
2268 // a context switch and vary the frequency. Of course we could also
2269 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
2270 // For a description of 'Adaptive spin-then-block mutual exclusion in
2271 // multi-threaded processing,' see U.S. Pat. No. 8046758.
2272 //
2273 // This implementation varies the duration "D", where D varies with
2274 // the success rate of recent spin attempts. (D is capped at approximately
2275 // length of a round-trip context switch). The success rate for recent
2276 // spin attempts is a good predictor of the success rate of future spin
2277 // attempts. The mechanism adapts automatically to varying critical
2278 // section length (lock modality), system load and degree of parallelism.
2279 // D is maintained per-monitor in _SpinDuration and is initialized
2280 // optimistically. Spin frequency is fixed at 100%.
2281 //
2282 // Note that _SpinDuration is volatile, but we update it without locks
2283 // or atomics. The code is designed so that _SpinDuration stays within
2284 // a reasonable range even in the presence of races. The arithmetic
2285 // operations on _SpinDuration are closed over the domain of legal values,
2286 // so at worst a race will install and older but still legal value.
2287 // At the very worst this introduces some apparent non-determinism.
2288 // We might spin when we shouldn't or vice-versa, but since the spin
2289 // count are relatively short, even in the worst case, the effect is harmless.
2290 //
2291 // Care must be taken that a low "D" value does not become an
2292 // an absorbing state. Transient spinning failures -- when spinning
2293 // is overall profitable -- should not cause the system to converge
2294 // on low "D" values. We want spinning to be stable and predictable
2295 // and fairly responsive to change and at the same time we don't want
2296 // it to oscillate, become metastable, be "too" non-deterministic,
2297 // or converge on or enter undesirable stable absorbing states.
2298 //
2299 // We implement a feedback-based control system -- using past behavior
2300 // to predict future behavior. We face two issues: (a) if the
2301 // input signal is random then the spin predictor won't provide optimal
2302 // results, and (b) if the signal frequency is too high then the control
2303 // system, which has some natural response lag, will "chase" the signal.
2304 // (b) can arise from multimodal lock hold times. Transient preemption
2305 // can also result in apparent bimodal lock hold times.
2306 // Although sub-optimal, neither condition is particularly harmful, as
2307 // in the worst-case we'll spin when we shouldn't or vice-versa.
2308 // The maximum spin duration is rather short so the failure modes aren't bad.
2309 // To be conservative, I've tuned the gain in system to bias toward
2310 // _not spinning. Relatedly, the system can sometimes enter a mode where it
2311 // "rings" or oscillates between spinning and not spinning. This happens
2312 // when spinning is just on the cusp of profitability, however, so the
2313 // situation is not dire. The state is benign -- there's no need to add
2314 // hysteresis control to damp the transition rate between spinning and
2315 // not spinning.
2316
2317 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool
2318
2319 static int Knob_Bonus = 100; // spin success bonus
2320 static int Knob_Penalty = 200; // spin failure penalty
2321 static int Knob_Poverty = 1000;
2322 static int Knob_FixedSpin = 0;
2323 static int Knob_PreSpin = 10; // 20-100 likely better, but it's not better in my testing.
2324
2325 inline static int adjust_up(int spin_duration) {
2326 int x = spin_duration;
2327 if (x < ObjectMonitor::Knob_SpinLimit) {
2328 if (x < Knob_Poverty) {
2329 x = Knob_Poverty;
2330 }
2331 return x + Knob_Bonus;
2332 } else {
2333 return spin_duration;
2334 }
2335 }
2336
2337 inline static int adjust_down(int spin_duration) {
2338 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2339 // AIMD is globally stable.
2340 int x = spin_duration;
2341 if (x > 0) {
2342 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2343 // This is globally sample and tends to damp the response.
2344 x -= Knob_Penalty;
2345 if (x < 0) { x = 0; }
2346 return x;
2347 } else {
2348 return spin_duration;
2349 }
2350 }
2351
2352 bool ObjectMonitor::short_fixed_spin(JavaThread* current, int spin_count, bool adapt) {
2353 for (int ctr = 0; ctr < spin_count; ctr++) {
2354 TryLockResult status = try_lock(current);
2355 if (status == TryLockResult::Success) {
2356 if (adapt) {
2357 _SpinDuration = adjust_up(_SpinDuration);
2358 }
2359 return true;
2360 } else if (status == TryLockResult::Interference) {
2361 break;
2362 }
2363 SpinPause();
2364 }
2365 return false;
2366 }
2367
2368 // Spinning: Fixed frequency (100%), vary duration
2369 bool ObjectMonitor::try_spin(JavaThread* current) {
2370
2371 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
2372 int knob_fixed_spin = Knob_FixedSpin; // 0 (don't spin: default), 2000 good test
2373 if (knob_fixed_spin > 0) {
2374 return short_fixed_spin(current, knob_fixed_spin, false);
2375 }
2376
2377 // Admission control - verify preconditions for spinning
2378 //
2379 // We always spin a little bit, just to prevent _SpinDuration == 0 from
2380 // becoming an absorbing state. Put another way, we spin briefly to
2381 // sample, just in case the system load, parallelism, contention, or lock
2382 // modality changed.
2383
2384 int knob_pre_spin = Knob_PreSpin; // 10 (default), 100, 1000 or 2000
2385 if (short_fixed_spin(current, knob_pre_spin, true)) {
2386 return true;
2387 }
2388
2389 //
2390 // Consider the following alternative:
2391 // Periodically set _SpinDuration = _SpinLimit and try a long/full
2392 // spin attempt. "Periodically" might mean after a tally of
2393 // the # of failed spin attempts (or iterations) reaches some threshold.
2394 // This takes us into the realm of 1-out-of-N spinning, where we
2395 // hold the duration constant but vary the frequency.
2396
2397 int ctr = _SpinDuration;
2398 if (ctr <= 0) return false;
2399
2400 // We're good to spin ... spin ingress.
2401 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
2402 // when preparing to LD...CAS _owner, etc and the CAS is likely
2403 // to succeed.
2404 if (!has_successor()) {
2405 set_successor(current);
2406 }
2407 int64_t prv = NO_OWNER;
2408
2409 // There are three ways to exit the following loop:
2410 // 1. A successful spin where this thread has acquired the lock.
2411 // 2. Spin failure with prejudice
2412 // 3. Spin failure without prejudice
2413
2414 while (--ctr >= 0) {
2415
2416 // Periodic polling -- Check for pending GC
2417 // Threads may spin while they're unsafe.
2418 // We don't want spinning threads to delay the JVM from reaching
2419 // a stop-the-world safepoint or to steal cycles from GC.
2420 // If we detect a pending safepoint we abort in order that
2421 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
2422 // this thread, if safe, doesn't steal cycles from GC.
2423 // This is in keeping with the "no loitering in runtime" rule.
2424 // We periodically check to see if there's a safepoint pending.
2425 if ((ctr & 0xFF) == 0) {
2426 // Can't call SafepointMechanism::should_process() since that
2427 // might update the poll values and we could be in a thread_blocked
2428 // state here which is not allowed so just check the poll.
2429 if (SafepointMechanism::local_poll_armed(current)) {
2430 break;
2431 }
2432 SpinPause();
2433 }
2434
2435 // Probe _owner with TATAS
2436 // If this thread observes the monitor transition or flicker
2437 // from locked to unlocked to locked, then the odds that this
2438 // thread will acquire the lock in this spin attempt go down
2439 // considerably. The same argument applies if the CAS fails
2440 // or if we observe _owner change from one non-null value to
2441 // another non-null value. In such cases we might abort
2442 // the spin without prejudice or apply a "penalty" to the
2443 // spin count-down variable "ctr", reducing it by 100, say.
2444
2445 int64_t ox = owner_raw();
2446 if (ox == NO_OWNER) {
2447 ox = try_set_owner_from(NO_OWNER, current);
2448 if (ox == NO_OWNER) {
2449 // The CAS succeeded -- this thread acquired ownership
2450 // Take care of some bookkeeping to exit spin state.
2451 if (has_successor(current)) {
2452 clear_successor();
2453 }
2454
2455 // Increase _SpinDuration :
2456 // The spin was successful (profitable) so we tend toward
2457 // longer spin attempts in the future.
2458 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2459 // If we acquired the lock early in the spin cycle it
2460 // makes sense to increase _SpinDuration proportionally.
2461 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2462 _SpinDuration = adjust_up(_SpinDuration);
2463 return true;
2464 }
2465
2466 // The CAS failed ... we can take any of the following actions:
2467 // * penalize: ctr -= CASPenalty
2468 // * exit spin with prejudice -- abort without adapting spinner
2469 // * exit spin without prejudice.
2470 // * Since CAS is high-latency, retry again immediately.
2471 break;
2472 }
2473
2474 // Did lock ownership change hands ?
2475 if (ox != prv && prv != NO_OWNER) {
2476 break;
2477 }
2478 prv = ox;
2479
2480 if (!has_successor()) {
2481 set_successor(current);
2482 }
2483 }
2484
2485 // Spin failed with prejudice -- reduce _SpinDuration.
2486 if (ctr < 0) {
2487 _SpinDuration = adjust_down(_SpinDuration);
2488 }
2489
2490 if (has_successor(current)) {
2491 clear_successor();
2492 // Invariant: after setting succ=null a contending thread
2493 // must recheck-retry _owner before parking. This usually happens
2494 // in the normal usage of try_spin(), but it's safest
2495 // to make try_spin() as foolproof as possible.
2496 OrderAccess::fence();
2497 if (try_lock(current) == TryLockResult::Success) {
2498 return true;
2499 }
2500 }
2501
2502 return false;
2503 }
2504
2505
2506 // -----------------------------------------------------------------------------
2507 // wait_set management ...
2508
2509 ObjectWaiter::ObjectWaiter(JavaThread* current) {
2510 _next = nullptr;
2511 _prev = nullptr;
2512 _thread = current;
2513 _monitor = nullptr;
2514 _notifier_tid = 0;
2515 _recursions = 0;
2516 TState = TS_RUN;
2517 _notified = false;
2518 _is_wait = false;
2519 _at_reenter = false;
2520 _interrupted = false;
2521 _active = false;
2522 }
2523
2524 ObjectWaiter::ObjectWaiter(oop vthread, ObjectMonitor* mon) : ObjectWaiter(nullptr) {
2525 assert(oopDesc::is_oop(vthread), "");
2526 _vthread = OopHandle(JavaThread::thread_oop_storage(), vthread);
2527 _monitor = mon;
2528 }
2529
2530 ObjectWaiter::~ObjectWaiter() {
2531 if (is_vthread()) {
2532 assert(vthread() != nullptr, "");
2533 _vthread.release(JavaThread::thread_oop_storage());
2534 }
2535 }
2536
2537 oop ObjectWaiter::vthread() const {
2538 return _vthread.resolve();
2539 }
2540
2541 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2542 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(_thread, mon);
2543 }
2544
2545 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2546 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(_thread, _active);
2547 }
2548
2549 inline void ObjectMonitor::add_waiter(ObjectWaiter* node) {
2550 assert(node != nullptr, "should not add null node");
2551 assert(node->_prev == nullptr, "node already in list");
2552 assert(node->_next == nullptr, "node already in list");
2553 // put node at end of queue (circular doubly linked list)
2554 if (_wait_set == nullptr) {
2555 _wait_set = node;
2556 node->_prev = node;
2557 node->_next = node;
2558 } else {
2559 ObjectWaiter* head = _wait_set;
2560 ObjectWaiter* tail = head->_prev;
2561 assert(tail->_next == head, "invariant check");
2562 tail->_next = node;
2563 head->_prev = node;
2564 node->_next = head;
2565 node->_prev = tail;
2566 }
2567 }
2568
2569 inline ObjectWaiter* ObjectMonitor::dequeue_waiter() {
2570 // dequeue the very first waiter
2571 ObjectWaiter* waiter = _wait_set;
2572 if (waiter) {
2573 dequeue_specific_waiter(waiter);
2574 }
2575 return waiter;
2576 }
2577
2578 inline void ObjectMonitor::dequeue_specific_waiter(ObjectWaiter* node) {
2579 assert(node != nullptr, "should not dequeue nullptr node");
2580 assert(node->_prev != nullptr, "node already removed from list");
2581 assert(node->_next != nullptr, "node already removed from list");
2582 // when the waiter has woken up because of interrupt,
2583 // timeout or other spurious wake-up, dequeue the
2584 // waiter from waiting list
2585 ObjectWaiter* next = node->_next;
2586 if (next == node) {
2587 assert(node->_prev == node, "invariant check");
2588 _wait_set = nullptr;
2589 } else {
2590 ObjectWaiter* prev = node->_prev;
2591 assert(prev->_next == node, "invariant check");
2592 assert(next->_prev == node, "invariant check");
2593 next->_prev = prev;
2594 prev->_next = next;
2595 if (_wait_set == node) {
2596 _wait_set = next;
2597 }
2598 }
2599 node->_next = nullptr;
2600 node->_prev = nullptr;
2601 }
2602
2603 // -----------------------------------------------------------------------------
2604
2605 // One-shot global initialization for the sync subsystem.
2606 // We could also defer initialization and initialize on-demand
2607 // the first time we call ObjectSynchronizer::inflate().
2608 // Initialization would be protected - like so many things - by
2609 // the MonitorCache_lock.
2610
2611 void ObjectMonitor::Initialize() {
2612 assert(!InitDone, "invariant");
2613
2614 if (!os::is_MP()) {
2615 Knob_SpinLimit = 0;
2616 Knob_PreSpin = 0;
2617 Knob_FixedSpin = -1;
2618 }
2619
2620 _oop_storage = OopStorageSet::create_weak("ObjectSynchronizer Weak", mtSynchronizer);
2621
2622 DEBUG_ONLY(InitDone = true;)
2623 }
2624
2625 // We can't call this during Initialize() because BarrierSet needs to be set.
2626 void ObjectMonitor::Initialize2() {
2627 _vthread_list_head = OopHandle(JavaThread::thread_oop_storage(), nullptr);
2628 _vthread_unparker_ParkEvent = ParkEvent::Allocate(nullptr);
2629 }
2630
2631 void ObjectMonitor::print_on(outputStream* st) const {
2632 // The minimal things to print for markWord printing, more can be added for debugging and logging.
2633 st->print("{contentions=0x%08x,waiters=0x%08x"
2634 ",recursions=%zd,owner=" INT64_FORMAT "}",
2635 contentions(), waiters(), recursions(),
2636 owner_raw());
2637 }
2638 void ObjectMonitor::print() const { print_on(tty); }
2639
2640 #ifdef ASSERT
2641 // Print the ObjectMonitor like a debugger would:
2642 //
2643 // (ObjectMonitor) 0x00007fdfb6012e40 = {
2644 // _metadata = 0x0000000000000001
2645 // _object = 0x000000070ff45fd0
2646 // _pad_buf0 = {
2647 // [0] = '\0'
2648 // ...
2649 // [43] = '\0'
2650 // }
2651 // _owner = 0x0000000000000000
2652 // _previous_owner_tid = 0
2653 // _pad_buf1 = {
2654 // [0] = '\0'
2655 // ...
2656 // [47] = '\0'
2657 // }
2658 // _next_om = 0x0000000000000000
2659 // _recursions = 0
2660 // _entry_list = 0x0000000000000000
2661 // _entry_list_tail = 0x0000000000000000
2662 // _succ = 0x0000000000000000
2663 // _SpinDuration = 5000
2664 // _contentions = 0
2665 // _wait_set = 0x0000700009756248
2666 // _waiters = 1
2667 // _wait_set_lock = 0
2668 // }
2669 //
2670 void ObjectMonitor::print_debug_style_on(outputStream* st) const {
2671 st->print_cr("(ObjectMonitor*) " INTPTR_FORMAT " = {", p2i(this));
2672 st->print_cr(" _metadata = " INTPTR_FORMAT, _metadata);
2673 st->print_cr(" _object = " INTPTR_FORMAT, p2i(object_peek()));
2674 st->print_cr(" _pad_buf0 = {");
2675 st->print_cr(" [0] = '\\0'");
2676 st->print_cr(" ...");
2677 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf0) - 1);
2678 st->print_cr(" }");
2679 st->print_cr(" _owner = " INT64_FORMAT, owner_raw());
2680 st->print_cr(" _previous_owner_tid = " UINT64_FORMAT, _previous_owner_tid);
2681 st->print_cr(" _pad_buf1 = {");
2682 st->print_cr(" [0] = '\\0'");
2683 st->print_cr(" ...");
2684 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf1) - 1);
2685 st->print_cr(" }");
2686 st->print_cr(" _next_om = " INTPTR_FORMAT, p2i(next_om()));
2687 st->print_cr(" _recursions = %zd", _recursions);
2688 st->print_cr(" _entry_list = " INTPTR_FORMAT, p2i(_entry_list));
2689 st->print_cr(" _entry_list_tail = " INTPTR_FORMAT, p2i(_entry_list_tail));
2690 st->print_cr(" _succ = " INT64_FORMAT, successor());
2691 st->print_cr(" _SpinDuration = %d", _SpinDuration);
2692 st->print_cr(" _contentions = %d", contentions());
2693 st->print_cr(" _unmounted_vthreads = " INT64_FORMAT, _unmounted_vthreads);
2694 st->print_cr(" _wait_set = " INTPTR_FORMAT, p2i(_wait_set));
2695 st->print_cr(" _waiters = %d", _waiters);
2696 st->print_cr(" _wait_set_lock = %d", _wait_set_lock);
2697 st->print_cr("}");
2698 }
2699 #endif