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 node->_prev = nullptr;
716 node->TState = ObjectWaiter::TS_ENTER;
717
718 for (;;) {
719 ObjectWaiter* head = Atomic::load(&_entry_list);
720 node->_next = head;
721 if (Atomic::cmpxchg(&_entry_list, head, node) == head) {
722 return false;
723 }
724
725 // Interference - the CAS failed because _entry_list changed. Before
726 // retrying the CAS retry taking the lock as it may now be free.
727 if (try_lock(current) == TryLockResult::Success) {
728 assert(!has_successor(current), "invariant");
729 assert(has_owner(current), "invariant");
730 return true;
731 }
732 }
733 }
734
735 static void post_monitor_deflate_event(EventJavaMonitorDeflate* event,
736 const oop obj) {
737 assert(event != nullptr, "invariant");
738 if (obj == nullptr) {
739 // Accept the case when obj was already garbage-collected.
740 // Emit the event anyway, but without details.
741 event->set_monitorClass(nullptr);
742 event->set_address(0);
743 } else {
744 const Klass* monitor_klass = obj->klass();
745 if (ObjectMonitor::is_jfr_excluded(monitor_klass)) {
746 return;
747 }
748 event->set_monitorClass(monitor_klass);
749 event->set_address((uintptr_t)(void*)obj);
750 }
751 event->commit();
752 }
753
754 // Deflate the specified ObjectMonitor if not in-use. Returns true if it
755 // was deflated and false otherwise.
756 //
757 // The async deflation protocol sets owner to DEFLATER_MARKER and
758 // makes contentions negative as signals to contending threads that
759 // an async deflation is in progress. There are a number of checks
760 // as part of the protocol to make sure that the calling thread has
761 // not lost the race to a contending thread.
762 //
763 // The ObjectMonitor has been successfully async deflated when:
764 // (contentions < 0)
765 // Contending threads that see that condition know to retry their operation.
766 //
767 bool ObjectMonitor::deflate_monitor(Thread* current) {
768 if (is_busy()) {
769 // Easy checks are first - the ObjectMonitor is busy so no deflation.
770 return false;
771 }
772
773 EventJavaMonitorDeflate event;
774
775 const oop obj = object_peek();
776
777 if (obj == nullptr) {
778 // If the object died, we can recycle the monitor without racing with
779 // Java threads. The GC already broke the association with the object.
780 set_owner_from_raw(NO_OWNER, DEFLATER_MARKER);
781 assert(contentions() >= 0, "must be non-negative: contentions=%d", contentions());
782 _contentions = INT_MIN; // minimum negative int
783 } else {
784 // Attempt async deflation protocol.
785
786 // Set a null owner to DEFLATER_MARKER to force any contending thread
787 // through the slow path. This is just the first part of the async
788 // deflation dance.
789 if (try_set_owner_from_raw(NO_OWNER, DEFLATER_MARKER) != NO_OWNER) {
790 // The owner field is no longer null so we lost the race since the
791 // ObjectMonitor is now busy.
792 return false;
793 }
794
795 if (contentions() > 0 || _waiters != 0) {
796 // Another thread has raced to enter the ObjectMonitor after
797 // is_busy() above or has already entered and waited on
798 // it which makes it busy so no deflation. Restore owner to
799 // null if it is still DEFLATER_MARKER.
800 if (try_set_owner_from_raw(DEFLATER_MARKER, NO_OWNER) != DEFLATER_MARKER) {
801 // Deferred decrement for the JT enter_internal() that cancelled the async deflation.
802 add_to_contentions(-1);
803 }
804 return false;
805 }
806
807 // Make a zero contentions field negative to force any contending threads
808 // to retry. This is the second part of the async deflation dance.
809 if (Atomic::cmpxchg(&_contentions, 0, INT_MIN) != 0) {
810 // Contentions was no longer 0 so we lost the race since the
811 // ObjectMonitor is now busy. Restore owner to null if it is
812 // still DEFLATER_MARKER:
813 if (try_set_owner_from_raw(DEFLATER_MARKER, NO_OWNER) != DEFLATER_MARKER) {
814 // Deferred decrement for the JT enter_internal() that cancelled the async deflation.
815 add_to_contentions(-1);
816 }
817 return false;
818 }
819 }
820
821 // Sanity checks for the races:
822 guarantee(owner_is_DEFLATER_MARKER(), "must be deflater marker");
823 guarantee(contentions() < 0, "must be negative: contentions=%d",
824 contentions());
825 guarantee(_waiters == 0, "must be 0: waiters=%d", _waiters);
826 ObjectWaiter* w = Atomic::load(&_entry_list);
827 guarantee(w == nullptr,
828 "must be no entering threads: entry_list=" INTPTR_FORMAT,
829 p2i(w));
830
831 if (obj != nullptr) {
832 if (log_is_enabled(Trace, monitorinflation)) {
833 ResourceMark rm;
834 log_trace(monitorinflation)("deflate_monitor: object=" INTPTR_FORMAT
835 ", mark=" INTPTR_FORMAT ", type='%s'",
836 p2i(obj), obj->mark().value(),
837 obj->klass()->external_name());
838 }
839 }
840
841 if (UseObjectMonitorTable) {
842 LightweightSynchronizer::deflate_monitor(current, obj, this);
843 } else if (obj != nullptr) {
844 // Install the old mark word if nobody else has already done it.
845 install_displaced_markword_in_object(obj);
846 }
847
848 if (event.should_commit()) {
849 post_monitor_deflate_event(&event, obj);
850 }
851
852 // We leave owner == DEFLATER_MARKER and contentions < 0
853 // to force any racing threads to retry.
854 return true; // Success, ObjectMonitor has been deflated.
855 }
856
857 // Install the displaced mark word (dmw) of a deflating ObjectMonitor
858 // into the header of the object associated with the monitor. This
859 // idempotent method is called by a thread that is deflating a
860 // monitor and by other threads that have detected a race with the
861 // deflation process.
862 void ObjectMonitor::install_displaced_markword_in_object(const oop obj) {
863 assert(!UseObjectMonitorTable, "ObjectMonitorTable has no dmw");
864 // This function must only be called when (owner == DEFLATER_MARKER
865 // && contentions <= 0), but we can't guarantee that here because
866 // those values could change when the ObjectMonitor gets moved from
867 // the global free list to a per-thread free list.
868
869 guarantee(obj != nullptr, "must be non-null");
870
871 // Separate loads in is_being_async_deflated(), which is almost always
872 // called before this function, from the load of dmw/header below.
873
874 // _contentions and dmw/header may get written by different threads.
875 // Make sure to observe them in the same order when having several observers.
876 OrderAccess::loadload_for_IRIW();
877
878 const oop l_object = object_peek();
879 if (l_object == nullptr) {
880 // ObjectMonitor's object ref has already been cleared by async
881 // deflation or GC so we're done here.
882 return;
883 }
884 assert(l_object == obj, "object=" INTPTR_FORMAT " must equal obj="
885 INTPTR_FORMAT, p2i(l_object), p2i(obj));
886
887 markWord dmw = header();
888 // The dmw has to be neutral (not null, not locked and not marked).
889 assert(dmw.is_neutral(), "must be neutral: dmw=" INTPTR_FORMAT, dmw.value());
890
891 // Install displaced mark word if the object's header still points
892 // to this ObjectMonitor. More than one racing caller to this function
893 // can rarely reach this point, but only one can win.
894 markWord res = obj->cas_set_mark(dmw, markWord::encode(this));
895 if (res != markWord::encode(this)) {
896 // This should be rare so log at the Info level when it happens.
897 log_info(monitorinflation)("install_displaced_markword_in_object: "
898 "failed cas_set_mark: new_mark=" INTPTR_FORMAT
899 ", old_mark=" INTPTR_FORMAT ", res=" INTPTR_FORMAT,
900 dmw.value(), markWord::encode(this).value(),
901 res.value());
902 }
903
904 // Note: It does not matter which thread restored the header/dmw
905 // into the object's header. The thread deflating the monitor just
906 // wanted the object's header restored and it is. The threads that
907 // detected a race with the deflation process also wanted the
908 // object's header restored before they retry their operation and
909 // because it is restored they will only retry once.
910 }
911
912 // Convert the fields used by is_busy() to a string that can be
913 // used for diagnostic output.
914 const char* ObjectMonitor::is_busy_to_string(stringStream* ss) {
915 ss->print("is_busy: waiters=%d"
916 ", contentions=%d"
917 ", owner=" INT64_FORMAT
918 ", entry_list=" PTR_FORMAT,
919 _waiters,
920 (contentions() > 0 ? contentions() : 0),
921 owner_is_DEFLATER_MARKER()
922 // We report null instead of DEFLATER_MARKER here because is_busy()
923 // ignores DEFLATER_MARKER values.
924 ? NO_OWNER
925 : owner_raw(),
926 p2i(_entry_list));
927 return ss->base();
928 }
929
930 void ObjectMonitor::enter_internal(JavaThread* current) {
931 assert(current->thread_state() == _thread_blocked, "invariant");
932
933 // Try the lock - TATAS
934 if (try_lock(current) == TryLockResult::Success) {
935 assert(!has_successor(current), "invariant");
936 assert(has_owner(current), "invariant");
937 return;
938 }
939
940 assert(InitDone, "Unexpectedly not initialized");
941
942 // We try one round of spinning *before* enqueueing current.
943 //
944 // If the _owner is ready but OFFPROC we could use a YieldTo()
945 // operation to donate the remainder of this thread's quantum
946 // to the owner. This has subtle but beneficial affinity
947 // effects.
948
949 if (try_spin(current)) {
950 assert(has_owner(current), "invariant");
951 assert(!has_successor(current), "invariant");
952 return;
953 }
954
955 // The Spin failed -- Enqueue and park the thread ...
956 assert(!has_successor(current), "invariant");
957 assert(!has_owner(current), "invariant");
958
959 // Enqueue "current" on ObjectMonitor's _entry_list.
960 //
961 // Node acts as a proxy for current.
962 // As an aside, if were to ever rewrite the synchronization code mostly
963 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
964 // Java objects. This would avoid awkward lifecycle and liveness issues,
965 // as well as eliminate a subset of ABA issues.
966 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
967
968 ObjectWaiter node(current);
969 current->_ParkEvent->reset();
970
971 if (try_lock_or_add_to_entry_list(current, &node)) {
972 return; // We got the lock.
973 }
974 // This thread is now added to the _entry_list.
975
976 // The lock might have been released while this thread was occupied queueing
977 // itself onto _entry_list. To close the race and avoid "stranding" and
978 // progress-liveness failure we must resample-retry _owner before parking.
979 // Note the Dekker/Lamport duality: ST _entry_list; MEMBAR; LD Owner.
980 // In this case the ST-MEMBAR is accomplished with CAS().
981 //
982 // TODO: Defer all thread state transitions until park-time.
983 // Since state transitions are heavy and inefficient we'd like
984 // to defer the state transitions until absolutely necessary,
985 // and in doing so avoid some transitions ...
986
987 // If there are unmounted virtual threads in the _entry_list do a timed-park
988 // instead to alleviate some deadlocks cases where one of them is picked as
989 // the successor but cannot run due to having run out of carriers. This can
990 // happen, for example, if this is a pinned virtual thread currently loading
991 // or initializining a class, and all other carriers have a pinned vthread
992 // waiting for said class to be loaded/initialized.
993 // Read counter *after* adding this thread to the _entry_list.
994 // Adding to _entry_list uses Atomic::cmpxchg() which already provides
995 // a fence that prevents this load from floating up previous store.
996 bool do_timed_parked = has_unmounted_vthreads();
997 static int MAX_RECHECK_INTERVAL = 1000;
998 int recheck_interval = 1;
999
1000 for (;;) {
1001
1002 if (try_lock(current) == TryLockResult::Success) {
1003 break;
1004 }
1005 assert(!has_owner(current), "invariant");
1006
1007 // park self
1008 if (do_timed_parked) {
1009 current->_ParkEvent->park((jlong) recheck_interval);
1010 // Increase the recheck_interval, but clamp the value.
1011 recheck_interval *= 8;
1012 if (recheck_interval > MAX_RECHECK_INTERVAL) {
1013 recheck_interval = MAX_RECHECK_INTERVAL;
1014 }
1015 } else {
1016 current->_ParkEvent->park();
1017 }
1018
1019 if (try_lock(current) == TryLockResult::Success) {
1020 break;
1021 }
1022
1023 // The lock is still contested.
1024
1025 // Assuming this is not a spurious wakeup we'll normally find _succ == current.
1026 // We can defer clearing _succ until after the spin completes
1027 // try_spin() must tolerate being called with _succ == current.
1028 // Try yet another round of adaptive spinning.
1029 if (try_spin(current)) {
1030 break;
1031 }
1032
1033 // We can find that we were unpark()ed and redesignated _succ while
1034 // we were spinning. That's harmless. If we iterate and call park(),
1035 // park() will consume the event and return immediately and we'll
1036 // just spin again. This pattern can repeat, leaving _succ to simply
1037 // spin on a CPU.
1038
1039 if (has_successor(current)) clear_successor();
1040
1041 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
1042 OrderAccess::fence();
1043 }
1044
1045 // Egress :
1046 // Current has acquired the lock -- Unlink current from the _entry_list.
1047 unlink_after_acquire(current, &node);
1048 if (has_successor(current)) {
1049 clear_successor();
1050 // Note that we don't need to do OrderAccess::fence() after clearing
1051 // _succ here, since we own the lock.
1052 }
1053
1054 // We've acquired ownership with CAS().
1055 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
1056 // But since the CAS() this thread may have also stored into _succ
1057 // or entry_list. These meta-data updates must be visible __before
1058 // this thread subsequently drops the lock.
1059 // Consider what could occur if we didn't enforce this constraint --
1060 // STs to monitor meta-data and user-data could reorder with (become
1061 // visible after) the ST in exit that drops ownership of the lock.
1062 // Some other thread could then acquire the lock, but observe inconsistent
1063 // or old monitor meta-data and heap data. That violates the JMM.
1064 // To that end, the exit() operation must have at least STST|LDST
1065 // "release" barrier semantics. Specifically, there must be at least a
1066 // STST|LDST barrier in exit() before the ST of null into _owner that drops
1067 // the lock. The barrier ensures that changes to monitor meta-data and data
1068 // protected by the lock will be visible before we release the lock, and
1069 // therefore before some other thread (CPU) has a chance to acquire the lock.
1070 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
1071 //
1072 // Critically, any prior STs to _succ or entry_list must be visible before
1073 // the ST of null into _owner in the *subsequent* (following) corresponding
1074 // monitorexit.
1075
1076 return;
1077 }
1078
1079 // reenter_internal() is a specialized inline form of the latter half of the
1080 // contended slow-path from enter_internal(). We use reenter_internal() only for
1081 // monitor reentry in wait().
1082 //
1083 // In the future we should reconcile enter_internal() and reenter_internal().
1084
1085 void ObjectMonitor::reenter_internal(JavaThread* current, ObjectWaiter* currentNode) {
1086 assert(current != nullptr, "invariant");
1087 assert(current->thread_state() != _thread_blocked, "invariant");
1088 assert(currentNode != nullptr, "invariant");
1089 assert(currentNode->_thread == current, "invariant");
1090 assert(_waiters > 0, "invariant");
1091 assert_mark_word_consistency();
1092
1093 // If there are unmounted virtual threads in the _entry_list do a timed-park
1094 // instead to alleviate some deadlocks cases where one of them is picked as
1095 // the successor but cannot run due to having run out of carriers. This can
1096 // happen, for example, if this is a pinned virtual thread (or plain carrier)
1097 // waiting for a class to be initialized.
1098 // In theory we only get here in the "notification" case where the thread has
1099 // already been added to the _entry_list. But if the thread happened to be interrupted
1100 // at the same time it was being notified, we could have read a state of TS_ENTER
1101 // that led us here but the thread hasn't been added yet to the queue. In that
1102 // case getting a false value from has_unmounted_vthreads() is not a guarantee
1103 // that vthreads weren't added before this thread to the _entry_list. We will live
1104 // with this corner case not only because it would be very rare, but also because
1105 // if there are several carriers blocked in this same situation, this would only
1106 // happen for the first one notified.
1107 bool do_timed_parked = has_unmounted_vthreads();
1108 static int MAX_RECHECK_INTERVAL = 1000;
1109 int recheck_interval = 1;
1110
1111 for (;;) {
1112 ObjectWaiter::TStates v = currentNode->TState;
1113 guarantee(v == ObjectWaiter::TS_ENTER, "invariant");
1114 assert(!has_owner(current), "invariant");
1115
1116 // This thread has been notified so try to reacquire the lock.
1117 if (try_lock(current) == TryLockResult::Success) {
1118 break;
1119 }
1120
1121 // If that fails, spin again. Note that spin count may be zero so the above TryLock
1122 // is necessary.
1123 if (try_spin(current)) {
1124 break;
1125 }
1126
1127 {
1128 OSThreadContendState osts(current->osthread());
1129
1130 assert(current->thread_state() == _thread_in_vm, "invariant");
1131
1132 {
1133 ClearSuccOnSuspend csos(this);
1134 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */);
1135 if (do_timed_parked) {
1136 current->_ParkEvent->park((jlong) recheck_interval);
1137 // Increase the recheck_interval, but clamp the value.
1138 recheck_interval *= 8;
1139 if (recheck_interval > MAX_RECHECK_INTERVAL) {
1140 recheck_interval = MAX_RECHECK_INTERVAL;
1141 }
1142 } else {
1143 current->_ParkEvent->park();
1144 }
1145 }
1146 }
1147
1148 // Try again, but just so we distinguish between futile wakeups and
1149 // successful wakeups. The following test isn't algorithmically
1150 // necessary, but it helps us maintain sensible statistics.
1151 if (try_lock(current) == TryLockResult::Success) {
1152 break;
1153 }
1154
1155 // The lock is still contested.
1156
1157 // Assuming this is not a spurious wakeup we'll normally
1158 // find that _succ == current.
1159 if (has_successor(current)) clear_successor();
1160
1161 // Invariant: after clearing _succ a contending thread
1162 // *must* retry _owner before parking.
1163 OrderAccess::fence();
1164 }
1165
1166 // Current has acquired the lock -- Unlink current from the _entry_list.
1167 assert(has_owner(current), "invariant");
1168 assert_mark_word_consistency();
1169 unlink_after_acquire(current, currentNode);
1170 if (has_successor(current)) clear_successor();
1171 assert(!has_successor(current), "invariant");
1172 currentNode->TState = ObjectWaiter::TS_RUN;
1173 OrderAccess::fence(); // see comments at the end of enter_internal()
1174 }
1175
1176 // This method is called from two places:
1177 // - On monitorenter contention with a null waiter.
1178 // - After Object.wait() times out or the target is interrupted to reenter the
1179 // monitor, with the existing waiter.
1180 // For the Object.wait() case we do not delete the ObjectWaiter in case we
1181 // succesfully acquire the monitor since we are going to need it on return.
1182 bool ObjectMonitor::vthread_monitor_enter(JavaThread* current, ObjectWaiter* waiter) {
1183 if (try_lock(current) == TryLockResult::Success) {
1184 assert(has_owner(current), "invariant");
1185 assert(!has_successor(current), "invariant");
1186 return true;
1187 }
1188
1189 oop vthread = current->vthread();
1190 ObjectWaiter* node = waiter != nullptr ? waiter : new ObjectWaiter(vthread, this);
1191
1192 // Increment counter *before* adding the vthread to the _entry_list.
1193 // Adding to _entry_list uses Atomic::cmpxchg() which already provides
1194 // a fence that prevents reordering of the stores.
1195 inc_unmounted_vthreads();
1196
1197 if (try_lock_or_add_to_entry_list(current, node)) {
1198 // We got the lock.
1199 if (waiter == nullptr) delete node; // for Object.wait() don't delete yet
1200 dec_unmounted_vthreads();
1201 return true;
1202 }
1203 // This thread is now added to the entry_list.
1204
1205 // We have to try once more since owner could have exited monitor and checked
1206 // _entry_list before we added the node to the queue.
1207 if (try_lock(current) == TryLockResult::Success) {
1208 assert(has_owner(current), "invariant");
1209 unlink_after_acquire(current, node);
1210 if (has_successor(current)) clear_successor();
1211 if (waiter == nullptr) delete node; // for Object.wait() don't delete yet
1212 dec_unmounted_vthreads();
1213 return true;
1214 }
1215
1216 assert(java_lang_VirtualThread::state(vthread) == java_lang_VirtualThread::RUNNING, "wrong state for vthread");
1217 java_lang_VirtualThread::set_state(vthread, java_lang_VirtualThread::BLOCKING);
1218
1219 // We didn't succeed in acquiring the monitor so increment _contentions and
1220 // save ObjectWaiter* in the vthread since we will need it when resuming execution.
1221 add_to_contentions(1);
1222 java_lang_VirtualThread::set_objectWaiter(vthread, node);
1223 return false;
1224 }
1225
1226 // Called from thaw code to resume the monitor operation that caused the vthread
1227 // to be unmounted. Method returns true if the monitor is successfully acquired,
1228 // which marks the end of the monitor operation, otherwise it returns false.
1229 bool ObjectMonitor::resume_operation(JavaThread* current, ObjectWaiter* node, ContinuationWrapper& cont) {
1230 assert(java_lang_VirtualThread::state(current->vthread()) == java_lang_VirtualThread::RUNNING, "wrong state for vthread");
1231 assert(!has_owner(current), "");
1232
1233 if (node->is_wait() && !node->at_reenter()) {
1234 bool acquired_monitor = vthread_wait_reenter(current, node, cont);
1235 if (acquired_monitor) return true;
1236 }
1237
1238 // Retry acquiring monitor...
1239
1240 int state = node->TState;
1241 guarantee(state == ObjectWaiter::TS_ENTER, "invariant");
1242
1243 if (try_lock(current) == TryLockResult::Success) {
1244 vthread_epilog(current, node);
1245 return true;
1246 }
1247
1248 oop vthread = current->vthread();
1249 if (has_successor(current)) clear_successor();
1250
1251 // Invariant: after clearing _succ a thread *must* retry acquiring the monitor.
1252 OrderAccess::fence();
1253
1254 if (try_lock(current) == TryLockResult::Success) {
1255 vthread_epilog(current, node);
1256 return true;
1257 }
1258
1259 // We will return to Continuation.run() and unmount so set the right state.
1260 java_lang_VirtualThread::set_state(vthread, java_lang_VirtualThread::BLOCKING);
1261
1262 return false;
1263 }
1264
1265 void ObjectMonitor::vthread_epilog(JavaThread* current, ObjectWaiter* node) {
1266 assert(has_owner(current), "invariant");
1267 add_to_contentions(-1);
1268 dec_unmounted_vthreads();
1269
1270 if (has_successor(current)) clear_successor();
1271
1272 guarantee(_recursions == 0, "invariant");
1273
1274 if (node->is_wait()) {
1275 _recursions = node->_recursions; // restore the old recursion count
1276 _waiters--; // decrement the number of waiters
1277
1278 if (node->_interrupted) {
1279 // We will throw at thaw end after finishing the mount transition.
1280 current->set_pending_interrupted_exception(true);
1281 }
1282 }
1283
1284 unlink_after_acquire(current, node);
1285 delete node;
1286
1287 // Clear the ObjectWaiter* from the vthread.
1288 java_lang_VirtualThread::set_objectWaiter(current->vthread(), nullptr);
1289
1290 if (JvmtiExport::should_post_monitor_contended_entered()) {
1291 // We are going to call thaw again after this and finish the VMTS
1292 // transition so no need to do it here. We will post the event there.
1293 current->set_contended_entered_monitor(this);
1294 }
1295 }
1296
1297 // Convert entry_list into a doubly linked list by assigning the prev
1298 // pointers and the entry_list_tail pointer (if needed). Within the
1299 // entry_list the next pointers always form a consistent singly linked
1300 // list. When this function is called, the entry_list will be either
1301 // singly linked, or starting as singly linked (at the head), but
1302 // ending as doubly linked (at the tail).
1303 void ObjectMonitor::entry_list_build_dll(JavaThread* current) {
1304 assert(has_owner(current), "invariant");
1305 ObjectWaiter* prev = nullptr;
1306 // Need acquire here to match the implicit release of the cmpxchg
1307 // that updated entry_list, so we can access w->prev().
1308 ObjectWaiter* w = Atomic::load_acquire(&_entry_list);
1309 assert(w != nullptr, "should only be called when entry list is not empty");
1310 while (w != nullptr) {
1311 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1312 assert(w->prev() == nullptr || w->prev() == prev, "invariant");
1313 if (w->prev() != nullptr) {
1314 break;
1315 }
1316 w->_prev = prev;
1317 prev = w;
1318 w = w->next();
1319 }
1320 if (w == nullptr) {
1321 // We converted the entire entry_list from a singly linked list
1322 // into a doubly linked list. Now we just need to set the tail
1323 // pointer.
1324 assert(prev != nullptr && prev->next() == nullptr, "invariant");
1325 assert(_entry_list_tail == nullptr || _entry_list_tail == prev, "invariant");
1326 _entry_list_tail = prev;
1327 } else {
1328 #ifdef ASSERT
1329 // We stopped iterating through the _entry_list when we found a
1330 // node that had its prev pointer set. I.e. we converted the first
1331 // part of the entry_list from a singly linked list into a doubly
1332 // linked list. Now we just want to make sure the rest of the list
1333 // is doubly linked. But first we check that we have a tail
1334 // pointer, because if the end of the entry_list is doubly linked
1335 // and we don't have the tail pointer, something is broken.
1336 assert(_entry_list_tail != nullptr, "invariant");
1337 while (w != nullptr) {
1338 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1339 assert(w->prev() == prev, "invariant");
1340 prev = w;
1341 w = w->next();
1342 }
1343 assert(_entry_list_tail == prev, "invariant");
1344 #endif
1345 }
1346 }
1347
1348 // Return the tail of the _entry_list. If the tail is currently not
1349 // known, it can be found by first calling entry_list_build_dll().
1350 ObjectWaiter* ObjectMonitor::entry_list_tail(JavaThread* current) {
1351 assert(has_owner(current), "invariant");
1352 ObjectWaiter* w = _entry_list_tail;
1353 if (w != nullptr) {
1354 return w;
1355 }
1356 entry_list_build_dll(current);
1357 w = _entry_list_tail;
1358 assert(w != nullptr, "invariant");
1359 return w;
1360 }
1361
1362 // By convention we unlink a contending thread from _entry_list
1363 // immediately after the thread acquires the lock in ::enter().
1364 // The head of _entry_list is volatile but the interior is stable.
1365 // In addition, current.TState is stable.
1366
1367 void ObjectMonitor::unlink_after_acquire(JavaThread* current, ObjectWaiter* currentNode) {
1368 assert(has_owner(current), "invariant");
1369 assert((!currentNode->is_vthread() && currentNode->thread() == current) ||
1370 (currentNode->is_vthread() && currentNode->vthread() == current->vthread()), "invariant");
1371
1372 // Check if we are unlinking the last element in the _entry_list.
1373 // This is by far the most common case.
1374 if (currentNode->next() == nullptr) {
1375 assert(_entry_list_tail == nullptr || _entry_list_tail == currentNode, "invariant");
1376
1377 ObjectWaiter* w = Atomic::load(&_entry_list);
1378 if (w == currentNode) {
1379 // The currentNode is the only element in _entry_list.
1380 if (Atomic::cmpxchg(&_entry_list, w, (ObjectWaiter*)nullptr) == w) {
1381 _entry_list_tail = nullptr;
1382 currentNode->set_bad_pointers();
1383 return;
1384 }
1385 // The CAS above can fail from interference IFF a contending
1386 // thread "pushed" itself onto entry_list. So fall-through to
1387 // building the doubly linked list.
1388 assert(currentNode->prev() == nullptr, "invariant");
1389 }
1390 if (currentNode->prev() == nullptr) {
1391 // Build the doubly linked list to get hold of
1392 // currentNode->prev().
1393 entry_list_build_dll(current);
1394 assert(currentNode->prev() != nullptr, "must be");
1395 assert(_entry_list_tail == currentNode, "must be");
1396 }
1397 // The currentNode is the last element in _entry_list and we know
1398 // which element is the previous one.
1399 assert(_entry_list != currentNode, "invariant");
1400 _entry_list_tail = currentNode->prev();
1401 _entry_list_tail->_next = nullptr;
1402 currentNode->set_bad_pointers();
1403 return;
1404 }
1405
1406 // If we get here it means the current thread enqueued itself on the
1407 // _entry_list but was then able to "steal" the lock before the
1408 // chosen successor was able to. Consequently currentNode must be an
1409 // interior node in the _entry_list, or the head.
1410 assert(currentNode->next() != nullptr, "invariant");
1411 assert(currentNode != _entry_list_tail, "invariant");
1412
1413 // Check if we are in the singly linked portion of the
1414 // _entry_list. If we are the head then we try to remove ourselves,
1415 // else we convert to the doubly linked list.
1416 if (currentNode->prev() == nullptr) {
1417 ObjectWaiter* w = Atomic::load(&_entry_list);
1418
1419 assert(w != nullptr, "invariant");
1420 if (w == currentNode) {
1421 ObjectWaiter* next = currentNode->next();
1422 // currentNode is at the head of _entry_list.
1423 if (Atomic::cmpxchg(&_entry_list, w, next) == w) {
1424 // The CAS above sucsessfully unlinked currentNode from the
1425 // head of the _entry_list.
1426 assert(_entry_list != w, "invariant");
1427 next->_prev = nullptr;
1428 currentNode->set_bad_pointers();
1429 return;
1430 } else {
1431 // The CAS above can fail from interference IFF a contending
1432 // thread "pushed" itself onto _entry_list, in which case
1433 // currentNode must now be in the interior of the
1434 // list. Fall-through to building the doubly linked list.
1435 assert(_entry_list != currentNode, "invariant");
1436 }
1437 }
1438 // Build the doubly linked list to get hold of currentNode->prev().
1439 entry_list_build_dll(current);
1440 assert(currentNode->prev() != nullptr, "must be");
1441 }
1442
1443 // We now know we are unlinking currentNode from the interior of a
1444 // doubly linked list.
1445 assert(currentNode->next() != nullptr, "");
1446 assert(currentNode->prev() != nullptr, "");
1447 assert(currentNode != _entry_list, "");
1448 assert(currentNode != _entry_list_tail, "");
1449
1450 ObjectWaiter* nxt = currentNode->next();
1451 ObjectWaiter* prv = currentNode->prev();
1452 assert(nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
1453 assert(prv->TState == ObjectWaiter::TS_ENTER, "invariant");
1454
1455 nxt->_prev = prv;
1456 prv->_next = nxt;
1457 currentNode->set_bad_pointers();
1458 }
1459
1460 // -----------------------------------------------------------------------------
1461 // Exit support
1462 //
1463 // exit()
1464 // ~~~~~~
1465 // Note that the collector can't reclaim the objectMonitor or deflate
1466 // the object out from underneath the thread calling ::exit() as the
1467 // thread calling ::exit() never transitions to a stable state.
1468 // This inhibits GC, which in turn inhibits asynchronous (and
1469 // inopportune) reclamation of "this".
1470 //
1471 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
1472 // There's one exception to the claim above, however. enter_internal() can call
1473 // exit() to drop a lock if the acquirer has been externally suspended.
1474 // In that case exit() is called with _thread_state == _thread_blocked,
1475 // but the monitor's _contentions field is > 0, which inhibits reclamation.
1476 //
1477 // This is the exit part of the locking protocol, often implemented in
1478 // C2_MacroAssembler::fast_unlock()
1479 //
1480 // 1. A release barrier ensures that changes to monitor meta-data
1481 // (_succ, _entry_list) and data protected by the lock will be
1482 // visible before we release the lock.
1483 // 2. Release the lock by clearing the owner.
1484 // 3. A storeload MEMBAR is needed between releasing the owner and
1485 // subsequently reading meta-data to safely determine if the lock is
1486 // contended (step 4) without an elected successor (step 5).
1487 // 4. If _entry_list is null, we are done, since there is no
1488 // other thread waiting on the lock to wake up. I.e. there is no
1489 // contention.
1490 // 5. If there is a successor (_succ is non-null), we are done. The
1491 // responsibility for guaranteeing progress-liveness has now implicitly
1492 // been moved from the exiting thread to the successor.
1493 // 6. There are waiters in the entry list (_entry_list is non-null),
1494 // but there is no successor (_succ is null), so we need to
1495 // wake up (unpark) a waiting thread to avoid stranding.
1496 //
1497 // Note that since only the current lock owner can manipulate the
1498 // _entry_list (except for pushing new threads to the head), we need to
1499 // reacquire the lock before we can wake up (unpark) a waiting thread.
1500 //
1501 // The CAS() in enter provides for safety and exclusion, while the
1502 // MEMBAR in exit provides for progress and avoids stranding.
1503 //
1504 // There is also the risk of a futile wake-up. If we drop the lock
1505 // another thread can reacquire the lock immediately, and we can
1506 // then wake a thread unnecessarily. This is benign, and we've
1507 // structured the code so the windows are short and the frequency
1508 // of such futile wakups is low.
1509
1510 void ObjectMonitor::exit(JavaThread* current, bool not_suspended) {
1511 if (!has_owner(current)) {
1512 // Apparent unbalanced locking ...
1513 // Naively we'd like to throw IllegalMonitorStateException.
1514 // As a practical matter we can neither allocate nor throw an
1515 // exception as ::exit() can be called from leaf routines.
1516 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
1517 // Upon deeper reflection, however, in a properly run JVM the only
1518 // way we should encounter this situation is in the presence of
1519 // unbalanced JNI locking. TODO: CheckJNICalls.
1520 // See also: CR4414101
1521 #ifdef ASSERT
1522 LogStreamHandle(Error, monitorinflation) lsh;
1523 lsh.print_cr("ERROR: ObjectMonitor::exit(): thread=" INTPTR_FORMAT
1524 " is exiting an ObjectMonitor it does not own.", p2i(current));
1525 lsh.print_cr("The imbalance is possibly caused by JNI locking.");
1526 print_debug_style_on(&lsh);
1527 assert(false, "Non-balanced monitor enter/exit!");
1528 #endif
1529 return;
1530 }
1531
1532 if (_recursions != 0) {
1533 _recursions--; // this is simple recursive enter
1534 return;
1535 }
1536
1537 #if INCLUDE_JFR
1538 // get the owner's thread id for the MonitorEnter event
1539 // if it is enabled and the thread isn't suspended
1540 if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
1541 _previous_owner_tid = JFR_THREAD_ID(current);
1542 }
1543 #endif
1544
1545 for (;;) {
1546 assert(has_owner(current), "invariant");
1547
1548 // Drop the lock.
1549 // release semantics: prior loads and stores from within the critical section
1550 // must not float (reorder) past the following store that drops the lock.
1551 // Uses a storeload to separate release_store(owner) from the
1552 // successor check. The try_set_owner_from() below uses cmpxchg() so
1553 // we get the fence down there.
1554 release_clear_owner(current);
1555 OrderAccess::storeload();
1556
1557 // Normally the exiting thread is responsible for ensuring succession,
1558 // but if this thread observes other successors are ready or other
1559 // entering threads are spinning after it has stored null into _owner
1560 // then it can exit without waking a successor. The existence of
1561 // spinners or ready successors guarantees proper succession (liveness).
1562 // Responsibility passes to the ready or running successors. The exiting
1563 // thread delegates the duty. More precisely, if a successor already
1564 // exists this thread is absolved of the responsibility of waking
1565 // (unparking) one.
1566
1567 // The _succ variable is critical to reducing futile wakeup frequency.
1568 // _succ identifies the "heir presumptive" thread that has been made
1569 // ready (unparked) but that has not yet run. We need only one such
1570 // successor thread to guarantee progress.
1571 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1572 // section 3.3 "Futile Wakeup Throttling" for details.
1573 //
1574 // Note that spinners in Enter() also set _succ non-null.
1575 // In the current implementation spinners opportunistically set
1576 // _succ so that exiting threads might avoid waking a successor.
1577 // Which means that the exiting thread could exit immediately without
1578 // waking a successor, if it observes a successor after it has dropped
1579 // the lock. Note that the dropped lock needs to become visible to the
1580 // spinner.
1581
1582 if (_entry_list == nullptr || has_successor()) {
1583 return;
1584 }
1585
1586 // Other threads are blocked trying to acquire the lock and there
1587 // is no successor, so it appears that an heir-presumptive
1588 // (successor) must be made ready. Only the current lock owner can
1589 // detach threads from the entry_list, therefore we need to
1590 // reacquire the lock. If we fail to reacquire the lock the
1591 // responsibility for ensuring succession falls to the new owner.
1592
1593 if (try_lock(current) != TryLockResult::Success) {
1594 // Some other thread acquired the lock (or the monitor was
1595 // deflated). Either way we are done.
1596 return;
1597 }
1598
1599 guarantee(has_owner(current), "invariant");
1600
1601 ObjectWaiter* w = nullptr;
1602
1603 w = Atomic::load(&_entry_list);
1604 if (w != nullptr) {
1605 w = entry_list_tail(current);
1606 // I'd like to write: guarantee (w->_thread != current).
1607 // But in practice an exiting thread may find itself on the entry_list.
1608 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1609 // then calls exit(). Exit release the lock by setting O._owner to null.
1610 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1611 // notify() operation moves T1 from O's waitset to O's entry_list. T2 then
1612 // release the lock "O". T1 resumes immediately after the ST of null into
1613 // _owner, above. T1 notices that the entry_list is populated, so it
1614 // reacquires the lock and then finds itself on the entry_list.
1615 // Given all that, we have to tolerate the circumstance where "w" is
1616 // associated with current.
1617 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1618 exit_epilog(current, w);
1619 return;
1620 }
1621 }
1622 }
1623
1624 void ObjectMonitor::exit_epilog(JavaThread* current, ObjectWaiter* Wakee) {
1625 assert(has_owner(current), "invariant");
1626
1627 // Exit protocol:
1628 // 1. ST _succ = wakee
1629 // 2. membar #loadstore|#storestore;
1630 // 2. ST _owner = nullptr
1631 // 3. unpark(wakee)
1632
1633 oop vthread = nullptr;
1634 ParkEvent * Trigger;
1635 if (!Wakee->is_vthread()) {
1636 JavaThread* t = Wakee->thread();
1637 assert(t != nullptr, "");
1638 Trigger = t->_ParkEvent;
1639 set_successor(t);
1640 } else {
1641 vthread = Wakee->vthread();
1642 assert(vthread != nullptr, "");
1643 Trigger = ObjectMonitor::vthread_unparker_ParkEvent();
1644 set_successor(vthread);
1645 }
1646
1647 // Hygiene -- once we've set _owner = nullptr we can't safely dereference Wakee again.
1648 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1649 // out-of-scope (non-extant).
1650 Wakee = nullptr;
1651
1652 // Drop the lock.
1653 // Uses a fence to separate release_store(owner) from the LD in unpark().
1654 release_clear_owner(current);
1655 OrderAccess::fence();
1656
1657 DTRACE_MONITOR_PROBE(contended__exit, this, object(), current);
1658
1659 if (vthread == nullptr) {
1660 // Platform thread case.
1661 Trigger->unpark();
1662 } else if (java_lang_VirtualThread::set_onWaitingList(vthread, vthread_list_head())) {
1663 // Virtual thread case.
1664 Trigger->unpark();
1665 }
1666 }
1667
1668 // Exits the monitor returning recursion count. _owner should
1669 // be set to current's owner_id, i.e. no ANONYMOUS_OWNER allowed.
1670 intx ObjectMonitor::complete_exit(JavaThread* current) {
1671 assert(InitDone, "Unexpectedly not initialized");
1672 guarantee(has_owner(current), "complete_exit not owner");
1673
1674 intx save = _recursions; // record the old recursion count
1675 _recursions = 0; // set the recursion level to be 0
1676 exit(current); // exit the monitor
1677 guarantee(!has_owner(current), "invariant");
1678 return save;
1679 }
1680
1681 // Checks that the current THREAD owns this monitor and causes an
1682 // immediate return if it doesn't. We don't use the CHECK macro
1683 // because we want the IMSE to be the only exception that is thrown
1684 // from the call site when false is returned. Any other pending
1685 // exception is ignored.
1686 #define CHECK_OWNER() \
1687 do { \
1688 if (!check_owner(THREAD)) { \
1689 assert(HAS_PENDING_EXCEPTION, "expected a pending IMSE here."); \
1690 return; \
1691 } \
1692 } while (false)
1693
1694 // Returns true if the specified thread owns the ObjectMonitor.
1695 // Otherwise returns false and throws IllegalMonitorStateException
1696 // (IMSE). If there is a pending exception and the specified thread
1697 // is not the owner, that exception will be replaced by the IMSE.
1698 bool ObjectMonitor::check_owner(TRAPS) {
1699 JavaThread* current = THREAD;
1700 int64_t cur = owner_raw();
1701 if (cur == owner_id_from(current)) {
1702 return true;
1703 }
1704 THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(),
1705 "current thread is not owner", false);
1706 }
1707
1708 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1709 ObjectMonitor* monitor,
1710 uint64_t notifier_tid,
1711 jlong timeout,
1712 bool timedout) {
1713 assert(event != nullptr, "invariant");
1714 assert(monitor != nullptr, "invariant");
1715 const Klass* monitor_klass = monitor->object()->klass();
1716 if (ObjectMonitor::is_jfr_excluded(monitor_klass)) {
1717 return;
1718 }
1719 event->set_monitorClass(monitor_klass);
1720 event->set_timeout(timeout);
1721 // Set an address that is 'unique enough', such that events close in
1722 // time and with the same address are likely (but not guaranteed) to
1723 // belong to the same object.
1724 event->set_address((uintptr_t)monitor);
1725 event->set_notifier(notifier_tid);
1726 event->set_timedOut(timedout);
1727 event->commit();
1728 }
1729
1730 static void vthread_monitor_waited_event(JavaThread* current, ObjectWaiter* node, ContinuationWrapper& cont, EventJavaMonitorWait* event, jboolean timed_out) {
1731 // Since we might safepoint set the anchor so that the stack can we walked.
1732 assert(current->last_continuation() != nullptr, "");
1733 JavaFrameAnchor* anchor = current->frame_anchor();
1734 anchor->set_last_Java_sp(current->last_continuation()->entry_sp());
1735 anchor->set_last_Java_pc(current->last_continuation()->entry_pc());
1736
1737 ContinuationWrapper::SafepointOp so(current, cont);
1738
1739 JRT_BLOCK
1740 if (event->should_commit()) {
1741 long timeout = java_lang_VirtualThread::timeout(current->vthread());
1742 post_monitor_wait_event(event, node->_monitor, node->_notifier_tid, timeout, timed_out);
1743 }
1744 if (JvmtiExport::should_post_monitor_waited()) {
1745 // We mark this call in case of an upcall to Java while posting the event.
1746 // If somebody walks the stack in that case, processing the enterSpecial
1747 // frame should not include processing callee arguments since there is no
1748 // actual callee (see nmethod::preserve_callee_argument_oops()).
1749 ThreadOnMonitorWaitedEvent tmwe(current);
1750 JvmtiExport::vthread_post_monitor_waited(current, node->_monitor, timed_out);
1751 }
1752 JRT_BLOCK_END
1753 current->frame_anchor()->clear();
1754 }
1755
1756 // -----------------------------------------------------------------------------
1757 // Wait/Notify/NotifyAll
1758 //
1759 // Note: a subset of changes to ObjectMonitor::wait()
1760 // will need to be replicated in complete_exit
1761 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1762 JavaThread* current = THREAD;
1763
1764 assert(InitDone, "Unexpectedly not initialized");
1765
1766 CHECK_OWNER(); // Throws IMSE if not owner.
1767
1768 EventJavaMonitorWait wait_event;
1769 EventVirtualThreadPinned vthread_pinned_event;
1770
1771 // check for a pending interrupt
1772 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1773 JavaThreadInObjectWaitState jtiows(current, millis != 0, interruptible);
1774
1775 if (JvmtiExport::should_post_monitor_wait()) {
1776 JvmtiExport::post_monitor_wait(current, object(), millis);
1777 }
1778 // post monitor waited event. Note that this is past-tense, we are done waiting.
1779 if (JvmtiExport::should_post_monitor_waited()) {
1780 // Note: 'false' parameter is passed here because the
1781 // wait was not timed out due to thread interrupt.
1782 JvmtiExport::post_monitor_waited(current, this, false);
1783
1784 // In this short circuit of the monitor wait protocol, the
1785 // current thread never drops ownership of the monitor and
1786 // never gets added to the wait queue so the current thread
1787 // cannot be made the successor. This means that the
1788 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1789 // consume an unpark() meant for the ParkEvent associated with
1790 // this ObjectMonitor.
1791 }
1792 if (wait_event.should_commit()) {
1793 post_monitor_wait_event(&wait_event, this, 0, millis, false);
1794 }
1795 THROW(vmSymbols::java_lang_InterruptedException());
1796 return;
1797 }
1798
1799 freeze_result result;
1800 ContinuationEntry* ce = current->last_continuation();
1801 bool is_virtual = ce != nullptr && ce->is_virtual_thread();
1802 if (is_virtual) {
1803 if (interruptible && JvmtiExport::should_post_monitor_wait()) {
1804 JvmtiExport::post_monitor_wait(current, object(), millis);
1805 }
1806 current->set_current_waiting_monitor(this);
1807 result = Continuation::try_preempt(current, ce->cont_oop(current));
1808 if (result == freeze_ok) {
1809 vthread_wait(current, millis, interruptible);
1810 current->set_current_waiting_monitor(nullptr);
1811 return;
1812 }
1813 }
1814 // The jtiows does nothing for non-interruptible.
1815 JavaThreadInObjectWaitState jtiows(current, millis != 0, interruptible);
1816
1817 if (!is_virtual) { // it was already set for virtual thread
1818 if (interruptible && JvmtiExport::should_post_monitor_wait()) {
1819 JvmtiExport::post_monitor_wait(current, object(), millis);
1820
1821 // The current thread already owns the monitor and it has not yet
1822 // been added to the wait queue so the current thread cannot be
1823 // made the successor. This means that the JVMTI_EVENT_MONITOR_WAIT
1824 // event handler cannot accidentally consume an unpark() meant for
1825 // the ParkEvent associated with this ObjectMonitor.
1826 }
1827 current->set_current_waiting_monitor(this);
1828 }
1829 // create a node to be put into the queue
1830 // Critically, after we reset() the event but prior to park(), we must check
1831 // for a pending interrupt.
1832 ObjectWaiter node(current);
1833 node.TState = ObjectWaiter::TS_WAIT;
1834 current->_ParkEvent->reset();
1835 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1836
1837 // Enter the waiting queue, which is a circular doubly linked list in this case
1838 // but it could be a priority queue or any data structure.
1839 // _wait_set_lock protects the wait queue. Normally the wait queue is accessed only
1840 // by the owner of the monitor *except* in the case where park()
1841 // returns because of a timeout of interrupt. Contention is exceptionally rare
1842 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1843
1844 Thread::SpinAcquire(&_wait_set_lock, "wait_set - add");
1845 add_waiter(&node);
1846 Thread::SpinRelease(&_wait_set_lock);
1847
1848 intx save = _recursions; // record the old recursion count
1849 _waiters++; // increment the number of waiters
1850 _recursions = 0; // set the recursion level to be 1
1851 exit(current); // exit the monitor
1852 guarantee(!has_owner(current), "invariant");
1853
1854 // The thread is on the wait_set list - now park() it.
1855 // On MP systems it's conceivable that a brief spin before we park
1856 // could be profitable.
1857 //
1858 // TODO-FIXME: change the following logic to a loop of the form
1859 // while (!timeout && !interrupted && _notified == 0) park()
1860
1861 int ret = OS_OK;
1862 int WasNotified = 0;
1863
1864 // Need to check interrupt state whilst still _thread_in_vm
1865 bool interrupted = interruptible && current->is_interrupted(false);
1866
1867 { // State transition wrappers
1868 OSThread* osthread = current->osthread();
1869 OSThreadWaitState osts(osthread, true);
1870
1871 assert(current->thread_state() == _thread_in_vm, "invariant");
1872
1873 {
1874 ClearSuccOnSuspend csos(this);
1875 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */);
1876 if (interrupted || HAS_PENDING_EXCEPTION) {
1877 // Intentionally empty
1878 } else if (!node._notified) {
1879 if (millis <= 0) {
1880 current->_ParkEvent->park();
1881 } else {
1882 ret = current->_ParkEvent->park(millis);
1883 }
1884 }
1885 }
1886
1887 // Node may be on the wait_set, or on the entry_list, or in transition
1888 // from the wait_set to the entry_list.
1889 // See if we need to remove Node from the wait_set.
1890 // We use double-checked locking to avoid grabbing _wait_set_lock
1891 // if the thread is not on the wait queue.
1892 //
1893 // Note that we don't need a fence before the fetch of TState.
1894 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1895 // written by the is thread. (perhaps the fetch might even be satisfied
1896 // by a look-aside into the processor's own store buffer, although given
1897 // the length of the code path between the prior ST and this load that's
1898 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1899 // then we'll acquire the lock and then re-fetch a fresh TState value.
1900 // That is, we fail toward safety.
1901
1902 if (node.TState == ObjectWaiter::TS_WAIT) {
1903 Thread::SpinAcquire(&_wait_set_lock, "wait_set - unlink");
1904 if (node.TState == ObjectWaiter::TS_WAIT) {
1905 dequeue_specific_waiter(&node); // unlink from wait_set
1906 assert(!node._notified, "invariant");
1907 node.TState = ObjectWaiter::TS_RUN;
1908 }
1909 Thread::SpinRelease(&_wait_set_lock);
1910 }
1911
1912 // The thread is now either on off-list (TS_RUN),
1913 // or on the entry_list (TS_ENTER).
1914 // The Node's TState variable is stable from the perspective of this thread.
1915 // No other threads will asynchronously modify TState.
1916 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1917 OrderAccess::loadload();
1918 if (has_successor(current)) clear_successor();
1919 WasNotified = node._notified;
1920
1921 // Reentry phase -- reacquire the monitor.
1922 // re-enter contended monitor after object.wait().
1923 // retain OBJECT_WAIT state until re-enter successfully completes
1924 // Thread state is thread_in_vm and oop access is again safe,
1925 // although the raw address of the object may have changed.
1926 // (Don't cache naked oops over safepoints, of course).
1927
1928 // post monitor waited event. Note that this is past-tense, we are done waiting.
1929 if (JvmtiExport::should_post_monitor_waited()) {
1930 JvmtiExport::post_monitor_waited(current, this, ret == OS_TIMEOUT);
1931
1932 if (node._notified && has_successor(current)) {
1933 // In this part of the monitor wait-notify-reenter protocol it
1934 // is possible (and normal) for another thread to do a fastpath
1935 // monitor enter-exit while this thread is still trying to get
1936 // to the reenter portion of the protocol.
1937 //
1938 // The ObjectMonitor was notified and the current thread is
1939 // the successor which also means that an unpark() has already
1940 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1941 // consume the unpark() that was done when the successor was
1942 // set because the same ParkEvent is shared between Java
1943 // monitors and JVM/TI RawMonitors (for now).
1944 //
1945 // We redo the unpark() to ensure forward progress, i.e., we
1946 // don't want all pending threads hanging (parked) with none
1947 // entering the unlocked monitor.
1948 current->_ParkEvent->unpark();
1949 }
1950 }
1951
1952 if (wait_event.should_commit()) {
1953 post_monitor_wait_event(&wait_event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1954 }
1955
1956 OrderAccess::fence();
1957
1958 assert(!has_owner(current), "invariant");
1959 ObjectWaiter::TStates v = node.TState;
1960 if (v == ObjectWaiter::TS_RUN) {
1961 // We use the NoPreemptMark for the very rare case where the previous
1962 // preempt attempt failed due to OOM. The preempt on monitor contention
1963 // could succeed but we can't unmount now.
1964 NoPreemptMark npm(current);
1965 enter(current);
1966 } else {
1967 guarantee(v == ObjectWaiter::TS_ENTER, "invariant");
1968 reenter_internal(current, &node);
1969 node.wait_reenter_end(this);
1970 }
1971
1972 // current has reacquired the lock.
1973 // Lifecycle - the node representing current must not appear on any queues.
1974 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1975 // want residual elements associated with this thread left on any lists.
1976 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1977 assert(has_owner(current), "invariant");
1978 assert(!has_successor(current), "invariant");
1979 } // OSThreadWaitState()
1980
1981 current->set_current_waiting_monitor(nullptr);
1982
1983 guarantee(_recursions == 0, "invariant");
1984 int relock_count = JvmtiDeferredUpdates::get_and_reset_relock_count_after_wait(current);
1985 _recursions = save // restore the old recursion count
1986 + relock_count; // increased by the deferred relock count
1987 current->inc_held_monitor_count(relock_count); // Deopt never entered these counts.
1988 _waiters--; // decrement the number of waiters
1989
1990 // Verify a few postconditions
1991 assert(has_owner(current), "invariant");
1992 assert(!has_successor(current), "invariant");
1993 assert_mark_word_consistency();
1994
1995 if (ce != nullptr && ce->is_virtual_thread()) {
1996 current->post_vthread_pinned_event(&vthread_pinned_event, "Object.wait", result);
1997 }
1998
1999 // check if the notification happened
2000 if (!WasNotified) {
2001 // no, it could be timeout or Thread.interrupt() or both
2002 // check for interrupt event, otherwise it is timeout
2003 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
2004 THROW(vmSymbols::java_lang_InterruptedException());
2005 }
2006 }
2007
2008 // NOTE: Spurious wake up will be consider as timeout.
2009 // Monitor notify has precedence over thread interrupt.
2010 }
2011
2012 // Consider:
2013 // If the lock is cool (entry_list == null && succ == null) and we're on an MP system
2014 // then instead of transferring a thread from the wait_set to the entry_list
2015 // we might just dequeue a thread from the wait_set and directly unpark() it.
2016
2017 bool ObjectMonitor::notify_internal(JavaThread* current) {
2018 bool did_notify = false;
2019 Thread::SpinAcquire(&_wait_set_lock, "wait_set - notify");
2020 ObjectWaiter* iterator = dequeue_waiter();
2021 if (iterator != nullptr) {
2022 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
2023 guarantee(!iterator->_notified, "invariant");
2024
2025 if (iterator->is_vthread()) {
2026 oop vthread = iterator->vthread();
2027 java_lang_VirtualThread::set_notified(vthread, true);
2028 int old_state = java_lang_VirtualThread::state(vthread);
2029 // If state is not WAIT/TIMED_WAIT then target could still be on
2030 // unmount transition, or wait could have already timed-out or target
2031 // could have been interrupted. In the first case, the target itself
2032 // will set the state to BLOCKED at the end of the unmount transition.
2033 // In the other cases the target would have been already unblocked so
2034 // there is nothing to do.
2035 if (old_state == java_lang_VirtualThread::WAIT ||
2036 old_state == java_lang_VirtualThread::TIMED_WAIT) {
2037 java_lang_VirtualThread::cmpxchg_state(vthread, old_state, java_lang_VirtualThread::BLOCKED);
2038 }
2039 // Increment counter *before* adding the vthread to the _entry_list.
2040 // Adding to _entry_list uses Atomic::cmpxchg() which already provides
2041 // a fence that prevents reordering of the stores.
2042 inc_unmounted_vthreads();
2043 }
2044
2045 iterator->_notified = true;
2046 iterator->_notifier_tid = JFR_THREAD_ID(current);
2047 did_notify = true;
2048 add_to_entry_list(current, iterator);
2049
2050 // _wait_set_lock protects the wait queue, not the entry_list. We could
2051 // move the add-to-entry_list operation, above, outside the critical section
2052 // protected by _wait_set_lock. In practice that's not useful. With the
2053 // exception of wait() timeouts and interrupts the monitor owner
2054 // is the only thread that grabs _wait_set_lock. There's almost no contention
2055 // on _wait_set_lock so it's not profitable to reduce the length of the
2056 // critical section.
2057
2058 if (!iterator->is_vthread()) {
2059 iterator->wait_reenter_begin(this);
2060
2061 // Read counter *after* adding the thread to the _entry_list.
2062 // Adding to _entry_list uses Atomic::cmpxchg() which already provides
2063 // a fence that prevents this load from floating up previous store.
2064 if (has_unmounted_vthreads()) {
2065 // Wake up the thread to alleviate some deadlocks cases where the successor
2066 // that will be picked up when this thread releases the monitor is an unmounted
2067 // virtual thread that cannot run due to having run out of carriers. Upon waking
2068 // up, the thread will call reenter_internal() which will use time-park in case
2069 // there is contention and there are still vthreads in the _entry_list.
2070 JavaThread* t = iterator->thread();
2071 t->_ParkEvent->unpark();
2072 }
2073 }
2074 }
2075 Thread::SpinRelease(&_wait_set_lock);
2076 return did_notify;
2077 }
2078
2079 static void post_monitor_notify_event(EventJavaMonitorNotify* event,
2080 ObjectMonitor* monitor,
2081 int notified_count) {
2082 assert(event != nullptr, "invariant");
2083 assert(monitor != nullptr, "invariant");
2084 const Klass* monitor_klass = monitor->object()->klass();
2085 if (ObjectMonitor::is_jfr_excluded(monitor_klass)) {
2086 return;
2087 }
2088 event->set_monitorClass(monitor_klass);
2089 // Set an address that is 'unique enough', such that events close in
2090 // time and with the same address are likely (but not guaranteed) to
2091 // belong to the same object.
2092 event->set_address((uintptr_t)monitor);
2093 event->set_notifiedCount(notified_count);
2094 event->commit();
2095 }
2096
2097 // Consider: a not-uncommon synchronization bug is to use notify() when
2098 // notifyAll() is more appropriate, potentially resulting in stranded
2099 // threads; this is one example of a lost wakeup. A useful diagnostic
2100 // option is to force all notify() operations to behave as notifyAll().
2101 //
2102 // Note: We can also detect many such problems with a "minimum wait".
2103 // When the "minimum wait" is set to a small non-zero timeout value
2104 // and the program does not hang whereas it did absent "minimum wait",
2105 // that suggests a lost wakeup bug.
2106
2107 void ObjectMonitor::notify(TRAPS) {
2108 JavaThread* current = THREAD;
2109 CHECK_OWNER(); // Throws IMSE if not owner.
2110 if (_wait_set == nullptr) {
2111 return;
2112 }
2113
2114 quick_notify(current);
2115 }
2116
2117 void ObjectMonitor::quick_notify(JavaThread* current) {
2118 assert(has_owner(current), "Precondition");
2119
2120 EventJavaMonitorNotify event;
2121 DTRACE_MONITOR_PROBE(notify, this, object(), current);
2122 int tally = notify_internal(current) ? 1 : 0;
2123
2124 if ((tally > 0) && event.should_commit()) {
2125 post_monitor_notify_event(&event, this, /* notified_count = */ tally);
2126 }
2127 }
2128
2129 // notifyAll() transfers the waiters one-at-a-time from the waitset to
2130 // the entry_list. If the waitset is "ABCD" (where A was added first
2131 // and D last) and the entry_list is ->X->Y->Z. After a notifyAll()
2132 // the waitset will be empty and the entry_list will be
2133 // ->D->C->B->A->X->Y->Z, and the next choosen successor will be Z.
2134
2135 void ObjectMonitor::notifyAll(TRAPS) {
2136 JavaThread* current = THREAD;
2137 CHECK_OWNER(); // Throws IMSE if not owner.
2138 if (_wait_set == nullptr) {
2139 return;
2140 }
2141
2142 quick_notifyAll(current);
2143 }
2144
2145 void ObjectMonitor::quick_notifyAll(JavaThread* current) {
2146 assert(has_owner(current), "Precondition");
2147
2148 EventJavaMonitorNotify event;
2149 DTRACE_MONITOR_PROBE(notifyAll, this, object(), current);
2150 int tally = 0;
2151 while (_wait_set != nullptr) {
2152 if (notify_internal(current)) {
2153 tally++;
2154 }
2155 }
2156
2157 if ((tally > 0) && event.should_commit()) {
2158 post_monitor_notify_event(&event, this, /* notified_count = */ tally);
2159 }
2160 }
2161
2162 void ObjectMonitor::vthread_wait(JavaThread* current, jlong millis, bool interruptible) {
2163 oop vthread = current->vthread();
2164 ObjectWaiter* node = new ObjectWaiter(vthread, this);
2165 node->_is_wait = true;
2166 node->_interruptible = interruptible;
2167 node->TState = ObjectWaiter::TS_WAIT;
2168 java_lang_VirtualThread::set_notified(vthread, false); // Reset notified flag
2169 java_lang_VirtualThread::set_interruptible_wait(vthread, interruptible);
2170
2171 // Enter the waiting queue, which is a circular doubly linked list in this case
2172 // but it could be a priority queue or any data structure.
2173 // _wait_set_lock protects the wait queue. Normally the wait queue is accessed only
2174 // by the owner of the monitor *except* in the case where park()
2175 // returns because of a timeout or interrupt. Contention is exceptionally rare
2176 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
2177
2178 Thread::SpinAcquire(&_wait_set_lock, "wait_set - add");
2179 add_waiter(node);
2180 Thread::SpinRelease(&_wait_set_lock);
2181
2182 node->_recursions = _recursions; // record the old recursion count
2183 _recursions = 0; // set the recursion level to be 0
2184 _waiters++; // increment the number of waiters
2185 exit(current); // exit the monitor
2186 guarantee(!has_owner(current), "invariant");
2187
2188 assert(java_lang_VirtualThread::state(vthread) == java_lang_VirtualThread::RUNNING, "wrong state for vthread");
2189 java_lang_VirtualThread::set_state(vthread, millis == 0 ? java_lang_VirtualThread::WAITING : java_lang_VirtualThread::TIMED_WAITING);
2190 java_lang_VirtualThread::set_timeout(vthread, millis);
2191
2192 // Save the ObjectWaiter* in the vthread since we will need it when resuming execution.
2193 java_lang_VirtualThread::set_objectWaiter(vthread, node);
2194 }
2195
2196 bool ObjectMonitor::vthread_wait_reenter(JavaThread* current, ObjectWaiter* node, ContinuationWrapper& cont) {
2197 // The first time we run after being preempted on Object.wait() we
2198 // need to check if we were interrupted or the wait timed-out, and
2199 // in that case remove ourselves from the _wait_set queue.
2200 if (node->TState == ObjectWaiter::TS_WAIT) {
2201 Thread::SpinAcquire(&_wait_set_lock, "wait_set - unlink");
2202 if (node->TState == ObjectWaiter::TS_WAIT) {
2203 dequeue_specific_waiter(node); // unlink from wait_set
2204 assert(!node->_notified, "invariant");
2205 node->TState = ObjectWaiter::TS_RUN;
2206 }
2207 Thread::SpinRelease(&_wait_set_lock);
2208 }
2209
2210 // If this was an interrupted case, set the _interrupted boolean so that
2211 // once we re-acquire the monitor we know if we need to throw IE or not.
2212 ObjectWaiter::TStates state = node->TState;
2213 bool was_notified = state == ObjectWaiter::TS_ENTER;
2214 assert(was_notified || state == ObjectWaiter::TS_RUN, "");
2215 node->_interrupted = node->_interruptible && !was_notified && current->is_interrupted(false);
2216
2217 // Post JFR and JVMTI events. If non-interruptible we are in
2218 // ObjectLocker case so we don't post anything.
2219 EventJavaMonitorWait wait_event;
2220 if (node->_interruptible && (wait_event.should_commit() || JvmtiExport::should_post_monitor_waited())) {
2221 vthread_monitor_waited_event(current, node, cont, &wait_event, !was_notified && !node->_interrupted);
2222 }
2223
2224 // Mark that we are at reenter so that we don't call this method again.
2225 node->_at_reenter = true;
2226
2227 if (!was_notified) {
2228 bool acquired = vthread_monitor_enter(current, node);
2229 if (acquired) {
2230 guarantee(_recursions == 0, "invariant");
2231 _recursions = node->_recursions; // restore the old recursion count
2232 _waiters--; // decrement the number of waiters
2233
2234 if (node->_interrupted) {
2235 // We will throw at thaw end after finishing the mount transition.
2236 current->set_pending_interrupted_exception(true);
2237 }
2238
2239 delete node;
2240 // Clear the ObjectWaiter* from the vthread.
2241 java_lang_VirtualThread::set_objectWaiter(current->vthread(), nullptr);
2242 return true;
2243 }
2244 } else {
2245 // Already moved to _entry_list by notifier, so just add to contentions.
2246 add_to_contentions(1);
2247 }
2248 return false;
2249 }
2250
2251 // -----------------------------------------------------------------------------
2252 // Adaptive Spinning Support
2253 //
2254 // Adaptive spin-then-block - rational spinning
2255 //
2256 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
2257 // algorithm.
2258 //
2259 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
2260 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
2261 // (duration) or we can fix the count at approximately the duration of
2262 // a context switch and vary the frequency. Of course we could also
2263 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
2264 // For a description of 'Adaptive spin-then-block mutual exclusion in
2265 // multi-threaded processing,' see U.S. Pat. No. 8046758.
2266 //
2267 // This implementation varies the duration "D", where D varies with
2268 // the success rate of recent spin attempts. (D is capped at approximately
2269 // length of a round-trip context switch). The success rate for recent
2270 // spin attempts is a good predictor of the success rate of future spin
2271 // attempts. The mechanism adapts automatically to varying critical
2272 // section length (lock modality), system load and degree of parallelism.
2273 // D is maintained per-monitor in _SpinDuration and is initialized
2274 // optimistically. Spin frequency is fixed at 100%.
2275 //
2276 // Note that _SpinDuration is volatile, but we update it without locks
2277 // or atomics. The code is designed so that _SpinDuration stays within
2278 // a reasonable range even in the presence of races. The arithmetic
2279 // operations on _SpinDuration are closed over the domain of legal values,
2280 // so at worst a race will install and older but still legal value.
2281 // At the very worst this introduces some apparent non-determinism.
2282 // We might spin when we shouldn't or vice-versa, but since the spin
2283 // count are relatively short, even in the worst case, the effect is harmless.
2284 //
2285 // Care must be taken that a low "D" value does not become an
2286 // an absorbing state. Transient spinning failures -- when spinning
2287 // is overall profitable -- should not cause the system to converge
2288 // on low "D" values. We want spinning to be stable and predictable
2289 // and fairly responsive to change and at the same time we don't want
2290 // it to oscillate, become metastable, be "too" non-deterministic,
2291 // or converge on or enter undesirable stable absorbing states.
2292 //
2293 // We implement a feedback-based control system -- using past behavior
2294 // to predict future behavior. We face two issues: (a) if the
2295 // input signal is random then the spin predictor won't provide optimal
2296 // results, and (b) if the signal frequency is too high then the control
2297 // system, which has some natural response lag, will "chase" the signal.
2298 // (b) can arise from multimodal lock hold times. Transient preemption
2299 // can also result in apparent bimodal lock hold times.
2300 // Although sub-optimal, neither condition is particularly harmful, as
2301 // in the worst-case we'll spin when we shouldn't or vice-versa.
2302 // The maximum spin duration is rather short so the failure modes aren't bad.
2303 // To be conservative, I've tuned the gain in system to bias toward
2304 // _not spinning. Relatedly, the system can sometimes enter a mode where it
2305 // "rings" or oscillates between spinning and not spinning. This happens
2306 // when spinning is just on the cusp of profitability, however, so the
2307 // situation is not dire. The state is benign -- there's no need to add
2308 // hysteresis control to damp the transition rate between spinning and
2309 // not spinning.
2310
2311 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool
2312
2313 static int Knob_Bonus = 100; // spin success bonus
2314 static int Knob_Penalty = 200; // spin failure penalty
2315 static int Knob_Poverty = 1000;
2316 static int Knob_FixedSpin = 0;
2317 static int Knob_PreSpin = 10; // 20-100 likely better, but it's not better in my testing.
2318
2319 inline static int adjust_up(int spin_duration) {
2320 int x = spin_duration;
2321 if (x < ObjectMonitor::Knob_SpinLimit) {
2322 if (x < Knob_Poverty) {
2323 x = Knob_Poverty;
2324 }
2325 return x + Knob_Bonus;
2326 } else {
2327 return spin_duration;
2328 }
2329 }
2330
2331 inline static int adjust_down(int spin_duration) {
2332 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2333 // AIMD is globally stable.
2334 int x = spin_duration;
2335 if (x > 0) {
2336 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2337 // This is globally sample and tends to damp the response.
2338 x -= Knob_Penalty;
2339 if (x < 0) { x = 0; }
2340 return x;
2341 } else {
2342 return spin_duration;
2343 }
2344 }
2345
2346 bool ObjectMonitor::short_fixed_spin(JavaThread* current, int spin_count, bool adapt) {
2347 for (int ctr = 0; ctr < spin_count; ctr++) {
2348 TryLockResult status = try_lock(current);
2349 if (status == TryLockResult::Success) {
2350 if (adapt) {
2351 _SpinDuration = adjust_up(_SpinDuration);
2352 }
2353 return true;
2354 } else if (status == TryLockResult::Interference) {
2355 break;
2356 }
2357 SpinPause();
2358 }
2359 return false;
2360 }
2361
2362 // Spinning: Fixed frequency (100%), vary duration
2363 bool ObjectMonitor::try_spin(JavaThread* current) {
2364
2365 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
2366 int knob_fixed_spin = Knob_FixedSpin; // 0 (don't spin: default), 2000 good test
2367 if (knob_fixed_spin > 0) {
2368 return short_fixed_spin(current, knob_fixed_spin, false);
2369 }
2370
2371 // Admission control - verify preconditions for spinning
2372 //
2373 // We always spin a little bit, just to prevent _SpinDuration == 0 from
2374 // becoming an absorbing state. Put another way, we spin briefly to
2375 // sample, just in case the system load, parallelism, contention, or lock
2376 // modality changed.
2377
2378 int knob_pre_spin = Knob_PreSpin; // 10 (default), 100, 1000 or 2000
2379 if (short_fixed_spin(current, knob_pre_spin, true)) {
2380 return true;
2381 }
2382
2383 //
2384 // Consider the following alternative:
2385 // Periodically set _SpinDuration = _SpinLimit and try a long/full
2386 // spin attempt. "Periodically" might mean after a tally of
2387 // the # of failed spin attempts (or iterations) reaches some threshold.
2388 // This takes us into the realm of 1-out-of-N spinning, where we
2389 // hold the duration constant but vary the frequency.
2390
2391 int ctr = _SpinDuration;
2392 if (ctr <= 0) return false;
2393
2394 // We're good to spin ... spin ingress.
2395 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
2396 // when preparing to LD...CAS _owner, etc and the CAS is likely
2397 // to succeed.
2398 if (!has_successor()) {
2399 set_successor(current);
2400 }
2401 int64_t prv = NO_OWNER;
2402
2403 // There are three ways to exit the following loop:
2404 // 1. A successful spin where this thread has acquired the lock.
2405 // 2. Spin failure with prejudice
2406 // 3. Spin failure without prejudice
2407
2408 while (--ctr >= 0) {
2409
2410 // Periodic polling -- Check for pending GC
2411 // Threads may spin while they're unsafe.
2412 // We don't want spinning threads to delay the JVM from reaching
2413 // a stop-the-world safepoint or to steal cycles from GC.
2414 // If we detect a pending safepoint we abort in order that
2415 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
2416 // this thread, if safe, doesn't steal cycles from GC.
2417 // This is in keeping with the "no loitering in runtime" rule.
2418 // We periodically check to see if there's a safepoint pending.
2419 if ((ctr & 0xFF) == 0) {
2420 // Can't call SafepointMechanism::should_process() since that
2421 // might update the poll values and we could be in a thread_blocked
2422 // state here which is not allowed so just check the poll.
2423 if (SafepointMechanism::local_poll_armed(current)) {
2424 break;
2425 }
2426 SpinPause();
2427 }
2428
2429 // Probe _owner with TATAS
2430 // If this thread observes the monitor transition or flicker
2431 // from locked to unlocked to locked, then the odds that this
2432 // thread will acquire the lock in this spin attempt go down
2433 // considerably. The same argument applies if the CAS fails
2434 // or if we observe _owner change from one non-null value to
2435 // another non-null value. In such cases we might abort
2436 // the spin without prejudice or apply a "penalty" to the
2437 // spin count-down variable "ctr", reducing it by 100, say.
2438
2439 int64_t ox = owner_raw();
2440 if (ox == NO_OWNER) {
2441 ox = try_set_owner_from(NO_OWNER, current);
2442 if (ox == NO_OWNER) {
2443 // The CAS succeeded -- this thread acquired ownership
2444 // Take care of some bookkeeping to exit spin state.
2445 if (has_successor(current)) {
2446 clear_successor();
2447 }
2448
2449 // Increase _SpinDuration :
2450 // The spin was successful (profitable) so we tend toward
2451 // longer spin attempts in the future.
2452 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2453 // If we acquired the lock early in the spin cycle it
2454 // makes sense to increase _SpinDuration proportionally.
2455 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2456 _SpinDuration = adjust_up(_SpinDuration);
2457 return true;
2458 }
2459
2460 // The CAS failed ... we can take any of the following actions:
2461 // * penalize: ctr -= CASPenalty
2462 // * exit spin with prejudice -- abort without adapting spinner
2463 // * exit spin without prejudice.
2464 // * Since CAS is high-latency, retry again immediately.
2465 break;
2466 }
2467
2468 // Did lock ownership change hands ?
2469 if (ox != prv && prv != NO_OWNER) {
2470 break;
2471 }
2472 prv = ox;
2473
2474 if (!has_successor()) {
2475 set_successor(current);
2476 }
2477 }
2478
2479 // Spin failed with prejudice -- reduce _SpinDuration.
2480 if (ctr < 0) {
2481 _SpinDuration = adjust_down(_SpinDuration);
2482 }
2483
2484 if (has_successor(current)) {
2485 clear_successor();
2486 // Invariant: after setting succ=null a contending thread
2487 // must recheck-retry _owner before parking. This usually happens
2488 // in the normal usage of try_spin(), but it's safest
2489 // to make try_spin() as foolproof as possible.
2490 OrderAccess::fence();
2491 if (try_lock(current) == TryLockResult::Success) {
2492 return true;
2493 }
2494 }
2495
2496 return false;
2497 }
2498
2499
2500 // -----------------------------------------------------------------------------
2501 // wait_set management ...
2502
2503 ObjectWaiter::ObjectWaiter(JavaThread* current) {
2504 _next = nullptr;
2505 _prev = nullptr;
2506 _thread = current;
2507 _monitor = nullptr;
2508 _notifier_tid = 0;
2509 _recursions = 0;
2510 TState = TS_RUN;
2511 _notified = false;
2512 _is_wait = false;
2513 _at_reenter = false;
2514 _interrupted = false;
2515 _active = false;
2516 }
2517
2518 ObjectWaiter::ObjectWaiter(oop vthread, ObjectMonitor* mon) : ObjectWaiter(nullptr) {
2519 assert(oopDesc::is_oop(vthread), "");
2520 _vthread = OopHandle(JavaThread::thread_oop_storage(), vthread);
2521 _monitor = mon;
2522 }
2523
2524 ObjectWaiter::~ObjectWaiter() {
2525 if (is_vthread()) {
2526 assert(vthread() != nullptr, "");
2527 _vthread.release(JavaThread::thread_oop_storage());
2528 }
2529 }
2530
2531 oop ObjectWaiter::vthread() const {
2532 return _vthread.resolve();
2533 }
2534
2535 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2536 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(_thread, mon);
2537 }
2538
2539 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2540 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(_thread, _active);
2541 }
2542
2543 inline void ObjectMonitor::add_waiter(ObjectWaiter* node) {
2544 assert(node != nullptr, "should not add null node");
2545 assert(node->_prev == nullptr, "node already in list");
2546 assert(node->_next == nullptr, "node already in list");
2547 // put node at end of queue (circular doubly linked list)
2548 if (_wait_set == nullptr) {
2549 _wait_set = node;
2550 node->_prev = node;
2551 node->_next = node;
2552 } else {
2553 ObjectWaiter* head = _wait_set;
2554 ObjectWaiter* tail = head->_prev;
2555 assert(tail->_next == head, "invariant check");
2556 tail->_next = node;
2557 head->_prev = node;
2558 node->_next = head;
2559 node->_prev = tail;
2560 }
2561 }
2562
2563 inline ObjectWaiter* ObjectMonitor::dequeue_waiter() {
2564 // dequeue the very first waiter
2565 ObjectWaiter* waiter = _wait_set;
2566 if (waiter) {
2567 dequeue_specific_waiter(waiter);
2568 }
2569 return waiter;
2570 }
2571
2572 inline void ObjectMonitor::dequeue_specific_waiter(ObjectWaiter* node) {
2573 assert(node != nullptr, "should not dequeue nullptr node");
2574 assert(node->_prev != nullptr, "node already removed from list");
2575 assert(node->_next != nullptr, "node already removed from list");
2576 // when the waiter has woken up because of interrupt,
2577 // timeout or other spurious wake-up, dequeue the
2578 // waiter from waiting list
2579 ObjectWaiter* next = node->_next;
2580 if (next == node) {
2581 assert(node->_prev == node, "invariant check");
2582 _wait_set = nullptr;
2583 } else {
2584 ObjectWaiter* prev = node->_prev;
2585 assert(prev->_next == node, "invariant check");
2586 assert(next->_prev == node, "invariant check");
2587 next->_prev = prev;
2588 prev->_next = next;
2589 if (_wait_set == node) {
2590 _wait_set = next;
2591 }
2592 }
2593 node->_next = nullptr;
2594 node->_prev = nullptr;
2595 }
2596
2597 // -----------------------------------------------------------------------------
2598
2599 // One-shot global initialization for the sync subsystem.
2600 // We could also defer initialization and initialize on-demand
2601 // the first time we call ObjectSynchronizer::inflate().
2602 // Initialization would be protected - like so many things - by
2603 // the MonitorCache_lock.
2604
2605 void ObjectMonitor::Initialize() {
2606 assert(!InitDone, "invariant");
2607
2608 if (!os::is_MP()) {
2609 Knob_SpinLimit = 0;
2610 Knob_PreSpin = 0;
2611 Knob_FixedSpin = -1;
2612 }
2613
2614 _oop_storage = OopStorageSet::create_weak("ObjectSynchronizer Weak", mtSynchronizer);
2615
2616 DEBUG_ONLY(InitDone = true;)
2617 }
2618
2619 // We can't call this during Initialize() because BarrierSet needs to be set.
2620 void ObjectMonitor::Initialize2() {
2621 _vthread_list_head = OopHandle(JavaThread::thread_oop_storage(), nullptr);
2622 _vthread_unparker_ParkEvent = ParkEvent::Allocate(nullptr);
2623 }
2624
2625 void ObjectMonitor::print_on(outputStream* st) const {
2626 // The minimal things to print for markWord printing, more can be added for debugging and logging.
2627 st->print("{contentions=0x%08x,waiters=0x%08x"
2628 ",recursions=%zd,owner=" INT64_FORMAT "}",
2629 contentions(), waiters(), recursions(),
2630 owner_raw());
2631 }
2632 void ObjectMonitor::print() const { print_on(tty); }
2633
2634 #ifdef ASSERT
2635 // Print the ObjectMonitor like a debugger would:
2636 //
2637 // (ObjectMonitor) 0x00007fdfb6012e40 = {
2638 // _metadata = 0x0000000000000001
2639 // _object = 0x000000070ff45fd0
2640 // _pad_buf0 = {
2641 // [0] = '\0'
2642 // ...
2643 // [43] = '\0'
2644 // }
2645 // _owner = 0x0000000000000000
2646 // _previous_owner_tid = 0
2647 // _pad_buf1 = {
2648 // [0] = '\0'
2649 // ...
2650 // [47] = '\0'
2651 // }
2652 // _next_om = 0x0000000000000000
2653 // _recursions = 0
2654 // _entry_list = 0x0000000000000000
2655 // _entry_list_tail = 0x0000000000000000
2656 // _succ = 0x0000000000000000
2657 // _SpinDuration = 5000
2658 // _contentions = 0
2659 // _wait_set = 0x0000700009756248
2660 // _waiters = 1
2661 // _wait_set_lock = 0
2662 // }
2663 //
2664 void ObjectMonitor::print_debug_style_on(outputStream* st) const {
2665 st->print_cr("(ObjectMonitor*) " INTPTR_FORMAT " = {", p2i(this));
2666 st->print_cr(" _metadata = " INTPTR_FORMAT, _metadata);
2667 st->print_cr(" _object = " INTPTR_FORMAT, p2i(object_peek()));
2668 st->print_cr(" _pad_buf0 = {");
2669 st->print_cr(" [0] = '\\0'");
2670 st->print_cr(" ...");
2671 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf0) - 1);
2672 st->print_cr(" }");
2673 st->print_cr(" _owner = " INT64_FORMAT, owner_raw());
2674 st->print_cr(" _previous_owner_tid = " UINT64_FORMAT, _previous_owner_tid);
2675 st->print_cr(" _pad_buf1 = {");
2676 st->print_cr(" [0] = '\\0'");
2677 st->print_cr(" ...");
2678 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf1) - 1);
2679 st->print_cr(" }");
2680 st->print_cr(" _next_om = " INTPTR_FORMAT, p2i(next_om()));
2681 st->print_cr(" _recursions = %zd", _recursions);
2682 st->print_cr(" _entry_list = " INTPTR_FORMAT, p2i(_entry_list));
2683 st->print_cr(" _entry_list_tail = " INTPTR_FORMAT, p2i(_entry_list_tail));
2684 st->print_cr(" _succ = " INT64_FORMAT, successor());
2685 st->print_cr(" _SpinDuration = %d", _SpinDuration);
2686 st->print_cr(" _contentions = %d", contentions());
2687 st->print_cr(" _unmounted_vthreads = " INT64_FORMAT, _unmounted_vthreads);
2688 st->print_cr(" _wait_set = " INTPTR_FORMAT, p2i(_wait_set));
2689 st->print_cr(" _waiters = %d", _waiters);
2690 st->print_cr(" _wait_set_lock = %d", _wait_set_lock);
2691 st->print_cr("}");
2692 }
2693 #endif