1 /*
2 * Copyright (c) 1998, 2025, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 #include "classfile/vmSymbols.hpp"
26 #include "gc/shared/oopStorage.hpp"
27 #include "gc/shared/oopStorageSet.hpp"
28 #include "jfr/jfrEvents.hpp"
29 #include "jfr/support/jfrThreadId.hpp"
30 #include "logging/log.hpp"
31 #include "logging/logStream.hpp"
32 #include "memory/allocation.inline.hpp"
33 #include "memory/resourceArea.hpp"
34 #include "oops/markWord.hpp"
35 #include "oops/oop.inline.hpp"
36 #include "oops/oopHandle.inline.hpp"
37 #include "oops/weakHandle.inline.hpp"
38 #include "prims/jvmtiDeferredUpdates.hpp"
39 #include "prims/jvmtiExport.hpp"
40 #include "runtime/atomicAccess.hpp"
41 #include "runtime/continuationWrapper.inline.hpp"
42 #include "runtime/globals.hpp"
43 #include "runtime/handles.inline.hpp"
44 #include "runtime/interfaceSupport.inline.hpp"
45 #include "runtime/javaThread.inline.hpp"
46 #include "runtime/lightweightSynchronizer.hpp"
47 #include "runtime/mutexLocker.hpp"
48 #include "runtime/objectMonitor.inline.hpp"
49 #include "runtime/orderAccess.hpp"
50 #include "runtime/osThread.hpp"
51 #include "runtime/safefetch.hpp"
52 #include "runtime/safepointMechanism.inline.hpp"
53 #include "runtime/sharedRuntime.hpp"
54 #include "runtime/threads.hpp"
55 #include "services/threadService.hpp"
56 #include "utilities/debug.hpp"
57 #include "utilities/dtrace.hpp"
58 #include "utilities/globalCounter.inline.hpp"
59 #include "utilities/globalDefinitions.hpp"
60 #include "utilities/macros.hpp"
61 #include "utilities/preserveException.hpp"
62 #if INCLUDE_JFR
63 #include "jfr/support/jfrFlush.hpp"
64 #endif
65
66 #ifdef DTRACE_ENABLED
67
68 // Only bother with this argument setup if dtrace is available
69 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
70
71
72 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
73 char* bytes = nullptr; \
74 int len = 0; \
75 jlong jtid = SharedRuntime::get_java_tid(thread); \
76 Symbol* klassname = obj->klass()->name(); \
77 if (klassname != nullptr) { \
78 bytes = (char*)klassname->bytes(); \
79 len = klassname->utf8_length(); \
80 }
81
82 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
83 { \
84 if (DTraceMonitorProbes) { \
85 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
86 HOTSPOT_MONITOR_WAIT(jtid, \
87 (monitor), bytes, len, (millis)); \
88 } \
89 }
90
91 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
92 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
93 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
94 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
95 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
96
97 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
98 { \
99 if (DTraceMonitorProbes) { \
100 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
101 HOTSPOT_MONITOR_##probe(jtid, \
102 (uintptr_t)(monitor), bytes, len); \
103 } \
104 }
105
106 #else // ndef DTRACE_ENABLED
107
108 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
109 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
110
111 #endif // ndef DTRACE_ENABLED
112
113 DEBUG_ONLY(static volatile bool InitDone = false;)
114
115 OopStorage* ObjectMonitor::_oop_storage = nullptr;
116
117 OopHandle ObjectMonitor::_vthread_list_head;
118 ParkEvent* ObjectMonitor::_vthread_unparker_ParkEvent = nullptr;
119
120 // -----------------------------------------------------------------------------
121 // Theory of operations -- Monitors lists, thread residency, etc:
122 //
123 // * A thread acquires ownership of a monitor by successfully
124 // CAS()ing the _owner field from NO_OWNER/DEFLATER_MARKER to
125 // its owner_id (return value from owner_id_from()).
126 //
127 // * Invariant: A thread appears on at most one monitor list --
128 // entry_list or wait_set -- at any one time.
129 //
130 // * Contending threads "push" themselves onto the entry_list with CAS
131 // and then spin/park.
132 // If the thread is a virtual thread it will first attempt to
133 // unmount itself. The virtual thread will first try to freeze
134 // all frames in the heap. If the operation fails it will just
135 // follow the regular path for platform threads. If the operation
136 // succeeds, it will push itself onto the entry_list with CAS and then
137 // return back to Java to continue the unmount logic.
138 //
139 // * After a contending thread eventually acquires the lock it must
140 // dequeue itself from the entry_list.
141 //
142 // * The exiting thread identifies and unparks an "heir presumptive"
143 // tentative successor thread on the entry_list. In case the successor
144 // is an unmounted virtual thread, the exiting thread will first try
145 // to add it to the list of vthreads waiting to be unblocked, and on
146 // success it will unpark the special unblocker thread instead, which
147 // will be in charge of submitting the vthread back to the scheduler
148 // queue. Critically, the exiting thread doesn't unlink the successor
149 // thread from the entry_list. After having been unparked/re-scheduled,
150 // the wakee will recontend for ownership of the monitor. The successor
151 // (wakee) will either acquire the lock or re-park/unmount itself.
152 //
153 // Succession is provided for by a policy of competitive handoff.
154 // The exiting thread does _not_ grant or pass ownership to the
155 // successor thread. (This is also referred to as "handoff succession").
156 // Instead the exiting thread releases ownership and possibly wakes
157 // a successor, so the successor can (re)compete for ownership of the lock.
158 //
159 // * The entry_list forms a queue of threads stalled trying to acquire
160 // the lock. Within the entry_list the next pointers always form a
161 // consistent singly linked list. At unlock-time when the unlocking
162 // thread notices that the tail of the entry_list is not known, we
163 // convert the singly linked entry_list into a doubly linked list by
164 // assigning the prev pointers and the entry_list_tail pointer.
165 //
166 // Example:
167 //
168 // The first contending thread that "pushed" itself onto entry_list,
169 // will be the last thread in the list. Each newly pushed thread in
170 // entry_list will be linked through its next pointer, and have its
171 // prev pointer set to null. Thus pushing six threads A-F (in that
172 // order) onto entry_list, will form a singly linked list, see 1)
173 // below.
174 //
175 // 1) entry_list ->F->E->D->C->B->A->null
176 // entry_list_tail ->null
177 //
178 // Since the successor is chosen in FIFO order, the exiting thread
179 // needs to find the tail of the entry_list. This is done by walking
180 // from the entry_list head. While walking the list we also assign
181 // the prev pointers of each thread, essentially forming a doubly
182 // linked list, see 2) below.
183 //
184 // 2) entry_list ->F<=>E<=>D<=>C<=>B<=>A->null
185 // entry_list_tail ----------------------^
186 //
187 // Once we have formed a doubly linked list it's easy to find the
188 // successor (A), wake it up, have it remove itself, and update the
189 // tail pointer, as seen in and 3) below.
190 //
191 // 3) entry_list ->F<=>E<=>D<=>C<=>B->null
192 // entry_list_tail ------------------^
193 //
194 // At any time new threads can add themselves to the entry_list, see
195 // 4) below.
196 //
197 // 4) entry_list ->I->H->G->F<=>E<=>D->null
198 // entry_list_tail -------------------^
199 //
200 // At some point in time the thread (F) that wants to remove itself
201 // from the end of the list, will not have any prev pointer, see 5)
202 // below.
203 //
204 // 5) entry_list ->I->H->G->F->null
205 // entry_list_tail -----------^
206 //
207 // To resolve this we just start walking from the entry_list head
208 // again, forming a new doubly linked list, before removing the
209 // thread (F), see 6) and 7) below.
210 //
211 // 6) entry_list ->I<=>H<=>G<=>F->null
212 // entry_list_tail --------------^
213 //
214 // 7) entry_list ->I<=>H<=>G->null
215 // entry_list_tail ----------^
216 //
217 // * The monitor itself protects all of the operations on the
218 // entry_list except for the CAS of a new arrival to the head. Only
219 // the monitor owner can read or write the prev links (e.g. to
220 // remove itself) or update the tail.
221 //
222 // * The monitor entry list operations avoid locks, but strictly speaking
223 // they're not lock-free. Enter is lock-free, exit is not.
224 // For a description of 'Methods and apparatus providing non-blocking access
225 // to a resource,' see U.S. Pat. No. 7844973.
226 //
227 // * The entry_list can have multiple concurrent "pushers" but only
228 // one concurrent detaching thread. There is no ABA-problem with
229 // this usage of CAS.
230 //
231 // * As long as the entry_list_tail is known the odds are good that we
232 // should be able to dequeue after acquisition (in the ::enter()
233 // epilogue) in constant-time. This is good since a key desideratum
234 // is to minimize queue & monitor metadata manipulation that occurs
235 // while holding the monitor lock -- that is, we want to minimize
236 // monitor lock holds times. Note that even a small amount of fixed
237 // spinning will greatly reduce the # of enqueue-dequeue operations
238 // on entry_list. That is, spinning relieves contention on the
239 // "inner" locks and monitor metadata.
240 //
241 // Insert and delete operations may not operate in constant-time if
242 // we have interference because some other thread is adding or
243 // removing the head element of entry_list or if we need to convert
244 // the singly linked entry_list into a doubly linked list to find the
245 // tail.
246 //
247 // * The monitor synchronization subsystem avoids the use of native
248 // synchronization primitives except for the narrow platform-specific
249 // park-unpark abstraction. See the comments in os_posix.cpp regarding
250 // the semantics of park-unpark. Put another way, this monitor implementation
251 // depends only on atomic operations and park-unpark.
252 //
253 // * Waiting threads reside on the wait_set list -- wait() puts
254 // the caller onto the wait_set.
255 //
256 // * notify() or notifyAll() simply transfers threads from the wait_set
257 // to the entry_list. Subsequent exit() operations will
258 // unpark/re-schedule the notifyee. Unparking/re-scheduling a
259 // notifyee in notify() is inefficient - it's likely the notifyee
260 // would simply impale itself on the lock held by the notifier.
261
262 // Check that object() and set_object() are called from the right context:
263 static void check_object_context() {
264 #ifdef ASSERT
265 Thread* self = Thread::current();
266 if (self->is_Java_thread()) {
267 // Mostly called from JavaThreads so sanity check the thread state.
268 JavaThread* jt = JavaThread::cast(self);
269 switch (jt->thread_state()) {
270 case _thread_in_vm: // the usual case
271 case _thread_in_Java: // during deopt
272 break;
273 default:
274 fatal("called from an unsafe thread state");
275 }
276 assert(jt->is_active_Java_thread(), "must be active JavaThread");
277 } else {
278 // However, ThreadService::get_current_contended_monitor()
279 // can call here via the VMThread so sanity check it.
280 assert(self->is_VM_thread(), "must be");
281 }
282 #endif // ASSERT
283 }
284
285 ObjectMonitor::ObjectMonitor(oop object) :
286 _metadata(0),
287 _object(_oop_storage, object),
288 _owner(NO_OWNER),
289 _previous_owner_tid(0),
290 _next_om(nullptr),
291 _recursions(0),
292 _entry_list(nullptr),
293 _entry_list_tail(nullptr),
294 _succ(NO_OWNER),
295 _SpinDuration(ObjectMonitor::Knob_SpinLimit),
296 _contentions(0),
297 _unmounted_vthreads(0),
298 _wait_set(nullptr),
299 _waiters(0),
300 _wait_set_lock(0)
301 { }
302
303 ObjectMonitor::~ObjectMonitor() {
304 _object.release(_oop_storage);
305 _object_strong.release(JavaThread::thread_oop_storage());
306 }
307
308 oop ObjectMonitor::object() const {
309 check_object_context();
310 return _object.resolve();
311 }
312
313 // Keep object protected during ObjectLocker preemption.
314 void ObjectMonitor::set_object_strong() {
315 check_object_context();
316 if (_object_strong.is_empty()) {
317 if (Thread::TrySpinAcquire(&_object_strong_lock)) {
318 if (_object_strong.is_empty()) {
319 assert(_object.resolve() != nullptr, "");
320 _object_strong = OopHandle(JavaThread::thread_oop_storage(), _object.resolve());
321 }
322 Thread::SpinRelease(&_object_strong_lock);
323 }
324 }
325 }
326
327 void ObjectMonitor::ExitOnSuspend::operator()(JavaThread* current) {
328 if (current->is_suspended()) {
329 _om->_recursions = 0;
330 _om->clear_successor();
331 // Don't need a full fence after clearing successor here because of the call to exit().
332 _om->exit(current, false /* not_suspended */);
333 _om_exited = true;
334
335 current->set_current_pending_monitor(_om);
336 }
337 }
338
339 void ObjectMonitor::ClearSuccOnSuspend::operator()(JavaThread* current) {
340 if (current->is_suspended()) {
341 if (_om->has_successor(current)) {
342 _om->clear_successor();
343 OrderAccess::fence(); // always do a full fence when successor is cleared
344 }
345 }
346 }
347
348 #define assert_mark_word_consistency() \
349 assert(UseObjectMonitorTable || object()->mark() == markWord::encode(this), \
350 "object mark must match encoded this: mark=" INTPTR_FORMAT \
351 ", encoded this=" INTPTR_FORMAT, object()->mark().value(), \
352 markWord::encode(this).value());
353
354 // -----------------------------------------------------------------------------
355 // Enter support
356
357 bool ObjectMonitor::enter_is_async_deflating() {
358 if (is_being_async_deflated()) {
359 if (!UseObjectMonitorTable) {
360 const oop l_object = object();
361 if (l_object != nullptr) {
362 // Attempt to restore the header/dmw to the object's header so that
363 // we only retry once if the deflater thread happens to be slow.
364 install_displaced_markword_in_object(l_object);
365 }
366 }
367 return true;
368 }
369
370 return false;
371 }
372
373 bool ObjectMonitor::try_lock_with_contention_mark(JavaThread* locking_thread, ObjectMonitorContentionMark& contention_mark) {
374 assert(contention_mark._monitor == this, "must be");
375 assert(!is_being_async_deflated(), "must be");
376
377 int64_t prev_owner = try_set_owner_from(NO_OWNER, locking_thread);
378 bool success = false;
379
380 if (prev_owner == NO_OWNER) {
381 assert(_recursions == 0, "invariant");
382 success = true;
383 } else if (prev_owner == owner_id_from(locking_thread)) {
384 _recursions++;
385 success = true;
386 } else if (prev_owner == DEFLATER_MARKER) {
387 // Racing with deflation.
388 prev_owner = try_set_owner_from(DEFLATER_MARKER, locking_thread);
389 if (prev_owner == DEFLATER_MARKER) {
390 // We successfully cancelled the in-progress async deflation by
391 // changing owner from DEFLATER_MARKER to current. We now extend
392 // the lifetime of the contention_mark (e.g. contentions++) here
393 // to prevent the deflater thread from winning the last part of
394 // the 2-part async deflation protocol after the regular
395 // decrement occurs when the contention_mark goes out of
396 // scope. ObjectMonitor::deflate_monitor() which is called by
397 // the deflater thread will decrement contentions after it
398 // recognizes that the async deflation was cancelled.
399 contention_mark.extend();
400 success = true;
401 } else if (prev_owner == NO_OWNER) {
402 // At this point we cannot race with deflation as we have both incremented
403 // contentions, seen contention > 0 and seen a DEFLATER_MARKER.
404 // success will only be false if this races with something other than
405 // deflation.
406 prev_owner = try_set_owner_from(NO_OWNER, locking_thread);
407 success = prev_owner == NO_OWNER;
408 }
409 }
410 assert(!success || has_owner(locking_thread), "must be");
411
412 return success;
413 }
414
415 void ObjectMonitor::enter_for_with_contention_mark(JavaThread* locking_thread, ObjectMonitorContentionMark& contention_mark) {
416 // Used by LightweightSynchronizer::inflate_and_enter in deoptimization path to enter for another thread.
417 // The monitor is private to or already owned by locking_thread which must be suspended.
418 // So this code may only contend with deflation.
419 assert(locking_thread == Thread::current() || locking_thread->is_obj_deopt_suspend(), "must be");
420 bool success = try_lock_with_contention_mark(locking_thread, contention_mark);
421
422 assert(success, "Failed to enter_for: locking_thread=" INTPTR_FORMAT
423 ", this=" INTPTR_FORMAT "{owner=" INT64_FORMAT "}",
424 p2i(locking_thread), p2i(this), owner_raw());
425 }
426
427 bool ObjectMonitor::enter_for(JavaThread* locking_thread) {
428 // Used by ObjectSynchronizer::enter_for() to enter for another thread.
429 // The monitor is private to or already owned by locking_thread which must be suspended.
430 // So this code may only contend with deflation.
431 assert(locking_thread == Thread::current() || locking_thread->is_obj_deopt_suspend(), "must be");
432
433 // Block out deflation as soon as possible.
434 ObjectMonitorContentionMark contention_mark(this);
435
436 // Check for deflation.
437 if (enter_is_async_deflating()) {
438 return false;
439 }
440
441 bool success = try_lock_with_contention_mark(locking_thread, contention_mark);
442
443 assert(success, "Failed to enter_for: locking_thread=" INTPTR_FORMAT
444 ", this=" INTPTR_FORMAT "{owner=" INT64_FORMAT "}",
445 p2i(locking_thread), p2i(this), owner_raw());
446 assert(has_owner(locking_thread), "must be");
447 return true;
448 }
449
450 bool ObjectMonitor::try_enter(JavaThread* current, bool check_for_recursion) {
451 // TryLock avoids the CAS and handles deflation.
452 TryLockResult r = try_lock(current);
453 if (r == TryLockResult::Success) {
454 assert(_recursions == 0, "invariant");
455 return true;
456 }
457
458 // If called from SharedRuntime::monitor_exit_helper(), we know that
459 // this thread doesn't already own the lock.
460 if (!check_for_recursion) {
461 return false;
462 }
463
464 if (r == TryLockResult::HasOwner && has_owner(current)) {
465 _recursions++;
466 return true;
467 }
468
469 return false;
470 }
471
472 bool ObjectMonitor::spin_enter(JavaThread* current) {
473 assert(current == JavaThread::current(), "must be");
474
475 // Check for recursion.
476 if (try_enter(current)) {
477 return true;
478 }
479
480 // Check for deflation.
481 if (enter_is_async_deflating()) {
482 return false;
483 }
484
485 // We've encountered genuine contention.
486
487 // Do one round of spinning.
488 // Note that if we acquire the monitor from an initial spin
489 // we forgo posting JVMTI events and firing DTRACE probes.
490 if (try_spin(current)) {
491 assert(has_owner(current), "must be current: owner=" INT64_FORMAT, owner_raw());
492 assert(_recursions == 0, "must be 0: recursions=%zd", _recursions);
493 assert_mark_word_consistency();
494 return true;
495 }
496
497 return false;
498 }
499
500 bool ObjectMonitor::enter(JavaThread* current) {
501 assert(current == JavaThread::current(), "must be");
502
503 if (spin_enter(current)) {
504 return true;
505 }
506
507 assert(!has_owner(current), "invariant");
508 assert(!has_successor(current), "invariant");
509 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
510 assert(current->thread_state() != _thread_blocked, "invariant");
511
512 // Keep is_being_async_deflated stable across the rest of enter
513 ObjectMonitorContentionMark contention_mark(this);
514
515 // Check for deflation.
516 if (enter_is_async_deflating()) {
517 return false;
518 }
519
520 // At this point this ObjectMonitor cannot be deflated, finish contended enter
521 enter_with_contention_mark(current, contention_mark);
522 return true;
523 }
524
525 void ObjectMonitor::notify_contended_enter(JavaThread* current) {
526 current->set_current_pending_monitor(this);
527
528 DTRACE_MONITOR_PROBE(contended__enter, this, object(), current);
529 if (JvmtiExport::should_post_monitor_contended_enter()) {
530 JvmtiExport::post_monitor_contended_enter(current, this);
531
532 // The current thread does not yet own the monitor and does not
533 // yet appear on any queues that would get it made the successor.
534 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
535 // handler cannot accidentally consume an unpark() meant for the
536 // ParkEvent associated with this ObjectMonitor.
537 }
538 }
539
540 void ObjectMonitor::enter_with_contention_mark(JavaThread* current, ObjectMonitorContentionMark &cm) {
541 assert(current == JavaThread::current(), "must be");
542 assert(!has_owner(current), "must be");
543 assert(cm._monitor == this, "must be");
544 assert(!is_being_async_deflated(), "must be");
545
546 JFR_ONLY(JfrConditionalFlush<EventJavaMonitorEnter> flush(current);)
547 EventJavaMonitorEnter enter_event;
548 if (enter_event.is_started()) {
549 enter_event.set_monitorClass(object()->klass());
550 // Set an address that is 'unique enough', such that events close in
551 // time and with the same address are likely (but not guaranteed) to
552 // belong to the same object.
553 enter_event.set_address((uintptr_t)this);
554 }
555 EventVirtualThreadPinned vthread_pinned_event;
556
557 freeze_result result;
558
559 assert(current->current_pending_monitor() == nullptr, "invariant");
560
561 ContinuationEntry* ce = current->last_continuation();
562 bool is_virtual = ce != nullptr && ce->is_virtual_thread();
563 if (is_virtual) {
564 notify_contended_enter(current);
565 result = Continuation::try_preempt(current, ce->cont_oop(current));
566 if (result == freeze_ok) {
567 bool acquired = vthread_monitor_enter(current);
568 if (acquired) {
569 // We actually acquired the monitor while trying to add the vthread to the
570 // _entry_list so cancel preemption. We will still go through the preempt stub
571 // but instead of unmounting we will call thaw to continue execution.
572 current->set_preemption_cancelled(true);
573 if (JvmtiExport::should_post_monitor_contended_entered()) {
574 // We are going to call thaw again after this and finish the VMTS
575 // transition so no need to do it here. We will post the event there.
576 current->set_contended_entered_monitor(this);
577 }
578 }
579 current->set_current_pending_monitor(nullptr);
580 DEBUG_ONLY(int state = java_lang_VirtualThread::state(current->vthread()));
581 assert((acquired && current->preemption_cancelled() && state == java_lang_VirtualThread::RUNNING) ||
582 (!acquired && !current->preemption_cancelled() && state == java_lang_VirtualThread::BLOCKING), "invariant");
583 return;
584 }
585 }
586
587 {
588 // Change java thread status to indicate blocked on monitor enter.
589 JavaThreadBlockedOnMonitorEnterState jtbmes(current, this);
590
591 if (!is_virtual) { // already notified contended_enter for virtual
592 notify_contended_enter(current);
593 }
594 OSThreadContendState osts(current->osthread());
595
596 assert(current->thread_state() == _thread_in_vm, "invariant");
597
598 for (;;) {
599 ExitOnSuspend eos(this);
600 {
601 ThreadBlockInVMPreprocess<ExitOnSuspend> tbivs(current, eos, true /* allow_suspend */);
602 enter_internal(current);
603 current->set_current_pending_monitor(nullptr);
604 // We can go to a safepoint at the end of this block. If we
605 // do a thread dump during that safepoint, then this thread will show
606 // as having "-locked" the monitor, but the OS and java.lang.Thread
607 // states will still report that the thread is blocked trying to
608 // acquire it.
609 // If there is a suspend request, ExitOnSuspend will exit the OM
610 // and set the OM as pending.
611 }
612 if (!eos.exited()) {
613 // ExitOnSuspend did not exit the OM
614 assert(has_owner(current), "invariant");
615 break;
616 }
617 }
618
619 // We've just gotten past the enter-check-for-suspend dance and we now own
620 // the monitor free and clear.
621 }
622
623 assert(contentions() >= 0, "must not be negative: contentions=%d", contentions());
624
625 // Must either set _recursions = 0 or ASSERT _recursions == 0.
626 assert(_recursions == 0, "invariant");
627 assert(has_owner(current), "invariant");
628 assert(!has_successor(current), "invariant");
629 assert_mark_word_consistency();
630
631 // The thread -- now the owner -- is back in vm mode.
632 // Report the glorious news via TI,DTrace and jvmstat.
633 // The probe effect is non-trivial. All the reportage occurs
634 // while we hold the monitor, increasing the length of the critical
635 // section. Amdahl's parallel speedup law comes vividly into play.
636 //
637 // Another option might be to aggregate the events (thread local or
638 // per-monitor aggregation) and defer reporting until a more opportune
639 // time -- such as next time some thread encounters contention but has
640 // yet to acquire the lock. While spinning that thread could
641 // spinning we could increment JVMStat counters, etc.
642
643 DTRACE_MONITOR_PROBE(contended__entered, this, object(), current);
644 if (JvmtiExport::should_post_monitor_contended_entered()) {
645 JvmtiExport::post_monitor_contended_entered(current, this);
646
647 // The current thread already owns the monitor and is not going to
648 // call park() for the remainder of the monitor enter protocol. So
649 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
650 // event handler consumed an unpark() issued by the thread that
651 // just exited the monitor.
652 }
653 if (enter_event.should_commit()) {
654 enter_event.set_previousOwner(_previous_owner_tid);
655 enter_event.commit();
656 }
657
658 if (current->current_waiting_monitor() == nullptr) {
659 ContinuationEntry* ce = current->last_continuation();
660 if (ce != nullptr && ce->is_virtual_thread()) {
661 current->post_vthread_pinned_event(&vthread_pinned_event, "Contended monitor enter", result);
662 }
663 }
664 }
665
666 // Caveat: try_lock() is not necessarily serializing if it returns failure.
667 // Callers must compensate as needed.
668
669 ObjectMonitor::TryLockResult ObjectMonitor::try_lock(JavaThread* current) {
670 int64_t own = owner_raw();
671 int64_t first_own = own;
672
673 for (;;) {
674 if (own == DEFLATER_MARKER) {
675 // Block out deflation as soon as possible.
676 ObjectMonitorContentionMark contention_mark(this);
677
678 // Check for deflation.
679 if (enter_is_async_deflating()) {
680 // Treat deflation as interference.
681 return TryLockResult::Interference;
682 }
683 if (try_lock_with_contention_mark(current, contention_mark)) {
684 assert(_recursions == 0, "invariant");
685 return TryLockResult::Success;
686 } else {
687 // Deflation won or change of owner; dont spin
688 break;
689 }
690 } else if (own == NO_OWNER) {
691 int64_t prev_own = try_set_owner_from(NO_OWNER, current);
692 if (prev_own == NO_OWNER) {
693 assert(_recursions == 0, "invariant");
694 return TryLockResult::Success;
695 } else {
696 // The lock had been free momentarily, but we lost the race to the lock.
697 own = prev_own;
698 }
699 } else {
700 // Retry doesn't make as much sense because the lock was just acquired.
701 break;
702 }
703 }
704 return first_own == own ? TryLockResult::HasOwner : TryLockResult::Interference;
705 }
706
707 // Push "current" onto the head of the _entry_list. Once on _entry_list,
708 // current stays on-queue until it acquires the lock.
709 void ObjectMonitor::add_to_entry_list(JavaThread* current, ObjectWaiter* node) {
710 node->_prev = nullptr;
711 node->TState = ObjectWaiter::TS_ENTER;
712
713 for (;;) {
714 ObjectWaiter* head = AtomicAccess::load(&_entry_list);
715 node->_next = head;
716 if (AtomicAccess::cmpxchg(&_entry_list, head, node) == head) {
717 return;
718 }
719 }
720 }
721
722 // Push "current" onto the head of the entry_list.
723 // If the _entry_list was changed during our push operation, we try to
724 // lock the monitor. Returns true if we locked the monitor, and false
725 // if we added current to _entry_list. Once on _entry_list, current
726 // stays on-queue until it acquires the lock.
727 bool ObjectMonitor::try_lock_or_add_to_entry_list(JavaThread* current, ObjectWaiter* node) {
728 assert(node->TState == ObjectWaiter::TS_RUN, "");
729 node->_prev = nullptr;
730 node->TState = ObjectWaiter::TS_ENTER;
731
732 for (;;) {
733 ObjectWaiter* head = AtomicAccess::load(&_entry_list);
734 node->_next = head;
735 if (AtomicAccess::cmpxchg(&_entry_list, head, node) == head) {
736 return false;
737 }
738
739 // Interference - the CAS failed because _entry_list changed. Before
740 // retrying the CAS retry taking the lock as it may now be free.
741 if (try_lock(current) == TryLockResult::Success) {
742 assert(!has_successor(current), "invariant");
743 assert(has_owner(current), "invariant");
744 node->TState = ObjectWaiter::TS_RUN;
745 return true;
746 }
747 }
748 }
749
750 static void post_monitor_deflate_event(EventJavaMonitorDeflate* event,
751 const oop obj) {
752 assert(event != nullptr, "invariant");
753 if (obj == nullptr) {
754 // Accept the case when obj was already garbage-collected.
755 // Emit the event anyway, but without details.
756 event->set_monitorClass(nullptr);
757 event->set_address(0);
758 } else {
759 const Klass* monitor_klass = obj->klass();
760 if (ObjectMonitor::is_jfr_excluded(monitor_klass)) {
761 return;
762 }
763 event->set_monitorClass(monitor_klass);
764 event->set_address((uintptr_t)(void*)obj);
765 }
766 event->commit();
767 }
768
769 // Deflate the specified ObjectMonitor if not in-use. Returns true if it
770 // was deflated and false otherwise.
771 //
772 // The async deflation protocol sets owner to DEFLATER_MARKER and
773 // makes contentions negative as signals to contending threads that
774 // an async deflation is in progress. There are a number of checks
775 // as part of the protocol to make sure that the calling thread has
776 // not lost the race to a contending thread.
777 //
778 // The ObjectMonitor has been successfully async deflated when:
779 // (contentions < 0)
780 // Contending threads that see that condition know to retry their operation.
781 //
782 bool ObjectMonitor::deflate_monitor(Thread* current) {
783 if (is_busy()) {
784 // Easy checks are first - the ObjectMonitor is busy so no deflation.
785 return false;
786 }
787
788 EventJavaMonitorDeflate event;
789
790 const oop obj = object_peek();
791
792 if (obj == nullptr) {
793 // If the object died, we can recycle the monitor without racing with
794 // Java threads. The GC already broke the association with the object.
795 set_owner_from_raw(NO_OWNER, DEFLATER_MARKER);
796 assert(contentions() >= 0, "must be non-negative: contentions=%d", contentions());
797 _contentions = INT_MIN; // minimum negative int
798 } else {
799 // Attempt async deflation protocol.
800
801 // Set a null owner to DEFLATER_MARKER to force any contending thread
802 // through the slow path. This is just the first part of the async
803 // deflation dance.
804 if (try_set_owner_from_raw(NO_OWNER, DEFLATER_MARKER) != NO_OWNER) {
805 // The owner field is no longer null so we lost the race since the
806 // ObjectMonitor is now busy.
807 return false;
808 }
809
810 if (contentions() > 0 || _waiters != 0) {
811 // Another thread has raced to enter the ObjectMonitor after
812 // is_busy() above or has already entered and waited on
813 // it which makes it busy so no deflation. Restore owner to
814 // null if it is still DEFLATER_MARKER.
815 if (try_set_owner_from_raw(DEFLATER_MARKER, NO_OWNER) != DEFLATER_MARKER) {
816 // Deferred decrement for the JT enter_internal() that cancelled the async deflation.
817 add_to_contentions(-1);
818 }
819 return false;
820 }
821
822 // Make a zero contentions field negative to force any contending threads
823 // to retry. This is the second part of the async deflation dance.
824 if (AtomicAccess::cmpxchg(&_contentions, 0, INT_MIN) != 0) {
825 // Contentions was no longer 0 so we lost the race since the
826 // ObjectMonitor is now busy. Restore owner to null if it is
827 // still DEFLATER_MARKER:
828 if (try_set_owner_from_raw(DEFLATER_MARKER, NO_OWNER) != DEFLATER_MARKER) {
829 // Deferred decrement for the JT enter_internal() that cancelled the async deflation.
830 add_to_contentions(-1);
831 }
832 return false;
833 }
834 }
835
836 // Sanity checks for the races:
837 guarantee(owner_is_DEFLATER_MARKER(), "must be deflater marker");
838 guarantee(contentions() < 0, "must be negative: contentions=%d",
839 contentions());
840 guarantee(_waiters == 0, "must be 0: waiters=%d", _waiters);
841 ObjectWaiter* w = AtomicAccess::load(&_entry_list);
842 guarantee(w == nullptr,
843 "must be no entering threads: entry_list=" INTPTR_FORMAT,
844 p2i(w));
845
846 if (obj != nullptr) {
847 if (log_is_enabled(Trace, monitorinflation)) {
848 ResourceMark rm;
849 log_trace(monitorinflation)("deflate_monitor: object=" INTPTR_FORMAT
850 ", mark=" INTPTR_FORMAT ", type='%s'",
851 p2i(obj), obj->mark().value(),
852 obj->klass()->external_name());
853 }
854 }
855
856 if (UseObjectMonitorTable) {
857 LightweightSynchronizer::deflate_monitor(current, obj, this);
858 } else if (obj != nullptr) {
859 // Install the old mark word if nobody else has already done it.
860 install_displaced_markword_in_object(obj);
861 }
862
863 if (event.should_commit()) {
864 post_monitor_deflate_event(&event, obj);
865 }
866
867 // We leave owner == DEFLATER_MARKER and contentions < 0
868 // to force any racing threads to retry.
869 return true; // Success, ObjectMonitor has been deflated.
870 }
871
872 // Install the displaced mark word (dmw) of a deflating ObjectMonitor
873 // into the header of the object associated with the monitor. This
874 // idempotent method is called by a thread that is deflating a
875 // monitor and by other threads that have detected a race with the
876 // deflation process.
877 void ObjectMonitor::install_displaced_markword_in_object(const oop obj) {
878 assert(!UseObjectMonitorTable, "ObjectMonitorTable has no dmw");
879 // This function must only be called when (owner == DEFLATER_MARKER
880 // && contentions <= 0), but we can't guarantee that here because
881 // those values could change when the ObjectMonitor gets moved from
882 // the global free list to a per-thread free list.
883
884 guarantee(obj != nullptr, "must be non-null");
885
886 // Separate loads in is_being_async_deflated(), which is almost always
887 // called before this function, from the load of dmw/header below.
888
889 // _contentions and dmw/header may get written by different threads.
890 // Make sure to observe them in the same order when having several observers.
891 OrderAccess::loadload_for_IRIW();
892
893 const oop l_object = object_peek();
894 if (l_object == nullptr) {
895 // ObjectMonitor's object ref has already been cleared by async
896 // deflation or GC so we're done here.
897 return;
898 }
899 assert(l_object == obj, "object=" INTPTR_FORMAT " must equal obj="
900 INTPTR_FORMAT, p2i(l_object), p2i(obj));
901
902 markWord dmw = header();
903 // The dmw has to be neutral (not null, not locked and not marked).
904 assert(dmw.is_neutral(), "must be neutral: dmw=" INTPTR_FORMAT, dmw.value());
905
906 // Install displaced mark word if the object's header still points
907 // to this ObjectMonitor. More than one racing caller to this function
908 // can rarely reach this point, but only one can win.
909 markWord res = obj->cas_set_mark(dmw, markWord::encode(this));
910 if (res != markWord::encode(this)) {
911 // This should be rare so log at the Info level when it happens.
912 log_info(monitorinflation)("install_displaced_markword_in_object: "
913 "failed cas_set_mark: new_mark=" INTPTR_FORMAT
914 ", old_mark=" INTPTR_FORMAT ", res=" INTPTR_FORMAT,
915 dmw.value(), markWord::encode(this).value(),
916 res.value());
917 }
918
919 // Note: It does not matter which thread restored the header/dmw
920 // into the object's header. The thread deflating the monitor just
921 // wanted the object's header restored and it is. The threads that
922 // detected a race with the deflation process also wanted the
923 // object's header restored before they retry their operation and
924 // because it is restored they will only retry once.
925 }
926
927 // Convert the fields used by is_busy() to a string that can be
928 // used for diagnostic output.
929 const char* ObjectMonitor::is_busy_to_string(stringStream* ss) {
930 ss->print("is_busy: waiters=%d"
931 ", contentions=%d"
932 ", owner=" INT64_FORMAT
933 ", entry_list=" PTR_FORMAT,
934 _waiters,
935 (contentions() > 0 ? contentions() : 0),
936 owner_is_DEFLATER_MARKER()
937 // We report null instead of DEFLATER_MARKER here because is_busy()
938 // ignores DEFLATER_MARKER values.
939 ? NO_OWNER
940 : owner_raw(),
941 p2i(_entry_list));
942 return ss->base();
943 }
944
945 void ObjectMonitor::enter_internal(JavaThread* current) {
946 assert(current->thread_state() == _thread_blocked, "invariant");
947
948 // Try the lock - TATAS
949 if (try_lock(current) == TryLockResult::Success) {
950 assert(!has_successor(current), "invariant");
951 assert(has_owner(current), "invariant");
952 return;
953 }
954
955 assert(InitDone, "Unexpectedly not initialized");
956
957 // We try one round of spinning *before* enqueueing current.
958 //
959 // If the _owner is ready but OFFPROC we could use a YieldTo()
960 // operation to donate the remainder of this thread's quantum
961 // to the owner. This has subtle but beneficial affinity
962 // effects.
963
964 if (try_spin(current)) {
965 assert(has_owner(current), "invariant");
966 assert(!has_successor(current), "invariant");
967 return;
968 }
969
970 // The Spin failed -- Enqueue and park the thread ...
971 assert(!has_successor(current), "invariant");
972 assert(!has_owner(current), "invariant");
973
974 // Enqueue "current" on ObjectMonitor's _entry_list.
975 //
976 // Node acts as a proxy for current.
977 // As an aside, if were to ever rewrite the synchronization code mostly
978 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
979 // Java objects. This would avoid awkward lifecycle and liveness issues,
980 // as well as eliminate a subset of ABA issues.
981 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
982
983 ObjectWaiter node(current);
984 current->_ParkEvent->reset();
985
986 if (try_lock_or_add_to_entry_list(current, &node)) {
987 return; // We got the lock.
988 }
989 // This thread is now added to the _entry_list.
990
991 // The lock might have been released while this thread was occupied queueing
992 // itself onto _entry_list. To close the race and avoid "stranding" and
993 // progress-liveness failure we must resample-retry _owner before parking.
994 // Note the Dekker/Lamport duality: ST _entry_list; MEMBAR; LD Owner.
995 // In this case the ST-MEMBAR is accomplished with CAS().
996 //
997 // TODO: Defer all thread state transitions until park-time.
998 // Since state transitions are heavy and inefficient we'd like
999 // to defer the state transitions until absolutely necessary,
1000 // and in doing so avoid some transitions ...
1001
1002 // If there are unmounted virtual threads in the _entry_list do a timed-park
1003 // instead to alleviate some deadlocks cases where one of them is picked as
1004 // the successor but cannot run due to having run out of carriers. This can
1005 // happen, for example, if this is a pinned virtual thread currently loading
1006 // or initializining a class, and all other carriers have a pinned vthread
1007 // waiting for said class to be loaded/initialized.
1008 // Read counter *after* adding this thread to the _entry_list.
1009 // Adding to _entry_list uses Atomic::cmpxchg() which already provides
1010 // a fence that prevents this load from floating up previous store.
1011 bool do_timed_parked = has_unmounted_vthreads();
1012 static int MAX_RECHECK_INTERVAL = 1000;
1013 int recheck_interval = 1;
1014
1015 for (;;) {
1016
1017 if (try_lock(current) == TryLockResult::Success) {
1018 break;
1019 }
1020 assert(!has_owner(current), "invariant");
1021
1022 // park self
1023 if (do_timed_parked) {
1024 current->_ParkEvent->park((jlong) recheck_interval);
1025 // Increase the recheck_interval, but clamp the value.
1026 recheck_interval *= 8;
1027 if (recheck_interval > MAX_RECHECK_INTERVAL) {
1028 recheck_interval = MAX_RECHECK_INTERVAL;
1029 }
1030 } else {
1031 current->_ParkEvent->park();
1032 }
1033
1034 if (try_lock(current) == TryLockResult::Success) {
1035 break;
1036 }
1037
1038 // The lock is still contested.
1039
1040 // Assuming this is not a spurious wakeup we'll normally find _succ == current.
1041 // We can defer clearing _succ until after the spin completes
1042 // try_spin() must tolerate being called with _succ == current.
1043 // Try yet another round of adaptive spinning.
1044 if (try_spin(current)) {
1045 break;
1046 }
1047
1048 // We can find that we were unpark()ed and redesignated _succ while
1049 // we were spinning. That's harmless. If we iterate and call park(),
1050 // park() will consume the event and return immediately and we'll
1051 // just spin again. This pattern can repeat, leaving _succ to simply
1052 // spin on a CPU.
1053
1054 if (has_successor(current)) clear_successor();
1055
1056 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
1057 OrderAccess::fence();
1058 }
1059
1060 // Egress :
1061 // Current has acquired the lock -- Unlink current from the _entry_list.
1062 unlink_after_acquire(current, &node);
1063 if (has_successor(current)) {
1064 clear_successor();
1065 // Note that we don't need to do OrderAccess::fence() after clearing
1066 // _succ here, since we own the lock.
1067 }
1068
1069 // We've acquired ownership with CAS().
1070 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
1071 // But since the CAS() this thread may have also stored into _succ
1072 // or entry_list. These meta-data updates must be visible __before
1073 // this thread subsequently drops the lock.
1074 // Consider what could occur if we didn't enforce this constraint --
1075 // STs to monitor meta-data and user-data could reorder with (become
1076 // visible after) the ST in exit that drops ownership of the lock.
1077 // Some other thread could then acquire the lock, but observe inconsistent
1078 // or old monitor meta-data and heap data. That violates the JMM.
1079 // To that end, the exit() operation must have at least STST|LDST
1080 // "release" barrier semantics. Specifically, there must be at least a
1081 // STST|LDST barrier in exit() before the ST of null into _owner that drops
1082 // the lock. The barrier ensures that changes to monitor meta-data and data
1083 // protected by the lock will be visible before we release the lock, and
1084 // therefore before some other thread (CPU) has a chance to acquire the lock.
1085 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
1086 //
1087 // Critically, any prior STs to _succ or entry_list must be visible before
1088 // the ST of null into _owner in the *subsequent* (following) corresponding
1089 // monitorexit.
1090
1091 return;
1092 }
1093
1094 // reenter_internal() is a specialized inline form of the latter half of the
1095 // contended slow-path from enter_internal(). We use reenter_internal() only for
1096 // monitor reentry in wait().
1097 //
1098 // In the future we should reconcile enter_internal() and reenter_internal().
1099
1100 void ObjectMonitor::reenter_internal(JavaThread* current, ObjectWaiter* currentNode) {
1101 assert(current != nullptr, "invariant");
1102 assert(current->thread_state() != _thread_blocked, "invariant");
1103 assert(currentNode != nullptr, "invariant");
1104 assert(currentNode->_thread == current, "invariant");
1105 assert(_waiters > 0, "invariant");
1106 assert_mark_word_consistency();
1107
1108 // If there are unmounted virtual threads in the _entry_list do a timed-park
1109 // instead to alleviate some deadlocks cases where one of them is picked as
1110 // the successor but cannot run due to having run out of carriers. This can
1111 // happen, for example, if this is a pinned virtual thread (or plain carrier)
1112 // waiting for a class to be initialized.
1113 bool do_timed_parked = has_unmounted_vthreads();
1114 static int MAX_RECHECK_INTERVAL = 1000;
1115 int recheck_interval = 1;
1116
1117 for (;;) {
1118 ObjectWaiter::TStates v = currentNode->TState;
1119 guarantee(v == ObjectWaiter::TS_ENTER, "invariant");
1120 assert(!has_owner(current), "invariant");
1121
1122 // This thread has been notified so try to reacquire the lock.
1123 if (try_lock(current) == TryLockResult::Success) {
1124 break;
1125 }
1126
1127 // If that fails, spin again. Note that spin count may be zero so the above TryLock
1128 // is necessary.
1129 if (try_spin(current)) {
1130 break;
1131 }
1132
1133 {
1134 OSThreadContendState osts(current->osthread());
1135
1136 assert(current->thread_state() == _thread_in_vm, "invariant");
1137
1138 {
1139 ClearSuccOnSuspend csos(this);
1140 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */);
1141 if (do_timed_parked) {
1142 current->_ParkEvent->park((jlong) recheck_interval);
1143 // Increase the recheck_interval, but clamp the value.
1144 recheck_interval *= 8;
1145 if (recheck_interval > MAX_RECHECK_INTERVAL) {
1146 recheck_interval = MAX_RECHECK_INTERVAL;
1147 }
1148 } else {
1149 current->_ParkEvent->park();
1150 }
1151 }
1152 }
1153
1154 // Try again, but just so we distinguish between futile wakeups and
1155 // successful wakeups. The following test isn't algorithmically
1156 // necessary, but it helps us maintain sensible statistics.
1157 if (try_lock(current) == TryLockResult::Success) {
1158 break;
1159 }
1160
1161 // The lock is still contested.
1162
1163 // Assuming this is not a spurious wakeup we'll normally
1164 // find that _succ == current.
1165 if (has_successor(current)) clear_successor();
1166
1167 // Invariant: after clearing _succ a contending thread
1168 // *must* retry _owner before parking.
1169 OrderAccess::fence();
1170
1171 // See comment in notify_internal
1172 do_timed_parked |= currentNode->_do_timed_park;
1173 }
1174
1175 // Current has acquired the lock -- Unlink current from the _entry_list.
1176 assert(has_owner(current), "invariant");
1177 assert_mark_word_consistency();
1178 unlink_after_acquire(current, currentNode);
1179 if (has_successor(current)) clear_successor();
1180 assert(!has_successor(current), "invariant");
1181 currentNode->TState = ObjectWaiter::TS_RUN;
1182 OrderAccess::fence(); // see comments at the end of enter_internal()
1183 }
1184
1185 // This method is called from two places:
1186 // - On monitorenter contention with a null waiter.
1187 // - After Object.wait() times out or the target is interrupted to reenter the
1188 // monitor, with the existing waiter.
1189 // For the Object.wait() case we do not delete the ObjectWaiter in case we
1190 // succesfully acquire the monitor since we are going to need it on return.
1191 bool ObjectMonitor::vthread_monitor_enter(JavaThread* current, ObjectWaiter* waiter) {
1192 if (try_lock(current) == TryLockResult::Success) {
1193 assert(has_owner(current), "invariant");
1194 assert(!has_successor(current), "invariant");
1195 return true;
1196 }
1197
1198 oop vthread = current->vthread();
1199 ObjectWaiter* node = waiter != nullptr ? waiter : new ObjectWaiter(vthread, this);
1200
1201 // Increment counter *before* adding the vthread to the _entry_list.
1202 // Adding to _entry_list uses Atomic::cmpxchg() which already provides
1203 // a fence that prevents reordering of the stores.
1204 inc_unmounted_vthreads();
1205
1206 if (try_lock_or_add_to_entry_list(current, node)) {
1207 // We got the lock.
1208 if (waiter == nullptr) delete node; // for Object.wait() don't delete yet
1209 dec_unmounted_vthreads();
1210 return true;
1211 }
1212 // This thread is now added to the entry_list.
1213
1214 // We have to try once more since owner could have exited monitor and checked
1215 // _entry_list before we added the node to the queue.
1216 if (try_lock(current) == TryLockResult::Success) {
1217 assert(has_owner(current), "invariant");
1218 unlink_after_acquire(current, node);
1219 if (has_successor(current)) clear_successor();
1220 if (waiter == nullptr) delete node; // for Object.wait() don't delete yet
1221 dec_unmounted_vthreads();
1222 return true;
1223 }
1224
1225 assert(java_lang_VirtualThread::state(vthread) == java_lang_VirtualThread::RUNNING, "wrong state for vthread");
1226 java_lang_VirtualThread::set_state(vthread, java_lang_VirtualThread::BLOCKING);
1227
1228 // We didn't succeed in acquiring the monitor so increment _contentions and
1229 // save ObjectWaiter* in the vthread since we will need it when resuming execution.
1230 add_to_contentions(1);
1231 java_lang_VirtualThread::set_objectWaiter(vthread, node);
1232 return false;
1233 }
1234
1235 // Called from thaw code to resume the monitor operation that caused the vthread
1236 // to be unmounted. Method returns true if the monitor is successfully acquired,
1237 // which marks the end of the monitor operation, otherwise it returns false.
1238 bool ObjectMonitor::resume_operation(JavaThread* current, ObjectWaiter* node, ContinuationWrapper& cont) {
1239 assert(java_lang_VirtualThread::state(current->vthread()) == java_lang_VirtualThread::RUNNING, "wrong state for vthread");
1240 assert(!has_owner(current), "");
1241
1242 if (node->is_wait() && !node->at_reenter()) {
1243 bool acquired_monitor = vthread_wait_reenter(current, node, cont);
1244 if (acquired_monitor) return true;
1245 }
1246
1247 // Retry acquiring monitor...
1248
1249 int state = node->TState;
1250 guarantee(state == ObjectWaiter::TS_ENTER, "invariant");
1251
1252 if (try_lock(current) == TryLockResult::Success) {
1253 vthread_epilog(current, node);
1254 return true;
1255 }
1256
1257 oop vthread = current->vthread();
1258 if (has_successor(current)) clear_successor();
1259
1260 // Invariant: after clearing _succ a thread *must* retry acquiring the monitor.
1261 OrderAccess::fence();
1262
1263 if (try_lock(current) == TryLockResult::Success) {
1264 vthread_epilog(current, node);
1265 return true;
1266 }
1267
1268 // We will return to Continuation.run() and unmount so set the right state.
1269 java_lang_VirtualThread::set_state(vthread, java_lang_VirtualThread::BLOCKING);
1270
1271 return false;
1272 }
1273
1274 void ObjectMonitor::vthread_epilog(JavaThread* current, ObjectWaiter* node) {
1275 assert(has_owner(current), "invariant");
1276 add_to_contentions(-1);
1277 dec_unmounted_vthreads();
1278
1279 if (has_successor(current)) clear_successor();
1280
1281 guarantee(_recursions == 0, "invariant");
1282
1283 if (node->is_wait()) {
1284 _recursions = node->_recursions; // restore the old recursion count
1285 _waiters--; // decrement the number of waiters
1286
1287 if (node->_interrupted) {
1288 // We will throw at thaw end after finishing the mount transition.
1289 current->set_pending_interrupted_exception(true);
1290 }
1291 }
1292
1293 unlink_after_acquire(current, node);
1294 delete node;
1295
1296 // Clear the ObjectWaiter* from the vthread.
1297 java_lang_VirtualThread::set_objectWaiter(current->vthread(), nullptr);
1298
1299 if (JvmtiExport::should_post_monitor_contended_entered()) {
1300 // We are going to call thaw again after this and finish the VMTS
1301 // transition so no need to do it here. We will post the event there.
1302 current->set_contended_entered_monitor(this);
1303 }
1304 }
1305
1306 // Convert entry_list into a doubly linked list by assigning the prev
1307 // pointers and the entry_list_tail pointer (if needed). Within the
1308 // entry_list the next pointers always form a consistent singly linked
1309 // list. When this function is called, the entry_list will be either
1310 // singly linked, or starting as singly linked (at the head), but
1311 // ending as doubly linked (at the tail).
1312 void ObjectMonitor::entry_list_build_dll(JavaThread* current) {
1313 assert(has_owner(current), "invariant");
1314 ObjectWaiter* prev = nullptr;
1315 // Need acquire here to match the implicit release of the cmpxchg
1316 // that updated entry_list, so we can access w->prev().
1317 ObjectWaiter* w = AtomicAccess::load_acquire(&_entry_list);
1318 assert(w != nullptr, "should only be called when entry list is not empty");
1319 while (w != nullptr) {
1320 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1321 assert(w->prev() == nullptr || w->prev() == prev, "invariant");
1322 if (w->prev() != nullptr) {
1323 break;
1324 }
1325 w->_prev = prev;
1326 prev = w;
1327 w = w->next();
1328 }
1329 if (w == nullptr) {
1330 // We converted the entire entry_list from a singly linked list
1331 // into a doubly linked list. Now we just need to set the tail
1332 // pointer.
1333 assert(prev != nullptr && prev->next() == nullptr, "invariant");
1334 assert(_entry_list_tail == nullptr || _entry_list_tail == prev, "invariant");
1335 _entry_list_tail = prev;
1336 } else {
1337 #ifdef ASSERT
1338 // We stopped iterating through the _entry_list when we found a
1339 // node that had its prev pointer set. I.e. we converted the first
1340 // part of the entry_list from a singly linked list into a doubly
1341 // linked list. Now we just want to make sure the rest of the list
1342 // is doubly linked. But first we check that we have a tail
1343 // pointer, because if the end of the entry_list is doubly linked
1344 // and we don't have the tail pointer, something is broken.
1345 assert(_entry_list_tail != nullptr, "invariant");
1346 while (w != nullptr) {
1347 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1348 assert(w->prev() == prev, "invariant");
1349 prev = w;
1350 w = w->next();
1351 }
1352 assert(_entry_list_tail == prev, "invariant");
1353 #endif
1354 }
1355 }
1356
1357 // Return the tail of the _entry_list. If the tail is currently not
1358 // known, it can be found by first calling entry_list_build_dll().
1359 ObjectWaiter* ObjectMonitor::entry_list_tail(JavaThread* current) {
1360 assert(has_owner(current), "invariant");
1361 ObjectWaiter* w = _entry_list_tail;
1362 if (w != nullptr) {
1363 return w;
1364 }
1365 entry_list_build_dll(current);
1366 w = _entry_list_tail;
1367 assert(w != nullptr, "invariant");
1368 return w;
1369 }
1370
1371 // By convention we unlink a contending thread from _entry_list
1372 // immediately after the thread acquires the lock in ::enter().
1373 // The head of _entry_list is volatile but the interior is stable.
1374 // In addition, current.TState is stable.
1375
1376 void ObjectMonitor::unlink_after_acquire(JavaThread* current, ObjectWaiter* currentNode) {
1377 assert(has_owner(current), "invariant");
1378 assert((!currentNode->is_vthread() && currentNode->thread() == current) ||
1379 (currentNode->is_vthread() && currentNode->vthread() == current->vthread()), "invariant");
1380
1381 // Check if we are unlinking the last element in the _entry_list.
1382 // This is by far the most common case.
1383 if (currentNode->next() == nullptr) {
1384 assert(_entry_list_tail == nullptr || _entry_list_tail == currentNode, "invariant");
1385
1386 ObjectWaiter* w = AtomicAccess::load(&_entry_list);
1387 if (w == currentNode) {
1388 // The currentNode is the only element in _entry_list.
1389 if (AtomicAccess::cmpxchg(&_entry_list, w, (ObjectWaiter*)nullptr) == w) {
1390 _entry_list_tail = nullptr;
1391 currentNode->set_bad_pointers();
1392 return;
1393 }
1394 // The CAS above can fail from interference IFF a contending
1395 // thread "pushed" itself onto entry_list. So fall-through to
1396 // building the doubly linked list.
1397 assert(currentNode->prev() == nullptr, "invariant");
1398 }
1399 if (currentNode->prev() == nullptr) {
1400 // Build the doubly linked list to get hold of
1401 // currentNode->prev().
1402 entry_list_build_dll(current);
1403 assert(currentNode->prev() != nullptr, "must be");
1404 assert(_entry_list_tail == currentNode, "must be");
1405 }
1406 // The currentNode is the last element in _entry_list and we know
1407 // which element is the previous one.
1408 assert(_entry_list != currentNode, "invariant");
1409 _entry_list_tail = currentNode->prev();
1410 _entry_list_tail->_next = nullptr;
1411 currentNode->set_bad_pointers();
1412 return;
1413 }
1414
1415 // If we get here it means the current thread enqueued itself on the
1416 // _entry_list but was then able to "steal" the lock before the
1417 // chosen successor was able to. Consequently currentNode must be an
1418 // interior node in the _entry_list, or the head.
1419 assert(currentNode->next() != nullptr, "invariant");
1420 assert(currentNode != _entry_list_tail, "invariant");
1421
1422 // Check if we are in the singly linked portion of the
1423 // _entry_list. If we are the head then we try to remove ourselves,
1424 // else we convert to the doubly linked list.
1425 if (currentNode->prev() == nullptr) {
1426 ObjectWaiter* w = AtomicAccess::load(&_entry_list);
1427
1428 assert(w != nullptr, "invariant");
1429 if (w == currentNode) {
1430 ObjectWaiter* next = currentNode->next();
1431 // currentNode is at the head of _entry_list.
1432 if (AtomicAccess::cmpxchg(&_entry_list, w, next) == w) {
1433 // The CAS above sucsessfully unlinked currentNode from the
1434 // head of the _entry_list.
1435 assert(_entry_list != w, "invariant");
1436 next->_prev = nullptr;
1437 currentNode->set_bad_pointers();
1438 return;
1439 } else {
1440 // The CAS above can fail from interference IFF a contending
1441 // thread "pushed" itself onto _entry_list, in which case
1442 // currentNode must now be in the interior of the
1443 // list. Fall-through to building the doubly linked list.
1444 assert(_entry_list != currentNode, "invariant");
1445 }
1446 }
1447 // Build the doubly linked list to get hold of currentNode->prev().
1448 entry_list_build_dll(current);
1449 assert(currentNode->prev() != nullptr, "must be");
1450 }
1451
1452 // We now know we are unlinking currentNode from the interior of a
1453 // doubly linked list.
1454 assert(currentNode->next() != nullptr, "");
1455 assert(currentNode->prev() != nullptr, "");
1456 assert(currentNode != _entry_list, "");
1457 assert(currentNode != _entry_list_tail, "");
1458
1459 ObjectWaiter* nxt = currentNode->next();
1460 ObjectWaiter* prv = currentNode->prev();
1461 assert(nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
1462 assert(prv->TState == ObjectWaiter::TS_ENTER, "invariant");
1463
1464 nxt->_prev = prv;
1465 prv->_next = nxt;
1466 currentNode->set_bad_pointers();
1467 }
1468
1469 // -----------------------------------------------------------------------------
1470 // Exit support
1471 //
1472 // exit()
1473 // ~~~~~~
1474 // Note that the collector can't reclaim the objectMonitor or deflate
1475 // the object out from underneath the thread calling ::exit() as the
1476 // thread calling ::exit() never transitions to a stable state.
1477 // This inhibits GC, which in turn inhibits asynchronous (and
1478 // inopportune) reclamation of "this".
1479 //
1480 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
1481 // There's one exception to the claim above, however. enter_internal() can call
1482 // exit() to drop a lock if the acquirer has been externally suspended.
1483 // In that case exit() is called with _thread_state == _thread_blocked,
1484 // but the monitor's _contentions field is > 0, which inhibits reclamation.
1485 //
1486 // This is the exit part of the locking protocol, often implemented in
1487 // C2_MacroAssembler::fast_unlock()
1488 //
1489 // 1. A release barrier ensures that changes to monitor meta-data
1490 // (_succ, _entry_list) and data protected by the lock will be
1491 // visible before we release the lock.
1492 // 2. Release the lock by clearing the owner.
1493 // 3. A storeload MEMBAR is needed between releasing the owner and
1494 // subsequently reading meta-data to safely determine if the lock is
1495 // contended (step 4) without an elected successor (step 5).
1496 // 4. If _entry_list is null, we are done, since there is no
1497 // other thread waiting on the lock to wake up. I.e. there is no
1498 // contention.
1499 // 5. If there is a successor (_succ is non-null), we are done. The
1500 // responsibility for guaranteeing progress-liveness has now implicitly
1501 // been moved from the exiting thread to the successor.
1502 // 6. There are waiters in the entry list (_entry_list is non-null),
1503 // but there is no successor (_succ is null), so we need to
1504 // wake up (unpark) a waiting thread to avoid stranding.
1505 //
1506 // Note that since only the current lock owner can manipulate the
1507 // _entry_list (except for pushing new threads to the head), we need to
1508 // reacquire the lock before we can wake up (unpark) a waiting thread.
1509 //
1510 // The CAS() in enter provides for safety and exclusion, while the
1511 // MEMBAR in exit provides for progress and avoids stranding.
1512 //
1513 // There is also the risk of a futile wake-up. If we drop the lock
1514 // another thread can reacquire the lock immediately, and we can
1515 // then wake a thread unnecessarily. This is benign, and we've
1516 // structured the code so the windows are short and the frequency
1517 // of such futile wakups is low.
1518
1519 void ObjectMonitor::exit(JavaThread* current, bool not_suspended) {
1520 if (!has_owner(current)) {
1521 // Apparent unbalanced locking ...
1522 // Naively we'd like to throw IllegalMonitorStateException.
1523 // As a practical matter we can neither allocate nor throw an
1524 // exception as ::exit() can be called from leaf routines.
1525 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
1526 // Upon deeper reflection, however, in a properly run JVM the only
1527 // way we should encounter this situation is in the presence of
1528 // unbalanced JNI locking. TODO: CheckJNICalls.
1529 // See also: CR4414101
1530 #ifdef ASSERT
1531 LogStreamHandle(Error, monitorinflation) lsh;
1532 lsh.print_cr("ERROR: ObjectMonitor::exit(): thread=" INTPTR_FORMAT
1533 " is exiting an ObjectMonitor it does not own.", p2i(current));
1534 lsh.print_cr("The imbalance is possibly caused by JNI locking.");
1535 print_debug_style_on(&lsh);
1536 assert(false, "Non-balanced monitor enter/exit!");
1537 #endif
1538 return;
1539 }
1540
1541 if (_recursions != 0) {
1542 _recursions--; // this is simple recursive enter
1543 return;
1544 }
1545
1546 #if INCLUDE_JFR
1547 // get the owner's thread id for the MonitorEnter event
1548 // if it is enabled and the thread isn't suspended
1549 if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
1550 _previous_owner_tid = JFR_THREAD_ID(current);
1551 }
1552 #endif
1553
1554 for (;;) {
1555 // If there is a successor we should release the lock as soon as
1556 // possible, so that the successor can acquire the lock. If there is
1557 // no successor, we might need to wake up a waiting thread.
1558 if (!has_successor()) {
1559 ObjectWaiter* w = AtomicAccess::load(&_entry_list);
1560 if (w != nullptr) {
1561 // Other threads are blocked trying to acquire the lock and
1562 // there is no successor, so it appears that an heir-
1563 // presumptive (successor) must be made ready. Since threads
1564 // are woken up in FIFO order, we need to find the tail of the
1565 // entry_list.
1566 w = entry_list_tail(current);
1567 // I'd like to write: guarantee (w->_thread != current).
1568 // But in practice an exiting thread may find itself on the entry_list.
1569 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1570 // then calls exit(). Exit release the lock by setting O._owner to null.
1571 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1572 // notify() operation moves T1 from O's waitset to O's entry_list. T2 then
1573 // release the lock "O". T1 resumes immediately after the ST of null into
1574 // _owner, above. T1 notices that the entry_list is populated, so it
1575 // reacquires the lock and then finds itself on the entry_list.
1576 // Given all that, we have to tolerate the circumstance where "w" is
1577 // associated with current.
1578 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1579 exit_epilog(current, w);
1580 return;
1581 }
1582 }
1583
1584 // Drop the lock.
1585 // release semantics: prior loads and stores from within the critical section
1586 // must not float (reorder) past the following store that drops the lock.
1587 // Uses a storeload to separate release_store(owner) from the
1588 // successor check. The try_set_owner_from() below uses cmpxchg() so
1589 // we get the fence down there.
1590 release_clear_owner(current);
1591 OrderAccess::storeload();
1592
1593 // Normally the exiting thread is responsible for ensuring succession,
1594 // but if this thread observes other successors are ready or other
1595 // entering threads are spinning after it has stored null into _owner
1596 // then it can exit without waking a successor. The existence of
1597 // spinners or ready successors guarantees proper succession (liveness).
1598 // Responsibility passes to the ready or running successors. The exiting
1599 // thread delegates the duty. More precisely, if a successor already
1600 // exists this thread is absolved of the responsibility of waking
1601 // (unparking) one.
1602
1603 // The _succ variable is critical to reducing futile wakeup frequency.
1604 // _succ identifies the "heir presumptive" thread that has been made
1605 // ready (unparked) but that has not yet run. We need only one such
1606 // successor thread to guarantee progress.
1607 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1608 // section 3.3 "Futile Wakeup Throttling" for details.
1609 //
1610 // Note that spinners in Enter() also set _succ non-null.
1611 // In the current implementation spinners opportunistically set
1612 // _succ so that exiting threads might avoid waking a successor.
1613 // Which means that the exiting thread could exit immediately without
1614 // waking a successor, if it observes a successor after it has dropped
1615 // the lock. Note that the dropped lock needs to become visible to the
1616 // spinner.
1617
1618 if (_entry_list == nullptr || has_successor()) {
1619 return;
1620 }
1621
1622 // Only the current lock owner can manipulate the entry_list
1623 // (except for pushing new threads to the head), therefore we need
1624 // to reacquire the lock. If we fail to reacquire the lock the
1625 // responsibility for ensuring succession falls to the new owner.
1626
1627 if (try_lock(current) != TryLockResult::Success) {
1628 // Some other thread acquired the lock (or the monitor was
1629 // deflated). Either way we are done.
1630 return;
1631 }
1632
1633 guarantee(has_owner(current), "invariant");
1634 }
1635 }
1636
1637 void ObjectMonitor::exit_epilog(JavaThread* current, ObjectWaiter* Wakee) {
1638 assert(has_owner(current), "invariant");
1639
1640 // Exit protocol:
1641 // 1. ST _succ = wakee
1642 // 2. membar #loadstore|#storestore;
1643 // 2. ST _owner = nullptr
1644 // 3. unpark(wakee)
1645
1646 oop vthread = nullptr;
1647 ParkEvent * Trigger;
1648 if (!Wakee->is_vthread()) {
1649 JavaThread* t = Wakee->thread();
1650 assert(t != nullptr, "");
1651 Trigger = t->_ParkEvent;
1652 set_successor(t);
1653 } else {
1654 vthread = Wakee->vthread();
1655 assert(vthread != nullptr, "");
1656 Trigger = ObjectMonitor::vthread_unparker_ParkEvent();
1657 set_successor(vthread);
1658 }
1659
1660 // Hygiene -- once we've set _owner = nullptr we can't safely dereference Wakee again.
1661 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1662 // out-of-scope (non-extant).
1663 Wakee = nullptr;
1664
1665 // Drop the lock.
1666 // Uses a fence to separate release_store(owner) from the LD in unpark().
1667 release_clear_owner(current);
1668 OrderAccess::fence();
1669
1670 DTRACE_MONITOR_PROBE(contended__exit, this, object(), current);
1671
1672 if (vthread == nullptr) {
1673 // Platform thread case.
1674 Trigger->unpark();
1675 } else if (java_lang_VirtualThread::set_onWaitingList(vthread, vthread_list_head())) {
1676 // Virtual thread case.
1677 Trigger->unpark();
1678 }
1679 }
1680
1681 // Exits the monitor returning recursion count. _owner should
1682 // be set to current's owner_id, i.e. no ANONYMOUS_OWNER allowed.
1683 intx ObjectMonitor::complete_exit(JavaThread* current) {
1684 assert(InitDone, "Unexpectedly not initialized");
1685 guarantee(has_owner(current), "complete_exit not owner");
1686
1687 intx save = _recursions; // record the old recursion count
1688 _recursions = 0; // set the recursion level to be 0
1689 exit(current); // exit the monitor
1690 guarantee(!has_owner(current), "invariant");
1691 return save;
1692 }
1693
1694 // Checks that the current THREAD owns this monitor and causes an
1695 // immediate return if it doesn't. We don't use the CHECK macro
1696 // because we want the IMSE to be the only exception that is thrown
1697 // from the call site when false is returned. Any other pending
1698 // exception is ignored.
1699 #define CHECK_OWNER() \
1700 do { \
1701 if (!check_owner(THREAD)) { \
1702 assert(HAS_PENDING_EXCEPTION, "expected a pending IMSE here."); \
1703 return; \
1704 } \
1705 } while (false)
1706
1707 // Returns true if the specified thread owns the ObjectMonitor.
1708 // Otherwise returns false and throws IllegalMonitorStateException
1709 // (IMSE). If there is a pending exception and the specified thread
1710 // is not the owner, that exception will be replaced by the IMSE.
1711 bool ObjectMonitor::check_owner(TRAPS) {
1712 JavaThread* current = THREAD;
1713 int64_t cur = owner_raw();
1714 if (cur == owner_id_from(current)) {
1715 return true;
1716 }
1717 THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(),
1718 "current thread is not owner", false);
1719 }
1720
1721 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1722 ObjectMonitor* monitor,
1723 uint64_t notifier_tid,
1724 jlong timeout,
1725 bool timedout) {
1726 assert(event != nullptr, "invariant");
1727 assert(monitor != nullptr, "invariant");
1728 const Klass* monitor_klass = monitor->object()->klass();
1729 if (ObjectMonitor::is_jfr_excluded(monitor_klass)) {
1730 return;
1731 }
1732 event->set_monitorClass(monitor_klass);
1733 event->set_timeout(timeout);
1734 // Set an address that is 'unique enough', such that events close in
1735 // time and with the same address are likely (but not guaranteed) to
1736 // belong to the same object.
1737 event->set_address((uintptr_t)monitor);
1738 event->set_notifier(notifier_tid);
1739 event->set_timedOut(timedout);
1740 event->commit();
1741 }
1742
1743 static void vthread_monitor_waited_event(JavaThread* current, ObjectWaiter* node, ContinuationWrapper& cont, EventJavaMonitorWait* event, jboolean timed_out) {
1744 // Since we might safepoint set the anchor so that the stack can we walked.
1745 assert(current->last_continuation() != nullptr, "");
1746 JavaFrameAnchor* anchor = current->frame_anchor();
1747 anchor->set_last_Java_sp(current->last_continuation()->entry_sp());
1748 anchor->set_last_Java_pc(current->last_continuation()->entry_pc());
1749
1750 ContinuationWrapper::SafepointOp so(current, cont);
1751
1752 JRT_BLOCK
1753 if (event->should_commit()) {
1754 long timeout = java_lang_VirtualThread::timeout(current->vthread());
1755 post_monitor_wait_event(event, node->_monitor, node->_notifier_tid, timeout, timed_out);
1756 }
1757 if (JvmtiExport::should_post_monitor_waited()) {
1758 // We mark this call in case of an upcall to Java while posting the event.
1759 // If somebody walks the stack in that case, processing the enterSpecial
1760 // frame should not include processing callee arguments since there is no
1761 // actual callee (see nmethod::preserve_callee_argument_oops()).
1762 ThreadOnMonitorWaitedEvent tmwe(current);
1763 JvmtiExport::vthread_post_monitor_waited(current, node->_monitor, timed_out);
1764 }
1765 JRT_BLOCK_END
1766 current->frame_anchor()->clear();
1767 }
1768
1769 // -----------------------------------------------------------------------------
1770 // Wait/Notify/NotifyAll
1771 //
1772 // Note: a subset of changes to ObjectMonitor::wait()
1773 // will need to be replicated in complete_exit
1774 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1775 JavaThread* current = THREAD;
1776
1777 assert(InitDone, "Unexpectedly not initialized");
1778
1779 CHECK_OWNER(); // Throws IMSE if not owner.
1780
1781 EventJavaMonitorWait wait_event;
1782 EventVirtualThreadPinned vthread_pinned_event;
1783
1784 // check for a pending interrupt
1785 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1786 JavaThreadInObjectWaitState jtiows(current, millis != 0, interruptible);
1787
1788 if (JvmtiExport::should_post_monitor_wait()) {
1789 JvmtiExport::post_monitor_wait(current, object(), millis);
1790 }
1791 // post monitor waited event. Note that this is past-tense, we are done waiting.
1792 if (JvmtiExport::should_post_monitor_waited()) {
1793 // Note: 'false' parameter is passed here because the
1794 // wait was not timed out due to thread interrupt.
1795 JvmtiExport::post_monitor_waited(current, this, false);
1796
1797 // In this short circuit of the monitor wait protocol, the
1798 // current thread never drops ownership of the monitor and
1799 // never gets added to the wait queue so the current thread
1800 // cannot be made the successor. This means that the
1801 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1802 // consume an unpark() meant for the ParkEvent associated with
1803 // this ObjectMonitor.
1804 }
1805 if (wait_event.should_commit()) {
1806 post_monitor_wait_event(&wait_event, this, 0, millis, false);
1807 }
1808 THROW(vmSymbols::java_lang_InterruptedException());
1809 return;
1810 }
1811
1812 freeze_result result;
1813 ContinuationEntry* ce = current->last_continuation();
1814 bool is_virtual = ce != nullptr && ce->is_virtual_thread();
1815 if (is_virtual) {
1816 if (interruptible && JvmtiExport::should_post_monitor_wait()) {
1817 JvmtiExport::post_monitor_wait(current, object(), millis);
1818 }
1819 current->set_current_waiting_monitor(this);
1820 result = Continuation::try_preempt(current, ce->cont_oop(current));
1821 if (result == freeze_ok) {
1822 vthread_wait(current, millis, interruptible);
1823 current->set_current_waiting_monitor(nullptr);
1824 return;
1825 }
1826 }
1827 // The jtiows does nothing for non-interruptible.
1828 JavaThreadInObjectWaitState jtiows(current, millis != 0, interruptible);
1829
1830 if (!is_virtual) { // it was already set for virtual thread
1831 if (interruptible && JvmtiExport::should_post_monitor_wait()) {
1832 JvmtiExport::post_monitor_wait(current, object(), millis);
1833
1834 // The current thread already owns the monitor and it has not yet
1835 // been added to the wait queue so the current thread cannot be
1836 // made the successor. This means that the JVMTI_EVENT_MONITOR_WAIT
1837 // event handler cannot accidentally consume an unpark() meant for
1838 // the ParkEvent associated with this ObjectMonitor.
1839 }
1840 current->set_current_waiting_monitor(this);
1841 }
1842 // create a node to be put into the queue
1843 // Critically, after we reset() the event but prior to park(), we must check
1844 // for a pending interrupt.
1845 ObjectWaiter node(current);
1846 node.TState = ObjectWaiter::TS_WAIT;
1847 current->_ParkEvent->reset();
1848 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1849
1850 // Enter the waiting queue, which is a circular doubly linked list in this case
1851 // but it could be a priority queue or any data structure.
1852 // _wait_set_lock protects the wait queue. Normally the wait queue is accessed only
1853 // by the owner of the monitor *except* in the case where park()
1854 // returns because of a timeout of interrupt. Contention is exceptionally rare
1855 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1856
1857 Thread::SpinAcquire(&_wait_set_lock);
1858 add_waiter(&node);
1859 Thread::SpinRelease(&_wait_set_lock);
1860
1861 intx save = _recursions; // record the old recursion count
1862 _waiters++; // increment the number of waiters
1863 _recursions = 0; // set the recursion level to be 1
1864 exit(current); // exit the monitor
1865 guarantee(!has_owner(current), "invariant");
1866
1867 // The thread is on the wait_set list - now park() it.
1868 // On MP systems it's conceivable that a brief spin before we park
1869 // could be profitable.
1870 //
1871 // TODO-FIXME: change the following logic to a loop of the form
1872 // while (!timeout && !interrupted && _notified == 0) park()
1873
1874 int ret = OS_OK;
1875 int WasNotified = 0;
1876
1877 // Need to check interrupt state whilst still _thread_in_vm
1878 bool interrupted = interruptible && current->is_interrupted(false);
1879
1880 { // State transition wrappers
1881 OSThread* osthread = current->osthread();
1882 OSThreadWaitState osts(osthread, true);
1883
1884 assert(current->thread_state() == _thread_in_vm, "invariant");
1885
1886 {
1887 ClearSuccOnSuspend csos(this);
1888 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */);
1889 if (interrupted || HAS_PENDING_EXCEPTION) {
1890 // Intentionally empty
1891 } else if (!node._notified) {
1892 if (millis <= 0) {
1893 current->_ParkEvent->park();
1894 } else {
1895 ret = current->_ParkEvent->park(millis);
1896 }
1897 }
1898 }
1899
1900 // Node may be on the wait_set, or on the entry_list, or in transition
1901 // from the wait_set to the entry_list.
1902 // See if we need to remove Node from the wait_set.
1903 // We use double-checked locking to avoid grabbing _wait_set_lock
1904 // if the thread is not on the wait queue.
1905 //
1906 // Note that we don't need a fence before the fetch of TState.
1907 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1908 // written by the is thread. (perhaps the fetch might even be satisfied
1909 // by a look-aside into the processor's own store buffer, although given
1910 // the length of the code path between the prior ST and this load that's
1911 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1912 // then we'll acquire the lock and then re-fetch a fresh TState value.
1913 // That is, we fail toward safety.
1914
1915 if (node.TState == ObjectWaiter::TS_WAIT) {
1916 Thread::SpinAcquire(&_wait_set_lock);
1917 if (node.TState == ObjectWaiter::TS_WAIT) {
1918 dequeue_specific_waiter(&node); // unlink from wait_set
1919 assert(!node._notified, "invariant");
1920 node.TState = ObjectWaiter::TS_RUN;
1921 }
1922 Thread::SpinRelease(&_wait_set_lock);
1923 }
1924
1925 // The thread is now either on off-list (TS_RUN),
1926 // or on the entry_list (TS_ENTER).
1927 // The Node's TState variable is stable from the perspective of this thread.
1928 // No other threads will asynchronously modify TState.
1929 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1930 OrderAccess::loadload();
1931 if (has_successor(current)) clear_successor();
1932 WasNotified = node._notified;
1933
1934 // Reentry phase -- reacquire the monitor.
1935 // re-enter contended monitor after object.wait().
1936 // retain OBJECT_WAIT state until re-enter successfully completes
1937 // Thread state is thread_in_vm and oop access is again safe,
1938 // although the raw address of the object may have changed.
1939 // (Don't cache naked oops over safepoints, of course).
1940
1941 // post monitor waited event. Note that this is past-tense, we are done waiting.
1942 if (JvmtiExport::should_post_monitor_waited()) {
1943 JvmtiExport::post_monitor_waited(current, this, ret == OS_TIMEOUT);
1944
1945 if (node._notified && has_successor(current)) {
1946 // In this part of the monitor wait-notify-reenter protocol it
1947 // is possible (and normal) for another thread to do a fastpath
1948 // monitor enter-exit while this thread is still trying to get
1949 // to the reenter portion of the protocol.
1950 //
1951 // The ObjectMonitor was notified and the current thread is
1952 // the successor which also means that an unpark() has already
1953 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1954 // consume the unpark() that was done when the successor was
1955 // set because the same ParkEvent is shared between Java
1956 // monitors and JVM/TI RawMonitors (for now).
1957 //
1958 // We redo the unpark() to ensure forward progress, i.e., we
1959 // don't want all pending threads hanging (parked) with none
1960 // entering the unlocked monitor.
1961 current->_ParkEvent->unpark();
1962 }
1963 }
1964
1965 if (wait_event.should_commit()) {
1966 post_monitor_wait_event(&wait_event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1967 }
1968
1969 OrderAccess::fence();
1970
1971 assert(!has_owner(current), "invariant");
1972 ObjectWaiter::TStates v = node.TState;
1973 if (v == ObjectWaiter::TS_RUN) {
1974 // We use the NoPreemptMark for the very rare case where the previous
1975 // preempt attempt failed due to OOM. The preempt on monitor contention
1976 // could succeed but we can't unmount now.
1977 NoPreemptMark npm(current);
1978 enter(current);
1979 } else {
1980 guarantee(v == ObjectWaiter::TS_ENTER, "invariant");
1981 reenter_internal(current, &node);
1982 node.wait_reenter_end(this);
1983 }
1984
1985 // current has reacquired the lock.
1986 // Lifecycle - the node representing current must not appear on any queues.
1987 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1988 // want residual elements associated with this thread left on any lists.
1989 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1990 assert(has_owner(current), "invariant");
1991 assert(!has_successor(current), "invariant");
1992 } // OSThreadWaitState()
1993
1994 current->set_current_waiting_monitor(nullptr);
1995
1996 guarantee(_recursions == 0, "invariant");
1997 int relock_count = JvmtiDeferredUpdates::get_and_reset_relock_count_after_wait(current);
1998 _recursions = save // restore the old recursion count
1999 + relock_count; // increased by the deferred relock count
2000 current->inc_held_monitor_count(relock_count); // Deopt never entered these counts.
2001 _waiters--; // decrement the number of waiters
2002
2003 // Verify a few postconditions
2004 assert(has_owner(current), "invariant");
2005 assert(!has_successor(current), "invariant");
2006 assert_mark_word_consistency();
2007
2008 if (ce != nullptr && ce->is_virtual_thread()) {
2009 current->post_vthread_pinned_event(&vthread_pinned_event, "Object.wait", result);
2010 }
2011
2012 // check if the notification happened
2013 if (!WasNotified) {
2014 // no, it could be timeout or Thread.interrupt() or both
2015 // check for interrupt event, otherwise it is timeout
2016 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
2017 THROW(vmSymbols::java_lang_InterruptedException());
2018 }
2019 }
2020
2021 // NOTE: Spurious wake up will be consider as timeout.
2022 // Monitor notify has precedence over thread interrupt.
2023 }
2024
2025 // Consider:
2026 // If the lock is cool (entry_list == null && succ == null) and we're on an MP system
2027 // then instead of transferring a thread from the wait_set to the entry_list
2028 // we might just dequeue a thread from the wait_set and directly unpark() it.
2029
2030 bool ObjectMonitor::notify_internal(JavaThread* current) {
2031 bool did_notify = false;
2032 Thread::SpinAcquire(&_wait_set_lock);
2033 ObjectWaiter* iterator = dequeue_waiter();
2034 if (iterator != nullptr) {
2035 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
2036 guarantee(!iterator->_notified, "invariant");
2037
2038 if (iterator->is_vthread()) {
2039 oop vthread = iterator->vthread();
2040 java_lang_VirtualThread::set_notified(vthread, true);
2041 int old_state = java_lang_VirtualThread::state(vthread);
2042 // If state is not WAIT/TIMED_WAIT then target could still be on
2043 // unmount transition, or wait could have already timed-out or target
2044 // could have been interrupted. In the first case, the target itself
2045 // will set the state to BLOCKED at the end of the unmount transition.
2046 // In the other cases the target would have been already unblocked so
2047 // there is nothing to do.
2048 if (old_state == java_lang_VirtualThread::WAIT ||
2049 old_state == java_lang_VirtualThread::TIMED_WAIT) {
2050 java_lang_VirtualThread::cmpxchg_state(vthread, old_state, java_lang_VirtualThread::BLOCKED);
2051 }
2052 // Increment counter *before* adding the vthread to the _entry_list.
2053 // Adding to _entry_list uses Atomic::cmpxchg() which already provides
2054 // a fence that prevents reordering of the stores.
2055 inc_unmounted_vthreads();
2056 }
2057
2058 iterator->_notified = true;
2059 iterator->_notifier_tid = JFR_THREAD_ID(current);
2060 did_notify = true;
2061 add_to_entry_list(current, iterator);
2062
2063 // _wait_set_lock protects the wait queue, not the entry_list. We could
2064 // move the add-to-entry_list operation, above, outside the critical section
2065 // protected by _wait_set_lock. In practice that's not useful. With the
2066 // exception of wait() timeouts and interrupts the monitor owner
2067 // is the only thread that grabs _wait_set_lock. There's almost no contention
2068 // on _wait_set_lock so it's not profitable to reduce the length of the
2069 // critical section.
2070
2071 if (!iterator->is_vthread()) {
2072 iterator->wait_reenter_begin(this);
2073
2074 // Read counter *after* adding the thread to the _entry_list.
2075 // Adding to _entry_list uses Atomic::cmpxchg() which already provides
2076 // a fence that prevents this load from floating up previous store.
2077 if (has_unmounted_vthreads()) {
2078 // Wake up the thread to alleviate some deadlocks cases where the successor
2079 // that will be picked up when this thread releases the monitor is an unmounted
2080 // virtual thread that cannot run due to having run out of carriers. Upon waking
2081 // up, the thread will call reenter_internal() which will use timed-park in case
2082 // there is contention and there are still vthreads in the _entry_list.
2083 // If the target was interrupted or the wait timed-out at the same time, it could
2084 // have reached reenter_internal and read a false value of has_unmounted_vthreads()
2085 // before we added it to the _entry_list above. To fix that, we set _do_timed_park
2086 // which will be read by the target on the next loop iteration in reenter_internal.
2087 iterator->_do_timed_park = true;
2088 JavaThread* t = iterator->thread();
2089 t->_ParkEvent->unpark();
2090 }
2091 }
2092 }
2093 Thread::SpinRelease(&_wait_set_lock);
2094 return did_notify;
2095 }
2096
2097 static void post_monitor_notify_event(EventJavaMonitorNotify* event,
2098 ObjectMonitor* monitor,
2099 int notified_count) {
2100 assert(event != nullptr, "invariant");
2101 assert(monitor != nullptr, "invariant");
2102 const Klass* monitor_klass = monitor->object()->klass();
2103 if (ObjectMonitor::is_jfr_excluded(monitor_klass)) {
2104 return;
2105 }
2106 event->set_monitorClass(monitor_klass);
2107 // Set an address that is 'unique enough', such that events close in
2108 // time and with the same address are likely (but not guaranteed) to
2109 // belong to the same object.
2110 event->set_address((uintptr_t)monitor);
2111 event->set_notifiedCount(notified_count);
2112 event->commit();
2113 }
2114
2115 // Consider: a not-uncommon synchronization bug is to use notify() when
2116 // notifyAll() is more appropriate, potentially resulting in stranded
2117 // threads; this is one example of a lost wakeup. A useful diagnostic
2118 // option is to force all notify() operations to behave as notifyAll().
2119 //
2120 // Note: We can also detect many such problems with a "minimum wait".
2121 // When the "minimum wait" is set to a small non-zero timeout value
2122 // and the program does not hang whereas it did absent "minimum wait",
2123 // that suggests a lost wakeup bug.
2124
2125 void ObjectMonitor::notify(TRAPS) {
2126 JavaThread* current = THREAD;
2127 CHECK_OWNER(); // Throws IMSE if not owner.
2128 if (_wait_set == nullptr) {
2129 return;
2130 }
2131
2132 quick_notify(current);
2133 }
2134
2135 void ObjectMonitor::quick_notify(JavaThread* current) {
2136 assert(has_owner(current), "Precondition");
2137
2138 EventJavaMonitorNotify event;
2139 DTRACE_MONITOR_PROBE(notify, this, object(), current);
2140 int tally = notify_internal(current) ? 1 : 0;
2141
2142 if ((tally > 0) && event.should_commit()) {
2143 post_monitor_notify_event(&event, this, /* notified_count = */ tally);
2144 }
2145 }
2146
2147 // notifyAll() transfers the waiters one-at-a-time from the waitset to
2148 // the entry_list. If the waitset is "ABCD" (where A was added first
2149 // and D last) and the entry_list is ->X->Y->Z. After a notifyAll()
2150 // the waitset will be empty and the entry_list will be
2151 // ->D->C->B->A->X->Y->Z, and the next choosen successor will be Z.
2152
2153 void ObjectMonitor::notifyAll(TRAPS) {
2154 JavaThread* current = THREAD;
2155 CHECK_OWNER(); // Throws IMSE if not owner.
2156 if (_wait_set == nullptr) {
2157 return;
2158 }
2159
2160 quick_notifyAll(current);
2161 }
2162
2163 void ObjectMonitor::quick_notifyAll(JavaThread* current) {
2164 assert(has_owner(current), "Precondition");
2165
2166 EventJavaMonitorNotify event;
2167 DTRACE_MONITOR_PROBE(notifyAll, this, object(), current);
2168 int tally = 0;
2169 while (_wait_set != nullptr) {
2170 if (notify_internal(current)) {
2171 tally++;
2172 }
2173 }
2174
2175 if ((tally > 0) && event.should_commit()) {
2176 post_monitor_notify_event(&event, this, /* notified_count = */ tally);
2177 }
2178 }
2179
2180 void ObjectMonitor::vthread_wait(JavaThread* current, jlong millis, bool interruptible) {
2181 oop vthread = current->vthread();
2182 ObjectWaiter* node = new ObjectWaiter(vthread, this);
2183 node->_is_wait = true;
2184 node->_interruptible = interruptible;
2185 node->TState = ObjectWaiter::TS_WAIT;
2186 java_lang_VirtualThread::set_notified(vthread, false); // Reset notified flag
2187 java_lang_VirtualThread::set_interruptible_wait(vthread, interruptible);
2188
2189 // Enter the waiting queue, which is a circular doubly linked list in this case
2190 // but it could be a priority queue or any data structure.
2191 // _wait_set_lock protects the wait queue. Normally the wait queue is accessed only
2192 // by the owner of the monitor *except* in the case where park()
2193 // returns because of a timeout or interrupt. Contention is exceptionally rare
2194 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
2195
2196 Thread::SpinAcquire(&_wait_set_lock);
2197 add_waiter(node);
2198 Thread::SpinRelease(&_wait_set_lock);
2199
2200 node->_recursions = _recursions; // record the old recursion count
2201 _recursions = 0; // set the recursion level to be 0
2202 _waiters++; // increment the number of waiters
2203 exit(current); // exit the monitor
2204 guarantee(!has_owner(current), "invariant");
2205
2206 assert(java_lang_VirtualThread::state(vthread) == java_lang_VirtualThread::RUNNING, "wrong state for vthread");
2207 java_lang_VirtualThread::set_state(vthread, millis == 0 ? java_lang_VirtualThread::WAITING : java_lang_VirtualThread::TIMED_WAITING);
2208 java_lang_VirtualThread::set_timeout(vthread, millis);
2209
2210 // Save the ObjectWaiter* in the vthread since we will need it when resuming execution.
2211 java_lang_VirtualThread::set_objectWaiter(vthread, node);
2212 }
2213
2214 bool ObjectMonitor::vthread_wait_reenter(JavaThread* current, ObjectWaiter* node, ContinuationWrapper& cont) {
2215 // The first time we run after being preempted on Object.wait() we
2216 // need to check if we were interrupted or the wait timed-out, and
2217 // in that case remove ourselves from the _wait_set queue.
2218 if (node->TState == ObjectWaiter::TS_WAIT) {
2219 Thread::SpinAcquire(&_wait_set_lock);
2220 if (node->TState == ObjectWaiter::TS_WAIT) {
2221 dequeue_specific_waiter(node); // unlink from wait_set
2222 assert(!node->_notified, "invariant");
2223 node->TState = ObjectWaiter::TS_RUN;
2224 }
2225 Thread::SpinRelease(&_wait_set_lock);
2226 }
2227
2228 // If this was an interrupted case, set the _interrupted boolean so that
2229 // once we re-acquire the monitor we know if we need to throw IE or not.
2230 ObjectWaiter::TStates state = node->TState;
2231 bool was_notified = state == ObjectWaiter::TS_ENTER;
2232 assert(was_notified || state == ObjectWaiter::TS_RUN, "");
2233 node->_interrupted = node->_interruptible && !was_notified && current->is_interrupted(false);
2234
2235 // Post JFR and JVMTI events. If non-interruptible we are in
2236 // ObjectLocker case so we don't post anything.
2237 EventJavaMonitorWait wait_event;
2238 if (node->_interruptible && (wait_event.should_commit() || JvmtiExport::should_post_monitor_waited())) {
2239 vthread_monitor_waited_event(current, node, cont, &wait_event, !was_notified && !node->_interrupted);
2240 }
2241
2242 // Mark that we are at reenter so that we don't call this method again.
2243 node->_at_reenter = true;
2244
2245 if (!was_notified) {
2246 bool acquired = vthread_monitor_enter(current, node);
2247 if (acquired) {
2248 guarantee(_recursions == 0, "invariant");
2249 _recursions = node->_recursions; // restore the old recursion count
2250 _waiters--; // decrement the number of waiters
2251
2252 if (node->_interrupted) {
2253 // We will throw at thaw end after finishing the mount transition.
2254 current->set_pending_interrupted_exception(true);
2255 }
2256
2257 delete node;
2258 // Clear the ObjectWaiter* from the vthread.
2259 java_lang_VirtualThread::set_objectWaiter(current->vthread(), nullptr);
2260 return true;
2261 }
2262 } else {
2263 // Already moved to _entry_list by notifier, so just add to contentions.
2264 add_to_contentions(1);
2265 }
2266 return false;
2267 }
2268
2269 // -----------------------------------------------------------------------------
2270 // Adaptive Spinning Support
2271 //
2272 // Adaptive spin-then-block - rational spinning
2273 //
2274 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
2275 // algorithm.
2276 //
2277 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
2278 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
2279 // (duration) or we can fix the count at approximately the duration of
2280 // a context switch and vary the frequency. Of course we could also
2281 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
2282 // For a description of 'Adaptive spin-then-block mutual exclusion in
2283 // multi-threaded processing,' see U.S. Pat. No. 8046758.
2284 //
2285 // This implementation varies the duration "D", where D varies with
2286 // the success rate of recent spin attempts. (D is capped at approximately
2287 // length of a round-trip context switch). The success rate for recent
2288 // spin attempts is a good predictor of the success rate of future spin
2289 // attempts. The mechanism adapts automatically to varying critical
2290 // section length (lock modality), system load and degree of parallelism.
2291 // D is maintained per-monitor in _SpinDuration and is initialized
2292 // optimistically. Spin frequency is fixed at 100%.
2293 //
2294 // Note that _SpinDuration is volatile, but we update it without locks
2295 // or atomics. The code is designed so that _SpinDuration stays within
2296 // a reasonable range even in the presence of races. The arithmetic
2297 // operations on _SpinDuration are closed over the domain of legal values,
2298 // so at worst a race will install and older but still legal value.
2299 // At the very worst this introduces some apparent non-determinism.
2300 // We might spin when we shouldn't or vice-versa, but since the spin
2301 // count are relatively short, even in the worst case, the effect is harmless.
2302 //
2303 // Care must be taken that a low "D" value does not become an
2304 // an absorbing state. Transient spinning failures -- when spinning
2305 // is overall profitable -- should not cause the system to converge
2306 // on low "D" values. We want spinning to be stable and predictable
2307 // and fairly responsive to change and at the same time we don't want
2308 // it to oscillate, become metastable, be "too" non-deterministic,
2309 // or converge on or enter undesirable stable absorbing states.
2310 //
2311 // We implement a feedback-based control system -- using past behavior
2312 // to predict future behavior. We face two issues: (a) if the
2313 // input signal is random then the spin predictor won't provide optimal
2314 // results, and (b) if the signal frequency is too high then the control
2315 // system, which has some natural response lag, will "chase" the signal.
2316 // (b) can arise from multimodal lock hold times. Transient preemption
2317 // can also result in apparent bimodal lock hold times.
2318 // Although sub-optimal, neither condition is particularly harmful, as
2319 // in the worst-case we'll spin when we shouldn't or vice-versa.
2320 // The maximum spin duration is rather short so the failure modes aren't bad.
2321 // To be conservative, I've tuned the gain in system to bias toward
2322 // _not spinning. Relatedly, the system can sometimes enter a mode where it
2323 // "rings" or oscillates between spinning and not spinning. This happens
2324 // when spinning is just on the cusp of profitability, however, so the
2325 // situation is not dire. The state is benign -- there's no need to add
2326 // hysteresis control to damp the transition rate between spinning and
2327 // not spinning.
2328
2329 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool
2330
2331 static int Knob_Bonus = 100; // spin success bonus
2332 static int Knob_Penalty = 200; // spin failure penalty
2333 static int Knob_Poverty = 1000;
2334 static int Knob_FixedSpin = 0;
2335 static int Knob_PreSpin = 10; // 20-100 likely better, but it's not better in my testing.
2336
2337 inline static int adjust_up(int spin_duration) {
2338 int x = spin_duration;
2339 if (x < ObjectMonitor::Knob_SpinLimit) {
2340 if (x < Knob_Poverty) {
2341 x = Knob_Poverty;
2342 }
2343 return x + Knob_Bonus;
2344 } else {
2345 return spin_duration;
2346 }
2347 }
2348
2349 inline static int adjust_down(int spin_duration) {
2350 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2351 // AIMD is globally stable.
2352 int x = spin_duration;
2353 if (x > 0) {
2354 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2355 // This is globally sample and tends to damp the response.
2356 x -= Knob_Penalty;
2357 if (x < 0) { x = 0; }
2358 return x;
2359 } else {
2360 return spin_duration;
2361 }
2362 }
2363
2364 bool ObjectMonitor::short_fixed_spin(JavaThread* current, int spin_count, bool adapt) {
2365 for (int ctr = 0; ctr < spin_count; ctr++) {
2366 TryLockResult status = try_lock(current);
2367 if (status == TryLockResult::Success) {
2368 if (adapt) {
2369 _SpinDuration = adjust_up(_SpinDuration);
2370 }
2371 return true;
2372 } else if (status == TryLockResult::Interference) {
2373 break;
2374 }
2375 SpinPause();
2376 }
2377 return false;
2378 }
2379
2380 // Spinning: Fixed frequency (100%), vary duration
2381 bool ObjectMonitor::try_spin(JavaThread* current) {
2382
2383 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
2384 int knob_fixed_spin = Knob_FixedSpin; // 0 (don't spin: default), 2000 good test
2385 if (knob_fixed_spin > 0) {
2386 return short_fixed_spin(current, knob_fixed_spin, false);
2387 }
2388
2389 // Admission control - verify preconditions for spinning
2390 //
2391 // We always spin a little bit, just to prevent _SpinDuration == 0 from
2392 // becoming an absorbing state. Put another way, we spin briefly to
2393 // sample, just in case the system load, parallelism, contention, or lock
2394 // modality changed.
2395
2396 int knob_pre_spin = Knob_PreSpin; // 10 (default), 100, 1000 or 2000
2397 if (short_fixed_spin(current, knob_pre_spin, true)) {
2398 return true;
2399 }
2400
2401 //
2402 // Consider the following alternative:
2403 // Periodically set _SpinDuration = _SpinLimit and try a long/full
2404 // spin attempt. "Periodically" might mean after a tally of
2405 // the # of failed spin attempts (or iterations) reaches some threshold.
2406 // This takes us into the realm of 1-out-of-N spinning, where we
2407 // hold the duration constant but vary the frequency.
2408
2409 int ctr = _SpinDuration;
2410 if (ctr <= 0) return false;
2411
2412 // We're good to spin ... spin ingress.
2413 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
2414 // when preparing to LD...CAS _owner, etc and the CAS is likely
2415 // to succeed.
2416 if (!has_successor()) {
2417 set_successor(current);
2418 }
2419 int64_t prv = NO_OWNER;
2420
2421 // There are three ways to exit the following loop:
2422 // 1. A successful spin where this thread has acquired the lock.
2423 // 2. Spin failure with prejudice
2424 // 3. Spin failure without prejudice
2425
2426 while (--ctr >= 0) {
2427
2428 // Periodic polling -- Check for pending GC
2429 // Threads may spin while they're unsafe.
2430 // We don't want spinning threads to delay the JVM from reaching
2431 // a stop-the-world safepoint or to steal cycles from GC.
2432 // If we detect a pending safepoint we abort in order that
2433 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
2434 // this thread, if safe, doesn't steal cycles from GC.
2435 // This is in keeping with the "no loitering in runtime" rule.
2436 // We periodically check to see if there's a safepoint pending.
2437 if ((ctr & 0xFF) == 0) {
2438 // Can't call SafepointMechanism::should_process() since that
2439 // might update the poll values and we could be in a thread_blocked
2440 // state here which is not allowed so just check the poll.
2441 if (SafepointMechanism::local_poll_armed(current)) {
2442 break;
2443 }
2444 SpinPause();
2445 }
2446
2447 // Probe _owner with TATAS
2448 // If this thread observes the monitor transition or flicker
2449 // from locked to unlocked to locked, then the odds that this
2450 // thread will acquire the lock in this spin attempt go down
2451 // considerably. The same argument applies if the CAS fails
2452 // or if we observe _owner change from one non-null value to
2453 // another non-null value. In such cases we might abort
2454 // the spin without prejudice or apply a "penalty" to the
2455 // spin count-down variable "ctr", reducing it by 100, say.
2456
2457 int64_t ox = owner_raw();
2458 if (ox == NO_OWNER) {
2459 ox = try_set_owner_from(NO_OWNER, current);
2460 if (ox == NO_OWNER) {
2461 // The CAS succeeded -- this thread acquired ownership
2462 // Take care of some bookkeeping to exit spin state.
2463 if (has_successor(current)) {
2464 clear_successor();
2465 }
2466
2467 // Increase _SpinDuration :
2468 // The spin was successful (profitable) so we tend toward
2469 // longer spin attempts in the future.
2470 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2471 // If we acquired the lock early in the spin cycle it
2472 // makes sense to increase _SpinDuration proportionally.
2473 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2474 _SpinDuration = adjust_up(_SpinDuration);
2475 return true;
2476 }
2477
2478 // The CAS failed ... we can take any of the following actions:
2479 // * penalize: ctr -= CASPenalty
2480 // * exit spin with prejudice -- abort without adapting spinner
2481 // * exit spin without prejudice.
2482 // * Since CAS is high-latency, retry again immediately.
2483 break;
2484 }
2485
2486 // Did lock ownership change hands ?
2487 if (ox != prv && prv != NO_OWNER) {
2488 break;
2489 }
2490 prv = ox;
2491
2492 if (!has_successor()) {
2493 set_successor(current);
2494 }
2495 }
2496
2497 // Spin failed with prejudice -- reduce _SpinDuration.
2498 if (ctr < 0) {
2499 _SpinDuration = adjust_down(_SpinDuration);
2500 }
2501
2502 if (has_successor(current)) {
2503 clear_successor();
2504 // Invariant: after setting succ=null a contending thread
2505 // must recheck-retry _owner before parking. This usually happens
2506 // in the normal usage of try_spin(), but it's safest
2507 // to make try_spin() as foolproof as possible.
2508 OrderAccess::fence();
2509 if (try_lock(current) == TryLockResult::Success) {
2510 return true;
2511 }
2512 }
2513
2514 return false;
2515 }
2516
2517
2518 // -----------------------------------------------------------------------------
2519 // wait_set management ...
2520
2521 ObjectWaiter::ObjectWaiter(JavaThread* current) {
2522 _next = nullptr;
2523 _prev = nullptr;
2524 _thread = current;
2525 _monitor = nullptr;
2526 _notifier_tid = 0;
2527 _recursions = 0;
2528 TState = TS_RUN;
2529 _notified = false;
2530 _is_wait = false;
2531 _at_reenter = false;
2532 _interrupted = false;
2533 _do_timed_park = false;
2534 _active = false;
2535 }
2536
2537 ObjectWaiter::ObjectWaiter(oop vthread, ObjectMonitor* mon) : ObjectWaiter(nullptr) {
2538 assert(oopDesc::is_oop(vthread), "");
2539 _vthread = OopHandle(JavaThread::thread_oop_storage(), vthread);
2540 _monitor = mon;
2541 }
2542
2543 ObjectWaiter::~ObjectWaiter() {
2544 if (is_vthread()) {
2545 assert(vthread() != nullptr, "");
2546 _vthread.release(JavaThread::thread_oop_storage());
2547 }
2548 }
2549
2550 oop ObjectWaiter::vthread() const {
2551 return _vthread.resolve();
2552 }
2553
2554 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2555 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(_thread, mon);
2556 }
2557
2558 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2559 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(_thread, _active);
2560 }
2561
2562 inline void ObjectMonitor::add_waiter(ObjectWaiter* node) {
2563 assert(node != nullptr, "should not add null node");
2564 assert(node->_prev == nullptr, "node already in list");
2565 assert(node->_next == nullptr, "node already in list");
2566 // put node at end of queue (circular doubly linked list)
2567 if (_wait_set == nullptr) {
2568 _wait_set = node;
2569 node->_prev = node;
2570 node->_next = node;
2571 } else {
2572 ObjectWaiter* head = _wait_set;
2573 ObjectWaiter* tail = head->_prev;
2574 assert(tail->_next == head, "invariant check");
2575 tail->_next = node;
2576 head->_prev = node;
2577 node->_next = head;
2578 node->_prev = tail;
2579 }
2580 }
2581
2582 inline ObjectWaiter* ObjectMonitor::dequeue_waiter() {
2583 // dequeue the very first waiter
2584 ObjectWaiter* waiter = _wait_set;
2585 if (waiter) {
2586 dequeue_specific_waiter(waiter);
2587 }
2588 return waiter;
2589 }
2590
2591 inline void ObjectMonitor::dequeue_specific_waiter(ObjectWaiter* node) {
2592 assert(node != nullptr, "should not dequeue nullptr node");
2593 assert(node->_prev != nullptr, "node already removed from list");
2594 assert(node->_next != nullptr, "node already removed from list");
2595 // when the waiter has woken up because of interrupt,
2596 // timeout or other spurious wake-up, dequeue the
2597 // waiter from waiting list
2598 ObjectWaiter* next = node->_next;
2599 if (next == node) {
2600 assert(node->_prev == node, "invariant check");
2601 _wait_set = nullptr;
2602 } else {
2603 ObjectWaiter* prev = node->_prev;
2604 assert(prev->_next == node, "invariant check");
2605 assert(next->_prev == node, "invariant check");
2606 next->_prev = prev;
2607 prev->_next = next;
2608 if (_wait_set == node) {
2609 _wait_set = next;
2610 }
2611 }
2612 node->_next = nullptr;
2613 node->_prev = nullptr;
2614 }
2615
2616 // -----------------------------------------------------------------------------
2617
2618 // One-shot global initialization for the sync subsystem.
2619 // We could also defer initialization and initialize on-demand
2620 // the first time we call ObjectSynchronizer::inflate().
2621 // Initialization would be protected - like so many things - by
2622 // the MonitorCache_lock.
2623
2624 void ObjectMonitor::Initialize() {
2625 assert(!InitDone, "invariant");
2626
2627 if (!os::is_MP()) {
2628 Knob_SpinLimit = 0;
2629 Knob_PreSpin = 0;
2630 Knob_FixedSpin = -1;
2631 }
2632
2633 _oop_storage = OopStorageSet::create_weak("ObjectSynchronizer Weak", mtSynchronizer);
2634
2635 DEBUG_ONLY(InitDone = true;)
2636 }
2637
2638 // We can't call this during Initialize() because BarrierSet needs to be set.
2639 void ObjectMonitor::Initialize2() {
2640 _vthread_list_head = OopHandle(JavaThread::thread_oop_storage(), nullptr);
2641 _vthread_unparker_ParkEvent = ParkEvent::Allocate(nullptr);
2642 }
2643
2644 void ObjectMonitor::print_on(outputStream* st) const {
2645 // The minimal things to print for markWord printing, more can be added for debugging and logging.
2646 st->print("{contentions=0x%08x,waiters=0x%08x"
2647 ",recursions=%zd,owner=" INT64_FORMAT "}",
2648 contentions(), waiters(), recursions(),
2649 owner_raw());
2650 }
2651 void ObjectMonitor::print() const { print_on(tty); }
2652
2653 #ifdef ASSERT
2654 // Print the ObjectMonitor like a debugger would:
2655 //
2656 // (ObjectMonitor) 0x00007fdfb6012e40 = {
2657 // _metadata = 0x0000000000000001
2658 // _object = 0x000000070ff45fd0
2659 // _pad_buf0 = {
2660 // [0] = '\0'
2661 // ...
2662 // [43] = '\0'
2663 // }
2664 // _owner = 0x0000000000000000
2665 // _previous_owner_tid = 0
2666 // _pad_buf1 = {
2667 // [0] = '\0'
2668 // ...
2669 // [47] = '\0'
2670 // }
2671 // _next_om = 0x0000000000000000
2672 // _recursions = 0
2673 // _entry_list = 0x0000000000000000
2674 // _entry_list_tail = 0x0000000000000000
2675 // _succ = 0x0000000000000000
2676 // _SpinDuration = 5000
2677 // _contentions = 0
2678 // _wait_set = 0x0000700009756248
2679 // _waiters = 1
2680 // _wait_set_lock = 0
2681 // }
2682 //
2683 void ObjectMonitor::print_debug_style_on(outputStream* st) const {
2684 st->print_cr("(ObjectMonitor*) " INTPTR_FORMAT " = {", p2i(this));
2685 st->print_cr(" _metadata = " INTPTR_FORMAT, _metadata);
2686 st->print_cr(" _object = " INTPTR_FORMAT, p2i(object_peek()));
2687 st->print_cr(" _pad_buf0 = {");
2688 st->print_cr(" [0] = '\\0'");
2689 st->print_cr(" ...");
2690 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf0) - 1);
2691 st->print_cr(" }");
2692 st->print_cr(" _owner = " INT64_FORMAT, owner_raw());
2693 st->print_cr(" _previous_owner_tid = " UINT64_FORMAT, _previous_owner_tid);
2694 st->print_cr(" _pad_buf1 = {");
2695 st->print_cr(" [0] = '\\0'");
2696 st->print_cr(" ...");
2697 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf1) - 1);
2698 st->print_cr(" }");
2699 st->print_cr(" _next_om = " INTPTR_FORMAT, p2i(next_om()));
2700 st->print_cr(" _recursions = %zd", _recursions);
2701 st->print_cr(" _entry_list = " INTPTR_FORMAT, p2i(_entry_list));
2702 st->print_cr(" _entry_list_tail = " INTPTR_FORMAT, p2i(_entry_list_tail));
2703 st->print_cr(" _succ = " INT64_FORMAT, successor());
2704 st->print_cr(" _SpinDuration = %d", _SpinDuration);
2705 st->print_cr(" _contentions = %d", contentions());
2706 st->print_cr(" _unmounted_vthreads = " INT64_FORMAT, _unmounted_vthreads);
2707 st->print_cr(" _wait_set = " INTPTR_FORMAT, p2i(_wait_set));
2708 st->print_cr(" _waiters = %d", _waiters);
2709 st->print_cr(" _wait_set_lock = %d", _wait_set_lock);
2710 st->print_cr("}");
2711 }
2712 #endif