1 /*
2 * Copyright (c) 1998, 2025, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 #include "classfile/vmSymbols.hpp"
26 #include "gc/shared/oopStorage.hpp"
27 #include "gc/shared/oopStorageSet.hpp"
28 #include "jfr/jfrEvents.hpp"
29 #include "jfr/support/jfrThreadId.hpp"
30 #include "logging/log.hpp"
31 #include "logging/logStream.hpp"
32 #include "memory/allocation.inline.hpp"
33 #include "memory/resourceArea.hpp"
34 #include "oops/markWord.hpp"
35 #include "oops/oop.inline.hpp"
36 #include "oops/oopHandle.inline.hpp"
37 #include "oops/weakHandle.inline.hpp"
38 #include "prims/jvmtiDeferredUpdates.hpp"
39 #include "prims/jvmtiExport.hpp"
40 #include "runtime/atomicAccess.hpp"
41 #include "runtime/continuationWrapper.inline.hpp"
42 #include "runtime/globals.hpp"
43 #include "runtime/handles.inline.hpp"
44 #include "runtime/interfaceSupport.inline.hpp"
45 #include "runtime/javaThread.inline.hpp"
46 #include "runtime/lightweightSynchronizer.hpp"
47 #include "runtime/mutexLocker.hpp"
48 #include "runtime/objectMonitor.inline.hpp"
49 #include "runtime/orderAccess.hpp"
50 #include "runtime/osThread.hpp"
51 #include "runtime/safefetch.hpp"
52 #include "runtime/safepointMechanism.inline.hpp"
53 #include "runtime/sharedRuntime.hpp"
54 #include "runtime/threads.hpp"
55 #include "services/threadService.hpp"
56 #include "utilities/debug.hpp"
57 #include "utilities/dtrace.hpp"
58 #include "utilities/globalCounter.inline.hpp"
59 #include "utilities/globalDefinitions.hpp"
60 #include "utilities/macros.hpp"
61 #include "utilities/preserveException.hpp"
62 #if INCLUDE_JFR
63 #include "jfr/support/jfrFlush.hpp"
64 #endif
65
66 #ifdef DTRACE_ENABLED
67
68 // Only bother with this argument setup if dtrace is available
69 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
70
71
72 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
73 char* bytes = nullptr; \
74 int len = 0; \
75 jlong jtid = SharedRuntime::get_java_tid(thread); \
76 Symbol* klassname = obj->klass()->name(); \
77 if (klassname != nullptr) { \
78 bytes = (char*)klassname->bytes(); \
79 len = klassname->utf8_length(); \
80 }
81
82 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
83 { \
84 if (DTraceMonitorProbes) { \
85 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
86 HOTSPOT_MONITOR_WAIT(jtid, \
87 (monitor), bytes, len, (millis)); \
88 } \
89 }
90
91 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
92 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
93 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
94 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
95 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
96
97 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
98 { \
99 if (DTraceMonitorProbes) { \
100 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
101 HOTSPOT_MONITOR_##probe(jtid, \
102 (uintptr_t)(monitor), bytes, len); \
103 } \
104 }
105
106 #else // ndef DTRACE_ENABLED
107
108 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
109 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
110
111 #endif // ndef DTRACE_ENABLED
112
113 DEBUG_ONLY(static volatile bool InitDone = false;)
114
115 OopStorage* ObjectMonitor::_oop_storage = nullptr;
116
117 OopHandle ObjectMonitor::_vthread_list_head;
118 ParkEvent* ObjectMonitor::_vthread_unparker_ParkEvent = nullptr;
119
120 static const jlong MAX_RECHECK_INTERVAL = 1000;
121
122 // -----------------------------------------------------------------------------
123 // Theory of operations -- Monitors lists, thread residency, etc:
124 //
125 // * A thread acquires ownership of a monitor by successfully
126 // CAS()ing the _owner field from NO_OWNER/DEFLATER_MARKER to
127 // its owner_id (return value from owner_id_from()).
128 //
129 // * Invariant: A thread appears on at most one monitor list --
130 // entry_list or wait_set -- at any one time.
131 //
132 // * Contending threads "push" themselves onto the entry_list with CAS
133 // and then spin/park.
134 // If the thread is a virtual thread it will first attempt to
135 // unmount itself. The virtual thread will first try to freeze
136 // all frames in the heap. If the operation fails it will just
137 // follow the regular path for platform threads. If the operation
138 // succeeds, it will push itself onto the entry_list with CAS and then
139 // return back to Java to continue the unmount logic.
140 //
141 // * After a contending thread eventually acquires the lock it must
142 // dequeue itself from the entry_list.
143 //
144 // * The exiting thread identifies and unparks an "heir presumptive"
145 // tentative successor thread on the entry_list. In case the successor
146 // is an unmounted virtual thread, the exiting thread will first try
147 // to add it to the list of vthreads waiting to be unblocked, and on
148 // success it will unpark the special unblocker thread instead, which
149 // will be in charge of submitting the vthread back to the scheduler
150 // queue. Critically, the exiting thread doesn't unlink the successor
151 // thread from the entry_list. After having been unparked/re-scheduled,
152 // the wakee will recontend for ownership of the monitor. The successor
153 // (wakee) will either acquire the lock or re-park/unmount itself.
154 //
155 // Succession is provided for by a policy of competitive handoff.
156 // The exiting thread does _not_ grant or pass ownership to the
157 // successor thread. (This is also referred to as "handoff succession").
158 // Instead the exiting thread releases ownership and possibly wakes
159 // a successor, so the successor can (re)compete for ownership of the lock.
160 //
161 // * The entry_list forms a queue of threads stalled trying to acquire
162 // the lock. Within the entry_list the next pointers always form a
163 // consistent singly linked list. At unlock-time when the unlocking
164 // thread notices that the tail of the entry_list is not known, we
165 // convert the singly linked entry_list into a doubly linked list by
166 // assigning the prev pointers and the entry_list_tail pointer.
167 //
168 // Example:
169 //
170 // The first contending thread that "pushed" itself onto entry_list,
171 // will be the last thread in the list. Each newly pushed thread in
172 // entry_list will be linked through its next pointer, and have its
173 // prev pointer set to null. Thus pushing six threads A-F (in that
174 // order) onto entry_list, will form a singly linked list, see 1)
175 // below.
176 //
177 // 1) entry_list ->F->E->D->C->B->A->null
178 // entry_list_tail ->null
179 //
180 // Since the successor is chosen in FIFO order, the exiting thread
181 // needs to find the tail of the entry_list. This is done by walking
182 // from the entry_list head. While walking the list we also assign
183 // the prev pointers of each thread, essentially forming a doubly
184 // linked list, see 2) below.
185 //
186 // 2) entry_list ->F<=>E<=>D<=>C<=>B<=>A->null
187 // entry_list_tail ----------------------^
188 //
189 // Once we have formed a doubly linked list it's easy to find the
190 // successor (A), wake it up, have it remove itself, and update the
191 // tail pointer, as seen in and 3) below.
192 //
193 // 3) entry_list ->F<=>E<=>D<=>C<=>B->null
194 // entry_list_tail ------------------^
195 //
196 // At any time new threads can add themselves to the entry_list, see
197 // 4) below.
198 //
199 // 4) entry_list ->I->H->G->F<=>E<=>D->null
200 // entry_list_tail -------------------^
201 //
202 // At some point in time the thread (F) that wants to remove itself
203 // from the end of the list, will not have any prev pointer, see 5)
204 // below.
205 //
206 // 5) entry_list ->I->H->G->F->null
207 // entry_list_tail -----------^
208 //
209 // To resolve this we just start walking from the entry_list head
210 // again, forming a new doubly linked list, before removing the
211 // thread (F), see 6) and 7) below.
212 //
213 // 6) entry_list ->I<=>H<=>G<=>F->null
214 // entry_list_tail --------------^
215 //
216 // 7) entry_list ->I<=>H<=>G->null
217 // entry_list_tail ----------^
218 //
219 // * The monitor itself protects all of the operations on the
220 // entry_list except for the CAS of a new arrival to the head. Only
221 // the monitor owner can read or write the prev links (e.g. to
222 // remove itself) or update the tail.
223 //
224 // * The monitor entry list operations avoid locks, but strictly speaking
225 // they're not lock-free. Enter is lock-free, exit is not.
226 // For a description of 'Methods and apparatus providing non-blocking access
227 // to a resource,' see U.S. Pat. No. 7844973.
228 //
229 // * The entry_list can have multiple concurrent "pushers" but only
230 // one concurrent detaching thread. There is no ABA-problem with
231 // this usage of CAS.
232 //
233 // * As long as the entry_list_tail is known the odds are good that we
234 // should be able to dequeue after acquisition (in the ::enter()
235 // epilogue) in constant-time. This is good since a key desideratum
236 // is to minimize queue & monitor metadata manipulation that occurs
237 // while holding the monitor lock -- that is, we want to minimize
238 // monitor lock holds times. Note that even a small amount of fixed
239 // spinning will greatly reduce the # of enqueue-dequeue operations
240 // on entry_list. That is, spinning relieves contention on the
241 // "inner" locks and monitor metadata.
242 //
243 // Insert and delete operations may not operate in constant-time if
244 // we have interference because some other thread is adding or
245 // removing the head element of entry_list or if we need to convert
246 // the singly linked entry_list into a doubly linked list to find the
247 // tail.
248 //
249 // * The monitor synchronization subsystem avoids the use of native
250 // synchronization primitives except for the narrow platform-specific
251 // park-unpark abstraction. See the comments in os_posix.cpp regarding
252 // the semantics of park-unpark. Put another way, this monitor implementation
253 // depends only on atomic operations and park-unpark.
254 //
255 // * Waiting threads reside on the wait_set list -- wait() puts
256 // the caller onto the wait_set.
257 //
258 // * notify() or notifyAll() simply transfers threads from the wait_set
259 // to the entry_list. Subsequent exit() operations will
260 // unpark/re-schedule the notifyee. Unparking/re-scheduling a
261 // notifyee in notify() is inefficient - it's likely the notifyee
262 // would simply impale itself on the lock held by the notifier.
263
264 // Check that object() and set_object() are called from the right context:
265 static void check_object_context() {
266 #ifdef ASSERT
267 Thread* self = Thread::current();
268 if (self->is_Java_thread()) {
269 // Mostly called from JavaThreads so sanity check the thread state.
270 JavaThread* jt = JavaThread::cast(self);
271 switch (jt->thread_state()) {
272 case _thread_in_vm: // the usual case
273 case _thread_in_Java: // during deopt
274 break;
275 default:
276 fatal("called from an unsafe thread state");
277 }
278 assert(jt->is_active_Java_thread(), "must be active JavaThread");
279 } else {
280 // However, ThreadService::get_current_contended_monitor()
281 // can call here via the VMThread so sanity check it.
282 assert(self->is_VM_thread(), "must be");
283 }
284 #endif // ASSERT
285 }
286
287 ObjectMonitor::ObjectMonitor(oop object) :
288 _metadata(0),
289 _object(_oop_storage, object),
290 _owner(NO_OWNER),
291 _previous_owner_tid(0),
292 _next_om(nullptr),
293 _recursions(0),
294 _entry_list(nullptr),
295 _entry_list_tail(nullptr),
296 _succ(NO_OWNER),
297 _SpinDuration(ObjectMonitor::Knob_SpinLimit),
298 _contentions(0),
299 _unmounted_vthreads(0),
300 _wait_set(nullptr),
301 _waiters(0),
302 _wait_set_lock(0)
303 { }
304
305 ObjectMonitor::~ObjectMonitor() {
306 _object.release(_oop_storage);
307 _object_strong.release(JavaThread::thread_oop_storage());
308 }
309
310 oop ObjectMonitor::object() const {
311 check_object_context();
312 return _object.resolve();
313 }
314
315 // Keep object protected during ObjectLocker preemption.
316 void ObjectMonitor::set_object_strong() {
317 check_object_context();
318 if (_object_strong.is_empty()) {
319 if (Thread::TrySpinAcquire(&_object_strong_lock)) {
320 if (_object_strong.is_empty()) {
321 assert(_object.resolve() != nullptr, "");
322 _object_strong = OopHandle(JavaThread::thread_oop_storage(), _object.resolve());
323 }
324 Thread::SpinRelease(&_object_strong_lock);
325 }
326 }
327 }
328
329 void ObjectMonitor::ExitOnSuspend::operator()(JavaThread* current) {
330 if (current->is_suspended()) {
331 _om->_recursions = 0;
332 _om->clear_successor();
333 // Don't need a full fence after clearing successor here because of the call to exit().
334 _om->exit(current, false /* not_suspended */);
335 _om_exited = true;
336
337 current->set_current_pending_monitor(_om);
338 }
339 }
340
341 void ObjectMonitor::ClearSuccOnSuspend::operator()(JavaThread* current) {
342 if (current->is_suspended()) {
343 if (_om->has_successor(current)) {
344 _om->clear_successor();
345 OrderAccess::fence(); // always do a full fence when successor is cleared
346 }
347 }
348 }
349
350 #define assert_mark_word_consistency() \
351 assert(UseObjectMonitorTable || object()->mark() == markWord::encode(this), \
352 "object mark must match encoded this: mark=" INTPTR_FORMAT \
353 ", encoded this=" INTPTR_FORMAT, object()->mark().value(), \
354 markWord::encode(this).value());
355
356 // -----------------------------------------------------------------------------
357 // Enter support
358
359 bool ObjectMonitor::enter_is_async_deflating() {
360 if (is_being_async_deflated()) {
361 if (!UseObjectMonitorTable) {
362 const oop l_object = object();
363 if (l_object != nullptr) {
364 // Attempt to restore the header/dmw to the object's header so that
365 // we only retry once if the deflater thread happens to be slow.
366 install_displaced_markword_in_object(l_object);
367 }
368 }
369 return true;
370 }
371
372 return false;
373 }
374
375 bool ObjectMonitor::try_lock_with_contention_mark(JavaThread* locking_thread, ObjectMonitorContentionMark& contention_mark) {
376 assert(contention_mark._monitor == this, "must be");
377 assert(!is_being_async_deflated(), "must be");
378
379 int64_t prev_owner = try_set_owner_from(NO_OWNER, locking_thread);
380 bool success = false;
381
382 if (prev_owner == NO_OWNER) {
383 assert(_recursions == 0, "invariant");
384 success = true;
385 } else if (prev_owner == owner_id_from(locking_thread)) {
386 _recursions++;
387 success = true;
388 } else if (prev_owner == DEFLATER_MARKER) {
389 // Racing with deflation.
390 prev_owner = try_set_owner_from(DEFLATER_MARKER, locking_thread);
391 if (prev_owner == DEFLATER_MARKER) {
392 // We successfully cancelled the in-progress async deflation by
393 // changing owner from DEFLATER_MARKER to current. We now extend
394 // the lifetime of the contention_mark (e.g. contentions++) here
395 // to prevent the deflater thread from winning the last part of
396 // the 2-part async deflation protocol after the regular
397 // decrement occurs when the contention_mark goes out of
398 // scope. ObjectMonitor::deflate_monitor() which is called by
399 // the deflater thread will decrement contentions after it
400 // recognizes that the async deflation was cancelled.
401 contention_mark.extend();
402 success = true;
403 } else if (prev_owner == NO_OWNER) {
404 // At this point we cannot race with deflation as we have both incremented
405 // contentions, seen contention > 0 and seen a DEFLATER_MARKER.
406 // success will only be false if this races with something other than
407 // deflation.
408 prev_owner = try_set_owner_from(NO_OWNER, locking_thread);
409 success = prev_owner == NO_OWNER;
410 }
411 }
412 assert(!success || has_owner(locking_thread), "must be");
413
414 return success;
415 }
416
417 void ObjectMonitor::enter_for_with_contention_mark(JavaThread* locking_thread, ObjectMonitorContentionMark& contention_mark) {
418 // Used by LightweightSynchronizer::inflate_and_enter in deoptimization path to enter for another thread.
419 // The monitor is private to or already owned by locking_thread which must be suspended.
420 // So this code may only contend with deflation.
421 assert(locking_thread == Thread::current() || locking_thread->is_obj_deopt_suspend(), "must be");
422 bool success = try_lock_with_contention_mark(locking_thread, contention_mark);
423
424 assert(success, "Failed to enter_for: locking_thread=" INTPTR_FORMAT
425 ", this=" INTPTR_FORMAT "{owner=" INT64_FORMAT "}",
426 p2i(locking_thread), p2i(this), owner_raw());
427 }
428
429 bool ObjectMonitor::enter_for(JavaThread* locking_thread) {
430 // Used by ObjectSynchronizer::enter_for() to enter for another thread.
431 // The monitor is private to or already owned by locking_thread which must be suspended.
432 // So this code may only contend with deflation.
433 assert(locking_thread == Thread::current() || locking_thread->is_obj_deopt_suspend(), "must be");
434
435 // Block out deflation as soon as possible.
436 ObjectMonitorContentionMark contention_mark(this);
437
438 // Check for deflation.
439 if (enter_is_async_deflating()) {
440 return false;
441 }
442
443 bool success = try_lock_with_contention_mark(locking_thread, contention_mark);
444
445 assert(success, "Failed to enter_for: locking_thread=" INTPTR_FORMAT
446 ", this=" INTPTR_FORMAT "{owner=" INT64_FORMAT "}",
447 p2i(locking_thread), p2i(this), owner_raw());
448 assert(has_owner(locking_thread), "must be");
449 return true;
450 }
451
452 bool ObjectMonitor::try_enter(JavaThread* current, bool check_for_recursion) {
453 // TryLock avoids the CAS and handles deflation.
454 TryLockResult r = try_lock(current);
455 if (r == TryLockResult::Success) {
456 assert(_recursions == 0, "invariant");
457 return true;
458 }
459
460 // If called from SharedRuntime::monitor_exit_helper(), we know that
461 // this thread doesn't already own the lock.
462 if (!check_for_recursion) {
463 return false;
464 }
465
466 if (r == TryLockResult::HasOwner && has_owner(current)) {
467 _recursions++;
468 return true;
469 }
470
471 return false;
472 }
473
474 bool ObjectMonitor::spin_enter(JavaThread* current) {
475 assert(current == JavaThread::current(), "must be");
476
477 // Check for recursion.
478 if (try_enter(current)) {
479 return true;
480 }
481
482 // Check for deflation.
483 if (enter_is_async_deflating()) {
484 return false;
485 }
486
487 // We've encountered genuine contention.
488
489 // Do one round of spinning.
490 // Note that if we acquire the monitor from an initial spin
491 // we forgo posting JVMTI events and firing DTRACE probes.
492 if (try_spin(current)) {
493 assert(has_owner(current), "must be current: owner=" INT64_FORMAT, owner_raw());
494 assert(_recursions == 0, "must be 0: recursions=%zd", _recursions);
495 assert_mark_word_consistency();
496 return true;
497 }
498
499 return false;
500 }
501
502 bool ObjectMonitor::enter(JavaThread* current) {
503 assert(current == JavaThread::current(), "must be");
504
505 if (spin_enter(current)) {
506 return true;
507 }
508
509 assert(!has_owner(current), "invariant");
510 assert(!has_successor(current), "invariant");
511 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
512 assert(current->thread_state() != _thread_blocked, "invariant");
513
514 // Keep is_being_async_deflated stable across the rest of enter
515 ObjectMonitorContentionMark contention_mark(this);
516
517 // Check for deflation.
518 if (enter_is_async_deflating()) {
519 return false;
520 }
521
522 // At this point this ObjectMonitor cannot be deflated, finish contended enter
523 enter_with_contention_mark(current, contention_mark);
524 return true;
525 }
526
527 void ObjectMonitor::notify_contended_enter(JavaThread* current) {
528 current->set_current_pending_monitor(this);
529
530 DTRACE_MONITOR_PROBE(contended__enter, this, object(), current);
531 if (JvmtiExport::should_post_monitor_contended_enter()) {
532 JvmtiExport::post_monitor_contended_enter(current, this);
533
534 // The current thread does not yet own the monitor and does not
535 // yet appear on any queues that would get it made the successor.
536 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
537 // handler cannot accidentally consume an unpark() meant for the
538 // ParkEvent associated with this ObjectMonitor.
539 }
540 }
541
542 void ObjectMonitor::enter_with_contention_mark(JavaThread* current, ObjectMonitorContentionMark &cm) {
543 assert(current == JavaThread::current(), "must be");
544 assert(!has_owner(current), "must be");
545 assert(cm._monitor == this, "must be");
546 assert(!is_being_async_deflated(), "must be");
547
548 JFR_ONLY(JfrConditionalFlush<EventJavaMonitorEnter> flush(current);)
549 EventJavaMonitorEnter enter_event;
550 if (enter_event.is_started()) {
551 enter_event.set_monitorClass(object()->klass());
552 // Set an address that is 'unique enough', such that events close in
553 // time and with the same address are likely (but not guaranteed) to
554 // belong to the same object.
555 enter_event.set_address((uintptr_t)this);
556 }
557 EventVirtualThreadPinned vthread_pinned_event;
558
559 freeze_result result;
560
561 assert(current->current_pending_monitor() == nullptr, "invariant");
562
563 ContinuationEntry* ce = current->last_continuation();
564 bool is_virtual = ce != nullptr && ce->is_virtual_thread();
565 if (is_virtual) {
566 notify_contended_enter(current);
567 result = Continuation::try_preempt(current, ce->cont_oop(current));
568 if (result == freeze_ok) {
569 bool acquired = vthread_monitor_enter(current);
570 if (acquired) {
571 // We actually acquired the monitor while trying to add the vthread to the
572 // _entry_list so cancel preemption. We will still go through the preempt stub
573 // but instead of unmounting we will call thaw to continue execution.
574 current->set_preemption_cancelled(true);
575 if (JvmtiExport::should_post_monitor_contended_entered()) {
576 // We are going to call thaw again after this and finish the VMTS
577 // transition so no need to do it here. We will post the event there.
578 current->set_contended_entered_monitor(this);
579 }
580 }
581 current->set_current_pending_monitor(nullptr);
582 DEBUG_ONLY(int state = java_lang_VirtualThread::state(current->vthread()));
583 assert((acquired && current->preemption_cancelled() && state == java_lang_VirtualThread::RUNNING) ||
584 (!acquired && !current->preemption_cancelled() && state == java_lang_VirtualThread::BLOCKING), "invariant");
585 return;
586 }
587 }
588
589 {
590 // Change java thread status to indicate blocked on monitor enter.
591 JavaThreadBlockedOnMonitorEnterState jtbmes(current, this);
592
593 if (!is_virtual) { // already notified contended_enter for virtual
594 notify_contended_enter(current);
595 }
596 OSThreadContendState osts(current->osthread());
597
598 assert(current->thread_state() == _thread_in_vm, "invariant");
599
600 for (;;) {
601 ExitOnSuspend eos(this);
602 {
603 ThreadBlockInVMPreprocess<ExitOnSuspend> tbivs(current, eos, true /* allow_suspend */);
604 enter_internal(current);
605 current->set_current_pending_monitor(nullptr);
606 // We can go to a safepoint at the end of this block. If we
607 // do a thread dump during that safepoint, then this thread will show
608 // as having "-locked" the monitor, but the OS and java.lang.Thread
609 // states will still report that the thread is blocked trying to
610 // acquire it.
611 // If there is a suspend request, ExitOnSuspend will exit the OM
612 // and set the OM as pending.
613 }
614 if (!eos.exited()) {
615 // ExitOnSuspend did not exit the OM
616 assert(has_owner(current), "invariant");
617 break;
618 }
619 }
620
621 // We've just gotten past the enter-check-for-suspend dance and we now own
622 // the monitor free and clear.
623 }
624
625 assert(contentions() >= 0, "must not be negative: contentions=%d", contentions());
626
627 // Must either set _recursions = 0 or ASSERT _recursions == 0.
628 assert(_recursions == 0, "invariant");
629 assert(has_owner(current), "invariant");
630 assert(!has_successor(current), "invariant");
631 assert_mark_word_consistency();
632
633 // The thread -- now the owner -- is back in vm mode.
634 // Report the glorious news via TI,DTrace and jvmstat.
635 // The probe effect is non-trivial. All the reportage occurs
636 // while we hold the monitor, increasing the length of the critical
637 // section. Amdahl's parallel speedup law comes vividly into play.
638 //
639 // Another option might be to aggregate the events (thread local or
640 // per-monitor aggregation) and defer reporting until a more opportune
641 // time -- such as next time some thread encounters contention but has
642 // yet to acquire the lock. While spinning that thread could
643 // spinning we could increment JVMStat counters, etc.
644
645 DTRACE_MONITOR_PROBE(contended__entered, this, object(), current);
646 if (JvmtiExport::should_post_monitor_contended_entered()) {
647 JvmtiExport::post_monitor_contended_entered(current, this);
648
649 // The current thread already owns the monitor and is not going to
650 // call park() for the remainder of the monitor enter protocol. So
651 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
652 // event handler consumed an unpark() issued by the thread that
653 // just exited the monitor.
654 }
655 if (enter_event.should_commit()) {
656 enter_event.set_previousOwner(_previous_owner_tid);
657 enter_event.commit();
658 }
659
660 if (current->current_waiting_monitor() == nullptr) {
661 ContinuationEntry* ce = current->last_continuation();
662 if (ce != nullptr && ce->is_virtual_thread()) {
663 current->post_vthread_pinned_event(&vthread_pinned_event, "Contended monitor enter", result);
664 }
665 }
666 }
667
668 // Caveat: try_lock() is not necessarily serializing if it returns failure.
669 // Callers must compensate as needed.
670
671 ObjectMonitor::TryLockResult ObjectMonitor::try_lock(JavaThread* current) {
672 int64_t own = owner_raw();
673 int64_t first_own = own;
674
675 for (;;) {
676 if (own == DEFLATER_MARKER) {
677 // Block out deflation as soon as possible.
678 ObjectMonitorContentionMark contention_mark(this);
679
680 // Check for deflation.
681 if (enter_is_async_deflating()) {
682 // Treat deflation as interference.
683 return TryLockResult::Interference;
684 }
685 if (try_lock_with_contention_mark(current, contention_mark)) {
686 assert(_recursions == 0, "invariant");
687 return TryLockResult::Success;
688 } else {
689 // Deflation won or change of owner; dont spin
690 break;
691 }
692 } else if (own == NO_OWNER) {
693 int64_t prev_own = try_set_owner_from(NO_OWNER, current);
694 if (prev_own == NO_OWNER) {
695 assert(_recursions == 0, "invariant");
696 return TryLockResult::Success;
697 } else {
698 // The lock had been free momentarily, but we lost the race to the lock.
699 own = prev_own;
700 }
701 } else {
702 // Retry doesn't make as much sense because the lock was just acquired.
703 break;
704 }
705 }
706 return first_own == own ? TryLockResult::HasOwner : TryLockResult::Interference;
707 }
708
709 // Push "current" onto the head of the _entry_list. Once on _entry_list,
710 // current stays on-queue until it acquires the lock.
711 void ObjectMonitor::add_to_entry_list(JavaThread* current, ObjectWaiter* node) {
712 node->_prev = nullptr;
713 node->TState = ObjectWaiter::TS_ENTER;
714
715 for (;;) {
716 ObjectWaiter* head = AtomicAccess::load(&_entry_list);
717 node->_next = head;
718 if (AtomicAccess::cmpxchg(&_entry_list, head, node) == head) {
719 return;
720 }
721 }
722 }
723
724 // Push "current" onto the head of the entry_list.
725 // If the _entry_list was changed during our push operation, we try to
726 // lock the monitor. Returns true if we locked the monitor, and false
727 // if we added current to _entry_list. Once on _entry_list, current
728 // stays on-queue until it acquires the lock.
729 bool ObjectMonitor::try_lock_or_add_to_entry_list(JavaThread* current, ObjectWaiter* node) {
730 assert(node->TState == ObjectWaiter::TS_RUN, "");
731 node->_prev = nullptr;
732 node->TState = ObjectWaiter::TS_ENTER;
733
734 for (;;) {
735 ObjectWaiter* head = AtomicAccess::load(&_entry_list);
736 node->_next = head;
737 if (AtomicAccess::cmpxchg(&_entry_list, head, node) == head) {
738 return false;
739 }
740
741 // Interference - the CAS failed because _entry_list changed. Before
742 // retrying the CAS retry taking the lock as it may now be free.
743 if (try_lock(current) == TryLockResult::Success) {
744 assert(!has_successor(current), "invariant");
745 assert(has_owner(current), "invariant");
746 node->TState = ObjectWaiter::TS_RUN;
747 return true;
748 }
749 }
750 }
751
752 static void post_monitor_deflate_event(EventJavaMonitorDeflate* event,
753 const oop obj) {
754 assert(event != nullptr, "invariant");
755 if (obj == nullptr) {
756 // Accept the case when obj was already garbage-collected.
757 // Emit the event anyway, but without details.
758 event->set_monitorClass(nullptr);
759 event->set_address(0);
760 } else {
761 const Klass* monitor_klass = obj->klass();
762 if (ObjectMonitor::is_jfr_excluded(monitor_klass)) {
763 return;
764 }
765 event->set_monitorClass(monitor_klass);
766 event->set_address((uintptr_t)(void*)obj);
767 }
768 event->commit();
769 }
770
771 // Deflate the specified ObjectMonitor if not in-use. Returns true if it
772 // was deflated and false otherwise.
773 //
774 // The async deflation protocol sets owner to DEFLATER_MARKER and
775 // makes contentions negative as signals to contending threads that
776 // an async deflation is in progress. There are a number of checks
777 // as part of the protocol to make sure that the calling thread has
778 // not lost the race to a contending thread.
779 //
780 // The ObjectMonitor has been successfully async deflated when:
781 // (contentions < 0)
782 // Contending threads that see that condition know to retry their operation.
783 //
784 bool ObjectMonitor::deflate_monitor(Thread* current) {
785 if (is_busy()) {
786 // Easy checks are first - the ObjectMonitor is busy so no deflation.
787 return false;
788 }
789
790 EventJavaMonitorDeflate event;
791
792 const oop obj = object_peek();
793
794 if (obj == nullptr) {
795 // If the object died, we can recycle the monitor without racing with
796 // Java threads. The GC already broke the association with the object.
797 set_owner_from_raw(NO_OWNER, DEFLATER_MARKER);
798 assert(contentions() >= 0, "must be non-negative: contentions=%d", contentions());
799 _contentions = INT_MIN; // minimum negative int
800 } else {
801 // Attempt async deflation protocol.
802
803 // Set a null owner to DEFLATER_MARKER to force any contending thread
804 // through the slow path. This is just the first part of the async
805 // deflation dance.
806 if (try_set_owner_from_raw(NO_OWNER, DEFLATER_MARKER) != NO_OWNER) {
807 // The owner field is no longer null so we lost the race since the
808 // ObjectMonitor is now busy.
809 return false;
810 }
811
812 if (contentions() > 0 || _waiters != 0) {
813 // Another thread has raced to enter the ObjectMonitor after
814 // is_busy() above or has already entered and waited on
815 // it which makes it busy so no deflation. Restore owner to
816 // null if it is still DEFLATER_MARKER.
817 if (try_set_owner_from_raw(DEFLATER_MARKER, NO_OWNER) != DEFLATER_MARKER) {
818 // Deferred decrement for the JT enter_internal() that cancelled the async deflation.
819 add_to_contentions(-1);
820 }
821 return false;
822 }
823
824 // Make a zero contentions field negative to force any contending threads
825 // to retry. This is the second part of the async deflation dance.
826 if (AtomicAccess::cmpxchg(&_contentions, 0, INT_MIN) != 0) {
827 // Contentions was no longer 0 so we lost the race since the
828 // ObjectMonitor is now busy. Restore owner to null if it is
829 // still DEFLATER_MARKER:
830 if (try_set_owner_from_raw(DEFLATER_MARKER, NO_OWNER) != DEFLATER_MARKER) {
831 // Deferred decrement for the JT enter_internal() that cancelled the async deflation.
832 add_to_contentions(-1);
833 }
834 return false;
835 }
836 }
837
838 // Sanity checks for the races:
839 guarantee(owner_is_DEFLATER_MARKER(), "must be deflater marker");
840 guarantee(contentions() < 0, "must be negative: contentions=%d",
841 contentions());
842 guarantee(_waiters == 0, "must be 0: waiters=%d", _waiters);
843 ObjectWaiter* w = AtomicAccess::load(&_entry_list);
844 guarantee(w == nullptr,
845 "must be no entering threads: entry_list=" INTPTR_FORMAT,
846 p2i(w));
847
848 if (obj != nullptr) {
849 if (log_is_enabled(Trace, monitorinflation)) {
850 ResourceMark rm;
851 log_trace(monitorinflation)("deflate_monitor: object=" INTPTR_FORMAT
852 ", mark=" INTPTR_FORMAT ", type='%s'",
853 p2i(obj), obj->mark().value(),
854 obj->klass()->external_name());
855 }
856 }
857
858 if (UseObjectMonitorTable) {
859 LightweightSynchronizer::deflate_monitor(current, obj, this);
860 } else if (obj != nullptr) {
861 // Install the old mark word if nobody else has already done it.
862 install_displaced_markword_in_object(obj);
863 }
864
865 if (event.should_commit()) {
866 post_monitor_deflate_event(&event, obj);
867 }
868
869 // We leave owner == DEFLATER_MARKER and contentions < 0
870 // to force any racing threads to retry.
871 return true; // Success, ObjectMonitor has been deflated.
872 }
873
874 // Install the displaced mark word (dmw) of a deflating ObjectMonitor
875 // into the header of the object associated with the monitor. This
876 // idempotent method is called by a thread that is deflating a
877 // monitor and by other threads that have detected a race with the
878 // deflation process.
879 void ObjectMonitor::install_displaced_markword_in_object(const oop obj) {
880 assert(!UseObjectMonitorTable, "ObjectMonitorTable has no dmw");
881 // This function must only be called when (owner == DEFLATER_MARKER
882 // && contentions <= 0), but we can't guarantee that here because
883 // those values could change when the ObjectMonitor gets moved from
884 // the global free list to a per-thread free list.
885
886 guarantee(obj != nullptr, "must be non-null");
887
888 // Separate loads in is_being_async_deflated(), which is almost always
889 // called before this function, from the load of dmw/header below.
890
891 // _contentions and dmw/header may get written by different threads.
892 // Make sure to observe them in the same order when having several observers.
893 OrderAccess::loadload_for_IRIW();
894
895 const oop l_object = object_peek();
896 if (l_object == nullptr) {
897 // ObjectMonitor's object ref has already been cleared by async
898 // deflation or GC so we're done here.
899 return;
900 }
901 assert(l_object == obj, "object=" INTPTR_FORMAT " must equal obj="
902 INTPTR_FORMAT, p2i(l_object), p2i(obj));
903
904 markWord dmw = header();
905 // The dmw has to be neutral (not null, not locked and not marked).
906 assert(dmw.is_neutral(), "must be neutral: dmw=" INTPTR_FORMAT, dmw.value());
907
908 // Install displaced mark word if the object's header still points
909 // to this ObjectMonitor. More than one racing caller to this function
910 // can rarely reach this point, but only one can win.
911 markWord res = obj->cas_set_mark(dmw, markWord::encode(this));
912 if (res != markWord::encode(this)) {
913 // This should be rare so log at the Info level when it happens.
914 log_info(monitorinflation)("install_displaced_markword_in_object: "
915 "failed cas_set_mark: new_mark=" INTPTR_FORMAT
916 ", old_mark=" INTPTR_FORMAT ", res=" INTPTR_FORMAT,
917 dmw.value(), markWord::encode(this).value(),
918 res.value());
919 }
920
921 // Note: It does not matter which thread restored the header/dmw
922 // into the object's header. The thread deflating the monitor just
923 // wanted the object's header restored and it is. The threads that
924 // detected a race with the deflation process also wanted the
925 // object's header restored before they retry their operation and
926 // because it is restored they will only retry once.
927 }
928
929 // Convert the fields used by is_busy() to a string that can be
930 // used for diagnostic output.
931 const char* ObjectMonitor::is_busy_to_string(stringStream* ss) {
932 ss->print("is_busy: waiters=%d"
933 ", contentions=%d"
934 ", owner=" INT64_FORMAT
935 ", entry_list=" PTR_FORMAT,
936 _waiters,
937 (contentions() > 0 ? contentions() : 0),
938 owner_is_DEFLATER_MARKER()
939 // We report null instead of DEFLATER_MARKER here because is_busy()
940 // ignores DEFLATER_MARKER values.
941 ? NO_OWNER
942 : owner_raw(),
943 p2i(_entry_list));
944 return ss->base();
945 }
946
947 void ObjectMonitor::enter_internal(JavaThread* current) {
948 assert(current->thread_state() == _thread_blocked, "invariant");
949
950 // Try the lock - TATAS
951 if (try_lock(current) == TryLockResult::Success) {
952 assert(!has_successor(current), "invariant");
953 assert(has_owner(current), "invariant");
954 return;
955 }
956
957 assert(InitDone, "Unexpectedly not initialized");
958
959 // We try one round of spinning *before* enqueueing current.
960 //
961 // If the _owner is ready but OFFPROC we could use a YieldTo()
962 // operation to donate the remainder of this thread's quantum
963 // to the owner. This has subtle but beneficial affinity
964 // effects.
965
966 if (try_spin(current)) {
967 assert(has_owner(current), "invariant");
968 assert(!has_successor(current), "invariant");
969 return;
970 }
971
972 // The Spin failed -- Enqueue and park the thread ...
973 assert(!has_successor(current), "invariant");
974 assert(!has_owner(current), "invariant");
975
976 // Enqueue "current" on ObjectMonitor's _entry_list.
977 //
978 // Node acts as a proxy for current.
979 // As an aside, if were to ever rewrite the synchronization code mostly
980 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
981 // Java objects. This would avoid awkward lifecycle and liveness issues,
982 // as well as eliminate a subset of ABA issues.
983 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
984
985 ObjectWaiter node(current);
986 current->_ParkEvent->reset();
987
988 if (try_lock_or_add_to_entry_list(current, &node)) {
989 return; // We got the lock.
990 }
991 // This thread is now added to the _entry_list.
992
993 // The lock might have been released while this thread was occupied queueing
994 // itself onto _entry_list. To close the race and avoid "stranding" and
995 // progress-liveness failure we must resample-retry _owner before parking.
996 // Note the Dekker/Lamport duality: ST _entry_list; MEMBAR; LD Owner.
997 // In this case the ST-MEMBAR is accomplished with CAS().
998 //
999 // TODO: Defer all thread state transitions until park-time.
1000 // Since state transitions are heavy and inefficient we'd like
1001 // to defer the state transitions until absolutely necessary,
1002 // and in doing so avoid some transitions ...
1003
1004 // If there are unmounted virtual threads ahead in the _entry_list we want
1005 // to do a timed-park instead to alleviate some deadlock cases where one
1006 // of them is picked as the successor but cannot run due to having run out
1007 // of carriers. This can happen, for example, if this is a pinned virtual
1008 // thread currently loading or initializining a class, and all other carriers
1009 // have a pinned vthread waiting for said class to be loaded/initialized.
1010 // Read counter *after* adding this thread to the _entry_list. Adding to
1011 // _entry_list uses Atomic::cmpxchg() which already provides a fence that
1012 // prevents this load from floating up previous store.
1013 // Note that we can have false positives where timed-park is not necessary.
1014 bool do_timed_parked = has_unmounted_vthreads();
1015 jlong recheck_interval = 1;
1016
1017 for (;;) {
1018
1019 if (try_lock(current) == TryLockResult::Success) {
1020 break;
1021 }
1022 assert(!has_owner(current), "invariant");
1023
1024 // park self
1025 if (do_timed_parked) {
1026 current->_ParkEvent->park(recheck_interval);
1027 // Increase the recheck_interval, but clamp the value.
1028 recheck_interval *= 8;
1029 if (recheck_interval > MAX_RECHECK_INTERVAL) {
1030 recheck_interval = MAX_RECHECK_INTERVAL;
1031 }
1032 } else {
1033 current->_ParkEvent->park();
1034 }
1035
1036 if (try_lock(current) == TryLockResult::Success) {
1037 break;
1038 }
1039
1040 // The lock is still contested.
1041
1042 // Assuming this is not a spurious wakeup we'll normally find _succ == current.
1043 // We can defer clearing _succ until after the spin completes
1044 // try_spin() must tolerate being called with _succ == current.
1045 // Try yet another round of adaptive spinning.
1046 if (try_spin(current)) {
1047 break;
1048 }
1049
1050 // We can find that we were unpark()ed and redesignated _succ while
1051 // we were spinning. That's harmless. If we iterate and call park(),
1052 // park() will consume the event and return immediately and we'll
1053 // just spin again. This pattern can repeat, leaving _succ to simply
1054 // spin on a CPU.
1055
1056 if (has_successor(current)) clear_successor();
1057
1058 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
1059 OrderAccess::fence();
1060 }
1061
1062 // Egress :
1063 // Current has acquired the lock -- Unlink current from the _entry_list.
1064 unlink_after_acquire(current, &node);
1065 if (has_successor(current)) {
1066 clear_successor();
1067 // Note that we don't need to do OrderAccess::fence() after clearing
1068 // _succ here, since we own the lock.
1069 }
1070
1071 // We've acquired ownership with CAS().
1072 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
1073 // But since the CAS() this thread may have also stored into _succ
1074 // or entry_list. These meta-data updates must be visible __before
1075 // this thread subsequently drops the lock.
1076 // Consider what could occur if we didn't enforce this constraint --
1077 // STs to monitor meta-data and user-data could reorder with (become
1078 // visible after) the ST in exit that drops ownership of the lock.
1079 // Some other thread could then acquire the lock, but observe inconsistent
1080 // or old monitor meta-data and heap data. That violates the JMM.
1081 // To that end, the exit() operation must have at least STST|LDST
1082 // "release" barrier semantics. Specifically, there must be at least a
1083 // STST|LDST barrier in exit() before the ST of null into _owner that drops
1084 // the lock. The barrier ensures that changes to monitor meta-data and data
1085 // protected by the lock will be visible before we release the lock, and
1086 // therefore before some other thread (CPU) has a chance to acquire the lock.
1087 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
1088 //
1089 // Critically, any prior STs to _succ or entry_list must be visible before
1090 // the ST of null into _owner in the *subsequent* (following) corresponding
1091 // monitorexit.
1092
1093 return;
1094 }
1095
1096 // reenter_internal() is a specialized inline form of the latter half of the
1097 // contended slow-path from enter_internal(). We use reenter_internal() only for
1098 // monitor reentry in wait().
1099 //
1100 // In the future we should reconcile enter_internal() and reenter_internal().
1101
1102 void ObjectMonitor::reenter_internal(JavaThread* current, ObjectWaiter* currentNode) {
1103 assert(current != nullptr, "invariant");
1104 assert(current->thread_state() != _thread_blocked, "invariant");
1105 assert(currentNode != nullptr, "invariant");
1106 assert(currentNode->_thread == current, "invariant");
1107 assert(_waiters > 0, "invariant");
1108 assert_mark_word_consistency();
1109
1110 // If there are unmounted virtual threads ahead in the _entry_list we want
1111 // to do a timed-park instead to alleviate some deadlock cases where one
1112 // of them is picked as the successor but cannot run due to having run out
1113 // of carriers. This can happen, for example, if a mixed of unmounted and
1114 // pinned vthreads taking up all the carriers are waiting for a class to be
1115 // initialized, and the selected successor is one of the unmounted vthreads.
1116 // Although this method is used for the "notification" case, it could be
1117 // that this thread reached here without been added to the _entry_list yet.
1118 // This can happen if it was interrupted or the wait timed-out at the same
1119 // time. In that case we rely on currentNode->_do_timed_park, which will be
1120 // read on the next loop iteration, after consuming the park permit set by
1121 // the notifier in notify_internal.
1122 // Note that we can have false positives where timed-park is not necessary.
1123 bool do_timed_parked = has_unmounted_vthreads();
1124 jlong recheck_interval = 1;
1125
1126 for (;;) {
1127 ObjectWaiter::TStates v = currentNode->TState;
1128 guarantee(v == ObjectWaiter::TS_ENTER, "invariant");
1129 assert(!has_owner(current), "invariant");
1130
1131 // This thread has been notified so try to reacquire the lock.
1132 if (try_lock(current) == TryLockResult::Success) {
1133 break;
1134 }
1135
1136 // If that fails, spin again. Note that spin count may be zero so the above TryLock
1137 // is necessary.
1138 if (try_spin(current)) {
1139 break;
1140 }
1141
1142 {
1143 OSThreadContendState osts(current->osthread());
1144
1145 assert(current->thread_state() == _thread_in_vm, "invariant");
1146
1147 {
1148 ClearSuccOnSuspend csos(this);
1149 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */);
1150 if (do_timed_parked) {
1151 current->_ParkEvent->park(recheck_interval);
1152 // Increase the recheck_interval, but clamp the value.
1153 recheck_interval *= 8;
1154 if (recheck_interval > MAX_RECHECK_INTERVAL) {
1155 recheck_interval = MAX_RECHECK_INTERVAL;
1156 }
1157 } else {
1158 current->_ParkEvent->park();
1159 }
1160 }
1161 }
1162
1163 // Try again, but just so we distinguish between futile wakeups and
1164 // successful wakeups. The following test isn't algorithmically
1165 // necessary, but it helps us maintain sensible statistics.
1166 if (try_lock(current) == TryLockResult::Success) {
1167 break;
1168 }
1169
1170 // The lock is still contested.
1171
1172 // Assuming this is not a spurious wakeup we'll normally
1173 // find that _succ == current.
1174 if (has_successor(current)) clear_successor();
1175
1176 // Invariant: after clearing _succ a contending thread
1177 // *must* retry _owner before parking.
1178 OrderAccess::fence();
1179
1180 // See comment in notify_internal
1181 do_timed_parked |= currentNode->_do_timed_park;
1182 }
1183
1184 // Current has acquired the lock -- Unlink current from the _entry_list.
1185 assert(has_owner(current), "invariant");
1186 assert_mark_word_consistency();
1187 unlink_after_acquire(current, currentNode);
1188 if (has_successor(current)) clear_successor();
1189 assert(!has_successor(current), "invariant");
1190 currentNode->TState = ObjectWaiter::TS_RUN;
1191 OrderAccess::fence(); // see comments at the end of enter_internal()
1192 }
1193
1194 // This method is called from two places:
1195 // - On monitorenter contention with a null waiter.
1196 // - After Object.wait() times out or the target is interrupted to reenter the
1197 // monitor, with the existing waiter.
1198 // For the Object.wait() case we do not delete the ObjectWaiter in case we
1199 // succesfully acquire the monitor since we are going to need it on return.
1200 bool ObjectMonitor::vthread_monitor_enter(JavaThread* current, ObjectWaiter* waiter) {
1201 if (try_lock(current) == TryLockResult::Success) {
1202 assert(has_owner(current), "invariant");
1203 assert(!has_successor(current), "invariant");
1204 return true;
1205 }
1206
1207 oop vthread = current->vthread();
1208 ObjectWaiter* node = waiter != nullptr ? waiter : new ObjectWaiter(vthread, this);
1209
1210 // Increment counter *before* adding the vthread to the _entry_list.
1211 // Adding to _entry_list uses Atomic::cmpxchg() which already provides
1212 // a fence that prevents reordering of the stores.
1213 inc_unmounted_vthreads();
1214
1215 if (try_lock_or_add_to_entry_list(current, node)) {
1216 // We got the lock.
1217 if (waiter == nullptr) delete node; // for Object.wait() don't delete yet
1218 dec_unmounted_vthreads();
1219 return true;
1220 }
1221 // This thread is now added to the entry_list.
1222
1223 // We have to try once more since owner could have exited monitor and checked
1224 // _entry_list before we added the node to the queue.
1225 if (try_lock(current) == TryLockResult::Success) {
1226 assert(has_owner(current), "invariant");
1227 unlink_after_acquire(current, node);
1228 if (has_successor(current)) clear_successor();
1229 if (waiter == nullptr) delete node; // for Object.wait() don't delete yet
1230 dec_unmounted_vthreads();
1231 return true;
1232 }
1233
1234 assert(java_lang_VirtualThread::state(vthread) == java_lang_VirtualThread::RUNNING, "wrong state for vthread");
1235 java_lang_VirtualThread::set_state(vthread, java_lang_VirtualThread::BLOCKING);
1236
1237 // We didn't succeed in acquiring the monitor so increment _contentions and
1238 // save ObjectWaiter* in the vthread since we will need it when resuming execution.
1239 add_to_contentions(1);
1240 java_lang_VirtualThread::set_objectWaiter(vthread, node);
1241 return false;
1242 }
1243
1244 // Called from thaw code to resume the monitor operation that caused the vthread
1245 // to be unmounted. Method returns true if the monitor is successfully acquired,
1246 // which marks the end of the monitor operation, otherwise it returns false.
1247 bool ObjectMonitor::resume_operation(JavaThread* current, ObjectWaiter* node, ContinuationWrapper& cont) {
1248 assert(java_lang_VirtualThread::state(current->vthread()) == java_lang_VirtualThread::RUNNING, "wrong state for vthread");
1249 assert(!has_owner(current), "");
1250
1251 if (node->is_wait() && !node->at_reenter()) {
1252 bool acquired_monitor = vthread_wait_reenter(current, node, cont);
1253 if (acquired_monitor) return true;
1254 }
1255
1256 // Retry acquiring monitor...
1257
1258 int state = node->TState;
1259 guarantee(state == ObjectWaiter::TS_ENTER, "invariant");
1260
1261 if (try_lock(current) == TryLockResult::Success) {
1262 vthread_epilog(current, node);
1263 return true;
1264 }
1265
1266 oop vthread = current->vthread();
1267 if (has_successor(current)) clear_successor();
1268
1269 // Invariant: after clearing _succ a thread *must* retry acquiring the monitor.
1270 OrderAccess::fence();
1271
1272 if (try_lock(current) == TryLockResult::Success) {
1273 vthread_epilog(current, node);
1274 return true;
1275 }
1276
1277 // We will return to Continuation.run() and unmount so set the right state.
1278 java_lang_VirtualThread::set_state(vthread, java_lang_VirtualThread::BLOCKING);
1279
1280 return false;
1281 }
1282
1283 void ObjectMonitor::vthread_epilog(JavaThread* current, ObjectWaiter* node) {
1284 assert(has_owner(current), "invariant");
1285 add_to_contentions(-1);
1286 dec_unmounted_vthreads();
1287
1288 if (has_successor(current)) clear_successor();
1289
1290 guarantee(_recursions == 0, "invariant");
1291
1292 if (node->is_wait()) {
1293 _recursions = node->_recursions; // restore the old recursion count
1294 _waiters--; // decrement the number of waiters
1295
1296 if (node->_interrupted) {
1297 // We will throw at thaw end after finishing the mount transition.
1298 current->set_pending_interrupted_exception(true);
1299 }
1300 }
1301
1302 unlink_after_acquire(current, node);
1303 delete node;
1304
1305 // Clear the ObjectWaiter* from the vthread.
1306 java_lang_VirtualThread::set_objectWaiter(current->vthread(), nullptr);
1307
1308 if (JvmtiExport::should_post_monitor_contended_entered()) {
1309 // We are going to call thaw again after this and finish the VMTS
1310 // transition so no need to do it here. We will post the event there.
1311 current->set_contended_entered_monitor(this);
1312 }
1313 }
1314
1315 // Convert entry_list into a doubly linked list by assigning the prev
1316 // pointers and the entry_list_tail pointer (if needed). Within the
1317 // entry_list the next pointers always form a consistent singly linked
1318 // list. When this function is called, the entry_list will be either
1319 // singly linked, or starting as singly linked (at the head), but
1320 // ending as doubly linked (at the tail).
1321 void ObjectMonitor::entry_list_build_dll(JavaThread* current) {
1322 assert(has_owner(current), "invariant");
1323 ObjectWaiter* prev = nullptr;
1324 // Need acquire here to match the implicit release of the cmpxchg
1325 // that updated entry_list, so we can access w->prev().
1326 ObjectWaiter* w = AtomicAccess::load_acquire(&_entry_list);
1327 assert(w != nullptr, "should only be called when entry list is not empty");
1328 while (w != nullptr) {
1329 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1330 assert(w->prev() == nullptr || w->prev() == prev, "invariant");
1331 if (w->prev() != nullptr) {
1332 break;
1333 }
1334 w->_prev = prev;
1335 prev = w;
1336 w = w->next();
1337 }
1338 if (w == nullptr) {
1339 // We converted the entire entry_list from a singly linked list
1340 // into a doubly linked list. Now we just need to set the tail
1341 // pointer.
1342 assert(prev != nullptr && prev->next() == nullptr, "invariant");
1343 assert(_entry_list_tail == nullptr || _entry_list_tail == prev, "invariant");
1344 _entry_list_tail = prev;
1345 } else {
1346 #ifdef ASSERT
1347 // We stopped iterating through the _entry_list when we found a
1348 // node that had its prev pointer set. I.e. we converted the first
1349 // part of the entry_list from a singly linked list into a doubly
1350 // linked list. Now we just want to make sure the rest of the list
1351 // is doubly linked. But first we check that we have a tail
1352 // pointer, because if the end of the entry_list is doubly linked
1353 // and we don't have the tail pointer, something is broken.
1354 assert(_entry_list_tail != nullptr, "invariant");
1355 while (w != nullptr) {
1356 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1357 assert(w->prev() == prev, "invariant");
1358 prev = w;
1359 w = w->next();
1360 }
1361 assert(_entry_list_tail == prev, "invariant");
1362 #endif
1363 }
1364 }
1365
1366 // Return the tail of the _entry_list. If the tail is currently not
1367 // known, it can be found by first calling entry_list_build_dll().
1368 ObjectWaiter* ObjectMonitor::entry_list_tail(JavaThread* current) {
1369 assert(has_owner(current), "invariant");
1370 ObjectWaiter* w = _entry_list_tail;
1371 if (w != nullptr) {
1372 return w;
1373 }
1374 entry_list_build_dll(current);
1375 w = _entry_list_tail;
1376 assert(w != nullptr, "invariant");
1377 return w;
1378 }
1379
1380 // By convention we unlink a contending thread from _entry_list
1381 // immediately after the thread acquires the lock in ::enter().
1382 // The head of _entry_list is volatile but the interior is stable.
1383 // In addition, current.TState is stable.
1384
1385 void ObjectMonitor::unlink_after_acquire(JavaThread* current, ObjectWaiter* currentNode) {
1386 assert(has_owner(current), "invariant");
1387 assert((!currentNode->is_vthread() && currentNode->thread() == current) ||
1388 (currentNode->is_vthread() && currentNode->vthread() == current->vthread()), "invariant");
1389
1390 // Check if we are unlinking the last element in the _entry_list.
1391 // This is by far the most common case.
1392 if (currentNode->next() == nullptr) {
1393 assert(_entry_list_tail == nullptr || _entry_list_tail == currentNode, "invariant");
1394
1395 ObjectWaiter* w = AtomicAccess::load(&_entry_list);
1396 if (w == currentNode) {
1397 // The currentNode is the only element in _entry_list.
1398 if (AtomicAccess::cmpxchg(&_entry_list, w, (ObjectWaiter*)nullptr) == w) {
1399 _entry_list_tail = nullptr;
1400 currentNode->set_bad_pointers();
1401 return;
1402 }
1403 // The CAS above can fail from interference IFF a contending
1404 // thread "pushed" itself onto entry_list. So fall-through to
1405 // building the doubly linked list.
1406 assert(currentNode->prev() == nullptr, "invariant");
1407 }
1408 if (currentNode->prev() == nullptr) {
1409 // Build the doubly linked list to get hold of
1410 // currentNode->prev().
1411 entry_list_build_dll(current);
1412 assert(currentNode->prev() != nullptr, "must be");
1413 assert(_entry_list_tail == currentNode, "must be");
1414 }
1415 // The currentNode is the last element in _entry_list and we know
1416 // which element is the previous one.
1417 assert(_entry_list != currentNode, "invariant");
1418 _entry_list_tail = currentNode->prev();
1419 _entry_list_tail->_next = nullptr;
1420 currentNode->set_bad_pointers();
1421 return;
1422 }
1423
1424 // If we get here it means the current thread enqueued itself on the
1425 // _entry_list but was then able to "steal" the lock before the
1426 // chosen successor was able to. Consequently currentNode must be an
1427 // interior node in the _entry_list, or the head.
1428 assert(currentNode->next() != nullptr, "invariant");
1429 assert(currentNode != _entry_list_tail, "invariant");
1430
1431 // Check if we are in the singly linked portion of the
1432 // _entry_list. If we are the head then we try to remove ourselves,
1433 // else we convert to the doubly linked list.
1434 if (currentNode->prev() == nullptr) {
1435 ObjectWaiter* w = AtomicAccess::load(&_entry_list);
1436
1437 assert(w != nullptr, "invariant");
1438 if (w == currentNode) {
1439 ObjectWaiter* next = currentNode->next();
1440 // currentNode is at the head of _entry_list.
1441 if (AtomicAccess::cmpxchg(&_entry_list, w, next) == w) {
1442 // The CAS above sucsessfully unlinked currentNode from the
1443 // head of the _entry_list.
1444 assert(_entry_list != w, "invariant");
1445 next->_prev = nullptr;
1446 currentNode->set_bad_pointers();
1447 return;
1448 } else {
1449 // The CAS above can fail from interference IFF a contending
1450 // thread "pushed" itself onto _entry_list, in which case
1451 // currentNode must now be in the interior of the
1452 // list. Fall-through to building the doubly linked list.
1453 assert(_entry_list != currentNode, "invariant");
1454 }
1455 }
1456 // Build the doubly linked list to get hold of currentNode->prev().
1457 entry_list_build_dll(current);
1458 assert(currentNode->prev() != nullptr, "must be");
1459 }
1460
1461 // We now know we are unlinking currentNode from the interior of a
1462 // doubly linked list.
1463 assert(currentNode->next() != nullptr, "");
1464 assert(currentNode->prev() != nullptr, "");
1465 assert(currentNode != _entry_list, "");
1466 assert(currentNode != _entry_list_tail, "");
1467
1468 ObjectWaiter* nxt = currentNode->next();
1469 ObjectWaiter* prv = currentNode->prev();
1470 assert(nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
1471 assert(prv->TState == ObjectWaiter::TS_ENTER, "invariant");
1472
1473 nxt->_prev = prv;
1474 prv->_next = nxt;
1475 currentNode->set_bad_pointers();
1476 }
1477
1478 // -----------------------------------------------------------------------------
1479 // Exit support
1480 //
1481 // exit()
1482 // ~~~~~~
1483 // Note that the collector can't reclaim the objectMonitor or deflate
1484 // the object out from underneath the thread calling ::exit() as the
1485 // thread calling ::exit() never transitions to a stable state.
1486 // This inhibits GC, which in turn inhibits asynchronous (and
1487 // inopportune) reclamation of "this".
1488 //
1489 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
1490 // There's one exception to the claim above, however. enter_internal() can call
1491 // exit() to drop a lock if the acquirer has been externally suspended.
1492 // In that case exit() is called with _thread_state == _thread_blocked,
1493 // but the monitor's _contentions field is > 0, which inhibits reclamation.
1494 //
1495 // This is the exit part of the locking protocol, often implemented in
1496 // C2_MacroAssembler::fast_unlock()
1497 //
1498 // 1. A release barrier ensures that changes to monitor meta-data
1499 // (_succ, _entry_list) and data protected by the lock will be
1500 // visible before we release the lock.
1501 // 2. Release the lock by clearing the owner.
1502 // 3. A storeload MEMBAR is needed between releasing the owner and
1503 // subsequently reading meta-data to safely determine if the lock is
1504 // contended (step 4) without an elected successor (step 5).
1505 // 4. If _entry_list is null, we are done, since there is no
1506 // other thread waiting on the lock to wake up. I.e. there is no
1507 // contention.
1508 // 5. If there is a successor (_succ is non-null), we are done. The
1509 // responsibility for guaranteeing progress-liveness has now implicitly
1510 // been moved from the exiting thread to the successor.
1511 // 6. There are waiters in the entry list (_entry_list is non-null),
1512 // but there is no successor (_succ is null), so we need to
1513 // wake up (unpark) a waiting thread to avoid stranding.
1514 //
1515 // Note that since only the current lock owner can manipulate the
1516 // _entry_list (except for pushing new threads to the head), we need to
1517 // reacquire the lock before we can wake up (unpark) a waiting thread.
1518 //
1519 // The CAS() in enter provides for safety and exclusion, while the
1520 // MEMBAR in exit provides for progress and avoids stranding.
1521 //
1522 // There is also the risk of a futile wake-up. If we drop the lock
1523 // another thread can reacquire the lock immediately, and we can
1524 // then wake a thread unnecessarily. This is benign, and we've
1525 // structured the code so the windows are short and the frequency
1526 // of such futile wakups is low.
1527
1528 void ObjectMonitor::exit(JavaThread* current, bool not_suspended) {
1529 if (!has_owner(current)) {
1530 // Apparent unbalanced locking ...
1531 // Naively we'd like to throw IllegalMonitorStateException.
1532 // As a practical matter we can neither allocate nor throw an
1533 // exception as ::exit() can be called from leaf routines.
1534 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
1535 // Upon deeper reflection, however, in a properly run JVM the only
1536 // way we should encounter this situation is in the presence of
1537 // unbalanced JNI locking. TODO: CheckJNICalls.
1538 // See also: CR4414101
1539 #ifdef ASSERT
1540 LogStreamHandle(Error, monitorinflation) lsh;
1541 lsh.print_cr("ERROR: ObjectMonitor::exit(): thread=" INTPTR_FORMAT
1542 " is exiting an ObjectMonitor it does not own.", p2i(current));
1543 lsh.print_cr("The imbalance is possibly caused by JNI locking.");
1544 print_debug_style_on(&lsh);
1545 assert(false, "Non-balanced monitor enter/exit!");
1546 #endif
1547 return;
1548 }
1549
1550 if (_recursions != 0) {
1551 _recursions--; // this is simple recursive enter
1552 return;
1553 }
1554
1555 #if INCLUDE_JFR
1556 // get the owner's thread id for the MonitorEnter event
1557 // if it is enabled and the thread isn't suspended
1558 if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
1559 _previous_owner_tid = JFR_THREAD_ID(current);
1560 }
1561 #endif
1562
1563 for (;;) {
1564 // If there is a successor we should release the lock as soon as
1565 // possible, so that the successor can acquire the lock. If there is
1566 // no successor, we might need to wake up a waiting thread.
1567 if (!has_successor()) {
1568 ObjectWaiter* w = AtomicAccess::load(&_entry_list);
1569 if (w != nullptr) {
1570 // Other threads are blocked trying to acquire the lock and
1571 // there is no successor, so it appears that an heir-
1572 // presumptive (successor) must be made ready. Since threads
1573 // are woken up in FIFO order, we need to find the tail of the
1574 // entry_list.
1575 w = entry_list_tail(current);
1576 // I'd like to write: guarantee (w->_thread != current).
1577 // But in practice an exiting thread may find itself on the entry_list.
1578 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1579 // then calls exit(). Exit release the lock by setting O._owner to null.
1580 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1581 // notify() operation moves T1 from O's waitset to O's entry_list. T2 then
1582 // release the lock "O". T1 resumes immediately after the ST of null into
1583 // _owner, above. T1 notices that the entry_list is populated, so it
1584 // reacquires the lock and then finds itself on the entry_list.
1585 // Given all that, we have to tolerate the circumstance where "w" is
1586 // associated with current.
1587 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1588 exit_epilog(current, w);
1589 return;
1590 }
1591 }
1592
1593 // Drop the lock.
1594 // release semantics: prior loads and stores from within the critical section
1595 // must not float (reorder) past the following store that drops the lock.
1596 // Uses a storeload to separate release_store(owner) from the
1597 // successor check. The try_set_owner_from() below uses cmpxchg() so
1598 // we get the fence down there.
1599 release_clear_owner(current);
1600 OrderAccess::storeload();
1601
1602 // Normally the exiting thread is responsible for ensuring succession,
1603 // but if this thread observes other successors are ready or other
1604 // entering threads are spinning after it has stored null into _owner
1605 // then it can exit without waking a successor. The existence of
1606 // spinners or ready successors guarantees proper succession (liveness).
1607 // Responsibility passes to the ready or running successors. The exiting
1608 // thread delegates the duty. More precisely, if a successor already
1609 // exists this thread is absolved of the responsibility of waking
1610 // (unparking) one.
1611
1612 // The _succ variable is critical to reducing futile wakeup frequency.
1613 // _succ identifies the "heir presumptive" thread that has been made
1614 // ready (unparked) but that has not yet run. We need only one such
1615 // successor thread to guarantee progress.
1616 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1617 // section 3.3 "Futile Wakeup Throttling" for details.
1618 //
1619 // Note that spinners in Enter() also set _succ non-null.
1620 // In the current implementation spinners opportunistically set
1621 // _succ so that exiting threads might avoid waking a successor.
1622 // Which means that the exiting thread could exit immediately without
1623 // waking a successor, if it observes a successor after it has dropped
1624 // the lock. Note that the dropped lock needs to become visible to the
1625 // spinner.
1626
1627 if (_entry_list == nullptr || has_successor()) {
1628 return;
1629 }
1630
1631 // Only the current lock owner can manipulate the entry_list
1632 // (except for pushing new threads to the head), therefore we need
1633 // to reacquire the lock. If we fail to reacquire the lock the
1634 // responsibility for ensuring succession falls to the new owner.
1635
1636 if (try_lock(current) != TryLockResult::Success) {
1637 // Some other thread acquired the lock (or the monitor was
1638 // deflated). Either way we are done.
1639 return;
1640 }
1641
1642 guarantee(has_owner(current), "invariant");
1643 }
1644 }
1645
1646 void ObjectMonitor::exit_epilog(JavaThread* current, ObjectWaiter* Wakee) {
1647 assert(has_owner(current), "invariant");
1648
1649 // Exit protocol:
1650 // 1. ST _succ = wakee
1651 // 2. membar #loadstore|#storestore;
1652 // 2. ST _owner = nullptr
1653 // 3. unpark(wakee)
1654
1655 oop vthread = nullptr;
1656 ParkEvent * Trigger;
1657 if (!Wakee->is_vthread()) {
1658 JavaThread* t = Wakee->thread();
1659 assert(t != nullptr, "");
1660 Trigger = t->_ParkEvent;
1661 set_successor(t);
1662 } else {
1663 vthread = Wakee->vthread();
1664 assert(vthread != nullptr, "");
1665 Trigger = ObjectMonitor::vthread_unparker_ParkEvent();
1666 set_successor(vthread);
1667 }
1668
1669 // Hygiene -- once we've set _owner = nullptr we can't safely dereference Wakee again.
1670 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1671 // out-of-scope (non-extant).
1672 Wakee = nullptr;
1673
1674 // Drop the lock.
1675 // Uses a fence to separate release_store(owner) from the LD in unpark().
1676 release_clear_owner(current);
1677 OrderAccess::fence();
1678
1679 DTRACE_MONITOR_PROBE(contended__exit, this, object(), current);
1680
1681 if (vthread == nullptr) {
1682 // Platform thread case.
1683 Trigger->unpark();
1684 } else if (java_lang_VirtualThread::set_onWaitingList(vthread, vthread_list_head())) {
1685 // Virtual thread case.
1686 Trigger->unpark();
1687 }
1688 }
1689
1690 // Exits the monitor returning recursion count. _owner should
1691 // be set to current's owner_id, i.e. no ANONYMOUS_OWNER allowed.
1692 intx ObjectMonitor::complete_exit(JavaThread* current) {
1693 assert(InitDone, "Unexpectedly not initialized");
1694 guarantee(has_owner(current), "complete_exit not owner");
1695
1696 intx save = _recursions; // record the old recursion count
1697 _recursions = 0; // set the recursion level to be 0
1698 exit(current); // exit the monitor
1699 guarantee(!has_owner(current), "invariant");
1700 return save;
1701 }
1702
1703 // Checks that the current THREAD owns this monitor and causes an
1704 // immediate return if it doesn't. We don't use the CHECK macro
1705 // because we want the IMSE to be the only exception that is thrown
1706 // from the call site when false is returned. Any other pending
1707 // exception is ignored.
1708 #define CHECK_OWNER() \
1709 do { \
1710 if (!check_owner(THREAD)) { \
1711 assert(HAS_PENDING_EXCEPTION, "expected a pending IMSE here."); \
1712 return; \
1713 } \
1714 } while (false)
1715
1716 // Returns true if the specified thread owns the ObjectMonitor.
1717 // Otherwise returns false and throws IllegalMonitorStateException
1718 // (IMSE). If there is a pending exception and the specified thread
1719 // is not the owner, that exception will be replaced by the IMSE.
1720 bool ObjectMonitor::check_owner(TRAPS) {
1721 JavaThread* current = THREAD;
1722 int64_t cur = owner_raw();
1723 if (cur == owner_id_from(current)) {
1724 return true;
1725 }
1726 THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(),
1727 "current thread is not owner", false);
1728 }
1729
1730 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1731 ObjectMonitor* monitor,
1732 uint64_t notifier_tid,
1733 jlong timeout,
1734 bool timedout) {
1735 assert(event != nullptr, "invariant");
1736 assert(monitor != nullptr, "invariant");
1737 const Klass* monitor_klass = monitor->object()->klass();
1738 if (ObjectMonitor::is_jfr_excluded(monitor_klass)) {
1739 return;
1740 }
1741 event->set_monitorClass(monitor_klass);
1742 event->set_timeout(timeout);
1743 // Set an address that is 'unique enough', such that events close in
1744 // time and with the same address are likely (but not guaranteed) to
1745 // belong to the same object.
1746 event->set_address((uintptr_t)monitor);
1747 event->set_notifier(notifier_tid);
1748 event->set_timedOut(timedout);
1749 event->commit();
1750 }
1751
1752 static void vthread_monitor_waited_event(JavaThread* current, ObjectWaiter* node, ContinuationWrapper& cont, EventJavaMonitorWait* event, jboolean timed_out) {
1753 // Since we might safepoint set the anchor so that the stack can we walked.
1754 assert(current->last_continuation() != nullptr, "");
1755 JavaFrameAnchor* anchor = current->frame_anchor();
1756 anchor->set_last_Java_sp(current->last_continuation()->entry_sp());
1757 anchor->set_last_Java_pc(current->last_continuation()->entry_pc());
1758
1759 ContinuationWrapper::SafepointOp so(current, cont);
1760
1761 JRT_BLOCK
1762 if (event->should_commit()) {
1763 long timeout = java_lang_VirtualThread::timeout(current->vthread());
1764 post_monitor_wait_event(event, node->_monitor, node->_notifier_tid, timeout, timed_out);
1765 }
1766 if (JvmtiExport::should_post_monitor_waited()) {
1767 // We mark this call in case of an upcall to Java while posting the event.
1768 // If somebody walks the stack in that case, processing the enterSpecial
1769 // frame should not include processing callee arguments since there is no
1770 // actual callee (see nmethod::preserve_callee_argument_oops()).
1771 ThreadOnMonitorWaitedEvent tmwe(current);
1772 JvmtiExport::vthread_post_monitor_waited(current, node->_monitor, timed_out);
1773 }
1774 JRT_BLOCK_END
1775 current->frame_anchor()->clear();
1776 }
1777
1778 // -----------------------------------------------------------------------------
1779 // Wait/Notify/NotifyAll
1780 //
1781 // Note: a subset of changes to ObjectMonitor::wait()
1782 // will need to be replicated in complete_exit
1783 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1784 JavaThread* current = THREAD;
1785
1786 assert(InitDone, "Unexpectedly not initialized");
1787
1788 CHECK_OWNER(); // Throws IMSE if not owner.
1789
1790 EventJavaMonitorWait wait_event;
1791 EventVirtualThreadPinned vthread_pinned_event;
1792
1793 // check for a pending interrupt
1794 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1795 JavaThreadInObjectWaitState jtiows(current, millis != 0, interruptible);
1796
1797 if (JvmtiExport::should_post_monitor_wait()) {
1798 JvmtiExport::post_monitor_wait(current, object(), millis);
1799 }
1800 // post monitor waited event. Note that this is past-tense, we are done waiting.
1801 if (JvmtiExport::should_post_monitor_waited()) {
1802 // Note: 'false' parameter is passed here because the
1803 // wait was not timed out due to thread interrupt.
1804 JvmtiExport::post_monitor_waited(current, this, false);
1805
1806 // In this short circuit of the monitor wait protocol, the
1807 // current thread never drops ownership of the monitor and
1808 // never gets added to the wait queue so the current thread
1809 // cannot be made the successor. This means that the
1810 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1811 // consume an unpark() meant for the ParkEvent associated with
1812 // this ObjectMonitor.
1813 }
1814 if (wait_event.should_commit()) {
1815 post_monitor_wait_event(&wait_event, this, 0, millis, false);
1816 }
1817 THROW(vmSymbols::java_lang_InterruptedException());
1818 return;
1819 }
1820
1821 freeze_result result;
1822 ContinuationEntry* ce = current->last_continuation();
1823 bool is_virtual = ce != nullptr && ce->is_virtual_thread();
1824 if (is_virtual) {
1825 if (interruptible && JvmtiExport::should_post_monitor_wait()) {
1826 JvmtiExport::post_monitor_wait(current, object(), millis);
1827 }
1828 current->set_current_waiting_monitor(this);
1829 result = Continuation::try_preempt(current, ce->cont_oop(current));
1830 if (result == freeze_ok) {
1831 vthread_wait(current, millis, interruptible);
1832 current->set_current_waiting_monitor(nullptr);
1833 return;
1834 }
1835 }
1836 // The jtiows does nothing for non-interruptible.
1837 JavaThreadInObjectWaitState jtiows(current, millis != 0, interruptible);
1838
1839 if (!is_virtual) { // it was already set for virtual thread
1840 if (interruptible && JvmtiExport::should_post_monitor_wait()) {
1841 JvmtiExport::post_monitor_wait(current, object(), millis);
1842
1843 // The current thread already owns the monitor and it has not yet
1844 // been added to the wait queue so the current thread cannot be
1845 // made the successor. This means that the JVMTI_EVENT_MONITOR_WAIT
1846 // event handler cannot accidentally consume an unpark() meant for
1847 // the ParkEvent associated with this ObjectMonitor.
1848 }
1849 current->set_current_waiting_monitor(this);
1850 }
1851 // create a node to be put into the queue
1852 // Critically, after we reset() the event but prior to park(), we must check
1853 // for a pending interrupt.
1854 ObjectWaiter node(current);
1855 node.TState = ObjectWaiter::TS_WAIT;
1856 current->_ParkEvent->reset();
1857 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1858
1859 // Enter the waiting queue, which is a circular doubly linked list in this case
1860 // but it could be a priority queue or any data structure.
1861 // _wait_set_lock protects the wait queue. Normally the wait queue is accessed only
1862 // by the owner of the monitor *except* in the case where park()
1863 // returns because of a timeout of interrupt. Contention is exceptionally rare
1864 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1865
1866 Thread::SpinAcquire(&_wait_set_lock);
1867 add_waiter(&node);
1868 Thread::SpinRelease(&_wait_set_lock);
1869
1870 intx save = _recursions; // record the old recursion count
1871 _waiters++; // increment the number of waiters
1872 _recursions = 0; // set the recursion level to be 1
1873 exit(current); // exit the monitor
1874 guarantee(!has_owner(current), "invariant");
1875
1876 // The thread is on the wait_set list - now park() it.
1877 // On MP systems it's conceivable that a brief spin before we park
1878 // could be profitable.
1879 //
1880 // TODO-FIXME: change the following logic to a loop of the form
1881 // while (!timeout && !interrupted && node.TState == TS_WAIT) park()
1882
1883 int ret = OS_OK;
1884 bool was_notified = false;
1885
1886 // Need to check interrupt state whilst still _thread_in_vm
1887 bool interrupted = interruptible && current->is_interrupted(false);
1888
1889 { // State transition wrappers
1890 OSThread* osthread = current->osthread();
1891 OSThreadWaitState osts(osthread, true);
1892
1893 assert(current->thread_state() == _thread_in_vm, "invariant");
1894
1895 {
1896 ClearSuccOnSuspend csos(this);
1897 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */);
1898 if (interrupted || HAS_PENDING_EXCEPTION) {
1899 // Intentionally empty
1900 } else if (node.TState == ObjectWaiter::TS_WAIT) {
1901 if (millis <= 0) {
1902 current->_ParkEvent->park();
1903 } else {
1904 ret = current->_ParkEvent->park(millis);
1905 }
1906 }
1907 }
1908
1909 // Node may be on the wait_set, or on the entry_list, or in transition
1910 // from the wait_set to the entry_list.
1911 // See if we need to remove Node from the wait_set.
1912 // We use double-checked locking to avoid grabbing _wait_set_lock
1913 // if the thread is not on the wait queue.
1914 //
1915 // Note that we don't need a fence before the fetch of TState.
1916 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1917 // written by the is thread. (perhaps the fetch might even be satisfied
1918 // by a look-aside into the processor's own store buffer, although given
1919 // the length of the code path between the prior ST and this load that's
1920 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1921 // then we'll acquire the lock and then re-fetch a fresh TState value.
1922 // That is, we fail toward safety.
1923
1924 if (node.TState == ObjectWaiter::TS_WAIT) {
1925 Thread::SpinAcquire(&_wait_set_lock);
1926 if (node.TState == ObjectWaiter::TS_WAIT) {
1927 dequeue_specific_waiter(&node); // unlink from wait_set
1928 node.TState = ObjectWaiter::TS_RUN;
1929 }
1930 Thread::SpinRelease(&_wait_set_lock);
1931 }
1932
1933 // The thread is now either on off-list (TS_RUN),
1934 // or on the entry_list (TS_ENTER).
1935 // The Node's TState variable is stable from the perspective of this thread.
1936 // No other threads will asynchronously modify TState.
1937 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1938 OrderAccess::loadload();
1939 if (has_successor(current)) clear_successor();
1940 was_notified = node.TState == ObjectWaiter::TS_ENTER;
1941
1942 // Reentry phase -- reacquire the monitor.
1943 // re-enter contended monitor after object.wait().
1944 // retain OBJECT_WAIT state until re-enter successfully completes
1945 // Thread state is thread_in_vm and oop access is again safe,
1946 // although the raw address of the object may have changed.
1947 // (Don't cache naked oops over safepoints, of course).
1948
1949 // post monitor waited event. Note that this is past-tense, we are done waiting.
1950 if (JvmtiExport::should_post_monitor_waited()) {
1951 JvmtiExport::post_monitor_waited(current, this, ret == OS_TIMEOUT);
1952
1953 if (was_notified && has_successor(current)) {
1954 // In this part of the monitor wait-notify-reenter protocol it
1955 // is possible (and normal) for another thread to do a fastpath
1956 // monitor enter-exit while this thread is still trying to get
1957 // to the reenter portion of the protocol.
1958 //
1959 // The ObjectMonitor was notified and the current thread is
1960 // the successor which also means that an unpark() has already
1961 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1962 // consume the unpark() that was done when the successor was
1963 // set because the same ParkEvent is shared between Java
1964 // monitors and JVM/TI RawMonitors (for now).
1965 //
1966 // We redo the unpark() to ensure forward progress, i.e., we
1967 // don't want all pending threads hanging (parked) with none
1968 // entering the unlocked monitor.
1969 current->_ParkEvent->unpark();
1970 }
1971 }
1972
1973 if (wait_event.should_commit()) {
1974 post_monitor_wait_event(&wait_event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1975 }
1976
1977 OrderAccess::fence();
1978
1979 assert(!has_owner(current), "invariant");
1980 ObjectWaiter::TStates v = node.TState;
1981 if (v == ObjectWaiter::TS_RUN) {
1982 // We use the NoPreemptMark for the very rare case where the previous
1983 // preempt attempt failed due to OOM. The preempt on monitor contention
1984 // could succeed but we can't unmount now.
1985 NoPreemptMark npm(current);
1986 enter(current);
1987 } else {
1988 guarantee(v == ObjectWaiter::TS_ENTER, "invariant");
1989 reenter_internal(current, &node);
1990 node.wait_reenter_end(this);
1991 }
1992
1993 // current has reacquired the lock.
1994 // Lifecycle - the node representing current must not appear on any queues.
1995 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1996 // want residual elements associated with this thread left on any lists.
1997 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1998 assert(has_owner(current), "invariant");
1999 assert(!has_successor(current), "invariant");
2000 } // OSThreadWaitState()
2001
2002 current->set_current_waiting_monitor(nullptr);
2003
2004 guarantee(_recursions == 0, "invariant");
2005 int relock_count = JvmtiDeferredUpdates::get_and_reset_relock_count_after_wait(current);
2006 _recursions = save // restore the old recursion count
2007 + relock_count; // increased by the deferred relock count
2008 _waiters--; // decrement the number of waiters
2009
2010 // Verify a few postconditions
2011 assert(has_owner(current), "invariant");
2012 assert(!has_successor(current), "invariant");
2013 assert_mark_word_consistency();
2014
2015 if (ce != nullptr && ce->is_virtual_thread()) {
2016 current->post_vthread_pinned_event(&vthread_pinned_event, "Object.wait", result);
2017 }
2018
2019 // check if the notification happened
2020 if (!was_notified) {
2021 // no, it could be timeout or Thread.interrupt() or both
2022 // check for interrupt event, otherwise it is timeout
2023 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
2024 THROW(vmSymbols::java_lang_InterruptedException());
2025 }
2026 }
2027
2028 // NOTE: Spurious wake up will be consider as timeout.
2029 // Monitor notify has precedence over thread interrupt.
2030 }
2031
2032 // Consider:
2033 // If the lock is cool (entry_list == null && succ == null) and we're on an MP system
2034 // then instead of transferring a thread from the wait_set to the entry_list
2035 // we might just dequeue a thread from the wait_set and directly unpark() it.
2036
2037 bool ObjectMonitor::notify_internal(JavaThread* current) {
2038 bool did_notify = false;
2039 Thread::SpinAcquire(&_wait_set_lock);
2040 ObjectWaiter* iterator = dequeue_waiter();
2041 if (iterator != nullptr) {
2042 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
2043
2044 if (iterator->is_vthread()) {
2045 oop vthread = iterator->vthread();
2046 java_lang_VirtualThread::set_notified(vthread, true);
2047 int old_state = java_lang_VirtualThread::state(vthread);
2048 // If state is not WAIT/TIMED_WAIT then target could still be on
2049 // unmount transition, or wait could have already timed-out or target
2050 // could have been interrupted. In the first case, the target itself
2051 // will set the state to BLOCKED at the end of the unmount transition.
2052 // In the other cases the target would have been already unblocked so
2053 // there is nothing to do.
2054 if (old_state == java_lang_VirtualThread::WAIT ||
2055 old_state == java_lang_VirtualThread::TIMED_WAIT) {
2056 java_lang_VirtualThread::cmpxchg_state(vthread, old_state, java_lang_VirtualThread::BLOCKED);
2057 }
2058 // Increment counter *before* adding the vthread to the _entry_list.
2059 // Adding to _entry_list uses Atomic::cmpxchg() which already provides
2060 // a fence that prevents reordering of the stores.
2061 inc_unmounted_vthreads();
2062 }
2063
2064 iterator->_notifier_tid = JFR_THREAD_ID(current);
2065 did_notify = true;
2066 add_to_entry_list(current, iterator);
2067
2068 // _wait_set_lock protects the wait queue, not the entry_list. We could
2069 // move the add-to-entry_list operation, above, outside the critical section
2070 // protected by _wait_set_lock. In practice that's not useful. With the
2071 // exception of wait() timeouts and interrupts the monitor owner
2072 // is the only thread that grabs _wait_set_lock. There's almost no contention
2073 // on _wait_set_lock so it's not profitable to reduce the length of the
2074 // critical section.
2075
2076 if (!iterator->is_vthread()) {
2077 iterator->wait_reenter_begin(this);
2078
2079 // Read counter *after* adding the thread to the _entry_list.
2080 // Adding to _entry_list uses Atomic::cmpxchg() which already provides
2081 // a fence that prevents this load from floating up previous store.
2082 if (has_unmounted_vthreads()) {
2083 // Wake up the thread to alleviate some deadlock cases where the successor
2084 // that will be picked up when this thread releases the monitor is an unmounted
2085 // virtual thread that cannot run due to having run out of carriers. Upon waking
2086 // up, the thread will call reenter_internal() which will use timed-park in case
2087 // there is contention and there are still vthreads in the _entry_list.
2088 // If the target was interrupted or the wait timed-out at the same time, it could
2089 // have reached reenter_internal and read a false value of has_unmounted_vthreads()
2090 // before we added it to the _entry_list above. To deal with that case, we set _do_timed_park
2091 // which will be read by the target on the next loop iteration in reenter_internal.
2092 iterator->_do_timed_park = true;
2093 JavaThread* t = iterator->thread();
2094 t->_ParkEvent->unpark();
2095 }
2096 }
2097 }
2098 Thread::SpinRelease(&_wait_set_lock);
2099 return did_notify;
2100 }
2101
2102 static void post_monitor_notify_event(EventJavaMonitorNotify* event,
2103 ObjectMonitor* monitor,
2104 int notified_count) {
2105 assert(event != nullptr, "invariant");
2106 assert(monitor != nullptr, "invariant");
2107 const Klass* monitor_klass = monitor->object()->klass();
2108 if (ObjectMonitor::is_jfr_excluded(monitor_klass)) {
2109 return;
2110 }
2111 event->set_monitorClass(monitor_klass);
2112 // Set an address that is 'unique enough', such that events close in
2113 // time and with the same address are likely (but not guaranteed) to
2114 // belong to the same object.
2115 event->set_address((uintptr_t)monitor);
2116 event->set_notifiedCount(notified_count);
2117 event->commit();
2118 }
2119
2120 // Consider: a not-uncommon synchronization bug is to use notify() when
2121 // notifyAll() is more appropriate, potentially resulting in stranded
2122 // threads; this is one example of a lost wakeup. A useful diagnostic
2123 // option is to force all notify() operations to behave as notifyAll().
2124 //
2125 // Note: We can also detect many such problems with a "minimum wait".
2126 // When the "minimum wait" is set to a small non-zero timeout value
2127 // and the program does not hang whereas it did absent "minimum wait",
2128 // that suggests a lost wakeup bug.
2129
2130 void ObjectMonitor::notify(TRAPS) {
2131 JavaThread* current = THREAD;
2132 CHECK_OWNER(); // Throws IMSE if not owner.
2133 if (_wait_set == nullptr) {
2134 return;
2135 }
2136
2137 quick_notify(current);
2138 }
2139
2140 void ObjectMonitor::quick_notify(JavaThread* current) {
2141 assert(has_owner(current), "Precondition");
2142
2143 EventJavaMonitorNotify event;
2144 DTRACE_MONITOR_PROBE(notify, this, object(), current);
2145 int tally = notify_internal(current) ? 1 : 0;
2146
2147 if ((tally > 0) && event.should_commit()) {
2148 post_monitor_notify_event(&event, this, /* notified_count = */ tally);
2149 }
2150 }
2151
2152 // notifyAll() transfers the waiters one-at-a-time from the waitset to
2153 // the entry_list. If the waitset is "ABCD" (where A was added first
2154 // and D last) and the entry_list is ->X->Y->Z. After a notifyAll()
2155 // the waitset will be empty and the entry_list will be
2156 // ->D->C->B->A->X->Y->Z, and the next choosen successor will be Z.
2157
2158 void ObjectMonitor::notifyAll(TRAPS) {
2159 JavaThread* current = THREAD;
2160 CHECK_OWNER(); // Throws IMSE if not owner.
2161 if (_wait_set == nullptr) {
2162 return;
2163 }
2164
2165 quick_notifyAll(current);
2166 }
2167
2168 void ObjectMonitor::quick_notifyAll(JavaThread* current) {
2169 assert(has_owner(current), "Precondition");
2170
2171 EventJavaMonitorNotify event;
2172 DTRACE_MONITOR_PROBE(notifyAll, this, object(), current);
2173 int tally = 0;
2174 while (_wait_set != nullptr) {
2175 if (notify_internal(current)) {
2176 tally++;
2177 }
2178 }
2179
2180 if ((tally > 0) && event.should_commit()) {
2181 post_monitor_notify_event(&event, this, /* notified_count = */ tally);
2182 }
2183 }
2184
2185 void ObjectMonitor::vthread_wait(JavaThread* current, jlong millis, bool interruptible) {
2186 oop vthread = current->vthread();
2187 ObjectWaiter* node = new ObjectWaiter(vthread, this);
2188 node->_is_wait = true;
2189 node->_interruptible = interruptible;
2190 node->TState = ObjectWaiter::TS_WAIT;
2191 java_lang_VirtualThread::set_notified(vthread, false); // Reset notified flag
2192 java_lang_VirtualThread::set_interruptible_wait(vthread, interruptible);
2193
2194 // Enter the waiting queue, which is a circular doubly linked list in this case
2195 // but it could be a priority queue or any data structure.
2196 // _wait_set_lock protects the wait queue. Normally the wait queue is accessed only
2197 // by the owner of the monitor *except* in the case where park()
2198 // returns because of a timeout or interrupt. Contention is exceptionally rare
2199 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
2200
2201 Thread::SpinAcquire(&_wait_set_lock);
2202 add_waiter(node);
2203 Thread::SpinRelease(&_wait_set_lock);
2204
2205 node->_recursions = _recursions; // record the old recursion count
2206 _recursions = 0; // set the recursion level to be 0
2207 _waiters++; // increment the number of waiters
2208 exit(current); // exit the monitor
2209 guarantee(!has_owner(current), "invariant");
2210
2211 assert(java_lang_VirtualThread::state(vthread) == java_lang_VirtualThread::RUNNING, "wrong state for vthread");
2212 java_lang_VirtualThread::set_state(vthread, millis == 0 ? java_lang_VirtualThread::WAITING : java_lang_VirtualThread::TIMED_WAITING);
2213 java_lang_VirtualThread::set_timeout(vthread, millis);
2214
2215 // Save the ObjectWaiter* in the vthread since we will need it when resuming execution.
2216 java_lang_VirtualThread::set_objectWaiter(vthread, node);
2217 }
2218
2219 bool ObjectMonitor::vthread_wait_reenter(JavaThread* current, ObjectWaiter* node, ContinuationWrapper& cont) {
2220 // The first time we run after being preempted on Object.wait() we
2221 // need to check if we were interrupted or the wait timed-out, and
2222 // in that case remove ourselves from the _wait_set queue.
2223 if (node->TState == ObjectWaiter::TS_WAIT) {
2224 Thread::SpinAcquire(&_wait_set_lock);
2225 if (node->TState == ObjectWaiter::TS_WAIT) {
2226 dequeue_specific_waiter(node); // unlink from wait_set
2227 node->TState = ObjectWaiter::TS_RUN;
2228 }
2229 Thread::SpinRelease(&_wait_set_lock);
2230 }
2231
2232 // If this was an interrupted case, set the _interrupted boolean so that
2233 // once we re-acquire the monitor we know if we need to throw IE or not.
2234 ObjectWaiter::TStates state = node->TState;
2235 bool was_notified = state == ObjectWaiter::TS_ENTER;
2236 assert(was_notified || state == ObjectWaiter::TS_RUN, "");
2237 node->_interrupted = node->_interruptible && !was_notified && current->is_interrupted(false);
2238
2239 // Post JFR and JVMTI events. If non-interruptible we are in
2240 // ObjectLocker case so we don't post anything.
2241 EventJavaMonitorWait wait_event;
2242 if (node->_interruptible && (wait_event.should_commit() || JvmtiExport::should_post_monitor_waited())) {
2243 vthread_monitor_waited_event(current, node, cont, &wait_event, !was_notified && !node->_interrupted);
2244 }
2245
2246 // Mark that we are at reenter so that we don't call this method again.
2247 node->_at_reenter = true;
2248
2249 if (!was_notified) {
2250 bool acquired = vthread_monitor_enter(current, node);
2251 if (acquired) {
2252 guarantee(_recursions == 0, "invariant");
2253 _recursions = node->_recursions; // restore the old recursion count
2254 _waiters--; // decrement the number of waiters
2255
2256 if (node->_interrupted) {
2257 // We will throw at thaw end after finishing the mount transition.
2258 current->set_pending_interrupted_exception(true);
2259 }
2260
2261 delete node;
2262 // Clear the ObjectWaiter* from the vthread.
2263 java_lang_VirtualThread::set_objectWaiter(current->vthread(), nullptr);
2264 return true;
2265 }
2266 } else {
2267 // Already moved to _entry_list by notifier, so just add to contentions.
2268 add_to_contentions(1);
2269 }
2270 return false;
2271 }
2272
2273 // -----------------------------------------------------------------------------
2274 // Adaptive Spinning Support
2275 //
2276 // Adaptive spin-then-block - rational spinning
2277 //
2278 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
2279 // algorithm.
2280 //
2281 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
2282 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
2283 // (duration) or we can fix the count at approximately the duration of
2284 // a context switch and vary the frequency. Of course we could also
2285 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
2286 // For a description of 'Adaptive spin-then-block mutual exclusion in
2287 // multi-threaded processing,' see U.S. Pat. No. 8046758.
2288 //
2289 // This implementation varies the duration "D", where D varies with
2290 // the success rate of recent spin attempts. (D is capped at approximately
2291 // length of a round-trip context switch). The success rate for recent
2292 // spin attempts is a good predictor of the success rate of future spin
2293 // attempts. The mechanism adapts automatically to varying critical
2294 // section length (lock modality), system load and degree of parallelism.
2295 // D is maintained per-monitor in _SpinDuration and is initialized
2296 // optimistically. Spin frequency is fixed at 100%.
2297 //
2298 // Note that _SpinDuration is volatile, but we update it without locks
2299 // or atomics. The code is designed so that _SpinDuration stays within
2300 // a reasonable range even in the presence of races. The arithmetic
2301 // operations on _SpinDuration are closed over the domain of legal values,
2302 // so at worst a race will install and older but still legal value.
2303 // At the very worst this introduces some apparent non-determinism.
2304 // We might spin when we shouldn't or vice-versa, but since the spin
2305 // count are relatively short, even in the worst case, the effect is harmless.
2306 //
2307 // Care must be taken that a low "D" value does not become an
2308 // an absorbing state. Transient spinning failures -- when spinning
2309 // is overall profitable -- should not cause the system to converge
2310 // on low "D" values. We want spinning to be stable and predictable
2311 // and fairly responsive to change and at the same time we don't want
2312 // it to oscillate, become metastable, be "too" non-deterministic,
2313 // or converge on or enter undesirable stable absorbing states.
2314 //
2315 // We implement a feedback-based control system -- using past behavior
2316 // to predict future behavior. We face two issues: (a) if the
2317 // input signal is random then the spin predictor won't provide optimal
2318 // results, and (b) if the signal frequency is too high then the control
2319 // system, which has some natural response lag, will "chase" the signal.
2320 // (b) can arise from multimodal lock hold times. Transient preemption
2321 // can also result in apparent bimodal lock hold times.
2322 // Although sub-optimal, neither condition is particularly harmful, as
2323 // in the worst-case we'll spin when we shouldn't or vice-versa.
2324 // The maximum spin duration is rather short so the failure modes aren't bad.
2325 // To be conservative, I've tuned the gain in system to bias toward
2326 // _not spinning. Relatedly, the system can sometimes enter a mode where it
2327 // "rings" or oscillates between spinning and not spinning. This happens
2328 // when spinning is just on the cusp of profitability, however, so the
2329 // situation is not dire. The state is benign -- there's no need to add
2330 // hysteresis control to damp the transition rate between spinning and
2331 // not spinning.
2332
2333 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool
2334
2335 static int Knob_Bonus = 100; // spin success bonus
2336 static int Knob_Penalty = 200; // spin failure penalty
2337 static int Knob_Poverty = 1000;
2338 static int Knob_FixedSpin = 0;
2339 static int Knob_PreSpin = 10; // 20-100 likely better, but it's not better in my testing.
2340
2341 inline static int adjust_up(int spin_duration) {
2342 int x = spin_duration;
2343 if (x < ObjectMonitor::Knob_SpinLimit) {
2344 if (x < Knob_Poverty) {
2345 x = Knob_Poverty;
2346 }
2347 return x + Knob_Bonus;
2348 } else {
2349 return spin_duration;
2350 }
2351 }
2352
2353 inline static int adjust_down(int spin_duration) {
2354 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2355 // AIMD is globally stable.
2356 int x = spin_duration;
2357 if (x > 0) {
2358 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2359 // This is globally sample and tends to damp the response.
2360 x -= Knob_Penalty;
2361 if (x < 0) { x = 0; }
2362 return x;
2363 } else {
2364 return spin_duration;
2365 }
2366 }
2367
2368 bool ObjectMonitor::short_fixed_spin(JavaThread* current, int spin_count, bool adapt) {
2369 for (int ctr = 0; ctr < spin_count; ctr++) {
2370 TryLockResult status = try_lock(current);
2371 if (status == TryLockResult::Success) {
2372 if (adapt) {
2373 _SpinDuration = adjust_up(_SpinDuration);
2374 }
2375 return true;
2376 } else if (status == TryLockResult::Interference) {
2377 break;
2378 }
2379 SpinPause();
2380 }
2381 return false;
2382 }
2383
2384 // Spinning: Fixed frequency (100%), vary duration
2385 bool ObjectMonitor::try_spin(JavaThread* current) {
2386
2387 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
2388 int knob_fixed_spin = Knob_FixedSpin; // 0 (don't spin: default), 2000 good test
2389 if (knob_fixed_spin > 0) {
2390 return short_fixed_spin(current, knob_fixed_spin, false);
2391 }
2392
2393 // Admission control - verify preconditions for spinning
2394 //
2395 // We always spin a little bit, just to prevent _SpinDuration == 0 from
2396 // becoming an absorbing state. Put another way, we spin briefly to
2397 // sample, just in case the system load, parallelism, contention, or lock
2398 // modality changed.
2399
2400 int knob_pre_spin = Knob_PreSpin; // 10 (default), 100, 1000 or 2000
2401 if (short_fixed_spin(current, knob_pre_spin, true)) {
2402 return true;
2403 }
2404
2405 //
2406 // Consider the following alternative:
2407 // Periodically set _SpinDuration = _SpinLimit and try a long/full
2408 // spin attempt. "Periodically" might mean after a tally of
2409 // the # of failed spin attempts (or iterations) reaches some threshold.
2410 // This takes us into the realm of 1-out-of-N spinning, where we
2411 // hold the duration constant but vary the frequency.
2412
2413 int ctr = _SpinDuration;
2414 if (ctr <= 0) return false;
2415
2416 // We're good to spin ... spin ingress.
2417 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
2418 // when preparing to LD...CAS _owner, etc and the CAS is likely
2419 // to succeed.
2420 if (!has_successor()) {
2421 set_successor(current);
2422 }
2423 int64_t prv = NO_OWNER;
2424
2425 // There are three ways to exit the following loop:
2426 // 1. A successful spin where this thread has acquired the lock.
2427 // 2. Spin failure with prejudice
2428 // 3. Spin failure without prejudice
2429
2430 while (--ctr >= 0) {
2431
2432 // Periodic polling -- Check for pending GC
2433 // Threads may spin while they're unsafe.
2434 // We don't want spinning threads to delay the JVM from reaching
2435 // a stop-the-world safepoint or to steal cycles from GC.
2436 // If we detect a pending safepoint we abort in order that
2437 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
2438 // this thread, if safe, doesn't steal cycles from GC.
2439 // This is in keeping with the "no loitering in runtime" rule.
2440 // We periodically check to see if there's a safepoint pending.
2441 if ((ctr & 0xFF) == 0) {
2442 // Can't call SafepointMechanism::should_process() since that
2443 // might update the poll values and we could be in a thread_blocked
2444 // state here which is not allowed so just check the poll.
2445 if (SafepointMechanism::local_poll_armed(current)) {
2446 break;
2447 }
2448 SpinPause();
2449 }
2450
2451 // Probe _owner with TATAS
2452 // If this thread observes the monitor transition or flicker
2453 // from locked to unlocked to locked, then the odds that this
2454 // thread will acquire the lock in this spin attempt go down
2455 // considerably. The same argument applies if the CAS fails
2456 // or if we observe _owner change from one non-null value to
2457 // another non-null value. In such cases we might abort
2458 // the spin without prejudice or apply a "penalty" to the
2459 // spin count-down variable "ctr", reducing it by 100, say.
2460
2461 int64_t ox = owner_raw();
2462 if (ox == NO_OWNER) {
2463 ox = try_set_owner_from(NO_OWNER, current);
2464 if (ox == NO_OWNER) {
2465 // The CAS succeeded -- this thread acquired ownership
2466 // Take care of some bookkeeping to exit spin state.
2467 if (has_successor(current)) {
2468 clear_successor();
2469 }
2470
2471 // Increase _SpinDuration :
2472 // The spin was successful (profitable) so we tend toward
2473 // longer spin attempts in the future.
2474 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2475 // If we acquired the lock early in the spin cycle it
2476 // makes sense to increase _SpinDuration proportionally.
2477 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2478 _SpinDuration = adjust_up(_SpinDuration);
2479 return true;
2480 }
2481
2482 // The CAS failed ... we can take any of the following actions:
2483 // * penalize: ctr -= CASPenalty
2484 // * exit spin with prejudice -- abort without adapting spinner
2485 // * exit spin without prejudice.
2486 // * Since CAS is high-latency, retry again immediately.
2487 break;
2488 }
2489
2490 // Did lock ownership change hands ?
2491 if (ox != prv && prv != NO_OWNER) {
2492 break;
2493 }
2494 prv = ox;
2495
2496 if (!has_successor()) {
2497 set_successor(current);
2498 }
2499 }
2500
2501 // Spin failed with prejudice -- reduce _SpinDuration.
2502 if (ctr < 0) {
2503 _SpinDuration = adjust_down(_SpinDuration);
2504 }
2505
2506 if (has_successor(current)) {
2507 clear_successor();
2508 // Invariant: after setting succ=null a contending thread
2509 // must recheck-retry _owner before parking. This usually happens
2510 // in the normal usage of try_spin(), but it's safest
2511 // to make try_spin() as foolproof as possible.
2512 OrderAccess::fence();
2513 if (try_lock(current) == TryLockResult::Success) {
2514 return true;
2515 }
2516 }
2517
2518 return false;
2519 }
2520
2521
2522 // -----------------------------------------------------------------------------
2523 // wait_set management ...
2524
2525 ObjectWaiter::ObjectWaiter(JavaThread* current) {
2526 _next = nullptr;
2527 _prev = nullptr;
2528 _thread = current;
2529 _monitor = nullptr;
2530 _notifier_tid = 0;
2531 _recursions = 0;
2532 TState = TS_RUN;
2533 _is_wait = false;
2534 _at_reenter = false;
2535 _interrupted = false;
2536 _do_timed_park = false;
2537 _active = false;
2538 }
2539
2540 ObjectWaiter::ObjectWaiter(oop vthread, ObjectMonitor* mon) : ObjectWaiter(nullptr) {
2541 assert(oopDesc::is_oop(vthread), "");
2542 _vthread = OopHandle(JavaThread::thread_oop_storage(), vthread);
2543 _monitor = mon;
2544 }
2545
2546 ObjectWaiter::~ObjectWaiter() {
2547 if (is_vthread()) {
2548 assert(vthread() != nullptr, "");
2549 _vthread.release(JavaThread::thread_oop_storage());
2550 }
2551 }
2552
2553 oop ObjectWaiter::vthread() const {
2554 return _vthread.resolve();
2555 }
2556
2557 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2558 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(_thread, mon);
2559 }
2560
2561 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2562 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(_thread, _active);
2563 }
2564
2565 inline void ObjectMonitor::add_waiter(ObjectWaiter* node) {
2566 assert(node != nullptr, "should not add null node");
2567 assert(node->_prev == nullptr, "node already in list");
2568 assert(node->_next == nullptr, "node already in list");
2569 // put node at end of queue (circular doubly linked list)
2570 if (_wait_set == nullptr) {
2571 _wait_set = node;
2572 node->_prev = node;
2573 node->_next = node;
2574 } else {
2575 ObjectWaiter* head = _wait_set;
2576 ObjectWaiter* tail = head->_prev;
2577 assert(tail->_next == head, "invariant check");
2578 tail->_next = node;
2579 head->_prev = node;
2580 node->_next = head;
2581 node->_prev = tail;
2582 }
2583 }
2584
2585 inline ObjectWaiter* ObjectMonitor::dequeue_waiter() {
2586 // dequeue the very first waiter
2587 ObjectWaiter* waiter = _wait_set;
2588 if (waiter) {
2589 dequeue_specific_waiter(waiter);
2590 }
2591 return waiter;
2592 }
2593
2594 inline void ObjectMonitor::dequeue_specific_waiter(ObjectWaiter* node) {
2595 assert(node != nullptr, "should not dequeue nullptr node");
2596 assert(node->_prev != nullptr, "node already removed from list");
2597 assert(node->_next != nullptr, "node already removed from list");
2598 // when the waiter has woken up because of interrupt,
2599 // timeout or other spurious wake-up, dequeue the
2600 // waiter from waiting list
2601 ObjectWaiter* next = node->_next;
2602 if (next == node) {
2603 assert(node->_prev == node, "invariant check");
2604 _wait_set = nullptr;
2605 } else {
2606 ObjectWaiter* prev = node->_prev;
2607 assert(prev->_next == node, "invariant check");
2608 assert(next->_prev == node, "invariant check");
2609 next->_prev = prev;
2610 prev->_next = next;
2611 if (_wait_set == node) {
2612 _wait_set = next;
2613 }
2614 }
2615 node->_next = nullptr;
2616 node->_prev = nullptr;
2617 }
2618
2619 // -----------------------------------------------------------------------------
2620
2621 // One-shot global initialization for the sync subsystem.
2622 // We could also defer initialization and initialize on-demand
2623 // the first time we call ObjectSynchronizer::inflate().
2624 // Initialization would be protected - like so many things - by
2625 // the MonitorCache_lock.
2626
2627 void ObjectMonitor::Initialize() {
2628 assert(!InitDone, "invariant");
2629
2630 if (!os::is_MP()) {
2631 Knob_SpinLimit = 0;
2632 Knob_PreSpin = 0;
2633 Knob_FixedSpin = -1;
2634 }
2635
2636 _oop_storage = OopStorageSet::create_weak("ObjectSynchronizer Weak", mtSynchronizer);
2637
2638 DEBUG_ONLY(InitDone = true;)
2639 }
2640
2641 // We can't call this during Initialize() because BarrierSet needs to be set.
2642 void ObjectMonitor::Initialize2() {
2643 _vthread_list_head = OopHandle(JavaThread::thread_oop_storage(), nullptr);
2644 _vthread_unparker_ParkEvent = ParkEvent::Allocate(nullptr);
2645 }
2646
2647 void ObjectMonitor::print_on(outputStream* st) const {
2648 // The minimal things to print for markWord printing, more can be added for debugging and logging.
2649 st->print("{contentions=0x%08x,waiters=0x%08x"
2650 ",recursions=%zd,owner=" INT64_FORMAT "}",
2651 contentions(), waiters(), recursions(),
2652 owner_raw());
2653 }
2654 void ObjectMonitor::print() const { print_on(tty); }
2655
2656 #ifdef ASSERT
2657 // Print the ObjectMonitor like a debugger would:
2658 //
2659 // (ObjectMonitor) 0x00007fdfb6012e40 = {
2660 // _metadata = 0x0000000000000001
2661 // _object = 0x000000070ff45fd0
2662 // _pad_buf0 = {
2663 // [0] = '\0'
2664 // ...
2665 // [43] = '\0'
2666 // }
2667 // _owner = 0x0000000000000000
2668 // _previous_owner_tid = 0
2669 // _pad_buf1 = {
2670 // [0] = '\0'
2671 // ...
2672 // [47] = '\0'
2673 // }
2674 // _next_om = 0x0000000000000000
2675 // _recursions = 0
2676 // _entry_list = 0x0000000000000000
2677 // _entry_list_tail = 0x0000000000000000
2678 // _succ = 0x0000000000000000
2679 // _SpinDuration = 5000
2680 // _contentions = 0
2681 // _wait_set = 0x0000700009756248
2682 // _waiters = 1
2683 // _wait_set_lock = 0
2684 // }
2685 //
2686 void ObjectMonitor::print_debug_style_on(outputStream* st) const {
2687 st->print_cr("(ObjectMonitor*) " INTPTR_FORMAT " = {", p2i(this));
2688 st->print_cr(" _metadata = " INTPTR_FORMAT, _metadata);
2689 st->print_cr(" _object = " INTPTR_FORMAT, p2i(object_peek()));
2690 st->print_cr(" _pad_buf0 = {");
2691 st->print_cr(" [0] = '\\0'");
2692 st->print_cr(" ...");
2693 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf0) - 1);
2694 st->print_cr(" }");
2695 st->print_cr(" _owner = " INT64_FORMAT, owner_raw());
2696 st->print_cr(" _previous_owner_tid = " UINT64_FORMAT, _previous_owner_tid);
2697 st->print_cr(" _pad_buf1 = {");
2698 st->print_cr(" [0] = '\\0'");
2699 st->print_cr(" ...");
2700 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf1) - 1);
2701 st->print_cr(" }");
2702 st->print_cr(" _next_om = " INTPTR_FORMAT, p2i(next_om()));
2703 st->print_cr(" _recursions = %zd", _recursions);
2704 st->print_cr(" _entry_list = " INTPTR_FORMAT, p2i(_entry_list));
2705 st->print_cr(" _entry_list_tail = " INTPTR_FORMAT, p2i(_entry_list_tail));
2706 st->print_cr(" _succ = " INT64_FORMAT, successor());
2707 st->print_cr(" _SpinDuration = %d", _SpinDuration);
2708 st->print_cr(" _contentions = %d", contentions());
2709 st->print_cr(" _unmounted_vthreads = " INT64_FORMAT, _unmounted_vthreads);
2710 st->print_cr(" _wait_set = " INTPTR_FORMAT, p2i(_wait_set));
2711 st->print_cr(" _waiters = %d", _waiters);
2712 st->print_cr(" _wait_set_lock = %d", _wait_set_lock);
2713 st->print_cr("}");
2714 }
2715 #endif