1 /*
2 * Copyright (c) 1998, 2024, 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 "precompiled.hpp"
26 #include "classfile/vmSymbols.hpp"
27 #include "gc/shared/oopStorage.hpp"
28 #include "gc/shared/oopStorageSet.hpp"
29 #include "jfr/jfrEvents.hpp"
30 #include "jfr/support/jfrThreadId.hpp"
31 #include "logging/log.hpp"
32 #include "logging/logStream.hpp"
33 #include "memory/allocation.inline.hpp"
34 #include "memory/resourceArea.hpp"
35 #include "oops/markWord.hpp"
36 #include "oops/oop.inline.hpp"
37 #include "oops/oopHandle.inline.hpp"
38 #include "oops/weakHandle.inline.hpp"
39 #include "prims/jvmtiDeferredUpdates.hpp"
40 #include "prims/jvmtiExport.hpp"
41 #include "runtime/atomic.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/mutexLocker.hpp"
47 #include "runtime/objectMonitor.hpp"
48 #include "runtime/objectMonitor.inline.hpp"
49 #include "runtime/orderAccess.hpp"
50 #include "runtime/osThread.hpp"
51 #include "runtime/perfData.hpp"
52 #include "runtime/safefetch.hpp"
53 #include "runtime/safepointMechanism.inline.hpp"
54 #include "runtime/sharedRuntime.hpp"
55 #include "runtime/threads.hpp"
56 #include "services/threadService.hpp"
57 #include "utilities/dtrace.hpp"
58 #include "utilities/globalDefinitions.hpp"
59 #include "utilities/macros.hpp"
60 #include "utilities/preserveException.hpp"
61 #if INCLUDE_JFR
62 #include "jfr/support/jfrFlush.hpp"
63 #endif
64
65 #ifdef DTRACE_ENABLED
66
67 // Only bother with this argument setup if dtrace is available
68 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
69
70
71 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
72 char* bytes = nullptr; \
73 int len = 0; \
74 jlong jtid = SharedRuntime::get_java_tid(thread); \
75 Symbol* klassname = obj->klass()->name(); \
76 if (klassname != nullptr) { \
77 bytes = (char*)klassname->bytes(); \
78 len = klassname->utf8_length(); \
79 }
80
81 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
82 { \
83 if (DTraceMonitorProbes) { \
84 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
85 HOTSPOT_MONITOR_WAIT(jtid, \
86 (monitor), bytes, len, (millis)); \
87 } \
88 }
89
90 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
91 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
92 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
93 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
94 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
95
96 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
97 { \
98 if (DTraceMonitorProbes) { \
99 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
100 HOTSPOT_MONITOR_##probe(jtid, \
101 (uintptr_t)(monitor), bytes, len); \
102 } \
103 }
104
105 #else // ndef DTRACE_ENABLED
106
107 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
108 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
109
110 #endif // ndef DTRACE_ENABLED
111
112 DEBUG_ONLY(static volatile bool InitDone = false;)
113
114 OopStorage* ObjectMonitor::_oop_storage = nullptr;
115
116 OopHandle ObjectMonitor::_vthread_cxq_head;
117 ParkEvent* ObjectMonitor::_vthread_unparker_ParkEvent = nullptr;
118
119 static void post_virtual_thread_pinned_event(JavaThread* current, const char* reason) {
120 EventVirtualThreadPinned e;
121 if (e.should_commit()) {
122 e.set_pinnedReason(reason);
123 e.set_carrierThread(JFR_JVM_THREAD_ID(current));
124 e.commit();
125 }
126 }
127
128 // -----------------------------------------------------------------------------
129 // Theory of operations -- Monitors lists, thread residency, etc:
130 //
131 // * A thread acquires ownership of a monitor by successfully
132 // CAS()ing the _owner field from null to non-null.
133 //
134 // * Invariant: A thread appears on at most one monitor list --
135 // cxq, EntryList or WaitSet -- at any one time.
136 //
137 // * Contending threads "push" themselves onto the cxq with CAS
138 // and then spin/park.
139 //
140 // * After a contending thread eventually acquires the lock it must
141 // dequeue itself from either the EntryList or the cxq.
142 //
143 // * The exiting thread identifies and unparks an "heir presumptive"
144 // tentative successor thread on the EntryList. Critically, the
145 // exiting thread doesn't unlink the successor thread from the EntryList.
146 // After having been unparked, the wakee will recontend for ownership of
147 // the monitor. The successor (wakee) will either acquire the lock or
148 // re-park itself.
149 //
150 // Succession is provided for by a policy of competitive handoff.
151 // The exiting thread does _not_ grant or pass ownership to the
152 // successor thread. (This is also referred to as "handoff" succession").
153 // Instead the exiting thread releases ownership and possibly wakes
154 // a successor, so the successor can (re)compete for ownership of the lock.
155 // If the EntryList is empty but the cxq is populated the exiting
156 // thread will drain the cxq into the EntryList. It does so by
157 // by detaching the cxq (installing null with CAS) and folding
158 // the threads from the cxq into the EntryList. The EntryList is
159 // doubly linked, while the cxq is singly linked because of the
160 // CAS-based "push" used to enqueue recently arrived threads (RATs).
161 //
162 // * Concurrency invariants:
163 //
164 // -- only the monitor owner may access or mutate the EntryList.
165 // The mutex property of the monitor itself protects the EntryList
166 // from concurrent interference.
167 // -- Only the monitor owner may detach the cxq.
168 //
169 // * The monitor entry list operations avoid locks, but strictly speaking
170 // they're not lock-free. Enter is lock-free, exit is not.
171 // For a description of 'Methods and apparatus providing non-blocking access
172 // to a resource,' see U.S. Pat. No. 7844973.
173 //
174 // * The cxq can have multiple concurrent "pushers" but only one concurrent
175 // detaching thread. This mechanism is immune from the ABA corruption.
176 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
177 //
178 // * Taken together, the cxq and the EntryList constitute or form a
179 // single logical queue of threads stalled trying to acquire the lock.
180 // We use two distinct lists to improve the odds of a constant-time
181 // dequeue operation after acquisition (in the ::enter() epilogue) and
182 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm).
183 // A key desideratum is to minimize queue & monitor metadata manipulation
184 // that occurs while holding the monitor lock -- that is, we want to
185 // minimize monitor lock holds times. Note that even a small amount of
186 // fixed spinning will greatly reduce the # of enqueue-dequeue operations
187 // on EntryList|cxq. That is, spinning relieves contention on the "inner"
188 // locks and monitor metadata.
189 //
190 // Cxq points to the set of Recently Arrived Threads attempting entry.
191 // Because we push threads onto _cxq with CAS, the RATs must take the form of
192 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when
193 // the unlocking thread notices that EntryList is null but _cxq is != null.
194 //
195 // The EntryList is ordered by the prevailing queue discipline and
196 // can be organized in any convenient fashion, such as a doubly-linked list or
197 // a circular doubly-linked list. Critically, we want insert and delete operations
198 // to operate in constant-time. If we need a priority queue then something akin
199 // to Solaris' sleepq would work nicely. Viz.,
200 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
201 // Queue discipline is enforced at ::exit() time, when the unlocking thread
202 // drains the cxq into the EntryList, and orders or reorders the threads on the
203 // EntryList accordingly.
204 //
205 // Barring "lock barging", this mechanism provides fair cyclic ordering,
206 // somewhat similar to an elevator-scan.
207 //
208 // * The monitor synchronization subsystem avoids the use of native
209 // synchronization primitives except for the narrow platform-specific
210 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
211 // the semantics of park-unpark. Put another way, this monitor implementation
212 // depends only on atomic operations and park-unpark. The monitor subsystem
213 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
214 // underlying OS manages the READY<->RUN transitions.
215 //
216 // * Waiting threads reside on the WaitSet list -- wait() puts
217 // the caller onto the WaitSet.
218 //
219 // * notify() or notifyAll() simply transfers threads from the WaitSet to
220 // either the EntryList or cxq. Subsequent exit() operations will
221 // unpark the notifyee. Unparking a notifee in notify() is inefficient -
222 // it's likely the notifyee would simply impale itself on the lock held
223 // by the notifier.
224 //
225 // * An interesting alternative is to encode cxq as (List,LockByte) where
226 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary
227 // variable, like _recursions, in the scheme. The threads or Events that form
228 // the list would have to be aligned in 256-byte addresses. A thread would
229 // try to acquire the lock or enqueue itself with CAS, but exiting threads
230 // could use a 1-0 protocol and simply STB to set the LockByte to 0.
231 // Note that is is *not* word-tearing, but it does presume that full-word
232 // CAS operations are coherent with intermix with STB operations. That's true
233 // on most common processors.
234 //
235 // * See also http://blogs.sun.com/dave
236
237
238 // Check that object() and set_object() are called from the right context:
239 static void check_object_context() {
240 #ifdef ASSERT
241 Thread* self = Thread::current();
242 if (self->is_Java_thread()) {
243 // Mostly called from JavaThreads so sanity check the thread state.
244 JavaThread* jt = JavaThread::cast(self);
245 switch (jt->thread_state()) {
246 case _thread_in_vm: // the usual case
247 case _thread_in_Java: // during deopt
248 break;
249 default:
250 fatal("called from an unsafe thread state");
251 }
252 assert(jt->is_active_Java_thread(), "must be active JavaThread");
253 } else {
254 // However, ThreadService::get_current_contended_monitor()
255 // can call here via the VMThread so sanity check it.
256 assert(self->is_VM_thread(), "must be");
257 }
258 #endif // ASSERT
259 }
260
261 ObjectMonitor::ObjectMonitor(oop object) :
262 _header(markWord::zero()),
263 _object(_oop_storage, object),
264 _owner(nullptr),
265 _stack_locker(nullptr),
266 _previous_owner_tid(0),
267 _next_om(nullptr),
268 _recursions(0),
269 _EntryList(nullptr),
270 _cxq(nullptr),
271 _succ(nullptr),
272 _Responsible(nullptr),
273 _SpinDuration(ObjectMonitor::Knob_SpinLimit),
274 _contentions(0),
275 _WaitSet(nullptr),
276 _waiters(0),
277 _WaitSetLock(0)
278 { }
279
280 ObjectMonitor::~ObjectMonitor() {
281 _object.release(_oop_storage);
282 }
283
284 oop ObjectMonitor::object() const {
285 check_object_context();
286 return _object.resolve();
287 }
288
289 oop ObjectMonitor::object_peek() const {
290 return _object.peek();
291 }
292
293 void ObjectMonitor::ExitOnSuspend::operator()(JavaThread* current) {
294 if (current->is_suspended()) {
295 _om->_recursions = 0;
296 _om->_succ = nullptr;
297 // Don't need a full fence after clearing successor here because of the call to exit().
298 _om->exit(current, false /* not_suspended */);
299 _om_exited = true;
300
301 current->set_current_pending_monitor(_om);
302 }
303 }
304
305 void ObjectMonitor::ClearSuccOnSuspend::operator()(JavaThread* current) {
306 if (current->is_suspended()) {
307 if (_om->_succ == current) {
308 _om->_succ = nullptr;
309 OrderAccess::fence(); // always do a full fence when successor is cleared
310 }
311 }
312 }
313
314 // -----------------------------------------------------------------------------
315 // Enter support
316
317 bool ObjectMonitor::enter_for(JavaThread* locking_thread) {
318 // Used by ObjectSynchronizer::enter_for to enter for another thread.
319 // The monitor is private to or already owned by locking_thread which must be suspended.
320 // So this code may only contend with deflation.
321 assert(locking_thread == Thread::current() || locking_thread->is_obj_deopt_suspend(), "must be");
322
323 // Block out deflation as soon as possible.
324 add_to_contentions(1);
325
326 bool success = false;
327 if (!is_being_async_deflated()) {
328 void* prev_owner = try_set_owner_from(nullptr, locking_thread);
329
330 if (prev_owner == nullptr) {
331 assert(_recursions == 0, "invariant");
332 success = true;
333 } else if (prev_owner == owner_for(locking_thread)) {
334 _recursions++;
335 success = true;
336 } else if (prev_owner == DEFLATER_MARKER) {
337 // Racing with deflation.
338 prev_owner = try_set_owner_from(DEFLATER_MARKER, locking_thread);
339 if (prev_owner == DEFLATER_MARKER) {
340 // Cancelled deflation. Increment contentions as part of the deflation protocol.
341 add_to_contentions(1);
342 success = true;
343 } else if (prev_owner == nullptr) {
344 // At this point we cannot race with deflation as we have both incremented
345 // contentions, seen contention > 0 and seen a DEFLATER_MARKER.
346 // success will only be false if this races with something other than
347 // deflation.
348 prev_owner = try_set_owner_from(nullptr, locking_thread);
349 success = prev_owner == nullptr;
350 }
351 }
352 assert(success, "Failed to enter_for: locking_thread=" INTPTR_FORMAT
353 ", this=" INTPTR_FORMAT "{owner=" INTPTR_FORMAT "}, observed owner: " INTPTR_FORMAT,
354 p2i(locking_thread), p2i(this), p2i(owner_raw()), p2i(prev_owner));
355 } else {
356 // Async deflation is in progress and our contentions increment
357 // above lost the race to async deflation. Undo the work and
358 // force the caller to retry.
359 const oop l_object = object();
360 if (l_object != nullptr) {
361 // Attempt to restore the header/dmw to the object's header so that
362 // we only retry once if the deflater thread happens to be slow.
363 install_displaced_markword_in_object(l_object);
364 }
365 }
366
367 add_to_contentions(-1);
368
369 assert(!success || is_owner(locking_thread), "must be");
370
371 return success;
372 }
373
374 bool ObjectMonitor::enter(JavaThread* current) {
375 assert(current == JavaThread::current(), "must be");
376 // The following code is ordered to check the most common cases first
377 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
378
379 void* cur = try_set_owner_from(nullptr, current);
380 if (cur == nullptr) {
381 assert(_recursions == 0, "invariant");
382 return true;
383 }
384
385 if (cur == owner_for(current)) {
386 // TODO-FIXME: check for integer overflow! BUGID 6557169.
387 _recursions++;
388 return true;
389 }
390
391 // We've encountered genuine contention.
392
393 // Try one round of spinning *before* enqueueing current
394 // and before going through the awkward and expensive state
395 // transitions. The following spin is strictly optional ...
396 // Note that if we acquire the monitor from an initial spin
397 // we forgo posting JVMTI events and firing DTRACE probes.
398 if (TrySpin(current)) {
399 assert(owner_raw() == owner_for(current), "must be current: owner=" INTPTR_FORMAT, p2i(owner_raw()));
400 assert(_recursions == 0, "must be 0: recursions=" INTX_FORMAT, _recursions);
401 assert(object()->mark() == markWord::encode(this),
402 "object mark must match encoded this: mark=" INTPTR_FORMAT
403 ", encoded this=" INTPTR_FORMAT, object()->mark().value(),
404 markWord::encode(this).value());
405 return true;
406 }
407
408 assert(owner_raw() != owner_for(current), "invariant");
409 assert(_succ != current, "invariant");
410 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
411 assert(current->thread_state() != _thread_blocked, "invariant");
412
413 // Keep track of contention for JVM/TI and M&M queries.
414 add_to_contentions(1);
415 if (is_being_async_deflated()) {
416 // Async deflation is in progress and our contentions increment
417 // above lost the race to async deflation. Undo the work and
418 // force the caller to retry.
419 const oop l_object = object();
420 if (l_object != nullptr) {
421 // Attempt to restore the header/dmw to the object's header so that
422 // we only retry once if the deflater thread happens to be slow.
423 install_displaced_markword_in_object(l_object);
424 }
425 add_to_contentions(-1);
426 return false;
427 }
428
429 JFR_ONLY(JfrConditionalFlush<EventJavaMonitorEnter> flush(current);)
430 EventJavaMonitorEnter event;
431 if (event.is_started()) {
432 event.set_monitorClass(object()->klass());
433 // Set an address that is 'unique enough', such that events close in
434 // time and with the same address are likely (but not guaranteed) to
435 // belong to the same object.
436 event.set_address((uintptr_t)this);
437 }
438
439 { // Change java thread status to indicate blocked on monitor enter.
440 JavaThreadBlockedOnMonitorEnterState jtbmes(current, this);
441
442 assert(current->current_pending_monitor() == nullptr, "invariant");
443 current->set_current_pending_monitor(this);
444
445 DTRACE_MONITOR_PROBE(contended__enter, this, object(), current);
446 if (JvmtiExport::should_post_monitor_contended_enter()) {
447 JvmtiExport::post_monitor_contended_enter(current, this);
448
449 // The current thread does not yet own the monitor and does not
450 // yet appear on any queues that would get it made the successor.
451 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
452 // handler cannot accidentally consume an unpark() meant for the
453 // ParkEvent associated with this ObjectMonitor.
454 }
455
456 #ifdef LOOM_MONITOR_SUPPORT
457 ContinuationEntry* ce = current->last_continuation();
458 if (ce != nullptr && ce->is_virtual_thread() && current->is_on_monitorenter()) {
459 int result = Continuation::try_preempt(current, ce->cont_oop(current));
460 if (result == freeze_ok) {
461 bool acquired = HandlePreemptedVThread(current);
462 DEBUG_ONLY(int state = java_lang_VirtualThread::state(current->vthread()));
463 assert((acquired && current->preemption_cancelled() && state == java_lang_VirtualThread::RUNNING) ||
464 (!acquired && !current->preemption_cancelled() && state == java_lang_VirtualThread::BLOCKING), "invariant");
465 return true;
466 }
467 if (result == freeze_pinned_native) {
468 post_virtual_thread_pinned_event(current, "Native frame or <clinit> on stack");
469 }
470 }
471 #endif
472
473 OSThreadContendState osts(current->osthread());
474
475 assert(current->thread_state() == _thread_in_vm, "invariant");
476
477 for (;;) {
478 ExitOnSuspend eos(this);
479 {
480 ThreadBlockInVMPreprocess<ExitOnSuspend> tbivs(current, eos, true /* allow_suspend */);
481 EnterI(current);
482 current->set_current_pending_monitor(nullptr);
483 // We can go to a safepoint at the end of this block. If we
484 // do a thread dump during that safepoint, then this thread will show
485 // as having "-locked" the monitor, but the OS and java.lang.Thread
486 // states will still report that the thread is blocked trying to
487 // acquire it.
488 // If there is a suspend request, ExitOnSuspend will exit the OM
489 // and set the OM as pending.
490 }
491 if (!eos.exited()) {
492 // ExitOnSuspend did not exit the OM
493 assert(owner_raw() == owner_for(current), "invariant");
494 break;
495 }
496 }
497
498 // We've just gotten past the enter-check-for-suspend dance and we now own
499 // the monitor free and clear.
500 }
501
502 add_to_contentions(-1);
503 assert(contentions() >= 0, "must not be negative: contentions=%d", contentions());
504
505 // Must either set _recursions = 0 or ASSERT _recursions == 0.
506 assert(_recursions == 0, "invariant");
507 assert(owner_raw() == owner_for(current), "invariant");
508 assert(_succ != current, "invariant");
509 assert(object()->mark() == markWord::encode(this), "invariant");
510
511 // The thread -- now the owner -- is back in vm mode.
512 // Report the glorious news via TI,DTrace and jvmstat.
513 // The probe effect is non-trivial. All the reportage occurs
514 // while we hold the monitor, increasing the length of the critical
515 // section. Amdahl's parallel speedup law comes vividly into play.
516 //
517 // Another option might be to aggregate the events (thread local or
518 // per-monitor aggregation) and defer reporting until a more opportune
519 // time -- such as next time some thread encounters contention but has
520 // yet to acquire the lock. While spinning that thread could
521 // spinning we could increment JVMStat counters, etc.
522
523 DTRACE_MONITOR_PROBE(contended__entered, this, object(), current);
524 if (JvmtiExport::should_post_monitor_contended_entered()) {
525 JvmtiExport::post_monitor_contended_entered(current, this);
526
527 // The current thread already owns the monitor and is not going to
528 // call park() for the remainder of the monitor enter protocol. So
529 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
530 // event handler consumed an unpark() issued by the thread that
531 // just exited the monitor.
532 }
533 if (event.should_commit()) {
534 event.set_previousOwner(_previous_owner_tid);
535 event.commit();
536 }
537 OM_PERFDATA_OP(ContendedLockAttempts, inc());
538 return true;
539 }
540
541 // Caveat: TryLock() is not necessarily serializing if it returns failure.
542 // Callers must compensate as needed.
543
544 ObjectMonitor::TryLockResult ObjectMonitor::TryLock(JavaThread* current) {
545 void* own = owner_raw();
546 if (own != nullptr) return TryLockResult::HasOwner;
547 if (try_set_owner_from(nullptr, current) == nullptr) {
548 assert(_recursions == 0, "invariant");
549 return TryLockResult::Success;
550 }
551 // The lock had been free momentarily, but we lost the race to the lock.
552 // Interference -- the CAS failed.
553 // We can either return -1 or retry.
554 // Retry doesn't make as much sense because the lock was just acquired.
555 return TryLockResult::Interference;
556 }
557
558 // Deflate the specified ObjectMonitor if not in-use. Returns true if it
559 // was deflated and false otherwise.
560 //
561 // The async deflation protocol sets owner to DEFLATER_MARKER and
562 // makes contentions negative as signals to contending threads that
563 // an async deflation is in progress. There are a number of checks
564 // as part of the protocol to make sure that the calling thread has
565 // not lost the race to a contending thread.
566 //
567 // The ObjectMonitor has been successfully async deflated when:
568 // (contentions < 0)
569 // Contending threads that see that condition know to retry their operation.
570 //
571 bool ObjectMonitor::deflate_monitor() {
572 if (is_busy()) {
573 // Easy checks are first - the ObjectMonitor is busy so no deflation.
574 return false;
575 }
576
577 const oop obj = object_peek();
578
579 if (obj == nullptr) {
580 // If the object died, we can recycle the monitor without racing with
581 // Java threads. The GC already broke the association with the object.
582 set_owner_from_raw(nullptr, DEFLATER_MARKER);
583 assert(contentions() >= 0, "must be non-negative: contentions=%d", contentions());
584 _contentions = INT_MIN; // minimum negative int
585 } else {
586 // Attempt async deflation protocol.
587
588 // Set a null owner to DEFLATER_MARKER to force any contending thread
589 // through the slow path. This is just the first part of the async
590 // deflation dance.
591 if (try_set_owner_from_raw(nullptr, DEFLATER_MARKER) != nullptr) {
592 // The owner field is no longer null so we lost the race since the
593 // ObjectMonitor is now busy.
594 return false;
595 }
596
597 if (contentions() > 0 || _waiters != 0) {
598 // Another thread has raced to enter the ObjectMonitor after
599 // is_busy() above or has already entered and waited on
600 // it which makes it busy so no deflation. Restore owner to
601 // null if it is still DEFLATER_MARKER.
602 if (try_set_owner_from_raw(DEFLATER_MARKER, nullptr) != DEFLATER_MARKER) {
603 // Deferred decrement for the JT EnterI() that cancelled the async deflation.
604 add_to_contentions(-1);
605 }
606 return false;
607 }
608
609 // Make a zero contentions field negative to force any contending threads
610 // to retry. This is the second part of the async deflation dance.
611 if (Atomic::cmpxchg(&_contentions, 0, INT_MIN) != 0) {
612 // Contentions was no longer 0 so we lost the race since the
613 // ObjectMonitor is now busy. Restore owner to null if it is
614 // still DEFLATER_MARKER:
615 if (try_set_owner_from_raw(DEFLATER_MARKER, nullptr) != DEFLATER_MARKER) {
616 // Deferred decrement for the JT EnterI() that cancelled the async deflation.
617 add_to_contentions(-1);
618 }
619 return false;
620 }
621 }
622
623 // Sanity checks for the races:
624 guarantee(owner_is_DEFLATER_MARKER(), "must be deflater marker");
625 guarantee(contentions() < 0, "must be negative: contentions=%d",
626 contentions());
627 guarantee(_waiters == 0, "must be 0: waiters=%d", _waiters);
628 guarantee(_cxq == nullptr, "must be no contending threads: cxq="
629 INTPTR_FORMAT, p2i(_cxq));
630 guarantee(_EntryList == nullptr,
631 "must be no entering threads: EntryList=" INTPTR_FORMAT,
632 p2i(_EntryList));
633
634 if (obj != nullptr) {
635 if (log_is_enabled(Trace, monitorinflation)) {
636 ResourceMark rm;
637 log_trace(monitorinflation)("deflate_monitor: object=" INTPTR_FORMAT
638 ", mark=" INTPTR_FORMAT ", type='%s'",
639 p2i(obj), obj->mark().value(),
640 obj->klass()->external_name());
641 }
642
643 // Install the old mark word if nobody else has already done it.
644 install_displaced_markword_in_object(obj);
645 }
646
647 // We leave owner == DEFLATER_MARKER and contentions < 0
648 // to force any racing threads to retry.
649 return true; // Success, ObjectMonitor has been deflated.
650 }
651
652 // Install the displaced mark word (dmw) of a deflating ObjectMonitor
653 // into the header of the object associated with the monitor. This
654 // idempotent method is called by a thread that is deflating a
655 // monitor and by other threads that have detected a race with the
656 // deflation process.
657 void ObjectMonitor::install_displaced_markword_in_object(const oop obj) {
658 // This function must only be called when (owner == DEFLATER_MARKER
659 // && contentions <= 0), but we can't guarantee that here because
660 // those values could change when the ObjectMonitor gets moved from
661 // the global free list to a per-thread free list.
662
663 guarantee(obj != nullptr, "must be non-null");
664
665 // Separate loads in is_being_async_deflated(), which is almost always
666 // called before this function, from the load of dmw/header below.
667
668 // _contentions and dmw/header may get written by different threads.
669 // Make sure to observe them in the same order when having several observers.
670 OrderAccess::loadload_for_IRIW();
671
672 const oop l_object = object_peek();
673 if (l_object == nullptr) {
674 // ObjectMonitor's object ref has already been cleared by async
675 // deflation or GC so we're done here.
676 return;
677 }
678 assert(l_object == obj, "object=" INTPTR_FORMAT " must equal obj="
679 INTPTR_FORMAT, p2i(l_object), p2i(obj));
680
681 markWord dmw = header();
682 // The dmw has to be neutral (not null, not locked and not marked).
683 assert(dmw.is_neutral(), "must be neutral: dmw=" INTPTR_FORMAT, dmw.value());
684
685 // Install displaced mark word if the object's header still points
686 // to this ObjectMonitor. More than one racing caller to this function
687 // can rarely reach this point, but only one can win.
688 markWord res = obj->cas_set_mark(dmw, markWord::encode(this));
689 if (res != markWord::encode(this)) {
690 // This should be rare so log at the Info level when it happens.
691 log_info(monitorinflation)("install_displaced_markword_in_object: "
692 "failed cas_set_mark: new_mark=" INTPTR_FORMAT
693 ", old_mark=" INTPTR_FORMAT ", res=" INTPTR_FORMAT,
694 dmw.value(), markWord::encode(this).value(),
695 res.value());
696 }
697
698 // Note: It does not matter which thread restored the header/dmw
699 // into the object's header. The thread deflating the monitor just
700 // wanted the object's header restored and it is. The threads that
701 // detected a race with the deflation process also wanted the
702 // object's header restored before they retry their operation and
703 // because it is restored they will only retry once.
704 }
705
706 // Convert the fields used by is_busy() to a string that can be
707 // used for diagnostic output.
708 const char* ObjectMonitor::is_busy_to_string(stringStream* ss) {
709 ss->print("is_busy: waiters=%d"
710 ", contentions=%d"
711 ", owner=" INTPTR_FORMAT
712 ", cxq=" PTR_FORMAT
713 ", EntryList=" PTR_FORMAT,
714 _waiters,
715 (contentions() > 0 ? contentions() : 0),
716 owner_is_DEFLATER_MARKER()
717 // We report null instead of DEFLATER_MARKER here because is_busy()
718 // ignores DEFLATER_MARKER values.
719 ? p2i(nullptr)
720 : p2i(owner_raw()),
721 p2i(_cxq),
722 p2i(_EntryList));
723 return ss->base();
724 }
725
726 #define MAX_RECHECK_INTERVAL 1000
727
728 void ObjectMonitor::EnterI(JavaThread* current) {
729 assert(current->thread_state() == _thread_blocked, "invariant");
730
731 // Try the lock - TATAS
732 if (TryLock(current) == TryLockResult::Success) {
733 assert(_succ != current, "invariant");
734 assert(owner_raw() == owner_for(current), "invariant");
735 assert(_Responsible != current, "invariant");
736 return;
737 }
738
739 if (try_set_owner_from(DEFLATER_MARKER, current) == DEFLATER_MARKER) {
740 // Cancelled the in-progress async deflation by changing owner from
741 // DEFLATER_MARKER to current. As part of the contended enter protocol,
742 // contentions was incremented to a positive value before EnterI()
743 // was called and that prevents the deflater thread from winning the
744 // last part of the 2-part async deflation protocol. After EnterI()
745 // returns to enter(), contentions is decremented because the caller
746 // now owns the monitor. We bump contentions an extra time here to
747 // prevent the deflater thread from winning the last part of the
748 // 2-part async deflation protocol after the regular decrement
749 // occurs in enter(). The deflater thread will decrement contentions
750 // after it recognizes that the async deflation was cancelled.
751 add_to_contentions(1);
752 assert(_succ != current, "invariant");
753 assert(_Responsible != current, "invariant");
754 return;
755 }
756
757 assert(InitDone, "Unexpectedly not initialized");
758
759 // We try one round of spinning *before* enqueueing current.
760 //
761 // If the _owner is ready but OFFPROC we could use a YieldTo()
762 // operation to donate the remainder of this thread's quantum
763 // to the owner. This has subtle but beneficial affinity
764 // effects.
765
766 if (TrySpin(current)) {
767 assert(owner_raw() == owner_for(current), "invariant");
768 assert(_succ != current, "invariant");
769 assert(_Responsible != current, "invariant");
770 return;
771 }
772
773 // The Spin failed -- Enqueue and park the thread ...
774 assert(_succ != current, "invariant");
775 assert(owner_raw() != owner_for(current), "invariant");
776 assert(_Responsible != current, "invariant");
777
778 // Enqueue "current" on ObjectMonitor's _cxq.
779 //
780 // Node acts as a proxy for current.
781 // As an aside, if were to ever rewrite the synchronization code mostly
782 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
783 // Java objects. This would avoid awkward lifecycle and liveness issues,
784 // as well as eliminate a subset of ABA issues.
785 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
786
787 ObjectWaiter node(current);
788 current->_ParkEvent->reset();
789 node._prev = (ObjectWaiter*) 0xBAD;
790 node.TState = ObjectWaiter::TS_CXQ;
791
792 // Push "current" onto the front of the _cxq.
793 // Once on cxq/EntryList, current stays on-queue until it acquires the lock.
794 // Note that spinning tends to reduce the rate at which threads
795 // enqueue and dequeue on EntryList|cxq.
796 ObjectWaiter* nxt;
797 for (;;) {
798 node._next = nxt = _cxq;
799 if (Atomic::cmpxchg(&_cxq, nxt, &node) == nxt) break;
800
801 // Interference - the CAS failed because _cxq changed. Just retry.
802 // As an optional optimization we retry the lock.
803 if (TryLock(current) == TryLockResult::Success) {
804 assert(_succ != current, "invariant");
805 assert(owner_raw() == owner_for(current), "invariant");
806 assert(_Responsible != current, "invariant");
807 return;
808 }
809 }
810
811 // Check for cxq|EntryList edge transition to non-null. This indicates
812 // the onset of contention. While contention persists exiting threads
813 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
814 // operations revert to the faster 1-0 mode. This enter operation may interleave
815 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
816 // arrange for one of the contending thread to use a timed park() operations
817 // to detect and recover from the race. (Stranding is form of progress failure
818 // where the monitor is unlocked but all the contending threads remain parked).
819 // That is, at least one of the contended threads will periodically poll _owner.
820 // One of the contending threads will become the designated "Responsible" thread.
821 // The Responsible thread uses a timed park instead of a normal indefinite park
822 // operation -- it periodically wakes and checks for and recovers from potential
823 // strandings admitted by 1-0 exit operations. We need at most one Responsible
824 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
825 // be responsible for a monitor.
826 //
827 // Currently, one of the contended threads takes on the added role of "Responsible".
828 // A viable alternative would be to use a dedicated "stranding checker" thread
829 // that periodically iterated over all the threads (or active monitors) and unparked
830 // successors where there was risk of stranding. This would help eliminate the
831 // timer scalability issues we see on some platforms as we'd only have one thread
832 // -- the checker -- parked on a timer.
833
834 if (nxt == nullptr && _EntryList == nullptr) {
835 // Try to assume the role of responsible thread for the monitor.
836 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=current }
837 Atomic::replace_if_null(&_Responsible, current);
838 }
839
840 // The lock might have been released while this thread was occupied queueing
841 // itself onto _cxq. To close the race and avoid "stranding" and
842 // progress-liveness failure we must resample-retry _owner before parking.
843 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
844 // In this case the ST-MEMBAR is accomplished with CAS().
845 //
846 // TODO: Defer all thread state transitions until park-time.
847 // Since state transitions are heavy and inefficient we'd like
848 // to defer the state transitions until absolutely necessary,
849 // and in doing so avoid some transitions ...
850
851 int nWakeups = 0;
852 int recheckInterval = 1;
853 bool do_timed_parked = false;
854
855 ContinuationEntry* ce = current->last_continuation();
856 if (ce != nullptr && ce->is_virtual_thread()) {
857 do_timed_parked = true;
858 }
859
860 for (;;) {
861
862 if (TryLock(current) == TryLockResult::Success) {
863 break;
864 }
865 assert(owner_raw() != owner_for(current), "invariant");
866
867 // park self
868 if (_Responsible == current || do_timed_parked) {
869 current->_ParkEvent->park((jlong) recheckInterval);
870 // Increase the recheckInterval, but clamp the value.
871 recheckInterval *= 8;
872 if (recheckInterval > MAX_RECHECK_INTERVAL) {
873 recheckInterval = MAX_RECHECK_INTERVAL;
874 }
875 } else {
876 current->_ParkEvent->park();
877 }
878
879 if (TryLock(current) == TryLockResult::Success) {
880 break;
881 }
882
883 if (try_set_owner_from(DEFLATER_MARKER, current) == DEFLATER_MARKER) {
884 // Cancelled the in-progress async deflation by changing owner from
885 // DEFLATER_MARKER to current. As part of the contended enter protocol,
886 // contentions was incremented to a positive value before EnterI()
887 // was called and that prevents the deflater thread from winning the
888 // last part of the 2-part async deflation protocol. After EnterI()
889 // returns to enter(), contentions is decremented because the caller
890 // now owns the monitor. We bump contentions an extra time here to
891 // prevent the deflater thread from winning the last part of the
892 // 2-part async deflation protocol after the regular decrement
893 // occurs in enter(). The deflater thread will decrement contentions
894 // after it recognizes that the async deflation was cancelled.
895 add_to_contentions(1);
896 break;
897 }
898
899 // The lock is still contested.
900 // Keep a tally of the # of futile wakeups.
901 // Note that the counter is not protected by a lock or updated by atomics.
902 // That is by design - we trade "lossy" counters which are exposed to
903 // races during updates for a lower probe effect.
904
905 // This PerfData object can be used in parallel with a safepoint.
906 // See the work around in PerfDataManager::destroy().
907 OM_PERFDATA_OP(FutileWakeups, inc());
908 ++nWakeups;
909
910 // Assuming this is not a spurious wakeup we'll normally find _succ == current.
911 // We can defer clearing _succ until after the spin completes
912 // TrySpin() must tolerate being called with _succ == current.
913 // Try yet another round of adaptive spinning.
914 if (TrySpin(current)) {
915 break;
916 }
917
918 // We can find that we were unpark()ed and redesignated _succ while
919 // we were spinning. That's harmless. If we iterate and call park(),
920 // park() will consume the event and return immediately and we'll
921 // just spin again. This pattern can repeat, leaving _succ to simply
922 // spin on a CPU.
923
924 if (_succ == current) _succ = nullptr;
925
926 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
927 OrderAccess::fence();
928 }
929
930 // Egress :
931 // current has acquired the lock -- Unlink current from the cxq or EntryList.
932 // Normally we'll find current on the EntryList .
933 // From the perspective of the lock owner (this thread), the
934 // EntryList is stable and cxq is prepend-only.
935 // The head of cxq is volatile but the interior is stable.
936 // In addition, current.TState is stable.
937
938 assert(owner_raw() == owner_for(current), "invariant");
939
940 UnlinkAfterAcquire(current, &node);
941 if (_succ == current) _succ = nullptr;
942
943 assert(_succ != current, "invariant");
944 if (_Responsible == current) {
945 _Responsible = nullptr;
946 OrderAccess::fence(); // Dekker pivot-point
947
948 // We may leave threads on cxq|EntryList without a designated
949 // "Responsible" thread. This is benign. When this thread subsequently
950 // exits the monitor it can "see" such preexisting "old" threads --
951 // threads that arrived on the cxq|EntryList before the fence, above --
952 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
953 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
954 // non-null and elect a new "Responsible" timer thread.
955 //
956 // This thread executes:
957 // ST Responsible=null; MEMBAR (in enter epilogue - here)
958 // LD cxq|EntryList (in subsequent exit)
959 //
960 // Entering threads in the slow/contended path execute:
961 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
962 // The (ST cxq; MEMBAR) is accomplished with CAS().
963 //
964 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
965 // exit operation from floating above the ST Responsible=null.
966 }
967
968 // We've acquired ownership with CAS().
969 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
970 // But since the CAS() this thread may have also stored into _succ,
971 // EntryList, cxq or Responsible. These meta-data updates must be
972 // visible __before this thread subsequently drops the lock.
973 // Consider what could occur if we didn't enforce this constraint --
974 // STs to monitor meta-data and user-data could reorder with (become
975 // visible after) the ST in exit that drops ownership of the lock.
976 // Some other thread could then acquire the lock, but observe inconsistent
977 // or old monitor meta-data and heap data. That violates the JMM.
978 // To that end, the 1-0 exit() operation must have at least STST|LDST
979 // "release" barrier semantics. Specifically, there must be at least a
980 // STST|LDST barrier in exit() before the ST of null into _owner that drops
981 // the lock. The barrier ensures that changes to monitor meta-data and data
982 // protected by the lock will be visible before we release the lock, and
983 // therefore before some other thread (CPU) has a chance to acquire the lock.
984 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
985 //
986 // Critically, any prior STs to _succ or EntryList must be visible before
987 // the ST of null into _owner in the *subsequent* (following) corresponding
988 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
989 // execute a serializing instruction.
990
991 return;
992 }
993
994 bool ObjectMonitor::HandlePreemptedVThread(JavaThread* current) {
995 // Either because we acquire the lock below or because we will preempt the
996 // vthread clear the _current_pending_monitor field from the current JavaThread.
997 current->set_current_pending_monitor(nullptr);
998
999 // Try once more after freezing the continuation.
1000 if (TryLock(current) == TryLockResult::Success) {
1001 assert(owner_raw() == owner_for(current), "invariant");
1002 assert(_succ != current, "invariant");
1003 assert(_Responsible != current, "invariant");
1004 current->set_preemption_cancelled(true);
1005 add_to_contentions(-1);
1006 return true;
1007 }
1008
1009 if (try_set_owner_from(DEFLATER_MARKER, current) == DEFLATER_MARKER) {
1010 // Cancelled the in-progress async deflation by changing owner from
1011 // DEFLATER_MARKER to current. As part of the contended enter protocol,
1012 // contentions was incremented to a positive value before this call to
1013 // HandlePreemptedVThread(). We avoid decrementing contentions to
1014 // prevent the deflater thread from winning the last part of the
1015 // 2-part async deflation protocol. The deflater thread will decrement
1016 // contentions after it recognizes that the async deflation was cancelled.
1017 assert(_succ != current, "invariant");
1018 assert(_Responsible != current, "invariant");
1019 current->set_preemption_cancelled(true);
1020 return true;
1021 }
1022
1023 oop vthread = current->vthread();
1024 assert(java_lang_VirtualThread::state(vthread) == java_lang_VirtualThread::RUNNING, "wrong state for vthread");
1025 java_lang_VirtualThread::set_state(vthread, java_lang_VirtualThread::BLOCKING);
1026
1027 ObjectWaiter* node = new ObjectWaiter(vthread);
1028 node->_prev = (ObjectWaiter*) 0xBAD;
1029 node->TState = ObjectWaiter::TS_CXQ;
1030
1031 // Push node associated with vthread onto the front of the _cxq.
1032 ObjectWaiter* nxt;
1033 for (;;) {
1034 node->_next = nxt = _cxq;
1035 if (Atomic::cmpxchg(&_cxq, nxt, node) == nxt) break;
1036
1037 // Interference - the CAS failed because _cxq changed. Just retry.
1038 // As an optional optimization we retry the lock.
1039 if (TryLock(current) == TryLockResult::Success) {
1040 assert(owner_raw() == owner_for(current), "invariant");
1041 assert(_succ != current, "invariant");
1042 assert(_Responsible != current, "invariant");
1043 current->set_preemption_cancelled(true);
1044 java_lang_VirtualThread::set_state(vthread, java_lang_VirtualThread::RUNNING);
1045 add_to_contentions(-1);
1046 delete node;
1047 return true;
1048 }
1049 }
1050
1051 // We have to try once more since owner could have exited monitor and checked
1052 // _cxq before we added the node to the queue.
1053 if (TryLock(current) == TryLockResult::Success) {
1054 assert(owner_raw() == owner_for(current), "invariant");
1055 assert(_Responsible != current, "invariant");
1056 current->set_preemption_cancelled(true);
1057 java_lang_VirtualThread::set_state(vthread, java_lang_VirtualThread::RUNNING);
1058 UnlinkAfterAcquire(current, node, vthread);
1059 delete node;
1060 if (_succ == (JavaThread*)java_lang_Thread::thread_id(vthread)) _succ = nullptr;
1061 add_to_contentions(-1);
1062 return true;
1063 }
1064
1065 if (nxt == nullptr && _EntryList == nullptr) {
1066 // The C2 unlock() fast path first checks if _cxq and _EntryList are empty and
1067 // if they are it just clears the _owner field. Since we always run the risk of
1068 // having that check happening before we added the node to _cxq and the release
1069 // of the monitor happening after the last TryLock attempt we need to do something
1070 // to avoid stranding. We set the _Responsible field which results in a timed-wait.
1071 if (Atomic::replace_if_null(&_Responsible, (JavaThread*)java_lang_Thread::thread_id(vthread))) {
1072 java_lang_VirtualThread::set_recheckInterval(vthread, 1);
1073 }
1074 }
1075
1076 return false;
1077 }
1078
1079 // ReenterI() is a specialized inline form of the latter half of the
1080 // contended slow-path from EnterI(). We use ReenterI() only for
1081 // monitor reentry in wait().
1082 //
1083 // In the future we should reconcile EnterI() and ReenterI().
1084
1085 void ObjectMonitor::ReenterI(JavaThread* current, ObjectWaiter* currentNode) {
1086 assert(current != nullptr, "invariant");
1087 assert(currentNode != nullptr, "invariant");
1088 assert(currentNode->_thread == current, "invariant");
1089 assert(_waiters > 0, "invariant");
1090 assert(object()->mark() == markWord::encode(this), "invariant");
1091
1092 assert(current->thread_state() != _thread_blocked, "invariant");
1093
1094 int nWakeups = 0;
1095 for (;;) {
1096 ObjectWaiter::TStates v = currentNode->TState;
1097 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1098 assert(owner_raw() != owner_for(current), "invariant");
1099
1100 if (TrySpin(current)) {
1101 break;
1102 }
1103
1104 {
1105 OSThreadContendState osts(current->osthread());
1106
1107 assert(current->thread_state() == _thread_in_vm, "invariant");
1108
1109 {
1110 ClearSuccOnSuspend csos(this);
1111 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */);
1112 current->_ParkEvent->park();
1113 }
1114 }
1115
1116 // Try again, but just so we distinguish between futile wakeups and
1117 // successful wakeups. The following test isn't algorithmically
1118 // necessary, but it helps us maintain sensible statistics.
1119 if (TryLock(current) == TryLockResult::Success) {
1120 break;
1121 }
1122
1123 // The lock is still contested.
1124 // Keep a tally of the # of futile wakeups.
1125 // Note that the counter is not protected by a lock or updated by atomics.
1126 // That is by design - we trade "lossy" counters which are exposed to
1127 // races during updates for a lower probe effect.
1128 ++nWakeups;
1129
1130 // Assuming this is not a spurious wakeup we'll normally
1131 // find that _succ == current.
1132 if (_succ == current) _succ = nullptr;
1133
1134 // Invariant: after clearing _succ a contending thread
1135 // *must* retry _owner before parking.
1136 OrderAccess::fence();
1137
1138 // This PerfData object can be used in parallel with a safepoint.
1139 // See the work around in PerfDataManager::destroy().
1140 OM_PERFDATA_OP(FutileWakeups, inc());
1141 }
1142
1143 // current has acquired the lock -- Unlink current from the cxq or EntryList .
1144 // Normally we'll find current on the EntryList.
1145 // Unlinking from the EntryList is constant-time and atomic-free.
1146 // From the perspective of the lock owner (this thread), the
1147 // EntryList is stable and cxq is prepend-only.
1148 // The head of cxq is volatile but the interior is stable.
1149 // In addition, current.TState is stable.
1150
1151 assert(owner_raw() == owner_for(current), "invariant");
1152 assert(object()->mark() == markWord::encode(this), "invariant");
1153 UnlinkAfterAcquire(current, currentNode);
1154 if (_succ == current) _succ = nullptr;
1155 assert(_succ != current, "invariant");
1156 currentNode->TState = ObjectWaiter::TS_RUN;
1157 OrderAccess::fence(); // see comments at the end of EnterI()
1158 }
1159
1160 void ObjectMonitor::redo_enter(JavaThread* current) {
1161 assert(java_lang_VirtualThread::state(current->vthread()) == java_lang_VirtualThread::RUNNING, "wrong state for vthread");
1162 assert(current->is_in_VTMS_transition(), "must be");
1163
1164 if (TryLock(current) == TryLockResult::Success) {
1165 VThreadEpilog(current);
1166 return;
1167 }
1168
1169 oop vthread = current->vthread();
1170 if (_succ == (JavaThread*)java_lang_Thread::thread_id(vthread)) _succ = nullptr;
1171
1172 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
1173 OrderAccess::fence();
1174
1175 if (TryLock(current) == TryLockResult::Success) {
1176 assert(owner_raw() == owner_for(current), "invariant");
1177 VThreadEpilog(current);
1178 return;
1179 }
1180
1181 // Fast preemption. The JT will read this variable on return to the
1182 // monitorenter_redo stub and will just remove enterSpecial frame
1183 // from the stack and return to Continuation.run()
1184 current->set_preempting(true);
1185
1186 java_lang_VirtualThread::set_state(vthread, java_lang_VirtualThread::BLOCKING);
1187 if (_Responsible == (JavaThread*)java_lang_Thread::thread_id(vthread)) {
1188 int recheckInterval = java_lang_VirtualThread::recheckInterval(vthread);
1189 assert(recheckInterval >= 1 && recheckInterval <= 6, "invariant");
1190 if (recheckInterval < 6) {
1191 recheckInterval++;
1192 java_lang_VirtualThread::set_recheckInterval(vthread, recheckInterval);
1193 }
1194 } else if (java_lang_VirtualThread::recheckInterval(vthread) > 0) {
1195 // No need to do timed park anymore
1196 java_lang_VirtualThread::set_recheckInterval(vthread, 0);
1197 }
1198 }
1199
1200 void ObjectMonitor::VThreadEpilog(JavaThread* current) {
1201 assert(owner_raw() == owner_for(current), "invariant");
1202 add_to_contentions(-1);
1203
1204 oop vthread = current->vthread();
1205 if (java_lang_VirtualThread::recheckInterval(vthread) > 0) {
1206 java_lang_VirtualThread::set_recheckInterval(vthread, 0);
1207 }
1208 int64_t threadid = java_lang_Thread::thread_id(vthread);
1209 if (_succ == (JavaThread*)threadid) _succ = nullptr;
1210 if (_Responsible == (JavaThread*)threadid) {
1211 _Responsible = nullptr;
1212 OrderAccess::fence(); // Dekker pivot-point
1213 }
1214 ObjectWaiter* node = LookupWaiter(threadid);
1215 UnlinkAfterAcquire(current, node, vthread);
1216 delete node;
1217 }
1218
1219 // By convention we unlink a contending thread from EntryList|cxq immediately
1220 // after the thread acquires the lock in ::enter(). Equally, we could defer
1221 // unlinking the thread until ::exit()-time.
1222
1223 void ObjectMonitor::UnlinkAfterAcquire(JavaThread* current, ObjectWaiter* currentNode, oop vthread) {
1224 assert(owner_raw() == owner_for(current), "invariant");
1225 assert((currentNode->_thread == current) || (currentNode->_thread == nullptr && currentNode->vthread() == vthread), "invariant");
1226
1227 if (currentNode->TState == ObjectWaiter::TS_ENTER) {
1228 // Normal case: remove current from the DLL EntryList .
1229 // This is a constant-time operation.
1230 ObjectWaiter* nxt = currentNode->_next;
1231 ObjectWaiter* prv = currentNode->_prev;
1232 if (nxt != nullptr) nxt->_prev = prv;
1233 if (prv != nullptr) prv->_next = nxt;
1234 if (currentNode == _EntryList) _EntryList = nxt;
1235 assert(nxt == nullptr || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
1236 assert(prv == nullptr || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
1237 } else {
1238 assert(currentNode->TState == ObjectWaiter::TS_CXQ, "invariant");
1239 // Inopportune interleaving -- current is still on the cxq.
1240 // This usually means the enqueue of self raced an exiting thread.
1241 // Normally we'll find current near the front of the cxq, so
1242 // dequeueing is typically fast. If needbe we can accelerate
1243 // this with some MCS/CHL-like bidirectional list hints and advisory
1244 // back-links so dequeueing from the interior will normally operate
1245 // in constant-time.
1246 // Dequeue current from either the head (with CAS) or from the interior
1247 // with a linear-time scan and normal non-atomic memory operations.
1248 // CONSIDER: if current is on the cxq then simply drain cxq into EntryList
1249 // and then unlink current from EntryList. We have to drain eventually,
1250 // so it might as well be now.
1251
1252 ObjectWaiter* v = _cxq;
1253 assert(v != nullptr, "invariant");
1254 if (v != currentNode || Atomic::cmpxchg(&_cxq, v, currentNode->_next) != v) {
1255 // The CAS above can fail from interference IFF a "RAT" arrived.
1256 // In that case current must be in the interior and can no longer be
1257 // at the head of cxq.
1258 if (v == currentNode) {
1259 assert(_cxq != v, "invariant");
1260 v = _cxq; // CAS above failed - start scan at head of list
1261 }
1262 ObjectWaiter* p;
1263 ObjectWaiter* q = nullptr;
1264 for (p = v; p != nullptr && p != currentNode; p = p->_next) {
1265 q = p;
1266 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
1267 }
1268 assert(v != currentNode, "invariant");
1269 assert(p == currentNode, "Node not found on cxq");
1270 assert(p != _cxq, "invariant");
1271 assert(q != nullptr, "invariant");
1272 assert(q->_next == p, "invariant");
1273 q->_next = p->_next;
1274 }
1275 }
1276
1277 #ifdef ASSERT
1278 // Diagnostic hygiene ...
1279 currentNode->_prev = (ObjectWaiter*) 0xBAD;
1280 currentNode->_next = (ObjectWaiter*) 0xBAD;
1281 currentNode->TState = ObjectWaiter::TS_RUN;
1282 #endif
1283 }
1284
1285 // Fix this. Save ObjectWaiter* when freezing. Or use hashtable.
1286 ObjectWaiter* ObjectMonitor::LookupWaiter(int64_t threadid) {
1287 ObjectWaiter* p;
1288 for (p = _EntryList; p != nullptr && (!p->is_vthread() || java_lang_Thread::thread_id(p->vthread()) != threadid); p = p->_next) {}
1289 if (p != nullptr) return p;
1290 for (p = _cxq; p != nullptr && (!p->is_vthread() || java_lang_Thread::thread_id(p->vthread()) != threadid); p = p->_next) {}
1291 assert(p != nullptr, "should be on either _cxq or _EntryList");
1292 return p;
1293 }
1294
1295 // -----------------------------------------------------------------------------
1296 // Exit support
1297 //
1298 // exit()
1299 // ~~~~~~
1300 // Note that the collector can't reclaim the objectMonitor or deflate
1301 // the object out from underneath the thread calling ::exit() as the
1302 // thread calling ::exit() never transitions to a stable state.
1303 // This inhibits GC, which in turn inhibits asynchronous (and
1304 // inopportune) reclamation of "this".
1305 //
1306 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
1307 // There's one exception to the claim above, however. EnterI() can call
1308 // exit() to drop a lock if the acquirer has been externally suspended.
1309 // In that case exit() is called with _thread_state == _thread_blocked,
1310 // but the monitor's _contentions field is > 0, which inhibits reclamation.
1311 //
1312 // 1-0 exit
1313 // ~~~~~~~~
1314 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
1315 // the fast-path operators have been optimized so the common ::exit()
1316 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
1317 // The code emitted by fast_unlock() elides the usual MEMBAR. This
1318 // greatly improves latency -- MEMBAR and CAS having considerable local
1319 // latency on modern processors -- but at the cost of "stranding". Absent the
1320 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
1321 // ::enter() path, resulting in the entering thread being stranding
1322 // and a progress-liveness failure. Stranding is extremely rare.
1323 // We use timers (timed park operations) & periodic polling to detect
1324 // and recover from stranding. Potentially stranded threads periodically
1325 // wake up and poll the lock. See the usage of the _Responsible variable.
1326 //
1327 // The CAS() in enter provides for safety and exclusion, while the CAS or
1328 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
1329 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
1330 // We detect and recover from stranding with timers.
1331 //
1332 // If a thread transiently strands it'll park until (a) another
1333 // thread acquires the lock and then drops the lock, at which time the
1334 // exiting thread will notice and unpark the stranded thread, or, (b)
1335 // the timer expires. If the lock is high traffic then the stranding latency
1336 // will be low due to (a). If the lock is low traffic then the odds of
1337 // stranding are lower, although the worst-case stranding latency
1338 // is longer. Critically, we don't want to put excessive load in the
1339 // platform's timer subsystem. We want to minimize both the timer injection
1340 // rate (timers created/sec) as well as the number of timers active at
1341 // any one time. (more precisely, we want to minimize timer-seconds, which is
1342 // the integral of the # of active timers at any instant over time).
1343 // Both impinge on OS scalability. Given that, at most one thread parked on
1344 // a monitor will use a timer.
1345 //
1346 // There is also the risk of a futile wake-up. If we drop the lock
1347 // another thread can reacquire the lock immediately, and we can
1348 // then wake a thread unnecessarily. This is benign, and we've
1349 // structured the code so the windows are short and the frequency
1350 // of such futile wakups is low.
1351
1352 void ObjectMonitor::exit(JavaThread* current, bool not_suspended) {
1353 void* cur = owner_raw();
1354 if (owner_for(current) != cur) {
1355 // Apparent unbalanced locking ...
1356 // Naively we'd like to throw IllegalMonitorStateException.
1357 // As a practical matter we can neither allocate nor throw an
1358 // exception as ::exit() can be called from leaf routines.
1359 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
1360 // Upon deeper reflection, however, in a properly run JVM the only
1361 // way we should encounter this situation is in the presence of
1362 // unbalanced JNI locking. TODO: CheckJNICalls.
1363 // See also: CR4414101
1364 #ifdef ASSERT
1365 LogStreamHandle(Error, monitorinflation) lsh;
1366 lsh.print_cr("ERROR: ObjectMonitor::exit(): thread=" INTPTR_FORMAT
1367 " is exiting an ObjectMonitor it does not own.", p2i(current));
1368 lsh.print_cr("The imbalance is possibly caused by JNI locking.");
1369 print_debug_style_on(&lsh);
1370 assert(false, "Non-balanced monitor enter/exit!");
1371 #endif
1372 return;
1373 }
1374
1375 if (_recursions != 0) {
1376 _recursions--; // this is simple recursive enter
1377 return;
1378 }
1379
1380 // Invariant: after setting Responsible=null an thread must execute
1381 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
1382 _Responsible = nullptr;
1383
1384 #if INCLUDE_JFR
1385 // get the owner's thread id for the MonitorEnter event
1386 // if it is enabled and the thread isn't suspended
1387 if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
1388 _previous_owner_tid = JFR_THREAD_ID(current);
1389 }
1390 #endif
1391
1392 for (;;) {
1393 assert(owner_for(current) == owner_raw(), "invariant");
1394
1395 // Drop the lock.
1396 // release semantics: prior loads and stores from within the critical section
1397 // must not float (reorder) past the following store that drops the lock.
1398 // Uses a storeload to separate release_store(owner) from the
1399 // successor check. The try_set_owner_from() below uses cmpxchg() so
1400 // we get the fence down there.
1401 release_clear_owner(current);
1402 OrderAccess::storeload();
1403
1404 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != nullptr) {
1405 return;
1406 }
1407 // Other threads are blocked trying to acquire the lock.
1408
1409 // Normally the exiting thread is responsible for ensuring succession,
1410 // but if other successors are ready or other entering threads are spinning
1411 // then this thread can simply store null into _owner and exit without
1412 // waking a successor. The existence of spinners or ready successors
1413 // guarantees proper succession (liveness). Responsibility passes to the
1414 // ready or running successors. The exiting thread delegates the duty.
1415 // More precisely, if a successor already exists this thread is absolved
1416 // of the responsibility of waking (unparking) one.
1417 //
1418 // The _succ variable is critical to reducing futile wakeup frequency.
1419 // _succ identifies the "heir presumptive" thread that has been made
1420 // ready (unparked) but that has not yet run. We need only one such
1421 // successor thread to guarantee progress.
1422 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1423 // section 3.3 "Futile Wakeup Throttling" for details.
1424 //
1425 // Note that spinners in Enter() also set _succ non-null.
1426 // In the current implementation spinners opportunistically set
1427 // _succ so that exiting threads might avoid waking a successor.
1428 // Another less appealing alternative would be for the exiting thread
1429 // to drop the lock and then spin briefly to see if a spinner managed
1430 // to acquire the lock. If so, the exiting thread could exit
1431 // immediately without waking a successor, otherwise the exiting
1432 // thread would need to dequeue and wake a successor.
1433 // (Note that we'd need to make the post-drop spin short, but no
1434 // shorter than the worst-case round-trip cache-line migration time.
1435 // The dropped lock needs to become visible to the spinner, and then
1436 // the acquisition of the lock by the spinner must become visible to
1437 // the exiting thread).
1438
1439 // It appears that an heir-presumptive (successor) must be made ready.
1440 // Only the current lock owner can manipulate the EntryList or
1441 // drain _cxq, so we need to reacquire the lock. If we fail
1442 // to reacquire the lock the responsibility for ensuring succession
1443 // falls to the new owner.
1444 //
1445 if (try_set_owner_from(nullptr, current) != nullptr) {
1446 return;
1447 }
1448
1449 guarantee(owner_raw() == owner_for(current), "invariant");
1450
1451 ObjectWaiter* w = nullptr;
1452
1453 w = _EntryList;
1454 if (w != nullptr) {
1455 // I'd like to write: guarantee (w->_thread != current).
1456 // But in practice an exiting thread may find itself on the EntryList.
1457 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1458 // then calls exit(). Exit release the lock by setting O._owner to null.
1459 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1460 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1461 // release the lock "O". T2 resumes immediately after the ST of null into
1462 // _owner, above. T2 notices that the EntryList is populated, so it
1463 // reacquires the lock and then finds itself on the EntryList.
1464 // Given all that, we have to tolerate the circumstance where "w" is
1465 // associated with current.
1466 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1467 ExitEpilog(current, w);
1468 return;
1469 }
1470
1471 // If we find that both _cxq and EntryList are null then just
1472 // re-run the exit protocol from the top.
1473 w = _cxq;
1474 if (w == nullptr) continue;
1475
1476 // Drain _cxq into EntryList - bulk transfer.
1477 // First, detach _cxq.
1478 // The following loop is tantamount to: w = swap(&cxq, nullptr)
1479 for (;;) {
1480 assert(w != nullptr, "Invariant");
1481 ObjectWaiter* u = Atomic::cmpxchg(&_cxq, w, (ObjectWaiter*)nullptr);
1482 if (u == w) break;
1483 w = u;
1484 }
1485
1486 assert(w != nullptr, "invariant");
1487 assert(_EntryList == nullptr, "invariant");
1488
1489 // Convert the LIFO SLL anchored by _cxq into a DLL.
1490 // The list reorganization step operates in O(LENGTH(w)) time.
1491 // It's critical that this step operate quickly as
1492 // "current" still holds the outer-lock, restricting parallelism
1493 // and effectively lengthening the critical section.
1494 // Invariant: s chases t chases u.
1495 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1496 // we have faster access to the tail.
1497
1498 _EntryList = w;
1499 ObjectWaiter* q = nullptr;
1500 ObjectWaiter* p;
1501 for (p = w; p != nullptr; p = p->_next) {
1502 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1503 p->TState = ObjectWaiter::TS_ENTER;
1504 p->_prev = q;
1505 q = p;
1506 }
1507
1508 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = nullptr
1509 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1510
1511 // See if we can abdicate to a spinner instead of waking a thread.
1512 // A primary goal of the implementation is to reduce the
1513 // context-switch rate.
1514 if (_succ != nullptr) continue;
1515
1516 w = _EntryList;
1517 if (w != nullptr) {
1518 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1519 ExitEpilog(current, w);
1520 return;
1521 }
1522 }
1523 }
1524
1525 void ObjectMonitor::ExitEpilog(JavaThread* current, ObjectWaiter* Wakee) {
1526 assert(owner_raw() == owner_for(current), "invariant");
1527
1528 // Exit protocol:
1529 // 1. ST _succ = wakee
1530 // 2. membar #loadstore|#storestore;
1531 // 2. ST _owner = nullptr
1532 // 3. unpark(wakee)
1533
1534 oop vthread = nullptr;
1535 if (Wakee->_thread != nullptr) {
1536 // Platform thread case
1537 _succ = Wakee->_thread;
1538 } else {
1539 assert(Wakee->vthread() != nullptr, "invariant");
1540 vthread = Wakee->vthread();
1541 _succ = (JavaThread*)java_lang_Thread::thread_id(vthread);
1542 }
1543 ParkEvent * Trigger = Wakee->_event;
1544
1545 // Hygiene -- once we've set _owner = nullptr we can't safely dereference Wakee again.
1546 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1547 // out-of-scope (non-extant).
1548 Wakee = nullptr;
1549
1550 // Drop the lock.
1551 // Uses a fence to separate release_store(owner) from the LD in unpark().
1552 release_clear_owner(current);
1553 OrderAccess::fence();
1554
1555 DTRACE_MONITOR_PROBE(contended__exit, this, object(), current);
1556
1557 if (vthread == nullptr) {
1558 // Platform thread case
1559 Trigger->unpark();
1560 } else if (java_lang_VirtualThread::set_onWaitingList(vthread, _vthread_cxq_head)) {
1561 Trigger->unpark();
1562 }
1563
1564 // Maintain stats and report events to JVMTI
1565 OM_PERFDATA_OP(Parks, inc());
1566 }
1567
1568 // complete_exit exits a lock returning recursion count
1569 // complete_exit requires an inflated monitor
1570 // The _owner field is not always the Thread addr even with an
1571 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1572 // thread due to contention.
1573 intx ObjectMonitor::complete_exit(JavaThread* current) {
1574 assert(InitDone, "Unexpectedly not initialized");
1575
1576 void* cur = owner_raw();
1577 if (owner_for(current) != cur) {
1578 if (LockingMode == LM_LEGACY && is_stack_locker(current)) {
1579 assert(_recursions == 0, "internal state error");
1580 set_owner_from_BasicLock(current); // Convert from BasicLock* to Thread*.
1581 _recursions = 0;
1582 }
1583 }
1584
1585 guarantee(owner_for(current) == owner_raw(), "complete_exit not owner");
1586 intx save = _recursions; // record the old recursion count
1587 _recursions = 0; // set the recursion level to be 0
1588 exit(current); // exit the monitor
1589 guarantee(owner_raw() != owner_for(current), "invariant");
1590 return save;
1591 }
1592
1593 // Checks that the current THREAD owns this monitor and causes an
1594 // immediate return if it doesn't. We don't use the CHECK macro
1595 // because we want the IMSE to be the only exception that is thrown
1596 // from the call site when false is returned. Any other pending
1597 // exception is ignored.
1598 #define CHECK_OWNER() \
1599 do { \
1600 if (!check_owner(THREAD)) { \
1601 assert(HAS_PENDING_EXCEPTION, "expected a pending IMSE here."); \
1602 return; \
1603 } \
1604 } while (false)
1605
1606 // Returns true if the specified thread owns the ObjectMonitor.
1607 // Otherwise returns false and throws IllegalMonitorStateException
1608 // (IMSE). If there is a pending exception and the specified thread
1609 // is not the owner, that exception will be replaced by the IMSE.
1610 bool ObjectMonitor::check_owner(TRAPS) {
1611 JavaThread* current = THREAD;
1612 void* cur = owner_raw();
1613 if (cur == owner_for(current)) {
1614 return true;
1615 }
1616 THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(),
1617 "current thread is not owner", false);
1618 }
1619
1620 static inline bool is_excluded(const Klass* monitor_klass) {
1621 assert(monitor_klass != nullptr, "invariant");
1622 NOT_JFR_RETURN_(false);
1623 JFR_ONLY(return vmSymbols::jfr_chunk_rotation_monitor() == monitor_klass->name();)
1624 }
1625
1626 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1627 ObjectMonitor* monitor,
1628 uint64_t notifier_tid,
1629 jlong timeout,
1630 bool timedout) {
1631 assert(event != nullptr, "invariant");
1632 assert(monitor != nullptr, "invariant");
1633 const Klass* monitor_klass = monitor->object()->klass();
1634 if (is_excluded(monitor_klass)) {
1635 return;
1636 }
1637 event->set_monitorClass(monitor_klass);
1638 event->set_timeout(timeout);
1639 // Set an address that is 'unique enough', such that events close in
1640 // time and with the same address are likely (but not guaranteed) to
1641 // belong to the same object.
1642 event->set_address((uintptr_t)monitor);
1643 event->set_notifier(notifier_tid);
1644 event->set_timedOut(timedout);
1645 event->commit();
1646 }
1647
1648 // -----------------------------------------------------------------------------
1649 // Wait/Notify/NotifyAll
1650 //
1651 // Note: a subset of changes to ObjectMonitor::wait()
1652 // will need to be replicated in complete_exit
1653 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1654 JavaThread* current = THREAD;
1655
1656 assert(InitDone, "Unexpectedly not initialized");
1657
1658 CHECK_OWNER(); // Throws IMSE if not owner.
1659
1660 EventJavaMonitorWait event;
1661
1662 // check for a pending interrupt
1663 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1664 // post monitor waited event. Note that this is past-tense, we are done waiting.
1665 if (JvmtiExport::should_post_monitor_waited()) {
1666 // Note: 'false' parameter is passed here because the
1667 // wait was not timed out due to thread interrupt.
1668 JvmtiExport::post_monitor_waited(current, this, false);
1669
1670 // In this short circuit of the monitor wait protocol, the
1671 // current thread never drops ownership of the monitor and
1672 // never gets added to the wait queue so the current thread
1673 // cannot be made the successor. This means that the
1674 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1675 // consume an unpark() meant for the ParkEvent associated with
1676 // this ObjectMonitor.
1677 }
1678 if (event.should_commit()) {
1679 post_monitor_wait_event(&event, this, 0, millis, false);
1680 }
1681 THROW(vmSymbols::java_lang_InterruptedException());
1682 return;
1683 }
1684
1685 ContinuationEntry* ce = current->last_continuation();
1686 if (ce != nullptr && ce->is_virtual_thread()) {
1687 const Klass* monitor_klass = object()->klass();
1688 if (!is_excluded(monitor_klass)) {
1689 ResourceMark rm;
1690 char reason[256];
1691 jio_snprintf(reason, sizeof reason, "Object.wait on object of klass %s", monitor_klass->external_name());
1692 post_virtual_thread_pinned_event(current, reason);
1693 }
1694 }
1695
1696 current->set_current_waiting_monitor(this);
1697
1698 // create a node to be put into the queue
1699 // Critically, after we reset() the event but prior to park(), we must check
1700 // for a pending interrupt.
1701 ObjectWaiter node(current);
1702 node.TState = ObjectWaiter::TS_WAIT;
1703 current->_ParkEvent->reset();
1704 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1705
1706 // Enter the waiting queue, which is a circular doubly linked list in this case
1707 // but it could be a priority queue or any data structure.
1708 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1709 // by the owner of the monitor *except* in the case where park()
1710 // returns because of a timeout of interrupt. Contention is exceptionally rare
1711 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1712
1713 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1714 AddWaiter(&node);
1715 Thread::SpinRelease(&_WaitSetLock);
1716
1717 _Responsible = nullptr;
1718
1719 intx save = _recursions; // record the old recursion count
1720 _waiters++; // increment the number of waiters
1721 _recursions = 0; // set the recursion level to be 1
1722 exit(current); // exit the monitor
1723 guarantee(owner_raw() != owner_for(current), "invariant");
1724
1725 // The thread is on the WaitSet list - now park() it.
1726 // On MP systems it's conceivable that a brief spin before we park
1727 // could be profitable.
1728 //
1729 // TODO-FIXME: change the following logic to a loop of the form
1730 // while (!timeout && !interrupted && _notified == 0) park()
1731
1732 int ret = OS_OK;
1733 int WasNotified = 0;
1734
1735 // Need to check interrupt state whilst still _thread_in_vm
1736 bool interrupted = interruptible && current->is_interrupted(false);
1737
1738 { // State transition wrappers
1739 OSThread* osthread = current->osthread();
1740 OSThreadWaitState osts(osthread, true);
1741
1742 assert(current->thread_state() == _thread_in_vm, "invariant");
1743
1744 {
1745 ClearSuccOnSuspend csos(this);
1746 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */);
1747 if (interrupted || HAS_PENDING_EXCEPTION) {
1748 // Intentionally empty
1749 } else if (node._notified == 0) {
1750 if (millis <= 0) {
1751 current->_ParkEvent->park();
1752 } else {
1753 ret = current->_ParkEvent->park(millis);
1754 }
1755 }
1756 }
1757
1758 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1759 // from the WaitSet to the EntryList.
1760 // See if we need to remove Node from the WaitSet.
1761 // We use double-checked locking to avoid grabbing _WaitSetLock
1762 // if the thread is not on the wait queue.
1763 //
1764 // Note that we don't need a fence before the fetch of TState.
1765 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1766 // written by the is thread. (perhaps the fetch might even be satisfied
1767 // by a look-aside into the processor's own store buffer, although given
1768 // the length of the code path between the prior ST and this load that's
1769 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1770 // then we'll acquire the lock and then re-fetch a fresh TState value.
1771 // That is, we fail toward safety.
1772
1773 if (node.TState == ObjectWaiter::TS_WAIT) {
1774 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1775 if (node.TState == ObjectWaiter::TS_WAIT) {
1776 DequeueSpecificWaiter(&node); // unlink from WaitSet
1777 assert(node._notified == 0, "invariant");
1778 node.TState = ObjectWaiter::TS_RUN;
1779 }
1780 Thread::SpinRelease(&_WaitSetLock);
1781 }
1782
1783 // The thread is now either on off-list (TS_RUN),
1784 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1785 // The Node's TState variable is stable from the perspective of this thread.
1786 // No other threads will asynchronously modify TState.
1787 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1788 OrderAccess::loadload();
1789 if (_succ == current) _succ = nullptr;
1790 WasNotified = node._notified;
1791
1792 // Reentry phase -- reacquire the monitor.
1793 // re-enter contended monitor after object.wait().
1794 // retain OBJECT_WAIT state until re-enter successfully completes
1795 // Thread state is thread_in_vm and oop access is again safe,
1796 // although the raw address of the object may have changed.
1797 // (Don't cache naked oops over safepoints, of course).
1798
1799 // post monitor waited event. Note that this is past-tense, we are done waiting.
1800 if (JvmtiExport::should_post_monitor_waited()) {
1801 JvmtiExport::post_monitor_waited(current, this, ret == OS_TIMEOUT);
1802
1803 if (node._notified != 0 && _succ == current) {
1804 // In this part of the monitor wait-notify-reenter protocol it
1805 // is possible (and normal) for another thread to do a fastpath
1806 // monitor enter-exit while this thread is still trying to get
1807 // to the reenter portion of the protocol.
1808 //
1809 // The ObjectMonitor was notified and the current thread is
1810 // the successor which also means that an unpark() has already
1811 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1812 // consume the unpark() that was done when the successor was
1813 // set because the same ParkEvent is shared between Java
1814 // monitors and JVM/TI RawMonitors (for now).
1815 //
1816 // We redo the unpark() to ensure forward progress, i.e., we
1817 // don't want all pending threads hanging (parked) with none
1818 // entering the unlocked monitor.
1819 node._event->unpark();
1820 }
1821 }
1822
1823 if (event.should_commit()) {
1824 post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1825 }
1826
1827 OrderAccess::fence();
1828
1829 assert(owner_raw() != owner_for(current), "invariant");
1830 ObjectWaiter::TStates v = node.TState;
1831 if (v == ObjectWaiter::TS_RUN) {
1832 enter(current);
1833 } else {
1834 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1835 ReenterI(current, &node);
1836 node.wait_reenter_end(this);
1837 }
1838
1839 // current has reacquired the lock.
1840 // Lifecycle - the node representing current must not appear on any queues.
1841 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1842 // want residual elements associated with this thread left on any lists.
1843 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1844 assert(owner_raw() == owner_for(current), "invariant");
1845 assert(_succ != current, "invariant");
1846 } // OSThreadWaitState()
1847
1848 current->set_current_waiting_monitor(nullptr);
1849
1850 guarantee(_recursions == 0, "invariant");
1851 int relock_count = JvmtiDeferredUpdates::get_and_reset_relock_count_after_wait(current);
1852 _recursions = save // restore the old recursion count
1853 + relock_count; // increased by the deferred relock count
1854 NOT_LOOM_MONITOR_SUPPORT(current->inc_held_monitor_count(relock_count);) // Deopt never entered these counts.
1855 _waiters--; // decrement the number of waiters
1856
1857 // Verify a few postconditions
1858 assert(owner_raw() == owner_for(current), "invariant");
1859 assert(_succ != current, "invariant");
1860 assert(object()->mark() == markWord::encode(this), "invariant");
1861
1862 // check if the notification happened
1863 if (!WasNotified) {
1864 // no, it could be timeout or Thread.interrupt() or both
1865 // check for interrupt event, otherwise it is timeout
1866 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1867 THROW(vmSymbols::java_lang_InterruptedException());
1868 }
1869 }
1870
1871 // NOTE: Spurious wake up will be consider as timeout.
1872 // Monitor notify has precedence over thread interrupt.
1873 }
1874
1875
1876 // Consider:
1877 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1878 // then instead of transferring a thread from the WaitSet to the EntryList
1879 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1880
1881 void ObjectMonitor::INotify(JavaThread* current) {
1882 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1883 ObjectWaiter* iterator = DequeueWaiter();
1884 if (iterator != nullptr) {
1885 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1886 guarantee(iterator->_notified == 0, "invariant");
1887 // Disposition - what might we do with iterator ?
1888 // a. add it directly to the EntryList - either tail (policy == 1)
1889 // or head (policy == 0).
1890 // b. push it onto the front of the _cxq (policy == 2).
1891 // For now we use (b).
1892
1893 iterator->TState = ObjectWaiter::TS_ENTER;
1894
1895 iterator->_notified = 1;
1896 iterator->_notifier_tid = JFR_THREAD_ID(current);
1897
1898 ObjectWaiter* list = _EntryList;
1899 if (list != nullptr) {
1900 assert(list->_prev == nullptr, "invariant");
1901 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1902 assert(list != iterator, "invariant");
1903 }
1904
1905 // prepend to cxq
1906 if (list == nullptr) {
1907 iterator->_next = iterator->_prev = nullptr;
1908 _EntryList = iterator;
1909 } else {
1910 iterator->TState = ObjectWaiter::TS_CXQ;
1911 for (;;) {
1912 ObjectWaiter* front = _cxq;
1913 iterator->_next = front;
1914 if (Atomic::cmpxchg(&_cxq, front, iterator) == front) {
1915 break;
1916 }
1917 }
1918 }
1919
1920 // _WaitSetLock protects the wait queue, not the EntryList. We could
1921 // move the add-to-EntryList operation, above, outside the critical section
1922 // protected by _WaitSetLock. In practice that's not useful. With the
1923 // exception of wait() timeouts and interrupts the monitor owner
1924 // is the only thread that grabs _WaitSetLock. There's almost no contention
1925 // on _WaitSetLock so it's not profitable to reduce the length of the
1926 // critical section.
1927
1928 iterator->wait_reenter_begin(this);
1929 }
1930 Thread::SpinRelease(&_WaitSetLock);
1931 }
1932
1933 // Consider: a not-uncommon synchronization bug is to use notify() when
1934 // notifyAll() is more appropriate, potentially resulting in stranded
1935 // threads; this is one example of a lost wakeup. A useful diagnostic
1936 // option is to force all notify() operations to behave as notifyAll().
1937 //
1938 // Note: We can also detect many such problems with a "minimum wait".
1939 // When the "minimum wait" is set to a small non-zero timeout value
1940 // and the program does not hang whereas it did absent "minimum wait",
1941 // that suggests a lost wakeup bug.
1942
1943 void ObjectMonitor::notify(TRAPS) {
1944 JavaThread* current = THREAD;
1945 CHECK_OWNER(); // Throws IMSE if not owner.
1946 if (_WaitSet == nullptr) {
1947 return;
1948 }
1949 DTRACE_MONITOR_PROBE(notify, this, object(), current);
1950 INotify(current);
1951 OM_PERFDATA_OP(Notifications, inc(1));
1952 }
1953
1954
1955 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1956 // from the waitset to the EntryList. This could be done more efficiently with a
1957 // single bulk transfer but in practice it's not time-critical. Beware too,
1958 // that in prepend-mode we invert the order of the waiters. Let's say that the
1959 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1960 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1961
1962 void ObjectMonitor::notifyAll(TRAPS) {
1963 JavaThread* current = THREAD;
1964 CHECK_OWNER(); // Throws IMSE if not owner.
1965 if (_WaitSet == nullptr) {
1966 return;
1967 }
1968
1969 DTRACE_MONITOR_PROBE(notifyAll, this, object(), current);
1970 int tally = 0;
1971 while (_WaitSet != nullptr) {
1972 tally++;
1973 INotify(current);
1974 }
1975
1976 OM_PERFDATA_OP(Notifications, inc(tally));
1977 }
1978
1979 // -----------------------------------------------------------------------------
1980 // Adaptive Spinning Support
1981 //
1982 // Adaptive spin-then-block - rational spinning
1983 //
1984 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1985 // algorithm. On high order SMP systems it would be better to start with
1986 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1987 // a contending thread could enqueue itself on the cxq and then spin locally
1988 // on a thread-specific variable such as its ParkEvent._Event flag.
1989 // That's left as an exercise for the reader. Note that global spinning is
1990 // not problematic on Niagara, as the L2 cache serves the interconnect and
1991 // has both low latency and massive bandwidth.
1992 //
1993 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1994 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1995 // (duration) or we can fix the count at approximately the duration of
1996 // a context switch and vary the frequency. Of course we could also
1997 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1998 // For a description of 'Adaptive spin-then-block mutual exclusion in
1999 // multi-threaded processing,' see U.S. Pat. No. 8046758.
2000 //
2001 // This implementation varies the duration "D", where D varies with
2002 // the success rate of recent spin attempts. (D is capped at approximately
2003 // length of a round-trip context switch). The success rate for recent
2004 // spin attempts is a good predictor of the success rate of future spin
2005 // attempts. The mechanism adapts automatically to varying critical
2006 // section length (lock modality), system load and degree of parallelism.
2007 // D is maintained per-monitor in _SpinDuration and is initialized
2008 // optimistically. Spin frequency is fixed at 100%.
2009 //
2010 // Note that _SpinDuration is volatile, but we update it without locks
2011 // or atomics. The code is designed so that _SpinDuration stays within
2012 // a reasonable range even in the presence of races. The arithmetic
2013 // operations on _SpinDuration are closed over the domain of legal values,
2014 // so at worst a race will install and older but still legal value.
2015 // At the very worst this introduces some apparent non-determinism.
2016 // We might spin when we shouldn't or vice-versa, but since the spin
2017 // count are relatively short, even in the worst case, the effect is harmless.
2018 //
2019 // Care must be taken that a low "D" value does not become an
2020 // an absorbing state. Transient spinning failures -- when spinning
2021 // is overall profitable -- should not cause the system to converge
2022 // on low "D" values. We want spinning to be stable and predictable
2023 // and fairly responsive to change and at the same time we don't want
2024 // it to oscillate, become metastable, be "too" non-deterministic,
2025 // or converge on or enter undesirable stable absorbing states.
2026 //
2027 // We implement a feedback-based control system -- using past behavior
2028 // to predict future behavior. We face two issues: (a) if the
2029 // input signal is random then the spin predictor won't provide optimal
2030 // results, and (b) if the signal frequency is too high then the control
2031 // system, which has some natural response lag, will "chase" the signal.
2032 // (b) can arise from multimodal lock hold times. Transient preemption
2033 // can also result in apparent bimodal lock hold times.
2034 // Although sub-optimal, neither condition is particularly harmful, as
2035 // in the worst-case we'll spin when we shouldn't or vice-versa.
2036 // The maximum spin duration is rather short so the failure modes aren't bad.
2037 // To be conservative, I've tuned the gain in system to bias toward
2038 // _not spinning. Relatedly, the system can sometimes enter a mode where it
2039 // "rings" or oscillates between spinning and not spinning. This happens
2040 // when spinning is just on the cusp of profitability, however, so the
2041 // situation is not dire. The state is benign -- there's no need to add
2042 // hysteresis control to damp the transition rate between spinning and
2043 // not spinning.
2044
2045 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool
2046
2047 static int Knob_Bonus = 100; // spin success bonus
2048 static int Knob_Penalty = 200; // spin failure penalty
2049 static int Knob_Poverty = 1000;
2050 static int Knob_FixedSpin = 0;
2051 static int Knob_PreSpin = 10; // 20-100 likely better, but it's not better in my testing.
2052
2053 inline static int adjust_up(int spin_duration) {
2054 int x = spin_duration;
2055 if (x < ObjectMonitor::Knob_SpinLimit) {
2056 if (x < Knob_Poverty) {
2057 x = Knob_Poverty;
2058 }
2059 return x + Knob_Bonus;
2060 } else {
2061 return spin_duration;
2062 }
2063 }
2064
2065 inline static int adjust_down(int spin_duration) {
2066 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2067 // AIMD is globally stable.
2068 int x = spin_duration;
2069 if (x > 0) {
2070 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2071 // This is globally sample and tends to damp the response.
2072 x -= Knob_Penalty;
2073 if (x < 0) { x = 0; }
2074 return x;
2075 } else {
2076 return spin_duration;
2077 }
2078 }
2079
2080 bool ObjectMonitor::short_fixed_spin(JavaThread* current, int spin_count, bool adapt) {
2081 for (int ctr = 0; ctr < spin_count; ctr++) {
2082 TryLockResult status = TryLock(current);
2083 if (status == TryLockResult::Success) {
2084 if (adapt) {
2085 _SpinDuration = adjust_up(_SpinDuration);
2086 }
2087 return true;
2088 } else if (status == TryLockResult::Interference) {
2089 break;
2090 }
2091 SpinPause();
2092 }
2093 return false;
2094 }
2095
2096 // Spinning: Fixed frequency (100%), vary duration
2097 bool ObjectMonitor::TrySpin(JavaThread* current) {
2098
2099 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
2100 int knob_fixed_spin = Knob_FixedSpin; // 0 (don't spin: default), 2000 good test
2101 if (knob_fixed_spin > 0) {
2102 return short_fixed_spin(current, knob_fixed_spin, false);
2103 }
2104
2105 // Admission control - verify preconditions for spinning
2106 //
2107 // We always spin a little bit, just to prevent _SpinDuration == 0 from
2108 // becoming an absorbing state. Put another way, we spin briefly to
2109 // sample, just in case the system load, parallelism, contention, or lock
2110 // modality changed.
2111
2112 int knob_pre_spin = Knob_PreSpin; // 10 (default), 100, 1000 or 2000
2113 if (short_fixed_spin(current, knob_pre_spin, true)) {
2114 return true;
2115 }
2116
2117 //
2118 // Consider the following alternative:
2119 // Periodically set _SpinDuration = _SpinLimit and try a long/full
2120 // spin attempt. "Periodically" might mean after a tally of
2121 // the # of failed spin attempts (or iterations) reaches some threshold.
2122 // This takes us into the realm of 1-out-of-N spinning, where we
2123 // hold the duration constant but vary the frequency.
2124
2125 int ctr = _SpinDuration;
2126 if (ctr <= 0) return false;
2127
2128 // We're good to spin ... spin ingress.
2129 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
2130 // when preparing to LD...CAS _owner, etc and the CAS is likely
2131 // to succeed.
2132 if (_succ == nullptr) {
2133 _succ = current;
2134 }
2135 void* prv = nullptr;
2136
2137 // There are three ways to exit the following loop:
2138 // 1. A successful spin where this thread has acquired the lock.
2139 // 2. Spin failure with prejudice
2140 // 3. Spin failure without prejudice
2141
2142 while (--ctr >= 0) {
2143
2144 // Periodic polling -- Check for pending GC
2145 // Threads may spin while they're unsafe.
2146 // We don't want spinning threads to delay the JVM from reaching
2147 // a stop-the-world safepoint or to steal cycles from GC.
2148 // If we detect a pending safepoint we abort in order that
2149 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
2150 // this thread, if safe, doesn't steal cycles from GC.
2151 // This is in keeping with the "no loitering in runtime" rule.
2152 // We periodically check to see if there's a safepoint pending.
2153 if ((ctr & 0xFF) == 0) {
2154 // Can't call SafepointMechanism::should_process() since that
2155 // might update the poll values and we could be in a thread_blocked
2156 // state here which is not allowed so just check the poll.
2157 if (SafepointMechanism::local_poll_armed(current)) {
2158 break;
2159 }
2160 SpinPause();
2161 }
2162
2163 // Probe _owner with TATAS
2164 // If this thread observes the monitor transition or flicker
2165 // from locked to unlocked to locked, then the odds that this
2166 // thread will acquire the lock in this spin attempt go down
2167 // considerably. The same argument applies if the CAS fails
2168 // or if we observe _owner change from one non-null value to
2169 // another non-null value. In such cases we might abort
2170 // the spin without prejudice or apply a "penalty" to the
2171 // spin count-down variable "ctr", reducing it by 100, say.
2172
2173 void* ox = owner_raw();
2174 if (ox == nullptr) {
2175 ox = try_set_owner_from(nullptr, current);
2176 if (ox == nullptr) {
2177 // The CAS succeeded -- this thread acquired ownership
2178 // Take care of some bookkeeping to exit spin state.
2179 if (_succ == current) {
2180 _succ = nullptr;
2181 }
2182
2183 // Increase _SpinDuration :
2184 // The spin was successful (profitable) so we tend toward
2185 // longer spin attempts in the future.
2186 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2187 // If we acquired the lock early in the spin cycle it
2188 // makes sense to increase _SpinDuration proportionally.
2189 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2190 _SpinDuration = adjust_up(_SpinDuration);
2191 return true;
2192 }
2193
2194 // The CAS failed ... we can take any of the following actions:
2195 // * penalize: ctr -= CASPenalty
2196 // * exit spin with prejudice -- abort without adapting spinner
2197 // * exit spin without prejudice.
2198 // * Since CAS is high-latency, retry again immediately.
2199 break;
2200 }
2201
2202 // Did lock ownership change hands ?
2203 if (ox != prv && prv != nullptr) {
2204 break;
2205 }
2206 prv = ox;
2207
2208 if (_succ == nullptr) {
2209 _succ = current;
2210 }
2211 }
2212
2213 // Spin failed with prejudice -- reduce _SpinDuration.
2214 if (ctr < 0) {
2215 _SpinDuration = adjust_down(_SpinDuration);
2216 }
2217
2218 if (_succ == current) {
2219 _succ = nullptr;
2220 // Invariant: after setting succ=null a contending thread
2221 // must recheck-retry _owner before parking. This usually happens
2222 // in the normal usage of TrySpin(), but it's safest
2223 // to make TrySpin() as foolproof as possible.
2224 OrderAccess::fence();
2225 if (TryLock(current) == TryLockResult::Success) {
2226 return true;
2227 }
2228 }
2229
2230 return false;
2231 }
2232
2233
2234 // -----------------------------------------------------------------------------
2235 // WaitSet management ...
2236
2237 ObjectWaiter::ObjectWaiter(JavaThread* current) {
2238 _next = nullptr;
2239 _prev = nullptr;
2240 _notified = 0;
2241 _notifier_tid = 0;
2242 TState = TS_RUN;
2243 _thread = current;
2244 _event = _thread != nullptr ? _thread->_ParkEvent : ObjectMonitor::vthread_unparker_ParkEvent();
2245 _active = false;
2246 assert(_event != nullptr, "invariant");
2247 }
2248
2249 ObjectWaiter::ObjectWaiter(oop vthread) : ObjectWaiter((JavaThread*)nullptr) {
2250 _vthread = OopHandle(JavaThread::thread_oop_storage(), vthread);
2251 }
2252
2253 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2254 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(_thread, mon);
2255 }
2256
2257 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2258 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(_thread, _active);
2259 }
2260
2261 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2262 assert(node != nullptr, "should not add null node");
2263 assert(node->_prev == nullptr, "node already in list");
2264 assert(node->_next == nullptr, "node already in list");
2265 // put node at end of queue (circular doubly linked list)
2266 if (_WaitSet == nullptr) {
2267 _WaitSet = node;
2268 node->_prev = node;
2269 node->_next = node;
2270 } else {
2271 ObjectWaiter* head = _WaitSet;
2272 ObjectWaiter* tail = head->_prev;
2273 assert(tail->_next == head, "invariant check");
2274 tail->_next = node;
2275 head->_prev = node;
2276 node->_next = head;
2277 node->_prev = tail;
2278 }
2279 }
2280
2281 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2282 // dequeue the very first waiter
2283 ObjectWaiter* waiter = _WaitSet;
2284 if (waiter) {
2285 DequeueSpecificWaiter(waiter);
2286 }
2287 return waiter;
2288 }
2289
2290 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2291 assert(node != nullptr, "should not dequeue nullptr node");
2292 assert(node->_prev != nullptr, "node already removed from list");
2293 assert(node->_next != nullptr, "node already removed from list");
2294 // when the waiter has woken up because of interrupt,
2295 // timeout or other spurious wake-up, dequeue the
2296 // waiter from waiting list
2297 ObjectWaiter* next = node->_next;
2298 if (next == node) {
2299 assert(node->_prev == node, "invariant check");
2300 _WaitSet = nullptr;
2301 } else {
2302 ObjectWaiter* prev = node->_prev;
2303 assert(prev->_next == node, "invariant check");
2304 assert(next->_prev == node, "invariant check");
2305 next->_prev = prev;
2306 prev->_next = next;
2307 if (_WaitSet == node) {
2308 _WaitSet = next;
2309 }
2310 }
2311 node->_next = nullptr;
2312 node->_prev = nullptr;
2313 }
2314
2315 // -----------------------------------------------------------------------------
2316 // PerfData support
2317 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = nullptr;
2318 PerfCounter * ObjectMonitor::_sync_FutileWakeups = nullptr;
2319 PerfCounter * ObjectMonitor::_sync_Parks = nullptr;
2320 PerfCounter * ObjectMonitor::_sync_Notifications = nullptr;
2321 PerfCounter * ObjectMonitor::_sync_Inflations = nullptr;
2322 PerfCounter * ObjectMonitor::_sync_Deflations = nullptr;
2323 PerfLongVariable * ObjectMonitor::_sync_MonExtant = nullptr;
2324
2325 // One-shot global initialization for the sync subsystem.
2326 // We could also defer initialization and initialize on-demand
2327 // the first time we call ObjectSynchronizer::inflate().
2328 // Initialization would be protected - like so many things - by
2329 // the MonitorCache_lock.
2330
2331 void ObjectMonitor::Initialize() {
2332 assert(!InitDone, "invariant");
2333
2334 if (!os::is_MP()) {
2335 Knob_SpinLimit = 0;
2336 Knob_PreSpin = 0;
2337 Knob_FixedSpin = -1;
2338 }
2339
2340 if (UsePerfData) {
2341 EXCEPTION_MARK;
2342 #define NEWPERFCOUNTER(n) \
2343 { \
2344 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \
2345 CHECK); \
2346 }
2347 #define NEWPERFVARIABLE(n) \
2348 { \
2349 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \
2350 CHECK); \
2351 }
2352 NEWPERFCOUNTER(_sync_Inflations);
2353 NEWPERFCOUNTER(_sync_Deflations);
2354 NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2355 NEWPERFCOUNTER(_sync_FutileWakeups);
2356 NEWPERFCOUNTER(_sync_Parks);
2357 NEWPERFCOUNTER(_sync_Notifications);
2358 NEWPERFVARIABLE(_sync_MonExtant);
2359 #undef NEWPERFCOUNTER
2360 #undef NEWPERFVARIABLE
2361 }
2362
2363 _oop_storage = OopStorageSet::create_weak("ObjectSynchronizer Weak", mtSynchronizer);
2364
2365 DEBUG_ONLY(InitDone = true;)
2366 }
2367
2368 void ObjectMonitor::Initialize2() {
2369 _vthread_cxq_head = OopHandle(JavaThread::thread_oop_storage(), nullptr);
2370 _vthread_unparker_ParkEvent = ParkEvent::Allocate(nullptr);
2371 }
2372
2373 void ObjectMonitor::print_on(outputStream* st) const {
2374 // The minimal things to print for markWord printing, more can be added for debugging and logging.
2375 st->print("{contentions=0x%08x,waiters=0x%08x"
2376 ",recursions=" INTX_FORMAT ",owner=" INTPTR_FORMAT "}",
2377 contentions(), waiters(), recursions(),
2378 p2i(owner()));
2379 }
2380 void ObjectMonitor::print() const { print_on(tty); }
2381
2382 #ifdef ASSERT
2383 // Print the ObjectMonitor like a debugger would:
2384 //
2385 // (ObjectMonitor) 0x00007fdfb6012e40 = {
2386 // _header = 0x0000000000000001
2387 // _object = 0x000000070ff45fd0
2388 // _pad_buf0 = {
2389 // [0] = '\0'
2390 // ...
2391 // [43] = '\0'
2392 // }
2393 // _owner = 0x0000000000000000
2394 // _previous_owner_tid = 0
2395 // _pad_buf1 = {
2396 // [0] = '\0'
2397 // ...
2398 // [47] = '\0'
2399 // }
2400 // _next_om = 0x0000000000000000
2401 // _recursions = 0
2402 // _EntryList = 0x0000000000000000
2403 // _cxq = 0x0000000000000000
2404 // _succ = 0x0000000000000000
2405 // _Responsible = 0x0000000000000000
2406 // _SpinDuration = 5000
2407 // _contentions = 0
2408 // _WaitSet = 0x0000700009756248
2409 // _waiters = 1
2410 // _WaitSetLock = 0
2411 // }
2412 //
2413 void ObjectMonitor::print_debug_style_on(outputStream* st) const {
2414 st->print_cr("(ObjectMonitor*) " INTPTR_FORMAT " = {", p2i(this));
2415 st->print_cr(" _header = " INTPTR_FORMAT, header().value());
2416 st->print_cr(" _object = " INTPTR_FORMAT, p2i(object_peek()));
2417 st->print_cr(" _pad_buf0 = {");
2418 st->print_cr(" [0] = '\\0'");
2419 st->print_cr(" ...");
2420 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf0) - 1);
2421 st->print_cr(" }");
2422 st->print_cr(" _owner = " INTPTR_FORMAT, p2i(owner_raw()));
2423 st->print_cr(" _previous_owner_tid = " UINT64_FORMAT, _previous_owner_tid);
2424 st->print_cr(" _pad_buf1 = {");
2425 st->print_cr(" [0] = '\\0'");
2426 st->print_cr(" ...");
2427 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf1) - 1);
2428 st->print_cr(" }");
2429 st->print_cr(" _next_om = " INTPTR_FORMAT, p2i(next_om()));
2430 st->print_cr(" _recursions = " INTX_FORMAT, _recursions);
2431 st->print_cr(" _EntryList = " INTPTR_FORMAT, p2i(_EntryList));
2432 st->print_cr(" _cxq = " INTPTR_FORMAT, p2i(_cxq));
2433 st->print_cr(" _succ = " INTPTR_FORMAT, p2i(_succ));
2434 st->print_cr(" _Responsible = " INTPTR_FORMAT, p2i(_Responsible));
2435 st->print_cr(" _SpinDuration = %d", _SpinDuration);
2436 st->print_cr(" _contentions = %d", contentions());
2437 st->print_cr(" _WaitSet = " INTPTR_FORMAT, p2i(_WaitSet));
2438 st->print_cr(" _waiters = %d", _waiters);
2439 st->print_cr(" _WaitSetLock = %d", _WaitSetLock);
2440 st->print_cr("}");
2441 }
2442 #endif