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), freeze_on_monitorenter);
460 if (result == freeze_ok) {
461 bool acquired = VThreadMonitorEnter(current);
462 if (acquired) {
463 current->set_preemption_cancelled(true);
464 if (JvmtiExport::should_post_monitor_contended_entered()) {
465 // We are going to call thaw again after this and finish the VMTS
466 // transition so no need to do it here. We will post the event there.
467 current->set_contended_entered_monitor(this);
468 }
469 }
470 current->set_current_pending_monitor(nullptr);
471 DEBUG_ONLY(int state = java_lang_VirtualThread::state(current->vthread()));
472 assert((acquired && current->preemption_cancelled() && state == java_lang_VirtualThread::RUNNING) ||
473 (!acquired && !current->preemption_cancelled() && state == java_lang_VirtualThread::BLOCKING), "invariant");
474 return true;
475 }
476 if (result == freeze_pinned_native) {
477 post_virtual_thread_pinned_event(current, "Native frame or <clinit> on stack");
478 }
479 }
480 #endif
481
482 OSThreadContendState osts(current->osthread());
483
484 assert(current->thread_state() == _thread_in_vm, "invariant");
485
486 for (;;) {
487 ExitOnSuspend eos(this);
488 {
489 ThreadBlockInVMPreprocess<ExitOnSuspend> tbivs(current, eos, true /* allow_suspend */);
490 EnterI(current);
491 current->set_current_pending_monitor(nullptr);
492 // We can go to a safepoint at the end of this block. If we
493 // do a thread dump during that safepoint, then this thread will show
494 // as having "-locked" the monitor, but the OS and java.lang.Thread
495 // states will still report that the thread is blocked trying to
496 // acquire it.
497 // If there is a suspend request, ExitOnSuspend will exit the OM
498 // and set the OM as pending.
499 }
500 if (!eos.exited()) {
501 // ExitOnSuspend did not exit the OM
502 assert(owner_raw() == owner_for(current), "invariant");
503 break;
504 }
505 }
506
507 // We've just gotten past the enter-check-for-suspend dance and we now own
508 // the monitor free and clear.
509 }
510
511 add_to_contentions(-1);
512 assert(contentions() >= 0, "must not be negative: contentions=%d", contentions());
513
514 // Must either set _recursions = 0 or ASSERT _recursions == 0.
515 assert(_recursions == 0, "invariant");
516 assert(owner_raw() == owner_for(current), "invariant");
517 assert(_succ != current, "invariant");
518 assert(object()->mark() == markWord::encode(this), "invariant");
519
520 // The thread -- now the owner -- is back in vm mode.
521 // Report the glorious news via TI,DTrace and jvmstat.
522 // The probe effect is non-trivial. All the reportage occurs
523 // while we hold the monitor, increasing the length of the critical
524 // section. Amdahl's parallel speedup law comes vividly into play.
525 //
526 // Another option might be to aggregate the events (thread local or
527 // per-monitor aggregation) and defer reporting until a more opportune
528 // time -- such as next time some thread encounters contention but has
529 // yet to acquire the lock. While spinning that thread could
530 // spinning we could increment JVMStat counters, etc.
531
532 DTRACE_MONITOR_PROBE(contended__entered, this, object(), current);
533 if (JvmtiExport::should_post_monitor_contended_entered()) {
534 JvmtiExport::post_monitor_contended_entered(current, this);
535
536 // The current thread already owns the monitor and is not going to
537 // call park() for the remainder of the monitor enter protocol. So
538 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
539 // event handler consumed an unpark() issued by the thread that
540 // just exited the monitor.
541 }
542 if (event.should_commit()) {
543 event.set_previousOwner(_previous_owner_tid);
544 event.commit();
545 }
546 OM_PERFDATA_OP(ContendedLockAttempts, inc());
547 return true;
548 }
549
550 // Caveat: TryLock() is not necessarily serializing if it returns failure.
551 // Callers must compensate as needed.
552
553 ObjectMonitor::TryLockResult ObjectMonitor::TryLock(JavaThread* current) {
554 void* own = owner_raw();
555 if (own != nullptr) return TryLockResult::HasOwner;
556 if (try_set_owner_from(nullptr, current) == nullptr) {
557 assert(_recursions == 0, "invariant");
558 return TryLockResult::Success;
559 }
560 // The lock had been free momentarily, but we lost the race to the lock.
561 // Interference -- the CAS failed.
562 // We can either return -1 or retry.
563 // Retry doesn't make as much sense because the lock was just acquired.
564 return TryLockResult::Interference;
565 }
566
567 // Deflate the specified ObjectMonitor if not in-use. Returns true if it
568 // was deflated and false otherwise.
569 //
570 // The async deflation protocol sets owner to DEFLATER_MARKER and
571 // makes contentions negative as signals to contending threads that
572 // an async deflation is in progress. There are a number of checks
573 // as part of the protocol to make sure that the calling thread has
574 // not lost the race to a contending thread.
575 //
576 // The ObjectMonitor has been successfully async deflated when:
577 // (contentions < 0)
578 // Contending threads that see that condition know to retry their operation.
579 //
580 bool ObjectMonitor::deflate_monitor() {
581 if (is_busy()) {
582 // Easy checks are first - the ObjectMonitor is busy so no deflation.
583 return false;
584 }
585
586 const oop obj = object_peek();
587
588 if (obj == nullptr) {
589 // If the object died, we can recycle the monitor without racing with
590 // Java threads. The GC already broke the association with the object.
591 set_owner_from_raw(nullptr, DEFLATER_MARKER);
592 assert(contentions() >= 0, "must be non-negative: contentions=%d", contentions());
593 _contentions = INT_MIN; // minimum negative int
594 } else {
595 // Attempt async deflation protocol.
596
597 // Set a null owner to DEFLATER_MARKER to force any contending thread
598 // through the slow path. This is just the first part of the async
599 // deflation dance.
600 if (try_set_owner_from_raw(nullptr, DEFLATER_MARKER) != nullptr) {
601 // The owner field is no longer null so we lost the race since the
602 // ObjectMonitor is now busy.
603 return false;
604 }
605
606 if (contentions() > 0 || _waiters != 0) {
607 // Another thread has raced to enter the ObjectMonitor after
608 // is_busy() above or has already entered and waited on
609 // it which makes it busy so no deflation. Restore owner to
610 // null if it is still DEFLATER_MARKER.
611 if (try_set_owner_from_raw(DEFLATER_MARKER, nullptr) != DEFLATER_MARKER) {
612 // Deferred decrement for the JT EnterI() that cancelled the async deflation.
613 add_to_contentions(-1);
614 }
615 return false;
616 }
617
618 // Make a zero contentions field negative to force any contending threads
619 // to retry. This is the second part of the async deflation dance.
620 if (Atomic::cmpxchg(&_contentions, 0, INT_MIN) != 0) {
621 // Contentions was no longer 0 so we lost the race since the
622 // ObjectMonitor is now busy. Restore owner to null if it is
623 // still DEFLATER_MARKER:
624 if (try_set_owner_from_raw(DEFLATER_MARKER, nullptr) != DEFLATER_MARKER) {
625 // Deferred decrement for the JT EnterI() that cancelled the async deflation.
626 add_to_contentions(-1);
627 }
628 return false;
629 }
630 }
631
632 // Sanity checks for the races:
633 guarantee(owner_is_DEFLATER_MARKER(), "must be deflater marker");
634 guarantee(contentions() < 0, "must be negative: contentions=%d",
635 contentions());
636 guarantee(_waiters == 0, "must be 0: waiters=%d", _waiters);
637 guarantee(_cxq == nullptr, "must be no contending threads: cxq="
638 INTPTR_FORMAT, p2i(_cxq));
639 guarantee(_EntryList == nullptr,
640 "must be no entering threads: EntryList=" INTPTR_FORMAT,
641 p2i(_EntryList));
642
643 if (obj != nullptr) {
644 if (log_is_enabled(Trace, monitorinflation)) {
645 ResourceMark rm;
646 log_trace(monitorinflation)("deflate_monitor: object=" INTPTR_FORMAT
647 ", mark=" INTPTR_FORMAT ", type='%s'",
648 p2i(obj), obj->mark().value(),
649 obj->klass()->external_name());
650 }
651
652 // Install the old mark word if nobody else has already done it.
653 install_displaced_markword_in_object(obj);
654 }
655
656 // We leave owner == DEFLATER_MARKER and contentions < 0
657 // to force any racing threads to retry.
658 return true; // Success, ObjectMonitor has been deflated.
659 }
660
661 // Install the displaced mark word (dmw) of a deflating ObjectMonitor
662 // into the header of the object associated with the monitor. This
663 // idempotent method is called by a thread that is deflating a
664 // monitor and by other threads that have detected a race with the
665 // deflation process.
666 void ObjectMonitor::install_displaced_markword_in_object(const oop obj) {
667 // This function must only be called when (owner == DEFLATER_MARKER
668 // && contentions <= 0), but we can't guarantee that here because
669 // those values could change when the ObjectMonitor gets moved from
670 // the global free list to a per-thread free list.
671
672 guarantee(obj != nullptr, "must be non-null");
673
674 // Separate loads in is_being_async_deflated(), which is almost always
675 // called before this function, from the load of dmw/header below.
676
677 // _contentions and dmw/header may get written by different threads.
678 // Make sure to observe them in the same order when having several observers.
679 OrderAccess::loadload_for_IRIW();
680
681 const oop l_object = object_peek();
682 if (l_object == nullptr) {
683 // ObjectMonitor's object ref has already been cleared by async
684 // deflation or GC so we're done here.
685 return;
686 }
687 assert(l_object == obj, "object=" INTPTR_FORMAT " must equal obj="
688 INTPTR_FORMAT, p2i(l_object), p2i(obj));
689
690 markWord dmw = header();
691 // The dmw has to be neutral (not null, not locked and not marked).
692 assert(dmw.is_neutral(), "must be neutral: dmw=" INTPTR_FORMAT, dmw.value());
693
694 // Install displaced mark word if the object's header still points
695 // to this ObjectMonitor. More than one racing caller to this function
696 // can rarely reach this point, but only one can win.
697 markWord res = obj->cas_set_mark(dmw, markWord::encode(this));
698 if (res != markWord::encode(this)) {
699 // This should be rare so log at the Info level when it happens.
700 log_info(monitorinflation)("install_displaced_markword_in_object: "
701 "failed cas_set_mark: new_mark=" INTPTR_FORMAT
702 ", old_mark=" INTPTR_FORMAT ", res=" INTPTR_FORMAT,
703 dmw.value(), markWord::encode(this).value(),
704 res.value());
705 }
706
707 // Note: It does not matter which thread restored the header/dmw
708 // into the object's header. The thread deflating the monitor just
709 // wanted the object's header restored and it is. The threads that
710 // detected a race with the deflation process also wanted the
711 // object's header restored before they retry their operation and
712 // because it is restored they will only retry once.
713 }
714
715 // Convert the fields used by is_busy() to a string that can be
716 // used for diagnostic output.
717 const char* ObjectMonitor::is_busy_to_string(stringStream* ss) {
718 ss->print("is_busy: waiters=%d"
719 ", contentions=%d"
720 ", owner=" INTPTR_FORMAT
721 ", cxq=" PTR_FORMAT
722 ", EntryList=" PTR_FORMAT,
723 _waiters,
724 (contentions() > 0 ? contentions() : 0),
725 owner_is_DEFLATER_MARKER()
726 // We report null instead of DEFLATER_MARKER here because is_busy()
727 // ignores DEFLATER_MARKER values.
728 ? p2i(nullptr)
729 : p2i(owner_raw()),
730 p2i(_cxq),
731 p2i(_EntryList));
732 return ss->base();
733 }
734
735 #define MAX_RECHECK_INTERVAL 1000
736
737 void ObjectMonitor::EnterI(JavaThread* current) {
738 assert(current->thread_state() == _thread_blocked, "invariant");
739
740 // Try the lock - TATAS
741 if (TryLock(current) == TryLockResult::Success) {
742 assert(_succ != current, "invariant");
743 assert(owner_raw() == owner_for(current), "invariant");
744 assert(_Responsible != current, "invariant");
745 return;
746 }
747
748 if (try_set_owner_from(DEFLATER_MARKER, current) == DEFLATER_MARKER) {
749 // Cancelled the in-progress async deflation by changing owner from
750 // DEFLATER_MARKER to current. As part of the contended enter protocol,
751 // contentions was incremented to a positive value before EnterI()
752 // was called and that prevents the deflater thread from winning the
753 // last part of the 2-part async deflation protocol. After EnterI()
754 // returns to enter(), contentions is decremented because the caller
755 // now owns the monitor. We bump contentions an extra time here to
756 // prevent the deflater thread from winning the last part of the
757 // 2-part async deflation protocol after the regular decrement
758 // occurs in enter(). The deflater thread will decrement contentions
759 // after it recognizes that the async deflation was cancelled.
760 add_to_contentions(1);
761 assert(_succ != current, "invariant");
762 assert(_Responsible != current, "invariant");
763 return;
764 }
765
766 assert(InitDone, "Unexpectedly not initialized");
767
768 // We try one round of spinning *before* enqueueing current.
769 //
770 // If the _owner is ready but OFFPROC we could use a YieldTo()
771 // operation to donate the remainder of this thread's quantum
772 // to the owner. This has subtle but beneficial affinity
773 // effects.
774
775 if (TrySpin(current)) {
776 assert(owner_raw() == owner_for(current), "invariant");
777 assert(_succ != current, "invariant");
778 assert(_Responsible != current, "invariant");
779 return;
780 }
781
782 // The Spin failed -- Enqueue and park the thread ...
783 assert(_succ != current, "invariant");
784 assert(owner_raw() != owner_for(current), "invariant");
785 assert(_Responsible != current, "invariant");
786
787 // Enqueue "current" on ObjectMonitor's _cxq.
788 //
789 // Node acts as a proxy for current.
790 // As an aside, if were to ever rewrite the synchronization code mostly
791 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
792 // Java objects. This would avoid awkward lifecycle and liveness issues,
793 // as well as eliminate a subset of ABA issues.
794 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
795
796 ObjectWaiter node(current);
797 current->_ParkEvent->reset();
798 node._prev = (ObjectWaiter*) 0xBAD;
799 node.TState = ObjectWaiter::TS_CXQ;
800
801 // Push "current" onto the front of the _cxq.
802 // Once on cxq/EntryList, current stays on-queue until it acquires the lock.
803 // Note that spinning tends to reduce the rate at which threads
804 // enqueue and dequeue on EntryList|cxq.
805 ObjectWaiter* nxt;
806 for (;;) {
807 node._next = nxt = _cxq;
808 if (Atomic::cmpxchg(&_cxq, nxt, &node) == nxt) break;
809
810 // Interference - the CAS failed because _cxq changed. Just retry.
811 // As an optional optimization we retry the lock.
812 if (TryLock(current) == TryLockResult::Success) {
813 assert(_succ != current, "invariant");
814 assert(owner_raw() == owner_for(current), "invariant");
815 assert(_Responsible != current, "invariant");
816 return;
817 }
818 }
819
820 // Check for cxq|EntryList edge transition to non-null. This indicates
821 // the onset of contention. While contention persists exiting threads
822 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
823 // operations revert to the faster 1-0 mode. This enter operation may interleave
824 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
825 // arrange for one of the contending thread to use a timed park() operations
826 // to detect and recover from the race. (Stranding is form of progress failure
827 // where the monitor is unlocked but all the contending threads remain parked).
828 // That is, at least one of the contended threads will periodically poll _owner.
829 // One of the contending threads will become the designated "Responsible" thread.
830 // The Responsible thread uses a timed park instead of a normal indefinite park
831 // operation -- it periodically wakes and checks for and recovers from potential
832 // strandings admitted by 1-0 exit operations. We need at most one Responsible
833 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
834 // be responsible for a monitor.
835 //
836 // Currently, one of the contended threads takes on the added role of "Responsible".
837 // A viable alternative would be to use a dedicated "stranding checker" thread
838 // that periodically iterated over all the threads (or active monitors) and unparked
839 // successors where there was risk of stranding. This would help eliminate the
840 // timer scalability issues we see on some platforms as we'd only have one thread
841 // -- the checker -- parked on a timer.
842
843 if (nxt == nullptr && _EntryList == nullptr) {
844 // Try to assume the role of responsible thread for the monitor.
845 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=current }
846 Atomic::replace_if_null(&_Responsible, current);
847 }
848
849 // The lock might have been released while this thread was occupied queueing
850 // itself onto _cxq. To close the race and avoid "stranding" and
851 // progress-liveness failure we must resample-retry _owner before parking.
852 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
853 // In this case the ST-MEMBAR is accomplished with CAS().
854 //
855 // TODO: Defer all thread state transitions until park-time.
856 // Since state transitions are heavy and inefficient we'd like
857 // to defer the state transitions until absolutely necessary,
858 // and in doing so avoid some transitions ...
859
860 int recheckInterval = 1;
861 bool do_timed_parked = false;
862
863 ContinuationEntry* ce = current->last_continuation();
864 if (ce != nullptr && ce->is_virtual_thread()) {
865 do_timed_parked = true;
866 }
867
868 for (;;) {
869
870 if (TryLock(current) == TryLockResult::Success) {
871 break;
872 }
873 assert(owner_raw() != owner_for(current), "invariant");
874
875 // park self
876 if (_Responsible == current || do_timed_parked) {
877 current->_ParkEvent->park((jlong) recheckInterval);
878 // Increase the recheckInterval, but clamp the value.
879 recheckInterval *= 8;
880 if (recheckInterval > MAX_RECHECK_INTERVAL) {
881 recheckInterval = MAX_RECHECK_INTERVAL;
882 }
883 } else {
884 current->_ParkEvent->park();
885 }
886
887 if (TryLock(current) == TryLockResult::Success) {
888 break;
889 }
890
891 if (try_set_owner_from(DEFLATER_MARKER, current) == DEFLATER_MARKER) {
892 // Cancelled the in-progress async deflation by changing owner from
893 // DEFLATER_MARKER to current. As part of the contended enter protocol,
894 // contentions was incremented to a positive value before EnterI()
895 // was called and that prevents the deflater thread from winning the
896 // last part of the 2-part async deflation protocol. After EnterI()
897 // returns to enter(), contentions is decremented because the caller
898 // now owns the monitor. We bump contentions an extra time here to
899 // prevent the deflater thread from winning the last part of the
900 // 2-part async deflation protocol after the regular decrement
901 // occurs in enter(). The deflater thread will decrement contentions
902 // after it recognizes that the async deflation was cancelled.
903 add_to_contentions(1);
904 break;
905 }
906
907 // The lock is still contested.
908
909 // Keep a tally of the # of futile wakeups.
910 // Note that the counter is not protected by a lock or updated by atomics.
911 // That is by design - we trade "lossy" counters which are exposed to
912 // races during updates for a lower probe effect.
913 // This PerfData object can be used in parallel with a safepoint.
914 // See the work around in PerfDataManager::destroy().
915 OM_PERFDATA_OP(FutileWakeups, inc());
916
917 // Assuming this is not a spurious wakeup we'll normally find _succ == current.
918 // We can defer clearing _succ until after the spin completes
919 // TrySpin() must tolerate being called with _succ == current.
920 // Try yet another round of adaptive spinning.
921 if (TrySpin(current)) {
922 break;
923 }
924
925 // We can find that we were unpark()ed and redesignated _succ while
926 // we were spinning. That's harmless. If we iterate and call park(),
927 // park() will consume the event and return immediately and we'll
928 // just spin again. This pattern can repeat, leaving _succ to simply
929 // spin on a CPU.
930
931 if (_succ == current) _succ = nullptr;
932
933 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
934 OrderAccess::fence();
935 }
936
937 // Egress :
938 // current has acquired the lock -- Unlink current from the cxq or EntryList.
939 // Normally we'll find current on the EntryList .
940 // From the perspective of the lock owner (this thread), the
941 // EntryList is stable and cxq is prepend-only.
942 // The head of cxq is volatile but the interior is stable.
943 // In addition, current.TState is stable.
944
945 assert(owner_raw() == owner_for(current), "invariant");
946
947 UnlinkAfterAcquire(current, &node);
948 if (_succ == current) _succ = nullptr;
949
950 assert(_succ != current, "invariant");
951 if (_Responsible == current) {
952 _Responsible = nullptr;
953 OrderAccess::fence(); // Dekker pivot-point
954
955 // We may leave threads on cxq|EntryList without a designated
956 // "Responsible" thread. This is benign. When this thread subsequently
957 // exits the monitor it can "see" such preexisting "old" threads --
958 // threads that arrived on the cxq|EntryList before the fence, above --
959 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
960 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
961 // non-null and elect a new "Responsible" timer thread.
962 //
963 // This thread executes:
964 // ST Responsible=null; MEMBAR (in enter epilogue - here)
965 // LD cxq|EntryList (in subsequent exit)
966 //
967 // Entering threads in the slow/contended path execute:
968 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
969 // The (ST cxq; MEMBAR) is accomplished with CAS().
970 //
971 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
972 // exit operation from floating above the ST Responsible=null.
973 }
974
975 // We've acquired ownership with CAS().
976 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
977 // But since the CAS() this thread may have also stored into _succ,
978 // EntryList, cxq or Responsible. These meta-data updates must be
979 // visible __before this thread subsequently drops the lock.
980 // Consider what could occur if we didn't enforce this constraint --
981 // STs to monitor meta-data and user-data could reorder with (become
982 // visible after) the ST in exit that drops ownership of the lock.
983 // Some other thread could then acquire the lock, but observe inconsistent
984 // or old monitor meta-data and heap data. That violates the JMM.
985 // To that end, the 1-0 exit() operation must have at least STST|LDST
986 // "release" barrier semantics. Specifically, there must be at least a
987 // STST|LDST barrier in exit() before the ST of null into _owner that drops
988 // the lock. The barrier ensures that changes to monitor meta-data and data
989 // protected by the lock will be visible before we release the lock, and
990 // therefore before some other thread (CPU) has a chance to acquire the lock.
991 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
992 //
993 // Critically, any prior STs to _succ or EntryList must be visible before
994 // the ST of null into _owner in the *subsequent* (following) corresponding
995 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
996 // execute a serializing instruction.
997
998 return;
999 }
1000
1001 // ReenterI() is a specialized inline form of the latter half of the
1002 // contended slow-path from EnterI(). We use ReenterI() only for
1003 // monitor reentry in wait().
1004 //
1005 // In the future we should reconcile EnterI() and ReenterI().
1006
1007 void ObjectMonitor::ReenterI(JavaThread* current, ObjectWaiter* currentNode) {
1008 assert(current != nullptr, "invariant");
1009 assert(currentNode != nullptr, "invariant");
1010 assert(currentNode->_thread == current, "invariant");
1011 assert(_waiters > 0, "invariant");
1012 assert(object()->mark() == markWord::encode(this), "invariant");
1013
1014 assert(current->thread_state() != _thread_blocked, "invariant");
1015
1016 for (;;) {
1017 uint8_t v = currentNode->TState;
1018 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1019 assert(owner_raw() != owner_for(current), "invariant");
1020
1021 // This thread has been notified so try to reacquire the lock.
1022 if (TryLock(current) == TryLockResult::Success) {
1023 break;
1024 }
1025
1026 // If that fails, spin again. Note that spin count may be zero so the above TryLock
1027 // is necessary.
1028 if (TrySpin(current)) {
1029 break;
1030 }
1031
1032 {
1033 OSThreadContendState osts(current->osthread());
1034
1035 assert(current->thread_state() == _thread_in_vm, "invariant");
1036
1037 {
1038 ClearSuccOnSuspend csos(this);
1039 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */);
1040 current->_ParkEvent->park();
1041 }
1042 }
1043
1044 // Try again, but just so we distinguish between futile wakeups and
1045 // successful wakeups. The following test isn't algorithmically
1046 // necessary, but it helps us maintain sensible statistics.
1047 if (TryLock(current) == TryLockResult::Success) {
1048 break;
1049 }
1050
1051 // The lock is still contested.
1052
1053 // Assuming this is not a spurious wakeup we'll normally
1054 // find that _succ == current.
1055 if (_succ == current) _succ = nullptr;
1056
1057 // Invariant: after clearing _succ a contending thread
1058 // *must* retry _owner before parking.
1059 OrderAccess::fence();
1060
1061 // Keep a tally of the # of futile wakeups.
1062 // Note that the counter is not protected by a lock or updated by atomics.
1063 // That is by design - we trade "lossy" counters which are exposed to
1064 // races during updates for a lower probe effect.
1065 // This PerfData object can be used in parallel with a safepoint.
1066 // See the work around in PerfDataManager::destroy().
1067 OM_PERFDATA_OP(FutileWakeups, inc());
1068 }
1069
1070 // current has acquired the lock -- Unlink current from the cxq or EntryList .
1071 // Normally we'll find current on the EntryList.
1072 // Unlinking from the EntryList is constant-time and atomic-free.
1073 // From the perspective of the lock owner (this thread), the
1074 // EntryList is stable and cxq is prepend-only.
1075 // The head of cxq is volatile but the interior is stable.
1076 // In addition, current.TState is stable.
1077
1078 assert(owner_raw() == owner_for(current), "invariant");
1079 assert(object()->mark() == markWord::encode(this), "invariant");
1080 UnlinkAfterAcquire(current, currentNode);
1081 if (_succ == current) _succ = nullptr;
1082 assert(_succ != current, "invariant");
1083 currentNode->TState = ObjectWaiter::TS_RUN;
1084 OrderAccess::fence(); // see comments at the end of EnterI()
1085 }
1086
1087 // This method is called from two places:
1088 // - On monitorenter contention with a null waiter.
1089 // - After Object.wait() times out or the target is interrupted to reenter the
1090 // monitor, with the existing waiter.
1091 // For the Object.wait() case we do not delete the ObjectWaiter in case we
1092 // succesfully acquire the monitor since we are going to need it on return.
1093 bool ObjectMonitor::VThreadMonitorEnter(JavaThread* current, ObjectWaiter* waiter) {
1094 if (TryLock(current) == TryLockResult::Success) {
1095 assert(owner_raw() == owner_for(current), "invariant");
1096 assert(_succ != current, "invariant");
1097 assert(_Responsible != current, "invariant");
1098 add_to_contentions(-1);
1099 return true;
1100 }
1101
1102 if (try_set_owner_from(DEFLATER_MARKER, current) == DEFLATER_MARKER) {
1103 // Cancelled the in-progress async deflation by changing owner from
1104 // DEFLATER_MARKER to current. As part of the contended enter protocol,
1105 // contentions was incremented to a positive value before this call to
1106 // VThreadMonitorEnter(). We avoid decrementing contentions to prevent
1107 // the deflater thread from winning the last part of the 2-part async
1108 // deflation protocol. The deflater thread will decrement contentions
1109 // after it recognizes that the async deflation was cancelled.
1110 assert(_succ != current, "invariant");
1111 assert(_Responsible != current, "invariant");
1112 assert(waiter != nullptr, "monitor currently in used marked for deflation??");
1113 return true;
1114 }
1115
1116 oop vthread = current->vthread();
1117 ObjectWaiter* node = waiter != nullptr ? waiter : new ObjectWaiter(vthread, this);
1118 node->_prev = (ObjectWaiter*) 0xBAD;
1119 node->TState = ObjectWaiter::TS_CXQ;
1120
1121 // Push node associated with vthread onto the front of the _cxq.
1122 ObjectWaiter* nxt;
1123 for (;;) {
1124 node->_next = nxt = _cxq;
1125 if (Atomic::cmpxchg(&_cxq, nxt, node) == nxt) break;
1126
1127 // Interference - the CAS failed because _cxq changed. Just retry.
1128 // As an optional optimization we retry the lock.
1129 if (TryLock(current) == TryLockResult::Success) {
1130 assert(owner_raw() == owner_for(current), "invariant");
1131 assert(_succ != current, "invariant");
1132 assert(_Responsible != current, "invariant");
1133 add_to_contentions(-1);
1134 if (waiter == nullptr) delete node; // for Object.wait() don't delete yet
1135 return true;
1136 }
1137 }
1138
1139 // We have to try once more since owner could have exited monitor and checked
1140 // _cxq before we added the node to the queue.
1141 if (TryLock(current) == TryLockResult::Success) {
1142 assert(owner_raw() == owner_for(current), "invariant");
1143 assert(_Responsible != current, "invariant");
1144 UnlinkAfterAcquire(current, node);
1145 if (_succ == (JavaThread*)java_lang_Thread::thread_id(vthread)) _succ = nullptr;
1146 add_to_contentions(-1);
1147 if (waiter == nullptr) delete node; // for Object.wait() don't delete yet
1148 return true;
1149 }
1150
1151 if (nxt == nullptr && _EntryList == nullptr) {
1152 // The C2 unlock() fast path first checks if _cxq and _EntryList are empty and
1153 // if they are it just clears the _owner field. Since we always run the risk of
1154 // having that check happening before we added the node to _cxq and the release
1155 // of the monitor happening after the last TryLock attempt we need to do something
1156 // to avoid stranding. We set the _Responsible field which results in a timed-wait.
1157 if (Atomic::replace_if_null(&_Responsible, (JavaThread*)java_lang_Thread::thread_id(vthread))) {
1158 java_lang_VirtualThread::set_recheckInterval(vthread, 1);
1159 }
1160 }
1161
1162 assert(java_lang_VirtualThread::state(vthread) == java_lang_VirtualThread::RUNNING, "wrong state for vthread");
1163 java_lang_VirtualThread::set_state(vthread, java_lang_VirtualThread::BLOCKING);
1164
1165 // We didn't succeed in acquiring the monitor so save ObjectWaiter*
1166 // in the chunk since we will need it when resuming execution.
1167 oop cont = java_lang_VirtualThread::continuation(vthread);
1168 stackChunkOop chunk = jdk_internal_vm_Continuation::tail(cont);
1169 chunk->set_object_waiter(node);
1170 return false;
1171 }
1172
1173 void ObjectMonitor::resume_operation(JavaThread* current, ObjectWaiter* node) {
1174 assert(java_lang_VirtualThread::state(current->vthread()) == java_lang_VirtualThread::RUNNING, "wrong state for vthread");
1175 assert(current->is_in_VTMS_transition(), "must be");
1176
1177 if (node->is_wait() && !node->at_reenter()) {
1178 bool notified = VThreadWaitReenter(current, node);
1179 if (!notified) return;
1180 // Notified case. We were already added to CXQ or TS_ENTER
1181 // by the notifier so just try to reenter the monitor.
1182 }
1183
1184 // Retry acquiring monitor...
1185
1186 int state = node->TState;
1187 guarantee(state == ObjectWaiter::TS_ENTER || state == ObjectWaiter::TS_CXQ, "invariant");
1188
1189 if (TryLock(current) == TryLockResult::Success) {
1190 VThreadEpilog(current, node);
1191 return;
1192 }
1193
1194 oop vthread = current->vthread();
1195 if (_succ == (JavaThread*)java_lang_Thread::thread_id(vthread)) _succ = nullptr;
1196
1197 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
1198 OrderAccess::fence();
1199
1200 if (TryLock(current) == TryLockResult::Success) {
1201 VThreadEpilog(current, node);
1202 return;
1203 }
1204
1205 // Update recheck interval in case we are the _Responsible.
1206 if (_Responsible == (JavaThread*)java_lang_Thread::thread_id(vthread)) {
1207 int recheckInterval = java_lang_VirtualThread::recheckInterval(vthread);
1208 assert(recheckInterval >= 1 && recheckInterval <= 6, "invariant");
1209 if (recheckInterval < 6) {
1210 recheckInterval++;
1211 java_lang_VirtualThread::set_recheckInterval(vthread, recheckInterval);
1212 }
1213 } else if (java_lang_VirtualThread::recheckInterval(vthread) > 0) {
1214 // No need to do timed park anymore
1215 java_lang_VirtualThread::set_recheckInterval(vthread, 0);
1216 }
1217
1218 // The JT will read this variable on return to the resume_monitor_operation stub
1219 // and will unmount (enterSpecial frame removed and return to Continuation.run()).
1220 current->set_preempting(true);
1221 java_lang_VirtualThread::set_state(vthread, java_lang_VirtualThread::BLOCKING);
1222 }
1223
1224 void ObjectMonitor::VThreadEpilog(JavaThread* current, ObjectWaiter* node) {
1225 assert(owner_raw() == owner_for(current), "invariant");
1226 add_to_contentions(-1);
1227
1228 oop vthread = current->vthread();
1229 if (java_lang_VirtualThread::recheckInterval(vthread) > 0) {
1230 java_lang_VirtualThread::set_recheckInterval(vthread, 0);
1231 }
1232 int64_t threadid = java_lang_Thread::thread_id(vthread);
1233 if (_succ == (JavaThread*)threadid) _succ = nullptr;
1234 if (_Responsible == (JavaThread*)threadid) {
1235 _Responsible = nullptr;
1236 OrderAccess::fence(); // Dekker pivot-point
1237 }
1238
1239 guarantee(_recursions == 0, "invariant");
1240
1241 if (node->is_wait()) {
1242 _recursions = node->_recursions; // restore the old recursion count
1243 _waiters--; // decrement the number of waiters
1244
1245 if (node->_interrupted) {
1246 // We will throw at thaw end after finishing the mount transition.
1247 current->set_pending_interrupted_exception(true);
1248 }
1249 }
1250
1251 assert(node->TState == ObjectWaiter::TS_ENTER || node->TState == ObjectWaiter::TS_CXQ, "");
1252 UnlinkAfterAcquire(current, node);
1253 delete node;
1254
1255 oop cont = java_lang_VirtualThread::continuation(vthread);
1256 stackChunkOop chunk = jdk_internal_vm_Continuation::tail(cont);
1257 chunk->set_object_waiter(nullptr);
1258
1259 if (JvmtiExport::should_post_monitor_contended_entered()) {
1260 // We are going to call thaw again after this and finish the VMTS
1261 // transition so no need to do it here. We will post the event there.
1262 current->set_contended_entered_monitor(this);
1263 }
1264 }
1265
1266 // By convention we unlink a contending thread from EntryList|cxq immediately
1267 // after the thread acquires the lock in ::enter(). Equally, we could defer
1268 // unlinking the thread until ::exit()-time.
1269
1270 void ObjectMonitor::UnlinkAfterAcquire(JavaThread* current, ObjectWaiter* currentNode) {
1271 assert(owner_raw() == owner_for(current), "invariant");
1272 assert((!currentNode->is_vthread() && currentNode->thread() == current) ||
1273 (currentNode->is_vthread() && currentNode->vthread() == current->vthread()), "invariant");
1274
1275 if (currentNode->TState == ObjectWaiter::TS_ENTER) {
1276 // Normal case: remove current from the DLL EntryList .
1277 // This is a constant-time operation.
1278 ObjectWaiter* nxt = currentNode->_next;
1279 ObjectWaiter* prv = currentNode->_prev;
1280 if (nxt != nullptr) nxt->_prev = prv;
1281 if (prv != nullptr) prv->_next = nxt;
1282 if (currentNode == _EntryList) _EntryList = nxt;
1283 assert(nxt == nullptr || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
1284 assert(prv == nullptr || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
1285 } else {
1286 assert(currentNode->TState == ObjectWaiter::TS_CXQ, "invariant");
1287 // Inopportune interleaving -- current is still on the cxq.
1288 // This usually means the enqueue of self raced an exiting thread.
1289 // Normally we'll find current near the front of the cxq, so
1290 // dequeueing is typically fast. If needbe we can accelerate
1291 // this with some MCS/CHL-like bidirectional list hints and advisory
1292 // back-links so dequeueing from the interior will normally operate
1293 // in constant-time.
1294 // Dequeue current from either the head (with CAS) or from the interior
1295 // with a linear-time scan and normal non-atomic memory operations.
1296 // CONSIDER: if current is on the cxq then simply drain cxq into EntryList
1297 // and then unlink current from EntryList. We have to drain eventually,
1298 // so it might as well be now.
1299
1300 ObjectWaiter* v = _cxq;
1301 assert(v != nullptr, "invariant");
1302 if (v != currentNode || Atomic::cmpxchg(&_cxq, v, currentNode->_next) != v) {
1303 // The CAS above can fail from interference IFF a "RAT" arrived.
1304 // In that case current must be in the interior and can no longer be
1305 // at the head of cxq.
1306 if (v == currentNode) {
1307 assert(_cxq != v, "invariant");
1308 v = _cxq; // CAS above failed - start scan at head of list
1309 }
1310 ObjectWaiter* p;
1311 ObjectWaiter* q = nullptr;
1312 for (p = v; p != nullptr && p != currentNode; p = p->_next) {
1313 q = p;
1314 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
1315 }
1316 assert(v != currentNode, "invariant");
1317 assert(p == currentNode, "Node not found on cxq");
1318 assert(p != _cxq, "invariant");
1319 assert(q != nullptr, "invariant");
1320 assert(q->_next == p, "invariant");
1321 q->_next = p->_next;
1322 }
1323 }
1324
1325 #ifdef ASSERT
1326 // Diagnostic hygiene ...
1327 currentNode->_prev = (ObjectWaiter*) 0xBAD;
1328 currentNode->_next = (ObjectWaiter*) 0xBAD;
1329 currentNode->TState = ObjectWaiter::TS_RUN;
1330 #endif
1331 }
1332
1333 // -----------------------------------------------------------------------------
1334 // Exit support
1335 //
1336 // exit()
1337 // ~~~~~~
1338 // Note that the collector can't reclaim the objectMonitor or deflate
1339 // the object out from underneath the thread calling ::exit() as the
1340 // thread calling ::exit() never transitions to a stable state.
1341 // This inhibits GC, which in turn inhibits asynchronous (and
1342 // inopportune) reclamation of "this".
1343 //
1344 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
1345 // There's one exception to the claim above, however. EnterI() can call
1346 // exit() to drop a lock if the acquirer has been externally suspended.
1347 // In that case exit() is called with _thread_state == _thread_blocked,
1348 // but the monitor's _contentions field is > 0, which inhibits reclamation.
1349 //
1350 // 1-0 exit
1351 // ~~~~~~~~
1352 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
1353 // the fast-path operators have been optimized so the common ::exit()
1354 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
1355 // The code emitted by fast_unlock() elides the usual MEMBAR. This
1356 // greatly improves latency -- MEMBAR and CAS having considerable local
1357 // latency on modern processors -- but at the cost of "stranding". Absent the
1358 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
1359 // ::enter() path, resulting in the entering thread being stranding
1360 // and a progress-liveness failure. Stranding is extremely rare.
1361 // We use timers (timed park operations) & periodic polling to detect
1362 // and recover from stranding. Potentially stranded threads periodically
1363 // wake up and poll the lock. See the usage of the _Responsible variable.
1364 //
1365 // The CAS() in enter provides for safety and exclusion, while the CAS or
1366 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
1367 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
1368 // We detect and recover from stranding with timers.
1369 //
1370 // If a thread transiently strands it'll park until (a) another
1371 // thread acquires the lock and then drops the lock, at which time the
1372 // exiting thread will notice and unpark the stranded thread, or, (b)
1373 // the timer expires. If the lock is high traffic then the stranding latency
1374 // will be low due to (a). If the lock is low traffic then the odds of
1375 // stranding are lower, although the worst-case stranding latency
1376 // is longer. Critically, we don't want to put excessive load in the
1377 // platform's timer subsystem. We want to minimize both the timer injection
1378 // rate (timers created/sec) as well as the number of timers active at
1379 // any one time. (more precisely, we want to minimize timer-seconds, which is
1380 // the integral of the # of active timers at any instant over time).
1381 // Both impinge on OS scalability. Given that, at most one thread parked on
1382 // a monitor will use a timer.
1383 //
1384 // There is also the risk of a futile wake-up. If we drop the lock
1385 // another thread can reacquire the lock immediately, and we can
1386 // then wake a thread unnecessarily. This is benign, and we've
1387 // structured the code so the windows are short and the frequency
1388 // of such futile wakups is low.
1389
1390 void ObjectMonitor::exit(JavaThread* current, bool not_suspended) {
1391 void* cur = owner_raw();
1392 if (owner_for(current) != cur) {
1393 // Apparent unbalanced locking ...
1394 // Naively we'd like to throw IllegalMonitorStateException.
1395 // As a practical matter we can neither allocate nor throw an
1396 // exception as ::exit() can be called from leaf routines.
1397 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
1398 // Upon deeper reflection, however, in a properly run JVM the only
1399 // way we should encounter this situation is in the presence of
1400 // unbalanced JNI locking. TODO: CheckJNICalls.
1401 // See also: CR4414101
1402 #ifdef ASSERT
1403 LogStreamHandle(Error, monitorinflation) lsh;
1404 lsh.print_cr("ERROR: ObjectMonitor::exit(): thread=" INTPTR_FORMAT
1405 " is exiting an ObjectMonitor it does not own.", p2i(current));
1406 lsh.print_cr("The imbalance is possibly caused by JNI locking.");
1407 print_debug_style_on(&lsh);
1408 assert(false, "Non-balanced monitor enter/exit!");
1409 #endif
1410 return;
1411 }
1412
1413 if (_recursions != 0) {
1414 _recursions--; // this is simple recursive enter
1415 return;
1416 }
1417
1418 // Invariant: after setting Responsible=null an thread must execute
1419 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
1420 _Responsible = nullptr;
1421
1422 #if INCLUDE_JFR
1423 // get the owner's thread id for the MonitorEnter event
1424 // if it is enabled and the thread isn't suspended
1425 if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
1426 _previous_owner_tid = JFR_THREAD_ID(current);
1427 }
1428 #endif
1429
1430 for (;;) {
1431 assert(owner_for(current) == owner_raw(), "invariant");
1432
1433 // Drop the lock.
1434 // release semantics: prior loads and stores from within the critical section
1435 // must not float (reorder) past the following store that drops the lock.
1436 // Uses a storeload to separate release_store(owner) from the
1437 // successor check. The try_set_owner_from() below uses cmpxchg() so
1438 // we get the fence down there.
1439 release_clear_owner(current);
1440 OrderAccess::storeload();
1441
1442 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != nullptr) {
1443 return;
1444 }
1445 // Other threads are blocked trying to acquire the lock.
1446
1447 // Normally the exiting thread is responsible for ensuring succession,
1448 // but if other successors are ready or other entering threads are spinning
1449 // then this thread can simply store null into _owner and exit without
1450 // waking a successor. The existence of spinners or ready successors
1451 // guarantees proper succession (liveness). Responsibility passes to the
1452 // ready or running successors. The exiting thread delegates the duty.
1453 // More precisely, if a successor already exists this thread is absolved
1454 // of the responsibility of waking (unparking) one.
1455 //
1456 // The _succ variable is critical to reducing futile wakeup frequency.
1457 // _succ identifies the "heir presumptive" thread that has been made
1458 // ready (unparked) but that has not yet run. We need only one such
1459 // successor thread to guarantee progress.
1460 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1461 // section 3.3 "Futile Wakeup Throttling" for details.
1462 //
1463 // Note that spinners in Enter() also set _succ non-null.
1464 // In the current implementation spinners opportunistically set
1465 // _succ so that exiting threads might avoid waking a successor.
1466 // Another less appealing alternative would be for the exiting thread
1467 // to drop the lock and then spin briefly to see if a spinner managed
1468 // to acquire the lock. If so, the exiting thread could exit
1469 // immediately without waking a successor, otherwise the exiting
1470 // thread would need to dequeue and wake a successor.
1471 // (Note that we'd need to make the post-drop spin short, but no
1472 // shorter than the worst-case round-trip cache-line migration time.
1473 // The dropped lock needs to become visible to the spinner, and then
1474 // the acquisition of the lock by the spinner must become visible to
1475 // the exiting thread).
1476
1477 // It appears that an heir-presumptive (successor) must be made ready.
1478 // Only the current lock owner can manipulate the EntryList or
1479 // drain _cxq, so we need to reacquire the lock. If we fail
1480 // to reacquire the lock the responsibility for ensuring succession
1481 // falls to the new owner.
1482 //
1483 if (try_set_owner_from(nullptr, current) != nullptr) {
1484 return;
1485 }
1486
1487 guarantee(owner_raw() == owner_for(current), "invariant");
1488
1489 ObjectWaiter* w = nullptr;
1490
1491 w = _EntryList;
1492 if (w != nullptr) {
1493 // I'd like to write: guarantee (w->_thread != current).
1494 // But in practice an exiting thread may find itself on the EntryList.
1495 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1496 // then calls exit(). Exit release the lock by setting O._owner to null.
1497 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1498 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1499 // release the lock "O". T2 resumes immediately after the ST of null into
1500 // _owner, above. T2 notices that the EntryList is populated, so it
1501 // reacquires the lock and then finds itself on the EntryList.
1502 // Given all that, we have to tolerate the circumstance where "w" is
1503 // associated with current.
1504 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1505 ExitEpilog(current, w);
1506 return;
1507 }
1508
1509 // If we find that both _cxq and EntryList are null then just
1510 // re-run the exit protocol from the top.
1511 w = _cxq;
1512 if (w == nullptr) continue;
1513
1514 // Drain _cxq into EntryList - bulk transfer.
1515 // First, detach _cxq.
1516 // The following loop is tantamount to: w = swap(&cxq, nullptr)
1517 for (;;) {
1518 assert(w != nullptr, "Invariant");
1519 ObjectWaiter* u = Atomic::cmpxchg(&_cxq, w, (ObjectWaiter*)nullptr);
1520 if (u == w) break;
1521 w = u;
1522 }
1523
1524 assert(w != nullptr, "invariant");
1525 assert(_EntryList == nullptr, "invariant");
1526
1527 // Convert the LIFO SLL anchored by _cxq into a DLL.
1528 // The list reorganization step operates in O(LENGTH(w)) time.
1529 // It's critical that this step operate quickly as
1530 // "current" still holds the outer-lock, restricting parallelism
1531 // and effectively lengthening the critical section.
1532 // Invariant: s chases t chases u.
1533 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1534 // we have faster access to the tail.
1535
1536 _EntryList = w;
1537 ObjectWaiter* q = nullptr;
1538 ObjectWaiter* p;
1539 for (p = w; p != nullptr; p = p->_next) {
1540 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1541 p->TState = ObjectWaiter::TS_ENTER;
1542 p->_prev = q;
1543 q = p;
1544 }
1545
1546 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = nullptr
1547 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1548
1549 // See if we can abdicate to a spinner instead of waking a thread.
1550 // A primary goal of the implementation is to reduce the
1551 // context-switch rate.
1552 if (_succ != nullptr) continue;
1553
1554 w = _EntryList;
1555 if (w != nullptr) {
1556 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1557 ExitEpilog(current, w);
1558 return;
1559 }
1560 }
1561 }
1562
1563 void ObjectMonitor::ExitEpilog(JavaThread* current, ObjectWaiter* Wakee) {
1564 assert(owner_raw() == owner_for(current), "invariant");
1565
1566 // Exit protocol:
1567 // 1. ST _succ = wakee
1568 // 2. membar #loadstore|#storestore;
1569 // 2. ST _owner = nullptr
1570 // 3. unpark(wakee)
1571
1572 oop vthread = nullptr;
1573 ParkEvent * Trigger;
1574 if (!Wakee->is_vthread()) {
1575 JavaThread* t = Wakee->thread();
1576 assert(t != nullptr, "");
1577 Trigger = t->_ParkEvent;
1578 _succ = t;
1579 } else {
1580 vthread = Wakee->vthread();
1581 assert(vthread != nullptr, "");
1582 Trigger = ObjectMonitor::vthread_unparker_ParkEvent();
1583 _succ = (JavaThread*)java_lang_Thread::thread_id(vthread);
1584 }
1585
1586 // Hygiene -- once we've set _owner = nullptr we can't safely dereference Wakee again.
1587 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1588 // out-of-scope (non-extant).
1589 Wakee = nullptr;
1590
1591 // Drop the lock.
1592 // Uses a fence to separate release_store(owner) from the LD in unpark().
1593 release_clear_owner(current);
1594 OrderAccess::fence();
1595
1596 DTRACE_MONITOR_PROBE(contended__exit, this, object(), current);
1597
1598 if (vthread == nullptr) {
1599 // Platform thread case
1600 Trigger->unpark();
1601 } else if (java_lang_VirtualThread::set_onWaitingList(vthread, _vthread_cxq_head)) {
1602 Trigger->unpark();
1603 }
1604
1605 // Maintain stats and report events to JVMTI
1606 OM_PERFDATA_OP(Parks, inc());
1607 }
1608
1609 // complete_exit exits a lock returning recursion count
1610 // complete_exit requires an inflated monitor
1611 // The _owner field is not always the Thread addr even with an
1612 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1613 // thread due to contention.
1614 intx ObjectMonitor::complete_exit(JavaThread* current) {
1615 assert(InitDone, "Unexpectedly not initialized");
1616
1617 void* cur = owner_raw();
1618 if (owner_for(current) != cur) {
1619 if (LockingMode == LM_LEGACY && is_stack_locker(current)) {
1620 assert(_recursions == 0, "internal state error");
1621 set_owner_from_BasicLock(current); // Convert from BasicLock* to Thread*.
1622 _recursions = 0;
1623 }
1624 }
1625
1626 guarantee(owner_for(current) == owner_raw(), "complete_exit not owner");
1627 intx save = _recursions; // record the old recursion count
1628 _recursions = 0; // set the recursion level to be 0
1629 exit(current); // exit the monitor
1630 guarantee(owner_raw() != owner_for(current), "invariant");
1631 return save;
1632 }
1633
1634 // Checks that the current THREAD owns this monitor and causes an
1635 // immediate return if it doesn't. We don't use the CHECK macro
1636 // because we want the IMSE to be the only exception that is thrown
1637 // from the call site when false is returned. Any other pending
1638 // exception is ignored.
1639 #define CHECK_OWNER() \
1640 do { \
1641 if (!check_owner(THREAD)) { \
1642 assert(HAS_PENDING_EXCEPTION, "expected a pending IMSE here."); \
1643 return; \
1644 } \
1645 } while (false)
1646
1647 // Returns true if the specified thread owns the ObjectMonitor.
1648 // Otherwise returns false and throws IllegalMonitorStateException
1649 // (IMSE). If there is a pending exception and the specified thread
1650 // is not the owner, that exception will be replaced by the IMSE.
1651 bool ObjectMonitor::check_owner(TRAPS) {
1652 JavaThread* current = THREAD;
1653 void* cur = owner_raw();
1654 if (cur == owner_for(current)) {
1655 return true;
1656 }
1657 THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(),
1658 "current thread is not owner", false);
1659 }
1660
1661 static inline bool is_excluded(const Klass* monitor_klass) {
1662 assert(monitor_klass != nullptr, "invariant");
1663 NOT_JFR_RETURN_(false);
1664 JFR_ONLY(return vmSymbols::jdk_jfr_internal_management_HiddenWait() == monitor_klass->name();)
1665 }
1666
1667 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1668 ObjectMonitor* monitor,
1669 uint64_t notifier_tid,
1670 jlong timeout,
1671 bool timedout) {
1672 assert(event != nullptr, "invariant");
1673 assert(monitor != nullptr, "invariant");
1674 const Klass* monitor_klass = monitor->object()->klass();
1675 if (is_excluded(monitor_klass)) {
1676 return;
1677 }
1678 event->set_monitorClass(monitor_klass);
1679 event->set_timeout(timeout);
1680 // Set an address that is 'unique enough', such that events close in
1681 // time and with the same address are likely (but not guaranteed) to
1682 // belong to the same object.
1683 event->set_address((uintptr_t)monitor);
1684 event->set_notifier(notifier_tid);
1685 event->set_timedOut(timedout);
1686 event->commit();
1687 }
1688
1689 static void vthread_monitor_waited_event(JavaThread *current, ObjectWaiter* node, EventJavaMonitorWait* event, jboolean timed_out) {
1690 // Since we might safepoint set the anchor so that the stack can we walked.
1691 assert(current->last_continuation() != nullptr, "");
1692 JavaFrameAnchor* anchor = current->frame_anchor();
1693 anchor->set_last_Java_sp(current->last_continuation()->entry_sp());
1694 anchor->set_last_Java_pc(current->last_continuation()->entry_pc());
1695
1696 JRT_BLOCK
1697 if (event->should_commit()) {
1698 long timeout = java_lang_VirtualThread::waitTimeout(current->vthread());
1699 post_monitor_wait_event(event, node->_monitor, node->_notifier_tid, timeout, timed_out);
1700 }
1701 if (JvmtiExport::should_post_monitor_waited()) {
1702 JvmtiExport::vthread_post_monitor_waited(current, node->_monitor, timed_out);
1703 }
1704 JRT_BLOCK_END
1705 current->frame_anchor()->clear();
1706 }
1707
1708 // -----------------------------------------------------------------------------
1709 // Wait/Notify/NotifyAll
1710 //
1711 // Note: a subset of changes to ObjectMonitor::wait()
1712 // will need to be replicated in complete_exit
1713 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1714 JavaThread* current = THREAD;
1715
1716 assert(InitDone, "Unexpectedly not initialized");
1717
1718 CHECK_OWNER(); // Throws IMSE if not owner.
1719
1720 EventJavaMonitorWait event;
1721
1722 // check for a pending interrupt
1723 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1724 // post monitor waited event. Note that this is past-tense, we are done waiting.
1725 if (JvmtiExport::should_post_monitor_waited()) {
1726 // Note: 'false' parameter is passed here because the
1727 // wait was not timed out due to thread interrupt.
1728 JvmtiExport::post_monitor_waited(current, this, false);
1729
1730 // In this short circuit of the monitor wait protocol, the
1731 // current thread never drops ownership of the monitor and
1732 // never gets added to the wait queue so the current thread
1733 // cannot be made the successor. This means that the
1734 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1735 // consume an unpark() meant for the ParkEvent associated with
1736 // this ObjectMonitor.
1737 }
1738 if (event.should_commit()) {
1739 post_monitor_wait_event(&event, this, 0, millis, false);
1740 }
1741 THROW(vmSymbols::java_lang_InterruptedException());
1742 return;
1743 }
1744
1745 current->set_current_waiting_monitor(this);
1746
1747 #ifdef LOOM_MONITOR_SUPPORT
1748 ContinuationEntry* ce = current->last_continuation();
1749 if (interruptible && ce != nullptr && ce->is_virtual_thread()) {
1750 int result = Continuation::try_preempt(current, ce->cont_oop(current), freeze_on_wait);
1751 if (result == freeze_ok) {
1752 VThreadWait(current, millis);
1753 current->set_current_waiting_monitor(nullptr);
1754 return;
1755 }
1756 if (result == freeze_pinned_native) {
1757 const Klass* monitor_klass = object()->klass();
1758 if (!is_excluded(monitor_klass)) {
1759 post_virtual_thread_pinned_event(current, "Native frame or <clinit> on stack");
1760 }
1761 }
1762 }
1763 #endif
1764
1765 // create a node to be put into the queue
1766 // Critically, after we reset() the event but prior to park(), we must check
1767 // for a pending interrupt.
1768 ObjectWaiter node(current);
1769 node.TState = ObjectWaiter::TS_WAIT;
1770 current->_ParkEvent->reset();
1771 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1772
1773 // Enter the waiting queue, which is a circular doubly linked list in this case
1774 // but it could be a priority queue or any data structure.
1775 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1776 // by the owner of the monitor *except* in the case where park()
1777 // returns because of a timeout of interrupt. Contention is exceptionally rare
1778 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1779
1780 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1781 AddWaiter(&node);
1782 Thread::SpinRelease(&_WaitSetLock);
1783
1784 _Responsible = nullptr;
1785
1786 intx save = _recursions; // record the old recursion count
1787 _waiters++; // increment the number of waiters
1788 _recursions = 0; // set the recursion level to be 1
1789 exit(current); // exit the monitor
1790 guarantee(owner_raw() != owner_for(current), "invariant");
1791
1792 // The thread is on the WaitSet list - now park() it.
1793 // On MP systems it's conceivable that a brief spin before we park
1794 // could be profitable.
1795 //
1796 // TODO-FIXME: change the following logic to a loop of the form
1797 // while (!timeout && !interrupted && _notified == 0) park()
1798
1799 int ret = OS_OK;
1800 int WasNotified = 0;
1801
1802 // Need to check interrupt state whilst still _thread_in_vm
1803 bool interrupted = interruptible && current->is_interrupted(false);
1804
1805 { // State transition wrappers
1806 OSThread* osthread = current->osthread();
1807 OSThreadWaitState osts(osthread, true);
1808
1809 assert(current->thread_state() == _thread_in_vm, "invariant");
1810
1811 {
1812 ClearSuccOnSuspend csos(this);
1813 ThreadBlockInVMPreprocess<ClearSuccOnSuspend> tbivs(current, csos, true /* allow_suspend */);
1814 if (interrupted || HAS_PENDING_EXCEPTION) {
1815 // Intentionally empty
1816 } else if (!node._notified) {
1817 if (millis <= 0) {
1818 current->_ParkEvent->park();
1819 } else {
1820 ret = current->_ParkEvent->park(millis);
1821 }
1822 }
1823 }
1824
1825 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1826 // from the WaitSet to the EntryList.
1827 // See if we need to remove Node from the WaitSet.
1828 // We use double-checked locking to avoid grabbing _WaitSetLock
1829 // if the thread is not on the wait queue.
1830 //
1831 // Note that we don't need a fence before the fetch of TState.
1832 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1833 // written by the is thread. (perhaps the fetch might even be satisfied
1834 // by a look-aside into the processor's own store buffer, although given
1835 // the length of the code path between the prior ST and this load that's
1836 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1837 // then we'll acquire the lock and then re-fetch a fresh TState value.
1838 // That is, we fail toward safety.
1839
1840 if (node.TState == ObjectWaiter::TS_WAIT) {
1841 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1842 if (node.TState == ObjectWaiter::TS_WAIT) {
1843 DequeueSpecificWaiter(&node); // unlink from WaitSet
1844 assert(!node._notified, "invariant");
1845 node.TState = ObjectWaiter::TS_RUN;
1846 }
1847 Thread::SpinRelease(&_WaitSetLock);
1848 }
1849
1850 // The thread is now either on off-list (TS_RUN),
1851 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1852 // The Node's TState variable is stable from the perspective of this thread.
1853 // No other threads will asynchronously modify TState.
1854 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1855 OrderAccess::loadload();
1856 if (_succ == current) _succ = nullptr;
1857 WasNotified = node._notified;
1858
1859 // Reentry phase -- reacquire the monitor.
1860 // re-enter contended monitor after object.wait().
1861 // retain OBJECT_WAIT state until re-enter successfully completes
1862 // Thread state is thread_in_vm and oop access is again safe,
1863 // although the raw address of the object may have changed.
1864 // (Don't cache naked oops over safepoints, of course).
1865
1866 // post monitor waited event. Note that this is past-tense, we are done waiting.
1867 if (JvmtiExport::should_post_monitor_waited()) {
1868 JvmtiExport::post_monitor_waited(current, this, ret == OS_TIMEOUT);
1869
1870 if (node._notified && _succ == current) {
1871 // In this part of the monitor wait-notify-reenter protocol it
1872 // is possible (and normal) for another thread to do a fastpath
1873 // monitor enter-exit while this thread is still trying to get
1874 // to the reenter portion of the protocol.
1875 //
1876 // The ObjectMonitor was notified and the current thread is
1877 // the successor which also means that an unpark() has already
1878 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1879 // consume the unpark() that was done when the successor was
1880 // set because the same ParkEvent is shared between Java
1881 // monitors and JVM/TI RawMonitors (for now).
1882 //
1883 // We redo the unpark() to ensure forward progress, i.e., we
1884 // don't want all pending threads hanging (parked) with none
1885 // entering the unlocked monitor.
1886 current->_ParkEvent->unpark();
1887 }
1888 }
1889
1890 if (event.should_commit()) {
1891 post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1892 }
1893
1894 OrderAccess::fence();
1895
1896 assert(owner_raw() != owner_for(current), "invariant");
1897 ObjectWaiter::TStates v = node.TState;
1898 if (v == ObjectWaiter::TS_RUN) {
1899 enter(current);
1900 } else {
1901 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1902 ReenterI(current, &node);
1903 node.wait_reenter_end(this);
1904 }
1905
1906 // current has reacquired the lock.
1907 // Lifecycle - the node representing current must not appear on any queues.
1908 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1909 // want residual elements associated with this thread left on any lists.
1910 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1911 assert(owner_raw() == owner_for(current), "invariant");
1912 assert(_succ != current, "invariant");
1913 } // OSThreadWaitState()
1914
1915 current->set_current_waiting_monitor(nullptr);
1916
1917 guarantee(_recursions == 0, "invariant");
1918 int relock_count = JvmtiDeferredUpdates::get_and_reset_relock_count_after_wait(current);
1919 _recursions = save // restore the old recursion count
1920 + relock_count; // increased by the deferred relock count
1921 NOT_LOOM_MONITOR_SUPPORT(current->inc_held_monitor_count(relock_count);) // Deopt never entered these counts.
1922 _waiters--; // decrement the number of waiters
1923
1924 // Verify a few postconditions
1925 assert(owner_raw() == owner_for(current), "invariant");
1926 assert(_succ != current, "invariant");
1927 assert(object()->mark() == markWord::encode(this), "invariant");
1928
1929 // check if the notification happened
1930 if (!WasNotified) {
1931 // no, it could be timeout or Thread.interrupt() or both
1932 // check for interrupt event, otherwise it is timeout
1933 if (interruptible && current->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1934 THROW(vmSymbols::java_lang_InterruptedException());
1935 }
1936 }
1937
1938 // NOTE: Spurious wake up will be consider as timeout.
1939 // Monitor notify has precedence over thread interrupt.
1940 }
1941
1942 // Consider:
1943 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1944 // then instead of transferring a thread from the WaitSet to the EntryList
1945 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1946
1947 void ObjectMonitor::INotify(JavaThread* current) {
1948 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1949 ObjectWaiter* iterator = DequeueWaiter();
1950 if (iterator != nullptr) {
1951 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1952 guarantee(!iterator->_notified, "invariant");
1953 // Disposition - what might we do with iterator ?
1954 // a. add it directly to the EntryList - either tail (policy == 1)
1955 // or head (policy == 0).
1956 // b. push it onto the front of the _cxq (policy == 2).
1957 // For now we use (b).
1958
1959 if (iterator->is_vthread()) {
1960 oop vthread = iterator->vthread();
1961 java_lang_VirtualThread::set_notified(vthread, true);
1962 int old_state = java_lang_VirtualThread::state(vthread);
1963 // If state is not WAIT/TIMED_WAIT then target could still be on
1964 // unmount transition, or wait could have already timed-out or target
1965 // could have been interrupted. In the first case, the target itself
1966 // will set the state to BLOCKED at the end of the unmount transition.
1967 // In the other cases the target would have been already unblocked so
1968 // there is nothing to do.
1969 if (old_state == java_lang_VirtualThread::WAIT ||
1970 old_state == java_lang_VirtualThread::TIMED_WAIT) {
1971 java_lang_VirtualThread::cmpxchg_state(vthread, old_state, java_lang_VirtualThread::BLOCKED);
1972 }
1973 }
1974
1975 iterator->TState = ObjectWaiter::TS_ENTER;
1976
1977 iterator->_notified = true;
1978 iterator->_notifier_tid = JFR_THREAD_ID(current);
1979
1980 ObjectWaiter* list = _EntryList;
1981 if (list != nullptr) {
1982 assert(list->_prev == nullptr, "invariant");
1983 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1984 assert(list != iterator, "invariant");
1985 }
1986
1987 // prepend to cxq
1988 if (list == nullptr) {
1989 iterator->_next = iterator->_prev = nullptr;
1990 _EntryList = iterator;
1991 } else {
1992 iterator->TState = ObjectWaiter::TS_CXQ;
1993 for (;;) {
1994 ObjectWaiter* front = _cxq;
1995 iterator->_next = front;
1996 if (Atomic::cmpxchg(&_cxq, front, iterator) == front) {
1997 break;
1998 }
1999 }
2000 }
2001
2002 // _WaitSetLock protects the wait queue, not the EntryList. We could
2003 // move the add-to-EntryList operation, above, outside the critical section
2004 // protected by _WaitSetLock. In practice that's not useful. With the
2005 // exception of wait() timeouts and interrupts the monitor owner
2006 // is the only thread that grabs _WaitSetLock. There's almost no contention
2007 // on _WaitSetLock so it's not profitable to reduce the length of the
2008 // critical section.
2009 if (!iterator->is_vthread()) {
2010 iterator->wait_reenter_begin(this);
2011 }
2012 }
2013 Thread::SpinRelease(&_WaitSetLock);
2014 }
2015
2016 // Consider: a not-uncommon synchronization bug is to use notify() when
2017 // notifyAll() is more appropriate, potentially resulting in stranded
2018 // threads; this is one example of a lost wakeup. A useful diagnostic
2019 // option is to force all notify() operations to behave as notifyAll().
2020 //
2021 // Note: We can also detect many such problems with a "minimum wait".
2022 // When the "minimum wait" is set to a small non-zero timeout value
2023 // and the program does not hang whereas it did absent "minimum wait",
2024 // that suggests a lost wakeup bug.
2025
2026 void ObjectMonitor::notify(TRAPS) {
2027 JavaThread* current = THREAD;
2028 CHECK_OWNER(); // Throws IMSE if not owner.
2029 if (_WaitSet == nullptr) {
2030 return;
2031 }
2032 DTRACE_MONITOR_PROBE(notify, this, object(), current);
2033 INotify(current);
2034 OM_PERFDATA_OP(Notifications, inc(1));
2035 }
2036
2037
2038 // The current implementation of notifyAll() transfers the waiters one-at-a-time
2039 // from the waitset to the EntryList. This could be done more efficiently with a
2040 // single bulk transfer but in practice it's not time-critical. Beware too,
2041 // that in prepend-mode we invert the order of the waiters. Let's say that the
2042 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
2043 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
2044
2045 void ObjectMonitor::notifyAll(TRAPS) {
2046 JavaThread* current = THREAD;
2047 CHECK_OWNER(); // Throws IMSE if not owner.
2048 if (_WaitSet == nullptr) {
2049 return;
2050 }
2051
2052 DTRACE_MONITOR_PROBE(notifyAll, this, object(), current);
2053 int tally = 0;
2054 while (_WaitSet != nullptr) {
2055 tally++;
2056 INotify(current);
2057 }
2058
2059 OM_PERFDATA_OP(Notifications, inc(tally));
2060 }
2061
2062 void ObjectMonitor::VThreadWait(JavaThread* current, jlong millis) {
2063 oop vthread = current->vthread();
2064 ObjectWaiter* node = new ObjectWaiter(vthread, this);
2065 node->_is_wait = true;
2066 node->TState = ObjectWaiter::TS_WAIT;
2067 java_lang_VirtualThread::set_notified(vthread, false); // Reset notified flag
2068
2069 // Enter the waiting queue, which is a circular doubly linked list in this case
2070 // but it could be a priority queue or any data structure.
2071 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
2072 // by the owner of the monitor *except* in the case where park()
2073 // returns because of a timeout or interrupt. Contention is exceptionally rare
2074 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
2075
2076 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
2077 AddWaiter(node);
2078 Thread::SpinRelease(&_WaitSetLock);
2079
2080 _Responsible = nullptr;
2081
2082 node->_recursions = _recursions; // record the old recursion count
2083 _recursions = 0; // set the recursion level to be 1
2084 _waiters++; // increment the number of waiters
2085 exit(current); // exit the monitor
2086 guarantee(owner_raw() != owner_for(current), "invariant");
2087
2088 assert(java_lang_VirtualThread::state(vthread) == java_lang_VirtualThread::RUNNING, "wrong state for vthread");
2089 java_lang_VirtualThread::set_state(vthread, millis == 0 ? java_lang_VirtualThread::WAITING : java_lang_VirtualThread::TIMED_WAITING);
2090 java_lang_VirtualThread::set_waitTimeout(vthread, millis);
2091
2092 // Save the ObjectWaiter* in the chunk since we will need it
2093 // when resuming execution.
2094 oop cont = java_lang_VirtualThread::continuation(vthread);
2095 stackChunkOop chunk = jdk_internal_vm_Continuation::tail(cont);
2096 chunk->set_object_waiter(node);
2097 }
2098
2099 bool ObjectMonitor::VThreadWaitReenter(JavaThread* current, ObjectWaiter* node) {
2100 // First time we run after being preempted on Object.wait().
2101 // We need to check if we were interrupted or wait() timed-out
2102 // and in that case remove ourselves from the _WaitSet queue.
2103 if (node->TState == ObjectWaiter::TS_WAIT) {
2104 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
2105 if (node->TState == ObjectWaiter::TS_WAIT) {
2106 DequeueSpecificWaiter(node); // unlink from WaitSet
2107 assert(!node->_notified, "invariant");
2108 node->TState = ObjectWaiter::TS_RUN;
2109 }
2110 Thread::SpinRelease(&_WaitSetLock);
2111 }
2112
2113 ObjectWaiter::TStates state = node->TState;
2114 bool was_notified = state == ObjectWaiter::TS_ENTER || state == ObjectWaiter::TS_CXQ;
2115 assert(was_notified || state == ObjectWaiter::TS_RUN, "");
2116
2117 // save it so that once we re-acquire the monitor we know if we need to throw IE.
2118 node->_interrupted = !was_notified && current->is_interrupted(false);
2119
2120 EventJavaMonitorWait event;
2121 if (event.should_commit() || JvmtiExport::should_post_monitor_waited()) {
2122 vthread_monitor_waited_event(current, node, &event, !was_notified && !node->_interrupted);
2123 }
2124
2125 node->_at_reenter = true;
2126 add_to_contentions(1);
2127 assert(owner_raw() != owner_for(current), "invariant");
2128
2129 if (!was_notified) {
2130 bool acquired = VThreadMonitorEnter(current, node);
2131 if (acquired) {
2132 guarantee(_recursions == 0, "invariant");
2133 _recursions = node->_recursions; // restore the old recursion count
2134 _waiters--; // decrement the number of waiters
2135
2136 if (node->_interrupted) {
2137 // We will throw at thaw end after finishing the mount transition.
2138 current->set_pending_interrupted_exception(true);
2139 }
2140
2141 delete node;
2142 oop cont = java_lang_VirtualThread::continuation(current->vthread());
2143 stackChunkOop chunk = jdk_internal_vm_Continuation::tail(cont);
2144 chunk->set_object_waiter(nullptr);
2145 } else {
2146 // The JT will read this variable on return to the resume_monitor_operation stub
2147 // and will unmount (enterSpecial frame removed and return to Continuation.run()).
2148 current->set_preempting(true);
2149 }
2150 }
2151 return was_notified;
2152 }
2153
2154 // -----------------------------------------------------------------------------
2155 // Adaptive Spinning Support
2156 //
2157 // Adaptive spin-then-block - rational spinning
2158 //
2159 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
2160 // algorithm. On high order SMP systems it would be better to start with
2161 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
2162 // a contending thread could enqueue itself on the cxq and then spin locally
2163 // on a thread-specific variable such as its ParkEvent._Event flag.
2164 // That's left as an exercise for the reader. Note that global spinning is
2165 // not problematic on Niagara, as the L2 cache serves the interconnect and
2166 // has both low latency and massive bandwidth.
2167 //
2168 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
2169 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
2170 // (duration) or we can fix the count at approximately the duration of
2171 // a context switch and vary the frequency. Of course we could also
2172 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
2173 // For a description of 'Adaptive spin-then-block mutual exclusion in
2174 // multi-threaded processing,' see U.S. Pat. No. 8046758.
2175 //
2176 // This implementation varies the duration "D", where D varies with
2177 // the success rate of recent spin attempts. (D is capped at approximately
2178 // length of a round-trip context switch). The success rate for recent
2179 // spin attempts is a good predictor of the success rate of future spin
2180 // attempts. The mechanism adapts automatically to varying critical
2181 // section length (lock modality), system load and degree of parallelism.
2182 // D is maintained per-monitor in _SpinDuration and is initialized
2183 // optimistically. Spin frequency is fixed at 100%.
2184 //
2185 // Note that _SpinDuration is volatile, but we update it without locks
2186 // or atomics. The code is designed so that _SpinDuration stays within
2187 // a reasonable range even in the presence of races. The arithmetic
2188 // operations on _SpinDuration are closed over the domain of legal values,
2189 // so at worst a race will install and older but still legal value.
2190 // At the very worst this introduces some apparent non-determinism.
2191 // We might spin when we shouldn't or vice-versa, but since the spin
2192 // count are relatively short, even in the worst case, the effect is harmless.
2193 //
2194 // Care must be taken that a low "D" value does not become an
2195 // an absorbing state. Transient spinning failures -- when spinning
2196 // is overall profitable -- should not cause the system to converge
2197 // on low "D" values. We want spinning to be stable and predictable
2198 // and fairly responsive to change and at the same time we don't want
2199 // it to oscillate, become metastable, be "too" non-deterministic,
2200 // or converge on or enter undesirable stable absorbing states.
2201 //
2202 // We implement a feedback-based control system -- using past behavior
2203 // to predict future behavior. We face two issues: (a) if the
2204 // input signal is random then the spin predictor won't provide optimal
2205 // results, and (b) if the signal frequency is too high then the control
2206 // system, which has some natural response lag, will "chase" the signal.
2207 // (b) can arise from multimodal lock hold times. Transient preemption
2208 // can also result in apparent bimodal lock hold times.
2209 // Although sub-optimal, neither condition is particularly harmful, as
2210 // in the worst-case we'll spin when we shouldn't or vice-versa.
2211 // The maximum spin duration is rather short so the failure modes aren't bad.
2212 // To be conservative, I've tuned the gain in system to bias toward
2213 // _not spinning. Relatedly, the system can sometimes enter a mode where it
2214 // "rings" or oscillates between spinning and not spinning. This happens
2215 // when spinning is just on the cusp of profitability, however, so the
2216 // situation is not dire. The state is benign -- there's no need to add
2217 // hysteresis control to damp the transition rate between spinning and
2218 // not spinning.
2219
2220 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool
2221
2222 static int Knob_Bonus = 100; // spin success bonus
2223 static int Knob_Penalty = 200; // spin failure penalty
2224 static int Knob_Poverty = 1000;
2225 static int Knob_FixedSpin = 0;
2226 static int Knob_PreSpin = 10; // 20-100 likely better, but it's not better in my testing.
2227
2228 inline static int adjust_up(int spin_duration) {
2229 int x = spin_duration;
2230 if (x < ObjectMonitor::Knob_SpinLimit) {
2231 if (x < Knob_Poverty) {
2232 x = Knob_Poverty;
2233 }
2234 return x + Knob_Bonus;
2235 } else {
2236 return spin_duration;
2237 }
2238 }
2239
2240 inline static int adjust_down(int spin_duration) {
2241 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
2242 // AIMD is globally stable.
2243 int x = spin_duration;
2244 if (x > 0) {
2245 // Consider an AIMD scheme like: x -= (x >> 3) + 100
2246 // This is globally sample and tends to damp the response.
2247 x -= Knob_Penalty;
2248 if (x < 0) { x = 0; }
2249 return x;
2250 } else {
2251 return spin_duration;
2252 }
2253 }
2254
2255 bool ObjectMonitor::short_fixed_spin(JavaThread* current, int spin_count, bool adapt) {
2256 for (int ctr = 0; ctr < spin_count; ctr++) {
2257 TryLockResult status = TryLock(current);
2258 if (status == TryLockResult::Success) {
2259 if (adapt) {
2260 _SpinDuration = adjust_up(_SpinDuration);
2261 }
2262 return true;
2263 } else if (status == TryLockResult::Interference) {
2264 break;
2265 }
2266 SpinPause();
2267 }
2268 return false;
2269 }
2270
2271 // Spinning: Fixed frequency (100%), vary duration
2272 bool ObjectMonitor::TrySpin(JavaThread* current) {
2273
2274 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
2275 int knob_fixed_spin = Knob_FixedSpin; // 0 (don't spin: default), 2000 good test
2276 if (knob_fixed_spin > 0) {
2277 return short_fixed_spin(current, knob_fixed_spin, false);
2278 }
2279
2280 // Admission control - verify preconditions for spinning
2281 //
2282 // We always spin a little bit, just to prevent _SpinDuration == 0 from
2283 // becoming an absorbing state. Put another way, we spin briefly to
2284 // sample, just in case the system load, parallelism, contention, or lock
2285 // modality changed.
2286
2287 int knob_pre_spin = Knob_PreSpin; // 10 (default), 100, 1000 or 2000
2288 if (short_fixed_spin(current, knob_pre_spin, true)) {
2289 return true;
2290 }
2291
2292 //
2293 // Consider the following alternative:
2294 // Periodically set _SpinDuration = _SpinLimit and try a long/full
2295 // spin attempt. "Periodically" might mean after a tally of
2296 // the # of failed spin attempts (or iterations) reaches some threshold.
2297 // This takes us into the realm of 1-out-of-N spinning, where we
2298 // hold the duration constant but vary the frequency.
2299
2300 int ctr = _SpinDuration;
2301 if (ctr <= 0) return false;
2302
2303 // We're good to spin ... spin ingress.
2304 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
2305 // when preparing to LD...CAS _owner, etc and the CAS is likely
2306 // to succeed.
2307 if (_succ == nullptr) {
2308 _succ = current;
2309 }
2310 void* prv = nullptr;
2311
2312 // There are three ways to exit the following loop:
2313 // 1. A successful spin where this thread has acquired the lock.
2314 // 2. Spin failure with prejudice
2315 // 3. Spin failure without prejudice
2316
2317 while (--ctr >= 0) {
2318
2319 // Periodic polling -- Check for pending GC
2320 // Threads may spin while they're unsafe.
2321 // We don't want spinning threads to delay the JVM from reaching
2322 // a stop-the-world safepoint or to steal cycles from GC.
2323 // If we detect a pending safepoint we abort in order that
2324 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
2325 // this thread, if safe, doesn't steal cycles from GC.
2326 // This is in keeping with the "no loitering in runtime" rule.
2327 // We periodically check to see if there's a safepoint pending.
2328 if ((ctr & 0xFF) == 0) {
2329 // Can't call SafepointMechanism::should_process() since that
2330 // might update the poll values and we could be in a thread_blocked
2331 // state here which is not allowed so just check the poll.
2332 if (SafepointMechanism::local_poll_armed(current)) {
2333 break;
2334 }
2335 SpinPause();
2336 }
2337
2338 // Probe _owner with TATAS
2339 // If this thread observes the monitor transition or flicker
2340 // from locked to unlocked to locked, then the odds that this
2341 // thread will acquire the lock in this spin attempt go down
2342 // considerably. The same argument applies if the CAS fails
2343 // or if we observe _owner change from one non-null value to
2344 // another non-null value. In such cases we might abort
2345 // the spin without prejudice or apply a "penalty" to the
2346 // spin count-down variable "ctr", reducing it by 100, say.
2347
2348 void* ox = owner_raw();
2349 if (ox == nullptr) {
2350 ox = try_set_owner_from(nullptr, current);
2351 if (ox == nullptr) {
2352 // The CAS succeeded -- this thread acquired ownership
2353 // Take care of some bookkeeping to exit spin state.
2354 if (_succ == current) {
2355 _succ = nullptr;
2356 }
2357
2358 // Increase _SpinDuration :
2359 // The spin was successful (profitable) so we tend toward
2360 // longer spin attempts in the future.
2361 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
2362 // If we acquired the lock early in the spin cycle it
2363 // makes sense to increase _SpinDuration proportionally.
2364 // Note that we don't clamp SpinDuration precisely at SpinLimit.
2365 _SpinDuration = adjust_up(_SpinDuration);
2366 return true;
2367 }
2368
2369 // The CAS failed ... we can take any of the following actions:
2370 // * penalize: ctr -= CASPenalty
2371 // * exit spin with prejudice -- abort without adapting spinner
2372 // * exit spin without prejudice.
2373 // * Since CAS is high-latency, retry again immediately.
2374 break;
2375 }
2376
2377 // Did lock ownership change hands ?
2378 if (ox != prv && prv != nullptr) {
2379 break;
2380 }
2381 prv = ox;
2382
2383 if (_succ == nullptr) {
2384 _succ = current;
2385 }
2386 }
2387
2388 // Spin failed with prejudice -- reduce _SpinDuration.
2389 if (ctr < 0) {
2390 _SpinDuration = adjust_down(_SpinDuration);
2391 }
2392
2393 if (_succ == current) {
2394 _succ = nullptr;
2395 // Invariant: after setting succ=null a contending thread
2396 // must recheck-retry _owner before parking. This usually happens
2397 // in the normal usage of TrySpin(), but it's safest
2398 // to make TrySpin() as foolproof as possible.
2399 OrderAccess::fence();
2400 if (TryLock(current) == TryLockResult::Success) {
2401 return true;
2402 }
2403 }
2404
2405 return false;
2406 }
2407
2408
2409 // -----------------------------------------------------------------------------
2410 // WaitSet management ...
2411
2412 ObjectWaiter::ObjectWaiter(JavaThread* current) {
2413 _next = nullptr;
2414 _prev = nullptr;
2415 _thread = current;
2416 _monitor = nullptr;
2417 _notifier_tid = 0;
2418 _recursions = 0;
2419 TState = TS_RUN;
2420 _notified = false;
2421 _is_wait = false;
2422 _at_reenter = false;
2423 _interrupted = false;
2424 _active = false;
2425 }
2426
2427 ObjectWaiter::ObjectWaiter(oop vthread, ObjectMonitor* mon) : ObjectWaiter(nullptr) {
2428 assert(oopDesc::is_oop(vthread), "");
2429 _vthread = OopHandle(JavaThread::thread_oop_storage(), vthread);
2430 _monitor = mon;
2431 }
2432
2433 ObjectWaiter::~ObjectWaiter() {
2434 if (is_vthread()) {
2435 assert(vthread() != nullptr, "");
2436 _vthread.release(JavaThread::thread_oop_storage());
2437 }
2438 }
2439
2440 oop ObjectWaiter::vthread() {
2441 return _vthread.resolve();
2442 }
2443
2444 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
2445 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(_thread, mon);
2446 }
2447
2448 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
2449 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(_thread, _active);
2450 }
2451
2452 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
2453 assert(node != nullptr, "should not add null node");
2454 assert(node->_prev == nullptr, "node already in list");
2455 assert(node->_next == nullptr, "node already in list");
2456 // put node at end of queue (circular doubly linked list)
2457 if (_WaitSet == nullptr) {
2458 _WaitSet = node;
2459 node->_prev = node;
2460 node->_next = node;
2461 } else {
2462 ObjectWaiter* head = _WaitSet;
2463 ObjectWaiter* tail = head->_prev;
2464 assert(tail->_next == head, "invariant check");
2465 tail->_next = node;
2466 head->_prev = node;
2467 node->_next = head;
2468 node->_prev = tail;
2469 }
2470 }
2471
2472 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
2473 // dequeue the very first waiter
2474 ObjectWaiter* waiter = _WaitSet;
2475 if (waiter) {
2476 DequeueSpecificWaiter(waiter);
2477 }
2478 return waiter;
2479 }
2480
2481 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
2482 assert(node != nullptr, "should not dequeue nullptr node");
2483 assert(node->_prev != nullptr, "node already removed from list");
2484 assert(node->_next != nullptr, "node already removed from list");
2485 // when the waiter has woken up because of interrupt,
2486 // timeout or other spurious wake-up, dequeue the
2487 // waiter from waiting list
2488 ObjectWaiter* next = node->_next;
2489 if (next == node) {
2490 assert(node->_prev == node, "invariant check");
2491 _WaitSet = nullptr;
2492 } else {
2493 ObjectWaiter* prev = node->_prev;
2494 assert(prev->_next == node, "invariant check");
2495 assert(next->_prev == node, "invariant check");
2496 next->_prev = prev;
2497 prev->_next = next;
2498 if (_WaitSet == node) {
2499 _WaitSet = next;
2500 }
2501 }
2502 node->_next = nullptr;
2503 node->_prev = nullptr;
2504 }
2505
2506 // -----------------------------------------------------------------------------
2507 // PerfData support
2508 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = nullptr;
2509 PerfCounter * ObjectMonitor::_sync_FutileWakeups = nullptr;
2510 PerfCounter * ObjectMonitor::_sync_Parks = nullptr;
2511 PerfCounter * ObjectMonitor::_sync_Notifications = nullptr;
2512 PerfCounter * ObjectMonitor::_sync_Inflations = nullptr;
2513 PerfCounter * ObjectMonitor::_sync_Deflations = nullptr;
2514 PerfLongVariable * ObjectMonitor::_sync_MonExtant = nullptr;
2515
2516 // One-shot global initialization for the sync subsystem.
2517 // We could also defer initialization and initialize on-demand
2518 // the first time we call ObjectSynchronizer::inflate().
2519 // Initialization would be protected - like so many things - by
2520 // the MonitorCache_lock.
2521
2522 void ObjectMonitor::Initialize() {
2523 assert(!InitDone, "invariant");
2524
2525 if (!os::is_MP()) {
2526 Knob_SpinLimit = 0;
2527 Knob_PreSpin = 0;
2528 Knob_FixedSpin = -1;
2529 }
2530
2531 if (UsePerfData) {
2532 EXCEPTION_MARK;
2533 #define NEWPERFCOUNTER(n) \
2534 { \
2535 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \
2536 CHECK); \
2537 }
2538 #define NEWPERFVARIABLE(n) \
2539 { \
2540 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \
2541 CHECK); \
2542 }
2543 NEWPERFCOUNTER(_sync_Inflations);
2544 NEWPERFCOUNTER(_sync_Deflations);
2545 NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2546 NEWPERFCOUNTER(_sync_FutileWakeups);
2547 NEWPERFCOUNTER(_sync_Parks);
2548 NEWPERFCOUNTER(_sync_Notifications);
2549 NEWPERFVARIABLE(_sync_MonExtant);
2550 #undef NEWPERFCOUNTER
2551 #undef NEWPERFVARIABLE
2552 }
2553
2554 _oop_storage = OopStorageSet::create_weak("ObjectSynchronizer Weak", mtSynchronizer);
2555
2556 DEBUG_ONLY(InitDone = true;)
2557 }
2558
2559 void ObjectMonitor::Initialize2() {
2560 _vthread_cxq_head = OopHandle(JavaThread::thread_oop_storage(), nullptr);
2561 _vthread_unparker_ParkEvent = ParkEvent::Allocate(nullptr);
2562 }
2563
2564 void ObjectMonitor::print_on(outputStream* st) const {
2565 // The minimal things to print for markWord printing, more can be added for debugging and logging.
2566 st->print("{contentions=0x%08x,waiters=0x%08x"
2567 ",recursions=" INTX_FORMAT ",owner=" INTPTR_FORMAT "}",
2568 contentions(), waiters(), recursions(),
2569 p2i(owner()));
2570 }
2571 void ObjectMonitor::print() const { print_on(tty); }
2572
2573 #ifdef ASSERT
2574 // Print the ObjectMonitor like a debugger would:
2575 //
2576 // (ObjectMonitor) 0x00007fdfb6012e40 = {
2577 // _header = 0x0000000000000001
2578 // _object = 0x000000070ff45fd0
2579 // _pad_buf0 = {
2580 // [0] = '\0'
2581 // ...
2582 // [43] = '\0'
2583 // }
2584 // _owner = 0x0000000000000000
2585 // _previous_owner_tid = 0
2586 // _pad_buf1 = {
2587 // [0] = '\0'
2588 // ...
2589 // [47] = '\0'
2590 // }
2591 // _next_om = 0x0000000000000000
2592 // _recursions = 0
2593 // _EntryList = 0x0000000000000000
2594 // _cxq = 0x0000000000000000
2595 // _succ = 0x0000000000000000
2596 // _Responsible = 0x0000000000000000
2597 // _SpinDuration = 5000
2598 // _contentions = 0
2599 // _WaitSet = 0x0000700009756248
2600 // _waiters = 1
2601 // _WaitSetLock = 0
2602 // }
2603 //
2604 void ObjectMonitor::print_debug_style_on(outputStream* st) const {
2605 st->print_cr("(ObjectMonitor*) " INTPTR_FORMAT " = {", p2i(this));
2606 st->print_cr(" _header = " INTPTR_FORMAT, header().value());
2607 st->print_cr(" _object = " INTPTR_FORMAT, p2i(object_peek()));
2608 st->print_cr(" _pad_buf0 = {");
2609 st->print_cr(" [0] = '\\0'");
2610 st->print_cr(" ...");
2611 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf0) - 1);
2612 st->print_cr(" }");
2613 st->print_cr(" _owner = " INTPTR_FORMAT, p2i(owner_raw()));
2614 st->print_cr(" _previous_owner_tid = " UINT64_FORMAT, _previous_owner_tid);
2615 st->print_cr(" _pad_buf1 = {");
2616 st->print_cr(" [0] = '\\0'");
2617 st->print_cr(" ...");
2618 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf1) - 1);
2619 st->print_cr(" }");
2620 st->print_cr(" _next_om = " INTPTR_FORMAT, p2i(next_om()));
2621 st->print_cr(" _recursions = " INTX_FORMAT, _recursions);
2622 st->print_cr(" _EntryList = " INTPTR_FORMAT, p2i(_EntryList));
2623 st->print_cr(" _cxq = " INTPTR_FORMAT, p2i(_cxq));
2624 st->print_cr(" _succ = " INTPTR_FORMAT, p2i(_succ));
2625 st->print_cr(" _Responsible = " INTPTR_FORMAT, p2i(_Responsible));
2626 st->print_cr(" _SpinDuration = %d", _SpinDuration);
2627 st->print_cr(" _contentions = %d", contentions());
2628 st->print_cr(" _WaitSet = " INTPTR_FORMAT, p2i(_WaitSet));
2629 st->print_cr(" _waiters = %d", _waiters);
2630 st->print_cr(" _WaitSetLock = %d", _WaitSetLock);
2631 st->print_cr("}");
2632 }
2633 #endif