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