1 /*
2 * Copyright (c) 1998, 2019, 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 "jfr/jfrEvents.hpp"
28 #include "jfr/support/jfrThreadId.hpp"
29 #include "memory/allocation.inline.hpp"
30 #include "memory/resourceArea.hpp"
31 #include "oops/markWord.hpp"
32 #include "oops/oop.inline.hpp"
33 #include "runtime/atomic.hpp"
34 #include "runtime/handles.inline.hpp"
35 #include "runtime/interfaceSupport.inline.hpp"
36 #include "runtime/mutexLocker.hpp"
37 #include "runtime/objectMonitor.hpp"
38 #include "runtime/objectMonitor.inline.hpp"
39 #include "runtime/orderAccess.hpp"
40 #include "runtime/osThread.hpp"
41 #include "runtime/safepointMechanism.inline.hpp"
42 #include "runtime/sharedRuntime.hpp"
43 #include "runtime/stubRoutines.hpp"
44 #include "runtime/thread.inline.hpp"
45 #include "services/threadService.hpp"
46 #include "utilities/dtrace.hpp"
47 #include "utilities/macros.hpp"
48 #include "utilities/preserveException.hpp"
49 #if INCLUDE_JFR
50 #include "jfr/support/jfrFlush.hpp"
51 #endif
52
53 #ifdef DTRACE_ENABLED
54
55 // Only bother with this argument setup if dtrace is available
56 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly.
57
58
59 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \
60 char* bytes = NULL; \
61 int len = 0; \
62 jlong jtid = SharedRuntime::get_java_tid(thread); \
63 Symbol* klassname = ((oop)obj)->klass()->name(); \
64 if (klassname != NULL) { \
65 bytes = (char*)klassname->bytes(); \
66 len = klassname->utf8_length(); \
67 }
68
69 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \
70 { \
71 if (DTraceMonitorProbes) { \
72 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
73 HOTSPOT_MONITOR_WAIT(jtid, \
74 (monitor), bytes, len, (millis)); \
75 } \
76 }
77
78 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER
79 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED
80 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT
81 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY
82 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL
83
84 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \
85 { \
86 if (DTraceMonitorProbes) { \
87 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \
88 HOTSPOT_MONITOR_##probe(jtid, \
89 (uintptr_t)(monitor), bytes, len); \
90 } \
91 }
92
93 #else // ndef DTRACE_ENABLED
94
95 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;}
96 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;}
97
98 #endif // ndef DTRACE_ENABLED
99
100 // Tunables ...
101 // The knob* variables are effectively final. Once set they should
102 // never be modified hence. Consider using __read_mostly with GCC.
103
104 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool -
105
106 static int Knob_Bonus = 100; // spin success bonus
107 static int Knob_BonusB = 100; // spin success bonus
108 static int Knob_Penalty = 200; // spin failure penalty
109 static int Knob_Poverty = 1000;
110 static int Knob_FixedSpin = 0;
111 static int Knob_PreSpin = 10; // 20-100 likely better
112
113 DEBUG_ONLY(static volatile bool InitDone = false;)
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 void* ObjectMonitor::operator new (size_t size) throw() {
226 return AllocateHeap(size, mtInternal);
227 }
228 void* ObjectMonitor::operator new[] (size_t size) throw() {
229 return operator new (size);
230 }
231 void ObjectMonitor::operator delete(void* p) {
232 FreeHeap(p);
233 }
234 void ObjectMonitor::operator delete[] (void *p) {
235 operator delete(p);
236 }
237
238 // -----------------------------------------------------------------------------
239 // Enter support
240
241 void ObjectMonitor::enter(TRAPS) {
242 ADIM_guarantee(ref_count() > 0, "must be positive: ref_count=%d", ref_count());
243
244 // The following code is ordered to check the most common cases first
245 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
246 Thread * const Self = THREAD;
247
248 void * cur = Atomic::cmpxchg(Self, &_owner, (void*)NULL);
249 if (cur == NULL) {
250 assert(_recursions == 0, "invariant");
251 return;
252 }
253
254 if (cur == Self) {
255 // TODO-FIXME: check for integer overflow! BUGID 6557169.
256 _recursions++;
257 return;
258 }
259
260 if (Self->is_lock_owned ((address)cur)) {
261 assert(_recursions == 0, "internal state error");
262 _recursions = 1;
263 // Commute owner from a thread-specific on-stack BasicLockObject address to
264 // a full-fledged "Thread *".
265 _owner = Self;
266 return;
267 }
268
269 if (AsyncDeflateIdleMonitors &&
270 Atomic::cmpxchg(Self, &_owner, DEFLATER_MARKER) == DEFLATER_MARKER) {
271 // The deflation protocol finished the first part (setting owner),
272 // but it failed the second part (making ref_count negative) and
273 // bailed. Or the ObjectMonitor was async deflated and reused.
274 // Acquired the monitor.
275 assert(_recursions == 0, "invariant");
276 return;
277 }
278
279 // We've encountered genuine contention.
280 assert(Self->_Stalled == 0, "invariant");
281 Self->_Stalled = intptr_t(this);
282
283 // Try one round of spinning *before* enqueueing Self
284 // and before going through the awkward and expensive state
285 // transitions. The following spin is strictly optional ...
286 // Note that if we acquire the monitor from an initial spin
287 // we forgo posting JVMTI events and firing DTRACE probes.
288 if (TrySpin(Self) > 0) {
289 assert(_owner == Self, "must be Self: owner=" INTPTR_FORMAT, p2i(_owner));
290 assert(_recursions == 0, "must be 0: recursions=" INTX_FORMAT, _recursions);
291 assert(((oop)object())->mark() == markWord::encode(this),
292 "object mark must match encoded this: mark=" INTPTR_FORMAT
293 ", encoded this=" INTPTR_FORMAT, ((oop)object())->mark().value(),
294 markWord::encode(this).value());
295 Self->_Stalled = 0;
296 return;
297 }
298
299 assert(_owner != Self, "invariant");
300 assert(_succ != Self, "invariant");
301 assert(Self->is_Java_thread(), "invariant");
302 JavaThread * jt = (JavaThread *) Self;
303 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
304 assert(jt->thread_state() != _thread_blocked, "invariant");
305 assert(AsyncDeflateIdleMonitors || this->object() != NULL, "invariant");
306 assert(_contentions >= 0, "must not be negative: contentions=%d", _contentions);
307
308 // Prevent deflation. See ObjectSynchronizer::deflate_monitor(),
309 // ObjectSynchronizer::deflate_monitor_using_JT() and is_busy().
310 // Ensure the object <-> monitor relationship remains stable while
311 // there's contention.
312 Atomic::add(1, &_contentions);
313
314 JFR_ONLY(JfrConditionalFlushWithStacktrace<EventJavaMonitorEnter> flush(jt);)
315 EventJavaMonitorEnter event;
316 if (event.should_commit()) {
317 event.set_monitorClass(((oop)this->object())->klass());
318 event.set_address((uintptr_t)(this->object_addr()));
319 }
320
321 { // Change java thread status to indicate blocked on monitor enter.
322 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
323
324 Self->set_current_pending_monitor(this);
325
326 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
327 if (JvmtiExport::should_post_monitor_contended_enter()) {
328 JvmtiExport::post_monitor_contended_enter(jt, this);
329
330 // The current thread does not yet own the monitor and does not
331 // yet appear on any queues that would get it made the successor.
332 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
333 // handler cannot accidentally consume an unpark() meant for the
334 // ParkEvent associated with this ObjectMonitor.
335 }
336
337 OSThreadContendState osts(Self->osthread());
338 ThreadBlockInVM tbivm(jt);
339
340 // TODO-FIXME: change the following for(;;) loop to straight-line code.
341 for (;;) {
342 jt->set_suspend_equivalent();
343 // cleared by handle_special_suspend_equivalent_condition()
344 // or java_suspend_self()
345
346 EnterI(THREAD);
347
348 if (!ExitSuspendEquivalent(jt)) break;
349
350 // We have acquired the contended monitor, but while we were
351 // waiting another thread suspended us. We don't want to enter
352 // the monitor while suspended because that would surprise the
353 // thread that suspended us.
354 //
355 _recursions = 0;
356 _succ = NULL;
357 exit(false, Self);
358
359 jt->java_suspend_self();
360 }
361 Self->set_current_pending_monitor(NULL);
362
363 // We cleared the pending monitor info since we've just gotten past
364 // the enter-check-for-suspend dance and we now own the monitor free
365 // and clear, i.e., it is no longer pending. The ThreadBlockInVM
366 // destructor can go to a safepoint at the end of this block. If we
367 // do a thread dump during that safepoint, then this thread will show
368 // as having "-locked" the monitor, but the OS and java.lang.Thread
369 // states will still report that the thread is blocked trying to
370 // acquire it.
371 }
372
373 Atomic::dec(&_contentions);
374 assert(_contentions >= 0, "must not be negative: contentions=%d", _contentions);
375 Self->_Stalled = 0;
376
377 // Must either set _recursions = 0 or ASSERT _recursions == 0.
378 assert(_recursions == 0, "invariant");
379 assert(_owner == Self, "invariant");
380 assert(_succ != Self, "invariant");
381 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant");
382
383 // The thread -- now the owner -- is back in vm mode.
384 // Report the glorious news via TI,DTrace and jvmstat.
385 // The probe effect is non-trivial. All the reportage occurs
386 // while we hold the monitor, increasing the length of the critical
387 // section. Amdahl's parallel speedup law comes vividly into play.
388 //
389 // Another option might be to aggregate the events (thread local or
390 // per-monitor aggregation) and defer reporting until a more opportune
391 // time -- such as next time some thread encounters contention but has
392 // yet to acquire the lock. While spinning that thread could
393 // spinning we could increment JVMStat counters, etc.
394
395 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
396 if (JvmtiExport::should_post_monitor_contended_entered()) {
397 JvmtiExport::post_monitor_contended_entered(jt, this);
398
399 // The current thread already owns the monitor and is not going to
400 // call park() for the remainder of the monitor enter protocol. So
401 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
402 // event handler consumed an unpark() issued by the thread that
403 // just exited the monitor.
404 }
405 if (event.should_commit()) {
406 event.set_previousOwner((uintptr_t)_previous_owner_tid);
407 event.commit();
408 }
409 OM_PERFDATA_OP(ContendedLockAttempts, inc());
410 }
411
412 // Caveat: TryLock() is not necessarily serializing if it returns failure.
413 // Callers must compensate as needed.
414
415 int ObjectMonitor::TryLock(Thread * Self) {
416 void * own = _owner;
417 if (own != NULL) return 0;
418 if (Atomic::replace_if_null(Self, &_owner)) {
419 assert(_recursions == 0, "invariant");
420 return 1;
421 }
422 // The lock had been free momentarily, but we lost the race to the lock.
423 // Interference -- the CAS failed.
424 // We can either return -1 or retry.
425 // Retry doesn't make as much sense because the lock was just acquired.
426 return -1;
427 }
428
429 // Install the displaced mark word (dmw) of a deflating ObjectMonitor
430 // into the header of the object associated with the monitor. This
431 // idempotent method is called by a thread that is deflating a
432 // monitor and by other threads that have detected a race with the
433 // deflation process.
434 void ObjectMonitor::install_displaced_markword_in_object(const oop obj) {
435 // This function must only be called when (owner == DEFLATER_MARKER
436 // && ref_count <= 0), but we can't guarantee that here because
437 // those values could change when the ObjectMonitor gets moved from
438 // the global free list to a per-thread free list.
439
440 guarantee(obj != NULL, "must be non-NULL");
441 if (object() != obj) {
442 // ObjectMonitor's object ref no longer refers to the target object
443 // so the object's header has already been restored.
444 return;
445 }
446
447 markWord dmw = header();
448 if (dmw.value() == 0) {
449 // ObjectMonitor's header/dmw has been cleared so the object's
450 // header has already been restored.
451 return;
452 }
453
454 // A non-NULL dmw has to be either neutral (not locked and not marked)
455 // or is already participating in this restoration protocol.
456 assert(dmw.is_neutral() || (dmw.is_marked() && dmw.hash() == 0),
457 "failed precondition: dmw=" INTPTR_FORMAT, dmw.value());
458
459 markWord marked_dmw = markWord::zero();
460 if (!dmw.is_marked() && dmw.hash() == 0) {
461 // This dmw has not yet started the restoration protocol so we
462 // mark a copy of the dmw to begin the protocol.
463 // Note: A dmw with a hashcode does not take this code path.
464 marked_dmw = dmw.set_marked();
465
466 // All of the callers to this function can be racing with each
467 // other trying to update the _header field.
468 dmw = (markWord) Atomic::cmpxchg(marked_dmw, &_header, dmw);
469 if (dmw.value() == 0) {
470 // ObjectMonitor's header/dmw has been cleared so the object's
471 // header has already been restored.
472 return;
473 }
474 // The _header field is now marked. The winner's 'dmw' variable
475 // contains the original, unmarked header/dmw value and any
476 // losers have a marked header/dmw value that will be cleaned
477 // up below.
478 }
479
480 if (dmw.is_marked()) {
481 // Clear the mark from the header/dmw copy in preparation for
482 // possible restoration from this thread.
483 assert(dmw.hash() == 0, "hashcode must be 0: dmw=" INTPTR_FORMAT,
484 dmw.value());
485 dmw = dmw.set_unmarked();
486 }
487 assert(dmw.is_neutral(), "must be neutral: dmw=" INTPTR_FORMAT, dmw.value());
488
489 // Install displaced mark word if the object's header still points
490 // to this ObjectMonitor. All racing callers to this function will
491 // reach this point, but only one can win.
492 obj->cas_set_mark(dmw, markWord::encode(this));
493
494 // Note: It does not matter which thread restored the header/dmw
495 // into the object's header. The thread deflating the monitor just
496 // wanted the object's header restored and it is. The threads that
497 // detected a race with the deflation process also wanted the
498 // object's header restored before they retry their operation and
499 // because it is restored they will only retry once.
500
501 if (marked_dmw.value() != 0) {
502 // Clear _header to NULL if it is still marked_dmw so a racing
503 // install_displaced_markword_in_object() can bail out sooner.
504 Atomic::cmpxchg(markWord::zero(), &_header, marked_dmw);
505 }
506 }
507
508 // Convert the fields used by is_busy() to a string that can be
509 // used for diagnostic output.
510 const char* ObjectMonitor::is_busy_to_string(stringStream* ss) {
511 ss->print("is_busy: contentions=%d, waiters=%d, ", _contentions, _waiters);
512 if (!AsyncDeflateIdleMonitors) {
513 ss->print("owner=" INTPTR_FORMAT, p2i(_owner));
514 } else if (_owner != DEFLATER_MARKER) {
515 ss->print("owner=" INTPTR_FORMAT, p2i(_owner));
516 } else {
517 ss->print("owner=" INTPTR_FORMAT, NULL);
518 }
519 ss->print(", cxq=" INTPTR_FORMAT ", EntryList=" INTPTR_FORMAT, p2i(_cxq),
520 p2i(_EntryList));
521 return ss->base();
522 }
523
524 #define MAX_RECHECK_INTERVAL 1000
525
526 void ObjectMonitor::EnterI(TRAPS) {
527 ADIM_guarantee(ref_count() > 0, "must be positive: ref_count=%d", ref_count());
528
529 Thread * const Self = THREAD;
530 assert(Self->is_Java_thread(), "invariant");
531 assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant");
532
533 // Try the lock - TATAS
534 if (TryLock (Self) > 0) {
535 assert(_succ != Self, "invariant");
536 assert(_owner == Self, "invariant");
537 assert(_Responsible != Self, "invariant");
538 return;
539 }
540
541 if (AsyncDeflateIdleMonitors &&
542 Atomic::cmpxchg(Self, &_owner, DEFLATER_MARKER) == DEFLATER_MARKER) {
543 // The deflation protocol finished the first part (setting owner),
544 // but it failed the second part (making ref_count negative) and
545 // bailed. Or the ObjectMonitor was async deflated and reused.
546 // Acquired the monitor.
547 assert(_succ != Self, "invariant");
548 assert(_Responsible != Self, "invariant");
549 return;
550 }
551
552 assert(InitDone, "Unexpectedly not initialized");
553
554 // We try one round of spinning *before* enqueueing Self.
555 //
556 // If the _owner is ready but OFFPROC we could use a YieldTo()
557 // operation to donate the remainder of this thread's quantum
558 // to the owner. This has subtle but beneficial affinity
559 // effects.
560
561 if (TrySpin(Self) > 0) {
562 assert(_owner == Self, "invariant");
563 assert(_succ != Self, "invariant");
564 assert(_Responsible != Self, "invariant");
565 return;
566 }
567
568 // The Spin failed -- Enqueue and park the thread ...
569 assert(_succ != Self, "invariant");
570 assert(_owner != Self, "invariant");
571 assert(_Responsible != Self, "invariant");
572
573 // Enqueue "Self" on ObjectMonitor's _cxq.
574 //
575 // Node acts as a proxy for Self.
576 // As an aside, if were to ever rewrite the synchronization code mostly
577 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
578 // Java objects. This would avoid awkward lifecycle and liveness issues,
579 // as well as eliminate a subset of ABA issues.
580 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
581
582 ObjectWaiter node(Self);
583 Self->_ParkEvent->reset();
584 node._prev = (ObjectWaiter *) 0xBAD;
585 node.TState = ObjectWaiter::TS_CXQ;
586
587 // Push "Self" onto the front of the _cxq.
588 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
589 // Note that spinning tends to reduce the rate at which threads
590 // enqueue and dequeue on EntryList|cxq.
591 ObjectWaiter * nxt;
592 for (;;) {
593 node._next = nxt = _cxq;
594 if (Atomic::cmpxchg(&node, &_cxq, nxt) == nxt) break;
595
596 // Interference - the CAS failed because _cxq changed. Just retry.
597 // As an optional optimization we retry the lock.
598 if (TryLock (Self) > 0) {
599 assert(_succ != Self, "invariant");
600 assert(_owner == Self, "invariant");
601 assert(_Responsible != Self, "invariant");
602 return;
603 }
604 }
605
606 // Check for cxq|EntryList edge transition to non-null. This indicates
607 // the onset of contention. While contention persists exiting threads
608 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
609 // operations revert to the faster 1-0 mode. This enter operation may interleave
610 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
611 // arrange for one of the contending thread to use a timed park() operations
612 // to detect and recover from the race. (Stranding is form of progress failure
613 // where the monitor is unlocked but all the contending threads remain parked).
614 // That is, at least one of the contended threads will periodically poll _owner.
615 // One of the contending threads will become the designated "Responsible" thread.
616 // The Responsible thread uses a timed park instead of a normal indefinite park
617 // operation -- it periodically wakes and checks for and recovers from potential
618 // strandings admitted by 1-0 exit operations. We need at most one Responsible
619 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
620 // be responsible for a monitor.
621 //
622 // Currently, one of the contended threads takes on the added role of "Responsible".
623 // A viable alternative would be to use a dedicated "stranding checker" thread
624 // that periodically iterated over all the threads (or active monitors) and unparked
625 // successors where there was risk of stranding. This would help eliminate the
626 // timer scalability issues we see on some platforms as we'd only have one thread
627 // -- the checker -- parked on a timer.
628
629 if (nxt == NULL && _EntryList == NULL) {
630 // Try to assume the role of responsible thread for the monitor.
631 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
632 Atomic::replace_if_null(Self, &_Responsible);
633 }
634
635 // The lock might have been released while this thread was occupied queueing
636 // itself onto _cxq. To close the race and avoid "stranding" and
637 // progress-liveness failure we must resample-retry _owner before parking.
638 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
639 // In this case the ST-MEMBAR is accomplished with CAS().
640 //
641 // TODO: Defer all thread state transitions until park-time.
642 // Since state transitions are heavy and inefficient we'd like
643 // to defer the state transitions until absolutely necessary,
644 // and in doing so avoid some transitions ...
645
646 int nWakeups = 0;
647 int recheckInterval = 1;
648
649 for (;;) {
650
651 if (TryLock(Self) > 0) break;
652 assert(_owner != Self, "invariant");
653
654 // park self
655 if (_Responsible == Self) {
656 Self->_ParkEvent->park((jlong) recheckInterval);
657 // Increase the recheckInterval, but clamp the value.
658 recheckInterval *= 8;
659 if (recheckInterval > MAX_RECHECK_INTERVAL) {
660 recheckInterval = MAX_RECHECK_INTERVAL;
661 }
662 } else {
663 Self->_ParkEvent->park();
664 }
665
666 if (TryLock(Self) > 0) break;
667
668 if (AsyncDeflateIdleMonitors &&
669 Atomic::cmpxchg(Self, &_owner, DEFLATER_MARKER) == DEFLATER_MARKER) {
670 // The deflation protocol finished the first part (setting owner),
671 // but it failed the second part (making ref_count negative) and
672 // bailed. Or the ObjectMonitor was async deflated and reused.
673 // Acquired the monitor.
674 break;
675 }
676
677 // The lock is still contested.
678 // Keep a tally of the # of futile wakeups.
679 // Note that the counter is not protected by a lock or updated by atomics.
680 // That is by design - we trade "lossy" counters which are exposed to
681 // races during updates for a lower probe effect.
682
683 // This PerfData object can be used in parallel with a safepoint.
684 // See the work around in PerfDataManager::destroy().
685 OM_PERFDATA_OP(FutileWakeups, inc());
686 ++nWakeups;
687
688 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
689 // We can defer clearing _succ until after the spin completes
690 // TrySpin() must tolerate being called with _succ == Self.
691 // Try yet another round of adaptive spinning.
692 if (TrySpin(Self) > 0) break;
693
694 // We can find that we were unpark()ed and redesignated _succ while
695 // we were spinning. That's harmless. If we iterate and call park(),
696 // park() will consume the event and return immediately and we'll
697 // just spin again. This pattern can repeat, leaving _succ to simply
698 // spin on a CPU.
699
700 if (_succ == Self) _succ = NULL;
701
702 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
703 OrderAccess::fence();
704 }
705
706 // Egress :
707 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
708 // Normally we'll find Self on the EntryList .
709 // From the perspective of the lock owner (this thread), the
710 // EntryList is stable and cxq is prepend-only.
711 // The head of cxq is volatile but the interior is stable.
712 // In addition, Self.TState is stable.
713
714 assert(_owner == Self, "invariant");
715 assert(object() != NULL, "invariant");
716 // I'd like to write:
717 // guarantee (((oop)(object()))->mark() == markWord::encode(this), "invariant") ;
718 // but as we're at a safepoint that's not safe.
719
720 UnlinkAfterAcquire(Self, &node);
721 if (_succ == Self) _succ = NULL;
722
723 assert(_succ != Self, "invariant");
724 if (_Responsible == Self) {
725 _Responsible = NULL;
726 OrderAccess::fence(); // Dekker pivot-point
727
728 // We may leave threads on cxq|EntryList without a designated
729 // "Responsible" thread. This is benign. When this thread subsequently
730 // exits the monitor it can "see" such preexisting "old" threads --
731 // threads that arrived on the cxq|EntryList before the fence, above --
732 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
733 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
734 // non-null and elect a new "Responsible" timer thread.
735 //
736 // This thread executes:
737 // ST Responsible=null; MEMBAR (in enter epilogue - here)
738 // LD cxq|EntryList (in subsequent exit)
739 //
740 // Entering threads in the slow/contended path execute:
741 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
742 // The (ST cxq; MEMBAR) is accomplished with CAS().
743 //
744 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
745 // exit operation from floating above the ST Responsible=null.
746 }
747
748 // We've acquired ownership with CAS().
749 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
750 // But since the CAS() this thread may have also stored into _succ,
751 // EntryList, cxq or Responsible. These meta-data updates must be
752 // visible __before this thread subsequently drops the lock.
753 // Consider what could occur if we didn't enforce this constraint --
754 // STs to monitor meta-data and user-data could reorder with (become
755 // visible after) the ST in exit that drops ownership of the lock.
756 // Some other thread could then acquire the lock, but observe inconsistent
757 // or old monitor meta-data and heap data. That violates the JMM.
758 // To that end, the 1-0 exit() operation must have at least STST|LDST
759 // "release" barrier semantics. Specifically, there must be at least a
760 // STST|LDST barrier in exit() before the ST of null into _owner that drops
761 // the lock. The barrier ensures that changes to monitor meta-data and data
762 // protected by the lock will be visible before we release the lock, and
763 // therefore before some other thread (CPU) has a chance to acquire the lock.
764 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
765 //
766 // Critically, any prior STs to _succ or EntryList must be visible before
767 // the ST of null into _owner in the *subsequent* (following) corresponding
768 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
769 // execute a serializing instruction.
770
771 return;
772 }
773
774 // ReenterI() is a specialized inline form of the latter half of the
775 // contended slow-path from EnterI(). We use ReenterI() only for
776 // monitor reentry in wait().
777 //
778 // In the future we should reconcile EnterI() and ReenterI().
779
780 void ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) {
781 ADIM_guarantee(ref_count() > 0, "must be positive: ref_count=%d", ref_count());
782
783 assert(Self != NULL, "invariant");
784 assert(SelfNode != NULL, "invariant");
785 assert(SelfNode->_thread == Self, "invariant");
786 assert(_waiters > 0, "invariant");
787 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant");
788 assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
789 JavaThread * jt = (JavaThread *) Self;
790
791 int nWakeups = 0;
792 for (;;) {
793 ObjectWaiter::TStates v = SelfNode->TState;
794 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
795 assert(_owner != Self, "invariant");
796
797 if (TryLock(Self) > 0) break;
798 if (TrySpin(Self) > 0) break;
799
800 if (AsyncDeflateIdleMonitors &&
801 Atomic::cmpxchg(Self, &_owner, DEFLATER_MARKER) == DEFLATER_MARKER) {
802 // The deflation protocol finished the first part (setting owner),
803 // but it failed the second part (making ref_count negative) and
804 // bailed. Or the ObjectMonitor was async deflated and reused.
805 // Acquired the monitor.
806 break;
807 }
808
809 // State transition wrappers around park() ...
810 // ReenterI() wisely defers state transitions until
811 // it's clear we must park the thread.
812 {
813 OSThreadContendState osts(Self->osthread());
814 ThreadBlockInVM tbivm(jt);
815
816 // cleared by handle_special_suspend_equivalent_condition()
817 // or java_suspend_self()
818 jt->set_suspend_equivalent();
819 Self->_ParkEvent->park();
820
821 // were we externally suspended while we were waiting?
822 for (;;) {
823 if (!ExitSuspendEquivalent(jt)) break;
824 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
825 jt->java_suspend_self();
826 jt->set_suspend_equivalent();
827 }
828 }
829
830 // Try again, but just so we distinguish between futile wakeups and
831 // successful wakeups. The following test isn't algorithmically
832 // necessary, but it helps us maintain sensible statistics.
833 if (TryLock(Self) > 0) break;
834
835 // The lock is still contested.
836 // Keep a tally of the # of futile wakeups.
837 // Note that the counter is not protected by a lock or updated by atomics.
838 // That is by design - we trade "lossy" counters which are exposed to
839 // races during updates for a lower probe effect.
840 ++nWakeups;
841
842 // Assuming this is not a spurious wakeup we'll normally
843 // find that _succ == Self.
844 if (_succ == Self) _succ = NULL;
845
846 // Invariant: after clearing _succ a contending thread
847 // *must* retry _owner before parking.
848 OrderAccess::fence();
849
850 // This PerfData object can be used in parallel with a safepoint.
851 // See the work around in PerfDataManager::destroy().
852 OM_PERFDATA_OP(FutileWakeups, inc());
853 }
854
855 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
856 // Normally we'll find Self on the EntryList.
857 // Unlinking from the EntryList is constant-time and atomic-free.
858 // From the perspective of the lock owner (this thread), the
859 // EntryList is stable and cxq is prepend-only.
860 // The head of cxq is volatile but the interior is stable.
861 // In addition, Self.TState is stable.
862
863 assert(_owner == Self, "invariant");
864 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant");
865 UnlinkAfterAcquire(Self, SelfNode);
866 if (_succ == Self) _succ = NULL;
867 assert(_succ != Self, "invariant");
868 SelfNode->TState = ObjectWaiter::TS_RUN;
869 OrderAccess::fence(); // see comments at the end of EnterI()
870 }
871
872 // By convention we unlink a contending thread from EntryList|cxq immediately
873 // after the thread acquires the lock in ::enter(). Equally, we could defer
874 // unlinking the thread until ::exit()-time.
875
876 void ObjectMonitor::UnlinkAfterAcquire(Thread *Self, ObjectWaiter *SelfNode) {
877 assert(_owner == Self, "invariant");
878 assert(SelfNode->_thread == Self, "invariant");
879
880 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
881 // Normal case: remove Self from the DLL EntryList .
882 // This is a constant-time operation.
883 ObjectWaiter * nxt = SelfNode->_next;
884 ObjectWaiter * prv = SelfNode->_prev;
885 if (nxt != NULL) nxt->_prev = prv;
886 if (prv != NULL) prv->_next = nxt;
887 if (SelfNode == _EntryList) _EntryList = nxt;
888 assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
889 assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
890 } else {
891 assert(SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant");
892 // Inopportune interleaving -- Self is still on the cxq.
893 // This usually means the enqueue of self raced an exiting thread.
894 // Normally we'll find Self near the front of the cxq, so
895 // dequeueing is typically fast. If needbe we can accelerate
896 // this with some MCS/CHL-like bidirectional list hints and advisory
897 // back-links so dequeueing from the interior will normally operate
898 // in constant-time.
899 // Dequeue Self from either the head (with CAS) or from the interior
900 // with a linear-time scan and normal non-atomic memory operations.
901 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
902 // and then unlink Self from EntryList. We have to drain eventually,
903 // so it might as well be now.
904
905 ObjectWaiter * v = _cxq;
906 assert(v != NULL, "invariant");
907 if (v != SelfNode || Atomic::cmpxchg(SelfNode->_next, &_cxq, v) != v) {
908 // The CAS above can fail from interference IFF a "RAT" arrived.
909 // In that case Self must be in the interior and can no longer be
910 // at the head of cxq.
911 if (v == SelfNode) {
912 assert(_cxq != v, "invariant");
913 v = _cxq; // CAS above failed - start scan at head of list
914 }
915 ObjectWaiter * p;
916 ObjectWaiter * q = NULL;
917 for (p = v; p != NULL && p != SelfNode; p = p->_next) {
918 q = p;
919 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
920 }
921 assert(v != SelfNode, "invariant");
922 assert(p == SelfNode, "Node not found on cxq");
923 assert(p != _cxq, "invariant");
924 assert(q != NULL, "invariant");
925 assert(q->_next == p, "invariant");
926 q->_next = p->_next;
927 }
928 }
929
930 #ifdef ASSERT
931 // Diagnostic hygiene ...
932 SelfNode->_prev = (ObjectWaiter *) 0xBAD;
933 SelfNode->_next = (ObjectWaiter *) 0xBAD;
934 SelfNode->TState = ObjectWaiter::TS_RUN;
935 #endif
936 }
937
938 // -----------------------------------------------------------------------------
939 // Exit support
940 //
941 // exit()
942 // ~~~~~~
943 // Note that the collector can't reclaim the objectMonitor or deflate
944 // the object out from underneath the thread calling ::exit() as the
945 // thread calling ::exit() never transitions to a stable state.
946 // This inhibits GC, which in turn inhibits asynchronous (and
947 // inopportune) reclamation of "this".
948 //
949 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
950 // There's one exception to the claim above, however. EnterI() can call
951 // exit() to drop a lock if the acquirer has been externally suspended.
952 // In that case exit() is called with _thread_state as _thread_blocked,
953 // but the monitor's _contentions field is > 0, which inhibits reclamation.
954 //
955 // 1-0 exit
956 // ~~~~~~~~
957 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
958 // the fast-path operators have been optimized so the common ::exit()
959 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
960 // The code emitted by fast_unlock() elides the usual MEMBAR. This
961 // greatly improves latency -- MEMBAR and CAS having considerable local
962 // latency on modern processors -- but at the cost of "stranding". Absent the
963 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
964 // ::enter() path, resulting in the entering thread being stranding
965 // and a progress-liveness failure. Stranding is extremely rare.
966 // We use timers (timed park operations) & periodic polling to detect
967 // and recover from stranding. Potentially stranded threads periodically
968 // wake up and poll the lock. See the usage of the _Responsible variable.
969 //
970 // The CAS() in enter provides for safety and exclusion, while the CAS or
971 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
972 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
973 // We detect and recover from stranding with timers.
974 //
975 // If a thread transiently strands it'll park until (a) another
976 // thread acquires the lock and then drops the lock, at which time the
977 // exiting thread will notice and unpark the stranded thread, or, (b)
978 // the timer expires. If the lock is high traffic then the stranding latency
979 // will be low due to (a). If the lock is low traffic then the odds of
980 // stranding are lower, although the worst-case stranding latency
981 // is longer. Critically, we don't want to put excessive load in the
982 // platform's timer subsystem. We want to minimize both the timer injection
983 // rate (timers created/sec) as well as the number of timers active at
984 // any one time. (more precisely, we want to minimize timer-seconds, which is
985 // the integral of the # of active timers at any instant over time).
986 // Both impinge on OS scalability. Given that, at most one thread parked on
987 // a monitor will use a timer.
988 //
989 // There is also the risk of a futile wake-up. If we drop the lock
990 // another thread can reacquire the lock immediately, and we can
991 // then wake a thread unnecessarily. This is benign, and we've
992 // structured the code so the windows are short and the frequency
993 // of such futile wakups is low.
994
995 void ObjectMonitor::exit(bool not_suspended, TRAPS) {
996 Thread * const Self = THREAD;
997 if (THREAD != _owner) {
998 if (THREAD->is_lock_owned((address) _owner)) {
999 // Transmute _owner from a BasicLock pointer to a Thread address.
1000 // We don't need to hold _mutex for this transition.
1001 // Non-null to Non-null is safe as long as all readers can
1002 // tolerate either flavor.
1003 assert(_recursions == 0, "invariant");
1004 _owner = THREAD;
1005 _recursions = 0;
1006 } else {
1007 // Apparent unbalanced locking ...
1008 // Naively we'd like to throw IllegalMonitorStateException.
1009 // As a practical matter we can neither allocate nor throw an
1010 // exception as ::exit() can be called from leaf routines.
1011 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
1012 // Upon deeper reflection, however, in a properly run JVM the only
1013 // way we should encounter this situation is in the presence of
1014 // unbalanced JNI locking. TODO: CheckJNICalls.
1015 // See also: CR4414101
1016 assert(false, "Non-balanced monitor enter/exit! Likely JNI locking: "
1017 "owner=" INTPTR_FORMAT, p2i(_owner));
1018 return;
1019 }
1020 }
1021
1022 if (_recursions != 0) {
1023 _recursions--; // this is simple recursive enter
1024 return;
1025 }
1026
1027 // Invariant: after setting Responsible=null an thread must execute
1028 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
1029 _Responsible = NULL;
1030
1031 #if INCLUDE_JFR
1032 // get the owner's thread id for the MonitorEnter event
1033 // if it is enabled and the thread isn't suspended
1034 if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
1035 _previous_owner_tid = JFR_THREAD_ID(Self);
1036 }
1037 #endif
1038
1039 for (;;) {
1040 assert(THREAD == _owner, "invariant");
1041
1042 // release semantics: prior loads and stores from within the critical section
1043 // must not float (reorder) past the following store that drops the lock.
1044 // On SPARC that requires MEMBAR #loadstore|#storestore.
1045 // But of course in TSO #loadstore|#storestore is not required.
1046 OrderAccess::release_store(&_owner, (void*)NULL); // drop the lock
1047 OrderAccess::storeload(); // See if we need to wake a successor
1048 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1049 return;
1050 }
1051 // Other threads are blocked trying to acquire the lock.
1052
1053 // Normally the exiting thread is responsible for ensuring succession,
1054 // but if other successors are ready or other entering threads are spinning
1055 // then this thread can simply store NULL into _owner and exit without
1056 // waking a successor. The existence of spinners or ready successors
1057 // guarantees proper succession (liveness). Responsibility passes to the
1058 // ready or running successors. The exiting thread delegates the duty.
1059 // More precisely, if a successor already exists this thread is absolved
1060 // of the responsibility of waking (unparking) one.
1061 //
1062 // The _succ variable is critical to reducing futile wakeup frequency.
1063 // _succ identifies the "heir presumptive" thread that has been made
1064 // ready (unparked) but that has not yet run. We need only one such
1065 // successor thread to guarantee progress.
1066 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1067 // section 3.3 "Futile Wakeup Throttling" for details.
1068 //
1069 // Note that spinners in Enter() also set _succ non-null.
1070 // In the current implementation spinners opportunistically set
1071 // _succ so that exiting threads might avoid waking a successor.
1072 // Another less appealing alternative would be for the exiting thread
1073 // to drop the lock and then spin briefly to see if a spinner managed
1074 // to acquire the lock. If so, the exiting thread could exit
1075 // immediately without waking a successor, otherwise the exiting
1076 // thread would need to dequeue and wake a successor.
1077 // (Note that we'd need to make the post-drop spin short, but no
1078 // shorter than the worst-case round-trip cache-line migration time.
1079 // The dropped lock needs to become visible to the spinner, and then
1080 // the acquisition of the lock by the spinner must become visible to
1081 // the exiting thread).
1082
1083 // It appears that an heir-presumptive (successor) must be made ready.
1084 // Only the current lock owner can manipulate the EntryList or
1085 // drain _cxq, so we need to reacquire the lock. If we fail
1086 // to reacquire the lock the responsibility for ensuring succession
1087 // falls to the new owner.
1088 //
1089 if (!Atomic::replace_if_null(THREAD, &_owner)) {
1090 return;
1091 }
1092
1093 guarantee(_owner == THREAD, "invariant");
1094
1095 ObjectWaiter * w = NULL;
1096
1097 w = _EntryList;
1098 if (w != NULL) {
1099 // I'd like to write: guarantee (w->_thread != Self).
1100 // But in practice an exiting thread may find itself on the EntryList.
1101 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1102 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1103 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1104 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1105 // release the lock "O". T2 resumes immediately after the ST of null into
1106 // _owner, above. T2 notices that the EntryList is populated, so it
1107 // reacquires the lock and then finds itself on the EntryList.
1108 // Given all that, we have to tolerate the circumstance where "w" is
1109 // associated with Self.
1110 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1111 ExitEpilog(Self, w);
1112 return;
1113 }
1114
1115 // If we find that both _cxq and EntryList are null then just
1116 // re-run the exit protocol from the top.
1117 w = _cxq;
1118 if (w == NULL) continue;
1119
1120 // Drain _cxq into EntryList - bulk transfer.
1121 // First, detach _cxq.
1122 // The following loop is tantamount to: w = swap(&cxq, NULL)
1123 for (;;) {
1124 assert(w != NULL, "Invariant");
1125 ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1126 if (u == w) break;
1127 w = u;
1128 }
1129
1130 assert(w != NULL, "invariant");
1131 assert(_EntryList == NULL, "invariant");
1132
1133 // Convert the LIFO SLL anchored by _cxq into a DLL.
1134 // The list reorganization step operates in O(LENGTH(w)) time.
1135 // It's critical that this step operate quickly as
1136 // "Self" still holds the outer-lock, restricting parallelism
1137 // and effectively lengthening the critical section.
1138 // Invariant: s chases t chases u.
1139 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1140 // we have faster access to the tail.
1141
1142 _EntryList = w;
1143 ObjectWaiter * q = NULL;
1144 ObjectWaiter * p;
1145 for (p = w; p != NULL; p = p->_next) {
1146 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1147 p->TState = ObjectWaiter::TS_ENTER;
1148 p->_prev = q;
1149 q = p;
1150 }
1151
1152 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1153 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1154
1155 // See if we can abdicate to a spinner instead of waking a thread.
1156 // A primary goal of the implementation is to reduce the
1157 // context-switch rate.
1158 if (_succ != NULL) continue;
1159
1160 w = _EntryList;
1161 if (w != NULL) {
1162 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1163 ExitEpilog(Self, w);
1164 return;
1165 }
1166 }
1167 }
1168
1169 // ExitSuspendEquivalent:
1170 // A faster alternate to handle_special_suspend_equivalent_condition()
1171 //
1172 // handle_special_suspend_equivalent_condition() unconditionally
1173 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1174 // operations have high latency. Note that in ::enter() we call HSSEC
1175 // while holding the monitor, so we effectively lengthen the critical sections.
1176 //
1177 // There are a number of possible solutions:
1178 //
1179 // A. To ameliorate the problem we might also defer state transitions
1180 // to as late as possible -- just prior to parking.
1181 // Given that, we'd call HSSEC after having returned from park(),
1182 // but before attempting to acquire the monitor. This is only a
1183 // partial solution. It avoids calling HSSEC while holding the
1184 // monitor (good), but it still increases successor reacquisition latency --
1185 // the interval between unparking a successor and the time the successor
1186 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1187 // If we use this technique we can also avoid EnterI()-exit() loop
1188 // in ::enter() where we iteratively drop the lock and then attempt
1189 // to reacquire it after suspending.
1190 //
1191 // B. In the future we might fold all the suspend bits into a
1192 // composite per-thread suspend flag and then update it with CAS().
1193 // Alternately, a Dekker-like mechanism with multiple variables
1194 // would suffice:
1195 // ST Self->_suspend_equivalent = false
1196 // MEMBAR
1197 // LD Self_>_suspend_flags
1198
1199 bool ObjectMonitor::ExitSuspendEquivalent(JavaThread * jSelf) {
1200 return jSelf->handle_special_suspend_equivalent_condition();
1201 }
1202
1203
1204 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) {
1205 assert(_owner == Self, "invariant");
1206
1207 // Exit protocol:
1208 // 1. ST _succ = wakee
1209 // 2. membar #loadstore|#storestore;
1210 // 2. ST _owner = NULL
1211 // 3. unpark(wakee)
1212
1213 _succ = Wakee->_thread;
1214 ParkEvent * Trigger = Wakee->_event;
1215
1216 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1217 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1218 // out-of-scope (non-extant).
1219 Wakee = NULL;
1220
1221 // Drop the lock
1222 OrderAccess::release_store(&_owner, (void*)NULL);
1223 OrderAccess::fence(); // ST _owner vs LD in unpark()
1224
1225 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1226 Trigger->unpark();
1227
1228 // Maintain stats and report events to JVMTI
1229 OM_PERFDATA_OP(Parks, inc());
1230 }
1231
1232
1233 // -----------------------------------------------------------------------------
1234 // Class Loader deadlock handling.
1235 //
1236 // complete_exit exits a lock returning recursion count
1237 // complete_exit/reenter operate as a wait without waiting
1238 // complete_exit requires an inflated monitor
1239 // The _owner field is not always the Thread addr even with an
1240 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1241 // thread due to contention.
1242 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1243 Thread * const Self = THREAD;
1244 assert(Self->is_Java_thread(), "Must be Java thread!");
1245 JavaThread *jt = (JavaThread *)THREAD;
1246
1247 assert(InitDone, "Unexpectedly not initialized");
1248
1249 if (THREAD != _owner) {
1250 if (THREAD->is_lock_owned ((address)_owner)) {
1251 assert(_recursions == 0, "internal state error");
1252 _owner = THREAD; // Convert from basiclock addr to Thread addr
1253 _recursions = 0;
1254 }
1255 }
1256
1257 guarantee(Self == _owner, "complete_exit not owner");
1258 intptr_t save = _recursions; // record the old recursion count
1259 _recursions = 0; // set the recursion level to be 0
1260 exit(true, Self); // exit the monitor
1261 guarantee(_owner != Self, "invariant");
1262 return save;
1263 }
1264
1265 // reenter() enters a lock and sets recursion count
1266 // complete_exit/reenter operate as a wait without waiting
1267 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1268 Thread * const Self = THREAD;
1269 assert(Self->is_Java_thread(), "Must be Java thread!");
1270 JavaThread *jt = (JavaThread *)THREAD;
1271
1272 guarantee(_owner != Self, "reenter already owner");
1273 enter(THREAD);
1274 // Entered the monitor.
1275 guarantee(_recursions == 0, "reenter recursion");
1276 _recursions = recursions;
1277 }
1278
1279 // Checks that the current THREAD owns this monitor and causes an
1280 // immediate return if it doesn't. We don't use the CHECK macro
1281 // because we want the IMSE to be the only exception that is thrown
1282 // from the call site when false is returned. Any other pending
1283 // exception is ignored.
1284 #define CHECK_OWNER() \
1285 do { \
1286 if (!check_owner(THREAD)) { \
1287 assert(HAS_PENDING_EXCEPTION, "expected a pending IMSE here."); \
1288 return; \
1289 } \
1290 } while (false)
1291
1292 // Returns true if the specified thread owns the ObjectMonitor.
1293 // Otherwise returns false and throws IllegalMonitorStateException
1294 // (IMSE). If there is a pending exception and the specified thread
1295 // is not the owner, that exception will be replaced by the IMSE.
1296 bool ObjectMonitor::check_owner(Thread* THREAD) {
1297 if (_owner == THREAD) {
1298 return true;
1299 }
1300 if (THREAD->is_lock_owned((address)_owner)) {
1301 _owner = THREAD; // convert from BasicLock addr to Thread addr
1302 _recursions = 0;
1303 return true;
1304 }
1305 THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(),
1306 "current thread is not owner", false);
1307 }
1308
1309 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1310 ObjectMonitor* monitor,
1311 jlong notifier_tid,
1312 jlong timeout,
1313 bool timedout) {
1314 assert(event != NULL, "invariant");
1315 assert(monitor != NULL, "invariant");
1316 event->set_monitorClass(((oop)monitor->object())->klass());
1317 event->set_timeout(timeout);
1318 event->set_address((uintptr_t)monitor->object_addr());
1319 event->set_notifier(notifier_tid);
1320 event->set_timedOut(timedout);
1321 event->commit();
1322 }
1323
1324 // -----------------------------------------------------------------------------
1325 // Wait/Notify/NotifyAll
1326 //
1327 // Note: a subset of changes to ObjectMonitor::wait()
1328 // will need to be replicated in complete_exit
1329 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1330 Thread * const Self = THREAD;
1331 assert(Self->is_Java_thread(), "Must be Java thread!");
1332 JavaThread *jt = (JavaThread *)THREAD;
1333
1334 assert(InitDone, "Unexpectedly not initialized");
1335
1336 CHECK_OWNER(); // Throws IMSE if not owner.
1337
1338 EventJavaMonitorWait event;
1339
1340 // check for a pending interrupt
1341 if (interruptible && jt->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1342 // post monitor waited event. Note that this is past-tense, we are done waiting.
1343 if (JvmtiExport::should_post_monitor_waited()) {
1344 // Note: 'false' parameter is passed here because the
1345 // wait was not timed out due to thread interrupt.
1346 JvmtiExport::post_monitor_waited(jt, this, false);
1347
1348 // In this short circuit of the monitor wait protocol, the
1349 // current thread never drops ownership of the monitor and
1350 // never gets added to the wait queue so the current thread
1351 // cannot be made the successor. This means that the
1352 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1353 // consume an unpark() meant for the ParkEvent associated with
1354 // this ObjectMonitor.
1355 }
1356 if (event.should_commit()) {
1357 post_monitor_wait_event(&event, this, 0, millis, false);
1358 }
1359 THROW(vmSymbols::java_lang_InterruptedException());
1360 return;
1361 }
1362
1363 assert(Self->_Stalled == 0, "invariant");
1364 Self->_Stalled = intptr_t(this);
1365 jt->set_current_waiting_monitor(this);
1366
1367 // create a node to be put into the queue
1368 // Critically, after we reset() the event but prior to park(), we must check
1369 // for a pending interrupt.
1370 ObjectWaiter node(Self);
1371 node.TState = ObjectWaiter::TS_WAIT;
1372 Self->_ParkEvent->reset();
1373 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1374
1375 // Enter the waiting queue, which is a circular doubly linked list in this case
1376 // but it could be a priority queue or any data structure.
1377 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1378 // by the the owner of the monitor *except* in the case where park()
1379 // returns because of a timeout of interrupt. Contention is exceptionally rare
1380 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1381
1382 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1383 AddWaiter(&node);
1384 Thread::SpinRelease(&_WaitSetLock);
1385
1386 _Responsible = NULL;
1387
1388 intptr_t save = _recursions; // record the old recursion count
1389 _waiters++; // increment the number of waiters
1390 _recursions = 0; // set the recursion level to be 1
1391 exit(true, Self); // exit the monitor
1392 guarantee(_owner != Self, "invariant");
1393
1394 // The thread is on the WaitSet list - now park() it.
1395 // On MP systems it's conceivable that a brief spin before we park
1396 // could be profitable.
1397 //
1398 // TODO-FIXME: change the following logic to a loop of the form
1399 // while (!timeout && !interrupted && _notified == 0) park()
1400
1401 int ret = OS_OK;
1402 int WasNotified = 0;
1403 { // State transition wrappers
1404 OSThread* osthread = Self->osthread();
1405 OSThreadWaitState osts(osthread, true);
1406 {
1407 ThreadBlockInVM tbivm(jt);
1408 // Thread is in thread_blocked state and oop access is unsafe.
1409 jt->set_suspend_equivalent();
1410
1411 if (interruptible && (jt->is_interrupted(false) || HAS_PENDING_EXCEPTION)) {
1412 // Intentionally empty
1413 } else if (node._notified == 0) {
1414 if (millis <= 0) {
1415 Self->_ParkEvent->park();
1416 } else {
1417 ret = Self->_ParkEvent->park(millis);
1418 }
1419 }
1420
1421 // were we externally suspended while we were waiting?
1422 if (ExitSuspendEquivalent (jt)) {
1423 // TODO-FIXME: add -- if succ == Self then succ = null.
1424 jt->java_suspend_self();
1425 }
1426
1427 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1428
1429 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1430 // from the WaitSet to the EntryList.
1431 // See if we need to remove Node from the WaitSet.
1432 // We use double-checked locking to avoid grabbing _WaitSetLock
1433 // if the thread is not on the wait queue.
1434 //
1435 // Note that we don't need a fence before the fetch of TState.
1436 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1437 // written by the is thread. (perhaps the fetch might even be satisfied
1438 // by a look-aside into the processor's own store buffer, although given
1439 // the length of the code path between the prior ST and this load that's
1440 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1441 // then we'll acquire the lock and then re-fetch a fresh TState value.
1442 // That is, we fail toward safety.
1443
1444 if (node.TState == ObjectWaiter::TS_WAIT) {
1445 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1446 if (node.TState == ObjectWaiter::TS_WAIT) {
1447 DequeueSpecificWaiter(&node); // unlink from WaitSet
1448 assert(node._notified == 0, "invariant");
1449 node.TState = ObjectWaiter::TS_RUN;
1450 }
1451 Thread::SpinRelease(&_WaitSetLock);
1452 }
1453
1454 // The thread is now either on off-list (TS_RUN),
1455 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1456 // The Node's TState variable is stable from the perspective of this thread.
1457 // No other threads will asynchronously modify TState.
1458 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1459 OrderAccess::loadload();
1460 if (_succ == Self) _succ = NULL;
1461 WasNotified = node._notified;
1462
1463 // Reentry phase -- reacquire the monitor.
1464 // re-enter contended monitor after object.wait().
1465 // retain OBJECT_WAIT state until re-enter successfully completes
1466 // Thread state is thread_in_vm and oop access is again safe,
1467 // although the raw address of the object may have changed.
1468 // (Don't cache naked oops over safepoints, of course).
1469
1470 // post monitor waited event. Note that this is past-tense, we are done waiting.
1471 if (JvmtiExport::should_post_monitor_waited()) {
1472 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1473
1474 if (node._notified != 0 && _succ == Self) {
1475 // In this part of the monitor wait-notify-reenter protocol it
1476 // is possible (and normal) for another thread to do a fastpath
1477 // monitor enter-exit while this thread is still trying to get
1478 // to the reenter portion of the protocol.
1479 //
1480 // The ObjectMonitor was notified and the current thread is
1481 // the successor which also means that an unpark() has already
1482 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1483 // consume the unpark() that was done when the successor was
1484 // set because the same ParkEvent is shared between Java
1485 // monitors and JVM/TI RawMonitors (for now).
1486 //
1487 // We redo the unpark() to ensure forward progress, i.e., we
1488 // don't want all pending threads hanging (parked) with none
1489 // entering the unlocked monitor.
1490 node._event->unpark();
1491 }
1492 }
1493
1494 if (event.should_commit()) {
1495 post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1496 }
1497
1498 OrderAccess::fence();
1499
1500 assert(Self->_Stalled != 0, "invariant");
1501 Self->_Stalled = 0;
1502
1503 assert(_owner != Self, "invariant");
1504 ObjectWaiter::TStates v = node.TState;
1505 if (v == ObjectWaiter::TS_RUN) {
1506 enter(Self);
1507 } else {
1508 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1509 ReenterI(Self, &node);
1510 node.wait_reenter_end(this);
1511 }
1512
1513 // Self has reacquired the lock.
1514 // Lifecycle - the node representing Self must not appear on any queues.
1515 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1516 // want residual elements associated with this thread left on any lists.
1517 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1518 assert(_owner == Self, "invariant");
1519 assert(_succ != Self, "invariant");
1520 } // OSThreadWaitState()
1521
1522 jt->set_current_waiting_monitor(NULL);
1523
1524 guarantee(_recursions == 0, "invariant");
1525 _recursions = save; // restore the old recursion count
1526 _waiters--; // decrement the number of waiters
1527
1528 // Verify a few postconditions
1529 assert(_owner == Self, "invariant");
1530 assert(_succ != Self, "invariant");
1531 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant");
1532
1533 // check if the notification happened
1534 if (!WasNotified) {
1535 // no, it could be timeout or Thread.interrupt() or both
1536 // check for interrupt event, otherwise it is timeout
1537 if (interruptible && jt->is_interrupted(true) && !HAS_PENDING_EXCEPTION) {
1538 THROW(vmSymbols::java_lang_InterruptedException());
1539 }
1540 }
1541
1542 // NOTE: Spurious wake up will be consider as timeout.
1543 // Monitor notify has precedence over thread interrupt.
1544 }
1545
1546
1547 // Consider:
1548 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1549 // then instead of transferring a thread from the WaitSet to the EntryList
1550 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1551
1552 void ObjectMonitor::INotify(Thread * Self) {
1553 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1554 ObjectWaiter * iterator = DequeueWaiter();
1555 if (iterator != NULL) {
1556 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1557 guarantee(iterator->_notified == 0, "invariant");
1558 // Disposition - what might we do with iterator ?
1559 // a. add it directly to the EntryList - either tail (policy == 1)
1560 // or head (policy == 0).
1561 // b. push it onto the front of the _cxq (policy == 2).
1562 // For now we use (b).
1563
1564 iterator->TState = ObjectWaiter::TS_ENTER;
1565
1566 iterator->_notified = 1;
1567 iterator->_notifier_tid = JFR_THREAD_ID(Self);
1568
1569 ObjectWaiter * list = _EntryList;
1570 if (list != NULL) {
1571 assert(list->_prev == NULL, "invariant");
1572 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1573 assert(list != iterator, "invariant");
1574 }
1575
1576 // prepend to cxq
1577 if (list == NULL) {
1578 iterator->_next = iterator->_prev = NULL;
1579 _EntryList = iterator;
1580 } else {
1581 iterator->TState = ObjectWaiter::TS_CXQ;
1582 for (;;) {
1583 ObjectWaiter * front = _cxq;
1584 iterator->_next = front;
1585 if (Atomic::cmpxchg(iterator, &_cxq, front) == front) {
1586 break;
1587 }
1588 }
1589 }
1590
1591 // _WaitSetLock protects the wait queue, not the EntryList. We could
1592 // move the add-to-EntryList operation, above, outside the critical section
1593 // protected by _WaitSetLock. In practice that's not useful. With the
1594 // exception of wait() timeouts and interrupts the monitor owner
1595 // is the only thread that grabs _WaitSetLock. There's almost no contention
1596 // on _WaitSetLock so it's not profitable to reduce the length of the
1597 // critical section.
1598
1599 iterator->wait_reenter_begin(this);
1600 }
1601 Thread::SpinRelease(&_WaitSetLock);
1602 }
1603
1604 // Consider: a not-uncommon synchronization bug is to use notify() when
1605 // notifyAll() is more appropriate, potentially resulting in stranded
1606 // threads; this is one example of a lost wakeup. A useful diagnostic
1607 // option is to force all notify() operations to behave as notifyAll().
1608 //
1609 // Note: We can also detect many such problems with a "minimum wait".
1610 // When the "minimum wait" is set to a small non-zero timeout value
1611 // and the program does not hang whereas it did absent "minimum wait",
1612 // that suggests a lost wakeup bug.
1613
1614 void ObjectMonitor::notify(TRAPS) {
1615 CHECK_OWNER(); // Throws IMSE if not owner.
1616 if (_WaitSet == NULL) {
1617 return;
1618 }
1619 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1620 INotify(THREAD);
1621 OM_PERFDATA_OP(Notifications, inc(1));
1622 }
1623
1624
1625 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1626 // from the waitset to the EntryList. This could be done more efficiently with a
1627 // single bulk transfer but in practice it's not time-critical. Beware too,
1628 // that in prepend-mode we invert the order of the waiters. Let's say that the
1629 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1630 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1631
1632 void ObjectMonitor::notifyAll(TRAPS) {
1633 CHECK_OWNER(); // Throws IMSE if not owner.
1634 if (_WaitSet == NULL) {
1635 return;
1636 }
1637
1638 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1639 int tally = 0;
1640 while (_WaitSet != NULL) {
1641 tally++;
1642 INotify(THREAD);
1643 }
1644
1645 OM_PERFDATA_OP(Notifications, inc(tally));
1646 }
1647
1648 // -----------------------------------------------------------------------------
1649 // Adaptive Spinning Support
1650 //
1651 // Adaptive spin-then-block - rational spinning
1652 //
1653 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1654 // algorithm. On high order SMP systems it would be better to start with
1655 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1656 // a contending thread could enqueue itself on the cxq and then spin locally
1657 // on a thread-specific variable such as its ParkEvent._Event flag.
1658 // That's left as an exercise for the reader. Note that global spinning is
1659 // not problematic on Niagara, as the L2 cache serves the interconnect and
1660 // has both low latency and massive bandwidth.
1661 //
1662 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1663 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1664 // (duration) or we can fix the count at approximately the duration of
1665 // a context switch and vary the frequency. Of course we could also
1666 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1667 // For a description of 'Adaptive spin-then-block mutual exclusion in
1668 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1669 //
1670 // This implementation varies the duration "D", where D varies with
1671 // the success rate of recent spin attempts. (D is capped at approximately
1672 // length of a round-trip context switch). The success rate for recent
1673 // spin attempts is a good predictor of the success rate of future spin
1674 // attempts. The mechanism adapts automatically to varying critical
1675 // section length (lock modality), system load and degree of parallelism.
1676 // D is maintained per-monitor in _SpinDuration and is initialized
1677 // optimistically. Spin frequency is fixed at 100%.
1678 //
1679 // Note that _SpinDuration is volatile, but we update it without locks
1680 // or atomics. The code is designed so that _SpinDuration stays within
1681 // a reasonable range even in the presence of races. The arithmetic
1682 // operations on _SpinDuration are closed over the domain of legal values,
1683 // so at worst a race will install and older but still legal value.
1684 // At the very worst this introduces some apparent non-determinism.
1685 // We might spin when we shouldn't or vice-versa, but since the spin
1686 // count are relatively short, even in the worst case, the effect is harmless.
1687 //
1688 // Care must be taken that a low "D" value does not become an
1689 // an absorbing state. Transient spinning failures -- when spinning
1690 // is overall profitable -- should not cause the system to converge
1691 // on low "D" values. We want spinning to be stable and predictable
1692 // and fairly responsive to change and at the same time we don't want
1693 // it to oscillate, become metastable, be "too" non-deterministic,
1694 // or converge on or enter undesirable stable absorbing states.
1695 //
1696 // We implement a feedback-based control system -- using past behavior
1697 // to predict future behavior. We face two issues: (a) if the
1698 // input signal is random then the spin predictor won't provide optimal
1699 // results, and (b) if the signal frequency is too high then the control
1700 // system, which has some natural response lag, will "chase" the signal.
1701 // (b) can arise from multimodal lock hold times. Transient preemption
1702 // can also result in apparent bimodal lock hold times.
1703 // Although sub-optimal, neither condition is particularly harmful, as
1704 // in the worst-case we'll spin when we shouldn't or vice-versa.
1705 // The maximum spin duration is rather short so the failure modes aren't bad.
1706 // To be conservative, I've tuned the gain in system to bias toward
1707 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1708 // "rings" or oscillates between spinning and not spinning. This happens
1709 // when spinning is just on the cusp of profitability, however, so the
1710 // situation is not dire. The state is benign -- there's no need to add
1711 // hysteresis control to damp the transition rate between spinning and
1712 // not spinning.
1713
1714 // Spinning: Fixed frequency (100%), vary duration
1715 int ObjectMonitor::TrySpin(Thread * Self) {
1716 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1717 int ctr = Knob_FixedSpin;
1718 if (ctr != 0) {
1719 while (--ctr >= 0) {
1720 if (TryLock(Self) > 0) return 1;
1721 SpinPause();
1722 }
1723 return 0;
1724 }
1725
1726 for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1727 if (TryLock(Self) > 0) {
1728 // Increase _SpinDuration ...
1729 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1730 // Raising _SpurDuration to the poverty line is key.
1731 int x = _SpinDuration;
1732 if (x < Knob_SpinLimit) {
1733 if (x < Knob_Poverty) x = Knob_Poverty;
1734 _SpinDuration = x + Knob_BonusB;
1735 }
1736 return 1;
1737 }
1738 SpinPause();
1739 }
1740
1741 // Admission control - verify preconditions for spinning
1742 //
1743 // We always spin a little bit, just to prevent _SpinDuration == 0 from
1744 // becoming an absorbing state. Put another way, we spin briefly to
1745 // sample, just in case the system load, parallelism, contention, or lock
1746 // modality changed.
1747 //
1748 // Consider the following alternative:
1749 // Periodically set _SpinDuration = _SpinLimit and try a long/full
1750 // spin attempt. "Periodically" might mean after a tally of
1751 // the # of failed spin attempts (or iterations) reaches some threshold.
1752 // This takes us into the realm of 1-out-of-N spinning, where we
1753 // hold the duration constant but vary the frequency.
1754
1755 ctr = _SpinDuration;
1756 if (ctr <= 0) return 0;
1757
1758 if (NotRunnable(Self, (Thread *) _owner)) {
1759 return 0;
1760 }
1761
1762 // We're good to spin ... spin ingress.
1763 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1764 // when preparing to LD...CAS _owner, etc and the CAS is likely
1765 // to succeed.
1766 if (_succ == NULL) {
1767 _succ = Self;
1768 }
1769 Thread * prv = NULL;
1770
1771 // There are three ways to exit the following loop:
1772 // 1. A successful spin where this thread has acquired the lock.
1773 // 2. Spin failure with prejudice
1774 // 3. Spin failure without prejudice
1775
1776 while (--ctr >= 0) {
1777
1778 // Periodic polling -- Check for pending GC
1779 // Threads may spin while they're unsafe.
1780 // We don't want spinning threads to delay the JVM from reaching
1781 // a stop-the-world safepoint or to steal cycles from GC.
1782 // If we detect a pending safepoint we abort in order that
1783 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1784 // this thread, if safe, doesn't steal cycles from GC.
1785 // This is in keeping with the "no loitering in runtime" rule.
1786 // We periodically check to see if there's a safepoint pending.
1787 if ((ctr & 0xFF) == 0) {
1788 if (SafepointMechanism::should_block(Self)) {
1789 goto Abort; // abrupt spin egress
1790 }
1791 SpinPause();
1792 }
1793
1794 // Probe _owner with TATAS
1795 // If this thread observes the monitor transition or flicker
1796 // from locked to unlocked to locked, then the odds that this
1797 // thread will acquire the lock in this spin attempt go down
1798 // considerably. The same argument applies if the CAS fails
1799 // or if we observe _owner change from one non-null value to
1800 // another non-null value. In such cases we might abort
1801 // the spin without prejudice or apply a "penalty" to the
1802 // spin count-down variable "ctr", reducing it by 100, say.
1803
1804 Thread * ox = (Thread *) _owner;
1805 if (ox == NULL) {
1806 ox = (Thread*)Atomic::cmpxchg(Self, &_owner, (void*)NULL);
1807 if (ox == NULL) {
1808 // The CAS succeeded -- this thread acquired ownership
1809 // Take care of some bookkeeping to exit spin state.
1810 if (_succ == Self) {
1811 _succ = NULL;
1812 }
1813
1814 // Increase _SpinDuration :
1815 // The spin was successful (profitable) so we tend toward
1816 // longer spin attempts in the future.
1817 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
1818 // If we acquired the lock early in the spin cycle it
1819 // makes sense to increase _SpinDuration proportionally.
1820 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1821 int x = _SpinDuration;
1822 if (x < Knob_SpinLimit) {
1823 if (x < Knob_Poverty) x = Knob_Poverty;
1824 _SpinDuration = x + Knob_Bonus;
1825 }
1826 return 1;
1827 }
1828
1829 // The CAS failed ... we can take any of the following actions:
1830 // * penalize: ctr -= CASPenalty
1831 // * exit spin with prejudice -- goto Abort;
1832 // * exit spin without prejudice.
1833 // * Since CAS is high-latency, retry again immediately.
1834 prv = ox;
1835 goto Abort;
1836 }
1837
1838 // Did lock ownership change hands ?
1839 if (ox != prv && prv != NULL) {
1840 goto Abort;
1841 }
1842 prv = ox;
1843
1844 // Abort the spin if the owner is not executing.
1845 // The owner must be executing in order to drop the lock.
1846 // Spinning while the owner is OFFPROC is idiocy.
1847 // Consider: ctr -= RunnablePenalty ;
1848 if (NotRunnable(Self, ox)) {
1849 goto Abort;
1850 }
1851 if (_succ == NULL) {
1852 _succ = Self;
1853 }
1854 }
1855
1856 // Spin failed with prejudice -- reduce _SpinDuration.
1857 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
1858 // AIMD is globally stable.
1859 {
1860 int x = _SpinDuration;
1861 if (x > 0) {
1862 // Consider an AIMD scheme like: x -= (x >> 3) + 100
1863 // This is globally sample and tends to damp the response.
1864 x -= Knob_Penalty;
1865 if (x < 0) x = 0;
1866 _SpinDuration = x;
1867 }
1868 }
1869
1870 Abort:
1871 if (_succ == Self) {
1872 _succ = NULL;
1873 // Invariant: after setting succ=null a contending thread
1874 // must recheck-retry _owner before parking. This usually happens
1875 // in the normal usage of TrySpin(), but it's safest
1876 // to make TrySpin() as foolproof as possible.
1877 OrderAccess::fence();
1878 if (TryLock(Self) > 0) return 1;
1879 }
1880 return 0;
1881 }
1882
1883 // NotRunnable() -- informed spinning
1884 //
1885 // Don't bother spinning if the owner is not eligible to drop the lock.
1886 // Spin only if the owner thread is _thread_in_Java or _thread_in_vm.
1887 // The thread must be runnable in order to drop the lock in timely fashion.
1888 // If the _owner is not runnable then spinning will not likely be
1889 // successful (profitable).
1890 //
1891 // Beware -- the thread referenced by _owner could have died
1892 // so a simply fetch from _owner->_thread_state might trap.
1893 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
1894 // Because of the lifecycle issues, the _thread_state values
1895 // observed by NotRunnable() might be garbage. NotRunnable must
1896 // tolerate this and consider the observed _thread_state value
1897 // as advisory.
1898 //
1899 // Beware too, that _owner is sometimes a BasicLock address and sometimes
1900 // a thread pointer.
1901 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
1902 // with the LSB of _owner. Another option would be to probabilistically probe
1903 // the putative _owner->TypeTag value.
1904 //
1905 // Checking _thread_state isn't perfect. Even if the thread is
1906 // in_java it might be blocked on a page-fault or have been preempted
1907 // and sitting on a ready/dispatch queue.
1908 //
1909 // The return value from NotRunnable() is *advisory* -- the
1910 // result is based on sampling and is not necessarily coherent.
1911 // The caller must tolerate false-negative and false-positive errors.
1912 // Spinning, in general, is probabilistic anyway.
1913
1914
1915 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) {
1916 // Check ox->TypeTag == 2BAD.
1917 if (ox == NULL) return 0;
1918
1919 // Avoid transitive spinning ...
1920 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
1921 // Immediately after T1 acquires L it's possible that T2, also
1922 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
1923 // This occurs transiently after T1 acquired L but before
1924 // T1 managed to clear T1.Stalled. T2 does not need to abort
1925 // its spin in this circumstance.
1926 intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
1927
1928 if (BlockedOn == 1) return 1;
1929 if (BlockedOn != 0) {
1930 return BlockedOn != intptr_t(this) && _owner == ox;
1931 }
1932
1933 assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant");
1934 int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);;
1935 // consider also: jst != _thread_in_Java -- but that's overspecific.
1936 return jst == _thread_blocked || jst == _thread_in_native;
1937 }
1938
1939
1940 // -----------------------------------------------------------------------------
1941 // WaitSet management ...
1942
1943 ObjectWaiter::ObjectWaiter(Thread* thread) {
1944 _next = NULL;
1945 _prev = NULL;
1946 _notified = 0;
1947 _notifier_tid = 0;
1948 TState = TS_RUN;
1949 _thread = thread;
1950 _event = thread->_ParkEvent;
1951 _active = false;
1952 assert(_event != NULL, "invariant");
1953 }
1954
1955 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
1956 JavaThread *jt = (JavaThread *)this->_thread;
1957 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
1958 }
1959
1960 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
1961 JavaThread *jt = (JavaThread *)this->_thread;
1962 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
1963 }
1964
1965 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
1966 assert(node != NULL, "should not add NULL node");
1967 assert(node->_prev == NULL, "node already in list");
1968 assert(node->_next == NULL, "node already in list");
1969 // put node at end of queue (circular doubly linked list)
1970 if (_WaitSet == NULL) {
1971 _WaitSet = node;
1972 node->_prev = node;
1973 node->_next = node;
1974 } else {
1975 ObjectWaiter* head = _WaitSet;
1976 ObjectWaiter* tail = head->_prev;
1977 assert(tail->_next == head, "invariant check");
1978 tail->_next = node;
1979 head->_prev = node;
1980 node->_next = head;
1981 node->_prev = tail;
1982 }
1983 }
1984
1985 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
1986 // dequeue the very first waiter
1987 ObjectWaiter* waiter = _WaitSet;
1988 if (waiter) {
1989 DequeueSpecificWaiter(waiter);
1990 }
1991 return waiter;
1992 }
1993
1994 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
1995 assert(node != NULL, "should not dequeue NULL node");
1996 assert(node->_prev != NULL, "node already removed from list");
1997 assert(node->_next != NULL, "node already removed from list");
1998 // when the waiter has woken up because of interrupt,
1999 // timeout or other spurious wake-up, dequeue the
2000 // waiter from waiting list
2001 ObjectWaiter* next = node->_next;
2002 if (next == node) {
2003 assert(node->_prev == node, "invariant check");
2004 _WaitSet = NULL;
2005 } else {
2006 ObjectWaiter* prev = node->_prev;
2007 assert(prev->_next == node, "invariant check");
2008 assert(next->_prev == node, "invariant check");
2009 next->_prev = prev;
2010 prev->_next = next;
2011 if (_WaitSet == node) {
2012 _WaitSet = next;
2013 }
2014 }
2015 node->_next = NULL;
2016 node->_prev = NULL;
2017 }
2018
2019 // -----------------------------------------------------------------------------
2020 // PerfData support
2021 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL;
2022 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL;
2023 PerfCounter * ObjectMonitor::_sync_Parks = NULL;
2024 PerfCounter * ObjectMonitor::_sync_Notifications = NULL;
2025 PerfCounter * ObjectMonitor::_sync_Inflations = NULL;
2026 PerfCounter * ObjectMonitor::_sync_Deflations = NULL;
2027 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL;
2028
2029 // One-shot global initialization for the sync subsystem.
2030 // We could also defer initialization and initialize on-demand
2031 // the first time we call ObjectSynchronizer::inflate().
2032 // Initialization would be protected - like so many things - by
2033 // the MonitorCache_lock.
2034
2035 void ObjectMonitor::Initialize() {
2036 assert(!InitDone, "invariant");
2037
2038 if (!os::is_MP()) {
2039 Knob_SpinLimit = 0;
2040 Knob_PreSpin = 0;
2041 Knob_FixedSpin = -1;
2042 }
2043
2044 if (UsePerfData) {
2045 EXCEPTION_MARK;
2046 #define NEWPERFCOUNTER(n) \
2047 { \
2048 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \
2049 CHECK); \
2050 }
2051 #define NEWPERFVARIABLE(n) \
2052 { \
2053 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \
2054 CHECK); \
2055 }
2056 NEWPERFCOUNTER(_sync_Inflations);
2057 NEWPERFCOUNTER(_sync_Deflations);
2058 NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2059 NEWPERFCOUNTER(_sync_FutileWakeups);
2060 NEWPERFCOUNTER(_sync_Parks);
2061 NEWPERFCOUNTER(_sync_Notifications);
2062 NEWPERFVARIABLE(_sync_MonExtant);
2063 #undef NEWPERFCOUNTER
2064 #undef NEWPERFVARIABLE
2065 }
2066
2067 DEBUG_ONLY(InitDone = true;)
2068 }
2069
2070 // For internal use by ObjectSynchronizer::monitors_iterate().
2071 ObjectMonitorHandle::ObjectMonitorHandle(ObjectMonitor * om_ptr) {
2072 om_ptr->inc_ref_count();
2073 _om_ptr = om_ptr;
2074 }
2075
2076 ObjectMonitorHandle::~ObjectMonitorHandle() {
2077 if (_om_ptr != NULL) {
2078 _om_ptr->dec_ref_count();
2079 _om_ptr = NULL;
2080 }
2081 }
2082
2083 // Save the ObjectMonitor* associated with the specified markWord and
2084 // increment the ref_count. This function should only be called if
2085 // the caller has verified mark.has_monitor() == true. The object
2086 // parameter is needed to verify that ObjectMonitor* has not been
2087 // deflated and reused for another object.
2088 //
2089 // This function returns true if the ObjectMonitor* has been safely
2090 // saved. This function returns false if we have lost a race with
2091 // async deflation; the caller should retry as appropriate.
2092 //
2093 bool ObjectMonitorHandle::save_om_ptr(oop object, markWord mark) {
2094 guarantee(mark.has_monitor(), "sanity check: mark=" INTPTR_FORMAT,
2095 mark.value());
2096
2097 ObjectMonitor * om_ptr = mark.monitor();
2098 om_ptr->inc_ref_count();
2099
2100 if (AsyncDeflateIdleMonitors) {
2101 // Race here if monitor is not owned! The above ref_count bump
2102 // will cause subsequent async deflation to skip it. However,
2103 // previous or concurrent async deflation is a race.
2104 if (om_ptr->owner_is_DEFLATER_MARKER() && om_ptr->ref_count() <= 0) {
2105 // Async deflation is in progress and our ref_count increment
2106 // above lost the race to async deflation. Attempt to restore
2107 // the header/dmw to the object's header so that we only retry
2108 // once if the deflater thread happens to be slow.
2109 om_ptr->install_displaced_markword_in_object(object);
2110 om_ptr->dec_ref_count();
2111 return false;
2112 }
2113 if (om_ptr->ref_count() <= 0) {
2114 // Async deflation is in the process of bailing out, but has not
2115 // yet restored the ref_count field so we return false to force
2116 // a retry. We want a positive ref_count value for a true return.
2117 om_ptr->dec_ref_count();
2118 return false;
2119 }
2120 // The ObjectMonitor could have been deflated and reused for
2121 // another object before we bumped the ref_count so make sure
2122 // our object still refers to this ObjectMonitor.
2123 const markWord tmp = object->mark();
2124 if (!tmp.has_monitor() || tmp.monitor() != om_ptr) {
2125 // Async deflation and reuse won the race so we have to retry.
2126 // Skip object header restoration since that's already done.
2127 om_ptr->dec_ref_count();
2128 return false;
2129 }
2130 }
2131
2132 ADIM_guarantee(_om_ptr == NULL, "sanity check: _om_ptr=" INTPTR_FORMAT,
2133 p2i(_om_ptr));
2134 _om_ptr = om_ptr;
2135 return true;
2136 }
2137
2138 // For internal use by ObjectSynchronizer::inflate().
2139 void ObjectMonitorHandle::set_om_ptr(ObjectMonitor * om_ptr) {
2140 if (_om_ptr == NULL) {
2141 ADIM_guarantee(om_ptr != NULL, "cannot clear an unset om_ptr");
2142 om_ptr->inc_ref_count();
2143 _om_ptr = om_ptr;
2144 } else {
2145 ADIM_guarantee(om_ptr == NULL, "can only clear a set om_ptr");
2146 _om_ptr->dec_ref_count();
2147 _om_ptr = NULL;
2148 }
2149 }
2150
2151 void ObjectMonitor::print_on(outputStream* st) const {
2152 // The minimal things to print for markWord printing, more can be added for debugging and logging.
2153 st->print("{contentions=0x%08x,waiters=0x%08x"
2154 ",recursions=" INTX_FORMAT ",owner=" INTPTR_FORMAT "}",
2155 contentions(), waiters(), recursions(),
2156 p2i(owner()));
2157 }
2158 void ObjectMonitor::print() const { print_on(tty); }