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=" INTPTR_FORMAT,
291 _recursions);
292 assert(((oop)object())->mark() == markWord::encode(this),
293 "object mark must match encoded this: mark=" INTPTR_FORMAT
294 ", encoded this=" INTPTR_FORMAT, ((oop)object())->mark().value(),
295 markWord::encode(this).value());
296 Self->_Stalled = 0;
297 return;
298 }
299
300 assert(_owner != Self, "invariant");
301 assert(_succ != Self, "invariant");
302 assert(Self->is_Java_thread(), "invariant");
303 JavaThread * jt = (JavaThread *) Self;
304 assert(!SafepointSynchronize::is_at_safepoint(), "invariant");
305 assert(jt->thread_state() != _thread_blocked, "invariant");
306 assert(AsyncDeflateIdleMonitors || this->object() != NULL, "invariant");
307 assert(_contentions >= 0, "must not be negative: contentions=%d", _contentions);
308
309 // Prevent deflation. See ObjectSynchronizer::deflate_monitor(),
310 // ObjectSynchronizer::deflate_monitor_using_JT() and is_busy().
311 // Ensure the object <-> monitor relationship remains stable while
312 // there's contention.
313 Atomic::add(1, &_contentions);
314
315 JFR_ONLY(JfrConditionalFlushWithStacktrace<EventJavaMonitorEnter> flush(jt);)
316 EventJavaMonitorEnter event;
317 if (event.should_commit()) {
318 event.set_monitorClass(((oop)this->object())->klass());
319 event.set_address((uintptr_t)(this->object_addr()));
320 }
321
322 { // Change java thread status to indicate blocked on monitor enter.
323 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
324
325 Self->set_current_pending_monitor(this);
326
327 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
328 if (JvmtiExport::should_post_monitor_contended_enter()) {
329 JvmtiExport::post_monitor_contended_enter(jt, this);
330
331 // The current thread does not yet own the monitor and does not
332 // yet appear on any queues that would get it made the successor.
333 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
334 // handler cannot accidentally consume an unpark() meant for the
335 // ParkEvent associated with this ObjectMonitor.
336 }
337
338 OSThreadContendState osts(Self->osthread());
339 ThreadBlockInVM tbivm(jt);
340
341 // TODO-FIXME: change the following for(;;) loop to straight-line code.
342 for (;;) {
343 jt->set_suspend_equivalent();
344 // cleared by handle_special_suspend_equivalent_condition()
345 // or java_suspend_self()
346
347 EnterI(THREAD);
348
349 if (!ExitSuspendEquivalent(jt)) break;
350
351 // We have acquired the contended monitor, but while we were
352 // waiting another thread suspended us. We don't want to enter
353 // the monitor while suspended because that would surprise the
354 // thread that suspended us.
355 //
356 _recursions = 0;
357 _succ = NULL;
358 exit(false, Self);
359
360 jt->java_suspend_self();
361 }
362 Self->set_current_pending_monitor(NULL);
363
364 // We cleared the pending monitor info since we've just gotten past
365 // the enter-check-for-suspend dance and we now own the monitor free
366 // and clear, i.e., it is no longer pending. The ThreadBlockInVM
367 // destructor can go to a safepoint at the end of this block. If we
368 // do a thread dump during that safepoint, then this thread will show
369 // as having "-locked" the monitor, but the OS and java.lang.Thread
370 // states will still report that the thread is blocked trying to
371 // acquire it.
372 }
373
374 Atomic::dec(&_contentions);
375 assert(_contentions >= 0, "must not be negative: contentions=%d", _contentions);
376 Self->_Stalled = 0;
377
378 // Must either set _recursions = 0 or ASSERT _recursions == 0.
379 assert(_recursions == 0, "invariant");
380 assert(_owner == Self, "invariant");
381 assert(_succ != Self, "invariant");
382 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant");
383
384 // The thread -- now the owner -- is back in vm mode.
385 // Report the glorious news via TI,DTrace and jvmstat.
386 // The probe effect is non-trivial. All the reportage occurs
387 // while we hold the monitor, increasing the length of the critical
388 // section. Amdahl's parallel speedup law comes vividly into play.
389 //
390 // Another option might be to aggregate the events (thread local or
391 // per-monitor aggregation) and defer reporting until a more opportune
392 // time -- such as next time some thread encounters contention but has
393 // yet to acquire the lock. While spinning that thread could
394 // spinning we could increment JVMStat counters, etc.
395
396 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
397 if (JvmtiExport::should_post_monitor_contended_entered()) {
398 JvmtiExport::post_monitor_contended_entered(jt, this);
399
400 // The current thread already owns the monitor and is not going to
401 // call park() for the remainder of the monitor enter protocol. So
402 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
403 // event handler consumed an unpark() issued by the thread that
404 // just exited the monitor.
405 }
406 if (event.should_commit()) {
407 event.set_previousOwner((uintptr_t)_previous_owner_tid);
408 event.commit();
409 }
410 OM_PERFDATA_OP(ContendedLockAttempts, inc());
411 }
412
413 // Caveat: TryLock() is not necessarily serializing if it returns failure.
414 // Callers must compensate as needed.
415
416 int ObjectMonitor::TryLock(Thread * Self) {
417 void * own = _owner;
418 if (own != NULL) return 0;
419 if (Atomic::replace_if_null(Self, &_owner)) {
420 assert(_recursions == 0, "invariant");
421 return 1;
422 }
423 // The lock had been free momentarily, but we lost the race to the lock.
424 // Interference -- the CAS failed.
425 // We can either return -1 or retry.
426 // Retry doesn't make as much sense because the lock was just acquired.
427 return -1;
428 }
429
430 // Install the displaced mark word (dmw) of a deflating ObjectMonitor
431 // into the header of the object associated with the monitor. This
432 // idempotent method is called by a thread that is deflating a
433 // monitor and by other threads that have detected a race with the
434 // deflation process.
435 void ObjectMonitor::install_displaced_markword_in_object(const oop obj) {
436 // This function must only be called when (owner == DEFLATER_MARKER
437 // && ref_count <= 0), but we can't guarantee that here because
438 // those values could change when the ObjectMonitor gets moved from
439 // the global free list to a per-thread free list.
440
441 guarantee(obj != NULL, "must be non-NULL");
442 if (object() != obj) {
443 // ObjectMonitor's object ref no longer refers to the target object
444 // so the object's header has already been restored.
445 return;
446 }
447
448 markWord dmw = header();
449 if (dmw.value() == 0) {
450 // ObjectMonitor's header/dmw has been cleared so the object's
451 // header has already been restored.
452 return;
453 }
454
455 // A non-NULL dmw has to be either neutral (not locked and not marked)
456 // or is already participating in this restoration protocol.
457 assert(dmw.is_neutral() || (dmw.is_marked() && dmw.hash() == 0),
458 "failed precondition: dmw=" INTPTR_FORMAT, dmw.value());
459
460 markWord marked_dmw = markWord::zero();
461 if (!dmw.is_marked() && dmw.hash() == 0) {
462 // This dmw has not yet started the restoration protocol so we
463 // mark a copy of the dmw to begin the protocol.
464 // Note: A dmw with a hashcode does not take this code path.
465 marked_dmw = dmw.set_marked();
466
467 // All of the callers to this function can be racing with each
468 // other trying to update the _header field.
469 dmw = (markWord) Atomic::cmpxchg(marked_dmw, &_header, dmw);
470 if (dmw.value() == 0) {
471 // ObjectMonitor's header/dmw has been cleared so the object's
472 // header has already been restored.
473 return;
474 }
475 // The _header field is now marked. The winner's 'dmw' variable
476 // contains the original, unmarked header/dmw value and any
477 // losers have a marked header/dmw value that will be cleaned
478 // up below.
479 }
480
481 if (dmw.is_marked()) {
482 // Clear the mark from the header/dmw copy in preparation for
483 // possible restoration from this thread.
484 assert(dmw.hash() == 0, "hashcode must be 0: dmw=" INTPTR_FORMAT,
485 dmw.value());
486 dmw = dmw.set_unmarked();
487 }
488 assert(dmw.is_neutral(), "must be neutral: dmw=" INTPTR_FORMAT, dmw.value());
489
490 // Install displaced mark word if the object's header still points
491 // to this ObjectMonitor. All racing callers to this function will
492 // reach this point, but only one can win.
493 obj->cas_set_mark(dmw, markWord::encode(this));
494
495 // Note: It does not matter which thread restored the header/dmw
496 // into the object's header. The thread deflating the monitor just
497 // wanted the object's header restored and it is. The threads that
498 // detected a race with the deflation process also wanted the
499 // object's header restored before they retry their operation and
500 // because it is restored they will only retry once.
501
502 if (marked_dmw.value() != 0) {
503 // Clear _header to NULL if it is still marked_dmw so a racing
504 // install_displaced_markword_in_object() can bail out sooner.
505 Atomic::cmpxchg(markWord::zero(), &_header, marked_dmw);
506 }
507 }
508
509 // Convert the fields used by is_busy() to a string that can be
510 // used for diagnostic output.
511 const char* ObjectMonitor::is_busy_to_string(stringStream* ss) {
512 ss->print("is_busy: contentions=%d, waiters=%d, ", _contentions, _waiters);
513 if (!AsyncDeflateIdleMonitors) {
514 ss->print("owner=" INTPTR_FORMAT, p2i(_owner));
515 } else if (_owner != DEFLATER_MARKER) {
516 ss->print("owner=" INTPTR_FORMAT, p2i(_owner));
517 } else {
518 ss->print("owner=" INTPTR_FORMAT, NULL);
519 }
520 ss->print(", cxq=" INTPTR_FORMAT ", EntryList=" INTPTR_FORMAT, p2i(_cxq),
521 p2i(_EntryList));
522 return ss->base();
523 }
524
525 #define MAX_RECHECK_INTERVAL 1000
526
527 void ObjectMonitor::EnterI(TRAPS) {
528 ADIM_guarantee(ref_count() > 0, "must be positive: ref_count=%d", ref_count());
529
530 Thread * const Self = THREAD;
531 assert(Self->is_Java_thread(), "invariant");
532 assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant");
533
534 // Try the lock - TATAS
535 if (TryLock (Self) > 0) {
536 assert(_succ != Self, "invariant");
537 assert(_owner == Self, "invariant");
538 assert(_Responsible != Self, "invariant");
539 return;
540 }
541
542 if (AsyncDeflateIdleMonitors &&
543 Atomic::cmpxchg(Self, &_owner, DEFLATER_MARKER) == DEFLATER_MARKER) {
544 // The deflation protocol finished the first part (setting owner),
545 // but it failed the second part (making ref_count negative) and
546 // bailed. Or the ObjectMonitor was async deflated and reused.
547 // Acquired the monitor.
548 assert(_succ != Self, "invariant");
549 assert(_Responsible != Self, "invariant");
550 return;
551 }
552
553 assert(InitDone, "Unexpectedly not initialized");
554
555 // We try one round of spinning *before* enqueueing Self.
556 //
557 // If the _owner is ready but OFFPROC we could use a YieldTo()
558 // operation to donate the remainder of this thread's quantum
559 // to the owner. This has subtle but beneficial affinity
560 // effects.
561
562 if (TrySpin(Self) > 0) {
563 assert(_owner == Self, "invariant");
564 assert(_succ != Self, "invariant");
565 assert(_Responsible != Self, "invariant");
566 return;
567 }
568
569 // The Spin failed -- Enqueue and park the thread ...
570 assert(_succ != Self, "invariant");
571 assert(_owner != Self, "invariant");
572 assert(_Responsible != Self, "invariant");
573
574 // Enqueue "Self" on ObjectMonitor's _cxq.
575 //
576 // Node acts as a proxy for Self.
577 // As an aside, if were to ever rewrite the synchronization code mostly
578 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class
579 // Java objects. This would avoid awkward lifecycle and liveness issues,
580 // as well as eliminate a subset of ABA issues.
581 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events.
582
583 ObjectWaiter node(Self);
584 Self->_ParkEvent->reset();
585 node._prev = (ObjectWaiter *) 0xBAD;
586 node.TState = ObjectWaiter::TS_CXQ;
587
588 // Push "Self" onto the front of the _cxq.
589 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock.
590 // Note that spinning tends to reduce the rate at which threads
591 // enqueue and dequeue on EntryList|cxq.
592 ObjectWaiter * nxt;
593 for (;;) {
594 node._next = nxt = _cxq;
595 if (Atomic::cmpxchg(&node, &_cxq, nxt) == nxt) break;
596
597 // Interference - the CAS failed because _cxq changed. Just retry.
598 // As an optional optimization we retry the lock.
599 if (TryLock (Self) > 0) {
600 assert(_succ != Self, "invariant");
601 assert(_owner == Self, "invariant");
602 assert(_Responsible != Self, "invariant");
603 return;
604 }
605 }
606
607 // Check for cxq|EntryList edge transition to non-null. This indicates
608 // the onset of contention. While contention persists exiting threads
609 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit
610 // operations revert to the faster 1-0 mode. This enter operation may interleave
611 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we
612 // arrange for one of the contending thread to use a timed park() operations
613 // to detect and recover from the race. (Stranding is form of progress failure
614 // where the monitor is unlocked but all the contending threads remain parked).
615 // That is, at least one of the contended threads will periodically poll _owner.
616 // One of the contending threads will become the designated "Responsible" thread.
617 // The Responsible thread uses a timed park instead of a normal indefinite park
618 // operation -- it periodically wakes and checks for and recovers from potential
619 // strandings admitted by 1-0 exit operations. We need at most one Responsible
620 // thread per-monitor at any given moment. Only threads on cxq|EntryList may
621 // be responsible for a monitor.
622 //
623 // Currently, one of the contended threads takes on the added role of "Responsible".
624 // A viable alternative would be to use a dedicated "stranding checker" thread
625 // that periodically iterated over all the threads (or active monitors) and unparked
626 // successors where there was risk of stranding. This would help eliminate the
627 // timer scalability issues we see on some platforms as we'd only have one thread
628 // -- the checker -- parked on a timer.
629
630 if (nxt == NULL && _EntryList == NULL) {
631 // Try to assume the role of responsible thread for the monitor.
632 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self }
633 Atomic::replace_if_null(Self, &_Responsible);
634 }
635
636 // The lock might have been released while this thread was occupied queueing
637 // itself onto _cxq. To close the race and avoid "stranding" and
638 // progress-liveness failure we must resample-retry _owner before parking.
639 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner.
640 // In this case the ST-MEMBAR is accomplished with CAS().
641 //
642 // TODO: Defer all thread state transitions until park-time.
643 // Since state transitions are heavy and inefficient we'd like
644 // to defer the state transitions until absolutely necessary,
645 // and in doing so avoid some transitions ...
646
647 int nWakeups = 0;
648 int recheckInterval = 1;
649
650 for (;;) {
651
652 if (TryLock(Self) > 0) break;
653 assert(_owner != Self, "invariant");
654
655 // park self
656 if (_Responsible == Self) {
657 Self->_ParkEvent->park((jlong) recheckInterval);
658 // Increase the recheckInterval, but clamp the value.
659 recheckInterval *= 8;
660 if (recheckInterval > MAX_RECHECK_INTERVAL) {
661 recheckInterval = MAX_RECHECK_INTERVAL;
662 }
663 } else {
664 Self->_ParkEvent->park();
665 }
666
667 if (TryLock(Self) > 0) break;
668
669 if (AsyncDeflateIdleMonitors &&
670 Atomic::cmpxchg(Self, &_owner, DEFLATER_MARKER) == DEFLATER_MARKER) {
671 // The deflation protocol finished the first part (setting owner),
672 // but it failed the second part (making ref_count negative) and
673 // bailed. Or the ObjectMonitor was async deflated and reused.
674 // Acquired the monitor.
675 break;
676 }
677
678 // The lock is still contested.
679 // Keep a tally of the # of futile wakeups.
680 // Note that the counter is not protected by a lock or updated by atomics.
681 // That is by design - we trade "lossy" counters which are exposed to
682 // races during updates for a lower probe effect.
683
684 // This PerfData object can be used in parallel with a safepoint.
685 // See the work around in PerfDataManager::destroy().
686 OM_PERFDATA_OP(FutileWakeups, inc());
687 ++nWakeups;
688
689 // Assuming this is not a spurious wakeup we'll normally find _succ == Self.
690 // We can defer clearing _succ until after the spin completes
691 // TrySpin() must tolerate being called with _succ == Self.
692 // Try yet another round of adaptive spinning.
693 if (TrySpin(Self) > 0) break;
694
695 // We can find that we were unpark()ed and redesignated _succ while
696 // we were spinning. That's harmless. If we iterate and call park(),
697 // park() will consume the event and return immediately and we'll
698 // just spin again. This pattern can repeat, leaving _succ to simply
699 // spin on a CPU.
700
701 if (_succ == Self) _succ = NULL;
702
703 // Invariant: after clearing _succ a thread *must* retry _owner before parking.
704 OrderAccess::fence();
705 }
706
707 // Egress :
708 // Self has acquired the lock -- Unlink Self from the cxq or EntryList.
709 // Normally we'll find Self on the EntryList .
710 // From the perspective of the lock owner (this thread), the
711 // EntryList is stable and cxq is prepend-only.
712 // The head of cxq is volatile but the interior is stable.
713 // In addition, Self.TState is stable.
714
715 assert(_owner == Self, "invariant");
716 assert(object() != NULL, "invariant");
717 // I'd like to write:
718 // guarantee (((oop)(object()))->mark() == markWord::encode(this), "invariant") ;
719 // but as we're at a safepoint that's not safe.
720
721 UnlinkAfterAcquire(Self, &node);
722 if (_succ == Self) _succ = NULL;
723
724 assert(_succ != Self, "invariant");
725 if (_Responsible == Self) {
726 _Responsible = NULL;
727 OrderAccess::fence(); // Dekker pivot-point
728
729 // We may leave threads on cxq|EntryList without a designated
730 // "Responsible" thread. This is benign. When this thread subsequently
731 // exits the monitor it can "see" such preexisting "old" threads --
732 // threads that arrived on the cxq|EntryList before the fence, above --
733 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads
734 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible
735 // non-null and elect a new "Responsible" timer thread.
736 //
737 // This thread executes:
738 // ST Responsible=null; MEMBAR (in enter epilogue - here)
739 // LD cxq|EntryList (in subsequent exit)
740 //
741 // Entering threads in the slow/contended path execute:
742 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog)
743 // The (ST cxq; MEMBAR) is accomplished with CAS().
744 //
745 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent
746 // exit operation from floating above the ST Responsible=null.
747 }
748
749 // We've acquired ownership with CAS().
750 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics.
751 // But since the CAS() this thread may have also stored into _succ,
752 // EntryList, cxq or Responsible. These meta-data updates must be
753 // visible __before this thread subsequently drops the lock.
754 // Consider what could occur if we didn't enforce this constraint --
755 // STs to monitor meta-data and user-data could reorder with (become
756 // visible after) the ST in exit that drops ownership of the lock.
757 // Some other thread could then acquire the lock, but observe inconsistent
758 // or old monitor meta-data and heap data. That violates the JMM.
759 // To that end, the 1-0 exit() operation must have at least STST|LDST
760 // "release" barrier semantics. Specifically, there must be at least a
761 // STST|LDST barrier in exit() before the ST of null into _owner that drops
762 // the lock. The barrier ensures that changes to monitor meta-data and data
763 // protected by the lock will be visible before we release the lock, and
764 // therefore before some other thread (CPU) has a chance to acquire the lock.
765 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html.
766 //
767 // Critically, any prior STs to _succ or EntryList must be visible before
768 // the ST of null into _owner in the *subsequent* (following) corresponding
769 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily
770 // execute a serializing instruction.
771
772 return;
773 }
774
775 // ReenterI() is a specialized inline form of the latter half of the
776 // contended slow-path from EnterI(). We use ReenterI() only for
777 // monitor reentry in wait().
778 //
779 // In the future we should reconcile EnterI() and ReenterI().
780
781 void ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) {
782 ADIM_guarantee(ref_count() > 0, "must be positive: ref_count=%d", ref_count());
783
784 assert(Self != NULL, "invariant");
785 assert(SelfNode != NULL, "invariant");
786 assert(SelfNode->_thread == Self, "invariant");
787 assert(_waiters > 0, "invariant");
788 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant");
789 assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant");
790 JavaThread * jt = (JavaThread *) Self;
791
792 int nWakeups = 0;
793 for (;;) {
794 ObjectWaiter::TStates v = SelfNode->TState;
795 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
796 assert(_owner != Self, "invariant");
797
798 if (TryLock(Self) > 0) break;
799 if (TrySpin(Self) > 0) break;
800
801 if (AsyncDeflateIdleMonitors &&
802 Atomic::cmpxchg(Self, &_owner, DEFLATER_MARKER) == DEFLATER_MARKER) {
803 // The deflation protocol finished the first part (setting owner),
804 // but it failed the second part (making ref_count negative) and
805 // bailed. Or the ObjectMonitor was async deflated and reused.
806 // Acquired the monitor.
807 break;
808 }
809
810 // State transition wrappers around park() ...
811 // ReenterI() wisely defers state transitions until
812 // it's clear we must park the thread.
813 {
814 OSThreadContendState osts(Self->osthread());
815 ThreadBlockInVM tbivm(jt);
816
817 // cleared by handle_special_suspend_equivalent_condition()
818 // or java_suspend_self()
819 jt->set_suspend_equivalent();
820 Self->_ParkEvent->park();
821
822 // were we externally suspended while we were waiting?
823 for (;;) {
824 if (!ExitSuspendEquivalent(jt)) break;
825 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); }
826 jt->java_suspend_self();
827 jt->set_suspend_equivalent();
828 }
829 }
830
831 // Try again, but just so we distinguish between futile wakeups and
832 // successful wakeups. The following test isn't algorithmically
833 // necessary, but it helps us maintain sensible statistics.
834 if (TryLock(Self) > 0) break;
835
836 // The lock is still contested.
837 // Keep a tally of the # of futile wakeups.
838 // Note that the counter is not protected by a lock or updated by atomics.
839 // That is by design - we trade "lossy" counters which are exposed to
840 // races during updates for a lower probe effect.
841 ++nWakeups;
842
843 // Assuming this is not a spurious wakeup we'll normally
844 // find that _succ == Self.
845 if (_succ == Self) _succ = NULL;
846
847 // Invariant: after clearing _succ a contending thread
848 // *must* retry _owner before parking.
849 OrderAccess::fence();
850
851 // This PerfData object can be used in parallel with a safepoint.
852 // See the work around in PerfDataManager::destroy().
853 OM_PERFDATA_OP(FutileWakeups, inc());
854 }
855
856 // Self has acquired the lock -- Unlink Self from the cxq or EntryList .
857 // Normally we'll find Self on the EntryList.
858 // Unlinking from the EntryList is constant-time and atomic-free.
859 // From the perspective of the lock owner (this thread), the
860 // EntryList is stable and cxq is prepend-only.
861 // The head of cxq is volatile but the interior is stable.
862 // In addition, Self.TState is stable.
863
864 assert(_owner == Self, "invariant");
865 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant");
866 UnlinkAfterAcquire(Self, SelfNode);
867 if (_succ == Self) _succ = NULL;
868 assert(_succ != Self, "invariant");
869 SelfNode->TState = ObjectWaiter::TS_RUN;
870 OrderAccess::fence(); // see comments at the end of EnterI()
871 }
872
873 // By convention we unlink a contending thread from EntryList|cxq immediately
874 // after the thread acquires the lock in ::enter(). Equally, we could defer
875 // unlinking the thread until ::exit()-time.
876
877 void ObjectMonitor::UnlinkAfterAcquire(Thread *Self, ObjectWaiter *SelfNode) {
878 assert(_owner == Self, "invariant");
879 assert(SelfNode->_thread == Self, "invariant");
880
881 if (SelfNode->TState == ObjectWaiter::TS_ENTER) {
882 // Normal case: remove Self from the DLL EntryList .
883 // This is a constant-time operation.
884 ObjectWaiter * nxt = SelfNode->_next;
885 ObjectWaiter * prv = SelfNode->_prev;
886 if (nxt != NULL) nxt->_prev = prv;
887 if (prv != NULL) prv->_next = nxt;
888 if (SelfNode == _EntryList) _EntryList = nxt;
889 assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant");
890 assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant");
891 } else {
892 assert(SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant");
893 // Inopportune interleaving -- Self is still on the cxq.
894 // This usually means the enqueue of self raced an exiting thread.
895 // Normally we'll find Self near the front of the cxq, so
896 // dequeueing is typically fast. If needbe we can accelerate
897 // this with some MCS/CHL-like bidirectional list hints and advisory
898 // back-links so dequeueing from the interior will normally operate
899 // in constant-time.
900 // Dequeue Self from either the head (with CAS) or from the interior
901 // with a linear-time scan and normal non-atomic memory operations.
902 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList
903 // and then unlink Self from EntryList. We have to drain eventually,
904 // so it might as well be now.
905
906 ObjectWaiter * v = _cxq;
907 assert(v != NULL, "invariant");
908 if (v != SelfNode || Atomic::cmpxchg(SelfNode->_next, &_cxq, v) != v) {
909 // The CAS above can fail from interference IFF a "RAT" arrived.
910 // In that case Self must be in the interior and can no longer be
911 // at the head of cxq.
912 if (v == SelfNode) {
913 assert(_cxq != v, "invariant");
914 v = _cxq; // CAS above failed - start scan at head of list
915 }
916 ObjectWaiter * p;
917 ObjectWaiter * q = NULL;
918 for (p = v; p != NULL && p != SelfNode; p = p->_next) {
919 q = p;
920 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant");
921 }
922 assert(v != SelfNode, "invariant");
923 assert(p == SelfNode, "Node not found on cxq");
924 assert(p != _cxq, "invariant");
925 assert(q != NULL, "invariant");
926 assert(q->_next == p, "invariant");
927 q->_next = p->_next;
928 }
929 }
930
931 #ifdef ASSERT
932 // Diagnostic hygiene ...
933 SelfNode->_prev = (ObjectWaiter *) 0xBAD;
934 SelfNode->_next = (ObjectWaiter *) 0xBAD;
935 SelfNode->TState = ObjectWaiter::TS_RUN;
936 #endif
937 }
938
939 // -----------------------------------------------------------------------------
940 // Exit support
941 //
942 // exit()
943 // ~~~~~~
944 // Note that the collector can't reclaim the objectMonitor or deflate
945 // the object out from underneath the thread calling ::exit() as the
946 // thread calling ::exit() never transitions to a stable state.
947 // This inhibits GC, which in turn inhibits asynchronous (and
948 // inopportune) reclamation of "this".
949 //
950 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ;
951 // There's one exception to the claim above, however. EnterI() can call
952 // exit() to drop a lock if the acquirer has been externally suspended.
953 // In that case exit() is called with _thread_state as _thread_blocked,
954 // but the monitor's _contentions field is > 0, which inhibits reclamation.
955 //
956 // 1-0 exit
957 // ~~~~~~~~
958 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of
959 // the fast-path operators have been optimized so the common ::exit()
960 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock().
961 // The code emitted by fast_unlock() elides the usual MEMBAR. This
962 // greatly improves latency -- MEMBAR and CAS having considerable local
963 // latency on modern processors -- but at the cost of "stranding". Absent the
964 // MEMBAR, a thread in fast_unlock() can race a thread in the slow
965 // ::enter() path, resulting in the entering thread being stranding
966 // and a progress-liveness failure. Stranding is extremely rare.
967 // We use timers (timed park operations) & periodic polling to detect
968 // and recover from stranding. Potentially stranded threads periodically
969 // wake up and poll the lock. See the usage of the _Responsible variable.
970 //
971 // The CAS() in enter provides for safety and exclusion, while the CAS or
972 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking
973 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding.
974 // We detect and recover from stranding with timers.
975 //
976 // If a thread transiently strands it'll park until (a) another
977 // thread acquires the lock and then drops the lock, at which time the
978 // exiting thread will notice and unpark the stranded thread, or, (b)
979 // the timer expires. If the lock is high traffic then the stranding latency
980 // will be low due to (a). If the lock is low traffic then the odds of
981 // stranding are lower, although the worst-case stranding latency
982 // is longer. Critically, we don't want to put excessive load in the
983 // platform's timer subsystem. We want to minimize both the timer injection
984 // rate (timers created/sec) as well as the number of timers active at
985 // any one time. (more precisely, we want to minimize timer-seconds, which is
986 // the integral of the # of active timers at any instant over time).
987 // Both impinge on OS scalability. Given that, at most one thread parked on
988 // a monitor will use a timer.
989 //
990 // There is also the risk of a futile wake-up. If we drop the lock
991 // another thread can reacquire the lock immediately, and we can
992 // then wake a thread unnecessarily. This is benign, and we've
993 // structured the code so the windows are short and the frequency
994 // of such futile wakups is low.
995
996 void ObjectMonitor::exit(bool not_suspended, TRAPS) {
997 Thread * const Self = THREAD;
998 if (THREAD != _owner) {
999 if (THREAD->is_lock_owned((address) _owner)) {
1000 // Transmute _owner from a BasicLock pointer to a Thread address.
1001 // We don't need to hold _mutex for this transition.
1002 // Non-null to Non-null is safe as long as all readers can
1003 // tolerate either flavor.
1004 assert(_recursions == 0, "invariant");
1005 _owner = THREAD;
1006 _recursions = 0;
1007 } else {
1008 // Apparent unbalanced locking ...
1009 // Naively we'd like to throw IllegalMonitorStateException.
1010 // As a practical matter we can neither allocate nor throw an
1011 // exception as ::exit() can be called from leaf routines.
1012 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties.
1013 // Upon deeper reflection, however, in a properly run JVM the only
1014 // way we should encounter this situation is in the presence of
1015 // unbalanced JNI locking. TODO: CheckJNICalls.
1016 // See also: CR4414101
1017 assert(false, "Non-balanced monitor enter/exit! Likely JNI locking: "
1018 "owner=" INTPTR_FORMAT, p2i(_owner));
1019 return;
1020 }
1021 }
1022
1023 if (_recursions != 0) {
1024 _recursions--; // this is simple recursive enter
1025 return;
1026 }
1027
1028 // Invariant: after setting Responsible=null an thread must execute
1029 // a MEMBAR or other serializing instruction before fetching EntryList|cxq.
1030 _Responsible = NULL;
1031
1032 #if INCLUDE_JFR
1033 // get the owner's thread id for the MonitorEnter event
1034 // if it is enabled and the thread isn't suspended
1035 if (not_suspended && EventJavaMonitorEnter::is_enabled()) {
1036 _previous_owner_tid = JFR_THREAD_ID(Self);
1037 }
1038 #endif
1039
1040 for (;;) {
1041 assert(THREAD == _owner, "invariant");
1042
1043 // release semantics: prior loads and stores from within the critical section
1044 // must not float (reorder) past the following store that drops the lock.
1045 // On SPARC that requires MEMBAR #loadstore|#storestore.
1046 // But of course in TSO #loadstore|#storestore is not required.
1047 OrderAccess::release_store(&_owner, (void*)NULL); // drop the lock
1048 OrderAccess::storeload(); // See if we need to wake a successor
1049 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) {
1050 return;
1051 }
1052 // Other threads are blocked trying to acquire the lock.
1053
1054 // Normally the exiting thread is responsible for ensuring succession,
1055 // but if other successors are ready or other entering threads are spinning
1056 // then this thread can simply store NULL into _owner and exit without
1057 // waking a successor. The existence of spinners or ready successors
1058 // guarantees proper succession (liveness). Responsibility passes to the
1059 // ready or running successors. The exiting thread delegates the duty.
1060 // More precisely, if a successor already exists this thread is absolved
1061 // of the responsibility of waking (unparking) one.
1062 //
1063 // The _succ variable is critical to reducing futile wakeup frequency.
1064 // _succ identifies the "heir presumptive" thread that has been made
1065 // ready (unparked) but that has not yet run. We need only one such
1066 // successor thread to guarantee progress.
1067 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
1068 // section 3.3 "Futile Wakeup Throttling" for details.
1069 //
1070 // Note that spinners in Enter() also set _succ non-null.
1071 // In the current implementation spinners opportunistically set
1072 // _succ so that exiting threads might avoid waking a successor.
1073 // Another less appealing alternative would be for the exiting thread
1074 // to drop the lock and then spin briefly to see if a spinner managed
1075 // to acquire the lock. If so, the exiting thread could exit
1076 // immediately without waking a successor, otherwise the exiting
1077 // thread would need to dequeue and wake a successor.
1078 // (Note that we'd need to make the post-drop spin short, but no
1079 // shorter than the worst-case round-trip cache-line migration time.
1080 // The dropped lock needs to become visible to the spinner, and then
1081 // the acquisition of the lock by the spinner must become visible to
1082 // the exiting thread).
1083
1084 // It appears that an heir-presumptive (successor) must be made ready.
1085 // Only the current lock owner can manipulate the EntryList or
1086 // drain _cxq, so we need to reacquire the lock. If we fail
1087 // to reacquire the lock the responsibility for ensuring succession
1088 // falls to the new owner.
1089 //
1090 if (!Atomic::replace_if_null(THREAD, &_owner)) {
1091 return;
1092 }
1093
1094 guarantee(_owner == THREAD, "invariant");
1095
1096 ObjectWaiter * w = NULL;
1097
1098 w = _EntryList;
1099 if (w != NULL) {
1100 // I'd like to write: guarantee (w->_thread != Self).
1101 // But in practice an exiting thread may find itself on the EntryList.
1102 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and
1103 // then calls exit(). Exit release the lock by setting O._owner to NULL.
1104 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The
1105 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then
1106 // release the lock "O". T2 resumes immediately after the ST of null into
1107 // _owner, above. T2 notices that the EntryList is populated, so it
1108 // reacquires the lock and then finds itself on the EntryList.
1109 // Given all that, we have to tolerate the circumstance where "w" is
1110 // associated with Self.
1111 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1112 ExitEpilog(Self, w);
1113 return;
1114 }
1115
1116 // If we find that both _cxq and EntryList are null then just
1117 // re-run the exit protocol from the top.
1118 w = _cxq;
1119 if (w == NULL) continue;
1120
1121 // Drain _cxq into EntryList - bulk transfer.
1122 // First, detach _cxq.
1123 // The following loop is tantamount to: w = swap(&cxq, NULL)
1124 for (;;) {
1125 assert(w != NULL, "Invariant");
1126 ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w);
1127 if (u == w) break;
1128 w = u;
1129 }
1130
1131 assert(w != NULL, "invariant");
1132 assert(_EntryList == NULL, "invariant");
1133
1134 // Convert the LIFO SLL anchored by _cxq into a DLL.
1135 // The list reorganization step operates in O(LENGTH(w)) time.
1136 // It's critical that this step operate quickly as
1137 // "Self" still holds the outer-lock, restricting parallelism
1138 // and effectively lengthening the critical section.
1139 // Invariant: s chases t chases u.
1140 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so
1141 // we have faster access to the tail.
1142
1143 _EntryList = w;
1144 ObjectWaiter * q = NULL;
1145 ObjectWaiter * p;
1146 for (p = w; p != NULL; p = p->_next) {
1147 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant");
1148 p->TState = ObjectWaiter::TS_ENTER;
1149 p->_prev = q;
1150 q = p;
1151 }
1152
1153 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL
1154 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog().
1155
1156 // See if we can abdicate to a spinner instead of waking a thread.
1157 // A primary goal of the implementation is to reduce the
1158 // context-switch rate.
1159 if (_succ != NULL) continue;
1160
1161 w = _EntryList;
1162 if (w != NULL) {
1163 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant");
1164 ExitEpilog(Self, w);
1165 return;
1166 }
1167 }
1168 }
1169
1170 // ExitSuspendEquivalent:
1171 // A faster alternate to handle_special_suspend_equivalent_condition()
1172 //
1173 // handle_special_suspend_equivalent_condition() unconditionally
1174 // acquires the SR_lock. On some platforms uncontended MutexLocker()
1175 // operations have high latency. Note that in ::enter() we call HSSEC
1176 // while holding the monitor, so we effectively lengthen the critical sections.
1177 //
1178 // There are a number of possible solutions:
1179 //
1180 // A. To ameliorate the problem we might also defer state transitions
1181 // to as late as possible -- just prior to parking.
1182 // Given that, we'd call HSSEC after having returned from park(),
1183 // but before attempting to acquire the monitor. This is only a
1184 // partial solution. It avoids calling HSSEC while holding the
1185 // monitor (good), but it still increases successor reacquisition latency --
1186 // the interval between unparking a successor and the time the successor
1187 // resumes and retries the lock. See ReenterI(), which defers state transitions.
1188 // If we use this technique we can also avoid EnterI()-exit() loop
1189 // in ::enter() where we iteratively drop the lock and then attempt
1190 // to reacquire it after suspending.
1191 //
1192 // B. In the future we might fold all the suspend bits into a
1193 // composite per-thread suspend flag and then update it with CAS().
1194 // Alternately, a Dekker-like mechanism with multiple variables
1195 // would suffice:
1196 // ST Self->_suspend_equivalent = false
1197 // MEMBAR
1198 // LD Self_>_suspend_flags
1199
1200 bool ObjectMonitor::ExitSuspendEquivalent(JavaThread * jSelf) {
1201 return jSelf->handle_special_suspend_equivalent_condition();
1202 }
1203
1204
1205 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) {
1206 assert(_owner == Self, "invariant");
1207
1208 // Exit protocol:
1209 // 1. ST _succ = wakee
1210 // 2. membar #loadstore|#storestore;
1211 // 2. ST _owner = NULL
1212 // 3. unpark(wakee)
1213
1214 _succ = Wakee->_thread;
1215 ParkEvent * Trigger = Wakee->_event;
1216
1217 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again.
1218 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be
1219 // out-of-scope (non-extant).
1220 Wakee = NULL;
1221
1222 // Drop the lock
1223 OrderAccess::release_store(&_owner, (void*)NULL);
1224 OrderAccess::fence(); // ST _owner vs LD in unpark()
1225
1226 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self);
1227 Trigger->unpark();
1228
1229 // Maintain stats and report events to JVMTI
1230 OM_PERFDATA_OP(Parks, inc());
1231 }
1232
1233
1234 // -----------------------------------------------------------------------------
1235 // Class Loader deadlock handling.
1236 //
1237 // complete_exit exits a lock returning recursion count
1238 // complete_exit/reenter operate as a wait without waiting
1239 // complete_exit requires an inflated monitor
1240 // The _owner field is not always the Thread addr even with an
1241 // inflated monitor, e.g. the monitor can be inflated by a non-owning
1242 // thread due to contention.
1243 intptr_t ObjectMonitor::complete_exit(TRAPS) {
1244 Thread * const Self = THREAD;
1245 assert(Self->is_Java_thread(), "Must be Java thread!");
1246 JavaThread *jt = (JavaThread *)THREAD;
1247
1248 assert(InitDone, "Unexpectedly not initialized");
1249
1250 if (THREAD != _owner) {
1251 if (THREAD->is_lock_owned ((address)_owner)) {
1252 assert(_recursions == 0, "internal state error");
1253 _owner = THREAD; // Convert from basiclock addr to Thread addr
1254 _recursions = 0;
1255 }
1256 }
1257
1258 guarantee(Self == _owner, "complete_exit not owner");
1259 intptr_t save = _recursions; // record the old recursion count
1260 _recursions = 0; // set the recursion level to be 0
1261 exit(true, Self); // exit the monitor
1262 guarantee(_owner != Self, "invariant");
1263 return save;
1264 }
1265
1266 // reenter() enters a lock and sets recursion count
1267 // complete_exit/reenter operate as a wait without waiting
1268 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) {
1269 Thread * const Self = THREAD;
1270 assert(Self->is_Java_thread(), "Must be Java thread!");
1271 JavaThread *jt = (JavaThread *)THREAD;
1272
1273 guarantee(_owner != Self, "reenter already owner");
1274 enter(THREAD);
1275 // Entered the monitor.
1276 guarantee(_recursions == 0, "reenter recursion");
1277 _recursions = recursions;
1278 }
1279
1280 // Checks that the current THREAD owns this monitor and causes an
1281 // immediate return if it doesn't. We don't use the CHECK macro
1282 // because we want the IMSE to be the only exception that is thrown
1283 // from the call site when false is returned. Any other pending
1284 // exception is ignored.
1285 #define CHECK_OWNER() \
1286 do { \
1287 if (!check_owner(THREAD)) { \
1288 assert(HAS_PENDING_EXCEPTION, "expected a pending IMSE here."); \
1289 return; \
1290 } \
1291 } while (false)
1292
1293 // Returns true if the specified thread owns the ObjectMonitor.
1294 // Otherwise returns false and throws IllegalMonitorStateException
1295 // (IMSE). If there is a pending exception and the specified thread
1296 // is not the owner, that exception will be replaced by the IMSE.
1297 bool ObjectMonitor::check_owner(Thread* THREAD) {
1298 if (_owner == THREAD) {
1299 return true;
1300 }
1301 if (THREAD->is_lock_owned((address)_owner)) {
1302 _owner = THREAD; // convert from BasicLock addr to Thread addr
1303 _recursions = 0;
1304 return true;
1305 }
1306 THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(),
1307 "current thread is not owner", false);
1308 }
1309
1310 static void post_monitor_wait_event(EventJavaMonitorWait* event,
1311 ObjectMonitor* monitor,
1312 jlong notifier_tid,
1313 jlong timeout,
1314 bool timedout) {
1315 assert(event != NULL, "invariant");
1316 assert(monitor != NULL, "invariant");
1317 event->set_monitorClass(((oop)monitor->object())->klass());
1318 event->set_timeout(timeout);
1319 event->set_address((uintptr_t)monitor->object_addr());
1320 event->set_notifier(notifier_tid);
1321 event->set_timedOut(timedout);
1322 event->commit();
1323 }
1324
1325 // -----------------------------------------------------------------------------
1326 // Wait/Notify/NotifyAll
1327 //
1328 // Note: a subset of changes to ObjectMonitor::wait()
1329 // will need to be replicated in complete_exit
1330 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
1331 Thread * const Self = THREAD;
1332 assert(Self->is_Java_thread(), "Must be Java thread!");
1333 JavaThread *jt = (JavaThread *)THREAD;
1334
1335 assert(InitDone, "Unexpectedly not initialized");
1336
1337 CHECK_OWNER(); // Throws IMSE if not owner.
1338
1339 EventJavaMonitorWait event;
1340
1341 // check for a pending interrupt
1342 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1343 // post monitor waited event. Note that this is past-tense, we are done waiting.
1344 if (JvmtiExport::should_post_monitor_waited()) {
1345 // Note: 'false' parameter is passed here because the
1346 // wait was not timed out due to thread interrupt.
1347 JvmtiExport::post_monitor_waited(jt, this, false);
1348
1349 // In this short circuit of the monitor wait protocol, the
1350 // current thread never drops ownership of the monitor and
1351 // never gets added to the wait queue so the current thread
1352 // cannot be made the successor. This means that the
1353 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
1354 // consume an unpark() meant for the ParkEvent associated with
1355 // this ObjectMonitor.
1356 }
1357 if (event.should_commit()) {
1358 post_monitor_wait_event(&event, this, 0, millis, false);
1359 }
1360 THROW(vmSymbols::java_lang_InterruptedException());
1361 return;
1362 }
1363
1364 assert(Self->_Stalled == 0, "invariant");
1365 Self->_Stalled = intptr_t(this);
1366 jt->set_current_waiting_monitor(this);
1367
1368 // create a node to be put into the queue
1369 // Critically, after we reset() the event but prior to park(), we must check
1370 // for a pending interrupt.
1371 ObjectWaiter node(Self);
1372 node.TState = ObjectWaiter::TS_WAIT;
1373 Self->_ParkEvent->reset();
1374 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
1375
1376 // Enter the waiting queue, which is a circular doubly linked list in this case
1377 // but it could be a priority queue or any data structure.
1378 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
1379 // by the the owner of the monitor *except* in the case where park()
1380 // returns because of a timeout of interrupt. Contention is exceptionally rare
1381 // so we use a simple spin-lock instead of a heavier-weight blocking lock.
1382
1383 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add");
1384 AddWaiter(&node);
1385 Thread::SpinRelease(&_WaitSetLock);
1386
1387 _Responsible = NULL;
1388
1389 intptr_t save = _recursions; // record the old recursion count
1390 _waiters++; // increment the number of waiters
1391 _recursions = 0; // set the recursion level to be 1
1392 exit(true, Self); // exit the monitor
1393 guarantee(_owner != Self, "invariant");
1394
1395 // The thread is on the WaitSet list - now park() it.
1396 // On MP systems it's conceivable that a brief spin before we park
1397 // could be profitable.
1398 //
1399 // TODO-FIXME: change the following logic to a loop of the form
1400 // while (!timeout && !interrupted && _notified == 0) park()
1401
1402 int ret = OS_OK;
1403 int WasNotified = 0;
1404 { // State transition wrappers
1405 OSThread* osthread = Self->osthread();
1406 OSThreadWaitState osts(osthread, true);
1407 {
1408 ThreadBlockInVM tbivm(jt);
1409 // Thread is in thread_blocked state and oop access is unsafe.
1410 jt->set_suspend_equivalent();
1411
1412 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
1413 // Intentionally empty
1414 } else if (node._notified == 0) {
1415 if (millis <= 0) {
1416 Self->_ParkEvent->park();
1417 } else {
1418 ret = Self->_ParkEvent->park(millis);
1419 }
1420 }
1421
1422 // were we externally suspended while we were waiting?
1423 if (ExitSuspendEquivalent (jt)) {
1424 // TODO-FIXME: add -- if succ == Self then succ = null.
1425 jt->java_suspend_self();
1426 }
1427
1428 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
1429
1430 // Node may be on the WaitSet, the EntryList (or cxq), or in transition
1431 // from the WaitSet to the EntryList.
1432 // See if we need to remove Node from the WaitSet.
1433 // We use double-checked locking to avoid grabbing _WaitSetLock
1434 // if the thread is not on the wait queue.
1435 //
1436 // Note that we don't need a fence before the fetch of TState.
1437 // In the worst case we'll fetch a old-stale value of TS_WAIT previously
1438 // written by the is thread. (perhaps the fetch might even be satisfied
1439 // by a look-aside into the processor's own store buffer, although given
1440 // the length of the code path between the prior ST and this load that's
1441 // highly unlikely). If the following LD fetches a stale TS_WAIT value
1442 // then we'll acquire the lock and then re-fetch a fresh TState value.
1443 // That is, we fail toward safety.
1444
1445 if (node.TState == ObjectWaiter::TS_WAIT) {
1446 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink");
1447 if (node.TState == ObjectWaiter::TS_WAIT) {
1448 DequeueSpecificWaiter(&node); // unlink from WaitSet
1449 assert(node._notified == 0, "invariant");
1450 node.TState = ObjectWaiter::TS_RUN;
1451 }
1452 Thread::SpinRelease(&_WaitSetLock);
1453 }
1454
1455 // The thread is now either on off-list (TS_RUN),
1456 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
1457 // The Node's TState variable is stable from the perspective of this thread.
1458 // No other threads will asynchronously modify TState.
1459 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant");
1460 OrderAccess::loadload();
1461 if (_succ == Self) _succ = NULL;
1462 WasNotified = node._notified;
1463
1464 // Reentry phase -- reacquire the monitor.
1465 // re-enter contended monitor after object.wait().
1466 // retain OBJECT_WAIT state until re-enter successfully completes
1467 // Thread state is thread_in_vm and oop access is again safe,
1468 // although the raw address of the object may have changed.
1469 // (Don't cache naked oops over safepoints, of course).
1470
1471 // post monitor waited event. Note that this is past-tense, we are done waiting.
1472 if (JvmtiExport::should_post_monitor_waited()) {
1473 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
1474
1475 if (node._notified != 0 && _succ == Self) {
1476 // In this part of the monitor wait-notify-reenter protocol it
1477 // is possible (and normal) for another thread to do a fastpath
1478 // monitor enter-exit while this thread is still trying to get
1479 // to the reenter portion of the protocol.
1480 //
1481 // The ObjectMonitor was notified and the current thread is
1482 // the successor which also means that an unpark() has already
1483 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
1484 // consume the unpark() that was done when the successor was
1485 // set because the same ParkEvent is shared between Java
1486 // monitors and JVM/TI RawMonitors (for now).
1487 //
1488 // We redo the unpark() to ensure forward progress, i.e., we
1489 // don't want all pending threads hanging (parked) with none
1490 // entering the unlocked monitor.
1491 node._event->unpark();
1492 }
1493 }
1494
1495 if (event.should_commit()) {
1496 post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT);
1497 }
1498
1499 OrderAccess::fence();
1500
1501 assert(Self->_Stalled != 0, "invariant");
1502 Self->_Stalled = 0;
1503
1504 assert(_owner != Self, "invariant");
1505 ObjectWaiter::TStates v = node.TState;
1506 if (v == ObjectWaiter::TS_RUN) {
1507 enter(Self);
1508 } else {
1509 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant");
1510 ReenterI(Self, &node);
1511 node.wait_reenter_end(this);
1512 }
1513
1514 // Self has reacquired the lock.
1515 // Lifecycle - the node representing Self must not appear on any queues.
1516 // Node is about to go out-of-scope, but even if it were immortal we wouldn't
1517 // want residual elements associated with this thread left on any lists.
1518 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant");
1519 assert(_owner == Self, "invariant");
1520 assert(_succ != Self, "invariant");
1521 } // OSThreadWaitState()
1522
1523 jt->set_current_waiting_monitor(NULL);
1524
1525 guarantee(_recursions == 0, "invariant");
1526 _recursions = save; // restore the old recursion count
1527 _waiters--; // decrement the number of waiters
1528
1529 // Verify a few postconditions
1530 assert(_owner == Self, "invariant");
1531 assert(_succ != Self, "invariant");
1532 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant");
1533
1534 // check if the notification happened
1535 if (!WasNotified) {
1536 // no, it could be timeout or Thread.interrupt() or both
1537 // check for interrupt event, otherwise it is timeout
1538 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
1539 THROW(vmSymbols::java_lang_InterruptedException());
1540 }
1541 }
1542
1543 // NOTE: Spurious wake up will be consider as timeout.
1544 // Monitor notify has precedence over thread interrupt.
1545 }
1546
1547
1548 // Consider:
1549 // If the lock is cool (cxq == null && succ == null) and we're on an MP system
1550 // then instead of transferring a thread from the WaitSet to the EntryList
1551 // we might just dequeue a thread from the WaitSet and directly unpark() it.
1552
1553 void ObjectMonitor::INotify(Thread * Self) {
1554 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify");
1555 ObjectWaiter * iterator = DequeueWaiter();
1556 if (iterator != NULL) {
1557 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant");
1558 guarantee(iterator->_notified == 0, "invariant");
1559 // Disposition - what might we do with iterator ?
1560 // a. add it directly to the EntryList - either tail (policy == 1)
1561 // or head (policy == 0).
1562 // b. push it onto the front of the _cxq (policy == 2).
1563 // For now we use (b).
1564
1565 iterator->TState = ObjectWaiter::TS_ENTER;
1566
1567 iterator->_notified = 1;
1568 iterator->_notifier_tid = JFR_THREAD_ID(Self);
1569
1570 ObjectWaiter * list = _EntryList;
1571 if (list != NULL) {
1572 assert(list->_prev == NULL, "invariant");
1573 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant");
1574 assert(list != iterator, "invariant");
1575 }
1576
1577 // prepend to cxq
1578 if (list == NULL) {
1579 iterator->_next = iterator->_prev = NULL;
1580 _EntryList = iterator;
1581 } else {
1582 iterator->TState = ObjectWaiter::TS_CXQ;
1583 for (;;) {
1584 ObjectWaiter * front = _cxq;
1585 iterator->_next = front;
1586 if (Atomic::cmpxchg(iterator, &_cxq, front) == front) {
1587 break;
1588 }
1589 }
1590 }
1591
1592 // _WaitSetLock protects the wait queue, not the EntryList. We could
1593 // move the add-to-EntryList operation, above, outside the critical section
1594 // protected by _WaitSetLock. In practice that's not useful. With the
1595 // exception of wait() timeouts and interrupts the monitor owner
1596 // is the only thread that grabs _WaitSetLock. There's almost no contention
1597 // on _WaitSetLock so it's not profitable to reduce the length of the
1598 // critical section.
1599
1600 iterator->wait_reenter_begin(this);
1601 }
1602 Thread::SpinRelease(&_WaitSetLock);
1603 }
1604
1605 // Consider: a not-uncommon synchronization bug is to use notify() when
1606 // notifyAll() is more appropriate, potentially resulting in stranded
1607 // threads; this is one example of a lost wakeup. A useful diagnostic
1608 // option is to force all notify() operations to behave as notifyAll().
1609 //
1610 // Note: We can also detect many such problems with a "minimum wait".
1611 // When the "minimum wait" is set to a small non-zero timeout value
1612 // and the program does not hang whereas it did absent "minimum wait",
1613 // that suggests a lost wakeup bug.
1614
1615 void ObjectMonitor::notify(TRAPS) {
1616 CHECK_OWNER(); // Throws IMSE if not owner.
1617 if (_WaitSet == NULL) {
1618 return;
1619 }
1620 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
1621 INotify(THREAD);
1622 OM_PERFDATA_OP(Notifications, inc(1));
1623 }
1624
1625
1626 // The current implementation of notifyAll() transfers the waiters one-at-a-time
1627 // from the waitset to the EntryList. This could be done more efficiently with a
1628 // single bulk transfer but in practice it's not time-critical. Beware too,
1629 // that in prepend-mode we invert the order of the waiters. Let's say that the
1630 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend
1631 // mode the waitset will be empty and the EntryList will be "DCBAXYZ".
1632
1633 void ObjectMonitor::notifyAll(TRAPS) {
1634 CHECK_OWNER(); // Throws IMSE if not owner.
1635 if (_WaitSet == NULL) {
1636 return;
1637 }
1638
1639 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
1640 int tally = 0;
1641 while (_WaitSet != NULL) {
1642 tally++;
1643 INotify(THREAD);
1644 }
1645
1646 OM_PERFDATA_OP(Notifications, inc(tally));
1647 }
1648
1649 // -----------------------------------------------------------------------------
1650 // Adaptive Spinning Support
1651 //
1652 // Adaptive spin-then-block - rational spinning
1653 //
1654 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS
1655 // algorithm. On high order SMP systems it would be better to start with
1656 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH,
1657 // a contending thread could enqueue itself on the cxq and then spin locally
1658 // on a thread-specific variable such as its ParkEvent._Event flag.
1659 // That's left as an exercise for the reader. Note that global spinning is
1660 // not problematic on Niagara, as the L2 cache serves the interconnect and
1661 // has both low latency and massive bandwidth.
1662 //
1663 // Broadly, we can fix the spin frequency -- that is, the % of contended lock
1664 // acquisition attempts where we opt to spin -- at 100% and vary the spin count
1665 // (duration) or we can fix the count at approximately the duration of
1666 // a context switch and vary the frequency. Of course we could also
1667 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor.
1668 // For a description of 'Adaptive spin-then-block mutual exclusion in
1669 // multi-threaded processing,' see U.S. Pat. No. 8046758.
1670 //
1671 // This implementation varies the duration "D", where D varies with
1672 // the success rate of recent spin attempts. (D is capped at approximately
1673 // length of a round-trip context switch). The success rate for recent
1674 // spin attempts is a good predictor of the success rate of future spin
1675 // attempts. The mechanism adapts automatically to varying critical
1676 // section length (lock modality), system load and degree of parallelism.
1677 // D is maintained per-monitor in _SpinDuration and is initialized
1678 // optimistically. Spin frequency is fixed at 100%.
1679 //
1680 // Note that _SpinDuration is volatile, but we update it without locks
1681 // or atomics. The code is designed so that _SpinDuration stays within
1682 // a reasonable range even in the presence of races. The arithmetic
1683 // operations on _SpinDuration are closed over the domain of legal values,
1684 // so at worst a race will install and older but still legal value.
1685 // At the very worst this introduces some apparent non-determinism.
1686 // We might spin when we shouldn't or vice-versa, but since the spin
1687 // count are relatively short, even in the worst case, the effect is harmless.
1688 //
1689 // Care must be taken that a low "D" value does not become an
1690 // an absorbing state. Transient spinning failures -- when spinning
1691 // is overall profitable -- should not cause the system to converge
1692 // on low "D" values. We want spinning to be stable and predictable
1693 // and fairly responsive to change and at the same time we don't want
1694 // it to oscillate, become metastable, be "too" non-deterministic,
1695 // or converge on or enter undesirable stable absorbing states.
1696 //
1697 // We implement a feedback-based control system -- using past behavior
1698 // to predict future behavior. We face two issues: (a) if the
1699 // input signal is random then the spin predictor won't provide optimal
1700 // results, and (b) if the signal frequency is too high then the control
1701 // system, which has some natural response lag, will "chase" the signal.
1702 // (b) can arise from multimodal lock hold times. Transient preemption
1703 // can also result in apparent bimodal lock hold times.
1704 // Although sub-optimal, neither condition is particularly harmful, as
1705 // in the worst-case we'll spin when we shouldn't or vice-versa.
1706 // The maximum spin duration is rather short so the failure modes aren't bad.
1707 // To be conservative, I've tuned the gain in system to bias toward
1708 // _not spinning. Relatedly, the system can sometimes enter a mode where it
1709 // "rings" or oscillates between spinning and not spinning. This happens
1710 // when spinning is just on the cusp of profitability, however, so the
1711 // situation is not dire. The state is benign -- there's no need to add
1712 // hysteresis control to damp the transition rate between spinning and
1713 // not spinning.
1714
1715 // Spinning: Fixed frequency (100%), vary duration
1716 int ObjectMonitor::TrySpin(Thread * Self) {
1717 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning.
1718 int ctr = Knob_FixedSpin;
1719 if (ctr != 0) {
1720 while (--ctr >= 0) {
1721 if (TryLock(Self) > 0) return 1;
1722 SpinPause();
1723 }
1724 return 0;
1725 }
1726
1727 for (ctr = Knob_PreSpin + 1; --ctr >= 0;) {
1728 if (TryLock(Self) > 0) {
1729 // Increase _SpinDuration ...
1730 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1731 // Raising _SpurDuration to the poverty line is key.
1732 int x = _SpinDuration;
1733 if (x < Knob_SpinLimit) {
1734 if (x < Knob_Poverty) x = Knob_Poverty;
1735 _SpinDuration = x + Knob_BonusB;
1736 }
1737 return 1;
1738 }
1739 SpinPause();
1740 }
1741
1742 // Admission control - verify preconditions for spinning
1743 //
1744 // We always spin a little bit, just to prevent _SpinDuration == 0 from
1745 // becoming an absorbing state. Put another way, we spin briefly to
1746 // sample, just in case the system load, parallelism, contention, or lock
1747 // modality changed.
1748 //
1749 // Consider the following alternative:
1750 // Periodically set _SpinDuration = _SpinLimit and try a long/full
1751 // spin attempt. "Periodically" might mean after a tally of
1752 // the # of failed spin attempts (or iterations) reaches some threshold.
1753 // This takes us into the realm of 1-out-of-N spinning, where we
1754 // hold the duration constant but vary the frequency.
1755
1756 ctr = _SpinDuration;
1757 if (ctr <= 0) return 0;
1758
1759 if (NotRunnable(Self, (Thread *) _owner)) {
1760 return 0;
1761 }
1762
1763 // We're good to spin ... spin ingress.
1764 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades
1765 // when preparing to LD...CAS _owner, etc and the CAS is likely
1766 // to succeed.
1767 if (_succ == NULL) {
1768 _succ = Self;
1769 }
1770 Thread * prv = NULL;
1771
1772 // There are three ways to exit the following loop:
1773 // 1. A successful spin where this thread has acquired the lock.
1774 // 2. Spin failure with prejudice
1775 // 3. Spin failure without prejudice
1776
1777 while (--ctr >= 0) {
1778
1779 // Periodic polling -- Check for pending GC
1780 // Threads may spin while they're unsafe.
1781 // We don't want spinning threads to delay the JVM from reaching
1782 // a stop-the-world safepoint or to steal cycles from GC.
1783 // If we detect a pending safepoint we abort in order that
1784 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b)
1785 // this thread, if safe, doesn't steal cycles from GC.
1786 // This is in keeping with the "no loitering in runtime" rule.
1787 // We periodically check to see if there's a safepoint pending.
1788 if ((ctr & 0xFF) == 0) {
1789 if (SafepointMechanism::should_block(Self)) {
1790 goto Abort; // abrupt spin egress
1791 }
1792 SpinPause();
1793 }
1794
1795 // Probe _owner with TATAS
1796 // If this thread observes the monitor transition or flicker
1797 // from locked to unlocked to locked, then the odds that this
1798 // thread will acquire the lock in this spin attempt go down
1799 // considerably. The same argument applies if the CAS fails
1800 // or if we observe _owner change from one non-null value to
1801 // another non-null value. In such cases we might abort
1802 // the spin without prejudice or apply a "penalty" to the
1803 // spin count-down variable "ctr", reducing it by 100, say.
1804
1805 Thread * ox = (Thread *) _owner;
1806 if (ox == NULL) {
1807 ox = (Thread*)Atomic::cmpxchg(Self, &_owner, (void*)NULL);
1808 if (ox == NULL) {
1809 // The CAS succeeded -- this thread acquired ownership
1810 // Take care of some bookkeeping to exit spin state.
1811 if (_succ == Self) {
1812 _succ = NULL;
1813 }
1814
1815 // Increase _SpinDuration :
1816 // The spin was successful (profitable) so we tend toward
1817 // longer spin attempts in the future.
1818 // CONSIDER: factor "ctr" into the _SpinDuration adjustment.
1819 // If we acquired the lock early in the spin cycle it
1820 // makes sense to increase _SpinDuration proportionally.
1821 // Note that we don't clamp SpinDuration precisely at SpinLimit.
1822 int x = _SpinDuration;
1823 if (x < Knob_SpinLimit) {
1824 if (x < Knob_Poverty) x = Knob_Poverty;
1825 _SpinDuration = x + Knob_Bonus;
1826 }
1827 return 1;
1828 }
1829
1830 // The CAS failed ... we can take any of the following actions:
1831 // * penalize: ctr -= CASPenalty
1832 // * exit spin with prejudice -- goto Abort;
1833 // * exit spin without prejudice.
1834 // * Since CAS is high-latency, retry again immediately.
1835 prv = ox;
1836 goto Abort;
1837 }
1838
1839 // Did lock ownership change hands ?
1840 if (ox != prv && prv != NULL) {
1841 goto Abort;
1842 }
1843 prv = ox;
1844
1845 // Abort the spin if the owner is not executing.
1846 // The owner must be executing in order to drop the lock.
1847 // Spinning while the owner is OFFPROC is idiocy.
1848 // Consider: ctr -= RunnablePenalty ;
1849 if (NotRunnable(Self, ox)) {
1850 goto Abort;
1851 }
1852 if (_succ == NULL) {
1853 _succ = Self;
1854 }
1855 }
1856
1857 // Spin failed with prejudice -- reduce _SpinDuration.
1858 // TODO: Use an AIMD-like policy to adjust _SpinDuration.
1859 // AIMD is globally stable.
1860 {
1861 int x = _SpinDuration;
1862 if (x > 0) {
1863 // Consider an AIMD scheme like: x -= (x >> 3) + 100
1864 // This is globally sample and tends to damp the response.
1865 x -= Knob_Penalty;
1866 if (x < 0) x = 0;
1867 _SpinDuration = x;
1868 }
1869 }
1870
1871 Abort:
1872 if (_succ == Self) {
1873 _succ = NULL;
1874 // Invariant: after setting succ=null a contending thread
1875 // must recheck-retry _owner before parking. This usually happens
1876 // in the normal usage of TrySpin(), but it's safest
1877 // to make TrySpin() as foolproof as possible.
1878 OrderAccess::fence();
1879 if (TryLock(Self) > 0) return 1;
1880 }
1881 return 0;
1882 }
1883
1884 // NotRunnable() -- informed spinning
1885 //
1886 // Don't bother spinning if the owner is not eligible to drop the lock.
1887 // Spin only if the owner thread is _thread_in_Java or _thread_in_vm.
1888 // The thread must be runnable in order to drop the lock in timely fashion.
1889 // If the _owner is not runnable then spinning will not likely be
1890 // successful (profitable).
1891 //
1892 // Beware -- the thread referenced by _owner could have died
1893 // so a simply fetch from _owner->_thread_state might trap.
1894 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state.
1895 // Because of the lifecycle issues, the _thread_state values
1896 // observed by NotRunnable() might be garbage. NotRunnable must
1897 // tolerate this and consider the observed _thread_state value
1898 // as advisory.
1899 //
1900 // Beware too, that _owner is sometimes a BasicLock address and sometimes
1901 // a thread pointer.
1902 // Alternately, we might tag the type (thread pointer vs basiclock pointer)
1903 // with the LSB of _owner. Another option would be to probabilistically probe
1904 // the putative _owner->TypeTag value.
1905 //
1906 // Checking _thread_state isn't perfect. Even if the thread is
1907 // in_java it might be blocked on a page-fault or have been preempted
1908 // and sitting on a ready/dispatch queue.
1909 //
1910 // The return value from NotRunnable() is *advisory* -- the
1911 // result is based on sampling and is not necessarily coherent.
1912 // The caller must tolerate false-negative and false-positive errors.
1913 // Spinning, in general, is probabilistic anyway.
1914
1915
1916 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) {
1917 // Check ox->TypeTag == 2BAD.
1918 if (ox == NULL) return 0;
1919
1920 // Avoid transitive spinning ...
1921 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L.
1922 // Immediately after T1 acquires L it's possible that T2, also
1923 // spinning on L, will see L.Owner=T1 and T1._Stalled=L.
1924 // This occurs transiently after T1 acquired L but before
1925 // T1 managed to clear T1.Stalled. T2 does not need to abort
1926 // its spin in this circumstance.
1927 intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1));
1928
1929 if (BlockedOn == 1) return 1;
1930 if (BlockedOn != 0) {
1931 return BlockedOn != intptr_t(this) && _owner == ox;
1932 }
1933
1934 assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant");
1935 int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);;
1936 // consider also: jst != _thread_in_Java -- but that's overspecific.
1937 return jst == _thread_blocked || jst == _thread_in_native;
1938 }
1939
1940
1941 // -----------------------------------------------------------------------------
1942 // WaitSet management ...
1943
1944 ObjectWaiter::ObjectWaiter(Thread* thread) {
1945 _next = NULL;
1946 _prev = NULL;
1947 _notified = 0;
1948 _notifier_tid = 0;
1949 TState = TS_RUN;
1950 _thread = thread;
1951 _event = thread->_ParkEvent;
1952 _active = false;
1953 assert(_event != NULL, "invariant");
1954 }
1955
1956 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) {
1957 JavaThread *jt = (JavaThread *)this->_thread;
1958 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon);
1959 }
1960
1961 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) {
1962 JavaThread *jt = (JavaThread *)this->_thread;
1963 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active);
1964 }
1965
1966 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
1967 assert(node != NULL, "should not add NULL node");
1968 assert(node->_prev == NULL, "node already in list");
1969 assert(node->_next == NULL, "node already in list");
1970 // put node at end of queue (circular doubly linked list)
1971 if (_WaitSet == NULL) {
1972 _WaitSet = node;
1973 node->_prev = node;
1974 node->_next = node;
1975 } else {
1976 ObjectWaiter* head = _WaitSet;
1977 ObjectWaiter* tail = head->_prev;
1978 assert(tail->_next == head, "invariant check");
1979 tail->_next = node;
1980 head->_prev = node;
1981 node->_next = head;
1982 node->_prev = tail;
1983 }
1984 }
1985
1986 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() {
1987 // dequeue the very first waiter
1988 ObjectWaiter* waiter = _WaitSet;
1989 if (waiter) {
1990 DequeueSpecificWaiter(waiter);
1991 }
1992 return waiter;
1993 }
1994
1995 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) {
1996 assert(node != NULL, "should not dequeue NULL node");
1997 assert(node->_prev != NULL, "node already removed from list");
1998 assert(node->_next != NULL, "node already removed from list");
1999 // when the waiter has woken up because of interrupt,
2000 // timeout or other spurious wake-up, dequeue the
2001 // waiter from waiting list
2002 ObjectWaiter* next = node->_next;
2003 if (next == node) {
2004 assert(node->_prev == node, "invariant check");
2005 _WaitSet = NULL;
2006 } else {
2007 ObjectWaiter* prev = node->_prev;
2008 assert(prev->_next == node, "invariant check");
2009 assert(next->_prev == node, "invariant check");
2010 next->_prev = prev;
2011 prev->_next = next;
2012 if (_WaitSet == node) {
2013 _WaitSet = next;
2014 }
2015 }
2016 node->_next = NULL;
2017 node->_prev = NULL;
2018 }
2019
2020 // -----------------------------------------------------------------------------
2021 // PerfData support
2022 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL;
2023 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL;
2024 PerfCounter * ObjectMonitor::_sync_Parks = NULL;
2025 PerfCounter * ObjectMonitor::_sync_Notifications = NULL;
2026 PerfCounter * ObjectMonitor::_sync_Inflations = NULL;
2027 PerfCounter * ObjectMonitor::_sync_Deflations = NULL;
2028 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL;
2029
2030 // One-shot global initialization for the sync subsystem.
2031 // We could also defer initialization and initialize on-demand
2032 // the first time we call ObjectSynchronizer::inflate().
2033 // Initialization would be protected - like so many things - by
2034 // the MonitorCache_lock.
2035
2036 void ObjectMonitor::Initialize() {
2037 assert(!InitDone, "invariant");
2038
2039 if (!os::is_MP()) {
2040 Knob_SpinLimit = 0;
2041 Knob_PreSpin = 0;
2042 Knob_FixedSpin = -1;
2043 }
2044
2045 if (UsePerfData) {
2046 EXCEPTION_MARK;
2047 #define NEWPERFCOUNTER(n) \
2048 { \
2049 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \
2050 CHECK); \
2051 }
2052 #define NEWPERFVARIABLE(n) \
2053 { \
2054 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \
2055 CHECK); \
2056 }
2057 NEWPERFCOUNTER(_sync_Inflations);
2058 NEWPERFCOUNTER(_sync_Deflations);
2059 NEWPERFCOUNTER(_sync_ContendedLockAttempts);
2060 NEWPERFCOUNTER(_sync_FutileWakeups);
2061 NEWPERFCOUNTER(_sync_Parks);
2062 NEWPERFCOUNTER(_sync_Notifications);
2063 NEWPERFVARIABLE(_sync_MonExtant);
2064 #undef NEWPERFCOUNTER
2065 #undef NEWPERFVARIABLE
2066 }
2067
2068 DEBUG_ONLY(InitDone = true;)
2069 }
2070
2071 // For internal use by ObjectSynchronizer::monitors_iterate().
2072 ObjectMonitorHandle::ObjectMonitorHandle(ObjectMonitor * om_ptr) {
2073 om_ptr->inc_ref_count();
2074 _om_ptr = om_ptr;
2075 }
2076
2077 ObjectMonitorHandle::~ObjectMonitorHandle() {
2078 if (_om_ptr != NULL) {
2079 _om_ptr->dec_ref_count();
2080 _om_ptr = NULL;
2081 }
2082 }
2083
2084 // Save the ObjectMonitor* associated with the specified markWord and
2085 // increment the ref_count. This function should only be called if
2086 // the caller has verified mark.has_monitor() == true. The object
2087 // parameter is needed to verify that ObjectMonitor* has not been
2088 // deflated and reused for another object.
2089 //
2090 // This function returns true if the ObjectMonitor* has been safely
2091 // saved. This function returns false if we have lost a race with
2092 // async deflation; the caller should retry as appropriate.
2093 //
2094 bool ObjectMonitorHandle::save_om_ptr(oop object, markWord mark) {
2095 guarantee(mark.has_monitor(), "sanity check: mark=" INTPTR_FORMAT,
2096 mark.value());
2097
2098 ObjectMonitor * om_ptr = mark.monitor();
2099 om_ptr->inc_ref_count();
2100
2101 if (AsyncDeflateIdleMonitors) {
2102 // Race here if monitor is not owned! The above ref_count bump
2103 // will cause subsequent async deflation to skip it. However,
2104 // previous or concurrent async deflation is a race.
2105 if (om_ptr->owner_is_DEFLATER_MARKER() && om_ptr->ref_count() <= 0) {
2106 // Async deflation is in progress and our ref_count increment
2107 // above lost the race to async deflation. Attempt to restore
2108 // the header/dmw to the object's header so that we only retry
2109 // once if the deflater thread happens to be slow.
2110 om_ptr->install_displaced_markword_in_object(object);
2111 om_ptr->dec_ref_count();
2112 return false;
2113 }
2114 if (om_ptr->ref_count() <= 0) {
2115 // Async deflation is in the process of bailing out, but has not
2116 // yet restored the ref_count field so we return false to force
2117 // a retry. We want a positive ref_count value for a true return.
2118 om_ptr->dec_ref_count();
2119 return false;
2120 }
2121 // The ObjectMonitor could have been deflated and reused for
2122 // another object before we bumped the ref_count so make sure
2123 // our object still refers to this ObjectMonitor.
2124 const markWord tmp = object->mark();
2125 if (!tmp.has_monitor() || tmp.monitor() != om_ptr) {
2126 // Async deflation and reuse won the race so we have to retry.
2127 // Skip object header restoration since that's already done.
2128 om_ptr->dec_ref_count();
2129 return false;
2130 }
2131 }
2132
2133 ADIM_guarantee(_om_ptr == NULL, "sanity check: _om_ptr=" INTPTR_FORMAT,
2134 p2i(_om_ptr));
2135 _om_ptr = om_ptr;
2136 return true;
2137 }
2138
2139 // For internal use by ObjectSynchronizer::inflate().
2140 void ObjectMonitorHandle::set_om_ptr(ObjectMonitor * om_ptr) {
2141 if (_om_ptr == NULL) {
2142 ADIM_guarantee(om_ptr != NULL, "cannot clear an unset om_ptr");
2143 om_ptr->inc_ref_count();
2144 _om_ptr = om_ptr;
2145 } else {
2146 ADIM_guarantee(om_ptr == NULL, "can only clear a set om_ptr");
2147 _om_ptr->dec_ref_count();
2148 _om_ptr = NULL;
2149 }
2150 }
2151
2152 void ObjectMonitor::print_on(outputStream* st) const {
2153 // The minimal things to print for markWord printing, more can be added for debugging and logging.
2154 st->print("{contentions=0x%08x,waiters=0x%08x"
2155 ",recursions=" INTPTR_FORMAT ",owner=" INTPTR_FORMAT "}",
2156 contentions(), waiters(), recursions(),
2157 p2i(owner()));
2158 }
2159 void ObjectMonitor::print() const { print_on(tty); }