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