1 /*
2 * Copyright (c) 1998, 2014, 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).
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23 */
24
25 #include "precompiled.hpp"
26 #include "runtime/atomic.inline.hpp"
27 #include "runtime/mutex.hpp"
28 #include "runtime/orderAccess.inline.hpp"
29 #include "runtime/osThread.hpp"
30 #include "runtime/thread.inline.hpp"
31 #include "utilities/events.hpp"
32 #ifdef TARGET_OS_FAMILY_linux
33 # include "mutex_linux.inline.hpp"
34 #endif
35 #ifdef TARGET_OS_FAMILY_solaris
36 # include "mutex_solaris.inline.hpp"
37 #endif
38 #ifdef TARGET_OS_FAMILY_windows
39 # include "mutex_windows.inline.hpp"
40 #endif
41 #ifdef TARGET_OS_FAMILY_bsd
42 # include "mutex_bsd.inline.hpp"
43 #endif
44
45 PRAGMA_FORMAT_MUTE_WARNINGS_FOR_GCC
46
47 // o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o
48 //
49 // Native Monitor-Mutex locking - theory of operations
50 //
51 // * Native Monitors are completely unrelated to Java-level monitors,
52 // although the "back-end" slow-path implementations share a common lineage.
53 // See objectMonitor:: in synchronizer.cpp.
54 // Native Monitors do *not* support nesting or recursion but otherwise
55 // they're basically Hoare-flavor monitors.
56 //
57 // * A thread acquires ownership of a Monitor/Mutex by CASing the LockByte
58 // in the _LockWord from zero to non-zero. Note that the _Owner field
59 // is advisory and is used only to verify that the thread calling unlock()
60 // is indeed the last thread to have acquired the lock.
61 //
62 // * Contending threads "push" themselves onto the front of the contention
63 // queue -- called the cxq -- with CAS and then spin/park.
64 // The _LockWord contains the LockByte as well as the pointer to the head
65 // of the cxq. Colocating the LockByte with the cxq precludes certain races.
66 //
67 // * Using a separately addressable LockByte allows for CAS:MEMBAR or CAS:0
68 // idioms. We currently use MEMBAR in the uncontended unlock() path, as
69 // MEMBAR often has less latency than CAS. If warranted, we could switch to
70 // a CAS:0 mode, using timers to close the resultant race, as is done
71 // with Java Monitors in synchronizer.cpp.
72 //
73 // See the following for a discussion of the relative cost of atomics (CAS)
74 // MEMBAR, and ways to eliminate such instructions from the common-case paths:
75 // -- http://blogs.sun.com/dave/entry/biased_locking_in_hotspot
76 // -- http://blogs.sun.com/dave/resource/MustangSync.pdf
77 // -- http://blogs.sun.com/dave/resource/synchronization-public2.pdf
78 // -- synchronizer.cpp
79 //
80 // * Overall goals - desiderata
81 // 1. Minimize context switching
82 // 2. Minimize lock migration
83 // 3. Minimize CPI -- affinity and locality
84 // 4. Minimize the execution of high-latency instructions such as CAS or MEMBAR
85 // 5. Minimize outer lock hold times
86 // 6. Behave gracefully on a loaded system
87 //
88 // * Thread flow and list residency:
89 //
90 // Contention queue --> EntryList --> OnDeck --> Owner --> !Owner
91 // [..resident on monitor list..]
92 // [...........contending..................]
93 //
94 // -- The contention queue (cxq) contains recently-arrived threads (RATs).
95 // Threads on the cxq eventually drain into the EntryList.
96 // -- Invariant: a thread appears on at most one list -- cxq, EntryList
97 // or WaitSet -- at any one time.
98 // -- For a given monitor there can be at most one "OnDeck" thread at any
99 // given time but if needbe this particular invariant could be relaxed.
100 //
101 // * The WaitSet and EntryList linked lists are composed of ParkEvents.
102 // I use ParkEvent instead of threads as ParkEvents are immortal and
103 // type-stable, meaning we can safely unpark() a possibly stale
104 // list element in the unlock()-path. (That's benign).
105 //
106 // * Succession policy - providing for progress:
107 //
108 // As necessary, the unlock()ing thread identifies, unlinks, and unparks
109 // an "heir presumptive" tentative successor thread from the EntryList.
110 // This becomes the so-called "OnDeck" thread, of which there can be only
111 // one at any given time for a given monitor. The wakee will recontend
112 // for ownership of monitor.
113 //
114 // Succession is provided for by a policy of competitive handoff.
115 // The exiting thread does _not_ grant or pass ownership to the
116 // successor thread. (This is also referred to as "handoff" succession").
117 // Instead the exiting thread releases ownership and possibly wakes
118 // a successor, so the successor can (re)compete for ownership of the lock.
119 //
120 // Competitive handoff provides excellent overall throughput at the expense
121 // of short-term fairness. If fairness is a concern then one remedy might
122 // be to add an AcquireCounter field to the monitor. After a thread acquires
123 // the lock it will decrement the AcquireCounter field. When the count
124 // reaches 0 the thread would reset the AcquireCounter variable, abdicate
125 // the lock directly to some thread on the EntryList, and then move itself to the
126 // tail of the EntryList.
127 //
128 // But in practice most threads engage or otherwise participate in resource
129 // bounded producer-consumer relationships, so lock domination is not usually
130 // a practical concern. Recall too, that in general it's easier to construct
131 // a fair lock from a fast lock, but not vice-versa.
132 //
133 // * The cxq can have multiple concurrent "pushers" but only one concurrent
134 // detaching thread. This mechanism is immune from the ABA corruption.
135 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
136 // We use OnDeck as a pseudo-lock to enforce the at-most-one detaching
137 // thread constraint.
138 //
139 // * Taken together, the cxq and the EntryList constitute or form a
140 // single logical queue of threads stalled trying to acquire the lock.
141 // We use two distinct lists to reduce heat on the list ends.
142 // Threads in lock() enqueue onto cxq while threads in unlock() will
143 // dequeue from the EntryList. (c.f. Michael Scott's "2Q" algorithm).
144 // A key desideratum is to minimize queue & monitor metadata manipulation
145 // that occurs while holding the "outer" monitor lock -- that is, we want to
146 // minimize monitor lock holds times.
147 //
148 // The EntryList is ordered by the prevailing queue discipline and
149 // can be organized in any convenient fashion, such as a doubly-linked list or
150 // a circular doubly-linked list. If we need a priority queue then something akin
151 // to Solaris' sleepq would work nicely. Viz.,
152 // -- http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
153 // -- http://cvs.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/uts/common/os/sleepq.c
154 // Queue discipline is enforced at ::unlock() time, when the unlocking thread
155 // drains the cxq into the EntryList, and orders or reorders the threads on the
156 // EntryList accordingly.
157 //
158 // Barring "lock barging", this mechanism provides fair cyclic ordering,
159 // somewhat similar to an elevator-scan.
160 //
161 // * OnDeck
162 // -- For a given monitor there can be at most one OnDeck thread at any given
163 // instant. The OnDeck thread is contending for the lock, but has been
164 // unlinked from the EntryList and cxq by some previous unlock() operations.
165 // Once a thread has been designated the OnDeck thread it will remain so
166 // until it manages to acquire the lock -- being OnDeck is a stable property.
167 // -- Threads on the EntryList or cxq are _not allowed to attempt lock acquisition.
168 // -- OnDeck also serves as an "inner lock" as follows. Threads in unlock() will, after
169 // having cleared the LockByte and dropped the outer lock, attempt to "trylock"
170 // OnDeck by CASing the field from null to non-null. If successful, that thread
171 // is then responsible for progress and succession and can use CAS to detach and
172 // drain the cxq into the EntryList. By convention, only this thread, the holder of
173 // the OnDeck inner lock, can manipulate the EntryList or detach and drain the
174 // RATs on the cxq into the EntryList. This avoids ABA corruption on the cxq as
175 // we allow multiple concurrent "push" operations but restrict detach concurrency
176 // to at most one thread. Having selected and detached a successor, the thread then
177 // changes the OnDeck to refer to that successor, and then unparks the successor.
178 // That successor will eventually acquire the lock and clear OnDeck. Beware
179 // that the OnDeck usage as a lock is asymmetric. A thread in unlock() transiently
180 // "acquires" OnDeck, performs queue manipulations, passes OnDeck to some successor,
181 // and then the successor eventually "drops" OnDeck. Note that there's never
182 // any sense of contention on the inner lock, however. Threads never contend
183 // or wait for the inner lock.
184 // -- OnDeck provides for futile wakeup throttling a described in section 3.3 of
185 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
186 // In a sense, OnDeck subsumes the ObjectMonitor _Succ and ObjectWaiter
187 // TState fields found in Java-level objectMonitors. (See synchronizer.cpp).
188 //
189 // * Waiting threads reside on the WaitSet list -- wait() puts
190 // the caller onto the WaitSet. Notify() or notifyAll() simply
191 // transfers threads from the WaitSet to either the EntryList or cxq.
192 // Subsequent unlock() operations will eventually unpark the notifyee.
193 // Unparking a notifee in notify() proper is inefficient - if we were to do so
194 // it's likely the notifyee would simply impale itself on the lock held
195 // by the notifier.
196 //
197 // * The mechanism is obstruction-free in that if the holder of the transient
198 // OnDeck lock in unlock() is preempted or otherwise stalls, other threads
199 // can still acquire and release the outer lock and continue to make progress.
200 // At worst, waking of already blocked contending threads may be delayed,
201 // but nothing worse. (We only use "trylock" operations on the inner OnDeck
202 // lock).
203 //
204 // * Note that thread-local storage must be initialized before a thread
205 // uses Native monitors or mutexes. The native monitor-mutex subsystem
206 // depends on Thread::current().
207 //
208 // * The monitor synchronization subsystem avoids the use of native
209 // synchronization primitives except for the narrow platform-specific
210 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
211 // the semantics of park-unpark. Put another way, this monitor implementation
212 // depends only on atomic operations and park-unpark. The monitor subsystem
213 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
214 // underlying OS manages the READY<->RUN transitions.
215 //
216 // * The memory consistency model provide by lock()-unlock() is at least as
217 // strong or stronger than the Java Memory model defined by JSR-133.
218 // That is, we guarantee at least entry consistency, if not stronger.
219 // See http://g.oswego.edu/dl/jmm/cookbook.html.
220 //
221 // * Thread:: currently contains a set of purpose-specific ParkEvents:
222 // _MutexEvent, _ParkEvent, etc. A better approach might be to do away with
223 // the purpose-specific ParkEvents and instead implement a general per-thread
224 // stack of available ParkEvents which we could provision on-demand. The
225 // stack acts as a local cache to avoid excessive calls to ParkEvent::Allocate()
226 // and ::Release(). A thread would simply pop an element from the local stack before it
227 // enqueued or park()ed. When the contention was over the thread would
228 // push the no-longer-needed ParkEvent back onto its stack.
229 //
230 // * A slightly reduced form of ILock() and IUnlock() have been partially
231 // model-checked (Murphi) for safety and progress at T=1,2,3 and 4.
232 // It'd be interesting to see if TLA/TLC could be useful as well.
233 //
234 // * Mutex-Monitor is a low-level "leaf" subsystem. That is, the monitor
235 // code should never call other code in the JVM that might itself need to
236 // acquire monitors or mutexes. That's true *except* in the case of the
237 // ThreadBlockInVM state transition wrappers. The ThreadBlockInVM DTOR handles
238 // mutator reentry (ingress) by checking for a pending safepoint in which case it will
239 // call SafepointSynchronize::block(), which in turn may call Safepoint_lock->lock(), etc.
240 // In that particular case a call to lock() for a given Monitor can end up recursively
241 // calling lock() on another monitor. While distasteful, this is largely benign
242 // as the calls come from jacket that wraps lock(), and not from deep within lock() itself.
243 //
244 // It's unfortunate that native mutexes and thread state transitions were convolved.
245 // They're really separate concerns and should have remained that way. Melding
246 // them together was facile -- a bit too facile. The current implementation badly
247 // conflates the two concerns.
248 //
249 // * TODO-FIXME:
250 //
251 // -- Add DTRACE probes for contended acquire, contended acquired, contended unlock
252 // We should also add DTRACE probes in the ParkEvent subsystem for
253 // Park-entry, Park-exit, and Unpark.
254 //
255 // -- We have an excess of mutex-like constructs in the JVM, namely:
256 // 1. objectMonitors for Java-level synchronization (synchronizer.cpp)
257 // 2. low-level muxAcquire and muxRelease
258 // 3. low-level spinAcquire and spinRelease
259 // 4. native Mutex:: and Monitor::
260 // 5. jvm_raw_lock() and _unlock()
261 // 6. JVMTI raw monitors -- distinct from (5) despite having a confusingly
262 // similar name.
263 //
264 // o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o
265
266
267 // CASPTR() uses the canonical argument order that dominates in the literature.
268 // Our internal cmpxchg_ptr() uses a bastardized ordering to accommodate Sun .il templates.
269
270 #define CASPTR(a, c, s) \
271 intptr_t(Atomic::cmpxchg_ptr((void *)(s), (void *)(a), (void *)(c)))
272 #define UNS(x) (uintptr_t(x))
273 #define TRACE(m) \
274 { \
275 static volatile int ctr = 0; \
276 int x = ++ctr; \
277 if ((x & (x - 1)) == 0) { \
278 ::printf("%d:%s\n", x, #m); \
279 ::fflush(stdout); \
280 } \
281 }
282
283 // Simplistic low-quality Marsaglia SHIFT-XOR RNG.
284 // Bijective except for the trailing mask operation.
285 // Useful for spin loops as the compiler can't optimize it away.
286
287 static inline jint MarsagliaXORV(jint x) {
288 if (x == 0) x = 1|os::random();
289 x ^= x << 6;
290 x ^= ((unsigned)x) >> 21;
291 x ^= x << 7;
292 return x & 0x7FFFFFFF;
293 }
294
295 static int Stall(int its) {
296 static volatile jint rv = 1;
297 volatile int OnFrame = 0;
298 jint v = rv ^ UNS(OnFrame);
299 while (--its >= 0) {
300 v = MarsagliaXORV(v);
301 }
302 // Make this impossible for the compiler to optimize away,
303 // but (mostly) avoid W coherency sharing on MP systems.
304 if (v == 0x12345) rv = v;
305 return v;
306 }
307
308 int Monitor::TryLock() {
309 intptr_t v = _LockWord.FullWord;
310 for (;;) {
311 if ((v & _LBIT) != 0) return 0;
312 const intptr_t u = CASPTR(&_LockWord, v, v|_LBIT);
313 if (v == u) return 1;
314 v = u;
315 }
316 }
317
318 int Monitor::TryFast() {
319 // Optimistic fast-path form ...
320 // Fast-path attempt for the common uncontended case.
321 // Avoid RTS->RTO $ coherence upgrade on typical SMP systems.
322 intptr_t v = CASPTR(&_LockWord, 0, _LBIT); // agro ...
323 if (v == 0) return 1;
324
325 for (;;) {
326 if ((v & _LBIT) != 0) return 0;
327 const intptr_t u = CASPTR(&_LockWord, v, v|_LBIT);
328 if (v == u) return 1;
329 v = u;
330 }
331 }
332
333 int Monitor::ILocked() {
334 const intptr_t w = _LockWord.FullWord & 0xFF;
335 assert(w == 0 || w == _LBIT, "invariant");
336 return w == _LBIT;
337 }
338
339 // Polite TATAS spinlock with exponential backoff - bounded spin.
340 // Ideally we'd use processor cycles, time or vtime to control
341 // the loop, but we currently use iterations.
342 // All the constants within were derived empirically but work over
343 // over the spectrum of J2SE reference platforms.
344 // On Niagara-class systems the back-off is unnecessary but
345 // is relatively harmless. (At worst it'll slightly retard
346 // acquisition times). The back-off is critical for older SMP systems
347 // where constant fetching of the LockWord would otherwise impair
348 // scalability.
349 //
350 // Clamp spinning at approximately 1/2 of a context-switch round-trip.
351 // See synchronizer.cpp for details and rationale.
352
353 int Monitor::TrySpin(Thread * const Self) {
354 if (TryLock()) return 1;
355 if (!os::is_MP()) return 0;
356
357 int Probes = 0;
358 int Delay = 0;
359 int Steps = 0;
360 int SpinMax = NativeMonitorSpinLimit;
361 int flgs = NativeMonitorFlags;
362 for (;;) {
363 intptr_t v = _LockWord.FullWord;
364 if ((v & _LBIT) == 0) {
365 if (CASPTR (&_LockWord, v, v|_LBIT) == v) {
366 return 1;
367 }
368 continue;
369 }
370
371 if ((flgs & 8) == 0) {
372 SpinPause();
373 }
374
375 // Periodically increase Delay -- variable Delay form
376 // conceptually: delay *= 1 + 1/Exponent
377 ++Probes;
378 if (Probes > SpinMax) return 0;
379
380 if ((Probes & 0x7) == 0) {
381 Delay = ((Delay << 1)|1) & 0x7FF;
382 // CONSIDER: Delay += 1 + (Delay/4); Delay &= 0x7FF ;
383 }
384
385 if (flgs & 2) continue;
386
387 // Consider checking _owner's schedctl state, if OFFPROC abort spin.
388 // If the owner is OFFPROC then it's unlike that the lock will be dropped
389 // in a timely fashion, which suggests that spinning would not be fruitful
390 // or profitable.
391
392 // Stall for "Delay" time units - iterations in the current implementation.
393 // Avoid generating coherency traffic while stalled.
394 // Possible ways to delay:
395 // PAUSE, SLEEP, MEMBAR #sync, MEMBAR #halt,
396 // wr %g0,%asi, gethrtime, rdstick, rdtick, rdtsc, etc. ...
397 // Note that on Niagara-class systems we want to minimize STs in the
398 // spin loop. N1 and brethren write-around the L1$ over the xbar into the L2$.
399 // Furthermore, they don't have a W$ like traditional SPARC processors.
400 // We currently use a Marsaglia Shift-Xor RNG loop.
401 Steps += Delay;
402 if (Self != NULL) {
403 jint rv = Self->rng[0];
404 for (int k = Delay; --k >= 0;) {
405 rv = MarsagliaXORV(rv);
406 if ((flgs & 4) == 0 && SafepointSynchronize::do_call_back()) return 0;
407 }
408 Self->rng[0] = rv;
409 } else {
410 Stall(Delay);
411 }
412 }
413 }
414
415 static int ParkCommon(ParkEvent * ev, jlong timo) {
416 // Diagnostic support - periodically unwedge blocked threads
417 intx nmt = NativeMonitorTimeout;
418 if (nmt > 0 && (nmt < timo || timo <= 0)) {
419 timo = nmt;
420 }
421 int err = OS_OK;
422 if (0 == timo) {
423 ev->park();
424 } else {
425 err = ev->park(timo);
426 }
427 return err;
428 }
429
430 inline int Monitor::AcquireOrPush(ParkEvent * ESelf) {
431 intptr_t v = _LockWord.FullWord;
432 for (;;) {
433 if ((v & _LBIT) == 0) {
434 const intptr_t u = CASPTR(&_LockWord, v, v|_LBIT);
435 if (u == v) return 1; // indicate acquired
436 v = u;
437 } else {
438 // Anticipate success ...
439 ESelf->ListNext = (ParkEvent *)(v & ~_LBIT);
440 const intptr_t u = CASPTR(&_LockWord, v, intptr_t(ESelf)|_LBIT);
441 if (u == v) return 0; // indicate pushed onto cxq
442 v = u;
443 }
444 // Interference - LockWord change - just retry
445 }
446 }
447
448 // ILock and IWait are the lowest level primitive internal blocking
449 // synchronization functions. The callers of IWait and ILock must have
450 // performed any needed state transitions beforehand.
451 // IWait and ILock may directly call park() without any concern for thread state.
452 // Note that ILock and IWait do *not* access _owner.
453 // _owner is a higher-level logical concept.
454
455 void Monitor::ILock(Thread * Self) {
456 assert(_OnDeck != Self->_MutexEvent, "invariant");
457
458 if (TryFast()) {
459 Exeunt:
460 assert(ILocked(), "invariant");
461 return;
462 }
463
464 ParkEvent * const ESelf = Self->_MutexEvent;
465 assert(_OnDeck != ESelf, "invariant");
466
467 // As an optimization, spinners could conditionally try to set ONDECK to _LBIT
468 // Synchronizer.cpp uses a similar optimization.
469 if (TrySpin(Self)) goto Exeunt;
470
471 // Slow-path - the lock is contended.
472 // Either Enqueue Self on cxq or acquire the outer lock.
473 // LockWord encoding = (cxq,LOCKBYTE)
474 ESelf->reset();
475 OrderAccess::fence();
476
477 // Optional optimization ... try barging on the inner lock
478 if ((NativeMonitorFlags & 32) && CASPTR (&_OnDeck, NULL, UNS(Self)) == 0) {
479 goto OnDeck_LOOP;
480 }
481
482 if (AcquireOrPush(ESelf)) goto Exeunt;
483
484 // At any given time there is at most one ondeck thread.
485 // ondeck implies not resident on cxq and not resident on EntryList
486 // Only the OnDeck thread can try to acquire -- contended for -- the lock.
487 // CONSIDER: use Self->OnDeck instead of m->OnDeck.
488 // Deschedule Self so that others may run.
489 while (_OnDeck != ESelf) {
490 ParkCommon(ESelf, 0);
491 }
492
493 // Self is now in the ONDECK position and will remain so until it
494 // manages to acquire the lock.
495 OnDeck_LOOP:
496 for (;;) {
497 assert(_OnDeck == ESelf, "invariant");
498 if (TrySpin(Self)) break;
499 // It's probably wise to spin only if we *actually* blocked
500 // CONSIDER: check the lockbyte, if it remains set then
501 // preemptively drain the cxq into the EntryList.
502 // The best place and time to perform queue operations -- lock metadata --
503 // is _before having acquired the outer lock, while waiting for the lock to drop.
504 ParkCommon(ESelf, 0);
505 }
506
507 assert(_OnDeck == ESelf, "invariant");
508 _OnDeck = NULL;
509
510 // Note that we current drop the inner lock (clear OnDeck) in the slow-path
511 // epilogue immediately after having acquired the outer lock.
512 // But instead we could consider the following optimizations:
513 // A. Shift or defer dropping the inner lock until the subsequent IUnlock() operation.
514 // This might avoid potential reacquisition of the inner lock in IUlock().
515 // B. While still holding the inner lock, attempt to opportunistically select
516 // and unlink the next ONDECK thread from the EntryList.
517 // If successful, set ONDECK to refer to that thread, otherwise clear ONDECK.
518 // It's critical that the select-and-unlink operation run in constant-time as
519 // it executes when holding the outer lock and may artificially increase the
520 // effective length of the critical section.
521 // Note that (A) and (B) are tantamount to succession by direct handoff for
522 // the inner lock.
523 goto Exeunt;
524 }
525
526 void Monitor::IUnlock(bool RelaxAssert) {
527 assert(ILocked(), "invariant");
528 // Conceptually we need a MEMBAR #storestore|#loadstore barrier or fence immediately
529 // before the store that releases the lock. Crucially, all the stores and loads in the
530 // critical section must be globally visible before the store of 0 into the lock-word
531 // that releases the lock becomes globally visible. That is, memory accesses in the
532 // critical section should not be allowed to bypass or overtake the following ST that
533 // releases the lock. As such, to prevent accesses within the critical section
534 // from "leaking" out, we need a release fence between the critical section and the
535 // store that releases the lock. In practice that release barrier is elided on
536 // platforms with strong memory models such as TSO.
537 //
538 // Note that the OrderAccess::storeload() fence that appears after unlock store
539 // provides for progress conditions and succession and is _not related to exclusion
540 // safety or lock release consistency.
541 OrderAccess::release_store(&_LockWord.Bytes[_LSBINDEX], 0); // drop outer lock
542
543 OrderAccess::storeload();
544 ParkEvent * const w = _OnDeck;
545 assert(RelaxAssert || w != Thread::current()->_MutexEvent, "invariant");
546 if (w != NULL) {
547 // Either we have a valid ondeck thread or ondeck is transiently "locked"
548 // by some exiting thread as it arranges for succession. The LSBit of
549 // OnDeck allows us to discriminate two cases. If the latter, the
550 // responsibility for progress and succession lies with that other thread.
551 // For good performance, we also depend on the fact that redundant unpark()
552 // operations are cheap. That is, repeated Unpark()ing of the ONDECK thread
553 // is inexpensive. This approach provides implicit futile wakeup throttling.
554 // Note that the referent "w" might be stale with respect to the lock.
555 // In that case the following unpark() is harmless and the worst that'll happen
556 // is a spurious return from a park() operation. Critically, if "w" _is stale,
557 // then progress is known to have occurred as that means the thread associated
558 // with "w" acquired the lock. In that case this thread need take no further
559 // action to guarantee progress.
560 if ((UNS(w) & _LBIT) == 0) w->unpark();
561 return;
562 }
563
564 intptr_t cxq = _LockWord.FullWord;
565 if (((cxq & ~_LBIT)|UNS(_EntryList)) == 0) {
566 return; // normal fast-path exit - cxq and EntryList both empty
567 }
568 if (cxq & _LBIT) {
569 // Optional optimization ...
570 // Some other thread acquired the lock in the window since this
571 // thread released it. Succession is now that thread's responsibility.
572 return;
573 }
574
575 Succession:
576 // Slow-path exit - this thread must ensure succession and progress.
577 // OnDeck serves as lock to protect cxq and EntryList.
578 // Only the holder of OnDeck can manipulate EntryList or detach the RATs from cxq.
579 // Avoid ABA - allow multiple concurrent producers (enqueue via push-CAS)
580 // but only one concurrent consumer (detacher of RATs).
581 // Consider protecting this critical section with schedctl on Solaris.
582 // Unlike a normal lock, however, the exiting thread "locks" OnDeck,
583 // picks a successor and marks that thread as OnDeck. That successor
584 // thread will then clear OnDeck once it eventually acquires the outer lock.
585 if (CASPTR (&_OnDeck, NULL, _LBIT) != UNS(NULL)) {
586 return;
587 }
588
589 ParkEvent * List = _EntryList;
590 if (List != NULL) {
591 // Transfer the head of the EntryList to the OnDeck position.
592 // Once OnDeck, a thread stays OnDeck until it acquires the lock.
593 // For a given lock there is at most OnDeck thread at any one instant.
594 WakeOne:
595 assert(List == _EntryList, "invariant");
596 ParkEvent * const w = List;
597 assert(RelaxAssert || w != Thread::current()->_MutexEvent, "invariant");
598 _EntryList = w->ListNext;
599 // as a diagnostic measure consider setting w->_ListNext = BAD
600 assert(UNS(_OnDeck) == _LBIT, "invariant");
601 _OnDeck = w; // pass OnDeck to w.
602 // w will clear OnDeck once it acquires the outer lock
603
604 // Another optional optimization ...
605 // For heavily contended locks it's not uncommon that some other
606 // thread acquired the lock while this thread was arranging succession.
607 // Try to defer the unpark() operation - Delegate the responsibility
608 // for unpark()ing the OnDeck thread to the current or subsequent owners
609 // That is, the new owner is responsible for unparking the OnDeck thread.
610 OrderAccess::storeload();
611 cxq = _LockWord.FullWord;
612 if (cxq & _LBIT) return;
613
614 w->unpark();
615 return;
616 }
617
618 cxq = _LockWord.FullWord;
619 if ((cxq & ~_LBIT) != 0) {
620 // The EntryList is empty but the cxq is populated.
621 // drain RATs from cxq into EntryList
622 // Detach RATs segment with CAS and then merge into EntryList
623 for (;;) {
624 // optional optimization - if locked, the owner is responsible for succession
625 if (cxq & _LBIT) goto Punt;
626 const intptr_t vfy = CASPTR(&_LockWord, cxq, cxq & _LBIT);
627 if (vfy == cxq) break;
628 cxq = vfy;
629 // Interference - LockWord changed - Just retry
630 // We can see concurrent interference from contending threads
631 // pushing themselves onto the cxq or from lock-unlock operations.
632 // From the perspective of this thread, EntryList is stable and
633 // the cxq is prepend-only -- the head is volatile but the interior
634 // of the cxq is stable. In theory if we encounter interference from threads
635 // pushing onto cxq we could simply break off the original cxq suffix and
636 // move that segment to the EntryList, avoiding a 2nd or multiple CAS attempts
637 // on the high-traffic LockWord variable. For instance lets say the cxq is "ABCD"
638 // when we first fetch cxq above. Between the fetch -- where we observed "A"
639 // -- and CAS -- where we attempt to CAS null over A -- "PQR" arrive,
640 // yielding cxq = "PQRABCD". In this case we could simply set A.ListNext
641 // null, leaving cxq = "PQRA" and transfer the "BCD" segment to the EntryList.
642 // Note too, that it's safe for this thread to traverse the cxq
643 // without taking any special concurrency precautions.
644 }
645
646 // We don't currently reorder the cxq segment as we move it onto
647 // the EntryList, but it might make sense to reverse the order
648 // or perhaps sort by thread priority. See the comments in
649 // synchronizer.cpp objectMonitor::exit().
650 assert(_EntryList == NULL, "invariant");
651 _EntryList = List = (ParkEvent *)(cxq & ~_LBIT);
652 assert(List != NULL, "invariant");
653 goto WakeOne;
654 }
655
656 // cxq|EntryList is empty.
657 // w == NULL implies that cxq|EntryList == NULL in the past.
658 // Possible race - rare inopportune interleaving.
659 // A thread could have added itself to cxq since this thread previously checked.
660 // Detect and recover by refetching cxq.
661 Punt:
662 assert(UNS(_OnDeck) == _LBIT, "invariant");
663 _OnDeck = NULL; // Release inner lock.
664 OrderAccess::storeload(); // Dekker duality - pivot point
665
666 // Resample LockWord/cxq to recover from possible race.
667 // For instance, while this thread T1 held OnDeck, some other thread T2 might
668 // acquire the outer lock. Another thread T3 might try to acquire the outer
669 // lock, but encounter contention and enqueue itself on cxq. T2 then drops the
670 // outer lock, but skips succession as this thread T1 still holds OnDeck.
671 // T1 is and remains responsible for ensuring succession of T3.
672 //
673 // Note that we don't need to recheck EntryList, just cxq.
674 // If threads moved onto EntryList since we dropped OnDeck
675 // that implies some other thread forced succession.
676 cxq = _LockWord.FullWord;
677 if ((cxq & ~_LBIT) != 0 && (cxq & _LBIT) == 0) {
678 goto Succession; // potential race -- re-run succession
679 }
680 return;
681 }
682
683 bool Monitor::notify() {
684 assert(_owner == Thread::current(), "invariant");
685 assert(ILocked(), "invariant");
686 if (_WaitSet == NULL) return true;
687 NotifyCount++;
688
689 // Transfer one thread from the WaitSet to the EntryList or cxq.
690 // Currently we just unlink the head of the WaitSet and prepend to the cxq.
691 // And of course we could just unlink it and unpark it, too, but
692 // in that case it'd likely impale itself on the reentry.
693 Thread::muxAcquire(_WaitLock, "notify:WaitLock");
694 ParkEvent * nfy = _WaitSet;
695 if (nfy != NULL) { // DCL idiom
696 _WaitSet = nfy->ListNext;
697 assert(nfy->Notified == 0, "invariant");
698 // push nfy onto the cxq
699 for (;;) {
700 const intptr_t v = _LockWord.FullWord;
701 assert((v & 0xFF) == _LBIT, "invariant");
702 nfy->ListNext = (ParkEvent *)(v & ~_LBIT);
703 if (CASPTR (&_LockWord, v, UNS(nfy)|_LBIT) == v) break;
704 // interference - _LockWord changed -- just retry
705 }
706 // Note that setting Notified before pushing nfy onto the cxq is
707 // also legal and safe, but the safety properties are much more
708 // subtle, so for the sake of code stewardship ...
709 OrderAccess::fence();
710 nfy->Notified = 1;
711 }
712 Thread::muxRelease(_WaitLock);
713 if (nfy != NULL && (NativeMonitorFlags & 16)) {
714 // Experimental code ... light up the wakee in the hope that this thread (the owner)
715 // will drop the lock just about the time the wakee comes ONPROC.
716 nfy->unpark();
717 }
718 assert(ILocked(), "invariant");
719 return true;
720 }
721
722 // Currently notifyAll() transfers the waiters one-at-a-time from the waitset
723 // to the cxq. This could be done more efficiently with a single bulk en-mass transfer,
724 // but in practice notifyAll() for large #s of threads is rare and not time-critical.
725 // Beware too, that we invert the order of the waiters. Lets say that the
726 // waitset is "ABCD" and the cxq is "XYZ". After a notifyAll() the waitset
727 // will be empty and the cxq will be "DCBAXYZ". This is benign, of course.
728
729 bool Monitor::notify_all() {
730 assert(_owner == Thread::current(), "invariant");
731 assert(ILocked(), "invariant");
732 while (_WaitSet != NULL) notify();
733 return true;
734 }
735
736 int Monitor::IWait(Thread * Self, jlong timo) {
737 assert(ILocked(), "invariant");
738
739 // Phases:
740 // 1. Enqueue Self on WaitSet - currently prepend
741 // 2. unlock - drop the outer lock
742 // 3. wait for either notification or timeout
743 // 4. lock - reentry - reacquire the outer lock
744
745 ParkEvent * const ESelf = Self->_MutexEvent;
746 ESelf->Notified = 0;
747 ESelf->reset();
748 OrderAccess::fence();
749
750 // Add Self to WaitSet
751 // Ideally only the holder of the outer lock would manipulate the WaitSet -
752 // That is, the outer lock would implicitly protect the WaitSet.
753 // But if a thread in wait() encounters a timeout it will need to dequeue itself
754 // from the WaitSet _before it becomes the owner of the lock. We need to dequeue
755 // as the ParkEvent -- which serves as a proxy for the thread -- can't reside
756 // on both the WaitSet and the EntryList|cxq at the same time.. That is, a thread
757 // on the WaitSet can't be allowed to compete for the lock until it has managed to
758 // unlink its ParkEvent from WaitSet. Thus the need for WaitLock.
759 // Contention on the WaitLock is minimal.
760 //
761 // Another viable approach would be add another ParkEvent, "WaitEvent" to the
762 // thread class. The WaitSet would be composed of WaitEvents. Only the
763 // owner of the outer lock would manipulate the WaitSet. A thread in wait()
764 // could then compete for the outer lock, and then, if necessary, unlink itself
765 // from the WaitSet only after having acquired the outer lock. More precisely,
766 // there would be no WaitLock. A thread in in wait() would enqueue its WaitEvent
767 // on the WaitSet; release the outer lock; wait for either notification or timeout;
768 // reacquire the inner lock; and then, if needed, unlink itself from the WaitSet.
769 //
770 // Alternatively, a 2nd set of list link fields in the ParkEvent might suffice.
771 // One set would be for the WaitSet and one for the EntryList.
772 // We could also deconstruct the ParkEvent into a "pure" event and add a
773 // new immortal/TSM "ListElement" class that referred to ParkEvents.
774 // In that case we could have one ListElement on the WaitSet and another
775 // on the EntryList, with both referring to the same pure Event.
776
777 Thread::muxAcquire(_WaitLock, "wait:WaitLock:Add");
778 ESelf->ListNext = _WaitSet;
779 _WaitSet = ESelf;
780 Thread::muxRelease(_WaitLock);
781
782 // Release the outer lock
783 // We call IUnlock (RelaxAssert=true) as a thread T1 might
784 // enqueue itself on the WaitSet, call IUnlock(), drop the lock,
785 // and then stall before it can attempt to wake a successor.
786 // Some other thread T2 acquires the lock, and calls notify(), moving
787 // T1 from the WaitSet to the cxq. T2 then drops the lock. T1 resumes,
788 // and then finds *itself* on the cxq. During the course of a normal
789 // IUnlock() call a thread should _never find itself on the EntryList
790 // or cxq, but in the case of wait() it's possible.
791 // See synchronizer.cpp objectMonitor::wait().
792 IUnlock(true);
793
794 // Wait for either notification or timeout
795 // Beware that in some circumstances we might propagate
796 // spurious wakeups back to the caller.
797
798 for (;;) {
799 if (ESelf->Notified) break;
800 int err = ParkCommon(ESelf, timo);
801 if (err == OS_TIMEOUT || (NativeMonitorFlags & 1)) break;
802 }
803
804 // Prepare for reentry - if necessary, remove ESelf from WaitSet
805 // ESelf can be:
806 // 1. Still on the WaitSet. This can happen if we exited the loop by timeout.
807 // 2. On the cxq or EntryList
808 // 3. Not resident on cxq, EntryList or WaitSet, but in the OnDeck position.
809
810 OrderAccess::fence();
811 int WasOnWaitSet = 0;
812 if (ESelf->Notified == 0) {
813 Thread::muxAcquire(_WaitLock, "wait:WaitLock:remove");
814 if (ESelf->Notified == 0) { // DCL idiom
815 assert(_OnDeck != ESelf, "invariant"); // can't be both OnDeck and on WaitSet
816 // ESelf is resident on the WaitSet -- unlink it.
817 // A doubly-linked list would be better here so we can unlink in constant-time.
818 // We have to unlink before we potentially recontend as ESelf might otherwise
819 // end up on the cxq|EntryList -- it can't be on two lists at once.
820 ParkEvent * p = _WaitSet;
821 ParkEvent * q = NULL; // classic q chases p
822 while (p != NULL && p != ESelf) {
823 q = p;
824 p = p->ListNext;
825 }
826 assert(p == ESelf, "invariant");
827 if (p == _WaitSet) { // found at head
828 assert(q == NULL, "invariant");
829 _WaitSet = p->ListNext;
830 } else { // found in interior
831 assert(q->ListNext == p, "invariant");
832 q->ListNext = p->ListNext;
833 }
834 WasOnWaitSet = 1; // We were *not* notified but instead encountered timeout
835 }
836 Thread::muxRelease(_WaitLock);
837 }
838
839 // Reentry phase - reacquire the lock
840 if (WasOnWaitSet) {
841 // ESelf was previously on the WaitSet but we just unlinked it above
842 // because of a timeout. ESelf is not resident on any list and is not OnDeck
843 assert(_OnDeck != ESelf, "invariant");
844 ILock(Self);
845 } else {
846 // A prior notify() operation moved ESelf from the WaitSet to the cxq.
847 // ESelf is now on the cxq, EntryList or at the OnDeck position.
848 // The following fragment is extracted from Monitor::ILock()
849 for (;;) {
850 if (_OnDeck == ESelf && TrySpin(Self)) break;
851 ParkCommon(ESelf, 0);
852 }
853 assert(_OnDeck == ESelf, "invariant");
854 _OnDeck = NULL;
855 }
856
857 assert(ILocked(), "invariant");
858 return WasOnWaitSet != 0; // return true IFF timeout
859 }
860
861
862 // ON THE VMTHREAD SNEAKING PAST HELD LOCKS:
863 // In particular, there are certain types of global lock that may be held
864 // by a Java thread while it is blocked at a safepoint but before it has
865 // written the _owner field. These locks may be sneakily acquired by the
866 // VM thread during a safepoint to avoid deadlocks. Alternatively, one should
867 // identify all such locks, and ensure that Java threads never block at
868 // safepoints while holding them (_no_safepoint_check_flag). While it
869 // seems as though this could increase the time to reach a safepoint
870 // (or at least increase the mean, if not the variance), the latter
871 // approach might make for a cleaner, more maintainable JVM design.
872 //
873 // Sneaking is vile and reprehensible and should be excised at the 1st
874 // opportunity. It's possible that the need for sneaking could be obviated
875 // as follows. Currently, a thread might (a) while TBIVM, call pthread_mutex_lock
876 // or ILock() thus acquiring the "physical" lock underlying Monitor/Mutex.
877 // (b) stall at the TBIVM exit point as a safepoint is in effect. Critically,
878 // it'll stall at the TBIVM reentry state transition after having acquired the
879 // underlying lock, but before having set _owner and having entered the actual
880 // critical section. The lock-sneaking facility leverages that fact and allowed the
881 // VM thread to logically acquire locks that had already be physically locked by mutators
882 // but where mutators were known blocked by the reentry thread state transition.
883 //
884 // If we were to modify the Monitor-Mutex so that TBIVM state transitions tightly
885 // wrapped calls to park(), then we could likely do away with sneaking. We'd
886 // decouple lock acquisition and parking. The critical invariant to eliminating
887 // sneaking is to ensure that we never "physically" acquire the lock while TBIVM.
888 // An easy way to accomplish this is to wrap the park calls in a narrow TBIVM jacket.
889 // One difficulty with this approach is that the TBIVM wrapper could recurse and
890 // call lock() deep from within a lock() call, while the MutexEvent was already enqueued.
891 // Using a stack (N=2 at minimum) of ParkEvents would take care of that problem.
892 //
893 // But of course the proper ultimate approach is to avoid schemes that require explicit
894 // sneaking or dependence on any any clever invariants or subtle implementation properties
895 // of Mutex-Monitor and instead directly address the underlying design flaw.
896
897 void Monitor::lock(Thread * Self) {
898 #ifdef CHECK_UNHANDLED_OOPS
899 // Clear unhandled oops so we get a crash right away. Only clear for non-vm
900 // or GC threads.
901 if (Self->is_Java_thread()) {
902 Self->clear_unhandled_oops();
903 }
904 #endif // CHECK_UNHANDLED_OOPS
905
906 debug_only(check_prelock_state(Self));
907 assert(_owner != Self, "invariant");
908 assert(_OnDeck != Self->_MutexEvent, "invariant");
909
910 if (TryFast()) {
911 Exeunt:
912 assert(ILocked(), "invariant");
913 assert(owner() == NULL, "invariant");
914 set_owner(Self);
915 return;
916 }
917
918 // The lock is contended ...
919
920 bool can_sneak = Self->is_VM_thread() && SafepointSynchronize::is_at_safepoint();
921 if (can_sneak && _owner == NULL) {
922 // a java thread has locked the lock but has not entered the
923 // critical region -- let's just pretend we've locked the lock
924 // and go on. we note this with _snuck so we can also
925 // pretend to unlock when the time comes.
926 _snuck = true;
927 goto Exeunt;
928 }
929
930 // Try a brief spin to avoid passing thru thread state transition ...
931 if (TrySpin(Self)) goto Exeunt;
932
933 check_block_state(Self);
934 if (Self->is_Java_thread()) {
935 // Horrible dictu - we suffer through a state transition
936 assert(rank() > Mutex::special, "Potential deadlock with special or lesser rank mutex");
937 ThreadBlockInVM tbivm((JavaThread *) Self);
938 ILock(Self);
939 } else {
940 // Mirabile dictu
941 ILock(Self);
942 }
943 goto Exeunt;
944 }
945
946 void Monitor::lock() {
947 this->lock(Thread::current());
948 }
949
950 // Lock without safepoint check - a degenerate variant of lock().
951 // Should ONLY be used by safepoint code and other code
952 // that is guaranteed not to block while running inside the VM. If this is called with
953 // thread state set to be in VM, the safepoint synchronization code will deadlock!
954
955 void Monitor::lock_without_safepoint_check(Thread * Self) {
956 assert(_owner != Self, "invariant");
957 ILock(Self);
958 assert(_owner == NULL, "invariant");
959 set_owner(Self);
960 }
961
962 void Monitor::lock_without_safepoint_check() {
963 lock_without_safepoint_check(Thread::current());
964 }
965
966
967 // Returns true if thread succeeds in grabbing the lock, otherwise false.
968
969 bool Monitor::try_lock() {
970 Thread * const Self = Thread::current();
971 debug_only(check_prelock_state(Self));
972 // assert(!thread->is_inside_signal_handler(), "don't lock inside signal handler");
973
974 // Special case, where all Java threads are stopped.
975 // The lock may have been acquired but _owner is not yet set.
976 // In that case the VM thread can safely grab the lock.
977 // It strikes me this should appear _after the TryLock() fails, below.
978 bool can_sneak = Self->is_VM_thread() && SafepointSynchronize::is_at_safepoint();
979 if (can_sneak && _owner == NULL) {
980 set_owner(Self); // Do not need to be atomic, since we are at a safepoint
981 _snuck = true;
982 return true;
983 }
984
985 if (TryLock()) {
986 // We got the lock
987 assert(_owner == NULL, "invariant");
988 set_owner(Self);
989 return true;
990 }
991 return false;
992 }
993
994 void Monitor::unlock() {
995 assert(_owner == Thread::current(), "invariant");
996 assert(_OnDeck != Thread::current()->_MutexEvent, "invariant");
997 set_owner(NULL);
998 if (_snuck) {
999 assert(SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread(), "sneak");
1000 _snuck = false;
1001 return;
1002 }
1003 IUnlock(false);
1004 }
1005
1006 // Yet another degenerate version of Monitor::lock() or lock_without_safepoint_check()
1007 // jvm_raw_lock() and _unlock() can be called by non-Java threads via JVM_RawMonitorEnter.
1008 //
1009 // There's no expectation that JVM_RawMonitors will interoperate properly with the native
1010 // Mutex-Monitor constructs. We happen to implement JVM_RawMonitors in terms of
1011 // native Mutex-Monitors simply as a matter of convenience. A simple abstraction layer
1012 // over a pthread_mutex_t would work equally as well, but require more platform-specific
1013 // code -- a "PlatformMutex". Alternatively, a simply layer over muxAcquire-muxRelease
1014 // would work too.
1015 //
1016 // Since the caller might be a foreign thread, we don't necessarily have a Thread.MutexEvent
1017 // instance available. Instead, we transiently allocate a ParkEvent on-demand if
1018 // we encounter contention. That ParkEvent remains associated with the thread
1019 // until it manages to acquire the lock, at which time we return the ParkEvent
1020 // to the global ParkEvent free list. This is correct and suffices for our purposes.
1021 //
1022 // Beware that the original jvm_raw_unlock() had a "_snuck" test but that
1023 // jvm_raw_lock() didn't have the corresponding test. I suspect that's an
1024 // oversight, but I've replicated the original suspect logic in the new code ...
1025
1026 void Monitor::jvm_raw_lock() {
1027 assert(rank() == native, "invariant");
1028
1029 if (TryLock()) {
1030 Exeunt:
1031 assert(ILocked(), "invariant");
1032 assert(_owner == NULL, "invariant");
1033 // This can potentially be called by non-java Threads. Thus, the ThreadLocalStorage
1034 // might return NULL. Don't call set_owner since it will break on an NULL owner
1035 // Consider installing a non-null "ANON" distinguished value instead of just NULL.
1036 _owner = ThreadLocalStorage::thread();
1037 return;
1038 }
1039
1040 if (TrySpin(NULL)) goto Exeunt;
1041
1042 // slow-path - apparent contention
1043 // Allocate a ParkEvent for transient use.
1044 // The ParkEvent remains associated with this thread until
1045 // the time the thread manages to acquire the lock.
1046 ParkEvent * const ESelf = ParkEvent::Allocate(NULL);
1047 ESelf->reset();
1048 OrderAccess::storeload();
1049
1050 // Either Enqueue Self on cxq or acquire the outer lock.
1051 if (AcquireOrPush (ESelf)) {
1052 ParkEvent::Release(ESelf); // surrender the ParkEvent
1053 goto Exeunt;
1054 }
1055
1056 // At any given time there is at most one ondeck thread.
1057 // ondeck implies not resident on cxq and not resident on EntryList
1058 // Only the OnDeck thread can try to acquire -- contended for -- the lock.
1059 // CONSIDER: use Self->OnDeck instead of m->OnDeck.
1060 for (;;) {
1061 if (_OnDeck == ESelf && TrySpin(NULL)) break;
1062 ParkCommon(ESelf, 0);
1063 }
1064
1065 assert(_OnDeck == ESelf, "invariant");
1066 _OnDeck = NULL;
1067 ParkEvent::Release(ESelf); // surrender the ParkEvent
1068 goto Exeunt;
1069 }
1070
1071 void Monitor::jvm_raw_unlock() {
1072 // Nearly the same as Monitor::unlock() ...
1073 // directly set _owner instead of using set_owner(null)
1074 _owner = NULL;
1075 if (_snuck) { // ???
1076 assert(SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread(), "sneak");
1077 _snuck = false;
1078 return;
1079 }
1080 IUnlock(false);
1081 }
1082
1083 bool Monitor::wait(bool no_safepoint_check, long timeout,
1084 bool as_suspend_equivalent) {
1085 Thread * const Self = Thread::current();
1086 assert(_owner == Self, "invariant");
1087 assert(ILocked(), "invariant");
1088
1089 // as_suspend_equivalent logically implies !no_safepoint_check
1090 guarantee(!as_suspend_equivalent || !no_safepoint_check, "invariant");
1091 // !no_safepoint_check logically implies java_thread
1092 guarantee(no_safepoint_check || Self->is_Java_thread(), "invariant");
1093
1094 #ifdef ASSERT
1095 Monitor * least = get_least_ranked_lock_besides_this(Self->owned_locks());
1096 assert(least != this, "Specification of get_least_... call above");
1097 if (least != NULL && least->rank() <= special) {
1098 tty->print("Attempting to wait on monitor %s/%d while holding"
1099 " lock %s/%d -- possible deadlock",
1100 name(), rank(), least->name(), least->rank());
1101 assert(false, "Shouldn't block(wait) while holding a lock of rank special");
1102 }
1103 #endif // ASSERT
1104
1105 int wait_status;
1106 // conceptually set the owner to NULL in anticipation of
1107 // abdicating the lock in wait
1108 set_owner(NULL);
1109 if (no_safepoint_check) {
1110 wait_status = IWait(Self, timeout);
1111 } else {
1112 assert(Self->is_Java_thread(), "invariant");
1113 JavaThread *jt = (JavaThread *)Self;
1114
1115 // Enter safepoint region - ornate and Rococo ...
1116 ThreadBlockInVM tbivm(jt);
1117 OSThreadWaitState osts(Self->osthread(), false /* not Object.wait() */);
1118
1119 if (as_suspend_equivalent) {
1120 jt->set_suspend_equivalent();
1121 // cleared by handle_special_suspend_equivalent_condition() or
1122 // java_suspend_self()
1123 }
1124
1125 wait_status = IWait(Self, timeout);
1126
1127 // were we externally suspended while we were waiting?
1128 if (as_suspend_equivalent && jt->handle_special_suspend_equivalent_condition()) {
1129 // Our event wait has finished and we own the lock, but
1130 // while we were waiting another thread suspended us. We don't
1131 // want to hold the lock while suspended because that
1132 // would surprise the thread that suspended us.
1133 assert(ILocked(), "invariant");
1134 IUnlock(true);
1135 jt->java_suspend_self();
1136 ILock(Self);
1137 assert(ILocked(), "invariant");
1138 }
1139 }
1140
1141 // Conceptually reestablish ownership of the lock.
1142 // The "real" lock -- the LockByte -- was reacquired by IWait().
1143 assert(ILocked(), "invariant");
1144 assert(_owner == NULL, "invariant");
1145 set_owner(Self);
1146 return wait_status != 0; // return true IFF timeout
1147 }
1148
1149 Monitor::~Monitor() {
1150 assert((UNS(_owner)|UNS(_LockWord.FullWord)|UNS(_EntryList)|UNS(_WaitSet)|UNS(_OnDeck)) == 0, "");
1151 }
1152
1153 void Monitor::ClearMonitor(Monitor * m, const char *name) {
1154 m->_owner = NULL;
1155 m->_snuck = false;
1156 if (name == NULL) {
1157 strcpy(m->_name, "UNKNOWN");
1158 } else {
1159 strncpy(m->_name, name, MONITOR_NAME_LEN - 1);
1160 m->_name[MONITOR_NAME_LEN - 1] = '\0';
1161 }
1162 m->_LockWord.FullWord = 0;
1163 m->_EntryList = NULL;
1164 m->_OnDeck = NULL;
1165 m->_WaitSet = NULL;
1166 m->_WaitLock[0] = 0;
1167 }
1168
1169 Monitor::Monitor() { ClearMonitor(this); }
1170
1171 Monitor::Monitor(int Rank, const char * name, bool allow_vm_block) {
1172 ClearMonitor(this, name);
1173 #ifdef ASSERT
1174 _allow_vm_block = allow_vm_block;
1175 _rank = Rank;
1176 #endif
1177 }
1178
1179 Mutex::~Mutex() {
1180 assert((UNS(_owner)|UNS(_LockWord.FullWord)|UNS(_EntryList)|UNS(_WaitSet)|UNS(_OnDeck)) == 0, "");
1181 }
1182
1183 Mutex::Mutex(int Rank, const char * name, bool allow_vm_block) {
1184 ClearMonitor((Monitor *) this, name);
1185 #ifdef ASSERT
1186 _allow_vm_block = allow_vm_block;
1187 _rank = Rank;
1188 #endif
1189 }
1190
1191 bool Monitor::owned_by_self() const {
1192 bool ret = _owner == Thread::current();
1193 assert(!ret || _LockWord.Bytes[_LSBINDEX] != 0, "invariant");
1194 return ret;
1195 }
1196
1197 void Monitor::print_on_error(outputStream* st) const {
1198 st->print("[" PTR_FORMAT, this);
1199 st->print("] %s", _name);
1200 st->print(" - owner thread: " PTR_FORMAT, _owner);
1201 }
1202
1203
1204
1205
1206 // ----------------------------------------------------------------------------------
1207 // Non-product code
1208
1209 #ifndef PRODUCT
1210 void Monitor::print_on(outputStream* st) const {
1211 st->print_cr("Mutex: [0x%lx/0x%lx] %s - owner: 0x%lx", this, _LockWord.FullWord, _name, _owner);
1212 }
1213 #endif
1214
1215 #ifndef PRODUCT
1216 #ifdef ASSERT
1217 Monitor * Monitor::get_least_ranked_lock(Monitor * locks) {
1218 Monitor *res, *tmp;
1219 for (res = tmp = locks; tmp != NULL; tmp = tmp->next()) {
1220 if (tmp->rank() < res->rank()) {
1221 res = tmp;
1222 }
1223 }
1224 if (!SafepointSynchronize::is_at_safepoint()) {
1225 // In this case, we expect the held locks to be
1226 // in increasing rank order (modulo any native ranks)
1227 for (tmp = locks; tmp != NULL; tmp = tmp->next()) {
1228 if (tmp->next() != NULL) {
1229 assert(tmp->rank() == Mutex::native ||
1230 tmp->rank() <= tmp->next()->rank(), "mutex rank anomaly?");
1231 }
1232 }
1233 }
1234 return res;
1235 }
1236
1237 Monitor* Monitor::get_least_ranked_lock_besides_this(Monitor* locks) {
1238 Monitor *res, *tmp;
1239 for (res = NULL, tmp = locks; tmp != NULL; tmp = tmp->next()) {
1240 if (tmp != this && (res == NULL || tmp->rank() < res->rank())) {
1241 res = tmp;
1242 }
1243 }
1244 if (!SafepointSynchronize::is_at_safepoint()) {
1245 // In this case, we expect the held locks to be
1246 // in increasing rank order (modulo any native ranks)
1247 for (tmp = locks; tmp != NULL; tmp = tmp->next()) {
1248 if (tmp->next() != NULL) {
1249 assert(tmp->rank() == Mutex::native ||
1250 tmp->rank() <= tmp->next()->rank(), "mutex rank anomaly?");
1251 }
1252 }
1253 }
1254 return res;
1255 }
1256
1257
1258 bool Monitor::contains(Monitor* locks, Monitor * lock) {
1259 for (; locks != NULL; locks = locks->next()) {
1260 if (locks == lock) {
1261 return true;
1262 }
1263 }
1264 return false;
1265 }
1266 #endif
1267
1268 // Called immediately after lock acquisition or release as a diagnostic
1269 // to track the lock-set of the thread and test for rank violations that
1270 // might indicate exposure to deadlock.
1271 // Rather like an EventListener for _owner (:>).
1272
1273 void Monitor::set_owner_implementation(Thread *new_owner) {
1274 // This function is solely responsible for maintaining
1275 // and checking the invariant that threads and locks
1276 // are in a 1/N relation, with some some locks unowned.
1277 // It uses the Mutex::_owner, Mutex::_next, and
1278 // Thread::_owned_locks fields, and no other function
1279 // changes those fields.
1280 // It is illegal to set the mutex from one non-NULL
1281 // owner to another--it must be owned by NULL as an
1282 // intermediate state.
1283
1284 if (new_owner != NULL) {
1285 // the thread is acquiring this lock
1286
1287 assert(new_owner == Thread::current(), "Should I be doing this?");
1288 assert(_owner == NULL, "setting the owner thread of an already owned mutex");
1289 _owner = new_owner; // set the owner
1290
1291 // link "this" into the owned locks list
1292
1293 #ifdef ASSERT // Thread::_owned_locks is under the same ifdef
1294 Monitor* locks = get_least_ranked_lock(new_owner->owned_locks());
1295 // Mutex::set_owner_implementation is a friend of Thread
1296
1297 assert(this->rank() >= 0, "bad lock rank");
1298
1299 // Deadlock avoidance rules require us to acquire Mutexes only in
1300 // a global total order. For example m1 is the lowest ranked mutex
1301 // that the thread holds and m2 is the mutex the thread is trying
1302 // to acquire, then deadlock avoidance rules require that the rank
1303 // of m2 be less than the rank of m1.
1304 // The rank Mutex::native is an exception in that it is not subject
1305 // to the verification rules.
1306 // Here are some further notes relating to mutex acquisition anomalies:
1307 // . under Solaris, the interrupt lock gets acquired when doing
1308 // profiling, so any lock could be held.
1309 // . it is also ok to acquire Safepoint_lock at the very end while we
1310 // already hold Terminator_lock - may happen because of periodic safepoints
1311 if (this->rank() != Mutex::native &&
1312 this->rank() != Mutex::suspend_resume &&
1313 locks != NULL && locks->rank() <= this->rank() &&
1314 !SafepointSynchronize::is_at_safepoint() &&
1315 this != Interrupt_lock && this != ProfileVM_lock &&
1316 !(this == Safepoint_lock && contains(locks, Terminator_lock) &&
1317 SafepointSynchronize::is_synchronizing())) {
1318 new_owner->print_owned_locks();
1319 fatal(err_msg("acquiring lock %s/%d out of order with lock %s/%d -- "
1320 "possible deadlock", this->name(), this->rank(),
1321 locks->name(), locks->rank()));
1322 }
1323
1324 this->_next = new_owner->_owned_locks;
1325 new_owner->_owned_locks = this;
1326 #endif
1327
1328 } else {
1329 // the thread is releasing this lock
1330
1331 Thread* old_owner = _owner;
1332 debug_only(_last_owner = old_owner);
1333
1334 assert(old_owner != NULL, "removing the owner thread of an unowned mutex");
1335 assert(old_owner == Thread::current(), "removing the owner thread of an unowned mutex");
1336
1337 _owner = NULL; // set the owner
1338
1339 #ifdef ASSERT
1340 Monitor *locks = old_owner->owned_locks();
1341
1342 // remove "this" from the owned locks list
1343
1344 Monitor *prev = NULL;
1345 bool found = false;
1346 for (; locks != NULL; prev = locks, locks = locks->next()) {
1347 if (locks == this) {
1348 found = true;
1349 break;
1350 }
1351 }
1352 assert(found, "Removing a lock not owned");
1353 if (prev == NULL) {
1354 old_owner->_owned_locks = _next;
1355 } else {
1356 prev->_next = _next;
1357 }
1358 _next = NULL;
1359 #endif
1360 }
1361 }
1362
1363
1364 // Factored out common sanity checks for locking mutex'es. Used by lock() and try_lock()
1365 void Monitor::check_prelock_state(Thread *thread) {
1366 assert((!thread->is_Java_thread() || ((JavaThread *)thread)->thread_state() == _thread_in_vm)
1367 || rank() == Mutex::special, "wrong thread state for using locks");
1368 if (StrictSafepointChecks) {
1369 if (thread->is_VM_thread() && !allow_vm_block()) {
1370 fatal(err_msg("VM thread using lock %s (not allowed to block on)",
1371 name()));
1372 }
1373 debug_only(if (rank() != Mutex::special) \
1374 thread->check_for_valid_safepoint_state(false);)
1375 }
1376 if (thread->is_Watcher_thread()) {
1377 assert(!WatcherThread::watcher_thread()->has_crash_protection(),
1378 "locking not allowed when crash protection is set");
1379 }
1380 }
1381
1382 void Monitor::check_block_state(Thread *thread) {
1383 if (!_allow_vm_block && thread->is_VM_thread()) {
1384 warning("VM thread blocked on lock");
1385 print();
1386 BREAKPOINT;
1387 }
1388 assert(_owner != thread, "deadlock: blocking on monitor owned by current thread");
1389 }
1390
1391 #endif // PRODUCT