1
2 /*
3 * Copyright (c) 1998, 2008, Oracle and/or its affiliates. All rights reserved.
4 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
5 *
6 * This code is free software; you can redistribute it and/or modify it
7 * under the terms of the GNU General Public License version 2 only, as
8 * published by the Free Software Foundation.
9 *
10 * This code is distributed in the hope that it will be useful, but WITHOUT
11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
13 * version 2 for more details (a copy is included in the LICENSE file that
14 * accompanied this code).
15 *
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24 */
25
26 # include "incls/_precompiled.incl"
27 # include "incls/_mutex.cpp.incl"
28
29 // 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
30 //
31 // Native Monitor-Mutex locking - theory of operations
32 //
33 // * Native Monitors are completely unrelated to Java-level monitors,
34 // although the "back-end" slow-path implementations share a common lineage.
35 // See objectMonitor:: in synchronizer.cpp.
36 // Native Monitors do *not* support nesting or recursion but otherwise
37 // they're basically Hoare-flavor monitors.
38 //
39 // * A thread acquires ownership of a Monitor/Mutex by CASing the LockByte
40 // in the _LockWord from zero to non-zero. Note that the _Owner field
41 // is advisory and is used only to verify that the thread calling unlock()
42 // is indeed the last thread to have acquired the lock.
43 //
44 // * Contending threads "push" themselves onto the front of the contention
45 // queue -- called the cxq -- with CAS and then spin/park.
46 // The _LockWord contains the LockByte as well as the pointer to the head
47 // of the cxq. Colocating the LockByte with the cxq precludes certain races.
48 //
49 // * Using a separately addressable LockByte allows for CAS:MEMBAR or CAS:0
50 // idioms. We currently use MEMBAR in the uncontended unlock() path, as
51 // MEMBAR often has less latency than CAS. If warranted, we could switch to
52 // a CAS:0 mode, using timers to close the resultant race, as is done
53 // with Java Monitors in synchronizer.cpp.
54 //
55 // See the following for a discussion of the relative cost of atomics (CAS)
56 // MEMBAR, and ways to eliminate such instructions from the common-case paths:
57 // -- http://blogs.sun.com/dave/entry/biased_locking_in_hotspot
58 // -- http://blogs.sun.com/dave/resource/MustangSync.pdf
59 // -- http://blogs.sun.com/dave/resource/synchronization-public2.pdf
60 // -- synchronizer.cpp
61 //
62 // * Overall goals - desiderata
63 // 1. Minimize context switching
64 // 2. Minimize lock migration
65 // 3. Minimize CPI -- affinity and locality
66 // 4. Minimize the execution of high-latency instructions such as CAS or MEMBAR
67 // 5. Minimize outer lock hold times
68 // 6. Behave gracefully on a loaded system
69 //
70 // * Thread flow and list residency:
71 //
72 // Contention queue --> EntryList --> OnDeck --> Owner --> !Owner
73 // [..resident on monitor list..]
74 // [...........contending..................]
75 //
76 // -- The contention queue (cxq) contains recently-arrived threads (RATs).
77 // Threads on the cxq eventually drain into the EntryList.
78 // -- Invariant: a thread appears on at most one list -- cxq, EntryList
79 // or WaitSet -- at any one time.
80 // -- For a given monitor there can be at most one "OnDeck" thread at any
81 // given time but if needbe this particular invariant could be relaxed.
82 //
83 // * The WaitSet and EntryList linked lists are composed of ParkEvents.
84 // I use ParkEvent instead of threads as ParkEvents are immortal and
85 // type-stable, meaning we can safely unpark() a possibly stale
86 // list element in the unlock()-path. (That's benign).
87 //
88 // * Succession policy - providing for progress:
89 //
90 // As necessary, the unlock()ing thread identifies, unlinks, and unparks
91 // an "heir presumptive" tentative successor thread from the EntryList.
92 // This becomes the so-called "OnDeck" thread, of which there can be only
93 // one at any given time for a given monitor. The wakee will recontend
94 // for ownership of monitor.
95 //
96 // Succession is provided for by a policy of competitive handoff.
97 // The exiting thread does _not_ grant or pass ownership to the
98 // successor thread. (This is also referred to as "handoff" succession").
99 // Instead the exiting thread releases ownership and possibly wakes
100 // a successor, so the successor can (re)compete for ownership of the lock.
101 //
102 // Competitive handoff provides excellent overall throughput at the expense
103 // of short-term fairness. If fairness is a concern then one remedy might
104 // be to add an AcquireCounter field to the monitor. After a thread acquires
105 // the lock it will decrement the AcquireCounter field. When the count
106 // reaches 0 the thread would reset the AcquireCounter variable, abdicate
107 // the lock directly to some thread on the EntryList, and then move itself to the
108 // tail of the EntryList.
109 //
110 // But in practice most threads engage or otherwise participate in resource
111 // bounded producer-consumer relationships, so lock domination is not usually
112 // a practical concern. Recall too, that in general it's easier to construct
113 // a fair lock from a fast lock, but not vice-versa.
114 //
115 // * The cxq can have multiple concurrent "pushers" but only one concurrent
116 // detaching thread. This mechanism is immune from the ABA corruption.
117 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
118 // We use OnDeck as a pseudo-lock to enforce the at-most-one detaching
119 // thread constraint.
120 //
121 // * Taken together, the cxq and the EntryList constitute or form a
122 // single logical queue of threads stalled trying to acquire the lock.
123 // We use two distinct lists to reduce heat on the list ends.
124 // Threads in lock() enqueue onto cxq while threads in unlock() will
125 // dequeue from the EntryList. (c.f. Michael Scott's "2Q" algorithm).
126 // A key desideratum is to minimize queue & monitor metadata manipulation
127 // that occurs while holding the "outer" monitor lock -- that is, we want to
128 // minimize monitor lock holds times.
129 //
130 // The EntryList is ordered by the prevailing queue discipline and
131 // can be organized in any convenient fashion, such as a doubly-linked list or
132 // a circular doubly-linked list. If we need a priority queue then something akin
133 // to Solaris' sleepq would work nicely. Viz.,
134 // -- http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
135 // -- http://cvs.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/uts/common/os/sleepq.c
136 // Queue discipline is enforced at ::unlock() time, when the unlocking thread
137 // drains the cxq into the EntryList, and orders or reorders the threads on the
138 // EntryList accordingly.
139 //
140 // Barring "lock barging", this mechanism provides fair cyclic ordering,
141 // somewhat similar to an elevator-scan.
142 //
143 // * OnDeck
144 // -- For a given monitor there can be at most one OnDeck thread at any given
145 // instant. The OnDeck thread is contending for the lock, but has been
146 // unlinked from the EntryList and cxq by some previous unlock() operations.
147 // Once a thread has been designated the OnDeck thread it will remain so
148 // until it manages to acquire the lock -- being OnDeck is a stable property.
149 // -- Threads on the EntryList or cxq are _not allowed to attempt lock acquisition.
150 // -- OnDeck also serves as an "inner lock" as follows. Threads in unlock() will, after
151 // having cleared the LockByte and dropped the outer lock, attempt to "trylock"
152 // OnDeck by CASing the field from null to non-null. If successful, that thread
153 // is then responsible for progress and succession and can use CAS to detach and
154 // drain the cxq into the EntryList. By convention, only this thread, the holder of
155 // the OnDeck inner lock, can manipulate the EntryList or detach and drain the
156 // RATs on the cxq into the EntryList. This avoids ABA corruption on the cxq as
157 // we allow multiple concurrent "push" operations but restrict detach concurrency
158 // to at most one thread. Having selected and detached a successor, the thread then
159 // changes the OnDeck to refer to that successor, and then unparks the successor.
160 // That successor will eventually acquire the lock and clear OnDeck. Beware
161 // that the OnDeck usage as a lock is asymmetric. A thread in unlock() transiently
162 // "acquires" OnDeck, performs queue manipulations, passes OnDeck to some successor,
163 // and then the successor eventually "drops" OnDeck. Note that there's never
164 // any sense of contention on the inner lock, however. Threads never contend
165 // or wait for the inner lock.
166 // -- OnDeck provides for futile wakeup throttling a described in section 3.3 of
167 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
168 // In a sense, OnDeck subsumes the ObjectMonitor _Succ and ObjectWaiter
169 // TState fields found in Java-level objectMonitors. (See synchronizer.cpp).
170 //
171 // * Waiting threads reside on the WaitSet list -- wait() puts
172 // the caller onto the WaitSet. Notify() or notifyAll() simply
173 // transfers threads from the WaitSet to either the EntryList or cxq.
174 // Subsequent unlock() operations will eventually unpark the notifyee.
175 // Unparking a notifee in notify() proper is inefficient - if we were to do so
176 // it's likely the notifyee would simply impale itself on the lock held
177 // by the notifier.
178 //
179 // * The mechanism is obstruction-free in that if the holder of the transient
180 // OnDeck lock in unlock() is preempted or otherwise stalls, other threads
181 // can still acquire and release the outer lock and continue to make progress.
182 // At worst, waking of already blocked contending threads may be delayed,
183 // but nothing worse. (We only use "trylock" operations on the inner OnDeck
184 // lock).
185 //
186 // * Note that thread-local storage must be initialized before a thread
187 // uses Native monitors or mutexes. The native monitor-mutex subsystem
188 // depends on Thread::current().
189 //
190 // * The monitor synchronization subsystem avoids the use of native
191 // synchronization primitives except for the narrow platform-specific
192 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
193 // the semantics of park-unpark. Put another way, this monitor implementation
194 // depends only on atomic operations and park-unpark. The monitor subsystem
195 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
196 // underlying OS manages the READY<->RUN transitions.
197 //
198 // * The memory consistency model provide by lock()-unlock() is at least as
199 // strong or stronger than the Java Memory model defined by JSR-133.
200 // That is, we guarantee at least entry consistency, if not stronger.
201 // See http://g.oswego.edu/dl/jmm/cookbook.html.
202 //
203 // * Thread:: currently contains a set of purpose-specific ParkEvents:
204 // _MutexEvent, _ParkEvent, etc. A better approach might be to do away with
205 // the purpose-specific ParkEvents and instead implement a general per-thread
206 // stack of available ParkEvents which we could provision on-demand. The
207 // stack acts as a local cache to avoid excessive calls to ParkEvent::Allocate()
208 // and ::Release(). A thread would simply pop an element from the local stack before it
209 // enqueued or park()ed. When the contention was over the thread would
210 // push the no-longer-needed ParkEvent back onto its stack.
211 //
212 // * A slightly reduced form of ILock() and IUnlock() have been partially
213 // model-checked (Murphi) for safety and progress at T=1,2,3 and 4.
214 // It'd be interesting to see if TLA/TLC could be useful as well.
215 //
216 // * Mutex-Monitor is a low-level "leaf" subsystem. That is, the monitor
217 // code should never call other code in the JVM that might itself need to
218 // acquire monitors or mutexes. That's true *except* in the case of the
219 // ThreadBlockInVM state transition wrappers. The ThreadBlockInVM DTOR handles
220 // mutator reentry (ingress) by checking for a pending safepoint in which case it will
221 // call SafepointSynchronize::block(), which in turn may call Safepoint_lock->lock(), etc.
222 // In that particular case a call to lock() for a given Monitor can end up recursively
223 // calling lock() on another monitor. While distasteful, this is largely benign
224 // as the calls come from jacket that wraps lock(), and not from deep within lock() itself.
225 //
226 // It's unfortunate that native mutexes and thread state transitions were convolved.
227 // They're really separate concerns and should have remained that way. Melding
228 // them together was facile -- a bit too facile. The current implementation badly
229 // conflates the two concerns.
230 //
231 // * TODO-FIXME:
232 //
233 // -- Add DTRACE probes for contended acquire, contended acquired, contended unlock
234 // We should also add DTRACE probes in the ParkEvent subsystem for
235 // Park-entry, Park-exit, and Unpark.
236 //
237 // -- We have an excess of mutex-like constructs in the JVM, namely:
238 // 1. objectMonitors for Java-level synchronization (synchronizer.cpp)
239 // 2. low-level muxAcquire and muxRelease
240 // 3. low-level spinAcquire and spinRelease
241 // 4. native Mutex:: and Monitor::
242 // 5. jvm_raw_lock() and _unlock()
243 // 6. JVMTI raw monitors -- distinct from (5) despite having a confusingly
244 // similar name.
245 //
246 // 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
247
248
249 // CASPTR() uses the canonical argument order that dominates in the literature.
250 // Our internal cmpxchg_ptr() uses a bastardized ordering to accommodate Sun .il templates.
251
252 #define CASPTR(a,c,s) intptr_t(Atomic::cmpxchg_ptr ((void *)(s),(void *)(a),(void *)(c)))
253 #define UNS(x) (uintptr_t(x))
254 #define TRACE(m) { static volatile int ctr = 0 ; int x = ++ctr ; if ((x & (x-1))==0) { ::printf ("%d:%s\n", x, #m); ::fflush(stdout); }}
255
256 // Simplistic low-quality Marsaglia SHIFT-XOR RNG.
257 // Bijective except for the trailing mask operation.
258 // Useful for spin loops as the compiler can't optimize it away.
259
260 static inline jint MarsagliaXORV (jint x) {
261 if (x == 0) x = 1|os::random() ;
262 x ^= x << 6;
263 x ^= ((unsigned)x) >> 21;
264 x ^= x << 7 ;
265 return x & 0x7FFFFFFF ;
266 }
267
268 static inline jint MarsagliaXOR (jint * const a) {
269 jint x = *a ;
270 if (x == 0) x = UNS(a)|1 ;
271 x ^= x << 6;
272 x ^= ((unsigned)x) >> 21;
273 x ^= x << 7 ;
274 *a = x ;
275 return x & 0x7FFFFFFF ;
276 }
277
278 static int Stall (int its) {
279 static volatile jint rv = 1 ;
280 volatile int OnFrame = 0 ;
281 jint v = rv ^ UNS(OnFrame) ;
282 while (--its >= 0) {
283 v = MarsagliaXORV (v) ;
284 }
285 // Make this impossible for the compiler to optimize away,
286 // but (mostly) avoid W coherency sharing on MP systems.
287 if (v == 0x12345) rv = v ;
288 return v ;
289 }
290
291 int Monitor::TryLock () {
292 intptr_t v = _LockWord.FullWord ;
293 for (;;) {
294 if ((v & _LBIT) != 0) return 0 ;
295 const intptr_t u = CASPTR (&_LockWord, v, v|_LBIT) ;
296 if (v == u) return 1 ;
297 v = u ;
298 }
299 }
300
301 int Monitor::TryFast () {
302 // Optimistic fast-path form ...
303 // Fast-path attempt for the common uncontended case.
304 // Avoid RTS->RTO $ coherence upgrade on typical SMP systems.
305 intptr_t v = CASPTR (&_LockWord, 0, _LBIT) ; // agro ...
306 if (v == 0) return 1 ;
307
308 for (;;) {
309 if ((v & _LBIT) != 0) return 0 ;
310 const intptr_t u = CASPTR (&_LockWord, v, v|_LBIT) ;
311 if (v == u) return 1 ;
312 v = u ;
313 }
314 }
315
316 int Monitor::ILocked () {
317 const intptr_t w = _LockWord.FullWord & 0xFF ;
318 assert (w == 0 || w == _LBIT, "invariant") ;
319 return w == _LBIT ;
320 }
321
322 // Polite TATAS spinlock with exponential backoff - bounded spin.
323 // Ideally we'd use processor cycles, time or vtime to control
324 // the loop, but we currently use iterations.
325 // All the constants within were derived empirically but work over
326 // over the spectrum of J2SE reference platforms.
327 // On Niagara-class systems the back-off is unnecessary but
328 // is relatively harmless. (At worst it'll slightly retard
329 // acquisition times). The back-off is critical for older SMP systems
330 // where constant fetching of the LockWord would otherwise impair
331 // scalability.
332 //
333 // Clamp spinning at approximately 1/2 of a context-switch round-trip.
334 // See synchronizer.cpp for details and rationale.
335
336 int Monitor::TrySpin (Thread * const Self) {
337 if (TryLock()) return 1 ;
338 if (!os::is_MP()) return 0 ;
339
340 int Probes = 0 ;
341 int Delay = 0 ;
342 int Steps = 0 ;
343 int SpinMax = NativeMonitorSpinLimit ;
344 int flgs = NativeMonitorFlags ;
345 for (;;) {
346 intptr_t v = _LockWord.FullWord;
347 if ((v & _LBIT) == 0) {
348 if (CASPTR (&_LockWord, v, v|_LBIT) == v) {
349 return 1 ;
350 }
351 continue ;
352 }
353
354 if ((flgs & 8) == 0) {
355 SpinPause () ;
356 }
357
358 // Periodically increase Delay -- variable Delay form
359 // conceptually: delay *= 1 + 1/Exponent
360 ++ Probes;
361 if (Probes > SpinMax) return 0 ;
362
363 if ((Probes & 0x7) == 0) {
364 Delay = ((Delay << 1)|1) & 0x7FF ;
365 // CONSIDER: Delay += 1 + (Delay/4); Delay &= 0x7FF ;
366 }
367
368 if (flgs & 2) continue ;
369
370 // Consider checking _owner's schedctl state, if OFFPROC abort spin.
371 // If the owner is OFFPROC then it's unlike that the lock will be dropped
372 // in a timely fashion, which suggests that spinning would not be fruitful
373 // or profitable.
374
375 // Stall for "Delay" time units - iterations in the current implementation.
376 // Avoid generating coherency traffic while stalled.
377 // Possible ways to delay:
378 // PAUSE, SLEEP, MEMBAR #sync, MEMBAR #halt,
379 // wr %g0,%asi, gethrtime, rdstick, rdtick, rdtsc, etc. ...
380 // Note that on Niagara-class systems we want to minimize STs in the
381 // spin loop. N1 and brethren write-around the L1$ over the xbar into the L2$.
382 // Furthermore, they don't have a W$ like traditional SPARC processors.
383 // We currently use a Marsaglia Shift-Xor RNG loop.
384 Steps += Delay ;
385 if (Self != NULL) {
386 jint rv = Self->rng[0] ;
387 for (int k = Delay ; --k >= 0; ) {
388 rv = MarsagliaXORV (rv) ;
389 if ((flgs & 4) == 0 && SafepointSynchronize::do_call_back()) return 0 ;
390 }
391 Self->rng[0] = rv ;
392 } else {
393 Stall (Delay) ;
394 }
395 }
396 }
397
398 static int ParkCommon (ParkEvent * ev, jlong timo) {
399 // Diagnostic support - periodically unwedge blocked threads
400 intx nmt = NativeMonitorTimeout ;
401 if (nmt > 0 && (nmt < timo || timo <= 0)) {
402 timo = nmt ;
403 }
404 int err = OS_OK ;
405 if (0 == timo) {
406 ev->park() ;
407 } else {
408 err = ev->park(timo) ;
409 }
410 return err ;
411 }
412
413 inline int Monitor::AcquireOrPush (ParkEvent * ESelf) {
414 intptr_t v = _LockWord.FullWord ;
415 for (;;) {
416 if ((v & _LBIT) == 0) {
417 const intptr_t u = CASPTR (&_LockWord, v, v|_LBIT) ;
418 if (u == v) return 1 ; // indicate acquired
419 v = u ;
420 } else {
421 // Anticipate success ...
422 ESelf->ListNext = (ParkEvent *) (v & ~_LBIT) ;
423 const intptr_t u = CASPTR (&_LockWord, v, intptr_t(ESelf)|_LBIT) ;
424 if (u == v) return 0 ; // indicate pushed onto cxq
425 v = u ;
426 }
427 // Interference - LockWord change - just retry
428 }
429 }
430
431 // ILock and IWait are the lowest level primitive internal blocking
432 // synchronization functions. The callers of IWait and ILock must have
433 // performed any needed state transitions beforehand.
434 // IWait and ILock may directly call park() without any concern for thread state.
435 // Note that ILock and IWait do *not* access _owner.
436 // _owner is a higher-level logical concept.
437
438 void Monitor::ILock (Thread * Self) {
439 assert (_OnDeck != Self->_MutexEvent, "invariant") ;
440
441 if (TryFast()) {
442 Exeunt:
443 assert (ILocked(), "invariant") ;
444 return ;
445 }
446
447 ParkEvent * const ESelf = Self->_MutexEvent ;
448 assert (_OnDeck != ESelf, "invariant") ;
449
450 // As an optimization, spinners could conditionally try to set ONDECK to _LBIT
451 // Synchronizer.cpp uses a similar optimization.
452 if (TrySpin (Self)) goto Exeunt ;
453
454 // Slow-path - the lock is contended.
455 // Either Enqueue Self on cxq or acquire the outer lock.
456 // LockWord encoding = (cxq,LOCKBYTE)
457 ESelf->reset() ;
458 OrderAccess::fence() ;
459
460 // Optional optimization ... try barging on the inner lock
461 if ((NativeMonitorFlags & 32) && CASPTR (&_OnDeck, NULL, UNS(Self)) == 0) {
462 goto OnDeck_LOOP ;
463 }
464
465 if (AcquireOrPush (ESelf)) goto Exeunt ;
466
467 // At any given time there is at most one ondeck thread.
468 // ondeck implies not resident on cxq and not resident on EntryList
469 // Only the OnDeck thread can try to acquire -- contended for -- the lock.
470 // CONSIDER: use Self->OnDeck instead of m->OnDeck.
471 // Deschedule Self so that others may run.
472 while (_OnDeck != ESelf) {
473 ParkCommon (ESelf, 0) ;
474 }
475
476 // Self is now in the ONDECK position and will remain so until it
477 // manages to acquire the lock.
478 OnDeck_LOOP:
479 for (;;) {
480 assert (_OnDeck == ESelf, "invariant") ;
481 if (TrySpin (Self)) break ;
482 // CONSIDER: if ESelf->TryPark() && TryLock() break ...
483 // It's probably wise to spin only if we *actually* blocked
484 // CONSIDER: check the lockbyte, if it remains set then
485 // preemptively drain the cxq into the EntryList.
486 // The best place and time to perform queue operations -- lock metadata --
487 // is _before having acquired the outer lock, while waiting for the lock to drop.
488 ParkCommon (ESelf, 0) ;
489 }
490
491 assert (_OnDeck == ESelf, "invariant") ;
492 _OnDeck = NULL ;
493
494 // Note that we current drop the inner lock (clear OnDeck) in the slow-path
495 // epilog immediately after having acquired the outer lock.
496 // But instead we could consider the following optimizations:
497 // A. Shift or defer dropping the inner lock until the subsequent IUnlock() operation.
498 // This might avoid potential reacquisition of the inner lock in IUlock().
499 // B. While still holding the inner lock, attempt to opportunistically select
500 // and unlink the next ONDECK thread from the EntryList.
501 // If successful, set ONDECK to refer to that thread, otherwise clear ONDECK.
502 // It's critical that the select-and-unlink operation run in constant-time as
503 // it executes when holding the outer lock and may artificially increase the
504 // effective length of the critical section.
505 // Note that (A) and (B) are tantamount to succession by direct handoff for
506 // the inner lock.
507 goto Exeunt ;
508 }
509
510 void Monitor::IUnlock (bool RelaxAssert) {
511 assert (ILocked(), "invariant") ;
512 _LockWord.Bytes[_LSBINDEX] = 0 ; // drop outer lock
513 OrderAccess::storeload ();
514 ParkEvent * const w = _OnDeck ;
515 assert (RelaxAssert || w != Thread::current()->_MutexEvent, "invariant") ;
516 if (w != NULL) {
517 // Either we have a valid ondeck thread or ondeck is transiently "locked"
518 // by some exiting thread as it arranges for succession. The LSBit of
519 // OnDeck allows us to discriminate two cases. If the latter, the
520 // responsibility for progress and succession lies with that other thread.
521 // For good performance, we also depend on the fact that redundant unpark()
522 // operations are cheap. That is, repeated Unpark()ing of the ONDECK thread
523 // is inexpensive. This approach provides implicit futile wakeup throttling.
524 // Note that the referent "w" might be stale with respect to the lock.
525 // In that case the following unpark() is harmless and the worst that'll happen
526 // is a spurious return from a park() operation. Critically, if "w" _is stale,
527 // then progress is known to have occurred as that means the thread associated
528 // with "w" acquired the lock. In that case this thread need take no further
529 // action to guarantee progress.
530 if ((UNS(w) & _LBIT) == 0) w->unpark() ;
531 return ;
532 }
533
534 intptr_t cxq = _LockWord.FullWord ;
535 if (((cxq & ~_LBIT)|UNS(_EntryList)) == 0) {
536 return ; // normal fast-path exit - cxq and EntryList both empty
537 }
538 if (cxq & _LBIT) {
539 // Optional optimization ...
540 // Some other thread acquired the lock in the window since this
541 // thread released it. Succession is now that thread's responsibility.
542 return ;
543 }
544
545 Succession:
546 // Slow-path exit - this thread must ensure succession and progress.
547 // OnDeck serves as lock to protect cxq and EntryList.
548 // Only the holder of OnDeck can manipulate EntryList or detach the RATs from cxq.
549 // Avoid ABA - allow multiple concurrent producers (enqueue via push-CAS)
550 // but only one concurrent consumer (detacher of RATs).
551 // Consider protecting this critical section with schedctl on Solaris.
552 // Unlike a normal lock, however, the exiting thread "locks" OnDeck,
553 // picks a successor and marks that thread as OnDeck. That successor
554 // thread will then clear OnDeck once it eventually acquires the outer lock.
555 if (CASPTR (&_OnDeck, NULL, _LBIT) != UNS(NULL)) {
556 return ;
557 }
558
559 ParkEvent * List = _EntryList ;
560 if (List != NULL) {
561 // Transfer the head of the EntryList to the OnDeck position.
562 // Once OnDeck, a thread stays OnDeck until it acquires the lock.
563 // For a given lock there is at most OnDeck thread at any one instant.
564 WakeOne:
565 assert (List == _EntryList, "invariant") ;
566 ParkEvent * const w = List ;
567 assert (RelaxAssert || w != Thread::current()->_MutexEvent, "invariant") ;
568 _EntryList = w->ListNext ;
569 // as a diagnostic measure consider setting w->_ListNext = BAD
570 assert (UNS(_OnDeck) == _LBIT, "invariant") ;
571 _OnDeck = w ; // pass OnDeck to w.
572 // w will clear OnDeck once it acquires the outer lock
573
574 // Another optional optimization ...
575 // For heavily contended locks it's not uncommon that some other
576 // thread acquired the lock while this thread was arranging succession.
577 // Try to defer the unpark() operation - Delegate the responsibility
578 // for unpark()ing the OnDeck thread to the current or subsequent owners
579 // That is, the new owner is responsible for unparking the OnDeck thread.
580 OrderAccess::storeload() ;
581 cxq = _LockWord.FullWord ;
582 if (cxq & _LBIT) return ;
583
584 w->unpark() ;
585 return ;
586 }
587
588 cxq = _LockWord.FullWord ;
589 if ((cxq & ~_LBIT) != 0) {
590 // The EntryList is empty but the cxq is populated.
591 // drain RATs from cxq into EntryList
592 // Detach RATs segment with CAS and then merge into EntryList
593 for (;;) {
594 // optional optimization - if locked, the owner is responsible for succession
595 if (cxq & _LBIT) goto Punt ;
596 const intptr_t vfy = CASPTR (&_LockWord, cxq, cxq & _LBIT) ;
597 if (vfy == cxq) break ;
598 cxq = vfy ;
599 // Interference - LockWord changed - Just retry
600 // We can see concurrent interference from contending threads
601 // pushing themselves onto the cxq or from lock-unlock operations.
602 // From the perspective of this thread, EntryList is stable and
603 // the cxq is prepend-only -- the head is volatile but the interior
604 // of the cxq is stable. In theory if we encounter interference from threads
605 // pushing onto cxq we could simply break off the original cxq suffix and
606 // move that segment to the EntryList, avoiding a 2nd or multiple CAS attempts
607 // on the high-traffic LockWord variable. For instance lets say the cxq is "ABCD"
608 // when we first fetch cxq above. Between the fetch -- where we observed "A"
609 // -- and CAS -- where we attempt to CAS null over A -- "PQR" arrive,
610 // yielding cxq = "PQRABCD". In this case we could simply set A.ListNext
611 // null, leaving cxq = "PQRA" and transfer the "BCD" segment to the EntryList.
612 // Note too, that it's safe for this thread to traverse the cxq
613 // without taking any special concurrency precautions.
614 }
615
616 // We don't currently reorder the cxq segment as we move it onto
617 // the EntryList, but it might make sense to reverse the order
618 // or perhaps sort by thread priority. See the comments in
619 // synchronizer.cpp objectMonitor::exit().
620 assert (_EntryList == NULL, "invariant") ;
621 _EntryList = List = (ParkEvent *)(cxq & ~_LBIT) ;
622 assert (List != NULL, "invariant") ;
623 goto WakeOne ;
624 }
625
626 // cxq|EntryList is empty.
627 // w == NULL implies that cxq|EntryList == NULL in the past.
628 // Possible race - rare inopportune interleaving.
629 // A thread could have added itself to cxq since this thread previously checked.
630 // Detect and recover by refetching cxq.
631 Punt:
632 assert (UNS(_OnDeck) == _LBIT, "invariant") ;
633 _OnDeck = NULL ; // Release inner lock.
634 OrderAccess::storeload(); // Dekker duality - pivot point
635
636 // Resample LockWord/cxq to recover from possible race.
637 // For instance, while this thread T1 held OnDeck, some other thread T2 might
638 // acquire the outer lock. Another thread T3 might try to acquire the outer
639 // lock, but encounter contention and enqueue itself on cxq. T2 then drops the
640 // outer lock, but skips succession as this thread T1 still holds OnDeck.
641 // T1 is and remains responsible for ensuring succession of T3.
642 //
643 // Note that we don't need to recheck EntryList, just cxq.
644 // If threads moved onto EntryList since we dropped OnDeck
645 // that implies some other thread forced succession.
646 cxq = _LockWord.FullWord ;
647 if ((cxq & ~_LBIT) != 0 && (cxq & _LBIT) == 0) {
648 goto Succession ; // potential race -- re-run succession
649 }
650 return ;
651 }
652
653 bool Monitor::notify() {
654 assert (_owner == Thread::current(), "invariant") ;
655 assert (ILocked(), "invariant") ;
656 if (_WaitSet == NULL) return true ;
657 NotifyCount ++ ;
658
659 // Transfer one thread from the WaitSet to the EntryList or cxq.
660 // Currently we just unlink the head of the WaitSet and prepend to the cxq.
661 // And of course we could just unlink it and unpark it, too, but
662 // in that case it'd likely impale itself on the reentry.
663 Thread::muxAcquire (_WaitLock, "notify:WaitLock") ;
664 ParkEvent * nfy = _WaitSet ;
665 if (nfy != NULL) { // DCL idiom
666 _WaitSet = nfy->ListNext ;
667 assert (nfy->Notified == 0, "invariant") ;
668 // push nfy onto the cxq
669 for (;;) {
670 const intptr_t v = _LockWord.FullWord ;
671 assert ((v & 0xFF) == _LBIT, "invariant") ;
672 nfy->ListNext = (ParkEvent *)(v & ~_LBIT);
673 if (CASPTR (&_LockWord, v, UNS(nfy)|_LBIT) == v) break;
674 // interference - _LockWord changed -- just retry
675 }
676 // Note that setting Notified before pushing nfy onto the cxq is
677 // also legal and safe, but the safety properties are much more
678 // subtle, so for the sake of code stewardship ...
679 OrderAccess::fence() ;
680 nfy->Notified = 1;
681 }
682 Thread::muxRelease (_WaitLock) ;
683 if (nfy != NULL && (NativeMonitorFlags & 16)) {
684 // Experimental code ... light up the wakee in the hope that this thread (the owner)
685 // will drop the lock just about the time the wakee comes ONPROC.
686 nfy->unpark() ;
687 }
688 assert (ILocked(), "invariant") ;
689 return true ;
690 }
691
692 // Currently notifyAll() transfers the waiters one-at-a-time from the waitset
693 // to the cxq. This could be done more efficiently with a single bulk en-mass transfer,
694 // but in practice notifyAll() for large #s of threads is rare and not time-critical.
695 // Beware too, that we invert the order of the waiters. Lets say that the
696 // waitset is "ABCD" and the cxq is "XYZ". After a notifyAll() the waitset
697 // will be empty and the cxq will be "DCBAXYZ". This is benign, of course.
698
699 bool Monitor::notify_all() {
700 assert (_owner == Thread::current(), "invariant") ;
701 assert (ILocked(), "invariant") ;
702 while (_WaitSet != NULL) notify() ;
703 return true ;
704 }
705
706 int Monitor::IWait (Thread * Self, jlong timo) {
707 assert (ILocked(), "invariant") ;
708
709 // Phases:
710 // 1. Enqueue Self on WaitSet - currently prepend
711 // 2. unlock - drop the outer lock
712 // 3. wait for either notification or timeout
713 // 4. lock - reentry - reacquire the outer lock
714
715 ParkEvent * const ESelf = Self->_MutexEvent ;
716 ESelf->Notified = 0 ;
717 ESelf->reset() ;
718 OrderAccess::fence() ;
719
720 // Add Self to WaitSet
721 // Ideally only the holder of the outer lock would manipulate the WaitSet -
722 // That is, the outer lock would implicitly protect the WaitSet.
723 // But if a thread in wait() encounters a timeout it will need to dequeue itself
724 // from the WaitSet _before it becomes the owner of the lock. We need to dequeue
725 // as the ParkEvent -- which serves as a proxy for the thread -- can't reside
726 // on both the WaitSet and the EntryList|cxq at the same time.. That is, a thread
727 // on the WaitSet can't be allowed to compete for the lock until it has managed to
728 // unlink its ParkEvent from WaitSet. Thus the need for WaitLock.
729 // Contention on the WaitLock is minimal.
730 //
731 // Another viable approach would be add another ParkEvent, "WaitEvent" to the
732 // thread class. The WaitSet would be composed of WaitEvents. Only the
733 // owner of the outer lock would manipulate the WaitSet. A thread in wait()
734 // could then compete for the outer lock, and then, if necessary, unlink itself
735 // from the WaitSet only after having acquired the outer lock. More precisely,
736 // there would be no WaitLock. A thread in in wait() would enqueue its WaitEvent
737 // on the WaitSet; release the outer lock; wait for either notification or timeout;
738 // reacquire the inner lock; and then, if needed, unlink itself from the WaitSet.
739 //
740 // Alternatively, a 2nd set of list link fields in the ParkEvent might suffice.
741 // One set would be for the WaitSet and one for the EntryList.
742 // We could also deconstruct the ParkEvent into a "pure" event and add a
743 // new immortal/TSM "ListElement" class that referred to ParkEvents.
744 // In that case we could have one ListElement on the WaitSet and another
745 // on the EntryList, with both referring to the same pure Event.
746
747 Thread::muxAcquire (_WaitLock, "wait:WaitLock:Add") ;
748 ESelf->ListNext = _WaitSet ;
749 _WaitSet = ESelf ;
750 Thread::muxRelease (_WaitLock) ;
751
752 // Release the outer lock
753 // We call IUnlock (RelaxAssert=true) as a thread T1 might
754 // enqueue itself on the WaitSet, call IUnlock(), drop the lock,
755 // and then stall before it can attempt to wake a successor.
756 // Some other thread T2 acquires the lock, and calls notify(), moving
757 // T1 from the WaitSet to the cxq. T2 then drops the lock. T1 resumes,
758 // and then finds *itself* on the cxq. During the course of a normal
759 // IUnlock() call a thread should _never find itself on the EntryList
760 // or cxq, but in the case of wait() it's possible.
761 // See synchronizer.cpp objectMonitor::wait().
762 IUnlock (true) ;
763
764 // Wait for either notification or timeout
765 // Beware that in some circumstances we might propagate
766 // spurious wakeups back to the caller.
767
768 for (;;) {
769 if (ESelf->Notified) break ;
770 int err = ParkCommon (ESelf, timo) ;
771 if (err == OS_TIMEOUT || (NativeMonitorFlags & 1)) break ;
772 }
773
774 // Prepare for reentry - if necessary, remove ESelf from WaitSet
775 // ESelf can be:
776 // 1. Still on the WaitSet. This can happen if we exited the loop by timeout.
777 // 2. On the cxq or EntryList
778 // 3. Not resident on cxq, EntryList or WaitSet, but in the OnDeck position.
779
780 OrderAccess::fence() ;
781 int WasOnWaitSet = 0 ;
782 if (ESelf->Notified == 0) {
783 Thread::muxAcquire (_WaitLock, "wait:WaitLock:remove") ;
784 if (ESelf->Notified == 0) { // DCL idiom
785 assert (_OnDeck != ESelf, "invariant") ; // can't be both OnDeck and on WaitSet
786 // ESelf is resident on the WaitSet -- unlink it.
787 // A doubly-linked list would be better here so we can unlink in constant-time.
788 // We have to unlink before we potentially recontend as ESelf might otherwise
789 // end up on the cxq|EntryList -- it can't be on two lists at once.
790 ParkEvent * p = _WaitSet ;
791 ParkEvent * q = NULL ; // classic q chases p
792 while (p != NULL && p != ESelf) {
793 q = p ;
794 p = p->ListNext ;
795 }
796 assert (p == ESelf, "invariant") ;
797 if (p == _WaitSet) { // found at head
798 assert (q == NULL, "invariant") ;
799 _WaitSet = p->ListNext ;
800 } else { // found in interior
801 assert (q->ListNext == p, "invariant") ;
802 q->ListNext = p->ListNext ;
803 }
804 WasOnWaitSet = 1 ; // We were *not* notified but instead encountered timeout
805 }
806 Thread::muxRelease (_WaitLock) ;
807 }
808
809 // Reentry phase - reacquire the lock
810 if (WasOnWaitSet) {
811 // ESelf was previously on the WaitSet but we just unlinked it above
812 // because of a timeout. ESelf is not resident on any list and is not OnDeck
813 assert (_OnDeck != ESelf, "invariant") ;
814 ILock (Self) ;
815 } else {
816 // A prior notify() operation moved ESelf from the WaitSet to the cxq.
817 // ESelf is now on the cxq, EntryList or at the OnDeck position.
818 // The following fragment is extracted from Monitor::ILock()
819 for (;;) {
820 if (_OnDeck == ESelf && TrySpin(Self)) break ;
821 ParkCommon (ESelf, 0) ;
822 }
823 assert (_OnDeck == ESelf, "invariant") ;
824 _OnDeck = NULL ;
825 }
826
827 assert (ILocked(), "invariant") ;
828 return WasOnWaitSet != 0 ; // return true IFF timeout
829 }
830
831
832 // ON THE VMTHREAD SNEAKING PAST HELD LOCKS:
833 // In particular, there are certain types of global lock that may be held
834 // by a Java thread while it is blocked at a safepoint but before it has
835 // written the _owner field. These locks may be sneakily acquired by the
836 // VM thread during a safepoint to avoid deadlocks. Alternatively, one should
837 // identify all such locks, and ensure that Java threads never block at
838 // safepoints while holding them (_no_safepoint_check_flag). While it
839 // seems as though this could increase the time to reach a safepoint
840 // (or at least increase the mean, if not the variance), the latter
841 // approach might make for a cleaner, more maintainable JVM design.
842 //
843 // Sneaking is vile and reprehensible and should be excised at the 1st
844 // opportunity. It's possible that the need for sneaking could be obviated
845 // as follows. Currently, a thread might (a) while TBIVM, call pthread_mutex_lock
846 // or ILock() thus acquiring the "physical" lock underlying Monitor/Mutex.
847 // (b) stall at the TBIVM exit point as a safepoint is in effect. Critically,
848 // it'll stall at the TBIVM reentry state transition after having acquired the
849 // underlying lock, but before having set _owner and having entered the actual
850 // critical section. The lock-sneaking facility leverages that fact and allowed the
851 // VM thread to logically acquire locks that had already be physically locked by mutators
852 // but where mutators were known blocked by the reentry thread state transition.
853 //
854 // If we were to modify the Monitor-Mutex so that TBIVM state transitions tightly
855 // wrapped calls to park(), then we could likely do away with sneaking. We'd
856 // decouple lock acquisition and parking. The critical invariant to eliminating
857 // sneaking is to ensure that we never "physically" acquire the lock while TBIVM.
858 // An easy way to accomplish this is to wrap the park calls in a narrow TBIVM jacket.
859 // One difficulty with this approach is that the TBIVM wrapper could recurse and
860 // call lock() deep from within a lock() call, while the MutexEvent was already enqueued.
861 // Using a stack (N=2 at minimum) of ParkEvents would take care of that problem.
862 //
863 // But of course the proper ultimate approach is to avoid schemes that require explicit
864 // sneaking or dependence on any any clever invariants or subtle implementation properties
865 // of Mutex-Monitor and instead directly address the underlying design flaw.
866
867 void Monitor::lock (Thread * Self) {
868 #ifdef CHECK_UNHANDLED_OOPS
869 // Clear unhandled oops so we get a crash right away. Only clear for non-vm
870 // or GC threads.
871 if (Self->is_Java_thread()) {
872 Self->clear_unhandled_oops();
873 }
874 #endif // CHECK_UNHANDLED_OOPS
875
876 debug_only(check_prelock_state(Self));
877 assert (_owner != Self , "invariant") ;
878 assert (_OnDeck != Self->_MutexEvent, "invariant") ;
879
880 if (TryFast()) {
881 Exeunt:
882 assert (ILocked(), "invariant") ;
883 assert (owner() == NULL, "invariant");
884 set_owner (Self);
885 return ;
886 }
887
888 // The lock is contended ...
889
890 bool can_sneak = Self->is_VM_thread() && SafepointSynchronize::is_at_safepoint();
891 if (can_sneak && _owner == NULL) {
892 // a java thread has locked the lock but has not entered the
893 // critical region -- let's just pretend we've locked the lock
894 // and go on. we note this with _snuck so we can also
895 // pretend to unlock when the time comes.
896 _snuck = true;
897 goto Exeunt ;
898 }
899
900 // Try a brief spin to avoid passing thru thread state transition ...
901 if (TrySpin (Self)) goto Exeunt ;
902
903 check_block_state(Self);
904 if (Self->is_Java_thread()) {
905 // Horribile dictu - we suffer through a state transition
906 assert(rank() > Mutex::special, "Potential deadlock with special or lesser rank mutex");
907 ThreadBlockInVM tbivm ((JavaThread *) Self) ;
908 ILock (Self) ;
909 } else {
910 // Mirabile dictu
911 ILock (Self) ;
912 }
913 goto Exeunt ;
914 }
915
916 void Monitor::lock() {
917 this->lock(Thread::current());
918 }
919
920 // Lock without safepoint check - a degenerate variant of lock().
921 // Should ONLY be used by safepoint code and other code
922 // that is guaranteed not to block while running inside the VM. If this is called with
923 // thread state set to be in VM, the safepoint synchronization code will deadlock!
924
925 void Monitor::lock_without_safepoint_check (Thread * Self) {
926 assert (_owner != Self, "invariant") ;
927 ILock (Self) ;
928 assert (_owner == NULL, "invariant");
929 set_owner (Self);
930 }
931
932 void Monitor::lock_without_safepoint_check () {
933 lock_without_safepoint_check (Thread::current()) ;
934 }
935
936
937 // Returns true if thread succeceed [sic] in grabbing the lock, otherwise false.
938
939 bool Monitor::try_lock() {
940 Thread * const Self = Thread::current();
941 debug_only(check_prelock_state(Self));
942 // assert(!thread->is_inside_signal_handler(), "don't lock inside signal handler");
943
944 // Special case, where all Java threads are stopped.
945 // The lock may have been acquired but _owner is not yet set.
946 // In that case the VM thread can safely grab the lock.
947 // It strikes me this should appear _after the TryLock() fails, below.
948 bool can_sneak = Self->is_VM_thread() && SafepointSynchronize::is_at_safepoint();
949 if (can_sneak && _owner == NULL) {
950 set_owner(Self); // Do not need to be atomic, since we are at a safepoint
951 _snuck = true;
952 return true;
953 }
954
955 if (TryLock()) {
956 // We got the lock
957 assert (_owner == NULL, "invariant");
958 set_owner (Self);
959 return true;
960 }
961 return false;
962 }
963
964 void Monitor::unlock() {
965 assert (_owner == Thread::current(), "invariant") ;
966 assert (_OnDeck != Thread::current()->_MutexEvent , "invariant") ;
967 set_owner (NULL) ;
968 if (_snuck) {
969 assert(SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread(), "sneak");
970 _snuck = false;
971 return ;
972 }
973 IUnlock (false) ;
974 }
975
976 // Yet another degenerate version of Monitor::lock() or lock_without_safepoint_check()
977 // jvm_raw_lock() and _unlock() can be called by non-Java threads via JVM_RawMonitorEnter.
978 //
979 // There's no expectation that JVM_RawMonitors will interoperate properly with the native
980 // Mutex-Monitor constructs. We happen to implement JVM_RawMonitors in terms of
981 // native Mutex-Monitors simply as a matter of convenience. A simple abstraction layer
982 // over a pthread_mutex_t would work equally as well, but require more platform-specific
983 // code -- a "PlatformMutex". Alternatively, a simply layer over muxAcquire-muxRelease
984 // would work too.
985 //
986 // Since the caller might be a foreign thread, we don't necessarily have a Thread.MutexEvent
987 // instance available. Instead, we transiently allocate a ParkEvent on-demand if
988 // we encounter contention. That ParkEvent remains associated with the thread
989 // until it manages to acquire the lock, at which time we return the ParkEvent
990 // to the global ParkEvent free list. This is correct and suffices for our purposes.
991 //
992 // Beware that the original jvm_raw_unlock() had a "_snuck" test but that
993 // jvm_raw_lock() didn't have the corresponding test. I suspect that's an
994 // oversight, but I've replicated the original suspect logic in the new code ...
995
996 void Monitor::jvm_raw_lock() {
997 assert(rank() == native, "invariant");
998
999 if (TryLock()) {
1000 Exeunt:
1001 assert (ILocked(), "invariant") ;
1002 assert (_owner == NULL, "invariant");
1003 // This can potentially be called by non-java Threads. Thus, the ThreadLocalStorage
1004 // might return NULL. Don't call set_owner since it will break on an NULL owner
1005 // Consider installing a non-null "ANON" distinguished value instead of just NULL.
1006 _owner = ThreadLocalStorage::thread();
1007 return ;
1008 }
1009
1010 if (TrySpin(NULL)) goto Exeunt ;
1011
1012 // slow-path - apparent contention
1013 // Allocate a ParkEvent for transient use.
1014 // The ParkEvent remains associated with this thread until
1015 // the time the thread manages to acquire the lock.
1016 ParkEvent * const ESelf = ParkEvent::Allocate(NULL) ;
1017 ESelf->reset() ;
1018 OrderAccess::storeload() ;
1019
1020 // Either Enqueue Self on cxq or acquire the outer lock.
1021 if (AcquireOrPush (ESelf)) {
1022 ParkEvent::Release (ESelf) ; // surrender the ParkEvent
1023 goto Exeunt ;
1024 }
1025
1026 // At any given time there is at most one ondeck thread.
1027 // ondeck implies not resident on cxq and not resident on EntryList
1028 // Only the OnDeck thread can try to acquire -- contended for -- the lock.
1029 // CONSIDER: use Self->OnDeck instead of m->OnDeck.
1030 for (;;) {
1031 if (_OnDeck == ESelf && TrySpin(NULL)) break ;
1032 ParkCommon (ESelf, 0) ;
1033 }
1034
1035 assert (_OnDeck == ESelf, "invariant") ;
1036 _OnDeck = NULL ;
1037 ParkEvent::Release (ESelf) ; // surrender the ParkEvent
1038 goto Exeunt ;
1039 }
1040
1041 void Monitor::jvm_raw_unlock() {
1042 // Nearly the same as Monitor::unlock() ...
1043 // directly set _owner instead of using set_owner(null)
1044 _owner = NULL ;
1045 if (_snuck) { // ???
1046 assert(SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread(), "sneak");
1047 _snuck = false;
1048 return ;
1049 }
1050 IUnlock(false) ;
1051 }
1052
1053 bool Monitor::wait(bool no_safepoint_check, long timeout, bool as_suspend_equivalent) {
1054 Thread * const Self = Thread::current() ;
1055 assert (_owner == Self, "invariant") ;
1056 assert (ILocked(), "invariant") ;
1057
1058 // as_suspend_equivalent logically implies !no_safepoint_check
1059 guarantee (!as_suspend_equivalent || !no_safepoint_check, "invariant") ;
1060 // !no_safepoint_check logically implies java_thread
1061 guarantee (no_safepoint_check || Self->is_Java_thread(), "invariant") ;
1062
1063 #ifdef ASSERT
1064 Monitor * least = get_least_ranked_lock_besides_this(Self->owned_locks());
1065 assert(least != this, "Specification of get_least_... call above");
1066 if (least != NULL && least->rank() <= special) {
1067 tty->print("Attempting to wait on monitor %s/%d while holding"
1068 " lock %s/%d -- possible deadlock",
1069 name(), rank(), least->name(), least->rank());
1070 assert(false, "Shouldn't block(wait) while holding a lock of rank special");
1071 }
1072 #endif // ASSERT
1073
1074 int wait_status ;
1075 // conceptually set the owner to NULL in anticipation of
1076 // abdicating the lock in wait
1077 set_owner(NULL);
1078 if (no_safepoint_check) {
1079 wait_status = IWait (Self, timeout) ;
1080 } else {
1081 assert (Self->is_Java_thread(), "invariant") ;
1082 JavaThread *jt = (JavaThread *)Self;
1083
1084 // Enter safepoint region - ornate and Rococo ...
1085 ThreadBlockInVM tbivm(jt);
1086 OSThreadWaitState osts(Self->osthread(), false /* not Object.wait() */);
1087
1088 if (as_suspend_equivalent) {
1089 jt->set_suspend_equivalent();
1090 // cleared by handle_special_suspend_equivalent_condition() or
1091 // java_suspend_self()
1092 }
1093
1094 wait_status = IWait (Self, timeout) ;
1095
1096 // were we externally suspended while we were waiting?
1097 if (as_suspend_equivalent && jt->handle_special_suspend_equivalent_condition()) {
1098 // Our event wait has finished and we own the lock, but
1099 // while we were waiting another thread suspended us. We don't
1100 // want to hold the lock while suspended because that
1101 // would surprise the thread that suspended us.
1102 assert (ILocked(), "invariant") ;
1103 IUnlock (true) ;
1104 jt->java_suspend_self();
1105 ILock (Self) ;
1106 assert (ILocked(), "invariant") ;
1107 }
1108 }
1109
1110 // Conceptually reestablish ownership of the lock.
1111 // The "real" lock -- the LockByte -- was reacquired by IWait().
1112 assert (ILocked(), "invariant") ;
1113 assert (_owner == NULL, "invariant") ;
1114 set_owner (Self) ;
1115 return wait_status != 0 ; // return true IFF timeout
1116 }
1117
1118 Monitor::~Monitor() {
1119 assert ((UNS(_owner)|UNS(_LockWord.FullWord)|UNS(_EntryList)|UNS(_WaitSet)|UNS(_OnDeck)) == 0, "") ;
1120 }
1121
1122 void Monitor::ClearMonitor (Monitor * m, const char *name) {
1123 m->_owner = NULL ;
1124 m->_snuck = false ;
1125 if (name == NULL) {
1126 strcpy(m->_name, "UNKNOWN") ;
1127 } else {
1128 strncpy(m->_name, name, MONITOR_NAME_LEN - 1);
1129 m->_name[MONITOR_NAME_LEN - 1] = '\0';
1130 }
1131 m->_LockWord.FullWord = 0 ;
1132 m->_EntryList = NULL ;
1133 m->_OnDeck = NULL ;
1134 m->_WaitSet = NULL ;
1135 m->_WaitLock[0] = 0 ;
1136 }
1137
1138 Monitor::Monitor() { ClearMonitor(this); }
1139
1140 Monitor::Monitor (int Rank, const char * name, bool allow_vm_block) {
1141 ClearMonitor (this, name) ;
1142 #ifdef ASSERT
1143 _allow_vm_block = allow_vm_block;
1144 _rank = Rank ;
1145 #endif
1146 }
1147
1148 Mutex::~Mutex() {
1149 assert ((UNS(_owner)|UNS(_LockWord.FullWord)|UNS(_EntryList)|UNS(_WaitSet)|UNS(_OnDeck)) == 0, "") ;
1150 }
1151
1152 Mutex::Mutex (int Rank, const char * name, bool allow_vm_block) {
1153 ClearMonitor ((Monitor *) this, name) ;
1154 #ifdef ASSERT
1155 _allow_vm_block = allow_vm_block;
1156 _rank = Rank ;
1157 #endif
1158 }
1159
1160 bool Monitor::owned_by_self() const {
1161 bool ret = _owner == Thread::current();
1162 assert (!ret || _LockWord.Bytes[_LSBINDEX] != 0, "invariant") ;
1163 return ret;
1164 }
1165
1166 void Monitor::print_on_error(outputStream* st) const {
1167 st->print("[" PTR_FORMAT, this);
1168 st->print("] %s", _name);
1169 st->print(" - owner thread: " PTR_FORMAT, _owner);
1170 }
1171
1172
1173
1174
1175 // ----------------------------------------------------------------------------------
1176 // Non-product code
1177
1178 #ifndef PRODUCT
1179 void Monitor::print_on(outputStream* st) const {
1180 st->print_cr("Mutex: [0x%lx/0x%lx] %s - owner: 0x%lx", this, _LockWord.FullWord, _name, _owner);
1181 }
1182 #endif
1183
1184 #ifndef PRODUCT
1185 #ifdef ASSERT
1186 Monitor * Monitor::get_least_ranked_lock(Monitor * locks) {
1187 Monitor *res, *tmp;
1188 for (res = tmp = locks; tmp != NULL; tmp = tmp->next()) {
1189 if (tmp->rank() < res->rank()) {
1190 res = tmp;
1191 }
1192 }
1193 if (!SafepointSynchronize::is_at_safepoint()) {
1194 // In this case, we expect the held locks to be
1195 // in increasing rank order (modulo any native ranks)
1196 for (tmp = locks; tmp != NULL; tmp = tmp->next()) {
1197 if (tmp->next() != NULL) {
1198 assert(tmp->rank() == Mutex::native ||
1199 tmp->rank() <= tmp->next()->rank(), "mutex rank anomaly?");
1200 }
1201 }
1202 }
1203 return res;
1204 }
1205
1206 Monitor* Monitor::get_least_ranked_lock_besides_this(Monitor* locks) {
1207 Monitor *res, *tmp;
1208 for (res = NULL, tmp = locks; tmp != NULL; tmp = tmp->next()) {
1209 if (tmp != this && (res == NULL || tmp->rank() < res->rank())) {
1210 res = tmp;
1211 }
1212 }
1213 if (!SafepointSynchronize::is_at_safepoint()) {
1214 // In this case, we expect the held locks to be
1215 // in increasing rank order (modulo any native ranks)
1216 for (tmp = locks; tmp != NULL; tmp = tmp->next()) {
1217 if (tmp->next() != NULL) {
1218 assert(tmp->rank() == Mutex::native ||
1219 tmp->rank() <= tmp->next()->rank(), "mutex rank anomaly?");
1220 }
1221 }
1222 }
1223 return res;
1224 }
1225
1226
1227 bool Monitor::contains(Monitor* locks, Monitor * lock) {
1228 for (; locks != NULL; locks = locks->next()) {
1229 if (locks == lock)
1230 return true;
1231 }
1232 return false;
1233 }
1234 #endif
1235
1236 // Called immediately after lock acquisition or release as a diagnostic
1237 // to track the lock-set of the thread and test for rank violations that
1238 // might indicate exposure to deadlock.
1239 // Rather like an EventListener for _owner (:>).
1240
1241 void Monitor::set_owner_implementation(Thread *new_owner) {
1242 // This function is solely responsible for maintaining
1243 // and checking the invariant that threads and locks
1244 // are in a 1/N relation, with some some locks unowned.
1245 // It uses the Mutex::_owner, Mutex::_next, and
1246 // Thread::_owned_locks fields, and no other function
1247 // changes those fields.
1248 // It is illegal to set the mutex from one non-NULL
1249 // owner to another--it must be owned by NULL as an
1250 // intermediate state.
1251
1252 if (new_owner != NULL) {
1253 // the thread is acquiring this lock
1254
1255 assert(new_owner == Thread::current(), "Should I be doing this?");
1256 assert(_owner == NULL, "setting the owner thread of an already owned mutex");
1257 _owner = new_owner; // set the owner
1258
1259 // link "this" into the owned locks list
1260
1261 #ifdef ASSERT // Thread::_owned_locks is under the same ifdef
1262 Monitor* locks = get_least_ranked_lock(new_owner->owned_locks());
1263 // Mutex::set_owner_implementation is a friend of Thread
1264
1265 assert(this->rank() >= 0, "bad lock rank");
1266
1267 if (LogMultipleMutexLocking && locks != NULL) {
1268 Events::log("thread " INTPTR_FORMAT " locks %s, already owns %s", new_owner, name(), locks->name());
1269 }
1270
1271 // Deadlock avoidance rules require us to acquire Mutexes only in
1272 // a global total order. For example m1 is the lowest ranked mutex
1273 // that the thread holds and m2 is the mutex the thread is trying
1274 // to acquire, then deadlock avoidance rules require that the rank
1275 // of m2 be less than the rank of m1.
1276 // The rank Mutex::native is an exception in that it is not subject
1277 // to the verification rules.
1278 // Here are some further notes relating to mutex acquisition anomalies:
1279 // . under Solaris, the interrupt lock gets acquired when doing
1280 // profiling, so any lock could be held.
1281 // . it is also ok to acquire Safepoint_lock at the very end while we
1282 // already hold Terminator_lock - may happen because of periodic safepoints
1283 if (this->rank() != Mutex::native &&
1284 this->rank() != Mutex::suspend_resume &&
1285 locks != NULL && locks->rank() <= this->rank() &&
1286 !SafepointSynchronize::is_at_safepoint() &&
1287 this != Interrupt_lock && this != ProfileVM_lock &&
1288 !(this == Safepoint_lock && contains(locks, Terminator_lock) &&
1289 SafepointSynchronize::is_synchronizing())) {
1290 new_owner->print_owned_locks();
1291 fatal(err_msg("acquiring lock %s/%d out of order with lock %s/%d -- "
1292 "possible deadlock", this->name(), this->rank(),
1293 locks->name(), locks->rank()));
1294 }
1295
1296 this->_next = new_owner->_owned_locks;
1297 new_owner->_owned_locks = this;
1298 #endif
1299
1300 } else {
1301 // the thread is releasing this lock
1302
1303 Thread* old_owner = _owner;
1304 debug_only(_last_owner = old_owner);
1305
1306 assert(old_owner != NULL, "removing the owner thread of an unowned mutex");
1307 assert(old_owner == Thread::current(), "removing the owner thread of an unowned mutex");
1308
1309 _owner = NULL; // set the owner
1310
1311 #ifdef ASSERT
1312 Monitor *locks = old_owner->owned_locks();
1313
1314 if (LogMultipleMutexLocking && locks != this) {
1315 Events::log("thread " INTPTR_FORMAT " unlocks %s, still owns %s", old_owner, this->name(), locks->name());
1316 }
1317
1318 // remove "this" from the owned locks list
1319
1320 Monitor *prev = NULL;
1321 bool found = false;
1322 for (; locks != NULL; prev = locks, locks = locks->next()) {
1323 if (locks == this) {
1324 found = true;
1325 break;
1326 }
1327 }
1328 assert(found, "Removing a lock not owned");
1329 if (prev == NULL) {
1330 old_owner->_owned_locks = _next;
1331 } else {
1332 prev->_next = _next;
1333 }
1334 _next = NULL;
1335 #endif
1336 }
1337 }
1338
1339
1340 // Factored out common sanity checks for locking mutex'es. Used by lock() and try_lock()
1341 void Monitor::check_prelock_state(Thread *thread) {
1342 assert((!thread->is_Java_thread() || ((JavaThread *)thread)->thread_state() == _thread_in_vm)
1343 || rank() == Mutex::special, "wrong thread state for using locks");
1344 if (StrictSafepointChecks) {
1345 if (thread->is_VM_thread() && !allow_vm_block()) {
1346 fatal(err_msg("VM thread using lock %s (not allowed to block on)",
1347 name()));
1348 }
1349 debug_only(if (rank() != Mutex::special) \
1350 thread->check_for_valid_safepoint_state(false);)
1351 }
1352 }
1353
1354 void Monitor::check_block_state(Thread *thread) {
1355 if (!_allow_vm_block && thread->is_VM_thread()) {
1356 warning("VM thread blocked on lock");
1357 print();
1358 BREAKPOINT;
1359 }
1360 assert(_owner != thread, "deadlock: blocking on monitor owned by current thread");
1361 }
1362
1363 #endif // PRODUCT