1
2 /*
3 * Copyright (c) 1998, 2014, 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 *
16 * You should have received a copy of the GNU General Public License version
17 * 2 along with this work; if not, write to the Free Software Foundation,
18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
21 * or visit www.oracle.com if you need additional information or have any
22 * questions.
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24 */
25
26 #include "precompiled.hpp"
27 #include "runtime/atomic.inline.hpp"
28 #include "runtime/mutex.hpp"
29 #include "runtime/orderAccess.inline.hpp"
30 #include "runtime/osThread.hpp"
31 #include "runtime/thread.inline.hpp"
32 #include "utilities/events.hpp"
33 #ifdef TARGET_OS_FAMILY_linux
34 # include "mutex_linux.inline.hpp"
35 #endif
36 #ifdef TARGET_OS_FAMILY_solaris
37 # include "mutex_solaris.inline.hpp"
38 #endif
39 #ifdef TARGET_OS_FAMILY_windows
40 # include "mutex_windows.inline.hpp"
41 #endif
42 #ifdef TARGET_OS_FAMILY_bsd
43 # include "mutex_bsd.inline.hpp"
44 #endif
45
46 PRAGMA_FORMAT_MUTE_WARNINGS_FOR_GCC
47
48 // 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
49 //
50 // Native Monitor-Mutex locking - theory of operations
51 //
52 // * Native Monitors are completely unrelated to Java-level monitors,
53 // although the "back-end" slow-path implementations share a common lineage.
54 // See objectMonitor:: in synchronizer.cpp.
55 // Native Monitors do *not* support nesting or recursion but otherwise
56 // they're basically Hoare-flavor monitors.
57 //
58 // * A thread acquires ownership of a Monitor/Mutex by CASing the LockByte
59 // in the _LockWord from zero to non-zero. Note that the _Owner field
60 // is advisory and is used only to verify that the thread calling unlock()
61 // is indeed the last thread to have acquired the lock.
62 //
63 // * Contending threads "push" themselves onto the front of the contention
64 // queue -- called the cxq -- with CAS and then spin/park.
65 // The _LockWord contains the LockByte as well as the pointer to the head
66 // of the cxq. Colocating the LockByte with the cxq precludes certain races.
67 //
68 // * Using a separately addressable LockByte allows for CAS:MEMBAR or CAS:0
69 // idioms. We currently use MEMBAR in the uncontended unlock() path, as
70 // MEMBAR often has less latency than CAS. If warranted, we could switch to
71 // a CAS:0 mode, using timers to close the resultant race, as is done
72 // with Java Monitors in synchronizer.cpp.
73 //
74 // See the following for a discussion of the relative cost of atomics (CAS)
75 // MEMBAR, and ways to eliminate such instructions from the common-case paths:
76 // -- http://blogs.sun.com/dave/entry/biased_locking_in_hotspot
77 // -- http://blogs.sun.com/dave/resource/MustangSync.pdf
78 // -- http://blogs.sun.com/dave/resource/synchronization-public2.pdf
79 // -- synchronizer.cpp
80 //
81 // * Overall goals - desiderata
82 // 1. Minimize context switching
83 // 2. Minimize lock migration
84 // 3. Minimize CPI -- affinity and locality
85 // 4. Minimize the execution of high-latency instructions such as CAS or MEMBAR
86 // 5. Minimize outer lock hold times
87 // 6. Behave gracefully on a loaded system
88 //
89 // * Thread flow and list residency:
90 //
91 // Contention queue --> EntryList --> OnDeck --> Owner --> !Owner
92 // [..resident on monitor list..]
93 // [...........contending..................]
94 //
95 // -- The contention queue (cxq) contains recently-arrived threads (RATs).
96 // Threads on the cxq eventually drain into the EntryList.
97 // -- Invariant: a thread appears on at most one list -- cxq, EntryList
98 // or WaitSet -- at any one time.
99 // -- For a given monitor there can be at most one "OnDeck" thread at any
100 // given time but if needbe this particular invariant could be relaxed.
101 //
102 // * The WaitSet and EntryList linked lists are composed of ParkEvents.
103 // I use ParkEvent instead of threads as ParkEvents are immortal and
104 // type-stable, meaning we can safely unpark() a possibly stale
105 // list element in the unlock()-path. (That's benign).
106 //
107 // * Succession policy - providing for progress:
108 //
109 // As necessary, the unlock()ing thread identifies, unlinks, and unparks
110 // an "heir presumptive" tentative successor thread from the EntryList.
111 // This becomes the so-called "OnDeck" thread, of which there can be only
112 // one at any given time for a given monitor. The wakee will recontend
113 // for ownership of monitor.
114 //
115 // Succession is provided for by a policy of competitive handoff.
116 // The exiting thread does _not_ grant or pass ownership to the
117 // successor thread. (This is also referred to as "handoff" succession").
118 // Instead the exiting thread releases ownership and possibly wakes
119 // a successor, so the successor can (re)compete for ownership of the lock.
120 //
121 // Competitive handoff provides excellent overall throughput at the expense
122 // of short-term fairness. If fairness is a concern then one remedy might
123 // be to add an AcquireCounter field to the monitor. After a thread acquires
124 // the lock it will decrement the AcquireCounter field. When the count
125 // reaches 0 the thread would reset the AcquireCounter variable, abdicate
126 // the lock directly to some thread on the EntryList, and then move itself to the
127 // tail of the EntryList.
128 //
129 // But in practice most threads engage or otherwise participate in resource
130 // bounded producer-consumer relationships, so lock domination is not usually
131 // a practical concern. Recall too, that in general it's easier to construct
132 // a fair lock from a fast lock, but not vice-versa.
133 //
134 // * The cxq can have multiple concurrent "pushers" but only one concurrent
135 // detaching thread. This mechanism is immune from the ABA corruption.
136 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
137 // We use OnDeck as a pseudo-lock to enforce the at-most-one detaching
138 // thread constraint.
139 //
140 // * Taken together, the cxq and the EntryList constitute or form a
141 // single logical queue of threads stalled trying to acquire the lock.
142 // We use two distinct lists to reduce heat on the list ends.
143 // Threads in lock() enqueue onto cxq while threads in unlock() will
144 // dequeue from the EntryList. (c.f. Michael Scott's "2Q" algorithm).
145 // A key desideratum is to minimize queue & monitor metadata manipulation
146 // that occurs while holding the "outer" monitor lock -- that is, we want to
147 // minimize monitor lock holds times.
148 //
149 // The EntryList is ordered by the prevailing queue discipline and
150 // can be organized in any convenient fashion, such as a doubly-linked list or
151 // a circular doubly-linked list. If we need a priority queue then something akin
152 // to Solaris' sleepq would work nicely. Viz.,
153 // -- http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
154 // -- http://cvs.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/uts/common/os/sleepq.c
155 // Queue discipline is enforced at ::unlock() time, when the unlocking thread
156 // drains the cxq into the EntryList, and orders or reorders the threads on the
157 // EntryList accordingly.
158 //
159 // Barring "lock barging", this mechanism provides fair cyclic ordering,
160 // somewhat similar to an elevator-scan.
161 //
162 // * OnDeck
163 // -- For a given monitor there can be at most one OnDeck thread at any given
164 // instant. The OnDeck thread is contending for the lock, but has been
165 // unlinked from the EntryList and cxq by some previous unlock() operations.
166 // Once a thread has been designated the OnDeck thread it will remain so
167 // until it manages to acquire the lock -- being OnDeck is a stable property.
168 // -- Threads on the EntryList or cxq are _not allowed to attempt lock acquisition.
169 // -- OnDeck also serves as an "inner lock" as follows. Threads in unlock() will, after
170 // having cleared the LockByte and dropped the outer lock, attempt to "trylock"
171 // OnDeck by CASing the field from null to non-null. If successful, that thread
172 // is then responsible for progress and succession and can use CAS to detach and
173 // drain the cxq into the EntryList. By convention, only this thread, the holder of
174 // the OnDeck inner lock, can manipulate the EntryList or detach and drain the
175 // RATs on the cxq into the EntryList. This avoids ABA corruption on the cxq as
176 // we allow multiple concurrent "push" operations but restrict detach concurrency
177 // to at most one thread. Having selected and detached a successor, the thread then
178 // changes the OnDeck to refer to that successor, and then unparks the successor.
179 // That successor will eventually acquire the lock and clear OnDeck. Beware
180 // that the OnDeck usage as a lock is asymmetric. A thread in unlock() transiently
181 // "acquires" OnDeck, performs queue manipulations, passes OnDeck to some successor,
182 // and then the successor eventually "drops" OnDeck. Note that there's never
183 // any sense of contention on the inner lock, however. Threads never contend
184 // or wait for the inner lock.
185 // -- OnDeck provides for futile wakeup throttling a described in section 3.3 of
186 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
187 // In a sense, OnDeck subsumes the ObjectMonitor _Succ and ObjectWaiter
188 // TState fields found in Java-level objectMonitors. (See synchronizer.cpp).
189 //
190 // * Waiting threads reside on the WaitSet list -- wait() puts
191 // the caller onto the WaitSet. Notify() or notifyAll() simply
192 // transfers threads from the WaitSet to either the EntryList or cxq.
193 // Subsequent unlock() operations will eventually unpark the notifyee.
194 // Unparking a notifee in notify() proper is inefficient - if we were to do so
195 // it's likely the notifyee would simply impale itself on the lock held
196 // by the notifier.
197 //
198 // * The mechanism is obstruction-free in that if the holder of the transient
199 // OnDeck lock in unlock() is preempted or otherwise stalls, other threads
200 // can still acquire and release the outer lock and continue to make progress.
201 // At worst, waking of already blocked contending threads may be delayed,
202 // but nothing worse. (We only use "trylock" operations on the inner OnDeck
203 // lock).
204 //
205 // * Note that thread-local storage must be initialized before a thread
206 // uses Native monitors or mutexes. The native monitor-mutex subsystem
207 // depends on Thread::current().
208 //
209 // * The monitor synchronization subsystem avoids the use of native
210 // synchronization primitives except for the narrow platform-specific
211 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
212 // the semantics of park-unpark. Put another way, this monitor implementation
213 // depends only on atomic operations and park-unpark. The monitor subsystem
214 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
215 // underlying OS manages the READY<->RUN transitions.
216 //
217 // * The memory consistency model provide by lock()-unlock() is at least as
218 // strong or stronger than the Java Memory model defined by JSR-133.
219 // That is, we guarantee at least entry consistency, if not stronger.
220 // See http://g.oswego.edu/dl/jmm/cookbook.html.
221 //
222 // * Thread:: currently contains a set of purpose-specific ParkEvents:
223 // _MutexEvent, _ParkEvent, etc. A better approach might be to do away with
224 // the purpose-specific ParkEvents and instead implement a general per-thread
225 // stack of available ParkEvents which we could provision on-demand. The
226 // stack acts as a local cache to avoid excessive calls to ParkEvent::Allocate()
227 // and ::Release(). A thread would simply pop an element from the local stack before it
228 // enqueued or park()ed. When the contention was over the thread would
229 // push the no-longer-needed ParkEvent back onto its stack.
230 //
231 // * A slightly reduced form of ILock() and IUnlock() have been partially
232 // model-checked (Murphi) for safety and progress at T=1,2,3 and 4.
233 // It'd be interesting to see if TLA/TLC could be useful as well.
234 //
235 // * Mutex-Monitor is a low-level "leaf" subsystem. That is, the monitor
236 // code should never call other code in the JVM that might itself need to
237 // acquire monitors or mutexes. That's true *except* in the case of the
238 // ThreadBlockInVM state transition wrappers. The ThreadBlockInVM DTOR handles
239 // mutator reentry (ingress) by checking for a pending safepoint in which case it will
240 // call SafepointSynchronize::block(), which in turn may call Safepoint_lock->lock(), etc.
241 // In that particular case a call to lock() for a given Monitor can end up recursively
242 // calling lock() on another monitor. While distasteful, this is largely benign
243 // as the calls come from jacket that wraps lock(), and not from deep within lock() itself.
244 //
245 // It's unfortunate that native mutexes and thread state transitions were convolved.
246 // They're really separate concerns and should have remained that way. Melding
247 // them together was facile -- a bit too facile. The current implementation badly
248 // conflates the two concerns.
249 //
250 // * TODO-FIXME:
251 //
252 // -- Add DTRACE probes for contended acquire, contended acquired, contended unlock
253 // We should also add DTRACE probes in the ParkEvent subsystem for
254 // Park-entry, Park-exit, and Unpark.
255 //
256 // -- We have an excess of mutex-like constructs in the JVM, namely:
257 // 1. objectMonitors for Java-level synchronization (synchronizer.cpp)
258 // 2. low-level muxAcquire and muxRelease
259 // 3. low-level spinAcquire and spinRelease
260 // 4. native Mutex:: and Monitor::
261 // 5. jvm_raw_lock() and _unlock()
262 // 6. JVMTI raw monitors -- distinct from (5) despite having a confusingly
263 // similar name.
264 //
265 // 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
266
267
268 // CASPTR() uses the canonical argument order that dominates in the literature.
269 // Our internal cmpxchg_ptr() uses a bastardized ordering to accommodate Sun .il templates.
270
271 #define CASPTR(a,c,s) intptr_t(Atomic::cmpxchg_ptr ((void *)(s),(void *)(a),(void *)(c)))
272 #define UNS(x) (uintptr_t(x))
273 #define TRACE(m) { static volatile int ctr = 0 ; int x = ++ctr ; if ((x & (x-1))==0) { ::printf ("%d:%s\n", x, #m); ::fflush(stdout); }}
274
275 // Simplistic low-quality Marsaglia SHIFT-XOR RNG.
276 // Bijective except for the trailing mask operation.
277 // Useful for spin loops as the compiler can't optimize it away.
278
279 static inline jint MarsagliaXORV (jint x) {
280 if (x == 0) x = 1|os::random() ;
281 x ^= x << 6;
282 x ^= ((unsigned)x) >> 21;
283 x ^= x << 7 ;
284 return x & 0x7FFFFFFF ;
285 }
286
287 static inline jint MarsagliaXOR (jint * const a) {
288 jint x = *a ;
289 if (x == 0) x = UNS(a)|1 ;
290 x ^= x << 6;
291 x ^= ((unsigned)x) >> 21;
292 x ^= x << 7 ;
293 *a = x ;
294 return x & 0x7FFFFFFF ;
295 }
296
297 static int Stall (int its) {
298 static volatile jint rv = 1 ;
299 volatile int OnFrame = 0 ;
300 jint v = rv ^ UNS(OnFrame) ;
301 while (--its >= 0) {
302 v = MarsagliaXORV (v) ;
303 }
304 // Make this impossible for the compiler to optimize away,
305 // but (mostly) avoid W coherency sharing on MP systems.
306 if (v == 0x12345) rv = v ;
307 return v ;
308 }
309
310 int Monitor::TryLock () {
311 intptr_t v = _LockWord.FullWord ;
312 for (;;) {
313 if ((v & _LBIT) != 0) return 0 ;
314 const intptr_t u = CASPTR (&_LockWord, v, v|_LBIT) ;
315 if (v == u) return 1 ;
316 v = u ;
317 }
318 }
319
320 int Monitor::TryFast () {
321 // Optimistic fast-path form ...
322 // Fast-path attempt for the common uncontended case.
323 // Avoid RTS->RTO $ coherence upgrade on typical SMP systems.
324 intptr_t v = CASPTR (&_LockWord, 0, _LBIT) ; // agro ...
325 if (v == 0) return 1 ;
326
327 for (;;) {
328 if ((v & _LBIT) != 0) return 0 ;
329 const intptr_t u = CASPTR (&_LockWord, v, v|_LBIT) ;
330 if (v == u) return 1 ;
331 v = u ;
332 }
333 }
334
335 int Monitor::ILocked () {
336 const intptr_t w = _LockWord.FullWord & 0xFF ;
337 assert (w == 0 || w == _LBIT, "invariant") ;
338 return w == _LBIT ;
339 }
340
341 // Polite TATAS spinlock with exponential backoff - bounded spin.
342 // Ideally we'd use processor cycles, time or vtime to control
343 // the loop, but we currently use iterations.
344 // All the constants within were derived empirically but work over
345 // over the spectrum of J2SE reference platforms.
346 // On Niagara-class systems the back-off is unnecessary but
347 // is relatively harmless. (At worst it'll slightly retard
348 // acquisition times). The back-off is critical for older SMP systems
349 // where constant fetching of the LockWord would otherwise impair
350 // scalability.
351 //
352 // Clamp spinning at approximately 1/2 of a context-switch round-trip.
353 // See synchronizer.cpp for details and rationale.
354
355 int Monitor::TrySpin (Thread * const Self) {
356 if (TryLock()) return 1 ;
357 if (!os::is_MP()) return 0 ;
358
359 int Probes = 0 ;
360 int Delay = 0 ;
361 int Steps = 0 ;
362 int SpinMax = NativeMonitorSpinLimit ;
363 int flgs = NativeMonitorFlags ;
364 for (;;) {
365 intptr_t v = _LockWord.FullWord;
366 if ((v & _LBIT) == 0) {
367 if (CASPTR (&_LockWord, v, v|_LBIT) == v) {
368 return 1 ;
369 }
370 continue ;
371 }
372
373 if ((flgs & 8) == 0) {
374 SpinPause () ;
375 }
376
377 // Periodically increase Delay -- variable Delay form
378 // conceptually: delay *= 1 + 1/Exponent
379 ++ Probes;
380 if (Probes > SpinMax) return 0 ;
381
382 if ((Probes & 0x7) == 0) {
383 Delay = ((Delay << 1)|1) & 0x7FF ;
384 // CONSIDER: Delay += 1 + (Delay/4); Delay &= 0x7FF ;
385 }
386
387 if (flgs & 2) continue ;
388
389 // Consider checking _owner's schedctl state, if OFFPROC abort spin.
390 // If the owner is OFFPROC then it's unlike that the lock will be dropped
391 // in a timely fashion, which suggests that spinning would not be fruitful
392 // or profitable.
393
394 // Stall for "Delay" time units - iterations in the current implementation.
395 // Avoid generating coherency traffic while stalled.
396 // Possible ways to delay:
397 // PAUSE, SLEEP, MEMBAR #sync, MEMBAR #halt,
398 // wr %g0,%asi, gethrtime, rdstick, rdtick, rdtsc, etc. ...
399 // Note that on Niagara-class systems we want to minimize STs in the
400 // spin loop. N1 and brethren write-around the L1$ over the xbar into the L2$.
401 // Furthermore, they don't have a W$ like traditional SPARC processors.
402 // We currently use a Marsaglia Shift-Xor RNG loop.
403 Steps += Delay ;
404 if (Self != NULL) {
405 jint rv = Self->rng[0] ;
406 for (int k = Delay ; --k >= 0; ) {
407 rv = MarsagliaXORV (rv) ;
408 if ((flgs & 4) == 0 && SafepointSynchronize::do_call_back()) return 0 ;
409 }
410 Self->rng[0] = rv ;
411 } else {
412 Stall (Delay) ;
413 }
414 }
415 }
416
417 static int ParkCommon (ParkEvent * ev, jlong timo) {
418 // Diagnostic support - periodically unwedge blocked threads
419 intx nmt = NativeMonitorTimeout ;
420 if (nmt > 0 && (nmt < timo || timo <= 0)) {
421 timo = nmt ;
422 }
423 int err = OS_OK ;
424 if (0 == timo) {
425 ev->park() ;
426 } else {
427 err = ev->park(timo) ;
428 }
429 return err ;
430 }
431
432 inline int Monitor::AcquireOrPush (ParkEvent * ESelf) {
433 intptr_t v = _LockWord.FullWord ;
434 for (;;) {
435 if ((v & _LBIT) == 0) {
436 const intptr_t u = CASPTR (&_LockWord, v, v|_LBIT) ;
437 if (u == v) return 1 ; // indicate acquired
438 v = u ;
439 } else {
440 // Anticipate success ...
441 ESelf->ListNext = (ParkEvent *) (v & ~_LBIT) ;
442 const intptr_t u = CASPTR (&_LockWord, v, intptr_t(ESelf)|_LBIT) ;
443 if (u == v) return 0 ; // indicate pushed onto cxq
444 v = u ;
445 }
446 // Interference - LockWord change - just retry
447 }
448 }
449
450 // ILock and IWait are the lowest level primitive internal blocking
451 // synchronization functions. The callers of IWait and ILock must have
452 // performed any needed state transitions beforehand.
453 // IWait and ILock may directly call park() without any concern for thread state.
454 // Note that ILock and IWait do *not* access _owner.
455 // _owner is a higher-level logical concept.
456
457 void Monitor::ILock (Thread * Self) {
458 assert (_OnDeck != Self->_MutexEvent, "invariant") ;
459
460 if (TryFast()) {
461 Exeunt:
462 assert (ILocked(), "invariant") ;
463 return ;
464 }
465
466 ParkEvent * const ESelf = Self->_MutexEvent ;
467 assert (_OnDeck != ESelf, "invariant") ;
468
469 // As an optimization, spinners could conditionally try to set ONDECK to _LBIT
470 // Synchronizer.cpp uses a similar optimization.
471 if (TrySpin (Self)) goto Exeunt ;
472
473 // Slow-path - the lock is contended.
474 // Either Enqueue Self on cxq or acquire the outer lock.
475 // LockWord encoding = (cxq,LOCKBYTE)
476 ESelf->reset() ;
477 OrderAccess::fence() ;
478
479 // Optional optimization ... try barging on the inner lock
480 if ((NativeMonitorFlags & 32) && CASPTR (&_OnDeck, NULL, UNS(Self)) == 0) {
481 goto OnDeck_LOOP ;
482 }
483
484 if (AcquireOrPush (ESelf)) goto Exeunt ;
485
486 // At any given time there is at most one ondeck thread.
487 // ondeck implies not resident on cxq and not resident on EntryList
488 // Only the OnDeck thread can try to acquire -- contended for -- the lock.
489 // CONSIDER: use Self->OnDeck instead of m->OnDeck.
490 // Deschedule Self so that others may run.
491 while (_OnDeck != ESelf) {
492 ParkCommon (ESelf, 0) ;
493 }
494
495 // Self is now in the ONDECK position and will remain so until it
496 // manages to acquire the lock.
497 OnDeck_LOOP:
498 for (;;) {
499 assert (_OnDeck == ESelf, "invariant") ;
500 if (TrySpin (Self)) break ;
501 // CONSIDER: if ESelf->TryPark() && TryLock() break ...
502 // It's probably wise to spin only if we *actually* blocked
503 // CONSIDER: check the lockbyte, if it remains set then
504 // preemptively drain the cxq into the EntryList.
505 // The best place and time to perform queue operations -- lock metadata --
506 // is _before having acquired the outer lock, while waiting for the lock to drop.
507 ParkCommon (ESelf, 0) ;
508 }
509
510 assert (_OnDeck == ESelf, "invariant") ;
511 _OnDeck = NULL ;
512
513 // Note that we current drop the inner lock (clear OnDeck) in the slow-path
514 // epilog immediately after having acquired the outer lock.
515 // But instead we could consider the following optimizations:
516 // A. Shift or defer dropping the inner lock until the subsequent IUnlock() operation.
517 // This might avoid potential reacquisition of the inner lock in IUlock().
518 // B. While still holding the inner lock, attempt to opportunistically select
519 // and unlink the next ONDECK thread from the EntryList.
520 // If successful, set ONDECK to refer to that thread, otherwise clear ONDECK.
521 // It's critical that the select-and-unlink operation run in constant-time as
522 // it executes when holding the outer lock and may artificially increase the
523 // effective length of the critical section.
524 // Note that (A) and (B) are tantamount to succession by direct handoff for
525 // the inner lock.
526 goto Exeunt ;
527 }
528
529 void Monitor::IUnlock (bool RelaxAssert) {
530 assert (ILocked(), "invariant") ;
531 // Conceptually we need a MEMBAR #storestore|#loadstore barrier or fence immediately
532 // before the store that releases the lock. Crucially, all the stores and loads in the
533 // critical section must be globally visible before the store of 0 into the lock-word
534 // that releases the lock becomes globally visible. That is, memory accesses in the
535 // critical section should not be allowed to bypass or overtake the following ST that
536 // releases the lock. As such, to prevent accesses within the critical section
537 // from "leaking" out, we need a release fence between the critical section and the
538 // store that releases the lock. In practice that release barrier is elided on
539 // platforms with strong memory models such as TSO.
540 //
541 // Note that the OrderAccess::storeload() fence that appears after unlock store
542 // provides for progress conditions and succession and is _not related to exclusion
543 // safety or lock release consistency.
544 OrderAccess::release_store(&_LockWord.Bytes[_LSBINDEX], 0); // drop outer lock
545
546 OrderAccess::storeload ();
547 ParkEvent * const w = _OnDeck ;
548 assert (RelaxAssert || w != Thread::current()->_MutexEvent, "invariant") ;
549 if (w != NULL) {
550 // Either we have a valid ondeck thread or ondeck is transiently "locked"
551 // by some exiting thread as it arranges for succession. The LSBit of
552 // OnDeck allows us to discriminate two cases. If the latter, the
553 // responsibility for progress and succession lies with that other thread.
554 // For good performance, we also depend on the fact that redundant unpark()
555 // operations are cheap. That is, repeated Unpark()ing of the ONDECK thread
556 // is inexpensive. This approach provides implicit futile wakeup throttling.
557 // Note that the referent "w" might be stale with respect to the lock.
558 // In that case the following unpark() is harmless and the worst that'll happen
559 // is a spurious return from a park() operation. Critically, if "w" _is stale,
560 // then progress is known to have occurred as that means the thread associated
561 // with "w" acquired the lock. In that case this thread need take no further
562 // action to guarantee progress.
563 if ((UNS(w) & _LBIT) == 0) w->unpark() ;
564 return ;
565 }
566
567 intptr_t cxq = _LockWord.FullWord ;
568 if (((cxq & ~_LBIT)|UNS(_EntryList)) == 0) {
569 return ; // normal fast-path exit - cxq and EntryList both empty
570 }
571 if (cxq & _LBIT) {
572 // Optional optimization ...
573 // Some other thread acquired the lock in the window since this
574 // thread released it. Succession is now that thread's responsibility.
575 return ;
576 }
577
578 Succession:
579 // Slow-path exit - this thread must ensure succession and progress.
580 // OnDeck serves as lock to protect cxq and EntryList.
581 // Only the holder of OnDeck can manipulate EntryList or detach the RATs from cxq.
582 // Avoid ABA - allow multiple concurrent producers (enqueue via push-CAS)
583 // but only one concurrent consumer (detacher of RATs).
584 // Consider protecting this critical section with schedctl on Solaris.
585 // Unlike a normal lock, however, the exiting thread "locks" OnDeck,
586 // picks a successor and marks that thread as OnDeck. That successor
587 // thread will then clear OnDeck once it eventually acquires the outer lock.
588 if (CASPTR (&_OnDeck, NULL, _LBIT) != UNS(NULL)) {
589 return ;
590 }
591
592 ParkEvent * List = _EntryList ;
593 if (List != NULL) {
594 // Transfer the head of the EntryList to the OnDeck position.
595 // Once OnDeck, a thread stays OnDeck until it acquires the lock.
596 // For a given lock there is at most OnDeck thread at any one instant.
597 WakeOne:
598 assert (List == _EntryList, "invariant") ;
599 ParkEvent * const w = List ;
600 assert (RelaxAssert || w != Thread::current()->_MutexEvent, "invariant") ;
601 _EntryList = w->ListNext ;
602 // as a diagnostic measure consider setting w->_ListNext = BAD
603 assert (UNS(_OnDeck) == _LBIT, "invariant") ;
604 _OnDeck = w ; // pass OnDeck to w.
605 // w will clear OnDeck once it acquires the outer lock
606
607 // Another optional optimization ...
608 // For heavily contended locks it's not uncommon that some other
609 // thread acquired the lock while this thread was arranging succession.
610 // Try to defer the unpark() operation - Delegate the responsibility
611 // for unpark()ing the OnDeck thread to the current or subsequent owners
612 // That is, the new owner is responsible for unparking the OnDeck thread.
613 OrderAccess::storeload() ;
614 cxq = _LockWord.FullWord ;
615 if (cxq & _LBIT) return ;
616
617 w->unpark() ;
618 return ;
619 }
620
621 cxq = _LockWord.FullWord ;
622 if ((cxq & ~_LBIT) != 0) {
623 // The EntryList is empty but the cxq is populated.
624 // drain RATs from cxq into EntryList
625 // Detach RATs segment with CAS and then merge into EntryList
626 for (;;) {
627 // optional optimization - if locked, the owner is responsible for succession
628 if (cxq & _LBIT) goto Punt ;
629 const intptr_t vfy = CASPTR (&_LockWord, cxq, cxq & _LBIT) ;
630 if (vfy == cxq) break ;
631 cxq = vfy ;
632 // Interference - LockWord changed - Just retry
633 // We can see concurrent interference from contending threads
634 // pushing themselves onto the cxq or from lock-unlock operations.
635 // From the perspective of this thread, EntryList is stable and
636 // the cxq is prepend-only -- the head is volatile but the interior
637 // of the cxq is stable. In theory if we encounter interference from threads
638 // pushing onto cxq we could simply break off the original cxq suffix and
639 // move that segment to the EntryList, avoiding a 2nd or multiple CAS attempts
640 // on the high-traffic LockWord variable. For instance lets say the cxq is "ABCD"
641 // when we first fetch cxq above. Between the fetch -- where we observed "A"
642 // -- and CAS -- where we attempt to CAS null over A -- "PQR" arrive,
643 // yielding cxq = "PQRABCD". In this case we could simply set A.ListNext
644 // null, leaving cxq = "PQRA" and transfer the "BCD" segment to the EntryList.
645 // Note too, that it's safe for this thread to traverse the cxq
646 // without taking any special concurrency precautions.
647 }
648
649 // We don't currently reorder the cxq segment as we move it onto
650 // the EntryList, but it might make sense to reverse the order
651 // or perhaps sort by thread priority. See the comments in
652 // synchronizer.cpp objectMonitor::exit().
653 assert (_EntryList == NULL, "invariant") ;
654 _EntryList = List = (ParkEvent *)(cxq & ~_LBIT) ;
655 assert (List != NULL, "invariant") ;
656 goto WakeOne ;
657 }
658
659 // cxq|EntryList is empty.
660 // w == NULL implies that cxq|EntryList == NULL in the past.
661 // Possible race - rare inopportune interleaving.
662 // A thread could have added itself to cxq since this thread previously checked.
663 // Detect and recover by refetching cxq.
664 Punt:
665 assert (UNS(_OnDeck) == _LBIT, "invariant") ;
666 _OnDeck = NULL ; // Release inner lock.
667 OrderAccess::storeload(); // Dekker duality - pivot point
668
669 // Resample LockWord/cxq to recover from possible race.
670 // For instance, while this thread T1 held OnDeck, some other thread T2 might
671 // acquire the outer lock. Another thread T3 might try to acquire the outer
672 // lock, but encounter contention and enqueue itself on cxq. T2 then drops the
673 // outer lock, but skips succession as this thread T1 still holds OnDeck.
674 // T1 is and remains responsible for ensuring succession of T3.
675 //
676 // Note that we don't need to recheck EntryList, just cxq.
677 // If threads moved onto EntryList since we dropped OnDeck
678 // that implies some other thread forced succession.
679 cxq = _LockWord.FullWord ;
680 if ((cxq & ~_LBIT) != 0 && (cxq & _LBIT) == 0) {
681 goto Succession ; // potential race -- re-run succession
682 }
683 return ;
684 }
685
686 bool Monitor::notify() {
687 assert (_owner == Thread::current(), "invariant") ;
688 assert (ILocked(), "invariant") ;
689 if (_WaitSet == NULL) return true ;
690 NotifyCount ++ ;
691
692 // Transfer one thread from the WaitSet to the EntryList or cxq.
693 // Currently we just unlink the head of the WaitSet and prepend to the cxq.
694 // And of course we could just unlink it and unpark it, too, but
695 // in that case it'd likely impale itself on the reentry.
696 Thread::muxAcquire (_WaitLock, "notify:WaitLock") ;
697 ParkEvent * nfy = _WaitSet ;
698 if (nfy != NULL) { // DCL idiom
699 _WaitSet = nfy->ListNext ;
700 assert (nfy->Notified == 0, "invariant") ;
701 // push nfy onto the cxq
702 for (;;) {
703 const intptr_t v = _LockWord.FullWord ;
704 assert ((v & 0xFF) == _LBIT, "invariant") ;
705 nfy->ListNext = (ParkEvent *)(v & ~_LBIT);
706 if (CASPTR (&_LockWord, v, UNS(nfy)|_LBIT) == v) break;
707 // interference - _LockWord changed -- just retry
708 }
709 // Note that setting Notified before pushing nfy onto the cxq is
710 // also legal and safe, but the safety properties are much more
711 // subtle, so for the sake of code stewardship ...
712 OrderAccess::fence() ;
713 nfy->Notified = 1;
714 }
715 Thread::muxRelease (_WaitLock) ;
716 if (nfy != NULL && (NativeMonitorFlags & 16)) {
717 // Experimental code ... light up the wakee in the hope that this thread (the owner)
718 // will drop the lock just about the time the wakee comes ONPROC.
719 nfy->unpark() ;
720 }
721 assert (ILocked(), "invariant") ;
722 return true ;
723 }
724
725 // Currently notifyAll() transfers the waiters one-at-a-time from the waitset
726 // to the cxq. This could be done more efficiently with a single bulk en-mass transfer,
727 // but in practice notifyAll() for large #s of threads is rare and not time-critical.
728 // Beware too, that we invert the order of the waiters. Lets say that the
729 // waitset is "ABCD" and the cxq is "XYZ". After a notifyAll() the waitset
730 // will be empty and the cxq will be "DCBAXYZ". This is benign, of course.
731
732 bool Monitor::notify_all() {
733 assert (_owner == Thread::current(), "invariant") ;
734 assert (ILocked(), "invariant") ;
735 while (_WaitSet != NULL) notify() ;
736 return true ;
737 }
738
739 int Monitor::IWait (Thread * Self, jlong timo) {
740 assert (ILocked(), "invariant") ;
741
742 // Phases:
743 // 1. Enqueue Self on WaitSet - currently prepend
744 // 2. unlock - drop the outer lock
745 // 3. wait for either notification or timeout
746 // 4. lock - reentry - reacquire the outer lock
747
748 ParkEvent * const ESelf = Self->_MutexEvent ;
749 ESelf->Notified = 0 ;
750 ESelf->reset() ;
751 OrderAccess::fence() ;
752
753 // Add Self to WaitSet
754 // Ideally only the holder of the outer lock would manipulate the WaitSet -
755 // That is, the outer lock would implicitly protect the WaitSet.
756 // But if a thread in wait() encounters a timeout it will need to dequeue itself
757 // from the WaitSet _before it becomes the owner of the lock. We need to dequeue
758 // as the ParkEvent -- which serves as a proxy for the thread -- can't reside
759 // on both the WaitSet and the EntryList|cxq at the same time.. That is, a thread
760 // on the WaitSet can't be allowed to compete for the lock until it has managed to
761 // unlink its ParkEvent from WaitSet. Thus the need for WaitLock.
762 // Contention on the WaitLock is minimal.
763 //
764 // Another viable approach would be add another ParkEvent, "WaitEvent" to the
765 // thread class. The WaitSet would be composed of WaitEvents. Only the
766 // owner of the outer lock would manipulate the WaitSet. A thread in wait()
767 // could then compete for the outer lock, and then, if necessary, unlink itself
768 // from the WaitSet only after having acquired the outer lock. More precisely,
769 // there would be no WaitLock. A thread in in wait() would enqueue its WaitEvent
770 // on the WaitSet; release the outer lock; wait for either notification or timeout;
771 // reacquire the inner lock; and then, if needed, unlink itself from the WaitSet.
772 //
773 // Alternatively, a 2nd set of list link fields in the ParkEvent might suffice.
774 // One set would be for the WaitSet and one for the EntryList.
775 // We could also deconstruct the ParkEvent into a "pure" event and add a
776 // new immortal/TSM "ListElement" class that referred to ParkEvents.
777 // In that case we could have one ListElement on the WaitSet and another
778 // on the EntryList, with both referring to the same pure Event.
779
780 Thread::muxAcquire (_WaitLock, "wait:WaitLock:Add") ;
781 ESelf->ListNext = _WaitSet ;
782 _WaitSet = ESelf ;
783 Thread::muxRelease (_WaitLock) ;
784
785 // Release the outer lock
786 // We call IUnlock (RelaxAssert=true) as a thread T1 might
787 // enqueue itself on the WaitSet, call IUnlock(), drop the lock,
788 // and then stall before it can attempt to wake a successor.
789 // Some other thread T2 acquires the lock, and calls notify(), moving
790 // T1 from the WaitSet to the cxq. T2 then drops the lock. T1 resumes,
791 // and then finds *itself* on the cxq. During the course of a normal
792 // IUnlock() call a thread should _never find itself on the EntryList
793 // or cxq, but in the case of wait() it's possible.
794 // See synchronizer.cpp objectMonitor::wait().
795 IUnlock (true) ;
796
797 // Wait for either notification or timeout
798 // Beware that in some circumstances we might propagate
799 // spurious wakeups back to the caller.
800
801 for (;;) {
802 if (ESelf->Notified) break ;
803 int err = ParkCommon (ESelf, timo) ;
804 if (err == OS_TIMEOUT || (NativeMonitorFlags & 1)) break ;
805 }
806
807 // Prepare for reentry - if necessary, remove ESelf from WaitSet
808 // ESelf can be:
809 // 1. Still on the WaitSet. This can happen if we exited the loop by timeout.
810 // 2. On the cxq or EntryList
811 // 3. Not resident on cxq, EntryList or WaitSet, but in the OnDeck position.
812
813 OrderAccess::fence() ;
814 int WasOnWaitSet = 0 ;
815 if (ESelf->Notified == 0) {
816 Thread::muxAcquire (_WaitLock, "wait:WaitLock:remove") ;
817 if (ESelf->Notified == 0) { // DCL idiom
818 assert (_OnDeck != ESelf, "invariant") ; // can't be both OnDeck and on WaitSet
819 // ESelf is resident on the WaitSet -- unlink it.
820 // A doubly-linked list would be better here so we can unlink in constant-time.
821 // We have to unlink before we potentially recontend as ESelf might otherwise
822 // end up on the cxq|EntryList -- it can't be on two lists at once.
823 ParkEvent * p = _WaitSet ;
824 ParkEvent * q = NULL ; // classic q chases p
825 while (p != NULL && p != ESelf) {
826 q = p ;
827 p = p->ListNext ;
828 }
829 assert (p == ESelf, "invariant") ;
830 if (p == _WaitSet) { // found at head
831 assert (q == NULL, "invariant") ;
832 _WaitSet = p->ListNext ;
833 } else { // found in interior
834 assert (q->ListNext == p, "invariant") ;
835 q->ListNext = p->ListNext ;
836 }
837 WasOnWaitSet = 1 ; // We were *not* notified but instead encountered timeout
838 }
839 Thread::muxRelease (_WaitLock) ;
840 }
841
842 // Reentry phase - reacquire the lock
843 if (WasOnWaitSet) {
844 // ESelf was previously on the WaitSet but we just unlinked it above
845 // because of a timeout. ESelf is not resident on any list and is not OnDeck
846 assert (_OnDeck != ESelf, "invariant") ;
847 ILock (Self) ;
848 } else {
849 // A prior notify() operation moved ESelf from the WaitSet to the cxq.
850 // ESelf is now on the cxq, EntryList or at the OnDeck position.
851 // The following fragment is extracted from Monitor::ILock()
852 for (;;) {
853 if (_OnDeck == ESelf && TrySpin(Self)) break ;
854 ParkCommon (ESelf, 0) ;
855 }
856 assert (_OnDeck == ESelf, "invariant") ;
857 _OnDeck = NULL ;
858 }
859
860 assert (ILocked(), "invariant") ;
861 return WasOnWaitSet != 0 ; // return true IFF timeout
862 }
863
864
865 // ON THE VMTHREAD SNEAKING PAST HELD LOCKS:
866 // In particular, there are certain types of global lock that may be held
867 // by a Java thread while it is blocked at a safepoint but before it has
868 // written the _owner field. These locks may be sneakily acquired by the
869 // VM thread during a safepoint to avoid deadlocks. Alternatively, one should
870 // identify all such locks, and ensure that Java threads never block at
871 // safepoints while holding them (_no_safepoint_check_flag). While it
872 // seems as though this could increase the time to reach a safepoint
873 // (or at least increase the mean, if not the variance), the latter
874 // approach might make for a cleaner, more maintainable JVM design.
875 //
876 // Sneaking is vile and reprehensible and should be excised at the 1st
877 // opportunity. It's possible that the need for sneaking could be obviated
878 // as follows. Currently, a thread might (a) while TBIVM, call pthread_mutex_lock
879 // or ILock() thus acquiring the "physical" lock underlying Monitor/Mutex.
880 // (b) stall at the TBIVM exit point as a safepoint is in effect. Critically,
881 // it'll stall at the TBIVM reentry state transition after having acquired the
882 // underlying lock, but before having set _owner and having entered the actual
883 // critical section. The lock-sneaking facility leverages that fact and allowed the
884 // VM thread to logically acquire locks that had already be physically locked by mutators
885 // but where mutators were known blocked by the reentry thread state transition.
886 //
887 // If we were to modify the Monitor-Mutex so that TBIVM state transitions tightly
888 // wrapped calls to park(), then we could likely do away with sneaking. We'd
889 // decouple lock acquisition and parking. The critical invariant to eliminating
890 // sneaking is to ensure that we never "physically" acquire the lock while TBIVM.
891 // An easy way to accomplish this is to wrap the park calls in a narrow TBIVM jacket.
892 // One difficulty with this approach is that the TBIVM wrapper could recurse and
893 // call lock() deep from within a lock() call, while the MutexEvent was already enqueued.
894 // Using a stack (N=2 at minimum) of ParkEvents would take care of that problem.
895 //
896 // But of course the proper ultimate approach is to avoid schemes that require explicit
897 // sneaking or dependence on any any clever invariants or subtle implementation properties
898 // of Mutex-Monitor and instead directly address the underlying design flaw.
899
900 void Monitor::lock (Thread * Self) {
901 #ifdef CHECK_UNHANDLED_OOPS
902 // Clear unhandled oops so we get a crash right away. Only clear for non-vm
903 // or GC threads.
904 if (Self->is_Java_thread()) {
905 Self->clear_unhandled_oops();
906 }
907 #endif // CHECK_UNHANDLED_OOPS
908
909 debug_only(check_prelock_state(Self));
910 assert (_owner != Self , "invariant") ;
911 assert (_OnDeck != Self->_MutexEvent, "invariant") ;
912
913 if (TryFast()) {
914 Exeunt:
915 assert (ILocked(), "invariant") ;
916 assert (owner() == NULL, "invariant");
917 set_owner (Self);
918 return ;
919 }
920
921 // The lock is contended ...
922
923 bool can_sneak = Self->is_VM_thread() && SafepointSynchronize::is_at_safepoint();
924 if (can_sneak && _owner == NULL) {
925 // a java thread has locked the lock but has not entered the
926 // critical region -- let's just pretend we've locked the lock
927 // and go on. we note this with _snuck so we can also
928 // pretend to unlock when the time comes.
929 _snuck = true;
930 goto Exeunt ;
931 }
932
933 // Try a brief spin to avoid passing thru thread state transition ...
934 if (TrySpin (Self)) goto Exeunt ;
935
936 check_block_state(Self);
937 if (Self->is_Java_thread()) {
938 // Horribile dictu - we suffer through a state transition
939 assert(rank() > Mutex::special, "Potential deadlock with special or lesser rank mutex");
940 ThreadBlockInVM tbivm ((JavaThread *) Self) ;
941 ILock (Self) ;
942 } else {
943 // Mirabile dictu
944 ILock (Self) ;
945 }
946 goto Exeunt ;
947 }
948
949 void Monitor::lock() {
950 this->lock(Thread::current());
951 }
952
953 // Lock without safepoint check - a degenerate variant of lock().
954 // Should ONLY be used by safepoint code and other code
955 // that is guaranteed not to block while running inside the VM. If this is called with
956 // thread state set to be in VM, the safepoint synchronization code will deadlock!
957
958 void Monitor::lock_without_safepoint_check (Thread * Self) {
959 assert (_owner != Self, "invariant") ;
960 ILock (Self) ;
961 assert (_owner == NULL, "invariant");
962 set_owner (Self);
963 }
964
965 void Monitor::lock_without_safepoint_check () {
966 lock_without_safepoint_check (Thread::current()) ;
967 }
968
969
970 // Returns true if thread succeceed [sic] in grabbing the lock, otherwise false.
971
972 bool Monitor::try_lock() {
973 Thread * const Self = Thread::current();
974 debug_only(check_prelock_state(Self));
975 // assert(!thread->is_inside_signal_handler(), "don't lock inside signal handler");
976
977 // Special case, where all Java threads are stopped.
978 // The lock may have been acquired but _owner is not yet set.
979 // In that case the VM thread can safely grab the lock.
980 // It strikes me this should appear _after the TryLock() fails, below.
981 bool can_sneak = Self->is_VM_thread() && SafepointSynchronize::is_at_safepoint();
982 if (can_sneak && _owner == NULL) {
983 set_owner(Self); // Do not need to be atomic, since we are at a safepoint
984 _snuck = true;
985 return true;
986 }
987
988 if (TryLock()) {
989 // We got the lock
990 assert (_owner == NULL, "invariant");
991 set_owner (Self);
992 return true;
993 }
994 return false;
995 }
996
997 void Monitor::unlock() {
998 assert (_owner == Thread::current(), "invariant") ;
999 assert (_OnDeck != Thread::current()->_MutexEvent , "invariant") ;
1000 set_owner (NULL) ;
1001 if (_snuck) {
1002 assert(SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread(), "sneak");
1003 _snuck = false;
1004 return ;
1005 }
1006 IUnlock (false) ;
1007 }
1008
1009 // Yet another degenerate version of Monitor::lock() or lock_without_safepoint_check()
1010 // jvm_raw_lock() and _unlock() can be called by non-Java threads via JVM_RawMonitorEnter.
1011 //
1012 // There's no expectation that JVM_RawMonitors will interoperate properly with the native
1013 // Mutex-Monitor constructs. We happen to implement JVM_RawMonitors in terms of
1014 // native Mutex-Monitors simply as a matter of convenience. A simple abstraction layer
1015 // over a pthread_mutex_t would work equally as well, but require more platform-specific
1016 // code -- a "PlatformMutex". Alternatively, a simply layer over muxAcquire-muxRelease
1017 // would work too.
1018 //
1019 // Since the caller might be a foreign thread, we don't necessarily have a Thread.MutexEvent
1020 // instance available. Instead, we transiently allocate a ParkEvent on-demand if
1021 // we encounter contention. That ParkEvent remains associated with the thread
1022 // until it manages to acquire the lock, at which time we return the ParkEvent
1023 // to the global ParkEvent free list. This is correct and suffices for our purposes.
1024 //
1025 // Beware that the original jvm_raw_unlock() had a "_snuck" test but that
1026 // jvm_raw_lock() didn't have the corresponding test. I suspect that's an
1027 // oversight, but I've replicated the original suspect logic in the new code ...
1028
1029 void Monitor::jvm_raw_lock() {
1030 assert(rank() == native, "invariant");
1031
1032 if (TryLock()) {
1033 Exeunt:
1034 assert (ILocked(), "invariant") ;
1035 assert (_owner == NULL, "invariant");
1036 // This can potentially be called by non-java Threads. Thus, the ThreadLocalStorage
1037 // might return NULL. Don't call set_owner since it will break on an NULL owner
1038 // Consider installing a non-null "ANON" distinguished value instead of just NULL.
1039 _owner = ThreadLocalStorage::thread();
1040 return ;
1041 }
1042
1043 if (TrySpin(NULL)) goto Exeunt ;
1044
1045 // slow-path - apparent contention
1046 // Allocate a ParkEvent for transient use.
1047 // The ParkEvent remains associated with this thread until
1048 // the time the thread manages to acquire the lock.
1049 ParkEvent * const ESelf = ParkEvent::Allocate(NULL) ;
1050 ESelf->reset() ;
1051 OrderAccess::storeload() ;
1052
1053 // Either Enqueue Self on cxq or acquire the outer lock.
1054 if (AcquireOrPush (ESelf)) {
1055 ParkEvent::Release (ESelf) ; // surrender the ParkEvent
1056 goto Exeunt ;
1057 }
1058
1059 // At any given time there is at most one ondeck thread.
1060 // ondeck implies not resident on cxq and not resident on EntryList
1061 // Only the OnDeck thread can try to acquire -- contended for -- the lock.
1062 // CONSIDER: use Self->OnDeck instead of m->OnDeck.
1063 for (;;) {
1064 if (_OnDeck == ESelf && TrySpin(NULL)) break ;
1065 ParkCommon (ESelf, 0) ;
1066 }
1067
1068 assert (_OnDeck == ESelf, "invariant") ;
1069 _OnDeck = NULL ;
1070 ParkEvent::Release (ESelf) ; // surrender the ParkEvent
1071 goto Exeunt ;
1072 }
1073
1074 void Monitor::jvm_raw_unlock() {
1075 // Nearly the same as Monitor::unlock() ...
1076 // directly set _owner instead of using set_owner(null)
1077 _owner = NULL ;
1078 if (_snuck) { // ???
1079 assert(SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread(), "sneak");
1080 _snuck = false;
1081 return ;
1082 }
1083 IUnlock(false) ;
1084 }
1085
1086 bool Monitor::wait(bool no_safepoint_check, long timeout, bool as_suspend_equivalent) {
1087 Thread * const Self = Thread::current() ;
1088 assert (_owner == Self, "invariant") ;
1089 assert (ILocked(), "invariant") ;
1090
1091 // as_suspend_equivalent logically implies !no_safepoint_check
1092 guarantee (!as_suspend_equivalent || !no_safepoint_check, "invariant") ;
1093 // !no_safepoint_check logically implies java_thread
1094 guarantee (no_safepoint_check || Self->is_Java_thread(), "invariant") ;
1095
1096 #ifdef ASSERT
1097 Monitor * least = get_least_ranked_lock_besides_this(Self->owned_locks());
1098 assert(least != this, "Specification of get_least_... call above");
1099 if (least != NULL && least->rank() <= special) {
1100 tty->print("Attempting to wait on monitor %s/%d while holding"
1101 " lock %s/%d -- possible deadlock",
1102 name(), rank(), least->name(), least->rank());
1103 assert(false, "Shouldn't block(wait) while holding a lock of rank special");
1104 }
1105 #endif // ASSERT
1106
1107 int wait_status ;
1108 // conceptually set the owner to NULL in anticipation of
1109 // abdicating the lock in wait
1110 set_owner(NULL);
1111 if (no_safepoint_check) {
1112 wait_status = IWait (Self, timeout) ;
1113 } else {
1114 assert (Self->is_Java_thread(), "invariant") ;
1115 JavaThread *jt = (JavaThread *)Self;
1116
1117 // Enter safepoint region - ornate and Rococo ...
1118 ThreadBlockInVM tbivm(jt);
1119 OSThreadWaitState osts(Self->osthread(), false /* not Object.wait() */);
1120
1121 if (as_suspend_equivalent) {
1122 jt->set_suspend_equivalent();
1123 // cleared by handle_special_suspend_equivalent_condition() or
1124 // java_suspend_self()
1125 }
1126
1127 wait_status = IWait (Self, timeout) ;
1128
1129 // were we externally suspended while we were waiting?
1130 if (as_suspend_equivalent && jt->handle_special_suspend_equivalent_condition()) {
1131 // Our event wait has finished and we own the lock, but
1132 // while we were waiting another thread suspended us. We don't
1133 // want to hold the lock while suspended because that
1134 // would surprise the thread that suspended us.
1135 assert (ILocked(), "invariant") ;
1136 IUnlock (true) ;
1137 jt->java_suspend_self();
1138 ILock (Self) ;
1139 assert (ILocked(), "invariant") ;
1140 }
1141 }
1142
1143 // Conceptually reestablish ownership of the lock.
1144 // The "real" lock -- the LockByte -- was reacquired by IWait().
1145 assert (ILocked(), "invariant") ;
1146 assert (_owner == NULL, "invariant") ;
1147 set_owner (Self) ;
1148 return wait_status != 0 ; // return true IFF timeout
1149 }
1150
1151 Monitor::~Monitor() {
1152 assert ((UNS(_owner)|UNS(_LockWord.FullWord)|UNS(_EntryList)|UNS(_WaitSet)|UNS(_OnDeck)) == 0, "") ;
1153 }
1154
1155 void Monitor::ClearMonitor (Monitor * m, const char *name) {
1156 m->_owner = NULL ;
1157 m->_snuck = false ;
1158 if (name == NULL) {
1159 strcpy(m->_name, "UNKNOWN") ;
1160 } else {
1161 strncpy(m->_name, name, MONITOR_NAME_LEN - 1);
1162 m->_name[MONITOR_NAME_LEN - 1] = '\0';
1163 }
1164 m->_LockWord.FullWord = 0 ;
1165 m->_EntryList = NULL ;
1166 m->_OnDeck = NULL ;
1167 m->_WaitSet = NULL ;
1168 m->_WaitLock[0] = 0 ;
1169 }
1170
1171 Monitor::Monitor() { ClearMonitor(this); }
1172
1173 Monitor::Monitor (int Rank, const char * name, bool allow_vm_block) {
1174 ClearMonitor (this, name) ;
1175 #ifdef ASSERT
1176 _allow_vm_block = allow_vm_block;
1177 _rank = Rank ;
1178 #endif
1179 }
1180
1181 Mutex::~Mutex() {
1182 assert ((UNS(_owner)|UNS(_LockWord.FullWord)|UNS(_EntryList)|UNS(_WaitSet)|UNS(_OnDeck)) == 0, "") ;
1183 }
1184
1185 Mutex::Mutex (int Rank, const char * name, bool allow_vm_block) {
1186 ClearMonitor ((Monitor *) this, name) ;
1187 #ifdef ASSERT
1188 _allow_vm_block = allow_vm_block;
1189 _rank = Rank ;
1190 #endif
1191 }
1192
1193 bool Monitor::owned_by_self() const {
1194 bool ret = _owner == Thread::current();
1195 assert (!ret || _LockWord.Bytes[_LSBINDEX] != 0, "invariant") ;
1196 return ret;
1197 }
1198
1199 void Monitor::print_on_error(outputStream* st) const {
1200 st->print("[" PTR_FORMAT, this);
1201 st->print("] %s", _name);
1202 st->print(" - owner thread: " PTR_FORMAT, _owner);
1203 }
1204
1205
1206
1207
1208 // ----------------------------------------------------------------------------------
1209 // Non-product code
1210
1211 #ifndef PRODUCT
1212 void Monitor::print_on(outputStream* st) const {
1213 st->print_cr("Mutex: [0x%lx/0x%lx] %s - owner: 0x%lx", this, _LockWord.FullWord, _name, _owner);
1214 }
1215 #endif
1216
1217 #ifndef PRODUCT
1218 #ifdef ASSERT
1219 Monitor * Monitor::get_least_ranked_lock(Monitor * locks) {
1220 Monitor *res, *tmp;
1221 for (res = tmp = locks; tmp != NULL; tmp = tmp->next()) {
1222 if (tmp->rank() < res->rank()) {
1223 res = tmp;
1224 }
1225 }
1226 if (!SafepointSynchronize::is_at_safepoint()) {
1227 // In this case, we expect the held locks to be
1228 // in increasing rank order (modulo any native ranks)
1229 for (tmp = locks; tmp != NULL; tmp = tmp->next()) {
1230 if (tmp->next() != NULL) {
1231 assert(tmp->rank() == Mutex::native ||
1232 tmp->rank() <= tmp->next()->rank(), "mutex rank anomaly?");
1233 }
1234 }
1235 }
1236 return res;
1237 }
1238
1239 Monitor* Monitor::get_least_ranked_lock_besides_this(Monitor* locks) {
1240 Monitor *res, *tmp;
1241 for (res = NULL, tmp = locks; tmp != NULL; tmp = tmp->next()) {
1242 if (tmp != this && (res == NULL || tmp->rank() < res->rank())) {
1243 res = tmp;
1244 }
1245 }
1246 if (!SafepointSynchronize::is_at_safepoint()) {
1247 // In this case, we expect the held locks to be
1248 // in increasing rank order (modulo any native ranks)
1249 for (tmp = locks; tmp != NULL; tmp = tmp->next()) {
1250 if (tmp->next() != NULL) {
1251 assert(tmp->rank() == Mutex::native ||
1252 tmp->rank() <= tmp->next()->rank(), "mutex rank anomaly?");
1253 }
1254 }
1255 }
1256 return res;
1257 }
1258
1259
1260 bool Monitor::contains(Monitor* locks, Monitor * lock) {
1261 for (; locks != NULL; locks = locks->next()) {
1262 if (locks == lock)
1263 return true;
1264 }
1265 return false;
1266 }
1267 #endif
1268
1269 // Called immediately after lock acquisition or release as a diagnostic
1270 // to track the lock-set of the thread and test for rank violations that
1271 // might indicate exposure to deadlock.
1272 // Rather like an EventListener for _owner (:>).
1273
1274 void Monitor::set_owner_implementation(Thread *new_owner) {
1275 // This function is solely responsible for maintaining
1276 // and checking the invariant that threads and locks
1277 // are in a 1/N relation, with some some locks unowned.
1278 // It uses the Mutex::_owner, Mutex::_next, and
1279 // Thread::_owned_locks fields, and no other function
1280 // changes those fields.
1281 // It is illegal to set the mutex from one non-NULL
1282 // owner to another--it must be owned by NULL as an
1283 // intermediate state.
1284
1285 if (new_owner != NULL) {
1286 // the thread is acquiring this lock
1287
1288 assert(new_owner == Thread::current(), "Should I be doing this?");
1289 assert(_owner == NULL, "setting the owner thread of an already owned mutex");
1290 _owner = new_owner; // set the owner
1291
1292 // link "this" into the owned locks list
1293
1294 #ifdef ASSERT // Thread::_owned_locks is under the same ifdef
1295 Monitor* locks = get_least_ranked_lock(new_owner->owned_locks());
1296 // Mutex::set_owner_implementation is a friend of Thread
1297
1298 assert(this->rank() >= 0, "bad lock rank");
1299
1300 // Deadlock avoidance rules require us to acquire Mutexes only in
1301 // a global total order. For example m1 is the lowest ranked mutex
1302 // that the thread holds and m2 is the mutex the thread is trying
1303 // to acquire, then deadlock avoidance rules require that the rank
1304 // of m2 be less than the rank of m1.
1305 // The rank Mutex::native is an exception in that it is not subject
1306 // to the verification rules.
1307 // Here are some further notes relating to mutex acquisition anomalies:
1308 // . under Solaris, the interrupt lock gets acquired when doing
1309 // profiling, so any lock could be held.
1310 // . it is also ok to acquire Safepoint_lock at the very end while we
1311 // already hold Terminator_lock - may happen because of periodic safepoints
1312 if (this->rank() != Mutex::native &&
1313 this->rank() != Mutex::suspend_resume &&
1314 locks != NULL && locks->rank() <= this->rank() &&
1315 !SafepointSynchronize::is_at_safepoint() &&
1316 this != Interrupt_lock && this != ProfileVM_lock &&
1317 !(this == Safepoint_lock && contains(locks, Terminator_lock) &&
1318 SafepointSynchronize::is_synchronizing())) {
1319 new_owner->print_owned_locks();
1320 fatal(err_msg("acquiring lock %s/%d out of order with lock %s/%d -- "
1321 "possible deadlock", this->name(), this->rank(),
1322 locks->name(), locks->rank()));
1323 }
1324
1325 this->_next = new_owner->_owned_locks;
1326 new_owner->_owned_locks = this;
1327 #endif
1328
1329 } else {
1330 // the thread is releasing this lock
1331
1332 Thread* old_owner = _owner;
1333 debug_only(_last_owner = old_owner);
1334
1335 assert(old_owner != NULL, "removing the owner thread of an unowned mutex");
1336 assert(old_owner == Thread::current(), "removing the owner thread of an unowned mutex");
1337
1338 _owner = NULL; // set the owner
1339
1340 #ifdef ASSERT
1341 Monitor *locks = old_owner->owned_locks();
1342
1343 // remove "this" from the owned locks list
1344
1345 Monitor *prev = NULL;
1346 bool found = false;
1347 for (; locks != NULL; prev = locks, locks = locks->next()) {
1348 if (locks == this) {
1349 found = true;
1350 break;
1351 }
1352 }
1353 assert(found, "Removing a lock not owned");
1354 if (prev == NULL) {
1355 old_owner->_owned_locks = _next;
1356 } else {
1357 prev->_next = _next;
1358 }
1359 _next = NULL;
1360 #endif
1361 }
1362 }
1363
1364
1365 // Factored out common sanity checks for locking mutex'es. Used by lock() and try_lock()
1366 void Monitor::check_prelock_state(Thread *thread) {
1367 assert((!thread->is_Java_thread() || ((JavaThread *)thread)->thread_state() == _thread_in_vm)
1368 || rank() == Mutex::special, "wrong thread state for using locks");
1369 if (StrictSafepointChecks) {
1370 if (thread->is_VM_thread() && !allow_vm_block()) {
1371 fatal(err_msg("VM thread using lock %s (not allowed to block on)",
1372 name()));
1373 }
1374 debug_only(if (rank() != Mutex::special) \
1375 thread->check_for_valid_safepoint_state(false);)
1376 }
1377 if (thread->is_Watcher_thread()) {
1378 assert(!WatcherThread::watcher_thread()->has_crash_protection(),
1379 "locking not allowed when crash protection is set");
1380 }
1381 }
1382
1383 void Monitor::check_block_state(Thread *thread) {
1384 if (!_allow_vm_block && thread->is_VM_thread()) {
1385 warning("VM thread blocked on lock");
1386 print();
1387 BREAKPOINT;
1388 }
1389 assert(_owner != thread, "deadlock: blocking on monitor owned by current thread");
1390 }
1391
1392 #endif // PRODUCT