Mercurial > hg > truffle
annotate src/share/vm/runtime/mutex.cpp @ 14680:2dfa56e10640
8027124: [TESTBUG] NonTieredLevelsTest: java.lang.RuntimeException: private TestCase$Helper(java.lang.Object) must be osr_compiled
Reviewed-by: kvn, roland
author | iignatyev |
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date | Thu, 06 Mar 2014 12:47:45 +0400 |
parents | c86519f8d826 |
children | 4ca6dc0799b6 |
rev | line source |
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0 | 1 |
2 /* | |
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3 * Copyright (c) 1998, 2013, Oracle and/or its affiliates. All rights reserved. |
0 | 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. | |
19 * | |
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20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA |
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21 * or visit www.oracle.com if you need additional information or have any |
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22 * questions. |
0 | 23 * |
24 */ | |
25 | |
1972 | 26 #include "precompiled.hpp" |
27 #include "runtime/mutex.hpp" | |
28 #include "runtime/osThread.hpp" | |
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29 #include "runtime/thread.inline.hpp" |
1972 | 30 #include "utilities/events.hpp" |
31 #ifdef TARGET_OS_FAMILY_linux | |
32 # include "mutex_linux.inline.hpp" | |
33 #endif | |
34 #ifdef TARGET_OS_FAMILY_solaris | |
35 # include "mutex_solaris.inline.hpp" | |
36 #endif | |
37 #ifdef TARGET_OS_FAMILY_windows | |
38 # include "mutex_windows.inline.hpp" | |
39 #endif | |
3960 | 40 #ifdef TARGET_OS_FAMILY_bsd |
41 # include "mutex_bsd.inline.hpp" | |
42 #endif | |
0 | 43 |
44 // 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 | |
45 // | |
46 // Native Monitor-Mutex locking - theory of operations | |
47 // | |
48 // * Native Monitors are completely unrelated to Java-level monitors, | |
49 // although the "back-end" slow-path implementations share a common lineage. | |
50 // See objectMonitor:: in synchronizer.cpp. | |
51 // Native Monitors do *not* support nesting or recursion but otherwise | |
52 // they're basically Hoare-flavor monitors. | |
53 // | |
54 // * A thread acquires ownership of a Monitor/Mutex by CASing the LockByte | |
55 // in the _LockWord from zero to non-zero. Note that the _Owner field | |
56 // is advisory and is used only to verify that the thread calling unlock() | |
57 // is indeed the last thread to have acquired the lock. | |
58 // | |
59 // * Contending threads "push" themselves onto the front of the contention | |
60 // queue -- called the cxq -- with CAS and then spin/park. | |
61 // The _LockWord contains the LockByte as well as the pointer to the head | |
62 // of the cxq. Colocating the LockByte with the cxq precludes certain races. | |
63 // | |
64 // * Using a separately addressable LockByte allows for CAS:MEMBAR or CAS:0 | |
65 // idioms. We currently use MEMBAR in the uncontended unlock() path, as | |
66 // MEMBAR often has less latency than CAS. If warranted, we could switch to | |
67 // a CAS:0 mode, using timers to close the resultant race, as is done | |
68 // with Java Monitors in synchronizer.cpp. | |
69 // | |
70 // See the following for a discussion of the relative cost of atomics (CAS) | |
71 // MEMBAR, and ways to eliminate such instructions from the common-case paths: | |
72 // -- http://blogs.sun.com/dave/entry/biased_locking_in_hotspot | |
73 // -- http://blogs.sun.com/dave/resource/MustangSync.pdf | |
74 // -- http://blogs.sun.com/dave/resource/synchronization-public2.pdf | |
75 // -- synchronizer.cpp | |
76 // | |
77 // * Overall goals - desiderata | |
78 // 1. Minimize context switching | |
79 // 2. Minimize lock migration | |
80 // 3. Minimize CPI -- affinity and locality | |
81 // 4. Minimize the execution of high-latency instructions such as CAS or MEMBAR | |
82 // 5. Minimize outer lock hold times | |
83 // 6. Behave gracefully on a loaded system | |
84 // | |
85 // * Thread flow and list residency: | |
86 // | |
87 // Contention queue --> EntryList --> OnDeck --> Owner --> !Owner | |
88 // [..resident on monitor list..] | |
89 // [...........contending..................] | |
90 // | |
91 // -- The contention queue (cxq) contains recently-arrived threads (RATs). | |
92 // Threads on the cxq eventually drain into the EntryList. | |
93 // -- Invariant: a thread appears on at most one list -- cxq, EntryList | |
94 // or WaitSet -- at any one time. | |
95 // -- For a given monitor there can be at most one "OnDeck" thread at any | |
96 // given time but if needbe this particular invariant could be relaxed. | |
97 // | |
98 // * The WaitSet and EntryList linked lists are composed of ParkEvents. | |
99 // I use ParkEvent instead of threads as ParkEvents are immortal and | |
100 // type-stable, meaning we can safely unpark() a possibly stale | |
101 // list element in the unlock()-path. (That's benign). | |
102 // | |
103 // * Succession policy - providing for progress: | |
104 // | |
105 // As necessary, the unlock()ing thread identifies, unlinks, and unparks | |
106 // an "heir presumptive" tentative successor thread from the EntryList. | |
107 // This becomes the so-called "OnDeck" thread, of which there can be only | |
108 // one at any given time for a given monitor. The wakee will recontend | |
109 // for ownership of monitor. | |
110 // | |
111 // Succession is provided for by a policy of competitive handoff. | |
112 // The exiting thread does _not_ grant or pass ownership to the | |
113 // successor thread. (This is also referred to as "handoff" succession"). | |
114 // Instead the exiting thread releases ownership and possibly wakes | |
115 // a successor, so the successor can (re)compete for ownership of the lock. | |
116 // | |
117 // Competitive handoff provides excellent overall throughput at the expense | |
118 // of short-term fairness. If fairness is a concern then one remedy might | |
119 // be to add an AcquireCounter field to the monitor. After a thread acquires | |
120 // the lock it will decrement the AcquireCounter field. When the count | |
121 // reaches 0 the thread would reset the AcquireCounter variable, abdicate | |
122 // the lock directly to some thread on the EntryList, and then move itself to the | |
123 // tail of the EntryList. | |
124 // | |
125 // But in practice most threads engage or otherwise participate in resource | |
126 // bounded producer-consumer relationships, so lock domination is not usually | |
127 // a practical concern. Recall too, that in general it's easier to construct | |
128 // a fair lock from a fast lock, but not vice-versa. | |
129 // | |
130 // * The cxq can have multiple concurrent "pushers" but only one concurrent | |
131 // detaching thread. This mechanism is immune from the ABA corruption. | |
132 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious. | |
133 // We use OnDeck as a pseudo-lock to enforce the at-most-one detaching | |
134 // thread constraint. | |
135 // | |
136 // * Taken together, the cxq and the EntryList constitute or form a | |
137 // single logical queue of threads stalled trying to acquire the lock. | |
138 // We use two distinct lists to reduce heat on the list ends. | |
139 // Threads in lock() enqueue onto cxq while threads in unlock() will | |
140 // dequeue from the EntryList. (c.f. Michael Scott's "2Q" algorithm). | |
141 // A key desideratum is to minimize queue & monitor metadata manipulation | |
142 // that occurs while holding the "outer" monitor lock -- that is, we want to | |
143 // minimize monitor lock holds times. | |
144 // | |
145 // The EntryList is ordered by the prevailing queue discipline and | |
146 // can be organized in any convenient fashion, such as a doubly-linked list or | |
147 // a circular doubly-linked list. If we need a priority queue then something akin | |
148 // to Solaris' sleepq would work nicely. Viz., | |
149 // -- http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c. | |
150 // -- http://cvs.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/uts/common/os/sleepq.c | |
151 // Queue discipline is enforced at ::unlock() time, when the unlocking thread | |
152 // drains the cxq into the EntryList, and orders or reorders the threads on the | |
153 // EntryList accordingly. | |
154 // | |
155 // Barring "lock barging", this mechanism provides fair cyclic ordering, | |
156 // somewhat similar to an elevator-scan. | |
157 // | |
158 // * OnDeck | |
159 // -- For a given monitor there can be at most one OnDeck thread at any given | |
160 // instant. The OnDeck thread is contending for the lock, but has been | |
161 // unlinked from the EntryList and cxq by some previous unlock() operations. | |
162 // Once a thread has been designated the OnDeck thread it will remain so | |
163 // until it manages to acquire the lock -- being OnDeck is a stable property. | |
164 // -- Threads on the EntryList or cxq are _not allowed to attempt lock acquisition. | |
165 // -- OnDeck also serves as an "inner lock" as follows. Threads in unlock() will, after | |
166 // having cleared the LockByte and dropped the outer lock, attempt to "trylock" | |
167 // OnDeck by CASing the field from null to non-null. If successful, that thread | |
168 // is then responsible for progress and succession and can use CAS to detach and | |
169 // drain the cxq into the EntryList. By convention, only this thread, the holder of | |
170 // the OnDeck inner lock, can manipulate the EntryList or detach and drain the | |
171 // RATs on the cxq into the EntryList. This avoids ABA corruption on the cxq as | |
172 // we allow multiple concurrent "push" operations but restrict detach concurrency | |
173 // to at most one thread. Having selected and detached a successor, the thread then | |
174 // changes the OnDeck to refer to that successor, and then unparks the successor. | |
175 // That successor will eventually acquire the lock and clear OnDeck. Beware | |
176 // that the OnDeck usage as a lock is asymmetric. A thread in unlock() transiently | |
177 // "acquires" OnDeck, performs queue manipulations, passes OnDeck to some successor, | |
178 // and then the successor eventually "drops" OnDeck. Note that there's never | |
179 // any sense of contention on the inner lock, however. Threads never contend | |
180 // or wait for the inner lock. | |
181 // -- OnDeck provides for futile wakeup throttling a described in section 3.3 of | |
182 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf | |
183 // In a sense, OnDeck subsumes the ObjectMonitor _Succ and ObjectWaiter | |
184 // TState fields found in Java-level objectMonitors. (See synchronizer.cpp). | |
185 // | |
186 // * Waiting threads reside on the WaitSet list -- wait() puts | |
187 // the caller onto the WaitSet. Notify() or notifyAll() simply | |
188 // transfers threads from the WaitSet to either the EntryList or cxq. | |
189 // Subsequent unlock() operations will eventually unpark the notifyee. | |
190 // Unparking a notifee in notify() proper is inefficient - if we were to do so | |
191 // it's likely the notifyee would simply impale itself on the lock held | |
192 // by the notifier. | |
193 // | |
194 // * The mechanism is obstruction-free in that if the holder of the transient | |
195 // OnDeck lock in unlock() is preempted or otherwise stalls, other threads | |
196 // can still acquire and release the outer lock and continue to make progress. | |
197 // At worst, waking of already blocked contending threads may be delayed, | |
198 // but nothing worse. (We only use "trylock" operations on the inner OnDeck | |
199 // lock). | |
200 // | |
201 // * Note that thread-local storage must be initialized before a thread | |
202 // uses Native monitors or mutexes. The native monitor-mutex subsystem | |
203 // depends on Thread::current(). | |
204 // | |
205 // * The monitor synchronization subsystem avoids the use of native | |
206 // synchronization primitives except for the narrow platform-specific | |
207 // park-unpark abstraction. See the comments in os_solaris.cpp regarding | |
208 // the semantics of park-unpark. Put another way, this monitor implementation | |
209 // depends only on atomic operations and park-unpark. The monitor subsystem | |
210 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the | |
211 // underlying OS manages the READY<->RUN transitions. | |
212 // | |
213 // * The memory consistency model provide by lock()-unlock() is at least as | |
214 // strong or stronger than the Java Memory model defined by JSR-133. | |
215 // That is, we guarantee at least entry consistency, if not stronger. | |
216 // See http://g.oswego.edu/dl/jmm/cookbook.html. | |
217 // | |
218 // * Thread:: currently contains a set of purpose-specific ParkEvents: | |
219 // _MutexEvent, _ParkEvent, etc. A better approach might be to do away with | |
220 // the purpose-specific ParkEvents and instead implement a general per-thread | |
221 // stack of available ParkEvents which we could provision on-demand. The | |
222 // stack acts as a local cache to avoid excessive calls to ParkEvent::Allocate() | |
223 // and ::Release(). A thread would simply pop an element from the local stack before it | |
224 // enqueued or park()ed. When the contention was over the thread would | |
225 // push the no-longer-needed ParkEvent back onto its stack. | |
226 // | |
227 // * A slightly reduced form of ILock() and IUnlock() have been partially | |
228 // model-checked (Murphi) for safety and progress at T=1,2,3 and 4. | |
229 // It'd be interesting to see if TLA/TLC could be useful as well. | |
230 // | |
231 // * Mutex-Monitor is a low-level "leaf" subsystem. That is, the monitor | |
232 // code should never call other code in the JVM that might itself need to | |
233 // acquire monitors or mutexes. That's true *except* in the case of the | |
234 // ThreadBlockInVM state transition wrappers. The ThreadBlockInVM DTOR handles | |
235 // mutator reentry (ingress) by checking for a pending safepoint in which case it will | |
236 // call SafepointSynchronize::block(), which in turn may call Safepoint_lock->lock(), etc. | |
237 // In that particular case a call to lock() for a given Monitor can end up recursively | |
238 // calling lock() on another monitor. While distasteful, this is largely benign | |
239 // as the calls come from jacket that wraps lock(), and not from deep within lock() itself. | |
240 // | |
241 // It's unfortunate that native mutexes and thread state transitions were convolved. | |
242 // They're really separate concerns and should have remained that way. Melding | |
243 // them together was facile -- a bit too facile. The current implementation badly | |
244 // conflates the two concerns. | |
245 // | |
246 // * TODO-FIXME: | |
247 // | |
248 // -- Add DTRACE probes for contended acquire, contended acquired, contended unlock | |
249 // We should also add DTRACE probes in the ParkEvent subsystem for | |
250 // Park-entry, Park-exit, and Unpark. | |
251 // | |
252 // -- We have an excess of mutex-like constructs in the JVM, namely: | |
253 // 1. objectMonitors for Java-level synchronization (synchronizer.cpp) | |
254 // 2. low-level muxAcquire and muxRelease | |
255 // 3. low-level spinAcquire and spinRelease | |
256 // 4. native Mutex:: and Monitor:: | |
257 // 5. jvm_raw_lock() and _unlock() | |
258 // 6. JVMTI raw monitors -- distinct from (5) despite having a confusingly | |
259 // similar name. | |
260 // | |
261 // 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 | |
262 | |
263 | |
264 // CASPTR() uses the canonical argument order that dominates in the literature. | |
265 // Our internal cmpxchg_ptr() uses a bastardized ordering to accommodate Sun .il templates. | |
266 | |
267 #define CASPTR(a,c,s) intptr_t(Atomic::cmpxchg_ptr ((void *)(s),(void *)(a),(void *)(c))) | |
268 #define UNS(x) (uintptr_t(x)) | |
269 #define TRACE(m) { static volatile int ctr = 0 ; int x = ++ctr ; if ((x & (x-1))==0) { ::printf ("%d:%s\n", x, #m); ::fflush(stdout); }} | |
270 | |
271 // Simplistic low-quality Marsaglia SHIFT-XOR RNG. | |
272 // Bijective except for the trailing mask operation. | |
273 // Useful for spin loops as the compiler can't optimize it away. | |
274 | |
275 static inline jint MarsagliaXORV (jint x) { | |
276 if (x == 0) x = 1|os::random() ; | |
277 x ^= x << 6; | |
278 x ^= ((unsigned)x) >> 21; | |
279 x ^= x << 7 ; | |
280 return x & 0x7FFFFFFF ; | |
281 } | |
282 | |
283 static int Stall (int its) { | |
284 static volatile jint rv = 1 ; | |
285 volatile int OnFrame = 0 ; | |
286 jint v = rv ^ UNS(OnFrame) ; | |
287 while (--its >= 0) { | |
288 v = MarsagliaXORV (v) ; | |
289 } | |
290 // Make this impossible for the compiler to optimize away, | |
291 // but (mostly) avoid W coherency sharing on MP systems. | |
292 if (v == 0x12345) rv = v ; | |
293 return v ; | |
294 } | |
295 | |
296 int Monitor::TryLock () { | |
297 intptr_t v = _LockWord.FullWord ; | |
298 for (;;) { | |
299 if ((v & _LBIT) != 0) return 0 ; | |
300 const intptr_t u = CASPTR (&_LockWord, v, v|_LBIT) ; | |
301 if (v == u) return 1 ; | |
302 v = u ; | |
303 } | |
304 } | |
305 | |
306 int Monitor::TryFast () { | |
307 // Optimistic fast-path form ... | |
308 // Fast-path attempt for the common uncontended case. | |
309 // Avoid RTS->RTO $ coherence upgrade on typical SMP systems. | |
310 intptr_t v = CASPTR (&_LockWord, 0, _LBIT) ; // agro ... | |
311 if (v == 0) return 1 ; | |
312 | |
313 for (;;) { | |
314 if ((v & _LBIT) != 0) return 0 ; | |
315 const intptr_t u = CASPTR (&_LockWord, v, v|_LBIT) ; | |
316 if (v == u) return 1 ; | |
317 v = u ; | |
318 } | |
319 } | |
320 | |
321 int Monitor::ILocked () { | |
322 const intptr_t w = _LockWord.FullWord & 0xFF ; | |
323 assert (w == 0 || w == _LBIT, "invariant") ; | |
324 return w == _LBIT ; | |
325 } | |
326 | |
327 // Polite TATAS spinlock with exponential backoff - bounded spin. | |
328 // Ideally we'd use processor cycles, time or vtime to control | |
329 // the loop, but we currently use iterations. | |
330 // All the constants within were derived empirically but work over | |
331 // over the spectrum of J2SE reference platforms. | |
332 // On Niagara-class systems the back-off is unnecessary but | |
333 // is relatively harmless. (At worst it'll slightly retard | |
334 // acquisition times). The back-off is critical for older SMP systems | |
335 // where constant fetching of the LockWord would otherwise impair | |
336 // scalability. | |
337 // | |
338 // Clamp spinning at approximately 1/2 of a context-switch round-trip. | |
339 // See synchronizer.cpp for details and rationale. | |
340 | |
341 int Monitor::TrySpin (Thread * const Self) { | |
342 if (TryLock()) return 1 ; | |
343 if (!os::is_MP()) return 0 ; | |
344 | |
345 int Probes = 0 ; | |
346 int Delay = 0 ; | |
347 int Steps = 0 ; | |
348 int SpinMax = NativeMonitorSpinLimit ; | |
349 int flgs = NativeMonitorFlags ; | |
350 for (;;) { | |
351 intptr_t v = _LockWord.FullWord; | |
352 if ((v & _LBIT) == 0) { | |
353 if (CASPTR (&_LockWord, v, v|_LBIT) == v) { | |
354 return 1 ; | |
355 } | |
356 continue ; | |
357 } | |
358 | |
359 if ((flgs & 8) == 0) { | |
360 SpinPause () ; | |
361 } | |
362 | |
363 // Periodically increase Delay -- variable Delay form | |
364 // conceptually: delay *= 1 + 1/Exponent | |
365 ++ Probes; | |
366 if (Probes > SpinMax) return 0 ; | |
367 | |
368 if ((Probes & 0x7) == 0) { | |
369 Delay = ((Delay << 1)|1) & 0x7FF ; | |
370 // CONSIDER: Delay += 1 + (Delay/4); Delay &= 0x7FF ; | |
371 } | |
372 | |
373 if (flgs & 2) continue ; | |
374 | |
375 // Consider checking _owner's schedctl state, if OFFPROC abort spin. | |
376 // If the owner is OFFPROC then it's unlike that the lock will be dropped | |
377 // in a timely fashion, which suggests that spinning would not be fruitful | |
378 // or profitable. | |
379 | |
380 // Stall for "Delay" time units - iterations in the current implementation. | |
381 // Avoid generating coherency traffic while stalled. | |
382 // Possible ways to delay: | |
383 // PAUSE, SLEEP, MEMBAR #sync, MEMBAR #halt, | |
384 // wr %g0,%asi, gethrtime, rdstick, rdtick, rdtsc, etc. ... | |
385 // Note that on Niagara-class systems we want to minimize STs in the | |
386 // spin loop. N1 and brethren write-around the L1$ over the xbar into the L2$. | |
387 // Furthermore, they don't have a W$ like traditional SPARC processors. | |
388 // We currently use a Marsaglia Shift-Xor RNG loop. | |
389 Steps += Delay ; | |
390 if (Self != NULL) { | |
391 jint rv = Self->rng[0] ; | |
392 for (int k = Delay ; --k >= 0; ) { | |
393 rv = MarsagliaXORV (rv) ; | |
394 if ((flgs & 4) == 0 && SafepointSynchronize::do_call_back()) return 0 ; | |
395 } | |
396 Self->rng[0] = rv ; | |
397 } else { | |
398 Stall (Delay) ; | |
399 } | |
400 } | |
401 } | |
402 | |
403 static int ParkCommon (ParkEvent * ev, jlong timo) { | |
404 // Diagnostic support - periodically unwedge blocked threads | |
405 intx nmt = NativeMonitorTimeout ; | |
406 if (nmt > 0 && (nmt < timo || timo <= 0)) { | |
407 timo = nmt ; | |
408 } | |
409 int err = OS_OK ; | |
410 if (0 == timo) { | |
411 ev->park() ; | |
412 } else { | |
413 err = ev->park(timo) ; | |
414 } | |
415 return err ; | |
416 } | |
417 | |
418 inline int Monitor::AcquireOrPush (ParkEvent * ESelf) { | |
419 intptr_t v = _LockWord.FullWord ; | |
420 for (;;) { | |
421 if ((v & _LBIT) == 0) { | |
422 const intptr_t u = CASPTR (&_LockWord, v, v|_LBIT) ; | |
423 if (u == v) return 1 ; // indicate acquired | |
424 v = u ; | |
425 } else { | |
426 // Anticipate success ... | |
427 ESelf->ListNext = (ParkEvent *) (v & ~_LBIT) ; | |
428 const intptr_t u = CASPTR (&_LockWord, v, intptr_t(ESelf)|_LBIT) ; | |
429 if (u == v) return 0 ; // indicate pushed onto cxq | |
430 v = u ; | |
431 } | |
432 // Interference - LockWord change - just retry | |
433 } | |
434 } | |
435 | |
436 // ILock and IWait are the lowest level primitive internal blocking | |
437 // synchronization functions. The callers of IWait and ILock must have | |
438 // performed any needed state transitions beforehand. | |
439 // IWait and ILock may directly call park() without any concern for thread state. | |
440 // Note that ILock and IWait do *not* access _owner. | |
441 // _owner is a higher-level logical concept. | |
442 | |
443 void Monitor::ILock (Thread * Self) { | |
444 assert (_OnDeck != Self->_MutexEvent, "invariant") ; | |
445 | |
446 if (TryFast()) { | |
447 Exeunt: | |
448 assert (ILocked(), "invariant") ; | |
449 return ; | |
450 } | |
451 | |
452 ParkEvent * const ESelf = Self->_MutexEvent ; | |
453 assert (_OnDeck != ESelf, "invariant") ; | |
454 | |
455 // As an optimization, spinners could conditionally try to set ONDECK to _LBIT | |
456 // Synchronizer.cpp uses a similar optimization. | |
457 if (TrySpin (Self)) goto Exeunt ; | |
458 | |
459 // Slow-path - the lock is contended. | |
460 // Either Enqueue Self on cxq or acquire the outer lock. | |
461 // LockWord encoding = (cxq,LOCKBYTE) | |
462 ESelf->reset() ; | |
463 OrderAccess::fence() ; | |
464 | |
465 // Optional optimization ... try barging on the inner lock | |
466 if ((NativeMonitorFlags & 32) && CASPTR (&_OnDeck, NULL, UNS(Self)) == 0) { | |
467 goto OnDeck_LOOP ; | |
468 } | |
469 | |
470 if (AcquireOrPush (ESelf)) goto Exeunt ; | |
471 | |
472 // At any given time there is at most one ondeck thread. | |
473 // ondeck implies not resident on cxq and not resident on EntryList | |
474 // Only the OnDeck thread can try to acquire -- contended for -- the lock. | |
475 // CONSIDER: use Self->OnDeck instead of m->OnDeck. | |
476 // Deschedule Self so that others may run. | |
477 while (_OnDeck != ESelf) { | |
478 ParkCommon (ESelf, 0) ; | |
479 } | |
480 | |
481 // Self is now in the ONDECK position and will remain so until it | |
482 // manages to acquire the lock. | |
483 OnDeck_LOOP: | |
484 for (;;) { | |
485 assert (_OnDeck == ESelf, "invariant") ; | |
486 if (TrySpin (Self)) break ; | |
487 // CONSIDER: if ESelf->TryPark() && TryLock() break ... | |
488 // It's probably wise to spin only if we *actually* blocked | |
489 // CONSIDER: check the lockbyte, if it remains set then | |
490 // preemptively drain the cxq into the EntryList. | |
491 // The best place and time to perform queue operations -- lock metadata -- | |
492 // is _before having acquired the outer lock, while waiting for the lock to drop. | |
493 ParkCommon (ESelf, 0) ; | |
494 } | |
495 | |
496 assert (_OnDeck == ESelf, "invariant") ; | |
497 _OnDeck = NULL ; | |
498 | |
499 // Note that we current drop the inner lock (clear OnDeck) in the slow-path | |
14309 | 500 // epilogue immediately after having acquired the outer lock. |
0 | 501 // But instead we could consider the following optimizations: |
502 // A. Shift or defer dropping the inner lock until the subsequent IUnlock() operation. | |
503 // This might avoid potential reacquisition of the inner lock in IUlock(). | |
504 // B. While still holding the inner lock, attempt to opportunistically select | |
505 // and unlink the next ONDECK thread from the EntryList. | |
506 // If successful, set ONDECK to refer to that thread, otherwise clear ONDECK. | |
507 // It's critical that the select-and-unlink operation run in constant-time as | |
508 // it executes when holding the outer lock and may artificially increase the | |
509 // effective length of the critical section. | |
510 // Note that (A) and (B) are tantamount to succession by direct handoff for | |
511 // the inner lock. | |
512 goto Exeunt ; | |
513 } | |
514 | |
515 void Monitor::IUnlock (bool RelaxAssert) { | |
516 assert (ILocked(), "invariant") ; | |
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517 // Conceptually we need a MEMBAR #storestore|#loadstore barrier or fence immediately |
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518 // before the store that releases the lock. Crucially, all the stores and loads in the |
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519 // critical section must be globally visible before the store of 0 into the lock-word |
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520 // that releases the lock becomes globally visible. That is, memory accesses in the |
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521 // critical section should not be allowed to bypass or overtake the following ST that |
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522 // releases the lock. As such, to prevent accesses within the critical section |
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523 // from "leaking" out, we need a release fence between the critical section and the |
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524 // store that releases the lock. In practice that release barrier is elided on |
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525 // platforms with strong memory models such as TSO. |
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526 // |
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527 // Note that the OrderAccess::storeload() fence that appears after unlock store |
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528 // provides for progress conditions and succession and is _not related to exclusion |
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529 // safety or lock release consistency. |
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530 OrderAccess::release_store(&_LockWord.Bytes[_LSBINDEX], 0); // drop outer lock |
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531 |
0 | 532 OrderAccess::storeload (); |
533 ParkEvent * const w = _OnDeck ; | |
534 assert (RelaxAssert || w != Thread::current()->_MutexEvent, "invariant") ; | |
535 if (w != NULL) { | |
536 // Either we have a valid ondeck thread or ondeck is transiently "locked" | |
537 // by some exiting thread as it arranges for succession. The LSBit of | |
538 // OnDeck allows us to discriminate two cases. If the latter, the | |
539 // responsibility for progress and succession lies with that other thread. | |
540 // For good performance, we also depend on the fact that redundant unpark() | |
541 // operations are cheap. That is, repeated Unpark()ing of the ONDECK thread | |
542 // is inexpensive. This approach provides implicit futile wakeup throttling. | |
543 // Note that the referent "w" might be stale with respect to the lock. | |
544 // In that case the following unpark() is harmless and the worst that'll happen | |
545 // is a spurious return from a park() operation. Critically, if "w" _is stale, | |
546 // then progress is known to have occurred as that means the thread associated | |
547 // with "w" acquired the lock. In that case this thread need take no further | |
548 // action to guarantee progress. | |
549 if ((UNS(w) & _LBIT) == 0) w->unpark() ; | |
550 return ; | |
551 } | |
552 | |
553 intptr_t cxq = _LockWord.FullWord ; | |
554 if (((cxq & ~_LBIT)|UNS(_EntryList)) == 0) { | |
555 return ; // normal fast-path exit - cxq and EntryList both empty | |
556 } | |
557 if (cxq & _LBIT) { | |
558 // Optional optimization ... | |
559 // Some other thread acquired the lock in the window since this | |
560 // thread released it. Succession is now that thread's responsibility. | |
561 return ; | |
562 } | |
563 | |
564 Succession: | |
565 // Slow-path exit - this thread must ensure succession and progress. | |
566 // OnDeck serves as lock to protect cxq and EntryList. | |
567 // Only the holder of OnDeck can manipulate EntryList or detach the RATs from cxq. | |
568 // Avoid ABA - allow multiple concurrent producers (enqueue via push-CAS) | |
569 // but only one concurrent consumer (detacher of RATs). | |
570 // Consider protecting this critical section with schedctl on Solaris. | |
571 // Unlike a normal lock, however, the exiting thread "locks" OnDeck, | |
572 // picks a successor and marks that thread as OnDeck. That successor | |
573 // thread will then clear OnDeck once it eventually acquires the outer lock. | |
574 if (CASPTR (&_OnDeck, NULL, _LBIT) != UNS(NULL)) { | |
575 return ; | |
576 } | |
577 | |
578 ParkEvent * List = _EntryList ; | |
579 if (List != NULL) { | |
580 // Transfer the head of the EntryList to the OnDeck position. | |
581 // Once OnDeck, a thread stays OnDeck until it acquires the lock. | |
582 // For a given lock there is at most OnDeck thread at any one instant. | |
583 WakeOne: | |
584 assert (List == _EntryList, "invariant") ; | |
585 ParkEvent * const w = List ; | |
586 assert (RelaxAssert || w != Thread::current()->_MutexEvent, "invariant") ; | |
587 _EntryList = w->ListNext ; | |
588 // as a diagnostic measure consider setting w->_ListNext = BAD | |
589 assert (UNS(_OnDeck) == _LBIT, "invariant") ; | |
590 _OnDeck = w ; // pass OnDeck to w. | |
591 // w will clear OnDeck once it acquires the outer lock | |
592 | |
593 // Another optional optimization ... | |
594 // For heavily contended locks it's not uncommon that some other | |
595 // thread acquired the lock while this thread was arranging succession. | |
596 // Try to defer the unpark() operation - Delegate the responsibility | |
597 // for unpark()ing the OnDeck thread to the current or subsequent owners | |
598 // That is, the new owner is responsible for unparking the OnDeck thread. | |
599 OrderAccess::storeload() ; | |
600 cxq = _LockWord.FullWord ; | |
601 if (cxq & _LBIT) return ; | |
602 | |
603 w->unpark() ; | |
604 return ; | |
605 } | |
606 | |
607 cxq = _LockWord.FullWord ; | |
608 if ((cxq & ~_LBIT) != 0) { | |
609 // The EntryList is empty but the cxq is populated. | |
610 // drain RATs from cxq into EntryList | |
611 // Detach RATs segment with CAS and then merge into EntryList | |
612 for (;;) { | |
613 // optional optimization - if locked, the owner is responsible for succession | |
614 if (cxq & _LBIT) goto Punt ; | |
615 const intptr_t vfy = CASPTR (&_LockWord, cxq, cxq & _LBIT) ; | |
616 if (vfy == cxq) break ; | |
617 cxq = vfy ; | |
618 // Interference - LockWord changed - Just retry | |
619 // We can see concurrent interference from contending threads | |
620 // pushing themselves onto the cxq or from lock-unlock operations. | |
621 // From the perspective of this thread, EntryList is stable and | |
622 // the cxq is prepend-only -- the head is volatile but the interior | |
623 // of the cxq is stable. In theory if we encounter interference from threads | |
624 // pushing onto cxq we could simply break off the original cxq suffix and | |
625 // move that segment to the EntryList, avoiding a 2nd or multiple CAS attempts | |
626 // on the high-traffic LockWord variable. For instance lets say the cxq is "ABCD" | |
627 // when we first fetch cxq above. Between the fetch -- where we observed "A" | |
628 // -- and CAS -- where we attempt to CAS null over A -- "PQR" arrive, | |
629 // yielding cxq = "PQRABCD". In this case we could simply set A.ListNext | |
630 // null, leaving cxq = "PQRA" and transfer the "BCD" segment to the EntryList. | |
631 // Note too, that it's safe for this thread to traverse the cxq | |
632 // without taking any special concurrency precautions. | |
633 } | |
634 | |
635 // We don't currently reorder the cxq segment as we move it onto | |
636 // the EntryList, but it might make sense to reverse the order | |
637 // or perhaps sort by thread priority. See the comments in | |
638 // synchronizer.cpp objectMonitor::exit(). | |
639 assert (_EntryList == NULL, "invariant") ; | |
640 _EntryList = List = (ParkEvent *)(cxq & ~_LBIT) ; | |
641 assert (List != NULL, "invariant") ; | |
642 goto WakeOne ; | |
643 } | |
644 | |
645 // cxq|EntryList is empty. | |
646 // w == NULL implies that cxq|EntryList == NULL in the past. | |
647 // Possible race - rare inopportune interleaving. | |
648 // A thread could have added itself to cxq since this thread previously checked. | |
649 // Detect and recover by refetching cxq. | |
650 Punt: | |
651 assert (UNS(_OnDeck) == _LBIT, "invariant") ; | |
652 _OnDeck = NULL ; // Release inner lock. | |
653 OrderAccess::storeload(); // Dekker duality - pivot point | |
654 | |
655 // Resample LockWord/cxq to recover from possible race. | |
656 // For instance, while this thread T1 held OnDeck, some other thread T2 might | |
657 // acquire the outer lock. Another thread T3 might try to acquire the outer | |
658 // lock, but encounter contention and enqueue itself on cxq. T2 then drops the | |
659 // outer lock, but skips succession as this thread T1 still holds OnDeck. | |
660 // T1 is and remains responsible for ensuring succession of T3. | |
661 // | |
662 // Note that we don't need to recheck EntryList, just cxq. | |
663 // If threads moved onto EntryList since we dropped OnDeck | |
664 // that implies some other thread forced succession. | |
665 cxq = _LockWord.FullWord ; | |
666 if ((cxq & ~_LBIT) != 0 && (cxq & _LBIT) == 0) { | |
667 goto Succession ; // potential race -- re-run succession | |
668 } | |
669 return ; | |
670 } | |
671 | |
672 bool Monitor::notify() { | |
673 assert (_owner == Thread::current(), "invariant") ; | |
674 assert (ILocked(), "invariant") ; | |
675 if (_WaitSet == NULL) return true ; | |
676 NotifyCount ++ ; | |
677 | |
678 // Transfer one thread from the WaitSet to the EntryList or cxq. | |
679 // Currently we just unlink the head of the WaitSet and prepend to the cxq. | |
680 // And of course we could just unlink it and unpark it, too, but | |
681 // in that case it'd likely impale itself on the reentry. | |
682 Thread::muxAcquire (_WaitLock, "notify:WaitLock") ; | |
683 ParkEvent * nfy = _WaitSet ; | |
684 if (nfy != NULL) { // DCL idiom | |
685 _WaitSet = nfy->ListNext ; | |
686 assert (nfy->Notified == 0, "invariant") ; | |
687 // push nfy onto the cxq | |
688 for (;;) { | |
689 const intptr_t v = _LockWord.FullWord ; | |
690 assert ((v & 0xFF) == _LBIT, "invariant") ; | |
691 nfy->ListNext = (ParkEvent *)(v & ~_LBIT); | |
692 if (CASPTR (&_LockWord, v, UNS(nfy)|_LBIT) == v) break; | |
693 // interference - _LockWord changed -- just retry | |
694 } | |
695 // Note that setting Notified before pushing nfy onto the cxq is | |
696 // also legal and safe, but the safety properties are much more | |
697 // subtle, so for the sake of code stewardship ... | |
698 OrderAccess::fence() ; | |
699 nfy->Notified = 1; | |
700 } | |
701 Thread::muxRelease (_WaitLock) ; | |
702 if (nfy != NULL && (NativeMonitorFlags & 16)) { | |
703 // Experimental code ... light up the wakee in the hope that this thread (the owner) | |
704 // will drop the lock just about the time the wakee comes ONPROC. | |
705 nfy->unpark() ; | |
706 } | |
707 assert (ILocked(), "invariant") ; | |
708 return true ; | |
709 } | |
710 | |
711 // Currently notifyAll() transfers the waiters one-at-a-time from the waitset | |
712 // to the cxq. This could be done more efficiently with a single bulk en-mass transfer, | |
713 // but in practice notifyAll() for large #s of threads is rare and not time-critical. | |
714 // Beware too, that we invert the order of the waiters. Lets say that the | |
715 // waitset is "ABCD" and the cxq is "XYZ". After a notifyAll() the waitset | |
716 // will be empty and the cxq will be "DCBAXYZ". This is benign, of course. | |
717 | |
718 bool Monitor::notify_all() { | |
719 assert (_owner == Thread::current(), "invariant") ; | |
720 assert (ILocked(), "invariant") ; | |
721 while (_WaitSet != NULL) notify() ; | |
722 return true ; | |
723 } | |
724 | |
725 int Monitor::IWait (Thread * Self, jlong timo) { | |
726 assert (ILocked(), "invariant") ; | |
727 | |
728 // Phases: | |
729 // 1. Enqueue Self on WaitSet - currently prepend | |
730 // 2. unlock - drop the outer lock | |
731 // 3. wait for either notification or timeout | |
732 // 4. lock - reentry - reacquire the outer lock | |
733 | |
734 ParkEvent * const ESelf = Self->_MutexEvent ; | |
735 ESelf->Notified = 0 ; | |
736 ESelf->reset() ; | |
737 OrderAccess::fence() ; | |
738 | |
739 // Add Self to WaitSet | |
740 // Ideally only the holder of the outer lock would manipulate the WaitSet - | |
741 // That is, the outer lock would implicitly protect the WaitSet. | |
742 // But if a thread in wait() encounters a timeout it will need to dequeue itself | |
743 // from the WaitSet _before it becomes the owner of the lock. We need to dequeue | |
744 // as the ParkEvent -- which serves as a proxy for the thread -- can't reside | |
745 // on both the WaitSet and the EntryList|cxq at the same time.. That is, a thread | |
746 // on the WaitSet can't be allowed to compete for the lock until it has managed to | |
747 // unlink its ParkEvent from WaitSet. Thus the need for WaitLock. | |
748 // Contention on the WaitLock is minimal. | |
749 // | |
750 // Another viable approach would be add another ParkEvent, "WaitEvent" to the | |
751 // thread class. The WaitSet would be composed of WaitEvents. Only the | |
752 // owner of the outer lock would manipulate the WaitSet. A thread in wait() | |
753 // could then compete for the outer lock, and then, if necessary, unlink itself | |
754 // from the WaitSet only after having acquired the outer lock. More precisely, | |
755 // there would be no WaitLock. A thread in in wait() would enqueue its WaitEvent | |
756 // on the WaitSet; release the outer lock; wait for either notification or timeout; | |
757 // reacquire the inner lock; and then, if needed, unlink itself from the WaitSet. | |
758 // | |
759 // Alternatively, a 2nd set of list link fields in the ParkEvent might suffice. | |
760 // One set would be for the WaitSet and one for the EntryList. | |
761 // We could also deconstruct the ParkEvent into a "pure" event and add a | |
762 // new immortal/TSM "ListElement" class that referred to ParkEvents. | |
763 // In that case we could have one ListElement on the WaitSet and another | |
764 // on the EntryList, with both referring to the same pure Event. | |
765 | |
766 Thread::muxAcquire (_WaitLock, "wait:WaitLock:Add") ; | |
767 ESelf->ListNext = _WaitSet ; | |
768 _WaitSet = ESelf ; | |
769 Thread::muxRelease (_WaitLock) ; | |
770 | |
771 // Release the outer lock | |
772 // We call IUnlock (RelaxAssert=true) as a thread T1 might | |
773 // enqueue itself on the WaitSet, call IUnlock(), drop the lock, | |
774 // and then stall before it can attempt to wake a successor. | |
775 // Some other thread T2 acquires the lock, and calls notify(), moving | |
776 // T1 from the WaitSet to the cxq. T2 then drops the lock. T1 resumes, | |
777 // and then finds *itself* on the cxq. During the course of a normal | |
778 // IUnlock() call a thread should _never find itself on the EntryList | |
779 // or cxq, but in the case of wait() it's possible. | |
780 // See synchronizer.cpp objectMonitor::wait(). | |
781 IUnlock (true) ; | |
782 | |
783 // Wait for either notification or timeout | |
784 // Beware that in some circumstances we might propagate | |
785 // spurious wakeups back to the caller. | |
786 | |
787 for (;;) { | |
788 if (ESelf->Notified) break ; | |
789 int err = ParkCommon (ESelf, timo) ; | |
790 if (err == OS_TIMEOUT || (NativeMonitorFlags & 1)) break ; | |
791 } | |
792 | |
793 // Prepare for reentry - if necessary, remove ESelf from WaitSet | |
794 // ESelf can be: | |
795 // 1. Still on the WaitSet. This can happen if we exited the loop by timeout. | |
796 // 2. On the cxq or EntryList | |
797 // 3. Not resident on cxq, EntryList or WaitSet, but in the OnDeck position. | |
798 | |
799 OrderAccess::fence() ; | |
800 int WasOnWaitSet = 0 ; | |
801 if (ESelf->Notified == 0) { | |
802 Thread::muxAcquire (_WaitLock, "wait:WaitLock:remove") ; | |
803 if (ESelf->Notified == 0) { // DCL idiom | |
804 assert (_OnDeck != ESelf, "invariant") ; // can't be both OnDeck and on WaitSet | |
805 // ESelf is resident on the WaitSet -- unlink it. | |
806 // A doubly-linked list would be better here so we can unlink in constant-time. | |
807 // We have to unlink before we potentially recontend as ESelf might otherwise | |
808 // end up on the cxq|EntryList -- it can't be on two lists at once. | |
809 ParkEvent * p = _WaitSet ; | |
810 ParkEvent * q = NULL ; // classic q chases p | |
811 while (p != NULL && p != ESelf) { | |
812 q = p ; | |
813 p = p->ListNext ; | |
814 } | |
815 assert (p == ESelf, "invariant") ; | |
816 if (p == _WaitSet) { // found at head | |
817 assert (q == NULL, "invariant") ; | |
818 _WaitSet = p->ListNext ; | |
819 } else { // found in interior | |
820 assert (q->ListNext == p, "invariant") ; | |
821 q->ListNext = p->ListNext ; | |
822 } | |
823 WasOnWaitSet = 1 ; // We were *not* notified but instead encountered timeout | |
824 } | |
825 Thread::muxRelease (_WaitLock) ; | |
826 } | |
827 | |
828 // Reentry phase - reacquire the lock | |
829 if (WasOnWaitSet) { | |
830 // ESelf was previously on the WaitSet but we just unlinked it above | |
831 // because of a timeout. ESelf is not resident on any list and is not OnDeck | |
832 assert (_OnDeck != ESelf, "invariant") ; | |
833 ILock (Self) ; | |
834 } else { | |
835 // A prior notify() operation moved ESelf from the WaitSet to the cxq. | |
836 // ESelf is now on the cxq, EntryList or at the OnDeck position. | |
837 // The following fragment is extracted from Monitor::ILock() | |
838 for (;;) { | |
839 if (_OnDeck == ESelf && TrySpin(Self)) break ; | |
840 ParkCommon (ESelf, 0) ; | |
841 } | |
842 assert (_OnDeck == ESelf, "invariant") ; | |
843 _OnDeck = NULL ; | |
844 } | |
845 | |
846 assert (ILocked(), "invariant") ; | |
847 return WasOnWaitSet != 0 ; // return true IFF timeout | |
848 } | |
849 | |
850 | |
851 // ON THE VMTHREAD SNEAKING PAST HELD LOCKS: | |
852 // In particular, there are certain types of global lock that may be held | |
853 // by a Java thread while it is blocked at a safepoint but before it has | |
854 // written the _owner field. These locks may be sneakily acquired by the | |
855 // VM thread during a safepoint to avoid deadlocks. Alternatively, one should | |
856 // identify all such locks, and ensure that Java threads never block at | |
857 // safepoints while holding them (_no_safepoint_check_flag). While it | |
858 // seems as though this could increase the time to reach a safepoint | |
859 // (or at least increase the mean, if not the variance), the latter | |
860 // approach might make for a cleaner, more maintainable JVM design. | |
861 // | |
862 // Sneaking is vile and reprehensible and should be excised at the 1st | |
863 // opportunity. It's possible that the need for sneaking could be obviated | |
864 // as follows. Currently, a thread might (a) while TBIVM, call pthread_mutex_lock | |
865 // or ILock() thus acquiring the "physical" lock underlying Monitor/Mutex. | |
866 // (b) stall at the TBIVM exit point as a safepoint is in effect. Critically, | |
867 // it'll stall at the TBIVM reentry state transition after having acquired the | |
868 // underlying lock, but before having set _owner and having entered the actual | |
869 // critical section. The lock-sneaking facility leverages that fact and allowed the | |
870 // VM thread to logically acquire locks that had already be physically locked by mutators | |
871 // but where mutators were known blocked by the reentry thread state transition. | |
872 // | |
873 // If we were to modify the Monitor-Mutex so that TBIVM state transitions tightly | |
874 // wrapped calls to park(), then we could likely do away with sneaking. We'd | |
875 // decouple lock acquisition and parking. The critical invariant to eliminating | |
876 // sneaking is to ensure that we never "physically" acquire the lock while TBIVM. | |
877 // An easy way to accomplish this is to wrap the park calls in a narrow TBIVM jacket. | |
878 // One difficulty with this approach is that the TBIVM wrapper could recurse and | |
879 // call lock() deep from within a lock() call, while the MutexEvent was already enqueued. | |
880 // Using a stack (N=2 at minimum) of ParkEvents would take care of that problem. | |
881 // | |
882 // But of course the proper ultimate approach is to avoid schemes that require explicit | |
883 // sneaking or dependence on any any clever invariants or subtle implementation properties | |
884 // of Mutex-Monitor and instead directly address the underlying design flaw. | |
885 | |
886 void Monitor::lock (Thread * Self) { | |
887 #ifdef CHECK_UNHANDLED_OOPS | |
888 // Clear unhandled oops so we get a crash right away. Only clear for non-vm | |
889 // or GC threads. | |
890 if (Self->is_Java_thread()) { | |
891 Self->clear_unhandled_oops(); | |
892 } | |
893 #endif // CHECK_UNHANDLED_OOPS | |
894 | |
895 debug_only(check_prelock_state(Self)); | |
896 assert (_owner != Self , "invariant") ; | |
897 assert (_OnDeck != Self->_MutexEvent, "invariant") ; | |
898 | |
899 if (TryFast()) { | |
900 Exeunt: | |
901 assert (ILocked(), "invariant") ; | |
902 assert (owner() == NULL, "invariant"); | |
903 set_owner (Self); | |
904 return ; | |
905 } | |
906 | |
907 // The lock is contended ... | |
908 | |
909 bool can_sneak = Self->is_VM_thread() && SafepointSynchronize::is_at_safepoint(); | |
910 if (can_sneak && _owner == NULL) { | |
911 // a java thread has locked the lock but has not entered the | |
912 // critical region -- let's just pretend we've locked the lock | |
913 // and go on. we note this with _snuck so we can also | |
914 // pretend to unlock when the time comes. | |
915 _snuck = true; | |
916 goto Exeunt ; | |
917 } | |
918 | |
919 // Try a brief spin to avoid passing thru thread state transition ... | |
920 if (TrySpin (Self)) goto Exeunt ; | |
921 | |
922 check_block_state(Self); | |
923 if (Self->is_Java_thread()) { | |
14309 | 924 // Horrible dictu - we suffer through a state transition |
0 | 925 assert(rank() > Mutex::special, "Potential deadlock with special or lesser rank mutex"); |
926 ThreadBlockInVM tbivm ((JavaThread *) Self) ; | |
927 ILock (Self) ; | |
928 } else { | |
929 // Mirabile dictu | |
930 ILock (Self) ; | |
931 } | |
932 goto Exeunt ; | |
933 } | |
934 | |
935 void Monitor::lock() { | |
936 this->lock(Thread::current()); | |
937 } | |
938 | |
939 // Lock without safepoint check - a degenerate variant of lock(). | |
940 // Should ONLY be used by safepoint code and other code | |
941 // that is guaranteed not to block while running inside the VM. If this is called with | |
942 // thread state set to be in VM, the safepoint synchronization code will deadlock! | |
943 | |
944 void Monitor::lock_without_safepoint_check (Thread * Self) { | |
945 assert (_owner != Self, "invariant") ; | |
946 ILock (Self) ; | |
947 assert (_owner == NULL, "invariant"); | |
948 set_owner (Self); | |
949 } | |
950 | |
951 void Monitor::lock_without_safepoint_check () { | |
952 lock_without_safepoint_check (Thread::current()) ; | |
953 } | |
954 | |
955 | |
14309 | 956 // Returns true if thread succeeds in grabbing the lock, otherwise false. |
0 | 957 |
958 bool Monitor::try_lock() { | |
959 Thread * const Self = Thread::current(); | |
960 debug_only(check_prelock_state(Self)); | |
961 // assert(!thread->is_inside_signal_handler(), "don't lock inside signal handler"); | |
962 | |
963 // Special case, where all Java threads are stopped. | |
964 // The lock may have been acquired but _owner is not yet set. | |
965 // In that case the VM thread can safely grab the lock. | |
966 // It strikes me this should appear _after the TryLock() fails, below. | |
967 bool can_sneak = Self->is_VM_thread() && SafepointSynchronize::is_at_safepoint(); | |
968 if (can_sneak && _owner == NULL) { | |
969 set_owner(Self); // Do not need to be atomic, since we are at a safepoint | |
970 _snuck = true; | |
971 return true; | |
972 } | |
973 | |
974 if (TryLock()) { | |
975 // We got the lock | |
976 assert (_owner == NULL, "invariant"); | |
977 set_owner (Self); | |
978 return true; | |
979 } | |
980 return false; | |
981 } | |
982 | |
983 void Monitor::unlock() { | |
984 assert (_owner == Thread::current(), "invariant") ; | |
985 assert (_OnDeck != Thread::current()->_MutexEvent , "invariant") ; | |
986 set_owner (NULL) ; | |
987 if (_snuck) { | |
988 assert(SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread(), "sneak"); | |
989 _snuck = false; | |
990 return ; | |
991 } | |
992 IUnlock (false) ; | |
993 } | |
994 | |
995 // Yet another degenerate version of Monitor::lock() or lock_without_safepoint_check() | |
996 // jvm_raw_lock() and _unlock() can be called by non-Java threads via JVM_RawMonitorEnter. | |
997 // | |
998 // There's no expectation that JVM_RawMonitors will interoperate properly with the native | |
999 // Mutex-Monitor constructs. We happen to implement JVM_RawMonitors in terms of | |
1000 // native Mutex-Monitors simply as a matter of convenience. A simple abstraction layer | |
1001 // over a pthread_mutex_t would work equally as well, but require more platform-specific | |
1002 // code -- a "PlatformMutex". Alternatively, a simply layer over muxAcquire-muxRelease | |
1003 // would work too. | |
1004 // | |
1005 // Since the caller might be a foreign thread, we don't necessarily have a Thread.MutexEvent | |
1006 // instance available. Instead, we transiently allocate a ParkEvent on-demand if | |
1007 // we encounter contention. That ParkEvent remains associated with the thread | |
1008 // until it manages to acquire the lock, at which time we return the ParkEvent | |
1009 // to the global ParkEvent free list. This is correct and suffices for our purposes. | |
1010 // | |
1011 // Beware that the original jvm_raw_unlock() had a "_snuck" test but that | |
1012 // jvm_raw_lock() didn't have the corresponding test. I suspect that's an | |
1013 // oversight, but I've replicated the original suspect logic in the new code ... | |
1014 | |
1015 void Monitor::jvm_raw_lock() { | |
1016 assert(rank() == native, "invariant"); | |
1017 | |
1018 if (TryLock()) { | |
1019 Exeunt: | |
1020 assert (ILocked(), "invariant") ; | |
1021 assert (_owner == NULL, "invariant"); | |
1022 // This can potentially be called by non-java Threads. Thus, the ThreadLocalStorage | |
1023 // might return NULL. Don't call set_owner since it will break on an NULL owner | |
1024 // Consider installing a non-null "ANON" distinguished value instead of just NULL. | |
1025 _owner = ThreadLocalStorage::thread(); | |
1026 return ; | |
1027 } | |
1028 | |
1029 if (TrySpin(NULL)) goto Exeunt ; | |
1030 | |
1031 // slow-path - apparent contention | |
1032 // Allocate a ParkEvent for transient use. | |
1033 // The ParkEvent remains associated with this thread until | |
1034 // the time the thread manages to acquire the lock. | |
1035 ParkEvent * const ESelf = ParkEvent::Allocate(NULL) ; | |
1036 ESelf->reset() ; | |
1037 OrderAccess::storeload() ; | |
1038 | |
1039 // Either Enqueue Self on cxq or acquire the outer lock. | |
1040 if (AcquireOrPush (ESelf)) { | |
1041 ParkEvent::Release (ESelf) ; // surrender the ParkEvent | |
1042 goto Exeunt ; | |
1043 } | |
1044 | |
1045 // At any given time there is at most one ondeck thread. | |
1046 // ondeck implies not resident on cxq and not resident on EntryList | |
1047 // Only the OnDeck thread can try to acquire -- contended for -- the lock. | |
1048 // CONSIDER: use Self->OnDeck instead of m->OnDeck. | |
1049 for (;;) { | |
1050 if (_OnDeck == ESelf && TrySpin(NULL)) break ; | |
1051 ParkCommon (ESelf, 0) ; | |
1052 } | |
1053 | |
1054 assert (_OnDeck == ESelf, "invariant") ; | |
1055 _OnDeck = NULL ; | |
1056 ParkEvent::Release (ESelf) ; // surrender the ParkEvent | |
1057 goto Exeunt ; | |
1058 } | |
1059 | |
1060 void Monitor::jvm_raw_unlock() { | |
1061 // Nearly the same as Monitor::unlock() ... | |
1062 // directly set _owner instead of using set_owner(null) | |
1063 _owner = NULL ; | |
1064 if (_snuck) { // ??? | |
1065 assert(SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread(), "sneak"); | |
1066 _snuck = false; | |
1067 return ; | |
1068 } | |
1069 IUnlock(false) ; | |
1070 } | |
1071 | |
1072 bool Monitor::wait(bool no_safepoint_check, long timeout, bool as_suspend_equivalent) { | |
1073 Thread * const Self = Thread::current() ; | |
1074 assert (_owner == Self, "invariant") ; | |
1075 assert (ILocked(), "invariant") ; | |
1076 | |
1077 // as_suspend_equivalent logically implies !no_safepoint_check | |
1078 guarantee (!as_suspend_equivalent || !no_safepoint_check, "invariant") ; | |
1079 // !no_safepoint_check logically implies java_thread | |
1080 guarantee (no_safepoint_check || Self->is_Java_thread(), "invariant") ; | |
1081 | |
1082 #ifdef ASSERT | |
1083 Monitor * least = get_least_ranked_lock_besides_this(Self->owned_locks()); | |
1084 assert(least != this, "Specification of get_least_... call above"); | |
1085 if (least != NULL && least->rank() <= special) { | |
1086 tty->print("Attempting to wait on monitor %s/%d while holding" | |
1087 " lock %s/%d -- possible deadlock", | |
1088 name(), rank(), least->name(), least->rank()); | |
1089 assert(false, "Shouldn't block(wait) while holding a lock of rank special"); | |
1090 } | |
1091 #endif // ASSERT | |
1092 | |
1093 int wait_status ; | |
1094 // conceptually set the owner to NULL in anticipation of | |
1095 // abdicating the lock in wait | |
1096 set_owner(NULL); | |
1097 if (no_safepoint_check) { | |
1098 wait_status = IWait (Self, timeout) ; | |
1099 } else { | |
1100 assert (Self->is_Java_thread(), "invariant") ; | |
1101 JavaThread *jt = (JavaThread *)Self; | |
1102 | |
1103 // Enter safepoint region - ornate and Rococo ... | |
1104 ThreadBlockInVM tbivm(jt); | |
1105 OSThreadWaitState osts(Self->osthread(), false /* not Object.wait() */); | |
1106 | |
1107 if (as_suspend_equivalent) { | |
1108 jt->set_suspend_equivalent(); | |
1109 // cleared by handle_special_suspend_equivalent_condition() or | |
1110 // java_suspend_self() | |
1111 } | |
1112 | |
1113 wait_status = IWait (Self, timeout) ; | |
1114 | |
1115 // were we externally suspended while we were waiting? | |
1116 if (as_suspend_equivalent && jt->handle_special_suspend_equivalent_condition()) { | |
1117 // Our event wait has finished and we own the lock, but | |
1118 // while we were waiting another thread suspended us. We don't | |
1119 // want to hold the lock while suspended because that | |
1120 // would surprise the thread that suspended us. | |
1121 assert (ILocked(), "invariant") ; | |
1122 IUnlock (true) ; | |
1123 jt->java_suspend_self(); | |
1124 ILock (Self) ; | |
1125 assert (ILocked(), "invariant") ; | |
1126 } | |
1127 } | |
1128 | |
1129 // Conceptually reestablish ownership of the lock. | |
1130 // The "real" lock -- the LockByte -- was reacquired by IWait(). | |
1131 assert (ILocked(), "invariant") ; | |
1132 assert (_owner == NULL, "invariant") ; | |
1133 set_owner (Self) ; | |
1134 return wait_status != 0 ; // return true IFF timeout | |
1135 } | |
1136 | |
1137 Monitor::~Monitor() { | |
1138 assert ((UNS(_owner)|UNS(_LockWord.FullWord)|UNS(_EntryList)|UNS(_WaitSet)|UNS(_OnDeck)) == 0, "") ; | |
1139 } | |
1140 | |
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1141 void Monitor::ClearMonitor (Monitor * m, const char *name) { |
0 | 1142 m->_owner = NULL ; |
1143 m->_snuck = false ; | |
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1144 if (name == NULL) { |
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1145 strcpy(m->_name, "UNKNOWN") ; |
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1146 } else { |
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1147 strncpy(m->_name, name, MONITOR_NAME_LEN - 1); |
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1148 m->_name[MONITOR_NAME_LEN - 1] = '\0'; |
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1149 } |
0 | 1150 m->_LockWord.FullWord = 0 ; |
1151 m->_EntryList = NULL ; | |
1152 m->_OnDeck = NULL ; | |
1153 m->_WaitSet = NULL ; | |
1154 m->_WaitLock[0] = 0 ; | |
1155 } | |
1156 | |
1157 Monitor::Monitor() { ClearMonitor(this); } | |
1158 | |
1159 Monitor::Monitor (int Rank, const char * name, bool allow_vm_block) { | |
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1160 ClearMonitor (this, name) ; |
0 | 1161 #ifdef ASSERT |
1162 _allow_vm_block = allow_vm_block; | |
1163 _rank = Rank ; | |
1164 #endif | |
1165 } | |
1166 | |
1167 Mutex::~Mutex() { | |
1168 assert ((UNS(_owner)|UNS(_LockWord.FullWord)|UNS(_EntryList)|UNS(_WaitSet)|UNS(_OnDeck)) == 0, "") ; | |
1169 } | |
1170 | |
1171 Mutex::Mutex (int Rank, const char * name, bool allow_vm_block) { | |
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1172 ClearMonitor ((Monitor *) this, name) ; |
0 | 1173 #ifdef ASSERT |
1174 _allow_vm_block = allow_vm_block; | |
1175 _rank = Rank ; | |
1176 #endif | |
1177 } | |
1178 | |
1179 bool Monitor::owned_by_self() const { | |
1180 bool ret = _owner == Thread::current(); | |
1181 assert (!ret || _LockWord.Bytes[_LSBINDEX] != 0, "invariant") ; | |
1182 return ret; | |
1183 } | |
1184 | |
1185 void Monitor::print_on_error(outputStream* st) const { | |
1186 st->print("[" PTR_FORMAT, this); | |
1187 st->print("] %s", _name); | |
1188 st->print(" - owner thread: " PTR_FORMAT, _owner); | |
1189 } | |
1190 | |
1191 | |
1192 | |
1193 | |
1194 // ---------------------------------------------------------------------------------- | |
1195 // Non-product code | |
1196 | |
1197 #ifndef PRODUCT | |
1198 void Monitor::print_on(outputStream* st) const { | |
1199 st->print_cr("Mutex: [0x%lx/0x%lx] %s - owner: 0x%lx", this, _LockWord.FullWord, _name, _owner); | |
1200 } | |
1201 #endif | |
1202 | |
1203 #ifndef PRODUCT | |
1204 #ifdef ASSERT | |
1205 Monitor * Monitor::get_least_ranked_lock(Monitor * locks) { | |
1206 Monitor *res, *tmp; | |
1207 for (res = tmp = locks; tmp != NULL; tmp = tmp->next()) { | |
1208 if (tmp->rank() < res->rank()) { | |
1209 res = tmp; | |
1210 } | |
1211 } | |
1212 if (!SafepointSynchronize::is_at_safepoint()) { | |
1213 // In this case, we expect the held locks to be | |
1214 // in increasing rank order (modulo any native ranks) | |
1215 for (tmp = locks; tmp != NULL; tmp = tmp->next()) { | |
1216 if (tmp->next() != NULL) { | |
1217 assert(tmp->rank() == Mutex::native || | |
1218 tmp->rank() <= tmp->next()->rank(), "mutex rank anomaly?"); | |
1219 } | |
1220 } | |
1221 } | |
1222 return res; | |
1223 } | |
1224 | |
1225 Monitor* Monitor::get_least_ranked_lock_besides_this(Monitor* locks) { | |
1226 Monitor *res, *tmp; | |
1227 for (res = NULL, tmp = locks; tmp != NULL; tmp = tmp->next()) { | |
1228 if (tmp != this && (res == NULL || tmp->rank() < res->rank())) { | |
1229 res = tmp; | |
1230 } | |
1231 } | |
1232 if (!SafepointSynchronize::is_at_safepoint()) { | |
1233 // In this case, we expect the held locks to be | |
1234 // in increasing rank order (modulo any native ranks) | |
1235 for (tmp = locks; tmp != NULL; tmp = tmp->next()) { | |
1236 if (tmp->next() != NULL) { | |
1237 assert(tmp->rank() == Mutex::native || | |
1238 tmp->rank() <= tmp->next()->rank(), "mutex rank anomaly?"); | |
1239 } | |
1240 } | |
1241 } | |
1242 return res; | |
1243 } | |
1244 | |
1245 | |
1246 bool Monitor::contains(Monitor* locks, Monitor * lock) { | |
1247 for (; locks != NULL; locks = locks->next()) { | |
1248 if (locks == lock) | |
1249 return true; | |
1250 } | |
1251 return false; | |
1252 } | |
1253 #endif | |
1254 | |
1255 // Called immediately after lock acquisition or release as a diagnostic | |
1256 // to track the lock-set of the thread and test for rank violations that | |
1257 // might indicate exposure to deadlock. | |
1258 // Rather like an EventListener for _owner (:>). | |
1259 | |
1260 void Monitor::set_owner_implementation(Thread *new_owner) { | |
1261 // This function is solely responsible for maintaining | |
1262 // and checking the invariant that threads and locks | |
1263 // are in a 1/N relation, with some some locks unowned. | |
1264 // It uses the Mutex::_owner, Mutex::_next, and | |
1265 // Thread::_owned_locks fields, and no other function | |
1266 // changes those fields. | |
1267 // It is illegal to set the mutex from one non-NULL | |
1268 // owner to another--it must be owned by NULL as an | |
1269 // intermediate state. | |
1270 | |
1271 if (new_owner != NULL) { | |
1272 // the thread is acquiring this lock | |
1273 | |
1274 assert(new_owner == Thread::current(), "Should I be doing this?"); | |
1275 assert(_owner == NULL, "setting the owner thread of an already owned mutex"); | |
1276 _owner = new_owner; // set the owner | |
1277 | |
1278 // link "this" into the owned locks list | |
1279 | |
1280 #ifdef ASSERT // Thread::_owned_locks is under the same ifdef | |
1281 Monitor* locks = get_least_ranked_lock(new_owner->owned_locks()); | |
1282 // Mutex::set_owner_implementation is a friend of Thread | |
1283 | |
1284 assert(this->rank() >= 0, "bad lock rank"); | |
1285 | |
1286 // Deadlock avoidance rules require us to acquire Mutexes only in | |
1287 // a global total order. For example m1 is the lowest ranked mutex | |
1288 // that the thread holds and m2 is the mutex the thread is trying | |
1289 // to acquire, then deadlock avoidance rules require that the rank | |
1290 // of m2 be less than the rank of m1. | |
1291 // The rank Mutex::native is an exception in that it is not subject | |
1292 // to the verification rules. | |
1293 // Here are some further notes relating to mutex acquisition anomalies: | |
1294 // . under Solaris, the interrupt lock gets acquired when doing | |
1295 // profiling, so any lock could be held. | |
1296 // . it is also ok to acquire Safepoint_lock at the very end while we | |
1297 // already hold Terminator_lock - may happen because of periodic safepoints | |
1298 if (this->rank() != Mutex::native && | |
1299 this->rank() != Mutex::suspend_resume && | |
1300 locks != NULL && locks->rank() <= this->rank() && | |
1301 !SafepointSynchronize::is_at_safepoint() && | |
1302 this != Interrupt_lock && this != ProfileVM_lock && | |
1303 !(this == Safepoint_lock && contains(locks, Terminator_lock) && | |
1304 SafepointSynchronize::is_synchronizing())) { | |
1305 new_owner->print_owned_locks(); | |
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1306 fatal(err_msg("acquiring lock %s/%d out of order with lock %s/%d -- " |
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1307 "possible deadlock", this->name(), this->rank(), |
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1308 locks->name(), locks->rank())); |
0 | 1309 } |
1310 | |
1311 this->_next = new_owner->_owned_locks; | |
1312 new_owner->_owned_locks = this; | |
1313 #endif | |
1314 | |
1315 } else { | |
1316 // the thread is releasing this lock | |
1317 | |
1318 Thread* old_owner = _owner; | |
1319 debug_only(_last_owner = old_owner); | |
1320 | |
1321 assert(old_owner != NULL, "removing the owner thread of an unowned mutex"); | |
1322 assert(old_owner == Thread::current(), "removing the owner thread of an unowned mutex"); | |
1323 | |
1324 _owner = NULL; // set the owner | |
1325 | |
1326 #ifdef ASSERT | |
1327 Monitor *locks = old_owner->owned_locks(); | |
1328 | |
1329 // remove "this" from the owned locks list | |
1330 | |
1331 Monitor *prev = NULL; | |
1332 bool found = false; | |
1333 for (; locks != NULL; prev = locks, locks = locks->next()) { | |
1334 if (locks == this) { | |
1335 found = true; | |
1336 break; | |
1337 } | |
1338 } | |
1339 assert(found, "Removing a lock not owned"); | |
1340 if (prev == NULL) { | |
1341 old_owner->_owned_locks = _next; | |
1342 } else { | |
1343 prev->_next = _next; | |
1344 } | |
1345 _next = NULL; | |
1346 #endif | |
1347 } | |
1348 } | |
1349 | |
1350 | |
1351 // Factored out common sanity checks for locking mutex'es. Used by lock() and try_lock() | |
1352 void Monitor::check_prelock_state(Thread *thread) { | |
1353 assert((!thread->is_Java_thread() || ((JavaThread *)thread)->thread_state() == _thread_in_vm) | |
1354 || rank() == Mutex::special, "wrong thread state for using locks"); | |
1355 if (StrictSafepointChecks) { | |
1356 if (thread->is_VM_thread() && !allow_vm_block()) { | |
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1357 fatal(err_msg("VM thread using lock %s (not allowed to block on)", |
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1358 name())); |
0 | 1359 } |
1360 debug_only(if (rank() != Mutex::special) \ | |
1361 thread->check_for_valid_safepoint_state(false);) | |
1362 } | |
11151 | 1363 if (thread->is_Watcher_thread()) { |
1364 assert(!WatcherThread::watcher_thread()->has_crash_protection(), | |
1365 "locking not allowed when crash protection is set"); | |
1366 } | |
0 | 1367 } |
1368 | |
1369 void Monitor::check_block_state(Thread *thread) { | |
1370 if (!_allow_vm_block && thread->is_VM_thread()) { | |
1371 warning("VM thread blocked on lock"); | |
1372 print(); | |
1373 BREAKPOINT; | |
1374 } | |
1375 assert(_owner != thread, "deadlock: blocking on monitor owned by current thread"); | |
1376 } | |
1377 | |
1378 #endif // PRODUCT |