Mercurial > hg > truffle
annotate src/share/vm/runtime/mutex.cpp @ 21573:ffdd2ec86f42
Truffle/Debugging: temporary patches so the DebugEngine will work (partially) with the new TruffleVM framework.
author | Michael Van De Vanter <michael.van.de.vanter@oracle.com> |
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date | Wed, 27 May 2015 20:52:21 -0700 |
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0 | 1 |
2 /* | |
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3 * Copyright (c) 1998, 2014, 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" | |
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28 #include "runtime/orderAccess.inline.hpp" |
1972 | 29 #include "runtime/osThread.hpp" |
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30 #include "runtime/thread.inline.hpp" |
1972 | 31 #include "utilities/events.hpp" |
32 #ifdef TARGET_OS_FAMILY_linux | |
33 # include "mutex_linux.inline.hpp" | |
34 #endif | |
35 #ifdef TARGET_OS_FAMILY_solaris | |
36 # include "mutex_solaris.inline.hpp" | |
37 #endif | |
38 #ifdef TARGET_OS_FAMILY_windows | |
39 # include "mutex_windows.inline.hpp" | |
40 #endif | |
3960 | 41 #ifdef TARGET_OS_FAMILY_bsd |
42 # include "mutex_bsd.inline.hpp" | |
43 #endif | |
0 | 44 |
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45 PRAGMA_FORMAT_MUTE_WARNINGS_FOR_GCC |
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46 |
0 | 47 // o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o |
48 // | |
49 // Native Monitor-Mutex locking - theory of operations | |
50 // | |
51 // * Native Monitors are completely unrelated to Java-level monitors, | |
52 // although the "back-end" slow-path implementations share a common lineage. | |
53 // See objectMonitor:: in synchronizer.cpp. | |
54 // Native Monitors do *not* support nesting or recursion but otherwise | |
55 // they're basically Hoare-flavor monitors. | |
56 // | |
57 // * A thread acquires ownership of a Monitor/Mutex by CASing the LockByte | |
58 // in the _LockWord from zero to non-zero. Note that the _Owner field | |
59 // is advisory and is used only to verify that the thread calling unlock() | |
60 // is indeed the last thread to have acquired the lock. | |
61 // | |
62 // * Contending threads "push" themselves onto the front of the contention | |
63 // queue -- called the cxq -- with CAS and then spin/park. | |
64 // The _LockWord contains the LockByte as well as the pointer to the head | |
65 // of the cxq. Colocating the LockByte with the cxq precludes certain races. | |
66 // | |
67 // * Using a separately addressable LockByte allows for CAS:MEMBAR or CAS:0 | |
68 // idioms. We currently use MEMBAR in the uncontended unlock() path, as | |
69 // MEMBAR often has less latency than CAS. If warranted, we could switch to | |
70 // a CAS:0 mode, using timers to close the resultant race, as is done | |
71 // with Java Monitors in synchronizer.cpp. | |
72 // | |
73 // See the following for a discussion of the relative cost of atomics (CAS) | |
74 // MEMBAR, and ways to eliminate such instructions from the common-case paths: | |
75 // -- http://blogs.sun.com/dave/entry/biased_locking_in_hotspot | |
76 // -- http://blogs.sun.com/dave/resource/MustangSync.pdf | |
77 // -- http://blogs.sun.com/dave/resource/synchronization-public2.pdf | |
78 // -- synchronizer.cpp | |
79 // | |
80 // * Overall goals - desiderata | |
81 // 1. Minimize context switching | |
82 // 2. Minimize lock migration | |
83 // 3. Minimize CPI -- affinity and locality | |
84 // 4. Minimize the execution of high-latency instructions such as CAS or MEMBAR | |
85 // 5. Minimize outer lock hold times | |
86 // 6. Behave gracefully on a loaded system | |
87 // | |
88 // * Thread flow and list residency: | |
89 // | |
90 // Contention queue --> EntryList --> OnDeck --> Owner --> !Owner | |
91 // [..resident on monitor list..] | |
92 // [...........contending..................] | |
93 // | |
94 // -- The contention queue (cxq) contains recently-arrived threads (RATs). | |
95 // Threads on the cxq eventually drain into the EntryList. | |
96 // -- Invariant: a thread appears on at most one list -- cxq, EntryList | |
97 // or WaitSet -- at any one time. | |
98 // -- For a given monitor there can be at most one "OnDeck" thread at any | |
99 // given time but if needbe this particular invariant could be relaxed. | |
100 // | |
101 // * The WaitSet and EntryList linked lists are composed of ParkEvents. | |
102 // I use ParkEvent instead of threads as ParkEvents are immortal and | |
103 // type-stable, meaning we can safely unpark() a possibly stale | |
104 // list element in the unlock()-path. (That's benign). | |
105 // | |
106 // * Succession policy - providing for progress: | |
107 // | |
108 // As necessary, the unlock()ing thread identifies, unlinks, and unparks | |
109 // an "heir presumptive" tentative successor thread from the EntryList. | |
110 // This becomes the so-called "OnDeck" thread, of which there can be only | |
111 // one at any given time for a given monitor. The wakee will recontend | |
112 // for ownership of monitor. | |
113 // | |
114 // Succession is provided for by a policy of competitive handoff. | |
115 // The exiting thread does _not_ grant or pass ownership to the | |
116 // successor thread. (This is also referred to as "handoff" succession"). | |
117 // Instead the exiting thread releases ownership and possibly wakes | |
118 // a successor, so the successor can (re)compete for ownership of the lock. | |
119 // | |
120 // Competitive handoff provides excellent overall throughput at the expense | |
121 // of short-term fairness. If fairness is a concern then one remedy might | |
122 // be to add an AcquireCounter field to the monitor. After a thread acquires | |
123 // the lock it will decrement the AcquireCounter field. When the count | |
124 // reaches 0 the thread would reset the AcquireCounter variable, abdicate | |
125 // the lock directly to some thread on the EntryList, and then move itself to the | |
126 // tail of the EntryList. | |
127 // | |
128 // But in practice most threads engage or otherwise participate in resource | |
129 // bounded producer-consumer relationships, so lock domination is not usually | |
130 // a practical concern. Recall too, that in general it's easier to construct | |
131 // a fair lock from a fast lock, but not vice-versa. | |
132 // | |
133 // * The cxq can have multiple concurrent "pushers" but only one concurrent | |
134 // detaching thread. This mechanism is immune from the ABA corruption. | |
135 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious. | |
136 // We use OnDeck as a pseudo-lock to enforce the at-most-one detaching | |
137 // thread constraint. | |
138 // | |
139 // * Taken together, the cxq and the EntryList constitute or form a | |
140 // single logical queue of threads stalled trying to acquire the lock. | |
141 // We use two distinct lists to reduce heat on the list ends. | |
142 // Threads in lock() enqueue onto cxq while threads in unlock() will | |
143 // dequeue from the EntryList. (c.f. Michael Scott's "2Q" algorithm). | |
144 // A key desideratum is to minimize queue & monitor metadata manipulation | |
145 // that occurs while holding the "outer" monitor lock -- that is, we want to | |
146 // minimize monitor lock holds times. | |
147 // | |
148 // The EntryList is ordered by the prevailing queue discipline and | |
149 // can be organized in any convenient fashion, such as a doubly-linked list or | |
150 // a circular doubly-linked list. If we need a priority queue then something akin | |
151 // to Solaris' sleepq would work nicely. Viz., | |
152 // -- http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c. | |
153 // -- http://cvs.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/uts/common/os/sleepq.c | |
154 // Queue discipline is enforced at ::unlock() time, when the unlocking thread | |
155 // drains the cxq into the EntryList, and orders or reorders the threads on the | |
156 // EntryList accordingly. | |
157 // | |
158 // Barring "lock barging", this mechanism provides fair cyclic ordering, | |
159 // somewhat similar to an elevator-scan. | |
160 // | |
161 // * OnDeck | |
162 // -- For a given monitor there can be at most one OnDeck thread at any given | |
163 // instant. The OnDeck thread is contending for the lock, but has been | |
164 // unlinked from the EntryList and cxq by some previous unlock() operations. | |
165 // Once a thread has been designated the OnDeck thread it will remain so | |
166 // until it manages to acquire the lock -- being OnDeck is a stable property. | |
167 // -- Threads on the EntryList or cxq are _not allowed to attempt lock acquisition. | |
168 // -- OnDeck also serves as an "inner lock" as follows. Threads in unlock() will, after | |
169 // having cleared the LockByte and dropped the outer lock, attempt to "trylock" | |
170 // OnDeck by CASing the field from null to non-null. If successful, that thread | |
171 // is then responsible for progress and succession and can use CAS to detach and | |
172 // drain the cxq into the EntryList. By convention, only this thread, the holder of | |
173 // the OnDeck inner lock, can manipulate the EntryList or detach and drain the | |
174 // RATs on the cxq into the EntryList. This avoids ABA corruption on the cxq as | |
175 // we allow multiple concurrent "push" operations but restrict detach concurrency | |
176 // to at most one thread. Having selected and detached a successor, the thread then | |
177 // changes the OnDeck to refer to that successor, and then unparks the successor. | |
178 // That successor will eventually acquire the lock and clear OnDeck. Beware | |
179 // that the OnDeck usage as a lock is asymmetric. A thread in unlock() transiently | |
180 // "acquires" OnDeck, performs queue manipulations, passes OnDeck to some successor, | |
181 // and then the successor eventually "drops" OnDeck. Note that there's never | |
182 // any sense of contention on the inner lock, however. Threads never contend | |
183 // or wait for the inner lock. | |
184 // -- OnDeck provides for futile wakeup throttling a described in section 3.3 of | |
185 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf | |
186 // In a sense, OnDeck subsumes the ObjectMonitor _Succ and ObjectWaiter | |
187 // TState fields found in Java-level objectMonitors. (See synchronizer.cpp). | |
188 // | |
189 // * Waiting threads reside on the WaitSet list -- wait() puts | |
190 // the caller onto the WaitSet. Notify() or notifyAll() simply | |
191 // transfers threads from the WaitSet to either the EntryList or cxq. | |
192 // Subsequent unlock() operations will eventually unpark the notifyee. | |
193 // Unparking a notifee in notify() proper is inefficient - if we were to do so | |
194 // it's likely the notifyee would simply impale itself on the lock held | |
195 // by the notifier. | |
196 // | |
197 // * The mechanism is obstruction-free in that if the holder of the transient | |
198 // OnDeck lock in unlock() is preempted or otherwise stalls, other threads | |
199 // can still acquire and release the outer lock and continue to make progress. | |
200 // At worst, waking of already blocked contending threads may be delayed, | |
201 // but nothing worse. (We only use "trylock" operations on the inner OnDeck | |
202 // lock). | |
203 // | |
204 // * Note that thread-local storage must be initialized before a thread | |
205 // uses Native monitors or mutexes. The native monitor-mutex subsystem | |
206 // depends on Thread::current(). | |
207 // | |
208 // * The monitor synchronization subsystem avoids the use of native | |
209 // synchronization primitives except for the narrow platform-specific | |
210 // park-unpark abstraction. See the comments in os_solaris.cpp regarding | |
211 // the semantics of park-unpark. Put another way, this monitor implementation | |
212 // depends only on atomic operations and park-unpark. The monitor subsystem | |
213 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the | |
214 // underlying OS manages the READY<->RUN transitions. | |
215 // | |
216 // * The memory consistency model provide by lock()-unlock() is at least as | |
217 // strong or stronger than the Java Memory model defined by JSR-133. | |
218 // That is, we guarantee at least entry consistency, if not stronger. | |
219 // See http://g.oswego.edu/dl/jmm/cookbook.html. | |
220 // | |
221 // * Thread:: currently contains a set of purpose-specific ParkEvents: | |
222 // _MutexEvent, _ParkEvent, etc. A better approach might be to do away with | |
223 // the purpose-specific ParkEvents and instead implement a general per-thread | |
224 // stack of available ParkEvents which we could provision on-demand. The | |
225 // stack acts as a local cache to avoid excessive calls to ParkEvent::Allocate() | |
226 // and ::Release(). A thread would simply pop an element from the local stack before it | |
227 // enqueued or park()ed. When the contention was over the thread would | |
228 // push the no-longer-needed ParkEvent back onto its stack. | |
229 // | |
230 // * A slightly reduced form of ILock() and IUnlock() have been partially | |
231 // model-checked (Murphi) for safety and progress at T=1,2,3 and 4. | |
232 // It'd be interesting to see if TLA/TLC could be useful as well. | |
233 // | |
234 // * Mutex-Monitor is a low-level "leaf" subsystem. That is, the monitor | |
235 // code should never call other code in the JVM that might itself need to | |
236 // acquire monitors or mutexes. That's true *except* in the case of the | |
237 // ThreadBlockInVM state transition wrappers. The ThreadBlockInVM DTOR handles | |
238 // mutator reentry (ingress) by checking for a pending safepoint in which case it will | |
239 // call SafepointSynchronize::block(), which in turn may call Safepoint_lock->lock(), etc. | |
240 // In that particular case a call to lock() for a given Monitor can end up recursively | |
241 // calling lock() on another monitor. While distasteful, this is largely benign | |
242 // as the calls come from jacket that wraps lock(), and not from deep within lock() itself. | |
243 // | |
244 // It's unfortunate that native mutexes and thread state transitions were convolved. | |
245 // They're really separate concerns and should have remained that way. Melding | |
246 // them together was facile -- a bit too facile. The current implementation badly | |
247 // conflates the two concerns. | |
248 // | |
249 // * TODO-FIXME: | |
250 // | |
251 // -- Add DTRACE probes for contended acquire, contended acquired, contended unlock | |
252 // We should also add DTRACE probes in the ParkEvent subsystem for | |
253 // Park-entry, Park-exit, and Unpark. | |
254 // | |
255 // -- We have an excess of mutex-like constructs in the JVM, namely: | |
256 // 1. objectMonitors for Java-level synchronization (synchronizer.cpp) | |
257 // 2. low-level muxAcquire and muxRelease | |
258 // 3. low-level spinAcquire and spinRelease | |
259 // 4. native Mutex:: and Monitor:: | |
260 // 5. jvm_raw_lock() and _unlock() | |
261 // 6. JVMTI raw monitors -- distinct from (5) despite having a confusingly | |
262 // similar name. | |
263 // | |
264 // o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o | |
265 | |
266 | |
267 // CASPTR() uses the canonical argument order that dominates in the literature. | |
268 // Our internal cmpxchg_ptr() uses a bastardized ordering to accommodate Sun .il templates. | |
269 | |
270 #define CASPTR(a,c,s) intptr_t(Atomic::cmpxchg_ptr ((void *)(s),(void *)(a),(void *)(c))) | |
271 #define UNS(x) (uintptr_t(x)) | |
272 #define TRACE(m) { static volatile int ctr = 0 ; int x = ++ctr ; if ((x & (x-1))==0) { ::printf ("%d:%s\n", x, #m); ::fflush(stdout); }} | |
273 | |
274 // Simplistic low-quality Marsaglia SHIFT-XOR RNG. | |
275 // Bijective except for the trailing mask operation. | |
276 // Useful for spin loops as the compiler can't optimize it away. | |
277 | |
278 static inline jint MarsagliaXORV (jint x) { | |
279 if (x == 0) x = 1|os::random() ; | |
280 x ^= x << 6; | |
281 x ^= ((unsigned)x) >> 21; | |
282 x ^= x << 7 ; | |
283 return x & 0x7FFFFFFF ; | |
284 } | |
285 | |
286 static int Stall (int its) { | |
287 static volatile jint rv = 1 ; | |
288 volatile int OnFrame = 0 ; | |
289 jint v = rv ^ UNS(OnFrame) ; | |
290 while (--its >= 0) { | |
291 v = MarsagliaXORV (v) ; | |
292 } | |
293 // Make this impossible for the compiler to optimize away, | |
294 // but (mostly) avoid W coherency sharing on MP systems. | |
295 if (v == 0x12345) rv = v ; | |
296 return v ; | |
297 } | |
298 | |
299 int Monitor::TryLock () { | |
300 intptr_t v = _LockWord.FullWord ; | |
301 for (;;) { | |
302 if ((v & _LBIT) != 0) return 0 ; | |
303 const intptr_t u = CASPTR (&_LockWord, v, v|_LBIT) ; | |
304 if (v == u) return 1 ; | |
305 v = u ; | |
306 } | |
307 } | |
308 | |
309 int Monitor::TryFast () { | |
310 // Optimistic fast-path form ... | |
311 // Fast-path attempt for the common uncontended case. | |
312 // Avoid RTS->RTO $ coherence upgrade on typical SMP systems. | |
313 intptr_t v = CASPTR (&_LockWord, 0, _LBIT) ; // agro ... | |
314 if (v == 0) return 1 ; | |
315 | |
316 for (;;) { | |
317 if ((v & _LBIT) != 0) return 0 ; | |
318 const intptr_t u = CASPTR (&_LockWord, v, v|_LBIT) ; | |
319 if (v == u) return 1 ; | |
320 v = u ; | |
321 } | |
322 } | |
323 | |
324 int Monitor::ILocked () { | |
325 const intptr_t w = _LockWord.FullWord & 0xFF ; | |
326 assert (w == 0 || w == _LBIT, "invariant") ; | |
327 return w == _LBIT ; | |
328 } | |
329 | |
330 // Polite TATAS spinlock with exponential backoff - bounded spin. | |
331 // Ideally we'd use processor cycles, time or vtime to control | |
332 // the loop, but we currently use iterations. | |
333 // All the constants within were derived empirically but work over | |
334 // over the spectrum of J2SE reference platforms. | |
335 // On Niagara-class systems the back-off is unnecessary but | |
336 // is relatively harmless. (At worst it'll slightly retard | |
337 // acquisition times). The back-off is critical for older SMP systems | |
338 // where constant fetching of the LockWord would otherwise impair | |
339 // scalability. | |
340 // | |
341 // Clamp spinning at approximately 1/2 of a context-switch round-trip. | |
342 // See synchronizer.cpp for details and rationale. | |
343 | |
344 int Monitor::TrySpin (Thread * const Self) { | |
345 if (TryLock()) return 1 ; | |
346 if (!os::is_MP()) return 0 ; | |
347 | |
348 int Probes = 0 ; | |
349 int Delay = 0 ; | |
350 int Steps = 0 ; | |
351 int SpinMax = NativeMonitorSpinLimit ; | |
352 int flgs = NativeMonitorFlags ; | |
353 for (;;) { | |
354 intptr_t v = _LockWord.FullWord; | |
355 if ((v & _LBIT) == 0) { | |
356 if (CASPTR (&_LockWord, v, v|_LBIT) == v) { | |
357 return 1 ; | |
358 } | |
359 continue ; | |
360 } | |
361 | |
362 if ((flgs & 8) == 0) { | |
363 SpinPause () ; | |
364 } | |
365 | |
366 // Periodically increase Delay -- variable Delay form | |
367 // conceptually: delay *= 1 + 1/Exponent | |
368 ++ Probes; | |
369 if (Probes > SpinMax) return 0 ; | |
370 | |
371 if ((Probes & 0x7) == 0) { | |
372 Delay = ((Delay << 1)|1) & 0x7FF ; | |
373 // CONSIDER: Delay += 1 + (Delay/4); Delay &= 0x7FF ; | |
374 } | |
375 | |
376 if (flgs & 2) continue ; | |
377 | |
378 // Consider checking _owner's schedctl state, if OFFPROC abort spin. | |
379 // If the owner is OFFPROC then it's unlike that the lock will be dropped | |
380 // in a timely fashion, which suggests that spinning would not be fruitful | |
381 // or profitable. | |
382 | |
383 // Stall for "Delay" time units - iterations in the current implementation. | |
384 // Avoid generating coherency traffic while stalled. | |
385 // Possible ways to delay: | |
386 // PAUSE, SLEEP, MEMBAR #sync, MEMBAR #halt, | |
387 // wr %g0,%asi, gethrtime, rdstick, rdtick, rdtsc, etc. ... | |
388 // Note that on Niagara-class systems we want to minimize STs in the | |
389 // spin loop. N1 and brethren write-around the L1$ over the xbar into the L2$. | |
390 // Furthermore, they don't have a W$ like traditional SPARC processors. | |
391 // We currently use a Marsaglia Shift-Xor RNG loop. | |
392 Steps += Delay ; | |
393 if (Self != NULL) { | |
394 jint rv = Self->rng[0] ; | |
395 for (int k = Delay ; --k >= 0; ) { | |
396 rv = MarsagliaXORV (rv) ; | |
397 if ((flgs & 4) == 0 && SafepointSynchronize::do_call_back()) return 0 ; | |
398 } | |
399 Self->rng[0] = rv ; | |
400 } else { | |
401 Stall (Delay) ; | |
402 } | |
403 } | |
404 } | |
405 | |
406 static int ParkCommon (ParkEvent * ev, jlong timo) { | |
407 // Diagnostic support - periodically unwedge blocked threads | |
408 intx nmt = NativeMonitorTimeout ; | |
409 if (nmt > 0 && (nmt < timo || timo <= 0)) { | |
410 timo = nmt ; | |
411 } | |
412 int err = OS_OK ; | |
413 if (0 == timo) { | |
414 ev->park() ; | |
415 } else { | |
416 err = ev->park(timo) ; | |
417 } | |
418 return err ; | |
419 } | |
420 | |
421 inline int Monitor::AcquireOrPush (ParkEvent * ESelf) { | |
422 intptr_t v = _LockWord.FullWord ; | |
423 for (;;) { | |
424 if ((v & _LBIT) == 0) { | |
425 const intptr_t u = CASPTR (&_LockWord, v, v|_LBIT) ; | |
426 if (u == v) return 1 ; // indicate acquired | |
427 v = u ; | |
428 } else { | |
429 // Anticipate success ... | |
430 ESelf->ListNext = (ParkEvent *) (v & ~_LBIT) ; | |
431 const intptr_t u = CASPTR (&_LockWord, v, intptr_t(ESelf)|_LBIT) ; | |
432 if (u == v) return 0 ; // indicate pushed onto cxq | |
433 v = u ; | |
434 } | |
435 // Interference - LockWord change - just retry | |
436 } | |
437 } | |
438 | |
439 // ILock and IWait are the lowest level primitive internal blocking | |
440 // synchronization functions. The callers of IWait and ILock must have | |
441 // performed any needed state transitions beforehand. | |
442 // IWait and ILock may directly call park() without any concern for thread state. | |
443 // Note that ILock and IWait do *not* access _owner. | |
444 // _owner is a higher-level logical concept. | |
445 | |
446 void Monitor::ILock (Thread * Self) { | |
447 assert (_OnDeck != Self->_MutexEvent, "invariant") ; | |
448 | |
449 if (TryFast()) { | |
450 Exeunt: | |
451 assert (ILocked(), "invariant") ; | |
452 return ; | |
453 } | |
454 | |
455 ParkEvent * const ESelf = Self->_MutexEvent ; | |
456 assert (_OnDeck != ESelf, "invariant") ; | |
457 | |
458 // As an optimization, spinners could conditionally try to set ONDECK to _LBIT | |
459 // Synchronizer.cpp uses a similar optimization. | |
460 if (TrySpin (Self)) goto Exeunt ; | |
461 | |
462 // Slow-path - the lock is contended. | |
463 // Either Enqueue Self on cxq or acquire the outer lock. | |
464 // LockWord encoding = (cxq,LOCKBYTE) | |
465 ESelf->reset() ; | |
466 OrderAccess::fence() ; | |
467 | |
468 // Optional optimization ... try barging on the inner lock | |
469 if ((NativeMonitorFlags & 32) && CASPTR (&_OnDeck, NULL, UNS(Self)) == 0) { | |
470 goto OnDeck_LOOP ; | |
471 } | |
472 | |
473 if (AcquireOrPush (ESelf)) goto Exeunt ; | |
474 | |
475 // At any given time there is at most one ondeck thread. | |
476 // ondeck implies not resident on cxq and not resident on EntryList | |
477 // Only the OnDeck thread can try to acquire -- contended for -- the lock. | |
478 // CONSIDER: use Self->OnDeck instead of m->OnDeck. | |
479 // Deschedule Self so that others may run. | |
480 while (_OnDeck != ESelf) { | |
481 ParkCommon (ESelf, 0) ; | |
482 } | |
483 | |
484 // Self is now in the ONDECK position and will remain so until it | |
485 // manages to acquire the lock. | |
486 OnDeck_LOOP: | |
487 for (;;) { | |
488 assert (_OnDeck == ESelf, "invariant") ; | |
489 if (TrySpin (Self)) break ; | |
490 // CONSIDER: if ESelf->TryPark() && TryLock() break ... | |
491 // It's probably wise to spin only if we *actually* blocked | |
492 // CONSIDER: check the lockbyte, if it remains set then | |
493 // preemptively drain the cxq into the EntryList. | |
494 // The best place and time to perform queue operations -- lock metadata -- | |
495 // is _before having acquired the outer lock, while waiting for the lock to drop. | |
496 ParkCommon (ESelf, 0) ; | |
497 } | |
498 | |
499 assert (_OnDeck == ESelf, "invariant") ; | |
500 _OnDeck = NULL ; | |
501 | |
502 // Note that we current drop the inner lock (clear OnDeck) in the slow-path | |
503 // epilog immediately after having acquired the outer lock. | |
504 // But instead we could consider the following optimizations: | |
505 // A. Shift or defer dropping the inner lock until the subsequent IUnlock() operation. | |
506 // This might avoid potential reacquisition of the inner lock in IUlock(). | |
507 // B. While still holding the inner lock, attempt to opportunistically select | |
508 // and unlink the next ONDECK thread from the EntryList. | |
509 // If successful, set ONDECK to refer to that thread, otherwise clear ONDECK. | |
510 // It's critical that the select-and-unlink operation run in constant-time as | |
511 // it executes when holding the outer lock and may artificially increase the | |
512 // effective length of the critical section. | |
513 // Note that (A) and (B) are tantamount to succession by direct handoff for | |
514 // the inner lock. | |
515 goto Exeunt ; | |
516 } | |
517 | |
518 void Monitor::IUnlock (bool RelaxAssert) { | |
519 assert (ILocked(), "invariant") ; | |
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520 // Conceptually we need a MEMBAR #storestore|#loadstore barrier or fence immediately |
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521 // before the store that releases the lock. Crucially, all the stores and loads in the |
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522 // critical section must be globally visible before the store of 0 into the lock-word |
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523 // that releases the lock becomes globally visible. That is, memory accesses in the |
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524 // critical section should not be allowed to bypass or overtake the following ST that |
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525 // releases the lock. As such, to prevent accesses within the critical section |
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526 // from "leaking" out, we need a release fence between the critical section and the |
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527 // store that releases the lock. In practice that release barrier is elided on |
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528 // platforms with strong memory models such as TSO. |
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529 // |
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530 // Note that the OrderAccess::storeload() fence that appears after unlock store |
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531 // provides for progress conditions and succession and is _not related to exclusion |
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532 // safety or lock release consistency. |
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533 OrderAccess::release_store(&_LockWord.Bytes[_LSBINDEX], 0); // drop outer lock |
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534 |
0 | 535 OrderAccess::storeload (); |
536 ParkEvent * const w = _OnDeck ; | |
537 assert (RelaxAssert || w != Thread::current()->_MutexEvent, "invariant") ; | |
538 if (w != NULL) { | |
539 // Either we have a valid ondeck thread or ondeck is transiently "locked" | |
540 // by some exiting thread as it arranges for succession. The LSBit of | |
541 // OnDeck allows us to discriminate two cases. If the latter, the | |
542 // responsibility for progress and succession lies with that other thread. | |
543 // For good performance, we also depend on the fact that redundant unpark() | |
544 // operations are cheap. That is, repeated Unpark()ing of the ONDECK thread | |
545 // is inexpensive. This approach provides implicit futile wakeup throttling. | |
546 // Note that the referent "w" might be stale with respect to the lock. | |
547 // In that case the following unpark() is harmless and the worst that'll happen | |
548 // is a spurious return from a park() operation. Critically, if "w" _is stale, | |
549 // then progress is known to have occurred as that means the thread associated | |
550 // with "w" acquired the lock. In that case this thread need take no further | |
551 // action to guarantee progress. | |
552 if ((UNS(w) & _LBIT) == 0) w->unpark() ; | |
553 return ; | |
554 } | |
555 | |
556 intptr_t cxq = _LockWord.FullWord ; | |
557 if (((cxq & ~_LBIT)|UNS(_EntryList)) == 0) { | |
558 return ; // normal fast-path exit - cxq and EntryList both empty | |
559 } | |
560 if (cxq & _LBIT) { | |
561 // Optional optimization ... | |
562 // Some other thread acquired the lock in the window since this | |
563 // thread released it. Succession is now that thread's responsibility. | |
564 return ; | |
565 } | |
566 | |
567 Succession: | |
568 // Slow-path exit - this thread must ensure succession and progress. | |
569 // OnDeck serves as lock to protect cxq and EntryList. | |
570 // Only the holder of OnDeck can manipulate EntryList or detach the RATs from cxq. | |
571 // Avoid ABA - allow multiple concurrent producers (enqueue via push-CAS) | |
572 // but only one concurrent consumer (detacher of RATs). | |
573 // Consider protecting this critical section with schedctl on Solaris. | |
574 // Unlike a normal lock, however, the exiting thread "locks" OnDeck, | |
575 // picks a successor and marks that thread as OnDeck. That successor | |
576 // thread will then clear OnDeck once it eventually acquires the outer lock. | |
577 if (CASPTR (&_OnDeck, NULL, _LBIT) != UNS(NULL)) { | |
578 return ; | |
579 } | |
580 | |
581 ParkEvent * List = _EntryList ; | |
582 if (List != NULL) { | |
583 // Transfer the head of the EntryList to the OnDeck position. | |
584 // Once OnDeck, a thread stays OnDeck until it acquires the lock. | |
585 // For a given lock there is at most OnDeck thread at any one instant. | |
586 WakeOne: | |
587 assert (List == _EntryList, "invariant") ; | |
588 ParkEvent * const w = List ; | |
589 assert (RelaxAssert || w != Thread::current()->_MutexEvent, "invariant") ; | |
590 _EntryList = w->ListNext ; | |
591 // as a diagnostic measure consider setting w->_ListNext = BAD | |
592 assert (UNS(_OnDeck) == _LBIT, "invariant") ; | |
593 _OnDeck = w ; // pass OnDeck to w. | |
594 // w will clear OnDeck once it acquires the outer lock | |
595 | |
596 // Another optional optimization ... | |
597 // For heavily contended locks it's not uncommon that some other | |
598 // thread acquired the lock while this thread was arranging succession. | |
599 // Try to defer the unpark() operation - Delegate the responsibility | |
600 // for unpark()ing the OnDeck thread to the current or subsequent owners | |
601 // That is, the new owner is responsible for unparking the OnDeck thread. | |
602 OrderAccess::storeload() ; | |
603 cxq = _LockWord.FullWord ; | |
604 if (cxq & _LBIT) return ; | |
605 | |
606 w->unpark() ; | |
607 return ; | |
608 } | |
609 | |
610 cxq = _LockWord.FullWord ; | |
611 if ((cxq & ~_LBIT) != 0) { | |
612 // The EntryList is empty but the cxq is populated. | |
613 // drain RATs from cxq into EntryList | |
614 // Detach RATs segment with CAS and then merge into EntryList | |
615 for (;;) { | |
616 // optional optimization - if locked, the owner is responsible for succession | |
617 if (cxq & _LBIT) goto Punt ; | |
618 const intptr_t vfy = CASPTR (&_LockWord, cxq, cxq & _LBIT) ; | |
619 if (vfy == cxq) break ; | |
620 cxq = vfy ; | |
621 // Interference - LockWord changed - Just retry | |
622 // We can see concurrent interference from contending threads | |
623 // pushing themselves onto the cxq or from lock-unlock operations. | |
624 // From the perspective of this thread, EntryList is stable and | |
625 // the cxq is prepend-only -- the head is volatile but the interior | |
626 // of the cxq is stable. In theory if we encounter interference from threads | |
627 // pushing onto cxq we could simply break off the original cxq suffix and | |
628 // move that segment to the EntryList, avoiding a 2nd or multiple CAS attempts | |
629 // on the high-traffic LockWord variable. For instance lets say the cxq is "ABCD" | |
630 // when we first fetch cxq above. Between the fetch -- where we observed "A" | |
631 // -- and CAS -- where we attempt to CAS null over A -- "PQR" arrive, | |
632 // yielding cxq = "PQRABCD". In this case we could simply set A.ListNext | |
633 // null, leaving cxq = "PQRA" and transfer the "BCD" segment to the EntryList. | |
634 // Note too, that it's safe for this thread to traverse the cxq | |
635 // without taking any special concurrency precautions. | |
636 } | |
637 | |
638 // We don't currently reorder the cxq segment as we move it onto | |
639 // the EntryList, but it might make sense to reverse the order | |
640 // or perhaps sort by thread priority. See the comments in | |
641 // synchronizer.cpp objectMonitor::exit(). | |
642 assert (_EntryList == NULL, "invariant") ; | |
643 _EntryList = List = (ParkEvent *)(cxq & ~_LBIT) ; | |
644 assert (List != NULL, "invariant") ; | |
645 goto WakeOne ; | |
646 } | |
647 | |
648 // cxq|EntryList is empty. | |
649 // w == NULL implies that cxq|EntryList == NULL in the past. | |
650 // Possible race - rare inopportune interleaving. | |
651 // A thread could have added itself to cxq since this thread previously checked. | |
652 // Detect and recover by refetching cxq. | |
653 Punt: | |
654 assert (UNS(_OnDeck) == _LBIT, "invariant") ; | |
655 _OnDeck = NULL ; // Release inner lock. | |
656 OrderAccess::storeload(); // Dekker duality - pivot point | |
657 | |
658 // Resample LockWord/cxq to recover from possible race. | |
659 // For instance, while this thread T1 held OnDeck, some other thread T2 might | |
660 // acquire the outer lock. Another thread T3 might try to acquire the outer | |
661 // lock, but encounter contention and enqueue itself on cxq. T2 then drops the | |
662 // outer lock, but skips succession as this thread T1 still holds OnDeck. | |
663 // T1 is and remains responsible for ensuring succession of T3. | |
664 // | |
665 // Note that we don't need to recheck EntryList, just cxq. | |
666 // If threads moved onto EntryList since we dropped OnDeck | |
667 // that implies some other thread forced succession. | |
668 cxq = _LockWord.FullWord ; | |
669 if ((cxq & ~_LBIT) != 0 && (cxq & _LBIT) == 0) { | |
670 goto Succession ; // potential race -- re-run succession | |
671 } | |
672 return ; | |
673 } | |
674 | |
675 bool Monitor::notify() { | |
676 assert (_owner == Thread::current(), "invariant") ; | |
677 assert (ILocked(), "invariant") ; | |
678 if (_WaitSet == NULL) return true ; | |
679 NotifyCount ++ ; | |
680 | |
681 // Transfer one thread from the WaitSet to the EntryList or cxq. | |
682 // Currently we just unlink the head of the WaitSet and prepend to the cxq. | |
683 // And of course we could just unlink it and unpark it, too, but | |
684 // in that case it'd likely impale itself on the reentry. | |
685 Thread::muxAcquire (_WaitLock, "notify:WaitLock") ; | |
686 ParkEvent * nfy = _WaitSet ; | |
687 if (nfy != NULL) { // DCL idiom | |
688 _WaitSet = nfy->ListNext ; | |
689 assert (nfy->Notified == 0, "invariant") ; | |
690 // push nfy onto the cxq | |
691 for (;;) { | |
692 const intptr_t v = _LockWord.FullWord ; | |
693 assert ((v & 0xFF) == _LBIT, "invariant") ; | |
694 nfy->ListNext = (ParkEvent *)(v & ~_LBIT); | |
695 if (CASPTR (&_LockWord, v, UNS(nfy)|_LBIT) == v) break; | |
696 // interference - _LockWord changed -- just retry | |
697 } | |
698 // Note that setting Notified before pushing nfy onto the cxq is | |
699 // also legal and safe, but the safety properties are much more | |
700 // subtle, so for the sake of code stewardship ... | |
701 OrderAccess::fence() ; | |
702 nfy->Notified = 1; | |
703 } | |
704 Thread::muxRelease (_WaitLock) ; | |
705 if (nfy != NULL && (NativeMonitorFlags & 16)) { | |
706 // Experimental code ... light up the wakee in the hope that this thread (the owner) | |
707 // will drop the lock just about the time the wakee comes ONPROC. | |
708 nfy->unpark() ; | |
709 } | |
710 assert (ILocked(), "invariant") ; | |
711 return true ; | |
712 } | |
713 | |
714 // Currently notifyAll() transfers the waiters one-at-a-time from the waitset | |
715 // to the cxq. This could be done more efficiently with a single bulk en-mass transfer, | |
716 // but in practice notifyAll() for large #s of threads is rare and not time-critical. | |
717 // Beware too, that we invert the order of the waiters. Lets say that the | |
718 // waitset is "ABCD" and the cxq is "XYZ". After a notifyAll() the waitset | |
719 // will be empty and the cxq will be "DCBAXYZ". This is benign, of course. | |
720 | |
721 bool Monitor::notify_all() { | |
722 assert (_owner == Thread::current(), "invariant") ; | |
723 assert (ILocked(), "invariant") ; | |
724 while (_WaitSet != NULL) notify() ; | |
725 return true ; | |
726 } | |
727 | |
728 int Monitor::IWait (Thread * Self, jlong timo) { | |
729 assert (ILocked(), "invariant") ; | |
730 | |
731 // Phases: | |
732 // 1. Enqueue Self on WaitSet - currently prepend | |
733 // 2. unlock - drop the outer lock | |
734 // 3. wait for either notification or timeout | |
735 // 4. lock - reentry - reacquire the outer lock | |
736 | |
737 ParkEvent * const ESelf = Self->_MutexEvent ; | |
738 ESelf->Notified = 0 ; | |
739 ESelf->reset() ; | |
740 OrderAccess::fence() ; | |
741 | |
742 // Add Self to WaitSet | |
743 // Ideally only the holder of the outer lock would manipulate the WaitSet - | |
744 // That is, the outer lock would implicitly protect the WaitSet. | |
745 // But if a thread in wait() encounters a timeout it will need to dequeue itself | |
746 // from the WaitSet _before it becomes the owner of the lock. We need to dequeue | |
747 // as the ParkEvent -- which serves as a proxy for the thread -- can't reside | |
748 // on both the WaitSet and the EntryList|cxq at the same time.. That is, a thread | |
749 // on the WaitSet can't be allowed to compete for the lock until it has managed to | |
750 // unlink its ParkEvent from WaitSet. Thus the need for WaitLock. | |
751 // Contention on the WaitLock is minimal. | |
752 // | |
753 // Another viable approach would be add another ParkEvent, "WaitEvent" to the | |
754 // thread class. The WaitSet would be composed of WaitEvents. Only the | |
755 // owner of the outer lock would manipulate the WaitSet. A thread in wait() | |
756 // could then compete for the outer lock, and then, if necessary, unlink itself | |
757 // from the WaitSet only after having acquired the outer lock. More precisely, | |
758 // there would be no WaitLock. A thread in in wait() would enqueue its WaitEvent | |
759 // on the WaitSet; release the outer lock; wait for either notification or timeout; | |
760 // reacquire the inner lock; and then, if needed, unlink itself from the WaitSet. | |
761 // | |
762 // Alternatively, a 2nd set of list link fields in the ParkEvent might suffice. | |
763 // One set would be for the WaitSet and one for the EntryList. | |
764 // We could also deconstruct the ParkEvent into a "pure" event and add a | |
765 // new immortal/TSM "ListElement" class that referred to ParkEvents. | |
766 // In that case we could have one ListElement on the WaitSet and another | |
767 // on the EntryList, with both referring to the same pure Event. | |
768 | |
769 Thread::muxAcquire (_WaitLock, "wait:WaitLock:Add") ; | |
770 ESelf->ListNext = _WaitSet ; | |
771 _WaitSet = ESelf ; | |
772 Thread::muxRelease (_WaitLock) ; | |
773 | |
774 // Release the outer lock | |
775 // We call IUnlock (RelaxAssert=true) as a thread T1 might | |
776 // enqueue itself on the WaitSet, call IUnlock(), drop the lock, | |
777 // and then stall before it can attempt to wake a successor. | |
778 // Some other thread T2 acquires the lock, and calls notify(), moving | |
779 // T1 from the WaitSet to the cxq. T2 then drops the lock. T1 resumes, | |
780 // and then finds *itself* on the cxq. During the course of a normal | |
781 // IUnlock() call a thread should _never find itself on the EntryList | |
782 // or cxq, but in the case of wait() it's possible. | |
783 // See synchronizer.cpp objectMonitor::wait(). | |
784 IUnlock (true) ; | |
785 | |
786 // Wait for either notification or timeout | |
787 // Beware that in some circumstances we might propagate | |
788 // spurious wakeups back to the caller. | |
789 | |
790 for (;;) { | |
791 if (ESelf->Notified) break ; | |
792 int err = ParkCommon (ESelf, timo) ; | |
793 if (err == OS_TIMEOUT || (NativeMonitorFlags & 1)) break ; | |
794 } | |
795 | |
796 // Prepare for reentry - if necessary, remove ESelf from WaitSet | |
797 // ESelf can be: | |
798 // 1. Still on the WaitSet. This can happen if we exited the loop by timeout. | |
799 // 2. On the cxq or EntryList | |
800 // 3. Not resident on cxq, EntryList or WaitSet, but in the OnDeck position. | |
801 | |
802 OrderAccess::fence() ; | |
803 int WasOnWaitSet = 0 ; | |
804 if (ESelf->Notified == 0) { | |
805 Thread::muxAcquire (_WaitLock, "wait:WaitLock:remove") ; | |
806 if (ESelf->Notified == 0) { // DCL idiom | |
807 assert (_OnDeck != ESelf, "invariant") ; // can't be both OnDeck and on WaitSet | |
808 // ESelf is resident on the WaitSet -- unlink it. | |
809 // A doubly-linked list would be better here so we can unlink in constant-time. | |
810 // We have to unlink before we potentially recontend as ESelf might otherwise | |
811 // end up on the cxq|EntryList -- it can't be on two lists at once. | |
812 ParkEvent * p = _WaitSet ; | |
813 ParkEvent * q = NULL ; // classic q chases p | |
814 while (p != NULL && p != ESelf) { | |
815 q = p ; | |
816 p = p->ListNext ; | |
817 } | |
818 assert (p == ESelf, "invariant") ; | |
819 if (p == _WaitSet) { // found at head | |
820 assert (q == NULL, "invariant") ; | |
821 _WaitSet = p->ListNext ; | |
822 } else { // found in interior | |
823 assert (q->ListNext == p, "invariant") ; | |
824 q->ListNext = p->ListNext ; | |
825 } | |
826 WasOnWaitSet = 1 ; // We were *not* notified but instead encountered timeout | |
827 } | |
828 Thread::muxRelease (_WaitLock) ; | |
829 } | |
830 | |
831 // Reentry phase - reacquire the lock | |
832 if (WasOnWaitSet) { | |
833 // ESelf was previously on the WaitSet but we just unlinked it above | |
834 // because of a timeout. ESelf is not resident on any list and is not OnDeck | |
835 assert (_OnDeck != ESelf, "invariant") ; | |
836 ILock (Self) ; | |
837 } else { | |
838 // A prior notify() operation moved ESelf from the WaitSet to the cxq. | |
839 // ESelf is now on the cxq, EntryList or at the OnDeck position. | |
840 // The following fragment is extracted from Monitor::ILock() | |
841 for (;;) { | |
842 if (_OnDeck == ESelf && TrySpin(Self)) break ; | |
843 ParkCommon (ESelf, 0) ; | |
844 } | |
845 assert (_OnDeck == ESelf, "invariant") ; | |
846 _OnDeck = NULL ; | |
847 } | |
848 | |
849 assert (ILocked(), "invariant") ; | |
850 return WasOnWaitSet != 0 ; // return true IFF timeout | |
851 } | |
852 | |
853 | |
854 // ON THE VMTHREAD SNEAKING PAST HELD LOCKS: | |
855 // In particular, there are certain types of global lock that may be held | |
856 // by a Java thread while it is blocked at a safepoint but before it has | |
857 // written the _owner field. These locks may be sneakily acquired by the | |
858 // VM thread during a safepoint to avoid deadlocks. Alternatively, one should | |
859 // identify all such locks, and ensure that Java threads never block at | |
860 // safepoints while holding them (_no_safepoint_check_flag). While it | |
861 // seems as though this could increase the time to reach a safepoint | |
862 // (or at least increase the mean, if not the variance), the latter | |
863 // approach might make for a cleaner, more maintainable JVM design. | |
864 // | |
865 // Sneaking is vile and reprehensible and should be excised at the 1st | |
866 // opportunity. It's possible that the need for sneaking could be obviated | |
867 // as follows. Currently, a thread might (a) while TBIVM, call pthread_mutex_lock | |
868 // or ILock() thus acquiring the "physical" lock underlying Monitor/Mutex. | |
869 // (b) stall at the TBIVM exit point as a safepoint is in effect. Critically, | |
870 // it'll stall at the TBIVM reentry state transition after having acquired the | |
871 // underlying lock, but before having set _owner and having entered the actual | |
872 // critical section. The lock-sneaking facility leverages that fact and allowed the | |
873 // VM thread to logically acquire locks that had already be physically locked by mutators | |
874 // but where mutators were known blocked by the reentry thread state transition. | |
875 // | |
876 // If we were to modify the Monitor-Mutex so that TBIVM state transitions tightly | |
877 // wrapped calls to park(), then we could likely do away with sneaking. We'd | |
878 // decouple lock acquisition and parking. The critical invariant to eliminating | |
879 // sneaking is to ensure that we never "physically" acquire the lock while TBIVM. | |
880 // An easy way to accomplish this is to wrap the park calls in a narrow TBIVM jacket. | |
881 // One difficulty with this approach is that the TBIVM wrapper could recurse and | |
882 // call lock() deep from within a lock() call, while the MutexEvent was already enqueued. | |
883 // Using a stack (N=2 at minimum) of ParkEvents would take care of that problem. | |
884 // | |
885 // But of course the proper ultimate approach is to avoid schemes that require explicit | |
886 // sneaking or dependence on any any clever invariants or subtle implementation properties | |
887 // of Mutex-Monitor and instead directly address the underlying design flaw. | |
888 | |
889 void Monitor::lock (Thread * Self) { | |
890 #ifdef CHECK_UNHANDLED_OOPS | |
891 // Clear unhandled oops so we get a crash right away. Only clear for non-vm | |
892 // or GC threads. | |
893 if (Self->is_Java_thread()) { | |
894 Self->clear_unhandled_oops(); | |
895 } | |
896 #endif // CHECK_UNHANDLED_OOPS | |
897 | |
898 debug_only(check_prelock_state(Self)); | |
899 assert (_owner != Self , "invariant") ; | |
900 assert (_OnDeck != Self->_MutexEvent, "invariant") ; | |
901 | |
902 if (TryFast()) { | |
903 Exeunt: | |
904 assert (ILocked(), "invariant") ; | |
905 assert (owner() == NULL, "invariant"); | |
906 set_owner (Self); | |
907 return ; | |
908 } | |
909 | |
910 // The lock is contended ... | |
911 | |
912 bool can_sneak = Self->is_VM_thread() && SafepointSynchronize::is_at_safepoint(); | |
913 if (can_sneak && _owner == NULL) { | |
914 // a java thread has locked the lock but has not entered the | |
915 // critical region -- let's just pretend we've locked the lock | |
916 // and go on. we note this with _snuck so we can also | |
917 // pretend to unlock when the time comes. | |
918 _snuck = true; | |
919 goto Exeunt ; | |
920 } | |
921 | |
922 // Try a brief spin to avoid passing thru thread state transition ... | |
923 if (TrySpin (Self)) goto Exeunt ; | |
924 | |
925 check_block_state(Self); | |
926 if (Self->is_Java_thread()) { | |
927 // Horribile dictu - we suffer through a state transition | |
928 assert(rank() > Mutex::special, "Potential deadlock with special or lesser rank mutex"); | |
929 ThreadBlockInVM tbivm ((JavaThread *) Self) ; | |
930 ILock (Self) ; | |
931 } else { | |
932 // Mirabile dictu | |
933 ILock (Self) ; | |
934 } | |
935 goto Exeunt ; | |
936 } | |
937 | |
938 void Monitor::lock() { | |
939 this->lock(Thread::current()); | |
940 } | |
941 | |
942 // Lock without safepoint check - a degenerate variant of lock(). | |
943 // Should ONLY be used by safepoint code and other code | |
944 // that is guaranteed not to block while running inside the VM. If this is called with | |
945 // thread state set to be in VM, the safepoint synchronization code will deadlock! | |
946 | |
947 void Monitor::lock_without_safepoint_check (Thread * Self) { | |
948 assert (_owner != Self, "invariant") ; | |
949 ILock (Self) ; | |
950 assert (_owner == NULL, "invariant"); | |
951 set_owner (Self); | |
952 } | |
953 | |
954 void Monitor::lock_without_safepoint_check () { | |
955 lock_without_safepoint_check (Thread::current()) ; | |
956 } | |
957 | |
958 | |
959 // Returns true if thread succeceed [sic] in grabbing the lock, otherwise false. | |
960 | |
961 bool Monitor::try_lock() { | |
962 Thread * const Self = Thread::current(); | |
963 debug_only(check_prelock_state(Self)); | |
964 // assert(!thread->is_inside_signal_handler(), "don't lock inside signal handler"); | |
965 | |
966 // Special case, where all Java threads are stopped. | |
967 // The lock may have been acquired but _owner is not yet set. | |
968 // In that case the VM thread can safely grab the lock. | |
969 // It strikes me this should appear _after the TryLock() fails, below. | |
970 bool can_sneak = Self->is_VM_thread() && SafepointSynchronize::is_at_safepoint(); | |
971 if (can_sneak && _owner == NULL) { | |
972 set_owner(Self); // Do not need to be atomic, since we are at a safepoint | |
973 _snuck = true; | |
974 return true; | |
975 } | |
976 | |
977 if (TryLock()) { | |
978 // We got the lock | |
979 assert (_owner == NULL, "invariant"); | |
980 set_owner (Self); | |
981 return true; | |
982 } | |
983 return false; | |
984 } | |
985 | |
986 void Monitor::unlock() { | |
987 assert (_owner == Thread::current(), "invariant") ; | |
988 assert (_OnDeck != Thread::current()->_MutexEvent , "invariant") ; | |
989 set_owner (NULL) ; | |
990 if (_snuck) { | |
991 assert(SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread(), "sneak"); | |
992 _snuck = false; | |
993 return ; | |
994 } | |
995 IUnlock (false) ; | |
996 } | |
997 | |
998 // Yet another degenerate version of Monitor::lock() or lock_without_safepoint_check() | |
999 // jvm_raw_lock() and _unlock() can be called by non-Java threads via JVM_RawMonitorEnter. | |
1000 // | |
1001 // There's no expectation that JVM_RawMonitors will interoperate properly with the native | |
1002 // Mutex-Monitor constructs. We happen to implement JVM_RawMonitors in terms of | |
1003 // native Mutex-Monitors simply as a matter of convenience. A simple abstraction layer | |
1004 // over a pthread_mutex_t would work equally as well, but require more platform-specific | |
1005 // code -- a "PlatformMutex". Alternatively, a simply layer over muxAcquire-muxRelease | |
1006 // would work too. | |
1007 // | |
1008 // Since the caller might be a foreign thread, we don't necessarily have a Thread.MutexEvent | |
1009 // instance available. Instead, we transiently allocate a ParkEvent on-demand if | |
1010 // we encounter contention. That ParkEvent remains associated with the thread | |
1011 // until it manages to acquire the lock, at which time we return the ParkEvent | |
1012 // to the global ParkEvent free list. This is correct and suffices for our purposes. | |
1013 // | |
1014 // Beware that the original jvm_raw_unlock() had a "_snuck" test but that | |
1015 // jvm_raw_lock() didn't have the corresponding test. I suspect that's an | |
1016 // oversight, but I've replicated the original suspect logic in the new code ... | |
1017 | |
1018 void Monitor::jvm_raw_lock() { | |
1019 assert(rank() == native, "invariant"); | |
1020 | |
1021 if (TryLock()) { | |
1022 Exeunt: | |
1023 assert (ILocked(), "invariant") ; | |
1024 assert (_owner == NULL, "invariant"); | |
1025 // This can potentially be called by non-java Threads. Thus, the ThreadLocalStorage | |
1026 // might return NULL. Don't call set_owner since it will break on an NULL owner | |
1027 // Consider installing a non-null "ANON" distinguished value instead of just NULL. | |
1028 _owner = ThreadLocalStorage::thread(); | |
1029 return ; | |
1030 } | |
1031 | |
1032 if (TrySpin(NULL)) goto Exeunt ; | |
1033 | |
1034 // slow-path - apparent contention | |
1035 // Allocate a ParkEvent for transient use. | |
1036 // The ParkEvent remains associated with this thread until | |
1037 // the time the thread manages to acquire the lock. | |
1038 ParkEvent * const ESelf = ParkEvent::Allocate(NULL) ; | |
1039 ESelf->reset() ; | |
1040 OrderAccess::storeload() ; | |
1041 | |
1042 // Either Enqueue Self on cxq or acquire the outer lock. | |
1043 if (AcquireOrPush (ESelf)) { | |
1044 ParkEvent::Release (ESelf) ; // surrender the ParkEvent | |
1045 goto Exeunt ; | |
1046 } | |
1047 | |
1048 // At any given time there is at most one ondeck thread. | |
1049 // ondeck implies not resident on cxq and not resident on EntryList | |
1050 // Only the OnDeck thread can try to acquire -- contended for -- the lock. | |
1051 // CONSIDER: use Self->OnDeck instead of m->OnDeck. | |
1052 for (;;) { | |
1053 if (_OnDeck == ESelf && TrySpin(NULL)) break ; | |
1054 ParkCommon (ESelf, 0) ; | |
1055 } | |
1056 | |
1057 assert (_OnDeck == ESelf, "invariant") ; | |
1058 _OnDeck = NULL ; | |
1059 ParkEvent::Release (ESelf) ; // surrender the ParkEvent | |
1060 goto Exeunt ; | |
1061 } | |
1062 | |
1063 void Monitor::jvm_raw_unlock() { | |
1064 // Nearly the same as Monitor::unlock() ... | |
1065 // directly set _owner instead of using set_owner(null) | |
1066 _owner = NULL ; | |
1067 if (_snuck) { // ??? | |
1068 assert(SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread(), "sneak"); | |
1069 _snuck = false; | |
1070 return ; | |
1071 } | |
1072 IUnlock(false) ; | |
1073 } | |
1074 | |
1075 bool Monitor::wait(bool no_safepoint_check, long timeout, bool as_suspend_equivalent) { | |
1076 Thread * const Self = Thread::current() ; | |
1077 assert (_owner == Self, "invariant") ; | |
1078 assert (ILocked(), "invariant") ; | |
1079 | |
1080 // as_suspend_equivalent logically implies !no_safepoint_check | |
1081 guarantee (!as_suspend_equivalent || !no_safepoint_check, "invariant") ; | |
1082 // !no_safepoint_check logically implies java_thread | |
1083 guarantee (no_safepoint_check || Self->is_Java_thread(), "invariant") ; | |
1084 | |
1085 #ifdef ASSERT | |
1086 Monitor * least = get_least_ranked_lock_besides_this(Self->owned_locks()); | |
1087 assert(least != this, "Specification of get_least_... call above"); | |
1088 if (least != NULL && least->rank() <= special) { | |
1089 tty->print("Attempting to wait on monitor %s/%d while holding" | |
1090 " lock %s/%d -- possible deadlock", | |
1091 name(), rank(), least->name(), least->rank()); | |
1092 assert(false, "Shouldn't block(wait) while holding a lock of rank special"); | |
1093 } | |
1094 #endif // ASSERT | |
1095 | |
1096 int wait_status ; | |
1097 // conceptually set the owner to NULL in anticipation of | |
1098 // abdicating the lock in wait | |
1099 set_owner(NULL); | |
1100 if (no_safepoint_check) { | |
1101 wait_status = IWait (Self, timeout) ; | |
1102 } else { | |
1103 assert (Self->is_Java_thread(), "invariant") ; | |
1104 JavaThread *jt = (JavaThread *)Self; | |
1105 | |
1106 // Enter safepoint region - ornate and Rococo ... | |
1107 ThreadBlockInVM tbivm(jt); | |
1108 OSThreadWaitState osts(Self->osthread(), false /* not Object.wait() */); | |
1109 | |
1110 if (as_suspend_equivalent) { | |
1111 jt->set_suspend_equivalent(); | |
1112 // cleared by handle_special_suspend_equivalent_condition() or | |
1113 // java_suspend_self() | |
1114 } | |
1115 | |
1116 wait_status = IWait (Self, timeout) ; | |
1117 | |
1118 // were we externally suspended while we were waiting? | |
1119 if (as_suspend_equivalent && jt->handle_special_suspend_equivalent_condition()) { | |
1120 // Our event wait has finished and we own the lock, but | |
1121 // while we were waiting another thread suspended us. We don't | |
1122 // want to hold the lock while suspended because that | |
1123 // would surprise the thread that suspended us. | |
1124 assert (ILocked(), "invariant") ; | |
1125 IUnlock (true) ; | |
1126 jt->java_suspend_self(); | |
1127 ILock (Self) ; | |
1128 assert (ILocked(), "invariant") ; | |
1129 } | |
1130 } | |
1131 | |
1132 // Conceptually reestablish ownership of the lock. | |
1133 // The "real" lock -- the LockByte -- was reacquired by IWait(). | |
1134 assert (ILocked(), "invariant") ; | |
1135 assert (_owner == NULL, "invariant") ; | |
1136 set_owner (Self) ; | |
1137 return wait_status != 0 ; // return true IFF timeout | |
1138 } | |
1139 | |
1140 Monitor::~Monitor() { | |
1141 assert ((UNS(_owner)|UNS(_LockWord.FullWord)|UNS(_EntryList)|UNS(_WaitSet)|UNS(_OnDeck)) == 0, "") ; | |
1142 } | |
1143 | |
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1144 void Monitor::ClearMonitor (Monitor * m, const char *name) { |
0 | 1145 m->_owner = NULL ; |
1146 m->_snuck = false ; | |
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1147 if (name == NULL) { |
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1148 strcpy(m->_name, "UNKNOWN") ; |
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1149 } else { |
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1150 strncpy(m->_name, name, MONITOR_NAME_LEN - 1); |
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1151 m->_name[MONITOR_NAME_LEN - 1] = '\0'; |
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1152 } |
0 | 1153 m->_LockWord.FullWord = 0 ; |
1154 m->_EntryList = NULL ; | |
1155 m->_OnDeck = NULL ; | |
1156 m->_WaitSet = NULL ; | |
1157 m->_WaitLock[0] = 0 ; | |
1158 } | |
1159 | |
1160 Monitor::Monitor() { ClearMonitor(this); } | |
1161 | |
1162 Monitor::Monitor (int Rank, const char * name, bool allow_vm_block) { | |
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1163 ClearMonitor (this, name) ; |
0 | 1164 #ifdef ASSERT |
1165 _allow_vm_block = allow_vm_block; | |
1166 _rank = Rank ; | |
1167 #endif | |
1168 } | |
1169 | |
1170 Mutex::~Mutex() { | |
1171 assert ((UNS(_owner)|UNS(_LockWord.FullWord)|UNS(_EntryList)|UNS(_WaitSet)|UNS(_OnDeck)) == 0, "") ; | |
1172 } | |
1173 | |
1174 Mutex::Mutex (int Rank, const char * name, bool allow_vm_block) { | |
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1175 ClearMonitor ((Monitor *) this, name) ; |
0 | 1176 #ifdef ASSERT |
1177 _allow_vm_block = allow_vm_block; | |
1178 _rank = Rank ; | |
1179 #endif | |
1180 } | |
1181 | |
1182 bool Monitor::owned_by_self() const { | |
1183 bool ret = _owner == Thread::current(); | |
1184 assert (!ret || _LockWord.Bytes[_LSBINDEX] != 0, "invariant") ; | |
1185 return ret; | |
1186 } | |
1187 | |
1188 void Monitor::print_on_error(outputStream* st) const { | |
1189 st->print("[" PTR_FORMAT, this); | |
1190 st->print("] %s", _name); | |
1191 st->print(" - owner thread: " PTR_FORMAT, _owner); | |
1192 } | |
1193 | |
1194 | |
1195 | |
1196 | |
1197 // ---------------------------------------------------------------------------------- | |
1198 // Non-product code | |
1199 | |
1200 #ifndef PRODUCT | |
1201 void Monitor::print_on(outputStream* st) const { | |
1202 st->print_cr("Mutex: [0x%lx/0x%lx] %s - owner: 0x%lx", this, _LockWord.FullWord, _name, _owner); | |
1203 } | |
1204 #endif | |
1205 | |
1206 #ifndef PRODUCT | |
1207 #ifdef ASSERT | |
1208 Monitor * Monitor::get_least_ranked_lock(Monitor * locks) { | |
1209 Monitor *res, *tmp; | |
1210 for (res = tmp = locks; tmp != NULL; tmp = tmp->next()) { | |
1211 if (tmp->rank() < res->rank()) { | |
1212 res = tmp; | |
1213 } | |
1214 } | |
1215 if (!SafepointSynchronize::is_at_safepoint()) { | |
1216 // In this case, we expect the held locks to be | |
1217 // in increasing rank order (modulo any native ranks) | |
1218 for (tmp = locks; tmp != NULL; tmp = tmp->next()) { | |
1219 if (tmp->next() != NULL) { | |
1220 assert(tmp->rank() == Mutex::native || | |
1221 tmp->rank() <= tmp->next()->rank(), "mutex rank anomaly?"); | |
1222 } | |
1223 } | |
1224 } | |
1225 return res; | |
1226 } | |
1227 | |
1228 Monitor* Monitor::get_least_ranked_lock_besides_this(Monitor* locks) { | |
1229 Monitor *res, *tmp; | |
1230 for (res = NULL, tmp = locks; tmp != NULL; tmp = tmp->next()) { | |
1231 if (tmp != this && (res == NULL || tmp->rank() < res->rank())) { | |
1232 res = tmp; | |
1233 } | |
1234 } | |
1235 if (!SafepointSynchronize::is_at_safepoint()) { | |
1236 // In this case, we expect the held locks to be | |
1237 // in increasing rank order (modulo any native ranks) | |
1238 for (tmp = locks; tmp != NULL; tmp = tmp->next()) { | |
1239 if (tmp->next() != NULL) { | |
1240 assert(tmp->rank() == Mutex::native || | |
1241 tmp->rank() <= tmp->next()->rank(), "mutex rank anomaly?"); | |
1242 } | |
1243 } | |
1244 } | |
1245 return res; | |
1246 } | |
1247 | |
1248 | |
1249 bool Monitor::contains(Monitor* locks, Monitor * lock) { | |
1250 for (; locks != NULL; locks = locks->next()) { | |
1251 if (locks == lock) | |
1252 return true; | |
1253 } | |
1254 return false; | |
1255 } | |
1256 #endif | |
1257 | |
1258 // Called immediately after lock acquisition or release as a diagnostic | |
1259 // to track the lock-set of the thread and test for rank violations that | |
1260 // might indicate exposure to deadlock. | |
1261 // Rather like an EventListener for _owner (:>). | |
1262 | |
1263 void Monitor::set_owner_implementation(Thread *new_owner) { | |
1264 // This function is solely responsible for maintaining | |
1265 // and checking the invariant that threads and locks | |
1266 // are in a 1/N relation, with some some locks unowned. | |
1267 // It uses the Mutex::_owner, Mutex::_next, and | |
1268 // Thread::_owned_locks fields, and no other function | |
1269 // changes those fields. | |
1270 // It is illegal to set the mutex from one non-NULL | |
1271 // owner to another--it must be owned by NULL as an | |
1272 // intermediate state. | |
1273 | |
1274 if (new_owner != NULL) { | |
1275 // the thread is acquiring this lock | |
1276 | |
1277 assert(new_owner == Thread::current(), "Should I be doing this?"); | |
1278 assert(_owner == NULL, "setting the owner thread of an already owned mutex"); | |
1279 _owner = new_owner; // set the owner | |
1280 | |
1281 // link "this" into the owned locks list | |
1282 | |
1283 #ifdef ASSERT // Thread::_owned_locks is under the same ifdef | |
1284 Monitor* locks = get_least_ranked_lock(new_owner->owned_locks()); | |
1285 // Mutex::set_owner_implementation is a friend of Thread | |
1286 | |
1287 assert(this->rank() >= 0, "bad lock rank"); | |
1288 | |
1289 // Deadlock avoidance rules require us to acquire Mutexes only in | |
1290 // a global total order. For example m1 is the lowest ranked mutex | |
1291 // that the thread holds and m2 is the mutex the thread is trying | |
1292 // to acquire, then deadlock avoidance rules require that the rank | |
1293 // of m2 be less than the rank of m1. | |
1294 // The rank Mutex::native is an exception in that it is not subject | |
1295 // to the verification rules. | |
1296 // Here are some further notes relating to mutex acquisition anomalies: | |
1297 // . under Solaris, the interrupt lock gets acquired when doing | |
1298 // profiling, so any lock could be held. | |
1299 // . it is also ok to acquire Safepoint_lock at the very end while we | |
1300 // already hold Terminator_lock - may happen because of periodic safepoints | |
1301 if (this->rank() != Mutex::native && | |
1302 this->rank() != Mutex::suspend_resume && | |
1303 locks != NULL && locks->rank() <= this->rank() && | |
1304 !SafepointSynchronize::is_at_safepoint() && | |
1305 this != Interrupt_lock && this != ProfileVM_lock && | |
1306 !(this == Safepoint_lock && contains(locks, Terminator_lock) && | |
1307 SafepointSynchronize::is_synchronizing())) { | |
1308 new_owner->print_owned_locks(); | |
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1309 fatal(err_msg("acquiring lock %s/%d out of order with lock %s/%d -- " |
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1310 "possible deadlock", this->name(), this->rank(), |
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1311 locks->name(), locks->rank())); |
0 | 1312 } |
1313 | |
1314 this->_next = new_owner->_owned_locks; | |
1315 new_owner->_owned_locks = this; | |
1316 #endif | |
1317 | |
1318 } else { | |
1319 // the thread is releasing this lock | |
1320 | |
1321 Thread* old_owner = _owner; | |
1322 debug_only(_last_owner = old_owner); | |
1323 | |
1324 assert(old_owner != NULL, "removing the owner thread of an unowned mutex"); | |
1325 assert(old_owner == Thread::current(), "removing the owner thread of an unowned mutex"); | |
1326 | |
1327 _owner = NULL; // set the owner | |
1328 | |
1329 #ifdef ASSERT | |
1330 Monitor *locks = old_owner->owned_locks(); | |
1331 | |
1332 // remove "this" from the owned locks list | |
1333 | |
1334 Monitor *prev = NULL; | |
1335 bool found = false; | |
1336 for (; locks != NULL; prev = locks, locks = locks->next()) { | |
1337 if (locks == this) { | |
1338 found = true; | |
1339 break; | |
1340 } | |
1341 } | |
1342 assert(found, "Removing a lock not owned"); | |
1343 if (prev == NULL) { | |
1344 old_owner->_owned_locks = _next; | |
1345 } else { | |
1346 prev->_next = _next; | |
1347 } | |
1348 _next = NULL; | |
1349 #endif | |
1350 } | |
1351 } | |
1352 | |
1353 | |
1354 // Factored out common sanity checks for locking mutex'es. Used by lock() and try_lock() | |
1355 void Monitor::check_prelock_state(Thread *thread) { | |
1356 assert((!thread->is_Java_thread() || ((JavaThread *)thread)->thread_state() == _thread_in_vm) | |
1357 || rank() == Mutex::special, "wrong thread state for using locks"); | |
1358 if (StrictSafepointChecks) { | |
1359 if (thread->is_VM_thread() && !allow_vm_block()) { | |
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1360 fatal(err_msg("VM thread using lock %s (not allowed to block on)", |
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1361 name())); |
0 | 1362 } |
1363 debug_only(if (rank() != Mutex::special) \ | |
1364 thread->check_for_valid_safepoint_state(false);) | |
1365 } | |
11151 | 1366 if (thread->is_Watcher_thread()) { |
1367 assert(!WatcherThread::watcher_thread()->has_crash_protection(), | |
1368 "locking not allowed when crash protection is set"); | |
1369 } | |
0 | 1370 } |
1371 | |
1372 void Monitor::check_block_state(Thread *thread) { | |
1373 if (!_allow_vm_block && thread->is_VM_thread()) { | |
1374 warning("VM thread blocked on lock"); | |
1375 print(); | |
1376 BREAKPOINT; | |
1377 } | |
1378 assert(_owner != thread, "deadlock: blocking on monitor owned by current thread"); | |
1379 } | |
1380 | |
1381 #endif // PRODUCT |