|
1 |
|
2 /* |
|
3 * Copyright (c) 1998, 2014, Oracle and/or its affiliates. All rights reserved. |
|
4 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
|
5 * |
|
6 * This code is free software; you can redistribute it and/or modify it |
|
7 * under the terms of the GNU General Public License version 2 only, as |
|
8 * published by the Free Software Foundation. |
|
9 * |
|
10 * This code is distributed in the hope that it will be useful, but WITHOUT |
|
11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
|
12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
|
13 * version 2 for more details (a copy is included in the LICENSE file that |
|
14 * accompanied this code). |
|
15 * |
|
16 * You should have received a copy of the GNU General Public License version |
|
17 * 2 along with this work; if not, write to the Free Software Foundation, |
|
18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
|
19 * |
|
20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA |
|
21 * or visit www.oracle.com if you need additional information or have any |
|
22 * questions. |
|
23 * |
|
24 */ |
|
25 |
|
26 #include "precompiled.hpp" |
|
27 #include "runtime/mutex.hpp" |
|
28 #include "runtime/osThread.hpp" |
|
29 #include "runtime/thread.inline.hpp" |
|
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 |
|
40 #ifdef TARGET_OS_FAMILY_bsd |
|
41 # include "mutex_bsd.inline.hpp" |
|
42 #endif |
|
43 |
|
44 PRAGMA_FORMAT_MUTE_WARNINGS_FOR_GCC |
|
45 |
|
46 // 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 |
|
47 // |
|
48 // Native Monitor-Mutex locking - theory of operations |
|
49 // |
|
50 // * Native Monitors are completely unrelated to Java-level monitors, |
|
51 // although the "back-end" slow-path implementations share a common lineage. |
|
52 // See objectMonitor:: in synchronizer.cpp. |
|
53 // Native Monitors do *not* support nesting or recursion but otherwise |
|
54 // they're basically Hoare-flavor monitors. |
|
55 // |
|
56 // * A thread acquires ownership of a Monitor/Mutex by CASing the LockByte |
|
57 // in the _LockWord from zero to non-zero. Note that the _Owner field |
|
58 // is advisory and is used only to verify that the thread calling unlock() |
|
59 // is indeed the last thread to have acquired the lock. |
|
60 // |
|
61 // * Contending threads "push" themselves onto the front of the contention |
|
62 // queue -- called the cxq -- with CAS and then spin/park. |
|
63 // The _LockWord contains the LockByte as well as the pointer to the head |
|
64 // of the cxq. Colocating the LockByte with the cxq precludes certain races. |
|
65 // |
|
66 // * Using a separately addressable LockByte allows for CAS:MEMBAR or CAS:0 |
|
67 // idioms. We currently use MEMBAR in the uncontended unlock() path, as |
|
68 // MEMBAR often has less latency than CAS. If warranted, we could switch to |
|
69 // a CAS:0 mode, using timers to close the resultant race, as is done |
|
70 // with Java Monitors in synchronizer.cpp. |
|
71 // |
|
72 // See the following for a discussion of the relative cost of atomics (CAS) |
|
73 // MEMBAR, and ways to eliminate such instructions from the common-case paths: |
|
74 // -- http://blogs.sun.com/dave/entry/biased_locking_in_hotspot |
|
75 // -- http://blogs.sun.com/dave/resource/MustangSync.pdf |
|
76 // -- http://blogs.sun.com/dave/resource/synchronization-public2.pdf |
|
77 // -- synchronizer.cpp |
|
78 // |
|
79 // * Overall goals - desiderata |
|
80 // 1. Minimize context switching |
|
81 // 2. Minimize lock migration |
|
82 // 3. Minimize CPI -- affinity and locality |
|
83 // 4. Minimize the execution of high-latency instructions such as CAS or MEMBAR |
|
84 // 5. Minimize outer lock hold times |
|
85 // 6. Behave gracefully on a loaded system |
|
86 // |
|
87 // * Thread flow and list residency: |
|
88 // |
|
89 // Contention queue --> EntryList --> OnDeck --> Owner --> !Owner |
|
90 // [..resident on monitor list..] |
|
91 // [...........contending..................] |
|
92 // |
|
93 // -- The contention queue (cxq) contains recently-arrived threads (RATs). |
|
94 // Threads on the cxq eventually drain into the EntryList. |
|
95 // -- Invariant: a thread appears on at most one list -- cxq, EntryList |
|
96 // or WaitSet -- at any one time. |
|
97 // -- For a given monitor there can be at most one "OnDeck" thread at any |
|
98 // given time but if needbe this particular invariant could be relaxed. |
|
99 // |
|
100 // * The WaitSet and EntryList linked lists are composed of ParkEvents. |
|
101 // I use ParkEvent instead of threads as ParkEvents are immortal and |
|
102 // type-stable, meaning we can safely unpark() a possibly stale |
|
103 // list element in the unlock()-path. (That's benign). |
|
104 // |
|
105 // * Succession policy - providing for progress: |
|
106 // |
|
107 // As necessary, the unlock()ing thread identifies, unlinks, and unparks |
|
108 // an "heir presumptive" tentative successor thread from the EntryList. |
|
109 // This becomes the so-called "OnDeck" thread, of which there can be only |
|
110 // one at any given time for a given monitor. The wakee will recontend |
|
111 // for ownership of monitor. |
|
112 // |
|
113 // Succession is provided for by a policy of competitive handoff. |
|
114 // The exiting thread does _not_ grant or pass ownership to the |
|
115 // successor thread. (This is also referred to as "handoff" succession"). |
|
116 // Instead the exiting thread releases ownership and possibly wakes |
|
117 // a successor, so the successor can (re)compete for ownership of the lock. |
|
118 // |
|
119 // Competitive handoff provides excellent overall throughput at the expense |
|
120 // of short-term fairness. If fairness is a concern then one remedy might |
|
121 // be to add an AcquireCounter field to the monitor. After a thread acquires |
|
122 // the lock it will decrement the AcquireCounter field. When the count |
|
123 // reaches 0 the thread would reset the AcquireCounter variable, abdicate |
|
124 // the lock directly to some thread on the EntryList, and then move itself to the |
|
125 // tail of the EntryList. |
|
126 // |
|
127 // But in practice most threads engage or otherwise participate in resource |
|
128 // bounded producer-consumer relationships, so lock domination is not usually |
|
129 // a practical concern. Recall too, that in general it's easier to construct |
|
130 // a fair lock from a fast lock, but not vice-versa. |
|
131 // |
|
132 // * The cxq can have multiple concurrent "pushers" but only one concurrent |
|
133 // detaching thread. This mechanism is immune from the ABA corruption. |
|
134 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious. |
|
135 // We use OnDeck as a pseudo-lock to enforce the at-most-one detaching |
|
136 // thread constraint. |
|
137 // |
|
138 // * Taken together, the cxq and the EntryList constitute or form a |
|
139 // single logical queue of threads stalled trying to acquire the lock. |
|
140 // We use two distinct lists to reduce heat on the list ends. |
|
141 // Threads in lock() enqueue onto cxq while threads in unlock() will |
|
142 // dequeue from the EntryList. (c.f. Michael Scott's "2Q" algorithm). |
|
143 // A key desideratum is to minimize queue & monitor metadata manipulation |
|
144 // that occurs while holding the "outer" monitor lock -- that is, we want to |
|
145 // minimize monitor lock holds times. |
|
146 // |
|
147 // The EntryList is ordered by the prevailing queue discipline and |
|
148 // can be organized in any convenient fashion, such as a doubly-linked list or |
|
149 // a circular doubly-linked list. If we need a priority queue then something akin |
|
150 // to Solaris' sleepq would work nicely. Viz., |
|
151 // -- http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c. |
|
152 // -- http://cvs.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/uts/common/os/sleepq.c |
|
153 // Queue discipline is enforced at ::unlock() time, when the unlocking thread |
|
154 // drains the cxq into the EntryList, and orders or reorders the threads on the |
|
155 // EntryList accordingly. |
|
156 // |
|
157 // Barring "lock barging", this mechanism provides fair cyclic ordering, |
|
158 // somewhat similar to an elevator-scan. |
|
159 // |
|
160 // * OnDeck |
|
161 // -- For a given monitor there can be at most one OnDeck thread at any given |
|
162 // instant. The OnDeck thread is contending for the lock, but has been |
|
163 // unlinked from the EntryList and cxq by some previous unlock() operations. |
|
164 // Once a thread has been designated the OnDeck thread it will remain so |
|
165 // until it manages to acquire the lock -- being OnDeck is a stable property. |
|
166 // -- Threads on the EntryList or cxq are _not allowed to attempt lock acquisition. |
|
167 // -- OnDeck also serves as an "inner lock" as follows. Threads in unlock() will, after |
|
168 // having cleared the LockByte and dropped the outer lock, attempt to "trylock" |
|
169 // OnDeck by CASing the field from null to non-null. If successful, that thread |
|
170 // is then responsible for progress and succession and can use CAS to detach and |
|
171 // drain the cxq into the EntryList. By convention, only this thread, the holder of |
|
172 // the OnDeck inner lock, can manipulate the EntryList or detach and drain the |
|
173 // RATs on the cxq into the EntryList. This avoids ABA corruption on the cxq as |
|
174 // we allow multiple concurrent "push" operations but restrict detach concurrency |
|
175 // to at most one thread. Having selected and detached a successor, the thread then |
|
176 // changes the OnDeck to refer to that successor, and then unparks the successor. |
|
177 // That successor will eventually acquire the lock and clear OnDeck. Beware |
|
178 // that the OnDeck usage as a lock is asymmetric. A thread in unlock() transiently |
|
179 // "acquires" OnDeck, performs queue manipulations, passes OnDeck to some successor, |
|
180 // and then the successor eventually "drops" OnDeck. Note that there's never |
|
181 // any sense of contention on the inner lock, however. Threads never contend |
|
182 // or wait for the inner lock. |
|
183 // -- OnDeck provides for futile wakeup throttling a described in section 3.3 of |
|
184 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf |
|
185 // In a sense, OnDeck subsumes the ObjectMonitor _Succ and ObjectWaiter |
|
186 // TState fields found in Java-level objectMonitors. (See synchronizer.cpp). |
|
187 // |
|
188 // * Waiting threads reside on the WaitSet list -- wait() puts |
|
189 // the caller onto the WaitSet. Notify() or notifyAll() simply |
|
190 // transfers threads from the WaitSet to either the EntryList or cxq. |
|
191 // Subsequent unlock() operations will eventually unpark the notifyee. |
|
192 // Unparking a notifee in notify() proper is inefficient - if we were to do so |
|
193 // it's likely the notifyee would simply impale itself on the lock held |
|
194 // by the notifier. |
|
195 // |
|
196 // * The mechanism is obstruction-free in that if the holder of the transient |
|
197 // OnDeck lock in unlock() is preempted or otherwise stalls, other threads |
|
198 // can still acquire and release the outer lock and continue to make progress. |
|
199 // At worst, waking of already blocked contending threads may be delayed, |
|
200 // but nothing worse. (We only use "trylock" operations on the inner OnDeck |
|
201 // lock). |
|
202 // |
|
203 // * Note that thread-local storage must be initialized before a thread |
|
204 // uses Native monitors or mutexes. The native monitor-mutex subsystem |
|
205 // depends on Thread::current(). |
|
206 // |
|
207 // * The monitor synchronization subsystem avoids the use of native |
|
208 // synchronization primitives except for the narrow platform-specific |
|
209 // park-unpark abstraction. See the comments in os_solaris.cpp regarding |
|
210 // the semantics of park-unpark. Put another way, this monitor implementation |
|
211 // depends only on atomic operations and park-unpark. The monitor subsystem |
|
212 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the |
|
213 // underlying OS manages the READY<->RUN transitions. |
|
214 // |
|
215 // * The memory consistency model provide by lock()-unlock() is at least as |
|
216 // strong or stronger than the Java Memory model defined by JSR-133. |
|
217 // That is, we guarantee at least entry consistency, if not stronger. |
|
218 // See http://g.oswego.edu/dl/jmm/cookbook.html. |
|
219 // |
|
220 // * Thread:: currently contains a set of purpose-specific ParkEvents: |
|
221 // _MutexEvent, _ParkEvent, etc. A better approach might be to do away with |
|
222 // the purpose-specific ParkEvents and instead implement a general per-thread |
|
223 // stack of available ParkEvents which we could provision on-demand. The |
|
224 // stack acts as a local cache to avoid excessive calls to ParkEvent::Allocate() |
|
225 // and ::Release(). A thread would simply pop an element from the local stack before it |
|
226 // enqueued or park()ed. When the contention was over the thread would |
|
227 // push the no-longer-needed ParkEvent back onto its stack. |
|
228 // |
|
229 // * A slightly reduced form of ILock() and IUnlock() have been partially |
|
230 // model-checked (Murphi) for safety and progress at T=1,2,3 and 4. |
|
231 // It'd be interesting to see if TLA/TLC could be useful as well. |
|
232 // |
|
233 // * Mutex-Monitor is a low-level "leaf" subsystem. That is, the monitor |
|
234 // code should never call other code in the JVM that might itself need to |
|
235 // acquire monitors or mutexes. That's true *except* in the case of the |
|
236 // ThreadBlockInVM state transition wrappers. The ThreadBlockInVM DTOR handles |
|
237 // mutator reentry (ingress) by checking for a pending safepoint in which case it will |
|
238 // call SafepointSynchronize::block(), which in turn may call Safepoint_lock->lock(), etc. |
|
239 // In that particular case a call to lock() for a given Monitor can end up recursively |
|
240 // calling lock() on another monitor. While distasteful, this is largely benign |
|
241 // as the calls come from jacket that wraps lock(), and not from deep within lock() itself. |
|
242 // |
|
243 // It's unfortunate that native mutexes and thread state transitions were convolved. |
|
244 // They're really separate concerns and should have remained that way. Melding |
|
245 // them together was facile -- a bit too facile. The current implementation badly |
|
246 // conflates the two concerns. |
|
247 // |
|
248 // * TODO-FIXME: |
|
249 // |
|
250 // -- Add DTRACE probes for contended acquire, contended acquired, contended unlock |
|
251 // We should also add DTRACE probes in the ParkEvent subsystem for |
|
252 // Park-entry, Park-exit, and Unpark. |
|
253 // |
|
254 // -- We have an excess of mutex-like constructs in the JVM, namely: |
|
255 // 1. objectMonitors for Java-level synchronization (synchronizer.cpp) |
|
256 // 2. low-level muxAcquire and muxRelease |
|
257 // 3. low-level spinAcquire and spinRelease |
|
258 // 4. native Mutex:: and Monitor:: |
|
259 // 5. jvm_raw_lock() and _unlock() |
|
260 // 6. JVMTI raw monitors -- distinct from (5) despite having a confusingly |
|
261 // similar name. |
|
262 // |
|
263 // 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 |
|
264 |
|
265 |
|
266 // CASPTR() uses the canonical argument order that dominates in the literature. |
|
267 // Our internal cmpxchg_ptr() uses a bastardized ordering to accommodate Sun .il templates. |
|
268 |
|
269 #define CASPTR(a,c,s) intptr_t(Atomic::cmpxchg_ptr ((void *)(s),(void *)(a),(void *)(c))) |
|
270 #define UNS(x) (uintptr_t(x)) |
|
271 #define TRACE(m) { static volatile int ctr = 0 ; int x = ++ctr ; if ((x & (x-1))==0) { ::printf ("%d:%s\n", x, #m); ::fflush(stdout); }} |
|
272 |
|
273 // Simplistic low-quality Marsaglia SHIFT-XOR RNG. |
|
274 // Bijective except for the trailing mask operation. |
|
275 // Useful for spin loops as the compiler can't optimize it away. |
|
276 |
|
277 static inline jint MarsagliaXORV (jint x) { |
|
278 if (x == 0) x = 1|os::random() ; |
|
279 x ^= x << 6; |
|
280 x ^= ((unsigned)x) >> 21; |
|
281 x ^= x << 7 ; |
|
282 return x & 0x7FFFFFFF ; |
|
283 } |
|
284 |
|
285 static inline jint MarsagliaXOR (jint * const a) { |
|
286 jint x = *a ; |
|
287 if (x == 0) x = UNS(a)|1 ; |
|
288 x ^= x << 6; |
|
289 x ^= ((unsigned)x) >> 21; |
|
290 x ^= x << 7 ; |
|
291 *a = x ; |
|
292 return x & 0x7FFFFFFF ; |
|
293 } |
|
294 |
|
295 static int Stall (int its) { |
|
296 static volatile jint rv = 1 ; |
|
297 volatile int OnFrame = 0 ; |
|
298 jint v = rv ^ UNS(OnFrame) ; |
|
299 while (--its >= 0) { |
|
300 v = MarsagliaXORV (v) ; |
|
301 } |
|
302 // Make this impossible for the compiler to optimize away, |
|
303 // but (mostly) avoid W coherency sharing on MP systems. |
|
304 if (v == 0x12345) rv = v ; |
|
305 return v ; |
|
306 } |
|
307 |
|
308 int Monitor::TryLock () { |
|
309 intptr_t v = _LockWord.FullWord ; |
|
310 for (;;) { |
|
311 if ((v & _LBIT) != 0) return 0 ; |
|
312 const intptr_t u = CASPTR (&_LockWord, v, v|_LBIT) ; |
|
313 if (v == u) return 1 ; |
|
314 v = u ; |
|
315 } |
|
316 } |
|
317 |
|
318 int Monitor::TryFast () { |
|
319 // Optimistic fast-path form ... |
|
320 // Fast-path attempt for the common uncontended case. |
|
321 // Avoid RTS->RTO $ coherence upgrade on typical SMP systems. |
|
322 intptr_t v = CASPTR (&_LockWord, 0, _LBIT) ; // agro ... |
|
323 if (v == 0) return 1 ; |
|
324 |
|
325 for (;;) { |
|
326 if ((v & _LBIT) != 0) return 0 ; |
|
327 const intptr_t u = CASPTR (&_LockWord, v, v|_LBIT) ; |
|
328 if (v == u) return 1 ; |
|
329 v = u ; |
|
330 } |
|
331 } |
|
332 |
|
333 int Monitor::ILocked () { |
|
334 const intptr_t w = _LockWord.FullWord & 0xFF ; |
|
335 assert (w == 0 || w == _LBIT, "invariant") ; |
|
336 return w == _LBIT ; |
|
337 } |
|
338 |
|
339 // Polite TATAS spinlock with exponential backoff - bounded spin. |
|
340 // Ideally we'd use processor cycles, time or vtime to control |
|
341 // the loop, but we currently use iterations. |
|
342 // All the constants within were derived empirically but work over |
|
343 // over the spectrum of J2SE reference platforms. |
|
344 // On Niagara-class systems the back-off is unnecessary but |
|
345 // is relatively harmless. (At worst it'll slightly retard |
|
346 // acquisition times). The back-off is critical for older SMP systems |
|
347 // where constant fetching of the LockWord would otherwise impair |
|
348 // scalability. |
|
349 // |
|
350 // Clamp spinning at approximately 1/2 of a context-switch round-trip. |
|
351 // See synchronizer.cpp for details and rationale. |
|
352 |
|
353 int Monitor::TrySpin (Thread * const Self) { |
|
354 if (TryLock()) return 1 ; |
|
355 if (!os::is_MP()) return 0 ; |
|
356 |
|
357 int Probes = 0 ; |
|
358 int Delay = 0 ; |
|
359 int Steps = 0 ; |
|
360 int SpinMax = NativeMonitorSpinLimit ; |
|
361 int flgs = NativeMonitorFlags ; |
|
362 for (;;) { |
|
363 intptr_t v = _LockWord.FullWord; |
|
364 if ((v & _LBIT) == 0) { |
|
365 if (CASPTR (&_LockWord, v, v|_LBIT) == v) { |
|
366 return 1 ; |
|
367 } |
|
368 continue ; |
|
369 } |
|
370 |
|
371 if ((flgs & 8) == 0) { |
|
372 SpinPause () ; |
|
373 } |
|
374 |
|
375 // Periodically increase Delay -- variable Delay form |
|
376 // conceptually: delay *= 1 + 1/Exponent |
|
377 ++ Probes; |
|
378 if (Probes > SpinMax) return 0 ; |
|
379 |
|
380 if ((Probes & 0x7) == 0) { |
|
381 Delay = ((Delay << 1)|1) & 0x7FF ; |
|
382 // CONSIDER: Delay += 1 + (Delay/4); Delay &= 0x7FF ; |
|
383 } |
|
384 |
|
385 if (flgs & 2) continue ; |
|
386 |
|
387 // Consider checking _owner's schedctl state, if OFFPROC abort spin. |
|
388 // If the owner is OFFPROC then it's unlike that the lock will be dropped |
|
389 // in a timely fashion, which suggests that spinning would not be fruitful |
|
390 // or profitable. |
|
391 |
|
392 // Stall for "Delay" time units - iterations in the current implementation. |
|
393 // Avoid generating coherency traffic while stalled. |
|
394 // Possible ways to delay: |
|
395 // PAUSE, SLEEP, MEMBAR #sync, MEMBAR #halt, |
|
396 // wr %g0,%asi, gethrtime, rdstick, rdtick, rdtsc, etc. ... |
|
397 // Note that on Niagara-class systems we want to minimize STs in the |
|
398 // spin loop. N1 and brethren write-around the L1$ over the xbar into the L2$. |
|
399 // Furthermore, they don't have a W$ like traditional SPARC processors. |
|
400 // We currently use a Marsaglia Shift-Xor RNG loop. |
|
401 Steps += Delay ; |
|
402 if (Self != NULL) { |
|
403 jint rv = Self->rng[0] ; |
|
404 for (int k = Delay ; --k >= 0; ) { |
|
405 rv = MarsagliaXORV (rv) ; |
|
406 if ((flgs & 4) == 0 && SafepointSynchronize::do_call_back()) return 0 ; |
|
407 } |
|
408 Self->rng[0] = rv ; |
|
409 } else { |
|
410 Stall (Delay) ; |
|
411 } |
|
412 } |
|
413 } |
|
414 |
|
415 static int ParkCommon (ParkEvent * ev, jlong timo) { |
|
416 // Diagnostic support - periodically unwedge blocked threads |
|
417 intx nmt = NativeMonitorTimeout ; |
|
418 if (nmt > 0 && (nmt < timo || timo <= 0)) { |
|
419 timo = nmt ; |
|
420 } |
|
421 int err = OS_OK ; |
|
422 if (0 == timo) { |
|
423 ev->park() ; |
|
424 } else { |
|
425 err = ev->park(timo) ; |
|
426 } |
|
427 return err ; |
|
428 } |
|
429 |
|
430 inline int Monitor::AcquireOrPush (ParkEvent * ESelf) { |
|
431 intptr_t v = _LockWord.FullWord ; |
|
432 for (;;) { |
|
433 if ((v & _LBIT) == 0) { |
|
434 const intptr_t u = CASPTR (&_LockWord, v, v|_LBIT) ; |
|
435 if (u == v) return 1 ; // indicate acquired |
|
436 v = u ; |
|
437 } else { |
|
438 // Anticipate success ... |
|
439 ESelf->ListNext = (ParkEvent *) (v & ~_LBIT) ; |
|
440 const intptr_t u = CASPTR (&_LockWord, v, intptr_t(ESelf)|_LBIT) ; |
|
441 if (u == v) return 0 ; // indicate pushed onto cxq |
|
442 v = u ; |
|
443 } |
|
444 // Interference - LockWord change - just retry |
|
445 } |
|
446 } |
|
447 |
|
448 // ILock and IWait are the lowest level primitive internal blocking |
|
449 // synchronization functions. The callers of IWait and ILock must have |
|
450 // performed any needed state transitions beforehand. |
|
451 // IWait and ILock may directly call park() without any concern for thread state. |
|
452 // Note that ILock and IWait do *not* access _owner. |
|
453 // _owner is a higher-level logical concept. |
|
454 |
|
455 void Monitor::ILock (Thread * Self) { |
|
456 assert (_OnDeck != Self->_MutexEvent, "invariant") ; |
|
457 |
|
458 if (TryFast()) { |
|
459 Exeunt: |
|
460 assert (ILocked(), "invariant") ; |
|
461 return ; |
|
462 } |
|
463 |
|
464 ParkEvent * const ESelf = Self->_MutexEvent ; |
|
465 assert (_OnDeck != ESelf, "invariant") ; |
|
466 |
|
467 // As an optimization, spinners could conditionally try to set ONDECK to _LBIT |
|
468 // Synchronizer.cpp uses a similar optimization. |
|
469 if (TrySpin (Self)) goto Exeunt ; |
|
470 |
|
471 // Slow-path - the lock is contended. |
|
472 // Either Enqueue Self on cxq or acquire the outer lock. |
|
473 // LockWord encoding = (cxq,LOCKBYTE) |
|
474 ESelf->reset() ; |
|
475 OrderAccess::fence() ; |
|
476 |
|
477 // Optional optimization ... try barging on the inner lock |
|
478 if ((NativeMonitorFlags & 32) && CASPTR (&_OnDeck, NULL, UNS(Self)) == 0) { |
|
479 goto OnDeck_LOOP ; |
|
480 } |
|
481 |
|
482 if (AcquireOrPush (ESelf)) goto Exeunt ; |
|
483 |
|
484 // At any given time there is at most one ondeck thread. |
|
485 // ondeck implies not resident on cxq and not resident on EntryList |
|
486 // Only the OnDeck thread can try to acquire -- contended for -- the lock. |
|
487 // CONSIDER: use Self->OnDeck instead of m->OnDeck. |
|
488 // Deschedule Self so that others may run. |
|
489 while (_OnDeck != ESelf) { |
|
490 ParkCommon (ESelf, 0) ; |
|
491 } |
|
492 |
|
493 // Self is now in the ONDECK position and will remain so until it |
|
494 // manages to acquire the lock. |
|
495 OnDeck_LOOP: |
|
496 for (;;) { |
|
497 assert (_OnDeck == ESelf, "invariant") ; |
|
498 if (TrySpin (Self)) break ; |
|
499 // CONSIDER: if ESelf->TryPark() && TryLock() break ... |
|
500 // It's probably wise to spin only if we *actually* blocked |
|
501 // CONSIDER: check the lockbyte, if it remains set then |
|
502 // preemptively drain the cxq into the EntryList. |
|
503 // The best place and time to perform queue operations -- lock metadata -- |
|
504 // is _before having acquired the outer lock, while waiting for the lock to drop. |
|
505 ParkCommon (ESelf, 0) ; |
|
506 } |
|
507 |
|
508 assert (_OnDeck == ESelf, "invariant") ; |
|
509 _OnDeck = NULL ; |
|
510 |
|
511 // Note that we current drop the inner lock (clear OnDeck) in the slow-path |
|
512 // epilog immediately after having acquired the outer lock. |
|
513 // But instead we could consider the following optimizations: |
|
514 // A. Shift or defer dropping the inner lock until the subsequent IUnlock() operation. |
|
515 // This might avoid potential reacquisition of the inner lock in IUlock(). |
|
516 // B. While still holding the inner lock, attempt to opportunistically select |
|
517 // and unlink the next ONDECK thread from the EntryList. |
|
518 // If successful, set ONDECK to refer to that thread, otherwise clear ONDECK. |
|
519 // It's critical that the select-and-unlink operation run in constant-time as |
|
520 // it executes when holding the outer lock and may artificially increase the |
|
521 // effective length of the critical section. |
|
522 // Note that (A) and (B) are tantamount to succession by direct handoff for |
|
523 // the inner lock. |
|
524 goto Exeunt ; |
|
525 } |
|
526 |
|
527 void Monitor::IUnlock (bool RelaxAssert) { |
|
528 assert (ILocked(), "invariant") ; |
|
529 // Conceptually we need a MEMBAR #storestore|#loadstore barrier or fence immediately |
|
530 // before the store that releases the lock. Crucially, all the stores and loads in the |
|
531 // critical section must be globally visible before the store of 0 into the lock-word |
|
532 // that releases the lock becomes globally visible. That is, memory accesses in the |
|
533 // critical section should not be allowed to bypass or overtake the following ST that |
|
534 // releases the lock. As such, to prevent accesses within the critical section |
|
535 // from "leaking" out, we need a release fence between the critical section and the |
|
536 // store that releases the lock. In practice that release barrier is elided on |
|
537 // platforms with strong memory models such as TSO. |
|
538 // |
|
539 // Note that the OrderAccess::storeload() fence that appears after unlock store |
|
540 // provides for progress conditions and succession and is _not related to exclusion |
|
541 // safety or lock release consistency. |
|
542 OrderAccess::release_store(&_LockWord.Bytes[_LSBINDEX], 0); // drop outer lock |
|
543 |
|
544 OrderAccess::storeload (); |
|
545 ParkEvent * const w = _OnDeck ; |
|
546 assert (RelaxAssert || w != Thread::current()->_MutexEvent, "invariant") ; |
|
547 if (w != NULL) { |
|
548 // Either we have a valid ondeck thread or ondeck is transiently "locked" |
|
549 // by some exiting thread as it arranges for succession. The LSBit of |
|
550 // OnDeck allows us to discriminate two cases. If the latter, the |
|
551 // responsibility for progress and succession lies with that other thread. |
|
552 // For good performance, we also depend on the fact that redundant unpark() |
|
553 // operations are cheap. That is, repeated Unpark()ing of the ONDECK thread |
|
554 // is inexpensive. This approach provides implicit futile wakeup throttling. |
|
555 // Note that the referent "w" might be stale with respect to the lock. |
|
556 // In that case the following unpark() is harmless and the worst that'll happen |
|
557 // is a spurious return from a park() operation. Critically, if "w" _is stale, |
|
558 // then progress is known to have occurred as that means the thread associated |
|
559 // with "w" acquired the lock. In that case this thread need take no further |
|
560 // action to guarantee progress. |
|
561 if ((UNS(w) & _LBIT) == 0) w->unpark() ; |
|
562 return ; |
|
563 } |
|
564 |
|
565 intptr_t cxq = _LockWord.FullWord ; |
|
566 if (((cxq & ~_LBIT)|UNS(_EntryList)) == 0) { |
|
567 return ; // normal fast-path exit - cxq and EntryList both empty |
|
568 } |
|
569 if (cxq & _LBIT) { |
|
570 // Optional optimization ... |
|
571 // Some other thread acquired the lock in the window since this |
|
572 // thread released it. Succession is now that thread's responsibility. |
|
573 return ; |
|
574 } |
|
575 |
|
576 Succession: |
|
577 // Slow-path exit - this thread must ensure succession and progress. |
|
578 // OnDeck serves as lock to protect cxq and EntryList. |
|
579 // Only the holder of OnDeck can manipulate EntryList or detach the RATs from cxq. |
|
580 // Avoid ABA - allow multiple concurrent producers (enqueue via push-CAS) |
|
581 // but only one concurrent consumer (detacher of RATs). |
|
582 // Consider protecting this critical section with schedctl on Solaris. |
|
583 // Unlike a normal lock, however, the exiting thread "locks" OnDeck, |
|
584 // picks a successor and marks that thread as OnDeck. That successor |
|
585 // thread will then clear OnDeck once it eventually acquires the outer lock. |
|
586 if (CASPTR (&_OnDeck, NULL, _LBIT) != UNS(NULL)) { |
|
587 return ; |
|
588 } |
|
589 |
|
590 ParkEvent * List = _EntryList ; |
|
591 if (List != NULL) { |
|
592 // Transfer the head of the EntryList to the OnDeck position. |
|
593 // Once OnDeck, a thread stays OnDeck until it acquires the lock. |
|
594 // For a given lock there is at most OnDeck thread at any one instant. |
|
595 WakeOne: |
|
596 assert (List == _EntryList, "invariant") ; |
|
597 ParkEvent * const w = List ; |
|
598 assert (RelaxAssert || w != Thread::current()->_MutexEvent, "invariant") ; |
|
599 _EntryList = w->ListNext ; |
|
600 // as a diagnostic measure consider setting w->_ListNext = BAD |
|
601 assert (UNS(_OnDeck) == _LBIT, "invariant") ; |
|
602 _OnDeck = w ; // pass OnDeck to w. |
|
603 // w will clear OnDeck once it acquires the outer lock |
|
604 |
|
605 // Another optional optimization ... |
|
606 // For heavily contended locks it's not uncommon that some other |
|
607 // thread acquired the lock while this thread was arranging succession. |
|
608 // Try to defer the unpark() operation - Delegate the responsibility |
|
609 // for unpark()ing the OnDeck thread to the current or subsequent owners |
|
610 // That is, the new owner is responsible for unparking the OnDeck thread. |
|
611 OrderAccess::storeload() ; |
|
612 cxq = _LockWord.FullWord ; |
|
613 if (cxq & _LBIT) return ; |
|
614 |
|
615 w->unpark() ; |
|
616 return ; |
|
617 } |
|
618 |
|
619 cxq = _LockWord.FullWord ; |
|
620 if ((cxq & ~_LBIT) != 0) { |
|
621 // The EntryList is empty but the cxq is populated. |
|
622 // drain RATs from cxq into EntryList |
|
623 // Detach RATs segment with CAS and then merge into EntryList |
|
624 for (;;) { |
|
625 // optional optimization - if locked, the owner is responsible for succession |
|
626 if (cxq & _LBIT) goto Punt ; |
|
627 const intptr_t vfy = CASPTR (&_LockWord, cxq, cxq & _LBIT) ; |
|
628 if (vfy == cxq) break ; |
|
629 cxq = vfy ; |
|
630 // Interference - LockWord changed - Just retry |
|
631 // We can see concurrent interference from contending threads |
|
632 // pushing themselves onto the cxq or from lock-unlock operations. |
|
633 // From the perspective of this thread, EntryList is stable and |
|
634 // the cxq is prepend-only -- the head is volatile but the interior |
|
635 // of the cxq is stable. In theory if we encounter interference from threads |
|
636 // pushing onto cxq we could simply break off the original cxq suffix and |
|
637 // move that segment to the EntryList, avoiding a 2nd or multiple CAS attempts |
|
638 // on the high-traffic LockWord variable. For instance lets say the cxq is "ABCD" |
|
639 // when we first fetch cxq above. Between the fetch -- where we observed "A" |
|
640 // -- and CAS -- where we attempt to CAS null over A -- "PQR" arrive, |
|
641 // yielding cxq = "PQRABCD". In this case we could simply set A.ListNext |
|
642 // null, leaving cxq = "PQRA" and transfer the "BCD" segment to the EntryList. |
|
643 // Note too, that it's safe for this thread to traverse the cxq |
|
644 // without taking any special concurrency precautions. |
|
645 } |
|
646 |
|
647 // We don't currently reorder the cxq segment as we move it onto |
|
648 // the EntryList, but it might make sense to reverse the order |
|
649 // or perhaps sort by thread priority. See the comments in |
|
650 // synchronizer.cpp objectMonitor::exit(). |
|
651 assert (_EntryList == NULL, "invariant") ; |
|
652 _EntryList = List = (ParkEvent *)(cxq & ~_LBIT) ; |
|
653 assert (List != NULL, "invariant") ; |
|
654 goto WakeOne ; |
|
655 } |
|
656 |
|
657 // cxq|EntryList is empty. |
|
658 // w == NULL implies that cxq|EntryList == NULL in the past. |
|
659 // Possible race - rare inopportune interleaving. |
|
660 // A thread could have added itself to cxq since this thread previously checked. |
|
661 // Detect and recover by refetching cxq. |
|
662 Punt: |
|
663 assert (UNS(_OnDeck) == _LBIT, "invariant") ; |
|
664 _OnDeck = NULL ; // Release inner lock. |
|
665 OrderAccess::storeload(); // Dekker duality - pivot point |
|
666 |
|
667 // Resample LockWord/cxq to recover from possible race. |
|
668 // For instance, while this thread T1 held OnDeck, some other thread T2 might |
|
669 // acquire the outer lock. Another thread T3 might try to acquire the outer |
|
670 // lock, but encounter contention and enqueue itself on cxq. T2 then drops the |
|
671 // outer lock, but skips succession as this thread T1 still holds OnDeck. |
|
672 // T1 is and remains responsible for ensuring succession of T3. |
|
673 // |
|
674 // Note that we don't need to recheck EntryList, just cxq. |
|
675 // If threads moved onto EntryList since we dropped OnDeck |
|
676 // that implies some other thread forced succession. |
|
677 cxq = _LockWord.FullWord ; |
|
678 if ((cxq & ~_LBIT) != 0 && (cxq & _LBIT) == 0) { |
|
679 goto Succession ; // potential race -- re-run succession |
|
680 } |
|
681 return ; |
|
682 } |
|
683 |
|
684 bool Monitor::notify() { |
|
685 assert (_owner == Thread::current(), "invariant") ; |
|
686 assert (ILocked(), "invariant") ; |
|
687 if (_WaitSet == NULL) return true ; |
|
688 NotifyCount ++ ; |
|
689 |
|
690 // Transfer one thread from the WaitSet to the EntryList or cxq. |
|
691 // Currently we just unlink the head of the WaitSet and prepend to the cxq. |
|
692 // And of course we could just unlink it and unpark it, too, but |
|
693 // in that case it'd likely impale itself on the reentry. |
|
694 Thread::muxAcquire (_WaitLock, "notify:WaitLock") ; |
|
695 ParkEvent * nfy = _WaitSet ; |
|
696 if (nfy != NULL) { // DCL idiom |
|
697 _WaitSet = nfy->ListNext ; |
|
698 assert (nfy->Notified == 0, "invariant") ; |
|
699 // push nfy onto the cxq |
|
700 for (;;) { |
|
701 const intptr_t v = _LockWord.FullWord ; |
|
702 assert ((v & 0xFF) == _LBIT, "invariant") ; |
|
703 nfy->ListNext = (ParkEvent *)(v & ~_LBIT); |
|
704 if (CASPTR (&_LockWord, v, UNS(nfy)|_LBIT) == v) break; |
|
705 // interference - _LockWord changed -- just retry |
|
706 } |
|
707 // Note that setting Notified before pushing nfy onto the cxq is |
|
708 // also legal and safe, but the safety properties are much more |
|
709 // subtle, so for the sake of code stewardship ... |
|
710 OrderAccess::fence() ; |
|
711 nfy->Notified = 1; |
|
712 } |
|
713 Thread::muxRelease (_WaitLock) ; |
|
714 if (nfy != NULL && (NativeMonitorFlags & 16)) { |
|
715 // Experimental code ... light up the wakee in the hope that this thread (the owner) |
|
716 // will drop the lock just about the time the wakee comes ONPROC. |
|
717 nfy->unpark() ; |
|
718 } |
|
719 assert (ILocked(), "invariant") ; |
|
720 return true ; |
|
721 } |
|
722 |
|
723 // Currently notifyAll() transfers the waiters one-at-a-time from the waitset |
|
724 // to the cxq. This could be done more efficiently with a single bulk en-mass transfer, |
|
725 // but in practice notifyAll() for large #s of threads is rare and not time-critical. |
|
726 // Beware too, that we invert the order of the waiters. Lets say that the |
|
727 // waitset is "ABCD" and the cxq is "XYZ". After a notifyAll() the waitset |
|
728 // will be empty and the cxq will be "DCBAXYZ". This is benign, of course. |
|
729 |
|
730 bool Monitor::notify_all() { |
|
731 assert (_owner == Thread::current(), "invariant") ; |
|
732 assert (ILocked(), "invariant") ; |
|
733 while (_WaitSet != NULL) notify() ; |
|
734 return true ; |
|
735 } |
|
736 |
|
737 int Monitor::IWait (Thread * Self, jlong timo) { |
|
738 assert (ILocked(), "invariant") ; |
|
739 |
|
740 // Phases: |
|
741 // 1. Enqueue Self on WaitSet - currently prepend |
|
742 // 2. unlock - drop the outer lock |
|
743 // 3. wait for either notification or timeout |
|
744 // 4. lock - reentry - reacquire the outer lock |
|
745 |
|
746 ParkEvent * const ESelf = Self->_MutexEvent ; |
|
747 ESelf->Notified = 0 ; |
|
748 ESelf->reset() ; |
|
749 OrderAccess::fence() ; |
|
750 |
|
751 // Add Self to WaitSet |
|
752 // Ideally only the holder of the outer lock would manipulate the WaitSet - |
|
753 // That is, the outer lock would implicitly protect the WaitSet. |
|
754 // But if a thread in wait() encounters a timeout it will need to dequeue itself |
|
755 // from the WaitSet _before it becomes the owner of the lock. We need to dequeue |
|
756 // as the ParkEvent -- which serves as a proxy for the thread -- can't reside |
|
757 // on both the WaitSet and the EntryList|cxq at the same time.. That is, a thread |
|
758 // on the WaitSet can't be allowed to compete for the lock until it has managed to |
|
759 // unlink its ParkEvent from WaitSet. Thus the need for WaitLock. |
|
760 // Contention on the WaitLock is minimal. |
|
761 // |
|
762 // Another viable approach would be add another ParkEvent, "WaitEvent" to the |
|
763 // thread class. The WaitSet would be composed of WaitEvents. Only the |
|
764 // owner of the outer lock would manipulate the WaitSet. A thread in wait() |
|
765 // could then compete for the outer lock, and then, if necessary, unlink itself |
|
766 // from the WaitSet only after having acquired the outer lock. More precisely, |
|
767 // there would be no WaitLock. A thread in in wait() would enqueue its WaitEvent |
|
768 // on the WaitSet; release the outer lock; wait for either notification or timeout; |
|
769 // reacquire the inner lock; and then, if needed, unlink itself from the WaitSet. |
|
770 // |
|
771 // Alternatively, a 2nd set of list link fields in the ParkEvent might suffice. |
|
772 // One set would be for the WaitSet and one for the EntryList. |
|
773 // We could also deconstruct the ParkEvent into a "pure" event and add a |
|
774 // new immortal/TSM "ListElement" class that referred to ParkEvents. |
|
775 // In that case we could have one ListElement on the WaitSet and another |
|
776 // on the EntryList, with both referring to the same pure Event. |
|
777 |
|
778 Thread::muxAcquire (_WaitLock, "wait:WaitLock:Add") ; |
|
779 ESelf->ListNext = _WaitSet ; |
|
780 _WaitSet = ESelf ; |
|
781 Thread::muxRelease (_WaitLock) ; |
|
782 |
|
783 // Release the outer lock |
|
784 // We call IUnlock (RelaxAssert=true) as a thread T1 might |
|
785 // enqueue itself on the WaitSet, call IUnlock(), drop the lock, |
|
786 // and then stall before it can attempt to wake a successor. |
|
787 // Some other thread T2 acquires the lock, and calls notify(), moving |
|
788 // T1 from the WaitSet to the cxq. T2 then drops the lock. T1 resumes, |
|
789 // and then finds *itself* on the cxq. During the course of a normal |
|
790 // IUnlock() call a thread should _never find itself on the EntryList |
|
791 // or cxq, but in the case of wait() it's possible. |
|
792 // See synchronizer.cpp objectMonitor::wait(). |
|
793 IUnlock (true) ; |
|
794 |
|
795 // Wait for either notification or timeout |
|
796 // Beware that in some circumstances we might propagate |
|
797 // spurious wakeups back to the caller. |
|
798 |
|
799 for (;;) { |
|
800 if (ESelf->Notified) break ; |
|
801 int err = ParkCommon (ESelf, timo) ; |
|
802 if (err == OS_TIMEOUT || (NativeMonitorFlags & 1)) break ; |
|
803 } |
|
804 |
|
805 // Prepare for reentry - if necessary, remove ESelf from WaitSet |
|
806 // ESelf can be: |
|
807 // 1. Still on the WaitSet. This can happen if we exited the loop by timeout. |
|
808 // 2. On the cxq or EntryList |
|
809 // 3. Not resident on cxq, EntryList or WaitSet, but in the OnDeck position. |
|
810 |
|
811 OrderAccess::fence() ; |
|
812 int WasOnWaitSet = 0 ; |
|
813 if (ESelf->Notified == 0) { |
|
814 Thread::muxAcquire (_WaitLock, "wait:WaitLock:remove") ; |
|
815 if (ESelf->Notified == 0) { // DCL idiom |
|
816 assert (_OnDeck != ESelf, "invariant") ; // can't be both OnDeck and on WaitSet |
|
817 // ESelf is resident on the WaitSet -- unlink it. |
|
818 // A doubly-linked list would be better here so we can unlink in constant-time. |
|
819 // We have to unlink before we potentially recontend as ESelf might otherwise |
|
820 // end up on the cxq|EntryList -- it can't be on two lists at once. |
|
821 ParkEvent * p = _WaitSet ; |
|
822 ParkEvent * q = NULL ; // classic q chases p |
|
823 while (p != NULL && p != ESelf) { |
|
824 q = p ; |
|
825 p = p->ListNext ; |
|
826 } |
|
827 assert (p == ESelf, "invariant") ; |
|
828 if (p == _WaitSet) { // found at head |
|
829 assert (q == NULL, "invariant") ; |
|
830 _WaitSet = p->ListNext ; |
|
831 } else { // found in interior |
|
832 assert (q->ListNext == p, "invariant") ; |
|
833 q->ListNext = p->ListNext ; |
|
834 } |
|
835 WasOnWaitSet = 1 ; // We were *not* notified but instead encountered timeout |
|
836 } |
|
837 Thread::muxRelease (_WaitLock) ; |
|
838 } |
|
839 |
|
840 // Reentry phase - reacquire the lock |
|
841 if (WasOnWaitSet) { |
|
842 // ESelf was previously on the WaitSet but we just unlinked it above |
|
843 // because of a timeout. ESelf is not resident on any list and is not OnDeck |
|
844 assert (_OnDeck != ESelf, "invariant") ; |
|
845 ILock (Self) ; |
|
846 } else { |
|
847 // A prior notify() operation moved ESelf from the WaitSet to the cxq. |
|
848 // ESelf is now on the cxq, EntryList or at the OnDeck position. |
|
849 // The following fragment is extracted from Monitor::ILock() |
|
850 for (;;) { |
|
851 if (_OnDeck == ESelf && TrySpin(Self)) break ; |
|
852 ParkCommon (ESelf, 0) ; |
|
853 } |
|
854 assert (_OnDeck == ESelf, "invariant") ; |
|
855 _OnDeck = NULL ; |
|
856 } |
|
857 |
|
858 assert (ILocked(), "invariant") ; |
|
859 return WasOnWaitSet != 0 ; // return true IFF timeout |
|
860 } |
|
861 |
|
862 |
|
863 // ON THE VMTHREAD SNEAKING PAST HELD LOCKS: |
|
864 // In particular, there are certain types of global lock that may be held |
|
865 // by a Java thread while it is blocked at a safepoint but before it has |
|
866 // written the _owner field. These locks may be sneakily acquired by the |
|
867 // VM thread during a safepoint to avoid deadlocks. Alternatively, one should |
|
868 // identify all such locks, and ensure that Java threads never block at |
|
869 // safepoints while holding them (_no_safepoint_check_flag). While it |
|
870 // seems as though this could increase the time to reach a safepoint |
|
871 // (or at least increase the mean, if not the variance), the latter |
|
872 // approach might make for a cleaner, more maintainable JVM design. |
|
873 // |
|
874 // Sneaking is vile and reprehensible and should be excised at the 1st |
|
875 // opportunity. It's possible that the need for sneaking could be obviated |
|
876 // as follows. Currently, a thread might (a) while TBIVM, call pthread_mutex_lock |
|
877 // or ILock() thus acquiring the "physical" lock underlying Monitor/Mutex. |
|
878 // (b) stall at the TBIVM exit point as a safepoint is in effect. Critically, |
|
879 // it'll stall at the TBIVM reentry state transition after having acquired the |
|
880 // underlying lock, but before having set _owner and having entered the actual |
|
881 // critical section. The lock-sneaking facility leverages that fact and allowed the |
|
882 // VM thread to logically acquire locks that had already be physically locked by mutators |
|
883 // but where mutators were known blocked by the reentry thread state transition. |
|
884 // |
|
885 // If we were to modify the Monitor-Mutex so that TBIVM state transitions tightly |
|
886 // wrapped calls to park(), then we could likely do away with sneaking. We'd |
|
887 // decouple lock acquisition and parking. The critical invariant to eliminating |
|
888 // sneaking is to ensure that we never "physically" acquire the lock while TBIVM. |
|
889 // An easy way to accomplish this is to wrap the park calls in a narrow TBIVM jacket. |
|
890 // One difficulty with this approach is that the TBIVM wrapper could recurse and |
|
891 // call lock() deep from within a lock() call, while the MutexEvent was already enqueued. |
|
892 // Using a stack (N=2 at minimum) of ParkEvents would take care of that problem. |
|
893 // |
|
894 // But of course the proper ultimate approach is to avoid schemes that require explicit |
|
895 // sneaking or dependence on any any clever invariants or subtle implementation properties |
|
896 // of Mutex-Monitor and instead directly address the underlying design flaw. |
|
897 |
|
898 void Monitor::lock (Thread * Self) { |
|
899 #ifdef CHECK_UNHANDLED_OOPS |
|
900 // Clear unhandled oops so we get a crash right away. Only clear for non-vm |
|
901 // or GC threads. |
|
902 if (Self->is_Java_thread()) { |
|
903 Self->clear_unhandled_oops(); |
|
904 } |
|
905 #endif // CHECK_UNHANDLED_OOPS |
|
906 |
|
907 debug_only(check_prelock_state(Self)); |
|
908 assert (_owner != Self , "invariant") ; |
|
909 assert (_OnDeck != Self->_MutexEvent, "invariant") ; |
|
910 |
|
911 if (TryFast()) { |
|
912 Exeunt: |
|
913 assert (ILocked(), "invariant") ; |
|
914 assert (owner() == NULL, "invariant"); |
|
915 set_owner (Self); |
|
916 return ; |
|
917 } |
|
918 |
|
919 // The lock is contended ... |
|
920 |
|
921 bool can_sneak = Self->is_VM_thread() && SafepointSynchronize::is_at_safepoint(); |
|
922 if (can_sneak && _owner == NULL) { |
|
923 // a java thread has locked the lock but has not entered the |
|
924 // critical region -- let's just pretend we've locked the lock |
|
925 // and go on. we note this with _snuck so we can also |
|
926 // pretend to unlock when the time comes. |
|
927 _snuck = true; |
|
928 goto Exeunt ; |
|
929 } |
|
930 |
|
931 // Try a brief spin to avoid passing thru thread state transition ... |
|
932 if (TrySpin (Self)) goto Exeunt ; |
|
933 |
|
934 check_block_state(Self); |
|
935 if (Self->is_Java_thread()) { |
|
936 // Horribile dictu - we suffer through a state transition |
|
937 assert(rank() > Mutex::special, "Potential deadlock with special or lesser rank mutex"); |
|
938 ThreadBlockInVM tbivm ((JavaThread *) Self) ; |
|
939 ILock (Self) ; |
|
940 } else { |
|
941 // Mirabile dictu |
|
942 ILock (Self) ; |
|
943 } |
|
944 goto Exeunt ; |
|
945 } |
|
946 |
|
947 void Monitor::lock() { |
|
948 this->lock(Thread::current()); |
|
949 } |
|
950 |
|
951 // Lock without safepoint check - a degenerate variant of lock(). |
|
952 // Should ONLY be used by safepoint code and other code |
|
953 // that is guaranteed not to block while running inside the VM. If this is called with |
|
954 // thread state set to be in VM, the safepoint synchronization code will deadlock! |
|
955 |
|
956 void Monitor::lock_without_safepoint_check (Thread * Self) { |
|
957 assert (_owner != Self, "invariant") ; |
|
958 ILock (Self) ; |
|
959 assert (_owner == NULL, "invariant"); |
|
960 set_owner (Self); |
|
961 } |
|
962 |
|
963 void Monitor::lock_without_safepoint_check () { |
|
964 lock_without_safepoint_check (Thread::current()) ; |
|
965 } |
|
966 |
|
967 |
|
968 // Returns true if thread succeceed [sic] in grabbing the lock, otherwise false. |
|
969 |
|
970 bool Monitor::try_lock() { |
|
971 Thread * const Self = Thread::current(); |
|
972 debug_only(check_prelock_state(Self)); |
|
973 // assert(!thread->is_inside_signal_handler(), "don't lock inside signal handler"); |
|
974 |
|
975 // Special case, where all Java threads are stopped. |
|
976 // The lock may have been acquired but _owner is not yet set. |
|
977 // In that case the VM thread can safely grab the lock. |
|
978 // It strikes me this should appear _after the TryLock() fails, below. |
|
979 bool can_sneak = Self->is_VM_thread() && SafepointSynchronize::is_at_safepoint(); |
|
980 if (can_sneak && _owner == NULL) { |
|
981 set_owner(Self); // Do not need to be atomic, since we are at a safepoint |
|
982 _snuck = true; |
|
983 return true; |
|
984 } |
|
985 |
|
986 if (TryLock()) { |
|
987 // We got the lock |
|
988 assert (_owner == NULL, "invariant"); |
|
989 set_owner (Self); |
|
990 return true; |
|
991 } |
|
992 return false; |
|
993 } |
|
994 |
|
995 void Monitor::unlock() { |
|
996 assert (_owner == Thread::current(), "invariant") ; |
|
997 assert (_OnDeck != Thread::current()->_MutexEvent , "invariant") ; |
|
998 set_owner (NULL) ; |
|
999 if (_snuck) { |
|
1000 assert(SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread(), "sneak"); |
|
1001 _snuck = false; |
|
1002 return ; |
|
1003 } |
|
1004 IUnlock (false) ; |
|
1005 } |
|
1006 |
|
1007 // Yet another degenerate version of Monitor::lock() or lock_without_safepoint_check() |
|
1008 // jvm_raw_lock() and _unlock() can be called by non-Java threads via JVM_RawMonitorEnter. |
|
1009 // |
|
1010 // There's no expectation that JVM_RawMonitors will interoperate properly with the native |
|
1011 // Mutex-Monitor constructs. We happen to implement JVM_RawMonitors in terms of |
|
1012 // native Mutex-Monitors simply as a matter of convenience. A simple abstraction layer |
|
1013 // over a pthread_mutex_t would work equally as well, but require more platform-specific |
|
1014 // code -- a "PlatformMutex". Alternatively, a simply layer over muxAcquire-muxRelease |
|
1015 // would work too. |
|
1016 // |
|
1017 // Since the caller might be a foreign thread, we don't necessarily have a Thread.MutexEvent |
|
1018 // instance available. Instead, we transiently allocate a ParkEvent on-demand if |
|
1019 // we encounter contention. That ParkEvent remains associated with the thread |
|
1020 // until it manages to acquire the lock, at which time we return the ParkEvent |
|
1021 // to the global ParkEvent free list. This is correct and suffices for our purposes. |
|
1022 // |
|
1023 // Beware that the original jvm_raw_unlock() had a "_snuck" test but that |
|
1024 // jvm_raw_lock() didn't have the corresponding test. I suspect that's an |
|
1025 // oversight, but I've replicated the original suspect logic in the new code ... |
|
1026 |
|
1027 void Monitor::jvm_raw_lock() { |
|
1028 assert(rank() == native, "invariant"); |
|
1029 |
|
1030 if (TryLock()) { |
|
1031 Exeunt: |
|
1032 assert (ILocked(), "invariant") ; |
|
1033 assert (_owner == NULL, "invariant"); |
|
1034 // This can potentially be called by non-java Threads. Thus, the ThreadLocalStorage |
|
1035 // might return NULL. Don't call set_owner since it will break on an NULL owner |
|
1036 // Consider installing a non-null "ANON" distinguished value instead of just NULL. |
|
1037 _owner = ThreadLocalStorage::thread(); |
|
1038 return ; |
|
1039 } |
|
1040 |
|
1041 if (TrySpin(NULL)) goto Exeunt ; |
|
1042 |
|
1043 // slow-path - apparent contention |
|
1044 // Allocate a ParkEvent for transient use. |
|
1045 // The ParkEvent remains associated with this thread until |
|
1046 // the time the thread manages to acquire the lock. |
|
1047 ParkEvent * const ESelf = ParkEvent::Allocate(NULL) ; |
|
1048 ESelf->reset() ; |
|
1049 OrderAccess::storeload() ; |
|
1050 |
|
1051 // Either Enqueue Self on cxq or acquire the outer lock. |
|
1052 if (AcquireOrPush (ESelf)) { |
|
1053 ParkEvent::Release (ESelf) ; // surrender the ParkEvent |
|
1054 goto Exeunt ; |
|
1055 } |
|
1056 |
|
1057 // At any given time there is at most one ondeck thread. |
|
1058 // ondeck implies not resident on cxq and not resident on EntryList |
|
1059 // Only the OnDeck thread can try to acquire -- contended for -- the lock. |
|
1060 // CONSIDER: use Self->OnDeck instead of m->OnDeck. |
|
1061 for (;;) { |
|
1062 if (_OnDeck == ESelf && TrySpin(NULL)) break ; |
|
1063 ParkCommon (ESelf, 0) ; |
|
1064 } |
|
1065 |
|
1066 assert (_OnDeck == ESelf, "invariant") ; |
|
1067 _OnDeck = NULL ; |
|
1068 ParkEvent::Release (ESelf) ; // surrender the ParkEvent |
|
1069 goto Exeunt ; |
|
1070 } |
|
1071 |
|
1072 void Monitor::jvm_raw_unlock() { |
|
1073 // Nearly the same as Monitor::unlock() ... |
|
1074 // directly set _owner instead of using set_owner(null) |
|
1075 _owner = NULL ; |
|
1076 if (_snuck) { // ??? |
|
1077 assert(SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread(), "sneak"); |
|
1078 _snuck = false; |
|
1079 return ; |
|
1080 } |
|
1081 IUnlock(false) ; |
|
1082 } |
|
1083 |
|
1084 bool Monitor::wait(bool no_safepoint_check, long timeout, bool as_suspend_equivalent) { |
|
1085 Thread * const Self = Thread::current() ; |
|
1086 assert (_owner == Self, "invariant") ; |
|
1087 assert (ILocked(), "invariant") ; |
|
1088 |
|
1089 // as_suspend_equivalent logically implies !no_safepoint_check |
|
1090 guarantee (!as_suspend_equivalent || !no_safepoint_check, "invariant") ; |
|
1091 // !no_safepoint_check logically implies java_thread |
|
1092 guarantee (no_safepoint_check || Self->is_Java_thread(), "invariant") ; |
|
1093 |
|
1094 #ifdef ASSERT |
|
1095 Monitor * least = get_least_ranked_lock_besides_this(Self->owned_locks()); |
|
1096 assert(least != this, "Specification of get_least_... call above"); |
|
1097 if (least != NULL && least->rank() <= special) { |
|
1098 tty->print("Attempting to wait on monitor %s/%d while holding" |
|
1099 " lock %s/%d -- possible deadlock", |
|
1100 name(), rank(), least->name(), least->rank()); |
|
1101 assert(false, "Shouldn't block(wait) while holding a lock of rank special"); |
|
1102 } |
|
1103 #endif // ASSERT |
|
1104 |
|
1105 int wait_status ; |
|
1106 // conceptually set the owner to NULL in anticipation of |
|
1107 // abdicating the lock in wait |
|
1108 set_owner(NULL); |
|
1109 if (no_safepoint_check) { |
|
1110 wait_status = IWait (Self, timeout) ; |
|
1111 } else { |
|
1112 assert (Self->is_Java_thread(), "invariant") ; |
|
1113 JavaThread *jt = (JavaThread *)Self; |
|
1114 |
|
1115 // Enter safepoint region - ornate and Rococo ... |
|
1116 ThreadBlockInVM tbivm(jt); |
|
1117 OSThreadWaitState osts(Self->osthread(), false /* not Object.wait() */); |
|
1118 |
|
1119 if (as_suspend_equivalent) { |
|
1120 jt->set_suspend_equivalent(); |
|
1121 // cleared by handle_special_suspend_equivalent_condition() or |
|
1122 // java_suspend_self() |
|
1123 } |
|
1124 |
|
1125 wait_status = IWait (Self, timeout) ; |
|
1126 |
|
1127 // were we externally suspended while we were waiting? |
|
1128 if (as_suspend_equivalent && jt->handle_special_suspend_equivalent_condition()) { |
|
1129 // Our event wait has finished and we own the lock, but |
|
1130 // while we were waiting another thread suspended us. We don't |
|
1131 // want to hold the lock while suspended because that |
|
1132 // would surprise the thread that suspended us. |
|
1133 assert (ILocked(), "invariant") ; |
|
1134 IUnlock (true) ; |
|
1135 jt->java_suspend_self(); |
|
1136 ILock (Self) ; |
|
1137 assert (ILocked(), "invariant") ; |
|
1138 } |
|
1139 } |
|
1140 |
|
1141 // Conceptually reestablish ownership of the lock. |
|
1142 // The "real" lock -- the LockByte -- was reacquired by IWait(). |
|
1143 assert (ILocked(), "invariant") ; |
|
1144 assert (_owner == NULL, "invariant") ; |
|
1145 set_owner (Self) ; |
|
1146 return wait_status != 0 ; // return true IFF timeout |
|
1147 } |
|
1148 |
|
1149 Monitor::~Monitor() { |
|
1150 assert ((UNS(_owner)|UNS(_LockWord.FullWord)|UNS(_EntryList)|UNS(_WaitSet)|UNS(_OnDeck)) == 0, "") ; |
|
1151 } |
|
1152 |
|
1153 void Monitor::ClearMonitor (Monitor * m, const char *name) { |
|
1154 m->_owner = NULL ; |
|
1155 m->_snuck = false ; |
|
1156 if (name == NULL) { |
|
1157 strcpy(m->_name, "UNKNOWN") ; |
|
1158 } else { |
|
1159 strncpy(m->_name, name, MONITOR_NAME_LEN - 1); |
|
1160 m->_name[MONITOR_NAME_LEN - 1] = '\0'; |
|
1161 } |
|
1162 m->_LockWord.FullWord = 0 ; |
|
1163 m->_EntryList = NULL ; |
|
1164 m->_OnDeck = NULL ; |
|
1165 m->_WaitSet = NULL ; |
|
1166 m->_WaitLock[0] = 0 ; |
|
1167 } |
|
1168 |
|
1169 Monitor::Monitor() { ClearMonitor(this); } |
|
1170 |
|
1171 Monitor::Monitor (int Rank, const char * name, bool allow_vm_block) { |
|
1172 ClearMonitor (this, name) ; |
|
1173 #ifdef ASSERT |
|
1174 _allow_vm_block = allow_vm_block; |
|
1175 _rank = Rank ; |
|
1176 #endif |
|
1177 } |
|
1178 |
|
1179 Mutex::~Mutex() { |
|
1180 assert ((UNS(_owner)|UNS(_LockWord.FullWord)|UNS(_EntryList)|UNS(_WaitSet)|UNS(_OnDeck)) == 0, "") ; |
|
1181 } |
|
1182 |
|
1183 Mutex::Mutex (int Rank, const char * name, bool allow_vm_block) { |
|
1184 ClearMonitor ((Monitor *) this, name) ; |
|
1185 #ifdef ASSERT |
|
1186 _allow_vm_block = allow_vm_block; |
|
1187 _rank = Rank ; |
|
1188 #endif |
|
1189 } |
|
1190 |
|
1191 bool Monitor::owned_by_self() const { |
|
1192 bool ret = _owner == Thread::current(); |
|
1193 assert (!ret || _LockWord.Bytes[_LSBINDEX] != 0, "invariant") ; |
|
1194 return ret; |
|
1195 } |
|
1196 |
|
1197 void Monitor::print_on_error(outputStream* st) const { |
|
1198 st->print("[" PTR_FORMAT, this); |
|
1199 st->print("] %s", _name); |
|
1200 st->print(" - owner thread: " PTR_FORMAT, _owner); |
|
1201 } |
|
1202 |
|
1203 |
|
1204 |
|
1205 |
|
1206 // ---------------------------------------------------------------------------------- |
|
1207 // Non-product code |
|
1208 |
|
1209 #ifndef PRODUCT |
|
1210 void Monitor::print_on(outputStream* st) const { |
|
1211 st->print_cr("Mutex: [0x%lx/0x%lx] %s - owner: 0x%lx", this, _LockWord.FullWord, _name, _owner); |
|
1212 } |
|
1213 #endif |
|
1214 |
|
1215 #ifndef PRODUCT |
|
1216 #ifdef ASSERT |
|
1217 Monitor * Monitor::get_least_ranked_lock(Monitor * locks) { |
|
1218 Monitor *res, *tmp; |
|
1219 for (res = tmp = locks; tmp != NULL; tmp = tmp->next()) { |
|
1220 if (tmp->rank() < res->rank()) { |
|
1221 res = tmp; |
|
1222 } |
|
1223 } |
|
1224 if (!SafepointSynchronize::is_at_safepoint()) { |
|
1225 // In this case, we expect the held locks to be |
|
1226 // in increasing rank order (modulo any native ranks) |
|
1227 for (tmp = locks; tmp != NULL; tmp = tmp->next()) { |
|
1228 if (tmp->next() != NULL) { |
|
1229 assert(tmp->rank() == Mutex::native || |
|
1230 tmp->rank() <= tmp->next()->rank(), "mutex rank anomaly?"); |
|
1231 } |
|
1232 } |
|
1233 } |
|
1234 return res; |
|
1235 } |
|
1236 |
|
1237 Monitor* Monitor::get_least_ranked_lock_besides_this(Monitor* locks) { |
|
1238 Monitor *res, *tmp; |
|
1239 for (res = NULL, tmp = locks; tmp != NULL; tmp = tmp->next()) { |
|
1240 if (tmp != this && (res == NULL || tmp->rank() < res->rank())) { |
|
1241 res = tmp; |
|
1242 } |
|
1243 } |
|
1244 if (!SafepointSynchronize::is_at_safepoint()) { |
|
1245 // In this case, we expect the held locks to be |
|
1246 // in increasing rank order (modulo any native ranks) |
|
1247 for (tmp = locks; tmp != NULL; tmp = tmp->next()) { |
|
1248 if (tmp->next() != NULL) { |
|
1249 assert(tmp->rank() == Mutex::native || |
|
1250 tmp->rank() <= tmp->next()->rank(), "mutex rank anomaly?"); |
|
1251 } |
|
1252 } |
|
1253 } |
|
1254 return res; |
|
1255 } |
|
1256 |
|
1257 |
|
1258 bool Monitor::contains(Monitor* locks, Monitor * lock) { |
|
1259 for (; locks != NULL; locks = locks->next()) { |
|
1260 if (locks == lock) |
|
1261 return true; |
|
1262 } |
|
1263 return false; |
|
1264 } |
|
1265 #endif |
|
1266 |
|
1267 // Called immediately after lock acquisition or release as a diagnostic |
|
1268 // to track the lock-set of the thread and test for rank violations that |
|
1269 // might indicate exposure to deadlock. |
|
1270 // Rather like an EventListener for _owner (:>). |
|
1271 |
|
1272 void Monitor::set_owner_implementation(Thread *new_owner) { |
|
1273 // This function is solely responsible for maintaining |
|
1274 // and checking the invariant that threads and locks |
|
1275 // are in a 1/N relation, with some some locks unowned. |
|
1276 // It uses the Mutex::_owner, Mutex::_next, and |
|
1277 // Thread::_owned_locks fields, and no other function |
|
1278 // changes those fields. |
|
1279 // It is illegal to set the mutex from one non-NULL |
|
1280 // owner to another--it must be owned by NULL as an |
|
1281 // intermediate state. |
|
1282 |
|
1283 if (new_owner != NULL) { |
|
1284 // the thread is acquiring this lock |
|
1285 |
|
1286 assert(new_owner == Thread::current(), "Should I be doing this?"); |
|
1287 assert(_owner == NULL, "setting the owner thread of an already owned mutex"); |
|
1288 _owner = new_owner; // set the owner |
|
1289 |
|
1290 // link "this" into the owned locks list |
|
1291 |
|
1292 #ifdef ASSERT // Thread::_owned_locks is under the same ifdef |
|
1293 Monitor* locks = get_least_ranked_lock(new_owner->owned_locks()); |
|
1294 // Mutex::set_owner_implementation is a friend of Thread |
|
1295 |
|
1296 assert(this->rank() >= 0, "bad lock rank"); |
|
1297 |
|
1298 // Deadlock avoidance rules require us to acquire Mutexes only in |
|
1299 // a global total order. For example m1 is the lowest ranked mutex |
|
1300 // that the thread holds and m2 is the mutex the thread is trying |
|
1301 // to acquire, then deadlock avoidance rules require that the rank |
|
1302 // of m2 be less than the rank of m1. |
|
1303 // The rank Mutex::native is an exception in that it is not subject |
|
1304 // to the verification rules. |
|
1305 // Here are some further notes relating to mutex acquisition anomalies: |
|
1306 // . under Solaris, the interrupt lock gets acquired when doing |
|
1307 // profiling, so any lock could be held. |
|
1308 // . it is also ok to acquire Safepoint_lock at the very end while we |
|
1309 // already hold Terminator_lock - may happen because of periodic safepoints |
|
1310 if (this->rank() != Mutex::native && |
|
1311 this->rank() != Mutex::suspend_resume && |
|
1312 locks != NULL && locks->rank() <= this->rank() && |
|
1313 !SafepointSynchronize::is_at_safepoint() && |
|
1314 this != Interrupt_lock && this != ProfileVM_lock && |
|
1315 !(this == Safepoint_lock && contains(locks, Terminator_lock) && |
|
1316 SafepointSynchronize::is_synchronizing())) { |
|
1317 new_owner->print_owned_locks(); |
|
1318 fatal(err_msg("acquiring lock %s/%d out of order with lock %s/%d -- " |
|
1319 "possible deadlock", this->name(), this->rank(), |
|
1320 locks->name(), locks->rank())); |
|
1321 } |
|
1322 |
|
1323 this->_next = new_owner->_owned_locks; |
|
1324 new_owner->_owned_locks = this; |
|
1325 #endif |
|
1326 |
|
1327 } else { |
|
1328 // the thread is releasing this lock |
|
1329 |
|
1330 Thread* old_owner = _owner; |
|
1331 debug_only(_last_owner = old_owner); |
|
1332 |
|
1333 assert(old_owner != NULL, "removing the owner thread of an unowned mutex"); |
|
1334 assert(old_owner == Thread::current(), "removing the owner thread of an unowned mutex"); |
|
1335 |
|
1336 _owner = NULL; // set the owner |
|
1337 |
|
1338 #ifdef ASSERT |
|
1339 Monitor *locks = old_owner->owned_locks(); |
|
1340 |
|
1341 // remove "this" from the owned locks list |
|
1342 |
|
1343 Monitor *prev = NULL; |
|
1344 bool found = false; |
|
1345 for (; locks != NULL; prev = locks, locks = locks->next()) { |
|
1346 if (locks == this) { |
|
1347 found = true; |
|
1348 break; |
|
1349 } |
|
1350 } |
|
1351 assert(found, "Removing a lock not owned"); |
|
1352 if (prev == NULL) { |
|
1353 old_owner->_owned_locks = _next; |
|
1354 } else { |
|
1355 prev->_next = _next; |
|
1356 } |
|
1357 _next = NULL; |
|
1358 #endif |
|
1359 } |
|
1360 } |
|
1361 |
|
1362 |
|
1363 // Factored out common sanity checks for locking mutex'es. Used by lock() and try_lock() |
|
1364 void Monitor::check_prelock_state(Thread *thread) { |
|
1365 assert((!thread->is_Java_thread() || ((JavaThread *)thread)->thread_state() == _thread_in_vm) |
|
1366 || rank() == Mutex::special, "wrong thread state for using locks"); |
|
1367 if (StrictSafepointChecks) { |
|
1368 if (thread->is_VM_thread() && !allow_vm_block()) { |
|
1369 fatal(err_msg("VM thread using lock %s (not allowed to block on)", |
|
1370 name())); |
|
1371 } |
|
1372 debug_only(if (rank() != Mutex::special) \ |
|
1373 thread->check_for_valid_safepoint_state(false);) |
|
1374 } |
|
1375 if (thread->is_Watcher_thread()) { |
|
1376 assert(!WatcherThread::watcher_thread()->has_crash_protection(), |
|
1377 "locking not allowed when crash protection is set"); |
|
1378 } |
|
1379 } |
|
1380 |
|
1381 void Monitor::check_block_state(Thread *thread) { |
|
1382 if (!_allow_vm_block && thread->is_VM_thread()) { |
|
1383 warning("VM thread blocked on lock"); |
|
1384 print(); |
|
1385 BREAKPOINT; |
|
1386 } |
|
1387 assert(_owner != thread, "deadlock: blocking on monitor owned by current thread"); |
|
1388 } |
|
1389 |
|
1390 #endif // PRODUCT |