1 /*
2 * Copyright (c) 1998, 2017, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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5 * This code is free software; you can redistribute it and/or modify it
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23 */
24
25 #include "precompiled.hpp"
26 #include "runtime/atomic.hpp"
27 #include "runtime/interfaceSupport.hpp"
28 #include "runtime/mutex.hpp"
29 #include "runtime/orderAccess.inline.hpp"
30 #include "runtime/osThread.hpp"
31 #include "runtime/safepointMechanism.inline.hpp"
32 #include "runtime/thread.inline.hpp"
33 #include "utilities/events.hpp"
34 #include "utilities/macros.hpp"
35
36 // 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
37 //
38 // Native Monitor-Mutex locking - theory of operations
39 //
40 // * Native Monitors are completely unrelated to Java-level monitors,
41 // although the "back-end" slow-path implementations share a common lineage.
42 // See objectMonitor:: in synchronizer.cpp.
43 // Native Monitors do *not* support nesting or recursion but otherwise
44 // they're basically Hoare-flavor monitors.
45 //
46 // * A thread acquires ownership of a Monitor/Mutex by CASing the LockByte
47 // in the _LockWord from zero to non-zero. Note that the _Owner field
48 // is advisory and is used only to verify that the thread calling unlock()
49 // is indeed the last thread to have acquired the lock.
50 //
51 // * Contending threads "push" themselves onto the front of the contention
52 // queue -- called the cxq -- with CAS and then spin/park.
53 // The _LockWord contains the LockByte as well as the pointer to the head
54 // of the cxq. Colocating the LockByte with the cxq precludes certain races.
55 //
56 // * Using a separately addressable LockByte allows for CAS:MEMBAR or CAS:0
57 // idioms. We currently use MEMBAR in the uncontended unlock() path, as
58 // MEMBAR often has less latency than CAS. If warranted, we could switch to
59 // a CAS:0 mode, using timers to close the resultant race, as is done
60 // with Java Monitors in synchronizer.cpp.
61 //
62 // See the following for a discussion of the relative cost of atomics (CAS)
63 // MEMBAR, and ways to eliminate such instructions from the common-case paths:
64 // -- http://blogs.sun.com/dave/entry/biased_locking_in_hotspot
65 // -- http://blogs.sun.com/dave/resource/MustangSync.pdf
66 // -- http://blogs.sun.com/dave/resource/synchronization-public2.pdf
67 // -- synchronizer.cpp
68 //
69 // * Overall goals - desiderata
70 // 1. Minimize context switching
71 // 2. Minimize lock migration
72 // 3. Minimize CPI -- affinity and locality
73 // 4. Minimize the execution of high-latency instructions such as CAS or MEMBAR
74 // 5. Minimize outer lock hold times
75 // 6. Behave gracefully on a loaded system
76 //
77 // * Thread flow and list residency:
78 //
79 // Contention queue --> EntryList --> OnDeck --> Owner --> !Owner
80 // [..resident on monitor list..]
81 // [...........contending..................]
82 //
83 // -- The contention queue (cxq) contains recently-arrived threads (RATs).
84 // Threads on the cxq eventually drain into the EntryList.
85 // -- Invariant: a thread appears on at most one list -- cxq, EntryList
86 // or WaitSet -- at any one time.
87 // -- For a given monitor there can be at most one "OnDeck" thread at any
88 // given time but if needbe this particular invariant could be relaxed.
89 //
90 // * The WaitSet and EntryList linked lists are composed of ParkEvents.
91 // I use ParkEvent instead of threads as ParkEvents are immortal and
92 // type-stable, meaning we can safely unpark() a possibly stale
93 // list element in the unlock()-path. (That's benign).
94 //
95 // * Succession policy - providing for progress:
96 //
97 // As necessary, the unlock()ing thread identifies, unlinks, and unparks
98 // an "heir presumptive" tentative successor thread from the EntryList.
99 // This becomes the so-called "OnDeck" thread, of which there can be only
100 // one at any given time for a given monitor. The wakee will recontend
101 // for ownership of monitor.
102 //
103 // Succession is provided for by a policy of competitive handoff.
104 // The exiting thread does _not_ grant or pass ownership to the
105 // successor thread. (This is also referred to as "handoff" succession").
106 // Instead the exiting thread releases ownership and possibly wakes
107 // a successor, so the successor can (re)compete for ownership of the lock.
108 //
109 // Competitive handoff provides excellent overall throughput at the expense
110 // of short-term fairness. If fairness is a concern then one remedy might
111 // be to add an AcquireCounter field to the monitor. After a thread acquires
112 // the lock it will decrement the AcquireCounter field. When the count
113 // reaches 0 the thread would reset the AcquireCounter variable, abdicate
114 // the lock directly to some thread on the EntryList, and then move itself to the
115 // tail of the EntryList.
116 //
117 // But in practice most threads engage or otherwise participate in resource
118 // bounded producer-consumer relationships, so lock domination is not usually
119 // a practical concern. Recall too, that in general it's easier to construct
120 // a fair lock from a fast lock, but not vice-versa.
121 //
122 // * The cxq can have multiple concurrent "pushers" but only one concurrent
123 // detaching thread. This mechanism is immune from the ABA corruption.
124 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious.
125 // We use OnDeck as a pseudo-lock to enforce the at-most-one detaching
126 // thread constraint.
127 //
128 // * Taken together, the cxq and the EntryList constitute or form a
129 // single logical queue of threads stalled trying to acquire the lock.
130 // We use two distinct lists to reduce heat on the list ends.
131 // Threads in lock() enqueue onto cxq while threads in unlock() will
132 // dequeue from the EntryList. (c.f. Michael Scott's "2Q" algorithm).
133 // A key desideratum is to minimize queue & monitor metadata manipulation
134 // that occurs while holding the "outer" monitor lock -- that is, we want to
135 // minimize monitor lock holds times.
136 //
137 // The EntryList is ordered by the prevailing queue discipline and
138 // can be organized in any convenient fashion, such as a doubly-linked list or
139 // a circular doubly-linked list. If we need a priority queue then something akin
140 // to Solaris' sleepq would work nicely. Viz.,
141 // -- http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c.
142 // -- http://cvs.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/uts/common/os/sleepq.c
143 // Queue discipline is enforced at ::unlock() time, when the unlocking thread
144 // drains the cxq into the EntryList, and orders or reorders the threads on the
145 // EntryList accordingly.
146 //
147 // Barring "lock barging", this mechanism provides fair cyclic ordering,
148 // somewhat similar to an elevator-scan.
149 //
150 // * OnDeck
151 // -- For a given monitor there can be at most one OnDeck thread at any given
152 // instant. The OnDeck thread is contending for the lock, but has been
153 // unlinked from the EntryList and cxq by some previous unlock() operations.
154 // Once a thread has been designated the OnDeck thread it will remain so
155 // until it manages to acquire the lock -- being OnDeck is a stable property.
156 // -- Threads on the EntryList or cxq are _not allowed to attempt lock acquisition.
157 // -- OnDeck also serves as an "inner lock" as follows. Threads in unlock() will, after
158 // having cleared the LockByte and dropped the outer lock, attempt to "trylock"
159 // OnDeck by CASing the field from null to non-null. If successful, that thread
160 // is then responsible for progress and succession and can use CAS to detach and
161 // drain the cxq into the EntryList. By convention, only this thread, the holder of
162 // the OnDeck inner lock, can manipulate the EntryList or detach and drain the
163 // RATs on the cxq into the EntryList. This avoids ABA corruption on the cxq as
164 // we allow multiple concurrent "push" operations but restrict detach concurrency
165 // to at most one thread. Having selected and detached a successor, the thread then
166 // changes the OnDeck to refer to that successor, and then unparks the successor.
167 // That successor will eventually acquire the lock and clear OnDeck. Beware
168 // that the OnDeck usage as a lock is asymmetric. A thread in unlock() transiently
169 // "acquires" OnDeck, performs queue manipulations, passes OnDeck to some successor,
170 // and then the successor eventually "drops" OnDeck. Note that there's never
171 // any sense of contention on the inner lock, however. Threads never contend
172 // or wait for the inner lock.
173 // -- OnDeck provides for futile wakeup throttling a described in section 3.3 of
174 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf
175 // In a sense, OnDeck subsumes the ObjectMonitor _Succ and ObjectWaiter
176 // TState fields found in Java-level objectMonitors. (See synchronizer.cpp).
177 //
178 // * Waiting threads reside on the WaitSet list -- wait() puts
179 // the caller onto the WaitSet. Notify() or notifyAll() simply
180 // transfers threads from the WaitSet to either the EntryList or cxq.
181 // Subsequent unlock() operations will eventually unpark the notifyee.
182 // Unparking a notifee in notify() proper is inefficient - if we were to do so
183 // it's likely the notifyee would simply impale itself on the lock held
184 // by the notifier.
185 //
186 // * The mechanism is obstruction-free in that if the holder of the transient
187 // OnDeck lock in unlock() is preempted or otherwise stalls, other threads
188 // can still acquire and release the outer lock and continue to make progress.
189 // At worst, waking of already blocked contending threads may be delayed,
190 // but nothing worse. (We only use "trylock" operations on the inner OnDeck
191 // lock).
192 //
193 // * Note that thread-local storage must be initialized before a thread
194 // uses Native monitors or mutexes. The native monitor-mutex subsystem
195 // depends on Thread::current().
196 //
197 // * The monitor synchronization subsystem avoids the use of native
198 // synchronization primitives except for the narrow platform-specific
199 // park-unpark abstraction. See the comments in os_solaris.cpp regarding
200 // the semantics of park-unpark. Put another way, this monitor implementation
201 // depends only on atomic operations and park-unpark. The monitor subsystem
202 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the
203 // underlying OS manages the READY<->RUN transitions.
204 //
205 // * The memory consistency model provide by lock()-unlock() is at least as
206 // strong or stronger than the Java Memory model defined by JSR-133.
207 // That is, we guarantee at least entry consistency, if not stronger.
208 // See http://g.oswego.edu/dl/jmm/cookbook.html.
209 //
210 // * Thread:: currently contains a set of purpose-specific ParkEvents:
211 // _MutexEvent, _ParkEvent, etc. A better approach might be to do away with
212 // the purpose-specific ParkEvents and instead implement a general per-thread
213 // stack of available ParkEvents which we could provision on-demand. The
214 // stack acts as a local cache to avoid excessive calls to ParkEvent::Allocate()
215 // and ::Release(). A thread would simply pop an element from the local stack before it
216 // enqueued or park()ed. When the contention was over the thread would
217 // push the no-longer-needed ParkEvent back onto its stack.
218 //
219 // * A slightly reduced form of ILock() and IUnlock() have been partially
220 // model-checked (Murphi) for safety and progress at T=1,2,3 and 4.
221 // It'd be interesting to see if TLA/TLC could be useful as well.
222 //
223 // * Mutex-Monitor is a low-level "leaf" subsystem. That is, the monitor
224 // code should never call other code in the JVM that might itself need to
225 // acquire monitors or mutexes. That's true *except* in the case of the
226 // ThreadBlockInVM state transition wrappers. The ThreadBlockInVM DTOR handles
227 // mutator reentry (ingress) by checking for a pending safepoint in which case it will
228 // call SafepointSynchronize::block(), which in turn may call Safepoint_lock->lock(), etc.
229 // In that particular case a call to lock() for a given Monitor can end up recursively
230 // calling lock() on another monitor. While distasteful, this is largely benign
231 // as the calls come from jacket that wraps lock(), and not from deep within lock() itself.
232 //
233 // It's unfortunate that native mutexes and thread state transitions were convolved.
234 // They're really separate concerns and should have remained that way. Melding
235 // them together was facile -- a bit too facile. The current implementation badly
236 // conflates the two concerns.
237 //
238 // * TODO-FIXME:
239 //
240 // -- Add DTRACE probes for contended acquire, contended acquired, contended unlock
241 // We should also add DTRACE probes in the ParkEvent subsystem for
242 // Park-entry, Park-exit, and Unpark.
243 //
244 // -- We have an excess of mutex-like constructs in the JVM, namely:
245 // 1. objectMonitors for Java-level synchronization (synchronizer.cpp)
246 // 2. low-level muxAcquire and muxRelease
247 // 3. low-level spinAcquire and spinRelease
248 // 4. native Mutex:: and Monitor::
249 // 5. jvm_raw_lock() and _unlock()
250 // 6. JVMTI raw monitors -- distinct from (5) despite having a confusingly
251 // similar name.
252 //
253 // 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
254
255 #define UNS(x) (uintptr_t(x))
256 #define TRACE(m) \
257 { \
258 static volatile int ctr = 0; \
259 int x = ++ctr; \
260 if ((x & (x - 1)) == 0) { \
261 ::printf("%d:%s\n", x, #m); \
262 ::fflush(stdout); \
263 } \
264 }
265
266 const intptr_t _LBIT = 1;
267
268 // Endian-ness ... index of least-significant byte in SplitWord.Bytes[]
269 #ifdef VM_LITTLE_ENDIAN
270 #define _LSBINDEX 0
271 #else
272 #define _LSBINDEX (sizeof(intptr_t)-1)
273 #endif
274
275 // Simplistic low-quality Marsaglia SHIFT-XOR RNG.
276 // Bijective except for the trailing mask operation.
277 // Useful for spin loops as the compiler can't optimize it away.
278
279 static inline jint MarsagliaXORV(jint x) {
280 if (x == 0) x = 1|os::random();
281 x ^= x << 6;
282 x ^= ((unsigned)x) >> 21;
283 x ^= x << 7;
284 return x & 0x7FFFFFFF;
285 }
286
287 static int Stall(int its) {
288 static volatile jint rv = 1;
289 volatile int OnFrame = 0;
290 jint v = rv ^ UNS(OnFrame);
291 while (--its >= 0) {
292 v = MarsagliaXORV(v);
293 }
294 // Make this impossible for the compiler to optimize away,
295 // but (mostly) avoid W coherency sharing on MP systems.
296 if (v == 0x12345) rv = v;
297 return v;
298 }
299
300 int Monitor::TryLock() {
301 intptr_t v = _LockWord.FullWord;
302 for (;;) {
303 if ((v & _LBIT) != 0) return 0;
304 const intptr_t u = Atomic::cmpxchg(v|_LBIT, &_LockWord.FullWord, v);
305 if (v == u) return 1;
306 v = u;
307 }
308 }
309
310 int Monitor::TryFast() {
311 // Optimistic fast-path form ...
312 // Fast-path attempt for the common uncontended case.
313 // Avoid RTS->RTO $ coherence upgrade on typical SMP systems.
314 intptr_t v = Atomic::cmpxchg(_LBIT, &_LockWord.FullWord, (intptr_t)0); // agro ...
315 if (v == 0) return 1;
316
317 for (;;) {
318 if ((v & _LBIT) != 0) return 0;
319 const intptr_t u = Atomic::cmpxchg(v|_LBIT, &_LockWord.FullWord, v);
320 if (v == u) return 1;
321 v = u;
322 }
323 }
324
325 int Monitor::ILocked() {
326 const intptr_t w = _LockWord.FullWord & 0xFF;
327 assert(w == 0 || w == _LBIT, "invariant");
328 return w == _LBIT;
329 }
330
331 // Polite TATAS spinlock with exponential backoff - bounded spin.
332 // Ideally we'd use processor cycles, time or vtime to control
333 // the loop, but we currently use iterations.
334 // All the constants within were derived empirically but work over
335 // over the spectrum of J2SE reference platforms.
336 // On Niagara-class systems the back-off is unnecessary but
337 // is relatively harmless. (At worst it'll slightly retard
338 // acquisition times). The back-off is critical for older SMP systems
339 // where constant fetching of the LockWord would otherwise impair
340 // scalability.
341 //
342 // Clamp spinning at approximately 1/2 of a context-switch round-trip.
343 // See synchronizer.cpp for details and rationale.
344
345 int Monitor::TrySpin(Thread * const Self) {
346 if (TryLock()) return 1;
347 if (!os::is_MP()) return 0;
348
349 int Probes = 0;
350 int Delay = 0;
351 int Steps = 0;
352 int SpinMax = NativeMonitorSpinLimit;
353 int flgs = NativeMonitorFlags;
354 for (;;) {
355 intptr_t v = _LockWord.FullWord;
356 if ((v & _LBIT) == 0) {
357 if (Atomic::cmpxchg (v|_LBIT, &_LockWord.FullWord, v) == v) {
358 return 1;
359 }
360 continue;
361 }
362
363 if ((flgs & 8) == 0) {
364 SpinPause();
365 }
366
367 // Periodically increase Delay -- variable Delay form
368 // conceptually: delay *= 1 + 1/Exponent
369 ++Probes;
370 if (Probes > SpinMax) return 0;
371
372 if ((Probes & 0x7) == 0) {
373 Delay = ((Delay << 1)|1) & 0x7FF;
374 // CONSIDER: Delay += 1 + (Delay/4); Delay &= 0x7FF ;
375 }
376
377 if (flgs & 2) continue;
378
379 // Consider checking _owner's schedctl state, if OFFPROC abort spin.
380 // If the owner is OFFPROC then it's unlike that the lock will be dropped
381 // in a timely fashion, which suggests that spinning would not be fruitful
382 // or profitable.
383
384 // Stall for "Delay" time units - iterations in the current implementation.
385 // Avoid generating coherency traffic while stalled.
386 // Possible ways to delay:
387 // PAUSE, SLEEP, MEMBAR #sync, MEMBAR #halt,
388 // wr %g0,%asi, gethrtime, rdstick, rdtick, rdtsc, etc. ...
389 // Note that on Niagara-class systems we want to minimize STs in the
390 // spin loop. N1 and brethren write-around the L1$ over the xbar into the L2$.
391 // Furthermore, they don't have a W$ like traditional SPARC processors.
392 // We currently use a Marsaglia Shift-Xor RNG loop.
393 Steps += Delay;
394 if (Self != NULL) {
395 jint rv = Self->rng[0];
396 for (int k = Delay; --k >= 0;) {
397 rv = MarsagliaXORV(rv);
398 if ((flgs & 4) == 0 && SafepointMechanism::poll(Self)) return 0;
399 }
400 Self->rng[0] = rv;
401 } else {
402 Stall(Delay);
403 }
404 }
405 }
406
407 static int ParkCommon(ParkEvent * ev, jlong timo) {
408 // Diagnostic support - periodically unwedge blocked threads
409 intx nmt = NativeMonitorTimeout;
410 if (nmt > 0 && (nmt < timo || timo <= 0)) {
411 timo = nmt;
412 }
413 int err = OS_OK;
414 if (0 == timo) {
415 ev->park();
416 } else {
417 err = ev->park(timo);
418 }
419 return err;
420 }
421
422 inline int Monitor::AcquireOrPush(ParkEvent * ESelf) {
423 intptr_t v = _LockWord.FullWord;
424 for (;;) {
425 if ((v & _LBIT) == 0) {
426 const intptr_t u = Atomic::cmpxchg(v|_LBIT, &_LockWord.FullWord, v);
427 if (u == v) return 1; // indicate acquired
428 v = u;
429 } else {
430 // Anticipate success ...
431 ESelf->ListNext = (ParkEvent *)(v & ~_LBIT);
432 const intptr_t u = Atomic::cmpxchg(intptr_t(ESelf)|_LBIT, &_LockWord.FullWord, v);
433 if (u == v) return 0; // indicate pushed onto cxq
434 v = u;
435 }
436 // Interference - LockWord change - just retry
437 }
438 }
439
440 // ILock and IWait are the lowest level primitive internal blocking
441 // synchronization functions. The callers of IWait and ILock must have
442 // performed any needed state transitions beforehand.
443 // IWait and ILock may directly call park() without any concern for thread state.
444 // Note that ILock and IWait do *not* access _owner.
445 // _owner is a higher-level logical concept.
446
447 void Monitor::ILock(Thread * Self) {
448 assert(_OnDeck != Self->_MutexEvent, "invariant");
449
450 if (TryFast()) {
451 Exeunt:
452 assert(ILocked(), "invariant");
453 return;
454 }
455
456 ParkEvent * const ESelf = Self->_MutexEvent;
457 assert(_OnDeck != ESelf, "invariant");
458
459 // As an optimization, spinners could conditionally try to set _OnDeck to _LBIT
460 // Synchronizer.cpp uses a similar optimization.
461 if (TrySpin(Self)) goto Exeunt;
462
463 // Slow-path - the lock is contended.
464 // Either Enqueue Self on cxq or acquire the outer lock.
465 // LockWord encoding = (cxq,LOCKBYTE)
466 ESelf->reset();
467 OrderAccess::fence();
468
469 // Optional optimization ... try barging on the inner lock
470 if ((NativeMonitorFlags & 32) && Atomic::replace_if_null(ESelf, &_OnDeck)) {
471 goto OnDeck_LOOP;
472 }
473
474 if (AcquireOrPush(ESelf)) goto Exeunt;
475
476 // At any given time there is at most one ondeck thread.
477 // ondeck implies not resident on cxq and not resident on EntryList
478 // Only the OnDeck thread can try to acquire -- contend for -- the lock.
479 // CONSIDER: use Self->OnDeck instead of m->OnDeck.
480 // Deschedule Self so that others may run.
481 while (OrderAccess::load_acquire(&_OnDeck) != ESelf) {
482 ParkCommon(ESelf, 0);
483 }
484
485 // Self is now in the OnDeck position and will remain so until it
486 // manages to acquire the lock.
487 OnDeck_LOOP:
488 for (;;) {
489 assert(_OnDeck == ESelf, "invariant");
490 if (TrySpin(Self)) 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 // epilogue 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");
520 // Conceptually we need a MEMBAR #storestore|#loadstore barrier or fence immediately
521 // before the store that releases the lock. Crucially, all the stores and loads in the
522 // critical section must be globally visible before the store of 0 into the lock-word
523 // that releases the lock becomes globally visible. That is, memory accesses in the
524 // critical section should not be allowed to bypass or overtake the following ST that
525 // releases the lock. As such, to prevent accesses within the critical section
526 // from "leaking" out, we need a release fence between the critical section and the
527 // store that releases the lock. In practice that release barrier is elided on
528 // platforms with strong memory models such as TSO.
529 //
530 // Note that the OrderAccess::storeload() fence that appears after unlock store
531 // provides for progress conditions and succession and is _not related to exclusion
532 // safety or lock release consistency.
533 OrderAccess::release_store(&_LockWord.Bytes[_LSBINDEX], jbyte(0)); // drop outer lock
534
535 OrderAccess::storeload();
536 ParkEvent * const w = _OnDeck; // raw load as we will just return if non-NULL
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 (!Atomic::replace_if_null((ParkEvent*)_LBIT, &_OnDeck)) {
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(intptr_t(_OnDeck) == _LBIT, "invariant");
593
594 // Pass OnDeck role to w, ensuring that _EntryList has been set first.
595 // w will clear _OnDeck once it acquires the outer lock.
596 // Note that once we set _OnDeck that thread can acquire the mutex, proceed
597 // with its critical section and then enter this code to unlock the mutex. So
598 // you can have multiple threads active in IUnlock at the same time.
599 OrderAccess::release_store(&_OnDeck, w);
600
601 // Another optional optimization ...
602 // For heavily contended locks it's not uncommon that some other
603 // thread acquired the lock while this thread was arranging succession.
604 // Try to defer the unpark() operation - Delegate the responsibility
605 // for unpark()ing the OnDeck thread to the current or subsequent owners
606 // That is, the new owner is responsible for unparking the OnDeck thread.
607 OrderAccess::storeload();
608 cxq = _LockWord.FullWord;
609 if (cxq & _LBIT) return;
610
611 w->unpark();
612 return;
613 }
614
615 cxq = _LockWord.FullWord;
616 if ((cxq & ~_LBIT) != 0) {
617 // The EntryList is empty but the cxq is populated.
618 // drain RATs from cxq into EntryList
619 // Detach RATs segment with CAS and then merge into EntryList
620 for (;;) {
621 // optional optimization - if locked, the owner is responsible for succession
622 if (cxq & _LBIT) goto Punt;
623 const intptr_t vfy = Atomic::cmpxchg(cxq & _LBIT, &_LockWord.FullWord, cxq);
624 if (vfy == cxq) break;
625 cxq = vfy;
626 // Interference - LockWord changed - Just retry
627 // We can see concurrent interference from contending threads
628 // pushing themselves onto the cxq or from lock-unlock operations.
629 // From the perspective of this thread, EntryList is stable and
630 // the cxq is prepend-only -- the head is volatile but the interior
631 // of the cxq is stable. In theory if we encounter interference from threads
632 // pushing onto cxq we could simply break off the original cxq suffix and
633 // move that segment to the EntryList, avoiding a 2nd or multiple CAS attempts
634 // on the high-traffic LockWord variable. For instance lets say the cxq is "ABCD"
635 // when we first fetch cxq above. Between the fetch -- where we observed "A"
636 // -- and CAS -- where we attempt to CAS null over A -- "PQR" arrive,
637 // yielding cxq = "PQRABCD". In this case we could simply set A.ListNext
638 // null, leaving cxq = "PQRA" and transfer the "BCD" segment to the EntryList.
639 // Note too, that it's safe for this thread to traverse the cxq
640 // without taking any special concurrency precautions.
641 }
642
643 // We don't currently reorder the cxq segment as we move it onto
644 // the EntryList, but it might make sense to reverse the order
645 // or perhaps sort by thread priority. See the comments in
646 // synchronizer.cpp objectMonitor::exit().
647 assert(_EntryList == NULL, "invariant");
648 _EntryList = List = (ParkEvent *)(cxq & ~_LBIT);
649 assert(List != NULL, "invariant");
650 goto WakeOne;
651 }
652
653 // cxq|EntryList is empty.
654 // w == NULL implies that cxq|EntryList == NULL in the past.
655 // Possible race - rare inopportune interleaving.
656 // A thread could have added itself to cxq since this thread previously checked.
657 // Detect and recover by refetching cxq.
658 Punt:
659 assert(intptr_t(_OnDeck) == _LBIT, "invariant");
660 _OnDeck = NULL; // Release inner lock.
661 OrderAccess::storeload(); // Dekker duality - pivot point
662
663 // Resample LockWord/cxq to recover from possible race.
664 // For instance, while this thread T1 held OnDeck, some other thread T2 might
665 // acquire the outer lock. Another thread T3 might try to acquire the outer
666 // lock, but encounter contention and enqueue itself on cxq. T2 then drops the
667 // outer lock, but skips succession as this thread T1 still holds OnDeck.
668 // T1 is and remains responsible for ensuring succession of T3.
669 //
670 // Note that we don't need to recheck EntryList, just cxq.
671 // If threads moved onto EntryList since we dropped OnDeck
672 // that implies some other thread forced succession.
673 cxq = _LockWord.FullWord;
674 if ((cxq & ~_LBIT) != 0 && (cxq & _LBIT) == 0) {
675 goto Succession; // potential race -- re-run succession
676 }
677 return;
678 }
679
680 bool Monitor::notify() {
681 assert(_owner == Thread::current(), "invariant");
682 assert(ILocked(), "invariant");
683 if (_WaitSet == NULL) return true;
684 NotifyCount++;
685
686 // Transfer one thread from the WaitSet to the EntryList or cxq.
687 // Currently we just unlink the head of the WaitSet and prepend to the cxq.
688 // And of course we could just unlink it and unpark it, too, but
689 // in that case it'd likely impale itself on the reentry.
690 Thread::muxAcquire(_WaitLock, "notify:WaitLock");
691 ParkEvent * nfy = _WaitSet;
692 if (nfy != NULL) { // DCL idiom
693 _WaitSet = nfy->ListNext;
694 assert(nfy->Notified == 0, "invariant");
695 // push nfy onto the cxq
696 for (;;) {
697 const intptr_t v = _LockWord.FullWord;
698 assert((v & 0xFF) == _LBIT, "invariant");
699 nfy->ListNext = (ParkEvent *)(v & ~_LBIT);
700 if (Atomic::cmpxchg(intptr_t(nfy)|_LBIT, &_LockWord.FullWord, v) == v) break;
701 // interference - _LockWord changed -- just retry
702 }
703 // Note that setting Notified before pushing nfy onto the cxq is
704 // also legal and safe, but the safety properties are much more
705 // subtle, so for the sake of code stewardship ...
706 OrderAccess::fence();
707 nfy->Notified = 1;
708 }
709 Thread::muxRelease(_WaitLock);
710 if (nfy != NULL && (NativeMonitorFlags & 16)) {
711 // Experimental code ... light up the wakee in the hope that this thread (the owner)
712 // will drop the lock just about the time the wakee comes ONPROC.
713 nfy->unpark();
714 }
715 assert(ILocked(), "invariant");
716 return true;
717 }
718
719 // Currently notifyAll() transfers the waiters one-at-a-time from the waitset
720 // to the cxq. This could be done more efficiently with a single bulk en-mass transfer,
721 // but in practice notifyAll() for large #s of threads is rare and not time-critical.
722 // Beware too, that we invert the order of the waiters. Lets say that the
723 // waitset is "ABCD" and the cxq is "XYZ". After a notifyAll() the waitset
724 // will be empty and the cxq will be "DCBAXYZ". This is benign, of course.
725
726 bool Monitor::notify_all() {
727 assert(_owner == Thread::current(), "invariant");
728 assert(ILocked(), "invariant");
729 while (_WaitSet != NULL) notify();
730 return true;
731 }
732
733 int Monitor::IWait(Thread * Self, jlong timo) {
734 assert(ILocked(), "invariant");
735
736 // Phases:
737 // 1. Enqueue Self on WaitSet - currently prepend
738 // 2. unlock - drop the outer lock
739 // 3. wait for either notification or timeout
740 // 4. lock - reentry - reacquire the outer lock
741
742 ParkEvent * const ESelf = Self->_MutexEvent;
743 ESelf->Notified = 0;
744 ESelf->reset();
745 OrderAccess::fence();
746
747 // Add Self to WaitSet
748 // Ideally only the holder of the outer lock would manipulate the WaitSet -
749 // That is, the outer lock would implicitly protect the WaitSet.
750 // But if a thread in wait() encounters a timeout it will need to dequeue itself
751 // from the WaitSet _before it becomes the owner of the lock. We need to dequeue
752 // as the ParkEvent -- which serves as a proxy for the thread -- can't reside
753 // on both the WaitSet and the EntryList|cxq at the same time.. That is, a thread
754 // on the WaitSet can't be allowed to compete for the lock until it has managed to
755 // unlink its ParkEvent from WaitSet. Thus the need for WaitLock.
756 // Contention on the WaitLock is minimal.
757 //
758 // Another viable approach would be add another ParkEvent, "WaitEvent" to the
759 // thread class. The WaitSet would be composed of WaitEvents. Only the
760 // owner of the outer lock would manipulate the WaitSet. A thread in wait()
761 // could then compete for the outer lock, and then, if necessary, unlink itself
762 // from the WaitSet only after having acquired the outer lock. More precisely,
763 // there would be no WaitLock. A thread in in wait() would enqueue its WaitEvent
764 // on the WaitSet; release the outer lock; wait for either notification or timeout;
765 // reacquire the inner lock; and then, if needed, unlink itself from the WaitSet.
766 //
767 // Alternatively, a 2nd set of list link fields in the ParkEvent might suffice.
768 // One set would be for the WaitSet and one for the EntryList.
769 // We could also deconstruct the ParkEvent into a "pure" event and add a
770 // new immortal/TSM "ListElement" class that referred to ParkEvents.
771 // In that case we could have one ListElement on the WaitSet and another
772 // on the EntryList, with both referring to the same pure Event.
773
774 Thread::muxAcquire(_WaitLock, "wait:WaitLock:Add");
775 ESelf->ListNext = _WaitSet;
776 _WaitSet = ESelf;
777 Thread::muxRelease(_WaitLock);
778
779 // Release the outer lock
780 // We call IUnlock (RelaxAssert=true) as a thread T1 might
781 // enqueue itself on the WaitSet, call IUnlock(), drop the lock,
782 // and then stall before it can attempt to wake a successor.
783 // Some other thread T2 acquires the lock, and calls notify(), moving
784 // T1 from the WaitSet to the cxq. T2 then drops the lock. T1 resumes,
785 // and then finds *itself* on the cxq. During the course of a normal
786 // IUnlock() call a thread should _never find itself on the EntryList
787 // or cxq, but in the case of wait() it's possible.
788 // See synchronizer.cpp objectMonitor::wait().
789 IUnlock(true);
790
791 // Wait for either notification or timeout
792 // Beware that in some circumstances we might propagate
793 // spurious wakeups back to the caller.
794
795 for (;;) {
796 if (ESelf->Notified) break;
797 int err = ParkCommon(ESelf, timo);
798 if (err == OS_TIMEOUT || (NativeMonitorFlags & 1)) break;
799 }
800
801 // Prepare for reentry - if necessary, remove ESelf from WaitSet
802 // ESelf can be:
803 // 1. Still on the WaitSet. This can happen if we exited the loop by timeout.
804 // 2. On the cxq or EntryList
805 // 3. Not resident on cxq, EntryList or WaitSet, but in the OnDeck position.
806
807 OrderAccess::fence();
808 int WasOnWaitSet = 0;
809 if (ESelf->Notified == 0) {
810 Thread::muxAcquire(_WaitLock, "wait:WaitLock:remove");
811 if (ESelf->Notified == 0) { // DCL idiom
812 assert(_OnDeck != ESelf, "invariant"); // can't be both OnDeck and on WaitSet
813 // ESelf is resident on the WaitSet -- unlink it.
814 // A doubly-linked list would be better here so we can unlink in constant-time.
815 // We have to unlink before we potentially recontend as ESelf might otherwise
816 // end up on the cxq|EntryList -- it can't be on two lists at once.
817 ParkEvent * p = _WaitSet;
818 ParkEvent * q = NULL; // classic q chases p
819 while (p != NULL && p != ESelf) {
820 q = p;
821 p = p->ListNext;
822 }
823 assert(p == ESelf, "invariant");
824 if (p == _WaitSet) { // found at head
825 assert(q == NULL, "invariant");
826 _WaitSet = p->ListNext;
827 } else { // found in interior
828 assert(q->ListNext == p, "invariant");
829 q->ListNext = p->ListNext;
830 }
831 WasOnWaitSet = 1; // We were *not* notified but instead encountered timeout
832 }
833 Thread::muxRelease(_WaitLock);
834 }
835
836 // Reentry phase - reacquire the lock
837 if (WasOnWaitSet) {
838 // ESelf was previously on the WaitSet but we just unlinked it above
839 // because of a timeout. ESelf is not resident on any list and is not OnDeck
840 assert(_OnDeck != ESelf, "invariant");
841 ILock(Self);
842 } else {
843 // A prior notify() operation moved ESelf from the WaitSet to the cxq.
844 // ESelf is now on the cxq, EntryList or at the OnDeck position.
845 // The following fragment is extracted from Monitor::ILock()
846 for (;;) {
847 if (OrderAccess::load_acquire(&_OnDeck) == ESelf && TrySpin(Self)) break;
848 ParkCommon(ESelf, 0);
849 }
850 assert(_OnDeck == ESelf, "invariant");
851 _OnDeck = NULL;
852 }
853
854 assert(ILocked(), "invariant");
855 return WasOnWaitSet != 0; // return true IFF timeout
856 }
857
858
859 // ON THE VMTHREAD SNEAKING PAST HELD LOCKS:
860 // In particular, there are certain types of global lock that may be held
861 // by a Java thread while it is blocked at a safepoint but before it has
862 // written the _owner field. These locks may be sneakily acquired by the
863 // VM thread during a safepoint to avoid deadlocks. Alternatively, one should
864 // identify all such locks, and ensure that Java threads never block at
865 // safepoints while holding them (_no_safepoint_check_flag). While it
866 // seems as though this could increase the time to reach a safepoint
867 // (or at least increase the mean, if not the variance), the latter
868 // approach might make for a cleaner, more maintainable JVM design.
869 //
870 // Sneaking is vile and reprehensible and should be excised at the 1st
871 // opportunity. It's possible that the need for sneaking could be obviated
872 // as follows. Currently, a thread might (a) while TBIVM, call pthread_mutex_lock
873 // or ILock() thus acquiring the "physical" lock underlying Monitor/Mutex.
874 // (b) stall at the TBIVM exit point as a safepoint is in effect. Critically,
875 // it'll stall at the TBIVM reentry state transition after having acquired the
876 // underlying lock, but before having set _owner and having entered the actual
877 // critical section. The lock-sneaking facility leverages that fact and allowed the
878 // VM thread to logically acquire locks that had already be physically locked by mutators
879 // but where mutators were known blocked by the reentry thread state transition.
880 //
881 // If we were to modify the Monitor-Mutex so that TBIVM state transitions tightly
882 // wrapped calls to park(), then we could likely do away with sneaking. We'd
883 // decouple lock acquisition and parking. The critical invariant to eliminating
884 // sneaking is to ensure that we never "physically" acquire the lock while TBIVM.
885 // An easy way to accomplish this is to wrap the park calls in a narrow TBIVM jacket.
886 // One difficulty with this approach is that the TBIVM wrapper could recurse and
887 // call lock() deep from within a lock() call, while the MutexEvent was already enqueued.
888 // Using a stack (N=2 at minimum) of ParkEvents would take care of that problem.
889 //
890 // But of course the proper ultimate approach is to avoid schemes that require explicit
891 // sneaking or dependence on any any clever invariants or subtle implementation properties
892 // of Mutex-Monitor and instead directly address the underlying design flaw.
893
894 void Monitor::lock(Thread * Self) {
895 // Ensure that the Monitor requires/allows safepoint checks.
896 assert(_safepoint_check_required != Monitor::_safepoint_check_never,
897 "This lock should never have a safepoint check: %s", name());
898
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 // Horrible 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 // Ensure that the Monitor does not require or allow safepoint checks.
958 assert(_safepoint_check_required != Monitor::_safepoint_check_always,
959 "This lock should always have a safepoint check: %s", name());
960 assert(_owner != Self, "invariant");
961 ILock(Self);
962 assert(_owner == NULL, "invariant");
963 set_owner(Self);
964 }
965
966 void Monitor::lock_without_safepoint_check() {
967 lock_without_safepoint_check(Thread::current());
968 }
969
970
971 // Returns true if thread succeeds in grabbing the lock, otherwise false.
972
973 bool Monitor::try_lock() {
974 Thread * const Self = Thread::current();
975 debug_only(check_prelock_state(Self));
976 // assert(!thread->is_inside_signal_handler(), "don't lock inside signal handler");
977
978 // Special case, where all Java threads are stopped.
979 // The lock may have been acquired but _owner is not yet set.
980 // In that case the VM thread can safely grab the lock.
981 // It strikes me this should appear _after the TryLock() fails, below.
982 bool can_sneak = Self->is_VM_thread() && SafepointSynchronize::is_at_safepoint();
983 if (can_sneak && _owner == NULL) {
984 set_owner(Self); // Do not need to be atomic, since we are at a safepoint
985 _snuck = true;
986 return true;
987 }
988
989 if (TryLock()) {
990 // We got the lock
991 assert(_owner == NULL, "invariant");
992 set_owner(Self);
993 return true;
994 }
995 return false;
996 }
997
998 void Monitor::unlock() {
999 assert(_owner == Thread::current(), "invariant");
1000 assert(_OnDeck != Thread::current()->_MutexEvent, "invariant");
1001 set_owner(NULL);
1002 if (_snuck) {
1003 assert(SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread(), "sneak");
1004 _snuck = false;
1005 return;
1006 }
1007 IUnlock(false);
1008 }
1009
1010 // Yet another degenerate version of Monitor::lock() or lock_without_safepoint_check()
1011 // jvm_raw_lock() and _unlock() can be called by non-Java threads via JVM_RawMonitorEnter.
1012 //
1013 // There's no expectation that JVM_RawMonitors will interoperate properly with the native
1014 // Mutex-Monitor constructs. We happen to implement JVM_RawMonitors in terms of
1015 // native Mutex-Monitors simply as a matter of convenience. A simple abstraction layer
1016 // over a pthread_mutex_t would work equally as well, but require more platform-specific
1017 // code -- a "PlatformMutex". Alternatively, a simply layer over muxAcquire-muxRelease
1018 // would work too.
1019 //
1020 // Since the caller might be a foreign thread, we don't necessarily have a Thread.MutexEvent
1021 // instance available. Instead, we transiently allocate a ParkEvent on-demand if
1022 // we encounter contention. That ParkEvent remains associated with the thread
1023 // until it manages to acquire the lock, at which time we return the ParkEvent
1024 // to the global ParkEvent free list. This is correct and suffices for our purposes.
1025 //
1026 // Beware that the original jvm_raw_unlock() had a "_snuck" test but that
1027 // jvm_raw_lock() didn't have the corresponding test. I suspect that's an
1028 // oversight, but I've replicated the original suspect logic in the new code ...
1029
1030 void Monitor::jvm_raw_lock() {
1031 assert(rank() == native, "invariant");
1032
1033 if (TryLock()) {
1034 Exeunt:
1035 assert(ILocked(), "invariant");
1036 assert(_owner == NULL, "invariant");
1037 // This can potentially be called by non-java Threads. Thus, the Thread::current_or_null()
1038 // might return NULL. Don't call set_owner since it will break on an NULL owner
1039 // Consider installing a non-null "ANON" distinguished value instead of just NULL.
1040 _owner = Thread::current_or_null();
1041 return;
1042 }
1043
1044 if (TrySpin(NULL)) goto Exeunt;
1045
1046 // slow-path - apparent contention
1047 // Allocate a ParkEvent for transient use.
1048 // The ParkEvent remains associated with this thread until
1049 // the time the thread manages to acquire the lock.
1050 ParkEvent * const ESelf = ParkEvent::Allocate(NULL);
1051 ESelf->reset();
1052 OrderAccess::storeload();
1053
1054 // Either Enqueue Self on cxq or acquire the outer lock.
1055 if (AcquireOrPush (ESelf)) {
1056 ParkEvent::Release(ESelf); // surrender the ParkEvent
1057 goto Exeunt;
1058 }
1059
1060 // At any given time there is at most one ondeck thread.
1061 // ondeck implies not resident on cxq and not resident on EntryList
1062 // Only the OnDeck thread can try to acquire -- contend for -- the lock.
1063 // CONSIDER: use Self->OnDeck instead of m->OnDeck.
1064 for (;;) {
1065 if (OrderAccess::load_acquire(&_OnDeck) == ESelf && TrySpin(NULL)) break;
1066 ParkCommon(ESelf, 0);
1067 }
1068
1069 assert(_OnDeck == ESelf, "invariant");
1070 _OnDeck = NULL;
1071 ParkEvent::Release(ESelf); // surrender the ParkEvent
1072 goto Exeunt;
1073 }
1074
1075 void Monitor::jvm_raw_unlock() {
1076 // Nearly the same as Monitor::unlock() ...
1077 // directly set _owner instead of using set_owner(null)
1078 _owner = NULL;
1079 if (_snuck) { // ???
1080 assert(SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread(), "sneak");
1081 _snuck = false;
1082 return;
1083 }
1084 IUnlock(false);
1085 }
1086
1087 bool Monitor::wait(bool no_safepoint_check, long timeout,
1088 bool as_suspend_equivalent) {
1089 // Make sure safepoint checking is used properly.
1090 assert(!(_safepoint_check_required == Monitor::_safepoint_check_never && no_safepoint_check == false),
1091 "This lock should never have a safepoint check: %s", name());
1092 assert(!(_safepoint_check_required == Monitor::_safepoint_check_always && no_safepoint_check == true),
1093 "This lock should always have a safepoint check: %s", name());
1094
1095 Thread * const Self = Thread::current();
1096 assert(_owner == Self, "invariant");
1097 assert(ILocked(), "invariant");
1098
1099 // as_suspend_equivalent logically implies !no_safepoint_check
1100 guarantee(!as_suspend_equivalent || !no_safepoint_check, "invariant");
1101 // !no_safepoint_check logically implies java_thread
1102 guarantee(no_safepoint_check || Self->is_Java_thread(), "invariant");
1103
1104 #ifdef ASSERT
1105 Monitor * least = get_least_ranked_lock_besides_this(Self->owned_locks());
1106 assert(least != this, "Specification of get_least_... call above");
1107 if (least != NULL && least->rank() <= special) {
1108 tty->print("Attempting to wait on monitor %s/%d while holding"
1109 " lock %s/%d -- possible deadlock",
1110 name(), rank(), least->name(), least->rank());
1111 assert(false, "Shouldn't block(wait) while holding a lock of rank special");
1112 }
1113 #endif // ASSERT
1114
1115 int wait_status;
1116 // conceptually set the owner to NULL in anticipation of
1117 // abdicating the lock in wait
1118 set_owner(NULL);
1119 if (no_safepoint_check) {
1120 wait_status = IWait(Self, timeout);
1121 } else {
1122 assert(Self->is_Java_thread(), "invariant");
1123 JavaThread *jt = (JavaThread *)Self;
1124
1125 // Enter safepoint region - ornate and Rococo ...
1126 ThreadBlockInVM tbivm(jt);
1127 OSThreadWaitState osts(Self->osthread(), false /* not Object.wait() */);
1128
1129 if (as_suspend_equivalent) {
1130 jt->set_suspend_equivalent();
1131 // cleared by handle_special_suspend_equivalent_condition() or
1132 // java_suspend_self()
1133 }
1134
1135 wait_status = IWait(Self, timeout);
1136
1137 // were we externally suspended while we were waiting?
1138 if (as_suspend_equivalent && jt->handle_special_suspend_equivalent_condition()) {
1139 // Our event wait has finished and we own the lock, but
1140 // while we were waiting another thread suspended us. We don't
1141 // want to hold the lock while suspended because that
1142 // would surprise the thread that suspended us.
1143 assert(ILocked(), "invariant");
1144 IUnlock(true);
1145 jt->java_suspend_self();
1146 ILock(Self);
1147 assert(ILocked(), "invariant");
1148 }
1149 }
1150
1151 // Conceptually reestablish ownership of the lock.
1152 // The "real" lock -- the LockByte -- was reacquired by IWait().
1153 assert(ILocked(), "invariant");
1154 assert(_owner == NULL, "invariant");
1155 set_owner(Self);
1156 return wait_status != 0; // return true IFF timeout
1157 }
1158
1159 Monitor::~Monitor() {
1160 #ifdef ASSERT
1161 uintptr_t owner = UNS(_owner);
1162 uintptr_t lockword = UNS(_LockWord.FullWord);
1163 uintptr_t entrylist = UNS(_EntryList);
1164 uintptr_t waitset = UNS(_WaitSet);
1165 uintptr_t ondeck = UNS(_OnDeck);
1166 // Print _name with precision limit, in case failure is due to memory
1167 // corruption that also trashed _name.
1168 assert((owner|lockword|entrylist|waitset|ondeck) == 0,
1169 "%.*s: _owner(" INTPTR_FORMAT ")|_LockWord(" INTPTR_FORMAT ")|_EntryList(" INTPTR_FORMAT ")|_WaitSet("
1170 INTPTR_FORMAT ")|_OnDeck(" INTPTR_FORMAT ") != 0",
1171 MONITOR_NAME_LEN, _name, owner, lockword, entrylist, waitset, ondeck);
1172 #endif
1173 }
1174
1175 void Monitor::ClearMonitor(Monitor * m, const char *name) {
1176 m->_owner = NULL;
1177 m->_snuck = false;
1178 if (name == NULL) {
1179 strcpy(m->_name, "UNKNOWN");
1180 } else {
1181 strncpy(m->_name, name, MONITOR_NAME_LEN - 1);
1182 m->_name[MONITOR_NAME_LEN - 1] = '\0';
1183 }
1184 m->_LockWord.FullWord = 0;
1185 m->_EntryList = NULL;
1186 m->_OnDeck = NULL;
1187 m->_WaitSet = NULL;
1188 m->_WaitLock[0] = 0;
1189 }
1190
1191 Monitor::Monitor() { ClearMonitor(this); }
1192
1193 Monitor::Monitor(int Rank, const char * name, bool allow_vm_block,
1194 SafepointCheckRequired safepoint_check_required) {
1195 ClearMonitor(this, name);
1196 #ifdef ASSERT
1197 _allow_vm_block = allow_vm_block;
1198 _rank = Rank;
1199 NOT_PRODUCT(_safepoint_check_required = safepoint_check_required;)
1200 #endif
1201 }
1202
1203 Mutex::Mutex(int Rank, const char * name, bool allow_vm_block,
1204 SafepointCheckRequired safepoint_check_required) {
1205 ClearMonitor((Monitor *) this, name);
1206 #ifdef ASSERT
1207 _allow_vm_block = allow_vm_block;
1208 _rank = Rank;
1209 NOT_PRODUCT(_safepoint_check_required = safepoint_check_required;)
1210 #endif
1211 }
1212
1213 bool Monitor::owned_by_self() const {
1214 bool ret = _owner == Thread::current();
1215 assert(!ret || _LockWord.Bytes[_LSBINDEX] != 0, "invariant");
1216 return ret;
1217 }
1218
1219 void Monitor::print_on_error(outputStream* st) const {
1220 st->print("[" PTR_FORMAT, p2i(this));
1221 st->print("] %s", _name);
1222 st->print(" - owner thread: " PTR_FORMAT, p2i(_owner));
1223 }
1224
1225
1226
1227
1228 // ----------------------------------------------------------------------------------
1229 // Non-product code
1230
1231 #ifndef PRODUCT
1232 void Monitor::print_on(outputStream* st) const {
1233 st->print_cr("Mutex: [" PTR_FORMAT "/" PTR_FORMAT "] %s - owner: " PTR_FORMAT,
1234 p2i(this), _LockWord.FullWord, _name, p2i(_owner));
1235 }
1236 #endif
1237
1238 #ifndef PRODUCT
1239 #ifdef ASSERT
1240 Monitor * Monitor::get_least_ranked_lock(Monitor * locks) {
1241 Monitor *res, *tmp;
1242 for (res = tmp = locks; tmp != NULL; tmp = tmp->next()) {
1243 if (tmp->rank() < res->rank()) {
1244 res = tmp;
1245 }
1246 }
1247 if (!SafepointSynchronize::is_at_safepoint()) {
1248 // In this case, we expect the held locks to be
1249 // in increasing rank order (modulo any native ranks)
1250 for (tmp = locks; tmp != NULL; tmp = tmp->next()) {
1251 if (tmp->next() != NULL) {
1252 assert(tmp->rank() == Mutex::native ||
1253 tmp->rank() <= tmp->next()->rank(), "mutex rank anomaly?");
1254 }
1255 }
1256 }
1257 return res;
1258 }
1259
1260 Monitor* Monitor::get_least_ranked_lock_besides_this(Monitor* locks) {
1261 Monitor *res, *tmp;
1262 for (res = NULL, tmp = locks; tmp != NULL; tmp = tmp->next()) {
1263 if (tmp != this && (res == NULL || tmp->rank() < res->rank())) {
1264 res = tmp;
1265 }
1266 }
1267 if (!SafepointSynchronize::is_at_safepoint()) {
1268 // In this case, we expect the held locks to be
1269 // in increasing rank order (modulo any native ranks)
1270 for (tmp = locks; tmp != NULL; tmp = tmp->next()) {
1271 if (tmp->next() != NULL) {
1272 assert(tmp->rank() == Mutex::native ||
1273 tmp->rank() <= tmp->next()->rank(), "mutex rank anomaly?");
1274 }
1275 }
1276 }
1277 return res;
1278 }
1279
1280
1281 bool Monitor::contains(Monitor* locks, Monitor * lock) {
1282 for (; locks != NULL; locks = locks->next()) {
1283 if (locks == lock) {
1284 return true;
1285 }
1286 }
1287 return false;
1288 }
1289 #endif
1290
1291 // Called immediately after lock acquisition or release as a diagnostic
1292 // to track the lock-set of the thread and test for rank violations that
1293 // might indicate exposure to deadlock.
1294 // Rather like an EventListener for _owner (:>).
1295
1296 void Monitor::set_owner_implementation(Thread *new_owner) {
1297 // This function is solely responsible for maintaining
1298 // and checking the invariant that threads and locks
1299 // are in a 1/N relation, with some some locks unowned.
1300 // It uses the Mutex::_owner, Mutex::_next, and
1301 // Thread::_owned_locks fields, and no other function
1302 // changes those fields.
1303 // It is illegal to set the mutex from one non-NULL
1304 // owner to another--it must be owned by NULL as an
1305 // intermediate state.
1306
1307 if (new_owner != NULL) {
1308 // the thread is acquiring this lock
1309
1310 assert(new_owner == Thread::current(), "Should I be doing this?");
1311 assert(_owner == NULL, "setting the owner thread of an already owned mutex");
1312 _owner = new_owner; // set the owner
1313
1314 // link "this" into the owned locks list
1315
1316 #ifdef ASSERT // Thread::_owned_locks is under the same ifdef
1317 Monitor* locks = get_least_ranked_lock(new_owner->owned_locks());
1318 // Mutex::set_owner_implementation is a friend of Thread
1319
1320 assert(this->rank() >= 0, "bad lock rank");
1321
1322 // Deadlock avoidance rules require us to acquire Mutexes only in
1323 // a global total order. For example m1 is the lowest ranked mutex
1324 // that the thread holds and m2 is the mutex the thread is trying
1325 // to acquire, then deadlock avoidance rules require that the rank
1326 // of m2 be less than the rank of m1.
1327 // The rank Mutex::native is an exception in that it is not subject
1328 // to the verification rules.
1329 // Here are some further notes relating to mutex acquisition anomalies:
1330 // . it is also ok to acquire Safepoint_lock at the very end while we
1331 // already hold Terminator_lock - may happen because of periodic safepoints
1332 if (this->rank() != Mutex::native &&
1333 this->rank() != Mutex::suspend_resume &&
1334 locks != NULL && locks->rank() <= this->rank() &&
1335 !SafepointSynchronize::is_at_safepoint() &&
1336 !(this == Safepoint_lock && contains(locks, Terminator_lock) &&
1337 SafepointSynchronize::is_synchronizing())) {
1338 new_owner->print_owned_locks();
1339 fatal("acquiring lock %s/%d out of order with lock %s/%d -- "
1340 "possible deadlock", this->name(), this->rank(),
1341 locks->name(), locks->rank());
1342 }
1343
1344 this->_next = new_owner->_owned_locks;
1345 new_owner->_owned_locks = this;
1346 #endif
1347
1348 } else {
1349 // the thread is releasing this lock
1350
1351 Thread* old_owner = _owner;
1352 debug_only(_last_owner = old_owner);
1353
1354 assert(old_owner != NULL, "removing the owner thread of an unowned mutex");
1355 assert(old_owner == Thread::current(), "removing the owner thread of an unowned mutex");
1356
1357 _owner = NULL; // set the owner
1358
1359 #ifdef ASSERT
1360 Monitor *locks = old_owner->owned_locks();
1361
1362 // remove "this" from the owned locks list
1363
1364 Monitor *prev = NULL;
1365 bool found = false;
1366 for (; locks != NULL; prev = locks, locks = locks->next()) {
1367 if (locks == this) {
1368 found = true;
1369 break;
1370 }
1371 }
1372 assert(found, "Removing a lock not owned");
1373 if (prev == NULL) {
1374 old_owner->_owned_locks = _next;
1375 } else {
1376 prev->_next = _next;
1377 }
1378 _next = NULL;
1379 #endif
1380 }
1381 }
1382
1383
1384 // Factored out common sanity checks for locking mutex'es. Used by lock() and try_lock()
1385 void Monitor::check_prelock_state(Thread *thread) {
1386 assert((!thread->is_Java_thread() || ((JavaThread *)thread)->thread_state() == _thread_in_vm)
1387 || rank() == Mutex::special, "wrong thread state for using locks");
1388 if (StrictSafepointChecks) {
1389 if (thread->is_VM_thread() && !allow_vm_block()) {
1390 fatal("VM thread using lock %s (not allowed to block on)", name());
1391 }
1392 debug_only(if (rank() != Mutex::special) \
1393 thread->check_for_valid_safepoint_state(false);)
1394 }
1395 assert(!os::ThreadCrashProtection::is_crash_protected(thread),
1396 "locking not allowed when crash protection is set");
1397 }
1398
1399 void Monitor::check_block_state(Thread *thread) {
1400 if (!_allow_vm_block && thread->is_VM_thread()) {
1401 warning("VM thread blocked on lock");
1402 print();
1403 BREAKPOINT;
1404 }
1405 assert(_owner != thread, "deadlock: blocking on monitor owned by current thread");
1406 }
1407
1408 #endif // PRODUCT