1 2 /* 3 * Copyright (c) 1998, 2008, 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 "incls/_precompiled.incl" 27 # include "incls/_mutex.cpp.incl" 28 29 // 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 30 // 31 // Native Monitor-Mutex locking - theory of operations 32 // 33 // * Native Monitors are completely unrelated to Java-level monitors, 34 // although the "back-end" slow-path implementations share a common lineage. 35 // See objectMonitor:: in synchronizer.cpp. 36 // Native Monitors do *not* support nesting or recursion but otherwise 37 // they're basically Hoare-flavor monitors. 38 // 39 // * A thread acquires ownership of a Monitor/Mutex by CASing the LockByte 40 // in the _LockWord from zero to non-zero. Note that the _Owner field 41 // is advisory and is used only to verify that the thread calling unlock() 42 // is indeed the last thread to have acquired the lock. 43 // 44 // * Contending threads "push" themselves onto the front of the contention 45 // queue -- called the cxq -- with CAS and then spin/park. 46 // The _LockWord contains the LockByte as well as the pointer to the head 47 // of the cxq. Colocating the LockByte with the cxq precludes certain races. 48 // 49 // * Using a separately addressable LockByte allows for CAS:MEMBAR or CAS:0 50 // idioms. We currently use MEMBAR in the uncontended unlock() path, as 51 // MEMBAR often has less latency than CAS. If warranted, we could switch to 52 // a CAS:0 mode, using timers to close the resultant race, as is done 53 // with Java Monitors in synchronizer.cpp. 54 // 55 // See the following for a discussion of the relative cost of atomics (CAS) 56 // MEMBAR, and ways to eliminate such instructions from the common-case paths: 57 // -- http://blogs.sun.com/dave/entry/biased_locking_in_hotspot 58 // -- http://blogs.sun.com/dave/resource/MustangSync.pdf 59 // -- http://blogs.sun.com/dave/resource/synchronization-public2.pdf 60 // -- synchronizer.cpp 61 // 62 // * Overall goals - desiderata 63 // 1. Minimize context switching 64 // 2. Minimize lock migration 65 // 3. Minimize CPI -- affinity and locality 66 // 4. Minimize the execution of high-latency instructions such as CAS or MEMBAR 67 // 5. Minimize outer lock hold times 68 // 6. Behave gracefully on a loaded system 69 // 70 // * Thread flow and list residency: 71 // 72 // Contention queue --> EntryList --> OnDeck --> Owner --> !Owner 73 // [..resident on monitor list..] 74 // [...........contending..................] 75 // 76 // -- The contention queue (cxq) contains recently-arrived threads (RATs). 77 // Threads on the cxq eventually drain into the EntryList. 78 // -- Invariant: a thread appears on at most one list -- cxq, EntryList 79 // or WaitSet -- at any one time. 80 // -- For a given monitor there can be at most one "OnDeck" thread at any 81 // given time but if needbe this particular invariant could be relaxed. 82 // 83 // * The WaitSet and EntryList linked lists are composed of ParkEvents. 84 // I use ParkEvent instead of threads as ParkEvents are immortal and 85 // type-stable, meaning we can safely unpark() a possibly stale 86 // list element in the unlock()-path. (That's benign). 87 // 88 // * Succession policy - providing for progress: 89 // 90 // As necessary, the unlock()ing thread identifies, unlinks, and unparks 91 // an "heir presumptive" tentative successor thread from the EntryList. 92 // This becomes the so-called "OnDeck" thread, of which there can be only 93 // one at any given time for a given monitor. The wakee will recontend 94 // for ownership of monitor. 95 // 96 // Succession is provided for by a policy of competitive handoff. 97 // The exiting thread does _not_ grant or pass ownership to the 98 // successor thread. (This is also referred to as "handoff" succession"). 99 // Instead the exiting thread releases ownership and possibly wakes 100 // a successor, so the successor can (re)compete for ownership of the lock. 101 // 102 // Competitive handoff provides excellent overall throughput at the expense 103 // of short-term fairness. If fairness is a concern then one remedy might 104 // be to add an AcquireCounter field to the monitor. After a thread acquires 105 // the lock it will decrement the AcquireCounter field. When the count 106 // reaches 0 the thread would reset the AcquireCounter variable, abdicate 107 // the lock directly to some thread on the EntryList, and then move itself to the 108 // tail of the EntryList. 109 // 110 // But in practice most threads engage or otherwise participate in resource 111 // bounded producer-consumer relationships, so lock domination is not usually 112 // a practical concern. Recall too, that in general it's easier to construct 113 // a fair lock from a fast lock, but not vice-versa. 114 // 115 // * The cxq can have multiple concurrent "pushers" but only one concurrent 116 // detaching thread. This mechanism is immune from the ABA corruption. 117 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious. 118 // We use OnDeck as a pseudo-lock to enforce the at-most-one detaching 119 // thread constraint. 120 // 121 // * Taken together, the cxq and the EntryList constitute or form a 122 // single logical queue of threads stalled trying to acquire the lock. 123 // We use two distinct lists to reduce heat on the list ends. 124 // Threads in lock() enqueue onto cxq while threads in unlock() will 125 // dequeue from the EntryList. (c.f. Michael Scott's "2Q" algorithm). 126 // A key desideratum is to minimize queue & monitor metadata manipulation 127 // that occurs while holding the "outer" monitor lock -- that is, we want to 128 // minimize monitor lock holds times. 129 // 130 // The EntryList is ordered by the prevailing queue discipline and 131 // can be organized in any convenient fashion, such as a doubly-linked list or 132 // a circular doubly-linked list. If we need a priority queue then something akin 133 // to Solaris' sleepq would work nicely. Viz., 134 // -- http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c. 135 // -- http://cvs.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/uts/common/os/sleepq.c 136 // Queue discipline is enforced at ::unlock() time, when the unlocking thread 137 // drains the cxq into the EntryList, and orders or reorders the threads on the 138 // EntryList accordingly. 139 // 140 // Barring "lock barging", this mechanism provides fair cyclic ordering, 141 // somewhat similar to an elevator-scan. 142 // 143 // * OnDeck 144 // -- For a given monitor there can be at most one OnDeck thread at any given 145 // instant. The OnDeck thread is contending for the lock, but has been 146 // unlinked from the EntryList and cxq by some previous unlock() operations. 147 // Once a thread has been designated the OnDeck thread it will remain so 148 // until it manages to acquire the lock -- being OnDeck is a stable property. 149 // -- Threads on the EntryList or cxq are _not allowed to attempt lock acquisition. 150 // -- OnDeck also serves as an "inner lock" as follows. Threads in unlock() will, after 151 // having cleared the LockByte and dropped the outer lock, attempt to "trylock" 152 // OnDeck by CASing the field from null to non-null. If successful, that thread 153 // is then responsible for progress and succession and can use CAS to detach and 154 // drain the cxq into the EntryList. By convention, only this thread, the holder of 155 // the OnDeck inner lock, can manipulate the EntryList or detach and drain the 156 // RATs on the cxq into the EntryList. This avoids ABA corruption on the cxq as 157 // we allow multiple concurrent "push" operations but restrict detach concurrency 158 // to at most one thread. Having selected and detached a successor, the thread then 159 // changes the OnDeck to refer to that successor, and then unparks the successor. 160 // That successor will eventually acquire the lock and clear OnDeck. Beware 161 // that the OnDeck usage as a lock is asymmetric. A thread in unlock() transiently 162 // "acquires" OnDeck, performs queue manipulations, passes OnDeck to some successor, 163 // and then the successor eventually "drops" OnDeck. Note that there's never 164 // any sense of contention on the inner lock, however. Threads never contend 165 // or wait for the inner lock. 166 // -- OnDeck provides for futile wakeup throttling a described in section 3.3 of 167 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf 168 // In a sense, OnDeck subsumes the ObjectMonitor _Succ and ObjectWaiter 169 // TState fields found in Java-level objectMonitors. (See synchronizer.cpp). 170 // 171 // * Waiting threads reside on the WaitSet list -- wait() puts 172 // the caller onto the WaitSet. Notify() or notifyAll() simply 173 // transfers threads from the WaitSet to either the EntryList or cxq. 174 // Subsequent unlock() operations will eventually unpark the notifyee. 175 // Unparking a notifee in notify() proper is inefficient - if we were to do so 176 // it's likely the notifyee would simply impale itself on the lock held 177 // by the notifier. 178 // 179 // * The mechanism is obstruction-free in that if the holder of the transient 180 // OnDeck lock in unlock() is preempted or otherwise stalls, other threads 181 // can still acquire and release the outer lock and continue to make progress. 182 // At worst, waking of already blocked contending threads may be delayed, 183 // but nothing worse. (We only use "trylock" operations on the inner OnDeck 184 // lock). 185 // 186 // * Note that thread-local storage must be initialized before a thread 187 // uses Native monitors or mutexes. The native monitor-mutex subsystem 188 // depends on Thread::current(). 189 // 190 // * The monitor synchronization subsystem avoids the use of native 191 // synchronization primitives except for the narrow platform-specific 192 // park-unpark abstraction. See the comments in os_solaris.cpp regarding 193 // the semantics of park-unpark. Put another way, this monitor implementation 194 // depends only on atomic operations and park-unpark. The monitor subsystem 195 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the 196 // underlying OS manages the READY<->RUN transitions. 197 // 198 // * The memory consistency model provide by lock()-unlock() is at least as 199 // strong or stronger than the Java Memory model defined by JSR-133. 200 // That is, we guarantee at least entry consistency, if not stronger. 201 // See http://g.oswego.edu/dl/jmm/cookbook.html. 202 // 203 // * Thread:: currently contains a set of purpose-specific ParkEvents: 204 // _MutexEvent, _ParkEvent, etc. A better approach might be to do away with 205 // the purpose-specific ParkEvents and instead implement a general per-thread 206 // stack of available ParkEvents which we could provision on-demand. The 207 // stack acts as a local cache to avoid excessive calls to ParkEvent::Allocate() 208 // and ::Release(). A thread would simply pop an element from the local stack before it 209 // enqueued or park()ed. When the contention was over the thread would 210 // push the no-longer-needed ParkEvent back onto its stack. 211 // 212 // * A slightly reduced form of ILock() and IUnlock() have been partially 213 // model-checked (Murphi) for safety and progress at T=1,2,3 and 4. 214 // It'd be interesting to see if TLA/TLC could be useful as well. 215 // 216 // * Mutex-Monitor is a low-level "leaf" subsystem. That is, the monitor 217 // code should never call other code in the JVM that might itself need to 218 // acquire monitors or mutexes. That's true *except* in the case of the 219 // ThreadBlockInVM state transition wrappers. The ThreadBlockInVM DTOR handles 220 // mutator reentry (ingress) by checking for a pending safepoint in which case it will 221 // call SafepointSynchronize::block(), which in turn may call Safepoint_lock->lock(), etc. 222 // In that particular case a call to lock() for a given Monitor can end up recursively 223 // calling lock() on another monitor. While distasteful, this is largely benign 224 // as the calls come from jacket that wraps lock(), and not from deep within lock() itself. 225 // 226 // It's unfortunate that native mutexes and thread state transitions were convolved. 227 // They're really separate concerns and should have remained that way. Melding 228 // them together was facile -- a bit too facile. The current implementation badly 229 // conflates the two concerns. 230 // 231 // * TODO-FIXME: 232 // 233 // -- Add DTRACE probes for contended acquire, contended acquired, contended unlock 234 // We should also add DTRACE probes in the ParkEvent subsystem for 235 // Park-entry, Park-exit, and Unpark. 236 // 237 // -- We have an excess of mutex-like constructs in the JVM, namely: 238 // 1. objectMonitors for Java-level synchronization (synchronizer.cpp) 239 // 2. low-level muxAcquire and muxRelease 240 // 3. low-level spinAcquire and spinRelease 241 // 4. native Mutex:: and Monitor:: 242 // 5. jvm_raw_lock() and _unlock() 243 // 6. JVMTI raw monitors -- distinct from (5) despite having a confusingly 244 // similar name. 245 // 246 // 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 247 248 249 // CASPTR() uses the canonical argument order that dominates in the literature. 250 // Our internal cmpxchg_ptr() uses a bastardized ordering to accommodate Sun .il templates. 251 252 #define CASPTR(a,c,s) intptr_t(Atomic::cmpxchg_ptr ((void *)(s),(void *)(a),(void *)(c))) 253 #define UNS(x) (uintptr_t(x)) 254 #define TRACE(m) { static volatile int ctr = 0 ; int x = ++ctr ; if ((x & (x-1))==0) { ::printf ("%d:%s\n", x, #m); ::fflush(stdout); }} 255 256 // Simplistic low-quality Marsaglia SHIFT-XOR RNG. 257 // Bijective except for the trailing mask operation. 258 // Useful for spin loops as the compiler can't optimize it away. 259 260 static inline jint MarsagliaXORV (jint x) { 261 if (x == 0) x = 1|os::random() ; 262 x ^= x << 6; 263 x ^= ((unsigned)x) >> 21; 264 x ^= x << 7 ; 265 return x & 0x7FFFFFFF ; 266 } 267 268 static inline jint MarsagliaXOR (jint * const a) { 269 jint x = *a ; 270 if (x == 0) x = UNS(a)|1 ; 271 x ^= x << 6; 272 x ^= ((unsigned)x) >> 21; 273 x ^= x << 7 ; 274 *a = x ; 275 return x & 0x7FFFFFFF ; 276 } 277 278 static int Stall (int its) { 279 static volatile jint rv = 1 ; 280 volatile int OnFrame = 0 ; 281 jint v = rv ^ UNS(OnFrame) ; 282 while (--its >= 0) { 283 v = MarsagliaXORV (v) ; 284 } 285 // Make this impossible for the compiler to optimize away, 286 // but (mostly) avoid W coherency sharing on MP systems. 287 if (v == 0x12345) rv = v ; 288 return v ; 289 } 290 291 int Monitor::TryLock () { 292 intptr_t v = _LockWord.FullWord ; 293 for (;;) { 294 if ((v & _LBIT) != 0) return 0 ; 295 const intptr_t u = CASPTR (&_LockWord, v, v|_LBIT) ; 296 if (v == u) return 1 ; 297 v = u ; 298 } 299 } 300 301 int Monitor::TryFast () { 302 // Optimistic fast-path form ... 303 // Fast-path attempt for the common uncontended case. 304 // Avoid RTS->RTO $ coherence upgrade on typical SMP systems. 305 intptr_t v = CASPTR (&_LockWord, 0, _LBIT) ; // agro ... 306 if (v == 0) return 1 ; 307 308 for (;;) { 309 if ((v & _LBIT) != 0) return 0 ; 310 const intptr_t u = CASPTR (&_LockWord, v, v|_LBIT) ; 311 if (v == u) return 1 ; 312 v = u ; 313 } 314 } 315 316 int Monitor::ILocked () { 317 const intptr_t w = _LockWord.FullWord & 0xFF ; 318 assert (w == 0 || w == _LBIT, "invariant") ; 319 return w == _LBIT ; 320 } 321 322 // Polite TATAS spinlock with exponential backoff - bounded spin. 323 // Ideally we'd use processor cycles, time or vtime to control 324 // the loop, but we currently use iterations. 325 // All the constants within were derived empirically but work over 326 // over the spectrum of J2SE reference platforms. 327 // On Niagara-class systems the back-off is unnecessary but 328 // is relatively harmless. (At worst it'll slightly retard 329 // acquisition times). The back-off is critical for older SMP systems 330 // where constant fetching of the LockWord would otherwise impair 331 // scalability. 332 // 333 // Clamp spinning at approximately 1/2 of a context-switch round-trip. 334 // See synchronizer.cpp for details and rationale. 335 336 int Monitor::TrySpin (Thread * const Self) { 337 if (TryLock()) return 1 ; 338 if (!os::is_MP()) return 0 ; 339 340 int Probes = 0 ; 341 int Delay = 0 ; 342 int Steps = 0 ; 343 int SpinMax = NativeMonitorSpinLimit ; 344 int flgs = NativeMonitorFlags ; 345 for (;;) { 346 intptr_t v = _LockWord.FullWord; 347 if ((v & _LBIT) == 0) { 348 if (CASPTR (&_LockWord, v, v|_LBIT) == v) { 349 return 1 ; 350 } 351 continue ; 352 } 353 354 if ((flgs & 8) == 0) { 355 SpinPause () ; 356 } 357 358 // Periodically increase Delay -- variable Delay form 359 // conceptually: delay *= 1 + 1/Exponent 360 ++ Probes; 361 if (Probes > SpinMax) return 0 ; 362 363 if ((Probes & 0x7) == 0) { 364 Delay = ((Delay << 1)|1) & 0x7FF ; 365 // CONSIDER: Delay += 1 + (Delay/4); Delay &= 0x7FF ; 366 } 367 368 if (flgs & 2) continue ; 369 370 // Consider checking _owner's schedctl state, if OFFPROC abort spin. 371 // If the owner is OFFPROC then it's unlike that the lock will be dropped 372 // in a timely fashion, which suggests that spinning would not be fruitful 373 // or profitable. 374 375 // Stall for "Delay" time units - iterations in the current implementation. 376 // Avoid generating coherency traffic while stalled. 377 // Possible ways to delay: 378 // PAUSE, SLEEP, MEMBAR #sync, MEMBAR #halt, 379 // wr %g0,%asi, gethrtime, rdstick, rdtick, rdtsc, etc. ... 380 // Note that on Niagara-class systems we want to minimize STs in the 381 // spin loop. N1 and brethren write-around the L1$ over the xbar into the L2$. 382 // Furthermore, they don't have a W$ like traditional SPARC processors. 383 // We currently use a Marsaglia Shift-Xor RNG loop. 384 Steps += Delay ; 385 if (Self != NULL) { 386 jint rv = Self->rng[0] ; 387 for (int k = Delay ; --k >= 0; ) { 388 rv = MarsagliaXORV (rv) ; 389 if ((flgs & 4) == 0 && SafepointSynchronize::do_call_back()) return 0 ; 390 } 391 Self->rng[0] = rv ; 392 } else { 393 Stall (Delay) ; 394 } 395 } 396 } 397 398 static int ParkCommon (ParkEvent * ev, jlong timo) { 399 // Diagnostic support - periodically unwedge blocked threads 400 intx nmt = NativeMonitorTimeout ; 401 if (nmt > 0 && (nmt < timo || timo <= 0)) { 402 timo = nmt ; 403 } 404 int err = OS_OK ; 405 if (0 == timo) { 406 ev->park() ; 407 } else { 408 err = ev->park(timo) ; 409 } 410 return err ; 411 } 412 413 inline int Monitor::AcquireOrPush (ParkEvent * ESelf) { 414 intptr_t v = _LockWord.FullWord ; 415 for (;;) { 416 if ((v & _LBIT) == 0) { 417 const intptr_t u = CASPTR (&_LockWord, v, v|_LBIT) ; 418 if (u == v) return 1 ; // indicate acquired 419 v = u ; 420 } else { 421 // Anticipate success ... 422 ESelf->ListNext = (ParkEvent *) (v & ~_LBIT) ; 423 const intptr_t u = CASPTR (&_LockWord, v, intptr_t(ESelf)|_LBIT) ; 424 if (u == v) return 0 ; // indicate pushed onto cxq 425 v = u ; 426 } 427 // Interference - LockWord change - just retry 428 } 429 } 430 431 // ILock and IWait are the lowest level primitive internal blocking 432 // synchronization functions. The callers of IWait and ILock must have 433 // performed any needed state transitions beforehand. 434 // IWait and ILock may directly call park() without any concern for thread state. 435 // Note that ILock and IWait do *not* access _owner. 436 // _owner is a higher-level logical concept. 437 438 void Monitor::ILock (Thread * Self) { 439 assert (_OnDeck != Self->_MutexEvent, "invariant") ; 440 441 if (TryFast()) { 442 Exeunt: 443 assert (ILocked(), "invariant") ; 444 return ; 445 } 446 447 ParkEvent * const ESelf = Self->_MutexEvent ; 448 assert (_OnDeck != ESelf, "invariant") ; 449 450 // As an optimization, spinners could conditionally try to set ONDECK to _LBIT 451 // Synchronizer.cpp uses a similar optimization. 452 if (TrySpin (Self)) goto Exeunt ; 453 454 // Slow-path - the lock is contended. 455 // Either Enqueue Self on cxq or acquire the outer lock. 456 // LockWord encoding = (cxq,LOCKBYTE) 457 ESelf->reset() ; 458 OrderAccess::fence() ; 459 460 // Optional optimization ... try barging on the inner lock 461 if ((NativeMonitorFlags & 32) && CASPTR (&_OnDeck, NULL, UNS(Self)) == 0) { 462 goto OnDeck_LOOP ; 463 } 464 465 if (AcquireOrPush (ESelf)) goto Exeunt ; 466 467 // At any given time there is at most one ondeck thread. 468 // ondeck implies not resident on cxq and not resident on EntryList 469 // Only the OnDeck thread can try to acquire -- contended for -- the lock. 470 // CONSIDER: use Self->OnDeck instead of m->OnDeck. 471 // Deschedule Self so that others may run. 472 while (_OnDeck != ESelf) { 473 ParkCommon (ESelf, 0) ; 474 } 475 476 // Self is now in the ONDECK position and will remain so until it 477 // manages to acquire the lock. 478 OnDeck_LOOP: 479 for (;;) { 480 assert (_OnDeck == ESelf, "invariant") ; 481 if (TrySpin (Self)) break ; 482 // CONSIDER: if ESelf->TryPark() && TryLock() break ... 483 // It's probably wise to spin only if we *actually* blocked 484 // CONSIDER: check the lockbyte, if it remains set then 485 // preemptively drain the cxq into the EntryList. 486 // The best place and time to perform queue operations -- lock metadata -- 487 // is _before having acquired the outer lock, while waiting for the lock to drop. 488 ParkCommon (ESelf, 0) ; 489 } 490 491 assert (_OnDeck == ESelf, "invariant") ; 492 _OnDeck = NULL ; 493 494 // Note that we current drop the inner lock (clear OnDeck) in the slow-path 495 // epilog immediately after having acquired the outer lock. 496 // But instead we could consider the following optimizations: 497 // A. Shift or defer dropping the inner lock until the subsequent IUnlock() operation. 498 // This might avoid potential reacquisition of the inner lock in IUlock(). 499 // B. While still holding the inner lock, attempt to opportunistically select 500 // and unlink the next ONDECK thread from the EntryList. 501 // If successful, set ONDECK to refer to that thread, otherwise clear ONDECK. 502 // It's critical that the select-and-unlink operation run in constant-time as 503 // it executes when holding the outer lock and may artificially increase the 504 // effective length of the critical section. 505 // Note that (A) and (B) are tantamount to succession by direct handoff for 506 // the inner lock. 507 goto Exeunt ; 508 } 509 510 void Monitor::IUnlock (bool RelaxAssert) { 511 assert (ILocked(), "invariant") ; 512 _LockWord.Bytes[_LSBINDEX] = 0 ; // drop outer lock 513 OrderAccess::storeload (); 514 ParkEvent * const w = _OnDeck ; 515 assert (RelaxAssert || w != Thread::current()->_MutexEvent, "invariant") ; 516 if (w != NULL) { 517 // Either we have a valid ondeck thread or ondeck is transiently "locked" 518 // by some exiting thread as it arranges for succession. The LSBit of 519 // OnDeck allows us to discriminate two cases. If the latter, the 520 // responsibility for progress and succession lies with that other thread. 521 // For good performance, we also depend on the fact that redundant unpark() 522 // operations are cheap. That is, repeated Unpark()ing of the ONDECK thread 523 // is inexpensive. This approach provides implicit futile wakeup throttling. 524 // Note that the referent "w" might be stale with respect to the lock. 525 // In that case the following unpark() is harmless and the worst that'll happen 526 // is a spurious return from a park() operation. Critically, if "w" _is stale, 527 // then progress is known to have occurred as that means the thread associated 528 // with "w" acquired the lock. In that case this thread need take no further 529 // action to guarantee progress. 530 if ((UNS(w) & _LBIT) == 0) w->unpark() ; 531 return ; 532 } 533 534 intptr_t cxq = _LockWord.FullWord ; 535 if (((cxq & ~_LBIT)|UNS(_EntryList)) == 0) { 536 return ; // normal fast-path exit - cxq and EntryList both empty 537 } 538 if (cxq & _LBIT) { 539 // Optional optimization ... 540 // Some other thread acquired the lock in the window since this 541 // thread released it. Succession is now that thread's responsibility. 542 return ; 543 } 544 545 Succession: 546 // Slow-path exit - this thread must ensure succession and progress. 547 // OnDeck serves as lock to protect cxq and EntryList. 548 // Only the holder of OnDeck can manipulate EntryList or detach the RATs from cxq. 549 // Avoid ABA - allow multiple concurrent producers (enqueue via push-CAS) 550 // but only one concurrent consumer (detacher of RATs). 551 // Consider protecting this critical section with schedctl on Solaris. 552 // Unlike a normal lock, however, the exiting thread "locks" OnDeck, 553 // picks a successor and marks that thread as OnDeck. That successor 554 // thread will then clear OnDeck once it eventually acquires the outer lock. 555 if (CASPTR (&_OnDeck, NULL, _LBIT) != UNS(NULL)) { 556 return ; 557 } 558 559 ParkEvent * List = _EntryList ; 560 if (List != NULL) { 561 // Transfer the head of the EntryList to the OnDeck position. 562 // Once OnDeck, a thread stays OnDeck until it acquires the lock. 563 // For a given lock there is at most OnDeck thread at any one instant. 564 WakeOne: 565 assert (List == _EntryList, "invariant") ; 566 ParkEvent * const w = List ; 567 assert (RelaxAssert || w != Thread::current()->_MutexEvent, "invariant") ; 568 _EntryList = w->ListNext ; 569 // as a diagnostic measure consider setting w->_ListNext = BAD 570 assert (UNS(_OnDeck) == _LBIT, "invariant") ; 571 _OnDeck = w ; // pass OnDeck to w. 572 // w will clear OnDeck once it acquires the outer lock 573 574 // Another optional optimization ... 575 // For heavily contended locks it's not uncommon that some other 576 // thread acquired the lock while this thread was arranging succession. 577 // Try to defer the unpark() operation - Delegate the responsibility 578 // for unpark()ing the OnDeck thread to the current or subsequent owners 579 // That is, the new owner is responsible for unparking the OnDeck thread. 580 OrderAccess::storeload() ; 581 cxq = _LockWord.FullWord ; 582 if (cxq & _LBIT) return ; 583 584 w->unpark() ; 585 return ; 586 } 587 588 cxq = _LockWord.FullWord ; 589 if ((cxq & ~_LBIT) != 0) { 590 // The EntryList is empty but the cxq is populated. 591 // drain RATs from cxq into EntryList 592 // Detach RATs segment with CAS and then merge into EntryList 593 for (;;) { 594 // optional optimization - if locked, the owner is responsible for succession 595 if (cxq & _LBIT) goto Punt ; 596 const intptr_t vfy = CASPTR (&_LockWord, cxq, cxq & _LBIT) ; 597 if (vfy == cxq) break ; 598 cxq = vfy ; 599 // Interference - LockWord changed - Just retry 600 // We can see concurrent interference from contending threads 601 // pushing themselves onto the cxq or from lock-unlock operations. 602 // From the perspective of this thread, EntryList is stable and 603 // the cxq is prepend-only -- the head is volatile but the interior 604 // of the cxq is stable. In theory if we encounter interference from threads 605 // pushing onto cxq we could simply break off the original cxq suffix and 606 // move that segment to the EntryList, avoiding a 2nd or multiple CAS attempts 607 // on the high-traffic LockWord variable. For instance lets say the cxq is "ABCD" 608 // when we first fetch cxq above. Between the fetch -- where we observed "A" 609 // -- and CAS -- where we attempt to CAS null over A -- "PQR" arrive, 610 // yielding cxq = "PQRABCD". In this case we could simply set A.ListNext 611 // null, leaving cxq = "PQRA" and transfer the "BCD" segment to the EntryList. 612 // Note too, that it's safe for this thread to traverse the cxq 613 // without taking any special concurrency precautions. 614 } 615 616 // We don't currently reorder the cxq segment as we move it onto 617 // the EntryList, but it might make sense to reverse the order 618 // or perhaps sort by thread priority. See the comments in 619 // synchronizer.cpp objectMonitor::exit(). 620 assert (_EntryList == NULL, "invariant") ; 621 _EntryList = List = (ParkEvent *)(cxq & ~_LBIT) ; 622 assert (List != NULL, "invariant") ; 623 goto WakeOne ; 624 } 625 626 // cxq|EntryList is empty. 627 // w == NULL implies that cxq|EntryList == NULL in the past. 628 // Possible race - rare inopportune interleaving. 629 // A thread could have added itself to cxq since this thread previously checked. 630 // Detect and recover by refetching cxq. 631 Punt: 632 assert (UNS(_OnDeck) == _LBIT, "invariant") ; 633 _OnDeck = NULL ; // Release inner lock. 634 OrderAccess::storeload(); // Dekker duality - pivot point 635 636 // Resample LockWord/cxq to recover from possible race. 637 // For instance, while this thread T1 held OnDeck, some other thread T2 might 638 // acquire the outer lock. Another thread T3 might try to acquire the outer 639 // lock, but encounter contention and enqueue itself on cxq. T2 then drops the 640 // outer lock, but skips succession as this thread T1 still holds OnDeck. 641 // T1 is and remains responsible for ensuring succession of T3. 642 // 643 // Note that we don't need to recheck EntryList, just cxq. 644 // If threads moved onto EntryList since we dropped OnDeck 645 // that implies some other thread forced succession. 646 cxq = _LockWord.FullWord ; 647 if ((cxq & ~_LBIT) != 0 && (cxq & _LBIT) == 0) { 648 goto Succession ; // potential race -- re-run succession 649 } 650 return ; 651 } 652 653 bool Monitor::notify() { 654 assert (_owner == Thread::current(), "invariant") ; 655 assert (ILocked(), "invariant") ; 656 if (_WaitSet == NULL) return true ; 657 NotifyCount ++ ; 658 659 // Transfer one thread from the WaitSet to the EntryList or cxq. 660 // Currently we just unlink the head of the WaitSet and prepend to the cxq. 661 // And of course we could just unlink it and unpark it, too, but 662 // in that case it'd likely impale itself on the reentry. 663 Thread::muxAcquire (_WaitLock, "notify:WaitLock") ; 664 ParkEvent * nfy = _WaitSet ; 665 if (nfy != NULL) { // DCL idiom 666 _WaitSet = nfy->ListNext ; 667 assert (nfy->Notified == 0, "invariant") ; 668 // push nfy onto the cxq 669 for (;;) { 670 const intptr_t v = _LockWord.FullWord ; 671 assert ((v & 0xFF) == _LBIT, "invariant") ; 672 nfy->ListNext = (ParkEvent *)(v & ~_LBIT); 673 if (CASPTR (&_LockWord, v, UNS(nfy)|_LBIT) == v) break; 674 // interference - _LockWord changed -- just retry 675 } 676 // Note that setting Notified before pushing nfy onto the cxq is 677 // also legal and safe, but the safety properties are much more 678 // subtle, so for the sake of code stewardship ... 679 OrderAccess::fence() ; 680 nfy->Notified = 1; 681 } 682 Thread::muxRelease (_WaitLock) ; 683 if (nfy != NULL && (NativeMonitorFlags & 16)) { 684 // Experimental code ... light up the wakee in the hope that this thread (the owner) 685 // will drop the lock just about the time the wakee comes ONPROC. 686 nfy->unpark() ; 687 } 688 assert (ILocked(), "invariant") ; 689 return true ; 690 } 691 692 // Currently notifyAll() transfers the waiters one-at-a-time from the waitset 693 // to the cxq. This could be done more efficiently with a single bulk en-mass transfer, 694 // but in practice notifyAll() for large #s of threads is rare and not time-critical. 695 // Beware too, that we invert the order of the waiters. Lets say that the 696 // waitset is "ABCD" and the cxq is "XYZ". After a notifyAll() the waitset 697 // will be empty and the cxq will be "DCBAXYZ". This is benign, of course. 698 699 bool Monitor::notify_all() { 700 assert (_owner == Thread::current(), "invariant") ; 701 assert (ILocked(), "invariant") ; 702 while (_WaitSet != NULL) notify() ; 703 return true ; 704 } 705 706 int Monitor::IWait (Thread * Self, jlong timo) { 707 assert (ILocked(), "invariant") ; 708 709 // Phases: 710 // 1. Enqueue Self on WaitSet - currently prepend 711 // 2. unlock - drop the outer lock 712 // 3. wait for either notification or timeout 713 // 4. lock - reentry - reacquire the outer lock 714 715 ParkEvent * const ESelf = Self->_MutexEvent ; 716 ESelf->Notified = 0 ; 717 ESelf->reset() ; 718 OrderAccess::fence() ; 719 720 // Add Self to WaitSet 721 // Ideally only the holder of the outer lock would manipulate the WaitSet - 722 // That is, the outer lock would implicitly protect the WaitSet. 723 // But if a thread in wait() encounters a timeout it will need to dequeue itself 724 // from the WaitSet _before it becomes the owner of the lock. We need to dequeue 725 // as the ParkEvent -- which serves as a proxy for the thread -- can't reside 726 // on both the WaitSet and the EntryList|cxq at the same time.. That is, a thread 727 // on the WaitSet can't be allowed to compete for the lock until it has managed to 728 // unlink its ParkEvent from WaitSet. Thus the need for WaitLock. 729 // Contention on the WaitLock is minimal. 730 // 731 // Another viable approach would be add another ParkEvent, "WaitEvent" to the 732 // thread class. The WaitSet would be composed of WaitEvents. Only the 733 // owner of the outer lock would manipulate the WaitSet. A thread in wait() 734 // could then compete for the outer lock, and then, if necessary, unlink itself 735 // from the WaitSet only after having acquired the outer lock. More precisely, 736 // there would be no WaitLock. A thread in in wait() would enqueue its WaitEvent 737 // on the WaitSet; release the outer lock; wait for either notification or timeout; 738 // reacquire the inner lock; and then, if needed, unlink itself from the WaitSet. 739 // 740 // Alternatively, a 2nd set of list link fields in the ParkEvent might suffice. 741 // One set would be for the WaitSet and one for the EntryList. 742 // We could also deconstruct the ParkEvent into a "pure" event and add a 743 // new immortal/TSM "ListElement" class that referred to ParkEvents. 744 // In that case we could have one ListElement on the WaitSet and another 745 // on the EntryList, with both referring to the same pure Event. 746 747 Thread::muxAcquire (_WaitLock, "wait:WaitLock:Add") ; 748 ESelf->ListNext = _WaitSet ; 749 _WaitSet = ESelf ; 750 Thread::muxRelease (_WaitLock) ; 751 752 // Release the outer lock 753 // We call IUnlock (RelaxAssert=true) as a thread T1 might 754 // enqueue itself on the WaitSet, call IUnlock(), drop the lock, 755 // and then stall before it can attempt to wake a successor. 756 // Some other thread T2 acquires the lock, and calls notify(), moving 757 // T1 from the WaitSet to the cxq. T2 then drops the lock. T1 resumes, 758 // and then finds *itself* on the cxq. During the course of a normal 759 // IUnlock() call a thread should _never find itself on the EntryList 760 // or cxq, but in the case of wait() it's possible. 761 // See synchronizer.cpp objectMonitor::wait(). 762 IUnlock (true) ; 763 764 // Wait for either notification or timeout 765 // Beware that in some circumstances we might propagate 766 // spurious wakeups back to the caller. 767 768 for (;;) { 769 if (ESelf->Notified) break ; 770 int err = ParkCommon (ESelf, timo) ; 771 if (err == OS_TIMEOUT || (NativeMonitorFlags & 1)) break ; 772 } 773 774 // Prepare for reentry - if necessary, remove ESelf from WaitSet 775 // ESelf can be: 776 // 1. Still on the WaitSet. This can happen if we exited the loop by timeout. 777 // 2. On the cxq or EntryList 778 // 3. Not resident on cxq, EntryList or WaitSet, but in the OnDeck position. 779 780 OrderAccess::fence() ; 781 int WasOnWaitSet = 0 ; 782 if (ESelf->Notified == 0) { 783 Thread::muxAcquire (_WaitLock, "wait:WaitLock:remove") ; 784 if (ESelf->Notified == 0) { // DCL idiom 785 assert (_OnDeck != ESelf, "invariant") ; // can't be both OnDeck and on WaitSet 786 // ESelf is resident on the WaitSet -- unlink it. 787 // A doubly-linked list would be better here so we can unlink in constant-time. 788 // We have to unlink before we potentially recontend as ESelf might otherwise 789 // end up on the cxq|EntryList -- it can't be on two lists at once. 790 ParkEvent * p = _WaitSet ; 791 ParkEvent * q = NULL ; // classic q chases p 792 while (p != NULL && p != ESelf) { 793 q = p ; 794 p = p->ListNext ; 795 } 796 assert (p == ESelf, "invariant") ; 797 if (p == _WaitSet) { // found at head 798 assert (q == NULL, "invariant") ; 799 _WaitSet = p->ListNext ; 800 } else { // found in interior 801 assert (q->ListNext == p, "invariant") ; 802 q->ListNext = p->ListNext ; 803 } 804 WasOnWaitSet = 1 ; // We were *not* notified but instead encountered timeout 805 } 806 Thread::muxRelease (_WaitLock) ; 807 } 808 809 // Reentry phase - reacquire the lock 810 if (WasOnWaitSet) { 811 // ESelf was previously on the WaitSet but we just unlinked it above 812 // because of a timeout. ESelf is not resident on any list and is not OnDeck 813 assert (_OnDeck != ESelf, "invariant") ; 814 ILock (Self) ; 815 } else { 816 // A prior notify() operation moved ESelf from the WaitSet to the cxq. 817 // ESelf is now on the cxq, EntryList or at the OnDeck position. 818 // The following fragment is extracted from Monitor::ILock() 819 for (;;) { 820 if (_OnDeck == ESelf && TrySpin(Self)) break ; 821 ParkCommon (ESelf, 0) ; 822 } 823 assert (_OnDeck == ESelf, "invariant") ; 824 _OnDeck = NULL ; 825 } 826 827 assert (ILocked(), "invariant") ; 828 return WasOnWaitSet != 0 ; // return true IFF timeout 829 } 830 831 832 // ON THE VMTHREAD SNEAKING PAST HELD LOCKS: 833 // In particular, there are certain types of global lock that may be held 834 // by a Java thread while it is blocked at a safepoint but before it has 835 // written the _owner field. These locks may be sneakily acquired by the 836 // VM thread during a safepoint to avoid deadlocks. Alternatively, one should 837 // identify all such locks, and ensure that Java threads never block at 838 // safepoints while holding them (_no_safepoint_check_flag). While it 839 // seems as though this could increase the time to reach a safepoint 840 // (or at least increase the mean, if not the variance), the latter 841 // approach might make for a cleaner, more maintainable JVM design. 842 // 843 // Sneaking is vile and reprehensible and should be excised at the 1st 844 // opportunity. It's possible that the need for sneaking could be obviated 845 // as follows. Currently, a thread might (a) while TBIVM, call pthread_mutex_lock 846 // or ILock() thus acquiring the "physical" lock underlying Monitor/Mutex. 847 // (b) stall at the TBIVM exit point as a safepoint is in effect. Critically, 848 // it'll stall at the TBIVM reentry state transition after having acquired the 849 // underlying lock, but before having set _owner and having entered the actual 850 // critical section. The lock-sneaking facility leverages that fact and allowed the 851 // VM thread to logically acquire locks that had already be physically locked by mutators 852 // but where mutators were known blocked by the reentry thread state transition. 853 // 854 // If we were to modify the Monitor-Mutex so that TBIVM state transitions tightly 855 // wrapped calls to park(), then we could likely do away with sneaking. We'd 856 // decouple lock acquisition and parking. The critical invariant to eliminating 857 // sneaking is to ensure that we never "physically" acquire the lock while TBIVM. 858 // An easy way to accomplish this is to wrap the park calls in a narrow TBIVM jacket. 859 // One difficulty with this approach is that the TBIVM wrapper could recurse and 860 // call lock() deep from within a lock() call, while the MutexEvent was already enqueued. 861 // Using a stack (N=2 at minimum) of ParkEvents would take care of that problem. 862 // 863 // But of course the proper ultimate approach is to avoid schemes that require explicit 864 // sneaking or dependence on any any clever invariants or subtle implementation properties 865 // of Mutex-Monitor and instead directly address the underlying design flaw. 866 867 void Monitor::lock (Thread * Self) { 868 #ifdef CHECK_UNHANDLED_OOPS 869 // Clear unhandled oops so we get a crash right away. Only clear for non-vm 870 // or GC threads. 871 if (Self->is_Java_thread()) { 872 Self->clear_unhandled_oops(); 873 } 874 #endif // CHECK_UNHANDLED_OOPS 875 876 debug_only(check_prelock_state(Self)); 877 assert (_owner != Self , "invariant") ; 878 assert (_OnDeck != Self->_MutexEvent, "invariant") ; 879 880 if (TryFast()) { 881 Exeunt: 882 assert (ILocked(), "invariant") ; 883 assert (owner() == NULL, "invariant"); 884 set_owner (Self); 885 return ; 886 } 887 888 // The lock is contended ... 889 890 bool can_sneak = Self->is_VM_thread() && SafepointSynchronize::is_at_safepoint(); 891 if (can_sneak && _owner == NULL) { 892 // a java thread has locked the lock but has not entered the 893 // critical region -- let's just pretend we've locked the lock 894 // and go on. we note this with _snuck so we can also 895 // pretend to unlock when the time comes. 896 _snuck = true; 897 goto Exeunt ; 898 } 899 900 // Try a brief spin to avoid passing thru thread state transition ... 901 if (TrySpin (Self)) goto Exeunt ; 902 903 check_block_state(Self); 904 if (Self->is_Java_thread()) { 905 // Horribile dictu - we suffer through a state transition 906 assert(rank() > Mutex::special, "Potential deadlock with special or lesser rank mutex"); 907 ThreadBlockInVM tbivm ((JavaThread *) Self) ; 908 ILock (Self) ; 909 } else { 910 // Mirabile dictu 911 ILock (Self) ; 912 } 913 goto Exeunt ; 914 } 915 916 void Monitor::lock() { 917 this->lock(Thread::current()); 918 } 919 920 // Lock without safepoint check - a degenerate variant of lock(). 921 // Should ONLY be used by safepoint code and other code 922 // that is guaranteed not to block while running inside the VM. If this is called with 923 // thread state set to be in VM, the safepoint synchronization code will deadlock! 924 925 void Monitor::lock_without_safepoint_check (Thread * Self) { 926 assert (_owner != Self, "invariant") ; 927 ILock (Self) ; 928 assert (_owner == NULL, "invariant"); 929 set_owner (Self); 930 } 931 932 void Monitor::lock_without_safepoint_check () { 933 lock_without_safepoint_check (Thread::current()) ; 934 } 935 936 937 // Returns true if thread succeceed [sic] in grabbing the lock, otherwise false. 938 939 bool Monitor::try_lock() { 940 Thread * const Self = Thread::current(); 941 debug_only(check_prelock_state(Self)); 942 // assert(!thread->is_inside_signal_handler(), "don't lock inside signal handler"); 943 944 // Special case, where all Java threads are stopped. 945 // The lock may have been acquired but _owner is not yet set. 946 // In that case the VM thread can safely grab the lock. 947 // It strikes me this should appear _after the TryLock() fails, below. 948 bool can_sneak = Self->is_VM_thread() && SafepointSynchronize::is_at_safepoint(); 949 if (can_sneak && _owner == NULL) { 950 set_owner(Self); // Do not need to be atomic, since we are at a safepoint 951 _snuck = true; 952 return true; 953 } 954 955 if (TryLock()) { 956 // We got the lock 957 assert (_owner == NULL, "invariant"); 958 set_owner (Self); 959 return true; 960 } 961 return false; 962 } 963 964 void Monitor::unlock() { 965 assert (_owner == Thread::current(), "invariant") ; 966 assert (_OnDeck != Thread::current()->_MutexEvent , "invariant") ; 967 set_owner (NULL) ; 968 if (_snuck) { 969 assert(SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread(), "sneak"); 970 _snuck = false; 971 return ; 972 } 973 IUnlock (false) ; 974 } 975 976 // Yet another degenerate version of Monitor::lock() or lock_without_safepoint_check() 977 // jvm_raw_lock() and _unlock() can be called by non-Java threads via JVM_RawMonitorEnter. 978 // 979 // There's no expectation that JVM_RawMonitors will interoperate properly with the native 980 // Mutex-Monitor constructs. We happen to implement JVM_RawMonitors in terms of 981 // native Mutex-Monitors simply as a matter of convenience. A simple abstraction layer 982 // over a pthread_mutex_t would work equally as well, but require more platform-specific 983 // code -- a "PlatformMutex". Alternatively, a simply layer over muxAcquire-muxRelease 984 // would work too. 985 // 986 // Since the caller might be a foreign thread, we don't necessarily have a Thread.MutexEvent 987 // instance available. Instead, we transiently allocate a ParkEvent on-demand if 988 // we encounter contention. That ParkEvent remains associated with the thread 989 // until it manages to acquire the lock, at which time we return the ParkEvent 990 // to the global ParkEvent free list. This is correct and suffices for our purposes. 991 // 992 // Beware that the original jvm_raw_unlock() had a "_snuck" test but that 993 // jvm_raw_lock() didn't have the corresponding test. I suspect that's an 994 // oversight, but I've replicated the original suspect logic in the new code ... 995 996 void Monitor::jvm_raw_lock() { 997 assert(rank() == native, "invariant"); 998 999 if (TryLock()) { 1000 Exeunt: 1001 assert (ILocked(), "invariant") ; 1002 assert (_owner == NULL, "invariant"); 1003 // This can potentially be called by non-java Threads. Thus, the ThreadLocalStorage 1004 // might return NULL. Don't call set_owner since it will break on an NULL owner 1005 // Consider installing a non-null "ANON" distinguished value instead of just NULL. 1006 _owner = ThreadLocalStorage::thread(); 1007 return ; 1008 } 1009 1010 if (TrySpin(NULL)) goto Exeunt ; 1011 1012 // slow-path - apparent contention 1013 // Allocate a ParkEvent for transient use. 1014 // The ParkEvent remains associated with this thread until 1015 // the time the thread manages to acquire the lock. 1016 ParkEvent * const ESelf = ParkEvent::Allocate(NULL) ; 1017 ESelf->reset() ; 1018 OrderAccess::storeload() ; 1019 1020 // Either Enqueue Self on cxq or acquire the outer lock. 1021 if (AcquireOrPush (ESelf)) { 1022 ParkEvent::Release (ESelf) ; // surrender the ParkEvent 1023 goto Exeunt ; 1024 } 1025 1026 // At any given time there is at most one ondeck thread. 1027 // ondeck implies not resident on cxq and not resident on EntryList 1028 // Only the OnDeck thread can try to acquire -- contended for -- the lock. 1029 // CONSIDER: use Self->OnDeck instead of m->OnDeck. 1030 for (;;) { 1031 if (_OnDeck == ESelf && TrySpin(NULL)) break ; 1032 ParkCommon (ESelf, 0) ; 1033 } 1034 1035 assert (_OnDeck == ESelf, "invariant") ; 1036 _OnDeck = NULL ; 1037 ParkEvent::Release (ESelf) ; // surrender the ParkEvent 1038 goto Exeunt ; 1039 } 1040 1041 void Monitor::jvm_raw_unlock() { 1042 // Nearly the same as Monitor::unlock() ... 1043 // directly set _owner instead of using set_owner(null) 1044 _owner = NULL ; 1045 if (_snuck) { // ??? 1046 assert(SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread(), "sneak"); 1047 _snuck = false; 1048 return ; 1049 } 1050 IUnlock(false) ; 1051 } 1052 1053 bool Monitor::wait(bool no_safepoint_check, long timeout, bool as_suspend_equivalent) { 1054 Thread * const Self = Thread::current() ; 1055 assert (_owner == Self, "invariant") ; 1056 assert (ILocked(), "invariant") ; 1057 1058 // as_suspend_equivalent logically implies !no_safepoint_check 1059 guarantee (!as_suspend_equivalent || !no_safepoint_check, "invariant") ; 1060 // !no_safepoint_check logically implies java_thread 1061 guarantee (no_safepoint_check || Self->is_Java_thread(), "invariant") ; 1062 1063 #ifdef ASSERT 1064 Monitor * least = get_least_ranked_lock_besides_this(Self->owned_locks()); 1065 assert(least != this, "Specification of get_least_... call above"); 1066 if (least != NULL && least->rank() <= special) { 1067 tty->print("Attempting to wait on monitor %s/%d while holding" 1068 " lock %s/%d -- possible deadlock", 1069 name(), rank(), least->name(), least->rank()); 1070 assert(false, "Shouldn't block(wait) while holding a lock of rank special"); 1071 } 1072 #endif // ASSERT 1073 1074 int wait_status ; 1075 // conceptually set the owner to NULL in anticipation of 1076 // abdicating the lock in wait 1077 set_owner(NULL); 1078 if (no_safepoint_check) { 1079 wait_status = IWait (Self, timeout) ; 1080 } else { 1081 assert (Self->is_Java_thread(), "invariant") ; 1082 JavaThread *jt = (JavaThread *)Self; 1083 1084 // Enter safepoint region - ornate and Rococo ... 1085 ThreadBlockInVM tbivm(jt); 1086 OSThreadWaitState osts(Self->osthread(), false /* not Object.wait() */); 1087 1088 if (as_suspend_equivalent) { 1089 jt->set_suspend_equivalent(); 1090 // cleared by handle_special_suspend_equivalent_condition() or 1091 // java_suspend_self() 1092 } 1093 1094 wait_status = IWait (Self, timeout) ; 1095 1096 // were we externally suspended while we were waiting? 1097 if (as_suspend_equivalent && jt->handle_special_suspend_equivalent_condition()) { 1098 // Our event wait has finished and we own the lock, but 1099 // while we were waiting another thread suspended us. We don't 1100 // want to hold the lock while suspended because that 1101 // would surprise the thread that suspended us. 1102 assert (ILocked(), "invariant") ; 1103 IUnlock (true) ; 1104 jt->java_suspend_self(); 1105 ILock (Self) ; 1106 assert (ILocked(), "invariant") ; 1107 } 1108 } 1109 1110 // Conceptually reestablish ownership of the lock. 1111 // The "real" lock -- the LockByte -- was reacquired by IWait(). 1112 assert (ILocked(), "invariant") ; 1113 assert (_owner == NULL, "invariant") ; 1114 set_owner (Self) ; 1115 return wait_status != 0 ; // return true IFF timeout 1116 } 1117 1118 Monitor::~Monitor() { 1119 assert ((UNS(_owner)|UNS(_LockWord.FullWord)|UNS(_EntryList)|UNS(_WaitSet)|UNS(_OnDeck)) == 0, "") ; 1120 } 1121 1122 void Monitor::ClearMonitor (Monitor * m, const char *name) { 1123 m->_owner = NULL ; 1124 m->_snuck = false ; 1125 if (name == NULL) { 1126 strcpy(m->_name, "UNKNOWN") ; 1127 } else { 1128 strncpy(m->_name, name, MONITOR_NAME_LEN - 1); 1129 m->_name[MONITOR_NAME_LEN - 1] = '\0'; 1130 } 1131 m->_LockWord.FullWord = 0 ; 1132 m->_EntryList = NULL ; 1133 m->_OnDeck = NULL ; 1134 m->_WaitSet = NULL ; 1135 m->_WaitLock[0] = 0 ; 1136 } 1137 1138 Monitor::Monitor() { ClearMonitor(this); } 1139 1140 Monitor::Monitor (int Rank, const char * name, bool allow_vm_block) { 1141 ClearMonitor (this, name) ; 1142 #ifdef ASSERT 1143 _allow_vm_block = allow_vm_block; 1144 _rank = Rank ; 1145 #endif 1146 } 1147 1148 Mutex::~Mutex() { 1149 assert ((UNS(_owner)|UNS(_LockWord.FullWord)|UNS(_EntryList)|UNS(_WaitSet)|UNS(_OnDeck)) == 0, "") ; 1150 } 1151 1152 Mutex::Mutex (int Rank, const char * name, bool allow_vm_block) { 1153 ClearMonitor ((Monitor *) this, name) ; 1154 #ifdef ASSERT 1155 _allow_vm_block = allow_vm_block; 1156 _rank = Rank ; 1157 #endif 1158 } 1159 1160 bool Monitor::owned_by_self() const { 1161 bool ret = _owner == Thread::current(); 1162 assert (!ret || _LockWord.Bytes[_LSBINDEX] != 0, "invariant") ; 1163 return ret; 1164 } 1165 1166 void Monitor::print_on_error(outputStream* st) const { 1167 st->print("[" PTR_FORMAT, this); 1168 st->print("] %s", _name); 1169 st->print(" - owner thread: " PTR_FORMAT, _owner); 1170 } 1171 1172 1173 1174 1175 // ---------------------------------------------------------------------------------- 1176 // Non-product code 1177 1178 #ifndef PRODUCT 1179 void Monitor::print_on(outputStream* st) const { 1180 st->print_cr("Mutex: [0x%lx/0x%lx] %s - owner: 0x%lx", this, _LockWord.FullWord, _name, _owner); 1181 } 1182 #endif 1183 1184 #ifndef PRODUCT 1185 #ifdef ASSERT 1186 Monitor * Monitor::get_least_ranked_lock(Monitor * locks) { 1187 Monitor *res, *tmp; 1188 for (res = tmp = locks; tmp != NULL; tmp = tmp->next()) { 1189 if (tmp->rank() < res->rank()) { 1190 res = tmp; 1191 } 1192 } 1193 if (!SafepointSynchronize::is_at_safepoint()) { 1194 // In this case, we expect the held locks to be 1195 // in increasing rank order (modulo any native ranks) 1196 for (tmp = locks; tmp != NULL; tmp = tmp->next()) { 1197 if (tmp->next() != NULL) { 1198 assert(tmp->rank() == Mutex::native || 1199 tmp->rank() <= tmp->next()->rank(), "mutex rank anomaly?"); 1200 } 1201 } 1202 } 1203 return res; 1204 } 1205 1206 Monitor* Monitor::get_least_ranked_lock_besides_this(Monitor* locks) { 1207 Monitor *res, *tmp; 1208 for (res = NULL, tmp = locks; tmp != NULL; tmp = tmp->next()) { 1209 if (tmp != this && (res == NULL || tmp->rank() < res->rank())) { 1210 res = tmp; 1211 } 1212 } 1213 if (!SafepointSynchronize::is_at_safepoint()) { 1214 // In this case, we expect the held locks to be 1215 // in increasing rank order (modulo any native ranks) 1216 for (tmp = locks; tmp != NULL; tmp = tmp->next()) { 1217 if (tmp->next() != NULL) { 1218 assert(tmp->rank() == Mutex::native || 1219 tmp->rank() <= tmp->next()->rank(), "mutex rank anomaly?"); 1220 } 1221 } 1222 } 1223 return res; 1224 } 1225 1226 1227 bool Monitor::contains(Monitor* locks, Monitor * lock) { 1228 for (; locks != NULL; locks = locks->next()) { 1229 if (locks == lock) 1230 return true; 1231 } 1232 return false; 1233 } 1234 #endif 1235 1236 // Called immediately after lock acquisition or release as a diagnostic 1237 // to track the lock-set of the thread and test for rank violations that 1238 // might indicate exposure to deadlock. 1239 // Rather like an EventListener for _owner (:>). 1240 1241 void Monitor::set_owner_implementation(Thread *new_owner) { 1242 // This function is solely responsible for maintaining 1243 // and checking the invariant that threads and locks 1244 // are in a 1/N relation, with some some locks unowned. 1245 // It uses the Mutex::_owner, Mutex::_next, and 1246 // Thread::_owned_locks fields, and no other function 1247 // changes those fields. 1248 // It is illegal to set the mutex from one non-NULL 1249 // owner to another--it must be owned by NULL as an 1250 // intermediate state. 1251 1252 if (new_owner != NULL) { 1253 // the thread is acquiring this lock 1254 1255 assert(new_owner == Thread::current(), "Should I be doing this?"); 1256 assert(_owner == NULL, "setting the owner thread of an already owned mutex"); 1257 _owner = new_owner; // set the owner 1258 1259 // link "this" into the owned locks list 1260 1261 #ifdef ASSERT // Thread::_owned_locks is under the same ifdef 1262 Monitor* locks = get_least_ranked_lock(new_owner->owned_locks()); 1263 // Mutex::set_owner_implementation is a friend of Thread 1264 1265 assert(this->rank() >= 0, "bad lock rank"); 1266 1267 if (LogMultipleMutexLocking && locks != NULL) { 1268 Events::log("thread " INTPTR_FORMAT " locks %s, already owns %s", new_owner, name(), locks->name()); 1269 } 1270 1271 // Deadlock avoidance rules require us to acquire Mutexes only in 1272 // a global total order. For example m1 is the lowest ranked mutex 1273 // that the thread holds and m2 is the mutex the thread is trying 1274 // to acquire, then deadlock avoidance rules require that the rank 1275 // of m2 be less than the rank of m1. 1276 // The rank Mutex::native is an exception in that it is not subject 1277 // to the verification rules. 1278 // Here are some further notes relating to mutex acquisition anomalies: 1279 // . under Solaris, the interrupt lock gets acquired when doing 1280 // profiling, so any lock could be held. 1281 // . it is also ok to acquire Safepoint_lock at the very end while we 1282 // already hold Terminator_lock - may happen because of periodic safepoints 1283 if (this->rank() != Mutex::native && 1284 this->rank() != Mutex::suspend_resume && 1285 locks != NULL && locks->rank() <= this->rank() && 1286 !SafepointSynchronize::is_at_safepoint() && 1287 this != Interrupt_lock && this != ProfileVM_lock && 1288 !(this == Safepoint_lock && contains(locks, Terminator_lock) && 1289 SafepointSynchronize::is_synchronizing())) { 1290 new_owner->print_owned_locks(); 1291 fatal(err_msg("acquiring lock %s/%d out of order with lock %s/%d -- " 1292 "possible deadlock", this->name(), this->rank(), 1293 locks->name(), locks->rank())); 1294 } 1295 1296 this->_next = new_owner->_owned_locks; 1297 new_owner->_owned_locks = this; 1298 #endif 1299 1300 } else { 1301 // the thread is releasing this lock 1302 1303 Thread* old_owner = _owner; 1304 debug_only(_last_owner = old_owner); 1305 1306 assert(old_owner != NULL, "removing the owner thread of an unowned mutex"); 1307 assert(old_owner == Thread::current(), "removing the owner thread of an unowned mutex"); 1308 1309 _owner = NULL; // set the owner 1310 1311 #ifdef ASSERT 1312 Monitor *locks = old_owner->owned_locks(); 1313 1314 if (LogMultipleMutexLocking && locks != this) { 1315 Events::log("thread " INTPTR_FORMAT " unlocks %s, still owns %s", old_owner, this->name(), locks->name()); 1316 } 1317 1318 // remove "this" from the owned locks list 1319 1320 Monitor *prev = NULL; 1321 bool found = false; 1322 for (; locks != NULL; prev = locks, locks = locks->next()) { 1323 if (locks == this) { 1324 found = true; 1325 break; 1326 } 1327 } 1328 assert(found, "Removing a lock not owned"); 1329 if (prev == NULL) { 1330 old_owner->_owned_locks = _next; 1331 } else { 1332 prev->_next = _next; 1333 } 1334 _next = NULL; 1335 #endif 1336 } 1337 } 1338 1339 1340 // Factored out common sanity checks for locking mutex'es. Used by lock() and try_lock() 1341 void Monitor::check_prelock_state(Thread *thread) { 1342 assert((!thread->is_Java_thread() || ((JavaThread *)thread)->thread_state() == _thread_in_vm) 1343 || rank() == Mutex::special, "wrong thread state for using locks"); 1344 if (StrictSafepointChecks) { 1345 if (thread->is_VM_thread() && !allow_vm_block()) { 1346 fatal(err_msg("VM thread using lock %s (not allowed to block on)", 1347 name())); 1348 } 1349 debug_only(if (rank() != Mutex::special) \ 1350 thread->check_for_valid_safepoint_state(false);) 1351 } 1352 } 1353 1354 void Monitor::check_block_state(Thread *thread) { 1355 if (!_allow_vm_block && thread->is_VM_thread()) { 1356 warning("VM thread blocked on lock"); 1357 print(); 1358 BREAKPOINT; 1359 } 1360 assert(_owner != thread, "deadlock: blocking on monitor owned by current thread"); 1361 } 1362 1363 #endif // PRODUCT