1 2 /* 3 * Copyright (c) 1998, 2012, Oracle and/or its affiliates. All rights reserved. 4 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 5 * 6 * This code is free software; you can redistribute it and/or modify it 7 * under the terms of the GNU General Public License version 2 only, as 8 * published by the Free Software Foundation. 9 * 10 * This code is distributed in the hope that it will be useful, but WITHOUT 11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 13 * version 2 for more details (a copy is included in the LICENSE file that 14 * accompanied this code). 15 * 16 * You should have received a copy of the GNU General Public License version 17 * 2 along with this work; if not, write to the Free Software Foundation, 18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 19 * 20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 21 * or visit www.oracle.com if you need additional information or have any 22 * questions. 23 * 24 */ 25 26 #include "precompiled.hpp" 27 #include "runtime/mutex.hpp" 28 #include "runtime/osThread.hpp" 29 #include "utilities/events.hpp" 30 #ifdef TARGET_OS_FAMILY_linux 31 # include "mutex_linux.inline.hpp" 32 # include "thread_linux.inline.hpp" 33 #endif 34 #ifdef TARGET_OS_FAMILY_solaris 35 # include "mutex_solaris.inline.hpp" 36 # include "thread_solaris.inline.hpp" 37 #endif 38 #ifdef TARGET_OS_FAMILY_windows 39 # include "mutex_windows.inline.hpp" 40 # include "thread_windows.inline.hpp" 41 #endif 42 #ifdef TARGET_OS_FAMILY_bsd 43 # include "mutex_bsd.inline.hpp" 44 # include "thread_bsd.inline.hpp" 45 #endif 46 47 // o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o 48 // 49 // Native Monitor-Mutex locking - theory of operations 50 // 51 // * Native Monitors are completely unrelated to Java-level monitors, 52 // although the "back-end" slow-path implementations share a common lineage. 53 // See objectMonitor:: in synchronizer.cpp. 54 // Native Monitors do *not* support nesting or recursion but otherwise 55 // they're basically Hoare-flavor monitors. 56 // 57 // * A thread acquires ownership of a Monitor/Mutex by CASing the LockByte 58 // in the _LockWord from zero to non-zero. Note that the _Owner field 59 // is advisory and is used only to verify that the thread calling unlock() 60 // is indeed the last thread to have acquired the lock. 61 // 62 // * Contending threads "push" themselves onto the front of the contention 63 // queue -- called the cxq -- with CAS and then spin/park. 64 // The _LockWord contains the LockByte as well as the pointer to the head 65 // of the cxq. Colocating the LockByte with the cxq precludes certain races. 66 // 67 // * Using a separately addressable LockByte allows for CAS:MEMBAR or CAS:0 68 // idioms. We currently use MEMBAR in the uncontended unlock() path, as 69 // MEMBAR often has less latency than CAS. If warranted, we could switch to 70 // a CAS:0 mode, using timers to close the resultant race, as is done 71 // with Java Monitors in synchronizer.cpp. 72 // 73 // See the following for a discussion of the relative cost of atomics (CAS) 74 // MEMBAR, and ways to eliminate such instructions from the common-case paths: 75 // -- http://blogs.sun.com/dave/entry/biased_locking_in_hotspot 76 // -- http://blogs.sun.com/dave/resource/MustangSync.pdf 77 // -- http://blogs.sun.com/dave/resource/synchronization-public2.pdf 78 // -- synchronizer.cpp 79 // 80 // * Overall goals - desiderata 81 // 1. Minimize context switching 82 // 2. Minimize lock migration 83 // 3. Minimize CPI -- affinity and locality 84 // 4. Minimize the execution of high-latency instructions such as CAS or MEMBAR 85 // 5. Minimize outer lock hold times 86 // 6. Behave gracefully on a loaded system 87 // 88 // * Thread flow and list residency: 89 // 90 // Contention queue --> EntryList --> OnDeck --> Owner --> !Owner 91 // [..resident on monitor list..] 92 // [...........contending..................] 93 // 94 // -- The contention queue (cxq) contains recently-arrived threads (RATs). 95 // Threads on the cxq eventually drain into the EntryList. 96 // -- Invariant: a thread appears on at most one list -- cxq, EntryList 97 // or WaitSet -- at any one time. 98 // -- For a given monitor there can be at most one "OnDeck" thread at any 99 // given time but if needbe this particular invariant could be relaxed. 100 // 101 // * The WaitSet and EntryList linked lists are composed of ParkEvents. 102 // I use ParkEvent instead of threads as ParkEvents are immortal and 103 // type-stable, meaning we can safely unpark() a possibly stale 104 // list element in the unlock()-path. (That's benign). 105 // 106 // * Succession policy - providing for progress: 107 // 108 // As necessary, the unlock()ing thread identifies, unlinks, and unparks 109 // an "heir presumptive" tentative successor thread from the EntryList. 110 // This becomes the so-called "OnDeck" thread, of which there can be only 111 // one at any given time for a given monitor. The wakee will recontend 112 // for ownership of monitor. 113 // 114 // Succession is provided for by a policy of competitive handoff. 115 // The exiting thread does _not_ grant or pass ownership to the 116 // successor thread. (This is also referred to as "handoff" succession"). 117 // Instead the exiting thread releases ownership and possibly wakes 118 // a successor, so the successor can (re)compete for ownership of the lock. 119 // 120 // Competitive handoff provides excellent overall throughput at the expense 121 // of short-term fairness. If fairness is a concern then one remedy might 122 // be to add an AcquireCounter field to the monitor. After a thread acquires 123 // the lock it will decrement the AcquireCounter field. When the count 124 // reaches 0 the thread would reset the AcquireCounter variable, abdicate 125 // the lock directly to some thread on the EntryList, and then move itself to the 126 // tail of the EntryList. 127 // 128 // But in practice most threads engage or otherwise participate in resource 129 // bounded producer-consumer relationships, so lock domination is not usually 130 // a practical concern. Recall too, that in general it's easier to construct 131 // a fair lock from a fast lock, but not vice-versa. 132 // 133 // * The cxq can have multiple concurrent "pushers" but only one concurrent 134 // detaching thread. This mechanism is immune from the ABA corruption. 135 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious. 136 // We use OnDeck as a pseudo-lock to enforce the at-most-one detaching 137 // thread constraint. 138 // 139 // * Taken together, the cxq and the EntryList constitute or form a 140 // single logical queue of threads stalled trying to acquire the lock. 141 // We use two distinct lists to reduce heat on the list ends. 142 // Threads in lock() enqueue onto cxq while threads in unlock() will 143 // dequeue from the EntryList. (c.f. Michael Scott's "2Q" algorithm). 144 // A key desideratum is to minimize queue & monitor metadata manipulation 145 // that occurs while holding the "outer" monitor lock -- that is, we want to 146 // minimize monitor lock holds times. 147 // 148 // The EntryList is ordered by the prevailing queue discipline and 149 // can be organized in any convenient fashion, such as a doubly-linked list or 150 // a circular doubly-linked list. If we need a priority queue then something akin 151 // to Solaris' sleepq would work nicely. Viz., 152 // -- http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c. 153 // -- http://cvs.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/uts/common/os/sleepq.c 154 // Queue discipline is enforced at ::unlock() time, when the unlocking thread 155 // drains the cxq into the EntryList, and orders or reorders the threads on the 156 // EntryList accordingly. 157 // 158 // Barring "lock barging", this mechanism provides fair cyclic ordering, 159 // somewhat similar to an elevator-scan. 160 // 161 // * OnDeck 162 // -- For a given monitor there can be at most one OnDeck thread at any given 163 // instant. The OnDeck thread is contending for the lock, but has been 164 // unlinked from the EntryList and cxq by some previous unlock() operations. 165 // Once a thread has been designated the OnDeck thread it will remain so 166 // until it manages to acquire the lock -- being OnDeck is a stable property. 167 // -- Threads on the EntryList or cxq are _not allowed to attempt lock acquisition. 168 // -- OnDeck also serves as an "inner lock" as follows. Threads in unlock() will, after 169 // having cleared the LockByte and dropped the outer lock, attempt to "trylock" 170 // OnDeck by CASing the field from null to non-null. If successful, that thread 171 // is then responsible for progress and succession and can use CAS to detach and 172 // drain the cxq into the EntryList. By convention, only this thread, the holder of 173 // the OnDeck inner lock, can manipulate the EntryList or detach and drain the 174 // RATs on the cxq into the EntryList. This avoids ABA corruption on the cxq as 175 // we allow multiple concurrent "push" operations but restrict detach concurrency 176 // to at most one thread. Having selected and detached a successor, the thread then 177 // changes the OnDeck to refer to that successor, and then unparks the successor. 178 // That successor will eventually acquire the lock and clear OnDeck. Beware 179 // that the OnDeck usage as a lock is asymmetric. A thread in unlock() transiently 180 // "acquires" OnDeck, performs queue manipulations, passes OnDeck to some successor, 181 // and then the successor eventually "drops" OnDeck. Note that there's never 182 // any sense of contention on the inner lock, however. Threads never contend 183 // or wait for the inner lock. 184 // -- OnDeck provides for futile wakeup throttling a described in section 3.3 of 185 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf 186 // In a sense, OnDeck subsumes the ObjectMonitor _Succ and ObjectWaiter 187 // TState fields found in Java-level objectMonitors. (See synchronizer.cpp). 188 // 189 // * Waiting threads reside on the WaitSet list -- wait() puts 190 // the caller onto the WaitSet. Notify() or notifyAll() simply 191 // transfers threads from the WaitSet to either the EntryList or cxq. 192 // Subsequent unlock() operations will eventually unpark the notifyee. 193 // Unparking a notifee in notify() proper is inefficient - if we were to do so 194 // it's likely the notifyee would simply impale itself on the lock held 195 // by the notifier. 196 // 197 // * The mechanism is obstruction-free in that if the holder of the transient 198 // OnDeck lock in unlock() is preempted or otherwise stalls, other threads 199 // can still acquire and release the outer lock and continue to make progress. 200 // At worst, waking of already blocked contending threads may be delayed, 201 // but nothing worse. (We only use "trylock" operations on the inner OnDeck 202 // lock). 203 // 204 // * Note that thread-local storage must be initialized before a thread 205 // uses Native monitors or mutexes. The native monitor-mutex subsystem 206 // depends on Thread::current(). 207 // 208 // * The monitor synchronization subsystem avoids the use of native 209 // synchronization primitives except for the narrow platform-specific 210 // park-unpark abstraction. See the comments in os_solaris.cpp regarding 211 // the semantics of park-unpark. Put another way, this monitor implementation 212 // depends only on atomic operations and park-unpark. The monitor subsystem 213 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the 214 // underlying OS manages the READY<->RUN transitions. 215 // 216 // * The memory consistency model provide by lock()-unlock() is at least as 217 // strong or stronger than the Java Memory model defined by JSR-133. 218 // That is, we guarantee at least entry consistency, if not stronger. 219 // See http://g.oswego.edu/dl/jmm/cookbook.html. 220 // 221 // * Thread:: currently contains a set of purpose-specific ParkEvents: 222 // _MutexEvent, _ParkEvent, etc. A better approach might be to do away with 223 // the purpose-specific ParkEvents and instead implement a general per-thread 224 // stack of available ParkEvents which we could provision on-demand. The 225 // stack acts as a local cache to avoid excessive calls to ParkEvent::Allocate() 226 // and ::Release(). A thread would simply pop an element from the local stack before it 227 // enqueued or park()ed. When the contention was over the thread would 228 // push the no-longer-needed ParkEvent back onto its stack. 229 // 230 // * A slightly reduced form of ILock() and IUnlock() have been partially 231 // model-checked (Murphi) for safety and progress at T=1,2,3 and 4. 232 // It'd be interesting to see if TLA/TLC could be useful as well. 233 // 234 // * Mutex-Monitor is a low-level "leaf" subsystem. That is, the monitor 235 // code should never call other code in the JVM that might itself need to 236 // acquire monitors or mutexes. That's true *except* in the case of the 237 // ThreadBlockInVM state transition wrappers. The ThreadBlockInVM DTOR handles 238 // mutator reentry (ingress) by checking for a pending safepoint in which case it will 239 // call SafepointSynchronize::block(), which in turn may call Safepoint_lock->lock(), etc. 240 // In that particular case a call to lock() for a given Monitor can end up recursively 241 // calling lock() on another monitor. While distasteful, this is largely benign 242 // as the calls come from jacket that wraps lock(), and not from deep within lock() itself. 243 // 244 // It's unfortunate that native mutexes and thread state transitions were convolved. 245 // They're really separate concerns and should have remained that way. Melding 246 // them together was facile -- a bit too facile. The current implementation badly 247 // conflates the two concerns. 248 // 249 // * TODO-FIXME: 250 // 251 // -- Add DTRACE probes for contended acquire, contended acquired, contended unlock 252 // We should also add DTRACE probes in the ParkEvent subsystem for 253 // Park-entry, Park-exit, and Unpark. 254 // 255 // -- We have an excess of mutex-like constructs in the JVM, namely: 256 // 1. objectMonitors for Java-level synchronization (synchronizer.cpp) 257 // 2. low-level muxAcquire and muxRelease 258 // 3. low-level spinAcquire and spinRelease 259 // 4. native Mutex:: and Monitor:: 260 // 5. jvm_raw_lock() and _unlock() 261 // 6. JVMTI raw monitors -- distinct from (5) despite having a confusingly 262 // similar name. 263 // 264 // o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o-o 265 266 267 // CASPTR() uses the canonical argument order that dominates in the literature. 268 // Our internal cmpxchg_ptr() uses a bastardized ordering to accommodate Sun .il templates. 269 270 #define CASPTR(a,c,s) intptr_t(Atomic::cmpxchg_ptr ((void *)(s),(void *)(a),(void *)(c))) 271 #define UNS(x) (uintptr_t(x)) 272 #define TRACE(m) { static volatile int ctr = 0 ; int x = ++ctr ; if ((x & (x-1))==0) { ::printf ("%d:%s\n", x, #m); ::fflush(stdout); }} 273 274 // Simplistic low-quality Marsaglia SHIFT-XOR RNG. 275 // Bijective except for the trailing mask operation. 276 // Useful for spin loops as the compiler can't optimize it away. 277 278 static inline jint MarsagliaXORV (jint x) { 279 if (x == 0) x = 1|os::random() ; 280 x ^= x << 6; 281 x ^= ((unsigned)x) >> 21; 282 x ^= x << 7 ; 283 return x & 0x7FFFFFFF ; 284 } 285 286 static inline jint MarsagliaXOR (jint * const a) { 287 jint x = *a ; 288 if (x == 0) x = UNS(a)|1 ; 289 x ^= x << 6; 290 x ^= ((unsigned)x) >> 21; 291 x ^= x << 7 ; 292 *a = x ; 293 return x & 0x7FFFFFFF ; 294 } 295 296 static int Stall (int its) { 297 static volatile jint rv = 1 ; 298 volatile int OnFrame = 0 ; 299 jint v = rv ^ UNS(OnFrame) ; 300 while (--its >= 0) { 301 v = MarsagliaXORV (v) ; 302 } 303 // Make this impossible for the compiler to optimize away, 304 // but (mostly) avoid W coherency sharing on MP systems. 305 if (v == 0x12345) rv = v ; 306 return v ; 307 } 308 309 int Monitor::TryLock () { 310 intptr_t v = _LockWord.FullWord ; 311 for (;;) { 312 if ((v & _LBIT) != 0) return 0 ; 313 const intptr_t u = CASPTR (&_LockWord, v, v|_LBIT) ; 314 if (v == u) return 1 ; 315 v = u ; 316 } 317 } 318 319 int Monitor::TryFast () { 320 // Optimistic fast-path form ... 321 // Fast-path attempt for the common uncontended case. 322 // Avoid RTS->RTO $ coherence upgrade on typical SMP systems. 323 intptr_t v = CASPTR (&_LockWord, 0, _LBIT) ; // agro ... 324 if (v == 0) return 1 ; 325 326 for (;;) { 327 if ((v & _LBIT) != 0) return 0 ; 328 const intptr_t u = CASPTR (&_LockWord, v, v|_LBIT) ; 329 if (v == u) return 1 ; 330 v = u ; 331 } 332 } 333 334 int Monitor::ILocked () { 335 const intptr_t w = _LockWord.FullWord & 0xFF ; 336 assert (w == 0 || w == _LBIT, "invariant") ; 337 return w == _LBIT ; 338 } 339 340 // Polite TATAS spinlock with exponential backoff - bounded spin. 341 // Ideally we'd use processor cycles, time or vtime to control 342 // the loop, but we currently use iterations. 343 // All the constants within were derived empirically but work over 344 // over the spectrum of J2SE reference platforms. 345 // On Niagara-class systems the back-off is unnecessary but 346 // is relatively harmless. (At worst it'll slightly retard 347 // acquisition times). The back-off is critical for older SMP systems 348 // where constant fetching of the LockWord would otherwise impair 349 // scalability. 350 // 351 // Clamp spinning at approximately 1/2 of a context-switch round-trip. 352 // See synchronizer.cpp for details and rationale. 353 354 int Monitor::TrySpin (Thread * const Self) { 355 if (TryLock()) return 1 ; 356 if (!os::is_MP()) return 0 ; 357 358 int Probes = 0 ; 359 int Delay = 0 ; 360 int Steps = 0 ; 361 int SpinMax = NativeMonitorSpinLimit ; 362 int flgs = NativeMonitorFlags ; 363 for (;;) { 364 intptr_t v = _LockWord.FullWord; 365 if ((v & _LBIT) == 0) { 366 if (CASPTR (&_LockWord, v, v|_LBIT) == v) { 367 return 1 ; 368 } 369 continue ; 370 } 371 372 if ((flgs & 8) == 0) { 373 SpinPause () ; 374 } 375 376 // Periodically increase Delay -- variable Delay form 377 // conceptually: delay *= 1 + 1/Exponent 378 ++ Probes; 379 if (Probes > SpinMax) return 0 ; 380 381 if ((Probes & 0x7) == 0) { 382 Delay = ((Delay << 1)|1) & 0x7FF ; 383 // CONSIDER: Delay += 1 + (Delay/4); Delay &= 0x7FF ; 384 } 385 386 if (flgs & 2) continue ; 387 388 // Consider checking _owner's schedctl state, if OFFPROC abort spin. 389 // If the owner is OFFPROC then it's unlike that the lock will be dropped 390 // in a timely fashion, which suggests that spinning would not be fruitful 391 // or profitable. 392 393 // Stall for "Delay" time units - iterations in the current implementation. 394 // Avoid generating coherency traffic while stalled. 395 // Possible ways to delay: 396 // PAUSE, SLEEP, MEMBAR #sync, MEMBAR #halt, 397 // wr %g0,%asi, gethrtime, rdstick, rdtick, rdtsc, etc. ... 398 // Note that on Niagara-class systems we want to minimize STs in the 399 // spin loop. N1 and brethren write-around the L1$ over the xbar into the L2$. 400 // Furthermore, they don't have a W$ like traditional SPARC processors. 401 // We currently use a Marsaglia Shift-Xor RNG loop. 402 Steps += Delay ; 403 if (Self != NULL) { 404 jint rv = Self->rng[0] ; 405 for (int k = Delay ; --k >= 0; ) { 406 rv = MarsagliaXORV (rv) ; 407 if ((flgs & 4) == 0 && SafepointSynchronize::do_call_back()) return 0 ; 408 } 409 Self->rng[0] = rv ; 410 } else { 411 Stall (Delay) ; 412 } 413 } 414 } 415 416 static int ParkCommon (ParkEvent * ev, jlong timo) { 417 // Diagnostic support - periodically unwedge blocked threads 418 intx nmt = NativeMonitorTimeout ; 419 if (nmt > 0 && (nmt < timo || timo <= 0)) { 420 timo = nmt ; 421 } 422 int err = OS_OK ; 423 if (0 == timo) { 424 ev->park() ; 425 } else { 426 err = ev->park(timo) ; 427 } 428 return err ; 429 } 430 431 inline int Monitor::AcquireOrPush (ParkEvent * ESelf) { 432 intptr_t v = _LockWord.FullWord ; 433 for (;;) { 434 if ((v & _LBIT) == 0) { 435 const intptr_t u = CASPTR (&_LockWord, v, v|_LBIT) ; 436 if (u == v) return 1 ; // indicate acquired 437 v = u ; 438 } else { 439 // Anticipate success ... 440 ESelf->ListNext = (ParkEvent *) (v & ~_LBIT) ; 441 const intptr_t u = CASPTR (&_LockWord, v, intptr_t(ESelf)|_LBIT) ; 442 if (u == v) return 0 ; // indicate pushed onto cxq 443 v = u ; 444 } 445 // Interference - LockWord change - just retry 446 } 447 } 448 449 // ILock and IWait are the lowest level primitive internal blocking 450 // synchronization functions. The callers of IWait and ILock must have 451 // performed any needed state transitions beforehand. 452 // IWait and ILock may directly call park() without any concern for thread state. 453 // Note that ILock and IWait do *not* access _owner. 454 // _owner is a higher-level logical concept. 455 456 void Monitor::ILock (Thread * Self) { 457 assert (_OnDeck != Self->_MutexEvent, "invariant") ; 458 459 if (TryFast()) { 460 Exeunt: 461 assert (ILocked(), "invariant") ; 462 return ; 463 } 464 465 ParkEvent * const ESelf = Self->_MutexEvent ; 466 assert (_OnDeck != ESelf, "invariant") ; 467 468 // As an optimization, spinners could conditionally try to set ONDECK to _LBIT 469 // Synchronizer.cpp uses a similar optimization. 470 if (TrySpin (Self)) goto Exeunt ; 471 472 // Slow-path - the lock is contended. 473 // Either Enqueue Self on cxq or acquire the outer lock. 474 // LockWord encoding = (cxq,LOCKBYTE) 475 ESelf->reset() ; 476 OrderAccess::fence() ; 477 478 // Optional optimization ... try barging on the inner lock 479 if ((NativeMonitorFlags & 32) && CASPTR (&_OnDeck, NULL, UNS(Self)) == 0) { 480 goto OnDeck_LOOP ; 481 } 482 483 if (AcquireOrPush (ESelf)) goto Exeunt ; 484 485 // At any given time there is at most one ondeck thread. 486 // ondeck implies not resident on cxq and not resident on EntryList 487 // Only the OnDeck thread can try to acquire -- contended for -- the lock. 488 // CONSIDER: use Self->OnDeck instead of m->OnDeck. 489 // Deschedule Self so that others may run. 490 while (_OnDeck != ESelf) { 491 ParkCommon (ESelf, 0) ; 492 } 493 494 // Self is now in the ONDECK position and will remain so until it 495 // manages to acquire the lock. 496 OnDeck_LOOP: 497 for (;;) { 498 assert (_OnDeck == ESelf, "invariant") ; 499 if (TrySpin (Self)) break ; 500 // CONSIDER: if ESelf->TryPark() && TryLock() break ... 501 // It's probably wise to spin only if we *actually* blocked 502 // CONSIDER: check the lockbyte, if it remains set then 503 // preemptively drain the cxq into the EntryList. 504 // The best place and time to perform queue operations -- lock metadata -- 505 // is _before having acquired the outer lock, while waiting for the lock to drop. 506 ParkCommon (ESelf, 0) ; 507 } 508 509 assert (_OnDeck == ESelf, "invariant") ; 510 _OnDeck = NULL ; 511 512 // Note that we current drop the inner lock (clear OnDeck) in the slow-path 513 // epilog immediately after having acquired the outer lock. 514 // But instead we could consider the following optimizations: 515 // A. Shift or defer dropping the inner lock until the subsequent IUnlock() operation. 516 // This might avoid potential reacquisition of the inner lock in IUlock(). 517 // B. While still holding the inner lock, attempt to opportunistically select 518 // and unlink the next ONDECK thread from the EntryList. 519 // If successful, set ONDECK to refer to that thread, otherwise clear ONDECK. 520 // It's critical that the select-and-unlink operation run in constant-time as 521 // it executes when holding the outer lock and may artificially increase the 522 // effective length of the critical section. 523 // Note that (A) and (B) are tantamount to succession by direct handoff for 524 // the inner lock. 525 goto Exeunt ; 526 } 527 528 void Monitor::IUnlock (bool RelaxAssert) { 529 assert (ILocked(), "invariant") ; 530 // Conceptually we need a MEMBAR #storestore|#loadstore barrier or fence immediately 531 // before the store that releases the lock. Crucially, all the stores and loads in the 532 // critical section must be globally visible before the store of 0 into the lock-word 533 // that releases the lock becomes globally visible. That is, memory accesses in the 534 // critical section should not be allowed to bypass or overtake the following ST that 535 // releases the lock. As such, to prevent accesses within the critical section 536 // from "leaking" out, we need a release fence between the critical section and the 537 // store that releases the lock. In practice that release barrier is elided on 538 // platforms with strong memory models such as TSO. 539 // 540 // Note that the OrderAccess::storeload() fence that appears after unlock store 541 // provides for progress conditions and succession and is _not related to exclusion 542 // safety or lock release consistency. 543 OrderAccess::release_store(&_LockWord.Bytes[_LSBINDEX], 0); // drop outer lock 544 545 OrderAccess::storeload (); 546 ParkEvent * const w = _OnDeck ; 547 assert (RelaxAssert || w != Thread::current()->_MutexEvent, "invariant") ; 548 if (w != NULL) { 549 // Either we have a valid ondeck thread or ondeck is transiently "locked" 550 // by some exiting thread as it arranges for succession. The LSBit of 551 // OnDeck allows us to discriminate two cases. If the latter, the 552 // responsibility for progress and succession lies with that other thread. 553 // For good performance, we also depend on the fact that redundant unpark() 554 // operations are cheap. That is, repeated Unpark()ing of the ONDECK thread 555 // is inexpensive. This approach provides implicit futile wakeup throttling. 556 // Note that the referent "w" might be stale with respect to the lock. 557 // In that case the following unpark() is harmless and the worst that'll happen 558 // is a spurious return from a park() operation. Critically, if "w" _is stale, 559 // then progress is known to have occurred as that means the thread associated 560 // with "w" acquired the lock. In that case this thread need take no further 561 // action to guarantee progress. 562 if ((UNS(w) & _LBIT) == 0) w->unpark() ; 563 return ; 564 } 565 566 intptr_t cxq = _LockWord.FullWord ; 567 if (((cxq & ~_LBIT)|UNS(_EntryList)) == 0) { 568 return ; // normal fast-path exit - cxq and EntryList both empty 569 } 570 if (cxq & _LBIT) { 571 // Optional optimization ... 572 // Some other thread acquired the lock in the window since this 573 // thread released it. Succession is now that thread's responsibility. 574 return ; 575 } 576 577 Succession: 578 // Slow-path exit - this thread must ensure succession and progress. 579 // OnDeck serves as lock to protect cxq and EntryList. 580 // Only the holder of OnDeck can manipulate EntryList or detach the RATs from cxq. 581 // Avoid ABA - allow multiple concurrent producers (enqueue via push-CAS) 582 // but only one concurrent consumer (detacher of RATs). 583 // Consider protecting this critical section with schedctl on Solaris. 584 // Unlike a normal lock, however, the exiting thread "locks" OnDeck, 585 // picks a successor and marks that thread as OnDeck. That successor 586 // thread will then clear OnDeck once it eventually acquires the outer lock. 587 if (CASPTR (&_OnDeck, NULL, _LBIT) != UNS(NULL)) { 588 return ; 589 } 590 591 ParkEvent * List = _EntryList ; 592 if (List != NULL) { 593 // Transfer the head of the EntryList to the OnDeck position. 594 // Once OnDeck, a thread stays OnDeck until it acquires the lock. 595 // For a given lock there is at most OnDeck thread at any one instant. 596 WakeOne: 597 assert (List == _EntryList, "invariant") ; 598 ParkEvent * const w = List ; 599 assert (RelaxAssert || w != Thread::current()->_MutexEvent, "invariant") ; 600 _EntryList = w->ListNext ; 601 // as a diagnostic measure consider setting w->_ListNext = BAD 602 assert (UNS(_OnDeck) == _LBIT, "invariant") ; 603 _OnDeck = w ; // pass OnDeck to w. 604 // w will clear OnDeck once it acquires the outer lock 605 606 // Another optional optimization ... 607 // For heavily contended locks it's not uncommon that some other 608 // thread acquired the lock while this thread was arranging succession. 609 // Try to defer the unpark() operation - Delegate the responsibility 610 // for unpark()ing the OnDeck thread to the current or subsequent owners 611 // That is, the new owner is responsible for unparking the OnDeck thread. 612 OrderAccess::storeload() ; 613 cxq = _LockWord.FullWord ; 614 if (cxq & _LBIT) return ; 615 616 w->unpark() ; 617 return ; 618 } 619 620 cxq = _LockWord.FullWord ; 621 if ((cxq & ~_LBIT) != 0) { 622 // The EntryList is empty but the cxq is populated. 623 // drain RATs from cxq into EntryList 624 // Detach RATs segment with CAS and then merge into EntryList 625 for (;;) { 626 // optional optimization - if locked, the owner is responsible for succession 627 if (cxq & _LBIT) goto Punt ; 628 const intptr_t vfy = CASPTR (&_LockWord, cxq, cxq & _LBIT) ; 629 if (vfy == cxq) break ; 630 cxq = vfy ; 631 // Interference - LockWord changed - Just retry 632 // We can see concurrent interference from contending threads 633 // pushing themselves onto the cxq or from lock-unlock operations. 634 // From the perspective of this thread, EntryList is stable and 635 // the cxq is prepend-only -- the head is volatile but the interior 636 // of the cxq is stable. In theory if we encounter interference from threads 637 // pushing onto cxq we could simply break off the original cxq suffix and 638 // move that segment to the EntryList, avoiding a 2nd or multiple CAS attempts 639 // on the high-traffic LockWord variable. For instance lets say the cxq is "ABCD" 640 // when we first fetch cxq above. Between the fetch -- where we observed "A" 641 // -- and CAS -- where we attempt to CAS null over A -- "PQR" arrive, 642 // yielding cxq = "PQRABCD". In this case we could simply set A.ListNext 643 // null, leaving cxq = "PQRA" and transfer the "BCD" segment to the EntryList. 644 // Note too, that it's safe for this thread to traverse the cxq 645 // without taking any special concurrency precautions. 646 } 647 648 // We don't currently reorder the cxq segment as we move it onto 649 // the EntryList, but it might make sense to reverse the order 650 // or perhaps sort by thread priority. See the comments in 651 // synchronizer.cpp objectMonitor::exit(). 652 assert (_EntryList == NULL, "invariant") ; 653 _EntryList = List = (ParkEvent *)(cxq & ~_LBIT) ; 654 assert (List != NULL, "invariant") ; 655 goto WakeOne ; 656 } 657 658 // cxq|EntryList is empty. 659 // w == NULL implies that cxq|EntryList == NULL in the past. 660 // Possible race - rare inopportune interleaving. 661 // A thread could have added itself to cxq since this thread previously checked. 662 // Detect and recover by refetching cxq. 663 Punt: 664 assert (UNS(_OnDeck) == _LBIT, "invariant") ; 665 _OnDeck = NULL ; // Release inner lock. 666 OrderAccess::storeload(); // Dekker duality - pivot point 667 668 // Resample LockWord/cxq to recover from possible race. 669 // For instance, while this thread T1 held OnDeck, some other thread T2 might 670 // acquire the outer lock. Another thread T3 might try to acquire the outer 671 // lock, but encounter contention and enqueue itself on cxq. T2 then drops the 672 // outer lock, but skips succession as this thread T1 still holds OnDeck. 673 // T1 is and remains responsible for ensuring succession of T3. 674 // 675 // Note that we don't need to recheck EntryList, just cxq. 676 // If threads moved onto EntryList since we dropped OnDeck 677 // that implies some other thread forced succession. 678 cxq = _LockWord.FullWord ; 679 if ((cxq & ~_LBIT) != 0 && (cxq & _LBIT) == 0) { 680 goto Succession ; // potential race -- re-run succession 681 } 682 return ; 683 } 684 685 bool Monitor::notify() { 686 assert (_owner == Thread::current(), "invariant") ; 687 assert (ILocked(), "invariant") ; 688 if (_WaitSet == NULL) return true ; 689 NotifyCount ++ ; 690 691 // Transfer one thread from the WaitSet to the EntryList or cxq. 692 // Currently we just unlink the head of the WaitSet and prepend to the cxq. 693 // And of course we could just unlink it and unpark it, too, but 694 // in that case it'd likely impale itself on the reentry. 695 Thread::muxAcquire (_WaitLock, "notify:WaitLock") ; 696 ParkEvent * nfy = _WaitSet ; 697 if (nfy != NULL) { // DCL idiom 698 _WaitSet = nfy->ListNext ; 699 assert (nfy->Notified == 0, "invariant") ; 700 // push nfy onto the cxq 701 for (;;) { 702 const intptr_t v = _LockWord.FullWord ; 703 assert ((v & 0xFF) == _LBIT, "invariant") ; 704 nfy->ListNext = (ParkEvent *)(v & ~_LBIT); 705 if (CASPTR (&_LockWord, v, UNS(nfy)|_LBIT) == v) break; 706 // interference - _LockWord changed -- just retry 707 } 708 // Note that setting Notified before pushing nfy onto the cxq is 709 // also legal and safe, but the safety properties are much more 710 // subtle, so for the sake of code stewardship ... 711 OrderAccess::fence() ; 712 nfy->Notified = 1; 713 } 714 Thread::muxRelease (_WaitLock) ; 715 if (nfy != NULL && (NativeMonitorFlags & 16)) { 716 // Experimental code ... light up the wakee in the hope that this thread (the owner) 717 // will drop the lock just about the time the wakee comes ONPROC. 718 nfy->unpark() ; 719 } 720 assert (ILocked(), "invariant") ; 721 return true ; 722 } 723 724 // Currently notifyAll() transfers the waiters one-at-a-time from the waitset 725 // to the cxq. This could be done more efficiently with a single bulk en-mass transfer, 726 // but in practice notifyAll() for large #s of threads is rare and not time-critical. 727 // Beware too, that we invert the order of the waiters. Lets say that the 728 // waitset is "ABCD" and the cxq is "XYZ". After a notifyAll() the waitset 729 // will be empty and the cxq will be "DCBAXYZ". This is benign, of course. 730 731 bool Monitor::notify_all() { 732 assert (_owner == Thread::current(), "invariant") ; 733 assert (ILocked(), "invariant") ; 734 while (_WaitSet != NULL) notify() ; 735 return true ; 736 } 737 738 int Monitor::IWait (Thread * Self, jlong timo) { 739 assert (ILocked(), "invariant") ; 740 741 // Phases: 742 // 1. Enqueue Self on WaitSet - currently prepend 743 // 2. unlock - drop the outer lock 744 // 3. wait for either notification or timeout 745 // 4. lock - reentry - reacquire the outer lock 746 747 ParkEvent * const ESelf = Self->_MutexEvent ; 748 ESelf->Notified = 0 ; 749 ESelf->reset() ; 750 OrderAccess::fence() ; 751 752 // Add Self to WaitSet 753 // Ideally only the holder of the outer lock would manipulate the WaitSet - 754 // That is, the outer lock would implicitly protect the WaitSet. 755 // But if a thread in wait() encounters a timeout it will need to dequeue itself 756 // from the WaitSet _before it becomes the owner of the lock. We need to dequeue 757 // as the ParkEvent -- which serves as a proxy for the thread -- can't reside 758 // on both the WaitSet and the EntryList|cxq at the same time.. That is, a thread 759 // on the WaitSet can't be allowed to compete for the lock until it has managed to 760 // unlink its ParkEvent from WaitSet. Thus the need for WaitLock. 761 // Contention on the WaitLock is minimal. 762 // 763 // Another viable approach would be add another ParkEvent, "WaitEvent" to the 764 // thread class. The WaitSet would be composed of WaitEvents. Only the 765 // owner of the outer lock would manipulate the WaitSet. A thread in wait() 766 // could then compete for the outer lock, and then, if necessary, unlink itself 767 // from the WaitSet only after having acquired the outer lock. More precisely, 768 // there would be no WaitLock. A thread in in wait() would enqueue its WaitEvent 769 // on the WaitSet; release the outer lock; wait for either notification or timeout; 770 // reacquire the inner lock; and then, if needed, unlink itself from the WaitSet. 771 // 772 // Alternatively, a 2nd set of list link fields in the ParkEvent might suffice. 773 // One set would be for the WaitSet and one for the EntryList. 774 // We could also deconstruct the ParkEvent into a "pure" event and add a 775 // new immortal/TSM "ListElement" class that referred to ParkEvents. 776 // In that case we could have one ListElement on the WaitSet and another 777 // on the EntryList, with both referring to the same pure Event. 778 779 Thread::muxAcquire (_WaitLock, "wait:WaitLock:Add") ; 780 ESelf->ListNext = _WaitSet ; 781 _WaitSet = ESelf ; 782 Thread::muxRelease (_WaitLock) ; 783 784 // Release the outer lock 785 // We call IUnlock (RelaxAssert=true) as a thread T1 might 786 // enqueue itself on the WaitSet, call IUnlock(), drop the lock, 787 // and then stall before it can attempt to wake a successor. 788 // Some other thread T2 acquires the lock, and calls notify(), moving 789 // T1 from the WaitSet to the cxq. T2 then drops the lock. T1 resumes, 790 // and then finds *itself* on the cxq. During the course of a normal 791 // IUnlock() call a thread should _never find itself on the EntryList 792 // or cxq, but in the case of wait() it's possible. 793 // See synchronizer.cpp objectMonitor::wait(). 794 IUnlock (true) ; 795 796 // Wait for either notification or timeout 797 // Beware that in some circumstances we might propagate 798 // spurious wakeups back to the caller. 799 800 for (;;) { 801 if (ESelf->Notified) break ; 802 int err = ParkCommon (ESelf, timo) ; 803 if (err == OS_TIMEOUT || (NativeMonitorFlags & 1)) break ; 804 } 805 806 // Prepare for reentry - if necessary, remove ESelf from WaitSet 807 // ESelf can be: 808 // 1. Still on the WaitSet. This can happen if we exited the loop by timeout. 809 // 2. On the cxq or EntryList 810 // 3. Not resident on cxq, EntryList or WaitSet, but in the OnDeck position. 811 812 OrderAccess::fence() ; 813 int WasOnWaitSet = 0 ; 814 if (ESelf->Notified == 0) { 815 Thread::muxAcquire (_WaitLock, "wait:WaitLock:remove") ; 816 if (ESelf->Notified == 0) { // DCL idiom 817 assert (_OnDeck != ESelf, "invariant") ; // can't be both OnDeck and on WaitSet 818 // ESelf is resident on the WaitSet -- unlink it. 819 // A doubly-linked list would be better here so we can unlink in constant-time. 820 // We have to unlink before we potentially recontend as ESelf might otherwise 821 // end up on the cxq|EntryList -- it can't be on two lists at once. 822 ParkEvent * p = _WaitSet ; 823 ParkEvent * q = NULL ; // classic q chases p 824 while (p != NULL && p != ESelf) { 825 q = p ; 826 p = p->ListNext ; 827 } 828 assert (p == ESelf, "invariant") ; 829 if (p == _WaitSet) { // found at head 830 assert (q == NULL, "invariant") ; 831 _WaitSet = p->ListNext ; 832 } else { // found in interior 833 assert (q->ListNext == p, "invariant") ; 834 q->ListNext = p->ListNext ; 835 } 836 WasOnWaitSet = 1 ; // We were *not* notified but instead encountered timeout 837 } 838 Thread::muxRelease (_WaitLock) ; 839 } 840 841 // Reentry phase - reacquire the lock 842 if (WasOnWaitSet) { 843 // ESelf was previously on the WaitSet but we just unlinked it above 844 // because of a timeout. ESelf is not resident on any list and is not OnDeck 845 assert (_OnDeck != ESelf, "invariant") ; 846 ILock (Self) ; 847 } else { 848 // A prior notify() operation moved ESelf from the WaitSet to the cxq. 849 // ESelf is now on the cxq, EntryList or at the OnDeck position. 850 // The following fragment is extracted from Monitor::ILock() 851 for (;;) { 852 if (_OnDeck == ESelf && TrySpin(Self)) break ; 853 ParkCommon (ESelf, 0) ; 854 } 855 assert (_OnDeck == ESelf, "invariant") ; 856 _OnDeck = NULL ; 857 } 858 859 assert (ILocked(), "invariant") ; 860 return WasOnWaitSet != 0 ; // return true IFF timeout 861 } 862 863 864 // ON THE VMTHREAD SNEAKING PAST HELD LOCKS: 865 // In particular, there are certain types of global lock that may be held 866 // by a Java thread while it is blocked at a safepoint but before it has 867 // written the _owner field. These locks may be sneakily acquired by the 868 // VM thread during a safepoint to avoid deadlocks. Alternatively, one should 869 // identify all such locks, and ensure that Java threads never block at 870 // safepoints while holding them (_no_safepoint_check_flag). While it 871 // seems as though this could increase the time to reach a safepoint 872 // (or at least increase the mean, if not the variance), the latter 873 // approach might make for a cleaner, more maintainable JVM design. 874 // 875 // Sneaking is vile and reprehensible and should be excised at the 1st 876 // opportunity. It's possible that the need for sneaking could be obviated 877 // as follows. Currently, a thread might (a) while TBIVM, call pthread_mutex_lock 878 // or ILock() thus acquiring the "physical" lock underlying Monitor/Mutex. 879 // (b) stall at the TBIVM exit point as a safepoint is in effect. Critically, 880 // it'll stall at the TBIVM reentry state transition after having acquired the 881 // underlying lock, but before having set _owner and having entered the actual 882 // critical section. The lock-sneaking facility leverages that fact and allowed the 883 // VM thread to logically acquire locks that had already be physically locked by mutators 884 // but where mutators were known blocked by the reentry thread state transition. 885 // 886 // If we were to modify the Monitor-Mutex so that TBIVM state transitions tightly 887 // wrapped calls to park(), then we could likely do away with sneaking. We'd 888 // decouple lock acquisition and parking. The critical invariant to eliminating 889 // sneaking is to ensure that we never "physically" acquire the lock while TBIVM. 890 // An easy way to accomplish this is to wrap the park calls in a narrow TBIVM jacket. 891 // One difficulty with this approach is that the TBIVM wrapper could recurse and 892 // call lock() deep from within a lock() call, while the MutexEvent was already enqueued. 893 // Using a stack (N=2 at minimum) of ParkEvents would take care of that problem. 894 // 895 // But of course the proper ultimate approach is to avoid schemes that require explicit 896 // sneaking or dependence on any any clever invariants or subtle implementation properties 897 // of Mutex-Monitor and instead directly address the underlying design flaw. 898 899 void Monitor::lock (Thread * Self) { 900 #ifdef CHECK_UNHANDLED_OOPS 901 // Clear unhandled oops so we get a crash right away. Only clear for non-vm 902 // or GC threads. 903 if (Self->is_Java_thread()) { 904 Self->clear_unhandled_oops(); 905 } 906 #endif // CHECK_UNHANDLED_OOPS 907 908 debug_only(check_prelock_state(Self)); 909 assert (_owner != Self , "invariant") ; 910 assert (_OnDeck != Self->_MutexEvent, "invariant") ; 911 912 if (TryFast()) { 913 Exeunt: 914 assert (ILocked(), "invariant") ; 915 assert (owner() == NULL, "invariant"); 916 set_owner (Self); 917 return ; 918 } 919 920 // The lock is contended ... 921 922 bool can_sneak = Self->is_VM_thread() && SafepointSynchronize::is_at_safepoint(); 923 if (can_sneak && _owner == NULL) { 924 // a java thread has locked the lock but has not entered the 925 // critical region -- let's just pretend we've locked the lock 926 // and go on. we note this with _snuck so we can also 927 // pretend to unlock when the time comes. 928 _snuck = true; 929 goto Exeunt ; 930 } 931 932 // Try a brief spin to avoid passing thru thread state transition ... 933 if (TrySpin (Self)) goto Exeunt ; 934 935 check_block_state(Self); 936 if (Self->is_Java_thread()) { 937 // Horribile dictu - we suffer through a state transition 938 assert(rank() > Mutex::special, "Potential deadlock with special or lesser rank mutex"); 939 ThreadBlockInVM tbivm ((JavaThread *) Self) ; 940 ILock (Self) ; 941 } else { 942 // Mirabile dictu 943 ILock (Self) ; 944 } 945 goto Exeunt ; 946 } 947 948 void Monitor::lock() { 949 this->lock(Thread::current()); 950 } 951 952 // Lock without safepoint check - a degenerate variant of lock(). 953 // Should ONLY be used by safepoint code and other code 954 // that is guaranteed not to block while running inside the VM. If this is called with 955 // thread state set to be in VM, the safepoint synchronization code will deadlock! 956 957 void Monitor::lock_without_safepoint_check (Thread * Self) { 958 assert (_owner != Self, "invariant") ; 959 ILock (Self) ; 960 assert (_owner == NULL, "invariant"); 961 set_owner (Self); 962 } 963 964 void Monitor::lock_without_safepoint_check () { 965 lock_without_safepoint_check (Thread::current()) ; 966 } 967 968 969 // Returns true if thread succeceed [sic] in grabbing the lock, otherwise false. 970 971 bool Monitor::try_lock() { 972 Thread * const Self = Thread::current(); 973 debug_only(check_prelock_state(Self)); 974 // assert(!thread->is_inside_signal_handler(), "don't lock inside signal handler"); 975 976 // Special case, where all Java threads are stopped. 977 // The lock may have been acquired but _owner is not yet set. 978 // In that case the VM thread can safely grab the lock. 979 // It strikes me this should appear _after the TryLock() fails, below. 980 bool can_sneak = Self->is_VM_thread() && SafepointSynchronize::is_at_safepoint(); 981 if (can_sneak && _owner == NULL) { 982 set_owner(Self); // Do not need to be atomic, since we are at a safepoint 983 _snuck = true; 984 return true; 985 } 986 987 if (TryLock()) { 988 // We got the lock 989 assert (_owner == NULL, "invariant"); 990 set_owner (Self); 991 return true; 992 } 993 return false; 994 } 995 996 void Monitor::unlock() { 997 assert (_owner == Thread::current(), "invariant") ; 998 assert (_OnDeck != Thread::current()->_MutexEvent , "invariant") ; 999 set_owner (NULL) ; 1000 if (_snuck) { 1001 assert(SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread(), "sneak"); 1002 _snuck = false; 1003 return ; 1004 } 1005 IUnlock (false) ; 1006 } 1007 1008 // Yet another degenerate version of Monitor::lock() or lock_without_safepoint_check() 1009 // jvm_raw_lock() and _unlock() can be called by non-Java threads via JVM_RawMonitorEnter. 1010 // 1011 // There's no expectation that JVM_RawMonitors will interoperate properly with the native 1012 // Mutex-Monitor constructs. We happen to implement JVM_RawMonitors in terms of 1013 // native Mutex-Monitors simply as a matter of convenience. A simple abstraction layer 1014 // over a pthread_mutex_t would work equally as well, but require more platform-specific 1015 // code -- a "PlatformMutex". Alternatively, a simply layer over muxAcquire-muxRelease 1016 // would work too. 1017 // 1018 // Since the caller might be a foreign thread, we don't necessarily have a Thread.MutexEvent 1019 // instance available. Instead, we transiently allocate a ParkEvent on-demand if 1020 // we encounter contention. That ParkEvent remains associated with the thread 1021 // until it manages to acquire the lock, at which time we return the ParkEvent 1022 // to the global ParkEvent free list. This is correct and suffices for our purposes. 1023 // 1024 // Beware that the original jvm_raw_unlock() had a "_snuck" test but that 1025 // jvm_raw_lock() didn't have the corresponding test. I suspect that's an 1026 // oversight, but I've replicated the original suspect logic in the new code ... 1027 1028 void Monitor::jvm_raw_lock() { 1029 assert(rank() == native, "invariant"); 1030 1031 if (TryLock()) { 1032 Exeunt: 1033 assert (ILocked(), "invariant") ; 1034 assert (_owner == NULL, "invariant"); 1035 // This can potentially be called by non-java Threads. Thus, the ThreadLocalStorage 1036 // might return NULL. Don't call set_owner since it will break on an NULL owner 1037 // Consider installing a non-null "ANON" distinguished value instead of just NULL. 1038 _owner = ThreadLocalStorage::thread(); 1039 return ; 1040 } 1041 1042 if (TrySpin(NULL)) goto Exeunt ; 1043 1044 // slow-path - apparent contention 1045 // Allocate a ParkEvent for transient use. 1046 // The ParkEvent remains associated with this thread until 1047 // the time the thread manages to acquire the lock. 1048 ParkEvent * const ESelf = ParkEvent::Allocate(NULL) ; 1049 ESelf->reset() ; 1050 OrderAccess::storeload() ; 1051 1052 // Either Enqueue Self on cxq or acquire the outer lock. 1053 if (AcquireOrPush (ESelf)) { 1054 ParkEvent::Release (ESelf) ; // surrender the ParkEvent 1055 goto Exeunt ; 1056 } 1057 1058 // At any given time there is at most one ondeck thread. 1059 // ondeck implies not resident on cxq and not resident on EntryList 1060 // Only the OnDeck thread can try to acquire -- contended for -- the lock. 1061 // CONSIDER: use Self->OnDeck instead of m->OnDeck. 1062 for (;;) { 1063 if (_OnDeck == ESelf && TrySpin(NULL)) break ; 1064 ParkCommon (ESelf, 0) ; 1065 } 1066 1067 assert (_OnDeck == ESelf, "invariant") ; 1068 _OnDeck = NULL ; 1069 ParkEvent::Release (ESelf) ; // surrender the ParkEvent 1070 goto Exeunt ; 1071 } 1072 1073 void Monitor::jvm_raw_unlock() { 1074 // Nearly the same as Monitor::unlock() ... 1075 // directly set _owner instead of using set_owner(null) 1076 _owner = NULL ; 1077 if (_snuck) { // ??? 1078 assert(SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread(), "sneak"); 1079 _snuck = false; 1080 return ; 1081 } 1082 IUnlock(false) ; 1083 } 1084 1085 bool Monitor::wait(bool no_safepoint_check, long timeout, bool as_suspend_equivalent) { 1086 Thread * const Self = Thread::current() ; 1087 assert (_owner == Self, "invariant") ; 1088 assert (ILocked(), "invariant") ; 1089 1090 // as_suspend_equivalent logically implies !no_safepoint_check 1091 guarantee (!as_suspend_equivalent || !no_safepoint_check, "invariant") ; 1092 // !no_safepoint_check logically implies java_thread 1093 guarantee (no_safepoint_check || Self->is_Java_thread(), "invariant") ; 1094 1095 #ifdef ASSERT 1096 Monitor * least = get_least_ranked_lock_besides_this(Self->owned_locks()); 1097 assert(least != this, "Specification of get_least_... call above"); 1098 if (least != NULL && least->rank() <= special) { 1099 tty->print("Attempting to wait on monitor %s/%d while holding" 1100 " lock %s/%d -- possible deadlock", 1101 name(), rank(), least->name(), least->rank()); 1102 assert(false, "Shouldn't block(wait) while holding a lock of rank special"); 1103 } 1104 #endif // ASSERT 1105 1106 int wait_status ; 1107 // conceptually set the owner to NULL in anticipation of 1108 // abdicating the lock in wait 1109 set_owner(NULL); 1110 if (no_safepoint_check) { 1111 wait_status = IWait (Self, timeout) ; 1112 } else { 1113 assert (Self->is_Java_thread(), "invariant") ; 1114 JavaThread *jt = (JavaThread *)Self; 1115 1116 // Enter safepoint region - ornate and Rococo ... 1117 ThreadBlockInVM tbivm(jt); 1118 OSThreadWaitState osts(Self->osthread(), false /* not Object.wait() */); 1119 1120 if (as_suspend_equivalent) { 1121 jt->set_suspend_equivalent(); 1122 // cleared by handle_special_suspend_equivalent_condition() or 1123 // java_suspend_self() 1124 } 1125 1126 wait_status = IWait (Self, timeout) ; 1127 1128 // were we externally suspended while we were waiting? 1129 if (as_suspend_equivalent && jt->handle_special_suspend_equivalent_condition()) { 1130 // Our event wait has finished and we own the lock, but 1131 // while we were waiting another thread suspended us. We don't 1132 // want to hold the lock while suspended because that 1133 // would surprise the thread that suspended us. 1134 assert (ILocked(), "invariant") ; 1135 IUnlock (true) ; 1136 jt->java_suspend_self(); 1137 ILock (Self) ; 1138 assert (ILocked(), "invariant") ; 1139 } 1140 } 1141 1142 // Conceptually reestablish ownership of the lock. 1143 // The "real" lock -- the LockByte -- was reacquired by IWait(). 1144 assert (ILocked(), "invariant") ; 1145 assert (_owner == NULL, "invariant") ; 1146 set_owner (Self) ; 1147 return wait_status != 0 ; // return true IFF timeout 1148 } 1149 1150 Monitor::~Monitor() { 1151 assert ((UNS(_owner)|UNS(_LockWord.FullWord)|UNS(_EntryList)|UNS(_WaitSet)|UNS(_OnDeck)) == 0, "") ; 1152 } 1153 1154 void Monitor::ClearMonitor (Monitor * m, const char *name) { 1155 m->_owner = NULL ; 1156 m->_snuck = false ; 1157 if (name == NULL) { 1158 strcpy(m->_name, "UNKNOWN") ; 1159 } else { 1160 strncpy(m->_name, name, MONITOR_NAME_LEN - 1); 1161 m->_name[MONITOR_NAME_LEN - 1] = '\0'; 1162 } 1163 m->_LockWord.FullWord = 0 ; 1164 m->_EntryList = NULL ; 1165 m->_OnDeck = NULL ; 1166 m->_WaitSet = NULL ; 1167 m->_WaitLock[0] = 0 ; 1168 } 1169 1170 Monitor::Monitor() { ClearMonitor(this); } 1171 1172 Monitor::Monitor (int Rank, const char * name, bool allow_vm_block) { 1173 ClearMonitor (this, name) ; 1174 #ifdef ASSERT 1175 _allow_vm_block = allow_vm_block; 1176 _rank = Rank ; 1177 #endif 1178 } 1179 1180 Mutex::~Mutex() { 1181 assert ((UNS(_owner)|UNS(_LockWord.FullWord)|UNS(_EntryList)|UNS(_WaitSet)|UNS(_OnDeck)) == 0, "") ; 1182 } 1183 1184 Mutex::Mutex (int Rank, const char * name, bool allow_vm_block) { 1185 ClearMonitor ((Monitor *) this, name) ; 1186 #ifdef ASSERT 1187 _allow_vm_block = allow_vm_block; 1188 _rank = Rank ; 1189 #endif 1190 } 1191 1192 bool Monitor::owned_by_self() const { 1193 bool ret = _owner == Thread::current(); 1194 assert (!ret || _LockWord.Bytes[_LSBINDEX] != 0, "invariant") ; 1195 return ret; 1196 } 1197 1198 void Monitor::print_on_error(outputStream* st) const { 1199 st->print("[" PTR_FORMAT, this); 1200 st->print("] %s", _name); 1201 st->print(" - owner thread: " PTR_FORMAT, _owner); 1202 } 1203 1204 1205 1206 1207 // ---------------------------------------------------------------------------------- 1208 // Non-product code 1209 1210 #ifndef PRODUCT 1211 void Monitor::print_on(outputStream* st) const { 1212 st->print_cr("Mutex: [0x%lx/0x%lx] %s - owner: 0x%lx", this, _LockWord.FullWord, _name, _owner); 1213 } 1214 #endif 1215 1216 #ifndef PRODUCT 1217 #ifdef ASSERT 1218 Monitor * Monitor::get_least_ranked_lock(Monitor * locks) { 1219 Monitor *res, *tmp; 1220 for (res = tmp = locks; tmp != NULL; tmp = tmp->next()) { 1221 if (tmp->rank() < res->rank()) { 1222 res = tmp; 1223 } 1224 } 1225 if (!SafepointSynchronize::is_at_safepoint()) { 1226 // In this case, we expect the held locks to be 1227 // in increasing rank order (modulo any native ranks) 1228 for (tmp = locks; tmp != NULL; tmp = tmp->next()) { 1229 if (tmp->next() != NULL) { 1230 assert(tmp->rank() == Mutex::native || 1231 tmp->rank() <= tmp->next()->rank(), "mutex rank anomaly?"); 1232 } 1233 } 1234 } 1235 return res; 1236 } 1237 1238 Monitor* Monitor::get_least_ranked_lock_besides_this(Monitor* locks) { 1239 Monitor *res, *tmp; 1240 for (res = NULL, tmp = locks; tmp != NULL; tmp = tmp->next()) { 1241 if (tmp != this && (res == NULL || tmp->rank() < res->rank())) { 1242 res = tmp; 1243 } 1244 } 1245 if (!SafepointSynchronize::is_at_safepoint()) { 1246 // In this case, we expect the held locks to be 1247 // in increasing rank order (modulo any native ranks) 1248 for (tmp = locks; tmp != NULL; tmp = tmp->next()) { 1249 if (tmp->next() != NULL) { 1250 assert(tmp->rank() == Mutex::native || 1251 tmp->rank() <= tmp->next()->rank(), "mutex rank anomaly?"); 1252 } 1253 } 1254 } 1255 return res; 1256 } 1257 1258 1259 bool Monitor::contains(Monitor* locks, Monitor * lock) { 1260 for (; locks != NULL; locks = locks->next()) { 1261 if (locks == lock) 1262 return true; 1263 } 1264 return false; 1265 } 1266 #endif 1267 1268 // Called immediately after lock acquisition or release as a diagnostic 1269 // to track the lock-set of the thread and test for rank violations that 1270 // might indicate exposure to deadlock. 1271 // Rather like an EventListener for _owner (:>). 1272 1273 void Monitor::set_owner_implementation(Thread *new_owner) { 1274 // This function is solely responsible for maintaining 1275 // and checking the invariant that threads and locks 1276 // are in a 1/N relation, with some some locks unowned. 1277 // It uses the Mutex::_owner, Mutex::_next, and 1278 // Thread::_owned_locks fields, and no other function 1279 // changes those fields. 1280 // It is illegal to set the mutex from one non-NULL 1281 // owner to another--it must be owned by NULL as an 1282 // intermediate state. 1283 1284 if (new_owner != NULL) { 1285 // the thread is acquiring this lock 1286 1287 assert(new_owner == Thread::current(), "Should I be doing this?"); 1288 assert(_owner == NULL, "setting the owner thread of an already owned mutex"); 1289 _owner = new_owner; // set the owner 1290 1291 // link "this" into the owned locks list 1292 1293 #ifdef ASSERT // Thread::_owned_locks is under the same ifdef 1294 Monitor* locks = get_least_ranked_lock(new_owner->owned_locks()); 1295 // Mutex::set_owner_implementation is a friend of Thread 1296 1297 assert(this->rank() >= 0, "bad lock rank"); 1298 1299 // Deadlock avoidance rules require us to acquire Mutexes only in 1300 // a global total order. For example m1 is the lowest ranked mutex 1301 // that the thread holds and m2 is the mutex the thread is trying 1302 // to acquire, then deadlock avoidance rules require that the rank 1303 // of m2 be less than the rank of m1. 1304 // The rank Mutex::native is an exception in that it is not subject 1305 // to the verification rules. 1306 // Here are some further notes relating to mutex acquisition anomalies: 1307 // . under Solaris, the interrupt lock gets acquired when doing 1308 // profiling, so any lock could be held. 1309 // . it is also ok to acquire Safepoint_lock at the very end while we 1310 // already hold Terminator_lock - may happen because of periodic safepoints 1311 if (this->rank() != Mutex::native && 1312 this->rank() != Mutex::suspend_resume && 1313 locks != NULL && locks->rank() <= this->rank() && 1314 !SafepointSynchronize::is_at_safepoint() && 1315 this != Interrupt_lock && 1316 !(this == Safepoint_lock && contains(locks, Terminator_lock) && 1317 SafepointSynchronize::is_synchronizing())) { 1318 new_owner->print_owned_locks(); 1319 fatal(err_msg("acquiring lock %s/%d out of order with lock %s/%d -- " 1320 "possible deadlock", this->name(), this->rank(), 1321 locks->name(), locks->rank())); 1322 } 1323 1324 this->_next = new_owner->_owned_locks; 1325 new_owner->_owned_locks = this; 1326 #endif 1327 1328 } else { 1329 // the thread is releasing this lock 1330 1331 Thread* old_owner = _owner; 1332 debug_only(_last_owner = old_owner); 1333 1334 assert(old_owner != NULL, "removing the owner thread of an unowned mutex"); 1335 assert(old_owner == Thread::current(), "removing the owner thread of an unowned mutex"); 1336 1337 _owner = NULL; // set the owner 1338 1339 #ifdef ASSERT 1340 Monitor *locks = old_owner->owned_locks(); 1341 1342 // remove "this" from the owned locks list 1343 1344 Monitor *prev = NULL; 1345 bool found = false; 1346 for (; locks != NULL; prev = locks, locks = locks->next()) { 1347 if (locks == this) { 1348 found = true; 1349 break; 1350 } 1351 } 1352 assert(found, "Removing a lock not owned"); 1353 if (prev == NULL) { 1354 old_owner->_owned_locks = _next; 1355 } else { 1356 prev->_next = _next; 1357 } 1358 _next = NULL; 1359 #endif 1360 } 1361 } 1362 1363 1364 // Factored out common sanity checks for locking mutex'es. Used by lock() and try_lock() 1365 void Monitor::check_prelock_state(Thread *thread) { 1366 assert((!thread->is_Java_thread() || ((JavaThread *)thread)->thread_state() == _thread_in_vm) 1367 || rank() == Mutex::special, "wrong thread state for using locks"); 1368 if (StrictSafepointChecks) { 1369 if (thread->is_VM_thread() && !allow_vm_block()) { 1370 fatal(err_msg("VM thread using lock %s (not allowed to block on)", 1371 name())); 1372 } 1373 debug_only(if (rank() != Mutex::special) \ 1374 thread->check_for_valid_safepoint_state(false);) 1375 } 1376 } 1377 1378 void Monitor::check_block_state(Thread *thread) { 1379 if (!_allow_vm_block && thread->is_VM_thread()) { 1380 warning("VM thread blocked on lock"); 1381 print(); 1382 BREAKPOINT; 1383 } 1384 assert(_owner != thread, "deadlock: blocking on monitor owned by current thread"); 1385 } 1386 1387 #endif // PRODUCT