1 2 /* 3 * Copyright (c) 1998, 2013, Oracle and/or its affiliates. All rights reserved. 4 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 5 * 6 * This code is free software; you can redistribute it and/or modify it 7 * under the terms of the GNU General Public License version 2 only, as 8 * published by the Free Software Foundation. 9 * 10 * This code is distributed in the hope that it will be useful, but WITHOUT 11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 13 * version 2 for more details (a copy is included in the LICENSE file that 14 * accompanied this code). 15 * 16 * You should have received a copy of the GNU General Public License version 17 * 2 along with this work; if not, write to the Free Software Foundation, 18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 19 * 20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 21 * or visit www.oracle.com if you need additional information or have any 22 * questions. 23 * 24 */ 25 26 #include "precompiled.hpp" 27 #include "runtime/mutex.hpp" 28 #include "runtime/osThread.hpp" 29 #include "runtime/thread.inline.hpp" 30 #include "utilities/events.hpp" 31 #ifdef TARGET_OS_FAMILY_linux 32 # include "mutex_linux.inline.hpp" 33 #endif 34 #ifdef TARGET_OS_FAMILY_solaris 35 # include "mutex_solaris.inline.hpp" 36 #endif 37 #ifdef TARGET_OS_FAMILY_windows 38 # include "mutex_windows.inline.hpp" 39 #endif 40 #ifdef TARGET_OS_FAMILY_bsd 41 # include "mutex_bsd.inline.hpp" 42 #endif 43 44 // 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 45 // 46 // Native Monitor-Mutex locking - theory of operations 47 // 48 // * Native Monitors are completely unrelated to Java-level monitors, 49 // although the "back-end" slow-path implementations share a common lineage. 50 // See objectMonitor:: in synchronizer.cpp. 51 // Native Monitors do *not* support nesting or recursion but otherwise 52 // they're basically Hoare-flavor monitors. 53 // 54 // * A thread acquires ownership of a Monitor/Mutex by CASing the LockByte 55 // in the _LockWord from zero to non-zero. Note that the _Owner field 56 // is advisory and is used only to verify that the thread calling unlock() 57 // is indeed the last thread to have acquired the lock. 58 // 59 // * Contending threads "push" themselves onto the front of the contention 60 // queue -- called the cxq -- with CAS and then spin/park. 61 // The _LockWord contains the LockByte as well as the pointer to the head 62 // of the cxq. Colocating the LockByte with the cxq precludes certain races. 63 // 64 // * Using a separately addressable LockByte allows for CAS:MEMBAR or CAS:0 65 // idioms. We currently use MEMBAR in the uncontended unlock() path, as 66 // MEMBAR often has less latency than CAS. If warranted, we could switch to 67 // a CAS:0 mode, using timers to close the resultant race, as is done 68 // with Java Monitors in synchronizer.cpp. 69 // 70 // See the following for a discussion of the relative cost of atomics (CAS) 71 // MEMBAR, and ways to eliminate such instructions from the common-case paths: 72 // -- http://blogs.sun.com/dave/entry/biased_locking_in_hotspot 73 // -- http://blogs.sun.com/dave/resource/MustangSync.pdf 74 // -- http://blogs.sun.com/dave/resource/synchronization-public2.pdf 75 // -- synchronizer.cpp 76 // 77 // * Overall goals - desiderata 78 // 1. Minimize context switching 79 // 2. Minimize lock migration 80 // 3. Minimize CPI -- affinity and locality 81 // 4. Minimize the execution of high-latency instructions such as CAS or MEMBAR 82 // 5. Minimize outer lock hold times 83 // 6. Behave gracefully on a loaded system 84 // 85 // * Thread flow and list residency: 86 // 87 // Contention queue --> EntryList --> OnDeck --> Owner --> !Owner 88 // [..resident on monitor list..] 89 // [...........contending..................] 90 // 91 // -- The contention queue (cxq) contains recently-arrived threads (RATs). 92 // Threads on the cxq eventually drain into the EntryList. 93 // -- Invariant: a thread appears on at most one list -- cxq, EntryList 94 // or WaitSet -- at any one time. 95 // -- For a given monitor there can be at most one "OnDeck" thread at any 96 // given time but if needbe this particular invariant could be relaxed. 97 // 98 // * The WaitSet and EntryList linked lists are composed of ParkEvents. 99 // I use ParkEvent instead of threads as ParkEvents are immortal and 100 // type-stable, meaning we can safely unpark() a possibly stale 101 // list element in the unlock()-path. (That's benign). 102 // 103 // * Succession policy - providing for progress: 104 // 105 // As necessary, the unlock()ing thread identifies, unlinks, and unparks 106 // an "heir presumptive" tentative successor thread from the EntryList. 107 // This becomes the so-called "OnDeck" thread, of which there can be only 108 // one at any given time for a given monitor. The wakee will recontend 109 // for ownership of monitor. 110 // 111 // Succession is provided for by a policy of competitive handoff. 112 // The exiting thread does _not_ grant or pass ownership to the 113 // successor thread. (This is also referred to as "handoff" succession"). 114 // Instead the exiting thread releases ownership and possibly wakes 115 // a successor, so the successor can (re)compete for ownership of the lock. 116 // 117 // Competitive handoff provides excellent overall throughput at the expense 118 // of short-term fairness. If fairness is a concern then one remedy might 119 // be to add an AcquireCounter field to the monitor. After a thread acquires 120 // the lock it will decrement the AcquireCounter field. When the count 121 // reaches 0 the thread would reset the AcquireCounter variable, abdicate 122 // the lock directly to some thread on the EntryList, and then move itself to the 123 // tail of the EntryList. 124 // 125 // But in practice most threads engage or otherwise participate in resource 126 // bounded producer-consumer relationships, so lock domination is not usually 127 // a practical concern. Recall too, that in general it's easier to construct 128 // a fair lock from a fast lock, but not vice-versa. 129 // 130 // * The cxq can have multiple concurrent "pushers" but only one concurrent 131 // detaching thread. This mechanism is immune from the ABA corruption. 132 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious. 133 // We use OnDeck as a pseudo-lock to enforce the at-most-one detaching 134 // thread constraint. 135 // 136 // * Taken together, the cxq and the EntryList constitute or form a 137 // single logical queue of threads stalled trying to acquire the lock. 138 // We use two distinct lists to reduce heat on the list ends. 139 // Threads in lock() enqueue onto cxq while threads in unlock() will 140 // dequeue from the EntryList. (c.f. Michael Scott's "2Q" algorithm). 141 // A key desideratum is to minimize queue & monitor metadata manipulation 142 // that occurs while holding the "outer" monitor lock -- that is, we want to 143 // minimize monitor lock holds times. 144 // 145 // The EntryList is ordered by the prevailing queue discipline and 146 // can be organized in any convenient fashion, such as a doubly-linked list or 147 // a circular doubly-linked list. If we need a priority queue then something akin 148 // to Solaris' sleepq would work nicely. Viz., 149 // -- http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c. 150 // -- http://cvs.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/uts/common/os/sleepq.c 151 // Queue discipline is enforced at ::unlock() time, when the unlocking thread 152 // drains the cxq into the EntryList, and orders or reorders the threads on the 153 // EntryList accordingly. 154 // 155 // Barring "lock barging", this mechanism provides fair cyclic ordering, 156 // somewhat similar to an elevator-scan. 157 // 158 // * OnDeck 159 // -- For a given monitor there can be at most one OnDeck thread at any given 160 // instant. The OnDeck thread is contending for the lock, but has been 161 // unlinked from the EntryList and cxq by some previous unlock() operations. 162 // Once a thread has been designated the OnDeck thread it will remain so 163 // until it manages to acquire the lock -- being OnDeck is a stable property. 164 // -- Threads on the EntryList or cxq are _not allowed to attempt lock acquisition. 165 // -- OnDeck also serves as an "inner lock" as follows. Threads in unlock() will, after 166 // having cleared the LockByte and dropped the outer lock, attempt to "trylock" 167 // OnDeck by CASing the field from null to non-null. If successful, that thread 168 // is then responsible for progress and succession and can use CAS to detach and 169 // drain the cxq into the EntryList. By convention, only this thread, the holder of 170 // the OnDeck inner lock, can manipulate the EntryList or detach and drain the 171 // RATs on the cxq into the EntryList. This avoids ABA corruption on the cxq as 172 // we allow multiple concurrent "push" operations but restrict detach concurrency 173 // to at most one thread. Having selected and detached a successor, the thread then 174 // changes the OnDeck to refer to that successor, and then unparks the successor. 175 // That successor will eventually acquire the lock and clear OnDeck. Beware 176 // that the OnDeck usage as a lock is asymmetric. A thread in unlock() transiently 177 // "acquires" OnDeck, performs queue manipulations, passes OnDeck to some successor, 178 // and then the successor eventually "drops" OnDeck. Note that there's never 179 // any sense of contention on the inner lock, however. Threads never contend 180 // or wait for the inner lock. 181 // -- OnDeck provides for futile wakeup throttling a described in section 3.3 of 182 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf 183 // In a sense, OnDeck subsumes the ObjectMonitor _Succ and ObjectWaiter 184 // TState fields found in Java-level objectMonitors. (See synchronizer.cpp). 185 // 186 // * Waiting threads reside on the WaitSet list -- wait() puts 187 // the caller onto the WaitSet. Notify() or notifyAll() simply 188 // transfers threads from the WaitSet to either the EntryList or cxq. 189 // Subsequent unlock() operations will eventually unpark the notifyee. 190 // Unparking a notifee in notify() proper is inefficient - if we were to do so 191 // it's likely the notifyee would simply impale itself on the lock held 192 // by the notifier. 193 // 194 // * The mechanism is obstruction-free in that if the holder of the transient 195 // OnDeck lock in unlock() is preempted or otherwise stalls, other threads 196 // can still acquire and release the outer lock and continue to make progress. 197 // At worst, waking of already blocked contending threads may be delayed, 198 // but nothing worse. (We only use "trylock" operations on the inner OnDeck 199 // lock). 200 // 201 // * Note that thread-local storage must be initialized before a thread 202 // uses Native monitors or mutexes. The native monitor-mutex subsystem 203 // depends on Thread::current(). 204 // 205 // * The monitor synchronization subsystem avoids the use of native 206 // synchronization primitives except for the narrow platform-specific 207 // park-unpark abstraction. See the comments in os_solaris.cpp regarding 208 // the semantics of park-unpark. Put another way, this monitor implementation 209 // depends only on atomic operations and park-unpark. The monitor subsystem 210 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the 211 // underlying OS manages the READY<->RUN transitions. 212 // 213 // * The memory consistency model provide by lock()-unlock() is at least as 214 // strong or stronger than the Java Memory model defined by JSR-133. 215 // That is, we guarantee at least entry consistency, if not stronger. 216 // See http://g.oswego.edu/dl/jmm/cookbook.html. 217 // 218 // * Thread:: currently contains a set of purpose-specific ParkEvents: 219 // _MutexEvent, _ParkEvent, etc. A better approach might be to do away with 220 // the purpose-specific ParkEvents and instead implement a general per-thread 221 // stack of available ParkEvents which we could provision on-demand. The 222 // stack acts as a local cache to avoid excessive calls to ParkEvent::Allocate() 223 // and ::Release(). A thread would simply pop an element from the local stack before it 224 // enqueued or park()ed. When the contention was over the thread would 225 // push the no-longer-needed ParkEvent back onto its stack. 226 // 227 // * A slightly reduced form of ILock() and IUnlock() have been partially 228 // model-checked (Murphi) for safety and progress at T=1,2,3 and 4. 229 // It'd be interesting to see if TLA/TLC could be useful as well. 230 // 231 // * Mutex-Monitor is a low-level "leaf" subsystem. That is, the monitor 232 // code should never call other code in the JVM that might itself need to 233 // acquire monitors or mutexes. That's true *except* in the case of the 234 // ThreadBlockInVM state transition wrappers. The ThreadBlockInVM DTOR handles 235 // mutator reentry (ingress) by checking for a pending safepoint in which case it will 236 // call SafepointSynchronize::block(), which in turn may call Safepoint_lock->lock(), etc. 237 // In that particular case a call to lock() for a given Monitor can end up recursively 238 // calling lock() on another monitor. While distasteful, this is largely benign 239 // as the calls come from jacket that wraps lock(), and not from deep within lock() itself. 240 // 241 // It's unfortunate that native mutexes and thread state transitions were convolved. 242 // They're really separate concerns and should have remained that way. Melding 243 // them together was facile -- a bit too facile. The current implementation badly 244 // conflates the two concerns. 245 // 246 // * TODO-FIXME: 247 // 248 // -- Add DTRACE probes for contended acquire, contended acquired, contended unlock 249 // We should also add DTRACE probes in the ParkEvent subsystem for 250 // Park-entry, Park-exit, and Unpark. 251 // 252 // -- We have an excess of mutex-like constructs in the JVM, namely: 253 // 1. objectMonitors for Java-level synchronization (synchronizer.cpp) 254 // 2. low-level muxAcquire and muxRelease 255 // 3. low-level spinAcquire and spinRelease 256 // 4. native Mutex:: and Monitor:: 257 // 5. jvm_raw_lock() and _unlock() 258 // 6. JVMTI raw monitors -- distinct from (5) despite having a confusingly 259 // similar name. 260 // 261 // 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 262 263 264 // CASPTR() uses the canonical argument order that dominates in the literature. 265 // Our internal cmpxchg_ptr() uses a bastardized ordering to accommodate Sun .il templates. 266 267 #define CASPTR(a,c,s) intptr_t(Atomic::cmpxchg_ptr ((void *)(s),(void *)(a),(void *)(c))) 268 #define UNS(x) (uintptr_t(x)) 269 #define TRACE(m) { static volatile int ctr = 0 ; int x = ++ctr ; if ((x & (x-1))==0) { ::printf ("%d:%s\n", x, #m); ::fflush(stdout); }} 270 271 // Simplistic low-quality Marsaglia SHIFT-XOR RNG. 272 // Bijective except for the trailing mask operation. 273 // Useful for spin loops as the compiler can't optimize it away. 274 275 static inline jint MarsagliaXORV (jint x) { 276 if (x == 0) x = 1|os::random() ; 277 x ^= x << 6; 278 x ^= ((unsigned)x) >> 21; 279 x ^= x << 7 ; 280 return x & 0x7FFFFFFF ; 281 } 282 283 static inline jint MarsagliaXOR (jint * const a) { 284 jint x = *a ; 285 if (x == 0) x = UNS(a)|1 ; 286 x ^= x << 6; 287 x ^= ((unsigned)x) >> 21; 288 x ^= x << 7 ; 289 *a = x ; 290 return x & 0x7FFFFFFF ; 291 } 292 293 static int Stall (int its) { 294 static volatile jint rv = 1 ; 295 volatile int OnFrame = 0 ; 296 jint v = rv ^ UNS(OnFrame) ; 297 while (--its >= 0) { 298 v = MarsagliaXORV (v) ; 299 } 300 // Make this impossible for the compiler to optimize away, 301 // but (mostly) avoid W coherency sharing on MP systems. 302 if (v == 0x12345) rv = v ; 303 return v ; 304 } 305 306 int Monitor::TryLock () { 307 intptr_t v = _LockWord.FullWord ; 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::TryFast () { 317 // Optimistic fast-path form ... 318 // Fast-path attempt for the common uncontended case. 319 // Avoid RTS->RTO $ coherence upgrade on typical SMP systems. 320 intptr_t v = CASPTR (&_LockWord, 0, _LBIT) ; // agro ... 321 if (v == 0) return 1 ; 322 323 for (;;) { 324 if ((v & _LBIT) != 0) return 0 ; 325 const intptr_t u = CASPTR (&_LockWord, v, v|_LBIT) ; 326 if (v == u) return 1 ; 327 v = u ; 328 } 329 } 330 331 int Monitor::ILocked () { 332 const intptr_t w = _LockWord.FullWord & 0xFF ; 333 assert (w == 0 || w == _LBIT, "invariant") ; 334 return w == _LBIT ; 335 } 336 337 // Polite TATAS spinlock with exponential backoff - bounded spin. 338 // Ideally we'd use processor cycles, time or vtime to control 339 // the loop, but we currently use iterations. 340 // All the constants within were derived empirically but work over 341 // over the spectrum of J2SE reference platforms. 342 // On Niagara-class systems the back-off is unnecessary but 343 // is relatively harmless. (At worst it'll slightly retard 344 // acquisition times). The back-off is critical for older SMP systems 345 // where constant fetching of the LockWord would otherwise impair 346 // scalability. 347 // 348 // Clamp spinning at approximately 1/2 of a context-switch round-trip. 349 // See synchronizer.cpp for details and rationale. 350 351 int Monitor::TrySpin (Thread * const Self) { 352 if (TryLock()) return 1 ; 353 if (!os::is_MP()) return 0 ; 354 355 int Probes = 0 ; 356 int Delay = 0 ; 357 int Steps = 0 ; 358 int SpinMax = NativeMonitorSpinLimit ; 359 int flgs = NativeMonitorFlags ; 360 for (;;) { 361 intptr_t v = _LockWord.FullWord; 362 if ((v & _LBIT) == 0) { 363 if (CASPTR (&_LockWord, v, v|_LBIT) == v) { 364 return 1 ; 365 } 366 continue ; 367 } 368 369 if ((flgs & 8) == 0) { 370 SpinPause () ; 371 } 372 373 // Periodically increase Delay -- variable Delay form 374 // conceptually: delay *= 1 + 1/Exponent 375 ++ Probes; 376 if (Probes > SpinMax) return 0 ; 377 378 if ((Probes & 0x7) == 0) { 379 Delay = ((Delay << 1)|1) & 0x7FF ; 380 // CONSIDER: Delay += 1 + (Delay/4); Delay &= 0x7FF ; 381 } 382 383 if (flgs & 2) continue ; 384 385 // Consider checking _owner's schedctl state, if OFFPROC abort spin. 386 // If the owner is OFFPROC then it's unlike that the lock will be dropped 387 // in a timely fashion, which suggests that spinning would not be fruitful 388 // or profitable. 389 390 // Stall for "Delay" time units - iterations in the current implementation. 391 // Avoid generating coherency traffic while stalled. 392 // Possible ways to delay: 393 // PAUSE, SLEEP, MEMBAR #sync, MEMBAR #halt, 394 // wr %g0,%asi, gethrtime, rdstick, rdtick, rdtsc, etc. ... 395 // Note that on Niagara-class systems we want to minimize STs in the 396 // spin loop. N1 and brethren write-around the L1$ over the xbar into the L2$. 397 // Furthermore, they don't have a W$ like traditional SPARC processors. 398 // We currently use a Marsaglia Shift-Xor RNG loop. 399 Steps += Delay ; 400 if (Self != NULL) { 401 jint rv = Self->rng[0] ; 402 for (int k = Delay ; --k >= 0; ) { 403 rv = MarsagliaXORV (rv) ; 404 if ((flgs & 4) == 0 && SafepointSynchronize::do_call_back()) return 0 ; 405 } 406 Self->rng[0] = rv ; 407 } else { 408 Stall (Delay) ; 409 } 410 } 411 } 412 413 static int ParkCommon (ParkEvent * ev, jlong timo) { 414 // Diagnostic support - periodically unwedge blocked threads 415 intx nmt = NativeMonitorTimeout ; 416 if (nmt > 0 && (nmt < timo || timo <= 0)) { 417 timo = nmt ; 418 } 419 int err = OS_OK ; 420 if (0 == timo) { 421 ev->park() ; 422 } else { 423 err = ev->park(timo) ; 424 } 425 return err ; 426 } 427 428 inline int Monitor::AcquireOrPush (ParkEvent * ESelf) { 429 intptr_t v = _LockWord.FullWord ; 430 for (;;) { 431 if ((v & _LBIT) == 0) { 432 const intptr_t u = CASPTR (&_LockWord, v, v|_LBIT) ; 433 if (u == v) return 1 ; // indicate acquired 434 v = u ; 435 } else { 436 // Anticipate success ... 437 ESelf->ListNext = (ParkEvent *) (v & ~_LBIT) ; 438 const intptr_t u = CASPTR (&_LockWord, v, intptr_t(ESelf)|_LBIT) ; 439 if (u == v) return 0 ; // indicate pushed onto cxq 440 v = u ; 441 } 442 // Interference - LockWord change - just retry 443 } 444 } 445 446 // ILock and IWait are the lowest level primitive internal blocking 447 // synchronization functions. The callers of IWait and ILock must have 448 // performed any needed state transitions beforehand. 449 // IWait and ILock may directly call park() without any concern for thread state. 450 // Note that ILock and IWait do *not* access _owner. 451 // _owner is a higher-level logical concept. 452 453 void Monitor::ILock (Thread * Self) { 454 assert (_OnDeck != Self->_MutexEvent, "invariant") ; 455 456 if (TryFast()) { 457 Exeunt: 458 assert (ILocked(), "invariant") ; 459 return ; 460 } 461 462 ParkEvent * const ESelf = Self->_MutexEvent ; 463 assert (_OnDeck != ESelf, "invariant") ; 464 465 // As an optimization, spinners could conditionally try to set ONDECK to _LBIT 466 // Synchronizer.cpp uses a similar optimization. 467 if (TrySpin (Self)) goto Exeunt ; 468 469 // Slow-path - the lock is contended. 470 // Either Enqueue Self on cxq or acquire the outer lock. 471 // LockWord encoding = (cxq,LOCKBYTE) 472 ESelf->reset() ; 473 OrderAccess::fence() ; 474 475 // Optional optimization ... try barging on the inner lock 476 if ((NativeMonitorFlags & 32) && CASPTR (&_OnDeck, NULL, UNS(Self)) == 0) { 477 goto OnDeck_LOOP ; 478 } 479 480 if (AcquireOrPush (ESelf)) goto Exeunt ; 481 482 // At any given time there is at most one ondeck thread. 483 // ondeck implies not resident on cxq and not resident on EntryList 484 // Only the OnDeck thread can try to acquire -- contended for -- the lock. 485 // CONSIDER: use Self->OnDeck instead of m->OnDeck. 486 // Deschedule Self so that others may run. 487 while (_OnDeck != ESelf) { 488 ParkCommon (ESelf, 0) ; 489 } 490 491 // Self is now in the ONDECK position and will remain so until it 492 // manages to acquire the lock. 493 OnDeck_LOOP: 494 for (;;) { 495 assert (_OnDeck == ESelf, "invariant") ; 496 if (TrySpin (Self)) break ; 497 // CONSIDER: if ESelf->TryPark() && TryLock() break ... 498 // It's probably wise to spin only if we *actually* blocked 499 // CONSIDER: check the lockbyte, if it remains set then 500 // preemptively drain the cxq into the EntryList. 501 // The best place and time to perform queue operations -- lock metadata -- 502 // is _before having acquired the outer lock, while waiting for the lock to drop. 503 ParkCommon (ESelf, 0) ; 504 } 505 506 assert (_OnDeck == ESelf, "invariant") ; 507 _OnDeck = NULL ; 508 509 // Note that we current drop the inner lock (clear OnDeck) in the slow-path 510 // epilog immediately after having acquired the outer lock. 511 // But instead we could consider the following optimizations: 512 // A. Shift or defer dropping the inner lock until the subsequent IUnlock() operation. 513 // This might avoid potential reacquisition of the inner lock in IUlock(). 514 // B. While still holding the inner lock, attempt to opportunistically select 515 // and unlink the next ONDECK thread from the EntryList. 516 // If successful, set ONDECK to refer to that thread, otherwise clear ONDECK. 517 // It's critical that the select-and-unlink operation run in constant-time as 518 // it executes when holding the outer lock and may artificially increase the 519 // effective length of the critical section. 520 // Note that (A) and (B) are tantamount to succession by direct handoff for 521 // the inner lock. 522 goto Exeunt ; 523 } 524 525 void Monitor::IUnlock (bool RelaxAssert) { 526 assert (ILocked(), "invariant") ; 527 // Conceptually we need a MEMBAR #storestore|#loadstore barrier or fence immediately 528 // before the store that releases the lock. Crucially, all the stores and loads in the 529 // critical section must be globally visible before the store of 0 into the lock-word 530 // that releases the lock becomes globally visible. That is, memory accesses in the 531 // critical section should not be allowed to bypass or overtake the following ST that 532 // releases the lock. As such, to prevent accesses within the critical section 533 // from "leaking" out, we need a release fence between the critical section and the 534 // store that releases the lock. In practice that release barrier is elided on 535 // platforms with strong memory models such as TSO. 536 // 537 // Note that the OrderAccess::storeload() fence that appears after unlock store 538 // provides for progress conditions and succession and is _not related to exclusion 539 // safety or lock release consistency. 540 OrderAccess::release_store(&_LockWord.Bytes[_LSBINDEX], 0); // drop outer lock 541 542 OrderAccess::storeload (); 543 ParkEvent * const w = _OnDeck ; 544 assert (RelaxAssert || w != Thread::current()->_MutexEvent, "invariant") ; 545 if (w != NULL) { 546 // Either we have a valid ondeck thread or ondeck is transiently "locked" 547 // by some exiting thread as it arranges for succession. The LSBit of 548 // OnDeck allows us to discriminate two cases. If the latter, the 549 // responsibility for progress and succession lies with that other thread. 550 // For good performance, we also depend on the fact that redundant unpark() 551 // operations are cheap. That is, repeated Unpark()ing of the ONDECK thread 552 // is inexpensive. This approach provides implicit futile wakeup throttling. 553 // Note that the referent "w" might be stale with respect to the lock. 554 // In that case the following unpark() is harmless and the worst that'll happen 555 // is a spurious return from a park() operation. Critically, if "w" _is stale, 556 // then progress is known to have occurred as that means the thread associated 557 // with "w" acquired the lock. In that case this thread need take no further 558 // action to guarantee progress. 559 if ((UNS(w) & _LBIT) == 0) w->unpark() ; 560 return ; 561 } 562 563 intptr_t cxq = _LockWord.FullWord ; 564 if (((cxq & ~_LBIT)|UNS(_EntryList)) == 0) { 565 return ; // normal fast-path exit - cxq and EntryList both empty 566 } 567 if (cxq & _LBIT) { 568 // Optional optimization ... 569 // Some other thread acquired the lock in the window since this 570 // thread released it. Succession is now that thread's responsibility. 571 return ; 572 } 573 574 Succession: 575 // Slow-path exit - this thread must ensure succession and progress. 576 // OnDeck serves as lock to protect cxq and EntryList. 577 // Only the holder of OnDeck can manipulate EntryList or detach the RATs from cxq. 578 // Avoid ABA - allow multiple concurrent producers (enqueue via push-CAS) 579 // but only one concurrent consumer (detacher of RATs). 580 // Consider protecting this critical section with schedctl on Solaris. 581 // Unlike a normal lock, however, the exiting thread "locks" OnDeck, 582 // picks a successor and marks that thread as OnDeck. That successor 583 // thread will then clear OnDeck once it eventually acquires the outer lock. 584 if (CASPTR (&_OnDeck, NULL, _LBIT) != UNS(NULL)) { 585 return ; 586 } 587 588 ParkEvent * List = _EntryList ; 589 if (List != NULL) { 590 // Transfer the head of the EntryList to the OnDeck position. 591 // Once OnDeck, a thread stays OnDeck until it acquires the lock. 592 // For a given lock there is at most OnDeck thread at any one instant. 593 WakeOne: 594 assert (List == _EntryList, "invariant") ; 595 ParkEvent * const w = List ; 596 assert (RelaxAssert || w != Thread::current()->_MutexEvent, "invariant") ; 597 _EntryList = w->ListNext ; 598 // as a diagnostic measure consider setting w->_ListNext = BAD 599 assert (UNS(_OnDeck) == _LBIT, "invariant") ; 600 _OnDeck = w ; // pass OnDeck to w. 601 // w will clear OnDeck once it acquires the outer lock 602 603 // Another optional optimization ... 604 // For heavily contended locks it's not uncommon that some other 605 // thread acquired the lock while this thread was arranging succession. 606 // Try to defer the unpark() operation - Delegate the responsibility 607 // for unpark()ing the OnDeck thread to the current or subsequent owners 608 // That is, the new owner is responsible for unparking the OnDeck thread. 609 OrderAccess::storeload() ; 610 cxq = _LockWord.FullWord ; 611 if (cxq & _LBIT) return ; 612 613 w->unpark() ; 614 return ; 615 } 616 617 cxq = _LockWord.FullWord ; 618 if ((cxq & ~_LBIT) != 0) { 619 // The EntryList is empty but the cxq is populated. 620 // drain RATs from cxq into EntryList 621 // Detach RATs segment with CAS and then merge into EntryList 622 for (;;) { 623 // optional optimization - if locked, the owner is responsible for succession 624 if (cxq & _LBIT) goto Punt ; 625 const intptr_t vfy = CASPTR (&_LockWord, cxq, cxq & _LBIT) ; 626 if (vfy == cxq) break ; 627 cxq = vfy ; 628 // Interference - LockWord changed - Just retry 629 // We can see concurrent interference from contending threads 630 // pushing themselves onto the cxq or from lock-unlock operations. 631 // From the perspective of this thread, EntryList is stable and 632 // the cxq is prepend-only -- the head is volatile but the interior 633 // of the cxq is stable. In theory if we encounter interference from threads 634 // pushing onto cxq we could simply break off the original cxq suffix and 635 // move that segment to the EntryList, avoiding a 2nd or multiple CAS attempts 636 // on the high-traffic LockWord variable. For instance lets say the cxq is "ABCD" 637 // when we first fetch cxq above. Between the fetch -- where we observed "A" 638 // -- and CAS -- where we attempt to CAS null over A -- "PQR" arrive, 639 // yielding cxq = "PQRABCD". In this case we could simply set A.ListNext 640 // null, leaving cxq = "PQRA" and transfer the "BCD" segment to the EntryList. 641 // Note too, that it's safe for this thread to traverse the cxq 642 // without taking any special concurrency precautions. 643 } 644 645 // We don't currently reorder the cxq segment as we move it onto 646 // the EntryList, but it might make sense to reverse the order 647 // or perhaps sort by thread priority. See the comments in 648 // synchronizer.cpp objectMonitor::exit(). 649 assert (_EntryList == NULL, "invariant") ; 650 _EntryList = List = (ParkEvent *)(cxq & ~_LBIT) ; 651 assert (List != NULL, "invariant") ; 652 goto WakeOne ; 653 } 654 655 // cxq|EntryList is empty. 656 // w == NULL implies that cxq|EntryList == NULL in the past. 657 // Possible race - rare inopportune interleaving. 658 // A thread could have added itself to cxq since this thread previously checked. 659 // Detect and recover by refetching cxq. 660 Punt: 661 assert (UNS(_OnDeck) == _LBIT, "invariant") ; 662 _OnDeck = NULL ; // Release inner lock. 663 OrderAccess::storeload(); // Dekker duality - pivot point 664 665 // Resample LockWord/cxq to recover from possible race. 666 // For instance, while this thread T1 held OnDeck, some other thread T2 might 667 // acquire the outer lock. Another thread T3 might try to acquire the outer 668 // lock, but encounter contention and enqueue itself on cxq. T2 then drops the 669 // outer lock, but skips succession as this thread T1 still holds OnDeck. 670 // T1 is and remains responsible for ensuring succession of T3. 671 // 672 // Note that we don't need to recheck EntryList, just cxq. 673 // If threads moved onto EntryList since we dropped OnDeck 674 // that implies some other thread forced succession. 675 cxq = _LockWord.FullWord ; 676 if ((cxq & ~_LBIT) != 0 && (cxq & _LBIT) == 0) { 677 goto Succession ; // potential race -- re-run succession 678 } 679 return ; 680 } 681 682 bool Monitor::notify() { 683 assert (_owner == Thread::current(), "invariant") ; 684 assert (ILocked(), "invariant") ; 685 if (_WaitSet == NULL) return true ; 686 NotifyCount ++ ; 687 688 // Transfer one thread from the WaitSet to the EntryList or cxq. 689 // Currently we just unlink the head of the WaitSet and prepend to the cxq. 690 // And of course we could just unlink it and unpark it, too, but 691 // in that case it'd likely impale itself on the reentry. 692 Thread::muxAcquire (_WaitLock, "notify:WaitLock") ; 693 ParkEvent * nfy = _WaitSet ; 694 if (nfy != NULL) { // DCL idiom 695 _WaitSet = nfy->ListNext ; 696 assert (nfy->Notified == 0, "invariant") ; 697 // push nfy onto the cxq 698 for (;;) { 699 const intptr_t v = _LockWord.FullWord ; 700 assert ((v & 0xFF) == _LBIT, "invariant") ; 701 nfy->ListNext = (ParkEvent *)(v & ~_LBIT); 702 if (CASPTR (&_LockWord, v, UNS(nfy)|_LBIT) == v) break; 703 // interference - _LockWord changed -- just retry 704 } 705 // Note that setting Notified before pushing nfy onto the cxq is 706 // also legal and safe, but the safety properties are much more 707 // subtle, so for the sake of code stewardship ... 708 OrderAccess::fence() ; 709 nfy->Notified = 1; 710 } 711 Thread::muxRelease (_WaitLock) ; 712 if (nfy != NULL && (NativeMonitorFlags & 16)) { 713 // Experimental code ... light up the wakee in the hope that this thread (the owner) 714 // will drop the lock just about the time the wakee comes ONPROC. 715 nfy->unpark() ; 716 } 717 assert (ILocked(), "invariant") ; 718 return true ; 719 } 720 721 // Currently notifyAll() transfers the waiters one-at-a-time from the waitset 722 // to the cxq. This could be done more efficiently with a single bulk en-mass transfer, 723 // but in practice notifyAll() for large #s of threads is rare and not time-critical. 724 // Beware too, that we invert the order of the waiters. Lets say that the 725 // waitset is "ABCD" and the cxq is "XYZ". After a notifyAll() the waitset 726 // will be empty and the cxq will be "DCBAXYZ". This is benign, of course. 727 728 bool Monitor::notify_all() { 729 assert (_owner == Thread::current(), "invariant") ; 730 assert (ILocked(), "invariant") ; 731 while (_WaitSet != NULL) notify() ; 732 return true ; 733 } 734 735 int Monitor::IWait (Thread * Self, jlong timo) { 736 assert (ILocked(), "invariant") ; 737 738 // Phases: 739 // 1. Enqueue Self on WaitSet - currently prepend 740 // 2. unlock - drop the outer lock 741 // 3. wait for either notification or timeout 742 // 4. lock - reentry - reacquire the outer lock 743 744 ParkEvent * const ESelf = Self->_MutexEvent ; 745 ESelf->Notified = 0 ; 746 ESelf->reset() ; 747 OrderAccess::fence() ; 748 749 // Add Self to WaitSet 750 // Ideally only the holder of the outer lock would manipulate the WaitSet - 751 // That is, the outer lock would implicitly protect the WaitSet. 752 // But if a thread in wait() encounters a timeout it will need to dequeue itself 753 // from the WaitSet _before it becomes the owner of the lock. We need to dequeue 754 // as the ParkEvent -- which serves as a proxy for the thread -- can't reside 755 // on both the WaitSet and the EntryList|cxq at the same time.. That is, a thread 756 // on the WaitSet can't be allowed to compete for the lock until it has managed to 757 // unlink its ParkEvent from WaitSet. Thus the need for WaitLock. 758 // Contention on the WaitLock is minimal. 759 // 760 // Another viable approach would be add another ParkEvent, "WaitEvent" to the 761 // thread class. The WaitSet would be composed of WaitEvents. Only the 762 // owner of the outer lock would manipulate the WaitSet. A thread in wait() 763 // could then compete for the outer lock, and then, if necessary, unlink itself 764 // from the WaitSet only after having acquired the outer lock. More precisely, 765 // there would be no WaitLock. A thread in in wait() would enqueue its WaitEvent 766 // on the WaitSet; release the outer lock; wait for either notification or timeout; 767 // reacquire the inner lock; and then, if needed, unlink itself from the WaitSet. 768 // 769 // Alternatively, a 2nd set of list link fields in the ParkEvent might suffice. 770 // One set would be for the WaitSet and one for the EntryList. 771 // We could also deconstruct the ParkEvent into a "pure" event and add a 772 // new immortal/TSM "ListElement" class that referred to ParkEvents. 773 // In that case we could have one ListElement on the WaitSet and another 774 // on the EntryList, with both referring to the same pure Event. 775 776 Thread::muxAcquire (_WaitLock, "wait:WaitLock:Add") ; 777 ESelf->ListNext = _WaitSet ; 778 _WaitSet = ESelf ; 779 Thread::muxRelease (_WaitLock) ; 780 781 // Release the outer lock 782 // We call IUnlock (RelaxAssert=true) as a thread T1 might 783 // enqueue itself on the WaitSet, call IUnlock(), drop the lock, 784 // and then stall before it can attempt to wake a successor. 785 // Some other thread T2 acquires the lock, and calls notify(), moving 786 // T1 from the WaitSet to the cxq. T2 then drops the lock. T1 resumes, 787 // and then finds *itself* on the cxq. During the course of a normal 788 // IUnlock() call a thread should _never find itself on the EntryList 789 // or cxq, but in the case of wait() it's possible. 790 // See synchronizer.cpp objectMonitor::wait(). 791 IUnlock (true) ; 792 793 // Wait for either notification or timeout 794 // Beware that in some circumstances we might propagate 795 // spurious wakeups back to the caller. 796 797 for (;;) { 798 if (ESelf->Notified) break ; 799 int err = ParkCommon (ESelf, timo) ; 800 if (err == OS_TIMEOUT || (NativeMonitorFlags & 1)) break ; 801 } 802 803 // Prepare for reentry - if necessary, remove ESelf from WaitSet 804 // ESelf can be: 805 // 1. Still on the WaitSet. This can happen if we exited the loop by timeout. 806 // 2. On the cxq or EntryList 807 // 3. Not resident on cxq, EntryList or WaitSet, but in the OnDeck position. 808 809 OrderAccess::fence() ; 810 int WasOnWaitSet = 0 ; 811 if (ESelf->Notified == 0) { 812 Thread::muxAcquire (_WaitLock, "wait:WaitLock:remove") ; 813 if (ESelf->Notified == 0) { // DCL idiom 814 assert (_OnDeck != ESelf, "invariant") ; // can't be both OnDeck and on WaitSet 815 // ESelf is resident on the WaitSet -- unlink it. 816 // A doubly-linked list would be better here so we can unlink in constant-time. 817 // We have to unlink before we potentially recontend as ESelf might otherwise 818 // end up on the cxq|EntryList -- it can't be on two lists at once. 819 ParkEvent * p = _WaitSet ; 820 ParkEvent * q = NULL ; // classic q chases p 821 while (p != NULL && p != ESelf) { 822 q = p ; 823 p = p->ListNext ; 824 } 825 assert (p == ESelf, "invariant") ; 826 if (p == _WaitSet) { // found at head 827 assert (q == NULL, "invariant") ; 828 _WaitSet = p->ListNext ; 829 } else { // found in interior 830 assert (q->ListNext == p, "invariant") ; 831 q->ListNext = p->ListNext ; 832 } 833 WasOnWaitSet = 1 ; // We were *not* notified but instead encountered timeout 834 } 835 Thread::muxRelease (_WaitLock) ; 836 } 837 838 // Reentry phase - reacquire the lock 839 if (WasOnWaitSet) { 840 // ESelf was previously on the WaitSet but we just unlinked it above 841 // because of a timeout. ESelf is not resident on any list and is not OnDeck 842 assert (_OnDeck != ESelf, "invariant") ; 843 ILock (Self) ; 844 } else { 845 // A prior notify() operation moved ESelf from the WaitSet to the cxq. 846 // ESelf is now on the cxq, EntryList or at the OnDeck position. 847 // The following fragment is extracted from Monitor::ILock() 848 for (;;) { 849 if (_OnDeck == ESelf && TrySpin(Self)) break ; 850 ParkCommon (ESelf, 0) ; 851 } 852 assert (_OnDeck == ESelf, "invariant") ; 853 _OnDeck = NULL ; 854 } 855 856 assert (ILocked(), "invariant") ; 857 return WasOnWaitSet != 0 ; // return true IFF timeout 858 } 859 860 861 // ON THE VMTHREAD SNEAKING PAST HELD LOCKS: 862 // In particular, there are certain types of global lock that may be held 863 // by a Java thread while it is blocked at a safepoint but before it has 864 // written the _owner field. These locks may be sneakily acquired by the 865 // VM thread during a safepoint to avoid deadlocks. Alternatively, one should 866 // identify all such locks, and ensure that Java threads never block at 867 // safepoints while holding them (_no_safepoint_check_flag). While it 868 // seems as though this could increase the time to reach a safepoint 869 // (or at least increase the mean, if not the variance), the latter 870 // approach might make for a cleaner, more maintainable JVM design. 871 // 872 // Sneaking is vile and reprehensible and should be excised at the 1st 873 // opportunity. It's possible that the need for sneaking could be obviated 874 // as follows. Currently, a thread might (a) while TBIVM, call pthread_mutex_lock 875 // or ILock() thus acquiring the "physical" lock underlying Monitor/Mutex. 876 // (b) stall at the TBIVM exit point as a safepoint is in effect. Critically, 877 // it'll stall at the TBIVM reentry state transition after having acquired the 878 // underlying lock, but before having set _owner and having entered the actual 879 // critical section. The lock-sneaking facility leverages that fact and allowed the 880 // VM thread to logically acquire locks that had already be physically locked by mutators 881 // but where mutators were known blocked by the reentry thread state transition. 882 // 883 // If we were to modify the Monitor-Mutex so that TBIVM state transitions tightly 884 // wrapped calls to park(), then we could likely do away with sneaking. We'd 885 // decouple lock acquisition and parking. The critical invariant to eliminating 886 // sneaking is to ensure that we never "physically" acquire the lock while TBIVM. 887 // An easy way to accomplish this is to wrap the park calls in a narrow TBIVM jacket. 888 // One difficulty with this approach is that the TBIVM wrapper could recurse and 889 // call lock() deep from within a lock() call, while the MutexEvent was already enqueued. 890 // Using a stack (N=2 at minimum) of ParkEvents would take care of that problem. 891 // 892 // But of course the proper ultimate approach is to avoid schemes that require explicit 893 // sneaking or dependence on any any clever invariants or subtle implementation properties 894 // of Mutex-Monitor and instead directly address the underlying design flaw. 895 896 void Monitor::lock (Thread * Self) { 897 #ifdef CHECK_UNHANDLED_OOPS 898 // Clear unhandled oops so we get a crash right away. Only clear for non-vm 899 // or GC threads. 900 if (Self->is_Java_thread()) { 901 Self->clear_unhandled_oops(); 902 } 903 #endif // CHECK_UNHANDLED_OOPS 904 905 debug_only(check_prelock_state(Self)); 906 assert (_owner != Self , "invariant") ; 907 assert (_OnDeck != Self->_MutexEvent, "invariant") ; 908 909 if (TryFast()) { 910 Exeunt: 911 assert (ILocked(), "invariant") ; 912 assert (owner() == NULL, "invariant"); 913 set_owner (Self); 914 return ; 915 } 916 917 // The lock is contended ... 918 919 bool can_sneak = Self->is_VM_thread() && SafepointSynchronize::is_at_safepoint(); 920 if (can_sneak && _owner == NULL) { 921 // a java thread has locked the lock but has not entered the 922 // critical region -- let's just pretend we've locked the lock 923 // and go on. we note this with _snuck so we can also 924 // pretend to unlock when the time comes. 925 _snuck = true; 926 goto Exeunt ; 927 } 928 929 // Try a brief spin to avoid passing thru thread state transition ... 930 if (TrySpin (Self)) goto Exeunt ; 931 932 check_block_state(Self); 933 if (Self->is_Java_thread()) { 934 // Horribile dictu - we suffer through a state transition 935 assert(rank() > Mutex::special, "Potential deadlock with special or lesser rank mutex"); 936 ThreadBlockInVM tbivm ((JavaThread *) Self) ; 937 ILock (Self) ; 938 } else { 939 // Mirabile dictu 940 ILock (Self) ; 941 } 942 goto Exeunt ; 943 } 944 945 void Monitor::lock() { 946 this->lock(Thread::current()); 947 } 948 949 // Lock without safepoint check - a degenerate variant of lock(). 950 // Should ONLY be used by safepoint code and other code 951 // that is guaranteed not to block while running inside the VM. If this is called with 952 // thread state set to be in VM, the safepoint synchronization code will deadlock! 953 954 void Monitor::lock_without_safepoint_check (Thread * Self) { 955 assert (_owner != Self, "invariant") ; 956 ILock (Self) ; 957 assert (_owner == NULL, "invariant"); 958 set_owner (Self); 959 } 960 961 void Monitor::lock_without_safepoint_check () { 962 lock_without_safepoint_check (Thread::current()) ; 963 } 964 965 966 // Returns true if thread succeceed [sic] in grabbing the lock, otherwise false. 967 968 bool Monitor::try_lock() { 969 Thread * const Self = Thread::current(); 970 debug_only(check_prelock_state(Self)); 971 // assert(!thread->is_inside_signal_handler(), "don't lock inside signal handler"); 972 973 // Special case, where all Java threads are stopped. 974 // The lock may have been acquired but _owner is not yet set. 975 // In that case the VM thread can safely grab the lock. 976 // It strikes me this should appear _after the TryLock() fails, below. 977 bool can_sneak = Self->is_VM_thread() && SafepointSynchronize::is_at_safepoint(); 978 if (can_sneak && _owner == NULL) { 979 set_owner(Self); // Do not need to be atomic, since we are at a safepoint 980 _snuck = true; 981 return true; 982 } 983 984 if (TryLock()) { 985 // We got the lock 986 assert (_owner == NULL, "invariant"); 987 set_owner (Self); 988 return true; 989 } 990 return false; 991 } 992 993 void Monitor::unlock() { 994 assert (_owner == Thread::current(), "invariant") ; 995 assert (_OnDeck != Thread::current()->_MutexEvent , "invariant") ; 996 set_owner (NULL) ; 997 if (_snuck) { 998 assert(SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread(), "sneak"); 999 _snuck = false; 1000 return ; 1001 } 1002 IUnlock (false) ; 1003 } 1004 1005 // Yet another degenerate version of Monitor::lock() or lock_without_safepoint_check() 1006 // jvm_raw_lock() and _unlock() can be called by non-Java threads via JVM_RawMonitorEnter. 1007 // 1008 // There's no expectation that JVM_RawMonitors will interoperate properly with the native 1009 // Mutex-Monitor constructs. We happen to implement JVM_RawMonitors in terms of 1010 // native Mutex-Monitors simply as a matter of convenience. A simple abstraction layer 1011 // over a pthread_mutex_t would work equally as well, but require more platform-specific 1012 // code -- a "PlatformMutex". Alternatively, a simply layer over muxAcquire-muxRelease 1013 // would work too. 1014 // 1015 // Since the caller might be a foreign thread, we don't necessarily have a Thread.MutexEvent 1016 // instance available. Instead, we transiently allocate a ParkEvent on-demand if 1017 // we encounter contention. That ParkEvent remains associated with the thread 1018 // until it manages to acquire the lock, at which time we return the ParkEvent 1019 // to the global ParkEvent free list. This is correct and suffices for our purposes. 1020 // 1021 // Beware that the original jvm_raw_unlock() had a "_snuck" test but that 1022 // jvm_raw_lock() didn't have the corresponding test. I suspect that's an 1023 // oversight, but I've replicated the original suspect logic in the new code ... 1024 1025 void Monitor::jvm_raw_lock() { 1026 assert(rank() == native, "invariant"); 1027 1028 if (TryLock()) { 1029 Exeunt: 1030 assert (ILocked(), "invariant") ; 1031 assert (_owner == NULL, "invariant"); 1032 // This can potentially be called by non-java Threads. Thus, the ThreadLocalStorage 1033 // might return NULL. Don't call set_owner since it will break on an NULL owner 1034 // Consider installing a non-null "ANON" distinguished value instead of just NULL. 1035 _owner = ThreadLocalStorage::thread(); 1036 return ; 1037 } 1038 1039 if (TrySpin(NULL)) goto Exeunt ; 1040 1041 // slow-path - apparent contention 1042 // Allocate a ParkEvent for transient use. 1043 // The ParkEvent remains associated with this thread until 1044 // the time the thread manages to acquire the lock. 1045 ParkEvent * const ESelf = ParkEvent::Allocate(NULL) ; 1046 ESelf->reset() ; 1047 OrderAccess::storeload() ; 1048 1049 // Either Enqueue Self on cxq or acquire the outer lock. 1050 if (AcquireOrPush (ESelf)) { 1051 ParkEvent::Release (ESelf) ; // surrender the ParkEvent 1052 goto Exeunt ; 1053 } 1054 1055 // At any given time there is at most one ondeck thread. 1056 // ondeck implies not resident on cxq and not resident on EntryList 1057 // Only the OnDeck thread can try to acquire -- contended for -- the lock. 1058 // CONSIDER: use Self->OnDeck instead of m->OnDeck. 1059 for (;;) { 1060 if (_OnDeck == ESelf && TrySpin(NULL)) break ; 1061 ParkCommon (ESelf, 0) ; 1062 } 1063 1064 assert (_OnDeck == ESelf, "invariant") ; 1065 _OnDeck = NULL ; 1066 ParkEvent::Release (ESelf) ; // surrender the ParkEvent 1067 goto Exeunt ; 1068 } 1069 1070 void Monitor::jvm_raw_unlock() { 1071 // Nearly the same as Monitor::unlock() ... 1072 // directly set _owner instead of using set_owner(null) 1073 _owner = NULL ; 1074 if (_snuck) { // ??? 1075 assert(SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread(), "sneak"); 1076 _snuck = false; 1077 return ; 1078 } 1079 IUnlock(false) ; 1080 } 1081 1082 bool Monitor::wait(bool no_safepoint_check, long timeout, bool as_suspend_equivalent) { 1083 Thread * const Self = Thread::current() ; 1084 assert (_owner == Self, "invariant") ; 1085 assert (ILocked(), "invariant") ; 1086 1087 // as_suspend_equivalent logically implies !no_safepoint_check 1088 guarantee (!as_suspend_equivalent || !no_safepoint_check, "invariant") ; 1089 // !no_safepoint_check logically implies java_thread 1090 guarantee (no_safepoint_check || Self->is_Java_thread(), "invariant") ; 1091 1092 #ifdef ASSERT 1093 Monitor * least = get_least_ranked_lock_besides_this(Self->owned_locks()); 1094 assert(least != this, "Specification of get_least_... call above"); 1095 if (least != NULL && least->rank() <= special) { 1096 tty->print("Attempting to wait on monitor %s/%d while holding" 1097 " lock %s/%d -- possible deadlock", 1098 name(), rank(), least->name(), least->rank()); 1099 assert(false, "Shouldn't block(wait) while holding a lock of rank special"); 1100 } 1101 #endif // ASSERT 1102 1103 int wait_status ; 1104 // conceptually set the owner to NULL in anticipation of 1105 // abdicating the lock in wait 1106 set_owner(NULL); 1107 if (no_safepoint_check) { 1108 wait_status = IWait (Self, timeout) ; 1109 } else { 1110 assert (Self->is_Java_thread(), "invariant") ; 1111 JavaThread *jt = (JavaThread *)Self; 1112 1113 // Enter safepoint region - ornate and Rococo ... 1114 ThreadBlockInVM tbivm(jt); 1115 OSThreadWaitState osts(Self->osthread(), false /* not Object.wait() */); 1116 1117 if (as_suspend_equivalent) { 1118 jt->set_suspend_equivalent(); 1119 // cleared by handle_special_suspend_equivalent_condition() or 1120 // java_suspend_self() 1121 } 1122 1123 wait_status = IWait (Self, timeout) ; 1124 1125 // were we externally suspended while we were waiting? 1126 if (as_suspend_equivalent && jt->handle_special_suspend_equivalent_condition()) { 1127 // Our event wait has finished and we own the lock, but 1128 // while we were waiting another thread suspended us. We don't 1129 // want to hold the lock while suspended because that 1130 // would surprise the thread that suspended us. 1131 assert (ILocked(), "invariant") ; 1132 IUnlock (true) ; 1133 jt->java_suspend_self(); 1134 ILock (Self) ; 1135 assert (ILocked(), "invariant") ; 1136 } 1137 } 1138 1139 // Conceptually reestablish ownership of the lock. 1140 // The "real" lock -- the LockByte -- was reacquired by IWait(). 1141 assert (ILocked(), "invariant") ; 1142 assert (_owner == NULL, "invariant") ; 1143 set_owner (Self) ; 1144 return wait_status != 0 ; // return true IFF timeout 1145 } 1146 1147 Monitor::~Monitor() { 1148 assert ((UNS(_owner)|UNS(_LockWord.FullWord)|UNS(_EntryList)|UNS(_WaitSet)|UNS(_OnDeck)) == 0, "") ; 1149 } 1150 1151 void Monitor::ClearMonitor (Monitor * m, const char *name) { 1152 m->_owner = NULL ; 1153 m->_snuck = false ; 1154 if (name == NULL) { 1155 strcpy(m->_name, "UNKNOWN") ; 1156 } else { 1157 strncpy(m->_name, name, MONITOR_NAME_LEN - 1); 1158 m->_name[MONITOR_NAME_LEN - 1] = '\0'; 1159 } 1160 m->_LockWord.FullWord = 0 ; 1161 m->_EntryList = NULL ; 1162 m->_OnDeck = NULL ; 1163 m->_WaitSet = NULL ; 1164 m->_WaitLock[0] = 0 ; 1165 } 1166 1167 Monitor::Monitor() { ClearMonitor(this); } 1168 1169 Monitor::Monitor (int Rank, const char * name, bool allow_vm_block) { 1170 ClearMonitor (this, name) ; 1171 #ifdef ASSERT 1172 _allow_vm_block = allow_vm_block; 1173 _rank = Rank ; 1174 #endif 1175 } 1176 1177 Mutex::~Mutex() { 1178 assert ((UNS(_owner)|UNS(_LockWord.FullWord)|UNS(_EntryList)|UNS(_WaitSet)|UNS(_OnDeck)) == 0, "") ; 1179 } 1180 1181 Mutex::Mutex (int Rank, const char * name, bool allow_vm_block) { 1182 ClearMonitor ((Monitor *) this, name) ; 1183 #ifdef ASSERT 1184 _allow_vm_block = allow_vm_block; 1185 _rank = Rank ; 1186 #endif 1187 } 1188 1189 bool Monitor::owned_by_self() const { 1190 bool ret = _owner == Thread::current(); 1191 assert (!ret || _LockWord.Bytes[_LSBINDEX] != 0, "invariant") ; 1192 return ret; 1193 } 1194 1195 void Monitor::print_on_error(outputStream* st) const { 1196 st->print("[" PTR_FORMAT, this); 1197 st->print("] %s", _name); 1198 st->print(" - owner thread: " PTR_FORMAT, _owner); 1199 } 1200 1201 1202 1203 1204 // ---------------------------------------------------------------------------------- 1205 // Non-product code 1206 1207 #ifndef PRODUCT 1208 void Monitor::print_on(outputStream* st) const { 1209 st->print_cr("Mutex: [0x%lx/0x%lx] %s - owner: 0x%lx", this, _LockWord.FullWord, _name, _owner); 1210 } 1211 #endif 1212 1213 #ifndef PRODUCT 1214 #ifdef ASSERT 1215 Monitor * Monitor::get_least_ranked_lock(Monitor * locks) { 1216 Monitor *res, *tmp; 1217 for (res = tmp = locks; tmp != NULL; tmp = tmp->next()) { 1218 if (tmp->rank() < res->rank()) { 1219 res = tmp; 1220 } 1221 } 1222 if (!SafepointSynchronize::is_at_safepoint()) { 1223 // In this case, we expect the held locks to be 1224 // in increasing rank order (modulo any native ranks) 1225 for (tmp = locks; tmp != NULL; tmp = tmp->next()) { 1226 if (tmp->next() != NULL) { 1227 assert(tmp->rank() == Mutex::native || 1228 tmp->rank() <= tmp->next()->rank(), "mutex rank anomaly?"); 1229 } 1230 } 1231 } 1232 return res; 1233 } 1234 1235 Monitor* Monitor::get_least_ranked_lock_besides_this(Monitor* locks) { 1236 Monitor *res, *tmp; 1237 for (res = NULL, tmp = locks; tmp != NULL; tmp = tmp->next()) { 1238 if (tmp != this && (res == NULL || tmp->rank() < res->rank())) { 1239 res = tmp; 1240 } 1241 } 1242 if (!SafepointSynchronize::is_at_safepoint()) { 1243 // In this case, we expect the held locks to be 1244 // in increasing rank order (modulo any native ranks) 1245 for (tmp = locks; tmp != NULL; tmp = tmp->next()) { 1246 if (tmp->next() != NULL) { 1247 assert(tmp->rank() == Mutex::native || 1248 tmp->rank() <= tmp->next()->rank(), "mutex rank anomaly?"); 1249 } 1250 } 1251 } 1252 return res; 1253 } 1254 1255 1256 bool Monitor::contains(Monitor* locks, Monitor * lock) { 1257 for (; locks != NULL; locks = locks->next()) { 1258 if (locks == lock) 1259 return true; 1260 } 1261 return false; 1262 } 1263 #endif 1264 1265 // Called immediately after lock acquisition or release as a diagnostic 1266 // to track the lock-set of the thread and test for rank violations that 1267 // might indicate exposure to deadlock. 1268 // Rather like an EventListener for _owner (:>). 1269 1270 void Monitor::set_owner_implementation(Thread *new_owner) { 1271 // This function is solely responsible for maintaining 1272 // and checking the invariant that threads and locks 1273 // are in a 1/N relation, with some some locks unowned. 1274 // It uses the Mutex::_owner, Mutex::_next, and 1275 // Thread::_owned_locks fields, and no other function 1276 // changes those fields. 1277 // It is illegal to set the mutex from one non-NULL 1278 // owner to another--it must be owned by NULL as an 1279 // intermediate state. 1280 1281 if (new_owner != NULL) { 1282 // the thread is acquiring this lock 1283 1284 assert(new_owner == Thread::current(), "Should I be doing this?"); 1285 assert(_owner == NULL, "setting the owner thread of an already owned mutex"); 1286 _owner = new_owner; // set the owner 1287 1288 // link "this" into the owned locks list 1289 1290 #ifdef ASSERT // Thread::_owned_locks is under the same ifdef 1291 Monitor* locks = get_least_ranked_lock(new_owner->owned_locks()); 1292 // Mutex::set_owner_implementation is a friend of Thread 1293 1294 assert(this->rank() >= 0, "bad lock rank"); 1295 1296 // Deadlock avoidance rules require us to acquire Mutexes only in 1297 // a global total order. For example m1 is the lowest ranked mutex 1298 // that the thread holds and m2 is the mutex the thread is trying 1299 // to acquire, then deadlock avoidance rules require that the rank 1300 // of m2 be less than the rank of m1. 1301 // The rank Mutex::native is an exception in that it is not subject 1302 // to the verification rules. 1303 // Here are some further notes relating to mutex acquisition anomalies: 1304 // . under Solaris, the interrupt lock gets acquired when doing 1305 // profiling, so any lock could be held. 1306 // . it is also ok to acquire Safepoint_lock at the very end while we 1307 // already hold Terminator_lock - may happen because of periodic safepoints 1308 if (this->rank() != Mutex::native && 1309 this->rank() != Mutex::suspend_resume && 1310 locks != NULL && locks->rank() <= this->rank() && 1311 !SafepointSynchronize::is_at_safepoint() && 1312 this != Interrupt_lock && this != ProfileVM_lock && 1313 !(this == Safepoint_lock && contains(locks, Terminator_lock) && 1314 SafepointSynchronize::is_synchronizing())) { 1315 new_owner->print_owned_locks(); 1316 fatal(err_msg("acquiring lock %s/%d out of order with lock %s/%d -- " 1317 "possible deadlock", this->name(), this->rank(), 1318 locks->name(), locks->rank())); 1319 } 1320 1321 this->_next = new_owner->_owned_locks; 1322 new_owner->_owned_locks = this; 1323 #endif 1324 1325 } else { 1326 // the thread is releasing this lock 1327 1328 Thread* old_owner = _owner; 1329 debug_only(_last_owner = old_owner); 1330 1331 assert(old_owner != NULL, "removing the owner thread of an unowned mutex"); 1332 assert(old_owner == Thread::current(), "removing the owner thread of an unowned mutex"); 1333 1334 _owner = NULL; // set the owner 1335 1336 #ifdef ASSERT 1337 Monitor *locks = old_owner->owned_locks(); 1338 1339 // remove "this" from the owned locks list 1340 1341 Monitor *prev = NULL; 1342 bool found = false; 1343 for (; locks != NULL; prev = locks, locks = locks->next()) { 1344 if (locks == this) { 1345 found = true; 1346 break; 1347 } 1348 } 1349 assert(found, "Removing a lock not owned"); 1350 if (prev == NULL) { 1351 old_owner->_owned_locks = _next; 1352 } else { 1353 prev->_next = _next; 1354 } 1355 _next = NULL; 1356 #endif 1357 } 1358 } 1359 1360 1361 // Factored out common sanity checks for locking mutex'es. Used by lock() and try_lock() 1362 void Monitor::check_prelock_state(Thread *thread) { 1363 assert((!thread->is_Java_thread() || ((JavaThread *)thread)->thread_state() == _thread_in_vm) 1364 || rank() == Mutex::special, "wrong thread state for using locks"); 1365 if (StrictSafepointChecks) { 1366 if (thread->is_VM_thread() && !allow_vm_block()) { 1367 fatal(err_msg("VM thread using lock %s (not allowed to block on)", 1368 name())); 1369 } 1370 debug_only(if (rank() != Mutex::special) \ 1371 thread->check_for_valid_safepoint_state(false);) 1372 } 1373 if (thread->is_Watcher_thread()) { 1374 assert(!WatcherThread::watcher_thread()->has_crash_protection(), 1375 "locking not allowed when crash protection is set"); 1376 } 1377 } 1378 1379 void Monitor::check_block_state(Thread *thread) { 1380 if (!_allow_vm_block && thread->is_VM_thread()) { 1381 warning("VM thread blocked on lock"); 1382 print(); 1383 BREAKPOINT; 1384 } 1385 assert(_owner != thread, "deadlock: blocking on monitor owned by current thread"); 1386 } 1387 1388 #endif // PRODUCT