1 2 /* 3 * Copyright (c) 1998, 2010, 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 _LockWord.Bytes[_LSBINDEX] = 0 ; // drop outer lock 531 OrderAccess::storeload (); 532 ParkEvent * const w = _OnDeck ; 533 assert (RelaxAssert || w != Thread::current()->_MutexEvent, "invariant") ; 534 if (w != NULL) { 535 // Either we have a valid ondeck thread or ondeck is transiently "locked" 536 // by some exiting thread as it arranges for succession. The LSBit of 537 // OnDeck allows us to discriminate two cases. If the latter, the 538 // responsibility for progress and succession lies with that other thread. 539 // For good performance, we also depend on the fact that redundant unpark() 540 // operations are cheap. That is, repeated Unpark()ing of the ONDECK thread 541 // is inexpensive. This approach provides implicit futile wakeup throttling. 542 // Note that the referent "w" might be stale with respect to the lock. 543 // In that case the following unpark() is harmless and the worst that'll happen 544 // is a spurious return from a park() operation. Critically, if "w" _is stale, 545 // then progress is known to have occurred as that means the thread associated 546 // with "w" acquired the lock. In that case this thread need take no further 547 // action to guarantee progress. 548 if ((UNS(w) & _LBIT) == 0) w->unpark() ; 549 return ; 550 } 551 552 intptr_t cxq = _LockWord.FullWord ; 553 if (((cxq & ~_LBIT)|UNS(_EntryList)) == 0) { 554 return ; // normal fast-path exit - cxq and EntryList both empty 555 } 556 if (cxq & _LBIT) { 557 // Optional optimization ... 558 // Some other thread acquired the lock in the window since this 559 // thread released it. Succession is now that thread's responsibility. 560 return ; 561 } 562 563 Succession: 564 // Slow-path exit - this thread must ensure succession and progress. 565 // OnDeck serves as lock to protect cxq and EntryList. 566 // Only the holder of OnDeck can manipulate EntryList or detach the RATs from cxq. 567 // Avoid ABA - allow multiple concurrent producers (enqueue via push-CAS) 568 // but only one concurrent consumer (detacher of RATs). 569 // Consider protecting this critical section with schedctl on Solaris. 570 // Unlike a normal lock, however, the exiting thread "locks" OnDeck, 571 // picks a successor and marks that thread as OnDeck. That successor 572 // thread will then clear OnDeck once it eventually acquires the outer lock. 573 if (CASPTR (&_OnDeck, NULL, _LBIT) != UNS(NULL)) { 574 return ; 575 } 576 577 ParkEvent * List = _EntryList ; 578 if (List != NULL) { 579 // Transfer the head of the EntryList to the OnDeck position. 580 // Once OnDeck, a thread stays OnDeck until it acquires the lock. 581 // For a given lock there is at most OnDeck thread at any one instant. 582 WakeOne: 583 assert (List == _EntryList, "invariant") ; 584 ParkEvent * const w = List ; 585 assert (RelaxAssert || w != Thread::current()->_MutexEvent, "invariant") ; 586 _EntryList = w->ListNext ; 587 // as a diagnostic measure consider setting w->_ListNext = BAD 588 assert (UNS(_OnDeck) == _LBIT, "invariant") ; 589 _OnDeck = w ; // pass OnDeck to w. 590 // w will clear OnDeck once it acquires the outer lock 591 592 // Another optional optimization ... 593 // For heavily contended locks it's not uncommon that some other 594 // thread acquired the lock while this thread was arranging succession. 595 // Try to defer the unpark() operation - Delegate the responsibility 596 // for unpark()ing the OnDeck thread to the current or subsequent owners 597 // That is, the new owner is responsible for unparking the OnDeck thread. 598 OrderAccess::storeload() ; 599 cxq = _LockWord.FullWord ; 600 if (cxq & _LBIT) return ; 601 602 w->unpark() ; 603 return ; 604 } 605 606 cxq = _LockWord.FullWord ; 607 if ((cxq & ~_LBIT) != 0) { 608 // The EntryList is empty but the cxq is populated. 609 // drain RATs from cxq into EntryList 610 // Detach RATs segment with CAS and then merge into EntryList 611 for (;;) { 612 // optional optimization - if locked, the owner is responsible for succession 613 if (cxq & _LBIT) goto Punt ; 614 const intptr_t vfy = CASPTR (&_LockWord, cxq, cxq & _LBIT) ; 615 if (vfy == cxq) break ; 616 cxq = vfy ; 617 // Interference - LockWord changed - Just retry 618 // We can see concurrent interference from contending threads 619 // pushing themselves onto the cxq or from lock-unlock operations. 620 // From the perspective of this thread, EntryList is stable and 621 // the cxq is prepend-only -- the head is volatile but the interior 622 // of the cxq is stable. In theory if we encounter interference from threads 623 // pushing onto cxq we could simply break off the original cxq suffix and 624 // move that segment to the EntryList, avoiding a 2nd or multiple CAS attempts 625 // on the high-traffic LockWord variable. For instance lets say the cxq is "ABCD" 626 // when we first fetch cxq above. Between the fetch -- where we observed "A" 627 // -- and CAS -- where we attempt to CAS null over A -- "PQR" arrive, 628 // yielding cxq = "PQRABCD". In this case we could simply set A.ListNext 629 // null, leaving cxq = "PQRA" and transfer the "BCD" segment to the EntryList. 630 // Note too, that it's safe for this thread to traverse the cxq 631 // without taking any special concurrency precautions. 632 } 633 634 // We don't currently reorder the cxq segment as we move it onto 635 // the EntryList, but it might make sense to reverse the order 636 // or perhaps sort by thread priority. See the comments in 637 // synchronizer.cpp objectMonitor::exit(). 638 assert (_EntryList == NULL, "invariant") ; 639 _EntryList = List = (ParkEvent *)(cxq & ~_LBIT) ; 640 assert (List != NULL, "invariant") ; 641 goto WakeOne ; 642 } 643 644 // cxq|EntryList is empty. 645 // w == NULL implies that cxq|EntryList == NULL in the past. 646 // Possible race - rare inopportune interleaving. 647 // A thread could have added itself to cxq since this thread previously checked. 648 // Detect and recover by refetching cxq. 649 Punt: 650 assert (UNS(_OnDeck) == _LBIT, "invariant") ; 651 _OnDeck = NULL ; // Release inner lock. 652 OrderAccess::storeload(); // Dekker duality - pivot point 653 654 // Resample LockWord/cxq to recover from possible race. 655 // For instance, while this thread T1 held OnDeck, some other thread T2 might 656 // acquire the outer lock. Another thread T3 might try to acquire the outer 657 // lock, but encounter contention and enqueue itself on cxq. T2 then drops the 658 // outer lock, but skips succession as this thread T1 still holds OnDeck. 659 // T1 is and remains responsible for ensuring succession of T3. 660 // 661 // Note that we don't need to recheck EntryList, just cxq. 662 // If threads moved onto EntryList since we dropped OnDeck 663 // that implies some other thread forced succession. 664 cxq = _LockWord.FullWord ; 665 if ((cxq & ~_LBIT) != 0 && (cxq & _LBIT) == 0) { 666 goto Succession ; // potential race -- re-run succession 667 } 668 return ; 669 } 670 671 bool Monitor::notify() { 672 assert (_owner == Thread::current(), "invariant") ; 673 assert (ILocked(), "invariant") ; 674 if (_WaitSet == NULL) return true ; 675 NotifyCount ++ ; 676 677 // Transfer one thread from the WaitSet to the EntryList or cxq. 678 // Currently we just unlink the head of the WaitSet and prepend to the cxq. 679 // And of course we could just unlink it and unpark it, too, but 680 // in that case it'd likely impale itself on the reentry. 681 Thread::muxAcquire (_WaitLock, "notify:WaitLock") ; 682 ParkEvent * nfy = _WaitSet ; 683 if (nfy != NULL) { // DCL idiom 684 _WaitSet = nfy->ListNext ; 685 assert (nfy->Notified == 0, "invariant") ; 686 // push nfy onto the cxq 687 for (;;) { 688 const intptr_t v = _LockWord.FullWord ; 689 assert ((v & 0xFF) == _LBIT, "invariant") ; 690 nfy->ListNext = (ParkEvent *)(v & ~_LBIT); 691 if (CASPTR (&_LockWord, v, UNS(nfy)|_LBIT) == v) break; 692 // interference - _LockWord changed -- just retry 693 } 694 // Note that setting Notified before pushing nfy onto the cxq is 695 // also legal and safe, but the safety properties are much more 696 // subtle, so for the sake of code stewardship ... 697 OrderAccess::fence() ; 698 nfy->Notified = 1; 699 } 700 Thread::muxRelease (_WaitLock) ; 701 if (nfy != NULL && (NativeMonitorFlags & 16)) { 702 // Experimental code ... light up the wakee in the hope that this thread (the owner) 703 // will drop the lock just about the time the wakee comes ONPROC. 704 nfy->unpark() ; 705 } 706 assert (ILocked(), "invariant") ; 707 return true ; 708 } 709 710 // Currently notifyAll() transfers the waiters one-at-a-time from the waitset 711 // to the cxq. This could be done more efficiently with a single bulk en-mass transfer, 712 // but in practice notifyAll() for large #s of threads is rare and not time-critical. 713 // Beware too, that we invert the order of the waiters. Lets say that the 714 // waitset is "ABCD" and the cxq is "XYZ". After a notifyAll() the waitset 715 // will be empty and the cxq will be "DCBAXYZ". This is benign, of course. 716 717 bool Monitor::notify_all() { 718 assert (_owner == Thread::current(), "invariant") ; 719 assert (ILocked(), "invariant") ; 720 while (_WaitSet != NULL) notify() ; 721 return true ; 722 } 723 724 int Monitor::IWait (Thread * Self, jlong timo) { 725 assert (ILocked(), "invariant") ; 726 727 // Phases: 728 // 1. Enqueue Self on WaitSet - currently prepend 729 // 2. unlock - drop the outer lock 730 // 3. wait for either notification or timeout 731 // 4. lock - reentry - reacquire the outer lock 732 733 ParkEvent * const ESelf = Self->_MutexEvent ; 734 ESelf->Notified = 0 ; 735 ESelf->reset() ; 736 OrderAccess::fence() ; 737 738 // Add Self to WaitSet 739 // Ideally only the holder of the outer lock would manipulate the WaitSet - 740 // That is, the outer lock would implicitly protect the WaitSet. 741 // But if a thread in wait() encounters a timeout it will need to dequeue itself 742 // from the WaitSet _before it becomes the owner of the lock. We need to dequeue 743 // as the ParkEvent -- which serves as a proxy for the thread -- can't reside 744 // on both the WaitSet and the EntryList|cxq at the same time.. That is, a thread 745 // on the WaitSet can't be allowed to compete for the lock until it has managed to 746 // unlink its ParkEvent from WaitSet. Thus the need for WaitLock. 747 // Contention on the WaitLock is minimal. 748 // 749 // Another viable approach would be add another ParkEvent, "WaitEvent" to the 750 // thread class. The WaitSet would be composed of WaitEvents. Only the 751 // owner of the outer lock would manipulate the WaitSet. A thread in wait() 752 // could then compete for the outer lock, and then, if necessary, unlink itself 753 // from the WaitSet only after having acquired the outer lock. More precisely, 754 // there would be no WaitLock. A thread in in wait() would enqueue its WaitEvent 755 // on the WaitSet; release the outer lock; wait for either notification or timeout; 756 // reacquire the inner lock; and then, if needed, unlink itself from the WaitSet. 757 // 758 // Alternatively, a 2nd set of list link fields in the ParkEvent might suffice. 759 // One set would be for the WaitSet and one for the EntryList. 760 // We could also deconstruct the ParkEvent into a "pure" event and add a 761 // new immortal/TSM "ListElement" class that referred to ParkEvents. 762 // In that case we could have one ListElement on the WaitSet and another 763 // on the EntryList, with both referring to the same pure Event. 764 765 Thread::muxAcquire (_WaitLock, "wait:WaitLock:Add") ; 766 ESelf->ListNext = _WaitSet ; 767 _WaitSet = ESelf ; 768 Thread::muxRelease (_WaitLock) ; 769 770 // Release the outer lock 771 // We call IUnlock (RelaxAssert=true) as a thread T1 might 772 // enqueue itself on the WaitSet, call IUnlock(), drop the lock, 773 // and then stall before it can attempt to wake a successor. 774 // Some other thread T2 acquires the lock, and calls notify(), moving 775 // T1 from the WaitSet to the cxq. T2 then drops the lock. T1 resumes, 776 // and then finds *itself* on the cxq. During the course of a normal 777 // IUnlock() call a thread should _never find itself on the EntryList 778 // or cxq, but in the case of wait() it's possible. 779 // See synchronizer.cpp objectMonitor::wait(). 780 IUnlock (true) ; 781 782 // Wait for either notification or timeout 783 // Beware that in some circumstances we might propagate 784 // spurious wakeups back to the caller. 785 786 for (;;) { 787 if (ESelf->Notified) break ; 788 int err = ParkCommon (ESelf, timo) ; 789 if (err == OS_TIMEOUT || (NativeMonitorFlags & 1)) break ; 790 } 791 792 // Prepare for reentry - if necessary, remove ESelf from WaitSet 793 // ESelf can be: 794 // 1. Still on the WaitSet. This can happen if we exited the loop by timeout. 795 // 2. On the cxq or EntryList 796 // 3. Not resident on cxq, EntryList or WaitSet, but in the OnDeck position. 797 798 OrderAccess::fence() ; 799 int WasOnWaitSet = 0 ; 800 if (ESelf->Notified == 0) { 801 Thread::muxAcquire (_WaitLock, "wait:WaitLock:remove") ; 802 if (ESelf->Notified == 0) { // DCL idiom 803 assert (_OnDeck != ESelf, "invariant") ; // can't be both OnDeck and on WaitSet 804 // ESelf is resident on the WaitSet -- unlink it. 805 // A doubly-linked list would be better here so we can unlink in constant-time. 806 // We have to unlink before we potentially recontend as ESelf might otherwise 807 // end up on the cxq|EntryList -- it can't be on two lists at once. 808 ParkEvent * p = _WaitSet ; 809 ParkEvent * q = NULL ; // classic q chases p 810 while (p != NULL && p != ESelf) { 811 q = p ; 812 p = p->ListNext ; 813 } 814 assert (p == ESelf, "invariant") ; 815 if (p == _WaitSet) { // found at head 816 assert (q == NULL, "invariant") ; 817 _WaitSet = p->ListNext ; 818 } else { // found in interior 819 assert (q->ListNext == p, "invariant") ; 820 q->ListNext = p->ListNext ; 821 } 822 WasOnWaitSet = 1 ; // We were *not* notified but instead encountered timeout 823 } 824 Thread::muxRelease (_WaitLock) ; 825 } 826 827 // Reentry phase - reacquire the lock 828 if (WasOnWaitSet) { 829 // ESelf was previously on the WaitSet but we just unlinked it above 830 // because of a timeout. ESelf is not resident on any list and is not OnDeck 831 assert (_OnDeck != ESelf, "invariant") ; 832 ILock (Self) ; 833 } else { 834 // A prior notify() operation moved ESelf from the WaitSet to the cxq. 835 // ESelf is now on the cxq, EntryList or at the OnDeck position. 836 // The following fragment is extracted from Monitor::ILock() 837 for (;;) { 838 if (_OnDeck == ESelf && TrySpin(Self)) break ; 839 ParkCommon (ESelf, 0) ; 840 } 841 assert (_OnDeck == ESelf, "invariant") ; 842 _OnDeck = NULL ; 843 } 844 845 assert (ILocked(), "invariant") ; 846 return WasOnWaitSet != 0 ; // return true IFF timeout 847 } 848 849 850 // ON THE VMTHREAD SNEAKING PAST HELD LOCKS: 851 // In particular, there are certain types of global lock that may be held 852 // by a Java thread while it is blocked at a safepoint but before it has 853 // written the _owner field. These locks may be sneakily acquired by the 854 // VM thread during a safepoint to avoid deadlocks. Alternatively, one should 855 // identify all such locks, and ensure that Java threads never block at 856 // safepoints while holding them (_no_safepoint_check_flag). While it 857 // seems as though this could increase the time to reach a safepoint 858 // (or at least increase the mean, if not the variance), the latter 859 // approach might make for a cleaner, more maintainable JVM design. 860 // 861 // Sneaking is vile and reprehensible and should be excised at the 1st 862 // opportunity. It's possible that the need for sneaking could be obviated 863 // as follows. Currently, a thread might (a) while TBIVM, call pthread_mutex_lock 864 // or ILock() thus acquiring the "physical" lock underlying Monitor/Mutex. 865 // (b) stall at the TBIVM exit point as a safepoint is in effect. Critically, 866 // it'll stall at the TBIVM reentry state transition after having acquired the 867 // underlying lock, but before having set _owner and having entered the actual 868 // critical section. The lock-sneaking facility leverages that fact and allowed the 869 // VM thread to logically acquire locks that had already be physically locked by mutators 870 // but where mutators were known blocked by the reentry thread state transition. 871 // 872 // If we were to modify the Monitor-Mutex so that TBIVM state transitions tightly 873 // wrapped calls to park(), then we could likely do away with sneaking. We'd 874 // decouple lock acquisition and parking. The critical invariant to eliminating 875 // sneaking is to ensure that we never "physically" acquire the lock while TBIVM. 876 // An easy way to accomplish this is to wrap the park calls in a narrow TBIVM jacket. 877 // One difficulty with this approach is that the TBIVM wrapper could recurse and 878 // call lock() deep from within a lock() call, while the MutexEvent was already enqueued. 879 // Using a stack (N=2 at minimum) of ParkEvents would take care of that problem. 880 // 881 // But of course the proper ultimate approach is to avoid schemes that require explicit 882 // sneaking or dependence on any any clever invariants or subtle implementation properties 883 // of Mutex-Monitor and instead directly address the underlying design flaw. 884 885 void Monitor::lock (Thread * Self) { 886 #ifdef CHECK_UNHANDLED_OOPS 887 // Clear unhandled oops so we get a crash right away. Only clear for non-vm 888 // or GC threads. 889 if (Self->is_Java_thread()) { 890 Self->clear_unhandled_oops(); 891 } 892 #endif // CHECK_UNHANDLED_OOPS 893 894 debug_only(check_prelock_state(Self)); 895 assert (_owner != Self , "invariant") ; 896 assert (_OnDeck != Self->_MutexEvent, "invariant") ; 897 898 if (TryFast()) { 899 Exeunt: 900 assert (ILocked(), "invariant") ; 901 assert (owner() == NULL, "invariant"); 902 set_owner (Self); 903 return ; 904 } 905 906 // The lock is contended ... 907 908 bool can_sneak = Self->is_VM_thread() && SafepointSynchronize::is_at_safepoint(); 909 if (can_sneak && _owner == NULL) { 910 // a java thread has locked the lock but has not entered the 911 // critical region -- let's just pretend we've locked the lock 912 // and go on. we note this with _snuck so we can also 913 // pretend to unlock when the time comes. 914 _snuck = true; 915 goto Exeunt ; 916 } 917 918 // Try a brief spin to avoid passing thru thread state transition ... 919 if (TrySpin (Self)) goto Exeunt ; 920 921 check_block_state(Self); 922 if (Self->is_Java_thread()) { 923 // Horribile dictu - we suffer through a state transition 924 assert(rank() > Mutex::special, "Potential deadlock with special or lesser rank mutex"); 925 ThreadBlockInVM tbivm ((JavaThread *) Self) ; 926 ILock (Self) ; 927 } else { 928 // Mirabile dictu 929 ILock (Self) ; 930 } 931 goto Exeunt ; 932 } 933 934 void Monitor::lock() { 935 this->lock(Thread::current()); 936 } 937 938 // Lock without safepoint check - a degenerate variant of lock(). 939 // Should ONLY be used by safepoint code and other code 940 // that is guaranteed not to block while running inside the VM. If this is called with 941 // thread state set to be in VM, the safepoint synchronization code will deadlock! 942 943 void Monitor::lock_without_safepoint_check (Thread * Self) { 944 assert (_owner != Self, "invariant") ; 945 ILock (Self) ; 946 assert (_owner == NULL, "invariant"); 947 set_owner (Self); 948 } 949 950 void Monitor::lock_without_safepoint_check () { 951 lock_without_safepoint_check (Thread::current()) ; 952 } 953 954 955 // Returns true if thread succeceed [sic] in grabbing the lock, otherwise false. 956 957 bool Monitor::try_lock() { 958 Thread * const Self = Thread::current(); 959 debug_only(check_prelock_state(Self)); 960 // assert(!thread->is_inside_signal_handler(), "don't lock inside signal handler"); 961 962 // Special case, where all Java threads are stopped. 963 // The lock may have been acquired but _owner is not yet set. 964 // In that case the VM thread can safely grab the lock. 965 // It strikes me this should appear _after the TryLock() fails, below. 966 bool can_sneak = Self->is_VM_thread() && SafepointSynchronize::is_at_safepoint(); 967 if (can_sneak && _owner == NULL) { 968 set_owner(Self); // Do not need to be atomic, since we are at a safepoint 969 _snuck = true; 970 return true; 971 } 972 973 if (TryLock()) { 974 // We got the lock 975 assert (_owner == NULL, "invariant"); 976 set_owner (Self); 977 return true; 978 } 979 return false; 980 } 981 982 void Monitor::unlock() { 983 assert (_owner == Thread::current(), "invariant") ; 984 assert (_OnDeck != Thread::current()->_MutexEvent , "invariant") ; 985 set_owner (NULL) ; 986 if (_snuck) { 987 assert(SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread(), "sneak"); 988 _snuck = false; 989 return ; 990 } 991 IUnlock (false) ; 992 } 993 994 // Yet another degenerate version of Monitor::lock() or lock_without_safepoint_check() 995 // jvm_raw_lock() and _unlock() can be called by non-Java threads via JVM_RawMonitorEnter. 996 // 997 // There's no expectation that JVM_RawMonitors will interoperate properly with the native 998 // Mutex-Monitor constructs. We happen to implement JVM_RawMonitors in terms of 999 // native Mutex-Monitors simply as a matter of convenience. A simple abstraction layer 1000 // over a pthread_mutex_t would work equally as well, but require more platform-specific 1001 // code -- a "PlatformMutex". Alternatively, a simply layer over muxAcquire-muxRelease 1002 // would work too. 1003 // 1004 // Since the caller might be a foreign thread, we don't necessarily have a Thread.MutexEvent 1005 // instance available. Instead, we transiently allocate a ParkEvent on-demand if 1006 // we encounter contention. That ParkEvent remains associated with the thread 1007 // until it manages to acquire the lock, at which time we return the ParkEvent 1008 // to the global ParkEvent free list. This is correct and suffices for our purposes. 1009 // 1010 // Beware that the original jvm_raw_unlock() had a "_snuck" test but that 1011 // jvm_raw_lock() didn't have the corresponding test. I suspect that's an 1012 // oversight, but I've replicated the original suspect logic in the new code ... 1013 1014 void Monitor::jvm_raw_lock() { 1015 assert(rank() == native, "invariant"); 1016 1017 if (TryLock()) { 1018 Exeunt: 1019 assert (ILocked(), "invariant") ; 1020 assert (_owner == NULL, "invariant"); 1021 // This can potentially be called by non-java Threads. Thus, the ThreadLocalStorage 1022 // might return NULL. Don't call set_owner since it will break on an NULL owner 1023 // Consider installing a non-null "ANON" distinguished value instead of just NULL. 1024 _owner = ThreadLocalStorage::thread(); 1025 return ; 1026 } 1027 1028 if (TrySpin(NULL)) goto Exeunt ; 1029 1030 // slow-path - apparent contention 1031 // Allocate a ParkEvent for transient use. 1032 // The ParkEvent remains associated with this thread until 1033 // the time the thread manages to acquire the lock. 1034 ParkEvent * const ESelf = ParkEvent::Allocate(NULL) ; 1035 ESelf->reset() ; 1036 OrderAccess::storeload() ; 1037 1038 // Either Enqueue Self on cxq or acquire the outer lock. 1039 if (AcquireOrPush (ESelf)) { 1040 ParkEvent::Release (ESelf) ; // surrender the ParkEvent 1041 goto Exeunt ; 1042 } 1043 1044 // At any given time there is at most one ondeck thread. 1045 // ondeck implies not resident on cxq and not resident on EntryList 1046 // Only the OnDeck thread can try to acquire -- contended for -- the lock. 1047 // CONSIDER: use Self->OnDeck instead of m->OnDeck. 1048 for (;;) { 1049 if (_OnDeck == ESelf && TrySpin(NULL)) break ; 1050 ParkCommon (ESelf, 0) ; 1051 } 1052 1053 assert (_OnDeck == ESelf, "invariant") ; 1054 _OnDeck = NULL ; 1055 ParkEvent::Release (ESelf) ; // surrender the ParkEvent 1056 goto Exeunt ; 1057 } 1058 1059 void Monitor::jvm_raw_unlock() { 1060 // Nearly the same as Monitor::unlock() ... 1061 // directly set _owner instead of using set_owner(null) 1062 _owner = NULL ; 1063 if (_snuck) { // ??? 1064 assert(SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread(), "sneak"); 1065 _snuck = false; 1066 return ; 1067 } 1068 IUnlock(false) ; 1069 } 1070 1071 bool Monitor::wait(bool no_safepoint_check, long timeout, bool as_suspend_equivalent) { 1072 Thread * const Self = Thread::current() ; 1073 assert (_owner == Self, "invariant") ; 1074 assert (ILocked(), "invariant") ; 1075 1076 // as_suspend_equivalent logically implies !no_safepoint_check 1077 guarantee (!as_suspend_equivalent || !no_safepoint_check, "invariant") ; 1078 // !no_safepoint_check logically implies java_thread 1079 guarantee (no_safepoint_check || Self->is_Java_thread(), "invariant") ; 1080 1081 #ifdef ASSERT 1082 Monitor * least = get_least_ranked_lock_besides_this(Self->owned_locks()); 1083 assert(least != this, "Specification of get_least_... call above"); 1084 if (least != NULL && least->rank() <= special) { 1085 tty->print("Attempting to wait on monitor %s/%d while holding" 1086 " lock %s/%d -- possible deadlock", 1087 name(), rank(), least->name(), least->rank()); 1088 assert(false, "Shouldn't block(wait) while holding a lock of rank special"); 1089 } 1090 #endif // ASSERT 1091 1092 int wait_status ; 1093 // conceptually set the owner to NULL in anticipation of 1094 // abdicating the lock in wait 1095 set_owner(NULL); 1096 if (no_safepoint_check) { 1097 wait_status = IWait (Self, timeout) ; 1098 } else { 1099 assert (Self->is_Java_thread(), "invariant") ; 1100 JavaThread *jt = (JavaThread *)Self; 1101 1102 // Enter safepoint region - ornate and Rococo ... 1103 ThreadBlockInVM tbivm(jt); 1104 OSThreadWaitState osts(Self->osthread(), false /* not Object.wait() */); 1105 1106 if (as_suspend_equivalent) { 1107 jt->set_suspend_equivalent(); 1108 // cleared by handle_special_suspend_equivalent_condition() or 1109 // java_suspend_self() 1110 } 1111 1112 wait_status = IWait (Self, timeout) ; 1113 1114 // were we externally suspended while we were waiting? 1115 if (as_suspend_equivalent && jt->handle_special_suspend_equivalent_condition()) { 1116 // Our event wait has finished and we own the lock, but 1117 // while we were waiting another thread suspended us. We don't 1118 // want to hold the lock while suspended because that 1119 // would surprise the thread that suspended us. 1120 assert (ILocked(), "invariant") ; 1121 IUnlock (true) ; 1122 jt->java_suspend_self(); 1123 ILock (Self) ; 1124 assert (ILocked(), "invariant") ; 1125 } 1126 } 1127 1128 // Conceptually reestablish ownership of the lock. 1129 // The "real" lock -- the LockByte -- was reacquired by IWait(). 1130 assert (ILocked(), "invariant") ; 1131 assert (_owner == NULL, "invariant") ; 1132 set_owner (Self) ; 1133 return wait_status != 0 ; // return true IFF timeout 1134 } 1135 1136 Monitor::~Monitor() { 1137 assert ((UNS(_owner)|UNS(_LockWord.FullWord)|UNS(_EntryList)|UNS(_WaitSet)|UNS(_OnDeck)) == 0, "") ; 1138 } 1139 1140 void Monitor::ClearMonitor (Monitor * m, const char *name) { 1141 m->_owner = NULL ; 1142 m->_snuck = false ; 1143 if (name == NULL) { 1144 strcpy(m->_name, "UNKNOWN") ; 1145 } else { 1146 strncpy(m->_name, name, MONITOR_NAME_LEN - 1); 1147 m->_name[MONITOR_NAME_LEN - 1] = '\0'; 1148 } 1149 m->_LockWord.FullWord = 0 ; 1150 m->_EntryList = NULL ; 1151 m->_OnDeck = NULL ; 1152 m->_WaitSet = NULL ; 1153 m->_WaitLock[0] = 0 ; 1154 } 1155 1156 Monitor::Monitor() { ClearMonitor(this); } 1157 1158 Monitor::Monitor (int Rank, const char * name, bool allow_vm_block) { 1159 ClearMonitor (this, name) ; 1160 #ifdef ASSERT 1161 _allow_vm_block = allow_vm_block; 1162 _rank = Rank ; 1163 #endif 1164 } 1165 1166 Mutex::~Mutex() { 1167 assert ((UNS(_owner)|UNS(_LockWord.FullWord)|UNS(_EntryList)|UNS(_WaitSet)|UNS(_OnDeck)) == 0, "") ; 1168 } 1169 1170 Mutex::Mutex (int Rank, const char * name, bool allow_vm_block) { 1171 ClearMonitor ((Monitor *) this, name) ; 1172 #ifdef ASSERT 1173 _allow_vm_block = allow_vm_block; 1174 _rank = Rank ; 1175 #endif 1176 } 1177 1178 bool Monitor::owned_by_self() const { 1179 bool ret = _owner == Thread::current(); 1180 assert (!ret || _LockWord.Bytes[_LSBINDEX] != 0, "invariant") ; 1181 return ret; 1182 } 1183 1184 void Monitor::print_on_error(outputStream* st) const { 1185 st->print("[" PTR_FORMAT, this); 1186 st->print("] %s", _name); 1187 st->print(" - owner thread: " PTR_FORMAT, _owner); 1188 } 1189 1190 1191 1192 1193 // ---------------------------------------------------------------------------------- 1194 // Non-product code 1195 1196 #ifndef PRODUCT 1197 void Monitor::print_on(outputStream* st) const { 1198 st->print_cr("Mutex: [0x%lx/0x%lx] %s - owner: 0x%lx", this, _LockWord.FullWord, _name, _owner); 1199 } 1200 #endif 1201 1202 #ifndef PRODUCT 1203 #ifdef ASSERT 1204 Monitor * Monitor::get_least_ranked_lock(Monitor * locks) { 1205 Monitor *res, *tmp; 1206 for (res = tmp = locks; tmp != NULL; tmp = tmp->next()) { 1207 if (tmp->rank() < res->rank()) { 1208 res = tmp; 1209 } 1210 } 1211 if (!SafepointSynchronize::is_at_safepoint()) { 1212 // In this case, we expect the held locks to be 1213 // in increasing rank order (modulo any native ranks) 1214 for (tmp = locks; tmp != NULL; tmp = tmp->next()) { 1215 if (tmp->next() != NULL) { 1216 assert(tmp->rank() == Mutex::native || 1217 tmp->rank() <= tmp->next()->rank(), "mutex rank anomaly?"); 1218 } 1219 } 1220 } 1221 return res; 1222 } 1223 1224 Monitor* Monitor::get_least_ranked_lock_besides_this(Monitor* locks) { 1225 Monitor *res, *tmp; 1226 for (res = NULL, tmp = locks; tmp != NULL; tmp = tmp->next()) { 1227 if (tmp != this && (res == NULL || tmp->rank() < res->rank())) { 1228 res = tmp; 1229 } 1230 } 1231 if (!SafepointSynchronize::is_at_safepoint()) { 1232 // In this case, we expect the held locks to be 1233 // in increasing rank order (modulo any native ranks) 1234 for (tmp = locks; tmp != NULL; tmp = tmp->next()) { 1235 if (tmp->next() != NULL) { 1236 assert(tmp->rank() == Mutex::native || 1237 tmp->rank() <= tmp->next()->rank(), "mutex rank anomaly?"); 1238 } 1239 } 1240 } 1241 return res; 1242 } 1243 1244 1245 bool Monitor::contains(Monitor* locks, Monitor * lock) { 1246 for (; locks != NULL; locks = locks->next()) { 1247 if (locks == lock) 1248 return true; 1249 } 1250 return false; 1251 } 1252 #endif 1253 1254 // Called immediately after lock acquisition or release as a diagnostic 1255 // to track the lock-set of the thread and test for rank violations that 1256 // might indicate exposure to deadlock. 1257 // Rather like an EventListener for _owner (:>). 1258 1259 void Monitor::set_owner_implementation(Thread *new_owner) { 1260 // This function is solely responsible for maintaining 1261 // and checking the invariant that threads and locks 1262 // are in a 1/N relation, with some some locks unowned. 1263 // It uses the Mutex::_owner, Mutex::_next, and 1264 // Thread::_owned_locks fields, and no other function 1265 // changes those fields. 1266 // It is illegal to set the mutex from one non-NULL 1267 // owner to another--it must be owned by NULL as an 1268 // intermediate state. 1269 1270 if (new_owner != NULL) { 1271 // the thread is acquiring this lock 1272 1273 assert(new_owner == Thread::current(), "Should I be doing this?"); 1274 assert(_owner == NULL, "setting the owner thread of an already owned mutex"); 1275 _owner = new_owner; // set the owner 1276 1277 // link "this" into the owned locks list 1278 1279 #ifdef ASSERT // Thread::_owned_locks is under the same ifdef 1280 Monitor* locks = get_least_ranked_lock(new_owner->owned_locks()); 1281 // Mutex::set_owner_implementation is a friend of Thread 1282 1283 assert(this->rank() >= 0, "bad lock rank"); 1284 1285 if (LogMultipleMutexLocking && locks != NULL) { 1286 Events::log("thread " INTPTR_FORMAT " locks %s, already owns %s", new_owner, name(), locks->name()); 1287 } 1288 1289 // Deadlock avoidance rules require us to acquire Mutexes only in 1290 // a global total order. For example m1 is the lowest ranked mutex 1291 // that the thread holds and m2 is the mutex the thread is trying 1292 // to acquire, then deadlock avoidance rules require that the rank 1293 // of m2 be less than the rank of m1. 1294 // The rank Mutex::native is an exception in that it is not subject 1295 // to the verification rules. 1296 // Here are some further notes relating to mutex acquisition anomalies: 1297 // . under Solaris, the interrupt lock gets acquired when doing 1298 // profiling, so any lock could be held. 1299 // . it is also ok to acquire Safepoint_lock at the very end while we 1300 // already hold Terminator_lock - may happen because of periodic safepoints 1301 if (this->rank() != Mutex::native && 1302 this->rank() != Mutex::suspend_resume && 1303 locks != NULL && locks->rank() <= this->rank() && 1304 !SafepointSynchronize::is_at_safepoint() && 1305 this != Interrupt_lock && this != ProfileVM_lock && 1306 !(this == Safepoint_lock && contains(locks, Terminator_lock) && 1307 SafepointSynchronize::is_synchronizing())) { 1308 new_owner->print_owned_locks(); 1309 fatal(err_msg("acquiring lock %s/%d out of order with lock %s/%d -- " 1310 "possible deadlock", this->name(), this->rank(), 1311 locks->name(), locks->rank())); 1312 } 1313 1314 this->_next = new_owner->_owned_locks; 1315 new_owner->_owned_locks = this; 1316 #endif 1317 1318 } else { 1319 // the thread is releasing this lock 1320 1321 Thread* old_owner = _owner; 1322 debug_only(_last_owner = old_owner); 1323 1324 assert(old_owner != NULL, "removing the owner thread of an unowned mutex"); 1325 assert(old_owner == Thread::current(), "removing the owner thread of an unowned mutex"); 1326 1327 _owner = NULL; // set the owner 1328 1329 #ifdef ASSERT 1330 Monitor *locks = old_owner->owned_locks(); 1331 1332 if (LogMultipleMutexLocking && locks != this) { 1333 Events::log("thread " INTPTR_FORMAT " unlocks %s, still owns %s", old_owner, this->name(), locks->name()); 1334 } 1335 1336 // remove "this" from the owned locks list 1337 1338 Monitor *prev = NULL; 1339 bool found = false; 1340 for (; locks != NULL; prev = locks, locks = locks->next()) { 1341 if (locks == this) { 1342 found = true; 1343 break; 1344 } 1345 } 1346 assert(found, "Removing a lock not owned"); 1347 if (prev == NULL) { 1348 old_owner->_owned_locks = _next; 1349 } else { 1350 prev->_next = _next; 1351 } 1352 _next = NULL; 1353 #endif 1354 } 1355 } 1356 1357 1358 // Factored out common sanity checks for locking mutex'es. Used by lock() and try_lock() 1359 void Monitor::check_prelock_state(Thread *thread) { 1360 assert((!thread->is_Java_thread() || ((JavaThread *)thread)->thread_state() == _thread_in_vm) 1361 || rank() == Mutex::special, "wrong thread state for using locks"); 1362 if (StrictSafepointChecks) { 1363 if (thread->is_VM_thread() && !allow_vm_block()) { 1364 fatal(err_msg("VM thread using lock %s (not allowed to block on)", 1365 name())); 1366 } 1367 debug_only(if (rank() != Mutex::special) \ 1368 thread->check_for_valid_safepoint_state(false);) 1369 } 1370 } 1371 1372 void Monitor::check_block_state(Thread *thread) { 1373 if (!_allow_vm_block && thread->is_VM_thread()) { 1374 warning("VM thread blocked on lock"); 1375 print(); 1376 BREAKPOINT; 1377 } 1378 assert(_owner != thread, "deadlock: blocking on monitor owned by current thread"); 1379 } 1380 1381 #endif // PRODUCT