1 /* 2 * Copyright (c) 1998, 2015, Oracle and/or its affiliates. All rights reserved. 3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 4 * 5 * This code is free software; you can redistribute it and/or modify it 6 * under the terms of the GNU General Public License version 2 only, as 7 * published by the Free Software Foundation. 8 * 9 * This code is distributed in the hope that it will be useful, but WITHOUT 10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 12 * version 2 for more details (a copy is included in the LICENSE file that 13 * accompanied this code). 14 * 15 * You should have received a copy of the GNU General Public License version 16 * 2 along with this work; if not, write to the Free Software Foundation, 17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 18 * 19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 20 * or visit www.oracle.com if you need additional information or have any 21 * questions. 22 * 23 */ 24 25 #include "precompiled.hpp" 26 #include "runtime/atomic.inline.hpp" 27 #include "runtime/mutex.hpp" 28 #include "runtime/orderAccess.inline.hpp" 29 #include "runtime/osThread.hpp" 30 #include "runtime/thread.inline.hpp" 31 #include "utilities/events.hpp" 32 #ifdef TARGET_OS_FAMILY_linux 33 # include "mutex_linux.inline.hpp" 34 #endif 35 #ifdef TARGET_OS_FAMILY_solaris 36 # include "mutex_solaris.inline.hpp" 37 #endif 38 #ifdef TARGET_OS_FAMILY_windows 39 # include "mutex_windows.inline.hpp" 40 #endif 41 #ifdef TARGET_OS_FAMILY_bsd 42 # include "mutex_bsd.inline.hpp" 43 #endif 44 45 // 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 46 // 47 // Native Monitor-Mutex locking - theory of operations 48 // 49 // * Native Monitors are completely unrelated to Java-level monitors, 50 // although the "back-end" slow-path implementations share a common lineage. 51 // See objectMonitor:: in synchronizer.cpp. 52 // Native Monitors do *not* support nesting or recursion but otherwise 53 // they're basically Hoare-flavor monitors. 54 // 55 // * A thread acquires ownership of a Monitor/Mutex by CASing the LockByte 56 // in the _LockWord from zero to non-zero. Note that the _Owner field 57 // is advisory and is used only to verify that the thread calling unlock() 58 // is indeed the last thread to have acquired the lock. 59 // 60 // * Contending threads "push" themselves onto the front of the contention 61 // queue -- called the cxq -- with CAS and then spin/park. 62 // The _LockWord contains the LockByte as well as the pointer to the head 63 // of the cxq. Colocating the LockByte with the cxq precludes certain races. 64 // 65 // * Using a separately addressable LockByte allows for CAS:MEMBAR or CAS:0 66 // idioms. We currently use MEMBAR in the uncontended unlock() path, as 67 // MEMBAR often has less latency than CAS. If warranted, we could switch to 68 // a CAS:0 mode, using timers to close the resultant race, as is done 69 // with Java Monitors in synchronizer.cpp. 70 // 71 // See the following for a discussion of the relative cost of atomics (CAS) 72 // MEMBAR, and ways to eliminate such instructions from the common-case paths: 73 // -- http://blogs.sun.com/dave/entry/biased_locking_in_hotspot 74 // -- http://blogs.sun.com/dave/resource/MustangSync.pdf 75 // -- http://blogs.sun.com/dave/resource/synchronization-public2.pdf 76 // -- synchronizer.cpp 77 // 78 // * Overall goals - desiderata 79 // 1. Minimize context switching 80 // 2. Minimize lock migration 81 // 3. Minimize CPI -- affinity and locality 82 // 4. Minimize the execution of high-latency instructions such as CAS or MEMBAR 83 // 5. Minimize outer lock hold times 84 // 6. Behave gracefully on a loaded system 85 // 86 // * Thread flow and list residency: 87 // 88 // Contention queue --> EntryList --> OnDeck --> Owner --> !Owner 89 // [..resident on monitor list..] 90 // [...........contending..................] 91 // 92 // -- The contention queue (cxq) contains recently-arrived threads (RATs). 93 // Threads on the cxq eventually drain into the EntryList. 94 // -- Invariant: a thread appears on at most one list -- cxq, EntryList 95 // or WaitSet -- at any one time. 96 // -- For a given monitor there can be at most one "OnDeck" thread at any 97 // given time but if needbe this particular invariant could be relaxed. 98 // 99 // * The WaitSet and EntryList linked lists are composed of ParkEvents. 100 // I use ParkEvent instead of threads as ParkEvents are immortal and 101 // type-stable, meaning we can safely unpark() a possibly stale 102 // list element in the unlock()-path. (That's benign). 103 // 104 // * Succession policy - providing for progress: 105 // 106 // As necessary, the unlock()ing thread identifies, unlinks, and unparks 107 // an "heir presumptive" tentative successor thread from the EntryList. 108 // This becomes the so-called "OnDeck" thread, of which there can be only 109 // one at any given time for a given monitor. The wakee will recontend 110 // for ownership of monitor. 111 // 112 // Succession is provided for by a policy of competitive handoff. 113 // The exiting thread does _not_ grant or pass ownership to the 114 // successor thread. (This is also referred to as "handoff" succession"). 115 // Instead the exiting thread releases ownership and possibly wakes 116 // a successor, so the successor can (re)compete for ownership of the lock. 117 // 118 // Competitive handoff provides excellent overall throughput at the expense 119 // of short-term fairness. If fairness is a concern then one remedy might 120 // be to add an AcquireCounter field to the monitor. After a thread acquires 121 // the lock it will decrement the AcquireCounter field. When the count 122 // reaches 0 the thread would reset the AcquireCounter variable, abdicate 123 // the lock directly to some thread on the EntryList, and then move itself to the 124 // tail of the EntryList. 125 // 126 // But in practice most threads engage or otherwise participate in resource 127 // bounded producer-consumer relationships, so lock domination is not usually 128 // a practical concern. Recall too, that in general it's easier to construct 129 // a fair lock from a fast lock, but not vice-versa. 130 // 131 // * The cxq can have multiple concurrent "pushers" but only one concurrent 132 // detaching thread. This mechanism is immune from the ABA corruption. 133 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious. 134 // We use OnDeck as a pseudo-lock to enforce the at-most-one detaching 135 // thread constraint. 136 // 137 // * Taken together, the cxq and the EntryList constitute or form a 138 // single logical queue of threads stalled trying to acquire the lock. 139 // We use two distinct lists to reduce heat on the list ends. 140 // Threads in lock() enqueue onto cxq while threads in unlock() will 141 // dequeue from the EntryList. (c.f. Michael Scott's "2Q" algorithm). 142 // A key desideratum is to minimize queue & monitor metadata manipulation 143 // that occurs while holding the "outer" monitor lock -- that is, we want to 144 // minimize monitor lock holds times. 145 // 146 // The EntryList is ordered by the prevailing queue discipline and 147 // can be organized in any convenient fashion, such as a doubly-linked list or 148 // a circular doubly-linked list. If we need a priority queue then something akin 149 // to Solaris' sleepq would work nicely. Viz., 150 // -- http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c. 151 // -- http://cvs.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/uts/common/os/sleepq.c 152 // Queue discipline is enforced at ::unlock() time, when the unlocking thread 153 // drains the cxq into the EntryList, and orders or reorders the threads on the 154 // EntryList accordingly. 155 // 156 // Barring "lock barging", this mechanism provides fair cyclic ordering, 157 // somewhat similar to an elevator-scan. 158 // 159 // * OnDeck 160 // -- For a given monitor there can be at most one OnDeck thread at any given 161 // instant. The OnDeck thread is contending for the lock, but has been 162 // unlinked from the EntryList and cxq by some previous unlock() operations. 163 // Once a thread has been designated the OnDeck thread it will remain so 164 // until it manages to acquire the lock -- being OnDeck is a stable property. 165 // -- Threads on the EntryList or cxq are _not allowed to attempt lock acquisition. 166 // -- OnDeck also serves as an "inner lock" as follows. Threads in unlock() will, after 167 // having cleared the LockByte and dropped the outer lock, attempt to "trylock" 168 // OnDeck by CASing the field from null to non-null. If successful, that thread 169 // is then responsible for progress and succession and can use CAS to detach and 170 // drain the cxq into the EntryList. By convention, only this thread, the holder of 171 // the OnDeck inner lock, can manipulate the EntryList or detach and drain the 172 // RATs on the cxq into the EntryList. This avoids ABA corruption on the cxq as 173 // we allow multiple concurrent "push" operations but restrict detach concurrency 174 // to at most one thread. Having selected and detached a successor, the thread then 175 // changes the OnDeck to refer to that successor, and then unparks the successor. 176 // That successor will eventually acquire the lock and clear OnDeck. Beware 177 // that the OnDeck usage as a lock is asymmetric. A thread in unlock() transiently 178 // "acquires" OnDeck, performs queue manipulations, passes OnDeck to some successor, 179 // and then the successor eventually "drops" OnDeck. Note that there's never 180 // any sense of contention on the inner lock, however. Threads never contend 181 // or wait for the inner lock. 182 // -- OnDeck provides for futile wakeup throttling a described in section 3.3 of 183 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf 184 // In a sense, OnDeck subsumes the ObjectMonitor _Succ and ObjectWaiter 185 // TState fields found in Java-level objectMonitors. (See synchronizer.cpp). 186 // 187 // * Waiting threads reside on the WaitSet list -- wait() puts 188 // the caller onto the WaitSet. Notify() or notifyAll() simply 189 // transfers threads from the WaitSet to either the EntryList or cxq. 190 // Subsequent unlock() operations will eventually unpark the notifyee. 191 // Unparking a notifee in notify() proper is inefficient - if we were to do so 192 // it's likely the notifyee would simply impale itself on the lock held 193 // by the notifier. 194 // 195 // * The mechanism is obstruction-free in that if the holder of the transient 196 // OnDeck lock in unlock() is preempted or otherwise stalls, other threads 197 // can still acquire and release the outer lock and continue to make progress. 198 // At worst, waking of already blocked contending threads may be delayed, 199 // but nothing worse. (We only use "trylock" operations on the inner OnDeck 200 // lock). 201 // 202 // * Note that thread-local storage must be initialized before a thread 203 // uses Native monitors or mutexes. The native monitor-mutex subsystem 204 // depends on Thread::current(). 205 // 206 // * The monitor synchronization subsystem avoids the use of native 207 // synchronization primitives except for the narrow platform-specific 208 // park-unpark abstraction. See the comments in os_solaris.cpp regarding 209 // the semantics of park-unpark. Put another way, this monitor implementation 210 // depends only on atomic operations and park-unpark. The monitor subsystem 211 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the 212 // underlying OS manages the READY<->RUN transitions. 213 // 214 // * The memory consistency model provide by lock()-unlock() is at least as 215 // strong or stronger than the Java Memory model defined by JSR-133. 216 // That is, we guarantee at least entry consistency, if not stronger. 217 // See http://g.oswego.edu/dl/jmm/cookbook.html. 218 // 219 // * Thread:: currently contains a set of purpose-specific ParkEvents: 220 // _MutexEvent, _ParkEvent, etc. A better approach might be to do away with 221 // the purpose-specific ParkEvents and instead implement a general per-thread 222 // stack of available ParkEvents which we could provision on-demand. The 223 // stack acts as a local cache to avoid excessive calls to ParkEvent::Allocate() 224 // and ::Release(). A thread would simply pop an element from the local stack before it 225 // enqueued or park()ed. When the contention was over the thread would 226 // push the no-longer-needed ParkEvent back onto its stack. 227 // 228 // * A slightly reduced form of ILock() and IUnlock() have been partially 229 // model-checked (Murphi) for safety and progress at T=1,2,3 and 4. 230 // It'd be interesting to see if TLA/TLC could be useful as well. 231 // 232 // * Mutex-Monitor is a low-level "leaf" subsystem. That is, the monitor 233 // code should never call other code in the JVM that might itself need to 234 // acquire monitors or mutexes. That's true *except* in the case of the 235 // ThreadBlockInVM state transition wrappers. The ThreadBlockInVM DTOR handles 236 // mutator reentry (ingress) by checking for a pending safepoint in which case it will 237 // call SafepointSynchronize::block(), which in turn may call Safepoint_lock->lock(), etc. 238 // In that particular case a call to lock() for a given Monitor can end up recursively 239 // calling lock() on another monitor. While distasteful, this is largely benign 240 // as the calls come from jacket that wraps lock(), and not from deep within lock() itself. 241 // 242 // It's unfortunate that native mutexes and thread state transitions were convolved. 243 // They're really separate concerns and should have remained that way. Melding 244 // them together was facile -- a bit too facile. The current implementation badly 245 // conflates the two concerns. 246 // 247 // * TODO-FIXME: 248 // 249 // -- Add DTRACE probes for contended acquire, contended acquired, contended unlock 250 // We should also add DTRACE probes in the ParkEvent subsystem for 251 // Park-entry, Park-exit, and Unpark. 252 // 253 // -- We have an excess of mutex-like constructs in the JVM, namely: 254 // 1. objectMonitors for Java-level synchronization (synchronizer.cpp) 255 // 2. low-level muxAcquire and muxRelease 256 // 3. low-level spinAcquire and spinRelease 257 // 4. native Mutex:: and Monitor:: 258 // 5. jvm_raw_lock() and _unlock() 259 // 6. JVMTI raw monitors -- distinct from (5) despite having a confusingly 260 // similar name. 261 // 262 // 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 263 264 265 // CASPTR() uses the canonical argument order that dominates in the literature. 266 // Our internal cmpxchg_ptr() uses a bastardized ordering to accommodate Sun .il templates. 267 268 #define CASPTR(a, c, s) \ 269 intptr_t(Atomic::cmpxchg_ptr((void *)(s), (void *)(a), (void *)(c))) 270 #define UNS(x) (uintptr_t(x)) 271 #define TRACE(m) \ 272 { \ 273 static volatile int ctr = 0; \ 274 int x = ++ctr; \ 275 if ((x & (x - 1)) == 0) { \ 276 ::printf("%d:%s\n", x, #m); \ 277 ::fflush(stdout); \ 278 } \ 279 } 280 281 // Simplistic low-quality Marsaglia SHIFT-XOR RNG. 282 // Bijective except for the trailing mask operation. 283 // Useful for spin loops as the compiler can't optimize it away. 284 285 static inline jint MarsagliaXORV(jint x) { 286 if (x == 0) x = 1|os::random(); 287 x ^= x << 6; 288 x ^= ((unsigned)x) >> 21; 289 x ^= x << 7; 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 // It's probably wise to spin only if we *actually* blocked 498 // CONSIDER: check the lockbyte, if it remains set then 499 // preemptively drain the cxq into the EntryList. 500 // The best place and time to perform queue operations -- lock metadata -- 501 // is _before having acquired the outer lock, while waiting for the lock to drop. 502 ParkCommon(ESelf, 0); 503 } 504 505 assert(_OnDeck == ESelf, "invariant"); 506 _OnDeck = NULL; 507 508 // Note that we current drop the inner lock (clear OnDeck) in the slow-path 509 // epilogue immediately after having acquired the outer lock. 510 // But instead we could consider the following optimizations: 511 // A. Shift or defer dropping the inner lock until the subsequent IUnlock() operation. 512 // This might avoid potential reacquisition of the inner lock in IUlock(). 513 // B. While still holding the inner lock, attempt to opportunistically select 514 // and unlink the next ONDECK thread from the EntryList. 515 // If successful, set ONDECK to refer to that thread, otherwise clear ONDECK. 516 // It's critical that the select-and-unlink operation run in constant-time as 517 // it executes when holding the outer lock and may artificially increase the 518 // effective length of the critical section. 519 // Note that (A) and (B) are tantamount to succession by direct handoff for 520 // the inner lock. 521 goto Exeunt; 522 } 523 524 void Monitor::IUnlock(bool RelaxAssert) { 525 assert(ILocked(), "invariant"); 526 // Conceptually we need a MEMBAR #storestore|#loadstore barrier or fence immediately 527 // before the store that releases the lock. Crucially, all the stores and loads in the 528 // critical section must be globally visible before the store of 0 into the lock-word 529 // that releases the lock becomes globally visible. That is, memory accesses in the 530 // critical section should not be allowed to bypass or overtake the following ST that 531 // releases the lock. As such, to prevent accesses within the critical section 532 // from "leaking" out, we need a release fence between the critical section and the 533 // store that releases the lock. In practice that release barrier is elided on 534 // platforms with strong memory models such as TSO. 535 // 536 // Note that the OrderAccess::storeload() fence that appears after unlock store 537 // provides for progress conditions and succession and is _not related to exclusion 538 // safety or lock release consistency. 539 OrderAccess::release_store(&_LockWord.Bytes[_LSBINDEX], 0); // drop outer lock 540 541 OrderAccess::storeload(); 542 ParkEvent * const w = _OnDeck; 543 assert(RelaxAssert || w != Thread::current()->_MutexEvent, "invariant"); 544 if (w != NULL) { 545 // Either we have a valid ondeck thread or ondeck is transiently "locked" 546 // by some exiting thread as it arranges for succession. The LSBit of 547 // OnDeck allows us to discriminate two cases. If the latter, the 548 // responsibility for progress and succession lies with that other thread. 549 // For good performance, we also depend on the fact that redundant unpark() 550 // operations are cheap. That is, repeated Unpark()ing of the ONDECK thread 551 // is inexpensive. This approach provides implicit futile wakeup throttling. 552 // Note that the referent "w" might be stale with respect to the lock. 553 // In that case the following unpark() is harmless and the worst that'll happen 554 // is a spurious return from a park() operation. Critically, if "w" _is stale, 555 // then progress is known to have occurred as that means the thread associated 556 // with "w" acquired the lock. In that case this thread need take no further 557 // action to guarantee progress. 558 if ((UNS(w) & _LBIT) == 0) w->unpark(); 559 return; 560 } 561 562 intptr_t cxq = _LockWord.FullWord; 563 if (((cxq & ~_LBIT)|UNS(_EntryList)) == 0) { 564 return; // normal fast-path exit - cxq and EntryList both empty 565 } 566 if (cxq & _LBIT) { 567 // Optional optimization ... 568 // Some other thread acquired the lock in the window since this 569 // thread released it. Succession is now that thread's responsibility. 570 return; 571 } 572 573 Succession: 574 // Slow-path exit - this thread must ensure succession and progress. 575 // OnDeck serves as lock to protect cxq and EntryList. 576 // Only the holder of OnDeck can manipulate EntryList or detach the RATs from cxq. 577 // Avoid ABA - allow multiple concurrent producers (enqueue via push-CAS) 578 // but only one concurrent consumer (detacher of RATs). 579 // Consider protecting this critical section with schedctl on Solaris. 580 // Unlike a normal lock, however, the exiting thread "locks" OnDeck, 581 // picks a successor and marks that thread as OnDeck. That successor 582 // thread will then clear OnDeck once it eventually acquires the outer lock. 583 if (CASPTR (&_OnDeck, NULL, _LBIT) != UNS(NULL)) { 584 return; 585 } 586 587 ParkEvent * List = _EntryList; 588 if (List != NULL) { 589 // Transfer the head of the EntryList to the OnDeck position. 590 // Once OnDeck, a thread stays OnDeck until it acquires the lock. 591 // For a given lock there is at most OnDeck thread at any one instant. 592 WakeOne: 593 assert(List == _EntryList, "invariant"); 594 ParkEvent * const w = List; 595 assert(RelaxAssert || w != Thread::current()->_MutexEvent, "invariant"); 596 _EntryList = w->ListNext; 597 // as a diagnostic measure consider setting w->_ListNext = BAD 598 assert(UNS(_OnDeck) == _LBIT, "invariant"); 599 _OnDeck = w; // pass OnDeck to w. 600 // w will clear OnDeck once it acquires the outer lock 601 602 // Another optional optimization ... 603 // For heavily contended locks it's not uncommon that some other 604 // thread acquired the lock while this thread was arranging succession. 605 // Try to defer the unpark() operation - Delegate the responsibility 606 // for unpark()ing the OnDeck thread to the current or subsequent owners 607 // That is, the new owner is responsible for unparking the OnDeck thread. 608 OrderAccess::storeload(); 609 cxq = _LockWord.FullWord; 610 if (cxq & _LBIT) return; 611 612 w->unpark(); 613 return; 614 } 615 616 cxq = _LockWord.FullWord; 617 if ((cxq & ~_LBIT) != 0) { 618 // The EntryList is empty but the cxq is populated. 619 // drain RATs from cxq into EntryList 620 // Detach RATs segment with CAS and then merge into EntryList 621 for (;;) { 622 // optional optimization - if locked, the owner is responsible for succession 623 if (cxq & _LBIT) goto Punt; 624 const intptr_t vfy = CASPTR(&_LockWord, cxq, cxq & _LBIT); 625 if (vfy == cxq) break; 626 cxq = vfy; 627 // Interference - LockWord changed - Just retry 628 // We can see concurrent interference from contending threads 629 // pushing themselves onto the cxq or from lock-unlock operations. 630 // From the perspective of this thread, EntryList is stable and 631 // the cxq is prepend-only -- the head is volatile but the interior 632 // of the cxq is stable. In theory if we encounter interference from threads 633 // pushing onto cxq we could simply break off the original cxq suffix and 634 // move that segment to the EntryList, avoiding a 2nd or multiple CAS attempts 635 // on the high-traffic LockWord variable. For instance lets say the cxq is "ABCD" 636 // when we first fetch cxq above. Between the fetch -- where we observed "A" 637 // -- and CAS -- where we attempt to CAS null over A -- "PQR" arrive, 638 // yielding cxq = "PQRABCD". In this case we could simply set A.ListNext 639 // null, leaving cxq = "PQRA" and transfer the "BCD" segment to the EntryList. 640 // Note too, that it's safe for this thread to traverse the cxq 641 // without taking any special concurrency precautions. 642 } 643 644 // We don't currently reorder the cxq segment as we move it onto 645 // the EntryList, but it might make sense to reverse the order 646 // or perhaps sort by thread priority. See the comments in 647 // synchronizer.cpp objectMonitor::exit(). 648 assert(_EntryList == NULL, "invariant"); 649 _EntryList = List = (ParkEvent *)(cxq & ~_LBIT); 650 assert(List != NULL, "invariant"); 651 goto WakeOne; 652 } 653 654 // cxq|EntryList is empty. 655 // w == NULL implies that cxq|EntryList == NULL in the past. 656 // Possible race - rare inopportune interleaving. 657 // A thread could have added itself to cxq since this thread previously checked. 658 // Detect and recover by refetching cxq. 659 Punt: 660 assert(UNS(_OnDeck) == _LBIT, "invariant"); 661 _OnDeck = NULL; // Release inner lock. 662 OrderAccess::storeload(); // Dekker duality - pivot point 663 664 // Resample LockWord/cxq to recover from possible race. 665 // For instance, while this thread T1 held OnDeck, some other thread T2 might 666 // acquire the outer lock. Another thread T3 might try to acquire the outer 667 // lock, but encounter contention and enqueue itself on cxq. T2 then drops the 668 // outer lock, but skips succession as this thread T1 still holds OnDeck. 669 // T1 is and remains responsible for ensuring succession of T3. 670 // 671 // Note that we don't need to recheck EntryList, just cxq. 672 // If threads moved onto EntryList since we dropped OnDeck 673 // that implies some other thread forced succession. 674 cxq = _LockWord.FullWord; 675 if ((cxq & ~_LBIT) != 0 && (cxq & _LBIT) == 0) { 676 goto Succession; // potential race -- re-run succession 677 } 678 return; 679 } 680 681 bool Monitor::notify() { 682 assert(_owner == Thread::current(), "invariant"); 683 assert(ILocked(), "invariant"); 684 if (_WaitSet == NULL) return true; 685 NotifyCount++; 686 687 // Transfer one thread from the WaitSet to the EntryList or cxq. 688 // Currently we just unlink the head of the WaitSet and prepend to the cxq. 689 // And of course we could just unlink it and unpark it, too, but 690 // in that case it'd likely impale itself on the reentry. 691 Thread::muxAcquire(_WaitLock, "notify:WaitLock"); 692 ParkEvent * nfy = _WaitSet; 693 if (nfy != NULL) { // DCL idiom 694 _WaitSet = nfy->ListNext; 695 assert(nfy->Notified == 0, "invariant"); 696 // push nfy onto the cxq 697 for (;;) { 698 const intptr_t v = _LockWord.FullWord; 699 assert((v & 0xFF) == _LBIT, "invariant"); 700 nfy->ListNext = (ParkEvent *)(v & ~_LBIT); 701 if (CASPTR (&_LockWord, v, UNS(nfy)|_LBIT) == v) break; 702 // interference - _LockWord changed -- just retry 703 } 704 // Note that setting Notified before pushing nfy onto the cxq is 705 // also legal and safe, but the safety properties are much more 706 // subtle, so for the sake of code stewardship ... 707 OrderAccess::fence(); 708 nfy->Notified = 1; 709 } 710 Thread::muxRelease(_WaitLock); 711 if (nfy != NULL && (NativeMonitorFlags & 16)) { 712 // Experimental code ... light up the wakee in the hope that this thread (the owner) 713 // will drop the lock just about the time the wakee comes ONPROC. 714 nfy->unpark(); 715 } 716 assert(ILocked(), "invariant"); 717 return true; 718 } 719 720 // Currently notifyAll() transfers the waiters one-at-a-time from the waitset 721 // to the cxq. This could be done more efficiently with a single bulk en-mass transfer, 722 // but in practice notifyAll() for large #s of threads is rare and not time-critical. 723 // Beware too, that we invert the order of the waiters. Lets say that the 724 // waitset is "ABCD" and the cxq is "XYZ". After a notifyAll() the waitset 725 // will be empty and the cxq will be "DCBAXYZ". This is benign, of course. 726 727 bool Monitor::notify_all() { 728 assert(_owner == Thread::current(), "invariant"); 729 assert(ILocked(), "invariant"); 730 while (_WaitSet != NULL) notify(); 731 return true; 732 } 733 734 int Monitor::IWait(Thread * Self, jlong timo) { 735 assert(ILocked(), "invariant"); 736 737 // Phases: 738 // 1. Enqueue Self on WaitSet - currently prepend 739 // 2. unlock - drop the outer lock 740 // 3. wait for either notification or timeout 741 // 4. lock - reentry - reacquire the outer lock 742 743 ParkEvent * const ESelf = Self->_MutexEvent; 744 ESelf->Notified = 0; 745 ESelf->reset(); 746 OrderAccess::fence(); 747 748 // Add Self to WaitSet 749 // Ideally only the holder of the outer lock would manipulate the WaitSet - 750 // That is, the outer lock would implicitly protect the WaitSet. 751 // But if a thread in wait() encounters a timeout it will need to dequeue itself 752 // from the WaitSet _before it becomes the owner of the lock. We need to dequeue 753 // as the ParkEvent -- which serves as a proxy for the thread -- can't reside 754 // on both the WaitSet and the EntryList|cxq at the same time.. That is, a thread 755 // on the WaitSet can't be allowed to compete for the lock until it has managed to 756 // unlink its ParkEvent from WaitSet. Thus the need for WaitLock. 757 // Contention on the WaitLock is minimal. 758 // 759 // Another viable approach would be add another ParkEvent, "WaitEvent" to the 760 // thread class. The WaitSet would be composed of WaitEvents. Only the 761 // owner of the outer lock would manipulate the WaitSet. A thread in wait() 762 // could then compete for the outer lock, and then, if necessary, unlink itself 763 // from the WaitSet only after having acquired the outer lock. More precisely, 764 // there would be no WaitLock. A thread in in wait() would enqueue its WaitEvent 765 // on the WaitSet; release the outer lock; wait for either notification or timeout; 766 // reacquire the inner lock; and then, if needed, unlink itself from the WaitSet. 767 // 768 // Alternatively, a 2nd set of list link fields in the ParkEvent might suffice. 769 // One set would be for the WaitSet and one for the EntryList. 770 // We could also deconstruct the ParkEvent into a "pure" event and add a 771 // new immortal/TSM "ListElement" class that referred to ParkEvents. 772 // In that case we could have one ListElement on the WaitSet and another 773 // on the EntryList, with both referring to the same pure Event. 774 775 Thread::muxAcquire(_WaitLock, "wait:WaitLock:Add"); 776 ESelf->ListNext = _WaitSet; 777 _WaitSet = ESelf; 778 Thread::muxRelease(_WaitLock); 779 780 // Release the outer lock 781 // We call IUnlock (RelaxAssert=true) as a thread T1 might 782 // enqueue itself on the WaitSet, call IUnlock(), drop the lock, 783 // and then stall before it can attempt to wake a successor. 784 // Some other thread T2 acquires the lock, and calls notify(), moving 785 // T1 from the WaitSet to the cxq. T2 then drops the lock. T1 resumes, 786 // and then finds *itself* on the cxq. During the course of a normal 787 // IUnlock() call a thread should _never find itself on the EntryList 788 // or cxq, but in the case of wait() it's possible. 789 // See synchronizer.cpp objectMonitor::wait(). 790 IUnlock(true); 791 792 // Wait for either notification or timeout 793 // Beware that in some circumstances we might propagate 794 // spurious wakeups back to the caller. 795 796 for (;;) { 797 if (ESelf->Notified) break; 798 int err = ParkCommon(ESelf, timo); 799 if (err == OS_TIMEOUT || (NativeMonitorFlags & 1)) break; 800 } 801 802 // Prepare for reentry - if necessary, remove ESelf from WaitSet 803 // ESelf can be: 804 // 1. Still on the WaitSet. This can happen if we exited the loop by timeout. 805 // 2. On the cxq or EntryList 806 // 3. Not resident on cxq, EntryList or WaitSet, but in the OnDeck position. 807 808 OrderAccess::fence(); 809 int WasOnWaitSet = 0; 810 if (ESelf->Notified == 0) { 811 Thread::muxAcquire(_WaitLock, "wait:WaitLock:remove"); 812 if (ESelf->Notified == 0) { // DCL idiom 813 assert(_OnDeck != ESelf, "invariant"); // can't be both OnDeck and on WaitSet 814 // ESelf is resident on the WaitSet -- unlink it. 815 // A doubly-linked list would be better here so we can unlink in constant-time. 816 // We have to unlink before we potentially recontend as ESelf might otherwise 817 // end up on the cxq|EntryList -- it can't be on two lists at once. 818 ParkEvent * p = _WaitSet; 819 ParkEvent * q = NULL; // classic q chases p 820 while (p != NULL && p != ESelf) { 821 q = p; 822 p = p->ListNext; 823 } 824 assert(p == ESelf, "invariant"); 825 if (p == _WaitSet) { // found at head 826 assert(q == NULL, "invariant"); 827 _WaitSet = p->ListNext; 828 } else { // found in interior 829 assert(q->ListNext == p, "invariant"); 830 q->ListNext = p->ListNext; 831 } 832 WasOnWaitSet = 1; // We were *not* notified but instead encountered timeout 833 } 834 Thread::muxRelease(_WaitLock); 835 } 836 837 // Reentry phase - reacquire the lock 838 if (WasOnWaitSet) { 839 // ESelf was previously on the WaitSet but we just unlinked it above 840 // because of a timeout. ESelf is not resident on any list and is not OnDeck 841 assert(_OnDeck != ESelf, "invariant"); 842 ILock(Self); 843 } else { 844 // A prior notify() operation moved ESelf from the WaitSet to the cxq. 845 // ESelf is now on the cxq, EntryList or at the OnDeck position. 846 // The following fragment is extracted from Monitor::ILock() 847 for (;;) { 848 if (_OnDeck == ESelf && TrySpin(Self)) break; 849 ParkCommon(ESelf, 0); 850 } 851 assert(_OnDeck == ESelf, "invariant"); 852 _OnDeck = NULL; 853 } 854 855 assert(ILocked(), "invariant"); 856 return WasOnWaitSet != 0; // return true IFF timeout 857 } 858 859 860 // ON THE VMTHREAD SNEAKING PAST HELD LOCKS: 861 // In particular, there are certain types of global lock that may be held 862 // by a Java thread while it is blocked at a safepoint but before it has 863 // written the _owner field. These locks may be sneakily acquired by the 864 // VM thread during a safepoint to avoid deadlocks. Alternatively, one should 865 // identify all such locks, and ensure that Java threads never block at 866 // safepoints while holding them (_no_safepoint_check_flag). While it 867 // seems as though this could increase the time to reach a safepoint 868 // (or at least increase the mean, if not the variance), the latter 869 // approach might make for a cleaner, more maintainable JVM design. 870 // 871 // Sneaking is vile and reprehensible and should be excised at the 1st 872 // opportunity. It's possible that the need for sneaking could be obviated 873 // as follows. Currently, a thread might (a) while TBIVM, call pthread_mutex_lock 874 // or ILock() thus acquiring the "physical" lock underlying Monitor/Mutex. 875 // (b) stall at the TBIVM exit point as a safepoint is in effect. Critically, 876 // it'll stall at the TBIVM reentry state transition after having acquired the 877 // underlying lock, but before having set _owner and having entered the actual 878 // critical section. The lock-sneaking facility leverages that fact and allowed the 879 // VM thread to logically acquire locks that had already be physically locked by mutators 880 // but where mutators were known blocked by the reentry thread state transition. 881 // 882 // If we were to modify the Monitor-Mutex so that TBIVM state transitions tightly 883 // wrapped calls to park(), then we could likely do away with sneaking. We'd 884 // decouple lock acquisition and parking. The critical invariant to eliminating 885 // sneaking is to ensure that we never "physically" acquire the lock while TBIVM. 886 // An easy way to accomplish this is to wrap the park calls in a narrow TBIVM jacket. 887 // One difficulty with this approach is that the TBIVM wrapper could recurse and 888 // call lock() deep from within a lock() call, while the MutexEvent was already enqueued. 889 // Using a stack (N=2 at minimum) of ParkEvents would take care of that problem. 890 // 891 // But of course the proper ultimate approach is to avoid schemes that require explicit 892 // sneaking or dependence on any any clever invariants or subtle implementation properties 893 // of Mutex-Monitor and instead directly address the underlying design flaw. 894 895 void Monitor::lock(Thread * Self) { 896 // Ensure that the Monitor requires/allows safepoint checks. 897 assert(_safepoint_check_required != Monitor::_safepoint_check_never, 898 "This lock should never have a safepoint check: %s", name()); 899 900 #ifdef CHECK_UNHANDLED_OOPS 901 // Clear unhandled oops so we get a crash right away. Only clear for non-vm 902 // or GC threads. 903 if (Self->is_Java_thread()) { 904 Self->clear_unhandled_oops(); 905 } 906 #endif // CHECK_UNHANDLED_OOPS 907 908 debug_only(check_prelock_state(Self)); 909 assert(_owner != Self, "invariant"); 910 assert(_OnDeck != Self->_MutexEvent, "invariant"); 911 912 if (TryFast()) { 913 Exeunt: 914 assert(ILocked(), "invariant"); 915 assert(owner() == NULL, "invariant"); 916 set_owner(Self); 917 return; 918 } 919 920 // The lock is contended ... 921 922 bool can_sneak = Self->is_VM_thread() && SafepointSynchronize::is_at_safepoint(); 923 if (can_sneak && _owner == NULL) { 924 // a java thread has locked the lock but has not entered the 925 // critical region -- let's just pretend we've locked the lock 926 // and go on. we note this with _snuck so we can also 927 // pretend to unlock when the time comes. 928 _snuck = true; 929 goto Exeunt; 930 } 931 932 // Try a brief spin to avoid passing thru thread state transition ... 933 if (TrySpin(Self)) goto Exeunt; 934 935 check_block_state(Self); 936 if (Self->is_Java_thread()) { 937 // Horrible dictu - we suffer through a state transition 938 assert(rank() > Mutex::special, "Potential deadlock with special or lesser rank mutex"); 939 ThreadBlockInVM tbivm((JavaThread *) Self); 940 ILock(Self); 941 } else { 942 // Mirabile dictu 943 ILock(Self); 944 } 945 goto Exeunt; 946 } 947 948 void Monitor::lock() { 949 this->lock(Thread::current()); 950 } 951 952 // Lock without safepoint check - a degenerate variant of lock(). 953 // Should ONLY be used by safepoint code and other code 954 // that is guaranteed not to block while running inside the VM. If this is called with 955 // thread state set to be in VM, the safepoint synchronization code will deadlock! 956 957 void Monitor::lock_without_safepoint_check(Thread * Self) { 958 // Ensure that the Monitor does not require or allow safepoint checks. 959 assert(_safepoint_check_required != Monitor::_safepoint_check_always, 960 "This lock should always have a safepoint check: %s", name()); 961 assert(_owner != Self, "invariant"); 962 ILock(Self); 963 assert(_owner == NULL, "invariant"); 964 set_owner(Self); 965 } 966 967 void Monitor::lock_without_safepoint_check() { 968 lock_without_safepoint_check(Thread::current()); 969 } 970 971 972 // Returns true if thread succeeds in grabbing the lock, otherwise false. 973 974 bool Monitor::try_lock() { 975 Thread * const Self = Thread::current(); 976 debug_only(check_prelock_state(Self)); 977 // assert(!thread->is_inside_signal_handler(), "don't lock inside signal handler"); 978 979 // Special case, where all Java threads are stopped. 980 // The lock may have been acquired but _owner is not yet set. 981 // In that case the VM thread can safely grab the lock. 982 // It strikes me this should appear _after the TryLock() fails, below. 983 bool can_sneak = Self->is_VM_thread() && SafepointSynchronize::is_at_safepoint(); 984 if (can_sneak && _owner == NULL) { 985 set_owner(Self); // Do not need to be atomic, since we are at a safepoint 986 _snuck = true; 987 return true; 988 } 989 990 if (TryLock()) { 991 // We got the lock 992 assert(_owner == NULL, "invariant"); 993 set_owner(Self); 994 return true; 995 } 996 return false; 997 } 998 999 void Monitor::unlock() { 1000 assert(_owner == Thread::current(), "invariant"); 1001 assert(_OnDeck != Thread::current()->_MutexEvent, "invariant"); 1002 set_owner(NULL); 1003 if (_snuck) { 1004 assert(SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread(), "sneak"); 1005 _snuck = false; 1006 return; 1007 } 1008 IUnlock(false); 1009 } 1010 1011 // Yet another degenerate version of Monitor::lock() or lock_without_safepoint_check() 1012 // jvm_raw_lock() and _unlock() can be called by non-Java threads via JVM_RawMonitorEnter. 1013 // 1014 // There's no expectation that JVM_RawMonitors will interoperate properly with the native 1015 // Mutex-Monitor constructs. We happen to implement JVM_RawMonitors in terms of 1016 // native Mutex-Monitors simply as a matter of convenience. A simple abstraction layer 1017 // over a pthread_mutex_t would work equally as well, but require more platform-specific 1018 // code -- a "PlatformMutex". Alternatively, a simply layer over muxAcquire-muxRelease 1019 // would work too. 1020 // 1021 // Since the caller might be a foreign thread, we don't necessarily have a Thread.MutexEvent 1022 // instance available. Instead, we transiently allocate a ParkEvent on-demand if 1023 // we encounter contention. That ParkEvent remains associated with the thread 1024 // until it manages to acquire the lock, at which time we return the ParkEvent 1025 // to the global ParkEvent free list. This is correct and suffices for our purposes. 1026 // 1027 // Beware that the original jvm_raw_unlock() had a "_snuck" test but that 1028 // jvm_raw_lock() didn't have the corresponding test. I suspect that's an 1029 // oversight, but I've replicated the original suspect logic in the new code ... 1030 1031 void Monitor::jvm_raw_lock() { 1032 assert(rank() == native, "invariant"); 1033 1034 if (TryLock()) { 1035 Exeunt: 1036 assert(ILocked(), "invariant"); 1037 assert(_owner == NULL, "invariant"); 1038 // This can potentially be called by non-java Threads. Thus, the Thread::current_or_null() 1039 // might return NULL. Don't call set_owner since it will break on an NULL owner 1040 // Consider installing a non-null "ANON" distinguished value instead of just NULL. 1041 _owner = Thread::current_or_null(); 1042 return; 1043 } 1044 1045 if (TrySpin(NULL)) goto Exeunt; 1046 1047 // slow-path - apparent contention 1048 // Allocate a ParkEvent for transient use. 1049 // The ParkEvent remains associated with this thread until 1050 // the time the thread manages to acquire the lock. 1051 ParkEvent * const ESelf = ParkEvent::Allocate(NULL); 1052 ESelf->reset(); 1053 OrderAccess::storeload(); 1054 1055 // Either Enqueue Self on cxq or acquire the outer lock. 1056 if (AcquireOrPush (ESelf)) { 1057 ParkEvent::Release(ESelf); // surrender the ParkEvent 1058 goto Exeunt; 1059 } 1060 1061 // At any given time there is at most one ondeck thread. 1062 // ondeck implies not resident on cxq and not resident on EntryList 1063 // Only the OnDeck thread can try to acquire -- contended for -- the lock. 1064 // CONSIDER: use Self->OnDeck instead of m->OnDeck. 1065 for (;;) { 1066 if (_OnDeck == ESelf && TrySpin(NULL)) break; 1067 ParkCommon(ESelf, 0); 1068 } 1069 1070 assert(_OnDeck == ESelf, "invariant"); 1071 _OnDeck = NULL; 1072 ParkEvent::Release(ESelf); // surrender the ParkEvent 1073 goto Exeunt; 1074 } 1075 1076 void Monitor::jvm_raw_unlock() { 1077 // Nearly the same as Monitor::unlock() ... 1078 // directly set _owner instead of using set_owner(null) 1079 _owner = NULL; 1080 if (_snuck) { // ??? 1081 assert(SafepointSynchronize::is_at_safepoint() && Thread::current()->is_VM_thread(), "sneak"); 1082 _snuck = false; 1083 return; 1084 } 1085 IUnlock(false); 1086 } 1087 1088 bool Monitor::wait(bool no_safepoint_check, long timeout, 1089 bool as_suspend_equivalent) { 1090 // Make sure safepoint checking is used properly. 1091 assert(!(_safepoint_check_required == Monitor::_safepoint_check_never && no_safepoint_check == false), 1092 "This lock should never have a safepoint check: %s", name()); 1093 assert(!(_safepoint_check_required == Monitor::_safepoint_check_always && no_safepoint_check == true), 1094 "This lock should always have a safepoint check: %s", name()); 1095 1096 Thread * const Self = Thread::current(); 1097 assert(_owner == Self, "invariant"); 1098 assert(ILocked(), "invariant"); 1099 1100 // as_suspend_equivalent logically implies !no_safepoint_check 1101 guarantee(!as_suspend_equivalent || !no_safepoint_check, "invariant"); 1102 // !no_safepoint_check logically implies java_thread 1103 guarantee(no_safepoint_check || Self->is_Java_thread(), "invariant"); 1104 1105 #ifdef ASSERT 1106 Monitor * least = get_least_ranked_lock_besides_this(Self->owned_locks()); 1107 assert(least != this, "Specification of get_least_... call above"); 1108 if (least != NULL && least->rank() <= special) { 1109 tty->print("Attempting to wait on monitor %s/%d while holding" 1110 " lock %s/%d -- possible deadlock", 1111 name(), rank(), least->name(), least->rank()); 1112 assert(false, "Shouldn't block(wait) while holding a lock of rank special"); 1113 } 1114 #endif // ASSERT 1115 1116 int wait_status; 1117 // conceptually set the owner to NULL in anticipation of 1118 // abdicating the lock in wait 1119 set_owner(NULL); 1120 if (no_safepoint_check) { 1121 wait_status = IWait(Self, timeout); 1122 } else { 1123 assert(Self->is_Java_thread(), "invariant"); 1124 JavaThread *jt = (JavaThread *)Self; 1125 1126 // Enter safepoint region - ornate and Rococo ... 1127 ThreadBlockInVM tbivm(jt); 1128 OSThreadWaitState osts(Self->osthread(), false /* not Object.wait() */); 1129 1130 if (as_suspend_equivalent) { 1131 jt->set_suspend_equivalent(); 1132 // cleared by handle_special_suspend_equivalent_condition() or 1133 // java_suspend_self() 1134 } 1135 1136 wait_status = IWait(Self, timeout); 1137 1138 // were we externally suspended while we were waiting? 1139 if (as_suspend_equivalent && jt->handle_special_suspend_equivalent_condition()) { 1140 // Our event wait has finished and we own the lock, but 1141 // while we were waiting another thread suspended us. We don't 1142 // want to hold the lock while suspended because that 1143 // would surprise the thread that suspended us. 1144 assert(ILocked(), "invariant"); 1145 IUnlock(true); 1146 jt->java_suspend_self(); 1147 ILock(Self); 1148 assert(ILocked(), "invariant"); 1149 } 1150 } 1151 1152 // Conceptually reestablish ownership of the lock. 1153 // The "real" lock -- the LockByte -- was reacquired by IWait(). 1154 assert(ILocked(), "invariant"); 1155 assert(_owner == NULL, "invariant"); 1156 set_owner(Self); 1157 return wait_status != 0; // return true IFF timeout 1158 } 1159 1160 Monitor::~Monitor() { 1161 assert((UNS(_owner)|UNS(_LockWord.FullWord)|UNS(_EntryList)|UNS(_WaitSet)|UNS(_OnDeck)) == 0, ""); 1162 } 1163 1164 void Monitor::ClearMonitor(Monitor * m, const char *name) { 1165 m->_owner = NULL; 1166 m->_snuck = false; 1167 if (name == NULL) { 1168 strcpy(m->_name, "UNKNOWN"); 1169 } else { 1170 strncpy(m->_name, name, MONITOR_NAME_LEN - 1); 1171 m->_name[MONITOR_NAME_LEN - 1] = '\0'; 1172 } 1173 m->_LockWord.FullWord = 0; 1174 m->_EntryList = NULL; 1175 m->_OnDeck = NULL; 1176 m->_WaitSet = NULL; 1177 m->_WaitLock[0] = 0; 1178 } 1179 1180 Monitor::Monitor() { ClearMonitor(this); } 1181 1182 Monitor::Monitor(int Rank, const char * name, bool allow_vm_block, 1183 SafepointCheckRequired safepoint_check_required) { 1184 ClearMonitor(this, name); 1185 #ifdef ASSERT 1186 _allow_vm_block = allow_vm_block; 1187 _rank = Rank; 1188 NOT_PRODUCT(_safepoint_check_required = safepoint_check_required;) 1189 #endif 1190 } 1191 1192 Mutex::~Mutex() { 1193 assert((UNS(_owner)|UNS(_LockWord.FullWord)|UNS(_EntryList)|UNS(_WaitSet)|UNS(_OnDeck)) == 0, ""); 1194 } 1195 1196 Mutex::Mutex(int Rank, const char * name, bool allow_vm_block, 1197 SafepointCheckRequired safepoint_check_required) { 1198 ClearMonitor((Monitor *) this, name); 1199 #ifdef ASSERT 1200 _allow_vm_block = allow_vm_block; 1201 _rank = Rank; 1202 NOT_PRODUCT(_safepoint_check_required = safepoint_check_required;) 1203 #endif 1204 } 1205 1206 bool Monitor::owned_by_self() const { 1207 bool ret = _owner == Thread::current(); 1208 assert(!ret || _LockWord.Bytes[_LSBINDEX] != 0, "invariant"); 1209 return ret; 1210 } 1211 1212 void Monitor::print_on_error(outputStream* st) const { 1213 st->print("[" PTR_FORMAT, p2i(this)); 1214 st->print("] %s", _name); 1215 st->print(" - owner thread: " PTR_FORMAT, p2i(_owner)); 1216 } 1217 1218 1219 1220 1221 // ---------------------------------------------------------------------------------- 1222 // Non-product code 1223 1224 #ifndef PRODUCT 1225 void Monitor::print_on(outputStream* st) const { 1226 st->print_cr("Mutex: [" PTR_FORMAT "/" PTR_FORMAT "] %s - owner: " PTR_FORMAT, 1227 p2i(this), _LockWord.FullWord, _name, p2i(_owner)); 1228 } 1229 #endif 1230 1231 #ifndef PRODUCT 1232 #ifdef ASSERT 1233 Monitor * Monitor::get_least_ranked_lock(Monitor * locks) { 1234 Monitor *res, *tmp; 1235 for (res = tmp = locks; tmp != NULL; tmp = tmp->next()) { 1236 if (tmp->rank() < res->rank()) { 1237 res = tmp; 1238 } 1239 } 1240 if (!SafepointSynchronize::is_at_safepoint()) { 1241 // In this case, we expect the held locks to be 1242 // in increasing rank order (modulo any native ranks) 1243 for (tmp = locks; tmp != NULL; tmp = tmp->next()) { 1244 if (tmp->next() != NULL) { 1245 assert(tmp->rank() == Mutex::native || 1246 tmp->rank() <= tmp->next()->rank(), "mutex rank anomaly?"); 1247 } 1248 } 1249 } 1250 return res; 1251 } 1252 1253 Monitor* Monitor::get_least_ranked_lock_besides_this(Monitor* locks) { 1254 Monitor *res, *tmp; 1255 for (res = NULL, tmp = locks; tmp != NULL; tmp = tmp->next()) { 1256 if (tmp != this && (res == NULL || tmp->rank() < res->rank())) { 1257 res = tmp; 1258 } 1259 } 1260 if (!SafepointSynchronize::is_at_safepoint()) { 1261 // In this case, we expect the held locks to be 1262 // in increasing rank order (modulo any native ranks) 1263 for (tmp = locks; tmp != NULL; tmp = tmp->next()) { 1264 if (tmp->next() != NULL) { 1265 assert(tmp->rank() == Mutex::native || 1266 tmp->rank() <= tmp->next()->rank(), "mutex rank anomaly?"); 1267 } 1268 } 1269 } 1270 return res; 1271 } 1272 1273 1274 bool Monitor::contains(Monitor* locks, Monitor * lock) { 1275 for (; locks != NULL; locks = locks->next()) { 1276 if (locks == lock) { 1277 return true; 1278 } 1279 } 1280 return false; 1281 } 1282 #endif 1283 1284 // Called immediately after lock acquisition or release as a diagnostic 1285 // to track the lock-set of the thread and test for rank violations that 1286 // might indicate exposure to deadlock. 1287 // Rather like an EventListener for _owner (:>). 1288 1289 void Monitor::set_owner_implementation(Thread *new_owner) { 1290 // This function is solely responsible for maintaining 1291 // and checking the invariant that threads and locks 1292 // are in a 1/N relation, with some some locks unowned. 1293 // It uses the Mutex::_owner, Mutex::_next, and 1294 // Thread::_owned_locks fields, and no other function 1295 // changes those fields. 1296 // It is illegal to set the mutex from one non-NULL 1297 // owner to another--it must be owned by NULL as an 1298 // intermediate state. 1299 1300 if (new_owner != NULL) { 1301 // the thread is acquiring this lock 1302 1303 assert(new_owner == Thread::current(), "Should I be doing this?"); 1304 assert(_owner == NULL, "setting the owner thread of an already owned mutex"); 1305 _owner = new_owner; // set the owner 1306 1307 // link "this" into the owned locks list 1308 1309 #ifdef ASSERT // Thread::_owned_locks is under the same ifdef 1310 Monitor* locks = get_least_ranked_lock(new_owner->owned_locks()); 1311 // Mutex::set_owner_implementation is a friend of Thread 1312 1313 assert(this->rank() >= 0, "bad lock rank"); 1314 1315 // Deadlock avoidance rules require us to acquire Mutexes only in 1316 // a global total order. For example m1 is the lowest ranked mutex 1317 // that the thread holds and m2 is the mutex the thread is trying 1318 // to acquire, then deadlock avoidance rules require that the rank 1319 // of m2 be less than the rank of m1. 1320 // The rank Mutex::native is an exception in that it is not subject 1321 // to the verification rules. 1322 // Here are some further notes relating to mutex acquisition anomalies: 1323 // . under Solaris, the interrupt lock gets acquired when doing 1324 // profiling, so any lock could be held. 1325 // . it is also ok to acquire Safepoint_lock at the very end while we 1326 // already hold Terminator_lock - may happen because of periodic safepoints 1327 if (this->rank() != Mutex::native && 1328 this->rank() != Mutex::suspend_resume && 1329 locks != NULL && locks->rank() <= this->rank() && 1330 !SafepointSynchronize::is_at_safepoint() && 1331 this != Interrupt_lock && this != ProfileVM_lock && 1332 !(this == Safepoint_lock && contains(locks, Terminator_lock) && 1333 SafepointSynchronize::is_synchronizing())) { 1334 new_owner->print_owned_locks(); 1335 fatal("acquiring lock %s/%d out of order with lock %s/%d -- " 1336 "possible deadlock", this->name(), this->rank(), 1337 locks->name(), locks->rank()); 1338 } 1339 1340 this->_next = new_owner->_owned_locks; 1341 new_owner->_owned_locks = this; 1342 #endif 1343 1344 } else { 1345 // the thread is releasing this lock 1346 1347 Thread* old_owner = _owner; 1348 debug_only(_last_owner = old_owner); 1349 1350 assert(old_owner != NULL, "removing the owner thread of an unowned mutex"); 1351 assert(old_owner == Thread::current(), "removing the owner thread of an unowned mutex"); 1352 1353 _owner = NULL; // set the owner 1354 1355 #ifdef ASSERT 1356 Monitor *locks = old_owner->owned_locks(); 1357 1358 // remove "this" from the owned locks list 1359 1360 Monitor *prev = NULL; 1361 bool found = false; 1362 for (; locks != NULL; prev = locks, locks = locks->next()) { 1363 if (locks == this) { 1364 found = true; 1365 break; 1366 } 1367 } 1368 assert(found, "Removing a lock not owned"); 1369 if (prev == NULL) { 1370 old_owner->_owned_locks = _next; 1371 } else { 1372 prev->_next = _next; 1373 } 1374 _next = NULL; 1375 #endif 1376 } 1377 } 1378 1379 1380 // Factored out common sanity checks for locking mutex'es. Used by lock() and try_lock() 1381 void Monitor::check_prelock_state(Thread *thread) { 1382 assert((!thread->is_Java_thread() || ((JavaThread *)thread)->thread_state() == _thread_in_vm) 1383 || rank() == Mutex::special, "wrong thread state for using locks"); 1384 if (StrictSafepointChecks) { 1385 if (thread->is_VM_thread() && !allow_vm_block()) { 1386 fatal("VM thread using lock %s (not allowed to block on)", name()); 1387 } 1388 debug_only(if (rank() != Mutex::special) \ 1389 thread->check_for_valid_safepoint_state(false);) 1390 } 1391 if (thread->is_Watcher_thread()) { 1392 assert(!WatcherThread::watcher_thread()->has_crash_protection(), 1393 "locking not allowed when crash protection is set"); 1394 } 1395 } 1396 1397 void Monitor::check_block_state(Thread *thread) { 1398 if (!_allow_vm_block && thread->is_VM_thread()) { 1399 warning("VM thread blocked on lock"); 1400 print(); 1401 BREAKPOINT; 1402 } 1403 assert(_owner != thread, "deadlock: blocking on monitor owned by current thread"); 1404 } 1405 1406 #endif // PRODUCT