1 /* 2 * Copyright (c) 1998, 2017, 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 "classfile/vmSymbols.hpp" 27 #include "memory/resourceArea.hpp" 28 #include "oops/markOop.hpp" 29 #include "oops/oop.inline.hpp" 30 #include "runtime/atomic.hpp" 31 #include "runtime/handles.inline.hpp" 32 #include "runtime/interfaceSupport.hpp" 33 #include "runtime/mutexLocker.hpp" 34 #include "runtime/objectMonitor.hpp" 35 #include "runtime/objectMonitor.inline.hpp" 36 #include "runtime/orderAccess.inline.hpp" 37 #include "runtime/osThread.hpp" 38 #include "runtime/stubRoutines.hpp" 39 #include "runtime/thread.inline.hpp" 40 #include "services/threadService.hpp" 41 #include "trace/tracing.hpp" 42 #include "trace/traceMacros.hpp" 43 #include "utilities/dtrace.hpp" 44 #include "utilities/macros.hpp" 45 #include "utilities/preserveException.hpp" 46 47 #ifdef DTRACE_ENABLED 48 49 // Only bother with this argument setup if dtrace is available 50 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly. 51 52 53 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \ 54 char* bytes = NULL; \ 55 int len = 0; \ 56 jlong jtid = SharedRuntime::get_java_tid(thread); \ 57 Symbol* klassname = ((oop)obj)->klass()->name(); \ 58 if (klassname != NULL) { \ 59 bytes = (char*)klassname->bytes(); \ 60 len = klassname->utf8_length(); \ 61 } 62 63 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \ 64 { \ 65 if (DTraceMonitorProbes) { \ 66 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \ 67 HOTSPOT_MONITOR_WAIT(jtid, \ 68 (monitor), bytes, len, (millis)); \ 69 } \ 70 } 71 72 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER 73 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED 74 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT 75 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY 76 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL 77 78 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \ 79 { \ 80 if (DTraceMonitorProbes) { \ 81 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \ 82 HOTSPOT_MONITOR_##probe(jtid, \ 83 (uintptr_t)(monitor), bytes, len); \ 84 } \ 85 } 86 87 #else // ndef DTRACE_ENABLED 88 89 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;} 90 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;} 91 92 #endif // ndef DTRACE_ENABLED 93 94 // Tunables ... 95 // The knob* variables are effectively final. Once set they should 96 // never be modified hence. Consider using __read_mostly with GCC. 97 98 int ObjectMonitor::Knob_ExitRelease = 0; 99 int ObjectMonitor::Knob_Verbose = 0; 100 int ObjectMonitor::Knob_VerifyInUse = 0; 101 int ObjectMonitor::Knob_VerifyMatch = 0; 102 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool - 103 static int Knob_LogSpins = 0; // enable jvmstat tally for spins 104 static int Knob_HandOff = 0; 105 static int Knob_ReportSettings = 0; 106 107 static int Knob_SpinBase = 0; // Floor AKA SpinMin 108 static int Knob_SpinBackOff = 0; // spin-loop backoff 109 static int Knob_CASPenalty = -1; // Penalty for failed CAS 110 static int Knob_OXPenalty = -1; // Penalty for observed _owner change 111 static int Knob_SpinSetSucc = 1; // spinners set the _succ field 112 static int Knob_SpinEarly = 1; 113 static int Knob_SuccEnabled = 1; // futile wake throttling 114 static int Knob_SuccRestrict = 0; // Limit successors + spinners to at-most-one 115 static int Knob_MaxSpinners = -1; // Should be a function of # CPUs 116 static int Knob_Bonus = 100; // spin success bonus 117 static int Knob_BonusB = 100; // spin success bonus 118 static int Knob_Penalty = 200; // spin failure penalty 119 static int Knob_Poverty = 1000; 120 static int Knob_SpinAfterFutile = 1; // Spin after returning from park() 121 static int Knob_FixedSpin = 0; 122 static int Knob_OState = 3; // Spinner checks thread state of _owner 123 static int Knob_UsePause = 1; 124 static int Knob_ExitPolicy = 0; 125 static int Knob_PreSpin = 10; // 20-100 likely better 126 static int Knob_ResetEvent = 0; 127 static int BackOffMask = 0; 128 129 static int Knob_FastHSSEC = 0; 130 static int Knob_MoveNotifyee = 2; // notify() - disposition of notifyee 131 static int Knob_QMode = 0; // EntryList-cxq policy - queue discipline 132 static volatile int InitDone = 0; 133 134 // ----------------------------------------------------------------------------- 135 // Theory of operations -- Monitors lists, thread residency, etc: 136 // 137 // * A thread acquires ownership of a monitor by successfully 138 // CAS()ing the _owner field from null to non-null. 139 // 140 // * Invariant: A thread appears on at most one monitor list -- 141 // cxq, EntryList or WaitSet -- at any one time. 142 // 143 // * Contending threads "push" themselves onto the cxq with CAS 144 // and then spin/park. 145 // 146 // * After a contending thread eventually acquires the lock it must 147 // dequeue itself from either the EntryList or the cxq. 148 // 149 // * The exiting thread identifies and unparks an "heir presumptive" 150 // tentative successor thread on the EntryList. Critically, the 151 // exiting thread doesn't unlink the successor thread from the EntryList. 152 // After having been unparked, the wakee will recontend for ownership of 153 // the monitor. The successor (wakee) will either acquire the lock or 154 // re-park itself. 155 // 156 // Succession is provided for by a policy of competitive handoff. 157 // The exiting thread does _not_ grant or pass ownership to the 158 // successor thread. (This is also referred to as "handoff" succession"). 159 // Instead the exiting thread releases ownership and possibly wakes 160 // a successor, so the successor can (re)compete for ownership of the lock. 161 // If the EntryList is empty but the cxq is populated the exiting 162 // thread will drain the cxq into the EntryList. It does so by 163 // by detaching the cxq (installing null with CAS) and folding 164 // the threads from the cxq into the EntryList. The EntryList is 165 // doubly linked, while the cxq is singly linked because of the 166 // CAS-based "push" used to enqueue recently arrived threads (RATs). 167 // 168 // * Concurrency invariants: 169 // 170 // -- only the monitor owner may access or mutate the EntryList. 171 // The mutex property of the monitor itself protects the EntryList 172 // from concurrent interference. 173 // -- Only the monitor owner may detach the cxq. 174 // 175 // * The monitor entry list operations avoid locks, but strictly speaking 176 // they're not lock-free. Enter is lock-free, exit is not. 177 // For a description of 'Methods and apparatus providing non-blocking access 178 // to a resource,' see U.S. Pat. No. 7844973. 179 // 180 // * The cxq can have multiple concurrent "pushers" but only one concurrent 181 // detaching thread. This mechanism is immune from the ABA corruption. 182 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious. 183 // 184 // * Taken together, the cxq and the EntryList constitute or form a 185 // single logical queue of threads stalled trying to acquire the lock. 186 // We use two distinct lists to improve the odds of a constant-time 187 // dequeue operation after acquisition (in the ::enter() epilogue) and 188 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm). 189 // A key desideratum is to minimize queue & monitor metadata manipulation 190 // that occurs while holding the monitor lock -- that is, we want to 191 // minimize monitor lock holds times. Note that even a small amount of 192 // fixed spinning will greatly reduce the # of enqueue-dequeue operations 193 // on EntryList|cxq. That is, spinning relieves contention on the "inner" 194 // locks and monitor metadata. 195 // 196 // Cxq points to the set of Recently Arrived Threads attempting entry. 197 // Because we push threads onto _cxq with CAS, the RATs must take the form of 198 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when 199 // the unlocking thread notices that EntryList is null but _cxq is != null. 200 // 201 // The EntryList is ordered by the prevailing queue discipline and 202 // can be organized in any convenient fashion, such as a doubly-linked list or 203 // a circular doubly-linked list. Critically, we want insert and delete operations 204 // to operate in constant-time. If we need a priority queue then something akin 205 // to Solaris' sleepq would work nicely. Viz., 206 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c. 207 // Queue discipline is enforced at ::exit() time, when the unlocking thread 208 // drains the cxq into the EntryList, and orders or reorders the threads on the 209 // EntryList accordingly. 210 // 211 // Barring "lock barging", this mechanism provides fair cyclic ordering, 212 // somewhat similar to an elevator-scan. 213 // 214 // * The monitor synchronization subsystem avoids the use of native 215 // synchronization primitives except for the narrow platform-specific 216 // park-unpark abstraction. See the comments in os_solaris.cpp regarding 217 // the semantics of park-unpark. Put another way, this monitor implementation 218 // depends only on atomic operations and park-unpark. The monitor subsystem 219 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the 220 // underlying OS manages the READY<->RUN transitions. 221 // 222 // * Waiting threads reside on the WaitSet list -- wait() puts 223 // the caller onto the WaitSet. 224 // 225 // * notify() or notifyAll() simply transfers threads from the WaitSet to 226 // either the EntryList or cxq. Subsequent exit() operations will 227 // unpark the notifyee. Unparking a notifee in notify() is inefficient - 228 // it's likely the notifyee would simply impale itself on the lock held 229 // by the notifier. 230 // 231 // * An interesting alternative is to encode cxq as (List,LockByte) where 232 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary 233 // variable, like _recursions, in the scheme. The threads or Events that form 234 // the list would have to be aligned in 256-byte addresses. A thread would 235 // try to acquire the lock or enqueue itself with CAS, but exiting threads 236 // could use a 1-0 protocol and simply STB to set the LockByte to 0. 237 // Note that is is *not* word-tearing, but it does presume that full-word 238 // CAS operations are coherent with intermix with STB operations. That's true 239 // on most common processors. 240 // 241 // * See also http://blogs.sun.com/dave 242 243 244 // ----------------------------------------------------------------------------- 245 // Enter support 246 247 void ObjectMonitor::enter(TRAPS) { 248 // The following code is ordered to check the most common cases first 249 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors. 250 Thread * const Self = THREAD; 251 252 void * cur = Atomic::cmpxchg((void*)Self, &_owner, (void*)NULL); 253 if (cur == NULL) { 254 // Either ASSERT _recursions == 0 or explicitly set _recursions = 0. 255 assert(_recursions == 0, "invariant"); 256 assert(_owner == Self, "invariant"); 257 return; 258 } 259 260 if (cur == Self) { 261 // TODO-FIXME: check for integer overflow! BUGID 6557169. 262 _recursions++; 263 return; 264 } 265 266 if (Self->is_lock_owned ((address)cur)) { 267 assert(_recursions == 0, "internal state error"); 268 _recursions = 1; 269 // Commute owner from a thread-specific on-stack BasicLockObject address to 270 // a full-fledged "Thread *". 271 _owner = Self; 272 return; 273 } 274 275 // We've encountered genuine contention. 276 assert(Self->_Stalled == 0, "invariant"); 277 Self->_Stalled = intptr_t(this); 278 279 // Try one round of spinning *before* enqueueing Self 280 // and before going through the awkward and expensive state 281 // transitions. The following spin is strictly optional ... 282 // Note that if we acquire the monitor from an initial spin 283 // we forgo posting JVMTI events and firing DTRACE probes. 284 if (Knob_SpinEarly && TrySpin (Self) > 0) { 285 assert(_owner == Self, "invariant"); 286 assert(_recursions == 0, "invariant"); 287 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant"); 288 Self->_Stalled = 0; 289 return; 290 } 291 292 assert(_owner != Self, "invariant"); 293 assert(_succ != Self, "invariant"); 294 assert(Self->is_Java_thread(), "invariant"); 295 JavaThread * jt = (JavaThread *) Self; 296 assert(!SafepointSynchronize::is_at_safepoint(), "invariant"); 297 assert(jt->thread_state() != _thread_blocked, "invariant"); 298 assert(this->object() != NULL, "invariant"); 299 assert(_count >= 0, "invariant"); 300 301 // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy(). 302 // Ensure the object-monitor relationship remains stable while there's contention. 303 Atomic::inc(&_count); 304 305 EventJavaMonitorEnter event; 306 307 { // Change java thread status to indicate blocked on monitor enter. 308 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this); 309 310 Self->set_current_pending_monitor(this); 311 312 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt); 313 if (JvmtiExport::should_post_monitor_contended_enter()) { 314 JvmtiExport::post_monitor_contended_enter(jt, this); 315 316 // The current thread does not yet own the monitor and does not 317 // yet appear on any queues that would get it made the successor. 318 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event 319 // handler cannot accidentally consume an unpark() meant for the 320 // ParkEvent associated with this ObjectMonitor. 321 } 322 323 OSThreadContendState osts(Self->osthread()); 324 ThreadBlockInVM tbivm(jt); 325 326 // TODO-FIXME: change the following for(;;) loop to straight-line code. 327 for (;;) { 328 jt->set_suspend_equivalent(); 329 // cleared by handle_special_suspend_equivalent_condition() 330 // or java_suspend_self() 331 332 EnterI(THREAD); 333 334 if (!ExitSuspendEquivalent(jt)) break; 335 336 // We have acquired the contended monitor, but while we were 337 // waiting another thread suspended us. We don't want to enter 338 // the monitor while suspended because that would surprise the 339 // thread that suspended us. 340 // 341 _recursions = 0; 342 _succ = NULL; 343 exit(false, Self); 344 345 jt->java_suspend_self(); 346 } 347 Self->set_current_pending_monitor(NULL); 348 349 // We cleared the pending monitor info since we've just gotten past 350 // the enter-check-for-suspend dance and we now own the monitor free 351 // and clear, i.e., it is no longer pending. The ThreadBlockInVM 352 // destructor can go to a safepoint at the end of this block. If we 353 // do a thread dump during that safepoint, then this thread will show 354 // as having "-locked" the monitor, but the OS and java.lang.Thread 355 // states will still report that the thread is blocked trying to 356 // acquire it. 357 } 358 359 Atomic::dec(&_count); 360 assert(_count >= 0, "invariant"); 361 Self->_Stalled = 0; 362 363 // Must either set _recursions = 0 or ASSERT _recursions == 0. 364 assert(_recursions == 0, "invariant"); 365 assert(_owner == Self, "invariant"); 366 assert(_succ != Self, "invariant"); 367 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant"); 368 369 // The thread -- now the owner -- is back in vm mode. 370 // Report the glorious news via TI,DTrace and jvmstat. 371 // The probe effect is non-trivial. All the reportage occurs 372 // while we hold the monitor, increasing the length of the critical 373 // section. Amdahl's parallel speedup law comes vividly into play. 374 // 375 // Another option might be to aggregate the events (thread local or 376 // per-monitor aggregation) and defer reporting until a more opportune 377 // time -- such as next time some thread encounters contention but has 378 // yet to acquire the lock. While spinning that thread could 379 // spinning we could increment JVMStat counters, etc. 380 381 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt); 382 if (JvmtiExport::should_post_monitor_contended_entered()) { 383 JvmtiExport::post_monitor_contended_entered(jt, this); 384 385 // The current thread already owns the monitor and is not going to 386 // call park() for the remainder of the monitor enter protocol. So 387 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED 388 // event handler consumed an unpark() issued by the thread that 389 // just exited the monitor. 390 } 391 392 if (event.should_commit()) { 393 event.set_monitorClass(((oop)this->object())->klass()); 394 event.set_previousOwner((TYPE_THREAD)_previous_owner_tid); 395 event.set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr())); 396 event.commit(); 397 } 398 399 OM_PERFDATA_OP(ContendedLockAttempts, inc()); 400 } 401 402 403 // Caveat: TryLock() is not necessarily serializing if it returns failure. 404 // Callers must compensate as needed. 405 406 int ObjectMonitor::TryLock(Thread * Self) { 407 void * own = _owner; 408 if (own != NULL) return 0; 409 if (Atomic::cmpxchg_if_null((void*)Self, &_owner)) { 410 // Either guarantee _recursions == 0 or set _recursions = 0. 411 assert(_recursions == 0, "invariant"); 412 assert(_owner == Self, "invariant"); 413 return 1; 414 } 415 // The lock had been free momentarily, but we lost the race to the lock. 416 // Interference -- the CAS failed. 417 // We can either return -1 or retry. 418 // Retry doesn't make as much sense because the lock was just acquired. 419 return -1; 420 } 421 422 #define MAX_RECHECK_INTERVAL 1000 423 424 void ObjectMonitor::EnterI(TRAPS) { 425 Thread * const Self = THREAD; 426 assert(Self->is_Java_thread(), "invariant"); 427 assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant"); 428 429 // Try the lock - TATAS 430 if (TryLock (Self) > 0) { 431 assert(_succ != Self, "invariant"); 432 assert(_owner == Self, "invariant"); 433 assert(_Responsible != Self, "invariant"); 434 return; 435 } 436 437 DeferredInitialize(); 438 439 // We try one round of spinning *before* enqueueing Self. 440 // 441 // If the _owner is ready but OFFPROC we could use a YieldTo() 442 // operation to donate the remainder of this thread's quantum 443 // to the owner. This has subtle but beneficial affinity 444 // effects. 445 446 if (TrySpin (Self) > 0) { 447 assert(_owner == Self, "invariant"); 448 assert(_succ != Self, "invariant"); 449 assert(_Responsible != Self, "invariant"); 450 return; 451 } 452 453 // The Spin failed -- Enqueue and park the thread ... 454 assert(_succ != Self, "invariant"); 455 assert(_owner != Self, "invariant"); 456 assert(_Responsible != Self, "invariant"); 457 458 // Enqueue "Self" on ObjectMonitor's _cxq. 459 // 460 // Node acts as a proxy for Self. 461 // As an aside, if were to ever rewrite the synchronization code mostly 462 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class 463 // Java objects. This would avoid awkward lifecycle and liveness issues, 464 // as well as eliminate a subset of ABA issues. 465 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events. 466 467 ObjectWaiter node(Self); 468 Self->_ParkEvent->reset(); 469 node._prev = (ObjectWaiter *) 0xBAD; 470 node.TState = ObjectWaiter::TS_CXQ; 471 472 // Push "Self" onto the front of the _cxq. 473 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock. 474 // Note that spinning tends to reduce the rate at which threads 475 // enqueue and dequeue on EntryList|cxq. 476 ObjectWaiter * nxt; 477 for (;;) { 478 node._next = nxt = _cxq; 479 if (Atomic::cmpxchg(&node, &_cxq, nxt) == nxt) break; 480 481 // Interference - the CAS failed because _cxq changed. Just retry. 482 // As an optional optimization we retry the lock. 483 if (TryLock (Self) > 0) { 484 assert(_succ != Self, "invariant"); 485 assert(_owner == Self, "invariant"); 486 assert(_Responsible != Self, "invariant"); 487 return; 488 } 489 } 490 491 // Check for cxq|EntryList edge transition to non-null. This indicates 492 // the onset of contention. While contention persists exiting threads 493 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit 494 // operations revert to the faster 1-0 mode. This enter operation may interleave 495 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we 496 // arrange for one of the contending thread to use a timed park() operations 497 // to detect and recover from the race. (Stranding is form of progress failure 498 // where the monitor is unlocked but all the contending threads remain parked). 499 // That is, at least one of the contended threads will periodically poll _owner. 500 // One of the contending threads will become the designated "Responsible" thread. 501 // The Responsible thread uses a timed park instead of a normal indefinite park 502 // operation -- it periodically wakes and checks for and recovers from potential 503 // strandings admitted by 1-0 exit operations. We need at most one Responsible 504 // thread per-monitor at any given moment. Only threads on cxq|EntryList may 505 // be responsible for a monitor. 506 // 507 // Currently, one of the contended threads takes on the added role of "Responsible". 508 // A viable alternative would be to use a dedicated "stranding checker" thread 509 // that periodically iterated over all the threads (or active monitors) and unparked 510 // successors where there was risk of stranding. This would help eliminate the 511 // timer scalability issues we see on some platforms as we'd only have one thread 512 // -- the checker -- parked on a timer. 513 514 if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) { 515 // Try to assume the role of responsible thread for the monitor. 516 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self } 517 Atomic::cmpxchg_if_null(Self, &_Responsible); 518 } 519 520 // The lock might have been released while this thread was occupied queueing 521 // itself onto _cxq. To close the race and avoid "stranding" and 522 // progress-liveness failure we must resample-retry _owner before parking. 523 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner. 524 // In this case the ST-MEMBAR is accomplished with CAS(). 525 // 526 // TODO: Defer all thread state transitions until park-time. 527 // Since state transitions are heavy and inefficient we'd like 528 // to defer the state transitions until absolutely necessary, 529 // and in doing so avoid some transitions ... 530 531 TEVENT(Inflated enter - Contention); 532 int nWakeups = 0; 533 int recheckInterval = 1; 534 535 for (;;) { 536 537 if (TryLock(Self) > 0) break; 538 assert(_owner != Self, "invariant"); 539 540 if ((SyncFlags & 2) && _Responsible == NULL) { 541 Atomic::cmpxchg_if_null(Self, &_Responsible); 542 } 543 544 // park self 545 if (_Responsible == Self || (SyncFlags & 1)) { 546 TEVENT(Inflated enter - park TIMED); 547 Self->_ParkEvent->park((jlong) recheckInterval); 548 // Increase the recheckInterval, but clamp the value. 549 recheckInterval *= 8; 550 if (recheckInterval > MAX_RECHECK_INTERVAL) { 551 recheckInterval = MAX_RECHECK_INTERVAL; 552 } 553 } else { 554 TEVENT(Inflated enter - park UNTIMED); 555 Self->_ParkEvent->park(); 556 } 557 558 if (TryLock(Self) > 0) break; 559 560 // The lock is still contested. 561 // Keep a tally of the # of futile wakeups. 562 // Note that the counter is not protected by a lock or updated by atomics. 563 // That is by design - we trade "lossy" counters which are exposed to 564 // races during updates for a lower probe effect. 565 TEVENT(Inflated enter - Futile wakeup); 566 // This PerfData object can be used in parallel with a safepoint. 567 // See the work around in PerfDataManager::destroy(). 568 OM_PERFDATA_OP(FutileWakeups, inc()); 569 ++nWakeups; 570 571 // Assuming this is not a spurious wakeup we'll normally find _succ == Self. 572 // We can defer clearing _succ until after the spin completes 573 // TrySpin() must tolerate being called with _succ == Self. 574 // Try yet another round of adaptive spinning. 575 if ((Knob_SpinAfterFutile & 1) && TrySpin(Self) > 0) break; 576 577 // We can find that we were unpark()ed and redesignated _succ while 578 // we were spinning. That's harmless. If we iterate and call park(), 579 // park() will consume the event and return immediately and we'll 580 // just spin again. This pattern can repeat, leaving _succ to simply 581 // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks(). 582 // Alternately, we can sample fired() here, and if set, forgo spinning 583 // in the next iteration. 584 585 if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) { 586 Self->_ParkEvent->reset(); 587 OrderAccess::fence(); 588 } 589 if (_succ == Self) _succ = NULL; 590 591 // Invariant: after clearing _succ a thread *must* retry _owner before parking. 592 OrderAccess::fence(); 593 } 594 595 // Egress : 596 // Self has acquired the lock -- Unlink Self from the cxq or EntryList. 597 // Normally we'll find Self on the EntryList . 598 // From the perspective of the lock owner (this thread), the 599 // EntryList is stable and cxq is prepend-only. 600 // The head of cxq is volatile but the interior is stable. 601 // In addition, Self.TState is stable. 602 603 assert(_owner == Self, "invariant"); 604 assert(object() != NULL, "invariant"); 605 // I'd like to write: 606 // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ; 607 // but as we're at a safepoint that's not safe. 608 609 UnlinkAfterAcquire(Self, &node); 610 if (_succ == Self) _succ = NULL; 611 612 assert(_succ != Self, "invariant"); 613 if (_Responsible == Self) { 614 _Responsible = NULL; 615 OrderAccess::fence(); // Dekker pivot-point 616 617 // We may leave threads on cxq|EntryList without a designated 618 // "Responsible" thread. This is benign. When this thread subsequently 619 // exits the monitor it can "see" such preexisting "old" threads -- 620 // threads that arrived on the cxq|EntryList before the fence, above -- 621 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads 622 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible 623 // non-null and elect a new "Responsible" timer thread. 624 // 625 // This thread executes: 626 // ST Responsible=null; MEMBAR (in enter epilogue - here) 627 // LD cxq|EntryList (in subsequent exit) 628 // 629 // Entering threads in the slow/contended path execute: 630 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog) 631 // The (ST cxq; MEMBAR) is accomplished with CAS(). 632 // 633 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent 634 // exit operation from floating above the ST Responsible=null. 635 } 636 637 // We've acquired ownership with CAS(). 638 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics. 639 // But since the CAS() this thread may have also stored into _succ, 640 // EntryList, cxq or Responsible. These meta-data updates must be 641 // visible __before this thread subsequently drops the lock. 642 // Consider what could occur if we didn't enforce this constraint -- 643 // STs to monitor meta-data and user-data could reorder with (become 644 // visible after) the ST in exit that drops ownership of the lock. 645 // Some other thread could then acquire the lock, but observe inconsistent 646 // or old monitor meta-data and heap data. That violates the JMM. 647 // To that end, the 1-0 exit() operation must have at least STST|LDST 648 // "release" barrier semantics. Specifically, there must be at least a 649 // STST|LDST barrier in exit() before the ST of null into _owner that drops 650 // the lock. The barrier ensures that changes to monitor meta-data and data 651 // protected by the lock will be visible before we release the lock, and 652 // therefore before some other thread (CPU) has a chance to acquire the lock. 653 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html. 654 // 655 // Critically, any prior STs to _succ or EntryList must be visible before 656 // the ST of null into _owner in the *subsequent* (following) corresponding 657 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily 658 // execute a serializing instruction. 659 660 if (SyncFlags & 8) { 661 OrderAccess::fence(); 662 } 663 return; 664 } 665 666 // ReenterI() is a specialized inline form of the latter half of the 667 // contended slow-path from EnterI(). We use ReenterI() only for 668 // monitor reentry in wait(). 669 // 670 // In the future we should reconcile EnterI() and ReenterI(), adding 671 // Knob_Reset and Knob_SpinAfterFutile support and restructuring the 672 // loop accordingly. 673 674 void ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) { 675 assert(Self != NULL, "invariant"); 676 assert(SelfNode != NULL, "invariant"); 677 assert(SelfNode->_thread == Self, "invariant"); 678 assert(_waiters > 0, "invariant"); 679 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant"); 680 assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant"); 681 JavaThread * jt = (JavaThread *) Self; 682 683 int nWakeups = 0; 684 for (;;) { 685 ObjectWaiter::TStates v = SelfNode->TState; 686 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant"); 687 assert(_owner != Self, "invariant"); 688 689 if (TryLock(Self) > 0) break; 690 if (TrySpin(Self) > 0) break; 691 692 TEVENT(Wait Reentry - parking); 693 694 // State transition wrappers around park() ... 695 // ReenterI() wisely defers state transitions until 696 // it's clear we must park the thread. 697 { 698 OSThreadContendState osts(Self->osthread()); 699 ThreadBlockInVM tbivm(jt); 700 701 // cleared by handle_special_suspend_equivalent_condition() 702 // or java_suspend_self() 703 jt->set_suspend_equivalent(); 704 if (SyncFlags & 1) { 705 Self->_ParkEvent->park((jlong)MAX_RECHECK_INTERVAL); 706 } else { 707 Self->_ParkEvent->park(); 708 } 709 710 // were we externally suspended while we were waiting? 711 for (;;) { 712 if (!ExitSuspendEquivalent(jt)) break; 713 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); } 714 jt->java_suspend_self(); 715 jt->set_suspend_equivalent(); 716 } 717 } 718 719 // Try again, but just so we distinguish between futile wakeups and 720 // successful wakeups. The following test isn't algorithmically 721 // necessary, but it helps us maintain sensible statistics. 722 if (TryLock(Self) > 0) break; 723 724 // The lock is still contested. 725 // Keep a tally of the # of futile wakeups. 726 // Note that the counter is not protected by a lock or updated by atomics. 727 // That is by design - we trade "lossy" counters which are exposed to 728 // races during updates for a lower probe effect. 729 TEVENT(Wait Reentry - futile wakeup); 730 ++nWakeups; 731 732 // Assuming this is not a spurious wakeup we'll normally 733 // find that _succ == Self. 734 if (_succ == Self) _succ = NULL; 735 736 // Invariant: after clearing _succ a contending thread 737 // *must* retry _owner before parking. 738 OrderAccess::fence(); 739 740 // This PerfData object can be used in parallel with a safepoint. 741 // See the work around in PerfDataManager::destroy(). 742 OM_PERFDATA_OP(FutileWakeups, inc()); 743 } 744 745 // Self has acquired the lock -- Unlink Self from the cxq or EntryList . 746 // Normally we'll find Self on the EntryList. 747 // Unlinking from the EntryList is constant-time and atomic-free. 748 // From the perspective of the lock owner (this thread), the 749 // EntryList is stable and cxq is prepend-only. 750 // The head of cxq is volatile but the interior is stable. 751 // In addition, Self.TState is stable. 752 753 assert(_owner == Self, "invariant"); 754 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant"); 755 UnlinkAfterAcquire(Self, SelfNode); 756 if (_succ == Self) _succ = NULL; 757 assert(_succ != Self, "invariant"); 758 SelfNode->TState = ObjectWaiter::TS_RUN; 759 OrderAccess::fence(); // see comments at the end of EnterI() 760 } 761 762 // By convention we unlink a contending thread from EntryList|cxq immediately 763 // after the thread acquires the lock in ::enter(). Equally, we could defer 764 // unlinking the thread until ::exit()-time. 765 766 void ObjectMonitor::UnlinkAfterAcquire(Thread *Self, ObjectWaiter *SelfNode) { 767 assert(_owner == Self, "invariant"); 768 assert(SelfNode->_thread == Self, "invariant"); 769 770 if (SelfNode->TState == ObjectWaiter::TS_ENTER) { 771 // Normal case: remove Self from the DLL EntryList . 772 // This is a constant-time operation. 773 ObjectWaiter * nxt = SelfNode->_next; 774 ObjectWaiter * prv = SelfNode->_prev; 775 if (nxt != NULL) nxt->_prev = prv; 776 if (prv != NULL) prv->_next = nxt; 777 if (SelfNode == _EntryList) _EntryList = nxt; 778 assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant"); 779 assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant"); 780 TEVENT(Unlink from EntryList); 781 } else { 782 assert(SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant"); 783 // Inopportune interleaving -- Self is still on the cxq. 784 // This usually means the enqueue of self raced an exiting thread. 785 // Normally we'll find Self near the front of the cxq, so 786 // dequeueing is typically fast. If needbe we can accelerate 787 // this with some MCS/CHL-like bidirectional list hints and advisory 788 // back-links so dequeueing from the interior will normally operate 789 // in constant-time. 790 // Dequeue Self from either the head (with CAS) or from the interior 791 // with a linear-time scan and normal non-atomic memory operations. 792 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList 793 // and then unlink Self from EntryList. We have to drain eventually, 794 // so it might as well be now. 795 796 ObjectWaiter * v = _cxq; 797 assert(v != NULL, "invariant"); 798 if (v != SelfNode || Atomic::cmpxchg(SelfNode->_next, &_cxq, v) != v) { 799 // The CAS above can fail from interference IFF a "RAT" arrived. 800 // In that case Self must be in the interior and can no longer be 801 // at the head of cxq. 802 if (v == SelfNode) { 803 assert(_cxq != v, "invariant"); 804 v = _cxq; // CAS above failed - start scan at head of list 805 } 806 ObjectWaiter * p; 807 ObjectWaiter * q = NULL; 808 for (p = v; p != NULL && p != SelfNode; p = p->_next) { 809 q = p; 810 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant"); 811 } 812 assert(v != SelfNode, "invariant"); 813 assert(p == SelfNode, "Node not found on cxq"); 814 assert(p != _cxq, "invariant"); 815 assert(q != NULL, "invariant"); 816 assert(q->_next == p, "invariant"); 817 q->_next = p->_next; 818 } 819 TEVENT(Unlink from cxq); 820 } 821 822 #ifdef ASSERT 823 // Diagnostic hygiene ... 824 SelfNode->_prev = (ObjectWaiter *) 0xBAD; 825 SelfNode->_next = (ObjectWaiter *) 0xBAD; 826 SelfNode->TState = ObjectWaiter::TS_RUN; 827 #endif 828 } 829 830 // ----------------------------------------------------------------------------- 831 // Exit support 832 // 833 // exit() 834 // ~~~~~~ 835 // Note that the collector can't reclaim the objectMonitor or deflate 836 // the object out from underneath the thread calling ::exit() as the 837 // thread calling ::exit() never transitions to a stable state. 838 // This inhibits GC, which in turn inhibits asynchronous (and 839 // inopportune) reclamation of "this". 840 // 841 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ; 842 // There's one exception to the claim above, however. EnterI() can call 843 // exit() to drop a lock if the acquirer has been externally suspended. 844 // In that case exit() is called with _thread_state as _thread_blocked, 845 // but the monitor's _count field is > 0, which inhibits reclamation. 846 // 847 // 1-0 exit 848 // ~~~~~~~~ 849 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of 850 // the fast-path operators have been optimized so the common ::exit() 851 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock(). 852 // The code emitted by fast_unlock() elides the usual MEMBAR. This 853 // greatly improves latency -- MEMBAR and CAS having considerable local 854 // latency on modern processors -- but at the cost of "stranding". Absent the 855 // MEMBAR, a thread in fast_unlock() can race a thread in the slow 856 // ::enter() path, resulting in the entering thread being stranding 857 // and a progress-liveness failure. Stranding is extremely rare. 858 // We use timers (timed park operations) & periodic polling to detect 859 // and recover from stranding. Potentially stranded threads periodically 860 // wake up and poll the lock. See the usage of the _Responsible variable. 861 // 862 // The CAS() in enter provides for safety and exclusion, while the CAS or 863 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking 864 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding. 865 // We detect and recover from stranding with timers. 866 // 867 // If a thread transiently strands it'll park until (a) another 868 // thread acquires the lock and then drops the lock, at which time the 869 // exiting thread will notice and unpark the stranded thread, or, (b) 870 // the timer expires. If the lock is high traffic then the stranding latency 871 // will be low due to (a). If the lock is low traffic then the odds of 872 // stranding are lower, although the worst-case stranding latency 873 // is longer. Critically, we don't want to put excessive load in the 874 // platform's timer subsystem. We want to minimize both the timer injection 875 // rate (timers created/sec) as well as the number of timers active at 876 // any one time. (more precisely, we want to minimize timer-seconds, which is 877 // the integral of the # of active timers at any instant over time). 878 // Both impinge on OS scalability. Given that, at most one thread parked on 879 // a monitor will use a timer. 880 // 881 // There is also the risk of a futile wake-up. If we drop the lock 882 // another thread can reacquire the lock immediately, and we can 883 // then wake a thread unnecessarily. This is benign, and we've 884 // structured the code so the windows are short and the frequency 885 // of such futile wakups is low. 886 887 void ObjectMonitor::exit(bool not_suspended, TRAPS) { 888 Thread * const Self = THREAD; 889 if (THREAD != _owner) { 890 if (THREAD->is_lock_owned((address) _owner)) { 891 // Transmute _owner from a BasicLock pointer to a Thread address. 892 // We don't need to hold _mutex for this transition. 893 // Non-null to Non-null is safe as long as all readers can 894 // tolerate either flavor. 895 assert(_recursions == 0, "invariant"); 896 _owner = THREAD; 897 _recursions = 0; 898 } else { 899 // Apparent unbalanced locking ... 900 // Naively we'd like to throw IllegalMonitorStateException. 901 // As a practical matter we can neither allocate nor throw an 902 // exception as ::exit() can be called from leaf routines. 903 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties. 904 // Upon deeper reflection, however, in a properly run JVM the only 905 // way we should encounter this situation is in the presence of 906 // unbalanced JNI locking. TODO: CheckJNICalls. 907 // See also: CR4414101 908 TEVENT(Exit - Throw IMSX); 909 assert(false, "Non-balanced monitor enter/exit! Likely JNI locking"); 910 return; 911 } 912 } 913 914 if (_recursions != 0) { 915 _recursions--; // this is simple recursive enter 916 TEVENT(Inflated exit - recursive); 917 return; 918 } 919 920 // Invariant: after setting Responsible=null an thread must execute 921 // a MEMBAR or other serializing instruction before fetching EntryList|cxq. 922 if ((SyncFlags & 4) == 0) { 923 _Responsible = NULL; 924 } 925 926 #if INCLUDE_TRACE 927 // get the owner's thread id for the MonitorEnter event 928 // if it is enabled and the thread isn't suspended 929 if (not_suspended && Tracing::is_event_enabled(TraceJavaMonitorEnterEvent)) { 930 _previous_owner_tid = THREAD_TRACE_ID(Self); 931 } 932 #endif 933 934 for (;;) { 935 assert(THREAD == _owner, "invariant"); 936 937 if (Knob_ExitPolicy == 0) { 938 // release semantics: prior loads and stores from within the critical section 939 // must not float (reorder) past the following store that drops the lock. 940 // On SPARC that requires MEMBAR #loadstore|#storestore. 941 // But of course in TSO #loadstore|#storestore is not required. 942 // I'd like to write one of the following: 943 // A. OrderAccess::release() ; _owner = NULL 944 // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL; 945 // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both 946 // store into a _dummy variable. That store is not needed, but can result 947 // in massive wasteful coherency traffic on classic SMP systems. 948 // Instead, I use release_store(), which is implemented as just a simple 949 // ST on x64, x86 and SPARC. 950 OrderAccess::release_store(&_owner, (void*)NULL); // drop the lock 951 OrderAccess::storeload(); // See if we need to wake a successor 952 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) { 953 TEVENT(Inflated exit - simple egress); 954 return; 955 } 956 TEVENT(Inflated exit - complex egress); 957 // Other threads are blocked trying to acquire the lock. 958 959 // Normally the exiting thread is responsible for ensuring succession, 960 // but if other successors are ready or other entering threads are spinning 961 // then this thread can simply store NULL into _owner and exit without 962 // waking a successor. The existence of spinners or ready successors 963 // guarantees proper succession (liveness). Responsibility passes to the 964 // ready or running successors. The exiting thread delegates the duty. 965 // More precisely, if a successor already exists this thread is absolved 966 // of the responsibility of waking (unparking) one. 967 // 968 // The _succ variable is critical to reducing futile wakeup frequency. 969 // _succ identifies the "heir presumptive" thread that has been made 970 // ready (unparked) but that has not yet run. We need only one such 971 // successor thread to guarantee progress. 972 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf 973 // section 3.3 "Futile Wakeup Throttling" for details. 974 // 975 // Note that spinners in Enter() also set _succ non-null. 976 // In the current implementation spinners opportunistically set 977 // _succ so that exiting threads might avoid waking a successor. 978 // Another less appealing alternative would be for the exiting thread 979 // to drop the lock and then spin briefly to see if a spinner managed 980 // to acquire the lock. If so, the exiting thread could exit 981 // immediately without waking a successor, otherwise the exiting 982 // thread would need to dequeue and wake a successor. 983 // (Note that we'd need to make the post-drop spin short, but no 984 // shorter than the worst-case round-trip cache-line migration time. 985 // The dropped lock needs to become visible to the spinner, and then 986 // the acquisition of the lock by the spinner must become visible to 987 // the exiting thread). 988 989 // It appears that an heir-presumptive (successor) must be made ready. 990 // Only the current lock owner can manipulate the EntryList or 991 // drain _cxq, so we need to reacquire the lock. If we fail 992 // to reacquire the lock the responsibility for ensuring succession 993 // falls to the new owner. 994 // 995 if (!Atomic::cmpxchg_if_null((void*)THREAD, &_owner)) { 996 return; 997 } 998 TEVENT(Exit - Reacquired); 999 } else { 1000 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) { 1001 OrderAccess::release_store(&_owner, (void*)NULL); // drop the lock 1002 OrderAccess::storeload(); 1003 // Ratify the previously observed values. 1004 if (_cxq == NULL || _succ != NULL) { 1005 TEVENT(Inflated exit - simple egress); 1006 return; 1007 } 1008 1009 // inopportune interleaving -- the exiting thread (this thread) 1010 // in the fast-exit path raced an entering thread in the slow-enter 1011 // path. 1012 // We have two choices: 1013 // A. Try to reacquire the lock. 1014 // If the CAS() fails return immediately, otherwise 1015 // we either restart/rerun the exit operation, or simply 1016 // fall-through into the code below which wakes a successor. 1017 // B. If the elements forming the EntryList|cxq are TSM 1018 // we could simply unpark() the lead thread and return 1019 // without having set _succ. 1020 if (!Atomic::cmpxchg_if_null((void*)THREAD, &_owner)) { 1021 TEVENT(Inflated exit - reacquired succeeded); 1022 return; 1023 } 1024 TEVENT(Inflated exit - reacquired failed); 1025 } else { 1026 TEVENT(Inflated exit - complex egress); 1027 } 1028 } 1029 1030 guarantee(_owner == THREAD, "invariant"); 1031 1032 ObjectWaiter * w = NULL; 1033 int QMode = Knob_QMode; 1034 1035 if (QMode == 2 && _cxq != NULL) { 1036 // QMode == 2 : cxq has precedence over EntryList. 1037 // Try to directly wake a successor from the cxq. 1038 // If successful, the successor will need to unlink itself from cxq. 1039 w = _cxq; 1040 assert(w != NULL, "invariant"); 1041 assert(w->TState == ObjectWaiter::TS_CXQ, "Invariant"); 1042 ExitEpilog(Self, w); 1043 return; 1044 } 1045 1046 if (QMode == 3 && _cxq != NULL) { 1047 // Aggressively drain cxq into EntryList at the first opportunity. 1048 // This policy ensure that recently-run threads live at the head of EntryList. 1049 // Drain _cxq into EntryList - bulk transfer. 1050 // First, detach _cxq. 1051 // The following loop is tantamount to: w = swap(&cxq, NULL) 1052 w = _cxq; 1053 for (;;) { 1054 assert(w != NULL, "Invariant"); 1055 ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w); 1056 if (u == w) break; 1057 w = u; 1058 } 1059 assert(w != NULL, "invariant"); 1060 1061 ObjectWaiter * q = NULL; 1062 ObjectWaiter * p; 1063 for (p = w; p != NULL; p = p->_next) { 1064 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant"); 1065 p->TState = ObjectWaiter::TS_ENTER; 1066 p->_prev = q; 1067 q = p; 1068 } 1069 1070 // Append the RATs to the EntryList 1071 // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time. 1072 ObjectWaiter * Tail; 1073 for (Tail = _EntryList; Tail != NULL && Tail->_next != NULL; 1074 Tail = Tail->_next) 1075 /* empty */; 1076 if (Tail == NULL) { 1077 _EntryList = w; 1078 } else { 1079 Tail->_next = w; 1080 w->_prev = Tail; 1081 } 1082 1083 // Fall thru into code that tries to wake a successor from EntryList 1084 } 1085 1086 if (QMode == 4 && _cxq != NULL) { 1087 // Aggressively drain cxq into EntryList at the first opportunity. 1088 // This policy ensure that recently-run threads live at the head of EntryList. 1089 1090 // Drain _cxq into EntryList - bulk transfer. 1091 // First, detach _cxq. 1092 // The following loop is tantamount to: w = swap(&cxq, NULL) 1093 w = _cxq; 1094 for (;;) { 1095 assert(w != NULL, "Invariant"); 1096 ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w); 1097 if (u == w) break; 1098 w = u; 1099 } 1100 assert(w != NULL, "invariant"); 1101 1102 ObjectWaiter * q = NULL; 1103 ObjectWaiter * p; 1104 for (p = w; p != NULL; p = p->_next) { 1105 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant"); 1106 p->TState = ObjectWaiter::TS_ENTER; 1107 p->_prev = q; 1108 q = p; 1109 } 1110 1111 // Prepend the RATs to the EntryList 1112 if (_EntryList != NULL) { 1113 q->_next = _EntryList; 1114 _EntryList->_prev = q; 1115 } 1116 _EntryList = w; 1117 1118 // Fall thru into code that tries to wake a successor from EntryList 1119 } 1120 1121 w = _EntryList; 1122 if (w != NULL) { 1123 // I'd like to write: guarantee (w->_thread != Self). 1124 // But in practice an exiting thread may find itself on the EntryList. 1125 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and 1126 // then calls exit(). Exit release the lock by setting O._owner to NULL. 1127 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The 1128 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then 1129 // release the lock "O". T2 resumes immediately after the ST of null into 1130 // _owner, above. T2 notices that the EntryList is populated, so it 1131 // reacquires the lock and then finds itself on the EntryList. 1132 // Given all that, we have to tolerate the circumstance where "w" is 1133 // associated with Self. 1134 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant"); 1135 ExitEpilog(Self, w); 1136 return; 1137 } 1138 1139 // If we find that both _cxq and EntryList are null then just 1140 // re-run the exit protocol from the top. 1141 w = _cxq; 1142 if (w == NULL) continue; 1143 1144 // Drain _cxq into EntryList - bulk transfer. 1145 // First, detach _cxq. 1146 // The following loop is tantamount to: w = swap(&cxq, NULL) 1147 for (;;) { 1148 assert(w != NULL, "Invariant"); 1149 ObjectWaiter * u = Atomic::cmpxchg((ObjectWaiter*)NULL, &_cxq, w); 1150 if (u == w) break; 1151 w = u; 1152 } 1153 TEVENT(Inflated exit - drain cxq into EntryList); 1154 1155 assert(w != NULL, "invariant"); 1156 assert(_EntryList == NULL, "invariant"); 1157 1158 // Convert the LIFO SLL anchored by _cxq into a DLL. 1159 // The list reorganization step operates in O(LENGTH(w)) time. 1160 // It's critical that this step operate quickly as 1161 // "Self" still holds the outer-lock, restricting parallelism 1162 // and effectively lengthening the critical section. 1163 // Invariant: s chases t chases u. 1164 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so 1165 // we have faster access to the tail. 1166 1167 if (QMode == 1) { 1168 // QMode == 1 : drain cxq to EntryList, reversing order 1169 // We also reverse the order of the list. 1170 ObjectWaiter * s = NULL; 1171 ObjectWaiter * t = w; 1172 ObjectWaiter * u = NULL; 1173 while (t != NULL) { 1174 guarantee(t->TState == ObjectWaiter::TS_CXQ, "invariant"); 1175 t->TState = ObjectWaiter::TS_ENTER; 1176 u = t->_next; 1177 t->_prev = u; 1178 t->_next = s; 1179 s = t; 1180 t = u; 1181 } 1182 _EntryList = s; 1183 assert(s != NULL, "invariant"); 1184 } else { 1185 // QMode == 0 or QMode == 2 1186 _EntryList = w; 1187 ObjectWaiter * q = NULL; 1188 ObjectWaiter * p; 1189 for (p = w; p != NULL; p = p->_next) { 1190 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant"); 1191 p->TState = ObjectWaiter::TS_ENTER; 1192 p->_prev = q; 1193 q = p; 1194 } 1195 } 1196 1197 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL 1198 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog(). 1199 1200 // See if we can abdicate to a spinner instead of waking a thread. 1201 // A primary goal of the implementation is to reduce the 1202 // context-switch rate. 1203 if (_succ != NULL) continue; 1204 1205 w = _EntryList; 1206 if (w != NULL) { 1207 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant"); 1208 ExitEpilog(Self, w); 1209 return; 1210 } 1211 } 1212 } 1213 1214 // ExitSuspendEquivalent: 1215 // A faster alternate to handle_special_suspend_equivalent_condition() 1216 // 1217 // handle_special_suspend_equivalent_condition() unconditionally 1218 // acquires the SR_lock. On some platforms uncontended MutexLocker() 1219 // operations have high latency. Note that in ::enter() we call HSSEC 1220 // while holding the monitor, so we effectively lengthen the critical sections. 1221 // 1222 // There are a number of possible solutions: 1223 // 1224 // A. To ameliorate the problem we might also defer state transitions 1225 // to as late as possible -- just prior to parking. 1226 // Given that, we'd call HSSEC after having returned from park(), 1227 // but before attempting to acquire the monitor. This is only a 1228 // partial solution. It avoids calling HSSEC while holding the 1229 // monitor (good), but it still increases successor reacquisition latency -- 1230 // the interval between unparking a successor and the time the successor 1231 // resumes and retries the lock. See ReenterI(), which defers state transitions. 1232 // If we use this technique we can also avoid EnterI()-exit() loop 1233 // in ::enter() where we iteratively drop the lock and then attempt 1234 // to reacquire it after suspending. 1235 // 1236 // B. In the future we might fold all the suspend bits into a 1237 // composite per-thread suspend flag and then update it with CAS(). 1238 // Alternately, a Dekker-like mechanism with multiple variables 1239 // would suffice: 1240 // ST Self->_suspend_equivalent = false 1241 // MEMBAR 1242 // LD Self_>_suspend_flags 1243 // 1244 // UPDATE 2007-10-6: since I've replaced the native Mutex/Monitor subsystem 1245 // with a more efficient implementation, the need to use "FastHSSEC" has 1246 // decreased. - Dave 1247 1248 1249 bool ObjectMonitor::ExitSuspendEquivalent(JavaThread * jSelf) { 1250 const int Mode = Knob_FastHSSEC; 1251 if (Mode && !jSelf->is_external_suspend()) { 1252 assert(jSelf->is_suspend_equivalent(), "invariant"); 1253 jSelf->clear_suspend_equivalent(); 1254 if (2 == Mode) OrderAccess::storeload(); 1255 if (!jSelf->is_external_suspend()) return false; 1256 // We raced a suspension -- fall thru into the slow path 1257 TEVENT(ExitSuspendEquivalent - raced); 1258 jSelf->set_suspend_equivalent(); 1259 } 1260 return jSelf->handle_special_suspend_equivalent_condition(); 1261 } 1262 1263 1264 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) { 1265 assert(_owner == Self, "invariant"); 1266 1267 // Exit protocol: 1268 // 1. ST _succ = wakee 1269 // 2. membar #loadstore|#storestore; 1270 // 2. ST _owner = NULL 1271 // 3. unpark(wakee) 1272 1273 _succ = Knob_SuccEnabled ? Wakee->_thread : NULL; 1274 ParkEvent * Trigger = Wakee->_event; 1275 1276 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again. 1277 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be 1278 // out-of-scope (non-extant). 1279 Wakee = NULL; 1280 1281 // Drop the lock 1282 OrderAccess::release_store(&_owner, (void*)NULL); 1283 OrderAccess::fence(); // ST _owner vs LD in unpark() 1284 1285 if (SafepointSynchronize::do_call_back()) { 1286 TEVENT(unpark before SAFEPOINT); 1287 } 1288 1289 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self); 1290 Trigger->unpark(); 1291 1292 // Maintain stats and report events to JVMTI 1293 OM_PERFDATA_OP(Parks, inc()); 1294 } 1295 1296 1297 // ----------------------------------------------------------------------------- 1298 // Class Loader deadlock handling. 1299 // 1300 // complete_exit exits a lock returning recursion count 1301 // complete_exit/reenter operate as a wait without waiting 1302 // complete_exit requires an inflated monitor 1303 // The _owner field is not always the Thread addr even with an 1304 // inflated monitor, e.g. the monitor can be inflated by a non-owning 1305 // thread due to contention. 1306 intptr_t ObjectMonitor::complete_exit(TRAPS) { 1307 Thread * const Self = THREAD; 1308 assert(Self->is_Java_thread(), "Must be Java thread!"); 1309 JavaThread *jt = (JavaThread *)THREAD; 1310 1311 DeferredInitialize(); 1312 1313 if (THREAD != _owner) { 1314 if (THREAD->is_lock_owned ((address)_owner)) { 1315 assert(_recursions == 0, "internal state error"); 1316 _owner = THREAD; // Convert from basiclock addr to Thread addr 1317 _recursions = 0; 1318 } 1319 } 1320 1321 guarantee(Self == _owner, "complete_exit not owner"); 1322 intptr_t save = _recursions; // record the old recursion count 1323 _recursions = 0; // set the recursion level to be 0 1324 exit(true, Self); // exit the monitor 1325 guarantee(_owner != Self, "invariant"); 1326 return save; 1327 } 1328 1329 // reenter() enters a lock and sets recursion count 1330 // complete_exit/reenter operate as a wait without waiting 1331 void ObjectMonitor::reenter(intptr_t recursions, TRAPS) { 1332 Thread * const Self = THREAD; 1333 assert(Self->is_Java_thread(), "Must be Java thread!"); 1334 JavaThread *jt = (JavaThread *)THREAD; 1335 1336 guarantee(_owner != Self, "reenter already owner"); 1337 enter(THREAD); // enter the monitor 1338 guarantee(_recursions == 0, "reenter recursion"); 1339 _recursions = recursions; 1340 return; 1341 } 1342 1343 1344 // ----------------------------------------------------------------------------- 1345 // A macro is used below because there may already be a pending 1346 // exception which should not abort the execution of the routines 1347 // which use this (which is why we don't put this into check_slow and 1348 // call it with a CHECK argument). 1349 1350 #define CHECK_OWNER() \ 1351 do { \ 1352 if (THREAD != _owner) { \ 1353 if (THREAD->is_lock_owned((address) _owner)) { \ 1354 _owner = THREAD; /* Convert from basiclock addr to Thread addr */ \ 1355 _recursions = 0; \ 1356 } else { \ 1357 TEVENT(Throw IMSX); \ 1358 THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \ 1359 } \ 1360 } \ 1361 } while (false) 1362 1363 // check_slow() is a misnomer. It's called to simply to throw an IMSX exception. 1364 // TODO-FIXME: remove check_slow() -- it's likely dead. 1365 1366 void ObjectMonitor::check_slow(TRAPS) { 1367 TEVENT(check_slow - throw IMSX); 1368 assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner"); 1369 THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner"); 1370 } 1371 1372 static int Adjust(volatile int * adr, int dx) { 1373 int v; 1374 for (v = *adr; Atomic::cmpxchg(v + dx, adr, v) != v; v = *adr) /* empty */; 1375 return v; 1376 } 1377 1378 // helper method for posting a monitor wait event 1379 void ObjectMonitor::post_monitor_wait_event(EventJavaMonitorWait* event, 1380 jlong notifier_tid, 1381 jlong timeout, 1382 bool timedout) { 1383 assert(event != NULL, "invariant"); 1384 event->set_monitorClass(((oop)this->object())->klass()); 1385 event->set_timeout(timeout); 1386 event->set_address((TYPE_ADDRESS)this->object_addr()); 1387 event->set_notifier(notifier_tid); 1388 event->set_timedOut(timedout); 1389 event->commit(); 1390 } 1391 1392 // ----------------------------------------------------------------------------- 1393 // Wait/Notify/NotifyAll 1394 // 1395 // Note: a subset of changes to ObjectMonitor::wait() 1396 // will need to be replicated in complete_exit 1397 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) { 1398 Thread * const Self = THREAD; 1399 assert(Self->is_Java_thread(), "Must be Java thread!"); 1400 JavaThread *jt = (JavaThread *)THREAD; 1401 1402 DeferredInitialize(); 1403 1404 // Throw IMSX or IEX. 1405 CHECK_OWNER(); 1406 1407 EventJavaMonitorWait event; 1408 1409 // check for a pending interrupt 1410 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) { 1411 // post monitor waited event. Note that this is past-tense, we are done waiting. 1412 if (JvmtiExport::should_post_monitor_waited()) { 1413 // Note: 'false' parameter is passed here because the 1414 // wait was not timed out due to thread interrupt. 1415 JvmtiExport::post_monitor_waited(jt, this, false); 1416 1417 // In this short circuit of the monitor wait protocol, the 1418 // current thread never drops ownership of the monitor and 1419 // never gets added to the wait queue so the current thread 1420 // cannot be made the successor. This means that the 1421 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally 1422 // consume an unpark() meant for the ParkEvent associated with 1423 // this ObjectMonitor. 1424 } 1425 if (event.should_commit()) { 1426 post_monitor_wait_event(&event, 0, millis, false); 1427 } 1428 TEVENT(Wait - Throw IEX); 1429 THROW(vmSymbols::java_lang_InterruptedException()); 1430 return; 1431 } 1432 1433 TEVENT(Wait); 1434 1435 assert(Self->_Stalled == 0, "invariant"); 1436 Self->_Stalled = intptr_t(this); 1437 jt->set_current_waiting_monitor(this); 1438 1439 // create a node to be put into the queue 1440 // Critically, after we reset() the event but prior to park(), we must check 1441 // for a pending interrupt. 1442 ObjectWaiter node(Self); 1443 node.TState = ObjectWaiter::TS_WAIT; 1444 Self->_ParkEvent->reset(); 1445 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag 1446 1447 // Enter the waiting queue, which is a circular doubly linked list in this case 1448 // but it could be a priority queue or any data structure. 1449 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only 1450 // by the the owner of the monitor *except* in the case where park() 1451 // returns because of a timeout of interrupt. Contention is exceptionally rare 1452 // so we use a simple spin-lock instead of a heavier-weight blocking lock. 1453 1454 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add"); 1455 AddWaiter(&node); 1456 Thread::SpinRelease(&_WaitSetLock); 1457 1458 if ((SyncFlags & 4) == 0) { 1459 _Responsible = NULL; 1460 } 1461 intptr_t save = _recursions; // record the old recursion count 1462 _waiters++; // increment the number of waiters 1463 _recursions = 0; // set the recursion level to be 1 1464 exit(true, Self); // exit the monitor 1465 guarantee(_owner != Self, "invariant"); 1466 1467 // The thread is on the WaitSet list - now park() it. 1468 // On MP systems it's conceivable that a brief spin before we park 1469 // could be profitable. 1470 // 1471 // TODO-FIXME: change the following logic to a loop of the form 1472 // while (!timeout && !interrupted && _notified == 0) park() 1473 1474 int ret = OS_OK; 1475 int WasNotified = 0; 1476 { // State transition wrappers 1477 OSThread* osthread = Self->osthread(); 1478 OSThreadWaitState osts(osthread, true); 1479 { 1480 ThreadBlockInVM tbivm(jt); 1481 // Thread is in thread_blocked state and oop access is unsafe. 1482 jt->set_suspend_equivalent(); 1483 1484 if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) { 1485 // Intentionally empty 1486 } else if (node._notified == 0) { 1487 if (millis <= 0) { 1488 Self->_ParkEvent->park(); 1489 } else { 1490 ret = Self->_ParkEvent->park(millis); 1491 } 1492 } 1493 1494 // were we externally suspended while we were waiting? 1495 if (ExitSuspendEquivalent (jt)) { 1496 // TODO-FIXME: add -- if succ == Self then succ = null. 1497 jt->java_suspend_self(); 1498 } 1499 1500 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm 1501 1502 // Node may be on the WaitSet, the EntryList (or cxq), or in transition 1503 // from the WaitSet to the EntryList. 1504 // See if we need to remove Node from the WaitSet. 1505 // We use double-checked locking to avoid grabbing _WaitSetLock 1506 // if the thread is not on the wait queue. 1507 // 1508 // Note that we don't need a fence before the fetch of TState. 1509 // In the worst case we'll fetch a old-stale value of TS_WAIT previously 1510 // written by the is thread. (perhaps the fetch might even be satisfied 1511 // by a look-aside into the processor's own store buffer, although given 1512 // the length of the code path between the prior ST and this load that's 1513 // highly unlikely). If the following LD fetches a stale TS_WAIT value 1514 // then we'll acquire the lock and then re-fetch a fresh TState value. 1515 // That is, we fail toward safety. 1516 1517 if (node.TState == ObjectWaiter::TS_WAIT) { 1518 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink"); 1519 if (node.TState == ObjectWaiter::TS_WAIT) { 1520 DequeueSpecificWaiter(&node); // unlink from WaitSet 1521 assert(node._notified == 0, "invariant"); 1522 node.TState = ObjectWaiter::TS_RUN; 1523 } 1524 Thread::SpinRelease(&_WaitSetLock); 1525 } 1526 1527 // The thread is now either on off-list (TS_RUN), 1528 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ). 1529 // The Node's TState variable is stable from the perspective of this thread. 1530 // No other threads will asynchronously modify TState. 1531 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant"); 1532 OrderAccess::loadload(); 1533 if (_succ == Self) _succ = NULL; 1534 WasNotified = node._notified; 1535 1536 // Reentry phase -- reacquire the monitor. 1537 // re-enter contended monitor after object.wait(). 1538 // retain OBJECT_WAIT state until re-enter successfully completes 1539 // Thread state is thread_in_vm and oop access is again safe, 1540 // although the raw address of the object may have changed. 1541 // (Don't cache naked oops over safepoints, of course). 1542 1543 // post monitor waited event. Note that this is past-tense, we are done waiting. 1544 if (JvmtiExport::should_post_monitor_waited()) { 1545 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT); 1546 1547 if (node._notified != 0 && _succ == Self) { 1548 // In this part of the monitor wait-notify-reenter protocol it 1549 // is possible (and normal) for another thread to do a fastpath 1550 // monitor enter-exit while this thread is still trying to get 1551 // to the reenter portion of the protocol. 1552 // 1553 // The ObjectMonitor was notified and the current thread is 1554 // the successor which also means that an unpark() has already 1555 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can 1556 // consume the unpark() that was done when the successor was 1557 // set because the same ParkEvent is shared between Java 1558 // monitors and JVM/TI RawMonitors (for now). 1559 // 1560 // We redo the unpark() to ensure forward progress, i.e., we 1561 // don't want all pending threads hanging (parked) with none 1562 // entering the unlocked monitor. 1563 node._event->unpark(); 1564 } 1565 } 1566 1567 if (event.should_commit()) { 1568 post_monitor_wait_event(&event, node._notifier_tid, millis, ret == OS_TIMEOUT); 1569 } 1570 1571 OrderAccess::fence(); 1572 1573 assert(Self->_Stalled != 0, "invariant"); 1574 Self->_Stalled = 0; 1575 1576 assert(_owner != Self, "invariant"); 1577 ObjectWaiter::TStates v = node.TState; 1578 if (v == ObjectWaiter::TS_RUN) { 1579 enter(Self); 1580 } else { 1581 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant"); 1582 ReenterI(Self, &node); 1583 node.wait_reenter_end(this); 1584 } 1585 1586 // Self has reacquired the lock. 1587 // Lifecycle - the node representing Self must not appear on any queues. 1588 // Node is about to go out-of-scope, but even if it were immortal we wouldn't 1589 // want residual elements associated with this thread left on any lists. 1590 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant"); 1591 assert(_owner == Self, "invariant"); 1592 assert(_succ != Self, "invariant"); 1593 } // OSThreadWaitState() 1594 1595 jt->set_current_waiting_monitor(NULL); 1596 1597 guarantee(_recursions == 0, "invariant"); 1598 _recursions = save; // restore the old recursion count 1599 _waiters--; // decrement the number of waiters 1600 1601 // Verify a few postconditions 1602 assert(_owner == Self, "invariant"); 1603 assert(_succ != Self, "invariant"); 1604 assert(((oop)(object()))->mark() == markOopDesc::encode(this), "invariant"); 1605 1606 if (SyncFlags & 32) { 1607 OrderAccess::fence(); 1608 } 1609 1610 // check if the notification happened 1611 if (!WasNotified) { 1612 // no, it could be timeout or Thread.interrupt() or both 1613 // check for interrupt event, otherwise it is timeout 1614 if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) { 1615 TEVENT(Wait - throw IEX from epilog); 1616 THROW(vmSymbols::java_lang_InterruptedException()); 1617 } 1618 } 1619 1620 // NOTE: Spurious wake up will be consider as timeout. 1621 // Monitor notify has precedence over thread interrupt. 1622 } 1623 1624 1625 // Consider: 1626 // If the lock is cool (cxq == null && succ == null) and we're on an MP system 1627 // then instead of transferring a thread from the WaitSet to the EntryList 1628 // we might just dequeue a thread from the WaitSet and directly unpark() it. 1629 1630 void ObjectMonitor::INotify(Thread * Self) { 1631 const int policy = Knob_MoveNotifyee; 1632 1633 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify"); 1634 ObjectWaiter * iterator = DequeueWaiter(); 1635 if (iterator != NULL) { 1636 TEVENT(Notify1 - Transfer); 1637 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant"); 1638 guarantee(iterator->_notified == 0, "invariant"); 1639 // Disposition - what might we do with iterator ? 1640 // a. add it directly to the EntryList - either tail (policy == 1) 1641 // or head (policy == 0). 1642 // b. push it onto the front of the _cxq (policy == 2). 1643 // For now we use (b). 1644 if (policy != 4) { 1645 iterator->TState = ObjectWaiter::TS_ENTER; 1646 } 1647 iterator->_notified = 1; 1648 iterator->_notifier_tid = THREAD_TRACE_ID(Self); 1649 1650 ObjectWaiter * list = _EntryList; 1651 if (list != NULL) { 1652 assert(list->_prev == NULL, "invariant"); 1653 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant"); 1654 assert(list != iterator, "invariant"); 1655 } 1656 1657 if (policy == 0) { // prepend to EntryList 1658 if (list == NULL) { 1659 iterator->_next = iterator->_prev = NULL; 1660 _EntryList = iterator; 1661 } else { 1662 list->_prev = iterator; 1663 iterator->_next = list; 1664 iterator->_prev = NULL; 1665 _EntryList = iterator; 1666 } 1667 } else if (policy == 1) { // append to EntryList 1668 if (list == NULL) { 1669 iterator->_next = iterator->_prev = NULL; 1670 _EntryList = iterator; 1671 } else { 1672 // CONSIDER: finding the tail currently requires a linear-time walk of 1673 // the EntryList. We can make tail access constant-time by converting to 1674 // a CDLL instead of using our current DLL. 1675 ObjectWaiter * tail; 1676 for (tail = list; tail->_next != NULL; tail = tail->_next) {} 1677 assert(tail != NULL && tail->_next == NULL, "invariant"); 1678 tail->_next = iterator; 1679 iterator->_prev = tail; 1680 iterator->_next = NULL; 1681 } 1682 } else if (policy == 2) { // prepend to cxq 1683 if (list == NULL) { 1684 iterator->_next = iterator->_prev = NULL; 1685 _EntryList = iterator; 1686 } else { 1687 iterator->TState = ObjectWaiter::TS_CXQ; 1688 for (;;) { 1689 ObjectWaiter * front = _cxq; 1690 iterator->_next = front; 1691 if (Atomic::cmpxchg(iterator, &_cxq, front) == front) { 1692 break; 1693 } 1694 } 1695 } 1696 } else if (policy == 3) { // append to cxq 1697 iterator->TState = ObjectWaiter::TS_CXQ; 1698 for (;;) { 1699 ObjectWaiter * tail = _cxq; 1700 if (tail == NULL) { 1701 iterator->_next = NULL; 1702 if (Atomic::cmpxchg_if_null(iterator, &_cxq)) { 1703 break; 1704 } 1705 } else { 1706 while (tail->_next != NULL) tail = tail->_next; 1707 tail->_next = iterator; 1708 iterator->_prev = tail; 1709 iterator->_next = NULL; 1710 break; 1711 } 1712 } 1713 } else { 1714 ParkEvent * ev = iterator->_event; 1715 iterator->TState = ObjectWaiter::TS_RUN; 1716 OrderAccess::fence(); 1717 ev->unpark(); 1718 } 1719 1720 // _WaitSetLock protects the wait queue, not the EntryList. We could 1721 // move the add-to-EntryList operation, above, outside the critical section 1722 // protected by _WaitSetLock. In practice that's not useful. With the 1723 // exception of wait() timeouts and interrupts the monitor owner 1724 // is the only thread that grabs _WaitSetLock. There's almost no contention 1725 // on _WaitSetLock so it's not profitable to reduce the length of the 1726 // critical section. 1727 1728 if (policy < 4) { 1729 iterator->wait_reenter_begin(this); 1730 } 1731 } 1732 Thread::SpinRelease(&_WaitSetLock); 1733 } 1734 1735 // Consider: a not-uncommon synchronization bug is to use notify() when 1736 // notifyAll() is more appropriate, potentially resulting in stranded 1737 // threads; this is one example of a lost wakeup. A useful diagnostic 1738 // option is to force all notify() operations to behave as notifyAll(). 1739 // 1740 // Note: We can also detect many such problems with a "minimum wait". 1741 // When the "minimum wait" is set to a small non-zero timeout value 1742 // and the program does not hang whereas it did absent "minimum wait", 1743 // that suggests a lost wakeup bug. The '-XX:SyncFlags=1' option uses 1744 // a "minimum wait" for all park() operations; see the recheckInterval 1745 // variable and MAX_RECHECK_INTERVAL. 1746 1747 void ObjectMonitor::notify(TRAPS) { 1748 CHECK_OWNER(); 1749 if (_WaitSet == NULL) { 1750 TEVENT(Empty-Notify); 1751 return; 1752 } 1753 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD); 1754 INotify(THREAD); 1755 OM_PERFDATA_OP(Notifications, inc(1)); 1756 } 1757 1758 1759 // The current implementation of notifyAll() transfers the waiters one-at-a-time 1760 // from the waitset to the EntryList. This could be done more efficiently with a 1761 // single bulk transfer but in practice it's not time-critical. Beware too, 1762 // that in prepend-mode we invert the order of the waiters. Let's say that the 1763 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend 1764 // mode the waitset will be empty and the EntryList will be "DCBAXYZ". 1765 1766 void ObjectMonitor::notifyAll(TRAPS) { 1767 CHECK_OWNER(); 1768 if (_WaitSet == NULL) { 1769 TEVENT(Empty-NotifyAll); 1770 return; 1771 } 1772 1773 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD); 1774 int tally = 0; 1775 while (_WaitSet != NULL) { 1776 tally++; 1777 INotify(THREAD); 1778 } 1779 1780 OM_PERFDATA_OP(Notifications, inc(tally)); 1781 } 1782 1783 // ----------------------------------------------------------------------------- 1784 // Adaptive Spinning Support 1785 // 1786 // Adaptive spin-then-block - rational spinning 1787 // 1788 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS 1789 // algorithm. On high order SMP systems it would be better to start with 1790 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH, 1791 // a contending thread could enqueue itself on the cxq and then spin locally 1792 // on a thread-specific variable such as its ParkEvent._Event flag. 1793 // That's left as an exercise for the reader. Note that global spinning is 1794 // not problematic on Niagara, as the L2 cache serves the interconnect and 1795 // has both low latency and massive bandwidth. 1796 // 1797 // Broadly, we can fix the spin frequency -- that is, the % of contended lock 1798 // acquisition attempts where we opt to spin -- at 100% and vary the spin count 1799 // (duration) or we can fix the count at approximately the duration of 1800 // a context switch and vary the frequency. Of course we could also 1801 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor. 1802 // For a description of 'Adaptive spin-then-block mutual exclusion in 1803 // multi-threaded processing,' see U.S. Pat. No. 8046758. 1804 // 1805 // This implementation varies the duration "D", where D varies with 1806 // the success rate of recent spin attempts. (D is capped at approximately 1807 // length of a round-trip context switch). The success rate for recent 1808 // spin attempts is a good predictor of the success rate of future spin 1809 // attempts. The mechanism adapts automatically to varying critical 1810 // section length (lock modality), system load and degree of parallelism. 1811 // D is maintained per-monitor in _SpinDuration and is initialized 1812 // optimistically. Spin frequency is fixed at 100%. 1813 // 1814 // Note that _SpinDuration is volatile, but we update it without locks 1815 // or atomics. The code is designed so that _SpinDuration stays within 1816 // a reasonable range even in the presence of races. The arithmetic 1817 // operations on _SpinDuration are closed over the domain of legal values, 1818 // so at worst a race will install and older but still legal value. 1819 // At the very worst this introduces some apparent non-determinism. 1820 // We might spin when we shouldn't or vice-versa, but since the spin 1821 // count are relatively short, even in the worst case, the effect is harmless. 1822 // 1823 // Care must be taken that a low "D" value does not become an 1824 // an absorbing state. Transient spinning failures -- when spinning 1825 // is overall profitable -- should not cause the system to converge 1826 // on low "D" values. We want spinning to be stable and predictable 1827 // and fairly responsive to change and at the same time we don't want 1828 // it to oscillate, become metastable, be "too" non-deterministic, 1829 // or converge on or enter undesirable stable absorbing states. 1830 // 1831 // We implement a feedback-based control system -- using past behavior 1832 // to predict future behavior. We face two issues: (a) if the 1833 // input signal is random then the spin predictor won't provide optimal 1834 // results, and (b) if the signal frequency is too high then the control 1835 // system, which has some natural response lag, will "chase" the signal. 1836 // (b) can arise from multimodal lock hold times. Transient preemption 1837 // can also result in apparent bimodal lock hold times. 1838 // Although sub-optimal, neither condition is particularly harmful, as 1839 // in the worst-case we'll spin when we shouldn't or vice-versa. 1840 // The maximum spin duration is rather short so the failure modes aren't bad. 1841 // To be conservative, I've tuned the gain in system to bias toward 1842 // _not spinning. Relatedly, the system can sometimes enter a mode where it 1843 // "rings" or oscillates between spinning and not spinning. This happens 1844 // when spinning is just on the cusp of profitability, however, so the 1845 // situation is not dire. The state is benign -- there's no need to add 1846 // hysteresis control to damp the transition rate between spinning and 1847 // not spinning. 1848 1849 // Spinning: Fixed frequency (100%), vary duration 1850 int ObjectMonitor::TrySpin(Thread * Self) { 1851 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning. 1852 int ctr = Knob_FixedSpin; 1853 if (ctr != 0) { 1854 while (--ctr >= 0) { 1855 if (TryLock(Self) > 0) return 1; 1856 SpinPause(); 1857 } 1858 return 0; 1859 } 1860 1861 for (ctr = Knob_PreSpin + 1; --ctr >= 0;) { 1862 if (TryLock(Self) > 0) { 1863 // Increase _SpinDuration ... 1864 // Note that we don't clamp SpinDuration precisely at SpinLimit. 1865 // Raising _SpurDuration to the poverty line is key. 1866 int x = _SpinDuration; 1867 if (x < Knob_SpinLimit) { 1868 if (x < Knob_Poverty) x = Knob_Poverty; 1869 _SpinDuration = x + Knob_BonusB; 1870 } 1871 return 1; 1872 } 1873 SpinPause(); 1874 } 1875 1876 // Admission control - verify preconditions for spinning 1877 // 1878 // We always spin a little bit, just to prevent _SpinDuration == 0 from 1879 // becoming an absorbing state. Put another way, we spin briefly to 1880 // sample, just in case the system load, parallelism, contention, or lock 1881 // modality changed. 1882 // 1883 // Consider the following alternative: 1884 // Periodically set _SpinDuration = _SpinLimit and try a long/full 1885 // spin attempt. "Periodically" might mean after a tally of 1886 // the # of failed spin attempts (or iterations) reaches some threshold. 1887 // This takes us into the realm of 1-out-of-N spinning, where we 1888 // hold the duration constant but vary the frequency. 1889 1890 ctr = _SpinDuration; 1891 if (ctr < Knob_SpinBase) ctr = Knob_SpinBase; 1892 if (ctr <= 0) return 0; 1893 1894 if (Knob_SuccRestrict && _succ != NULL) return 0; 1895 if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) { 1896 TEVENT(Spin abort - notrunnable [TOP]); 1897 return 0; 1898 } 1899 1900 int MaxSpin = Knob_MaxSpinners; 1901 if (MaxSpin >= 0) { 1902 if (_Spinner > MaxSpin) { 1903 TEVENT(Spin abort -- too many spinners); 1904 return 0; 1905 } 1906 // Slightly racy, but benign ... 1907 Adjust(&_Spinner, 1); 1908 } 1909 1910 // We're good to spin ... spin ingress. 1911 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades 1912 // when preparing to LD...CAS _owner, etc and the CAS is likely 1913 // to succeed. 1914 int hits = 0; 1915 int msk = 0; 1916 int caspty = Knob_CASPenalty; 1917 int oxpty = Knob_OXPenalty; 1918 int sss = Knob_SpinSetSucc; 1919 if (sss && _succ == NULL) _succ = Self; 1920 Thread * prv = NULL; 1921 1922 // There are three ways to exit the following loop: 1923 // 1. A successful spin where this thread has acquired the lock. 1924 // 2. Spin failure with prejudice 1925 // 3. Spin failure without prejudice 1926 1927 while (--ctr >= 0) { 1928 1929 // Periodic polling -- Check for pending GC 1930 // Threads may spin while they're unsafe. 1931 // We don't want spinning threads to delay the JVM from reaching 1932 // a stop-the-world safepoint or to steal cycles from GC. 1933 // If we detect a pending safepoint we abort in order that 1934 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b) 1935 // this thread, if safe, doesn't steal cycles from GC. 1936 // This is in keeping with the "no loitering in runtime" rule. 1937 // We periodically check to see if there's a safepoint pending. 1938 if ((ctr & 0xFF) == 0) { 1939 if (SafepointSynchronize::do_call_back()) { 1940 TEVENT(Spin: safepoint); 1941 goto Abort; // abrupt spin egress 1942 } 1943 if (Knob_UsePause & 1) SpinPause(); 1944 } 1945 1946 if (Knob_UsePause & 2) SpinPause(); 1947 1948 // Exponential back-off ... Stay off the bus to reduce coherency traffic. 1949 // This is useful on classic SMP systems, but is of less utility on 1950 // N1-style CMT platforms. 1951 // 1952 // Trade-off: lock acquisition latency vs coherency bandwidth. 1953 // Lock hold times are typically short. A histogram 1954 // of successful spin attempts shows that we usually acquire 1955 // the lock early in the spin. That suggests we want to 1956 // sample _owner frequently in the early phase of the spin, 1957 // but then back-off and sample less frequently as the spin 1958 // progresses. The back-off makes a good citizen on SMP big 1959 // SMP systems. Oversampling _owner can consume excessive 1960 // coherency bandwidth. Relatedly, if we _oversample _owner we 1961 // can inadvertently interfere with the the ST m->owner=null. 1962 // executed by the lock owner. 1963 if (ctr & msk) continue; 1964 ++hits; 1965 if ((hits & 0xF) == 0) { 1966 // The 0xF, above, corresponds to the exponent. 1967 // Consider: (msk+1)|msk 1968 msk = ((msk << 2)|3) & BackOffMask; 1969 } 1970 1971 // Probe _owner with TATAS 1972 // If this thread observes the monitor transition or flicker 1973 // from locked to unlocked to locked, then the odds that this 1974 // thread will acquire the lock in this spin attempt go down 1975 // considerably. The same argument applies if the CAS fails 1976 // or if we observe _owner change from one non-null value to 1977 // another non-null value. In such cases we might abort 1978 // the spin without prejudice or apply a "penalty" to the 1979 // spin count-down variable "ctr", reducing it by 100, say. 1980 1981 Thread * ox = (Thread *) _owner; 1982 if (ox == NULL) { 1983 ox = (Thread*)Atomic::cmpxchg((void*)Self, &_owner, (void*)NULL); 1984 if (ox == NULL) { 1985 // The CAS succeeded -- this thread acquired ownership 1986 // Take care of some bookkeeping to exit spin state. 1987 if (sss && _succ == Self) { 1988 _succ = NULL; 1989 } 1990 if (MaxSpin > 0) Adjust(&_Spinner, -1); 1991 1992 // Increase _SpinDuration : 1993 // The spin was successful (profitable) so we tend toward 1994 // longer spin attempts in the future. 1995 // CONSIDER: factor "ctr" into the _SpinDuration adjustment. 1996 // If we acquired the lock early in the spin cycle it 1997 // makes sense to increase _SpinDuration proportionally. 1998 // Note that we don't clamp SpinDuration precisely at SpinLimit. 1999 int x = _SpinDuration; 2000 if (x < Knob_SpinLimit) { 2001 if (x < Knob_Poverty) x = Knob_Poverty; 2002 _SpinDuration = x + Knob_Bonus; 2003 } 2004 return 1; 2005 } 2006 2007 // The CAS failed ... we can take any of the following actions: 2008 // * penalize: ctr -= Knob_CASPenalty 2009 // * exit spin with prejudice -- goto Abort; 2010 // * exit spin without prejudice. 2011 // * Since CAS is high-latency, retry again immediately. 2012 prv = ox; 2013 TEVENT(Spin: cas failed); 2014 if (caspty == -2) break; 2015 if (caspty == -1) goto Abort; 2016 ctr -= caspty; 2017 continue; 2018 } 2019 2020 // Did lock ownership change hands ? 2021 if (ox != prv && prv != NULL) { 2022 TEVENT(spin: Owner changed) 2023 if (oxpty == -2) break; 2024 if (oxpty == -1) goto Abort; 2025 ctr -= oxpty; 2026 } 2027 prv = ox; 2028 2029 // Abort the spin if the owner is not executing. 2030 // The owner must be executing in order to drop the lock. 2031 // Spinning while the owner is OFFPROC is idiocy. 2032 // Consider: ctr -= RunnablePenalty ; 2033 if (Knob_OState && NotRunnable (Self, ox)) { 2034 TEVENT(Spin abort - notrunnable); 2035 goto Abort; 2036 } 2037 if (sss && _succ == NULL) _succ = Self; 2038 } 2039 2040 // Spin failed with prejudice -- reduce _SpinDuration. 2041 // TODO: Use an AIMD-like policy to adjust _SpinDuration. 2042 // AIMD is globally stable. 2043 TEVENT(Spin failure); 2044 { 2045 int x = _SpinDuration; 2046 if (x > 0) { 2047 // Consider an AIMD scheme like: x -= (x >> 3) + 100 2048 // This is globally sample and tends to damp the response. 2049 x -= Knob_Penalty; 2050 if (x < 0) x = 0; 2051 _SpinDuration = x; 2052 } 2053 } 2054 2055 Abort: 2056 if (MaxSpin >= 0) Adjust(&_Spinner, -1); 2057 if (sss && _succ == Self) { 2058 _succ = NULL; 2059 // Invariant: after setting succ=null a contending thread 2060 // must recheck-retry _owner before parking. This usually happens 2061 // in the normal usage of TrySpin(), but it's safest 2062 // to make TrySpin() as foolproof as possible. 2063 OrderAccess::fence(); 2064 if (TryLock(Self) > 0) return 1; 2065 } 2066 return 0; 2067 } 2068 2069 // NotRunnable() -- informed spinning 2070 // 2071 // Don't bother spinning if the owner is not eligible to drop the lock. 2072 // Peek at the owner's schedctl.sc_state and Thread._thread_values and 2073 // spin only if the owner thread is _thread_in_Java or _thread_in_vm. 2074 // The thread must be runnable in order to drop the lock in timely fashion. 2075 // If the _owner is not runnable then spinning will not likely be 2076 // successful (profitable). 2077 // 2078 // Beware -- the thread referenced by _owner could have died 2079 // so a simply fetch from _owner->_thread_state might trap. 2080 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state. 2081 // Because of the lifecycle issues the schedctl and _thread_state values 2082 // observed by NotRunnable() might be garbage. NotRunnable must 2083 // tolerate this and consider the observed _thread_state value 2084 // as advisory. 2085 // 2086 // Beware too, that _owner is sometimes a BasicLock address and sometimes 2087 // a thread pointer. 2088 // Alternately, we might tag the type (thread pointer vs basiclock pointer) 2089 // with the LSB of _owner. Another option would be to probablistically probe 2090 // the putative _owner->TypeTag value. 2091 // 2092 // Checking _thread_state isn't perfect. Even if the thread is 2093 // in_java it might be blocked on a page-fault or have been preempted 2094 // and sitting on a ready/dispatch queue. _thread state in conjunction 2095 // with schedctl.sc_state gives us a good picture of what the 2096 // thread is doing, however. 2097 // 2098 // TODO: check schedctl.sc_state. 2099 // We'll need to use SafeFetch32() to read from the schedctl block. 2100 // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/ 2101 // 2102 // The return value from NotRunnable() is *advisory* -- the 2103 // result is based on sampling and is not necessarily coherent. 2104 // The caller must tolerate false-negative and false-positive errors. 2105 // Spinning, in general, is probabilistic anyway. 2106 2107 2108 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) { 2109 // Check ox->TypeTag == 2BAD. 2110 if (ox == NULL) return 0; 2111 2112 // Avoid transitive spinning ... 2113 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L. 2114 // Immediately after T1 acquires L it's possible that T2, also 2115 // spinning on L, will see L.Owner=T1 and T1._Stalled=L. 2116 // This occurs transiently after T1 acquired L but before 2117 // T1 managed to clear T1.Stalled. T2 does not need to abort 2118 // its spin in this circumstance. 2119 intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1)); 2120 2121 if (BlockedOn == 1) return 1; 2122 if (BlockedOn != 0) { 2123 return BlockedOn != intptr_t(this) && _owner == ox; 2124 } 2125 2126 assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant"); 2127 int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);; 2128 // consider also: jst != _thread_in_Java -- but that's overspecific. 2129 return jst == _thread_blocked || jst == _thread_in_native; 2130 } 2131 2132 2133 // ----------------------------------------------------------------------------- 2134 // WaitSet management ... 2135 2136 ObjectWaiter::ObjectWaiter(Thread* thread) { 2137 _next = NULL; 2138 _prev = NULL; 2139 _notified = 0; 2140 TState = TS_RUN; 2141 _thread = thread; 2142 _event = thread->_ParkEvent; 2143 _active = false; 2144 assert(_event != NULL, "invariant"); 2145 } 2146 2147 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) { 2148 JavaThread *jt = (JavaThread *)this->_thread; 2149 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon); 2150 } 2151 2152 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) { 2153 JavaThread *jt = (JavaThread *)this->_thread; 2154 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active); 2155 } 2156 2157 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) { 2158 assert(node != NULL, "should not add NULL node"); 2159 assert(node->_prev == NULL, "node already in list"); 2160 assert(node->_next == NULL, "node already in list"); 2161 // put node at end of queue (circular doubly linked list) 2162 if (_WaitSet == NULL) { 2163 _WaitSet = node; 2164 node->_prev = node; 2165 node->_next = node; 2166 } else { 2167 ObjectWaiter* head = _WaitSet; 2168 ObjectWaiter* tail = head->_prev; 2169 assert(tail->_next == head, "invariant check"); 2170 tail->_next = node; 2171 head->_prev = node; 2172 node->_next = head; 2173 node->_prev = tail; 2174 } 2175 } 2176 2177 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() { 2178 // dequeue the very first waiter 2179 ObjectWaiter* waiter = _WaitSet; 2180 if (waiter) { 2181 DequeueSpecificWaiter(waiter); 2182 } 2183 return waiter; 2184 } 2185 2186 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) { 2187 assert(node != NULL, "should not dequeue NULL node"); 2188 assert(node->_prev != NULL, "node already removed from list"); 2189 assert(node->_next != NULL, "node already removed from list"); 2190 // when the waiter has woken up because of interrupt, 2191 // timeout or other spurious wake-up, dequeue the 2192 // waiter from waiting list 2193 ObjectWaiter* next = node->_next; 2194 if (next == node) { 2195 assert(node->_prev == node, "invariant check"); 2196 _WaitSet = NULL; 2197 } else { 2198 ObjectWaiter* prev = node->_prev; 2199 assert(prev->_next == node, "invariant check"); 2200 assert(next->_prev == node, "invariant check"); 2201 next->_prev = prev; 2202 prev->_next = next; 2203 if (_WaitSet == node) { 2204 _WaitSet = next; 2205 } 2206 } 2207 node->_next = NULL; 2208 node->_prev = NULL; 2209 } 2210 2211 // ----------------------------------------------------------------------------- 2212 // PerfData support 2213 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL; 2214 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL; 2215 PerfCounter * ObjectMonitor::_sync_Parks = NULL; 2216 PerfCounter * ObjectMonitor::_sync_EmptyNotifications = NULL; 2217 PerfCounter * ObjectMonitor::_sync_Notifications = NULL; 2218 PerfCounter * ObjectMonitor::_sync_PrivateA = NULL; 2219 PerfCounter * ObjectMonitor::_sync_PrivateB = NULL; 2220 PerfCounter * ObjectMonitor::_sync_SlowExit = NULL; 2221 PerfCounter * ObjectMonitor::_sync_SlowEnter = NULL; 2222 PerfCounter * ObjectMonitor::_sync_SlowNotify = NULL; 2223 PerfCounter * ObjectMonitor::_sync_SlowNotifyAll = NULL; 2224 PerfCounter * ObjectMonitor::_sync_FailedSpins = NULL; 2225 PerfCounter * ObjectMonitor::_sync_SuccessfulSpins = NULL; 2226 PerfCounter * ObjectMonitor::_sync_MonInCirculation = NULL; 2227 PerfCounter * ObjectMonitor::_sync_MonScavenged = NULL; 2228 PerfCounter * ObjectMonitor::_sync_Inflations = NULL; 2229 PerfCounter * ObjectMonitor::_sync_Deflations = NULL; 2230 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL; 2231 2232 // One-shot global initialization for the sync subsystem. 2233 // We could also defer initialization and initialize on-demand 2234 // the first time we call inflate(). Initialization would 2235 // be protected - like so many things - by the MonitorCache_lock. 2236 2237 void ObjectMonitor::Initialize() { 2238 static int InitializationCompleted = 0; 2239 assert(InitializationCompleted == 0, "invariant"); 2240 InitializationCompleted = 1; 2241 if (UsePerfData) { 2242 EXCEPTION_MARK; 2243 #define NEWPERFCOUNTER(n) \ 2244 { \ 2245 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \ 2246 CHECK); \ 2247 } 2248 #define NEWPERFVARIABLE(n) \ 2249 { \ 2250 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \ 2251 CHECK); \ 2252 } 2253 NEWPERFCOUNTER(_sync_Inflations); 2254 NEWPERFCOUNTER(_sync_Deflations); 2255 NEWPERFCOUNTER(_sync_ContendedLockAttempts); 2256 NEWPERFCOUNTER(_sync_FutileWakeups); 2257 NEWPERFCOUNTER(_sync_Parks); 2258 NEWPERFCOUNTER(_sync_EmptyNotifications); 2259 NEWPERFCOUNTER(_sync_Notifications); 2260 NEWPERFCOUNTER(_sync_SlowEnter); 2261 NEWPERFCOUNTER(_sync_SlowExit); 2262 NEWPERFCOUNTER(_sync_SlowNotify); 2263 NEWPERFCOUNTER(_sync_SlowNotifyAll); 2264 NEWPERFCOUNTER(_sync_FailedSpins); 2265 NEWPERFCOUNTER(_sync_SuccessfulSpins); 2266 NEWPERFCOUNTER(_sync_PrivateA); 2267 NEWPERFCOUNTER(_sync_PrivateB); 2268 NEWPERFCOUNTER(_sync_MonInCirculation); 2269 NEWPERFCOUNTER(_sync_MonScavenged); 2270 NEWPERFVARIABLE(_sync_MonExtant); 2271 #undef NEWPERFCOUNTER 2272 #undef NEWPERFVARIABLE 2273 } 2274 } 2275 2276 static char * kvGet(char * kvList, const char * Key) { 2277 if (kvList == NULL) return NULL; 2278 size_t n = strlen(Key); 2279 char * Search; 2280 for (Search = kvList; *Search; Search += strlen(Search) + 1) { 2281 if (strncmp (Search, Key, n) == 0) { 2282 if (Search[n] == '=') return Search + n + 1; 2283 if (Search[n] == 0) return(char *) "1"; 2284 } 2285 } 2286 return NULL; 2287 } 2288 2289 static int kvGetInt(char * kvList, const char * Key, int Default) { 2290 char * v = kvGet(kvList, Key); 2291 int rslt = v ? ::strtol(v, NULL, 0) : Default; 2292 if (Knob_ReportSettings && v != NULL) { 2293 tty->print_cr("INFO: SyncKnob: %s %d(%d)", Key, rslt, Default) ; 2294 tty->flush(); 2295 } 2296 return rslt; 2297 } 2298 2299 void ObjectMonitor::DeferredInitialize() { 2300 if (InitDone > 0) return; 2301 if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) { 2302 while (InitDone != 1) /* empty */; 2303 return; 2304 } 2305 2306 // One-shot global initialization ... 2307 // The initialization is idempotent, so we don't need locks. 2308 // In the future consider doing this via os::init_2(). 2309 // SyncKnobs consist of <Key>=<Value> pairs in the style 2310 // of environment variables. Start by converting ':' to NUL. 2311 2312 if (SyncKnobs == NULL) SyncKnobs = ""; 2313 2314 size_t sz = strlen(SyncKnobs); 2315 char * knobs = (char *) malloc(sz + 2); 2316 if (knobs == NULL) { 2317 vm_exit_out_of_memory(sz + 2, OOM_MALLOC_ERROR, "Parse SyncKnobs"); 2318 guarantee(0, "invariant"); 2319 } 2320 strcpy(knobs, SyncKnobs); 2321 knobs[sz+1] = 0; 2322 for (char * p = knobs; *p; p++) { 2323 if (*p == ':') *p = 0; 2324 } 2325 2326 #define SETKNOB(x) { Knob_##x = kvGetInt(knobs, #x, Knob_##x); } 2327 SETKNOB(ReportSettings); 2328 SETKNOB(ExitRelease); 2329 SETKNOB(Verbose); 2330 SETKNOB(VerifyInUse); 2331 SETKNOB(VerifyMatch); 2332 SETKNOB(FixedSpin); 2333 SETKNOB(SpinLimit); 2334 SETKNOB(SpinBase); 2335 SETKNOB(SpinBackOff); 2336 SETKNOB(CASPenalty); 2337 SETKNOB(OXPenalty); 2338 SETKNOB(LogSpins); 2339 SETKNOB(SpinSetSucc); 2340 SETKNOB(SuccEnabled); 2341 SETKNOB(SuccRestrict); 2342 SETKNOB(Penalty); 2343 SETKNOB(Bonus); 2344 SETKNOB(BonusB); 2345 SETKNOB(Poverty); 2346 SETKNOB(SpinAfterFutile); 2347 SETKNOB(UsePause); 2348 SETKNOB(SpinEarly); 2349 SETKNOB(OState); 2350 SETKNOB(MaxSpinners); 2351 SETKNOB(PreSpin); 2352 SETKNOB(ExitPolicy); 2353 SETKNOB(QMode); 2354 SETKNOB(ResetEvent); 2355 SETKNOB(MoveNotifyee); 2356 SETKNOB(FastHSSEC); 2357 #undef SETKNOB 2358 2359 if (Knob_Verbose) { 2360 sanity_checks(); 2361 } 2362 2363 if (os::is_MP()) { 2364 BackOffMask = (1 << Knob_SpinBackOff) - 1; 2365 if (Knob_ReportSettings) { 2366 tty->print_cr("INFO: BackOffMask=0x%X", BackOffMask); 2367 } 2368 // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1) 2369 } else { 2370 Knob_SpinLimit = 0; 2371 Knob_SpinBase = 0; 2372 Knob_PreSpin = 0; 2373 Knob_FixedSpin = -1; 2374 } 2375 2376 if (Knob_LogSpins == 0) { 2377 ObjectMonitor::_sync_FailedSpins = NULL; 2378 } 2379 2380 free(knobs); 2381 OrderAccess::fence(); 2382 InitDone = 1; 2383 } 2384 2385 void ObjectMonitor::sanity_checks() { 2386 int error_cnt = 0; 2387 int warning_cnt = 0; 2388 bool verbose = Knob_Verbose != 0 NOT_PRODUCT(|| VerboseInternalVMTests); 2389 2390 if (verbose) { 2391 tty->print_cr("INFO: sizeof(ObjectMonitor)=" SIZE_FORMAT, 2392 sizeof(ObjectMonitor)); 2393 tty->print_cr("INFO: sizeof(PaddedEnd<ObjectMonitor>)=" SIZE_FORMAT, 2394 sizeof(PaddedEnd<ObjectMonitor>)); 2395 } 2396 2397 uint cache_line_size = VM_Version::L1_data_cache_line_size(); 2398 if (verbose) { 2399 tty->print_cr("INFO: L1_data_cache_line_size=%u", cache_line_size); 2400 } 2401 2402 ObjectMonitor dummy; 2403 u_char *addr_begin = (u_char*)&dummy; 2404 u_char *addr_header = (u_char*)&dummy._header; 2405 u_char *addr_owner = (u_char*)&dummy._owner; 2406 2407 uint offset_header = (uint)(addr_header - addr_begin); 2408 if (verbose) tty->print_cr("INFO: offset(_header)=%u", offset_header); 2409 2410 uint offset_owner = (uint)(addr_owner - addr_begin); 2411 if (verbose) tty->print_cr("INFO: offset(_owner)=%u", offset_owner); 2412 2413 if ((uint)(addr_header - addr_begin) != 0) { 2414 tty->print_cr("ERROR: offset(_header) must be zero (0)."); 2415 error_cnt++; 2416 } 2417 2418 if (cache_line_size != 0) { 2419 // We were able to determine the L1 data cache line size so 2420 // do some cache line specific sanity checks 2421 2422 if ((offset_owner - offset_header) < cache_line_size) { 2423 tty->print_cr("WARNING: the _header and _owner fields are closer " 2424 "than a cache line which permits false sharing."); 2425 warning_cnt++; 2426 } 2427 2428 if ((sizeof(PaddedEnd<ObjectMonitor>) % cache_line_size) != 0) { 2429 tty->print_cr("WARNING: PaddedEnd<ObjectMonitor> size is not a " 2430 "multiple of a cache line which permits false sharing."); 2431 warning_cnt++; 2432 } 2433 } 2434 2435 ObjectSynchronizer::sanity_checks(verbose, cache_line_size, &error_cnt, 2436 &warning_cnt); 2437 2438 if (verbose || error_cnt != 0 || warning_cnt != 0) { 2439 tty->print_cr("INFO: error_cnt=%d", error_cnt); 2440 tty->print_cr("INFO: warning_cnt=%d", warning_cnt); 2441 } 2442 2443 guarantee(error_cnt == 0, 2444 "Fatal error(s) found in ObjectMonitor::sanity_checks()"); 2445 } 2446 2447 #ifndef PRODUCT 2448 void ObjectMonitor_test() { 2449 ObjectMonitor::sanity_checks(); 2450 } 2451 #endif