1 /* 2 * Copyright (c) 1998, 2019, 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 "jfr/jfrEvents.hpp" 28 #include "jfr/support/jfrThreadId.hpp" 29 #include "logging/log.hpp" 30 #include "logging/logStream.hpp" 31 #include "memory/allocation.inline.hpp" 32 #include "memory/resourceArea.hpp" 33 #include "oops/markWord.hpp" 34 #include "oops/oop.inline.hpp" 35 #include "runtime/atomic.hpp" 36 #include "runtime/handles.inline.hpp" 37 #include "runtime/interfaceSupport.inline.hpp" 38 #include "runtime/mutexLocker.hpp" 39 #include "runtime/objectMonitor.hpp" 40 #include "runtime/objectMonitor.inline.hpp" 41 #include "runtime/orderAccess.hpp" 42 #include "runtime/osThread.hpp" 43 #include "runtime/safepointMechanism.inline.hpp" 44 #include "runtime/sharedRuntime.hpp" 45 #include "runtime/stubRoutines.hpp" 46 #include "runtime/thread.inline.hpp" 47 #include "services/threadService.hpp" 48 #include "utilities/dtrace.hpp" 49 #include "utilities/macros.hpp" 50 #include "utilities/preserveException.hpp" 51 #if INCLUDE_JFR 52 #include "jfr/support/jfrFlush.hpp" 53 #endif 54 55 #ifdef DTRACE_ENABLED 56 57 // Only bother with this argument setup if dtrace is available 58 // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly. 59 60 61 #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \ 62 char* bytes = NULL; \ 63 int len = 0; \ 64 jlong jtid = SharedRuntime::get_java_tid(thread); \ 65 Symbol* klassname = ((oop)obj)->klass()->name(); \ 66 if (klassname != NULL) { \ 67 bytes = (char*)klassname->bytes(); \ 68 len = klassname->utf8_length(); \ 69 } 70 71 #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \ 72 { \ 73 if (DTraceMonitorProbes) { \ 74 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \ 75 HOTSPOT_MONITOR_WAIT(jtid, \ 76 (monitor), bytes, len, (millis)); \ 77 } \ 78 } 79 80 #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER 81 #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED 82 #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT 83 #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY 84 #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL 85 86 #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \ 87 { \ 88 if (DTraceMonitorProbes) { \ 89 DTRACE_MONITOR_PROBE_COMMON(obj, thread); \ 90 HOTSPOT_MONITOR_##probe(jtid, \ 91 (uintptr_t)(monitor), bytes, len); \ 92 } \ 93 } 94 95 #else // ndef DTRACE_ENABLED 96 97 #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;} 98 #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;} 99 100 #endif // ndef DTRACE_ENABLED 101 102 // Tunables ... 103 // The knob* variables are effectively final. Once set they should 104 // never be modified hence. Consider using __read_mostly with GCC. 105 106 int ObjectMonitor::Knob_SpinLimit = 5000; // derived by an external tool - 107 108 static int Knob_Bonus = 100; // spin success bonus 109 static int Knob_BonusB = 100; // spin success bonus 110 static int Knob_Penalty = 200; // spin failure penalty 111 static int Knob_Poverty = 1000; 112 static int Knob_FixedSpin = 0; 113 static int Knob_PreSpin = 10; // 20-100 likely better 114 115 DEBUG_ONLY(static volatile bool InitDone = false;) 116 117 // ----------------------------------------------------------------------------- 118 // Theory of operations -- Monitors lists, thread residency, etc: 119 // 120 // * A thread acquires ownership of a monitor by successfully 121 // CAS()ing the _owner field from null to non-null. 122 // 123 // * Invariant: A thread appears on at most one monitor list -- 124 // cxq, EntryList or WaitSet -- at any one time. 125 // 126 // * Contending threads "push" themselves onto the cxq with CAS 127 // and then spin/park. 128 // 129 // * After a contending thread eventually acquires the lock it must 130 // dequeue itself from either the EntryList or the cxq. 131 // 132 // * The exiting thread identifies and unparks an "heir presumptive" 133 // tentative successor thread on the EntryList. Critically, the 134 // exiting thread doesn't unlink the successor thread from the EntryList. 135 // After having been unparked, the wakee will recontend for ownership of 136 // the monitor. The successor (wakee) will either acquire the lock or 137 // re-park itself. 138 // 139 // Succession is provided for by a policy of competitive handoff. 140 // The exiting thread does _not_ grant or pass ownership to the 141 // successor thread. (This is also referred to as "handoff" succession"). 142 // Instead the exiting thread releases ownership and possibly wakes 143 // a successor, so the successor can (re)compete for ownership of the lock. 144 // If the EntryList is empty but the cxq is populated the exiting 145 // thread will drain the cxq into the EntryList. It does so by 146 // by detaching the cxq (installing null with CAS) and folding 147 // the threads from the cxq into the EntryList. The EntryList is 148 // doubly linked, while the cxq is singly linked because of the 149 // CAS-based "push" used to enqueue recently arrived threads (RATs). 150 // 151 // * Concurrency invariants: 152 // 153 // -- only the monitor owner may access or mutate the EntryList. 154 // The mutex property of the monitor itself protects the EntryList 155 // from concurrent interference. 156 // -- Only the monitor owner may detach the cxq. 157 // 158 // * The monitor entry list operations avoid locks, but strictly speaking 159 // they're not lock-free. Enter is lock-free, exit is not. 160 // For a description of 'Methods and apparatus providing non-blocking access 161 // to a resource,' see U.S. Pat. No. 7844973. 162 // 163 // * The cxq can have multiple concurrent "pushers" but only one concurrent 164 // detaching thread. This mechanism is immune from the ABA corruption. 165 // More precisely, the CAS-based "push" onto cxq is ABA-oblivious. 166 // 167 // * Taken together, the cxq and the EntryList constitute or form a 168 // single logical queue of threads stalled trying to acquire the lock. 169 // We use two distinct lists to improve the odds of a constant-time 170 // dequeue operation after acquisition (in the ::enter() epilogue) and 171 // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm). 172 // A key desideratum is to minimize queue & monitor metadata manipulation 173 // that occurs while holding the monitor lock -- that is, we want to 174 // minimize monitor lock holds times. Note that even a small amount of 175 // fixed spinning will greatly reduce the # of enqueue-dequeue operations 176 // on EntryList|cxq. That is, spinning relieves contention on the "inner" 177 // locks and monitor metadata. 178 // 179 // Cxq points to the set of Recently Arrived Threads attempting entry. 180 // Because we push threads onto _cxq with CAS, the RATs must take the form of 181 // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when 182 // the unlocking thread notices that EntryList is null but _cxq is != null. 183 // 184 // The EntryList is ordered by the prevailing queue discipline and 185 // can be organized in any convenient fashion, such as a doubly-linked list or 186 // a circular doubly-linked list. Critically, we want insert and delete operations 187 // to operate in constant-time. If we need a priority queue then something akin 188 // to Solaris' sleepq would work nicely. Viz., 189 // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c. 190 // Queue discipline is enforced at ::exit() time, when the unlocking thread 191 // drains the cxq into the EntryList, and orders or reorders the threads on the 192 // EntryList accordingly. 193 // 194 // Barring "lock barging", this mechanism provides fair cyclic ordering, 195 // somewhat similar to an elevator-scan. 196 // 197 // * The monitor synchronization subsystem avoids the use of native 198 // synchronization primitives except for the narrow platform-specific 199 // park-unpark abstraction. See the comments in os_solaris.cpp regarding 200 // the semantics of park-unpark. Put another way, this monitor implementation 201 // depends only on atomic operations and park-unpark. The monitor subsystem 202 // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the 203 // underlying OS manages the READY<->RUN transitions. 204 // 205 // * Waiting threads reside on the WaitSet list -- wait() puts 206 // the caller onto the WaitSet. 207 // 208 // * notify() or notifyAll() simply transfers threads from the WaitSet to 209 // either the EntryList or cxq. Subsequent exit() operations will 210 // unpark the notifyee. Unparking a notifee in notify() is inefficient - 211 // it's likely the notifyee would simply impale itself on the lock held 212 // by the notifier. 213 // 214 // * An interesting alternative is to encode cxq as (List,LockByte) where 215 // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary 216 // variable, like _recursions, in the scheme. The threads or Events that form 217 // the list would have to be aligned in 256-byte addresses. A thread would 218 // try to acquire the lock or enqueue itself with CAS, but exiting threads 219 // could use a 1-0 protocol and simply STB to set the LockByte to 0. 220 // Note that is is *not* word-tearing, but it does presume that full-word 221 // CAS operations are coherent with intermix with STB operations. That's true 222 // on most common processors. 223 // 224 // * See also http://blogs.sun.com/dave 225 226 227 void* ObjectMonitor::operator new (size_t size) throw() { 228 return AllocateHeap(size, mtInternal); 229 } 230 void* ObjectMonitor::operator new[] (size_t size) throw() { 231 return operator new (size); 232 } 233 void ObjectMonitor::operator delete(void* p) { 234 FreeHeap(p); 235 } 236 void ObjectMonitor::operator delete[] (void *p) { 237 operator delete(p); 238 } 239 240 // ----------------------------------------------------------------------------- 241 // Enter support 242 243 void ObjectMonitor::enter(TRAPS) { 244 jint l_ref_count = ref_count(); 245 ADIM_guarantee(l_ref_count > 0, "must be positive: l_ref_count=%d, ref_count=%d", l_ref_count, ref_count()); 246 247 // The following code is ordered to check the most common cases first 248 // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors. 249 Thread * const Self = THREAD; 250 251 void* cur = try_set_owner_from(Self, NULL); 252 if (cur == NULL) { 253 assert(_recursions == 0, "invariant"); 254 return; 255 } 256 257 if (cur == Self) { 258 // TODO-FIXME: check for integer overflow! BUGID 6557169. 259 _recursions++; 260 return; 261 } 262 263 if (Self->is_lock_owned((address)cur)) { 264 assert(_recursions == 0, "internal state error"); 265 _recursions = 1; 266 simply_set_owner_from_BasicLock(Self, cur); // Convert from BasicLock* to Thread*. 267 return; 268 } 269 270 if (AsyncDeflateIdleMonitors && 271 try_set_owner_from(Self, DEFLATER_MARKER) == DEFLATER_MARKER) { 272 // The deflation protocol finished the first part (setting owner), 273 // but it failed the second part (making ref_count negative) and 274 // bailed. Or the ObjectMonitor was async deflated and reused. 275 // Acquired the monitor. 276 assert(_recursions == 0, "invariant"); 277 return; 278 } 279 280 // We've encountered genuine contention. 281 assert(Self->_Stalled == 0, "invariant"); 282 Self->_Stalled = intptr_t(this); 283 284 // Try one round of spinning *before* enqueueing Self 285 // and before going through the awkward and expensive state 286 // transitions. The following spin is strictly optional ... 287 // Note that if we acquire the monitor from an initial spin 288 // we forgo posting JVMTI events and firing DTRACE probes. 289 if (TrySpin(Self) > 0) { 290 assert(_owner == Self, "must be Self: owner=" INTPTR_FORMAT, p2i(_owner)); 291 assert(_recursions == 0, "must be 0: recursions=" INTX_FORMAT, _recursions); 292 assert(((oop)object())->mark() == markWord::encode(this), 293 "object mark must match encoded this: mark=" INTPTR_FORMAT 294 ", encoded this=" INTPTR_FORMAT, ((oop)object())->mark().value(), 295 markWord::encode(this).value()); 296 Self->_Stalled = 0; 297 return; 298 } 299 300 assert(_owner != Self, "invariant"); 301 assert(_succ != Self, "invariant"); 302 assert(Self->is_Java_thread(), "invariant"); 303 JavaThread * jt = (JavaThread *) Self; 304 assert(!SafepointSynchronize::is_at_safepoint(), "invariant"); 305 assert(jt->thread_state() != _thread_blocked, "invariant"); 306 assert(this->object() != NULL, "invariant"); 307 assert(_contentions >= 0, "must not be negative: contentions=%d", _contentions); 308 309 // Keep track of contention for JVM/TI and M&M queries. 310 Atomic::inc(&_contentions); 311 312 JFR_ONLY(JfrConditionalFlushWithStacktrace<EventJavaMonitorEnter> flush(jt);) 313 EventJavaMonitorEnter event; 314 if (event.should_commit()) { 315 event.set_monitorClass(((oop)this->object())->klass()); 316 event.set_address((uintptr_t)(this->object_addr())); 317 } 318 319 { // Change java thread status to indicate blocked on monitor enter. 320 JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this); 321 322 Self->set_current_pending_monitor(this); 323 324 DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt); 325 if (JvmtiExport::should_post_monitor_contended_enter()) { 326 JvmtiExport::post_monitor_contended_enter(jt, this); 327 328 // The current thread does not yet own the monitor and does not 329 // yet appear on any queues that would get it made the successor. 330 // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event 331 // handler cannot accidentally consume an unpark() meant for the 332 // ParkEvent associated with this ObjectMonitor. 333 } 334 335 OSThreadContendState osts(Self->osthread()); 336 ThreadBlockInVM tbivm(jt); 337 338 // TODO-FIXME: change the following for(;;) loop to straight-line code. 339 for (;;) { 340 jt->set_suspend_equivalent(); 341 // cleared by handle_special_suspend_equivalent_condition() 342 // or java_suspend_self() 343 344 EnterI(THREAD); 345 346 if (!ExitSuspendEquivalent(jt)) break; 347 348 // We have acquired the contended monitor, but while we were 349 // waiting another thread suspended us. We don't want to enter 350 // the monitor while suspended because that would surprise the 351 // thread that suspended us. 352 // 353 _recursions = 0; 354 _succ = NULL; 355 exit(false, Self); 356 357 jt->java_suspend_self(); 358 } 359 Self->set_current_pending_monitor(NULL); 360 361 // We cleared the pending monitor info since we've just gotten past 362 // the enter-check-for-suspend dance and we now own the monitor free 363 // and clear, i.e., it is no longer pending. The ThreadBlockInVM 364 // destructor can go to a safepoint at the end of this block. If we 365 // do a thread dump during that safepoint, then this thread will show 366 // as having "-locked" the monitor, but the OS and java.lang.Thread 367 // states will still report that the thread is blocked trying to 368 // acquire it. 369 } 370 371 Atomic::dec(&_contentions); 372 assert(_contentions >= 0, "must not be negative: contentions=%d", _contentions); 373 Self->_Stalled = 0; 374 375 // Must either set _recursions = 0 or ASSERT _recursions == 0. 376 assert(_recursions == 0, "invariant"); 377 assert(_owner == Self, "invariant"); 378 assert(_succ != Self, "invariant"); 379 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant"); 380 381 // The thread -- now the owner -- is back in vm mode. 382 // Report the glorious news via TI,DTrace and jvmstat. 383 // The probe effect is non-trivial. All the reportage occurs 384 // while we hold the monitor, increasing the length of the critical 385 // section. Amdahl's parallel speedup law comes vividly into play. 386 // 387 // Another option might be to aggregate the events (thread local or 388 // per-monitor aggregation) and defer reporting until a more opportune 389 // time -- such as next time some thread encounters contention but has 390 // yet to acquire the lock. While spinning that thread could 391 // spinning we could increment JVMStat counters, etc. 392 393 DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt); 394 if (JvmtiExport::should_post_monitor_contended_entered()) { 395 JvmtiExport::post_monitor_contended_entered(jt, this); 396 397 // The current thread already owns the monitor and is not going to 398 // call park() for the remainder of the monitor enter protocol. So 399 // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED 400 // event handler consumed an unpark() issued by the thread that 401 // just exited the monitor. 402 } 403 if (event.should_commit()) { 404 event.set_previousOwner((uintptr_t)_previous_owner_tid); 405 event.commit(); 406 } 407 OM_PERFDATA_OP(ContendedLockAttempts, inc()); 408 } 409 410 // Caveat: TryLock() is not necessarily serializing if it returns failure. 411 // Callers must compensate as needed. 412 413 int ObjectMonitor::TryLock(Thread * Self) { 414 void * own = _owner; 415 if (own != NULL) return 0; 416 if (try_set_owner_from(Self, NULL) == NULL) { 417 assert(_recursions == 0, "invariant"); 418 return 1; 419 } 420 // The lock had been free momentarily, but we lost the race to the lock. 421 // Interference -- the CAS failed. 422 // We can either return -1 or retry. 423 // Retry doesn't make as much sense because the lock was just acquired. 424 return -1; 425 } 426 427 // Install the displaced mark word (dmw) of a deflating ObjectMonitor 428 // into the header of the object associated with the monitor. This 429 // idempotent method is called by a thread that is deflating a 430 // monitor and by other threads that have detected a race with the 431 // deflation process. 432 void ObjectMonitor::install_displaced_markword_in_object(const oop obj) { 433 // This function must only be called when (owner == DEFLATER_MARKER 434 // && ref_count <= 0), but we can't guarantee that here because 435 // those values could change when the ObjectMonitor gets moved from 436 // the global free list to a per-thread free list. 437 438 guarantee(obj != NULL, "must be non-NULL"); 439 if (object() != obj) { 440 // ObjectMonitor's object ref no longer refers to the target object 441 // so the object's header has already been restored. 442 return; 443 } 444 445 markWord dmw = header(); 446 if (dmw.value() == 0) { 447 // ObjectMonitor's header/dmw has been cleared so the ObjectMonitor 448 // has been deflated and taken off the global free list. 449 return; 450 } 451 452 // A non-NULL dmw has to be either neutral (not locked and not marked) 453 // or is already participating in this restoration protocol. 454 assert(dmw.is_neutral() || (dmw.is_marked() && dmw.hash() == 0), 455 "failed precondition: dmw=" INTPTR_FORMAT, dmw.value()); 456 457 markWord marked_dmw = markWord::zero(); 458 if (!dmw.is_marked() && dmw.hash() == 0) { 459 // This dmw has not yet started the restoration protocol so we 460 // mark a copy of the dmw to begin the protocol. 461 // Note: A dmw with a hashcode does not take this code path. 462 marked_dmw = dmw.set_marked(); 463 464 // All of the callers to this function can be racing with each 465 // other trying to update the _header field. 466 dmw = (markWord) Atomic::cmpxchg(&_header, dmw, marked_dmw); 467 if (dmw.value() == 0) { 468 // ObjectMonitor's header/dmw has been cleared so the object's 469 // header has already been restored. 470 return; 471 } 472 // The _header field is now marked. The winner's 'dmw' variable 473 // contains the original, unmarked header/dmw value and any 474 // losers have a marked header/dmw value that will be cleaned 475 // up below. 476 } 477 478 if (dmw.is_marked()) { 479 // Clear the mark from the header/dmw copy in preparation for 480 // possible restoration from this thread. 481 assert(dmw.hash() == 0, "hashcode must be 0: dmw=" INTPTR_FORMAT, 482 dmw.value()); 483 dmw = dmw.set_unmarked(); 484 } 485 assert(dmw.is_neutral(), "must be neutral: dmw=" INTPTR_FORMAT, dmw.value()); 486 487 // Install displaced mark word if the object's header still points 488 // to this ObjectMonitor. All racing callers to this function will 489 // reach this point, but only one can win. 490 obj->cas_set_mark(dmw, markWord::encode(this)); 491 492 // Note: It does not matter which thread restored the header/dmw 493 // into the object's header. The thread deflating the monitor just 494 // wanted the object's header restored and it is. The threads that 495 // detected a race with the deflation process also wanted the 496 // object's header restored before they retry their operation and 497 // because it is restored they will only retry once. 498 } 499 500 // Convert the fields used by is_busy() to a string that can be 501 // used for diagnostic output. 502 const char* ObjectMonitor::is_busy_to_string(stringStream* ss) { 503 ss->print("is_busy: contentions=%d, waiters=%d, ", _contentions, _waiters); 504 if (!AsyncDeflateIdleMonitors) { 505 ss->print("owner=" INTPTR_FORMAT, p2i(_owner)); 506 } else if (_owner != DEFLATER_MARKER) { 507 ss->print("owner=" INTPTR_FORMAT, p2i(_owner)); 508 } else { 509 // We report NULL instead of DEFLATER_MARKER here because is_busy() 510 // ignores DEFLATER_MARKER values. 511 ss->print("owner=" INTPTR_FORMAT, NULL); 512 } 513 ss->print(", cxq=" INTPTR_FORMAT ", EntryList=" INTPTR_FORMAT, p2i(_cxq), 514 p2i(_EntryList)); 515 return ss->base(); 516 } 517 518 #define MAX_RECHECK_INTERVAL 1000 519 520 void ObjectMonitor::EnterI(TRAPS) { 521 jint l_ref_count = ref_count(); 522 ADIM_guarantee(l_ref_count > 0, "must be positive: l_ref_count=%d, ref_count=%d", l_ref_count, ref_count()); 523 524 Thread * const Self = THREAD; 525 assert(Self->is_Java_thread(), "invariant"); 526 assert(((JavaThread *) Self)->thread_state() == _thread_blocked, "invariant"); 527 528 // Try the lock - TATAS 529 if (TryLock (Self) > 0) { 530 assert(_succ != Self, "invariant"); 531 assert(_owner == Self, "invariant"); 532 assert(_Responsible != Self, "invariant"); 533 return; 534 } 535 536 if (AsyncDeflateIdleMonitors && 537 try_set_owner_from(Self, DEFLATER_MARKER) == DEFLATER_MARKER) { 538 // The deflation protocol finished the first part (setting owner), 539 // but it failed the second part (making ref_count negative) and 540 // bailed. Or the ObjectMonitor was async deflated and reused. 541 // Acquired the monitor. 542 assert(_succ != Self, "invariant"); 543 assert(_Responsible != Self, "invariant"); 544 return; 545 } 546 547 assert(InitDone, "Unexpectedly not initialized"); 548 549 // We try one round of spinning *before* enqueueing Self. 550 // 551 // If the _owner is ready but OFFPROC we could use a YieldTo() 552 // operation to donate the remainder of this thread's quantum 553 // to the owner. This has subtle but beneficial affinity 554 // effects. 555 556 if (TrySpin(Self) > 0) { 557 assert(_owner == Self, "invariant"); 558 assert(_succ != Self, "invariant"); 559 assert(_Responsible != Self, "invariant"); 560 return; 561 } 562 563 // The Spin failed -- Enqueue and park the thread ... 564 assert(_succ != Self, "invariant"); 565 assert(_owner != Self, "invariant"); 566 assert(_Responsible != Self, "invariant"); 567 568 // Enqueue "Self" on ObjectMonitor's _cxq. 569 // 570 // Node acts as a proxy for Self. 571 // As an aside, if were to ever rewrite the synchronization code mostly 572 // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class 573 // Java objects. This would avoid awkward lifecycle and liveness issues, 574 // as well as eliminate a subset of ABA issues. 575 // TODO: eliminate ObjectWaiter and enqueue either Threads or Events. 576 577 ObjectWaiter node(Self); 578 Self->_ParkEvent->reset(); 579 node._prev = (ObjectWaiter *) 0xBAD; 580 node.TState = ObjectWaiter::TS_CXQ; 581 582 // Push "Self" onto the front of the _cxq. 583 // Once on cxq/EntryList, Self stays on-queue until it acquires the lock. 584 // Note that spinning tends to reduce the rate at which threads 585 // enqueue and dequeue on EntryList|cxq. 586 ObjectWaiter * nxt; 587 for (;;) { 588 node._next = nxt = _cxq; 589 if (Atomic::cmpxchg(&_cxq, nxt, &node) == nxt) break; 590 591 // Interference - the CAS failed because _cxq changed. Just retry. 592 // As an optional optimization we retry the lock. 593 if (TryLock (Self) > 0) { 594 assert(_succ != Self, "invariant"); 595 assert(_owner == Self, "invariant"); 596 assert(_Responsible != Self, "invariant"); 597 return; 598 } 599 } 600 601 // Check for cxq|EntryList edge transition to non-null. This indicates 602 // the onset of contention. While contention persists exiting threads 603 // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit 604 // operations revert to the faster 1-0 mode. This enter operation may interleave 605 // (race) a concurrent 1-0 exit operation, resulting in stranding, so we 606 // arrange for one of the contending thread to use a timed park() operations 607 // to detect and recover from the race. (Stranding is form of progress failure 608 // where the monitor is unlocked but all the contending threads remain parked). 609 // That is, at least one of the contended threads will periodically poll _owner. 610 // One of the contending threads will become the designated "Responsible" thread. 611 // The Responsible thread uses a timed park instead of a normal indefinite park 612 // operation -- it periodically wakes and checks for and recovers from potential 613 // strandings admitted by 1-0 exit operations. We need at most one Responsible 614 // thread per-monitor at any given moment. Only threads on cxq|EntryList may 615 // be responsible for a monitor. 616 // 617 // Currently, one of the contended threads takes on the added role of "Responsible". 618 // A viable alternative would be to use a dedicated "stranding checker" thread 619 // that periodically iterated over all the threads (or active monitors) and unparked 620 // successors where there was risk of stranding. This would help eliminate the 621 // timer scalability issues we see on some platforms as we'd only have one thread 622 // -- the checker -- parked on a timer. 623 624 if (nxt == NULL && _EntryList == NULL) { 625 // Try to assume the role of responsible thread for the monitor. 626 // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self } 627 Atomic::replace_if_null(&_Responsible, Self); 628 } 629 630 // The lock might have been released while this thread was occupied queueing 631 // itself onto _cxq. To close the race and avoid "stranding" and 632 // progress-liveness failure we must resample-retry _owner before parking. 633 // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner. 634 // In this case the ST-MEMBAR is accomplished with CAS(). 635 // 636 // TODO: Defer all thread state transitions until park-time. 637 // Since state transitions are heavy and inefficient we'd like 638 // to defer the state transitions until absolutely necessary, 639 // and in doing so avoid some transitions ... 640 641 int nWakeups = 0; 642 int recheckInterval = 1; 643 644 for (;;) { 645 646 if (TryLock(Self) > 0) break; 647 assert(_owner != Self, "invariant"); 648 649 // park self 650 if (_Responsible == Self) { 651 Self->_ParkEvent->park((jlong) recheckInterval); 652 // Increase the recheckInterval, but clamp the value. 653 recheckInterval *= 8; 654 if (recheckInterval > MAX_RECHECK_INTERVAL) { 655 recheckInterval = MAX_RECHECK_INTERVAL; 656 } 657 } else { 658 Self->_ParkEvent->park(); 659 } 660 661 if (TryLock(Self) > 0) break; 662 663 if (AsyncDeflateIdleMonitors && 664 try_set_owner_from(Self, DEFLATER_MARKER) == DEFLATER_MARKER) { 665 // The deflation protocol finished the first part (setting owner), 666 // but it failed the second part (making ref_count negative) and 667 // bailed. Or the ObjectMonitor was async deflated and reused. 668 // Acquired the monitor. 669 break; 670 } 671 672 // The lock is still contested. 673 // Keep a tally of the # of futile wakeups. 674 // Note that the counter is not protected by a lock or updated by atomics. 675 // That is by design - we trade "lossy" counters which are exposed to 676 // races during updates for a lower probe effect. 677 678 // This PerfData object can be used in parallel with a safepoint. 679 // See the work around in PerfDataManager::destroy(). 680 OM_PERFDATA_OP(FutileWakeups, inc()); 681 ++nWakeups; 682 683 // Assuming this is not a spurious wakeup we'll normally find _succ == Self. 684 // We can defer clearing _succ until after the spin completes 685 // TrySpin() must tolerate being called with _succ == Self. 686 // Try yet another round of adaptive spinning. 687 if (TrySpin(Self) > 0) break; 688 689 // We can find that we were unpark()ed and redesignated _succ while 690 // we were spinning. That's harmless. If we iterate and call park(), 691 // park() will consume the event and return immediately and we'll 692 // just spin again. This pattern can repeat, leaving _succ to simply 693 // spin on a CPU. 694 695 if (_succ == Self) _succ = NULL; 696 697 // Invariant: after clearing _succ a thread *must* retry _owner before parking. 698 OrderAccess::fence(); 699 } 700 701 // Egress : 702 // Self has acquired the lock -- Unlink Self from the cxq or EntryList. 703 // Normally we'll find Self on the EntryList . 704 // From the perspective of the lock owner (this thread), the 705 // EntryList is stable and cxq is prepend-only. 706 // The head of cxq is volatile but the interior is stable. 707 // In addition, Self.TState is stable. 708 709 assert(_owner == Self, "invariant"); 710 assert(object() != NULL, "invariant"); 711 // I'd like to write: 712 // guarantee (((oop)(object()))->mark() == markWord::encode(this), "invariant") ; 713 // but as we're at a safepoint that's not safe. 714 715 UnlinkAfterAcquire(Self, &node); 716 if (_succ == Self) _succ = NULL; 717 718 assert(_succ != Self, "invariant"); 719 if (_Responsible == Self) { 720 _Responsible = NULL; 721 OrderAccess::fence(); // Dekker pivot-point 722 723 // We may leave threads on cxq|EntryList without a designated 724 // "Responsible" thread. This is benign. When this thread subsequently 725 // exits the monitor it can "see" such preexisting "old" threads -- 726 // threads that arrived on the cxq|EntryList before the fence, above -- 727 // by LDing cxq|EntryList. Newly arrived threads -- that is, threads 728 // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible 729 // non-null and elect a new "Responsible" timer thread. 730 // 731 // This thread executes: 732 // ST Responsible=null; MEMBAR (in enter epilogue - here) 733 // LD cxq|EntryList (in subsequent exit) 734 // 735 // Entering threads in the slow/contended path execute: 736 // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog) 737 // The (ST cxq; MEMBAR) is accomplished with CAS(). 738 // 739 // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent 740 // exit operation from floating above the ST Responsible=null. 741 } 742 743 // We've acquired ownership with CAS(). 744 // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics. 745 // But since the CAS() this thread may have also stored into _succ, 746 // EntryList, cxq or Responsible. These meta-data updates must be 747 // visible __before this thread subsequently drops the lock. 748 // Consider what could occur if we didn't enforce this constraint -- 749 // STs to monitor meta-data and user-data could reorder with (become 750 // visible after) the ST in exit that drops ownership of the lock. 751 // Some other thread could then acquire the lock, but observe inconsistent 752 // or old monitor meta-data and heap data. That violates the JMM. 753 // To that end, the 1-0 exit() operation must have at least STST|LDST 754 // "release" barrier semantics. Specifically, there must be at least a 755 // STST|LDST barrier in exit() before the ST of null into _owner that drops 756 // the lock. The barrier ensures that changes to monitor meta-data and data 757 // protected by the lock will be visible before we release the lock, and 758 // therefore before some other thread (CPU) has a chance to acquire the lock. 759 // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html. 760 // 761 // Critically, any prior STs to _succ or EntryList must be visible before 762 // the ST of null into _owner in the *subsequent* (following) corresponding 763 // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily 764 // execute a serializing instruction. 765 766 return; 767 } 768 769 // ReenterI() is a specialized inline form of the latter half of the 770 // contended slow-path from EnterI(). We use ReenterI() only for 771 // monitor reentry in wait(). 772 // 773 // In the future we should reconcile EnterI() and ReenterI(). 774 775 void ObjectMonitor::ReenterI(Thread * Self, ObjectWaiter * SelfNode) { 776 jint l_ref_count = ref_count(); 777 ADIM_guarantee(l_ref_count > 0, "must be positive: l_ref_count=%d, ref_count=%d", l_ref_count, ref_count()); 778 779 assert(Self != NULL, "invariant"); 780 assert(SelfNode != NULL, "invariant"); 781 assert(SelfNode->_thread == Self, "invariant"); 782 assert(_waiters > 0, "invariant"); 783 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant"); 784 assert(((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant"); 785 JavaThread * jt = (JavaThread *) Self; 786 787 int nWakeups = 0; 788 for (;;) { 789 ObjectWaiter::TStates v = SelfNode->TState; 790 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant"); 791 assert(_owner != Self, "invariant"); 792 793 if (TryLock(Self) > 0) break; 794 if (TrySpin(Self) > 0) break; 795 796 if (AsyncDeflateIdleMonitors && 797 try_set_owner_from(Self, DEFLATER_MARKER) == DEFLATER_MARKER) { 798 // The deflation protocol finished the first part (setting owner), 799 // but it failed the second part (making ref_count negative) and 800 // bailed. Or the ObjectMonitor was async deflated and reused. 801 // Acquired the monitor. 802 break; 803 } 804 805 // State transition wrappers around park() ... 806 // ReenterI() wisely defers state transitions until 807 // it's clear we must park the thread. 808 { 809 OSThreadContendState osts(Self->osthread()); 810 ThreadBlockInVM tbivm(jt); 811 812 // cleared by handle_special_suspend_equivalent_condition() 813 // or java_suspend_self() 814 jt->set_suspend_equivalent(); 815 Self->_ParkEvent->park(); 816 817 // were we externally suspended while we were waiting? 818 for (;;) { 819 if (!ExitSuspendEquivalent(jt)) break; 820 if (_succ == Self) { _succ = NULL; OrderAccess::fence(); } 821 jt->java_suspend_self(); 822 jt->set_suspend_equivalent(); 823 } 824 } 825 826 // Try again, but just so we distinguish between futile wakeups and 827 // successful wakeups. The following test isn't algorithmically 828 // necessary, but it helps us maintain sensible statistics. 829 if (TryLock(Self) > 0) break; 830 831 // The lock is still contested. 832 // Keep a tally of the # of futile wakeups. 833 // Note that the counter is not protected by a lock or updated by atomics. 834 // That is by design - we trade "lossy" counters which are exposed to 835 // races during updates for a lower probe effect. 836 ++nWakeups; 837 838 // Assuming this is not a spurious wakeup we'll normally 839 // find that _succ == Self. 840 if (_succ == Self) _succ = NULL; 841 842 // Invariant: after clearing _succ a contending thread 843 // *must* retry _owner before parking. 844 OrderAccess::fence(); 845 846 // This PerfData object can be used in parallel with a safepoint. 847 // See the work around in PerfDataManager::destroy(). 848 OM_PERFDATA_OP(FutileWakeups, inc()); 849 } 850 851 // Self has acquired the lock -- Unlink Self from the cxq or EntryList . 852 // Normally we'll find Self on the EntryList. 853 // Unlinking from the EntryList is constant-time and atomic-free. 854 // From the perspective of the lock owner (this thread), the 855 // EntryList is stable and cxq is prepend-only. 856 // The head of cxq is volatile but the interior is stable. 857 // In addition, Self.TState is stable. 858 859 assert(_owner == Self, "invariant"); 860 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant"); 861 UnlinkAfterAcquire(Self, SelfNode); 862 if (_succ == Self) _succ = NULL; 863 assert(_succ != Self, "invariant"); 864 SelfNode->TState = ObjectWaiter::TS_RUN; 865 OrderAccess::fence(); // see comments at the end of EnterI() 866 } 867 868 // By convention we unlink a contending thread from EntryList|cxq immediately 869 // after the thread acquires the lock in ::enter(). Equally, we could defer 870 // unlinking the thread until ::exit()-time. 871 872 void ObjectMonitor::UnlinkAfterAcquire(Thread *Self, ObjectWaiter *SelfNode) { 873 assert(_owner == Self, "invariant"); 874 assert(SelfNode->_thread == Self, "invariant"); 875 876 if (SelfNode->TState == ObjectWaiter::TS_ENTER) { 877 // Normal case: remove Self from the DLL EntryList . 878 // This is a constant-time operation. 879 ObjectWaiter * nxt = SelfNode->_next; 880 ObjectWaiter * prv = SelfNode->_prev; 881 if (nxt != NULL) nxt->_prev = prv; 882 if (prv != NULL) prv->_next = nxt; 883 if (SelfNode == _EntryList) _EntryList = nxt; 884 assert(nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant"); 885 assert(prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant"); 886 } else { 887 assert(SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant"); 888 // Inopportune interleaving -- Self is still on the cxq. 889 // This usually means the enqueue of self raced an exiting thread. 890 // Normally we'll find Self near the front of the cxq, so 891 // dequeueing is typically fast. If needbe we can accelerate 892 // this with some MCS/CHL-like bidirectional list hints and advisory 893 // back-links so dequeueing from the interior will normally operate 894 // in constant-time. 895 // Dequeue Self from either the head (with CAS) or from the interior 896 // with a linear-time scan and normal non-atomic memory operations. 897 // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList 898 // and then unlink Self from EntryList. We have to drain eventually, 899 // so it might as well be now. 900 901 ObjectWaiter * v = _cxq; 902 assert(v != NULL, "invariant"); 903 if (v != SelfNode || Atomic::cmpxchg(&_cxq, v, SelfNode->_next) != v) { 904 // The CAS above can fail from interference IFF a "RAT" arrived. 905 // In that case Self must be in the interior and can no longer be 906 // at the head of cxq. 907 if (v == SelfNode) { 908 assert(_cxq != v, "invariant"); 909 v = _cxq; // CAS above failed - start scan at head of list 910 } 911 ObjectWaiter * p; 912 ObjectWaiter * q = NULL; 913 for (p = v; p != NULL && p != SelfNode; p = p->_next) { 914 q = p; 915 assert(p->TState == ObjectWaiter::TS_CXQ, "invariant"); 916 } 917 assert(v != SelfNode, "invariant"); 918 assert(p == SelfNode, "Node not found on cxq"); 919 assert(p != _cxq, "invariant"); 920 assert(q != NULL, "invariant"); 921 assert(q->_next == p, "invariant"); 922 q->_next = p->_next; 923 } 924 } 925 926 #ifdef ASSERT 927 // Diagnostic hygiene ... 928 SelfNode->_prev = (ObjectWaiter *) 0xBAD; 929 SelfNode->_next = (ObjectWaiter *) 0xBAD; 930 SelfNode->TState = ObjectWaiter::TS_RUN; 931 #endif 932 } 933 934 // ----------------------------------------------------------------------------- 935 // Exit support 936 // 937 // exit() 938 // ~~~~~~ 939 // Note that the collector can't reclaim the objectMonitor or deflate 940 // the object out from underneath the thread calling ::exit() as the 941 // thread calling ::exit() never transitions to a stable state. 942 // This inhibits GC, which in turn inhibits asynchronous (and 943 // inopportune) reclamation of "this". 944 // 945 // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ; 946 // There's one exception to the claim above, however. EnterI() can call 947 // exit() to drop a lock if the acquirer has been externally suspended. 948 // In that case exit() is called with _thread_state == _thread_blocked, 949 // but the monitor's ref_count is > 0, which inhibits reclamation. 950 // 951 // 1-0 exit 952 // ~~~~~~~~ 953 // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of 954 // the fast-path operators have been optimized so the common ::exit() 955 // operation is 1-0, e.g., see macroAssembler_x86.cpp: fast_unlock(). 956 // The code emitted by fast_unlock() elides the usual MEMBAR. This 957 // greatly improves latency -- MEMBAR and CAS having considerable local 958 // latency on modern processors -- but at the cost of "stranding". Absent the 959 // MEMBAR, a thread in fast_unlock() can race a thread in the slow 960 // ::enter() path, resulting in the entering thread being stranding 961 // and a progress-liveness failure. Stranding is extremely rare. 962 // We use timers (timed park operations) & periodic polling to detect 963 // and recover from stranding. Potentially stranded threads periodically 964 // wake up and poll the lock. See the usage of the _Responsible variable. 965 // 966 // The CAS() in enter provides for safety and exclusion, while the CAS or 967 // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking 968 // eliminates the CAS/MEMBAR from the exit path, but it admits stranding. 969 // We detect and recover from stranding with timers. 970 // 971 // If a thread transiently strands it'll park until (a) another 972 // thread acquires the lock and then drops the lock, at which time the 973 // exiting thread will notice and unpark the stranded thread, or, (b) 974 // the timer expires. If the lock is high traffic then the stranding latency 975 // will be low due to (a). If the lock is low traffic then the odds of 976 // stranding are lower, although the worst-case stranding latency 977 // is longer. Critically, we don't want to put excessive load in the 978 // platform's timer subsystem. We want to minimize both the timer injection 979 // rate (timers created/sec) as well as the number of timers active at 980 // any one time. (more precisely, we want to minimize timer-seconds, which is 981 // the integral of the # of active timers at any instant over time). 982 // Both impinge on OS scalability. Given that, at most one thread parked on 983 // a monitor will use a timer. 984 // 985 // There is also the risk of a futile wake-up. If we drop the lock 986 // another thread can reacquire the lock immediately, and we can 987 // then wake a thread unnecessarily. This is benign, and we've 988 // structured the code so the windows are short and the frequency 989 // of such futile wakups is low. 990 991 void ObjectMonitor::exit(bool not_suspended, TRAPS) { 992 Thread * const Self = THREAD; 993 if (THREAD != _owner) { 994 void* cur = _owner; 995 if (THREAD->is_lock_owned((address)cur)) { 996 assert(_recursions == 0, "invariant"); 997 simply_set_owner_from_BasicLock(Self, cur); // Convert from BasicLock* to Thread*. 998 _recursions = 0; 999 } else { 1000 // Apparent unbalanced locking ... 1001 // Naively we'd like to throw IllegalMonitorStateException. 1002 // As a practical matter we can neither allocate nor throw an 1003 // exception as ::exit() can be called from leaf routines. 1004 // see x86_32.ad Fast_Unlock() and the I1 and I2 properties. 1005 // Upon deeper reflection, however, in a properly run JVM the only 1006 // way we should encounter this situation is in the presence of 1007 // unbalanced JNI locking. TODO: CheckJNICalls. 1008 // See also: CR4414101 1009 #ifdef ASSERT 1010 LogStreamHandle(Error, monitorinflation) lsh; 1011 lsh.print_cr("ERROR: ObjectMonitor::exit(): thread=" INTPTR_FORMAT 1012 " is exiting an ObjectMonitor it does not own.", p2i(THREAD)); 1013 lsh.print_cr("The imbalance is possibly caused by JNI locking."); 1014 print_debug_style_on(&lsh); 1015 #endif 1016 assert(false, "Non-balanced monitor enter/exit!"); 1017 return; 1018 } 1019 } 1020 1021 if (_recursions != 0) { 1022 _recursions--; // this is simple recursive enter 1023 return; 1024 } 1025 1026 // Invariant: after setting Responsible=null an thread must execute 1027 // a MEMBAR or other serializing instruction before fetching EntryList|cxq. 1028 _Responsible = NULL; 1029 1030 #if INCLUDE_JFR 1031 // get the owner's thread id for the MonitorEnter event 1032 // if it is enabled and the thread isn't suspended 1033 if (not_suspended && EventJavaMonitorEnter::is_enabled()) { 1034 _previous_owner_tid = JFR_THREAD_ID(Self); 1035 } 1036 #endif 1037 1038 for (;;) { 1039 assert(THREAD == _owner, "invariant"); 1040 1041 // Drop the lock. 1042 // release semantics: prior loads and stores from within the critical section 1043 // must not float (reorder) past the following store that drops the lock. 1044 // Uses a storeload to separate release_store(owner) from the 1045 // successor check. The try_set_owner() below uses cmpxchg() so 1046 // we get the fence down there. 1047 release_clear_owner_with_barrier(Self, /* needs_fence */ false); 1048 1049 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) { 1050 return; 1051 } 1052 // Other threads are blocked trying to acquire the lock. 1053 1054 // Normally the exiting thread is responsible for ensuring succession, 1055 // but if other successors are ready or other entering threads are spinning 1056 // then this thread can simply store NULL into _owner and exit without 1057 // waking a successor. The existence of spinners or ready successors 1058 // guarantees proper succession (liveness). Responsibility passes to the 1059 // ready or running successors. The exiting thread delegates the duty. 1060 // More precisely, if a successor already exists this thread is absolved 1061 // of the responsibility of waking (unparking) one. 1062 // 1063 // The _succ variable is critical to reducing futile wakeup frequency. 1064 // _succ identifies the "heir presumptive" thread that has been made 1065 // ready (unparked) but that has not yet run. We need only one such 1066 // successor thread to guarantee progress. 1067 // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf 1068 // section 3.3 "Futile Wakeup Throttling" for details. 1069 // 1070 // Note that spinners in Enter() also set _succ non-null. 1071 // In the current implementation spinners opportunistically set 1072 // _succ so that exiting threads might avoid waking a successor. 1073 // Another less appealing alternative would be for the exiting thread 1074 // to drop the lock and then spin briefly to see if a spinner managed 1075 // to acquire the lock. If so, the exiting thread could exit 1076 // immediately without waking a successor, otherwise the exiting 1077 // thread would need to dequeue and wake a successor. 1078 // (Note that we'd need to make the post-drop spin short, but no 1079 // shorter than the worst-case round-trip cache-line migration time. 1080 // The dropped lock needs to become visible to the spinner, and then 1081 // the acquisition of the lock by the spinner must become visible to 1082 // the exiting thread). 1083 1084 // It appears that an heir-presumptive (successor) must be made ready. 1085 // Only the current lock owner can manipulate the EntryList or 1086 // drain _cxq, so we need to reacquire the lock. If we fail 1087 // to reacquire the lock the responsibility for ensuring succession 1088 // falls to the new owner. 1089 // 1090 if (try_set_owner_from(Self, NULL) != NULL) { 1091 return; 1092 } 1093 1094 guarantee(_owner == THREAD, "invariant"); 1095 1096 ObjectWaiter * w = NULL; 1097 1098 w = _EntryList; 1099 if (w != NULL) { 1100 // I'd like to write: guarantee (w->_thread != Self). 1101 // But in practice an exiting thread may find itself on the EntryList. 1102 // Let's say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and 1103 // then calls exit(). Exit release the lock by setting O._owner to NULL. 1104 // Let's say T1 then stalls. T2 acquires O and calls O.notify(). The 1105 // notify() operation moves T1 from O's waitset to O's EntryList. T2 then 1106 // release the lock "O". T2 resumes immediately after the ST of null into 1107 // _owner, above. T2 notices that the EntryList is populated, so it 1108 // reacquires the lock and then finds itself on the EntryList. 1109 // Given all that, we have to tolerate the circumstance where "w" is 1110 // associated with Self. 1111 assert(w->TState == ObjectWaiter::TS_ENTER, "invariant"); 1112 ExitEpilog(Self, w); 1113 return; 1114 } 1115 1116 // If we find that both _cxq and EntryList are null then just 1117 // re-run the exit protocol from the top. 1118 w = _cxq; 1119 if (w == NULL) continue; 1120 1121 // Drain _cxq into EntryList - bulk transfer. 1122 // First, detach _cxq. 1123 // The following loop is tantamount to: w = swap(&cxq, NULL) 1124 for (;;) { 1125 assert(w != NULL, "Invariant"); 1126 ObjectWaiter * u = Atomic::cmpxchg(&_cxq, w, (ObjectWaiter*)NULL); 1127 if (u == w) break; 1128 w = u; 1129 } 1130 1131 assert(w != NULL, "invariant"); 1132 assert(_EntryList == NULL, "invariant"); 1133 1134 // Convert the LIFO SLL anchored by _cxq into a DLL. 1135 // The list reorganization step operates in O(LENGTH(w)) time. 1136 // It's critical that this step operate quickly as 1137 // "Self" still holds the outer-lock, restricting parallelism 1138 // and effectively lengthening the critical section. 1139 // Invariant: s chases t chases u. 1140 // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so 1141 // we have faster access to the tail. 1142 1143 _EntryList = w; 1144 ObjectWaiter * q = NULL; 1145 ObjectWaiter * p; 1146 for (p = w; p != NULL; p = p->_next) { 1147 guarantee(p->TState == ObjectWaiter::TS_CXQ, "Invariant"); 1148 p->TState = ObjectWaiter::TS_ENTER; 1149 p->_prev = q; 1150 q = p; 1151 } 1152 1153 // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL 1154 // The MEMBAR is satisfied by the release_store() operation in ExitEpilog(). 1155 1156 // See if we can abdicate to a spinner instead of waking a thread. 1157 // A primary goal of the implementation is to reduce the 1158 // context-switch rate. 1159 if (_succ != NULL) continue; 1160 1161 w = _EntryList; 1162 if (w != NULL) { 1163 guarantee(w->TState == ObjectWaiter::TS_ENTER, "invariant"); 1164 ExitEpilog(Self, w); 1165 return; 1166 } 1167 } 1168 } 1169 1170 // ExitSuspendEquivalent: 1171 // A faster alternate to handle_special_suspend_equivalent_condition() 1172 // 1173 // handle_special_suspend_equivalent_condition() unconditionally 1174 // acquires the SR_lock. On some platforms uncontended MutexLocker() 1175 // operations have high latency. Note that in ::enter() we call HSSEC 1176 // while holding the monitor, so we effectively lengthen the critical sections. 1177 // 1178 // There are a number of possible solutions: 1179 // 1180 // A. To ameliorate the problem we might also defer state transitions 1181 // to as late as possible -- just prior to parking. 1182 // Given that, we'd call HSSEC after having returned from park(), 1183 // but before attempting to acquire the monitor. This is only a 1184 // partial solution. It avoids calling HSSEC while holding the 1185 // monitor (good), but it still increases successor reacquisition latency -- 1186 // the interval between unparking a successor and the time the successor 1187 // resumes and retries the lock. See ReenterI(), which defers state transitions. 1188 // If we use this technique we can also avoid EnterI()-exit() loop 1189 // in ::enter() where we iteratively drop the lock and then attempt 1190 // to reacquire it after suspending. 1191 // 1192 // B. In the future we might fold all the suspend bits into a 1193 // composite per-thread suspend flag and then update it with CAS(). 1194 // Alternately, a Dekker-like mechanism with multiple variables 1195 // would suffice: 1196 // ST Self->_suspend_equivalent = false 1197 // MEMBAR 1198 // LD Self_>_suspend_flags 1199 1200 bool ObjectMonitor::ExitSuspendEquivalent(JavaThread * jSelf) { 1201 return jSelf->handle_special_suspend_equivalent_condition(); 1202 } 1203 1204 1205 void ObjectMonitor::ExitEpilog(Thread * Self, ObjectWaiter * Wakee) { 1206 assert(_owner == Self, "invariant"); 1207 1208 // Exit protocol: 1209 // 1. ST _succ = wakee 1210 // 2. membar #loadstore|#storestore; 1211 // 2. ST _owner = NULL 1212 // 3. unpark(wakee) 1213 1214 _succ = Wakee->_thread; 1215 ParkEvent * Trigger = Wakee->_event; 1216 1217 // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again. 1218 // The thread associated with Wakee may have grabbed the lock and "Wakee" may be 1219 // out-of-scope (non-extant). 1220 Wakee = NULL; 1221 1222 // Drop the lock. 1223 // Uses a fence to separate release_store(owner) from the LD in unpark(). 1224 release_clear_owner_with_barrier(Self, /* needs_fence */ true); 1225 1226 DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self); 1227 Trigger->unpark(); 1228 1229 // Maintain stats and report events to JVMTI 1230 OM_PERFDATA_OP(Parks, inc()); 1231 } 1232 1233 1234 // ----------------------------------------------------------------------------- 1235 // Class Loader deadlock handling. 1236 // 1237 // complete_exit exits a lock returning recursion count 1238 // complete_exit/reenter operate as a wait without waiting 1239 // complete_exit requires an inflated monitor 1240 // The _owner field is not always the Thread addr even with an 1241 // inflated monitor, e.g. the monitor can be inflated by a non-owning 1242 // thread due to contention. 1243 intx ObjectMonitor::complete_exit(TRAPS) { 1244 Thread * const Self = THREAD; 1245 assert(Self->is_Java_thread(), "Must be Java thread!"); 1246 JavaThread *jt = (JavaThread *)THREAD; 1247 1248 assert(InitDone, "Unexpectedly not initialized"); 1249 1250 if (THREAD != _owner) { 1251 void* cur = _owner; 1252 if (THREAD->is_lock_owned((address)cur)) { 1253 assert(_recursions == 0, "internal state error"); 1254 simply_set_owner_from_BasicLock(Self, cur); // Convert from BasicLock* to Thread*. 1255 _recursions = 0; 1256 } 1257 } 1258 1259 guarantee(Self == _owner, "complete_exit not owner"); 1260 intx save = _recursions; // record the old recursion count 1261 _recursions = 0; // set the recursion level to be 0 1262 exit(true, Self); // exit the monitor 1263 guarantee(_owner != Self, "invariant"); 1264 return save; 1265 } 1266 1267 // reenter() enters a lock and sets recursion count 1268 // complete_exit/reenter operate as a wait without waiting 1269 void ObjectMonitor::reenter(intx recursions, TRAPS) { 1270 Thread * const Self = THREAD; 1271 assert(Self->is_Java_thread(), "Must be Java thread!"); 1272 JavaThread *jt = (JavaThread *)THREAD; 1273 1274 guarantee(_owner != Self, "reenter already owner"); 1275 enter(THREAD); 1276 // Entered the monitor. 1277 guarantee(_recursions == 0, "reenter recursion"); 1278 _recursions = recursions; 1279 } 1280 1281 // Checks that the current THREAD owns this monitor and causes an 1282 // immediate return if it doesn't. We don't use the CHECK macro 1283 // because we want the IMSE to be the only exception that is thrown 1284 // from the call site when false is returned. Any other pending 1285 // exception is ignored. 1286 #define CHECK_OWNER() \ 1287 do { \ 1288 if (!check_owner(THREAD)) { \ 1289 assert(HAS_PENDING_EXCEPTION, "expected a pending IMSE here."); \ 1290 return; \ 1291 } \ 1292 } while (false) 1293 1294 // Returns true if the specified thread owns the ObjectMonitor. 1295 // Otherwise returns false and throws IllegalMonitorStateException 1296 // (IMSE). If there is a pending exception and the specified thread 1297 // is not the owner, that exception will be replaced by the IMSE. 1298 bool ObjectMonitor::check_owner(Thread* THREAD) { 1299 if (_owner == THREAD) { 1300 return true; 1301 } 1302 void* cur = _owner; 1303 if (THREAD->is_lock_owned((address)cur)) { 1304 simply_set_owner_from_BasicLock(THREAD, cur); // Convert from BasicLock* to Thread*. 1305 _recursions = 0; 1306 return true; 1307 } 1308 THROW_MSG_(vmSymbols::java_lang_IllegalMonitorStateException(), 1309 "current thread is not owner", false); 1310 } 1311 1312 static void post_monitor_wait_event(EventJavaMonitorWait* event, 1313 ObjectMonitor* monitor, 1314 jlong notifier_tid, 1315 jlong timeout, 1316 bool timedout) { 1317 assert(event != NULL, "invariant"); 1318 assert(monitor != NULL, "invariant"); 1319 event->set_monitorClass(((oop)monitor->object())->klass()); 1320 event->set_timeout(timeout); 1321 event->set_address((uintptr_t)monitor->object_addr()); 1322 event->set_notifier(notifier_tid); 1323 event->set_timedOut(timedout); 1324 event->commit(); 1325 } 1326 1327 // ----------------------------------------------------------------------------- 1328 // Wait/Notify/NotifyAll 1329 // 1330 // Note: a subset of changes to ObjectMonitor::wait() 1331 // will need to be replicated in complete_exit 1332 void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) { 1333 Thread * const Self = THREAD; 1334 assert(Self->is_Java_thread(), "Must be Java thread!"); 1335 JavaThread *jt = (JavaThread *)THREAD; 1336 1337 assert(InitDone, "Unexpectedly not initialized"); 1338 1339 CHECK_OWNER(); // Throws IMSE if not owner. 1340 1341 EventJavaMonitorWait event; 1342 1343 // check for a pending interrupt 1344 if (interruptible && jt->is_interrupted(true) && !HAS_PENDING_EXCEPTION) { 1345 // post monitor waited event. Note that this is past-tense, we are done waiting. 1346 if (JvmtiExport::should_post_monitor_waited()) { 1347 // Note: 'false' parameter is passed here because the 1348 // wait was not timed out due to thread interrupt. 1349 JvmtiExport::post_monitor_waited(jt, this, false); 1350 1351 // In this short circuit of the monitor wait protocol, the 1352 // current thread never drops ownership of the monitor and 1353 // never gets added to the wait queue so the current thread 1354 // cannot be made the successor. This means that the 1355 // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally 1356 // consume an unpark() meant for the ParkEvent associated with 1357 // this ObjectMonitor. 1358 } 1359 if (event.should_commit()) { 1360 post_monitor_wait_event(&event, this, 0, millis, false); 1361 } 1362 THROW(vmSymbols::java_lang_InterruptedException()); 1363 return; 1364 } 1365 1366 assert(Self->_Stalled == 0, "invariant"); 1367 Self->_Stalled = intptr_t(this); 1368 jt->set_current_waiting_monitor(this); 1369 1370 // create a node to be put into the queue 1371 // Critically, after we reset() the event but prior to park(), we must check 1372 // for a pending interrupt. 1373 ObjectWaiter node(Self); 1374 node.TState = ObjectWaiter::TS_WAIT; 1375 Self->_ParkEvent->reset(); 1376 OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag 1377 1378 // Enter the waiting queue, which is a circular doubly linked list in this case 1379 // but it could be a priority queue or any data structure. 1380 // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only 1381 // by the the owner of the monitor *except* in the case where park() 1382 // returns because of a timeout of interrupt. Contention is exceptionally rare 1383 // so we use a simple spin-lock instead of a heavier-weight blocking lock. 1384 1385 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - add"); 1386 AddWaiter(&node); 1387 Thread::SpinRelease(&_WaitSetLock); 1388 1389 _Responsible = NULL; 1390 1391 intx save = _recursions; // record the old recursion count 1392 _waiters++; // increment the number of waiters 1393 _recursions = 0; // set the recursion level to be 1 1394 exit(true, Self); // exit the monitor 1395 guarantee(_owner != Self, "invariant"); 1396 1397 // The thread is on the WaitSet list - now park() it. 1398 // On MP systems it's conceivable that a brief spin before we park 1399 // could be profitable. 1400 // 1401 // TODO-FIXME: change the following logic to a loop of the form 1402 // while (!timeout && !interrupted && _notified == 0) park() 1403 1404 int ret = OS_OK; 1405 int WasNotified = 0; 1406 1407 // Need to check interrupt state whilst still _thread_in_vm 1408 bool interrupted = interruptible && jt->is_interrupted(false); 1409 1410 { // State transition wrappers 1411 OSThread* osthread = Self->osthread(); 1412 OSThreadWaitState osts(osthread, true); 1413 { 1414 ThreadBlockInVM tbivm(jt); 1415 // Thread is in thread_blocked state and oop access is unsafe. 1416 jt->set_suspend_equivalent(); 1417 1418 if (interrupted || HAS_PENDING_EXCEPTION) { 1419 // Intentionally empty 1420 } else if (node._notified == 0) { 1421 if (millis <= 0) { 1422 Self->_ParkEvent->park(); 1423 } else { 1424 ret = Self->_ParkEvent->park(millis); 1425 } 1426 } 1427 1428 // were we externally suspended while we were waiting? 1429 if (ExitSuspendEquivalent (jt)) { 1430 // TODO-FIXME: add -- if succ == Self then succ = null. 1431 jt->java_suspend_self(); 1432 } 1433 1434 } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm 1435 1436 // Node may be on the WaitSet, the EntryList (or cxq), or in transition 1437 // from the WaitSet to the EntryList. 1438 // See if we need to remove Node from the WaitSet. 1439 // We use double-checked locking to avoid grabbing _WaitSetLock 1440 // if the thread is not on the wait queue. 1441 // 1442 // Note that we don't need a fence before the fetch of TState. 1443 // In the worst case we'll fetch a old-stale value of TS_WAIT previously 1444 // written by the is thread. (perhaps the fetch might even be satisfied 1445 // by a look-aside into the processor's own store buffer, although given 1446 // the length of the code path between the prior ST and this load that's 1447 // highly unlikely). If the following LD fetches a stale TS_WAIT value 1448 // then we'll acquire the lock and then re-fetch a fresh TState value. 1449 // That is, we fail toward safety. 1450 1451 if (node.TState == ObjectWaiter::TS_WAIT) { 1452 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - unlink"); 1453 if (node.TState == ObjectWaiter::TS_WAIT) { 1454 DequeueSpecificWaiter(&node); // unlink from WaitSet 1455 assert(node._notified == 0, "invariant"); 1456 node.TState = ObjectWaiter::TS_RUN; 1457 } 1458 Thread::SpinRelease(&_WaitSetLock); 1459 } 1460 1461 // The thread is now either on off-list (TS_RUN), 1462 // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ). 1463 // The Node's TState variable is stable from the perspective of this thread. 1464 // No other threads will asynchronously modify TState. 1465 guarantee(node.TState != ObjectWaiter::TS_WAIT, "invariant"); 1466 OrderAccess::loadload(); 1467 if (_succ == Self) _succ = NULL; 1468 WasNotified = node._notified; 1469 1470 // Reentry phase -- reacquire the monitor. 1471 // re-enter contended monitor after object.wait(). 1472 // retain OBJECT_WAIT state until re-enter successfully completes 1473 // Thread state is thread_in_vm and oop access is again safe, 1474 // although the raw address of the object may have changed. 1475 // (Don't cache naked oops over safepoints, of course). 1476 1477 // post monitor waited event. Note that this is past-tense, we are done waiting. 1478 if (JvmtiExport::should_post_monitor_waited()) { 1479 JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT); 1480 1481 if (node._notified != 0 && _succ == Self) { 1482 // In this part of the monitor wait-notify-reenter protocol it 1483 // is possible (and normal) for another thread to do a fastpath 1484 // monitor enter-exit while this thread is still trying to get 1485 // to the reenter portion of the protocol. 1486 // 1487 // The ObjectMonitor was notified and the current thread is 1488 // the successor which also means that an unpark() has already 1489 // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can 1490 // consume the unpark() that was done when the successor was 1491 // set because the same ParkEvent is shared between Java 1492 // monitors and JVM/TI RawMonitors (for now). 1493 // 1494 // We redo the unpark() to ensure forward progress, i.e., we 1495 // don't want all pending threads hanging (parked) with none 1496 // entering the unlocked monitor. 1497 node._event->unpark(); 1498 } 1499 } 1500 1501 if (event.should_commit()) { 1502 post_monitor_wait_event(&event, this, node._notifier_tid, millis, ret == OS_TIMEOUT); 1503 } 1504 1505 OrderAccess::fence(); 1506 1507 assert(Self->_Stalled != 0, "invariant"); 1508 Self->_Stalled = 0; 1509 1510 assert(_owner != Self, "invariant"); 1511 ObjectWaiter::TStates v = node.TState; 1512 if (v == ObjectWaiter::TS_RUN) { 1513 enter(Self); 1514 } else { 1515 guarantee(v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant"); 1516 ReenterI(Self, &node); 1517 node.wait_reenter_end(this); 1518 } 1519 1520 // Self has reacquired the lock. 1521 // Lifecycle - the node representing Self must not appear on any queues. 1522 // Node is about to go out-of-scope, but even if it were immortal we wouldn't 1523 // want residual elements associated with this thread left on any lists. 1524 guarantee(node.TState == ObjectWaiter::TS_RUN, "invariant"); 1525 assert(_owner == Self, "invariant"); 1526 assert(_succ != Self, "invariant"); 1527 } // OSThreadWaitState() 1528 1529 jt->set_current_waiting_monitor(NULL); 1530 1531 guarantee(_recursions == 0, "invariant"); 1532 _recursions = save; // restore the old recursion count 1533 _waiters--; // decrement the number of waiters 1534 1535 // Verify a few postconditions 1536 assert(_owner == Self, "invariant"); 1537 assert(_succ != Self, "invariant"); 1538 assert(((oop)(object()))->mark() == markWord::encode(this), "invariant"); 1539 1540 // check if the notification happened 1541 if (!WasNotified) { 1542 // no, it could be timeout or Thread.interrupt() or both 1543 // check for interrupt event, otherwise it is timeout 1544 if (interruptible && jt->is_interrupted(true) && !HAS_PENDING_EXCEPTION) { 1545 THROW(vmSymbols::java_lang_InterruptedException()); 1546 } 1547 } 1548 1549 // NOTE: Spurious wake up will be consider as timeout. 1550 // Monitor notify has precedence over thread interrupt. 1551 } 1552 1553 1554 // Consider: 1555 // If the lock is cool (cxq == null && succ == null) and we're on an MP system 1556 // then instead of transferring a thread from the WaitSet to the EntryList 1557 // we might just dequeue a thread from the WaitSet and directly unpark() it. 1558 1559 void ObjectMonitor::INotify(Thread * Self) { 1560 Thread::SpinAcquire(&_WaitSetLock, "WaitSet - notify"); 1561 ObjectWaiter * iterator = DequeueWaiter(); 1562 if (iterator != NULL) { 1563 guarantee(iterator->TState == ObjectWaiter::TS_WAIT, "invariant"); 1564 guarantee(iterator->_notified == 0, "invariant"); 1565 // Disposition - what might we do with iterator ? 1566 // a. add it directly to the EntryList - either tail (policy == 1) 1567 // or head (policy == 0). 1568 // b. push it onto the front of the _cxq (policy == 2). 1569 // For now we use (b). 1570 1571 iterator->TState = ObjectWaiter::TS_ENTER; 1572 1573 iterator->_notified = 1; 1574 iterator->_notifier_tid = JFR_THREAD_ID(Self); 1575 1576 ObjectWaiter * list = _EntryList; 1577 if (list != NULL) { 1578 assert(list->_prev == NULL, "invariant"); 1579 assert(list->TState == ObjectWaiter::TS_ENTER, "invariant"); 1580 assert(list != iterator, "invariant"); 1581 } 1582 1583 // prepend to cxq 1584 if (list == NULL) { 1585 iterator->_next = iterator->_prev = NULL; 1586 _EntryList = iterator; 1587 } else { 1588 iterator->TState = ObjectWaiter::TS_CXQ; 1589 for (;;) { 1590 ObjectWaiter * front = _cxq; 1591 iterator->_next = front; 1592 if (Atomic::cmpxchg(&_cxq, front, iterator) == front) { 1593 break; 1594 } 1595 } 1596 } 1597 1598 // _WaitSetLock protects the wait queue, not the EntryList. We could 1599 // move the add-to-EntryList operation, above, outside the critical section 1600 // protected by _WaitSetLock. In practice that's not useful. With the 1601 // exception of wait() timeouts and interrupts the monitor owner 1602 // is the only thread that grabs _WaitSetLock. There's almost no contention 1603 // on _WaitSetLock so it's not profitable to reduce the length of the 1604 // critical section. 1605 1606 iterator->wait_reenter_begin(this); 1607 } 1608 Thread::SpinRelease(&_WaitSetLock); 1609 } 1610 1611 // Consider: a not-uncommon synchronization bug is to use notify() when 1612 // notifyAll() is more appropriate, potentially resulting in stranded 1613 // threads; this is one example of a lost wakeup. A useful diagnostic 1614 // option is to force all notify() operations to behave as notifyAll(). 1615 // 1616 // Note: We can also detect many such problems with a "minimum wait". 1617 // When the "minimum wait" is set to a small non-zero timeout value 1618 // and the program does not hang whereas it did absent "minimum wait", 1619 // that suggests a lost wakeup bug. 1620 1621 void ObjectMonitor::notify(TRAPS) { 1622 CHECK_OWNER(); // Throws IMSE if not owner. 1623 if (_WaitSet == NULL) { 1624 return; 1625 } 1626 DTRACE_MONITOR_PROBE(notify, this, object(), THREAD); 1627 INotify(THREAD); 1628 OM_PERFDATA_OP(Notifications, inc(1)); 1629 } 1630 1631 1632 // The current implementation of notifyAll() transfers the waiters one-at-a-time 1633 // from the waitset to the EntryList. This could be done more efficiently with a 1634 // single bulk transfer but in practice it's not time-critical. Beware too, 1635 // that in prepend-mode we invert the order of the waiters. Let's say that the 1636 // waitset is "ABCD" and the EntryList is "XYZ". After a notifyAll() in prepend 1637 // mode the waitset will be empty and the EntryList will be "DCBAXYZ". 1638 1639 void ObjectMonitor::notifyAll(TRAPS) { 1640 CHECK_OWNER(); // Throws IMSE if not owner. 1641 if (_WaitSet == NULL) { 1642 return; 1643 } 1644 1645 DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD); 1646 int tally = 0; 1647 while (_WaitSet != NULL) { 1648 tally++; 1649 INotify(THREAD); 1650 } 1651 1652 OM_PERFDATA_OP(Notifications, inc(tally)); 1653 } 1654 1655 // ----------------------------------------------------------------------------- 1656 // Adaptive Spinning Support 1657 // 1658 // Adaptive spin-then-block - rational spinning 1659 // 1660 // Note that we spin "globally" on _owner with a classic SMP-polite TATAS 1661 // algorithm. On high order SMP systems it would be better to start with 1662 // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH, 1663 // a contending thread could enqueue itself on the cxq and then spin locally 1664 // on a thread-specific variable such as its ParkEvent._Event flag. 1665 // That's left as an exercise for the reader. Note that global spinning is 1666 // not problematic on Niagara, as the L2 cache serves the interconnect and 1667 // has both low latency and massive bandwidth. 1668 // 1669 // Broadly, we can fix the spin frequency -- that is, the % of contended lock 1670 // acquisition attempts where we opt to spin -- at 100% and vary the spin count 1671 // (duration) or we can fix the count at approximately the duration of 1672 // a context switch and vary the frequency. Of course we could also 1673 // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor. 1674 // For a description of 'Adaptive spin-then-block mutual exclusion in 1675 // multi-threaded processing,' see U.S. Pat. No. 8046758. 1676 // 1677 // This implementation varies the duration "D", where D varies with 1678 // the success rate of recent spin attempts. (D is capped at approximately 1679 // length of a round-trip context switch). The success rate for recent 1680 // spin attempts is a good predictor of the success rate of future spin 1681 // attempts. The mechanism adapts automatically to varying critical 1682 // section length (lock modality), system load and degree of parallelism. 1683 // D is maintained per-monitor in _SpinDuration and is initialized 1684 // optimistically. Spin frequency is fixed at 100%. 1685 // 1686 // Note that _SpinDuration is volatile, but we update it without locks 1687 // or atomics. The code is designed so that _SpinDuration stays within 1688 // a reasonable range even in the presence of races. The arithmetic 1689 // operations on _SpinDuration are closed over the domain of legal values, 1690 // so at worst a race will install and older but still legal value. 1691 // At the very worst this introduces some apparent non-determinism. 1692 // We might spin when we shouldn't or vice-versa, but since the spin 1693 // count are relatively short, even in the worst case, the effect is harmless. 1694 // 1695 // Care must be taken that a low "D" value does not become an 1696 // an absorbing state. Transient spinning failures -- when spinning 1697 // is overall profitable -- should not cause the system to converge 1698 // on low "D" values. We want spinning to be stable and predictable 1699 // and fairly responsive to change and at the same time we don't want 1700 // it to oscillate, become metastable, be "too" non-deterministic, 1701 // or converge on or enter undesirable stable absorbing states. 1702 // 1703 // We implement a feedback-based control system -- using past behavior 1704 // to predict future behavior. We face two issues: (a) if the 1705 // input signal is random then the spin predictor won't provide optimal 1706 // results, and (b) if the signal frequency is too high then the control 1707 // system, which has some natural response lag, will "chase" the signal. 1708 // (b) can arise from multimodal lock hold times. Transient preemption 1709 // can also result in apparent bimodal lock hold times. 1710 // Although sub-optimal, neither condition is particularly harmful, as 1711 // in the worst-case we'll spin when we shouldn't or vice-versa. 1712 // The maximum spin duration is rather short so the failure modes aren't bad. 1713 // To be conservative, I've tuned the gain in system to bias toward 1714 // _not spinning. Relatedly, the system can sometimes enter a mode where it 1715 // "rings" or oscillates between spinning and not spinning. This happens 1716 // when spinning is just on the cusp of profitability, however, so the 1717 // situation is not dire. The state is benign -- there's no need to add 1718 // hysteresis control to damp the transition rate between spinning and 1719 // not spinning. 1720 1721 // Spinning: Fixed frequency (100%), vary duration 1722 int ObjectMonitor::TrySpin(Thread * Self) { 1723 // Dumb, brutal spin. Good for comparative measurements against adaptive spinning. 1724 int ctr = Knob_FixedSpin; 1725 if (ctr != 0) { 1726 while (--ctr >= 0) { 1727 if (TryLock(Self) > 0) return 1; 1728 SpinPause(); 1729 } 1730 return 0; 1731 } 1732 1733 for (ctr = Knob_PreSpin + 1; --ctr >= 0;) { 1734 if (TryLock(Self) > 0) { 1735 // Increase _SpinDuration ... 1736 // Note that we don't clamp SpinDuration precisely at SpinLimit. 1737 // Raising _SpurDuration to the poverty line is key. 1738 int x = _SpinDuration; 1739 if (x < Knob_SpinLimit) { 1740 if (x < Knob_Poverty) x = Knob_Poverty; 1741 _SpinDuration = x + Knob_BonusB; 1742 } 1743 return 1; 1744 } 1745 SpinPause(); 1746 } 1747 1748 // Admission control - verify preconditions for spinning 1749 // 1750 // We always spin a little bit, just to prevent _SpinDuration == 0 from 1751 // becoming an absorbing state. Put another way, we spin briefly to 1752 // sample, just in case the system load, parallelism, contention, or lock 1753 // modality changed. 1754 // 1755 // Consider the following alternative: 1756 // Periodically set _SpinDuration = _SpinLimit and try a long/full 1757 // spin attempt. "Periodically" might mean after a tally of 1758 // the # of failed spin attempts (or iterations) reaches some threshold. 1759 // This takes us into the realm of 1-out-of-N spinning, where we 1760 // hold the duration constant but vary the frequency. 1761 1762 ctr = _SpinDuration; 1763 if (ctr <= 0) return 0; 1764 1765 if (NotRunnable(Self, (Thread *) _owner)) { 1766 return 0; 1767 } 1768 1769 // We're good to spin ... spin ingress. 1770 // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades 1771 // when preparing to LD...CAS _owner, etc and the CAS is likely 1772 // to succeed. 1773 if (_succ == NULL) { 1774 _succ = Self; 1775 } 1776 Thread * prv = NULL; 1777 1778 // There are three ways to exit the following loop: 1779 // 1. A successful spin where this thread has acquired the lock. 1780 // 2. Spin failure with prejudice 1781 // 3. Spin failure without prejudice 1782 1783 while (--ctr >= 0) { 1784 1785 // Periodic polling -- Check for pending GC 1786 // Threads may spin while they're unsafe. 1787 // We don't want spinning threads to delay the JVM from reaching 1788 // a stop-the-world safepoint or to steal cycles from GC. 1789 // If we detect a pending safepoint we abort in order that 1790 // (a) this thread, if unsafe, doesn't delay the safepoint, and (b) 1791 // this thread, if safe, doesn't steal cycles from GC. 1792 // This is in keeping with the "no loitering in runtime" rule. 1793 // We periodically check to see if there's a safepoint pending. 1794 if ((ctr & 0xFF) == 0) { 1795 if (SafepointMechanism::should_block(Self)) { 1796 goto Abort; // abrupt spin egress 1797 } 1798 SpinPause(); 1799 } 1800 1801 // Probe _owner with TATAS 1802 // If this thread observes the monitor transition or flicker 1803 // from locked to unlocked to locked, then the odds that this 1804 // thread will acquire the lock in this spin attempt go down 1805 // considerably. The same argument applies if the CAS fails 1806 // or if we observe _owner change from one non-null value to 1807 // another non-null value. In such cases we might abort 1808 // the spin without prejudice or apply a "penalty" to the 1809 // spin count-down variable "ctr", reducing it by 100, say. 1810 1811 Thread * ox = (Thread *) _owner; 1812 if (ox == NULL) { 1813 ox = (Thread*)try_set_owner_from(Self, NULL); 1814 if (ox == NULL) { 1815 // The CAS succeeded -- this thread acquired ownership 1816 // Take care of some bookkeeping to exit spin state. 1817 if (_succ == Self) { 1818 _succ = NULL; 1819 } 1820 1821 // Increase _SpinDuration : 1822 // The spin was successful (profitable) so we tend toward 1823 // longer spin attempts in the future. 1824 // CONSIDER: factor "ctr" into the _SpinDuration adjustment. 1825 // If we acquired the lock early in the spin cycle it 1826 // makes sense to increase _SpinDuration proportionally. 1827 // Note that we don't clamp SpinDuration precisely at SpinLimit. 1828 int x = _SpinDuration; 1829 if (x < Knob_SpinLimit) { 1830 if (x < Knob_Poverty) x = Knob_Poverty; 1831 _SpinDuration = x + Knob_Bonus; 1832 } 1833 return 1; 1834 } 1835 1836 // The CAS failed ... we can take any of the following actions: 1837 // * penalize: ctr -= CASPenalty 1838 // * exit spin with prejudice -- goto Abort; 1839 // * exit spin without prejudice. 1840 // * Since CAS is high-latency, retry again immediately. 1841 prv = ox; 1842 goto Abort; 1843 } 1844 1845 // Did lock ownership change hands ? 1846 if (ox != prv && prv != NULL) { 1847 goto Abort; 1848 } 1849 prv = ox; 1850 1851 // Abort the spin if the owner is not executing. 1852 // The owner must be executing in order to drop the lock. 1853 // Spinning while the owner is OFFPROC is idiocy. 1854 // Consider: ctr -= RunnablePenalty ; 1855 if (NotRunnable(Self, ox)) { 1856 goto Abort; 1857 } 1858 if (_succ == NULL) { 1859 _succ = Self; 1860 } 1861 } 1862 1863 // Spin failed with prejudice -- reduce _SpinDuration. 1864 // TODO: Use an AIMD-like policy to adjust _SpinDuration. 1865 // AIMD is globally stable. 1866 { 1867 int x = _SpinDuration; 1868 if (x > 0) { 1869 // Consider an AIMD scheme like: x -= (x >> 3) + 100 1870 // This is globally sample and tends to damp the response. 1871 x -= Knob_Penalty; 1872 if (x < 0) x = 0; 1873 _SpinDuration = x; 1874 } 1875 } 1876 1877 Abort: 1878 if (_succ == Self) { 1879 _succ = NULL; 1880 // Invariant: after setting succ=null a contending thread 1881 // must recheck-retry _owner before parking. This usually happens 1882 // in the normal usage of TrySpin(), but it's safest 1883 // to make TrySpin() as foolproof as possible. 1884 OrderAccess::fence(); 1885 if (TryLock(Self) > 0) return 1; 1886 } 1887 return 0; 1888 } 1889 1890 // NotRunnable() -- informed spinning 1891 // 1892 // Don't bother spinning if the owner is not eligible to drop the lock. 1893 // Spin only if the owner thread is _thread_in_Java or _thread_in_vm. 1894 // The thread must be runnable in order to drop the lock in timely fashion. 1895 // If the _owner is not runnable then spinning will not likely be 1896 // successful (profitable). 1897 // 1898 // Beware -- the thread referenced by _owner could have died 1899 // so a simply fetch from _owner->_thread_state might trap. 1900 // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state. 1901 // Because of the lifecycle issues, the _thread_state values 1902 // observed by NotRunnable() might be garbage. NotRunnable must 1903 // tolerate this and consider the observed _thread_state value 1904 // as advisory. 1905 // 1906 // Beware too, that _owner is sometimes a BasicLock address and sometimes 1907 // a thread pointer. 1908 // Alternately, we might tag the type (thread pointer vs basiclock pointer) 1909 // with the LSB of _owner. Another option would be to probabilistically probe 1910 // the putative _owner->TypeTag value. 1911 // 1912 // Checking _thread_state isn't perfect. Even if the thread is 1913 // in_java it might be blocked on a page-fault or have been preempted 1914 // and sitting on a ready/dispatch queue. 1915 // 1916 // The return value from NotRunnable() is *advisory* -- the 1917 // result is based on sampling and is not necessarily coherent. 1918 // The caller must tolerate false-negative and false-positive errors. 1919 // Spinning, in general, is probabilistic anyway. 1920 1921 1922 int ObjectMonitor::NotRunnable(Thread * Self, Thread * ox) { 1923 // Check ox->TypeTag == 2BAD. 1924 if (ox == NULL) return 0; 1925 1926 // Avoid transitive spinning ... 1927 // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L. 1928 // Immediately after T1 acquires L it's possible that T2, also 1929 // spinning on L, will see L.Owner=T1 and T1._Stalled=L. 1930 // This occurs transiently after T1 acquired L but before 1931 // T1 managed to clear T1.Stalled. T2 does not need to abort 1932 // its spin in this circumstance. 1933 intptr_t BlockedOn = SafeFetchN((intptr_t *) &ox->_Stalled, intptr_t(1)); 1934 1935 if (BlockedOn == 1) return 1; 1936 if (BlockedOn != 0) { 1937 return BlockedOn != intptr_t(this) && _owner == ox; 1938 } 1939 1940 assert(sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant"); 1941 int jst = SafeFetch32((int *) &((JavaThread *) ox)->_thread_state, -1);; 1942 // consider also: jst != _thread_in_Java -- but that's overspecific. 1943 return jst == _thread_blocked || jst == _thread_in_native; 1944 } 1945 1946 1947 // ----------------------------------------------------------------------------- 1948 // WaitSet management ... 1949 1950 ObjectWaiter::ObjectWaiter(Thread* thread) { 1951 _next = NULL; 1952 _prev = NULL; 1953 _notified = 0; 1954 _notifier_tid = 0; 1955 TState = TS_RUN; 1956 _thread = thread; 1957 _event = thread->_ParkEvent; 1958 _active = false; 1959 assert(_event != NULL, "invariant"); 1960 } 1961 1962 void ObjectWaiter::wait_reenter_begin(ObjectMonitor * const mon) { 1963 JavaThread *jt = (JavaThread *)this->_thread; 1964 _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon); 1965 } 1966 1967 void ObjectWaiter::wait_reenter_end(ObjectMonitor * const mon) { 1968 JavaThread *jt = (JavaThread *)this->_thread; 1969 JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active); 1970 } 1971 1972 inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) { 1973 assert(node != NULL, "should not add NULL node"); 1974 assert(node->_prev == NULL, "node already in list"); 1975 assert(node->_next == NULL, "node already in list"); 1976 // put node at end of queue (circular doubly linked list) 1977 if (_WaitSet == NULL) { 1978 _WaitSet = node; 1979 node->_prev = node; 1980 node->_next = node; 1981 } else { 1982 ObjectWaiter* head = _WaitSet; 1983 ObjectWaiter* tail = head->_prev; 1984 assert(tail->_next == head, "invariant check"); 1985 tail->_next = node; 1986 head->_prev = node; 1987 node->_next = head; 1988 node->_prev = tail; 1989 } 1990 } 1991 1992 inline ObjectWaiter* ObjectMonitor::DequeueWaiter() { 1993 // dequeue the very first waiter 1994 ObjectWaiter* waiter = _WaitSet; 1995 if (waiter) { 1996 DequeueSpecificWaiter(waiter); 1997 } 1998 return waiter; 1999 } 2000 2001 inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) { 2002 assert(node != NULL, "should not dequeue NULL node"); 2003 assert(node->_prev != NULL, "node already removed from list"); 2004 assert(node->_next != NULL, "node already removed from list"); 2005 // when the waiter has woken up because of interrupt, 2006 // timeout or other spurious wake-up, dequeue the 2007 // waiter from waiting list 2008 ObjectWaiter* next = node->_next; 2009 if (next == node) { 2010 assert(node->_prev == node, "invariant check"); 2011 _WaitSet = NULL; 2012 } else { 2013 ObjectWaiter* prev = node->_prev; 2014 assert(prev->_next == node, "invariant check"); 2015 assert(next->_prev == node, "invariant check"); 2016 next->_prev = prev; 2017 prev->_next = next; 2018 if (_WaitSet == node) { 2019 _WaitSet = next; 2020 } 2021 } 2022 node->_next = NULL; 2023 node->_prev = NULL; 2024 } 2025 2026 // ----------------------------------------------------------------------------- 2027 // PerfData support 2028 PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL; 2029 PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL; 2030 PerfCounter * ObjectMonitor::_sync_Parks = NULL; 2031 PerfCounter * ObjectMonitor::_sync_Notifications = NULL; 2032 PerfCounter * ObjectMonitor::_sync_Inflations = NULL; 2033 PerfCounter * ObjectMonitor::_sync_Deflations = NULL; 2034 PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL; 2035 2036 // One-shot global initialization for the sync subsystem. 2037 // We could also defer initialization and initialize on-demand 2038 // the first time we call ObjectSynchronizer::inflate(). 2039 // Initialization would be protected - like so many things - by 2040 // the MonitorCache_lock. 2041 2042 void ObjectMonitor::Initialize() { 2043 assert(!InitDone, "invariant"); 2044 2045 if (!os::is_MP()) { 2046 Knob_SpinLimit = 0; 2047 Knob_PreSpin = 0; 2048 Knob_FixedSpin = -1; 2049 } 2050 2051 if (UsePerfData) { 2052 EXCEPTION_MARK; 2053 #define NEWPERFCOUNTER(n) \ 2054 { \ 2055 n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events, \ 2056 CHECK); \ 2057 } 2058 #define NEWPERFVARIABLE(n) \ 2059 { \ 2060 n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events, \ 2061 CHECK); \ 2062 } 2063 NEWPERFCOUNTER(_sync_Inflations); 2064 NEWPERFCOUNTER(_sync_Deflations); 2065 NEWPERFCOUNTER(_sync_ContendedLockAttempts); 2066 NEWPERFCOUNTER(_sync_FutileWakeups); 2067 NEWPERFCOUNTER(_sync_Parks); 2068 NEWPERFCOUNTER(_sync_Notifications); 2069 NEWPERFVARIABLE(_sync_MonExtant); 2070 #undef NEWPERFCOUNTER 2071 #undef NEWPERFVARIABLE 2072 } 2073 2074 DEBUG_ONLY(InitDone = true;) 2075 } 2076 2077 ObjectMonitorHandle::~ObjectMonitorHandle() { 2078 if (_om_ptr != NULL) { 2079 _om_ptr->dec_ref_count(); 2080 _om_ptr = NULL; 2081 } 2082 } 2083 2084 // Save the ObjectMonitor* associated with the specified markWord and 2085 // increment the ref_count. This function should only be called if 2086 // the caller has verified mark.has_monitor() == true. The object 2087 // parameter is needed to verify that ObjectMonitor* has not been 2088 // deflated and reused for another object. 2089 // 2090 // This function returns true if the ObjectMonitor* has been safely 2091 // saved. This function returns false if we have lost a race with 2092 // async deflation; the caller should retry as appropriate. 2093 // 2094 bool ObjectMonitorHandle::save_om_ptr(oop object, markWord mark) { 2095 // is_marked() is a superset of has_monitor() so make sure we 2096 // are called with the proper markWord value. 2097 guarantee(mark.has_monitor() && !mark.is_marked(), "sanity check: mark=" 2098 INTPTR_FORMAT, mark.value()); 2099 2100 ObjectMonitor* om_ptr = mark.monitor(); 2101 om_ptr->inc_ref_count(); 2102 2103 if (AsyncDeflateIdleMonitors) { 2104 // Race here if monitor is not owned! The above ref_count bump 2105 // will cause subsequent async deflation to skip it. However, 2106 // previous or concurrent async deflation is a race. 2107 if (om_ptr->owner_is_DEFLATER_MARKER() && om_ptr->ref_count() <= 0) { 2108 // Async deflation is in progress and our ref_count increment 2109 // above lost the race to async deflation. Attempt to restore 2110 // the header/dmw to the object's header so that we only retry 2111 // once if the deflater thread happens to be slow. 2112 om_ptr->install_displaced_markword_in_object(object); 2113 om_ptr->dec_ref_count(); 2114 return false; 2115 } 2116 if (om_ptr->ref_count() <= 0) { 2117 // Async deflation is in the process of bailing out, but has not 2118 // yet restored the ref_count field so we return false to force 2119 // a retry. We want a positive ref_count value for a true return. 2120 om_ptr->dec_ref_count(); 2121 return false; 2122 } 2123 // The ObjectMonitor could have been deflated and reused for 2124 // another object before we bumped the ref_count so make sure 2125 // our object still refers to this ObjectMonitor. 2126 const markWord tmp = object->mark(); 2127 if (!tmp.has_monitor() || tmp.monitor() != om_ptr) { 2128 // Async deflation and reuse won the race so we have to retry. 2129 // Skip object header restoration since that's already done. 2130 om_ptr->dec_ref_count(); 2131 return false; 2132 } 2133 } 2134 2135 ADIM_guarantee(_om_ptr == NULL, "sanity check: _om_ptr=" INTPTR_FORMAT, 2136 p2i(_om_ptr)); 2137 _om_ptr = om_ptr; 2138 return true; 2139 } 2140 2141 // For internal use by ObjectSynchronizer::inflate(). 2142 // This function is only used when we don't have to worry about async 2143 // deflation of the specified ObjectMonitor*. 2144 void ObjectMonitorHandle::set_om_ptr(ObjectMonitor* om_ptr) { 2145 if (_om_ptr == NULL) { 2146 ADIM_guarantee(om_ptr != NULL, "cannot clear an unset om_ptr"); 2147 om_ptr->inc_ref_count(); 2148 _om_ptr = om_ptr; 2149 } else { 2150 ADIM_guarantee(om_ptr == NULL, "can only clear a set om_ptr"); 2151 _om_ptr->dec_ref_count(); 2152 _om_ptr = NULL; 2153 } 2154 } 2155 2156 // Save the specified ObjectMonitor* if it is safe, i.e., not being 2157 // async deflated. 2158 // 2159 // This function returns true if the ObjectMonitor* has been safely 2160 // saved. This function returns false if the specified ObjectMonitor* 2161 // is NULL or if we have lost a race with async deflation; the caller 2162 // can retry as appropriate. 2163 bool ObjectMonitorHandle::set_om_ptr_if_safe(ObjectMonitor* om_ptr) { 2164 if (om_ptr == NULL) { 2165 return false; // Nothing to save if input is NULL 2166 } 2167 2168 om_ptr->inc_ref_count(); 2169 2170 if (AsyncDeflateIdleMonitors) { 2171 if (om_ptr->owner_is_DEFLATER_MARKER() && om_ptr->ref_count() <= 0) { 2172 // Async deflation is in progress and our ref_count increment 2173 // above lost the race to async deflation. 2174 om_ptr->dec_ref_count(); 2175 return false; 2176 } 2177 if (om_ptr->ref_count() <= 0) { 2178 // Async deflation is in the process of bailing out, but has not 2179 // yet restored the ref_count field so we return false to force 2180 // a retry. We want a positive ref_count value for a true return. 2181 om_ptr->dec_ref_count(); 2182 return false; 2183 } 2184 // Unlike save_om_ptr(), we don't have context to determine if 2185 // the ObjectMonitor has been deflated and reused for another 2186 // object. 2187 } 2188 2189 ADIM_guarantee(_om_ptr == NULL, "sanity check: _om_ptr=" INTPTR_FORMAT, 2190 p2i(_om_ptr)); 2191 _om_ptr = om_ptr; 2192 return true; 2193 } 2194 2195 // Unset the _om_ptr field and decrement the ref_count field. 2196 void ObjectMonitorHandle::unset_om_ptr() { 2197 ADIM_guarantee(_om_ptr != NULL, "_om_ptr must not be NULL"); 2198 _om_ptr->dec_ref_count(); 2199 _om_ptr = NULL; 2200 } 2201 2202 void ObjectMonitor::print_on(outputStream* st) const { 2203 // The minimal things to print for markWord printing, more can be added for debugging and logging. 2204 st->print("{contentions=0x%08x,waiters=0x%08x" 2205 ",recursions=" INTX_FORMAT ",owner=" INTPTR_FORMAT "}", 2206 contentions(), waiters(), recursions(), 2207 p2i(owner())); 2208 } 2209 void ObjectMonitor::print() const { print_on(tty); } 2210 2211 #ifdef ASSERT 2212 // Print the ObjectMonitor like a debugger would: 2213 // 2214 // (ObjectMonitor) 0x00007fdfb6012e40 = { 2215 // _header = 0x0000000000000001 2216 // _object = 0x000000070ff45fd0 2217 // _allocation_state = Old 2218 // _pad_buf0 = { 2219 // [0] = '\0' 2220 // ... 2221 // [43] = '\0' 2222 // } 2223 // _owner = 0x0000000000000000 2224 // _previous_owner_tid = 0 2225 // _pad_buf1 = { 2226 // [0] = '\0' 2227 // ... 2228 // [47] = '\0' 2229 // } 2230 // _ref_count = 1 2231 // _pad_buf2 = { 2232 // [0] = '\0' 2233 // ... 2234 // [47] = '\0' 2235 // } 2236 // _next_om = 0x0000000000000000 2237 // _recursions = 0 2238 // _EntryList = 0x0000000000000000 2239 // _cxq = 0x0000000000000000 2240 // _succ = 0x0000000000000000 2241 // _Responsible = 0x0000000000000000 2242 // _Spinner = 0 2243 // _SpinDuration = 5000 2244 // _contentions = 0 2245 // _WaitSet = 0x0000700009756248 2246 // _waiters = 1 2247 // _WaitSetLock = 0 2248 // } 2249 // 2250 void ObjectMonitor::print_debug_style_on(outputStream* st) const { 2251 st->print_cr("(ObjectMonitor*) " INTPTR_FORMAT " = {", p2i(this)); 2252 st->print_cr(" _header = " INTPTR_FORMAT, header().value()); 2253 st->print_cr(" _object = " INTPTR_FORMAT, p2i(_object)); 2254 st->print(" _allocation_state = "); 2255 if (is_free()) { 2256 st->print("Free"); 2257 } else if (is_old()) { 2258 st->print("Old"); 2259 } else if (is_new()) { 2260 st->print("New"); 2261 } else { 2262 st->print("unknown=%d", _allocation_state); 2263 } 2264 st->cr(); 2265 st->print_cr(" _pad_buf0 = {"); 2266 st->print_cr(" [0] = '\\0'"); 2267 st->print_cr(" ..."); 2268 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf0) - 1); 2269 st->print_cr(" }"); 2270 st->print_cr(" _owner = " INTPTR_FORMAT, p2i(_owner)); 2271 st->print_cr(" _previous_owner_tid = " JLONG_FORMAT, _previous_owner_tid); 2272 st->print_cr(" _pad_buf1 = {"); 2273 st->print_cr(" [0] = '\\0'"); 2274 st->print_cr(" ..."); 2275 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf1) - 1); 2276 st->print_cr(" }"); 2277 st->print_cr(" _ref_count = %d", ref_count()); 2278 st->print_cr(" _pad_buf2 = {"); 2279 st->print_cr(" [0] = '\\0'"); 2280 st->print_cr(" ..."); 2281 st->print_cr(" [%d] = '\\0'", (int)sizeof(_pad_buf1) - 1); 2282 st->print_cr(" }"); 2283 st->print_cr(" _next_om = " INTPTR_FORMAT, p2i(Atomic::load(&_next_om))); 2284 st->print_cr(" _recursions = " INTX_FORMAT, _recursions); 2285 st->print_cr(" _EntryList = " INTPTR_FORMAT, p2i(_EntryList)); 2286 st->print_cr(" _cxq = " INTPTR_FORMAT, p2i(_cxq)); 2287 st->print_cr(" _succ = " INTPTR_FORMAT, p2i(_succ)); 2288 st->print_cr(" _Responsible = " INTPTR_FORMAT, p2i(_Responsible)); 2289 st->print_cr(" _Spinner = %d", _Spinner); 2290 st->print_cr(" _SpinDuration = %d", _SpinDuration); 2291 st->print_cr(" _contentions = %d", _contentions); 2292 st->print_cr(" _WaitSet = " INTPTR_FORMAT, p2i(_WaitSet)); 2293 st->print_cr(" _waiters = %d", _waiters); 2294 st->print_cr(" _WaitSetLock = %d", _WaitSetLock); 2295 st->print_cr("}"); 2296 } 2297 #endif