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