/* * Copyright (c) 1998, 2014, Oracle and/or its affiliates. All rights reserved. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. * */ #include "precompiled.hpp" #include "classfile/vmSymbols.hpp" #include "memory/resourceArea.hpp" #include "oops/markOop.hpp" #include "oops/oop.inline.hpp" #include "runtime/handles.inline.hpp" #include "runtime/interfaceSupport.hpp" #include "runtime/mutexLocker.hpp" #include "runtime/objectMonitor.hpp" #include "runtime/objectMonitor.inline.hpp" #include "runtime/osThread.hpp" #include "runtime/stubRoutines.hpp" #include "runtime/thread.inline.hpp" #include "services/threadService.hpp" #include "trace/tracing.hpp" #include "trace/traceMacros.hpp" #include "utilities/dtrace.hpp" #include "utilities/macros.hpp" #include "utilities/preserveException.hpp" #ifdef TARGET_OS_FAMILY_linux # include "os_linux.inline.hpp" #endif #ifdef TARGET_OS_FAMILY_solaris # include "os_solaris.inline.hpp" #endif #ifdef TARGET_OS_FAMILY_windows # include "os_windows.inline.hpp" #endif #ifdef TARGET_OS_FAMILY_bsd # include "os_bsd.inline.hpp" #endif #if defined(__GNUC__) && !defined(IA64) && !defined(PPC64) // Need to inhibit inlining for older versions of GCC to avoid build-time failures #define ATTR __attribute__((noinline)) #else #define ATTR #endif #ifdef DTRACE_ENABLED // Only bother with this argument setup if dtrace is available // TODO-FIXME: probes should not fire when caller is _blocked. assert() accordingly. #define DTRACE_MONITOR_PROBE_COMMON(obj, thread) \ char* bytes = NULL; \ int len = 0; \ jlong jtid = SharedRuntime::get_java_tid(thread); \ Symbol* klassname = ((oop)obj)->klass()->name(); \ if (klassname != NULL) { \ bytes = (char*)klassname->bytes(); \ len = klassname->utf8_length(); \ } #ifndef USDT2 HS_DTRACE_PROBE_DECL4(hotspot, monitor__notify, jlong, uintptr_t, char*, int); HS_DTRACE_PROBE_DECL4(hotspot, monitor__notifyAll, jlong, uintptr_t, char*, int); HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__enter, jlong, uintptr_t, char*, int); HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__entered, jlong, uintptr_t, char*, int); HS_DTRACE_PROBE_DECL4(hotspot, monitor__contended__exit, jlong, uintptr_t, char*, int); #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \ { \ if (DTraceMonitorProbes) { \ DTRACE_MONITOR_PROBE_COMMON(obj, thread); \ HS_DTRACE_PROBE5(hotspot, monitor__wait, jtid, \ (monitor), bytes, len, (millis)); \ } \ } #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \ { \ if (DTraceMonitorProbes) { \ DTRACE_MONITOR_PROBE_COMMON(obj, thread); \ HS_DTRACE_PROBE4(hotspot, monitor__##probe, jtid, \ (uintptr_t)(monitor), bytes, len); \ } \ } #else /* USDT2 */ #define DTRACE_MONITOR_WAIT_PROBE(monitor, obj, thread, millis) \ { \ if (DTraceMonitorProbes) { \ DTRACE_MONITOR_PROBE_COMMON(obj, thread); \ HOTSPOT_MONITOR_WAIT(jtid, \ (monitor), bytes, len, (millis)); \ } \ } #define HOTSPOT_MONITOR_contended__enter HOTSPOT_MONITOR_CONTENDED_ENTER #define HOTSPOT_MONITOR_contended__entered HOTSPOT_MONITOR_CONTENDED_ENTERED #define HOTSPOT_MONITOR_contended__exit HOTSPOT_MONITOR_CONTENDED_EXIT #define HOTSPOT_MONITOR_notify HOTSPOT_MONITOR_NOTIFY #define HOTSPOT_MONITOR_notifyAll HOTSPOT_MONITOR_NOTIFYALL #define DTRACE_MONITOR_PROBE(probe, monitor, obj, thread) \ { \ if (DTraceMonitorProbes) { \ DTRACE_MONITOR_PROBE_COMMON(obj, thread); \ HOTSPOT_MONITOR_##probe(jtid, \ (uintptr_t)(monitor), bytes, len); \ } \ } #endif /* USDT2 */ #else // ndef DTRACE_ENABLED #define DTRACE_MONITOR_WAIT_PROBE(obj, thread, millis, mon) {;} #define DTRACE_MONITOR_PROBE(probe, obj, thread, mon) {;} #endif // ndef DTRACE_ENABLED // Tunables ... // The knob* variables are effectively final. Once set they should // never be modified hence. Consider using __read_mostly with GCC. int ObjectMonitor::Knob_Verbose = 0 ; int ObjectMonitor::Knob_SpinLimit = 5000 ; // derived by an external tool - static int Knob_LogSpins = 0 ; // enable jvmstat tally for spins static int Knob_HandOff = 0 ; static int Knob_ReportSettings = 0 ; static int Knob_SpinBase = 0 ; // Floor AKA SpinMin static int Knob_SpinBackOff = 0 ; // spin-loop backoff static int Knob_CASPenalty = -1 ; // Penalty for failed CAS static int Knob_OXPenalty = -1 ; // Penalty for observed _owner change static int Knob_SpinSetSucc = 1 ; // spinners set the _succ field static int Knob_SpinEarly = 1 ; static int Knob_SuccEnabled = 1 ; // futile wake throttling static int Knob_SuccRestrict = 0 ; // Limit successors + spinners to at-most-one static int Knob_MaxSpinners = -1 ; // Should be a function of # CPUs static int Knob_Bonus = 100 ; // spin success bonus static int Knob_BonusB = 100 ; // spin success bonus static int Knob_Penalty = 200 ; // spin failure penalty static int Knob_Poverty = 1000 ; static int Knob_SpinAfterFutile = 1 ; // Spin after returning from park() static int Knob_FixedSpin = 0 ; static int Knob_OState = 3 ; // Spinner checks thread state of _owner static int Knob_UsePause = 1 ; static int Knob_ExitPolicy = 0 ; static int Knob_PreSpin = 10 ; // 20-100 likely better static int Knob_ResetEvent = 0 ; static int BackOffMask = 0 ; static int Knob_FastHSSEC = 0 ; static int Knob_MoveNotifyee = 2 ; // notify() - disposition of notifyee static int Knob_QMode = 0 ; // EntryList-cxq policy - queue discipline static volatile int InitDone = 0 ; #define TrySpin TrySpin_VaryDuration // ----------------------------------------------------------------------------- // Theory of operations -- Monitors lists, thread residency, etc: // // * A thread acquires ownership of a monitor by successfully // CAS()ing the _owner field from null to non-null. // // * Invariant: A thread appears on at most one monitor list -- // cxq, EntryList or WaitSet -- at any one time. // // * Contending threads "push" themselves onto the cxq with CAS // and then spin/park. // // * After a contending thread eventually acquires the lock it must // dequeue itself from either the EntryList or the cxq. // // * The exiting thread identifies and unparks an "heir presumptive" // tentative successor thread on the EntryList. Critically, the // exiting thread doesn't unlink the successor thread from the EntryList. // After having been unparked, the wakee will recontend for ownership of // the monitor. The successor (wakee) will either acquire the lock or // re-park itself. // // Succession is provided for by a policy of competitive handoff. // The exiting thread does _not_ grant or pass ownership to the // successor thread. (This is also referred to as "handoff" succession"). // Instead the exiting thread releases ownership and possibly wakes // a successor, so the successor can (re)compete for ownership of the lock. // If the EntryList is empty but the cxq is populated the exiting // thread will drain the cxq into the EntryList. It does so by // by detaching the cxq (installing null with CAS) and folding // the threads from the cxq into the EntryList. The EntryList is // doubly linked, while the cxq is singly linked because of the // CAS-based "push" used to enqueue recently arrived threads (RATs). // // * Concurrency invariants: // // -- only the monitor owner may access or mutate the EntryList. // The mutex property of the monitor itself protects the EntryList // from concurrent interference. // -- Only the monitor owner may detach the cxq. // // * The monitor entry list operations avoid locks, but strictly speaking // they're not lock-free. Enter is lock-free, exit is not. // See http://j2se.east/~dice/PERSIST/040825-LockFreeQueues.html // // * The cxq can have multiple concurrent "pushers" but only one concurrent // detaching thread. This mechanism is immune from the ABA corruption. // More precisely, the CAS-based "push" onto cxq is ABA-oblivious. // // * Taken together, the cxq and the EntryList constitute or form a // single logical queue of threads stalled trying to acquire the lock. // We use two distinct lists to improve the odds of a constant-time // dequeue operation after acquisition (in the ::enter() epilog) and // to reduce heat on the list ends. (c.f. Michael Scott's "2Q" algorithm). // A key desideratum is to minimize queue & monitor metadata manipulation // that occurs while holding the monitor lock -- that is, we want to // minimize monitor lock holds times. Note that even a small amount of // fixed spinning will greatly reduce the # of enqueue-dequeue operations // on EntryList|cxq. That is, spinning relieves contention on the "inner" // locks and monitor metadata. // // Cxq points to the the set of Recently Arrived Threads attempting entry. // Because we push threads onto _cxq with CAS, the RATs must take the form of // a singly-linked LIFO. We drain _cxq into EntryList at unlock-time when // the unlocking thread notices that EntryList is null but _cxq is != null. // // The EntryList is ordered by the prevailing queue discipline and // can be organized in any convenient fashion, such as a doubly-linked list or // a circular doubly-linked list. Critically, we want insert and delete operations // to operate in constant-time. If we need a priority queue then something akin // to Solaris' sleepq would work nicely. Viz., // http://agg.eng/ws/on10_nightly/source/usr/src/uts/common/os/sleepq.c. // Queue discipline is enforced at ::exit() time, when the unlocking thread // drains the cxq into the EntryList, and orders or reorders the threads on the // EntryList accordingly. // // Barring "lock barging", this mechanism provides fair cyclic ordering, // somewhat similar to an elevator-scan. // // * The monitor synchronization subsystem avoids the use of native // synchronization primitives except for the narrow platform-specific // park-unpark abstraction. See the comments in os_solaris.cpp regarding // the semantics of park-unpark. Put another way, this monitor implementation // depends only on atomic operations and park-unpark. The monitor subsystem // manages all RUNNING->BLOCKED and BLOCKED->READY transitions while the // underlying OS manages the READY<->RUN transitions. // // * Waiting threads reside on the WaitSet list -- wait() puts // the caller onto the WaitSet. // // * notify() or notifyAll() simply transfers threads from the WaitSet to // either the EntryList or cxq. Subsequent exit() operations will // unpark the notifyee. Unparking a notifee in notify() is inefficient - // it's likely the notifyee would simply impale itself on the lock held // by the notifier. // // * An interesting alternative is to encode cxq as (List,LockByte) where // the LockByte is 0 iff the monitor is owned. _owner is simply an auxiliary // variable, like _recursions, in the scheme. The threads or Events that form // the list would have to be aligned in 256-byte addresses. A thread would // try to acquire the lock or enqueue itself with CAS, but exiting threads // could use a 1-0 protocol and simply STB to set the LockByte to 0. // Note that is is *not* word-tearing, but it does presume that full-word // CAS operations are coherent with intermix with STB operations. That's true // on most common processors. // // * See also http://blogs.sun.com/dave // ----------------------------------------------------------------------------- // Enter support bool ObjectMonitor::try_enter(Thread* THREAD) { if (THREAD != _owner) { if (THREAD->is_lock_owned ((address)_owner)) { assert(_recursions == 0, "internal state error"); _owner = THREAD ; _recursions = 1 ; OwnerIsThread = 1 ; return true; } if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) { return false; } return true; } else { _recursions++; return true; } } void ATTR ObjectMonitor::enter(TRAPS) { // The following code is ordered to check the most common cases first // and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors. Thread * const Self = THREAD ; void * cur ; cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ; if (cur == NULL) { // Either ASSERT _recursions == 0 or explicitly set _recursions = 0. assert (_recursions == 0 , "invariant") ; assert (_owner == Self, "invariant") ; // CONSIDER: set or assert OwnerIsThread == 1 return ; } if (cur == Self) { // TODO-FIXME: check for integer overflow! BUGID 6557169. _recursions ++ ; return ; } if (Self->is_lock_owned ((address)cur)) { assert (_recursions == 0, "internal state error"); _recursions = 1 ; // Commute owner from a thread-specific on-stack BasicLockObject address to // a full-fledged "Thread *". _owner = Self ; OwnerIsThread = 1 ; return ; } // We've encountered genuine contention. assert (Self->_Stalled == 0, "invariant") ; Self->_Stalled = intptr_t(this) ; // Try one round of spinning *before* enqueueing Self // and before going through the awkward and expensive state // transitions. The following spin is strictly optional ... // Note that if we acquire the monitor from an initial spin // we forgo posting JVMTI events and firing DTRACE probes. if (Knob_SpinEarly && TrySpin (Self) > 0) { assert (_owner == Self , "invariant") ; assert (_recursions == 0 , "invariant") ; assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ; Self->_Stalled = 0 ; return ; } assert (_owner != Self , "invariant") ; assert (_succ != Self , "invariant") ; assert (Self->is_Java_thread() , "invariant") ; JavaThread * jt = (JavaThread *) Self ; assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ; assert (jt->thread_state() != _thread_blocked , "invariant") ; assert (this->object() != NULL , "invariant") ; assert (_count >= 0, "invariant") ; // Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy(). // Ensure the object-monitor relationship remains stable while there's contention. Atomic::inc_ptr(&_count); EventJavaMonitorEnter event; { // Change java thread status to indicate blocked on monitor enter. JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this); DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt); if (JvmtiExport::should_post_monitor_contended_enter()) { JvmtiExport::post_monitor_contended_enter(jt, this); // The current thread does not yet own the monitor and does not // yet appear on any queues that would get it made the successor. // This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event // handler cannot accidentally consume an unpark() meant for the // ParkEvent associated with this ObjectMonitor. } OSThreadContendState osts(Self->osthread()); ThreadBlockInVM tbivm(jt); Self->set_current_pending_monitor(this); // TODO-FIXME: change the following for(;;) loop to straight-line code. for (;;) { jt->set_suspend_equivalent(); // cleared by handle_special_suspend_equivalent_condition() // or java_suspend_self() EnterI (THREAD) ; if (!ExitSuspendEquivalent(jt)) break ; // // We have acquired the contended monitor, but while we were // waiting another thread suspended us. We don't want to enter // the monitor while suspended because that would surprise the // thread that suspended us. // _recursions = 0 ; _succ = NULL ; exit (false, Self) ; jt->java_suspend_self(); } Self->set_current_pending_monitor(NULL); // We cleared the pending monitor info since we've just gotten past // the enter-check-for-suspend dance and we now own the monitor free // and clear, i.e., it is no longer pending. The ThreadBlockInVM // destructor can go to a safepoint at the end of this block. If we // do a thread dump during that safepoint, then this thread will show // as having "-locked" the monitor, but the OS and java.lang.Thread // states will still report that the thread is blocked trying to // acquire it. } Atomic::dec_ptr(&_count); assert (_count >= 0, "invariant") ; Self->_Stalled = 0 ; // Must either set _recursions = 0 or ASSERT _recursions == 0. assert (_recursions == 0 , "invariant") ; assert (_owner == Self , "invariant") ; assert (_succ != Self , "invariant") ; assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ; // The thread -- now the owner -- is back in vm mode. // Report the glorious news via TI,DTrace and jvmstat. // The probe effect is non-trivial. All the reportage occurs // while we hold the monitor, increasing the length of the critical // section. Amdahl's parallel speedup law comes vividly into play. // // Another option might be to aggregate the events (thread local or // per-monitor aggregation) and defer reporting until a more opportune // time -- such as next time some thread encounters contention but has // yet to acquire the lock. While spinning that thread could // spinning we could increment JVMStat counters, etc. DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt); if (JvmtiExport::should_post_monitor_contended_entered()) { JvmtiExport::post_monitor_contended_entered(jt, this); // The current thread already owns the monitor and is not going to // call park() for the remainder of the monitor enter protocol. So // it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED // event handler consumed an unpark() issued by the thread that // just exited the monitor. } if (event.should_commit()) { event.set_klass(((oop)this->object())->klass()); event.set_previousOwner((TYPE_JAVALANGTHREAD)_previous_owner_tid); event.set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr())); event.commit(); } if (ObjectMonitor::_sync_ContendedLockAttempts != NULL) { ObjectMonitor::_sync_ContendedLockAttempts->inc() ; } } // Caveat: TryLock() is not necessarily serializing if it returns failure. // Callers must compensate as needed. int ObjectMonitor::TryLock (Thread * Self) { for (;;) { void * own = _owner ; if (own != NULL) return 0 ; if (Atomic::cmpxchg_ptr (Self, &_owner, NULL) == NULL) { // Either guarantee _recursions == 0 or set _recursions = 0. assert (_recursions == 0, "invariant") ; assert (_owner == Self, "invariant") ; // CONSIDER: set or assert that OwnerIsThread == 1 return 1 ; } // The lock had been free momentarily, but we lost the race to the lock. // Interference -- the CAS failed. // We can either return -1 or retry. // Retry doesn't make as much sense because the lock was just acquired. if (true) return -1 ; } } void ATTR ObjectMonitor::EnterI (TRAPS) { Thread * Self = THREAD ; assert (Self->is_Java_thread(), "invariant") ; assert (((JavaThread *) Self)->thread_state() == _thread_blocked , "invariant") ; // Try the lock - TATAS if (TryLock (Self) > 0) { assert (_succ != Self , "invariant") ; assert (_owner == Self , "invariant") ; assert (_Responsible != Self , "invariant") ; return ; } DeferredInitialize () ; // We try one round of spinning *before* enqueueing Self. // // If the _owner is ready but OFFPROC we could use a YieldTo() // operation to donate the remainder of this thread's quantum // to the owner. This has subtle but beneficial affinity // effects. if (TrySpin (Self) > 0) { assert (_owner == Self , "invariant") ; assert (_succ != Self , "invariant") ; assert (_Responsible != Self , "invariant") ; return ; } // The Spin failed -- Enqueue and park the thread ... assert (_succ != Self , "invariant") ; assert (_owner != Self , "invariant") ; assert (_Responsible != Self , "invariant") ; // Enqueue "Self" on ObjectMonitor's _cxq. // // Node acts as a proxy for Self. // As an aside, if were to ever rewrite the synchronization code mostly // in Java, WaitNodes, ObjectMonitors, and Events would become 1st-class // Java objects. This would avoid awkward lifecycle and liveness issues, // as well as eliminate a subset of ABA issues. // TODO: eliminate ObjectWaiter and enqueue either Threads or Events. // ObjectWaiter node(Self) ; Self->_ParkEvent->reset() ; node._prev = (ObjectWaiter *) 0xBAD ; node.TState = ObjectWaiter::TS_CXQ ; // Push "Self" onto the front of the _cxq. // Once on cxq/EntryList, Self stays on-queue until it acquires the lock. // Note that spinning tends to reduce the rate at which threads // enqueue and dequeue on EntryList|cxq. ObjectWaiter * nxt ; for (;;) { node._next = nxt = _cxq ; if (Atomic::cmpxchg_ptr (&node, &_cxq, nxt) == nxt) break ; // Interference - the CAS failed because _cxq changed. Just retry. // As an optional optimization we retry the lock. if (TryLock (Self) > 0) { assert (_succ != Self , "invariant") ; assert (_owner == Self , "invariant") ; assert (_Responsible != Self , "invariant") ; return ; } } // Check for cxq|EntryList edge transition to non-null. This indicates // the onset of contention. While contention persists exiting threads // will use a ST:MEMBAR:LD 1-1 exit protocol. When contention abates exit // operations revert to the faster 1-0 mode. This enter operation may interleave // (race) a concurrent 1-0 exit operation, resulting in stranding, so we // arrange for one of the contending thread to use a timed park() operations // to detect and recover from the race. (Stranding is form of progress failure // where the monitor is unlocked but all the contending threads remain parked). // That is, at least one of the contended threads will periodically poll _owner. // One of the contending threads will become the designated "Responsible" thread. // The Responsible thread uses a timed park instead of a normal indefinite park // operation -- it periodically wakes and checks for and recovers from potential // strandings admitted by 1-0 exit operations. We need at most one Responsible // thread per-monitor at any given moment. Only threads on cxq|EntryList may // be responsible for a monitor. // // Currently, one of the contended threads takes on the added role of "Responsible". // A viable alternative would be to use a dedicated "stranding checker" thread // that periodically iterated over all the threads (or active monitors) and unparked // successors where there was risk of stranding. This would help eliminate the // timer scalability issues we see on some platforms as we'd only have one thread // -- the checker -- parked on a timer. if ((SyncFlags & 16) == 0 && nxt == NULL && _EntryList == NULL) { // Try to assume the role of responsible thread for the monitor. // CONSIDER: ST vs CAS vs { if (Responsible==null) Responsible=Self } Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ; } // The lock have been released while this thread was occupied queueing // itself onto _cxq. To close the race and avoid "stranding" and // progress-liveness failure we must resample-retry _owner before parking. // Note the Dekker/Lamport duality: ST cxq; MEMBAR; LD Owner. // In this case the ST-MEMBAR is accomplished with CAS(). // // TODO: Defer all thread state transitions until park-time. // Since state transitions are heavy and inefficient we'd like // to defer the state transitions until absolutely necessary, // and in doing so avoid some transitions ... TEVENT (Inflated enter - Contention) ; int nWakeups = 0 ; int RecheckInterval = 1 ; for (;;) { if (TryLock (Self) > 0) break ; assert (_owner != Self, "invariant") ; if ((SyncFlags & 2) && _Responsible == NULL) { Atomic::cmpxchg_ptr (Self, &_Responsible, NULL) ; } // park self if (_Responsible == Self || (SyncFlags & 1)) { TEVENT (Inflated enter - park TIMED) ; Self->_ParkEvent->park ((jlong) RecheckInterval) ; // Increase the RecheckInterval, but clamp the value. RecheckInterval *= 8 ; if (RecheckInterval > 1000) RecheckInterval = 1000 ; } else { TEVENT (Inflated enter - park UNTIMED) ; Self->_ParkEvent->park() ; } if (TryLock(Self) > 0) break ; // The lock is still contested. // Keep a tally of the # of futile wakeups. // Note that the counter is not protected by a lock or updated by atomics. // That is by design - we trade "lossy" counters which are exposed to // races during updates for a lower probe effect. TEVENT (Inflated enter - Futile wakeup) ; if (ObjectMonitor::_sync_FutileWakeups != NULL) { ObjectMonitor::_sync_FutileWakeups->inc() ; } ++ nWakeups ; // Assuming this is not a spurious wakeup we'll normally find _succ == Self. // We can defer clearing _succ until after the spin completes // TrySpin() must tolerate being called with _succ == Self. // Try yet another round of adaptive spinning. if ((Knob_SpinAfterFutile & 1) && TrySpin (Self) > 0) break ; // We can find that we were unpark()ed and redesignated _succ while // we were spinning. That's harmless. If we iterate and call park(), // park() will consume the event and return immediately and we'll // just spin again. This pattern can repeat, leaving _succ to simply // spin on a CPU. Enable Knob_ResetEvent to clear pending unparks(). // Alternately, we can sample fired() here, and if set, forgo spinning // in the next iteration. if ((Knob_ResetEvent & 1) && Self->_ParkEvent->fired()) { Self->_ParkEvent->reset() ; OrderAccess::fence() ; } if (_succ == Self) _succ = NULL ; // Invariant: after clearing _succ a thread *must* retry _owner before parking. OrderAccess::fence() ; } // Egress : // Self has acquired the lock -- Unlink Self from the cxq or EntryList. // Normally we'll find Self on the EntryList . // From the perspective of the lock owner (this thread), the // EntryList is stable and cxq is prepend-only. // The head of cxq is volatile but the interior is stable. // In addition, Self.TState is stable. assert (_owner == Self , "invariant") ; assert (object() != NULL , "invariant") ; // I'd like to write: // guarantee (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ; // but as we're at a safepoint that's not safe. UnlinkAfterAcquire (Self, &node) ; if (_succ == Self) _succ = NULL ; assert (_succ != Self, "invariant") ; if (_Responsible == Self) { _Responsible = NULL ; OrderAccess::fence(); // Dekker pivot-point // We may leave threads on cxq|EntryList without a designated // "Responsible" thread. This is benign. When this thread subsequently // exits the monitor it can "see" such preexisting "old" threads -- // threads that arrived on the cxq|EntryList before the fence, above -- // by LDing cxq|EntryList. Newly arrived threads -- that is, threads // that arrive on cxq after the ST:MEMBAR, above -- will set Responsible // non-null and elect a new "Responsible" timer thread. // // This thread executes: // ST Responsible=null; MEMBAR (in enter epilog - here) // LD cxq|EntryList (in subsequent exit) // // Entering threads in the slow/contended path execute: // ST cxq=nonnull; MEMBAR; LD Responsible (in enter prolog) // The (ST cxq; MEMBAR) is accomplished with CAS(). // // The MEMBAR, above, prevents the LD of cxq|EntryList in the subsequent // exit operation from floating above the ST Responsible=null. } // We've acquired ownership with CAS(). // CAS is serializing -- it has MEMBAR/FENCE-equivalent semantics. // But since the CAS() this thread may have also stored into _succ, // EntryList, cxq or Responsible. These meta-data updates must be // visible __before this thread subsequently drops the lock. // Consider what could occur if we didn't enforce this constraint -- // STs to monitor meta-data and user-data could reorder with (become // visible after) the ST in exit that drops ownership of the lock. // Some other thread could then acquire the lock, but observe inconsistent // or old monitor meta-data and heap data. That violates the JMM. // To that end, the 1-0 exit() operation must have at least STST|LDST // "release" barrier semantics. Specifically, there must be at least a // STST|LDST barrier in exit() before the ST of null into _owner that drops // the lock. The barrier ensures that changes to monitor meta-data and data // protected by the lock will be visible before we release the lock, and // therefore before some other thread (CPU) has a chance to acquire the lock. // See also: http://gee.cs.oswego.edu/dl/jmm/cookbook.html. // // Critically, any prior STs to _succ or EntryList must be visible before // the ST of null into _owner in the *subsequent* (following) corresponding // monitorexit. Recall too, that in 1-0 mode monitorexit does not necessarily // execute a serializing instruction. if (SyncFlags & 8) { OrderAccess::fence() ; } return ; } // ReenterI() is a specialized inline form of the latter half of the // contended slow-path from EnterI(). We use ReenterI() only for // monitor reentry in wait(). // // In the future we should reconcile EnterI() and ReenterI(), adding // Knob_Reset and Knob_SpinAfterFutile support and restructuring the // loop accordingly. void ATTR ObjectMonitor::ReenterI (Thread * Self, ObjectWaiter * SelfNode) { assert (Self != NULL , "invariant") ; assert (SelfNode != NULL , "invariant") ; assert (SelfNode->_thread == Self , "invariant") ; assert (_waiters > 0 , "invariant") ; assert (((oop)(object()))->mark() == markOopDesc::encode(this) , "invariant") ; assert (((JavaThread *)Self)->thread_state() != _thread_blocked, "invariant") ; JavaThread * jt = (JavaThread *) Self ; int nWakeups = 0 ; for (;;) { ObjectWaiter::TStates v = SelfNode->TState ; guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ; assert (_owner != Self, "invariant") ; if (TryLock (Self) > 0) break ; if (TrySpin (Self) > 0) break ; TEVENT (Wait Reentry - parking) ; // State transition wrappers around park() ... // ReenterI() wisely defers state transitions until // it's clear we must park the thread. { OSThreadContendState osts(Self->osthread()); ThreadBlockInVM tbivm(jt); // cleared by handle_special_suspend_equivalent_condition() // or java_suspend_self() jt->set_suspend_equivalent(); if (SyncFlags & 1) { Self->_ParkEvent->park ((jlong)1000) ; } else { Self->_ParkEvent->park () ; } // were we externally suspended while we were waiting? for (;;) { if (!ExitSuspendEquivalent (jt)) break ; if (_succ == Self) { _succ = NULL; OrderAccess::fence(); } jt->java_suspend_self(); jt->set_suspend_equivalent(); } } // Try again, but just so we distinguish between futile wakeups and // successful wakeups. The following test isn't algorithmically // necessary, but it helps us maintain sensible statistics. if (TryLock(Self) > 0) break ; // The lock is still contested. // Keep a tally of the # of futile wakeups. // Note that the counter is not protected by a lock or updated by atomics. // That is by design - we trade "lossy" counters which are exposed to // races during updates for a lower probe effect. TEVENT (Wait Reentry - futile wakeup) ; ++ nWakeups ; // Assuming this is not a spurious wakeup we'll normally // find that _succ == Self. if (_succ == Self) _succ = NULL ; // Invariant: after clearing _succ a contending thread // *must* retry _owner before parking. OrderAccess::fence() ; if (ObjectMonitor::_sync_FutileWakeups != NULL) { ObjectMonitor::_sync_FutileWakeups->inc() ; } } // Self has acquired the lock -- Unlink Self from the cxq or EntryList . // Normally we'll find Self on the EntryList. // Unlinking from the EntryList is constant-time and atomic-free. // From the perspective of the lock owner (this thread), the // EntryList is stable and cxq is prepend-only. // The head of cxq is volatile but the interior is stable. // In addition, Self.TState is stable. assert (_owner == Self, "invariant") ; assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ; UnlinkAfterAcquire (Self, SelfNode) ; if (_succ == Self) _succ = NULL ; assert (_succ != Self, "invariant") ; SelfNode->TState = ObjectWaiter::TS_RUN ; OrderAccess::fence() ; // see comments at the end of EnterI() } // after the thread acquires the lock in ::enter(). Equally, we could defer // unlinking the thread until ::exit()-time. void ObjectMonitor::UnlinkAfterAcquire (Thread * Self, ObjectWaiter * SelfNode) { assert (_owner == Self, "invariant") ; assert (SelfNode->_thread == Self, "invariant") ; if (SelfNode->TState == ObjectWaiter::TS_ENTER) { // Normal case: remove Self from the DLL EntryList . // This is a constant-time operation. ObjectWaiter * nxt = SelfNode->_next ; ObjectWaiter * prv = SelfNode->_prev ; if (nxt != NULL) nxt->_prev = prv ; if (prv != NULL) prv->_next = nxt ; if (SelfNode == _EntryList ) _EntryList = nxt ; assert (nxt == NULL || nxt->TState == ObjectWaiter::TS_ENTER, "invariant") ; assert (prv == NULL || prv->TState == ObjectWaiter::TS_ENTER, "invariant") ; TEVENT (Unlink from EntryList) ; } else { guarantee (SelfNode->TState == ObjectWaiter::TS_CXQ, "invariant") ; // Inopportune interleaving -- Self is still on the cxq. // This usually means the enqueue of self raced an exiting thread. // Normally we'll find Self near the front of the cxq, so // dequeueing is typically fast. If needbe we can accelerate // this with some MCS/CHL-like bidirectional list hints and advisory // back-links so dequeueing from the interior will normally operate // in constant-time. // Dequeue Self from either the head (with CAS) or from the interior // with a linear-time scan and normal non-atomic memory operations. // CONSIDER: if Self is on the cxq then simply drain cxq into EntryList // and then unlink Self from EntryList. We have to drain eventually, // so it might as well be now. ObjectWaiter * v = _cxq ; assert (v != NULL, "invariant") ; if (v != SelfNode || Atomic::cmpxchg_ptr (SelfNode->_next, &_cxq, v) != v) { // The CAS above can fail from interference IFF a "RAT" arrived. // In that case Self must be in the interior and can no longer be // at the head of cxq. if (v == SelfNode) { assert (_cxq != v, "invariant") ; v = _cxq ; // CAS above failed - start scan at head of list } ObjectWaiter * p ; ObjectWaiter * q = NULL ; for (p = v ; p != NULL && p != SelfNode; p = p->_next) { q = p ; assert (p->TState == ObjectWaiter::TS_CXQ, "invariant") ; } assert (v != SelfNode, "invariant") ; assert (p == SelfNode, "Node not found on cxq") ; assert (p != _cxq, "invariant") ; assert (q != NULL, "invariant") ; assert (q->_next == p, "invariant") ; q->_next = p->_next ; } TEVENT (Unlink from cxq) ; } // Diagnostic hygiene ... SelfNode->_prev = (ObjectWaiter *) 0xBAD ; SelfNode->_next = (ObjectWaiter *) 0xBAD ; SelfNode->TState = ObjectWaiter::TS_RUN ; } // ----------------------------------------------------------------------------- // Exit support // // exit() // ~~~~~~ // Note that the collector can't reclaim the objectMonitor or deflate // the object out from underneath the thread calling ::exit() as the // thread calling ::exit() never transitions to a stable state. // This inhibits GC, which in turn inhibits asynchronous (and // inopportune) reclamation of "this". // // We'd like to assert that: (THREAD->thread_state() != _thread_blocked) ; // There's one exception to the claim above, however. EnterI() can call // exit() to drop a lock if the acquirer has been externally suspended. // In that case exit() is called with _thread_state as _thread_blocked, // but the monitor's _count field is > 0, which inhibits reclamation. // // 1-0 exit // ~~~~~~~~ // ::exit() uses a canonical 1-1 idiom with a MEMBAR although some of // the fast-path operators have been optimized so the common ::exit() // operation is 1-0. See i486.ad fast_unlock(), for instance. // The code emitted by fast_unlock() elides the usual MEMBAR. This // greatly improves latency -- MEMBAR and CAS having considerable local // latency on modern processors -- but at the cost of "stranding". Absent the // MEMBAR, a thread in fast_unlock() can race a thread in the slow // ::enter() path, resulting in the entering thread being stranding // and a progress-liveness failure. Stranding is extremely rare. // We use timers (timed park operations) & periodic polling to detect // and recover from stranding. Potentially stranded threads periodically // wake up and poll the lock. See the usage of the _Responsible variable. // // The CAS() in enter provides for safety and exclusion, while the CAS or // MEMBAR in exit provides for progress and avoids stranding. 1-0 locking // eliminates the CAS/MEMBAR from the exist path, but it admits stranding. // We detect and recover from stranding with timers. // // If a thread transiently strands it'll park until (a) another // thread acquires the lock and then drops the lock, at which time the // exiting thread will notice and unpark the stranded thread, or, (b) // the timer expires. If the lock is high traffic then the stranding latency // will be low due to (a). If the lock is low traffic then the odds of // stranding are lower, although the worst-case stranding latency // is longer. Critically, we don't want to put excessive load in the // platform's timer subsystem. We want to minimize both the timer injection // rate (timers created/sec) as well as the number of timers active at // any one time. (more precisely, we want to minimize timer-seconds, which is // the integral of the # of active timers at any instant over time). // Both impinge on OS scalability. Given that, at most one thread parked on // a monitor will use a timer. void ATTR ObjectMonitor::exit(bool not_suspended, TRAPS) { Thread * Self = THREAD ; if (THREAD != _owner) { if (THREAD->is_lock_owned((address) _owner)) { // Transmute _owner from a BasicLock pointer to a Thread address. // We don't need to hold _mutex for this transition. // Non-null to Non-null is safe as long as all readers can // tolerate either flavor. assert (_recursions == 0, "invariant") ; _owner = THREAD ; _recursions = 0 ; OwnerIsThread = 1 ; } else { // NOTE: we need to handle unbalanced monitor enter/exit // in native code by throwing an exception. // TODO: Throw an IllegalMonitorStateException ? TEVENT (Exit - Throw IMSX) ; assert(false, "Non-balanced monitor enter/exit!"); if (false) { THROW(vmSymbols::java_lang_IllegalMonitorStateException()); } return; } } if (_recursions != 0) { _recursions--; // this is simple recursive enter TEVENT (Inflated exit - recursive) ; return ; } // Invariant: after setting Responsible=null an thread must execute // a MEMBAR or other serializing instruction before fetching EntryList|cxq. if ((SyncFlags & 4) == 0) { _Responsible = NULL ; } #if INCLUDE_TRACE // get the owner's thread id for the MonitorEnter event // if it is enabled and the thread isn't suspended if (not_suspended && Tracing::is_event_enabled(TraceJavaMonitorEnterEvent)) { _previous_owner_tid = SharedRuntime::get_java_tid(Self); } #endif for (;;) { assert (THREAD == _owner, "invariant") ; if (Knob_ExitPolicy == 0) { // release semantics: prior loads and stores from within the critical section // must not float (reorder) past the following store that drops the lock. // On SPARC that requires MEMBAR #loadstore|#storestore. // But of course in TSO #loadstore|#storestore is not required. // I'd like to write one of the following: // A. OrderAccess::release() ; _owner = NULL // B. OrderAccess::loadstore(); OrderAccess::storestore(); _owner = NULL; // Unfortunately OrderAccess::release() and OrderAccess::loadstore() both // store into a _dummy variable. That store is not needed, but can result // in massive wasteful coherency traffic on classic SMP systems. // Instead, I use release_store(), which is implemented as just a simple // ST on x64, x86 and SPARC. OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock OrderAccess::storeload() ; // See if we need to wake a successor if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) { TEVENT (Inflated exit - simple egress) ; return ; } TEVENT (Inflated exit - complex egress) ; // Normally the exiting thread is responsible for ensuring succession, // but if other successors are ready or other entering threads are spinning // then this thread can simply store NULL into _owner and exit without // waking a successor. The existence of spinners or ready successors // guarantees proper succession (liveness). Responsibility passes to the // ready or running successors. The exiting thread delegates the duty. // More precisely, if a successor already exists this thread is absolved // of the responsibility of waking (unparking) one. // // The _succ variable is critical to reducing futile wakeup frequency. // _succ identifies the "heir presumptive" thread that has been made // ready (unparked) but that has not yet run. We need only one such // successor thread to guarantee progress. // See http://www.usenix.org/events/jvm01/full_papers/dice/dice.pdf // section 3.3 "Futile Wakeup Throttling" for details. // // Note that spinners in Enter() also set _succ non-null. // In the current implementation spinners opportunistically set // _succ so that exiting threads might avoid waking a successor. // Another less appealing alternative would be for the exiting thread // to drop the lock and then spin briefly to see if a spinner managed // to acquire the lock. If so, the exiting thread could exit // immediately without waking a successor, otherwise the exiting // thread would need to dequeue and wake a successor. // (Note that we'd need to make the post-drop spin short, but no // shorter than the worst-case round-trip cache-line migration time. // The dropped lock needs to become visible to the spinner, and then // the acquisition of the lock by the spinner must become visible to // the exiting thread). // // It appears that an heir-presumptive (successor) must be made ready. // Only the current lock owner can manipulate the EntryList or // drain _cxq, so we need to reacquire the lock. If we fail // to reacquire the lock the responsibility for ensuring succession // falls to the new owner. // if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) { return ; } TEVENT (Exit - Reacquired) ; } else { if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) { OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock OrderAccess::storeload() ; // Ratify the previously observed values. if (_cxq == NULL || _succ != NULL) { TEVENT (Inflated exit - simple egress) ; return ; } // inopportune interleaving -- the exiting thread (this thread) // in the fast-exit path raced an entering thread in the slow-enter // path. // We have two choices: // A. Try to reacquire the lock. // If the CAS() fails return immediately, otherwise // we either restart/rerun the exit operation, or simply // fall-through into the code below which wakes a successor. // B. If the elements forming the EntryList|cxq are TSM // we could simply unpark() the lead thread and return // without having set _succ. if (Atomic::cmpxchg_ptr (THREAD, &_owner, NULL) != NULL) { TEVENT (Inflated exit - reacquired succeeded) ; return ; } TEVENT (Inflated exit - reacquired failed) ; } else { TEVENT (Inflated exit - complex egress) ; } } guarantee (_owner == THREAD, "invariant") ; ObjectWaiter * w = NULL ; int QMode = Knob_QMode ; if (QMode == 2 && _cxq != NULL) { // QMode == 2 : cxq has precedence over EntryList. // Try to directly wake a successor from the cxq. // If successful, the successor will need to unlink itself from cxq. w = _cxq ; assert (w != NULL, "invariant") ; assert (w->TState == ObjectWaiter::TS_CXQ, "Invariant") ; ExitEpilog (Self, w) ; return ; } if (QMode == 3 && _cxq != NULL) { // Aggressively drain cxq into EntryList at the first opportunity. // This policy ensure that recently-run threads live at the head of EntryList. // Drain _cxq into EntryList - bulk transfer. // First, detach _cxq. // The following loop is tantamount to: w = swap (&cxq, NULL) w = _cxq ; for (;;) { assert (w != NULL, "Invariant") ; ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ; if (u == w) break ; w = u ; } assert (w != NULL , "invariant") ; ObjectWaiter * q = NULL ; ObjectWaiter * p ; for (p = w ; p != NULL ; p = p->_next) { guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ; p->TState = ObjectWaiter::TS_ENTER ; p->_prev = q ; q = p ; } // Append the RATs to the EntryList // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time. ObjectWaiter * Tail ; for (Tail = _EntryList ; Tail != NULL && Tail->_next != NULL ; Tail = Tail->_next) ; if (Tail == NULL) { _EntryList = w ; } else { Tail->_next = w ; w->_prev = Tail ; } // Fall thru into code that tries to wake a successor from EntryList } if (QMode == 4 && _cxq != NULL) { // Aggressively drain cxq into EntryList at the first opportunity. // This policy ensure that recently-run threads live at the head of EntryList. // Drain _cxq into EntryList - bulk transfer. // First, detach _cxq. // The following loop is tantamount to: w = swap (&cxq, NULL) w = _cxq ; for (;;) { assert (w != NULL, "Invariant") ; ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ; if (u == w) break ; w = u ; } assert (w != NULL , "invariant") ; ObjectWaiter * q = NULL ; ObjectWaiter * p ; for (p = w ; p != NULL ; p = p->_next) { guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ; p->TState = ObjectWaiter::TS_ENTER ; p->_prev = q ; q = p ; } // Prepend the RATs to the EntryList if (_EntryList != NULL) { q->_next = _EntryList ; _EntryList->_prev = q ; } _EntryList = w ; // Fall thru into code that tries to wake a successor from EntryList } w = _EntryList ; if (w != NULL) { // I'd like to write: guarantee (w->_thread != Self). // But in practice an exiting thread may find itself on the EntryList. // Lets say thread T1 calls O.wait(). Wait() enqueues T1 on O's waitset and // then calls exit(). Exit release the lock by setting O._owner to NULL. // Lets say T1 then stalls. T2 acquires O and calls O.notify(). The // notify() operation moves T1 from O's waitset to O's EntryList. T2 then // release the lock "O". T2 resumes immediately after the ST of null into // _owner, above. T2 notices that the EntryList is populated, so it // reacquires the lock and then finds itself on the EntryList. // Given all that, we have to tolerate the circumstance where "w" is // associated with Self. assert (w->TState == ObjectWaiter::TS_ENTER, "invariant") ; ExitEpilog (Self, w) ; return ; } // If we find that both _cxq and EntryList are null then just // re-run the exit protocol from the top. w = _cxq ; if (w == NULL) continue ; // Drain _cxq into EntryList - bulk transfer. // First, detach _cxq. // The following loop is tantamount to: w = swap (&cxq, NULL) for (;;) { assert (w != NULL, "Invariant") ; ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ; if (u == w) break ; w = u ; } TEVENT (Inflated exit - drain cxq into EntryList) ; assert (w != NULL , "invariant") ; assert (_EntryList == NULL , "invariant") ; // Convert the LIFO SLL anchored by _cxq into a DLL. // The list reorganization step operates in O(LENGTH(w)) time. // It's critical that this step operate quickly as // "Self" still holds the outer-lock, restricting parallelism // and effectively lengthening the critical section. // Invariant: s chases t chases u. // TODO-FIXME: consider changing EntryList from a DLL to a CDLL so // we have faster access to the tail. if (QMode == 1) { // QMode == 1 : drain cxq to EntryList, reversing order // We also reverse the order of the list. ObjectWaiter * s = NULL ; ObjectWaiter * t = w ; ObjectWaiter * u = NULL ; while (t != NULL) { guarantee (t->TState == ObjectWaiter::TS_CXQ, "invariant") ; t->TState = ObjectWaiter::TS_ENTER ; u = t->_next ; t->_prev = u ; t->_next = s ; s = t; t = u ; } _EntryList = s ; assert (s != NULL, "invariant") ; } else { // QMode == 0 or QMode == 2 _EntryList = w ; ObjectWaiter * q = NULL ; ObjectWaiter * p ; for (p = w ; p != NULL ; p = p->_next) { guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ; p->TState = ObjectWaiter::TS_ENTER ; p->_prev = q ; q = p ; } } // In 1-0 mode we need: ST EntryList; MEMBAR #storestore; ST _owner = NULL // The MEMBAR is satisfied by the release_store() operation in ExitEpilog(). // See if we can abdicate to a spinner instead of waking a thread. // A primary goal of the implementation is to reduce the // context-switch rate. if (_succ != NULL) continue; w = _EntryList ; if (w != NULL) { guarantee (w->TState == ObjectWaiter::TS_ENTER, "invariant") ; ExitEpilog (Self, w) ; return ; } } } // ExitSuspendEquivalent: // A faster alternate to handle_special_suspend_equivalent_condition() // // handle_special_suspend_equivalent_condition() unconditionally // acquires the SR_lock. On some platforms uncontended MutexLocker() // operations have high latency. Note that in ::enter() we call HSSEC // while holding the monitor, so we effectively lengthen the critical sections. // // There are a number of possible solutions: // // A. To ameliorate the problem we might also defer state transitions // to as late as possible -- just prior to parking. // Given that, we'd call HSSEC after having returned from park(), // but before attempting to acquire the monitor. This is only a // partial solution. It avoids calling HSSEC while holding the // monitor (good), but it still increases successor reacquisition latency -- // the interval between unparking a successor and the time the successor // resumes and retries the lock. See ReenterI(), which defers state transitions. // If we use this technique we can also avoid EnterI()-exit() loop // in ::enter() where we iteratively drop the lock and then attempt // to reacquire it after suspending. // // B. In the future we might fold all the suspend bits into a // composite per-thread suspend flag and then update it with CAS(). // Alternately, a Dekker-like mechanism with multiple variables // would suffice: // ST Self->_suspend_equivalent = false // MEMBAR // LD Self_>_suspend_flags // bool ObjectMonitor::ExitSuspendEquivalent (JavaThread * jSelf) { int Mode = Knob_FastHSSEC ; if (Mode && !jSelf->is_external_suspend()) { assert (jSelf->is_suspend_equivalent(), "invariant") ; jSelf->clear_suspend_equivalent() ; if (2 == Mode) OrderAccess::storeload() ; if (!jSelf->is_external_suspend()) return false ; // We raced a suspension -- fall thru into the slow path TEVENT (ExitSuspendEquivalent - raced) ; jSelf->set_suspend_equivalent() ; } return jSelf->handle_special_suspend_equivalent_condition() ; } void ObjectMonitor::ExitEpilog (Thread * Self, ObjectWaiter * Wakee) { assert (_owner == Self, "invariant") ; // Exit protocol: // 1. ST _succ = wakee // 2. membar #loadstore|#storestore; // 2. ST _owner = NULL // 3. unpark(wakee) _succ = Knob_SuccEnabled ? Wakee->_thread : NULL ; ParkEvent * Trigger = Wakee->_event ; // Hygiene -- once we've set _owner = NULL we can't safely dereference Wakee again. // The thread associated with Wakee may have grabbed the lock and "Wakee" may be // out-of-scope (non-extant). Wakee = NULL ; // Drop the lock OrderAccess::release_store_ptr (&_owner, NULL) ; OrderAccess::fence() ; // ST _owner vs LD in unpark() if (SafepointSynchronize::do_call_back()) { TEVENT (unpark before SAFEPOINT) ; } DTRACE_MONITOR_PROBE(contended__exit, this, object(), Self); Trigger->unpark() ; // Maintain stats and report events to JVMTI if (ObjectMonitor::_sync_Parks != NULL) { ObjectMonitor::_sync_Parks->inc() ; } } // ----------------------------------------------------------------------------- // Class Loader deadlock handling. // // complete_exit exits a lock returning recursion count // complete_exit/reenter operate as a wait without waiting // complete_exit requires an inflated monitor // The _owner field is not always the Thread addr even with an // inflated monitor, e.g. the monitor can be inflated by a non-owning // thread due to contention. intptr_t ObjectMonitor::complete_exit(TRAPS) { Thread * const Self = THREAD; assert(Self->is_Java_thread(), "Must be Java thread!"); JavaThread *jt = (JavaThread *)THREAD; DeferredInitialize(); if (THREAD != _owner) { if (THREAD->is_lock_owned ((address)_owner)) { assert(_recursions == 0, "internal state error"); _owner = THREAD ; /* Convert from basiclock addr to Thread addr */ _recursions = 0 ; OwnerIsThread = 1 ; } } guarantee(Self == _owner, "complete_exit not owner"); intptr_t save = _recursions; // record the old recursion count _recursions = 0; // set the recursion level to be 0 exit (true, Self) ; // exit the monitor guarantee (_owner != Self, "invariant"); return save; } // reenter() enters a lock and sets recursion count // complete_exit/reenter operate as a wait without waiting void ObjectMonitor::reenter(intptr_t recursions, TRAPS) { Thread * const Self = THREAD; assert(Self->is_Java_thread(), "Must be Java thread!"); JavaThread *jt = (JavaThread *)THREAD; guarantee(_owner != Self, "reenter already owner"); enter (THREAD); // enter the monitor guarantee (_recursions == 0, "reenter recursion"); _recursions = recursions; return; } // ----------------------------------------------------------------------------- // A macro is used below because there may already be a pending // exception which should not abort the execution of the routines // which use this (which is why we don't put this into check_slow and // call it with a CHECK argument). #define CHECK_OWNER() \ do { \ if (THREAD != _owner) { \ if (THREAD->is_lock_owned((address) _owner)) { \ _owner = THREAD ; /* Convert from basiclock addr to Thread addr */ \ _recursions = 0; \ OwnerIsThread = 1 ; \ } else { \ TEVENT (Throw IMSX) ; \ THROW(vmSymbols::java_lang_IllegalMonitorStateException()); \ } \ } \ } while (false) // check_slow() is a misnomer. It's called to simply to throw an IMSX exception. // TODO-FIXME: remove check_slow() -- it's likely dead. void ObjectMonitor::check_slow(TRAPS) { TEVENT (check_slow - throw IMSX) ; assert(THREAD != _owner && !THREAD->is_lock_owned((address) _owner), "must not be owner"); THROW_MSG(vmSymbols::java_lang_IllegalMonitorStateException(), "current thread not owner"); } static int Adjust (volatile int * adr, int dx) { int v ; for (v = *adr ; Atomic::cmpxchg (v + dx, adr, v) != v; v = *adr) ; return v ; } // helper method for posting a monitor wait event void ObjectMonitor::post_monitor_wait_event(EventJavaMonitorWait* event, jlong notifier_tid, jlong timeout, bool timedout) { event->set_klass(((oop)this->object())->klass()); event->set_timeout((TYPE_ULONG)timeout); event->set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr())); event->set_notifier((TYPE_OSTHREAD)notifier_tid); event->set_timedOut((TYPE_BOOLEAN)timedout); event->commit(); } // ----------------------------------------------------------------------------- // Wait/Notify/NotifyAll // // Note: a subset of changes to ObjectMonitor::wait() // will need to be replicated in complete_exit above void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) { Thread * const Self = THREAD ; assert(Self->is_Java_thread(), "Must be Java thread!"); JavaThread *jt = (JavaThread *)THREAD; DeferredInitialize () ; // Throw IMSX or IEX. CHECK_OWNER(); EventJavaMonitorWait event; // check for a pending interrupt if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) { // post monitor waited event. Note that this is past-tense, we are done waiting. if (JvmtiExport::should_post_monitor_waited()) { // Note: 'false' parameter is passed here because the // wait was not timed out due to thread interrupt. JvmtiExport::post_monitor_waited(jt, this, false); // In this short circuit of the monitor wait protocol, the // current thread never drops ownership of the monitor and // never gets added to the wait queue so the current thread // cannot be made the successor. This means that the // JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally // consume an unpark() meant for the ParkEvent associated with // this ObjectMonitor. } if (event.should_commit()) { post_monitor_wait_event(&event, 0, millis, false); } TEVENT (Wait - Throw IEX) ; THROW(vmSymbols::java_lang_InterruptedException()); return ; } TEVENT (Wait) ; assert (Self->_Stalled == 0, "invariant") ; Self->_Stalled = intptr_t(this) ; jt->set_current_waiting_monitor(this); // create a node to be put into the queue // Critically, after we reset() the event but prior to park(), we must check // for a pending interrupt. ObjectWaiter node(Self); node.TState = ObjectWaiter::TS_WAIT ; Self->_ParkEvent->reset() ; OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag // Enter the waiting queue, which is a circular doubly linked list in this case // but it could be a priority queue or any data structure. // _WaitSetLock protects the wait queue. Normally the wait queue is accessed only // by the the owner of the monitor *except* in the case where park() // returns because of a timeout of interrupt. Contention is exceptionally rare // so we use a simple spin-lock instead of a heavier-weight blocking lock. Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ; AddWaiter (&node) ; Thread::SpinRelease (&_WaitSetLock) ; if ((SyncFlags & 4) == 0) { _Responsible = NULL ; } intptr_t save = _recursions; // record the old recursion count _waiters++; // increment the number of waiters _recursions = 0; // set the recursion level to be 1 exit (true, Self) ; // exit the monitor guarantee (_owner != Self, "invariant") ; // The thread is on the WaitSet list - now park() it. // On MP systems it's conceivable that a brief spin before we park // could be profitable. // // TODO-FIXME: change the following logic to a loop of the form // while (!timeout && !interrupted && _notified == 0) park() int ret = OS_OK ; int WasNotified = 0 ; { // State transition wrappers OSThread* osthread = Self->osthread(); OSThreadWaitState osts(osthread, true); { ThreadBlockInVM tbivm(jt); // Thread is in thread_blocked state and oop access is unsafe. jt->set_suspend_equivalent(); if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) { // Intentionally empty } else if (node._notified == 0) { if (millis <= 0) { Self->_ParkEvent->park () ; } else { ret = Self->_ParkEvent->park (millis) ; } } // were we externally suspended while we were waiting? if (ExitSuspendEquivalent (jt)) { // TODO-FIXME: add -- if succ == Self then succ = null. jt->java_suspend_self(); } } // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm // Node may be on the WaitSet, the EntryList (or cxq), or in transition // from the WaitSet to the EntryList. // See if we need to remove Node from the WaitSet. // We use double-checked locking to avoid grabbing _WaitSetLock // if the thread is not on the wait queue. // // Note that we don't need a fence before the fetch of TState. // In the worst case we'll fetch a old-stale value of TS_WAIT previously // written by the is thread. (perhaps the fetch might even be satisfied // by a look-aside into the processor's own store buffer, although given // the length of the code path between the prior ST and this load that's // highly unlikely). If the following LD fetches a stale TS_WAIT value // then we'll acquire the lock and then re-fetch a fresh TState value. // That is, we fail toward safety. if (node.TState == ObjectWaiter::TS_WAIT) { Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ; if (node.TState == ObjectWaiter::TS_WAIT) { DequeueSpecificWaiter (&node) ; // unlink from WaitSet assert(node._notified == 0, "invariant"); node.TState = ObjectWaiter::TS_RUN ; } Thread::SpinRelease (&_WaitSetLock) ; } // The thread is now either on off-list (TS_RUN), // on the EntryList (TS_ENTER), or on the cxq (TS_CXQ). // The Node's TState variable is stable from the perspective of this thread. // No other threads will asynchronously modify TState. guarantee (node.TState != ObjectWaiter::TS_WAIT, "invariant") ; OrderAccess::loadload() ; if (_succ == Self) _succ = NULL ; WasNotified = node._notified ; // Reentry phase -- reacquire the monitor. // re-enter contended monitor after object.wait(). // retain OBJECT_WAIT state until re-enter successfully completes // Thread state is thread_in_vm and oop access is again safe, // although the raw address of the object may have changed. // (Don't cache naked oops over safepoints, of course). // post monitor waited event. Note that this is past-tense, we are done waiting. if (JvmtiExport::should_post_monitor_waited()) { JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT); if (node._notified != 0 && _succ == Self) { // In this part of the monitor wait-notify-reenter protocol it // is possible (and normal) for another thread to do a fastpath // monitor enter-exit while this thread is still trying to get // to the reenter portion of the protocol. // // The ObjectMonitor was notified and the current thread is // the successor which also means that an unpark() has already // been done. The JVMTI_EVENT_MONITOR_WAITED event handler can // consume the unpark() that was done when the successor was // set because the same ParkEvent is shared between Java // monitors and JVM/TI RawMonitors (for now). // // We redo the unpark() to ensure forward progress, i.e., we // don't want all pending threads hanging (parked) with none // entering the unlocked monitor. node._event->unpark(); } } if (event.should_commit()) { post_monitor_wait_event(&event, node._notifier_tid, millis, ret == OS_TIMEOUT); } OrderAccess::fence() ; assert (Self->_Stalled != 0, "invariant") ; Self->_Stalled = 0 ; assert (_owner != Self, "invariant") ; ObjectWaiter::TStates v = node.TState ; if (v == ObjectWaiter::TS_RUN) { enter (Self) ; } else { guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ; ReenterI (Self, &node) ; node.wait_reenter_end(this); } // Self has reacquired the lock. // Lifecycle - the node representing Self must not appear on any queues. // Node is about to go out-of-scope, but even if it were immortal we wouldn't // want residual elements associated with this thread left on any lists. guarantee (node.TState == ObjectWaiter::TS_RUN, "invariant") ; assert (_owner == Self, "invariant") ; assert (_succ != Self , "invariant") ; } // OSThreadWaitState() jt->set_current_waiting_monitor(NULL); guarantee (_recursions == 0, "invariant") ; _recursions = save; // restore the old recursion count _waiters--; // decrement the number of waiters // Verify a few postconditions assert (_owner == Self , "invariant") ; assert (_succ != Self , "invariant") ; assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ; if (SyncFlags & 32) { OrderAccess::fence() ; } // check if the notification happened if (!WasNotified) { // no, it could be timeout or Thread.interrupt() or both // check for interrupt event, otherwise it is timeout if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) { TEVENT (Wait - throw IEX from epilog) ; THROW(vmSymbols::java_lang_InterruptedException()); } } // NOTE: Spurious wake up will be consider as timeout. // Monitor notify has precedence over thread interrupt. } // Consider: // If the lock is cool (cxq == null && succ == null) and we're on an MP system // then instead of transferring a thread from the WaitSet to the EntryList // we might just dequeue a thread from the WaitSet and directly unpark() it. void ObjectMonitor::notify(TRAPS) { CHECK_OWNER(); if (_WaitSet == NULL) { TEVENT (Empty-Notify) ; return ; } DTRACE_MONITOR_PROBE(notify, this, object(), THREAD); int Policy = Knob_MoveNotifyee ; Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ; ObjectWaiter * iterator = DequeueWaiter() ; if (iterator != NULL) { TEVENT (Notify1 - Transfer) ; guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ; guarantee (iterator->_notified == 0, "invariant") ; if (Policy != 4) { iterator->TState = ObjectWaiter::TS_ENTER ; } iterator->_notified = 1 ; Thread * Self = THREAD; iterator->_notifier_tid = Self->osthread()->thread_id(); ObjectWaiter * List = _EntryList ; if (List != NULL) { assert (List->_prev == NULL, "invariant") ; assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ; assert (List != iterator, "invariant") ; } if (Policy == 0) { // prepend to EntryList if (List == NULL) { iterator->_next = iterator->_prev = NULL ; _EntryList = iterator ; } else { List->_prev = iterator ; iterator->_next = List ; iterator->_prev = NULL ; _EntryList = iterator ; } } else if (Policy == 1) { // append to EntryList if (List == NULL) { iterator->_next = iterator->_prev = NULL ; _EntryList = iterator ; } else { // CONSIDER: finding the tail currently requires a linear-time walk of // the EntryList. We can make tail access constant-time by converting to // a CDLL instead of using our current DLL. ObjectWaiter * Tail ; for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ; assert (Tail != NULL && Tail->_next == NULL, "invariant") ; Tail->_next = iterator ; iterator->_prev = Tail ; iterator->_next = NULL ; } } else if (Policy == 2) { // prepend to cxq // prepend to cxq if (List == NULL) { iterator->_next = iterator->_prev = NULL ; _EntryList = iterator ; } else { iterator->TState = ObjectWaiter::TS_CXQ ; for (;;) { ObjectWaiter * Front = _cxq ; iterator->_next = Front ; if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) { break ; } } } } else if (Policy == 3) { // append to cxq iterator->TState = ObjectWaiter::TS_CXQ ; for (;;) { ObjectWaiter * Tail ; Tail = _cxq ; if (Tail == NULL) { iterator->_next = NULL ; if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) { break ; } } else { while (Tail->_next != NULL) Tail = Tail->_next ; Tail->_next = iterator ; iterator->_prev = Tail ; iterator->_next = NULL ; break ; } } } else { ParkEvent * ev = iterator->_event ; iterator->TState = ObjectWaiter::TS_RUN ; OrderAccess::fence() ; ev->unpark() ; } if (Policy < 4) { iterator->wait_reenter_begin(this); } // _WaitSetLock protects the wait queue, not the EntryList. We could // move the add-to-EntryList operation, above, outside the critical section // protected by _WaitSetLock. In practice that's not useful. With the // exception of wait() timeouts and interrupts the monitor owner // is the only thread that grabs _WaitSetLock. There's almost no contention // on _WaitSetLock so it's not profitable to reduce the length of the // critical section. } Thread::SpinRelease (&_WaitSetLock) ; if (iterator != NULL && ObjectMonitor::_sync_Notifications != NULL) { ObjectMonitor::_sync_Notifications->inc() ; } } void ObjectMonitor::notifyAll(TRAPS) { CHECK_OWNER(); ObjectWaiter* iterator; if (_WaitSet == NULL) { TEVENT (Empty-NotifyAll) ; return ; } DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD); int Policy = Knob_MoveNotifyee ; int Tally = 0 ; Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notifyall") ; for (;;) { iterator = DequeueWaiter () ; if (iterator == NULL) break ; TEVENT (NotifyAll - Transfer1) ; ++Tally ; // Disposition - what might we do with iterator ? // a. add it directly to the EntryList - either tail or head. // b. push it onto the front of the _cxq. // For now we use (a). guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ; guarantee (iterator->_notified == 0, "invariant") ; iterator->_notified = 1 ; Thread * Self = THREAD; iterator->_notifier_tid = Self->osthread()->thread_id(); if (Policy != 4) { iterator->TState = ObjectWaiter::TS_ENTER ; } ObjectWaiter * List = _EntryList ; if (List != NULL) { assert (List->_prev == NULL, "invariant") ; assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ; assert (List != iterator, "invariant") ; } if (Policy == 0) { // prepend to EntryList if (List == NULL) { iterator->_next = iterator->_prev = NULL ; _EntryList = iterator ; } else { List->_prev = iterator ; iterator->_next = List ; iterator->_prev = NULL ; _EntryList = iterator ; } } else if (Policy == 1) { // append to EntryList if (List == NULL) { iterator->_next = iterator->_prev = NULL ; _EntryList = iterator ; } else { // CONSIDER: finding the tail currently requires a linear-time walk of // the EntryList. We can make tail access constant-time by converting to // a CDLL instead of using our current DLL. ObjectWaiter * Tail ; for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ; assert (Tail != NULL && Tail->_next == NULL, "invariant") ; Tail->_next = iterator ; iterator->_prev = Tail ; iterator->_next = NULL ; } } else if (Policy == 2) { // prepend to cxq // prepend to cxq iterator->TState = ObjectWaiter::TS_CXQ ; for (;;) { ObjectWaiter * Front = _cxq ; iterator->_next = Front ; if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) { break ; } } } else if (Policy == 3) { // append to cxq iterator->TState = ObjectWaiter::TS_CXQ ; for (;;) { ObjectWaiter * Tail ; Tail = _cxq ; if (Tail == NULL) { iterator->_next = NULL ; if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) { break ; } } else { while (Tail->_next != NULL) Tail = Tail->_next ; Tail->_next = iterator ; iterator->_prev = Tail ; iterator->_next = NULL ; break ; } } } else { ParkEvent * ev = iterator->_event ; iterator->TState = ObjectWaiter::TS_RUN ; OrderAccess::fence() ; ev->unpark() ; } if (Policy < 4) { iterator->wait_reenter_begin(this); } // _WaitSetLock protects the wait queue, not the EntryList. We could // move the add-to-EntryList operation, above, outside the critical section // protected by _WaitSetLock. In practice that's not useful. With the // exception of wait() timeouts and interrupts the monitor owner // is the only thread that grabs _WaitSetLock. There's almost no contention // on _WaitSetLock so it's not profitable to reduce the length of the // critical section. } Thread::SpinRelease (&_WaitSetLock) ; if (Tally != 0 && ObjectMonitor::_sync_Notifications != NULL) { ObjectMonitor::_sync_Notifications->inc(Tally) ; } } // ----------------------------------------------------------------------------- // Adaptive Spinning Support // // Adaptive spin-then-block - rational spinning // // Note that we spin "globally" on _owner with a classic SMP-polite TATAS // algorithm. On high order SMP systems it would be better to start with // a brief global spin and then revert to spinning locally. In the spirit of MCS/CLH, // a contending thread could enqueue itself on the cxq and then spin locally // on a thread-specific variable such as its ParkEvent._Event flag. // That's left as an exercise for the reader. Note that global spinning is // not problematic on Niagara, as the L2$ serves the interconnect and has both // low latency and massive bandwidth. // // Broadly, we can fix the spin frequency -- that is, the % of contended lock // acquisition attempts where we opt to spin -- at 100% and vary the spin count // (duration) or we can fix the count at approximately the duration of // a context switch and vary the frequency. Of course we could also // vary both satisfying K == Frequency * Duration, where K is adaptive by monitor. // See http://j2se.east/~dice/PERSIST/040824-AdaptiveSpinning.html. // // This implementation varies the duration "D", where D varies with // the success rate of recent spin attempts. (D is capped at approximately // length of a round-trip context switch). The success rate for recent // spin attempts is a good predictor of the success rate of future spin // attempts. The mechanism adapts automatically to varying critical // section length (lock modality), system load and degree of parallelism. // D is maintained per-monitor in _SpinDuration and is initialized // optimistically. Spin frequency is fixed at 100%. // // Note that _SpinDuration is volatile, but we update it without locks // or atomics. The code is designed so that _SpinDuration stays within // a reasonable range even in the presence of races. The arithmetic // operations on _SpinDuration are closed over the domain of legal values, // so at worst a race will install and older but still legal value. // At the very worst this introduces some apparent non-determinism. // We might spin when we shouldn't or vice-versa, but since the spin // count are relatively short, even in the worst case, the effect is harmless. // // Care must be taken that a low "D" value does not become an // an absorbing state. Transient spinning failures -- when spinning // is overall profitable -- should not cause the system to converge // on low "D" values. We want spinning to be stable and predictable // and fairly responsive to change and at the same time we don't want // it to oscillate, become metastable, be "too" non-deterministic, // or converge on or enter undesirable stable absorbing states. // // We implement a feedback-based control system -- using past behavior // to predict future behavior. We face two issues: (a) if the // input signal is random then the spin predictor won't provide optimal // results, and (b) if the signal frequency is too high then the control // system, which has some natural response lag, will "chase" the signal. // (b) can arise from multimodal lock hold times. Transient preemption // can also result in apparent bimodal lock hold times. // Although sub-optimal, neither condition is particularly harmful, as // in the worst-case we'll spin when we shouldn't or vice-versa. // The maximum spin duration is rather short so the failure modes aren't bad. // To be conservative, I've tuned the gain in system to bias toward // _not spinning. Relatedly, the system can sometimes enter a mode where it // "rings" or oscillates between spinning and not spinning. This happens // when spinning is just on the cusp of profitability, however, so the // situation is not dire. The state is benign -- there's no need to add // hysteresis control to damp the transition rate between spinning and // not spinning. // intptr_t ObjectMonitor::SpinCallbackArgument = 0 ; int (*ObjectMonitor::SpinCallbackFunction)(intptr_t, int) = NULL ; // Spinning: Fixed frequency (100%), vary duration int ObjectMonitor::TrySpin_VaryDuration (Thread * Self) { // Dumb, brutal spin. Good for comparative measurements against adaptive spinning. int ctr = Knob_FixedSpin ; if (ctr != 0) { while (--ctr >= 0) { if (TryLock (Self) > 0) return 1 ; SpinPause () ; } return 0 ; } for (ctr = Knob_PreSpin + 1; --ctr >= 0 ; ) { if (TryLock(Self) > 0) { // Increase _SpinDuration ... // Note that we don't clamp SpinDuration precisely at SpinLimit. // Raising _SpurDuration to the poverty line is key. int x = _SpinDuration ; if (x < Knob_SpinLimit) { if (x < Knob_Poverty) x = Knob_Poverty ; _SpinDuration = x + Knob_BonusB ; } return 1 ; } SpinPause () ; } // Admission control - verify preconditions for spinning // // We always spin a little bit, just to prevent _SpinDuration == 0 from // becoming an absorbing state. Put another way, we spin briefly to // sample, just in case the system load, parallelism, contention, or lock // modality changed. // // Consider the following alternative: // Periodically set _SpinDuration = _SpinLimit and try a long/full // spin attempt. "Periodically" might mean after a tally of // the # of failed spin attempts (or iterations) reaches some threshold. // This takes us into the realm of 1-out-of-N spinning, where we // hold the duration constant but vary the frequency. ctr = _SpinDuration ; if (ctr < Knob_SpinBase) ctr = Knob_SpinBase ; if (ctr <= 0) return 0 ; if (Knob_SuccRestrict && _succ != NULL) return 0 ; if (Knob_OState && NotRunnable (Self, (Thread *) _owner)) { TEVENT (Spin abort - notrunnable [TOP]); return 0 ; } int MaxSpin = Knob_MaxSpinners ; if (MaxSpin >= 0) { if (_Spinner > MaxSpin) { TEVENT (Spin abort -- too many spinners) ; return 0 ; } // Slighty racy, but benign ... Adjust (&_Spinner, 1) ; } // We're good to spin ... spin ingress. // CONSIDER: use Prefetch::write() to avoid RTS->RTO upgrades // when preparing to LD...CAS _owner, etc and the CAS is likely // to succeed. int hits = 0 ; int msk = 0 ; int caspty = Knob_CASPenalty ; int oxpty = Knob_OXPenalty ; int sss = Knob_SpinSetSucc ; if (sss && _succ == NULL ) _succ = Self ; Thread * prv = NULL ; // There are three ways to exit the following loop: // 1. A successful spin where this thread has acquired the lock. // 2. Spin failure with prejudice // 3. Spin failure without prejudice while (--ctr >= 0) { // Periodic polling -- Check for pending GC // Threads may spin while they're unsafe. // We don't want spinning threads to delay the JVM from reaching // a stop-the-world safepoint or to steal cycles from GC. // If we detect a pending safepoint we abort in order that // (a) this thread, if unsafe, doesn't delay the safepoint, and (b) // this thread, if safe, doesn't steal cycles from GC. // This is in keeping with the "no loitering in runtime" rule. // We periodically check to see if there's a safepoint pending. if ((ctr & 0xFF) == 0) { if (SafepointSynchronize::do_call_back()) { TEVENT (Spin: safepoint) ; goto Abort ; // abrupt spin egress } if (Knob_UsePause & 1) SpinPause () ; int (*scb)(intptr_t,int) = SpinCallbackFunction ; if (hits > 50 && scb != NULL) { int abend = (*scb)(SpinCallbackArgument, 0) ; } } if (Knob_UsePause & 2) SpinPause() ; // Exponential back-off ... Stay off the bus to reduce coherency traffic. // This is useful on classic SMP systems, but is of less utility on // N1-style CMT platforms. // // Trade-off: lock acquisition latency vs coherency bandwidth. // Lock hold times are typically short. A histogram // of successful spin attempts shows that we usually acquire // the lock early in the spin. That suggests we want to // sample _owner frequently in the early phase of the spin, // but then back-off and sample less frequently as the spin // progresses. The back-off makes a good citizen on SMP big // SMP systems. Oversampling _owner can consume excessive // coherency bandwidth. Relatedly, if we _oversample _owner we // can inadvertently interfere with the the ST m->owner=null. // executed by the lock owner. if (ctr & msk) continue ; ++hits ; if ((hits & 0xF) == 0) { // The 0xF, above, corresponds to the exponent. // Consider: (msk+1)|msk msk = ((msk << 2)|3) & BackOffMask ; } // Probe _owner with TATAS // If this thread observes the monitor transition or flicker // from locked to unlocked to locked, then the odds that this // thread will acquire the lock in this spin attempt go down // considerably. The same argument applies if the CAS fails // or if we observe _owner change from one non-null value to // another non-null value. In such cases we might abort // the spin without prejudice or apply a "penalty" to the // spin count-down variable "ctr", reducing it by 100, say. Thread * ox = (Thread *) _owner ; if (ox == NULL) { ox = (Thread *) Atomic::cmpxchg_ptr (Self, &_owner, NULL) ; if (ox == NULL) { // The CAS succeeded -- this thread acquired ownership // Take care of some bookkeeping to exit spin state. if (sss && _succ == Self) { _succ = NULL ; } if (MaxSpin > 0) Adjust (&_Spinner, -1) ; // Increase _SpinDuration : // The spin was successful (profitable) so we tend toward // longer spin attempts in the future. // CONSIDER: factor "ctr" into the _SpinDuration adjustment. // If we acquired the lock early in the spin cycle it // makes sense to increase _SpinDuration proportionally. // Note that we don't clamp SpinDuration precisely at SpinLimit. int x = _SpinDuration ; if (x < Knob_SpinLimit) { if (x < Knob_Poverty) x = Knob_Poverty ; _SpinDuration = x + Knob_Bonus ; } return 1 ; } // The CAS failed ... we can take any of the following actions: // * penalize: ctr -= Knob_CASPenalty // * exit spin with prejudice -- goto Abort; // * exit spin without prejudice. // * Since CAS is high-latency, retry again immediately. prv = ox ; TEVENT (Spin: cas failed) ; if (caspty == -2) break ; if (caspty == -1) goto Abort ; ctr -= caspty ; continue ; } // Did lock ownership change hands ? if (ox != prv && prv != NULL ) { TEVENT (spin: Owner changed) if (oxpty == -2) break ; if (oxpty == -1) goto Abort ; ctr -= oxpty ; } prv = ox ; // Abort the spin if the owner is not executing. // The owner must be executing in order to drop the lock. // Spinning while the owner is OFFPROC is idiocy. // Consider: ctr -= RunnablePenalty ; if (Knob_OState && NotRunnable (Self, ox)) { TEVENT (Spin abort - notrunnable); goto Abort ; } if (sss && _succ == NULL ) _succ = Self ; } // Spin failed with prejudice -- reduce _SpinDuration. // TODO: Use an AIMD-like policy to adjust _SpinDuration. // AIMD is globally stable. TEVENT (Spin failure) ; { int x = _SpinDuration ; if (x > 0) { // Consider an AIMD scheme like: x -= (x >> 3) + 100 // This is globally sample and tends to damp the response. x -= Knob_Penalty ; if (x < 0) x = 0 ; _SpinDuration = x ; } } Abort: if (MaxSpin >= 0) Adjust (&_Spinner, -1) ; if (sss && _succ == Self) { _succ = NULL ; // Invariant: after setting succ=null a contending thread // must recheck-retry _owner before parking. This usually happens // in the normal usage of TrySpin(), but it's safest // to make TrySpin() as foolproof as possible. OrderAccess::fence() ; if (TryLock(Self) > 0) return 1 ; } return 0 ; } // NotRunnable() -- informed spinning // // Don't bother spinning if the owner is not eligible to drop the lock. // Peek at the owner's schedctl.sc_state and Thread._thread_values and // spin only if the owner thread is _thread_in_Java or _thread_in_vm. // The thread must be runnable in order to drop the lock in timely fashion. // If the _owner is not runnable then spinning will not likely be // successful (profitable). // // Beware -- the thread referenced by _owner could have died // so a simply fetch from _owner->_thread_state might trap. // Instead, we use SafeFetchXX() to safely LD _owner->_thread_state. // Because of the lifecycle issues the schedctl and _thread_state values // observed by NotRunnable() might be garbage. NotRunnable must // tolerate this and consider the observed _thread_state value // as advisory. // // Beware too, that _owner is sometimes a BasicLock address and sometimes // a thread pointer. We differentiate the two cases with OwnerIsThread. // Alternately, we might tag the type (thread pointer vs basiclock pointer) // with the LSB of _owner. Another option would be to probablistically probe // the putative _owner->TypeTag value. // // Checking _thread_state isn't perfect. Even if the thread is // in_java it might be blocked on a page-fault or have been preempted // and sitting on a ready/dispatch queue. _thread state in conjunction // with schedctl.sc_state gives us a good picture of what the // thread is doing, however. // // TODO: check schedctl.sc_state. // We'll need to use SafeFetch32() to read from the schedctl block. // See RFE #5004247 and http://sac.sfbay.sun.com/Archives/CaseLog/arc/PSARC/2005/351/ // // The return value from NotRunnable() is *advisory* -- the // result is based on sampling and is not necessarily coherent. // The caller must tolerate false-negative and false-positive errors. // Spinning, in general, is probabilistic anyway. int ObjectMonitor::NotRunnable (Thread * Self, Thread * ox) { // Check either OwnerIsThread or ox->TypeTag == 2BAD. if (!OwnerIsThread) return 0 ; if (ox == NULL) return 0 ; // Avoid transitive spinning ... // Say T1 spins or blocks trying to acquire L. T1._Stalled is set to L. // Immediately after T1 acquires L it's possible that T2, also // spinning on L, will see L.Owner=T1 and T1._Stalled=L. // This occurs transiently after T1 acquired L but before // T1 managed to clear T1.Stalled. T2 does not need to abort // its spin in this circumstance. intptr_t BlockedOn = SafeFetchN ((intptr_t *) &ox->_Stalled, intptr_t(1)) ; if (BlockedOn == 1) return 1 ; if (BlockedOn != 0) { return BlockedOn != intptr_t(this) && _owner == ox ; } assert (sizeof(((JavaThread *)ox)->_thread_state == sizeof(int)), "invariant") ; int jst = SafeFetch32 ((int *) &((JavaThread *) ox)->_thread_state, -1) ; ; // consider also: jst != _thread_in_Java -- but that's overspecific. return jst == _thread_blocked || jst == _thread_in_native ; } // ----------------------------------------------------------------------------- // WaitSet management ... ObjectWaiter::ObjectWaiter(Thread* thread) { _next = NULL; _prev = NULL; _notified = 0; TState = TS_RUN ; _thread = thread; _event = thread->_ParkEvent ; _active = false; assert (_event != NULL, "invariant") ; } void ObjectWaiter::wait_reenter_begin(ObjectMonitor *mon) { JavaThread *jt = (JavaThread *)this->_thread; _active = JavaThreadBlockedOnMonitorEnterState::wait_reenter_begin(jt, mon); } void ObjectWaiter::wait_reenter_end(ObjectMonitor *mon) { JavaThread *jt = (JavaThread *)this->_thread; JavaThreadBlockedOnMonitorEnterState::wait_reenter_end(jt, _active); } inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) { assert(node != NULL, "should not dequeue NULL node"); assert(node->_prev == NULL, "node already in list"); assert(node->_next == NULL, "node already in list"); // put node at end of queue (circular doubly linked list) if (_WaitSet == NULL) { _WaitSet = node; node->_prev = node; node->_next = node; } else { ObjectWaiter* head = _WaitSet ; ObjectWaiter* tail = head->_prev; assert(tail->_next == head, "invariant check"); tail->_next = node; head->_prev = node; node->_next = head; node->_prev = tail; } } inline ObjectWaiter* ObjectMonitor::DequeueWaiter() { // dequeue the very first waiter ObjectWaiter* waiter = _WaitSet; if (waiter) { DequeueSpecificWaiter(waiter); } return waiter; } inline void ObjectMonitor::DequeueSpecificWaiter(ObjectWaiter* node) { assert(node != NULL, "should not dequeue NULL node"); assert(node->_prev != NULL, "node already removed from list"); assert(node->_next != NULL, "node already removed from list"); // when the waiter has woken up because of interrupt, // timeout or other spurious wake-up, dequeue the // waiter from waiting list ObjectWaiter* next = node->_next; if (next == node) { assert(node->_prev == node, "invariant check"); _WaitSet = NULL; } else { ObjectWaiter* prev = node->_prev; assert(prev->_next == node, "invariant check"); assert(next->_prev == node, "invariant check"); next->_prev = prev; prev->_next = next; if (_WaitSet == node) { _WaitSet = next; } } node->_next = NULL; node->_prev = NULL; } // ----------------------------------------------------------------------------- // PerfData support PerfCounter * ObjectMonitor::_sync_ContendedLockAttempts = NULL ; PerfCounter * ObjectMonitor::_sync_FutileWakeups = NULL ; PerfCounter * ObjectMonitor::_sync_Parks = NULL ; PerfCounter * ObjectMonitor::_sync_EmptyNotifications = NULL ; PerfCounter * ObjectMonitor::_sync_Notifications = NULL ; PerfCounter * ObjectMonitor::_sync_PrivateA = NULL ; PerfCounter * ObjectMonitor::_sync_PrivateB = NULL ; PerfCounter * ObjectMonitor::_sync_SlowExit = NULL ; PerfCounter * ObjectMonitor::_sync_SlowEnter = NULL ; PerfCounter * ObjectMonitor::_sync_SlowNotify = NULL ; PerfCounter * ObjectMonitor::_sync_SlowNotifyAll = NULL ; PerfCounter * ObjectMonitor::_sync_FailedSpins = NULL ; PerfCounter * ObjectMonitor::_sync_SuccessfulSpins = NULL ; PerfCounter * ObjectMonitor::_sync_MonInCirculation = NULL ; PerfCounter * ObjectMonitor::_sync_MonScavenged = NULL ; PerfCounter * ObjectMonitor::_sync_Inflations = NULL ; PerfCounter * ObjectMonitor::_sync_Deflations = NULL ; PerfLongVariable * ObjectMonitor::_sync_MonExtant = NULL ; // One-shot global initialization for the sync subsystem. // We could also defer initialization and initialize on-demand // the first time we call inflate(). Initialization would // be protected - like so many things - by the MonitorCache_lock. void ObjectMonitor::Initialize () { static int InitializationCompleted = 0 ; assert (InitializationCompleted == 0, "invariant") ; InitializationCompleted = 1 ; if (UsePerfData) { EXCEPTION_MARK ; #define NEWPERFCOUNTER(n) {n = PerfDataManager::create_counter(SUN_RT, #n, PerfData::U_Events,CHECK); } #define NEWPERFVARIABLE(n) {n = PerfDataManager::create_variable(SUN_RT, #n, PerfData::U_Events,CHECK); } NEWPERFCOUNTER(_sync_Inflations) ; NEWPERFCOUNTER(_sync_Deflations) ; NEWPERFCOUNTER(_sync_ContendedLockAttempts) ; NEWPERFCOUNTER(_sync_FutileWakeups) ; NEWPERFCOUNTER(_sync_Parks) ; NEWPERFCOUNTER(_sync_EmptyNotifications) ; NEWPERFCOUNTER(_sync_Notifications) ; NEWPERFCOUNTER(_sync_SlowEnter) ; NEWPERFCOUNTER(_sync_SlowExit) ; NEWPERFCOUNTER(_sync_SlowNotify) ; NEWPERFCOUNTER(_sync_SlowNotifyAll) ; NEWPERFCOUNTER(_sync_FailedSpins) ; NEWPERFCOUNTER(_sync_SuccessfulSpins) ; NEWPERFCOUNTER(_sync_PrivateA) ; NEWPERFCOUNTER(_sync_PrivateB) ; NEWPERFCOUNTER(_sync_MonInCirculation) ; NEWPERFCOUNTER(_sync_MonScavenged) ; NEWPERFVARIABLE(_sync_MonExtant) ; #undef NEWPERFCOUNTER } } // Compile-time asserts // When possible, it's better to catch errors deterministically at // compile-time than at runtime. The down-side to using compile-time // asserts is that error message -- often something about negative array // indices -- is opaque. #define CTASSERT(x) { int tag[1-(2*!(x))]; printf ("Tag @" INTPTR_FORMAT "\n", (intptr_t)tag); } void ObjectMonitor::ctAsserts() { CTASSERT(offset_of (ObjectMonitor, _header) == 0); } static char * kvGet (char * kvList, const char * Key) { if (kvList == NULL) return NULL ; size_t n = strlen (Key) ; char * Search ; for (Search = kvList ; *Search ; Search += strlen(Search) + 1) { if (strncmp (Search, Key, n) == 0) { if (Search[n] == '=') return Search + n + 1 ; if (Search[n] == 0) return (char *) "1" ; } } return NULL ; } static int kvGetInt (char * kvList, const char * Key, int Default) { char * v = kvGet (kvList, Key) ; int rslt = v ? ::strtol (v, NULL, 0) : Default ; if (Knob_ReportSettings && v != NULL) { ::printf (" SyncKnob: %s %d(%d)\n", Key, rslt, Default) ; ::fflush (stdout) ; } return rslt ; } void ObjectMonitor::DeferredInitialize () { if (InitDone > 0) return ; if (Atomic::cmpxchg (-1, &InitDone, 0) != 0) { while (InitDone != 1) ; return ; } // One-shot global initialization ... // The initialization is idempotent, so we don't need locks. // In the future consider doing this via os::init_2(). // SyncKnobs consist of = pairs in the style // of environment variables. Start by converting ':' to NUL. if (SyncKnobs == NULL) SyncKnobs = "" ; size_t sz = strlen (SyncKnobs) ; char * knobs = (char *) malloc (sz + 2) ; if (knobs == NULL) { vm_exit_out_of_memory (sz + 2, OOM_MALLOC_ERROR, "Parse SyncKnobs") ; guarantee (0, "invariant") ; } strcpy (knobs, SyncKnobs) ; knobs[sz+1] = 0 ; for (char * p = knobs ; *p ; p++) { if (*p == ':') *p = 0 ; } #define SETKNOB(x) { Knob_##x = kvGetInt (knobs, #x, Knob_##x); } SETKNOB(ReportSettings) ; SETKNOB(Verbose) ; SETKNOB(FixedSpin) ; SETKNOB(SpinLimit) ; SETKNOB(SpinBase) ; SETKNOB(SpinBackOff); SETKNOB(CASPenalty) ; SETKNOB(OXPenalty) ; SETKNOB(LogSpins) ; SETKNOB(SpinSetSucc) ; SETKNOB(SuccEnabled) ; SETKNOB(SuccRestrict) ; SETKNOB(Penalty) ; SETKNOB(Bonus) ; SETKNOB(BonusB) ; SETKNOB(Poverty) ; SETKNOB(SpinAfterFutile) ; SETKNOB(UsePause) ; SETKNOB(SpinEarly) ; SETKNOB(OState) ; SETKNOB(MaxSpinners) ; SETKNOB(PreSpin) ; SETKNOB(ExitPolicy) ; SETKNOB(QMode); SETKNOB(ResetEvent) ; SETKNOB(MoveNotifyee) ; SETKNOB(FastHSSEC) ; #undef SETKNOB if (os::is_MP()) { BackOffMask = (1 << Knob_SpinBackOff) - 1 ; if (Knob_ReportSettings) ::printf ("BackOffMask=%X\n", BackOffMask) ; // CONSIDER: BackOffMask = ROUNDUP_NEXT_POWER2 (ncpus-1) } else { Knob_SpinLimit = 0 ; Knob_SpinBase = 0 ; Knob_PreSpin = 0 ; Knob_FixedSpin = -1 ; } if (Knob_LogSpins == 0) { ObjectMonitor::_sync_FailedSpins = NULL ; } free (knobs) ; OrderAccess::fence() ; InitDone = 1 ; } #ifndef PRODUCT void ObjectMonitor::verify() { } void ObjectMonitor::print() { } #endif