--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/hotspot/src/share/vm/runtime/objectMonitor.cpp Fri Oct 22 15:59:34 2010 -0400
@@ -0,0 +1,2421 @@
+/*
+ * Copyright (c) 1998, 2009, 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 "incls/_precompiled.incl"
+# include "incls/_objectMonitor.cpp.incl"
+
+#if defined(__GNUC__) && !defined(IA64)
+ // 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.
+
+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_PROBE_COMMON(klassOop, thread) \
+ char* bytes = NULL; \
+ int len = 0; \
+ jlong jtid = SharedRuntime::get_java_tid(thread); \
+ symbolOop klassname = ((oop)(klassOop))->klass()->klass_part()->name(); \
+ if (klassname != NULL) { \
+ bytes = (char*)klassname->bytes(); \
+ len = klassname->utf8_length(); \
+ }
+
+#define DTRACE_MONITOR_WAIT_PROBE(monitor, klassOop, thread, millis) \
+ { \
+ if (DTraceMonitorProbes) { \
+ DTRACE_MONITOR_PROBE_COMMON(klassOop, thread); \
+ HS_DTRACE_PROBE5(hotspot, monitor__wait, jtid, \
+ (monitor), bytes, len, (millis)); \
+ } \
+ }
+
+#define DTRACE_MONITOR_PROBE(probe, monitor, klassOop, thread) \
+ { \
+ if (DTraceMonitorProbes) { \
+ DTRACE_MONITOR_PROBE_COMMON(klassOop, thread); \
+ HS_DTRACE_PROBE4(hotspot, monitor__##probe, jtid, \
+ (uintptr_t)(monitor), bytes, len); \
+ } \
+ }
+
+#else // ndef DTRACE_ENABLED
+
+#define DTRACE_MONITOR_WAIT_PROBE(klassOop, thread, millis, mon) {;}
+#define DTRACE_MONITOR_PROBE(probe, klassOop, 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);
+
+ { // 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);
+ }
+
+ 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 (Self) ;
+
+ jt->java_suspend_self();
+ }
+ Self->set_current_pending_monitor(NULL);
+ }
+
+ 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);
+ }
+ 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 ;
+ // Dekker pivot-point.
+ // Consider OrderAccess::storeload() here
+
+ // 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.
+ //
+ // In *practice* however, EnterI() is always followed by some atomic
+ // operation such as the decrement of _count in ::enter(). Those atomics
+ // obviate the need for the explicit MEMBAR, above.
+ }
+
+ // 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(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 ;
+ }
+
+ 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 (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 ;
+}
+// -----------------------------------------------------------------------------
+// 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();
+
+ // 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);
+ }
+ 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 (Self) ; // exit the monitor
+ guarantee (_owner != Self, "invariant") ;
+
+ // As soon as the ObjectMonitor's ownership is dropped in the exit()
+ // call above, another thread can enter() the ObjectMonitor, do the
+ // notify(), and exit() the ObjectMonitor. If the other thread's
+ // exit() call chooses this thread as the successor and the unpark()
+ // call happens to occur while this thread is posting a
+ // MONITOR_CONTENDED_EXIT event, then we run the risk of the event
+ // handler using RawMonitors and consuming the unpark().
+ //
+ // To avoid the problem, we re-post the event. This does no harm
+ // even if the original unpark() was not consumed because we are the
+ // chosen successor for this monitor.
+ if (node._notified != 0 && _succ == Self) {
+ node._event->unpark();
+ }
+
+ // 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);
+ }
+ 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 ;
+
+ 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 ;
+ 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 <Key>=<Value> 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, "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