Java example source code file (objectMonitor.cpp)
The objectMonitor.cpp Java example source code
/*
* Copyright (c) 1998, 2013, 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)
// 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);
}
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);
}
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 (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);
}
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") ;
// 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);
}
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 <Key>=
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