bilibili-Java併發學習筆記4 openjdk源碼
基於 java 1.8.0
P16_通過openjdk源碼分析ObjectMonitor底層實現
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openjdk
Mercurial
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http://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/d2c2cd90513e/src/share/vm/runtime/objectMonitor.hpp
class ObjectMonitor {
public:
enum {
OM_OK, // no error
OM_SYSTEM_ERROR, // operating system error
OM_ILLEGAL_MONITOR_STATE, // IllegalMonitorStateException
OM_INTERRUPTED, // Thread.interrupt()
OM_TIMED_OUT // Object.wait() timed out
};
public:
// TODO-FIXME: the "offset" routines should return a type of off_t instead of int ...
// ByteSize would also be an appropriate type.
static int header_offset_in_bytes() { return offset_of(ObjectMonitor, _header); }
static int object_offset_in_bytes() { return offset_of(ObjectMonitor, _object); }
static int owner_offset_in_bytes() { return offset_of(ObjectMonitor, _owner); }
static int count_offset_in_bytes() { return offset_of(ObjectMonitor, _count); }
static int recursions_offset_in_bytes() { return offset_of(ObjectMonitor, _recursions); }
static int cxq_offset_in_bytes() { return offset_of(ObjectMonitor, _cxq) ; }
static int succ_offset_in_bytes() { return offset_of(ObjectMonitor, _succ) ; }
static int EntryList_offset_in_bytes() { return offset_of(ObjectMonitor, _EntryList); }
static int FreeNext_offset_in_bytes() { return offset_of(ObjectMonitor, FreeNext); }
static int WaitSet_offset_in_bytes() { return offset_of(ObjectMonitor, _WaitSet) ; }
static int Responsible_offset_in_bytes() { return offset_of(ObjectMonitor, _Responsible);}
static int Spinner_offset_in_bytes() { return offset_of(ObjectMonitor, _Spinner); }
public:
// Eventaully we'll make provisions for multiple callbacks, but
// now one will suffice.
static int (*SpinCallbackFunction)(intptr_t, int) ;
static intptr_t SpinCallbackArgument ;
public:
markOop header() const;
void set_header(markOop hdr);
intptr_t is_busy() const {
// TODO-FIXME: merge _count and _waiters.
// TODO-FIXME: assert _owner == null implies _recursions = 0
// TODO-FIXME: assert _WaitSet != null implies _count > 0
return _count|_waiters|intptr_t(_owner)|intptr_t(_cxq)|intptr_t(_EntryList ) ;
}
intptr_t is_entered(Thread* current) const;
void* owner() const;
void set_owner(void* owner);
intptr_t waiters() const;
intptr_t count() const;
void set_count(intptr_t count);
intptr_t contentions() const ;
intptr_t recursions() const { return _recursions; }
// JVM/DI GetMonitorInfo() needs this
ObjectWaiter* first_waiter() { return _WaitSet; }
ObjectWaiter* next_waiter(ObjectWaiter* o) { return o->_next; }
Thread* thread_of_waiter(ObjectWaiter* o) { return o->_thread; }
// initialize the monitor, exception the semaphore, all other fields
// are simple integers or pointers
ObjectMonitor() {
_header = NULL;
_count = 0;
_waiters = 0,
_recursions = 0;
_object = NULL;
_owner = NULL;
_WaitSet = NULL;
_WaitSetLock = 0 ;
_Responsible = NULL ;
_succ = NULL ;
_cxq = NULL ;
FreeNext = NULL ;
_EntryList = NULL ;
_SpinFreq = 0 ;
_SpinClock = 0 ;
OwnerIsThread = 0 ;
_previous_owner_tid = 0;
}
~ObjectMonitor() {
// TODO: Add asserts ...
// _cxq == 0 _succ == NULL _owner == NULL _waiters == 0
// _count == 0 _EntryList == NULL etc
}
private:
void Recycle () {
// TODO: add stronger asserts ...
// _cxq == 0 _succ == NULL _owner == NULL _waiters == 0
// _count == 0 EntryList == NULL
// _recursions == 0 _WaitSet == NULL
// TODO: assert (is_busy()|_recursions) == 0
_succ = NULL ;
_EntryList = NULL ;
_cxq = NULL ;
_WaitSet = NULL ;
_recursions = 0 ;
_SpinFreq = 0 ;
_SpinClock = 0 ;
OwnerIsThread = 0 ;
}
public:
void* object() const;
void* object_addr();
void set_object(void* obj);
bool check(TRAPS); // true if the thread owns the monitor.
void check_slow(TRAPS);
void clear();
#ifndef PRODUCT
void verify();
void print();
#endif
bool try_enter (TRAPS) ;
void enter(TRAPS);
void exit(bool not_suspended, TRAPS);
void wait(jlong millis, bool interruptable, TRAPS);
void notify(TRAPS);
void notifyAll(TRAPS);
// Use the following at your own risk
intptr_t complete_exit(TRAPS);
void reenter(intptr_t recursions, TRAPS);
private:
void AddWaiter (ObjectWaiter * waiter) ;
static void DeferredInitialize();
ObjectWaiter * DequeueWaiter () ;
void DequeueSpecificWaiter (ObjectWaiter * waiter) ;
void EnterI (TRAPS) ;
void ReenterI (Thread * Self, ObjectWaiter * SelfNode) ;
void UnlinkAfterAcquire (Thread * Self, ObjectWaiter * SelfNode) ;
int TryLock (Thread * Self) ;
int NotRunnable (Thread * Self, Thread * Owner) ;
int TrySpin_Fixed (Thread * Self) ;
int TrySpin_VaryFrequency (Thread * Self) ;
int TrySpin_VaryDuration (Thread * Self) ;
void ctAsserts () ;
void ExitEpilog (Thread * Self, ObjectWaiter * Wakee) ;
bool ExitSuspendEquivalent (JavaThread * Self) ;
void post_monitor_wait_event(EventJavaMonitorWait * event,
jlong notifier_tid,
jlong timeout,
bool timedout);
private:
friend class ObjectSynchronizer;
friend class ObjectWaiter;
friend class VMStructs;
// WARNING: this must be the very first word of ObjectMonitor
// This means this class can't use any virtual member functions.
// TODO-FIXME: assert that offsetof(_header) is 0 or get rid of the
// implicit 0 offset in emitted code.
volatile markOop _header; // displaced object header word - mark
void* volatile _object; // backward object pointer - strong root
double SharingPad [1] ; // temp to reduce false sharing
// All the following fields must be machine word aligned
// The VM assumes write ordering wrt these fields, which can be
// read from other threads.
protected: // protected for jvmtiRawMonitor
void * volatile _owner; // pointer to owning thread OR BasicLock
volatile jlong _previous_owner_tid; // thread id of the previous owner of the monitor
volatile intptr_t _recursions; // recursion count, 0 for first entry
private:
int OwnerIsThread ; // _owner is (Thread *) vs SP/BasicLock
ObjectWaiter * volatile _cxq ; // LL of recently-arrived threads blocked on entry.
// The list is actually composed of WaitNodes, acting
// as proxies for Threads.
protected:
ObjectWaiter * volatile _EntryList ; // Threads blocked on entry or reentry.
private:
Thread * volatile _succ ; // Heir presumptive thread - used for futile wakeup throttling
Thread * volatile _Responsible ;
int _PromptDrain ; // rqst to drain cxq into EntryList ASAP
volatile int _Spinner ; // for exit->spinner handoff optimization
volatile int _SpinFreq ; // Spin 1-out-of-N attempts: success rate
volatile int _SpinClock ;
volatile int _SpinDuration ;
volatile intptr_t _SpinState ; // MCS/CLH list of spinners
// TODO-FIXME: _count, _waiters and _recursions should be of
// type int, or int32_t but not intptr_t. There's no reason
// to use 64-bit fields for these variables on a 64-bit JVM.
volatile intptr_t _count; // reference count to prevent reclaimation/deflation
// at stop-the-world time. See deflate_idle_monitors().
// _count is approximately |_WaitSet| + |_EntryList|
protected:
volatile intptr_t _waiters; // number of waiting threads
private:
protected:
ObjectWaiter * volatile _WaitSet; // LL of threads wait()ing on the monitor
private:
volatile int _WaitSetLock; // protects Wait Queue - simple spinlock
public:
int _QMix ; // Mixed prepend queue discipline
ObjectMonitor * FreeNext ; // Free list linkage
intptr_t StatA, StatsB ;
public:
static void Initialize () ;
static PerfCounter * _sync_ContendedLockAttempts ;
static PerfCounter * _sync_FutileWakeups ;
static PerfCounter * _sync_Parks ;
static PerfCounter * _sync_EmptyNotifications ;
static PerfCounter * _sync_Notifications ;
static PerfCounter * _sync_SlowEnter ;
static PerfCounter * _sync_SlowExit ;
static PerfCounter * _sync_SlowNotify ;
static PerfCounter * _sync_SlowNotifyAll ;
static PerfCounter * _sync_FailedSpins ;
static PerfCounter * _sync_SuccessfulSpins ;
static PerfCounter * _sync_PrivateA ;
static PerfCounter * _sync_PrivateB ;
static PerfCounter * _sync_MonInCirculation ;
static PerfCounter * _sync_MonScavenged ;
static PerfCounter * _sync_Inflations ;
static PerfCounter * _sync_Deflations ;
static PerfLongVariable * _sync_MonExtant ;
public:
static int Knob_Verbose;
static int Knob_SpinLimit;
void* operator new (size_t size) throw() {
return AllocateHeap(size, mtInternal);
}
void* operator new[] (size_t size) throw() {
return operator new (size);
}
void operator delete(void* p) {
FreeHeap(p, mtInternal);
}
void operator delete[] (void *p) {
operator delete(p);
}
};
P17_透過openjdk源碼分析wait與notify方法的本地實現
http://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/d2c2cd90513e/src/share/vm/runtime/objectMonitor.cpp
// -----------------------------------------------------------------------------
// Wait/Notify/NotifyAll
//
// Note: a subset of changes to ObjectMonitor::wait()
// will need to be replicated in complete_exit above
void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS) {
Thread * const Self = THREAD ;
assert(Self->is_Java_thread(), "Must be Java thread!");
JavaThread *jt = (JavaThread *)THREAD;
DeferredInitialize () ;
// Throw IMSX or IEX.
CHECK_OWNER();
EventJavaMonitorWait event;
// check for a pending interrupt
if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
// post monitor waited event. Note that this is past-tense, we are done waiting.
if (JvmtiExport::should_post_monitor_waited()) {
// Note: 'false' parameter is passed here because the
// wait was not timed out due to thread interrupt.
JvmtiExport::post_monitor_waited(jt, this, false);
// In this short circuit of the monitor wait protocol, the
// current thread never drops ownership of the monitor and
// never gets added to the wait queue so the current thread
// cannot be made the successor. This means that the
// JVMTI_EVENT_MONITOR_WAITED event handler cannot accidentally
// consume an unpark() meant for the ParkEvent associated with
// this ObjectMonitor.
}
if (event.should_commit()) {
post_monitor_wait_event(&event, 0, millis, false);
}
TEVENT (Wait - Throw IEX) ;
THROW(vmSymbols::java_lang_InterruptedException());
return ;
}
TEVENT (Wait) ;
assert (Self->_Stalled == 0, "invariant") ;
Self->_Stalled = intptr_t(this) ;
jt->set_current_waiting_monitor(this);
// create a node to be put into the queue
// Critically, after we reset() the event but prior to park(), we must check
// for a pending interrupt.
ObjectWaiter node(Self);
node.TState = ObjectWaiter::TS_WAIT ;
Self->_ParkEvent->reset() ;
OrderAccess::fence(); // ST into Event; membar ; LD interrupted-flag
// Enter the waiting queue, which is a circular doubly linked list in this case
// but it could be a priority queue or any data structure.
// _WaitSetLock protects the wait queue. Normally the wait queue is accessed only
// by the the owner of the monitor *except* in the case where park()
// returns because of a timeout of interrupt. Contention is exceptionally rare
// so we use a simple spin-lock instead of a heavier-weight blocking lock.
Thread::SpinAcquire (&_WaitSetLock, "WaitSet - add") ;
AddWaiter (&node) ;
Thread::SpinRelease (&_WaitSetLock) ;
if ((SyncFlags & 4) == 0) {
_Responsible = NULL ;
}
intptr_t save = _recursions; // record the old recursion count
_waiters++; // increment the number of waiters
_recursions = 0; // set the recursion level to be 1
exit (true, Self) ; // exit the monitor
guarantee (_owner != Self, "invariant") ;
// The thread is on the WaitSet list - now park() it.
// On MP systems it's conceivable that a brief spin before we park
// could be profitable.
//
// TODO-FIXME: change the following logic to a loop of the form
// while (!timeout && !interrupted && _notified == 0) park()
int ret = OS_OK ;
int WasNotified = 0 ;
{ // State transition wrappers
OSThread* osthread = Self->osthread();
OSThreadWaitState osts(osthread, true);
{
ThreadBlockInVM tbivm(jt);
// Thread is in thread_blocked state and oop access is unsafe.
jt->set_suspend_equivalent();
if (interruptible && (Thread::is_interrupted(THREAD, false) || HAS_PENDING_EXCEPTION)) {
// Intentionally empty
} else
if (node._notified == 0) {
if (millis <= 0) {
Self->_ParkEvent->park () ;
} else {
ret = Self->_ParkEvent->park (millis) ;
}
}
// were we externally suspended while we were waiting?
if (ExitSuspendEquivalent (jt)) {
// TODO-FIXME: add -- if succ == Self then succ = null.
jt->java_suspend_self();
}
} // Exit thread safepoint: transition _thread_blocked -> _thread_in_vm
// Node may be on the WaitSet, the EntryList (or cxq), or in transition
// from the WaitSet to the EntryList.
// See if we need to remove Node from the WaitSet.
// We use double-checked locking to avoid grabbing _WaitSetLock
// if the thread is not on the wait queue.
//
// Note that we don't need a fence before the fetch of TState.
// In the worst case we'll fetch a old-stale value of TS_WAIT previously
// written by the is thread. (perhaps the fetch might even be satisfied
// by a look-aside into the processor's own store buffer, although given
// the length of the code path between the prior ST and this load that's
// highly unlikely). If the following LD fetches a stale TS_WAIT value
// then we'll acquire the lock and then re-fetch a fresh TState value.
// That is, we fail toward safety.
if (node.TState == ObjectWaiter::TS_WAIT) {
Thread::SpinAcquire (&_WaitSetLock, "WaitSet - unlink") ;
if (node.TState == ObjectWaiter::TS_WAIT) {
DequeueSpecificWaiter (&node) ; // unlink from WaitSet
assert(node._notified == 0, "invariant");
node.TState = ObjectWaiter::TS_RUN ;
}
Thread::SpinRelease (&_WaitSetLock) ;
}
// The thread is now either on off-list (TS_RUN),
// on the EntryList (TS_ENTER), or on the cxq (TS_CXQ).
// The Node's TState variable is stable from the perspective of this thread.
// No other threads will asynchronously modify TState.
guarantee (node.TState != ObjectWaiter::TS_WAIT, "invariant") ;
OrderAccess::loadload() ;
if (_succ == Self) _succ = NULL ;
WasNotified = node._notified ;
// Reentry phase -- reacquire the monitor.
// re-enter contended monitor after object.wait().
// retain OBJECT_WAIT state until re-enter successfully completes
// Thread state is thread_in_vm and oop access is again safe,
// although the raw address of the object may have changed.
// (Don't cache naked oops over safepoints, of course).
// post monitor waited event. Note that this is past-tense, we are done waiting.
if (JvmtiExport::should_post_monitor_waited()) {
JvmtiExport::post_monitor_waited(jt, this, ret == OS_TIMEOUT);
if (node._notified != 0 && _succ == Self) {
// In this part of the monitor wait-notify-reenter protocol it
// is possible (and normal) for another thread to do a fastpath
// monitor enter-exit while this thread is still trying to get
// to the reenter portion of the protocol.
//
// The ObjectMonitor was notified and the current thread is
// the successor which also means that an unpark() has already
// been done. The JVMTI_EVENT_MONITOR_WAITED event handler can
// consume the unpark() that was done when the successor was
// set because the same ParkEvent is shared between Java
// monitors and JVM/TI RawMonitors (for now).
//
// We redo the unpark() to ensure forward progress, i.e., we
// don't want all pending threads hanging (parked) with none
// entering the unlocked monitor.
node._event->unpark();
}
}
if (event.should_commit()) {
post_monitor_wait_event(&event, node._notifier_tid, millis, ret == OS_TIMEOUT);
}
OrderAccess::fence() ;
assert (Self->_Stalled != 0, "invariant") ;
Self->_Stalled = 0 ;
assert (_owner != Self, "invariant") ;
ObjectWaiter::TStates v = node.TState ;
if (v == ObjectWaiter::TS_RUN) {
enter (Self) ;
} else {
guarantee (v == ObjectWaiter::TS_ENTER || v == ObjectWaiter::TS_CXQ, "invariant") ;
ReenterI (Self, &node) ;
node.wait_reenter_end(this);
}
// Self has reacquired the lock.
// Lifecycle - the node representing Self must not appear on any queues.
// Node is about to go out-of-scope, but even if it were immortal we wouldn't
// want residual elements associated with this thread left on any lists.
guarantee (node.TState == ObjectWaiter::TS_RUN, "invariant") ;
assert (_owner == Self, "invariant") ;
assert (_succ != Self , "invariant") ;
} // OSThreadWaitState()
jt->set_current_waiting_monitor(NULL);
guarantee (_recursions == 0, "invariant") ;
_recursions = save; // restore the old recursion count
_waiters--; // decrement the number of waiters
// Verify a few postconditions
assert (_owner == Self , "invariant") ;
assert (_succ != Self , "invariant") ;
assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
if (SyncFlags & 32) {
OrderAccess::fence() ;
}
// check if the notification happened
if (!WasNotified) {
// no, it could be timeout or Thread.interrupt() or both
// check for interrupt event, otherwise it is timeout
if (interruptible && Thread::is_interrupted(Self, true) && !HAS_PENDING_EXCEPTION) {
TEVENT (Wait - throw IEX from epilog) ;
THROW(vmSymbols::java_lang_InterruptedException());
}
}
// NOTE: Spurious wake up will be consider as timeout.
// Monitor notify has precedence over thread interrupt.
}
// Consider:
// If the lock is cool (cxq == null && succ == null) and we're on an MP system
// then instead of transferring a thread from the WaitSet to the EntryList
// we might just dequeue a thread from the WaitSet and directly unpark() it.
void ObjectMonitor::notify(TRAPS) {
CHECK_OWNER();
if (_WaitSet == NULL) {
TEVENT (Empty-Notify) ;
return ;
}
DTRACE_MONITOR_PROBE(notify, this, object(), THREAD);
int Policy = Knob_MoveNotifyee ;
Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ;
ObjectWaiter * iterator = DequeueWaiter() ;
if (iterator != NULL) {
TEVENT (Notify1 - Transfer) ;
guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
guarantee (iterator->_notified == 0, "invariant") ;
if (Policy != 4) {
iterator->TState = ObjectWaiter::TS_ENTER ;
}
iterator->_notified = 1 ;
Thread * Self = THREAD;
iterator->_notifier_tid = Self->osthread()->thread_id();
ObjectWaiter * List = _EntryList ;
if (List != NULL) {
assert (List->_prev == NULL, "invariant") ;
assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
assert (List != iterator, "invariant") ;
}
if (Policy == 0) { // prepend to EntryList
if (List == NULL) {
iterator->_next = iterator->_prev = NULL ;
_EntryList = iterator ;
} else {
List->_prev = iterator ;
iterator->_next = List ;
iterator->_prev = NULL ;
_EntryList = iterator ;
}
} else
if (Policy == 1) { // append to EntryList
if (List == NULL) {
iterator->_next = iterator->_prev = NULL ;
_EntryList = iterator ;
} else {
// CONSIDER: finding the tail currently requires a linear-time walk of
// the EntryList. We can make tail access constant-time by converting to
// a CDLL instead of using our current DLL.
ObjectWaiter * Tail ;
for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
Tail->_next = iterator ;
iterator->_prev = Tail ;
iterator->_next = NULL ;
}
} else
if (Policy == 2) { // prepend to cxq
// prepend to cxq
if (List == NULL) {
iterator->_next = iterator->_prev = NULL ;
_EntryList = iterator ;
} else {
iterator->TState = ObjectWaiter::TS_CXQ ;
for (;;) {
ObjectWaiter * Front = _cxq ;
iterator->_next = Front ;
if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
break ;
}
}
}
} else
if (Policy == 3) { // append to cxq
iterator->TState = ObjectWaiter::TS_CXQ ;
for (;;) {
ObjectWaiter * Tail ;
Tail = _cxq ;
if (Tail == NULL) {
iterator->_next = NULL ;
if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
break ;
}
} else {
while (Tail->_next != NULL) Tail = Tail->_next ;
Tail->_next = iterator ;
iterator->_prev = Tail ;
iterator->_next = NULL ;
break ;
}
}
} else {
ParkEvent * ev = iterator->_event ;
iterator->TState = ObjectWaiter::TS_RUN ;
OrderAccess::fence() ;
ev->unpark() ;
}
if (Policy < 4) {
iterator->wait_reenter_begin(this);
}
// _WaitSetLock protects the wait queue, not the EntryList. We could
// move the add-to-EntryList operation, above, outside the critical section
// protected by _WaitSetLock. In practice that's not useful. With the
// exception of wait() timeouts and interrupts the monitor owner
// is the only thread that grabs _WaitSetLock. There's almost no contention
// on _WaitSetLock so it's not profitable to reduce the length of the
// critical section.
}
Thread::SpinRelease (&_WaitSetLock) ;
if (iterator != NULL && ObjectMonitor::_sync_Notifications != NULL) {
ObjectMonitor::_sync_Notifications->inc() ;
}
}
void ObjectMonitor::notifyAll(TRAPS) {
CHECK_OWNER();
ObjectWaiter* iterator;
if (_WaitSet == NULL) {
TEVENT (Empty-NotifyAll) ;
return ;
}
DTRACE_MONITOR_PROBE(notifyAll, this, object(), THREAD);
int Policy = Knob_MoveNotifyee ;
int Tally = 0 ;
Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notifyall") ;
for (;;) {
iterator = DequeueWaiter () ;
if (iterator == NULL) break ;
TEVENT (NotifyAll - Transfer1) ;
++Tally ;
// Disposition - what might we do with iterator ?
// a. add it directly to the EntryList - either tail or head.
// b. push it onto the front of the _cxq.
// For now we use (a).
guarantee (iterator->TState == ObjectWaiter::TS_WAIT, "invariant") ;
guarantee (iterator->_notified == 0, "invariant") ;
iterator->_notified = 1 ;
Thread * Self = THREAD;
iterator->_notifier_tid = Self->osthread()->thread_id();
if (Policy != 4) {
iterator->TState = ObjectWaiter::TS_ENTER ;
}
ObjectWaiter * List = _EntryList ;
if (List != NULL) {
assert (List->_prev == NULL, "invariant") ;
assert (List->TState == ObjectWaiter::TS_ENTER, "invariant") ;
assert (List != iterator, "invariant") ;
}
if (Policy == 0) { // prepend to EntryList
if (List == NULL) {
iterator->_next = iterator->_prev = NULL ;
_EntryList = iterator ;
} else {
List->_prev = iterator ;
iterator->_next = List ;
iterator->_prev = NULL ;
_EntryList = iterator ;
}
} else
if (Policy == 1) { // append to EntryList
if (List == NULL) {
iterator->_next = iterator->_prev = NULL ;
_EntryList = iterator ;
} else {
// CONSIDER: finding the tail currently requires a linear-time walk of
// the EntryList. We can make tail access constant-time by converting to
// a CDLL instead of using our current DLL.
ObjectWaiter * Tail ;
for (Tail = List ; Tail->_next != NULL ; Tail = Tail->_next) ;
assert (Tail != NULL && Tail->_next == NULL, "invariant") ;
Tail->_next = iterator ;
iterator->_prev = Tail ;
iterator->_next = NULL ;
}
} else
if (Policy == 2) { // prepend to cxq
// prepend to cxq
iterator->TState = ObjectWaiter::TS_CXQ ;
for (;;) {
ObjectWaiter * Front = _cxq ;
iterator->_next = Front ;
if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
break ;
}
}
} else
if (Policy == 3) { // append to cxq
iterator->TState = ObjectWaiter::TS_CXQ ;
for (;;) {
ObjectWaiter * Tail ;
Tail = _cxq ;
if (Tail == NULL) {
iterator->_next = NULL ;
if (Atomic::cmpxchg_ptr (iterator, &_cxq, NULL) == NULL) {
break ;
}
} else {
while (Tail->_next != NULL) Tail = Tail->_next ;
Tail->_next = iterator ;
iterator->_prev = Tail ;
iterator->_next = NULL ;
break ;
}
}
} else {
ParkEvent * ev = iterator->_event ;
iterator->TState = ObjectWaiter::TS_RUN ;
OrderAccess::fence() ;
ev->unpark() ;
}
if (Policy < 4) {
iterator->wait_reenter_begin(this);
}
// _WaitSetLock protects the wait queue, not the EntryList. We could
// move the add-to-EntryList operation, above, outside the critical section
// protected by _WaitSetLock. In practice that's not useful. With the
// exception of wait() timeouts and interrupts the monitor owner
// is the only thread that grabs _WaitSetLock. There's almost no contention
// on _WaitSetLock so it's not profitable to reduce the length of the
// critical section.
}
Thread::SpinRelease (&_WaitSetLock) ;
if (Tally != 0 && ObjectMonitor::_sync_Notifications != NULL) {
ObjectMonitor::_sync_Notifications->inc(Tally) ;
}
}
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;
}
}
void ATTR ObjectMonitor::enter(TRAPS) {
// The following code is ordered to check the most common cases first
// and to reduce RTS->RTO cache line upgrades on SPARC and IA32 processors.
Thread * const Self = THREAD ;
void * cur ;
cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
if (cur == NULL) {
// Either ASSERT _recursions == 0 or explicitly set _recursions = 0.
assert (_recursions == 0 , "invariant") ;
assert (_owner == Self, "invariant") ;
// CONSIDER: set or assert OwnerIsThread == 1
return ;
}
if (cur == Self) {
// TODO-FIXME: check for integer overflow! BUGID 6557169.
_recursions ++ ;
return ;
}
if (Self->is_lock_owned ((address)cur)) {
assert (_recursions == 0, "internal state error");
_recursions = 1 ;
// Commute owner from a thread-specific on-stack BasicLockObject address to
// a full-fledged "Thread *".
_owner = Self ;
OwnerIsThread = 1 ;
return ;
}
// We've encountered genuine contention.
assert (Self->_Stalled == 0, "invariant") ;
Self->_Stalled = intptr_t(this) ;
// Try one round of spinning *before* enqueueing Self
// and before going through the awkward and expensive state
// transitions. The following spin is strictly optional ...
// Note that if we acquire the monitor from an initial spin
// we forgo posting JVMTI events and firing DTRACE probes.
if (Knob_SpinEarly && TrySpin (Self) > 0) {
assert (_owner == Self , "invariant") ;
assert (_recursions == 0 , "invariant") ;
assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
Self->_Stalled = 0 ;
return ;
}
assert (_owner != Self , "invariant") ;
assert (_succ != Self , "invariant") ;
assert (Self->is_Java_thread() , "invariant") ;
JavaThread * jt = (JavaThread *) Self ;
assert (!SafepointSynchronize::is_at_safepoint(), "invariant") ;
assert (jt->thread_state() != _thread_blocked , "invariant") ;
assert (this->object() != NULL , "invariant") ;
assert (_count >= 0, "invariant") ;
// Prevent deflation at STW-time. See deflate_idle_monitors() and is_busy().
// Ensure the object-monitor relationship remains stable while there's contention.
Atomic::inc_ptr(&_count);
EventJavaMonitorEnter event;
{ // Change java thread status to indicate blocked on monitor enter.
JavaThreadBlockedOnMonitorEnterState jtbmes(jt, this);
DTRACE_MONITOR_PROBE(contended__enter, this, object(), jt);
if (JvmtiExport::should_post_monitor_contended_enter()) {
JvmtiExport::post_monitor_contended_enter(jt, this);
// The current thread does not yet own the monitor and does not
// yet appear on any queues that would get it made the successor.
// This means that the JVMTI_EVENT_MONITOR_CONTENDED_ENTER event
// handler cannot accidentally consume an unpark() meant for the
// ParkEvent associated with this ObjectMonitor.
}
OSThreadContendState osts(Self->osthread());
ThreadBlockInVM tbivm(jt);
Self->set_current_pending_monitor(this);
// TODO-FIXME: change the following for(;;) loop to straight-line code.
for (;;) {
jt->set_suspend_equivalent();
// cleared by handle_special_suspend_equivalent_condition()
// or java_suspend_self()
EnterI (THREAD) ;
if (!ExitSuspendEquivalent(jt)) break ;
//
// We have acquired the contended monitor, but while we were
// waiting another thread suspended us. We don't want to enter
// the monitor while suspended because that would surprise the
// thread that suspended us.
//
_recursions = 0 ;
_succ = NULL ;
exit (false, Self) ;
jt->java_suspend_self();
}
Self->set_current_pending_monitor(NULL);
// We cleared the pending monitor info since we've just gotten past
// the enter-check-for-suspend dance and we now own the monitor free
// and clear, i.e., it is no longer pending. The ThreadBlockInVM
// destructor can go to a safepoint at the end of this block. If we
// do a thread dump during that safepoint, then this thread will show
// as having "-locked" the monitor, but the OS and java.lang.Thread
// states will still report that the thread is blocked trying to
// acquire it.
}
Atomic::dec_ptr(&_count);
assert (_count >= 0, "invariant") ;
Self->_Stalled = 0 ;
// Must either set _recursions = 0 or ASSERT _recursions == 0.
assert (_recursions == 0 , "invariant") ;
assert (_owner == Self , "invariant") ;
assert (_succ != Self , "invariant") ;
assert (((oop)(object()))->mark() == markOopDesc::encode(this), "invariant") ;
// The thread -- now the owner -- is back in vm mode.
// Report the glorious news via TI,DTrace and jvmstat.
// The probe effect is non-trivial. All the reportage occurs
// while we hold the monitor, increasing the length of the critical
// section. Amdahl's parallel speedup law comes vividly into play.
//
// Another option might be to aggregate the events (thread local or
// per-monitor aggregation) and defer reporting until a more opportune
// time -- such as next time some thread encounters contention but has
// yet to acquire the lock. While spinning that thread could
// spinning we could increment JVMStat counters, etc.
DTRACE_MONITOR_PROBE(contended__entered, this, object(), jt);
if (JvmtiExport::should_post_monitor_contended_entered()) {
JvmtiExport::post_monitor_contended_entered(jt, this);
// The current thread already owns the monitor and is not going to
// call park() for the remainder of the monitor enter protocol. So
// it doesn't matter if the JVMTI_EVENT_MONITOR_CONTENDED_ENTERED
// event handler consumed an unpark() issued by the thread that
// just exited the monitor.
}
if (event.should_commit()) {
event.set_klass(((oop)this->object())->klass());
event.set_previousOwner((TYPE_JAVALANGTHREAD)_previous_owner_tid);
event.set_address((TYPE_ADDRESS)(uintptr_t)(this->object_addr()));
event.commit();
}
if (ObjectMonitor::_sync_ContendedLockAttempts != NULL) {
ObjectMonitor::_sync_ContendedLockAttempts->inc() ;
}
}
// -----------------------------------------------------------------------------
// 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 ;
}
}
}
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