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root/jsr166/jsr166/src/jsr166y/ForkJoinWorkerThread.java

Comparing jsr166/src/jsr166y/ForkJoinWorkerThread.java (file contents):
Revision 1.33 by dl, Thu May 27 16:46:49 2010 UTC vs.
Revision 1.39 by dl, Sat Jul 24 20:28:18 2010 UTC

#	Line 83 | Line 83 | public class ForkJoinWorkerThread extend
83		* by the ForkJoinPool).  This allows use in message-passing
84		* frameworks in which tasks are never joined.
85		*
86	<	* Efficient implementation of this approach currently relies on
87	<	* an uncomfortable amount of "Unsafe" mechanics. To maintain
86	>	* When a worker would otherwise be blocked waiting to join a
87	>	* task, it first tries a form of linear helping: Each worker
88	>	* records (in field currentSteal) the most recent task it stole
89	>	* from some other worker. Plus, it records (in field currentJoin)
90	>	* the task it is currently actively joining. Method joinTask uses
91	>	* these markers to try to find a worker to help (i.e., steal back
92	>	* a task from and execute it) that could hasten completion of the
93	>	* actively joined task. In essence, the joiner executes a task
94	>	* that would be on its own local deque had the to-be-joined task
95	>	* not been stolen. This may be seen as a conservative variant of
96	>	* the approach in Wagner & Calder "Leapfrogging: a portable
97	>	* technique for implementing efficient futures" SIGPLAN Notices,
98	>	* 1993 (http://portal.acm.org/citation.cfm?id=155354). It differs
99	>	* in that: (1) We only maintain dependency links across workers
100	>	* upon steals, rather than use per-task bookkeeping.  This may
101	>	* require a linear scan of workers array to locate stealers, but
102	>	* usually doesn't because stealers leave hints (that may become
103	>	* stale/wrong) of where to locate them. This isolates cost to
104	>	* when it is needed, rather than adding to per-task overhead.
105	>	* (2) It is "shallow", ignoring nesting and potentially cyclic
106	>	* mutual steals.  (3) It is intentionally racy: field currentJoin
107	>	* is updated only while actively joining, which means that we
108	>	* miss links in the chain during long-lived tasks, GC stalls etc
109	>	* (which is OK since blocking in such cases is usually a good
110	>	* idea).  (4) We bound the number of attempts to find work (see
111	>	* MAX_HELP_DEPTH) and fall back to suspending the worker and if
112	>	* necessary replacing it with a spare (see
113	>	* ForkJoinPool.tryAwaitJoin).
114	>	*
115	>	* Efficient implementation of these algorithms currently relies
116	>	* on an uncomfortable amount of "Unsafe" mechanics. To maintain
117		* correct orderings, reads and writes of variable base require
118		* volatile ordering.  Variable sp does not require volatile
119		* writes but still needs store-ordering, which we accomplish by
120		* pre-incrementing sp before filling the slot with an ordered
121		* store.  (Pre-incrementing also enables backouts used in
122	<	* scanWhileJoining.)  Because they are protected by volatile base
123	<	* reads, reads of the queue array and its slots by other threads
124	<	* do not need volatile load semantics, but writes (in push)
125	<	* require store order and CASes (in pop and deq) require
126	<	* (volatile) CAS semantics.  (Michael, Saraswat, and Vechev's
127	<	* algorithm has similar properties, but without support for
128	<	* nulling slots.)  Since these combinations aren't supported
129	<	* using ordinary volatiles, the only way to accomplish these
130	<	* efficiently is to use direct Unsafe calls. (Using external
131	<	* AtomicIntegers and AtomicReferenceArrays for the indices and
132	<	* array is significantly slower because of memory locality and
133	<	* indirection effects.)
122	>	* joinTask.)  Because they are protected by volatile base reads,
123	>	* reads of the queue array and its slots by other threads do not
124	>	* need volatile load semantics, but writes (in push) require
125	>	* store order and CASes (in pop and deq) require (volatile) CAS
126	>	* semantics.  (Michael, Saraswat, and Vechev's algorithm has
127	>	* similar properties, but without support for nulling slots.)
128	>	* Since these combinations aren't supported using ordinary
129	>	* volatiles, the only way to accomplish these efficiently is to
130	>	* use direct Unsafe calls. (Using external AtomicIntegers and
131	>	* AtomicReferenceArrays for the indices and array is
132	>	* significantly slower because of memory locality and indirection
133	>	* effects.)
134		*
135		* Further, performance on most platforms is very sensitive to
136		* placement and sizing of the (resizable) queue array.  Even
#	Line 137 | Line 166 | public class ForkJoinWorkerThread extend
166		5L * 1000L * 1000L * 1000L; // 5 secs
167
168		/**
169	+	* The maximum stolen->joining link depth allowed in helpJoinTask.
170	+	* Depths for legitimate chains are unbounded, but we use a fixed
171	+	* constant to avoid (otherwise unchecked) cycles and bound
172	+	* staleness of traversal parameters at the expense of sometimes
173	+	* blocking when we could be helping.
174	+	*/
175	+	private static final int MAX_HELP_DEPTH = 8;
176	+
177	+	/**
178		* Capacity of work-stealing queue array upon initialization.
179	<	* Must be a power of two. Initial size must be at least 2, but is
179	>	* Must be a power of two. Initial size must be at least 4, but is
180		* padded to minimize cache effects.
181		*/
182		private static final int INITIAL_QUEUE_CAPACITY = 1 << 13;
#	Line 178 | Line 216 | public class ForkJoinWorkerThread extend
216		private int sp;
217
218		/**
219	+	* The index of most recent stealer, used as a hint to avoid
220	+	* traversal in method helpJoinTask. This is only a hint because a
221	+	* worker might have had multiple steals and this only holds one
222	+	* of them (usually the most current). Declared non-volatile,
223	+	* relying on other prevailing sync to keep reasonably current.
224	+	*/
225	+	private int stealHint;
226	+
227	+	/**
228		* Run state of this worker. In addition to the usual run levels,
229		* tracks if this worker is suspended as a spare, and if it was
230		* killed (trimmed) while suspended. However, "active" status is
#	Line 196 | Line 243 | public class ForkJoinWorkerThread extend
243		* currently not exported but included because volatile write upon
244		* park also provides a workaround for a JVM bug.
245		*/
246	<	private volatile int parkCount;
246	>	volatile int parkCount;
247
248		/**
249		* Number of steals, transferred and reset in pool callbacks pool
#	Line 218 | Line 265 | public class ForkJoinWorkerThread extend
265
266		/**
267		* True if use local fifo, not default lifo, for local polling.
268	<	* Shadows value from ForkJoinPool, which resets it if changed
222	<	* pool-wide.
268	>	* Shadows value from ForkJoinPool.
269		*/
270	<	private boolean locallyFifo;
270	>	private final boolean locallyFifo;
271
272		/**
273		* Index of this worker in pool array. Set once by pool before
#	Line 243 | Line 289 | public class ForkJoinWorkerThread extend
289		volatile long nextWaiter;
290
291		/**
292	+	* The task currently being joined, set only when actively trying
293	+	* to helpStealer. Written only by current thread, but read by
294	+	* others.
295	+	*/
296	+	private volatile ForkJoinTask<?> currentJoin;
297	+
298	+	/**
299	+	* The task most recently stolen from another worker (or
300	+	* submission queue).  Not volatile because always read/written in
301	+	* presence of related volatiles in those cases where it matters.
302	+	*/
303	+	private ForkJoinTask<?> currentSteal;
304	+
305	+	/**
306		* Creates a ForkJoinWorkerThread operating in the given pool.
307		*
308		* @param pool the pool this thread works in
309		* @throws NullPointerException if pool is null
310		*/
311		protected ForkJoinWorkerThread(ForkJoinPool pool) {
252	–	if (pool == null) throw new NullPointerException();
312		this.pool = pool;
313	+	this.locallyFifo = pool.locallyFifo;
314		// To avoid exposing construction details to subclasses,
315		// remaining initialization is in start() and onStart()
316		}
#	Line 258 | Line 318 | public class ForkJoinWorkerThread extend
318		/**
319		* Performs additional initialization and starts this thread
320		*/
321	<	final void start(int poolIndex, boolean locallyFifo,
262	<	UncaughtExceptionHandler ueh) {
321	>	final void start(int poolIndex, UncaughtExceptionHandler ueh) {
322		this.poolIndex = poolIndex;
323	<	this.locallyFifo = locallyFifo;
323	>	setDaemon(true);
324		if (ueh != null)
325		setUncaughtExceptionHandler(ueh);
267	–	setDaemon(true);
326		start();
327		}
328
#	Line 305 | Line 363 | public class ForkJoinWorkerThread extend
363		int rs = seedGenerator.nextInt();
364		seed = rs == 0? 1 : rs; // seed must be nonzero
365
366	<	// Allocate name string and queue array in this thread
366	>	// Allocate name string and arrays in this thread
367		String pid = Integer.toString(pool.getPoolNumber());
368		String wid = Integer.toString(poolIndex);
369		setName("ForkJoinPool-" + pid + "-worker-" + wid);
#	Line 358 | Line 416 | public class ForkJoinWorkerThread extend
416		* Find and execute tasks and check status while running
417		*/
418		private void mainLoop() {
419	<	boolean ran = false; // true if ran task on previous step
419	>	int emptyScans = 0; // consecutive times failed to find work
420		ForkJoinPool p = pool;
421		for (;;) {
422	<	p.preStep(this, ran);
422	>	p.preStep(this, emptyScans);
423		if (runState != 0)
424		return;
425		ForkJoinTask<?> t; // try to get and run stolen or submitted task
426	<	if (ran = (t = scan()) != null || (t = pollSubmission()) != null) {
426	>	if ((t = scan()) != null || (t = pollSubmission()) != null) {
427		t.tryExec();
428		if (base != sp)
429		runLocalTasks();
430	+	currentSteal = null;
431	+	emptyScans = 0;
432		}
433	+	else
434	+	++emptyScans;
435		}
436		}
437
#	Line 397 | Line 459 | public class ForkJoinWorkerThread extend
459		while (p.hasQueuedSubmissions()) {
460		if (active || (active = p.tryIncrementActiveCount())) {
461		ForkJoinTask<?> t = p.pollSubmission();
462	<	return t != null ? t : scan(); // if missed, rescan
462	>	if (t != null) {
463	>	currentSteal = t;
464	>	return t;
465	>	}
466	>	return scan(); // if missed, rescan
467		}
468		}
469		return null;
#	Line 447 | Line 513 | public class ForkJoinWorkerThread extend
513		* @param t the task. Caller must ensure non-null.
514		*/
515		final void pushTask(ForkJoinTask<?> t) {
450	–	int s;
516		ForkJoinTask<?>[] q = queue;
517		int mask = q.length - 1; // implicit assert q != null
518	<	UNSAFE.putOrderedObject(q, (((s = sp++) & mask) << qShift) + qBase, t);
519	<	if ((s -= base) <= 0)
520	<	pool.signalWork();
521	<	else if (s + 1 >= mask)
522	<	growQueue();
518	>	int s = sp++;            // ok to increment sp before slot write
519	>	UNSAFE.putOrderedObject(q, ((s & mask) << qShift) + qBase, t);
520	>	if ((s -= base) == 0)
521	>	pool.signalWork();   // was empty
522	>	else if (s == mask)
523	>	growQueue();         // is full
524		}
525
526		/**
527		* Tries to take a task from the base of the queue, failing if
528		* empty or contended. Note: Specializations of this code appear
529	<	* in scan and scanWhileJoining.
529	>	* in locallyDeqTask and elsewhere.
530		*
531		* @return a task, or null if none or contended
532		*/
#	Line 470 | Line 536 | public class ForkJoinWorkerThread extend
536		int b, i;
537		if ((b = base) != sp &&
538		(q = queue) != null && // must read q after b
539	<	(t = q[i = (q.length - 1) & b]) != null &&
539	>	(t = q[i = (q.length - 1) & b]) != null && base == b &&
540		UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase, t, null)) {
541		base = b + 1;
542		return t;
#	Line 490 | Line 556 | public class ForkJoinWorkerThread extend
556		ForkJoinTask<?> t;
557		int b, i;
558		while (sp != (b = base)) {
559	<	if ((t = q[i = (q.length - 1) & b]) != null &&
559	>	if ((t = q[i = (q.length - 1) & b]) != null && base == b &&
560		UNSAFE.compareAndSwapObject(q, (i << qShift) + qBase,
561		t, null)) {
562		base = b + 1;
#	Line 503 | Line 569 | public class ForkJoinWorkerThread extend
569
570		/**
571		* Returns a popped task, or null if empty. Assumes active status.
572	<	* Called only by current thread. (Note: a specialization of this
507	<	* code appears in popWhileJoining.)
572	>	* Called only by current thread.
573		*/
574		final ForkJoinTask<?> popTask() {
575		int s;
#	Line 623 | Line 688 | public class ForkJoinWorkerThread extend
688		ForkJoinWorkerThread v = ws[k & mask];
689		r ^= r << 13; r ^= r >>> 17; r ^= r << 5; // inline xorshift
690		if (v != null && v.base != v.sp) {
691	<	int b, i;             // inline specialized deqTask
692	<	ForkJoinTask<?>[] q;
693	<	ForkJoinTask<?> t;
694	<	if ((canSteal ||      // ensure active status
695	<	(canSteal = active = p.tryIncrementActiveCount())) &&
696	<	(q = v.queue) != null &&
697	<	(t = q[i = (q.length - 1) & (b = v.base)]) != null &&
698	<	UNSAFE.compareAndSwapObject
699	<	(q, (i << qShift) + qBase, t, null)) {
700	<	v.base = b + 1;
701	<	seed = r;
702	<	++stealCount;
703	<	return t;
691	>	if (canSteal ||       // ensure active status
692	>	(canSteal = active = p.tryIncrementActiveCount())) {
693	>	int b = v.base;   // inline specialized deqTask
694	>	ForkJoinTask<?>[] q;
695	>	if (b != v.sp && (q = v.queue) != null) {
696	>	ForkJoinTask<?> t;
697	>	int i = (q.length - 1) & b;
698	>	long u = (i << qShift) + qBase; // raw offset
699	>	if ((t = q[i]) != null && v.base == b &&
700	>	UNSAFE.compareAndSwapObject(q, u, t, null)) {
701	>	currentSteal = t;
702	>	v.stealHint = poolIndex;
703	>	v.base = b + 1;
704	>	seed = r;
705	>	++stealCount;
706	>	return t;
707	>	}
708	>	}
709		}
710		j = -n;
711		k = r;                // restart on contention
#	Line 690 | Line 760 | public class ForkJoinWorkerThread extend
760		}
761
762		/**
763	<	* Instrumented version of park. Also used by ForkJoinPool.awaitEvent
763	>	* Instrumented version of park used by ForkJoinPool.eventSync
764		*/
765		final void doPark() {
766		++parkCount;
#	Line 698 | Line 768 | public class ForkJoinWorkerThread extend
768		}
769
770		/**
771	<	* If suspended, tries to set status to unsuspended.
702	<	* Caller must unpark to actually resume
771	>	* If suspended, tries to set status to unsuspended and unparks.
772		*
773		* @return true if successful
774		*/
775	<	final boolean tryUnsuspend() {
776	<	int s;
777	<	return (((s = runState) & SUSPENDED) != 0 &&
778	<	UNSAFE.compareAndSwapInt(this, runStateOffset, s,
779	<	s & ~SUSPENDED));
775	>	final boolean tryResumeSpare() {
776	>	int s = runState;
777	>	if ((s & SUSPENDED) != 0 &&
778	>	UNSAFE.compareAndSwapInt(this, runStateOffset, s,
779	>	s & ~SUSPENDED)) {
780	>	LockSupport.unpark(this);
781	>	return true;
782	>	}
783	>	return false;
784		}
785
786		/**
#	Line 728 | Line 801 | public class ForkJoinWorkerThread extend
801		s | SUSPENDED))
802		break;
803		}
804	+	int pc = pool.parallelism;
805	+	pool.accumulateStealCount(this);
806	+	boolean timed;
807	+	long nanos;
808	+	long startTime;
809	+	if (poolIndex < pc) { // untimed wait for core threads
810	+	timed = false;
811	+	nanos = 0L;
812	+	startTime = 0L;
813	+	}
814	+	else {                // timed wait for added threads
815	+	timed = true;
816	+	nanos = SPARE_KEEPALIVE_NANOS;
817	+	startTime = System.nanoTime();
818	+	}
819		lastEventCount = 0;      // reset upon resume
732	–	ForkJoinPool p = pool;
733	–	p.releaseWaiters();      // help others progress
734	–	p.accumulateStealCount(this);
820		interrupted();           // clear/ignore interrupts
736	–	if (poolIndex < p.getParallelism()) { // untimed wait
737	–	while ((runState & SUSPENDED) != 0)
738	–	doPark();
739	–	return true;
740	–	}
741	–	return timedSuspend();   // timed wait if apparently non-core
742	–	}
743	–
744	–	/**
745	–	* Blocks as spare until resumed or timed out
746	–	* @return false if trimmed
747	–	*/
748	–	private boolean timedSuspend() {
749	–	long nanos = SPARE_KEEPALIVE_NANOS;
750	–	long startTime = System.nanoTime();
821		while ((runState & SUSPENDED) != 0) {
822		++parkCount;
823	<	if ((nanos -= (System.nanoTime() - startTime)) > 0)
823	>	if (!timed)
824	>	LockSupport.park(this);
825	>	else if ((nanos -= (System.nanoTime() - startTime)) > 0)
826		LockSupport.parkNanos(this, nanos);
827		else { // try to trim on timeout
828		int s = runState;
#	Line 774 | Line 846 | public class ForkJoinWorkerThread extend
846		}
847
848		/**
777	–	* Set locallyFifo mode. Called only by ForkJoinPool
778	–	*/
779	–	final void setAsyncMode(boolean async) {
780	–	locallyFifo = async;
781	–	}
782	–
783	–	/**
849		* Removes and cancels all tasks in queue.  Can be called from any
850		* thread.
851		*/
852		final void cancelTasks() {
853	+	ForkJoinTask<?> cj = currentJoin; // try to kill live tasks
854	+	if (cj != null) {
855	+	currentJoin = null;
856	+	cj.cancelIgnoringExceptions();
857	+	}
858	+	ForkJoinTask<?> cs = currentSteal;
859	+	if (cs != null) {
860	+	currentSteal = null;
861	+	cs.cancelIgnoringExceptions();
862	+	}
863		while (base != sp) {
864		ForkJoinTask<?> t = deqTask();
865		if (t != null)
#	Line 812 | Line 887 | public class ForkJoinWorkerThread extend
887		// Support methods for ForkJoinTask
888
889		/**
890	+	* Gets and removes a local task.
891	+	*
892	+	* @return a task, if available
893	+	*/
894	+	final ForkJoinTask<?> pollLocalTask() {
895	+	while (sp != base) {
896	+	if (active || (active = pool.tryIncrementActiveCount()))
897	+	return locallyFifo? locallyDeqTask() : popTask();
898	+	}
899	+	return null;
900	+	}
901	+
902	+	/**
903	+	* Gets and removes a local or stolen task.
904	+	*
905	+	* @return a task, if available
906	+	*/
907	+	final ForkJoinTask<?> pollTask() {
908	+	ForkJoinTask<?> t = pollLocalTask();
909	+	if (t == null) {
910	+	t = scan();
911	+	currentSteal = null; // cannot retain/track
912	+	}
913	+	return t;
914	+	}
915	+
916	+	/**
917	+	* Possibly runs some tasks and/or blocks, until task is done.
918	+	* The main body is basically a big spinloop, alternating between
919	+	* calls to helpJoinTask and pool.tryAwaitJoin with increased
920	+	* patience parameters until either the task is done without
921	+	* waiting, or we have, if necessary, created or resumed a
922	+	* replacement for this thread while it blocks.
923	+	*
924	+	* @param joinMe the task to join
925	+	* @return task status on exit
926	+	*/
927	+	final int joinTask(ForkJoinTask<?> joinMe) {
928	+	int stat;
929	+	ForkJoinTask<?> prevJoin = currentJoin;
930	+	// Only written by this thread; only need ordered store
931	+	UNSAFE.putOrderedObject(this, currentJoinOffset, joinMe);
932	+	if ((stat = joinMe.status) >= 0 &&
933	+	(sp == base || (stat = localHelpJoinTask(joinMe)) >= 0)) {
934	+	for (int retries = 0; ; ++retries) {
935	+	helpJoinTask(joinMe, retries);
936	+	if ((stat = joinMe.status) < 0)
937	+	break;
938	+	pool.tryAwaitJoin(joinMe, retries);
939	+	if ((stat = joinMe.status) < 0)
940	+	break;
941	+	Thread.yield(); // tame unbounded loop
942	+	}
943	+	}
944	+	UNSAFE.putOrderedObject(this, currentJoinOffset, prevJoin);
945	+	return stat;
946	+	}
947	+
948	+	/**
949	+	* Run tasks in local queue until given task is done.
950	+	*
951	+	* @param joinMe the task to join
952	+	* @return task status on exit
953	+	*/
954	+	private int localHelpJoinTask(ForkJoinTask<?> joinMe) {
955	+	int stat, s;
956	+	ForkJoinTask<?>[] q;
957	+	while ((stat = joinMe.status) >= 0 &&
958	+	base != (s = sp) && (q = queue) != null) {
959	+	ForkJoinTask<?> t;
960	+	int i = (q.length - 1) & --s;
961	+	long u = (i << qShift) + qBase; // raw offset
962	+	if ((t = q[i]) != null &&
963	+	UNSAFE.compareAndSwapObject(q, u, t, null)) {
964	+	/*
965	+	* This recheck (and similarly in helpJoinTask)
966	+	* handles cases where joinMe is independently
967	+	* cancelled or forced even though there is other work
968	+	* available. Back out of the pop by putting t back
969	+	* into slot before we commit by writing sp.
970	+	*/
971	+	if ((stat = joinMe.status) < 0) {
972	+	UNSAFE.putObjectVolatile(q, u, t);
973	+	break;
974	+	}
975	+	sp = s;
976	+	t.tryExec();
977	+	}
978	+	}
979	+	return stat;
980	+	}
981	+
982	+	/**
983	+	* Tries to locate and help perform tasks for a stealer of the
984	+	* given task, or in turn one of its stealers.  Traces
985	+	* currentSteal->currentJoin links looking for a thread working on
986	+	* a descendant of the given task and with a non-empty queue to
987	+	* steal back and execute tasks from. Restarts search upon
988	+	* encountering chains that are stale, unknown, or of length
989	+	* greater than MAX_HELP_DEPTH links, to avoid unbounded cycles.
990	+	*
991	+	* The implementation is very branchy to cope with the restart
992	+	* cases.  Returns void, not task status (which must be reread by
993	+	* caller anyway) to slightly simplify control paths.
994	+	*
995	+	* @param joinMe the task to join
996	+	* @param rescans the number of times to recheck for work
997	+	*/
998	+	private void helpJoinTask(ForkJoinTask<?> joinMe, int rescans) {
999	+	ForkJoinWorkerThread[] ws = pool.workers;
1000	+	int n;
1001	+	if (ws == null || (n = ws.length) <= 1)
1002	+	return;                   // need at least 2 workers
1003	+	restart:while (rescans-- >= 0 && joinMe.status >= 0) {
1004	+	ForkJoinTask<?> task = joinMe;        // base of chain
1005	+	ForkJoinWorkerThread thread = this;   // thread with stolen task
1006	+	for (int depth = 0; depth < MAX_HELP_DEPTH; ++depth) {
1007	+	// Try to find v, the stealer of task, by first using hint
1008	+	ForkJoinWorkerThread v = ws[thread.stealHint & (n - 1)];
1009	+	if (v == null || v.currentSteal != task) {
1010	+	for (int j = 0; ; ++j) {      // search array
1011	+	if (task.status < 0 || j == n)
1012	+	continue restart;     // stale or no stealer
1013	+	if ((v = ws[j]) != null && v.currentSteal == task) {
1014	+	thread.stealHint = j; // save for next time
1015	+	break;
1016	+	}
1017	+	}
1018	+	}
1019	+	// Try to help v, using specialized form of deqTask
1020	+	int b;
1021	+	ForkJoinTask<?>[] q;
1022	+	while ((b = v.base) != v.sp && (q = v.queue) != null) {
1023	+	int i = (q.length - 1) & b;
1024	+	long u = (i << qShift) + qBase;
1025	+	ForkJoinTask<?> t = q[i];
1026	+	if (task.status < 0)          // stale
1027	+	continue restart;
1028	+	if (t != null) {
1029	+	if (v.base == b &&
1030	+	UNSAFE.compareAndSwapObject(q, u, t, null)) {
1031	+	if (joinMe.status < 0) {
1032	+	UNSAFE.putObjectVolatile(q, u, t);
1033	+	return;           // back out on cancel
1034	+	}
1035	+	ForkJoinTask<?> prevSteal = currentSteal;
1036	+	currentSteal = t;
1037	+	v.stealHint = poolIndex;
1038	+	v.base = b + 1;
1039	+	t.tryExec();
1040	+	currentSteal = prevSteal;
1041	+	}
1042	+	}
1043	+	else if (v.base == b)          // producer stalled
1044	+	continue restart;          // retry via restart
1045	+	if (joinMe.status < 0)
1046	+	return;
1047	+	}
1048	+	// Try to descend to find v's stealer
1049	+	ForkJoinTask<?> next = v.currentJoin;
1050	+	if (next == null || next == task || task.status < 0)
1051	+	continue restart;             // no descendent or stale
1052	+	if (joinMe.status < 0)
1053	+	return;
1054	+	task = next;
1055	+	thread = v;
1056	+	}
1057	+	}
1058	+	}
1059	+
1060	+	/**
1061		* Returns an estimate of the number of tasks, offset by a
1062		* function of number of idle workers.
1063		*
#	Line 863 | Line 1109 | public class ForkJoinWorkerThread extend
1109		}
1110
1111		/**
866	–	* Gets and removes a local task.
867	–	*
868	–	* @return a task, if available
869	–	*/
870	–	final ForkJoinTask<?> pollLocalTask() {
871	–	while (base != sp) {
872	–	if (active || (active = pool.tryIncrementActiveCount()))
873	–	return locallyFifo? locallyDeqTask() : popTask();
874	–	}
875	–	return null;
876	–	}
877	–
878	–	/**
879	–	* Gets and removes a local or stolen task.
880	–	*
881	–	* @return a task, if available
882	–	*/
883	–	final ForkJoinTask<?> pollTask() {
884	–	ForkJoinTask<?> t;
885	–	return (t = pollLocalTask()) != null ? t : scan();
886	–	}
887	–
888	–	/**
889	–	* Executes or processes other tasks awaiting the given task
890	–	* @return task completion status
891	–	*/
892	–	final int execWhileJoining(ForkJoinTask<?> joinMe) {
893	–	int s;
894	–	while ((s = joinMe.status) >= 0) {
895	–	ForkJoinTask<?> t = base != sp?
896	–	popWhileJoining(joinMe) :
897	–	scanWhileJoining(joinMe);
898	–	if (t != null)
899	–	t.tryExec();
900	–	}
901	–	return s;
902	–	}
903	–
904	–	/**
905	–	* Returns or stolen task, if available, unless joinMe is done
906	–	*
907	–	* This method is intrinsically nonmodular. To maintain the
908	–	* property that tasks are never stolen if the awaited task is
909	–	* ready, we must interleave mechanics of scan with status
910	–	* checks. We rely here on the commit points of deq that allow us
911	–	* to cancel a steal even after CASing slot to null, but before
912	–	* adjusting base index: If, after the CAS, we see that joinMe is
913	–	* ready, we can back out by placing the task back into the slot,
914	–	* without adjusting index. The loop is otherwise a variant of the
915	–	* one in scan().
916	–	*
917	–	*/
918	–	private ForkJoinTask<?> scanWhileJoining(ForkJoinTask<?> joinMe) {
919	–	int r = seed;
920	–	ForkJoinPool p = pool;
921	–	ForkJoinWorkerThread[] ws;
922	–	int n;
923	–	outer:while ((ws = p.workers) != null && (n = ws.length) > 1) {
924	–	int mask = n - 1;
925	–	int k = r;
926	–	boolean contended = false; // to retry loop if deq contends
927	–	for (int j = -n; j <= n; ++j) {
928	–	if (joinMe.status < 0)
929	–	break outer;
930	–	int b;
931	–	ForkJoinTask<?>[] q;
932	–	ForkJoinWorkerThread v = ws[k & mask];
933	–	r ^= r << 13; r ^= r >>> 17; r ^= r << 5; // xorshift
934	–	if (v != null && (b=v.base) != v.sp && (q=v.queue) != null) {
935	–	int i = (q.length - 1) & b;
936	–	ForkJoinTask<?> t = q[i];
937	–	if (t != null && UNSAFE.compareAndSwapObject
938	–	(q, (i << qShift) + qBase, t, null)) {
939	–	if (joinMe.status >= 0) {
940	–	v.base = b + 1;
941	–	seed = r;
942	–	++stealCount;
943	–	return t;
944	–	}
945	–	UNSAFE.putObjectVolatile(q, (i<<qShift)+qBase, t);
946	–	break outer; // back out
947	–	}
948	–	contended = true;
949	–	}
950	–	k = j < 0 ? r : (k + ((n >>> 1) | 1));
951	–	}
952	–	if (!contended && p.tryAwaitBusyJoin(joinMe))
953	–	break;
954	–	}
955	–	return null;
956	–	}
957	–
958	–	/**
959	–	* Version of popTask with join checks surrounding extraction.
960	–	* Uses the same backout strategy as helpJoinTask. Note that
961	–	* we ignore locallyFifo flag for local tasks here since helping
962	–	* joins only make sense in LIFO mode.
963	–	*
964	–	* @return a popped task, if available, unless joinMe is done
965	–	*/
966	–	private ForkJoinTask<?> popWhileJoining(ForkJoinTask<?> joinMe) {
967	–	int s;
968	–	ForkJoinTask<?>[] q;
969	–	while ((s = sp) != base && (q = queue) != null && joinMe.status >= 0) {
970	–	int i = (q.length - 1) & --s;
971	–	ForkJoinTask<?> t = q[i];
972	–	if (t != null && UNSAFE.compareAndSwapObject
973	–	(q, (i << qShift) + qBase, t, null)) {
974	–	if (joinMe.status >= 0) {
975	–	sp = s;
976	–	return t;
977	–	}
978	–	UNSAFE.putObjectVolatile(q, (i << qShift) + qBase, t);
979	–	break;  // back out
980	–	}
981	–	}
982	–	return null;
983	–	}
984	–
985	–	/**
1112		* Runs tasks until {@code pool.isQuiescent()}.
1113		*/
1114		final void helpQuiescePool() {
1115		for (;;) {
1116		ForkJoinTask<?> t = pollLocalTask();
1117	<	if (t != null || (t = scan()) != null)
1117	>	if (t != null || (t = scan()) != null) {
1118		t.tryExec();
1119	+	currentSteal = null;
1120	+	}
1121		else {
1122		ForkJoinPool p = pool;
1123		if (active) {
#	Line 1010 | Line 1138 | public class ForkJoinWorkerThread extend
1138		private static final sun.misc.Unsafe UNSAFE = getUnsafe();
1139		private static final long runStateOffset =
1140		objectFieldOffset("runState", ForkJoinWorkerThread.class);
1141	+	private static final long currentJoinOffset =
1142	+	objectFieldOffset("currentJoin", ForkJoinWorkerThread.class);
1143		private static final long qBase =
1144		UNSAFE.arrayBaseOffset(ForkJoinTask[].class);
1145		private static final int qShift;

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