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root/jsr166/jsr166/src/jsr166y/LinkedTransferQueue.java

Comparing jsr166/src/jsr166y/LinkedTransferQueue.java (file contents):
Revision 1.51 by dl, Sat Oct 24 14:33:29 2009 UTC vs.
Revision 1.70 by jsr166, Sun Nov 15 02:29:12 2009 UTC

#	Line 206 | Line 206 | public class LinkedTransferQueue<E> exte
206		* additional GC bookkeeping ("write barriers") that are sometimes
207		* more costly than the writes themselves because of contention).
208		*
209	–	* Removal of interior nodes (due to timed out or interrupted
210	–	* waits, or calls to remove(x) or Iterator.remove) can use a
211	–	* scheme roughly similar to that described in Scherer, Lea, and
212	–	* Scott's SynchronousQueue. Given a predecessor, we can unsplice
213	–	* any node except the (actual) tail of the queue. To avoid
214	–	* build-up of cancelled trailing nodes, upon a request to remove
215	–	* a trailing node, it is placed in field "cleanMe" to be
216	–	* unspliced upon the next call to unsplice any other node.
217	–	* Situations needing such mechanics are not common but do occur
218	–	* in practice; for example when an unbounded series of short
219	–	* timed calls to poll repeatedly time out but never otherwise
220	–	* fall off the list because of an untimed call to take at the
221	–	* front of the queue. Note that maintaining field cleanMe does
222	–	* not otherwise much impact garbage retention even if never
223	–	* cleared by some other call because the held node will
224	–	* eventually either directly or indirectly lead to a self-link
225	–	* once off the list.
226	–	*
209		* *** Overview of implementation ***
210		*
211		* We use a threshold-based approach to updates, with a slack
#	Line 239 | Line 221 | public class LinkedTransferQueue<E> exte
221		* per-thread one available, but even ThreadLocalRandom is too
222		* heavy for these purposes.
223		*
224	<	* With such a small slack threshold value, it is rarely
225	<	* worthwhile to augment this with path short-circuiting; i.e.,
226	<	* unsplicing nodes between head and the first unmatched node, or
227	<	* similarly for tail, rather than advancing head or tail
246	<	* proper. However, it is used (in awaitMatch) immediately before
247	<	* a waiting thread starts to block, as a final bit of helping at
248	<	* a point when contention with others is extremely unlikely
249	<	* (since if other threads that could release it are operating,
250	<	* then the current thread wouldn't be blocking).
224	>	* With such a small slack threshold value, it is not worthwhile
225	>	* to augment this with path short-circuiting (i.e., unsplicing
226	>	* interior nodes) except in the case of cancellation/removal (see
227	>	* below).
228		*
229		* We allow both the head and tail fields to be null before any
230		* nodes are enqueued; initializing upon first append.  This
#	Line 329 | Line 306 | public class LinkedTransferQueue<E> exte
306		*    versa) compared to their predecessors receive additional
307		*    chained spins, reflecting longer paths typically required to
308		*    unblock threads during phase changes.
309	+	*
310	+	*
311	+	* ** Unlinking removed interior nodes **
312	+	*
313	+	* In addition to minimizing garbage retention via self-linking
314	+	* described above, we also unlink removed interior nodes. These
315	+	* may arise due to timed out or interrupted waits, or calls to
316	+	* remove(x) or Iterator.remove.  Normally, given a node that was
317	+	* at one time known to be the predecessor of some node s that is
318	+	* to be removed, we can unsplice s by CASing the next field of
319	+	* its predecessor if it still points to s (otherwise s must
320	+	* already have been removed or is now offlist). But there are two
321	+	* situations in which we cannot guarantee to make node s
322	+	* unreachable in this way: (1) If s is the trailing node of list
323	+	* (i.e., with null next), then it is pinned as the target node
324	+	* for appends, so can only be removed later when other nodes are
325	+	* appended. (2) We cannot necessarily unlink s given a
326	+	* predecessor node that is matched (including the case of being
327	+	* cancelled): the predecessor may already be unspliced, in which
328	+	* case some previous reachable node may still point to s.
329	+	* (For further explanation see Herlihy & Shavit "The Art of
330	+	* Multiprocessor Programming" chapter 9).  Although, in both
331	+	* cases, we can rule out the need for further action if either s
332	+	* or its predecessor are (or can be made to be) at, or fall off
333	+	* from, the head of list.
334	+	*
335	+	* Without taking these into account, it would be possible for an
336	+	* unbounded number of supposedly removed nodes to remain
337	+	* reachable.  Situations leading to such buildup are uncommon but
338	+	* can occur in practice; for example when a series of short timed
339	+	* calls to poll repeatedly time out but never otherwise fall off
340	+	* the list because of an untimed call to take at the front of the
341	+	* queue.
342	+	*
343	+	* When these cases arise, rather than always retraversing the
344	+	* entire list to find an actual predecessor to unlink (which
345	+	* won't help for case (1) anyway), we record a conservative
346	+	* estimate of possible unsplice failures (in "sweepVotes").  We
347	+	* trigger a full sweep when the estimate exceeds a threshold
348	+	* indicating the maximum number of estimated removal failures to
349	+	* tolerate before sweeping through, unlinking cancelled nodes
350	+	* that were not unlinked upon initial removal. We perform sweeps
351	+	* by the thread hitting threshold (rather than background threads
352	+	* or by spreading work to other threads) because in the main
353	+	* contexts in which removal occurs, the caller is already
354	+	* timed-out, cancelled, or performing a potentially O(n)
355	+	* operation (i.e., remove(x)), none of which are time-critical
356	+	* enough to warrant the overhead that alternatives would impose
357	+	* on other threads.
358	+	*
359	+	* Because the sweepVotes estimate is conservative, and because
360	+	* nodes become unlinked "naturally" as they fall off the head of
361	+	* the queue, and because we allow votes to accumulate even while
362	+	* sweeps are in progress, there are typically significantly fewer
363	+	* such nodes than estimated.  Choice of a threshold value
364	+	* balances the likelihood of wasted effort and contention, versus
365	+	* providing a worst-case bound on retention of interior nodes in
366	+	* quiescent queues. The value defined below was chosen
367	+	* empirically to balance these under various timeout scenarios.
368	+	*
369	+	* Note that we cannot self-link unlinked interior nodes during
370	+	* sweeps. However, the associated garbage chains terminate when
371	+	* some successor ultimately falls off the head of the list and is
372	+	* self-linked.
373		*/
374
375		/** True if on multiprocessor */
#	Line 355 | Line 396 | public class LinkedTransferQueue<E> exte
396		private static final int CHAINED_SPINS = FRONT_SPINS >>> 1;
397
398		/**
399	+	* The maximum number of estimated removal failures (sweepVotes)
400	+	* to tolerate before sweeping through the queue unlinking
401	+	* cancelled nodes that were not unlinked upon initial
402	+	* removal. See above for explanation. The value must be at least
403	+	* two to avoid useless sweeps when removing trailing nodes.
404	+	*/
405	+	static final int SWEEP_THRESHOLD = 32;
406	+
407	+	/**
408		* Queue nodes. Uses Object, not E, for items to allow forgetting
409		* them after use.  Relies heavily on Unsafe mechanics to minimize
410	<	* unnecessary ordering constraints: Writes that intrinsically
411	<	* precede or follow CASes use simple relaxed forms.  Other
362	<	* cleanups use releasing/lazy writes.
410	>	* unnecessary ordering constraints: Writes that are intrinsically
411	>	* ordered wrt other accesses or CASes use simple relaxed forms.
412		*/
413		static final class Node {
414		final boolean isData;   // false if this is a request node
#	Line 373 | Line 422 | public class LinkedTransferQueue<E> exte
422		}
423
424		final boolean casItem(Object cmp, Object val) {
425	+	assert cmp == null || cmp.getClass() != Node.class;
426		return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
427		}
428
#	Line 394 | Line 444 | public class LinkedTransferQueue<E> exte
444		}
445
446		/**
447	<	* Sets item to self (using a releasing/lazy write) and waiter
448	<	* to null, to avoid garbage retention after extracting or
449	<	* cancelling.
447	>	* Sets item to self and waiter to null, to avoid garbage
448	>	* retention after matching or cancelling. Uses relaxed writes
449	>	* bacause order is already constrained in the only calling
450	>	* contexts: item is forgotten only after volatile/atomic
451	>	* mechanics that extract items.  Similarly, clearing waiter
452	>	* follows either CAS or return from park (if ever parked;
453	>	* else we don't care).
454		*/
455		final void forgetContents() {
456	<	UNSAFE.putOrderedObject(this, itemOffset, this);
457	<	UNSAFE.putOrderedObject(this, waiterOffset, null);
456	>	UNSAFE.putObject(this, itemOffset, this);
457	>	UNSAFE.putObject(this, waiterOffset, null);
458		}
459
460		/**
#	Line 409 | Line 463 | public class LinkedTransferQueue<E> exte
463		*/
464		final boolean isMatched() {
465		Object x = item;
466	<	return x == this || (x != null) != isData;
466	>	return (x == this) || ((x == null) == isData);
467	>	}
468	>
469	>	/**
470	>	* Returns true if this is an unmatched request node.
471	>	*/
472	>	final boolean isUnmatchedRequest() {
473	>	return !isData && item == null;
474		}
475
476		/**
#	Line 427 | Line 488 | public class LinkedTransferQueue<E> exte
488		* Tries to artificially match a data node -- used by remove.
489		*/
490		final boolean tryMatchData() {
491	+	assert isData;
492		Object x = item;
493		if (x != null && x != this && casItem(x, null)) {
494		LockSupport.unpark(waiter);
#	Line 448 | Line 510 | public class LinkedTransferQueue<E> exte
510		}
511
512		/** head of the queue; null until first enqueue */
513	<	private transient volatile Node head;
452	<
453	<	/** predecessor of dangling unspliceable node */
454	<	private transient volatile Node cleanMe; // decl here to reduce contention
513	>	transient volatile Node head;
514
515		/** tail of the queue; null until first append */
516		private transient volatile Node tail;
517
518	+	/** The number of apparent failures to unsplice removed nodes */
519	+	private transient volatile int sweepVotes;
520	+
521		// CAS methods for fields
522		private boolean casTail(Node cmp, Node val) {
523		return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val);
#	Line 465 | Line 527 | public class LinkedTransferQueue<E> exte
527		return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val);
528		}
529
530	<	private boolean casCleanMe(Node cmp, Node val) {
531	<	return UNSAFE.compareAndSwapObject(this, cleanMeOffset, cmp, val);
530	>	private boolean casSweepVotes(int cmp, int val) {
531	>	return UNSAFE.compareAndSwapInt(this, sweepVotesOffset, cmp, val);
532		}
533
534		/*
535	<	* Possible values for "how" argument in xfer method. Beware that
474	<	* the order of assigned numerical values matters.
535	>	* Possible values for "how" argument in xfer method.
536		*/
537	<	private static final int NOW     = 0; // for untimed poll, tryTransfer
538	<	private static final int ASYNC   = 1; // for offer, put, add
539	<	private static final int SYNC    = 2; // for transfer, take
540	<	private static final int TIMEOUT = 3; // for timed poll, tryTransfer
537	>	private static final int NOW   = 0; // for untimed poll, tryTransfer
538	>	private static final int ASYNC = 1; // for offer, put, add
539	>	private static final int SYNC  = 2; // for transfer, take
540	>	private static final int TIMED = 3; // for timed poll, tryTransfer
541	>
542	>	@SuppressWarnings("unchecked")
543	>	static <E> E cast(Object item) {
544	>	assert item == null || item.getClass() != Node.class;
545	>	return (E) item;
546	>	}
547
548		/**
549		* Implements all queuing methods. See above for explanation.
550		*
551		* @param e the item or null for take
552		* @param haveData true if this is a put, else a take
553	<	* @param how NOW, ASYNC, SYNC, or TIMEOUT
554	<	* @param nanos timeout in nanosecs, used only if mode is TIMEOUT
553	>	* @param how NOW, ASYNC, SYNC, or TIMED
554	>	* @param nanos timeout in nanosecs, used only if mode is TIMED
555		* @return an item if matched, else e
556		* @throws NullPointerException if haveData mode but e is null
557		*/
558	<	private Object xfer(Object e, boolean haveData, int how, long nanos) {
558	>	private E xfer(E e, boolean haveData, int how, long nanos) {
559		if (haveData && (e == null))
560		throw new NullPointerException();
561		Node s = null;                        // the node to append, if needed
#	Line 502 | Line 569 | public class LinkedTransferQueue<E> exte
569		if (isData == haveData)   // can't match
570		break;
571		if (p.casItem(item, e)) { // match
572	<	LockSupport.unpark(p.waiter);
573	<	while (p != h) {      // update head
574	<	Node n = p.next;  // by 2 unless singleton
508	<	if (n != null)
509	<	p = n;
510	<	if (head == h && casHead(h, p)) {
572	>	for (Node q = p; q != h;) {
573	>	Node n = q.next;  // update by 2 unless singleton
574	>	if (head == h && casHead(h, n == null? q : n)) {
575		h.forgetNext();
576		break;
577		}                 // advance and retry
578		if ((h = head)   == null ||
579	<	(p = h.next) == null || !p.isMatched())
579	>	(q = h.next) == null || !q.isMatched())
580		break;        // unless slack < 2
581		}
582	<	return item;
582	>	LockSupport.unpark(p.waiter);
583	>	return this.<E>cast(item);
584		}
585		}
586		Node n = p.next;
587		p = (p != n) ? n : (h = head); // Use head if p offlist
588		}
589
590	<	if (how >= ASYNC) {               // No matches available
590	>	if (how != NOW) {                 // No matches available
591		if (s == null)
592		s = new Node(e, haveData);
593		Node pred = tryAppend(s, haveData);
594		if (pred == null)
595		continue retry;           // lost race vs opposite mode
596	<	if (how >= SYNC)
597	<	return awaitMatch(s, pred, e, how, nanos);
596	>	if (how != ASYNC)
597	>	return awaitMatch(s, pred, e, (how == TIMED), nanos);
598		}
599		return e; // not waiting
600		}
#	Line 545 | Line 610 | public class LinkedTransferQueue<E> exte
610		* predecessor
611		*/
612		private Node tryAppend(Node s, boolean haveData) {
613	<	for (Node t = tail, p = t;;) { // move p to last node and append
613	>	for (Node t = tail, p = t;;) {        // move p to last node and append
614		Node n, u;                        // temps for reads of next & tail
615		if (p == null && (p = head) == null) {
616		if (casHead(null, s))
#	Line 578 | Line 643 | public class LinkedTransferQueue<E> exte
643		* predecessor, or null if unknown (the null case does not occur
644		* in any current calls but may in possible future extensions)
645		* @param e the comparison value for checking match
646	<	* @param how either SYNC or TIMEOUT
647	<	* @param nanos timeout value
646	>	* @param timed if true, wait only until timeout elapses
647	>	* @param nanos timeout in nanosecs, used only if timed is true
648		* @return matched item, or e if unmatched on interrupt or timeout
649		*/
650	<	private Object awaitMatch(Node s, Node pred, Object e,
651	<	int how, long nanos) {
587	<	long lastTime = (how == TIMEOUT) ? System.nanoTime() : 0L;
650	>	private E awaitMatch(Node s, Node pred, E e, boolean timed, long nanos) {
651	>	long lastTime = timed ? System.nanoTime() : 0L;
652		Thread w = Thread.currentThread();
653		int spins = -1; // initialized after first item and cancel checks
654		ThreadLocalRandom randomYields = null; // bound if needed
#	Line 592 | Line 656 | public class LinkedTransferQueue<E> exte
656		for (;;) {
657		Object item = s.item;
658		if (item != e) {                  // matched
659	+	assert item != s;
660		s.forgetContents();           // avoid garbage
661	<	return item;
661	>	return this.<E>cast(item);
662		}
663	<	if ((w.isInterrupted() || (how == TIMEOUT && nanos <= 0)) &&
664	<	s.casItem(e, s)) {       // cancel
663	>	if ((w.isInterrupted() || (timed && nanos <= 0)) &&
664	>	s.casItem(e, s)) {        // cancel
665		unsplice(pred, s);
666		return e;
667		}
#	Line 606 | Line 671 | public class LinkedTransferQueue<E> exte
671		randomYields = ThreadLocalRandom.current();
672		}
673		else if (spins > 0) {             // spin
674	<	if (--spins == 0)
675	<	shortenHeadPath();        // reduce slack before blocking
611	<	else if (randomYields.nextInt(CHAINED_SPINS) == 0)
674	>	--spins;
675	>	if (randomYields.nextInt(CHAINED_SPINS) == 0)
676		Thread.yield();           // occasionally yield
677		}
678		else if (s.waiter == null) {
679		s.waiter = w;                 // request unpark then recheck
680		}
681	<	else if (how == TIMEOUT) {
681	>	else if (timed) {
682		long now = System.nanoTime();
683		if ((nanos -= now - lastTime) > 0)
684		LockSupport.parkNanos(this, nanos);
#	Line 622 | Line 686 | public class LinkedTransferQueue<E> exte
686		}
687		else {
688		LockSupport.park(this);
625	–	s.waiter = null;
626	–	spins = -1;                   // spin if front upon wakeup
689		}
690		}
691		}
#	Line 644 | Line 706 | public class LinkedTransferQueue<E> exte
706		return 0;
707		}
708
709	+	/* -------------- Traversal methods -------------- */
710	+
711		/**
712	<	* Tries (once) to unsplice nodes between head and first unmatched
713	<	* or trailing node; failing on contention.
714	<	*/
715	<	private void shortenHeadPath() {
716	<	Node h, hn, p, q;
717	<	if ((p = h = head) != null && h.isMatched() &&
718	<	(q = hn = h.next) != null) {
655	<	Node n;
656	<	while ((n = q.next) != q) {
657	<	if (n == null || !q.isMatched()) {
658	<	if (hn != q && h.next == hn)
659	<	h.casNext(hn, q);
660	<	break;
661	<	}
662	<	p = q;
663	<	q = n;
664	<	}
665	<	}
712	>	* Returns the successor of p, or the head node if p.next has been
713	>	* linked to self, which will only be true if traversing with a
714	>	* stale pointer that is now off the list.
715	>	*/
716	>	final Node succ(Node p) {
717	>	Node next = p.next;
718	>	return (p == next) ? head : next;
719		}
720
668	–	/* -------------- Traversal methods -------------- */
669	–
721		/**
722		* Returns the first unmatched node of the given mode, or null if
723		* none.  Used by methods isEmpty, hasWaitingConsumer.
724		*/
725	<	private Node firstOfMode(boolean data) {
726	<	for (Node p = head; p != null; ) {
725	>	private Node firstOfMode(boolean isData) {
726	>	for (Node p = head; p != null; p = succ(p)) {
727		if (!p.isMatched())
728	<	return (p.isData == data) ? p : null;
678	<	Node n = p.next;
679	<	p = (n != p) ? n : head;
728	>	return (p.isData == isData) ? p : null;
729		}
730		return null;
731		}
732
733		/**
734		* Returns the item in the first unmatched node with isData; or
735	<	* null if none. Used by peek.
735	>	* null if none.  Used by peek.
736		*/
737	<	private Object firstDataItem() {
738	<	for (Node p = head; p != null; ) {
690	<	boolean isData = p.isData;
737	>	private E firstDataItem() {
738	>	for (Node p = head; p != null; p = succ(p)) {
739		Object item = p.item;
740	<	if (item != p && (item != null) == isData)
741	<	return isData ? item : null;
742	<	Node n = p.next;
743	<	p = (n != p) ? n : head;
740	>	if (p.isData) {
741	>	if (item != null && item != p)
742	>	return this.<E>cast(item);
743	>	}
744	>	else if (item == null)
745	>	return null;
746		}
747		return null;
748		}
#	Line 723 | Line 773 | public class LinkedTransferQueue<E> exte
773
774		final class Itr implements Iterator<E> {
775		private Node nextNode;   // next node to return item for
776	<	private Object nextItem; // the corresponding item
776	>	private E nextItem;      // the corresponding item
777		private Node lastRet;    // last returned node, to support remove
778	+	private Node lastPred;   // predecessor to unlink lastRet
779
780		/**
781		* Moves to next node after prev, or first node if prev null.
782		*/
783		private void advance(Node prev) {
784	+	lastPred = lastRet;
785		lastRet = prev;
786	<	Node p;
787	<	if (prev == null || (p = prev.next) == prev)
736	<	p = head;
737	<	while (p != null) {
786	>	for (Node p = (prev == null) ? head : succ(prev);
787	>	p != null; p = succ(p)) {
788		Object item = p.item;
789		if (p.isData) {
790		if (item != null && item != p) {
791	<	nextItem = item;
791	>	nextItem = LinkedTransferQueue.this.<E>cast(item);
792		nextNode = p;
793		return;
794		}
795		}
796		else if (item == null)
797		break;
748	–	Node n = p.next;
749	–	p = (n != p) ? n : head;
798		}
799		nextNode = null;
800		}
#	Line 762 | Line 810 | public class LinkedTransferQueue<E> exte
810		public final E next() {
811		Node p = nextNode;
812		if (p == null) throw new NoSuchElementException();
813	<	Object e = nextItem;
813	>	E e = nextItem;
814		advance(p);
815	<	return (E) e;
815	>	return e;
816		}
817
818		public final void remove() {
819		Node p = lastRet;
820		if (p == null) throw new IllegalStateException();
821	<	lastRet = null;
822	<	findAndRemoveNode(p);
821	>	if (p.tryMatchData())
822	>	unsplice(lastPred, p);
823		}
824		}
825
#	Line 781 | Line 829 | public class LinkedTransferQueue<E> exte
829		* Unsplices (now or later) the given deleted/cancelled node with
830		* the given predecessor.
831		*
832	<	* @param pred predecessor of node to be unspliced
832	>	* @param pred a node that was at one time known to be the
833	>	* predecessor of s, or null or s itself if s is/was at head
834		* @param s the node to be unspliced
835		*/
836	<	private void unsplice(Node pred, Node s) {
837	<	s.forgetContents(); // clear unneeded fields
836	>	final void unsplice(Node pred, Node s) {
837	>	s.forgetContents(); // forget unneeded fields
838		/*
839	<	* At any given time, exactly one node on list cannot be
840	<	* unlinked -- the last inserted node. To accommodate this, if
841	<	* we cannot unlink s, we save its predecessor as "cleanMe",
842	<	* processing the previously saved version first. Because only
843	<	* one node in the list can have a null next, at least one of
795	<	* node s or the node previously saved can always be
796	<	* processed, so this always terminates.
839	>	* See above for rationale. Briefly: if pred still points to
840	>	* s, try to unlink s.  If s cannot be unlinked, because it is
841	>	* trailing node or pred might be unlinked, and neither pred
842	>	* nor s are head or offlist, add to sweepVotes, and if enough
843	>	* votes have accumulated, sweep.
844		*/
845	<	if (pred != null && pred != s) {
846	<	while (pred.next == s) {
847	<	Node oldpred = (cleanMe == null) ? null : reclean();
848	<	Node n = s.next;
849	<	if (n != null) {
850	<	if (n != s)
851	<	pred.casNext(s, n);
852	<	break;
845	>	if (pred != null && pred != s && pred.next == s) {
846	>	Node n = s.next;
847	>	if (n == null ||
848	>	(n != s && pred.casNext(s, n) && pred.isMatched())) {
849	>	for (;;) {               // check if at, or could be, head
850	>	Node h = head;
851	>	if (h == pred || h == s || h == null)
852	>	return;          // at head or list empty
853	>	if (!h.isMatched())
854	>	break;
855	>	Node hn = h.next;
856	>	if (hn == null)
857	>	return;          // now empty
858	>	if (hn != h && casHead(h, hn))
859	>	h.forgetNext();  // advance head
860	>	}
861	>	if (pred.next != pred && s.next != s) { // recheck if offlist
862	>	for (;;) {           // sweep now if enough votes
863	>	int v = sweepVotes;
864	>	if (v < SWEEP_THRESHOLD) {
865	>	if (casSweepVotes(v, v + 1))
866	>	break;
867	>	}
868	>	else if (casSweepVotes(v, 0)) {
869	>	sweep();
870	>	break;
871	>	}
872	>	}
873		}
807	–	if (oldpred == pred ||      // Already saved
808	–	(oldpred == null && casCleanMe(null, pred)))
809	–	break;                  // Postpone cleaning
874		}
875		}
876		}
877
878		/**
879	<	* Tries to unsplice the deleted/cancelled node held in cleanMe
816	<	* that was previously uncleanable because it was at tail.
817	<	*
818	<	* @return current cleanMe node (or null)
879	>	* Unlinks matched nodes encountered in a traversal from head.
880		*/
881	<	private Node reclean() {
882	<	/*
883	<	* cleanMe is, or at one time was, predecessor of a cancelled
884	<	* node s that was the tail so could not be unspliced.  If it
885	<	* is no longer the tail, try to unsplice if necessary and
886	<	* make cleanMe slot available.  This differs from similar
887	<	* code in unsplice() because we must check that pred still
827	<	* points to a matched node that can be unspliced -- if not,
828	<	* we can (must) clear cleanMe without unsplicing.  This can
829	<	* loop only due to contention.
830	<	*/
831	<	Node pred;
832	<	while ((pred = cleanMe) != null) {
833	<	Node s = pred.next;
834	<	Node n;
835	<	if (s == null || s == pred || !s.isMatched())
836	<	casCleanMe(pred, null); // already gone
837	<	else if ((n = s.next) != null) {
838	<	if (n != s)
839	<	pred.casNext(s, n);
840	<	casCleanMe(pred, null);
841	<	}
881	>	private void sweep() {
882	>	Node p = head, s, n;
883	>	while (p != null && (s = p.next) != null && (n = s.next) != null) {
884	>	if (p == s || s == n)
885	>	p = head; // stale
886	>	else if (s.isMatched())
887	>	p.casNext(s, n);
888		else
889	<	break;
844	<	}
845	<	return pred;
846	<	}
847	<
848	<	/**
849	<	* Main implementation of Iterator.remove(). Find
850	<	* and unsplice the given node.
851	<	*/
852	<	final void findAndRemoveNode(Node s) {
853	<	if (s.tryMatchData()) {
854	<	Node pred = null;
855	<	Node p = head;
856	<	while (p != null) {
857	<	if (p == s) {
858	<	unsplice(pred, p);
859	<	break;
860	<	}
861	<	if (!p.isData && !p.isMatched())
862	<	break;
863	<	pred = p;
864	<	if ((p = p.next) == pred) { // stale
865	<	pred = null;
866	<	p = head;
867	<	}
868	<	}
889	>	p = s;
890		}
891		}
892
#	Line 874 | Line 895 | public class LinkedTransferQueue<E> exte
895		*/
896		private boolean findAndRemove(Object e) {
897		if (e != null) {
898	<	Node pred = null;
878	<	Node p = head;
879	<	while (p != null) {
898	>	for (Node pred = null, p = head; p != null; ) {
899		Object item = p.item;
900		if (p.isData) {
901		if (item != null && item != p && e.equals(item) &&
#	Line 888 | Line 907 | public class LinkedTransferQueue<E> exte
907		else if (item == null)
908		break;
909		pred = p;
910	<	if ((p = p.next) == pred) {
910	>	if ((p = p.next) == pred) { // stale
911		pred = null;
912		p = head;
913		}
#	Line 1016 | Line 1035 | public class LinkedTransferQueue<E> exte
1035		*/
1036		public boolean tryTransfer(E e, long timeout, TimeUnit unit)
1037		throws InterruptedException {
1038	<	if (xfer(e, true, TIMEOUT, unit.toNanos(timeout)) == null)
1038	>	if (xfer(e, true, TIMED, unit.toNanos(timeout)) == null)
1039		return true;
1040		if (!Thread.interrupted())
1041		return false;
#	Line 1024 | Line 1043 | public class LinkedTransferQueue<E> exte
1043		}
1044
1045		public E take() throws InterruptedException {
1046	<	Object e = xfer(null, false, SYNC, 0);
1046	>	E e = xfer(null, false, SYNC, 0);
1047		if (e != null)
1048	<	return (E)e;
1048	>	return e;
1049		Thread.interrupted();
1050		throw new InterruptedException();
1051		}
1052
1053		public E poll(long timeout, TimeUnit unit) throws InterruptedException {
1054	<	Object e = xfer(null, false, TIMEOUT, unit.toNanos(timeout));
1054	>	E e = xfer(null, false, TIMED, unit.toNanos(timeout));
1055		if (e != null || !Thread.interrupted())
1056	<	return (E)e;
1056	>	return e;
1057		throw new InterruptedException();
1058		}
1059
1060		public E poll() {
1061	<	return (E)xfer(null, false, NOW, 0);
1061	>	return xfer(null, false, NOW, 0);
1062		}
1063
1064		/**
#	Line 1096 | Line 1115 | public class LinkedTransferQueue<E> exte
1115		}
1116
1117		public E peek() {
1118	<	return (E) firstDataItem();
1118	>	return firstDataItem();
1119		}
1120
1121		/**
#	Line 1192 | Line 1211 | public class LinkedTransferQueue<E> exte
1211		}
1212		}
1213
1195	–
1214		// Unsafe mechanics
1215
1216		private static final sun.misc.Unsafe UNSAFE = getUnsafe();
#	Line 1200 | Line 1218 | public class LinkedTransferQueue<E> exte
1218		objectFieldOffset(UNSAFE, "head", LinkedTransferQueue.class);
1219		private static final long tailOffset =
1220		objectFieldOffset(UNSAFE, "tail", LinkedTransferQueue.class);
1221	<	private static final long cleanMeOffset =
1222	<	objectFieldOffset(UNSAFE, "cleanMe", LinkedTransferQueue.class);
1221	>	private static final long sweepVotesOffset =
1222	>	objectFieldOffset(UNSAFE, "sweepVotes", LinkedTransferQueue.class);
1223
1224		static long objectFieldOffset(sun.misc.Unsafe UNSAFE,
1225		String field, Class<?> klazz) {
#	Line 1215 | Line 1233 | public class LinkedTransferQueue<E> exte
1233		}
1234		}
1235
1236	<	private static sun.misc.Unsafe getUnsafe() {
1236	>	/**
1237	>	* Returns a sun.misc.Unsafe.  Suitable for use in a 3rd party package.
1238	>	* Replace with a simple call to Unsafe.getUnsafe when integrating
1239	>	* into a jdk.
1240	>	*
1241	>	* @return a sun.misc.Unsafe
1242	>	*/
1243	>	static sun.misc.Unsafe getUnsafe() {
1244		try {
1245		return sun.misc.Unsafe.getUnsafe();
1246		} catch (SecurityException se) {

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