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

Comparing jsr166/src/jsr166y/LinkedTransferQueue.java (file contents):
Revision 1.53 by jsr166, Tue Oct 27 19:59:43 2009 UTC vs.
Revision 1.94 by jsr166, Sat Oct 3 18:17:51 2015 UTC

#	Line 1 | Line 1
1		/*
2		* Written by Doug Lea with assistance from members of JCP JSR-166
3		* Expert Group and released to the public domain, as explained at
4	<	* http://creativecommons.org/licenses/publicdomain
4	>	* http://creativecommons.org/publicdomain/zero/1.0/
5		*/
6
7		package jsr166y;
8
9	–	import java.util.concurrent.*;
10	–
9		import java.util.AbstractQueue;
10		import java.util.Collection;
13	–	import java.util.ConcurrentModificationException;
11		import java.util.Iterator;
12		import java.util.NoSuchElementException;
13		import java.util.Queue;
14	+	import java.util.concurrent.TimeUnit;
15		import java.util.concurrent.locks.LockSupport;
16	+
17		/**
18		* An unbounded {@link TransferQueue} based on linked nodes.
19		* This queue orders elements FIFO (first-in-first-out) with respect
#	Line 23 | Line 22 | import java.util.concurrent.locks.LockSu
22		* producer.  The <em>tail</em> of the queue is that element that has
23		* been on the queue the shortest time for some producer.
24		*
25	<	* <p>Beware that, unlike in most collections, the {@code size}
26	<	* method is <em>NOT</em> a constant-time operation. Because of the
25	>	* <p>Beware that, unlike in most collections, the {@code size} method
26	>	* is <em>NOT</em> a constant-time operation. Because of the
27		* asynchronous nature of these queues, determining the current number
28	<	* of elements requires a traversal of the elements.
28	>	* of elements requires a traversal of the elements, and so may report
29	>	* inaccurate results if this collection is modified during traversal.
30	>	* Additionally, the bulk operations {@code addAll},
31	>	* {@code removeAll}, {@code retainAll}, {@code containsAll},
32	>	* {@code equals}, and {@code toArray} are <em>not</em> guaranteed
33	>	* to be performed atomically. For example, an iterator operating
34	>	* concurrently with an {@code addAll} operation might view only some
35	>	* of the added elements.
36		*
37		* <p>This class and its iterator implement all of the
38		* <em>optional</em> methods of the {@link Collection} and {@link
#	Line 70 | Line 76 | public class LinkedTransferQueue<E> exte
76		*
77		* A FIFO dual queue may be implemented using a variation of the
78		* Michael & Scott (M&S) lock-free queue algorithm
79	<	* (http://www.cs.rochester.edu/u/scott/papers/1996_PODC_queues.pdf).
79	>	* (http://www.cs.rochester.edu/~scott/papers/1996_PODC_queues.pdf).
80		* It maintains two pointer fields, "head", pointing to a
81		* (matched) node that in turn points to the first actual
82		* (unmatched) queue node (or null if empty); and "tail" that
#	Line 206 | Line 212 | public class LinkedTransferQueue<E> exte
212		* additional GC bookkeeping ("write barriers") that are sometimes
213		* more costly than the writes themselves because of contention).
214		*
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	–	*
215		* *** Overview of implementation ***
216		*
217		* We use a threshold-based approach to updates, with a slack
#	Line 239 | Line 227 | public class LinkedTransferQueue<E> exte
227		* per-thread one available, but even ThreadLocalRandom is too
228		* heavy for these purposes.
229		*
230	<	* With such a small slack threshold value, it is rarely
231	<	* worthwhile to augment this with path short-circuiting; i.e.,
232	<	* unsplicing nodes between head and the first unmatched node, or
233	<	* 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).
230	>	* With such a small slack threshold value, it is not worthwhile
231	>	* to augment this with path short-circuiting (i.e., unsplicing
232	>	* interior nodes) except in the case of cancellation/removal (see
233	>	* below).
234		*
235		* We allow both the head and tail fields to be null before any
236		* nodes are enqueued; initializing upon first append.  This
#	Line 318 | Line 301 | public class LinkedTransferQueue<E> exte
301		*    of less-contended queues.  During spins threads check their
302		*    interrupt status and generate a thread-local random number
303		*    to decide to occasionally perform a Thread.yield. While
304	<	*    yield has underdefined specs, we assume that might it help,
305	<	*    and will not hurt in limiting impact of spinning on busy
304	>	*    yield has underdefined specs, we assume that it might help,
305	>	*    and will not hurt, in limiting impact of spinning on busy
306		*    systems.  We also use smaller (1/2) spins for nodes that are
307		*    not known to be front but whose predecessors have not
308		*    blocked -- these "chained" spins avoid artifacts of
#	Line 329 | Line 312 | public class LinkedTransferQueue<E> exte
312		*    versa) compared to their predecessors receive additional
313		*    chained spins, reflecting longer paths typically required to
314		*    unblock threads during phase changes.
315	+	*
316	+	*
317	+	* ** Unlinking removed interior nodes **
318	+	*
319	+	* In addition to minimizing garbage retention via self-linking
320	+	* described above, we also unlink removed interior nodes. These
321	+	* may arise due to timed out or interrupted waits, or calls to
322	+	* remove(x) or Iterator.remove.  Normally, given a node that was
323	+	* at one time known to be the predecessor of some node s that is
324	+	* to be removed, we can unsplice s by CASing the next field of
325	+	* its predecessor if it still points to s (otherwise s must
326	+	* already have been removed or is now offlist). But there are two
327	+	* situations in which we cannot guarantee to make node s
328	+	* unreachable in this way: (1) If s is the trailing node of list
329	+	* (i.e., with null next), then it is pinned as the target node
330	+	* for appends, so can only be removed later after other nodes are
331	+	* appended. (2) We cannot necessarily unlink s given a
332	+	* predecessor node that is matched (including the case of being
333	+	* cancelled): the predecessor may already be unspliced, in which
334	+	* case some previous reachable node may still point to s.
335	+	* (For further explanation see Herlihy & Shavit "The Art of
336	+	* Multiprocessor Programming" chapter 9).  Although, in both
337	+	* cases, we can rule out the need for further action if either s
338	+	* or its predecessor are (or can be made to be) at, or fall off
339	+	* from, the head of list.
340	+	*
341	+	* Without taking these into account, it would be possible for an
342	+	* unbounded number of supposedly removed nodes to remain
343	+	* reachable.  Situations leading to such buildup are uncommon but
344	+	* can occur in practice; for example when a series of short timed
345	+	* calls to poll repeatedly time out but never otherwise fall off
346	+	* the list because of an untimed call to take at the front of the
347	+	* queue.
348	+	*
349	+	* When these cases arise, rather than always retraversing the
350	+	* entire list to find an actual predecessor to unlink (which
351	+	* won't help for case (1) anyway), we record a conservative
352	+	* estimate of possible unsplice failures (in "sweepVotes").
353	+	* We trigger a full sweep when the estimate exceeds a threshold
354	+	* ("SWEEP_THRESHOLD") indicating the maximum number of estimated
355	+	* removal failures to tolerate before sweeping through, unlinking
356	+	* cancelled nodes that were not unlinked upon initial removal.
357	+	* We perform sweeps by the thread hitting threshold (rather than
358	+	* background threads or by spreading work to other threads)
359	+	* because in the main contexts in which removal occurs, the
360	+	* caller is already timed-out, cancelled, or performing a
361	+	* potentially O(n) operation (e.g. remove(x)), none of which are
362	+	* time-critical enough to warrant the overhead that alternatives
363	+	* would impose on other threads.
364	+	*
365	+	* Because the sweepVotes estimate is conservative, and because
366	+	* nodes become unlinked "naturally" as they fall off the head of
367	+	* the queue, and because we allow votes to accumulate even while
368	+	* sweeps are in progress, there are typically significantly fewer
369	+	* such nodes than estimated.  Choice of a threshold value
370	+	* balances the likelihood of wasted effort and contention, versus
371	+	* providing a worst-case bound on retention of interior nodes in
372	+	* quiescent queues. The value defined below was chosen
373	+	* empirically to balance these under various timeout scenarios.
374	+	*
375	+	* Note that we cannot self-link unlinked interior nodes during
376	+	* sweeps. However, the associated garbage chains terminate when
377	+	* some successor ultimately falls off the head of the list and is
378	+	* self-linked.
379		*/
380
381		/** True if on multiprocessor */
#	Line 355 | Line 402 | public class LinkedTransferQueue<E> exte
402		private static final int CHAINED_SPINS = FRONT_SPINS >>> 1;
403
404		/**
405	+	* The maximum number of estimated removal failures (sweepVotes)
406	+	* to tolerate before sweeping through the queue unlinking
407	+	* cancelled nodes that were not unlinked upon initial
408	+	* removal. See above for explanation. The value must be at least
409	+	* two to avoid useless sweeps when removing trailing nodes.
410	+	*/
411	+	static final int SWEEP_THRESHOLD = 32;
412	+
413	+	/**
414		* Queue nodes. Uses Object, not E, for items to allow forgetting
415		* them after use.  Relies heavily on Unsafe mechanics to minimize
416	<	* unnecessary ordering constraints: Writes that intrinsically
417	<	* precede or follow CASes use simple relaxed forms.  Other
362	<	* cleanups use releasing/lazy writes.
416	>	* unnecessary ordering constraints: Writes that are intrinsically
417	>	* ordered wrt other accesses or CASes use simple relaxed forms.
418		*/
419		static final class Node {
420		final boolean isData;   // false if this is a request node
#	Line 373 | Line 428 | public class LinkedTransferQueue<E> exte
428		}
429
430		final boolean casItem(Object cmp, Object val) {
431	+	// assert cmp == null || cmp.getClass() != Node.class;
432		return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
433		}
434
435		/**
436	<	* Creates a new node. Uses relaxed write because item can only
437	<	* be seen if followed by CAS.
436	>	* Constructs a new node.  Uses relaxed write because item can
437	>	* only be seen after publication via casNext.
438		*/
439		Node(Object item, boolean isData) {
440		UNSAFE.putObject(this, itemOffset, item); // relaxed write
#	Line 394 | Line 450 | public class LinkedTransferQueue<E> exte
450		}
451
452		/**
453	<	* Sets item to self (using a releasing/lazy write) and waiter
454	<	* to null, to avoid garbage retention after extracting or
455	<	* cancelling.
453	>	* Sets item to self and waiter to null, to avoid garbage
454	>	* retention after matching or cancelling. Uses relaxed writes
455	>	* because order is already constrained in the only calling
456	>	* contexts: item is forgotten only after volatile/atomic
457	>	* mechanics that extract items.  Similarly, clearing waiter
458	>	* follows either CAS or return from park (if ever parked;
459	>	* else we don't care).
460		*/
461		final void forgetContents() {
462	<	UNSAFE.putOrderedObject(this, itemOffset, this);
463	<	UNSAFE.putOrderedObject(this, waiterOffset, null);
462	>	UNSAFE.putObject(this, itemOffset, this);
463	>	UNSAFE.putObject(this, waiterOffset, null);
464		}
465
466		/**
#	Line 409 | Line 469 | public class LinkedTransferQueue<E> exte
469		*/
470		final boolean isMatched() {
471		Object x = item;
472	<	return x == this || (x != null) != isData;
472	>	return (x == this) || ((x == null) == isData);
473	>	}
474	>
475	>	/**
476	>	* Returns true if this is an unmatched request node.
477	>	*/
478	>	final boolean isUnmatchedRequest() {
479	>	return !isData && item == null;
480		}
481
482		/**
#	Line 427 | Line 494 | public class LinkedTransferQueue<E> exte
494		* Tries to artificially match a data node -- used by remove.
495		*/
496		final boolean tryMatchData() {
497	+	// assert isData;
498		Object x = item;
499		if (x != null && x != this && casItem(x, null)) {
500		LockSupport.unpark(waiter);
#	Line 435 | Line 503 | public class LinkedTransferQueue<E> exte
503		return false;
504		}
505
438	–	// Unsafe mechanics
439	–	private static final sun.misc.Unsafe UNSAFE = getUnsafe();
440	–	private static final long nextOffset =
441	–	objectFieldOffset(UNSAFE, "next", Node.class);
442	–	private static final long itemOffset =
443	–	objectFieldOffset(UNSAFE, "item", Node.class);
444	–	private static final long waiterOffset =
445	–	objectFieldOffset(UNSAFE, "waiter", Node.class);
446	–
506		private static final long serialVersionUID = -3375979862319811754L;
507	+
508	+	// Unsafe mechanics
509	+	private static final sun.misc.Unsafe UNSAFE;
510	+	private static final long itemOffset;
511	+	private static final long nextOffset;
512	+	private static final long waiterOffset;
513	+	static {
514	+	try {
515	+	UNSAFE = getUnsafe();
516	+	Class<?> k = Node.class;
517	+	itemOffset = UNSAFE.objectFieldOffset
518	+	(k.getDeclaredField("item"));
519	+	nextOffset = UNSAFE.objectFieldOffset
520	+	(k.getDeclaredField("next"));
521	+	waiterOffset = UNSAFE.objectFieldOffset
522	+	(k.getDeclaredField("waiter"));
523	+	} catch (Exception e) {
524	+	throw new Error(e);
525	+	}
526	+	}
527		}
528
529		/** head of the queue; null until first enqueue */
530	<	private transient volatile Node head;
452	<
453	<	/** predecessor of dangling unspliceable node */
454	<	private transient volatile Node cleanMe; // decl here to reduce contention
530	>	transient volatile Node head;
531
532		/** tail of the queue; null until first append */
533		private transient volatile Node tail;
534
535	+	/** The number of apparent failures to unsplice removed nodes */
536	+	private transient volatile int sweepVotes;
537	+
538		// CAS methods for fields
539		private boolean casTail(Node cmp, Node val) {
540		return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val);
#	Line 465 | Line 544 | public class LinkedTransferQueue<E> exte
544		return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val);
545		}
546
547	<	private boolean casCleanMe(Node cmp, Node val) {
548	<	return UNSAFE.compareAndSwapObject(this, cleanMeOffset, cmp, val);
547	>	private boolean casSweepVotes(int cmp, int val) {
548	>	return UNSAFE.compareAndSwapInt(this, sweepVotesOffset, cmp, val);
549		}
550
551		/*
552	<	* Possible values for "how" argument in xfer method. Beware that
474	<	* the order of assigned numerical values matters.
552	>	* Possible values for "how" argument in xfer method.
553		*/
554	<	private static final int NOW     = 0; // for untimed poll, tryTransfer
555	<	private static final int ASYNC   = 1; // for offer, put, add
556	<	private static final int SYNC    = 2; // for transfer, take
557	<	private static final int TIMEOUT = 3; // for timed poll, tryTransfer
554	>	private static final int NOW   = 0; // for untimed poll, tryTransfer
555	>	private static final int ASYNC = 1; // for offer, put, add
556	>	private static final int SYNC  = 2; // for transfer, take
557	>	private static final int TIMED = 3; // for timed poll, tryTransfer
558	>
559	>	@SuppressWarnings("unchecked")
560	>	static <E> E cast(Object item) {
561	>	// assert item == null || item.getClass() != Node.class;
562	>	return (E) item;
563	>	}
564
565		/**
566		* Implements all queuing methods. See above for explanation.
567		*
568		* @param e the item or null for take
569		* @param haveData true if this is a put, else a take
570	<	* @param how NOW, ASYNC, SYNC, or TIMEOUT
571	<	* @param nanos timeout in nanosecs, used only if mode is TIMEOUT
570	>	* @param how NOW, ASYNC, SYNC, or TIMED
571	>	* @param nanos timeout in nanosecs, used only if mode is TIMED
572		* @return an item if matched, else e
573		* @throws NullPointerException if haveData mode but e is null
574		*/
575	<	private Object xfer(Object e, boolean haveData, int how, long nanos) {
575	>	private E xfer(E e, boolean haveData, int how, long nanos) {
576		if (haveData && (e == null))
577		throw new NullPointerException();
578		Node s = null;                        // the node to append, if needed
579
580	<	retry: for (;;) {                     // restart on append race
580	>	retry:
581	>	for (;;) {                            // restart on append race
582
583		for (Node h = head, p = h; p != null;) { // find & match first node
584		boolean isData = p.isData;
#	Line 503 | Line 588 | public class LinkedTransferQueue<E> exte
588		break;
589		if (p.casItem(item, e)) { // match
590		for (Node q = p; q != h;) {
591	<	Node n = q.next;  // update head by 2
592	<	if (n != null)    // unless singleton
508	<	q = n;
509	<	if (head == h && casHead(h, q)) {
591	>	Node n = q.next;  // update by 2 unless singleton
592	>	if (head == h && casHead(h, n == null ? q : n)) {
593		h.forgetNext();
594		break;
595		}                 // advance and retry
#	Line 515 | Line 598 | public class LinkedTransferQueue<E> exte
598		break;        // unless slack < 2
599		}
600		LockSupport.unpark(p.waiter);
601	<	return item;
601	>	return LinkedTransferQueue.<E>cast(item);
602		}
603		}
604		Node n = p.next;
605		p = (p != n) ? n : (h = head); // Use head if p offlist
606		}
607
608	<	if (how >= ASYNC) {               // No matches available
608	>	if (how != NOW) {                 // No matches available
609		if (s == null)
610		s = new Node(e, haveData);
611		Node pred = tryAppend(s, haveData);
612		if (pred == null)
613		continue retry;           // lost race vs opposite mode
614	<	if (how >= SYNC)
615	<	return awaitMatch(s, pred, e, how, nanos);
614	>	if (how != ASYNC)
615	>	return awaitMatch(s, pred, e, (how == TIMED), nanos);
616		}
617		return e; // not waiting
618		}
#	Line 545 | Line 628 | public class LinkedTransferQueue<E> exte
628		* predecessor
629		*/
630		private Node tryAppend(Node s, boolean haveData) {
631	<	for (Node t = tail, p = t;;) { // move p to last node and append
631	>	for (Node t = tail, p = t;;) {        // move p to last node and append
632		Node n, u;                        // temps for reads of next & tail
633		if (p == null && (p = head) == null) {
634		if (casHead(null, s))
#	Line 578 | Line 661 | public class LinkedTransferQueue<E> exte
661		* predecessor, or null if unknown (the null case does not occur
662		* in any current calls but may in possible future extensions)
663		* @param e the comparison value for checking match
664	<	* @param how either SYNC or TIMEOUT
665	<	* @param nanos timeout value
664	>	* @param timed if true, wait only until timeout elapses
665	>	* @param nanos timeout in nanosecs, used only if timed is true
666		* @return matched item, or e if unmatched on interrupt or timeout
667		*/
668	<	private Object awaitMatch(Node s, Node pred, Object e,
669	<	int how, long nanos) {
587	<	long lastTime = (how == TIMEOUT) ? System.nanoTime() : 0L;
668	>	private E awaitMatch(Node s, Node pred, E e, boolean timed, long nanos) {
669	>	long lastTime = timed ? System.nanoTime() : 0L;
670		Thread w = Thread.currentThread();
671		int spins = -1; // initialized after first item and cancel checks
672		ThreadLocalRandom randomYields = null; // bound if needed
#	Line 592 | Line 674 | public class LinkedTransferQueue<E> exte
674		for (;;) {
675		Object item = s.item;
676		if (item != e) {                  // matched
677	+	// assert item != s;
678		s.forgetContents();           // avoid garbage
679	<	return item;
679	>	return LinkedTransferQueue.<E>cast(item);
680		}
681	<	if ((w.isInterrupted() || (how == TIMEOUT && nanos <= 0)) &&
682	<	s.casItem(e, s)) {       // cancel
681	>	if ((w.isInterrupted() || (timed && nanos <= 0L)) &&
682	>	s.casItem(e, s)) {        // cancel
683		unsplice(pred, s);
684		return e;
685		}
#	Line 606 | Line 689 | public class LinkedTransferQueue<E> exte
689		randomYields = ThreadLocalRandom.current();
690		}
691		else if (spins > 0) {             // spin
692	<	if (--spins == 0)
693	<	shortenHeadPath();        // reduce slack before blocking
611	<	else if (randomYields.nextInt(CHAINED_SPINS) == 0)
692	>	--spins;
693	>	if (randomYields.nextInt(CHAINED_SPINS) == 0)
694		Thread.yield();           // occasionally yield
695		}
696		else if (s.waiter == null) {
697		s.waiter = w;                 // request unpark then recheck
698		}
699	<	else if (how == TIMEOUT) {
699	>	else if (timed) {
700		long now = System.nanoTime();
701		if ((nanos -= now - lastTime) > 0)
702		LockSupport.parkNanos(this, nanos);
#	Line 622 | Line 704 | public class LinkedTransferQueue<E> exte
704		}
705		else {
706		LockSupport.park(this);
625	–	s.waiter = null;
626	–	spins = -1;                   // spin if front upon wakeup
707		}
708		}
709		}
#	Line 644 | Line 724 | public class LinkedTransferQueue<E> exte
724		return 0;
725		}
726
727	+	/* -------------- Traversal methods -------------- */
728	+
729		/**
730	<	* Tries (once) to unsplice nodes between head and first unmatched
731	<	* or trailing node; failing on contention.
732	<	*/
733	<	private void shortenHeadPath() {
734	<	Node h, hn, p, q;
735	<	if ((p = h = head) != null && h.isMatched() &&
736	<	(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	<	}
730	>	* Returns the successor of p, or the head node if p.next has been
731	>	* linked to self, which will only be true if traversing with a
732	>	* stale pointer that is now off the list.
733	>	*/
734	>	final Node succ(Node p) {
735	>	Node next = p.next;
736	>	return (p == next) ? head : next;
737		}
738
668	–	/* -------------- Traversal methods -------------- */
669	–
739		/**
740		* Returns the first unmatched node of the given mode, or null if
741		* none.  Used by methods isEmpty, hasWaitingConsumer.
742		*/
743	<	private Node firstOfMode(boolean data) {
744	<	for (Node p = head; p != null; ) {
743	>	private Node firstOfMode(boolean isData) {
744	>	for (Node p = head; p != null; p = succ(p)) {
745		if (!p.isMatched())
746	<	return (p.isData == data) ? p : null;
678	<	Node n = p.next;
679	<	p = (n != p) ? n : head;
746	>	return (p.isData == isData) ? p : null;
747		}
748		return null;
749		}
750
751		/**
752		* Returns the item in the first unmatched node with isData; or
753	<	* null if none. Used by peek.
753	>	* null if none.  Used by peek.
754		*/
755	<	private Object firstDataItem() {
756	<	for (Node p = head; p != null; ) {
690	<	boolean isData = p.isData;
755	>	private E firstDataItem() {
756	>	for (Node p = head; p != null; p = succ(p)) {
757		Object item = p.item;
758	<	if (item != p && (item != null) == isData)
759	<	return isData ? item : null;
760	<	Node n = p.next;
761	<	p = (n != p) ? n : head;
758	>	if (p.isData) {
759	>	if (item != null && item != p)
760	>	return LinkedTransferQueue.<E>cast(item);
761	>	}
762	>	else if (item == null)
763	>	return null;
764		}
765		return null;
766		}
#	Line 723 | Line 791 | public class LinkedTransferQueue<E> exte
791
792		final class Itr implements Iterator<E> {
793		private Node nextNode;   // next node to return item for
794	<	private Object nextItem; // the corresponding item
794	>	private E nextItem;      // the corresponding item
795		private Node lastRet;    // last returned node, to support remove
796	+	private Node lastPred;   // predecessor to unlink lastRet
797
798		/**
799		* Moves to next node after prev, or first node if prev null.
800		*/
801		private void advance(Node prev) {
802	<	lastRet = prev;
803	<	Node p;
804	<	if (prev == null || (p = prev.next) == prev)
805	<	p = head;
806	<	while (p != null) {
807	<	Object item = p.item;
808	<	if (p.isData) {
809	<	if (item != null && item != p) {
810	<	nextItem = item;
811	<	nextNode = p;
802	>	/*
803	>	* To track and avoid buildup of deleted nodes in the face
804	>	* of calls to both Queue.remove and Itr.remove, we must
805	>	* include variants of unsplice and sweep upon each
806	>	* advance: Upon Itr.remove, we may need to catch up links
807	>	* from lastPred, and upon other removes, we might need to
808	>	* skip ahead from stale nodes and unsplice deleted ones
809	>	* found while advancing.
810	>	*/
811	>
812	>	Node r, b; // reset lastPred upon possible deletion of lastRet
813	>	if ((r = lastRet) != null && !r.isMatched())
814	>	lastPred = r;    // next lastPred is old lastRet
815	>	else if ((b = lastPred) == null || b.isMatched())
816	>	lastPred = null; // at start of list
817	>	else {
818	>	Node s, n;       // help with removal of lastPred.next
819	>	while ((s = b.next) != null &&
820	>	s != b && s.isMatched() &&
821	>	(n = s.next) != null && n != s)
822	>	b.casNext(s, n);
823	>	}
824	>
825	>	this.lastRet = prev;
826	>
827	>	for (Node p = prev, s, n;;) {
828	>	s = (p == null) ? head : p.next;
829	>	if (s == null)
830	>	break;
831	>	else if (s == p) {
832	>	p = null;
833	>	continue;
834	>	}
835	>	Object item = s.item;
836	>	if (s.isData) {
837	>	if (item != null && item != s) {
838	>	nextItem = LinkedTransferQueue.<E>cast(item);
839	>	nextNode = s;
840		return;
841		}
842		}
843		else if (item == null)
844		break;
845	<	Node n = p.next;
846	<	p = (n != p) ? n : head;
845	>	// assert s.isMatched();
846	>	if (p == null)
847	>	p = s;
848	>	else if ((n = s.next) == null)
849	>	break;
850	>	else if (s == n)
851	>	p = null;
852	>	else
853	>	p.casNext(s, n);
854		}
855		nextNode = null;
856	+	nextItem = null;
857		}
858
859		Itr() {
#	Line 762 | Line 867 | public class LinkedTransferQueue<E> exte
867		public final E next() {
868		Node p = nextNode;
869		if (p == null) throw new NoSuchElementException();
870	<	Object e = nextItem;
870	>	E e = nextItem;
871		advance(p);
872	<	return (E) e;
872	>	return e;
873		}
874
875		public final void remove() {
876	<	Node p = lastRet;
877	<	if (p == null) throw new IllegalStateException();
878	<	lastRet = null;
879	<	findAndRemoveNode(p);
876	>	final Node lastRet = this.lastRet;
877	>	if (lastRet == null)
878	>	throw new IllegalStateException();
879	>	this.lastRet = null;
880	>	if (lastRet.tryMatchData())
881	>	unsplice(lastPred, lastRet);
882		}
883		}
884
#	Line 781 | Line 888 | public class LinkedTransferQueue<E> exte
888		* Unsplices (now or later) the given deleted/cancelled node with
889		* the given predecessor.
890		*
891	<	* @param pred predecessor of node to be unspliced
891	>	* @param pred a node that was at one time known to be the
892	>	* predecessor of s, or null or s itself if s is/was at head
893		* @param s the node to be unspliced
894		*/
895	<	private void unsplice(Node pred, Node s) {
896	<	s.forgetContents(); // clear unneeded fields
895	>	final void unsplice(Node pred, Node s) {
896	>	s.forgetContents(); // forget unneeded fields
897		/*
898	<	* At any given time, exactly one node on list cannot be
899	<	* unlinked -- the last inserted node. To accommodate this, if
900	<	* we cannot unlink s, we save its predecessor as "cleanMe",
901	<	* processing the previously saved version first. Because only
902	<	* 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.
898	>	* See above for rationale. Briefly: if pred still points to
899	>	* s, try to unlink s.  If s cannot be unlinked, because it is
900	>	* trailing node or pred might be unlinked, and neither pred
901	>	* nor s are head or offlist, add to sweepVotes, and if enough
902	>	* votes have accumulated, sweep.
903		*/
904	<	if (pred != null && pred != s) {
905	<	while (pred.next == s) {
906	<	Node oldpred = (cleanMe == null) ? null : reclean();
907	<	Node n = s.next;
908	<	if (n != null) {
909	<	if (n != s)
910	<	pred.casNext(s, n);
911	<	break;
904	>	if (pred != null && pred != s && pred.next == s) {
905	>	Node n = s.next;
906	>	if (n == null ||
907	>	(n != s && pred.casNext(s, n) && pred.isMatched())) {
908	>	for (;;) {               // check if at, or could be, head
909	>	Node h = head;
910	>	if (h == pred || h == s || h == null)
911	>	return;          // at head or list empty
912	>	if (!h.isMatched())
913	>	break;
914	>	Node hn = h.next;
915	>	if (hn == null)
916	>	return;          // now empty
917	>	if (hn != h && casHead(h, hn))
918	>	h.forgetNext();  // advance head
919	>	}
920	>	if (pred.next != pred && s.next != s) { // recheck if offlist
921	>	for (;;) {           // sweep now if enough votes
922	>	int v = sweepVotes;
923	>	if (v < SWEEP_THRESHOLD) {
924	>	if (casSweepVotes(v, v + 1))
925	>	break;
926	>	}
927	>	else if (casSweepVotes(v, 0)) {
928	>	sweep();
929	>	break;
930	>	}
931	>	}
932		}
807	–	if (oldpred == pred ||      // Already saved
808	–	(oldpred == null && casCleanMe(null, pred)))
809	–	break;                  // Postpone cleaning
933		}
934		}
935		}
936
937		/**
938	<	* Tries to unsplice the deleted/cancelled node held in cleanMe
939	<	* that was previously uncleanable because it was at tail.
817	<	*
818	<	* @return current cleanMe node (or null)
938	>	* Unlinks matched (typically cancelled) nodes encountered in a
939	>	* traversal from head.
940		*/
941	<	private Node reclean() {
942	<	/*
943	<	* cleanMe is, or at one time was, predecessor of a cancelled
944	<	* node s that was the tail so could not be unspliced.  If it
945	<	* is no longer the tail, try to unsplice if necessary and
946	<	* make cleanMe slot available.  This differs from similar
826	<	* 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	<	}
842	<	else
941	>	private void sweep() {
942	>	for (Node p = head, s, n; p != null && (s = p.next) != null; ) {
943	>	if (!s.isMatched())
944	>	// Unmatched nodes are never self-linked
945	>	p = s;
946	>	else if ((n = s.next) == null) // trailing node is pinned
947		break;
948	<	}
949	<	return pred;
950	<	}
951	<
952	<	/**
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	<	}
948	>	else if (s == n)    // stale
949	>	// No need to also check for p == s, since that implies s == n
950	>	p = head;
951	>	else
952	>	p.casNext(s, n);
953		}
954		}
955
#	Line 874 | Line 958 | public class LinkedTransferQueue<E> exte
958		*/
959		private boolean findAndRemove(Object e) {
960		if (e != null) {
961	<	Node pred = null;
878	<	Node p = head;
879	<	while (p != null) {
961	>	for (Node pred = null, p = head; p != null; ) {
962		Object item = p.item;
963		if (p.isData) {
964		if (item != null && item != p && e.equals(item) &&
#	Line 888 | Line 970 | public class LinkedTransferQueue<E> exte
970		else if (item == null)
971		break;
972		pred = p;
973	<	if ((p = p.next) == pred) {
973	>	if ((p = p.next) == pred) { // stale
974		pred = null;
975		p = head;
976		}
#	Line 934 | Line 1016 | public class LinkedTransferQueue<E> exte
1016		* return {@code false}.
1017		*
1018		* @return {@code true} (as specified by
1019	<	*  {@link BlockingQueue#offer(Object,long,TimeUnit) BlockingQueue.offer})
1019	>	*  {@link java.util.concurrent.BlockingQueue#offer(Object,long,TimeUnit)
1020	>	*  BlockingQueue.offer})
1021		* @throws NullPointerException if the specified element is null
1022		*/
1023		public boolean offer(E e, long timeout, TimeUnit unit) {
#	Line 946 | Line 1029 | public class LinkedTransferQueue<E> exte
1029		* Inserts the specified element at the tail of this queue.
1030		* As the queue is unbounded, this method will never return {@code false}.
1031		*
1032	<	* @return {@code true} (as specified by
950	<	*         {@link BlockingQueue#offer(Object) BlockingQueue.offer})
1032	>	* @return {@code true} (as specified by {@link Queue#offer})
1033		* @throws NullPointerException if the specified element is null
1034		*/
1035		public boolean offer(E e) {
#	Line 1016 | Line 1098 | public class LinkedTransferQueue<E> exte
1098		*/
1099		public boolean tryTransfer(E e, long timeout, TimeUnit unit)
1100		throws InterruptedException {
1101	<	if (xfer(e, true, TIMEOUT, unit.toNanos(timeout)) == null)
1101	>	if (xfer(e, true, TIMED, unit.toNanos(timeout)) == null)
1102		return true;
1103		if (!Thread.interrupted())
1104		return false;
#	Line 1024 | Line 1106 | public class LinkedTransferQueue<E> exte
1106		}
1107
1108		public E take() throws InterruptedException {
1109	<	Object e = xfer(null, false, SYNC, 0);
1109	>	E e = xfer(null, false, SYNC, 0);
1110		if (e != null)
1111	<	return (E)e;
1111	>	return e;
1112		Thread.interrupted();
1113		throw new InterruptedException();
1114		}
1115
1116		public E poll(long timeout, TimeUnit unit) throws InterruptedException {
1117	<	Object e = xfer(null, false, TIMEOUT, unit.toNanos(timeout));
1117	>	E e = xfer(null, false, TIMED, unit.toNanos(timeout));
1118		if (e != null || !Thread.interrupted())
1119	<	return (E)e;
1119	>	return e;
1120		throw new InterruptedException();
1121		}
1122
1123		public E poll() {
1124	<	return (E)xfer(null, false, NOW, 0);
1124	>	return xfer(null, false, NOW, 0);
1125		}
1126
1127		/**
#	Line 1052 | Line 1134 | public class LinkedTransferQueue<E> exte
1134		if (c == this)
1135		throw new IllegalArgumentException();
1136		int n = 0;
1137	<	E e;
1056	<	while ( (e = poll()) != null) {
1137	>	for (E e; (e = poll()) != null;) {
1138		c.add(e);
1139		++n;
1140		}
#	Line 1070 | Line 1151 | public class LinkedTransferQueue<E> exte
1151		if (c == this)
1152		throw new IllegalArgumentException();
1153		int n = 0;
1154	<	E e;
1074	<	while (n < maxElements && (e = poll()) != null) {
1154	>	for (E e; n < maxElements && (e = poll()) != null;) {
1155		c.add(e);
1156		++n;
1157		}
#	Line 1079 | Line 1159 | public class LinkedTransferQueue<E> exte
1159		}
1160
1161		/**
1162	<	* Returns an iterator over the elements in this queue in proper
1163	<	* sequence, from head to tail.
1162	>	* Returns an iterator over the elements in this queue in proper sequence.
1163	>	* The elements will be returned in order from first (head) to last (tail).
1164		*
1165		* <p>The returned iterator is a "weakly consistent" iterator that
1166	<	* will never throw
1167	<	* {@link ConcurrentModificationException ConcurrentModificationException},
1168	<	* and guarantees to traverse elements as they existed upon
1169	<	* construction of the iterator, and may (but is not guaranteed
1170	<	* to) reflect any modifications subsequent to construction.
1166	>	* will never throw {@link java.util.ConcurrentModificationException
1167	>	* ConcurrentModificationException}, and guarantees to traverse
1168	>	* elements as they existed upon construction of the iterator, and
1169	>	* may (but is not guaranteed to) reflect any modifications
1170	>	* subsequent to construction.
1171		*
1172		* @return an iterator over the elements in this queue in proper sequence
1173		*/
#	Line 1096 | Line 1176 | public class LinkedTransferQueue<E> exte
1176		}
1177
1178		public E peek() {
1179	<	return (E) firstDataItem();
1179	>	return firstDataItem();
1180		}
1181
1182		/**
#	Line 1105 | Line 1185 | public class LinkedTransferQueue<E> exte
1185		* @return {@code true} if this queue contains no elements
1186		*/
1187		public boolean isEmpty() {
1188	<	return firstOfMode(true) == null;
1188	>	for (Node p = head; p != null; p = succ(p)) {
1189	>	if (!p.isMatched())
1190	>	return !p.isData;
1191	>	}
1192	>	return true;
1193		}
1194
1195		public boolean hasWaitingConsumer() {
#	Line 1148 | Line 1232 | public class LinkedTransferQueue<E> exte
1232		}
1233
1234		/**
1235	+	* Returns {@code true} if this queue contains the specified element.
1236	+	* More formally, returns {@code true} if and only if this queue contains
1237	+	* at least one element {@code e} such that {@code o.equals(e)}.
1238	+	*
1239	+	* @param o object to be checked for containment in this queue
1240	+	* @return {@code true} if this queue contains the specified element
1241	+	*/
1242	+	public boolean contains(Object o) {
1243	+	if (o == null) return false;
1244	+	for (Node p = head; p != null; p = succ(p)) {
1245	+	Object item = p.item;
1246	+	if (p.isData) {
1247	+	if (item != null && item != p && o.equals(item))
1248	+	return true;
1249	+	}
1250	+	else if (item == null)
1251	+	break;
1252	+	}
1253	+	return false;
1254	+	}
1255	+
1256	+	/**
1257		* Always returns {@code Integer.MAX_VALUE} because a
1258		* {@code LinkedTransferQueue} is not capacity constrained.
1259		*
1260		* @return {@code Integer.MAX_VALUE} (as specified by
1261	<	*         {@link BlockingQueue#remainingCapacity()})
1261	>	*         {@link java.util.concurrent.BlockingQueue#remainingCapacity()
1262	>	*         BlockingQueue.remainingCapacity})
1263		*/
1264		public int remainingCapacity() {
1265		return Integer.MAX_VALUE;
#	Line 1184 | Line 1291 | public class LinkedTransferQueue<E> exte
1291		throws java.io.IOException, ClassNotFoundException {
1292		s.defaultReadObject();
1293		for (;;) {
1294	<	@SuppressWarnings("unchecked") E item = (E) s.readObject();
1294	>	@SuppressWarnings("unchecked")
1295	>	E item = (E) s.readObject();
1296		if (item == null)
1297		break;
1298		else
#	Line 1194 | Line 1302 | public class LinkedTransferQueue<E> exte
1302
1303		// Unsafe mechanics
1304
1305	<	private static final sun.misc.Unsafe UNSAFE = getUnsafe();
1306	<	private static final long headOffset =
1307	<	objectFieldOffset(UNSAFE, "head", LinkedTransferQueue.class);
1308	<	private static final long tailOffset =
1309	<	objectFieldOffset(UNSAFE, "tail", LinkedTransferQueue.class);
1202	<	private static final long cleanMeOffset =
1203	<	objectFieldOffset(UNSAFE, "cleanMe", LinkedTransferQueue.class);
1204	<
1205	<	static long objectFieldOffset(sun.misc.Unsafe UNSAFE,
1206	<	String field, Class<?> klazz) {
1305	>	private static final sun.misc.Unsafe UNSAFE;
1306	>	private static final long headOffset;
1307	>	private static final long tailOffset;
1308	>	private static final long sweepVotesOffset;
1309	>	static {
1310		try {
1311	<	return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
1312	<	} catch (NoSuchFieldException e) {
1313	<	// Convert Exception to corresponding Error
1314	<	NoSuchFieldError error = new NoSuchFieldError(field);
1315	<	error.initCause(e);
1316	<	throw error;
1311	>	UNSAFE = getUnsafe();
1312	>	Class<?> k = LinkedTransferQueue.class;
1313	>	headOffset = UNSAFE.objectFieldOffset
1314	>	(k.getDeclaredField("head"));
1315	>	tailOffset = UNSAFE.objectFieldOffset
1316	>	(k.getDeclaredField("tail"));
1317	>	sweepVotesOffset = UNSAFE.objectFieldOffset
1318	>	(k.getDeclaredField("sweepVotes"));
1319	>	} catch (Exception e) {
1320	>	throw new Error(e);
1321		}
1322		}
1323
#	Line 1221 | Line 1328 | public class LinkedTransferQueue<E> exte
1328		*
1329		* @return a sun.misc.Unsafe
1330		*/
1331	<	private static sun.misc.Unsafe getUnsafe() {
1331	>	static sun.misc.Unsafe getUnsafe() {
1332		try {
1333		return sun.misc.Unsafe.getUnsafe();
1334	<	} catch (SecurityException se) {
1335	<	try {
1336	<	return java.security.AccessController.doPrivileged
1337	<	(new java.security
1338	<	.PrivilegedExceptionAction<sun.misc.Unsafe>() {
1339	<	public sun.misc.Unsafe run() throws Exception {
1340	<	java.lang.reflect.Field f = sun.misc
1341	<	.Unsafe.class.getDeclaredField("theUnsafe");
1342	<	f.setAccessible(true);
1343	<	return (sun.misc.Unsafe) f.get(null);
1344	<	}});
1345	<	} catch (java.security.PrivilegedActionException e) {
1346	<	throw new RuntimeException("Could not initialize intrinsics",
1347	<	e.getCause());
1348	<	}
1334	>	} catch (SecurityException tryReflectionInstead) {}
1335	>	try {
1336	>	return java.security.AccessController.doPrivileged
1337	>	(new java.security.PrivilegedExceptionAction<sun.misc.Unsafe>() {
1338	>	public sun.misc.Unsafe run() throws Exception {
1339	>	Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class;
1340	>	for (java.lang.reflect.Field f : k.getDeclaredFields()) {
1341	>	f.setAccessible(true);
1342	>	Object x = f.get(null);
1343	>	if (k.isInstance(x))
1344	>	return k.cast(x);
1345	>	}
1346	>	throw new NoSuchFieldError("the Unsafe");
1347	>	}});
1348	>	} catch (java.security.PrivilegedActionException e) {
1349	>	throw new RuntimeException("Could not initialize intrinsics",
1350	>	e.getCause());
1351		}
1352		}
1244	–
1353		}

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