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

Comparing jsr166/src/jsr166y/LinkedTransferQueue.java (file contents):
Revision 1.6 by jsr166, Fri Jul 25 18:13:15 2008 UTC vs.
Revision 1.15 by dl, Wed Mar 25 13:43:42 2009 UTC

#	Line 10 | Line 10 | import java.util.concurrent.locks.*;
10		import java.util.concurrent.atomic.*;
11		import java.util.*;
12		import java.io.*;
13	+	import sun.misc.Unsafe;
14	+	import java.lang.reflect.*;
15
16		/**
17		* An unbounded {@linkplain TransferQueue} based on linked nodes.
#	Line 19 | Line 21 | import java.io.*;
21		* producer.  The <em>tail</em> of the queue is that element that has
22		* been on the queue the shortest time for some producer.
23		*
24	<	* <p>Beware that, unlike in most collections, the <tt>size</tt>
24	>	* <p>Beware that, unlike in most collections, the {@code size}
25		* method is <em>NOT</em> a constant-time operation. Because of the
26		* asynchronous nature of these queues, determining the current number
27		* of elements requires a traversal of the elements.
#	Line 49 | Line 51 | public class LinkedTransferQueue<E> exte
51		private static final long serialVersionUID = -3223113410248163686L;
52
53		/*
52	–	* This is still a work in progress...
53	–	*
54		* This class extends the approach used in FIFO-mode
55		* SynchronousQueues. See the internal documentation, as well as
56		* the PPoPP 2006 paper "Scalable Synchronous Queues" by Scherer,
57		* Lea & Scott
58		* (http://www.cs.rice.edu/~wns1/papers/2006-PPoPP-SQ.pdf)
59		*
60	<	* The main extension is to provide different Wait modes
61	<	* for the main "xfer" method that puts or takes items.
62	<	* These don't impact the basic dual-queue logic, but instead
63	<	* control whether or how threads block upon insertion
64	<	* of request or data nodes into the dual queue.
60	>	* The main extension is to provide different Wait modes for the
61	>	* main "xfer" method that puts or takes items.  These don't
62	>	* impact the basic dual-queue logic, but instead control whether
63	>	* or how threads block upon insertion of request or data nodes
64	>	* into the dual queue. It also uses slightly different
65	>	* conventions for tracking whether nodes are off-list or
66	>	* cancelled.
67		*/
68
69		// Wait modes for xfer method
#	Line 95 | Line 97 | public class LinkedTransferQueue<E> exte
97		static final long spinForTimeoutThreshold = 1000L;
98
99		/**
100	<	* Node class for LinkedTransferQueue. Opportunistically subclasses from
101	<	* AtomicReference to represent item. Uses Object, not E, to allow
102	<	* setting item to "this" after use, to avoid garbage
103	<	* retention. Similarly, setting the next field to this is used as
104	<	* sentinel that node is off list.
100	>	* Node class for LinkedTransferQueue. Opportunistically
101	>	* subclasses from AtomicReference to represent item. Uses Object,
102	>	* not E, to allow setting item to "this" after use, to avoid
103	>	* garbage retention. Similarly, setting the next field to this is
104	>	* used as sentinel that node is off list.
105		*/
106		static final class QNode extends AtomicReference<Object> {
107		volatile QNode next;
#	Line 114 | Line 116 | public class LinkedTransferQueue<E> exte
116		nextUpdater = AtomicReferenceFieldUpdater.newUpdater
117		(QNode.class, QNode.class, "next");
118
119	<	boolean casNext(QNode cmp, QNode val) {
119	>	final boolean casNext(QNode cmp, QNode val) {
120		return nextUpdater.compareAndSet(this, cmp, val);
121		}
122	+
123	+	final void clearNext() {
124	+	nextUpdater.lazySet(this, this);
125	+	}
126	+
127		}
128
129		/**
#	Line 131 | Line 138 | public class LinkedTransferQueue<E> exte
138		}
139
140
141	<	private final QNode dummy = new QNode(null, false);
142	<	private final PaddedAtomicReference<QNode> head =
143	<	new PaddedAtomicReference<QNode>(dummy);
144	<	private final PaddedAtomicReference<QNode> tail =
138	<	new PaddedAtomicReference<QNode>(dummy);
141	>	/** head of the queue */
142	>	private transient final PaddedAtomicReference<QNode> head;
143	>	/** tail of the queue */
144	>	private transient final PaddedAtomicReference<QNode> tail;
145
146		/**
147		* Reference to a cancelled node that might not yet have been
148		* unlinked from queue because it was the last inserted node
149		* when it cancelled.
150		*/
151	<	private final PaddedAtomicReference<QNode> cleanMe =
146	<	new PaddedAtomicReference<QNode>(null);
151	>	private transient final PaddedAtomicReference<QNode> cleanMe;
152
153		/**
154		* Tries to cas nh as new head; if successful, unlink
#	Line 151 | Line 156 | public class LinkedTransferQueue<E> exte
156		*/
157		private boolean advanceHead(QNode h, QNode nh) {
158		if (h == head.get() && head.compareAndSet(h, nh)) {
159	<	h.next = h; // forget old next
159	>	h.clearNext(); // forget old next
160		return true;
161		}
162		return false;
#	Line 159 | Line 164 | public class LinkedTransferQueue<E> exte
164
165		/**
166		* Puts or takes an item. Used for most queue operations (except
167	<	* poll() and tryTransfer())
167	>	* poll() and tryTransfer()). See the similar code in
168	>	* SynchronousQueue for detailed explanation.
169		* @param e the item or if null, signifies that this is a take
170		* @param mode the wait mode: NOWAIT, TIMEOUT, WAIT
171		* @param nanos timeout in nanosecs, used only if mode is TIMEOUT
#	Line 266 | Line 272 | public class LinkedTransferQueue<E> exte
272		Object x = s.get();
273		if (x != e) {                 // Node was matched or cancelled
274		advanceHead(pred, s);     // unlink if head
275	<	if (x == s)               // was cancelled
276	<	return clean(pred, s);
275	>	if (x == s) {              // was cancelled
276	>	clean(pred, s);
277	>	return null;
278	>	}
279		else if (x != null) {
280		s.set(s);             // avoid garbage retention
281		return x;
#	Line 275 | Line 283 | public class LinkedTransferQueue<E> exte
283		else
284		return e;
285		}
278	–
286		if (mode == TIMEOUT) {
287		long now = System.nanoTime();
288		nanos -= now - lastTime;
#	Line 296 | Line 303 | public class LinkedTransferQueue<E> exte
303		else if (s.waiter == null)
304		s.waiter = w;
305		else if (mode != TIMEOUT) {
306	<	//                LockSupport.park(this);
300	<	LockSupport.park(); // allows run on java5
306	>	LockSupport.park(this);
307		s.waiter = null;
308		spins = -1;
309		}
310		else if (nanos > spinForTimeoutThreshold) {
311	<	//                LockSupport.parkNanos(this, nanos);
306	<	LockSupport.parkNanos(nanos);
311	>	LockSupport.parkNanos(this, nanos);
312		s.waiter = null;
313		spins = -1;
314		}
#	Line 311 | Line 316 | public class LinkedTransferQueue<E> exte
316		}
317
318		/**
319	+	* Returns validated tail for use in cleaning methods
320	+	*/
321	+	private QNode getValidatedTail() {
322	+	for (;;) {
323	+	QNode h = head.get();
324	+	QNode first = h.next;
325	+	if (first != null && first.next == first) { // help advance
326	+	advanceHead(h, first);
327	+	continue;
328	+	}
329	+	QNode t = tail.get();
330	+	QNode last = t.next;
331	+	if (t == tail.get()) {
332	+	if (last != null)
333	+	tail.compareAndSet(t, last); // help advance
334	+	else
335	+	return t;
336	+	}
337	+	}
338	+	}
339	+
340	+	/**
341		* Gets rid of cancelled node s with original predecessor pred.
342	<	* @return null (to simplify use by callers)
342	>	* @param pred predecessor of cancelled node
343	>	* @param s the cancelled node
344		*/
345	<	private Object clean(QNode pred, QNode s) {
345	>	private void clean(QNode pred, QNode s) {
346		Thread w = s.waiter;
347		if (w != null) {             // Wake up thread
348		s.waiter = null;
#	Line 322 | Line 350 | public class LinkedTransferQueue<E> exte
350		LockSupport.unpark(w);
351		}
352
353	<	for (;;) {
354	<	if (pred.next != s) // already cleaned
355	<	return null;
356	<	QNode h = head.get();
357	<	QNode hn = h.next;   // Absorb cancelled first node as head
358	<	if (hn != null && hn.next == hn) {
359	<	advanceHead(h, hn);
360	<	continue;
361	<	}
362	<	QNode t = tail.get();      // Ensure consistent read for tail
363	<	if (t == h)
364	<	return null;
365	<	QNode tn = t.next;
366	<	if (t != tail.get())
367	<	continue;
368	<	if (tn != null) {          // Help advance tail
341	<	tail.compareAndSet(t, tn);
342	<	continue;
343	<	}
344	<	if (s != t) {             // If not tail, try to unsplice
345	<	QNode sn = s.next;
353	>	if (pred == null)
354	>	return;
355	>
356	>	/*
357	>	* At any given time, exactly one node on list cannot be
358	>	* deleted -- the last inserted node. To accommodate this, if
359	>	* we cannot delete s, we save its predecessor as "cleanMe",
360	>	* processing the previously saved version first. At least one
361	>	* of node s or the node previously saved can always be
362	>	* processed, so this always terminates.
363	>	*/
364	>	while (pred.next == s) {
365	>	QNode oldpred = reclean();  // First, help get rid of cleanMe
366	>	QNode t = getValidatedTail();
367	>	if (s != t) {               // If not tail, try to unsplice
368	>	QNode sn = s.next;      // s.next == s means s already off list
369		if (sn == s || pred.casNext(s, sn))
370	<	return null;
370	>	break;
371		}
372	<	QNode dp = cleanMe.get();
373	<	if (dp != null) {    // Try unlinking previous cancelled node
374	<	QNode d = dp.next;
375	<	QNode dn;
376	<	if (d == null ||               // d is gone or
377	<	d == dp ||                 // d is off list or
378	<	d.get() != d ||            // d not cancelled or
379	<	(d != t &&                 // d not tail and
380	<	(dn = d.next) != null &&  //   has successor
381	<	dn != d &&                //   that is on list
382	<	dp.casNext(d, dn)))       // d unspliced
383	<	cleanMe.compareAndSet(dp, null);
384	<	if (dp == pred)
385	<	return null;      // s is already saved node
372	>	else if (oldpred == pred || // Already saved
373	>	(oldpred == null && cleanMe.compareAndSet(null, pred)))
374	>	break;                  // Postpone cleaning
375	>	}
376	>	}
377	>
378	>	/**
379	>	* Tries to unsplice the cancelled node held in cleanMe that was
380	>	* previously uncleanable because it was at tail.
381	>	* @return current cleanMe node (or null)
382	>	*/
383	>	private QNode reclean() {
384	>	/*
385	>	* cleanMe is, or at one time was, predecessor of cancelled
386	>	* node s that was the tail so could not be unspliced.  If s
387	>	* is no longer the tail, try to unsplice if necessary and
388	>	* make cleanMe slot available.  This differs from similar
389	>	* code in clean() because we must check that pred still
390	>	* points to a cancelled node that must be unspliced -- if
391	>	* not, we can (must) clear cleanMe without unsplicing.
392	>	* This can loop only due to contention on casNext or
393	>	* clearing cleanMe.
394	>	*/
395	>	QNode pred;
396	>	while ((pred = cleanMe.get()) != null) {
397	>	QNode t = getValidatedTail();
398	>	QNode s = pred.next;
399	>	if (s != t) {
400	>	QNode sn;
401	>	if (s == null || s == pred || s.get() != s ||
402	>	(sn = s.next) == s || pred.casNext(s, sn))
403	>	cleanMe.compareAndSet(pred, null);
404		}
405	<	else if (cleanMe.compareAndSet(null, pred))
406	<	return null;          // Postpone cleaning s
405	>	else // s is still tail; cannot clean
406	>	break;
407		}
408	+	return pred;
409		}
410
411		/**
412	<	* Creates an initially empty <tt>LinkedTransferQueue</tt>.
412	>	* Creates an initially empty {@code LinkedTransferQueue}.
413		*/
414		public LinkedTransferQueue() {
415	+	QNode dummy = new QNode(null, false);
416	+	head = new PaddedAtomicReference<QNode>(dummy);
417	+	tail = new PaddedAtomicReference<QNode>(dummy);
418	+	cleanMe = new PaddedAtomicReference<QNode>(null);
419		}
420
421		/**
422	<	* Creates a <tt>LinkedTransferQueue</tt>
422	>	* Creates a {@code LinkedTransferQueue}
423		* initially containing the elements of the given collection,
424		* added in traversal order of the collection's iterator.
425		* @param c the collection of elements to initially contain
#	Line 381 | Line 427 | public class LinkedTransferQueue<E> exte
427		*         of its elements are null
428		*/
429		public LinkedTransferQueue(Collection<? extends E> c) {
430	+	this();
431		addAll(c);
432		}
433
#	Line 404 | Line 451 | public class LinkedTransferQueue<E> exte
451		return true;
452		}
453
454	+	public boolean add(E e) {
455	+	if (e == null) throw new NullPointerException();
456	+	xfer(e, NOWAIT, 0);
457	+	return true;
458	+	}
459	+
460		public void transfer(E e) throws InterruptedException {
461		if (e == null) throw new NullPointerException();
462		if (xfer(e, WAIT, 0) == null) {
#	Line 500 | Line 553 | public class LinkedTransferQueue<E> exte
553		return h;
554		}
555		}
556	+	reclean();
557		}
558		}
559
#	Line 516 | Line 570 | public class LinkedTransferQueue<E> exte
570		* if subsequently removed.
571		*/
572		class Itr implements Iterator<E> {
573	<	QNode nextNode;    // Next node to return next
574	<	QNode currentNode; // last returned node, for remove()
575	<	QNode prevNode;    // predecessor of last returned node
573	>	QNode next;        // node to return next
574	>	QNode pnext;       // predecessor of next
575	>	QNode snext;       // successor of next
576	>	QNode curr;        // last returned node, for remove()
577	>	QNode pcurr;       // predecessor of curr, for remove()
578		E nextItem;        // Cache of next item, once commited to in next
579
580		Itr() {
581	<	nextNode = traversalHead();
526	<	advance();
581	>	findNext();
582		}
583
584	<	E advance() {
585	<	prevNode = currentNode;
586	<	currentNode = nextNode;
587	<	E x = nextItem;
533	<
534	<	QNode p = nextNode.next;
584	>	/**
585	>	* Ensure next points to next valid node, or null if none.
586	>	*/
587	>	void findNext() {
588		for (;;) {
589	<	if (p == null || !p.isData) {
590	<	nextNode = null;
591	<	nextItem = null;
592	<	return x;
589	>	QNode pred = pnext;
590	>	QNode q = next;
591	>	if (pred == null || pred == q) {
592	>	pred = traversalHead();
593	>	q = pred.next;
594		}
595	<	Object item = p.get();
596	<	if (item != p && item != null) {
597	<	nextNode = p;
544	<	nextItem = (E)item;
545	<	return x;
595	>	if (q == null || !q.isData) {
596	>	next = null;
597	>	return;
598		}
599	<	prevNode = p;
600	<	p = p.next;
599	>	Object x = q.get();
600	>	QNode s = q.next;
601	>	if (x != null && q != x && q != s) {
602	>	nextItem = (E)x;
603	>	snext = s;
604	>	pnext = pred;
605	>	next = q;
606	>	return;
607	>	}
608	>	pnext = q;
609	>	next = s;
610		}
611		}
612
613		public boolean hasNext() {
614	<	return nextNode != null;
614	>	return next != null;
615		}
616
617		public E next() {
618	<	if (nextNode == null) throw new NoSuchElementException();
619	<	return advance();
618	>	if (next == null) throw new NoSuchElementException();
619	>	pcurr = pnext;
620	>	curr = next;
621	>	pnext = next;
622	>	next = snext;
623	>	E x = nextItem;
624	>	findNext();
625	>	return x;
626		}
627
628		public void remove() {
629	<	QNode p = currentNode;
630	<	QNode prev = prevNode;
564	<	if (prev == null || p == null)
629	>	QNode p = curr;
630	>	if (p == null)
631		throw new IllegalStateException();
632		Object x = p.get();
633		if (x != null && x != p && p.compareAndSet(x, p))
634	<	clean(prev, p);
634	>	clean(pcurr, p);
635		}
636		}
637
#	Line 615 | Line 681 | public class LinkedTransferQueue<E> exte
681
682		/**
683		* Returns the number of elements in this queue.  If this queue
684	<	* contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
685	<	* <tt>Integer.MAX_VALUE</tt>.
684	>	* contains more than {@code Integer.MAX_VALUE} elements, returns
685	>	* {@code Integer.MAX_VALUE}.
686		*
687		* <p>Beware that, unlike in most collections, this method is
688		* <em>NOT</em> a constant-time operation. Because of the
#	Line 654 | Line 720 | public class LinkedTransferQueue<E> exte
720		return Integer.MAX_VALUE;
721		}
722
723	+	public boolean remove(Object o) {
724	+	if (o == null)
725	+	return false;
726	+	for (;;) {
727	+	QNode pred = traversalHead();
728	+	for (;;) {
729	+	QNode q = pred.next;
730	+	if (q == null || !q.isData)
731	+	return false;
732	+	if (q == pred) // restart
733	+	break;
734	+	Object x = q.get();
735	+	if (x != null && x != q && o.equals(x) &&
736	+	q.compareAndSet(x, q)) {
737	+	clean(pred, q);
738	+	return true;
739	+	}
740	+	pred = q;
741	+	}
742	+	}
743	+	}
744	+
745		/**
746		* Save the state to a stream (that is, serialize it).
747		*
748	<	* @serialData All of the elements (each an <tt>E</tt>) in
748	>	* @serialData All of the elements (each an {@code E}) in
749		* the proper order, followed by a null
750		* @param s the stream
751		*/
#	Line 678 | Line 766 | public class LinkedTransferQueue<E> exte
766		private void readObject(java.io.ObjectInputStream s)
767		throws java.io.IOException, ClassNotFoundException {
768		s.defaultReadObject();
769	+	resetHeadAndTail();
770		for (;;) {
771		E item = (E)s.readObject();
772		if (item == null)
#	Line 686 | Line 775 | public class LinkedTransferQueue<E> exte
775		offer(item);
776		}
777		}
778	+
779	+
780	+	// Support for resetting head/tail while deserializing
781	+	private void resetHeadAndTail() {
782	+	QNode dummy = new QNode(null, false);
783	+	_unsafe.putObjectVolatile(this, headOffset,
784	+	new PaddedAtomicReference<QNode>(dummy));
785	+	_unsafe.putObjectVolatile(this, tailOffset,
786	+	new PaddedAtomicReference<QNode>(dummy));
787	+	_unsafe.putObjectVolatile(this, cleanMeOffset,
788	+	new PaddedAtomicReference<QNode>(null));
789	+	}
790	+
791	+	// Temporary Unsafe mechanics for preliminary release
792	+	private static Unsafe getUnsafe() throws Throwable {
793	+	try {
794	+	return Unsafe.getUnsafe();
795	+	} catch (SecurityException se) {
796	+	try {
797	+	return java.security.AccessController.doPrivileged
798	+	(new java.security.PrivilegedExceptionAction<Unsafe>() {
799	+	public Unsafe run() throws Exception {
800	+	return getUnsafePrivileged();
801	+	}});
802	+	} catch (java.security.PrivilegedActionException e) {
803	+	throw e.getCause();
804	+	}
805	+	}
806	+	}
807	+
808	+	private static Unsafe getUnsafePrivileged()
809	+	throws NoSuchFieldException, IllegalAccessException {
810	+	Field f = Unsafe.class.getDeclaredField("theUnsafe");
811	+	f.setAccessible(true);
812	+	return (Unsafe) f.get(null);
813	+	}
814	+
815	+	private static long fieldOffset(String fieldName)
816	+	throws NoSuchFieldException {
817	+	return _unsafe.objectFieldOffset
818	+	(LinkedTransferQueue.class.getDeclaredField(fieldName));
819	+	}
820	+
821	+	private static final Unsafe _unsafe;
822	+	private static final long headOffset;
823	+	private static final long tailOffset;
824	+	private static final long cleanMeOffset;
825	+	static {
826	+	try {
827	+	_unsafe = getUnsafe();
828	+	headOffset = fieldOffset("head");
829	+	tailOffset = fieldOffset("tail");
830	+	cleanMeOffset = fieldOffset("cleanMe");
831	+	} catch (Throwable e) {
832	+	throw new RuntimeException("Could not initialize intrinsics", e);
833	+	}
834	+	}
835	+
836		}

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