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

Comparing jsr166/src/jsr166y/LinkedTransferQueue.java (file contents):
Revision 1.32 by jsr166, Wed Jul 29 02:19:56 2009 UTC vs.
Revision 1.46 by jsr166, Thu Oct 22 08:19:44 2009 UTC

#	Line 10 | Line 10 | import java.util.concurrent.*;
10
11		import java.util.AbstractQueue;
12		import java.util.Collection;
13	+	import java.util.ConcurrentModificationException;
14		import java.util.Iterator;
15		import java.util.NoSuchElementException;
16	+	import java.util.Queue;
17		import java.util.concurrent.locks.LockSupport;
16	–	import java.util.concurrent.atomic.AtomicReference;
17	–
18		/**
19	<	* An unbounded {@linkplain TransferQueue} based on linked nodes.
19	>	* An unbounded {@link TransferQueue} based on linked nodes.
20		* This queue orders elements FIFO (first-in-first-out) with respect
21		* to any given producer.  The <em>head</em> of the queue is that
22		* element that has been on the queue the longest time for some
#	Line 52 | Line 52 | public class LinkedTransferQueue<E> exte
52		private static final long serialVersionUID = -3223113410248163686L;
53
54		/*
55	<	* This class extends the approach used in FIFO-mode
56	<	* SynchronousQueues. See the internal documentation, as well as
57	<	* the PPoPP 2006 paper "Scalable Synchronous Queues" by Scherer,
58	<	* Lea & Scott
59	<	* (http://www.cs.rice.edu/~wns1/papers/2006-PPoPP-SQ.pdf)
55	>	* *** Overview of Dual Queues with Slack ***
56		*
57	<	* The main extension is to provide different Wait modes for the
58	<	* main "xfer" method that puts or takes items.  These don't
59	<	* impact the basic dual-queue logic, but instead control whether
60	<	* or how threads block upon insertion of request or data nodes
61	<	* into the dual queue. It also uses slightly different
62	<	* conventions for tracking whether nodes are off-list or
63	<	* cancelled.
64	<	*/
65	<
66	<	// Wait modes for xfer method
67	<	static final int NOWAIT  = 0;
68	<	static final int TIMEOUT = 1;
69	<	static final int WAIT    = 2;
70	<
71	<	/** The number of CPUs, for spin control */
72	<	static final int NCPUS = Runtime.getRuntime().availableProcessors();
73	<
74	<	/**
75	<	* The number of times to spin before blocking in timed waits.
76	<	* The value is empirically derived -- it works well across a
77	<	* variety of processors and OSes. Empirically, the best value
78	<	* seems not to vary with number of CPUs (beyond 2) so is just
79	<	* a constant.
80	<	*/
81	<	static final int maxTimedSpins = (NCPUS < 2) ? 0 : 32;
82	<
83	<	/**
84	<	* The number of times to spin before blocking in untimed waits.
85	<	* This is greater than timed value because untimed waits spin
86	<	* faster since they don't need to check times on each spin.
87	<	*/
88	<	static final int maxUntimedSpins = maxTimedSpins * 16;
57	>	* Dual Queues, introduced by Scherer and Scott
58	>	* (http://www.cs.rice.edu/~wns1/papers/2004-DISC-DDS.pdf) are
59	>	* (linked) queues in which nodes may represent either data or
60	>	* requests.  When a thread tries to enqueue a data node, but
61	>	* encounters a request node, it instead "matches" and removes it;
62	>	* and vice versa for enqueuing requests. Blocking Dual Queues
63	>	* arrange that threads enqueuing unmatched requests block until
64	>	* other threads provide the match. Dual Synchronous Queues (see
65	>	* Scherer, Lea, & Scott
66	>	* http://www.cs.rochester.edu/u/scott/papers/2009_Scherer_CACM_SSQ.pdf)
67	>	* additionally arrange that threads enqueuing unmatched data also
68	>	* block.  Dual Transfer Queues support all of these modes, as
69	>	* dictated by callers.
70	>	*
71	>	* A FIFO dual queue may be implemented using a variation of the
72	>	* Michael & Scott (M&S) lock-free queue algorithm
73	>	* (http://www.cs.rochester.edu/u/scott/papers/1996_PODC_queues.pdf).
74	>	* It maintains two pointer fields, "head", pointing to a
75	>	* (matched) node that in turn points to the first actual
76	>	* (unmatched) queue node (or null if empty); and "tail" that
77	>	* points to the last node on the queue (or again null if
78	>	* empty). For example, here is a possible queue with four data
79	>	* elements:
80	>	*
81	>	*  head                tail
82	>	*    |                   |
83	>	*    v                   v
84	>	*    M -> U -> U -> U -> U
85	>	*
86	>	* The M&S queue algorithm is known to be prone to scalability and
87	>	* overhead limitations when maintaining (via CAS) these head and
88	>	* tail pointers. This has led to the development of
89	>	* contention-reducing variants such as elimination arrays (see
90	>	* Moir et al http://portal.acm.org/citation.cfm?id=1074013) and
91	>	* optimistic back pointers (see Ladan-Mozes & Shavit
92	>	* http://people.csail.mit.edu/edya/publications/OptimisticFIFOQueue-journal.pdf).
93	>	* However, the nature of dual queues enables a simpler tactic for
94	>	* improving M&S-style implementations when dual-ness is needed.
95	>	*
96	>	* In a dual queue, each node must atomically maintain its match
97	>	* status. While there are other possible variants, we implement
98	>	* this here as: for a data-mode node, matching entails CASing an
99	>	* "item" field from a non-null data value to null upon match, and
100	>	* vice-versa for request nodes, CASing from null to a data
101	>	* value. (Note that the linearization properties of this style of
102	>	* queue are easy to verify -- elements are made available by
103	>	* linking, and unavailable by matching.) Compared to plain M&S
104	>	* queues, this property of dual queues requires one additional
105	>	* successful atomic operation per enq/deq pair. But it also
106	>	* enables lower cost variants of queue maintenance mechanics. (A
107	>	* variation of this idea applies even for non-dual queues that
108	>	* support deletion of embedded elements, such as
109	>	* j.u.c.ConcurrentLinkedQueue.)
110	>	*
111	>	* Once a node is matched, its item can never again change.  We
112	>	* may thus arrange that the linked list of them contains a prefix
113	>	* of zero or more matched nodes, followed by a suffix of zero or
114	>	* more unmatched nodes. (Note that we allow both the prefix and
115	>	* suffix to be zero length, which in turn means that we do not
116	>	* use a dummy header.)  If we were not concerned with either time
117	>	* or space efficiency, we could correctly perform enqueue and
118	>	* dequeue operations by traversing from a pointer to the initial
119	>	* node; CASing the item of the first unmatched node on match and
120	>	* CASing the next field of the trailing node on appends.  While
121	>	* this would be a terrible idea in itself, it does have the
122	>	* benefit of not requiring ANY atomic updates on head/tail
123	>	* fields.
124	>	*
125	>	* We introduce here an approach that lies between the extremes of
126	>	* never versus always updating queue (head and tail) pointers
127	>	* that reflects the tradeoff of sometimes requiring extra traversal
128	>	* steps to locate the first and/or last unmatched nodes, versus
129	>	* the reduced overhead and contention of fewer updates to queue
130	>	* pointers. For example, a possible snapshot of a queue is:
131	>	*
132	>	*  head           tail
133	>	*    |              |
134	>	*    v              v
135	>	*    M -> M -> U -> U -> U -> U
136	>	*
137	>	* The best value for this "slack" (the targeted maximum distance
138	>	* between the value of "head" and the first unmatched node, and
139	>	* similarly for "tail") is an empirical matter. We have found
140	>	* that using very small constants in the range of 1-3 work best
141	>	* over a range of platforms. Larger values introduce increasing
142	>	* costs of cache misses and risks of long traversal chains.
143	>	*
144	>	* Dual queues with slack differ from plain M&S dual queues by
145	>	* virtue of only sometimes updating head or tail pointers when
146	>	* matching, appending, or even traversing nodes; in order to
147	>	* maintain a targeted slack.  The idea of "sometimes" may be
148	>	* operationalized in several ways. The simplest is to use a
149	>	* per-operation counter incremented on each traversal step, and
150	>	* to try (via CAS) to update the associated queue pointer
151	>	* whenever the count exceeds a threshold. Another, that requires
152	>	* more overhead, is to use random number generators to update
153	>	* with a given probability per traversal step.
154	>	*
155	>	* In any strategy along these lines, because CASes updating
156	>	* fields may fail, the actual slack may exceed targeted
157	>	* slack. However, they may be retried at any time to maintain
158	>	* targets.  Even when using very small slack values, this
159	>	* approach works well for dual queues because it allows all
160	>	* operations up to the point of matching or appending an item
161	>	* (hence potentially releasing another thread) to be read-only,
162	>	* thus not introducing any further contention. As described
163	>	* below, we implement this by performing slack maintenance
164	>	* retries only after these points.
165	>	*
166	>	* As an accompaniment to such techniques, traversal overhead can
167	>	* be further reduced without increasing contention of head
168	>	* pointer updates.  During traversals, threads may sometimes
169	>	* shortcut the "next" link path from the current "head" node to
170	>	* be closer to the currently known first unmatched node. Again,
171	>	* this may be triggered with using thresholds or randomization.
172	>	*
173	>	* These ideas must be further extended to avoid unbounded amounts
174	>	* of costly-to-reclaim garbage caused by the sequential "next"
175	>	* links of nodes starting at old forgotten head nodes: As first
176	>	* described in detail by Boehm
177	>	* (http://portal.acm.org/citation.cfm?doid=503272.503282) if a GC
178	>	* delays noticing that any arbitrarily old node has become
179	>	* garbage, all newer dead nodes will also be unreclaimed.
180	>	* (Similar issues arise in non-GC environments.)  To cope with
181	>	* this in our implementation, upon CASing to advance the head
182	>	* pointer, we set the "next" link of the previous head to point
183	>	* only to itself; thus limiting the length of connected dead lists.
184	>	* (We also take similar care to wipe out possibly garbage
185	>	* retaining values held in other Node fields.)  However, doing so
186	>	* adds some further complexity to traversal: If any "next"
187	>	* pointer links to itself, it indicates that the current thread
188	>	* has lagged behind a head-update, and so the traversal must
189	>	* continue from the "head".  Traversals trying to find the
190	>	* current tail starting from "tail" may also encounter
191	>	* self-links, in which case they also continue at "head".
192	>	*
193	>	* It is tempting in slack-based scheme to not even use CAS for
194	>	* updates (similarly to Ladan-Mozes & Shavit). However, this
195	>	* cannot be done for head updates under the above link-forgetting
196	>	* mechanics because an update may leave head at a detached node.
197	>	* And while direct writes are possible for tail updates, they
198	>	* increase the risk of long retraversals, and hence long garbage
199	>	* chains which can be much more costly than is worthwhile
200	>	* considering that the cost difference of performing a CAS vs
201	>	* write is smaller when they are not triggered on each operation
202	>	* (especially considering that writes and CASes equally require
203	>	* additional GC bookkeeping ("write barriers") that are sometimes
204	>	* more costly than the writes themselves because of contention).
205	>	*
206	>	* Removal of internal nodes (due to timed out or interrupted
207	>	* waits, or calls to remove or Iterator.remove) uses a scheme
208	>	* roughly similar to that in Scherer, Lea, and Scott
209	>	* SynchronousQueue. Given a predecessor, we can unsplice any node
210	>	* except the (actual) tail of the queue. To avoid build-up of
211	>	* cancelled trailing nodes, upon a request to remove a trailing
212	>	* node, it is placed in field "cleanMe" to be unspliced later.
213	>	*
214	>	* *** Overview of implementation ***
215	>	*
216	>	* We use a threshold-based approach to updates, with a target
217	>	* slack of two.  The slack value is hard-wired: a path greater
218	>	* than one is naturally implemented by checking equality of
219	>	* traversal pointers except when the list has only one element,
220	>	* in which case we keep max slack at one. Avoiding tracking
221	>	* explicit counts across situations slightly simplifies an
222	>	* already-messy implementation. Using randomization would
223	>	* probably work better if there were a low-quality dirt-cheap
224	>	* per-thread one available, but even ThreadLocalRandom is too
225	>	* heavy for these purposes.
226	>	*
227	>	* With such a small slack value, path short-circuiting is rarely
228	>	* worthwhile. However, it is used (in awaitMatch) immediately
229	>	* before a waiting thread starts to block, as a final bit of
230	>	* helping at a point when contention with others is extremely
231	>	* unlikely (since if other threads that could release it are
232	>	* operating, then the current thread wouldn't be blocking).
233	>	*
234	>	* All enqueue/dequeue operations are handled by the single method
235	>	* "xfer" with parameters indicating whether to act as some form
236	>	* of offer, put, poll, take, or transfer (each possibly with
237	>	* timeout). The relative complexity of using one monolithic
238	>	* method outweighs the code bulk and maintenance problems of
239	>	* using nine separate methods.
240	>	*
241	>	* Operation consists of up to three phases. The first is
242	>	* implemented within method xfer, the second in tryAppend, and
243	>	* the third in method awaitMatch.
244	>	*
245	>	* 1. Try to match an existing node
246	>	*
247	>	*    Starting at head, skip already-matched nodes until finding
248	>	*    an unmatched node of opposite mode, if one exists, in which
249	>	*    case matching it and returning, also if necessary updating
250	>	*    head to one past the matched node (or the node itself if the
251	>	*    list has no other unmatched nodes). If the CAS misses, then
252	>	*    a retry loops until the slack is at most two. Traversals
253	>	*    also check if the initial head is now off-list, in which
254	>	*    case they start at the new head.
255	>	*
256	>	*    If no candidates are found and the call was untimed
257	>	*    poll/offer, (argument "how" is NOW) return.
258	>	*
259	>	* 2. Try to append a new node (method tryAppend)
260	>	*
261	>	*    Starting at current tail pointer, try to append a new node
262	>	*    to the list (or if head was null, establish the first
263	>	*    node). Nodes can be appended only if their predecessors are
264	>	*    either already matched or are of the same mode. If we detect
265	>	*    otherwise, then a new node with opposite mode must have been
266	>	*    appended during traversal, so must restart at phase 1. The
267	>	*    traversal and update steps are otherwise similar to phase 1:
268	>	*    Retrying upon CAS misses and checking for staleness.  In
269	>	*    particular, if a self-link is encountered, then we can
270	>	*    safely jump to a node on the list by continuing the
271	>	*    traversal at current head.
272	>	*
273	>	*    On successful append, if the call was ASYNC, return.
274	>	*
275	>	* 3. Await match or cancellation (method awaitMatch)
276	>	*
277	>	*    Wait for another thread to match node; instead cancelling if
278	>	*    current thread was interrupted or the wait timed out. On
279	>	*    multiprocessors, we use front-of-queue spinning: If a node
280	>	*    appears to be the first unmatched node in the queue, it
281	>	*    spins a bit before blocking. In either case, before blocking
282	>	*    it tries to unsplice any nodes between the current "head"
283	>	*    and the first unmatched node.
284	>	*
285	>	*    Front-of-queue spinning vastly improves performance of
286	>	*    heavily contended queues. And so long as it is relatively
287	>	*    brief and "quiet", spinning does not much impact performance
288	>	*    of less-contended queues.  During spins threads check their
289	>	*    interrupt status and generate a thread-local random number
290	>	*    to decide to occasionally perform a Thread.yield. While
291	>	*    yield has underdefined specs, we assume that might it help,
292	>	*    and will not hurt in limiting impact of spinning on busy
293	>	*    systems.  We also use much smaller (1/4) spins for nodes
294	>	*    that are not known to be front but whose predecessors have
295	>	*    not blocked -- these "chained" spins avoid artifacts of
296	>	*    front-of-queue rules which otherwise lead to alternating
297	>	*    nodes spinning vs blocking. Further, front threads that
298	>	*    represent phase changes (from data to request node or vice
299	>	*    versa) compared to their predecessors receive additional
300	>	*    spins, reflecting the longer code path lengths necessary to
301	>	*    release them under contention.
302	>	*/
303	>
304	>	/** True if on multiprocessor */
305	>	private static final boolean MP =
306	>	Runtime.getRuntime().availableProcessors() > 1;
307	>
308	>	/**
309	>	* The number of times to spin (with on average one randomly
310	>	* interspersed call to Thread.yield) on multiprocessor before
311	>	* blocking when a node is apparently the first waiter in the
312	>	* queue.  See above for explanation. Must be a power of two. The
313	>	* value is empirically derived -- it works pretty well across a
314	>	* variety of processors, numbers of CPUs, and OSes.
315	>	*/
316	>	private static final int FRONT_SPINS   = 1 << 7;
317	>
318	>	/**
319	>	* The number of times to spin before blocking when a node is
320	>	* preceded by another node that is apparently spinning.
321	>	*/
322	>	private static final int CHAINED_SPINS = FRONT_SPINS >>> 2;
323	>
324	>	/**
325	>	* Queue nodes. Uses Object, not E, for items to allow forgetting
326	>	* them after use.  Relies heavily on Unsafe mechanics to minimize
327	>	* unnecessary ordering constraints: Writes that intrinsically
328	>	* precede or follow CASes use simple relaxed forms.  Other
329	>	* cleanups use releasing/lazy writes.
330	>	*/
331	>	static final class Node {
332	>	final boolean isData;   // false if this is a request node
333	>	volatile Object item;   // initially non-null if isData; CASed to match
334	>	volatile Node next;
335	>	volatile Thread waiter; // null until waiting
336
337	<	/**
338	<	* The number of nanoseconds for which it is faster to spin
339	<	* rather than to use timed park. A rough estimate suffices.
340	<	*/
98	<	static final long spinForTimeoutThreshold = 1000L;
337	>	// CAS methods for fields
338	>	final boolean casNext(Node cmp, Node val) {
339	>	return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
340	>	}
341
342	<	/**
343	<	* Node class for LinkedTransferQueue. Opportunistically
344	<	* subclasses from AtomicReference to represent item. Uses Object,
103	<	* not E, to allow setting item to "this" after use, to avoid
104	<	* garbage retention. Similarly, setting the next field to this is
105	<	* used as sentinel that node is off list.
106	<	*/
107	<	static final class Node<E> extends AtomicReference<Object> {
108	<	volatile Node<E> next;
109	<	volatile Thread waiter;       // to control park/unpark
110	<	final boolean isData;
342	>	final boolean casItem(Object cmp, Object val) {
343	>	return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
344	>	}
345
346	<	Node(E item, boolean isData) {
347	<	super(item);
346	>	/**
347	>	* Creates a new node. Uses relaxed write because item can only
348	>	* be seen if followed by CAS.
349	>	*/
350	>	Node(Object item, boolean isData) {
351	>	UNSAFE.putObject(this, itemOffset, item); // relaxed write
352		this.isData = isData;
353		}
354
355	<	// Unsafe mechanics
355	>	/**
356	>	* Links node to itself to avoid garbage retention.  Called
357	>	* only after CASing head field, so uses relaxed write.
358	>	*/
359	>	final void forgetNext() {
360	>	UNSAFE.putObject(this, nextOffset, this);
361	>	}
362
363	<	private static final sun.misc.Unsafe UNSAFE = getUnsafe();
364	<	private static final long nextOffset =
365	<	objectFieldOffset(UNSAFE, "next", Node.class);
363	>	/**
364	>	* Sets item to self (using a releasing/lazy write) and waiter
365	>	* to null, to avoid garbage retention after extracting or
366	>	* cancelling.
367	>	*/
368	>	final void forgetContents() {
369	>	UNSAFE.putOrderedObject(this, itemOffset, this);
370	>	UNSAFE.putOrderedObject(this, waiterOffset, null);
371	>	}
372
373	<	final boolean casNext(Node<E> cmp, Node<E> val) {
374	<	return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
373	>	/**
374	>	* Returns true if this node has been matched, including the
375	>	* case of artificial matches due to cancellation.
376	>	*/
377	>	final boolean isMatched() {
378	>	Object x = item;
379	>	return x == this || (x != null) != isData;
380		}
381
382	<	final void clearNext() {
383	<	UNSAFE.putOrderedObject(this, nextOffset, this);
382	>	/**
383	>	* Returns true if a node with the given mode cannot be
384	>	* appended to this node because this node is unmatched and
385	>	* has opposite data mode.
386	>	*/
387	>	final boolean cannotPrecede(boolean haveData) {
388	>	boolean d = isData;
389	>	Object x;
390	>	return d != haveData && (x = item) != this && (x != null) == d;
391		}
392
393		/**
394	<	* Returns a sun.misc.Unsafe.  Suitable for use in a 3rd party package.
133	<	* Replace with a simple call to Unsafe.getUnsafe when integrating
134	<	* into a jdk.
135	<	*
136	<	* @return a sun.misc.Unsafe
394	>	* Tries to artificially match a data node -- used by remove.
395		*/
396	<	private static sun.misc.Unsafe getUnsafe() {
397	<	try {
398	<	return sun.misc.Unsafe.getUnsafe();
399	<	} catch (SecurityException se) {
400	<	try {
143	<	return java.security.AccessController.doPrivileged
144	<	(new java.security
145	<	.PrivilegedExceptionAction<sun.misc.Unsafe>() {
146	<	public sun.misc.Unsafe run() throws Exception {
147	<	java.lang.reflect.Field f = sun.misc
148	<	.Unsafe.class.getDeclaredField("theUnsafe");
149	<	f.setAccessible(true);
150	<	return (sun.misc.Unsafe) f.get(null);
151	<	}});
152	<	} catch (java.security.PrivilegedActionException e) {
153	<	throw new RuntimeException("Could not initialize intrinsics",
154	<	e.getCause());
155	<	}
396	>	final boolean tryMatchData() {
397	>	Object x = item;
398	>	if (x != null && x != this && casItem(x, null)) {
399	>	LockSupport.unpark(waiter);
400	>	return true;
401		}
402	+	return false;
403		}
404
405	+	// Unsafe mechanics
406	+	private static final sun.misc.Unsafe UNSAFE = getUnsafe();
407	+	private static final long nextOffset =
408	+	objectFieldOffset(UNSAFE, "next", Node.class);
409	+	private static final long itemOffset =
410	+	objectFieldOffset(UNSAFE, "item", Node.class);
411	+	private static final long waiterOffset =
412	+	objectFieldOffset(UNSAFE, "waiter", Node.class);
413	+
414		private static final long serialVersionUID = -3375979862319811754L;
415		}
416
417	<	/**
418	<	* Padded version of AtomicReference used for head, tail and
164	<	* cleanMe, to alleviate contention across threads CASing one vs
165	<	* the other.
166	<	*/
167	<	static final class PaddedAtomicReference<T> extends AtomicReference<T> {
168	<	// enough padding for 64bytes with 4byte refs
169	<	Object p0, p1, p2, p3, p4, p5, p6, p7, p8, p9, pa, pb, pc, pd, pe;
170	<	PaddedAtomicReference(T r) { super(r); }
171	<	private static final long serialVersionUID = 8170090609809740854L;
172	<	}
417	>	/** head of the queue; null until first enqueue */
418	>	private transient volatile Node head;
419
420	+	/** predecessor of dangling unspliceable node */
421	+	private transient volatile Node cleanMe; // decl here to reduce contention
422
423	<	/** head of the queue */
424	<	private transient final PaddedAtomicReference<Node<E>> head;
423	>	/** tail of the queue; null until first append */
424	>	private transient volatile Node tail;
425
426	<	/** tail of the queue */
427	<	private transient final PaddedAtomicReference<Node<E>> tail;
426	>	// CAS methods for fields
427	>	private boolean casTail(Node cmp, Node val) {
428	>	return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val);
429	>	}
430
431	<	/**
432	<	* Reference to a cancelled node that might not yet have been
433	<	* unlinked from queue because it was the last inserted node
184	<	* when it cancelled.
185	<	*/
186	<	private transient final PaddedAtomicReference<Node<E>> cleanMe;
431	>	private boolean casHead(Node cmp, Node val) {
432	>	return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val);
433	>	}
434
435	<	/**
436	<	* Tries to cas nh as new head; if successful, unlink
190	<	* old head's next node to avoid garbage retention.
191	<	*/
192	<	private boolean advanceHead(Node<E> h, Node<E> nh) {
193	<	if (h == head.get() && head.compareAndSet(h, nh)) {
194	<	h.clearNext(); // forget old next
195	<	return true;
196	<	}
197	<	return false;
435	>	private boolean casCleanMe(Node cmp, Node val) {
436	>	return UNSAFE.compareAndSwapObject(this, cleanMeOffset, cmp, val);
437		}
438
439	+	/*
440	+	* Possible values for "how" argument in xfer method. Beware that
441	+	* the order of assigned numerical values matters.
442	+	*/
443	+	private static final int NOW     = 0; // for untimed poll, tryTransfer
444	+	private static final int ASYNC   = 1; // for offer, put, add
445	+	private static final int SYNC    = 2; // for transfer, take
446	+	private static final int TIMEOUT = 3; // for timed poll, tryTransfer
447	+
448		/**
449	<	* Puts or takes an item. Used for most queue operations (except
202	<	* poll() and tryTransfer()). See the similar code in
203	<	* SynchronousQueue for detailed explanation.
449	>	* Implements all queuing methods. See above for explanation.
450		*
451	<	* @param e the item or if null, signifies that this is a take
452	<	* @param mode the wait mode: NOWAIT, TIMEOUT, WAIT
451	>	* @param e the item or null for take
452	>	* @param haveData true if this is a put, else a take
453	>	* @param how NOW, ASYNC, SYNC, or TIMEOUT
454		* @param nanos timeout in nanosecs, used only if mode is TIMEOUT
455	<	* @return an item, or null on failure
455	>	* @return an item if matched, else e
456	>	* @throws NullPointerException if haveData mode but e is null
457		*/
458	<	private E xfer(E e, int mode, long nanos) {
459	<	boolean isData = (e != null);
460	<	Node<E> s = null;
461	<	final PaddedAtomicReference<Node<E>> head = this.head;
214	<	final PaddedAtomicReference<Node<E>> tail = this.tail;
458	>	private Object xfer(Object e, boolean haveData, int how, long nanos) {
459	>	if (haveData && (e == null))
460	>	throw new NullPointerException();
461	>	Node s = null;                        // the node to append, if needed
462
463	<	for (;;) {
217	<	Node<E> t = tail.get();
218	<	Node<E> h = head.get();
463	>	retry: for (;;) {                     // restart on append race
464
465	<	if (t != null && (t == h || t.isData == isData)) {
466	<	if (s == null)
467	<	s = new Node<E>(e, isData);
468	<	Node<E> last = t.next;
469	<	if (last != null) {
470	<	if (t == tail.get())
471	<	tail.compareAndSet(t, last);
472	<	}
473	<	else if (t.casNext(null, s)) {
474	<	tail.compareAndSet(t, s);
475	<	return awaitFulfill(t, s, e, mode, nanos);
465	>	for (Node h = head, p = h; p != null;) { // find & match first node
466	>	boolean isData = p.isData;
467	>	Object item = p.item;
468	>	if (item != p && (item != null) == isData) { // unmatched
469	>	if (isData == haveData)   // can't match
470	>	break;
471	>	if (p.casItem(item, e)) { // match
472	>	Thread w = p.waiter;
473	>	while (p != h) {      // update head
474	>	Node n = p.next;  // by 2 unless singleton
475	>	if (n != null)
476	>	p = n;
477	>	if (head == h && casHead(h, p)) {
478	>	h.forgetNext();
479	>	break;
480	>	}                 // advance and retry
481	>	if ((h = head)   == null ||
482	>	(p = h.next) == null || !p.isMatched())
483	>	break;        // unless slack < 2
484	>	}
485	>	LockSupport.unpark(w);
486	>	return item;
487	>	}
488		}
489	+	Node n = p.next;
490	+	p = p != n ? n : (h = head);  // Use head if p offlist
491		}
492
493	<	else if (h != null) {
494	<	Node<E> first = h.next;
495	<	if (t == tail.get() && first != null &&
496	<	advanceHead(h, first)) {
497	<	Object x = first.get();
498	<	if (x != first && first.compareAndSet(x, e)) {
499	<	LockSupport.unpark(first.waiter);
500	<	return isData ? e : (E) x;
242	<	}
243	<	}
493	>	if (how >= ASYNC) {               // No matches available
494	>	if (s == null)
495	>	s = new Node(e, haveData);
496	>	Node pred = tryAppend(s, haveData);
497	>	if (pred == null)
498	>	continue retry;           // lost race vs opposite mode
499	>	if (how >= SYNC)
500	>	return awaitMatch(pred, s, e, how, nanos);
501		}
502	+	return e; // not waiting
503		}
504		}
505
248	–
506		/**
507	<	* Version of xfer for poll() and tryTransfer, which
508	<	* simplifies control paths both here and in xfer.
509	<	*/
510	<	private E fulfill(E e) {
511	<	boolean isData = (e != null);
512	<	final PaddedAtomicReference<Node<E>> head = this.head;
513	<	final PaddedAtomicReference<Node<E>> tail = this.tail;
514	<
515	<	for (;;) {
516	<	Node<E> t = tail.get();
517	<	Node<E> h = head.get();
518	<
519	<	if (t != null && (t == h || t.isData == isData)) {
520	<	Node<E> last = t.next;
521	<	if (t == tail.get()) {
522	<	if (last != null)
523	<	tail.compareAndSet(t, last);
524	<	else
525	<	return null;
526	<	}
527	<	}
528	<	else if (h != null) {
529	<	Node<E> first = h.next;
530	<	if (t == tail.get() &&
531	<	first != null &&
532	<	advanceHead(h, first)) {
533	<	Object x = first.get();
534	<	if (x != first && first.compareAndSet(x, e)) {
278	<	LockSupport.unpark(first.waiter);
279	<	return isData ? e : (E) x;
280	<	}
507	>	* Tries to append node s as tail.
508	>	*
509	>	* @param haveData true if appending in data mode
510	>	* @param s the node to append
511	>	* @return null on failure due to losing race with append in
512	>	* different mode, else s's predecessor, or s itself if no
513	>	* predecessor
514	>	*/
515	>	private Node tryAppend(Node s, boolean haveData) {
516	>	for (Node t = tail, p = t;;) { // move p to actual tail and append
517	>	Node n, u;                        // temps for reads of next & tail
518	>	if (p == null && (p = head) == null) {
519	>	if (casHead(null, s))
520	>	return s;                 // initialize
521	>	}
522	>	else if (p.cannotPrecede(haveData))
523	>	return null;                  // lost race vs opposite mode
524	>	else if ((n = p.next) != null)    // Not tail; keep traversing
525	>	p = p != t && t != (u = tail) ? (t = u) : // stale tail
526	>	p != n ? n : null;        // restart if off list
527	>	else if (!p.casNext(null, s))
528	>	p = p.next;                   // re-read on CAS failure
529	>	else {
530	>	if (p != t) {                 // Update if slack now >= 2
531	>	while ((tail != t || !casTail(t, s)) &&
532	>	(t = tail)   != null &&
533	>	(s = t.next) != null && // advance and retry
534	>	(s = s.next) != null && s != t);
535		}
536	+	return p;
537		}
538		}
539		}
540
541		/**
542	<	* Spins/blocks until node s is fulfilled or caller gives up,
288	<	* depending on wait mode.
542	>	* Spins/yields/blocks until node s is matched or caller gives up.
543		*
544	<	* @param pred the predecessor of waiting node
544	>	* @param pred the predecessor of s or s or null if none
545		* @param s the waiting node
546		* @param e the comparison value for checking match
547	<	* @param mode mode
547	>	* @param how either SYNC or TIMEOUT
548		* @param nanos timeout value
549	<	* @return matched item, or s if cancelled
549	>	* @return matched item, or e if unmatched on interrupt or timeout
550		*/
551	<	private E awaitFulfill(Node<E> pred, Node<E> s, E e,
552	<	int mode, long nanos) {
553	<	if (mode == NOWAIT)
300	<	return null;
301	<
302	<	long lastTime = (mode == TIMEOUT) ? System.nanoTime() : 0;
551	>	private Object awaitMatch(Node pred, Node s, Object e,
552	>	int how, long nanos) {
553	>	long lastTime = (how == TIMEOUT) ? System.nanoTime() : 0L;
554		Thread w = Thread.currentThread();
555	<	int spins = -1; // set to desired spin count below
555	>	int spins = -1; // initialized after first item and cancel checks
556	>	ThreadLocalRandom randomYields = null; // bound if needed
557	>
558		for (;;) {
559	<	if (w.isInterrupted())
560	<	s.compareAndSet(e, s);
561	<	Object x = s.get();
562	<	if (x != e) {                 // Node was matched or cancelled
563	<	advanceHead(pred, s);     // unlink if head
564	<	if (x == s) {             // was cancelled
565	<	clean(pred, s);
566	<	return null;
567	<	}
568	<	else if (x != null) {
569	<	s.set(s);             // avoid garbage retention
570	<	return (E) x;
571	<	}
572	<	else
573	<	return e;
559	>	Object item = s.item;
560	>	if (item != e) {                  // matched
561	>	s.forgetContents();           // avoid garbage
562	>	return item;
563	>	}
564	>	if ((w.isInterrupted() || (how == TIMEOUT && nanos <= 0)) &&
565	>	s.casItem(e, s)) {       // cancel
566	>	unsplice(pred, s);
567	>	return e;
568	>	}
569	>
570	>	if (spins < 0) {                  // establish spins at/near front
571	>	if ((spins = spinsFor(pred, s.isData)) > 0)
572	>	randomYields = ThreadLocalRandom.current();
573	>	}
574	>	else if (spins > 0) {             // spin, occasionally yield
575	>	if (randomYields.nextInt(FRONT_SPINS) == 0)
576	>	Thread.yield();
577	>	--spins;
578	>	}
579	>	else if (s.waiter == null) {
580	>	shortenHeadPath();            // reduce slack before blocking
581	>	s.waiter = w;                 // request unpark
582		}
583	<	if (mode == TIMEOUT) {
583	>	else if (how == TIMEOUT) {
584		long now = System.nanoTime();
585	<	nanos -= now - lastTime;
585	>	if ((nanos -= now - lastTime) > 0)
586	>	LockSupport.parkNanos(this, nanos);
587		lastTime = now;
326	–	if (nanos <= 0) {
327	–	s.compareAndSet(e, s); // try to cancel
328	–	continue;
329	–	}
330	–	}
331	–	if (spins < 0) {
332	–	Node<E> h = head.get(); // only spin if at head
333	–	spins = ((h != null && h.next == s) ?
334	–	((mode == TIMEOUT) ?
335	–	maxTimedSpins : maxUntimedSpins) : 0);
588		}
589	<	if (spins > 0)
338	<	--spins;
339	<	else if (s.waiter == null)
340	<	s.waiter = w;
341	<	else if (mode != TIMEOUT) {
589	>	else {
590		LockSupport.park(this);
591	<	s.waiter = null;
344	<	spins = -1;
345	<	}
346	<	else if (nanos > spinForTimeoutThreshold) {
347	<	LockSupport.parkNanos(this, nanos);
348	<	s.waiter = null;
349	<	spins = -1;
591	>	spins = -1;                   // spin if front upon wakeup
592		}
593		}
594		}
595
596		/**
597	<	* Returns validated tail for use in cleaning methods.
597	>	* Returns spin/yield value for a node with given predecessor and
598	>	* data mode. See above for explanation.
599		*/
600	<	private Node<E> getValidatedTail() {
601	<	for (;;) {
602	<	Node<E> h = head.get();
603	<	Node<E> first = h.next;
604	<	if (first != null && first.next == first) { // help advance
605	<	advanceHead(h, first);
606	<	continue;
607	<	}
608	<	Node<E> t = tail.get();
609	<	Node<E> last = t.next;
610	<	if (t == tail.get()) {
611	<	if (last != null)
612	<	tail.compareAndSet(t, last); // help advance
613	<	else
614	<	return t;
600	>	private static int spinsFor(Node pred, boolean haveData) {
601	>	if (MP && pred != null) {
602	>	boolean predData = pred.isData;
603	>	if (predData != haveData)         // front and phase change
604	>	return FRONT_SPINS + (FRONT_SPINS >>> 1);
605	>	if (predData != (pred.item != null)) // probably at front
606	>	return FRONT_SPINS;
607	>	if (pred.waiter == null)          // pred apparently spinning
608	>	return CHAINED_SPINS;
609	>	}
610	>	return 0;
611	>	}
612	>
613	>	/**
614	>	* Tries (once) to unsplice nodes between head and first unmatched
615	>	* or trailing node; failing on contention.
616	>	*/
617	>	private void shortenHeadPath() {
618	>	Node h, hn, p, q;
619	>	if ((p = h = head) != null && h.isMatched() &&
620	>	(q = hn = h.next) != null) {
621	>	Node n;
622	>	while ((n = q.next) != q) {
623	>	if (n == null || !q.isMatched()) {
624	>	if (hn != q && h.next == hn)
625	>	h.casNext(hn, q);
626	>	break;
627	>	}
628	>	p = q;
629	>	q = n;
630		}
631		}
632		}
633
634	+	/* -------------- Traversal methods -------------- */
635	+
636		/**
637	<	* Gets rid of cancelled node s with original predecessor pred.
638	<	*
639	<	* @param pred predecessor of cancelled node
640	<	* @param s the cancelled node
637	>	* Returns the first unmatched node of the given mode, or null if
638	>	* none.  Used by methods isEmpty, hasWaitingConsumer.
639	>	*/
640	>	private Node firstOfMode(boolean data) {
641	>	for (Node p = head; p != null; ) {
642	>	if (!p.isMatched())
643	>	return p.isData == data? p : null;
644	>	Node n = p.next;
645	>	p = n != p ? n : head;
646	>	}
647	>	return null;
648	>	}
649	>
650	>	/**
651	>	* Returns the item in the first unmatched node with isData; or
652	>	* null if none. Used by peek.
653		*/
654	<	private void clean(Node<E> pred, Node<E> s) {
655	<	Thread w = s.waiter;
656	<	if (w != null) {             // Wake up thread
657	<	s.waiter = null;
658	<	if (w != Thread.currentThread())
659	<	LockSupport.unpark(w);
654	>	private Object firstDataItem() {
655	>	for (Node p = head; p != null; ) {
656	>	boolean isData = p.isData;
657	>	Object item = p.item;
658	>	if (item != p && (item != null) == isData)
659	>	return isData ? item : null;
660	>	Node n = p.next;
661	>	p = n != p ? n : head;
662		}
663	+	return null;
664	+	}
665	+
666	+	/**
667	+	* Traverses and counts unmatched nodes of the given mode.
668	+	* Used by methods size and getWaitingConsumerCount.
669	+	*/
670	+	private int countOfMode(boolean data) {
671	+	int count = 0;
672	+	for (Node p = head; p != null; ) {
673	+	if (!p.isMatched()) {
674	+	if (p.isData != data)
675	+	return 0;
676	+	if (++count == Integer.MAX_VALUE) // saturated
677	+	break;
678	+	}
679	+	Node n = p.next;
680	+	if (n != p)
681	+	p = n;
682	+	else {
683	+	count = 0;
684	+	p = head;
685	+	}
686	+	}
687	+	return count;
688	+	}
689
690	<	if (pred == null)
691	<	return;
690	>	final class Itr implements Iterator<E> {
691	>	private Node nextNode;   // next node to return item for
692	>	private Object nextItem; // the corresponding item
693	>	private Node lastRet;    // last returned node, to support remove
694
695	+	/**
696	+	* Moves to next node after prev, or first node if prev null.
697	+	*/
698	+	private void advance(Node prev) {
699	+	lastRet = prev;
700	+	Node p;
701	+	if (prev == null || (p = prev.next) == prev)
702	+	p = head;
703	+	while (p != null) {
704	+	Object item = p.item;
705	+	if (p.isData) {
706	+	if (item != null && item != p) {
707	+	nextItem = item;
708	+	nextNode = p;
709	+	return;
710	+	}
711	+	}
712	+	else if (item == null)
713	+	break;
714	+	Node n = p.next;
715	+	p = n != p ? n : head;
716	+	}
717	+	nextNode = null;
718	+	}
719	+
720	+	Itr() {
721	+	advance(null);
722	+	}
723	+
724	+	public final boolean hasNext() {
725	+	return nextNode != null;
726	+	}
727	+
728	+	public final E next() {
729	+	Node p = nextNode;
730	+	if (p == null) throw new NoSuchElementException();
731	+	Object e = nextItem;
732	+	advance(p);
733	+	return (E) e;
734	+	}
735	+
736	+	public final void remove() {
737	+	Node p = lastRet;
738	+	if (p == null) throw new IllegalStateException();
739	+	lastRet = null;
740	+	findAndRemoveNode(p);
741	+	}
742	+	}
743	+
744	+	/* -------------- Removal methods -------------- */
745	+
746	+	/**
747	+	* Unsplices (now or later) the given deleted/cancelled node with
748	+	* the given predecessor.
749	+	*
750	+	* @param pred predecessor of node to be unspliced
751	+	* @param s the node to be unspliced
752	+	*/
753	+	private void unsplice(Node pred, Node s) {
754	+	s.forgetContents(); // clear unneeded fields
755		/*
756		* At any given time, exactly one node on list cannot be
757		* deleted -- the last inserted node. To accommodate this, if
758		* we cannot delete s, we save its predecessor as "cleanMe",
759	<	* processing the previously saved version first. At least one
760	<	* of node s or the node previously saved can always be
759	>	* processing the previously saved version first. Because only
760	>	* one node in the list can have a null next, at least one of
761	>	* node s or the node previously saved can always be
762		* processed, so this always terminates.
763		*/
764	<	while (pred.next == s) {
765	<	Node<E> oldpred = reclean();  // First, help get rid of cleanMe
766	<	Node<E> t = getValidatedTail();
767	<	if (s != t) {               // If not tail, try to unsplice
768	<	Node<E> sn = s.next;      // s.next == s means s already off list
769	<	if (sn == s || pred.casNext(s, sn))
764	>	if (pred != null && pred != s) {
765	>	while (pred.next == s) {
766	>	Node oldpred = cleanMe == null? null : reclean();
767	>	Node n = s.next;
768	>	if (n != null) {
769	>	if (n != s)
770	>	pred.casNext(s, n);
771		break;
772	+	}
773	+	if (oldpred == pred ||      // Already saved
774	+	(oldpred == null && casCleanMe(null, pred)))
775	+	break;                  // Postpone cleaning
776		}
409	–	else if (oldpred == pred || // Already saved
410	–	(oldpred == null && cleanMe.compareAndSet(null, pred)))
411	–	break;                  // Postpone cleaning
777		}
778		}
779
780		/**
781	<	* Tries to unsplice the cancelled node held in cleanMe that was
782	<	* previously uncleanable because it was at tail.
781	>	* Tries to unsplice the deleted/cancelled node held in cleanMe
782	>	* that was previously uncleanable because it was at tail.
783		*
784		* @return current cleanMe node (or null)
785		*/
786	<	private Node<E> reclean() {
786	>	private Node reclean() {
787		/*
788	<	* cleanMe is, or at one time was, predecessor of cancelled
789	<	* node s that was the tail so could not be unspliced.  If s
788	>	* cleanMe is, or at one time was, predecessor of a cancelled
789	>	* node s that was the tail so could not be unspliced.  If it
790		* is no longer the tail, try to unsplice if necessary and
791		* make cleanMe slot available.  This differs from similar
792	<	* code in clean() because we must check that pred still
793	<	* points to a cancelled node that must be unspliced -- if
794	<	* not, we can (must) clear cleanMe without unsplicing.
795	<	* This can loop only due to contention on casNext or
431	<	* clearing cleanMe.
792	>	* code in unsplice() because we must check that pred still
793	>	* points to a matched node that can be unspliced -- if not,
794	>	* we can (must) clear cleanMe without unsplicing.  This can
795	>	* loop only due to contention.
796		*/
797	<	Node<E> pred;
798	<	while ((pred = cleanMe.get()) != null) {
799	<	Node<E> t = getValidatedTail();
800	<	Node<E> s = pred.next;
801	<	if (s != t) {
802	<	Node<E> sn;
803	<	if (s == null || s == pred || s.get() != s ||
804	<	(sn = s.next) == s || pred.casNext(s, sn))
805	<	cleanMe.compareAndSet(pred, null);
797	>	Node pred;
798	>	while ((pred = cleanMe) != null) {
799	>	Node s = pred.next;
800	>	Node n;
801	>	if (s == null || s == pred || !s.isMatched())
802	>	casCleanMe(pred, null); // already gone
803	>	else if ((n = s.next) != null) {
804	>	if (n != s)
805	>	pred.casNext(s, n);
806	>	casCleanMe(pred, null);
807		}
808	<	else // s is still tail; cannot clean
808	>	else
809		break;
810		}
811		return pred;
812		}
813
814		/**
815	+	* Main implementation of Iterator.remove(). Find
816	+	* and unsplice the given node.
817	+	*/
818	+	final void findAndRemoveNode(Node s) {
819	+	if (s.tryMatchData()) {
820	+	Node pred = null;
821	+	Node p = head;
822	+	while (p != null) {
823	+	if (p == s) {
824	+	unsplice(pred, p);
825	+	break;
826	+	}
827	+	if (!p.isData && !p.isMatched())
828	+	break;
829	+	pred = p;
830	+	if ((p = p.next) == pred) { // stale
831	+	pred = null;
832	+	p = head;
833	+	}
834	+	}
835	+	}
836	+	}
837	+
838	+	/**
839	+	* Main implementation of remove(Object)
840	+	*/
841	+	private boolean findAndRemove(Object e) {
842	+	if (e != null) {
843	+	Node pred = null;
844	+	Node p = head;
845	+	while (p != null) {
846	+	Object item = p.item;
847	+	if (p.isData) {
848	+	if (item != null && item != p && e.equals(item) &&
849	+	p.tryMatchData()) {
850	+	unsplice(pred, p);
851	+	return true;
852	+	}
853	+	}
854	+	else if (item == null)
855	+	break;
856	+	pred = p;
857	+	if ((p = p.next) == pred) {
858	+	pred = null;
859	+	p = head;
860	+	}
861	+	}
862	+	}
863	+	return false;
864	+	}
865	+
866	+
867	+	/**
868		* Creates an initially empty {@code LinkedTransferQueue}.
869		*/
870		public LinkedTransferQueue() {
453	–	Node<E> dummy = new Node<E>(null, false);
454	–	head = new PaddedAtomicReference<Node<E>>(dummy);
455	–	tail = new PaddedAtomicReference<Node<E>>(dummy);
456	–	cleanMe = new PaddedAtomicReference<Node<E>>(null);
871		}
872
873		/**
#	Line 471 | Line 885 | public class LinkedTransferQueue<E> exte
885		}
886
887		/**
888	<	* @throws InterruptedException {@inheritDoc}
889	<	* @throws NullPointerException {@inheritDoc}
888	>	* Inserts the specified element at the tail of this queue.
889	>	* As the queue is unbounded, this method will never block.
890	>	*
891	>	* @throws NullPointerException if the specified element is null
892		*/
893	<	public void put(E e) throws InterruptedException {
894	<	if (e == null) throw new NullPointerException();
479	<	if (Thread.interrupted()) throw new InterruptedException();
480	<	xfer(e, NOWAIT, 0);
893	>	public void put(E e) {
894	>	xfer(e, true, ASYNC, 0);
895		}
896
897		/**
898	<	* @throws InterruptedException {@inheritDoc}
899	<	* @throws NullPointerException {@inheritDoc}
898	>	* Inserts the specified element at the tail of this queue.
899	>	* As the queue is unbounded, this method will never block or
900	>	* return {@code false}.
901	>	*
902	>	* @return {@code true} (as specified by
903	>	*  {@link BlockingQueue#offer(Object,long,TimeUnit) BlockingQueue.offer})
904	>	* @throws NullPointerException if the specified element is null
905		*/
906	<	public boolean offer(E e, long timeout, TimeUnit unit)
907	<	throws InterruptedException {
489	<	if (e == null) throw new NullPointerException();
490	<	if (Thread.interrupted()) throw new InterruptedException();
491	<	xfer(e, NOWAIT, 0);
906	>	public boolean offer(E e, long timeout, TimeUnit unit) {
907	>	xfer(e, true, ASYNC, 0);
908		return true;
909		}
910
911		/**
912	<	* @throws NullPointerException {@inheritDoc}
912	>	* Inserts the specified element at the tail of this queue.
913	>	* As the queue is unbounded, this method will never return {@code false}.
914	>	*
915	>	* @return {@code true} (as specified by
916	>	*         {@link BlockingQueue#offer(Object) BlockingQueue.offer})
917	>	* @throws NullPointerException if the specified element is null
918		*/
919		public boolean offer(E e) {
920	<	if (e == null) throw new NullPointerException();
500	<	xfer(e, NOWAIT, 0);
920	>	xfer(e, true, ASYNC, 0);
921		return true;
922		}
923
924		/**
925	<	* @throws NullPointerException {@inheritDoc}
925	>	* Inserts the specified element at the tail of this queue.
926	>	* As the queue is unbounded, this method will never throw
927	>	* {@link IllegalStateException} or return {@code false}.
928	>	*
929	>	* @return {@code true} (as specified by {@link Collection#add})
930	>	* @throws NullPointerException if the specified element is null
931		*/
932		public boolean add(E e) {
933	<	if (e == null) throw new NullPointerException();
509	<	xfer(e, NOWAIT, 0);
933	>	xfer(e, true, ASYNC, 0);
934		return true;
935		}
936
937		/**
938	<	* @throws InterruptedException {@inheritDoc}
939	<	* @throws NullPointerException {@inheritDoc}
938	>	* Transfers the element to a waiting consumer immediately, if possible.
939	>	*
940	>	* <p>More precisely, transfers the specified element immediately
941	>	* if there exists a consumer already waiting to receive it (in
942	>	* {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
943	>	* otherwise returning {@code false} without enqueuing the element.
944	>	*
945	>	* @throws NullPointerException if the specified element is null
946	>	*/
947	>	public boolean tryTransfer(E e) {
948	>	return xfer(e, true, NOW, 0) == null;
949	>	}
950	>
951	>	/**
952	>	* Transfers the element to a consumer, waiting if necessary to do so.
953	>	*
954	>	* <p>More precisely, transfers the specified element immediately
955	>	* if there exists a consumer already waiting to receive it (in
956	>	* {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
957	>	* else inserts the specified element at the tail of this queue
958	>	* and waits until the element is received by a consumer.
959	>	*
960	>	* @throws NullPointerException if the specified element is null
961		*/
962		public void transfer(E e) throws InterruptedException {
963	<	if (e == null) throw new NullPointerException();
964	<	if (xfer(e, WAIT, 0) == null) {
520	<	Thread.interrupted();
963	>	if (xfer(e, true, SYNC, 0) != null) {
964	>	Thread.interrupted(); // failure possible only due to interrupt
965		throw new InterruptedException();
966		}
967		}
968
969		/**
970	<	* @throws InterruptedException {@inheritDoc}
971	<	* @throws NullPointerException {@inheritDoc}
970	>	* Transfers the element to a consumer if it is possible to do so
971	>	* before the timeout elapses.
972	>	*
973	>	* <p>More precisely, transfers the specified element immediately
974	>	* if there exists a consumer already waiting to receive it (in
975	>	* {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
976	>	* else inserts the specified element at the tail of this queue
977	>	* and waits until the element is received by a consumer,
978	>	* returning {@code false} if the specified wait time elapses
979	>	* before the element can be transferred.
980	>	*
981	>	* @throws NullPointerException if the specified element is null
982		*/
983		public boolean tryTransfer(E e, long timeout, TimeUnit unit)
984		throws InterruptedException {
985	<	if (e == null) throw new NullPointerException();
532	<	if (xfer(e, TIMEOUT, unit.toNanos(timeout)) != null)
985	>	if (xfer(e, true, TIMEOUT, unit.toNanos(timeout)) == null)
986		return true;
987		if (!Thread.interrupted())
988		return false;
989		throw new InterruptedException();
990		}
991
539	–	/**
540	–	* @throws NullPointerException {@inheritDoc}
541	–	*/
542	–	public boolean tryTransfer(E e) {
543	–	if (e == null) throw new NullPointerException();
544	–	return fulfill(e) != null;
545	–	}
546	–
547	–	/**
548	–	* @throws InterruptedException {@inheritDoc}
549	–	*/
992		public E take() throws InterruptedException {
993	<	Object e = xfer(null, WAIT, 0);
993	>	Object e = xfer(null, false, SYNC, 0);
994		if (e != null)
995	<	return (E) e;
995	>	return (E)e;
996		Thread.interrupted();
997		throw new InterruptedException();
998		}
999
558	–	/**
559	–	* @throws InterruptedException {@inheritDoc}
560	–	*/
1000		public E poll(long timeout, TimeUnit unit) throws InterruptedException {
1001	<	Object e = xfer(null, TIMEOUT, unit.toNanos(timeout));
1001	>	Object e = xfer(null, false, TIMEOUT, unit.toNanos(timeout));
1002		if (e != null || !Thread.interrupted())
1003	<	return (E) e;
1003	>	return (E)e;
1004		throw new InterruptedException();
1005		}
1006
1007		public E poll() {
1008	<	return fulfill(null);
1008	>	return (E)xfer(null, false, NOW, 0);
1009		}
1010
1011		/**
#	Line 605 | Line 1044 | public class LinkedTransferQueue<E> exte
1044		return n;
1045		}
1046
608	–	// Traversal-based methods
609	–
1047		/**
1048	<	* Returns head after performing any outstanding helping steps.
1048	>	* Returns an iterator over the elements in this queue in proper
1049	>	* sequence, from head to tail.
1050	>	*
1051	>	* <p>The returned iterator is a "weakly consistent" iterator that
1052	>	* will never throw
1053	>	* {@link ConcurrentModificationException ConcurrentModificationException},
1054	>	* and guarantees to traverse elements as they existed upon
1055	>	* construction of the iterator, and may (but is not guaranteed
1056	>	* to) reflect any modifications subsequent to construction.
1057	>	*
1058	>	* @return an iterator over the elements in this queue in proper sequence
1059		*/
613	–	private Node<E> traversalHead() {
614	–	for (;;) {
615	–	Node<E> t = tail.get();
616	–	Node<E> h = head.get();
617	–	if (h != null && t != null) {
618	–	Node<E> last = t.next;
619	–	Node<E> first = h.next;
620	–	if (t == tail.get()) {
621	–	if (last != null)
622	–	tail.compareAndSet(t, last);
623	–	else if (first != null) {
624	–	Object x = first.get();
625	–	if (x == first)
626	–	advanceHead(h, first);
627	–	else
628	–	return h;
629	–	}
630	–	else
631	–	return h;
632	–	}
633	–	}
634	–	reclean();
635	–	}
636	–	}
637	–
638	–
1060		public Iterator<E> iterator() {
1061		return new Itr();
1062		}
1063
643	–	/**
644	–	* Iterators. Basic strategy is to traverse list, treating
645	–	* non-data (i.e., request) nodes as terminating list.
646	–	* Once a valid data node is found, the item is cached
647	–	* so that the next call to next() will return it even
648	–	* if subsequently removed.
649	–	*/
650	–	class Itr implements Iterator<E> {
651	–	Node<E> next;        // node to return next
652	–	Node<E> pnext;       // predecessor of next
653	–	Node<E> snext;       // successor of next
654	–	Node<E> curr;        // last returned node, for remove()
655	–	Node<E> pcurr;       // predecessor of curr, for remove()
656	–	E nextItem;        // Cache of next item, once committed to in next
657	–
658	–	Itr() {
659	–	findNext();
660	–	}
661	–
662	–	/**
663	–	* Ensures next points to next valid node, or null if none.
664	–	*/
665	–	void findNext() {
666	–	for (;;) {
667	–	Node<E> pred = pnext;
668	–	Node<E> q = next;
669	–	if (pred == null || pred == q) {
670	–	pred = traversalHead();
671	–	q = pred.next;
672	–	}
673	–	if (q == null || !q.isData) {
674	–	next = null;
675	–	return;
676	–	}
677	–	Object x = q.get();
678	–	Node<E> s = q.next;
679	–	if (x != null && q != x && q != s) {
680	–	nextItem = (E) x;
681	–	snext = s;
682	–	pnext = pred;
683	–	next = q;
684	–	return;
685	–	}
686	–	pnext = q;
687	–	next = s;
688	–	}
689	–	}
690	–
691	–	public boolean hasNext() {
692	–	return next != null;
693	–	}
694	–
695	–	public E next() {
696	–	if (next == null) throw new NoSuchElementException();
697	–	pcurr = pnext;
698	–	curr = next;
699	–	pnext = next;
700	–	next = snext;
701	–	E x = nextItem;
702	–	findNext();
703	–	return x;
704	–	}
705	–
706	–	public void remove() {
707	–	Node<E> p = curr;
708	–	if (p == null)
709	–	throw new IllegalStateException();
710	–	Object x = p.get();
711	–	if (x != null && x != p && p.compareAndSet(x, p))
712	–	clean(pcurr, p);
713	–	}
714	–	}
715	–
1064		public E peek() {
1065	<	for (;;) {
718	<	Node<E> h = traversalHead();
719	<	Node<E> p = h.next;
720	<	if (p == null)
721	<	return null;
722	<	Object x = p.get();
723	<	if (p != x) {
724	<	if (!p.isData)
725	<	return null;
726	<	if (x != null)
727	<	return (E) x;
728	<	}
729	<	}
1065	>	return (E) firstDataItem();
1066		}
1067
1068	+	/**
1069	+	* Returns {@code true} if this queue contains no elements.
1070	+	*
1071	+	* @return {@code true} if this queue contains no elements
1072	+	*/
1073		public boolean isEmpty() {
1074	<	for (;;) {
734	<	Node<E> h = traversalHead();
735	<	Node<E> p = h.next;
736	<	if (p == null)
737	<	return true;
738	<	Object x = p.get();
739	<	if (p != x) {
740	<	if (!p.isData)
741	<	return true;
742	<	if (x != null)
743	<	return false;
744	<	}
745	<	}
1074	>	return firstOfMode(true) == null;
1075		}
1076
1077		public boolean hasWaitingConsumer() {
1078	<	for (;;) {
750	<	Node<E> h = traversalHead();
751	<	Node<E> p = h.next;
752	<	if (p == null)
753	<	return false;
754	<	Object x = p.get();
755	<	if (p != x)
756	<	return !p.isData;
757	<	}
1078	>	return firstOfMode(false) != null;
1079		}
1080
1081		/**
#	Line 770 | Line 1091 | public class LinkedTransferQueue<E> exte
1091		* @return the number of elements in this queue
1092		*/
1093		public int size() {
1094	<	int count = 0;
774	<	Node<E> h = traversalHead();
775	<	for (Node<E> p = h.next; p != null && p.isData; p = p.next) {
776	<	Object x = p.get();
777	<	if (x != null && x != p) {
778	<	if (++count == Integer.MAX_VALUE) // saturated
779	<	break;
780	<	}
781	<	}
782	<	return count;
1094	>	return countOfMode(true);
1095		}
1096
1097		public int getWaitingConsumerCount() {
1098	<	int count = 0;
787	<	Node<E> h = traversalHead();
788	<	for (Node<E> p = h.next; p != null && !p.isData; p = p.next) {
789	<	if (p.get() == null) {
790	<	if (++count == Integer.MAX_VALUE)
791	<	break;
792	<	}
793	<	}
794	<	return count;
1098	>	return countOfMode(false);
1099		}
1100
1101	<	public int remainingCapacity() {
1102	<	return Integer.MAX_VALUE;
1101	>	/**
1102	>	* Removes a single instance of the specified element from this queue,
1103	>	* if it is present.  More formally, removes an element {@code e} such
1104	>	* that {@code o.equals(e)}, if this queue contains one or more such
1105	>	* elements.
1106	>	* Returns {@code true} if this queue contained the specified element
1107	>	* (or equivalently, if this queue changed as a result of the call).
1108	>	*
1109	>	* @param o element to be removed from this queue, if present
1110	>	* @return {@code true} if this queue changed as a result of the call
1111	>	*/
1112	>	public boolean remove(Object o) {
1113	>	return findAndRemove(o);
1114		}
1115
1116	<	public boolean remove(Object o) {
1117	<	if (o == null)
1118	<	return false;
1119	<	for (;;) {
1120	<	Node<E> pred = traversalHead();
1121	<	for (;;) {
1122	<	Node<E> q = pred.next;
1123	<	if (q == null || !q.isData)
1124	<	return false;
810	<	if (q == pred) // restart
811	<	break;
812	<	Object x = q.get();
813	<	if (x != null && x != q && o.equals(x) &&
814	<	q.compareAndSet(x, q)) {
815	<	clean(pred, q);
816	<	return true;
817	<	}
818	<	pred = q;
819	<	}
820	<	}
1116	>	/**
1117	>	* Always returns {@code Integer.MAX_VALUE} because a
1118	>	* {@code LinkedTransferQueue} is not capacity constrained.
1119	>	*
1120	>	* @return {@code Integer.MAX_VALUE} (as specified by
1121	>	*         {@link BlockingQueue#remainingCapacity()})
1122	>	*/
1123	>	public int remainingCapacity() {
1124	>	return Integer.MAX_VALUE;
1125		}
1126
1127		/**
1128	<	* Save the state to a stream (that is, serialize it).
1128	>	* Saves the state to a stream (that is, serializes it).
1129		*
1130		* @serialData All of the elements (each an {@code E}) in
1131		* the proper order, followed by a null
#	Line 837 | Line 1141 | public class LinkedTransferQueue<E> exte
1141		}
1142
1143		/**
1144	<	* Reconstitute the Queue instance from a stream (that is,
1145	<	* deserialize it).
1144	>	* Reconstitutes the Queue instance from a stream (that is,
1145	>	* deserializes it).
1146		*
1147		* @param s the stream
1148		*/
1149		private void readObject(java.io.ObjectInputStream s)
1150		throws java.io.IOException, ClassNotFoundException {
1151		s.defaultReadObject();
848	–	resetHeadAndTail();
1152		for (;;) {
1153		@SuppressWarnings("unchecked") E item = (E) s.readObject();
1154		if (item == null)
#	Line 855 | Line 1158 | public class LinkedTransferQueue<E> exte
1158		}
1159		}
1160
858	–	// Support for resetting head/tail while deserializing
859	–	private void resetHeadAndTail() {
860	–	Node<E> dummy = new Node<E>(null, false);
861	–	UNSAFE.putObjectVolatile(this, headOffset,
862	–	new PaddedAtomicReference<Node<E>>(dummy));
863	–	UNSAFE.putObjectVolatile(this, tailOffset,
864	–	new PaddedAtomicReference<Node<E>>(dummy));
865	–	UNSAFE.putObjectVolatile(this, cleanMeOffset,
866	–	new PaddedAtomicReference<Node<E>>(null));
867	–	}
1161
1162		// Unsafe mechanics
1163
#	Line 876 | Line 1169 | public class LinkedTransferQueue<E> exte
1169		private static final long cleanMeOffset =
1170		objectFieldOffset(UNSAFE, "cleanMe", LinkedTransferQueue.class);
1171
879	–
1172		static long objectFieldOffset(sun.misc.Unsafe UNSAFE,
1173		String field, Class<?> klazz) {
1174		try {
#	Line 889 | Line 1181 | public class LinkedTransferQueue<E> exte
1181		}
1182		}
1183
892	–	/**
893	–	* Returns a sun.misc.Unsafe.  Suitable for use in a 3rd party package.
894	–	* Replace with a simple call to Unsafe.getUnsafe when integrating
895	–	* into a jdk.
896	–	*
897	–	* @return a sun.misc.Unsafe
898	–	*/
1184		private static sun.misc.Unsafe getUnsafe() {
1185		try {
1186		return sun.misc.Unsafe.getUnsafe();
#	Line 916 | Line 1201 | public class LinkedTransferQueue<E> exte
1201		}
1202		}
1203		}
1204	+
1205		}

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