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Comparing jsr166/src/jsr166y/LinkedTransferQueue.java (file contents):
Revision 1.23 by jsr166, Thu Jul 23 23:07:57 2009 UTC vs.
Revision 1.56 by jsr166, Wed Oct 28 00:14:03 2009 UTC

#	Line 5 | Line 5
5		*/
6
7		package jsr166y;
8	+
9		import java.util.concurrent.*;
9	–	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.*;
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;
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 50 | 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
54	<	* SynchronousQueues. See the internal documentation, as well as
55	<	* the PPoPP 2006 paper "Scalable Synchronous Queues" by Scherer,
56	<	* Lea & Scott
57	<	* (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();
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 interior elements, such as
109	>	* j.u.c.ConcurrentLinkedQueue.)
110	>	*
111	>	* Once a node is matched, its match status can never again
112	>	* change.  We may thus arrange that the linked list of them
113	>	* contain a prefix of zero or more matched nodes, followed by a
114	>	* suffix of zero or more unmatched nodes. (Note that we allow
115	>	* both the prefix and suffix to be zero length, which in turn
116	>	* means that we do not use a dummy header.)  If we were not
117	>	* concerned with either time or space efficiency, we could
118	>	* correctly perform enqueue and dequeue operations by traversing
119	>	* from a pointer to the initial node; CASing the item of the
120	>	* first unmatched node on match and CASing the next field of the
121	>	* trailing node on appends. (Plus some special-casing when
122	>	* initially empty).  While this would be a terrible idea in
123	>	* itself, it does have the benefit of not requiring ANY atomic
124	>	* updates on head/tail fields.
125	>	*
126	>	* We introduce here an approach that lies between the extremes of
127	>	* never versus always updating queue (head and tail) pointers.
128	>	* This offers a tradeoff between sometimes requiring extra
129	>	* traversal steps to locate the first and/or last unmatched
130	>	* nodes, versus the reduced overhead and contention of fewer
131	>	* updates to queue pointers. For example, a possible snapshot of
132	>	* a queue is:
133	>	*
134	>	*  head           tail
135	>	*    |              |
136	>	*    v              v
137	>	*    M -> M -> U -> U -> U -> U
138	>	*
139	>	* The best value for this "slack" (the targeted maximum distance
140	>	* between the value of "head" and the first unmatched node, and
141	>	* similarly for "tail") is an empirical matter. We have found
142	>	* that using very small constants in the range of 1-3 work best
143	>	* over a range of platforms. Larger values introduce increasing
144	>	* costs of cache misses and risks of long traversal chains, while
145	>	* smaller values increase CAS contention and overhead.
146	>	*
147	>	* Dual queues with slack differ from plain M&S dual queues by
148	>	* virtue of only sometimes updating head or tail pointers when
149	>	* matching, appending, or even traversing nodes; in order to
150	>	* maintain a targeted slack.  The idea of "sometimes" may be
151	>	* operationalized in several ways. The simplest is to use a
152	>	* per-operation counter incremented on each traversal step, and
153	>	* to try (via CAS) to update the associated queue pointer
154	>	* whenever the count exceeds a threshold. Another, that requires
155	>	* more overhead, is to use random number generators to update
156	>	* with a given probability per traversal step.
157	>	*
158	>	* In any strategy along these lines, because CASes updating
159	>	* fields may fail, the actual slack may exceed targeted
160	>	* slack. However, they may be retried at any time to maintain
161	>	* targets.  Even when using very small slack values, this
162	>	* approach works well for dual queues because it allows all
163	>	* operations up to the point of matching or appending an item
164	>	* (hence potentially allowing progress by another thread) to be
165	>	* read-only, thus not introducing any further contention. As
166	>	* described below, we implement this by performing slack
167	>	* maintenance retries only after these points.
168	>	*
169	>	* As an accompaniment to such techniques, traversal overhead can
170	>	* be further reduced without increasing contention of head
171	>	* pointer updates: Threads may sometimes shortcut the "next" link
172	>	* path from the current "head" node to be closer to the currently
173	>	* known first unmatched node, and similarly for tail. Again, this
174	>	* may be triggered with using thresholds or randomization.
175	>	*
176	>	* These ideas must be further extended to avoid unbounded amounts
177	>	* of costly-to-reclaim garbage caused by the sequential "next"
178	>	* links of nodes starting at old forgotten head nodes: As first
179	>	* described in detail by Boehm
180	>	* (http://portal.acm.org/citation.cfm?doid=503272.503282) if a GC
181	>	* delays noticing that any arbitrarily old node has become
182	>	* garbage, all newer dead nodes will also be unreclaimed.
183	>	* (Similar issues arise in non-GC environments.)  To cope with
184	>	* this in our implementation, upon CASing to advance the head
185	>	* pointer, we set the "next" link of the previous head to point
186	>	* only to itself; thus limiting the length of connected dead lists.
187	>	* (We also take similar care to wipe out possibly garbage
188	>	* retaining values held in other Node fields.)  However, doing so
189	>	* adds some further complexity to traversal: If any "next"
190	>	* pointer links to itself, it indicates that the current thread
191	>	* has lagged behind a head-update, and so the traversal must
192	>	* continue from the "head".  Traversals trying to find the
193	>	* current tail starting from "tail" may also encounter
194	>	* self-links, in which case they also continue at "head".
195	>	*
196	>	* It is tempting in slack-based scheme to not even use CAS for
197	>	* updates (similarly to Ladan-Mozes & Shavit). However, this
198	>	* cannot be done for head updates under the above link-forgetting
199	>	* mechanics because an update may leave head at a detached node.
200	>	* And while direct writes are possible for tail updates, they
201	>	* increase the risk of long retraversals, and hence long garbage
202	>	* chains, which can be much more costly than is worthwhile
203	>	* considering that the cost difference of performing a CAS vs
204	>	* write is smaller when they are not triggered on each operation
205	>	* (especially considering that writes and CASes equally require
206	>	* additional GC bookkeeping ("write barriers") that are sometimes
207	>	* more costly than the writes themselves because of contention).
208	>	*
209	>	* Removal of interior nodes (due to timed out or interrupted
210	>	* waits, or calls to remove(x) or Iterator.remove) can use a
211	>	* scheme roughly similar to that described in Scherer, Lea, and
212	>	* Scott's SynchronousQueue. Given a predecessor, we can unsplice
213	>	* any node except the (actual) tail of the queue. To avoid
214	>	* build-up of cancelled trailing nodes, upon a request to remove
215	>	* a trailing node, it is placed in field "cleanMe" to be
216	>	* unspliced upon the next call to unsplice any other node.
217	>	* Situations needing such mechanics are not common but do occur
218	>	* in practice; for example when an unbounded series of short
219	>	* timed calls to poll repeatedly time out but never otherwise
220	>	* fall off the list because of an untimed call to take at the
221	>	* front of the queue. Note that maintaining field cleanMe does
222	>	* not otherwise much impact garbage retention even if never
223	>	* cleared by some other call because the held node will
224	>	* eventually either directly or indirectly lead to a self-link
225	>	* once off the list.
226	>	*
227	>	* *** Overview of implementation ***
228	>	*
229	>	* We use a threshold-based approach to updates, with a slack
230	>	* threshold of two -- that is, we update head/tail when the
231	>	* current pointer appears to be two or more steps away from the
232	>	* first/last node. The slack value is hard-wired: a path greater
233	>	* than one is naturally implemented by checking equality of
234	>	* traversal pointers except when the list has only one element,
235	>	* in which case we keep slack threshold at one. Avoiding tracking
236	>	* explicit counts across method calls slightly simplifies an
237	>	* already-messy implementation. Using randomization would
238	>	* probably work better if there were a low-quality dirt-cheap
239	>	* per-thread one available, but even ThreadLocalRandom is too
240	>	* heavy for these purposes.
241	>	*
242	>	* With such a small slack threshold value, it is rarely
243	>	* worthwhile to augment this with path short-circuiting; i.e.,
244	>	* unsplicing nodes between head and the first unmatched node, or
245	>	* 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).
251	>	*
252	>	* We allow both the head and tail fields to be null before any
253	>	* nodes are enqueued; initializing upon first append.  This
254	>	* simplifies some other logic, as well as providing more
255	>	* efficient explicit control paths instead of letting JVMs insert
256	>	* implicit NullPointerExceptions when they are null.  While not
257	>	* currently fully implemented, we also leave open the possibility
258	>	* of re-nulling these fields when empty (which is complicated to
259	>	* arrange, for little benefit.)
260	>	*
261	>	* All enqueue/dequeue operations are handled by the single method
262	>	* "xfer" with parameters indicating whether to act as some form
263	>	* of offer, put, poll, take, or transfer (each possibly with
264	>	* timeout). The relative complexity of using one monolithic
265	>	* method outweighs the code bulk and maintenance problems of
266	>	* using separate methods for each case.
267	>	*
268	>	* Operation consists of up to three phases. The first is
269	>	* implemented within method xfer, the second in tryAppend, and
270	>	* the third in method awaitMatch.
271	>	*
272	>	* 1. Try to match an existing node
273	>	*
274	>	*    Starting at head, skip already-matched nodes until finding
275	>	*    an unmatched node of opposite mode, if one exists, in which
276	>	*    case matching it and returning, also if necessary updating
277	>	*    head to one past the matched node (or the node itself if the
278	>	*    list has no other unmatched nodes). If the CAS misses, then
279	>	*    a loop retries advancing head by two steps until either
280	>	*    success or the slack is at most two. By requiring that each
281	>	*    attempt advances head by two (if applicable), we ensure that
282	>	*    the slack does not grow without bound. Traversals also check
283	>	*    if the initial head is now off-list, in which case they
284	>	*    start at the new head.
285	>	*
286	>	*    If no candidates are found and the call was untimed
287	>	*    poll/offer, (argument "how" is NOW) return.
288	>	*
289	>	* 2. Try to append a new node (method tryAppend)
290	>	*
291	>	*    Starting at current tail pointer, find the actual last node
292	>	*    and try to append a new node (or if head was null, establish
293	>	*    the first node). Nodes can be appended only if their
294	>	*    predecessors are either already matched or are of the same
295	>	*    mode. If we detect otherwise, then a new node with opposite
296	>	*    mode must have been appended during traversal, so we must
297	>	*    restart at phase 1. The traversal and update steps are
298	>	*    otherwise similar to phase 1: Retrying upon CAS misses and
299	>	*    checking for staleness.  In particular, if a self-link is
300	>	*    encountered, then we can safely jump to a node on the list
301	>	*    by continuing the traversal at current head.
302	>	*
303	>	*    On successful append, if the call was ASYNC, return.
304	>	*
305	>	* 3. Await match or cancellation (method awaitMatch)
306	>	*
307	>	*    Wait for another thread to match node; instead cancelling if
308	>	*    the current thread was interrupted or the wait timed out. On
309	>	*    multiprocessors, we use front-of-queue spinning: If a node
310	>	*    appears to be the first unmatched node in the queue, it
311	>	*    spins a bit before blocking. In either case, before blocking
312	>	*    it tries to unsplice any nodes between the current "head"
313	>	*    and the first unmatched node.
314	>	*
315	>	*    Front-of-queue spinning vastly improves performance of
316	>	*    heavily contended queues. And so long as it is relatively
317	>	*    brief and "quiet", spinning does not much impact performance
318	>	*    of less-contended queues.  During spins threads check their
319	>	*    interrupt status and generate a thread-local random number
320	>	*    to decide to occasionally perform a Thread.yield. While
321	>	*    yield has underdefined specs, we assume that might it help,
322	>	*    and will not hurt in limiting impact of spinning on busy
323	>	*    systems.  We also use smaller (1/2) spins for nodes that are
324	>	*    not known to be front but whose predecessors have not
325	>	*    blocked -- these "chained" spins avoid artifacts of
326	>	*    front-of-queue rules which otherwise lead to alternating
327	>	*    nodes spinning vs blocking. Further, front threads that
328	>	*    represent phase changes (from data to request node or vice
329	>	*    versa) compared to their predecessors receive additional
330	>	*    chained spins, reflecting longer paths typically required to
331	>	*    unblock threads during phase changes.
332	>	*/
333	>
334	>	/** True if on multiprocessor */
335	>	private static final boolean MP =
336	>	Runtime.getRuntime().availableProcessors() > 1;
337
338		/**
339	<	* The number of times to spin before blocking in timed waits.
340	<	* The value is empirically derived -- it works well across a
341	<	* variety of processors and OSes. Empirically, the best value
342	<	* seems not to vary with number of CPUs (beyond 2) so is just
343	<	* a constant.
339	>	* The number of times to spin (with randomly interspersed calls
340	>	* to Thread.yield) on multiprocessor before blocking when a node
341	>	* is apparently the first waiter in the queue.  See above for
342	>	* explanation. Must be a power of two. The value is empirically
343	>	* derived -- it works pretty well across a variety of processors,
344	>	* numbers of CPUs, and OSes.
345		*/
346	<	static final int maxTimedSpins = (NCPUS < 2) ? 0 : 32;
346	>	private static final int FRONT_SPINS   = 1 << 7;
347
348		/**
349	<	* The number of times to spin before blocking in untimed waits.
350	<	* This is greater than timed value because untimed waits spin
351	<	* faster since they don't need to check times on each spin.
349	>	* The number of times to spin before blocking when a node is
350	>	* preceded by another node that is apparently spinning.  Also
351	>	* serves as an increment to FRONT_SPINS on phase changes, and as
352	>	* base average frequency for yielding during spins. Must be a
353	>	* power of two.
354		*/
355	<	static final int maxUntimedSpins = maxTimedSpins * 16;
355	>	private static final int CHAINED_SPINS = FRONT_SPINS >>> 1;
356
357		/**
358	<	* The number of nanoseconds for which it is faster to spin
359	<	* rather than to use timed park. A rough estimate suffices.
360	<	*/
361	<	static final long spinForTimeoutThreshold = 1000L;
358	>	* Queue nodes. Uses Object, not E, for items to allow forgetting
359	>	* them after use.  Relies heavily on Unsafe mechanics to minimize
360	>	* unnecessary ordering constraints: Writes that intrinsically
361	>	* precede or follow CASes use simple relaxed forms.  Other
362	>	* cleanups use releasing/lazy writes.
363	>	*/
364	>	static final class Node<E> {
365	>	final boolean isData;   // false if this is a request node
366	>	volatile Object item;   // initially non-null if isData; CASed to match
367	>	volatile Node<E> next;
368	>	volatile Thread waiter; // null until waiting
369	>
370	>	// CAS methods for fields
371	>	final boolean casNext(Node<E> cmp, Node<E> val) {
372	>	return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
373	>	}
374	>
375	>	final boolean casItem(Object cmp, Object val) {
376	>	assert cmp == null || cmp.getClass() != Node.class;
377	>	return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
378	>	}
379
380	<	/**
381	<	* Node class for LinkedTransferQueue. Opportunistically
382	<	* subclasses from AtomicReference to represent item. Uses Object,
383	<	* not E, to allow setting item to "this" after use, to avoid
384	<	* garbage retention. Similarly, setting the next field to this is
385	<	* used as sentinel that node is off list.
104	<	*/
105	<	static final class QNode extends AtomicReference<Object> {
106	<	volatile QNode next;
107	<	volatile Thread waiter;       // to control park/unpark
108	<	final boolean isData;
109	<	QNode(Object item, boolean isData) {
110	<	super(item);
380	>	/**
381	>	* Creates a new node. Uses relaxed write because item can only
382	>	* be seen if followed by CAS.
383	>	*/
384	>	Node(E item, boolean isData) {
385	>	UNSAFE.putObject(this, itemOffset, item); // relaxed write
386		this.isData = isData;
387		}
388
389	<	static final AtomicReferenceFieldUpdater<QNode, QNode>
390	<	nextUpdater = AtomicReferenceFieldUpdater.newUpdater
391	<	(QNode.class, QNode.class, "next");
389	>	/**
390	>	* Links node to itself to avoid garbage retention.  Called
391	>	* only after CASing head field, so uses relaxed write.
392	>	*/
393	>	final void forgetNext() {
394	>	UNSAFE.putObject(this, nextOffset, this);
395	>	}
396
397	<	final boolean casNext(QNode cmp, QNode val) {
398	<	return nextUpdater.compareAndSet(this, cmp, val);
397	>	/**
398	>	* Sets item to self (using a releasing/lazy write) and waiter
399	>	* to null, to avoid garbage retention after extracting or
400	>	* cancelling.
401	>	*/
402	>	final void forgetContents() {
403	>	UNSAFE.putOrderedObject(this, itemOffset, this);
404	>	UNSAFE.putOrderedObject(this, waiterOffset, null);
405		}
406
407	<	final void clearNext() {
408	<	nextUpdater.lazySet(this, this);
407	>	/**
408	>	* Returns true if this node has been matched, including the
409	>	* case of artificial matches due to cancellation.
410	>	*/
411	>	final boolean isMatched() {
412	>	Object x = item;
413	>	return x == this || (x != null) != isData;
414		}
415
416	<	}
416	>	/**
417	>	* Returns true if a node with the given mode cannot be
418	>	* appended to this node because this node is unmatched and
419	>	* has opposite data mode.
420	>	*/
421	>	final boolean cannotPrecede(boolean haveData) {
422	>	boolean d = isData;
423	>	Object x;
424	>	return d != haveData && (x = item) != this && (x != null) == d;
425	>	}
426
427	<	/**
428	<	* Padded version of AtomicReference used for head, tail and
429	<	* cleanMe, to alleviate contention across threads CASing one vs
430	<	* the other.
431	<	*/
432	<	static final class PaddedAtomicReference<T> extends AtomicReference<T> {
433	<	// enough padding for 64bytes with 4byte refs
434	<	Object p0, p1, p2, p3, p4, p5, p6, p7, p8, p9, pa, pb, pc, pd, pe;
435	<	PaddedAtomicReference(T r) { super(r); }
427	>	/**
428	>	* Tries to artificially match a data node -- used by remove.
429	>	*/
430	>	final boolean tryMatchData() {
431	>	Object x = item;
432	>	if (x != null && x != this && casItem(x, null)) {
433	>	LockSupport.unpark(waiter);
434	>	return true;
435	>	}
436	>	return false;
437	>	}
438	>
439	>	// Unsafe mechanics
440	>	private static final sun.misc.Unsafe UNSAFE = getUnsafe();
441	>	private static final long nextOffset =
442	>	objectFieldOffset(UNSAFE, "next", Node.class);
443	>	private static final long itemOffset =
444	>	objectFieldOffset(UNSAFE, "item", Node.class);
445	>	private static final long waiterOffset =
446	>	objectFieldOffset(UNSAFE, "waiter", Node.class);
447	>
448	>	private static final long serialVersionUID = -3375979862319811754L;
449		}
450
451	+	/** head of the queue; null until first enqueue */
452	+	transient volatile Node<E> head;
453
454	<	/** head of the queue */
455	<	private transient final PaddedAtomicReference<QNode> head;
454	>	/** predecessor of dangling unspliceable node */
455	>	private transient volatile Node<E> cleanMe; // decl here reduces contention
456
457	<	/** tail of the queue */
458	<	private transient final PaddedAtomicReference<QNode> tail;
457	>	/** tail of the queue; null until first append */
458	>	private transient volatile Node<E> tail;
459
460	<	/**
461	<	* Reference to a cancelled node that might not yet have been
462	<	* unlinked from queue because it was the last inserted node
463	<	* when it cancelled.
150	<	*/
151	<	private transient final PaddedAtomicReference<QNode> cleanMe;
460	>	// CAS methods for fields
461	>	private boolean casTail(Node<E> cmp, Node<E> val) {
462	>	return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val);
463	>	}
464
465	<	/**
466	<	* Tries to cas nh as new head; if successful, unlink
467	<	* old head's next node to avoid garbage retention.
465	>	private boolean casHead(Node<E> cmp, Node<E> val) {
466	>	return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val);
467	>	}
468	>
469	>	private boolean casCleanMe(Node<E> cmp, Node<E> val) {
470	>	return UNSAFE.compareAndSwapObject(this, cleanMeOffset, cmp, val);
471	>	}
472	>
473	>	/*
474	>	* Possible values for "how" argument in xfer method. Beware that
475	>	* the order of assigned numerical values matters.
476		*/
477	<	private boolean advanceHead(QNode h, QNode nh) {
478	<	if (h == head.get() && head.compareAndSet(h, nh)) {
479	<	h.clearNext(); // forget old next
480	<	return true;
481	<	}
482	<	return false;
477	>	private static final int NOW     = 0; // for untimed poll, tryTransfer
478	>	private static final int ASYNC   = 1; // for offer, put, add
479	>	private static final int SYNC    = 2; // for transfer, take
480	>	private static final int TIMEOUT = 3; // for timed poll, tryTransfer
481	>
482	>	@SuppressWarnings("unchecked")
483	>	static <E> E cast(Object item) {
484	>	assert item == null || item.getClass() != Node.class;
485	>	return (E) item;
486		}
487
488		/**
489	<	* Puts or takes an item. Used for most queue operations (except
167	<	* poll() and tryTransfer()). See the similar code in
168	<	* SynchronousQueue for detailed explanation.
489	>	* Implements all queuing methods. See above for explanation.
490		*
491	<	* @param e the item or if null, signifies that this is a take
492	<	* @param mode the wait mode: NOWAIT, TIMEOUT, WAIT
491	>	* @param e the item or null for take
492	>	* @param haveData true if this is a put, else a take
493	>	* @param how NOW, ASYNC, SYNC, or TIMEOUT
494		* @param nanos timeout in nanosecs, used only if mode is TIMEOUT
495	<	* @return an item, or null on failure
495	>	* @return an item if matched, else e
496	>	* @throws NullPointerException if haveData mode but e is null
497		*/
498	<	private Object xfer(Object e, int mode, long nanos) {
499	<	boolean isData = (e != null);
500	<	QNode s = null;
501	<	final PaddedAtomicReference<QNode> head = this.head;
179	<	final PaddedAtomicReference<QNode> tail = this.tail;
498	>	private E xfer(E e, boolean haveData, int how, long nanos) {
499	>	if (haveData && (e == null))
500	>	throw new NullPointerException();
501	>	Node<E> s = null;                     // the node to append, if needed
502
503	<	for (;;) {
182	<	QNode t = tail.get();
183	<	QNode h = head.get();
503	>	retry: for (;;) {                     // restart on append race
504
505	<	if (t != null && (t == h || t.isData == isData)) {
506	<	if (s == null)
507	<	s = new QNode(e, isData);
508	<	QNode last = t.next;
509	<	if (last != null) {
510	<	if (t == tail.get())
511	<	tail.compareAndSet(t, last);
512	<	}
513	<	else if (t.casNext(null, s)) {
514	<	tail.compareAndSet(t, s);
515	<	return awaitFulfill(t, s, e, mode, nanos);
505	>	for (Node<E> h = head, p = h; p != null;) {
506	>	// find & match first node
507	>	boolean isData = p.isData;
508	>	Object item = p.item;
509	>	if (item != p && (item != null) == isData) { // unmatched
510	>	if (isData == haveData)   // can't match
511	>	break;
512	>	if (p.casItem(item, e)) { // match
513	>	for (Node<E> q = p; q != h;) {
514	>	Node<E> n = q.next; // update head by 2
515	>	if (n != null)    // unless singleton
516	>	q = n;
517	>	if (head == h && casHead(h, q)) {
518	>	h.forgetNext();
519	>	break;
520	>	}                 // advance and retry
521	>	if ((h = head)   == null ||
522	>	(q = h.next) == null || !q.isMatched())
523	>	break;        // unless slack < 2
524	>	}
525	>	LockSupport.unpark(p.waiter);
526	>	return this.<E>cast(item);
527	>	}
528		}
529	+	Node<E> n = p.next;
530	+	p = (p != n) ? n : (h = head); // Use head if p offlist
531		}
532
533	<	else if (h != null) {
534	<	QNode first = h.next;
535	<	if (t == tail.get() && first != null &&
536	<	advanceHead(h, first)) {
537	<	Object x = first.get();
538	<	if (x != first && first.compareAndSet(x, e)) {
539	<	LockSupport.unpark(first.waiter);
540	<	return isData ? e : x;
207	<	}
208	<	}
533	>	if (how >= ASYNC) {               // No matches available
534	>	if (s == null)
535	>	s = new Node<E>(e, haveData);
536	>	Node<E> pred = tryAppend(s, haveData);
537	>	if (pred == null)
538	>	continue retry;           // lost race vs opposite mode
539	>	if (how >= SYNC)
540	>	return awaitMatch(s, pred, e, how, nanos);
541		}
542	+	return e; // not waiting
543		}
544		}
545
213	–
546		/**
547	<	* Version of xfer for poll() and tryTransfer, which
548	<	* simplifies control paths both here and in xfer.
549	<	*/
550	<	private Object fulfill(Object e) {
551	<	boolean isData = (e != null);
552	<	final PaddedAtomicReference<QNode> head = this.head;
553	<	final PaddedAtomicReference<QNode> tail = this.tail;
554	<
555	<	for (;;) {
556	<	QNode t = tail.get();
557	<	QNode h = head.get();
558	<
559	<	if (t != null && (t == h || t.isData == isData)) {
560	<	QNode last = t.next;
561	<	if (t == tail.get()) {
562	<	if (last != null)
563	<	tail.compareAndSet(t, last);
564	<	else
565	<	return null;
566	<	}
567	<	}
568	<	else if (h != null) {
569	<	QNode first = h.next;
570	<	if (t == tail.get() &&
571	<	first != null &&
572	<	advanceHead(h, first)) {
573	<	Object x = first.get();
574	<	if (x != first && first.compareAndSet(x, e)) {
243	<	LockSupport.unpark(first.waiter);
244	<	return isData ? e : x;
245	<	}
547	>	* Tries to append node s as tail.
548	>	*
549	>	* @param s the node to append
550	>	* @param haveData true if appending in data mode
551	>	* @return null on failure due to losing race with append in
552	>	* different mode, else s's predecessor, or s itself if no
553	>	* predecessor
554	>	*/
555	>	private Node<E> tryAppend(Node<E> s, boolean haveData) {
556	>	for (Node<E> t = tail, p = t;;) { // move p to last node and append
557	>	Node<E> n, u;                     // temps for reads of next & tail
558	>	if (p == null && (p = head) == null) {
559	>	if (casHead(null, s))
560	>	return s;                 // initialize
561	>	}
562	>	else if (p.cannotPrecede(haveData))
563	>	return null;                  // lost race vs opposite mode
564	>	else if ((n = p.next) != null)    // not last; keep traversing
565	>	p = p != t && t != (u = tail) ? (t = u) : // stale tail
566	>	(p != n) ? n : null;      // restart if off list
567	>	else if (!p.casNext(null, s))
568	>	p = p.next;                   // re-read on CAS failure
569	>	else {
570	>	if (p != t) {                 // update if slack now >= 2
571	>	while ((tail != t || !casTail(t, s)) &&
572	>	(t = tail)   != null &&
573	>	(s = t.next) != null && // advance and retry
574	>	(s = s.next) != null && s != t);
575		}
576	+	return p;
577		}
578		}
579		}
580
581		/**
582	<	* Spins/blocks until node s is fulfilled or caller gives up,
253	<	* depending on wait mode.
582	>	* Spins/yields/blocks until node s is matched or caller gives up.
583		*
255	–	* @param pred the predecessor of waiting node
584		* @param s the waiting node
585	+	* @param pred the predecessor of s, or s itself if it has no
586	+	* predecessor, or null if unknown (the null case does not occur
587	+	* in any current calls but may in possible future extensions)
588		* @param e the comparison value for checking match
589	<	* @param mode mode
589	>	* @param how either SYNC or TIMEOUT
590		* @param nanos timeout value
591	<	* @return matched item, or s if cancelled
591	>	* @return matched item, or e if unmatched on interrupt or timeout
592		*/
593	<	private Object awaitFulfill(QNode pred, QNode s, Object e,
594	<	int mode, long nanos) {
264	<	if (mode == NOWAIT)
265	<	return null;
266	<
267	<	long lastTime = (mode == TIMEOUT) ? System.nanoTime() : 0;
593	>	private E awaitMatch(Node<E> s, Node<E> pred, E e, int how, long nanos) {
594	>	long lastTime = (how == TIMEOUT) ? System.nanoTime() : 0L;
595		Thread w = Thread.currentThread();
596	<	int spins = -1; // set to desired spin count below
596	>	int spins = -1; // initialized after first item and cancel checks
597	>	ThreadLocalRandom randomYields = null; // bound if needed
598	>
599		for (;;) {
600	<	if (w.isInterrupted())
601	<	s.compareAndSet(e, s);
602	<	Object x = s.get();
603	<	if (x != e) {                 // Node was matched or cancelled
604	<	advanceHead(pred, s);     // unlink if head
605	<	if (x == s) {             // was cancelled
606	<	clean(pred, s);
607	<	return null;
608	<	}
609	<	else if (x != null) {
610	<	s.set(s);             // avoid garbage retention
611	<	return x;
612	<	}
613	<	else
614	<	return e;
600	>	Object item = s.item;
601	>	if (item != e) {                  // matched
602	>	assert item != s;
603	>	s.forgetContents();           // avoid garbage
604	>	return this.<E>cast(item);
605	>	}
606	>	if ((w.isInterrupted() || (how == TIMEOUT && nanos <= 0)) &&
607	>	s.casItem(e, s)) {       // cancel
608	>	unsplice(pred, s);
609	>	return e;
610	>	}
611	>
612	>	if (spins < 0) {                  // establish spins at/near front
613	>	if ((spins = spinsFor(pred, s.isData)) > 0)
614	>	randomYields = ThreadLocalRandom.current();
615	>	}
616	>	else if (spins > 0) {             // spin
617	>	if (--spins == 0)
618	>	shortenHeadPath();        // reduce slack before blocking
619	>	else if (randomYields.nextInt(CHAINED_SPINS) == 0)
620	>	Thread.yield();           // occasionally yield
621	>	}
622	>	else if (s.waiter == null) {
623	>	s.waiter = w;                 // request unpark then recheck
624		}
625	<	if (mode == TIMEOUT) {
625	>	else if (how == TIMEOUT) {
626		long now = System.nanoTime();
627	<	nanos -= now - lastTime;
627	>	if ((nanos -= now - lastTime) > 0)
628	>	LockSupport.parkNanos(this, nanos);
629		lastTime = now;
291	–	if (nanos <= 0) {
292	–	s.compareAndSet(e, s); // try to cancel
293	–	continue;
294	–	}
630		}
631	<	if (spins < 0) {
297	<	QNode h = head.get(); // only spin if at head
298	<	spins = ((h != null && h.next == s) ?
299	<	((mode == TIMEOUT) ?
300	<	maxTimedSpins : maxUntimedSpins) : 0);
301	<	}
302	<	if (spins > 0)
303	<	--spins;
304	<	else if (s.waiter == null)
305	<	s.waiter = w;
306	<	else if (mode != TIMEOUT) {
631	>	else {
632		LockSupport.park(this);
633		s.waiter = null;
634	<	spins = -1;
310	<	}
311	<	else if (nanos > spinForTimeoutThreshold) {
312	<	LockSupport.parkNanos(this, nanos);
313	<	s.waiter = null;
314	<	spins = -1;
634	>	spins = -1;                   // spin if front upon wakeup
635		}
636		}
637		}
638
639		/**
640	<	* Returns validated tail for use in cleaning methods.
640	>	* Returns spin/yield value for a node with given predecessor and
641	>	* data mode. See above for explanation.
642		*/
643	<	private QNode getValidatedTail() {
644	<	for (;;) {
645	<	QNode h = head.get();
646	<	QNode first = h.next;
647	<	if (first != null && first.next == first) { // help advance
648	<	advanceHead(h, first);
649	<	continue;
650	<	}
651	<	QNode t = tail.get();
652	<	QNode last = t.next;
653	<	if (t == tail.get()) {
654	<	if (last != null)
655	<	tail.compareAndSet(t, last); // help advance
656	<	else
657	<	return t;
643	>	private static int spinsFor(Node<?> pred, boolean haveData) {
644	>	if (MP && pred != null) {
645	>	if (pred.isData != haveData)      // phase change
646	>	return FRONT_SPINS + CHAINED_SPINS;
647	>	if (pred.isMatched())             // probably at front
648	>	return FRONT_SPINS;
649	>	if (pred.waiter == null)          // pred apparently spinning
650	>	return CHAINED_SPINS;
651	>	}
652	>	return 0;
653	>	}
654	>
655	>	/**
656	>	* Tries (once) to unsplice nodes between head and first unmatched
657	>	* or trailing node; failing on contention.
658	>	*/
659	>	private void shortenHeadPath() {
660	>	Node<E> h, hn, p, q;
661	>	if ((p = h = head) != null && h.isMatched() &&
662	>	(q = hn = h.next) != null) {
663	>	Node<E> n;
664	>	while ((n = q.next) != q) {
665	>	if (n == null || !q.isMatched()) {
666	>	if (hn != q && h.next == hn)
667	>	h.casNext(hn, q);
668	>	break;
669	>	}
670	>	p = q;
671	>	q = n;
672		}
673		}
674		}
675
676	+	/* -------------- Traversal methods -------------- */
677	+
678		/**
679	<	* Gets rid of cancelled node s with original predecessor pred.
680	<	*
344	<	* @param pred predecessor of cancelled node
345	<	* @param s the cancelled node
679	>	* Returns the first unmatched node of the given mode, or null if
680	>	* none.  Used by methods isEmpty, hasWaitingConsumer.
681		*/
682	<	private void clean(QNode pred, QNode s) {
683	<	Thread w = s.waiter;
684	<	if (w != null) {             // Wake up thread
685	<	s.waiter = null;
686	<	if (w != Thread.currentThread())
687	<	LockSupport.unpark(w);
682	>	private Node<E> firstOfMode(boolean data) {
683	>	for (Node<E> p = head; p != null; ) {
684	>	if (!p.isMatched())
685	>	return (p.isData == data) ? p : null;
686	>	Node<E> n = p.next;
687	>	p = (n != p) ? n : head;
688		}
689	+	return null;
690	+	}
691
692	<	if (pred == null)
693	<	return;
692	>	/**
693	>	* Returns the item in the first unmatched node with isData; or
694	>	* null if none.  Used by peek.
695	>	*/
696	>	private E firstDataItem() {
697	>	for (Node<E> p = head; p != null; ) {
698	>	boolean isData = p.isData;
699	>	Object item = p.item;
700	>	if (item != p && (item != null) == isData)
701	>	return isData ? this.<E>cast(item) : null;
702	>	Node<E> n = p.next;
703	>	p = (n != p) ? n : head;
704	>	}
705	>	return null;
706	>	}
707	>
708	>	/**
709	>	* Traverses and counts unmatched nodes of the given mode.
710	>	* Used by methods size and getWaitingConsumerCount.
711	>	*/
712	>	private int countOfMode(boolean data) {
713	>	int count = 0;
714	>	for (Node<E> p = head; p != null; ) {
715	>	if (!p.isMatched()) {
716	>	if (p.isData != data)
717	>	return 0;
718	>	if (++count == Integer.MAX_VALUE) // saturated
719	>	break;
720	>	}
721	>	Node<E> n = p.next;
722	>	if (n != p)
723	>	p = n;
724	>	else {
725	>	count = 0;
726	>	p = head;
727	>	}
728	>	}
729	>	return count;
730	>	}
731	>
732	>	final class Itr implements Iterator<E> {
733	>	private Node<E> nextNode;   // next node to return item for
734	>	private E nextItem;         // the corresponding item
735	>	private Node<E> lastRet;    // last returned node, to support remove
736	>
737	>	/**
738	>	* Moves to next node after prev, or first node if prev null.
739	>	*/
740	>	private void advance(Node<E> prev) {
741	>	lastRet = prev;
742	>	Node<E> p;
743	>	if (prev == null || (p = prev.next) == prev)
744	>	p = head;
745	>	while (p != null) {
746	>	Object item = p.item;
747	>	if (p.isData) {
748	>	if (item != null && item != p) {
749	>	nextItem = LinkedTransferQueue.this.<E>cast(item);
750	>	nextNode = p;
751	>	return;
752	>	}
753	>	}
754	>	else if (item == null)
755	>	break;
756	>	Node<E> n = p.next;
757	>	p = (n != p) ? n : head;
758	>	}
759	>	nextNode = null;
760	>	}
761	>
762	>	Itr() {
763	>	advance(null);
764	>	}
765	>
766	>	public final boolean hasNext() {
767	>	return nextNode != null;
768	>	}
769	>
770	>	public final E next() {
771	>	Node<E> p = nextNode;
772	>	if (p == null) throw new NoSuchElementException();
773	>	E e = nextItem;
774	>	advance(p);
775	>	return e;
776	>	}
777
778	+	public final void remove() {
779	+	Node<E> p = lastRet;
780	+	if (p == null) throw new IllegalStateException();
781	+	lastRet = null;
782	+	findAndRemoveNode(p);
783	+	}
784	+	}
785	+
786	+	/* -------------- Removal methods -------------- */
787	+
788	+	/**
789	+	* Unsplices (now or later) the given deleted/cancelled node with
790	+	* the given predecessor.
791	+	*
792	+	* @param pred predecessor of node to be unspliced
793	+	* @param s the node to be unspliced
794	+	*/
795	+	private void unsplice(Node<E> pred, Node<E> s) {
796	+	s.forgetContents(); // clear unneeded fields
797		/*
798		* At any given time, exactly one node on list cannot be
799	<	* deleted -- the last inserted node. To accommodate this, if
800	<	* we cannot delete s, we save its predecessor as "cleanMe",
801	<	* processing the previously saved version first. At least one
802	<	* of node s or the node previously saved can always be
799	>	* unlinked -- the last inserted node. To accommodate this, if
800	>	* we cannot unlink s, we save its predecessor as "cleanMe",
801	>	* processing the previously saved version first. Because only
802	>	* one node in the list can have a null next, at least one of
803	>	* node s or the node previously saved can always be
804		* processed, so this always terminates.
805		*/
806	<	while (pred.next == s) {
807	<	QNode oldpred = reclean();  // First, help get rid of cleanMe
808	<	QNode t = getValidatedTail();
809	<	if (s != t) {               // If not tail, try to unsplice
810	<	QNode sn = s.next;      // s.next == s means s already off list
811	<	if (sn == s || pred.casNext(s, sn))
806	>	if (pred != null && pred != s) {
807	>	while (pred.next == s) {
808	>	Node<E> oldpred = (cleanMe == null) ? null : reclean();
809	>	Node<E> n = s.next;
810	>	if (n != null) {
811	>	if (n != s)
812	>	pred.casNext(s, n);
813		break;
814	+	}
815	+	if (oldpred == pred ||      // Already saved
816	+	(oldpred == null && casCleanMe(null, pred)))
817	+	break;                  // Postpone cleaning
818		}
374	–	else if (oldpred == pred || // Already saved
375	–	(oldpred == null && cleanMe.compareAndSet(null, pred)))
376	–	break;                  // Postpone cleaning
819		}
820		}
821
822		/**
823	<	* Tries to unsplice the cancelled node held in cleanMe that was
824	<	* previously uncleanable because it was at tail.
823	>	* Tries to unsplice the deleted/cancelled node held in cleanMe
824	>	* that was previously uncleanable because it was at tail.
825		*
826		* @return current cleanMe node (or null)
827		*/
828	<	private QNode reclean() {
828	>	private Node<E> reclean() {
829		/*
830	<	* cleanMe is, or at one time was, predecessor of cancelled
831	<	* node s that was the tail so could not be unspliced.  If s
830	>	* cleanMe is, or at one time was, predecessor of a cancelled
831	>	* node s that was the tail so could not be unspliced.  If it
832		* is no longer the tail, try to unsplice if necessary and
833		* make cleanMe slot available.  This differs from similar
834	<	* code in clean() because we must check that pred still
835	<	* points to a cancelled node that must be unspliced -- if
836	<	* not, we can (must) clear cleanMe without unsplicing.
837	<	* This can loop only due to contention on casNext or
396	<	* clearing cleanMe.
834	>	* code in unsplice() because we must check that pred still
835	>	* points to a matched node that can be unspliced -- if not,
836	>	* we can (must) clear cleanMe without unsplicing.  This can
837	>	* loop only due to contention.
838		*/
839	<	QNode pred;
840	<	while ((pred = cleanMe.get()) != null) {
841	<	QNode t = getValidatedTail();
842	<	QNode s = pred.next;
843	<	if (s != t) {
844	<	QNode sn;
845	<	if (s == null || s == pred || s.get() != s ||
846	<	(sn = s.next) == s || pred.casNext(s, sn))
847	<	cleanMe.compareAndSet(pred, null);
839	>	Node<E> pred;
840	>	while ((pred = cleanMe) != null) {
841	>	Node<E> s = pred.next;
842	>	Node<E> n;
843	>	if (s == null || s == pred || !s.isMatched())
844	>	casCleanMe(pred, null); // already gone
845	>	else if ((n = s.next) != null) {
846	>	if (n != s)
847	>	pred.casNext(s, n);
848	>	casCleanMe(pred, null);
849		}
850	<	else // s is still tail; cannot clean
850	>	else
851		break;
852		}
853		return pred;
854		}
855
856		/**
857	+	* Main implementation of Iterator.remove(). Find
858	+	* and unsplice the given node.
859	+	*/
860	+	final void findAndRemoveNode(Node<E> s) {
861	+	if (s.tryMatchData()) {
862	+	Node<E> pred = null;
863	+	Node<E> p = head;
864	+	while (p != null) {
865	+	if (p == s) {
866	+	unsplice(pred, p);
867	+	break;
868	+	}
869	+	if (!p.isData && !p.isMatched())
870	+	break;
871	+	pred = p;
872	+	if ((p = p.next) == pred) { // stale
873	+	pred = null;
874	+	p = head;
875	+	}
876	+	}
877	+	}
878	+	}
879	+
880	+	/**
881	+	* Main implementation of remove(Object)
882	+	*/
883	+	private boolean findAndRemove(Object e) {
884	+	if (e != null) {
885	+	Node<E> pred = null;
886	+	Node<E> p = head;
887	+	while (p != null) {
888	+	Object item = p.item;
889	+	if (p.isData) {
890	+	if (item != null && item != p && e.equals(item) &&
891	+	p.tryMatchData()) {
892	+	unsplice(pred, p);
893	+	return true;
894	+	}
895	+	}
896	+	else if (item == null)
897	+	break;
898	+	pred = p;
899	+	if ((p = p.next) == pred) {
900	+	pred = null;
901	+	p = head;
902	+	}
903	+	}
904	+	}
905	+	return false;
906	+	}
907	+
908	+
909	+	/**
910		* Creates an initially empty {@code LinkedTransferQueue}.
911		*/
912		public LinkedTransferQueue() {
418	–	QNode dummy = new QNode(null, false);
419	–	head = new PaddedAtomicReference<QNode>(dummy);
420	–	tail = new PaddedAtomicReference<QNode>(dummy);
421	–	cleanMe = new PaddedAtomicReference<QNode>(null);
913		}
914
915		/**
#	Line 435 | Line 926 | public class LinkedTransferQueue<E> exte
926		addAll(c);
927		}
928
929	<	public void put(E e) throws InterruptedException {
930	<	if (e == null) throw new NullPointerException();
931	<	if (Thread.interrupted()) throw new InterruptedException();
932	<	xfer(e, NOWAIT, 0);
929	>	/**
930	>	* Inserts the specified element at the tail of this queue.
931	>	* As the queue is unbounded, this method will never block.
932	>	*
933	>	* @throws NullPointerException if the specified element is null
934	>	*/
935	>	public void put(E e) {
936	>	xfer(e, true, ASYNC, 0);
937		}
938
939	<	public boolean offer(E e, long timeout, TimeUnit unit)
940	<	throws InterruptedException {
941	<	if (e == null) throw new NullPointerException();
942	<	if (Thread.interrupted()) throw new InterruptedException();
943	<	xfer(e, NOWAIT, 0);
939	>	/**
940	>	* Inserts the specified element at the tail of this queue.
941	>	* As the queue is unbounded, this method will never block or
942	>	* return {@code false}.
943	>	*
944	>	* @return {@code true} (as specified by
945	>	*  {@link BlockingQueue#offer(Object,long,TimeUnit) BlockingQueue.offer})
946	>	* @throws NullPointerException if the specified element is null
947	>	*/
948	>	public boolean offer(E e, long timeout, TimeUnit unit) {
949	>	xfer(e, true, ASYNC, 0);
950		return true;
951		}
952
953	+	/**
954	+	* Inserts the specified element at the tail of this queue.
955	+	* As the queue is unbounded, this method will never return {@code false}.
956	+	*
957	+	* @return {@code true} (as specified by
958	+	*         {@link BlockingQueue#offer(Object) BlockingQueue.offer})
959	+	* @throws NullPointerException if the specified element is null
960	+	*/
961		public boolean offer(E e) {
962	<	if (e == null) throw new NullPointerException();
454	<	xfer(e, NOWAIT, 0);
962	>	xfer(e, true, ASYNC, 0);
963		return true;
964		}
965
966	+	/**
967	+	* Inserts the specified element at the tail of this queue.
968	+	* As the queue is unbounded, this method will never throw
969	+	* {@link IllegalStateException} or return {@code false}.
970	+	*
971	+	* @return {@code true} (as specified by {@link Collection#add})
972	+	* @throws NullPointerException if the specified element is null
973	+	*/
974		public boolean add(E e) {
975	<	if (e == null) throw new NullPointerException();
460	<	xfer(e, NOWAIT, 0);
975	>	xfer(e, true, ASYNC, 0);
976		return true;
977		}
978
979	+	/**
980	+	* Transfers the element to a waiting consumer immediately, if possible.
981	+	*
982	+	* <p>More precisely, transfers the specified element immediately
983	+	* if there exists a consumer already waiting to receive it (in
984	+	* {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
985	+	* otherwise returning {@code false} without enqueuing the element.
986	+	*
987	+	* @throws NullPointerException if the specified element is null
988	+	*/
989	+	public boolean tryTransfer(E e) {
990	+	return xfer(e, true, NOW, 0) == null;
991	+	}
992	+
993	+	/**
994	+	* Transfers the element to a consumer, waiting if necessary to do so.
995	+	*
996	+	* <p>More precisely, transfers the specified element immediately
997	+	* if there exists a consumer already waiting to receive it (in
998	+	* {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
999	+	* else inserts the specified element at the tail of this queue
1000	+	* and waits until the element is received by a consumer.
1001	+	*
1002	+	* @throws NullPointerException if the specified element is null
1003	+	*/
1004		public void transfer(E e) throws InterruptedException {
1005	<	if (e == null) throw new NullPointerException();
1006	<	if (xfer(e, WAIT, 0) == null) {
467	<	Thread.interrupted();
1005	>	if (xfer(e, true, SYNC, 0) != null) {
1006	>	Thread.interrupted(); // failure possible only due to interrupt
1007		throw new InterruptedException();
1008		}
1009		}
1010
1011	+	/**
1012	+	* Transfers the element to a consumer if it is possible to do so
1013	+	* before the timeout elapses.
1014	+	*
1015	+	* <p>More precisely, transfers the specified element immediately
1016	+	* if there exists a consumer already waiting to receive it (in
1017	+	* {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
1018	+	* else inserts the specified element at the tail of this queue
1019	+	* and waits until the element is received by a consumer,
1020	+	* returning {@code false} if the specified wait time elapses
1021	+	* before the element can be transferred.
1022	+	*
1023	+	* @throws NullPointerException if the specified element is null
1024	+	*/
1025		public boolean tryTransfer(E e, long timeout, TimeUnit unit)
1026		throws InterruptedException {
1027	<	if (e == null) throw new NullPointerException();
475	<	if (xfer(e, TIMEOUT, unit.toNanos(timeout)) != null)
1027	>	if (xfer(e, true, TIMEOUT, unit.toNanos(timeout)) == null)
1028		return true;
1029		if (!Thread.interrupted())
1030		return false;
1031		throw new InterruptedException();
1032		}
1033
482	–	public boolean tryTransfer(E e) {
483	–	if (e == null) throw new NullPointerException();
484	–	return fulfill(e) != null;
485	–	}
486	–
1034		public E take() throws InterruptedException {
1035	<	Object e = xfer(null, WAIT, 0);
1035	>	E e = xfer(null, false, SYNC, 0);
1036		if (e != null)
1037	<	return (E) e;
1037	>	return e;
1038		Thread.interrupted();
1039		throw new InterruptedException();
1040		}
1041
1042		public E poll(long timeout, TimeUnit unit) throws InterruptedException {
1043	<	Object e = xfer(null, TIMEOUT, unit.toNanos(timeout));
1043	>	E e = xfer(null, false, TIMEOUT, unit.toNanos(timeout));
1044		if (e != null || !Thread.interrupted())
1045	<	return (E) e;
1045	>	return e;
1046		throw new InterruptedException();
1047		}
1048
1049		public E poll() {
1050	<	return (E) fulfill(null);
1050	>	return xfer(null, false, NOW, 0);
1051		}
1052
1053	+	/**
1054	+	* @throws NullPointerException     {@inheritDoc}
1055	+	* @throws IllegalArgumentException {@inheritDoc}
1056	+	*/
1057		public int drainTo(Collection<? super E> c) {
1058		if (c == null)
1059		throw new NullPointerException();
#	Line 517 | Line 1068 | public class LinkedTransferQueue<E> exte
1068		return n;
1069		}
1070
1071	+	/**
1072	+	* @throws NullPointerException     {@inheritDoc}
1073	+	* @throws IllegalArgumentException {@inheritDoc}
1074	+	*/
1075		public int drainTo(Collection<? super E> c, int maxElements) {
1076		if (c == null)
1077		throw new NullPointerException();
#	Line 531 | Line 1086 | public class LinkedTransferQueue<E> exte
1086		return n;
1087		}
1088
534	–	// Traversal-based methods
535	–
1089		/**
1090	<	* Returns head after performing any outstanding helping steps.
1090	>	* Returns an iterator over the elements in this queue in proper
1091	>	* sequence, from head to tail.
1092	>	*
1093	>	* <p>The returned iterator is a "weakly consistent" iterator that
1094	>	* will never throw
1095	>	* {@link ConcurrentModificationException ConcurrentModificationException},
1096	>	* and guarantees to traverse elements as they existed upon
1097	>	* construction of the iterator, and may (but is not guaranteed
1098	>	* to) reflect any modifications subsequent to construction.
1099	>	*
1100	>	* @return an iterator over the elements in this queue in proper sequence
1101		*/
539	–	private QNode traversalHead() {
540	–	for (;;) {
541	–	QNode t = tail.get();
542	–	QNode h = head.get();
543	–	if (h != null && t != null) {
544	–	QNode last = t.next;
545	–	QNode first = h.next;
546	–	if (t == tail.get()) {
547	–	if (last != null)
548	–	tail.compareAndSet(t, last);
549	–	else if (first != null) {
550	–	Object x = first.get();
551	–	if (x == first)
552	–	advanceHead(h, first);
553	–	else
554	–	return h;
555	–	}
556	–	else
557	–	return h;
558	–	}
559	–	}
560	–	reclean();
561	–	}
562	–	}
563	–
564	–
1102		public Iterator<E> iterator() {
1103		return new Itr();
1104		}
1105
569	–	/**
570	–	* Iterators. Basic strategy is to traverse list, treating
571	–	* non-data (i.e., request) nodes as terminating list.
572	–	* Once a valid data node is found, the item is cached
573	–	* so that the next call to next() will return it even
574	–	* if subsequently removed.
575	–	*/
576	–	class Itr implements Iterator<E> {
577	–	QNode next;        // node to return next
578	–	QNode pnext;       // predecessor of next
579	–	QNode snext;       // successor of next
580	–	QNode curr;        // last returned node, for remove()
581	–	QNode pcurr;       // predecessor of curr, for remove()
582	–	E nextItem;        // Cache of next item, once committed to in next
583	–
584	–	Itr() {
585	–	findNext();
586	–	}
587	–
588	–	/**
589	–	* Ensures next points to next valid node, or null if none.
590	–	*/
591	–	void findNext() {
592	–	for (;;) {
593	–	QNode pred = pnext;
594	–	QNode q = next;
595	–	if (pred == null || pred == q) {
596	–	pred = traversalHead();
597	–	q = pred.next;
598	–	}
599	–	if (q == null || !q.isData) {
600	–	next = null;
601	–	return;
602	–	}
603	–	Object x = q.get();
604	–	QNode s = q.next;
605	–	if (x != null && q != x && q != s) {
606	–	nextItem = (E) x;
607	–	snext = s;
608	–	pnext = pred;
609	–	next = q;
610	–	return;
611	–	}
612	–	pnext = q;
613	–	next = s;
614	–	}
615	–	}
616	–
617	–	public boolean hasNext() {
618	–	return next != null;
619	–	}
620	–
621	–	public E next() {
622	–	if (next == null) throw new NoSuchElementException();
623	–	pcurr = pnext;
624	–	curr = next;
625	–	pnext = next;
626	–	next = snext;
627	–	E x = nextItem;
628	–	findNext();
629	–	return x;
630	–	}
631	–
632	–	public void remove() {
633	–	QNode p = curr;
634	–	if (p == null)
635	–	throw new IllegalStateException();
636	–	Object x = p.get();
637	–	if (x != null && x != p && p.compareAndSet(x, p))
638	–	clean(pcurr, p);
639	–	}
640	–	}
641	–
1106		public E peek() {
1107	<	for (;;) {
644	<	QNode h = traversalHead();
645	<	QNode p = h.next;
646	<	if (p == null)
647	<	return null;
648	<	Object x = p.get();
649	<	if (p != x) {
650	<	if (!p.isData)
651	<	return null;
652	<	if (x != null)
653	<	return (E) x;
654	<	}
655	<	}
1107	>	return firstDataItem();
1108		}
1109
1110	+	/**
1111	+	* Returns {@code true} if this queue contains no elements.
1112	+	*
1113	+	* @return {@code true} if this queue contains no elements
1114	+	*/
1115		public boolean isEmpty() {
1116	<	for (;;) {
660	<	QNode h = traversalHead();
661	<	QNode p = h.next;
662	<	if (p == null)
663	<	return true;
664	<	Object x = p.get();
665	<	if (p != x) {
666	<	if (!p.isData)
667	<	return true;
668	<	if (x != null)
669	<	return false;
670	<	}
671	<	}
1116	>	return firstOfMode(true) == null;
1117		}
1118
1119		public boolean hasWaitingConsumer() {
1120	<	for (;;) {
676	<	QNode h = traversalHead();
677	<	QNode p = h.next;
678	<	if (p == null)
679	<	return false;
680	<	Object x = p.get();
681	<	if (p != x)
682	<	return !p.isData;
683	<	}
1120	>	return firstOfMode(false) != null;
1121		}
1122
1123		/**
#	Line 696 | Line 1133 | public class LinkedTransferQueue<E> exte
1133		* @return the number of elements in this queue
1134		*/
1135		public int size() {
1136	<	int count = 0;
700	<	QNode h = traversalHead();
701	<	for (QNode p = h.next; p != null && p.isData; p = p.next) {
702	<	Object x = p.get();
703	<	if (x != null && x != p) {
704	<	if (++count == Integer.MAX_VALUE) // saturated
705	<	break;
706	<	}
707	<	}
708	<	return count;
1136	>	return countOfMode(true);
1137		}
1138
1139		public int getWaitingConsumerCount() {
1140	<	int count = 0;
713	<	QNode h = traversalHead();
714	<	for (QNode p = h.next; p != null && !p.isData; p = p.next) {
715	<	if (p.get() == null) {
716	<	if (++count == Integer.MAX_VALUE)
717	<	break;
718	<	}
719	<	}
720	<	return count;
1140	>	return countOfMode(false);
1141		}
1142
1143	<	public int remainingCapacity() {
1144	<	return Integer.MAX_VALUE;
1143	>	/**
1144	>	* Removes a single instance of the specified element from this queue,
1145	>	* if it is present.  More formally, removes an element {@code e} such
1146	>	* that {@code o.equals(e)}, if this queue contains one or more such
1147	>	* elements.
1148	>	* Returns {@code true} if this queue contained the specified element
1149	>	* (or equivalently, if this queue changed as a result of the call).
1150	>	*
1151	>	* @param o element to be removed from this queue, if present
1152	>	* @return {@code true} if this queue changed as a result of the call
1153	>	*/
1154	>	public boolean remove(Object o) {
1155	>	return findAndRemove(o);
1156		}
1157
1158	<	public boolean remove(Object o) {
1159	<	if (o == null)
1160	<	return false;
1161	<	for (;;) {
1162	<	QNode pred = traversalHead();
1163	<	for (;;) {
1164	<	QNode q = pred.next;
1165	<	if (q == null || !q.isData)
1166	<	return false;
736	<	if (q == pred) // restart
737	<	break;
738	<	Object x = q.get();
739	<	if (x != null && x != q && o.equals(x) &&
740	<	q.compareAndSet(x, q)) {
741	<	clean(pred, q);
742	<	return true;
743	<	}
744	<	pred = q;
745	<	}
746	<	}
1158	>	/**
1159	>	* Always returns {@code Integer.MAX_VALUE} because a
1160	>	* {@code LinkedTransferQueue} is not capacity constrained.
1161	>	*
1162	>	* @return {@code Integer.MAX_VALUE} (as specified by
1163	>	*         {@link BlockingQueue#remainingCapacity()})
1164	>	*/
1165	>	public int remainingCapacity() {
1166	>	return Integer.MAX_VALUE;
1167		}
1168
1169		/**
1170	<	* Save the state to a stream (that is, serialize it).
1170	>	* Saves the state to a stream (that is, serializes it).
1171		*
1172		* @serialData All of the elements (each an {@code E}) in
1173		* the proper order, followed by a null
#	Line 763 | Line 1183 | public class LinkedTransferQueue<E> exte
1183		}
1184
1185		/**
1186	<	* Reconstitute the Queue instance from a stream (that is,
1187	<	* deserialize it).
1186	>	* Reconstitutes the Queue instance from a stream (that is,
1187	>	* deserializes it).
1188		*
1189		* @param s the stream
1190		*/
1191		private void readObject(java.io.ObjectInputStream s)
1192		throws java.io.IOException, ClassNotFoundException {
1193		s.defaultReadObject();
774	–	resetHeadAndTail();
1194		for (;;) {
1195	<	E item = (E) s.readObject();
1195	>	@SuppressWarnings("unchecked") E item = (E) s.readObject();
1196		if (item == null)
1197		break;
1198		else
#	Line 781 | Line 1200 | public class LinkedTransferQueue<E> exte
1200		}
1201		}
1202
1203	+	// Unsafe mechanics
1204	+
1205	+	private static final sun.misc.Unsafe UNSAFE = getUnsafe();
1206	+	private static final long headOffset =
1207	+	objectFieldOffset(UNSAFE, "head", LinkedTransferQueue.class);
1208	+	private static final long tailOffset =
1209	+	objectFieldOffset(UNSAFE, "tail", LinkedTransferQueue.class);
1210	+	private static final long cleanMeOffset =
1211	+	objectFieldOffset(UNSAFE, "cleanMe", LinkedTransferQueue.class);
1212
1213	<	// Support for resetting head/tail while deserializing
1214	<	private void resetHeadAndTail() {
1215	<	QNode dummy = new QNode(null, false);
1216	<	UNSAFE.putObjectVolatile(this, headOffset,
1217	<	new PaddedAtomicReference<QNode>(dummy));
1218	<	UNSAFE.putObjectVolatile(this, tailOffset,
1219	<	new PaddedAtomicReference<QNode>(dummy));
1220	<	UNSAFE.putObjectVolatile(this, cleanMeOffset,
1221	<	new PaddedAtomicReference<QNode>(null));
1213	>	static long objectFieldOffset(sun.misc.Unsafe UNSAFE,
1214	>	String field, Class<?> klazz) {
1215	>	try {
1216	>	return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
1217	>	} catch (NoSuchFieldException e) {
1218	>	// Convert Exception to corresponding Error
1219	>	NoSuchFieldError error = new NoSuchFieldError(field);
1220	>	error.initCause(e);
1221	>	throw error;
1222	>	}
1223		}
1224
1225	<	// Temporary Unsafe mechanics for preliminary release
1226	<	private static Unsafe getUnsafe() throws Throwable {
1225	>	/**
1226	>	* Returns a sun.misc.Unsafe.  Suitable for use in a 3rd party package.
1227	>	* Replace with a simple call to Unsafe.getUnsafe when integrating
1228	>	* into a jdk.
1229	>	*
1230	>	* @return a sun.misc.Unsafe
1231	>	*/
1232	>	static sun.misc.Unsafe getUnsafe() {
1233		try {
1234	<	return Unsafe.getUnsafe();
1234	>	return sun.misc.Unsafe.getUnsafe();
1235		} catch (SecurityException se) {
1236		try {
1237		return java.security.AccessController.doPrivileged
1238	<	(new java.security.PrivilegedExceptionAction<Unsafe>() {
1239	<	public Unsafe run() throws Exception {
1240	<	return getUnsafePrivileged();
1238	>	(new java.security
1239	>	.PrivilegedExceptionAction<sun.misc.Unsafe>() {
1240	>	public sun.misc.Unsafe run() throws Exception {
1241	>	java.lang.reflect.Field f = sun.misc
1242	>	.Unsafe.class.getDeclaredField("theUnsafe");
1243	>	f.setAccessible(true);
1244	>	return (sun.misc.Unsafe) f.get(null);
1245		}});
1246		} catch (java.security.PrivilegedActionException e) {
1247	<	throw e.getCause();
1247	>	throw new RuntimeException("Could not initialize intrinsics",
1248	>	e.getCause());
1249		}
1250		}
1251		}
1252
813	–	private static Unsafe getUnsafePrivileged()
814	–	throws NoSuchFieldException, IllegalAccessException {
815	–	Field f = Unsafe.class.getDeclaredField("theUnsafe");
816	–	f.setAccessible(true);
817	–	return (Unsafe) f.get(null);
818	–	}
819	–
820	–	private static long fieldOffset(String fieldName)
821	–	throws NoSuchFieldException {
822	–	return UNSAFE.objectFieldOffset
823	–	(LinkedTransferQueue.class.getDeclaredField(fieldName));
824	–	}
825	–
826	–	private static final Unsafe UNSAFE;
827	–	private static final long headOffset;
828	–	private static final long tailOffset;
829	–	private static final long cleanMeOffset;
830	–	static {
831	–	try {
832	–	UNSAFE = getUnsafe();
833	–	headOffset = fieldOffset("head");
834	–	tailOffset = fieldOffset("tail");
835	–	cleanMeOffset = fieldOffset("cleanMe");
836	–	} catch (Throwable e) {
837	–	throw new RuntimeException("Could not initialize intrinsics", e);
838	–	}
839	–	}
840	–
1253		}

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