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Comparing jsr166/src/jsr166y/LinkedTransferQueue.java (file contents):
Revision 1.24 by jsr166, Thu Jul 23 23:23:41 2009 UTC vs.
Revision 1.58 by jsr166, Wed Oct 28 10:23:38 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	+	/**
417	+	* Returns true if this is an unmatched request node.
418	+	*/
419	+	final boolean isUnmatchedRequest() {
420	+	return !isData && item == null;
421	+	}
422	+
423	+	/**
424	+	* Returns true if a node with the given mode cannot be
425	+	* appended to this node because this node is unmatched and
426	+	* has opposite data mode.
427	+	*/
428	+	final boolean cannotPrecede(boolean haveData) {
429	+	boolean d = isData;
430	+	Object x;
431	+	return d != haveData && (x = item) != this && (x != null) == d;
432	+	}
433	+
434	+	/**
435	+	* Tries to artificially match a data node -- used by remove.
436	+	*/
437	+	final boolean tryMatchData() {
438	+	assert isData;
439	+	Object x = item;
440	+	if (x != null && x != this && casItem(x, null)) {
441	+	LockSupport.unpark(waiter);
442	+	return true;
443	+	}
444	+	return false;
445	+	}
446	+
447	+	// Unsafe mechanics
448	+	private static final sun.misc.Unsafe UNSAFE = getUnsafe();
449	+	private static final long nextOffset =
450	+	objectFieldOffset(UNSAFE, "next", Node.class);
451	+	private static final long itemOffset =
452	+	objectFieldOffset(UNSAFE, "item", Node.class);
453	+	private static final long waiterOffset =
454	+	objectFieldOffset(UNSAFE, "waiter", Node.class);
455	+
456		private static final long serialVersionUID = -3375979862319811754L;
457		}
458
459	<	/**
460	<	* Padded version of AtomicReference used for head, tail and
131	<	* cleanMe, to alleviate contention across threads CASing one vs
132	<	* the other.
133	<	*/
134	<	static final class PaddedAtomicReference<T> extends AtomicReference<T> {
135	<	// enough padding for 64bytes with 4byte refs
136	<	Object p0, p1, p2, p3, p4, p5, p6, p7, p8, p9, pa, pb, pc, pd, pe;
137	<	PaddedAtomicReference(T r) { super(r); }
138	<	private static final long serialVersionUID = 8170090609809740854L;
139	<	}
459	>	/** head of the queue; null until first enqueue */
460	>	transient volatile Node<E> head;
461
462	+	/** predecessor of dangling unspliceable node */
463	+	private transient volatile Node<E> cleanMe; // decl here reduces contention
464
465	<	/** head of the queue */
466	<	private transient final PaddedAtomicReference<QNode> head;
465	>	/** tail of the queue; null until first append */
466	>	private transient volatile Node<E> tail;
467
468	<	/** tail of the queue */
469	<	private transient final PaddedAtomicReference<QNode> tail;
468	>	// CAS methods for fields
469	>	private boolean casTail(Node<E> cmp, Node<E> val) {
470	>	return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val);
471	>	}
472
473	<	/**
474	<	* Reference to a cancelled node that might not yet have been
475	<	* unlinked from queue because it was the last inserted node
151	<	* when it cancelled.
152	<	*/
153	<	private transient final PaddedAtomicReference<QNode> cleanMe;
473	>	private boolean casHead(Node<E> cmp, Node<E> val) {
474	>	return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val);
475	>	}
476
477	<	/**
478	<	* Tries to cas nh as new head; if successful, unlink
479	<	* old head's next node to avoid garbage retention.
477	>	private boolean casCleanMe(Node<E> cmp, Node<E> val) {
478	>	return UNSAFE.compareAndSwapObject(this, cleanMeOffset, cmp, val);
479	>	}
480	>
481	>	/*
482	>	* Possible values for "how" argument in xfer method. Beware that
483	>	* the order of assigned numerical values matters.
484		*/
485	<	private boolean advanceHead(QNode h, QNode nh) {
486	<	if (h == head.get() && head.compareAndSet(h, nh)) {
487	<	h.clearNext(); // forget old next
488	<	return true;
489	<	}
490	<	return false;
485	>	private static final int NOW     = 0; // for untimed poll, tryTransfer
486	>	private static final int ASYNC   = 1; // for offer, put, add
487	>	private static final int SYNC    = 2; // for transfer, take
488	>	private static final int TIMEOUT = 3; // for timed poll, tryTransfer
489	>
490	>	@SuppressWarnings("unchecked")
491	>	static <E> E cast(Object item) {
492	>	assert item == null || item.getClass() != Node.class;
493	>	return (E) item;
494		}
495
496		/**
497	<	* Puts or takes an item. Used for most queue operations (except
169	<	* poll() and tryTransfer()). See the similar code in
170	<	* SynchronousQueue for detailed explanation.
497	>	* Implements all queuing methods. See above for explanation.
498		*
499	<	* @param e the item or if null, signifies that this is a take
500	<	* @param mode the wait mode: NOWAIT, TIMEOUT, WAIT
499	>	* @param e the item or null for take
500	>	* @param haveData true if this is a put, else a take
501	>	* @param how NOW, ASYNC, SYNC, or TIMEOUT
502		* @param nanos timeout in nanosecs, used only if mode is TIMEOUT
503	<	* @return an item, or null on failure
503	>	* @return an item if matched, else e
504	>	* @throws NullPointerException if haveData mode but e is null
505		*/
506	<	private Object xfer(Object e, int mode, long nanos) {
507	<	boolean isData = (e != null);
508	<	QNode s = null;
509	<	final PaddedAtomicReference<QNode> head = this.head;
181	<	final PaddedAtomicReference<QNode> tail = this.tail;
506	>	private E xfer(E e, boolean haveData, int how, long nanos) {
507	>	if (haveData && (e == null))
508	>	throw new NullPointerException();
509	>	Node<E> s = null;                     // the node to append, if needed
510
511	<	for (;;) {
184	<	QNode t = tail.get();
185	<	QNode h = head.get();
511	>	retry: for (;;) {                     // restart on append race
512
513	<	if (t != null && (t == h || t.isData == isData)) {
514	<	if (s == null)
515	<	s = new QNode(e, isData);
516	<	QNode last = t.next;
517	<	if (last != null) {
518	<	if (t == tail.get())
519	<	tail.compareAndSet(t, last);
520	<	}
521	<	else if (t.casNext(null, s)) {
522	<	tail.compareAndSet(t, s);
523	<	return awaitFulfill(t, s, e, mode, nanos);
513	>	for (Node<E> h = head, p = h; p != null;) {
514	>	// find & match first node
515	>	boolean isData = p.isData;
516	>	Object item = p.item;
517	>	if (item != p && (item != null) == isData) { // unmatched
518	>	if (isData == haveData)   // can't match
519	>	break;
520	>	if (p.casItem(item, e)) { // match
521	>	for (Node<E> q = p; q != h;) {
522	>	Node<E> n = q.next; // update head by 2
523	>	if (n != null)    // unless singleton
524	>	q = n;
525	>	if (head == h && casHead(h, q)) {
526	>	h.forgetNext();
527	>	break;
528	>	}                 // advance and retry
529	>	if ((h = head)   == null ||
530	>	(q = h.next) == null || !q.isMatched())
531	>	break;        // unless slack < 2
532	>	}
533	>	LockSupport.unpark(p.waiter);
534	>	return this.<E>cast(item);
535	>	}
536		}
537	+	Node<E> n = p.next;
538	+	p = (p != n) ? n : (h = head); // Use head if p offlist
539		}
540
541	<	else if (h != null) {
542	<	QNode first = h.next;
543	<	if (t == tail.get() && first != null &&
544	<	advanceHead(h, first)) {
545	<	Object x = first.get();
546	<	if (x != first && first.compareAndSet(x, e)) {
547	<	LockSupport.unpark(first.waiter);
548	<	return isData ? e : x;
209	<	}
210	<	}
541	>	if (how >= ASYNC) {               // No matches available
542	>	if (s == null)
543	>	s = new Node<E>(e, haveData);
544	>	Node<E> pred = tryAppend(s, haveData);
545	>	if (pred == null)
546	>	continue retry;           // lost race vs opposite mode
547	>	if (how >= SYNC)
548	>	return awaitMatch(s, pred, e, how, nanos);
549		}
550	+	return e; // not waiting
551		}
552		}
553
215	–
554		/**
555	<	* Version of xfer for poll() and tryTransfer, which
556	<	* simplifies control paths both here and in xfer.
557	<	*/
558	<	private Object fulfill(Object e) {
559	<	boolean isData = (e != null);
560	<	final PaddedAtomicReference<QNode> head = this.head;
561	<	final PaddedAtomicReference<QNode> tail = this.tail;
562	<
563	<	for (;;) {
564	<	QNode t = tail.get();
565	<	QNode h = head.get();
566	<
567	<	if (t != null && (t == h || t.isData == isData)) {
568	<	QNode last = t.next;
569	<	if (t == tail.get()) {
570	<	if (last != null)
571	<	tail.compareAndSet(t, last);
572	<	else
573	<	return null;
574	<	}
575	<	}
576	<	else if (h != null) {
577	<	QNode first = h.next;
578	<	if (t == tail.get() &&
579	<	first != null &&
580	<	advanceHead(h, first)) {
581	<	Object x = first.get();
582	<	if (x != first && first.compareAndSet(x, e)) {
245	<	LockSupport.unpark(first.waiter);
246	<	return isData ? e : x;
247	<	}
555	>	* Tries to append node s as tail.
556	>	*
557	>	* @param s the node to append
558	>	* @param haveData true if appending in data mode
559	>	* @return null on failure due to losing race with append in
560	>	* different mode, else s's predecessor, or s itself if no
561	>	* predecessor
562	>	*/
563	>	private Node<E> tryAppend(Node<E> s, boolean haveData) {
564	>	for (Node<E> t = tail, p = t;;) { // move p to last node and append
565	>	Node<E> n, u;                     // temps for reads of next & tail
566	>	if (p == null && (p = head) == null) {
567	>	if (casHead(null, s))
568	>	return s;                 // initialize
569	>	}
570	>	else if (p.cannotPrecede(haveData))
571	>	return null;                  // lost race vs opposite mode
572	>	else if ((n = p.next) != null)    // not last; keep traversing
573	>	p = p != t && t != (u = tail) ? (t = u) : // stale tail
574	>	(p != n) ? n : null;      // restart if off list
575	>	else if (!p.casNext(null, s))
576	>	p = p.next;                   // re-read on CAS failure
577	>	else {
578	>	if (p != t) {                 // update if slack now >= 2
579	>	while ((tail != t || !casTail(t, s)) &&
580	>	(t = tail)   != null &&
581	>	(s = t.next) != null && // advance and retry
582	>	(s = s.next) != null && s != t);
583		}
584	+	return p;
585		}
586		}
587		}
588
589		/**
590	<	* Spins/blocks until node s is fulfilled or caller gives up,
255	<	* depending on wait mode.
590	>	* Spins/yields/blocks until node s is matched or caller gives up.
591		*
257	–	* @param pred the predecessor of waiting node
592		* @param s the waiting node
593	+	* @param pred the predecessor of s, or s itself if it has no
594	+	* predecessor, or null if unknown (the null case does not occur
595	+	* in any current calls but may in possible future extensions)
596		* @param e the comparison value for checking match
597	<	* @param mode mode
597	>	* @param how either SYNC or TIMEOUT
598		* @param nanos timeout value
599	<	* @return matched item, or s if cancelled
599	>	* @return matched item, or e if unmatched on interrupt or timeout
600		*/
601	<	private Object awaitFulfill(QNode pred, QNode s, Object e,
602	<	int mode, long nanos) {
266	<	if (mode == NOWAIT)
267	<	return null;
268	<
269	<	long lastTime = (mode == TIMEOUT) ? System.nanoTime() : 0;
601	>	private E awaitMatch(Node<E> s, Node<E> pred, E e, int how, long nanos) {
602	>	long lastTime = (how == TIMEOUT) ? System.nanoTime() : 0L;
603		Thread w = Thread.currentThread();
604	<	int spins = -1; // set to desired spin count below
604	>	int spins = -1; // initialized after first item and cancel checks
605	>	ThreadLocalRandom randomYields = null; // bound if needed
606	>
607		for (;;) {
608	<	if (w.isInterrupted())
609	<	s.compareAndSet(e, s);
610	<	Object x = s.get();
611	<	if (x != e) {                 // Node was matched or cancelled
612	<	advanceHead(pred, s);     // unlink if head
613	<	if (x == s) {             // was cancelled
614	<	clean(pred, s);
615	<	return null;
616	<	}
617	<	else if (x != null) {
618	<	s.set(s);             // avoid garbage retention
619	<	return x;
620	<	}
621	<	else
622	<	return e;
608	>	Object item = s.item;
609	>	if (item != e) {                  // matched
610	>	assert item != s;
611	>	s.forgetContents();           // avoid garbage
612	>	return this.<E>cast(item);
613	>	}
614	>	if ((w.isInterrupted() || (how == TIMEOUT && nanos <= 0)) &&
615	>	s.casItem(e, s)) {       // cancel
616	>	unsplice(pred, s);
617	>	return e;
618	>	}
619	>
620	>	if (spins < 0) {                  // establish spins at/near front
621	>	if ((spins = spinsFor(pred, s.isData)) > 0)
622	>	randomYields = ThreadLocalRandom.current();
623	>	}
624	>	else if (spins > 0) {             // spin
625	>	if (--spins == 0)
626	>	shortenHeadPath();        // reduce slack before blocking
627	>	else if (randomYields.nextInt(CHAINED_SPINS) == 0)
628	>	Thread.yield();           // occasionally yield
629	>	}
630	>	else if (s.waiter == null) {
631	>	s.waiter = w;                 // request unpark then recheck
632		}
633	<	if (mode == TIMEOUT) {
633	>	else if (how == TIMEOUT) {
634		long now = System.nanoTime();
635	<	nanos -= now - lastTime;
635	>	if ((nanos -= now - lastTime) > 0)
636	>	LockSupport.parkNanos(this, nanos);
637		lastTime = now;
293	–	if (nanos <= 0) {
294	–	s.compareAndSet(e, s); // try to cancel
295	–	continue;
296	–	}
638		}
639	<	if (spins < 0) {
299	<	QNode h = head.get(); // only spin if at head
300	<	spins = ((h != null && h.next == s) ?
301	<	((mode == TIMEOUT) ?
302	<	maxTimedSpins : maxUntimedSpins) : 0);
303	<	}
304	<	if (spins > 0)
305	<	--spins;
306	<	else if (s.waiter == null)
307	<	s.waiter = w;
308	<	else if (mode != TIMEOUT) {
639	>	else {
640		LockSupport.park(this);
641		s.waiter = null;
642	<	spins = -1;
312	<	}
313	<	else if (nanos > spinForTimeoutThreshold) {
314	<	LockSupport.parkNanos(this, nanos);
315	<	s.waiter = null;
316	<	spins = -1;
642	>	spins = -1;                   // spin if front upon wakeup
643		}
644		}
645		}
646
647		/**
648	<	* Returns validated tail for use in cleaning methods.
648	>	* Returns spin/yield value for a node with given predecessor and
649	>	* data mode. See above for explanation.
650		*/
651	<	private QNode getValidatedTail() {
652	<	for (;;) {
653	<	QNode h = head.get();
654	<	QNode first = h.next;
655	<	if (first != null && first.next == first) { // help advance
656	<	advanceHead(h, first);
657	<	continue;
658	<	}
659	<	QNode t = tail.get();
660	<	QNode last = t.next;
661	<	if (t == tail.get()) {
662	<	if (last != null)
663	<	tail.compareAndSet(t, last); // help advance
664	<	else
665	<	return t;
651	>	private static int spinsFor(Node<?> pred, boolean haveData) {
652	>	if (MP && pred != null) {
653	>	if (pred.isData != haveData)      // phase change
654	>	return FRONT_SPINS + CHAINED_SPINS;
655	>	if (pred.isMatched())             // probably at front
656	>	return FRONT_SPINS;
657	>	if (pred.waiter == null)          // pred apparently spinning
658	>	return CHAINED_SPINS;
659	>	}
660	>	return 0;
661	>	}
662	>
663	>	/**
664	>	* Tries (once) to unsplice nodes between head and first unmatched
665	>	* or trailing node; failing on contention.
666	>	*/
667	>	private void shortenHeadPath() {
668	>	Node<E> h, hn, p, q;
669	>	if ((p = h = head) != null && h.isMatched() &&
670	>	(q = hn = h.next) != null) {
671	>	Node<E> n;
672	>	while ((n = q.next) != q) {
673	>	if (n == null || !q.isMatched()) {
674	>	if (hn != q && h.next == hn)
675	>	h.casNext(hn, q);
676	>	break;
677	>	}
678	>	p = q;
679	>	q = n;
680		}
681		}
682		}
683
684	+	/* -------------- Traversal methods -------------- */
685	+
686	+	/**
687	+	* Returns the first unmatched node of the given mode, or null if
688	+	* none.  Used by methods isEmpty, hasWaitingConsumer.
689	+	*/
690	+	private Node<E> firstOfMode(boolean data) {
691	+	for (Node<E> p = head; p != null; ) {
692	+	if (!p.isMatched())
693	+	return (p.isData == data) ? p : null;
694	+	Node<E> n = p.next;
695	+	p = (n != p) ? n : head;
696	+	}
697	+	return null;
698	+	}
699	+
700		/**
701	<	* Gets rid of cancelled node s with original predecessor pred.
702	<	*
346	<	* @param pred predecessor of cancelled node
347	<	* @param s the cancelled node
701	>	* Returns the item in the first unmatched node with isData; or
702	>	* null if none.  Used by peek.
703		*/
704	<	private void clean(QNode pred, QNode s) {
705	<	Thread w = s.waiter;
706	<	if (w != null) {             // Wake up thread
707	<	s.waiter = null;
708	<	if (w != Thread.currentThread())
709	<	LockSupport.unpark(w);
704	>	private E firstDataItem() {
705	>	for (Node<E> p = head; p != null; ) {
706	>	boolean isData = p.isData;
707	>	Object item = p.item;
708	>	if (item != p && (item != null) == isData)
709	>	return isData ? this.<E>cast(item) : null;
710	>	Node<E> n = p.next;
711	>	p = (n != p) ? n : head;
712		}
713	+	return null;
714	+	}
715	+
716	+	/**
717	+	* Traverses and counts unmatched nodes of the given mode.
718	+	* Used by methods size and getWaitingConsumerCount.
719	+	*/
720	+	private int countOfMode(boolean data) {
721	+	int count = 0;
722	+	for (Node<E> p = head; p != null; ) {
723	+	if (!p.isMatched()) {
724	+	if (p.isData != data)
725	+	return 0;
726	+	if (++count == Integer.MAX_VALUE) // saturated
727	+	break;
728	+	}
729	+	Node<E> n = p.next;
730	+	if (n != p)
731	+	p = n;
732	+	else {
733	+	count = 0;
734	+	p = head;
735	+	}
736	+	}
737	+	return count;
738	+	}
739	+
740	+	final class Itr implements Iterator<E> {
741	+	private Node<E> nextNode;   // next node to return item for
742	+	private E nextItem;         // the corresponding item
743	+	private Node<E> lastRet;    // last returned node, to support remove
744	+
745	+	/**
746	+	* Moves to next node after prev, or first node if prev null.
747	+	*/
748	+	private void advance(Node<E> prev) {
749	+	lastRet = prev;
750	+	Node<E> p;
751	+	if (prev == null || (p = prev.next) == prev)
752	+	p = head;
753	+	while (p != null) {
754	+	Object item = p.item;
755	+	if (p.isData) {
756	+	if (item != null && item != p) {
757	+	nextItem = LinkedTransferQueue.this.<E>cast(item);
758	+	nextNode = p;
759	+	return;
760	+	}
761	+	}
762	+	else if (item == null)
763	+	break;
764	+	Node<E> n = p.next;
765	+	p = (n != p) ? n : head;
766	+	}
767	+	nextNode = null;
768	+	}
769	+
770	+	Itr() {
771	+	advance(null);
772	+	}
773	+
774	+	public final boolean hasNext() {
775	+	return nextNode != null;
776	+	}
777	+
778	+	public final E next() {
779	+	Node<E> p = nextNode;
780	+	if (p == null) throw new NoSuchElementException();
781	+	E e = nextItem;
782	+	advance(p);
783	+	return e;
784	+	}
785	+
786	+	public final void remove() {
787	+	Node<E> p = lastRet;
788	+	if (p == null) throw new IllegalStateException();
789	+	lastRet = null;
790	+	findAndRemoveDataNode(p);
791	+	}
792	+	}
793
794	<	if (pred == null)
358	<	return;
794	>	/* -------------- Removal methods -------------- */
795
796	+	/**
797	+	* Unsplices (now or later) the given deleted/cancelled node with
798	+	* the given predecessor.
799	+	*
800	+	* @param pred predecessor of node to be unspliced
801	+	* @param s the node to be unspliced
802	+	*/
803	+	private void unsplice(Node<E> pred, Node<E> s) {
804	+	s.forgetContents(); // clear unneeded fields
805		/*
806		* At any given time, exactly one node on list cannot be
807	<	* deleted -- the last inserted node. To accommodate this, if
808	<	* we cannot delete s, we save its predecessor as "cleanMe",
809	<	* processing the previously saved version first. At least one
810	<	* of node s or the node previously saved can always be
807	>	* unlinked -- the last inserted node. To accommodate this, if
808	>	* we cannot unlink s, we save its predecessor as "cleanMe",
809	>	* processing the previously saved version first. Because only
810	>	* one node in the list can have a null next, at least one of
811	>	* node s or the node previously saved can always be
812		* processed, so this always terminates.
813		*/
814	<	while (pred.next == s) {
815	<	QNode oldpred = reclean();  // First, help get rid of cleanMe
816	<	QNode t = getValidatedTail();
817	<	if (s != t) {               // If not tail, try to unsplice
818	<	QNode sn = s.next;      // s.next == s means s already off list
819	<	if (sn == s || pred.casNext(s, sn))
814	>	if (pred != null && pred != s) {
815	>	while (pred.next == s) {
816	>	Node<E> oldpred = (cleanMe == null) ? null : reclean();
817	>	Node<E> n = s.next;
818	>	if (n != null) {
819	>	if (n != s)
820	>	pred.casNext(s, n);
821		break;
822	+	}
823	+	if (oldpred == pred ||      // Already saved
824	+	(oldpred == null && casCleanMe(null, pred)))
825	+	break;                  // Postpone cleaning
826		}
376	–	else if (oldpred == pred || // Already saved
377	–	(oldpred == null && cleanMe.compareAndSet(null, pred)))
378	–	break;                  // Postpone cleaning
827		}
828		}
829
830		/**
831	<	* Tries to unsplice the cancelled node held in cleanMe that was
832	<	* previously uncleanable because it was at tail.
831	>	* Tries to unsplice the deleted/cancelled node held in cleanMe
832	>	* that was previously uncleanable because it was at tail.
833		*
834		* @return current cleanMe node (or null)
835		*/
836	<	private QNode reclean() {
836	>	private Node<E> reclean() {
837		/*
838	<	* cleanMe is, or at one time was, predecessor of cancelled
839	<	* node s that was the tail so could not be unspliced.  If s
838	>	* cleanMe is, or at one time was, predecessor of a cancelled
839	>	* node s that was the tail so could not be unspliced.  If it
840		* is no longer the tail, try to unsplice if necessary and
841		* make cleanMe slot available.  This differs from similar
842	<	* code in clean() because we must check that pred still
843	<	* points to a cancelled node that must be unspliced -- if
844	<	* not, we can (must) clear cleanMe without unsplicing.
845	<	* This can loop only due to contention on casNext or
398	<	* clearing cleanMe.
842	>	* code in unsplice() because we must check that pred still
843	>	* points to a matched node that can be unspliced -- if not,
844	>	* we can (must) clear cleanMe without unsplicing.  This can
845	>	* loop only due to contention.
846		*/
847	<	QNode pred;
848	<	while ((pred = cleanMe.get()) != null) {
849	<	QNode t = getValidatedTail();
850	<	QNode s = pred.next;
851	<	if (s != t) {
852	<	QNode sn;
853	<	if (s == null || s == pred || s.get() != s ||
854	<	(sn = s.next) == s || pred.casNext(s, sn))
855	<	cleanMe.compareAndSet(pred, null);
847	>	Node<E> pred;
848	>	while ((pred = cleanMe) != null) {
849	>	Node<E> s = pred.next;
850	>	Node<E> n;
851	>	if (s == null || s == pred || !s.isMatched())
852	>	casCleanMe(pred, null); // already gone
853	>	else if ((n = s.next) != null) {
854	>	if (n != s)
855	>	pred.casNext(s, n);
856	>	casCleanMe(pred, null);
857		}
858	<	else // s is still tail; cannot clean
858	>	else
859		break;
860		}
861		return pred;
862		}
863
864		/**
865	+	* Main implementation of Iterator.remove(). Find
866	+	* and unsplice the given data node.
867	+	*/
868	+	final void findAndRemoveDataNode(Node<E> s) {
869	+	assert s.isData;
870	+	if (s.tryMatchData()) {
871	+	for (Node<E> pred = null, p = head; p != null; ) {
872	+	if (p == s) {
873	+	unsplice(pred, p);
874	+	break;
875	+	}
876	+	if (p.isUnmatchedRequest())
877	+	break;
878	+	pred = p;
879	+	if ((p = p.next) == pred) { // stale
880	+	pred = null;
881	+	p = head;
882	+	}
883	+	}
884	+	}
885	+	}
886	+
887	+	/**
888	+	* Main implementation of remove(Object)
889	+	*/
890	+	private boolean findAndRemove(Object e) {
891	+	if (e != null) {
892	+	for (Node<E> pred = null, p = head; p != null; ) {
893	+	Object item = p.item;
894	+	if (p.isData) {
895	+	if (item != null && item != p && e.equals(item) &&
896	+	p.tryMatchData()) {
897	+	unsplice(pred, p);
898	+	return true;
899	+	}
900	+	}
901	+	else if (item == null)
902	+	break;
903	+	pred = p;
904	+	if ((p = p.next) == pred) { // stale
905	+	pred = null;
906	+	p = head;
907	+	}
908	+	}
909	+	}
910	+	return false;
911	+	}
912	+
913	+
914	+	/**
915		* Creates an initially empty {@code LinkedTransferQueue}.
916		*/
917		public LinkedTransferQueue() {
420	–	QNode dummy = new QNode(null, false);
421	–	head = new PaddedAtomicReference<QNode>(dummy);
422	–	tail = new PaddedAtomicReference<QNode>(dummy);
423	–	cleanMe = new PaddedAtomicReference<QNode>(null);
918		}
919
920		/**
#	Line 437 | Line 931 | public class LinkedTransferQueue<E> exte
931		addAll(c);
932		}
933
934	<	public void put(E e) throws InterruptedException {
935	<	if (e == null) throw new NullPointerException();
936	<	if (Thread.interrupted()) throw new InterruptedException();
937	<	xfer(e, NOWAIT, 0);
934	>	/**
935	>	* Inserts the specified element at the tail of this queue.
936	>	* As the queue is unbounded, this method will never block.
937	>	*
938	>	* @throws NullPointerException if the specified element is null
939	>	*/
940	>	public void put(E e) {
941	>	xfer(e, true, ASYNC, 0);
942		}
943
944	<	public boolean offer(E e, long timeout, TimeUnit unit)
945	<	throws InterruptedException {
946	<	if (e == null) throw new NullPointerException();
947	<	if (Thread.interrupted()) throw new InterruptedException();
948	<	xfer(e, NOWAIT, 0);
944	>	/**
945	>	* Inserts the specified element at the tail of this queue.
946	>	* As the queue is unbounded, this method will never block or
947	>	* return {@code false}.
948	>	*
949	>	* @return {@code true} (as specified by
950	>	*  {@link BlockingQueue#offer(Object,long,TimeUnit) BlockingQueue.offer})
951	>	* @throws NullPointerException if the specified element is null
952	>	*/
953	>	public boolean offer(E e, long timeout, TimeUnit unit) {
954	>	xfer(e, true, ASYNC, 0);
955		return true;
956		}
957
958	+	/**
959	+	* Inserts the specified element at the tail of this queue.
960	+	* As the queue is unbounded, this method will never return {@code false}.
961	+	*
962	+	* @return {@code true} (as specified by
963	+	*         {@link BlockingQueue#offer(Object) BlockingQueue.offer})
964	+	* @throws NullPointerException if the specified element is null
965	+	*/
966		public boolean offer(E e) {
967	<	if (e == null) throw new NullPointerException();
456	<	xfer(e, NOWAIT, 0);
967	>	xfer(e, true, ASYNC, 0);
968		return true;
969		}
970
971	+	/**
972	+	* Inserts the specified element at the tail of this queue.
973	+	* As the queue is unbounded, this method will never throw
974	+	* {@link IllegalStateException} or return {@code false}.
975	+	*
976	+	* @return {@code true} (as specified by {@link Collection#add})
977	+	* @throws NullPointerException if the specified element is null
978	+	*/
979		public boolean add(E e) {
980	<	if (e == null) throw new NullPointerException();
462	<	xfer(e, NOWAIT, 0);
980	>	xfer(e, true, ASYNC, 0);
981		return true;
982		}
983
984	+	/**
985	+	* Transfers the element to a waiting consumer immediately, if possible.
986	+	*
987	+	* <p>More precisely, transfers the specified element immediately
988	+	* if there exists a consumer already waiting to receive it (in
989	+	* {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
990	+	* otherwise returning {@code false} without enqueuing the element.
991	+	*
992	+	* @throws NullPointerException if the specified element is null
993	+	*/
994	+	public boolean tryTransfer(E e) {
995	+	return xfer(e, true, NOW, 0) == null;
996	+	}
997	+
998	+	/**
999	+	* Transfers the element to a consumer, waiting if necessary to do so.
1000	+	*
1001	+	* <p>More precisely, transfers the specified element immediately
1002	+	* if there exists a consumer already waiting to receive it (in
1003	+	* {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
1004	+	* else inserts the specified element at the tail of this queue
1005	+	* and waits until the element is received by a consumer.
1006	+	*
1007	+	* @throws NullPointerException if the specified element is null
1008	+	*/
1009		public void transfer(E e) throws InterruptedException {
1010	<	if (e == null) throw new NullPointerException();
1011	<	if (xfer(e, WAIT, 0) == null) {
469	<	Thread.interrupted();
1010	>	if (xfer(e, true, SYNC, 0) != null) {
1011	>	Thread.interrupted(); // failure possible only due to interrupt
1012		throw new InterruptedException();
1013		}
1014		}
1015
1016	+	/**
1017	+	* Transfers the element to a consumer if it is possible to do so
1018	+	* before the timeout elapses.
1019	+	*
1020	+	* <p>More precisely, transfers the specified element immediately
1021	+	* if there exists a consumer already waiting to receive it (in
1022	+	* {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
1023	+	* else inserts the specified element at the tail of this queue
1024	+	* and waits until the element is received by a consumer,
1025	+	* returning {@code false} if the specified wait time elapses
1026	+	* before the element can be transferred.
1027	+	*
1028	+	* @throws NullPointerException if the specified element is null
1029	+	*/
1030		public boolean tryTransfer(E e, long timeout, TimeUnit unit)
1031		throws InterruptedException {
1032	<	if (e == null) throw new NullPointerException();
477	<	if (xfer(e, TIMEOUT, unit.toNanos(timeout)) != null)
1032	>	if (xfer(e, true, TIMEOUT, unit.toNanos(timeout)) == null)
1033		return true;
1034		if (!Thread.interrupted())
1035		return false;
1036		throw new InterruptedException();
1037		}
1038
484	–	public boolean tryTransfer(E e) {
485	–	if (e == null) throw new NullPointerException();
486	–	return fulfill(e) != null;
487	–	}
488	–
1039		public E take() throws InterruptedException {
1040	<	Object e = xfer(null, WAIT, 0);
1040	>	E e = xfer(null, false, SYNC, 0);
1041		if (e != null)
1042	<	return (E) e;
1042	>	return e;
1043		Thread.interrupted();
1044		throw new InterruptedException();
1045		}
1046
1047		public E poll(long timeout, TimeUnit unit) throws InterruptedException {
1048	<	Object e = xfer(null, TIMEOUT, unit.toNanos(timeout));
1048	>	E e = xfer(null, false, TIMEOUT, unit.toNanos(timeout));
1049		if (e != null || !Thread.interrupted())
1050	<	return (E) e;
1050	>	return e;
1051		throw new InterruptedException();
1052		}
1053
1054		public E poll() {
1055	<	return (E) fulfill(null);
1055	>	return xfer(null, false, NOW, 0);
1056		}
1057
1058	+	/**
1059	+	* @throws NullPointerException     {@inheritDoc}
1060	+	* @throws IllegalArgumentException {@inheritDoc}
1061	+	*/
1062		public int drainTo(Collection<? super E> c) {
1063		if (c == null)
1064		throw new NullPointerException();
#	Line 519 | Line 1073 | public class LinkedTransferQueue<E> exte
1073		return n;
1074		}
1075
1076	+	/**
1077	+	* @throws NullPointerException     {@inheritDoc}
1078	+	* @throws IllegalArgumentException {@inheritDoc}
1079	+	*/
1080		public int drainTo(Collection<? super E> c, int maxElements) {
1081		if (c == null)
1082		throw new NullPointerException();
#	Line 533 | Line 1091 | public class LinkedTransferQueue<E> exte
1091		return n;
1092		}
1093
536	–	// Traversal-based methods
537	–
1094		/**
1095	<	* Returns head after performing any outstanding helping steps.
1095	>	* Returns an iterator over the elements in this queue in proper
1096	>	* sequence, from head to tail.
1097	>	*
1098	>	* <p>The returned iterator is a "weakly consistent" iterator that
1099	>	* will never throw
1100	>	* {@link ConcurrentModificationException ConcurrentModificationException},
1101	>	* and guarantees to traverse elements as they existed upon
1102	>	* construction of the iterator, and may (but is not guaranteed
1103	>	* to) reflect any modifications subsequent to construction.
1104	>	*
1105	>	* @return an iterator over the elements in this queue in proper sequence
1106		*/
541	–	private QNode traversalHead() {
542	–	for (;;) {
543	–	QNode t = tail.get();
544	–	QNode h = head.get();
545	–	if (h != null && t != null) {
546	–	QNode last = t.next;
547	–	QNode first = h.next;
548	–	if (t == tail.get()) {
549	–	if (last != null)
550	–	tail.compareAndSet(t, last);
551	–	else if (first != null) {
552	–	Object x = first.get();
553	–	if (x == first)
554	–	advanceHead(h, first);
555	–	else
556	–	return h;
557	–	}
558	–	else
559	–	return h;
560	–	}
561	–	}
562	–	reclean();
563	–	}
564	–	}
565	–
566	–
1107		public Iterator<E> iterator() {
1108		return new Itr();
1109		}
1110
571	–	/**
572	–	* Iterators. Basic strategy is to traverse list, treating
573	–	* non-data (i.e., request) nodes as terminating list.
574	–	* Once a valid data node is found, the item is cached
575	–	* so that the next call to next() will return it even
576	–	* if subsequently removed.
577	–	*/
578	–	class Itr implements Iterator<E> {
579	–	QNode next;        // node to return next
580	–	QNode pnext;       // predecessor of next
581	–	QNode snext;       // successor of next
582	–	QNode curr;        // last returned node, for remove()
583	–	QNode pcurr;       // predecessor of curr, for remove()
584	–	E nextItem;        // Cache of next item, once committed to in next
585	–
586	–	Itr() {
587	–	findNext();
588	–	}
589	–
590	–	/**
591	–	* Ensures next points to next valid node, or null if none.
592	–	*/
593	–	void findNext() {
594	–	for (;;) {
595	–	QNode pred = pnext;
596	–	QNode q = next;
597	–	if (pred == null || pred == q) {
598	–	pred = traversalHead();
599	–	q = pred.next;
600	–	}
601	–	if (q == null || !q.isData) {
602	–	next = null;
603	–	return;
604	–	}
605	–	Object x = q.get();
606	–	QNode s = q.next;
607	–	if (x != null && q != x && q != s) {
608	–	nextItem = (E) x;
609	–	snext = s;
610	–	pnext = pred;
611	–	next = q;
612	–	return;
613	–	}
614	–	pnext = q;
615	–	next = s;
616	–	}
617	–	}
618	–
619	–	public boolean hasNext() {
620	–	return next != null;
621	–	}
622	–
623	–	public E next() {
624	–	if (next == null) throw new NoSuchElementException();
625	–	pcurr = pnext;
626	–	curr = next;
627	–	pnext = next;
628	–	next = snext;
629	–	E x = nextItem;
630	–	findNext();
631	–	return x;
632	–	}
633	–
634	–	public void remove() {
635	–	QNode p = curr;
636	–	if (p == null)
637	–	throw new IllegalStateException();
638	–	Object x = p.get();
639	–	if (x != null && x != p && p.compareAndSet(x, p))
640	–	clean(pcurr, p);
641	–	}
642	–	}
643	–
1111		public E peek() {
1112	<	for (;;) {
646	<	QNode h = traversalHead();
647	<	QNode p = h.next;
648	<	if (p == null)
649	<	return null;
650	<	Object x = p.get();
651	<	if (p != x) {
652	<	if (!p.isData)
653	<	return null;
654	<	if (x != null)
655	<	return (E) x;
656	<	}
657	<	}
1112	>	return firstDataItem();
1113		}
1114
1115	+	/**
1116	+	* Returns {@code true} if this queue contains no elements.
1117	+	*
1118	+	* @return {@code true} if this queue contains no elements
1119	+	*/
1120		public boolean isEmpty() {
1121	<	for (;;) {
662	<	QNode h = traversalHead();
663	<	QNode p = h.next;
664	<	if (p == null)
665	<	return true;
666	<	Object x = p.get();
667	<	if (p != x) {
668	<	if (!p.isData)
669	<	return true;
670	<	if (x != null)
671	<	return false;
672	<	}
673	<	}
1121	>	return firstOfMode(true) == null;
1122		}
1123
1124		public boolean hasWaitingConsumer() {
1125	<	for (;;) {
678	<	QNode h = traversalHead();
679	<	QNode p = h.next;
680	<	if (p == null)
681	<	return false;
682	<	Object x = p.get();
683	<	if (p != x)
684	<	return !p.isData;
685	<	}
1125	>	return firstOfMode(false) != null;
1126		}
1127
1128		/**
#	Line 698 | Line 1138 | public class LinkedTransferQueue<E> exte
1138		* @return the number of elements in this queue
1139		*/
1140		public int size() {
1141	<	int count = 0;
702	<	QNode h = traversalHead();
703	<	for (QNode p = h.next; p != null && p.isData; p = p.next) {
704	<	Object x = p.get();
705	<	if (x != null && x != p) {
706	<	if (++count == Integer.MAX_VALUE) // saturated
707	<	break;
708	<	}
709	<	}
710	<	return count;
1141	>	return countOfMode(true);
1142		}
1143
1144		public int getWaitingConsumerCount() {
1145	<	int count = 0;
715	<	QNode h = traversalHead();
716	<	for (QNode p = h.next; p != null && !p.isData; p = p.next) {
717	<	if (p.get() == null) {
718	<	if (++count == Integer.MAX_VALUE)
719	<	break;
720	<	}
721	<	}
722	<	return count;
1145	>	return countOfMode(false);
1146		}
1147
1148	<	public int remainingCapacity() {
1149	<	return Integer.MAX_VALUE;
1148	>	/**
1149	>	* Removes a single instance of the specified element from this queue,
1150	>	* if it is present.  More formally, removes an element {@code e} such
1151	>	* that {@code o.equals(e)}, if this queue contains one or more such
1152	>	* elements.
1153	>	* Returns {@code true} if this queue contained the specified element
1154	>	* (or equivalently, if this queue changed as a result of the call).
1155	>	*
1156	>	* @param o element to be removed from this queue, if present
1157	>	* @return {@code true} if this queue changed as a result of the call
1158	>	*/
1159	>	public boolean remove(Object o) {
1160	>	return findAndRemove(o);
1161		}
1162
1163	<	public boolean remove(Object o) {
1164	<	if (o == null)
1165	<	return false;
1166	<	for (;;) {
1167	<	QNode pred = traversalHead();
1168	<	for (;;) {
1169	<	QNode q = pred.next;
1170	<	if (q == null || !q.isData)
1171	<	return false;
738	<	if (q == pred) // restart
739	<	break;
740	<	Object x = q.get();
741	<	if (x != null && x != q && o.equals(x) &&
742	<	q.compareAndSet(x, q)) {
743	<	clean(pred, q);
744	<	return true;
745	<	}
746	<	pred = q;
747	<	}
748	<	}
1163	>	/**
1164	>	* Always returns {@code Integer.MAX_VALUE} because a
1165	>	* {@code LinkedTransferQueue} is not capacity constrained.
1166	>	*
1167	>	* @return {@code Integer.MAX_VALUE} (as specified by
1168	>	*         {@link BlockingQueue#remainingCapacity()})
1169	>	*/
1170	>	public int remainingCapacity() {
1171	>	return Integer.MAX_VALUE;
1172		}
1173
1174		/**
1175	<	* Save the state to a stream (that is, serialize it).
1175	>	* Saves the state to a stream (that is, serializes it).
1176		*
1177		* @serialData All of the elements (each an {@code E}) in
1178		* the proper order, followed by a null
#	Line 765 | Line 1188 | public class LinkedTransferQueue<E> exte
1188		}
1189
1190		/**
1191	<	* Reconstitute the Queue instance from a stream (that is,
1192	<	* deserialize it).
1191	>	* Reconstitutes the Queue instance from a stream (that is,
1192	>	* deserializes it).
1193		*
1194		* @param s the stream
1195		*/
1196		private void readObject(java.io.ObjectInputStream s)
1197		throws java.io.IOException, ClassNotFoundException {
1198		s.defaultReadObject();
776	–	resetHeadAndTail();
1199		for (;;) {
1200	<	E item = (E) s.readObject();
1200	>	@SuppressWarnings("unchecked") E item = (E) s.readObject();
1201		if (item == null)
1202		break;
1203		else
#	Line 783 | Line 1205 | public class LinkedTransferQueue<E> exte
1205		}
1206		}
1207
1208	+	// Unsafe mechanics
1209	+
1210	+	private static final sun.misc.Unsafe UNSAFE = getUnsafe();
1211	+	private static final long headOffset =
1212	+	objectFieldOffset(UNSAFE, "head", LinkedTransferQueue.class);
1213	+	private static final long tailOffset =
1214	+	objectFieldOffset(UNSAFE, "tail", LinkedTransferQueue.class);
1215	+	private static final long cleanMeOffset =
1216	+	objectFieldOffset(UNSAFE, "cleanMe", LinkedTransferQueue.class);
1217
1218	<	// Support for resetting head/tail while deserializing
1219	<	private void resetHeadAndTail() {
1220	<	QNode dummy = new QNode(null, false);
1221	<	UNSAFE.putObjectVolatile(this, headOffset,
1222	<	new PaddedAtomicReference<QNode>(dummy));
1223	<	UNSAFE.putObjectVolatile(this, tailOffset,
1224	<	new PaddedAtomicReference<QNode>(dummy));
1225	<	UNSAFE.putObjectVolatile(this, cleanMeOffset,
1226	<	new PaddedAtomicReference<QNode>(null));
1218	>	static long objectFieldOffset(sun.misc.Unsafe UNSAFE,
1219	>	String field, Class<?> klazz) {
1220	>	try {
1221	>	return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
1222	>	} catch (NoSuchFieldException e) {
1223	>	// Convert Exception to corresponding Error
1224	>	NoSuchFieldError error = new NoSuchFieldError(field);
1225	>	error.initCause(e);
1226	>	throw error;
1227	>	}
1228		}
1229
1230	<	// Temporary Unsafe mechanics for preliminary release
1231	<	private static Unsafe getUnsafe() throws Throwable {
1230	>	/**
1231	>	* Returns a sun.misc.Unsafe.  Suitable for use in a 3rd party package.
1232	>	* Replace with a simple call to Unsafe.getUnsafe when integrating
1233	>	* into a jdk.
1234	>	*
1235	>	* @return a sun.misc.Unsafe
1236	>	*/
1237	>	static sun.misc.Unsafe getUnsafe() {
1238		try {
1239	<	return Unsafe.getUnsafe();
1239	>	return sun.misc.Unsafe.getUnsafe();
1240		} catch (SecurityException se) {
1241		try {
1242		return java.security.AccessController.doPrivileged
1243	<	(new java.security.PrivilegedExceptionAction<Unsafe>() {
1244	<	public Unsafe run() throws Exception {
1245	<	return getUnsafePrivileged();
1243	>	(new java.security
1244	>	.PrivilegedExceptionAction<sun.misc.Unsafe>() {
1245	>	public sun.misc.Unsafe run() throws Exception {
1246	>	java.lang.reflect.Field f = sun.misc
1247	>	.Unsafe.class.getDeclaredField("theUnsafe");
1248	>	f.setAccessible(true);
1249	>	return (sun.misc.Unsafe) f.get(null);
1250		}});
1251		} catch (java.security.PrivilegedActionException e) {
1252	<	throw e.getCause();
1252	>	throw new RuntimeException("Could not initialize intrinsics",
1253	>	e.getCause());
1254		}
1255		}
1256		}
1257
815	–	private static Unsafe getUnsafePrivileged()
816	–	throws NoSuchFieldException, IllegalAccessException {
817	–	Field f = Unsafe.class.getDeclaredField("theUnsafe");
818	–	f.setAccessible(true);
819	–	return (Unsafe) f.get(null);
820	–	}
821	–
822	–	private static long fieldOffset(String fieldName)
823	–	throws NoSuchFieldException {
824	–	return UNSAFE.objectFieldOffset
825	–	(LinkedTransferQueue.class.getDeclaredField(fieldName));
826	–	}
827	–
828	–	private static final Unsafe UNSAFE;
829	–	private static final long headOffset;
830	–	private static final long tailOffset;
831	–	private static final long cleanMeOffset;
832	–	static {
833	–	try {
834	–	UNSAFE = getUnsafe();
835	–	headOffset = fieldOffset("head");
836	–	tailOffset = fieldOffset("tail");
837	–	cleanMeOffset = fieldOffset("cleanMe");
838	–	} catch (Throwable e) {
839	–	throw new RuntimeException("Could not initialize intrinsics", e);
840	–	}
841	–	}
842	–
1258		}

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