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Comparing jsr166/src/jsr166y/LinkedTransferQueue.java (file contents):
Revision 1.14 by jsr166, Thu Mar 19 05:10:42 2009 UTC vs.
Revision 1.51 by dl, Sat Oct 24 14:33:29 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 44 | Line 46 | import java.lang.reflect.*;
46		* @since 1.7
47		* @author Doug Lea
48		* @param <E> the type of elements held in this collection
47	–	*
49		*/
50		public class LinkedTransferQueue<E> extends AbstractQueue<E>
51		implements TransferQueue<E>, java.io.Serializable {
52		private static final long serialVersionUID = -3223113410248163686L;
53
54		/*
55	<	* This class extends the approach used in FIFO-mode
55	<	* SynchronousQueues. See the internal documentation, as well as
56	<	* the PPoPP 2006 paper "Scalable Synchronous Queues" by Scherer,
57	<	* Lea & Scott
58	<	* (http://www.cs.rice.edu/~wns1/papers/2006-PPoPP-SQ.pdf)
55	>	* *** Overview of Dual Queues with Slack ***
56		*
57	<	* The main extension is to provide different Wait modes for the
58	<	* main "xfer" method that puts or takes items.  These don't
59	<	* impact the basic dual-queue logic, but instead control whether
60	<	* or how threads block upon insertion of request or data nodes
61	<	* into the dual queue. It also uses slightly different
62	<	* conventions for tracking whether nodes are off-list or
63	<	* cancelled.
64	<	*/
65	<
66	<	// Wait modes for xfer method
67	<	static final int NOWAIT  = 0;
68	<	static final int TIMEOUT = 1;
69	<	static final int WAIT    = 2;
70	<
71	<	/** The number of CPUs, for spin control */
72	<	static final int NCPUS = Runtime.getRuntime().availableProcessors();
73	<
74	<	/**
75	<	* The number of times to spin before blocking in timed waits.
76	<	* The value is empirically derived -- it works well across a
77	<	* variety of processors and OSes. Empirically, the best value
78	<	* seems not to vary with number of CPUs (beyond 2) so is just
79	<	* a constant.
80	<	*/
81	<	static final int maxTimedSpins = (NCPUS < 2)? 0 : 32;
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 untimed waits.
340	<	* This is greater than timed value because untimed waits spin
341	<	* faster since they don't need to check times on each spin.
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 maxUntimedSpins = maxTimedSpins * 16;
346	>	private static final int FRONT_SPINS   = 1 << 7;
347
348		/**
349	<	* The number of nanoseconds for which it is faster to spin
350	<	* rather than to use timed park. A rough estimate suffices.
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 long spinForTimeoutThreshold = 1000L;
355	>	private static final int CHAINED_SPINS = FRONT_SPINS >>> 1;
356
357		/**
358	<	* Node class for LinkedTransferQueue. Opportunistically
359	<	* subclasses from AtomicReference to represent item. Uses Object,
360	<	* not E, to allow setting item to "this" after use, to avoid
361	<	* garbage retention. Similarly, setting the next field to this is
362	<	* used as sentinel that node is off list.
363	<	*/
364	<	static final class QNode extends AtomicReference<Object> {
365	<	volatile QNode next;
366	<	volatile Thread waiter;       // to control park/unpark
367	<	final boolean isData;
368	<	QNode(Object item, boolean isData) {
369	<	super(item);
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 {
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 next;
368	>	volatile Thread waiter; // null until waiting
369	>
370	>	// CAS methods for fields
371	>	final boolean casNext(Node cmp, Node val) {
372	>	return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
373	>	}
374	>
375	>	final boolean casItem(Object cmp, Object val) {
376	>	return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
377	>	}
378	>
379	>	/**
380	>	* Creates a new node. Uses relaxed write because item can only
381	>	* be seen if followed by CAS.
382	>	*/
383	>	Node(Object item, boolean isData) {
384	>	UNSAFE.putObject(this, itemOffset, item); // relaxed write
385		this.isData = isData;
386		}
387
388	<	static final AtomicReferenceFieldUpdater<QNode, QNode>
389	<	nextUpdater = AtomicReferenceFieldUpdater.newUpdater
390	<	(QNode.class, QNode.class, "next");
388	>	/**
389	>	* Links node to itself to avoid garbage retention.  Called
390	>	* only after CASing head field, so uses relaxed write.
391	>	*/
392	>	final void forgetNext() {
393	>	UNSAFE.putObject(this, nextOffset, this);
394	>	}
395
396	<	boolean casNext(QNode cmp, QNode val) {
397	<	return nextUpdater.compareAndSet(this, cmp, val);
396	>	/**
397	>	* Sets item to self (using a releasing/lazy write) and waiter
398	>	* to null, to avoid garbage retention after extracting or
399	>	* cancelling.
400	>	*/
401	>	final void forgetContents() {
402	>	UNSAFE.putOrderedObject(this, itemOffset, this);
403	>	UNSAFE.putOrderedObject(this, waiterOffset, null);
404		}
122	–	}
405
406	<	/**
407	<	* Padded version of AtomicReference used for head, tail and
408	<	* cleanMe, to alleviate contention across threads CASing one vs
409	<	* the other.
410	<	*/
411	<	static final class PaddedAtomicReference<T> extends AtomicReference<T> {
412	<	// enough padding for 64bytes with 4byte refs
413	<	Object p0, p1, p2, p3, p4, p5, p6, p7, p8, p9, pa, pb, pc, pd, pe;
414	<	PaddedAtomicReference(T r) { super(r); }
406	>	/**
407	>	* Returns true if this node has been matched, including the
408	>	* case of artificial matches due to cancellation.
409	>	*/
410	>	final boolean isMatched() {
411	>	Object x = item;
412	>	return x == this || (x != null) != isData;
413	>	}
414	>
415	>	/**
416	>	* Returns true if a node with the given mode cannot be
417	>	* appended to this node because this node is unmatched and
418	>	* has opposite data mode.
419	>	*/
420	>	final boolean cannotPrecede(boolean haveData) {
421	>	boolean d = isData;
422	>	Object x;
423	>	return d != haveData && (x = item) != this && (x != null) == d;
424	>	}
425	>
426	>	/**
427	>	* Tries to artificially match a data node -- used by remove.
428	>	*/
429	>	final boolean tryMatchData() {
430	>	Object x = item;
431	>	if (x != null && x != this && casItem(x, null)) {
432	>	LockSupport.unpark(waiter);
433	>	return true;
434	>	}
435	>	return false;
436	>	}
437	>
438	>	// Unsafe mechanics
439	>	private static final sun.misc.Unsafe UNSAFE = getUnsafe();
440	>	private static final long nextOffset =
441	>	objectFieldOffset(UNSAFE, "next", Node.class);
442	>	private static final long itemOffset =
443	>	objectFieldOffset(UNSAFE, "item", Node.class);
444	>	private static final long waiterOffset =
445	>	objectFieldOffset(UNSAFE, "waiter", Node.class);
446	>
447	>	private static final long serialVersionUID = -3375979862319811754L;
448		}
449
450	+	/** head of the queue; null until first enqueue */
451	+	private transient volatile Node head;
452
453	<	/** head of the queue */
454	<	private transient final PaddedAtomicReference<QNode> head;
138	<	/** tail of the queue */
139	<	private transient final PaddedAtomicReference<QNode> tail;
453	>	/** predecessor of dangling unspliceable node */
454	>	private transient volatile Node cleanMe; // decl here to reduce contention
455
456	<	/**
457	<	* Reference to a cancelled node that might not yet have been
143	<	* unlinked from queue because it was the last inserted node
144	<	* when it cancelled.
145	<	*/
146	<	private transient final PaddedAtomicReference<QNode> cleanMe;
456	>	/** tail of the queue; null until first append */
457	>	private transient volatile Node tail;
458
459	<	/**
460	<	* Tries to cas nh as new head; if successful, unlink
461	<	* old head's next node to avoid garbage retention.
462	<	*/
463	<	private boolean advanceHead(QNode h, QNode nh) {
464	<	if (h == head.get() && head.compareAndSet(h, nh)) {
465	<	h.next = h; // forget old next
466	<	return true;
467	<	}
468	<	return false;
459	>	// CAS methods for fields
460	>	private boolean casTail(Node cmp, Node val) {
461	>	return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val);
462	>	}
463	>
464	>	private boolean casHead(Node cmp, Node val) {
465	>	return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val);
466	>	}
467	>
468	>	private boolean casCleanMe(Node cmp, Node val) {
469	>	return UNSAFE.compareAndSwapObject(this, cleanMeOffset, cmp, val);
470		}
471
472	+	/*
473	+	* Possible values for "how" argument in xfer method. Beware that
474	+	* the order of assigned numerical values matters.
475	+	*/
476	+	private static final int NOW     = 0; // for untimed poll, tryTransfer
477	+	private static final int ASYNC   = 1; // for offer, put, add
478	+	private static final int SYNC    = 2; // for transfer, take
479	+	private static final int TIMEOUT = 3; // for timed poll, tryTransfer
480	+
481		/**
482	<	* Puts or takes an item. Used for most queue operations (except
483	<	* poll() and tryTransfer()). See the similar code in
484	<	* SynchronousQueue for detailed explanation.
485	<	* @param e the item or if null, signifies that this is a take
486	<	* @param mode the wait mode: NOWAIT, TIMEOUT, WAIT
482	>	* Implements all queuing methods. See above for explanation.
483	>	*
484	>	* @param e the item or null for take
485	>	* @param haveData true if this is a put, else a take
486	>	* @param how NOW, ASYNC, SYNC, or TIMEOUT
487		* @param nanos timeout in nanosecs, used only if mode is TIMEOUT
488	<	* @return an item, or null on failure
488	>	* @return an item if matched, else e
489	>	* @throws NullPointerException if haveData mode but e is null
490		*/
491	<	private Object xfer(Object e, int mode, long nanos) {
492	<	boolean isData = (e != null);
493	<	QNode s = null;
494	<	final PaddedAtomicReference<QNode> head = this.head;
173	<	final PaddedAtomicReference<QNode> tail = this.tail;
491	>	private Object xfer(Object e, boolean haveData, int how, long nanos) {
492	>	if (haveData && (e == null))
493	>	throw new NullPointerException();
494	>	Node s = null;                        // the node to append, if needed
495
496	<	for (;;) {
176	<	QNode t = tail.get();
177	<	QNode h = head.get();
496	>	retry: for (;;) {                     // restart on append race
497
498	<	if (t != null && (t == h || t.isData == isData)) {
499	<	if (s == null)
500	<	s = new QNode(e, isData);
501	<	QNode last = t.next;
502	<	if (last != null) {
503	<	if (t == tail.get())
504	<	tail.compareAndSet(t, last);
505	<	}
506	<	else if (t.casNext(null, s)) {
507	<	tail.compareAndSet(t, s);
508	<	return awaitFulfill(t, s, e, mode, nanos);
498	>	for (Node h = head, p = h; p != null;) { // find & match first node
499	>	boolean isData = p.isData;
500	>	Object item = p.item;
501	>	if (item != p && (item != null) == isData) { // unmatched
502	>	if (isData == haveData)   // can't match
503	>	break;
504	>	if (p.casItem(item, e)) { // match
505	>	LockSupport.unpark(p.waiter);
506	>	while (p != h) {      // update head
507	>	Node n = p.next;  // by 2 unless singleton
508	>	if (n != null)
509	>	p = n;
510	>	if (head == h && casHead(h, p)) {
511	>	h.forgetNext();
512	>	break;
513	>	}                 // advance and retry
514	>	if ((h = head)   == null ||
515	>	(p = h.next) == null || !p.isMatched())
516	>	break;        // unless slack < 2
517	>	}
518	>	return item;
519	>	}
520		}
521	+	Node n = p.next;
522	+	p = (p != n) ? n : (h = head); // Use head if p offlist
523		}
524
525	<	else if (h != null) {
526	<	QNode first = h.next;
527	<	if (t == tail.get() && first != null &&
528	<	advanceHead(h, first)) {
529	<	Object x = first.get();
530	<	if (x != first && first.compareAndSet(x, e)) {
531	<	LockSupport.unpark(first.waiter);
532	<	return isData? e : x;
201	<	}
202	<	}
525	>	if (how >= ASYNC) {               // No matches available
526	>	if (s == null)
527	>	s = new Node(e, haveData);
528	>	Node pred = tryAppend(s, haveData);
529	>	if (pred == null)
530	>	continue retry;           // lost race vs opposite mode
531	>	if (how >= SYNC)
532	>	return awaitMatch(s, pred, e, how, nanos);
533		}
534	+	return e; // not waiting
535		}
536		}
537
207	–
538		/**
539	<	* Version of xfer for poll() and tryTransfer, which
540	<	* simplifies control paths both here and in xfer
541	<	*/
542	<	private Object fulfill(Object e) {
543	<	boolean isData = (e != null);
544	<	final PaddedAtomicReference<QNode> head = this.head;
545	<	final PaddedAtomicReference<QNode> tail = this.tail;
546	<
547	<	for (;;) {
548	<	QNode t = tail.get();
549	<	QNode h = head.get();
550	<
551	<	if (t != null && (t == h || t.isData == isData)) {
552	<	QNode last = t.next;
223	<	if (t == tail.get()) {
224	<	if (last != null)
225	<	tail.compareAndSet(t, last);
226	<	else
227	<	return null;
228	<	}
539	>	* Tries to append node s as tail.
540	>	*
541	>	* @param s the node to append
542	>	* @param haveData true if appending in data mode
543	>	* @return null on failure due to losing race with append in
544	>	* different mode, else s's predecessor, or s itself if no
545	>	* predecessor
546	>	*/
547	>	private Node tryAppend(Node s, boolean haveData) {
548	>	for (Node t = tail, p = t;;) { // move p to last node and append
549	>	Node n, u;                        // temps for reads of next & tail
550	>	if (p == null && (p = head) == null) {
551	>	if (casHead(null, s))
552	>	return s;                 // initialize
553		}
554	<	else if (h != null) {
555	<	QNode first = h.next;
556	<	if (t == tail.get() &&
557	<	first != null &&
558	<	advanceHead(h, first)) {
559	<	Object x = first.get();
560	<	if (x != first && first.compareAndSet(x, e)) {
561	<	LockSupport.unpark(first.waiter);
562	<	return isData? e : x;
563	<	}
554	>	else if (p.cannotPrecede(haveData))
555	>	return null;                  // lost race vs opposite mode
556	>	else if ((n = p.next) != null)    // not last; keep traversing
557	>	p = p != t && t != (u = tail) ? (t = u) : // stale tail
558	>	(p != n) ? n : null;      // restart if off list
559	>	else if (!p.casNext(null, s))
560	>	p = p.next;                   // re-read on CAS failure
561	>	else {
562	>	if (p != t) {                 // update if slack now >= 2
563	>	while ((tail != t || !casTail(t, s)) &&
564	>	(t = tail)   != null &&
565	>	(s = t.next) != null && // advance and retry
566	>	(s = s.next) != null && s != t);
567		}
568	+	return p;
569		}
570		}
571		}
572
573		/**
574	<	* Spins/blocks until node s is fulfilled or caller gives up,
247	<	* depending on wait mode.
574	>	* Spins/yields/blocks until node s is matched or caller gives up.
575		*
249	–	* @param pred the predecessor of waiting node
576		* @param s the waiting node
577	+	* @param pred the predecessor of s, or s itself if it has no
578	+	* predecessor, or null if unknown (the null case does not occur
579	+	* in any current calls but may in possible future extensions)
580		* @param e the comparison value for checking match
581	<	* @param mode mode
581	>	* @param how either SYNC or TIMEOUT
582		* @param nanos timeout value
583	<	* @return matched item, or s if cancelled
583	>	* @return matched item, or e if unmatched on interrupt or timeout
584		*/
585	<	private Object awaitFulfill(QNode pred, QNode s, Object e,
586	<	int mode, long nanos) {
587	<	if (mode == NOWAIT)
259	<	return null;
260	<
261	<	long lastTime = (mode == TIMEOUT)? System.nanoTime() : 0;
585	>	private Object awaitMatch(Node s, Node pred, Object e,
586	>	int how, long nanos) {
587	>	long lastTime = (how == TIMEOUT) ? System.nanoTime() : 0L;
588		Thread w = Thread.currentThread();
589	<	int spins = -1; // set to desired spin count below
589	>	int spins = -1; // initialized after first item and cancel checks
590	>	ThreadLocalRandom randomYields = null; // bound if needed
591	>
592		for (;;) {
593	<	if (w.isInterrupted())
594	<	s.compareAndSet(e, s);
595	<	Object x = s.get();
596	<	if (x != e) {                 // Node was matched or cancelled
597	<	advanceHead(pred, s);     // unlink if head
598	<	if (x == s) {              // was cancelled
599	<	clean(pred, s);
600	<	return null;
601	<	}
602	<	else if (x != null) {
603	<	s.set(s);             // avoid garbage retention
604	<	return x;
605	<	}
606	<	else
279	<	return e;
593	>	Object item = s.item;
594	>	if (item != e) {                  // matched
595	>	s.forgetContents();           // avoid garbage
596	>	return item;
597	>	}
598	>	if ((w.isInterrupted() || (how == TIMEOUT && nanos <= 0)) &&
599	>	s.casItem(e, s)) {       // cancel
600	>	unsplice(pred, s);
601	>	return e;
602	>	}
603	>
604	>	if (spins < 0) {                  // establish spins at/near front
605	>	if ((spins = spinsFor(pred, s.isData)) > 0)
606	>	randomYields = ThreadLocalRandom.current();
607		}
608	<	if (mode == TIMEOUT) {
608	>	else if (spins > 0) {             // spin
609	>	if (--spins == 0)
610	>	shortenHeadPath();        // reduce slack before blocking
611	>	else if (randomYields.nextInt(CHAINED_SPINS) == 0)
612	>	Thread.yield();           // occasionally yield
613	>	}
614	>	else if (s.waiter == null) {
615	>	s.waiter = w;                 // request unpark then recheck
616	>	}
617	>	else if (how == TIMEOUT) {
618		long now = System.nanoTime();
619	<	nanos -= now - lastTime;
619	>	if ((nanos -= now - lastTime) > 0)
620	>	LockSupport.parkNanos(this, nanos);
621		lastTime = now;
285	–	if (nanos <= 0) {
286	–	s.compareAndSet(e, s); // try to cancel
287	–	continue;
288	–	}
622		}
623	<	if (spins < 0) {
291	<	QNode h = head.get(); // only spin if at head
292	<	spins = ((h != null && h.next == s) ?
293	<	(mode == TIMEOUT?
294	<	maxTimedSpins : maxUntimedSpins) : 0);
295	<	}
296	<	if (spins > 0)
297	<	--spins;
298	<	else if (s.waiter == null)
299	<	s.waiter = w;
300	<	else if (mode != TIMEOUT) {
623	>	else {
624		LockSupport.park(this);
625		s.waiter = null;
626	<	spins = -1;
304	<	}
305	<	else if (nanos > spinForTimeoutThreshold) {
306	<	LockSupport.parkNanos(this, nanos);
307	<	s.waiter = null;
308	<	spins = -1;
626	>	spins = -1;                   // spin if front upon wakeup
627		}
628		}
629		}
630
631		/**
632	<	* Returns validated tail for use in cleaning methods
632	>	* Returns spin/yield value for a node with given predecessor and
633	>	* data mode. See above for explanation.
634		*/
635	<	private QNode getValidatedTail() {
636	<	for (;;) {
637	<	QNode h = head.get();
638	<	QNode first = h.next;
639	<	if (first != null && first.next == first) { // help advance
640	<	advanceHead(h, first);
641	<	continue;
642	<	}
643	<	QNode t = tail.get();
644	<	QNode last = t.next;
645	<	if (t == tail.get()) {
646	<	if (last != null)
647	<	tail.compareAndSet(t, last); // help advance
648	<	else
649	<	return t;
635	>	private static int spinsFor(Node pred, boolean haveData) {
636	>	if (MP && pred != null) {
637	>	if (pred.isData != haveData)      // phase change
638	>	return FRONT_SPINS + CHAINED_SPINS;
639	>	if (pred.isMatched())             // probably at front
640	>	return FRONT_SPINS;
641	>	if (pred.waiter == null)          // pred apparently spinning
642	>	return CHAINED_SPINS;
643	>	}
644	>	return 0;
645	>	}
646	>
647	>	/**
648	>	* Tries (once) to unsplice nodes between head and first unmatched
649	>	* or trailing node; failing on contention.
650	>	*/
651	>	private void shortenHeadPath() {
652	>	Node h, hn, p, q;
653	>	if ((p = h = head) != null && h.isMatched() &&
654	>	(q = hn = h.next) != null) {
655	>	Node n;
656	>	while ((n = q.next) != q) {
657	>	if (n == null || !q.isMatched()) {
658	>	if (hn != q && h.next == hn)
659	>	h.casNext(hn, q);
660	>	break;
661	>	}
662	>	p = q;
663	>	q = n;
664		}
665		}
666		}
667
668	+	/* -------------- Traversal methods -------------- */
669	+
670	+	/**
671	+	* Returns the first unmatched node of the given mode, or null if
672	+	* none.  Used by methods isEmpty, hasWaitingConsumer.
673	+	*/
674	+	private Node firstOfMode(boolean data) {
675	+	for (Node p = head; p != null; ) {
676	+	if (!p.isMatched())
677	+	return (p.isData == data) ? p : null;
678	+	Node n = p.next;
679	+	p = (n != p) ? n : head;
680	+	}
681	+	return null;
682	+	}
683	+
684	+	/**
685	+	* Returns the item in the first unmatched node with isData; or
686	+	* null if none. Used by peek.
687	+	*/
688	+	private Object firstDataItem() {
689	+	for (Node p = head; p != null; ) {
690	+	boolean isData = p.isData;
691	+	Object item = p.item;
692	+	if (item != p && (item != null) == isData)
693	+	return isData ? item : null;
694	+	Node 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	<	* @param pred predecessor of cancelled node
703	<	* @param s the cancelled node
704	<	*/
705	<	private void clean(QNode pred, QNode s) {
706	<	Thread w = s.waiter;
707	<	if (w != null) {             // Wake up thread
708	<	s.waiter = null;
709	<	if (w != Thread.currentThread())
710	<	LockSupport.unpark(w);
701	>	* Traverses and counts unmatched nodes of the given mode.
702	>	* Used by methods size and getWaitingConsumerCount.
703	>	*/
704	>	private int countOfMode(boolean data) {
705	>	int count = 0;
706	>	for (Node p = head; p != null; ) {
707	>	if (!p.isMatched()) {
708	>	if (p.isData != data)
709	>	return 0;
710	>	if (++count == Integer.MAX_VALUE) // saturated
711	>	break;
712	>	}
713	>	Node n = p.next;
714	>	if (n != p)
715	>	p = n;
716	>	else {
717	>	count = 0;
718	>	p = head;
719	>	}
720	>	}
721	>	return count;
722	>	}
723	>
724	>	final class Itr implements Iterator<E> {
725	>	private Node nextNode;   // next node to return item for
726	>	private Object nextItem; // the corresponding item
727	>	private Node lastRet;    // last returned node, to support remove
728	>
729	>	/**
730	>	* Moves to next node after prev, or first node if prev null.
731	>	*/
732	>	private void advance(Node prev) {
733	>	lastRet = prev;
734	>	Node p;
735	>	if (prev == null || (p = prev.next) == prev)
736	>	p = head;
737	>	while (p != null) {
738	>	Object item = p.item;
739	>	if (p.isData) {
740	>	if (item != null && item != p) {
741	>	nextItem = item;
742	>	nextNode = p;
743	>	return;
744	>	}
745	>	}
746	>	else if (item == null)
747	>	break;
748	>	Node n = p.next;
749	>	p = (n != p) ? n : head;
750	>	}
751	>	nextNode = null;
752		}
753	+
754	+	Itr() {
755	+	advance(null);
756	+	}
757	+
758	+	public final boolean hasNext() {
759	+	return nextNode != null;
760	+	}
761	+
762	+	public final E next() {
763	+	Node p = nextNode;
764	+	if (p == null) throw new NoSuchElementException();
765	+	Object e = nextItem;
766	+	advance(p);
767	+	return (E) e;
768	+	}
769	+
770	+	public final void remove() {
771	+	Node p = lastRet;
772	+	if (p == null) throw new IllegalStateException();
773	+	lastRet = null;
774	+	findAndRemoveNode(p);
775	+	}
776	+	}
777	+
778	+	/* -------------- Removal methods -------------- */
779	+
780	+	/**
781	+	* Unsplices (now or later) the given deleted/cancelled node with
782	+	* the given predecessor.
783	+	*
784	+	* @param pred predecessor of node to be unspliced
785	+	* @param s the node to be unspliced
786	+	*/
787	+	private void unsplice(Node pred, Node s) {
788	+	s.forgetContents(); // clear unneeded fields
789		/*
790		* At any given time, exactly one node on list cannot be
791	<	* deleted -- the last inserted node. To accommodate this, if
792	<	* we cannot delete s, we save its predecessor as "cleanMe",
793	<	* processing the previously saved version first. At least one
794	<	* of node s or the node previously saved can always be
791	>	* unlinked -- the last inserted node. To accommodate this, if
792	>	* we cannot unlink s, we save its predecessor as "cleanMe",
793	>	* processing the previously saved version first. Because only
794	>	* one node in the list can have a null next, at least one of
795	>	* node s or the node previously saved can always be
796		* processed, so this always terminates.
797		*/
798	<	while (pred.next == s) {
799	<	QNode oldpred = reclean();  // First, help get rid of cleanMe
800	<	QNode t = getValidatedTail();
801	<	if (s != t) {               // If not tail, try to unsplice
802	<	QNode sn = s.next;      // s.next == s means s already off list
803	<	if (sn == s || pred.casNext(s, sn))
798	>	if (pred != null && pred != s) {
799	>	while (pred.next == s) {
800	>	Node oldpred = (cleanMe == null) ? null : reclean();
801	>	Node n = s.next;
802	>	if (n != null) {
803	>	if (n != s)
804	>	pred.casNext(s, n);
805		break;
806	+	}
807	+	if (oldpred == pred ||      // Already saved
808	+	(oldpred == null && casCleanMe(null, pred)))
809	+	break;                  // Postpone cleaning
810		}
363	–	else if (oldpred == pred || // Already saved
364	–	(oldpred == null && cleanMe.compareAndSet(null, pred)))
365	–	break;                  // Postpone cleaning
811		}
812		}
813
814		/**
815	<	* Tries to unsplice the cancelled node held in cleanMe that was
816	<	* previously uncleanable because it was at tail.
815	>	* Tries to unsplice the deleted/cancelled node held in cleanMe
816	>	* that was previously uncleanable because it was at tail.
817	>	*
818		* @return current cleanMe node (or null)
819		*/
820	<	private QNode reclean() {
820	>	private Node reclean() {
821		/*
822	<	* cleanMe is, or at one time was, predecessor of cancelled
823	<	* node s that was the tail so could not be unspliced.  If s
822	>	* cleanMe is, or at one time was, predecessor of a cancelled
823	>	* node s that was the tail so could not be unspliced.  If it
824		* is no longer the tail, try to unsplice if necessary and
825		* make cleanMe slot available.  This differs from similar
826	<	* code in clean() because we must check that pred still
827	<	* points to a cancelled node that must be unspliced -- if
828	<	* not, we can (must) clear cleanMe without unsplicing.
829	<	* This can loop only due to contention on casNext or
384	<	* clearing cleanMe.
826	>	* code in unsplice() because we must check that pred still
827	>	* points to a matched node that can be unspliced -- if not,
828	>	* we can (must) clear cleanMe without unsplicing.  This can
829	>	* loop only due to contention.
830		*/
831	<	QNode pred;
832	<	while ((pred = cleanMe.get()) != null) {
833	<	QNode t = getValidatedTail();
834	<	QNode s = pred.next;
835	<	if (s != t) {
836	<	QNode sn;
837	<	if (s == null || s == pred || s.get() != s ||
838	<	(sn = s.next) == s || pred.casNext(s, sn))
839	<	cleanMe.compareAndSet(pred, null);
831	>	Node pred;
832	>	while ((pred = cleanMe) != null) {
833	>	Node s = pred.next;
834	>	Node n;
835	>	if (s == null || s == pred || !s.isMatched())
836	>	casCleanMe(pred, null); // already gone
837	>	else if ((n = s.next) != null) {
838	>	if (n != s)
839	>	pred.casNext(s, n);
840	>	casCleanMe(pred, null);
841		}
842	<	else // s is still tail; cannot clean
842	>	else
843		break;
844		}
845		return pred;
846		}
847
848		/**
849	+	* Main implementation of Iterator.remove(). Find
850	+	* and unsplice the given node.
851	+	*/
852	+	final void findAndRemoveNode(Node s) {
853	+	if (s.tryMatchData()) {
854	+	Node pred = null;
855	+	Node p = head;
856	+	while (p != null) {
857	+	if (p == s) {
858	+	unsplice(pred, p);
859	+	break;
860	+	}
861	+	if (!p.isData && !p.isMatched())
862	+	break;
863	+	pred = p;
864	+	if ((p = p.next) == pred) { // stale
865	+	pred = null;
866	+	p = head;
867	+	}
868	+	}
869	+	}
870	+	}
871	+
872	+	/**
873	+	* Main implementation of remove(Object)
874	+	*/
875	+	private boolean findAndRemove(Object e) {
876	+	if (e != null) {
877	+	Node pred = null;
878	+	Node p = head;
879	+	while (p != null) {
880	+	Object item = p.item;
881	+	if (p.isData) {
882	+	if (item != null && item != p && e.equals(item) &&
883	+	p.tryMatchData()) {
884	+	unsplice(pred, p);
885	+	return true;
886	+	}
887	+	}
888	+	else if (item == null)
889	+	break;
890	+	pred = p;
891	+	if ((p = p.next) == pred) {
892	+	pred = null;
893	+	p = head;
894	+	}
895	+	}
896	+	}
897	+	return false;
898	+	}
899	+
900	+
901	+	/**
902		* Creates an initially empty {@code LinkedTransferQueue}.
903		*/
904		public LinkedTransferQueue() {
406	–	QNode dummy = new QNode(null, false);
407	–	head = new PaddedAtomicReference<QNode>(dummy);
408	–	tail = new PaddedAtomicReference<QNode>(dummy);
409	–	cleanMe = new PaddedAtomicReference<QNode>(null);
905		}
906
907		/**
908		* Creates a {@code LinkedTransferQueue}
909		* initially containing the elements of the given collection,
910		* added in traversal order of the collection's iterator.
911	+	*
912		* @param c the collection of elements to initially contain
913		* @throws NullPointerException if the specified collection or any
914		*         of its elements are null
#	Line 422 | Line 918 | public class LinkedTransferQueue<E> exte
918		addAll(c);
919		}
920
921	<	public void put(E e) throws InterruptedException {
922	<	if (e == null) throw new NullPointerException();
923	<	if (Thread.interrupted()) throw new InterruptedException();
924	<	xfer(e, NOWAIT, 0);
921	>	/**
922	>	* Inserts the specified element at the tail of this queue.
923	>	* As the queue is unbounded, this method will never block.
924	>	*
925	>	* @throws NullPointerException if the specified element is null
926	>	*/
927	>	public void put(E e) {
928	>	xfer(e, true, ASYNC, 0);
929		}
930
931	<	public boolean offer(E e, long timeout, TimeUnit unit)
932	<	throws InterruptedException {
933	<	if (e == null) throw new NullPointerException();
934	<	if (Thread.interrupted()) throw new InterruptedException();
935	<	xfer(e, NOWAIT, 0);
931	>	/**
932	>	* Inserts the specified element at the tail of this queue.
933	>	* As the queue is unbounded, this method will never block or
934	>	* return {@code false}.
935	>	*
936	>	* @return {@code true} (as specified by
937	>	*  {@link BlockingQueue#offer(Object,long,TimeUnit) BlockingQueue.offer})
938	>	* @throws NullPointerException if the specified element is null
939	>	*/
940	>	public boolean offer(E e, long timeout, TimeUnit unit) {
941	>	xfer(e, true, ASYNC, 0);
942		return true;
943		}
944
945	+	/**
946	+	* Inserts the specified element at the tail of this queue.
947	+	* As the queue is unbounded, this method will never return {@code false}.
948	+	*
949	+	* @return {@code true} (as specified by
950	+	*         {@link BlockingQueue#offer(Object) BlockingQueue.offer})
951	+	* @throws NullPointerException if the specified element is null
952	+	*/
953		public boolean offer(E e) {
954	<	if (e == null) throw new NullPointerException();
955	<	xfer(e, NOWAIT, 0);
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 throw
961	>	* {@link IllegalStateException} or return {@code false}.
962	>	*
963	>	* @return {@code true} (as specified by {@link Collection#add})
964	>	* @throws NullPointerException if the specified element is null
965	>	*/
966	>	public boolean add(E e) {
967	>	xfer(e, true, ASYNC, 0);
968		return true;
969		}
970
971	+	/**
972	+	* Transfers the element to a waiting consumer immediately, if possible.
973	+	*
974	+	* <p>More precisely, transfers the specified element immediately
975	+	* if there exists a consumer already waiting to receive it (in
976	+	* {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
977	+	* otherwise returning {@code false} without enqueuing the element.
978	+	*
979	+	* @throws NullPointerException if the specified element is null
980	+	*/
981	+	public boolean tryTransfer(E e) {
982	+	return xfer(e, true, NOW, 0) == null;
983	+	}
984	+
985	+	/**
986	+	* Transfers the element to a consumer, waiting if necessary to do so.
987	+	*
988	+	* <p>More precisely, transfers the specified element immediately
989	+	* if there exists a consumer already waiting to receive it (in
990	+	* {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
991	+	* else inserts the specified element at the tail of this queue
992	+	* and waits until the element is received by a consumer.
993	+	*
994	+	* @throws NullPointerException if the specified element is null
995	+	*/
996		public void transfer(E e) throws InterruptedException {
997	<	if (e == null) throw new NullPointerException();
998	<	if (xfer(e, WAIT, 0) == null) {
448	<	Thread.interrupted();
997	>	if (xfer(e, true, SYNC, 0) != null) {
998	>	Thread.interrupted(); // failure possible only due to interrupt
999		throw new InterruptedException();
1000		}
1001		}
1002
1003	+	/**
1004	+	* Transfers the element to a consumer if it is possible to do so
1005	+	* before the timeout elapses.
1006	+	*
1007	+	* <p>More precisely, transfers the specified element immediately
1008	+	* if there exists a consumer already waiting to receive it (in
1009	+	* {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
1010	+	* else inserts the specified element at the tail of this queue
1011	+	* and waits until the element is received by a consumer,
1012	+	* returning {@code false} if the specified wait time elapses
1013	+	* before the element can be transferred.
1014	+	*
1015	+	* @throws NullPointerException if the specified element is null
1016	+	*/
1017		public boolean tryTransfer(E e, long timeout, TimeUnit unit)
1018		throws InterruptedException {
1019	<	if (e == null) throw new NullPointerException();
456	<	if (xfer(e, TIMEOUT, unit.toNanos(timeout)) != null)
1019	>	if (xfer(e, true, TIMEOUT, unit.toNanos(timeout)) == null)
1020		return true;
1021		if (!Thread.interrupted())
1022		return false;
1023		throw new InterruptedException();
1024		}
1025
463	–	public boolean tryTransfer(E e) {
464	–	if (e == null) throw new NullPointerException();
465	–	return fulfill(e) != null;
466	–	}
467	–
1026		public E take() throws InterruptedException {
1027	<	Object e = xfer(null, WAIT, 0);
1027	>	Object e = xfer(null, false, SYNC, 0);
1028		if (e != null)
1029		return (E)e;
1030		Thread.interrupted();
#	Line 474 | Line 1032 | public class LinkedTransferQueue<E> exte
1032		}
1033
1034		public E poll(long timeout, TimeUnit unit) throws InterruptedException {
1035	<	Object e = xfer(null, TIMEOUT, unit.toNanos(timeout));
1035	>	Object e = xfer(null, false, TIMEOUT, unit.toNanos(timeout));
1036		if (e != null || !Thread.interrupted())
1037		return (E)e;
1038		throw new InterruptedException();
1039		}
1040
1041		public E poll() {
1042	<	return (E)fulfill(null);
1042	>	return (E)xfer(null, false, NOW, 0);
1043		}
1044
1045	+	/**
1046	+	* @throws NullPointerException     {@inheritDoc}
1047	+	* @throws IllegalArgumentException {@inheritDoc}
1048	+	*/
1049		public int drainTo(Collection<? super E> c) {
1050		if (c == null)
1051		throw new NullPointerException();
#	Line 498 | Line 1060 | public class LinkedTransferQueue<E> exte
1060		return n;
1061		}
1062
1063	+	/**
1064	+	* @throws NullPointerException     {@inheritDoc}
1065	+	* @throws IllegalArgumentException {@inheritDoc}
1066	+	*/
1067		public int drainTo(Collection<? super E> c, int maxElements) {
1068		if (c == null)
1069		throw new NullPointerException();
#	Line 512 | Line 1078 | public class LinkedTransferQueue<E> exte
1078		return n;
1079		}
1080
515	–	// Traversal-based methods
516	–
1081		/**
1082	<	* Return head after performing any outstanding helping steps
1082	>	* Returns an iterator over the elements in this queue in proper
1083	>	* sequence, from head to tail.
1084	>	*
1085	>	* <p>The returned iterator is a "weakly consistent" iterator that
1086	>	* will never throw
1087	>	* {@link ConcurrentModificationException ConcurrentModificationException},
1088	>	* and guarantees to traverse elements as they existed upon
1089	>	* construction of the iterator, and may (but is not guaranteed
1090	>	* to) reflect any modifications subsequent to construction.
1091	>	*
1092	>	* @return an iterator over the elements in this queue in proper sequence
1093		*/
520	–	private QNode traversalHead() {
521	–	for (;;) {
522	–	QNode t = tail.get();
523	–	QNode h = head.get();
524	–	if (h != null && t != null) {
525	–	QNode last = t.next;
526	–	QNode first = h.next;
527	–	if (t == tail.get()) {
528	–	if (last != null)
529	–	tail.compareAndSet(t, last);
530	–	else if (first != null) {
531	–	Object x = first.get();
532	–	if (x == first)
533	–	advanceHead(h, first);
534	–	else
535	–	return h;
536	–	}
537	–	else
538	–	return h;
539	–	}
540	–	}
541	–	}
542	–	}
543	–
544	–
1094		public Iterator<E> iterator() {
1095		return new Itr();
1096		}
1097
549	–	/**
550	–	* Iterators. Basic strategy is to traverse list, treating
551	–	* non-data (i.e., request) nodes as terminating list.
552	–	* Once a valid data node is found, the item is cached
553	–	* so that the next call to next() will return it even
554	–	* if subsequently removed.
555	–	*/
556	–	class Itr implements Iterator<E> {
557	–	QNode nextNode;    // Next node to return next
558	–	QNode currentNode; // last returned node, for remove()
559	–	QNode prevNode;    // predecessor of last returned node
560	–	E nextItem;        // Cache of next item, once commited to in next
561	–
562	–	Itr() {
563	–	nextNode = traversalHead();
564	–	advance();
565	–	}
566	–
567	–	E advance() {
568	–	prevNode = currentNode;
569	–	currentNode = nextNode;
570	–	E x = nextItem;
571	–
572	–	QNode p = nextNode.next;
573	–	for (;;) {
574	–	if (p == null || !p.isData) {
575	–	nextNode = null;
576	–	nextItem = null;
577	–	return x;
578	–	}
579	–	Object item = p.get();
580	–	if (item != p && item != null) {
581	–	nextNode = p;
582	–	nextItem = (E)item;
583	–	return x;
584	–	}
585	–	prevNode = p;
586	–	p = p.next;
587	–	}
588	–	}
589	–
590	–	public boolean hasNext() {
591	–	return nextNode != null;
592	–	}
593	–
594	–	public E next() {
595	–	if (nextNode == null) throw new NoSuchElementException();
596	–	return advance();
597	–	}
598	–
599	–	public void remove() {
600	–	QNode p = currentNode;
601	–	QNode prev = prevNode;
602	–	if (prev == null || p == null)
603	–	throw new IllegalStateException();
604	–	Object x = p.get();
605	–	if (x != null && x != p && p.compareAndSet(x, p))
606	–	clean(prev, p);
607	–	}
608	–	}
609	–
1098		public E peek() {
1099	<	for (;;) {
612	<	QNode h = traversalHead();
613	<	QNode p = h.next;
614	<	if (p == null)
615	<	return null;
616	<	Object x = p.get();
617	<	if (p != x) {
618	<	if (!p.isData)
619	<	return null;
620	<	if (x != null)
621	<	return (E)x;
622	<	}
623	<	}
1099	>	return (E) firstDataItem();
1100		}
1101
1102	+	/**
1103	+	* Returns {@code true} if this queue contains no elements.
1104	+	*
1105	+	* @return {@code true} if this queue contains no elements
1106	+	*/
1107		public boolean isEmpty() {
1108	<	for (;;) {
628	<	QNode h = traversalHead();
629	<	QNode p = h.next;
630	<	if (p == null)
631	<	return true;
632	<	Object x = p.get();
633	<	if (p != x) {
634	<	if (!p.isData)
635	<	return true;
636	<	if (x != null)
637	<	return false;
638	<	}
639	<	}
1108	>	return firstOfMode(true) == null;
1109		}
1110
1111		public boolean hasWaitingConsumer() {
1112	<	for (;;) {
644	<	QNode h = traversalHead();
645	<	QNode p = h.next;
646	<	if (p == null)
647	<	return false;
648	<	Object x = p.get();
649	<	if (p != x)
650	<	return !p.isData;
651	<	}
1112	>	return firstOfMode(false) != null;
1113		}
1114
1115		/**
#	Line 664 | Line 1125 | public class LinkedTransferQueue<E> exte
1125		* @return the number of elements in this queue
1126		*/
1127		public int size() {
1128	<	int count = 0;
668	<	QNode h = traversalHead();
669	<	for (QNode p = h.next; p != null && p.isData; p = p.next) {
670	<	Object x = p.get();
671	<	if (x != null && x != p) {
672	<	if (++count == Integer.MAX_VALUE) // saturated
673	<	break;
674	<	}
675	<	}
676	<	return count;
1128	>	return countOfMode(true);
1129		}
1130
1131		public int getWaitingConsumerCount() {
1132	<	int count = 0;
1133	<	QNode h = traversalHead();
1134	<	for (QNode p = h.next; p != null && !p.isData; p = p.next) {
1135	<	if (p.get() == null) {
1136	<	if (++count == Integer.MAX_VALUE)
1137	<	break;
1138	<	}
1139	<	}
1140	<	return count;
1132	>	return countOfMode(false);
1133	>	}
1134	>
1135	>	/**
1136	>	* Removes a single instance of the specified element from this queue,
1137	>	* if it is present.  More formally, removes an element {@code e} such
1138	>	* that {@code o.equals(e)}, if this queue contains one or more such
1139	>	* elements.
1140	>	* Returns {@code true} if this queue contained the specified element
1141	>	* (or equivalently, if this queue changed as a result of the call).
1142	>	*
1143	>	* @param o element to be removed from this queue, if present
1144	>	* @return {@code true} if this queue changed as a result of the call
1145	>	*/
1146	>	public boolean remove(Object o) {
1147	>	return findAndRemove(o);
1148		}
1149
1150	+	/**
1151	+	* Always returns {@code Integer.MAX_VALUE} because a
1152	+	* {@code LinkedTransferQueue} is not capacity constrained.
1153	+	*
1154	+	* @return {@code Integer.MAX_VALUE} (as specified by
1155	+	*         {@link BlockingQueue#remainingCapacity()})
1156	+	*/
1157		public int remainingCapacity() {
1158		return Integer.MAX_VALUE;
1159		}
1160
1161		/**
1162	<	* Save the state to a stream (that is, serialize it).
1162	>	* Saves the state to a stream (that is, serializes it).
1163		*
1164		* @serialData All of the elements (each an {@code E}) in
1165		* the proper order, followed by a null
#	Line 702 | Line 1168 | public class LinkedTransferQueue<E> exte
1168		private void writeObject(java.io.ObjectOutputStream s)
1169		throws java.io.IOException {
1170		s.defaultWriteObject();
1171	<	for (Iterator<E> it = iterator(); it.hasNext(); )
1172	<	s.writeObject(it.next());
1171	>	for (E e : this)
1172	>	s.writeObject(e);
1173		// Use trailing null as sentinel
1174		s.writeObject(null);
1175		}
1176
1177		/**
1178	<	* Reconstitute the Queue instance from a stream (that is,
1179	<	* deserialize it).
1178	>	* Reconstitutes the Queue instance from a stream (that is,
1179	>	* deserializes it).
1180	>	*
1181		* @param s the stream
1182		*/
1183		private void readObject(java.io.ObjectInputStream s)
1184		throws java.io.IOException, ClassNotFoundException {
1185		s.defaultReadObject();
719	–	resetHeadAndTail();
1186		for (;;) {
1187	<	E item = (E)s.readObject();
1187	>	@SuppressWarnings("unchecked") E item = (E) s.readObject();
1188		if (item == null)
1189		break;
1190		else
#	Line 727 | Line 1193 | public class LinkedTransferQueue<E> exte
1193		}
1194
1195
1196	<	// Support for resetting head/tail while deserializing
1197	<	private void resetHeadAndTail() {
1198	<	QNode dummy = new QNode(null, false);
1199	<	_unsafe.putObjectVolatile(this, headOffset,
1200	<	new PaddedAtomicReference<QNode>(dummy));
1201	<	_unsafe.putObjectVolatile(this, tailOffset,
1202	<	new PaddedAtomicReference<QNode>(dummy));
1203	<	_unsafe.putObjectVolatile(this, cleanMeOffset,
1204	<	new PaddedAtomicReference<QNode>(null));
1196	>	// Unsafe mechanics
1197	>
1198	>	private static final sun.misc.Unsafe UNSAFE = getUnsafe();
1199	>	private static final long headOffset =
1200	>	objectFieldOffset(UNSAFE, "head", LinkedTransferQueue.class);
1201	>	private static final long tailOffset =
1202	>	objectFieldOffset(UNSAFE, "tail", LinkedTransferQueue.class);
1203	>	private static final long cleanMeOffset =
1204	>	objectFieldOffset(UNSAFE, "cleanMe", LinkedTransferQueue.class);
1205	>
1206	>	static long objectFieldOffset(sun.misc.Unsafe UNSAFE,
1207	>	String field, Class<?> klazz) {
1208	>	try {
1209	>	return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
1210	>	} catch (NoSuchFieldException e) {
1211	>	// Convert Exception to corresponding Error
1212	>	NoSuchFieldError error = new NoSuchFieldError(field);
1213	>	error.initCause(e);
1214	>	throw error;
1215	>	}
1216		}
1217
1218	<	// Temporary Unsafe mechanics for preliminary release
742	<	private static Unsafe getUnsafe() throws Throwable {
1218	>	private static sun.misc.Unsafe getUnsafe() {
1219		try {
1220	<	return Unsafe.getUnsafe();
1220	>	return sun.misc.Unsafe.getUnsafe();
1221		} catch (SecurityException se) {
1222		try {
1223		return java.security.AccessController.doPrivileged
1224	<	(new java.security.PrivilegedExceptionAction<Unsafe>() {
1225	<	public Unsafe run() throws Exception {
1226	<	return getUnsafePrivileged();
1224	>	(new java.security
1225	>	.PrivilegedExceptionAction<sun.misc.Unsafe>() {
1226	>	public sun.misc.Unsafe run() throws Exception {
1227	>	java.lang.reflect.Field f = sun.misc
1228	>	.Unsafe.class.getDeclaredField("theUnsafe");
1229	>	f.setAccessible(true);
1230	>	return (sun.misc.Unsafe) f.get(null);
1231		}});
1232		} catch (java.security.PrivilegedActionException e) {
1233	<	throw e.getCause();
1233	>	throw new RuntimeException("Could not initialize intrinsics",
1234	>	e.getCause());
1235		}
1236		}
1237		}
1238
758	–	private static Unsafe getUnsafePrivileged()
759	–	throws NoSuchFieldException, IllegalAccessException {
760	–	Field f = Unsafe.class.getDeclaredField("theUnsafe");
761	–	f.setAccessible(true);
762	–	return (Unsafe) f.get(null);
763	–	}
764	–
765	–	private static long fieldOffset(String fieldName)
766	–	throws NoSuchFieldException {
767	–	return _unsafe.objectFieldOffset
768	–	(LinkedTransferQueue.class.getDeclaredField(fieldName));
769	–	}
770	–
771	–	private static final Unsafe _unsafe;
772	–	private static final long headOffset;
773	–	private static final long tailOffset;
774	–	private static final long cleanMeOffset;
775	–	static {
776	–	try {
777	–	_unsafe = getUnsafe();
778	–	headOffset = fieldOffset("head");
779	–	tailOffset = fieldOffset("tail");
780	–	cleanMeOffset = fieldOffset("cleanMe");
781	–	} catch (Throwable e) {
782	–	throw new RuntimeException("Could not initialize intrinsics", e);
783	–	}
784	–	}
785	–
1239		}

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