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

Comparing jsr166/src/jsr166y/LinkedTransferQueue.java (file contents):
Revision 1.41 by jsr166, Sat Aug 1 20:44:05 2009 UTC vs.
Revision 1.50 by dl, Sat Oct 24 12:29:57 2009 UTC

#	Line 15 | Line 15 | import java.util.Iterator;
15		import java.util.NoSuchElementException;
16		import java.util.Queue;
17		import java.util.concurrent.locks.LockSupport;
18	–	import java.util.concurrent.atomic.AtomicReference;
19	–
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 54 | 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
58	<	* SynchronousQueues. See the internal documentation, as well as
59	<	* the PPoPP 2006 paper "Scalable Synchronous Queues" by Scherer,
60	<	* Lea & Scott
61	<	* (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 (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	>	private static final int FRONT_SPINS   = 1 << 7;
347	>
348	>	/**
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	>	private static final int CHAINED_SPINS = FRONT_SPINS >>> 1;
356	>
357	>	/**
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	<	/**
371	<	* The number of times to spin before blocking in untimed waits.
372	<	* This is greater than timed value because untimed waits spin
373	<	* faster since they don't need to check times on each spin.
93	<	*/
94	<	static final int maxUntimedSpins = maxTimedSpins * 16;
95	<
96	<	/**
97	<	* The number of nanoseconds for which it is faster to spin
98	<	* rather than to use timed park. A rough estimate suffices.
99	<	*/
100	<	static final long spinForTimeoutThreshold = 1000L;
370	>	// CAS methods for fields
371	>	final boolean casNext(Node cmp, Node val) {
372	>	return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
373	>	}
374
375	<	/**
376	<	* Node class for LinkedTransferQueue. Opportunistically
377	<	* subclasses from AtomicReference to represent item. Uses Object,
105	<	* not E, to allow setting item to "this" after use, to avoid
106	<	* garbage retention. Similarly, setting the next field to this is
107	<	* used as sentinel that node is off list.
108	<	*/
109	<	static final class Node<E> extends AtomicReference<Object> {
110	<	volatile Node<E> next;
111	<	volatile Thread waiter;       // to control park/unpark
112	<	final boolean isData;
375	>	final boolean casItem(Object cmp, Object val) {
376	>	return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val);
377	>	}
378
379	<	Node(E item, boolean isData) {
380	<	super(item);
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	<	// Unsafe mechanics
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	<	private static final sun.misc.Unsafe UNSAFE = getUnsafe();
397	<	private static final long nextOffset =
398	<	objectFieldOffset(UNSAFE, "next", Node.class);
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	>	}
405
406	<	final boolean casNext(Node<E> cmp, Node<E> val) {
407	<	return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
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	<	final void clearNext() {
416	<	UNSAFE.putOrderedObject(this, nextOffset, this);
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	<	* Returns a sun.misc.Unsafe.  Suitable for use in a 3rd party package.
135	<	* Replace with a simple call to Unsafe.getUnsafe when integrating
136	<	* into a jdk.
137	<	*
138	<	* @return a sun.misc.Unsafe
427	>	* Tries to artificially match a data node -- used by remove.
428		*/
429	<	private static sun.misc.Unsafe getUnsafe() {
430	<	try {
431	<	return sun.misc.Unsafe.getUnsafe();
432	<	} catch (SecurityException se) {
433	<	try {
145	<	return java.security.AccessController.doPrivileged
146	<	(new java.security
147	<	.PrivilegedExceptionAction<sun.misc.Unsafe>() {
148	<	public sun.misc.Unsafe run() throws Exception {
149	<	java.lang.reflect.Field f = sun.misc
150	<	.Unsafe.class.getDeclaredField("theUnsafe");
151	<	f.setAccessible(true);
152	<	return (sun.misc.Unsafe) f.get(null);
153	<	}});
154	<	} catch (java.security.PrivilegedActionException e) {
155	<	throw new RuntimeException("Could not initialize intrinsics",
156	<	e.getCause());
157	<	}
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	<	/**
451	<	* Padded version of AtomicReference used for head, tail and
166	<	* cleanMe, to alleviate contention across threads CASing one vs
167	<	* the other.
168	<	*/
169	<	static final class PaddedAtomicReference<T> extends AtomicReference<T> {
170	<	// enough padding for 64bytes with 4byte refs
171	<	Object p0, p1, p2, p3, p4, p5, p6, p7, p8, p9, pa, pb, pc, pd, pe;
172	<	PaddedAtomicReference(T r) { super(r); }
173	<	private static final long serialVersionUID = 8170090609809740854L;
174	<	}
450	>	/** head of the queue; null until first enqueue */
451	>	private transient volatile Node head;
452
453	+	/** predecessor of dangling unspliceable node */
454	+	private transient volatile Node cleanMe; // decl here to reduce contention
455
456	<	/** head of the queue */
457	<	private transient final PaddedAtomicReference<Node<E>> head;
456	>	/** tail of the queue; null until first append */
457	>	private transient volatile Node tail;
458
459	<	/** tail of the queue */
460	<	private transient final PaddedAtomicReference<Node<E>> tail;
459	>	// CAS methods for fields
460	>	private boolean casTail(Node cmp, Node val) {
461	>	return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val);
462	>	}
463
464	<	/**
465	<	* Reference to a cancelled node that might not yet have been
466	<	* unlinked from queue because it was the last inserted node
186	<	* when it cancelled.
187	<	*/
188	<	private transient final PaddedAtomicReference<Node<E>> cleanMe;
464	>	private boolean casHead(Node cmp, Node val) {
465	>	return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val);
466	>	}
467
468	<	/**
469	<	* Tries to cas nh as new head; if successful, unlink
192	<	* old head's next node to avoid garbage retention.
193	<	*/
194	<	private boolean advanceHead(Node<E> h, Node<E> nh) {
195	<	if (h == head.get() && head.compareAndSet(h, nh)) {
196	<	h.clearNext(); // forget old next
197	<	return true;
198	<	}
199	<	return false;
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
204	<	* poll() and tryTransfer()). See the similar code in
205	<	* SynchronousQueue for detailed explanation.
482	>	* Implements all queuing methods. See above for explanation.
483		*
484	<	* @param e the item or if null, signifies that this is a take
485	<	* @param mode the wait mode: NOWAIT, TIMEOUT, WAIT
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 E xfer(E e, int mode, long nanos) {
492	<	boolean isData = (e != null);
493	<	Node<E> s = null;
494	<	final PaddedAtomicReference<Node<E>> head = this.head;
216	<	final PaddedAtomicReference<Node<E>> tail = this.tail;
217	<
218	<	for (;;) {
219	<	Node<E> t = tail.get();
220	<	Node<E> h = head.get();
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	<	if (t != null && (t == h || t.isData == isData)) {
223	<	if (s == null)
224	<	s = new Node<E>(e, isData);
225	<	Node<E> last = t.next;
226	<	if (last != null) {
227	<	if (t == tail.get())
228	<	tail.compareAndSet(t, last);
229	<	}
230	<	else if (t.casNext(null, s)) {
231	<	tail.compareAndSet(t, s);
232	<	return awaitFulfill(t, s, e, mode, nanos);
233	<	}
234	<	}
496	>	retry: for (;;) {                     // restart on append race
497
498	<	else if (h != null) {
499	<	Node<E> first = h.next;
500	<	if (t == tail.get() && first != null &&
501	<	advanceHead(h, first)) {
502	<	Object x = first.get();
503	<	if (x != first && first.compareAndSet(x, e)) {
504	<	LockSupport.unpark(first.waiter);
505	<	return isData ? e : (E) x;
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	>	Thread w = 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	>	LockSupport.unpark(w);
519	>	return item;
520		}
521		}
522	+	Node n = p.next;
523	+	p = (p != n) ? n : (h = head); // Use head if p offlist
524		}
525	+
526	+	if (how >= ASYNC) {               // No matches available
527	+	if (s == null)
528	+	s = new Node(e, haveData);
529	+	Node pred = tryAppend(s, haveData);
530	+	if (pred == null)
531	+	continue retry;           // lost race vs opposite mode
532	+	if (how >= SYNC)
533	+	return awaitMatch(s, pred, e, how, nanos);
534	+	}
535	+	return e; // not waiting
536		}
537		}
538
250	–
539		/**
540	<	* Version of xfer for poll() and tryTransfer, which
541	<	* simplifies control paths both here and in xfer.
542	<	*/
543	<	private E fulfill(E e) {
544	<	boolean isData = (e != null);
545	<	final PaddedAtomicReference<Node<E>> head = this.head;
546	<	final PaddedAtomicReference<Node<E>> tail = this.tail;
547	<
548	<	for (;;) {
549	<	Node<E> t = tail.get();
550	<	Node<E> h = head.get();
551	<
552	<	if (t != null && (t == h || t.isData == isData)) {
553	<	Node<E> last = t.next;
554	<	if (t == tail.get()) {
555	<	if (last != null)
556	<	tail.compareAndSet(t, last);
557	<	else
558	<	return null;
559	<	}
560	<	}
561	<	else if (h != null) {
562	<	Node<E> first = h.next;
563	<	if (t == tail.get() &&
564	<	first != null &&
565	<	advanceHead(h, first)) {
566	<	Object x = first.get();
567	<	if (x != first && first.compareAndSet(x, e)) {
280	<	LockSupport.unpark(first.waiter);
281	<	return isData ? e : (E) x;
282	<	}
540	>	* Tries to append node s as tail.
541	>	*
542	>	* @param s the node to append
543	>	* @param haveData true if appending in data mode
544	>	* @return null on failure due to losing race with append in
545	>	* different mode, else s's predecessor, or s itself if no
546	>	* predecessor
547	>	*/
548	>	private Node tryAppend(Node s, boolean haveData) {
549	>	for (Node t = tail, p = t;;) { // move p to last node and append
550	>	Node n, u;                        // temps for reads of next & tail
551	>	if (p == null && (p = head) == null) {
552	>	if (casHead(null, s))
553	>	return s;                 // initialize
554	>	}
555	>	else if (p.cannotPrecede(haveData))
556	>	return null;                  // lost race vs opposite mode
557	>	else if ((n = p.next) != null)    // not last; keep traversing
558	>	p = p != t && t != (u = tail) ? (t = u) : // stale tail
559	>	(p != n) ? n : null;      // restart if off list
560	>	else if (!p.casNext(null, s))
561	>	p = p.next;                   // re-read on CAS failure
562	>	else {
563	>	if (p != t) {                 // update if slack now >= 2
564	>	while ((tail != t || !casTail(t, s)) &&
565	>	(t = tail)   != null &&
566	>	(s = t.next) != null && // advance and retry
567	>	(s = s.next) != null && s != t);
568		}
569	+	return p;
570		}
571		}
572		}
573
574		/**
575	<	* Spins/blocks until node s is fulfilled or caller gives up,
290	<	* depending on wait mode.
575	>	* Spins/yields/blocks until node s is matched or caller gives up.
576		*
292	–	* @param pred the predecessor of waiting node
577		* @param s the waiting node
578	+	* @param pred the predecessor of s, or s itself if it has no
579	+	* predecessor, or null if unknown (the null case does not occur
580	+	* in any current calls but may in possible future extensions)
581		* @param e the comparison value for checking match
582	<	* @param mode mode
582	>	* @param how either SYNC or TIMEOUT
583		* @param nanos timeout value
584	<	* @return matched item, or null if cancelled
584	>	* @return matched item, or e if unmatched on interrupt or timeout
585		*/
586	<	private E awaitFulfill(Node<E> pred, Node<E> s, E e,
587	<	int mode, long nanos) {
588	<	if (mode == NOWAIT)
302	<	return null;
303	<
304	<	long lastTime = (mode == TIMEOUT) ? System.nanoTime() : 0;
586	>	private Object awaitMatch(Node s, Node pred, Object e,
587	>	int how, long nanos) {
588	>	long lastTime = (how == TIMEOUT) ? System.nanoTime() : 0L;
589		Thread w = Thread.currentThread();
590	<	int spins = -1; // set to desired spin count below
590	>	int spins = -1; // initialized after first item and cancel checks
591	>	ThreadLocalRandom randomYields = null; // bound if needed
592	>
593		for (;;) {
594	<	if (w.isInterrupted())
595	<	s.compareAndSet(e, s);
596	<	Object x = s.get();
597	<	if (x != e) {                 // Node was matched or cancelled
598	<	advanceHead(pred, s);     // unlink if head
599	<	if (x == s) {             // was cancelled
600	<	clean(pred, s);
601	<	return null;
602	<	}
603	<	else if (x != null) {
604	<	s.set(s);             // avoid garbage retention
605	<	return (E) x;
606	<	}
607	<	else
608	<	return e;
594	>	Object item = s.item;
595	>	if (item != e) {                  // matched
596	>	s.forgetContents();           // avoid garbage
597	>	return item;
598	>	}
599	>	if ((w.isInterrupted() || (how == TIMEOUT && nanos <= 0)) &&
600	>	s.casItem(e, s)) {       // cancel
601	>	unsplice(pred, s);
602	>	return e;
603	>	}
604	>
605	>	if (spins < 0) {                  // establish spins at/near front
606	>	if ((spins = spinsFor(pred, s.isData)) > 0)
607	>	randomYields = ThreadLocalRandom.current();
608	>	}
609	>	else if (spins > 0) {             // spin
610	>	if (--spins == 0)
611	>	shortenHeadPath();        // reduce slack before blocking
612	>	else if (randomYields.nextInt(CHAINED_SPINS) == 0)
613	>	Thread.yield();           // occasionally yield
614	>	}
615	>	else if (s.waiter == null) {
616	>	s.waiter = w;                 // request unpark
617		}
618	<	if (mode == TIMEOUT) {
618	>	else if (how == TIMEOUT) {
619		long now = System.nanoTime();
620	<	nanos -= now - lastTime;
620	>	if ((nanos -= now - lastTime) > 0)
621	>	LockSupport.parkNanos(this, nanos);
622		lastTime = now;
328	–	if (nanos <= 0) {
329	–	s.compareAndSet(e, s); // try to cancel
330	–	continue;
331	–	}
623		}
624	<	if (spins < 0) {
334	<	Node<E> h = head.get(); // only spin if at head
335	<	spins = ((h != null && h.next == s) ?
336	<	((mode == TIMEOUT) ?
337	<	maxTimedSpins : maxUntimedSpins) : 0);
338	<	}
339	<	if (spins > 0)
340	<	--spins;
341	<	else if (s.waiter == null)
342	<	s.waiter = w;
343	<	else if (mode != TIMEOUT) {
624	>	else {
625		LockSupport.park(this);
626	<	s.waiter = null;
346	<	spins = -1;
347	<	}
348	<	else if (nanos > spinForTimeoutThreshold) {
349	<	LockSupport.parkNanos(this, nanos);
350	<	s.waiter = null;
351	<	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 Node<E> getValidatedTail() {
636	<	for (;;) {
637	<	Node<E> h = head.get();
638	<	Node<E> first = h.next;
639	<	if (first != null && first.get() == first) { // help advance
640	<	advanceHead(h, first);
641	<	continue;
642	<	}
643	<	Node<E> t = tail.get();
644	<	Node<E> 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	<	* Gets rid of cancelled node s with original predecessor pred.
672	<	*
381	<	* @param pred predecessor of cancelled node
382	<	* @param s the cancelled node
671	>	* Returns the first unmatched node of the given mode, or null if
672	>	* none.  Used by methods isEmpty, hasWaitingConsumer.
673		*/
674	<	private void clean(Node<E> pred, Node<E> s) {
675	<	Thread w = s.waiter;
676	<	if (w != null) {             // Wake up thread
677	<	s.waiter = null;
678	<	if (w != Thread.currentThread())
679	<	LockSupport.unpark(w);
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	>	* 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	<	if (pred == null)
393	<	return;
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	<	Node<E> oldpred = reclean();  // First, help get rid of cleanMe
800	<	Node<E> t = getValidatedTail();
801	<	if (s != t) {               // If not tail, try to unsplice
802	<	Node<E> 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		}
411	–	else if (oldpred == pred || // Already saved
412	–	(oldpred == null && cleanMe.compareAndSet(null, pred)))
413	–	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 Node<E> 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
433	<	* 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	<	Node<E> pred;
832	<	while ((pred = cleanMe.get()) != null) {
833	<	Node<E> t = getValidatedTail();
834	<	Node<E> s = pred.next;
835	<	if (s != t) {
836	<	Node<E> 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() {
455	–	Node<E> dummy = new Node<E>(null, false);
456	–	head = new PaddedAtomicReference<Node<E>>(dummy);
457	–	tail = new PaddedAtomicReference<Node<E>>(dummy);
458	–	cleanMe = new PaddedAtomicReference<Node<E>>(null);
905		}
906
907		/**
#	Line 479 | Line 925 | public class LinkedTransferQueue<E> exte
925		* @throws NullPointerException if the specified element is null
926		*/
927		public void put(E e) {
928	<	offer(e);
928	>	xfer(e, true, ASYNC, 0);
929		}
930
931		/**
#	Line 492 | Line 938 | public class LinkedTransferQueue<E> exte
938		* @throws NullPointerException if the specified element is null
939		*/
940		public boolean offer(E e, long timeout, TimeUnit unit) {
941	<	return offer(e);
941	>	xfer(e, true, ASYNC, 0);
942	>	return true;
943		}
944
945		/**
#	Line 504 | Line 951 | public class LinkedTransferQueue<E> exte
951		* @throws NullPointerException if the specified element is null
952		*/
953		public boolean offer(E e) {
954	<	if (e == null) throw new NullPointerException();
508	<	xfer(e, NOWAIT, 0);
954	>	xfer(e, true, ASYNC, 0);
955		return true;
956		}
957
#	Line 518 | Line 964 | public class LinkedTransferQueue<E> exte
964		* @throws NullPointerException if the specified element is null
965		*/
966		public boolean add(E e) {
967	<	return offer(e);
967	>	xfer(e, true, ASYNC, 0);
968	>	return true;
969		}
970
971		/**
#	Line 532 | Line 979 | public class LinkedTransferQueue<E> exte
979		* @throws NullPointerException if the specified element is null
980		*/
981		public boolean tryTransfer(E e) {
982	<	if (e == null) throw new NullPointerException();
536	<	return fulfill(e) != null;
982	>	return xfer(e, true, NOW, 0) == null;
983		}
984
985		/**
#	Line 548 | Line 994 | public class LinkedTransferQueue<E> exte
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) {
553	<	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		}
#	Line 571 | Line 1016 | public class LinkedTransferQueue<E> exte
1016		*/
1017		public boolean tryTransfer(E e, long timeout, TimeUnit unit)
1018		throws InterruptedException {
1019	<	if (e == null) throw new NullPointerException();
575	<	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;
#	Line 580 | Line 1024 | public class LinkedTransferQueue<E> exte
1024		}
1025
1026		public E take() throws InterruptedException {
1027	<	E e = xfer(null, WAIT, 0);
1027	>	Object e = xfer(null, false, SYNC, 0);
1028		if (e != null)
1029	<	return e;
1029	>	return (E)e;
1030		Thread.interrupted();
1031		throw new InterruptedException();
1032		}
1033
1034		public E poll(long timeout, TimeUnit unit) throws InterruptedException {
1035	<	E 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;
1037	>	return (E)e;
1038		throw new InterruptedException();
1039		}
1040
1041		public E poll() {
1042	<	return fulfill(null);
1042	>	return (E)xfer(null, false, NOW, 0);
1043		}
1044
1045		/**
#	Line 634 | Line 1078 | public class LinkedTransferQueue<E> exte
1078		return n;
1079		}
1080
637	–	// Traversal-based methods
638	–
639	–	/**
640	–	* Returns head after performing any outstanding helping steps.
641	–	*/
642	–	private Node<E> traversalHead() {
643	–	for (;;) {
644	–	Node<E> t = tail.get();
645	–	Node<E> h = head.get();
646	–	if (h != null && t != null) {
647	–	Node<E> last = t.next;
648	–	Node<E> first = h.next;
649	–	if (t == tail.get()) {
650	–	if (last != null)
651	–	tail.compareAndSet(t, last);
652	–	else if (first != null) {
653	–	Object x = first.get();
654	–	if (x == first)
655	–	advanceHead(h, first);
656	–	else
657	–	return h;
658	–	}
659	–	else
660	–	return h;
661	–	}
662	–	}
663	–	reclean();
664	–	}
665	–	}
666	–
1081		/**
1082		* Returns an iterator over the elements in this queue in proper
1083		* sequence, from head to tail.
#	Line 681 | Line 1095 | public class LinkedTransferQueue<E> exte
1095		return new Itr();
1096		}
1097
684	–	/**
685	–	* Iterators. Basic strategy is to traverse list, treating
686	–	* non-data (i.e., request) nodes as terminating list.
687	–	* Once a valid data node is found, the item is cached
688	–	* so that the next call to next() will return it even
689	–	* if subsequently removed.
690	–	*/
691	–	class Itr implements Iterator<E> {
692	–	Node<E> next;        // node to return next
693	–	Node<E> pnext;       // predecessor of next
694	–	Node<E> curr;        // last returned node, for remove()
695	–	Node<E> pcurr;       // predecessor of curr, for remove()
696	–	E nextItem;          // Cache of next item, once committed to in next
697	–
698	–	Itr() {
699	–	advance();
700	–	}
701	–
702	–	/**
703	–	* Moves to next valid node and returns item to return for
704	–	* next(), or null if no such.
705	–	*/
706	–	private E advance() {
707	–	pcurr = pnext;
708	–	curr = next;
709	–	E item = nextItem;
710	–
711	–	for (;;) {
712	–	pnext = (next == null) ? traversalHead() : next;
713	–	next = pnext.next;
714	–	if (next == pnext) {
715	–	next = null;
716	–	continue;  // restart
717	–	}
718	–	if (next == null)
719	–	break;
720	–	Object x = next.get();
721	–	if (x != null && x != next) {
722	–	nextItem = (E) x;
723	–	break;
724	–	}
725	–	}
726	–	return item;
727	–	}
728	–
729	–	public boolean hasNext() {
730	–	return next != null;
731	–	}
732	–
733	–	public E next() {
734	–	if (next == null)
735	–	throw new NoSuchElementException();
736	–	return advance();
737	–	}
738	–
739	–	public void remove() {
740	–	Node<E> p = curr;
741	–	if (p == null)
742	–	throw new IllegalStateException();
743	–	Object x = p.get();
744	–	if (x != null && x != p && p.compareAndSet(x, p))
745	–	clean(pcurr, p);
746	–	}
747	–	}
748	–
1098		public E peek() {
1099	<	for (;;) {
751	<	Node<E> h = traversalHead();
752	<	Node<E> p = h.next;
753	<	if (p == null)
754	<	return null;
755	<	Object x = p.get();
756	<	if (p != x) {
757	<	if (!p.isData)
758	<	return null;
759	<	if (x != null)
760	<	return (E) x;
761	<	}
762	<	}
1099	>	return (E) firstDataItem();
1100		}
1101
1102		/**
#	Line 768 | Line 1105 | public class LinkedTransferQueue<E> exte
1105		* @return {@code true} if this queue contains no elements
1106		*/
1107		public boolean isEmpty() {
1108	<	for (;;) {
772	<	Node<E> h = traversalHead();
773	<	Node<E> p = h.next;
774	<	if (p == null)
775	<	return true;
776	<	Object x = p.get();
777	<	if (p != x) {
778	<	if (!p.isData)
779	<	return true;
780	<	if (x != null)
781	<	return false;
782	<	}
783	<	}
1108	>	return firstOfMode(true) == null;
1109		}
1110
1111		public boolean hasWaitingConsumer() {
1112	<	for (;;) {
788	<	Node<E> h = traversalHead();
789	<	Node<E> p = h.next;
790	<	if (p == null)
791	<	return false;
792	<	Object x = p.get();
793	<	if (p != x)
794	<	return !p.isData;
795	<	}
1112	>	return firstOfMode(false) != null;
1113		}
1114
1115		/**
#	Line 808 | Line 1125 | public class LinkedTransferQueue<E> exte
1125		* @return the number of elements in this queue
1126		*/
1127		public int size() {
1128	<	for (;;) {
812	<	int count = 0;
813	<	Node<E> pred = traversalHead();
814	<	for (;;) {
815	<	Node<E> q = pred.next;
816	<	if (q == pred) // restart
817	<	break;
818	<	if (q == null || !q.isData)
819	<	return count;
820	<	Object x = q.get();
821	<	if (x != null && x != q) {
822	<	if (++count == Integer.MAX_VALUE) // saturated
823	<	return count;
824	<	}
825	<	pred = q;
826	<	}
827	<	}
1128	>	return countOfMode(true);
1129		}
1130
1131		public int getWaitingConsumerCount() {
1132	<	// converse of size -- count valid non-data nodes
832	<	for (;;) {
833	<	int count = 0;
834	<	Node<E> pred = traversalHead();
835	<	for (;;) {
836	<	Node<E> q = pred.next;
837	<	if (q == pred) // restart
838	<	break;
839	<	if (q == null || q.isData)
840	<	return count;
841	<	Object x = q.get();
842	<	if (x == null) {
843	<	if (++count == Integer.MAX_VALUE) // saturated
844	<	return count;
845	<	}
846	<	pred = q;
847	<	}
848	<	}
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	<	if (o == null)
853	<	return false;
854	<	for (;;) {
855	<	Node<E> pred = traversalHead();
856	<	for (;;) {
857	<	Node<E> q = pred.next;
858	<	if (q == pred) // restart
859	<	break;
860	<	if (q == null || !q.isData)
861	<	return false;
862	<	Object x = q.get();
863	<	if (x != null && x != q && o.equals(x) &&
864	<	q.compareAndSet(x, q)) {
865	<	clean(pred, q);
866	<	return true;
867	<	}
868	<	pred = q;
869	<	}
870	<	}
1147	>	return findAndRemove(o);
1148		}
1149
1150		/**
#	Line 882 | Line 1159 | public class LinkedTransferQueue<E> exte
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 898 | Line 1175 | public class LinkedTransferQueue<E> exte
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();
909	–	resetHeadAndTail();
1186		for (;;) {
1187		@SuppressWarnings("unchecked") E item = (E) s.readObject();
1188		if (item == null)
#	Line 916 | Line 1192 | public class LinkedTransferQueue<E> exte
1192		}
1193		}
1194
919	–	// Support for resetting head/tail while deserializing
920	–	private void resetHeadAndTail() {
921	–	Node<E> dummy = new Node<E>(null, false);
922	–	UNSAFE.putObjectVolatile(this, headOffset,
923	–	new PaddedAtomicReference<Node<E>>(dummy));
924	–	UNSAFE.putObjectVolatile(this, tailOffset,
925	–	new PaddedAtomicReference<Node<E>>(dummy));
926	–	UNSAFE.putObjectVolatile(this, cleanMeOffset,
927	–	new PaddedAtomicReference<Node<E>>(null));
928	–	}
1195
1196		// Unsafe mechanics
1197
#	Line 937 | Line 1203 | public class LinkedTransferQueue<E> exte
1203		private static final long cleanMeOffset =
1204		objectFieldOffset(UNSAFE, "cleanMe", LinkedTransferQueue.class);
1205
940	–
1206		static long objectFieldOffset(sun.misc.Unsafe UNSAFE,
1207		String field, Class<?> klazz) {
1208		try {
#	Line 950 | Line 1215 | public class LinkedTransferQueue<E> exte
1215		}
1216		}
1217
953	–	/**
954	–	* Returns a sun.misc.Unsafe.  Suitable for use in a 3rd party package.
955	–	* Replace with a simple call to Unsafe.getUnsafe when integrating
956	–	* into a jdk.
957	–	*
958	–	* @return a sun.misc.Unsafe
959	–	*/
1218		private static sun.misc.Unsafe getUnsafe() {
1219		try {
1220		return sun.misc.Unsafe.getUnsafe();
#	Line 977 | Line 1235 | public class LinkedTransferQueue<E> exte
1235		}
1236		}
1237		}
1238	+
1239		}

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