105 |
|
* successful atomic operation per enq/deq pair. But it also |
106 |
|
* enables lower cost variants of queue maintenance mechanics. (A |
107 |
|
* variation of this idea applies even for non-dual queues that |
108 |
< |
* support deletion of embedded elements, such as |
108 |
> |
* support deletion of interior elements, such as |
109 |
|
* j.u.c.ConcurrentLinkedQueue.) |
110 |
|
* |
111 |
< |
* Once a node is matched, its item can never again change. We |
112 |
< |
* may thus arrange that the linked list of them contains a prefix |
113 |
< |
* of zero or more matched nodes, followed by a suffix of zero or |
114 |
< |
* more unmatched nodes. (Note that we allow both the prefix and |
115 |
< |
* suffix to be zero length, which in turn means that we do not |
116 |
< |
* use a dummy header.) If we were not concerned with either time |
117 |
< |
* or space efficiency, we could correctly perform enqueue and |
118 |
< |
* dequeue operations by traversing from a pointer to the initial |
119 |
< |
* node; CASing the item of the first unmatched node on match and |
120 |
< |
* CASing the next field of the trailing node on appends. While |
121 |
< |
* this would be a terrible idea in itself, it does have the |
122 |
< |
* benefit of not requiring ANY atomic updates on head/tail |
123 |
< |
* fields. |
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 |
< |
* that reflects the tradeoff of sometimes requiring extra traversal |
129 |
< |
* steps to locate the first and/or last unmatched nodes, versus |
130 |
< |
* the reduced overhead and contention of fewer updates to queue |
131 |
< |
* pointers. For example, a possible snapshot of a queue is: |
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 |
|
* | | |
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. |
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 |
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 releasing another thread) to be read-only, |
165 |
< |
* thus not introducing any further contention. As described |
166 |
< |
* below, we implement this by performing slack maintenance |
167 |
< |
* retries only after these points. |
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. During traversals, threads may sometimes |
172 |
< |
* shortcut the "next" link path from the current "head" node to |
173 |
< |
* be closer to the currently known first unmatched node. Again, |
174 |
< |
* this may be triggered with using thresholds or randomization. |
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" |
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 |
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 internal nodes (due to timed out or interrupted |
210 |
< |
* waits, or calls to remove or Iterator.remove) uses a scheme |
211 |
< |
* roughly similar to that in Scherer, Lea, and Scott |
212 |
< |
* SynchronousQueue. Given a predecessor, we can unsplice any node |
213 |
< |
* except the (actual) tail of the queue. To avoid build-up of |
214 |
< |
* cancelled trailing nodes, upon a request to remove a trailing |
215 |
< |
* node, it is placed in field "cleanMe" to be unspliced later. |
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 target |
230 |
< |
* slack of two. The slack value is hard-wired: a path greater |
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 max slack at one. Avoiding tracking |
236 |
< |
* explicit counts across situations slightly simplifies an |
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 value, path short-circuiting is rarely |
243 |
< |
* worthwhile. However, it is used (in awaitMatch) immediately |
244 |
< |
* before a waiting thread starts to block, as a final bit of |
245 |
< |
* helping at a point when contention with others is extremely |
246 |
< |
* unlikely (since if other threads that could release it are |
247 |
< |
* operating, then the current thread wouldn't be blocking). |
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 nine separate methods. |
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 |
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 retry loops until the slack is at most two. Traversals |
280 |
< |
* also check if the initial head is now off-list, in which |
281 |
< |
* case they start at the new head. |
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, try to append a new node |
292 |
< |
* to the list (or if head was null, establish the first |
293 |
< |
* node). Nodes can be appended only if their predecessors are |
294 |
< |
* either already matched or are of the same mode. If we detect |
295 |
< |
* otherwise, then a new node with opposite mode must have been |
296 |
< |
* appended during traversal, so must restart at phase 1. The |
297 |
< |
* traversal and update steps are otherwise similar to phase 1: |
298 |
< |
* Retrying upon CAS misses and checking for staleness. In |
299 |
< |
* particular, if a self-link is encountered, then we can |
300 |
< |
* safely jump to a node on the list by continuing the |
301 |
< |
* traversal at current head. |
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 |
< |
* current thread was interrupted or the wait timed out. On |
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 |
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 much smaller (1/4) spins for nodes |
324 |
< |
* that are not known to be front but whose predecessors have |
325 |
< |
* not blocked -- these "chained" spins avoid artifacts of |
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 |
< |
* spins, reflecting the longer code path lengths necessary to |
331 |
< |
* release them under contention. |
330 |
> |
* chained spins, reflecting longer paths typically required to |
331 |
> |
* unblock threads during phase changes. |
332 |
|
*/ |
333 |
|
|
334 |
|
/** True if on multiprocessor */ |
336 |
|
Runtime.getRuntime().availableProcessors() > 1; |
337 |
|
|
338 |
|
/** |
339 |
< |
* The number of times to spin (with on average one randomly |
340 |
< |
* interspersed call to Thread.yield) on multiprocessor before |
341 |
< |
* blocking when a node is apparently the first waiter in the |
342 |
< |
* queue. See above for explanation. Must be a power of two. The |
343 |
< |
* value is empirically derived -- it works pretty well across a |
344 |
< |
* variety of processors, numbers of CPUs, and OSes. |
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. |
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 >>> 2; |
355 |
> |
private static final int CHAINED_SPINS = FRONT_SPINS >>> 1; |
356 |
|
|
357 |
|
/** |
358 |
|
* Queue nodes. Uses Object, not E, for items to allow forgetting |
373 |
|
} |
374 |
|
|
375 |
|
final boolean casItem(Object cmp, Object val) { |
376 |
+ |
assert cmp == null || cmp.getClass() != Node.class; |
377 |
|
return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val); |
378 |
|
} |
379 |
|
|
410 |
|
*/ |
411 |
|
final boolean isMatched() { |
412 |
|
Object x = item; |
413 |
< |
return x == this || (x != null) != isData; |
413 |
> |
return (x == this) || ((x == null) == isData); |
414 |
> |
} |
415 |
> |
|
416 |
> |
/** |
417 |
> |
* Returns true if this is an unmatched request node. |
418 |
> |
*/ |
419 |
> |
final boolean isUnmatchedRequest() { |
420 |
> |
return !isData && item == null; |
421 |
|
} |
422 |
|
|
423 |
|
/** |
435 |
|
* Tries to artificially match a data node -- used by remove. |
436 |
|
*/ |
437 |
|
final boolean tryMatchData() { |
438 |
+ |
assert isData; |
439 |
|
Object x = item; |
440 |
|
if (x != null && x != this && casItem(x, null)) { |
441 |
|
LockSupport.unpark(waiter); |
457 |
|
} |
458 |
|
|
459 |
|
/** head of the queue; null until first enqueue */ |
460 |
< |
private transient volatile Node head; |
460 |
> |
transient volatile Node head; |
461 |
|
|
462 |
|
/** predecessor of dangling unspliceable node */ |
463 |
< |
private transient volatile Node cleanMe; // decl here to reduce contention |
463 |
> |
private transient volatile Node cleanMe; // decl here reduces contention |
464 |
|
|
465 |
|
/** tail of the queue; null until first append */ |
466 |
|
private transient volatile Node tail; |
479 |
|
} |
480 |
|
|
481 |
|
/* |
482 |
< |
* Possible values for "how" argument in xfer method. Beware that |
441 |
< |
* the order of assigned numerical values matters. |
482 |
> |
* Possible values for "how" argument in xfer method. |
483 |
|
*/ |
484 |
< |
private static final int NOW = 0; // for untimed poll, tryTransfer |
485 |
< |
private static final int ASYNC = 1; // for offer, put, add |
486 |
< |
private static final int SYNC = 2; // for transfer, take |
487 |
< |
private static final int TIMEOUT = 3; // for timed poll, tryTransfer |
484 |
> |
private static final int NOW = 0; // for untimed poll, tryTransfer |
485 |
> |
private static final int ASYNC = 1; // for offer, put, add |
486 |
> |
private static final int SYNC = 2; // for transfer, take |
487 |
> |
private static final int TIMED = 3; // for timed poll, tryTransfer |
488 |
> |
|
489 |
> |
@SuppressWarnings("unchecked") |
490 |
> |
static <E> E cast(Object item) { |
491 |
> |
assert item == null || item.getClass() != Node.class; |
492 |
> |
return (E) item; |
493 |
> |
} |
494 |
|
|
495 |
|
/** |
496 |
|
* Implements all queuing methods. See above for explanation. |
497 |
|
* |
498 |
|
* @param e the item or null for take |
499 |
|
* @param haveData true if this is a put, else a take |
500 |
< |
* @param how NOW, ASYNC, SYNC, or TIMEOUT |
501 |
< |
* @param nanos timeout in nanosecs, used only if mode is TIMEOUT |
500 |
> |
* @param how NOW, ASYNC, SYNC, or TIMED |
501 |
> |
* @param nanos timeout in nanosecs, used only if mode is TIMED |
502 |
|
* @return an item if matched, else e |
503 |
|
* @throws NullPointerException if haveData mode but e is null |
504 |
|
*/ |
505 |
< |
private Object xfer(Object e, boolean haveData, int how, long nanos) { |
505 |
> |
private E xfer(E e, boolean haveData, int how, long nanos) { |
506 |
|
if (haveData && (e == null)) |
507 |
|
throw new NullPointerException(); |
508 |
|
Node s = null; // the node to append, if needed |
516 |
|
if (isData == haveData) // can't match |
517 |
|
break; |
518 |
|
if (p.casItem(item, e)) { // match |
519 |
< |
Thread w = p.waiter; |
520 |
< |
while (p != h) { // update head |
521 |
< |
Node n = p.next; // by 2 unless singleton |
522 |
< |
if (n != null) |
523 |
< |
p = n; |
477 |
< |
if (head == h && casHead(h, p)) { |
519 |
> |
for (Node q = p; q != h;) { |
520 |
> |
Node n = q.next; // update head by 2 |
521 |
> |
if (n != null) // unless singleton |
522 |
> |
q = n; |
523 |
> |
if (head == h && casHead(h, q)) { |
524 |
|
h.forgetNext(); |
525 |
|
break; |
526 |
|
} // advance and retry |
527 |
|
if ((h = head) == null || |
528 |
< |
(p = h.next) == null || !p.isMatched()) |
528 |
> |
(q = h.next) == null || !q.isMatched()) |
529 |
|
break; // unless slack < 2 |
530 |
|
} |
531 |
< |
LockSupport.unpark(w); |
532 |
< |
return item; |
531 |
> |
LockSupport.unpark(p.waiter); |
532 |
> |
return this.<E>cast(item); |
533 |
|
} |
534 |
|
} |
535 |
|
Node n = p.next; |
536 |
|
p = (p != n) ? n : (h = head); // Use head if p offlist |
537 |
|
} |
538 |
|
|
539 |
< |
if (how >= ASYNC) { // No matches available |
539 |
> |
if (how != NOW) { // No matches available |
540 |
|
if (s == null) |
541 |
|
s = new Node(e, haveData); |
542 |
|
Node pred = tryAppend(s, haveData); |
543 |
|
if (pred == null) |
544 |
|
continue retry; // lost race vs opposite mode |
545 |
< |
if (how >= SYNC) |
546 |
< |
return awaitMatch(pred, s, e, how, nanos); |
545 |
> |
if (how != ASYNC) |
546 |
> |
return awaitMatch(s, pred, e, (how == TIMED), nanos); |
547 |
|
} |
548 |
|
return e; // not waiting |
549 |
|
} |
552 |
|
/** |
553 |
|
* Tries to append node s as tail. |
554 |
|
* |
509 |
– |
* @param haveData true if appending in data mode |
555 |
|
* @param s the node to append |
556 |
+ |
* @param haveData true if appending in data mode |
557 |
|
* @return null on failure due to losing race with append in |
558 |
|
* different mode, else s's predecessor, or s itself if no |
559 |
|
* predecessor |
560 |
|
*/ |
561 |
|
private Node tryAppend(Node s, boolean haveData) { |
562 |
< |
for (Node t = tail, p = t;;) { // move p to actual tail and append |
562 |
> |
for (Node t = tail, p = t;;) { // move p to last node and append |
563 |
|
Node n, u; // temps for reads of next & tail |
564 |
|
if (p == null && (p = head) == null) { |
565 |
|
if (casHead(null, s)) |
567 |
|
} |
568 |
|
else if (p.cannotPrecede(haveData)) |
569 |
|
return null; // lost race vs opposite mode |
570 |
< |
else if ((n = p.next) != null) // Not tail; keep traversing |
570 |
> |
else if ((n = p.next) != null) // not last; keep traversing |
571 |
|
p = p != t && t != (u = tail) ? (t = u) : // stale tail |
572 |
|
(p != n) ? n : null; // restart if off list |
573 |
|
else if (!p.casNext(null, s)) |
574 |
|
p = p.next; // re-read on CAS failure |
575 |
|
else { |
576 |
< |
if (p != t) { // Update if slack now >= 2 |
576 |
> |
if (p != t) { // update if slack now >= 2 |
577 |
|
while ((tail != t || !casTail(t, s)) && |
578 |
|
(t = tail) != null && |
579 |
|
(s = t.next) != null && // advance and retry |
587 |
|
/** |
588 |
|
* Spins/yields/blocks until node s is matched or caller gives up. |
589 |
|
* |
544 |
– |
* @param pred the predecessor of s or s or null if none |
590 |
|
* @param s the waiting node |
591 |
+ |
* @param pred the predecessor of s, or s itself if it has no |
592 |
+ |
* predecessor, or null if unknown (the null case does not occur |
593 |
+ |
* in any current calls but may in possible future extensions) |
594 |
|
* @param e the comparison value for checking match |
595 |
< |
* @param how either SYNC or TIMEOUT |
596 |
< |
* @param nanos timeout value |
595 |
> |
* @param timed if true, wait only until timeout elapses |
596 |
> |
* @param nanos timeout in nanosecs, used only if timed is true |
597 |
|
* @return matched item, or e if unmatched on interrupt or timeout |
598 |
|
*/ |
599 |
< |
private Object awaitMatch(Node pred, Node s, Object e, |
600 |
< |
int how, long nanos) { |
553 |
< |
long lastTime = (how == TIMEOUT) ? System.nanoTime() : 0L; |
599 |
> |
private E awaitMatch(Node s, Node pred, E e, boolean timed, long nanos) { |
600 |
> |
long lastTime = timed ? System.nanoTime() : 0L; |
601 |
|
Thread w = Thread.currentThread(); |
602 |
|
int spins = -1; // initialized after first item and cancel checks |
603 |
|
ThreadLocalRandom randomYields = null; // bound if needed |
605 |
|
for (;;) { |
606 |
|
Object item = s.item; |
607 |
|
if (item != e) { // matched |
608 |
+ |
assert item != s; |
609 |
|
s.forgetContents(); // avoid garbage |
610 |
< |
return item; |
610 |
> |
return this.<E>cast(item); |
611 |
|
} |
612 |
< |
if ((w.isInterrupted() || (how == TIMEOUT && nanos <= 0)) && |
613 |
< |
s.casItem(e, s)) { // cancel |
612 |
> |
if ((w.isInterrupted() || (timed && nanos <= 0)) && |
613 |
> |
s.casItem(e, s)) { // cancel |
614 |
|
unsplice(pred, s); |
615 |
|
return e; |
616 |
|
} |
619 |
|
if ((spins = spinsFor(pred, s.isData)) > 0) |
620 |
|
randomYields = ThreadLocalRandom.current(); |
621 |
|
} |
622 |
< |
else if (spins > 0) { // spin, occasionally yield |
623 |
< |
if (randomYields.nextInt(FRONT_SPINS) == 0) |
624 |
< |
Thread.yield(); |
625 |
< |
--spins; |
622 |
> |
else if (spins > 0) { // spin |
623 |
> |
if (--spins == 0) |
624 |
> |
shortenHeadPath(); // reduce slack before blocking |
625 |
> |
else if (randomYields.nextInt(CHAINED_SPINS) == 0) |
626 |
> |
Thread.yield(); // occasionally yield |
627 |
|
} |
628 |
|
else if (s.waiter == null) { |
629 |
< |
shortenHeadPath(); // reduce slack before blocking |
581 |
< |
s.waiter = w; // request unpark |
629 |
> |
s.waiter = w; // request unpark then recheck |
630 |
|
} |
631 |
< |
else if (how == TIMEOUT) { |
631 |
> |
else if (timed) { |
632 |
|
long now = System.nanoTime(); |
633 |
|
if ((nanos -= now - lastTime) > 0) |
634 |
|
LockSupport.parkNanos(this, nanos); |
636 |
|
} |
637 |
|
else { |
638 |
|
LockSupport.park(this); |
639 |
+ |
s.waiter = null; |
640 |
|
spins = -1; // spin if front upon wakeup |
641 |
|
} |
642 |
|
} |
648 |
|
*/ |
649 |
|
private static int spinsFor(Node pred, boolean haveData) { |
650 |
|
if (MP && pred != null) { |
651 |
< |
boolean predData = pred.isData; |
652 |
< |
if (predData != haveData) // front and phase change |
653 |
< |
return FRONT_SPINS + (FRONT_SPINS >>> 1); |
605 |
< |
if (predData != (pred.item != null)) // probably at front |
651 |
> |
if (pred.isData != haveData) // phase change |
652 |
> |
return FRONT_SPINS + CHAINED_SPINS; |
653 |
> |
if (pred.isMatched()) // probably at front |
654 |
|
return FRONT_SPINS; |
655 |
|
if (pred.waiter == null) // pred apparently spinning |
656 |
|
return CHAINED_SPINS; |
682 |
|
/* -------------- Traversal methods -------------- */ |
683 |
|
|
684 |
|
/** |
685 |
+ |
* Returns the successor of p, or the head node if p.next has been |
686 |
+ |
* linked to self, which will only be true if traversing with a |
687 |
+ |
* stale pointer that is now off the list. |
688 |
+ |
*/ |
689 |
+ |
final Node succ(Node p) { |
690 |
+ |
Node next = p.next; |
691 |
+ |
return (p == next) ? head : next; |
692 |
+ |
} |
693 |
+ |
|
694 |
+ |
/** |
695 |
|
* Returns the first unmatched node of the given mode, or null if |
696 |
|
* none. Used by methods isEmpty, hasWaitingConsumer. |
697 |
|
*/ |
698 |
< |
private Node firstOfMode(boolean data) { |
699 |
< |
for (Node p = head; p != null; ) { |
698 |
> |
private Node firstOfMode(boolean isData) { |
699 |
> |
for (Node p = head; p != null; p = succ(p)) { |
700 |
|
if (!p.isMatched()) |
701 |
< |
return (p.isData == data) ? p : null; |
644 |
< |
Node n = p.next; |
645 |
< |
p = (n != p) ? n : head; |
701 |
> |
return (p.isData == isData) ? p : null; |
702 |
|
} |
703 |
|
return null; |
704 |
|
} |
705 |
|
|
706 |
|
/** |
707 |
|
* Returns the item in the first unmatched node with isData; or |
708 |
< |
* null if none. Used by peek. |
708 |
> |
* null if none. Used by peek. |
709 |
|
*/ |
710 |
< |
private Object firstDataItem() { |
711 |
< |
for (Node p = head; p != null; ) { |
656 |
< |
boolean isData = p.isData; |
710 |
> |
private E firstDataItem() { |
711 |
> |
for (Node p = head; p != null; p = succ(p)) { |
712 |
|
Object item = p.item; |
713 |
< |
if (item != p && (item != null) == isData) |
714 |
< |
return isData ? item : null; |
715 |
< |
Node n = p.next; |
716 |
< |
p = (n != p) ? n : head; |
713 |
> |
if (p.isData) { |
714 |
> |
if (item != null && item != p) |
715 |
> |
return this.<E>cast(item); |
716 |
> |
} |
717 |
> |
else if (item == null) |
718 |
> |
return null; |
719 |
|
} |
720 |
|
return null; |
721 |
|
} |
746 |
|
|
747 |
|
final class Itr implements Iterator<E> { |
748 |
|
private Node nextNode; // next node to return item for |
749 |
< |
private Object nextItem; // the corresponding item |
749 |
> |
private E nextItem; // the corresponding item |
750 |
|
private Node lastRet; // last returned node, to support remove |
751 |
+ |
private Node lastPred; // predecessor to unlink lastRet |
752 |
|
|
753 |
|
/** |
754 |
|
* Moves to next node after prev, or first node if prev null. |
755 |
|
*/ |
756 |
|
private void advance(Node prev) { |
757 |
+ |
lastPred = lastRet; |
758 |
|
lastRet = prev; |
759 |
< |
Node p; |
760 |
< |
if (prev == null || (p = prev.next) == prev) |
702 |
< |
p = head; |
703 |
< |
while (p != null) { |
759 |
> |
for (Node p = (prev == null) ? head : succ(prev); |
760 |
> |
p != null; p = succ(p)) { |
761 |
|
Object item = p.item; |
762 |
|
if (p.isData) { |
763 |
|
if (item != null && item != p) { |
764 |
< |
nextItem = item; |
764 |
> |
nextItem = LinkedTransferQueue.this.<E>cast(item); |
765 |
|
nextNode = p; |
766 |
|
return; |
767 |
|
} |
768 |
|
} |
769 |
|
else if (item == null) |
770 |
|
break; |
714 |
– |
Node n = p.next; |
715 |
– |
p = (n != p) ? n : head; |
771 |
|
} |
772 |
|
nextNode = null; |
773 |
|
} |
783 |
|
public final E next() { |
784 |
|
Node p = nextNode; |
785 |
|
if (p == null) throw new NoSuchElementException(); |
786 |
< |
Object e = nextItem; |
786 |
> |
E e = nextItem; |
787 |
|
advance(p); |
788 |
< |
return (E) e; |
788 |
> |
return e; |
789 |
|
} |
790 |
|
|
791 |
|
public final void remove() { |
792 |
|
Node p = lastRet; |
793 |
|
if (p == null) throw new IllegalStateException(); |
794 |
< |
lastRet = null; |
740 |
< |
findAndRemoveNode(p); |
794 |
> |
findAndRemoveDataNode(lastPred, p); |
795 |
|
} |
796 |
|
} |
797 |
|
|
808 |
|
s.forgetContents(); // clear unneeded fields |
809 |
|
/* |
810 |
|
* At any given time, exactly one node on list cannot be |
811 |
< |
* deleted -- the last inserted node. To accommodate this, if |
812 |
< |
* we cannot delete s, we save its predecessor as "cleanMe", |
811 |
> |
* unlinked -- the last inserted node. To accommodate this, if |
812 |
> |
* we cannot unlink s, we save its predecessor as "cleanMe", |
813 |
|
* processing the previously saved version first. Because only |
814 |
|
* one node in the list can have a null next, at least one of |
815 |
|
* node s or the node previously saved can always be |
825 |
|
break; |
826 |
|
} |
827 |
|
if (oldpred == pred || // Already saved |
828 |
< |
(oldpred == null && casCleanMe(null, pred))) |
829 |
< |
break; // Postpone cleaning |
828 |
> |
((oldpred == null || oldpred.next == s) && |
829 |
> |
casCleanMe(oldpred, pred))) { |
830 |
> |
break; |
831 |
> |
} |
832 |
|
} |
833 |
|
} |
834 |
|
} |
868 |
|
} |
869 |
|
|
870 |
|
/** |
871 |
< |
* Main implementation of Iterator.remove(). Find |
872 |
< |
* and unsplice the given node. |
871 |
> |
* Main implementation of Iterator.remove(). Finds |
872 |
> |
* and unsplices the given data node. |
873 |
> |
* |
874 |
> |
* @param possiblePred possible predecessor of s |
875 |
> |
* @param s the node to remove |
876 |
|
*/ |
877 |
< |
final void findAndRemoveNode(Node s) { |
877 |
> |
final void findAndRemoveDataNode(Node possiblePred, Node s) { |
878 |
> |
assert s.isData; |
879 |
|
if (s.tryMatchData()) { |
880 |
< |
Node pred = null; |
881 |
< |
Node p = head; |
882 |
< |
while (p != null) { |
883 |
< |
if (p == s) { |
884 |
< |
unsplice(pred, p); |
885 |
< |
break; |
886 |
< |
} |
887 |
< |
if (!p.isData && !p.isMatched()) |
888 |
< |
break; |
889 |
< |
pred = p; |
890 |
< |
if ((p = p.next) == pred) { // stale |
891 |
< |
pred = null; |
892 |
< |
p = head; |
880 |
> |
if (possiblePred != null && possiblePred.next == s) |
881 |
> |
unsplice(possiblePred, s); // was actual predecessor |
882 |
> |
else { |
883 |
> |
for (Node pred = null, p = head; p != null; ) { |
884 |
> |
if (p == s) { |
885 |
> |
unsplice(pred, p); |
886 |
> |
break; |
887 |
> |
} |
888 |
> |
if (p.isUnmatchedRequest()) |
889 |
> |
break; |
890 |
> |
pred = p; |
891 |
> |
if ((p = p.next) == pred) { // stale |
892 |
> |
pred = null; |
893 |
> |
p = head; |
894 |
> |
} |
895 |
|
} |
896 |
|
} |
897 |
|
} |
902 |
|
*/ |
903 |
|
private boolean findAndRemove(Object e) { |
904 |
|
if (e != null) { |
905 |
< |
Node pred = null; |
844 |
< |
Node p = head; |
845 |
< |
while (p != null) { |
905 |
> |
for (Node pred = null, p = head; p != null; ) { |
906 |
|
Object item = p.item; |
907 |
|
if (p.isData) { |
908 |
|
if (item != null && item != p && e.equals(item) && |
914 |
|
else if (item == null) |
915 |
|
break; |
916 |
|
pred = p; |
917 |
< |
if ((p = p.next) == pred) { |
917 |
> |
if ((p = p.next) == pred) { // stale |
918 |
|
pred = null; |
919 |
|
p = head; |
920 |
|
} |
1042 |
|
*/ |
1043 |
|
public boolean tryTransfer(E e, long timeout, TimeUnit unit) |
1044 |
|
throws InterruptedException { |
1045 |
< |
if (xfer(e, true, TIMEOUT, unit.toNanos(timeout)) == null) |
1045 |
> |
if (xfer(e, true, TIMED, unit.toNanos(timeout)) == null) |
1046 |
|
return true; |
1047 |
|
if (!Thread.interrupted()) |
1048 |
|
return false; |
1050 |
|
} |
1051 |
|
|
1052 |
|
public E take() throws InterruptedException { |
1053 |
< |
Object e = xfer(null, false, SYNC, 0); |
1053 |
> |
E e = xfer(null, false, SYNC, 0); |
1054 |
|
if (e != null) |
1055 |
< |
return (E)e; |
1055 |
> |
return e; |
1056 |
|
Thread.interrupted(); |
1057 |
|
throw new InterruptedException(); |
1058 |
|
} |
1059 |
|
|
1060 |
|
public E poll(long timeout, TimeUnit unit) throws InterruptedException { |
1061 |
< |
Object e = xfer(null, false, TIMEOUT, unit.toNanos(timeout)); |
1061 |
> |
E e = xfer(null, false, TIMED, unit.toNanos(timeout)); |
1062 |
|
if (e != null || !Thread.interrupted()) |
1063 |
< |
return (E)e; |
1063 |
> |
return e; |
1064 |
|
throw new InterruptedException(); |
1065 |
|
} |
1066 |
|
|
1067 |
|
public E poll() { |
1068 |
< |
return (E)xfer(null, false, NOW, 0); |
1068 |
> |
return xfer(null, false, NOW, 0); |
1069 |
|
} |
1070 |
|
|
1071 |
|
/** |
1122 |
|
} |
1123 |
|
|
1124 |
|
public E peek() { |
1125 |
< |
return (E) firstDataItem(); |
1125 |
> |
return firstDataItem(); |
1126 |
|
} |
1127 |
|
|
1128 |
|
/** |
1218 |
|
} |
1219 |
|
} |
1220 |
|
|
1161 |
– |
|
1221 |
|
// Unsafe mechanics |
1222 |
|
|
1223 |
|
private static final sun.misc.Unsafe UNSAFE = getUnsafe(); |
1240 |
|
} |
1241 |
|
} |
1242 |
|
|
1243 |
< |
private static sun.misc.Unsafe getUnsafe() { |
1243 |
> |
/** |
1244 |
> |
* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package. |
1245 |
> |
* Replace with a simple call to Unsafe.getUnsafe when integrating |
1246 |
> |
* into a jdk. |
1247 |
> |
* |
1248 |
> |
* @return a sun.misc.Unsafe |
1249 |
> |
*/ |
1250 |
> |
static sun.misc.Unsafe getUnsafe() { |
1251 |
|
try { |
1252 |
|
return sun.misc.Unsafe.getUnsafe(); |
1253 |
|
} catch (SecurityException se) { |