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 |
– |
* |
209 |
|
* *** Overview of implementation *** |
210 |
|
* |
211 |
|
* We use a threshold-based approach to updates, with a slack |
221 |
|
* per-thread one available, but even ThreadLocalRandom is too |
222 |
|
* heavy for these purposes. |
223 |
|
* |
224 |
< |
* With such a small slack threshold value, it is rarely |
225 |
< |
* worthwhile to augment this with path short-circuiting; i.e., |
226 |
< |
* unsplicing nodes between head and the first unmatched node, or |
227 |
< |
* 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). |
224 |
> |
* With such a small slack threshold value, it is not worthwhile |
225 |
> |
* to augment this with path short-circuiting (i.e., unsplicing |
226 |
> |
* interior nodes) except in the case of cancellation/removal (see |
227 |
> |
* below). |
228 |
|
* |
229 |
|
* We allow both the head and tail fields to be null before any |
230 |
|
* nodes are enqueued; initializing upon first append. This |
306 |
|
* versa) compared to their predecessors receive additional |
307 |
|
* chained spins, reflecting longer paths typically required to |
308 |
|
* unblock threads during phase changes. |
309 |
+ |
* |
310 |
+ |
* |
311 |
+ |
* ** Unlinking removed interior nodes ** |
312 |
+ |
* |
313 |
+ |
* In addition to minimizing garbage retention via self-linking |
314 |
+ |
* described above, we also unlink removed interior nodes. These |
315 |
+ |
* may arise due to timed out or interrupted waits, or calls to |
316 |
+ |
* remove(x) or Iterator.remove. Normally, given a node that was |
317 |
+ |
* at one time known to be the predecessor of some node s that is |
318 |
+ |
* to be removed, we can unsplice s by CASing the next field of |
319 |
+ |
* its predecessor if it still points to s (otherwise s must |
320 |
+ |
* already have been removed or is now offlist). But there are two |
321 |
+ |
* situations in which we cannot guarantee to make node s |
322 |
+ |
* unreachable in this way: (1) If s is the trailing node of list |
323 |
+ |
* (i.e., with null next), then it is pinned as the target node |
324 |
+ |
* for appends, so can only be removed later when other nodes are |
325 |
+ |
* appended. (2) We cannot necessarily unlink s given a |
326 |
+ |
* predecessor node that is matched (including the case of being |
327 |
+ |
* cancelled): the predecessor may already be unspliced, in which |
328 |
+ |
* case some previous reachable node may still point to s. |
329 |
+ |
* (For further explanation see Herlihy & Shavit "The Art of |
330 |
+ |
* Multiprocessor Programming" chapter 9). Although, in both |
331 |
+ |
* cases, we can rule out the need for further action if either s |
332 |
+ |
* or its predecessor are (or can be made to be) at, or fall off |
333 |
+ |
* from, the head of list. |
334 |
+ |
* |
335 |
+ |
* Without taking these into account, it would be possible for an |
336 |
+ |
* unbounded number of supposedly removed nodes to remain |
337 |
+ |
* reachable. Situations leading to such buildup are uncommon but |
338 |
+ |
* can occur in practice; for example when a series of short timed |
339 |
+ |
* calls to poll repeatedly time out but never otherwise fall off |
340 |
+ |
* the list because of an untimed call to take at the front of the |
341 |
+ |
* queue. |
342 |
+ |
* |
343 |
+ |
* When these cases arise, rather than always retraversing the |
344 |
+ |
* entire list to find an actual predecessor to unlink (which |
345 |
+ |
* won't help for case (1) anyway), we record a conservative |
346 |
+ |
* estimate of possible unsplice failures (in "sweepVotes"). We |
347 |
+ |
* trigger a full sweep when the estimate exceeds a threshold |
348 |
+ |
* indicating the maximum number of estimated removal failures to |
349 |
+ |
* tolerate before sweeping through, unlinking cancelled nodes |
350 |
+ |
* that were not unlinked upon initial removal. We perform sweeps |
351 |
+ |
* by the thread hitting threshold (rather than background threads |
352 |
+ |
* or by spreading work to other threads) because in the main |
353 |
+ |
* contexts in which removal occurs, the caller is already |
354 |
+ |
* timed-out, cancelled, or performing a potentially O(n) |
355 |
+ |
* operation (i.e., remove(x)), none of which are time-critical |
356 |
+ |
* enough to warrant the overhead that alternatives would impose |
357 |
+ |
* on other threads. |
358 |
+ |
* |
359 |
+ |
* Because the sweepVotes estimate is conservative, and because |
360 |
+ |
* nodes become unlinked "naturally" as they fall off the head of |
361 |
+ |
* the queue, and because we allow votes to accumulate even while |
362 |
+ |
* sweeps are in progress, there are typically significantly fewer |
363 |
+ |
* such nodes than estimated. Choice of a threshold value |
364 |
+ |
* balances the likelihood of wasted effort and contention, versus |
365 |
+ |
* providing a worst-case bound on retention of interior nodes in |
366 |
+ |
* quiescent queues. The value defined below was chosen |
367 |
+ |
* empirically to balance these under various timeout scenarios. |
368 |
+ |
* |
369 |
+ |
* Note that we cannot self-link unlinked interior nodes during |
370 |
+ |
* sweeps. However, the associated garbage chains terminate when |
371 |
+ |
* some successor ultimately falls off the head of the list and is |
372 |
+ |
* self-linked. |
373 |
|
*/ |
374 |
|
|
375 |
|
/** True if on multiprocessor */ |
396 |
|
private static final int CHAINED_SPINS = FRONT_SPINS >>> 1; |
397 |
|
|
398 |
|
/** |
399 |
+ |
* The maximum number of estimated removal failures (sweepVotes) |
400 |
+ |
* to tolerate before sweeping through the queue unlinking |
401 |
+ |
* cancelled nodes that were not unlinked upon initial |
402 |
+ |
* removal. See above for explanation. The value must be at least |
403 |
+ |
* two to avoid useless sweeps when removing trailing nodes. |
404 |
+ |
*/ |
405 |
+ |
static final int SWEEP_THRESHOLD = 32; |
406 |
+ |
|
407 |
+ |
/** |
408 |
|
* Queue nodes. Uses Object, not E, for items to allow forgetting |
409 |
|
* them after use. Relies heavily on Unsafe mechanics to minimize |
410 |
< |
* unnecessary ordering constraints: Writes that intrinsically |
411 |
< |
* precede or follow CASes use simple relaxed forms. Other |
362 |
< |
* cleanups use releasing/lazy writes. |
410 |
> |
* unnecessary ordering constraints: Writes that are intrinsically |
411 |
> |
* ordered wrt other accesses or CASes use simple relaxed forms. |
412 |
|
*/ |
413 |
|
static final class Node { |
414 |
|
final boolean isData; // false if this is a request node |
422 |
|
} |
423 |
|
|
424 |
|
final boolean casItem(Object cmp, Object val) { |
425 |
+ |
assert cmp == null || cmp.getClass() != Node.class; |
426 |
|
return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val); |
427 |
|
} |
428 |
|
|
444 |
|
} |
445 |
|
|
446 |
|
/** |
447 |
< |
* Sets item to self (using a releasing/lazy write) and waiter |
448 |
< |
* to null, to avoid garbage retention after extracting or |
449 |
< |
* cancelling. |
447 |
> |
* Sets item to self and waiter to null, to avoid garbage |
448 |
> |
* retention after matching or cancelling. Uses relaxed writes |
449 |
> |
* bacause order is already constrained in the only calling |
450 |
> |
* contexts: item is forgotten only after volatile/atomic |
451 |
> |
* mechanics that extract items. Similarly, clearing waiter |
452 |
> |
* follows either CAS or return from park (if ever parked; |
453 |
> |
* else we don't care). |
454 |
|
*/ |
455 |
|
final void forgetContents() { |
456 |
< |
UNSAFE.putOrderedObject(this, itemOffset, this); |
457 |
< |
UNSAFE.putOrderedObject(this, waiterOffset, null); |
456 |
> |
UNSAFE.putObject(this, itemOffset, this); |
457 |
> |
UNSAFE.putObject(this, waiterOffset, null); |
458 |
|
} |
459 |
|
|
460 |
|
/** |
463 |
|
*/ |
464 |
|
final boolean isMatched() { |
465 |
|
Object x = item; |
466 |
< |
return x == this || (x != null) != isData; |
466 |
> |
return (x == this) || ((x == null) == isData); |
467 |
> |
} |
468 |
> |
|
469 |
> |
/** |
470 |
> |
* Returns true if this is an unmatched request node. |
471 |
> |
*/ |
472 |
> |
final boolean isUnmatchedRequest() { |
473 |
> |
return !isData && item == null; |
474 |
|
} |
475 |
|
|
476 |
|
/** |
488 |
|
* Tries to artificially match a data node -- used by remove. |
489 |
|
*/ |
490 |
|
final boolean tryMatchData() { |
491 |
+ |
assert isData; |
492 |
|
Object x = item; |
493 |
|
if (x != null && x != this && casItem(x, null)) { |
494 |
|
LockSupport.unpark(waiter); |
510 |
|
} |
511 |
|
|
512 |
|
/** head of the queue; null until first enqueue */ |
513 |
< |
private transient volatile Node head; |
452 |
< |
|
453 |
< |
/** predecessor of dangling unspliceable node */ |
454 |
< |
private transient volatile Node cleanMe; // decl here to reduce contention |
513 |
> |
transient volatile Node head; |
514 |
|
|
515 |
|
/** tail of the queue; null until first append */ |
516 |
|
private transient volatile Node tail; |
517 |
|
|
518 |
+ |
/** The number of apparent failures to unsplice removed nodes */ |
519 |
+ |
private transient volatile int sweepVotes; |
520 |
+ |
|
521 |
|
// CAS methods for fields |
522 |
|
private boolean casTail(Node cmp, Node val) { |
523 |
|
return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val); |
527 |
|
return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val); |
528 |
|
} |
529 |
|
|
530 |
< |
private boolean casCleanMe(Node cmp, Node val) { |
531 |
< |
return UNSAFE.compareAndSwapObject(this, cleanMeOffset, cmp, val); |
530 |
> |
private boolean casSweepVotes(int cmp, int val) { |
531 |
> |
return UNSAFE.compareAndSwapInt(this, sweepVotesOffset, cmp, val); |
532 |
|
} |
533 |
|
|
534 |
|
/* |
535 |
< |
* Possible values for "how" argument in xfer method. Beware that |
474 |
< |
* the order of assigned numerical values matters. |
535 |
> |
* Possible values for "how" argument in xfer method. |
536 |
|
*/ |
537 |
< |
private static final int NOW = 0; // for untimed poll, tryTransfer |
538 |
< |
private static final int ASYNC = 1; // for offer, put, add |
539 |
< |
private static final int SYNC = 2; // for transfer, take |
540 |
< |
private static final int TIMEOUT = 3; // for timed poll, tryTransfer |
537 |
> |
private static final int NOW = 0; // for untimed poll, tryTransfer |
538 |
> |
private static final int ASYNC = 1; // for offer, put, add |
539 |
> |
private static final int SYNC = 2; // for transfer, take |
540 |
> |
private static final int TIMED = 3; // for timed poll, tryTransfer |
541 |
> |
|
542 |
> |
@SuppressWarnings("unchecked") |
543 |
> |
static <E> E cast(Object item) { |
544 |
> |
assert item == null || item.getClass() != Node.class; |
545 |
> |
return (E) item; |
546 |
> |
} |
547 |
|
|
548 |
|
/** |
549 |
|
* Implements all queuing methods. See above for explanation. |
550 |
|
* |
551 |
|
* @param e the item or null for take |
552 |
|
* @param haveData true if this is a put, else a take |
553 |
< |
* @param how NOW, ASYNC, SYNC, or TIMEOUT |
554 |
< |
* @param nanos timeout in nanosecs, used only if mode is TIMEOUT |
553 |
> |
* @param how NOW, ASYNC, SYNC, or TIMED |
554 |
> |
* @param nanos timeout in nanosecs, used only if mode is TIMED |
555 |
|
* @return an item if matched, else e |
556 |
|
* @throws NullPointerException if haveData mode but e is null |
557 |
|
*/ |
558 |
< |
private Object xfer(Object e, boolean haveData, int how, long nanos) { |
558 |
> |
private E xfer(E e, boolean haveData, int how, long nanos) { |
559 |
|
if (haveData && (e == null)) |
560 |
|
throw new NullPointerException(); |
561 |
|
Node s = null; // the node to append, if needed |
569 |
|
if (isData == haveData) // can't match |
570 |
|
break; |
571 |
|
if (p.casItem(item, e)) { // match |
572 |
< |
LockSupport.unpark(p.waiter); |
573 |
< |
while (p != h) { // update head |
574 |
< |
Node n = p.next; // by 2 unless singleton |
508 |
< |
if (n != null) |
509 |
< |
p = n; |
510 |
< |
if (head == h && casHead(h, p)) { |
572 |
> |
for (Node q = p; q != h;) { |
573 |
> |
Node n = q.next; // update by 2 unless singleton |
574 |
> |
if (head == h && casHead(h, n == null? q : n)) { |
575 |
|
h.forgetNext(); |
576 |
|
break; |
577 |
|
} // advance and retry |
578 |
|
if ((h = head) == null || |
579 |
< |
(p = h.next) == null || !p.isMatched()) |
579 |
> |
(q = h.next) == null || !q.isMatched()) |
580 |
|
break; // unless slack < 2 |
581 |
|
} |
582 |
< |
return item; |
582 |
> |
LockSupport.unpark(p.waiter); |
583 |
> |
return this.<E>cast(item); |
584 |
|
} |
585 |
|
} |
586 |
|
Node n = p.next; |
587 |
|
p = (p != n) ? n : (h = head); // Use head if p offlist |
588 |
|
} |
589 |
|
|
590 |
< |
if (how >= ASYNC) { // No matches available |
590 |
> |
if (how != NOW) { // No matches available |
591 |
|
if (s == null) |
592 |
|
s = new Node(e, haveData); |
593 |
|
Node pred = tryAppend(s, haveData); |
594 |
|
if (pred == null) |
595 |
|
continue retry; // lost race vs opposite mode |
596 |
< |
if (how >= SYNC) |
597 |
< |
return awaitMatch(s, pred, e, how, nanos); |
596 |
> |
if (how != ASYNC) |
597 |
> |
return awaitMatch(s, pred, e, (how == TIMED), nanos); |
598 |
|
} |
599 |
|
return e; // not waiting |
600 |
|
} |
610 |
|
* predecessor |
611 |
|
*/ |
612 |
|
private Node tryAppend(Node s, boolean haveData) { |
613 |
< |
for (Node t = tail, p = t;;) { // move p to last node and append |
613 |
> |
for (Node t = tail, p = t;;) { // move p to last node and append |
614 |
|
Node n, u; // temps for reads of next & tail |
615 |
|
if (p == null && (p = head) == null) { |
616 |
|
if (casHead(null, s)) |
643 |
|
* predecessor, or null if unknown (the null case does not occur |
644 |
|
* in any current calls but may in possible future extensions) |
645 |
|
* @param e the comparison value for checking match |
646 |
< |
* @param how either SYNC or TIMEOUT |
647 |
< |
* @param nanos timeout value |
646 |
> |
* @param timed if true, wait only until timeout elapses |
647 |
> |
* @param nanos timeout in nanosecs, used only if timed is true |
648 |
|
* @return matched item, or e if unmatched on interrupt or timeout |
649 |
|
*/ |
650 |
< |
private Object awaitMatch(Node s, Node pred, Object e, |
651 |
< |
int how, long nanos) { |
587 |
< |
long lastTime = (how == TIMEOUT) ? System.nanoTime() : 0L; |
650 |
> |
private E awaitMatch(Node s, Node pred, E e, boolean timed, long nanos) { |
651 |
> |
long lastTime = timed ? System.nanoTime() : 0L; |
652 |
|
Thread w = Thread.currentThread(); |
653 |
|
int spins = -1; // initialized after first item and cancel checks |
654 |
|
ThreadLocalRandom randomYields = null; // bound if needed |
656 |
|
for (;;) { |
657 |
|
Object item = s.item; |
658 |
|
if (item != e) { // matched |
659 |
+ |
assert item != s; |
660 |
|
s.forgetContents(); // avoid garbage |
661 |
< |
return item; |
661 |
> |
return this.<E>cast(item); |
662 |
|
} |
663 |
< |
if ((w.isInterrupted() || (how == TIMEOUT && nanos <= 0)) && |
664 |
< |
s.casItem(e, s)) { // cancel |
663 |
> |
if ((w.isInterrupted() || (timed && nanos <= 0)) && |
664 |
> |
s.casItem(e, s)) { // cancel |
665 |
|
unsplice(pred, s); |
666 |
|
return e; |
667 |
|
} |
671 |
|
randomYields = ThreadLocalRandom.current(); |
672 |
|
} |
673 |
|
else if (spins > 0) { // spin |
674 |
< |
if (--spins == 0) |
675 |
< |
shortenHeadPath(); // reduce slack before blocking |
611 |
< |
else if (randomYields.nextInt(CHAINED_SPINS) == 0) |
674 |
> |
--spins; |
675 |
> |
if (randomYields.nextInt(CHAINED_SPINS) == 0) |
676 |
|
Thread.yield(); // occasionally yield |
677 |
|
} |
678 |
|
else if (s.waiter == null) { |
679 |
|
s.waiter = w; // request unpark then recheck |
680 |
|
} |
681 |
< |
else if (how == TIMEOUT) { |
681 |
> |
else if (timed) { |
682 |
|
long now = System.nanoTime(); |
683 |
|
if ((nanos -= now - lastTime) > 0) |
684 |
|
LockSupport.parkNanos(this, nanos); |
686 |
|
} |
687 |
|
else { |
688 |
|
LockSupport.park(this); |
625 |
– |
s.waiter = null; |
626 |
– |
spins = -1; // spin if front upon wakeup |
689 |
|
} |
690 |
|
} |
691 |
|
} |
706 |
|
return 0; |
707 |
|
} |
708 |
|
|
709 |
+ |
/* -------------- Traversal methods -------------- */ |
710 |
+ |
|
711 |
|
/** |
712 |
< |
* Tries (once) to unsplice nodes between head and first unmatched |
713 |
< |
* or trailing node; failing on contention. |
714 |
< |
*/ |
715 |
< |
private void shortenHeadPath() { |
716 |
< |
Node h, hn, p, q; |
717 |
< |
if ((p = h = head) != null && h.isMatched() && |
718 |
< |
(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 |
< |
} |
712 |
> |
* Returns the successor of p, or the head node if p.next has been |
713 |
> |
* linked to self, which will only be true if traversing with a |
714 |
> |
* stale pointer that is now off the list. |
715 |
> |
*/ |
716 |
> |
final Node succ(Node p) { |
717 |
> |
Node next = p.next; |
718 |
> |
return (p == next) ? head : next; |
719 |
|
} |
720 |
|
|
668 |
– |
/* -------------- Traversal methods -------------- */ |
669 |
– |
|
721 |
|
/** |
722 |
|
* Returns the first unmatched node of the given mode, or null if |
723 |
|
* none. Used by methods isEmpty, hasWaitingConsumer. |
724 |
|
*/ |
725 |
< |
private Node firstOfMode(boolean data) { |
726 |
< |
for (Node p = head; p != null; ) { |
725 |
> |
private Node firstOfMode(boolean isData) { |
726 |
> |
for (Node p = head; p != null; p = succ(p)) { |
727 |
|
if (!p.isMatched()) |
728 |
< |
return (p.isData == data) ? p : null; |
678 |
< |
Node n = p.next; |
679 |
< |
p = (n != p) ? n : head; |
728 |
> |
return (p.isData == isData) ? p : null; |
729 |
|
} |
730 |
|
return null; |
731 |
|
} |
732 |
|
|
733 |
|
/** |
734 |
|
* Returns the item in the first unmatched node with isData; or |
735 |
< |
* null if none. Used by peek. |
735 |
> |
* null if none. Used by peek. |
736 |
|
*/ |
737 |
< |
private Object firstDataItem() { |
738 |
< |
for (Node p = head; p != null; ) { |
690 |
< |
boolean isData = p.isData; |
737 |
> |
private E firstDataItem() { |
738 |
> |
for (Node p = head; p != null; p = succ(p)) { |
739 |
|
Object item = p.item; |
740 |
< |
if (item != p && (item != null) == isData) |
741 |
< |
return isData ? item : null; |
742 |
< |
Node n = p.next; |
743 |
< |
p = (n != p) ? n : head; |
740 |
> |
if (p.isData) { |
741 |
> |
if (item != null && item != p) |
742 |
> |
return this.<E>cast(item); |
743 |
> |
} |
744 |
> |
else if (item == null) |
745 |
> |
return null; |
746 |
|
} |
747 |
|
return null; |
748 |
|
} |
773 |
|
|
774 |
|
final class Itr implements Iterator<E> { |
775 |
|
private Node nextNode; // next node to return item for |
776 |
< |
private Object nextItem; // the corresponding item |
776 |
> |
private E nextItem; // the corresponding item |
777 |
|
private Node lastRet; // last returned node, to support remove |
778 |
+ |
private Node lastPred; // predecessor to unlink lastRet |
779 |
|
|
780 |
|
/** |
781 |
|
* Moves to next node after prev, or first node if prev null. |
782 |
|
*/ |
783 |
|
private void advance(Node prev) { |
784 |
+ |
lastPred = lastRet; |
785 |
|
lastRet = prev; |
786 |
< |
Node p; |
787 |
< |
if (prev == null || (p = prev.next) == prev) |
736 |
< |
p = head; |
737 |
< |
while (p != null) { |
786 |
> |
for (Node p = (prev == null) ? head : succ(prev); |
787 |
> |
p != null; p = succ(p)) { |
788 |
|
Object item = p.item; |
789 |
|
if (p.isData) { |
790 |
|
if (item != null && item != p) { |
791 |
< |
nextItem = item; |
791 |
> |
nextItem = LinkedTransferQueue.this.<E>cast(item); |
792 |
|
nextNode = p; |
793 |
|
return; |
794 |
|
} |
795 |
|
} |
796 |
|
else if (item == null) |
797 |
|
break; |
748 |
– |
Node n = p.next; |
749 |
– |
p = (n != p) ? n : head; |
798 |
|
} |
799 |
|
nextNode = null; |
800 |
|
} |
810 |
|
public final E next() { |
811 |
|
Node p = nextNode; |
812 |
|
if (p == null) throw new NoSuchElementException(); |
813 |
< |
Object e = nextItem; |
813 |
> |
E e = nextItem; |
814 |
|
advance(p); |
815 |
< |
return (E) e; |
815 |
> |
return e; |
816 |
|
} |
817 |
|
|
818 |
|
public final void remove() { |
819 |
|
Node p = lastRet; |
820 |
|
if (p == null) throw new IllegalStateException(); |
821 |
< |
lastRet = null; |
822 |
< |
findAndRemoveNode(p); |
821 |
> |
if (p.tryMatchData()) |
822 |
> |
unsplice(lastPred, p); |
823 |
|
} |
824 |
|
} |
825 |
|
|
829 |
|
* Unsplices (now or later) the given deleted/cancelled node with |
830 |
|
* the given predecessor. |
831 |
|
* |
832 |
< |
* @param pred predecessor of node to be unspliced |
832 |
> |
* @param pred a node that was at one time known to be the |
833 |
> |
* predecessor of s, or null or s itself if s is/was at head |
834 |
|
* @param s the node to be unspliced |
835 |
|
*/ |
836 |
< |
private void unsplice(Node pred, Node s) { |
837 |
< |
s.forgetContents(); // clear unneeded fields |
836 |
> |
final void unsplice(Node pred, Node s) { |
837 |
> |
s.forgetContents(); // forget unneeded fields |
838 |
|
/* |
839 |
< |
* At any given time, exactly one node on list cannot be |
840 |
< |
* unlinked -- the last inserted node. To accommodate this, if |
841 |
< |
* we cannot unlink s, we save its predecessor as "cleanMe", |
842 |
< |
* processing the previously saved version first. Because only |
843 |
< |
* 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. |
839 |
> |
* See above for rationale. Briefly: if pred still points to |
840 |
> |
* s, try to unlink s. If s cannot be unlinked, because it is |
841 |
> |
* trailing node or pred might be unlinked, and neither pred |
842 |
> |
* nor s are head or offlist, add to sweepVotes, and if enough |
843 |
> |
* votes have accumulated, sweep. |
844 |
|
*/ |
845 |
< |
if (pred != null && pred != s) { |
846 |
< |
while (pred.next == s) { |
847 |
< |
Node oldpred = (cleanMe == null) ? null : reclean(); |
848 |
< |
Node n = s.next; |
849 |
< |
if (n != null) { |
850 |
< |
if (n != s) |
851 |
< |
pred.casNext(s, n); |
852 |
< |
break; |
845 |
> |
if (pred != null && pred != s && pred.next == s) { |
846 |
> |
Node n = s.next; |
847 |
> |
if (n == null || |
848 |
> |
(n != s && pred.casNext(s, n) && pred.isMatched())) { |
849 |
> |
for (;;) { // check if at, or could be, head |
850 |
> |
Node h = head; |
851 |
> |
if (h == pred || h == s || h == null) |
852 |
> |
return; // at head or list empty |
853 |
> |
if (!h.isMatched()) |
854 |
> |
break; |
855 |
> |
Node hn = h.next; |
856 |
> |
if (hn == null) |
857 |
> |
return; // now empty |
858 |
> |
if (hn != h && casHead(h, hn)) |
859 |
> |
h.forgetNext(); // advance head |
860 |
> |
} |
861 |
> |
if (pred.next != pred && s.next != s) { // recheck if offlist |
862 |
> |
for (;;) { // sweep now if enough votes |
863 |
> |
int v = sweepVotes; |
864 |
> |
if (v < SWEEP_THRESHOLD) { |
865 |
> |
if (casSweepVotes(v, v + 1)) |
866 |
> |
break; |
867 |
> |
} |
868 |
> |
else if (casSweepVotes(v, 0)) { |
869 |
> |
sweep(); |
870 |
> |
break; |
871 |
> |
} |
872 |
> |
} |
873 |
|
} |
807 |
– |
if (oldpred == pred || // Already saved |
808 |
– |
(oldpred == null && casCleanMe(null, pred))) |
809 |
– |
break; // Postpone cleaning |
874 |
|
} |
875 |
|
} |
876 |
|
} |
877 |
|
|
878 |
|
/** |
879 |
< |
* 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) |
879 |
> |
* Unlinks matched nodes encountered in a traversal from head. |
880 |
|
*/ |
881 |
< |
private Node reclean() { |
882 |
< |
/* |
883 |
< |
* cleanMe is, or at one time was, predecessor of a cancelled |
884 |
< |
* node s that was the tail so could not be unspliced. If it |
885 |
< |
* is no longer the tail, try to unsplice if necessary and |
886 |
< |
* make cleanMe slot available. This differs from similar |
887 |
< |
* 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 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 |
< |
} |
881 |
> |
private void sweep() { |
882 |
> |
Node p = head, s, n; |
883 |
> |
while (p != null && (s = p.next) != null && (n = s.next) != null) { |
884 |
> |
if (p == s || s == n) |
885 |
> |
p = head; // stale |
886 |
> |
else if (s.isMatched()) |
887 |
> |
p.casNext(s, n); |
888 |
|
else |
889 |
< |
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 |
< |
} |
889 |
> |
p = s; |
890 |
|
} |
891 |
|
} |
892 |
|
|
895 |
|
*/ |
896 |
|
private boolean findAndRemove(Object e) { |
897 |
|
if (e != null) { |
898 |
< |
Node pred = null; |
878 |
< |
Node p = head; |
879 |
< |
while (p != null) { |
898 |
> |
for (Node pred = null, p = head; p != null; ) { |
899 |
|
Object item = p.item; |
900 |
|
if (p.isData) { |
901 |
|
if (item != null && item != p && e.equals(item) && |
907 |
|
else if (item == null) |
908 |
|
break; |
909 |
|
pred = p; |
910 |
< |
if ((p = p.next) == pred) { |
910 |
> |
if ((p = p.next) == pred) { // stale |
911 |
|
pred = null; |
912 |
|
p = head; |
913 |
|
} |
1035 |
|
*/ |
1036 |
|
public boolean tryTransfer(E e, long timeout, TimeUnit unit) |
1037 |
|
throws InterruptedException { |
1038 |
< |
if (xfer(e, true, TIMEOUT, unit.toNanos(timeout)) == null) |
1038 |
> |
if (xfer(e, true, TIMED, unit.toNanos(timeout)) == null) |
1039 |
|
return true; |
1040 |
|
if (!Thread.interrupted()) |
1041 |
|
return false; |
1043 |
|
} |
1044 |
|
|
1045 |
|
public E take() throws InterruptedException { |
1046 |
< |
Object e = xfer(null, false, SYNC, 0); |
1046 |
> |
E e = xfer(null, false, SYNC, 0); |
1047 |
|
if (e != null) |
1048 |
< |
return (E)e; |
1048 |
> |
return e; |
1049 |
|
Thread.interrupted(); |
1050 |
|
throw new InterruptedException(); |
1051 |
|
} |
1052 |
|
|
1053 |
|
public E poll(long timeout, TimeUnit unit) throws InterruptedException { |
1054 |
< |
Object e = xfer(null, false, TIMEOUT, unit.toNanos(timeout)); |
1054 |
> |
E e = xfer(null, false, TIMED, unit.toNanos(timeout)); |
1055 |
|
if (e != null || !Thread.interrupted()) |
1056 |
< |
return (E)e; |
1056 |
> |
return e; |
1057 |
|
throw new InterruptedException(); |
1058 |
|
} |
1059 |
|
|
1060 |
|
public E poll() { |
1061 |
< |
return (E)xfer(null, false, NOW, 0); |
1061 |
> |
return xfer(null, false, NOW, 0); |
1062 |
|
} |
1063 |
|
|
1064 |
|
/** |
1115 |
|
} |
1116 |
|
|
1117 |
|
public E peek() { |
1118 |
< |
return (E) firstDataItem(); |
1118 |
> |
return firstDataItem(); |
1119 |
|
} |
1120 |
|
|
1121 |
|
/** |
1211 |
|
} |
1212 |
|
} |
1213 |
|
|
1195 |
– |
|
1214 |
|
// Unsafe mechanics |
1215 |
|
|
1216 |
|
private static final sun.misc.Unsafe UNSAFE = getUnsafe(); |
1218 |
|
objectFieldOffset(UNSAFE, "head", LinkedTransferQueue.class); |
1219 |
|
private static final long tailOffset = |
1220 |
|
objectFieldOffset(UNSAFE, "tail", LinkedTransferQueue.class); |
1221 |
< |
private static final long cleanMeOffset = |
1222 |
< |
objectFieldOffset(UNSAFE, "cleanMe", LinkedTransferQueue.class); |
1221 |
> |
private static final long sweepVotesOffset = |
1222 |
> |
objectFieldOffset(UNSAFE, "sweepVotes", LinkedTransferQueue.class); |
1223 |
|
|
1224 |
|
static long objectFieldOffset(sun.misc.Unsafe UNSAFE, |
1225 |
|
String field, Class<?> klazz) { |
1233 |
|
} |
1234 |
|
} |
1235 |
|
|
1236 |
< |
private static sun.misc.Unsafe getUnsafe() { |
1236 |
> |
/** |
1237 |
> |
* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package. |
1238 |
> |
* Replace with a simple call to Unsafe.getUnsafe when integrating |
1239 |
> |
* into a jdk. |
1240 |
> |
* |
1241 |
> |
* @return a sun.misc.Unsafe |
1242 |
> |
*/ |
1243 |
> |
static sun.misc.Unsafe getUnsafe() { |
1244 |
|
try { |
1245 |
|
return sun.misc.Unsafe.getUnsafe(); |
1246 |
|
} catch (SecurityException se) { |