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import java.util.NoSuchElementException; |
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import java.util.Queue; |
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import java.util.concurrent.locks.LockSupport; |
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/** |
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* An unbounded {@link TransferQueue} based on linked nodes. |
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* This queue orders elements FIFO (first-in-first-out) with respect |
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* situations in which we cannot guarantee to make node s |
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* unreachable in this way: (1) If s is the trailing node of list |
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* (i.e., with null next), then it is pinned as the target node |
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* for appends, so can only be removed later when other nodes are |
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* for appends, so can only be removed later after other nodes are |
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* appended. (2) We cannot necessarily unlink s given a |
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* predecessor node that is matched (including the case of being |
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* cancelled): the predecessor may already be already unspliced, |
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* in which case some previous reachable node may still point to |
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* s. (For further explanation see Herlihy & Shavit "The Art of |
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* cancelled): the predecessor may already be unspliced, in which |
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* case some previous reachable node may still point to s. |
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* (For further explanation see Herlihy & Shavit "The Art of |
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* Multiprocessor Programming" chapter 9). Although, in both |
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* cases, we can rule out the need for further action if either s |
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* or its predecessor are (or can be made to be) at, or fall off |
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* When these cases arise, rather than always retraversing the |
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* entire list to find an actual predecessor to unlink (which |
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* won't help for case (1) anyway), we record a conservative |
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* estimate of possible unsplice failures (in "sweepVotes). We |
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* trigger a full sweep when the estimate exceeds a threshold |
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* indicating the maximum number of estimated removal failures to |
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* tolerate before sweeping through, unlinking cancelled nodes |
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* that were not unlinked upon initial removal. We perform sweeps |
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* by the thread hitting threshold (rather than background threads |
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* or by spreading work to other threads) because in the main |
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* contexts in which removal occurs, the caller is already |
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* timed-out, cancelled, or performing a potentially O(n) |
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* operation (i.e., remove(x)), none of which are time-critical |
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* enough to warrant the overhead that alternatives would impose |
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* on other threads. |
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* estimate of possible unsplice failures (in "sweepVotes"). |
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* We trigger a full sweep when the estimate exceeds a threshold |
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* ("SWEEP_THRESHOLD") indicating the maximum number of estimated |
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* removal failures to tolerate before sweeping through, unlinking |
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* cancelled nodes that were not unlinked upon initial removal. |
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* We perform sweeps by the thread hitting threshold (rather than |
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* background threads or by spreading work to other threads) |
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* because in the main contexts in which removal occurs, the |
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* caller is already timed-out, cancelled, or performing a |
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* potentially O(n) operation (e.g. remove(x)), none of which are |
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* time-critical enough to warrant the overhead that alternatives |
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* would impose on other threads. |
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* |
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* Because the sweepVotes estimate is conservative, and because |
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* nodes become unlinked "naturally" as they fall off the head of |
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* the queue, and because we allow votes to accumulate even while |
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* sweeps are in progress, there are typically signficantly fewer |
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* sweeps are in progress, there are typically significantly fewer |
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* such nodes than estimated. Choice of a threshold value |
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* balances the likelihood of wasted effort and contention, versus |
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* providing a worst-case bound on retention of interior nodes in |
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} |
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final boolean casItem(Object cmp, Object val) { |
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assert cmp == null || cmp.getClass() != Node.class; |
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// assert cmp == null || cmp.getClass() != Node.class; |
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return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val); |
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} |
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|
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/** |
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* Creates a new node. Uses relaxed write because item can only |
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* be seen if followed by CAS. |
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* Constructs a new node. Uses relaxed write because item can |
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* only be seen after publication via casNext. |
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*/ |
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Node(Object item, boolean isData) { |
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UNSAFE.putObject(this, itemOffset, item); // relaxed write |
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/** |
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* Sets item to self and waiter to null, to avoid garbage |
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* retention after matching or cancelling. Uses relaxed writes |
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* bacause order is already constrained in the only calling |
| 450 |
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* because order is already constrained in the only calling |
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* contexts: item is forgotten only after volatile/atomic |
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* mechanics that extract items. Similarly, clearing waiter |
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* follows either CAS or return from park (if ever parked; |
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* Tries to artificially match a data node -- used by remove. |
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*/ |
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final boolean tryMatchData() { |
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assert isData; |
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// assert isData; |
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Object x = item; |
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if (x != null && x != this && casItem(x, null)) { |
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LockSupport.unpark(waiter); |
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@SuppressWarnings("unchecked") |
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static <E> E cast(Object item) { |
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assert item == null || item.getClass() != Node.class; |
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// assert item == null || item.getClass() != Node.class; |
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return (E) item; |
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} |
| 548 |
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|
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for (;;) { |
| 658 |
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Object item = s.item; |
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if (item != e) { // matched |
| 660 |
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assert item != s; |
| 660 |
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// assert item != s; |
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s.forgetContents(); // avoid garbage |
| 662 |
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return this.<E>cast(item); |
| 663 |
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} |
| 877 |
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} |
| 878 |
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|
| 879 |
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/** |
| 880 |
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* Unlink matched nodes encountered in a traversal from head |
| 880 |
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* Unlinks matched (typically cancelled) nodes encountered in a |
| 881 |
> |
* traversal from head. |
| 882 |
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*/ |
| 883 |
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private void sweep() { |
| 884 |
< |
Node p = head, s, n; |
| 885 |
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while (p != null && (s = p.next) != null && (n = s.next) != null) { |
| 886 |
< |
if (p == s || s == n) |
| 887 |
< |
p = head; // stale |
| 886 |
< |
else if (s.isMatched()) |
| 887 |
< |
p.casNext(s, n); |
| 888 |
< |
else |
| 884 |
> |
for (Node p = head, s, n; p != null && (s = p.next) != null; ) { |
| 885 |
> |
if (p == s) // stale |
| 886 |
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p = head; |
| 887 |
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else if (!s.isMatched()) |
| 888 |
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p = s; |
| 889 |
+ |
else if ((n = s.next) == null) // trailing node is pinned |
| 890 |
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break; |
| 891 |
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else |
| 892 |
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p.casNext(s, n); |
| 893 |
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} |
| 894 |
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} |
| 895 |
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|
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* @return {@code true} if this queue contains no elements |
| 1128 |
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*/ |
| 1129 |
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public boolean isEmpty() { |
| 1130 |
< |
return firstOfMode(true) == null; |
| 1130 |
> |
for (Node p = head; p != null; p = succ(p)) { |
| 1131 |
> |
if (!p.isMatched()) |
| 1132 |
> |
return !p.isData; |
| 1133 |
> |
} |
| 1134 |
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return true; |
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} |
| 1136 |
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| 1137 |
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public boolean hasWaitingConsumer() { |
