| 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 already unspliced, |
| 328 |
< |
* in which case some previous reachable node may still point to |
| 329 |
< |
* s. (For further explanation see Herlihy & Shavit "The Art of |
| 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 |
| 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 |
| 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 |
| 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 signficantly fewer |
| 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 |
| 876 |
|
} |
| 877 |
|
|
| 878 |
|
/** |
| 879 |
< |
* Unlink matched nodes encountered in a traversal from head |
| 879 |
> |
* Unlinks matched nodes encountered in a traversal from head. |
| 880 |
|
*/ |
| 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 |
| 882 |
> |
for (Node p = head, s, n; p != null && (s = p.next) != null; ) { |
| 883 |
> |
if (p == s) // stale |
| 884 |
> |
p = head; |
| 885 |
> |
else if (!s.isMatched()) |
| 886 |
|
p = s; |
| 887 |
+ |
else if ((n = s.next) == null) // trailing node is pinned |
| 888 |
+ |
break; |
| 889 |
+ |
else |
| 890 |
+ |
p.casNext(s, n); |
| 891 |
|
} |
| 892 |
|
} |
| 893 |
|
|
