| 51 |
|
private static final long serialVersionUID = -3223113410248163686L; |
| 52 |
|
|
| 53 |
|
/* |
| 54 |
– |
* This is still a work in progress... |
| 55 |
– |
* |
| 54 |
|
* This class extends the approach used in FIFO-mode |
| 55 |
|
* SynchronousQueues. See the internal documentation, as well as |
| 56 |
|
* the PPoPP 2006 paper "Scalable Synchronous Queues" by Scherer, |
| 57 |
|
* Lea & Scott |
| 58 |
|
* (http://www.cs.rice.edu/~wns1/papers/2006-PPoPP-SQ.pdf) |
| 59 |
|
* |
| 60 |
< |
* The main extension is to provide different Wait modes |
| 61 |
< |
* for the main "xfer" method that puts or takes items. |
| 62 |
< |
* These don't impact the basic dual-queue logic, but instead |
| 63 |
< |
* control whether or how threads block upon insertion |
| 64 |
< |
* of request or data nodes into the dual queue. |
| 60 |
> |
* The main extension is to provide different Wait modes for the |
| 61 |
> |
* main "xfer" method that puts or takes items. These don't |
| 62 |
> |
* impact the basic dual-queue logic, but instead control whether |
| 63 |
> |
* or how threads block upon insertion of request or data nodes |
| 64 |
> |
* into the dual queue. It also uses slightly different |
| 65 |
> |
* conventions for tracking whether nodes are off-list or |
| 66 |
> |
* cancelled. |
| 67 |
|
*/ |
| 68 |
|
|
| 69 |
|
// Wait modes for xfer method |
| 97 |
|
static final long spinForTimeoutThreshold = 1000L; |
| 98 |
|
|
| 99 |
|
/** |
| 100 |
< |
* Node class for LinkedTransferQueue. Opportunistically subclasses from |
| 101 |
< |
* AtomicReference to represent item. Uses Object, not E, to allow |
| 102 |
< |
* setting item to "this" after use, to avoid garbage |
| 103 |
< |
* retention. Similarly, setting the next field to this is used as |
| 104 |
< |
* sentinel that node is off list. |
| 100 |
> |
* Node class for LinkedTransferQueue. Opportunistically |
| 101 |
> |
* subclasses from AtomicReference to represent item. Uses Object, |
| 102 |
> |
* not E, to allow setting item to "this" after use, to avoid |
| 103 |
> |
* garbage retention. Similarly, setting the next field to this is |
| 104 |
> |
* used as sentinel that node is off list. |
| 105 |
|
*/ |
| 106 |
|
static final class QNode extends AtomicReference<Object> { |
| 107 |
|
volatile QNode next; |
| 159 |
|
|
| 160 |
|
/** |
| 161 |
|
* Puts or takes an item. Used for most queue operations (except |
| 162 |
< |
* poll() and tryTransfer()) |
| 162 |
> |
* poll() and tryTransfer()). See the similar code in |
| 163 |
> |
* SynchronousQueue for detailed explanation. |
| 164 |
|
* @param e the item or if null, signifies that this is a take |
| 165 |
|
* @param mode the wait mode: NOWAIT, TIMEOUT, WAIT |
| 166 |
|
* @param nanos timeout in nanosecs, used only if mode is TIMEOUT |
| 267 |
|
Object x = s.get(); |
| 268 |
|
if (x != e) { // Node was matched or cancelled |
| 269 |
|
advanceHead(pred, s); // unlink if head |
| 270 |
< |
if (x == s) // was cancelled |
| 271 |
< |
return clean(pred, s); |
| 270 |
> |
if (x == s) { // was cancelled |
| 271 |
> |
clean(pred, s); |
| 272 |
> |
return null; |
| 273 |
> |
} |
| 274 |
|
else if (x != null) { |
| 275 |
|
s.set(s); // avoid garbage retention |
| 276 |
|
return x; |
| 278 |
|
else |
| 279 |
|
return e; |
| 280 |
|
} |
| 278 |
– |
|
| 281 |
|
if (mode == TIMEOUT) { |
| 282 |
|
long now = System.nanoTime(); |
| 283 |
|
nanos -= now - lastTime; |
| 313 |
|
} |
| 314 |
|
|
| 315 |
|
/** |
| 316 |
+ |
* Returns validated tail for use in cleaning methods |
| 317 |
+ |
*/ |
| 318 |
+ |
private QNode getValidatedTail() { |
| 319 |
+ |
for (;;) { |
| 320 |
+ |
QNode h = head.get(); |
| 321 |
+ |
QNode first = h.next; |
| 322 |
+ |
if (first != null && first.next == first) { // help advance |
| 323 |
+ |
advanceHead(h, first); |
| 324 |
+ |
continue; |
| 325 |
+ |
} |
| 326 |
+ |
QNode t = tail.get(); |
| 327 |
+ |
QNode last = t.next; |
| 328 |
+ |
if (t == tail.get()) { |
| 329 |
+ |
if (last != null) |
| 330 |
+ |
tail.compareAndSet(t, last); // help advance |
| 331 |
+ |
else |
| 332 |
+ |
return t; |
| 333 |
+ |
} |
| 334 |
+ |
} |
| 335 |
+ |
} |
| 336 |
+ |
|
| 337 |
+ |
/** |
| 338 |
|
* Gets rid of cancelled node s with original predecessor pred. |
| 339 |
< |
* @return null (to simplify use by callers) |
| 339 |
> |
* @param pred predecessor of cancelled node |
| 340 |
> |
* @param s the cancelled node |
| 341 |
|
*/ |
| 342 |
< |
private Object clean(QNode pred, QNode s) { |
| 342 |
> |
private void clean(QNode pred, QNode s) { |
| 343 |
|
Thread w = s.waiter; |
| 344 |
|
if (w != null) { // Wake up thread |
| 345 |
|
s.waiter = null; |
| 346 |
|
if (w != Thread.currentThread()) |
| 347 |
|
LockSupport.unpark(w); |
| 348 |
|
} |
| 349 |
+ |
/* |
| 350 |
+ |
* At any given time, exactly one node on list cannot be |
| 351 |
+ |
* deleted -- the last inserted node. To accommodate this, if |
| 352 |
+ |
* we cannot delete s, we save its predecessor as "cleanMe", |
| 353 |
+ |
* processing the previously saved version first. At least one |
| 354 |
+ |
* of node s or the node previously saved can always be |
| 355 |
+ |
* processed, so this always terminates. |
| 356 |
+ |
*/ |
| 357 |
+ |
while (pred.next == s) { |
| 358 |
+ |
QNode oldpred = reclean(); // First, help get rid of cleanMe |
| 359 |
+ |
QNode t = getValidatedTail(); |
| 360 |
+ |
if (s != t) { // If not tail, try to unsplice |
| 361 |
+ |
QNode sn = s.next; // s.next == s means s already off list |
| 362 |
+ |
if (sn == s || pred.casNext(s, sn)) |
| 363 |
+ |
break; |
| 364 |
+ |
} |
| 365 |
+ |
else if (oldpred == pred || // Already saved |
| 366 |
+ |
(oldpred == null && cleanMe.compareAndSet(null, pred))) |
| 367 |
+ |
break; // Postpone cleaning |
| 368 |
+ |
} |
| 369 |
+ |
} |
| 370 |
|
|
| 371 |
< |
for (;;) { |
| 372 |
< |
if (pred.next != s) // already cleaned |
| 373 |
< |
return null; |
| 374 |
< |
QNode h = head.get(); |
| 375 |
< |
QNode hn = h.next; // Absorb cancelled first node as head |
| 376 |
< |
if (hn != null && hn.next == hn) { |
| 377 |
< |
advanceHead(h, hn); |
| 378 |
< |
continue; |
| 379 |
< |
} |
| 380 |
< |
QNode t = tail.get(); // Ensure consistent read for tail |
| 381 |
< |
if (t == h) |
| 382 |
< |
return null; |
| 383 |
< |
QNode tn = t.next; |
| 384 |
< |
if (t != tail.get()) |
| 385 |
< |
continue; |
| 386 |
< |
if (tn != null) { // Help advance tail |
| 387 |
< |
tail.compareAndSet(t, tn); |
| 388 |
< |
continue; |
| 389 |
< |
} |
| 390 |
< |
if (s != t) { // If not tail, try to unsplice |
| 391 |
< |
QNode sn = s.next; |
| 392 |
< |
if (sn == s || pred.casNext(s, sn)) |
| 393 |
< |
return null; |
| 394 |
< |
} |
| 395 |
< |
QNode dp = cleanMe.get(); |
| 396 |
< |
if (dp != null) { // Try unlinking previous cancelled node |
| 351 |
< |
QNode d = dp.next; |
| 352 |
< |
QNode dn; |
| 353 |
< |
if (d == null || // d is gone or |
| 354 |
< |
d == dp || // d is off list or |
| 355 |
< |
d.get() != d || // d not cancelled or |
| 356 |
< |
(d != t && // d not tail and |
| 357 |
< |
(dn = d.next) != null && // has successor |
| 358 |
< |
dn != d && // that is on list |
| 359 |
< |
dp.casNext(d, dn))) // d unspliced |
| 360 |
< |
cleanMe.compareAndSet(dp, null); |
| 361 |
< |
if (dp == pred) |
| 362 |
< |
return null; // s is already saved node |
| 371 |
> |
/** |
| 372 |
> |
* Tries to unsplice the cancelled node held in cleanMe that was |
| 373 |
> |
* previously uncleanable because it was at tail. |
| 374 |
> |
* @return current cleanMe node (or null) |
| 375 |
> |
*/ |
| 376 |
> |
private QNode reclean() { |
| 377 |
> |
/* |
| 378 |
> |
* cleanMe is, or at one time was, predecessor of cancelled |
| 379 |
> |
* node s that was the tail so could not be unspliced. If s |
| 380 |
> |
* is no longer the tail, try to unsplice if necessary and |
| 381 |
> |
* make cleanMe slot available. This differs from similar |
| 382 |
> |
* code in clean() because we must check that pred still |
| 383 |
> |
* points to a cancelled node that must be unspliced -- if |
| 384 |
> |
* not, we can (must) clear cleanMe without unsplicing. |
| 385 |
> |
* This can loop only due to contention on casNext or |
| 386 |
> |
* clearing cleanMe. |
| 387 |
> |
*/ |
| 388 |
> |
QNode pred; |
| 389 |
> |
while ((pred = cleanMe.get()) != null) { |
| 390 |
> |
QNode t = getValidatedTail(); |
| 391 |
> |
QNode s = pred.next; |
| 392 |
> |
if (s != t) { |
| 393 |
> |
QNode sn; |
| 394 |
> |
if (s == null || s == pred || s.get() != s || |
| 395 |
> |
(sn = s.next) == s || pred.casNext(s, sn)) |
| 396 |
> |
cleanMe.compareAndSet(pred, null); |
| 397 |
|
} |
| 398 |
< |
else if (cleanMe.compareAndSet(null, pred)) |
| 399 |
< |
return null; // Postpone cleaning s |
| 398 |
> |
else // s is still tail; cannot clean |
| 399 |
> |
break; |
| 400 |
|
} |
| 401 |
+ |
return pred; |
| 402 |
|
} |
| 403 |
|
|
| 404 |
|
/** |
