| 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 |
| 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 or Iterator.remove) uses a scheme |
| 211 |
< |
* roughly similar to that in Scherer, Lea, and Scott's |
| 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 upon the |
| 216 |
< |
* next call to unsplice any other node. Situations needing such |
| 217 |
< |
* mechanics are not common but do occur in practice; for example |
| 218 |
< |
* when an unbounded series of short timed calls to poll |
| 219 |
< |
* repeatedly time out but never otherwise fall off the list |
| 220 |
< |
* because of an untimed call to take at the front of the |
| 221 |
< |
* queue. (Note that maintaining field cleanMe does not otherwise |
| 222 |
< |
* much impact garbage retention even if never cleared by some |
| 223 |
< |
* other call because the held node will eventually either |
| 224 |
< |
* directly or indirectly lead to a self-link once off the list.) |
| 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 target slack at one. Avoiding tracking |
| 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 target slack value, it is rarely worthwhile |
| 243 |
< |
* to augment this with path short-circuiting; i.e., unsplicing |
| 244 |
< |
* nodes between head and the first unmatched node, or similarly |
| 245 |
< |
* for tail, rather than advancing head or tail proper. However, |
| 246 |
< |
* it is used (in awaitMatch) immediately before a waiting thread |
| 247 |
< |
* starts to block, as a final bit of helping at a point when |
| 248 |
< |
* contention with others is extremely unlikely (since if other |
| 249 |
< |
* threads that could release it are operating, then the current |
| 250 |
< |
* 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 |
| 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 |
| 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 |
| 361 |
|
* precede or follow CASes use simple relaxed forms. Other |
| 362 |
|
* cleanups use releasing/lazy writes. |
| 363 |
|
*/ |
| 364 |
< |
static final class Node { |
| 364 |
> |
static final class Node<E> { |
| 365 |
|
final boolean isData; // false if this is a request node |
| 366 |
|
volatile Object item; // initially non-null if isData; CASed to match |
| 367 |
< |
volatile Node next; |
| 367 |
> |
volatile Node<E> next; |
| 368 |
|
volatile Thread waiter; // null until waiting |
| 369 |
|
|
| 370 |
|
// CAS methods for fields |
| 371 |
< |
final boolean casNext(Node cmp, Node val) { |
| 371 |
> |
final boolean casNext(Node<E> cmp, Node<E> val) { |
| 372 |
|
return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val); |
| 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 |
|
|
| 381 |
|
* Creates a new node. Uses relaxed write because item can only |
| 382 |
|
* be seen if followed by CAS. |
| 383 |
|
*/ |
| 384 |
< |
Node(Object item, boolean isData) { |
| 384 |
> |
Node(E item, boolean isData) { |
| 385 |
|
UNSAFE.putObject(this, itemOffset, item); // relaxed write |
| 386 |
|
this.isData = isData; |
| 387 |
|
} |
| 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<E> head; |
| 461 |
|
|
| 462 |
|
/** predecessor of dangling unspliceable node */ |
| 463 |
< |
private transient volatile Node cleanMe; // decl here to reduce contention |
| 463 |
> |
private transient volatile Node<E> cleanMe; // decl here reduces contention |
| 464 |
|
|
| 465 |
|
/** tail of the queue; null until first append */ |
| 466 |
< |
private transient volatile Node tail; |
| 466 |
> |
private transient volatile Node<E> tail; |
| 467 |
|
|
| 468 |
|
// CAS methods for fields |
| 469 |
< |
private boolean casTail(Node cmp, Node val) { |
| 469 |
> |
private boolean casTail(Node<E> cmp, Node<E> val) { |
| 470 |
|
return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val); |
| 471 |
|
} |
| 472 |
|
|
| 473 |
< |
private boolean casHead(Node cmp, Node val) { |
| 473 |
> |
private boolean casHead(Node<E> cmp, Node<E> val) { |
| 474 |
|
return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val); |
| 475 |
|
} |
| 476 |
|
|
| 477 |
< |
private boolean casCleanMe(Node cmp, Node val) { |
| 477 |
> |
private boolean casCleanMe(Node<E> cmp, Node<E> val) { |
| 478 |
|
return UNSAFE.compareAndSwapObject(this, cleanMeOffset, cmp, val); |
| 479 |
|
} |
| 480 |
|
|
| 487 |
|
private static final int SYNC = 2; // for transfer, take |
| 488 |
|
private static final int TIMEOUT = 3; // for timed poll, tryTransfer |
| 489 |
|
|
| 490 |
+ |
@SuppressWarnings("unchecked") |
| 491 |
+ |
static <E> E cast(Object item) { |
| 492 |
+ |
assert item == null || item.getClass() != Node.class; |
| 493 |
+ |
return (E) item; |
| 494 |
+ |
} |
| 495 |
+ |
|
| 496 |
|
/** |
| 497 |
|
* Implements all queuing methods. See above for explanation. |
| 498 |
|
* |
| 503 |
|
* @return an item if matched, else e |
| 504 |
|
* @throws NullPointerException if haveData mode but e is null |
| 505 |
|
*/ |
| 506 |
< |
private Object xfer(Object e, boolean haveData, int how, long nanos) { |
| 506 |
> |
private E xfer(E e, boolean haveData, int how, long nanos) { |
| 507 |
|
if (haveData && (e == null)) |
| 508 |
|
throw new NullPointerException(); |
| 509 |
< |
Node s = null; // the node to append, if needed |
| 509 |
> |
Node<E> s = null; // the node to append, if needed |
| 510 |
|
|
| 511 |
|
retry: for (;;) { // restart on append race |
| 512 |
|
|
| 513 |
< |
for (Node h = head, p = h; p != null;) { // find & match first node |
| 513 |
> |
for (Node<E> h = head, p = h; p != null;) { |
| 514 |
> |
// find & match first node |
| 515 |
|
boolean isData = p.isData; |
| 516 |
|
Object item = p.item; |
| 517 |
|
if (item != p && (item != null) == isData) { // unmatched |
| 518 |
|
if (isData == haveData) // can't match |
| 519 |
|
break; |
| 520 |
|
if (p.casItem(item, e)) { // match |
| 521 |
< |
Thread w = p.waiter; |
| 522 |
< |
while (p != h) { // update head |
| 523 |
< |
Node n = p.next; // by 2 unless singleton |
| 524 |
< |
if (n != null) |
| 525 |
< |
p = n; |
| 504 |
< |
if (head == h && casHead(h, p)) { |
| 521 |
> |
for (Node<E> q = p; q != h;) { |
| 522 |
> |
Node<E> n = q.next; // update head by 2 |
| 523 |
> |
if (n != null) // unless singleton |
| 524 |
> |
q = n; |
| 525 |
> |
if (head == h && casHead(h, q)) { |
| 526 |
|
h.forgetNext(); |
| 527 |
|
break; |
| 528 |
|
} // advance and retry |
| 529 |
|
if ((h = head) == null || |
| 530 |
< |
(p = h.next) == null || !p.isMatched()) |
| 530 |
> |
(q = h.next) == null || !q.isMatched()) |
| 531 |
|
break; // unless slack < 2 |
| 532 |
|
} |
| 533 |
< |
LockSupport.unpark(w); |
| 534 |
< |
return item; |
| 533 |
> |
LockSupport.unpark(p.waiter); |
| 534 |
> |
return this.<E>cast(item); |
| 535 |
|
} |
| 536 |
|
} |
| 537 |
< |
Node n = p.next; |
| 537 |
> |
Node<E> n = p.next; |
| 538 |
|
p = (p != n) ? n : (h = head); // Use head if p offlist |
| 539 |
|
} |
| 540 |
|
|
| 541 |
|
if (how >= ASYNC) { // No matches available |
| 542 |
|
if (s == null) |
| 543 |
< |
s = new Node(e, haveData); |
| 544 |
< |
Node pred = tryAppend(s, haveData); |
| 543 |
> |
s = new Node<E>(e, haveData); |
| 544 |
> |
Node<E> pred = tryAppend(s, haveData); |
| 545 |
|
if (pred == null) |
| 546 |
|
continue retry; // lost race vs opposite mode |
| 547 |
|
if (how >= SYNC) |
| 548 |
< |
return awaitMatch(pred, s, e, how, nanos); |
| 548 |
> |
return awaitMatch(s, pred, e, how, nanos); |
| 549 |
|
} |
| 550 |
|
return e; // not waiting |
| 551 |
|
} |
| 560 |
|
* different mode, else s's predecessor, or s itself if no |
| 561 |
|
* predecessor |
| 562 |
|
*/ |
| 563 |
< |
private Node tryAppend(Node s, boolean haveData) { |
| 564 |
< |
for (Node t = tail, p = t;;) { // move p to last node and append |
| 565 |
< |
Node n, u; // temps for reads of next & tail |
| 563 |
> |
private Node<E> tryAppend(Node<E> s, boolean haveData) { |
| 564 |
> |
for (Node<E> t = tail, p = t;;) { // move p to last node and append |
| 565 |
> |
Node<E> n, u; // temps for reads of next & tail |
| 566 |
|
if (p == null && (p = head) == null) { |
| 567 |
|
if (casHead(null, s)) |
| 568 |
|
return s; // initialize |
| 589 |
|
/** |
| 590 |
|
* Spins/yields/blocks until node s is matched or caller gives up. |
| 591 |
|
* |
| 571 |
– |
* @param pred the predecessor of s, or s or null if none |
| 592 |
|
* @param s the waiting node |
| 593 |
+ |
* @param pred the predecessor of s, or s itself if it has no |
| 594 |
+ |
* predecessor, or null if unknown (the null case does not occur |
| 595 |
+ |
* in any current calls but may in possible future extensions) |
| 596 |
|
* @param e the comparison value for checking match |
| 597 |
|
* @param how either SYNC or TIMEOUT |
| 598 |
|
* @param nanos timeout value |
| 599 |
|
* @return matched item, or e if unmatched on interrupt or timeout |
| 600 |
|
*/ |
| 601 |
< |
private Object awaitMatch(Node pred, Node s, Object e, |
| 579 |
< |
int how, long nanos) { |
| 601 |
> |
private E awaitMatch(Node<E> s, Node<E> pred, E e, int how, long nanos) { |
| 602 |
|
long lastTime = (how == TIMEOUT) ? System.nanoTime() : 0L; |
| 603 |
|
Thread w = Thread.currentThread(); |
| 604 |
|
int spins = -1; // initialized after first item and cancel checks |
| 607 |
|
for (;;) { |
| 608 |
|
Object item = s.item; |
| 609 |
|
if (item != e) { // matched |
| 610 |
+ |
assert item != s; |
| 611 |
|
s.forgetContents(); // avoid garbage |
| 612 |
< |
return item; |
| 612 |
> |
return this.<E>cast(item); |
| 613 |
|
} |
| 614 |
|
if ((w.isInterrupted() || (how == TIMEOUT && nanos <= 0)) && |
| 615 |
< |
s.casItem(e, s)) { // cancel |
| 615 |
> |
s.casItem(e, s)) { // cancel |
| 616 |
|
unsplice(pred, s); |
| 617 |
|
return e; |
| 618 |
|
} |
| 621 |
|
if ((spins = spinsFor(pred, s.isData)) > 0) |
| 622 |
|
randomYields = ThreadLocalRandom.current(); |
| 623 |
|
} |
| 624 |
< |
else if (spins > 0) { // spin, occasionally yield |
| 625 |
< |
if (randomYields.nextInt(FRONT_SPINS) == 0) |
| 626 |
< |
Thread.yield(); |
| 627 |
< |
--spins; |
| 624 |
> |
else if (spins > 0) { // spin |
| 625 |
> |
if (--spins == 0) |
| 626 |
> |
shortenHeadPath(); // reduce slack before blocking |
| 627 |
> |
else if (randomYields.nextInt(CHAINED_SPINS) == 0) |
| 628 |
> |
Thread.yield(); // occasionally yield |
| 629 |
|
} |
| 630 |
|
else if (s.waiter == null) { |
| 631 |
< |
shortenHeadPath(); // reduce slack before blocking |
| 608 |
< |
s.waiter = w; // request unpark |
| 631 |
> |
s.waiter = w; // request unpark then recheck |
| 632 |
|
} |
| 633 |
|
else if (how == TIMEOUT) { |
| 634 |
|
long now = System.nanoTime(); |
| 638 |
|
} |
| 639 |
|
else { |
| 640 |
|
LockSupport.park(this); |
| 641 |
+ |
s.waiter = null; |
| 642 |
|
spins = -1; // spin if front upon wakeup |
| 643 |
|
} |
| 644 |
|
} |
| 648 |
|
* Returns spin/yield value for a node with given predecessor and |
| 649 |
|
* data mode. See above for explanation. |
| 650 |
|
*/ |
| 651 |
< |
private static int spinsFor(Node pred, boolean haveData) { |
| 651 |
> |
private static int spinsFor(Node<?> pred, boolean haveData) { |
| 652 |
|
if (MP && pred != null) { |
| 653 |
< |
boolean predData = pred.isData; |
| 654 |
< |
if (predData != haveData) // front and phase change |
| 655 |
< |
return FRONT_SPINS + (FRONT_SPINS >>> 1); |
| 632 |
< |
if (predData != (pred.item != null)) // probably at front |
| 653 |
> |
if (pred.isData != haveData) // phase change |
| 654 |
> |
return FRONT_SPINS + CHAINED_SPINS; |
| 655 |
> |
if (pred.isMatched()) // probably at front |
| 656 |
|
return FRONT_SPINS; |
| 657 |
|
if (pred.waiter == null) // pred apparently spinning |
| 658 |
|
return CHAINED_SPINS; |
| 665 |
|
* or trailing node; failing on contention. |
| 666 |
|
*/ |
| 667 |
|
private void shortenHeadPath() { |
| 668 |
< |
Node h, hn, p, q; |
| 668 |
> |
Node<E> h, hn, p, q; |
| 669 |
|
if ((p = h = head) != null && h.isMatched() && |
| 670 |
|
(q = hn = h.next) != null) { |
| 671 |
< |
Node n; |
| 671 |
> |
Node<E> n; |
| 672 |
|
while ((n = q.next) != q) { |
| 673 |
|
if (n == null || !q.isMatched()) { |
| 674 |
|
if (hn != q && h.next == hn) |
| 687 |
|
* Returns the first unmatched node of the given mode, or null if |
| 688 |
|
* none. Used by methods isEmpty, hasWaitingConsumer. |
| 689 |
|
*/ |
| 690 |
< |
private Node firstOfMode(boolean data) { |
| 691 |
< |
for (Node p = head; p != null; ) { |
| 690 |
> |
private Node<E> firstOfMode(boolean data) { |
| 691 |
> |
for (Node<E> p = head; p != null; ) { |
| 692 |
|
if (!p.isMatched()) |
| 693 |
|
return (p.isData == data) ? p : null; |
| 694 |
< |
Node n = p.next; |
| 694 |
> |
Node<E> n = p.next; |
| 695 |
|
p = (n != p) ? n : head; |
| 696 |
|
} |
| 697 |
|
return null; |
| 699 |
|
|
| 700 |
|
/** |
| 701 |
|
* Returns the item in the first unmatched node with isData; or |
| 702 |
< |
* null if none. Used by peek. |
| 702 |
> |
* null if none. Used by peek. |
| 703 |
|
*/ |
| 704 |
< |
private Object firstDataItem() { |
| 705 |
< |
for (Node p = head; p != null; ) { |
| 704 |
> |
private E firstDataItem() { |
| 705 |
> |
for (Node<E> p = head; p != null; ) { |
| 706 |
|
boolean isData = p.isData; |
| 707 |
|
Object item = p.item; |
| 708 |
|
if (item != p && (item != null) == isData) |
| 709 |
< |
return isData ? item : null; |
| 710 |
< |
Node n = p.next; |
| 709 |
> |
return isData ? this.<E>cast(item) : null; |
| 710 |
> |
Node<E> n = p.next; |
| 711 |
|
p = (n != p) ? n : head; |
| 712 |
|
} |
| 713 |
|
return null; |
| 719 |
|
*/ |
| 720 |
|
private int countOfMode(boolean data) { |
| 721 |
|
int count = 0; |
| 722 |
< |
for (Node p = head; p != null; ) { |
| 722 |
> |
for (Node<E> p = head; p != null; ) { |
| 723 |
|
if (!p.isMatched()) { |
| 724 |
|
if (p.isData != data) |
| 725 |
|
return 0; |
| 726 |
|
if (++count == Integer.MAX_VALUE) // saturated |
| 727 |
|
break; |
| 728 |
|
} |
| 729 |
< |
Node n = p.next; |
| 729 |
> |
Node<E> n = p.next; |
| 730 |
|
if (n != p) |
| 731 |
|
p = n; |
| 732 |
|
else { |
| 738 |
|
} |
| 739 |
|
|
| 740 |
|
final class Itr implements Iterator<E> { |
| 741 |
< |
private Node nextNode; // next node to return item for |
| 742 |
< |
private Object nextItem; // the corresponding item |
| 743 |
< |
private Node lastRet; // last returned node, to support remove |
| 741 |
> |
private Node<E> nextNode; // next node to return item for |
| 742 |
> |
private E nextItem; // the corresponding item |
| 743 |
> |
private Node<E> lastRet; // last returned node, to support remove |
| 744 |
|
|
| 745 |
|
/** |
| 746 |
|
* Moves to next node after prev, or first node if prev null. |
| 747 |
|
*/ |
| 748 |
< |
private void advance(Node prev) { |
| 748 |
> |
private void advance(Node<E> prev) { |
| 749 |
|
lastRet = prev; |
| 750 |
< |
Node p; |
| 750 |
> |
Node<E> p; |
| 751 |
|
if (prev == null || (p = prev.next) == prev) |
| 752 |
|
p = head; |
| 753 |
|
while (p != null) { |
| 754 |
|
Object item = p.item; |
| 755 |
|
if (p.isData) { |
| 756 |
|
if (item != null && item != p) { |
| 757 |
< |
nextItem = item; |
| 757 |
> |
nextItem = LinkedTransferQueue.this.<E>cast(item); |
| 758 |
|
nextNode = p; |
| 759 |
|
return; |
| 760 |
|
} |
| 761 |
|
} |
| 762 |
|
else if (item == null) |
| 763 |
|
break; |
| 764 |
< |
Node n = p.next; |
| 764 |
> |
Node<E> n = p.next; |
| 765 |
|
p = (n != p) ? n : head; |
| 766 |
|
} |
| 767 |
|
nextNode = null; |
| 776 |
|
} |
| 777 |
|
|
| 778 |
|
public final E next() { |
| 779 |
< |
Node p = nextNode; |
| 779 |
> |
Node<E> p = nextNode; |
| 780 |
|
if (p == null) throw new NoSuchElementException(); |
| 781 |
< |
Object e = nextItem; |
| 781 |
> |
E e = nextItem; |
| 782 |
|
advance(p); |
| 783 |
< |
return (E) e; |
| 783 |
> |
return e; |
| 784 |
|
} |
| 785 |
|
|
| 786 |
|
public final void remove() { |
| 787 |
< |
Node p = lastRet; |
| 787 |
> |
Node<E> p = lastRet; |
| 788 |
|
if (p == null) throw new IllegalStateException(); |
| 789 |
|
lastRet = null; |
| 790 |
< |
findAndRemoveNode(p); |
| 790 |
> |
findAndRemoveDataNode(p); |
| 791 |
|
} |
| 792 |
|
} |
| 793 |
|
|
| 800 |
|
* @param pred predecessor of node to be unspliced |
| 801 |
|
* @param s the node to be unspliced |
| 802 |
|
*/ |
| 803 |
< |
private void unsplice(Node pred, Node s) { |
| 803 |
> |
private void unsplice(Node<E> pred, Node<E> s) { |
| 804 |
|
s.forgetContents(); // clear unneeded fields |
| 805 |
|
/* |
| 806 |
|
* At any given time, exactly one node on list cannot be |
| 813 |
|
*/ |
| 814 |
|
if (pred != null && pred != s) { |
| 815 |
|
while (pred.next == s) { |
| 816 |
< |
Node oldpred = (cleanMe == null) ? null : reclean(); |
| 817 |
< |
Node n = s.next; |
| 816 |
> |
Node<E> oldpred = (cleanMe == null) ? null : reclean(); |
| 817 |
> |
Node<E> n = s.next; |
| 818 |
|
if (n != null) { |
| 819 |
|
if (n != s) |
| 820 |
|
pred.casNext(s, n); |
| 821 |
|
break; |
| 822 |
|
} |
| 823 |
|
if (oldpred == pred || // Already saved |
| 824 |
< |
(oldpred == null && casCleanMe(null, pred))) |
| 825 |
< |
break; // Postpone cleaning |
| 824 |
> |
((oldpred == null || oldpred.next == s) && |
| 825 |
> |
casCleanMe(oldpred, pred))) { |
| 826 |
> |
break; |
| 827 |
> |
} |
| 828 |
|
} |
| 829 |
|
} |
| 830 |
|
} |
| 835 |
|
* |
| 836 |
|
* @return current cleanMe node (or null) |
| 837 |
|
*/ |
| 838 |
< |
private Node reclean() { |
| 838 |
> |
private Node<E> reclean() { |
| 839 |
|
/* |
| 840 |
|
* cleanMe is, or at one time was, predecessor of a cancelled |
| 841 |
|
* node s that was the tail so could not be unspliced. If it |
| 846 |
|
* we can (must) clear cleanMe without unsplicing. This can |
| 847 |
|
* loop only due to contention. |
| 848 |
|
*/ |
| 849 |
< |
Node pred; |
| 849 |
> |
Node<E> pred; |
| 850 |
|
while ((pred = cleanMe) != null) { |
| 851 |
< |
Node s = pred.next; |
| 852 |
< |
Node n; |
| 851 |
> |
Node<E> s = pred.next; |
| 852 |
> |
Node<E> n; |
| 853 |
|
if (s == null || s == pred || !s.isMatched()) |
| 854 |
|
casCleanMe(pred, null); // already gone |
| 855 |
|
else if ((n = s.next) != null) { |
| 865 |
|
|
| 866 |
|
/** |
| 867 |
|
* Main implementation of Iterator.remove(). Find |
| 868 |
< |
* and unsplice the given node. |
| 868 |
> |
* and unsplice the given data node. |
| 869 |
|
*/ |
| 870 |
< |
final void findAndRemoveNode(Node s) { |
| 870 |
> |
final void findAndRemoveDataNode(Node<E> s) { |
| 871 |
> |
assert s.isData; |
| 872 |
|
if (s.tryMatchData()) { |
| 873 |
< |
Node pred = null; |
| 848 |
< |
Node p = head; |
| 849 |
< |
while (p != null) { |
| 873 |
> |
for (Node<E> pred = null, p = head; p != null; ) { |
| 874 |
|
if (p == s) { |
| 875 |
|
unsplice(pred, p); |
| 876 |
|
break; |
| 877 |
|
} |
| 878 |
< |
if (!p.isData && !p.isMatched()) |
| 878 |
> |
if (p.isUnmatchedRequest()) |
| 879 |
|
break; |
| 880 |
|
pred = p; |
| 881 |
|
if ((p = p.next) == pred) { // stale |
| 891 |
|
*/ |
| 892 |
|
private boolean findAndRemove(Object e) { |
| 893 |
|
if (e != null) { |
| 894 |
< |
Node pred = null; |
| 871 |
< |
Node p = head; |
| 872 |
< |
while (p != null) { |
| 894 |
> |
for (Node<E> pred = null, p = head; p != null; ) { |
| 895 |
|
Object item = p.item; |
| 896 |
|
if (p.isData) { |
| 897 |
|
if (item != null && item != p && e.equals(item) && |
| 903 |
|
else if (item == null) |
| 904 |
|
break; |
| 905 |
|
pred = p; |
| 906 |
< |
if ((p = p.next) == pred) { |
| 906 |
> |
if ((p = p.next) == pred) { // stale |
| 907 |
|
pred = null; |
| 908 |
|
p = head; |
| 909 |
|
} |
| 1039 |
|
} |
| 1040 |
|
|
| 1041 |
|
public E take() throws InterruptedException { |
| 1042 |
< |
Object e = xfer(null, false, SYNC, 0); |
| 1042 |
> |
E e = xfer(null, false, SYNC, 0); |
| 1043 |
|
if (e != null) |
| 1044 |
< |
return (E)e; |
| 1044 |
> |
return e; |
| 1045 |
|
Thread.interrupted(); |
| 1046 |
|
throw new InterruptedException(); |
| 1047 |
|
} |
| 1048 |
|
|
| 1049 |
|
public E poll(long timeout, TimeUnit unit) throws InterruptedException { |
| 1050 |
< |
Object e = xfer(null, false, TIMEOUT, unit.toNanos(timeout)); |
| 1050 |
> |
E e = xfer(null, false, TIMEOUT, unit.toNanos(timeout)); |
| 1051 |
|
if (e != null || !Thread.interrupted()) |
| 1052 |
< |
return (E)e; |
| 1052 |
> |
return e; |
| 1053 |
|
throw new InterruptedException(); |
| 1054 |
|
} |
| 1055 |
|
|
| 1056 |
|
public E poll() { |
| 1057 |
< |
return (E)xfer(null, false, NOW, 0); |
| 1057 |
> |
return xfer(null, false, NOW, 0); |
| 1058 |
|
} |
| 1059 |
|
|
| 1060 |
|
/** |
| 1111 |
|
} |
| 1112 |
|
|
| 1113 |
|
public E peek() { |
| 1114 |
< |
return (E) firstDataItem(); |
| 1114 |
> |
return firstDataItem(); |
| 1115 |
|
} |
| 1116 |
|
|
| 1117 |
|
/** |
| 1207 |
|
} |
| 1208 |
|
} |
| 1209 |
|
|
| 1188 |
– |
|
| 1210 |
|
// Unsafe mechanics |
| 1211 |
|
|
| 1212 |
|
private static final sun.misc.Unsafe UNSAFE = getUnsafe(); |
| 1229 |
|
} |
| 1230 |
|
} |
| 1231 |
|
|
| 1232 |
< |
private static sun.misc.Unsafe getUnsafe() { |
| 1232 |
> |
/** |
| 1233 |
> |
* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package. |
| 1234 |
> |
* Replace with a simple call to Unsafe.getUnsafe when integrating |
| 1235 |
> |
* into a jdk. |
| 1236 |
> |
* |
| 1237 |
> |
* @return a sun.misc.Unsafe |
| 1238 |
> |
*/ |
| 1239 |
> |
static sun.misc.Unsafe getUnsafe() { |
| 1240 |
|
try { |
| 1241 |
|
return sun.misc.Unsafe.getUnsafe(); |
| 1242 |
|
} catch (SecurityException se) { |
