1 /* 2 * Copyright 2012 The Netty Project 3 * 4 * The Netty Project licenses this file to you under the Apache License, 5 * version 2.0 (the "License"); you may not use this file except in compliance 6 * with the License. You may obtain a copy of the License at: 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT 12 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the 13 * License for the specific language governing permissions and limitations 14 * under the License. 15 */ 16 17 /* 18 * Written by Doug Lea with assistance from members of JCP JSR-166 19 * Expert Group and released to the public domain, as explained at 20 * http://creativecommons.org/licenses/publicdomain 21 */ 22 23 package org.jboss.netty.util.internal; 24 25 import java.util.AbstractQueue; 26 import java.util.Collection; 27 import java.util.ConcurrentModificationException; 28 import java.util.Iterator; 29 import java.util.NoSuchElementException; 30 import java.util.concurrent.BlockingQueue; 31 import java.util.concurrent.TimeUnit; 32 import java.util.concurrent.atomic.AtomicIntegerFieldUpdater; 33 import java.util.concurrent.atomic.AtomicReferenceFieldUpdater; 34 import java.util.concurrent.locks.LockSupport; 35 36 /** 37 * <strong> 38 * This version does work even if sun.misc.Unsafe is not found in the classpath. So this is kept for compatibility reasons. 39 * Please use {@link QueueFactory} to create a Queue as it will use the "optimal" implementation depending on the JVM 40 * </strong> 41 * <br> 42 * <br> 43 * 44 * An unbounded {@link BlockingQueue} based on linked nodes. 45 * This queue orders elements FIFO (first-in-first-out) with respect 46 * to any given producer. The <em>head</em> of the queue is that 47 * element that has been on the queue the longest time for some 48 * producer. The <em>tail</em> of the queue is that element that has 49 * been on the queue the shortest time for some producer. 50 * 51 * <p>Beware that, unlike in most collections, the {@code size} 52 * method is <em>NOT</em> a constant-time operation. Because of the 53 * asynchronous nature of these queues, determining the current number 54 * of elements requires a traversal of the elements. 55 * 56 * <p>This class and its iterator implement all of the 57 * <em>optional</em> methods of the {@link Collection} and {@link 58 * Iterator} interfaces. 59 * 60 * <p>Memory consistency effects: As with other concurrent 61 * collections, actions in a thread prior to placing an object into a 62 * {@code LinkedTransferQueue} 63 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> 64 * actions subsequent to the access or removal of that element from 65 * the {@code LinkedTransferQueue} in another thread. 66 * 67 * <p>This class is a member of the 68 * <a href="{@docRoot}/../technotes/guides/collections/index.html"> 69 * Java Collections Framework</a>. 70 * @param <E> the type of elements held in this collection 71 */ 72 @Deprecated 73 public class LegacyLinkedTransferQueue<E> extends AbstractQueue<E> 74 implements BlockingQueue<E>, java.io.Serializable { 75 private static final long serialVersionUID = -3223113410248163686L; 76 77 /* 78 * *** Overview of Dual Queues with Slack *** 79 * 80 * Dual Queues, introduced by Scherer and Scott 81 * (http://www.cs.rice.edu/~wns1/papers/2004-DISC-DDS.pdf) are 82 * (linked) queues in which nodes may represent either data or 83 * requests. When a thread tries to enqueue a data node, but 84 * encounters a request node, it instead "matches" and removes it; 85 * and vice versa for enqueuing requests. Blocking Dual Queues 86 * arrange that threads enqueuing unmatched requests block until 87 * other threads provide the match. Dual Synchronous Queues (see 88 * Scherer, Lea, & Scott 89 * http://www.cs.rochester.edu/u/scott/papers/2009_Scherer_CACM_SSQ.pdf) 90 * additionally arrange that threads enqueuing unmatched data also 91 * block. Dual Transfer Queues support all of these modes, as 92 * dictated by callers. 93 * 94 * A FIFO dual queue may be implemented using a variation of the 95 * Michael & Scott (M&S) lock-free queue algorithm 96 * (http://www.cs.rochester.edu/u/scott/papers/1996_PODC_queues.pdf). 97 * It maintains two pointer fields, "head", pointing to a 98 * (matched) node that in turn points to the first actual 99 * (unmatched) queue node (or null if empty); and "tail" that 100 * points to the last node on the queue (or again null if 101 * empty). For example, here is a possible queue with four data 102 * elements: 103 * 104 * head tail 105 * | | 106 * v v 107 * M -> U -> U -> U -> U 108 * 109 * The M&S queue algorithm is known to be prone to scalability and 110 * overhead limitations when maintaining (via CAS) these head and 111 * tail pointers. This has led to the development of 112 * contention-reducing variants such as elimination arrays (see 113 * Moir et al http://portal.acm.org/citation.cfm?id=1074013) and 114 * optimistic back pointers (see Ladan-Mozes & Shavit 115 * http://people.csail.mit.edu/edya/publications/OptimisticFIFOQueue-journal.pdf). 116 * However, the nature of dual queues enables a simpler tactic for 117 * improving M&S-style implementations when dual-ness is needed. 118 * 119 * In a dual queue, each node must atomically maintain its match 120 * status. While there are other possible variants, we implement 121 * this here as: for a data-mode node, matching entails CASing an 122 * "item" field from a non-null data value to null upon match, and 123 * vice-versa for request nodes, CASing from null to a data 124 * value. (Note that the linearization properties of this style of 125 * queue are easy to verify -- elements are made available by 126 * linking, and unavailable by matching.) Compared to plain M&S 127 * queues, this property of dual queues requires one additional 128 * successful atomic operation per enq/deq pair. But it also 129 * enables lower cost variants of queue maintenance mechanics. (A 130 * variation of this idea applies even for non-dual queues that 131 * support deletion of interior elements, such as 132 * j.u.c.ConcurrentLinkedQueue.) 133 * 134 * Once a node is matched, its match status can never again 135 * change. We may thus arrange that the linked list of them 136 * contain a prefix of zero or more matched nodes, followed by a 137 * suffix of zero or more unmatched nodes. (Note that we allow 138 * both the prefix and suffix to be zero length, which in turn 139 * means that we do not use a dummy header.) If we were not 140 * concerned with either time or space efficiency, we could 141 * correctly perform enqueue and dequeue operations by traversing 142 * from a pointer to the initial node; CASing the item of the 143 * first unmatched node on match and CASing the next field of the 144 * trailing node on appends. (Plus some special-casing when 145 * initially empty). While this would be a terrible idea in 146 * itself, it does have the benefit of not requiring ANY atomic 147 * updates on head/tail fields. 148 * 149 * We introduce here an approach that lies between the extremes of 150 * never versus always updating queue (head and tail) pointers. 151 * This offers a tradeoff between sometimes requiring extra 152 * traversal steps to locate the first and/or last unmatched 153 * nodes, versus the reduced overhead and contention of fewer 154 * updates to queue pointers. For example, a possible snapshot of 155 * a queue is: 156 * 157 * head tail 158 * | | 159 * v v 160 * M -> M -> U -> U -> U -> U 161 * 162 * The best value for this "slack" (the targeted maximum distance 163 * between the value of "head" and the first unmatched node, and 164 * similarly for "tail") is an empirical matter. We have found 165 * that using very small constants in the range of 1-3 work best 166 * over a range of platforms. Larger values introduce increasing 167 * costs of cache misses and risks of long traversal chains, while 168 * smaller values increase CAS contention and overhead. 169 * 170 * Dual queues with slack differ from plain M&S dual queues by 171 * virtue of only sometimes updating head or tail pointers when 172 * matching, appending, or even traversing nodes; in order to 173 * maintain a targeted slack. The idea of "sometimes" may be 174 * operationalized in several ways. The simplest is to use a 175 * per-operation counter incremented on each traversal step, and 176 * to try (via CAS) to update the associated queue pointer 177 * whenever the count exceeds a threshold. Another, that requires 178 * more overhead, is to use random number generators to update 179 * with a given probability per traversal step. 180 * 181 * In any strategy along these lines, because CASes updating 182 * fields may fail, the actual slack may exceed targeted 183 * slack. However, they may be retried at any time to maintain 184 * targets. Even when using very small slack values, this 185 * approach works well for dual queues because it allows all 186 * operations up to the point of matching or appending an item 187 * (hence potentially allowing progress by another thread) to be 188 * read-only, thus not introducing any further contention. As 189 * described below, we implement this by performing slack 190 * maintenance retries only after these points. 191 * 192 * As an accompaniment to such techniques, traversal overhead can 193 * be further reduced without increasing contention of head 194 * pointer updates: Threads may sometimes shortcut the "next" link 195 * path from the current "head" node to be closer to the currently 196 * known first unmatched node, and similarly for tail. Again, this 197 * may be triggered with using thresholds or randomization. 198 * 199 * These ideas must be further extended to avoid unbounded amounts 200 * of costly-to-reclaim garbage caused by the sequential "next" 201 * links of nodes starting at old forgotten head nodes: As first 202 * described in detail by Boehm 203 * (http://portal.acm.org/citation.cfm?doid=503272.503282) if a GC 204 * delays noticing that any arbitrarily old node has become 205 * garbage, all newer dead nodes will also be unreclaimed. 206 * (Similar issues arise in non-GC environments.) To cope with 207 * this in our implementation, upon CASing to advance the head 208 * pointer, we set the "next" link of the previous head to point 209 * only to itself; thus limiting the length of connected dead lists. 210 * (We also take similar care to wipe out possibly garbage 211 * retaining values held in other Node fields.) However, doing so 212 * adds some further complexity to traversal: If any "next" 213 * pointer links to itself, it indicates that the current thread 214 * has lagged behind a head-update, and so the traversal must 215 * continue from the "head". Traversals trying to find the 216 * current tail starting from "tail" may also encounter 217 * self-links, in which case they also continue at "head". 218 * 219 * It is tempting in slack-based scheme to not even use CAS for 220 * updates (similarly to Ladan-Mozes & Shavit). However, this 221 * cannot be done for head updates under the above link-forgetting 222 * mechanics because an update may leave head at a detached node. 223 * And while direct writes are possible for tail updates, they 224 * increase the risk of long retraversals, and hence long garbage 225 * chains, which can be much more costly than is worthwhile 226 * considering that the cost difference of performing a CAS vs 227 * write is smaller when they are not triggered on each operation 228 * (especially considering that writes and CASes equally require 229 * additional GC bookkeeping ("write barriers") that are sometimes 230 * more costly than the writes themselves because of contention). 231 * 232 * *** Overview of implementation *** 233 * 234 * We use a threshold-based approach to updates, with a slack 235 * threshold of two -- that is, we update head/tail when the 236 * current pointer appears to be two or more steps away from the 237 * first/last node. The slack value is hard-wired: a path greater 238 * than one is naturally implemented by checking equality of 239 * traversal pointers except when the list has only one element, 240 * in which case we keep slack threshold at one. Avoiding tracking 241 * explicit counts across method calls slightly simplifies an 242 * already-messy implementation. Using randomization would 243 * probably work better if there were a low-quality dirt-cheap 244 * per-thread one available, but even ThreadLocalRandom is too 245 * heavy for these purposes. 246 * 247 * With such a small slack threshold value, it is not worthwhile 248 * to augment this with path short-circuiting (i.e., unsplicing 249 * interior nodes) except in the case of cancellation/removal (see 250 * below). 251 * 252 * We allow both the head and tail fields to be null before any 253 * nodes are enqueued; initializing upon first append. This 254 * simplifies some other logic, as well as providing more 255 * efficient explicit control paths instead of letting JVMs insert 256 * implicit NullPointerExceptions when they are null. While not 257 * currently fully implemented, we also leave open the possibility 258 * of re-nulling these fields when empty (which is complicated to 259 * arrange, for little benefit.) 260 * 261 * All enqueue/dequeue operations are handled by the single method 262 * "xfer" with parameters indicating whether to act as some form 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 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 270 * the third in method awaitMatch. 271 * 272 * 1. Try to match an existing node 273 * 274 * Starting at head, skip already-matched nodes until finding 275 * an unmatched node of opposite mode, if one exists, in which 276 * case matching it and returning, also if necessary updating 277 * head to one past the matched node (or the node itself if the 278 * list has no other unmatched nodes). If the CAS misses, then 279 * a loop retries advancing head by two steps until either 280 * success or the slack is at most two. By requiring that each 281 * attempt advances head by two (if applicable), we ensure that 282 * the slack does not grow without bound. Traversals also check 283 * if the initial head is now off-list, in which case they 284 * start at the new head. 285 * 286 * If no candidates are found and the call was untimed 287 * poll/offer, (argument "how" is NOW) return. 288 * 289 * 2. Try to append a new node (method tryAppend) 290 * 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 * 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 312 * it tries to unsplice any nodes between the current "head" 313 * and the first unmatched node. 314 * 315 * Front-of-queue spinning vastly improves performance of 316 * heavily contended queues. And so long as it is relatively 317 * brief and "quiet", spinning does not much impact performance 318 * of less-contended queues. During spins threads check their 319 * interrupt status and generate a thread-local random number 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 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 * chained spins, reflecting longer paths typically required to 331 * unblock threads during phase changes. 332 * ** Unlinking removed interior nodes ** 333 * 334 * In addition to minimizing garbage retention via self-linking 335 * described above, we also unlink removed interior nodes. These 336 * may arise due to timed out or interrupted waits, or calls to 337 * remove(x) or Iterator.remove. Normally, given a node that was 338 * at one time known to be the predecessor of some node s that is 339 * to be removed, we can unsplice s by CASing the next field of 340 * its predecessor if it still points to s (otherwise s must 341 * already have been removed or is now offlist). But there are two 342 * situations in which we cannot guarantee to make node s 343 * unreachable in this way: (1) If s is the trailing node of list 344 * (i.e., with null next), then it is pinned as the target node 345 * for appends, so can only be removed later after other nodes are 346 * appended. (2) We cannot necessarily unlink s given a 347 * predecessor node that is matched (including the case of being 348 * cancelled): the predecessor may already be unspliced, in which 349 * case some previous reachable node may still point to s. 350 * (For further explanation see Herlihy & Shavit "The Art of 351 * Multiprocessor Programming" chapter 9). Although, in both 352 * cases, we can rule out the need for further action if either s 353 * or its predecessor are (or can be made to be) at, or fall off 354 * from, the head of list. 355 * 356 * Without taking these into account, it would be possible for an 357 * unbounded number of supposedly removed nodes to remain 358 * reachable. Situations leading to such buildup are uncommon but 359 * can occur in practice; for example when a series of short timed 360 * calls to poll repeatedly time out but never otherwise fall off 361 * the list because of an untimed call to take at the front of the 362 * queue. 363 * 364 * When these cases arise, rather than always retraversing the 365 * entire list to find an actual predecessor to unlink (which 366 * won't help for case (1) anyway), we record a conservative 367 * estimate of possible unsplice failures (in "sweepVotes"). 368 * We trigger a full sweep when the estimate exceeds a threshold 369 * ("SWEEP_THRESHOLD") indicating the maximum number of estimated 370 * removal failures to tolerate before sweeping through, unlinking 371 * cancelled nodes that were not unlinked upon initial removal. 372 * We perform sweeps by the thread hitting threshold (rather than 373 * background threads or by spreading work to other threads) 374 * because in the main contexts in which removal occurs, the 375 * caller is already timed-out, cancelled, or performing a 376 * potentially O(n) operation (e.g. remove(x)), none of which are 377 * time-critical enough to warrant the overhead that alternatives 378 * would impose on other threads. 379 * 380 * Because the sweepVotes estimate is conservative, and because 381 * nodes become unlinked "naturally" as they fall off the head of 382 * the queue, and because we allow votes to accumulate even while 383 * sweeps are in progress, there are typically significantly fewer 384 * such nodes than estimated. Choice of a threshold value 385 * balances the likelihood of wasted effort and contention, versus 386 * providing a worst-case bound on retention of interior nodes in 387 * quiescent queues. The value defined below was chosen 388 * empirically to balance these under various timeout scenarios. 389 * 390 * Note that we cannot self-link unlinked interior nodes during 391 * sweeps. However, the associated garbage chains terminate when 392 * some successor ultimately falls off the head of the list and is 393 * self-linked. 394 */ 395 396 /** True if on multiprocessor */ 397 private static final boolean MP = 398 Runtime.getRuntime().availableProcessors() > 1; 399 400 /** 401 * The number of times to spin (with randomly interspersed calls 402 * to Thread.yield) on multiprocessor before blocking when a node 403 * is apparently the first waiter in the queue. See above for 404 * explanation. Must be a power of two. The value is empirically 405 * derived -- it works pretty well across a variety of processors, 406 * numbers of CPUs, and OSes. 407 */ 408 private static final int FRONT_SPINS = 1 << 7; 409 410 /** 411 * The number of times to spin before blocking when a node is 412 * preceded by another node that is apparently spinning. Also 413 * serves as an increment to FRONT_SPINS on phase changes, and as 414 * base average frequency for yielding during spins. Must be a 415 * power of two. 416 */ 417 private static final int CHAINED_SPINS = FRONT_SPINS >>> 1; 418 419 /** 420 * The maximum number of estimated removal failures (sweepVotes) 421 * to tolerate before sweeping through the queue unlinking 422 * cancelled nodes that were not unlinked upon initial 423 * removal. See above for explanation. The value must be at least 424 * two to avoid useless sweeps when removing trailing nodes. 425 */ 426 static final int SWEEP_THRESHOLD = 32; 427 428 /** 429 * Queue nodes. Uses Object, not E, for items to allow forgetting 430 * them after use. Relies heavily on Unsafe mechanics to minimize 431 * unnecessary ordering constraints: Writes that are intrinsically 432 * ordered wrt other accesses or CASes use simple relaxed forms. 433 */ 434 static final class Node { 435 final boolean isData; // false if this is a request node 436 volatile Object item; // initially non-null if isData; CASed to match 437 volatile Node next; 438 volatile Thread waiter; // null until waiting 439 440 // CAS methods for fields 441 boolean casNext(Node cmp, Node val) { 442 if (AtomicFieldUpdaterUtil.isAvailable()) { 443 return nextUpdater.compareAndSet(this, cmp, val); 444 } else { 445 synchronized (this) { 446 if (next == cmp) { 447 next = val; 448 return true; 449 } else { 450 return false; 451 } 452 } 453 } 454 } 455 456 boolean casItem(Object cmp, Object val) { 457 // assert cmp == null || cmp.getClass() != Node.class; 458 if (AtomicFieldUpdaterUtil.isAvailable()) { 459 return itemUpdater.compareAndSet(this, cmp, val); 460 } else { 461 synchronized (this) { 462 if (item == cmp) { 463 item = val; 464 return true; 465 } else { 466 return false; 467 } 468 } 469 } 470 } 471 472 /** 473 * Constructs a new node. Uses relaxed write because item can 474 * only be seen after publication via casNext. 475 */ 476 Node(Object item, boolean isData) { 477 this.item = item; 478 this.isData = isData; 479 } 480 481 /** 482 * Links node to itself to avoid garbage retention. Called 483 * only after CASing head field, so uses relaxed write. 484 */ 485 void forgetNext() { 486 this.next = this; 487 } 488 489 /** 490 * Sets item to self and waiter to null, to avoid garbage 491 * retention after matching or cancelling. Uses relaxed writes 492 * bacause order is already constrained in the only calling 493 * contexts: item is forgotten only after volatile/atomic 494 * mechanics that extract items. Similarly, clearing waiter 495 * follows either CAS or return from park (if ever parked; 496 * else we don't care). 497 */ 498 void forgetContents() { 499 this.item = this; 500 this.waiter = null; 501 } 502 503 /** 504 * Returns true if this node has been matched, including the 505 * case of artificial matches due to cancellation. 506 */ 507 boolean isMatched() { 508 Object x = item; 509 return x == this || x == null == isData; 510 } 511 512 /** 513 * Returns true if this is an unmatched request node. 514 */ 515 boolean isUnmatchedRequest() { 516 return !isData && item == null; 517 } 518 519 /** 520 * Returns true if a node with the given mode cannot be 521 * appended to this node because this node is unmatched and 522 * has opposite data mode. 523 */ 524 boolean cannotPrecede(boolean haveData) { 525 boolean d = isData; 526 Object x; 527 return d != haveData && (x = item) != this && x != null == d; 528 } 529 530 /** 531 * Tries to artificially match a data node -- used by remove. 532 */ 533 boolean tryMatchData() { 534 // assert isData; 535 Object x = item; 536 if (x != null && x != this && casItem(x, null)) { 537 LockSupport.unpark(waiter); 538 return true; 539 } 540 return false; 541 } 542 543 private static final AtomicReferenceFieldUpdater<Node, Node> nextUpdater = 544 AtomicFieldUpdaterUtil.newRefUpdater(Node.class, Node.class, "next"); 545 private static final AtomicReferenceFieldUpdater<Node, Object> itemUpdater = 546 AtomicFieldUpdaterUtil.newRefUpdater(Node.class, Object.class, "item"); 547 548 } 549 550 /** head of the queue; null until first enqueue */ 551 transient volatile Node head; 552 553 /** tail of the queue; null until first append */ 554 transient volatile Node tail; 555 556 /** The number of apparent failures to unsplice removed nodes */ 557 transient volatile int sweepVotes; 558 559 // CAS methods for fields 560 private boolean casTail(Node cmp, Node val) { 561 if (AtomicFieldUpdaterUtil.isAvailable()) { 562 return tailUpdater.compareAndSet(this, cmp, val); 563 } else { 564 synchronized (this) { 565 if (tail == cmp) { 566 tail = val; 567 return true; 568 } else { 569 return false; 570 } 571 } 572 } 573 } 574 575 private boolean casHead(Node cmp, Node val) { 576 if (AtomicFieldUpdaterUtil.isAvailable()) { 577 return headUpdater.compareAndSet(this, cmp, val); 578 } else { 579 synchronized (this) { 580 if (head == cmp) { 581 head = val; 582 return true; 583 } else { 584 return false; 585 } 586 } 587 } 588 } 589 590 private boolean casSweepVotes(int cmp, int val) { 591 if (AtomicFieldUpdaterUtil.isAvailable()) { 592 return sweepVotesUpdater.compareAndSet(this, cmp, val); 593 } else { 594 synchronized (this) { 595 if (sweepVotes == cmp) { 596 sweepVotes = val; 597 return true; 598 } else { 599 return false; 600 } 601 } 602 } 603 } 604 605 /* 606 * Possible values for "how" argument in xfer method. 607 */ 608 private static final int NOW = 0; // for untimed poll, tryTransfer 609 private static final int ASYNC = 1; // for offer, put, add 610 private static final int SYNC = 2; // for transfer, take 611 private static final int TIMED = 3; // for timed poll, tryTransfer 612 613 @SuppressWarnings("unchecked") 614 static <E> E cast(Object item) { 615 // assert item == null || item.getClass() != Node.class; 616 // Explicit cast, see http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6302954 617 return (E) item; 618 } 619 620 /** 621 * Implements all queuing methods. See above for explanation. 622 * 623 * @param e the item or null for take 624 * @param haveData true if this is a put, else a take 625 * @param how NOW, ASYNC, SYNC, or TIMED 626 * @param nanos timeout in nanosecs, used only if mode is TIMED 627 * @return an item if matched, else e 628 * @throws NullPointerException if haveData mode but e is null 629 */ 630 private E xfer(E e, boolean haveData, int how, long nanos) { 631 if (haveData && e == null) { 632 throw new NullPointerException(); 633 } 634 Node s = null; // the node to append, if needed 635 636 retry: for (;;) { // restart on append race 637 638 for (Node h = head, p = h; p != null;) { // find & match first node 639 boolean isData = p.isData; 640 Object item = p.item; 641 if (item != p && item != null == isData) { // unmatched 642 if (isData == haveData) { // can't match 643 break; 644 } 645 if (p.casItem(item, e)) { // match 646 for (Node q = p; q != h;) { 647 Node n = q.next; // update by 2 unless singleton 648 if (head == h && casHead(h, n == null? q : n)) { 649 h.forgetNext(); 650 break; 651 } // advance and retry 652 if ((h = head) == null || 653 (q = h.next) == null || !q.isMatched()) { 654 break; // unless slack < 2 655 } 656 } 657 LockSupport.unpark(p.waiter); 658 // Explicit cast, see http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=6302954 659 return LegacyLinkedTransferQueue.cast(item); 660 } 661 } 662 Node n = p.next; 663 p = p != n ? n : (h = head); // Use head if p offlist 664 } 665 666 if (how != NOW) { // No matches available 667 if (s == null) { 668 s = new Node(e, haveData); 669 } 670 Node pred = tryAppend(s, haveData); 671 if (pred == null) { 672 continue retry; // lost race vs opposite mode 673 } 674 if (how != ASYNC) { 675 return awaitMatch(s, pred, e, how == TIMED, nanos); 676 } 677 } 678 return e; // not waiting 679 } 680 } 681 682 /** 683 * Tries to append node s as tail. 684 * 685 * @param s the node to append 686 * @param haveData true if appending in data mode 687 * @return null on failure due to losing race with append in 688 * different mode, else s's predecessor, or s itself if no 689 * predecessor 690 */ 691 private Node tryAppend(Node s, boolean haveData) { 692 for (Node t = tail, p = t;;) { // move p to last node and append 693 Node n, u; // temps for reads of next & tail 694 if (p == null && (p = head) == null) { 695 if (casHead(null, s)) { 696 return s; // initialize 697 } 698 } 699 else if (p.cannotPrecede(haveData)) { 700 return null; // lost race vs opposite mode 701 } else if ((n = p.next) != null) { // not last; keep traversing 702 p = p != t && t != (u = tail) ? (t = u) : // stale tail 703 p != n ? n : null; // restart if off list 704 } else if (!p.casNext(null, s)) { 705 p = p.next; // re-read on CAS failure 706 } else { 707 if (p != t) { // update if slack now >= 2 708 while ((tail != t || !casTail(t, s)) && 709 (t = tail) != null && 710 (s = t.next) != null && // advance and retry 711 (s = s.next) != null && s != t) { 712 continue; 713 } 714 } 715 return p; 716 } 717 } 718 } 719 720 /** 721 * Spins/yields/blocks until node s is matched or caller gives up. 722 * 723 * @param s the waiting node 724 * @param pred the predecessor of s, or s itself if it has no 725 * predecessor, or null if unknown (the null case does not occur 726 * in any current calls but may in possible future extensions) 727 * @param e the comparison value for checking match 728 * @param timed if true, wait only until timeout elapses 729 * @param nanos timeout in nanosecs, used only if timed is true 730 * @return matched item, or e if unmatched on interrupt or timeout 731 */ 732 private E awaitMatch(Node s, Node pred, E e, boolean timed, long nanos) { 733 long lastTime = timed ? System.nanoTime() : 0L; 734 Thread w = Thread.currentThread(); 735 int spins = -1; // initialized after first item and cancel checks 736 ThreadLocalRandom randomYields = null; // bound if needed 737 738 for (;;) { 739 Object item = s.item; 740 if (item != e) { // matched 741 // assert item != s; 742 s.forgetContents(); // avoid garbage 743 return LegacyLinkedTransferQueue.cast(item); 744 } 745 if ((w.isInterrupted() || timed && nanos <= 0) && 746 s.casItem(e, s)) { // cancel 747 unsplice(pred, s); 748 return e; 749 } 750 751 if (spins < 0) { // establish spins at/near front 752 if ((spins = spinsFor(pred, s.isData)) > 0) { 753 randomYields = ThreadLocalRandom.current(); 754 } 755 } 756 else if (spins > 0) { // spin 757 --spins; 758 if (randomYields.nextInt(CHAINED_SPINS) == 0) { 759 Thread.yield(); // occasionally yield 760 } 761 } 762 else if (s.waiter == null) { 763 s.waiter = w; // request unpark then recheck 764 } 765 else if (timed) { 766 long now = System.nanoTime(); 767 if ((nanos -= now - lastTime) > 0) { 768 LockSupport.parkNanos(nanos); 769 } 770 lastTime = now; 771 } 772 else { 773 LockSupport.park(); 774 } 775 } 776 } 777 778 /** 779 * Returns spin/yield value for a node with given predecessor and 780 * data mode. See above for explanation. 781 */ 782 private static int spinsFor(Node pred, boolean haveData) { 783 if (MP && pred != null) { 784 if (pred.isData != haveData) { // phase change 785 return FRONT_SPINS + CHAINED_SPINS; 786 } 787 if (pred.isMatched()) { // probably at front 788 return FRONT_SPINS; 789 } 790 if (pred.waiter == null) { // pred apparently spinning 791 return CHAINED_SPINS; 792 } 793 } 794 return 0; 795 } 796 797 /* -------------- Traversal methods -------------- */ 798 799 /** 800 * Returns the successor of p, or the head node if p.next has been 801 * linked to self, which will only be true if traversing with a 802 * stale pointer that is now off the list. 803 */ 804 final Node succ(Node p) { 805 Node next = p.next; 806 return p == next ? head : next; 807 } 808 809 /** 810 * Returns the first unmatched node of the given mode, or null if 811 * none. Used by methods isEmpty, hasWaitingConsumer. 812 */ 813 private Node firstOfMode(boolean isData) { 814 for (Node p = head; p != null; p = succ(p)) { 815 if (!p.isMatched()) { 816 return p.isData == isData ? p : null; 817 } 818 } 819 return null; 820 } 821 822 /** 823 * Returns the item in the first unmatched node with isData; or 824 * null if none. Used by peek. 825 */ 826 private E firstDataItem() { 827 for (Node p = head; p != null; p = succ(p)) { 828 Object item = p.item; 829 if (p.isData) { 830 if (item != null && item != p) { 831 return LegacyLinkedTransferQueue.cast(item); 832 } 833 } 834 else if (item == null) { 835 return null; 836 } 837 } 838 return null; 839 } 840 841 /** 842 * Traverses and counts unmatched nodes of the given mode. 843 * Used by methods size and getWaitingConsumerCount. 844 */ 845 private int countOfMode(boolean data) { 846 int count = 0; 847 for (Node p = head; p != null; ) { 848 if (!p.isMatched()) { 849 if (p.isData != data) { 850 return 0; 851 } 852 if (++count == Integer.MAX_VALUE) { // saturated 853 break; 854 } 855 } 856 Node n = p.next; 857 if (n != p) { 858 p = n; 859 } else { 860 count = 0; 861 p = head; 862 } 863 } 864 return count; 865 } 866 867 final class Itr implements Iterator<E> { 868 private Node nextNode; // next node to return item for 869 private E nextItem; // the corresponding item 870 private Node lastRet; // last returned node, to support remove 871 private Node lastPred; // predecessor to unlink lastRet 872 873 /** 874 * Moves to next node after prev, or first node if prev null. 875 */ 876 private void advance(Node prev) { 877 lastPred = lastRet; 878 lastRet = prev; 879 for (Node p = prev == null ? head : succ(prev); 880 p != null; p = succ(p)) { 881 Object item = p.item; 882 if (p.isData) { 883 if (item != null && item != p) { 884 nextItem = LegacyLinkedTransferQueue.cast(item); 885 nextNode = p; 886 return; 887 } 888 } 889 else if (item == null) { 890 break; 891 } 892 } 893 nextNode = null; 894 } 895 896 Itr() { 897 advance(null); 898 } 899 900 public boolean hasNext() { 901 return nextNode != null; 902 } 903 904 public E next() { 905 Node p = nextNode; 906 if (p == null) { 907 throw new NoSuchElementException(); 908 } 909 E e = nextItem; 910 advance(p); 911 return e; 912 } 913 914 public void remove() { 915 Node p = lastRet; 916 if (p == null) { 917 throw new IllegalStateException(); 918 } 919 if (p.tryMatchData()) { 920 unsplice(lastPred, p); 921 } 922 } 923 } 924 925 /* -------------- Removal methods -------------- */ 926 927 /** 928 * Unsplices (now or later) the given deleted/cancelled node with 929 * the given predecessor. 930 * 931 * @param pred a node that was at one time known to be the 932 * predecessor of s, or null or s itself if s is/was at head 933 * @param s the node to be unspliced 934 */ 935 final void unsplice(Node pred, Node s) { 936 s.forgetContents(); // forget unneeded fields 937 /* 938 * See above for rationale. Briefly: if pred still points to 939 * s, try to unlink s. If s cannot be unlinked, because it is 940 * trailing node or pred might be unlinked, and neither pred 941 * nor s are head or offlist, add to sweepVotes, and if enough 942 * votes have accumulated, sweep. 943 */ 944 if (pred != null && pred != s && pred.next == s) { 945 Node n = s.next; 946 if (n == null || 947 n != s && pred.casNext(s, n) && pred.isMatched()) { 948 for (;;) { // check if at, or could be, head 949 Node h = head; 950 if (h == pred || h == s || h == null) { 951 return; // at head or list empty 952 } 953 if (!h.isMatched()) { 954 break; 955 } 956 Node hn = h.next; 957 if (hn == null) { 958 return; // now empty 959 } 960 if (hn != h && casHead(h, hn)) { 961 h.forgetNext(); // advance head 962 } 963 } 964 if (pred.next != pred && s.next != s) { // recheck if offlist 965 for (;;) { // sweep now if enough votes 966 int v = sweepVotes; 967 if (v < SWEEP_THRESHOLD) { 968 if (casSweepVotes(v, v + 1)) { 969 break; 970 } 971 } 972 else if (casSweepVotes(v, 0)) { 973 sweep(); 974 break; 975 } 976 } 977 } 978 } 979 } 980 } 981 982 /** 983 * Unlinks matched (typically cancelled) nodes encountered in a 984 * traversal from head. 985 */ 986 private void sweep() { 987 for (Node p = head, s, n; p != null && (s = p.next) != null; ) { 988 if (!s.isMatched()) { 989 // Unmatched nodes are never self-linked 990 p = s; 991 } else if ((n = s.next) == null) { // trailing node is pinned 992 break; 993 } else if (s == n) { // stale 994 // No need to also check for p == s, since that implies s == n 995 p = head; 996 } else { 997 p.casNext(s, n); 998 } 999 } 1000 } 1001 1002 /** 1003 * Main implementation of remove(Object) 1004 */ 1005 private boolean findAndRemove(Object e) { 1006 if (e != null) { 1007 for (Node pred = null, p = head; p != null; ) { 1008 Object item = p.item; 1009 if (p.isData) { 1010 if (item != null && item != p && e.equals(item) && 1011 p.tryMatchData()) { 1012 unsplice(pred, p); 1013 return true; 1014 } 1015 } 1016 else if (item == null) { 1017 break; 1018 } 1019 pred = p; 1020 if ((p = p.next) == pred) { // stale 1021 pred = null; 1022 p = head; 1023 } 1024 } 1025 } 1026 return false; 1027 } 1028 1029 1030 /** 1031 * Creates an initially empty {@code LinkedTransferQueue}. 1032 */ 1033 public LegacyLinkedTransferQueue() { 1034 } 1035 1036 /** 1037 * Creates a {@code LinkedTransferQueue} 1038 * initially containing the elements of the given collection, 1039 * added in traversal order of the collection's iterator. 1040 * 1041 * @param c the collection of elements to initially contain 1042 * @throws NullPointerException if the specified collection or any 1043 * of its elements are null 1044 */ 1045 public LegacyLinkedTransferQueue(Collection<? extends E> c) { 1046 this(); 1047 addAll(c); 1048 } 1049 1050 /** 1051 * Inserts the specified element at the tail of this queue. 1052 * As the queue is unbounded, this method will never block. 1053 * 1054 * @throws NullPointerException if the specified element is null 1055 */ 1056 public void put(E e) { 1057 xfer(e, true, ASYNC, 0); 1058 } 1059 1060 /** 1061 * Inserts the specified element at the tail of this queue. 1062 * As the queue is unbounded, this method will never block or 1063 * return {@code false}. 1064 * 1065 * @return {@code true} (as specified by 1066 * {@link BlockingQueue#offer(Object,long,TimeUnit) BlockingQueue.offer}) 1067 * @throws NullPointerException if the specified element is null 1068 */ 1069 public boolean offer(E e, long timeout, TimeUnit unit) { 1070 xfer(e, true, ASYNC, 0); 1071 return true; 1072 } 1073 1074 /** 1075 * Inserts the specified element at the tail of this queue. 1076 * As the queue is unbounded, this method will never return {@code false}. 1077 * 1078 * @return {@code true} (as specified by 1079 * {@link BlockingQueue#offer(Object) BlockingQueue.offer}) 1080 * @throws NullPointerException if the specified element is null 1081 */ 1082 public boolean offer(E e) { 1083 xfer(e, true, ASYNC, 0); 1084 return true; 1085 } 1086 1087 /** 1088 * Inserts the specified element at the tail of this queue. 1089 * As the queue is unbounded, this method will never throw 1090 * {@link IllegalStateException} or return {@code false}. 1091 * 1092 * @return {@code true} (as specified by {@link Collection#add}) 1093 * @throws NullPointerException if the specified element is null 1094 */ 1095 @Override 1096 public boolean add(E e) { 1097 xfer(e, true, ASYNC, 0); 1098 return true; 1099 } 1100 1101 /** 1102 * Transfers the element to a waiting consumer immediately, if possible. 1103 * 1104 * <p>More precisely, transfers the specified element immediately 1105 * if there exists a consumer already waiting to receive it (in 1106 * {@link #take} or timed {@link #poll(long,TimeUnit) poll}), 1107 * otherwise returning {@code false} without enqueuing the element. 1108 * 1109 * @throws NullPointerException if the specified element is null 1110 */ 1111 public boolean tryTransfer(E e) { 1112 return xfer(e, true, NOW, 0) == null; 1113 } 1114 1115 /** 1116 * Transfers the element to a consumer, waiting if necessary to do so. 1117 * 1118 * <p>More precisely, transfers the specified element immediately 1119 * if there exists a consumer already waiting to receive it (in 1120 * {@link #take} or timed {@link #poll(long,TimeUnit) poll}), 1121 * else inserts the specified element at the tail of this queue 1122 * and waits until the element is received by a consumer. 1123 * 1124 * @throws NullPointerException if the specified element is null 1125 */ 1126 public void transfer(E e) throws InterruptedException { 1127 if (xfer(e, true, SYNC, 0) != null) { 1128 Thread.interrupted(); // failure possible only due to interrupt 1129 throw new InterruptedException(); 1130 } 1131 } 1132 1133 /** 1134 * Transfers the element to a consumer if it is possible to do so 1135 * before the timeout elapses. 1136 * 1137 * <p>More precisely, transfers the specified element immediately 1138 * if there exists a consumer already waiting to receive it (in 1139 * {@link #take} or timed {@link #poll(long,TimeUnit) poll}), 1140 * else inserts the specified element at the tail of this queue 1141 * and waits until the element is received by a consumer, 1142 * returning {@code false} if the specified wait time elapses 1143 * before the element can be transferred. 1144 * 1145 * @throws NullPointerException if the specified element is null 1146 */ 1147 public boolean tryTransfer(E e, long timeout, TimeUnit unit) 1148 throws InterruptedException { 1149 if (xfer(e, true, TIMED, unit.toNanos(timeout)) == null) { 1150 return true; 1151 } 1152 if (!Thread.interrupted()) { 1153 return false; 1154 } 1155 throw new InterruptedException(); 1156 } 1157 1158 public E take() throws InterruptedException { 1159 E e = xfer(null, false, SYNC, 0); 1160 if (e != null) { 1161 return e; 1162 } 1163 Thread.interrupted(); 1164 throw new InterruptedException(); 1165 } 1166 1167 public E poll(long timeout, TimeUnit unit) throws InterruptedException { 1168 E e = xfer(null, false, TIMED, unit.toNanos(timeout)); 1169 if (e != null || !Thread.interrupted()) { 1170 return e; 1171 } 1172 throw new InterruptedException(); 1173 } 1174 1175 public E poll() { 1176 return xfer(null, false, NOW, 0); 1177 } 1178 1179 /** 1180 * @throws NullPointerException {@inheritDoc} 1181 * @throws IllegalArgumentException {@inheritDoc} 1182 */ 1183 public int drainTo(Collection<? super E> c) { 1184 if (c == null) { 1185 throw new NullPointerException(); 1186 } 1187 if (c == this) { 1188 throw new IllegalArgumentException(); 1189 } 1190 int n = 0; 1191 E e; 1192 while ( (e = poll()) != null) { 1193 c.add(e); 1194 ++n; 1195 } 1196 return n; 1197 } 1198 1199 /** 1200 * @throws NullPointerException {@inheritDoc} 1201 * @throws IllegalArgumentException {@inheritDoc} 1202 */ 1203 public int drainTo(Collection<? super E> c, int maxElements) { 1204 if (c == null) { 1205 throw new NullPointerException(); 1206 } 1207 if (c == this) { 1208 throw new IllegalArgumentException(); 1209 } 1210 int n = 0; 1211 E e; 1212 while (n < maxElements && (e = poll()) != null) { 1213 c.add(e); 1214 ++n; 1215 } 1216 return n; 1217 } 1218 1219 /** 1220 * Returns an iterator over the elements in this queue in proper 1221 * sequence, from head to tail. 1222 * 1223 * <p>The returned iterator is a "weakly consistent" iterator that 1224 * will never throw 1225 * {@link ConcurrentModificationException ConcurrentModificationException}, 1226 * and guarantees to traverse elements as they existed upon 1227 * construction of the iterator, and may (but is not guaranteed 1228 * to) reflect any modifications subsequent to construction. 1229 * 1230 * @return an iterator over the elements in this queue in proper sequence 1231 */ 1232 @Override 1233 public Iterator<E> iterator() { 1234 return new Itr(); 1235 } 1236 1237 public E peek() { 1238 return firstDataItem(); 1239 } 1240 1241 /** 1242 * Returns {@code true} if this queue contains no elements. 1243 * 1244 * @return {@code true} if this queue contains no elements 1245 */ 1246 @Override 1247 public boolean isEmpty() { 1248 for (Node p = head; p != null; p = succ(p)) { 1249 if (!p.isMatched()) { 1250 return !p.isData; 1251 } 1252 } 1253 return true; 1254 } 1255 1256 public boolean hasWaitingConsumer() { 1257 return firstOfMode(false) != null; 1258 } 1259 1260 /** 1261 * Returns the number of elements in this queue. If this queue 1262 * contains more than {@code Integer.MAX_VALUE} elements, returns 1263 * {@code Integer.MAX_VALUE}. 1264 * 1265 * <p>Beware that, unlike in most collections, this method is 1266 * <em>NOT</em> a constant-time operation. Because of the 1267 * asynchronous nature of these queues, determining the current 1268 * number of elements requires an O(n) traversal. 1269 * 1270 * @return the number of elements in this queue 1271 */ 1272 @Override 1273 public int size() { 1274 return countOfMode(true); 1275 } 1276 1277 public int getWaitingConsumerCount() { 1278 return countOfMode(false); 1279 } 1280 1281 /** 1282 * Removes a single instance of the specified element from this queue, 1283 * if it is present. More formally, removes an element {@code e} such 1284 * that {@code o.equals(e)}, if this queue contains one or more such 1285 * elements. 1286 * Returns {@code true} if this queue contained the specified element 1287 * (or equivalently, if this queue changed as a result of the call). 1288 * 1289 * @param o element to be removed from this queue, if present 1290 * @return {@code true} if this queue changed as a result of the call 1291 */ 1292 @Override 1293 public boolean remove(Object o) { 1294 return findAndRemove(o); 1295 } 1296 1297 /** 1298 * Always returns {@code Integer.MAX_VALUE} because a 1299 * {@code LinkedTransferQueue} is not capacity constrained. 1300 * 1301 * @return {@code Integer.MAX_VALUE} (as specified by 1302 * {@link BlockingQueue#remainingCapacity()}) 1303 */ 1304 public int remainingCapacity() { 1305 return Integer.MAX_VALUE; 1306 } 1307 1308 /** 1309 * Saves the state to a stream (that is, serializes it). 1310 * 1311 * @serialData All of the elements (each an {@code E}) in 1312 * the proper order, followed by a null 1313 * @param s the stream 1314 */ 1315 private void writeObject(java.io.ObjectOutputStream s) 1316 throws java.io.IOException { 1317 s.defaultWriteObject(); 1318 for (E e : this) { 1319 s.writeObject(e); 1320 } 1321 // Use trailing null as sentinel 1322 s.writeObject(null); 1323 } 1324 1325 /** 1326 * Reconstitutes the Queue instance from a stream (that is, 1327 * deserializes it). 1328 * 1329 * @param s the stream 1330 */ 1331 private void readObject(java.io.ObjectInputStream s) 1332 throws java.io.IOException, ClassNotFoundException { 1333 s.defaultReadObject(); 1334 for (;;) { 1335 E item = (E) s.readObject(); 1336 if (item == null) { 1337 break; 1338 } else { 1339 offer(item); 1340 } 1341 } 1342 } 1343 1344 @SuppressWarnings("rawtypes") 1345 private static final AtomicReferenceFieldUpdater<LegacyLinkedTransferQueue, Node> headUpdater = 1346 AtomicFieldUpdaterUtil.newRefUpdater(LegacyLinkedTransferQueue.class, Node.class, "head"); 1347 @SuppressWarnings("rawtypes") 1348 private static final AtomicReferenceFieldUpdater<LegacyLinkedTransferQueue, Node> tailUpdater = 1349 AtomicFieldUpdaterUtil.newRefUpdater(LegacyLinkedTransferQueue.class, Node.class, "tail"); 1350 @SuppressWarnings("rawtypes") 1351 private static final AtomicIntegerFieldUpdater<LegacyLinkedTransferQueue> sweepVotesUpdater = 1352 AtomicFieldUpdaterUtil.newIntUpdater(LegacyLinkedTransferQueue.class, "sweepVotes"); 1353 } 1354