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| 1 | /* | ||
| 2 | * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. | ||
| 3 | * | ||
| 4 | * This code is free software; you can redistribute it and/or modify it | ||
| 5 | * under the terms of the GNU General Public License version 2 only, as | ||
| 6 | * published by the Free Software Foundation. Oracle designates this | ||
| 7 | * particular file as subject to the "Classpath" exception as provided | ||
| 8 | * by Oracle in the LICENSE file that accompanied this code. | ||
| 9 | * | ||
| 10 | * This code is distributed in the hope that it will be useful, but WITHOUT | ||
| 11 | * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or | ||
| 12 | * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License | ||
| 13 | * version 2 for more details (a copy is included in the LICENSE file that | ||
| 14 | * accompanied this code). | ||
| 15 | * | ||
| 16 | * You should have received a copy of the GNU General Public License version | ||
| 17 | * 2 along with this work; if not, write to the Free Software Foundation, | ||
| 18 | * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. | ||
| 19 | * | ||
| 20 | * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA | ||
| 21 | * or visit www.oracle.com if you need additional information or have any | ||
| 22 | * questions. | ||
| 23 | */ | ||
| 24 | |||
| 25 | /* | ||
| 26 | * This file is available under and governed by the GNU General Public | ||
| 27 | * License version 2 only, as published by the Free Software Foundation. | ||
| 28 | * However, the following notice accompanied the original version of this | ||
| 29 | * file: | ||
| 30 | * | ||
| 31 | * Written by Doug Lea with assistance from members of JCP JSR-166 | ||
| 32 | * Expert Group and released to the public domain, as explained at | ||
| 33 | * http://creativecommons.org/publicdomain/zero/1.0/ | ||
| 34 | */ | ||
| 35 | |||
| 36 | package java.util.concurrent; | ||
| 37 | |||
| 38 | import java.util.AbstractQueue; | ||
| 39 | import java.util.Collection; | ||
| 40 | import java.util.Iterator; | ||
| 41 | import java.util.NoSuchElementException; | ||
| 42 | import java.util.Queue; | ||
| 43 | import java.util.concurrent.TimeUnit; | ||
| 44 | import java.util.concurrent.locks.LockSupport; | ||
| 45 | |||
| 46 | /** | ||
| 47 | * An unbounded {@link TransferQueue} based on linked nodes. | ||
| 48 | * This queue orders elements FIFO (first-in-first-out) with respect | ||
| 49 | * to any given producer. The <em>head</em> of the queue is that | ||
| 50 | * element that has been on the queue the longest time for some | ||
| 51 | * producer. The <em>tail</em> of the queue is that element that has | ||
| 52 | * been on the queue the shortest time for some producer. | ||
| 53 | * | ||
| 54 | * <p>Beware that, unlike in most collections, the {@code size} method | ||
| 55 | * is <em>NOT</em> a constant-time operation. Because of the | ||
| 56 | * asynchronous nature of these queues, determining the current number | ||
| 57 | * of elements requires a traversal of the elements, and so may report | ||
| 58 | * inaccurate results if this collection is modified during traversal. | ||
| 59 | * Additionally, the bulk operations {@code addAll}, | ||
| 60 | * {@code removeAll}, {@code retainAll}, {@code containsAll}, | ||
| 61 | * {@code equals}, and {@code toArray} are <em>not</em> guaranteed | ||
| 62 | * to be performed atomically. For example, an iterator operating | ||
| 63 | * concurrently with an {@code addAll} operation might view only some | ||
| 64 | * of the added elements. | ||
| 65 | * | ||
| 66 | * <p>This class and its iterator implement all of the | ||
| 67 | * <em>optional</em> methods of the {@link Collection} and {@link | ||
| 68 | * Iterator} interfaces. | ||
| 69 | * | ||
| 70 | * <p>Memory consistency effects: As with other concurrent | ||
| 71 | * collections, actions in a thread prior to placing an object into a | ||
| 72 | * {@code LinkedTransferQueue} | ||
| 73 | * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> | ||
| 74 | * actions subsequent to the access or removal of that element from | ||
| 75 | * the {@code LinkedTransferQueue} in another thread. | ||
| 76 | * | ||
| 77 | * <p>This class is a member of the | ||
| 78 | * <a href="{@docRoot}/../technotes/guides/collections/index.html"> | ||
| 79 | * Java Collections Framework</a>. | ||
| 80 | * | ||
| 81 | * @since 1.7 | ||
| 82 | * @author Doug Lea | ||
| 83 | * @param <E> the type of elements held in this collection | ||
| 84 | */ | ||
| 85 | public class LinkedTransferQueue<E> extends AbstractQueue<E> | ||
| 86 | implements TransferQueue<E>, java.io.Serializable { | ||
| 87 | private static final long serialVersionUID = -3223113410248163686L; | ||
| 88 | |||
| 89 | /* | ||
| 90 | * *** Overview of Dual Queues with Slack *** | ||
| 91 | * | ||
| 92 | * Dual Queues, introduced by Scherer and Scott | ||
| 93 | * (http://www.cs.rice.edu/~wns1/papers/2004-DISC-DDS.pdf) are | ||
| 94 | * (linked) queues in which nodes may represent either data or | ||
| 95 | * requests. When a thread tries to enqueue a data node, but | ||
| 96 | * encounters a request node, it instead "matches" and removes it; | ||
| 97 | * and vice versa for enqueuing requests. Blocking Dual Queues | ||
| 98 | * arrange that threads enqueuing unmatched requests block until | ||
| 99 | * other threads provide the match. Dual Synchronous Queues (see | ||
| 100 | * Scherer, Lea, & Scott | ||
| 101 | * http://www.cs.rochester.edu/u/scott/papers/2009_Scherer_CACM_SSQ.pdf) | ||
| 102 | * additionally arrange that threads enqueuing unmatched data also | ||
| 103 | * block. Dual Transfer Queues support all of these modes, as | ||
| 104 | * dictated by callers. | ||
| 105 | * | ||
| 106 | * A FIFO dual queue may be implemented using a variation of the | ||
| 107 | * Michael & Scott (M&S) lock-free queue algorithm | ||
| 108 | * (http://www.cs.rochester.edu/u/scott/papers/1996_PODC_queues.pdf). | ||
| 109 | * It maintains two pointer fields, "head", pointing to a | ||
| 110 | * (matched) node that in turn points to the first actual | ||
| 111 | * (unmatched) queue node (or null if empty); and "tail" that | ||
| 112 | * points to the last node on the queue (or again null if | ||
| 113 | * empty). For example, here is a possible queue with four data | ||
| 114 | * elements: | ||
| 115 | * | ||
| 116 | * head tail | ||
| 117 | * | | | ||
| 118 | * v v | ||
| 119 | * M -> U -> U -> U -> U | ||
| 120 | * | ||
| 121 | * The M&S queue algorithm is known to be prone to scalability and | ||
| 122 | * overhead limitations when maintaining (via CAS) these head and | ||
| 123 | * tail pointers. This has led to the development of | ||
| 124 | * contention-reducing variants such as elimination arrays (see | ||
| 125 | * Moir et al http://portal.acm.org/citation.cfm?id=1074013) and | ||
| 126 | * optimistic back pointers (see Ladan-Mozes & Shavit | ||
| 127 | * http://people.csail.mit.edu/edya/publications/OptimisticFIFOQueue-journal.pdf). | ||
| 128 | * However, the nature of dual queues enables a simpler tactic for | ||
| 129 | * improving M&S-style implementations when dual-ness is needed. | ||
| 130 | * | ||
| 131 | * In a dual queue, each node must atomically maintain its match | ||
| 132 | * status. While there are other possible variants, we implement | ||
| 133 | * this here as: for a data-mode node, matching entails CASing an | ||
| 134 | * "item" field from a non-null data value to null upon match, and | ||
| 135 | * vice-versa for request nodes, CASing from null to a data | ||
| 136 | * value. (Note that the linearization properties of this style of | ||
| 137 | * queue are easy to verify -- elements are made available by | ||
| 138 | * linking, and unavailable by matching.) Compared to plain M&S | ||
| 139 | * queues, this property of dual queues requires one additional | ||
| 140 | * successful atomic operation per enq/deq pair. But it also | ||
| 141 | * enables lower cost variants of queue maintenance mechanics. (A | ||
| 142 | * variation of this idea applies even for non-dual queues that | ||
| 143 | * support deletion of interior elements, such as | ||
| 144 | * j.u.c.ConcurrentLinkedQueue.) | ||
| 145 | * | ||
| 146 | * Once a node is matched, its match status can never again | ||
| 147 | * change. We may thus arrange that the linked list of them | ||
| 148 | * contain a prefix of zero or more matched nodes, followed by a | ||
| 149 | * suffix of zero or more unmatched nodes. (Note that we allow | ||
| 150 | * both the prefix and suffix to be zero length, which in turn | ||
| 151 | * means that we do not use a dummy header.) If we were not | ||
| 152 | * concerned with either time or space efficiency, we could | ||
| 153 | * correctly perform enqueue and dequeue operations by traversing | ||
| 154 | * from a pointer to the initial node; CASing the item of the | ||
| 155 | * first unmatched node on match and CASing the next field of the | ||
| 156 | * trailing node on appends. (Plus some special-casing when | ||
| 157 | * initially empty). While this would be a terrible idea in | ||
| 158 | * itself, it does have the benefit of not requiring ANY atomic | ||
| 159 | * updates on head/tail fields. | ||
| 160 | * | ||
| 161 | * We introduce here an approach that lies between the extremes of | ||
| 162 | * never versus always updating queue (head and tail) pointers. | ||
| 163 | * This offers a tradeoff between sometimes requiring extra | ||
| 164 | * traversal steps to locate the first and/or last unmatched | ||
| 165 | * nodes, versus the reduced overhead and contention of fewer | ||
| 166 | * updates to queue pointers. For example, a possible snapshot of | ||
| 167 | * a queue is: | ||
| 168 | * | ||
| 169 | * head tail | ||
| 170 | * | | | ||
| 171 | * v v | ||
| 172 | * M -> M -> U -> U -> U -> U | ||
| 173 | * | ||
| 174 | * The best value for this "slack" (the targeted maximum distance | ||
| 175 | * between the value of "head" and the first unmatched node, and | ||
| 176 | * similarly for "tail") is an empirical matter. We have found | ||
| 177 | * that using very small constants in the range of 1-3 work best | ||
| 178 | * over a range of platforms. Larger values introduce increasing | ||
| 179 | * costs of cache misses and risks of long traversal chains, while | ||
| 180 | * smaller values increase CAS contention and overhead. | ||
| 181 | * | ||
| 182 | * Dual queues with slack differ from plain M&S dual queues by | ||
| 183 | * virtue of only sometimes updating head or tail pointers when | ||
| 184 | * matching, appending, or even traversing nodes; in order to | ||
| 185 | * maintain a targeted slack. The idea of "sometimes" may be | ||
| 186 | * operationalized in several ways. The simplest is to use a | ||
| 187 | * per-operation counter incremented on each traversal step, and | ||
| 188 | * to try (via CAS) to update the associated queue pointer | ||
| 189 | * whenever the count exceeds a threshold. Another, that requires | ||
| 190 | * more overhead, is to use random number generators to update | ||
| 191 | * with a given probability per traversal step. | ||
| 192 | * | ||
| 193 | * In any strategy along these lines, because CASes updating | ||
| 194 | * fields may fail, the actual slack may exceed targeted | ||
| 195 | * slack. However, they may be retried at any time to maintain | ||
| 196 | * targets. Even when using very small slack values, this | ||
| 197 | * approach works well for dual queues because it allows all | ||
| 198 | * operations up to the point of matching or appending an item | ||
| 199 | * (hence potentially allowing progress by another thread) to be | ||
| 200 | * read-only, thus not introducing any further contention. As | ||
| 201 | * described below, we implement this by performing slack | ||
| 202 | * maintenance retries only after these points. | ||
| 203 | * | ||
| 204 | * As an accompaniment to such techniques, traversal overhead can | ||
| 205 | * be further reduced without increasing contention of head | ||
| 206 | * pointer updates: Threads may sometimes shortcut the "next" link | ||
| 207 | * path from the current "head" node to be closer to the currently | ||
| 208 | * known first unmatched node, and similarly for tail. Again, this | ||
| 209 | * may be triggered with using thresholds or randomization. | ||
| 210 | * | ||
| 211 | * These ideas must be further extended to avoid unbounded amounts | ||
| 212 | * of costly-to-reclaim garbage caused by the sequential "next" | ||
| 213 | * links of nodes starting at old forgotten head nodes: As first | ||
| 214 | * described in detail by Boehm | ||
| 215 | * (http://portal.acm.org/citation.cfm?doid=503272.503282) if a GC | ||
| 216 | * delays noticing that any arbitrarily old node has become | ||
| 217 | * garbage, all newer dead nodes will also be unreclaimed. | ||
| 218 | * (Similar issues arise in non-GC environments.) To cope with | ||
| 219 | * this in our implementation, upon CASing to advance the head | ||
| 220 | * pointer, we set the "next" link of the previous head to point | ||
| 221 | * only to itself; thus limiting the length of connected dead lists. | ||
| 222 | * (We also take similar care to wipe out possibly garbage | ||
| 223 | * retaining values held in other Node fields.) However, doing so | ||
| 224 | * adds some further complexity to traversal: If any "next" | ||
| 225 | * pointer links to itself, it indicates that the current thread | ||
| 226 | * has lagged behind a head-update, and so the traversal must | ||
| 227 | * continue from the "head". Traversals trying to find the | ||
| 228 | * current tail starting from "tail" may also encounter | ||
| 229 | * self-links, in which case they also continue at "head". | ||
| 230 | * | ||
| 231 | * It is tempting in slack-based scheme to not even use CAS for | ||
| 232 | * updates (similarly to Ladan-Mozes & Shavit). However, this | ||
| 233 | * cannot be done for head updates under the above link-forgetting | ||
| 234 | * mechanics because an update may leave head at a detached node. | ||
| 235 | * And while direct writes are possible for tail updates, they | ||
| 236 | * increase the risk of long retraversals, and hence long garbage | ||
| 237 | * chains, which can be much more costly than is worthwhile | ||
| 238 | * considering that the cost difference of performing a CAS vs | ||
| 239 | * write is smaller when they are not triggered on each operation | ||
| 240 | * (especially considering that writes and CASes equally require | ||
| 241 | * additional GC bookkeeping ("write barriers") that are sometimes | ||
| 242 | * more costly than the writes themselves because of contention). | ||
| 243 | * | ||
| 244 | * *** Overview of implementation *** | ||
| 245 | * | ||
| 246 | * We use a threshold-based approach to updates, with a slack | ||
| 247 | * threshold of two -- that is, we update head/tail when the | ||
| 248 | * current pointer appears to be two or more steps away from the | ||
| 249 | * first/last node. The slack value is hard-wired: a path greater | ||
| 250 | * than one is naturally implemented by checking equality of | ||
| 251 | * traversal pointers except when the list has only one element, | ||
| 252 | * in which case we keep slack threshold at one. Avoiding tracking | ||
| 253 | * explicit counts across method calls slightly simplifies an | ||
| 254 | * already-messy implementation. Using randomization would | ||
| 255 | * probably work better if there were a low-quality dirt-cheap | ||
| 256 | * per-thread one available, but even ThreadLocalRandom is too | ||
| 257 | * heavy for these purposes. | ||
| 258 | * | ||
| 259 | * With such a small slack threshold value, it is not worthwhile | ||
| 260 | * to augment this with path short-circuiting (i.e., unsplicing | ||
| 261 | * interior nodes) except in the case of cancellation/removal (see | ||
| 262 | * below). | ||
| 263 | * | ||
| 264 | * We allow both the head and tail fields to be null before any | ||
| 265 | * nodes are enqueued; initializing upon first append. This | ||
| 266 | * simplifies some other logic, as well as providing more | ||
| 267 | * efficient explicit control paths instead of letting JVMs insert | ||
| 268 | * implicit NullPointerExceptions when they are null. While not | ||
| 269 | * currently fully implemented, we also leave open the possibility | ||
| 270 | * of re-nulling these fields when empty (which is complicated to | ||
| 271 | * arrange, for little benefit.) | ||
| 272 | * | ||
| 273 | * All enqueue/dequeue operations are handled by the single method | ||
| 274 | * "xfer" with parameters indicating whether to act as some form | ||
| 275 | * of offer, put, poll, take, or transfer (each possibly with | ||
| 276 | * timeout). The relative complexity of using one monolithic | ||
| 277 | * method outweighs the code bulk and maintenance problems of | ||
| 278 | * using separate methods for each case. | ||
| 279 | * | ||
| 280 | * Operation consists of up to three phases. The first is | ||
| 281 | * implemented within method xfer, the second in tryAppend, and | ||
| 282 | * the third in method awaitMatch. | ||
| 283 | * | ||
| 284 | * 1. Try to match an existing node | ||
| 285 | * | ||
| 286 | * Starting at head, skip already-matched nodes until finding | ||
| 287 | * an unmatched node of opposite mode, if one exists, in which | ||
| 288 | * case matching it and returning, also if necessary updating | ||
| 289 | * head to one past the matched node (or the node itself if the | ||
| 290 | * list has no other unmatched nodes). If the CAS misses, then | ||
| 291 | * a loop retries advancing head by two steps until either | ||
| 292 | * success or the slack is at most two. By requiring that each | ||
| 293 | * attempt advances head by two (if applicable), we ensure that | ||
| 294 | * the slack does not grow without bound. Traversals also check | ||
| 295 | * if the initial head is now off-list, in which case they | ||
| 296 | * start at the new head. | ||
| 297 | * | ||
| 298 | * If no candidates are found and the call was untimed | ||
| 299 | * poll/offer, (argument "how" is NOW) return. | ||
| 300 | * | ||
| 301 | * 2. Try to append a new node (method tryAppend) | ||
| 302 | * | ||
| 303 | * Starting at current tail pointer, find the actual last node | ||
| 304 | * and try to append a new node (or if head was null, establish | ||
| 305 | * the first node). Nodes can be appended only if their | ||
| 306 | * predecessors are either already matched or are of the same | ||
| 307 | * mode. If we detect otherwise, then a new node with opposite | ||
| 308 | * mode must have been appended during traversal, so we must | ||
| 309 | * restart at phase 1. The traversal and update steps are | ||
| 310 | * otherwise similar to phase 1: Retrying upon CAS misses and | ||
| 311 | * checking for staleness. In particular, if a self-link is | ||
| 312 | * encountered, then we can safely jump to a node on the list | ||
| 313 | * by continuing the traversal at current head. | ||
| 314 | * | ||
| 315 | * On successful append, if the call was ASYNC, return. | ||
| 316 | * | ||
| 317 | * 3. Await match or cancellation (method awaitMatch) | ||
| 318 | * | ||
| 319 | * Wait for another thread to match node; instead cancelling if | ||
| 320 | * the current thread was interrupted or the wait timed out. On | ||
| 321 | * multiprocessors, we use front-of-queue spinning: If a node | ||
| 322 | * appears to be the first unmatched node in the queue, it | ||
| 323 | * spins a bit before blocking. In either case, before blocking | ||
| 324 | * it tries to unsplice any nodes between the current "head" | ||
| 325 | * and the first unmatched node. | ||
| 326 | * | ||
| 327 | * Front-of-queue spinning vastly improves performance of | ||
| 328 | * heavily contended queues. And so long as it is relatively | ||
| 329 | * brief and "quiet", spinning does not much impact performance | ||
| 330 | * of less-contended queues. During spins threads check their | ||
| 331 | * interrupt status and generate a thread-local random number | ||
| 332 | * to decide to occasionally perform a Thread.yield. While | ||
| 333 | * yield has underdefined specs, we assume that might it help, | ||
| 334 | * and will not hurt in limiting impact of spinning on busy | ||
| 335 | * systems. We also use smaller (1/2) spins for nodes that are | ||
| 336 | * not known to be front but whose predecessors have not | ||
| 337 | * blocked -- these "chained" spins avoid artifacts of | ||
| 338 | * front-of-queue rules which otherwise lead to alternating | ||
| 339 | * nodes spinning vs blocking. Further, front threads that | ||
| 340 | * represent phase changes (from data to request node or vice | ||
| 341 | * versa) compared to their predecessors receive additional | ||
| 342 | * chained spins, reflecting longer paths typically required to | ||
| 343 | * unblock threads during phase changes. | ||
| 344 | * | ||
| 345 | * | ||
| 346 | * ** Unlinking removed interior nodes ** | ||
| 347 | * | ||
| 348 | * In addition to minimizing garbage retention via self-linking | ||
| 349 | * described above, we also unlink removed interior nodes. These | ||
| 350 | * may arise due to timed out or interrupted waits, or calls to | ||
| 351 | * remove(x) or Iterator.remove. Normally, given a node that was | ||
| 352 | * at one time known to be the predecessor of some node s that is | ||
| 353 | * to be removed, we can unsplice s by CASing the next field of | ||
| 354 | * its predecessor if it still points to s (otherwise s must | ||
| 355 | * already have been removed or is now offlist). But there are two | ||
| 356 | * situations in which we cannot guarantee to make node s | ||
| 357 | * unreachable in this way: (1) If s is the trailing node of list | ||
| 358 | * (i.e., with null next), then it is pinned as the target node | ||
| 359 | * for appends, so can only be removed later after other nodes are | ||
| 360 | * appended. (2) We cannot necessarily unlink s given a | ||
| 361 | * predecessor node that is matched (including the case of being | ||
| 362 | * cancelled): the predecessor may already be unspliced, in which | ||
| 363 | * case some previous reachable node may still point to s. | ||
| 364 | * (For further explanation see Herlihy & Shavit "The Art of | ||
| 365 | * Multiprocessor Programming" chapter 9). Although, in both | ||
| 366 | * cases, we can rule out the need for further action if either s | ||
| 367 | * or its predecessor are (or can be made to be) at, or fall off | ||
| 368 | * from, the head of list. | ||
| 369 | * | ||
| 370 | * Without taking these into account, it would be possible for an | ||
| 371 | * unbounded number of supposedly removed nodes to remain | ||
| 372 | * reachable. Situations leading to such buildup are uncommon but | ||
| 373 | * can occur in practice; for example when a series of short timed | ||
| 374 | * calls to poll repeatedly time out but never otherwise fall off | ||
| 375 | * the list because of an untimed call to take at the front of the | ||
| 376 | * queue. | ||
| 377 | * | ||
| 378 | * When these cases arise, rather than always retraversing the | ||
| 379 | * entire list to find an actual predecessor to unlink (which | ||
| 380 | * won't help for case (1) anyway), we record a conservative | ||
| 381 | * estimate of possible unsplice failures (in "sweepVotes"). | ||
| 382 | * We trigger a full sweep when the estimate exceeds a threshold | ||
| 383 | * ("SWEEP_THRESHOLD") indicating the maximum number of estimated | ||
| 384 | * removal failures to tolerate before sweeping through, unlinking | ||
| 385 | * cancelled nodes that were not unlinked upon initial removal. | ||
| 386 | * We perform sweeps by the thread hitting threshold (rather than | ||
| 387 | * background threads or by spreading work to other threads) | ||
| 388 | * because in the main contexts in which removal occurs, the | ||
| 389 | * caller is already timed-out, cancelled, or performing a | ||
| 390 | * potentially O(n) operation (e.g. remove(x)), none of which are | ||
| 391 | * time-critical enough to warrant the overhead that alternatives | ||
| 392 | * would impose on other threads. | ||
| 393 | * | ||
| 394 | * Because the sweepVotes estimate is conservative, and because | ||
| 395 | * nodes become unlinked "naturally" as they fall off the head of | ||
| 396 | * the queue, and because we allow votes to accumulate even while | ||
| 397 | * sweeps are in progress, there are typically significantly fewer | ||
| 398 | * such nodes than estimated. Choice of a threshold value | ||
| 399 | * balances the likelihood of wasted effort and contention, versus | ||
| 400 | * providing a worst-case bound on retention of interior nodes in | ||
| 401 | * quiescent queues. The value defined below was chosen | ||
| 402 | * empirically to balance these under various timeout scenarios. | ||
| 403 | * | ||
| 404 | * Note that we cannot self-link unlinked interior nodes during | ||
| 405 | * sweeps. However, the associated garbage chains terminate when | ||
| 406 | * some successor ultimately falls off the head of the list and is | ||
| 407 | * self-linked. | ||
| 408 | */ | ||
| 409 | |||
| 410 | /** True if on multiprocessor */ | ||
| 411 | private static final boolean MP = | ||
| 412 | Runtime.getRuntime().availableProcessors() > 1; | ||
| 413 | |||
| 414 | /** | ||
| 415 | * The number of times to spin (with randomly interspersed calls | ||
| 416 | * to Thread.yield) on multiprocessor before blocking when a node | ||
| 417 | * is apparently the first waiter in the queue. See above for | ||
| 418 | * explanation. Must be a power of two. The value is empirically | ||
| 419 | * derived -- it works pretty well across a variety of processors, | ||
| 420 | * numbers of CPUs, and OSes. | ||
| 421 | */ | ||
| 422 | private static final int FRONT_SPINS = 1 << 7; | ||
| 423 | |||
| 424 | /** | ||
| 425 | * The number of times to spin before blocking when a node is | ||
| 426 | * preceded by another node that is apparently spinning. Also | ||
| 427 | * serves as an increment to FRONT_SPINS on phase changes, and as | ||
| 428 | * base average frequency for yielding during spins. Must be a | ||
| 429 | * power of two. | ||
| 430 | */ | ||
| 431 | private static final int CHAINED_SPINS = FRONT_SPINS >>> 1; | ||
| 432 | |||
| 433 | /** | ||
| 434 | * The maximum number of estimated removal failures (sweepVotes) | ||
| 435 | * to tolerate before sweeping through the queue unlinking | ||
| 436 | * cancelled nodes that were not unlinked upon initial | ||
| 437 | * removal. See above for explanation. The value must be at least | ||
| 438 | * two to avoid useless sweeps when removing trailing nodes. | ||
| 439 | */ | ||
| 440 | static final int SWEEP_THRESHOLD = 32; | ||
| 441 | |||
| 442 | /** | ||
| 443 | * Queue nodes. Uses Object, not E, for items to allow forgetting | ||
| 444 | * them after use. Relies heavily on Unsafe mechanics to minimize | ||
| 445 | * unnecessary ordering constraints: Writes that are intrinsically | ||
| 446 | * ordered wrt other accesses or CASes use simple relaxed forms. | ||
| 447 | */ | ||
| 448 | static final class Node { | ||
| 449 | final boolean isData; // false if this is a request node | ||
| 450 | volatile Object item; // initially non-null if isData; CASed to match | ||
| 451 | volatile Node next; | ||
| 452 | volatile Thread waiter; // null until waiting | ||
| 453 | |||
| 454 | // CAS methods for fields | ||
| 455 | final boolean casNext(Node cmp, Node val) { | ||
| 456 | return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val); | ||
| 457 | } | ||
| 458 | |||
| 459 | final boolean casItem(Object cmp, Object val) { | ||
| 460 | // assert cmp == null || cmp.getClass() != Node.class; | ||
| 461 | return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val); | ||
| 462 | } | ||
| 463 | |||
| 464 | /** | ||
| 465 | * Constructs a new node. Uses relaxed write because item can | ||
| 466 | * only be seen after publication via casNext. | ||
| 467 | */ | ||
| 468 | Node(Object item, boolean isData) { | ||
| 469 | UNSAFE.putObject(this, itemOffset, item); // relaxed write | ||
| 470 | this.isData = isData; | ||
| 471 | } | ||
| 472 | |||
| 473 | /** | ||
| 474 | * Links node to itself to avoid garbage retention. Called | ||
| 475 | * only after CASing head field, so uses relaxed write. | ||
| 476 | */ | ||
| 477 | final void forgetNext() { | ||
| 478 | UNSAFE.putObject(this, nextOffset, this); | ||
| 479 | } | ||
| 480 | |||
| 481 | /** | ||
| 482 | * Sets item to self and waiter to null, to avoid garbage | ||
| 483 | * retention after matching or cancelling. Uses relaxed writes | ||
| 484 | * because order is already constrained in the only calling | ||
| 485 | * contexts: item is forgotten only after volatile/atomic | ||
| 486 | * mechanics that extract items. Similarly, clearing waiter | ||
| 487 | * follows either CAS or return from park (if ever parked; | ||
| 488 | * else we don't care). | ||
| 489 | */ | ||
| 490 | final void forgetContents() { | ||
| 491 | UNSAFE.putObject(this, itemOffset, this); | ||
| 492 | UNSAFE.putObject(this, waiterOffset, null); | ||
| 493 | } | ||
| 494 | |||
| 495 | /** | ||
| 496 | * Returns true if this node has been matched, including the | ||
| 497 | * case of artificial matches due to cancellation. | ||
| 498 | */ | ||
| 499 | final boolean isMatched() { | ||
| 500 | Object x = item; | ||
| 501 | return (x == this) || ((x == null) == isData); | ||
| 502 | } | ||
| 503 | |||
| 504 | /** | ||
| 505 | * Returns true if this is an unmatched request node. | ||
| 506 | */ | ||
| 507 | final boolean isUnmatchedRequest() { | ||
| 508 | return !isData && item == null; | ||
| 509 | } | ||
| 510 | |||
| 511 | /** | ||
| 512 | * Returns true if a node with the given mode cannot be | ||
| 513 | * appended to this node because this node is unmatched and | ||
| 514 | * has opposite data mode. | ||
| 515 | */ | ||
| 516 | final boolean cannotPrecede(boolean haveData) { | ||
| 517 | boolean d = isData; | ||
| 518 | Object x; | ||
| 519 | return d != haveData && (x = item) != this && (x != null) == d; | ||
| 520 | } | ||
| 521 | |||
| 522 | /** | ||
| 523 | * Tries to artificially match a data node -- used by remove. | ||
| 524 | */ | ||
| 525 | final boolean tryMatchData() { | ||
| 526 | // assert isData; | ||
| 527 | Object x = item; | ||
| 528 | if (x != null && x != this && casItem(x, null)) { | ||
| 529 | LockSupport.unpark(waiter); | ||
| 530 | return true; | ||
| 531 | } | ||
| 532 | return false; | ||
| 533 | } | ||
| 534 | |||
| 535 | private static final long serialVersionUID = -3375979862319811754L; | ||
| 536 | |||
| 537 | // Unsafe mechanics | ||
| 538 | private static final sun.misc.Unsafe UNSAFE; | ||
| 539 | private static final long itemOffset; | ||
| 540 | private static final long nextOffset; | ||
| 541 | private static final long waiterOffset; | ||
| 542 | static { | ||
| 543 | try { | ||
| 544 | UNSAFE = sun.misc.Unsafe.getUnsafe(); | ||
| 545 | Class k = Node.class; | ||
| 546 | itemOffset = UNSAFE.objectFieldOffset | ||
| 547 | (k.getDeclaredField("item")); | ||
| 548 | nextOffset = UNSAFE.objectFieldOffset | ||
| 549 | (k.getDeclaredField("next")); | ||
| 550 | waiterOffset = UNSAFE.objectFieldOffset | ||
| 551 | (k.getDeclaredField("waiter")); | ||
| 552 | } catch (Exception e) { | ||
| 553 | throw new Error(e); | ||
| 554 | } | ||
| 555 | } | ||
| 556 | } | ||
| 557 | |||
| 558 | /** head of the queue; null until first enqueue */ | ||
| 559 | transient volatile Node head; | ||
| 560 | |||
| 561 | /** tail of the queue; null until first append */ | ||
| 562 | private transient volatile Node tail; | ||
| 563 | |||
| 564 | /** The number of apparent failures to unsplice removed nodes */ | ||
| 565 | private transient volatile int sweepVotes; | ||
| 566 | |||
| 567 | // CAS methods for fields | ||
| 568 | private boolean casTail(Node cmp, Node val) { | ||
| 569 | return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val); | ||
| 570 | } | ||
| 571 | |||
| 572 | private boolean casHead(Node cmp, Node val) { | ||
| 573 | return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val); | ||
| 574 | } | ||
| 575 | |||
| 576 | private boolean casSweepVotes(int cmp, int val) { | ||
| 577 | return UNSAFE.compareAndSwapInt(this, sweepVotesOffset, cmp, val); | ||
| 578 | } | ||
| 579 | |||
| 580 | /* | ||
| 581 | * Possible values for "how" argument in xfer method. | ||
| 582 | */ | ||
| 583 | private static final int NOW = 0; // for untimed poll, tryTransfer | ||
| 584 | private static final int ASYNC = 1; // for offer, put, add | ||
| 585 | private static final int SYNC = 2; // for transfer, take | ||
| 586 | private static final int TIMED = 3; // for timed poll, tryTransfer | ||
| 587 | |||
| 588 | @SuppressWarnings("unchecked") | ||
| 589 | static <E> E cast(Object item) { | ||
| 590 | // assert item == null || item.getClass() != Node.class; | ||
| 591 | return (E) item; | ||
| 592 | } | ||
| 593 | |||
| 594 | /** | ||
| 595 | * Implements all queuing methods. See above for explanation. | ||
| 596 | * | ||
| 597 | * @param e the item or null for take | ||
| 598 | * @param haveData true if this is a put, else a take | ||
| 599 | * @param how NOW, ASYNC, SYNC, or TIMED | ||
| 600 | * @param nanos timeout in nanosecs, used only if mode is TIMED | ||
| 601 | * @return an item if matched, else e | ||
| 602 | * @throws NullPointerException if haveData mode but e is null | ||
| 603 | */ | ||
| 604 | private E xfer(E e, boolean haveData, int how, long nanos) { | ||
| 605 | if (haveData && (e == null)) | ||
| 606 | throw new NullPointerException(); | ||
| 607 | Node s = null; // the node to append, if needed | ||
| 608 | |||
| 609 | retry: | ||
| 610 | for (;;) { // restart on append race | ||
| 611 | |||
| 612 | for (Node h = head, p = h; p != null;) { // find & match first node | ||
| 613 | boolean isData = p.isData; | ||
| 614 | Object item = p.item; | ||
| 615 | if (item != p && (item != null) == isData) { // unmatched | ||
| 616 | if (isData == haveData) // can't match | ||
| 617 | break; | ||
| 618 | if (p.casItem(item, e)) { // match | ||
| 619 | for (Node q = p; q != h;) { | ||
| 620 | Node n = q.next; // update by 2 unless singleton | ||
| 621 | if (head == h && casHead(h, n == null ? q : n)) { | ||
| 622 | h.forgetNext(); | ||
| 623 | break; | ||
| 624 | } // advance and retry | ||
| 625 | if ((h = head) == null || | ||
| 626 | (q = h.next) == null || !q.isMatched()) | ||
| 627 | break; // unless slack < 2 | ||
| 628 | } | ||
| 629 | LockSupport.unpark(p.waiter); | ||
| 630 | return this.<E>cast(item); | ||
| 631 | } | ||
| 632 | } | ||
| 633 | Node n = p.next; | ||
| 634 | p = (p != n) ? n : (h = head); // Use head if p offlist | ||
| 635 | } | ||
| 636 | |||
| 637 | if (how != NOW) { // No matches available | ||
| 638 | if (s == null) | ||
| 639 | s = new Node(e, haveData); | ||
| 640 | Node pred = tryAppend(s, haveData); | ||
| 641 | if (pred == null) | ||
| 642 | continue retry; // lost race vs opposite mode | ||
| 643 | if (how != ASYNC) | ||
| 644 | return awaitMatch(s, pred, e, (how == TIMED), nanos); | ||
| 645 | } | ||
| 646 | return e; // not waiting | ||
| 647 | } | ||
| 648 | } | ||
| 649 | |||
| 650 | /** | ||
| 651 | * Tries to append node s as tail. | ||
| 652 | * | ||
| 653 | * @param s the node to append | ||
| 654 | * @param haveData true if appending in data mode | ||
| 655 | * @return null on failure due to losing race with append in | ||
| 656 | * different mode, else s's predecessor, or s itself if no | ||
| 657 | * predecessor | ||
| 658 | */ | ||
| 659 | private Node tryAppend(Node s, boolean haveData) { | ||
| 660 | for (Node t = tail, p = t;;) { // move p to last node and append | ||
| 661 | Node n, u; // temps for reads of next & tail | ||
| 662 | if (p == null && (p = head) == null) { | ||
| 663 | if (casHead(null, s)) | ||
| 664 | return s; // initialize | ||
| 665 | } | ||
| 666 | else if (p.cannotPrecede(haveData)) | ||
| 667 | return null; // lost race vs opposite mode | ||
| 668 | else if ((n = p.next) != null) // not last; keep traversing | ||
| 669 | p = p != t && t != (u = tail) ? (t = u) : // stale tail | ||
| 670 | (p != n) ? n : null; // restart if off list | ||
| 671 | else if (!p.casNext(null, s)) | ||
| 672 | p = p.next; // re-read on CAS failure | ||
| 673 | else { | ||
| 674 | if (p != t) { // update if slack now >= 2 | ||
| 675 | while ((tail != t || !casTail(t, s)) && | ||
| 676 | (t = tail) != null && | ||
| 677 | (s = t.next) != null && // advance and retry | ||
| 678 | (s = s.next) != null && s != t); | ||
| 679 | } | ||
| 680 | return p; | ||
| 681 | } | ||
| 682 | } | ||
| 683 | } | ||
| 684 | |||
| 685 | /** | ||
| 686 | * Spins/yields/blocks until node s is matched or caller gives up. | ||
| 687 | * | ||
| 688 | * @param s the waiting node | ||
| 689 | * @param pred the predecessor of s, or s itself if it has no | ||
| 690 | * predecessor, or null if unknown (the null case does not occur | ||
| 691 | * in any current calls but may in possible future extensions) | ||
| 692 | * @param e the comparison value for checking match | ||
| 693 | * @param timed if true, wait only until timeout elapses | ||
| 694 | * @param nanos timeout in nanosecs, used only if timed is true | ||
| 695 | * @return matched item, or e if unmatched on interrupt or timeout | ||
| 696 | */ | ||
| 697 | private E awaitMatch(Node s, Node pred, E e, boolean timed, long nanos) { | ||
| 698 | long lastTime = timed ? System.nanoTime() : 0L; | ||
| 699 | Thread w = Thread.currentThread(); | ||
| 700 | int spins = -1; // initialized after first item and cancel checks | ||
| 701 | ThreadLocalRandom randomYields = null; // bound if needed | ||
| 702 | |||
| 703 | for (;;) { | ||
| 704 | Object item = s.item; | ||
| 705 | if (item != e) { // matched | ||
| 706 | // assert item != s; | ||
| 707 | s.forgetContents(); // avoid garbage | ||
| 708 | return this.<E>cast(item); | ||
| 709 | } | ||
| 710 | if ((w.isInterrupted() || (timed && nanos <= 0)) && | ||
| 711 | s.casItem(e, s)) { // cancel | ||
| 712 | unsplice(pred, s); | ||
| 713 | return e; | ||
| 714 | } | ||
| 715 | |||
| 716 | if (spins < 0) { // establish spins at/near front | ||
| 717 | if ((spins = spinsFor(pred, s.isData)) > 0) | ||
| 718 | randomYields = ThreadLocalRandom.current(); | ||
| 719 | } | ||
| 720 | else if (spins > 0) { // spin | ||
| 721 | --spins; | ||
| 722 | if (randomYields.nextInt(CHAINED_SPINS) == 0) | ||
| 723 | Thread.yield(); // occasionally yield | ||
| 724 | } | ||
| 725 | else if (s.waiter == null) { | ||
| 726 | s.waiter = w; // request unpark then recheck | ||
| 727 | } | ||
| 728 | else if (timed) { | ||
| 729 | long now = System.nanoTime(); | ||
| 730 | if ((nanos -= now - lastTime) > 0) | ||
| 731 | LockSupport.parkNanos(this, nanos); | ||
| 732 | lastTime = now; | ||
| 733 | } | ||
| 734 | else { | ||
| 735 | LockSupport.park(this); | ||
| 736 | } | ||
| 737 | } | ||
| 738 | } | ||
| 739 | |||
| 740 | /** | ||
| 741 | * Returns spin/yield value for a node with given predecessor and | ||
| 742 | * data mode. See above for explanation. | ||
| 743 | */ | ||
| 744 | private static int spinsFor(Node pred, boolean haveData) { | ||
| 745 | if (MP && pred != null) { | ||
| 746 | if (pred.isData != haveData) // phase change | ||
| 747 | return FRONT_SPINS + CHAINED_SPINS; | ||
| 748 | if (pred.isMatched()) // probably at front | ||
| 749 | return FRONT_SPINS; | ||
| 750 | if (pred.waiter == null) // pred apparently spinning | ||
| 751 | return CHAINED_SPINS; | ||
| 752 | } | ||
| 753 | return 0; | ||
| 754 | } | ||
| 755 | |||
| 756 | /* -------------- Traversal methods -------------- */ | ||
| 757 | |||
| 758 | /** | ||
| 759 | * Returns the successor of p, or the head node if p.next has been | ||
| 760 | * linked to self, which will only be true if traversing with a | ||
| 761 | * stale pointer that is now off the list. | ||
| 762 | */ | ||
| 763 | final Node succ(Node p) { | ||
| 764 | Node next = p.next; | ||
| 765 | return (p == next) ? head : next; | ||
| 766 | } | ||
| 767 | |||
| 768 | /** | ||
| 769 | * Returns the first unmatched node of the given mode, or null if | ||
| 770 | * none. Used by methods isEmpty, hasWaitingConsumer. | ||
| 771 | */ | ||
| 772 | private Node firstOfMode(boolean isData) { | ||
| 773 | for (Node p = head; p != null; p = succ(p)) { | ||
| 774 | if (!p.isMatched()) | ||
| 775 | return (p.isData == isData) ? p : null; | ||
| 776 | } | ||
| 777 | return null; | ||
| 778 | } | ||
| 779 | |||
| 780 | /** | ||
| 781 | * Returns the item in the first unmatched node with isData; or | ||
| 782 | * null if none. Used by peek. | ||
| 783 | */ | ||
| 784 | private E firstDataItem() { | ||
| 785 | for (Node p = head; p != null; p = succ(p)) { | ||
| 786 | Object item = p.item; | ||
| 787 | if (p.isData) { | ||
| 788 | if (item != null && item != p) | ||
| 789 | return this.<E>cast(item); | ||
| 790 | } | ||
| 791 | else if (item == null) | ||
| 792 | return null; | ||
| 793 | } | ||
| 794 | return null; | ||
| 795 | } | ||
| 796 | |||
| 797 | /** | ||
| 798 | * Traverses and counts unmatched nodes of the given mode. | ||
| 799 | * Used by methods size and getWaitingConsumerCount. | ||
| 800 | */ | ||
| 801 | private int countOfMode(boolean data) { | ||
| 802 | int count = 0; | ||
| 803 | for (Node p = head; p != null; ) { | ||
| 804 | if (!p.isMatched()) { | ||
| 805 | if (p.isData != data) | ||
| 806 | return 0; | ||
| 807 | if (++count == Integer.MAX_VALUE) // saturated | ||
| 808 | break; | ||
| 809 | } | ||
| 810 | Node n = p.next; | ||
| 811 | if (n != p) | ||
| 812 | p = n; | ||
| 813 | else { | ||
| 814 | count = 0; | ||
| 815 | p = head; | ||
| 816 | } | ||
| 817 | } | ||
| 818 | return count; | ||
| 819 | } | ||
| 820 | |||
| 821 | final class Itr implements Iterator<E> { | ||
| 822 | private Node nextNode; // next node to return item for | ||
| 823 | private E nextItem; // the corresponding item | ||
| 824 | private Node lastRet; // last returned node, to support remove | ||
| 825 | private Node lastPred; // predecessor to unlink lastRet | ||
| 826 | |||
| 827 | /** | ||
| 828 | * Moves to next node after prev, or first node if prev null. | ||
| 829 | */ | ||
| 830 | private void advance(Node prev) { | ||
| 831 | /* | ||
| 832 | * To track and avoid buildup of deleted nodes in the face | ||
| 833 | * of calls to both Queue.remove and Itr.remove, we must | ||
| 834 | * include variants of unsplice and sweep upon each | ||
| 835 | * advance: Upon Itr.remove, we may need to catch up links | ||
| 836 | * from lastPred, and upon other removes, we might need to | ||
| 837 | * skip ahead from stale nodes and unsplice deleted ones | ||
| 838 | * found while advancing. | ||
| 839 | */ | ||
| 840 | |||
| 841 | Node r, b; // reset lastPred upon possible deletion of lastRet | ||
| 842 | if ((r = lastRet) != null && !r.isMatched()) | ||
| 843 | lastPred = r; // next lastPred is old lastRet | ||
| 844 | else if ((b = lastPred) == null || b.isMatched()) | ||
| 845 | lastPred = null; // at start of list | ||
| 846 | else { | ||
| 847 | Node s, n; // help with removal of lastPred.next | ||
| 848 | while ((s = b.next) != null && | ||
| 849 | s != b && s.isMatched() && | ||
| 850 | (n = s.next) != null && n != s) | ||
| 851 | b.casNext(s, n); | ||
| 852 | } | ||
| 853 | |||
| 854 | this.lastRet = prev; | ||
| 855 | |||
| 856 | for (Node p = prev, s, n;;) { | ||
| 857 | s = (p == null) ? head : p.next; | ||
| 858 | if (s == null) | ||
| 859 | break; | ||
| 860 | else if (s == p) { | ||
| 861 | p = null; | ||
| 862 | continue; | ||
| 863 | } | ||
| 864 | Object item = s.item; | ||
| 865 | if (s.isData) { | ||
| 866 | if (item != null && item != s) { | ||
| 867 | nextItem = LinkedTransferQueue.<E>cast(item); | ||
| 868 | nextNode = s; | ||
| 869 | return; | ||
| 870 | } | ||
| 871 | } | ||
| 872 | else if (item == null) | ||
| 873 | break; | ||
| 874 | // assert s.isMatched(); | ||
| 875 | if (p == null) | ||
| 876 | p = s; | ||
| 877 | else if ((n = s.next) == null) | ||
| 878 | break; | ||
| 879 | else if (s == n) | ||
| 880 | p = null; | ||
| 881 | else | ||
| 882 | p.casNext(s, n); | ||
| 883 | } | ||
| 884 | nextNode = null; | ||
| 885 | nextItem = null; | ||
| 886 | } | ||
| 887 | |||
| 888 | Itr() { | ||
| 889 | advance(null); | ||
| 890 | } | ||
| 891 | |||
| 892 | public final boolean hasNext() { | ||
| 893 | return nextNode != null; | ||
| 894 | } | ||
| 895 | |||
| 896 | public final E next() { | ||
| 897 | Node p = nextNode; | ||
| 898 | if (p == null) throw new NoSuchElementException(); | ||
| 899 | E e = nextItem; | ||
| 900 | advance(p); | ||
| 901 | return e; | ||
| 902 | } | ||
| 903 | |||
| 904 | public final void remove() { | ||
| 905 | final Node lastRet = this.lastRet; | ||
| 906 | if (lastRet == null) | ||
| 907 | throw new IllegalStateException(); | ||
| 908 | this.lastRet = null; | ||
| 909 | if (lastRet.tryMatchData()) | ||
| 910 | unsplice(lastPred, lastRet); | ||
| 911 | } | ||
| 912 | } | ||
| 913 | |||
| 914 | /* -------------- Removal methods -------------- */ | ||
| 915 | |||
| 916 | /** | ||
| 917 | * Unsplices (now or later) the given deleted/cancelled node with | ||
| 918 | * the given predecessor. | ||
| 919 | * | ||
| 920 | * @param pred a node that was at one time known to be the | ||
| 921 | * predecessor of s, or null or s itself if s is/was at head | ||
| 922 | * @param s the node to be unspliced | ||
| 923 | */ | ||
| 924 | final void unsplice(Node pred, Node s) { | ||
| 925 | s.forgetContents(); // forget unneeded fields | ||
| 926 | /* | ||
| 927 | * See above for rationale. Briefly: if pred still points to | ||
| 928 | * s, try to unlink s. If s cannot be unlinked, because it is | ||
| 929 | * trailing node or pred might be unlinked, and neither pred | ||
| 930 | * nor s are head or offlist, add to sweepVotes, and if enough | ||
| 931 | * votes have accumulated, sweep. | ||
| 932 | */ | ||
| 933 | if (pred != null && pred != s && pred.next == s) { | ||
| 934 | Node n = s.next; | ||
| 935 | if (n == null || | ||
| 936 | (n != s && pred.casNext(s, n) && pred.isMatched())) { | ||
| 937 | for (;;) { // check if at, or could be, head | ||
| 938 | Node h = head; | ||
| 939 | if (h == pred || h == s || h == null) | ||
| 940 | return; // at head or list empty | ||
| 941 | if (!h.isMatched()) | ||
| 942 | break; | ||
| 943 | Node hn = h.next; | ||
| 944 | if (hn == null) | ||
| 945 | return; // now empty | ||
| 946 | if (hn != h && casHead(h, hn)) | ||
| 947 | h.forgetNext(); // advance head | ||
| 948 | } | ||
| 949 | if (pred.next != pred && s.next != s) { // recheck if offlist | ||
| 950 | for (;;) { // sweep now if enough votes | ||
| 951 | int v = sweepVotes; | ||
| 952 | if (v < SWEEP_THRESHOLD) { | ||
| 953 | if (casSweepVotes(v, v + 1)) | ||
| 954 | break; | ||
| 955 | } | ||
| 956 | else if (casSweepVotes(v, 0)) { | ||
| 957 | sweep(); | ||
| 958 | break; | ||
| 959 | } | ||
| 960 | } | ||
| 961 | } | ||
| 962 | } | ||
| 963 | } | ||
| 964 | } | ||
| 965 | |||
| 966 | /** | ||
| 967 | * Unlinks matched (typically cancelled) nodes encountered in a | ||
| 968 | * traversal from head. | ||
| 969 | */ | ||
| 970 | private void sweep() { | ||
| 971 | for (Node p = head, s, n; p != null && (s = p.next) != null; ) { | ||
| 972 | if (!s.isMatched()) | ||
| 973 | // Unmatched nodes are never self-linked | ||
| 974 | p = s; | ||
| 975 | else if ((n = s.next) == null) // trailing node is pinned | ||
| 976 | break; | ||
| 977 | else if (s == n) // stale | ||
| 978 | // No need to also check for p == s, since that implies s == n | ||
| 979 | p = head; | ||
| 980 | else | ||
| 981 | p.casNext(s, n); | ||
| 982 | } | ||
| 983 | } | ||
| 984 | |||
| 985 | /** | ||
| 986 | * Main implementation of remove(Object) | ||
| 987 | */ | ||
| 988 | private boolean findAndRemove(Object e) { | ||
| 989 | if (e != null) { | ||
| 990 | for (Node pred = null, p = head; p != null; ) { | ||
| 991 | Object item = p.item; | ||
| 992 | if (p.isData) { | ||
| 993 | if (item != null && item != p && e.equals(item) && | ||
| 994 | p.tryMatchData()) { | ||
| 995 | unsplice(pred, p); | ||
| 996 | return true; | ||
| 997 | } | ||
| 998 | } | ||
| 999 | else if (item == null) | ||
| 1000 | break; | ||
| 1001 | pred = p; | ||
| 1002 | if ((p = p.next) == pred) { // stale | ||
| 1003 | pred = null; | ||
| 1004 | p = head; | ||
| 1005 | } | ||
| 1006 | } | ||
| 1007 | } | ||
| 1008 | return false; | ||
| 1009 | } | ||
| 1010 | |||
| 1011 | |||
| 1012 | /** | ||
| 1013 | * Creates an initially empty {@code LinkedTransferQueue}. | ||
| 1014 | */ | ||
| 1015 | public LinkedTransferQueue() { | ||
| 1016 | } | ||
| 1017 | |||
| 1018 | /** | ||
| 1019 | * Creates a {@code LinkedTransferQueue} | ||
| 1020 | * initially containing the elements of the given collection, | ||
| 1021 | * added in traversal order of the collection's iterator. | ||
| 1022 | * | ||
| 1023 | * @param c the collection of elements to initially contain | ||
| 1024 | * @throws NullPointerException if the specified collection or any | ||
| 1025 | * of its elements are null | ||
| 1026 | */ | ||
| 1027 | public LinkedTransferQueue(Collection<? extends E> c) { | ||
| 1028 | this(); | ||
| 1029 | addAll(c); | ||
| 1030 | } | ||
| 1031 | |||
| 1032 | /** | ||
| 1033 | * Inserts the specified element at the tail of this queue. | ||
| 1034 | * As the queue is unbounded, this method will never block. | ||
| 1035 | * | ||
| 1036 | * @throws NullPointerException if the specified element is null | ||
| 1037 | */ | ||
| 1038 | public void put(E e) { | ||
| 1039 | xfer(e, true, ASYNC, 0); | ||
| 1040 | } | ||
| 1041 | |||
| 1042 | /** | ||
| 1043 | * Inserts the specified element at the tail of this queue. | ||
| 1044 | * As the queue is unbounded, this method will never block or | ||
| 1045 | * return {@code false}. | ||
| 1046 | * | ||
| 1047 | * @return {@code true} (as specified by | ||
| 1048 | * {@link BlockingQueue#offer(Object,long,TimeUnit) BlockingQueue.offer}) | ||
| 1049 | * @throws NullPointerException if the specified element is null | ||
| 1050 | */ | ||
| 1051 | public boolean offer(E e, long timeout, TimeUnit unit) { | ||
| 1052 | xfer(e, true, ASYNC, 0); | ||
| 1053 | return true; | ||
| 1054 | } | ||
| 1055 | |||
| 1056 | /** | ||
| 1057 | * Inserts the specified element at the tail of this queue. | ||
| 1058 | * As the queue is unbounded, this method will never return {@code false}. | ||
| 1059 | * | ||
| 1060 | * @return {@code true} (as specified by {@link Queue#offer}) | ||
| 1061 | * @throws NullPointerException if the specified element is null | ||
| 1062 | */ | ||
| 1063 | public boolean offer(E e) { | ||
| 1064 | xfer(e, true, ASYNC, 0); | ||
| 1065 | return true; | ||
| 1066 | } | ||
| 1067 | |||
| 1068 | /** | ||
| 1069 | * Inserts the specified element at the tail of this queue. | ||
| 1070 | * As the queue is unbounded, this method will never throw | ||
| 1071 | * {@link IllegalStateException} or return {@code false}. | ||
| 1072 | * | ||
| 1073 | * @return {@code true} (as specified by {@link Collection#add}) | ||
| 1074 | * @throws NullPointerException if the specified element is null | ||
| 1075 | */ | ||
| 1076 | public boolean add(E e) { | ||
| 1077 | xfer(e, true, ASYNC, 0); | ||
| 1078 | return true; | ||
| 1079 | } | ||
| 1080 | |||
| 1081 | /** | ||
| 1082 | * Transfers the element to a waiting consumer immediately, if possible. | ||
| 1083 | * | ||
| 1084 | * <p>More precisely, transfers the specified element immediately | ||
| 1085 | * if there exists a consumer already waiting to receive it (in | ||
| 1086 | * {@link #take} or timed {@link #poll(long,TimeUnit) poll}), | ||
| 1087 | * otherwise returning {@code false} without enqueuing the element. | ||
| 1088 | * | ||
| 1089 | * @throws NullPointerException if the specified element is null | ||
| 1090 | */ | ||
| 1091 | public boolean tryTransfer(E e) { | ||
| 1092 | return xfer(e, true, NOW, 0) == null; | ||
| 1093 | } | ||
| 1094 | |||
| 1095 | /** | ||
| 1096 | * Transfers the element to a consumer, waiting if necessary to do so. | ||
| 1097 | * | ||
| 1098 | * <p>More precisely, transfers the specified element immediately | ||
| 1099 | * if there exists a consumer already waiting to receive it (in | ||
| 1100 | * {@link #take} or timed {@link #poll(long,TimeUnit) poll}), | ||
| 1101 | * else inserts the specified element at the tail of this queue | ||
| 1102 | * and waits until the element is received by a consumer. | ||
| 1103 | * | ||
| 1104 | * @throws NullPointerException if the specified element is null | ||
| 1105 | */ | ||
| 1106 | public void transfer(E e) throws InterruptedException { | ||
| 1107 | if (xfer(e, true, SYNC, 0) != null) { | ||
| 1108 | Thread.interrupted(); // failure possible only due to interrupt | ||
| 1109 | throw new InterruptedException(); | ||
| 1110 | } | ||
| 1111 | } | ||
| 1112 | |||
| 1113 | /** | ||
| 1114 | * Transfers the element to a consumer if it is possible to do so | ||
| 1115 | * before the timeout elapses. | ||
| 1116 | * | ||
| 1117 | * <p>More precisely, transfers the specified element immediately | ||
| 1118 | * if there exists a consumer already waiting to receive it (in | ||
| 1119 | * {@link #take} or timed {@link #poll(long,TimeUnit) poll}), | ||
| 1120 | * else inserts the specified element at the tail of this queue | ||
| 1121 | * and waits until the element is received by a consumer, | ||
| 1122 | * returning {@code false} if the specified wait time elapses | ||
| 1123 | * before the element can be transferred. | ||
| 1124 | * | ||
| 1125 | * @throws NullPointerException if the specified element is null | ||
| 1126 | */ | ||
| 1127 | public boolean tryTransfer(E e, long timeout, TimeUnit unit) | ||
| 1128 | throws InterruptedException { | ||
| 1129 | if (xfer(e, true, TIMED, unit.toNanos(timeout)) == null) | ||
| 1130 | return true; | ||
| 1131 | if (!Thread.interrupted()) | ||
| 1132 | return false; | ||
| 1133 | throw new InterruptedException(); | ||
| 1134 | } | ||
| 1135 | |||
| 1136 | public E take() throws InterruptedException { | ||
| 1137 | E e = xfer(null, false, SYNC, 0); | ||
| 1138 | if (e != null) | ||
| 1139 | return e; | ||
| 1140 | Thread.interrupted(); | ||
| 1141 | throw new InterruptedException(); | ||
| 1142 | } | ||
| 1143 | |||
| 1144 | public E poll(long timeout, TimeUnit unit) throws InterruptedException { | ||
| 1145 | E e = xfer(null, false, TIMED, unit.toNanos(timeout)); | ||
| 1146 | if (e != null || !Thread.interrupted()) | ||
| 1147 | return e; | ||
| 1148 | throw new InterruptedException(); | ||
| 1149 | } | ||
| 1150 | |||
| 1151 | public E poll() { | ||
| 1152 | return xfer(null, false, NOW, 0); | ||
| 1153 | } | ||
| 1154 | |||
| 1155 | /** | ||
| 1156 | * @throws NullPointerException {@inheritDoc} | ||
| 1157 | * @throws IllegalArgumentException {@inheritDoc} | ||
| 1158 | */ | ||
| 1159 | public int drainTo(Collection<? super E> c) { | ||
| 1160 | if (c == null) | ||
| 1161 | throw new NullPointerException(); | ||
| 1162 | if (c == this) | ||
| 1163 | throw new IllegalArgumentException(); | ||
| 1164 | int n = 0; | ||
| 1165 | E e; | ||
| 1166 | while ( (e = poll()) != null) { | ||
| 1167 | c.add(e); | ||
| 1168 | ++n; | ||
| 1169 | } | ||
| 1170 | return n; | ||
| 1171 | } | ||
| 1172 | |||
| 1173 | /** | ||
| 1174 | * @throws NullPointerException {@inheritDoc} | ||
| 1175 | * @throws IllegalArgumentException {@inheritDoc} | ||
| 1176 | */ | ||
| 1177 | public int drainTo(Collection<? super E> c, int maxElements) { | ||
| 1178 | if (c == null) | ||
| 1179 | throw new NullPointerException(); | ||
| 1180 | if (c == this) | ||
| 1181 | throw new IllegalArgumentException(); | ||
| 1182 | int n = 0; | ||
| 1183 | E e; | ||
| 1184 | while (n < maxElements && (e = poll()) != null) { | ||
| 1185 | c.add(e); | ||
| 1186 | ++n; | ||
| 1187 | } | ||
| 1188 | return n; | ||
| 1189 | } | ||
| 1190 | |||
| 1191 | /** | ||
| 1192 | * Returns an iterator over the elements in this queue in proper sequence. | ||
| 1193 | * The elements will be returned in order from first (head) to last (tail). | ||
| 1194 | * | ||
| 1195 | * <p>The returned iterator is a "weakly consistent" iterator that | ||
| 1196 | * will never throw {@link java.util.ConcurrentModificationException | ||
| 1197 | * ConcurrentModificationException}, and guarantees to traverse | ||
| 1198 | * elements as they existed upon construction of the iterator, and | ||
| 1199 | * may (but is not guaranteed to) reflect any modifications | ||
| 1200 | * subsequent to construction. | ||
| 1201 | * | ||
| 1202 | * @return an iterator over the elements in this queue in proper sequence | ||
| 1203 | */ | ||
| 1204 | public Iterator<E> iterator() { | ||
| 1205 | return new Itr(); | ||
| 1206 | } | ||
| 1207 | |||
| 1208 | public E peek() { | ||
| 1209 | return firstDataItem(); | ||
| 1210 | } | ||
| 1211 | |||
| 1212 | /** | ||
| 1213 | * Returns {@code true} if this queue contains no elements. | ||
| 1214 | * | ||
| 1215 | * @return {@code true} if this queue contains no elements | ||
| 1216 | */ | ||
| 1217 | public boolean isEmpty() { | ||
| 1218 | for (Node p = head; p != null; p = succ(p)) { | ||
| 1219 | if (!p.isMatched()) | ||
| 1220 | return !p.isData; | ||
| 1221 | } | ||
| 1222 | return true; | ||
| 1223 | } | ||
| 1224 | |||
| 1225 | public boolean hasWaitingConsumer() { | ||
| 1226 | return firstOfMode(false) != null; | ||
| 1227 | } | ||
| 1228 | |||
| 1229 | /** | ||
| 1230 | * Returns the number of elements in this queue. If this queue | ||
| 1231 | * contains more than {@code Integer.MAX_VALUE} elements, returns | ||
| 1232 | * {@code Integer.MAX_VALUE}. | ||
| 1233 | * | ||
| 1234 | * <p>Beware that, unlike in most collections, this method is | ||
| 1235 | * <em>NOT</em> a constant-time operation. Because of the | ||
| 1236 | * asynchronous nature of these queues, determining the current | ||
| 1237 | * number of elements requires an O(n) traversal. | ||
| 1238 | * | ||
| 1239 | * @return the number of elements in this queue | ||
| 1240 | */ | ||
| 1241 | public int size() { | ||
| 1242 | return countOfMode(true); | ||
| 1243 | } | ||
| 1244 | |||
| 1245 | public int getWaitingConsumerCount() { | ||
| 1246 | return countOfMode(false); | ||
| 1247 | } | ||
| 1248 | |||
| 1249 | /** | ||
| 1250 | * Removes a single instance of the specified element from this queue, | ||
| 1251 | * if it is present. More formally, removes an element {@code e} such | ||
| 1252 | * that {@code o.equals(e)}, if this queue contains one or more such | ||
| 1253 | * elements. | ||
| 1254 | * Returns {@code true} if this queue contained the specified element | ||
| 1255 | * (or equivalently, if this queue changed as a result of the call). | ||
| 1256 | * | ||
| 1257 | * @param o element to be removed from this queue, if present | ||
| 1258 | * @return {@code true} if this queue changed as a result of the call | ||
| 1259 | */ | ||
| 1260 | public boolean remove(Object o) { | ||
| 1261 | return findAndRemove(o); | ||
| 1262 | } | ||
| 1263 | |||
| 1264 | /** | ||
| 1265 | * Returns {@code true} if this queue contains the specified element. | ||
| 1266 | * More formally, returns {@code true} if and only if this queue contains | ||
| 1267 | * at least one element {@code e} such that {@code o.equals(e)}. | ||
| 1268 | * | ||
| 1269 | * @param o object to be checked for containment in this queue | ||
| 1270 | * @return {@code true} if this queue contains the specified element | ||
| 1271 | */ | ||
| 1272 | public boolean contains(Object o) { | ||
| 1273 | if (o == null) return false; | ||
| 1274 | for (Node p = head; p != null; p = succ(p)) { | ||
| 1275 | Object item = p.item; | ||
| 1276 | if (p.isData) { | ||
| 1277 | if (item != null && item != p && o.equals(item)) | ||
| 1278 | return true; | ||
| 1279 | } | ||
| 1280 | else if (item == null) | ||
| 1281 | break; | ||
| 1282 | } | ||
| 1283 | return false; | ||
| 1284 | } | ||
| 1285 | |||
| 1286 | /** | ||
| 1287 | * Always returns {@code Integer.MAX_VALUE} because a | ||
| 1288 | * {@code LinkedTransferQueue} is not capacity constrained. | ||
| 1289 | * | ||
| 1290 | * @return {@code Integer.MAX_VALUE} (as specified by | ||
| 1291 | * {@link BlockingQueue#remainingCapacity()}) | ||
| 1292 | */ | ||
| 1293 | public int remainingCapacity() { | ||
| 1294 | return Integer.MAX_VALUE; | ||
| 1295 | } | ||
| 1296 | |||
| 1297 | /** | ||
| 1298 | * Saves the state to a stream (that is, serializes it). | ||
| 1299 | * | ||
| 1300 | * @serialData All of the elements (each an {@code E}) in | ||
| 1301 | * the proper order, followed by a null | ||
| 1302 | * @param s the stream | ||
| 1303 | */ | ||
| 1304 | private void writeObject(java.io.ObjectOutputStream s) | ||
| 1305 | throws java.io.IOException { | ||
| 1306 | s.defaultWriteObject(); | ||
| 1307 | for (E e : this) | ||
| 1308 | s.writeObject(e); | ||
| 1309 | // Use trailing null as sentinel | ||
| 1310 | s.writeObject(null); | ||
| 1311 | } | ||
| 1312 | |||
| 1313 | /** | ||
| 1314 | * Reconstitutes the Queue instance from a stream (that is, | ||
| 1315 | * deserializes it). | ||
| 1316 | * | ||
| 1317 | * @param s the stream | ||
| 1318 | */ | ||
| 1319 | private void readObject(java.io.ObjectInputStream s) | ||
| 1320 | throws java.io.IOException, ClassNotFoundException { | ||
| 1321 | s.defaultReadObject(); | ||
| 1322 | for (;;) { | ||
| 1323 | @SuppressWarnings("unchecked") E item = (E) s.readObject(); | ||
| 1324 | if (item == null) | ||
| 1325 | break; | ||
| 1326 | else | ||
| 1327 | offer(item); | ||
| 1328 | } | ||
| 1329 | } | ||
| 1330 | |||
| 1331 | // Unsafe mechanics | ||
| 1332 | |||
| 1333 | private static final sun.misc.Unsafe UNSAFE; | ||
| 1334 | private static final long headOffset; | ||
| 1335 | private static final long tailOffset; | ||
| 1336 | private static final long sweepVotesOffset; | ||
| 1337 | static { | ||
| 1338 | try { | ||
| 1339 | UNSAFE = sun.misc.Unsafe.getUnsafe(); | ||
| 1340 | Class k = LinkedTransferQueue.class; | ||
| 1341 | headOffset = UNSAFE.objectFieldOffset | ||
| 1342 | (k.getDeclaredField("head")); | ||
| 1343 | tailOffset = UNSAFE.objectFieldOffset | ||
| 1344 | (k.getDeclaredField("tail")); | ||
| 1345 | sweepVotesOffset = UNSAFE.objectFieldOffset | ||
| 1346 | (k.getDeclaredField("sweepVotes")); | ||
| 1347 | } catch (Exception e) { | ||
| 1348 | throw new Error(e); | ||
| 1349 | } | ||
| 1350 | } | ||
| 1351 | } | ||