1 /* 2 * Written by Doug Lea with assistance from members of JCP JSR-166 3 * Expert Group and released to the public domain, as explained at 4 * http://creativecommons.org/publicdomain/zero/1.0/ 5 */ 6 7 package java.util.concurrent; 8 9 import java.util.concurrent.TimeUnit; 10 import java.util.concurrent.TimeoutException; 11 import java.util.concurrent.atomic.AtomicReference; 12 import java.util.concurrent.locks.LockSupport; 13 14 /** 15 * A reusable synchronization barrier, similar in functionality to 16 * {@link java.util.concurrent.CyclicBarrier CyclicBarrier} and 17 * {@link java.util.concurrent.CountDownLatch CountDownLatch} 18 * but supporting more flexible usage. 19 * 20 * <p><b>Registration.</b> Unlike the case for other barriers, the 21 * number of parties <em>registered</em> to synchronize on a phaser 22 * may vary over time. Tasks may be registered at any time (using 23 * methods {@link #register}, {@link #bulkRegister}, or forms of 24 * constructors establishing initial numbers of parties), and 25 * optionally deregistered upon any arrival (using {@link 26 * #arriveAndDeregister}). As is the case with most basic 27 * synchronization constructs, registration and deregistration affect 28 * only internal counts; they do not establish any further internal 29 * bookkeeping, so tasks cannot query whether they are registered. 30 * (However, you can introduce such bookkeeping by subclassing this 31 * class.) 32 * 33 * <p><b>Synchronization.</b> Like a {@code CyclicBarrier}, a {@code 34 * Phaser} may be repeatedly awaited. Method {@link 35 * #arriveAndAwaitAdvance} has effect analogous to {@link 36 * java.util.concurrent.CyclicBarrier#await CyclicBarrier.await}. Each 37 * generation of a phaser has an associated phase number. The phase 38 * number starts at zero, and advances when all parties arrive at the 39 * phaser, wrapping around to zero after reaching {@code 40 * Integer.MAX_VALUE}. The use of phase numbers enables independent 41 * control of actions upon arrival at a phaser and upon awaiting 42 * others, via two kinds of methods that may be invoked by any 43 * registered party: 44 * 45 * <ul> 46 * 47 * <li> <b>Arrival.</b> Methods {@link #arrive} and 48 * {@link #arriveAndDeregister} record arrival. These methods 49 * do not block, but return an associated <em>arrival phase 50 * number</em>; that is, the phase number of the phaser to which 51 * the arrival applied. When the final party for a given phase 52 * arrives, an optional action is performed and the phase 53 * advances. These actions are performed by the party 54 * triggering a phase advance, and are arranged by overriding 55 * method {@link #onAdvance(int, int)}, which also controls 56 * termination. Overriding this method is similar to, but more 57 * flexible than, providing a barrier action to a {@code 58 * CyclicBarrier}. 59 * 60 * <li> <b>Waiting.</b> Method {@link #awaitAdvance} requires an 61 * argument indicating an arrival phase number, and returns when 62 * the phaser advances to (or is already at) a different phase. 63 * Unlike similar constructions using {@code CyclicBarrier}, 64 * method {@code awaitAdvance} continues to wait even if the 65 * waiting thread is interrupted. Interruptible and timeout 66 * versions are also available, but exceptions encountered while 67 * tasks wait interruptibly or with timeout do not change the 68 * state of the phaser. If necessary, you can perform any 69 * associated recovery within handlers of those exceptions, 70 * often after invoking {@code forceTermination}. Phasers may 71 * also be used by tasks executing in a {@link ForkJoinPool}, 72 * which will ensure sufficient parallelism to execute tasks 73 * when others are blocked waiting for a phase to advance. 74 * 75 * </ul> 76 * 77 * <p><b>Termination.</b> A phaser may enter a <em>termination</em> 78 * state, that may be checked using method {@link #isTerminated}. Upon 79 * termination, all synchronization methods immediately return without 80 * waiting for advance, as indicated by a negative return value. 81 * Similarly, attempts to register upon termination have no effect. 82 * Termination is triggered when an invocation of {@code onAdvance} 83 * returns {@code true}. The default implementation returns {@code 84 * true} if a deregistration has caused the number of registered 85 * parties to become zero. As illustrated below, when phasers control 86 * actions with a fixed number of iterations, it is often convenient 87 * to override this method to cause termination when the current phase 88 * number reaches a threshold. Method {@link #forceTermination} is 89 * also available to abruptly release waiting threads and allow them 90 * to terminate. 91 * 92 * <p><b>Tiering.</b> Phasers may be <em>tiered</em> (i.e., 93 * constructed in tree structures) to reduce contention. Phasers with 94 * large numbers of parties that would otherwise experience heavy 95 * synchronization contention costs may instead be set up so that 96 * groups of sub-phasers share a common parent. This may greatly 97 * increase throughput even though it incurs greater per-operation 98 * overhead. 99 * 100 * <p>In a tree of tiered phasers, registration and deregistration of 101 * child phasers with their parent are managed automatically. 102 * Whenever the number of registered parties of a child phaser becomes 103 * non-zero (as established in the {@link #Phaser(Phaser,int)} 104 * constructor, {@link #register}, or {@link #bulkRegister}), the 105 * child phaser is registered with its parent. Whenever the number of 106 * registered parties becomes zero as the result of an invocation of 107 * {@link #arriveAndDeregister}, the child phaser is deregistered 108 * from its parent. 109 * 110 * <p><b>Monitoring.</b> While synchronization methods may be invoked 111 * only by registered parties, the current state of a phaser may be 112 * monitored by any caller. At any given moment there are {@link 113 * #getRegisteredParties} parties in total, of which {@link 114 * #getArrivedParties} have arrived at the current phase ({@link 115 * #getPhase}). When the remaining ({@link #getUnarrivedParties}) 116 * parties arrive, the phase advances. The values returned by these 117 * methods may reflect transient states and so are not in general 118 * useful for synchronization control. Method {@link #toString} 119 * returns snapshots of these state queries in a form convenient for 120 * informal monitoring. 121 * 122 * <p><b>Sample usages:</b> 123 * 124 * <p>A {@code Phaser} may be used instead of a {@code CountDownLatch} 125 * to control a one-shot action serving a variable number of parties. 126 * The typical idiom is for the method setting this up to first 127 * register, then start the actions, then deregister, as in: 128 * 129 * <pre> {@code 130 * void runTasks(List<Runnable> tasks) { 131 * final Phaser phaser = new Phaser(1); // "1" to register self 132 * // create and start threads 133 * for (final Runnable task : tasks) { 134 * phaser.register(); 135 * new Thread() { 136 * public void run() { 137 * phaser.arriveAndAwaitAdvance(); // await all creation 138 * task.run(); 139 * } 140 * }.start(); 141 * } 142 * 143 * // allow threads to start and deregister self 144 * phaser.arriveAndDeregister(); 145 * }}</pre> 146 * 147 * <p>One way to cause a set of threads to repeatedly perform actions 148 * for a given number of iterations is to override {@code onAdvance}: 149 * 150 * <pre> {@code 151 * void startTasks(List<Runnable> tasks, final int iterations) { 152 * final Phaser phaser = new Phaser() { 153 * protected boolean onAdvance(int phase, int registeredParties) { 154 * return phase >= iterations || registeredParties == 0; 155 * } 156 * }; 157 * phaser.register(); 158 * for (final Runnable task : tasks) { 159 * phaser.register(); 160 * new Thread() { 161 * public void run() { 162 * do { 163 * task.run(); 164 * phaser.arriveAndAwaitAdvance(); 165 * } while (!phaser.isTerminated()); 166 * } 167 * }.start(); 168 * } 169 * phaser.arriveAndDeregister(); // deregister self, don't wait 170 * }}</pre> 171 * 172 * If the main task must later await termination, it 173 * may re-register and then execute a similar loop: 174 * <pre> {@code 175 * // ... 176 * phaser.register(); 177 * while (!phaser.isTerminated()) 178 * phaser.arriveAndAwaitAdvance();}</pre> 179 * 180 * <p>Related constructions may be used to await particular phase numbers 181 * in contexts where you are sure that the phase will never wrap around 182 * {@code Integer.MAX_VALUE}. For example: 183 * 184 * <pre> {@code 185 * void awaitPhase(Phaser phaser, int phase) { 186 * int p = phaser.register(); // assumes caller not already registered 187 * while (p < phase) { 188 * if (phaser.isTerminated()) 189 * // ... deal with unexpected termination 190 * else 191 * p = phaser.arriveAndAwaitAdvance(); 192 * } 193 * phaser.arriveAndDeregister(); 194 * }}</pre> 195 * 196 * 197 * <p>To create a set of {@code n} tasks using a tree of phasers, you 198 * could use code of the following form, assuming a Task class with a 199 * constructor accepting a {@code Phaser} that it registers with upon 200 * construction. After invocation of {@code build(new Task[n], 0, n, 201 * new Phaser())}, these tasks could then be started, for example by 202 * submitting to a pool: 203 * 204 * <pre> {@code 205 * void build(Task[] tasks, int lo, int hi, Phaser ph) { 206 * if (hi - lo > TASKS_PER_PHASER) { 207 * for (int i = lo; i < hi; i += TASKS_PER_PHASER) { 208 * int j = Math.min(i + TASKS_PER_PHASER, hi); 209 * build(tasks, i, j, new Phaser(ph)); 210 * } 211 * } else { 212 * for (int i = lo; i < hi; ++i) 213 * tasks[i] = new Task(ph); 214 * // assumes new Task(ph) performs ph.register() 215 * } 216 * }}</pre> 217 * 218 * The best value of {@code TASKS_PER_PHASER} depends mainly on 219 * expected synchronization rates. A value as low as four may 220 * be appropriate for extremely small per-phase task bodies (thus 221 * high rates), or up to hundreds for extremely large ones. 222 * 223 * <p><b>Implementation notes</b>: This implementation restricts the 224 * maximum number of parties to 65535. Attempts to register additional 225 * parties result in {@code IllegalStateException}. However, you can and 226 * should create tiered phasers to accommodate arbitrarily large sets 227 * of participants. 228 * 229 * @since 1.7 230 * @author Doug Lea 231 */ 232 public class Phaser { 233 /* 234 * This class implements an extension of X10 "clocks". Thanks to 235 * Vijay Saraswat for the idea, and to Vivek Sarkar for 236 * enhancements to extend functionality. 237 */ 238 239 /** 240 * Primary state representation, holding four bit-fields: 241 * 242 * unarrived -- the number of parties yet to hit barrier (bits 0-15) 243 * parties -- the number of parties to wait (bits 16-31) 244 * phase -- the generation of the barrier (bits 32-62) 245 * terminated -- set if barrier is terminated (bit 63 / sign) 246 * 247 * Except that a phaser with no registered parties is 248 * distinguished by the otherwise illegal state of having zero 249 * parties and one unarrived parties (encoded as EMPTY below). 250 * 251 * To efficiently maintain atomicity, these values are packed into 252 * a single (atomic) long. Good performance relies on keeping 253 * state decoding and encoding simple, and keeping race windows 254 * short. 255 * 256 * All state updates are performed via CAS except initial 257 * registration of a sub-phaser (i.e., one with a non-null 258 * parent). In this (relatively rare) case, we use built-in 259 * synchronization to lock while first registering with its 260 * parent. 261 * 262 * The phase of a subphaser is allowed to lag that of its 263 * ancestors until it is actually accessed -- see method 264 * reconcileState. 265 */ 266 private volatile long state; 267 268 private static final int MAX_PARTIES = 0xffff; 269 private static final int MAX_PHASE = Integer.MAX_VALUE; 270 private static final int PARTIES_SHIFT = 16; 271 private static final int PHASE_SHIFT = 32; 272 private static final int UNARRIVED_MASK = 0xffff; // to mask ints 273 private static final long PARTIES_MASK = 0xffff0000L; // to mask longs 274 private static final long COUNTS_MASK = 0xffffffffL; 275 private static final long TERMINATION_BIT = 1L << 63; 276 277 // some special values 278 private static final int ONE_ARRIVAL = 1; 279 private static final int ONE_PARTY = 1 << PARTIES_SHIFT; 280 private static final int ONE_DEREGISTER = ONE_ARRIVAL|ONE_PARTY; 281 private static final int EMPTY = 1; 282 283 // The following unpacking methods are usually manually inlined 284 285 private static int unarrivedOf(long s) { 286 int counts = (int)s; 287 return (counts == EMPTY) ? 0 : (counts & UNARRIVED_MASK); 288 } 289 290 private static int partiesOf(long s) { 291 return (int)s >>> PARTIES_SHIFT; 292 } 293 294 private static int phaseOf(long s) { 295 return (int)(s >>> PHASE_SHIFT); 296 } 297 298 private static int arrivedOf(long s) { 299 int counts = (int)s; 300 return (counts == EMPTY) ? 0 : 301 (counts >>> PARTIES_SHIFT) - (counts & UNARRIVED_MASK); 302 } 303 304 /** 305 * The parent of this phaser, or null if none 306 */ 307 private final Phaser parent; 308 309 /** 310 * The root of phaser tree. Equals this if not in a tree. 311 */ 312 private final Phaser root; 313 314 /** 315 * Heads of Treiber stacks for waiting threads. To eliminate 316 * contention when releasing some threads while adding others, we 317 * use two of them, alternating across even and odd phases. 318 * Subphasers share queues with root to speed up releases. 319 */ 320 private final AtomicReference<QNode> evenQ; 321 private final AtomicReference<QNode> oddQ; 322 323 private AtomicReference<QNode> queueFor(int phase) { 324 return ((phase & 1) == 0) ? evenQ : oddQ; 325 } 326 327 /** 328 * Returns message string for bounds exceptions on arrival. 329 */ 330 private String badArrive(long s) { 331 return "Attempted arrival of unregistered party for " + 332 stateToString(s); 333 } 334 335 /** 336 * Returns message string for bounds exceptions on registration. 337 */ 338 private String badRegister(long s) { 339 return "Attempt to register more than " + 340 MAX_PARTIES + " parties for " + stateToString(s); 341 } 342 343 /** 344 * Main implementation for methods arrive and arriveAndDeregister. 345 * Manually tuned to speed up and minimize race windows for the 346 * common case of just decrementing unarrived field. 347 * 348 * @param adjust value to subtract from state; 349 * ONE_ARRIVAL for arrive, 350 * ONE_DEREGISTER for arriveAndDeregister 351 */ 352 private int doArrive(int adjust) { 353 final Phaser root = this.root; 354 for (;;) { 355 long s = (root == this) ? state : reconcileState(); 356 int phase = (int)(s >>> PHASE_SHIFT); 357 if (phase < 0) 358 return phase; 359 int counts = (int)s; 360 int unarrived = (counts == EMPTY) ? 0 : (counts & UNARRIVED_MASK); 361 if (unarrived <= 0) 362 throw new IllegalStateException(badArrive(s)); 363 if (UNSAFE.compareAndSwapLong(this, stateOffset, s, s-=adjust)) { 364 if (unarrived == 1) { 365 long n = s & PARTIES_MASK; // base of next state 366 int nextUnarrived = (int)n >>> PARTIES_SHIFT; 367 if (root == this) { 368 if (onAdvance(phase, nextUnarrived)) 369 n |= TERMINATION_BIT; 370 else if (nextUnarrived == 0) 371 n |= EMPTY; 372 else 373 n |= nextUnarrived; 374 int nextPhase = (phase + 1) & MAX_PHASE; 375 n |= (long)nextPhase << PHASE_SHIFT; 376 UNSAFE.compareAndSwapLong(this, stateOffset, s, n); 377 releaseWaiters(phase); 378 } 379 else if (nextUnarrived == 0) { // propagate deregistration 380 phase = parent.doArrive(ONE_DEREGISTER); 381 UNSAFE.compareAndSwapLong(this, stateOffset, 382 s, s | EMPTY); 383 } 384 else 385 phase = parent.doArrive(ONE_ARRIVAL); 386 } 387 return phase; 388 } 389 } 390 } 391 392 /** 393 * Implementation of register, bulkRegister 394 * 395 * @param registrations number to add to both parties and 396 * unarrived fields. Must be greater than zero. 397 */ 398 private int doRegister(int registrations) { 399 // adjustment to state 400 long adjust = ((long)registrations << PARTIES_SHIFT) | registrations; 401 final Phaser parent = this.parent; 402 int phase; 403 for (;;) { 404 long s = (parent == null) ? state : reconcileState(); 405 int counts = (int)s; 406 int parties = counts >>> PARTIES_SHIFT; 407 int unarrived = counts & UNARRIVED_MASK; 408 if (registrations > MAX_PARTIES - parties) 409 throw new IllegalStateException(badRegister(s)); 410 phase = (int)(s >>> PHASE_SHIFT); 411 if (phase < 0) 412 break; 413 if (counts != EMPTY) { // not 1st registration 414 if (parent == null || reconcileState() == s) { 415 if (unarrived == 0) // wait out advance 416 root.internalAwaitAdvance(phase, null); 417 else if (UNSAFE.compareAndSwapLong(this, stateOffset, 418 s, s + adjust)) 419 break; 420 } 421 } 422 else if (parent == null) { // 1st root registration 423 long next = ((long)phase << PHASE_SHIFT) | adjust; 424 if (UNSAFE.compareAndSwapLong(this, stateOffset, s, next)) 425 break; 426 } 427 else { 428 synchronized (this) { // 1st sub registration 429 if (state == s) { // recheck under lock 430 phase = parent.doRegister(1); 431 if (phase < 0) 432 break; 433 // finish registration whenever parent registration 434 // succeeded, even when racing with termination, 435 // since these are part of the same "transaction". 436 while (!UNSAFE.compareAndSwapLong 437 (this, stateOffset, s, 438 ((long)phase << PHASE_SHIFT) | adjust)) { 439 s = state; 440 phase = (int)(root.state >>> PHASE_SHIFT); 441 // assert (int)s == EMPTY; 442 } 443 break; 444 } 445 } 446 } 447 } 448 return phase; 449 } 450 451 /** 452 * Resolves lagged phase propagation from root if necessary. 453 * Reconciliation normally occurs when root has advanced but 454 * subphasers have not yet done so, in which case they must finish 455 * their own advance by setting unarrived to parties (or if 456 * parties is zero, resetting to unregistered EMPTY state). 457 * 458 * @return reconciled state 459 */ 460 private long reconcileState() { 461 final Phaser root = this.root; 462 long s = state; 463 if (root != this) { 464 int phase, p; 465 // CAS to root phase with current parties, tripping unarrived 466 while ((phase = (int)(root.state >>> PHASE_SHIFT)) != 467 (int)(s >>> PHASE_SHIFT) && 468 !UNSAFE.compareAndSwapLong 469 (this, stateOffset, s, 470 s = (((long)phase << PHASE_SHIFT) | 471 ((phase < 0) ? (s & COUNTS_MASK) : 472 (((p = (int)s >>> PARTIES_SHIFT) == 0) ? EMPTY : 473 ((s & PARTIES_MASK) | p)))))) 474 s = state; 475 } 476 return s; 477 } 478 479 /** 480 * Creates a new phaser with no initially registered parties, no 481 * parent, and initial phase number 0. Any thread using this 482 * phaser will need to first register for it. 483 */ 484 public Phaser() { 485 this(null, 0); 486 } 487 488 /** 489 * Creates a new phaser with the given number of registered 490 * unarrived parties, no parent, and initial phase number 0. 491 * 492 * @param parties the number of parties required to advance to the 493 * next phase 494 * @throws IllegalArgumentException if parties less than zero 495 * or greater than the maximum number of parties supported 496 */ 497 public Phaser(int parties) { 498 this(null, parties); 499 } 500 501 /** 502 * Equivalent to {@link #Phaser(Phaser, int) Phaser(parent, 0)}. 503 * 504 * @param parent the parent phaser 505 */ 506 public Phaser(Phaser parent) { 507 this(parent, 0); 508 } 509 510 /** 511 * Creates a new phaser with the given parent and number of 512 * registered unarrived parties. When the given parent is non-null 513 * and the given number of parties is greater than zero, this 514 * child phaser is registered with its parent. 515 * 516 * @param parent the parent phaser 517 * @param parties the number of parties required to advance to the 518 * next phase 519 * @throws IllegalArgumentException if parties less than zero 520 * or greater than the maximum number of parties supported 521 */ 522 public Phaser(Phaser parent, int parties) { 523 if (parties >>> PARTIES_SHIFT != 0) 524 throw new IllegalArgumentException("Illegal number of parties"); 525 int phase = 0; 526 this.parent = parent; 527 if (parent != null) { 528 final Phaser root = parent.root; 529 this.root = root; 530 this.evenQ = root.evenQ; 531 this.oddQ = root.oddQ; 532 if (parties != 0) 533 phase = parent.doRegister(1); 534 } 535 else { 536 this.root = this; 537 this.evenQ = new AtomicReference<QNode>(); 538 this.oddQ = new AtomicReference<QNode>(); 539 } 540 this.state = (parties == 0) ? (long)EMPTY : 541 ((long)phase << PHASE_SHIFT) | 542 ((long)parties << PARTIES_SHIFT) | 543 ((long)parties); 544 } 545 546 /** 547 * Adds a new unarrived party to this phaser. If an ongoing 548 * invocation of {@link #onAdvance} is in progress, this method 549 * may await its completion before returning. If this phaser has 550 * a parent, and this phaser previously had no registered parties, 551 * this child phaser is also registered with its parent. If 552 * this phaser is terminated, the attempt to register has 553 * no effect, and a negative value is returned. 554 * 555 * @return the arrival phase number to which this registration 556 * applied. If this value is negative, then this phaser has 557 * terminated, in which case registration has no effect. 558 * @throws IllegalStateException if attempting to register more 559 * than the maximum supported number of parties 560 */ 561 public int register() { 562 return doRegister(1); 563 } 564 565 /** 566 * Adds the given number of new unarrived parties to this phaser. 567 * If an ongoing invocation of {@link #onAdvance} is in progress, 568 * this method may await its completion before returning. If this 569 * phaser has a parent, and the given number of parties is greater 570 * than zero, and this phaser previously had no registered 571 * parties, this child phaser is also registered with its parent. 572 * If this phaser is terminated, the attempt to register has no 573 * effect, and a negative value is returned. 574 * 575 * @param parties the number of additional parties required to 576 * advance to the next phase 577 * @return the arrival phase number to which this registration 578 * applied. If this value is negative, then this phaser has 579 * terminated, in which case registration has no effect. 580 * @throws IllegalStateException if attempting to register more 581 * than the maximum supported number of parties 582 * @throws IllegalArgumentException if {@code parties < 0} 583 */ 584 public int bulkRegister(int parties) { 585 if (parties < 0) 586 throw new IllegalArgumentException(); 587 if (parties == 0) 588 return getPhase(); 589 return doRegister(parties); 590 } 591 592 /** 593 * Arrives at this phaser, without waiting for others to arrive. 594 * 595 * <p>It is a usage error for an unregistered party to invoke this 596 * method. However, this error may result in an {@code 597 * IllegalStateException} only upon some subsequent operation on 598 * this phaser, if ever. 599 * 600 * @return the arrival phase number, or a negative value if terminated 601 * @throws IllegalStateException if not terminated and the number 602 * of unarrived parties would become negative 603 */ 604 public int arrive() { 605 return doArrive(ONE_ARRIVAL); 606 } 607 608 /** 609 * Arrives at this phaser and deregisters from it without waiting 610 * for others to arrive. Deregistration reduces the number of 611 * parties required to advance in future phases. If this phaser 612 * has a parent, and deregistration causes this phaser to have 613 * zero parties, this phaser is also deregistered from its parent. 614 * 615 * <p>It is a usage error for an unregistered party to invoke this 616 * method. However, this error may result in an {@code 617 * IllegalStateException} only upon some subsequent operation on 618 * this phaser, if ever. 619 * 620 * @return the arrival phase number, or a negative value if terminated 621 * @throws IllegalStateException if not terminated and the number 622 * of registered or unarrived parties would become negative 623 */ 624 public int arriveAndDeregister() { 625 return doArrive(ONE_DEREGISTER); 626 } 627 628 /** 629 * Arrives at this phaser and awaits others. Equivalent in effect 630 * to {@code awaitAdvance(arrive())}. If you need to await with 631 * interruption or timeout, you can arrange this with an analogous 632 * construction using one of the other forms of the {@code 633 * awaitAdvance} method. If instead you need to deregister upon 634 * arrival, use {@code awaitAdvance(arriveAndDeregister())}. 635 * 636 * <p>It is a usage error for an unregistered party to invoke this 637 * method. However, this error may result in an {@code 638 * IllegalStateException} only upon some subsequent operation on 639 * this phaser, if ever. 640 * 641 * @return the arrival phase number, or the (negative) 642 * {@linkplain #getPhase() current phase} if terminated 643 * @throws IllegalStateException if not terminated and the number 644 * of unarrived parties would become negative 645 */ 646 public int arriveAndAwaitAdvance() { 647 // Specialization of doArrive+awaitAdvance eliminating some reads/paths 648 final Phaser root = this.root; 649 for (;;) { 650 long s = (root == this) ? state : reconcileState(); 651 int phase = (int)(s >>> PHASE_SHIFT); 652 if (phase < 0) 653 return phase; 654 int counts = (int)s; 655 int unarrived = (counts == EMPTY) ? 0 : (counts & UNARRIVED_MASK); 656 if (unarrived <= 0) 657 throw new IllegalStateException(badArrive(s)); 658 if (UNSAFE.compareAndSwapLong(this, stateOffset, s, 659 s -= ONE_ARRIVAL)) { 660 if (unarrived > 1) 661 return root.internalAwaitAdvance(phase, null); 662 if (root != this) 663 return parent.arriveAndAwaitAdvance(); 664 long n = s & PARTIES_MASK; // base of next state 665 int nextUnarrived = (int)n >>> PARTIES_SHIFT; 666 if (onAdvance(phase, nextUnarrived)) 667 n |= TERMINATION_BIT; 668 else if (nextUnarrived == 0) 669 n |= EMPTY; 670 else 671 n |= nextUnarrived; 672 int nextPhase = (phase + 1) & MAX_PHASE; 673 n |= (long)nextPhase << PHASE_SHIFT; 674 if (!UNSAFE.compareAndSwapLong(this, stateOffset, s, n)) 675 return (int)(state >>> PHASE_SHIFT); // terminated 676 releaseWaiters(phase); 677 return nextPhase; 678 } 679 } 680 } 681 682 /** 683 * Awaits the phase of this phaser to advance from the given phase 684 * value, returning immediately if the current phase is not equal 685 * to the given phase value or this phaser is terminated. 686 * 687 * @param phase an arrival phase number, or negative value if 688 * terminated; this argument is normally the value returned by a 689 * previous call to {@code arrive} or {@code arriveAndDeregister}. 690 * @return the next arrival phase number, or the argument if it is 691 * negative, or the (negative) {@linkplain #getPhase() current phase} 692 * if terminated 693 */ 694 public int awaitAdvance(int phase) { 695 final Phaser root = this.root; 696 long s = (root == this) ? state : reconcileState(); 697 int p = (int)(s >>> PHASE_SHIFT); 698 if (phase < 0) 699 return phase; 700 if (p == phase) 701 return root.internalAwaitAdvance(phase, null); 702 return p; 703 } 704 705 /** 706 * Awaits the phase of this phaser to advance from the given phase 707 * value, throwing {@code InterruptedException} if interrupted 708 * while waiting, or returning immediately if the current phase is 709 * not equal to the given phase value or this phaser is 710 * terminated. 711 * 712 * @param phase an arrival phase number, or negative value if 713 * terminated; this argument is normally the value returned by a 714 * previous call to {@code arrive} or {@code arriveAndDeregister}. 715 * @return the next arrival phase number, or the argument if it is 716 * negative, or the (negative) {@linkplain #getPhase() current phase} 717 * if terminated 718 * @throws InterruptedException if thread interrupted while waiting 719 */ 720 public int awaitAdvanceInterruptibly(int phase) 721 throws InterruptedException { 722 final Phaser root = this.root; 723 long s = (root == this) ? state : reconcileState(); 724 int p = (int)(s >>> PHASE_SHIFT); 725 if (phase < 0) 726 return phase; 727 if (p == phase) { 728 QNode node = new QNode(this, phase, true, false, 0L); 729 p = root.internalAwaitAdvance(phase, node); 730 if (node.wasInterrupted) 731 throw new InterruptedException(); 732 } 733 return p; 734 } 735 736 /** 737 * Awaits the phase of this phaser to advance from the given phase 738 * value or the given timeout to elapse, throwing {@code 739 * InterruptedException} if interrupted while waiting, or 740 * returning immediately if the current phase is not equal to the 741 * given phase value or this phaser is terminated. 742 * 743 * @param phase an arrival phase number, or negative value if 744 * terminated; this argument is normally the value returned by a 745 * previous call to {@code arrive} or {@code arriveAndDeregister}. 746 * @param timeout how long to wait before giving up, in units of 747 * {@code unit} 748 * @param unit a {@code TimeUnit} determining how to interpret the 749 * {@code timeout} parameter 750 * @return the next arrival phase number, or the argument if it is 751 * negative, or the (negative) {@linkplain #getPhase() current phase} 752 * if terminated 753 * @throws InterruptedException if thread interrupted while waiting 754 * @throws TimeoutException if timed out while waiting 755 */ 756 public int awaitAdvanceInterruptibly(int phase, 757 long timeout, TimeUnit unit) 758 throws InterruptedException, TimeoutException { 759 long nanos = unit.toNanos(timeout); 760 final Phaser root = this.root; 761 long s = (root == this) ? state : reconcileState(); 762 int p = (int)(s >>> PHASE_SHIFT); 763 if (phase < 0) 764 return phase; 765 if (p == phase) { 766 QNode node = new QNode(this, phase, true, true, nanos); 767 p = root.internalAwaitAdvance(phase, node); 768 if (node.wasInterrupted) 769 throw new InterruptedException(); 770 else if (p == phase) 771 throw new TimeoutException(); 772 } 773 return p; 774 } 775 776 /** 777 * Forces this phaser to enter termination state. Counts of 778 * registered parties are unaffected. If this phaser is a member 779 * of a tiered set of phasers, then all of the phasers in the set 780 * are terminated. If this phaser is already terminated, this 781 * method has no effect. This method may be useful for 782 * coordinating recovery after one or more tasks encounter 783 * unexpected exceptions. 784 */ 785 public void forceTermination() { 786 // Only need to change root state 787 final Phaser root = this.root; 788 long s; 789 while ((s = root.state) >= 0) { 790 if (UNSAFE.compareAndSwapLong(root, stateOffset, 791 s, s | TERMINATION_BIT)) { 792 // signal all threads 793 releaseWaiters(0); // Waiters on evenQ 794 releaseWaiters(1); // Waiters on oddQ 795 return; 796 } 797 } 798 } 799 800 /** 801 * Returns the current phase number. The maximum phase number is 802 * {@code Integer.MAX_VALUE}, after which it restarts at 803 * zero. Upon termination, the phase number is negative, 804 * in which case the prevailing phase prior to termination 805 * may be obtained via {@code getPhase() + Integer.MIN_VALUE}. 806 * 807 * @return the phase number, or a negative value if terminated 808 */ 809 public final int getPhase() { 810 return (int)(root.state >>> PHASE_SHIFT); 811 } 812 813 /** 814 * Returns the number of parties registered at this phaser. 815 * 816 * @return the number of parties 817 */ 818 public int getRegisteredParties() { 819 return partiesOf(state); 820 } 821 822 /** 823 * Returns the number of registered parties that have arrived at 824 * the current phase of this phaser. If this phaser has terminated, 825 * the returned value is meaningless and arbitrary. 826 * 827 * @return the number of arrived parties 828 */ 829 public int getArrivedParties() { 830 return arrivedOf(reconcileState()); 831 } 832 833 /** 834 * Returns the number of registered parties that have not yet 835 * arrived at the current phase of this phaser. If this phaser has 836 * terminated, the returned value is meaningless and arbitrary. 837 * 838 * @return the number of unarrived parties 839 */ 840 public int getUnarrivedParties() { 841 return unarrivedOf(reconcileState()); 842 } 843 844 /** 845 * Returns the parent of this phaser, or {@code null} if none. 846 * 847 * @return the parent of this phaser, or {@code null} if none 848 */ 849 public Phaser getParent() { 850 return parent; 851 } 852 853 /** 854 * Returns the root ancestor of this phaser, which is the same as 855 * this phaser if it has no parent. 856 * 857 * @return the root ancestor of this phaser 858 */ 859 public Phaser getRoot() { 860 return root; 861 } 862 863 /** 864 * Returns {@code true} if this phaser has been terminated. 865 * 866 * @return {@code true} if this phaser has been terminated 867 */ 868 public boolean isTerminated() { 869 return root.state < 0L; 870 } 871 872 /** 873 * Overridable method to perform an action upon impending phase 874 * advance, and to control termination. This method is invoked 875 * upon arrival of the party advancing this phaser (when all other 876 * waiting parties are dormant). If this method returns {@code 877 * true}, this phaser will be set to a final termination state 878 * upon advance, and subsequent calls to {@link #isTerminated} 879 * will return true. Any (unchecked) Exception or Error thrown by 880 * an invocation of this method is propagated to the party 881 * attempting to advance this phaser, in which case no advance 882 * occurs. 883 * 884 * <p>The arguments to this method provide the state of the phaser 885 * prevailing for the current transition. The effects of invoking 886 * arrival, registration, and waiting methods on this phaser from 887 * within {@code onAdvance} are unspecified and should not be 888 * relied on. 889 * 890 * <p>If this phaser is a member of a tiered set of phasers, then 891 * {@code onAdvance} is invoked only for its root phaser on each 892 * advance. 893 * 894 * <p>To support the most common use cases, the default 895 * implementation of this method returns {@code true} when the 896 * number of registered parties has become zero as the result of a 897 * party invoking {@code arriveAndDeregister}. You can disable 898 * this behavior, thus enabling continuation upon future 899 * registrations, by overriding this method to always return 900 * {@code false}: 901 * 902 * <pre> {@code 903 * Phaser phaser = new Phaser() { 904 * protected boolean onAdvance(int phase, int parties) { return false; } 905 * }}</pre> 906 * 907 * @param phase the current phase number on entry to this method, 908 * before this phaser is advanced 909 * @param registeredParties the current number of registered parties 910 * @return {@code true} if this phaser should terminate 911 */ 912 protected boolean onAdvance(int phase, int registeredParties) { 913 return registeredParties == 0; 914 } 915 916 /** 917 * Returns a string identifying this phaser, as well as its 918 * state. The state, in brackets, includes the String {@code 919 * "phase = "} followed by the phase number, {@code "parties = "} 920 * followed by the number of registered parties, and {@code 921 * "arrived = "} followed by the number of arrived parties. 922 * 923 * @return a string identifying this phaser, as well as its state 924 */ 925 public String toString() { 926 return stateToString(reconcileState()); 927 } 928 929 /** 930 * Implementation of toString and string-based error messages 931 */ 932 private String stateToString(long s) { 933 return super.toString() + 934 "[phase = " + phaseOf(s) + 935 " parties = " + partiesOf(s) + 936 " arrived = " + arrivedOf(s) + "]"; 937 } 938 939 // Waiting mechanics 940 941 /** 942 * Removes and signals threads from queue for phase. 943 */ 944 private void releaseWaiters(int phase) { 945 QNode q; // first element of queue 946 Thread t; // its thread 947 AtomicReference<QNode> head = (phase & 1) == 0 ? evenQ : oddQ; 948 while ((q = head.get()) != null && 949 q.phase != (int)(root.state >>> PHASE_SHIFT)) { 950 if (head.compareAndSet(q, q.next) && 951 (t = q.thread) != null) { 952 q.thread = null; 953 LockSupport.unpark(t); 954 } 955 } 956 } 957 958 /** 959 * Variant of releaseWaiters that additionally tries to remove any 960 * nodes no longer waiting for advance due to timeout or 961 * interrupt. Currently, nodes are removed only if they are at 962 * head of queue, which suffices to reduce memory footprint in 963 * most usages. 964 * 965 * @return current phase on exit 966 */ 967 private int abortWait(int phase) { 968 AtomicReference<QNode> head = (phase & 1) == 0 ? evenQ : oddQ; 969 for (;;) { 970 Thread t; 971 QNode q = head.get(); 972 int p = (int)(root.state >>> PHASE_SHIFT); 973 if (q == null || ((t = q.thread) != null && q.phase == p)) 974 return p; 975 if (head.compareAndSet(q, q.next) && t != null) { 976 q.thread = null; 977 LockSupport.unpark(t); 978 } 979 } 980 } 981 982 /** The number of CPUs, for spin control */ 983 private static final int NCPU = Runtime.getRuntime().availableProcessors(); 984 985 /** 986 * The number of times to spin before blocking while waiting for 987 * advance, per arrival while waiting. On multiprocessors, fully 988 * blocking and waking up a large number of threads all at once is 989 * usually a very slow process, so we use rechargeable spins to 990 * avoid it when threads regularly arrive: When a thread in 991 * internalAwaitAdvance notices another arrival before blocking, 992 * and there appear to be enough CPUs available, it spins 993 * SPINS_PER_ARRIVAL more times before blocking. The value trades 994 * off good-citizenship vs big unnecessary slowdowns. 995 */ 996 static final int SPINS_PER_ARRIVAL = (NCPU < 2) ? 1 : 1 << 8; 997 998 /** 999 * Possibly blocks and waits for phase to advance unless aborted. 1000 * Call only on root phaser. 1001 * 1002 * @param phase current phase 1003 * @param node if non-null, the wait node to track interrupt and timeout; 1004 * if null, denotes noninterruptible wait 1005 * @return current phase 1006 */ 1007 private int internalAwaitAdvance(int phase, QNode node) { 1008 // assert root == this; 1009 releaseWaiters(phase-1); // ensure old queue clean 1010 boolean queued = false; // true when node is enqueued 1011 int lastUnarrived = 0; // to increase spins upon change 1012 int spins = SPINS_PER_ARRIVAL; 1013 long s; 1014 int p; 1015 while ((p = (int)((s = state) >>> PHASE_SHIFT)) == phase) { 1016 if (node == null) { // spinning in noninterruptible mode 1017 int unarrived = (int)s & UNARRIVED_MASK; 1018 if (unarrived != lastUnarrived && 1019 (lastUnarrived = unarrived) < NCPU) 1020 spins += SPINS_PER_ARRIVAL; 1021 boolean interrupted = Thread.interrupted(); 1022 if (interrupted || --spins < 0) { // need node to record intr 1023 node = new QNode(this, phase, false, false, 0L); 1024 node.wasInterrupted = interrupted; 1025 } 1026 } 1027 else if (node.isReleasable()) // done or aborted 1028 break; 1029 else if (!queued) { // push onto queue 1030 AtomicReference<QNode> head = (phase & 1) == 0 ? evenQ : oddQ; 1031 QNode q = node.next = head.get(); 1032 if ((q == null || q.phase == phase) && 1033 (int)(state >>> PHASE_SHIFT) == phase) // avoid stale enq 1034 queued = head.compareAndSet(q, node); 1035 } 1036 else { 1037 try { 1038 ForkJoinPool.managedBlock(node); 1039 } catch (InterruptedException ie) { 1040 node.wasInterrupted = true; 1041 } 1042 } 1043 } 1044 1045 if (node != null) { 1046 if (node.thread != null) 1047 node.thread = null; // avoid need for unpark() 1048 if (node.wasInterrupted && !node.interruptible) 1049 Thread.currentThread().interrupt(); 1050 if (p == phase && (p = (int)(state >>> PHASE_SHIFT)) == phase) 1051 return abortWait(phase); // possibly clean up on abort 1052 } 1053 releaseWaiters(phase); 1054 return p; 1055 } 1056 1057 /** 1058 * Wait nodes for Treiber stack representing wait queue 1059 */ 1060 static final class QNode implements ForkJoinPool.ManagedBlocker { 1061 final Phaser phaser; 1062 final int phase; 1063 final boolean interruptible; 1064 final boolean timed; 1065 boolean wasInterrupted; 1066 long nanos; 1067 final long deadline; 1068 volatile Thread thread; // nulled to cancel wait 1069 QNode next; 1070 1071 QNode(Phaser phaser, int phase, boolean interruptible, 1072 boolean timed, long nanos) { 1073 this.phaser = phaser; 1074 this.phase = phase; 1075 this.interruptible = interruptible; 1076 this.nanos = nanos; 1077 this.timed = timed; 1078 this.deadline = timed ? System.nanoTime() + nanos : 0L; 1079 thread = Thread.currentThread(); 1080 } 1081 1082 public boolean isReleasable() { 1083 if (thread == null) 1084 return true; 1085 if (phaser.getPhase() != phase) { 1086 thread = null; 1087 return true; 1088 } 1089 if (Thread.interrupted()) 1090 wasInterrupted = true; 1091 if (wasInterrupted && interruptible) { 1092 thread = null; 1093 return true; 1094 } 1095 if (timed) { 1096 if (nanos > 0L) { 1097 nanos = deadline - System.nanoTime(); 1098 } 1099 if (nanos <= 0L) { 1100 thread = null; 1101 return true; 1102 } 1103 } 1104 return false; 1105 } 1106 1107 public boolean block() { 1108 if (isReleasable()) 1109 return true; 1110 else if (!timed) 1111 LockSupport.park(this); 1112 else if (nanos > 0L) 1113 LockSupport.parkNanos(this, nanos); 1114 return isReleasable(); 1115 } 1116 } 1117 1118 // Unsafe mechanics 1119 1120 private static final sun.misc.Unsafe UNSAFE; 1121 private static final long stateOffset; 1122 static { 1123 try { 1124 UNSAFE = sun.misc.Unsafe.getUnsafe(); 1125 Class<?> k = Phaser.class; 1126 stateOffset = UNSAFE.objectFieldOffset 1127 (k.getDeclaredField("state")); 1128 } catch (Exception e) { 1129 throw new Error(e); 1130 } 1131 } 1132 } 1133
