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.concurrent.locks.Condition; 39 import java.util.concurrent.locks.ReentrantLock; 40 41 /** 42 * A synchronization aid that allows a set of threads to all wait for 43 * each other to reach a common barrier point. CyclicBarriers are 44 * useful in programs involving a fixed sized party of threads that 45 * must occasionally wait for each other. The barrier is called 46 * <em>cyclic</em> because it can be re-used after the waiting threads 47 * are released. 48 * 49 * <p>A {@code CyclicBarrier} supports an optional {@link Runnable} command 50 * that is run once per barrier point, after the last thread in the party 51 * arrives, but before any threads are released. 52 * This <em>barrier action</em> is useful 53 * for updating shared-state before any of the parties continue. 54 * 55 * <p><b>Sample usage:</b> Here is an example of using a barrier in a 56 * parallel decomposition design: 57 * 58 * <pre> {@code 59 * class Solver { 60 * final int N; 61 * final float[][] data; 62 * final CyclicBarrier barrier; 63 * 64 * class Worker implements Runnable { 65 * int myRow; 66 * Worker(int row) { myRow = row; } 67 * public void run() { 68 * while (!done()) { 69 * processRow(myRow); 70 * 71 * try { 72 * barrier.await(); 73 * } catch (InterruptedException ex) { 74 * return; 75 * } catch (BrokenBarrierException ex) { 76 * return; 77 * } 78 * } 79 * } 80 * } 81 * 82 * public Solver(float[][] matrix) { 83 * data = matrix; 84 * N = matrix.length; 85 * Runnable barrierAction = () -> mergeRows(...); 86 * barrier = new CyclicBarrier(N, barrierAction); 87 * 88 * List<Thread> threads = new ArrayList<>(N); 89 * for (int i = 0; i < N; i++) { 90 * Thread thread = new Thread(new Worker(i)); 91 * threads.add(thread); 92 * thread.start(); 93 * } 94 * 95 * // wait until done 96 * for (Thread thread : threads) 97 * try { 98 * thread.join(); 99 * } catch (InterruptedException ex) { } 100 * } 101 * }}</pre> 102 * 103 * Here, each worker thread processes a row of the matrix, then waits at the 104 * barrier until all rows have been processed. When all rows are processed the 105 * supplied {@link Runnable} barrier action is executed and merges the rows. 106 * If the merger determines that a solution has been found then {@code done()} 107 * will return {@code true} and each worker will terminate. 108 * 109 * <p>If the barrier action does not rely on the parties being suspended when 110 * it is executed, then any of the threads in the party could execute that 111 * action when it is released. To facilitate this, each invocation of 112 * {@link #await} returns the arrival index of that thread at the barrier. 113 * You can then choose which thread should execute the barrier action, for 114 * example: 115 * <pre> {@code 116 * if (barrier.await() == 0) { 117 * // log the completion of this iteration 118 * }}</pre> 119 * 120 * <p>The {@code CyclicBarrier} uses an all-or-none breakage model 121 * for failed synchronization attempts: If a thread leaves a barrier 122 * point prematurely because of interruption, failure, or timeout, all 123 * other threads waiting at that barrier point will also leave 124 * abnormally via {@link BrokenBarrierException} (or 125 * {@link InterruptedException} if they too were interrupted at about 126 * the same time). 127 * 128 * <p>Memory consistency effects: Actions in a thread prior to calling 129 * {@code await()} 130 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> 131 * actions that are part of the barrier action, which in turn 132 * <i>happen-before</i> actions following a successful return from the 133 * corresponding {@code await()} in other threads. 134 * 135 * @see CountDownLatch 136 * @see Phaser 137 * 138 * @author Doug Lea 139 * @since 1.5 140 */ 141 public class CyclicBarrier { 142 /** 143 * Each use of the barrier is represented as a generation instance. 144 * The generation changes whenever the barrier is tripped, or 145 * is reset. There can be many generations associated with threads 146 * using the barrier - due to the non-deterministic way the lock 147 * may be allocated to waiting threads - but only one of these 148 * can be active at a time (the one to which {@code count} applies) 149 * and all the rest are either broken or tripped. 150 * There need not be an active generation if there has been a break 151 * but no subsequent reset. 152 */ 153 private static class Generation { Generation()154 Generation() {} // prevent access constructor creation 155 boolean broken; // initially false 156 } 157 158 /** The lock for guarding barrier entry */ 159 private final ReentrantLock lock = new ReentrantLock(); 160 /** Condition to wait on until tripped */ 161 private final Condition trip = lock.newCondition(); 162 /** The number of parties */ 163 private final int parties; 164 /** The command to run when tripped */ 165 private final Runnable barrierCommand; 166 /** The current generation */ 167 private Generation generation = new Generation(); 168 169 /** 170 * Number of parties still waiting. Counts down from parties to 0 171 * on each generation. It is reset to parties on each new 172 * generation or when broken. 173 */ 174 private int count; 175 176 /** 177 * Updates state on barrier trip and wakes up everyone. 178 * Called only while holding lock. 179 */ nextGeneration()180 private void nextGeneration() { 181 // signal completion of last generation 182 trip.signalAll(); 183 // set up next generation 184 count = parties; 185 generation = new Generation(); 186 } 187 188 /** 189 * Sets current barrier generation as broken and wakes up everyone. 190 * Called only while holding lock. 191 */ breakBarrier()192 private void breakBarrier() { 193 generation.broken = true; 194 count = parties; 195 trip.signalAll(); 196 } 197 198 /** 199 * Main barrier code, covering the various policies. 200 */ dowait(boolean timed, long nanos)201 private int dowait(boolean timed, long nanos) 202 throws InterruptedException, BrokenBarrierException, 203 TimeoutException { 204 final ReentrantLock lock = this.lock; 205 lock.lock(); 206 try { 207 final Generation g = generation; 208 209 if (g.broken) 210 throw new BrokenBarrierException(); 211 212 if (Thread.interrupted()) { 213 breakBarrier(); 214 throw new InterruptedException(); 215 } 216 217 int index = --count; 218 if (index == 0) { // tripped 219 Runnable command = barrierCommand; 220 if (command != null) { 221 try { 222 command.run(); 223 } catch (Throwable ex) { 224 breakBarrier(); 225 throw ex; 226 } 227 } 228 nextGeneration(); 229 return 0; 230 } 231 232 // loop until tripped, broken, interrupted, or timed out 233 for (;;) { 234 try { 235 if (!timed) 236 trip.await(); 237 else if (nanos > 0L) 238 nanos = trip.awaitNanos(nanos); 239 } catch (InterruptedException ie) { 240 if (g == generation && ! g.broken) { 241 breakBarrier(); 242 throw ie; 243 } else { 244 // We're about to finish waiting even if we had not 245 // been interrupted, so this interrupt is deemed to 246 // "belong" to subsequent execution. 247 Thread.currentThread().interrupt(); 248 } 249 } 250 251 if (g.broken) 252 throw new BrokenBarrierException(); 253 254 if (g != generation) 255 return index; 256 257 if (timed && nanos <= 0L) { 258 breakBarrier(); 259 throw new TimeoutException(); 260 } 261 } 262 } finally { 263 lock.unlock(); 264 } 265 } 266 267 /** 268 * Creates a new {@code CyclicBarrier} that will trip when the 269 * given number of parties (threads) are waiting upon it, and which 270 * will execute the given barrier action when the barrier is tripped, 271 * performed by the last thread entering the barrier. 272 * 273 * @param parties the number of threads that must invoke {@link #await} 274 * before the barrier is tripped 275 * @param barrierAction the command to execute when the barrier is 276 * tripped, or {@code null} if there is no action 277 * @throws IllegalArgumentException if {@code parties} is less than 1 278 */ CyclicBarrier(int parties, Runnable barrierAction)279 public CyclicBarrier(int parties, Runnable barrierAction) { 280 if (parties <= 0) throw new IllegalArgumentException(); 281 this.parties = parties; 282 this.count = parties; 283 this.barrierCommand = barrierAction; 284 } 285 286 /** 287 * Creates a new {@code CyclicBarrier} that will trip when the 288 * given number of parties (threads) are waiting upon it, and 289 * does not perform a predefined action when the barrier is tripped. 290 * 291 * @param parties the number of threads that must invoke {@link #await} 292 * before the barrier is tripped 293 * @throws IllegalArgumentException if {@code parties} is less than 1 294 */ CyclicBarrier(int parties)295 public CyclicBarrier(int parties) { 296 this(parties, null); 297 } 298 299 /** 300 * Returns the number of parties required to trip this barrier. 301 * 302 * @return the number of parties required to trip this barrier 303 */ getParties()304 public int getParties() { 305 return parties; 306 } 307 308 /** 309 * Waits until all {@linkplain #getParties parties} have invoked 310 * {@code await} on this barrier. 311 * 312 * <p>If the current thread is not the last to arrive then it is 313 * disabled for thread scheduling purposes and lies dormant until 314 * one of the following things happens: 315 * <ul> 316 * <li>The last thread arrives; or 317 * <li>Some other thread {@linkplain Thread#interrupt interrupts} 318 * the current thread; or 319 * <li>Some other thread {@linkplain Thread#interrupt interrupts} 320 * one of the other waiting threads; or 321 * <li>Some other thread times out while waiting for barrier; or 322 * <li>Some other thread invokes {@link #reset} on this barrier. 323 * </ul> 324 * 325 * <p>If the current thread: 326 * <ul> 327 * <li>has its interrupted status set on entry to this method; or 328 * <li>is {@linkplain Thread#interrupt interrupted} while waiting 329 * </ul> 330 * then {@link InterruptedException} is thrown and the current thread's 331 * interrupted status is cleared. 332 * 333 * <p>If the barrier is {@link #reset} while any thread is waiting, 334 * or if the barrier {@linkplain #isBroken is broken} when 335 * {@code await} is invoked, or while any thread is waiting, then 336 * {@link BrokenBarrierException} is thrown. 337 * 338 * <p>If any thread is {@linkplain Thread#interrupt interrupted} while waiting, 339 * then all other waiting threads will throw 340 * {@link BrokenBarrierException} and the barrier is placed in the broken 341 * state. 342 * 343 * <p>If the current thread is the last thread to arrive, and a 344 * non-null barrier action was supplied in the constructor, then the 345 * current thread runs the action before allowing the other threads to 346 * continue. 347 * If an exception occurs during the barrier action then that exception 348 * will be propagated in the current thread and the barrier is placed in 349 * the broken state. 350 * 351 * @return the arrival index of the current thread, where index 352 * {@code getParties() - 1} indicates the first 353 * to arrive and zero indicates the last to arrive 354 * @throws InterruptedException if the current thread was interrupted 355 * while waiting 356 * @throws BrokenBarrierException if <em>another</em> thread was 357 * interrupted or timed out while the current thread was 358 * waiting, or the barrier was reset, or the barrier was 359 * broken when {@code await} was called, or the barrier 360 * action (if present) failed due to an exception 361 */ await()362 public int await() throws InterruptedException, BrokenBarrierException { 363 try { 364 return dowait(false, 0L); 365 } catch (TimeoutException toe) { 366 throw new Error(toe); // cannot happen 367 } 368 } 369 370 /** 371 * Waits until all {@linkplain #getParties parties} have invoked 372 * {@code await} on this barrier, or the specified waiting time elapses. 373 * 374 * <p>If the current thread is not the last to arrive then it is 375 * disabled for thread scheduling purposes and lies dormant until 376 * one of the following things happens: 377 * <ul> 378 * <li>The last thread arrives; or 379 * <li>The specified timeout elapses; or 380 * <li>Some other thread {@linkplain Thread#interrupt interrupts} 381 * the current thread; or 382 * <li>Some other thread {@linkplain Thread#interrupt interrupts} 383 * one of the other waiting threads; or 384 * <li>Some other thread times out while waiting for barrier; or 385 * <li>Some other thread invokes {@link #reset} on this barrier. 386 * </ul> 387 * 388 * <p>If the current thread: 389 * <ul> 390 * <li>has its interrupted status set on entry to this method; or 391 * <li>is {@linkplain Thread#interrupt interrupted} while waiting 392 * </ul> 393 * then {@link InterruptedException} is thrown and the current thread's 394 * interrupted status is cleared. 395 * 396 * <p>If the specified waiting time elapses then {@link TimeoutException} 397 * is thrown. If the time is less than or equal to zero, the 398 * method will not wait at all. 399 * 400 * <p>If the barrier is {@link #reset} while any thread is waiting, 401 * or if the barrier {@linkplain #isBroken is broken} when 402 * {@code await} is invoked, or while any thread is waiting, then 403 * {@link BrokenBarrierException} is thrown. 404 * 405 * <p>If any thread is {@linkplain Thread#interrupt interrupted} while 406 * waiting, then all other waiting threads will throw {@link 407 * BrokenBarrierException} and the barrier is placed in the broken 408 * state. 409 * 410 * <p>If the current thread is the last thread to arrive, and a 411 * non-null barrier action was supplied in the constructor, then the 412 * current thread runs the action before allowing the other threads to 413 * continue. 414 * If an exception occurs during the barrier action then that exception 415 * will be propagated in the current thread and the barrier is placed in 416 * the broken state. 417 * 418 * @param timeout the time to wait for the barrier 419 * @param unit the time unit of the timeout parameter 420 * @return the arrival index of the current thread, where index 421 * {@code getParties() - 1} indicates the first 422 * to arrive and zero indicates the last to arrive 423 * @throws InterruptedException if the current thread was interrupted 424 * while waiting 425 * @throws TimeoutException if the specified timeout elapses. 426 * In this case the barrier will be broken. 427 * @throws BrokenBarrierException if <em>another</em> thread was 428 * interrupted or timed out while the current thread was 429 * waiting, or the barrier was reset, or the barrier was broken 430 * when {@code await} was called, or the barrier action (if 431 * present) failed due to an exception 432 */ await(long timeout, TimeUnit unit)433 public int await(long timeout, TimeUnit unit) 434 throws InterruptedException, 435 BrokenBarrierException, 436 TimeoutException { 437 return dowait(true, unit.toNanos(timeout)); 438 } 439 440 /** 441 * Queries if this barrier is in a broken state. 442 * 443 * @return {@code true} if one or more parties broke out of this 444 * barrier due to interruption or timeout since 445 * construction or the last reset, or a barrier action 446 * failed due to an exception; {@code false} otherwise. 447 */ isBroken()448 public boolean isBroken() { 449 final ReentrantLock lock = this.lock; 450 lock.lock(); 451 try { 452 return generation.broken; 453 } finally { 454 lock.unlock(); 455 } 456 } 457 458 /** 459 * Resets the barrier to its initial state. If any parties are 460 * currently waiting at the barrier, they will return with a 461 * {@link BrokenBarrierException}. Note that resets <em>after</em> 462 * a breakage has occurred for other reasons can be complicated to 463 * carry out; threads need to re-synchronize in some other way, 464 * and choose one to perform the reset. It may be preferable to 465 * instead create a new barrier for subsequent use. 466 */ reset()467 public void reset() { 468 final ReentrantLock lock = this.lock; 469 lock.lock(); 470 try { 471 breakBarrier(); // break the current generation 472 nextGeneration(); // start a new generation 473 } finally { 474 lock.unlock(); 475 } 476 } 477 478 /** 479 * Returns the number of parties currently waiting at the barrier. 480 * This method is primarily useful for debugging and assertions. 481 * 482 * @return the number of parties currently blocked in {@link #await} 483 */ getNumberWaiting()484 public int getNumberWaiting() { 485 final ReentrantLock lock = this.lock; 486 lock.lock(); 487 try { 488 return parties - count; 489 } finally { 490 lock.unlock(); 491 } 492 } 493 } 494