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