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.locks; 37 38 import jdk.internal.misc.Unsafe; 39 40 /** 41 * Basic thread blocking primitives for creating locks and other 42 * synchronization classes. 43 * 44 * <p>This class associates, with each thread that uses it, a permit 45 * (in the sense of the {@link java.util.concurrent.Semaphore 46 * Semaphore} class). A call to {@code park} will return immediately 47 * if the permit is available, consuming it in the process; otherwise 48 * it <em>may</em> block. A call to {@code unpark} makes the permit 49 * available, if it was not already available. (Unlike with Semaphores 50 * though, permits do not accumulate. There is at most one.) 51 * Reliable usage requires the use of volatile (or atomic) variables 52 * to control when to park or unpark. Orderings of calls to these 53 * methods are maintained with respect to volatile variable accesses, 54 * but not necessarily non-volatile variable accesses. 55 * 56 * <p>Methods {@code park} and {@code unpark} provide efficient 57 * means of blocking and unblocking threads that do not encounter the 58 * problems that cause the deprecated methods {@code Thread.suspend} 59 * and {@code Thread.resume} to be unusable for such purposes: Races 60 * between one thread invoking {@code park} and another thread trying 61 * to {@code unpark} it will preserve liveness, due to the 62 * permit. Additionally, {@code park} will return if the caller's 63 * thread was interrupted, and timeout versions are supported. The 64 * {@code park} method may also return at any other time, for "no 65 * reason", so in general must be invoked within a loop that rechecks 66 * conditions upon return. In this sense {@code park} serves as an 67 * optimization of a "busy wait" that does not waste as much time 68 * spinning, but must be paired with an {@code unpark} to be 69 * effective. 70 * 71 * <p>The three forms of {@code park} each also support a 72 * {@code blocker} object parameter. This object is recorded while 73 * the thread is blocked to permit monitoring and diagnostic tools to 74 * identify the reasons that threads are blocked. (Such tools may 75 * access blockers using method {@link #getBlocker(Thread)}.) 76 * The use of these forms rather than the original forms without this 77 * parameter is strongly encouraged. The normal argument to supply as 78 * a {@code blocker} within a lock implementation is {@code this}. 79 * 80 * <p>These methods are designed to be used as tools for creating 81 * higher-level synchronization utilities, and are not in themselves 82 * useful for most concurrency control applications. The {@code park} 83 * method is designed for use only in constructions of the form: 84 * 85 * <pre> {@code 86 * while (!canProceed()) { 87 * // ensure request to unpark is visible to other threads 88 * ... 89 * LockSupport.park(this); 90 * }}</pre> 91 * 92 * where no actions by the thread publishing a request to unpark, 93 * prior to the call to {@code park}, entail locking or blocking. 94 * Because only one permit is associated with each thread, any 95 * intermediary uses of {@code park}, including implicitly via class 96 * loading, could lead to an unresponsive thread (a "lost unpark"). 97 * 98 * <p><b>Sample Usage.</b> Here is a sketch of a first-in-first-out 99 * non-reentrant lock class: 100 * <pre> {@code 101 * class FIFOMutex { 102 * private final AtomicBoolean locked = new AtomicBoolean(false); 103 * private final Queue<Thread> waiters 104 * = new ConcurrentLinkedQueue<>(); 105 * 106 * public void lock() { 107 * boolean wasInterrupted = false; 108 * // publish current thread for unparkers 109 * waiters.add(Thread.currentThread()); 110 * 111 * // Block while not first in queue or cannot acquire lock 112 * while (waiters.peek() != Thread.currentThread() || 113 * !locked.compareAndSet(false, true)) { 114 * LockSupport.park(this); 115 * // ignore interrupts while waiting 116 * if (Thread.interrupted()) 117 * wasInterrupted = true; 118 * } 119 * 120 * waiters.remove(); 121 * // ensure correct interrupt status on return 122 * if (wasInterrupted) 123 * Thread.currentThread().interrupt(); 124 * } 125 * 126 * public void unlock() { 127 * locked.set(false); 128 * LockSupport.unpark(waiters.peek()); 129 * } 130 * 131 * static { 132 * // Reduce the risk of "lost unpark" due to classloading 133 * Class<?> ensureLoaded = LockSupport.class; 134 * } 135 * }}</pre> 136 * 137 * @since 1.5 138 */ 139 public class LockSupport { 140 private LockSupport() {} // Cannot be instantiated. 141 142 private static void setBlocker(Thread t, Object arg) { 143 // Even though volatile, hotspot doesn't need a write barrier here. 144 U.putReference(t, PARKBLOCKER, arg); 145 } 146 147 /** 148 * Makes available the permit for the given thread, if it 149 * was not already available. If the thread was blocked on 150 * {@code park} then it will unblock. Otherwise, its next call 151 * to {@code park} is guaranteed not to block. This operation 152 * is not guaranteed to have any effect at all if the given 153 * thread has not been started. 154 * 155 * @param thread the thread to unpark, or {@code null}, in which case 156 * this operation has no effect 157 */ 158 public static void unpark(Thread thread) { 159 if (thread != null) 160 U.unpark(thread); 161 } 162 163 /** 164 * Disables the current thread for thread scheduling purposes unless the 165 * permit is available. 166 * 167 * <p>If the permit is available then it is consumed and the call returns 168 * immediately; otherwise 169 * the current thread becomes disabled for thread scheduling 170 * purposes and lies dormant until one of three things happens: 171 * 172 * <ul> 173 * <li>Some other thread invokes {@link #unpark unpark} with the 174 * current thread as the target; or 175 * 176 * <li>Some other thread {@linkplain Thread#interrupt interrupts} 177 * the current thread; or 178 * 179 * <li>The call spuriously (that is, for no reason) returns. 180 * </ul> 181 * 182 * <p>This method does <em>not</em> report which of these caused the 183 * method to return. Callers should re-check the conditions which caused 184 * the thread to park in the first place. Callers may also determine, 185 * for example, the interrupt status of the thread upon return. 186 * 187 * @param blocker the synchronization object responsible for this 188 * thread parking 189 * @since 1.6 190 */ 191 public static void park(Object blocker) { 192 Thread t = Thread.currentThread(); 193 setBlocker(t, blocker); 194 U.park(false, 0L); 195 setBlocker(t, null); 196 } 197 198 /** 199 * Disables the current thread for thread scheduling purposes, for up to 200 * the specified waiting time, unless the permit is available. 201 * 202 * <p>If the specified waiting time is zero or negative, the 203 * method does nothing. Otherwise, if the permit is available then 204 * it is consumed and the call returns immediately; otherwise the 205 * current thread becomes disabled for thread scheduling purposes 206 * and lies dormant until one of four things happens: 207 * 208 * <ul> 209 * <li>Some other thread invokes {@link #unpark unpark} with the 210 * current thread as the target; or 211 * 212 * <li>Some other thread {@linkplain Thread#interrupt interrupts} 213 * the current thread; or 214 * 215 * <li>The specified waiting time elapses; or 216 * 217 * <li>The call spuriously (that is, for no reason) returns. 218 * </ul> 219 * 220 * <p>This method does <em>not</em> report which of these caused the 221 * method to return. Callers should re-check the conditions which caused 222 * the thread to park in the first place. Callers may also determine, 223 * for example, the interrupt status of the thread, or the elapsed time 224 * upon return. 225 * 226 * @param blocker the synchronization object responsible for this 227 * thread parking 228 * @param nanos the maximum number of nanoseconds to wait 229 * @since 1.6 230 */ 231 public static void parkNanos(Object blocker, long nanos) { 232 if (nanos > 0) { 233 Thread t = Thread.currentThread(); 234 setBlocker(t, blocker); 235 U.park(false, nanos); 236 setBlocker(t, null); 237 } 238 } 239 240 /** 241 * Disables the current thread for thread scheduling purposes, until 242 * the specified deadline, unless the permit is available. 243 * 244 * <p>If the permit is available then it is consumed and the call 245 * returns immediately; otherwise the current thread becomes disabled 246 * for thread scheduling purposes and lies dormant until one of four 247 * things happens: 248 * 249 * <ul> 250 * <li>Some other thread invokes {@link #unpark unpark} with the 251 * current thread as the target; or 252 * 253 * <li>Some other thread {@linkplain Thread#interrupt interrupts} the 254 * current thread; or 255 * 256 * <li>The specified deadline passes; or 257 * 258 * <li>The call spuriously (that is, for no reason) returns. 259 * </ul> 260 * 261 * <p>This method does <em>not</em> report which of these caused the 262 * method to return. Callers should re-check the conditions which caused 263 * the thread to park in the first place. Callers may also determine, 264 * for example, the interrupt status of the thread, or the current time 265 * upon return. 266 * 267 * @param blocker the synchronization object responsible for this 268 * thread parking 269 * @param deadline the absolute time, in milliseconds from the Epoch, 270 * to wait until 271 * @since 1.6 272 */ 273 public static void parkUntil(Object blocker, long deadline) { 274 Thread t = Thread.currentThread(); 275 setBlocker(t, blocker); 276 U.park(true, deadline); 277 setBlocker(t, null); 278 } 279 280 /** 281 * Returns the blocker object supplied to the most recent 282 * invocation of a park method that has not yet unblocked, or null 283 * if not blocked. The value returned is just a momentary 284 * snapshot -- the thread may have since unblocked or blocked on a 285 * different blocker object. 286 * 287 * @param t the thread 288 * @return the blocker 289 * @throws NullPointerException if argument is null 290 * @since 1.6 291 */ 292 public static Object getBlocker(Thread t) { 293 if (t == null) 294 throw new NullPointerException(); 295 return U.getReferenceVolatile(t, PARKBLOCKER); 296 } 297 298 /** 299 * Disables the current thread for thread scheduling purposes unless the 300 * permit is available. 301 * 302 * <p>If the permit is available then it is consumed and the call 303 * returns immediately; otherwise the current thread becomes disabled 304 * for thread scheduling purposes and lies dormant until one of three 305 * things happens: 306 * 307 * <ul> 308 * 309 * <li>Some other thread invokes {@link #unpark unpark} with the 310 * current thread as the target; or 311 * 312 * <li>Some other thread {@linkplain Thread#interrupt interrupts} 313 * the current thread; or 314 * 315 * <li>The call spuriously (that is, for no reason) returns. 316 * </ul> 317 * 318 * <p>This method does <em>not</em> report which of these caused the 319 * method to return. Callers should re-check the conditions which caused 320 * the thread to park in the first place. Callers may also determine, 321 * for example, the interrupt status of the thread upon return. 322 */ 323 public static void park() { 324 U.park(false, 0L); 325 } 326 327 /** 328 * Disables the current thread for thread scheduling purposes, for up to 329 * the specified waiting time, unless the permit is available. 330 * 331 * <p>If the specified waiting time is zero or negative, the 332 * method does nothing. Otherwise, if the permit is available then 333 * it is consumed and the call returns immediately; otherwise the 334 * current thread becomes disabled for thread scheduling purposes 335 * and lies dormant until one of four things happens: 336 * 337 * <ul> 338 * <li>Some other thread invokes {@link #unpark unpark} with the 339 * current thread as the target; or 340 * 341 * <li>Some other thread {@linkplain Thread#interrupt interrupts} 342 * the current thread; or 343 * 344 * <li>The specified waiting time elapses; or 345 * 346 * <li>The call spuriously (that is, for no reason) returns. 347 * </ul> 348 * 349 * <p>This method does <em>not</em> report which of these caused the 350 * method to return. Callers should re-check the conditions which caused 351 * the thread to park in the first place. Callers may also determine, 352 * for example, the interrupt status of the thread, or the elapsed time 353 * upon return. 354 * 355 * @param nanos the maximum number of nanoseconds to wait 356 */ 357 public static void parkNanos(long nanos) { 358 if (nanos > 0) 359 U.park(false, nanos); 360 } 361 362 /** 363 * Disables the current thread for thread scheduling purposes, until 364 * the specified deadline, unless the permit is available. 365 * 366 * <p>If the permit is available then it is consumed and the call 367 * returns immediately; otherwise the current thread becomes disabled 368 * for thread scheduling purposes and lies dormant until one of four 369 * things happens: 370 * 371 * <ul> 372 * <li>Some other thread invokes {@link #unpark unpark} with the 373 * current thread as the target; or 374 * 375 * <li>Some other thread {@linkplain Thread#interrupt interrupts} 376 * the current thread; or 377 * 378 * <li>The specified deadline passes; or 379 * 380 * <li>The call spuriously (that is, for no reason) returns. 381 * </ul> 382 * 383 * <p>This method does <em>not</em> report which of these caused the 384 * method to return. Callers should re-check the conditions which caused 385 * the thread to park in the first place. Callers may also determine, 386 * for example, the interrupt status of the thread, or the current time 387 * upon return. 388 * 389 * @param deadline the absolute time, in milliseconds from the Epoch, 390 * to wait until 391 */ 392 public static void parkUntil(long deadline) { 393 U.park(true, deadline); 394 } 395 396 /** 397 * Returns the pseudo-randomly initialized or updated secondary seed. 398 * Copied from ThreadLocalRandom due to package access restrictions. 399 */ 400 static final int nextSecondarySeed() { 401 int r; 402 Thread t = Thread.currentThread(); 403 if ((r = U.getInt(t, SECONDARY)) != 0) { 404 r ^= r << 13; // xorshift 405 r ^= r >>> 17; 406 r ^= r << 5; 407 } 408 else if ((r = java.util.concurrent.ThreadLocalRandom.current().nextInt()) == 0) 409 r = 1; // avoid zero 410 U.putInt(t, SECONDARY, r); 411 return r; 412 } 413 414 /** 415 * Returns the thread id for the given thread. We must access 416 * this directly rather than via method Thread.getId() because 417 * getId() has been known to be overridden in ways that do not 418 * preserve unique mappings. 419 */ 420 static final long getThreadId(Thread thread) { 421 return U.getLong(thread, TID); 422 } 423 424 // Hotspot implementation via intrinsics API 425 private static final Unsafe U = Unsafe.getUnsafe(); 426 private static final long PARKBLOCKER = U.objectFieldOffset 427 (Thread.class, "parkBlocker"); 428 private static final long SECONDARY = U.objectFieldOffset 429 (Thread.class, "threadLocalRandomSecondarySeed"); 430 private static final long TID = U.objectFieldOffset 431 (Thread.class, "tid"); 432 433 }