1 /*
   2  * Copyright (c) 2018, 2022, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 package java.lang;
  26 
  27 import java.lang.ref.Reference;
  28 import java.security.AccessController;
  29 import java.security.PrivilegedAction;
  30 import java.util.Locale;
  31 import java.util.Objects;
  32 import java.util.concurrent.CountDownLatch;
  33 import java.util.concurrent.Executor;
  34 import java.util.concurrent.Executors;
  35 import java.util.concurrent.ForkJoinPool;
  36 import java.util.concurrent.ForkJoinPool.ForkJoinWorkerThreadFactory;
  37 import java.util.concurrent.ForkJoinTask;
  38 import java.util.concurrent.ForkJoinWorkerThread;
  39 import java.util.concurrent.Future;
  40 import java.util.concurrent.RejectedExecutionException;
  41 import java.util.concurrent.ScheduledExecutorService;
  42 import java.util.concurrent.ScheduledThreadPoolExecutor;
  43 import jdk.internal.event.ThreadSleepEvent;
  44 import jdk.internal.event.VirtualThreadEndEvent;
  45 import jdk.internal.event.VirtualThreadPinnedEvent;
  46 import jdk.internal.event.VirtualThreadStartEvent;
  47 import jdk.internal.event.VirtualThreadSubmitFailedEvent;
  48 import jdk.internal.misc.CarrierThread;
  49 import jdk.internal.misc.InnocuousThread;
  50 import jdk.internal.misc.Unsafe;
  51 import jdk.internal.vm.Continuation;
  52 import jdk.internal.vm.ContinuationScope;
  53 import jdk.internal.vm.StackableScope;
  54 import jdk.internal.vm.ThreadContainer;
  55 import jdk.internal.vm.ThreadContainers;
  56 import jdk.internal.vm.annotation.ChangesCurrentThread;
  57 import jdk.internal.vm.annotation.ForceInline;
  58 import jdk.internal.vm.annotation.Hidden;
  59 import jdk.internal.vm.annotation.JvmtiMountTransition;
  60 import sun.nio.ch.Interruptible;
  61 import sun.security.action.GetPropertyAction;
  62 import static java.util.concurrent.TimeUnit.*;
  63 
  64 /**
  65  * A thread that is scheduled by the Java virtual machine rather than the operating
  66  * system.
  67  */
  68 final class VirtualThread extends BaseVirtualThread {
  69     private static final Unsafe U = Unsafe.getUnsafe();
  70     private static final ContinuationScope VTHREAD_SCOPE = new ContinuationScope("VirtualThreads");
  71     private static final ForkJoinPool DEFAULT_SCHEDULER = createDefaultScheduler();
  72     private static final ScheduledExecutorService UNPARKER = createDelayedTaskScheduler();
  73     private static final int TRACE_PINNING_MODE = tracePinningMode();
  74 
  75     private static final long STATE = U.objectFieldOffset(VirtualThread.class, "state");
  76     private static final long PARK_PERMIT = U.objectFieldOffset(VirtualThread.class, "parkPermit");
  77     private static final long CARRIER_THREAD = U.objectFieldOffset(VirtualThread.class, "carrierThread");
  78     private static final long TERMINATION = U.objectFieldOffset(VirtualThread.class, "termination");
  79 
  80     // scheduler and continuation
  81     private final Executor scheduler;
  82     private final Continuation cont;
  83     private final Runnable runContinuation;
  84 
  85     // virtual thread state, accessed by VM
  86     private volatile int state;
  87 
  88     /*
  89      * Virtual thread state and transitions:
  90      *
  91      *      NEW -> STARTED         // Thread.start
  92      *  STARTED -> TERMINATED      // failed to start
  93      *  STARTED -> RUNNING         // first run
  94      *
  95      *  RUNNING -> PARKING         // Thread attempts to park
  96      *  PARKING -> PARKED          // cont.yield successful, thread is parked
  97      *  PARKING -> PINNED          // cont.yield failed, thread is pinned
  98      *
  99      *   PARKED -> RUNNABLE        // unpark or interrupted
 100      *   PINNED -> RUNNABLE        // unpark or interrupted
 101      *
 102      * RUNNABLE -> RUNNING         // continue execution
 103      *
 104      *  RUNNING -> YIELDING        // Thread.yield
 105      * YIELDING -> RUNNABLE        // yield successful
 106      * YIELDING -> RUNNING         // yield failed
 107      *
 108      *  RUNNING -> TERMINATED      // done
 109      */
 110     private static final int NEW      = 0;
 111     private static final int STARTED  = 1;
 112     private static final int RUNNABLE = 2;     // runnable-unmounted
 113     private static final int RUNNING  = 3;     // runnable-mounted
 114     private static final int PARKING  = 4;
 115     private static final int PARKED   = 5;     // unmounted
 116     private static final int PINNED   = 6;     // mounted
 117     private static final int YIELDING = 7;     // Thread.yield
 118     private static final int TERMINATED = 99;  // final state
 119 
 120     // can be suspended from scheduling when unmounted
 121     private static final int SUSPENDED = 1 << 8;
 122     private static final int RUNNABLE_SUSPENDED = (RUNNABLE | SUSPENDED);
 123     private static final int PARKED_SUSPENDED   = (PARKED | SUSPENDED);
 124 
 125     // parking permit
 126     private volatile boolean parkPermit;
 127 
 128     // carrier thread when mounted, accessed by VM
 129     private volatile Thread carrierThread;
 130 
 131     // termination object when joining, created lazily if needed
 132     private volatile CountDownLatch termination;
 133 
 134     /**
 135      * Returns the continuation scope used for virtual threads.
 136      */
 137     static ContinuationScope continuationScope() {
 138         return VTHREAD_SCOPE;
 139     }
 140 
 141     /**
 142      * Creates a new {@code VirtualThread} to run the given task with the given
 143      * scheduler. If the given scheduler is {@code null} and the current thread
 144      * is a platform thread then the newly created virtual thread will use the
 145      * default scheduler. If given scheduler is {@code null} and the current
 146      * thread is a virtual thread then the current thread's scheduler is used.
 147      *
 148      * @param scheduler the scheduler or null
 149      * @param name thread name
 150      * @param characteristics characteristics
 151      * @param task the task to execute
 152      */
 153     VirtualThread(Executor scheduler, String name, int characteristics, Runnable task) {
 154         super(name, characteristics, /*bound*/ false);
 155         Objects.requireNonNull(task);
 156 
 157         // choose scheduler if not specified
 158         if (scheduler == null) {
 159             Thread parent = Thread.currentThread();
 160             if (parent instanceof VirtualThread vparent) {
 161                 scheduler = vparent.scheduler;
 162             } else {
 163                 scheduler = DEFAULT_SCHEDULER;
 164             }
 165         }
 166 
 167         this.scheduler = scheduler;
 168         this.cont = new VThreadContinuation(this, task);
 169         this.runContinuation = this::runContinuation;
 170     }
 171 
 172     /**
 173      * The continuation that a virtual thread executes.
 174      */
 175     private static class VThreadContinuation extends Continuation {
 176         VThreadContinuation(VirtualThread vthread, Runnable task) {
 177             super(VTHREAD_SCOPE, () -> vthread.run(task));
 178         }
 179         @Override
 180         protected void onPinned(Continuation.Pinned reason) {
 181             if (TRACE_PINNING_MODE > 0) {
 182                 boolean printAll = (TRACE_PINNING_MODE == 1);
 183                 PinnedThreadPrinter.printStackTrace(System.out, printAll);
 184             }
 185         }
 186     }
 187 
 188     /**
 189      * Runs or continues execution of the continuation on the current thread.
 190      */
 191     private void runContinuation() {
 192         // the carrier must be a platform thread
 193         if (Thread.currentThread().isVirtual()) {
 194             throw new WrongThreadException();
 195         }
 196 
 197         // set state to RUNNING
 198         boolean firstRun;
 199         int initialState = state();
 200         if (initialState == STARTED && compareAndSetState(STARTED, RUNNING)) {
 201             // first run
 202             firstRun = true;
 203         } else if (initialState == RUNNABLE && compareAndSetState(RUNNABLE, RUNNING)) {
 204             // consume parking permit
 205             setParkPermit(false);
 206             firstRun = false;
 207         } else {
 208             // not runnable
 209             return;
 210         }
 211 
 212         // notify JVMTI before mount
 213         if (notifyJvmtiEvents) notifyJvmtiMountBegin(firstRun);
 214 
 215         try {
 216             cont.run();
 217         } finally {
 218             if (cont.isDone()) {
 219                 afterTerminate(/*executed*/ true);
 220             } else {
 221                 afterYield();
 222             }
 223         }
 224     }
 225 
 226     /**
 227      * Submits the runContinuation task to the scheduler. For the default scheduler,
 228      * and calling it on a worker thread, the task will be pushed to the local queue,
 229      * otherwise it will be pushed to a submission queue.
 230      *
 231      * @throws RejectedExecutionException
 232      */
 233     private void submitRunContinuation() {
 234         try {
 235             scheduler.execute(runContinuation);
 236         } catch (RejectedExecutionException ree) {
 237             submitFailed(ree);
 238             throw ree;
 239         }
 240     }
 241 
 242     /**
 243      * Submits the runContinuation task to the scheduler with a lazy submit.
 244      * @throws RejectedExecutionException
 245      * @see ForkJoinPool#lazySubmit(ForkJoinTask)
 246      */
 247     private void lazySubmitRunContinuation(ForkJoinPool pool) {
 248         try {
 249             pool.lazySubmit(ForkJoinTask.adapt(runContinuation));
 250         } catch (RejectedExecutionException ree) {
 251             submitFailed(ree);
 252             throw ree;
 253         }
 254     }
 255 
 256     /**
 257      * Submits the runContinuation task to the scheduler as an external submit.
 258      * @throws RejectedExecutionException
 259      * @see ForkJoinPool#externalSubmit(ForkJoinTask)
 260      */
 261     private void externalSubmitRunContinuation(ForkJoinPool pool) {
 262         try {
 263             pool.externalSubmit(ForkJoinTask.adapt(runContinuation));
 264         } catch (RejectedExecutionException ree) {
 265             submitFailed(ree);
 266             throw ree;
 267         }
 268     }
 269 
 270     /**
 271      * If enabled, emits a JFR VirtualThreadSubmitFailedEvent.
 272      */
 273     private void submitFailed(RejectedExecutionException ree) {
 274         var event = new VirtualThreadSubmitFailedEvent();
 275         if (event.isEnabled()) {
 276             event.javaThreadId = threadId();
 277             event.exceptionMessage = ree.getMessage();
 278             event.commit();
 279         }
 280     }
 281 
 282     /**
 283      * Runs a task in the context of this virtual thread. The virtual thread is
 284      * mounted on the current (carrier) thread before the task runs. It unmounts
 285      * from its carrier thread when the task completes.
 286      */
 287     @ChangesCurrentThread
 288     private void run(Runnable task) {
 289         assert state == RUNNING;
 290         boolean notifyJvmti = notifyJvmtiEvents;
 291 
 292         // first mount
 293         mount();
 294         if (notifyJvmti) notifyJvmtiMountEnd(true);
 295 
 296         // emit JFR event if enabled
 297         if (VirtualThreadStartEvent.isTurnedOn()) {
 298             var event = new VirtualThreadStartEvent();
 299             event.javaThreadId = threadId();
 300             event.commit();
 301         }
 302 
 303         Object bindings = scopedValueBindings();
 304         try {
 305             runWith(bindings, task);
 306         } catch (Throwable exc) {
 307             dispatchUncaughtException(exc);
 308         } finally {
 309             try {
 310                 // pop any remaining scopes from the stack, this may block
 311                 StackableScope.popAll();
 312 
 313                 // emit JFR event if enabled
 314                 if (VirtualThreadEndEvent.isTurnedOn()) {
 315                     var event = new VirtualThreadEndEvent();
 316                     event.javaThreadId = threadId();
 317                     event.commit();
 318                 }
 319 
 320             } finally {
 321                 // last unmount
 322                 if (notifyJvmti) notifyJvmtiUnmountBegin(true);
 323                 unmount();
 324 
 325                 // final state
 326                 setState(TERMINATED);
 327             }
 328         }
 329     }
 330 
 331     @Hidden
 332     @ForceInline
 333     private void runWith(Object bindings, Runnable op) {
 334         ensureMaterializedForStackWalk(bindings);
 335         op.run();
 336         Reference.reachabilityFence(bindings);
 337     }
 338 
 339     /**
 340      * Mounts this virtual thread onto the current platform thread. On
 341      * return, the current thread is the virtual thread.
 342      */
 343     @ChangesCurrentThread
 344     private void mount() {
 345         // sets the carrier thread
 346         Thread carrier = Thread.currentCarrierThread();
 347         setCarrierThread(carrier);
 348 
 349         // sync up carrier thread interrupt status if needed
 350         if (interrupted) {
 351             carrier.setInterrupt();
 352         } else if (carrier.isInterrupted()) {
 353             synchronized (interruptLock) {
 354                 // need to recheck interrupt status
 355                 if (!interrupted) {
 356                     carrier.clearInterrupt();
 357                 }
 358             }
 359         }
 360 
 361         // set Thread.currentThread() to return this virtual thread
 362         carrier.setCurrentThread(this);
 363     }
 364 
 365     /**
 366      * Unmounts this virtual thread from the carrier. On return, the
 367      * current thread is the current platform thread.
 368      */
 369     @ChangesCurrentThread
 370     private void unmount() {
 371         // set Thread.currentThread() to return the platform thread
 372         Thread carrier = this.carrierThread;
 373         carrier.setCurrentThread(carrier);
 374 
 375         // break connection to carrier thread, synchronized with interrupt
 376         synchronized (interruptLock) {
 377             setCarrierThread(null);
 378         }
 379         carrier.clearInterrupt();
 380     }
 381 
 382     /**
 383      * Sets the current thread to the current carrier thread.
 384      * @return true if JVMTI was notified
 385      */
 386     @ChangesCurrentThread
 387     @JvmtiMountTransition
 388     private boolean switchToCarrierThread() {
 389         boolean notifyJvmti = notifyJvmtiEvents;
 390         if (notifyJvmti) {
 391             notifyJvmtiHideFrames(true);
 392         }
 393         Thread carrier = this.carrierThread;
 394         assert Thread.currentThread() == this
 395                 && carrier == Thread.currentCarrierThread();
 396         carrier.setCurrentThread(carrier);
 397         return notifyJvmti;
 398     }
 399 
 400     /**
 401      * Sets the current thread to the given virtual thread.
 402      * If {@code notifyJvmti} is true then JVMTI is notified.
 403      */
 404     @ChangesCurrentThread
 405     @JvmtiMountTransition
 406     private void switchToVirtualThread(VirtualThread vthread, boolean notifyJvmti) {
 407         Thread carrier = vthread.carrierThread;
 408         assert carrier == Thread.currentCarrierThread();
 409         carrier.setCurrentThread(vthread);
 410         if (notifyJvmti) {
 411             notifyJvmtiHideFrames(false);
 412         }
 413     }
 414 
 415     /**
 416      * Unmounts this virtual thread, invokes Continuation.yield, and re-mounts the
 417      * thread when continued. When enabled, JVMTI must be notified from this method.
 418      * @return true if the yield was successful
 419      */
 420     @ChangesCurrentThread
 421     private boolean yieldContinuation() {
 422         boolean notifyJvmti = notifyJvmtiEvents;
 423 
 424         // unmount
 425         if (notifyJvmti) notifyJvmtiUnmountBegin(false);
 426         unmount();
 427         try {
 428             return Continuation.yield(VTHREAD_SCOPE);
 429         } finally {
 430             // re-mount
 431             mount();
 432             if (notifyJvmti) notifyJvmtiMountEnd(false);
 433         }
 434     }
 435 
 436     /**
 437      * Invoked after the continuation yields. If parking then it sets the state
 438      * and also re-submits the task to continue if unparked while parking.
 439      * If yielding due to Thread.yield then it just submits the task to continue.
 440      */
 441     private void afterYield() {
 442         int s = state();
 443         assert (s == PARKING || s == YIELDING) && (carrierThread == null);
 444 
 445         if (s == PARKING) {
 446             setState(PARKED);
 447 
 448             // notify JVMTI that unmount has completed, thread is parked
 449             if (notifyJvmtiEvents) notifyJvmtiUnmountEnd(false);
 450 
 451             // may have been unparked while parking
 452             if (parkPermit && compareAndSetState(PARKED, RUNNABLE)) {
 453                 // lazy submit to continue on the current thread as carrier if possible
 454                 if (currentThread() instanceof CarrierThread ct) {
 455                     lazySubmitRunContinuation(ct.getPool());
 456                 } else {
 457                     submitRunContinuation();
 458                 }
 459 
 460             }
 461         } else if (s == YIELDING) {   // Thread.yield
 462             setState(RUNNABLE);
 463 
 464             // notify JVMTI that unmount has completed, thread is runnable
 465             if (notifyJvmtiEvents) notifyJvmtiUnmountEnd(false);
 466 
 467             // external submit if there are no tasks in the local task queue
 468             if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
 469                 externalSubmitRunContinuation(ct.getPool());
 470             } else {
 471                 submitRunContinuation();
 472             }
 473         }
 474     }
 475 
 476     /**
 477      * Invoked after the thread terminates (or start failed). This method
 478      * notifies anyone waiting for the thread to terminate.
 479      *
 480      * @param executed true if the thread executed, false if it failed to start
 481      */
 482     private void afterTerminate(boolean executed) {
 483         assert (state() == TERMINATED) && (carrierThread == null);
 484 
 485         if (executed) {
 486             if (notifyJvmtiEvents) notifyJvmtiUnmountEnd(true);
 487         }
 488 
 489         // notify anyone waiting for this virtual thread to terminate
 490         CountDownLatch termination = this.termination;
 491         if (termination != null) {
 492             assert termination.getCount() == 1;
 493             termination.countDown();
 494         }
 495 
 496         if (executed) {
 497             // notify container if thread executed
 498             threadContainer().onExit(this);
 499 
 500             // clear references to thread locals
 501             clearReferences();
 502         }
 503     }
 504 
 505     /**
 506      * Schedules this {@code VirtualThread} to execute.
 507      *
 508      * @throws IllegalStateException if the container is shutdown or closed
 509      * @throws IllegalThreadStateException if the thread has already been started
 510      * @throws RejectedExecutionException if the scheduler cannot accept a task
 511      */
 512     @Override
 513     void start(ThreadContainer container) {
 514         if (!compareAndSetState(NEW, STARTED)) {
 515             throw new IllegalThreadStateException("Already started");
 516         }
 517 
 518         // bind thread to container
 519         setThreadContainer(container);
 520 
 521         // start thread
 522         boolean started = false;
 523         container.onStart(this); // may throw
 524         try {
 525             // scoped values may be inherited
 526             inheritScopedValueBindings(container);
 527 
 528             // submit task to run thread
 529             submitRunContinuation();
 530             started = true;
 531         } finally {
 532             if (!started) {
 533                 setState(TERMINATED);
 534                 container.onExit(this);
 535                 afterTerminate(/*executed*/ false);
 536             }
 537         }
 538     }
 539 
 540     @Override
 541     public void start() {
 542         start(ThreadContainers.root());
 543     }
 544 
 545     @Override
 546     public void run() {
 547         // do nothing
 548     }
 549 
 550     /**
 551      * Parks until unparked or interrupted. If already unparked then the parking
 552      * permit is consumed and this method completes immediately (meaning it doesn't
 553      * yield). It also completes immediately if the interrupt status is set.
 554      */
 555     @Override
 556     void park() {
 557         assert Thread.currentThread() == this;
 558 
 559         // complete immediately if parking permit available or interrupted
 560         if (getAndSetParkPermit(false) || interrupted)
 561             return;
 562 
 563         // park the thread
 564         setState(PARKING);
 565         try {
 566             if (!yieldContinuation()) {
 567                 // park on the carrier thread when pinned
 568                 parkOnCarrierThread(false, 0);
 569             }
 570         } finally {
 571             assert (Thread.currentThread() == this) && (state() == RUNNING);
 572         }
 573     }
 574 
 575     /**
 576      * Parks up to the given waiting time or until unparked or interrupted.
 577      * If already unparked then the parking permit is consumed and this method
 578      * completes immediately (meaning it doesn't yield). It also completes immediately
 579      * if the interrupt status is set or the waiting time is {@code <= 0}.
 580      *
 581      * @param nanos the maximum number of nanoseconds to wait.
 582      */
 583     @Override
 584     void parkNanos(long nanos) {
 585         assert Thread.currentThread() == this;
 586 
 587         // complete immediately if parking permit available or interrupted
 588         if (getAndSetParkPermit(false) || interrupted)
 589             return;
 590 
 591         // park the thread for the waiting time
 592         if (nanos > 0) {
 593             long startTime = System.nanoTime();
 594 
 595             boolean yielded;
 596             Future<?> unparker = scheduleUnpark(this::unpark, nanos);
 597             setState(PARKING);
 598             try {
 599                 yielded = yieldContinuation();
 600             } finally {
 601                 assert (Thread.currentThread() == this)
 602                         && (state() == RUNNING || state() == PARKING);
 603                 cancel(unparker);
 604             }
 605 
 606             // park on carrier thread for remaining time when pinned
 607             if (!yielded) {
 608                 long deadline = startTime + nanos;
 609                 if (deadline < 0L)
 610                     deadline = Long.MAX_VALUE;
 611                 parkOnCarrierThread(true, deadline - System.nanoTime());
 612             }
 613         }
 614     }
 615 
 616     /**
 617      * Parks the current carrier thread up to the given waiting time or until
 618      * unparked or interrupted. If the virtual thread is interrupted then the
 619      * interrupt status will be propagated to the carrier thread.
 620      * @param timed true for a timed park, false for untimed
 621      * @param nanos the waiting time in nanoseconds
 622      */
 623     private void parkOnCarrierThread(boolean timed, long nanos) {
 624         assert state() == PARKING;
 625 
 626         var pinnedEvent = new VirtualThreadPinnedEvent();
 627         pinnedEvent.begin();
 628 
 629         setState(PINNED);
 630         try {
 631             if (!parkPermit) {
 632                 if (!timed) {
 633                     U.park(false, 0);
 634                 } else if (nanos > 0) {
 635                     U.park(false, nanos);
 636                 }
 637             }
 638         } finally {
 639             setState(RUNNING);
 640         }
 641 
 642         // consume parking permit
 643         setParkPermit(false);
 644 
 645         pinnedEvent.commit();
 646     }
 647 
 648     /**
 649      * Schedule an unpark task to run after a given delay.
 650      */
 651     @ChangesCurrentThread
 652     private Future<?> scheduleUnpark(Runnable unparker, long nanos) {
 653         // need to switch to current carrier thread to avoid nested parking
 654         boolean notifyJvmti = switchToCarrierThread();
 655         try {
 656             return UNPARKER.schedule(unparker, nanos, NANOSECONDS);
 657         } finally {
 658             switchToVirtualThread(this, notifyJvmti);
 659         }
 660     }
 661 
 662     /**
 663      * Cancels a task if it has not completed.
 664      */
 665     @ChangesCurrentThread
 666     private void cancel(Future<?> future) {
 667         if (!future.isDone()) {
 668             // need to switch to current carrier thread to avoid nested parking
 669             boolean notifyJvmti = switchToCarrierThread();
 670             try {
 671                 future.cancel(false);
 672             } finally {
 673                 switchToVirtualThread(this, notifyJvmti);
 674             }
 675         }
 676     }
 677 
 678     /**
 679      * Re-enables this virtual thread for scheduling. If the virtual thread was
 680      * {@link #park() parked} then it will be unblocked, otherwise its next call
 681      * to {@code park} or {@linkplain #parkNanos(long) parkNanos} is guaranteed
 682      * not to block.
 683      * @throws RejectedExecutionException if the scheduler cannot accept a task
 684      */
 685     @Override
 686     @ChangesCurrentThread
 687     void unpark() {
 688         Thread currentThread = Thread.currentThread();
 689         if (!getAndSetParkPermit(true) && currentThread != this) {
 690             int s = state();
 691             if (s == PARKED && compareAndSetState(PARKED, RUNNABLE)) {
 692                 if (currentThread instanceof VirtualThread vthread) {
 693                     boolean notifyJvmti = vthread.switchToCarrierThread();
 694                     try {
 695                         submitRunContinuation();
 696                     } finally {
 697                         switchToVirtualThread(vthread, notifyJvmti);
 698                     }
 699                 } else {
 700                     submitRunContinuation();
 701                 }
 702             } else if (s == PINNED) {
 703                 // unpark carrier thread when pinned.
 704                 synchronized (carrierThreadAccessLock()) {
 705                     Thread carrier = carrierThread;
 706                     if (carrier != null && state() == PINNED) {
 707                         U.unpark(carrier);
 708                     }
 709                 }
 710             }
 711         }
 712     }
 713 
 714     /**
 715      * Attempts to yield the current virtual thread (Thread.yield).
 716      */
 717     void tryYield() {
 718         assert Thread.currentThread() == this;
 719         setState(YIELDING);
 720         try {
 721             yieldContinuation();
 722         } finally {
 723             assert Thread.currentThread() == this;
 724             if (state() != RUNNING) {
 725                 assert state() == YIELDING;
 726                 setState(RUNNING);
 727             }
 728         }
 729     }
 730 
 731     /**
 732      * Sleep the current virtual thread for the given sleep time.
 733      *
 734      * @param nanos the maximum number of nanoseconds to sleep
 735      * @throws InterruptedException if interrupted while sleeping
 736      */
 737     void sleepNanos(long nanos) throws InterruptedException {
 738         assert Thread.currentThread() == this;
 739         if (nanos >= 0) {
 740             if (ThreadSleepEvent.isTurnedOn()) {
 741                 ThreadSleepEvent event = new ThreadSleepEvent();
 742                 try {
 743                     event.time = nanos;
 744                     event.begin();
 745                     doSleepNanos(nanos);
 746                 } finally {
 747                     event.commit();
 748                 }
 749             } else {
 750                 doSleepNanos(nanos);
 751             }
 752         }
 753     }
 754 
 755     /**
 756      * Sleep the current thread for the given sleep time (in nanoseconds). If
 757      * nanos is 0 then the thread will attempt to yield.
 758      *
 759      * @implNote This implementation parks the thread for the given sleeping time
 760      * and will therefore be observed in PARKED state during the sleep. Parking
 761      * will consume the parking permit so this method makes available the parking
 762      * permit after the sleep. This may be observed as a spurious, but benign,
 763      * wakeup when the thread subsequently attempts to park.
 764      */
 765     private void doSleepNanos(long nanos) throws InterruptedException {
 766         assert nanos >= 0;
 767         if (getAndClearInterrupt())
 768             throw new InterruptedException();
 769         if (nanos == 0) {
 770             tryYield();
 771         } else {
 772             // park for the sleep time
 773             try {
 774                 long remainingNanos = nanos;
 775                 long startNanos = System.nanoTime();
 776                 while (remainingNanos > 0) {
 777                     parkNanos(remainingNanos);
 778                     if (getAndClearInterrupt()) {
 779                         throw new InterruptedException();
 780                     }
 781                     remainingNanos = nanos - (System.nanoTime() - startNanos);
 782                 }
 783             } finally {
 784                 // may have been unparked while sleeping
 785                 setParkPermit(true);
 786             }
 787         }
 788     }
 789 
 790     /**
 791      * Waits up to {@code nanos} nanoseconds for this virtual thread to terminate.
 792      * A timeout of {@code 0} means to wait forever.
 793      *
 794      * @throws InterruptedException if interrupted while waiting
 795      * @return true if the thread has terminated
 796      */
 797     boolean joinNanos(long nanos) throws InterruptedException {
 798         if (state() == TERMINATED)
 799             return true;
 800 
 801         // ensure termination object exists, then re-check state
 802         CountDownLatch termination = getTermination();
 803         if (state() == TERMINATED)
 804             return true;
 805 
 806         // wait for virtual thread to terminate
 807         if (nanos == 0) {
 808             termination.await();
 809         } else {
 810             boolean terminated = termination.await(nanos, NANOSECONDS);
 811             if (!terminated) {
 812                 // waiting time elapsed
 813                 return false;
 814             }
 815         }
 816         assert state() == TERMINATED;
 817         return true;
 818     }
 819 
 820     @Override
 821     @SuppressWarnings("removal")
 822     public void interrupt() {
 823         if (Thread.currentThread() != this) {
 824             checkAccess();
 825             synchronized (interruptLock) {
 826                 interrupted = true;
 827                 Interruptible b = nioBlocker;
 828                 if (b != null) {
 829                     b.interrupt(this);
 830                 }
 831 
 832                 // interrupt carrier thread if mounted
 833                 Thread carrier = carrierThread;
 834                 if (carrier != null) carrier.setInterrupt();
 835             }
 836         } else {
 837             interrupted = true;
 838             carrierThread.setInterrupt();
 839         }
 840         unpark();
 841     }
 842 
 843     @Override
 844     public boolean isInterrupted() {
 845         return interrupted;
 846     }
 847 
 848     @Override
 849     boolean getAndClearInterrupt() {
 850         assert Thread.currentThread() == this;
 851         synchronized (interruptLock) {
 852             boolean oldValue = interrupted;
 853             if (oldValue)
 854                 interrupted = false;
 855             carrierThread.clearInterrupt();
 856             return oldValue;
 857         }
 858     }
 859 
 860     @Override
 861     Thread.State threadState() {
 862         switch (state()) {
 863             case NEW:
 864                 return Thread.State.NEW;
 865             case STARTED:
 866                 // return NEW if thread container not yet set
 867                 if (threadContainer() == null) {
 868                     return Thread.State.NEW;
 869                 } else {
 870                     return Thread.State.RUNNABLE;
 871                 }
 872             case RUNNABLE:
 873             case RUNNABLE_SUSPENDED:
 874                 // runnable, not mounted
 875                 return Thread.State.RUNNABLE;
 876             case RUNNING:
 877                 // if mounted then return state of carrier thread
 878                 synchronized (carrierThreadAccessLock()) {
 879                     Thread carrierThread = this.carrierThread;
 880                     if (carrierThread != null) {
 881                         return carrierThread.threadState();
 882                     }
 883                 }
 884                 // runnable, mounted
 885                 return Thread.State.RUNNABLE;
 886             case PARKING:
 887             case YIELDING:
 888                 // runnable, mounted, not yet waiting
 889                 return Thread.State.RUNNABLE;
 890             case PARKED:
 891             case PARKED_SUSPENDED:
 892             case PINNED:
 893                 return Thread.State.WAITING;
 894             case TERMINATED:
 895                 return Thread.State.TERMINATED;
 896             default:
 897                 throw new InternalError();
 898         }
 899     }
 900 
 901     @Override
 902     boolean alive() {
 903         int s = state;
 904         return (s != NEW && s != TERMINATED);
 905     }
 906 
 907     @Override
 908     boolean isTerminated() {
 909         return (state == TERMINATED);
 910     }
 911 
 912     @Override
 913     StackTraceElement[] asyncGetStackTrace() {
 914         StackTraceElement[] stackTrace;
 915         do {
 916             stackTrace = (carrierThread != null)
 917                     ? super.asyncGetStackTrace()  // mounted
 918                     : tryGetStackTrace();         // unmounted
 919             if (stackTrace == null) {
 920                 Thread.yield();
 921             }
 922         } while (stackTrace == null);
 923         return stackTrace;
 924     }
 925 
 926     /**
 927      * Returns the stack trace for this virtual thread if it is unmounted.
 928      * Returns null if the thread is in another state.
 929      */
 930     private StackTraceElement[] tryGetStackTrace() {
 931         int initialState = state();
 932         return switch (initialState) {
 933             case RUNNABLE, PARKED -> {
 934                 int suspendedState = initialState | SUSPENDED;
 935                 if (compareAndSetState(initialState, suspendedState)) {
 936                     try {
 937                         yield cont.getStackTrace();
 938                     } finally {
 939                         assert state == suspendedState;
 940                         setState(initialState);
 941 
 942                         // re-submit if runnable
 943                         // re-submit if unparked while suspended
 944                         if (initialState == RUNNABLE
 945                             || (parkPermit && compareAndSetState(PARKED, RUNNABLE))) {
 946                             try {
 947                                 submitRunContinuation();
 948                             } catch (RejectedExecutionException ignore) { }
 949                         }
 950                     }
 951                 }
 952                 yield null;
 953             }
 954             case NEW, STARTED, TERMINATED ->  new StackTraceElement[0];  // empty stack
 955             default -> null;
 956         };
 957     }
 958 
 959     @Override
 960     public String toString() {
 961         StringBuilder sb = new StringBuilder("VirtualThread[#");
 962         sb.append(threadId());
 963         String name = getName();
 964         if (!name.isEmpty()) {
 965             sb.append(",");
 966             sb.append(name);
 967         }
 968         sb.append("]/");
 969         Thread carrier = carrierThread;
 970         if (carrier != null) {
 971             // include the carrier thread state and name when mounted
 972             synchronized (carrierThreadAccessLock()) {
 973                 carrier = carrierThread;
 974                 if (carrier != null) {
 975                     String stateAsString = carrier.threadState().toString();
 976                     sb.append(stateAsString.toLowerCase(Locale.ROOT));
 977                     sb.append('@');
 978                     sb.append(carrier.getName());
 979                 }
 980             }
 981         }
 982         // include virtual thread state when not mounted
 983         if (carrier == null) {
 984             String stateAsString = threadState().toString();
 985             sb.append(stateAsString.toLowerCase(Locale.ROOT));
 986         }
 987         return sb.toString();
 988     }
 989 
 990     @Override
 991     public int hashCode() {
 992         return (int) threadId();
 993     }
 994 
 995     @Override
 996     public boolean equals(Object obj) {
 997         return obj == this;
 998     }
 999 
1000     /**
1001      * Returns the termination object, creating it if needed.
1002      */
1003     private CountDownLatch getTermination() {
1004         CountDownLatch termination = this.termination;
1005         if (termination == null) {
1006             termination = new CountDownLatch(1);
1007             if (!U.compareAndSetReference(this, TERMINATION, null, termination)) {
1008                 termination = this.termination;
1009             }
1010         }
1011         return termination;
1012     }
1013 
1014     /**
1015      * Returns the lock object to synchronize on when accessing carrierThread.
1016      * The lock prevents carrierThread from being reset to null during unmount.
1017      */
1018     private Object carrierThreadAccessLock() {
1019         // return interruptLock as unmount has to coordinate with interrupt
1020         return interruptLock;
1021     }
1022 
1023     // -- wrappers for get/set of state, parking permit, and carrier thread --
1024 
1025     private int state() {
1026         return state;  // volatile read
1027     }
1028 
1029     private void setState(int newValue) {
1030         state = newValue;  // volatile write
1031     }
1032 
1033     private boolean compareAndSetState(int expectedValue, int newValue) {
1034         return U.compareAndSetInt(this, STATE, expectedValue, newValue);
1035     }
1036 
1037     private void setParkPermit(boolean newValue) {
1038         if (parkPermit != newValue) {
1039             parkPermit = newValue;
1040         }
1041     }
1042 
1043     private boolean getAndSetParkPermit(boolean newValue) {
1044         if (parkPermit != newValue) {
1045             return U.getAndSetBoolean(this, PARK_PERMIT, newValue);
1046         } else {
1047             return newValue;
1048         }
1049     }
1050 
1051     private void setCarrierThread(Thread carrier) {
1052         // U.putReferenceRelease(this, CARRIER_THREAD, carrier);
1053         this.carrierThread = carrier;
1054     }
1055 
1056     // -- JVM TI support --
1057 
1058     private static volatile boolean notifyJvmtiEvents;  // set by VM
1059 
1060     @JvmtiMountTransition
1061     private native void notifyJvmtiMountBegin(boolean firstMount);
1062 
1063     @JvmtiMountTransition
1064     private native void notifyJvmtiMountEnd(boolean firstMount);
1065 
1066     @JvmtiMountTransition
1067     private native void notifyJvmtiUnmountBegin(boolean lastUnmount);
1068 
1069     @JvmtiMountTransition
1070     private native void notifyJvmtiUnmountEnd(boolean lastUnmount);
1071 
1072     @JvmtiMountTransition
1073     private native void notifyJvmtiHideFrames(boolean hide);
1074 
1075     private static native void registerNatives();
1076     static {
1077         registerNatives();
1078     }
1079 
1080     /**
1081      * Creates the default scheduler.
1082      */
1083     @SuppressWarnings("removal")
1084     private static ForkJoinPool createDefaultScheduler() {
1085         ForkJoinWorkerThreadFactory factory = pool -> {
1086             PrivilegedAction<ForkJoinWorkerThread> pa = () -> new CarrierThread(pool);
1087             return AccessController.doPrivileged(pa);
1088         };
1089         PrivilegedAction<ForkJoinPool> pa = () -> {
1090             int parallelism, maxPoolSize, minRunnable;
1091             String parallelismValue = System.getProperty("jdk.virtualThreadScheduler.parallelism");
1092             String maxPoolSizeValue = System.getProperty("jdk.virtualThreadScheduler.maxPoolSize");
1093             String minRunnableValue = System.getProperty("jdk.virtualThreadScheduler.minRunnable");
1094             if (parallelismValue != null) {
1095                 parallelism = Integer.parseInt(parallelismValue);
1096             } else {
1097                 parallelism = Runtime.getRuntime().availableProcessors();
1098             }
1099             if (maxPoolSizeValue != null) {
1100                 maxPoolSize = Integer.parseInt(maxPoolSizeValue);
1101                 parallelism = Integer.min(parallelism, maxPoolSize);
1102             } else {
1103                 maxPoolSize = Integer.max(parallelism, 256);
1104             }
1105             if (minRunnableValue != null) {
1106                 minRunnable = Integer.parseInt(minRunnableValue);
1107             } else {
1108                 minRunnable = Integer.max(parallelism / 2, 1);
1109             }
1110             Thread.UncaughtExceptionHandler handler = (t, e) -> { };
1111             boolean asyncMode = true; // FIFO
1112             return new ForkJoinPool(parallelism, factory, handler, asyncMode,
1113                          0, maxPoolSize, minRunnable, pool -> true, 30, SECONDS);
1114         };
1115         return AccessController.doPrivileged(pa);
1116     }
1117 
1118     /**
1119      * Creates the ScheduledThreadPoolExecutor used for timed unpark.
1120      */
1121     private static ScheduledExecutorService createDelayedTaskScheduler() {
1122         String propValue = GetPropertyAction.privilegedGetProperty("jdk.unparker.maxPoolSize");
1123         int poolSize;
1124         if (propValue != null) {
1125             poolSize = Integer.parseInt(propValue);
1126         } else {
1127             poolSize = 1;
1128         }
1129         ScheduledThreadPoolExecutor stpe = (ScheduledThreadPoolExecutor)
1130             Executors.newScheduledThreadPool(poolSize, task -> {
1131                 return InnocuousThread.newThread("VirtualThread-unparker", task);
1132             });
1133         stpe.setRemoveOnCancelPolicy(true);
1134         return stpe;
1135     }
1136 
1137     /**
1138      * Reads the value of the jdk.tracePinnedThreads property to determine if stack
1139      * traces should be printed when a carrier thread is pinned when a virtual thread
1140      * attempts to park.
1141      */
1142     private static int tracePinningMode() {
1143         String propValue = GetPropertyAction.privilegedGetProperty("jdk.tracePinnedThreads");
1144         if (propValue != null) {
1145             if (propValue.length() == 0 || "full".equalsIgnoreCase(propValue))
1146                 return 1;
1147             if ("short".equalsIgnoreCase(propValue))
1148                 return 2;
1149         }
1150         return 0;
1151     }
1152 }
--- EOF ---