openjdk/jdk/src/share/classes/java/util/concurrent/ForkJoinPool.java

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 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
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/*
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file:
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.Random;
import java.util.concurrent.AbstractExecutorService;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Future;
import java.util.concurrent.RejectedExecutionException;
import java.util.concurrent.RunnableFuture;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.locks.LockSupport;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.locks.Condition;

/**
 * An {@link ExecutorService} for running {@link ForkJoinTask}s.
 * A {@code ForkJoinPool} provides the entry point for submissions
 * from non-{@code ForkJoinTask} clients, as well as management and
 * monitoring operations.
 *
 * <p>A {@code ForkJoinPool} differs from other kinds of {@link
 * ExecutorService} mainly by virtue of employing
 * <em>work-stealing</em>: all threads in the pool attempt to find and
 * execute subtasks created by other active tasks (eventually blocking
 * waiting for work if none exist). This enables efficient processing
 * when most tasks spawn other subtasks (as do most {@code
 * ForkJoinTask}s). When setting <em>asyncMode</em> to true in
 * constructors, {@code ForkJoinPool}s may also be appropriate for use
 * with event-style tasks that are never joined.
 *
 * <p>A {@code ForkJoinPool} is constructed with a given target
 * parallelism level; by default, equal to the number of available
 * processors. The pool attempts to maintain enough active (or
 * available) threads by dynamically adding, suspending, or resuming
 * internal worker threads, even if some tasks are stalled waiting to
 * join others. However, no such adjustments are guaranteed in the
 * face of blocked IO or other unmanaged synchronization. The nested
 * {@link ManagedBlocker} interface enables extension of the kinds of
 * synchronization accommodated.
 *
 * <p>In addition to execution and lifecycle control methods, this
 * class provides status check methods (for example
 * {@link #getStealCount}) that are intended to aid in developing,
 * tuning, and monitoring fork/join applications. Also, method
 * {@link #toString} returns indications of pool state in a
 * convenient form for informal monitoring.
 *
 * <p> As is the case with other ExecutorServices, there are three
 * main task execution methods summarized in the following
 * table. These are designed to be used by clients not already engaged
 * in fork/join computations in the current pool.  The main forms of
 * these methods accept instances of {@code ForkJoinTask}, but
 * overloaded forms also allow mixed execution of plain {@code
 * Runnable}- or {@code Callable}- based activities as well.  However,
 * tasks that are already executing in a pool should normally
 * <em>NOT</em> use these pool execution methods, but instead use the
 * within-computation forms listed in the table.
 *
 * <table BORDER CELLPADDING=3 CELLSPACING=1>
 *  <tr>
 *    <td></td>
 *    <td ALIGN=CENTER> <b>Call from non-fork/join clients</b></td>
 *    <td ALIGN=CENTER> <b>Call from within fork/join computations</b></td>
 *  </tr>
 *  <tr>
 *    <td> <b>Arrange async execution</td>
 *    <td> {@link #execute(ForkJoinTask)}</td>
 *    <td> {@link ForkJoinTask#fork}</td>
 *  </tr>
 *  <tr>
 *    <td> <b>Await and obtain result</td>
 *    <td> {@link #invoke(ForkJoinTask)}</td>
 *    <td> {@link ForkJoinTask#invoke}</td>
 *  </tr>
 *  <tr>
 *    <td> <b>Arrange exec and obtain Future</td>
 *    <td> {@link #submit(ForkJoinTask)}</td>
 *    <td> {@link ForkJoinTask#fork} (ForkJoinTasks <em>are</em> Futures)</td>
 *  </tr>
 * </table>
 *
 * <p><b>Sample Usage.</b> Normally a single {@code ForkJoinPool} is
 * used for all parallel task execution in a program or subsystem.
 * Otherwise, use would not usually outweigh the construction and
 * bookkeeping overhead of creating a large set of threads. For
 * example, a common pool could be used for the {@code SortTasks}
 * illustrated in {@link RecursiveAction}. Because {@code
 * ForkJoinPool} uses threads in {@linkplain java.lang.Thread#isDaemon
 * daemon} mode, there is typically no need to explicitly {@link
 * #shutdown} such a pool upon program exit.
 *
 * <pre>
 * static final ForkJoinPool mainPool = new ForkJoinPool();
 * ...
 * public void sort(long[] array) {
 *   mainPool.invoke(new SortTask(array, 0, array.length));
 * }
 * </pre>
 *
 * <p><b>Implementation notes</b>: This implementation restricts the
 * maximum number of running threads to 32767. Attempts to create
 * pools with greater than the maximum number result in
 * {@code IllegalArgumentException}.
 *
 * <p>This implementation rejects submitted tasks (that is, by throwing
 * {@link RejectedExecutionException}) only when the pool is shut down
 * or internal resources have been exhausted.
 *
 * @since 1.7
 * @author Doug Lea
 */
public class ForkJoinPool extends AbstractExecutorService {

    /*
     * Implementation Overview
     *
     * This class provides the central bookkeeping and control for a
     * set of worker threads: Submissions from non-FJ threads enter
     * into a submission queue. Workers take these tasks and typically
     * split them into subtasks that may be stolen by other workers.
     * Preference rules give first priority to processing tasks from
     * their own queues (LIFO or FIFO, depending on mode), then to
     * randomized FIFO steals of tasks in other worker queues, and
     * lastly to new submissions.
     *
     * The main throughput advantages of work-stealing stem from
     * decentralized control -- workers mostly take tasks from
     * themselves or each other. We cannot negate this in the
     * implementation of other management responsibilities. The main
     * tactic for avoiding bottlenecks is packing nearly all
     * essentially atomic control state into a single 64bit volatile
     * variable ("ctl"). This variable is read on the order of 10-100
     * times as often as it is modified (always via CAS). (There is
     * some additional control state, for example variable "shutdown"
     * for which we can cope with uncoordinated updates.)  This
     * streamlines synchronization and control at the expense of messy
     * constructions needed to repack status bits upon updates.
     * Updates tend not to contend with each other except during
     * bursts while submitted tasks begin or end.  In some cases when
     * they do contend, threads can instead do something else
     * (usually, scan for tasks) until contention subsides.
     *
     * To enable packing, we restrict maximum parallelism to (1<<15)-1
     * (which is far in excess of normal operating range) to allow
     * ids, counts, and their negations (used for thresholding) to fit
     * into 16bit fields.
     *
     * Recording Workers.  Workers are recorded in the "workers" array
     * that is created upon pool construction and expanded if (rarely)
     * necessary.  This is an array as opposed to some other data
     * structure to support index-based random steals by workers.
     * Updates to the array recording new workers and unrecording
     * terminated ones are protected from each other by a seqLock
     * (scanGuard) but the array is otherwise concurrently readable,
     * and accessed directly by workers. To simplify index-based
     * operations, the array size is always a power of two, and all
     * readers must tolerate null slots. To avoid flailing during
     * start-up, the array is presized to hold twice #parallelism
     * workers (which is unlikely to need further resizing during
     * execution). But to avoid dealing with so many null slots,
     * variable scanGuard includes a mask for the nearest power of two
     * that contains all current workers.  All worker thread creation
     * is on-demand, triggered by task submissions, replacement of
     * terminated workers, and/or compensation for blocked
     * workers. However, all other support code is set up to work with
     * other policies.  To ensure that we do not hold on to worker
     * references that would prevent GC, ALL accesses to workers are
     * via indices into the workers array (which is one source of some
     * of the messy code constructions here). In essence, the workers
     * array serves as a weak reference mechanism. Thus for example
     * the wait queue field of ctl stores worker indices, not worker
     * references.  Access to the workers in associated methods (for
     * example signalWork) must both index-check and null-check the
     * IDs. All such accesses ignore bad IDs by returning out early
     * from what they are doing, since this can only be associated
     * with termination, in which case it is OK to give up.
     *
     * All uses of the workers array, as well as queue arrays, check
     * that the array is non-null (even if previously non-null). This
     * allows nulling during termination, which is currently not
     * necessary, but remains an option for resource-revocation-based
     * shutdown schemes.
     *
     * Wait Queuing. Unlike HPC work-stealing frameworks, we cannot
     * let workers spin indefinitely scanning for tasks when none can
     * be found immediately, and we cannot start/resume workers unless
     * there appear to be tasks available.  On the other hand, we must
     * quickly prod them into action when new tasks are submitted or
     * generated.  We park/unpark workers after placing in an event
     * wait queue when they cannot find work. This "queue" is actually
     * a simple Treiber stack, headed by the "id" field of ctl, plus a
     * 15bit counter value to both wake up waiters (by advancing their
     * count) and avoid ABA effects. Successors are held in worker
     * field "nextWait".  Queuing deals with several intrinsic races,
     * mainly that a task-producing thread can miss seeing (and
     * signalling) another thread that gave up looking for work but
     * has not yet entered the wait queue. We solve this by requiring
     * a full sweep of all workers both before (in scan()) and after
     * (in tryAwaitWork()) a newly waiting worker is added to the wait
     * queue. During a rescan, the worker might release some other
     * queued worker rather than itself, which has the same net
     * effect. Because enqueued workers may actually be rescanning
     * rather than waiting, we set and clear the "parked" field of
     * ForkJoinWorkerThread to reduce unnecessary calls to unpark.
     * (Use of the parked field requires a secondary recheck to avoid
     * missed signals.)
     *
     * Signalling.  We create or wake up workers only when there
     * appears to be at least one task they might be able to find and
     * execute.  When a submission is added or another worker adds a
     * task to a queue that previously had two or fewer tasks, they
     * signal waiting workers (or trigger creation of new ones if
     * fewer than the given parallelism level -- see signalWork).
     * These primary signals are buttressed by signals during rescans
     * as well as those performed when a worker steals a task and
     * notices that there are more tasks too; together these cover the
     * signals needed in cases when more than two tasks are pushed
     * but untaken.
     *
     * Trimming workers. To release resources after periods of lack of
     * use, a worker starting to wait when the pool is quiescent will
     * time out and terminate if the pool has remained quiescent for
     * SHRINK_RATE nanosecs. This will slowly propagate, eventually
     * terminating all workers after long periods of non-use.
     *
     * Submissions. External submissions are maintained in an
     * array-based queue that is structured identically to
     * ForkJoinWorkerThread queues except for the use of
     * submissionLock in method addSubmission. Unlike the case for
     * worker queues, multiple external threads can add new
     * submissions, so adding requires a lock.
     *
     * Compensation. Beyond work-stealing support and lifecycle
     * control, the main responsibility of this framework is to take
     * actions when one worker is waiting to join a task stolen (or
     * always held by) another.  Because we are multiplexing many
     * tasks on to a pool of workers, we can't just let them block (as
     * in Thread.join).  We also cannot just reassign the joiner's
     * run-time stack with another and replace it later, which would
     * be a form of "continuation", that even if possible is not
     * necessarily a good idea since we sometimes need both an
     * unblocked task and its continuation to progress. Instead we
     * combine two tactics:
     *
     *   Helping: Arranging for the joiner to execute some task that it
     *      would be running if the steal had not occurred.  Method
     *      ForkJoinWorkerThread.joinTask tracks joining->stealing
     *      links to try to find such a task.
     *
     *   Compensating: Unless there are already enough live threads,
     *      method tryPreBlock() may create or re-activate a spare
     *      thread to compensate for blocked joiners until they
     *      unblock.
     *
     * The ManagedBlocker extension API can't use helping so relies
     * only on compensation in method awaitBlocker.
     *
     * It is impossible to keep exactly the target parallelism number
     * of threads running at any given time.  Determining the
     * existence of conservatively safe helping targets, the
     * availability of already-created spares, and the apparent need
     * to create new spares are all racy and require heuristic
     * guidance, so we rely on multiple retries of each.  Currently,
     * in keeping with on-demand signalling policy, we compensate only
     * if blocking would leave less than one active (non-waiting,
     * non-blocked) worker. Additionally, to avoid some false alarms
     * due to GC, lagging counters, system activity, etc, compensated
     * blocking for joins is only attempted after rechecks stabilize
     * (retries are interspersed with Thread.yield, for good
     * citizenship).  The variable blockedCount, incremented before
     * blocking and decremented after, is sometimes needed to
     * distinguish cases of waiting for work vs blocking on joins or
     * other managed sync. Both cases are equivalent for most pool
     * control, so we can update non-atomically. (Additionally,
     * contention on blockedCount alleviates some contention on ctl).
     *
     * Shutdown and Termination. A call to shutdownNow atomically sets
     * the ctl stop bit and then (non-atomically) sets each workers
     * "terminate" status, cancels all unprocessed tasks, and wakes up
     * all waiting workers.  Detecting whether termination should
     * commence after a non-abrupt shutdown() call requires more work
     * and bookkeeping. We need consensus about quiesence (i.e., that
     * there is no more work) which is reflected in active counts so
     * long as there are no current blockers, as well as possible
     * re-evaluations during independent changes in blocking or
     * quiescing workers.
     *
     * Style notes: There is a lot of representation-level coupling
     * among classes ForkJoinPool, ForkJoinWorkerThread, and
     * ForkJoinTask.  Most fields of ForkJoinWorkerThread maintain
     * data structures managed by ForkJoinPool, so are directly
     * accessed.  Conversely we allow access to "workers" array by
     * workers, and direct access to ForkJoinTask.status by both
     * ForkJoinPool and ForkJoinWorkerThread.  There is little point
     * trying to reduce this, since any associated future changes in
     * representations will need to be accompanied by algorithmic
     * changes anyway. All together, these low-level implementation
     * choices produce as much as a factor of 4 performance
     * improvement compared to naive implementations, and enable the
     * processing of billions of tasks per second, at the expense of
     * some ugliness.
     *
     * Methods signalWork() and scan() are the main bottlenecks so are
     * especially heavily micro-optimized/mangled.  There are lots of
     * inline assignments (of form "while ((local = field) != 0)")
     * which are usually the simplest way to ensure the required read
     * orderings (which are sometimes critical). This leads to a
     * "C"-like style of listing declarations of these locals at the
     * heads of methods or blocks.  There are several occurrences of
     * the unusual "do {} while (!cas...)"  which is the simplest way
     * to force an update of a CAS'ed variable. There are also other
     * coding oddities that help some methods perform reasonably even
     * when interpreted (not compiled).
     *
     * The order of declarations in this file is: (1) declarations of
     * statics (2) fields (along with constants used when unpacking
     * some of them), listed in an order that tends to reduce
     * contention among them a bit under most JVMs.  (3) internal
     * control methods (4) callbacks and other support for
     * ForkJoinTask and ForkJoinWorkerThread classes, (5) exported
     * methods (plus a few little helpers). (6) static block
     * initializing all statics in a minimally dependent order.
     */

    /**
     * Factory for creating new {@link ForkJoinWorkerThread}s.
     * A {@code ForkJoinWorkerThreadFactory} must be defined and used
     * for {@code ForkJoinWorkerThread} subclasses that extend base
     * functionality or initialize threads with different contexts.
     */
    public static interface ForkJoinWorkerThreadFactory {
        /**
         * Returns a new worker thread operating in the given pool.
         *
         * @param pool the pool this thread works in
         * @throws NullPointerException if the pool is null
         */
        public ForkJoinWorkerThread newThread(ForkJoinPool pool);
    }

    /**
     * Default ForkJoinWorkerThreadFactory implementation; creates a
     * new ForkJoinWorkerThread.
     */
    static class DefaultForkJoinWorkerThreadFactory
        implements ForkJoinWorkerThreadFactory {
        public ForkJoinWorkerThread newThread(ForkJoinPool pool) {
            return new ForkJoinWorkerThread(pool);
        }
    }

    /**
     * Creates a new ForkJoinWorkerThread. This factory is used unless
     * overridden in ForkJoinPool constructors.
     */
    public static final ForkJoinWorkerThreadFactory
        defaultForkJoinWorkerThreadFactory;

    /**
     * Permission required for callers of methods that may start or
     * kill threads.
     */
    private static final RuntimePermission modifyThreadPermission;

    /**
     * If there is a security manager, makes sure caller has
     * permission to modify threads.
     */
    private static void checkPermission() {
        SecurityManager security = System.getSecurityManager();
        if (security != null)
            security.checkPermission(modifyThreadPermission);
    }

    /**
     * Generator for assigning sequence numbers as pool names.
     */
    private static final AtomicInteger poolNumberGenerator;

    /**
     * Generator for initial random seeds for worker victim
     * selection. This is used only to create initial seeds. Random
     * steals use a cheaper xorshift generator per steal attempt. We
     * don't expect much contention on seedGenerator, so just use a
     * plain Random.
     */
    static final Random workerSeedGenerator;

    /**
     * Array holding all worker threads in the pool.  Initialized upon
     * construction. Array size must be a power of two.  Updates and
     * replacements are protected by scanGuard, but the array is
     * always kept in a consistent enough state to be randomly
     * accessed without locking by workers performing work-stealing,
     * as well as other traversal-based methods in this class, so long
     * as reads memory-acquire by first reading ctl. All readers must
     * tolerate that some array slots may be null.
     */
    ForkJoinWorkerThread[] workers;

    /**
     * Initial size for submission queue array. Must be a power of
     * two.  In many applications, these always stay small so we use a
     * small initial cap.
     */
    private static final int INITIAL_QUEUE_CAPACITY = 8;

    /**
     * Maximum size for submission queue array. Must be a power of two
     * less than or equal to 1 << (31 - width of array entry) to
     * ensure lack of index wraparound, but is capped at a lower
     * value to help users trap runaway computations.
     */
    private static final int MAXIMUM_QUEUE_CAPACITY = 1 << 24; // 16M

    /**
     * Array serving as submission queue. Initialized upon construction.
     */
    private ForkJoinTask<?>[] submissionQueue;

    /**
     * Lock protecting submissions array for addSubmission
     */
    private final ReentrantLock submissionLock;

    /**
     * Condition for awaitTermination, using submissionLock for
     * convenience.
     */
    private final Condition termination;

    /**
     * Creation factory for worker threads.
     */
    private final ForkJoinWorkerThreadFactory factory;

    /**
     * The uncaught exception handler used when any worker abruptly
     * terminates.
     */
    final Thread.UncaughtExceptionHandler ueh;

    /**
     * Prefix for assigning names to worker threads
     */
    private final String workerNamePrefix;

    /**
     * Sum of per-thread steal counts, updated only when threads are
     * idle or terminating.
     */
    private volatile long stealCount;

    /**
     * Main pool control -- a long packed with:
     * AC: Number of active running workers minus target parallelism (16 bits)
     * TC: Number of total workers minus target parallelism (16bits)
     * ST: true if pool is terminating (1 bit)
     * EC: the wait count of top waiting thread (15 bits)
     * ID: ~poolIndex of top of Treiber stack of waiting threads (16 bits)
     *
     * When convenient, we can extract the upper 32 bits of counts and
     * the lower 32 bits of queue state, u = (int)(ctl >>> 32) and e =
     * (int)ctl.  The ec field is never accessed alone, but always
     * together with id and st. The offsets of counts by the target
     * parallelism and the positionings of fields makes it possible to
     * perform the most common checks via sign tests of fields: When
     * ac is negative, there are not enough active workers, when tc is
     * negative, there are not enough total workers, when id is
     * negative, there is at least one waiting worker, and when e is
     * negative, the pool is terminating.  To deal with these possibly
     * negative fields, we use casts in and out of "short" and/or
     * signed shifts to maintain signedness.
     */
    volatile long ctl;

    // bit positions/shifts for fields
    private static final int  AC_SHIFT   = 48;
    private static final int  TC_SHIFT   = 32;
    private static final int  ST_SHIFT   = 31;
    private static final int  EC_SHIFT   = 16;

    // bounds
    private static final int  MAX_ID     = 0x7fff;  // max poolIndex
    private static final int  SMASK      = 0xffff;  // mask short bits
    private static final int  SHORT_SIGN = 1 << 15;
    private static final int  INT_SIGN   = 1 << 31;

    // masks
    private static final long STOP_BIT   = 0x0001L << ST_SHIFT;
    private static final long AC_MASK    = ((long)SMASK) << AC_SHIFT;
    private static final long TC_MASK    = ((long)SMASK) << TC_SHIFT;

    // units for incrementing and decrementing
    private static final long TC_UNIT    = 1L << TC_SHIFT;
    private static final long AC_UNIT    = 1L << AC_SHIFT;

    // masks and units for dealing with u = (int)(ctl >>> 32)
    private static final int  UAC_SHIFT  = AC_SHIFT - 32;
    private static final int  UTC_SHIFT  = TC_SHIFT - 32;
    private static final int  UAC_MASK   = SMASK << UAC_SHIFT;
    private static final int  UTC_MASK   = SMASK << UTC_SHIFT;
    private static final int  UAC_UNIT   = 1 << UAC_SHIFT;
    private static final int  UTC_UNIT   = 1 << UTC_SHIFT;

    // masks and units for dealing with e = (int)ctl
    private static final int  E_MASK     = 0x7fffffff; // no STOP_BIT
    private static final int  EC_UNIT    = 1 << EC_SHIFT;

    /**
     * The target parallelism level.
     */
    final int parallelism;

    /**
     * Index (mod submission queue length) of next element to take
     * from submission queue. Usage is identical to that for
     * per-worker queues -- see ForkJoinWorkerThread internal
     * documentation.
     */
    volatile int queueBase;

    /**
     * Index (mod submission queue length) of next element to add
     * in submission queue. Usage is identical to that for
     * per-worker queues -- see ForkJoinWorkerThread internal
     * documentation.
     */
    int queueTop;

    /**
     * True when shutdown() has been called.
     */
    volatile boolean shutdown;

    /**
     * True if use local fifo, not default lifo, for local polling
     * Read by, and replicated by ForkJoinWorkerThreads
     */
    final boolean locallyFifo;

    /**
     * The number of threads in ForkJoinWorkerThreads.helpQuiescePool.
     * When non-zero, suppresses automatic shutdown when active
     * counts become zero.
     */
    volatile int quiescerCount;

    /**
     * The number of threads blocked in join.
     */
    volatile int blockedCount;

    /**
     * Counter for worker Thread names (unrelated to their poolIndex)
     */
    private volatile int nextWorkerNumber;

    /**
     * The index for the next created worker. Accessed under scanGuard.
     */
    private int nextWorkerIndex;

    /**
     * SeqLock and index masking for updates to workers array.  Locked
     * when SG_UNIT is set. Unlocking clears bit by adding
     * SG_UNIT. Staleness of read-only operations can be checked by
     * comparing scanGuard to value before the reads. The low 16 bits
     * (i.e, anding with SMASK) hold (the smallest power of two
     * covering all worker indices, minus one, and is used to avoid
     * dealing with large numbers of null slots when the workers array
     * is overallocated.
     */
    volatile int scanGuard;

    private static final int SG_UNIT = 1 << 16;

    /**
     * The wakeup interval (in nanoseconds) for a worker waiting for a
     * task when the pool is quiescent to instead try to shrink the
     * number of workers.  The exact value does not matter too
     * much. It must be short enough to release resources during
     * sustained periods of idleness, but not so short that threads
     * are continually re-created.
     */
    private static final long SHRINK_RATE =
        4L * 1000L * 1000L * 1000L; // 4 seconds

    /**
     * Top-level loop for worker threads: On each step: if the
     * previous step swept through all queues and found no tasks, or
     * there are excess threads, then possibly blocks. Otherwise,
     * scans for and, if found, executes a task. Returns when pool
     * and/or worker terminate.
     *
     * @param w the worker
     */
    final void work(ForkJoinWorkerThread w) {
        boolean swept = false;                // true on empty scans
        long c;
        while (!w.terminate && (int)(c = ctl) >= 0) {
            int a;                            // active count
            if (!swept && (a = (int)(c >> AC_SHIFT)) <= 0)
                swept = scan(w, a);
            else if (tryAwaitWork(w, c))
                swept = false;
        }
    }

    // Signalling

    /**
     * Wakes up or creates a worker.
     */
    final void signalWork() {
        /*
         * The while condition is true if: (there is are too few total
         * workers OR there is at least one waiter) AND (there are too
         * few active workers OR the pool is terminating).  The value
         * of e distinguishes the remaining cases: zero (no waiters)
         * for create, negative if terminating (in which case do
         * nothing), else release a waiter. The secondary checks for
         * release (non-null array etc) can fail if the pool begins
         * terminating after the test, and don't impose any added cost
         * because JVMs must perform null and bounds checks anyway.
         */
        long c; int e, u;
        while ((((e = (int)(c = ctl)) | (u = (int)(c >>> 32))) &
                (INT_SIGN|SHORT_SIGN)) == (INT_SIGN|SHORT_SIGN) && e >= 0) {
            if (e > 0) {                         // release a waiting worker
                int i; ForkJoinWorkerThread w; ForkJoinWorkerThread[] ws;
                if ((ws = workers) == null ||
                    (i = ~e & SMASK) >= ws.length ||
                    (w = ws[i]) == null)
                    break;
                long nc = (((long)(w.nextWait & E_MASK)) |
                           ((long)(u + UAC_UNIT) << 32));
                if (w.eventCount == e &&
                    UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) {
                    w.eventCount = (e + EC_UNIT) & E_MASK;
                    if (w.parked)
                        UNSAFE.unpark(w);
                    break;
                }
            }
            else if (UNSAFE.compareAndSwapLong
                     (this, ctlOffset, c,
                      (long)(((u + UTC_UNIT) & UTC_MASK) |
                             ((u + UAC_UNIT) & UAC_MASK)) << 32)) {
                addWorker();
                break;
            }
        }
    }

    /**
     * Variant of signalWork to help release waiters on rescans.
     * Tries once to release a waiter if active count < 0.
     *
     * @return false if failed due to contention, else true
     */
    private boolean tryReleaseWaiter() {
        long c; int e, i; ForkJoinWorkerThread w; ForkJoinWorkerThread[] ws;
        if ((e = (int)(c = ctl)) > 0 &&
            (int)(c >> AC_SHIFT) < 0 &&
            (ws = workers) != null &&
            (i = ~e & SMASK) < ws.length &&
            (w = ws[i]) != null) {
            long nc = ((long)(w.nextWait & E_MASK) |
                       ((c + AC_UNIT) & (AC_MASK|TC_MASK)));
            if (w.eventCount != e ||
                !UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc))
                return false;
            w.eventCount = (e + EC_UNIT) & E_MASK;
            if (w.parked)
                UNSAFE.unpark(w);
        }
        return true;
    }

    // Scanning for tasks

    /**
     * Scans for and, if found, executes one task. Scans start at a
     * random index of workers array, and randomly select the first
     * (2*#workers)-1 probes, and then, if all empty, resort to 2
     * circular sweeps, which is necessary to check quiescence. and
     * taking a submission only if no stealable tasks were found.  The
     * steal code inside the loop is a specialized form of
     * ForkJoinWorkerThread.deqTask, followed bookkeeping to support
     * helpJoinTask and signal propagation. The code for submission
     * queues is almost identical. On each steal, the worker completes
     * not only the task, but also all local tasks that this task may
     * have generated. On detecting staleness or contention when
     * trying to take a task, this method returns without finishing
     * sweep, which allows global state rechecks before retry.
     *
     * @param w the worker
     * @param a the number of active workers
     * @return true if swept all queues without finding a task
     */
    private boolean scan(ForkJoinWorkerThread w, int a) {
        int g = scanGuard; // mask 0 avoids useless scans if only one active
        int m = (parallelism == 1 - a && blockedCount == 0) ? 0 : g & SMASK;
        ForkJoinWorkerThread[] ws = workers;
        if (ws == null || ws.length <= m)         // staleness check
            return false;
        for (int r = w.seed, k = r, j = -(m + m); j <= m + m; ++j) {
            ForkJoinTask<?> t; ForkJoinTask<?>[] q; int b, i;
            ForkJoinWorkerThread v = ws[k & m];
            if (v != null && (b = v.queueBase) != v.queueTop &&
                (q = v.queue) != null && (i = (q.length - 1) & b) >= 0) {
                long u = (i << ASHIFT) + ABASE;
                if ((t = q[i]) != null && v.queueBase == b &&
                    UNSAFE.compareAndSwapObject(q, u, t, null)) {
                    int d = (v.queueBase = b + 1) - v.queueTop;
                    v.stealHint = w.poolIndex;
                    if (d != 0)
                        signalWork();             // propagate if nonempty
                    w.execTask(t);
                }
                r ^= r << 13; r ^= r >>> 17; w.seed = r ^ (r << 5);
                return false;                     // store next seed
            }
            else if (j < 0) {                     // xorshift
                r ^= r << 13; r ^= r >>> 17; k = r ^= r << 5;
            }
            else
                ++k;
        }
        if (scanGuard != g)                       // staleness check
            return false;
        else {                                    // try to take submission
            ForkJoinTask<?> t; ForkJoinTask<?>[] q; int b, i;
            if ((b = queueBase) != queueTop &&
                (q = submissionQueue) != null &&
                (i = (q.length - 1) & b) >= 0) {
                long u = (i << ASHIFT) + ABASE;
                if ((t = q[i]) != null && queueBase == b &&
                    UNSAFE.compareAndSwapObject(q, u, t, null)) {
                    queueBase = b + 1;
                    w.execTask(t);
                }
                return false;
            }
            return true;                         // all queues empty
        }
    }

    /**
     * Tries to enqueue worker w in wait queue and await change in
     * worker's eventCount.  If the pool is quiescent and there is
     * more than one worker, possibly terminates worker upon exit.
     * Otherwise, before blocking, rescans queues to avoid missed
     * signals.  Upon finding work, releases at least one worker
     * (which may be the current worker). Rescans restart upon
     * detected staleness or failure to release due to
     * contention. Note the unusual conventions about Thread.interrupt
     * here and elsewhere: Because interrupts are used solely to alert
     * threads to check termination, which is checked here anyway, we
     * clear status (using Thread.interrupted) before any call to
     * park, so that park does not immediately return due to status
     * being set via some other unrelated call to interrupt in user
     * code.
     *
     * @param w the calling worker
     * @param c the ctl value on entry
     * @return true if waited or another thread was released upon enq
     */
    private boolean tryAwaitWork(ForkJoinWorkerThread w, long c) {
        int v = w.eventCount;
        w.nextWait = (int)c;                      // w's successor record
        long nc = (long)(v & E_MASK) | ((c - AC_UNIT) & (AC_MASK|TC_MASK));
        if (ctl != c || !UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) {
            long d = ctl; // return true if lost to a deq, to force scan
            return (int)d != (int)c && ((d - c) & AC_MASK) >= 0L;
        }
        for (int sc = w.stealCount; sc != 0;) {   // accumulate stealCount
            long s = stealCount;
            if (UNSAFE.compareAndSwapLong(this, stealCountOffset, s, s + sc))
                sc = w.stealCount = 0;
            else if (w.eventCount != v)
                return true;                      // update next time
        }
        if ((!shutdown || !tryTerminate(false)) &&
            (int)c != 0 && parallelism + (int)(nc >> AC_SHIFT) == 0 &&
            blockedCount == 0 && quiescerCount == 0)
            idleAwaitWork(w, nc, c, v);           // quiescent
        for (boolean rescanned = false;;) {
            if (w.eventCount != v)
                return true;
            if (!rescanned) {
                int g = scanGuard, m = g & SMASK;
                ForkJoinWorkerThread[] ws = workers;
                if (ws != null && m < ws.length) {
                    rescanned = true;
                    for (int i = 0; i <= m; ++i) {
                        ForkJoinWorkerThread u = ws[i];
                        if (u != null) {
                            if (u.queueBase != u.queueTop &&
                                !tryReleaseWaiter())
                                rescanned = false; // contended
                            if (w.eventCount != v)
                                return true;
                        }
                    }
                }
                if (scanGuard != g ||              // stale
                    (queueBase != queueTop && !tryReleaseWaiter()))
                    rescanned = false;
                if (!rescanned)
                    Thread.yield();                // reduce contention
                else
                    Thread.interrupted();          // clear before park
            }
            else {
                w.parked = true;                   // must recheck
                if (w.eventCount != v) {
                    w.parked = false;
                    return true;
                }
                LockSupport.park(this);
                rescanned = w.parked = false;
            }
        }
    }

    /**
     * If inactivating worker w has caused pool to become
     * quiescent, check for pool termination, and wait for event
     * for up to SHRINK_RATE nanosecs (rescans are unnecessary in
     * this case because quiescence reflects consensus about lack
     * of work). On timeout, if ctl has not changed, terminate the
     * worker. Upon its termination (see deregisterWorker), it may
     * wake up another worker to possibly repeat this process.
     *
     * @param w the calling worker
     * @param currentCtl the ctl value after enqueuing w
     * @param prevCtl the ctl value if w terminated
     * @param v the eventCount w awaits change
     */
    private void idleAwaitWork(ForkJoinWorkerThread w, long currentCtl,
                               long prevCtl, int v) {
        if (w.eventCount == v) {
            if (shutdown)
                tryTerminate(false);
            ForkJoinTask.helpExpungeStaleExceptions(); // help clean weak refs
            while (ctl == currentCtl) {
                long startTime = System.nanoTime();
                w.parked = true;
                if (w.eventCount == v)             // must recheck
                    LockSupport.parkNanos(this, SHRINK_RATE);
                w.parked = false;
                if (w.eventCount != v)
                    break;
                else if (System.nanoTime() - startTime <
                         SHRINK_RATE - (SHRINK_RATE / 10)) // timing slop
                    Thread.interrupted();          // spurious wakeup
                else if (UNSAFE.compareAndSwapLong(this, ctlOffset,
                                                   currentCtl, prevCtl)) {
                    w.terminate = true;            // restore previous
                    w.eventCount = ((int)currentCtl + EC_UNIT) & E_MASK;
                    break;
                }
            }
        }
    }

    // Submissions

    /**
     * Enqueues the given task in the submissionQueue.  Same idea as
     * ForkJoinWorkerThread.pushTask except for use of submissionLock.
     *
     * @param t the task
     */
    private void addSubmission(ForkJoinTask<?> t) {
        final ReentrantLock lock = this.submissionLock;
        lock.lock();
        try {
            ForkJoinTask<?>[] q; int s, m;
            if ((q = submissionQueue) != null) {    // ignore if queue removed
                long u = (((s = queueTop) & (m = q.length-1)) << ASHIFT)+ABASE;
                UNSAFE.putOrderedObject(q, u, t);
                queueTop = s + 1;
                if (s - queueBase == m)
                    growSubmissionQueue();
            }
        } finally {
            lock.unlock();
        }
        signalWork();
    }

    //  (pollSubmission is defined below with exported methods)

    /**
     * Creates or doubles submissionQueue array.
     * Basically identical to ForkJoinWorkerThread version.
     */
    private void growSubmissionQueue() {
        ForkJoinTask<?>[] oldQ = submissionQueue;
        int size = oldQ != null ? oldQ.length << 1 : INITIAL_QUEUE_CAPACITY;
        if (size > MAXIMUM_QUEUE_CAPACITY)
            throw new RejectedExecutionException("Queue capacity exceeded");
        if (size < INITIAL_QUEUE_CAPACITY)
            size = INITIAL_QUEUE_CAPACITY;
        ForkJoinTask<?>[] q = submissionQueue = new ForkJoinTask<?>[size];
        int mask = size - 1;
        int top = queueTop;
        int oldMask;
        if (oldQ != null && (oldMask = oldQ.length - 1) >= 0) {
            for (int b = queueBase; b != top; ++b) {
                long u = ((b & oldMask) << ASHIFT) + ABASE;
                Object x = UNSAFE.getObjectVolatile(oldQ, u);
                if (x != null && UNSAFE.compareAndSwapObject(oldQ, u, x, null))
                    UNSAFE.putObjectVolatile
                        (q, ((b & mask) << ASHIFT) + ABASE, x);
            }
        }
    }

    // Blocking support

    /**
     * Tries to increment blockedCount, decrement active count
     * (sometimes implicitly) and possibly release or create a
     * compensating worker in preparation for blocking. Fails
     * on contention or termination.
     *
     * @return true if the caller can block, else should recheck and retry
     */
    private boolean tryPreBlock() {
        int b = blockedCount;
        if (UNSAFE.compareAndSwapInt(this, blockedCountOffset, b, b + 1)) {
            int pc = parallelism;
            do {
                ForkJoinWorkerThread[] ws; ForkJoinWorkerThread w;
                int e, ac, tc, rc, i;
                long c = ctl;
                int u = (int)(c >>> 32);
                if ((e = (int)c) < 0) {
                                                 // skip -- terminating
                }
                else if ((ac = (u >> UAC_SHIFT)) <= 0 && e != 0 &&
                         (ws = workers) != null &&
                         (i = ~e & SMASK) < ws.length &&
                         (w = ws[i]) != null) {
                    long nc = ((long)(w.nextWait & E_MASK) |
                               (c & (AC_MASK|TC_MASK)));
                    if (w.eventCount == e &&
                        UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) {
                        w.eventCount = (e + EC_UNIT) & E_MASK;
                        if (w.parked)
                            UNSAFE.unpark(w);
                        return true;             // release an idle worker
                    }
                }
                else if ((tc = (short)(u >>> UTC_SHIFT)) >= 0 && ac + pc > 1) {
                    long nc = ((c - AC_UNIT) & AC_MASK) | (c & ~AC_MASK);
                    if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc))
                        return true;             // no compensation needed
                }
                else if (tc + pc < MAX_ID) {
                    long nc = ((c + TC_UNIT) & TC_MASK) | (c & ~TC_MASK);
                    if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) {
                        addWorker();
                        return true;            // create a replacement
                    }
                }
                // try to back out on any failure and let caller retry
            } while (!UNSAFE.compareAndSwapInt(this, blockedCountOffset,
                                               b = blockedCount, b - 1));
        }
        return false;
    }

    /**
     * Decrements blockedCount and increments active count
     */
    private void postBlock() {
        long c;
        do {} while (!UNSAFE.compareAndSwapLong(this, ctlOffset,  // no mask
                                                c = ctl, c + AC_UNIT));
        int b;
        do {} while (!UNSAFE.compareAndSwapInt(this, blockedCountOffset,
                                               b = blockedCount, b - 1));
    }

    /**
     * Possibly blocks waiting for the given task to complete, or
     * cancels the task if terminating.  Fails to wait if contended.
     *
     * @param joinMe the task
     */
    final void tryAwaitJoin(ForkJoinTask<?> joinMe) {
        int s;
        Thread.interrupted(); // clear interrupts before checking termination
        if (joinMe.status >= 0) {
            if (tryPreBlock()) {
                joinMe.tryAwaitDone(0L);
                postBlock();
            }
            else if ((ctl & STOP_BIT) != 0L)
                joinMe.cancelIgnoringExceptions();
        }
    }

    /**
     * Possibly blocks the given worker waiting for joinMe to
     * complete or timeout
     *
     * @param joinMe the task
     * @param millis the wait time for underlying Object.wait
     */
    final void timedAwaitJoin(ForkJoinTask<?> joinMe, long nanos) {
        while (joinMe.status >= 0) {
            Thread.interrupted();
            if ((ctl & STOP_BIT) != 0L) {
                joinMe.cancelIgnoringExceptions();
                break;
            }
            if (tryPreBlock()) {
                long last = System.nanoTime();
                while (joinMe.status >= 0) {
                    long millis = TimeUnit.NANOSECONDS.toMillis(nanos);
                    if (millis <= 0)
                        break;
                    joinMe.tryAwaitDone(millis);
                    if (joinMe.status < 0)
                        break;
                    if ((ctl & STOP_BIT) != 0L) {
                        joinMe.cancelIgnoringExceptions();
                        break;
                    }
                    long now = System.nanoTime();
                    nanos -= now - last;
                    last = now;
                }
                postBlock();
                break;
            }
        }
    }

    /**
     * If necessary, compensates for blocker, and blocks
     */
    private void awaitBlocker(ManagedBlocker blocker)
        throws InterruptedException {
        while (!blocker.isReleasable()) {
            if (tryPreBlock()) {
                try {
                    do {} while (!blocker.isReleasable() && !blocker.block());
                } finally {
                    postBlock();
                }
                break;
            }
        }
    }

    // Creating, registering and deregistring workers

    /**
     * Tries to create and start a worker; minimally rolls back counts
     * on failure.
     */
    private void addWorker() {
        Throwable ex = null;
        ForkJoinWorkerThread t = null;
        try {
            t = factory.newThread(this);
        } catch (Throwable e) {
            ex = e;
        }
        if (t == null) {  // null or exceptional factory return
            long c;       // adjust counts
            do {} while (!UNSAFE.compareAndSwapLong
                         (this, ctlOffset, c = ctl,
                          (((c - AC_UNIT) & AC_MASK) |
                           ((c - TC_UNIT) & TC_MASK) |
                           (c & ~(AC_MASK|TC_MASK)))));
            // Propagate exception if originating from an external caller
            if (!tryTerminate(false) && ex != null &&
                !(Thread.currentThread() instanceof ForkJoinWorkerThread))
                UNSAFE.throwException(ex);
        }
        else
            t.start();
    }

    /**
     * Callback from ForkJoinWorkerThread constructor to assign a
     * public name
     */
    final String nextWorkerName() {
        for (int n;;) {
            if (UNSAFE.compareAndSwapInt(this, nextWorkerNumberOffset,
                                         n = nextWorkerNumber, ++n))
                return workerNamePrefix + n;
        }
    }

    /**
     * Callback from ForkJoinWorkerThread constructor to
     * determine its poolIndex and record in workers array.
     *
     * @param w the worker
     * @return the worker's pool index
     */
    final int registerWorker(ForkJoinWorkerThread w) {
        /*
         * In the typical case, a new worker acquires the lock, uses
         * next available index and returns quickly.  Since we should
         * not block callers (ultimately from signalWork or
         * tryPreBlock) waiting for the lock needed to do this, we
         * instead help release other workers while waiting for the
         * lock.
         */
        for (int g;;) {
            ForkJoinWorkerThread[] ws;
            if (((g = scanGuard) & SG_UNIT) == 0 &&
                UNSAFE.compareAndSwapInt(this, scanGuardOffset,
                                         g, g | SG_UNIT)) {
                int k = nextWorkerIndex;
                try {
                    if ((ws = workers) != null) { // ignore on shutdown
                        int n = ws.length;
                        if (k < 0 || k >= n || ws[k] != null) {
                            for (k = 0; k < n && ws[k] != null; ++k)
                                ;
                            if (k == n)
                                ws = workers = Arrays.copyOf(ws, n << 1);
                        }
                        ws[k] = w;
                        nextWorkerIndex = k + 1;
                        int m = g & SMASK;
                        g = (k > m) ? ((m << 1) + 1) & SMASK : g + (SG_UNIT<<1);
                    }
                } finally {
                    scanGuard = g;
                }
                return k;
            }
            else if ((ws = workers) != null) { // help release others
                for (ForkJoinWorkerThread u : ws) {
                    if (u != null && u.queueBase != u.queueTop) {
                        if (tryReleaseWaiter())
                            break;
                    }
                }
            }
        }
    }

    /**
     * Final callback from terminating worker.  Removes record of
     * worker from array, and adjusts counts. If pool is shutting
     * down, tries to complete termination.
     *
     * @param w the worker
     */
    final void deregisterWorker(ForkJoinWorkerThread w, Throwable ex) {
        int idx = w.poolIndex;
        int sc = w.stealCount;
        int steps = 0;
        // Remove from array, adjust worker counts and collect steal count.
        // We can intermix failed removes or adjusts with steal updates
        do {
            long s, c;
            int g;
            if (steps == 0 && ((g = scanGuard) & SG_UNIT) == 0 &&
                UNSAFE.compareAndSwapInt(this, scanGuardOffset,
                                         g, g |= SG_UNIT)) {
                ForkJoinWorkerThread[] ws = workers;
                if (ws != null && idx >= 0 &&
                    idx < ws.length && ws[idx] == w)
                    ws[idx] = null;    // verify
                nextWorkerIndex = idx;
                scanGuard = g + SG_UNIT;
                steps = 1;
            }
            if (steps == 1 &&
                UNSAFE.compareAndSwapLong(this, ctlOffset, c = ctl,
                                          (((c - AC_UNIT) & AC_MASK) |
                                           ((c - TC_UNIT) & TC_MASK) |
                                           (c & ~(AC_MASK|TC_MASK)))))
                steps = 2;
            if (sc != 0 &&
                UNSAFE.compareAndSwapLong(this, stealCountOffset,
                                          s = stealCount, s + sc))
                sc = 0;
        } while (steps != 2 || sc != 0);
        if (!tryTerminate(false)) {
            if (ex != null)   // possibly replace if died abnormally
                signalWork();
            else
                tryReleaseWaiter();
        }
    }

    // Shutdown and termination

    /**
     * Possibly initiates and/or completes termination.
     *
     * @param now if true, unconditionally terminate, else only
     * if shutdown and empty queue and no active workers
     * @return true if now terminating or terminated
     */
    private boolean tryTerminate(boolean now) {
        long c;
        while (((c = ctl) & STOP_BIT) == 0) {
            if (!now) {
                if ((int)(c >> AC_SHIFT) != -parallelism)
                    return false;
                if (!shutdown || blockedCount != 0 || quiescerCount != 0 ||
                    queueBase != queueTop) {
                    if (ctl == c) // staleness check
                        return false;
                    continue;
                }
            }
            if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, c | STOP_BIT))
                startTerminating();
        }
        if ((short)(c >>> TC_SHIFT) == -parallelism) { // signal when 0 workers
            final ReentrantLock lock = this.submissionLock;
            lock.lock();
            try {
                termination.signalAll();
            } finally {
                lock.unlock();
            }
        }
        return true;
    }

    /**
     * Runs up to three passes through workers: (0) Setting
     * termination status for each worker, followed by wakeups up to
     * queued workers; (1) helping cancel tasks; (2) interrupting
     * lagging threads (likely in external tasks, but possibly also
     * blocked in joins).  Each pass repeats previous steps because of
     * potential lagging thread creation.
     */
    private void startTerminating() {
        cancelSubmissions();
        for (int pass = 0; pass < 3; ++pass) {
            ForkJoinWorkerThread[] ws = workers;
            if (ws != null) {
                for (ForkJoinWorkerThread w : ws) {
                    if (w != null) {
                        w.terminate = true;
                        if (pass > 0) {
                            w.cancelTasks();
                            if (pass > 1 && !w.isInterrupted()) {
                                try {
                                    w.interrupt();
                                } catch (SecurityException ignore) {
                                }
                            }
                        }
                    }
                }
                terminateWaiters();
            }
        }
    }

    /**
     * Polls and cancels all submissions. Called only during termination.
     */
    private void cancelSubmissions() {
        while (queueBase != queueTop) {
            ForkJoinTask<?> task = pollSubmission();
            if (task != null) {
                try {
                    task.cancel(false);
                } catch (Throwable ignore) {
                }
            }
        }
    }

    /**
     * Tries to set the termination status of waiting workers, and
     * then wakes them up (after which they will terminate).
     */
    private void terminateWaiters() {
        ForkJoinWorkerThread[] ws = workers;
        if (ws != null) {
            ForkJoinWorkerThread w; long c; int i, e;
            int n = ws.length;
            while ((i = ~(e = (int)(c = ctl)) & SMASK) < n &&
                   (w = ws[i]) != null && w.eventCount == (e & E_MASK)) {
                if (UNSAFE.compareAndSwapLong(this, ctlOffset, c,
                                              (long)(w.nextWait & E_MASK) |
                                              ((c + AC_UNIT) & AC_MASK) |
                                              (c & (TC_MASK|STOP_BIT)))) {
                    w.terminate = true;
                    w.eventCount = e + EC_UNIT;
                    if (w.parked)
                        UNSAFE.unpark(w);
                }
            }
        }
    }

    // misc ForkJoinWorkerThread support

    /**
     * Increment or decrement quiescerCount. Needed only to prevent
     * triggering shutdown if a worker is transiently inactive while
     * checking quiescence.
     *
     * @param delta 1 for increment, -1 for decrement
     */
    final void addQuiescerCount(int delta) {
        int c;
        do {} while (!UNSAFE.compareAndSwapInt(this, quiescerCountOffset,
                                               c = quiescerCount, c + delta));
    }

    /**
     * Directly increment or decrement active count without
     * queuing. This method is used to transiently assert inactivation
     * while checking quiescence.
     *
     * @param delta 1 for increment, -1 for decrement
     */
    final void addActiveCount(int delta) {
        long d = delta < 0 ? -AC_UNIT : AC_UNIT;
        long c;
        do {} while (!UNSAFE.compareAndSwapLong(this, ctlOffset, c = ctl,
                                                ((c + d) & AC_MASK) |
                                                (c & ~AC_MASK)));
    }

    /**
     * Returns the approximate (non-atomic) number of idle threads per
     * active thread.
     */
    final int idlePerActive() {
        // Approximate at powers of two for small values, saturate past 4
        int p = parallelism;
        int a = p + (int)(ctl >> AC_SHIFT);
        return (a > (p >>>= 1) ? 0 :
                a > (p >>>= 1) ? 1 :
                a > (p >>>= 1) ? 2 :
                a > (p >>>= 1) ? 4 :
                8);
    }

    // Exported methods

    // Constructors

    /**
     * Creates a {@code ForkJoinPool} with parallelism equal to {@link
     * java.lang.Runtime#availableProcessors}, using the {@linkplain
     * #defaultForkJoinWorkerThreadFactory default thread factory},
     * no UncaughtExceptionHandler, and non-async LIFO processing mode.
     *
     * @throws SecurityException if a security manager exists and
     *         the caller is not permitted to modify threads
     *         because it does not hold {@link
     *         java.lang.RuntimePermission}{@code ("modifyThread")}
     */
    public ForkJoinPool() {
        this(Runtime.getRuntime().availableProcessors(),
             defaultForkJoinWorkerThreadFactory, null, false);
    }

    /**
     * Creates a {@code ForkJoinPool} with the indicated parallelism
     * level, the {@linkplain
     * #defaultForkJoinWorkerThreadFactory default thread factory},
     * no UncaughtExceptionHandler, and non-async LIFO processing mode.
     *
     * @param parallelism the parallelism level
     * @throws IllegalArgumentException if parallelism less than or
     *         equal to zero, or greater than implementation limit
     * @throws SecurityException if a security manager exists and
     *         the caller is not permitted to modify threads
     *         because it does not hold {@link
     *         java.lang.RuntimePermission}{@code ("modifyThread")}
     */
    public ForkJoinPool(int parallelism) {
        this(parallelism, defaultForkJoinWorkerThreadFactory, null, false);
    }

    /**
     * Creates a {@code ForkJoinPool} with the given parameters.
     *
     * @param parallelism the parallelism level. For default value,
     * use {@link java.lang.Runtime#availableProcessors}.
     * @param factory the factory for creating new threads. For default value,
     * use {@link #defaultForkJoinWorkerThreadFactory}.
     * @param handler the handler for internal worker threads that
     * terminate due to unrecoverable errors encountered while executing
     * tasks. For default value, use {@code null}.
     * @param asyncMode if true,
     * establishes local first-in-first-out scheduling mode for forked
     * tasks that are never joined. This mode may be more appropriate
     * than default locally stack-based mode in applications in which
     * worker threads only process event-style asynchronous tasks.
     * For default value, use {@code false}.
     * @throws IllegalArgumentException if parallelism less than or
     *         equal to zero, or greater than implementation limit
     * @throws NullPointerException if the factory is null
     * @throws SecurityException if a security manager exists and
     *         the caller is not permitted to modify threads
     *         because it does not hold {@link
     *         java.lang.RuntimePermission}{@code ("modifyThread")}
     */
    public ForkJoinPool(int parallelism,
                        ForkJoinWorkerThreadFactory factory,
                        Thread.UncaughtExceptionHandler handler,
                        boolean asyncMode) {
        checkPermission();
        if (factory == null)
            throw new NullPointerException();
        if (parallelism <= 0 || parallelism > MAX_ID)
            throw new IllegalArgumentException();
        this.parallelism = parallelism;
        this.factory = factory;
        this.ueh = handler;
        this.locallyFifo = asyncMode;
        long np = (long)(-parallelism); // offset ctl counts
        this.ctl = ((np << AC_SHIFT) & AC_MASK) | ((np << TC_SHIFT) & TC_MASK);
        this.submissionQueue = new ForkJoinTask<?>[INITIAL_QUEUE_CAPACITY];
        // initialize workers array with room for 2*parallelism if possible
        int n = parallelism << 1;
        if (n >= MAX_ID)
            n = MAX_ID;
        else { // See Hackers Delight, sec 3.2, where n < (1 << 16)
            n |= n >>> 1; n |= n >>> 2; n |= n >>> 4; n |= n >>> 8;
        }
        workers = new ForkJoinWorkerThread[n + 1];
        this.submissionLock = new ReentrantLock();
        this.termination = submissionLock.newCondition();
        StringBuilder sb = new StringBuilder("ForkJoinPool-");
        sb.append(poolNumberGenerator.incrementAndGet());
        sb.append("-worker-");
        this.workerNamePrefix = sb.toString();
    }

    // Execution methods

    /**
     * Performs the given task, returning its result upon completion.
     * If the computation encounters an unchecked Exception or Error,
     * it is rethrown as the outcome of this invocation.  Rethrown
     * exceptions behave in the same way as regular exceptions, but,
     * when possible, contain stack traces (as displayed for example
     * using {@code ex.printStackTrace()}) of both the current thread
     * as well as the thread actually encountering the exception;
     * minimally only the latter.
     *
     * @param task the task
     * @return the task's result
     * @throws NullPointerException if the task is null
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     */
    public <T> T invoke(ForkJoinTask<T> task) {
        Thread t = Thread.currentThread();
        if (task == null)
            throw new NullPointerException();
        if (shutdown)
            throw new RejectedExecutionException();
        if ((t instanceof ForkJoinWorkerThread) &&
            ((ForkJoinWorkerThread)t).pool == this)
            return task.invoke();  // bypass submit if in same pool
        else {
            addSubmission(task);
            return task.join();
        }
    }

    /**
     * Unless terminating, forks task if within an ongoing FJ
     * computation in the current pool, else submits as external task.
     */
    private <T> void forkOrSubmit(ForkJoinTask<T> task) {
        ForkJoinWorkerThread w;
        Thread t = Thread.currentThread();
        if (shutdown)
            throw new RejectedExecutionException();
        if ((t instanceof ForkJoinWorkerThread) &&
            (w = (ForkJoinWorkerThread)t).pool == this)
            w.pushTask(task);
        else
            addSubmission(task);
    }

    /**
     * Arranges for (asynchronous) execution of the given task.
     *
     * @param task the task
     * @throws NullPointerException if the task is null
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     */
    public void execute(ForkJoinTask<?> task) {
        if (task == null)
            throw new NullPointerException();
        forkOrSubmit(task);
    }

    // AbstractExecutorService methods

    /**
     * @throws NullPointerException if the task is null
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     */
    public void execute(Runnable task) {
        if (task == null)
            throw new NullPointerException();
        ForkJoinTask<?> job;
        if (task instanceof ForkJoinTask<?>) // avoid re-wrap
            job = (ForkJoinTask<?>) task;
        else
            job = ForkJoinTask.adapt(task, null);
        forkOrSubmit(job);
    }

    /**
     * Submits a ForkJoinTask for execution.
     *
     * @param task the task to submit
     * @return the task
     * @throws NullPointerException if the task is null
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     */
    public <T> ForkJoinTask<T> submit(ForkJoinTask<T> task) {
        if (task == null)
            throw new NullPointerException();
        forkOrSubmit(task);
        return task;
    }

    /**
     * @throws NullPointerException if the task is null
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     */
    public <T> ForkJoinTask<T> submit(Callable<T> task) {
        if (task == null)
            throw new NullPointerException();
        ForkJoinTask<T> job = ForkJoinTask.adapt(task);
        forkOrSubmit(job);
        return job;
    }

    /**
     * @throws NullPointerException if the task is null
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     */
    public <T> ForkJoinTask<T> submit(Runnable task, T result) {
        if (task == null)
            throw new NullPointerException();
        ForkJoinTask<T> job = ForkJoinTask.adapt(task, result);
        forkOrSubmit(job);
        return job;
    }

    /**
     * @throws NullPointerException if the task is null
     * @throws RejectedExecutionException if the task cannot be
     *         scheduled for execution
     */
    public ForkJoinTask<?> submit(Runnable task) {
        if (task == null)
            throw new NullPointerException();
        ForkJoinTask<?> job;
        if (task instanceof ForkJoinTask<?>) // avoid re-wrap
            job = (ForkJoinTask<?>) task;
        else
            job = ForkJoinTask.adapt(task, null);
        forkOrSubmit(job);
        return job;
    }

    /**
     * @throws NullPointerException       {@inheritDoc}
     * @throws RejectedExecutionException {@inheritDoc}
     */
    public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks) {
        ArrayList<ForkJoinTask<T>> forkJoinTasks =
            new ArrayList<ForkJoinTask<T>>(tasks.size());
        for (Callable<T> task : tasks)
            forkJoinTasks.add(ForkJoinTask.adapt(task));
        invoke(new InvokeAll<T>(forkJoinTasks));

        @SuppressWarnings({"unchecked", "rawtypes"})
            List<Future<T>> futures = (List<Future<T>>) (List) forkJoinTasks;
        return futures;
    }

    static final class InvokeAll<T> extends RecursiveAction {
        final ArrayList<ForkJoinTask<T>> tasks;
        InvokeAll(ArrayList<ForkJoinTask<T>> tasks) { this.tasks = tasks; }
        public void compute() {
            try { invokeAll(tasks); }
            catch (Exception ignore) {}
        }
        private static final long serialVersionUID = -7914297376763021607L;
    }

    /**
     * Returns the factory used for constructing new workers.
     *
     * @return the factory used for constructing new workers
     */
    public ForkJoinWorkerThreadFactory getFactory() {
        return factory;
    }

    /**
     * Returns the handler for internal worker threads that terminate
     * due to unrecoverable errors encountered while executing tasks.
     *
     * @return the handler, or {@code null} if none
     */
    public Thread.UncaughtExceptionHandler getUncaughtExceptionHandler() {
        return ueh;
    }

    /**
     * Returns the targeted parallelism level of this pool.
     *
     * @return the targeted parallelism level of this pool
     */
    public int getParallelism() {
        return parallelism;
    }

    /**
     * Returns the number of worker threads that have started but not
     * yet terminated.  The result returned by this method may differ
     * from {@link #getParallelism} when threads are created to
     * maintain parallelism when others are cooperatively blocked.
     *
     * @return the number of worker threads
     */
    public int getPoolSize() {
        return parallelism + (short)(ctl >>> TC_SHIFT);
    }

    /**
     * Returns {@code true} if this pool uses local first-in-first-out
     * scheduling mode for forked tasks that are never joined.
     *
     * @return {@code true} if this pool uses async mode
     */
    public boolean getAsyncMode() {
        return locallyFifo;
    }

    /**
     * Returns an estimate of the number of worker threads that are
     * not blocked waiting to join tasks or for other managed
     * synchronization. This method may overestimate the
     * number of running threads.
     *
     * @return the number of worker threads
     */
    public int getRunningThreadCount() {
        int r = parallelism + (int)(ctl >> AC_SHIFT);
        return (r <= 0) ? 0 : r; // suppress momentarily negative values
    }

    /**
     * Returns an estimate of the number of threads that are currently
     * stealing or executing tasks. This method may overestimate the
     * number of active threads.
     *
     * @return the number of active threads
     */
    public int getActiveThreadCount() {
        int r = parallelism + (int)(ctl >> AC_SHIFT) + blockedCount;
        return (r <= 0) ? 0 : r; // suppress momentarily negative values
    }

    /**
     * Returns {@code true} if all worker threads are currently idle.
     * An idle worker is one that cannot obtain a task to execute
     * because none are available to steal from other threads, and
     * there are no pending submissions to the pool. This method is
     * conservative; it might not return {@code true} immediately upon
     * idleness of all threads, but will eventually become true if
     * threads remain inactive.
     *
     * @return {@code true} if all threads are currently idle
     */
    public boolean isQuiescent() {
        return parallelism + (int)(ctl >> AC_SHIFT) + blockedCount == 0;
    }

    /**
     * Returns an estimate of the total number of tasks stolen from
     * one thread's work queue by another. The reported value
     * underestimates the actual total number of steals when the pool
     * is not quiescent. This value may be useful for monitoring and
     * tuning fork/join programs: in general, steal counts should be
     * high enough to keep threads busy, but low enough to avoid
     * overhead and contention across threads.
     *
     * @return the number of steals
     */
    public long getStealCount() {
        return stealCount;
    }

    /**
     * Returns an estimate of the total number of tasks currently held
     * in queues by worker threads (but not including tasks submitted
     * to the pool that have not begun executing). This value is only
     * an approximation, obtained by iterating across all threads in
     * the pool. This method may be useful for tuning task
     * granularities.
     *
     * @return the number of queued tasks
     */
    public long getQueuedTaskCount() {
        long count = 0;
        ForkJoinWorkerThread[] ws;
        if ((short)(ctl >>> TC_SHIFT) > -parallelism &&
            (ws = workers) != null) {
            for (ForkJoinWorkerThread w : ws)
                if (w != null)
                    count -= w.queueBase - w.queueTop; // must read base first
        }
        return count;
    }

    /**
     * Returns an estimate of the number of tasks submitted to this
     * pool that have not yet begun executing.  This method may take
     * time proportional to the number of submissions.
     *
     * @return the number of queued submissions
     */
    public int getQueuedSubmissionCount() {
        return -queueBase + queueTop;
    }

    /**
     * Returns {@code true} if there are any tasks submitted to this
     * pool that have not yet begun executing.
     *
     * @return {@code true} if there are any queued submissions
     */
    public boolean hasQueuedSubmissions() {
        return queueBase != queueTop;
    }

    /**
     * Removes and returns the next unexecuted submission if one is
     * available.  This method may be useful in extensions to this
     * class that re-assign work in systems with multiple pools.
     *
     * @return the next submission, or {@code null} if none
     */
    protected ForkJoinTask<?> pollSubmission() {
        ForkJoinTask<?> t; ForkJoinTask<?>[] q; int b, i;
        while ((b = queueBase) != queueTop &&
               (q = submissionQueue) != null &&
               (i = (q.length - 1) & b) >= 0) {
            long u = (i << ASHIFT) + ABASE;
            if ((t = q[i]) != null &&
                queueBase == b &&
                UNSAFE.compareAndSwapObject(q, u, t, null)) {
                queueBase = b + 1;
                return t;
            }
        }
        return null;
    }

    /**
     * Removes all available unexecuted submitted and forked tasks
     * from scheduling queues and adds them to the given collection,
     * without altering their execution status. These may include
     * artificially generated or wrapped tasks. This method is
     * designed to be invoked only when the pool is known to be
     * quiescent. Invocations at other times may not remove all
     * tasks. A failure encountered while attempting to add elements
     * to collection {@code c} may result in elements being in
     * neither, either or both collections when the associated
     * exception is thrown.  The behavior of this operation is
     * undefined if the specified collection is modified while the
     * operation is in progress.
     *
     * @param c the collection to transfer elements into
     * @return the number of elements transferred
     */
    protected int drainTasksTo(Collection<? super ForkJoinTask<?>> c) {
        int count = 0;
        while (queueBase != queueTop) {
            ForkJoinTask<?> t = pollSubmission();
            if (t != null) {
                c.add(t);
                ++count;
            }
        }
        ForkJoinWorkerThread[] ws;
        if ((short)(ctl >>> TC_SHIFT) > -parallelism &&
            (ws = workers) != null) {
            for (ForkJoinWorkerThread w : ws)
                if (w != null)
                    count += w.drainTasksTo(c);
        }
        return count;
    }

    /**
     * Returns a string identifying this pool, as well as its state,
     * including indications of run state, parallelism level, and
     * worker and task counts.
     *
     * @return a string identifying this pool, as well as its state
     */
    public String toString() {
        long st = getStealCount();
        long qt = getQueuedTaskCount();
        long qs = getQueuedSubmissionCount();
        int pc = parallelism;
        long c = ctl;
        int tc = pc + (short)(c >>> TC_SHIFT);
        int rc = pc + (int)(c >> AC_SHIFT);
        if (rc < 0) // ignore transient negative
            rc = 0;
        int ac = rc + blockedCount;
        String level;
        if ((c & STOP_BIT) != 0)
            level = (tc == 0) ? "Terminated" : "Terminating";
        else
            level = shutdown ? "Shutting down" : "Running";
        return super.toString() +
            "[" + level +
            ", parallelism = " + pc +
            ", size = " + tc +
            ", active = " + ac +
            ", running = " + rc +
            ", steals = " + st +
            ", tasks = " + qt +
            ", submissions = " + qs +
            "]";
    }

    /**
     * Initiates an orderly shutdown in which previously submitted
     * tasks are executed, but no new tasks will be accepted.
     * Invocation has no additional effect if already shut down.
     * Tasks that are in the process of being submitted concurrently
     * during the course of this method may or may not be rejected.
     *
     * @throws SecurityException if a security manager exists and
     *         the caller is not permitted to modify threads
     *         because it does not hold {@link
     *         java.lang.RuntimePermission}{@code ("modifyThread")}
     */
    public void shutdown() {
        checkPermission();
        shutdown = true;
        tryTerminate(false);
    }

    /**
     * Attempts to cancel and/or stop all tasks, and reject all
     * subsequently submitted tasks.  Tasks that are in the process of
     * being submitted or executed concurrently during the course of
     * this method may or may not be rejected. This method cancels
     * both existing and unexecuted tasks, in order to permit
     * termination in the presence of task dependencies. So the method
     * always returns an empty list (unlike the case for some other
     * Executors).
     *
     * @return an empty list
     * @throws SecurityException if a security manager exists and
     *         the caller is not permitted to modify threads
     *         because it does not hold {@link
     *         java.lang.RuntimePermission}{@code ("modifyThread")}
     */
    public List<Runnable> shutdownNow() {
        checkPermission();
        shutdown = true;
        tryTerminate(true);
        return Collections.emptyList();
    }

    /**
     * Returns {@code true} if all tasks have completed following shut down.
     *
     * @return {@code true} if all tasks have completed following shut down
     */
    public boolean isTerminated() {
        long c = ctl;
        return ((c & STOP_BIT) != 0L &&
                (short)(c >>> TC_SHIFT) == -parallelism);
    }

    /**
     * Returns {@code true} if the process of termination has
     * commenced but not yet completed.  This method may be useful for
     * debugging. A return of {@code true} reported a sufficient
     * period after shutdown may indicate that submitted tasks have
     * ignored or suppressed interruption, or are waiting for IO,
     * causing this executor not to properly terminate. (See the
     * advisory notes for class {@link ForkJoinTask} stating that
     * tasks should not normally entail blocking operations.  But if
     * they do, they must abort them on interrupt.)
     *
     * @return {@code true} if terminating but not yet terminated
     */
    public boolean isTerminating() {
        long c = ctl;
        return ((c & STOP_BIT) != 0L &&
                (short)(c >>> TC_SHIFT) != -parallelism);
    }

    /**
     * Returns true if terminating or terminated. Used by ForkJoinWorkerThread.
     */
    final boolean isAtLeastTerminating() {
        return (ctl & STOP_BIT) != 0L;
    }

    /**
     * Returns {@code true} if this pool has been shut down.
     *
     * @return {@code true} if this pool has been shut down
     */
    public boolean isShutdown() {
        return shutdown;
    }

    /**
     * Blocks until all tasks have completed execution after a shutdown
     * request, or the timeout occurs, or the current thread is
     * interrupted, whichever happens first.
     *
     * @param timeout the maximum time to wait
     * @param unit the time unit of the timeout argument
     * @return {@code true} if this executor terminated and
     *         {@code false} if the timeout elapsed before termination
     * @throws InterruptedException if interrupted while waiting
     */
    public boolean awaitTermination(long timeout, TimeUnit unit)
        throws InterruptedException {
        long nanos = unit.toNanos(timeout);
        final ReentrantLock lock = this.submissionLock;
        lock.lock();
        try {
            for (;;) {
                if (isTerminated())
                    return true;
                if (nanos <= 0)
                    return false;
                nanos = termination.awaitNanos(nanos);
            }
        } finally {
            lock.unlock();
        }
    }

    /**
     * Interface for extending managed parallelism for tasks running
     * in {@link ForkJoinPool}s.
     *
     * <p>A {@code ManagedBlocker} provides two methods.  Method
     * {@code isReleasable} must return {@code true} if blocking is
     * not necessary. Method {@code block} blocks the current thread
     * if necessary (perhaps internally invoking {@code isReleasable}
     * before actually blocking). These actions are performed by any
     * thread invoking {@link ForkJoinPool#managedBlock}.  The
     * unusual methods in this API accommodate synchronizers that may,
     * but don't usually, block for long periods. Similarly, they
     * allow more efficient internal handling of cases in which
     * additional workers may be, but usually are not, needed to
     * ensure sufficient parallelism.  Toward this end,
     * implementations of method {@code isReleasable} must be amenable
     * to repeated invocation.
     *
     * <p>For example, here is a ManagedBlocker based on a
     * ReentrantLock:
     *  <pre> {@code
     * class ManagedLocker implements ManagedBlocker {
     *   final ReentrantLock lock;
     *   boolean hasLock = false;
     *   ManagedLocker(ReentrantLock lock) { this.lock = lock; }
     *   public boolean block() {
     *     if (!hasLock)
     *       lock.lock();
     *     return true;
     *   }
     *   public boolean isReleasable() {
     *     return hasLock || (hasLock = lock.tryLock());
     *   }
     * }}</pre>
     *
     * <p>Here is a class that possibly blocks waiting for an
     * item on a given queue:
     *  <pre> {@code
     * class QueueTaker<E> implements ManagedBlocker {
     *   final BlockingQueue<E> queue;
     *   volatile E item = null;
     *   QueueTaker(BlockingQueue<E> q) { this.queue = q; }
     *   public boolean block() throws InterruptedException {
     *     if (item == null)
     *       item = queue.take();
     *     return true;
     *   }
     *   public boolean isReleasable() {
     *     return item != null || (item = queue.poll()) != null;
     *   }
     *   public E getItem() { // call after pool.managedBlock completes
     *     return item;
     *   }
     * }}</pre>
     */
    public static interface ManagedBlocker {
        /**
         * Possibly blocks the current thread, for example waiting for
         * a lock or condition.
         *
         * @return {@code true} if no additional blocking is necessary
         * (i.e., if isReleasable would return true)
         * @throws InterruptedException if interrupted while waiting
         * (the method is not required to do so, but is allowed to)
         */
        boolean block() throws InterruptedException;

        /**
         * Returns {@code true} if blocking is unnecessary.
         */
        boolean isReleasable();
    }

    /**
     * Blocks in accord with the given blocker.  If the current thread
     * is a {@link ForkJoinWorkerThread}, this method possibly
     * arranges for a spare thread to be activated if necessary to
     * ensure sufficient parallelism while the current thread is blocked.
     *
     * <p>If the caller is not a {@link ForkJoinTask}, this method is
     * behaviorally equivalent to
     *  <pre> {@code
     * while (!blocker.isReleasable())
     *   if (blocker.block())
     *     return;
     * }</pre>
     *
     * If the caller is a {@code ForkJoinTask}, then the pool may
     * first be expanded to ensure parallelism, and later adjusted.
     *
     * @param blocker the blocker
     * @throws InterruptedException if blocker.block did so
     */
    public static void managedBlock(ManagedBlocker blocker)
        throws InterruptedException {
        Thread t = Thread.currentThread();
        if (t instanceof ForkJoinWorkerThread) {
            ForkJoinWorkerThread w = (ForkJoinWorkerThread) t;
            w.pool.awaitBlocker(blocker);
        }
        else {
            do {} while (!blocker.isReleasable() && !blocker.block());
        }
    }

    // AbstractExecutorService overrides.  These rely on undocumented
    // fact that ForkJoinTask.adapt returns ForkJoinTasks that also
    // implement RunnableFuture.

    protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
        return (RunnableFuture<T>) ForkJoinTask.adapt(runnable, value);
    }

    protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
        return (RunnableFuture<T>) ForkJoinTask.adapt(callable);
    }

    // Unsafe mechanics
    private static final sun.misc.Unsafe UNSAFE;
    private static final long ctlOffset;
    private static final long stealCountOffset;
    private static final long blockedCountOffset;
    private static final long quiescerCountOffset;
    private static final long scanGuardOffset;
    private static final long nextWorkerNumberOffset;
    private static final long ABASE;
    private static final int ASHIFT;

    static {
        poolNumberGenerator = new AtomicInteger();
        workerSeedGenerator = new Random();
        modifyThreadPermission = new RuntimePermission("modifyThread");
        defaultForkJoinWorkerThreadFactory =
            new DefaultForkJoinWorkerThreadFactory();
        int s;
        try {
            UNSAFE = sun.misc.Unsafe.getUnsafe();
            Class<?> k = ForkJoinPool.class;
            ctlOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("ctl"));
            stealCountOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("stealCount"));
            blockedCountOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("blockedCount"));
            quiescerCountOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("quiescerCount"));
            scanGuardOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("scanGuard"));
            nextWorkerNumberOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("nextWorkerNumber"));
            Class<?> a = ForkJoinTask[].class;
            ABASE = UNSAFE.arrayBaseOffset(a);
            s = UNSAFE.arrayIndexScale(a);
        } catch (Exception e) {
            throw new Error(e);
        }
        if ((s & (s-1)) != 0)
            throw new Error("data type scale not a power of two");
        ASHIFT = 31 - Integer.numberOfLeadingZeros(s);
    }

}