/*
 * 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/licenses/publicdomain
 */

package jsr166y;
import java.util.*;
import java.util.concurrent.*;
import java.util.concurrent.locks.*;
import java.util.concurrent.atomic.*;
import sun.misc.Unsafe;
import java.lang.reflect.*;

/**
 * An {@link ExecutorService} for running {@link ForkJoinTask}s.  A
 * ForkJoinPool provides the entry point for submissions from
 * non-ForkJoinTasks, as well as management and monitoring operations.
 * Normally a single ForkJoinPool is used for a large number of
 * submitted tasks. Otherwise, use would not usually outweigh the
 * construction and bookkeeping overhead of creating a large set of
 * threads.
 *
 * <p>ForkJoinPools differ from other kinds of Executors mainly in
 * that they provide <em>work-stealing</em>: all threads in the pool
 * attempt to find and execute subtasks created by other active tasks
 * (eventually blocking if none exist). This makes them efficient when
 * most tasks spawn other subtasks (as do most ForkJoinTasks), as well
 * as the mixed execution of some plain Runnable- or Callable- based
 * activities along with ForkJoinTasks. Otherwise, other
 * ExecutorService implementations are typically more appropriate
 * choices.
 *
 * <p>A ForkJoinPool may be constructed with a given parallelism level
 * (target pool size), which it attempts to maintain by dynamically
 * adding, suspending, or resuming threads, even if some tasks are
 * waiting to join others. However, no such adjustments are performed
 * in the face of blocked IO or other unmanaged synchronization. The
 * nested <code>ManagedBlocker</code> interface enables extension of
 * the kinds of synchronization accommodated.  The target parallelism
 * level may also be changed dynamically (<code>setParallelism</code>)
 * and dynamically thread construction can be limited using methods
 * <code>setMaximumPoolSize</code> and/or
 * <code>setMaintainsParallelism</code>.
 *
 * <p>In addition to execution and lifecycle control methods, this
 * class provides status check methods (for example
 * <code>getStealCount</code>) that are intended to aid in developing,
 * tuning, and monitoring fork/join applications. Also, method
 * <code>toString</code> returns indications of pool state in a
 * convenient form for informal monitoring.
 *
 * <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 result in
 * IllegalArgumentExceptions.
 */
public class ForkJoinPool extends AbstractExecutorService {

    /*
     * See the extended comments interspersed below for design,
     * rationale, and walkthroughs.
     */

    /** Mask for packing and unpacking shorts */
    private static final int  shortMask = 0xffff;

    /** Max pool size -- must be a power of two minus 1 */
    private static final int MAX_THREADS =  0x7FFF;

    /**
     * Factory for creating new ForkJoinWorkerThreads.  A
     * ForkJoinWorkerThreadFactory must be defined and used for
     * 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 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) {
            try {
                return new ForkJoinWorkerThread(pool);
            } catch (OutOfMemoryError oom)  {
                return null;
            }
        }
    }

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

    /**
     * Permission required for callers of methods that may start or
     * kill threads.
     */
    private static final RuntimePermission modifyThreadPermission =
        new RuntimePermission("modifyThread");

    /**
     * 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 =
        new AtomicInteger();

    /**
     * Array holding all worker threads in the pool. Array size must
     * be a power of two.  Updates and replacements are protected by
     * workerLock, but it is always kept in a consistent enough state
     * to be randomly accessed without locking by workers performing
     * work-stealing.
     */
    volatile ForkJoinWorkerThread[] workers;

    /**
     * Lock protecting access to workers.
     */
    private final ReentrantLock workerLock;

    /**
     * Condition for awaitTermination.
     */
    private final Condition termination;

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

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

    /**
     * Head of stack of threads that were created to maintain
     * parallelism when other threads blocked, but have since
     * suspended when the parallelism level rose.
     */
    private volatile WaitQueueNode spareStack;

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

    /**
     * Queue for external submissions.
     */
    private final LinkedTransferQueue<ForkJoinTask<?>> submissionQueue;

    /**
     * Head of Treiber stack for barrier sync. See below for explanation
     */
    private volatile WaitQueueNode barrierStack;

    /**
     * The count for event barrier
     */
    private volatile long eventCount;

    /**
     * Pool number, just for assigning useful names to worker threads
     */
    private final int poolNumber;

    /**
     * The maximum allowed pool size
     */
    private volatile int maxPoolSize;

    /**
     * The desired parallelism level, updated only under workerLock.
     */
    private volatile int parallelism;

    /**
     * Holds number of total (i.e., created and not yet terminated)
     * and running (i.e., not blocked on joins or other managed sync)
     * threads, packed into one int to ensure consistent snapshot when
     * making decisions about creating and suspending spare
     * threads. Updated only by CAS.  Note: CASes in
     * updateRunningCount and preJoin running active count is in low
     * word, so need to be modified if this changes
     */
    private volatile int workerCounts;

    private static int totalCountOf(int s)           { return s >>> 16;  }
    private static int runningCountOf(int s)         { return s & shortMask; }
    private static int workerCountsFor(int t, int r) { return (t << 16) + r; }

    /**
     * Add delta (which may be negative) to running count.  This must
     * be called before (with negative arg) and after (with positive)
     * any managed synchronization (i.e., mainly, joins)
     * @param delta the number to add
     */
    final void updateRunningCount(int delta) {
        int s;
        do;while (!casWorkerCounts(s = workerCounts, s + delta));
    }

    /**
     * Add delta (which may be negative) to both total and running
     * count.  This must be called upon creation and termination of
     * worker threads.
     * @param delta the number to add
     */
    private void updateWorkerCount(int delta) {
        int d = delta + (delta << 16); // add to both lo and hi parts
        int s;
        do;while (!casWorkerCounts(s = workerCounts, s + d));
    }

    /**
     * Lifecycle control. High word contains runState, low word
     * contains the number of workers that are (probably) executing
     * tasks. This value is atomically incremented before a worker
     * gets a task to run, and decremented when worker has no tasks
     * and cannot find any. These two fields are bundled together to
     * support correct termination triggering.  Note: activeCount
     * CAS'es cheat by assuming active count is in low word, so need
     * to be modified if this changes
     */
    private volatile int runControl;

    // RunState values. Order among values matters
    private static final int RUNNING     = 0;
    private static final int SHUTDOWN    = 1;
    private static final int TERMINATING = 2;
    private static final int TERMINATED  = 3;

    private static int runStateOf(int c)             { return c >>> 16; }
    private static int activeCountOf(int c)          { return c & shortMask; }
    private static int runControlFor(int r, int a)   { return (r << 16) + a; }

    /**
     * Increment active count. Called by workers before/during
     * executing tasks.
     */
    final void incrementActiveCount() {
        int c;
        do;while (!casRunControl(c = runControl, c+1));
    }

    /**
     * Decrement active count; possibly trigger termination.
     * Called by workers when they can't find tasks.
     */
    final void decrementActiveCount() {
        int c, nextc;
        do;while (!casRunControl(c = runControl, nextc = c-1));
        if (canTerminateOnShutdown(nextc))
            terminateOnShutdown();
    }

    /**
     * Return true if argument represents zero active count and
     * nonzero runstate, which is the triggering condition for
     * terminating on shutdown.
     */
    private static boolean canTerminateOnShutdown(int c) {
        return ((c & -c) >>> 16) != 0; // i.e. least bit is nonzero runState bit
    }

    /**
     * Transition run state to at least the given state. Return true
     * if not already at least given state.
     */
    private boolean transitionRunStateTo(int state) {
        for (;;) {
            int c = runControl;
            if (runStateOf(c) >= state)
                return false;
            if (casRunControl(c, runControlFor(state, activeCountOf(c))))
                return true;
        }
    }

    /**
     * Controls whether to add spares to maintain parallelism
     */
    private volatile boolean maintainsParallelism;

    // Constructors

    /**
     * Creates a ForkJoinPool with a pool size equal to the number of
     * processors available on the system and using the default
     * ForkJoinWorkerThreadFactory,
     * @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")</code>,
     */
    public ForkJoinPool() {
        this(Runtime.getRuntime().availableProcessors(),
             defaultForkJoinWorkerThreadFactory);
    }

    /**
     * Creates a ForkJoinPool with the indicated parellelism level
     * threads, and using the default ForkJoinWorkerThreadFactory,
     * @param parallelism the number of worker threads
     * @throws IllegalArgumentException if parallelism less than or
     * equal to zero
     * @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")</code>,
     */
    public ForkJoinPool(int parallelism) {
        this(parallelism, defaultForkJoinWorkerThreadFactory);
    }

    /**
     * Creates a ForkJoinPool with parallelism equal to the number of
     * processors available on the system and using the given
     * ForkJoinWorkerThreadFactory,
     * @param factory the factory for creating new threads
     * @throws NullPointerException if 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")</code>,
     */
    public ForkJoinPool(ForkJoinWorkerThreadFactory factory) {
        this(Runtime.getRuntime().availableProcessors(), factory);
    }

    /**
     * Creates a ForkJoinPool with the given parallelism and factory.
     *
     * @param parallelism the targeted number of worker threads
     * @param factory the factory for creating new threads
     * @throws IllegalArgumentException if parallelism less than or
     * equal to zero, or greater than implementation limit.
     * @throws NullPointerException if 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")</code>,
     */
    public ForkJoinPool(int parallelism, ForkJoinWorkerThreadFactory factory) {
        if (parallelism <= 0 || parallelism > MAX_THREADS)
            throw new IllegalArgumentException();
        if (factory == null)
            throw new NullPointerException();
        checkPermission();
        this.factory = factory;
        this.parallelism = parallelism;
        this.maxPoolSize = MAX_THREADS;
        this.maintainsParallelism = true;
        this.poolNumber = poolNumberGenerator.incrementAndGet();
        this.workerLock = new ReentrantLock();
        this.termination = workerLock.newCondition();
        this.stealCount = new AtomicLong();
        this.submissionQueue = new LinkedTransferQueue<ForkJoinTask<?>>();
        createAndStartInitialWorkers(parallelism);
    }

    /**
     * Create new worker using factory.
     * @param index the index to assign worker
     * @return new worker, or null of factory failed
     */
    private ForkJoinWorkerThread createWorker(int index) {
        Thread.UncaughtExceptionHandler h = ueh;
        ForkJoinWorkerThread w = factory.newThread(this);
        if (w != null) {
            w.poolIndex = index;
            w.setDaemon(true);
            w.setName("ForkJoinPool-" + poolNumber + "-worker-" + index);
            if (h != null)
                w.setUncaughtExceptionHandler(h);
        }
        return w;
    }

    /**
     * Return a good size for worker array given pool size.
     * Currently requires size to be a power of two.
     */
    private static int arraySizeFor(int ps) {
        return ps <= 1? 1 : (1 << (32 - Integer.numberOfLeadingZeros(ps-1)));
    }

    /**
     * Create or resize array if necessary to hold newLength
     * @return the array
     */
    private ForkJoinWorkerThread[] ensureWorkerArrayCapacity(int newLength) {
        ForkJoinWorkerThread[] ws = workers;
        if (ws == null)
            return workers = new ForkJoinWorkerThread[arraySizeFor(newLength)];
        else if (newLength > ws.length)
            return workers = Arrays.copyOf(ws, arraySizeFor(newLength));
        else
            return ws;
    }

    /**
     * Try to shrink workers into smaller array after one or more terminate
     */
    private void tryShrinkWorkerArray() {
        ForkJoinWorkerThread[] ws = workers;
        int len = ws.length;
        int last = len - 1;
        while (last >= 0 && ws[last] == null)
            --last;
        int newLength = arraySizeFor(last+1);
        if (newLength < len)
            workers = Arrays.copyOf(ws, newLength);
    }

    /**
     * Initial worker array and worker creation and startup. (This
     * must be done under lock to avoid interference by some of the
     * newly started threads while creating others.)
     */
    private void createAndStartInitialWorkers(int ps) {
        final ReentrantLock lock = this.workerLock;
        lock.lock();
        try {
            ForkJoinWorkerThread[] ws = ensureWorkerArrayCapacity(ps);
            for (int i = 0; i < ps; ++i) {
                ForkJoinWorkerThread w = createWorker(i);
                if (w != null) {
                    ws[i] = w;
                    w.start();
                    updateWorkerCount(1);
                }
            }
        } finally {
            lock.unlock();
        }
    }

    /**
     * Worker creation and startup for threads added via setParallelism.
     */
    private void createAndStartAddedWorkers() {
        resumeAllSpares();  // Allow spares to convert to nonspare
        int ps = parallelism;
        ForkJoinWorkerThread[] ws = ensureWorkerArrayCapacity(ps);
        int len = ws.length;
        // Sweep through slots, to keep lowest indices most populated
        int k = 0;
        while (k < len) {
            if (ws[k] != null) {
                ++k;
                continue;
            }
            int s = workerCounts;
            int tc = totalCountOf(s);
            int rc = runningCountOf(s);
            if (rc >= ps || tc >= ps)
                break;
            if (casWorkerCounts (s, workerCountsFor(tc+1, rc+1))) {
                ForkJoinWorkerThread w = createWorker(k);
                if (w != null) {
                    ws[k++] = w;
                    w.start();
                }
                else {
                    updateWorkerCount(-1); // back out on failed creation
                    break;
                }
            }
        }
    }

    // Execution methods

    /**
     * Common code for execute, invoke and submit
     */
    private <T> void doSubmit(ForkJoinTask<T> task) {
        if (isShutdown())
            throw new RejectedExecutionException();
        submissionQueue.offer(task);
        signalIdleWorkers(true);
    }

    /**
     * Performs the given task; returning its result upon completion
     * @param task the task
     * @return the task's result
     * @throws NullPointerException if task is null
     * @throws RejectedExecutionException if pool is shut down
     */
    public <T> T invoke(ForkJoinTask<T> task) {
        doSubmit(task);
        return task.join();
    }

    /**
     * Arranges for (asynchronous) execution of the given task.
     * @param task the task
     * @throws NullPointerException if task is null
     * @throws RejectedExecutionException if pool is shut down
     */
    public <T> void execute(ForkJoinTask<T> task) {
        doSubmit(task);
    }

    // AbstractExecutorService methods

    public void execute(Runnable task) {
        doSubmit(new AdaptedRunnable<Void>(task, null));
    }

    public <T> ForkJoinTask<T> submit(Callable<T> task) {
        ForkJoinTask<T> job = new AdaptedCallable<T>(task);
        doSubmit(job);
        return job;
    }

    public <T> ForkJoinTask<T> submit(Runnable task, T result) {
        ForkJoinTask<T> job = new AdaptedRunnable<T>(task, result);
        doSubmit(job);
        return job;
    }

    public ForkJoinTask<?> submit(Runnable task) {
        ForkJoinTask<Void> job = new AdaptedRunnable<Void>(task, null);
        doSubmit(job);
        return job;
    }

    /**
     * Adaptor for Runnables. This implements RunnableFuture
     * to be compliant with AbstractExecutorService constraints
     */
    static final class AdaptedRunnable<T> extends ForkJoinTask<T>
        implements RunnableFuture<T> {
        final Runnable runnable;
        final T resultOnCompletion;
        T result;
        AdaptedRunnable(Runnable runnable, T result) {
            if (runnable == null) throw new NullPointerException();
            this.runnable = runnable;
            this.resultOnCompletion = result;
        }
        public T getRawResult() { return result; }
        public void setRawResult(T v) { result = v; }
        public boolean exec() {
            runnable.run();
            result = resultOnCompletion;
            return true;
        }
        public void run() { invoke(); }
    }

    /**
     * Adaptor for Callables
     */
    static final class AdaptedCallable<T> extends ForkJoinTask<T>
        implements RunnableFuture<T> {
        final Callable<T> callable;
        T result;
        AdaptedCallable(Callable<T> callable) {
            if (callable == null) throw new NullPointerException();
            this.callable = callable;
        }
        public T getRawResult() { return result; }
        public void setRawResult(T v) { result = v; }
        public boolean exec() {
            try {
                result = callable.call();
                return true;
            } catch (Error err) {
                throw err;
            } catch (RuntimeException rex) {
                throw rex;
            } catch (Exception ex) {
                throw new RuntimeException(ex);
            }
        }
        public void run() { invoke(); }
    }

    public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks) {
        ArrayList<ForkJoinTask<T>> ts =
            new ArrayList<ForkJoinTask<T>>(tasks.size());
        for (Callable<T> c : tasks)
            ts.add(new AdaptedCallable<T>(c));
        invoke(new InvokeAll<T>(ts));
        return (List<Future<T>>)(List)ts;
    }

    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) {}
        }
    }

    // Configuration and status settings and queries

    /**
     * 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 null if none
     */
    public Thread.UncaughtExceptionHandler getUncaughtExceptionHandler() {
        Thread.UncaughtExceptionHandler h;
        final ReentrantLock lock = this.workerLock;
        lock.lock();
        try {
            h = ueh;
        } finally {
            lock.unlock();
        }
        return h;
    }

    /**
     * Sets the handler for internal worker threads that terminate due
     * to unrecoverable errors encountered while executing tasks.
     * Unless set, the current default or ThreadGroup handler is used
     * as handler.
     *
     * @param h the new handler
     * @return the old handler, or null if none
     * @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")</code>,
     */
    public Thread.UncaughtExceptionHandler
        setUncaughtExceptionHandler(Thread.UncaughtExceptionHandler h) {
        checkPermission();
        Thread.UncaughtExceptionHandler old = null;
        final ReentrantLock lock = this.workerLock;
        lock.lock();
        try {
            old = ueh;
            ueh = h;
            ForkJoinWorkerThread[] ws = workers;
            for (int i = 0; i < ws.length; ++i) {
                ForkJoinWorkerThread w = ws[i];
                if (w != null)
                    w.setUncaughtExceptionHandler(h);
            }
        } finally {
            lock.unlock();
        }
        return old;
    }


    /**
     * Sets the target paralleism level of this pool.
     * @param parallelism the target parallelism
     * @throws IllegalArgumentException if parallelism less than or
     * equal to zero or greater than maximum size bounds.
     * @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")</code>,
     */
    public void setParallelism(int parallelism) {
        checkPermission();
        if (parallelism <= 0 || parallelism > maxPoolSize)
            throw new IllegalArgumentException();
        final ReentrantLock lock = this.workerLock;
        lock.lock();
        try {
            if (!isTerminating()) {
                int p = this.parallelism;
                this.parallelism = parallelism;
                if (parallelism > p)
                    createAndStartAddedWorkers();
                else
                    trimSpares();
            }
        } finally {
            lock.unlock();
        }
        signalIdleWorkers(false);
    }

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

    /**
     * Returns the number of worker threads that have started but not
     * yet terminated.  This result returned by this method may differ
     * from <code>getParallelism</code> when threads are created to
     * maintain parallelism when others are cooperatively blocked.
     *
     * @return the number of worker threads
     */
    public int getPoolSize() {
        return totalCountOf(workerCounts);
    }

    /**
     * Returns the maximum number of threads allowed to exist in the
     * pool, even if there are insufficient unblocked running threads.
     * @return the maximum
     */
    public int getMaximumPoolSize() {
        return maxPoolSize;
    }

    /**
     * Sets the maximum number of threads allowed to exist in the
     * pool, even if there are insufficient unblocked running threads.
     * Setting this value has no effect on current pool size. It
     * controls construction of new threads.
     * @throws IllegalArgumentException if negative or greater then
     * internal implementation limit.
     */
    public void setMaximumPoolSize(int newMax) {
        if (newMax < 0 || newMax > MAX_THREADS)
            throw new IllegalArgumentException();
        maxPoolSize = newMax;
    }


    /**
     * Returns true if this pool dynamically maintains its target
     * parallelism level. If false, new threads are added only to
     * avoid possible starvation.
     * This setting is by default true;
     * @return true if maintains parallelism
     */
    public boolean getMaintainsParallelism() {
        return maintainsParallelism;
    }

    /**
     * Sets whether this pool dynamically maintains its target
     * parallelism level. If false, new threads are added only to
     * avoid possible starvation.
     * @param enable true to maintains parallelism
     */
    public void setMaintainsParallelism(boolean enable) {
        maintainsParallelism = enable;
    }

    /**
     * Returns an estimate of the number of worker threads that are
     * not blocked waiting to join tasks or for other managed
     * synchronization.
     *
     * @return the number of worker threads
     */
    public int getRunningThreadCount() {
        return runningCountOf(workerCounts);
    }

    /**
     * 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() {
        return activeCountOf(runControl);
    }

    /**
     * Returns an estimate of the number of threads that are currently
     * idle waiting for tasks. This method may underestimate the
     * number of idle threads.
     * @return the number of idle threads.
     */
    final int getIdleThreadCount() {
        int c = runningCountOf(workerCounts) - activeCountOf(runControl);
        return (c <= 0)? 0 : c;
    }

    /**
     * Returns 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 true immediately upon idleness of all
     * threads, but will eventually become true if threads remain
     * inactive.
     * @return true if all threads are currently idle
     */
    public boolean isQuiescent() {
        return activeCountOf(runControl) == 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.get();
    }

    /**
     * Accumulate steal count from a worker. Call only
     * when worker known to be idle.
     */
    private void updateStealCount(ForkJoinWorkerThread w) {
        int sc = w.getAndClearStealCount();
        if (sc != 0)
            stealCount.addAndGet(sc);
    }

    /**
     * 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 = workers;
        for (int i = 0; i < ws.length; ++i) {
            ForkJoinWorkerThread t = ws[i];
            if (t != null)
                count += t.getQueueSize();
        }
        return count;
    }

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

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

    /**
     * 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 null if none
     */
    protected ForkJoinTask<?> pollSubmission() {
        return submissionQueue.poll();
    }

    /**
     * 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() {
        int ps = parallelism;
        int wc = workerCounts;
        int rc = runControl;
        long st = getStealCount();
        long qt = getQueuedTaskCount();
        long qs = getQueuedSubmissionCount();
        return super.toString() +
            "[" + runStateToString(runStateOf(rc)) +
            ", parallelism = " + ps +
            ", size = " + totalCountOf(wc) +
            ", active = " + activeCountOf(rc) +
            ", running = " + runningCountOf(wc) +
            ", steals = " + st +
            ", tasks = " + qt +
            ", submissions = " + qs +
            "]";
    }

    private static String runStateToString(int rs) {
        switch(rs) {
        case RUNNING: return "Running";
        case SHUTDOWN: return "Shutting down";
        case TERMINATING: return "Terminating";
        case TERMINATED: return "Terminated";
        default: throw new Error("Unknown run state");
        }
    }

    // lifecycle control

    /**
     * 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")</code>,
     */
    public void shutdown() {
        checkPermission();
        transitionRunStateTo(SHUTDOWN);
        if (canTerminateOnShutdown(runControl))
            terminateOnShutdown();
    }

    /**
     * Attempts to stop all actively executing tasks, and cancels all
     * waiting 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. Unlike some other executors,
     * this method cancels rather than collects non-executed tasks,
     * so always returns an empty list.
     * @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")</code>,
     */
    public List<Runnable> shutdownNow() {
        checkPermission();
        terminate();
        return Collections.emptyList();
    }

    /**
     * Returns <code>true</code> if all tasks have completed following shut down.
     *
     * @return <code>true</code> if all tasks have completed following shut down
     */
    public boolean isTerminated() {
        return runStateOf(runControl) == TERMINATED;
    }

    /**
     * Returns <code>true</code> if the process of termination has
     * commenced but possibly not yet completed.
     *
     * @return <code>true</code> if terminating
     */
    public boolean isTerminating() {
        return runStateOf(runControl) >= TERMINATING;
    }

    /**
     * Returns <code>true</code> if this pool has been shut down.
     *
     * @return <code>true</code> if this pool has been shut down
     */
    public boolean isShutdown() {
        return runStateOf(runControl) >= 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</code> if this executor terminated and
     *         <code>false</code> 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.workerLock;
        lock.lock();
        try {
            for (;;) {
                if (isTerminated())
                    return true;
                if (nanos <= 0)
                    return false;
                nanos = termination.awaitNanos(nanos);
            }
        } finally {
            lock.unlock();
        }
    }

    // Shutdown and termination support

    /**
     * Callback from terminating worker. Null out the corresponding
     * workers slot, and if terminating, try to terminate, else try to
     * shrink workers array.
     * @param w the worker
     */
    final void workerTerminated(ForkJoinWorkerThread w) {
        updateStealCount(w);
        updateWorkerCount(-1);
        final ReentrantLock lock = this.workerLock;
        lock.lock();
        try {
            ForkJoinWorkerThread[] ws = workers;
            int idx = w.poolIndex;
            if (idx >= 0 && idx < ws.length && ws[idx] == w)
                ws[idx] = null;
            if (totalCountOf(workerCounts) == 0) {
                terminate(); // no-op if already terminating
                transitionRunStateTo(TERMINATED);
                termination.signalAll();
            }
            else if (!isTerminating()) {
                tryShrinkWorkerArray();
                tryResumeSpare(true); // allow replacement
            }
        } finally {
            lock.unlock();
        }
        signalIdleWorkers(false);
    }

    /**
     * Initiate termination.
     */
    private void terminate() {
        if (transitionRunStateTo(TERMINATING)) {
            stopAllWorkers();
            resumeAllSpares();
            signalIdleWorkers(true);
            cancelQueuedSubmissions();
            cancelQueuedWorkerTasks();
            interruptUnterminatedWorkers();
            signalIdleWorkers(true); // resignal after interrupt
        }
    }

    /**
     * Possibly terminate when on shutdown state
     */
    private void terminateOnShutdown() {
        if (!hasQueuedSubmissions() && canTerminateOnShutdown(runControl))
            terminate();
    }

    /**
     * Clear out and cancel submissions
     */
    private void cancelQueuedSubmissions() {
        ForkJoinTask<?> task;
        while ((task = pollSubmission()) != null)
            task.cancel(false);
    }

    /**
     * Clean out worker queues.
     */
    private void cancelQueuedWorkerTasks() {
        final ReentrantLock lock = this.workerLock;
        lock.lock();
        try {
            ForkJoinWorkerThread[] ws = workers;
            for (int i = 0; i < ws.length; ++i) {
                ForkJoinWorkerThread t = ws[i];
                if (t != null)
                    t.cancelTasks();
            }
        } finally {
            lock.unlock();
        }
    }

    /**
     * Set each worker's status to terminating. Requires lock to avoid
     * conflicts with add/remove
     */
    private void stopAllWorkers() {
        final ReentrantLock lock = this.workerLock;
        lock.lock();
        try {
            ForkJoinWorkerThread[] ws = workers;
            for (int i = 0; i < ws.length; ++i) {
                ForkJoinWorkerThread t = ws[i];
                if (t != null)
                    t.shutdownNow();
            }
        } finally {
            lock.unlock();
        }
    }

    /**
     * Interrupt all unterminated workers.  This is not required for
     * sake of internal control, but may help unstick user code during
     * shutdown.
     */
    private void interruptUnterminatedWorkers() {
        final ReentrantLock lock = this.workerLock;
        lock.lock();
        try {
            ForkJoinWorkerThread[] ws = workers;
            for (int i = 0; i < ws.length; ++i) {
                ForkJoinWorkerThread t = ws[i];
                if (t != null && !t.isTerminated()) {
                    try {
                        t.interrupt();
                    } catch (SecurityException ignore) {
                    }
                }
            }
        } finally {
            lock.unlock();
        }
    }


    /*
     * Nodes for event barrier to manage idle threads.
     *
     * The event barrier has an event count and a wait queue (actually
     * a Treiber stack).  Workers are enabled to look for work when
     * the eventCount is incremented. If they fail to find some,
     * they may wait for next count. Synchronization events occur only
     * in enough contexts to maintain overall liveness:
     *
     *   - Submission of a new task to the pool
     *   - Creation or termination of a worker
     *   - pool termination
     *   - A worker pushing a task on an empty queue
     *
     * The last case (pushing a task) occurs often enough, and is
     * heavy enough compared to simple stack pushes to require some
     * special handling: Method signalNonEmptyWorkerQueue returns
     * without advancing count if the queue appears to be empty.  This
     * would ordinarily result in races causing some queued waiters
     * not to be woken up. To avoid this, a worker in sync
     * rescans for tasks after being enqueued if it was the first to
     * enqueue, and aborts the wait if finding one, also helping to
     * signal others. This works well because the worker has nothing
     * better to do anyway, and so might as well help alleviate the
     * overhead and contention on the threads actually doing work.
     *
     * Queue nodes are basic Treiber stack nodes, also used for spare
     * stack.
     */
    static final class WaitQueueNode {
        WaitQueueNode next; // only written before enqueued
        volatile ForkJoinWorkerThread thread; // nulled to cancel wait
        final long count; // unused for spare stack
        WaitQueueNode(ForkJoinWorkerThread w, long c) {
            count = c;
            thread = w;
        }
        final boolean signal() {
            ForkJoinWorkerThread t = thread;
            thread = null;
            if (t != null) {
                LockSupport.unpark(t);
                return true;
            }
            return false;
        }
    }

    /**
     * Release at least one thread waiting for event count to advance,
     * if one exists. If initial attempt fails, release all threads.
     * @param all if false, at first try to only release one thread
     * @return current event
     */
    private long releaseIdleWorkers(boolean all) {
        long c;
        for (;;) {
            WaitQueueNode q = barrierStack;
            c = eventCount;
            long qc;
            if (q == null || (qc = q.count) >= c)
                break;
            if (!all) {
                if (casBarrierStack(q, q.next) && q.signal())
                    break;
                all = true;
            }
            else if (casBarrierStack(q, null)) {
                do {
                 q.signal();
                } while ((q = q.next) != null);
                break;
            }
        }
        return c;
    }

    /**
     * Returns current barrier event count
     * @return current barrier event count
     */
    final long getEventCount() {
        long ec = eventCount;
        releaseIdleWorkers(true); // release to ensure accurate result
        return ec;
    }

    /**
     * Increment event count and release at least one waiting thread,
     * if one exists (released threads will in turn wake up others).
     * @param all if true, try to wake up all
     */
    final void signalIdleWorkers(boolean all) {
        long c;
        do;while (!casEventCount(c = eventCount, c+1));
        releaseIdleWorkers(all);
    }

    /**
     * Wake up threads waiting to steal a task. Because method
     * sync rechecks availability, it is OK to only proceed if
     * queue appears to be non-empty.
     */
    final void signalNonEmptyWorkerQueue() {
        // If CAS fails another signaller must have succeeded
        long c;
        if (barrierStack != null && casEventCount(c = eventCount, c+1))
            releaseIdleWorkers(false);
    }

    /**
     * Waits until event count advances from count, or some thread is
     * waiting on a previous count, or there is stealable work
     * available. Help wake up others on release.
     * @param w the calling worker thread
     * @param prev previous value returned by sync (or 0)
     * @return current event count
     */
    final long sync(ForkJoinWorkerThread w, long prev) {
        updateStealCount(w);

        while (!w.isShutdown() && !isTerminating() &&
               (parallelism >= runningCountOf(workerCounts) ||
                !suspendIfSpare(w))) { // prefer suspend to waiting here
            WaitQueueNode node = null;
            boolean queued = false;
            for (;;) {
                if (!queued) {
                    if (eventCount != prev)
                        break;
                    WaitQueueNode h = barrierStack;
                    if (h != null && h.count != prev)
                        break; // release below and maybe retry
                    if (node == null)
                        node = new WaitQueueNode(w, prev);
                    queued = casBarrierStack(node.next = h, node);
                }
                else if (Thread.interrupted() ||
                         node.thread == null ||
                         (node.next == null && w.prescan()) ||
                         eventCount != prev) {
                    node.thread = null;
                    if (eventCount == prev) // help trigger
                        casEventCount(prev, prev+1);
                    break;
                }
                else
                    LockSupport.park(this);
            }
            long ec = eventCount;
            if (releaseIdleWorkers(false) != prev)
                return ec;
        }
        return prev; // return old count if aborted
    }

    //  Parallelism maintenance

    /**
     * Decrement running count; if too low, add spare.
     *
     * Conceptually, all we need to do here is add or resume a
     * spare thread when one is about to block (and remove or
     * suspend it later when unblocked -- see suspendIfSpare).
     * However, implementing this idea requires coping with
     * several problems: We have imperfect information about the
     * states of threads. Some count updates can and usually do
     * lag run state changes, despite arrangements to keep them
     * accurate (for example, when possible, updating counts
     * before signalling or resuming), especially when running on
     * dynamic JVMs that don't optimize the infrequent paths that
     * update counts. Generating too many threads can make these
     * problems become worse, because excess threads are more
     * likely to be context-switched with others, slowing them all
     * down, especially if there is no work available, so all are
     * busy scanning or idling.  Also, excess spare threads can
     * only be suspended or removed when they are idle, not
     * immediately when they aren't needed. So adding threads will
     * raise parallelism level for longer than necessary.  Also,
     * FJ applications often enounter highly transient peaks when
     * many threads are blocked joining, but for less time than it
     * takes to create or resume spares.
     *
     * @param joinMe if non-null, return early if done
     * @param maintainParallelism if true, try to stay within
     * target counts, else create only to avoid starvation
     * @return true if joinMe known to be done
     */
    final boolean preJoin(ForkJoinTask<?> joinMe, boolean maintainParallelism) {
        maintainParallelism &= maintainsParallelism; // overrride
        boolean dec = false;  // true when running count decremented
        while (spareStack == null || !tryResumeSpare(dec)) {
            int counts = workerCounts;
            if (dec || (dec = casWorkerCounts(counts, --counts))) { // CAS cheat
                if (!needSpare(counts, maintainParallelism))
                    break;
                if (joinMe.status < 0)
                    return true;
                if (tryAddSpare(counts))
                    break;
            }
        }
        return false;
    }

    /**
     * Same idea as preJoin
     */
    final boolean preBlock(ManagedBlocker blocker, boolean maintainParallelism){
        maintainParallelism &= maintainsParallelism;
        boolean dec = false;
        while (spareStack == null || !tryResumeSpare(dec)) {
            int counts = workerCounts;
            if (dec || (dec = casWorkerCounts(counts, --counts))) {
                if (!needSpare(counts, maintainParallelism))
                    break;
                if (blocker.isReleasable())
                    return true;
                if (tryAddSpare(counts))
                    break;
            }
        }
        return false;
    }

    /**
     * Returns true if a spare thread appears to be needed.  If
     * maintaining parallelism, returns true when the deficit in
     * running threads is more than the surplus of total threads, and
     * there is apparently some work to do.  This self-limiting rule
     * means that the more threads that have already been added, the
     * less parallelism we will tolerate before adding another.
     * @param counts current worker counts
     * @param maintainParallelism try to maintain parallelism
     */
    private boolean needSpare(int counts, boolean maintainParallelism) {
        int ps = parallelism;
        int rc = runningCountOf(counts);
        int tc = totalCountOf(counts);
        int runningDeficit = ps - rc;
        int totalSurplus = tc - ps;
        return (tc < maxPoolSize &&
                (rc == 0 || totalSurplus < 0 ||
                 (maintainParallelism &&
                  runningDeficit > totalSurplus && mayHaveQueuedWork())));
    }

    /**
     * Returns true if at least one worker queue appears to be
     * nonempty. This is expensive but not often called. It is not
     * critical that this be accurate, but if not, more or fewer
     * running threads than desired might be maintained.
     */
    private boolean mayHaveQueuedWork() {
        ForkJoinWorkerThread[] ws = workers;
        int len = ws.length;
        ForkJoinWorkerThread v;
        for (int i = 0; i < len; ++i) {
            if ((v = ws[i]) != null && v.getRawQueueSize() > 0) {
                releaseIdleWorkers(false); // help wake up stragglers
                return true;
            }
        }
        return false;
    }

    /**
     * Add a spare worker if lock available and no more than the
     * expected numbers of threads exist
     * @return true if successful
     */
    private boolean tryAddSpare(int expectedCounts) {
        final ReentrantLock lock = this.workerLock;
        int expectedRunning = runningCountOf(expectedCounts);
        int expectedTotal = totalCountOf(expectedCounts);
        boolean success = false;
        boolean locked = false;
        // confirm counts while locking; CAS after obtaining lock
        try {
            for (;;) {
                int s = workerCounts;
                int tc = totalCountOf(s);
                int rc = runningCountOf(s);
                if (rc > expectedRunning || tc > expectedTotal)
                    break;
                if (!locked && !(locked = lock.tryLock()))
                    break;
                if (casWorkerCounts(s, workerCountsFor(tc+1, rc+1))) {
                    createAndStartSpare(tc);
                    success = true;
                    break;
                }
            }
        } finally {
            if (locked)
                lock.unlock();
        }
        return success;
    }

    /**
     * Add the kth spare worker. On entry, pool coounts are already
     * adjusted to reflect addition.
     */
    private void createAndStartSpare(int k) {
        ForkJoinWorkerThread w = null;
        ForkJoinWorkerThread[] ws = ensureWorkerArrayCapacity(k + 1);
        int len = ws.length;
        // Probably, we can place at slot k. If not, find empty slot
        if (k < len && ws[k] != null) {
            for (k = 0; k < len && ws[k] != null; ++k)
                ;
        }
        if (k < len && !isTerminating() && (w = createWorker(k)) != null) {
            ws[k] = w;
            w.start();
        }
        else
            updateWorkerCount(-1); // adjust on failure
        signalIdleWorkers(false);
    }

    /**
     * Suspend calling thread w if there are excess threads.  Called
     * only from sync.  Spares are enqueued in a Treiber stack
     * using the same WaitQueueNodes as barriers.  They are resumed
     * mainly in preJoin, but are also woken on pool events that
     * require all threads to check run state.
     * @param w the caller
     */
    private boolean suspendIfSpare(ForkJoinWorkerThread w) {
        WaitQueueNode node = null;
        int s;
        while (parallelism < runningCountOf(s = workerCounts)) {
            if (node == null)
                node = new WaitQueueNode(w, 0);
            if (casWorkerCounts(s, s-1)) { // representation-dependent
                // push onto stack
                do;while (!casSpareStack(node.next = spareStack, node));

                // block until released by resumeSpare
                while (node.thread != null) {
                    if (!Thread.interrupted())
                        LockSupport.park(this);
                }
                w.activate(); // help warm up
                return true;
            }
        }
        return false;
    }

    /**
     * Try to pop and resume a spare thread.
     * @param updateCount if true, increment running count on success
     * @return true if successful
     */
    private boolean tryResumeSpare(boolean updateCount) {
        WaitQueueNode q;
        while ((q = spareStack) != null) {
            if (casSpareStack(q, q.next)) {
                if (updateCount)
                    updateRunningCount(1);
                q.signal();
                return true;
            }
        }
        return false;
    }

    /**
     * Pop and resume all spare threads. Same idea as
     * releaseIdleWorkers.
     * @return true if any spares released
     */
    private boolean resumeAllSpares() {
        WaitQueueNode q;
        while ( (q = spareStack) != null) {
            if (casSpareStack(q, null)) {
                do {
                    updateRunningCount(1);
                    q.signal();
                } while ((q = q.next) != null);
                return true;
            }
        }
        return false;
    }

    /**
     * Pop and shutdown excessive spare threads. Call only while
     * holding lock. This is not guaranteed to eliminate all excess
     * threads, only those suspended as spares, which are the ones
     * unlikely to be needed in the future.
     */
    private void trimSpares() {
        int surplus = totalCountOf(workerCounts) - parallelism;
        WaitQueueNode q;
        while (surplus > 0 && (q = spareStack) != null) {
            if (casSpareStack(q, null)) {
                do {
                    updateRunningCount(1);
                    ForkJoinWorkerThread w = q.thread;
                    if (w != null && surplus > 0 &&
                        runningCountOf(workerCounts) > 0 && w.shutdown())
                        --surplus;
                    q.signal();
                } while ((q = q.next) != null);
            }
        }
    }

    /**
     * Returns approximate number of spares, just for diagnostics.
     */
    private int countSpares() {
        int sum = 0;
        for (WaitQueueNode q = spareStack; q != null; q = q.next)
            ++sum;
        return sum;
    }

    /**
     * Interface for extending managed parallelism for tasks running
     * in ForkJoinPools. A ManagedBlocker provides two methods.
     * Method <code>isReleasable</code> must return true if blocking is not
     * necessary. Method <code>block</code> blocks the current thread
     * if necessary (perhaps internally invoking isReleasable before
     * actually blocking.).
     * <p>For example, here is a ManagedBlocker based on a
     * ReentrantLock:
     * <pre>
     *   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>
     */
    public static interface ManagedBlocker {
        /**
         * Possibly blocks the current thread, for example waiting for
         * a lock or condition.
         * @return 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 allowe to).
         */
        boolean block() throws InterruptedException;

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

    /**
     * Blocks in accord with the given blocker.  If the current thread
     * is a ForkJoinWorkerThread, this method possibly arranges for a
     * spare thread to be activated if necessary to ensure parallelism
     * while the current thread is blocked.  If
     * <code>maintainParallelism</code> is true and the pool supports
     * it ({@link #getMaintainsParallelism}), this method attempts to
     * maintain the pool's nominal parallelism. Otherwise if activates
     * a thread only if necessary to avoid complete starvation. This
     * option may be preferable when blockages use timeouts, or are
     * almost always brief.
     *
     * <p> If the caller is not a ForkJoinTask, this method is behaviorally
     * equivalent to
     * <pre>
     *   while (!blocker.isReleasable())
     *      if (blocker.block())
     *         return;
     * </pre>
     * If the caller is a ForkJoinTask, then the pool may first
     * be expanded to ensure parallelism, and later adjusted.
     *
     * @param blocker the blocker
     * @param maintainParallelism if true and supported by this pool,
     * attempt to maintain the pool's nominal parallelism; otherwise
     * activate a thread only if necessary to avoid complete
     * starvation.
     * @throws InterruptedException if blocker.block did so.
     */
    public static void managedBlock(ManagedBlocker blocker,
                                    boolean maintainParallelism)
        throws InterruptedException {
        Thread t = Thread.currentThread();
        ForkJoinPool pool = (t instanceof ForkJoinWorkerThread?
                             ((ForkJoinWorkerThread)t).pool : null);
        if (!blocker.isReleasable()) {
            try {
                if (pool == null ||
                    !pool.preBlock(blocker, maintainParallelism))
                    awaitBlocker(blocker);
            } finally {
                if (pool != null)
                    pool.updateRunningCount(1);
            }
        }
    }

    private static void awaitBlocker(ManagedBlocker blocker)
        throws InterruptedException {
        do;while (!blocker.isReleasable() && !blocker.block());
    }

    // AbstractExecutorService overrides

    protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
        return new AdaptedRunnable(runnable, value);
    }

    protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
        return new AdaptedCallable(callable);
    }


    // Temporary Unsafe mechanics for preliminary release

    static final Unsafe _unsafe;
    static final long eventCountOffset;
    static final long workerCountsOffset;
    static final long runControlOffset;
    static final long barrierStackOffset;
    static final long spareStackOffset;

    static {
        try {
            if (ForkJoinPool.class.getClassLoader() != null) {
                Field f = Unsafe.class.getDeclaredField("theUnsafe");
                f.setAccessible(true);
                _unsafe = (Unsafe)f.get(null);
            }
            else
                _unsafe = Unsafe.getUnsafe();
            eventCountOffset = _unsafe.objectFieldOffset
                (ForkJoinPool.class.getDeclaredField("eventCount"));
            workerCountsOffset = _unsafe.objectFieldOffset
                (ForkJoinPool.class.getDeclaredField("workerCounts"));
            runControlOffset = _unsafe.objectFieldOffset
                (ForkJoinPool.class.getDeclaredField("runControl"));
            barrierStackOffset = _unsafe.objectFieldOffset
                (ForkJoinPool.class.getDeclaredField("barrierStack"));
            spareStackOffset = _unsafe.objectFieldOffset
                (ForkJoinPool.class.getDeclaredField("spareStack"));
        } catch (Exception e) {
            throw new RuntimeException("Could not initialize intrinsics", e);
        }
    }

    private boolean casEventCount(long cmp, long val) {
        return _unsafe.compareAndSwapLong(this, eventCountOffset, cmp, val);
    }
    private boolean casWorkerCounts(int cmp, int val) {
        return _unsafe.compareAndSwapInt(this, workerCountsOffset, cmp, val);
    }
    private boolean casRunControl(int cmp, int val) {
        return _unsafe.compareAndSwapInt(this, runControlOffset, cmp, val);
    }
    private boolean casSpareStack(WaitQueueNode cmp, WaitQueueNode val) {
        return _unsafe.compareAndSwapObject(this, spareStackOffset, cmp, val);
    }
    private boolean casBarrierStack(WaitQueueNode cmp, WaitQueueNode val) {
        return _unsafe.compareAndSwapObject(this, barrierStackOffset, cmp, val);
    }
}