--- jsr166/src/jsr166y/Phaser.java	2008/07/07 16:53:30	1.1
+++ jsr166/src/jsr166y/Phaser.java	2008/10/28 23:03:24	1.6
@@ -5,40 +5,43 @@
  */
 
 package jsr166y;
-import jsr166y.forkjoin.*;
 import java.util.concurrent.*;
 import java.util.concurrent.atomic.*;
 import java.util.concurrent.locks.LockSupport;
+import sun.misc.Unsafe;
+import java.lang.reflect.*;
 
 /**
  * A reusable synchronization barrier, similar in functionality to a
- * {@link java.util.concurrent.CyclicBarrier}, but supporting more
- * flexible usage.
+ * {@link java.util.concurrent.CyclicBarrier} and {@link
+ * java.util.concurrent.CountDownLatch} but supporting more flexible
+ * usage.
  *
  * <ul>
  *
- * <li> The number of parties synchronizing on the barrier may vary
- * over time.  A task may register to be a party in a barrier at any
- * time, and may deregister upon arriving at the barrier.  As is the
- * case with most basic synchronization constructs, registration
- * and deregistration affect only internal counts; they do not
- * establish any further internal bookkeeping, so tasks cannot query
- * whether they are registered.
+ * <li> The number of parties synchronizing on a phaser may vary over
+ * time.  A task may register to be a party at any time, and may
+ * deregister upon arriving at the barrier.  As is the case with most
+ * basic synchronization constructs, registration and deregistration
+ * affect only internal counts; they do not establish any further
+ * internal bookkeeping, so tasks cannot query whether they are
+ * registered. (However, you can introduce such bookkeeping in by
+ * subclassing this class.)
  *
  * <li> Each generation has an associated phase value, starting at
  * zero, and advancing when all parties reach the barrier (wrapping
  * around to zero after reaching <tt>Integer.MAX_VALUE</tt>).
- * 
+ *
  * <li> Like a CyclicBarrier, a Phaser may be repeatedly awaited.
  * Method <tt>arriveAndAwaitAdvance</tt> has effect analogous to
  * <tt>CyclicBarrier.await</tt>.  However, Phasers separate two
- * aspects of coordination, that may be invoked independently:
+ * aspects of coordination, that may also be invoked independently:
  *
  * <ul>
  *
  *   <li> Arriving at a barrier. Methods <tt>arrive</tt> and
- *       <tt>arriveAndDeregister</tt> do not block, but return 
- *       the phase value on entry to the method.
+ *       <tt>arriveAndDeregister</tt> do not block, but return
+ *       the phase value current upon entry to the method.
  *
  *   <li> Awaiting others. Method <tt>awaitAdvance</tt> requires an
  *       argument indicating the entry phase, and returns when the
@@ -49,100 +52,156 @@ import java.util.concurrent.locks.LockSu
  * <li> Barrier actions, performed by the task triggering a phase
  * advance while others may be waiting, are arranged by overriding
  * method <tt>onAdvance</tt>, that also controls termination.
+ * Overriding this method may be used to similar but more flexible
+ * effect as providing a barrier action to a CyclicBarrier.
  *
  * <li> Phasers may enter a <em>termination</em> state in which all
  * await actions immediately return, indicating (via a negative phase
  * value) that execution is complete.  Termination is triggered by
  * executing the overridable <tt>onAdvance</tt> method that is invoked
- * each time the barrier is tripped. When a Phaser is controlling an
- * action with a fixed number of iterations, it is often convenient to
- * override this method to cause termination when the current phase
- * number reaches a threshold.  Method <tt>forceTermination</tt> is
- * also available to assist recovery actions upon failure.
- *
- * <li> Unlike most synchronizers, a Phaser may also be used with 
- * ForkJoinTasks (as well as plain threads).
- * 
+ * each time the barrier is about to be tripped. When a Phaser is
+ * controlling an action with a fixed number of iterations, it is
+ * often convenient to override this method to cause termination when
+ * the current phase number reaches a threshold. Method
+ * <tt>forceTermination</tt> is also available to abruptly release
+ * waiting threads and allow them to terminate.
+ *
+ * <li> Phasers may be tiered to reduce contention. Phasers with large
+ * numbers of parties that would otherwise experience heavy
+ * synchronization contention costs may instead be arranged in trees.
+ * This will typically greatly increase throughput even though it
+ * incurs somewhat greater per-operation overhead.
+ *
  * <li> By default, <tt>awaitAdvance</tt> continues to wait even if
- * the current thread is interrupted. And unlike the case in
+ * the waiting thread is interrupted. And unlike the case in
  * CyclicBarriers, exceptions encountered while tasks wait
  * interruptibly or with timeout do not change the state of the
  * barrier. If necessary, you can perform any associated recovery
- * within handlers of those exceptions.
+ * within handlers of those exceptions, often after invoking
+ * <tt>forceTermination</tt>.
  *
  * </ul>
  *
- * <p><b>Sample usage:</b> 
+ * <p><b>Sample usages:</b>
  *
- * <p>[todo: non-FJ example]
- *
- * <p> A Phaser may be used to support a style of programming in
- * which a task waits for others to complete, without otherwise
- * needing to keep track of which tasks it is waiting for. This is
- * similar to the "sync" construct in Cilk and "clocks" in X10.
- * Special constructions based on such barriers are available using
- * the <tt>LinkedAsyncAction</tt> and <tt>CyclicAction</tt> classes,
- * but they can be useful in other contexts as well.  For a simple
- * (but not very useful) example, here is a variant of Fibonacci:
+ * <p>A Phaser may be used instead of a <tt>CountdownLatch</tt> to control
+ * a one-shot action serving a variable number of parties. The typical
+ * idiom is for the method setting this up to first register, then
+ * start the actions, then deregister, as in:
  *
  * <pre>
- * class BarrierFibonacci extends RecursiveAction {
- *   int argument, result;
- *   final Phaser parentBarrier;
- *   BarrierFibonacci(int n, Phaser parentBarrier) {
- *     this.argument = n;
- *     this.parentBarrier = parentBarrier;
- *     parentBarrier.register();
+ *  void runTasks(List&lt;Runnable&gt; list) {
+ *    final Phaser phaser = new Phaser(1); // "1" to register self
+ *    for (Runnable r : list) {
+ *      phaser.register();
+ *      new Thread() {
+ *        public void run() {
+ *          phaser.arriveAndAwaitAdvance(); // await all creation
+ *          r.run();
+ *          phaser.arriveAndDeregister();   // signal completion
+ *        }
+ *      }.start();
  *   }
- *   protected void compute() {
- *     int n = argument;
- *     if (n &lt;= 1)
- *        result = n;
- *     else {
- *        Phaser childBarrier = new Phaser(1);
- *        BarrierFibonacci f1 = new BarrierFibonacci(n - 1, childBarrier);
- *        BarrierFibonacci f2 = new BarrierFibonacci(n - 2, childBarrier);
- *        f1.fork();
- *        f2.fork();
- *        childBarrier.arriveAndAwait();
- *        result = f1.result + f2.result;
- *     }
- *     parentBarrier.arriveAndDeregister();
+ *
+ *   doSomethingOnBehalfOfWorkers();
+ *   phaser.arrive(); // allow threads to start
+ *   int p = phaser.arriveAndDeregister(); // deregister self  ...
+ *   p = phaser.awaitAdvance(p); // ... and await arrival
+ *   otherActions(); // do other things while tasks execute
+ *   phaser.awaitAdvance(p); // awit final completion
+ * }
+ * </pre>
+ *
+ * <p>One way to cause a set of threads to repeatedly perform actions
+ * for a given number of iterations is to override <tt>onAdvance</tt>:
+ *
+ * <pre>
+ *  void startTasks(List&lt;Runnable&gt; list, final int iterations) {
+ *    final Phaser phaser = new Phaser() {
+ *       public boolean onAdvance(int phase, int registeredParties) {
+ *         return phase &gt;= iterations || registeredParties == 0;
+ *       }
+ *    };
+ *    phaser.register();
+ *    for (Runnable r : list) {
+ *      phaser.register();
+ *      new Thread() {
+ *        public void run() {
+ *           do {
+ *             r.run();
+ *             phaser.arriveAndAwaitAdvance();
+ *           } while(!phaser.isTerminated();
+ *        }
+ *      }.start();
  *   }
+ *   phaser.arriveAndDeregister(); // deregister self, don't wait
  * }
  * </pre>
  *
+ * <p> To create a set of tasks using a tree of Phasers,
+ * you could use code of the following form, assuming a
+ * Task class with a constructor accepting a Phaser that
+ * it registers for upon construction:
+ * <pre>
+ *  void build(Task[] actions, int lo, int hi, Phaser b) {
+ *    int step = (hi - lo) / TASKS_PER_PHASER;
+ *    if (step &gt; 1) {
+ *       int i = lo;
+ *       while (i &lt; hi) {
+ *         int r = Math.min(i + step, hi);
+ *         build(actions, i, r, new Phaser(b));
+ *         i = r;
+ *       }
+ *    }
+ *    else {
+ *      for (int i = lo; i &lt; hi; ++i)
+ *        actions[i] = new Task(b);
+ *        // assumes new Task(b) performs b.register()
+ *    }
+ *  }
+ *  // .. initially called, for n tasks via
+ *  build(new Task[n], 0, n, new Phaser());
+ * </pre>
+ *
+ * The best value of <tt>TASKS_PER_PHASER</tt> depends mainly on
+ * expected barrier synchronization rates. A value as low as four may
+ * be appropriate for extremely small per-barrier task bodies (thus
+ * high rates), or up to hundreds for extremely large ones.
+ *
+ * </pre>
+ *
  * <p><b>Implementation notes</b>: This implementation restricts the
- * maximum number of parties to 65535. Attempts to register
- * additional parties result in IllegalStateExceptions.  
+ * maximum number of parties to 65535. Attempts to register additional
+ * parties result in IllegalStateExceptions. However, you can and
+ * should create tiered phasers to accommodate arbitrarily large sets
+ * of participants.
  */
 public class Phaser {
     /*
      * This class implements an extension of X10 "clocks".  Thanks to
-     * Vijay Saraswat for the idea of applying it to ForkJoinTasks,
-     * and to Vivek Sarkar for enhancements to extend functionality.
+     * Vijay Saraswat for the idea, and to Vivek Sarkar for
+     * enhancements to extend functionality.
      */
 
     /**
      * Barrier state representation. Conceptually, a barrier contains
      * four values:
-     * 
+     *
      * * parties -- the number of parties to wait (16 bits)
      * * unarrived -- the number of parties yet to hit barrier (16 bits)
      * * phase -- the generation of the barrier (31 bits)
      * * terminated -- set if barrier is terminated (1 bit)
      *
      * However, to efficiently maintain atomicity, these values are
-     * packed into a single AtomicLong. Termination uses the sign bit
-     * of 32 bit representation of phase, so phase is set to -1 on
-     * termination.
-     */
-    private final AtomicLong state;
-
-    /**
-     * Head of Treiber stack for waiting nonFJ threads.
+     * packed into a single (atomic) long. Termination uses the sign
+     * bit of 32 bit representation of phase, so phase is set to -1 on
+     * termination. Good performace relies on keeping state decoding
+     * and encoding simple, and keeping race windows short.
+     *
+     * Note: there are some cheats in arrive() that rely on unarrived
+     * being lowest 16 bits.
      */
-    private final AtomicReference<QNode> head = new AtomicReference<QNode>();
+    private volatile long state;
 
     private static final int ushortBits = 16;
     private static final int ushortMask =  (1 << ushortBits) - 1;
@@ -168,30 +227,132 @@ public class Phaser {
         return (((long)phase) << 32) | ((parties << 16) | unarrived);
     }
 
+    private static long trippedStateFor(int phase, int parties) {
+        return (((long)phase) << 32) | ((parties << 16) | parties);
+    }
+
     private static IllegalStateException badBounds(int parties, int unarrived) {
-        return new IllegalStateException("Attempt to set " + unarrived +
-                                         " unarrived of " + parties + " parties");
+        return new IllegalStateException
+            ("Attempt to set " + unarrived +
+             " unarrived of " + parties + " parties");
+    }
+
+    /**
+     * The parent of this phaser, or null if none
+     */
+    private final Phaser parent;
+
+    /**
+     * The root of Phaser tree. Equals this if not in a tree.  Used to
+     * support faster state push-down.
+     */
+    private final Phaser root;
+
+    // Wait queues
+
+    /**
+     * Heads of Treiber stacks waiting for nonFJ threads. To eliminate
+     * contention while releasing some threads while adding others, we
+     * use two of them, alternating across even and odd phases.
+     */
+    private final AtomicReference<QNode> evenQ = new AtomicReference<QNode>();
+    private final AtomicReference<QNode> oddQ  = new AtomicReference<QNode>();
+
+    private AtomicReference<QNode> queueFor(int phase) {
+        return (phase & 1) == 0? evenQ : oddQ;
+    }
+
+    /**
+     * Returns current state, first resolving lagged propagation from
+     * root if necessary.
+     */
+    private long getReconciledState() {
+        return parent == null? state : reconcileState();
+    }
+
+    /**
+     * Recursively resolves state.
+     */
+    private long reconcileState() {
+        Phaser p = parent;
+        long s = state;
+        if (p != null) {
+            while (unarrivedOf(s) == 0 && phaseOf(s) != phaseOf(root.state)) {
+                long parentState = p.getReconciledState();
+                int parentPhase = phaseOf(parentState);
+                int phase = phaseOf(s = state);
+                if (phase != parentPhase) {
+                    long next = trippedStateFor(parentPhase, partiesOf(s));
+                    if (casState(s, next)) {
+                        releaseWaiters(phase);
+                        s = next;
+                    }
+                }
+            }
+        }
+        return s;
     }
 
     /**
      * Creates a new Phaser without any initially registered parties,
-     * and initial phase number 0.
+     * initial phase number 0, and no parent.
      */
     public Phaser() {
-        state = new AtomicLong(stateFor(0, 0, 0));
+        this(null);
     }
 
     /**
      * Creates a new Phaser with the given numbers of registered
-     * unarrived parties and initial phase number 0.
+     * unarrived parties, initial phase number 0, and no parent.
      * @param parties the number of parties required to trip barrier.
      * @throws IllegalArgumentException if parties less than zero
      * or greater than the maximum number of parties supported.
      */
     public Phaser(int parties) {
+        this(null, parties);
+    }
+
+    /**
+     * Creates a new Phaser with the given parent, without any
+     * initially registered parties. If parent is non-null this phaser
+     * is registered with the parent and its initial phase number is
+     * the same as that of parent phaser.
+     * @param parent the parent phaser.
+     */
+    public Phaser(Phaser parent) {
+        int phase = 0;
+        this.parent = parent;
+        if (parent != null) {
+            this.root = parent.root;
+            phase = parent.register();
+        }
+        else
+            this.root = this;
+        this.state = trippedStateFor(phase, 0);
+    }
+
+    /**
+     * Creates a new Phaser with the given parent and numbers of
+     * registered unarrived parties. If parent is non-null this phaser
+     * is registered with the parent and its initial phase number is
+     * the same as that of parent phaser.
+     * @param parent the parent phaser.
+     * @param parties the number of parties required to trip barrier.
+     * @throws IllegalArgumentException if parties less than zero
+     * or greater than the maximum number of parties supported.
+     */
+    public Phaser(Phaser parent, int parties) {
         if (parties < 0 || parties > ushortMask)
             throw new IllegalArgumentException("Illegal number of parties");
-        state = new AtomicLong(stateFor(0, parties, parties));
+        int phase = 0;
+        this.parent = parent;
+        if (parent != null) {
+            this.root = parent.root;
+            phase = parent.register();
+        }
+        else
+            this.root = this;
+        this.state = trippedStateFor(phase, parties);
     }
 
     /**
@@ -200,124 +361,189 @@ public class Phaser {
      * @throws IllegalStateException if attempting to register more
      * than the maximum supported number of parties.
      */
-    public int register() { // increment both parties and unarrived
-        final AtomicLong state = this.state;
+    public int register() {
+        return doRegister(1);
+    }
+
+    /**
+     * Adds the given number of new unarrived parties to this phaser.
+     * @param parties the number of parties required to trip barrier.
+     * @return the current barrier phase number upon registration
+     * @throws IllegalStateException if attempting to register more
+     * than the maximum supported number of parties.
+     */
+    public int bulkRegister(int parties) {
+        if (parties < 0)
+            throw new IllegalArgumentException();
+        if (parties == 0)
+            return getPhase();
+        return doRegister(parties);
+    }
+
+    /**
+     * Shared code for register, bulkRegister
+     */
+    private int doRegister(int registrations) {
+        int phase;
         for (;;) {
-            long s = state.get();
-            int phase = phaseOf(s);
-            int parties = partiesOf(s) + 1;
-            int unarrived = unarrivedOf(s) + 1;
+            long s = getReconciledState();
+            phase = phaseOf(s);
+            int unarrived = unarrivedOf(s) + registrations;
+            int parties = partiesOf(s) + registrations;
+            if (phase < 0)
+                break;
             if (parties > ushortMask || unarrived > ushortMask)
                 throw badBounds(parties, unarrived);
-            if (state.compareAndSet(s, stateFor(phase, parties, unarrived)))
-                return phase;
+            if (phase == phaseOf(root.state) &&
+                casState(s, stateFor(phase, parties, unarrived)))
+                break;
         }
+        return phase;
     }
 
     /**
      * Arrives at the barrier, but does not wait for others.  (You can
      * in turn wait for others via {@link #awaitAdvance}).
      *
-     * @return the current barrier phase number upon entry to
-     * this method, or a negative value if terminated;
-     * @throws IllegalStateException if the number of unarrived
-     * parties would become negative.
+     * @return the barrier phase number upon entry to this method, or a
+     * negative value if terminated;
+     * @throws IllegalStateException if not terminated and the number
+     * of unarrived parties would become negative.
      */
-    public int arrive() { // decrement unarrived. If zero, trip
-        final AtomicLong state = this.state;
+    public int arrive() {
+        int phase;
         for (;;) {
-            long s = state.get();
-            int phase = phaseOf(s);
+            long s = state;
+            phase = phaseOf(s);
             int parties = partiesOf(s);
             int unarrived = unarrivedOf(s) - 1;
-            if (unarrived < 0)
-                throw badBounds(parties, unarrived);
-            if (unarrived == 0 && phase >= 0) {
-                trip(phase, parties);
-                return phase;
+            if (unarrived > 0) {        // Not the last arrival
+                if (casState(s, s - 1)) // s-1 adds one arrival
+                    break;
+            }
+            else if (unarrived == 0) {  // the last arrival
+                Phaser par = parent;
+                if (par == null) {      // directly trip
+                    if (casState
+                        (s,
+                         trippedStateFor(onAdvance(phase, parties)? -1 :
+                                         ((phase + 1) & phaseMask), parties))) {
+                        releaseWaiters(phase);
+                        break;
+                    }
+                }
+                else {                  // cascade to parent
+                    if (casState(s, s - 1)) { // zeroes unarrived
+                        par.arrive();
+                        reconcileState();
+                        break;
+                    }
+                }
             }
-            if (state.compareAndSet(s, stateFor(phase, parties, unarrived)))
-                return phase;
+            else if (phase < 0) // Don't throw exception if terminated
+                break;
+            else if (phase != phaseOf(root.state)) // or if unreconciled
+                reconcileState();
+            else
+                throw badBounds(parties, unarrived);
         }
+        return phase;
     }
 
     /**
      * Arrives at the barrier, and deregisters from it, without
-     * waiting for others.
+     * waiting for others. Deregistration reduces number of parties
+     * required to trip the barrier in future phases.  If this phaser
+     * has a parent, and deregistration causes this phaser to have
+     * zero parties, this phaser is also deregistered from its parent.
      *
      * @return the current barrier phase number upon entry to
      * this method, or a negative value if terminated;
-     * @throws IllegalStateException if the number of registered or
-     * unarrived parties would become negative.
+     * @throws IllegalStateException if not terminated and the number
+     * of registered or unarrived parties would become negative.
      */
-    public int arriveAndDeregister() { // Same as arrive, plus decrement parties
-        final AtomicLong state = this.state;
+    public int arriveAndDeregister() {
+        // similar code to arrive, but too different to merge
+        Phaser par = parent;
+        int phase;
         for (;;) {
-            long s = state.get();
-            int phase = phaseOf(s);
+            long s = state;
+            phase = phaseOf(s);
             int parties = partiesOf(s) - 1;
             int unarrived = unarrivedOf(s) - 1;
-            if (parties < 0 || unarrived < 0)
-                throw badBounds(parties, unarrived);
-            if (unarrived == 0 && phase >= 0) {
-                trip(phase, parties);
-                return phase;
+            if (parties >= 0) {
+                if (unarrived > 0 || (unarrived == 0 && par != null)) {
+                    if (casState
+                        (s,
+                         stateFor(phase, parties, unarrived))) {
+                        if (unarrived == 0) {
+                            par.arriveAndDeregister();
+                            reconcileState();
+                        }
+                        break;
+                    }
+                    continue;
+                }
+                if (unarrived == 0) {
+                    if (casState
+                        (s,
+                         trippedStateFor(onAdvance(phase, parties)? -1 :
+                                         ((phase + 1) & phaseMask), parties))) {
+                        releaseWaiters(phase);
+                        break;
+                    }
+                    continue;
+                }
+                if (phase < 0)
+                    break;
+                if (par != null && phase != phaseOf(root.state)) {
+                    reconcileState();
+                    continue;
+                }
             }
-            if (state.compareAndSet(s, stateFor(phase, parties, unarrived)))
-                return phase;
+            throw badBounds(parties, unarrived);
         }
+        return phase;
     }
 
     /**
-     * Arrives at the barrier and awaits others. Unlike other arrival
-     * methods, this method returns the arrival index of the
-     * caller. The caller tripping the barrier returns zero, the
-     * previous caller 1, and so on.
-     * @return the arrival index
-     * @throws IllegalStateException if the number of unarrived
-     * parties would become negative.
+     * Arrives at the barrier and awaits others. Equivalent in effect
+     * to <tt>awaitAdvance(arrive())</tt>.  If you instead need to
+     * await with interruption of timeout, and/or deregister upon
+     * arrival, you can arrange them using analogous constructions.
+     * @return the phase on entry to this method
+     * @throws IllegalStateException if not terminated and the number
+     * of unarrived parties would become negative.
      */
     public int arriveAndAwaitAdvance() {
-        final AtomicLong state = this.state;
-        for (;;) {
-            long s = state.get();
-            int phase = phaseOf(s);
-            int parties = partiesOf(s);
-            int unarrived = unarrivedOf(s) - 1;
-            if (unarrived < 0)
-                throw badBounds(parties, unarrived);
-            if (unarrived == 0 && phase >= 0) {
-                trip(phase, parties);
-                return 0;
-            }
-            if (state.compareAndSet(s, stateFor(phase, parties, unarrived))) {
-                awaitAdvance(phase);
-                return unarrived;
-            }
-        }
+        return awaitAdvance(arrive());
     }
 
     /**
      * Awaits the phase of the barrier to advance from the given
-     * value, or returns immediately if this barrier is terminated
+     * value, or returns immediately if argument is negative or this
+     * barrier is terminated.
      * @param phase the phase on entry to this method
      * @return the phase on exit from this method
      */
     public int awaitAdvance(int phase) {
         if (phase < 0)
             return phase;
-        Thread current = Thread.currentThread();
-        if (current instanceof ForkJoinWorkerThread) 
-            return helpingWait(phase);
-        if (untimedWait(current, phase, false))
-            current.interrupt();
-        return phaseOf(state.get());
+        long s = getReconciledState();
+        int p = phaseOf(s);
+        if (p != phase)
+            return p;
+        if (unarrivedOf(s) == 0)
+            parent.awaitAdvance(phase);
+        // Fall here even if parent waited, to reconcile and help release
+        return untimedWait(phase);
     }
 
     /**
      * Awaits the phase of the barrier to advance from the given
-     * value, or returns immediately if this barrier is terminated, or
-     * throws InterruptedException if interrupted while waiting.
+     * value, or returns immediately if argumet is negative or this
+     * barrier is terminated, or throws InterruptedException if
+     * interrupted while waiting.
      * @param phase the phase on entry to this method
      * @return the phase on exit from this method
      * @throws InterruptedException if thread interrupted while waiting
@@ -325,83 +551,78 @@ public class Phaser {
     public int awaitAdvanceInterruptibly(int phase) throws InterruptedException {
         if (phase < 0)
             return phase;
-        Thread current = Thread.currentThread();
-        if (current instanceof ForkJoinWorkerThread) 
-            return helpingWait(phase);
-        else if (Thread.interrupted() || untimedWait(current, phase, true))
-            throw new InterruptedException();
-        else
-            return phaseOf(state.get());
+        long s = getReconciledState();
+        int p = phaseOf(s);
+        if (p != phase)
+            return p;
+        if (unarrivedOf(s) != 0)
+            parent.awaitAdvanceInterruptibly(phase);
+        return interruptibleWait(phase);
     }
 
     /**
      * Awaits the phase of the barrier to advance from the given value
-     * or the given timeout elapses, or returns immediately if this
-     * barrier is terminated
+     * or the given timeout elapses, or returns immediately if
+     * argument is negative or this barrier is terminated.
      * @param phase the phase on entry to this method
      * @return the phase on exit from this method
      * @throws InterruptedException if thread interrupted while waiting
      * @throws TimeoutException if timed out while waiting
      */
-    public int awaitAdvanceInterruptibly(int phase, long timeout, TimeUnit unit) 
+    public int awaitAdvanceInterruptibly(int phase, long timeout, TimeUnit unit)
         throws InterruptedException, TimeoutException {
         if (phase < 0)
             return phase;
-        long nanos = unit.toNanos(timeout);
-        Thread current = Thread.currentThread();
-        if (current instanceof ForkJoinWorkerThread) 
-            return timedHelpingWait(phase, nanos);
-        timedWait(current, phase, nanos);
-        return phaseOf(state.get());
+        long s = getReconciledState();
+        int p = phaseOf(s);
+        if (p != phase)
+            return p;
+        if (unarrivedOf(s) == 0)
+            parent.awaitAdvanceInterruptibly(phase, timeout, unit);
+        return timedWait(phase, unit.toNanos(timeout));
     }
 
     /**
      * Forces this barrier to enter termination state. Counts of
-     * arrived and registered parties are unaffected. This method may
-     * be useful for coordinating recovery after one or more tasks
-     * encounter unexpected exceptions.
+     * arrived and registered parties are unaffected. If this phaser
+     * has a parent, it too is terminated. This method may be useful
+     * for coordinating recovery after one or more tasks encounter
+     * unexpected exceptions.
      */
     public void forceTermination() {
-        final AtomicLong state = this.state;
         for (;;) {
-            long s = state.get();
+            long s = getReconciledState();
             int phase = phaseOf(s);
             int parties = partiesOf(s);
             int unarrived = unarrivedOf(s);
             if (phase < 0 ||
-                state.compareAndSet(s, stateFor(-1, parties, unarrived))) {
-                if (head.get() != null)
-                    releaseWaiters(-1);
+                casState(s, stateFor(-1, parties, unarrived))) {
+                releaseWaiters(0);
+                releaseWaiters(1);
+                if (parent != null)
+                    parent.forceTermination();
                 return;
             }
         }
     }
 
     /**
-     * Resets the barrier with the given numbers of registered unarrived
-     * parties and phase number 0. This method allows repeated reuse
-     * of this barrier, but only if it is somehow known not to be in
-     * use for other purposes.
-     * @param parties the number of parties required to trip barrier.
-     * @throws IllegalArgumentException if parties less than zero
-     * or greater than the maximum number of parties supported.
-     */
-    public void reset(int parties) {
-        if (parties < 0 || parties > ushortMask)
-            throw new IllegalArgumentException("Illegal number of parties");
-        state.set(stateFor(0, parties, parties));
-        if (head.get() != null)
-            releaseWaiters(0);
-    }
-
-    /**
      * Returns the current phase number. The maximum phase number is
      * <tt>Integer.MAX_VALUE</tt>, after which it restarts at
      * zero. Upon termination, the phase number is negative.
      * @return the phase number, or a negative value if terminated
      */
-    public int getPhase() {
-        return phaseOf(state.get());
+    public final int getPhase() {
+        return phaseOf(getReconciledState());
+    }
+
+    /**
+     * Returns true if the current phase number equals the given phase.
+     * @param phase the phase
+     * @return true if the current phase number equals the given phase.
+     */
+    public final boolean hasPhase(int phase) {
+        return phaseOf(getReconciledState()) == phase;
     }
 
     /**
@@ -409,7 +630,7 @@ public class Phaser {
      * @return the number of parties
      */
     public int getRegisteredParties() {
-        return partiesOf(state.get());
+        return partiesOf(state);
     }
 
     /**
@@ -418,7 +639,7 @@ public class Phaser {
      * @return the number of arrived parties
      */
     public int getArrivedParties() {
-        return arrivedOf(state.get());
+        return arrivedOf(state);
     }
 
     /**
@@ -427,15 +648,32 @@ public class Phaser {
      * @return the number of unarrived parties
      */
     public int getUnarrivedParties() {
-        return unarrivedOf(state.get());
+        return unarrivedOf(state);
+    }
+
+    /**
+     * Returns the parent of this phaser, or null if none.
+     * @return the parent of this phaser, or null if none.
+     */
+    public Phaser getParent() {
+        return parent;
     }
 
     /**
-     * Returns true if this barrier has been terminated
+     * Returns the root ancestor of this phaser, which is the same as
+     * this phaser if it has no parent.
+     * @return the root ancestor of this phaser.
+     */
+    public Phaser getRoot() {
+        return root;
+    }
+
+    /**
+     * Returns true if this barrier has been terminated.
      * @return true if this barrier has been terminated
      */
     public boolean isTerminated() {
-        return phaseOf(state.get()) < 0;
+        return getPhase() < 0;
     }
 
     /**
@@ -446,12 +684,23 @@ public class Phaser {
      * phase number, this barrier will be set to a final termination
      * state, and subsequent calls to <tt>isTerminated</tt> will
      * return true.
-     * 
+     *
      * <p> The default version returns true when the number of
      * registered parties is zero. Normally, overrides that arrange
      * termination for other reasons should also preserve this
      * property.
      *
+     * <p> You may override this method to perform an action with side
+     * effects visible to participating tasks, but it is in general
+     * only sensible to do so in designs where all parties register
+     * before any arrive, and all <tt>awaitAdvance</tt> at each phase.
+     * Otherwise, you cannot ensure lack of interference. In
+     * particular, this method may be invoked more than once per
+     * transition if other parties successfully register while the
+     * invocation of this method is in progress, thus postponing the
+     * transition until those parties also arrive, re-triggering this
+     * method.
+     *
      * @param phase the phase number on entering the barrier
      * @param registeredParties the current number of registered
      * parties.
@@ -462,7 +711,7 @@ public class Phaser {
     }
 
     /**
-     * Returns a string identifying this barrier, as well as its
+     * Returns a string identifying this phaser, as well as its
      * state.  The state, in brackets, includes the String {@code
      * "phase ="} followed by the phase number, {@code "parties ="}
      * followed by the number of registered parties, and {@code
@@ -471,105 +720,20 @@ public class Phaser {
      * @return a string identifying this barrier, as well as its state
      */
     public String toString() {
-        long s = state.get();
+        long s = getReconciledState();
         return super.toString() + "[phase = " + phaseOf(s) + " parties = " + partiesOf(s) + " arrived = " + arrivedOf(s) + "]";
     }
 
-    // methods for tripping and waiting
-
-    /**
-     * Advance the current phase (or terminate)
-     */
-    private void trip(int phase, int parties) {
-        int next = onAdvance(phase, parties)? -1 : ((phase + 1) & phaseMask);
-        state.set(stateFor(next, parties, parties));
-        if (head.get() != null)
-            releaseWaiters(next);
-    }
-
-    private int helpingWait(int phase) {
-        final AtomicLong state = this.state;
-        int p;
-        while ((p = phaseOf(state.get())) == phase) {
-            ForkJoinTask<?> t = ForkJoinWorkerThread.pollTask();
-            if (t != null) {
-                if ((p = phaseOf(state.get())) == phase)
-                    t.exec();
-                else {   // push task and exit if barrier advanced
-                    t.fork();
-                    break;
-                }
-            }
-        }
-        return p;
-    }
-
-    private int timedHelpingWait(int phase, long nanos) throws TimeoutException {
-        final AtomicLong state = this.state;
-        long lastTime = System.nanoTime();
-        int p;
-        while ((p = phaseOf(state.get())) == phase) {
-            long now = System.nanoTime();
-            nanos -= now - lastTime;
-            lastTime = now;
-            if (nanos <= 0) {
-                if ((p = phaseOf(state.get())) == phase)
-                    throw new TimeoutException();
-                else
-                    break;
-            }
-            ForkJoinTask<?> t = ForkJoinWorkerThread.pollTask();
-            if (t != null) {
-                if ((p = phaseOf(state.get())) == phase)
-                    t.exec();
-                else {   // push task and exit if barrier advanced
-                    t.fork();
-                    break;
-                }
-            }
-        }
-        return p;
-    }
-
-    /**
-     * Wait nodes for Treiber stack representing wait queue for non-FJ
-     * tasks. The waiting scheme is an adaptation of the one used in
-     * forkjoin.PoolBarrier.
-     */
-    static final class QNode {
-        QNode next;
-        volatile Thread thread; // nulled to cancel wait
-        final int phase;
-        QNode(Thread t, int c) {
-            thread = t;
-            phase = c;
-        }
-    }
-
-    private void releaseWaiters(int currentPhase) {
-        final AtomicReference<QNode> head = this.head;
-        QNode p;
-        while ((p = head.get()) != null && p.phase != currentPhase) {
-            if (head.compareAndSet(p, null)) {
-                do {
-                    Thread t = p.thread;
-                    if (t != null) {
-                        p.thread = null;
-                        LockSupport.unpark(t);
-                    }
-                } while ((p = p.next) != null);
-            }
-        }
-    }
+    // methods for waiting
 
     /** The number of CPUs, for spin control */
     static final int NCPUS = Runtime.getRuntime().availableProcessors();
 
     /**
      * The number of times to spin before blocking in timed waits.
-     * The value is empirically derived 
+     * The value is empirically derived.
      */
-    static final int maxTimedSpins = (NCPUS < 2)? 0 : 32; 
+    static final int maxTimedSpins = (NCPUS < 2)? 0 : 32;
 
     /**
      * The number of times to spin before blocking in untimed waits.
@@ -585,111 +749,168 @@ public class Phaser {
     static final long spinForTimeoutThreshold = 1000L;
 
     /**
+     * Wait nodes for Treiber stack representing wait queue for non-FJ
+     * tasks.
+     */
+    static final class QNode {
+        QNode next;
+        volatile Thread thread; // nulled to cancel wait
+        QNode() {
+            thread = Thread.currentThread();
+        }
+        void signal() {
+            Thread t = thread;
+            if (t != null) {
+                thread = null;
+                LockSupport.unpark(t);
+            }
+        }
+    }
+
+    /**
+     * Removes and signals waiting threads from wait queue
+     */
+    private void releaseWaiters(int phase) {
+        AtomicReference<QNode> head = queueFor(phase);
+        QNode q;
+        while ((q = head.get()) != null) {
+            if (head.compareAndSet(q, q.next))
+                q.signal();
+        }
+    }
+
+    /**
      * Enqueues node and waits unless aborted or signalled.
      */
-    private boolean untimedWait(Thread thread, int currentPhase, 
-                               boolean abortOnInterrupt) {
-        final AtomicReference<QNode> head = this.head;
-        final AtomicLong state = this.state;
-        boolean wasInterrupted = false;
+    private int untimedWait(int phase) {
+        int spins = maxUntimedSpins;
         QNode node = null;
+        boolean interrupted = false;
         boolean queued = false;
-        int spins = maxUntimedSpins;
-        while (phaseOf(state.get()) == currentPhase) {
-            QNode h;
-            if (node != null && queued) {
-                if (node.thread != null) {
-                    LockSupport.park();
-                    if (Thread.interrupted()) {
-                        wasInterrupted = true;
-                        if (abortOnInterrupt) 
-                            break;
-                    }
+        int p;
+        while ((p = getPhase()) == phase) {
+            interrupted = Thread.interrupted();
+            if (node != null) {
+                if (!queued) {
+                    AtomicReference<QNode> head = queueFor(phase);
+                    queued = head.compareAndSet(node.next = head.get(), node);
                 }
+                else if (node.thread != null)
+                    LockSupport.park(this);
             }
-            else if ((h = head.get()) != null && h.phase != currentPhase) {
-                if (phaseOf(state.get()) == currentPhase) { // must recheck
-                    if (head.compareAndSet(h, h.next)) {
-                        Thread t = h.thread; // help clear out old waiters
-                        if (t != null) {
-                            h.thread = null;
-                            LockSupport.unpark(t);
-                        }
-                    }
+            else if (spins <= 0)
+                node = new QNode();
+            else
+                --spins;
+        }
+        if (node != null)
+            node.thread = null;
+        if (interrupted)
+            Thread.currentThread().interrupt();
+        releaseWaiters(phase);
+        return p;
+    }
+
+    /**
+     * Messier interruptible version
+     */
+    private int interruptibleWait(int phase) throws InterruptedException {
+        int spins = maxUntimedSpins;
+        QNode node = null;
+        boolean queued = false;
+        boolean interrupted = false;
+        int p;
+        while ((p = getPhase()) == phase) {
+            if (interrupted = Thread.interrupted())
+                break;
+            if (node != null) {
+                if (!queued) {
+                    AtomicReference<QNode> head = queueFor(phase);
+                    queued = head.compareAndSet(node.next = head.get(), node);
                 }
-                else
-                    break;
+                else if (node.thread != null)
+                    LockSupport.park(this);
             }
-            else if (node != null)
-                queued = head.compareAndSet(node.next = h, node);
             else if (spins <= 0)
-                node = new QNode(thread, currentPhase);
+                node = new QNode();
             else
                 --spins;
         }
         if (node != null)
             node.thread = null;
-        return wasInterrupted;
+        if (interrupted)
+            throw new InterruptedException();
+        releaseWaiters(phase);
+        return p;
     }
 
     /**
-     * Messier timeout version
+     * Even messier timeout version.
      */
-    private void timedWait(Thread thread, int currentPhase, long nanos) 
+    private int timedWait(int phase, long nanos)
         throws InterruptedException, TimeoutException {
-        final AtomicReference<QNode> head = this.head;
-        final AtomicLong state = this.state;
-        long lastTime = System.nanoTime();
-        QNode node = null;
-        boolean queued = false;
-        int spins = maxTimedSpins;
-        while (phaseOf(state.get()) == currentPhase) {
-            QNode h;
-            long now = System.nanoTime();
-            nanos -= now - lastTime;
-            lastTime = now;
-            if (nanos <= 0) {
-                if (node != null)
-                    node.thread = null;
-                if (phaseOf(state.get()) == currentPhase)
-                    throw new TimeoutException();
-                else
+        int p;
+        if ((p = getPhase()) == phase) {
+            long lastTime = System.nanoTime();
+            int spins = maxTimedSpins;
+            QNode node = null;
+            boolean queued = false;
+            boolean interrupted = false;
+            while ((p = getPhase()) == phase) {
+                if (interrupted = Thread.interrupted())
                     break;
-            }
-            else if (node != null && queued) {
-                if (node.thread != null &&
-                    nanos > spinForTimeoutThreshold) {
-                    //                LockSupport.parkNanos(this, nanos);
-                    LockSupport.parkNanos(nanos);
-                    if (Thread.interrupted()) {
-                        node.thread = null;
-                        throw new InterruptedException();
+                long now = System.nanoTime();
+                if ((nanos -= now - lastTime) <= 0)
+                    break;
+                lastTime = now;
+                if (node != null) {
+                    if (!queued) {
+                        AtomicReference<QNode> head = queueFor(phase);
+                        queued = head.compareAndSet(node.next = head.get(), node);
                     }
-                }
-            }
-            else if ((h = head.get()) != null && h.phase != currentPhase) {
-                if (phaseOf(state.get()) == currentPhase) { // must recheck
-                    if (head.compareAndSet(h, h.next)) {
-                        Thread t = h.thread; // help clear out old waiters
-                        if (t != null) {
-                            h.thread = null;
-                            LockSupport.unpark(t);
-                        }
+                    else if (node.thread != null &&
+                             nanos > spinForTimeoutThreshold) {
+                        LockSupport.parkNanos(this, nanos);
                     }
                 }
+                else if (spins <= 0)
+                    node = new QNode();
                 else
-                    break;
+                    --spins;
+            }
+            if (node != null)
+                node.thread = null;
+            if (interrupted)
+                throw new InterruptedException();
+            if (p == phase && (p = getPhase()) == phase)
+                throw new TimeoutException();
+        }
+        releaseWaiters(phase);
+        return p;
+    }
+
+    // Temporary Unsafe mechanics for preliminary release
+
+    static final Unsafe _unsafe;
+    static final long stateOffset;
+
+    static {
+        try {
+            if (Phaser.class.getClassLoader() != null) {
+                Field f = Unsafe.class.getDeclaredField("theUnsafe");
+                f.setAccessible(true);
+                _unsafe = (Unsafe)f.get(null);
             }
-            else if (node != null)
-                queued = head.compareAndSet(node.next = h, node);
-            else if (spins <= 0)
-                node = new QNode(thread, currentPhase);
             else
-                --spins;
+                _unsafe = Unsafe.getUnsafe();
+            stateOffset = _unsafe.objectFieldOffset
+                (Phaser.class.getDeclaredField("state"));
+        } catch (Exception e) {
+            throw new RuntimeException("Could not initialize intrinsics", e);
         }
-        if (node != null)
-            node.thread = null;
     }
 
+    final boolean casState(long cmp, long val) {
+        return _unsafe.compareAndSwapLong(this, stateOffset, cmp, val);
+    }
 }
-