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From vfor...@apache.org
Subject [21/50] [abbrv] incubator-geode git commit: GEODE-378: use SynchronousQueue to prevent deadlock
Date Wed, 25 Nov 2015 19:07:34 GMT
GEODE-378: use SynchronousQueue to prevent deadlock

The SynchronousQueueNoSpin class has been completely removed.
The JDK SynchronousQueue is now used in its place.


Project: http://git-wip-us.apache.org/repos/asf/incubator-geode/repo
Commit: http://git-wip-us.apache.org/repos/asf/incubator-geode/commit/9e4a94e0
Tree: http://git-wip-us.apache.org/repos/asf/incubator-geode/tree/9e4a94e0
Diff: http://git-wip-us.apache.org/repos/asf/incubator-geode/diff/9e4a94e0

Branch: refs/heads/feature/GEODE-78
Commit: 9e4a94e06377094336cb31882b96e8aff2ebe8dd
Parents: 35d3f06
Author: Darrel Schneider <dschneider@pivotal.io>
Authored: Thu Oct 1 11:45:37 2015 -0700
Committer: Darrel Schneider <dschneider@pivotal.io>
Committed: Fri Oct 2 09:36:45 2015 -0700

----------------------------------------------------------------------
 .../FunctionExecutionPooledExecutor.java        |    7 +-
 .../internal/PooledExecutorWithDMStats.java     |    7 +-
 .../util/concurrent/SynchronousQueueNoSpin.java | 1144 ------------------
 3 files changed, 6 insertions(+), 1152 deletions(-)
----------------------------------------------------------------------


http://git-wip-us.apache.org/repos/asf/incubator-geode/blob/9e4a94e0/gemfire-core/src/main/java/com/gemstone/gemfire/distributed/internal/FunctionExecutionPooledExecutor.java
----------------------------------------------------------------------
diff --git a/gemfire-core/src/main/java/com/gemstone/gemfire/distributed/internal/FunctionExecutionPooledExecutor.java b/gemfire-core/src/main/java/com/gemstone/gemfire/distributed/internal/FunctionExecutionPooledExecutor.java
index 501b83c..698bc86 100644
--- a/gemfire-core/src/main/java/com/gemstone/gemfire/distributed/internal/FunctionExecutionPooledExecutor.java
+++ b/gemfire-core/src/main/java/com/gemstone/gemfire/distributed/internal/FunctionExecutionPooledExecutor.java
@@ -21,7 +21,6 @@ import com.gemstone.gemfire.i18n.LogWriterI18n;
 import com.gemstone.gemfire.internal.i18n.LocalizedStrings;
 import com.gemstone.gemfire.internal.logging.LocalLogWriter;
 import com.gemstone.gemfire.internal.logging.LogWriterImpl;
-import com.gemstone.java.util.concurrent.SynchronousQueueNoSpin;
 
 import java.util.List;
 
@@ -75,10 +74,10 @@ public class FunctionExecutionPooledExecutor extends ThreadPoolExecutor {
   Thread bufferConsumer;
   
   private static BlockingQueue<Runnable> initQ(BlockingQueue<Runnable> q) {
-    if (q instanceof SynchronousQueue || q instanceof SynchronousQueueNoSpin) {
+    if (q instanceof SynchronousQueue) {
       return q;
     } else {
-      return new SynchronousQueueNoSpin<Runnable>();
+      return new SynchronousQueue<Runnable>();
     }
   }
 
@@ -124,7 +123,7 @@ public class FunctionExecutionPooledExecutor extends ThreadPoolExecutor {
     }
     else {
 
-      if (q instanceof SynchronousQueue || q instanceof SynchronousQueueNoSpin) {
+      if (q instanceof SynchronousQueue) {
         return new CallerRunsPolicy();
         // return new BlockHandler();
       }

http://git-wip-us.apache.org/repos/asf/incubator-geode/blob/9e4a94e0/gemfire-core/src/main/java/com/gemstone/gemfire/distributed/internal/PooledExecutorWithDMStats.java
----------------------------------------------------------------------
diff --git a/gemfire-core/src/main/java/com/gemstone/gemfire/distributed/internal/PooledExecutorWithDMStats.java b/gemfire-core/src/main/java/com/gemstone/gemfire/distributed/internal/PooledExecutorWithDMStats.java
index 05f9dd5..a50e594 100644
--- a/gemfire-core/src/main/java/com/gemstone/gemfire/distributed/internal/PooledExecutorWithDMStats.java
+++ b/gemfire-core/src/main/java/com/gemstone/gemfire/distributed/internal/PooledExecutorWithDMStats.java
@@ -17,7 +17,6 @@ import java.util.concurrent.TimeUnit;
 import java.util.concurrent.ThreadFactory;
 import com.gemstone.gemfire.SystemFailure;
 import com.gemstone.gemfire.internal.i18n.LocalizedStrings;
-import com.gemstone.java.util.concurrent.SynchronousQueueNoSpin;
 
 import java.util.List;
 
@@ -55,15 +54,15 @@ public class PooledExecutorWithDMStats extends ThreadPoolExecutor {
   private Thread bufferConsumer;
   
   private static BlockingQueue<Runnable> initQ(BlockingQueue<Runnable> q) {
-    if (q instanceof SynchronousQueue || q instanceof SynchronousQueueNoSpin) {
+    if (q instanceof SynchronousQueue) {
       return q;
     } else {
-      return new SynchronousQueueNoSpin<Runnable>();
+      return new SynchronousQueue<Runnable>();
     }
   }
 
   private static RejectedExecutionHandler initREH(BlockingQueue<Runnable> q) {
-    if (q instanceof SynchronousQueue || q instanceof SynchronousQueueNoSpin) {
+    if (q instanceof SynchronousQueue) {
       return new CallerRunsPolicy();
       //return new BlockHandler();
     } else {

http://git-wip-us.apache.org/repos/asf/incubator-geode/blob/9e4a94e0/gemfire-core/src/main/java/com/gemstone/java/util/concurrent/SynchronousQueueNoSpin.java
----------------------------------------------------------------------
diff --git a/gemfire-core/src/main/java/com/gemstone/java/util/concurrent/SynchronousQueueNoSpin.java b/gemfire-core/src/main/java/com/gemstone/java/util/concurrent/SynchronousQueueNoSpin.java
deleted file mode 100644
index ca2c1d9..0000000
--- a/gemfire-core/src/main/java/com/gemstone/java/util/concurrent/SynchronousQueueNoSpin.java
+++ /dev/null
@@ -1,1144 +0,0 @@
-/*
- * Written by Doug Lea, Bill Scherer, and Michael Scott 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 com.gemstone.java.util.concurrent;
-import java.util.AbstractQueue;
-import java.util.Collection;
-import java.util.Collections;
-import java.util.Iterator;
-import java.util.concurrent.BlockingQueue;
-import java.util.concurrent.TimeUnit;
-import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
-import java.util.concurrent.locks.LockSupport;
-import java.util.concurrent.locks.ReentrantLock;
-
-/**
- * A {@linkplain BlockingQueue blocking queue} in which each insert
- * operation must wait for a corresponding remove operation by another
- * thread, and vice versa.  A synchronous queue does not have any
- * internal capacity, not even a capacity of one.  You cannot
- * <tt>peek</tt> at a synchronous queue because an element is only
- * present when you try to remove it; you cannot insert an element
- * (using any method) unless another thread is trying to remove it;
- * you cannot iterate as there is nothing to iterate.  The
- * <em>head</em> of the queue is the element that the first queued
- * inserting thread is trying to add to the queue; if there is no such
- * queued thread then no element is available for removal and
- * <tt>poll()</tt> will return <tt>null</tt>.  For purposes of other
- * <tt>Collection</tt> methods (for example <tt>contains</tt>), a
- * <tt>SynchronousQueue</tt> acts as an empty collection.  This queue
- * does not permit <tt>null</tt> elements.
- *
- * <p>Synchronous queues are similar to rendezvous channels used in
- * CSP and Ada. They are well suited for handoff designs, in which an
- * object running in one thread must sync up with an object running
- * in another thread in order to hand it some information, event, or
- * task.
- *
- * <p> This class supports an optional fairness policy for ordering
- * waiting producer and consumer threads.  By default, this ordering
- * is not guaranteed. However, a queue constructed with fairness set
- * to <tt>true</tt> grants threads access in FIFO order.
- *
- * <p>This class and its iterator implement all of the
- * <em>optional</em> methods of the {@link Collection} and {@link
- * Iterator} interfaces.
- *
- * <p>This class is a member of the
- * <a href="{@docRoot}/../technotes/guides/collections/index.html">
- * Java Collections Framework</a>.
- *
- * @since 1.5
- * @author Doug Lea and Bill Scherer and Michael Scott
- * @param <E> the type of elements held in this collection
- * 
- * GemstoneAddition
- * ------------------
- * This version was taken from the jsr166 CVS repository
- * http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/main/java/util/concurrent/SynchronousQueue.java revision 1.68
- * 
- * The JSR version is identical java 1.6_10 version except for formatting
- * and licensing.
- * 
- * It has been modified by gemstone to remove the spin waiting which is hard coded
- * into the java 1.6 version. We found through performance testing
- * that this spin waiting was causing up to a 50% throughput decrease
- * for some of our perf tests (eg perf081).
- * 
- * In addition, the calls to LockSupport.park(Object) have been changed
- * to LockSupport.park() to compile with java 1.5. 
- */
-public class SynchronousQueueNoSpin<E> extends AbstractQueue<E>
-    implements BlockingQueue<E>, java.io.Serializable {
-    private static final long serialVersionUID = -3223113410248163686L;
-
-    /*
-     * This class implements extensions of the dual stack and dual
-     * queue algorithms described in "Nonblocking Concurrent Objects
-     * with Condition Synchronization", by W. N. Scherer III and
-     * M. L. Scott.  18th Annual Conf. on Distributed Computing,
-     * Oct. 2004 (see also
-     * http://www.cs.rochester.edu/u/scott/synchronization/pseudocode/duals.html).
-     * The (Lifo) stack is used for non-fair mode, and the (Fifo)
-     * queue for fair mode. The performance of the two is generally
-     * similar. Fifo usually supports higher throughput under
-     * contention but Lifo maintains higher thread locality in common
-     * applications.
-     *
-     * A dual queue (and similarly stack) is one that at any given
-     * time either holds "data" -- items provided by put operations,
-     * or "requests" -- slots representing take operations, or is
-     * empty. A call to "fulfill" (i.e., a call requesting an item
-     * from a queue holding data or vice versa) dequeues a
-     * complementary node.  The most interesting feature of these
-     * queues is that any operation can figure out which mode the
-     * queue is in, and act accordingly without needing locks.
-     *
-     * Both the queue and stack extend abstract class Transferer
-     * defining the single method transfer that does a put or a
-     * take. These are unified into a single method because in dual
-     * data structures, the put and take operations are symmetrical,
-     * so nearly all code can be combined. The resulting transfer
-     * methods are on the long side, but are easier to follow than
-     * they would be if broken up into nearly-duplicated parts.
-     *
-     * The queue and stack data structures share many conceptual
-     * similarities but very few concrete details. For simplicity,
-     * they are kept distinct so that they can later evolve
-     * separately.
-     *
-     * The algorithms here differ from the versions in the above paper
-     * in extending them for use in synchronous queues, as well as
-     * dealing with cancellation. The main differences include:
-     *
-     *  1. The original algorithms used bit-marked pointers, but
-     *     the ones here use mode bits in nodes, leading to a number
-     *     of further adaptations.
-     *  2. SynchronousQueues must block threads waiting to become
-     *     fulfilled.
-     *  3. Support for cancellation via timeout and interrupts,
-     *     including cleaning out cancelled nodes/threads
-     *     from lists to avoid garbage retention and memory depletion.
-     *
-     * Blocking is mainly accomplished using LockSupport park/unpark,
-     * except that nodes that appear to be the next ones to become
-     * fulfilled first spin a bit (on multiprocessors only). On very
-     * busy synchronous queues, spinning can dramatically improve
-     * throughput. And on less busy ones, the amount of spinning is
-     * small enough not to be noticeable.
-     *
-     * Cleaning is done in different ways in queues vs stacks.  For
-     * queues, we can almost always remove a node immediately in O(1)
-     * time (modulo retries for consistency checks) when it is
-     * cancelled. But if it may be pinned as the current tail, it must
-     * wait until some subsequent cancellation. For stacks, we need a
-     * potentially O(n) traversal to be sure that we can remove the
-     * node, but this can run concurrently with other threads
-     * accessing the stack.
-     *
-     * While garbage collection takes care of most node reclamation
-     * issues that otherwise complicate nonblocking algorithms, care
-     * is taken to "forget" references to data, other nodes, and
-     * threads that might be held on to long-term by blocked
-     * threads. In cases where setting to null would otherwise
-     * conflict with main algorithms, this is done by changing a
-     * node's link to now point to the node itself. This doesn't arise
-     * much for Stack nodes (because blocked threads do not hang on to
-     * old head pointers), but references in Queue nodes must be
-     * aggressively forgotten to avoid reachability of everything any
-     * node has ever referred to since arrival.
-     */
-
-    /**
-     * Shared internal API for dual stacks and queues.
-     */
-    static abstract class Transferer {
-        /**
-         * Performs a put or take.
-         *
-         * @param e if non-null, the item to be handed to a consumer;
-         *          if null, requests that transfer return an item
-         *          offered by producer.
-         * @param timed if this operation should timeout
-         * @param nanos the timeout, in nanoseconds
-         * @return if non-null, the item provided or received; if null,
-         *         the operation failed due to timeout or interrupt --
-         *         the caller can distinguish which of these occurred
-         *         by checking Thread.interrupted.
-         */
-        abstract Object transfer(Object e, boolean timed, long nanos);
-    }
-
-    /** 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 -- it works well across a
-     * variety of processors and OSes. Empirically, the best value
-     * seems not to vary with number of CPUs (beyond 2) so is just
-     * a constant.
-     */
-    //@GemstoneAddition
-    //static final int maxTimedSpins = (NCPUS < 2)? 0 : 32;
-    static final int maxTimedSpins = 0;
-
-    /**
-     * The number of times to spin before blocking in untimed waits.
-     * This is greater than timed value because untimed waits spin
-     * faster since they don't need to check times on each spin.
-     */
-    static final int maxUntimedSpins = maxTimedSpins * 16;
-
-    /**
-     * The number of nanoseconds for which it is faster to spin
-     * rather than to use timed park. A rough estimate suffices.
-     */
-    //@GemstoneAddition
-    //static final long spinForTimeoutThreshold = 1000L;
-    static final long spinForTimeoutThreshold = 0L;
-
-    /** Dual stack */
-    static final class TransferStack extends Transferer {
-        /*
-         * This extends Scherer-Scott dual stack algorithm, differing,
-         * among other ways, by using "covering" nodes rather than
-         * bit-marked pointers: Fulfilling operations push on marker
-         * nodes (with FULFILLING bit set in mode) to reserve a spot
-         * to match a waiting node.
-         */
-
-        /* Modes for SNodes, ORed together in node fields */
-        /** Node represents an unfulfilled consumer */
-        static final int REQUEST    = 0;
-        /** Node represents an unfulfilled producer */
-        static final int DATA       = 1;
-        /** Node is fulfilling another unfulfilled DATA or REQUEST */
-        static final int FULFILLING = 2;
-
-        /** Return true if m has fulfilling bit set */
-        static boolean isFulfilling(int m) { return (m & FULFILLING) != 0; }
-
-        /** Node class for TransferStacks. */
-        static final class SNode {
-            volatile SNode next;        // next node in stack
-            volatile SNode match;       // the node matched to this
-            volatile Thread waiter;     // to control park/unpark
-            Object item;                // data; or null for REQUESTs
-            int mode;
-            // Note: item and mode fields don't need to be volatile
-            // since they are always written before, and read after,
-            // other volatile/atomic operations.
-
-            SNode(Object item) {
-                this.item = item;
-            }
-
-            static final AtomicReferenceFieldUpdater<SNode, SNode>
-                nextUpdater = AtomicReferenceFieldUpdater.newUpdater
-                (SNode.class, SNode.class, "next");
-
-            boolean casNext(SNode cmp, SNode val) {
-                return (cmp == next &&
-                        nextUpdater.compareAndSet(this, cmp, val));
-            }
-
-            static final AtomicReferenceFieldUpdater<SNode, SNode>
-                matchUpdater = AtomicReferenceFieldUpdater.newUpdater
-                (SNode.class, SNode.class, "match");
-
-            /**
-             * Tries to match node s to this node, if so, waking up thread.
-             * Fulfillers call tryMatch to identify their waiters.
-             * Waiters block until they have been matched.
-             *
-             * @param s the node to match
-             * @return true if successfully matched to s
-             */
-            boolean tryMatch(SNode s) {
-                if (match == null &&
-                    matchUpdater.compareAndSet(this, null, s)) {
-                    Thread w = waiter;
-                    if (w != null) {    // waiters need at most one unpark
-                        waiter = null;
-                        LockSupport.unpark(w);
-                    }
-                    return true;
-                }
-                return match == s;
-            }
-
-            /**
-             * Tries to cancel a wait by matching node to itself.
-             */
-            void tryCancel() {
-                matchUpdater.compareAndSet(this, null, this);
-            }
-
-            boolean isCancelled() {
-                return match == this;
-            }
-        }
-
-        /** The head (top) of the stack */
-        volatile SNode head;
-
-        static final AtomicReferenceFieldUpdater<TransferStack, SNode>
-            headUpdater = AtomicReferenceFieldUpdater.newUpdater
-            (TransferStack.class,  SNode.class, "head");
-
-        boolean casHead(SNode h, SNode nh) {
-            return h == head && headUpdater.compareAndSet(this, h, nh);
-        }
-
-        /**
-         * Creates or resets fields of a node. Called only from transfer
-         * where the node to push on stack is lazily created and
-         * reused when possible to help reduce intervals between reads
-         * and CASes of head and to avoid surges of garbage when CASes
-         * to push nodes fail due to contention.
-         */
-        static SNode snode(SNode s, Object e, SNode next, int mode) {
-            if (s == null) s = new SNode(e);
-            s.mode = mode;
-            s.next = next;
-            return s;
-        }
-
-        /**
-         * Puts or takes an item.
-         */
-        Object transfer(Object e, boolean timed, long nanos) {
-            /*
-             * Basic algorithm is to loop trying one of three actions:
-             *
-             * 1. If apparently empty or already containing nodes of same
-             *    mode, try to push node on stack and wait for a match,
-             *    returning it, or null if cancelled.
-             *
-             * 2. If apparently containing node of complementary mode,
-             *    try to push a fulfilling node on to stack, match
-             *    with corresponding waiting node, pop both from
-             *    stack, and return matched item. The matching or
-             *    unlinking might not actually be necessary because of
-             *    other threads performing action 3:
-             *
-             * 3. If top of stack already holds another fulfilling node,
-             *    help it out by doing its match and/or pop
-             *    operations, and then continue. The code for helping
-             *    is essentially the same as for fulfilling, except
-             *    that it doesn't return the item.
-             */
-
-            SNode s = null; // constructed/reused as needed
-            int mode = (e == null)? REQUEST : DATA;
-
-            for (;;) {
-                SNode h = head;
-                if (h == null || h.mode == mode) {  // empty or same-mode
-                    if (timed && nanos <= 0) {      // can't wait
-                        if (h != null && h.isCancelled())
-                            casHead(h, h.next);     // pop cancelled node
-                        else
-                            return null;
-                    } else if (casHead(h, s = snode(s, e, h, mode))) {
-                        SNode m = awaitFulfill(s, timed, nanos);
-                        if (m == s) {               // wait was cancelled
-                            clean(s);
-                            return null;
-                        }
-                        if ((h = head) != null && h.next == s)
-                            casHead(h, s.next);     // help s's fulfiller
-                        return mode == REQUEST? m.item : s.item;
-                    }
-                } else if (!isFulfilling(h.mode)) { // try to fulfill
-                    if (h.isCancelled())            // already cancelled
-                        casHead(h, h.next);         // pop and retry
-                    else if (casHead(h, s=snode(s, e, h, FULFILLING|mode))) {
-                        for (;;) { // loop until matched or waiters disappear
-                            SNode m = s.next;       // m is s's match
-                            if (m == null) {        // all waiters are gone
-                                casHead(s, null);   // pop fulfill node
-                                s = null;           // use new node next time
-                                break;              // restart main loop
-                            }
-                            SNode mn = m.next;
-                            if (m.tryMatch(s)) {
-                                casHead(s, mn);     // pop both s and m
-                                return (mode == REQUEST)? m.item : s.item;
-                            } else                  // lost match
-                                s.casNext(m, mn);   // help unlink
-                        }
-                    }
-                } else {                            // help a fulfiller
-                    SNode m = h.next;               // m is h's match
-                    if (m == null)                  // waiter is gone
-                        casHead(h, null);           // pop fulfilling node
-                    else {
-                        SNode mn = m.next;
-                        if (m.tryMatch(h))          // help match
-                            casHead(h, mn);         // pop both h and m
-                        else                        // lost match
-                            h.casNext(m, mn);       // help unlink
-                    }
-                }
-            }
-        }
-
-        /**
-         * Spins/blocks until node s is matched by a fulfill operation.
-         *
-         * @param s the waiting node
-         * @param timed true if timed wait
-         * @param nanos timeout value
-         * @return matched node, or s if cancelled
-         */
-        SNode awaitFulfill(SNode s, boolean timed, long nanos) {
-            /*
-             * When a node/thread is about to block, it sets its waiter
-             * field and then rechecks state at least one more time
-             * before actually parking, thus covering race vs
-             * fulfiller noticing that waiter is non-null so should be
-             * woken.
-             *
-             * When invoked by nodes that appear at the point of call
-             * to be at the head of the stack, calls to park are
-             * preceded by spins to avoid blocking when producers and
-             * consumers are arriving very close in time.  This can
-             * happen enough to bother only on multiprocessors.
-             *
-             * The order of checks for returning out of main loop
-             * reflects fact that interrupts have precedence over
-             * normal returns, which have precedence over
-             * timeouts. (So, on timeout, one last check for match is
-             * done before giving up.) Except that calls from untimed
-             * SynchronousQueue.{poll/offer} don't check interrupts
-             * and don't wait at all, so are trapped in transfer
-             * method rather than calling awaitFulfill.
-             */
-            long lastTime = (timed)? System.nanoTime() : 0;
-            Thread w = Thread.currentThread();
-            SNode h = head;
-            int spins = (shouldSpin(s)?
-                         (timed? maxTimedSpins : maxUntimedSpins) : 0);
-            for (;;) {
-                if (w.isInterrupted())
-                    s.tryCancel();
-                SNode m = s.match;
-                if (m != null)
-                    return m;
-                if (timed) {
-                    long now = System.nanoTime();
-                    nanos -= now - lastTime;
-                    lastTime = now;
-                    if (nanos <= 0) {
-                        s.tryCancel();
-                        continue;
-                    }
-                }
-                if (spins > 0)
-                    spins = shouldSpin(s)? (spins-1) : 0;
-                else if (s.waiter == null)
-                    s.waiter = w; // establish waiter so can park next iter
-                else if (!timed)
-                    LockSupport.park();
-                else if (nanos > spinForTimeoutThreshold)
-                    LockSupport.parkNanos(nanos);
-            }
-        }
-
-        /**
-         * Returns true if node s is at head or there is an active
-         * fulfiller.
-         */
-        boolean shouldSpin(SNode s) {
-            SNode h = head;
-            return (h == s || h == null || isFulfilling(h.mode));
-        }
-
-        /**
-         * Unlinks s from the stack.
-         */
-        void clean(SNode s) {
-            s.item = null;   // forget item
-            s.waiter = null; // forget thread
-
-            /*
-             * At worst we may need to traverse entire stack to unlink
-             * s. If there are multiple concurrent calls to clean, we
-             * might not see s if another thread has already removed
-             * it. But we can stop when we see any node known to
-             * follow s. We use s.next unless it too is cancelled, in
-             * which case we try the node one past. We don't check any
-             * further because we don't want to doubly traverse just to
-             * find sentinel.
-             */
-
-            SNode past = s.next;
-            if (past != null && past.isCancelled())
-                past = past.next;
-
-            // Absorb cancelled nodes at head
-            SNode p;
-            while ((p = head) != null && p != past && p.isCancelled())
-                casHead(p, p.next);
-
-            // Unsplice embedded nodes
-            while (p != null && p != past) {
-                SNode n = p.next;
-                if (n != null && n.isCancelled())
-                    p.casNext(n, n.next);
-                else
-                    p = n;
-            }
-        }
-    }
-
-    /** Dual Queue */
-    static final class TransferQueue extends Transferer {
-        /*
-         * This extends Scherer-Scott dual queue algorithm, differing,
-         * among other ways, by using modes within nodes rather than
-         * marked pointers. The algorithm is a little simpler than
-         * that for stacks because fulfillers do not need explicit
-         * nodes, and matching is done by CAS'ing QNode.item field
-         * from non-null to null (for put) or vice versa (for take).
-         */
-
-        /** Node class for TransferQueue. */
-        static final class QNode {
-            volatile QNode next;          // next node in queue
-            volatile Object item;         // CAS'ed to or from null
-            volatile Thread waiter;       // to control park/unpark
-            final boolean isData;
-
-            QNode(Object item, boolean isData) {
-                this.item = item;
-                this.isData = isData;
-            }
-
-            static final AtomicReferenceFieldUpdater<QNode, QNode>
-                nextUpdater = AtomicReferenceFieldUpdater.newUpdater
-                (QNode.class, QNode.class, "next");
-
-            boolean casNext(QNode cmp, QNode val) {
-                return (next == cmp &&
-                        nextUpdater.compareAndSet(this, cmp, val));
-            }
-
-            static final AtomicReferenceFieldUpdater<QNode, Object>
-                itemUpdater = AtomicReferenceFieldUpdater.newUpdater
-                (QNode.class, Object.class, "item");
-
-            boolean casItem(Object cmp, Object val) {
-                return (item == cmp &&
-                        itemUpdater.compareAndSet(this, cmp, val));
-            }
-
-            /**
-             * Tries to cancel by CAS'ing ref to this as item.
-             */
-            void tryCancel(Object cmp) {
-                itemUpdater.compareAndSet(this, cmp, this);
-            }
-
-            boolean isCancelled() {
-                return item == this;
-            }
-
-            /**
-             * Returns true if this node is known to be off the queue
-             * because its next pointer has been forgotten due to
-             * an advanceHead operation.
-             */
-            boolean isOffList() {
-                return next == this;
-            }
-        }
-
-        /** Head of queue */
-        transient volatile QNode head;
-        /** Tail of queue */
-        transient volatile QNode tail;
-        /**
-         * Reference to a cancelled node that might not yet have been
-         * unlinked from queue because it was the last inserted node
-         * when it cancelled.
-         */
-        transient volatile QNode cleanMe;
-
-        TransferQueue() {
-            QNode h = new QNode(null, false); // initialize to dummy node.
-            head = h;
-            tail = h;
-        }
-
-        static final AtomicReferenceFieldUpdater<TransferQueue, QNode>
-            headUpdater = AtomicReferenceFieldUpdater.newUpdater
-            (TransferQueue.class,  QNode.class, "head");
-
-        /**
-         * Tries to cas nh as new head; if successful, unlink
-         * old head's next node to avoid garbage retention.
-         */
-        void advanceHead(QNode h, QNode nh) {
-            if (h == head && headUpdater.compareAndSet(this, h, nh))
-                h.next = h; // forget old next
-        }
-
-        static final AtomicReferenceFieldUpdater<TransferQueue, QNode>
-            tailUpdater = AtomicReferenceFieldUpdater.newUpdater
-            (TransferQueue.class, QNode.class, "tail");
-
-        /**
-         * Tries to cas nt as new tail.
-         */
-        void advanceTail(QNode t, QNode nt) {
-            if (tail == t)
-                tailUpdater.compareAndSet(this, t, nt);
-        }
-
-        static final AtomicReferenceFieldUpdater<TransferQueue, QNode>
-            cleanMeUpdater = AtomicReferenceFieldUpdater.newUpdater
-            (TransferQueue.class, QNode.class, "cleanMe");
-
-        /**
-         * Tries to CAS cleanMe slot.
-         */
-        boolean casCleanMe(QNode cmp, QNode val) {
-            return (cleanMe == cmp &&
-                    cleanMeUpdater.compareAndSet(this, cmp, val));
-        }
-
-        /**
-         * Puts or takes an item.
-         */
-        Object transfer(Object e, boolean timed, long nanos) {
-            /* Basic algorithm is to loop trying to take either of
-             * two actions:
-             *
-             * 1. If queue apparently empty or holding same-mode nodes,
-             *    try to add node to queue of waiters, wait to be
-             *    fulfilled (or cancelled) and return matching item.
-             *
-             * 2. If queue apparently contains waiting items, and this
-             *    call is of complementary mode, try to fulfill by CAS'ing
-             *    item field of waiting node and dequeuing it, and then
-             *    returning matching item.
-             *
-             * In each case, along the way, check for and try to help
-             * advance head and tail on behalf of other stalled/slow
-             * threads.
-             *
-             * The loop starts off with a null check guarding against
-             * seeing uninitialized head or tail values. This never
-             * happens in current SynchronousQueue, but could if
-             * callers held non-volatile/final ref to the
-             * transferer. The check is here anyway because it places
-             * null checks at top of loop, which is usually faster
-             * than having them implicitly interspersed.
-             */
-
-            QNode s = null; // constructed/reused as needed
-            boolean isData = (e != null);
-
-            for (;;) {
-                QNode t = tail;
-                QNode h = head;
-                if (t == null || h == null)         // saw uninitialized value
-                    continue;                       // spin
-
-                if (h == t || t.isData == isData) { // empty or same-mode
-                    QNode tn = t.next;
-                    if (t != tail)                  // inconsistent read
-                        continue;
-                    if (tn != null) {               // lagging tail
-                        advanceTail(t, tn);
-                        continue;
-                    }
-                    if (timed && nanos <= 0)        // can't wait
-                        return null;
-                    if (s == null)
-                        s = new QNode(e, isData);
-                    if (!t.casNext(null, s))        // failed to link in
-                        continue;
-
-                    advanceTail(t, s);              // swing tail and wait
-                    Object x = awaitFulfill(s, e, timed, nanos);
-                    if (x == s) {                   // wait was cancelled
-                        clean(t, s);
-                        return null;
-                    }
-
-                    if (!s.isOffList()) {           // not already unlinked
-                        advanceHead(t, s);          // unlink if head
-                        if (x != null)              // and forget fields
-                            s.item = s;
-                        s.waiter = null;
-                    }
-                    return (x != null)? x : e;
-
-                } else {                            // complementary-mode
-                    QNode m = h.next;               // node to fulfill
-                    if (t != tail || m == null || h != head)
-                        continue;                   // inconsistent read
-
-                    Object x = m.item;
-                    if (isData == (x != null) ||    // m already fulfilled
-                        x == m ||                   // m cancelled
-                        !m.casItem(x, e)) {         // lost CAS
-                        advanceHead(h, m);          // dequeue and retry
-                        continue;
-                    }
-
-                    advanceHead(h, m);              // successfully fulfilled
-                    LockSupport.unpark(m.waiter);
-                    return (x != null)? x : e;
-                }
-            }
-        }
-
-        /**
-         * Spins/blocks until node s is fulfilled.
-         *
-         * @param s the waiting node
-         * @param e the comparison value for checking match
-         * @param timed true if timed wait
-         * @param nanos timeout value
-         * @return matched item, or s if cancelled
-         */
-        Object awaitFulfill(QNode s, Object e, boolean timed, long nanos) {
-            /* Same idea as TransferStack.awaitFulfill */
-            long lastTime = (timed)? System.nanoTime() : 0;
-            Thread w = Thread.currentThread();
-            int spins = ((head.next == s) ?
-                         (timed? maxTimedSpins : maxUntimedSpins) : 0);
-            for (;;) {
-                if (w.isInterrupted())
-                    s.tryCancel(e);
-                Object x = s.item;
-                if (x != e)
-                    return x;
-                if (timed) {
-                    long now = System.nanoTime();
-                    nanos -= now - lastTime;
-                    lastTime = now;
-                    if (nanos <= 0) {
-                        s.tryCancel(e);
-                        continue;
-                    }
-                }
-                if (spins > 0)
-                    --spins;
-                else if (s.waiter == null)
-                    s.waiter = w;
-                else if (!timed)
-                    LockSupport.park();
-                else if (nanos > spinForTimeoutThreshold)
-                    LockSupport.parkNanos(nanos);
-            }
-        }
-
-        /**
-         * Gets rid of cancelled node s with original predecessor pred.
-         */
-        void clean(QNode pred, QNode s) {
-            s.waiter = null; // forget thread
-            /*
-             * At any given time, exactly one node on list cannot be
-             * deleted -- the last inserted node. To accommodate this,
-             * if we cannot delete s, we save its predecessor as
-             * "cleanMe", deleting the previously saved version
-             * first. At least one of node s or the node previously
-             * saved can always be deleted, so this always terminates.
-             */
-            while (pred.next == s) { // Return early if already unlinked
-                QNode h = head;
-                QNode hn = h.next;   // Absorb cancelled first node as head
-                if (hn != null && hn.isCancelled()) {
-                    advanceHead(h, hn);
-                    continue;
-                }
-                QNode t = tail;      // Ensure consistent read for tail
-                if (t == h)
-                    return;
-                QNode tn = t.next;
-                if (t != tail)
-                    continue;
-                if (tn != null) {
-                    advanceTail(t, tn);
-                    continue;
-                }
-                if (s != t) {        // If not tail, try to unsplice
-                    QNode sn = s.next;
-                    if (sn == s || pred.casNext(s, sn))
-                        return;
-                }
-                QNode dp = cleanMe;
-                if (dp != null) {    // Try unlinking previous cancelled node
-                    QNode d = dp.next;
-                    QNode dn;
-                    if (d == null ||               // d is gone or
-                        d == dp ||                 // d is off list or
-                        !d.isCancelled() ||        // d not cancelled or
-                        (d != t &&                 // d not tail and
-                         (dn = d.next) != null &&  //   has successor
-                         dn != d &&                //   that is on list
-                         dp.casNext(d, dn)))       // d unspliced
-                        casCleanMe(dp, null);
-                    if (dp == pred)
-                        return;      // s is already saved node
-                } else if (casCleanMe(null, pred))
-                    return;          // Postpone cleaning s
-            }
-        }
-    }
-
-    /**
-     * The transferer. Set only in constructor, but cannot be declared
-     * as final without further complicating serialization.  Since
-     * this is accessed only at most once per public method, there
-     * isn't a noticeable performance penalty for using volatile
-     * instead of final here.
-     */
-    private transient volatile Transferer transferer;
-
-    /**
-     * Creates a <tt>SynchronousQueue</tt> with nonfair access policy.
-     */
-    public SynchronousQueueNoSpin() {
-        this(false);
-    }
-
-    /**
-     * Creates a <tt>SynchronousQueue</tt> with the specified fairness policy.
-     *
-     * @param fair if true, waiting threads contend in FIFO order for
-     *        access; otherwise the order is unspecified.
-     */
-    public SynchronousQueueNoSpin(boolean fair) {
-        transferer = (fair)? new TransferQueue() : new TransferStack();
-    }
-
-    /**
-     * Adds the specified element to this queue, waiting if necessary for
-     * another thread to receive it.
-     *
-     * @throws InterruptedException {@inheritDoc}
-     * @throws NullPointerException {@inheritDoc}
-     */
-    public void put(E o) throws InterruptedException {
-        if (o == null) throw new NullPointerException();
-        if (transferer.transfer(o, false, 0) == null) {
-            Thread.interrupted();
-            throw new InterruptedException();
-        }
-    }
-
-    /**
-     * Inserts the specified element into this queue, waiting if necessary
-     * up to the specified wait time for another thread to receive it.
-     *
-     * @return <tt>true</tt> if successful, or <tt>false</tt> if the
-     *         specified waiting time elapses before a consumer appears.
-     * @throws InterruptedException {@inheritDoc}
-     * @throws NullPointerException {@inheritDoc}
-     */
-    public boolean offer(E o, long timeout, TimeUnit unit)
-        throws InterruptedException {
-        if (o == null) throw new NullPointerException();
-        if (transferer.transfer(o, true, unit.toNanos(timeout)) != null)
-            return true;
-        if (!Thread.interrupted())
-            return false;
-        throw new InterruptedException();
-    }
-
-    /**
-     * Inserts the specified element into this queue, if another thread is
-     * waiting to receive it.
-     *
-     * @param e the element to add
-     * @return <tt>true</tt> if the element was added to this queue, else
-     *         <tt>false</tt>
-     * @throws NullPointerException if the specified element is null
-     */
-    public boolean offer(E e) {
-        if (e == null) throw new NullPointerException();
-        return transferer.transfer(e, true, 0) != null;
-    }
-
-    /**
-     * Retrieves and removes the head of this queue, waiting if necessary
-     * for another thread to insert it.
-     *
-     * @return the head of this queue
-     * @throws InterruptedException {@inheritDoc}
-     */
-    public E take() throws InterruptedException {
-        Object e = transferer.transfer(null, false, 0);
-        if (e != null)
-            return (E)e;
-        Thread.interrupted();
-        throw new InterruptedException();
-    }
-
-    /**
-     * Retrieves and removes the head of this queue, waiting
-     * if necessary up to the specified wait time, for another thread
-     * to insert it.
-     *
-     * @return the head of this queue, or <tt>null</tt> if the
-     *         specified waiting time elapses before an element is present.
-     * @throws InterruptedException {@inheritDoc}
-     */
-    public E poll(long timeout, TimeUnit unit) throws InterruptedException {
-        Object e = transferer.transfer(null, true, unit.toNanos(timeout));
-        if (e != null || !Thread.interrupted())
-            return (E)e;
-        throw new InterruptedException();
-    }
-
-    /**
-     * Retrieves and removes the head of this queue, if another thread
-     * is currently making an element available.
-     *
-     * @return the head of this queue, or <tt>null</tt> if no
-     *         element is available.
-     */
-    public E poll() {
-        return (E)transferer.transfer(null, true, 0);
-    }
-
-    /**
-     * Always returns <tt>true</tt>.
-     * A <tt>SynchronousQueue</tt> has no internal capacity.
-     *
-     * @return <tt>true</tt>
-     */
-    public boolean isEmpty() {
-        return true;
-    }
-
-    /**
-     * Always returns zero.
-     * A <tt>SynchronousQueue</tt> has no internal capacity.
-     *
-     * @return zero.
-     */
-    public int size() {
-        return 0;
-    }
-
-    /**
-     * Always returns zero.
-     * A <tt>SynchronousQueue</tt> has no internal capacity.
-     *
-     * @return zero.
-     */
-    public int remainingCapacity() {
-        return 0;
-    }
-
-    /**
-     * Does nothing.
-     * A <tt>SynchronousQueue</tt> has no internal capacity.
-     */
-    public void clear() {
-    }
-
-    /**
-     * Always returns <tt>false</tt>.
-     * A <tt>SynchronousQueue</tt> has no internal capacity.
-     *
-     * @param o the element
-     * @return <tt>false</tt>
-     */
-    public boolean contains(Object o) {
-        return false;
-    }
-
-    /**
-     * Always returns <tt>false</tt>.
-     * A <tt>SynchronousQueue</tt> has no internal capacity.
-     *
-     * @param o the element to remove
-     * @return <tt>false</tt>
-     */
-    public boolean remove(Object o) {
-        return false;
-    }
-
-    /**
-     * Returns <tt>false</tt> unless the given collection is empty.
-     * A <tt>SynchronousQueue</tt> has no internal capacity.
-     *
-     * @param c the collection
-     * @return <tt>false</tt> unless given collection is empty
-     */
-    public boolean containsAll(Collection<?> c) {
-        return c.isEmpty();
-    }
-
-    /**
-     * Always returns <tt>false</tt>.
-     * A <tt>SynchronousQueue</tt> has no internal capacity.
-     *
-     * @param c the collection
-     * @return <tt>false</tt>
-     */
-    public boolean removeAll(Collection<?> c) {
-        return false;
-    }
-
-    /**
-     * Always returns <tt>false</tt>.
-     * A <tt>SynchronousQueue</tt> has no internal capacity.
-     *
-     * @param c the collection
-     * @return <tt>false</tt>
-     */
-    public boolean retainAll(Collection<?> c) {
-        return false;
-    }
-
-    /**
-     * Always returns <tt>null</tt>.
-     * A <tt>SynchronousQueue</tt> does not return elements
-     * unless actively waited on.
-     *
-     * @return <tt>null</tt>
-     */
-    public E peek() {
-        return null;
-    }
-
-    /**
-     * Returns an empty iterator in which <tt>hasNext</tt> always returns
-     * <tt>false</tt>.
-     *
-     * @return an empty iterator
-     */
-    public Iterator<E> iterator() {
-        return Collections.<E>emptyList().iterator();
-    }
-
-    /**
-     * Returns a zero-length array.
-     * @return a zero-length array
-     */
-    public Object[] toArray() {
-        return new Object[0];
-    }
-
-    /**
-     * Sets the zeroeth element of the specified array to <tt>null</tt>
-     * (if the array has non-zero length) and returns it.
-     *
-     * @param a the array
-     * @return the specified array
-     * @throws NullPointerException if the specified array is null
-     */
-    public <T> T[] toArray(T[] a) {
-        if (a.length > 0)
-            a[0] = null;
-        return a;
-    }
-
-    /**
-     * @throws UnsupportedOperationException {@inheritDoc}
-     * @throws ClassCastException            {@inheritDoc}
-     * @throws NullPointerException          {@inheritDoc}
-     * @throws IllegalArgumentException      {@inheritDoc}
-     */
-    public int drainTo(Collection<? super E> c) {
-        if (c == null)
-            throw new NullPointerException();
-        if (c == this)
-            throw new IllegalArgumentException();
-        int n = 0;
-        E e;
-        while ( (e = poll()) != null) {
-            c.add(e);
-            ++n;
-        }
-        return n;
-    }
-
-    /**
-     * @throws UnsupportedOperationException {@inheritDoc}
-     * @throws ClassCastException            {@inheritDoc}
-     * @throws NullPointerException          {@inheritDoc}
-     * @throws IllegalArgumentException      {@inheritDoc}
-     */
-    public int drainTo(Collection<? super E> c, int maxElements) {
-        if (c == null)
-            throw new NullPointerException();
-        if (c == this)
-            throw new IllegalArgumentException();
-        int n = 0;
-        E e;
-        while (n < maxElements && (e = poll()) != null) {
-            c.add(e);
-            ++n;
-        }
-        return n;
-    }
-
-    /*
-     * To cope with serialization strategy in the 1.5 version of
-     * SynchronousQueue, we declare some unused classes and fields
-     * that exist solely to enable serializability across versions.
-     * These fields are never used, so are initialized only if this
-     * object is ever serialized or deserialized.
-     */
-
-    static class WaitQueue implements java.io.Serializable {
-        private static final long serialVersionUID = 423369940180943459L;
-    }
-    static class LifoWaitQueue extends WaitQueue {
-        private static final long serialVersionUID = -3633113410248163686L;
-    }
-    static class FifoWaitQueue extends WaitQueue {
-        private static final long serialVersionUID = -3623113410248163686L;
-    }
-    private ReentrantLock qlock;
-    private WaitQueue waitingProducers;
-    private WaitQueue waitingConsumers;
-
-    /**
-     * Save the state to a stream (that is, serialize it).
-     *
-     * @param s the stream
-     */
-    private void writeObject(java.io.ObjectOutputStream s)
-        throws java.io.IOException {
-        boolean fair = transferer instanceof TransferQueue;
-        if (fair) {
-            qlock = new ReentrantLock(true);
-            waitingProducers = new FifoWaitQueue();
-            waitingConsumers = new FifoWaitQueue();
-        }
-        else {
-            qlock = new ReentrantLock();
-            waitingProducers = new LifoWaitQueue();
-            waitingConsumers = new LifoWaitQueue();
-        }
-        s.defaultWriteObject();
-    }
-
-    private void readObject(final java.io.ObjectInputStream s)
-        throws java.io.IOException, ClassNotFoundException {
-        s.defaultReadObject();
-        if (waitingProducers instanceof FifoWaitQueue)
-            transferer = new TransferQueue();
-        else
-            transferer = new TransferStack();
-    }
-
-}


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