util/concurrent/Exchanger.java

/*
 * 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 java.util.concurrent;
import java.util.concurrent.*; // for javadoc (till 6280605 is fixed)
import java.util.concurrent.locks.*;
import java.util.concurrent.atomic.*;
import java.util.Random;

/**
 * A synchronization point at which threads can pair and swap elements
 * within pairs.  Each thread presents some object on entry to the
 * {@link #exchange exchange} method, matches with a partner thread,
 * and receives its partner's object on return.
 *
 * <p><b>Sample Usage:</b>
 * Here are the highlights of a class that uses an {@code Exchanger}
 * to swap buffers between threads so that the thread filling the
 * buffer gets a freshly emptied one when it needs it, handing off the
 * filled one to the thread emptying the buffer.
 * <pre>{@code
 * class FillAndEmpty {
 *   Exchanger<DataBuffer> exchanger = new Exchanger<DataBuffer>();
 *   DataBuffer initialEmptyBuffer = ... a made-up type
 *   DataBuffer initialFullBuffer = ...
 *
 *   class FillingLoop implements Runnable {
 *     public void run() {
 *       DataBuffer currentBuffer = initialEmptyBuffer;
 *       try {
 *         while (currentBuffer != null) {
 *           addToBuffer(currentBuffer);
 *           if (currentBuffer.isFull())
 *             currentBuffer = exchanger.exchange(currentBuffer);
 *         }
 *       } catch (InterruptedException ex) { ... handle ... }
 *     }
 *   }
 *
 *   class EmptyingLoop implements Runnable {
 *     public void run() {
 *       DataBuffer currentBuffer = initialFullBuffer;
 *       try {
 *         while (currentBuffer != null) {
 *           takeFromBuffer(currentBuffer);
 *           if (currentBuffer.isEmpty())
 *             currentBuffer = exchanger.exchange(currentBuffer);
 *         }
 *       } catch (InterruptedException ex) { ... handle ...}
 *     }
 *   }
 *
 *   void start() {
 *     new Thread(new FillingLoop()).start();
 *     new Thread(new EmptyingLoop()).start();
 *   }
 * }
 * }</pre>
 *
 * <p>Memory consistency effects: For each pair of threads that
 * successfully exchange objects via an {@code Exchanger}, actions
 * prior to the {@code exchange()} in each thread
 * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
 * those subsequent to a return from the corresponding {@code exchange()}
 * in the other thread.
 *
 * @since 1.5
 * @author Doug Lea and Bill Scherer and Michael Scott
 * @param <V> The type of objects that may be exchanged
 */
public class Exchanger<V> {
    /*
     * The underlying idea is to use a stack to hold nodes containing
     * pairs of items to be exchanged. Except that:
     *
     *  *  Only one element of the pair is known on creation by a
     *     first-arriving thread; the other is a "hole" waiting to be
     *     filled in. This is a degenerate form of the dual stacks
     *     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.
     *     It is "degenerate" in that both the items and the holes
     *     are shared in the same nodes.
     *
     *  *  There isn't really a stack here! There can't be -- if two
     *     nodes were both in the stack, they should cancel themselves
     *     out by combining. So that's what we do. The 0th element of
     *     the "arena" array serves only as the top of stack.  The
     *     remainder of the array is a form of the elimination backoff
     *     collision array described in "A Scalable Lock-free Stack
     *     Algorithm", by D. Hendler, N. Shavit, and L. Yerushalmi.
     *     16th ACM Symposium on Parallelism in Algorithms and
     *     Architectures, June 2004. Here, threads spin (using short
     *     timed waits with exponential backoff) looking for each
     *     other.  If they fail to find others waiting, they try the
     *     top spot again.  As shown in that paper, this always
     *     converges.
     *
     * The backoff elimination mechanics never come into play in
     * common usages where only two threads ever meet to exchange
     * items, but they prevent contention bottlenecks when an
     * exchanger is used by a large number of threads.
     *
     * For more details, see the paper "A Scalable Elimination-based
     * Exchange Channel" by William Scherer, Doug Lea, and Michael
     * Scott in Proceedings of SCOOL05 workshop. Available at:
     * http://hdl.handle.net/1802/2104
     */

    /**
     * Size of collision space. Using a size of half the number of
     * CPUs provides enough space for threads to find each other but
     * not so much that it would always require one or more to time
     * out to become unstuck. Note that the arena array holds SIZE+1
     * elements, to include the top-of-stack slot.
     */
    private static final int SIZE =
        (Runtime.getRuntime().availableProcessors() + 1) / 2;

    /**
     * Base unit in nanoseconds for backoffs. Must be a power of two.
     * Should be small because backoffs exponentially increase from
     * base.
     */
    private static final long BACKOFF_BASE = 128L;

    /**
     * Sentinel item representing cancellation.  This value is placed
     * in holes on cancellation, and used as a return value from Node
     * methods to indicate failure to set or get hole.
     */
    static final Object FAIL = new Object();

    /**
     * The collision arena. arena[0] is used as the top of the stack.
     * The remainder is used as the collision elimination space.
     * Each slot holds an AtomicReference<Node>, but this cannot be
     * expressed for arrays, so elements are casted on each use.
     */
    private final AtomicReference<Node>[] arena;

    /** Generator for random backoffs and delays. */
    private final Random random = new Random();

    /**
     * Creates a new Exchanger.
     */
    public Exchanger() {
        arena = (AtomicReference<Node>[]) new AtomicReference[SIZE + 1];
        for (int i = 0; i < arena.length; ++i)
            arena[i] = new AtomicReference<Node>();
    }

    /**
     * Main exchange function, handling the different policy variants.
     * Uses Object, not "V" as argument and return value to simplify
     * handling of internal sentinel values. Callers from public
     * methods cast accordingly.
     *
     * @param item the item to exchange
     * @param timed true if the wait is timed
     * @param nanos if timed, the maximum wait time
     * @return the other thread's item
     */
    private Object doExchange(Object item, boolean timed, long nanos)
        throws InterruptedException, TimeoutException {
        Node me = new Node(item);
        long lastTime = timed ? System.nanoTime() : 0;
        int idx = 0;     // start out at slot representing top
        int backoff = 0; // increases on failure to occupy a slot

        for (;;) {
            AtomicReference<Node> slot = arena[idx];

            // If this slot is already occupied, there is a waiting item...
            Node you = slot.get();
            if (you != null) {
                Object v = you.fillHole(item);
                slot.compareAndSet(you, null);
                if (v != FAIL)       // ... unless it was cancelled
                    return v;
            }

            // Try to occupy this slot
            if (slot.compareAndSet(null, me)) {
                // If this is top slot, use regular wait, else backoff-wait
                Object v = ((idx == 0)?
                            me.waitForHole(timed, nanos) :
                            me.waitForHole(true, randomDelay(backoff)));
                slot.compareAndSet(me, null);
                if (v != FAIL)
                    return v;
                if (Thread.interrupted())
                    throw new InterruptedException();
                if (timed) {
                    long now = System.nanoTime();
                    nanos -= now - lastTime;
                    lastTime = now;
                    if (nanos <= 0)
                        throw new TimeoutException();
                }

                me = new Node(item);      // Throw away nodes on failure
                if (backoff < SIZE - 1)   // Increase or stay saturated
                    ++backoff;
                idx = 0;                  // Restart at top
            }

            else // Retry with a random non-top slot <= backoff
                idx = 1 + random.nextInt(backoff + 1);

        }
    }

    /**
     * Returns a random delay less than (base times (2 raised to backoff))
     */
    private long randomDelay(int backoff) {
        return ((BACKOFF_BASE << backoff) - 1) & random.nextInt();
    }

    /**
     * Nodes hold partially exchanged data. This class
     * opportunistically subclasses AtomicReference to represent the
     * hole. So get() returns hole, and compareAndSet CAS'es value
     * into hole.  Note that this class cannot be parameterized as V
     * because the sentinel value FAIL is only of type Object.
     */
    static final class Node extends AtomicReference<Object> {
        private static final long serialVersionUID = -3221313401284163686L;

        /** The element offered by the Thread creating this node. */
        final Object item;
        /** The Thread creating this node. */
        final Thread waiter;

        /**
         * Creates node with given item and empty hole.
         * @param item the item.
         */
        Node(Object item) {
            this.item = item;
            waiter = Thread.currentThread();
        }

        /**
         * Tries to fill in hole. On success, wakes up the waiter.
         * @param val the value to place in hole.
         * @return on success, the item; on failure, FAIL.
         */
        Object fillHole(Object val) {
            if (compareAndSet(null, val)) {
                LockSupport.unpark(waiter);
                return item;
            }
            return FAIL;
        }

        /**
         * Waits for and gets the hole filled in by another thread.
         * Fails if timed out or interrupted before hole filled.
         *
         * @param timed true if the wait is timed
         * @param nanos if timed, the maximum wait time
         * @return on success, the hole; on failure, FAIL
         */
        Object waitForHole(boolean timed, long nanos) {
            long lastTime = timed ? System.nanoTime() : 0;
            Object h;
            while ((h = get()) == null) {
                // If interrupted or timed out, try to cancel by
                // CASing FAIL as hole value.
                if (Thread.currentThread().isInterrupted() ||
                    (timed && nanos <= 0)) {
                    if (compareAndSet(null, FAIL))
                        return FAIL;
                }
                else if (!timed)
                    LockSupport.park();
                else {
                    LockSupport.parkNanos(nanos);
                    long now = System.nanoTime();
                    nanos -= now - lastTime;
                    lastTime = now;
                }
            }
            return h;
        }
    }

    /**
     * Waits for another thread to arrive at this exchange point (unless
     * it is {@link Thread#interrupt interrupted}),
     * and then transfers the given object to it, receiving its object
     * in return.
     *
     * <p>If another thread is already waiting at the exchange point then
     * it is resumed for thread scheduling purposes and receives the object
     * passed in by the current thread. The current thread returns immediately,
     * receiving the object passed to the exchange by that other thread.
     *
     * <p>If no other thread is already waiting at the exchange then the
     * current thread is disabled for thread scheduling purposes and lies
     * dormant until one of two things happens:
     * <ul>
     * <li>Some other thread enters the exchange; or
     * <li>Some other thread {@link Thread#interrupt interrupts} the current
     * thread.
     * </ul>
     * <p>If the current thread:
     * <ul>
     * <li>has its interrupted status set on entry to this method; or
     * <li>is {@link Thread#interrupt interrupted} while waiting
     * for the exchange,
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     *
     * @param x the object to exchange
     * @return the object provided by the other thread
     * @throws InterruptedException if the current thread was
     *         interrupted while waiting
     */
    public V exchange(V x) throws InterruptedException {
        try {
            return (V)doExchange(x, false, 0);
        } catch (TimeoutException cannotHappen) {
            throw new Error(cannotHappen);
        }
    }

    /**
     * Waits for another thread to arrive at this exchange point (unless
     * it is {@link Thread#interrupt interrupted}, or the specified waiting
     * time elapses),
     * and then transfers the given object to it, receiving its object
     * in return.
     *
     * <p>If another thread is already waiting at the exchange point then
     * it is resumed for thread scheduling purposes and receives the object
     * passed in by the current thread. The current thread returns immediately,
     * receiving the object passed to the exchange by that other thread.
     *
     * <p>If no other thread is already waiting at the exchange then the
     * current thread is disabled for thread scheduling purposes and lies
     * dormant until one of three things happens:
     * <ul>
     * <li>Some other thread enters the exchange; or
     * <li>Some other thread {@link Thread#interrupt interrupts} the current
     * thread; or
     * <li>The specified waiting time elapses.
     * </ul>
     * <p>If the current thread:
     * <ul>
     * <li>has its interrupted status set on entry to this method; or
     * <li>is {@link Thread#interrupt interrupted} while waiting
     * for the exchange,
     * </ul>
     * then {@link InterruptedException} is thrown and the current thread's
     * interrupted status is cleared.
     *
     * <p>If the specified waiting time elapses then {@link TimeoutException}
     * is thrown.
     * If the time is
     * less than or equal to zero, the method will not wait at all.
     *
     * @param x the object to exchange
     * @param timeout the maximum time to wait
     * @param unit the time unit of the <tt>timeout</tt> argument
     * @return the object provided by the other thread
     * @throws InterruptedException if the current thread was
     *         interrupted while waiting
     * @throws TimeoutException if the specified waiting time elapses
     *         before another thread enters the exchange
     */
    public V exchange(V x, long timeout, TimeUnit unit)
        throws InterruptedException, TimeoutException {
        return (V)doExchange(x, true, unit.toNanos(timeout));
    }
}
Revision:	1.30
Committed:	Sun Nov 6 15:30:24 2005 UTC (18 years, 7 months ago) by dl
Branch:	MAIN
Changes since 1.29:	+35 -26 lines
Log Message:	Incorporate review suggestions
#	User	Rev	Content
1	dl	1.2	/*
2	dl	1.16	* Written by Doug Lea, Bill Scherer, and Michael Scott with
3			* assistance from members of JCP JSR-166 Expert Group and released to
4			* the public domain, as explained at
5	dl	1.14	* http://creativecommons.org/licenses/publicdomain
6	dl	1.2	*/
7
8	tim	1.1	package java.util.concurrent;
9	jsr166	1.21	import java.util.concurrent.*; // for javadoc (till 6280605 is fixed)
10	dl	1.4	import java.util.concurrent.locks.*;
11	dl	1.16	import java.util.concurrent.atomic.*;
12			import java.util.Random;
13	tim	1.1
14			/**
15	dl	1.28	* A synchronization point at which threads can pair and swap elements
16			* within pairs. Each thread presents some object on entry to the
17			* {@link #exchange exchange} method, matches with a partner thread,
18			* and receives its partner's object on return.
19	tim	1.1	*
20			* <p><b>Sample Usage:</b>
21	jsr166	1.29	* Here are the highlights of a class that uses an {@code Exchanger}
22			* to swap buffers between threads so that the thread filling the
23			* buffer gets a freshly emptied one when it needs it, handing off the
24			* filled one to the thread emptying the buffer.
25			* <pre>{@code
26	tim	1.1	* class FillAndEmpty {
27	jsr166	1.29	* Exchanger<DataBuffer> exchanger = new Exchanger<DataBuffer>();
28	dl	1.9	* DataBuffer initialEmptyBuffer = ... a made-up type
29			* DataBuffer initialFullBuffer = ...
30	tim	1.1	*
31			* class FillingLoop implements Runnable {
32			* public void run() {
33	dl	1.9	* DataBuffer currentBuffer = initialEmptyBuffer;
34	tim	1.1	* try {
35			* while (currentBuffer != null) {
36			* addToBuffer(currentBuffer);
37	dl	1.30	* if (currentBuffer.isFull())
38	tim	1.1	* currentBuffer = exchanger.exchange(currentBuffer);
39			* }
40	tim	1.7	* } catch (InterruptedException ex) { ... handle ... }
41	tim	1.1	* }
42			* }
43			*
44			* class EmptyingLoop implements Runnable {
45			* public void run() {
46	dl	1.9	* DataBuffer currentBuffer = initialFullBuffer;
47	tim	1.1	* try {
48			* while (currentBuffer != null) {
49			* takeFromBuffer(currentBuffer);
50	dl	1.30	* if (currentBuffer.isEmpty())
51	tim	1.1	* currentBuffer = exchanger.exchange(currentBuffer);
52			* }
53	tim	1.7	* } catch (InterruptedException ex) { ... handle ...}
54	tim	1.1	* }
55			* }
56			*
57			* void start() {
58			* new Thread(new FillingLoop()).start();
59			* new Thread(new EmptyingLoop()).start();
60			* }
61			* }
62	jsr166	1.29	* }</pre>
63	tim	1.1	*
64	jsr166	1.27	* <p>Memory consistency effects: For each pair of threads that
65			* successfully exchange objects via an {@code Exchanger}, actions
66			* prior to the {@code exchange()} in each thread
67			* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
68			* those subsequent to a return from the corresponding {@code exchange()}
69			* in the other thread.
70	brian	1.22	*
71	tim	1.1	* @since 1.5
72	dl	1.16	* @author Doug Lea and Bill Scherer and Michael Scott
73	dl	1.11	* @param <V> The type of objects that may be exchanged
74	tim	1.1	*/
75			public class Exchanger<V> {
76	dl	1.16	/*
77			* The underlying idea is to use a stack to hold nodes containing
78			* pairs of items to be exchanged. Except that:
79			*
80			* * Only one element of the pair is known on creation by a
81			* first-arriving thread; the other is a "hole" waiting to be
82			* filled in. This is a degenerate form of the dual stacks
83			* described in "Nonblocking Concurrent Objects with Condition
84			* Synchronization", by W. N. Scherer III and M. L. Scott.
85			* 18th Annual Conf. on Distributed Computing, Oct. 2004.
86			* It is "degenerate" in that both the items and the holes
87			* are shared in the same nodes.
88			*
89			* * There isn't really a stack here! There can't be -- if two
90			* nodes were both in the stack, they should cancel themselves
91			* out by combining. So that's what we do. The 0th element of
92			* the "arena" array serves only as the top of stack. The
93			* remainder of the array is a form of the elimination backoff
94			* collision array described in "A Scalable Lock-free Stack
95			* Algorithm", by D. Hendler, N. Shavit, and L. Yerushalmi.
96			* 16th ACM Symposium on Parallelism in Algorithms and
97			* Architectures, June 2004. Here, threads spin (using short
98			* timed waits with exponential backoff) looking for each
99			* other. If they fail to find others waiting, they try the
100			* top spot again. As shown in that paper, this always
101			* converges.
102			*
103			* The backoff elimination mechanics never come into play in
104			* common usages where only two threads ever meet to exchange
105			* items, but they prevent contention bottlenecks when an
106			* exchanger is used by a large number of threads.
107	dl	1.30	*
108			* For more details, see the paper "A Scalable Elimination-based
109			* Exchange Channel" by William Scherer, Doug Lea, and Michael
110			* Scott in Proceedings of SCOOL05 workshop. Available at:
111			* http://hdl.handle.net/1802/2104
112	dl	1.16	*/
113	dl	1.2
114	jsr166	1.17	/**
115	dl	1.16	* Size of collision space. Using a size of half the number of
116			* CPUs provides enough space for threads to find each other but
117			* not so much that it would always require one or more to time
118			* out to become unstuck. Note that the arena array holds SIZE+1
119			* elements, to include the top-of-stack slot.
120			*/
121	jsr166	1.17	private static final int SIZE =
122	dl	1.16	(Runtime.getRuntime().availableProcessors() + 1) / 2;
123	jsr166	1.15
124	dl	1.2	/**
125	dl	1.16	* Base unit in nanoseconds for backoffs. Must be a power of two.
126			* Should be small because backoffs exponentially increase from
127			* base.
128	dl	1.2	*/
129	dl	1.16	private static final long BACKOFF_BASE = 128L;
130	dl	1.2
131	jsr166	1.17	/**
132	dl	1.16	* Sentinel item representing cancellation. This value is placed
133			* in holes on cancellation, and used as a return value from Node
134			* methods to indicate failure to set or get hole.
135	jsr166	1.17	*/
136	dl	1.16	static final Object FAIL = new Object();
137
138	jsr166	1.17	/**
139	dl	1.16	* The collision arena. arena[0] is used as the top of the stack.
140			* The remainder is used as the collision elimination space.
141	jsr166	1.17	* Each slot holds an AtomicReference<Node>, but this cannot be
142	dl	1.16	* expressed for arrays, so elements are casted on each use.
143	dl	1.3	*/
144	dl	1.30	private final AtomicReference<Node>[] arena;
145	dl	1.5
146	dl	1.16	/** Generator for random backoffs and delays. */
147			private final Random random = new Random();
148	dl	1.5
149	dl	1.16	/**
150			* Creates a new Exchanger.
151			*/
152			public Exchanger() {
153	dl	1.30	arena = (AtomicReference<Node>[]) new AtomicReference[SIZE + 1];
154	dl	1.16	for (int i = 0; i < arena.length; ++i)
155	dl	1.30	arena[i] = new AtomicReference<Node>();
156	dl	1.16	}
157	dl	1.2
158	dl	1.16	/**
159			* Main exchange function, handling the different policy variants.
160			* Uses Object, not "V" as argument and return value to simplify
161			* handling of internal sentinel values. Callers from public
162			* methods cast accordingly.
163	dl	1.30	*
164			* @param item the item to exchange
165			* @param timed true if the wait is timed
166			* @param nanos if timed, the maximum wait time
167			* @return the other thread's item
168	dl	1.16	*/
169			private Object doExchange(Object item, boolean timed, long nanos)
170			throws InterruptedException, TimeoutException {
171			Node me = new Node(item);
172	dl	1.30	long lastTime = timed ? System.nanoTime() : 0;
173	dl	1.16	int idx = 0; // start out at slot representing top
174			int backoff = 0; // increases on failure to occupy a slot
175
176			for (;;) {
177	dl	1.30	AtomicReference<Node> slot = arena[idx];
178	dl	1.16
179			// If this slot is already occupied, there is a waiting item...
180			Node you = slot.get();
181			if (you != null) {
182			Object v = you.fillHole(item);
183			slot.compareAndSet(you, null);
184			if (v != FAIL) // ... unless it was cancelled
185			return v;
186	dl	1.2	}
187
188	dl	1.16	// Try to occupy this slot
189			if (slot.compareAndSet(null, me)) {
190			// If this is top slot, use regular wait, else backoff-wait
191			Object v = ((idx == 0)?
192			me.waitForHole(timed, nanos) :
193			me.waitForHole(true, randomDelay(backoff)));
194			slot.compareAndSet(me, null);
195			if (v != FAIL)
196			return v;
197			if (Thread.interrupted())
198			throw new InterruptedException();
199			if (timed) {
200			long now = System.nanoTime();
201			nanos -= now - lastTime;
202			lastTime = now;
203			if (nanos <= 0)
204			throw new TimeoutException();
205			}
206	dl	1.2
207	dl	1.16	me = new Node(item); // Throw away nodes on failure
208			if (backoff < SIZE - 1) // Increase or stay saturated
209			++backoff;
210			idx = 0; // Restart at top
211	dl	1.2	}
212
213	dl	1.16	else // Retry with a random non-top slot <= backoff
214			idx = 1 + random.nextInt(backoff + 1);
215	dl	1.2
216			}
217			}
218	tim	1.1
219			/**
220	dl	1.16	* Returns a random delay less than (base times (2 raised to backoff))
221			*/
222			private long randomDelay(int backoff) {
223			return ((BACKOFF_BASE << backoff) - 1) & random.nextInt();
224			}
225
226			/**
227			* Nodes hold partially exchanged data. This class
228			* opportunistically subclasses AtomicReference to represent the
229			* hole. So get() returns hole, and compareAndSet CAS'es value
230			* into hole. Note that this class cannot be parameterized as V
231			* because the sentinel value FAIL is only of type Object.
232	jsr166	1.15	*/
233	dl	1.16	static final class Node extends AtomicReference<Object> {
234	dl	1.20	private static final long serialVersionUID = -3221313401284163686L;
235	jsr166	1.21
236	dl	1.16	/** The element offered by the Thread creating this node. */
237			final Object item;
238			/** The Thread creating this node. */
239			final Thread waiter;
240
241			/**
242			* Creates node with given item and empty hole.
243			* @param item the item.
244			*/
245			Node(Object item) {
246			this.item = item;
247			waiter = Thread.currentThread();
248			}
249
250			/**
251			* Tries to fill in hole. On success, wakes up the waiter.
252			* @param val the value to place in hole.
253			* @return on success, the item; on failure, FAIL.
254			*/
255			Object fillHole(Object val) {
256			if (compareAndSet(null, val)) {
257			LockSupport.unpark(waiter);
258			return item;
259			}
260			return FAIL;
261			}
262
263			/**
264	jsr166	1.17	* Waits for and gets the hole filled in by another thread.
265			* Fails if timed out or interrupted before hole filled.
266	dl	1.30	*
267			* @param timed true if the wait is timed
268			* @param nanos if timed, the maximum wait time
269			* @return on success, the hole; on failure, FAIL
270	dl	1.16	*/
271			Object waitForHole(boolean timed, long nanos) {
272	dl	1.30	long lastTime = timed ? System.nanoTime() : 0;
273	dl	1.18	Object h;
274			while ((h = get()) == null) {
275			// If interrupted or timed out, try to cancel by
276			// CASing FAIL as hole value.
277			if (Thread.currentThread().isInterrupted() \|\|
278	dl	1.30	(timed && nanos <= 0)) {
279			if (compareAndSet(null, FAIL))
280			return FAIL;
281			}
282	dl	1.18	else if (!timed)
283	dl	1.16	LockSupport.park();
284			else {
285			LockSupport.parkNanos(nanos);
286			long now = System.nanoTime();
287			nanos -= now - lastTime;
288			lastTime = now;
289			}
290			}
291			return h;
292			}
293	tim	1.1	}
294
295			/**
296			* Waits for another thread to arrive at this exchange point (unless
297			* it is {@link Thread#interrupt interrupted}),
298			* and then transfers the given object to it, receiving its object
299			* in return.
300	jsr166	1.17	*
301	tim	1.1	* <p>If another thread is already waiting at the exchange point then
302			* it is resumed for thread scheduling purposes and receives the object
303			* passed in by the current thread. The current thread returns immediately,
304			* receiving the object passed to the exchange by that other thread.
305	jsr166	1.17	*
306	jsr166	1.15	* <p>If no other thread is already waiting at the exchange then the
307	tim	1.1	* current thread is disabled for thread scheduling purposes and lies
308			* dormant until one of two things happens:
309			* <ul>
310			* <li>Some other thread enters the exchange; or
311			* <li>Some other thread {@link Thread#interrupt interrupts} the current
312			* thread.
313			* </ul>
314			* <p>If the current thread:
315			* <ul>
316	jsr166	1.15	* <li>has its interrupted status set on entry to this method; or
317	tim	1.1	* <li>is {@link Thread#interrupt interrupted} while waiting
318	jsr166	1.15	* for the exchange,
319	tim	1.1	* </ul>
320	jsr166	1.15	* then {@link InterruptedException} is thrown and the current thread's
321			* interrupted status is cleared.
322	tim	1.1	*
323			* @param x the object to exchange
324	dl	1.30	* @return the object provided by the other thread
325			* @throws InterruptedException if the current thread was
326			* interrupted while waiting
327	jsr166	1.15	*/
328	tim	1.1	public V exchange(V x) throws InterruptedException {
329	dl	1.2	try {
330	dl	1.16	return (V)doExchange(x, false, 0);
331	jsr166	1.15	} catch (TimeoutException cannotHappen) {
332	dl	1.2	throw new Error(cannotHappen);
333			}
334	tim	1.1	}
335
336			/**
337			* Waits for another thread to arrive at this exchange point (unless
338			* it is {@link Thread#interrupt interrupted}, or the specified waiting
339			* time elapses),
340			* and then transfers the given object to it, receiving its object
341			* in return.
342			*
343			* <p>If another thread is already waiting at the exchange point then
344			* it is resumed for thread scheduling purposes and receives the object
345			* passed in by the current thread. The current thread returns immediately,
346			* receiving the object passed to the exchange by that other thread.
347			*
348	jsr166	1.15	* <p>If no other thread is already waiting at the exchange then the
349	tim	1.1	* current thread is disabled for thread scheduling purposes and lies
350			* dormant until one of three things happens:
351			* <ul>
352			* <li>Some other thread enters the exchange; or
353			* <li>Some other thread {@link Thread#interrupt interrupts} the current
354			* thread; or
355			* <li>The specified waiting time elapses.
356			* </ul>
357			* <p>If the current thread:
358			* <ul>
359	jsr166	1.15	* <li>has its interrupted status set on entry to this method; or
360	tim	1.1	* <li>is {@link Thread#interrupt interrupted} while waiting
361	jsr166	1.15	* for the exchange,
362	tim	1.1	* </ul>
363	jsr166	1.15	* then {@link InterruptedException} is thrown and the current thread's
364			* interrupted status is cleared.
365	tim	1.1	*
366			* <p>If the specified waiting time elapses then {@link TimeoutException}
367			* is thrown.
368	jsr166	1.15	* If the time is
369	tim	1.1	* less than or equal to zero, the method will not wait at all.
370			*
371			* @param x the object to exchange
372			* @param timeout the maximum time to wait
373	dl	1.30	* @param unit the time unit of the <tt>timeout</tt> argument
374			* @return the object provided by the other thread
375			* @throws InterruptedException if the current thread was
376			* interrupted while waiting
377			* @throws TimeoutException if the specified waiting time elapses
378			* before another thread enters the exchange
379	jsr166	1.15	*/
380			public V exchange(V x, long timeout, TimeUnit unit)
381	tim	1.1	throws InterruptedException, TimeoutException {
382	dl	1.16	return (V)doExchange(x, true, unit.toNanos(timeout));
383	tim	1.1	}
384			}