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
     */

    /** The number of CPUs, for sizing and spin control */
    static final int NCPUS = Runtime.getRuntime().availableProcessors();

    /**
     * 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 = (NCPUS + 1) / 2;

    /**
     * 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.
     */
    static final int maxTimedSpins = (NCPUS < 2)? 0 : 16;

    /**
     * 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 * 32;

    /**
     * The number of nanoseconds for which it is faster to spin
     * rather than to use timed park. A rough estimate suffices.
     */
    static final long spinForTimeoutThreshold = 1000L;

    /**
     * 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.
     */
    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;
            int spins = timed? maxTimedSpins : maxUntimedSpins;
            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) {
                        long now = System.nanoTime();
                        nanos -= now - lastTime;
                        lastTime = now;
                    }
                    if (spins > 0)
                        --spins;
                    else if (!timed) 
                        LockSupport.park();
                    else if (nanos > spinForTimeoutThreshold)
                        LockSupport.parkNanos(nanos);
                }
            }
            return h;
        }
    }

    /**
     * Waits for another thread to arrive at this exchange point (unless
     * the current thread 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
     * the current thread 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:	1.32
Committed:	Sat Dec 10 20:09:28 2005 UTC (18 years, 5 months ago) by dl
Branch:	MAIN
Changes since 1.31:	+39 -10 lines
Log Message:	Use same spin control as SynchronousQueue
#	Content
1	/*
2	* 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	* http://creativecommons.org/licenses/publicdomain
6	*/
7
8	package java.util.concurrent;
9	import java.util.concurrent.*; // for javadoc (till 6280605 is fixed)
10	import java.util.concurrent.locks.*;
11	import java.util.concurrent.atomic.*;
12	import java.util.Random;
13
14	/**
15	* 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	*
20	* <p><b>Sample Usage:</b>
21	* 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	* class FillAndEmpty {
27	* Exchanger<DataBuffer> exchanger = new Exchanger<DataBuffer>();
28	* DataBuffer initialEmptyBuffer = ... a made-up type
29	* DataBuffer initialFullBuffer = ...
30	*
31	* class FillingLoop implements Runnable {
32	* public void run() {
33	* DataBuffer currentBuffer = initialEmptyBuffer;
34	* try {
35	* while (currentBuffer != null) {
36	* addToBuffer(currentBuffer);
37	* if (currentBuffer.isFull())
38	* currentBuffer = exchanger.exchange(currentBuffer);
39	* }
40	* } catch (InterruptedException ex) { ... handle ... }
41	* }
42	* }
43	*
44	* class EmptyingLoop implements Runnable {
45	* public void run() {
46	* DataBuffer currentBuffer = initialFullBuffer;
47	* try {
48	* while (currentBuffer != null) {
49	* takeFromBuffer(currentBuffer);
50	* if (currentBuffer.isEmpty())
51	* currentBuffer = exchanger.exchange(currentBuffer);
52	* }
53	* } catch (InterruptedException ex) { ... handle ...}
54	* }
55	* }
56	*
57	* void start() {
58	* new Thread(new FillingLoop()).start();
59	* new Thread(new EmptyingLoop()).start();
60	* }
61	* }
62	* }</pre>
63	*
64	* <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	*
71	* @since 1.5
72	* @author Doug Lea and Bill Scherer and Michael Scott
73	* @param <V> The type of objects that may be exchanged
74	*/
75	public class Exchanger<V> {
76	/*
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	*
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	*/
113
114	/** The number of CPUs, for sizing and spin control */
115	static final int NCPUS = Runtime.getRuntime().availableProcessors();
116
117	/**
118	* Size of collision space. Using a size of half the number of
119	* CPUs provides enough space for threads to find each other but
120	* not so much that it would always require one or more to time
121	* out to become unstuck. Note that the arena array holds SIZE+1
122	* elements, to include the top-of-stack slot.
123	*/
124	private static final int SIZE = (NCPUS + 1) / 2;
125
126	/**
127	* The number of times to spin before blocking in timed waits.
128	* The value is empirically derived -- it works well across a
129	* variety of processors and OSes. Empirically, the best value
130	* seems not to vary with number of CPUs (beyond 2) so is just
131	* a constant.
132	*/
133	static final int maxTimedSpins = (NCPUS < 2)? 0 : 16;
134
135	/**
136	* The number of times to spin before blocking in untimed waits.
137	* This is greater than timed value because untimed waits spin
138	* faster since they don't need to check times on each spin.
139	*/
140	static final int maxUntimedSpins = maxTimedSpins * 32;
141
142	/**
143	* The number of nanoseconds for which it is faster to spin
144	* rather than to use timed park. A rough estimate suffices.
145	*/
146	static final long spinForTimeoutThreshold = 1000L;
147
148	/**
149	* Base unit in nanoseconds for backoffs. Must be a power of two.
150	* Should be small because backoffs exponentially increase from
151	* base.
152	*/
153	private static final long BACKOFF_BASE = 128L;
154
155	/**
156	* Sentinel item representing cancellation. This value is placed
157	* in holes on cancellation, and used as a return value from Node
158	* methods to indicate failure to set or get hole.
159	*/
160	static final Object FAIL = new Object();
161
162	/**
163	* The collision arena. arena[0] is used as the top of the stack.
164	* The remainder is used as the collision elimination space.
165	*/
166	private final AtomicReference<Node>[] arena;
167
168	/** Generator for random backoffs and delays. */
169	private final Random random = new Random();
170
171	/**
172	* Creates a new Exchanger.
173	*/
174	public Exchanger() {
175	arena = (AtomicReference<Node>[]) new AtomicReference[SIZE + 1];
176	for (int i = 0; i < arena.length; ++i)
177	arena[i] = new AtomicReference<Node>();
178	}
179
180	/**
181	* Main exchange function, handling the different policy variants.
182	* Uses Object, not "V" as argument and return value to simplify
183	* handling of internal sentinel values. Callers from public
184	* methods cast accordingly.
185	*
186	* @param item the item to exchange
187	* @param timed true if the wait is timed
188	* @param nanos if timed, the maximum wait time
189	* @return the other thread's item
190	*/
191	private Object doExchange(Object item, boolean timed, long nanos)
192	throws InterruptedException, TimeoutException {
193	Node me = new Node(item);
194	long lastTime = timed ? System.nanoTime() : 0;
195	int idx = 0; // start out at slot representing top
196	int backoff = 0; // increases on failure to occupy a slot
197
198	for (;;) {
199	AtomicReference<Node> slot = arena[idx];
200
201	// If this slot is already occupied, there is a waiting item...
202	Node you = slot.get();
203	if (you != null) {
204	Object v = you.fillHole(item);
205	slot.compareAndSet(you, null);
206	if (v != FAIL) // ... unless it was cancelled
207	return v;
208	}
209
210	// Try to occupy this slot
211	if (slot.compareAndSet(null, me)) {
212	// If this is top slot, use regular wait, else backoff-wait
213	Object v = ((idx == 0)?
214	me.waitForHole(timed, nanos) :
215	me.waitForHole(true, randomDelay(backoff)));
216	slot.compareAndSet(me, null);
217	if (v != FAIL)
218	return v;
219	if (Thread.interrupted())
220	throw new InterruptedException();
221	if (timed) {
222	long now = System.nanoTime();
223	nanos -= now - lastTime;
224	lastTime = now;
225	if (nanos <= 0)
226	throw new TimeoutException();
227	}
228
229	me = new Node(item); // Throw away nodes on failure
230	if (backoff < SIZE - 1) // Increase or stay saturated
231	++backoff;
232	idx = 0; // Restart at top
233	}
234
235	else // Retry with a random non-top slot <= backoff
236	idx = 1 + random.nextInt(backoff + 1);
237
238	}
239	}
240
241	/**
242	* Returns a random delay less than (base times (2 raised to backoff)).
243	*/
244	private long randomDelay(int backoff) {
245	return ((BACKOFF_BASE << backoff) - 1) & random.nextInt();
246	}
247
248	/**
249	* Nodes hold partially exchanged data. This class
250	* opportunistically subclasses AtomicReference to represent the
251	* hole. So get() returns hole, and compareAndSet CAS'es value
252	* into hole. Note that this class cannot be parameterized as V
253	* because the sentinel value FAIL is only of type Object.
254	*/
255	static final class Node extends AtomicReference<Object> {
256	private static final long serialVersionUID = -3221313401284163686L;
257
258	/** The element offered by the Thread creating this node. */
259	final Object item;
260
261	/** The Thread creating this node. */
262	final Thread waiter;
263
264	/**
265	* Creates node with given item and empty hole.
266	*
267	* @param item the item
268	*/
269	Node(Object item) {
270	this.item = item;
271	waiter = Thread.currentThread();
272	}
273
274	/**
275	* Tries to fill in hole. On success, wakes up the waiter.
276	*
277	* @param val the value to place in hole
278	* @return on success, the item; on failure, FAIL
279	*/
280	Object fillHole(Object val) {
281	if (compareAndSet(null, val)) {
282	LockSupport.unpark(waiter);
283	return item;
284	}
285	return FAIL;
286	}
287
288	/**
289	* Waits for and gets the hole filled in by another thread.
290	* Fails if timed out or interrupted before hole filled.
291	*
292	* @param timed true if the wait is timed
293	* @param nanos if timed, the maximum wait time
294	* @return on success, the hole; on failure, FAIL
295	*/
296	Object waitForHole(boolean timed, long nanos) {
297	long lastTime = timed ? System.nanoTime() : 0;
298	int spins = timed? maxTimedSpins : maxUntimedSpins;
299	Object h;
300	while ((h = get()) == null) {
301	// If interrupted or timed out, try to cancel by
302	// CASing FAIL as hole value.
303	if (Thread.currentThread().isInterrupted() \|\|
304	(timed && nanos <= 0)) {
305	if (compareAndSet(null, FAIL))
306	return FAIL;
307	} else {
308	if (timed) {
309	long now = System.nanoTime();
310	nanos -= now - lastTime;
311	lastTime = now;
312	}
313	if (spins > 0)
314	--spins;
315	else if (!timed)
316	LockSupport.park();
317	else if (nanos > spinForTimeoutThreshold)
318	LockSupport.parkNanos(nanos);
319	}
320	}
321	return h;
322	}
323	}
324
325	/**
326	* Waits for another thread to arrive at this exchange point (unless
327	* the current thread is {@link Thread#interrupt interrupted}),
328	* and then transfers the given object to it, receiving its object
329	* in return.
330	*
331	* <p>If another thread is already waiting at the exchange point then
332	* it is resumed for thread scheduling purposes and receives the object
333	* passed in by the current thread. The current thread returns immediately,
334	* receiving the object passed to the exchange by that other thread.
335	*
336	* <p>If no other thread is already waiting at the exchange then the
337	* current thread is disabled for thread scheduling purposes and lies
338	* dormant until one of two things happens:
339	* <ul>
340	* <li>Some other thread enters the exchange; or
341	* <li>Some other thread {@link Thread#interrupt interrupts} the current
342	* thread.
343	* </ul>
344	* <p>If the current thread:
345	* <ul>
346	* <li>has its interrupted status set on entry to this method; or
347	* <li>is {@link Thread#interrupt interrupted} while waiting
348	* for the exchange,
349	* </ul>
350	* then {@link InterruptedException} is thrown and the current thread's
351	* interrupted status is cleared.
352	*
353	* @param x the object to exchange
354	* @return the object provided by the other thread
355	* @throws InterruptedException if the current thread was
356	* interrupted while waiting
357	*/
358	public V exchange(V x) throws InterruptedException {
359	try {
360	return (V)doExchange(x, false, 0);
361	} catch (TimeoutException cannotHappen) {
362	throw new Error(cannotHappen);
363	}
364	}
365
366	/**
367	* Waits for another thread to arrive at this exchange point (unless
368	* the current thread is {@link Thread#interrupt interrupted} or
369	* the specified waiting time elapses), and then transfers the given
370	* object to it, receiving its object in return.
371	*
372	* <p>If another thread is already waiting at the exchange point then
373	* it is resumed for thread scheduling purposes and receives the object
374	* passed in by the current thread. The current thread returns immediately,
375	* receiving the object passed to the exchange by that other thread.
376	*
377	* <p>If no other thread is already waiting at the exchange then the
378	* current thread is disabled for thread scheduling purposes and lies
379	* dormant until one of three things happens:
380	* <ul>
381	* <li>Some other thread enters the exchange; or
382	* <li>Some other thread {@link Thread#interrupt interrupts} the current
383	* thread; or
384	* <li>The specified waiting time elapses.
385	* </ul>
386	* <p>If the current thread:
387	* <ul>
388	* <li>has its interrupted status set on entry to this method; or
389	* <li>is {@link Thread#interrupt interrupted} while waiting
390	* for the exchange,
391	* </ul>
392	* then {@link InterruptedException} is thrown and the current thread's
393	* interrupted status is cleared.
394	*
395	* <p>If the specified waiting time elapses then {@link TimeoutException}
396	* is thrown.
397	* If the time is
398	* less than or equal to zero, the method will not wait at all.
399	*
400	* @param x the object to exchange
401	* @param timeout the maximum time to wait
402	* @param unit the time unit of the <tt>timeout</tt> argument
403	* @return the object provided by the other thread
404	* @throws InterruptedException if the current thread was
405	* interrupted while waiting
406	* @throws TimeoutException if the specified waiting time elapses
407	* before another thread enters the exchange
408	*/
409	public V exchange(V x, long timeout, TimeUnit unit)
410	throws InterruptedException, TimeoutException {
411	return (V)doExchange(x, true, unit.toNanos(timeout));
412	}
413	}