Imported Upstream version 1.0.3upstream/1.0.3

1 files changed, 1419 insertions, 0 deletions

diff --git a/src/main/java/jsr166e/StampedLock.java b/src/main/java/jsr166e/StampedLock.java
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+++ b/src/main/java/jsr166e/StampedLock.java
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+/*
+ * Written by Doug Lea with assistance from members of JCP JSR-166
+ * Expert Group and released to the public domain, as explained at
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+package jsr166e;
+import jsr166y.ThreadLocalRandom;
+
+import java.util.concurrent.TimeUnit;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.Condition;
+import java.util.concurrent.locks.ReadWriteLock;
+import java.util.concurrent.locks.LockSupport;
+
+/**
+ * A capability-based lock with three modes for controlling read/write
+ * access.  The state of a StampedLock consists of a version and mode.
+ * Lock acquisition methods return a stamp that represents and
+ * controls access with respect to a lock state; "try" versions of
+ * these methods may instead return the special value zero to
+ * represent failure to acquire access. Lock release and conversion
+ * methods require stamps as arguments, and fail if they do not match
+ * the state of the lock. The three modes are:
+ *
+ * <ul>
+ *
+ *  <li><b>Writing.</b> Method {@link #writeLock} possibly blocks
+ *   waiting for exclusive access, returning a stamp that can be used
+ *   in method {@link #unlockWrite} to release the lock. Untimed and
+ *   timed versions of {@code tryWriteLock} are also provided. When
+ *   the lock is held in write mode, no read locks may be obtained,
+ *   and all optimistic read validations will fail.  </li>
+ *
+ *  <li><b>Reading.</b> Method {@link #readLock} possibly blocks
+ *   waiting for non-exclusive access, returning a stamp that can be
+ *   used in method {@link #unlockRead} to release the lock. Untimed
+ *   and timed versions of {@code tryReadLock} are also provided. </li>
+ *
+ *  <li><b>Optimistic Reading.</b> Method {@link #tryOptimisticRead}
+ *   returns a non-zero stamp only if the lock is not currently held
+ *   in write mode. Method {@link #validate} returns true if the lock
+ *   has not been acquired in write mode since obtaining a given
+ *   stamp.  This mode can be thought of as an extremely weak version
+ *   of a read-lock, that can be broken by a writer at any time.  The
+ *   use of optimistic mode for short read-only code segments often
+ *   reduces contention and improves throughput.  However, its use is
+ *   inherently fragile.  Optimistic read sections should only read
+ *   fields and hold them in local variables for later use after
+ *   validation. Fields read while in optimistic mode may be wildly
+ *   inconsistent, so usage applies only when you are familiar enough
+ *   with data representations to check consistency and/or repeatedly
+ *   invoke method {@code validate()}.  For example, such steps are
+ *   typically required when first reading an object or array
+ *   reference, and then accessing one of its fields, elements or
+ *   methods. </li>
+ *
+ * </ul>
+ *
+ * <p>This class also supports methods that conditionally provide
+ * conversions across the three modes. For example, method {@link
+ * #tryConvertToWriteLock} attempts to "upgrade" a mode, returning
+ * a valid write stamp if (1) already in writing mode (2) in reading
+ * mode and there are no other readers or (3) in optimistic mode and
+ * the lock is available. The forms of these methods are designed to
+ * help reduce some of the code bloat that otherwise occurs in
+ * retry-based designs.
+ *
+ * <p>StampedLocks are designed for use as internal utilities in the
+ * development of thread-safe components. Their use relies on
+ * knowledge of the internal properties of the data, objects, and
+ * methods they are protecting.  They are not reentrant, so locked
+ * bodies should not call other unknown methods that may try to
+ * re-acquire locks (although you may pass a stamp to other methods
+ * that can use or convert it).  The use of read lock modes relies on
+ * the associated code sections being side-effect-free.  Unvalidated
+ * optimistic read sections cannot call methods that are not known to
+ * tolerate potential inconsistencies.  Stamps use finite
+ * representations, and are not cryptographically secure (i.e., a
+ * valid stamp may be guessable). Stamp values may recycle after (no
+ * sooner than) one year of continuous operation. A stamp held without
+ * use or validation for longer than this period may fail to validate
+ * correctly.  StampedLocks are serializable, but always deserialize
+ * into initial unlocked state, so they are not useful for remote
+ * locking.
+ *
+ * <p>The scheduling policy of StampedLock does not consistently
+ * prefer readers over writers or vice versa.  All "try" methods are
+ * best-effort and do not necessarily conform to any scheduling or
+ * fairness policy. A zero return from any "try" method for acquiring
+ * or converting locks does not carry any information about the state
+ * of the lock; a subsequent invocation may succeed.
+ *
+ * <p>Because it supports coordinated usage across multiple lock
+ * modes, this class does not directly implement the {@link Lock} or
+ * {@link ReadWriteLock} interfaces. However, a StampedLock may be
+ * viewed {@link #asReadLock()}, {@link #asWriteLock()}, or {@link
+ * #asReadWriteLock()} in applications requiring only the associated
+ * set of functionality.
+ *
+ * <p><b>Sample Usage.</b> The following illustrates some usage idioms
+ * in a class that maintains simple two-dimensional points. The sample
+ * code illustrates some try/catch conventions even though they are
+ * not strictly needed here because no exceptions can occur in their
+ * bodies.<br>
+ *
+ *  <pre>{@code
+ * class Point {
+ *   private double x, y;
+ *   private final StampedLock sl = new StampedLock();
+ *
+ *   void move(double deltaX, double deltaY) { // an exclusively locked method
+ *     long stamp = sl.writeLock();
+ *     try {
+ *       x += deltaX;
+ *       y += deltaY;
+ *     } finally {
+ *       sl.unlockWrite(stamp);
+ *     }
+ *   }
+ *
+ *   double distanceFromOrigin() { // A read-only method
+ *     long stamp = sl.tryOptimisticRead();
+ *     double currentX = x, currentY = y;
+ *     if (!sl.validate(stamp)) {
+ *        stamp = sl.readLock();
+ *        try {
+ *          currentX = x;
+ *          currentY = y;
+ *        } finally {
+ *           sl.unlockRead(stamp);
+ *        }
+ *     }
+ *     return Math.sqrt(currentX * currentX + currentY * currentY);
+ *   }
+ *
+ *   void moveIfAtOrigin(double newX, double newY) { // upgrade
+ *     // Could instead start with optimistic, not read mode
+ *     long stamp = sl.readLock();
+ *     try {
+ *       while (x == 0.0 && y == 0.0) {
+ *         long ws = sl.tryConvertToWriteLock(stamp);
+ *         if (ws != 0L) {
+ *           stamp = ws;
+ *           x = newX;
+ *           y = newY;
+ *           break;
+ *         }
+ *         else {
+ *           sl.unlockRead(stamp);
+ *           stamp = sl.writeLock();
+ *         }
+ *       }
+ *     } finally {
+ *       sl.unlock(stamp);
+ *     }
+ *   }
+ * }}</pre>
+ *
+ * @since 1.8
+ * @author Doug Lea
+ */
+public class StampedLock implements java.io.Serializable {
+    /*
+     * Algorithmic notes:
+     *
+     * The design employs elements of Sequence locks
+     * (as used in linux kernels; see Lameter's
+     * http://www.lameter.com/gelato2005.pdf
+     * and elsewhere; see
+     * Boehm's http://www.hpl.hp.com/techreports/2012/HPL-2012-68.html)
+     * and Ordered RW locks (see Shirako et al
+     * http://dl.acm.org/citation.cfm?id=2312015)
+     *
+     * Conceptually, the primary state of the lock includes a sequence
+     * number that is odd when write-locked and even otherwise.
+     * However, this is offset by a reader count that is non-zero when
+     * read-locked.  The read count is ignored when validating
+     * "optimistic" seqlock-reader-style stamps.  Because we must use
+     * a small finite number of bits (currently 7) for readers, a
+     * supplementary reader overflow word is used when the number of
+     * readers exceeds the count field. We do this by treating the max
+     * reader count value (RBITS) as a spinlock protecting overflow
+     * updates.
+     *
+     * Waiters use a modified form of CLH lock used in
+     * AbstractQueuedSynchronizer (see its internal documentation for
+     * a fuller account), where each node is tagged (field mode) as
+     * either a reader or writer. Sets of waiting readers are grouped
+     * (linked) under a common node (field cowait) so act as a single
+     * node with respect to most CLH mechanics.  By virtue of the
+     * queue structure, wait nodes need not actually carry sequence
+     * numbers; we know each is greater than its predecessor.  This
+     * simplifies the scheduling policy to a mainly-FIFO scheme that
+     * incorporates elements of Phase-Fair locks (see Brandenburg &
+     * Anderson, especially http://www.cs.unc.edu/~bbb/diss/).  In
+     * particular, we use the phase-fair anti-barging rule: If an
+     * incoming reader arrives while read lock is held but there is a
+     * queued writer, this incoming reader is queued.  (This rule is
+     * responsible for some of the complexity of method acquireRead,
+     * but without it, the lock becomes highly unfair.) Method release
+     * does not (and sometimes cannot) itself wake up cowaiters. This
+     * is done by the primary thread, but helped by any other threads
+     * with nothing better to do in methods acquireRead and
+     * acquireWrite.
+     *
+     * These rules apply to threads actually queued. All tryLock forms
+     * opportunistically try to acquire locks regardless of preference
+     * rules, and so may "barge" their way in.  Randomized spinning is
+     * used in the acquire methods to reduce (increasingly expensive)
+     * context switching while also avoiding sustained memory
+     * thrashing among many threads.  We limit spins to the head of
+     * queue. A thread spin-waits up to SPINS times (where each
+     * iteration decreases spin count with 50% probability) before
+     * blocking. If, upon wakening it fails to obtain lock, and is
+     * still (or becomes) the first waiting thread (which indicates
+     * that some other thread barged and obtained lock), it escalates
+     * spins (up to MAX_HEAD_SPINS) to reduce the likelihood of
+     * continually losing to barging threads.
+     *
+     * Nearly all of these mechanics are carried out in methods
+     * acquireWrite and acquireRead, that, as typical of such code,
+     * sprawl out because actions and retries rely on consistent sets
+     * of locally cached reads.
+     *
+     * As noted in Boehm's paper (above), sequence validation (mainly
+     * method validate()) requires stricter ordering rules than apply
+     * to normal volatile reads (of "state").  In the absence of (but
+     * continual hope for) explicit JVM support of intrinsics with
+     * double-sided reordering prohibition, or corresponding fence
+     * intrinsics, we for now uncomfortably rely on the fact that the
+     * Unsafe.getXVolatile intrinsic must have this property
+     * (syntactic volatile reads do not) for internal purposes anyway,
+     * even though it is not documented.
+     *
+     * The memory layout keeps lock state and queue pointers together
+     * (normally on the same cache line). This usually works well for
+     * read-mostly loads. In most other cases, the natural tendency of
+     * adaptive-spin CLH locks to reduce memory contention lessens
+     * motivation to further spread out contended locations, but might
+     * be subject to future improvements.
+     */
+
+    private static final long serialVersionUID = -6001602636862214147L;
+
+    /** Number of processors, for spin control */
+    private static final int NCPU = Runtime.getRuntime().availableProcessors();
+
+    /** Maximum number of retries before enqueuing on acquisition */
+    private static final int SPINS = (NCPU > 1) ? 1 << 6 : 0;
+
+    /** Maximum number of retries before blocking at head on acquisition */
+    private static final int HEAD_SPINS = (NCPU > 1) ? 1 << 10 : 0;
+
+    /** Maximum number of retries before re-blocking */
+    private static final int MAX_HEAD_SPINS = (NCPU > 1) ? 1 << 16 : 0;
+
+    /** The period for yielding when waiting for overflow spinlock */
+    private static final int OVERFLOW_YIELD_RATE = 7; // must be power 2 - 1
+
+    /** The number of bits to use for reader count before overflowing */
+    private static final int LG_READERS = 7;
+
+    // Values for lock state and stamp operations
+    private static final long RUNIT = 1L;
+    private static final long WBIT  = 1L << LG_READERS;
+    private static final long RBITS = WBIT - 1L;
+    private static final long RFULL = RBITS - 1L;
+    private static final long ABITS = RBITS | WBIT;
+    private static final long SBITS = ~RBITS; // note overlap with ABITS
+
+    // Initial value for lock state; avoid failure value zero
+    private static final long ORIGIN = WBIT << 1;
+
+    // Special value from cancelled acquire methods so caller can throw IE
+    private static final long INTERRUPTED = 1L;
+
+    // Values for node status; order matters
+    private static final int WAITING   = -1;
+    private static final int CANCELLED =  1;
+
+    // Modes for nodes (int not boolean to allow arithmetic)
+    private static final int RMODE = 0;
+    private static final int WMODE = 1;
+
+    /** Wait nodes */
+    static final class WNode {
+        volatile WNode prev;
+        volatile WNode next;
+        volatile WNode cowait;    // list of linked readers
+        volatile Thread thread;   // non-null while possibly parked
+        volatile int status;      // 0, WAITING, or CANCELLED
+        final int mode;           // RMODE or WMODE
+        WNode(int m, WNode p) { mode = m; prev = p; }
+    }
+
+    /** Head of CLH queue */
+    private transient volatile WNode whead;
+    /** Tail (last) of CLH queue */
+    private transient volatile WNode wtail;
+
+    // views
+    transient ReadLockView readLockView;
+    transient WriteLockView writeLockView;
+    transient ReadWriteLockView readWriteLockView;
+
+    /** Lock sequence/state */
+    private transient volatile long state;
+    /** extra reader count when state read count saturated */
+    private transient int readerOverflow;
+
+    /**
+     * Creates a new lock, initially in unlocked state.
+     */
+    public StampedLock() {
+        state = ORIGIN;
+    }
+
+    /**
+     * Exclusively acquires the lock, blocking if necessary
+     * until available.
+     *
+     * @return a stamp that can be used to unlock or convert mode
+     */
+    public long writeLock() {
+        long s, next;  // bypass acquireWrite in fully unlocked case only
+        return ((((s = state) & ABITS) == 0L &&
+                 U.compareAndSwapLong(this, STATE, s, next = s + WBIT)) ?
+                next : acquireWrite(false, 0L));
+    }
+
+    /**
+     * Exclusively acquires the lock if it is immediately available.
+     *
+     * @return a stamp that can be used to unlock or convert mode,
+     * or zero if the lock is not available
+     */
+    public long tryWriteLock() {
+        long s, next;
+        return ((((s = state) & ABITS) == 0L &&
+                 U.compareAndSwapLong(this, STATE, s, next = s + WBIT)) ?
+                next : 0L);
+    }
+
+    /**
+     * Exclusively acquires the lock if it is available within the
+     * given time and the current thread has not been interrupted.
+     * Behavior under timeout and interruption matches that specified
+     * for method {@link Lock#tryLock(long,TimeUnit)}.
+     *
+     * @param time the maximum time to wait for the lock
+     * @param unit the time unit of the {@code time} argument
+     * @return a stamp that can be used to unlock or convert mode,
+     * or zero if the lock is not available
+     * @throws InterruptedException if the current thread is interrupted
+     * before acquiring the lock
+     */
+    public long tryWriteLock(long time, TimeUnit unit)
+        throws InterruptedException {
+        long nanos = unit.toNanos(time);
+        if (!Thread.interrupted()) {
+            long next, deadline;
+            if ((next = tryWriteLock()) != 0L)
+                return next;
+            if (nanos <= 0L)
+                return 0L;
+            if ((deadline = System.nanoTime() + nanos) == 0L)
+                deadline = 1L;
+            if ((next = acquireWrite(true, deadline)) != INTERRUPTED)
+                return next;
+        }
+        throw new InterruptedException();
+    }
+
+    /**
+     * Exclusively acquires the lock, blocking if necessary
+     * until available or the current thread is interrupted.
+     * Behavior under interruption matches that specified
+     * for method {@link Lock#lockInterruptibly()}.
+     *
+     * @return a stamp that can be used to unlock or convert mode
+     * @throws InterruptedException if the current thread is interrupted
+     * before acquiring the lock
+     */
+    public long writeLockInterruptibly() throws InterruptedException {
+        long next;
+        if (!Thread.interrupted() &&
+            (next = acquireWrite(true, 0L)) != INTERRUPTED)
+            return next;
+        throw new InterruptedException();
+    }
+
+    /**
+     * Non-exclusively acquires the lock, blocking if necessary
+     * until available.
+     *
+     * @return a stamp that can be used to unlock or convert mode
+     */
+    public long readLock() {
+        long s = state, next;  // bypass acquireRead on common uncontended case
+        return ((whead == wtail && (s & ABITS) < RFULL &&
+                 U.compareAndSwapLong(this, STATE, s, next = s + RUNIT)) ?
+                next : acquireRead(false, 0L));
+    }
+
+    /**
+     * Non-exclusively acquires the lock if it is immediately available.
+     *
+     * @return a stamp that can be used to unlock or convert mode,
+     * or zero if the lock is not available
+     */
+    public long tryReadLock() {
+        for (;;) {
+            long s, m, next;
+            if ((m = (s = state) & ABITS) == WBIT)
+                return 0L;
+            else if (m < RFULL) {
+                if (U.compareAndSwapLong(this, STATE, s, next = s + RUNIT))
+                    return next;
+            }
+            else if ((next = tryIncReaderOverflow(s)) != 0L)
+                return next;
+        }
+    }
+
+    /**
+     * Non-exclusively acquires the lock if it is available within the
+     * given time and the current thread has not been interrupted.
+     * Behavior under timeout and interruption matches that specified
+     * for method {@link Lock#tryLock(long,TimeUnit)}.
+     *
+     * @param time the maximum time to wait for the lock
+     * @param unit the time unit of the {@code time} argument
+     * @return a stamp that can be used to unlock or convert mode,
+     * or zero if the lock is not available
+     * @throws InterruptedException if the current thread is interrupted
+     * before acquiring the lock
+     */
+    public long tryReadLock(long time, TimeUnit unit)
+        throws InterruptedException {
+        long s, m, next, deadline;
+        long nanos = unit.toNanos(time);
+        if (!Thread.interrupted()) {
+            if ((m = (s = state) & ABITS) != WBIT) {
+                if (m < RFULL) {
+                    if (U.compareAndSwapLong(this, STATE, s, next = s + RUNIT))
+                        return next;
+                }
+                else if ((next = tryIncReaderOverflow(s)) != 0L)
+                    return next;
+            }
+            if (nanos <= 0L)
+                return 0L;
+            if ((deadline = System.nanoTime() + nanos) == 0L)
+                deadline = 1L;
+            if ((next = acquireRead(true, deadline)) != INTERRUPTED)
+                return next;
+        }
+        throw new InterruptedException();
+    }
+
+    /**
+     * Non-exclusively acquires the lock, blocking if necessary
+     * until available or the current thread is interrupted.
+     * Behavior under interruption matches that specified
+     * for method {@link Lock#lockInterruptibly()}.
+     *
+     * @return a stamp that can be used to unlock or convert mode
+     * @throws InterruptedException if the current thread is interrupted
+     * before acquiring the lock
+     */
+    public long readLockInterruptibly() throws InterruptedException {
+        long next;
+        if (!Thread.interrupted() &&
+            (next = acquireRead(true, 0L)) != INTERRUPTED)
+            return next;
+        throw new InterruptedException();
+    }
+
+    /**
+     * Returns a stamp that can later be validated, or zero
+     * if exclusively locked.
+     *
+     * @return a stamp, or zero if exclusively locked
+     */
+    public long tryOptimisticRead() {
+        long s;
+        return (((s = state) & WBIT) == 0L) ? (s & SBITS) : 0L;
+    }
+
+    /**
+     * Returns true if the lock has not been exclusively acquired
+     * since issuance of the given stamp. Always returns false if the
+     * stamp is zero. Always returns true if the stamp represents a
+     * currently held lock. Invoking this method with a value not
+     * obtained from {@link #tryOptimisticRead} or a locking method
+     * for this lock has no defined effect or result.
+     *
+     * @param stamp a stamp
+     * @return {@code true} if the lock has not been exclusively acquired
+     * since issuance of the given stamp; else false
+     */
+    public boolean validate(long stamp) {
+        // See above about current use of getLongVolatile here
+        return (stamp & SBITS) == (U.getLongVolatile(this, STATE) & SBITS);
+    }
+
+    /**
+     * If the lock state matches the given stamp, releases the
+     * exclusive lock.
+     *
+     * @param stamp a stamp returned by a write-lock operation
+     * @throws IllegalMonitorStateException if the stamp does
+     * not match the current state of this lock
+     */
+    public void unlockWrite(long stamp) {
+        WNode h;
+        if (state != stamp || (stamp & WBIT) == 0L)
+            throw new IllegalMonitorStateException();
+        state = (stamp += WBIT) == 0L ? ORIGIN : stamp;
+        if ((h = whead) != null && h.status != 0)
+            release(h);
+    }
+
+    /**
+     * If the lock state matches the given stamp, releases the
+     * non-exclusive lock.
+     *
+     * @param stamp a stamp returned by a read-lock operation
+     * @throws IllegalMonitorStateException if the stamp does
+     * not match the current state of this lock
+     */
+    public void unlockRead(long stamp) {
+        long s, m; WNode h;
+        for (;;) {
+            if (((s = state) & SBITS) != (stamp & SBITS) ||
+                (stamp & ABITS) == 0L || (m = s & ABITS) == 0L || m == WBIT)
+                throw new IllegalMonitorStateException();
+            if (m < RFULL) {
+                if (U.compareAndSwapLong(this, STATE, s, s - RUNIT)) {
+                    if (m == RUNIT && (h = whead) != null && h.status != 0)
+                        release(h);
+                    break;
+                }
+            }
+            else if (tryDecReaderOverflow(s) != 0L)
+                break;
+        }
+    }
+
+    /**
+     * If the lock state matches the given stamp, releases the
+     * corresponding mode of the lock.
+     *
+     * @param stamp a stamp returned by a lock operation
+     * @throws IllegalMonitorStateException if the stamp does
+     * not match the current state of this lock
+     */
+    public void unlock(long stamp) {
+        long a = stamp & ABITS, m, s; WNode h;
+        while (((s = state) & SBITS) == (stamp & SBITS)) {
+            if ((m = s & ABITS) == 0L)
+                break;
+            else if (m == WBIT) {
+                if (a != m)
+                    break;
+                state = (s += WBIT) == 0L ? ORIGIN : s;
+                if ((h = whead) != null && h.status != 0)
+                    release(h);
+                return;
+            }
+            else if (a == 0L || a >= WBIT)
+                break;
+            else if (m < RFULL) {
+                if (U.compareAndSwapLong(this, STATE, s, s - RUNIT)) {
+                    if (m == RUNIT && (h = whead) != null && h.status != 0)
+                        release(h);
+                    return;
+                }
+            }
+            else if (tryDecReaderOverflow(s) != 0L)
+                return;
+        }
+        throw new IllegalMonitorStateException();
+    }
+
+    /**
+     * If the lock state matches the given stamp, performs one of
+     * the following actions. If the stamp represents holding a write
+     * lock, returns it.  Or, if a read lock, if the write lock is
+     * available, releases the read lock and returns a write stamp.
+     * Or, if an optimistic read, returns a write stamp only if
+     * immediately available. This method returns zero in all other
+     * cases.
+     *
+     * @param stamp a stamp
+     * @return a valid write stamp, or zero on failure
+     */
+    public long tryConvertToWriteLock(long stamp) {
+        long a = stamp & ABITS, m, s, next;
+        while (((s = state) & SBITS) == (stamp & SBITS)) {
+            if ((m = s & ABITS) == 0L) {
+                if (a != 0L)
+                    break;
+                if (U.compareAndSwapLong(this, STATE, s, next = s + WBIT))
+                    return next;
+            }
+            else if (m == WBIT) {
+                if (a != m)
+                    break;
+                return stamp;
+            }
+            else if (m == RUNIT && a != 0L) {
+                if (U.compareAndSwapLong(this, STATE, s,
+                                         next = s - RUNIT + WBIT))
+                    return next;
+            }
+            else
+                break;
+        }
+        return 0L;
+    }
+
+    /**
+     * If the lock state matches the given stamp, performs one of
+     * the following actions. If the stamp represents holding a write
+     * lock, releases it and obtains a read lock.  Or, if a read lock,
+     * returns it. Or, if an optimistic read, acquires a read lock and
+     * returns a read stamp only if immediately available. This method
+     * returns zero in all other cases.
+     *
+     * @param stamp a stamp
+     * @return a valid read stamp, or zero on failure
+     */
+    public long tryConvertToReadLock(long stamp) {
+        long a = stamp & ABITS, m, s, next; WNode h;
+        while (((s = state) & SBITS) == (stamp & SBITS)) {
+            if ((m = s & ABITS) == 0L) {
+                if (a != 0L)
+                    break;
+                else if (m < RFULL) {
+                    if (U.compareAndSwapLong(this, STATE, s, next = s + RUNIT))
+                        return next;
+                }
+                else if ((next = tryIncReaderOverflow(s)) != 0L)
+                    return next;
+            }
+            else if (m == WBIT) {
+                if (a != m)
+                    break;
+                state = next = s + (WBIT + RUNIT);
+                if ((h = whead) != null && h.status != 0)
+                    release(h);
+                return next;
+            }
+            else if (a != 0L && a < WBIT)
+                return stamp;
+            else
+                break;
+        }
+        return 0L;
+    }
+
+    /**
+     * If the lock state matches the given stamp then, if the stamp
+     * represents holding a lock, releases it and returns an
+     * observation stamp.  Or, if an optimistic read, returns it if
+     * validated. This method returns zero in all other cases, and so
+     * may be useful as a form of "tryUnlock".
+     *
+     * @param stamp a stamp
+     * @return a valid optimistic read stamp, or zero on failure
+     */
+    public long tryConvertToOptimisticRead(long stamp) {
+        long a = stamp & ABITS, m, s, next; WNode h;
+        for (;;) {
+            s = U.getLongVolatile(this, STATE); // see above
+            if (((s = state) & SBITS) != (stamp & SBITS))
+                break;
+            if ((m = s & ABITS) == 0L) {
+                if (a != 0L)
+                    break;
+                return s;
+            }
+            else if (m == WBIT) {
+                if (a != m)
+                    break;
+                state = next = (s += WBIT) == 0L ? ORIGIN : s;
+                if ((h = whead) != null && h.status != 0)
+                    release(h);
+                return next;
+            }
+            else if (a == 0L || a >= WBIT)
+                break;
+            else if (m < RFULL) {
+                if (U.compareAndSwapLong(this, STATE, s, next = s - RUNIT)) {
+                    if (m == RUNIT && (h = whead) != null && h.status != 0)
+                        release(h);
+                    return next & SBITS;
+                }
+            }
+            else if ((next = tryDecReaderOverflow(s)) != 0L)
+                return next & SBITS;
+        }
+        return 0L;
+    }
+
+    /**
+     * Releases the write lock if it is held, without requiring a
+     * stamp value. This method may be useful for recovery after
+     * errors.
+     *
+     * @return {@code true} if the lock was held, else false
+     */
+    public boolean tryUnlockWrite() {
+        long s; WNode h;
+        if (((s = state) & WBIT) != 0L) {
+            state = (s += WBIT) == 0L ? ORIGIN : s;
+            if ((h = whead) != null && h.status != 0)
+                release(h);
+            return true;
+        }
+        return false;
+    }
+
+    /**
+     * Releases one hold of the read lock if it is held, without
+     * requiring a stamp value. This method may be useful for recovery
+     * after errors.
+     *
+     * @return {@code true} if the read lock was held, else false
+     */
+    public boolean tryUnlockRead() {
+        long s, m; WNode h;
+        while ((m = (s = state) & ABITS) != 0L && m < WBIT) {
+            if (m < RFULL) {
+                if (U.compareAndSwapLong(this, STATE, s, s - RUNIT)) {
+                    if (m == RUNIT && (h = whead) != null && h.status != 0)
+                        release(h);
+                    return true;
+                }
+            }
+            else if (tryDecReaderOverflow(s) != 0L)
+                return true;
+        }
+        return false;
+    }
+
+    // status monitoring methods
+
+    /**
+     * Returns combined state-held and overflow read count for given
+     * state s.
+     */
+    private int getReadLockCount(long s) {
+        long readers;
+        if ((readers = s & RBITS) >= RFULL)
+            readers = RFULL + readerOverflow;
+        return (int) readers;
+    }
+
+    /**
+     * Returns {@code true} if the lock is currently held exclusively.
+     *
+     * @return {@code true} if the lock is currently held exclusively
+     */
+    public boolean isWriteLocked() {
+        return (state & WBIT) != 0L;
+    }
+
+    /**
+     * Returns {@code true} if the lock is currently held non-exclusively.
+     *
+     * @return {@code true} if the lock is currently held non-exclusively
+     */
+    public boolean isReadLocked() {
+        return (state & RBITS) != 0L;
+    }
+
+    /**
+     * Queries the number of read locks held for this lock. This
+     * method is designed for use in monitoring system state, not for
+     * synchronization control.
+     * @return the number of read locks held
+     */
+    public int getReadLockCount() {
+        return getReadLockCount(state);
+    }
+
+    /**
+     * Returns a string identifying this lock, as well as its lock
+     * state.  The state, in brackets, includes the String {@code
+     * "Unlocked"} or the String {@code "Write-locked"} or the String
+     * {@code "Read-locks:"} followed by the current number of
+     * read-locks held.
+     *
+     * @return a string identifying this lock, as well as its lock state
+     */
+    public String toString() {
+        long s = state;
+        return super.toString() +
+            ((s & ABITS) == 0L ? "[Unlocked]" :
+             (s & WBIT) != 0L ? "[Write-locked]" :
+             "[Read-locks:" + getReadLockCount(s) + "]");
+    }
+
+    // views
+
+    /**
+     * Returns a plain {@link Lock} view of this StampedLock in which
+     * the {@link Lock#lock} method is mapped to {@link #readLock},
+     * and similarly for other methods. The returned Lock does not
+     * support a {@link Condition}; method {@link
+     * Lock#newCondition()} throws {@code
+     * UnsupportedOperationException}.
+     *
+     * @return the lock
+     */
+    public Lock asReadLock() {
+        ReadLockView v;
+        return ((v = readLockView) != null ? v :
+                (readLockView = new ReadLockView()));
+    }
+
+    /**
+     * Returns a plain {@link Lock} view of this StampedLock in which
+     * the {@link Lock#lock} method is mapped to {@link #writeLock},
+     * and similarly for other methods. The returned Lock does not
+     * support a {@link Condition}; method {@link
+     * Lock#newCondition()} throws {@code
+     * UnsupportedOperationException}.
+     *
+     * @return the lock
+     */
+    public Lock asWriteLock() {
+        WriteLockView v;
+        return ((v = writeLockView) != null ? v :
+                (writeLockView = new WriteLockView()));
+    }
+
+    /**
+     * Returns a {@link ReadWriteLock} view of this StampedLock in
+     * which the {@link ReadWriteLock#readLock()} method is mapped to
+     * {@link #asReadLock()}, and {@link ReadWriteLock#writeLock()} to
+     * {@link #asWriteLock()}.
+     *
+     * @return the lock
+     */
+    public ReadWriteLock asReadWriteLock() {
+        ReadWriteLockView v;
+        return ((v = readWriteLockView) != null ? v :
+                (readWriteLockView = new ReadWriteLockView()));
+    }
+
+    // view classes
+
+    final class ReadLockView implements Lock {
+        public void lock() { readLock(); }
+        public void lockInterruptibly() throws InterruptedException {
+            readLockInterruptibly();
+        }
+        public boolean tryLock() { return tryReadLock() != 0L; }
+        public boolean tryLock(long time, TimeUnit unit)
+            throws InterruptedException {
+            return tryReadLock(time, unit) != 0L;
+        }
+        public void unlock() { unstampedUnlockRead(); }
+        public Condition newCondition() {
+            throw new UnsupportedOperationException();
+        }
+    }
+
+    final class WriteLockView implements Lock {
+        public void lock() { writeLock(); }
+        public void lockInterruptibly() throws InterruptedException {
+            writeLockInterruptibly();
+        }
+        public boolean tryLock() { return tryWriteLock() != 0L; }
+        public boolean tryLock(long time, TimeUnit unit)
+            throws InterruptedException {
+            return tryWriteLock(time, unit) != 0L;
+        }
+        public void unlock() { unstampedUnlockWrite(); }
+        public Condition newCondition() {
+            throw new UnsupportedOperationException();
+        }
+    }
+
+    final class ReadWriteLockView implements ReadWriteLock {
+        public Lock readLock() { return asReadLock(); }
+        public Lock writeLock() { return asWriteLock(); }
+    }
+
+    // Unlock methods without stamp argument checks for view classes.
+    // Needed because view-class lock methods throw away stamps.
+
+    final void unstampedUnlockWrite() {
+        WNode h; long s;
+        if (((s = state) & WBIT) == 0L)
+            throw new IllegalMonitorStateException();
+        state = (s += WBIT) == 0L ? ORIGIN : s;
+        if ((h = whead) != null && h.status != 0)
+            release(h);
+    }
+
+    final void unstampedUnlockRead() {
+        for (;;) {
+            long s, m; WNode h;
+            if ((m = (s = state) & ABITS) == 0L || m >= WBIT)
+                throw new IllegalMonitorStateException();
+            else if (m < RFULL) {
+                if (U.compareAndSwapLong(this, STATE, s, s - RUNIT)) {
+                    if (m == RUNIT && (h = whead) != null && h.status != 0)
+                        release(h);
+                    break;
+                }
+            }
+            else if (tryDecReaderOverflow(s) != 0L)
+                break;
+        }
+    }
+
+    private void readObject(java.io.ObjectInputStream s)
+        throws java.io.IOException, ClassNotFoundException {
+        s.defaultReadObject();
+        state = ORIGIN; // reset to unlocked state
+    }
+
+    // internals
+
+    /**
+     * Tries to increment readerOverflow by first setting state
+     * access bits value to RBITS, indicating hold of spinlock,
+     * then updating, then releasing.
+     *
+     * @param s a reader overflow stamp: (s & ABITS) >= RFULL
+     * @return new stamp on success, else zero
+     */
+    private long tryIncReaderOverflow(long s) {
+        // assert (s & ABITS) >= RFULL;
+        if ((s & ABITS) == RFULL) {
+            if (U.compareAndSwapLong(this, STATE, s, s | RBITS)) {
+                ++readerOverflow;
+                state = s;
+                return s;
+            }
+        }
+        else if ((ThreadLocalRandom.current().nextInt() &
+                  OVERFLOW_YIELD_RATE) == 0)
+            Thread.yield();
+        return 0L;
+    }
+
+    /**
+     * Tries to decrement readerOverflow.
+     *
+     * @param s a reader overflow stamp: (s & ABITS) >= RFULL
+     * @return new stamp on success, else zero
+     */
+    private long tryDecReaderOverflow(long s) {
+        // assert (s & ABITS) >= RFULL;
+        if ((s & ABITS) == RFULL) {
+            if (U.compareAndSwapLong(this, STATE, s, s | RBITS)) {
+                int r; long next;
+                if ((r = readerOverflow) > 0) {
+                    readerOverflow = r - 1;
+                    next = s;
+                }
+                else
+                    next = s - RUNIT;
+                 state = next;
+                 return next;
+            }
+        }
+        else if ((ThreadLocalRandom.current().nextInt() &
+                  OVERFLOW_YIELD_RATE) == 0)
+            Thread.yield();
+        return 0L;
+    }
+
+    /**
+     * Wakes up the successor of h (normally whead). This is normally
+     * just h.next, but may require traversal from wtail if next
+     * pointers are lagging. This may fail to wake up an acquiring
+     * thread when one or more have been cancelled, but the cancel
+     * methods themselves provide extra safeguards to ensure liveness.
+     */
+    private void release(WNode h) {
+        if (h != null) {
+            WNode q; Thread w;
+            U.compareAndSwapInt(h, WSTATUS, WAITING, 0);
+            if ((q = h.next) == null || q.status == CANCELLED) {
+                for (WNode t = wtail; t != null && t != h; t = t.prev)
+                    if (t.status <= 0)
+                        q = t;
+            }
+            if (q != null && (w = q.thread) != null)
+                U.unpark(w);
+        }
+    }
+
+    /**
+     * See above for explanation.
+     *
+     * @param interruptible true if should check interrupts and if so
+     * return INTERRUPTED
+     * @param deadline if nonzero, the System.nanoTime value to timeout
+     * at (and return zero)
+     * @return next state, or INTERRUPTED
+     */
+    private long acquireWrite(boolean interruptible, long deadline) {
+        WNode node = null, p;
+        for (int spins = -1;;) { // spin while enqueuing
+            long m, s, ns;
+            if ((m = (s = state) & ABITS) == 0L) {
+                if (U.compareAndSwapLong(this, STATE, s, ns = s + WBIT))
+                    return ns;
+            }
+            else if (spins < 0)
+                spins = (m == WBIT && wtail == whead) ? SPINS : 0;
+            else if (spins > 0) {
+                if (ThreadLocalRandom.current().nextInt() >= 0)
+                    --spins;
+            }
+            else if ((p = wtail) == null) { // initialize queue
+                WNode hd = new WNode(WMODE, null);
+                if (U.compareAndSwapObject(this, WHEAD, null, hd))
+                    wtail = hd;
+            }
+            else if (node == null)
+                node = new WNode(WMODE, p);
+            else if (node.prev != p)
+                node.prev = p;
+            else if (U.compareAndSwapObject(this, WTAIL, p, node)) {
+                p.next = node;
+                break;
+            }
+        }
+
+        for (int spins = -1;;) {
+            WNode h, np, pp; int ps;
+            if ((h = whead) == p) {
+                if (spins < 0)
+                    spins = HEAD_SPINS;
+                else if (spins < MAX_HEAD_SPINS)
+                    spins <<= 1;
+                for (int k = spins;;) { // spin at head
+                    long s, ns;
+                    if (((s = state) & ABITS) == 0L) {
+                        if (U.compareAndSwapLong(this, STATE, s,
+                                                 ns = s + WBIT)) {
+                            whead = node;
+                            node.prev = null;
+                            return ns;
+                        }
+                    }
+                    else if (ThreadLocalRandom.current().nextInt() >= 0 &&
+                             --k <= 0)
+                        break;
+                }
+            }
+            else if (h != null) { // help release stale waiters
+                WNode c; Thread w;
+                while ((c = h.cowait) != null) {
+                    if (U.compareAndSwapObject(h, WCOWAIT, c, c.cowait) &&
+                        (w = c.thread) != null)
+                        U.unpark(w);
+                }
+            }
+            if (whead == h) {
+                if ((np = node.prev) != p) {
+                    if (np != null)
+                        (p = np).next = node;   // stale
+                }
+                else if ((ps = p.status) == 0)
+                    U.compareAndSwapInt(p, WSTATUS, 0, WAITING);
+                else if (ps == CANCELLED) {
+                    if ((pp = p.prev) != null) {
+                        node.prev = pp;
+                        pp.next = node;
+                    }
+                }
+                else {
+                    long time; // 0 argument to park means no timeout
+                    if (deadline == 0L)
+                        time = 0L;
+                    else if ((time = deadline - System.nanoTime()) <= 0L)
+                        return cancelWaiter(node, node, false);
+                    Thread wt = Thread.currentThread();
+                    U.putObject(wt, PARKBLOCKER, this);
+                    node.thread = wt;
+                    if (p.status < 0 && (p != h || (state & ABITS) != 0L) &&
+                        whead == h && node.prev == p)
+                        U.park(false, time);  // emulate LockSupport.park
+                    node.thread = null;
+                    U.putObject(wt, PARKBLOCKER, null);
+                    if (interruptible && Thread.interrupted())
+                        return cancelWaiter(node, node, true);
+                }
+            }
+        }
+    }
+
+    /**
+     * See above for explanation.
+     *
+     * @param interruptible true if should check interrupts and if so
+     * return INTERRUPTED
+     * @param deadline if nonzero, the System.nanoTime value to timeout
+     * at (and return zero)
+     * @return next state, or INTERRUPTED
+     */
+    private long acquireRead(boolean interruptible, long deadline) {
+        WNode node = null, p;
+        for (int spins = -1;;) {
+            WNode h;
+            if ((h = whead) == (p = wtail)) {
+                for (long m, s, ns;;) {
+                    if ((m = (s = state) & ABITS) < RFULL ?
+                        U.compareAndSwapLong(this, STATE, s, ns = s + RUNIT) :
+                        (m < WBIT && (ns = tryIncReaderOverflow(s)) != 0L))
+                        return ns;
+                    else if (m >= WBIT) {
+                        if (spins > 0) {
+                            if (ThreadLocalRandom.current().nextInt() >= 0)
+                                --spins;
+                        }
+                        else {
+                            if (spins == 0) {
+                                WNode nh = whead, np = wtail;
+                                if ((nh == h && np == p) || (h = nh) != (p = np))
+                                    break;
+                            }
+                            spins = SPINS;
+                        }
+                    }
+                }
+            }
+            if (p == null) { // initialize queue
+                WNode hd = new WNode(WMODE, null);
+                if (U.compareAndSwapObject(this, WHEAD, null, hd))
+                    wtail = hd;
+            }
+            else if (node == null)
+                node = new WNode(RMODE, p);
+            else if (h == p || p.mode != RMODE) {
+                if (node.prev != p)
+                    node.prev = p;
+                else if (U.compareAndSwapObject(this, WTAIL, p, node)) {
+                    p.next = node;
+                    break;
+                }
+            }
+            else if (!U.compareAndSwapObject(p, WCOWAIT,
+                                             node.cowait = p.cowait, node))
+                node.cowait = null;
+            else {
+                for (;;) {
+                    WNode pp, c; Thread w;
+                    if ((h = whead) != null && (c = h.cowait) != null &&
+                        U.compareAndSwapObject(h, WCOWAIT, c, c.cowait) &&
+                        (w = c.thread) != null) // help release
+                        U.unpark(w);
+                    if (h == (pp = p.prev) || h == p || pp == null) {
+                        long m, s, ns;
+                        do {
+                            if ((m = (s = state) & ABITS) < RFULL ?
+                                U.compareAndSwapLong(this, STATE, s,
+                                                     ns = s + RUNIT) :
+                                (m < WBIT &&
+                                 (ns = tryIncReaderOverflow(s)) != 0L))
+                                return ns;
+                        } while (m < WBIT);
+                    }
+                    if (whead == h && p.prev == pp) {
+                        long time;
+                        if (pp == null || h == p || p.status > 0) {
+                            node = null; // throw away
+                            break;
+                        }
+                        if (deadline == 0L)
+                            time = 0L;
+                        else if ((time = deadline - System.nanoTime()) <= 0L)
+                            return cancelWaiter(node, p, false);
+                        Thread wt = Thread.currentThread();
+                        U.putObject(wt, PARKBLOCKER, this);
+                        node.thread = wt;
+                        if ((h != pp || (state & ABITS) == WBIT) &&
+                            whead == h && p.prev == pp)
+                            U.park(false, time);
+                        node.thread = null;
+                        U.putObject(wt, PARKBLOCKER, null);
+                        if (interruptible && Thread.interrupted())
+                            return cancelWaiter(node, p, true);
+                    }
+                }
+            }
+        }
+
+        for (int spins = -1;;) {
+            WNode h, np, pp; int ps;
+            if ((h = whead) == p) {
+                if (spins < 0)
+                    spins = HEAD_SPINS;
+                else if (spins < MAX_HEAD_SPINS)
+                    spins <<= 1;
+                for (int k = spins;;) { // spin at head
+                    long m, s, ns;
+                    if ((m = (s = state) & ABITS) < RFULL ?
+                        U.compareAndSwapLong(this, STATE, s, ns = s + RUNIT) :
+                        (m < WBIT && (ns = tryIncReaderOverflow(s)) != 0L)) {
+                        WNode c; Thread w;
+                        whead = node;
+                        node.prev = null;
+                        while ((c = node.cowait) != null) {
+                            if (U.compareAndSwapObject(node, WCOWAIT,
+                                                       c, c.cowait) &&
+                                (w = c.thread) != null)
+                                U.unpark(w);
+                        }
+                        return ns;
+                    }
+                    else if (m >= WBIT &&
+                             ThreadLocalRandom.current().nextInt() >= 0 && --k <= 0)
+                        break;
+                }
+            }
+            else if (h != null) {
+                WNode c; Thread w;
+                while ((c = h.cowait) != null) {
+                    if (U.compareAndSwapObject(h, WCOWAIT, c, c.cowait) &&
+                        (w = c.thread) != null)
+                        U.unpark(w);
+                }
+            }
+            if (whead == h) {
+                if ((np = node.prev) != p) {
+                    if (np != null)
+                        (p = np).next = node;   // stale
+                }
+                else if ((ps = p.status) == 0)
+                    U.compareAndSwapInt(p, WSTATUS, 0, WAITING);
+                else if (ps == CANCELLED) {
+                    if ((pp = p.prev) != null) {
+                        node.prev = pp;
+                        pp.next = node;
+                    }
+                }
+                else {
+                    long time;
+                    if (deadline == 0L)
+                        time = 0L;
+                    else if ((time = deadline - System.nanoTime()) <= 0L)
+                        return cancelWaiter(node, node, false);
+                    Thread wt = Thread.currentThread();
+                    U.putObject(wt, PARKBLOCKER, this);
+                    node.thread = wt;
+                    if (p.status < 0 &&
+                        (p != h || (state & ABITS) == WBIT) &&
+                        whead == h && node.prev == p)
+                        U.park(false, time);
+                    node.thread = null;
+                    U.putObject(wt, PARKBLOCKER, null);
+                    if (interruptible && Thread.interrupted())
+                        return cancelWaiter(node, node, true);
+                }
+            }
+        }
+    }
+
+    /**
+     * If node non-null, forces cancel status and unsplices it from
+     * queue if possible and wakes up any cowaiters (of the node, or
+     * group, as applicable), and in any case helps release current
+     * first waiter if lock is free. (Calling with null arguments
+     * serves as a conditional form of release, which is not currently
+     * needed but may be needed under possible future cancellation
+     * policies). This is a variant of cancellation methods in
+     * AbstractQueuedSynchronizer (see its detailed explanation in AQS
+     * internal documentation).
+     *
+     * @param node if nonnull, the waiter
+     * @param group either node or the group node is cowaiting with
+     * @param interrupted if already interrupted
+     * @return INTERRUPTED if interrupted or Thread.interrupted, else zero
+     */
+    private long cancelWaiter(WNode node, WNode group, boolean interrupted) {
+        if (node != null && group != null) {
+            Thread w;
+            node.status = CANCELLED;
+            // unsplice cancelled nodes from group
+            for (WNode p = group, q; (q = p.cowait) != null;) {
+                if (q.status == CANCELLED) {
+                    U.compareAndSwapObject(p, WCOWAIT, q, q.cowait);
+                    p = group; // restart
+                }
+                else
+                    p = q;
+            }
+            if (group == node) {
+                for (WNode r = group.cowait; r != null; r = r.cowait) {
+                    if ((w = r.thread) != null)
+                        U.unpark(w);       // wake up uncancelled co-waiters
+                }
+                for (WNode pred = node.prev; pred != null; ) { // unsplice
+                    WNode succ, pp;        // find valid successor
+                    while ((succ = node.next) == null ||
+                           succ.status == CANCELLED) {
+                        WNode q = null;    // find successor the slow way
+                        for (WNode t = wtail; t != null && t != node; t = t.prev)
+                            if (t.status != CANCELLED)
+                                q = t;     // don't link if succ cancelled
+                        if (succ == q ||   // ensure accurate successor
+                            U.compareAndSwapObject(node, WNEXT,
+                                                   succ, succ = q)) {
+                            if (succ == null && node == wtail)
+                                U.compareAndSwapObject(this, WTAIL, node, pred);
+                            break;
+                        }
+                    }
+                    if (pred.next == node) // unsplice pred link
+                        U.compareAndSwapObject(pred, WNEXT, node, succ);
+                    if (succ != null && (w = succ.thread) != null) {
+                        succ.thread = null;
+                        U.unpark(w);       // wake up succ to observe new pred
+                    }
+                    if (pred.status != CANCELLED || (pp = pred.prev) == null)
+                        break;
+                    node.prev = pp;        // repeat if new pred wrong/cancelled
+                    U.compareAndSwapObject(pp, WNEXT, pred, succ);
+                    pred = pp;
+                }
+            }
+        }
+        WNode h; // Possibly release first waiter
+        while ((h = whead) != null) {
+            long s; WNode q; // similar to release() but check eligibility
+            if ((q = h.next) == null || q.status == CANCELLED) {
+                for (WNode t = wtail; t != null && t != h; t = t.prev)
+                    if (t.status <= 0)
+                        q = t;
+            }
+            if (h == whead) {
+                if (q != null && h.status == 0 &&
+                    ((s = state) & ABITS) != WBIT && // waiter is eligible
+                    (s == 0L || q.mode == RMODE))
+                    release(h);
+                break;
+            }
+        }
+        return (interrupted || Thread.interrupted()) ? INTERRUPTED : 0L;
+    }
+
+    // Unsafe mechanics
+    private static final sun.misc.Unsafe U;
+    private static final long STATE;
+    private static final long WHEAD;
+    private static final long WTAIL;
+    private static final long WNEXT;
+    private static final long WSTATUS;
+    private static final long WCOWAIT;
+    private static final long PARKBLOCKER;
+
+    static {
+        try {
+            U = getUnsafe();
+            Class<?> k = StampedLock.class;
+            Class<?> wk = WNode.class;
+            STATE = U.objectFieldOffset
+                (k.getDeclaredField("state"));
+            WHEAD = U.objectFieldOffset
+                (k.getDeclaredField("whead"));
+            WTAIL = U.objectFieldOffset
+                (k.getDeclaredField("wtail"));
+            WSTATUS = U.objectFieldOffset
+                (wk.getDeclaredField("status"));
+            WNEXT = U.objectFieldOffset
+                (wk.getDeclaredField("next"));
+            WCOWAIT = U.objectFieldOffset
+                (wk.getDeclaredField("cowait"));
+            Class<?> tk = Thread.class;
+            PARKBLOCKER = U.objectFieldOffset
+                (tk.getDeclaredField("parkBlocker"));
+
+        } catch (Exception e) {
+            throw new Error(e);
+        }
+    }
+
+    /**
+     * Returns a sun.misc.Unsafe.  Suitable for use in a 3rd party package.
+     * Replace with a simple call to Unsafe.getUnsafe when integrating
+     * into a jdk.
+     *
+     * @return a sun.misc.Unsafe
+     */
+    private static sun.misc.Unsafe getUnsafe() {
+        try {
+            return sun.misc.Unsafe.getUnsafe();
+        } catch (SecurityException tryReflectionInstead) {}
+        try {
+            return java.security.AccessController.doPrivileged
+            (new java.security.PrivilegedExceptionAction<sun.misc.Unsafe>() {
+                public sun.misc.Unsafe run() throws Exception {
+                    Class<sun.misc.Unsafe> k = sun.misc.Unsafe.class;
+                    for (java.lang.reflect.Field f : k.getDeclaredFields()) {
+                        f.setAccessible(true);
+                        Object x = f.get(null);
+                        if (k.isInstance(x))
+                            return k.cast(x);
+                    }
+                    throw new NoSuchFieldError("the Unsafe");
+                }});
+        } catch (java.security.PrivilegedActionException e) {
+            throw new RuntimeException("Could not initialize intrinsics",
+                                       e.getCause());
+        }
+    }
+}