/*

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

 * or visit www.oracle.com if you need additional information or have any

 * questions.

 */

/*

 * This file is available under and governed by the GNU General Public

 * License version 2 only, as published by the Free Software Foundation.

 * However, the following notice accompanied the original version of this

 * file:

 *

 * 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 java.util.concurrent.locks;

import java.util.Collection;

import java.util.concurrent.TimeUnit;

import jdk.internal.vm.annotation.ReservedStackAccess;

/**

 * An implementation of {@link ReadWriteLock} supporting similar

 * semantics to {@link ReentrantLock}.

 * <p>This class has the following properties:

 *

 * <ul>

 * <li><b>Acquisition order</b>

 *

 * <p>This class does not impose a reader or writer preference

 * ordering for lock access.  However, it does support an optional

 * <em>fairness</em> policy.

 *

 * <dl>

 * <dt><b><i>Non-fair mode (default)</i></b>

 * <dd>When constructed as non-fair (the default), the order of entry

 * to the read and write lock is unspecified, subject to reentrancy

 * constraints.  A nonfair lock that is continuously contended may

 * indefinitely postpone one or more reader or writer threads, but

 * will normally have higher throughput than a fair lock.

 *

 * <dt><b><i>Fair mode</i></b>

 * <dd>When constructed as fair, threads contend for entry using an

 * approximately arrival-order policy. When the currently held lock

 * is released, either the longest-waiting single writer thread will

 * be assigned the write lock, or if there is a group of reader threads

 * waiting longer than all waiting writer threads, that group will be

 * assigned the read lock.

 *

 * <p>A thread that tries to acquire a fair read lock (non-reentrantly)

 * will block if either the write lock is held, or there is a waiting

 * writer thread. The thread will not acquire the read lock until

 * after the oldest currently waiting writer thread has acquired and

 * released the write lock. Of course, if a waiting writer abandons

 * its wait, leaving one or more reader threads as the longest waiters

 * in the queue with the write lock free, then those readers will be

 * assigned the read lock.

 *

 * <p>A thread that tries to acquire a fair write lock (non-reentrantly)

 * will block unless both the read lock and write lock are free (which

 * implies there are no waiting threads).  (Note that the non-blocking

 * {@link ReadLock#tryLock()} and {@link WriteLock#tryLock()} methods

 * do not honor this fair setting and will immediately acquire the lock

 * if it is possible, regardless of waiting threads.)

 * </dl>

 *

 * <li><b>Reentrancy</b>

 *

 * <p>This lock allows both readers and writers to reacquire read or

 * write locks in the style of a {@link ReentrantLock}. Non-reentrant

 * readers are not allowed until all write locks held by the writing

 * thread have been released.

 *

 * <p>Additionally, a writer can acquire the read lock, but not

 * vice-versa.  Among other applications, reentrancy can be useful

 * when write locks are held during calls or callbacks to methods that

 * perform reads under read locks.  If a reader tries to acquire the

 * write lock it will never succeed.

 *

 * <li><b>Lock downgrading</b>

 * <p>Reentrancy also allows downgrading from the write lock to a read lock,

 * by acquiring the write lock, then the read lock and then releasing the

 * write lock. However, upgrading from a read lock to the write lock is

 * <b>not</b> possible.

 *

 * <li><b>Interruption of lock acquisition</b>

 * <p>The read lock and write lock both support interruption during lock

 * acquisition.

 *

 * <li><b>{@link Condition} support</b>

 * <p>The write lock provides a {@link Condition} implementation that

 * behaves in the same way, with respect to the write lock, as the

 * {@link Condition} implementation provided by

 * {@link ReentrantLock#newCondition} does for {@link ReentrantLock}.

 * This {@link Condition} can, of course, only be used with the write lock.

 *

 * <p>The read lock does not support a {@link Condition} and

 * {@code readLock().newCondition()} throws

 * {@code UnsupportedOperationException}.

 *

 * <li><b>Instrumentation</b>

 * <p>This class supports methods to determine whether locks

 * are held or contended. These methods are designed for monitoring

 * system state, not for synchronization control.

 * </ul>

 *

 * <p>Serialization of this class behaves in the same way as built-in

 * locks: a deserialized lock is in the unlocked state, regardless of

 * its state when serialized.

 *

 * <p><b>Sample usages</b>. Here is a code sketch showing how to perform

 * lock downgrading after updating a cache (exception handling is

 * particularly tricky when handling multiple locks in a non-nested

 * fashion):

 *

 * <pre> {@code

 * class CachedData {

 *   Object data;

 *   boolean cacheValid;

 *   final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();

 *

 *   void processCachedData() {

 *     rwl.readLock().lock();

 *     if (!cacheValid) {

 *       // Must release read lock before acquiring write lock

 *       rwl.readLock().unlock();

 *       rwl.writeLock().lock();

 *       try {

 *         // Recheck state because another thread might have

 *         // acquired write lock and changed state before we did.

 *         if (!cacheValid) {

 *           data = ...

 *           cacheValid = true;

 *         }

 *         // Downgrade by acquiring read lock before releasing write lock

 *         rwl.readLock().lock();

 *       } finally {

 *         rwl.writeLock().unlock(); // Unlock write, still hold read

 *       }

 *     }

 *

 *     try {

 *       use(data);

 *     } finally {

 *       rwl.readLock().unlock();

 *     }

 *   }

 * }}</pre>

 *

 * ReentrantReadWriteLocks can be used to improve concurrency in some

 * uses of some kinds of Collections. This is typically worthwhile

 * only when the collections are expected to be large, accessed by

 * more reader threads than writer threads, and entail operations with

 * overhead that outweighs synchronization overhead. For example, here

 * is a class using a TreeMap that is expected to be large and

 * concurrently accessed.

 *

 * <pre> {@code

 * class RWDictionary {

 *   private final Map<String, Data> m = new TreeMap<>();

 *   private final ReentrantReadWriteLock rwl = new ReentrantReadWriteLock();

 *   private final Lock r = rwl.readLock();

 *   private final Lock w = rwl.writeLock();

 *

 *   public Data get(String key) {

 *     r.lock();

 *     try { return m.get(key); }

 *     finally { r.unlock(); }

 *   }

 *   public List<String> allKeys() {

 *     r.lock();

 *     try { return new ArrayList<>(m.keySet()); }

 *     finally { r.unlock(); }

 *   }

 *   public Data put(String key, Data value) {

 *     w.lock();

 *     try { return m.put(key, value); }

 *     finally { w.unlock(); }

 *   }

 *   public void clear() {

 *     w.lock();

 *     try { m.clear(); }

 *     finally { w.unlock(); }

 *   }

 * }}</pre>

 *

 *

 * <p>This lock supports a maximum of 65535 recursive write locks

 * and 65535 read locks. Attempts to exceed these limits result in

 * {@link Error} throws from locking methods.

 *

 * @since 1.5

 * @author Doug Lea

 */

public class ReentrantReadWriteLock

        implements ReadWriteLock, java.io.Serializable {

    private static final long serialVersionUID = -6992448646407690164L;

    /** Inner class providing readlock */

    private final ReentrantReadWriteLock.ReadLock readerLock;

    /** Inner class providing writelock */

    private final ReentrantReadWriteLock.WriteLock writerLock;

    /** Performs all synchronization mechanics */

    final Sync sync;

    /**

     * Creates a new {@code ReentrantReadWriteLock} with

     * default (nonfair) ordering properties.

     */

    public ReentrantReadWriteLock() {

        this(false);

    }

    /**

     * Creates a new {@code ReentrantReadWriteLock} with

     * the given fairness policy.

     *

     * @param fair {@code true} if this lock should use a fair ordering policy

     */

    public ReentrantReadWriteLock(boolean fair) {

        sync = fair ? new FairSync() : new NonfairSync();

        readerLock = new ReadLock(this);

        writerLock = new WriteLock(this);

    }

    public ReentrantReadWriteLock.WriteLock writeLock() { return writerLock; }

    public ReentrantReadWriteLock.ReadLock  readLock()  { return readerLock; }

    /**

     * Synchronization implementation for ReentrantReadWriteLock.

     * Subclassed into fair and nonfair versions.

     */

    abstract static class Sync extends AbstractQueuedSynchronizer {

        private static final long serialVersionUID = 6317671515068378041L;

        /*

         * Read vs write count extraction constants and functions.

         * Lock state is logically divided into two unsigned shorts:

         * The lower one representing the exclusive (writer) lock hold count,

         * and the upper the shared (reader) hold count.

         */

        static final int SHARED_SHIFT   = 16;

        static final int SHARED_UNIT    = (1 << SHARED_SHIFT);

        static final int MAX_COUNT      = (1 << SHARED_SHIFT) - 1;

        static final int EXCLUSIVE_MASK = (1 << SHARED_SHIFT) - 1;

        /** Returns the number of shared holds represented in count. */

        static int sharedCount(int c)    { return c >>> SHARED_SHIFT; }

        /** Returns the number of exclusive holds represented in count. */

        static int exclusiveCount(int c) { return c & EXCLUSIVE_MASK; }

        /**

         * A counter for per-thread read hold counts.

         * Maintained as a ThreadLocal; cached in cachedHoldCounter.

         */

        static final class HoldCounter {

            int count;          // initially 0

            // Use id, not reference, to avoid garbage retention

            final long tid = LockSupport.getThreadId(Thread.currentThread());

        }

        /**

         * ThreadLocal subclass. Easiest to explicitly define for sake

         * of deserialization mechanics.

         */

        static final class ThreadLocalHoldCounter

            extends ThreadLocal<HoldCounter> {

            public HoldCounter initialValue() {

                return new HoldCounter();

            }

        }

        /**

         * The number of reentrant read locks held by current thread.

         * Initialized only in constructor and readObject.

         * Removed whenever a thread's read hold count drops to 0.

         */

        private transient ThreadLocalHoldCounter readHolds;

        /**

         * The hold count of the last thread to successfully acquire

         * readLock. This saves ThreadLocal lookup in the common case

         * where the next thread to release is the last one to

         * acquire. This is non-volatile since it is just used

         * as a heuristic, and would be great for threads to cache.

         *

         * <p>Can outlive the Thread for which it is caching the read

         * hold count, but avoids garbage retention by not retaining a

         * reference to the Thread.

         *

         * <p>Accessed via a benign data race; relies on the memory

         * model's final field and out-of-thin-air guarantees.

         */

        private transient HoldCounter cachedHoldCounter;

        /**

         * firstReader is the first thread to have acquired the read lock.

         * firstReaderHoldCount is firstReader's hold count.

         *

         * <p>More precisely, firstReader is the unique thread that last

         * changed the shared count from 0 to 1, and has not released the

         * read lock since then; null if there is no such thread.

         *

         * <p>Cannot cause garbage retention unless the thread terminated

         * without relinquishing its read locks, since tryReleaseShared

         * sets it to null.

         *

         * <p>Accessed via a benign data race; relies on the memory

         * model's out-of-thin-air guarantees for references.

         *

         * <p>This allows tracking of read holds for uncontended read

         * locks to be very cheap.

         */

        private transient Thread firstReader;

        private transient int firstReaderHoldCount;

        Sync() {

            readHolds = new ThreadLocalHoldCounter();

            setState(getState()); // ensures visibility of readHolds

        }

        /*

         * Acquires and releases use the same code for fair and

         * nonfair locks, but differ in whether/how they allow barging

         * when queues are non-empty.

         */

        /**

         * Returns true if the current thread, when trying to acquire

         * the read lock, and otherwise eligible to do so, should block

         * because of policy for overtaking other waiting threads.

         */

        abstract boolean readerShouldBlock();

        /**

         * Returns true if the current thread, when trying to acquire

         * the write lock, and otherwise eligible to do so, should block

         * because of policy for overtaking other waiting threads.

         */

        abstract boolean writerShouldBlock();

        /*

         * Note that tryRelease and tryAcquire can be called by

         * Conditions. So it is possible that their arguments contain

         * both read and write holds that are all released during a

         * condition wait and re-established in tryAcquire.

         */

        @ReservedStackAccess

        protected final boolean tryRelease(int releases) {

            if (!isHeldExclusively())

                throw new IllegalMonitorStateException();

            int nextc = getState() - releases;

            boolean free = exclusiveCount(nextc) == 0;

            if (free)

                setExclusiveOwnerThread(null);

            setState(nextc);

            return free;

        }

        @ReservedStackAccess

        protected final boolean tryAcquire(int acquires) {

            /*

             * Walkthrough:

             * 1. If read count nonzero or write count nonzero

             *    and owner is a different thread, fail.

             * 2. If count would saturate, fail. (This can only

             *    happen if count is already nonzero.)

             * 3. Otherwise, this thread is eligible for lock if

             *    it is either a reentrant acquire or

             *    queue policy allows it. If so, update state

             *    and set owner.

             */

            Thread current = Thread.currentThread();

            int c = getState();

            int w = exclusiveCount(c);

            if (c != 0) {

                // (Note: if c != 0 and w == 0 then shared count != 0)

                if (w == 0 || current != getExclusiveOwnerThread())

                    return false;

                if (w + exclusiveCount(acquires) > MAX_COUNT)

                    throw new Error("Maximum lock count exceeded");

                // Reentrant acquire

                setState(c + acquires);

                return true;

            }

            if (writerShouldBlock() ||

                !compareAndSetState(c, c + acquires))

                return false;

            setExclusiveOwnerThread(current);

            return true;

        }

        @ReservedStackAccess

        protected final boolean tryReleaseShared(int unused) {

            Thread current = Thread.currentThread();

            if (firstReader == current) {

                // assert firstReaderHoldCount > 0;

                if (firstReaderHoldCount == 1)

                    firstReader = null;

                else

                    firstReaderHoldCount--;

            } else {

                HoldCounter rh = cachedHoldCounter;

                if (rh == null ||

                    rh.tid != LockSupport.getThreadId(current))

                    rh = readHolds.get();

                int count = rh.count;

                if (count <= 1) {

                    readHolds.remove();

                    if (count <= 0)

                        throw unmatchedUnlockException();

                }

                --rh.count;

            }

            for (;;) {

                int c = getState();

                int nextc = c - SHARED_UNIT;

                if (compareAndSetState(c, nextc))

                    // Releasing the read lock has no effect on readers,

                    // but it may allow waiting writers to proceed if

                    // both read and write locks are now free.

                    return nextc == 0;

            }

        }

        private static IllegalMonitorStateException unmatchedUnlockException() {

            return new IllegalMonitorStateException(

                "attempt to unlock read lock, not locked by current thread");

        }

        @ReservedStackAccess

        protected final int tryAcquireShared(int unused) {

            /*

             * Walkthrough:

             * 1. If write lock held by another thread, fail.

             * 2. Otherwise, this thread is eligible for

             *    lock wrt state, so ask if it should block

             *    because of queue policy. If not, try

             *    to grant by CASing state and updating count.

             *    Note that step does not check for reentrant

             *    acquires, which is postponed to full version

             *    to avoid having to check hold count in

             *    the more typical non-reentrant case.

             * 3. If step 2 fails either because thread

             *    apparently not eligible or CAS fails or count

             *    saturated, chain to version with full retry loop.

             */

            Thread current = Thread.currentThread();

            int c = getState();

            if (exclusiveCount(c) != 0 &&

                getExclusiveOwnerThread() != current)

                return -1;

            int r = sharedCount(c);

            if (!readerShouldBlock() &&

                r < MAX_COUNT &&

                compareAndSetState(c, c + SHARED_UNIT)) {

                if (r == 0) {

                    firstReader = current;

                    firstReaderHoldCount = 1;

                } else if (firstReader == current) {

                    firstReaderHoldCount++;

                } else {

                    HoldCounter rh = cachedHoldCounter;

                    if (rh == null ||

                        rh.tid != LockSupport.getThreadId(current))

                        cachedHoldCounter = rh = readHolds.get();

                    else if (rh.count == 0)

                        readHolds.set(rh);

                    rh.count++;

                }

                return 1;

            }

            return fullTryAcquireShared(current);

        }

        /**

         * Full version of acquire for reads, that handles CAS misses

         * and reentrant reads not dealt with in tryAcquireShared.

         */

        final int fullTryAcquireShared(Thread current) {

            /*

             * This code is in part redundant with that in

             * tryAcquireShared but is simpler overall by not

             * complicating tryAcquireShared with interactions between

             * retries and lazily reading hold counts.

             */

            HoldCounter rh = null;

            for (;;) {

                int c = getState();

                if (exclusiveCount(c) != 0) {