Class core.sync.rwmutex.ReadWriteMutex

Please note that this mutex is not recursive and is intended to guard access to data only. Also, no deadlock checking is in place because doing so would require dynamic memory allocation, which would reduce performance by an unacceptable amount. As a result, any attempt to recursively acquire this mutex may well deadlock the caller, particularly if a write lock is acquired while holding a read lock, or vice-versa. In practice, this should not be an issue however, because it is uncommon to call deeply into unknown code while holding a lock that simply protects data.

Inherits from

• Object (base class)

Constructors

Properties

Methods

Inner classes

Enums

Example

import core.atomic, core.thread, core.sync.semaphore;

static void runTest(ReadWriteMutex.Policy policy)
{
    scope mutex = new ReadWriteMutex(policy);
    scope rdSemA = new Semaphore, rdSemB = new Semaphore,
          wrSemA = new Semaphore, wrSemB = new Semaphore;
    shared size_t numReaders, numWriters;

    void readerFn()
    {
        synchronized (mutex.reader)
        {
            atomicOp!"+="(numReaders, 1);
            rdSemA.notify();
            rdSemB.wait();
            atomicOp!"-="(numReaders, 1);
        }
    }

    void writerFn()
    {
        synchronized (mutex.writer)
        {
            atomicOp!"+="(numWriters, 1);
            wrSemA.notify();
            wrSemB.wait();
            atomicOp!"-="(numWriters, 1);
        }
    }

    void waitQueued(size_t queuedReaders, size_t queuedWriters)
    {
        for (;;)
        {
            synchronized (mutex.m_commonMutex)
            {
                if (mutex.m_numQueuedReaders == queuedReaders &&
                    mutex.m_numQueuedWriters == queuedWriters)
                    break;
            }
            Thread.yield();
        }
    }

    scope group = new ThreadGroup;

    // 2 simultaneous readers
    group.create(&readerFn); group.create(&readerFn);
    rdSemA.wait(); rdSemA.wait();
    assert(numReaders == 2);
    rdSemB.notify(); rdSemB.notify();
    group.joinAll();
    assert(numReaders == 0);
    foreach (t; group) group.remove(t);

    // 1 writer at a time
    group.create(&writerFn); group.create(&writerFn);
    wrSemA.wait();
    assert(!wrSemA.tryWait());
    assert(numWriters == 1);
    wrSemB.notify();
    wrSemA.wait();
    assert(numWriters == 1);
    wrSemB.notify();
    group.joinAll();
    assert(numWriters == 0);
    foreach (t; group) group.remove(t);

    // reader and writer are mutually exclusive
    group.create(&readerFn);
    rdSemA.wait();
    group.create(&writerFn);
    waitQueued(0, 1);
    assert(!wrSemA.tryWait());
    assert(numReaders == 1 && numWriters == 0);
    rdSemB.notify();
    wrSemA.wait();
    assert(numReaders == 0 && numWriters == 1);
    wrSemB.notify();
    group.joinAll();
    assert(numReaders == 0 && numWriters == 0);
    foreach (t; group) group.remove(t);

    // writer and reader are mutually exclusive
    group.create(&writerFn);
    wrSemA.wait();
    group.create(&readerFn);
    waitQueued(1, 0);
    assert(!rdSemA.tryWait());
    assert(numReaders == 0 && numWriters == 1);
    wrSemB.notify();
    rdSemA.wait();
    assert(numReaders == 1 && numWriters == 0);
    rdSemB.notify();
    group.joinAll();
    assert(numReaders == 0 && numWriters == 0);
    foreach (t; group) group.remove(t);

    // policy determines whether queued reader or writers progress first
    group.create(&writerFn);
    wrSemA.wait();
    group.create(&readerFn);
    group.create(&writerFn);
    waitQueued(1, 1);
    assert(numReaders == 0 && numWriters == 1);
    wrSemB.notify();

    if (policy == ReadWriteMutex.Policy.PREFER_READERS)
    {
        rdSemA.wait();
        assert(numReaders == 1 && numWriters == 0);
        rdSemB.notify();
        wrSemA.wait();
        assert(numReaders == 0 && numWriters == 1);
        wrSemB.notify();
    }
    else if (policy == ReadWriteMutex.Policy.PREFER_WRITERS)
    {
        wrSemA.wait();
        assert(numReaders == 0 && numWriters == 1);
        wrSemB.notify();
        rdSemA.wait();
        assert(numReaders == 1 && numWriters == 0);
        rdSemB.notify();
    }
    group.joinAll();
    assert(numReaders == 0 && numWriters == 0);
    foreach (t; group) group.remove(t);
}
runTest(ReadWriteMutex.Policy.PREFER_READERS);
runTest(ReadWriteMutex.Policy.PREFER_WRITERS);