Read-write synchronization class implementation

Question

I am writing a read-write synchronization class, and would like some advice on what I to do next. For some reason, it sometimes allows a Read to happen in the middle of a Write, and I cannot find the reason.

This is what I want from this class:

• Reads not allowed at the same time as writes.
• Multiples reads can happen at the same time.
• Only one write can happen at a time.
• When a write is needed, all already executing reads continue, no new reads are allowed, when all reads finish the write executes.

I know that .Net framework has a class to do this... but what I want is to understand and to reproduce something like that. I'm not reinventing the wheel, I am trying to understand it by making my own wheel... happens that my wheel is kinda squared a bit.

What I have currently is this:

public class ReadWriteSync
{
    private ManualResetEvent read = new ManualResetEvent(true);
    private volatile int readingBlocks = 0;
    private AutoResetEvent write = new AutoResetEvent(true);
    private object locker = new object();

    public IDisposable ReadLock()
    {
        lock (this.locker)
        {
            this.write.Reset();
            Interlocked.Increment(ref this.readingBlocks);
            this.read.WaitOne();
        }

        return new Disposer(() =>
        {
            if (Interlocked.Decrement(ref this.readingBlocks) == 0)
                this.write.Set();
        });
    }

    public IDisposable WriteLock()
    {
        lock (this.locker)
        {
            this.read.Reset();
            this.write.WaitOne();
        }

        return new Disposer(() =>
        {
            this.read.Set();
            if (this.readingBlocks == 0)
                this.write.Set();
        });
    }

    class Disposer : IDisposable
    {
        Action disposer;
        public Disposer(Action disposer) { this.disposer = disposer; }
        public void Dispose() { this.disposer(); }
    }
}

This is my test program... when something goes wrong it prints the lines in red.

class Program
{
    static ReadWriteSync sync = new ReadWriteSync();

    static void Main(string[] args)
    {
        Console.BackgroundColor = ConsoleColor.DarkGray;
        Console.ForegroundColor = ConsoleColor.Gray;
        Console.Clear();

        Task readTask1 = new Task(() => DoReads("A", 20));
        Task readTask2 = new Task(() => DoReads("B", 30));
        Task readTask3 = new Task(() => DoReads("C", 40));
        Task readTask4 = new Task(() => DoReads("D", 50));

        Task writeTask1 = new Task(() => DoWrites("E", 500));
        Task writeTask2 = new Task(() => DoWrites("F", 200));

        readTask1.Start();
        readTask2.Start();
        readTask3.Start();
        readTask4.Start();

        writeTask1.Start();
        writeTask2.Start();

        Task.WaitAll(
            readTask1, readTask2, readTask3, readTask4,
            writeTask1, writeTask2);
    }

    static volatile bool reading;
    static volatile bool writing;

    static void DoWrites(string name, int interval)
    {
        for (int i = 1; i < int.MaxValue; i += 2)
        {
            using (sync.WriteLock())
            {
                Console.ForegroundColor = (writing || reading) ? ConsoleColor.Red : ConsoleColor.Gray;
                writing = true;
                Console.WriteLine("WRITE {1}-{0} BEGIN", i, name);
                Thread.Sleep(interval);
                Console.WriteLine("WRITE {1}-{0} END", i, name);
                writing = false;
            }

            Thread.Sleep(interval);
        }
    }

    static void DoReads(string name, int interval)
    {
        for (int i = 0; i < int.MaxValue; i += 2)
        {
            using (sync.ReadLock())
            {
                Console.ForegroundColor = (writing) ? ConsoleColor.Red : ConsoleColor.Gray;
                reading = true;
                Console.WriteLine("READ {1}-{0} BEGIN", i, name);
                Thread.Sleep(interval * 3);
                Console.WriteLine("READ {1}-{0} END", i, name);
                reading = false;
            }

            Thread.Sleep(interval);
        }
    }
}

What is wrong with all this... any advice on how to do it correctly?

Answer 1

The primary issue that I see is that you are trying to make reset events encompass both the meanings of a read/write and the handling of their current state, without synchronizing in a consistent manner.

Here's an example of how the inconsistent synchronization may bite you in your specific code.

• A write is disposing and a read is coming in.
• The read acquires the lock
• The write sets the read ManualResetEvent (MRE)
• The write checks the current read count, finding 0
• The read resets the write AutoResetEvent (ARE)
• The read increments the read count
• The read finds its MRE has been set and begins to read

All is fine so far, but the write hasn't finished yet...

• A second write comes in and acquires the lock
• The second write resets the read MRE
• The first write finishes by setting the write ARE
• The second write finds its ARE has been set and begins to write

When thinking about multiple threads, unless you are within a lock of some sort, you must take the view that all other data is wildly fluctuating and cannot be trusted.

A naive implementation of this may split out the queueing logic from the state logic and synchronize appropriately.

    public class ReadWrite
    {
        private static int readerCount = 0;
        private static int writerCount = 0;
        private int pendingReaderCount = 0;
        private int pendingWriterCount = 0;
        private readonly object decision = new object();

        private class WakeLock:IDisposable
        {
            private readonly object wakeLock;
            public WakeLock(object wakeLock) { this.wakeLock = wakeLock; }
            public virtual void Dispose() { lock(this.wakeLock) Monitor.PulseAll(this.wakeLock); }
        }
        private class ReadLock:WakeLock
        {
            public ReadLock(object wakeLock) : base(wakeLock) { Interlocked.Increment(ref readerCount); }
            public override void Dispose()
            {
                Interlocked.Decrement(ref readerCount);
                base.Dispose();
            }
        }            
        private class WriteLock:WakeLock
        {
            public WriteLock(object wakeLock) : base(wakeLock) { Interlocked.Increment(ref writerCount); }
            public override void Dispose()
            {
                Interlocked.Decrement(ref writerCount);
                base.Dispose();
            }
        }

        public IDisposable TakeReadLock()
        {
            lock(decision)
            {
                pendingReaderCount++;
                while (pendingWriterCount > 0 || Thread.VolatileRead(ref writerCount) > 0)
                    Monitor.Wait(decision);
                pendingReaderCount--;
                return new ReadLock(this.decision);
            }
        }

        public IDisposable TakeWriteLock()
        {
            lock(decision)
            {
                pendingWriterCount++;
                while (Thread.VolatileRead(ref readerCount) > 0 || Thread.VolatileRead(ref writerCount) > 0)
                    Monitor.Wait(decision);
                pendingWriterCount--;
                return new WriteLock(this.decision);
            }
        }
    }