Locking with nested async calls

Question

I am working on a multi threaded WindowsPhone8 app that has critical sections within async methods.

Does anyone know of a way to properly use semaphores / mutexes in C

Answer 1

You can use the System.Threading.ReaderWriterLockSlim (doc), which has a support recursion flag:

ReaderWriterLockSlim _lock = new ReaderWriterLockSlim(LockRecursionPolicy.SupportsRecursion);

async Task Bar()
{
    try
    {
        _lock.EnterReadLock();
        await BarInternal();
    }
    finally
    {
        if (_lock.IsReadLockHeld)
            _lock.ExitReadLock();
    }
}

async Task BarInternal()
{
    try
    {
        _lock.EnterReadLock();
        await Task.Delay(1000);
    }
    finally
    {
        if (_lock.IsReadLockHeld)
            _lock.ExitReadLock();
    }
}

Still you should be very careful with recursion because it is very difficult to control which thread took a lock and when.

The code in the question will be result in a deadlock because it tries to acquire the lock twice, something like:

await _lock.WaitAsync();
await _lock.WaitAsync(); --> Will result in exception.

While flagging the ReaderWriterLockSlim in SupportsRecursion will not throw an exception for this weird code:

 _lock.EnterReadLock();
 _lock.EnterReadLock();