How to protect resources that may be used in a multi-threaded or async environment?

Question

I am working on a C# API that is used by a variety of consumers. This API provides access to a shared resource (in my case hardware that does serial communication) , that will often have a few different actors trying to use it concurrently.

The issue I have is that some of my consumers will want to use this in a multi-threaded environment - each actor works independently and try to use the resource. A simple lock works fine here. But some of my consumers would prefer to use async-await and time-slice the resource. (As I understand it) this requires an asynchronous lock to yield the timeslice back to other tasks; blocking at a lock would halt that whole thread.

And I imagine that having serial locks is unperformant at best, and a potential race condition or deadlock at worst.

So how can I protect this shared resource in a common codebase for both potential concurrency usages?

Answer 1

You can use SemaphoreSlim with 1 as the number of requests. SemaphoreSlim allows to lock in both an async fashion using WaitAsync and the old synchronous way:

await _semphore.WaitAsync()
try
{
    ... use shared resource.
}
finally
{
    _semphore.Release()
}

You can also write your own AsyncLock based on Stephen Toub's great post Building Async Coordination Primitives, Part 6: AsyncLock. I did it in my application and allowed for both synchronous and asynchronous locks on the same construct.

Usage:

// Async
using (await _asyncLock.LockAsync())
{
    ... use shared resource.
}

// Synchronous
using (_asyncLock.Lock())
{
    ... use shared resource.
}

Implementation:

class AsyncLock
{
    private readonly Task<IDisposable> _releaserTask;
    private readonly SemaphoreSlim _semaphore = new SemaphoreSlim(1, 1);
    private readonly IDisposable _releaser;

    public AsyncLock()
    {
        _releaser = new Releaser(_semaphore);
        _releaserTask = Task.FromResult(_releaser);
    }
    public IDisposable Lock()
    {
        _semaphore.Wait();
        return _releaser;
    }
    public Task<IDisposable> LockAsync()
    {
        var waitTask = _semaphore.WaitAsync();
        return waitTask.IsCompleted
            ? _releaserTask
            : waitTask.ContinueWith(
                (_, releaser) => (IDisposable) releaser,
                _releaser,
                CancellationToken.None,
                TaskContinuationOptions.ExecuteSynchronously,
                TaskScheduler.Default);
    }
    private class Releaser : IDisposable
    {
        private readonly SemaphoreSlim _semaphore;
        public Releaser(SemaphoreSlim semaphore)
        {
            _semaphore = semaphore;
        }
        public void Dispose()
        {
            _semaphore.Release();
        }
    }
}