Potential Deadlock in `waitEither`’s Implementation: Unraveling the Mystery in “Parallel and Concurrent Programming in Haskell” Book

Are you an avid Haskell enthusiast, delving into the realms of parallel and concurrent programming? Have you stumbled upon the `waitEither` function in the “Parallel and Concurrent Programming in Haskell” book, only to be left bewildered by the potential deadlock in its implementation? Fear not, dear reader, for this article is designed to guide you through the labyrinth of concurrent Haskell programming, shedding light on the intricacies of `waitEither` and its potential pitfalls.

Understanding `waitEither`

Before we dive into the depths of the potential deadlock, let’s first grasp the fundamentals of `waitEither`. This function is a part of the `Control.Concurrent` module in Haskell, which facilitates concurrent programming. `waitEither` takes two arguments: `_ivar1` and `ivar2`, both of type `MVar a`, where `a` is the type of the value stored in the `MVar`. The function blocks until one of the `MVar`s is filled, and then returns the value from that `MVar`.

waitEither :: MVar a -> MVar a -> IO a
waitEither ivar1 ivar2 = do
  m <- newEmptyMVar
  forkIO (putMVar m =<< takeMVar ivar1)
  forkIO (putMVar m =<< takeMVar ivar2)
  takeMVar m

Potential Deadlock Scenario

Now, let's imagine a scenario where `waitEither` is used to wait on two `MVar`s, `m1` and `m2`. Suppose we have two threads, `t1` and `t2`, created using `forkIO`. Both threads attempt to fill one of the `MVar`s, as shown below:

t1 = forkIO (putMVar m1 1)
t2 = forkIO (putMVar m2 2)

wait Either m1 m2

In this scenario, `waitEither` will block until one of the `MVar`s is filled. However, what happens if both threads, `t1` and `t2`, are blocked, waiting for the other thread to fill its respective `MVar`? This is where the potential deadlock arises.

Here's a step-by-step breakdown of the deadlock scenario:

1. `t1` is blocked, waiting for `m2` to be filled, and `t2` is blocked, waiting for `m1` to be filled.
2. Neither thread can proceed, as they are awaiting the other thread to fill its `MVar`.
3. The `waitEither` function is also blocked, waiting for one of the `MVar`s to be filled.
4. The program is stuck in an infinite loop, with all threads waiting for each other.

Why Does This Deadlock Occur?

The root cause of this deadlock lies in the implementation of `waitEither`. When `waitEither` creates two new threads to fill the `MVar`s, it doesn't guarantee that one of the threads will succeed in filling its `MVar` before the other thread blocks. This leads to a situation where both threads are blocked, waiting for each other to fill their respective `MVar`s.

In addition, the `forkIO` function, used to create the threads, does not provide any guarantees about the order in which the threads will be executed. This lack of determinism in thread scheduling exacerbates the likelihood of the deadlock.

Avoiding the Deadlock

To avoid this potential deadlock, we can modify the implementation of `waitEither` to ensure that one of the threads is guaranteed to fill its `MVar` before the other thread blocks. One approach is to use `tryTakeMVar` instead of `takeMVar` in the forked threads:

waitEither :: MVar a -> MVar a -> IO a
waitEither ivar1 ivar2 = do
  m <- newEmptyMVar
  forkIO (putMVar m =<< tryTakeMVar ivar1)
  forkIO (putMVar m =<< tryTakeMVar ivar2)
  takeMVar m

By using `tryTakeMVar`, we ensure that if one thread fails to take the value from its `MVar`, it will immediately retry, rather than blocking indefinitely. This breaks the deadlock scenario, as one of the threads will eventually succeed in filling its `MVar`.

Conclusion

The `waitEither` function, although convenient for waiting on multiple `MVar`s, harbors a potential deadlock in its implementation. This article has demonstrated the scenario under which the deadlock arises and provided a solution to avoid it. By understanding the intricacies of concurrent Haskell programming and being aware of the potential pitfalls, we can write more robust and efficient parallel and concurrent programs.

Remember, in the realm of concurrent programming, attention to detail and a deep understanding of the underlying mechanics are crucial in avoiding deadlocks and ensuring program correctness.

Function	Description
waitEither	Waits on two MVars and returns the value from the first one that is filled.
forkIO	Creates a new thread that runs in parallel with the current thread.
putMVar	Puts a value into an MVar.
takeMVar	Takes the value from an MVar, blocking if the MVar is empty.
tryTakeMVar	Tries to take the value from an MVar without blocking.

For more information on concurrent programming in Haskell, refer to the "Parallel and Concurrent Programming in Haskell" book, which provides an in-depth exploration of the subject.

• Simon Marlow, Parallel and Concurrent Programming in Haskell, Cambridge University Press, 2013.

By acknowledging the potential deadlock in `waitEither`'s implementation and applying the solutions outlined in this article, you'll be well-equipped to tackle the complexities of parallel and concurrent programming in Haskell.

Frequently Asked Question

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