Hello,
recently, I and a few colleagues have been wondering about the interaction between IORefs and MVars, and we can't seem to find any explicit documentation stating the behavior, so I was wondering if anyone might know the answer.
The question is: is it safe to use IORefs in a multi-threaded program, provided that the uses are within a "critical section" implemented with MVars. Here is a simple program to illustrate the situation: we have two threads, each thread takes a lock, increments a counter, then releases the lock:
> import Control.Concurrent
> import Data.IORef
>
> main :: IO ()
> main =
> do lock <- newMVar ()
> counter <- newIORef 0
> forkIO (thread lock counter)
> thread lock counter
>
> thread :: MVar () -> IORef Integer -> IO a
> thread lock counter =
> do takeMVar lock
> value <- readIORef counter
> print value
> writeIORef counter (value + 1)
> putMVar lock ()
> thread lock counter
The question is if this program has a race condition or not, due to the use of IORefs? More explicitly, the concern is if a write to an IORef in one thread is guaranteed to be seen by a read from the same IORef in another thread, provided that there is proper synchronization between the two.
-Iavor