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

_______________________________________________
ghc-devs mailing list
ghc-devs@haskell.org
http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs