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