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
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