Hi Iavor, You might be interested in what Edward has written about this: http://blog.ezyang.com/2014/01/so-you-want-to-add-a-new-concurrency-primitiv... I would say when we do have a memory model for GHC the program you gave will almost certainly be correct. MVar operations should be full synchronization operations. There are some bugs on relaxed systems with initialization that I think are being addressed. I can't find the tickets at the moment. Ryan On Fri, Oct 21, 2016 at 1:19 PM, Iavor Diatchki wrote:

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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