[Haskell-cafe] Race Condition in threads

[Serguey Zefirov]

2012/12/18 mukesh tiwari <mukeshtiwari.iiitm at gmail.com>:
> Hello All
> I have two questions.
> 1. I wrote this code to create 10  simultaneous threads. Could some one
> please tell me if this is correct or not ?
>
> incr_count :: MVar () -> MVar Int -> IO ()
> incr_count m n = ( forM_ [ 1..10000 ] $ \_ -> modifyMVar_ n ( return . ( +
> 10 ) ) ) >> putMVar m ()
>
> main :: IO()
> main = do
>       count <- newMVar 0
>       list <- forM [1..10] $ \_ -> newEmptyMVar
>       forM_ list $ \var -> forkIO . incr_count var $ count
>       forM_ list $ \var ->  takeMVar var
>       val <- takeMVar count
>       print val

It is pretty much correct (some comments would be nice, though).


> 2. I am trying to create  race condition which makes the variable in
> inconsistent state. Could some one please tell me how to achieve this ? I
> look at IORef but it does not have function like modifyMVar_.

MVars are "atomic" in the sense that they have empty state and mutator
can wait until variable will have a value. So you have to operate with
something else, either IORef or with readMVar instead of takeMVar or
modifyMVar_.

> Mukesh Tiwari
>
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