Hi Bertram,
On 12.10.2011, at 13:24, Bertram Felgenhauer wrote:
> This has nothing to do with Data.Typeable though - it appears to be some
> interaction between unsaferPerformIO and MVars that I do not understand.
> The following program occasionally terminates with "thread blocked
> indefinitely in an MVar operation", too (tested on ghc 7.0.3 and 7.2.1):
>
> import Control.Concurrent
> import Control.Exception
> import Control.Monad
> import System.IO.Unsafe
>
> main :: IO ()
> main = do
> -- evaluate lock -- adding this line fixes the problem
>
> fin1 <- newEmptyMVar
> fin2 <- newEmptyMVar
>
> forkIO $ ping >>= putMVar fin1
> forkIO $ ping >>= putMVar fin2
>
> takeMVar fin1
> takeMVar fin2
>
> {-# NOINLINE lock #-}
> lock :: MVar ()
> lock = unsafePerformIO $ newMVar ()
>
> ping = do
> () <- takeMVar lock
> putMVar lock ()
>
> Since I don't yet understand why this blocks, I cannot say whether it
> should work or not.
I've seen blocks with GHC 6.12.1, but I never got one with 7.0.3 and 7.2.1.
Probably, bad luck on my side. ;-)
Cheers,
Jean
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