Is this a concurrency bug in base?

[David Brown]

On Sun, Oct 09, 2011 at 03:30:20PM +0200, Jean-Marie Gaillourdet wrote:
>Hi Daniel,
>
>On 09.10.2011, at 14:45, Daniel Fischer wrote:
>
>> On Sunday 09 October 2011, 13:52:47, Jean-Marie Gaillourdet wrote:
>>> This seems to be a Heisenbug as it is extremely fragile, when adding a
>>> "| grep 1" to the while loop it seems to disappears. At least on my
>>> computers.
>>
>> Still produces 1s here with a grep.
>
>Well, it may have been bad luck on my site.

The program below will occasionally print "1 /= 0" or "0 /= 1" on
x86_64 linux with the Debian testing 7.0.4 ghc.

$ ghc bug -rtsopts -threaded
$ while true; do ./bug +RTS -N; done

> module Main where
> 
> import Control.Monad
> import Control.Concurrent
> import Data.Typeable
> 
> main :: IO ()
> main = do
>    fin1 <- newEmptyMVar
>    fin2 <- newEmptyMVar
> 
>    forkIO $ child fin1
>    forkIO $ child fin2
> 
>    a <- takeMVar fin1
>    b <- takeMVar fin2
>    when (a /= b) $
>       putStrLn $ show a ++ " /= " ++ show b
> 
> child :: MVar Int -> IO ()
> child var = do
>    key <- typeRepKey (typeOf ())
>    putMVar var key