Is this a concurrency bug in base?

[Bertram Felgenhauer]

Simon Marlow wrote:
> >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 think it should work.  Could you make a ticket for it please?

http://hackage.haskell.org/trac/ghc/ticket/5558

It occurred to me that normally, MVars will always be fully evaluated,
since they are supposedly created by a previous IO action. This
property is violated in this program. Perhaps that's the reason?

Bertram