[Haskell-cafe] Possible bug in Control.Concurrent

[Krzysztof Skrzętnicki]

Shame on me, I forgot to include the software versions I use:

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 6.12.3

$ uname -a
Linux raptor 2.6.37-ARCH #1 SMP PREEMPT Sat Jan 29 20:00:33 CET 2011 x86_64
Intel(R) Core(TM) i7 CPU 870 @ 2.93GHz GenuineIntel GNU/Linux

This is a normal Arch Linux setup with GHC installed via pacman.

Best regards,
Krzysztof Skrzętnicki

2011/2/9 Krzysztof Skrzętnicki <gtener at gmail.com>

> Hello Cafe,
>
> Here is a simple program that yields strange results:
>
> module Main where
>
> import Control.Concurrent
> import Control.Concurrent.Chan
> import Control.Monad
>
> main = do
>  c <- newChan
>  writeChan c 1
>  forkIO $ forever $ do
>    i <- readChan c
>    print ("forkio",i)
> isEmptyChan c >>= print
>
> First of all, if we try to run it via runhaskell, it will hang:
>
> runhaskell deadlock.hs
> ("forkio",1)
> -- no more output --
>
>
> Compiled version OTOH behaves differently dependent on compilation flags.
> Without -threaded:
> ./deadlock
> ("forkio",1)
> False
>
> With -threaded:
> ./deadlock
> False
>
> Now, this is strange thing: we put single element into the channel. We take
> it out. And then we see the channel isn't really empty. Perhaps there is a
> race condition here? So we put an delay, so that we will be sure the check
> for empty channel occurs 1 second later than the channel is emptied.
>
> import Control.Concurrent
> import Control.Concurrent.Chan
> import Control.Monad
>
> main = do
>  c <- newChan
>  writeChan c 1
>  forkIO $ forever $ do
>    i <- readChan c
>    print ("forkio",i)
>   threadDelay 1000000
>  isEmptyChan c >>= print
>
> This program will misbehave. Invariably of -threaded flag it will go like
> this:
>
> ./deadlock
> ("forkio",1)
> deadlock: thread blocked indefinitely in an MVar operation
>
> I have no idea what is the problem here. Perhaps I'm not using the library
> in the right way. Does anyone has any idea what's going on here?
>
> Best regards,
> Krzysztof Skrzętnicki
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/haskell-cafe/attachments/20110209/136d3c9e/attachment.htm>