Broken beyond repair: Control.Concurrent.SampleVar
haskell at list.mightyreason.com
Sun Apr 12 16:56:41 EDT 2009
and I am dumb, sorry. Yes, you are right. In fact the code I had posted on the
wiki at http://haskell.org/haskellwiki/SafeConcurrent#SampleVar is
> writeSampleVar :: SampleVar a -> a -> IO ()
> writeSampleVar svar value = do
> withMVar (lockedStore svar) $ \ store -> do
> _ <- tryTakeMVar store
> putMVar store value
which is functionally identical to yours. I should not write so hastily on my
way out the door...
Felipe Lessa wrote:
> On Sun, Apr 12, 2009 at 05:12:15PM +0100, Chris Kuklewicz wrote:
>> Felipe Lessa wrote:
>>>> writeSampleVar :: SampleVar a -> a -> IO ()
>>>> writeSampleVar s x = block $ withMVar (svLock s) $ const $ do
>>>> tryTakeMVar (svData s)
>>>> putMVar (svData s) x
>> That is obvious, but 'block $' is not atomic.
>> Threads 1 and 2 get past the tryTakeMVar.
> That's why there is a "withMVar (svLock s)", the "const $ do"
> part is executed serially. It is impossible to have two threads
> past the "tryTakeMVar".
>> Then thread 1 succeeds with putMVar and thread 2 blocks.
>> Thread 2 is not supposed to block, so this is a failure to agree with the
> Well, it doesn't block. In the worst case I can imagine a thread
> could be killed after the 'tryTakeMVar', leaving the 'SampleVar'
> empty, but I guess the 'block' would prevent that.
More information about the Libraries