[Haskell-cafe] Race condition possible?

Don Stewart dons at galois.com
Thu Feb 12 18:26:25 EST 2009

> Consider the following code
> stamp v x = do
>   t <- getCurrentTime 
>   putMVar v (x,t)
> Is it possible - with GHC - that a thread switch happens after the t <-
> getCurrentTime and the putMVar v (x,t)? 

Yes. if 't' is heap allocated, there could be a context switch.
> If so, how would it be possible to make sure that the operation of reading the
> current time and writing the pair to the MVar is an "atomic" operation, in the
> sense that no thread switch can happen between the two? Would this require STM?

Using 'atomically' and TVars in STM, perhaps? Else, use withMVar?   Or a
modifyMVar in IO?

-- Don

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