[Haskell-cafe] Race condition possible?

Paul Johnson paul at cogito.org.uk
Sat Feb 14 14:01:39 EST 2009

Peter Verswyvelen wrote:
> 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)? 
> 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?
> Thanks again for sharing your wisdom with me :)
> Peter
I'm not entirely sure what you are trying to achieve here.  Presumably 
you want v to contain the (value, time) pair as soon as possible after 
time "t".  Of course it won't be instantaneous.  So another thread could 
observe that at time (t+delta) the variable "v" does not yet contain 
(x,t).  Is this a problem?

Atomic transactions won't work because "getCurrentTime" is of type "IO 
Time", whereas anything inside "atomic" has to be of type "STM a".

In theory you could lock out context switches by messing around with the 
GHC runtime, but if you are running on a multicore machine then that 
won't work either.


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