[Haskell-cafe] Race condition possible?

[Duncan Coutts]

On Thu, 2009-02-12 at 22:58 -0500, David Menendez wrote:
> On Thu, Feb 12, 2009 at 6:26 PM, Don Stewart <dons at galois.com> wrote:
> > bugfact:
> >> 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?
> 
> As I understand it, withMVar or modifyMVar will protect you from
> extraneous exceptions, but they won't prevent another thread from
> writing to the MVar between the take and the put.

You have to cooperate with the other users of the MVar.

If each thread is using readMVar, withMVar or modifyMVar then it's fine.
The read/with/modify actions do a takeMVar followed by a putMVar. If one
thread is using withMVar and another is doing putMVar directly then the
exclusion scheme does not work.

Duncan