Re: Optimization of conditional access to globals: thread-unsafe?

[Tomash Brechko <tomash dot brechko at gmail dot com>]

On Mon, Oct 22, 2007 at 12:07:20 +0100, Dave Korn wrote:
> And even volatile wouldn't help if the code said
> 
>       if (i > x)
>         var += i;
> 
> instead of a simple assignment.  The race in fact *does* exist in the original
> program, but is hidden by the fact that you don't care which of two operations
> that overwrite the previous value complete in which order, but you're assuming
> the operation that modifies var is atomic, and there's nothing to innately
> guarantee that in the original program.  The race condition *is* already
> there.

Why?  For that example, if executed verbatim, it is either i > x
always false, or the mutex is properly acquired.  No one is assuming
atomic update.



-- 
   Tomash Brechko