Optimization of conditional access to globals: thread-unsafe?

[Darryl Miles]

Michael Matz wrote:
>   if (condition)
>     *p = value;
> 
> (i.e. without any synchronization primitive or in fact anything else after 
> the store in the control region) and expect that the store indeed only 
> happens in that control region.  And this expectation is misguided.  Had 
> they written it like:
> 
>   if (condition) {
>     *p = value;
>     membarrier();
>   }
> 
> it would have worked just fine.


Don't you need the barrier before.  This is to ensure it completed the 
condition test completely first before it then processed the assignment 
expression.

if(condition) {
  somebarrier();
  *p = value;
}

The issue is not that the store is done too late, but that a 
write-access is done too early.


Darryl