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


On 22 October 2007 12:17, Tomash Brechko wrote:

> 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.

  *What* mutex are you referring to?  There is no mutex in that code.

    cheers,
      DaveK
-- 
Can't think of a witty .sigline today....


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