Cairo exports and more (was Re: [cairo] Re: Munging header files for export (and other) attributes)

[Bill Spitzak]

mental at rydia.net wrote:
> Quoting Bill Spitzak <spitzak at d2.com>:
> 
> 
>>_cairo_backend_foo() {
>>   static char been_here = false;
>>   if (!been_here) {
>>     lock(backend_internal_mutex);
>>     if (!been_here) {
>>       initialize_stuff();
>>       been_here = true;
>>     }
>>     unlock(backend_internal_mutex);
>>   }
>>}
> 
> 
> I got my knuckles rapped for doing that kind of thing once.
> 
> Turns out, that outer test of !been_here isn't safe on
> multi-processor systems:  without the memory barrier presumably
> introduced by lock(), you can't assume that the value of been_here
> will reflect the value visible to the other CPU(s) due to cache
> issues, or that the compiler won't play nasty tricks on you.
> 
> Now, especially if you make been_here volatile, you can _get away
> with it_ on some compilers and platforms.  But it's not portable or
> guaranteed.
> 
> Basically you must only test been_here within the protection of the
> lock.

It's true that it should be marked violatile, but I think you missed the 
fact that I re-tested the variable inside the lock.