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

[Alexander Larsson]

On Fri, 2005-09-16 at 15:02 -0700, Bill Spitzak wrote:
> 
> 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.

This method is called double checked locking, and it doesn't in general
work for all hardware architectures. See e.g. 
http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html

glib uses this on architectures where it works. Looking at its
configure.in it seems it doesn't work on old sparcs, alpha, ppc and
ia64. 

=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
 Alexander Larsson                                            Red Hat, Inc 
                   alexl at redhat.com    alla at lysator.liu.se 
He's a short-sighted coffee-fuelled werewolf whom everyone believes is mad. 
She's a cold-hearted punk femme fatale with an incredible destiny. They fight 
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