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SubjectRe: Uses for memory barriers
Date
Am Samstag, 9. September 2006 00:25 schrieb Alan Stern:
> On Fri, 8 Sep 2006, Oliver Neukum wrote:
>
> > > Again you have misunderstood. The original code was _not_ incorrect. I
> > > was asking: Given the code as stated, would the assertion ever fail?
> >
> > I claim the right to call code that fails its own assertions incorrect. :-)
>
> Touche!
>
> > > The code _was_ correct for my purposes, namely, to illustrate a technical
> > > point about the behavior of memory barriers.
> >
> > I would say that the code may fail the assertion purely based
> > on the formal definition of a memory barrier. And do so in a subtle
> > and inobvious way.
>
> But what _is_ the formal definition of a memory barrier? I've never seen
> one that was complete and correct.

I' d say "mb();" is "rmb();wmb();"

and they work so that:

CPU 0

a = TRUE;
wmb();
b = TRUE;

CPU 1

if (b) {
rmb();
assert(a);
}
is correct. Possibly that is not a complete definition though.

Regards
Oliver
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