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SubjectRe: [patch 00/21] mutex subsystem, -V14


On Thu, 5 Jan 2006, Ingo Molnar wrote:
>
> [ long details removed ]
>
> to sum it up: atomic_dec/inc_return() alone is not enough to implement
> critical sections, on a number of architectures. atomic_xchg() seems to
> have similar problems too.

Yes.

> the patch below adds the smp_mb() barriers to the generic headers, which
> should now fulfill all the ordering requirements, on every architecture.
> It only relies on one property of the atomic primitives: that they wont
> get reordered with respect to themselves, so an atomic_inc_ret() and an
> atomic_dec_ret() cannot switch place.
>
> Can you see any hole in this reasoning?

No. The alternative is to just make the ordering requirements
for "atomic_dec_return()" and "atomic_xchg()" be absolute. Say that they
have to be full memory barriers, and push the problem into the low-level
architecture.

I _think_ your patch is the right approach, because most architectures are
likely to do their own fast-paths for mutexes, and as such the generic
ones are more of a template for how to do it, and hopefilly aren't that
performance critical.

Linus
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