I wonder why (on an SP machine) you don't just disable interrupts for a few
instructions and give yourself an atomic dec?




On Wed, Aug 12, 2009 at 9:30 AM, Jamie Lokier <ja...@shareable.org> wrote:

> Michael Schnell wrote:
> > >> Seemingly the best instruction to handle these bits is "atomic compare
> > >> and exchange" (e.g. provided by the X86 CPU).
> > >
> > > It's actually not the best, because when it returns "did not match"
> > > you have to loop and try again.
> >
> > The supposedly "best" user space implementation for the X86 I found (in
> > "Futexes are Tricky") uses both "atomic_compare_and_exchange" and
> > "atomic_exchange" for the lock part and "atomic_dec" for the unlock part.
>
> That's right.  It's possible to use atomic_compare_and_exchange for
> unlock; it's correct.  But you get that extra bit of spinning with SMP,
> so atomic_dec is better.
>
> -- Jamie
>
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