RE: Synchronizing Bit operations V2

[Chen, Kenneth W]

Christoph Lameter wrote on Thursday, March 30, 2006 4:43 PM
> > > Note that the current semantics for bitops IA64 are broken. Both
> > > smp_mb__after/before_clear_bit are now set to full memory barriers
> > > to compensate
> > 
> > Why you say that?  clear_bit has built-in acq or rel semantic depends
> > on how you define it. I think only one of smp_mb__after/before need to
> > be smp_mb?
> 
> clear_bit has no barrier semantics just acquire. Therefore both smp_mb_* 
> need to be barriers or they need to add some form of "release".

We are talking about arch specific implementation of clear_bit and smp_mb_*.
Yes, for generic code, clear_bit has no implication of memory ordering, but
for arch specific code, one should optimize those three functions with the
architecture knowledge of exactly what's happening under the hood.

- Ken
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