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SubjectRe: [PATCH] Profile likely/unlikely macros
On Wed, 26 April 2006 11:08:09 +1000, Nick Piggin wrote:
> Daniel Walker wrote:
>
> Ah, I see. Then you should be OK with either your current scheme, or
> Andrew's suggestion, so long as you have a memory barrier before the
> unlock (eg. smp_mb__before_clear_bit()).
>
> >I'm not exactly sure what you mean by "release consistency" ?
>
> Without a barrier, the stores to the linked list may be visible to another
> CPU after the store that unlocks the atomic_t. Ie. the critical section can
> leak out of the lock.

Admitted, I'm a bit slow at times. But why does this matter?
According to my fairly limited brain, you take a potentially expensive
barrier, so you pay with a bit of runtime. What you buy is a smaller
critical section, so you can save some runtime on other cpus. When
optimizing for the common case, which is one cpu, this is a net loss.

There must be some correctness issue hidden that I cannot see. Can
you explain that to me?

Jörn

--
A victorious army first wins and then seeks battle.
-- Sun Tzu
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