Re: [PATCH 3/4] x86,asm: Re-work smp_store_mb()

[Peter Zijlstra]

On Wed, Oct 28, 2015 at 06:33:56AM +0900, Linus Torvalds wrote:
> On Wed, Oct 28, 2015 at 4:53 AM, Davidlohr Bueso <dave@xxxxxxxxxxxx> wrote:
> >
> > Note that this might affect callers that could/would rely on the
> > atomicity semantics, but there are no guarantees of that for
> > smp_store_mb() mentioned anywhere, plus most archs use this anyway.
> > Thus we continue to be consistent with the memory-barriers.txt file,
> > and more importantly, maintain the semantics of the smp_ nature.
> 

> So with this patch, the whole thing becomes pointless, I feel. (Ok, so
> it may have been pointless before too, but at least before this patch
> it generated special code, now it doesn't). So why carry it along at
> all?

So I suppose this boils down to if: XCHG ends up being cheaper than
MOV+FENCE.

PeterA, any idea?
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