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

[Davidlohr Bueso]

On Tue, 27 Oct 2015, Peter Zijlstra wrote:

> 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.

So I ran some experiments on an IvyBridge (2.8GHz) and the cost of XCHG is
constantly cheaper (by at least half the latency) than MFENCE. While there
was a decent amount of variation, this difference remained rather constant.

Then again, I'm not sure this matters. Thoughts?

Thanks,
Davidlohr
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