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

[Linus Torvalds]

On Mon, Nov 2, 2015 at 12:15 PM, Davidlohr Bueso <dave@xxxxxxxxxxxx> wrote:
>
> 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.

Mind testing "lock addq $0,0(%rsp)" instead of mfence? That's what we
use on old cpu's without one (ie 32-bit).

I'm not actually convinced that mfence is necessarily a good idea. I
could easily see it being microcode, for example.

At least on my Haswell, the "lock addq" is pretty much exactly half
the cost of "mfence".

                     Linus
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