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

[Andy Lutomirski]

On Tue, Jan 12, 2016 at 12:54 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Tue, Jan 12, 2016 at 12:30 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>>
>> I recall reading somewhere that lock addl $0, 32(%rsp) or so (maybe even 64)
>> was better because it avoided stomping on very-likely-to-be-hot write
>> buffers.
>
> I suspect it could go either way. You want a small constant (for the
> isntruction size), but any small constant is likely to be within the
> current stack frame anyway. I don't think 0(%rsp) is particularly
> likely to have a spill on it right then and there, but who knows..
>
> And 64(%rsp) is  possibly going to be cold in the L1 cache, especially
> if it's just after a deep function call. Which it might be. So it
> might work the other way.
>
> So my guess would be that you wouldn't be able to measure the
> difference. It might be there, but probably too small to really see in
> any noise.
>
> But numbers talk, bullshit walks. It would be interesting to be proven wrong.

Here's an article with numbers:

http://shipilev.net/blog/2014/on-the-fence-with-dependencies/

I think they're suggesting using a negative offset, which is safe as
long as it doesn't page fault, even though we have the redzone
disabled.

--Andy