Re: [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu_ptr()

[Linus Torvalds]

On Wed, 18 Oct 2023 at 09:03, Nadav Amit <namit@xxxxxxxxxx> wrote:
>
> Having said that, I am not sure what other usages you have in mind.
> “current” is a pretty obvious straight forward case with considerable
> impact on code generation. There may be additional variables, but it is
> likely that there would be more functions/TU in which they would not be
> constant and would require more refined techniques to avoid mistakes
> such as the use of stale cached values.

Yeah, I don't think there really are other cases.

We do have things that could be considered stable (like
"smp_processor_id()" which is stable as long as preemption or
migration is disabled (or it's in an irq-off section).

And it might be lovely to optimize those too, *BUT* that would require
that there be a barrier against that optimization that works.

And if there is anything that this thread has made clear, it's that
the whole 'load from a constant section' doesn't seem to have any sane
barriers.

So while the CSE for inline asm statements is a bit too weak with that
whole "only CSE within a basic block" thing, the CSE of "load a
constant value from memory" is too *strong*, in that we don't seem to
have _any_ sane way to say "now you need to reload".

The traditional way we've done that is with our "barrier()" macro,
which does the whole inline asm with a memory clobber, but even that
doesn't act as a barrier for gcc optimizing the constant load.

Which means that while we'd probably love for the compiere to optimize
smp_processor_id() a bit more, we can't use the 'stable memory
location' trick for it.

Because I can't think of anything but 'current' that would be _that_
stable as far as C code is concerned.

                Linus