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

[Linus Torvalds]

On Thu, 19 Oct 2023 at 11:14, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> But are you really saying this_cpu_read() should not imply READ_ONCE()?

Well, Uros is saying that we may be *forced* to have that implication,
much as I really hate it (and wonder at the competence of a compiler
that forces the code-pessimizing 'volatile').

And the "it's not volatile" is actually our historical behavior. The
volatile really is new, and didn't exist before your commit
b59167ac7baf ("x86/percpu: Fix this_cpu_read()").

So the whole "implies READ_ONCE()" really seems to be due to that
*one* mistake in our percpu sequence locking code.

Yes, it's been that way for 5 years now, but it was the other way
around for the preceding decade....

                    Linus