linux-kernel - Re: [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu

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Message-ID: <CAHk-=wgHE-6z1CYGrZx0XSrPif+s7MuA+JZKCWWL5CvbdqWK+A@mail.gmail.com>
Date:   Thu, 19 Oct 2023 11:22:38 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Uros Bizjak <ubizjak@...il.com>, Nadav Amit <namit@...are.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu_ptr()

On Thu, 19 Oct 2023 at 11:14, Peter Zijlstra <peterz@...radead.org> 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