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

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Message-ID: <20231019085432.GQ33217@noisy.programming.kicks-ass.net>
Date:   Thu, 19 Oct 2023 10:54:32 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.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 Wed, Oct 18, 2023 at 03:40:05PM -0700, Linus Torvalds wrote:

> Look at the *REAL* sequence counter code in <linux/seqlock.h>. Notice
> how in raw_read_seqcount_begin() we have
> 
>         unsigned _seq = __read_seqcount_begin(s);
>         smp_rmb();
> 
> because it actually does the proper barriers. Notice how the garbage
> code in __cyc2ns_read() doesn't have them - and how it was buggy as a
> result.
> 
> (Also notice how this all predates our "we should use load_acquire()
> instead of smb_rmb()", but whatever).

seqlock actually wants rmb even today, the pattern is:

    do {
	seq = load-seq
	rmb
	load-data
	rmb
    } while (seq != re-load-seq)

we specifically only care about loads, and the data loads must be
between the sequence number loads.

As such, load-acquire is not a natural match.