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Date:   Thu, 19 Oct 2023 10:44:59 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Uros Bizjak <ubizjak@gmail.com>, Nadav Amit <namit@vmware.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Brian Gerst <brgerst@gmail.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Nick Desaulniers <ndesaulniers@google.com>
Subject: Re: [PATCH v2 -tip] x86/percpu: Use C for arch_raw_cpu_ptr()
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On Wed, Oct 18, 2023 at 03:40:05PM -0700, Linus Torvalds wrote:

> Side note: the code that caused that problem is this:
> 
>   __always_inline void __cyc2ns_read(struct cyc2ns_data *data)
>   {
>         int seq, idx;
> 
>         do {
>                 seq = this_cpu_read(cyc2ns.seq.seqcount.sequence);
>                 ...
>         } while (unlikely(seq != this_cpu_read(cyc2ns.seq.seqcount.sequence)));
>   }
> 
> where the issue is that the this_cpu_read() of that sequence number
> needs to be ordered.

I have very vague memories of other code also relying on this_cpu_read()
implying READ_ONCE().

And that code really only is buggy if you do not have that. Since it is
cpu local, the smp_rmb() would be confusing, as would smp_load_acquire()
be -- there is no cross-cpu data ordering.

The other option is of couse adding explicit barrier(), but that's
entirely superfluous when all the loads are READ_ONCE().


If you want to make this_cpu_read() not imply READ_ONCE(), then we
should go audit all users :/ Can be done ofc.