linux-kernel - Re: [PATCH v2 8/9] atomic,x86: Alternative atomic_*

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Date:   Mon, 13 Dec 2021 10:11:28 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Will Deacon <will@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Marco Elver <elver@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Christoph Hellwig <hch@...radead.org>,
        Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH v2 8/9] atomic,x86: Alternative atomic_*_overflow() scheme

On Mon, Dec 13, 2021 at 8:43 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> So Marco was expressing doubt about this exact interface for the
> atomic_*_overflow() functions, since it's extremely easy to get the
> whole ATOMIC_OVERFLOW_OFFSET thing wrong.

I missed that discussion (maybe it was on irc? Or maybe I just get too
much email).

Anyway, my preferred solution would simply be to make the ref-counting
atomics use a different type.

Voilà, problem solved. You can't really misuse them by mistake,
because you can't access it by mistake.

Sure, it could be a wrapper around 'atomic_t' on architectures that
end up using the generic fallback, so it might be as simple as

   typedef atomic_t atomic_ref_t;

in some asm-generic implementation, although I suspect that you'd want
type safety even there, and do

  typedef struct { atomic_t atomic_val; } atomic_ref_t;

But then on x86 - and other architectures that might prefer to use
that offset trick because they have flags - I'm not sure it even makes
sense to have anything to do with 'atomic_t' at all, since there would
basically be zero overlap with the regular atomic operations (partly
due to the offset, but partly simply because the 'ref' operations are
simply different).

(Wrt naming: I do think this is more about the "ref" part than the
"overflow" part - thus I'd suggest the "atomic_ref_t" rather than your
ofl naming).

            Linus