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From: Uros Bizjak <ubizjak@gmail.com>
Date: Tue, 9 Apr 2024 14:50:19 +0200
Message-ID: <CAFULd4Z8JTc_5p8apO680es0fYO5ke9yYPpPUfp=15XrRbKESQ@mail.gmail.com>
Subject: Re: [PATCH 2/6] locking/atomic/x86: Rewrite x86_32
 arch_atomic64_{,fetch}_{and,or,xor}() functions
To: Mark Rutland <mark.rutland@arm.com>
Cc: x86@kernel.org, linux-kernel@vger.kernel.org, 
	Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@alien8.de>, 
	Dave Hansen <dave.hansen@linux.intel.com>, "H. Peter Anvin" <hpa@zytor.com>, 
	Peter Zijlstra <peterz@infradead.org>
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On Tue, Apr 9, 2024 at 2:03=E2=80=AFPM Uros Bizjak <ubizjak@gmail.com> wrot=
e:
>
> On Tue, Apr 9, 2024 at 1:13=E2=80=AFPM Mark Rutland <mark.rutland@arm.com=
> wrote:
>
> > >  static __always_inline void arch_atomic64_and(s64 i, atomic64_t *v)
> > >  {
> > > -     s64 old, c =3D 0;
> > > +     s64 val =3D __READ_ONCE(v->counter);
> >
> > I reckon it's worth placing this in a helper with a big comment, e.g.
> >
> > static __always_inline s64 arch_atomic64_read_tearable(atomic64_t *v)
> > {
> >         /*
> >          * TODO: explain that this might be torn, but it occurs *once*,=
 and can
> >          * safely be consumed by atomic64_try_cmpxchg().
> >          *
> >          * TODO: point to the existing commentary regarding why we use
> >          * __READ_ONCE() for KASAN reasons.
> >          */
> >         return __READ_ONCE(v->counter);
> > }
> >
> > ... and then use that in each of the instances below.
> >
> > That way the subtlety is clearly documented, and it'd more clearly alig=
n with
> > the x86_64 verions.
>
> This is an excellent idea. The separate definitions needs to be placed
> in atomic64_32.h and atomic_64_64.h (due to use of atomic64_t
> typedef), but it will allow the same unification of functions between
> x64_32 and x64_64 as the approach with __READ_ONCE().

Something like this:

--cut here--
/*
 * This function is intended to preload the value from atomic64_t
 * location in a non-atomic way. The read might be torn, but can
 * safely be consumed by the compare-and-swap loop.
 */
static __always_inline s64 arch_atomic64_read_tearable(atomic64_t *v)
{
    /*
     * See the comment in arch_atomic_read() on why we use
     * __READ_ONCE() instead of READ_ONCE_NOCHECK() here.
     */
    return __READ_ONCE(v->counter);
}
--cut here--

Thanks,
Uros.