linux-kernel - Re: [PATCH 2/6] locking/atomic/x86: Rewrite x86_32 arch_atomic64_{,fetch}

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Date: Tue, 9 Apr 2024 18:39:32 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: Mark Rutland <mark.rutland@....com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, 
	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>, 
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 2/6] locking/atomic/x86: Rewrite x86_32
 arch_atomic64_{,fetch}_{and,or,xor}() functions

On Tue, Apr 9, 2024 at 6:34 PM Mark Rutland <mark.rutland@....com> wrote:

> > > > ... and then use that in each of the instances below.
> > > >
> > > > That way the subtlety is clearly documented, and it'd more clearly align 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.
>
> Yeah, something of that shape.
>
> Having thought for a bit longer, it's probably better to use '_torn' rather
> than '_tearable' (i.e. name this arch_atomic64_read_torn()).
>
> It'd be nice if we could specify the usage restrictions a bit more clearly,
> since this can only be used for compare-and-swap loops that implement
> unconditional atomics. (e.g. arch_atomic64_and(), but not
> arch_atomic_add_unless()).
>
> So I'd suggest:

Eh, just sent a v2 a second before I received your mail. I'll respin
the patchset tomorrow to include your suggested text. Please note that
v2 patch set avoids all cosmetic  changes.

Thanks,
Uros.

>
> /*
>  * Read an atomic64_t non-atomically.
>  *
>  * This is intended to be used in cases where a subsequent atomic operation
>  * will handle the torn value, and can be used to prime the first iteration of
>  * unconditional try_cmpxchg() loops, e.g.
>  *
>  *      s64 val = arch_atomic64_read_torn(v);
>  *      do { } while (!arch_atomic_try_cmpxchg(v, &val, val OP i);
>  *
>  * This is NOT safe to use where the value is not always checked by a
>  * subsequent atomic operation, such as in conditional try_cmpxchg() loops that
>  * can break before the atomic, e.g.
>  *
>  *      s64 val = arch_atomic64_read_torn(v);
>  *      do {
>  *              if (condition(val))
>  *                      break;
>  *      } while (!arch_atomic_try_cmpxchg(v, &val, val OP i);
>  */
> static __always_inline s64 arch_atomic64_read_torn(atomic64_t *v)
> {
>     /* See comment in arch_atomic_read() */
>     return __READ_ONCE(v->counter);
> }
>
> Mark.