linux-kernel - Re: [PATCH 2/7] atomics/treewide: rework ordering barriers

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Message-ID: <20180605135616.GV12198@hirez.programming.kicks-ass.net>
Date:   Tue, 5 Jun 2018 15:56:16 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-kernel@...r.kernel.org, Boqun Feng <boqun.feng@...il.com>,
        Will Deacon <will.deacon@....com>,
        Andrea Parri <parri.andrea@...il.com>
Subject: Re: [PATCH 2/7] atomics/treewide: rework ordering barriers

On Tue, Jun 05, 2018 at 02:28:02PM +0100, Mark Rutland wrote:
> On Tue, Jun 05, 2018 at 02:16:23PM +0200, Peter Zijlstra wrote:
> > and simply using smp_mb__{before,after}_atomic for the full fence, its
> > exactly what they were made for.
> 
> The snag is arch/alpha, whare we have:
> 
> /*
>  * To ensure dependency ordering is preserved for the _relaxed and
>  * _release atomics, an smp_read_barrier_depends() is unconditionally
>  * inserted into the _relaxed variants, which are used to build the
>  * barriered versions. To avoid redundant back-to-back fences, we can
>  * define the _acquire and _fence versions explicitly.
>  */
> #define __atomic_op_acquire(op, args...)        op##_relaxed(args)
> #define __atomic_op_fence                       __atomic_op_release
> 
> ... where alpha's smp_read_barrier_depends() is the same as
> smp_mb_after_atomic().
> 
> Since alpha's non-value-returning atomics do not have the
> smp_read_barrier_depends(), I can't just define an empty
> smp_mb_after_atomic().
> 
> Thoughts?

Bah, of course there had to be a misfit.

Something along these lines then:

 __atomic_acquire_fence
 __atomic_release_fence
 __atomic_mb_before
 __atomic_mb_after

?