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Date:   Thu, 25 Jun 2020 13:29:26 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Marco Elver <elver@google.com>
Cc:     "Paul E. McKenney" <paulmck@kernel.org>,
        kernel test robot <lkp@intel.com>, kbuild-all@lists.01.org,
        LKML <linux-kernel@vger.kernel.org>,
        Will Deacon <will@kernel.org>
Subject: Re: [rcu:rcu/next 35/35] kernel/rcu/tree.c:251:8: error: implicit
 declaration of function 'arch_atomic_add_return'; did you mean
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On Thu, Jun 25, 2020 at 11:55:13AM +0200, Marco Elver wrote:
> On Wed, 24 Jun 2020 at 22:30, Paul E. McKenney <paulmck@kernel.org> wrote:

> > diff --git a/include/linux/atomic-fallback.h b/include/linux/atomic-fallback.h
> > index 2c4927b..b7935857 100644
> > --- a/include/linux/atomic-fallback.h
> > +++ b/include/linux/atomic-fallback.h
> > @@ -133,6 +133,7 @@ atomic_add_return(int i, atomic_t *v)
> >         return ret;
> >  }
> >  #define atomic_add_return atomic_add_return
> > +#define arch_atomic_add_return atomic_add_return
> >  #endif
> >
> >  #endif /* atomic_add_return_relaxed */
> >
> > ------------------------------------------------------------------------
> >
> > And of course similar for arch_atomic_andnot() and arch_atomic_read().
> >
> > Another way would be to define a noinstr_atomic_add_return() that
> > was defined something like this:
> >
> > ------------------------------------------------------------------------
> >
> > #ifdef CONFIG_HAVE_ARCH_KCSAN
> > # define noinstr_atomic_add_return arch_atomic_add_return
> > #else
> > # define noinstr_atomic_add_return atomic_add_return
> > #endif
> 
> noinstr also needs to apply to KASAN & co, so this won't quite work.
> Every architecture that defines arch_atomic_* has #define ARCH_ATOMIC,
> so that could be used instead.

Right. And my bad for forgetting arch_atomic_ isn't generally available
:/

> > ------------------------------------------------------------------------
> >
> > And again similarly for the others.
> >
> > Left to myself, I would take the second option just because it provably
> > leaves unaltered anything that isn't using the new API.  That said,
> > there has to be a better Kconfig option to key this off of.
> >
> > Thoughts?
> 
> I think 'arch_atomic_*' is already the noinstr variant, and your first
> suggestion of adding arch-defines to atomic-fallback.h seems cleaner,
> as it avoids introducing new interfaces. But that also depends on if
> it's a one-off, only for RCU, or if the use of 'arch_atomic'
> proliferates outside of arch/. My guess is that, unfortunately, other
> places will want 'arch_atomic' as well eventually.

I fear the same. Let me see if I can quickly modify the atomic scripts
to generate the required fallbacks.