linux-kernel - Re: [PATCH 02/18] compiler.h: Split {READ,WRITE}

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Date:   Wed, 1 Jul 2020 11:16:04 +0100
From:   Will Deacon <will@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Marco Elver <elver@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Matt Turner <mattst88@...il.com>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Richard Henderson <rth@...ddle.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Alan Stern <stern@...land.harvard.edu>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        alpha <linux-alpha@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [PATCH 02/18] compiler.h: Split {READ,WRITE}_ONCE definitions
 out into rwonce.h

Hi Arnd,

On Tue, Jun 30, 2020 at 09:11:32PM +0200, Arnd Bergmann wrote:
> On Tue, Jun 30, 2020 at 7:37 PM Will Deacon <will@...nel.org> wrote:
> >
> > In preparation for allowing architectures to define their own
> > implementation of the READ_ONCE() macro, move the generic
> > {READ,WRITE}_ONCE() definitions out of the unwieldy 'linux/compiler.h'
> > file and into a new 'rwonce.h' header under 'asm-generic'.
> >
> > Acked-by: Paul E. McKenney <paulmck@...nel.org>
> > Signed-off-by: Will Deacon <will@...nel.org>
> > ---
> >  include/asm-generic/Kbuild   |  1 +
> >  include/asm-generic/rwonce.h | 91 ++++++++++++++++++++++++++++++++++++
> >  include/linux/compiler.h     | 83 +-------------------------------
> 
> Very nice, this has the added benefit of allowing us to stop including
> asm/barrier.h once linux/compiler.h gets changed to not include
> asm/rwonce.h.

Yeah, with this series linux/compiler.h _does_ include asm/rwonce.h because
otherwise there are many callers to fix up, but that could be addressed
subsequently, I suppose.

> The asm/barrier.h header has a circular dependency, pulling in
> linux/compiler.h itself.

Hmm. Once smp_read_barrier_depends() disappears, I could actually remove
the include of <asm/barrier.h> from asm-generic/rwonce.h. It would have to
remain for arch/alpha/, however, since we need the barrier definitions to
implement READ_ONCE(). I can probably also replace the include of
<linux/compiler.h> in asm-generic/barrier.h with <asm/rwonce.h> too (so it's
still circular, but at least a lot simpler).

I'll have a play...

Will