Linux Virtualization - Jul 2020 - [PATCH 02/18] compiler.h: Split {READ,WRITE}_ONCE definitions out into rwonce.h

On Tue, Jun 30, 2020 at 7:37 PM Will Deacon <will at kernel.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 at kernel.org>
> Signed-off-by: Will Deacon <will at kernel.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.

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

       Arnd

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 at kernel.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 at kernel.org>
> > Signed-off-by: Will Deacon <will at kernel.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

On Wed, Jul 1, 2020 at 12:16 PM Will Deacon <will at kernel.org>
wrote:
> 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 at kernel.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 at kernel.org>
> > > Signed-off-by: Will Deacon <will at kernel.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.
Right, I didn't mean you should change that right away. I actually
have a work-in-progress patch series that removes a ton of
indirect header inclusions to improve build speed, and a script to
add the explicit includes into .c files where needed.
> > 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...
I think this would require the same kind of fixup for anything that depends on
asm/barrier.h being included implicitly at the moment.

      Arnd