linux-kernel - Re: [PATCH 13/13] rcutorture: formal: prepare for ACCESS

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Date:   Tue, 10 Oct 2017 13:50:39 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH 13/13] rcutorture: formal: prepare for ACCESS_ONCE()
 removal

On Tue, Oct 10, 2017 at 05:47:12AM -0700, Paul E. McKenney wrote:
> On Tue, Oct 10, 2017 at 10:54:14AM +0100, Mark Rutland wrote:
> > On Mon, Oct 09, 2017 at 12:51:12PM -0700, Paul E. McKenney wrote:
> > > On Mon, Oct 09, 2017 at 07:28:50PM +0100, Mark Rutland wrote:
> > > > diff --git a/tools/testing/selftests/rcutorture/formal/srcu-cbmc/src/barriers.h b/tools/testing/selftests/rcutorture/formal/srcu-cbmc/src/barriers.h
> > > > index 6687acc..ee4e4f8 100644
> > > > --- a/tools/testing/selftests/rcutorture/formal/srcu-cbmc/src/barriers.h
> > > > +++ b/tools/testing/selftests/rcutorture/formal/srcu-cbmc/src/barriers.h
> > > > @@ -34,8 +34,9 @@
> > > >  #define rs_smp_mb() do {} while (0)
> > > >  #endif
> > > > 
> > > > -#define ACCESS_ONCE(x) (*(volatile typeof(x) *) &(x))
> > > > -#define READ_ONCE(x) ACCESS_ONCE(x)
> > > > -#define WRITE_ONCE(x, val) (ACCESS_ONCE(x) = (val))
> > > > +#define __ACCESS_ONCE(x) (*(volatile typeof(x) *) &(x))
> > > > +#define ACCESS_ONCE(x) __ACCESS_ONCE(x)
> > > > +#define READ_ONCE(x) __ACCESS_ONCE(x)
> > > > +#define WRITE_ONCE(x, val) (__ACCESS_ONCE(x) = (val))
> > > 
> > > How about something like the following?
> > > 
> > > #define READ_ONCE(x) (*(volatile typeof(x) *) &(x))
> > > #define WRITE_ONCE(x) ((*(volatile typeof(x) *) &(x)) = (val))
> > 
> > Sure; folded in and pushed out. :)
> 
> Thank you!
> 
> > I've assumed that the ACCESS_ONCE() definition needs to be kept until
> > that's ripped out treewide. Please shout if that's not the case!
> 
> You have it right.  This case is an exception because this code is
> used only by RCU, which has long since had ACCESS_ONCE() ripped out.

Sorry; I meant that in this case, I leave this code as:

#define ACCESS_ONCE(x) (*(volatile typeof(x) *) &(x))
#define READ_ONCE(x) (*(volatile typeof(x) *) &(x))
#define WRITE_ONCE(x) ((*(volatile typeof(x) *) &(x)) = (val))

... if you mean that we can drop ACCESS_ONCE() in this case, then I can
rip that out.

Thanks,
Mark.