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

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Date:   Mon, 9 Oct 2017 12:51:12 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH 13/13] rcutorture: formal: prepare for ACCESS_ONCE()
 removal

On Mon, Oct 09, 2017 at 07:28:50PM +0100, Mark Rutland wrote:
> For several reasons, it is desirable to use {READ,WRITE}_ONCE() in
> preference to ACCESS_ONCE(), and new code is expected to use one of the
> former. So far, there's been no reason to change most existing uses of
> ACCESS_ONCE(), as these aren't currently harmful.
> 
> However, for some features it is necessary to instrument reads and
> writes separately, which is not possible with ACCESS_ONCE(). This
> distinction is critical to correct operation.
> 
> The bulk of the kernel code can be transformed via Coccinelle to use
> {READ,WRITE}_ONCE(), though this only modifies users of ACCESS_ONCE(),
> and not the implementation itself. As such, it has the potential to
> break homebrew ACCESS_ONCE() macros seen in some user code in the kernel
> tree (e.g. the virtio code, as fixed in commit ea9156fb3b71d9f7).
> 
> To avoid fragility if/when that transformation occurs, this patch
> reworks the rcutorture formal tests to use an intermediate
> __ACCESS_ONCE() helper, which will avoid {READ,WRITE}_ONCE() being
> turned into tautological definitions. There should be no functional
> change as a result of this patch.
> 
> Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> Signed-off-by: Mark Rutland <mark.rutland@....com>
> ---
>  tools/testing/selftests/rcutorture/formal/srcu-cbmc/src/barriers.h | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> 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))


							Thanx, Paul