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SubjectRe: [PATCH 2/2] compiler: Prevent evaluation of WRITE_ONCE()

This would be better titled as:

compiler: don't return a value from WRITE_ONCE()

... since we do want the WRITE_ONCE() itself to be evaluated.

On Fri, May 24, 2019 at 12:35:36PM +0200, Andrea Parri wrote:
> Now that there's no single use of the value of WRITE_ONCE(), change
> the implementation to eliminate it.

I hope that's the case, but it's possible that some macros might be
relying on this, so it's probably worth waiting to see if the kbuild
test robot screams.

Otherwise, I agree that WRITE_ONCE() returning a value is surprising,
and unnecessary. IIRC you said that trying to suport that in other
implementations was painful, so aligning on a non-returning version
sounds reasonable to me.

>
> Suggested-by: Mark Rutland <mark.rutland@arm.com>
> Signed-off-by: Andrea Parri <andrea.parri@amarulasolutions.com>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Cc: Jorgen Hansen <jhansen@vmware.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Will Deacon <will.deacon@arm.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: "Paul E. McKenney" <paulmck@linux.ibm.com>
> ---
> include/linux/compiler.h | 5 ++---
> 1 file changed, 2 insertions(+), 3 deletions(-)
>
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 8aaf7cd026b06..4024c809a6c63 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -277,12 +277,11 @@ unsigned long read_word_at_a_time(const void *addr)
> }
>
> #define WRITE_ONCE(x, val) \
> -({ \
> +do { \
> union { typeof(x) __val; char __c[1]; } __u = \
> { .__val = (__force typeof(x)) (val) }; \
> __write_once_size(&(x), __u.__c, sizeof(x)); \
> - __u.__val; \
> -})
> +} while (0)

With the title fixed, and assuming that the kbuild test robot doesn't
find uses we've missed:

Acked-by: Mark Rutland <mark.rutland@arm.com>

Thanks,
Mark.


>
> #endif /* __KERNEL__ */
>
> --
> 2.7.4
>

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