linux-kernel - Re: [PATCH 2/2] compiler: Prevent evaluation of WRITE

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Message-ID: <20190524105339.GC12796@lakrids.cambridge.arm.com>
Date:   Fri, 24 May 2019 11:53:40 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Andrea Parri <andrea.parri@...rulasolutions.com>
Cc:     linux-kernel@...r.kernel.org, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jorgen Hansen <jhansen@...are.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>
Subject: Re: [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@....com>
> Signed-off-by: Andrea Parri <andrea.parri@...rulasolutions.com>
> Cc: Arnd Bergmann <arnd@...db.de>
> Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Cc: Jorgen Hansen <jhansen@...are.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Will Deacon <will.deacon@....com>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: "Paul E. McKenney" <paulmck@...ux.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@....com>

Thanks,
Mark.


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