Re: [PATCH] tools: restore READ_ONCE() C++ compatibility

[Mark Rutland]

Hi Arnaldo,

As Sandipan gave a Tested-by for this, are you happy to pick it up as a
fix for v4.17?

Or would you prefer that I resend this?

Thanks,
Mark.

On Wed, Apr 04, 2018 at 05:34:45PM +0100, Mark Rutland wrote:
> Our userspace <linux/compiler.h> defines READ_ONCE() in a way that clang
> doesn't like, as we have an anonymous union in which neither field is
> initialized.
> 
> WRITE_ONCE() is fine since it initializes the __val field. For
> READ_ONCE() we can keep clang and GCC happy with a dummy initialization
> of the __c field, so let's do that.
> 
> At the same time, let's split READ_ONCE() and WRITE_ONCE() over several
> lines for legibility, as we do in the in-kernel <linux/compiler.h>.
> 
> Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
> Fixes: 6aa7de059173a986 ("locking/atomics: COCCINELLE/treewide: Convert trivial ACCESS_ONCE() patterns to READ_ONCE()/WRITE_ONCE()")
> Reported-by: Li Zhijian <lizhijian@xxxxxxxxxxxxxx>
> Reported-by: Sandipan Das <sandipan@xxxxxxxxxxxxxxxxxx>
> Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> ---
>  tools/include/linux/compiler.h | 20 +++++++++++++++-----
>  1 file changed, 15 insertions(+), 5 deletions(-)
> 
> Hi,
> 
> This is fallout from my automated ACCESS_ONCE() removal, and I'm not that
> familiar with using clang for perf.
> 
> In local testing, this fixes READ_ONCE() when compiling with clang, but I
> subsequently hit some other issues which I believe are down to LLVM API
> changes.
> 
> Zhijian, Sandipan, does this patch work for you?
> 
> Thanks,
> Mark.
> 
> diff --git a/tools/include/linux/compiler.h b/tools/include/linux/compiler.h
> index 04e32f965ad7..1827c2f973f9 100644
> --- a/tools/include/linux/compiler.h
> +++ b/tools/include/linux/compiler.h
> @@ -151,11 +151,21 @@ static __always_inline void __write_once_size(volatile void *p, void *res, int s
>   * required ordering.
>   */
>  
> -#define READ_ONCE(x) \
> -	({ union { typeof(x) __val; char __c[1]; } __u; __read_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
> -
> -#define WRITE_ONCE(x, val) \
> -	({ union { typeof(x) __val; char __c[1]; } __u = { .__val = (val) }; __write_once_size(&(x), __u.__c, sizeof(x)); __u.__val; })
> +#define READ_ONCE(x)					\
> +({							\
> +	union { typeof(x) __val; char __c[1]; } __u =	\
> +		{ .__c = { 0 } };			\
> +	__read_once_size(&(x), __u.__c, sizeof(x));	\
> +	__u.__val;					\
> +})
> +
> +#define WRITE_ONCE(x, val)				\
> +({							\
> +	union { typeof(x) __val; char __c[1]; } __u =	\
> +		{ .__val = (val) }; 			\
> +	__write_once_size(&(x), __u.__c, sizeof(x));	\
> +	__u.__val;					\
> +})
>  
>  
>  #ifndef __fallthrough
> -- 
> 2.11.0
>