linux-kernel - Re: [PATCH -tip v2 09/11] data_race: Avoid nested statement expression

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Message-ID: <20200521133150.GB6608@willie-the-truck>
Date:   Thu, 21 May 2020 14:31:50 +0100
From:   Will Deacon <will@...nel.org>
To:     Marco Elver <elver@...gle.com>
Cc:     paulmck@...nel.org, dvyukov@...gle.com, glider@...gle.com,
        andreyknvl@...gle.com, kasan-dev@...glegroups.com,
        linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...nel.org,
        peterz@...radead.org, clang-built-linux@...glegroups.com,
        bp@...en8.de
Subject: Re: [PATCH -tip v2 09/11] data_race: Avoid nested statement
 expression

On Thu, May 21, 2020 at 01:08:52PM +0200, Marco Elver wrote:
> It appears that compilers have trouble with nested statements
> expressions, as such make the data_race() macro be only a single
> statement expression. This will help us avoid potential problems in
> future as its usage increases.
> 
> Link: https://lkml.kernel.org/r/20200520221712.GA21166@zn.tnic
> Signed-off-by: Marco Elver <elver@...gle.com>
> ---
> v2:
> * Add patch to series in response to above linked discussion.
> ---
>  include/linux/compiler.h | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 7444f026eead..1f9bd9f35368 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -211,12 +211,11 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
>   */
>  #define data_race(expr)							\
>  ({									\
> +	__unqual_scalar_typeof(({ expr; })) __v;			\
>  	__kcsan_disable_current();					\
> -	({								\
> -		__unqual_scalar_typeof(({ expr; })) __v = ({ expr; });	\
> -		__kcsan_enable_current();				\
> -		__v;							\
> -	});								\
> +	__v = ({ expr; });						\
> +	__kcsan_enable_current();					\
> +	__v;								\

Hopefully it doesn't matter, but this will run into issues with 'const'
non-scalar expressions.

Will