linux-kernel - Re: [PATCH] compiler.h: simplify data

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Date: Mon, 24 Jun 2024 14:27:43 +0200
From: Marco Elver <elver@...gle.com>
To: Alexey Dobriyan <adobriyan@...il.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org, 
	akpm@...ux-foundation.org
Subject: Re: [PATCH] compiler.h: simplify data_race() macro

On Mon, 24 Jun 2024 at 13:49, Alexey Dobriyan <adobriyan@...il.com> wrote:
>
> Use auto type deduction and comma expression to decrease macro expansion
> size.
>
> __unqual_scalar_typeof() is quite wordy macro by itself.
>
> "expr" can be arbitrary complex so not expanding it twice is good.
> Should be faster too because type is deduced only once
> from the initializer.
>
> Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>

Thanks for cleaning up.  That code certainly predates the availability
of __auto_type. Although if I recall correctly, __unqual_scalar_typeof
became the first user of _Generic (the first C11 keyword we used in
the kernel?), but we used some ifdef to still support ancient
compilers initially (that definitely also didn't have __auto_type).

Reviewed-by: Marco Elver <elver@...gle.com>

Which tree is this for?

> ---
>
>  include/linux/compiler.h |    5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
>
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -200,10 +200,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
>   */
>  #define data_race(expr)                                                        \
>  ({                                                                     \
> -       __unqual_scalar_typeof(({ expr; })) __v = ({                    \
> -               __kcsan_disable_current();                              \
> -               expr;                                                   \
> -       });                                                             \
> +       __auto_type __v = (__kcsan_disable_current(), expr);            \
>         __kcsan_enable_current();                                       \
>         __v;                                                            \
>  })