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From:   Nick Desaulniers <ndesaulniers@google.com>
Date:   Fri, 15 Oct 2021 10:33:50 -0700
Message-ID: <CAKwvOdm6LUFBC=as-9fTxYM=H2U=JuzquanFYwasn2Vu3TgsxA@mail.gmail.com>
Subject: Re: [PATCH] linux/container_of.h: switch to static_assert
To:     Rasmus Villemoes <linux@rasmusvillemoes.dk>
Cc:     akpm@linux-foundation.org,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        linux-kernel@vger.kernel.org
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On Fri, Oct 15, 2021 at 2:05 AM Rasmus Villemoes
<linux@rasmusvillemoes.dk> wrote:
>
> _Static_assert() is evaluated already in the compiler's frontend, and
> gives a somehat more to-the-point error, compared to the BUILD_BUG_ON
> macro, which only fires after the optimizer has had a chance to
> eliminate calls to functions marked with
> __attribute__((error)). In theory, this might make builds a tiny bit
> faster.
>
> There's also a little less gunk in the error message emitted:
>
> lib/sort.c: In function =E2=80=98foo=E2=80=99:
> ./include/linux/build_bug.h:78:41: error: static assertion failed: "point=
er type mismatch in container_of()"
>    78 | #define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
>
> compared to
>
> lib/sort.c: In function =E2=80=98foo=E2=80=99:
> ././include/linux/compiler_types.h:322:38: error: call to =E2=80=98__comp=
iletime_assert_2=E2=80=99 declared with attribute error: pointer type misma=
tch in container_of()
>   322 |  _compiletime_assert(condition, msg, __compiletime_assert_, __COU=
NTER__)
>
> While at it, fix the copy-pasto in container_of_safe().
>
> Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>

Thanks for the patch!
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

> ---
> akpm: This is obviously on top of Andy's kernel.h splitup series, so
> should go along with those if acked.
>
>  include/linux/container_of.h | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/container_of.h b/include/linux/container_of.h
> index dd56019838c6..2f4944b791b8 100644
> --- a/include/linux/container_of.h
> +++ b/include/linux/container_of.h
> @@ -16,9 +16,9 @@
>   */
>  #define container_of(ptr, type, member) ({                             \
>         void *__mptr =3D (void *)(ptr);                                  =
 \
> -       BUILD_BUG_ON_MSG(!__same_type(*(ptr), ((type *)0)->member) &&   \
> -                        !__same_type(*(ptr), void),                    \
> -                        "pointer type mismatch in container_of()");    \
> +       static_assert(__same_type(*(ptr), ((type *)0)->member) ||       \
> +                     __same_type(*(ptr), void),                        \
> +                     "pointer type mismatch in container_of()");       \
>         ((type *)(__mptr - offsetof(type, member))); })
>
>  /**
> @@ -31,9 +31,9 @@
>   */
>  #define container_of_safe(ptr, type, member) ({                         =
       \
>         void *__mptr =3D (void *)(ptr);                                  =
 \
> -       BUILD_BUG_ON_MSG(!__same_type(*(ptr), ((type *)0)->member) &&   \
> -                        !__same_type(*(ptr), void),                    \
> -                        "pointer type mismatch in container_of()");    \
> +       static_assert(__same_type(*(ptr), ((type *)0)->member) ||       \
> +                     __same_type(*(ptr), void),                        \
> +                     "pointer type mismatch in container_of_safe()");  \
>         IS_ERR_OR_NULL(__mptr) ? ERR_CAST(__mptr) :                     \
>                 ((type *)(__mptr - offsetof(type, member))); })
>
> --
> 2.31.1
>


--=20
Thanks,
~Nick Desaulniers