linux-kernel - Re: [PATCH 3/4] Documentation/process: Add fallthrough pseudo-keyword

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Message-ID: <CANiq72=KMcYmcHL442OKwDBJj3czey-XtjtOBTLqh_HAsoJAzA@mail.gmail.com>
Date:   Sat, 5 Oct 2019 19:47:57 +0200
From:   Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To:     Joe Perches <joe@...ches.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Pavel Machek <pavel@....cz>,
        "Gustavo A . R . Silva" <gustavo@...eddedor.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Jiri Olsa <jolsa@...hat.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Shawn Landden <shawn@....icu>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Miller <davem@...emloft.net>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Jonathan Corbet <corbet@....net>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>
Subject: Re: [PATCH 3/4] Documentation/process: Add fallthrough pseudo-keyword

On Sat, Oct 5, 2019 at 6:47 PM Joe Perches <joe@...ches.com> wrote:
>
> diff --git a/Documentation/process/deprecated.rst b/Documentation/process/deprecated.rst
> index 56280e108d5a..a0ffdc8daef3 100644
> --- a/Documentation/process/deprecated.rst
> +++ b/Documentation/process/deprecated.rst
> @@ -122,14 +122,27 @@ memory adjacent to the stack (when built without `CONFIG_VMAP_STACK=y`)
>
>  Implicit switch case fall-through
>  ---------------------------------
> -The C language allows switch cases to "fall through" when
> -a "break" statement is missing at the end of a case. This,
> -however, introduces ambiguity in the code, as it's not always
> -clear if the missing break is intentional or a bug. As there
> -have been a long list of flaws `due to missing "break" statements
> +The C language allows switch cases to "fall-through" when a "break" statement
> +is missing at the end of a case. This, however, introduces ambiguity in the
> +code, as it's not always clear if the missing break is intentional or a bug.
> +
> +As there have been a long list of flaws `due to missing "break" statements
>  <https://cwe.mitre.org/data/definitions/484.html>`_, we no longer allow
> -"implicit fall-through". In order to identify an intentional fall-through
> -case, we have adopted the marking used by static analyzers: a comment
> -saying `/* Fall through */`. Once the C++17 `__attribute__((fallthrough))`
> -is more widely handled by C compilers, static analyzers, and IDEs, we can
> -switch to using that instead.
> +"implicit fall-through".
> +
> +In order to identify intentional fall-through cases, we have adopted a
> +pseudo-keyword macro 'fallthrough' which expands to gcc's extension
> +__attribute__((__fallthrough__)).  `Statement Attributes
> +<https://gcc.gnu.org/onlinedocs/gcc/Statement-Attributes.html>`_
> +
> +When the C17/C18  [[fallthrough]] syntax is more commonly supported by

Note that C17/C18 does not have [[fallthrough]]. C++17 introduced it,
as it is mentioned above. I would keep the
__attribute__((fallthrough)) -> [[fallthrough]] change you did,
though, since that is indeed the standard syntax (given the paragraph
references C++17).

I was told by Aaron Ballman (who is proposing them for C) that it is
more or less likely that it becomes standardized in C2x. However, it
is still not added to the draft (other attributes are already,
though). See N2268 and N2269:

    http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2268.pdf (fallthrough)
    http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2269.pdf
(attributes in general)

Cheers,
Miguel