linux-kernel - Re: [RFC PATCH] compiler_attributes.h: Add 'fallthrough' pseudo keyword for switch/case use

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Date:   Sun, 4 Aug 2019 11:09:19 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Joe Perches <joe@...ches.com>
Cc:     Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, 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>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Neil Horman <nhorman@...driver.com>,
        David Miller <davem@...emloft.net>
Subject: Re: [RFC PATCH] compiler_attributes.h: Add 'fallthrough' pseudo
 keyword for switch/case use

On Sun, Aug 4, 2019 at 11:01 AM Joe Perches <joe@...ches.com> wrote:
>
> Linus?  Do you have an opinion about this RFC/patch?

So my only real concern is that the comment approach has always been
the really traditional one, going back all the way to 'lint' days.

And you obviously cannot use a #define to create a comment, so this
whole keyword model will never be able to do that.

At the same time, all the modern tools we care about do seem to be
happy with it, either through the gcc attribute, the clang
[[clang:fallthrough]] or the (eventual) standard C [[fallthrough]]
model.

So I'm ok with just saying "the comment model may be traditional, but
it's not very good".

I didn't look at all the patches, but the one I *did* see had a few issues:

 - it didn't seem to handle clang

 - we'd need to make -Wimplicit-fallthrough be dependent on the
compiler actually supporting the attribute, not just on supporting the
flag.

without those changes, nobody can actually start doing any
conversions. But I assume such patches exist somewhere, and I've just
missed them.

               Linus