linux-kernel - Re: [PATCH] mm: Use fallthrough;

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Date:   Tue, 10 Mar 2020 04:10:29 -0700
From:   Joe Perches <joe@...ches.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Hugh Dickins <hughd@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        Nitin Gupta <ngupta@...are.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org
Subject: Re: [PATCH] mm: Use fallthrough;

On Mon, 2020-03-09 at 15:48 +0900, Sergey Senozhatsky wrote:
> On (20/03/09 15:20), Sergey Senozhatsky wrote:
> [..]
> > > <shrug, maybe>  I've no real opinion about that necessity.
> > > 
> > > fallthrough commments are relatively rarely used as a
> > > separating element between case labels.
> > > 
> > > It's by far most common to just have consecutive case labels
> > > without any other content.
> > > 
> > > It's somewhere between 500:1 to 1000:1 in the kernel.
> > 
> > I thought that those labels were used by some static code analysis
> > tools, so that the removal of some labels raised questions. But I
> > don't think I have opinions otherwise.
> 
> ... I guess GCC counts as a static code analysis tool :)
> 
> Looking at previous commits, people wanted to have proper 'fall through'
> 
> 
>     Replace "fallthru" with a proper "fall through" annotation.
>     This fix is part of the ongoing efforts to enabling
>     -Wimplicit-fallthrough
> 
> ---
> 
> -       case ZPOOL_MM_RW: /* fallthru */
> +       case ZPOOL_MM_RW: /* fall through */

That conversion was unnecessary.
(there are still 6 /* fallthru */ comments in today's kernel)

There are tens of thousands of consecutive case labels without
interleaving fallthrough comments in the kernel like:

	switch (foo) {
	case BAR:
	case BAZ:
		do_something();
		break;
	default:
		something_else();
		break;
	}

So gcc and clang handle consecutive cases without fallthrough
without uselessly emitting warnings just fine.