linux-kernel - Re: [PATCH] nfs: remove incorrect fallthrough label

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Date:   Tue, 15 Sep 2020 17:33:52 -0700
From:   Joe Perches <joe@...ches.com>
To:     "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>
Cc:     Nathan Chancellor <natechancellor@...il.com>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Hongxiang Lou <louhongxiang@...wei.com>,
        linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] nfs: remove incorrect fallthrough label

On Tue, 2020-09-15 at 19:01 -0500, Gustavo A. R. Silva wrote:
> 
> On 9/15/20 18:51, Gustavo A. R. Silva wrote:
> > 
> > On 9/15/20 18:29, Joe Perches wrote:
> > > On Tue, 2020-09-15 at 15:57 -0700, Nick Desaulniers wrote:
> > > > There is no case after the default from which to fallthrough to. Clang
> > > > will error in this case (unhelpfully without context, see link below)
> > > > and GCC will with -Wswitch-unreachable.
> > > > 
> > > > The previous commit should have just removed the comment.
> > > []
> > > > diff --git a/fs/nfs/super.c b/fs/nfs/super.c
> > > []
> > > > @@ -889,7 +889,6 @@ static struct nfs_server *nfs_try_mount_request(struct fs_context *fc)
> > > >  		default:
> > > >  			if (rpcauth_get_gssinfo(flavor, &info) != 0)
> > > >  				continue;
> > > > -			fallthrough;
> > > 
> > > My preference would be to convert the fallthrough
> > > to a break here so if someone ever adds another
> > > label after default: for any reason, the code would
> > > still work as expected.
> > 
> > I agree with Joe.
> 
> Actually, this is part of the work I plan to do to enable -Wimplicit-fallthrough
> for Clang: audit every place where we could use a break instead of a fallthrough.
> 
> I'm on vacation this week. So, I'll get back to this next week.

Nice, thanks Gustavo.

As part of that work, perhaps you could also find all the

	switch (<foo>) {
	[cases...]
		[code...];
		break;

	default:
		[code...]
		(no break)
	}

uawa where the last label/default block does _not_ have a break
statement and add one too.

Also see:  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91432

gcc does _not_ warn on

	switch (<foo>) {
	case BAR:
		[code];
		(no fallthrough)

	case BAZ:
		break;
	}

It might be good to add the appropriate fallthrough
for those case blocks too.