linux-kernel - Re: [PATCH v3] nfs: remove incorrect fallthrough label

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Date:   Thu, 24 Sep 2020 10:40:55 -0700
From:   Joe Perches <joe@...ches.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "Gustavo A . R . Silva" <gustavo@...eddedor.com>,
        "Gustavo A . R . Silva" <gustavoars@...nel.org>,
        Miaohe Lin <linmiaohe@...wei.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Hongxiang Lou <louhongxiang@...wei.com>,
        linux-nfs@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mark Brown <broonie@...nel.org>
Subject: Re: [PATCH v3] nfs: remove incorrect fallthrough label

On Thu, 2020-09-24 at 10:19 -0700, Nick Desaulniers wrote:
> Hello maintainers,
> Would you mind please picking up this patch?  KernelCI has been
> erroring for over a week without it.

As it's trivial and necessary, why not go through Linus directly?

https://lore.kernel.org/patchwork/patch/1307549/

From: Nick Desaulniers <ndesaulniers@...gle.com>

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 replaced the comment with a break
statement.

If we consider implicit fallthrough to be a design mistake of C, then
all case statements should be terminated with one of the following
statements:
* break
* continue
* return
* fallthrough
* goto
* (call of function with __attribute__(__noreturn__))

Fixes: 2a1390c95a69 ("nfs: Convert to use the preferred fallthrough macro")
Link: https://bugs.llvm.org/show_bug.cgi?id=47539
Acked-by: Gustavo A. R. Silva <gustavoars@...nel.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@...nel.org>
Reviewed-by: Miaohe Lin <linmiaohe@...wei.com>
Reviewed-by: Nathan Chancellor <natechancellor@...il.com>
Suggested-by: Joe Perches <joe@...ches.com>
Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
---
Changes v3:
* update the commit message as per Joe.
* collect tags.

Changes v2:
* add break rather than no terminating statement as per Joe.
* add Joe's suggested by tag.
* add blurb about acceptable terminal statements.

 fs/nfs/super.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/nfs/super.c b/fs/nfs/super.c
index d20326ee0475..eb2401079b04 100644
--- a/fs/nfs/super.c
+++ b/fs/nfs/super.c
@@ -889,7 +889,7 @@  static struct nfs_server *nfs_try_mount_request(struct fs_context *fc)
 		default:
 			if (rpcauth_get_gssinfo(flavor, &info) != 0)
 				continue;
-			fallthrough;
+			break;
 		}
 		dfprintk(MOUNT, "NFS: attempting to use auth flavor %u\n", flavor);
 		ctx->selected_flavor = flavor;