linux-kernel - Re: [PATCH v2] nfs: remove incorrect fallthrough label

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Message-ID: <20200917043521.GA3355283@ubuntu-n2-xlarge-x86>
Date:   Wed, 16 Sep 2020 21:35:21 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>,
        "Gustavo A . R . Silva" <gustavo@...eddedor.com>,
        Joe Perches <joe@...ches.com>,
        Hongxiang Lou <louhongxiang@...wei.com>,
        Miaohe Lin <linmiaohe@...wei.com>, linux-nfs@...r.kernel.org,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH v2] nfs: remove incorrect fallthrough label

On Wed, Sep 16, 2020 at 01:02:55PM -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 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
> * __attribute__(__fallthrough__)
> * goto (plz no)
> * (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
> Suggested-by: Joe Perches <joe@...ches.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>

Reviewed-by: Nathan Chancellor <natechancellor@...il.com>

> ---
> 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;
> -- 
> 2.28.0.618.gf4bc123cb7-goog
>