linux-kernel - Re: [PATCH] ext4: Remove unreachable code

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Message-ID: <20210130013505.GB2709570@localhost>
Date:   Fri, 29 Jan 2021 18:35:05 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Vinicius Tinti <viniciustinti@...il.com>
Cc:     Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] ext4: Remove unreachable code

On Fri, Jan 29, 2021 at 06:58:56PM +0000, Vinicius Tinti wrote:
> By enabling -Wunreachable-code-aggressive on Clang the following code
> paths are unreachable.
> 
> Commit dd73b5d5cb67 ("ext4: convert dx_probe() to use the ERR_PTR
> convention")
> Commit ac27a0ec112a ("[PATCH] ext4: initial copy of files from ext3")
> 
> Clang warns:
> 
> fs/ext4/namei.c:831:17: warning: code will never be executed
> [-Wunreachable-code]
>                         unsigned n = count - 1;
>                                      ^~~~~
> fs/ext4/namei.c:830:7: note: silence by adding parentheses to mark code as
> explicitly dead
>                 if (0) { // linear search cross check
>                     ^
>                     /* DISABLES CODE */ ( )

The commit message might be a little clearer if it were restructured a
bit, maybe something like so?

By enabling -Wunreachable-code-aggressive on Clang, the following code
paths are unreachable:

fs/ext4/namei.c:831:17: warning: code will never be executed
[-Wunreachable-code]
                        unsigned n = count - 1;
                                     ^~~~~
fs/ext4/namei.c:830:7: note: silence by adding parentheses to mark code as
explicitly dead
                if (0) { // linear search cross check
                    ^
                    /* DISABLES CODE */ ( )

This has been present since commit ac27a0ec112a ("[PATCH] ext4: initial
copy of files from ext3") and fs/ext3 had it present at the beginning of
git history so it is safe to remove.

> Signed-off-by: Vinicius Tinti <viniciustinti@...il.com>

Regardless of the commit message, I believe this is the right way to get
rid of the warning:

Reviewed-by: Nathan Chancellor <nathan@...nel.org>

> ---
>  fs/ext4/namei.c | 15 ---------------
>  1 file changed, 15 deletions(-)
> 
> diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
> index cf652ba3e74d..1f64dbd7237b 100644
> --- a/fs/ext4/namei.c
> +++ b/fs/ext4/namei.c
> @@ -827,21 +827,6 @@ dx_probe(struct ext4_filename *fname, struct inode *dir,
>  				p = m + 1;
>  		}
>  
> -		if (0) { // linear search cross check
> -			unsigned n = count - 1;
> -			at = entries;
> -			while (n--)
> -			{
> -				dxtrace(printk(KERN_CONT ","));
> -				if (dx_get_hash(++at) > hash)
> -				{
> -					at--;
> -					break;
> -				}
> -			}
> -			ASSERT(at == p - 1);
> -		}
> -
>  		at = p - 1;
>  		dxtrace(printk(KERN_CONT " %x->%u\n",
>  			       at == entries ? 0 : dx_get_hash(at),
> -- 
> 2.25.1
>