linux-ext4 - Re: [PATCH v2] ext4: Enable code path when DX

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Date:   Sun, 31 Jan 2021 17:48:20 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Vinicius Tinti <viniciustinti@...il.com>
Cc:     Andreas Dilger <adilger.kernel@...ger.ca>,
        Nathan Chancellor <natechancellor@...il.com>,
        Theodore Ts'o <tytso@....edu>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH v2] ext4: Enable code path when DX_DEBUG is set

On Mon, Feb 01, 2021 at 12:31:25AM +0000, Vinicius Tinti wrote:
> By enabling -Wunreachable-code-aggressive on Clang the following code
> paths are unreachable.
> 
> 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. It has not been changed since.
> 
> 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 */ ( )
> 
> Signed-off-by: Vinicius Tinti <viniciustinti@...il.com>
> ---
>  fs/ext4/namei.c | 23 ++++++++++++-----------
>  1 file changed, 12 insertions(+), 11 deletions(-)
> 
> diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
> index cf652ba3e74d..46ae6a4e4be5 100644
> --- a/fs/ext4/namei.c
> +++ b/fs/ext4/namei.c
> @@ -827,20 +827,21 @@ 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--)
> +#ifdef DX_DEBUG
> +		// linear search cross check
> +		unsigned n = count - 1;

You are going to introduce an instance of -Wdeclaration-after-statement
here.

fs/ext4/namei.c:834:12: warning: ISO C90 forbids mixing declarations and
code [-Wdeclaration-after-statement]
                unsigned n = count - 1;
                         ^
1 warning generated.

The quick hack would be changing the

if (0) {

to

#ifdef DX_DEBUG
    if (1) {

but that seems kind of ugly.

Other options would be turning DX_DEBUG into a proper Kconfig symbol so
that IS_ENABLED could be used or maybe combine it into
CONFIG_EXT4_DEBUG.

Cheers,
Nathan