linux-kernel - Re: [PATCH v3] ext4: Enable code path when DX

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Message-ID: <CALD9WKz-Jgmz7inOghv37ZwzD_8sVPT_-YZUX5FpQE5aKrDJJg@mail.gmail.com>
Date:   Wed, 3 Feb 2021 06:51:26 -0300
From:   Vinicius Tinti <viniciustinti@...il.com>
To:     "Theodore Ts'o" <tytso@....edu>
Cc:     Christoph Hellwig <hch@...radead.org>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH v3] ext4: Enable code path when DX_DEBUG is set

On Wed, Feb 3, 2021 at 2:39 AM Theodore Ts'o <tytso@....edu> wrote:
>
> On Tue, Feb 02, 2021 at 04:28:37PM +0000, Vinicius Tinti wrote:
> > Clang with -Wunreachable-code-aggressive is being used to try to find
> > unreachable code that could cause potential bugs. There is no plan to
> > enable it by default.
> >
> > The following code was detected as 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. It has not been changed since.
> >
> > This patch moves the code to a new function `htree_rep_invariant_check`
> > which only performs the check when DX_DEBUG is set. This allows the
> > function to be used in other parts of the code.
> >
> > Suggestions from: Andreas, Christoph, Nathan, Nick and Ted.
> >
> > Signed-off-by: Vinicius Tinti <viniciustinti@...il.com>
>
> Thanks, applied, although I rewrote the commit description:
>
>     ext4: factor out htree rep invariant check
>
>     This patch moves some debugging code which is used to validate the
>     hash tree node when doing a binary search of an htree node into a
>     separate function, which is disabled by default (since it is only used
>     by developers when they are modifying the htree code paths).
>
>     In addition to cleaning up the code to make it more maintainable, it
>     silences a Clang compiler warning when -Wunreachable-code-aggressive
>     is enabled.  (There is no plan to enable this warning by default, since
>     there it has far too many false positives; nevertheless, this commit
>     reduces one of the many false positives by one.)
>
> The original description buried the lede, in terms of the primary
> reason why I think the change was worthwhile (although I know you have
> different priorities than mine :-).
>
> Thanks for working to find a way to improve the code in a way that
> makes both of us happy!

Thanks for the feedback.

And thanks for all the ones who reviewed.

>                                         - Ted