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

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Date:   Mon, 1 Feb 2021 12:13:52 -0500
From:   "Theodore Ts'o" <tytso@....edu>
To:     Vinicius Tinti <viniciustinti@...il.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        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 v2] ext4: Enable code path when DX_DEBUG is set

On Mon, Feb 01, 2021 at 01:15:29PM -0300, Vinicius Tinti wrote:
> On Mon, Feb 1, 2021 at 9:49 AM Christoph Hellwig <hch@...radead.org> wrote:
> >
> > DX_DEBUG is completely dead code, so either kill it off or make it an
> > actual CONFIG_* symbol through Kconfig if it seems useful.
> 
> About the unreachable code in "if (0)" I think it could be removed.
> It seems to be doing an extra check.
> 
> About the DX_DEBUG I think I can do another patch adding it to Kconfig
> as you and Nathan suggested.

Yes, it's doing another check which is useful in terms of early
detection of bugs when a developer has the code open for
modifications.  It slows down performance under normal circumstances,
and assuming the code is bug-free(tm), it's entirely unnecessary ---
which is why it's under an "if (0)".

However, if there *is* a bug, having an early detection that the
representation invariant of the data structure has been violated can
be useful in root causing a bug.  This would probably be clearer if
the code was pulled out into a separate function with comments
explaining that this is a rep invariant check.

The main thing about DX_DEBUG right now is that it is **super**
verbose.  Unwary users who enable it.... will be sorry.  If we want to
make it to be a first-class feature enabled via CONFIG_EXT4_DEBUG, we
should convert all of the dx_trace calls to use pr_debug so they are
enabled only if dynamic debug enables those pr_debug() statements.
And this should absolutely be a separate patch.

Cheers,

						- Ted