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From: "Theodore Ts'o" <tytso@mit.edu>
To: Vinicius Tinti <viniciustinti@gmail.com>
Cc: Christoph Hellwig <hch@infradead.org>,
	Andreas Dilger <adilger.kernel@dilger.ca>,
	Nathan Chancellor <natechancellor@gmail.com>,
	Nick Desaulniers <ndesaulniers@google.com>,
	linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org,
	clang-built-linux@googlegroups.com
Subject: Re: [PATCH v2] ext4: Enable code path when DX_DEBUG is set
Date: Mon, 1 Feb 2021 12:13:52 -0500	[thread overview]
Message-ID: <YBg20AuSC3/9w2zz@mit.edu> (raw)
In-Reply-To: <CALD9WKxc0kMPCHSoikko+qYk2+ZLUy73+ryKGW9qMSpyzAobLA@mail.gmail.com>

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@infradead.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

  reply	other threads:[~2021-02-01 17:15 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-29 18:58 [PATCH] ext4: Remove unreachable code Vinicius Tinti
2021-01-30  1:35 ` Nathan Chancellor
2021-01-30  6:42 ` Andreas Dilger
2021-02-01  0:31   ` [PATCH v2] ext4: Enable code path when DX_DEBUG is set Vinicius Tinti
2021-02-01  0:48     ` Nathan Chancellor
2021-02-01 12:49     ` Christoph Hellwig
2021-02-01 16:15       ` Vinicius Tinti
2021-02-01 17:13         ` Theodore Ts'o [this message]
2021-02-01 18:41           ` Vinicius Tinti
2021-02-01 20:45             ` Andreas Dilger
2021-02-01 21:09             ` Theodore Ts'o
2021-02-01 21:16               ` Nick Desaulniers
2021-02-01 21:38                 ` Theodore Ts'o
2021-02-01 21:41                   ` Nick Desaulniers
2021-02-01 22:05                     ` Vinicius Tinti
2021-02-01 22:48                       ` Theodore Ts'o
2021-02-01 23:09                         ` Nick Desaulniers
2021-02-02  8:05           ` Christoph Hellwig
2021-02-02 16:28             ` [PATCH v3] " Vinicius Tinti
2021-02-03  5:39               ` Theodore Ts'o
2021-02-03  9:51                 ` Vinicius Tinti
2021-02-01 16:58       ` [PATCH v2] " Theodore Ts'o

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