From: Andreas Dilger <adilger@dilger.ca>
To: Vinicius Tinti <viniciustinti@gmail.com>
Cc: Theodore Ts'o <tytso@mit.edu>,
Christoph Hellwig <hch@infradead.org>,
Nathan Chancellor <natechancellor@gmail.com>,
Nick Desaulniers <ndesaulniers@google.com>,
Ext4 Developers List <linux-ext4@vger.kernel.org>,
Linux Kernel Mailing List <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 13:45:11 -0700 [thread overview]
Message-ID: <9CA2BADA-9CD9-49DC-A5E2-063C8A593E57@dilger.ca> (raw)
In-Reply-To: <CALD9WKzO53AXQW-qQ82VZ41H5=cGdLTUiEoz3X6BmPkb6XaTag@mail.gmail.com>
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On Feb 1, 2021, at 11:41 AM, Vinicius Tinti <viniciustinti@gmail.com> wrote:
>
> On Mon, Feb 1, 2021 at 2:13 PM Theodore Ts'o <tytso@mit.edu> wrote:
>>
>> 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)".
>
> My goal is to avoid having a dead code. Three options come to mind.
>
> The first would be to add another #ifdef SOMETHING (suggest a name).
> But this doesn't remove the code and someone could enable it by accident.
I don't see anything wrong with the original suggestion to use "DX_DEBUG".
If we want to get rid of "if (0)" in the code it could be changed like:
#define DX_DEBUG 0
#if DX_DEBUG
#define dxtrace(command) command
#else
#define dxtrace(command)
#endif
Then in the code check this directly (and fix the //-style comment also):
if (DX_DEBUG) { /* linear search cross check */
:
}
That will hopefully avoid the "dead code" warning, while keeping the code
visible for syntax checking by the compiler (unlike if it was under #ifdef).
Alternately, if we want to keep the "#ifdef DX_DEBUG" check intact:
#ifdef DX_DEBUG
#define dxtrace(command) command
#define DX_DEBUG_CROSSCHECK true
#else
#define dxtrace(command)
#define DX_DEBUG_CROSSCHECK false
#endif
if (DX_DEBUG_CROSSCHECK) { /* linear search cross check */
:
}
Cheers, Andreas
>
> The second would be to add the code in a block comment. Then document
> that it is for checking an invariant. This will make it harder to cause
> problems.
>
> The third would be to remove the code and explain the invariant in a block
> comment. Maybe add a pseudo code too in the comment.
>
> What do you think?
>
>> 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.
>
> Good idea. I will do that too.
>
>> 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.
>
> Agreed. For now I only want to focus on the "if (0)".
>
> Regards,
> Vinicius
>
>> Cheers,
>>
>> - Ted
Cheers, Andreas
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next prev parent reply other threads:[~2021-02-01 20:46 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
2021-02-01 18:41 ` Vinicius Tinti
2021-02-01 20:45 ` Andreas Dilger [this message]
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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