Received-SPF: pass (google.com: domain of linux-kernel+bounces-98456-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45e3:2400::1 as permitted sender) client-ip=2604:1380:45e3:2400::1;
Sender: Guenter Roeck <groeck7@gmail.com>
Date: Sun, 10 Mar 2024 21:36:10 -0700
From: Guenter Roeck <linux@roeck-us.net>
To: linux-kselftest@vger.kernel.org
Cc: David Airlie <airlied@gmail.com>, Arnd Bergmann <arnd@arndb.de>,
	=?iso-8859-1?Q?Ma=EDra?= Canal <mcanal@igalia.com>,
	Dan Carpenter <dan.carpenter@linaro.org>,
	Kees Cook <keescook@chromium.org>,
	Daniel Diaz <daniel.diaz@linaro.org>,
	David Gow <davidgow@google.com>,
	Arthur Grillo <arthurgrillo@riseup.net>,
	Brendan Higgins <brendan.higgins@linux.dev>,
	Naresh Kamboju <naresh.kamboju@linaro.org>,
	Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Maxime Ripard <mripard@kernel.org>,
	Ville =?iso-8859-1?Q?Syrj=E4l=E4?= <ville.syrjala@linux.intel.com>,
	Daniel Vetter <daniel@ffwll.ch>,
	Thomas Zimmermann <tzimmermann@suse.de>,
	dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org,
	loongarch@lists.linux.dev, linux-arm-kernel@lists.infradead.org,
	kunit-dev@googlegroups.com, linux-arch@vger.kernel.org
Subject: Re: [RFC PATCH 0/5] Add support for suppressing warning backtraces
Message-ID: <d24f147c-a304-4395-aefc-bed7490278b6@roeck-us.net>
References: <20240305184033.425294-1-linux@roeck-us.net>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20240305184033.425294-1-linux@roeck-us.net>

On Tue, Mar 05, 2024 at 10:40:28AM -0800, Guenter Roeck wrote:
> Some unit tests intentionally trigger warning backtraces by passing bad
> parameters to kernel API functions. Such unit tests typically check the
> return value from such calls, not the existence of the warning backtrace.
> 
> Such intentionally generated warning backtraces are neither desirable
> nor useful for a number of reasons.
> - They can result in overlooked real problems.
> - A warning that suddenly starts to show up in unit tests needs to be
>   investigated and has to be marked to be ignored, for example by
>   adjusting filter scripts. Such filters are ad-hoc because there is
>   no real standard format for warnings. On top of that, such filter
>   scripts would require constant maintenance.
> 
> One option to address problem would be to add messages such as "expected
> warning backtraces start / end here" to the kernel log.  However, that
> would again require filter scripts, it might result in missing real
> problematic warning backtraces triggered while the test is running, and
> the irrelevant backtrace(s) would still clog the kernel log.
> 
> Solve the problem by providing a means to identify and suppress specific
> warning backtraces while executing test code. Support suppressing multiple
> backtraces while at the same time limiting changes to generic code to the
> absolute minimum. Architecture specific changes are kept at minimum by
> retaining function names only if both CONFIG_DEBUG_BUGVERBOSE and
> CONFIG_KUNIT are enabled.
> 
> The first patch of the series introduces the necessary infrastructure.
> The second patch marks the warning message in drm_calc_scale() in the DRM
> subsystem as intentional where warranted. This patch is intended to serve
> as an example for the use of the functionality introduced with this series.
> The last three patches in the series introduce the necessary architecture
> specific changes for x86, arm64, and loongarch.
> 
> This series is based on the RFC patch and subsequent discussion at
> https://patchwork.kernel.org/project/linux-kselftest/patch/02546e59-1afe-4b08-ba81-d94f3b691c9a@moroto.mountain/
> and offers a more comprehensive solution of the problem discussed there.
> 
> Checkpatch note:
>   Remaining checkpatch errors and warnings were deliberately ignored.
>   Some are triggered by matching coding style or by comments interpreted
>   as code, others by assembler macros which are disliked by checkpatch.
>   Suggestions for improvements are welcome.
> 
> Some questions:
> 
> - Is the general approach promising ? If not, are there other possible
>   solutions ?
> - Function pointers are only added to the __bug_table section if both
>   CONFIG_KUNIT and CONFIG_DEBUG_BUGVERBOSE are enabled. This avoids image
>   size increases if CONFIG_KUNIT=n. Downside is slightly more complex
>   architecture specific assembler code. If function pointers were always
>   added to the __bug_table section, vmlinux image size would increase by
>   approximately 0.6-0.7%. Is the increased complexity in assembler code
>   worth the reduced image size ? I think so, but I would like to hear
>   other opinions.
> - There are additional possibilities associated with storing the bug
>   function name in the __bug_table section. It could be independent of
>   KUNIT, it could be a configuration flag, and/or it could be used to
>   display the name of the offending function in BUG/WARN messages.
>   Is any of those of interest ?
> 

I am ready to send a full version of this series with support for
all affected architectures. I am undecided if I should send it now,
based on v6.8, and send v2 after rebasing it to v6.9-rc1, or if I
should just wait for v6.9-rc1.

I understand that some maintainers dislike getting new patch series
while the commit window is is open. On the ther side, I tested the
series thoroughly on top of v6.8-rc7, and initial v6.9 release candidates
may have their own problems. Given that, I tend to send the series now.

Any thoughts ? Unless there is strong negative feedback, I'll likely
do that in a day or two.

Thanks,
Guenter