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docs: bug-bisect: rewrite to better match the other bisecting text

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Rewrite the short document on bisecting kernel bugs. The new text
improves .config handling, brings a mention of 'git bisect skip', and
explains what to do after the bisection finished -- including trying a
revert to verify the result. The rewrite at the same time removes the
unrelated and outdated section on 'Devices not appearing' and replaces
some sentences about bug reporting with a pointer to the document
covering that topic in detail.

This overall brings the approach close to the one in the recently added
text Documentation/admin-guide/verify-bugs-and-bisect-regressions.rst.
As those two texts serve a similar purpose for different audiences,
mention that document in the head of this one and outline when the
other might be the better one to follow.

Signed-off-by: Thorsten Leemhuis <linux@leemhuis.info>
Reviewed-by: Petr Tesarik <petr@tesarici.cz>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/74dc0137dcc3e2c05648e885a7bc31ffd39a0890.1724312119.git.linux@leemhuis.info
This commit is contained in:
Thorsten Leemhuis 2024-08-22 09:35:33 +02:00 committed by Jonathan Corbet
parent a4931bb8b0
commit cbbdb6c625
2 changed files with 118 additions and 49 deletions
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166
Documentation/admin-guide/bug-bisect.rst
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@ -1,76 +1,144 @@
Bisecting a bug
+++++++++++++++
.. SPDX-License-Identifier: (GPL-2.0+ OR CC-BY-4.0)
.. [see the bottom of this file for redistribution information]
Last updated: 28 October 2016
======================
Bisecting a regression
======================
Introduction
============
This document describes how to use a ``git bisect`` to find the source code
change that broke something -- for example when some functionality stopped
working after upgrading from Linux 6.0 to 6.1.
Always try the latest kernel from kernel.org and build from source. If you are
not confident in doing that please report the bug to your distribution vendor
instead of to a kernel developer.
The text focuses on the gist of the process. If you are new to bisecting the
kernel, better follow Documentation/admin-guide/verify-bugs-and-bisect-regressions.rst
instead: it depicts everything from start to finish while covering multiple
aspects even kernel developers occasionally forget. This includes detecting
situations early where a bisection would be a waste of time, as nobody would
care about the result -- for example, because the problem happens after the
kernel marked itself as 'tainted', occurs in an abandoned version, was already
fixed, or is caused by a .config change you or your Linux distributor performed.
Finding bugs is not always easy. Have a go though. If you can't find it don't
give up. Report as much as you have found to the relevant maintainer. See
MAINTAINERS for who that is for the subsystem you have worked on.
Finding the change causing a kernel issue using a bisection
===========================================================
Before you submit a bug report read
'Documentation/admin-guide/reporting-issues.rst'.
*Note: the following process assumes you prepared everything for a bisection.
This includes having a Git clone with the appropriate sources, installing the
software required to build and install kernels, as well as a .config file stored
in a safe place (the following example assumes '~/prepared_kernel_.config') to
use as pristine base at each bisection step; ideally, you have also worked out
a fully reliable and straight-forward way to reproduce the regression, too.*
Devices not appearing
=====================
* Preparation: start the bisection and tell Git about the points in the history
you consider to be working and broken, which Git calls 'good' and 'bad'::
Often this is caused by udev/systemd. Check that first before blaming it
on the kernel.
git bisect start
git bisect good v6.0
git bisect bad v6.1
Finding patch that caused a bug
===============================
Instead of Git tags like 'v6.0' and 'v6.1' you can specify commit-ids, too.
Using the provided tools with ``git`` makes finding bugs easy provided the bug
is reproducible.
1. Copy your prepared .config into the build directory and adjust it to the
needs of the codebase Git checked out for testing::
Steps to do it:
cp ~/prepared_kernel_.config .config
make olddefconfig
- build the Kernel from its git source
- start bisect with [#f1]_::
2. Now build, install, and boot a kernel. This might fail for unrelated reasons,
for example, when a compile error happens at the current stage of the
bisection a later change resolves. In such cases run ``git bisect skip`` and
go back to step 1.
$ git bisect start
3. Check if the functionality that regressed works in the kernel you just built.
- mark the broken changeset with::
If it works, execute::
$ git bisect bad [commit]
git bisect good
- mark a changeset where the code is known to work with::
If it is broken, run::
$ git bisect good [commit]
git bisect bad
- rebuild the Kernel and test
- interact with git bisect by using either::
Note, getting this wrong just once will send the rest of the bisection
totally off course. To prevent having to start anew later you thus want to
ensure what you tell Git is correct; it is thus often wise to spend a few
minutes more on testing in case your reproducer is unreliable.
$ git bisect good
After issuing one of these two commands, Git will usually check out another
bisection point and print something like 'Bisecting: 675 revisions left to
test after this (roughly 10 steps)'. In that case go back to step 1.
or::
If Git instead prints something like 'cafecaca0c0dacafecaca0c0dacafecaca0c0da
is the first bad commit', then you have finished the bisection. In that case
move to the next point below. Note, right after displaying that line Git will
show some details about the culprit including its patch description; this can
easily fill your terminal, so you might need to scroll up to see the message
mentioning the culprit's commit-id.
$ git bisect bad
In case you missed Git's output, you can always run ``git bisect log`` to
print the status: it will show how many steps remain or mention the result of
the bisection.
depending if the bug happened on the changeset you're testing
- After some interactions, git bisect will give you the changeset that
likely caused the bug.
* Recommended complementary task: put the bisection log and the current .config
file aside for the bug report; furthermore tell Git to reset the sources to
the state before the bisection::
- For example, if you know that the current version is bad, and version
4.8 is good, you could do::
git bisect log > ~/bisection-log
cp .config ~/bisection-config-culprit
git bisect reset
$ git bisect start
$ git bisect bad # Current version is bad
$ git bisect good v4.8
* Recommended optional task: try reverting the culprit on top of the latest
codebase and check if that fixes your bug; if that is the case, it validates
the bisection and enables developers to resolve the regression through a
revert.
To try this, update your clone and check out latest mainline. Then tell Git
to revert the change by specifying its commit-id::
git revert --no-edit cafec0cacaca0
Git might reject this, for example when the bisection landed on a merge
commit. In that case, abandon the attempt. Do the same, if Git fails to revert
the culprit on its own because later changes depend on it -- at least unless
you bisected a stable or longterm kernel series, in which case you want to
check out its latest codebase and try a revert there.
If a revert succeeds, build and test another kernel to check if reverting
resolved your regression.
With that the process is complete. Now report the regression as described by
Documentation/admin-guide/reporting-issues.rst.
.. [#f1] You can, optionally, provide both good and bad arguments at git
start with ``git bisect start [BAD] [GOOD]``
Additional reading material
---------------------------
For further references, please read:
* The `man page for 'git bisect' <https://git-scm.com/docs/git-bisect>`_ and
`fighting regressions with 'git bisect' <https://git-scm.com/docs/git-bisect-lk2009.html>`_
in the Git documentation.
* `Working with git bisect <https://nathanchance.dev/posts/working-with-git-bisect/>`_
from kernel developer Nathan Chancellor.
* `Using Git bisect to figure out when brokenness was introduced <http://webchick.net/node/99>`_.
* `Fully automated bisecting with 'git bisect run' <https://lwn.net/Articles/317154>`_.
- The man page for ``git-bisect``
- `Fighting regressions with git bisect <https://www.kernel.org/pub/software/scm/git/docs/git-bisect-lk2009.html>`_
- `Fully automated bisecting with "git bisect run" <https://lwn.net/Articles/317154>`_
- `Using Git bisect to figure out when brokenness was introduced <http://webchick.net/node/99>`_
..
end-of-content
..
This document is maintained by Thorsten Leemhuis <linux@leemhuis.info>. If
you spot a typo or small mistake, feel free to let him know directly and
he'll fix it. You are free to do the same in a mostly informal way if you
want to contribute changes to the text -- but for copyright reasons please CC
linux-doc@vger.kernel.org and 'sign-off' your contribution as
Documentation/process/submitting-patches.rst explains in the section 'Sign
your work - the Developer's Certificate of Origin'.
..
This text is available under GPL-2.0+ or CC-BY-4.0, as stated at the top
of the file. If you want to distribute this text under CC-BY-4.0 only,
please use 'The Linux kernel development community' for author attribution
and link this as source:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/plain/Documentation/admin-guide/bug-bisect.rst
..
Note: Only the content of this RST file as found in the Linux kernel sources
is available under CC-BY-4.0, as versions of this text that were processed
(for example by the kernel's build system) might contain content taken from
files which use a more restrictive license.

1
MAINTAINERS
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@ -6719,6 +6719,7 @@ DOCUMENTATION REPORTING ISSUES
M: Thorsten Leemhuis <linux@leemhuis.info>
L: linux-doc@vger.kernel.org
S: Maintained
F: Documentation/admin-guide/bug-bisect.rst
F: Documentation/admin-guide/quickly-build-trimmed-linux.rst
F: Documentation/admin-guide/reporting-issues.rst
F: Documentation/admin-guide/verify-bugs-and-bisect-regressions.rst