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Pull documentation update from Jonathan Corbet: "Another relatively mundane cycle for docs: - The beginning of an EEVDF scheduler document - More Chinese translations - A rethrashing of our bisection documentation ...plus the usual array of smaller fixes, and more than the usual number of typo fixes" * tag 'docs-6.12' of git://git.lwn.net/linux: (48 commits) Remove duplicate "and" in 'Linux NVMe docs. docs:filesystems: fix spelling and grammar mistakes docs:filesystem: fix mispelled words on autofs page docs:mm: fixed spelling and grammar mistakes on vmalloc kernel stack page Documentation: PCI: fix typo in pci.rst docs/zh_CN: add the translation of kbuild/gcc-plugins.rst docs/process: fix typos docs:mm: fix spelling mistakes in heterogeneous memory management page accel/qaic: Fix a typo docs/zh_CN: update the translation of security-bugs docs: block: Fix grammar and spelling mistakes in bfq-iosched.rst Documentation: Fix spelling mistakes Documentation/gpu: Fix typo in Documentation/gpu/komeda-kms.rst scripts: sphinx-pre-install: remove unnecessary double check for $cur_version Loongarch: KVM: Add KVM hypercalls documentation for LoongArch Documentation: Document the kernel flag bdev_allow_write_mounted docs: scheduler: completion: Update member of struct completion docs: kerneldoc-preamble.sty: Suppress extra spaces in CJK literal blocks docs: submitting-patches: Advertise b4 docs: update dev-tools/kcsan.rst url about KTSAN ...
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Original file line number | Diff line number | Diff line change |
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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.* | ||
|
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Devices not appearing | ||
===================== | ||
|
||
Often this is caused by udev/systemd. Check that first before blaming it | ||
on the kernel. | ||
|
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Finding patch that caused a bug | ||
=============================== | ||
|
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Using the provided tools with ``git`` makes finding bugs easy provided the bug | ||
is reproducible. | ||
|
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Steps to do it: | ||
|
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- build the Kernel from its git source | ||
- start bisect with [#f1]_:: | ||
|
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$ git bisect start | ||
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- mark the broken changeset with:: | ||
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$ git bisect bad [commit] | ||
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- mark a changeset where the code is known to work with:: | ||
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$ git bisect good [commit] | ||
|
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- rebuild the Kernel and test | ||
- interact with git bisect by using either:: | ||
|
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$ git bisect good | ||
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or:: | ||
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$ git bisect bad | ||
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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. | ||
|
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- For example, if you know that the current version is bad, and version | ||
4.8 is good, you could do:: | ||
|
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$ git bisect start | ||
$ git bisect bad # Current version is bad | ||
$ git bisect good v4.8 | ||
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|
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.. [#f1] You can, optionally, provide both good and bad arguments at git | ||
start with ``git bisect start [BAD] [GOOD]`` | ||
For further references, please read: | ||
|
||
- 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>`_ | ||
* 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':: | ||
|
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git bisect start | ||
git bisect good v6.0 | ||
git bisect bad v6.1 | ||
|
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Instead of Git tags like 'v6.0' and 'v6.1' you can specify commit-ids, too. | ||
|
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1. Copy your prepared .config into the build directory and adjust it to the | ||
needs of the codebase Git checked out for testing:: | ||
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cp ~/prepared_kernel_.config .config | ||
make olddefconfig | ||
|
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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. | ||
|
||
3. Check if the functionality that regressed works in the kernel you just built. | ||
|
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If it works, execute:: | ||
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git bisect good | ||
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If it is broken, run:: | ||
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git bisect bad | ||
|
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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. | ||
|
||
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. | ||
|
||
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. | ||
|
||
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. | ||
|
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* 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:: | ||
|
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git bisect log > ~/bisection-log | ||
cp .config ~/bisection-config-culprit | ||
git bisect reset | ||
|
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* 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. | ||
|
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To try this, update your clone and check out latest mainline. Then tell Git | ||
to revert the change by specifying its commit-id:: | ||
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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. | ||
|
||
|
||
Additional reading material | ||
--------------------------- | ||
|
||
* 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>`_. | ||
|
||
.. | ||
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. |
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