docs: add two documents about regression handling

Create two documents explaining various aspects around regression
handling and tracking; one is aimed at users, the other targets
developers.

The texts among others describes the first rule of Linux kernel
development and what it means in practice. They also explain what a
regression actually is and how to report one properly.

Both texts additionally provide a brief introduction to the bot the
kernel's regression tracker uses to facilitate the work, but mention the
use is optional.

To sum things up, provide a few quotes from Linus in the document for
developers to show how serious we take regressions.

Signed-off-by: Thorsten Leemhuis <linux@leemhuis.info>
Link: https://lore.kernel.org/r/34e56d3588f22d7e0b4d635ef9c9c3b33ca4ac04.1644994117.git.linux@leemhuis.info
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 659 insertions, 0 deletions

diff --git a/Documentation/process/handling-regressions.rst b/Documentation/process/handling-regressions.rst
new file mode 100644
index 000000000000..e1102a3207e3
--- /dev/null
+++ b/Documentation/process/handling-regressions.rst
@@ -0,0 +1,659 @@
+.. SPDX-License-Identifier: (GPL-2.0+ OR CC-BY-4.0)
+.. See the bottom of this file for additional redistribution information.
+
+Handling regressions
+++++++++++++++++++++
+
+*We don't cause regressions* -- this document describes what this "first rule of
+Linux kernel development" means in practice for developers. It complements
+Documentation/admin-guide/reporting-regressions.rst, which covers the topic from a
+user's point of view; if you never read that text, go and at least skim over it
+before continuing here.
+
+The important bits (aka "The TL;DR")
+====================================
+
+#. Ensure subscribers of the `regression mailing list <https://lore.kernel.org/regressions/>`_
+   (regressions@lists.linux.dev) quickly become aware of any new regression
+   report:
+
+    * When receiving a mailed report that did not CC the list, bring it into the
+      loop by immediately sending at least a brief "Reply-all" with the list
+      CCed.
+
+    * Forward or bounce any reports submitted in bug trackers to the list.
+
+#. Make the Linux kernel regression tracking bot "regzbot" track the issue (this
+   is optional, but recommended):
+
+    * For mailed reports, check if the reporter included a line like ``#regzbot
+      introduced v5.13..v5.14-rc1``. If not, send a reply (with the regressions
+      list in CC) containing a paragraph like the following, which tells regzbot
+      when the issue started to happen::
+
+       #regzbot ^introduced 1f2e3d4c5b6a
+
+    * When forwarding reports from a bug tracker to the regressions list (see
+      above), include a paragraph like the following::
+
+       #regzbot introduced: v5.13..v5.14-rc1
+       #regzbot from: Some N. Ice Human <some.human@example.com>
+       #regzbot monitor: http://some.bugtracker.example.com/ticket?id=123456789
+
+#. When submitting fixes for regressions, add "Link:" tags to the patch
+   description pointing to all places where the issue was reported, as
+   mandated by Documentation/process/submitting-patches.rst and
+   :ref:`Documentation/process/5.Posting.rst <development_posting>`.
+
+
+All the details on Linux kernel regressions relevant for developers
+===================================================================
+
+
+The important basics in more detail
+-----------------------------------
+
+
+What to do when receiving regression reports
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Ensure the Linux kernel's regression tracker and others subscribers of the
+`regression mailing list <https://lore.kernel.org/regressions/>`_
+(regressions@lists.linux.dev) become aware of any newly reported regression:
+
+ * When you receive a report by mail that did not CC the list, immediately bring
+   it into the loop by sending at least a brief "Reply-all" with the list CCed;
+   try to ensure it gets CCed again in case you reply to a reply that omitted
+   the list.
+
+ * If a report submitted in a bug tracker hits your Inbox, forward or bounce it
+   to the list. Consider checking the list archives beforehand, if the reporter
+   already forwarded the report as instructed by
+   Documentation/admin-guide/reporting-issues.rst.
+
+When doing either, consider making the Linux kernel regression tracking bot
+"regzbot" immediately start tracking the issue:
+
+ * For mailed reports, check if the reporter included a "regzbot command" like
+   ``#regzbot introduced 1f2e3d4c5b6a``. If not, send a reply (with the
+   regressions list in CC) with a paragraph like the following:::
+
+       #regzbot ^introduced: v5.13..v5.14-rc1
+
+   This tells regzbot the version range in which the issue started to happen;
+   you can specify a range using commit-ids as well or state a single commit-id
+   in case the reporter bisected the culprit.
+
+   Note the caret (^) before the "introduced": it tells regzbot to treat the
+   parent mail (the one you reply to) as the initial report for the regression
+   you want to see tracked; that's important, as regzbot will later look out
+   for patches with "Link:" tags pointing to the report in the archives on
+   lore.kernel.org.
+
+ * When forwarding a regressions reported to a bug tracker, include a paragraph
+   with these regzbot commands::
+
+       #regzbot introduced: 1f2e3d4c5b6a
+       #regzbot from: Some N. Ice Human <some.human@example.com>
+       #regzbot monitor: http://some.bugtracker.example.com/ticket?id=123456789
+
+   Regzbot will then automatically associate patches with the report that
+   contain "Link:" tags pointing to your mail or the mentioned ticket.
+
+What's important when fixing regressions
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+You don't need to do anything special when submitting fixes for regression, just
+remember to do what Documentation/process/submitting-patches.rst,
+:ref:`Documentation/process/5.Posting.rst <development_posting>`, and
+Documentation/process/stable-kernel-rules.rst already explain in more detail:
+
+ * Point to all places where the issue was reported using "Link:" tags::
+
+       Link: https://lore.kernel.org/r/30th.anniversary.repost@klaava.Helsinki.FI/
+       Link: https://bugzilla.kernel.org/show_bug.cgi?id=1234567890
+
+ * Add a "Fixes:" tag to specify the commit causing the regression.
+
+ * If the culprit was merged in an earlier development cycle, explicitly mark
+   the fix for backporting using the ``Cc: stable@vger.kernel.org`` tag.
+
+All this is expected from you and important when it comes to regression, as
+these tags are of great value for everyone (you included) that might be looking
+into the issue weeks, months, or years later. These tags are also crucial for
+tools and scripts used by other kernel developers or Linux distributions; one of
+these tools is regzbot, which heavily relies on the "Link:" tags to associate
+reports for regression with changes resolving them.
+
+
+More aspects regarding regressions developers should be aware of
+----------------------------------------------------------------
+
+
+How to deal with changes where a risk of regression is known
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Evaluate how big the risk of regressions is, for example by performing a code
+search in Linux distributions and Git forges. Also consider asking other
+developers or projects likely to be affected to evaluate or even test the
+proposed change; if problems surface, maybe some solution acceptable for all
+can be found.
+
+If the risk of regressions in the end seems to be relatively small, go ahead
+with the change, but let all involved parties know about the risk. Hence, make
+sure your patch description makes this aspect obvious. Once the change is
+merged, tell the Linux kernel's regression tracker and the regressions mailing
+list about the risk, so everyone has the change on the radar in case reports
+trickle in. Depending on the risk, you also might want to ask the subsystem
+maintainer to mention the issue in his mainline pull request.
+
+What else is there to known about regressions?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Check out Documentation/admin-guide/reporting-regressions.rst, it covers a lot
+of other aspects you want might want to be aware of:
+
+ * the purpose of the "no regressions rule"
+
+ * what issues actually qualify as regression
+
+ * who's in charge for finding the root cause of a regression
+
+ * how to handle tricky situations, e.g. when a regression is caused by a
+   security fix or when fixing a regression might cause another one
+
+Whom to ask for advice when it comes to regressions
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Send a mail to the regressions mailing list (regressions@lists.linux.dev) while
+CCing the Linux kernel's regression tracker (regressions@leemhuis.info); if the
+issue might better be dealt with in private, feel free to omit the list.
+
+
+More about regression tracking and regzbot
+------------------------------------------
+
+
+Why the Linux kernel has a regression tracker, and why is regzbot used?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Rules like "no regressions" need someone to ensure they are followed, otherwise
+they are broken either accidentally or on purpose. History has shown this to be
+true for the Linux kernel as well. That's why Thorsten Leemhuis volunteered to
+keep an eye on things as the Linux kernel's regression tracker, who's
+occasionally helped by other people. Neither of them are paid to do this,
+that's why regression tracking is done on a best effort basis.
+
+Earlier attempts to manually track regressions have shown it's an exhausting and
+frustrating work, which is why they were abandoned after a while. To prevent
+this from happening again, Thorsten developed regzbot to facilitate the work,
+with the long term goal to automate regression tracking as much as possible for
+everyone involved.
+
+How does regression tracking work with regzbot?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The bot watches for replies to reports of tracked regressions. Additionally,
+it's looking out for posted or committed patches referencing such reports
+with "Link:" tags; replies to such patch postings are tracked as well.
+Combined this data provides good insights into the current state of the fixing
+process.
+
+Regzbot tries to do its job with as little overhead as possible for both
+reporters and developers. In fact, only reporters are burdened with an extra
+duty: they need to tell regzbot about the regression report using the ``#regzbot
+introduced`` command outlined above; if they don't do that, someone else can
+take care of that using ``#regzbot ^introduced``.
+
+For developers there normally is no extra work involved, they just need to make
+sure to do something that was expected long before regzbot came to light: add
+"Link:" tags to the patch description pointing to all reports about the issue
+fixed.
+
+Do I have to use regzbot?
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+It's in the interest of everyone if you do, as kernel maintainers like Linus
+Torvalds partly rely on regzbot's tracking in their work -- for example when
+deciding to release a new version or extend the development phase. For this they
+need to be aware of all unfixed regression; to do that, Linus is known to look
+into the weekly reports sent by regzbot.
+
+Do I have to tell regzbot about every regression I stumble upon?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Ideally yes: we are all humans and easily forget problems when something more
+important unexpectedly comes up -- for example a bigger problem in the Linux
+kernel or something in real life that's keeping us away from keyboards for a
+while. Hence, it's best to tell regzbot about every regression, except when you
+immediately write a fix and commit it to a tree regularly merged to the affected
+kernel series.
+
+How to see which regressions regzbot tracks currently?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Check `regzbot's web-interface <https://linux-regtracking.leemhuis.info/regzbot/>`_
+for the latest info; alternatively, `search for the latest regression report
+<https://lore.kernel.org/lkml/?q=%22Linux+regressions+report%22+f%3Aregzbot>`_,
+which regzbot normally sends out once a week on Sunday evening (UTC), which is a
+few hours before Linus usually publishes new (pre-)releases.
+
+What places is regzbot monitoring?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Regzbot is watching the most important Linux mailing lists as well as the git
+repositories of linux-next, mainline, and stable/longterm.
+
+What kind of issues are supposed to be tracked by regzbot?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The bot is meant to track regressions, hence please don't involve regzbot for
+regular issues. But it's okay for the Linux kernel's regression tracker if you
+use regzbot to track severe issues, like reports about hangs, corrupted data,
+or internal errors (Panic, Oops, BUG(), warning, ...).
+
+Can I add regressions found by CI systems to regzbot's tracking?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Feel free to do so, if the particular regression likely has impact on practical
+use cases and thus might be noticed by users; hence, please don't involve
+regzbot for theoretical regressions unlikely to show themselves in real world
+usage.
+
+How to interact with regzbot?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+By using a 'regzbot command' in a direct or indirect reply to the mail with the
+regression report. These commands need to be in their own paragraph (IOW: they
+need to be separated from the rest of the mail using blank lines).
+
+One such command is ``#regzbot introduced <version or commit>``, which makes
+regzbot consider your mail as a regressions report added to the tracking, as
+already described above; ``#regzbot ^introduced <version or commit>`` is another
+such command, which makes regzbot consider the parent mail as a report for a
+regression which it starts to track.
+
+Once one of those two commands has been utilized, other regzbot commands can be
+used in direct or indirect replies to the report. You can write them below one
+of the `introduced` commands or in replies to the mail that used one of them
+or itself is a reply to that mail:
+
+ * Set or update the title::
+
+       #regzbot title: foo
+
+ * Monitor a discussion or bugzilla.kernel.org ticket where additions aspects of
+   the issue or a fix are discussed -- for example the posting of a patch fixing
+   the regression::
+
+       #regzbot monitor: https://lore.kernel.org/all/30th.anniversary.repost@klaava.Helsinki.FI/
+
+   Monitoring only works for lore.kernel.org and bugzilla.kernel.org; regzbot
+   will consider all messages in that thread or ticket as related to the fixing
+   process.
+
+ * Point to a place with further details of interest, like a mailing list post
+   or a ticket in a bug tracker that are slightly related, but about a different
+   topic::
+
+       #regzbot link: https://bugzilla.kernel.org/show_bug.cgi?id=123456789
+
+ * Mark a regression as fixed by a commit that is heading upstream or already
+   landed::
+
+       #regzbot fixed-by: 1f2e3d4c5d
+
+ * Mark a regression as a duplicate of another one already tracked by regzbot::
+
+       #regzbot dup-of: https://lore.kernel.org/all/30th.anniversary.repost@klaava.Helsinki.FI/
+
+ * Mark a regression as invalid::
+
+       #regzbot invalid: wasn't a regression, problem has always existed
+
+Is there more to tell about regzbot and its commands?
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+More detailed and up-to-date information about the Linux
+kernel's regression tracking bot can be found on its
+`project page <https://gitlab.com/knurd42/regzbot>`_, which among others
+contains a `getting started guide <https://gitlab.com/knurd42/regzbot/-/blob/main/docs/getting_started.md>`_
+and `reference documentation <https://gitlab.com/knurd42/regzbot/-/blob/main/docs/reference.md>`_
+which both cover more details than the above section.
+
+Quotes from Linus about regression
+----------------------------------
+
+Find below a few real life examples of how Linus Torvalds expects regressions to
+be handled:
+
+ * From `2017-10-26 (1/2)
+   <https://lore.kernel.org/lkml/CA+55aFwiiQYJ+YoLKCXjN_beDVfu38mg=Ggg5LFOcqHE8Qi7Zw@mail.gmail.com/>`_::
+
+       If you break existing user space setups THAT IS A REGRESSION.
+
+       It's not ok to say "but we'll fix the user space setup".
+
+       Really. NOT OK.
+
+       [...]
+
+       The first rule is:
+
+        - we don't cause regressions
+
+       and the corollary is that when regressions *do* occur, we admit to
+       them and fix them, instead of blaming user space.
+
+       The fact that you have apparently been denying the regression now for
+       three weeks means that I will revert, and I will stop pulling apparmor
+       requests until the people involved understand how kernel development
+       is done.
+
+ * From `2017-10-26 (2/2)
+   <https://lore.kernel.org/lkml/CA+55aFxW7NMAMvYhkvz1UPbUTUJewRt6Yb51QAx5RtrWOwjebg@mail.gmail.com/>`_::
+
+       People should basically always feel like they can update their kernel
+       and simply not have to worry about it.
+
+       I refuse to introduce "you can only update the kernel if you also
+       update that other program" kind of limitations. If the kernel used to
+       work for you, the rule is that it continues to work for you.
+
+       There have been exceptions, but they are few and far between, and they
+       generally have some major and fundamental reasons for having happened,
+       that were basically entirely unavoidable, and people _tried_hard_ to
+       avoid them. Maybe we can't practically support the hardware any more
+       after it is decades old and nobody uses it with modern kernels any
+       more. Maybe there's a serious security issue with how we did things,
+       and people actually depended on that fundamentally broken model. Maybe
+       there was some fundamental other breakage that just _had_ to have a
+       flag day for very core and fundamental reasons.
+
+       And notice that this is very much about *breaking* peoples environments.
+
+       Behavioral changes happen, and maybe we don't even support some
+       feature any more. There's a number of fields in /proc/<pid>/stat that
+       are printed out as zeroes, simply because they don't even *exist* in
+       the kernel any more, or because showing them was a mistake (typically
+       an information leak). But the numbers got replaced by zeroes, so that
+       the code that used to parse the fields still works. The user might not
+       see everything they used to see, and so behavior is clearly different,
+       but things still _work_, even if they might no longer show sensitive
+       (or no longer relevant) information.
+
+       But if something actually breaks, then the change must get fixed or
+       reverted. And it gets fixed in the *kernel*. Not by saying "well, fix
+       your user space then". It was a kernel change that exposed the
+       problem, it needs to be the kernel that corrects for it, because we
+       have a "upgrade in place" model. We don't have a "upgrade with new
+       user space".
+
+       And I seriously will refuse to take code from people who do not
+       understand and honor this very simple rule.
+
+       This rule is also not going to change.
+
+       And yes, I realize that the kernel is "special" in this respect. I'm
+       proud of it.
+
+       I have seen, and can point to, lots of projects that go "We need to
+       break that use case in order to make progress" or "you relied on
+       undocumented behavior, it sucks to be you" or "there's a better way to
+       do what you want to do, and you have to change to that new better
+       way", and I simply don't think that's acceptable outside of very early
+       alpha releases that have experimental users that know what they signed
+       up for. The kernel hasn't been in that situation for the last two
+       decades.
+
+       We do API breakage _inside_ the kernel all the time. We will fix
+       internal problems by saying "you now need to do XYZ", but then it's
+       about internal kernel API's, and the people who do that then also
+       obviously have to fix up all the in-kernel users of that API. Nobody
+       can say "I now broke the API you used, and now _you_ need to fix it
+       up". Whoever broke something gets to fix it too.
+
+       And we simply do not break user space.
+
+ * From `2020-05-21
+   <https://lore.kernel.org/all/CAHk-=wiVi7mSrsMP=fLXQrXK_UimybW=ziLOwSzFTtoXUacWVQ@mail.gmail.com/>`_::
+
+       The rules about regressions have never been about any kind of
+       documented behavior, or where the code lives.
+
+       The rules about regressions are always about "breaks user workflow".
+
+       Users are literally the _only_ thing that matters.
+
+       No amount of "you shouldn't have used this" or "that behavior was
+       undefined, it's your own fault your app broke" or "that used to work
+       simply because of a kernel bug" is at all relevant.
+
+       Now, reality is never entirely black-and-white. So we've had things
+       like "serious security issue" etc that just forces us to make changes
+       that may break user space. But even then the rule is that we don't
+       really have other options that would allow things to continue.
+
+       And obviously, if users take years to even notice that something
+       broke, or if we have sane ways to work around the breakage that
+       doesn't make for too much trouble for users (ie "ok, there are a
+       handful of users, and they can use a kernel command line to work
+       around it" kind of things) we've also been a bit less strict.
+
+       But no, "that was documented to be broken" (whether it's because the
+       code was in staging or because the man-page said something else) is
+       irrelevant. If staging code is so useful that people end up using it,
+       that means that it's basically regular kernel code with a flag saying
+       "please clean this up".
+
+       The other side of the coin is that people who talk about "API
+       stability" are entirely wrong. API's don't matter either. You can make
+       any changes to an API you like - as long as nobody notices.
+
+       Again, the regression rule is not about documentation, not about
+       API's, and not about the phase of the moon.
+
+       It's entirely about "we caused problems for user space that used to work".
+
+ * From `2017-11-05
+   <https://lore.kernel.org/all/CA+55aFzUvbGjD8nQ-+3oiMBx14c_6zOj2n7KLN3UsJ-qsd4Dcw@mail.gmail.com/>`_::
+
+       And our regression rule has never been "behavior doesn't change".
+       That would mean that we could never make any changes at all.
+
+       For example, we do things like add new error handling etc all the
+       time, which we then sometimes even add tests for in our kselftest
+       directory.
+
+       So clearly behavior changes all the time and we don't consider that a
+       regression per se.
+
+       The rule for a regression for the kernel is that some real user
+       workflow breaks. Not some test. Not a "look, I used to be able to do
+       X, now I can't".
+
+ * From `2018-08-03
+   <https://lore.kernel.org/all/CA+55aFwWZX=CXmWDTkDGb36kf12XmTehmQjbiMPCqCRG2hi9kw@mail.gmail.com/>`_::
+
+       YOU ARE MISSING THE #1 KERNEL RULE.
+
+       We do not regress, and we do not regress exactly because your are 100% wrong.
+
+       And the reason you state for your opinion is in fact exactly *WHY* you
+       are wrong.
+
+       Your "good reasons" are pure and utter garbage.
+
+       The whole point of "we do not regress" is so that people can upgrade
+       the kernel and never have to worry about it.
+
+       > Kernel had a bug which has been fixed
+
+       That is *ENTIRELY* immaterial.
+
+       Guys, whether something was buggy or not DOES NOT MATTER.
+
+       Why?
+
+       Bugs happen. That's a fact of life. Arguing that "we had to break
+       something because we were fixing a bug" is completely insane. We fix
+       tens of bugs every single day, thinking that "fixing a bug" means that
+       we can break something is simply NOT TRUE.
+
+       So bugs simply aren't even relevant to the discussion. They happen,
+       they get found, they get fixed, and it has nothing to do with "we
+       break users".
+
+       Because the only thing that matters IS THE USER.
+
+       How hard is that to understand?
+
+       Anybody who uses "but it was buggy" as an argument is entirely missing
+       the point. As far as the USER was concerned, it wasn't buggy - it
+       worked for him/her.
+
+       Maybe it worked *because* the user had taken the bug into account,
+       maybe it worked because the user didn't notice - again, it doesn't
+       matter. It worked for the user.
+
+       Breaking a user workflow for a "bug" is absolutely the WORST reason
+       for breakage you can imagine.
+
+       It's basically saying "I took something that worked, and I broke it,
+       but now it's better". Do you not see how f*cking insane that statement
+       is?
+
+       And without users, your program is not a program, it's a pointless
+       piece of code that you might as well throw away.
+
+       Seriously. This is *why* the #1 rule for kernel development is "we
+       don't break users". Because "I fixed a bug" is absolutely NOT AN
+       ARGUMENT if that bug fix broke a user setup. You actually introduced a
+       MUCH BIGGER bug by "fixing" something that the user clearly didn't
+       even care about.
+
+       And dammit, we upgrade the kernel ALL THE TIME without upgrading any
+       other programs at all. It is absolutely required, because flag-days
+       and dependencies are horribly bad.
+
+       And it is also required simply because I as a kernel developer do not
+       upgrade random other tools that I don't even care about as I develop
+       the kernel, and I want any of my users to feel safe doing the same
+       time.
+
+       So no. Your rule is COMPLETELY wrong. If you cannot upgrade a kernel
+       without upgrading some other random binary, then we have a problem.
+
+ * From `2021-06-05
+   <https://lore.kernel.org/all/CAHk-=wiUVqHN76YUwhkjZzwTdjMMJf_zN4+u7vEJjmEGh3recw@mail.gmail.com/>`_::
+
+       THERE ARE NO VALID ARGUMENTS FOR REGRESSIONS.
+
+       Honestly, security people need to understand that "not working" is not
+       a success case of security. It's a failure case.
+
+       Yes, "not working" may be secure. But security in that case is *pointless*.
+
+ * From `2011-05-06 (1/3)
+   <https://lore.kernel.org/all/BANLkTim9YvResB+PwRp7QTK-a5VNg2PvmQ@mail.gmail.com/>`_::
+
+       Binary compatibility is more important.
+
+       And if binaries don't use the interface to parse the format (or just
+       parse it wrongly - see the fairly recent example of adding uuid's to
+       /proc/self/mountinfo), then it's a regression.
+
+       And regressions get reverted, unless there are security issues or
+       similar that makes us go "Oh Gods, we really have to break things".
+
+       I don't understand why this simple logic is so hard for some kernel
+       developers to understand. Reality matters. Your personal wishes matter
+       NOT AT ALL.
+
+       If you made an interface that can be used without parsing the
+       interface description, then we're stuck with the interface. Theory
+       simply doesn't matter.
+
+       You could help fix the tools, and try to avoid the compatibility
+       issues that way. There aren't that many of them.
+
+   From `2011-05-06 (2/3)
+   <https://lore.kernel.org/all/BANLkTi=KVXjKR82sqsz4gwjr+E0vtqCmvA@mail.gmail.com/>`_::
+
+       it's clearly NOT an internal tracepoint. By definition. It's being
+       used by powertop.
+
+   From `2011-05-06 (3/3)
+   <https://lore.kernel.org/all/BANLkTinazaXRdGovYL7rRVp+j6HbJ7pzhg@mail.gmail.com/>`_::
+
+       We have programs that use that ABI and thus it's a regression if they break.
+
+ * From `2012-07-06 <https://lore.kernel.org/all/CA+55aFwnLJ+0sjx92EGREGTWOx84wwKaraSzpTNJwPVV8edw8g@mail.gmail.com/>`_::
+
+       > Now this got me wondering if Debian _unstable_ actually qualifies as a
+       > standard distro userspace.
+
+       Oh, if the kernel breaks some standard user space, that counts. Tons
+       of people run Debian unstable
+
+ * From `2019-09-15
+   <https://lore.kernel.org/lkml/CAHk-=wiP4K8DRJWsCo=20hn_6054xBamGKF2kPgUzpB5aMaofA@mail.gmail.com/>`_::
+
+       One _particularly_ last-minute revert is the top-most commit (ignoring
+       the version change itself) done just before the release, and while
+       it's very annoying, it's perhaps also instructive.
+
+       What's instructive about it is that I reverted a commit that wasn't
+       actually buggy. In fact, it was doing exactly what it set out to do,
+       and did it very well. In fact it did it _so_ well that the much
+       improved IO patterns it caused then ended up revealing a user-visible
+       regression due to a real bug in a completely unrelated area.
+
+       The actual details of that regression are not the reason I point that
+       revert out as instructive, though. It's more that it's an instructive
+       example of what counts as a regression, and what the whole "no
+       regressions" kernel rule means. The reverted commit didn't change any
+       API's, and it didn't introduce any new bugs. But it ended up exposing
+       another problem, and as such caused a kernel upgrade to fail for a
+       user. So it got reverted.
+
+       The point here being that we revert based on user-reported _behavior_,
+       not based on some "it changes the ABI" or "it caused a bug" concept.
+       The problem was really pre-existing, and it just didn't happen to
+       trigger before. The better IO patterns introduced by the change just
+       happened to expose an old bug, and people had grown to depend on the
+       previously benign behavior of that old issue.
+
+       And never fear, we'll re-introduce the fix that improved on the IO
+       patterns once we've decided just how to handle the fact that we had a
+       bad interaction with an interface that people had then just happened
+       to rely on incidental behavior for before. It's just that we'll have
+       to hash through how to do that (there are no less than three different
+       patches by three different developers being discussed, and there might
+       be more coming...). In the meantime, I reverted the thing that exposed
+       the problem to users for this release, even if I hope it will be
+       re-introduced (perhaps even backported as a stable patch) once we have
+       consensus about the issue it exposed.
+
+       Take-away from the whole thing: it's not about whether you change the
+       kernel-userspace ABI, or fix a bug, or about whether the old code
+       "should never have worked in the first place". It's about whether
+       something breaks existing users' workflow.
+
+       Anyway, that was my little aside on the whole regression thing.  Since
+       it's that "first rule of kernel programming", I felt it is perhaps
+       worth just bringing it up every once in a while
+
+..
+   end-of-content
+..
+   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 developers" for author attribution and link
+   this as source:
+   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/plain/Documentation/process/handling-regressions.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.