1.. SPDX-License-Identifier: (GPL-2.0+ OR CC-BY-4.0)
2.. See the bottom of this file for additional redistribution information.
3
4Handling regressions
5++++++++++++++++++++
6
7*We don't cause regressions* -- this document describes what this "first rule of
8Linux kernel development" means in practice for developers. It complements
9Documentation/admin-guide/reporting-regressions.rst, which covers the topic from a
10user's point of view; if you never read that text, go and at least skim over it
11before continuing here.
12
13The important bits (aka "The TL;DR")
14====================================
15
16#. Ensure subscribers of the `regression mailing list <https://lore.kernel.org/regressions/>`_
17   (regressions@lists.linux.dev) quickly become aware of any new regression
18   report:
19
20    * When receiving a mailed report that did not CC the list, bring it into the
21      loop by immediately sending at least a brief "Reply-all" with the list
22      CCed.
23
24    * Forward or bounce any reports submitted in bug trackers to the list.
25
26#. Make the Linux kernel regression tracking bot "regzbot" track the issue (this
27   is optional, but recommended):
28
29    * For mailed reports, check if the reporter included a line like ``#regzbot
30      introduced v5.13..v5.14-rc1``. If not, send a reply (with the regressions
31      list in CC) containing a paragraph like the following, which tells regzbot
32      when the issue started to happen::
33
34       #regzbot ^introduced 1f2e3d4c5b6a
35
36    * When forwarding reports from a bug tracker to the regressions list (see
37      above), include a paragraph like the following::
38
39       #regzbot introduced: v5.13..v5.14-rc1
40       #regzbot from: Some N. Ice Human <some.human@example.com>
41       #regzbot monitor: http://some.bugtracker.example.com/ticket?id=123456789
42
43#. When submitting fixes for regressions, add "Link:" tags to the patch
44   description pointing to all places where the issue was reported, as
45   mandated by Documentation/process/submitting-patches.rst and
46   :ref:`Documentation/process/5.Posting.rst <development_posting>`.
47
48#. Try to fix regressions quickly once the culprit has been identified; fixes
49   for most regressions should be merged within two weeks, but some need to be
50   resolved within two or three days.
51
52
53All the details on Linux kernel regressions relevant for developers
54===================================================================
55
56
57The important basics in more detail
58-----------------------------------
59
60
61What to do when receiving regression reports
62~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
63
64Ensure the Linux kernel's regression tracker and others subscribers of the
65`regression mailing list <https://lore.kernel.org/regressions/>`_
66(regressions@lists.linux.dev) become aware of any newly reported regression:
67
68 * When you receive a report by mail that did not CC the list, immediately bring
69   it into the loop by sending at least a brief "Reply-all" with the list CCed;
70   try to ensure it gets CCed again in case you reply to a reply that omitted
71   the list.
72
73 * If a report submitted in a bug tracker hits your Inbox, forward or bounce it
74   to the list. Consider checking the list archives beforehand, if the reporter
75   already forwarded the report as instructed by
76   Documentation/admin-guide/reporting-issues.rst.
77
78When doing either, consider making the Linux kernel regression tracking bot
79"regzbot" immediately start tracking the issue:
80
81 * For mailed reports, check if the reporter included a "regzbot command" like
82   ``#regzbot introduced 1f2e3d4c5b6a``. If not, send a reply (with the
83   regressions list in CC) with a paragraph like the following:::
84
85       #regzbot ^introduced: v5.13..v5.14-rc1
86
87   This tells regzbot the version range in which the issue started to happen;
88   you can specify a range using commit-ids as well or state a single commit-id
89   in case the reporter bisected the culprit.
90
91   Note the caret (^) before the "introduced": it tells regzbot to treat the
92   parent mail (the one you reply to) as the initial report for the regression
93   you want to see tracked; that's important, as regzbot will later look out
94   for patches with "Link:" tags pointing to the report in the archives on
95   lore.kernel.org.
96
97 * When forwarding a regressions reported to a bug tracker, include a paragraph
98   with these regzbot commands::
99
100       #regzbot introduced: 1f2e3d4c5b6a
101       #regzbot from: Some N. Ice Human <some.human@example.com>
102       #regzbot monitor: http://some.bugtracker.example.com/ticket?id=123456789
103
104   Regzbot will then automatically associate patches with the report that
105   contain "Link:" tags pointing to your mail or the mentioned ticket.
106
107What's important when fixing regressions
108~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
109
110You don't need to do anything special when submitting fixes for regression, just
111remember to do what Documentation/process/submitting-patches.rst,
112:ref:`Documentation/process/5.Posting.rst <development_posting>`, and
113Documentation/process/stable-kernel-rules.rst already explain in more detail:
114
115 * Point to all places where the issue was reported using "Link:" tags::
116
117       Link: https://lore.kernel.org/r/30th.anniversary.repost@klaava.Helsinki.FI/
118       Link: https://bugzilla.kernel.org/show_bug.cgi?id=1234567890
119
120 * Add a "Fixes:" tag to specify the commit causing the regression.
121
122 * If the culprit was merged in an earlier development cycle, explicitly mark
123   the fix for backporting using the ``Cc: stable@vger.kernel.org`` tag.
124
125All this is expected from you and important when it comes to regression, as
126these tags are of great value for everyone (you included) that might be looking
127into the issue weeks, months, or years later. These tags are also crucial for
128tools and scripts used by other kernel developers or Linux distributions; one of
129these tools is regzbot, which heavily relies on the "Link:" tags to associate
130reports for regression with changes resolving them.
131
132Expectations and best practices for fixing regressions
133~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
134
135As a Linux kernel developer, you are expected to give your best to prevent
136situations where a regression caused by a recent change of yours leaves users
137only these options:
138
139 * Run a kernel with a regression that impacts usage.
140
141 * Switch to an older or newer kernel series.
142
143 * Continue running an outdated and thus potentially insecure kernel for more
144   than three weeks after the regression's culprit was identified. Ideally it
145   should be less than two. And it ought to be just a few days, if the issue is
146   severe or affects many users -- either in general or in prevalent
147   environments.
148
149How to realize that in practice depends on various factors. Use the following
150rules of thumb as a guide.
151
152In general:
153
154 * Prioritize work on regressions over all other Linux kernel work, unless the
155   latter concerns a severe issue (e.g. acute security vulnerability, data loss,
156   bricked hardware, ...).
157
158 * Expedite fixing mainline regressions that recently made it into a proper
159   mainline, stable, or longterm release (either directly or via backport).
160
161 * Do not consider regressions from the current cycle as something that can wait
162   till the end of the cycle, as the issue might discourage or prevent users and
163   CI systems from testing mainline now or generally.
164
165 * Work with the required care to avoid additional or bigger damage, even if
166   resolving an issue then might take longer than outlined below.
167
168On timing once the culprit of a regression is known:
169
170 * Aim to mainline a fix within two or three days, if the issue is severe or
171   bothering many users -- either in general or in prevalent conditions like a
172   particular hardware environment, distribution, or stable/longterm series.
173
174 * Aim to mainline a fix by Sunday after the next, if the culprit made it
175   into a recent mainline, stable, or longterm release (either directly or via
176   backport); if the culprit became known early during a week and is simple to
177   resolve, try to mainline the fix within the same week.
178
179 * For other regressions, aim to mainline fixes before the hindmost Sunday
180   within the next three weeks. One or two Sundays later are acceptable, if the
181   regression is something people can live with easily for a while -- like a
182   mild performance regression.
183
184 * It's strongly discouraged to delay mainlining regression fixes till the next
185   merge window, except when the fix is extraordinarily risky or when the
186   culprit was mainlined more than a year ago.
187
188On procedure:
189
190 * Always consider reverting the culprit, as it's often the quickest and least
191   dangerous way to fix a regression. Don't worry about mainlining a fixed
192   variant later: that should be straight-forward, as most of the code went
193   through review once already.
194
195 * Try to resolve any regressions introduced in mainline during the past
196   twelve months before the current development cycle ends: Linus wants such
197   regressions to be handled like those from the current cycle, unless fixing
198   bears unusual risks.
199
200 * Consider CCing Linus on discussions or patch review, if a regression seems
201   tangly. Do the same in precarious or urgent cases -- especially if the
202   subsystem maintainer might be unavailable. Also CC the stable team, when you
203   know such a regression made it into a mainline, stable, or longterm release.
204
205 * For urgent regressions, consider asking Linus to pick up the fix straight
206   from the mailing list: he is totally fine with that for uncontroversial
207   fixes. Ideally though such requests should happen in accordance with the
208   subsystem maintainers or come directly from them.
209
210 * In case you are unsure if a fix is worth the risk applying just days before
211   a new mainline release, send Linus a mail with the usual lists and people in
212   CC; in it, summarize the situation while asking him to consider picking up
213   the fix straight from the list. He then himself can make the call and when
214   needed even postpone the release. Such requests again should ideally happen
215   in accordance with the subsystem maintainers or come directly from them.
216
217Regarding stable and longterm kernels:
218
219 * You are free to leave regressions to the stable team, if they at no point in
220   time occurred with mainline or were fixed there already.
221
222 * If a regression made it into a proper mainline release during the past
223   twelve months, ensure to tag the fix with "Cc: stable@vger.kernel.org", as a
224   "Fixes:" tag alone does not guarantee a backport. Please add the same tag,
225   in case you know the culprit was backported to stable or longterm kernels.
226
227 * When receiving reports about regressions in recent stable or longterm kernel
228   series, please evaluate at least briefly if the issue might happen in current
229   mainline as well -- and if that seems likely, take hold of the report. If in
230   doubt, ask the reporter to check mainline.
231
232 * Whenever you want to swiftly resolve a regression that recently also made it
233   into a proper mainline, stable, or longterm release, fix it quickly in
234   mainline; when appropriate thus involve Linus to fast-track the fix (see
235   above). That's because the stable team normally does neither revert nor fix
236   any changes that cause the same problems in mainline.
237
238 * In case of urgent regression fixes you might want to ensure prompt
239   backporting by dropping the stable team a note once the fix was mainlined;
240   this is especially advisable during merge windows and shortly thereafter, as
241   the fix otherwise might land at the end of a huge patch queue.
242
243On patch flow:
244
245 * Developers, when trying to reach the time periods mentioned above, remember
246   to account for the time it takes to get fixes tested, reviewed, and merged by
247   Linus, ideally with them being in linux-next at least briefly. Hence, if a
248   fix is urgent, make it obvious to ensure others handle it appropriately.
249
250 * Reviewers, you are kindly asked to assist developers in reaching the time
251   periods mentioned above by reviewing regression fixes in a timely manner.
252
253 * Subsystem maintainers, you likewise are encouraged to expedite the handling
254   of regression fixes. Thus evaluate if skipping linux-next is an option for
255   the particular fix. Also consider sending git pull requests more often than
256   usual when needed. And try to avoid holding onto regression fixes over
257   weekends -- especially when the fix is marked for backporting.
258
259
260More aspects regarding regressions developers should be aware of
261----------------------------------------------------------------
262
263
264How to deal with changes where a risk of regression is known
265~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
266
267Evaluate how big the risk of regressions is, for example by performing a code
268search in Linux distributions and Git forges. Also consider asking other
269developers or projects likely to be affected to evaluate or even test the
270proposed change; if problems surface, maybe some solution acceptable for all
271can be found.
272
273If the risk of regressions in the end seems to be relatively small, go ahead
274with the change, but let all involved parties know about the risk. Hence, make
275sure your patch description makes this aspect obvious. Once the change is
276merged, tell the Linux kernel's regression tracker and the regressions mailing
277list about the risk, so everyone has the change on the radar in case reports
278trickle in. Depending on the risk, you also might want to ask the subsystem
279maintainer to mention the issue in his mainline pull request.
280
281What else is there to known about regressions?
282~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
283
284Check out Documentation/admin-guide/reporting-regressions.rst, it covers a lot
285of other aspects you want might want to be aware of:
286
287 * the purpose of the "no regressions rule"
288
289 * what issues actually qualify as regression
290
291 * who's in charge for finding the root cause of a regression
292
293 * how to handle tricky situations, e.g. when a regression is caused by a
294   security fix or when fixing a regression might cause another one
295
296Whom to ask for advice when it comes to regressions
297~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
298
299Send a mail to the regressions mailing list (regressions@lists.linux.dev) while
300CCing the Linux kernel's regression tracker (regressions@leemhuis.info); if the
301issue might better be dealt with in private, feel free to omit the list.
302
303
304More about regression tracking and regzbot
305------------------------------------------
306
307
308Why the Linux kernel has a regression tracker, and why is regzbot used?
309~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
310
311Rules like "no regressions" need someone to ensure they are followed, otherwise
312they are broken either accidentally or on purpose. History has shown this to be
313true for the Linux kernel as well. That's why Thorsten Leemhuis volunteered to
314keep an eye on things as the Linux kernel's regression tracker, who's
315occasionally helped by other people. Neither of them are paid to do this,
316that's why regression tracking is done on a best effort basis.
317
318Earlier attempts to manually track regressions have shown it's an exhausting and
319frustrating work, which is why they were abandoned after a while. To prevent
320this from happening again, Thorsten developed regzbot to facilitate the work,
321with the long term goal to automate regression tracking as much as possible for
322everyone involved.
323
324How does regression tracking work with regzbot?
325~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
326
327The bot watches for replies to reports of tracked regressions. Additionally,
328it's looking out for posted or committed patches referencing such reports
329with "Link:" tags; replies to such patch postings are tracked as well.
330Combined this data provides good insights into the current state of the fixing
331process.
332
333Regzbot tries to do its job with as little overhead as possible for both
334reporters and developers. In fact, only reporters are burdened with an extra
335duty: they need to tell regzbot about the regression report using the ``#regzbot
336introduced`` command outlined above; if they don't do that, someone else can
337take care of that using ``#regzbot ^introduced``.
338
339For developers there normally is no extra work involved, they just need to make
340sure to do something that was expected long before regzbot came to light: add
341"Link:" tags to the patch description pointing to all reports about the issue
342fixed.
343
344Do I have to use regzbot?
345~~~~~~~~~~~~~~~~~~~~~~~~~
346
347It's in the interest of everyone if you do, as kernel maintainers like Linus
348Torvalds partly rely on regzbot's tracking in their work -- for example when
349deciding to release a new version or extend the development phase. For this they
350need to be aware of all unfixed regression; to do that, Linus is known to look
351into the weekly reports sent by regzbot.
352
353Do I have to tell regzbot about every regression I stumble upon?
354~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
355
356Ideally yes: we are all humans and easily forget problems when something more
357important unexpectedly comes up -- for example a bigger problem in the Linux
358kernel or something in real life that's keeping us away from keyboards for a
359while. Hence, it's best to tell regzbot about every regression, except when you
360immediately write a fix and commit it to a tree regularly merged to the affected
361kernel series.
362
363How to see which regressions regzbot tracks currently?
364~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
365
366Check `regzbot's web-interface <https://linux-regtracking.leemhuis.info/regzbot/>`_
367for the latest info; alternatively, `search for the latest regression report
368<https://lore.kernel.org/lkml/?q=%22Linux+regressions+report%22+f%3Aregzbot>`_,
369which regzbot normally sends out once a week on Sunday evening (UTC), which is a
370few hours before Linus usually publishes new (pre-)releases.
371
372What places is regzbot monitoring?
373~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
374
375Regzbot is watching the most important Linux mailing lists as well as the git
376repositories of linux-next, mainline, and stable/longterm.
377
378What kind of issues are supposed to be tracked by regzbot?
379~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
380
381The bot is meant to track regressions, hence please don't involve regzbot for
382regular issues. But it's okay for the Linux kernel's regression tracker if you
383use regzbot to track severe issues, like reports about hangs, corrupted data,
384or internal errors (Panic, Oops, BUG(), warning, ...).
385
386Can I add regressions found by CI systems to regzbot's tracking?
387~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
388
389Feel free to do so, if the particular regression likely has impact on practical
390use cases and thus might be noticed by users; hence, please don't involve
391regzbot for theoretical regressions unlikely to show themselves in real world
392usage.
393
394How to interact with regzbot?
395~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
396
397By using a 'regzbot command' in a direct or indirect reply to the mail with the
398regression report. These commands need to be in their own paragraph (IOW: they
399need to be separated from the rest of the mail using blank lines).
400
401One such command is ``#regzbot introduced <version or commit>``, which makes
402regzbot consider your mail as a regressions report added to the tracking, as
403already described above; ``#regzbot ^introduced <version or commit>`` is another
404such command, which makes regzbot consider the parent mail as a report for a
405regression which it starts to track.
406
407Once one of those two commands has been utilized, other regzbot commands can be
408used in direct or indirect replies to the report. You can write them below one
409of the `introduced` commands or in replies to the mail that used one of them
410or itself is a reply to that mail:
411
412 * Set or update the title::
413
414       #regzbot title: foo
415
416 * Monitor a discussion or bugzilla.kernel.org ticket where additions aspects of
417   the issue or a fix are discussed -- for example the posting of a patch fixing
418   the regression::
419
420       #regzbot monitor: https://lore.kernel.org/all/30th.anniversary.repost@klaava.Helsinki.FI/
421
422   Monitoring only works for lore.kernel.org and bugzilla.kernel.org; regzbot
423   will consider all messages in that thread or ticket as related to the fixing
424   process.
425
426 * Point to a place with further details of interest, like a mailing list post
427   or a ticket in a bug tracker that are slightly related, but about a different
428   topic::
429
430       #regzbot link: https://bugzilla.kernel.org/show_bug.cgi?id=123456789
431
432 * Mark a regression as fixed by a commit that is heading upstream or already
433   landed::
434
435       #regzbot fixed-by: 1f2e3d4c5d
436
437 * Mark a regression as a duplicate of another one already tracked by regzbot::
438
439       #regzbot dup-of: https://lore.kernel.org/all/30th.anniversary.repost@klaava.Helsinki.FI/
440
441 * Mark a regression as invalid::
442
443       #regzbot invalid: wasn't a regression, problem has always existed
444
445Is there more to tell about regzbot and its commands?
446~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
447
448More detailed and up-to-date information about the Linux
449kernel's regression tracking bot can be found on its
450`project page <https://gitlab.com/knurd42/regzbot>`_, which among others
451contains a `getting started guide <https://gitlab.com/knurd42/regzbot/-/blob/main/docs/getting_started.md>`_
452and `reference documentation <https://gitlab.com/knurd42/regzbot/-/blob/main/docs/reference.md>`_
453which both cover more details than the above section.
454
455Quotes from Linus about regression
456----------------------------------
457
458Find below a few real life examples of how Linus Torvalds expects regressions to
459be handled:
460
461 * From `2017-10-26 (1/2)
462   <https://lore.kernel.org/lkml/CA+55aFwiiQYJ+YoLKCXjN_beDVfu38mg=Ggg5LFOcqHE8Qi7Zw@mail.gmail.com/>`_::
463
464       If you break existing user space setups THAT IS A REGRESSION.
465
466       It's not ok to say "but we'll fix the user space setup".
467
468       Really. NOT OK.
469
470       [...]
471
472       The first rule is:
473
474        - we don't cause regressions
475
476       and the corollary is that when regressions *do* occur, we admit to
477       them and fix them, instead of blaming user space.
478
479       The fact that you have apparently been denying the regression now for
480       three weeks means that I will revert, and I will stop pulling apparmor
481       requests until the people involved understand how kernel development
482       is done.
483
484 * From `2017-10-26 (2/2)
485   <https://lore.kernel.org/lkml/CA+55aFxW7NMAMvYhkvz1UPbUTUJewRt6Yb51QAx5RtrWOwjebg@mail.gmail.com/>`_::
486
487       People should basically always feel like they can update their kernel
488       and simply not have to worry about it.
489
490       I refuse to introduce "you can only update the kernel if you also
491       update that other program" kind of limitations. If the kernel used to
492       work for you, the rule is that it continues to work for you.
493
494       There have been exceptions, but they are few and far between, and they
495       generally have some major and fundamental reasons for having happened,
496       that were basically entirely unavoidable, and people _tried_hard_ to
497       avoid them. Maybe we can't practically support the hardware any more
498       after it is decades old and nobody uses it with modern kernels any
499       more. Maybe there's a serious security issue with how we did things,
500       and people actually depended on that fundamentally broken model. Maybe
501       there was some fundamental other breakage that just _had_ to have a
502       flag day for very core and fundamental reasons.
503
504       And notice that this is very much about *breaking* peoples environments.
505
506       Behavioral changes happen, and maybe we don't even support some
507       feature any more. There's a number of fields in /proc/<pid>/stat that
508       are printed out as zeroes, simply because they don't even *exist* in
509       the kernel any more, or because showing them was a mistake (typically
510       an information leak). But the numbers got replaced by zeroes, so that
511       the code that used to parse the fields still works. The user might not
512       see everything they used to see, and so behavior is clearly different,
513       but things still _work_, even if they might no longer show sensitive
514       (or no longer relevant) information.
515
516       But if something actually breaks, then the change must get fixed or
517       reverted. And it gets fixed in the *kernel*. Not by saying "well, fix
518       your user space then". It was a kernel change that exposed the
519       problem, it needs to be the kernel that corrects for it, because we
520       have a "upgrade in place" model. We don't have a "upgrade with new
521       user space".
522
523       And I seriously will refuse to take code from people who do not
524       understand and honor this very simple rule.
525
526       This rule is also not going to change.
527
528       And yes, I realize that the kernel is "special" in this respect. I'm
529       proud of it.
530
531       I have seen, and can point to, lots of projects that go "We need to
532       break that use case in order to make progress" or "you relied on
533       undocumented behavior, it sucks to be you" or "there's a better way to
534       do what you want to do, and you have to change to that new better
535       way", and I simply don't think that's acceptable outside of very early
536       alpha releases that have experimental users that know what they signed
537       up for. The kernel hasn't been in that situation for the last two
538       decades.
539
540       We do API breakage _inside_ the kernel all the time. We will fix
541       internal problems by saying "you now need to do XYZ", but then it's
542       about internal kernel API's, and the people who do that then also
543       obviously have to fix up all the in-kernel users of that API. Nobody
544       can say "I now broke the API you used, and now _you_ need to fix it
545       up". Whoever broke something gets to fix it too.
546
547       And we simply do not break user space.
548
549 * From `2020-05-21
550   <https://lore.kernel.org/all/CAHk-=wiVi7mSrsMP=fLXQrXK_UimybW=ziLOwSzFTtoXUacWVQ@mail.gmail.com/>`_::
551
552       The rules about regressions have never been about any kind of
553       documented behavior, or where the code lives.
554
555       The rules about regressions are always about "breaks user workflow".
556
557       Users are literally the _only_ thing that matters.
558
559       No amount of "you shouldn't have used this" or "that behavior was
560       undefined, it's your own fault your app broke" or "that used to work
561       simply because of a kernel bug" is at all relevant.
562
563       Now, reality is never entirely black-and-white. So we've had things
564       like "serious security issue" etc that just forces us to make changes
565       that may break user space. But even then the rule is that we don't
566       really have other options that would allow things to continue.
567
568       And obviously, if users take years to even notice that something
569       broke, or if we have sane ways to work around the breakage that
570       doesn't make for too much trouble for users (ie "ok, there are a
571       handful of users, and they can use a kernel command line to work
572       around it" kind of things) we've also been a bit less strict.
573
574       But no, "that was documented to be broken" (whether it's because the
575       code was in staging or because the man-page said something else) is
576       irrelevant. If staging code is so useful that people end up using it,
577       that means that it's basically regular kernel code with a flag saying
578       "please clean this up".
579
580       The other side of the coin is that people who talk about "API
581       stability" are entirely wrong. API's don't matter either. You can make
582       any changes to an API you like - as long as nobody notices.
583
584       Again, the regression rule is not about documentation, not about
585       API's, and not about the phase of the moon.
586
587       It's entirely about "we caused problems for user space that used to work".
588
589 * From `2017-11-05
590   <https://lore.kernel.org/all/CA+55aFzUvbGjD8nQ-+3oiMBx14c_6zOj2n7KLN3UsJ-qsd4Dcw@mail.gmail.com/>`_::
591
592       And our regression rule has never been "behavior doesn't change".
593       That would mean that we could never make any changes at all.
594
595       For example, we do things like add new error handling etc all the
596       time, which we then sometimes even add tests for in our kselftest
597       directory.
598
599       So clearly behavior changes all the time and we don't consider that a
600       regression per se.
601
602       The rule for a regression for the kernel is that some real user
603       workflow breaks. Not some test. Not a "look, I used to be able to do
604       X, now I can't".
605
606 * From `2018-08-03
607   <https://lore.kernel.org/all/CA+55aFwWZX=CXmWDTkDGb36kf12XmTehmQjbiMPCqCRG2hi9kw@mail.gmail.com/>`_::
608
609       YOU ARE MISSING THE #1 KERNEL RULE.
610
611       We do not regress, and we do not regress exactly because your are 100% wrong.
612
613       And the reason you state for your opinion is in fact exactly *WHY* you
614       are wrong.
615
616       Your "good reasons" are pure and utter garbage.
617
618       The whole point of "we do not regress" is so that people can upgrade
619       the kernel and never have to worry about it.
620
621       > Kernel had a bug which has been fixed
622
623       That is *ENTIRELY* immaterial.
624
625       Guys, whether something was buggy or not DOES NOT MATTER.
626
627       Why?
628
629       Bugs happen. That's a fact of life. Arguing that "we had to break
630       something because we were fixing a bug" is completely insane. We fix
631       tens of bugs every single day, thinking that "fixing a bug" means that
632       we can break something is simply NOT TRUE.
633
634       So bugs simply aren't even relevant to the discussion. They happen,
635       they get found, they get fixed, and it has nothing to do with "we
636       break users".
637
638       Because the only thing that matters IS THE USER.
639
640       How hard is that to understand?
641
642       Anybody who uses "but it was buggy" as an argument is entirely missing
643       the point. As far as the USER was concerned, it wasn't buggy - it
644       worked for him/her.
645
646       Maybe it worked *because* the user had taken the bug into account,
647       maybe it worked because the user didn't notice - again, it doesn't
648       matter. It worked for the user.
649
650       Breaking a user workflow for a "bug" is absolutely the WORST reason
651       for breakage you can imagine.
652
653       It's basically saying "I took something that worked, and I broke it,
654       but now it's better". Do you not see how f*cking insane that statement
655       is?
656
657       And without users, your program is not a program, it's a pointless
658       piece of code that you might as well throw away.
659
660       Seriously. This is *why* the #1 rule for kernel development is "we
661       don't break users". Because "I fixed a bug" is absolutely NOT AN
662       ARGUMENT if that bug fix broke a user setup. You actually introduced a
663       MUCH BIGGER bug by "fixing" something that the user clearly didn't
664       even care about.
665
666       And dammit, we upgrade the kernel ALL THE TIME without upgrading any
667       other programs at all. It is absolutely required, because flag-days
668       and dependencies are horribly bad.
669
670       And it is also required simply because I as a kernel developer do not
671       upgrade random other tools that I don't even care about as I develop
672       the kernel, and I want any of my users to feel safe doing the same
673       time.
674
675       So no. Your rule is COMPLETELY wrong. If you cannot upgrade a kernel
676       without upgrading some other random binary, then we have a problem.
677
678 * From `2021-06-05
679   <https://lore.kernel.org/all/CAHk-=wiUVqHN76YUwhkjZzwTdjMMJf_zN4+u7vEJjmEGh3recw@mail.gmail.com/>`_::
680
681       THERE ARE NO VALID ARGUMENTS FOR REGRESSIONS.
682
683       Honestly, security people need to understand that "not working" is not
684       a success case of security. It's a failure case.
685
686       Yes, "not working" may be secure. But security in that case is *pointless*.
687
688 * From `2011-05-06 (1/3)
689   <https://lore.kernel.org/all/BANLkTim9YvResB+PwRp7QTK-a5VNg2PvmQ@mail.gmail.com/>`_::
690
691       Binary compatibility is more important.
692
693       And if binaries don't use the interface to parse the format (or just
694       parse it wrongly - see the fairly recent example of adding uuid's to
695       /proc/self/mountinfo), then it's a regression.
696
697       And regressions get reverted, unless there are security issues or
698       similar that makes us go "Oh Gods, we really have to break things".
699
700       I don't understand why this simple logic is so hard for some kernel
701       developers to understand. Reality matters. Your personal wishes matter
702       NOT AT ALL.
703
704       If you made an interface that can be used without parsing the
705       interface description, then we're stuck with the interface. Theory
706       simply doesn't matter.
707
708       You could help fix the tools, and try to avoid the compatibility
709       issues that way. There aren't that many of them.
710
711   From `2011-05-06 (2/3)
712   <https://lore.kernel.org/all/BANLkTi=KVXjKR82sqsz4gwjr+E0vtqCmvA@mail.gmail.com/>`_::
713
714       it's clearly NOT an internal tracepoint. By definition. It's being
715       used by powertop.
716
717   From `2011-05-06 (3/3)
718   <https://lore.kernel.org/all/BANLkTinazaXRdGovYL7rRVp+j6HbJ7pzhg@mail.gmail.com/>`_::
719
720       We have programs that use that ABI and thus it's a regression if they break.
721
722 * From `2012-07-06 <https://lore.kernel.org/all/CA+55aFwnLJ+0sjx92EGREGTWOx84wwKaraSzpTNJwPVV8edw8g@mail.gmail.com/>`_::
723
724       > Now this got me wondering if Debian _unstable_ actually qualifies as a
725       > standard distro userspace.
726
727       Oh, if the kernel breaks some standard user space, that counts. Tons
728       of people run Debian unstable
729
730 * From `2019-09-15
731   <https://lore.kernel.org/lkml/CAHk-=wiP4K8DRJWsCo=20hn_6054xBamGKF2kPgUzpB5aMaofA@mail.gmail.com/>`_::
732
733       One _particularly_ last-minute revert is the top-most commit (ignoring
734       the version change itself) done just before the release, and while
735       it's very annoying, it's perhaps also instructive.
736
737       What's instructive about it is that I reverted a commit that wasn't
738       actually buggy. In fact, it was doing exactly what it set out to do,
739       and did it very well. In fact it did it _so_ well that the much
740       improved IO patterns it caused then ended up revealing a user-visible
741       regression due to a real bug in a completely unrelated area.
742
743       The actual details of that regression are not the reason I point that
744       revert out as instructive, though. It's more that it's an instructive
745       example of what counts as a regression, and what the whole "no
746       regressions" kernel rule means. The reverted commit didn't change any
747       API's, and it didn't introduce any new bugs. But it ended up exposing
748       another problem, and as such caused a kernel upgrade to fail for a
749       user. So it got reverted.
750
751       The point here being that we revert based on user-reported _behavior_,
752       not based on some "it changes the ABI" or "it caused a bug" concept.
753       The problem was really pre-existing, and it just didn't happen to
754       trigger before. The better IO patterns introduced by the change just
755       happened to expose an old bug, and people had grown to depend on the
756       previously benign behavior of that old issue.
757
758       And never fear, we'll re-introduce the fix that improved on the IO
759       patterns once we've decided just how to handle the fact that we had a
760       bad interaction with an interface that people had then just happened
761       to rely on incidental behavior for before. It's just that we'll have
762       to hash through how to do that (there are no less than three different
763       patches by three different developers being discussed, and there might
764       be more coming...). In the meantime, I reverted the thing that exposed
765       the problem to users for this release, even if I hope it will be
766       re-introduced (perhaps even backported as a stable patch) once we have
767       consensus about the issue it exposed.
768
769       Take-away from the whole thing: it's not about whether you change the
770       kernel-userspace ABI, or fix a bug, or about whether the old code
771       "should never have worked in the first place". It's about whether
772       something breaks existing users' workflow.
773
774       Anyway, that was my little aside on the whole regression thing.  Since
775       it's that "first rule of kernel programming", I felt it is perhaps
776       worth just bringing it up every once in a while
777
778..
779   end-of-content
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784   this as source:
785   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/plain/Documentation/process/handling-regressions.rst
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791