1 .. SPDX-License-Identifier: (GPL-2.0+ OR CC-BY 1 .. SPDX-License-Identifier: (GPL-2.0+ OR CC-BY-4.0)
2 .. See the bottom of this file for additional 2 .. See the bottom of this file for additional redistribution information.
3 3
4 Handling regressions 4 Handling regressions
5 ++++++++++++++++++++ 5 ++++++++++++++++++++
6 6
7 *We don't cause regressions* -- this document 7 *We don't cause regressions* -- this document describes what this "first rule of
8 Linux kernel development" means in practice fo 8 Linux kernel development" means in practice for developers. It complements
9 Documentation/admin-guide/reporting-regression 9 Documentation/admin-guide/reporting-regressions.rst, which covers the topic from a
10 user's point of view; if you never read that t 10 user's point of view; if you never read that text, go and at least skim over it
11 before continuing here. 11 before continuing here.
12 12
13 The important bits (aka "The TL;DR") 13 The important bits (aka "The TL;DR")
14 ==================================== 14 ====================================
15 15
16 #. Ensure subscribers of the `regression maili 16 #. Ensure subscribers of the `regression mailing list <https://lore.kernel.org/regressions/>`_
17 (regressions@lists.linux.dev) quickly becom 17 (regressions@lists.linux.dev) quickly become aware of any new regression
18 report: 18 report:
19 19
20 * When receiving a mailed report that did 20 * When receiving a mailed report that did not CC the list, bring it into the
21 loop by immediately sending at least a b 21 loop by immediately sending at least a brief "Reply-all" with the list
22 CCed. 22 CCed.
23 23
24 * Forward or bounce any reports submitted 24 * Forward or bounce any reports submitted in bug trackers to the list.
25 25
26 #. Make the Linux kernel regression tracking b 26 #. Make the Linux kernel regression tracking bot "regzbot" track the issue (this
27 is optional, but recommended): 27 is optional, but recommended):
28 28
29 * For mailed reports, check if the reporte 29 * For mailed reports, check if the reporter included a line like ``#regzbot
30 introduced: v5.13..v5.14-rc1``. If not, !! 30 introduced v5.13..v5.14-rc1``. If not, send a reply (with the regressions
31 list in CC) containing a paragraph like 31 list in CC) containing a paragraph like the following, which tells regzbot
32 when the issue started to happen:: 32 when the issue started to happen::
33 33
34 #regzbot ^introduced: 1f2e3d4c5b6a !! 34 #regzbot ^introduced 1f2e3d4c5b6a
35 35
36 * When forwarding reports from a bug track 36 * When forwarding reports from a bug tracker to the regressions list (see
37 above), include a paragraph like the fol 37 above), include a paragraph like the following::
38 38
39 #regzbot introduced: v5.13..v5.14-rc1 39 #regzbot introduced: v5.13..v5.14-rc1
40 #regzbot from: Some N. Ice Human <some.h 40 #regzbot from: Some N. Ice Human <some.human@example.com>
41 #regzbot monitor: http://some.bugtracke 41 #regzbot monitor: http://some.bugtracker.example.com/ticket?id=123456789
42 42
43 #. When submitting fixes for regressions, add !! 43 #. When submitting fixes for regressions, add "Link:" tags to the patch
44 description pointing to all places where th 44 description pointing to all places where the issue was reported, as
45 mandated by Documentation/process/submittin 45 mandated by Documentation/process/submitting-patches.rst and
46 :ref:`Documentation/process/5.Posting.rst < !! 46 :ref:`Documentation/process/5.Posting.rst <development_posting>`.
47 only fixing part of the issue that caused t <<
48 "Link:" tags instead. regzbot currently mak <<
49 two. <<
50 47
51 #. Try to fix regressions quickly once the cul 48 #. Try to fix regressions quickly once the culprit has been identified; fixes
52 for most regressions should be merged withi 49 for most regressions should be merged within two weeks, but some need to be
53 resolved within two or three days. 50 resolved within two or three days.
54 51
55 52
56 All the details on Linux kernel regressions re 53 All the details on Linux kernel regressions relevant for developers
57 ============================================== 54 ===================================================================
58 55
59 56
60 The important basics in more detail 57 The important basics in more detail
61 ----------------------------------- 58 -----------------------------------
62 59
63 60
64 What to do when receiving regression reports 61 What to do when receiving regression reports
65 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 62 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
66 63
67 Ensure the Linux kernel's regression tracker a 64 Ensure the Linux kernel's regression tracker and others subscribers of the
68 `regression mailing list <https://lore.kernel. 65 `regression mailing list <https://lore.kernel.org/regressions/>`_
69 (regressions@lists.linux.dev) become aware of 66 (regressions@lists.linux.dev) become aware of any newly reported regression:
70 67
71 * When you receive a report by mail that did 68 * When you receive a report by mail that did not CC the list, immediately bring
72 it into the loop by sending at least a brie 69 it into the loop by sending at least a brief "Reply-all" with the list CCed;
73 try to ensure it gets CCed again in case yo 70 try to ensure it gets CCed again in case you reply to a reply that omitted
74 the list. 71 the list.
75 72
76 * If a report submitted in a bug tracker hits 73 * If a report submitted in a bug tracker hits your Inbox, forward or bounce it
77 to the list. Consider checking the list arc 74 to the list. Consider checking the list archives beforehand, if the reporter
78 already forwarded the report as instructed 75 already forwarded the report as instructed by
79 Documentation/admin-guide/reporting-issues. 76 Documentation/admin-guide/reporting-issues.rst.
80 77
81 When doing either, consider making the Linux k 78 When doing either, consider making the Linux kernel regression tracking bot
82 "regzbot" immediately start tracking the issue 79 "regzbot" immediately start tracking the issue:
83 80
84 * For mailed reports, check if the reporter i 81 * For mailed reports, check if the reporter included a "regzbot command" like
85 ``#regzbot introduced: 1f2e3d4c5b6a``. If n !! 82 ``#regzbot introduced 1f2e3d4c5b6a``. If not, send a reply (with the
86 regressions list in CC) with a paragraph li 83 regressions list in CC) with a paragraph like the following:::
87 84
88 #regzbot ^introduced: v5.13..v5.14-rc1 85 #regzbot ^introduced: v5.13..v5.14-rc1
89 86
90 This tells regzbot the version range in whi 87 This tells regzbot the version range in which the issue started to happen;
91 you can specify a range using commit-ids as 88 you can specify a range using commit-ids as well or state a single commit-id
92 in case the reporter bisected the culprit. 89 in case the reporter bisected the culprit.
93 90
94 Note the caret (^) before the "introduced": 91 Note the caret (^) before the "introduced": it tells regzbot to treat the
95 parent mail (the one you reply to) as the i 92 parent mail (the one you reply to) as the initial report for the regression
96 you want to see tracked; that's important, 93 you want to see tracked; that's important, as regzbot will later look out
97 for patches with "Closes:" tags pointing to !! 94 for patches with "Link:" tags pointing to the report in the archives on
98 lore.kernel.org. 95 lore.kernel.org.
99 96
100 * When forwarding a regression reported to a !! 97 * When forwarding a regressions reported to a bug tracker, include a paragraph
101 with these regzbot commands:: 98 with these regzbot commands::
102 99
103 #regzbot introduced: 1f2e3d4c5b6a 100 #regzbot introduced: 1f2e3d4c5b6a
104 #regzbot from: Some N. Ice Human <some.h 101 #regzbot from: Some N. Ice Human <some.human@example.com>
105 #regzbot monitor: http://some.bugtracke 102 #regzbot monitor: http://some.bugtracker.example.com/ticket?id=123456789
106 103
107 Regzbot will then automatically associate p 104 Regzbot will then automatically associate patches with the report that
108 contain "Closes:" tags pointing to your mai !! 105 contain "Link:" tags pointing to your mail or the mentioned ticket.
109 106
110 What's important when fixing regressions 107 What's important when fixing regressions
111 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 108 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
112 109
113 You don't need to do anything special when sub 110 You don't need to do anything special when submitting fixes for regression, just
114 remember to do what Documentation/process/subm 111 remember to do what Documentation/process/submitting-patches.rst,
115 :ref:`Documentation/process/5.Posting.rst <dev 112 :ref:`Documentation/process/5.Posting.rst <development_posting>`, and
116 Documentation/process/stable-kernel-rules.rst 113 Documentation/process/stable-kernel-rules.rst already explain in more detail:
117 114
118 * Point to all places where the issue was rep !! 115 * Point to all places where the issue was reported using "Link:" tags::
119 116
120 Closes: https://lore.kernel.org/r/30th. !! 117 Link: https://lore.kernel.org/r/30th.anniversary.repost@klaava.Helsinki.FI/
121 Closes: https://bugzilla.kernel.org/sho !! 118 Link: https://bugzilla.kernel.org/show_bug.cgi?id=1234567890
122 <<
123 If you are only fixing part of the issue, y <<
124 described in the first document mentioned a <<
125 both of these equivalently and considers th <<
126 119
127 * Add a "Fixes:" tag to specify the commit ca 120 * Add a "Fixes:" tag to specify the commit causing the regression.
128 121
129 * If the culprit was merged in an earlier dev 122 * If the culprit was merged in an earlier development cycle, explicitly mark
130 the fix for backporting using the ``Cc: sta 123 the fix for backporting using the ``Cc: stable@vger.kernel.org`` tag.
131 124
132 All this is expected from you and important wh 125 All this is expected from you and important when it comes to regression, as
133 these tags are of great value for everyone (yo 126 these tags are of great value for everyone (you included) that might be looking
134 into the issue weeks, months, or years later. 127 into the issue weeks, months, or years later. These tags are also crucial for
135 tools and scripts used by other kernel develop 128 tools and scripts used by other kernel developers or Linux distributions; one of
136 these tools is regzbot, which heavily relies o !! 129 these tools is regzbot, which heavily relies on the "Link:" tags to associate
137 reports for regression with changes resolving 130 reports for regression with changes resolving them.
138 131
139 Expectations and best practices for fixing reg !! 132 Prioritize work on fixing regressions
140 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ !! 133 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
141 134
142 As a Linux kernel developer, you are expected !! 135 You should fix any reported regression as quickly as possible, to provide
143 situations where a regression caused by a rece !! 136 affected users with a solution in a timely manner and prevent more users from
144 only these options: !! 137 running into the issue; nevertheless developers need to take enough time and
145 !! 138 care to ensure regression fixes do not cause additional damage.
146 * Run a kernel with a regression that impacts !! 139
147 !! 140 In the end though, developers should give their best to prevent users from
148 * Switch to an older or newer kernel series. !! 141 running into situations where a regression leaves them only three options: "run
149 !! 142 a kernel with a regression that seriously impacts usage", "continue running an
150 * Continue running an outdated and thus poten !! 143 outdated and thus potentially insecure kernel version for more than two weeks
151 than three weeks after the regression's cul !! 144 after a regression's culprit was identified", and "downgrade to a still
152 should be less than two. And it ought to be !! 145 supported kernel series that lack required features".
153 severe or affects many users -- either in g !! 146
154 environments. !! 147 How to realize this depends a lot on the situation. Here are a few rules of
155 !! 148 thumb for you, in order or importance:
156 How to realize that in practice depends on var !! 149
157 rules of thumb as a guide. !! 150 * Prioritize work on handling regression reports and fixing regression over all
158 !! 151 other Linux kernel work, unless the latter concerns acute security issues or
159 In general: !! 152 bugs causing data loss or damage.
160 !! 153
161 * Prioritize work on regressions over all oth !! 154 * Always consider reverting the culprit commits and reapplying them later
162 latter concerns a severe issue (e.g. acute !! 155 together with necessary fixes, as this might be the least dangerous and
163 bricked hardware, ...). !! 156 quickest way to fix a regression.
164 !! 157
165 * Expedite fixing mainline regressions that r !! 158 * Developers should handle regressions in all supported kernel series, but are
166 mainline, stable, or longterm release (eith !! 159 free to delegate the work to the stable team, if the issue probably at no
167 !! 160 point in time occurred with mainline.
168 * Do not consider regressions from the curren !! 161
169 till the end of the cycle, as the issue mig !! 162 * Try to resolve any regressions introduced in the current development before
170 CI systems from testing mainline now or gen !! 163 its end. If you fear a fix might be too risky to apply only days before a new
171 !! 164 mainline release, let Linus decide: submit the fix separately to him as soon
172 * Work with the required care to avoid additi !! 165 as possible with the explanation of the situation. He then can make a call
173 resolving an issue then might take longer t !! 166 and postpone the release if necessary, for example if multiple such changes
174 !! 167 show up in his inbox.
175 On timing once the culprit of a regression is !! 168
176 !! 169 * Address regressions in stable, longterm, or proper mainline releases with
177 * Aim to mainline a fix within two or three d !! 170 more urgency than regressions in mainline pre-releases. That changes after
178 bothering many users -- either in general o !! 171 the release of the fifth pre-release, aka "-rc5": mainline then becomes as
179 particular hardware environment, distributi !! 172 important, to ensure all the improvements and fixes are ideally tested
180 !! 173 together for at least one week before Linus releases a new mainline version.
181 * Aim to mainline a fix by Sunday after the n !! 174
182 into a recent mainline, stable, or longterm !! 175 * Fix regressions within two or three days, if they are critical for some
183 backport); if the culprit became known earl !! 176 reason -- for example, if the issue is likely to affect many users of the
184 resolve, try to mainline the fix within the !! 177 kernel series in question on all or certain architectures. Note, this
185 !! 178 includes mainline, as issues like compile errors otherwise might prevent many
186 * For other regressions, aim to mainline fixe !! 179 testers or continuous integration systems from testing the series.
187 within the next three weeks. One or two Sun !! 180
188 regression is something people can live wit !! 181 * Aim to fix regressions within one week after the culprit was identified, if
189 mild performance regression. !! 182 the issue was introduced in either:
190 !! 183
191 * It's strongly discouraged to delay mainlini !! 184 * a recent stable/longterm release
192 merge window, except when the fix is extrao !! 185
193 culprit was mainlined more than a year ago. !! 186 * the development cycle of the latest proper mainline release
194 !! 187
195 On procedure: !! 188 In the latter case (say Linux v5.14), try to address regressions even
196 !! 189 quicker, if the stable series for the predecessor (v5.13) will be abandoned
197 * Always consider reverting the culprit, as i !! 190 soon or already was stamped "End-of-Life" (EOL) -- this usually happens about
198 dangerous way to fix a regression. Don't wo !! 191 three to four weeks after a new mainline release.
199 variant later: that should be straight-forw !! 192
200 through review once already. !! 193 * Try to fix all other regressions within two weeks after the culprit was
201 !! 194 found. Two or three additional weeks are acceptable for performance
202 * Try to resolve any regressions introduced i !! 195 regressions and other issues which are annoying, but don't prevent anyone
203 twelve months before the current developmen !! 196 from running Linux (unless it's an issue in the current development cycle,
204 regressions to be handled like those from t !! 197 as those should ideally be addressed before the release). A few weeks in
205 bears unusual risks. !! 198 total are acceptable if a regression can only be fixed with a risky change
206 !! 199 and at the same time is affecting only a few users; as much time is
207 * Consider CCing Linus on discussions or patc !! 200 also okay if the regression is already present in the second newest longterm
208 tangly. Do the same in precarious or urgent !! 201 kernel series.
209 subsystem maintainer might be unavailable. !! 202
210 know such a regression made it into a mainl !! 203 Note: The aforementioned time frames for resolving regressions are meant to
211 !! 204 include getting the fix tested, reviewed, and merged into mainline, ideally with
212 * For urgent regressions, consider asking Lin !! 205 the fix being in linux-next at least briefly. This leads to delays you need to
213 from the mailing list: he is totally fine w !! 206 account for.
214 fixes. Ideally though such requests should !! 207
215 subsystem maintainers or come directly from !! 208 Subsystem maintainers are expected to assist in reaching those periods by doing
216 !! 209 timely reviews and quick handling of accepted patches. They thus might have to
217 * In case you are unsure if a fix is worth th !! 210 send git-pull requests earlier or more often than usual; depending on the fix,
218 a new mainline release, send Linus a mail w !! 211 it might even be acceptable to skip testing in linux-next. Especially fixes for
219 CC; in it, summarize the situation while as !! 212 regressions in stable and longterm kernels need to be handled quickly, as fixes
220 the fix straight from the list. He then him !! 213 need to be merged in mainline before they can be backported to older series.
221 needed even postpone the release. Such requ <<
222 in accordance with the subsystem maintainer <<
223 <<
224 Regarding stable and longterm kernels: <<
225 <<
226 * You are free to leave regressions to the st <<
227 time occurred with mainline or were fixed t <<
228 <<
229 * If a regression made it into a proper mainl <<
230 twelve months, ensure to tag the fix with " <<
231 "Fixes:" tag alone does not guarantee a bac <<
232 in case you know the culprit was backported <<
233 <<
234 * When receiving reports about regressions in <<
235 series, please evaluate at least briefly if <<
236 mainline as well -- and if that seems likel <<
237 doubt, ask the reporter to check mainline. <<
238 <<
239 * Whenever you want to swiftly resolve a regr <<
240 into a proper mainline, stable, or longterm <<
241 mainline; when appropriate thus involve Lin <<
242 above). That's because the stable team norm <<
243 any changes that cause the same problems in <<
244 <<
245 * In case of urgent regression fixes you migh <<
246 backporting by dropping the stable team a n <<
247 this is especially advisable during merge w <<
248 the fix otherwise might land at the end of <<
249 <<
250 On patch flow: <<
251 <<
252 * Developers, when trying to reach the time p <<
253 to account for the time it takes to get fix <<
254 Linus, ideally with them being in linux-nex <<
255 fix is urgent, make it obvious to ensure ot <<
256 <<
257 * Reviewers, you are kindly asked to assist d <<
258 periods mentioned above by reviewing regres <<
259 <<
260 * Subsystem maintainers, you likewise are enc <<
261 of regression fixes. Thus evaluate if skipp <<
262 the particular fix. Also consider sending g <<
263 usual when needed. And try to avoid holding <<
264 weekends -- especially when the fix is mark <<
265 214
266 215
267 More aspects regarding regressions developers 216 More aspects regarding regressions developers should be aware of
268 ---------------------------------------------- 217 ----------------------------------------------------------------
269 218
270 219
271 How to deal with changes where a risk of regre 220 How to deal with changes where a risk of regression is known
272 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 221 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
273 222
274 Evaluate how big the risk of regressions is, f 223 Evaluate how big the risk of regressions is, for example by performing a code
275 search in Linux distributions and Git forges. 224 search in Linux distributions and Git forges. Also consider asking other
276 developers or projects likely to be affected t 225 developers or projects likely to be affected to evaluate or even test the
277 proposed change; if problems surface, maybe so 226 proposed change; if problems surface, maybe some solution acceptable for all
278 can be found. 227 can be found.
279 228
280 If the risk of regressions in the end seems to 229 If the risk of regressions in the end seems to be relatively small, go ahead
281 with the change, but let all involved parties 230 with the change, but let all involved parties know about the risk. Hence, make
282 sure your patch description makes this aspect 231 sure your patch description makes this aspect obvious. Once the change is
283 merged, tell the Linux kernel's regression tra 232 merged, tell the Linux kernel's regression tracker and the regressions mailing
284 list about the risk, so everyone has the chang 233 list about the risk, so everyone has the change on the radar in case reports
285 trickle in. Depending on the risk, you also mi 234 trickle in. Depending on the risk, you also might want to ask the subsystem
286 maintainer to mention the issue in his mainlin 235 maintainer to mention the issue in his mainline pull request.
287 236
288 What else is there to known about regressions? 237 What else is there to known about regressions?
289 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 238 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
290 239
291 Check out Documentation/admin-guide/reporting- 240 Check out Documentation/admin-guide/reporting-regressions.rst, it covers a lot
292 of other aspects you want might want to be awa 241 of other aspects you want might want to be aware of:
293 242
294 * the purpose of the "no regressions" rule !! 243 * the purpose of the "no regressions rule"
295 244
296 * what issues actually qualify as regression 245 * what issues actually qualify as regression
297 246
298 * who's in charge for finding the root cause 247 * who's in charge for finding the root cause of a regression
299 248
300 * how to handle tricky situations, e.g. when 249 * how to handle tricky situations, e.g. when a regression is caused by a
301 security fix or when fixing a regression mi 250 security fix or when fixing a regression might cause another one
302 251
303 Whom to ask for advice when it comes to regres 252 Whom to ask for advice when it comes to regressions
304 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 253 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
305 254
306 Send a mail to the regressions mailing list (r 255 Send a mail to the regressions mailing list (regressions@lists.linux.dev) while
307 CCing the Linux kernel's regression tracker (r 256 CCing the Linux kernel's regression tracker (regressions@leemhuis.info); if the
308 issue might better be dealt with in private, f 257 issue might better be dealt with in private, feel free to omit the list.
309 258
310 259
311 More about regression tracking and regzbot 260 More about regression tracking and regzbot
312 ------------------------------------------ 261 ------------------------------------------
313 262
314 263
315 Why the Linux kernel has a regression tracker, 264 Why the Linux kernel has a regression tracker, and why is regzbot used?
316 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 265 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
317 266
318 Rules like "no regressions" need someone to en 267 Rules like "no regressions" need someone to ensure they are followed, otherwise
319 they are broken either accidentally or on purp 268 they are broken either accidentally or on purpose. History has shown this to be
320 true for the Linux kernel as well. That's why 269 true for the Linux kernel as well. That's why Thorsten Leemhuis volunteered to
321 keep an eye on things as the Linux kernel's re 270 keep an eye on things as the Linux kernel's regression tracker, who's
322 occasionally helped by other people. Neither o 271 occasionally helped by other people. Neither of them are paid to do this,
323 that's why regression tracking is done on a be 272 that's why regression tracking is done on a best effort basis.
324 273
325 Earlier attempts to manually track regressions 274 Earlier attempts to manually track regressions have shown it's an exhausting and
326 frustrating work, which is why they were aband 275 frustrating work, which is why they were abandoned after a while. To prevent
327 this from happening again, Thorsten developed 276 this from happening again, Thorsten developed regzbot to facilitate the work,
328 with the long term goal to automate regression 277 with the long term goal to automate regression tracking as much as possible for
329 everyone involved. 278 everyone involved.
330 279
331 How does regression tracking work with regzbot 280 How does regression tracking work with regzbot?
332 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 281 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
333 282
334 The bot watches for replies to reports of trac 283 The bot watches for replies to reports of tracked regressions. Additionally,
335 it's looking out for posted or committed patch 284 it's looking out for posted or committed patches referencing such reports
336 with "Closes:" tags; replies to such patch pos !! 285 with "Link:" tags; replies to such patch postings are tracked as well.
337 Combined this data provides good insights into 286 Combined this data provides good insights into the current state of the fixing
338 process. 287 process.
339 288
340 Regzbot tries to do its job with as little ove 289 Regzbot tries to do its job with as little overhead as possible for both
341 reporters and developers. In fact, only report 290 reporters and developers. In fact, only reporters are burdened with an extra
342 duty: they need to tell regzbot about the regr 291 duty: they need to tell regzbot about the regression report using the ``#regzbot
343 introduced`` command outlined above; if they d 292 introduced`` command outlined above; if they don't do that, someone else can
344 take care of that using ``#regzbot ^introduced 293 take care of that using ``#regzbot ^introduced``.
345 294
346 For developers there normally is no extra work 295 For developers there normally is no extra work involved, they just need to make
347 sure to do something that was expected long be 296 sure to do something that was expected long before regzbot came to light: add
348 links to the patch description pointing to all !! 297 "Link:" tags to the patch description pointing to all reports about the issue
>> 298 fixed.
349 299
350 Do I have to use regzbot? 300 Do I have to use regzbot?
351 ~~~~~~~~~~~~~~~~~~~~~~~~~ 301 ~~~~~~~~~~~~~~~~~~~~~~~~~
352 302
353 It's in the interest of everyone if you do, as 303 It's in the interest of everyone if you do, as kernel maintainers like Linus
354 Torvalds partly rely on regzbot's tracking in 304 Torvalds partly rely on regzbot's tracking in their work -- for example when
355 deciding to release a new version or extend th 305 deciding to release a new version or extend the development phase. For this they
356 need to be aware of all unfixed regression; to 306 need to be aware of all unfixed regression; to do that, Linus is known to look
357 into the weekly reports sent by regzbot. 307 into the weekly reports sent by regzbot.
358 308
359 Do I have to tell regzbot about every regressi 309 Do I have to tell regzbot about every regression I stumble upon?
360 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 310 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
361 311
362 Ideally yes: we are all humans and easily forg 312 Ideally yes: we are all humans and easily forget problems when something more
363 important unexpectedly comes up -- for example 313 important unexpectedly comes up -- for example a bigger problem in the Linux
364 kernel or something in real life that's keepin 314 kernel or something in real life that's keeping us away from keyboards for a
365 while. Hence, it's best to tell regzbot about 315 while. Hence, it's best to tell regzbot about every regression, except when you
366 immediately write a fix and commit it to a tre 316 immediately write a fix and commit it to a tree regularly merged to the affected
367 kernel series. 317 kernel series.
368 318
369 How to see which regressions regzbot tracks cu 319 How to see which regressions regzbot tracks currently?
370 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 320 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
371 321
372 Check `regzbot's web-interface <https://linux- 322 Check `regzbot's web-interface <https://linux-regtracking.leemhuis.info/regzbot/>`_
373 for the latest info; alternatively, `search fo 323 for the latest info; alternatively, `search for the latest regression report
374 <https://lore.kernel.org/lkml/?q=%22Linux+regr 324 <https://lore.kernel.org/lkml/?q=%22Linux+regressions+report%22+f%3Aregzbot>`_,
375 which regzbot normally sends out once a week o 325 which regzbot normally sends out once a week on Sunday evening (UTC), which is a
376 few hours before Linus usually publishes new ( 326 few hours before Linus usually publishes new (pre-)releases.
377 327
378 What places is regzbot monitoring? 328 What places is regzbot monitoring?
379 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 329 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
380 330
381 Regzbot is watching the most important Linux m 331 Regzbot is watching the most important Linux mailing lists as well as the git
382 repositories of linux-next, mainline, and stab 332 repositories of linux-next, mainline, and stable/longterm.
383 333
384 What kind of issues are supposed to be tracked 334 What kind of issues are supposed to be tracked by regzbot?
385 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 335 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
386 336
387 The bot is meant to track regressions, hence p 337 The bot is meant to track regressions, hence please don't involve regzbot for
388 regular issues. But it's okay for the Linux ke 338 regular issues. But it's okay for the Linux kernel's regression tracker if you
389 use regzbot to track severe issues, like repor 339 use regzbot to track severe issues, like reports about hangs, corrupted data,
390 or internal errors (Panic, Oops, BUG(), warnin 340 or internal errors (Panic, Oops, BUG(), warning, ...).
391 341
392 Can I add regressions found by CI systems to r 342 Can I add regressions found by CI systems to regzbot's tracking?
393 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 343 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
394 344
395 Feel free to do so, if the particular regressi 345 Feel free to do so, if the particular regression likely has impact on practical
396 use cases and thus might be noticed by users; 346 use cases and thus might be noticed by users; hence, please don't involve
397 regzbot for theoretical regressions unlikely t 347 regzbot for theoretical regressions unlikely to show themselves in real world
398 usage. 348 usage.
399 349
400 How to interact with regzbot? 350 How to interact with regzbot?
401 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 351 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
402 352
403 By using a 'regzbot command' in a direct or in 353 By using a 'regzbot command' in a direct or indirect reply to the mail with the
404 regression report. These commands need to be i 354 regression report. These commands need to be in their own paragraph (IOW: they
405 need to be separated from the rest of the mail 355 need to be separated from the rest of the mail using blank lines).
406 356
407 One such command is ``#regzbot introduced: <ve !! 357 One such command is ``#regzbot introduced <version or commit>``, which makes
408 regzbot consider your mail as a regressions re 358 regzbot consider your mail as a regressions report added to the tracking, as
409 already described above; ``#regzbot ^introduce !! 359 already described above; ``#regzbot ^introduced <version or commit>`` is another
410 such command, which makes regzbot consider the 360 such command, which makes regzbot consider the parent mail as a report for a
411 regression which it starts to track. 361 regression which it starts to track.
412 362
413 Once one of those two commands has been utiliz 363 Once one of those two commands has been utilized, other regzbot commands can be
414 used in direct or indirect replies to the repo 364 used in direct or indirect replies to the report. You can write them below one
415 of the `introduced` commands or in replies to 365 of the `introduced` commands or in replies to the mail that used one of them
416 or itself is a reply to that mail: 366 or itself is a reply to that mail:
417 367
418 * Set or update the title:: 368 * Set or update the title::
419 369
420 #regzbot title: foo 370 #regzbot title: foo
421 371
422 * Monitor a discussion or bugzilla.kernel.org 372 * Monitor a discussion or bugzilla.kernel.org ticket where additions aspects of
423 the issue or a fix are discussed -- for exa 373 the issue or a fix are discussed -- for example the posting of a patch fixing
424 the regression:: 374 the regression::
425 375
426 #regzbot monitor: https://lore.kernel.o 376 #regzbot monitor: https://lore.kernel.org/all/30th.anniversary.repost@klaava.Helsinki.FI/
427 377
428 Monitoring only works for lore.kernel.org a 378 Monitoring only works for lore.kernel.org and bugzilla.kernel.org; regzbot
429 will consider all messages in that thread o 379 will consider all messages in that thread or ticket as related to the fixing
430 process. 380 process.
431 381
432 * Point to a place with further details of in 382 * Point to a place with further details of interest, like a mailing list post
433 or a ticket in a bug tracker that are sligh 383 or a ticket in a bug tracker that are slightly related, but about a different
434 topic:: 384 topic::
435 385
436 #regzbot link: https://bugzilla.kernel. 386 #regzbot link: https://bugzilla.kernel.org/show_bug.cgi?id=123456789
437 387
438 * Mark a regression as fixed by a commit that 388 * Mark a regression as fixed by a commit that is heading upstream or already
439 landed:: 389 landed::
440 390
441 #regzbot fix: 1f2e3d4c5d !! 391 #regzbot fixed-by: 1f2e3d4c5d
442 392
443 * Mark a regression as a duplicate of another 393 * Mark a regression as a duplicate of another one already tracked by regzbot::
444 394
445 #regzbot dup-of: https://lore.kernel.or 395 #regzbot dup-of: https://lore.kernel.org/all/30th.anniversary.repost@klaava.Helsinki.FI/
446 396
447 * Mark a regression as invalid:: 397 * Mark a regression as invalid::
448 398
449 #regzbot invalid: wasn't a regression, 399 #regzbot invalid: wasn't a regression, problem has always existed
450 400
451 Is there more to tell about regzbot and its co 401 Is there more to tell about regzbot and its commands?
452 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 402 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
453 403
454 More detailed and up-to-date information about 404 More detailed and up-to-date information about the Linux
455 kernel's regression tracking bot can be found 405 kernel's regression tracking bot can be found on its
456 `project page <https://gitlab.com/knurd42/regz 406 `project page <https://gitlab.com/knurd42/regzbot>`_, which among others
457 contains a `getting started guide <https://git 407 contains a `getting started guide <https://gitlab.com/knurd42/regzbot/-/blob/main/docs/getting_started.md>`_
458 and `reference documentation <https://gitlab.c 408 and `reference documentation <https://gitlab.com/knurd42/regzbot/-/blob/main/docs/reference.md>`_
459 which both cover more details than the above s 409 which both cover more details than the above section.
460 410
461 Quotes from Linus about regression 411 Quotes from Linus about regression
462 ---------------------------------- 412 ----------------------------------
463 413
464 Find below a few real life examples of how Lin 414 Find below a few real life examples of how Linus Torvalds expects regressions to
465 be handled: 415 be handled:
466 416
467 * From `2017-10-26 (1/2) 417 * From `2017-10-26 (1/2)
468 <https://lore.kernel.org/lkml/CA+55aFwiiQYJ+ 418 <https://lore.kernel.org/lkml/CA+55aFwiiQYJ+YoLKCXjN_beDVfu38mg=Ggg5LFOcqHE8Qi7Zw@mail.gmail.com/">https://lore.kernel.org/lkml/CA+55aFwiiQYJ+YoLKCXjN_beDVfu38mg=Ggg5LFOcqHE8Qi7Zw@mail.gmail.com/>`_::
469 419
470 If you break existing user space setups 420 If you break existing user space setups THAT IS A REGRESSION.
471 421
472 It's not ok to say "but we'll fix the u 422 It's not ok to say "but we'll fix the user space setup".
473 423
474 Really. NOT OK. 424 Really. NOT OK.
475 425
476 [...] 426 [...]
477 427
478 The first rule is: 428 The first rule is:
479 429
480 - we don't cause regressions 430 - we don't cause regressions
481 431
482 and the corollary is that when regressi 432 and the corollary is that when regressions *do* occur, we admit to
483 them and fix them, instead of blaming u 433 them and fix them, instead of blaming user space.
484 434
485 The fact that you have apparently been 435 The fact that you have apparently been denying the regression now for
486 three weeks means that I will revert, a 436 three weeks means that I will revert, and I will stop pulling apparmor
487 requests until the people involved unde 437 requests until the people involved understand how kernel development
488 is done. 438 is done.
489 439
490 * From `2017-10-26 (2/2) 440 * From `2017-10-26 (2/2)
491 <https://lore.kernel.org/lkml/CA+55aFxW7NMAM 441 <https://lore.kernel.org/lkml/CA+55aFxW7NMAMvYhkvz1UPbUTUJewRt6Yb51QAx5RtrWOwjebg@mail.gmail.com/">https://lore.kernel.org/lkml/CA+55aFxW7NMAMvYhkvz1UPbUTUJewRt6Yb51QAx5RtrWOwjebg@mail.gmail.com/>`_::
492 442
493 People should basically always feel lik 443 People should basically always feel like they can update their kernel
494 and simply not have to worry about it. 444 and simply not have to worry about it.
495 445
496 I refuse to introduce "you can only upd 446 I refuse to introduce "you can only update the kernel if you also
497 update that other program" kind of limi 447 update that other program" kind of limitations. If the kernel used to
498 work for you, the rule is that it conti 448 work for you, the rule is that it continues to work for you.
499 449
500 There have been exceptions, but they ar 450 There have been exceptions, but they are few and far between, and they
501 generally have some major and fundament 451 generally have some major and fundamental reasons for having happened,
502 that were basically entirely unavoidabl 452 that were basically entirely unavoidable, and people _tried_hard_ to
503 avoid them. Maybe we can't practically 453 avoid them. Maybe we can't practically support the hardware any more
504 after it is decades old and nobody uses 454 after it is decades old and nobody uses it with modern kernels any
505 more. Maybe there's a serious security 455 more. Maybe there's a serious security issue with how we did things,
506 and people actually depended on that fu 456 and people actually depended on that fundamentally broken model. Maybe
507 there was some fundamental other breaka 457 there was some fundamental other breakage that just _had_ to have a
508 flag day for very core and fundamental 458 flag day for very core and fundamental reasons.
509 459
510 And notice that this is very much about 460 And notice that this is very much about *breaking* peoples environments.
511 461
512 Behavioral changes happen, and maybe we 462 Behavioral changes happen, and maybe we don't even support some
513 feature any more. There's a number of f 463 feature any more. There's a number of fields in /proc/<pid>/stat that
514 are printed out as zeroes, simply becau 464 are printed out as zeroes, simply because they don't even *exist* in
515 the kernel any more, or because showing 465 the kernel any more, or because showing them was a mistake (typically
516 an information leak). But the numbers g 466 an information leak). But the numbers got replaced by zeroes, so that
517 the code that used to parse the fields 467 the code that used to parse the fields still works. The user might not
518 see everything they used to see, and so 468 see everything they used to see, and so behavior is clearly different,
519 but things still _work_, even if they m 469 but things still _work_, even if they might no longer show sensitive
520 (or no longer relevant) information. 470 (or no longer relevant) information.
521 471
522 But if something actually breaks, then 472 But if something actually breaks, then the change must get fixed or
523 reverted. And it gets fixed in the *ker 473 reverted. And it gets fixed in the *kernel*. Not by saying "well, fix
524 your user space then". It was a kernel 474 your user space then". It was a kernel change that exposed the
525 problem, it needs to be the kernel that 475 problem, it needs to be the kernel that corrects for it, because we
526 have a "upgrade in place" model. We don 476 have a "upgrade in place" model. We don't have a "upgrade with new
527 user space". 477 user space".
528 478
529 And I seriously will refuse to take cod 479 And I seriously will refuse to take code from people who do not
530 understand and honor this very simple r 480 understand and honor this very simple rule.
531 481
532 This rule is also not going to change. 482 This rule is also not going to change.
533 483
534 And yes, I realize that the kernel is " 484 And yes, I realize that the kernel is "special" in this respect. I'm
535 proud of it. 485 proud of it.
536 486
537 I have seen, and can point to, lots of 487 I have seen, and can point to, lots of projects that go "We need to
538 break that use case in order to make pr 488 break that use case in order to make progress" or "you relied on
539 undocumented behavior, it sucks to be y 489 undocumented behavior, it sucks to be you" or "there's a better way to
540 do what you want to do, and you have to 490 do what you want to do, and you have to change to that new better
541 way", and I simply don't think that's a 491 way", and I simply don't think that's acceptable outside of very early
542 alpha releases that have experimental u 492 alpha releases that have experimental users that know what they signed
543 up for. The kernel hasn't been in that 493 up for. The kernel hasn't been in that situation for the last two
544 decades. 494 decades.
545 495
546 We do API breakage _inside_ the kernel 496 We do API breakage _inside_ the kernel all the time. We will fix
547 internal problems by saying "you now ne 497 internal problems by saying "you now need to do XYZ", but then it's
548 about internal kernel API's, and the pe 498 about internal kernel API's, and the people who do that then also
549 obviously have to fix up all the in-ker 499 obviously have to fix up all the in-kernel users of that API. Nobody
550 can say "I now broke the API you used, 500 can say "I now broke the API you used, and now _you_ need to fix it
551 up". Whoever broke something gets to fi 501 up". Whoever broke something gets to fix it too.
552 502
553 And we simply do not break user space. 503 And we simply do not break user space.
554 504
555 * From `2020-05-21 505 * From `2020-05-21
556 <https://lore.kernel.org/all/CAHk-=wiVi7mSrs 506 <https://lore.kernel.org/all/CAHk-=wiVi7mSrsMP=fLXQrXK_UimybW=ziLOwSzFTtoXUacWVQ@mail.gmail.com/">https://lore.kernel.org/all/CAHk-=wiVi7mSrsMP=fLXQrXK_UimybW=ziLOwSzFTtoXUacWVQ@mail.gmail.com/>`_::
557 507
558 The rules about regressions have never 508 The rules about regressions have never been about any kind of
559 documented behavior, or where the code 509 documented behavior, or where the code lives.
560 510
561 The rules about regressions are always 511 The rules about regressions are always about "breaks user workflow".
562 512
563 Users are literally the _only_ thing th 513 Users are literally the _only_ thing that matters.
564 514
565 No amount of "you shouldn't have used t 515 No amount of "you shouldn't have used this" or "that behavior was
566 undefined, it's your own fault your app 516 undefined, it's your own fault your app broke" or "that used to work
567 simply because of a kernel bug" is at a 517 simply because of a kernel bug" is at all relevant.
568 518
569 Now, reality is never entirely black-an 519 Now, reality is never entirely black-and-white. So we've had things
570 like "serious security issue" etc that 520 like "serious security issue" etc that just forces us to make changes
571 that may break user space. But even the 521 that may break user space. But even then the rule is that we don't
572 really have other options that would al 522 really have other options that would allow things to continue.
573 523
574 And obviously, if users take years to e 524 And obviously, if users take years to even notice that something
575 broke, or if we have sane ways to work 525 broke, or if we have sane ways to work around the breakage that
576 doesn't make for too much trouble for u 526 doesn't make for too much trouble for users (ie "ok, there are a
577 handful of users, and they can use a ke 527 handful of users, and they can use a kernel command line to work
578 around it" kind of things) we've also b 528 around it" kind of things) we've also been a bit less strict.
579 529
580 But no, "that was documented to be brok 530 But no, "that was documented to be broken" (whether it's because the
581 code was in staging or because the man- 531 code was in staging or because the man-page said something else) is
582 irrelevant. If staging code is so usefu 532 irrelevant. If staging code is so useful that people end up using it,
583 that means that it's basically regular 533 that means that it's basically regular kernel code with a flag saying
584 "please clean this up". 534 "please clean this up".
585 535
586 The other side of the coin is that peop 536 The other side of the coin is that people who talk about "API
587 stability" are entirely wrong. API's do 537 stability" are entirely wrong. API's don't matter either. You can make
588 any changes to an API you like - as lon 538 any changes to an API you like - as long as nobody notices.
589 539
590 Again, the regression rule is not about 540 Again, the regression rule is not about documentation, not about
591 API's, and not about the phase of the m 541 API's, and not about the phase of the moon.
592 542
593 It's entirely about "we caused problems 543 It's entirely about "we caused problems for user space that used to work".
594 544
595 * From `2017-11-05 545 * From `2017-11-05
596 <https://lore.kernel.org/all/CA+55aFzUvbGjD8 546 <https://lore.kernel.org/all/CA+55aFzUvbGjD8nQ-+3oiMBx14c_6zOj2n7KLN3UsJ-qsd4Dcw@mail.gmail.com/">https://lore.kernel.org/all/CA+55aFzUvbGjD8nQ-+3oiMBx14c_6zOj2n7KLN3UsJ-qsd4Dcw@mail.gmail.com/>`_::
597 547
598 And our regression rule has never been 548 And our regression rule has never been "behavior doesn't change".
599 That would mean that we could never mak 549 That would mean that we could never make any changes at all.
600 550
601 For example, we do things like add new 551 For example, we do things like add new error handling etc all the
602 time, which we then sometimes even add 552 time, which we then sometimes even add tests for in our kselftest
603 directory. 553 directory.
604 554
605 So clearly behavior changes all the tim 555 So clearly behavior changes all the time and we don't consider that a
606 regression per se. 556 regression per se.
607 557
608 The rule for a regression for the kerne 558 The rule for a regression for the kernel is that some real user
609 workflow breaks. Not some test. Not a " 559 workflow breaks. Not some test. Not a "look, I used to be able to do
610 X, now I can't". 560 X, now I can't".
611 561
612 * From `2018-08-03 562 * From `2018-08-03
613 <https://lore.kernel.org/all/CA+55aFwWZX=CXm 563 <https://lore.kernel.org/all/CA+55aFwWZX=CXmWDTkDGb36kf12XmTehmQjbiMPCqCRG2hi9kw@mail.gmail.com/">https://lore.kernel.org/all/CA+55aFwWZX=CXmWDTkDGb36kf12XmTehmQjbiMPCqCRG2hi9kw@mail.gmail.com/>`_::
614 564
615 YOU ARE MISSING THE #1 KERNEL RULE. 565 YOU ARE MISSING THE #1 KERNEL RULE.
616 566
617 We do not regress, and we do not regres 567 We do not regress, and we do not regress exactly because your are 100% wrong.
618 568
619 And the reason you state for your opini 569 And the reason you state for your opinion is in fact exactly *WHY* you
620 are wrong. 570 are wrong.
621 571
622 Your "good reasons" are pure and utter 572 Your "good reasons" are pure and utter garbage.
623 573
624 The whole point of "we do not regress" 574 The whole point of "we do not regress" is so that people can upgrade
625 the kernel and never have to worry abou 575 the kernel and never have to worry about it.
626 576
627 > Kernel had a bug which has been fixed 577 > Kernel had a bug which has been fixed
628 578
629 That is *ENTIRELY* immaterial. 579 That is *ENTIRELY* immaterial.
630 580
631 Guys, whether something was buggy or no 581 Guys, whether something was buggy or not DOES NOT MATTER.
632 582
633 Why? 583 Why?
634 584
635 Bugs happen. That's a fact of life. Arg 585 Bugs happen. That's a fact of life. Arguing that "we had to break
636 something because we were fixing a bug" 586 something because we were fixing a bug" is completely insane. We fix
637 tens of bugs every single day, thinking 587 tens of bugs every single day, thinking that "fixing a bug" means that
638 we can break something is simply NOT TR 588 we can break something is simply NOT TRUE.
639 589
640 So bugs simply aren't even relevant to 590 So bugs simply aren't even relevant to the discussion. They happen,
641 they get found, they get fixed, and it 591 they get found, they get fixed, and it has nothing to do with "we
642 break users". 592 break users".
643 593
644 Because the only thing that matters IS 594 Because the only thing that matters IS THE USER.
645 595
646 How hard is that to understand? 596 How hard is that to understand?
647 597
648 Anybody who uses "but it was buggy" as 598 Anybody who uses "but it was buggy" as an argument is entirely missing
649 the point. As far as the USER was conce 599 the point. As far as the USER was concerned, it wasn't buggy - it
650 worked for him/her. 600 worked for him/her.
651 601
652 Maybe it worked *because* the user had 602 Maybe it worked *because* the user had taken the bug into account,
653 maybe it worked because the user didn't 603 maybe it worked because the user didn't notice - again, it doesn't
654 matter. It worked for the user. 604 matter. It worked for the user.
655 605
656 Breaking a user workflow for a "bug" is 606 Breaking a user workflow for a "bug" is absolutely the WORST reason
657 for breakage you can imagine. 607 for breakage you can imagine.
658 608
659 It's basically saying "I took something 609 It's basically saying "I took something that worked, and I broke it,
660 but now it's better". Do you not see ho 610 but now it's better". Do you not see how f*cking insane that statement
661 is? 611 is?
662 612
663 And without users, your program is not 613 And without users, your program is not a program, it's a pointless
664 piece of code that you might as well th 614 piece of code that you might as well throw away.
665 615
666 Seriously. This is *why* the #1 rule fo 616 Seriously. This is *why* the #1 rule for kernel development is "we
667 don't break users". Because "I fixed a 617 don't break users". Because "I fixed a bug" is absolutely NOT AN
668 ARGUMENT if that bug fix broke a user s 618 ARGUMENT if that bug fix broke a user setup. You actually introduced a
669 MUCH BIGGER bug by "fixing" something t 619 MUCH BIGGER bug by "fixing" something that the user clearly didn't
670 even care about. 620 even care about.
671 621
672 And dammit, we upgrade the kernel ALL T 622 And dammit, we upgrade the kernel ALL THE TIME without upgrading any
673 other programs at all. It is absolutely 623 other programs at all. It is absolutely required, because flag-days
674 and dependencies are horribly bad. 624 and dependencies are horribly bad.
675 625
676 And it is also required simply because 626 And it is also required simply because I as a kernel developer do not
677 upgrade random other tools that I don't 627 upgrade random other tools that I don't even care about as I develop
678 the kernel, and I want any of my users 628 the kernel, and I want any of my users to feel safe doing the same
679 time. 629 time.
680 630
681 So no. Your rule is COMPLETELY wrong. I 631 So no. Your rule is COMPLETELY wrong. If you cannot upgrade a kernel
682 without upgrading some other random bin 632 without upgrading some other random binary, then we have a problem.
683 633
684 * From `2021-06-05 634 * From `2021-06-05
685 <https://lore.kernel.org/all/CAHk-=wiUVqHN76 635 <https://lore.kernel.org/all/CAHk-=wiUVqHN76YUwhkjZzwTdjMMJf_zN4+u7vEJjmEGh3recw@mail.gmail.com/">https://lore.kernel.org/all/CAHk-=wiUVqHN76YUwhkjZzwTdjMMJf_zN4+u7vEJjmEGh3recw@mail.gmail.com/>`_::
686 636
687 THERE ARE NO VALID ARGUMENTS FOR REGRES 637 THERE ARE NO VALID ARGUMENTS FOR REGRESSIONS.
688 638
689 Honestly, security people need to under 639 Honestly, security people need to understand that "not working" is not
690 a success case of security. It's a fail 640 a success case of security. It's a failure case.
691 641
692 Yes, "not working" may be secure. But s 642 Yes, "not working" may be secure. But security in that case is *pointless*.
693 643
694 * From `2011-05-06 (1/3) 644 * From `2011-05-06 (1/3)
695 <https://lore.kernel.org/all/BANLkTim9YvResB 645 <https://lore.kernel.org/all/BANLkTim9YvResB+PwRp7QTK-a5VNg2PvmQ@mail.gmail.com/">https://lore.kernel.org/all/BANLkTim9YvResB+PwRp7QTK-a5VNg2PvmQ@mail.gmail.com/>`_::
696 646
697 Binary compatibility is more important. 647 Binary compatibility is more important.
698 648
699 And if binaries don't use the interface 649 And if binaries don't use the interface to parse the format (or just
700 parse it wrongly - see the fairly recen 650 parse it wrongly - see the fairly recent example of adding uuid's to
701 /proc/self/mountinfo), then it's a regr 651 /proc/self/mountinfo), then it's a regression.
702 652
703 And regressions get reverted, unless th 653 And regressions get reverted, unless there are security issues or
704 similar that makes us go "Oh Gods, we r 654 similar that makes us go "Oh Gods, we really have to break things".
705 655
706 I don't understand why this simple logi 656 I don't understand why this simple logic is so hard for some kernel
707 developers to understand. Reality matte 657 developers to understand. Reality matters. Your personal wishes matter
708 NOT AT ALL. 658 NOT AT ALL.
709 659
710 If you made an interface that can be us 660 If you made an interface that can be used without parsing the
711 interface description, then we're stuck 661 interface description, then we're stuck with the interface. Theory
712 simply doesn't matter. 662 simply doesn't matter.
713 663
714 You could help fix the tools, and try t 664 You could help fix the tools, and try to avoid the compatibility
715 issues that way. There aren't that many 665 issues that way. There aren't that many of them.
716 666
717 From `2011-05-06 (2/3) 667 From `2011-05-06 (2/3)
718 <https://lore.kernel.org/all/BANLkTi=KVXjKR8 668 <https://lore.kernel.org/all/BANLkTi=KVXjKR82sqsz4gwjr+E0vtqCmvA@mail.gmail.com/">https://lore.kernel.org/all/BANLkTi=KVXjKR82sqsz4gwjr+E0vtqCmvA@mail.gmail.com/>`_::
719 669
720 it's clearly NOT an internal tracepoint 670 it's clearly NOT an internal tracepoint. By definition. It's being
721 used by powertop. 671 used by powertop.
722 672
723 From `2011-05-06 (3/3) 673 From `2011-05-06 (3/3)
724 <https://lore.kernel.org/all/BANLkTinazaXRdG 674 <https://lore.kernel.org/all/BANLkTinazaXRdGovYL7rRVp+j6HbJ7pzhg@mail.gmail.com/">https://lore.kernel.org/all/BANLkTinazaXRdGovYL7rRVp+j6HbJ7pzhg@mail.gmail.com/>`_::
725 675
726 We have programs that use that ABI and 676 We have programs that use that ABI and thus it's a regression if they break.
727 677
728 * From `2012-07-06 <https://lore.kernel.org/al 678 * From `2012-07-06 <https://lore.kernel.org/all/CA+55aFwnLJ+0sjx92EGREGTWOx84wwKaraSzpTNJwPVV8edw8g@mail.gmail.com/">https://lore.kernel.org/all/CA+55aFwnLJ+0sjx92EGREGTWOx84wwKaraSzpTNJwPVV8edw8g@mail.gmail.com/>`_::
729 679
730 > Now this got me wondering if Debian _ 680 > Now this got me wondering if Debian _unstable_ actually qualifies as a
731 > standard distro userspace. 681 > standard distro userspace.
732 682
733 Oh, if the kernel breaks some standard 683 Oh, if the kernel breaks some standard user space, that counts. Tons
734 of people run Debian unstable 684 of people run Debian unstable
735 685
736 * From `2019-09-15 686 * From `2019-09-15
737 <https://lore.kernel.org/lkml/CAHk-=wiP4K8DR 687 <https://lore.kernel.org/lkml/CAHk-=wiP4K8DRJWsCo=20hn_6054xBamGKF2kPgUzpB5aMaofA@mail.gmail.com/">https://lore.kernel.org/lkml/CAHk-=wiP4K8DRJWsCo=20hn_6054xBamGKF2kPgUzpB5aMaofA@mail.gmail.com/>`_::
738 688
739 One _particularly_ last-minute revert i 689 One _particularly_ last-minute revert is the top-most commit (ignoring
740 the version change itself) done just be 690 the version change itself) done just before the release, and while
741 it's very annoying, it's perhaps also i 691 it's very annoying, it's perhaps also instructive.
742 692
743 What's instructive about it is that I r 693 What's instructive about it is that I reverted a commit that wasn't
744 actually buggy. In fact, it was doing e 694 actually buggy. In fact, it was doing exactly what it set out to do,
745 and did it very well. In fact it did it 695 and did it very well. In fact it did it _so_ well that the much
746 improved IO patterns it caused then end 696 improved IO patterns it caused then ended up revealing a user-visible
747 regression due to a real bug in a compl 697 regression due to a real bug in a completely unrelated area.
748 698
749 The actual details of that regression a 699 The actual details of that regression are not the reason I point that
750 revert out as instructive, though. It's 700 revert out as instructive, though. It's more that it's an instructive
751 example of what counts as a regression, 701 example of what counts as a regression, and what the whole "no
752 regressions" kernel rule means. The rev 702 regressions" kernel rule means. The reverted commit didn't change any
753 API's, and it didn't introduce any new 703 API's, and it didn't introduce any new bugs. But it ended up exposing
754 another problem, and as such caused a k 704 another problem, and as such caused a kernel upgrade to fail for a
755 user. So it got reverted. 705 user. So it got reverted.
756 706
757 The point here being that we revert bas 707 The point here being that we revert based on user-reported _behavior_,
758 not based on some "it changes the ABI" 708 not based on some "it changes the ABI" or "it caused a bug" concept.
759 The problem was really pre-existing, an 709 The problem was really pre-existing, and it just didn't happen to
760 trigger before. The better IO patterns 710 trigger before. The better IO patterns introduced by the change just
761 happened to expose an old bug, and peop 711 happened to expose an old bug, and people had grown to depend on the
762 previously benign behavior of that old 712 previously benign behavior of that old issue.
763 713
764 And never fear, we'll re-introduce the 714 And never fear, we'll re-introduce the fix that improved on the IO
765 patterns once we've decided just how to 715 patterns once we've decided just how to handle the fact that we had a
766 bad interaction with an interface that 716 bad interaction with an interface that people had then just happened
767 to rely on incidental behavior for befo 717 to rely on incidental behavior for before. It's just that we'll have
768 to hash through how to do that (there a 718 to hash through how to do that (there are no less than three different
769 patches by three different developers b 719 patches by three different developers being discussed, and there might
770 be more coming...). In the meantime, I 720 be more coming...). In the meantime, I reverted the thing that exposed
771 the problem to users for this release, 721 the problem to users for this release, even if I hope it will be
772 re-introduced (perhaps even backported 722 re-introduced (perhaps even backported as a stable patch) once we have
773 consensus about the issue it exposed. 723 consensus about the issue it exposed.
774 724
775 Take-away from the whole thing: it's no 725 Take-away from the whole thing: it's not about whether you change the
776 kernel-userspace ABI, or fix a bug, or 726 kernel-userspace ABI, or fix a bug, or about whether the old code
777 "should never have worked in the first 727 "should never have worked in the first place". It's about whether
778 something breaks existing users' workfl 728 something breaks existing users' workflow.
779 729
780 Anyway, that was my little aside on the 730 Anyway, that was my little aside on the whole regression thing. Since
781 it's that "first rule of kernel program 731 it's that "first rule of kernel programming", I felt it is perhaps
782 worth just bringing it up every once in 732 worth just bringing it up every once in a while
783 733
784 .. 734 ..
785 end-of-content 735 end-of-content
786 .. 736 ..
787 This text is available under GPL-2.0+ or CC 737 This text is available under GPL-2.0+ or CC-BY-4.0, as stated at the top
788 of the file. If you want to distribute this 738 of the file. If you want to distribute this text under CC-BY-4.0 only,
789 please use "The Linux kernel developers" fo 739 please use "The Linux kernel developers" for author attribution and link
790 this as source: 740 this as source:
791 https://git.kernel.org/pub/scm/linux/kernel 741 https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/plain/Documentation/process/handling-regressions.rst
792 .. 742 ..
793 Note: Only the content of this RST file as 743 Note: Only the content of this RST file as found in the Linux kernel sources
794 is available under CC-BY-4.0, as versions o 744 is available under CC-BY-4.0, as versions of this text that were processed
795 (for example by the kernel's build system) 745 (for example by the kernel's build system) might contain content taken from
796 files which use a more restrictive license. 746 files which use a more restrictive license.
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