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