[RFC PATCH v2 2/2] docs: regressions.rst: rules of thumb for handling regressions

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Add a section with a few rules of thumb about how quickly regressions
should be fixed. They are written after studying the quotes from Linus
found in the modified document and especially influenced by statements
like "Users are literally the _only_ thing that matters" and "without
users, your program is not a program, it's a pointless piece of code
that you might as well throw away". The author interpreted those in
perspective to how the various Linux kernel series are maintained and
what those practices might mean for users running into a regression when

That for example lead to the paragraph starting with "Aim to get fixes
for regressions mainlined within one week after identifying the culprit,
if the regression was introduced in a stable/longterm release or the
devel cycle for the latest mainline release". This is a pretty high bar,
but on the other hand needed to not leave users out in the cold for to
long. This can quickly happen, as the previous stable series is normally
stamped "End of Life" about three or four weeks after a new mainline
release, which makes a lot of users switch during this timeframe. Any of
them risk running into regressions not promptly fixed; even worse, once
the previous stable series is EOLed for real, users that face a
regression might be left with only three options:

 (1) continue running an outdated and thus potentially insecure kernel
     version from an abandoned stable series

 (2) run the kernel with the regression

 (3) downgrade to an earlier longterm series still supported

This is better avoided, as (1) puts users and their data in danger, (2)
will only be possible if it's a minor regression that doesn't interfere
with booting or serious usage, and (3) might be regression itself or
impossible on the particular machine, as some users will require drivers
or features only introduced after the latest longterm series took of.

In the end this lead to the "Aim to fix regression within one week"
part. It also is the reason for the "Try to resolve any regressions
introduced in the current development cycle before its end.".

Signed-off-by: Thorsten Leemhuis <linux@xxxxxxxxxxxxx>
CC: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
CC: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

Hi! A lot of developers are doing a good job in fixing regressions, but
I noticed it sometimes takes many weeks to get even simple fixes for
regressions merged. Most of the time this is due to one of these

 * it takes a long time to get the fix ready, as some developers
   apparently don't prioritize work on fixing regressions

 * fully developed fixes linger in git trees of maintainers for weeks,
   sometimes even without the fix being in linux-next

This afaics is especially a problem for regressions introduced in
mainline, but only found after a recent release or a stable kernel
series derived from it. Sometimes fixes for these regressions are even
left lying around for weeks until the next merge window, which
contributes to a huge pile of fixes getting backported to stable and
longterm releases during or shortly after merge windows. Asking
developers to speed things up rarely helped, as people have different
options on how fast regression fixes need to be developed and merged

That's why it would be a great help to my work as regression tracker if
we had some rough written down guidelines for handling regressions, as
proposed by the patch below. I'm well aware that the text sets a pretty
high bar. That's because I approached the problem primarily from the
point of a user, as can be seen by the patch description.

The text added by this patch likely will lead to some discussions,
that's why I submit it separately from the rest of the new document on
regressions, which is found in patch 1/2; I also CCed Linus on this
patch and hope they state his opinion or ACK is. In the end I can
easily tone this down or write something totally different: that's
totally fine for me, I'm mainly interested in having some expectations
roughly documented to get everyone on the same page.
 Documentation/admin-guide/regressions.rst | 79 +++++++++++++++++++++++
 1 file changed, 79 insertions(+)

diff --git a/Documentation/admin-guide/regressions.rst b/Documentation/admin-guide/regressions.rst
index 6eb8d9784a1f..17db7be110c1 100644
--- a/Documentation/admin-guide/regressions.rst
+++ b/Documentation/admin-guide/regressions.rst
@@ -64,6 +64,10 @@ list; add the aforementioned paragraph, just omit the caret (the ^) before the
 ``introduced``, which makes regzbot treat your mail (and not the one you reply
 to) as the report.
+Try to fix regressions quickly once the culprit has been identified. Fixes for
+most regressions should be mainlined within two weeks, but some should be
+addressed within two or three days.
 When submitting fixes for regressions, always include 'Link:' tags in the commit
 message that point to all places where the issue was reported, as explained in
 `Documentation/process/submitting-patches.rst` and
@@ -235,6 +239,81 @@ Alternatively to all the above you can just forward or bounce the report to the
 Linux kernel's regression tracker, but consider the downsides already outlined
 in the previous section.
+How quickly should regressions be fixed?
+Developers should fix any reported regression as quickly as possible, to provide
+affected users with a solution in a timely manner and prevent more users from
+running into the issue; nevertheless developers need to take enough time and
+care to ensure regression fixes do not cause additional damage.
+In the end though, developers should give their best to prevent users from
+running into situations where a regression leaves them only three options: 'run
+a kernel with a regression that seriously impacts usage', 'continue running an
+outdated and thus potentially insecure kernel version for more than two weeks
+after a regression's culprit was identified', and 'downgrade to a still
+supported kernel series that's missing required features'.
+How to realize this depends a lot on the situation. Here are a few rules of
+thumb for developers, in order or importance:
+ * Prioritize work on handling reports about regression and fixing them over all
+   other Linux kernel work, unless the latter concerns acute security issues or
+   bugs causing data loss or damage.
+ * Always consider reverting the culprit commits and reapplying them later
+   together with necessary fixes, as this might be the least dangerous and
+   quickest way to fix a regression.
+ * Try to resolve any regressions introduced in the current development before
+   its end. If you fear a fix might be too risky to apply only days before a new
+   mainline release, let Linus decide: submit the fix separately to him as soon
+   as possible with the explanation of the situation. He then can make a call
+   and postpone the release if necessary, for example if multiple such changes
+   show up in his inbox.
+ * Address regressions in stable, longterm, or proper mainline releases with
+   more urgency than regressions in mainline pre-releases. That changes after
+   the release of the fifth pre-release, aka '-rc5': mainline then becomes as
+   important, to ensure all the improvements and fixes are ideally tested
+   together for at least one week before Linus releases a new mainline version.
+ * Fix regressions within two or three days, if they are critical for some
+   reason -- for example, if the issue is likely to affect many users of the
+   kernel series in question on all or certain architectures. This thus includes
+   fixes for compile errors in mainline, as they might prevent testers and
+   continuous integration systems from doing their work.
+ * Aim to merge regression fixes into mainline within one week after the culprit
+   was identified, if the regression was introduced in a stable/longterm release
+   or the development cycle for the latest mainline release (say v5.14). If
+   possible, try to address the issue even quicker, if the previous stable
+   series (v5.13.y) will be abandoned soon or already was stamped 'End-of-Life'
+   (EOL) -- this usually happens about three to four weeks after a new mainline
+   release.
+ * Try to fix all other regressions within two weeks after the culprit was
+   found. Two or three additional weeks are acceptable for performance
+   regressions and other issues which are annoying, but don't prevent anyone
+   from running Linux -- unless it's an issue in the current development cycle,
+   which should be addressed before the release. A few weeks in total are also
+   acceptable if a regression can only be fixed with a risky change and at the
+   same time is affecting only a few users; as much time is also acceptable if
+   the regression is already present in the second newest longterm kernel
+   series.
+Note: The aforementioned time frames for resolving regressions are meant to
+include getting the fix tested, reviewed, and merged into mainline, ideally with
+the fix being in linux-next for two days. Developers need to keep in mind that
+each of these steps takes some time.
+Subsystem maintainers are expected to assist in reaching those periods by doing
+timely reviews and quick handling of accepted patches. They thus might have to
+send git-pull requests earlier or more often than usual; depending on the fix,
+it might even be acceptable to skip testing in linux-next. Especially fixes for
+regressions in stable and longterm kernels need to be handled quickly, as fixes
+need to be merged in mainline before they can be backported to older series.
 Are really all regressions fixed?

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