ubuntu-x-swat team mailing list archive

[Bryce Harrington <124406@xxxxxxxxxxxxxxxxxx>]

Heh, I see my comment about this being a minor annoyance got people in a
huff.  ;-)

Anyway, as to whether it's a kernel or X problem, it's sort of a bit of
both.  (The best bugs live in the cracks between two codebases.)  X and
the kernel communicate key events as signals rather than via threading.
And it only handles one signal at a time, so if for some reason the key
up signal was fired while another signal was being handled (e.g. from
another device in your system) then it can get lost.  That's why hitting
a key a second time (to fire a new up-key signal) makes things work.
The way signals behave is a kernel thing, so this is why it's partly
kernel, partly X.  Essentially it's a race condition.  For deeper
information see http://ajaxxx.livejournal.com/62378.html

Really, it sounds like it's an upstream design flaw in X.  Ajax appears
to be thinking that the whole system should be ripped out and replaced
with a threading system.  My own experience with threading is that
sometimes the cure can be worse than the disease... threading can be
quite hard to get right and sometimes has nasty side effects.  Needless
to say, such a change is not trivial and not something we'd do at the
Ubuntu distro level - definitely work that needs done upstream.  Maybe
Wayland will gain a better system for handling keyboard events, and
that's where efforts today should be directed?  Don't know.

As an aside, you guys are right that there could conceivably be some
rare scenarios where this bug could cause some severe issue like a stuck
delete key deleting files or whatnot.  Maybe some of you have even
experienced something like that.  But for the vast majority of cases,
the issue will exhibit itself as extraneous characters when you're
typing documents and some such - definitely a lot less severe than
random GPU lockups or sudden X crashes back to the login screen or your
monitor suddenly turning tie died.  These latter issues are
unfortunately not as uncommon as I'd like, and until they are I tend to
judge anything less severe as a "minor annoyance".  ;-)

But annoyances are bad.  While I don't think this issue is one we're
likely to work on in Ubuntu at the distro level, I can give some advice
about how to go forward with it, if you're wanting to pursue it
yourself.  (And FSM bless you!)

Due to the nature of the issue, it's frequency and severity are going to
vary from hardware to hardware.  Due to timings in hardware interrupts
and signal generation, and even interactions with software, you might
see it only with a particular combination of keyboard, motherboard, and
mouse.  Or it might go away after turning off your wireless.  Or might
go away for 3 Ubuntu releases and then suddenly and quite mysteriously
reappear.  Most of our typical keyboard debugging tools such as xev are
going to be of limited value in investigating it; it may tell you that
the release signal didn't show up, but that doesn't explain why.  There
are kernel debugging interfaces that will show what's going on there,
but that gives limited insights as well.

The first thing I would look at is obtaining a reliable repetitive test
case.  Get together hardware and a set of steps that lets you reliably
reproduce the issue on command, or with a sane enough frequency (like
steps that let you reproduce >25% of the time).  This makes testing less
time consuming and also gives you a strong way to determine it's
definitely fixed.

Next, it would be smart to ensure someone hasn't already fixed it.  That
could save you a lot of work.  We provide xorg-edgers and kernel ppas to
facilitate doing this.  See https://launchpad.net/~xorg-
edgers/+archive/ppa and http://kernel.ubuntu.com/~kernel-ppa/mainline/.

Finally the hard part, which will require code hacking.  Touch base
about this with Ajax to find if he has any experimental branches, or if
he knows if anyone else is working on the problem, and how you can help.
>From his blog post, it sounds like the implementation of threaded input
handling might be the real way forward, and that could probably benefit
from extra testers and/or coders.

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https://bugs.launchpad.net/bugs/124406

Title:
  Keyboard keys get stuck and repeat