KDBUS and freezing of DBUS-querying applications

["Мисбах-Соловьёв Вадим" <mva@mva.name>]

Hello!
I'm experiencing a strange issue, when using KDBUS, but I don't know how to 
properly report it and which info I should proide in addition to th one below.

In short, some userspace applications I'm using are freezing for about a 
minute or so, when thy trying to query system bus.


Mostly it hurts me (in my daily workflow) when I trying to open some network 
resource with kde-open5 (or even target application), because KF5 seavily 
depends on DBUS.

But some time ago I even experienced this with "sudo", and even when I tried 
to query system bus with qdbusviwer, it freezed too.


It is pretty hard to trace entire KF5, but talking about issue with sudo, when 
I straced it, I saw frreze on the:

> ppol(fd=9, ...

syscall

and when I looked in lsof, I noticed that fd9 is /sys/fs/kdbus/0-system/bus.

KF5 things also freezing on sending queries to dbus, but I just can't get 
exactly application that freezing (although, if I just open http-links in 
konsole kpart in either yakuake, konsole or kdevelop — any of them also 
freezing on kdbus fd).

To be honest, I'm not traced qdbusviewer, but I'm pretty sure it will show the 
same.


When I recompiling kernel without KDBUS (or move the module somewhere, so that 
SystemD can't autoload it), all that userspace things starting to work like a 
charm, without a freezes and timeouts.

When I enable KDBUS back (either as module or by compile it in the monolithic 
kernel), all the things reproduces again.


I'm tried with both SystemD-216 and SystemD-226.


P.S. I'll be happy to provide any additional info if you'll ask me.


-- 
Best regards,
mva