[systemd-devel] Zombie process still exists after stopping gdm.service

[Andrei Borzenkov]

В Fri, 17 Apr 2015 14:04:18 -0600
Daniel Drake <drake at endlessm.com> пишет:

> Hi,
> 
> I'm investigating why "systemctl stop gdm; Xorg" usually fails. The
> new X process complains that X is still running.
> 
> Here's what I think is happening:
> 
> 1. systemd sends SIGTERM to gdm to stop the service
> 
> 2. gdm exits - it has a simple SIGTERM handler which just quits the
> mainloop without doing any cleanup (as far as I can see, it doesn't
> make any attempt to kill the child X server)
> 
> 3. X exits because of PR_SET_PDEATHSIG (i.e. it's set to be
> automatically killed when the parent goes away). The killed process
> enters defunct state and is reparented to PID 1, presumably also
> moving it out of the gdm cgroup.
> 

No, it remains in cgroup. Otherwise systemd service management would
not be possible at all ...

> 4. systemd notes that gdm's cgroup is empty and decides that gdm is
> now successfully stopped.
> 

I looked at display-manager.service here and it sets KillMode=process.
That is better explanation to your observation.

> 5. systemctl returns and now Xorg is launched immediately. Xorg reads
> the PID of the old Xorg process from /tmp, and notices that that PID
> is still in use (it is still an unreaped zombie) because kill()
> doesn't return an error. Xorg aborts thinking that it is already
> running.
> 
> 6. Moments later, systemd reaps the zombie. Oops, too late.
> 
> 
> Does that make sense?
> I wonder how it is best to fix this. Is it a bug that systemd decided
> that gdm.service had stopped before it had reaped zombie processes
> that originally belonged to gdm?
> 
> Is it a gdm bug that killing gdm doesn't make any attempt to reap X
> before going away itself? (they chose PR_SET_PDEATHSIG to do something
> similar, but maybe we have to argue that it is not quite sufficient)
> 
> Thanks
> Daniel
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