Re: X hang with quirk VT switches

[Takashi Iwai <tiwai@suse.de>]

At Wed, 3 Dec 2014 18:31:45 +0000,
Chris Wilson wrote:
> 
> On Wed, Dec 03, 2014 at 03:45:35PM +0100, Takashi Iwai wrote:
> > Hi,
> > 
> > while checking the reported bug about VT switch hang on openSUSE 13.2,
> > I also could reproduce a similar issue as reported: namely, X hangs
> > when repeatedly switching VT quickly.
> > 
> > For example, running the following on KDE results in the stall of X.
> > 
> > 	% for i in $(seq 1 100); do chvt 1; chvt 7; done
> > 
> > Looking at the sysrq-t output, it stalls at drm_read().  And after
> > putting some debug prints at event handling codes, it shows like:
> > 
> >  drm_queue_vblank_event event_space=4064
> >  send_vblank_event event_space=4064
> >  drm_poll ENTER event_space=4064
> >  drm_poll mask=0x41 event_space=4064
> >  drm_poll ENTER event_space=4064
> >  drm_poll mask=0x41 event_space=4064
> >  drm_read ENTER event_space=4064
> >  drm_read total=32 event_space=4096
> >  drm_poll ENTER event_space=4096
> >  drm_poll mask=0x0 event_space=4096
> >  drm_read ENTER event_space=4096
> >  drm_read ENTER event_space=4096
> >  drm_read ENTER event_space=4096
> > 
> > So, after a vblank event, two poll calls succeeded, followed by one
> > drm_read().  After that, there were one poll call without event,
> > followed by three(!) drm_read() calls.  The last three drm_read()
> > never exited, thus X stalled.  So, this looks like a race or a
> > refcount issue somewhere.
> 
> The key question is how did you get 3 calls to drm_read that each didn't
> return?

I asked this to myself, too...

> The only place where we call drm_read without first doing a poll
> is in the WakeupHandler with the drm fd flagged for reads. This is
> broken in ZaphodHeads as the drm fd is not O_NONBLOCK without
> 
> commit bd008e5b2953186fc0c6633a885ade95e7043800
> Author: Chris Wilson <chris@chris-wilson.co.uk>
> Date:   Tue Oct 7 14:13:51 2014 +0100
> 
>     drm: Implement O_NONBLOCK support on /dev/dri/cardN
> 
> I assume that isn't the case as I expect you would have mentioned using
> ZaphodHeads.

Right, no extra option is used.  And, eDP output only.

> Maarten found a very similar issue using radeon:
> 
> https://bugs.freedesktop.org/show_bug.cgi?id=77885
> 
> where the supposition was that the DRM_MASTER was not being
> relinquished.

Interesting.  In the original bug report we've got, the symptom
happened at switching two X sessions, as the bfo entry above.
	http://bugzilla.opensuse.org/show_bug.cgi?id=904097

The reporter mentioned there that his issue was fixed with 3.17.x
kernel, so it's likely a different problem from what I've seen.
The bug couldn't be reproduced on my machine, so I tried the simple VT
quick switches, then hits a new bug now.

In anyway, I'll poke a bit more deeply later (maybe on Friday).


But, before that, I'd like to ask: can anyone reproduce this problem
at all...?


thanks,

Takashi
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