[Intel-gfx] X hang with quirk VT switches

[Chris Wilson]

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? 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 at 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.

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.
-Chris

-- 
Chris Wilson, Intel Open Source Technology Centre