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@xxxxxxxxxxxxxxxxxx>
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
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