Possible fb ref count issue with drm_plane_force_disable()

[Daniel Vetter]

On Thu, Apr 10, 2014 at 02:47:52PM +0300, Tomi Valkeinen wrote:
> Hi,
> 
> I've been debugging omapdrm issues on top of the latest drm mainline
> changes. Sometimes a drm_framebuffer ref count drops to -1 when aborting
> a drm application, or unloading the modules.
> 
> The setup is very basic, just a single crtc with the crtc's primary plane.
> 
> What seems to happen is:
> 
> - App is started
> 
> - fb is created, and taken into use by omapdrm. omapdrm takes a ref to
> the fb.
> 
> - the app is starts to shut down
> 
> - drm_framebuffer_remove is called
> 
> - fb->refcount.refcount > 1, so it goes to disable stuff
> 
> - drm_plane_force_disable is called for the primary plane
> 
> - drm_plane_force_disable does plane->disable_plane, which on omapdrm
> puts stuff on a workqueue as plane cannot be disabled immediately
> 
> - drm_plane_force_disable calls __drm_framebuffer_unreference()
> 
> - at the end of drm_framebuffer_remove(), there's
> drm_framebuffer_unreference, which causes ref count to go to zero, and
> the fb to be destroyed
> 
> - a bit later, the queued work is ran, which does
> drm_framebuffer_unreference(), and ref count goes to -1. Here omapdrm is
> removing the ref that had been taken in the beginning.
> 
> 
> So the explicit unref done by drm_plane_force_disable() seems a bit out
> of place. I can't figure out which drm_framebuffer_reference() would be
> the matching one for the unref done by drm_plane_force_disable().
> 
> Any ideas what ref is that? Or is the __drm_framebuffer_unreference()
> extra in drm_plane_force_disable()?

plane->fb is holding a reference which should have been acquired in
drm_mode_setplane. It's not really explicit since drm_framebuffer_lookup
implicitly grabs a ref for you.

Hence when we disable the plane and set plane->fb = NULL we must drop that
ref again. So that's not the bug, but clearly something goes wrong
somewhere.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch