[PATCH] drm/i915: Fix race condition in intel_dp_destroy_mst_connector()

[Daniel Vetter]

On Wed, Mar 16, 2016 at 09:39:49PM +0200, Ville Syrjälä wrote:
> On Wed, Mar 16, 2016 at 03:18:04PM -0400, Lyude wrote:
> > After unplugging a DP MST display from the system, we have to go through
> > and destroy all of the DRM connectors associated with it since none of
> > them are valid anymore. Unfortunately, intel_dp_destroy_mst_connector()
> > doesn't do a good enough job of ensuring that throughout the destruction
> > process that no modesettings can be done with the connectors. As it is
> > right now, intel_dp_destroy_mst_connector() works like this:
> > 
> > * Take all modeset locks
> > * Clear the configuration of the crtc on the connector, if there is one
> > * Drop all modeset locks, this is required because of circular
> >   dependency issues that arise with trying to remove the connector from
> >   sysfs with modeset locks held
> > * Unregister the connector
> > * Take all modeset locks, again
> > * Do the rest of the required cleaning for destroying the connector
> > * Finally drop all modeset locks for good
> 
> So pretty much what I suspected
> https://lists.freedesktop.org/archives/intel-gfx/2016-February/087734.html
> 
> > 
> > This only works sometimes. During the destruction process, it's very
> > possible that a userspace application will attempt to do a modesetting
> > using the connector. When we drop the modeset locks, an ioctl handler
> > such as drm_mode_setcrtc has the oppurtunity to take all of the modeset
> > locks from us. When this happens, one thing leads to another and
> > eventually we end up committing a mode with the non-existent connector:
> > 
> > 	[drm:intel_dp_link_training_clock_recovery [i915]] *ERROR* failed to enable link training
> > 	[drm:intel_dp_aux_ch] dp_aux_ch timeout status 0x7cf0001f
> > 	[drm:intel_dp_start_link_train [i915]] *ERROR* failed to start channel equalization
> > 	[drm:intel_dp_aux_ch] dp_aux_ch timeout status 0x7cf0001f
> > 	[drm:intel_mst_pre_enable_dp [i915]] *ERROR* failed to allocate vcpi
> > 
> > And in some cases, such as with the T460s using an MST dock, this
> > results in breaking modesetting and/or panicking the system.
> 
> Are these just kernel oopses etc.? If the hardware gets upset from
> modesetting when the sink is gone, well, then we still have a problem
> because the user can of course yank the cable while the modeset is already
> underway.
> 
> > 
> > To work around this, we now unregister the connector at the very
> > beginning of intel_dp_destroy_mst_connector(), grab all the modesetting
> > locks, and then hold them until we finish the rest of the function.
> > 
> > CC: stable at vger.kernel.org
> > Signed-off-by: Lyude <cpaul at redhat.com>
> > Signed-off-by: Rob Clark <rclark at redhat.com>
> 
> These sobs don't make much sense to me.
> 
> Patch itself does make sense to me, so 
> Reviewed-by: Ville Syrjälä <ville.syrjala at linux.intel.com>

Applied, thanks.
-Daniel

> 
> > ---
> >  drivers/gpu/drm/i915/intel_dp_mst.c | 6 ++----
> >  1 file changed, 2 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/intel_dp_mst.c b/drivers/gpu/drm/i915/intel_dp_mst.c
> > index fa0dabf..b21ac88 100644
> > --- a/drivers/gpu/drm/i915/intel_dp_mst.c
> > +++ b/drivers/gpu/drm/i915/intel_dp_mst.c
> > @@ -499,6 +499,8 @@ static void intel_dp_destroy_mst_connector(struct drm_dp_mst_topology_mgr *mgr,
> >  	struct intel_connector *intel_connector = to_intel_connector(connector);
> >  	struct drm_device *dev = connector->dev;
> >  
> > +	intel_connector->unregister(intel_connector);
> > +
> >  	/* need to nuke the connector */
> >  	drm_modeset_lock_all(dev);
> >  	if (connector->state->crtc) {
> > @@ -512,11 +514,7 @@ static void intel_dp_destroy_mst_connector(struct drm_dp_mst_topology_mgr *mgr,
> >  
> >  		WARN(ret, "Disabling mst crtc failed with %i\n", ret);
> >  	}
> > -	drm_modeset_unlock_all(dev);
> >  
> > -	intel_connector->unregister(intel_connector);
> > -
> > -	drm_modeset_lock_all(dev);
> >  	intel_connector_remove_from_fbdev(intel_connector);
> >  	drm_connector_cleanup(connector);
> >  	drm_modeset_unlock_all(dev);
> > -- 
> > 2.5.0
> > 
> > _______________________________________________
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> > dri-devel at lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/dri-devel
> 
> -- 
> Ville Syrjälä
> Intel OTC
> _______________________________________________
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> dri-devel at lists.freedesktop.org
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch