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

[Ville Syrjälä]

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>

> ---
>  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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-- 
Ville Syrjälä
Intel OTC