linux-kernel - [PATCH 4.2.y-ckt 002/206] drm/i915: Fix race condition in intel_dp_destroy_mst

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Message-Id: <1465507015-23052-3-git-send-email-kamal@canonical.com>
Date:	Thu,  9 Jun 2016 14:13:31 -0700
From:	Kamal Mostafa <kamal@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	Lyude <cpaul@...hat.com>, Rob Clark <rclark@...hat.com>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	Kamal Mostafa <kamal@...onical.com>
Subject: [PATCH 4.2.y-ckt 002/206] drm/i915: Fix race condition in intel_dp_destroy_mst_connector()

4.2.8-ckt12 -stable review patch.  If anyone has any objections, please let me know.

---8<------------------------------------------------------------

From: Lyude <cpaul@...hat.com>

commit 1f7717552ef1306be3b7ed28c66c6eff550e3a23 upstream.

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

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.

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.

Signed-off-by: Lyude <cpaul@...hat.com>
Signed-off-by: Rob Clark <rclark@...hat.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@...ux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@...ll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1458155884-13877-1-git-send-email-cpaul@redhat.com
[ kamal: backport to 4.2-stable: context ]
Signed-off-by: Kamal Mostafa <kamal@...onical.com>
---
 drivers/gpu/drm/i915/intel_dp_mst.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/i915/intel_dp_mst.c b/drivers/gpu/drm/i915/intel_dp_mst.c
index 8c127201..7feec0f 100644
--- a/drivers/gpu/drm/i915/intel_dp_mst.c
+++ b/drivers/gpu/drm/i915/intel_dp_mst.c
@@ -470,14 +470,13 @@ 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;
-	/* need to nuke the connector */
-	mutex_lock(&dev->mode_config.mutex);
-	intel_connector_dpms(connector, DRM_MODE_DPMS_OFF);
-	mutex_unlock(&dev->mode_config.mutex);

 	intel_connector->unregister(intel_connector);

+	/* need to nuke the connector */
 	mutex_lock(&dev->mode_config.mutex);
+	intel_connector_dpms(connector, DRM_MODE_DPMS_OFF);
+
 	intel_connector_remove_from_fbdev(intel_connector);
 	drm_connector_cleanup(connector);
 	mutex_unlock(&dev->mode_config.mutex);
-- 
2.7.4