Patch "drm/i915: Fix race condition in intel_dp_destroy_mst_connector()" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

drm/i915: Fix race condition in intel_dp_destroy_mst_connector()

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
drm-i915-fix-race-condition-in-intel_dp_destroy_mst_connector.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Lyude <[email protected]>
Date: Wed, 16 Mar 2016 15:18:04 -0400
Subject: drm/i915: Fix race condition in intel_dp_destroy_mst_connector()
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From: Lyude <[email protected]>

commit 9e60290dbafdf577766e5fc5f2fdb3be450cf9a6 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.

---
 drivers/gpu/drm/i915/intel_dp_mst.c |    6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

@@ -477,6 +477,8 @@ static void intel_dp_destroy_mst_connect
 	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) {

@@ -490,11 +492,7 @@ static void intel_dp_destroy_mst_connect
 
 		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);

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/drm-i915-fix-race-condition-in-intel_dp_destroy_mst_connector.patch