[PATCH] drm/atomic-helper: Grab connection_mutex while duplicating state

[Matt Roper]

Callers of drm_atomic_helper_duplicate_state() may not have grabbed
locks before calling this function.  We're not supposed to iterate over
connectors without holding connection_mutex (since MST allows new
connectors to be spawned at hotplug), so make sure we grab that
ourselves before invoking drm_for_each_connector().  Failure to grab
this lock would cause us to stumble over the assertion added in commit:

        commit 7a3f3d6667f5f9ffd1517f6b21d64bbf5312042c
        Author: Daniel Vetter <daniel.vetter at ffwll.ch>
        Date:   Thu Jul 9 23:44:28 2015 +0200

            drm: Check locking in drm_for_each_connector

Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper at intel.com>
---
 drivers/gpu/drm/drm_atomic_helper.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/gpu/drm/drm_atomic_helper.c b/drivers/gpu/drm/drm_atomic_helper.c
index 3731a26..e5d0b21 100644
--- a/drivers/gpu/drm/drm_atomic_helper.c
+++ b/drivers/gpu/drm/drm_atomic_helper.c
@@ -2478,6 +2478,10 @@ drm_atomic_helper_duplicate_state(struct drm_device *dev,
 		}
 	}
 
+	err = drm_modeset_lock(&dev->mode_config.connection_mutex, ctx);
+	if (err)
+		goto free;
+
 	drm_for_each_connector(conn, dev) {
 		struct drm_connector_state *conn_state;
 
-- 
2.1.4