drm: fix mutex leak in drm_dp_get_mst_branch_device

In Linux 4.3-rc5, there is an error case in drm_dp_get_branch_device
that returns without releasing mgr->lock, resulting a spew of kernel
messages about a kernel work function possibly having leaked a mutex
and presumably more serious adverse consequences later.  This patch
changes the error to "goto out" to unlock the mutex before returning.

[airlied: grabbed from drm-next as it fixes something we've seen]

Signed-off-by: Adam J. Richter <[email protected]>
Cc: [email protected]
Signed-off-by: Daniel Vetter <[email protected]>
Signed-off-by: Dave Airlie <[email protected]>

1 files changed, 4 insertions, 3 deletions

diff --git a/drivers/gpu/drm/drm_dp_mst_topology.c b/drivers/gpu/drm/drm_dp_mst_topology.c
index 5bca390d9ae2..809959d56d78 100644
--- a/drivers/gpu/drm/drm_dp_mst_topology.c
+++ b/drivers/gpu/drm/drm_dp_mst_topology.c
@@ -1194,17 +1194,18 @@ static struct drm_dp_mst_branch *drm_dp_get_mst_branch_device(struct drm_dp_mst_
 
 		list_for_each_entry(port, &mstb->ports, next) {
 			if (port->port_num == port_num) {
-				if (!port->mstb) {
+				mstb = port->mstb;
+				if (!mstb) {
 					DRM_ERROR("failed to lookup MSTB with lct %d, rad %02x\n", lct, rad[0]);
-					return NULL;
+					goto out;
 				}
 
-				mstb = port->mstb;
 				break;
 			}
 		}
 	}
 	kref_get(&mstb->kref);
+out:
 	mutex_unlock(&mgr->lock);
 	return mstb;
 }