[PATCH v11 12/74] drm/bridge: Fix unlocked list_del in drm_bridge_add()

When re-adding a bridge that was previously removed, drm_bridge_add()
drops it from bridge_lingering_list without holding bridge_lock.

Both bridge_list and bridge_lingering_list are protected by bridge_lock,
as they are concurrently modified by drm_bridge_remove() and
__drm_bridge_free(), and walked by the debugfs 'bridges' file.  Running
the list_empty() test and the list_del_init() outside of the lock may
therefore corrupt either list.

Perform both under bridge_lock.

Fixes: 17805a15d175 ("drm/bridge: add list of removed refcounted bridges")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://lore.kernel.org/all/20260723015004.1F5711F000E9@smtp.kernel.org/ (local)
Signed-off-by: Cristian Ciocaltea <redacted>
---
 drivers/gpu/drm/drm_bridge.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/gpu/drm/drm_bridge.c b/drivers/gpu/drm/drm_bridge.c
index afaae272347c..2c457ad74f3b 100644
--- a/drivers/gpu/drm/drm_bridge.c
+++ b/drivers/gpu/drm/drm_bridge.c

@@ -454,8 +454,10 @@ void drm_bridge_add(struct drm_bridge *bridge)
 	 * in bridge_lingering_list. Remove it or bridge_lingering_list will be
 	 * corrupted when adding this bridge to bridge_list below.
 	 */
+	mutex_lock(&bridge_lock);
 	if (!list_empty(&bridge->list))
 		list_del_init(&bridge->list);
+	mutex_unlock(&bridge_lock);
 
 	mutex_init(&bridge->hpd_state_mutex);
 	mutex_init(&bridge->hpd_mutex);

-- 
2.55.0