There's a race condition between the list_del_init in the
v4l2_ctrl_request_complete, and the list_add_tail in the
v4l2_ctrl_request_queue, since they can be called in different thread
and the requests_queued list is not protected by a lock. This can lead
to that the v4l2_ctrl_handler is still in the requests_queued list while
the request_is_queued is already set to false, which would cause
use-after-free if the v4l2_ctrl_handler is later released.

Fix this by locking the ->lock of main_hdl (which is the owner of the
requests_queued list) when doing list operations on the
->requests_queued list.
Signed-off-by: NPi-Hsun Shih <pihsun@chromium.org>
Signed-off-by: NHans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: NMauro Carvalho Chehab <mchehab@kernel.org>