The problem is that kobject_add_internal() first adds a kobject to the
kset and then try to create sysfs directory for it. If the creation
fails, it remove the kobject from the kset. get_device_parent()
accesses class_dirs kset while only holding class_dirs.list_lock to
see whether the cuse class dir exists. But when it exists, it may not
have finished initialization yet or may fail and get removed soon. In
the above case, the former happened so the second one ends up trying
to create subdirectory under NULL sysfs_dirent.
Fix it by grabbing a mutex in get_device_parent().