IMHO, it doesn't really make sense to mix and match between knlist_mtx_{,un,assert_}lock and other functions filling those roles, unless all three are replaced (e.g., by knlist_rw_{,un,assert_}lock), so it's a little strange that knlist_init() allows its function pointer parameters to be NULL or not, independently. Indeed, glancing at all the in-tree knlist_init() calls, it appears that only knlist_init_mtx() took knlist_init() up on its offer of NULL-signaled defaults. Instead, explicitly pass in the _mtx variants in knlist_init_mtx() and assert() against NULL values in knlist_init().