callback_entry
callback_register(struct callback_head *ch, struct callback_entry *ce,
void *obj, int (*fn)(struct callback_entry *, void *, void *))
callback_unregister(struct callback_head *ch, struct callback_entry *ce)
struct callback_entry *ce;
TAILQ_ENTRY(callback_entry) ce_q;
int (*ce_func)(struct callback_entry *, void *, void *);
TAILQ_HEAD(, callback_entry) ch_q;
struct callback_entry *ch_next;
void callback_register(struct callback_head *, struct callback_entry *,
void *, int (*)(struct callback_entry *, void *, void *));
void callback_unregister(struct callback_head *, struct callback_entry *);