callback_head
struct callback_head *rcu;
STAILQ_HEAD(, callback_head) tmp_head;
rcu = container_of(node, struct callback_head, node);
struct callback_head *rcu;
rcu = (struct callback_head *)context;
STAILQ_ENTRY(callback_head) entry;
CTASSERT(sizeof(struct rcu_head) == sizeof(struct callback_head));