Man page 9 : sk_wait_data

sk_wait_data - wait for data to arrive at sk_receive_queue

int sk_wait_data(struct sock *  sk , long *  timeo );

sk
    sock to wait on

timeo
    for how long

Now socket state including sk->sk_err is changed only under lock, hence we may omit checks after joining wait queue. We check receive queue before schedule only as optimization; it is very likely that release_sock added new data.