keepalive_probes
u8 keepalive_probes; /* num of allowed keep alive probes */
val = READ_ONCE(tp->keepalive_probes);
WRITE_ONCE(tcp_sk(sk)->keepalive_probes, val);
val = keepalive_probes(tp);
icsk->icsk_probes_out >= keepalive_probes(tp))) {