tcp: introduce tcp_rto_delta_us() helper for xmit timer fix

Pure refactor. This helper will be required in the xmit timer fix
later in the patch series. (Because the TLP logic will want to make
this calculation.)

Fixes: 6ba8a3b19e ("tcp: Tail loss probe (TLP)")
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Nandita Dukkipati <nanditad@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/tcp.h

@@ -1916,6 +1916,16 @@ extern void tcp_rack_advance(struct tcp_sock *tp, u8 sacked, u32 end_seq,
 			     u64 xmit_time);
 extern void tcp_rack_reo_timeout(struct sock *sk);
 
+/* At how many usecs into the future should the RTO fire? */
+static inline s64 tcp_rto_delta_us(const struct sock *sk)
+{
+	const struct sk_buff *skb = tcp_write_queue_head(sk);
+	u32 rto = inet_csk(sk)->icsk_rto;
+	u64 rto_time_stamp_us = skb->skb_mstamp + jiffies_to_usecs(rto);
+
+	return rto_time_stamp_us - tcp_sk(sk)->tcp_mstamp;
+}
+
 /*
  * Save and compile IPv4 options, return a pointer to it
  */

net/ipv4/tcp_input.c

@@ -3004,10 +3004,7 @@ void tcp_rearm_rto(struct sock *sk)
 		/* Offset the time elapsed after installing regular RTO */
 		if (icsk->icsk_pending == ICSK_TIME_REO_TIMEOUT ||
 		    icsk->icsk_pending == ICSK_TIME_LOSS_PROBE) {
-			struct sk_buff *skb = tcp_write_queue_head(sk);
-			u64 rto_time_stamp = skb->skb_mstamp +
-					     jiffies_to_usecs(rto);
-			s64 delta_us = rto_time_stamp - tp->tcp_mstamp;
+			s64 delta_us = tcp_rto_delta_us(sk);
 			/* delta_us may not be positive if the socket is locked
 			 * when the retrans timer fires and is rescheduled.
 			 */