[PATCH net 1/3] tcp: introduce tcp_rto_delta_us() helper for xmit timer fix

[Neal Cardwell <ncardwell@google.com>]

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: 6ba8a3b19e76 ("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>
---
 include/net/tcp.h    | 10 ++++++++++
 net/ipv4/tcp_input.c |  5 +----
 2 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 70483296157f..ada65e767b28 100644
--- a/include/net/tcp.h
+++ b/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
  */
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 2920e0cb09f8..345febf0a46e 100644
--- a/net/ipv4/tcp_input.c
+++ b/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.
 			 */
-- 
2.14.0.rc0.400.g1c36432dff-goog