tcp: fix TCP socket rehash stats mis-accounting

The previous commit 32efcc06d2a1 ("tcp: export count for rehash attempts")
would mis-account rehashing SNMP and socket stats:

a. During handshake of an active open, only counts the first
SYN timeout

b. After handshake of passive and active open, stop updating
after (roughly) TCP_RETRIES1 recurring RTOs

c. After the socket aborts, over count timeout_rehash by 1

This patch fixes this by checking the rehash result from sk_rethink_txhash.

Fixes: 32efcc06d2a1 ("tcp: export count for rehash attempts")
Signed-off-by: Yuchung Cheng <[e-mail]>
Signed-off-by: Eric Dumazet <[e-mail]>
Signed-off-by: Neal Cardwell <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

include/net/sock.h

···
 	sk->sk_txhash = net_tx_rndhash();
 }
 
-static inline void sk_rethink_txhash(struct sock *sk)
+static inline bool sk_rethink_txhash(struct sock *sk)
 {
-	if (sk->sk_txhash)
+	if (sk->sk_txhash) {
 		sk_set_txhash(sk);
+		return true;
+	}
+	return false;
 }
 
 static inline struct dst_entry *
···
 	return dst;
 }
 
-static inline void dst_negative_advice(struct sock *sk)
+static inline void __dst_negative_advice(struct sock *sk)
 {
 	struct dst_entry *ndst, *dst = __sk_dst_get(sk);
-
-	sk_rethink_txhash(sk);
 
 	if (dst && dst->ops->negative_advice) {
 		ndst = dst->ops->negative_advice(dst);
···
 			sk->sk_dst_pending_confirm = 0;
 		}
 	}
+}
+
+static inline void dst_negative_advice(struct sock *sk)
+{
+	sk_rethink_txhash(sk);
+	__dst_negative_advice(sk);
 }
 
 static inline void

net/ipv4/tcp_input.c

···
 	 * The receiver remembers and reflects via DSACKs. Leverage the
 	 * DSACK state and change the txhash to re-route speculatively.
 	 */
-	if (TCP_SKB_CB(skb)->seq == tcp_sk(sk)->duplicate_sack[0].start_seq) {
-		sk_rethink_txhash(sk);
+	if (TCP_SKB_CB(skb)->seq == tcp_sk(sk)->duplicate_sack[0].start_seq &&
+	    sk_rethink_txhash(sk))
 		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPDUPLICATEDATAREHASH);
-	}
 }
 
 static void tcp_send_dupack(struct sock *sk, const struct sk_buff *skb)

net/ipv4/tcp_timer.c

···
 	int retry_until;
 
 	if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV)) {
-		if (icsk->icsk_retransmits) {
-			dst_negative_advice(sk);
-		} else {
-			sk_rethink_txhash(sk);
-			tp->timeout_rehash++;
-			__NET_INC_STATS(sock_net(sk),
-					LINUX_MIB_TCPTIMEOUTREHASH);
-		}
+		if (icsk->icsk_retransmits)
+			__dst_negative_advice(sk);
 		retry_until = icsk->icsk_syn_retries ? : net->ipv4.sysctl_tcp_syn_retries;
 		expired = icsk->icsk_retransmits >= retry_until;
 	} else {
···
 			/* Black hole detection */
 			tcp_mtu_probing(icsk, sk);
 
-			dst_negative_advice(sk);
-		} else {
-			sk_rethink_txhash(sk);
-			tp->timeout_rehash++;
-			__NET_INC_STATS(sock_net(sk),
-					LINUX_MIB_TCPTIMEOUTREHASH);
+			__dst_negative_advice(sk);
 		}
 
 		retry_until = net->ipv4.sysctl_tcp_retries2;
···
 		/* Has it gone just too far? */
 		tcp_write_err(sk);
 		return 1;
+	}
+
+	if (sk_rethink_txhash(sk)) {
+		tp->timeout_rehash++;
+		__NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPTIMEOUTREHASH);
 	}
 
 	return 0;