3 files changed, 33 insertions, 6 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 38b6a811edf..c49a4fc175b 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5652,6 +5652,7 @@ static bool tcp_rcv_fastopen_synack(struct sock *sk, struct sk_buff *synack,
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct sk_buff *data = tcp_write_queue_head(sk);
 	u16 mss = tp->rx_opt.mss_clamp;
+	bool syn_drop;
 
 	if (mss == tp->rx_opt.user_mss) {
 		struct tcp_options_received opt;
@@ -5664,7 +5665,14 @@ static bool tcp_rcv_fastopen_synack(struct sock *sk, struct sk_buff *synack,
 		mss = opt.mss_clamp;
 	}
 
-	tcp_fastopen_cache_set(sk, mss, cookie);
+	/* The SYN-ACK neither has cookie nor acknowledges the data. Presumably
+	 * the remote receives only the retransmitted (regular) SYNs: either
+	 * the original SYN-data or the corresponding SYN-ACK is lost.
+	 */
+	syn_drop = (cookie->len <= 0 && data &&
+		    inet_csk(sk)->icsk_retransmits);
+
+	tcp_fastopen_cache_set(sk, mss, cookie, syn_drop);
 
 	if (data) { /* Retransmit unacked data in SYN */
 		tcp_retransmit_skb(sk, data);
diff --git a/net/ipv4/tcp_metrics.c b/net/ipv4/tcp_metrics.c
index d02ff377778..99779ae44f6 100644
--- a/net/ipv4/tcp_metrics.c
+++ b/net/ipv4/tcp_metrics.c
@@ -32,6 +32,8 @@ enum tcp_metric_index {
 
 struct tcp_fastopen_metrics {
 	u16	mss;
+	u16	syn_loss:10;		/* Recurring Fast Open SYN losses */
+	unsigned long	last_syn_loss;	/* Last Fast Open SYN loss */
 	struct	tcp_fastopen_cookie	cookie;
 };
 
@@ -125,6 +127,7 @@ static void tcpm_suck_dst(struct tcp_metrics_block *tm, struct dst_entry *dst)
 	tm->tcpm_ts = 0;
 	tm->tcpm_ts_stamp = 0;
 	tm->tcpm_fastopen.mss = 0;
+	tm->tcpm_fastopen.syn_loss = 0;
 	tm->tcpm_fastopen.cookie.len = 0;
 }
 
@@ -644,7 +647,8 @@ bool tcp_tw_remember_stamp(struct inet_timewait_sock *tw)
 static DEFINE_SEQLOCK(fastopen_seqlock);
 
 void tcp_fastopen_cache_get(struct sock *sk, u16 *mss,
-			    struct tcp_fastopen_cookie *cookie)
+			    struct tcp_fastopen_cookie *cookie,
+			    int *syn_loss, unsigned long *last_syn_loss)
 {
 	struct tcp_metrics_block *tm;
 
@@ -659,14 +663,15 @@ void tcp_fastopen_cache_get(struct sock *sk, u16 *mss,
 			if (tfom->mss)
 				*mss = tfom->mss;
 			*cookie = tfom->cookie;
+			*syn_loss = tfom->syn_loss;
+			*last_syn_loss = *syn_loss ? tfom->last_syn_loss : 0;
 		} while (read_seqretry(&fastopen_seqlock, seq));
 	}
 	rcu_read_unlock();
 }
 
-
 void tcp_fastopen_cache_set(struct sock *sk, u16 mss,
-			    struct tcp_fastopen_cookie *cookie)
+			    struct tcp_fastopen_cookie *cookie, bool syn_lost)
 {
 	struct tcp_metrics_block *tm;
 
@@ -679,6 +684,11 @@ void tcp_fastopen_cache_set(struct sock *sk, u16 mss,
 		tfom->mss = mss;
 		if (cookie->len > 0)
 			tfom->cookie = *cookie;
+		if (syn_lost) {
+			++tfom->syn_loss;
+			tfom->last_syn_loss = jiffies;
+		} else
+			tfom->syn_loss = 0;
 		write_sequnlock_bh(&fastopen_seqlock);
 	}
 	rcu_read_unlock();
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index 88693281da4..c5cfd5ec318 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -2860,10 +2860,19 @@ static int tcp_send_syn_data(struct sock *sk, struct sk_buff *syn)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct tcp_fastopen_request *fo = tp->fastopen_req;
-	int space, i, err = 0, iovlen = fo->data->msg_iovlen;
+	int syn_loss = 0, space, i, err = 0, iovlen = fo->data->msg_iovlen;
 	struct sk_buff *syn_data = NULL, *data;
+	unsigned long last_syn_loss = 0;
+
+	tcp_fastopen_cache_get(sk, &tp->rx_opt.mss_clamp, &fo->cookie,
+			       &syn_loss, &last_syn_loss);
+	/* Recurring FO SYN losses: revert to regular handshake temporarily */
+	if (syn_loss > 1 &&
+	    time_before(jiffies, last_syn_loss + (60*HZ << syn_loss))) {
+		fo->cookie.len = -1;
+		goto fallback;
+	}
 
-	tcp_fastopen_cache_get(sk, &tp->rx_opt.mss_clamp, &fo->cookie);
 	if (fo->cookie.len <= 0)
 		goto fallback;