3 files changed, 34 insertions, 1 deletions

diff --git a/net/ipv4/sysctl_net_ipv4.c b/net/ipv4/sysctl_net_ipv4.c
index 7e3712ce3994..e6a2460587d4 100644
--- a/net/ipv4/sysctl_net_ipv4.c
+++ b/net/ipv4/sysctl_net_ipv4.c
@@ -576,6 +576,13 @@ static struct ctl_table ipv4_table[] = {
                 .proc_handler   = proc_dointvec
         },
         {
+                .procname       = "tcp_thin_linear_timeouts",
+                .data           = &sysctl_tcp_thin_linear_timeouts,
+                .maxlen         = sizeof(int),
+                .mode           = 0644,
+                .proc_handler   = proc_dointvec
+        },
+        {
                 .procname       = "udp_mem",
                 .data           = &sysctl_udp_mem,
                 .maxlen         = sizeof(sysctl_udp_mem),
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index e471d037fcc9..21bae9afefea 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2229,6 +2229,13 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
                 }
                 break;
 
+        case TCP_THIN_LINEAR_TIMEOUTS:
+                if (val < 0 || val > 1)
+                        err = -EINVAL;
+                else
+                        tp->thin_lto = val;
+                break;
+
         case TCP_CORK:
                 /* When set indicates to always queue non-full frames.
                  * Later the user clears this option and we transmit
diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index de7d1bf9114f..a17629b8912e 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -29,6 +29,7 @@ int sysctl_tcp_keepalive_intvl __read_mostly = TCP_KEEPALIVE_INTVL;
 int sysctl_tcp_retries1 __read_mostly = TCP_RETR1;
 int sysctl_tcp_retries2 __read_mostly = TCP_RETR2;
 int sysctl_tcp_orphan_retries __read_mostly;
+int sysctl_tcp_thin_linear_timeouts __read_mostly;
 
 static void tcp_write_timer(unsigned long);
 static void tcp_delack_timer(unsigned long);
@@ -415,7 +416,25 @@ void tcp_retransmit_timer(struct sock *sk)
         icsk->icsk_retransmits++;
 
 out_reset_timer:
-        icsk->icsk_rto = min(icsk->icsk_rto << 1, TCP_RTO_MAX);
+        /* If stream is thin, use linear timeouts. Since 'icsk_backoff' is
+         * used to reset timer, set to 0. Recalculate 'icsk_rto' as this
+         * might be increased if the stream oscillates between thin and thick,
+         * thus the old value might already be too high compared to the value
+         * set by 'tcp_set_rto' in tcp_input.c which resets the rto without
+         * backoff. Limit to TCP_THIN_LINEAR_RETRIES before initiating
+         * exponential backoff behaviour to avoid continue hammering
+         * linear-timeout retransmissions into a black hole
+         */
+        if (sk->sk_state == TCP_ESTABLISHED &&
+            (tp->thin_lto || sysctl_tcp_thin_linear_timeouts) &&
+            tcp_stream_is_thin(tp) &&
+            icsk->icsk_retransmits <= TCP_THIN_LINEAR_RETRIES) {
+                icsk->icsk_backoff = 0;
+                icsk->icsk_rto = min(__tcp_set_rto(tp), TCP_RTO_MAX);
+        } else {
+                /* Use normal (exponential) backoff */
+                icsk->icsk_rto = min(icsk->icsk_rto << 1, TCP_RTO_MAX);
+        }
         inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, icsk->icsk_rto, TCP_RTO_MAX);
         if (retransmits_timed_out(sk, sysctl_tcp_retries1 + 1))
                 __sk_dst_reset(sk);