tcp: fix delayed acks behavior for SO_RCVLOWAT

We should not delay acks if there are not enough bytes
in receive queue to satisfy SO_RCVLOWAT.

Since [E]POLLIN event is not going to be generated, there is little
hope for a delayed ack to be useful.

In fact, delaying ACK prevents sender from completing
the transfer.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 5 insertions, 2 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 367def6ddeda..d854363a4387 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5026,9 +5026,12 @@ static void __tcp_ack_snd_check(struct sock *sk, int ofo_possible)
             /* More than one full frame received... */
         if (((tp->rcv_nxt - tp->rcv_wup) > inet_csk(sk)->icsk_ack.rcv_mss &&
              /* ... and right edge of window advances far enough.
-              * (tcp_recvmsg() will send ACK otherwise). Or...
+              * (tcp_recvmsg() will send ACK otherwise).
+              * If application uses SO_RCVLOWAT, we want send ack now if
+              * we have not received enough bytes to satisfy the condition.
               */
-             __tcp_select_window(sk) >= tp->rcv_wnd) ||
+            (tp->rcv_nxt - tp->copied_seq < sk->sk_rcvlowat ||
+             __tcp_select_window(sk) >= tp->rcv_wnd)) ||
             /* We ACK each frame or... */
             tcp_in_quickack_mode(sk) ||
             /* We have out of order data. */