[PATCH v3 net-next 3/4] tcp: make tcp_space() aware of socket backlog

[Eric Dumazet <edumazet@google.com>]

Jean-Louis Dupond reported poor iscsi TCP receive performance
that we tracked to backlog drops.

Apparently we fail to send window updates reflecting the
fact that we are under stress.

Note that we might lack a proper window increase when
backlog is fully processed, since __release_sock() clears
sk->sk_backlog.len _after_ all skbs have been processed.

This should not matter in practice. If we had a significant
load through socket backlog, we are in a dangerous
situation.

Reported-by: Jean-Louis Dupond <jean-louis@dupond.be>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Yuchung Cheng <ycheng@google.com>
---
 include/net/tcp.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 0c61bf0a06dac95268c26b6302a2afbaef4c88b3..3b522259da7d5a54d7d3730ddd8d8c9ef24313e1 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1368,7 +1368,7 @@ static inline int tcp_win_from_space(const struct sock *sk, int space)
 /* Note: caller must be prepared to deal with negative returns */
 static inline int tcp_space(const struct sock *sk)
 {
-	return tcp_win_from_space(sk, sk->sk_rcvbuf -
+	return tcp_win_from_space(sk, sk->sk_rcvbuf - sk->sk_backlog.len -
 				  atomic_read(&sk->sk_rmem_alloc));
 }
 
-- 
2.20.0.rc0.387.gc7a69e6b6c-goog