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Date:   Sat, 31 Oct 2020 12:35:19 +0100
From:   Greg Kroah-Hartman <>
Cc:     Greg Kroah-Hartman <>,, Arjun Roy <>,
        Soheil Hassas Yeganeh <>,
        Neal Cardwell <>,
        Eric Dumazet <>,
        Jakub Kicinski <>
Subject: [PATCH 5.4 23/49] tcp: Prevent low rmem stalls with SO_RCVLOWAT.

From: Arjun Roy <>

[ Upstream commit 435ccfa894e35e3d4a1799e6ac030e48a7b69ef5 ]

With SO_RCVLOWAT, under memory pressure,
it is possible to enter a state where:

1. We have not received enough bytes to satisfy SO_RCVLOWAT.
2. We have not entered buffer pressure (see tcp_rmem_pressure()).
3. But, we do not have enough buffer space to accept more packets.

In this case, we advertise 0 rwnd (due to #3) but the application does
not drain the receive queue (no wakeup because of #1 and #2) so the
flow stalls.

Modify the heuristic for SO_RCVLOWAT so that, if we are advertising
rwnd<=rcv_mss, force a wakeup to prevent a stall.

Without this patch, setting tcp_rmem to 6143 and disabling TCP
autotune causes a stalled flow. With this patch, no stall occurs. This
is with RPC-style traffic with large messages.

Fixes: 03f45c883c6f ("tcp: avoid extra wakeups for SO_RCVLOWAT users")
Signed-off-by: Arjun Roy <>
Acked-by: Soheil Hassas Yeganeh <>
Acked-by: Neal Cardwell <>
Signed-off-by: Eric Dumazet <>
Signed-off-by: Jakub Kicinski <>
Signed-off-by: Greg Kroah-Hartman <>
 net/ipv4/tcp.c       |    2 ++
 net/ipv4/tcp_input.c |    3 ++-
 2 files changed, 4 insertions(+), 1 deletion(-)

--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -484,6 +484,8 @@ static inline bool tcp_stream_is_readabl
 			return true;
 		if (tcp_rmem_pressure(sk))
 			return true;
+		if (tcp_receive_window(tp) <= inet_csk(sk)->icsk_ack.rcv_mss)
+			return true;
 	if (sk->sk_prot->stream_memory_read)
 		return sk->sk_prot->stream_memory_read(sk);
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -4774,7 +4774,8 @@ void tcp_data_ready(struct sock *sk)
 	int avail = tp->rcv_nxt - tp->copied_seq;
 	if (avail < sk->sk_rcvlowat && !tcp_rmem_pressure(sk) &&
-	    !sock_flag(sk, SOCK_DONE))
+	    !sock_flag(sk, SOCK_DONE) &&
+	    tcp_receive_window(tp) > inet_csk(sk)->icsk_ack.rcv_mss)

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