4.4-stable review patch. If anyone has any objections, please let me know.
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From: Yuchung Cheng ycheng@google.com
[ Upstream commit b0c05d0e99d98d7f0cd41efc1eeec94efdc3325d ]
Previously, when a data segment was sent an ACK was piggybacked on the data segment without generating a CA_EVENT_NON_DELAYED_ACK event to notify congestion control modules. So the DCTCP ca->delayed_ack_reserved flag could incorrectly stay set when in fact there were no delayed ACKs being reserved. This could result in sending a special ECN notification ACK that carries an older ACK sequence, when in fact there was no need for such an ACK. DCTCP keeps track of the delayed ACK status with its own separate state ca->delayed_ack_reserved. Previously it may accidentally cancel the delayed ACK without updating this field upon sending a special ACK that carries a older ACK sequence. This inconsistency would lead to DCTCP receiver never acknowledging the latest data until the sender times out and retry in some cases.
Packetdrill script (provided by Larry Brakmo)
0.000 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3 0.000 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0 0.000 setsockopt(3, SOL_TCP, TCP_CONGESTION, "dctcp", 5) = 0 0.000 bind(3, ..., ...) = 0 0.000 listen(3, 1) = 0
0.100 < [ect0] SEW 0:0(0) win 32792 <mss 1000,sackOK,nop,nop,nop,wscale 7> 0.100 > SE. 0:0(0) ack 1 <mss 1460,nop,nop,sackOK,nop,wscale 8> 0.110 < [ect0] . 1:1(0) ack 1 win 257 0.200 accept(3, ..., ...) = 4
0.200 < [ect0] . 1:1001(1000) ack 1 win 257 0.200 > [ect01] . 1:1(0) ack 1001
0.200 write(4, ..., 1) = 1 0.200 > [ect01] P. 1:2(1) ack 1001
0.200 < [ect0] . 1001:2001(1000) ack 2 win 257 0.200 write(4, ..., 1) = 1 0.200 > [ect01] P. 2:3(1) ack 2001
0.200 < [ect0] . 2001:3001(1000) ack 3 win 257 0.200 < [ect0] . 3001:4001(1000) ack 3 win 257 0.200 > [ect01] . 3:3(0) ack 4001
0.210 < [ce] P. 4001:4501(500) ack 3 win 257
+0.001 read(4, ..., 4500) = 4500 +0 write(4, ..., 1) = 1 +0 > [ect01] PE. 3:4(1) ack 4501
+0.010 < [ect0] W. 4501:5501(1000) ack 4 win 257 // Previously the ACK sequence below would be 4501, causing a long RTO +0.040~+0.045 > [ect01] . 4:4(0) ack 5501 // delayed ack
+0.311 < [ect0] . 5501:6501(1000) ack 4 win 257 // More data +0 > [ect01] . 4:4(0) ack 6501 // now acks everything
+0.500 < F. 9501:9501(0) ack 4 win 257
Reported-by: Larry Brakmo brakmo@fb.com Signed-off-by: Yuchung Cheng ycheng@google.com Signed-off-by: Eric Dumazet edumazet@google.com Acked-by: Neal Cardwell ncardwell@google.com Acked-by: Lawrence Brakmo brakmo@fb.com Signed-off-by: David S. Miller davem@davemloft.net Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- net/ipv4/tcp_dctcp.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-)
--- a/net/ipv4/tcp_dctcp.c +++ b/net/ipv4/tcp_dctcp.c @@ -134,7 +134,8 @@ static void dctcp_ce_state_0_to_1(struct /* State has changed from CE=0 to CE=1 and delayed * ACK has not sent yet. */ - if (!ca->ce_state && ca->delayed_ack_reserved) { + if (!ca->ce_state && + inet_csk(sk)->icsk_ack.pending & ICSK_ACK_TIMER) { u32 tmp_rcv_nxt;
/* Save current rcv_nxt. */ @@ -164,7 +165,8 @@ static void dctcp_ce_state_1_to_0(struct /* State has changed from CE=1 to CE=0 and delayed * ACK has not sent yet. */ - if (ca->ce_state && ca->delayed_ack_reserved) { + if (ca->ce_state && + inet_csk(sk)->icsk_ack.pending & ICSK_ACK_TIMER) { u32 tmp_rcv_nxt;
/* Save current rcv_nxt. */