From: Neal Cardwell ncardwell@google.com
[ Upstream commit 3d2af9cce3133b3bc596a9d065c6f9d93419ccfb ]
This commit fixes a bug that can cause a TCP data sender to repeatedly defer RTOs when encountering SACK reneging.
The bug is that when we're in fast recovery in a scenario with SACK reneging, every time we get an ACK we call tcp_check_sack_reneging() and it can note the apparent SACK reneging and rearm the RTO timer for srtt/2 into the future. In some SACK reneging scenarios that can happen repeatedly until the receive window fills up, at which point the sender can't send any more, the ACKs stop arriving, and the RTO fires at srtt/2 after the last ACK. But that can take far too long (O(10 secs)), since the connection is stuck in fast recovery with a low cwnd that cannot grow beyond ssthresh, even if more bandwidth is available.
This fix changes the logic in tcp_check_sack_reneging() to only rearm the RTO timer if data is cumulatively ACKed, indicating forward progress. This avoids this kind of nearly infinite loop of RTO timer re-arming. In addition, this meets the goals of tcp_check_sack_reneging() in handling Windows TCP behavior that looks temporarily like SACK reneging but is not really.
Many thanks to Jakub Kicinski and Neil Spring, who reported this issue and provided critical packet traces that enabled root-causing this issue. Also, many thanks to Jakub Kicinski for testing this fix.
Fixes: 5ae344c949e7 ("tcp: reduce spurious retransmits due to transient SACK reneging") Reported-by: Jakub Kicinski kuba@kernel.org Reported-by: Neil Spring ntspring@fb.com Signed-off-by: Neal Cardwell ncardwell@google.com Reviewed-by: Eric Dumazet edumazet@google.com Cc: Yuchung Cheng ycheng@google.com Tested-by: Jakub Kicinski kuba@kernel.org Link: https://lore.kernel.org/r/20221021170821.1093930-1-ncardwell.kernel@gmail.co... Signed-off-by: Jakub Kicinski kuba@kernel.org Signed-off-by: Sasha Levin sashal@kernel.org --- net/ipv4/tcp_input.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index 079f150e480d..11716780667c 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -2035,7 +2035,8 @@ void tcp_enter_loss(struct sock *sk) */ static bool tcp_check_sack_reneging(struct sock *sk, int flag) { - if (flag & FLAG_SACK_RENEGING) { + if (flag & FLAG_SACK_RENEGING && + flag & FLAG_SND_UNA_ADVANCED) { struct tcp_sock *tp = tcp_sk(sk); unsigned long delay = max(usecs_to_jiffies(tp->srtt_us >> 4), msecs_to_jiffies(10));