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net-tcp_bbr: v2: adjust skb tx.in_flight upon merge in tcp_shifted_skb()
When tcp_shifted_skb() updates state as adjacent SACKed skbs are coalesced, previously the tx.in_flight was not adjusted, so we could get contradictory state where the skb's recorded pcount was bigger than the tx.in_flight (the number of segments that were in_flight after sending the skb). Normally have a SACKed skb with contradictory pcount/tx.in_flight would not matter. However, with SACK reneging, the SACKed bit is removed, and an skb once again becomes eligible for retransmitting, fragmenting, SACKing, etc. Packetdrill testing verified the following sequence is possible in a kernel that does not have this commit: - skb N is SACKed - skb N+1 is SACKed and combined with skb N using tcp_shifted_skb() - tcp_shifted_skb() will increase the pcount of prev, but leave tx.in_flight as-is - so prev skb can have pcount > tx.in_flight - RTO, tcp_timeout_mark_lost(), detect reneg, remove "SACKed" bit, mark skb N as lost - find pcount of skb N is greater than its tx.in_flight I suspect this issue iw what caused the bbr2_inflight_hi_from_lost_skb(): WARN_ON_ONCE(inflight_prev < 0) to fire in production machines using bbr2. Effort: net-tcp_bbr Origin-9xx-SHA1: 1a3e997e613d2dcf32b947992882854ebe873715 Change-Id: I1b0b75c27519953430c7db51c6f358f104c7af55 Signed-off-by: Alexandre Frade <kernel@xanmod.org>
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