Re: [PATCH net-next] tcp: better retrans tracking for defer-accept

From: Eric Dumazet <redacted>
Date: Sat, 27 Oct 2012 16:18:57 +0200

From: Eric Dumazet <redacted>

For passive TCP connections using TCP_DEFER_ACCEPT facility,
we incorrectly increment req->retrans each time timeout triggers
while no SYNACK is sent.

SYNACK are not sent for TCP_DEFER_ACCEPT that were established (for wich
we received the ACK from client). Only the last SYNACK is
sent so that we can receive again an ACK from client, to move the
req into accept queue. We plan to change this later to avoid
the useless retransmit (and potential problem as this SYNACK could be
lost)

TCP_INFO later gives wrong information to user, claiming imaginary
retransmits.

Decouple req->retrans field into two independent fields :

num_retrans : number of retransmit
num_timeout : number of timeouts

num_timeout is the counter that is incremented at each timeout,
regardless of actual SYNACK being sent or not, and used to
compute the exponential timeout.

Introduce inet_rtx_syn_ack() helper to increment num_retrans
only if ->rtx_syn_ack() succeeded.

Use inet_rtx_syn_ack() from tcp_check_req() to increment num_retrans
when we re-send a SYNACK in answer to a (retransmitted) SYN.
Prior to this patch, we were not counting these retransmits.

Change tcp_v[46]_rtx_synack() to increment TCP_MIB_RETRANSSEGS
only if a synack packet was successfully queued.

Reported-by: Yuchung Cheng <redacted>
Signed-off-by: Eric Dumazet <redacted>

Applied, thanks Eric.