netdev - Warnings generated from tcp_sacktag_write

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Date:   Tue, 25 Jun 2019 18:37:07 +0530
From:   Chinmay Agarwal <chinagar@...eaurora.org>
To:     netdev@...r.kernel.org
Cc:     sharathv@...eaurora.org, kapandey@...eaurora.org
Subject: Warnings generated from tcp_sacktag_write_queue.

Dear All,

We are hitting the following WARN_ON condition:

	WARN_ON((int)tcp_packets_in_flight(tp) < 0);

	tcp_packets_in_flight =  packets_out –( lost_out +
	sacked_out ) + retrans_out  (This value is coming -ve)

The tcp socket being used is in fin_wait_1 state.
The values for variables just before the crash:
packets_out = 0,
retrans_out = 28,
lost_out = 38,
sacked_out = 8

The only place I can find the packets_out value being set as 0 is:

void tcp_write_queue_purge(struct sock *sk)
{
...

	tcp_sk(sk)->packets_out = 0;
        inet_csk(sk)->icsk_backoff = 0;
}

Is there some code flow where packets_out can be set to 0 and other
values can remain unchanged?
If not, is there some scenario which may lead to "tcp_write_queue_purge"
called and not followed up by "tcp_clear_retrans"?

According to my understanding we should call "tcp_clear_retrans" after
setting packets_out to 0.

[ 1950.556150] Call trace:
[ 1950.558689] tcp_sacktag_write_queue+0x704/0x72c
[ 1950.561313] init: Untracked pid 10745 exited with status 0
[ 1950.563441] tcp_ack+0x3a4/0xd40
[ 1950.563447] tcp_rcv_state_process+0x1e8/0xbbc
[ 1950.563457] tcp_v4_do_rcv+0x18c/0x1cc
[ 1950.563461] tcp_v4_rcv+0x84c/0x8a8
[ 1950.563471] ip_protocol_deliver_rcu+0xdc/0x190
[ 1950.563474] ip_local_deliver_finish+0x64/0x80
[ 1950.563479] ip_local_deliver+0xc4/0xf8
[ 1950.563482] ip_rcv_finish+0x214/0x2e0
[ 1950.563486] ip_rcv+0x2fc/0x39c
[ 1950.563496] __netif_receive_skb_core+0x698/0x84c
[ 1950.563499] __netif_receive_skb+0x3c/0x7c
[ 1950.563503] process_backlog+0x98/0x148
[ 1950.563506] net_rx_action+0x128/0x388
[ 1950.563519] __do_softirq+0x20c/0x3f0
[ 1950.563528] irq_exit+0x9c/0xa8
[ 1950.563536] handle_IPI+0x174/0x278
[ 1950.563540] gic_handle_irq+0x124/0x1c0
[ 1950.563544] el1_irq+0xb4/0x12c
[ 1950.563556] lpm_cpuidle_enter+0x3f4/0x430
[ 1950.563561] cpuidle_enter_state+0x124/0x25c
[ 1950.563565] cpuidle_enter+0x30/0x40
[ 1950.563575] call_cpuidle+0x3c/0x60
[ 1950.563579] do_idle+0x190/0x228
[ 1950.563583] cpu_startup_entry+0x24/0x28
[ 1950.563588] secondary_start_kernel+0x12c/0x138

Thanks and Regards
Chinmay Agarwal