Subject: WARN_ON in TLP causing RT throttling

From: [email-masked]
Date: Wed, 26 Sep 2018 17:46:27 -0600

Hi Eric,

Someone recently reported a crash to us on the 4.14.62 kernel where 
excessive
WARNING prints were spamming the logs and causing watchdog bites. The 
kernel
does have the following commit by Soheil:
bffd168c3fc5 "tcp: clear tp->packets_out when purging write queue"

Before this bug we see over 1 second of continuous WARN_ON prints from
tcp_send_loss_probe() like so:

7795.530450:   <2>  tcp_send_loss_probe+0x194/0x1b8
7795.534833:   <2>  tcp_write_timer_handler+0xf8/0x1c4
7795.539492:   <2>  tcp_write_timer+0x4c/0x74
7795.543348:   <2>  call_timer_fn+0xc0/0x1b4
7795.547113:   <2>  run_timer_softirq+0x248/0x81c

Specifically, the prints come from the following check:

	/* Retransmit last segment. */
	if (WARN_ON(!skb))
		goto rearm_timer;

Since skb is always NULL, we know there's nothing on the write queue or 
the
retransmit queue, so we just keep resetting the timer, waiting for more 
data
to be queued. However, we were able to determine that the TCP socket is 
in the
TCP_FIN_WAIT1 state, so we will no longer be sending any data and these 
queues
remain empty.

Would it be appropriate to stop resetting the TLP timer if we detect 
that the
connection is starting to close and we have no more data to send the 
probe with,
or is there some way that this scenario should already be handled?

Unfortunately, we don't have a reproducer for this crash.

Thanks,
Sean