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Date: Fri, 22 Dec 2017 16:28:07 -0200
From: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
To: Dmitry Vyukov <dvyukov@google.com>
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Subject: Re: INFO: task hung in bpf_exit_net
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On Fri, Dec 22, 2017 at 11:58:08AM +0100, Dmitry Vyukov wrote:
...
> > Same with this one, perhaps related to / fixed by:
> >     http://patchwork.ozlabs.org/patch/850957/
> >
> 
> 
> 
> Looking at the log, this one seems to be an infinite loop in SCTP code
> with console output in it. Kernel is busy printing gazilion of:
> 
> [  176.491099] sctp: sctp_transport_update_pmtu: Reported pmtu 508 too
> low, using default minimum of 512
> ** 110 printk messages dropped **
> [  176.503409] sctp: sctp_transport_update_pmtu: Reported pmtu 508 too
> low, using default minimum of 512
> ** 103 printk messages dropped **
> ...
> [  246.742374] sctp: sctp_transport_update_pmtu: Reported pmtu 508 too
> low, using default minimum of 512
> [  246.742484] sctp: sctp_transport_update_pmtu: Reported pmtu 508 too
> low, using default minimum of 512
> [  246.742590] sctp: sctp_transport_update_pmtu: Reported pmtu 508 too
> low, using default minimum of 512
> 
> Looks like a different issue.
> 

Oh. I guess this is caused by the interface having a MTU smaller than
SCTP_DEFAULT_MINSEGMENT (512), as the icmp frag needed handler
(sctp_icmp_frag_needed) will trigger an instant retransmission.
But as the MTU is smaller, SCTP won't update it, but will issue the
retransmission anyway.

I will test this soon. Should be fairly easy to trigger it.

  Marcelo