[Bug 252958] [tcp] Kernel panic in tcp_prr_partialack()

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=252958

--- Comment #4 from Richard Scheffenegger <rscheff at freebsd.org> ---
I'm not sure how to end up with a PRR_partialack, without recover_fs being
initialized.

Potentially with ACK reordering (unlikely), or spurious RTO rollback (where
TF_FASTRECOVERY may be set, but recover_fs was cleared already.

Do you observe non-zero "data packets unnecessarily retransmitted" in 
the output of netstat -snp tcp?

https://reviews.freebsd.org/D28326 has a patch to fix the div/0 in that section
of code, although knowing explicitly the system ends up going there would
be interesting.

If the frequence of the above counter incrementing (and no more panics with
that patch) matches the typical runtime when it happend, it's like to have to
do with RTO rollbacks.

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