Bug 178079

Summary:	[tcp] Switching TCP CC algorithm panics on sparc64 with a complaint from the MMU
Product:	Base System	Reporter:	jau
Component:	kern	Assignee:	freebsd-net (Nobody) <net>
Status:	Closed FIXED
Severity:	Affects Only Me	CC:	mmoll
Priority:	Normal
Version:	9.1-STABLE
Hardware:	Any
OS:	Any

Description jau 2013-04-23 12:10:01 UTC

An attempt to switch the TCP congestion control algorithm on a sparc64 system
causes an instant panic.
The message shown on the console mentions something about NewReno and then
something about the MMU. Though I do not have the exact text shown to quote
here, it looked to me quite a lot like one of those MMU complaints one gets
for improperly aligned data.

Notice:
I have never seen anything remotely similar happening on an amd64.
So, it is supposedly impossible to replicate the problem using a CPU
which does not care about the data alignment.

Fix: 

Not the foggiest idea yet.

I will keep a crash dump around for a while just in case someone wishes to have
a chance to see it. I cannot promise to save it for very long, though.

OTOH with the consistency I have been getting new crash dumps when touching
the CC algorithm on sparc64 it should be easy enough for anyone to generate
ones own crash dump if needs be. ;-)
How-To-Repeat: On a sparc64 system follow these steps:

First make sure you have the alternate CC modules loaded...

kldload /boot/kernel/cc_*.ko

Then try this...

sysctl net.inet.tcp.cc.algorithm=htcp

At least in my case this has been a guaranteed trick to panic
the system with a complaint from the MMU.

Comment 1 jau 2013-04-23 14:48:54 UTC

In a sudden burst of inspiration I decided to check the same thing
on PowerPC as well...

# sysctl net.inet.tcp.cc.algorithm=htcp
net.inet.tcp.cc.algorithm: newreno
sysctl: net.inet.tcp.cc.algorithm=htcp: No such process

If I replace htcp with e.g. cubic the result is quite analogous.
So, the CC code does not seem all that healthy on PowerPC either.

This PowerPC test was done using a 3 week old 9.1-STABLE...

FreeBSD yggdrasil 9.1-STABLE FreeBSD 9.1-STABLE #0 r248939M: Mon Apr  1
20:22:53 EEST 2013     root@yggdrasil:/usr/obj/usr/src/sys/GENERIC  powerpc


--jau

Comment 2 Mark Linimon freebsd_committer

2013-05-20 04:30:59 UTC

Responsible Changed
From-To: freebsd-bugs->freebsd-net

Over to maintainer(s).

Comment 3 Michael Moll freebsd_committer

2015-11-30 22:26:07 UTC

not reproducible anymore with a fresh -CURRENT on sparc64