On Tue, Mar 22, 2016 at 06:31:54AM +0100, Borislav Petkov wrote:
> On Mon, Mar 21, 2016 at 09:42:09PM +0000, Chris Bainbridge wrote:
> > Hi,
> > 
> > I was testing something on an old server (Dell T105 opteron) and noticed
> > packet loss after updating the kernel from 3.10 to 4.5. The test was:
> > 
> > On Dell run: iperf -s
> > On another system: iperf3 -c dell -u -b 20M -l 1k -t 1000
> > 
> > This sends a 20mbit UDP stream to the Dell. It works fine normally (0%
> > packet loss), but when CONFIG_PROVE_LOCKING is enabled there is high
> > (35%) packet loss. (DEBUG_LOCKDEP also seems to cause packet loss)
> > 
> > The packet loss bisected back to:
> > 
> > commit 88d84ac97378c2f1d5fec9af1e8b7d9a662d6b00
> > Author: Borislav Petkov <bp@suse.de>
> > Date:   Fri Jul 19 12:28:25 2013 +0200
> > 
> >     EDAC: Fix lockdep splat
> 
> Hmm, how would that cause a packet loss?!

Good question. The patch looks pretty innocuous but it is for a lockdep
issue and the bug only appears when lockdep config options are enabled.
Could it somehow have broken a lock used to synchronise packet rx?

> > I have confirmed that the commit preceding this (v3.11-rc1) is fine and
> > that 88d84a introduced the bug.
> 
> Did you revert this commit ontop of 4.5 and reproduce again? Do you see
> the same packet loss?

Reverting over 4.5 does fix the packet loss issue.

> What kind of hw is that target system, can you send full dmesg and
> .config?

It is https://www.suse.com/yes/123682.htm with a slightly faster CPU
(Opteron 1212 2Ghz) and 4GB RAM.

dmesg and .config attached.