Re: net: hang in unregister_netdevice: waiting for lo to become free

[Tommi Rantala]

On 20.02.2018 18:26, Neil Horman wrote:
> On Tue, Feb 20, 2018 at 09:14:41AM +0100, Dmitry Vyukov wrote:
> > On Tue, Feb 20, 2018 at 8:56 AM, Tommi Rantala
> > <tommi.t.rantala@xxxxxxxxx> wrote:
> > > On 19.02.2018 20:59, Dmitry Vyukov wrote:
> > > > Is this meant to be fixed already? I am still seeing this on the
> > > > latest upstream tree.
> > >
> > > These two commits are in v4.16-rc1:
> > >
> > > commit 4a31a6b19f9ddf498c81f5c9b089742b7472a6f8
> > > Author: Tommi Rantala <tommi.t.rantala@xxxxxxxxx>
> > > Date:   Mon Feb 5 21:48:14 2018 +0200
> > >
> > >      sctp: fix dst refcnt leak in sctp_v4_get_dst
> > > ...
> > >      Fixes: 410f03831 ("sctp: add routing output fallback")
> > >      Fixes: 0ca50d12f ("sctp: fix src address selection if using secondary
> > > addresses")
> > >
> > >
> > > commit 957d761cf91cdbb175ad7d8f5472336a4d54dbf2
> > > Author: Alexey Kodanev <alexey.kodanev@xxxxxxxxxx>
> > > Date:   Mon Feb 5 15:10:35 2018 +0300
> > >
> > >      sctp: fix dst refcnt leak in sctp_v6_get_dst()
> > > ...
> > >      Fixes: dbc2b5e9a09e ("sctp: fix src address selection if using secondary
> > > addresses for ipv6")
> > >
> > >
> > > I guess we missed something if it's still reproducible.
> > >
> > > I can check it later this week, unless someone else beat me to it.
> >
> > Hi Tommi,
> >
> > Hmmm, I can't claim that it's exactly the same bug. Perhaps it's
> > another one then. But I am still seeing these:
> >
> > [   58.799130] unregister_netdevice: waiting for lo to become free.
> > Usage count = 4
> > [   60.847138] unregister_netdevice: waiting for lo to become free.
> > Usage count = 4
> > [   62.895093] unregister_netdevice: waiting for lo to become free.
> > Usage count = 4
> > [   64.943103] unregister_netdevice: waiting for lo to become free.
> > Usage count = 4
> >
> > on upstream tree pulled ~12 hours ago.
> Can you write a systemtap script to probe dev_hold, and dev_put, printing out a
> backtrace if the device name matches "lo".  That should tell us definitively if
> the problem is in the same location or not

Hi Dmitry, I tested with the reproducer and the kernel .config file that 
you sent in the first email in this thread:

With 4.16-rc2 unable to reproduce.

With 4.15-rc9 bug reproducible, and I get "unregister_netdevice: waiting 
for lo to become free. Usage count = 3"

With 4.15-rc9 and Alexey's "sctp: fix dst refcnt leak in 
sctp_v6_get_dst()" cherry-picked on top, unable to reproduce.


Is syzkaller doing something else now to trigger the bug...?
Can you still trigger the bug with the same reproducer?


Tommi