Re: Long stalls creating a new netns after a netns with a SMB client exits

Hello,

On Fri, Jul 28, 2017 at 9:47 AM, Rolf Neugebauer
[off-list ref] wrote:

Creating the new namespace is stalling for around 200 seconds and
there 20 odd messages on the console, like:

[   67.372603] unregister_netdevice: waiting for lo to become free.
Usage count = 1

Sounds like another netdev refcnt leak.

Adding a 'sleep 1' before deleting the original network namespace
"solves" the issue, but that doesn't sound like a good fix. Not using
unmount also does not help (understandable).

Interesting, if sleeping for 1sec help, why did you see the stall for
200sec? The "leak" should go away eventually without 'sleep 1',
right?

While the creation of the new namespace is stalled, I used 'sysrq' a
few times to dump the work queues. There is an example below. Also,
the hung task detection kicks in after 120 seconds (also below)

Yeah, the net_mutex is held by cleanup_net().

I can readily reproduce this on 4.9.39, 4.11.12 and another user
repro-ed it on 4.12.3. It seems to happen every time. At least one
user reported issues with NFS mounts as well, but we were not able to
reproduce it. It's not clear to me if this is directly related to
'mount.cifs' or if that just happens to reliably repro it.

OK, so commit d747a7a51b00984127a88113c does not help this case
either.

It would be great if someone more familiar with the code could take a
look. I'm happy to provide additional info (perf traces etc) or test
patches if needed.

The last time I debugged this kind of netdev refcnt leak problem,
I added a few trace_printk() to dev_hold() and dev_put(),
so you can try it too. I will see if I can use your reproducer
here.

Thanks.