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

From: Tommi Rantala <tommi.t.rantala@nokia.com>
To: Neil Horman <nhorman@tuxdriver.com>, Dmitry Vyukov <dvyukov@google.com>
Cc: Xin Long <lucien.xin@gmail.com>, David Ahern <dsahern@gmail.com>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Cong Wang <xiyou.wangcong@gmail.com>,
	David Miller <davem@davemloft.net>,
	Eric Dumazet <edumazet@google.com>,
	Willem de Bruijn <willemb@google.com>,
	Jakub Kicinski <jakub.kicinski@netronome.com>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	netdev <netdev@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
	Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
	syzkaller <syzkaller@googlegroups.com>,
	Dan Streetman <ddstreet@canonical.com>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Alexey Kodanev <alexey.kodanev@oracle.com>,
	Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>,
	linux-sctp@vger.kernel.org
Subject: Re: net: hang in unregister_netdevice: waiting for lo to become free
Date: Wed, 21 Feb 2018 16:53:54 +0200	[thread overview]
Message-ID: <7d98027d-e810-a079-49c5-0bf8beef390e@nokia.com> (raw)
In-Reply-To: <20180220162622.GA32068@hmswarspite.think-freely.org>

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@nokia.com> 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@nokia.com>
>>> 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@oracle.com>
>>> 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