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Subject: Re: net: hang in unregister_netdevice: waiting for lo to become free
To: Daniel Borkmann <daniel@iogearbox.net>,
        Cong Wang <xiyou.wangcong@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.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>
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From: David Ahern <dsahern@gmail.com>
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Date: Tue, 30 Jan 2018 13:57:35 -0700
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On 1/30/18 1:08 PM, Daniel Borkmann wrote:
> On 01/30/2018 07:32 PM, Cong Wang wrote:
>> On Tue, Jan 30, 2018 at 4:09 AM, Dmitry Vyukov <dvyukov@google.com> wrote:
>>> Hello,
>>>
>>> The following program creates a hang in unregister_netdevice.
>>> cleanup_net work hangs there forever periodically printing
>>> "unregister_netdevice: waiting for lo to become free. Usage count = 3"
>>> and creation of any new network namespaces hangs forever.
>>
>> Interestingly, this is not reproducible on net-next.
> 
> The most recent change on netns refcnt was 4ee806d51176 ("net: tcp: close
> sock if net namespace is exiting") in net/net-next from 5 days ago, maybe
> fixed due to that?
> 

This appears to be the commit introducing the refcnt leak:

$ git bisect bad
dbc2b5e9a09e9a6664679a667ff81cff6e5f2641 is the first bad commit
commit dbc2b5e9a09e9a6664679a667ff81cff6e5f2641
Author: Xin Long <lucien.xin@gmail.com>
Date:   Fri May 12 14:39:52 2017 +0800

    sctp: fix src address selection if using secondary addresses for ipv6


v4.14 is bad. Running bisect in the background while doing other things....