From mboxrd@z Thu Jan  1 00:00:00 1970
From: Kirill Tkhai <ktkhai@virtuozzo.com>
Subject: Re: INFO: task hung in ip6gre_exit_batch_net
Date: Tue, 5 Jun 2018 16:55:20 +0300
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Cc: syzbot <syzbot+bf78a74f82c1cf19069e@syzkaller.appspotmail.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        David Miller <davem@davemloft.net>,
        David Ahern <dsahern@gmail.com>,
        Florian Westphal <fw@strlen.de>, Jiri Benc <jbenc@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Xin Long <lucien.xin@gmail.com>,
        mschiffer@universe-factory.net, netdev <netdev@vger.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Vladislav Yasevich <vyasevich@gmail.com>
To: Dmitry Vyukov <dvyukov@google.com>
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On 05.06.2018 12:36, Dmitry Vyukov wrote:
> On Tue, Jun 5, 2018 at 11:03 AM, Kirill Tkhai <ktkhai@virtuozzo.com> wrote:
>> Hi, Dmirty!
>>
>> On 04.06.2018 18:22, Dmitry Vyukov wrote:
>>> On Mon, Jun 4, 2018 at 5:03 PM, syzbot
>>> <syzbot+bf78a74f82c1cf19069e@syzkaller.appspotmail.com> wrote:
>>>> Hello,
>>>>
>>>> syzbot found the following crash on:
>>>>
>>>> HEAD commit:    bc2dbc5420e8 Merge branch 'akpm' (patches from Andrew)
>>>> git tree:       upstream
>>>> console output: https://syzkaller.appspot.com/x/log.txt?x=164e42b7800000
>>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=982e2df1b9e60b02
>>>> dashboard link: https://syzkaller.appspot.com/bug?extid=bf78a74f82c1cf19069e
>>>> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
>>>>
>>>> Unfortunately, I don't have any reproducer for this crash yet.
>>>>
>>>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>>>> Reported-by: syzbot+bf78a74f82c1cf19069e@syzkaller.appspotmail.com
>>>
>>> Another hang on rtnl lock:
>>>
>>> #syz dup: INFO: task hung in netdev_run_todo
>>>
>>> May be related to "unregister_netdevice: waiting for DEV to become free":
>>> https://syzkaller.appspot.com/bug?id=1a97a5bd119fd97995f752819fd87840ab9479a9
> 
> netdev_wait_allrefs does not hold rtnl lock during waiting, so it must
> be something different.
> 
> 
>>> Any other explanations for massive hangs on rtnl lock for minutes?
>>
>> To exclude the situation, when a task exists with rtnl_mutex held:
>>
>> would the pr_warn() from print_held_locks_bug() be included in the console output
>> if they appear?
> 
> Yes, everything containing "WARNING:" is detected as bug.

OK, then dead task not releasing the lock is excluded.

One more assumption: someone corrupted memory around rtnl_mutex and it looks like locked.
(I track lockdep "(rtnl_mutex){+.+.}" prints in initial message as "nobody owns rtnl_mutex").
There may help a crash dump of the VM.

Also, there may be a locking code BUG, but this seems the least probable for me.

Kirill