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

[Dmitry Vyukov]

On Fri, Apr 13, 2018 at 5:54 PM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> On Fri, Apr 13, 2018 at 2:43 PM, Dan Streetman <ddstreet@xxxxxxxx> wrote:
>> On Thu, Apr 12, 2018 at 8:15 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
>>> On Wed, Feb 21, 2018 at 3:53 PM, Tommi Rantala
>>> <tommi.t.rantala@xxxxxxxxx> wrote:
>>>> 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?
>>>
>>> Hi Neil, Tommi,
>>>
>>> Reviving this old thread about "unregister_netdevice: waiting for lo
>>> to become free. Usage count = 3" hangs.
>>> I still did not have time to deep dive into what happens there (too
>>> many bugs coming from syzbot). But this still actively happens and I
>>> suspect accounts to a significant portion of various hang reports,
>>> which are quite unpleasant.
>>>
>>> One idea that could make it all simpler:
>>>
>>> Is this wait loop in netdev_wait_allrefs() supposed to wait for any
>>> prolonged periods of time under any non-buggy conditions? E.g. more
>>> than 1-2 minutes?
>>> If it only supposed to wait briefly for things that already supposed
>>> to be shutting down, and we add a WARNING there after some timeout,
>>> then syzbot will report all info how/when it happens, hopefully
>>> extracting reproducers, and all the nice things.
>>> But this WARNING should not have any false positives under any
>>> realistic conditions (e.g. waiting for arrival of remote packets with
>>> large timeouts).
>>>
>>> Looking at some task hung reports, it seems that this code holds some
>>> mutexes, takes workqueue thread and prevents any progress with
>>> destruction of other devices (and net namespace creation/destruction),
>>> so I guess it should not wait for any indefinite periods of time?
>>
>> I'm working on this currently:
>> https://bugs.launchpad.net/ubuntu/zesty/+source/linux/+bug/1711407
>>
>> I added a summary of what I've found to be the cause (or at least, one
>> possible cause) of this:
>> https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1711407/comments/72
>>
>> I'm working on a patch to work around the main side-effect of this,
>> which is hanging while holding the global net mutex.  Hangs will still
>> happen (e.g. if a dst leaks) but should not affect anything else,
>> other than a leak of the dst and its net namespace.
>>
>> Fixing the dst leaks is important too, of course, but a dst leak (or
>> other cause) shouldn't break the entire system.
>
> Leaking some memory is definitely better than hanging the system.
>
> So I've made syzkaller to recognize "unregister_netdevice: waiting for
> (.*) to become free" as a kernel bug:
> https://github.com/google/syzkaller/commit/7a67784ca8bdc3b26cce2f0ec9a40d2dd9ec9396
> Unfortunately it does not make it catch these bugs because creating a
> net namespace per test is too damn slow, so namespaces are reused for
> lots of tests and when/if it's eventually destroyed it's already too
> late to find root cause.
>
> But I've run a one-off experiment with prompt net namespace
> destruction and syzkaller was able to easily extract a C reproducer:
> https://gist.githubusercontent.com/dvyukov/d571e8fff24e127ca48a8c4790d42bfa/raw/52050e93ba9afbb5126b9d7bb39b7e71a82af016/gistfile1.txt
>
> On upstream 16e205cf42da1f497b10a4a24f563e6c0d574eec with this config:
> https://gist.githubusercontent.com/dvyukov/9663c57443adb21f2795b92ef0829d62/raw/bbea0652e23746096dd56855a28f6c681aebcdee/gistfile1.txt
>
> this gives me:
>
> [   83.183198] unregister_netdevice: waiting for lo to become free.
> Usage count = 9
> [   85.231202] unregister_netdevice: waiting for lo to become free.
> Usage count = 9
> ...
> [  523.511205] unregister_netdevice: waiting for lo to become free.
> Usage count = 9
> ...
>
> This is generated from this syzkaller program:
>
> r0 = socket$inet6(0xa, 0x1, 0x84)
> setsockopt$inet6_IPV6_XFRM_POLICY(r0, 0x29, 0x23,
> &(0x7f0000000380)={{{@in6=@remote={0xfe, 0x80, [], 0xbb},
> @in=@dev={0xac, 0x14, 0x14}, 0x0, 0x0, 0x0, 0x0, 0xa}, {}, {}, 0x0,
> 0x0, 0x1}, {{@in=@local={0xac, 0x14, 0x14, 0xaa}, 0x0, 0x32}, 0x0,
> @in=@local={0xac, 0x14, 0x14, 0xaa}, 0x3504}}, 0xe8)
> bind$inet6(r0, &(0x7f0000000000)={0xa, 0x4e20}, 0x1c)
> connect$inet(r0, &(0x7f0000000040)={0x2, 0x4e20, @dev={0xac, 0x14,
> 0x14, 0xd}}, 0x10)
> syz_emit_ethernet(0x3e, &(0x7f00000001c0)={@local={[0xaa, 0xaa, 0xaa,
> 0xaa, 0xaa], 0xaa}, @dev={[0xaa, 0xaa, 0xaa, 0xaa, 0xaa]}, [],
> {@ipv6={0x86dd, {0x0, 0x6, "50a09c", 0x8, 0xffffff11, 0x0,
> @remote={0xfe, 0x80, [], 0xbb}, @local={0xfe, 0x80, [], 0xaa}, {[],
> @udp={0x0, 0x4e20, 0x8}}}}}}, &(0x7f0000000040))
>
> So this seems to be related to IPv6 and/or xfrm and is potentially
> caused by external packets (that syz_emit_ethernet call).



Here is another repro which seems to be a different bug (note that it
requires fault injection):

https://gist.githubusercontent.com/dvyukov/1c56623016cc4c24a69d433c5114ad5b/raw/530478f571b195193101b912aa646948528baa8e/gistfile1.txt

Dan, do you mind taking a look at them? Fixing these should eliminate
root causes of these hangs/leaks.