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

[Dmitry Vyukov]

On Thu, May 10, 2018 at 12:23 PM, Dan Streetman <ddstreet@xxxxxxxx> wrote:
>>>>>>> <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.
>>>
>>> Yep I will look at them, thanks for the reproducers.
>>
>> Hi Dan,
>>
>> Any updates on this? syzbot is hitting this all the time.
>
> Sorry, the recent changes from net_mutex -> net_rwsem/pernet_ops_rwsem
> have complicated what I had done to workaround this, but I'm still
> working on it.  Apologies for the delay.

Are you looking at the mitigation? Or the bugs that trigger it? Or both?