Re: INFO: task hung in ip6gre_exit_batch_net

From: Dmitry Vyukov
Date: Fri Jun 08 2018 - 04:18:48 EST

On Thu, Jun 7, 2018 at 9:59 PM, Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> wrote:
> On 07.06.2018 22:03, Dmitry Vyukov wrote:
>> On Thu, Jun 7, 2018 at 8:54 PM, Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> wrote:
>>>>>>> Hi, Dmirty!
>>>>>>> On 04.06.2018 18:22, Dmitry Vyukov wrote:
>>>>>>>> On Mon, Jun 4, 2018 at 5:03 PM, syzbot
>>>>>>>> <syzbot+bf78a74f82c1cf19069e@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote:
>>>>>>>>> Hello,
>>>>>>>>> syzbot found the following crash on:
>>>>>>>>> HEAD commit: bc2dbc5420e8 Merge branch 'akpm' (patches from Andrew)
>>>>>>>>> git tree: upstream
>>>>>>>>> console output:
>>>>>>>>> kernel config:
>>>>>>>>> dashboard link:
>>>>>>>>> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
>>>>>>>> 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":
>>>>>> 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.
>>>> I can't find any legend for these +'s and .'s, but {+.+.} is present
>>>> in large amounts in just any task hung report for different mutexes,
>>>> so I would not expect that it means corruption.
>>>> Are dozens of known corruptions that syzkaller can trigger. But
>>>> usually they are reliably caught by KASAN. If any of them would lead
>>>> to silent memory corruption, we would got dozens of assorted crashes
>>>> throughout the kernel. We've seen that at some points, but not
>>>> recently. So I would assume that memory is not corrupted in all these
>>>> cases:
>>> This BUG clarifies the {+.+.}:
>>> 4 locks held by kworker/0:145/381:
>>> #0: ((wq_completion)"hwsim_wq"){+.+.}, at: [<000000003f9487f0>] work_static include/linux/workqueue.h:198 [inline]
>>> #0: ((wq_completion)"hwsim_wq"){+.+.}, at: [<000000003f9487f0>] set_work_data kernel/workqueue.c:619 [inline]
>>> #0: ((wq_completion)"hwsim_wq"){+.+.}, at: [<000000003f9487f0>] set_work_pool_and_clear_pending kernel/workqueue.c:646 [inline]
>>> #0: ((wq_completion)"hwsim_wq"){+.+.}, at: [<000000003f9487f0>] process_one_work+0xb12/0x1bb0 kernel/workqueue.c:2084
>>> #1: ((work_completion)(&data->destroy_work)){+.+.}, at: [<00000000bbdd2115>] process_one_work+0xb89/0x1bb0 kernel/workqueue.c:2088
>>> #2: (rtnl_mutex){+.+.}, at: [<000000009c9d14f8>] rtnl_lock+0x17/0x20 net/core/rtnetlink.c:74
>>> #3: (rcu_sched_state.exp_mutex){+.+.}, at: [<000000001ba1a807>] exp_funnel_lock kernel/rcu/tree_exp.h:272 [inline]
>>> #3: (rcu_sched_state.exp_mutex){+.+.}, at: [<000000001ba1a807>] _synchronize_rcu_expedited.constprop.72+0x9fa/0xac0 kernel/rcu/tree_exp.h:596
>>> There we have rtnl_mutex locked and the {..} is like above. It's definitely locked
>>> since there is one more lock after it.
>>> This BUG happen because of there are many rtnl_mutex waiters while owner
>>> is synchronizing RCU. Rather clear for me in comparison to the topic's hung.
>> You mean that it's not hanged, but rather needs more than 2 minutes to
>> complete, right?
> Yeah, I think, this is possible. I've seen the situations like that.
> Let synchronize_rcu_expedited() is executed for X seconds. Then,
> it's need just 120/x calls of "this function under rtnl_mutex" to make
> a soft lockup of someone else who wants the mutex too.
> Also, despite the CFS is fair scheduler, in case of the calls are
> made from workqueue, every work will cause sleep. So, every work
> will be executed in separate worker task. Every worker will haved its
> own time slice. This increases the probability these tasks will
> take cpu time before the task in the header of the hang.

OK, let's stick with this theory for now. Looking at the crash frequencies here:
I can actually believe that this is just flakes due to too slow execution.

I've noted that we need either reduce load and/or increase timeouts:

Let's keep duping new such reports onto "INFO: task hung in
netdev_run_todo" so that they are all collected at a single location.

Thanks for help