[Ocfs2-devel] [Ocfs2-dev] BUG: deadlock with umount and ocfs2 workqueue triggered by ocfs2rec thread

[Changwei Ge]

Hi Jun,

On 2018/1/19 11:59, piaojun wrote:
> Hi Changwei,
> 
> On 2018/1/19 11:38, Changwei Ge wrote:
>> Hi Jun,
>>
>> On 2018/1/19 11:17, piaojun wrote:
>>> Hi Jan, Eric and Changwei,
>>>
>>> Could we use another mutex lock to protect quota recovery? Sharing the
>>> lock with VFS-layer probably seems a little weird.
>>
>> I am afraid that we can't since quota need ::s_umount and we indeed need
>> ::s_umount to get rid of race that quota has freed structs that will be
>> used by quota recovery in ocfs2.
>>
> Could you explain which 'structs' used by quota recovery? Do you mean
> 'struct super_block'?
I am not pointing to super_block.

Sure.
You can refer to
ocfs2_finish_quota_recovery
   ocfs2_recover_local_quota_file -> here, operations on quota happens

Thanks,
Changwei

> 
> thanks,
> Jun
> 
>>>
>>> On 2018/1/19 9:48, Changwei Ge wrote:
>>>> Hi Jan,
>>>>
>>>> On 2018/1/18 0:03, Jan Kara wrote:
>>>>> On Wed 17-01-18 16:21:35, Jan Kara wrote:
>>>>>> Hello,
>>>>>>
>>>>>> On Fri 12-01-18 16:25:56, Eric Ren wrote:
>>>>>>> On 01/12/2018 11:43 AM, Shichangkuo wrote:
>>>>>>>> Hi all,
>>>>>>>> 　　Now we are testing ocfs2 with 4.14 kernel, and we finding a deadlock with umount and ocfs2 workqueue triggered by ocfs2rec thread. The stack as follows:
>>>>>>>> journal recovery work:
>>>>>>>> [<ffffffff8a8c0694>] call_rwsem_down_read_failed+0x14/0x30
>>>>>>>> [<ffffffffc0d5d652>] ocfs2_finish_quota_recovery+0x62/0x450 [ocfs2]
>>>>>>>> [<ffffffffc0d21221>] ocfs2_complete_recovery+0xc1/0x440 [ocfs2]
>>>>>>>> [<ffffffff8a09a1f0>] process_one_work+0x130/0x350
>>>>>>>> [<ffffffff8a09a946>] worker_thread+0x46/0x3b0
>>>>>>>> [<ffffffff8a0a0e51>] kthread+0x101/0x140
>>>>>>>> [<ffffffff8aa002ff>] ret_from_fork+0x1f/0x30
>>>>>>>> [<ffffffffffffffff>] 0xffffffffffffffff
>>>>>>>>
>>>>>>>> /bin/umount:
>>>>>>>> [<ffffffff8a099b24>] flush_workqueue+0x104/0x3e0
>>>>>>>> [<ffffffffc0cf18db>] ocfs2_truncate_log_shutdown+0x3b/0xc0 [ocfs2]
>>>>>>>> [<ffffffffc0d4fd6c>] ocfs2_dismount_volume+0x8c/0x3d0 [ocfs2]
>>>>>>>> [<ffffffffc0d500e1>] ocfs2_put_super+0x31/0xa0 [ocfs2]
>>>>>>>> [<ffffffff8a2445bd>] generic_shutdown_super+0x6d/0x120
>>>>>>>> [<ffffffff8a24469d>] kill_block_super+0x2d/0x60
>>>>>>>> [<ffffffff8a244e71>] deactivate_locked_super+0x51/0x90
>>>>>>>> [<ffffffff8a263a1b>] cleanup_mnt+0x3b/0x70
>>>>>>>> [<ffffffff8a09e9c6>] task_work_run+0x86/0xa0
>>>>>>>> [<ffffffff8a003d70>] exit_to_usermode_loop+0x6d/0xa9
>>>>>>>> [<ffffffff8a003a2d>] do_syscall_64+0x11d/0x130
>>>>>>>> [<ffffffff8aa00113>] entry_SYSCALL64_slow_path+0x25/0x25
>>>>>>>> [<ffffffffffffffff>] 0xffffffffffffffff
>>>>>>>> 　　
>>>>>>>> Function ocfs2_finish_quota_recovery try to get sb->s_umount, which was already locked by umount thread, then get a deadlock.
>>>>>>>
>>>>>>> Good catch, thanks for reporting.  Is it reproducible? Can you please share
>>>>>>> the steps for reproducing this issue?
>>>>>>>> This issue was introduced by c3b004460d77bf3f980d877be539016f2df4df12 and 5f530de63cfc6ca8571cbdf58af63fb166cc6517.
>>>>>>>> I think we cannot use :: s_umount, but the mutex ::dqonoff_mutex was already removed.
>>>>>>>> Shall we add a new mutex?
>>>>>>>
>>>>>>> @Jan, I don't look into the code yet, could you help me understand why we
>>>>>>> need to get sb->s_umount in ocfs2_finish_quota_recovery?
>>>>>>> Is it because that the quota recovery process will start at umounting? or
>>>>>>> some where else?
>>>>>>
>>>>>> I was refreshing my memory wrt how ocfs2 quota recovery works. The problem
>>>>>> is the following: We load information about all quota information that
>>>>>> needs recovering (this is possibly for other nodes) in
>>>>>> ocfs2_begin_quota_recovery() that gets called during mount. Real quota
>>>>>> recovery happens from the recovery thread in ocfs2_finish_quota_recovery().
>>>>>> We need to protect code running there from dquot_disable() calls as that
>>>>>> will free structures we use for updating quota information etc. Currently
>>>>>> we use sb->s_umount for that protection.
>>>>>>
>>>>>> The problem above apparently happens when someone calls umount before the
>>>>>> recovery thread can finish quota recovery. I will think more about how to
>>>>>> fix the locking so that this lock inversion does not happen...
>>>>>
>>>>> So could we move ocfs2_recovery_exit() call in ocfs2_dismount_volume() up
>>>>> before ocfs2_disable_quotas()? It seems possible to me, I'm just not sure
>>>>> if there are not some hidden dependencies on recovery being shut down only
>>>>> after truncate log / local alloc. If we can do that, we could remove
>>>>> s_umount protection from ocfs2_finish_quota_recovery() and thus resolve the
>>>>> race.
>>>>>
>>>>> 								Honza
>>>>
>>>> Thanks for looking into this.
>>>> I am not quite familiar with quota part.:)
>>>>
>>>> Or can we move ocfs2_disable_quotas() in ocfs2_dismount_volume down
>>>> after ocfs2_recovery_exit() with ::invoking down_read(&sb->s_umount)
>>>> eliminated?
>>>>
>>>> Another way I can figure out is:
>>>> I think we might get inspired from qsync_work_fn().
>>>> In that function if current work is under running context of umount with
>>>> ::s_umount held, it just delays current work to next time.
>>>>
>>>> So can we also _try lock_ in ocfs2_finish_quota_recovery() and recover
>>>> quota by other ocfs2 cluster member nodes or local node's next time of
>>>> mount?
>>>>
>>> I guess we need analyse the impact of _try lock_. Such as no other node
>>> will help recovering quota when I'm the only node in cluster.
>>
>> I don't see any risk for now. I will think about it more, later.
>>
>> Thanks,
>> Changwei
>>>
>>> thanks,
>>> Jun
>>>
>>>> Thanks,
>>>> Changwei
>>>>
>>>>
>>>>>
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>>>>
>>>
>> .
>>
>