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

[piaojun <piaojun@huawei.com>]

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.

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.

thanks,
Jun

> Thanks,
> Changwei
> 
> 
>>
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