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

[Changwei Ge]

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?

Thanks,
Changwei


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