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Subject: Re: [dm-devel] [PATCH -next v2 4/6] md: refactor
 idle/frozen_sync_thread() to fix deadlock
To:     Xiao Ni <xni@redhat.com>, Yu Kuai <yukuai1@huaweicloud.com>
Cc:     guoqing.jiang@linux.dev, agk@redhat.com, snitzer@kernel.org,
        dm-devel@redhat.com, song@kernel.org, linux-raid@vger.kernel.org,
        yangerkun@huawei.com, linux-kernel@vger.kernel.org,
        yi.zhang@huawei.com, "yukuai (C)" <yukuai3@huawei.com>
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 <CALTww28UapJnK+Xfx7O9uEd5ZH2E7ufPT_7pKY6YYuzTZ0Fbdw@mail.gmail.com>
From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <b96ec15b-6102-17bb-2c18-a487f224865b@huaweicloud.com>
Date:   Wed, 14 Jun 2023 16:28:54 +0800
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Hi,

在 2023/06/14 15:57, Xiao Ni 写道:
> On Wed, Jun 14, 2023 at 3:38 PM Yu Kuai <yukuai1@huaweicloud.com> wrote:
>>
>> Hi,
>>
>> 在 2023/06/14 15:12, Xiao Ni 写道:
>>> On Wed, Jun 14, 2023 at 10:04 AM Yu Kuai <yukuai1@huaweicloud.com> wrote:
>>>>
>>>> Hi,
>>>>
>>>> 在 2023/06/14 9:48, Yu Kuai 写道:
>>>>
>>>>
>>>>>>
>>>>>> In the patch, sync_seq is added in md_reap_sync_thread. In
>>>>>> idle_sync_thread, if sync_seq isn't equal
>>>>>>
>>>>>> mddev->sync_seq, it should mean there is someone that stops the sync
>>>>>> thread already, right? Why do
>>>>>>
>>>>>> you say 'new started sync thread' here?
>>>>
>>>> If someone stops the sync thread, and new sync thread is not started,
>>>> then this sync_seq won't make a difference, above wait_event() will not
>>>> wait because !test_bit(MD_RECOVERY_RUNNING, &mddev->recovery) will pass.
>>>> So 'sync_seq' is only used when the old sync thread stops and new sync
>>>> thread starts, add 'sync_seq' will bypass this case.
>>>
>>> Hi
>>>
>>> If a new sync thread starts, why can sync_seq be different? sync_seq
>>> is only added in md_reap_sync_thread. And when a new sync request
>>> starts, it can't stop the sync request again?
>>>
>>> Af first, the sync_seq is 0
>>>
>>> admin1
>>> echo idle > sync_action
>>> idle_sync_thread(sync_seq is 1)
>>
>> Wait, I'm confused here, how can sync_seq to be 1 here? I suppose you
>> mean that there is a sync_thread just finished?
> 
> Hi Kuai
> 
> Yes. Because idle_sync_thread needs to wait until md_reap_sync_thread
> finishes. And md_reap_sync_thread adds sync_seq. Do I understand your
> patch right?

Yes, noted that idle_sync_thread() will only wait if MD_RECOVERY_RUNNING
is set.

> 
>>
>> Then the problem is that idle_sync_thread() read sync_seq after the old
>> sync_thread is done, and new sync_thread start before wait_event() is
>> called, should we wait for this new sync_thread?
>>
>> My answer here is that we should, but I'm also ok to not wait this new
>> sync_thread, I don't think this behaviour matters. The key point here
>> is that once wait_event() is called from idle_sync_thread(), this
>> wait_event() should not wait for new sync_thread...
> 
> I think we should wait. If we don't wait for it, there is a problem.
> One person echos idle to sync_action and it doesn't work sometimes.
> It's a strange thing.
> 

Ok. I'll add new comment to emphasize that idle_sync_thread() won't wait
for new sync_thread that is started after wait_event().

>>
>>> echo resync > sync_action (new sync)
>>
>> If this is behind "echo idle > sync_action", idle_sync_thread should not
>> see that MD_RECOVERY_RUNNING is set and wait_event() won't wait at all.
> 
> `echo resync > sync_action` can't change the sync_seq. So 'echo idle >
> sync_action' still waits until MD_RECOVERY_RUNNING is cleared?

This is not accurate, if `echo resync > sync_action` triggers a new
sync_thread, then sync_seq is updated when this sync_thread is done,
during this period, MD_RECOVERY_RUNNING is still set, so `echo idle
 >sync_action` will wait for sync_thread to be done.

Thanks,
Kuai
> 
> Regards
> Xiao
> 
>>
>> Thanks,
>> Kuai
>>>
>>> Then admin2 echos idle > sync_action, sync_seq is still 1
>>>
>>> Regards
>>> Xiao
>>>
>>>>
>>>> Thanks,
>>>> Kuai
>>>>
>>>
>>> .
>>>
>>
> 
> .
>