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Subject: Re: [PATCH] Remove blkg node after destroying blkg
To:     Yu Kuai <yukuai1@huaweicloud.com>, Tao Su <tao1.su@linux.intel.com>
Cc:     linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        tj@kernel.org, josef@toxicpanda.com, axboe@kernel.dk,
        "yukuai (C)" <yukuai3@huawei.com>
References: <20230425075911.839539-1-tao1.su@linux.intel.com>
 <aa5de32c-c92b-d032-e9bb-83d2436ff72c@huawei.com>
 <ZEegQCCZ96ij6mw5@linux.bj.intel.com>
 <6d486f85-87ed-fa35-00cb-4c37fef17536@huaweicloud.com>
From:   Yu Kuai <yukuai1@huaweicloud.com>
Message-ID: <9f5eeba2-fbc9-3b56-c7ed-d8ecc1c888b3@huaweicloud.com>
Date:   Wed, 26 Apr 2023 09:13:08 +0800
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Hi,

在 2023/04/25 19:09, Yu Kuai 写道:
> Hi,
> 
> 在 2023/04/25 17:41, Tao Su 写道:
>> On Tue, Apr 25, 2023 at 04:09:34PM +0800, Yu Kuai wrote:
>>> Hi,
>>>
>>> 在 2023/04/25 15:59, Tao Su 写道:
>>>> Kernel hang when poweroff or reboot, due to infinite restart in 
>>>> function
>>>> blkg_destroy_all. It will goto restart label when a batch of blkgs are
>>>> destroyed, but not remove blkg node in blkg_list. So the blkg_list is
>>>> same in every 'restart' and result in kernel hang.
>>>>
>>>> By adding list_del to remove blkg node after destroying, can solve this
>>>> kernel hang issue and satisfy the previous will to 'restart'.
>>>>
>>>> Reported-by: Xiangfei Ma <xiangfeix.ma@intel.com>
>>>> Tested-by: Xiangfei Ma <xiangfeix.ma@intel.com>
>>>> Tested-by: Farrah Chen <farrah.chen@intel.com>
>>>> Signed-off-by: Tao Su <tao1.su@linux.intel.com>
>>>> ---
>>>>    block/blk-cgroup.c | 1 +
>>>>    1 file changed, 1 insertion(+)
>>>>
>>>> diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
>>>> index bd50b55bdb61..960eb538a704 100644
>>>> --- a/block/blk-cgroup.c
>>>> +++ b/block/blk-cgroup.c
>>>> @@ -530,6 +530,7 @@ static void blkg_destroy_all(struct gendisk *disk)
>>>>            spin_lock(&blkcg->lock);
>>>>            blkg_destroy(blkg);
>>>> +        list_del(&blkg->q_node);
>>>
>>> blkg should stay on the queue list until blkg_free_workfn(), otherwise
>>> parent blkg can be freed before child, which will cause some known
>>> issue.
>>
>> Yes, directly removing blkg node is not appropriate, which I noticed some
>> comments in blkg_destroy(), thanks for pointing out this issue.
>>
>>>
>>> I think this hung happens when total blkg is greater than
>>> BLKG_DESTROY_BATCH_SIZE, right?
>>
>> Yes, you are right.
>>
>>>
>>> Can you try if following patch fix your problem?
>>
>> This patch can also fix my problem, and indeed is a more secure way.
> 
> Thanks for the test, for a better solution, I think 'blkcg_mutex' can
> be used to protect 'blkg->q_node' list instead of 'queue_lock', so that
> the 'restart' can be removed because softlockup can be avoided.
> 

I looked into this, and I found that this is not a easy thing to do.

Anyway, feel free to submit a new patch based on my orignial suggestion.

Thanks,
Kuai