linux-kernel - Re: [PATCH 1/3] blk-mq: remove redundant call to blk_freeze_queue_start in blk_mq_destroy

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Message-ID: <036eb44a-fd6a-4460-49d0-5f596452d1e1@acm.org>
Date:   Sun, 30 Oct 2022 13:19:10 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Jinlong Chen <nickyc975@....edu.cn>
Cc:     axboe@...nel.dk, hch@....de, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org, Ming Lei <ming.lei@...hat.com>
Subject: Re: [PATCH 1/3] blk-mq: remove redundant call to
 blk_freeze_queue_start in blk_mq_destroy_queue

On 10/30/22 07:55, Jinlong Chen wrote:
>>> So I think there is a redundant call to blk_freeze_queue_start(), we
>>> just need to call blk_mq_freeze_queue_wait() after calling
>>> blk_queue_start_drain().
>>
>> I think it is on purpose that blk_queue_start_drain() freezes the
>> request queue and never unfreezes it. So if you want to change this
>> behavior it's up to you to motivate why you want to change this behavior
>> and also why it is safe to make that change. See also commit
>> d3cfb2a0ac0b ("block: block new I/O just after queue is set as dying").
> 
> I think there might be some misunderstanding. I didn't touch
> blk_queue_start_drain(), so its behavior is not changed. What I have done
> is just replacing blk_freeze_queue() with blk_mq_freeze_queue_wait() in
> blk_mq_destroy_queue().

Hi Jinlong,

Does this mean that you want me to provide more information about what I 
wrote? Without this patch, blk_mq_destroy_queue() uses two mechanisms to 
block future I/O requests:
1. Set the flag QUEUE_FLAG_DYING.
2. Freeze the request queue and leave it frozen.

Your patch modifies blk_mq_destroy_queue() such that it unfreezes the 
request queue after I/O has been quiesced instead of leaving it frozen. 
I would appreciate it if Ming Lei (Cc-ed) could comment on this change 
since I think that Ming introduced (2) in blk_mq_destroy_queue() 
(formerly called blk_cleanup_queue()).

Thanks,

Bart.