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

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Date:   Sun, 30 Oct 2022 07:25:45 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Jinlong Chen <nickyc975@....edu.cn>
Cc:     axboe@...nel.dk, kbusch@...nel.org, hch@....de, sagi@...mberg.me,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-nvme@...ts.infradead.org
Subject: Re: [PATCH 1/3] blk-mq: remove redundant call to
 blk_freeze_queue_start in blk_mq_destroy_queue

On 10/29/22 19:27, Jinlong Chen wrote:
>> I think this patch introduces a hang for every caller of
>> blk_mq_destroy_queue() other than blk_queue_start_drain().
 >> I don't see why the patch introduces a hang. The calling relationship in
> blk_mq_destroy_queue is as follows: [ ... ]

Agreed - what I wrote is wrong.

> 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").

Bart.