linux-kernel - Re: [PATCH] blk-mq: put the reference of the io scheduler module after switching back

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Message-ID: <b79a8187-5342-8ff3-73fc-0324bdfb67fb@huaweicloud.com>
Date:   Thu, 13 Oct 2022 21:22:41 +0800
From:   Yu Kuai <yukuai1@...weicloud.com>
To:     Jinlong Chen <chenjinlong2016@...look.com>, axboe@...nel.dk
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH] blk-mq: put the reference of the io scheduler module
 after switching back

Hi,

在 2022/10/13 21:03, Jinlong Chen 写道:
> Sorry for the disturbance.
> 
> This patch is just wrong. elevator_switch_mq does not increase the reference
>   count of the io scheduler module to which we are switching. Hence, we do not
>   need to put the reference back manually.

I'm confused here, cause I do think this patch make sense.

blk_mq_update_nr_hw_queues
  // for each queue using the tagset
  blk_mq_freeze_queue
  // if current elevator is not none, swith to none
  blk_mq_elv_switch_none
   // elevator need to be switched back, got a reference to
   // prevent module to be removed.
   __module_get
   elevator_switch(q, NULL);

  // switch back from none elevator
  blk_mq_elv_switch_back
   -> should release the module reference here
  blk_mq_unfreeze_queue

By the way, I do not test yet, but I think problem can be reporduced:

1. modprobe bfq
2. switch elevator to bfq
3. trigger blk_mq_update_nr_hw_queues
4. switch elevator to none
5. rmmod bfq will fail

Thanks,
Kuai
> 
> Sincerely
> Jinlong Chen
> .
>