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

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Date:   Thu, 13 Oct 2022 21:47:33 +0800
From:   Jinlong Chen <chenjinlong2016@...look.com>
To:     yukuai1@...weicloud.com
Cc:     axboe@...nel.dk, chenjinlong2016@...look.com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        yukuai3@...wei.com
Subject: Re: [PATCH] blk-mq: put the reference of the io scheduler module after switching back

Hi, Yu Kuai!

> 
> 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
> 

We need to release the reference only if blk_mq_elv_switch_back got its own
 reference. However, blk_mq_elv_switch_back (precisely elevator_switch_mq)
 does not increase the reference of the module it is switching to. 
 Hence, the reference got in blk_mq_elv_switch_none does not need to be released,
 or we'll have to re-increase the reference count manually.

> 
> 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
> 

I tried to reproduce the problem but failed, so I found my mistake.

Sincerely,
Jinlong Chen