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Sender: Tejun Heo <htejun@gmail.com>
Date:   Wed, 30 Nov 2022 10:50:27 -1000
From:   Tejun Heo <tj@kernel.org>
To:     Li Nan <linan122@huawei.com>
Cc:     josef@toxicpanda.com, axboe@kernel.dk, cgroups@vger.kernel.org,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        yukuai3@huawei.com, yi.zhang@huawei.com
Subject: Re: [PATCH -next v2 8/9] block: fix null-pointer dereference in
 ioc_pd_init
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References: <20221130132156.2836184-1-linan122@huawei.com>
 <20221130132156.2836184-9-linan122@huawei.com>
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On Wed, Nov 30, 2022 at 09:21:55PM +0800, Li Nan wrote:
> Remove block device when iocost is initializing may cause
> null-pointer dereference:
> 
> 	CPU1				   CPU2
>   ioc_qos_write
>    blkcg_conf_open_bdev
>     blkdev_get_no_open
>      kobject_get_unless_zero
>     blk_iocost_init
>      rq_qos_add
>   					del_gendisk
>   					 rq_qos_exit
>   					  q->rq_qos = rqos->next
>   					   //iocost is removed from q->roqs
>       blkcg_activate_policy
>        pd_init_fn
>         ioc_pd_init
>   	 ioc = q_to_ioc(blkg->q)
>  	  //cant find iocost and return null
> 
> Fix problem by moving rq_qos_exit() to disk_release(). ioc_qos_write() get
> bd_device.kobj in blkcg_conf_open_bdev(), so disk_release will not be
> actived until iocost initialization is complited.

I think it'd be better to make del_gendisk wait for these in-flight
operations because this might fix the above particular issue but now all the
policies are exposed to request_queue in a state it never expected before.

del_gendisk() is quiescing the queue around rq_qos_exit(), so maybe we can
piggyback on that and update blkcg_conf_open_bdev() to provide such
exclusion?

Thanks.

-- 
tejun