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Sender: Tejun Heo <htejun@gmail.com>
Date:   Wed, 4 Jan 2023 11:39:47 -1000
From:   Tejun Heo <tj@kernel.org>
To:     Yu Kuai <yukuai1@huaweicloud.com>
Cc:     hch@infradead.org, 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, yangerkun@huawei.com
Subject: Re: [PATCH -next 3/4] block/rq_qos: use a global mutex to protect
 rq_qos apis
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On Wed, Jan 04, 2023 at 04:53:53PM +0800, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@huawei.com>
> 
> This patch fix following problems:
> 
> 1) rq_qos_add() and rq_qos_del() is protected, while rq_qos_exit() is
>    not.

This part makes sense.

> 2) rq_qos_add() and blkcg_activate_policy() is not atomic, if
>    rq_qos_exit() is done before blkcg_activate_policy(),
>    null-ptr-deference can be triggered.

I'm not sure this part does. I think it'd be better to guarantee that device
destruction is blocked while these configuration operations are in progress
which can be built into blkg_conf helpers.

Thanks.

-- 
tejun