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Sender: Tejun Heo <htejun@gmail.com>
Date:   Mon, 22 May 2023 10:59:31 -1000
From:   Tejun Heo <tj@kernel.org>
To:     Yu Kuai <yukuai1@huaweicloud.com>
Cc:     hch@lst.de, 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 for-6.4/block] block/rq_qos: protect rq_qos apis with a
 new lock
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References: <20230414084008.2085155-1-yukuai1@huaweicloud.com>
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On Fri, Apr 14, 2023 at 04:40:08PM +0800, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@huawei.com>
> 
> commit 50e34d78815e ("block: disable the elevator int del_gendisk")
> move rq_qos_exit() from disk_release() to del_gendisk(), this will
> introduce some problems:
> 
> 1) If rq_qos_add() is triggered by enabling iocost/iolatency through
>    cgroupfs, then it can concurrent with del_gendisk(), it's not safe to
>    write 'q->rq_qos' concurrently.
> 
> 2) Activate cgroup policy that is relied on rq_qos will call
>    rq_qos_add() and blkcg_activate_policy(), and if rq_qos_exit() is
>    called in the middle, null-ptr-dereference will be triggered in
>    blkcg_activate_policy().
> 
> 3) blkg_conf_open_bdev() can call blkdev_get_no_open() first to find the
>    disk, then if rq_qos_exit() from del_gendisk() is done before
>    rq_qos_add(), then memory will be leaked.
> 
> This patch add a new disk level mutex 'rq_qos_mutex':
> 
> 1) The lock will protect rq_qos_exit() directly.
> 
> 2) For wbt that doesn't relied on blk-cgroup, rq_qos_add() can only be
>    called from disk initialization for now because wbt can't be
>    destructed until rq_qos_exit(), so it's safe not to protect wbt for
>    now. Hoever, in case that rq_qos dynamically destruction is supported
>    in the furture, this patch also protect rq_qos_add() from wbt_init()
>    directly, this is enough because blk-sysfs already synchronize
>    writers with disk removal.
> 
> 3) For iocost and iolatency, in order to synchronize disk removal and
>    cgroup configuration, the lock is held after blkdev_get_no_open()
>    from blkg_conf_open_bdev(), and is released in blkg_conf_exit().
>    In order to fix the above memory leak, disk_live() is checked after
>    holding the new lock.
> 
> Fixes: 50e34d78815e ("block: disable the elevator int del_gendisk")
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>

Acked-by: Tejun Heo <tj@kernel.org>

Thanks.

-- 
tejun