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Subject: Re: [PATCH] mtd: fix lock hierarchy in deregister_mtd_blktrans
To:     miquel.raynal@bootlin.com
Cc:     richard@nod.at, vigneshr@ti.com, linux-mtd@lists.infradead.org,
        linux-kernel@vger.kernel.org, skhan@linuxfoundation.org,
        gregkh@linuxfoundation.org,
        linux-kernel-mentees@lists.linuxfoundation.org,
        Hillf Danton <hdanton@sina.com>
References: <20210717100719.728829-1-desmondcheongzx@gmail.com>
From:   Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Message-ID: <999b1450-949e-fd90-6bf2-7fd4452158bb@gmail.com>
Date:   Mon, 2 Aug 2021 19:16:39 +0800
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On 17/7/21 6:07 pm, Desmond Cheong Zhi Xi wrote:
> There is a lock hierarchy of major_names_lock --> mtd_table_mutex. One
> existing chain is as follows:
> 
> 1. major_names_lock --> loop_ctl_mutex (when blk_request_module calls
> loop_probe)
> 
> 2. loop_ctl_mutex --> bdev->bd_mutex (when loop_control_ioctl calls
> loop_remove, which then calls del_gendisk)
> 
> 3. bdev->bd_mutex --> mtd_table_mutex (when blkdev_get_by_dev calls
> __blkdev_get, which then calls blktrans_open)
> 
> Since unregister_blkdev grabs the major_names_lock, we need to call it
> outside the critical section for mtd_table_mutex, otherwise we invert
> the lock hierarchy.
> 
> Reported-by: Hillf Danton <hdanton@sina.com>
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
> ---
>   drivers/mtd/mtd_blkdevs.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/mtd/mtd_blkdevs.c b/drivers/mtd/mtd_blkdevs.c
> index 7d26cfe24d05..87e26788ef60 100644
> --- a/drivers/mtd/mtd_blkdevs.c
> +++ b/drivers/mtd/mtd_blkdevs.c
> @@ -563,8 +563,8 @@ int deregister_mtd_blktrans(struct mtd_blktrans_ops *tr)
>   	list_for_each_entry_safe(dev, next, &tr->devs, list)
>   		tr->remove_dev(dev);
>   
> -	unregister_blkdev(tr->major, tr->name);
>   	mutex_unlock(&mtd_table_mutex);
> +	unregister_blkdev(tr->major, tr->name);
>   
>   	BUG_ON(!list_empty(&tr->devs));
>   	return 0;
> 

Hi Miqu?l,

Just a friendly ping, this patch is part 2 of fixing the lock hierarchy 
inversion between major_names_lock and mtd_table_mutex that was 
identified by Hillf Danton.

Best wishes,
Desmond