# Re: \[PATCH\] mtd: fix lock hierarchy in deregister\_mtd\_blktrans

- From: Desmond Cheong Zhi Xi \<hidden\>
- Date: 2021-08-02 11:16:47 
- List: linux-kernel-mentees
- Page: https://ratatoskr.run/linux-kernel-mentees/2021/08/1464830
- Thread: https://ratatoskr.run/lkml/2021/07/1448101/t.md

```text
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 <redacted>
> Signed-off-by: Desmond Cheong Zhi Xi <redacted>
> ---
>   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 Miquèl,

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
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```