linux-kernel - Re: [PATCH] mtd: fix lock hierarchy in deregister_mtd

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Date:   Fri,  6 Aug 2021 21:45:56 +0200
From:   Miquel Raynal <miquel.raynal@...tlin.com>
To:     Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>,
        miquel.raynal@...tlin.com, richard@....at, vigneshr@...com
Cc:     linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org,
        skhan@...uxfoundation.org, gregkh@...uxfoundation.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        Hillf Danton <hdanton@...a.com>
Subject: Re: [PATCH] mtd: fix lock hierarchy in deregister_mtd_blktrans

On Sat, 2021-07-17 at 10:07:19 UTC, 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@...a.com>
> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>

Applied to https://git.kernel.org/pub/scm/linux/kernel/git/mtd/linux.git mtd/fixes, thanks.

Miquel