linux-kernel - [BUG] fs: super: possible ABBA deadlocks in do_thaw_all_callback() and freeze

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Date:   Mon, 27 Dec 2021 10:03:35 +0800
From:   Jia-Ju Bai <baijiaju1990@...il.com>
To:     viro@...iv.linux.org.uk, Jens Axboe <axboe@...nel.dk>,
        hch@...radead.org
Cc:     linux-fsdevel@...r.kernel.org, linux-block@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: [BUG] fs: super: possible ABBA deadlocks in do_thaw_all_callback()
 and freeze_bdev()

Hello,

My static analysis tool reports several possible ABBA deadlocks in Linux 
5.10:

do_thaw_all_callback()
   down_write(&sb->s_umount); --> Line 1028 (Lock A)
   emergency_thaw_bdev()
     thaw_bdev()
       mutex_lock(&bdev->bd_fsfreeze_mutex); --> Line 602 (Lock B)

freeze_bdev()
   mutex_lock(&bdev->bd_fsfreeze_mutex); --> Line 556 (Lock B)
   freeze_super()
     down_write(&sb->s_umount); --> Line 1716 (Lock A)
     down_write(&sb->s_umount); --> Line 1738 (Lock A)
   deactivate_super()
     down_write(&s->s_umount); --> Line 365 (Lock A)

When do_thaw_all_callback() and freeze_bdev() are concurrently executed, 
the deadlocks can occur.

I am not quite sure whether these possible deadlocks are real and how to 
fix them if them are real.
Any feedback would be appreciated, thanks :)

Reported-by: TOTE Robot <oslab@...nghua.edu.cn>


Best wishes,
Jia-Ju Bai