From:   Jia-Ju Bai <baijiaju1990@gmail.com>
Subject: [BUG] fs: possible ABBA deadlocks in do_thaw_all_callback() and
 freeze_bdev()
To:     viro@zeniv.linux.org.uk, andrea@suse.de, axboe@kernel.dk,
        hch@infradead.org
Cc:     linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-block@vger.kernel.org
Message-ID: <bcc39dfd-3734-bb82-b327-8445aedef605@gmail.com>
Date:   Fri, 10 Dec 2021 12:03:26 +0800
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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@tsinghua.edu.cn>


Best wishes,
Jia-Ju Bai