From: Jia-Ju Bai <baijiaju1990@gmail.com>
To: "Theodore Y. Ts'o" <tytso@mit.edu>
Cc: adilger.kernel@dilger.ca, linux-ext4@vger.kernel.org,
linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [BUG] fs: ext4: possible ABBA deadlock in ext4_inline_data_truncate() and ext4_punch_hole()
Date: Fri, 10 Dec 2021 10:03:37 +0800 [thread overview]
Message-ID: <c9f25d5a-0963-5b2d-b1f0-e7c7454f7153@gmail.com> (raw)
In-Reply-To: <YbI2IEzCVo+A6GTi@mit.edu>
On 2021/12/10 1:00, Theodore Y. Ts'o wrote:
> On Thu, Dec 09, 2021 at 07:10:44PM +0800, Jia-Ju Bai wrote:
>> Hello,
>>
>> My static analysis tool reports a possible ABBA deadlock in the ext4 module
>> in Linux 5.10:
>>
>> ext4_inline_data_truncate()
>> down_write(&EXT4_I(inode)->i_data_sem); --> Line 1895 (Lock A)
>> ext4_xattr_ibody_get()
>> ext4_xattr_inode_get()
>> ext4_xattr_inode_iget()
>> inode_lock(inode); --> Line 427 (Lock B)
>>
>> ext4_punch_hole()
>> inode_lock(inode); --> Line 4018 (Lock B)
>> ext4_update_disksize_before_punch()
>> ext4_update_i_disksize()
>> down_write(&EXT4_I(inode)->i_data_sem); --> Line 3248 (Lock A)
>>
>> When ext4_inline_data_truncate() and ext4_punch_hole() are concurrently
>> executed, the deadlock can occur.
>>
>> I am not quite sure whether this possible deadlock is real and how to fix it
>> if it is real.
> Hi,
>
> Thanks for the report. I don't believe this is deadlock is possible,
> because the first thing ext4_punch_hole() does is to check to see if
> the inode has inline data --- and if so, it calls
> ext4_convert_inline_data() to convert it to a normal file. In
> ext4_convert_inline_data(), we take the xattr lock, and then do the
> conversion, and then drop the xattr lock. So by the time
> ext4_punch_hole() starts doing its work, the inode is not an inline
> data file.
>
> In ext4_inline_data_truncate(), we take the xattr lock, and once we
> have the xattr lock, we check to see if inode is still an inline data
> file. If it has been converted, we then bail out.
>
> Hence, the ABBA deadlock that your static analysis tool has pointed
> shouldn't happen in practice.
Hi Ted,
Thank you very much for the detailed explanation!
I will improve my static analysis tool for this point.
Best wishes,
Jia-Ju Bai
next prev parent reply other threads:[~2021-12-10 2:03 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-12-09 11:10 [BUG] fs: ext4: possible ABBA deadlock in ext4_inline_data_truncate() and ext4_punch_hole() Jia-Ju Bai
2021-12-09 17:00 ` Theodore Y. Ts'o
2021-12-10 2:03 ` Jia-Ju Bai [this message]
2021-12-10 18:05 ` Theodore Y. Ts'o
2021-12-12 12:56 ` Jia-Ju Bai
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