Subject: Re: [BUG] fs: btrfs: several possible ABBA deadlocks
To:     Josef Bacik <josef@toxicpanda.com>
Cc:     clm@fb.com, dsterba@suse.com, linux-btrfs@vger.kernel.org,
        linux-kernel <linux-kernel@vger.kernel.org>
References: <44b385ca-f00d-0b47-e370-bd7d97cb1be3@gmail.com>
 <YaQgFhuaQHsND/jr@localhost.localdomain>
From:   Jia-Ju Bai <baijiaju1990@gmail.com>
Message-ID: <f0ea2d7e-c82d-6749-6e90-48661f5e1dfd@gmail.com>
Date:   Mon, 29 Nov 2021 09:48:09 +0800
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On 2021/11/29 8:34, Josef Bacik wrote:
> On Sat, Nov 27, 2021 at 04:23:37PM +0800, Jia-Ju Bai wrote:
>> Hello,
>>
>> My static analysis tool reports several possible ABBA deadlocks in the btrfs
>> module in Linux 5.10:
>>
>> # DEADLOCK 1:
>> __clear_extent_bit()
>>    spin_lock(&tree->lock); --> Line 733 (Lock A)
>>    split_state()
>>      btrfs_split_delalloc_extent()
>>        spin_lock(&BTRFS_I(inode)->lock); --> Line 1870 (Lock B)
>>
>> btrfs_inode_safe_disk_i_size_write()
>>    spin_lock(&BTRFS_I(inode)->lock); --> Line 53 (Lock B)
>>    find_contiguous_extent_bit()
>>      spin_lock(&tree->lock); --> Line 1620 (Lock A)
>>
>> When __clear_extent_bit() and btrfs_inode_safe_disk_i_size_write() are
>> concurrently executed, the deadlock can occur.
>>
>> # DEADLOCK 2:
>> __set_extent_bit()
>>    spin_lock(&tree->lock); --> Line 995 (Lock A)
>>    set_state_bits()
>>      btrfs_set_delalloc_extent()
>>        spin_lock(&BTRFS_I(inode)->lock); --> Line 2007 or 2017 or 2029 (Lock
>> B)
>>
>> btrfs_inode_safe_disk_i_size_write()
>>    spin_lock(&BTRFS_I(inode)->lock); --> Line 53 (Lock B)
>>    find_contiguous_extent_bit()
>>      spin_lock(&tree->lock); --> Line 1620 (Lock A)
>>
>> When __set_extent_bit() and btrfs_inode_safe_disk_i_size_write() are
>> concurrently executed, the deadlock can occur.
>>
>> # DEADLOCK 3:
>> convert_extent_bit()
>>    spin_lock(&tree->lock); --> Line 1241 (Lock A)
>>    set_state_bits()
>>      btrfs_set_delalloc_extent()
>>        spin_lock(&BTRFS_I(inode)->lock); --> Line 2007 or 2017 or 2029 (Lock
>> B)
>>
>> btrfs_inode_safe_disk_i_size_write()
>>    spin_lock(&BTRFS_I(inode)->lock); --> Line 53 (Lock B)
>>    find_contiguous_extent_bit()
>>      spin_lock(&tree->lock); --> Line 1620 (Lock A)
>>
>> When convert_extent_bit() and btrfs_inode_safe_disk_i_size_write() are
>> concurrently executed, the deadlock can occur.
>>
>> I am not quite sure whether these possible deadlocks are real and how to fix
>> them if they are real.
>> Any feedback would be appreciated, thanks :)
>>
> Hey Jia-Ju,
>
> This is pretty good work, unfortunately it's wrong but it's in a subtle way that
> a tool wouldn't be able to catch.  The btrfs_inode_safe_disk_i_size_write()
> helper only messes with BTRFS_I(inode)->file_extent_tree, which is separate from
> the BTRFS_I(inode)->io_tree.  io_tree gets the btrfs_set_delalloc_extent() stuff
> called on it, but the file_extent_tree does not.  The file_extent_tree has
> inode->lock -> tree->lock as the locking order, whereas the file_extent_tree has
> inode->lock -> tree->lock as the locking order.

Okay, thanks for your explanation :)


Best wishes,
Jia-Ju Bai