[BUG] fs: btrfs: several possible ABBA deadlocks

[BUG] fs: btrfs: several possible ABBA deadlocks

[Jia-Ju Bai]

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 :)

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


Best wishes,
Jia-Ju Bai

Re: [BUG] fs: btrfs: several possible ABBA deadlocks

[Josef Bacik]

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.  Thanks,

Josef

Re: [BUG] fs: btrfs: several possible ABBA deadlocks

[Jia-Ju Bai]

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

Re: [BUG] fs: btrfs: several possible ABBA deadlocks

[Nikolay Borisov]

On 29.11.21 г. 2:34, 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.  Thanks,

nit: did you mean to reverse tree->lock ->inode->lock for the
file_extent_tree?

> 
> Josef
>