[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@xxxxxxxxxxxxxxx>


Best wishes,
Jia-Ju Bai