On 29.11.21 г. 2:34, Josef Bacik wrote:

On Sat, Nov 27, 2021 at 04:23:37PM +0800, Jia-Ju Bai wrote:

quoted

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