[BUG] fs: btrfs: several possible ABBA deadlocks

Jia-Ju Bai
2021-11-27T08:25:51Z

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