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