Re: [PATCH v2] libbtrfsutil: fix race between subvolume iterator and deletion

[David Sterba <dsterba@suse.cz>]

On Wed, Jul 28, 2021 at 12:04:45PM -0700, Omar Sandoval wrote:
> From: Omar Sandoval <osandov@fb.com>
> 
> Subvolume iteration has a window between when we get a root ref (with
> BTRFS_IOC_TREE_SEARCH or BTRFS_IOC_GET_SUBVOL_ROOTREF) and when we look
> up the path of the parent directory (with BTRFS_IOC_INO_LOOKUP{,_USER}).
> If the subvolume is moved or deleted and its old parent directory is
> deleted during that window, then BTRFS_IOC_INO_LOOKUP{,_USER} will fail
> with ENOENT. The iteration will then fail with ENOENT as well.
> 
> We originally encountered this bug with an application that called
> `btrfs subvolume show` (which iterates subvolumes to find snapshots) in
> parallel with other threads creating and deleting subvolumes. It can be
> reproduced almost instantly with the included test cases.
> 
> Subvolume iteration should be robust against concurrent modifications to
> subvolumes. So, if a subvolume's parent directory no longer exists, just
> skip the subvolume, as it must have been deleted or moved elsewhere.
> 
> Reviewed-by: Neal Gompa <ngompa13@gmail.com>
> Signed-off-by: Omar Sandoval <osandov@fb.com>
> ---
> Changes from v1 -> v2:
> 
> - Added Neal's reviewed-by.
> - Added test cases.

Replaced in devel, thanks.