Btrfs: fix wrong dentries after fsync of file that got its parent replaced

Btrfs: fix wrong dentries after fsync of file that got its parent replaced

In a scenario like the following:

  mkdir /mnt/A               # inode 258
  mkdir /mnt/B               # inode 259
  touch /mnt/B/bar           # inode 260

  sync

  mv /mnt/B/bar /mnt/A/bar
  mv -T /mnt/A /mnt/B
  fsync /mnt/B/bar

  <power fail>

After replaying the log we end up with file bar having 2 hard links, both
with the name 'bar' and one in the directory with inode number 258 and the
other in the directory with inode number 259. Also, we end up with the
directory inode 259 still existing and with the directory inode 258 still
named as 'A', instead of 'B'. In this scenario, file 'bar' should only
have one hard link, located at directory inode 258, the directory inode
259 should not exist anymore and the name for directory inode 258 should
be 'B'.

This incorrect behaviour happens because when attempting to log the old
parents of an inode, we skip any parents that no longer exist. Fix this
by forcing a full commit if an old parent no longer exists.

A test case for fstests follows soon.

CC: stable@vger.kernel.org # 4.4+
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>