btrfs: fix barrier usage in btrfs_record_root_in_trans()

The barrier usage in btrfs_record_root_in_trans() is wrong, as the writer
side, in record_root_in_trans(), sets BTRFS_ROOT_IN_TRANS_SETUP, does a
write barrier and then sets the root's last transaction. This means the
reader side must check the root's last transaction, issue a read barrier
and then check for BTRFS_ROOT_IN_TRANS_SETUP. However, currently we issue
a read barrier and then check the root's last transaction and the bit
BTRFS_ROOT_IN_TRANS_SETUP, which can be problematic because the CPU is
free to reorder the checks and the following can happen:

1) Before reading the root's last_trans, it checks that
   BTRFS_ROOT_IN_TRANS_SETUP is not set.

2) A writer sets BTRFS_ROOT_IN_TRANS_SETUP, does smp_wmb() and updates
   the root's last_trans.

3) The reader then sees the root's last_trans matches the current
   transaction and falsely concludes the root setup is completes and
   returns without waiting for the writer task to complete the setup
   (calling btrfs_init_reloc_root()).

So fix the reading ordered as previously described: check the root's
last_trans, issue read barrier and then check BTRFS_ROOT_IN_TRANS_SETUP
(the reverse of what the writer side does).

Assisted-by: LLM
Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

0 files changed, 0 insertions, 0 deletions