Per comment, record_root_in_trans orders the writes of the root->state
and root->last_trans:
      set_bit(BTRFS_ROOT_IN_TRANS_SETUP, &root->state);
      smp_wmb();
      root->last_trans = trans->transid;

But the barrier that enforces the order on the read side is misplaced:
     smp_rmb(); <-- misplaced
     if (root->last_trans == trans->transid &&
    <-- missing barrier here -->
            !test_bit(BTRFS_ROOT_IN_TRANS_SETUP, &root->state))

This patches fixes the ordering and wraps the racy accesses with
READ_ONCE and WRITE_ONCE calls to avoid load/store tearing.

Fixes: 7585717f304f5 ("Btrfs: fix relocation races")
Signed-off-by: Baptiste Lepers <redacted>
---
 fs/btrfs/transaction.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 14b9fdc8aaa9..a609222e6704 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c

@@ -437,7 +437,7 @@ static int record_root_in_trans(struct btrfs_trans_handle *trans,
 				   (unsigned long)root->root_key.objectid,
 				   BTRFS_ROOT_TRANS_TAG);
 		spin_unlock(&fs_info->fs_roots_radix_lock);
-		root->last_trans = trans->transid;
+		WRITE_ONCE(root->last_trans, trans->transid);
 
 		/* this is pretty tricky.  We don't want to
 		 * take the relocation lock in btrfs_record_root_in_trans

@@ -489,7 +489,7 @@ int btrfs_record_root_in_trans(struct btrfs_trans_handle *trans,
 			       struct btrfs_root *root)
 {
 	struct btrfs_fs_info *fs_info = root->fs_info;
-	int ret;
+	int ret, last_trans;
 
 	if (!test_bit(BTRFS_ROOT_SHAREABLE, &root->state))
 		return 0;

@@ -498,8 +498,9 @@ int btrfs_record_root_in_trans(struct btrfs_trans_handle *trans,
 	 * see record_root_in_trans for comments about IN_TRANS_SETUP usage
 	 * and barriers
 	 */
+	last_trans = READ_ONCE(root->last_trans);
 	smp_rmb();
-	if (root->last_trans == trans->transid &&
+	if (last_trans == trans->transid &&
 	    !test_bit(BTRFS_ROOT_IN_TRANS_SETUP, &root->state))