+int xfs_iunlink_init(struct xfs_perag *pag);
+void xfs_iunlink_destroy(struct xfs_perag *pag);
+xfs_agino_t xfs_iunlink_lookup_backref(struct xfs_perag *pag,
+		xfs_agino_t agino);
+int xfs_iunlink_add_backref(struct xfs_perag *pag, xfs_agino_t prev_agino,
+		xfs_agino_t this_agino);
+int xfs_iunlink_change_backref(struct xfs_perag *pag, xfs_agino_t prev_agino,
+		xfs_agino_t this_agino);

xfs_iunlink_lookup_backref and xfs_iunlink_change_backref aren't
used outside of xfs_inode.c and should be marked static.

+	/*
+	 * Make sure the in-core data knows about this unlinked inode.  Since
+	 * our iunlinks recovery basically just deletes the head of a bucket
+	 * list until the bucket is empty, we need only to add the backref from
+	 * the current list item to the next one, if this isn't the list tail.
+	 */
 	pag = xfs_perag_get(mp, agno);
 	pag->pagi_unlinked_count++;
+	if (agino != NULLAGINO)
+		error = xfs_iunlink_add_backref(pag, XFS_INO_TO_AGINO(mp, ino),
+				agino);
 	xfs_perag_put(pag);
+	if (error)
+		goto fail_iput;

Note that the previos agino that we recaculate above is actually passed
to the function as an argument.  I think we should just add a new
next_agino variable for the one we read from the dinode and return and
reuse the argument here instead of recaculating it.

Question: what lock now protects the rhastable modifications?  Maybe
we need to add some lockdep asserts to document that.