ceph: use READ_ONCE to access d_parent in RCU critical section

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

fs/ceph/mds_client.c

@@ -913,7 +913,7 @@ static int __choose_mds(struct ceph_mds_client *mdsc,
 		struct inode *dir;
 
 		rcu_read_lock();
-		parent = req->r_dentry->d_parent;
+		parent = READ_ONCE(req->r_dentry->d_parent);
 		dir = req->r_parent ? : d_inode_rcu(parent);
 
 		if (!dir || dir->i_sb != mdsc->fsc->sb) {
@@ -2145,7 +2145,7 @@ char *ceph_mdsc_build_path(struct dentry *dentry, int *plen, u64 *pbase,
 			memcpy(path + pos, temp->d_name.name, temp->d_name.len);
 		}
 		spin_unlock(&temp->d_lock);
-		temp = temp->d_parent;
+		temp = READ_ONCE(temp->d_parent);
 
 		/* Are we at the root? */
 		if (IS_ROOT(temp))