NFS: Fix an Oops in NFS unmount

Ensure that the dummy 'root dentry' is invisible to d_find_alias(). If not,
then it may be spliced into the tree if a parent directory from the same
filesystem gets mounted at a later time.
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/getroot.c

@@ -57,6 +57,17 @@ static int nfs_superblock_set_dummy_root(struct super_block *sb, struct inode *i
 		}
 		/* Circumvent igrab(): we know the inode is not being freed */
 		atomic_inc(&inode->i_count);
+		/*
+		 * Ensure that this dentry is invisible to d_find_alias().
+		 * Otherwise, it may be spliced into the tree by
+		 * d_materialise_unique if a parent directory from the same
+		 * filesystem gets mounted at a later time.
+		 * This again causes shrink_dcache_for_umount_subtree() to
+		 * Oops, since the test for IS_ROOT() will fail.
+		 */
+		spin_lock(&dcache_lock);
+		list_del_init(&sb->s_root->d_alias);
+		spin_unlock(&dcache_lock);
 	}
 	return 0;
 }