NFS: allow close-to-open cache semantics to apply to root of NFS filesystem

To obey NFS cache semantics, the client must verify the cached
attributes when a file is opened.  In most cases this is done by a call to
d_validate as one of the last steps in path_walk.

However for the root of a filesystem, d_validate is only ever called
on the mounted-on filesystem (except when the path ends '.' or '..').
So NFS has no chance to validate the attributes.

So, in nfs_opendir, we revalidate the attributes if the opened
directory is the mountpoint.  This may cause double-validation for "."
and ".." lookups, but that is better than missing regular /path/name
lookups completely.
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/dir.c

@@ -140,6 +140,13 @@ nfs_opendir(struct inode *inode, struct file *filp)
 
 	/* Call generic open code in order to cache credentials */
 	res = nfs_open(inode, filp);
+	if (filp->f_path.dentry == filp->f_path.mnt->mnt_root) {
+		/* This is a mountpoint, so d_revalidate will never
+		 * have been called, so we need to refresh the
+		 * inode (for close-open consistency) ourselves.
+		 */
+		__nfs_revalidate_inode(NFS_SERVER(inode), inode);
+	}
 	return res;
 }