cifs: don't instantiate new dentries in readdir for inodes that need to be revalidated immediately

commit 757c4f62 upstream.

David reported that commit c2b93e06 (cifs: only set ops for inodes in
I_NEW state) caused a regression with mfsymlinks. Prior to that patch,
if a mfsymlink dentry was instantiated at readdir time, the inode would
get a new set of ops when it was revalidated. After that patch, this
did not occur.

This patch addresses this by simply skipping instantiating dentries in
the readdir codepath when we know that they will need to be immediately
revalidated. The next attempt to use that dentry will cause a new lookup
to occur (which is basically what we want to happen anyway).

Cc: "Stefan (metze) Metzmacher" <metze@samba.org>
Cc: Sachin Prabhu <sprabhu@redhat.com>
Reported-and-Tested-by: David McBride <dwm37@cam.ac.uk>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>
[bwh: Backported to 3.2: need to return NULL]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Cc: Rui Xiang <rui.xiang@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/cifs/readdir.c

@@ -96,6 +96,14 @@ cifs_readdir_lookup(struct dentry *parent, struct qstr *name,
 		dput(dentry);
 	}
 
+	/*
+	 * If we know that the inode will need to be revalidated immediately,
+	 * then don't create a new dentry for it. We'll end up doing an on
+	 * the wire call either way and this spares us an invalidation.
+	 */
+	if (fattr->cf_flags & CIFS_FATTR_NEED_REVAL)
+		return NULL;
+
 	dentry = d_alloc(parent, name);
 	if (dentry == NULL)
 		return NULL;