From: Steve Dickson <SteveD@redhat.com>
Date: Sat, 18 Oct 2008 08:25:42 -0400
Subject: [nfs] remove recoverable BUG_ON
Message-id: 48F9D5C6.6010602@RedHat.com
O-Subject: [PATCH] [RHEL5.3 exception] NFS: Remove recoverable BUG_ON()
Bugzilla: 458774
RH-Acked-by: Jeff Layton <jlayton@redhat.com>

The attached patch removes a BUG_ON() that can occurs when the client
transfers a different mount points on the server. I've personally
have tripped over this a number of times in the Fedora kernel
in some recent server work. The point being, the BUG_ON() can
pop fairly easily.

The point of the patch is when the client transverse a mount
point and the dirent is not a root, we don't need to bring
down the machine. Returning ESTALE is much more appropriate.

The patch will be (or has been) committed to 2.6.27.

The bz is: https://bugzilla.redhat.com/show_bug.cgi?id=458774
which was accepted as a exception for 5.3

steved.

commit 44d5759d3fdad660f000ef319f0ec33a6ac6ae28
Author: Denis V. Lunev <den@openvz.org>
Date:   Mon Aug 11 12:02:34 2008 +0400

    nfs: BUG_ON in nfs_follow_mountpoint

    Unfortunately, BUG_ON(IS_ROOT(dentry)) can happen inside
    nfs_follow_mountpoint with NFS running Fedora 8 using a
    specific setup.
    https://bugzilla.redhat.com/show_bug.cgi?id=458622

    So, the situation should be handled on NFS client gracefully.

    Signed-off-by: Denis V. Lunev <den@openvz.org>
    CC: Trond Myklebust <Trond.Myklebust@netapp.com>
    CC: J. Bruce Fields <bfields@fieldses.org>
    Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

diff --git a/fs/nfs/namespace.c b/fs/nfs/namespace.c
index f4763c1..f69d82a 100644
--- a/fs/nfs/namespace.c
+++ b/fs/nfs/namespace.c
@@ -100,7 +100,10 @@ static void * nfs_follow_mountpoint(struct dentry *dentry, struct nameidata *nd)
 
 	dprintk("--> nfs_follow_mountpoint()\n");
 
-	BUG_ON(IS_ROOT(dentry));
+	err = -ESTALE;
+	if (IS_ROOT(dentry))
+		goto out_err;
+
 	dprintk("%s: enter\n", __FUNCTION__);
 	dput(nd->dentry);
 	nd->dentry = dget(dentry);