NFS: fix two problems in lookup_revalidate in RCU-walk

1/ rcu_dereference isn't correct: that field isn't
   RCU protected.   It could potentially change at any time
   so ACCESS_ONCE might be justified.

   changes to ->d_parent are protected by ->d_seq.  However
   that isn't always checked after ->d_revalidate is called,
   so it is safest to keep the double-check that ->d_parent
   hasn't changed at the end of these functions.

2/ in nfs4_lookup_revalidate, "->d_parent" was forgotten.
   So 'parent' was not the parent of 'dentry'.
   This fails safe is the context is that dentry->d_inode is
   NULL, and the result of parent->d_inode being NULL is
   that ECHILD is returned, which is always safe.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/dir.c

@@ -1102,7 +1102,7 @@ static int nfs_lookup_revalidate(struct dentry *dentry, unsigned int flags)
 	int error;
 
 	if (flags & LOOKUP_RCU) {
-		parent = rcu_dereference(dentry->d_parent);
+		parent = ACCESS_ONCE(dentry->d_parent);
 		dir = ACCESS_ONCE(parent->d_inode);
 		if (!dir)
 			return -ECHILD;
@@ -1184,7 +1184,7 @@ static int nfs_lookup_revalidate(struct dentry *dentry, unsigned int flags)
 	nfs_advise_use_readdirplus(dir);
  out_valid_noent:
 	if (flags & LOOKUP_RCU) {
-		if (parent != rcu_dereference(dentry->d_parent))
+		if (parent != ACCESS_ONCE(dentry->d_parent))
 			return -ECHILD;
 	} else
 		dput(parent);
@@ -1585,7 +1585,7 @@ static int nfs4_lookup_revalidate(struct dentry *dentry, unsigned int flags)
 		struct inode *dir;
 
 		if (flags & LOOKUP_RCU) {
-			parent = rcu_dereference(dentry);
+			parent = ACCESS_ONCE(dentry->d_parent);
 			dir = ACCESS_ONCE(parent->d_inode);
 			if (!dir)
 				return -ECHILD;
@@ -1599,7 +1599,7 @@ static int nfs4_lookup_revalidate(struct dentry *dentry, unsigned int flags)
 			ret = -ECHILD;
 		if (!(flags & LOOKUP_RCU))
 			dput(parent);
-		else if (parent != rcu_dereference(dentry))
+		else if (parent != ACCESS_ONCE(dentry->d_parent))
 			return -ECHILD;
 		goto out;
 	}