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>

1 files changed, 4 insertions, 4 deletions

diff --git a/fs/nfs/dir.c b/fs/nfs/dir.c
index 1b5f38f48dab..36d921f0c602 100644
--- a/fs/nfs/dir.c
+++ b/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 @@ out_set_verifier:
         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;
         }