NFS: Dont clobber more uptodate values in nfs_set_verifier()

nfs_lookup_revalidate and friends are not serialised, so it is currently
quite possible for the dentry to be revalidated, and then have the
updated verifier replaced with an older value by another process.
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/dir.c

@@ -637,7 +637,7 @@ int nfs_fsync_dir(struct file *filp, struct dentry *dentry, int datasync)
  * In the case it has, we assume that the dentries are untrustworthy
  * and may need to be looked up again.
  */
-static inline int nfs_check_verifier(struct inode *dir, struct dentry *dentry)
+static int nfs_check_verifier(struct inode *dir, struct dentry *dentry)
 {
 	if (IS_ROOT(dentry))
 		return 1;
@@ -652,6 +652,12 @@ static inline void nfs_set_verifier(struct dentry * dentry, unsigned long verf)
 	dentry->d_fsdata = (void *)verf;
 }
 
+static void nfs_refresh_verifier(struct dentry * dentry, unsigned long verf)
+{
+	if (time_after(verf, (unsigned long)dentry->d_fsdata))
+		nfs_set_verifier(dentry, verf);
+}
+
 /*
  * Whenever an NFS operation succeeds, we know that the dentry
  * is valid, so we update the revalidation timestamp.
@@ -785,7 +791,7 @@ static int nfs_lookup_revalidate(struct dentry * dentry, struct nameidata *nd)
 		goto out_bad;
 
 	nfs_renew_times(dentry);
-	nfs_set_verifier(dentry, verifier);
+	nfs_refresh_verifier(dentry, verifier);
  out_valid:
 	unlock_kernel();
 	dput(parent);
@@ -1085,7 +1091,7 @@ static int nfs_open_revalidate(struct dentry *dentry, struct nameidata *nd)
 	verifier = nfs_save_change_attribute(dir);
 	ret = nfs4_open_revalidate(dir, dentry, openflags, nd);
 	if (!ret)
-		nfs_set_verifier(dentry, verifier);
+		nfs_refresh_verifier(dentry, verifier);
 	unlock_kernel();
 out:
 	dput(parent);
@@ -1159,10 +1165,13 @@ static struct dentry *nfs_readdir_lookup(nfs_readdir_descriptor_t *desc)
 		dentry = alias;
 	}
 
-out_renew:
 	nfs_renew_times(dentry);
 	nfs_set_verifier(dentry, nfs_save_change_attribute(dir));
 	return dentry;
+out_renew:
+	nfs_renew_times(dentry);
+	nfs_refresh_verifier(dentry, nfs_save_change_attribute(dir));
+	return dentry;
 }
 
 /*
@@ -1700,7 +1709,7 @@ static int nfs_rename(struct inode *old_dir, struct dentry *old_dentry,
 	if (!error) {
 		d_move(old_dentry, new_dentry);
 		nfs_renew_times(new_dentry);
-		nfs_set_verifier(new_dentry, nfs_save_change_attribute(new_dir));
+		nfs_refresh_verifier(new_dentry, nfs_save_change_attribute(new_dir));
 	}
 
 	/* new dentry created? */