Revert "fs: use RCU read side protection in d_validate"

This reverts commit 3825bdb.

You cannot dget() a dentry without having a reference, or holding
a lock that guarantees it remains valid.

Signed-off-by: Nick Piggin <npiggin@kernel.dk>

fs/dcache.c

@@ -1491,26 +1491,33 @@ struct dentry *d_hash_and_lookup(struct dentry *dir, struct qstr *name)
  * This is used by ncpfs in its readdir implementation.
  * Zero is returned in the dentry is invalid.
  */
-int d_validate(struct dentry *dentry, struct dentry *parent)
+
+int d_validate(struct dentry *dentry, struct dentry *dparent)
 {
-	struct hlist_head *head = d_hash(parent, dentry->d_name.hash);
-	struct hlist_node *node;
-	struct dentry *d;
+	struct hlist_head *base;
+	struct hlist_node *lhp;
 
 	/* Check whether the ptr might be valid at all.. */
 	if (!kmem_ptr_validate(dentry_cache, dentry))
-		return 0;
-	if (dentry->d_parent != parent)
-		return 0;
+		goto out;
 
-	rcu_read_lock();
-	hlist_for_each_entry_rcu(d, node, head, d_hash) {
-		if (d == dentry) {
-			dget(dentry);
+	if (dentry->d_parent != dparent)
+		goto out;
+
+	spin_lock(&dcache_lock);
+	base = d_hash(dparent, dentry->d_name.hash);
+	hlist_for_each(lhp,base) { 
+		/* hlist_for_each_entry_rcu() not required for d_hash list
+		 * as it is parsed under dcache_lock
+		 */
+		if (dentry == hlist_entry(lhp, struct dentry, d_hash)) {
+			__dget_locked(dentry);
+			spin_unlock(&dcache_lock);
 			return 1;
 		}
 	}
-	rcu_read_unlock();
+	spin_unlock(&dcache_lock);
+out:
 	return 0;
 }
 EXPORT_SYMBOL(d_validate);