gre: Simplify gre protocol registration locking.

Use cmpxchg() for atomic protocol registration which saves
code and data space.
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/gre.c

@@ -26,46 +26,32 @@
 
 
 static const struct gre_protocol __rcu *gre_proto[GREPROTO_MAX] __read_mostly;
-static DEFINE_SPINLOCK(gre_proto_lock);
 
 int gre_add_protocol(const struct gre_protocol *proto, u8 version)
 {
 	if (version >= GREPROTO_MAX)
-		goto err_out;
-
-	spin_lock(&gre_proto_lock);
-	if (gre_proto[version])
-		goto err_out_unlock;
-
-	RCU_INIT_POINTER(gre_proto[version], proto);
-	spin_unlock(&gre_proto_lock);
-	return 0;
+		return -EINVAL;
 
-err_out_unlock:
-	spin_unlock(&gre_proto_lock);
-err_out:
-	return -1;
+	return (cmpxchg((const struct gre_protocol **)&gre_proto[version], NULL, proto) == NULL) ?
+		0 : -EBUSY;
 }
 EXPORT_SYMBOL_GPL(gre_add_protocol);
 
 int gre_del_protocol(const struct gre_protocol *proto, u8 version)
 {
+	int ret;
+
 	if (version >= GREPROTO_MAX)
-		goto err_out;
-
-	spin_lock(&gre_proto_lock);
-	if (rcu_dereference_protected(gre_proto[version],
-			lockdep_is_held(&gre_proto_lock)) != proto)
-		goto err_out_unlock;
-	RCU_INIT_POINTER(gre_proto[version], NULL);
-	spin_unlock(&gre_proto_lock);
+		return -EINVAL;
+
+	ret = (cmpxchg((const struct gre_protocol **)&gre_proto[version], proto, NULL) == proto) ?
+		0 : -EBUSY;
+
+	if (ret)
+		return ret;
+
 	synchronize_rcu();
 	return 0;
-
-err_out_unlock:
-	spin_unlock(&gre_proto_lock);
-err_out:
-	return -1;
 }
 EXPORT_SYMBOL_GPL(gre_del_protocol);