garp: remove last synchronize_rcu() call

When removing last vlan from a device, garp_uninit_applicant() calls
synchronize_rcu() to make sure no user can still manipulate struct
garp_applicant before we free it.

Use call_rcu() instead, as a step to further net_device dismantle
optimizations.

Add the temporary garp_cleanup_module() function to make sure no pending
call_rcu() are left at module unload time [ this will be removed when
kfree_rcu() is available ]

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/garp.h

@@ -104,6 +104,7 @@ struct garp_applicant {
 	struct sk_buff_head	queue;
 	struct sk_buff		*pdu;
 	struct rb_root		gid;
+	struct rcu_head		rcu;
 };
 
 struct garp_port {

net/802/garp.c

@@ -603,6 +603,11 @@ err1:
 }
 EXPORT_SYMBOL_GPL(garp_init_applicant);
 
+static void garp_app_kfree_rcu(struct rcu_head *head)
+{
+	kfree(container_of(head, struct garp_applicant, rcu));
+}
+
 void garp_uninit_applicant(struct net_device *dev, struct garp_application *appl)
 {
 	struct garp_port *port = rtnl_dereference(dev->garp_port);
@@ -611,7 +616,6 @@ void garp_uninit_applicant(struct net_device *dev, struct garp_application *appl
 	ASSERT_RTNL();
 
 	rcu_assign_pointer(port->applicants[appl->type], NULL);
-	synchronize_rcu();
 
 	/* Delete timer and generate a final TRANSMIT_PDU event to flush out
 	 * all pending messages before the applicant is gone. */
@@ -621,7 +625,7 @@ void garp_uninit_applicant(struct net_device *dev, struct garp_application *appl
 	garp_queue_xmit(app);
 
 	dev_mc_del(dev, appl->proto.group_address);
-	kfree(app);
+	call_rcu(&app->rcu, garp_app_kfree_rcu);
 	garp_release_port(dev);
 }
 EXPORT_SYMBOL_GPL(garp_uninit_applicant);
@@ -639,3 +643,9 @@ void garp_unregister_application(struct garp_application *appl)
 	stp_proto_unregister(&appl->proto);
 }
 EXPORT_SYMBOL_GPL(garp_unregister_application);
+
+static void __exit garp_cleanup_module(void)
+{
+	rcu_barrier(); /* Wait for completion of call_rcu()'s */
+}
+module_exit(garp_cleanup_module);