net: bridge: shorten ageing time on topology change

802.1D [1] specifies that the bridges must use a short value to age out
dynamic entries in the Filtering Database for a period, once a topology
change has been communicated by the root bridge.

Add a bridge_ageing_time member in the net_bridge structure to store the
bridge ageing time value configured by the user (ioctl/netlink/sysfs).

If we are using in-kernel STP, shorten the ageing time value to twice
the forward delay used by the topology when the topology change flag is
set. When the flag is cleared, restore the configured ageing time.

[1] "8.3.5 Notifying topology changes ",
    http://profesores.elo.utfsm.cl/~agv/elo309/doc/802.1D-1998.pdfSigned-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/bridge/br_device.c

@@ -409,7 +409,7 @@ void br_dev_setup(struct net_device *dev)
 	br->bridge_max_age = br->max_age = 20 * HZ;
 	br->bridge_hello_time = br->hello_time = 2 * HZ;
 	br->bridge_forward_delay = br->forward_delay = 15 * HZ;
-	br->ageing_time = BR_DEFAULT_AGEING_TIME;
+	br->bridge_ageing_time = br->ageing_time = BR_DEFAULT_AGEING_TIME;
 	dev->max_mtu = ETH_MAX_MTU;
 
 	br_netfilter_rtable_init(br);

net/bridge/br_private.h

@@ -300,10 +300,11 @@ struct net_bridge
 	unsigned long			max_age;
 	unsigned long			hello_time;
 	unsigned long			forward_delay;
-	unsigned long			bridge_max_age;
 	unsigned long			ageing_time;
+	unsigned long			bridge_max_age;
 	unsigned long			bridge_hello_time;
 	unsigned long			bridge_forward_delay;
+	unsigned long			bridge_ageing_time;
 
 	u8				group_addr[ETH_ALEN];
 	bool				group_addr_set;

net/bridge/br_stp.c

@@ -597,7 +597,11 @@ int br_set_ageing_time(struct net_bridge *br, clock_t ageing_time)
 	if (err)
 		return err;
 
+	spin_lock_bh(&br->lock);
+	br->bridge_ageing_time = t;
 	br->ageing_time = t;
+	spin_unlock_bh(&br->lock);
+
 	mod_timer(&br->gc_timer, jiffies);
 
 	return 0;
@@ -606,6 +610,29 @@ int br_set_ageing_time(struct net_bridge *br, clock_t ageing_time)
 /* called under bridge lock */
 void __br_set_topology_change(struct net_bridge *br, unsigned char val)
 {
+	unsigned long t;
+	int err;
+
+	if (br->stp_enabled == BR_KERNEL_STP && br->topology_change != val) {
+		/* On topology change, set the bridge ageing time to twice the
+		 * forward delay. Otherwise, restore its default ageing time.
+		 */
+
+		if (val) {
+			t = 2 * br->forward_delay;
+			br_debug(br, "decreasing ageing time to %lu\n", t);
+		} else {
+			t = br->bridge_ageing_time;
+			br_debug(br, "restoring ageing time to %lu\n", t);
+		}
+
+		err = __set_ageing_time(br->dev, t);
+		if (err)
+			br_warn(br, "error offloading ageing time\n");
+		else
+			br->ageing_time = t;
+	}
+
 	br->topology_change = val;
 }