igmp: call ip_mc_clear_src() only when we have no users of ip_mc_list

In igmp_group_dropped() we call ip_mc_clear_src(), which resets the number
of source filters per mulitcast. However, igmp_group_dropped() is also
called on NETDEV_DOWN, NETDEV_PRE_TYPE_CHANGE and NETDEV_UNREGISTER, which
means that the group might get added back on NETDEV_UP, NETDEV_REGISTER and
NETDEV_POST_TYPE_CHANGE respectively, leaving us with broken source
filters.

To fix that, we must clear the source filters only when there are no users
in the ip_mc_list, i.e. in ip_mc_dec_group() and on device destroy.
Acked-by: NDavid L Stevens <dlstevens@us.ibm.com>
Signed-off-by: NVeaceslav Falico <vfalico@redhat.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/igmp.c

@@ -1155,20 +1155,18 @@ static void igmp_group_dropped(struct ip_mc_list *im)
 
 	if (!in_dev->dead) {
 		if (IGMP_V1_SEEN(in_dev))
-			goto done;
+			return;
 		if (IGMP_V2_SEEN(in_dev)) {
 			if (reporter)
 				igmp_send_report(in_dev, im, IGMP_HOST_LEAVE_MESSAGE);
-			goto done;
+			return;
 		}
 		/* IGMPv3 */
 		igmpv3_add_delrec(in_dev, im);
 
 		igmp_ifc_event(in_dev);
 	}
-done:
 #endif
-	ip_mc_clear_src(im);
 }
 
 static void igmp_group_added(struct ip_mc_list *im)
@@ -1305,6 +1303,7 @@ void ip_mc_dec_group(struct in_device *in_dev, __be32 addr)
 				*ip = i->next_rcu;
 				in_dev->mc_count--;
 				igmp_group_dropped(i);
+				ip_mc_clear_src(i);
 
 				if (!in_dev->dead)
 					ip_rt_multicast_event(in_dev);
@@ -1414,7 +1413,8 @@ void ip_mc_destroy_dev(struct in_device *in_dev)
 		in_dev->mc_list = i->next_rcu;
 		in_dev->mc_count--;
 
-		igmp_group_dropped(i);
+		/* We've dropped the groups in ip_mc_down already */
+		ip_mc_clear_src(i);
 		ip_ma_put(i);
 	}
 }