team: avoid possible underflow of count_pending value for notify_peers and mcast_rejoin

This patch is fixing a race condition that may cause setting
count_pending to -1, which results in unwanted big bulk of arp messages
(in case of "notify peers").

Consider following scenario:

count_pending == 2
   CPU0                                           CPU1
					team_notify_peers_work
					  atomic_dec_and_test (dec count_pending to 1)
					  schedule_delayed_work
 team_notify_peers
   atomic_add (adding 1 to count_pending)
					team_notify_peers_work
					  atomic_dec_and_test (dec count_pending to 1)
					  schedule_delayed_work
					team_notify_peers_work
					  atomic_dec_and_test (dec count_pending to 0)
   schedule_delayed_work
					team_notify_peers_work
					  atomic_dec_and_test (dec count_pending to -1)

Fix this race by using atomic_dec_if_positive - that will prevent
count_pending running under 0.

Fixes: fc423ff ("team: add peer notification")
Fixes: 492b200  ("team: add support for sending multicast rejoins")
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: jpirko

drivers/net/team/team.c

@@ -629,16 +629,22 @@ static int team_change_mode(struct team *team, const char *kind)
 static void team_notify_peers_work(struct work_struct *work)
 {
 	struct team *team;
+	int val;
 
 	team = container_of(work, struct team, notify_peers.dw.work);
 
 	if (!rtnl_trylock()) {
 		schedule_delayed_work(&team->notify_peers.dw, 0);
 		return;
 	}
+	val = atomic_dec_if_positive(&team->notify_peers.count_pending);
+	if (val < 0) {
+		rtnl_unlock();
+		return;
+	}
 	call_netdevice_notifiers(NETDEV_NOTIFY_PEERS, team->dev);
 	rtnl_unlock();
-	if (!atomic_dec_and_test(&team->notify_peers.count_pending))
+	if (val)
 		schedule_delayed_work(&team->notify_peers.dw,
 				      msecs_to_jiffies(team->notify_peers.interval));
 }
@@ -669,16 +675,22 @@ static void team_notify_peers_fini(struct team *team)
 static void team_mcast_rejoin_work(struct work_struct *work)
 {
 	struct team *team;
+	int val;
 
 	team = container_of(work, struct team, mcast_rejoin.dw.work);
 
 	if (!rtnl_trylock()) {
 		schedule_delayed_work(&team->mcast_rejoin.dw, 0);
 		return;
 	}
+	val = atomic_dec_if_positive(&team->mcast_rejoin.count_pending);
+	if (val < 0) {
+		rtnl_unlock();
+		return;
+	}
 	call_netdevice_notifiers(NETDEV_RESEND_IGMP, team->dev);
 	rtnl_unlock();
-	if (!atomic_dec_and_test(&team->mcast_rejoin.count_pending))
+	if (val)
 		schedule_delayed_work(&team->mcast_rejoin.dw,
 				      msecs_to_jiffies(team->mcast_rejoin.interval));
 }