bonding: restructure locking of bond_ab_arp_probe()

Currently we're calling it from under RCU context, however we're using some
functions that require rtnl to be held.

Fix this by restructuring the locking - don't call it under any locks,
aquire rcu_read_lock() if we're sending _only_ (i.e. we have the active
slave present), and use rtnl locking otherwise - if we need to modify
(in)active flags of a slave.

CC: Jay Vosburgh <fubar@us.ibm.com>
CC: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: Veaceslav Falico <vfalico@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/bonding/bond_main.c

@@ -2599,17 +2599,18 @@ static void bond_ab_arp_commit(struct bonding *bond)
 
 /*
  * Send ARP probes for active-backup mode ARP monitor.
- *
- * Called with rcu_read_lock hold.
  */
-static void bond_ab_arp_probe(struct bonding *bond)
+static bool bond_ab_arp_probe(struct bonding *bond)
 {
 	struct slave *slave, *before = NULL, *new_slave = NULL,
-		     *curr_arp_slave = rcu_dereference(bond->current_arp_slave),
-		     *curr_active_slave = rcu_dereference(bond->curr_active_slave);
+		     *curr_arp_slave, *curr_active_slave;
 	struct list_head *iter;
 	bool found = false;
 
+	rcu_read_lock();
+	curr_arp_slave = rcu_dereference(bond->current_arp_slave);
+	curr_active_slave = rcu_dereference(bond->curr_active_slave);
+
 	if (curr_arp_slave && curr_active_slave)
 		pr_info("PROBE: c_arp %s && cas %s BAD\n",
 			curr_arp_slave->dev->name,
@@ -2617,23 +2618,32 @@ static void bond_ab_arp_probe(struct bonding *bond)
 
 	if (curr_active_slave) {
 		bond_arp_send_all(bond, curr_active_slave);
-		return;
+		rcu_read_unlock();
+		return true;
 	}
+	rcu_read_unlock();
 
 	/* if we don't have a curr_active_slave, search for the next available
 	 * backup slave from the current_arp_slave and make it the candidate
 	 * for becoming the curr_active_slave
 	 */
 
+	if (!rtnl_trylock())
+		return false;
+	/* curr_arp_slave might have gone away */
+	curr_arp_slave = ACCESS_ONCE(bond->current_arp_slave);
+
 	if (!curr_arp_slave) {
-		curr_arp_slave = bond_first_slave_rcu(bond);
-		if (!curr_arp_slave)
-			return;
+		curr_arp_slave = bond_first_slave(bond);
+		if (!curr_arp_slave) {
+			rtnl_unlock();
+			return true;
+		}
 	}
 
 	bond_set_slave_inactive_flags(curr_arp_slave);
 
-	bond_for_each_slave_rcu(bond, slave, iter) {
+	bond_for_each_slave(bond, slave, iter) {
 		if (!found && !before && IS_UP(slave->dev))
 			before = slave;
 
@@ -2663,21 +2673,26 @@ static void bond_ab_arp_probe(struct bonding *bond)
 	if (!new_slave && before)
 		new_slave = before;
 
-	if (!new_slave)
-		return;
+	if (!new_slave) {
+		rtnl_unlock();
+		return true;
+	}
 
 	new_slave->link = BOND_LINK_BACK;
 	bond_set_slave_active_flags(new_slave);
 	bond_arp_send_all(bond, new_slave);
 	new_slave->jiffies = jiffies;
 	rcu_assign_pointer(bond->current_arp_slave, new_slave);
+	rtnl_unlock();
+
+	return true;
 }
 
 static void bond_activebackup_arp_mon(struct work_struct *work)
 {
 	struct bonding *bond = container_of(work, struct bonding,
 					    arp_work.work);
-	bool should_notify_peers = false;
+	bool should_notify_peers = false, should_commit = false;
 	int delta_in_ticks;
 
 	delta_in_ticks = msecs_to_jiffies(bond->params.arp_interval);
@@ -2686,12 +2701,11 @@ static void bond_activebackup_arp_mon(struct work_struct *work)
 		goto re_arm;
 
 	rcu_read_lock();
-
 	should_notify_peers = bond_should_notify_peers(bond);
+	should_commit = bond_ab_arp_inspect(bond);
+	rcu_read_unlock();
 
-	if (bond_ab_arp_inspect(bond)) {
-		rcu_read_unlock();
-
+	if (should_commit) {
 		/* Race avoidance with bond_close flush of workqueue */
 		if (!rtnl_trylock()) {
 			delta_in_ticks = 1;
@@ -2700,13 +2714,14 @@ static void bond_activebackup_arp_mon(struct work_struct *work)
 		}
 
 		bond_ab_arp_commit(bond);
-
 		rtnl_unlock();
-		rcu_read_lock();
 	}
 
-	bond_ab_arp_probe(bond);
-	rcu_read_unlock();
+	if (!bond_ab_arp_probe(bond)) {
+		/* rtnl locking failed, re-arm */
+		delta_in_ticks = 1;
+		should_notify_peers = false;
+	}
 
 re_arm:
 	if (bond->params.arp_interval)