bonding: add rtnl protection for bonding_store_fail_over_mac

We need rtnl protection while reading slave_cnt and updating
the .fail_over_mac, and it also follows the logic "don't change
anything slave-related without rtnl". :)

Signed-off-by: Ding Tianhong <dingtianhong@huawei.com>
Cc: Jay Vosburgh <fubar@us.ibm.com>
Cc: Andy Gospodarek <andy@greyhouse.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/bonding/bond_sysfs.c

@@ -501,28 +501,35 @@ static ssize_t bonding_store_fail_over_mac(struct device *d,
 					   struct device_attribute *attr,
 					   const char *buf, size_t count)
 {
-	int new_value;
+	int new_value, ret = count;
 	struct bonding *bond = to_bond(d);
 
+	if (!rtnl_trylock())
+		return restart_syscall();
+
 	if (bond->slave_cnt != 0) {
 		pr_err("%s: Can't alter fail_over_mac with slaves in bond.\n",
 		       bond->dev->name);
-		return -EPERM;
+		ret = -EPERM;
+		goto out;
 	}
 
 	new_value = bond_parse_parm(buf, fail_over_mac_tbl);
 	if (new_value < 0) {
 		pr_err("%s: Ignoring invalid fail_over_mac value %s.\n",
 		       bond->dev->name, buf);
-		return -EINVAL;
+		ret = -EINVAL;
+		goto out;
 	}
 
 	bond->params.fail_over_mac = new_value;
 	pr_info("%s: Setting fail_over_mac to %s (%d).\n",
 		bond->dev->name, fail_over_mac_tbl[new_value].modename,
 		new_value);
 
-	return count;
+out:
+	rtnl_unlock();
+	return ret;
 }
 
 static DEVICE_ATTR(fail_over_mac, S_IRUGO | S_IWUSR,