batman-adv: Avoid deadlock between rtnl_lock and s_active

The hard_if_event is called by the notifier with rtnl_lock and tries to
remove sysfs entries when a NETDEV_UNREGISTER event is received. This
will automatically take the s_active lock.

The s_active lock is also used when a new interface is added to a meshif
through sysfs. In that situation we cannot wait for the rntl_lock before
creating the actual batman-adv interface to prevent a deadlock. It is
still possible to try to get the rtnl_lock and immediately abort the
current operation when the trylock call failed.
Signed-off-by: Sven Eckelmann <sven@narfation.org>

net/batman-adv/bat_sysfs.c

@@ -488,24 +488,24 @@ static ssize_t store_mesh_iface(struct kobject *kobj, struct attribute *attr,
 	    (strncmp(hard_iface->soft_iface->name, buff, IFNAMSIZ) == 0))
 		goto out;
 
+	if (!rtnl_trylock()) {
+		ret = -ERESTARTSYS;
+		goto out;
+	}
+
 	if (status_tmp == IF_NOT_IN_USE) {
-		rtnl_lock();
 		hardif_disable_interface(hard_iface);
-		rtnl_unlock();
-		goto out;
+		goto unlock;
 	}
 
 	/* if the interface already is in use */
-	if (hard_iface->if_status != IF_NOT_IN_USE) {
-		rtnl_lock();
+	if (hard_iface->if_status != IF_NOT_IN_USE)
 		hardif_disable_interface(hard_iface);
-		rtnl_unlock();
-	}
 
-	rtnl_lock();
 	ret = hardif_enable_interface(hard_iface, buff);
-	rtnl_unlock();
 
+unlock:
+	rtnl_unlock();
 out:
 	hardif_free_ref(hard_iface);
 	return ret;