Staging: batman-adv: Use synchronize_rcu instead of call_rcu

It is recommended [1] to use synchronize_rcu to simplify the code -
especially when otherwise extra locking is needed to protect other code
from picking stale elements. It also protects us for emitting to many
callbacks which may results in OOM conditions.

The only reason not to use it, would be in performance critical sections
or when we are not allowed to block.

[1] Documentation/RCU/checklist.txt
Signed-off-by: Sven Eckelmann <sven.eckelmann@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/batman-adv/TODO

  * Rework usage of RCU
     - don't leak pointers from rcu out of rcu critical area which may
       get freed
-    - check were synchronize_rcu must be used
     - go through Documentation/RCU/checklist.txt
  * Request a new review
  * Process the comments from the review

drivers/staging/batman-adv/hard-interface.c

@@ -418,13 +418,6 @@ static struct batman_if *hardif_add_interface(struct net_device *net_dev)
 	return NULL;
 }
 
-static void hardif_free_interface(struct rcu_head *rcu)
-{
-	struct batman_if *batman_if = container_of(rcu, struct batman_if, rcu);
-
-	kfree(batman_if);
-}
-
 static void hardif_remove_interface(struct batman_if *batman_if)
 {
 	/* first deactivate interface */
@@ -438,9 +431,10 @@ static void hardif_remove_interface(struct batman_if *batman_if)
 
 	/* caller must take if_list_lock */
 	list_del_rcu(&batman_if->list);
+	synchronize_rcu();
 	sysfs_del_hardif(&batman_if->hardif_obj);
 	dev_put(batman_if->net_dev);
-	call_rcu(&batman_if->rcu, hardif_free_interface);
+	kfree(batman_if);
 }
 
 void hardif_remove_interfaces(void)

drivers/staging/batman-adv/types.h

@@ -44,7 +44,6 @@ struct batman_if {
 	unsigned char *packet_buff;
 	int packet_len;
 	struct kobject *hardif_obj;
-	struct rcu_head rcu;
 	struct packet_type batman_adv_ptype;
 	struct net_device *soft_iface;
 };