net: Add comment about pernet_operations methods and synchronization

Make locking scheme be visible for users, and provide
a comment what for we are need exit_batch() methods,
and when it should be used.
Signed-off-by: NKirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

include/net/net_namespace.h

@@ -312,6 +312,20 @@ struct net *get_net_ns_by_id(struct net *net, int id);
 
 struct pernet_operations {
 	struct list_head list;
+	/*
+	 * Below methods are called without any exclusive locks.
+	 * More than one net may be constructed and destructed
+	 * in parallel on several cpus. Every pernet_operations
+	 * have to keep in mind all other pernet_operations and
+	 * to introduce a locking, if they share common resources.
+	 *
+	 * Exit methods using blocking RCU primitives, such as
+	 * synchronize_rcu(), should be implemented via exit_batch.
+	 * Then, destruction of a group of net requires single
+	 * synchronize_rcu() related to these pernet_operations,
+	 * instead of separate synchronize_rcu() for every net.
+	 * Please, avoid synchronize_rcu() at all, where it's possible.
+	 */
 	int (*init)(struct net *net);
 	void (*exit)(struct net *net);
 	void (*exit_batch)(struct list_head *net_exit_list);