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: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/include/net/net_namespace.h b/include/net/net_namespace.h
index d441749..71abc8d 100644 (file)
--- a/include/net/net_namespace.h
+++ b/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);