netdev - [PATCH net-next] net: Add comment about pernet_operations methods and synchronization

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Message-ID: <152093850734.26057.10545422384266241590.stgit@localhost.localdomain>
Date:   Tue, 13 Mar 2018 13:55:55 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     davem@...emloft.net, ktkhai@...tuozzo.com, netdev@...r.kernel.org
Subject: [PATCH net-next] 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@...tuozzo.com>
---
 include/net/net_namespace.h |   14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/include/net/net_namespace.h b/include/net/net_namespace.h
index d4417495773a..71abc8d79178 100644
--- 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);