netdev - Re: [PATCH] wireless extensions: play with netns

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Date:	Thu, 18 Jun 2009 02:39:01 +0200
From:	Johannes Berg <johannes@...solutions.net>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	John Linville <linville@...driver.com>,
	Netdev <netdev@...r.kernel.org>,
	linux-wireless <linux-wireless@...r.kernel.org>,
	"Eric W. Biederman" <ebiederm@...stanetworks.com>,
	Alexey Dobriyan <adobriyan@...il.com>
Subject: Re: [PATCH] wireless extensions: play with netns

On Wed, 2009-06-17 at 17:30 -0700, Eric W. Biederman wrote:

> >> This of course gives you a network namespace that can be found by for_each_net rcu
> >> while the per net exit functions are running.  I think that opens up to races
> >> that I don't want to think about.
> >
> > Indeed. Can we move the rcu_barrier() up? Or we could insert a
> > synchronize_rcu() (which is sufficient for rcu_read_lock) before the
> > exit functions are run?
> 
> I don't think we can move the barrier.   But adding an extra synchronize_rcu
> should be fine.  We are talking about the slowest of the slow paths here.
> It is so slow it even gets it's own kernel thread.

Ok :) New patch below.

Yes, you're right, we can't move it, it might still be used from exit
for net_assign_generic or so.

johannes

Subject: net: make namespace iteration possible under RCU

We already call rcu_barrier(), so all we need to take
care of is using proper RCU list add/del primitives.

Signed-off-by: Johannes Berg <johannes@...solutions.net>
---
 include/net/net_namespace.h |    3 +++
 net/core/net_namespace.c    |   10 +++++++---
 2 files changed, 10 insertions(+), 3 deletions(-)

--- wireless-testing.orig/include/net/net_namespace.h	2009-06-18 01:36:26.000000000 +0200
+++ wireless-testing/include/net/net_namespace.h	2009-06-18 02:17:14.000000000 +0200
@@ -211,6 +211,9 @@ static inline struct net *read_pnet(stru
 #define for_each_net(VAR)				\
 	list_for_each_entry(VAR, &net_namespace_list, list)
 
+#define for_each_net_rcu(VAR)				\
+	list_for_each_entry_rcu(VAR, &net_namespace_list, list)
+
 #ifdef CONFIG_NET_NS
 #define __net_init
 #define __net_exit
--- wireless-testing.orig/net/core/net_namespace.c	2009-06-18 01:36:39.000000000 +0200
+++ wireless-testing/net/core/net_namespace.c	2009-06-18 02:03:06.000000000 +0200
@@ -6,6 +6,7 @@
 #include <linux/delay.h>
 #include <linux/sched.h>
 #include <linux/idr.h>
+#include <linux/rculist.h>
 #include <net/net_namespace.h>
 #include <net/netns/generic.h>
 
@@ -134,7 +135,7 @@ struct net *copy_net_ns(unsigned long fl
 	err = setup_net(new_net);
 	if (!err) {
 		rtnl_lock();
-		list_add_tail(&new_net->list, &net_namespace_list);
+		list_add_tail_rcu(&new_net->list, &net_namespace_list);
 		rtnl_unlock();
 	}
 	mutex_unlock(&net_mutex);
@@ -163,9 +164,12 @@ static void cleanup_net(struct work_stru
 
 	/* Don't let anyone else find us. */
 	rtnl_lock();
-	list_del(&net->list);
+	list_del_rcu(&net->list);
 	rtnl_unlock();
 
+	/* if somebody is rcu-iterating the list, wait */
+	synchronize_rcu();
+
 	/* Run all of the network namespace exit methods */
 	list_for_each_entry_reverse(ops, &pernet_list, list) {
 		if (ops->exit)
@@ -227,7 +231,7 @@ static int __init net_ns_init(void)
 	err = setup_net(&init_net);
 
 	rtnl_lock();
-	list_add_tail(&init_net.list, &net_namespace_list);
+	list_add_tail_rcu(&init_net.list, &net_namespace_list);
 	rtnl_unlock();
 
 	mutex_unlock(&net_mutex);


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