Johannes Berg <johannes@xxxxxxxxxxxxxxxx> writes:
> 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.
Except for the changelog which is wrong about not needing any rcu
primitives I can't spot any bugs.
> 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@xxxxxxxxxxxxxxxx>
> ---
> 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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