Re: [PATCH v5 2/2] sock: Move the socket inuse to namespace.

[Tonghao Zhang <xiangxia.m.yue@gmail.com>]

hi all. we can add synchronize_rcu and rcu_barrier in sock_inuse_exit_net to
ensure there are no outstanding rcu callbacks using this network namespace.
we will not have to test if net->core.sock_inuse is NULL or not from
sock_inuse_add(). :)

 static void __net_exit sock_inuse_exit_net(struct net *net)
 {
        free_percpu(net->core.prot_inuse);
+
+       synchronize_rcu();
+       rcu_barrier();
+
+       free_percpu(net->core.sock_inuse);
 }


On Fri, Dec 8, 2017 at 5:52 PM, Tonghao Zhang <xiangxia.m.yue@gmail.com> wrote:
> On Fri, Dec 8, 2017 at 1:40 PM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
>> On Fri, 2017-12-08 at 13:28 +0800, Tonghao Zhang wrote:
>>> On Fri, Dec 8, 2017 at 1:20 AM, Eric Dumazet <eric.dumazet@gmail.com>
>>> wrote:
>>> > On Thu, 2017-12-07 at 08:45 -0800, Tonghao Zhang wrote:
>>> > > In some case, we want to know how many sockets are in use in
>>> > > different _net_ namespaces. It's a key resource metric.
>>> > >
>>> >
>>> > ...
>>> >
>>> > > +static void sock_inuse_add(struct net *net, int val)
>>> > > +{
>>> > > +     if (net->core.prot_inuse)
>>> > > +             this_cpu_add(*net->core.sock_inuse, val);
>>> > > +}
>>> >
>>> > This is very confusing.
>>> >
>>> > Why testing net->core.prot_inuse for NULL is needed at all ?
>>> >
>>> > Why not testing net->core.sock_inuse instead ?
>>> >
>>>
>>> Hi Eric and Cong, oh it's a typo. it's net->core.sock_inuse there.
>>> Why
>>> we should check the net->core.sock_inuse
>>> Now show you the code:
>>>
>>> cleanup_net will call all of the network namespace exit methods,
>>> rcu_barrier, and then remove the _net_ namespace.
>>>
>>> cleanup_net:
>>>     list_for_each_entry_reverse(ops, &pernet_list, list)
>>>          ops_exit_list(ops, &net_exit_list);
>>>
>>>     rcu_barrier(); /* for netlink sock, the ‘deferred_put_nlk_sk’
>>> will
>>> be called. But sock_inuse has been released. */
>>
>>
>> Thats would be a bug.
>>
>> Please find another way, but we want ultimately to check that before
>> net->core.sock_inuse is freed, folding the inuse count on all cpus is
>> 0, to make sure we do not have a bug somewhere.
>
> Yes, I am aware of this issue even we will destroy the network namespace.
> By the way, we can counter the socket-inuse in sock_alloc or sock_release.
> In this way, we have to hold the network namespace again(via
> get_net()) while sock
> may hold it.
>
> what do you think of this idea?
>
>> We should not have to test if net->core.sock_inuse is NULL or not from
>> sock_inuse_add(). Pointer must be there all the time.
>>
>> The freeing should only happen once we are sure sock_inuse_add() can
>> not be called anymore.
>>
>>>
>>>
>>>     /* Finally it is safe to free my network namespace structure */
>>>     list_for_each_entry_safe(net, tmp, &net_exit_list, exit_list) {}
>>>
>>>
>>>
>>> Release the netlink sock created in kernel(not hold the _net_
>>> namespace):
>>>
>>> netlink_release
>>>        call_rcu(&nlk->rcu, deferred_put_nlk_sk);
>>>
>>> deferred_put_nlk_sk
>>>        sk_free(sk);
>>>
>>>
>>> I may add a comment for sock_inuse_add in v6.
>>
>>