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

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Date:   Fri, 8 Dec 2017 17:52:06 +0800
From:   Tonghao Zhang <xiangxia.m.yue@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        Cong Wang <xiyou.wangcong@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH v5 2/2] sock: Move the socket inuse to namespace.

On Fri, Dec 8, 2017 at 1:40 PM, Eric Dumazet <eric.dumazet@...il.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@...il.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.
>
>