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

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Message-ID: <1512711658.25033.23.camel@gmail.com>
Date:   Thu, 07 Dec 2017 21:40:58 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Tonghao Zhang <xiangxia.m.yue@...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, 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.

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.