Re: [Patch net-next v2 3/3] igmp: convert RTNL lock to a spinlock

[Cong Wang <amwang@redhat.com>]

On Wed, 2013-06-05 at 17:38 +0800, Cong Wang wrote:
> -       while ((i = rtnl_dereference(in_dev->mc_list)) != NULL) {
> -               in_dev->mc_list = i->next_rcu;
> +       spin_lock(&ipv4_sk_mc_lock);
> +       while ((i = rcu_dereference_protected(in_dev->mc_list,
> +                       lockdep_is_held(&ipv4_sk_mc_lock))) != NULL) {
> +               rcu_assign_pointer(in_dev->mc_list, i->next_rcu);
>                 in_dev->mc_count--;
>  
>                 /* We've dropped the groups in ip_mc_down already */
>                 ip_mc_clear_src(i);
>                 ip_ma_put(i);
>         }
> +       spin_unlock(&ipv4_sk_mc_lock);
> +       synchronize_rcu(); 

Hmm, it seems this kind synchronize_rcu() is not needed, since we don't
free anything here, not very sure if it is okay that readers still have
the old value of ->mc_list... I think probably yes.