Subject: Re: [PATCH] net: Convert net_mutex into rw_semaphore and down read it on net->init/->exit

From: [email-masked] (Eric W. Biederman)
Date: Tue, 14 Nov 2017 21:19:50 -0600

Kirill Tkhai <[email-masked]> writes:

> On 14.11.2017 21:39, Cong Wang wrote:
>> On Tue, Nov 14, 2017 at 5:53 AM, Kirill Tkhai <[email-masked]> wrote:
>>> @@ -406,7 +406,7 @@ struct net *copy_net_ns(unsigned long flags,
>>>
>>>         get_user_ns(user_ns);
>>>
>>> -       rv = mutex_lock_killable(&net_mutex);
>>> +       rv = down_read_killable(&net_sem);
>>>         if (rv < 0) {
>>>                 net_free(net);
>>>                 dec_net_namespaces(ucounts);
>>> @@ -421,7 +421,7 @@ struct net *copy_net_ns(unsigned long flags,
>>>                 list_add_tail_rcu(&net->list, &net_namespace_list);
>>>                 rtnl_unlock();
>>>         }
>>> -       mutex_unlock(&net_mutex);
>>> +       up_read(&net_sem);
>>>         if (rv < 0) {
>>>                 dec_net_namespaces(ucounts);
>>>                 put_user_ns(user_ns);
>>> @@ -446,7 +446,7 @@ static void cleanup_net(struct work_struct *work)
>>>         list_replace_init(&cleanup_list, &net_kill_list);
>>>         spin_unlock_irq(&cleanup_list_lock);
>>>
>>> -       mutex_lock(&net_mutex);
>>> +       down_read(&net_sem);
>>>
>>>         /* Don't let anyone else find us. */
>>>         rtnl_lock();
>>> @@ -486,7 +486,7 @@ static void cleanup_net(struct work_struct *work)
>>>         list_for_each_entry_reverse(ops, &pernet_list, list)
>>>                 ops_free_list(ops, &net_exit_list);
>>>
>>> -       mutex_unlock(&net_mutex);
>>> +       up_read(&net_sem);
>> 
>> After your patch setup_net() could run concurrently with cleanup_net(),
>> given that ops_exit_list() is called on error path of setup_net() too,
>> it means ops->exit() now could run concurrently if it doesn't have its
>> own lock. Not sure if this breaks any existing user.
>
> Yes, there will be possible concurrent ops->init() for a net namespace,
> and ops->exit() for another one. I hadn't found pernet operations, which
> have a problem with that. If they exist, they are hidden and not clear seen.
> The pernet operations in general do not touch someone else's memory.
> If suddenly there is one, KASAN should show it after a while.

Certainly the use of hash tables shared between multiple network
namespaces would count.  I don't rembmer how many of these we have but
there used to be quite a few.

Eric