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    SubjectRe: [PATCH] net: Convert net_mutex into rw_semaphore and down read it on net->init/->exit
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    On 15.11.2017 19:31, Eric W. Biederman wrote:
    > Kirill Tkhai <ktkhai@virtuozzo.com> writes:
    >
    >> On 15.11.2017 12:51, Kirill Tkhai wrote:
    >>> On 15.11.2017 06:19, Eric W. Biederman wrote:
    >>>> Kirill Tkhai <ktkhai@virtuozzo.com> writes:
    >>>>
    >>>>> On 14.11.2017 21:39, Cong Wang wrote:
    >>>>>> On Tue, Nov 14, 2017 at 5:53 AM, Kirill Tkhai <ktkhai@virtuozzo.com> 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.
    >>>
    >>> Could you please provide an example of hash tables, you mean?
    >>
    >> Ah, I see, it's dccp_hashinfo etc.

    JFI, I've checked dccp_hashinfo, and it seems to be safe.

    >
    > The big one used to be the route cache. With resizable hash tables
    > things may be getting better in that regard.

    I've checked some fib-related things, and wasn't able to find that.
    Excuse me, could you please clarify, if it's an assumption, or
    there is exactly a problem hash table, you know? Could you please
    point it me more exactly, if it's so.

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