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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 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.

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