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

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Date:   Tue, 14 Nov 2017 10:39:20 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Kirill Tkhai <ktkhai@...tuozzo.com>
Cc:     David Miller <davem@...emloft.net>, vyasevic@...hat.com,
        kstewart@...uxfoundation.org, pombredanne@...b.com,
        Vladislav Yasevich <vyasevich@...il.com>, mark.rutland@....com,
        Greg KH <gregkh@...uxfoundation.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Florian Westphal <fw@...len.de>,
        Nicolas Dichtel <nicolas.dichtel@...nd.com>,
        roman.kapl@...go.com, Paul Moore <paul@...l-moore.com>,
        David Ahern <dsahern@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        lucien xin <lucien.xin@...il.com>,
        Matthias Schiffer <mschiffer@...verse-factory.net>,
        rshearma@...cade.com, LKML <linux-kernel@...r.kernel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>, avagin@...tuozzo.com,
        gorcunov@...tuozzo.com
Subject: Re: [PATCH] net: Convert net_mutex into rw_semaphore and down read it
 on net->init/->exit

On Tue, Nov 14, 2017 at 5:53 AM, Kirill Tkhai <ktkhai@...tuozzo.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.