From: Fan Du <fan.du@windriver•com>
To: "Eric W. Biederman" <ebiederm@xmission•com>
Cc: <serge.hallyn@canonical•com>, <davem@davemloft•net>,
<netdev@vger•kernel.org>
Subject: Re: [PATCH] net namespace: wrap for_each_net with rtnl_lock
Date: Thu, 4 Jul 2013 15:57:14 +0800 [thread overview]
Message-ID: <51D52ADA.5020905@windriver.com> (raw)
In-Reply-To: <87txkaolfo.fsf@xmission.com>
Hi, Eric
Thanks for your reply!
On 2013年07月04日 15:04, Eric W. Biederman wrote:
> Fan Du<fan.du@windriver•com> writes:
>
>> The read access to net_namespace_list with for_each_net should always
>> be protected with rtnl_lock agiast any adding/removing operation from
>> the list.
>
> That is not correct. The rtnl_lock does not protect the
> net_namespace_list. The net_mutex provides that protection.
>
> Modifications to the net_namespace_list are under both the rtnl_lock
> and the net_mutex which removes the need of grabbing the net_mutex when
> you just need to traverse the list of network namespaces. This avoids a
> lock ordering problem as most places it is desirable to traverse the
> net namespace list the rtnl_lock is already held.
By my understanding, net_mutex protects operations on pernet_list, and rtln_lock
protects net_namespace_list. net_mutex has side effects on net_namespace_list,
because we try to hold rtnl_lock to modify net_namespace_list after already holding
net_mutex(copy_net_ns). Sorry, I cann't understand the necessity by doing so.
(1)
mutex_lock(&net_mutex);
rv = setup_net(net, user_ns);
if (rv == 0) {
rtnl_lock();
list_add_tail_rcu(&net->list, &net_namespace_list);
rtnl_unlock();
}
mutex_unlock(&net_mutex);
why could we do it separately as below?
(2)
mutex_lock(&net_mutex);
rv = setup_net(net, user_ns);
mutex_unlock(&net_mutex);
if (rv == 0) {
rtnl_lock();
list_add_tail_rcu(&net->list, &net_namespace_list);
rtnl_unlock();
}
>
> In general the init methods will deadlock if you call them with the
> rtnl_lock held, as they grab the rtnl_lock when creating network devices
> etc.
Yes, I understand. This is reason why we do it in (1) style.
> The methods you change are protected by the net_mutex so I don't see any
> problems here.
> Was this patch inspired by code review or was there an actual problem
> that inspired it?
It's code review, no real alarm :)
>> Signed-off-by: Fan Du<fan.du@windriver•com>
>> ---
>> net/core/net_namespace.c | 4 ++++
>> 1 files changed, 4 insertions(+), 0 deletions(-)
>>
>> diff --git a/net/core/net_namespace.c b/net/core/net_namespace.c
>> index f976520..f3808ff 100644
>> --- a/net/core/net_namespace.c
>> +++ b/net/core/net_namespace.c
>> @@ -445,12 +445,14 @@ static int __register_pernet_operations(struct list_head *list,
>>
>> list_add_tail(&ops->list, list);
>> if (ops->init || (ops->id&& ops->size)) {
>> + rtnl_lock();
>> for_each_net(net) {
>> error = ops_init(ops, net);
>> if (error)
>> goto out_undo;
>> list_add_tail(&net->exit_list,&net_exit_list);
>> }
>> + rtnl_unlock();
>> }
>> return 0;
>>
>> @@ -468,8 +470,10 @@ static void __unregister_pernet_operations(struct pernet_operations *ops)
>> LIST_HEAD(net_exit_list);
>>
>> list_del(&ops->list);
>> + rtnl_lock();
>> for_each_net(net)
>> list_add_tail(&net->exit_list,&net_exit_list);
>> + rtnl_unlock();
>> ops_exit_list(ops,&net_exit_list);
>> ops_free_list(ops,&net_exit_list);
>> }
>
--
浮沉随浪只记今朝笑
--fan
prev parent reply other threads:[~2013-07-04 7:57 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-07-04 6:20 [PATCH] net namespace: wrap for_each_net with rtnl_lock Fan Du
2013-07-04 7:04 ` Eric W. Biederman
2013-07-04 7:57 ` Fan Du [this message]
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