netdev - Re: [PATCH] net: dev: fix the incorrect hold of net namespace's lo device

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Date:	Thu, 23 Aug 2012 11:09:41 +0800
From:	Gao feng <gaofeng@...fujitsu.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	ebiederm@...ssion.com, davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [PATCH] net: dev: fix the incorrect hold of net namespace's lo
 device

于 2012年08月22日 19:24, Eric Dumazet 写道:
> On Wed, 2012-08-22 at 19:00 +0800, Gao feng wrote:
> 
>> Hi Eric
>>
>> I saw your patch and think this patch is clear and doesn't change too much logic.
>>
>> I test your patch, it not fix this problem.
>>
>> In my test case,when moving a net device to another net namespace,
>> Because you patch delete NETDEV_UNREGISTER event from dst_dev_event,
>> we will just put dst entries into the dst garbage list in event
>> NETDEV_DOWN,without call dst_ifdown to change these dst entries' device
>> to the lo device,and now this net device belongs to the new net namespace.
>>
> 
> Then fix the "moving a net device to another net namespace", instead
> of slowing down other common operations.
> 

okay, I will send a patch to fix this problem after your patch beeing applied.

> dev_change_net_namespace() is probably a better place to put your patch
> 
>> After the net device beeing moved to another net namespace, I rmmod this
>> net device's driver,this will trigger the new added event NETDEV_UNREGISTER_FINISH,
>> so in dst_dev_event,we will change these dst entries's device to the new net
>> namespace's lo device,and this will make the referenct count of the new net namespace's
>> lo device incorrect. when we exit the new net namespace,this emg message is still exist.
>>
>> Message from syslogd@...key at Aug 22 18:50:13 ...
>>  kernel:[ 1161.979036] unregister_netdevice: waiting for lo to become free. Usage count = 1
>>
>> And because net_mutex is locked here,so we can't create new net namespace.
>>
>>> rcu_barrier() at this place will kill some workloads.
>>>
>>
>> I think this will only add some workloads when unregister a net device.
>> Do I miss something?
> 
> Yes, rcu_barrier() at this point is killing performance, because we hold
> RTNL.
> 
> We worked hard to batch things, your patch is a huge step backward.
> 

Get it,thanks for your explanation.

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