Re: [PATCH v2] vxlan: remove the redundant gro_cells_destroy() calling.

[Eric Dumazet <eric.dumazet@gmail.com>]

On 03/15/2019 02:08 PM, Stefano Brivio wrote:
> On Fri, 15 Mar 2019 11:56:01 -0700
> Eric Dumazet <eric.dumazet@gmail.com> wrote:
> 
>> On 03/15/2019 11:02 AM, David Miller wrote:
>>> From: Eric Dumazet <eric.dumazet@gmail.com>
>>> Date: Fri, 15 Mar 2019 09:06:25 -0700
>>>   
>>>>
>>>>
>>>> On 03/15/2019 08:28 AM, Stefano Brivio wrote:  
>>>>> On Fri, 15 Mar 2019 23:18:52 +0800
>>>>> Zhiqiang Liu <liuzhiqiang26@huawei.com> wrote:
>>>>>  
>>>>>> In vxlan_destroy_tunnels func, unregister_netdevice_queue is called after
>>>>>> gro_cells_destroy func. However, in unregister_netdevice_queue func, the
>>>>>> gro_cells_destroy func will also call the gro_cells_destroy func as the
>>>>>> following routine:
>>>>>> unregister_netdevice_many() -> rollback_registered_many()  
>>>>>> -> ndo_uninit() -> gro_cells_destroy()    
>>>>>>
>>>>>> Signed-off-by: Suanming.Mou <mousuanming@huawei.com>
>>>>>> Reviewed-by: Zhiqiang Liu <liuzhiqiang26@huawei.com>
>>>>>> Reviewed-by: Stefano Brivio <sbrivio@redhat.com>  
>>>>>
>>>>> NACK, please read my and Eric's comments to v1 -- giving me more than 23
>>>>> minutes to answer would have been a nice touch as well :)
>>>>>  
>>>>
>>>> Sorry for the confusion, I forgot to add the question marks to my sentences.
>>>>
>>>> In fact, this is a bug fix, that we missed in the previous fix.
>>>>
>>>> Technically the bug is older.  
>>>
>>> Please elaborate.
>>>   
>>
>> Commit ad6c9986bcb62
>> ("vxlan: Fix GRO cells race condition between receive and link delete")
>>
>> fixed a race condition for the typical case a vxlan device is dismantled from the
>> current netns.
>>
>> But if a netns is dismantled, we call vxlan_destroy_tunnels()
>> to schedule a unregister_netdevice_queue() of all the vxlan tunnels
>> that are related to this netns.
> 
> Won't that happen via ops_exit_list() only after synchronize_rcu() is
> called by cleanup_net(), though? Is there another path I missed?

Just look at vxlan_destroy_tunnels().

The call to gro_cells_destroy(&vxlan->gro_cells);
is done _before_ 
unregister_netdevice_queue(vxlan->dev, head);

So packets can still fly, the RCU grace period has not yet started.