netdev - Re: [CFT][PATCH] net: Delay default_device_exit_batch until no devices are unregistering

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Date:	Tue, 17 Sep 2013 23:52:47 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	fruggeri@...stanetworks.com
Cc:	ebiederm@...ssion.com, edumazet@...gle.com, jiri@...nulli.us,
	alexander.h.duyck@...el.com, amwang@...hat.com,
	netdev@...r.kernel.org
Subject: Re: [CFT][PATCH] net: Delay default_device_exit_batch until no
 devices are unregistering

From: Francesco Ruggeri <fruggeri@...stanetworks.com>
Date: Tue, 17 Sep 2013 20:50:41 -0700

>>> List/count I don't much care but currently we don't have a list of
>>> all of the devices that are unregistering.
>>>
>>> The problem with this is that netdev_run_todo moves all of the
>>> devices to a local list, so they are only visible from a list_head
>>> on the stack.  Which makes sense as we run this all in the context
>>> of rtnl_unlock.
>>
>> And when that local list is processed (the while loop completes and
>> has iterated over the entire list), either the global todo list is
>> empty, or it is not empty.
>>
>> And the waked up code will check for this.
>>
>> I really don't see what the problem is.
> 
> I am not sure that would work.
> list_empty(&net_todo_list) does not guarantee that there are no
> unregistering devices still in flight.
> Another process may have copied net_todo_run into its local list, left
> net_todo_list empty and still be in the middle of processing
> unregistering devices (without the rtnl lock) when
> default_device_exit_batch starts executing.

Ok, now I understand.

Eric B., when you get a chance can you resubmit your patch and perhaps
elaborate on the situation in the commit message.

If I was confused I'm sure other people will be if they look into
this in the future.
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