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

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Date:	Tue, 17 Sep 2013 20:50:41 -0700
From:	Francesco Ruggeri <fruggeri@...stanetworks.com>
To:	David Miller <davem@...emloft.net>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Jiří Pírko <jiri@...nulli.us>,
	Alexander Duyck <alexander.h.duyck@...el.com>,
	Cong Wang <amwang@...hat.com>, netdev@...r.kernel.org
Subject: Re: [CFT][PATCH] net: Delay default_device_exit_batch until no
 devices are unregistering

>> 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.

> The count doesn't necessarily need to be atomic as it can be protected
> by the rtnl_lock.

If we use the rtnl_lock then we could have per-net wait queues. While
not strictly needed (since only one instance of
default_device_exit_batch can execute at a time) it would avoid
unnecessarily waking up the waiting code.

Francesco
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