netdev - Re: [patch net-next V2] net: introduce ethernet teaming device

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Date:	Sun, 23 Oct 2011 14:51:01 +0200
From:	Jiri Pirko <jpirko@...hat.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net,
	bhutchings@...arflare.com, shemminger@...tta.com, fubar@...ibm.com,
	andy@...yhouse.net, tgraf@...radead.org, ebiederm@...ssion.com,
	mirqus@...il.com, kaber@...sh.net, greearb@...delatech.com,
	jesse@...ira.com, fbl@...hat.com, benjamin.poirier@...il.com,
	jzupka@...hat.com
Subject: Re: [patch net-next V2] net: introduce ethernet teaming device

Sun, Oct 23, 2011 at 12:45:09PM CEST, eric.dumazet@...il.com wrote:
>Le dimanche 23 octobre 2011 à 10:25 +0200, Jiri Pirko a écrit :
>
>> Please forgive me, it's possible I'm missing something. But I see no way how
>> team->mode_ops.receive can change during team_handle_frame (holding
>> rcu_read_lock) for the reason I stated earlier.
>> 
>> team_port_del() calls netdev_rx_handler_unregister() and after that it
>> calls synchronize_rcu(). That ensures that by the finish of team_port_del()
>> run, team_handle_frame() is not called for this port anymore.
>> 
>> And this combined with "if (!list_empty(&team->port_list))" check in
>> team_change_mode() ensures safety.
>> 
>> Of course team_port_del() and team_change_mode() are both protected by
>> team->lock so they are mutually excluded.
>
>Oh well.
>
>Jirka, I believe I do understand how RCU is working ;)

With this I do not argue :)
But note that team->mode_ops.receive is not rcu-protected pointer per
say (no rcu_assign, rcu_dereference).

>
>There is an obvious problem I pointed to you, but you persist leaving
>this potential bug.
>
>After netdev_rx_handler_unregister(), you can still have other cpus
>calling your handler and reading/using previous memory content. Only
>after synchronize_rcu() you can be safe. But in your patch the bug
>window is _exactly_ _before_ synchronize_rcu() returns.

Yes. And team->mode_ops.receive can change only after synchronize_rcu is
done. It's not possible it changes within the window you are talking about.

>
>Your spinlock wont help you at all, since readers dont take it.
>Spinlock only protects writers.

Sure. team->lock ensures that team_change_mode() is not called withing
the "bug window" but only after team_port_del() finishes.

>
>
>So a reader, even holding rcu lock, can really see two different
>mode_ops.receive values for the :
>
>if (team->mode_ops.receive)
>	res = team->mode_ops.receive(team, port, skb);
>
>
>rcu_lock() doesnt mean the reader can see an unique .receive value,	

I'm very well aware of this.

>
>I am afraid you misunderstood the point.
>
>Real point of RCU here is that the _writer_ wont returns from
>synchronize_rcu() if at least one reader is still running the handler.
>
>No problem with me, I'll just post a patch later, I just cant Ack your
>work as is.
>
>
>
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