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

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Message-ID: <20111023082545.GA15908@minipsycho.orion>
Date:	Sun, 23 Oct 2011 10:25:47 +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

Sat, Oct 22, 2011 at 06:51:22PM CEST, eric.dumazet@...il.com wrote:
>Le samedi 22 octobre 2011 à 17:13 +0200, Jiri Pirko a écrit :
>> >> +
>> >> +/************************
>> >> + * Rx path frame handler
>> >> + ************************/
>> >> +
>> >> +/* note: already called with rcu_read_lock */
>> >> +static rx_handler_result_t team_handle_frame(struct sk_buff **pskb)
>> >> +{
>> >> +	struct sk_buff *skb = *pskb;
>> >> +	struct team_port *port;
>> >> +	struct team *team;
>> >> +	rx_handler_result_t res = RX_HANDLER_ANOTHER;
>> >> +
>> >> +	skb = skb_share_check(skb, GFP_ATOMIC);
>> >> +	if (!skb)
>> >> +		return RX_HANDLER_CONSUMED;
>> >> +
>> >> +	*pskb = skb;
>> >> +
>> >> +	port = team_port_get_rcu(skb->dev);
>> >> +	team = port->team;
>> >> +
>> >> +	if (team->mode_ops.receive)
>> >
>> >Hmm, you need ACCESS_ONCE() here or rcu_dereference()
>> >
>> >See commit 4d97480b1806e883eb (bonding: use local function pointer of
>> >bond->recv_probe in bond_handle_frame) for reference
>> 
>> I do not think so. Because mode_ops.receive changes only from
>> __team_change_mode() and this can be called only in case no ports are in
>> team. And team_port_del() calls synchronize_rcu().
>> 
>
>
>
>We are used to code following this template :
>
>if (ops->handler)
>	ops->handler(arguments);
>
>But this is valid only because ops points to constant memory.
>
>
>In your case, we really see its not true, dont try to pretend its safe.

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.

Jirka.

>
>
>
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