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

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Message-ID: <20111023085217.GB15908@minipsycho.orion>
Date:	Sun, 23 Oct 2011 10:52:18 +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 10:43:57AM CEST, eric.dumazet@...il.com wrote:
>Le dimanche 23 octobre 2011 à 10:25 +0200, Jiri Pirko a écrit :
>> 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.
>
>Then, why even testing (team->mode_ops.receive) being NULL at the first
>place, if you are sure no packets can flight meeting this NULL pointer ?
>
>Something is flawed in the logic.

It's not :) The test is simply because a mode may not implement this
callback (actually "roundrobin" mode doesn't have this implemented).

Jirka
>
>
>
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