netdev - Re: [PATCH v2 net-next 4/4] net/sched: act

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Date:   Wed, 28 Sep 2016 00:42:44 +0300
From:   Shmulik Ladkani <shmulik.ladkani@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        Jamal Hadi Salim <jhs@...atatu.com>,
        WANG Cong <xiyou.wangcong@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Florian Westphal <fw@...len.de>, netdev@...r.kernel.org
Subject: Re: [PATCH v2 net-next 4/4] net/sched: act_mirred: Implement
 ingress actions

Hi,

On Tue, 27 Sep 2016 14:27:13 -0700 Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Tue, 2016-09-27 at 23:59 +0300, Shmulik Ladkani wrote:
> > Up until now, 'action mirred' supported only egress actions (either
> > TCA_EGRESS_REDIR or TCA_EGRESS_MIRROR).
> > 
> > This patch implements the corresponding ingress actions
> > TCA_INGRESS_REDIR and TCA_INGRESS_MIRROR.  
> 
> 
> > -		if (m->tcfm_mac_header_xmit)
> > +	/* If action's target direction differs than filter's direction,
> > +	 * and devices expect a mac header on xmit, then mac push/pull is
> > +	 * needed.
> > +	 */
> > +	if (at != tcf_mirred_act_direction(m->tcfm_eaction) &&  
> 
> Note that m->tcfm_eaction is read here.
> 
> > +	    m->tcfm_mac_header_xmit) {
> > +		if (at & AT_EGRESS) {
> > +			/* caught at egress, act ingress: pull mac */
> > +			mac_len = skb_network_header(skb) - skb_mac_header(skb);
> > +			skb_pull_rcsum(skb2, mac_len);
> > +		} else {
> > +			/* caught at ingress, act egress: push mac */
> >  			skb_push_rcsum(skb2, skb->mac_len);
> > +		}
> >  	}
> >  
> >  	/* mirror is always swallowed */
> > -	if (m->tcfm_eaction != TCA_EGRESS_MIRROR)
> > +	if (tcf_mirred_is_act_redirect(m->tcfm_eaction))
> >  		skb2->tc_verd = SET_TC_FROM(skb2->tc_verd, at);
> >  
> >  	skb2->skb_iif = skb->dev->ifindex;
> >  	skb2->dev = dev;
> > -	err = dev_queue_xmit(skb2);  
> 
> Note that m->tcfm_eaction is read another time here.
> 
> > +	if (tcf_mirred_act_direction(m->tcfm_eaction) & AT_EGRESS)
> > +		err = dev_queue_xmit(skb2);
> > +	else
> > +		netif_receive_skb(skb2);
> >    
> 
> Since this runs lockless, another cpu might change m->tcfm_eaction in
> the middle, and you could call dev_queue_xmit(skb2) while the skb2 was
> prepared for the opposite action.

Thanks Eric.

I assume adding a READ_ONCE(m->tcfm_eaction) at beggining of section,
and using the read value, will solve this specific inconsistency?