netdev - Re: [PATCH] act_mirred: don't go back.

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Date:	Mon, 9 Nov 2009 20:33:07 +0800
From:	Changli Gao <xiaosuo@...il.com>
To:	hadi@...erus.ca
Cc:	Stephen Hemminger <shemminger@...tta.com>,
	"David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org
Subject: Re: [PATCH] act_mirred: don't go back.

On Mon, Nov 9, 2009 at 6:54 PM, jamal <hadi@...erus.ca> wrote:
> On Mon, 2009-11-09 at 16:31 +0800, Changli Gao wrote:
>
>> code cleanup! :)
>
> I dont really see it as a cleanup to be honest. The code
> is not less ugly even after the change;->
>

static int tcf_mirred(struct sk_buff *skb, struct tc_action *a,
                      struct tcf_result *res)
{
        struct net_device *dev;
        struct sk_buff *skb2;
        u32 at;
        struct tcf_mirred *m = a->priv;
        int retval, err = 1;

        spin_lock(&m->tcf_lock);
        m->tcf_tm.lastuse = jiffies;
        if (m->tcfm_eaction != TCA_EGRESS_MIRROR &&
            m->tcfm_eaction != TCA_EGRESS_REDIR) {
                if (net_ratelimit())
                        printk("tcf_mirred unknown action %d\n",
                               m->tcfm_eaction);
                goto out;
        }

        dev = m->tcfm_dev;
        if (!(dev->flags&IFF_UP) ) {
                if (net_ratelimit())
                        printk("mirred to Houston: device %s is gone!\n",
                               dev->name);
                goto out;
        }

        skb2 = skb_act_clone(skb, GFP_ATOMIC);
        if (skb2 == NULL)
                goto out;

        m->tcf_bstats.bytes += qdisc_pkt_len(skb2);
        m->tcf_bstats.packets++;
        at = G_TC_AT(skb->tc_verd);
        if (!(at & AT_EGRESS)) {
                if (m->tcfm_ok_push)
                        skb_push(skb2, skb2->dev->hard_header_len);
        }

        /* mirror is always swallowed */
        if (m->tcfm_eaction != TCA_EGRESS_MIRROR)
                skb2->tc_verd = SET_TC_FROM(skb2->tc_verd, at);

        skb2->dev = dev;
        skb2->iif = skb->dev->ifindex;
        dev_queue_xmit(skb2);
        err = 0;

out:
        if (err) {
                m->tcf_qstats.overlimits++;
                m->tcf_bstats.bytes += qdisc_pkt_len(skb);
                m->tcf_bstats.packets++;
                /* should we be asking for packet to be dropped?
                 * may make sense for redirect case only
                 */
                retval = TC_ACT_SHOT;
        } else {
                retval = m->tcf_action;
        }
        spin_unlock(&m->tcf_lock);

        return retval;
}

How about this version.

1. move skb_act_clone() after all the necessary checks, and it can
eliminate unnecessary skb_act_clone() if tcfm_eaction isn't correct.
2. there is one exit of the critical section.
3. jump forward instead of backward.


-- 
Regards，
Changli Gao(xiaosuo@...il.com)
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