netdev - Re: [PATCH RFC v3 net 2/2] ipv6: Avoid restarting fib6_lookup() for RTF

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Message-ID: <20141009041903.GA27904@devbig242.prn2.facebook.com>
Date:	Wed, 8 Oct 2014 21:19:04 -0700
From:	Martin Lau <kafai@...com>
To:	David Miller <davem@...emloft.net>
CC:	<netdev@...r.kernel.org>, <hannes@...essinduktion.org>
Subject: Re: [PATCH RFC v3 net 2/2] ipv6: Avoid restarting fib6_lookup() for
 RTF_CACHE hit case

Hi, 

> Ok this looks fine, and I can see how there is a dependency with patch
> #1.
>
> But I also want to point out how this change show my point about
> BACKTRACK() even more.  Read this function after this patch is
> applied and someone auditing might say "oh, 'out' label is now
> unused, we can remove it"
> 
> Again, hidden control flow is really bad, and we've had very serious
> bugs in the past because of it (which we've fixed by ditching the
> side effect causing macros in favor of properly designed inline
> functions).

Agree.

> Trying to be constructive, why don't we go in a direction like
> the following?

Looks good.  There is another place calling BACKTRACK. Similar change
also needs to happen there or some more re-factoring is needed.

I also have another idea to further reduce the number of fib6_lookup() in
the CONFIG_IPV6_MULTIPLE_TABLES case.  I will give it another attempt together.

Thanks,
Martin

> 
> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
> index a318dd89..99612c5 100644
> --- a/net/ipv6/route.c
> +++ b/net/ipv6/route.c
> @@ -772,6 +772,26 @@ int rt6_route_rcv(struct net_device *dev, u8 *opt, int len,
>  }
>  #endif
>  
> +static struct fib6_node *rt6_backtrack(struct net *net, struct rt6_info *rt, struct fib6_node *fn, const struct in6_addr *saddr)
> +{
> +	if (rt == net->ipv6.ip6_null_entry) {
> +		struct fib6_node *pn;
> +
> +		while (1) {
> +			if (fn->fn_flags & RTN_TL_ROOT)
> +				break;
> +			pn = fn->parent;
> +			if (FIB6_SUBTREE(pn) && FIB6_SUBTREE(pn) != fn)
> +				fn = fib6_lookup(FIB6_SUBTREE(pn), NULL, saddr);
> +			else
> +				fn = pn;
> +			if (fn->fn_flags & RTN_RTINFO)
> +				break;
> +		}
> +	}
> +	return fn;
> +}
> +
>  #define BACKTRACK(__net, saddr)			\
>  do { \
>  	if (rt == __net->ipv6.ip6_null_entry) {	\
> @@ -934,10 +954,15 @@ restart:
>  	rt = rt6_select(fn, oif, strict | reachable);
>  	if (rt->rt6i_nsiblings)
>  		rt = rt6_multipath_select(rt, fl6, oif, strict | reachable);
> -	BACKTRACK(net, &fl6->saddr);
> -	if (rt == net->ipv6.ip6_null_entry ||
> -	    rt->rt6i_flags & RTF_CACHE)
> -		goto out;
> +	fn = rt6_backtrack(net, rt, fn, &fl6->saddr);
> +	if (rt == net->ipv6.ip6_null_entry) {
> +		if (fn->fn_flags & RTN_TL_ROOT)
> +			goto out;
> +		if (fn->fn_flags & RTN_RTINFO)
> +			goto restart;
> +	}
> +	if (rt->rt6i_flags & RTF_CACHE)
> +		goto out1;
>  
>  	dst_hold(&rt->dst);
>  	read_unlock_bh(&table->tb6_lock);
> @@ -974,6 +999,7 @@ out:
>  		reachable = 0;
>  		goto restart_2;
>  	}
> +out1:
>  	dst_hold(&rt->dst);
>  	read_unlock_bh(&table->tb6_lock);
>  out2:
--
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