From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Miller <davem@davemloft.net>
Subject: Re: [PATCH 09/15] ipv4: Cache output routes in fib_info nexthops.
Date: Wed, 18 Jul 2012 13:36:47 -0700 (PDT)
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From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Wed, 18 Jul 2012 22:34:19 +0200

> On Wed, 2012-07-18 at 11:24 -0700, David Miller wrote:
>> If we have an output route that lacks nexthop exceptions, we can cache
>> it in the FIB info nexthop.
>> 
>> Such routes will have DST_HOST cleared because such routes refer to a
>> family of destinations, rather than just one.
> 
> 
>> -			   const struct fib_result *res,
>> +static void rt_cache_route(struct fib_nh *nh, struct rtable *rt)
>> +{
>> +	static DEFINE_SPINLOCK(fib_cache_lock);
>> +	struct rtable **p = &nh->nh_rth_output;
>> +
>> +	if (*p)
>> +		return;
>> +
>> +	spin_lock_bh(&fib_cache_lock);
>> +	if (!*p) {
>> +		*p = rt;
>> +		dst_clone(&rt->dst);
>> +	}
>> +	spin_unlock_bh(&fib_cache_lock);
>> +}
>> +
> 
> This probably should use cmpxchg()

Do you really think it's faster/better in this case?  I did consider
it.