netdev - Re: [PATCH 00/16] Remove the ipv4 routing cache

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Date:	Thu, 26 Jul 2012 01:18:53 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	eric.dumazet@...il.com
Cc:	alexander.duyck@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH 00/16] Remove the ipv4 routing cache

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Thu, 26 Jul 2012 10:13:34 +0200

> On Wed, 2012-07-25 at 17:54 -0700, David Miller wrote:
>> @@ -1216,9 +1215,15 @@ static void rt_cache_route(struct fib_nh *nh, struct rtable *rt)
>>  
>>  	prev = cmpxchg(p, orig, rt);
>>  	if (prev == orig) {
>> -		dst_clone(&rt->dst);
>>  		if (orig)
>> -			call_rcu_bh(&orig->dst.rcu_head, rt_release_rcu);
>> +			rt_free(orig);
>> +	} else {
>> +		/* Routes we intend to cache in the FIB nexthop have
>> +		 * the DST_NOCACHE bit set.  However, if we are
>> +		 * unsuccessful at storing this route into the cache
>> +		 * we really need to clear that bit.
>> +		 */
>> +		rt->dst.flags &= ~DST_NOCACHE;
>>  	}
>>  }
>>  
> 
> Not sure why you removed the dst_clone(&rt->dst) ?

Because "cached" dst objects have no reference count.  When such a
cached dst is "released", it is dst_free()'d instead of
dst_release()'d.

> If it is not needed, we might need to release a reference in the else {}
> clause, no ?

Nope.  'rt' has a reference count of one, which is for the caller, not
for this cached location.
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