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

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Date:	Thu, 26 Jul 2012 10:27:58 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	alexander.duyck@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH 00/16] Remove the ipv4 routing cache

On Thu, 2012-07-26 at 01:18 -0700, David Miller wrote:
> 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.

But isnt DST_NOCACHE intent reverted then ?

like you meant :

+       } else {
+               /* Routes we intend to cache in the FIB nexthop have
+                * the DST_NOCACHE bit unset.  However, if we are
+                * unsuccessful at storing this route into the cache
+                * we really need to set that bit.
+                */
+               rt->dst.flags |= DST_NOCACHE;
        }



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