Subject: Re: [PATCH] ipv4 routing: Fixes to allow route cache entries to work when route caching is disabled

From: Jarek Poplawski <[email-masked]>
Date: Mon, 22 Jun 2009 18:51:45 +0200

On Mon, Jun 22, 2009 at 11:23:14AM -0400, Neil Horman wrote:
> Hey all-
> 	As we've been discussing recently, There are a few bugs with routing if
> we exceed our route cache rebuild count, and subsequently disable route caching.
> An oops was reported to me, which has been subsequently fixed, and then
> subsequently a route cache leak and failure to forward frames was reported to me
> when rt_caching returns false.  I've reproduced these on a local system, and
> tracked down the cause.  This patch fixes both of these problems for me on my
> test system.
> 
> 
>     Ensure that route cache entries are usable and reclaimable when caching is off
>     
>     When route caching is disabled (rt_caching returns false), We still use route
>     cache entries that are created and passed into rt_intern_hash once.  These
>     routes need to be made usable for the one call path that holds a reference to
>     them, and they need to be reclaimed when they're finished with their use.  To be
>     made usable, they need to be associated with a neighbor table entry (which they
>     currently are not), otherwise iproute_finish2 just discards the packet, since we
>     don't know which L2 peer to send the packet to.  To do this binding, we need to
>     follow the path a bit higher up in rt_intern_hash, which calls
>     arp_bind_neighbour, but not assign the route entry to the hash table.
>     Currently, if caching is off, we simply assign the route to the rp pointer and
>     are reutrn success.  This patch associates us with a neighbor entry first.
>     
>     Secondly, we need to make sure that any single use routes like this are known to
>     the garbage collector when caching is off.  If caching is off, and we try to
>     hash in a route, it will leak when its refcount reaches zero.  To avoid this,
>     this patch calls rt_free on the route cache entry passed into rt_intern_hash.
>     This places us on the gc list for the route cache garbage collector, so that
>     when its refcount reaches zero, it will be reclaimed (Thanks to Alexey for this
>     suggestion).
>     
>     I've tested this on a local system here, and with these patches in place, I'm
>     able to maintain routed connectivity to remote systems, even if I set
>     /proc/sys/net/ipv4/rt_cache_rebuild_count to -1, which forces rt_caching to
>     return false.
>     
>     Best
>     Neil
>     
>     Signed-off-by: Neil Horman <[email-masked]>
>     Reported-by: Jarek Poplawski <[email-masked]>
>     Reported-by: Maxime Bizon <[email-masked]>
> 
> 
>  route.c |   44 ++++++++++++++++++++++++++------------------
>  1 file changed, 26 insertions(+), 18 deletions(-)
> 
> diff --git a/net/ipv4/route.c b/net/ipv4/route.c
> index 65b3a8b..4b21513 100644
> --- a/net/ipv4/route.c
> +++ b/net/ipv4/route.c
> @@ -1076,6 +1076,7 @@ static int rt_intern_hash(unsigned hash, struct rtable *rt,
>  	u32 		min_score;
>  	int		chain_length;
>  	int attempts = !in_softirq();
> +	int caching = rt_caching(dev_net(rt->u.dst.dev));
>  
>  restart:
>  	chain_length = 0;
> @@ -1084,7 +1085,7 @@ restart:
>  	candp = NULL;
>  	now = jiffies;
>  
> -	if (!rt_caching(dev_net(rt->u.dst.dev))) {
> +	if (!caching) {
>  		/*
>  		 * If we're not caching, just tell the caller we
>  		 * were successful and don't touch the route.  The
> @@ -1093,8 +1094,12 @@ restart:
>  		 * If we drop it here, the callers have no way to resolve routes
>  		 * when we're not caching.  Instead, just point *rp at rt, so
>  		 * the caller gets a single use out of the route
> +		 * Note that we do rt_free on this new route entry, so that
> +		 * once its refcount hits zero, we are still able to reap it
> +		 * (Thanks Alexey)

I hope Alexey Dobriyan won't be confused... 

>  		 */
> -		goto report_and_exit;
> +		rt_free(rt);

To save some coulds & woulds in the future I'd(!) prefer here
dst_free() yet.

> +		goto skip_hashing;

Aren't we jumping over a spin_lock here?

Jarek P.

>  	}
>  
>  	rthp = &rt_hash_table[hash].chain;
> @@ -1174,6 +1179,7 @@ restart:
>  	/* Try to bind route to arp only if it is output
>  	   route or unicast forwarding path.
>  	 */
> +skip_hashing:
>  	if (rt->rt_type == RTN_UNICAST || rt->fl.iif == 0) {
>  		int err = arp_bind_neighbour(&rt->u.dst);
>  		if (err) {
> @@ -1206,27 +1212,29 @@ restart:
>  		}
>  	}
>  
> -	rt->u.dst.rt_next = rt_hash_table[hash].chain;
> +	if (caching) {
> +		rt->u.dst.rt_next = rt_hash_table[hash].chain;
>  
>  #if RT_CACHE_DEBUG >= 2
> -	if (rt->u.dst.rt_next) {
> -		struct rtable *trt;
> -		printk(KERN_DEBUG "rt_cache @%02x: %pI4", hash, &rt->rt_dst);
> -		for (trt = rt->u.dst.rt_next; trt; trt = trt->u.dst.rt_next)
> -			printk(" . %pI4", &trt->rt_dst);
> -		printk("\n");
> -	}
> +		if (rt->u.dst.rt_next) {
> +			struct rtable *trt;
> +			printk(KERN_DEBUG "rt_cache @%02x: %pI4",
> +			       hash, &rt->rt_dst);
> +			for (trt = rt->u.dst.rt_next; trt; trt = trt->u.dst.rt_next)
> +				printk(" . %pI4", &trt->rt_dst);
> +			printk("\n");
> +		}
>  #endif
> -	/*
> -	 * Since lookup is lockfree, we must make sure
> -	 * previous writes to rt are comitted to memory
> -	 * before making rt visible to other CPUS.
> -	 */
> -	rcu_assign_pointer(rt_hash_table[hash].chain, rt);
> +		/*
> +		 * Since lookup is lockfree, we must make sure
> +		 * previous writes to rt are comitted to memory
> +		 * before making rt visible to other CPUS.
> +		 */
> +		rcu_assign_pointer(rt_hash_table[hash].chain, rt);
>  
> -	spin_unlock_bh(rt_hash_lock_addr(hash));
> +		spin_unlock_bh(rt_hash_lock_addr(hash));
> +	}
>  
> -report_and_exit:
>  	if (rp)
>  		*rp = rt;
>  	else