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Date:	Thu, 11 Sep 2014 16:47:22 +0200
From:	Hannes Frederic Sowa <hannes@...essinduktion.org>
To:	Vlad Yasevich <vyasevich@...il.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	eric.dumazet@...il.com, nicolas.dichtel@...nd.com
Subject: Re: [PATCH net-next] ipv6: implement rt_genid_bump_ipv6 with
 fn_sernum and remove rt6i_genid

On Do, 2014-09-11 at 10:44 -0400, Vlad Yasevich wrote:
> On 09/11/2014 10:32 AM, Hannes Frederic Sowa wrote:
> > On Do, 2014-09-11 at 10:19 -0400, Vlad Yasevich wrote:
> >> On 09/11/2014 08:05 AM, Hannes Frederic Sowa wrote:
> >>> On Mi, 2014-09-10 at 13:09 -0700, David Miller wrote:
> >>>> From: Hannes Frederic Sowa <hannes@...essinduktion.org>
> >>>> Date: Wed, 10 Sep 2014 11:31:28 +0200
> >>>>
> >>>>> In case we need to force the sockets to relookup the routes we now
> >>>>> increase the fn_sernum on all fibnodes in the routing tree. This is a
> >>>>> costly operation but should only happen if we have major routing/policy
> >>>>> changes in the kernel (e.g. manual route adding/removal, xfrm policy
> >>>>> changes).
> >>>>
> >>>> Core routers can update thousands of route updates per second, and they
> >>>> do this via what you refer to as "manual route adding/removal".
> >>>>
> >>>> I don't think we want to put such a scalability problem into the tree.
> >>>>
> >>>> There has to be a lightweight way to address this.
> >>>
> >>> An alternative approach without traversing the routing table, but each
> >>> newly inserted route (even only cached ones) might bump all other routes
> >>> out of the per-socket caches:
> >>>
> >>> diff --git a/include/net/ip6_fib.h b/include/net/ip6_fib.h
> >>> index 9bcb220..a7e45b9 100644
> >>> --- a/include/net/ip6_fib.h
> >>> +++ b/include/net/ip6_fib.h
> >>> @@ -119,8 +119,6 @@ struct rt6_info {
> >>>  	struct inet6_dev		*rt6i_idev;
> >>>  	unsigned long			_rt6i_peer;
> >>>  
> >>> -	u32				rt6i_genid;
> >>> -
> >>>  	/* more non-fragment space at head required */
> >>>  	unsigned short			rt6i_nfheader_len;
> >>>  
> >>> diff --git a/include/net/net_namespace.h b/include/net/net_namespace.h
> >>> index 361d260..428fdcb 100644
> >>> --- a/include/net/net_namespace.h
> >>> +++ b/include/net/net_namespace.h
> >>> @@ -358,18 +358,28 @@ static inline int rt_genid_ipv6(struct net *net)
> >>>  	return atomic_read(&net->ipv6.rt_genid);
> >>>  }
> >>>  
> >>> -static inline void rt_genid_bump_ipv6(struct net *net)
> >>> +static inline int rt_genid_bump_ipv6(struct net *net)
> >>>  {
> >>> -	atomic_inc(&net->ipv6.rt_genid);
> >>> +	int new, old;
> >>> +
> >>> +	do {
> >>> +		old = atomic_read(&net->ipv6.rt_genid);
> >>> +		new = old + 1;
> >>> +		if (new <= 0)
> >>> +			new = 1;
> >>> +	} while (atomic_cmpxchg(&net->ipv6.rt_genid, old, new) != old);
> >>> +	return new;
> >>> +
> >>>  }
> >>>  #else
> >>>  static inline int rt_genid_ipv6(struct net *net)
> >>>  {
> >>> -	return 0;
> >>> +	return 1;
> >>>  }
> >>>  
> >>> -static inline void rt_genid_bump_ipv6(struct net *net)
> >>> +static inline int rt_genid_bump_ipv6(struct net *net)
> >>>  {
> >>> +	return 1;
> >>>  }
> >>>  #endif
> >>>  
> >>> diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
> >>> index 76b7f5e..4a2f130 100644
> >>> --- a/net/ipv6/ip6_fib.c
> >>> +++ b/net/ipv6/ip6_fib.c
> >>> @@ -84,7 +84,10 @@ static int fib6_walk_continue(struct fib6_walker_t *w);
> >>>   *	result of redirects, path MTU changes, etc.
> >>>   */
> >>>  
> >>> -static __u32 rt_sernum;
> >>> +static int fib6_new_sernum(struct net *net)
> >>> +{
> >>> +	return rt_genid_bump_ipv6(net);
> >>> +}
> >>>  
> >>>  static void fib6_gc_timer_cb(unsigned long arg);
> >>>  
> >>> @@ -104,13 +107,6 @@ static inline void fib6_walker_unlink(struct fib6_walker_t *w)
> >>>  	list_del(&w->lh);
> >>>  	write_unlock_bh(&fib6_walker_lock);
> >>>  }
> >>> -static __inline__ u32 fib6_new_sernum(void)
> >>> -{
> >>> -	u32 n = ++rt_sernum;
> >>> -	if ((__s32)n <= 0)
> >>> -		rt_sernum = n = 1;
> >>> -	return n;
> >>> -}
> >>>  
> >>>  /*
> >>>   *	Auxiliary address test functions for the radix tree.
> >>> @@ -421,16 +417,15 @@ out:
> >>>   */
> >>>  
> >>>  static struct fib6_node *fib6_add_1(struct fib6_node *root,
> >>> -				     struct in6_addr *addr, int plen,
> >>> -				     int offset, int allow_create,
> >>> -				     int replace_required)
> >>> +				    struct in6_addr *addr, int plen,
> >>> +				    int offset, int allow_create,
> >>> +				    int replace_required, int sernum)
> >>>  {
> >>>  	struct fib6_node *fn, *in, *ln;
> >>>  	struct fib6_node *pn = NULL;
> >>>  	struct rt6key *key;
> >>>  	int	bit;
> >>>  	__be32	dir = 0;
> >>> -	__u32	sernum = fib6_new_sernum();
> >>>  
> >>>  	RT6_TRACE("fib6_add_1\n");
> >>>  
> >>> @@ -844,6 +839,7 @@ void fib6_force_start_gc(struct net *net)
> >>>  int fib6_add(struct fib6_node *root, struct rt6_info *rt, struct nl_info *info,
> >>>  	     struct nlattr *mx, int mx_len)
> >>>  {
> >>> +	struct net *net = dev_net(rt->dst.dev);
> >>>  	struct fib6_node *fn, *pn = NULL;
> >>>  	int err = -ENOMEM;
> >>>  	int allow_create = 1;
> >>> @@ -860,7 +856,7 @@ int fib6_add(struct fib6_node *root, struct rt6_info *rt, struct nl_info *info,
> >>>  
> >>>  	fn = fib6_add_1(root, &rt->rt6i_dst.addr, rt->rt6i_dst.plen,
> >>>  			offsetof(struct rt6_info, rt6i_dst), allow_create,
> >>> -			replace_required);
> >>> +			replace_required, fib6_new_sernum(net));
> >>>  	if (IS_ERR(fn)) {
> >>>  		err = PTR_ERR(fn);
> >>>  		fn = NULL;
> >>> @@ -894,14 +890,15 @@ int fib6_add(struct fib6_node *root, struct rt6_info *rt, struct nl_info *info,
> >>>  			sfn->leaf = info->nl_net->ipv6.ip6_null_entry;
> >>>  			atomic_inc(&info->nl_net->ipv6.ip6_null_entry->rt6i_ref);
> >>>  			sfn->fn_flags = RTN_ROOT;
> >>> -			sfn->fn_sernum = fib6_new_sernum();
> >>> +			sfn->fn_sernum = fib6_new_sernum(net);
> >>>  
> >>>  			/* Now add the first leaf node to new subtree */
> >>>  
> >>>  			sn = fib6_add_1(sfn, &rt->rt6i_src.addr,
> >>>  					rt->rt6i_src.plen,
> >>>  					offsetof(struct rt6_info, rt6i_src),
> >>> -					allow_create, replace_required);
> >>> +					allow_create, replace_required,
> >>> +					fib6_new_sernum(net));
> >>>  
> >>>  			if (IS_ERR(sn)) {
> >>>  				/* If it is failed, discard just allocated
> >>> @@ -920,7 +917,8 @@ int fib6_add(struct fib6_node *root, struct rt6_info *rt, struct nl_info *info,
> >>>  			sn = fib6_add_1(fn->subtree, &rt->rt6i_src.addr,
> >>>  					rt->rt6i_src.plen,
> >>>  					offsetof(struct rt6_info, rt6i_src),
> >>> -					allow_create, replace_required);
> >>> +					allow_create, replace_required,
> >>> +					fib6_new_sernum(net));
> >>>  
> >>>  			if (IS_ERR(sn)) {
> >>>  				err = PTR_ERR(sn);
> >>> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
> >>> index f74b041..54b7d81 100644
> >>> --- a/net/ipv6/route.c
> >>> +++ b/net/ipv6/route.c
> >>> @@ -314,7 +314,6 @@ static inline struct rt6_info *ip6_dst_alloc(struct net *net,
> >>>  
> >>>  		memset(dst + 1, 0, sizeof(*rt) - sizeof(*dst));
> >>>  		rt6_init_peer(rt, table ? &table->tb6_peers : net->ipv6.peers);
> >>> -		rt->rt6i_genid = rt_genid_ipv6(net);
> >>>  		INIT_LIST_HEAD(&rt->rt6i_siblings);
> >>>  	}
> >>>  	return rt;
> >>> @@ -1096,10 +1095,7 @@ static struct dst_entry *ip6_dst_check(struct dst_entry *dst, u32 cookie)
> >>>  	 * DST_OBSOLETE_FORCE_CHK which forces validation calls down
> >>>  	 * into this function always.
> >>>  	 */
> >>> -	if (rt->rt6i_genid != rt_genid_ipv6(dev_net(rt->dst.dev)))
> >>> -		return NULL;
> >>> -
> >>> -	if (!rt->rt6i_node || (rt->rt6i_node->fn_sernum != cookie))
> >>> +	if (!rt->rt6i_node || rt_genid_ipv6(dev_net(rt->dst.dev)) != cookie)
> >>>  		return NULL;
> >>>  
> >>>  	if (rt6_check_expired(rt))
> >>>
> >>
> >> Ok, so now we bump the gen_id every time we add a route and use that as fn_sernum for
> >> that route.  But this doesn't solve the problem that we are seeing in that a re-lookup
> >> of the route still gives us an older route with an older gen_id.
> > 
> > Hmm, this patch completely removes rt6i_genid from rt6_info (first
> > hunk). We decide if we need to do the relookup based on the socket's
> > cookie and the per-netns serial number of the ipv6 routing tables.
> > 
> 
> Hi Hannes
> 
> Right, by you still compare the per-netns serial number to the cookie.  The cookie is
> typically taken from the route.  The route takes it from the per-netns serial number.
> So, if you add a route with serial=1 and when the socket looks up that route, it
> stashes 1 into the cookie.
> Then you add another route and serial = 2.  Now the dst_check on the socket fails
> (cookie != rt_getid_ipv6(..)) and socket has to re-lookup a route.  It gets the same old
> route as before with fn_sernum = 1.  That's stashed in the cookie again.  Next dst_check
> is done, the route is invalidated again!.
> 
> Am I missing something?

No, you're right. I wanted to initialize the socket cookie with
rt_genid_ipv6() in this patch not with the value from the tree.

Thanks! :)


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