ipvs: avoid rcu_barrier during netns cleanup

commit 578bc3ef ("ipvs: reorganize dest trash") added
rcu_barrier() on cleanup to wait dest users and schedulers
like LBLC and LBLCR to put their last dest reference.
Using rcu_barrier with many namespaces is problematic.

Trying to fix it by freeing dest with kfree_rcu is not
a solution, RCU callbacks can run in parallel and execution
order is random.

Fix it by creating new function ip_vs_dest_put_and_free()
which is heavier than ip_vs_dest_put(). We will use it just
for schedulers like LBLC, LBLCR that can delay their dest
release.

By default, dests reference is above 0 if they are present in
service and it is 0 when deleted but still in trash list.
Change the dest trash code to use ip_vs_dest_put_and_free(),
so that refcnt -1 can be used for freeing. As result,
such checks remain in slow path and the rcu_barrier() from
netns cleanup can be removed.
Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>

include/net/ip_vs.h

@@ -1442,6 +1442,12 @@ static inline void ip_vs_dest_put(struct ip_vs_dest *dest)
 	atomic_dec(&dest->refcnt);
 }
 
+static inline void ip_vs_dest_put_and_free(struct ip_vs_dest *dest)
+{
+	if (atomic_dec_return(&dest->refcnt) < 0)
+		kfree(dest);
+}
+
 /*
  *      IPVS sync daemon data and function prototypes
  *      (from ip_vs_sync.c)

net/netfilter/ipvs/ip_vs_ctl.c

@@ -704,7 +704,7 @@ static void ip_vs_dest_free(struct ip_vs_dest *dest)
 	__ip_vs_dst_cache_reset(dest);
 	__ip_vs_svc_put(svc, false);
 	free_percpu(dest->stats.cpustats);
-	kfree(dest);
+	ip_vs_dest_put_and_free(dest);
 }
 
 /*
@@ -3820,10 +3820,6 @@ void __net_exit ip_vs_control_net_cleanup(struct net *net)
 {
 	struct netns_ipvs *ipvs = net_ipvs(net);
 
-	/* Some dest can be in grace period even before cleanup, we have to
-	 * defer ip_vs_trash_cleanup until ip_vs_dest_wait_readers is called.
-	 */
-	rcu_barrier();
 	ip_vs_trash_cleanup(net);
 	ip_vs_stop_estimator(net, &ipvs->tot_stats);
 	ip_vs_control_net_cleanup_sysctl(net);

net/netfilter/ipvs/ip_vs_lblc.c

@@ -136,7 +136,7 @@ static void ip_vs_lblc_rcu_free(struct rcu_head *head)
 						   struct ip_vs_lblc_entry,
 						   rcu_head);
 
-	ip_vs_dest_put(en->dest);
+	ip_vs_dest_put_and_free(en->dest);
 	kfree(en);
 }
 

net/netfilter/ipvs/ip_vs_lblcr.c

@@ -130,7 +130,7 @@ static void ip_vs_lblcr_elem_rcu_free(struct rcu_head *head)
 	struct ip_vs_dest_set_elem *e;
 
 	e = container_of(head, struct ip_vs_dest_set_elem, rcu_head);
-	ip_vs_dest_put(e->dest);
+	ip_vs_dest_put_and_free(e->dest);
 	kfree(e);
 }