ipvs: free ip_vs_dest structs when refcnt=0

Currently, the ip_vs_dest cache frees ip_vs_dest objects when their
reference count becomes < 0.  Aside from not being semantically sound,
this is problematic for the new type refcount_t, which will be introduced
shortly in a separate patch. refcount_t is the new kernel type for
holding reference counts, and provides overflow protection and a
constrained interface relative to atomic_t (the type currently being
used for kernel reference counts).

Per Julian Anastasov: "The problem is that dest_trash currently holds
deleted dests (unlinked from RCU lists) with refcnt=0."  Changing
dest_trash to hold dest with refcnt=1 will allow us to free ip_vs_dest
structs when their refcnt=0, in ip_vs_dest_put_and_free().

Signed-off-by: David Windsor <dwindsor@gmail.com>
Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

include/net/ip_vs.h

@@ -1421,7 +1421,7 @@ static inline void ip_vs_dest_put(struct ip_vs_dest *dest)
 
 static inline void ip_vs_dest_put_and_free(struct ip_vs_dest *dest)
 {
-	if (atomic_dec_return(&dest->refcnt) < 0)
+	if (atomic_dec_and_test(&dest->refcnt))
 		kfree(dest);
 }
 

net/netfilter/ipvs/ip_vs_ctl.c

@@ -711,7 +711,6 @@ ip_vs_trash_get_dest(struct ip_vs_service *svc, int dest_af,
 		      dest->vport == svc->port))) {
 			/* HIT */
 			list_del(&dest->t_list);
-			ip_vs_dest_hold(dest);
 			goto out;
 		}
 	}
@@ -741,7 +740,7 @@ static void ip_vs_dest_free(struct ip_vs_dest *dest)
  *  When the ip_vs_control_clearup is activated by ipvs module exit,
  *  the service tables must have been flushed and all the connections
  *  are expired, and the refcnt of each destination in the trash must
- *  be 0, so we simply release them here.
+ *  be 1, so we simply release them here.
  */
 static void ip_vs_trash_cleanup(struct netns_ipvs *ipvs)
 {
@@ -1080,11 +1079,10 @@ static void __ip_vs_del_dest(struct netns_ipvs *ipvs, struct ip_vs_dest *dest,
 	if (list_empty(&ipvs->dest_trash) && !cleanup)
 		mod_timer(&ipvs->dest_trash_timer,
 			  jiffies + (IP_VS_DEST_TRASH_PERIOD >> 1));
-	/* dest lives in trash without reference */
+	/* dest lives in trash with reference */
 	list_add(&dest->t_list, &ipvs->dest_trash);
 	dest->idle_start = 0;
 	spin_unlock_bh(&ipvs->dest_trash_lock);
-	ip_vs_dest_put(dest);
 }
 
 
@@ -1160,7 +1158,7 @@ static void ip_vs_dest_trash_expire(unsigned long data)
 
 	spin_lock(&ipvs->dest_trash_lock);
 	list_for_each_entry_safe(dest, next, &ipvs->dest_trash, t_list) {
-		if (atomic_read(&dest->refcnt) > 0)
+		if (atomic_read(&dest->refcnt) > 1)
 			continue;
 		if (dest->idle_start) {
 			if (time_before(now, dest->idle_start +