Revert "ipv6: make ip6_rt_gc_expire an atomic_t"

This reverts commit b4cfbea which is
commit 9cb7c013420f98fa6fd12fc6a5dc055170c108db upstream.

It breaks the Android kernel abi and can be brought back in the future
in an abi-safe way if it is really needed.

Bug: 161946584
Change-Id: Idad46376e176f54a1d6781bdb432131bed7844f2
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>

include/net/netns/ipv6.h

@@ -72,8 +72,8 @@ struct netns_ipv6 {
 	struct dst_ops		ip6_dst_ops;
 	rwlock_t		fib6_walker_lock;
 	spinlock_t		fib6_gc_lock;
-	atomic_t		ip6_rt_gc_expire;
-	unsigned long		ip6_rt_last_gc;
+	unsigned int		 ip6_rt_gc_expire;
+	unsigned long		 ip6_rt_last_gc;
 #ifdef CONFIG_IPV6_MULTIPLE_TABLES
 	unsigned int		fib6_rules_require_fldissect;
 	bool			fib6_has_custom_rules;

net/ipv6/route.c

@@ -2775,7 +2775,6 @@ static int ip6_dst_gc(struct dst_ops *ops)
 	int rt_elasticity = net->ipv6.sysctl.ip6_rt_gc_elasticity;
 	int rt_gc_timeout = net->ipv6.sysctl.ip6_rt_gc_timeout;
 	unsigned long rt_last_gc = net->ipv6.ip6_rt_last_gc;
-	unsigned int val;
 	int entries;
 
 	entries = dst_entries_get_fast(ops);
@@ -2786,13 +2785,13 @@ static int ip6_dst_gc(struct dst_ops *ops)
 	    entries <= rt_max_size)
 		goto out;
 
-	fib6_run_gc(atomic_inc_return(&net->ipv6.ip6_rt_gc_expire), net, true);
+	net->ipv6.ip6_rt_gc_expire++;
+	fib6_run_gc(net->ipv6.ip6_rt_gc_expire, net, true);
 	entries = dst_entries_get_slow(ops);
 	if (entries < ops->gc_thresh)
-		atomic_set(&net->ipv6.ip6_rt_gc_expire, rt_gc_timeout >> 1);
+		net->ipv6.ip6_rt_gc_expire = rt_gc_timeout>>1;
 out:
-	val = atomic_read(&net->ipv6.ip6_rt_gc_expire);
-	atomic_set(&net->ipv6.ip6_rt_gc_expire, val - (val >> rt_elasticity));
+	net->ipv6.ip6_rt_gc_expire -= net->ipv6.ip6_rt_gc_expire>>rt_elasticity;
 	return entries > rt_max_size;
 }
 
@@ -5321,7 +5320,7 @@ static int __net_init ip6_route_net_init(struct net *net)
 	net->ipv6.sysctl.ip6_rt_mtu_expires = 10*60*HZ;
 	net->ipv6.sysctl.ip6_rt_min_advmss = IPV6_MIN_MTU - 20 - 40;
 
-	atomic_set(&net->ipv6.ip6_rt_gc_expire, 30*HZ);
+	net->ipv6.ip6_rt_gc_expire = 30*HZ;
 
 	ret = 0;
 out: