ipv4: Restore old dst_free() behavior.

commit 404e0a8b6a55 (net: ipv4: fix RCU races on dst refcounts) tried
to solve a race but added a problem at device/fib dismantle time :

We really want to call dst_free() as soon as possible, even if sockets
still have dst in their cache.
dst_release() calls in free_fib_info_rcu() are not welcomed.

Root of the problem was that now we also cache output routes (in
nh_rth_output), we must use call_rcu() instead of call_rcu_bh() in
rt_free(), because output route lookups are done in process context.

Based on feedback and initial patch from David Miller (adding another
call_rcu_bh() call in fib, but it appears it was not the right fix)

I left the inet_sk_rx_dst_set() helper and added __rcu attributes
to nh_rth_output and nh_rth_input to better document what is going on in
this code.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 17 insertions, 9 deletions

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index d6eabcfe8a90..2bd107477469 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1199,23 +1199,31 @@ restart:
 	fnhe->fnhe_stamp = jiffies;
 }
 
+static inline void rt_free(struct rtable *rt)
+{
+	call_rcu(&rt->dst.rcu_head, dst_rcu_free);
+}
+
 static void rt_cache_route(struct fib_nh *nh, struct rtable *rt)
 {
-	struct rtable *orig, *prev, **p = &nh->nh_rth_output;
+	struct rtable *orig, *prev, **p = (struct rtable **)&nh->nh_rth_output;
 
 	if (rt_is_input_route(rt))
-		p = &nh->nh_rth_input;
+		p = (struct rtable **)&nh->nh_rth_input;
 
 	orig = *p;
 
-	rt->dst.flags |= DST_RCU_FREE;
-	dst_hold(&rt->dst);
 	prev = cmpxchg(p, orig, rt);
 	if (prev == orig) {
 		if (orig)
-			dst_release(&orig->dst);
+			rt_free(orig);
 	} else {
-		dst_release(&rt->dst);
+		/* Routes we intend to cache in the FIB nexthop have
+		 * the DST_NOCACHE bit clear.  However, if we are
+		 * unsuccessful at storing this route into the cache
+		 * we really need to set it.
+		 */
+		rt->dst.flags |= DST_NOCACHE;
 	}
 }
 
@@ -1412,7 +1420,7 @@ static int __mkroute_input(struct sk_buff *skb,
 	do_cache = false;
 	if (res->fi) {
 		if (!itag) {
-			rth = FIB_RES_NH(*res).nh_rth_input;
+			rth = rcu_dereference(FIB_RES_NH(*res).nh_rth_input);
 			if (rt_cache_valid(rth)) {
 				skb_dst_set_noref(skb, &rth->dst);
 				goto out;
@@ -1574,7 +1582,7 @@ local_input:
 	do_cache = false;
 	if (res.fi) {
 		if (!itag) {
-			rth = FIB_RES_NH(res).nh_rth_input;
+			rth = rcu_dereference(FIB_RES_NH(res).nh_rth_input);
 			if (rt_cache_valid(rth)) {
 				skb_dst_set_noref(skb, &rth->dst);
 				err = 0;
@@ -1742,7 +1750,7 @@ static struct rtable *__mkroute_output(const struct fib_result *res,
 	if (fi) {
 		fnhe = find_exception(&FIB_RES_NH(*res), fl4->daddr);
 		if (!fnhe) {
-			rth = FIB_RES_NH(*res).nh_rth_output;
+			rth = rcu_dereference(FIB_RES_NH(*res).nh_rth_output);
 			if (rt_cache_valid(rth)) {
 				dst_hold(&rth->dst);
 				return rth;