[PATCH 1/2] ipv4: igmp: annotate data-races around in_dev->mc_count

[Yuyang Huang <yuyanghuang@google.com>]

/proc/net/igmp walks the multicast list for IPv4 interfaces locklessly
under RCU and prints state->in_dev->mc_count. Concurrently, device
init/destruction and multicast join/leave paths update the count
under the RTNL lock. Fix this intentional lockless snapshot by
annotating the read with READ_ONCE() and the updates with WRITE_ONCE().

Fixes: 1d7138de878d ("igmp: RCU conversion of in_dev->mc_list")
Signed-off-by: Yuyang Huang <yuyanghuang@google.com>
---
 net/ipv4/igmp.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/net/ipv4/igmp.c b/net/ipv4/igmp.c
index f2aca659b29c..fd0faf042fa6 100644
--- a/net/ipv4/igmp.c
+++ b/net/ipv4/igmp.c
@@ -1566,7 +1566,7 @@ static void ____ip_mc_inc_group(struct in_device *in_dev, __be32 addr,
 #endif
 
 	im->next_rcu = in_dev->mc_list;
-	in_dev->mc_count++;
+	WRITE_ONCE(in_dev->mc_count, in_dev->mc_count + 1);
 	rcu_assign_pointer(in_dev->mc_list, im);
 
 	ip_mc_hash_add(in_dev, im);
@@ -1790,7 +1790,8 @@ void __ip_mc_dec_group(struct in_device *in_dev, __be32 addr, gfp_t gfp)
 			if (new_users == 0) {
 				ip_mc_hash_remove(in_dev, i);
 				*ip = i->next_rcu;
-				in_dev->mc_count--;
+				WRITE_ONCE(in_dev->mc_count,
+					   in_dev->mc_count - 1);
 				__igmp_group_dropped(i, gfp);
 				inet_ifmcaddr_notify(in_dev->dev, i,
 						     RTM_DELMULTICAST);
@@ -1922,7 +1923,7 @@ void ip_mc_destroy_dev(struct in_device *in_dev)
 
 	while ((i = rtnl_dereference(in_dev->mc_list)) != NULL) {
 		in_dev->mc_list = i->next_rcu;
-		in_dev->mc_count--;
+		WRITE_ONCE(in_dev->mc_count, in_dev->mc_count - 1);
 		ip_mc_clear_src(i);
 		ip_ma_put(i);
 	}
@@ -2974,7 +2975,9 @@ static int igmp_mc_seq_show(struct seq_file *seq, void *v)
 
 		if (rcu_access_pointer(state->in_dev->mc_list) == im) {
 			seq_printf(seq, "%d\t%-10s: %5d %7s\n",
-				   state->dev->ifindex, state->dev->name, state->in_dev->mc_count, querier);
+				   state->dev->ifindex, state->dev->name,
+				   READ_ONCE(state->in_dev->mc_count),
+				   querier);
 		}
 
 		delta = im->timer.expires - jiffies;
-- 
2.54.0.823.g6e5bcc1fc9-goog