[PATCH net-next] ipv6: mcast: annotate data-races around mca_users

[PATCH net-next] ipv6: mcast: annotate data-races around mca_users

[Yuyang Huang]

/proc/net/igmp6 walks IPv6 multicast memberships under RCU and prints
mca_users without holding idev->mc_lock, while multicast join and leave
paths update the field while holding idev->mc_lock. Annotate this
intentional lockless snapshot with READ_ONCE() and the matching writers
with WRITE_ONCE().

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Yuyang Huang <sigefriedhyy@gmail.com>
---
 net/ipv6/mcast.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/net/ipv6/mcast.c b/net/ipv6/mcast.c
index 6ddc18ac59b9..1297047fedb2 100644
--- a/net/ipv6/mcast.c
+++ b/net/ipv6/mcast.c
@@ -876,7 +876,7 @@ static struct ifmcaddr6 *mca_alloc(struct inet6_dev *idev,
 
 	mc->mca_addr = *addr;
 	mc->idev = idev; /* reference taken by caller */
-	mc->mca_users = 1;
+	WRITE_ONCE(mc->mca_users, 1);
 	/* mca_stamp should be updated upon changes */
 	mc->mca_cstamp = mc->mca_tstamp = jiffies;
 	refcount_set(&mc->mca_refcnt, 1);
@@ -950,7 +950,7 @@ static int __ipv6_dev_mc_inc(struct net_device *dev,
 
 	for_each_mc_mclock(idev, mc) {
 		if (ipv6_addr_equal(&mc->mca_addr, addr)) {
-			mc->mca_users++;
+			WRITE_ONCE(mc->mca_users, mc->mca_users + 1);
 			ip6_mc_add_src(idev, &mc->mca_addr, mode, 0, NULL, 0);
 			mutex_unlock(&idev->mc_lock);
 			in6_dev_put(idev);
@@ -995,7 +995,10 @@ int __ipv6_dev_mc_dec(struct inet6_dev *idev, const struct in6_addr *addr)
 	     (ma = mc_dereference(*map, idev));
 	     map = &ma->next) {
 		if (ipv6_addr_equal(&ma->mca_addr, addr)) {
-			if (--ma->mca_users == 0) {
+			int new_users = ma->mca_users - 1;
+
+			WRITE_ONCE(ma->mca_users, new_users);
+			if (new_users == 0) {
 				*map = ma->next;
 
 				igmp6_group_dropped(ma);
@@ -2974,7 +2977,7 @@ static int igmp6_mc_seq_show(struct seq_file *seq, void *v)
 		   "%-4d %-15s %pi6 %5d %08X %ld\n",
 		   state->dev->ifindex, state->dev->name,
 		   &im->mca_addr,
-		   im->mca_users, im->mca_flags,
+		   READ_ONCE(im->mca_users), im->mca_flags,
 		   (im->mca_flags & MAF_TIMER_RUNNING) ?
 		   jiffies_to_clock_t(im->mca_work.timer.expires - jiffies) : 0);
 	return 0;
-- 
2.43.0

Re: [PATCH net-next] ipv6: mcast: annotate data-races around mca_users

[Ido Schimmel]

On Sun, May 24, 2026 at 11:24:56AM +0900, Yuyang Huang wrote:
> /proc/net/igmp6 walks IPv6 multicast memberships under RCU and prints
> mca_users without holding idev->mc_lock, while multicast join and leave
> paths update the field while holding idev->mc_lock. Annotate this
> intentional lockless snapshot with READ_ONCE() and the matching writers
> with WRITE_ONCE().
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Signed-off-by: Yuyang Huang <sigefriedhyy@gmail.com>

Reviewed-by: Ido Schimmel <idosch@nvidia.com>