ipv6: mcast: annotate data-races around mca_users

/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().

Signed-off-by: Yuyang Huang <sigefriedhyy@gmail.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260524022456.20689-1-sigefriedhyy@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/ipv6/mcast.c b/net/ipv6/mcast.c
index 6ddc18ac59b951e84a49fcce7972bc1989ef2dae..1297047fedb2b5ce70b5c3460f2bfe87af720ee0 100644 (file)
--- 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;