From: Yuyang Huang <sigefriedhyy@gmail.com>
To: Yuyang Huang <sigefriedhyy@gmail.com>
Cc: Jiayuan Chen <jiayuan.chen@linux.dev>,
	"David S. Miller" <davem@davemloft.net>,
	David Ahern <dsahern@kernel.org>,
	Eric Dumazet <edumazet@google.com>,
	Ido Schimmel <idosch@nvidia.com>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	Simon Horman <horms@kernel.org>,
	linux-kernel@vger.kernel.org, netdev@vger.kernel.org
Subject: [PATCH net-next] ipv6: mcast: annotate data-races around mca_users
Date: Sun, 24 May 2026 11:24:56 +0900	[thread overview]
Message-ID: <20260524022456.20689-1-sigefriedhyy@gmail.com> (raw)

/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

next             reply	other threads:[~2026-05-24  2:25 UTC|newest]

Thread overview: 3+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2026-05-24  2:24 Yuyang Huang [this message]
2026-05-27  7:15 ` [PATCH net-next] ipv6: mcast: annotate data-races around mca_users Ido Schimmel
2026-05-28  0:10 ` patchwork-bot+netdevbpf

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