Both sysctl(2) and IPv6 stack accept negative value for
`ip6_neighborgcthresh' to disable garbage collector of NDP entries. No
limits is bad design, however I don't want to mix unlocking with
behavior change.

Not used in ramdisk.

Index: sys/netinet6/in6_proto.c
===================================================================
RCS file: /cvs/src/sys/netinet6/in6_proto.c,v
retrieving revision 1.146
diff -u -p -r1.146 in6_proto.c
--- sys/netinet6/in6_proto.c	25 Jul 2025 22:24:06 -0000	1.146
+++ sys/netinet6/in6_proto.c	25 Jul 2025 23:02:15 -0000
@@ -363,7 +363,7 @@ int	ip6_dad_pending;	/* number of curren
 int	ip6_auto_flowlabel = 1;	/* [a] */
 int	ip6_use_deprecated = 1;	/* [a] allow deprecated addr (RFC2462 5.5.4) */
 int	ip6_mcast_pmtu = 0;	/* [a] enable pMTU discovery for multicast? */
-int	ip6_neighborgcthresh = 2048; /* Threshold # of NDP entries for GC */
+int	ip6_neighborgcthresh = 2048; /* [a] Threshold # of NDP entries for GC */
 int	ip6_maxdynroutes = 4096; /* Max # of routes created via redirect */
 time_t	ip6_log_time = (time_t)0L;
 
Index: sys/netinet6/ip6_input.c
===================================================================
RCS file: /cvs/src/sys/netinet6/ip6_input.c,v
retrieving revision 1.291
diff -u -p -r1.291 ip6_input.c
--- sys/netinet6/ip6_input.c	25 Jul 2025 22:24:06 -0000	1.291
+++ sys/netinet6/ip6_input.c	25 Jul 2025 23:02:15 -0000
@@ -1458,10 +1458,10 @@ const struct sysctl_bounded_args ipv6ctl
 	{ IPV6CTL_MAXFRAGS, &ip6_maxfrags, 0, 1000 },
 	{ IPV6CTL_MFORWARDING, &ip6_mforwarding, 0, 1 },
 	{ IPV6CTL_MCAST_PMTU, &ip6_mcast_pmtu, 0, 1 },
+	{ IPV6CTL_NEIGHBORGCTHRESH, &ip6_neighborgcthresh, -1, 5 * 2048 },
 };
 
 const struct sysctl_bounded_args ipv6ctl_vars[] = {
-	{ IPV6CTL_NEIGHBORGCTHRESH, &ip6_neighborgcthresh, -1, 5 * 2048 },
 	{ IPV6CTL_MAXDYNROUTES, &ip6_maxdynroutes, -1, 5 * 4096 },
 };
 
@@ -1578,6 +1578,7 @@ ip6_sysctl(int *name, u_int namelen, voi
 	case IPV6CTL_MAXFRAGS:
 	case IPV6CTL_MFORWARDING:
 	case IPV6CTL_MCAST_PMTU:
+	case IPV6CTL_NEIGHBORGCTHRESH:
 		return (sysctl_bounded_arr(
 		    ipv6ctl_vars_unlocked, nitems(ipv6ctl_vars_unlocked),
 		    name, namelen, oldp, oldlenp, newp, newlen));
Index: sys/netinet6/nd6.c
===================================================================
RCS file: /cvs/src/sys/netinet6/nd6.c,v
retrieving revision 1.292
diff -u -p -r1.292 nd6.c
--- sys/netinet6/nd6.c	8 Jul 2025 00:47:41 -0000	1.292
+++ sys/netinet6/nd6.c	25 Jul 2025 23:02:15 -0000
@@ -729,6 +729,7 @@ nd6_rtrequest(struct ifnet *ifp, int req
 	struct llinfo_nd6 *ln;
 	struct ifaddr *ifa;
 	struct in6_ifaddr *ifa6;
+	int ip6_neighborgcthresh_local;
 
 	if (ISSET(rt->rt_flags, RTF_GATEWAY|RTF_MULTICAST|RTF_MPLS))
 		return;
@@ -829,8 +830,11 @@ nd6_rtrequest(struct ifnet *ifp, int req
 		 * cause re-entering rtable related routines triggering
 		 * lock-order-reversal problems.
 		 */
-		if (ip6_neighborgcthresh >= 0 &&
-		    nd6_inuse >= ip6_neighborgcthresh) {
+		ip6_neighborgcthresh_local =
+		    atomic_load_int(&ip6_neighborgcthresh);
+
+		if (ip6_neighborgcthresh_local >= 0 &&
+		    nd6_inuse >= ip6_neighborgcthresh_local) {
 			int i;
 
 			for (i = 0; i < 10; i++) {