ipv6: release nexthop on device removal

JIRA: https://issues.redhat.com/browse/RHEL-73281
Upstream Status: net.git commit eb02688

Conflicts: context conflicts due to missing b4cb4a1
("net: use unrcu_pointer() helper"). Use unrcu_pointer() helper in the
new code since we have it on rhel9.

commit eb02688
Author: Paolo Abeni <pabeni@redhat.com>
Date:   Tue Nov 5 19:23:50 2024 +0100

    ipv6: release nexthop on device removal

    The CI is hitting some aperiodic hangup at device removal time in the
    pmtu.sh self-test:

    unregister_netdevice: waiting for veth_A-R1 to become free. Usage count = 6
    ref_tracker: veth_A-R1@ffff888013df15d8 has 1/5 users at
            dst_init+0x84/0x4a0
            dst_alloc+0x97/0x150
            ip6_dst_alloc+0x23/0x90
            ip6_rt_pcpu_alloc+0x1e6/0x520
            ip6_pol_route+0x56f/0x840
            fib6_rule_lookup+0x334/0x630
            ip6_route_output_flags+0x259/0x480
            ip6_dst_lookup_tail.constprop.0+0x5c2/0x940
            ip6_dst_lookup_flow+0x88/0x190
            udp_tunnel6_dst_lookup+0x2a7/0x4c0
            vxlan_xmit_one+0xbde/0x4a50 [vxlan]
            vxlan_xmit+0x9ad/0xf20 [vxlan]
            dev_hard_start_xmit+0x10e/0x360
            __dev_queue_xmit+0xf95/0x18c0
            arp_solicit+0x4a2/0xe00
            neigh_probe+0xaa/0xf0

    While the first suspect is the dst_cache, explicitly tracking the dst
    owing the last device reference via probes proved such dst is held by
    the nexthop in the originating fib6_info.

    Similar to commit f5b51fe ("ipv6: route: purge exception on
    removal"), we need to explicitly release the originating fib info when
    disconnecting a to-be-removed device from a live ipv6 dst: move the
    fib6_info cleanup into ip6_dst_ifdown().

    Tested running:

    ./pmtu.sh cleanup_ipv6_exception

    in a tight loop for more than 400 iterations with no spat, running an
    unpatched kernel  I observed a splat every ~10 iterations.

    Fixes: f88d8ea ("ipv6: Plumb support for nexthop object in a fib6_info")
    Signed-off-by: Paolo Abeni <pabeni@redhat.com>
    Reviewed-by: Eric Dumazet <edumazet@google.com>
    Reviewed-by: David Ahern <dsahern@kernel.org>
    Link: https://patch.msgid.link/604c45c188c609b732286b47ac2a451a40f6cf6d.1730828007.git.pabeni@redhat.com
    Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Signed-off-by: Hangbin Liu <haliu@redhat.com>

Author: Hangbin Liu

net/ipv6/route.c

@@ -377,6 +377,7 @@ static void ip6_dst_ifdown(struct dst_entry *dst, struct net_device *dev,
 {
 	struct rt6_info *rt = (struct rt6_info *)dst;
 	struct inet6_dev *idev = rt->rt6i_idev;
+	struct fib6_info *from;
 
 	if (idev && idev->dev != blackhole_netdev) {
 		struct inet6_dev *blackhole_idev = in6_dev_get(blackhole_netdev);
@@ -386,6 +387,8 @@ static void ip6_dst_ifdown(struct dst_entry *dst, struct net_device *dev,
 			in6_dev_put(idev);
 		}
 	}
+	from = unrcu_pointer(xchg(&rt->from, NULL));
+	fib6_info_release(from);
 }
 
 static bool __rt6_check_expired(const struct rt6_info *rt)
@@ -1449,7 +1452,6 @@ static DEFINE_SPINLOCK(rt6_exception_lock);
 static void rt6_remove_exception(struct rt6_exception_bucket *bucket,
 				 struct rt6_exception *rt6_ex)
 {
-	struct fib6_info *from;
 	struct net *net;
 
 	if (!bucket || !rt6_ex)
@@ -1461,8 +1463,6 @@ static void rt6_remove_exception(struct rt6_exception_bucket *bucket,
 	/* purge completely the exception to allow releasing the held resources:
 	 * some [sk] cache may keep the dst around for unlimited time
 	 */
-	from = xchg((__force struct fib6_info **)&rt6_ex->rt6i->from, NULL);
-	fib6_info_release(from);
 	dst_dev_put(&rt6_ex->rt6i->dst);
 
 	hlist_del_rcu(&rt6_ex->hlist);