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Taehee Yoo

2021-Apr-25 15:57 UTC

[Bridge] [PATCH net v2 0/2] net: fix lockdep false positive splat

This patchset is to fix lockdep false positive splat in multicast part of
the bridge.

When mdb or multicast config is changed, it acquires multicast_lock,
which is a per-interface(bridge) lock.
So, spin_lock_nested() should be used instead of spin_lock() because
interfaces can be recursive.
The spin_lock_nested() needs 'subclass' parameter.

The first patch adds a new helper function, which returns the nest_level
variable under RCU. The nest_level variable can be used as 'subclass'
parameter of spin_lock_nested().
The second patch fix lockdep false positive splat in the bridge multicast
part by using netdev_get_nest_level_rcu().

v2:
 - Remove dupliate Subject

Taehee Yoo (2):
  net: core: make bond_get_lowest_level_rcu() generic
  net: bridge: fix lockdep multicast_lock false positive splat

 drivers/net/bonding/bond_main.c |  45 +---------
 include/linux/netdevice.h       |   1 +
 net/bridge/br_mdb.c             |  12 +--
 net/bridge/br_multicast.c       | 146 ++++++++++++++++++++------------
 net/bridge/br_multicast_eht.c   |  18 ++--
 net/bridge/br_private.h         |  48 +++++++++++
 net/core/dev.c                  |  44 ++++++++++
 7 files changed, 204 insertions(+), 110 deletions(-)

-- 
2.17.1

Taehee Yoo

2021-Apr-25 15:57 UTC

head link

[Bridge] [PATCH net v2 1/2] net: core: make bond_get_lowest_level_rcu() generic

The purpose of bond_get_lowest_level_rcu() is to get nested_level under
RCU. Because dev->nested_level is protected by RTNL, so it shouldn't be
used without RTNL. But bonding module needs this value under RCU without
RTNL.
So, this function was added.

But, there is another module, which needs this function.
So, make this function generic.
the new name is netdev_get_nest_level_rcu().

Signed-off-by: Taehee Yoo <ap420073 at gmail.com>
---

v2:
 - No change

 drivers/net/bonding/bond_main.c | 45 +--------------------------------
 include/linux/netdevice.h       |  1 +
 net/core/dev.c                  | 44 ++++++++++++++++++++++++++++++++
 3 files changed, 46 insertions(+), 44 deletions(-)

diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
index 83ef62db6285..a9feb039ffa6 100644
--- a/drivers/net/bonding/bond_main.c
+++ b/drivers/net/bonding/bond_main.c
@@ -3754,47 +3754,6 @@ static void bond_fold_stats(struct rtnl_link_stats64
*_res,
 	}
 }
 
-#ifdef CONFIG_LOCKDEP
-static int bond_get_lowest_level_rcu(struct net_device *dev)
-{
-	struct net_device *ldev, *next, *now, *dev_stack[MAX_NEST_DEV + 1];
-	struct list_head *niter, *iter, *iter_stack[MAX_NEST_DEV + 1];
-	int cur = 0, max = 0;
-
-	now = dev;
-	iter = &dev->adj_list.lower;
-
-	while (1) {
-		next = NULL;
-		while (1) {
-			ldev = netdev_next_lower_dev_rcu(now, &iter);
-			if (!ldev)
-				break;
-
-			next = ldev;
-			niter = &ldev->adj_list.lower;
-			dev_stack[cur] = now;
-			iter_stack[cur++] = iter;
-			if (max <= cur)
-				max = cur;
-			break;
-		}
-
-		if (!next) {
-			if (!cur)
-				return max;
-			next = dev_stack[--cur];
-			niter = iter_stack[cur];
-		}
-
-		now = next;
-		iter = niter;
-	}
-
-	return max;
-}
-#endif
-
 static void bond_get_stats(struct net_device *bond_dev,
 			   struct rtnl_link_stats64 *stats)
 {
@@ -3806,9 +3765,7 @@ static void bond_get_stats(struct net_device *bond_dev,
 
 
 	rcu_read_lock();
-#ifdef CONFIG_LOCKDEP
-	nest_level = bond_get_lowest_level_rcu(bond_dev);
-#endif
+	nest_level = netdev_get_nest_level_rcu(bond_dev);
 
 	spin_lock_nested(&bond->stats_lock, nest_level);
 	memcpy(stats, &bond->bond_stats, sizeof(*stats));
diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 87a5d186faff..507c06bf5dba 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -4699,6 +4699,7 @@ int netdev_walk_all_lower_dev(struct net_device *dev,
 			      int (*fn)(struct net_device *lower_dev,
 					struct netdev_nested_priv *priv),
 			      struct netdev_nested_priv *priv);
+int netdev_get_nest_level_rcu(struct net_device *dev);
 int netdev_walk_all_lower_dev_rcu(struct net_device *dev,
 				  int (*fn)(struct net_device *lower_dev,
 					    struct netdev_nested_priv *priv),
diff --git a/net/core/dev.c b/net/core/dev.c
index 15fe36332fb8..efc2bf88eafd 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -7709,6 +7709,50 @@ static int __netdev_update_lower_level(struct net_device
*dev,
 	return 0;
 }
 
+int netdev_get_nest_level_rcu(struct net_device *dev)
+{
+#ifdef CONFIG_LOCKDEP
+	struct net_device *ldev, *next, *now, *dev_stack[MAX_NEST_DEV + 1];
+	struct list_head *niter, *iter, *iter_stack[MAX_NEST_DEV + 1];
+	int cur = 0, max = 0;
+
+	now = dev;
+	iter = &dev->adj_list.lower;
+
+	while (1) {
+		next = NULL;
+		while (1) {
+			ldev = netdev_next_lower_dev_rcu(now, &iter);
+			if (!ldev)
+				break;
+
+			next = ldev;
+			niter = &ldev->adj_list.lower;
+			dev_stack[cur] = now;
+			iter_stack[cur++] = iter;
+			if (max <= cur)
+				max = cur;
+			break;
+		}
+
+		if (!next) {
+			if (!cur)
+				return max;
+			next = dev_stack[--cur];
+			niter = iter_stack[cur];
+		}
+
+		now = next;
+		iter = niter;
+	}
+
+	return max;
+#else
+	return 0;
+#endif
+}
+EXPORT_SYMBOL_GPL(netdev_get_nest_level_rcu);
+
 int netdev_walk_all_lower_dev_rcu(struct net_device *dev,
 				  int (*fn)(struct net_device *dev,
 					    struct netdev_nested_priv *priv),
-- 
2.17.1

Taehee Yoo

2021-Apr-25 15:57 UTC

head link

[Bridge] [PATCH net 2/2] net: bridge: fix lockdep multicast_lock false positive splat

multicast_lock is a per-interface(bridge) lock.
This lock can be used recursively because interfaces can be used
recursively. So, it should use spin_lock_nested() but it doesn't.
So lockdep false positive splat occurred.

Some inline helper functions are added.
These functions internally get 'subclass' variable, which is used as
parameter of spin_lock_nested() and use spin_lock_nested() with a
subclass parameter.

Test commands:
    ip link add br0 type bridge
    ip link add bond0 type bond
    ip link add br1 type bridge
    ip link set br0 master bond0
    ip link set bond0 up
    ip link set bond0 master br1
    ip link set br0 up
    ip link set br1 up
    ip link set br0 type bridge mcast_router 1 mcast_querier 1
    ip link set br1 type bridge mcast_querier 1 mcast_router 1

Splat looks like:
===========================================WARNING: possible recursive locking
detected
5.12.0-rc7+ #855 Not tainted
--------------------------------------------
kworker/5:1/56 is trying to acquire lock:
ffff88810f833000 (&br->multicast_lock){+.-.}-{2:2}, at:
br_multicast_rcv+0x1484/0x5280 [bridge]

but task is already holding lock:
ffff888116e63000 (&br->multicast_lock){+.-.}-{2:2}, at:
br_multicast_port_query_expired+0x40/0x140 [bridge]

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(&br->multicast_lock);
  lock(&br->multicast_lock);

 *** DEADLOCK ***

 May be due to missing lock nesting notation

9 locks held by kworker/5:1/56:
 #0: ffff888100079b48 ((wq_completion)events){+.+.}-{0:0}, at:
process_one_work+0x761/0x1440
 #1: ffff888100c6fe00 ((linkwatch_work).work){+.+.}-{0:0}, at:
process_one_work+0x795/0x1440
 #2: ffffffffa1278f30 (rtnl_mutex){+.+.}-{3:3}, at:
linkwatch_event+0xa/0x50
 #3: ffff88811a009dc8 ((&port->ip4_own_query.timer)){+.-.}-{0:0}, at:
call_timer_fn+0xc8/0x460
 #4: ffff888116e63000 (&br->multicast_lock){+.-.}-{2:2}, at:
br_multicast_port_query_expired+0x40/0x140 [bridge]
 #5: ffffffffa0e308a0 (rcu_read_lock_bh){....}-{1:2}, at:
__dev_queue_xmit+0x1fd/0x2990
 #6: ffffffffa0e30900 (rcu_read_lock){....}-{1:2}, at:
bond_start_xmit+0x7b/0x1160 [bonding]
 #7: ffffffffa0e308a0 (rcu_read_lock_bh){....}-{1:2}, at:
__dev_queue_xmit+0x1fd/0x2990
 #8: ffffffffa0e30900 (rcu_read_lock){....}-{1:2}, at:
br_dev_xmit+0x5/0x1350 [bridge]

stack backtrace:
CPU: 5 PID: 56 Comm: kworker/5:1 Not tainted 5.12.0-rc7+ #855
Workqueue: events linkwatch_event
Call Trace:
 <IRQ>
 dump_stack+0xa4/0xe5
 __lock_acquire.cold.76+0x1f2/0x3d7
 ? register_lock_class+0x1750/0x1750
 ? module_assert_mutex_or_preempt+0x41/0x70
 ? rcu_read_lock_sched_held+0x91/0xc0
 ? rcu_read_lock_bh_held+0xa0/0xa0
 lock_acquire+0x191/0x720
 ? br_multicast_rcv+0x1484/0x5280 [bridge]
 ? lockdep_hardirqs_on_prepare+0x3e0/0x3e0
 ? skb_checksum_trimmed+0x247/0x360
 ? ip_mc_check_igmp+0x423/0xf70
 _raw_spin_lock+0x30/0x70
 ? br_multicast_rcv+0x1484/0x5280 [bridge]
 br_multicast_rcv+0x1484/0x5280 [bridge]
 ? print_usage_bug+0x80/0x80
 ? register_lock_class+0x1750/0x1750
 ? br_ip6_multicast_leave_group.part.62+0x1b0/0x1b0 [bridge]
 ? br_handle_vlan+0xa50/0xa50 [bridge]
 ? lockdep_hardirqs_on_prepare+0x3e0/0x3e0
 ? rcu_read_lock_held+0x91/0xa0
 ? rcu_read_lock_sched_held+0xc0/0xc0
 br_dev_xmit+0x5fe/0x1350 [bridge]
 ? br_dev_open+0xd0/0xd0 [bridge]
 ? rcu_read_lock_sched_held+0x91/0xc0
 ? skb_csum_hwoffload_help+0x100/0x100
 ? rcu_read_lock_sched_held+0x91/0xc0
 ? rcu_read_lock_bh_held+0xa0/0xa0
 dev_hard_start_xmit+0x1ae/0x6e0
 __dev_queue_xmit+0x1ef8/0x2990
 ? netdev_core_pick_tx+0x290/0x290
 ? check_chain_key+0x244/0x5f0
 ? rcu_read_lock_sched_held+0x91/0xc0
 ? rcu_read_lock_bh_held+0xa0/0xa0
 ? rcu_read_lock_held+0x91/0xa0
 ? rcu_read_lock_sched_held+0xc0/0xc0
 ? register_lock_class+0x1750/0x1750
 ? bond_start_xmit+0x274/0x1160 [bonding]
 bond_start_xmit+0x274/0x1160 [bonding]
 ? skb_csum_hwoffload_help+0x100/0x100
 ? bond_xmit_get_slave+0x2c0/0x2c0 [bonding]
 ? rcu_read_lock_sched_held+0x91/0xc0
 ? rcu_read_lock_bh_held+0xa0/0xa0
 ? netif_skb_features+0x296/0x9c0
 dev_hard_start_xmit+0x1ae/0x6e0
 __dev_queue_xmit+0x1ef8/0x2990
 ? check_object+0xb3/0x290
 ? init_object+0x4e/0x80
 ? netdev_core_pick_tx+0x290/0x290
 ? mark_held_locks+0xb7/0x120
 ? rcu_read_lock_sched_held+0x91/0xc0
 ? rcu_read_lock_bh_held+0xa0/0xa0
 ? slab_post_alloc_hook+0x43/0x430
 ? trace_kmalloc_node+0xb0/0xe0
 ? __kmalloc_node_track_caller+0x11c/0x2b0
 ? __alloc_skb+0xc5/0x290
 ? memset+0x1f/0x40
 ? br_dev_queue_push_xmit+0x1ad/0x610 [bridge]
 br_dev_queue_push_xmit+0x1ad/0x610 [bridge]
 [...]

Fixes: eb1d16414339 ("bridge: Add core IGMP snooping support")
Signed-off-by: Taehee Yoo <ap420073 at gmail.com>
---

v2:
 - No change

 net/bridge/br_mdb.c           |  12 +--
 net/bridge/br_multicast.c     | 146 +++++++++++++++++++++-------------
 net/bridge/br_multicast_eht.c |  18 +++--
 net/bridge/br_private.h       |  48 +++++++++++
 4 files changed, 158 insertions(+), 66 deletions(-)

diff --git a/net/bridge/br_mdb.c b/net/bridge/br_mdb.c
index 8846c5bcd075..0a905f397ba9 100644
--- a/net/bridge/br_mdb.c
+++ b/net/bridge/br_mdb.c
@@ -490,7 +490,7 @@ static void br_mdb_complete(struct net_device *dev, int err,
void *priv)
 	if (err)
 		goto err;
 
-	spin_lock_bh(&br->multicast_lock);
+	br_multicast_lock_rtnl_bh(br);
 	mp = br_mdb_ip_get(br, &data->ip);
 	if (!mp)
 		goto out;
@@ -501,7 +501,7 @@ static void br_mdb_complete(struct net_device *dev, int err,
void *priv)
 		p->flags |= MDB_PG_FLAGS_OFFLOAD;
 	}
 out:
-	spin_unlock_bh(&br->multicast_lock);
+	br_multicast_unlock_bh(br);
 err:
 	kfree(priv);
 }
@@ -949,9 +949,9 @@ static int __br_mdb_add(struct net *net, struct net_bridge
*br,
 {
 	int ret;
 
-	spin_lock_bh(&br->multicast_lock);
+	br_multicast_lock_rtnl_bh(br);
 	ret = br_mdb_add_group(br, p, entry, mdb_attrs, extack);
-	spin_unlock_bh(&br->multicast_lock);
+	br_multicast_unlock_bh(br);
 
 	return ret;
 }
@@ -1042,7 +1042,7 @@ static int __br_mdb_del(struct net_bridge *br, struct
br_mdb_entry *entry,
 
 	__mdb_entry_to_br_ip(entry, &ip, mdb_attrs);
 
-	spin_lock_bh(&br->multicast_lock);
+	br_multicast_lock_rtnl_bh(br);
 	mp = br_mdb_ip_get(br, &ip);
 	if (!mp)
 		goto unlock;
@@ -1072,7 +1072,7 @@ static int __br_mdb_del(struct net_bridge *br, struct
br_mdb_entry *entry,
 	}
 
 unlock:
-	spin_unlock_bh(&br->multicast_lock);
+	br_multicast_unlock_bh(br);
 	return err;
 }
 
diff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c
index 9d265447d654..f25ac316242b 100644
--- a/net/bridge/br_multicast.c
+++ b/net/bridge/br_multicast.c
@@ -538,7 +538,8 @@ static void br_multicast_group_expired(struct timer_list *t)
 	struct net_bridge_mdb_entry *mp = from_timer(mp, t, timer);
 	struct net_bridge *br = mp->br;
 
-	spin_lock(&br->multicast_lock);
+	rcu_read_lock();
+	br_multicast_lock_rcu(br);
 	if (hlist_unhashed(&mp->mdb_node) || !netif_running(br->dev) ||
 	    timer_pending(&mp->timer))
 		goto out;
@@ -549,7 +550,8 @@ static void br_multicast_group_expired(struct timer_list *t)
 		goto out;
 	br_multicast_del_mdb_entry(mp);
 out:
-	spin_unlock(&br->multicast_lock);
+	br_multicast_unlock(br);
+	rcu_read_unlock();
 }
 
 static void br_multicast_destroy_group_src(struct net_bridge_mcast_gc *gc)
@@ -648,7 +650,8 @@ static void br_multicast_port_group_expired(struct
timer_list *t)
 	struct hlist_node *tmp;
 	bool changed;
 
-	spin_lock(&br->multicast_lock);
+	rcu_read_lock();
+	br_multicast_lock_rcu(br);
 	if (!netif_running(br->dev) || timer_pending(&pg->timer) ||
 	    hlist_unhashed(&pg->mglist) || pg->flags &
MDB_PG_FLAGS_PERMANENT)
 		goto out;
@@ -675,7 +678,8 @@ static void br_multicast_port_group_expired(struct
timer_list *t)
 		br_mdb_notify(br->dev, mp, pg, RTM_NEWMDB);
 	}
 out:
-	spin_unlock(&br->multicast_lock);
+	br_multicast_unlock(br);
+	rcu_read_unlock();
 }
 
 static void br_multicast_gc(struct hlist_head *head)
@@ -1077,7 +1081,8 @@ static void br_multicast_group_src_expired(struct
timer_list *t)
 	struct net_bridge_port_group *pg;
 	struct net_bridge *br = src->br;
 
-	spin_lock(&br->multicast_lock);
+	rcu_read_lock();
+	br_multicast_lock_rcu(br);
 	if (hlist_unhashed(&src->node) || !netif_running(br->dev) ||
 	    timer_pending(&src->timer))
 		goto out;
@@ -1093,7 +1098,8 @@ static void br_multicast_group_src_expired(struct
timer_list *t)
 	}
 
 out:
-	spin_unlock(&br->multicast_lock);
+	br_multicast_unlock(br);
+	rcu_read_unlock();
 }
 
 struct net_bridge_group_src *
@@ -1296,12 +1302,12 @@ static int br_multicast_add_group(struct net_bridge *br,
 	struct net_bridge_port_group *pg;
 	int err;
 
-	spin_lock(&br->multicast_lock);
+	br_multicast_lock_rcu(br);
 	pg = __br_multicast_add_group(br, port, group, src, filter_mode,
 				      igmpv2_mldv1, false);
 	/* NULL is considered valid for host joined groups */
 	err = PTR_ERR_OR_ZERO(pg);
-	spin_unlock(&br->multicast_lock);
+	br_multicast_unlock(br);
 
 	return err;
 }
@@ -1360,7 +1366,8 @@ static void br_multicast_router_expired(struct timer_list
*t)
 			from_timer(port, t, multicast_router_timer);
 	struct net_bridge *br = port->br;
 
-	spin_lock(&br->multicast_lock);
+	rcu_read_lock();
+	br_multicast_lock_rcu(br);
 	if (port->multicast_router == MDB_RTR_TYPE_DISABLED ||
 	    port->multicast_router == MDB_RTR_TYPE_PERM ||
 	    timer_pending(&port->multicast_router_timer))
@@ -1368,7 +1375,8 @@ static void br_multicast_router_expired(struct timer_list
*t)
 
 	__del_port_router(port);
 out:
-	spin_unlock(&br->multicast_lock);
+	br_multicast_unlock(br);
+	rcu_read_unlock();
 }
 
 static void br_mc_router_state_change(struct net_bridge *p,
@@ -1388,7 +1396,8 @@ static void br_multicast_local_router_expired(struct
timer_list *t)
 {
 	struct net_bridge *br = from_timer(br, t, multicast_router_timer);
 
-	spin_lock(&br->multicast_lock);
+	rcu_read_lock();
+	br_multicast_lock_rcu(br);
 	if (br->multicast_router == MDB_RTR_TYPE_DISABLED ||
 	    br->multicast_router == MDB_RTR_TYPE_PERM ||
 	    timer_pending(&br->multicast_router_timer))
@@ -1396,20 +1405,23 @@ static void br_multicast_local_router_expired(struct
timer_list *t)
 
 	br_mc_router_state_change(br, false);
 out:
-	spin_unlock(&br->multicast_lock);
+	br_multicast_unlock(br);
+	rcu_read_unlock();
 }
 
 static void br_multicast_querier_expired(struct net_bridge *br,
 					 struct bridge_mcast_own_query *query)
 {
-	spin_lock(&br->multicast_lock);
+	rcu_read_lock();
+	br_multicast_lock_rcu(br);
 	if (!netif_running(br->dev) || !br_opt_get(br, BROPT_MULTICAST_ENABLED))
 		goto out;
 
 	br_multicast_start_querier(br, query);
 
 out:
-	spin_unlock(&br->multicast_lock);
+	br_multicast_unlock(br);
+	rcu_read_unlock();
 }
 
 static void br_ip4_multicast_querier_expired(struct timer_list *t)
@@ -1525,7 +1537,8 @@ br_multicast_port_query_expired(struct net_bridge_port
*port,
 {
 	struct net_bridge *br = port->br;
 
-	spin_lock(&br->multicast_lock);
+	rcu_read_lock();
+	br_multicast_lock_rcu(br);
 	if (port->state == BR_STATE_DISABLED ||
 	    port->state == BR_STATE_BLOCKING)
 		goto out;
@@ -1536,7 +1549,8 @@ br_multicast_port_query_expired(struct net_bridge_port
*port,
 	br_multicast_send_query(port->br, port, query);
 
 out:
-	spin_unlock(&br->multicast_lock);
+	br_multicast_unlock(br);
+	rcu_read_unlock();
 }
 
 static void br_ip4_multicast_port_query_expired(struct timer_list *t)
@@ -1562,7 +1576,8 @@ static void br_multicast_port_group_rexmit(struct
timer_list *t)
 	struct net_bridge *br = pg->key.port->br;
 	bool need_rexmit = false;
 
-	spin_lock(&br->multicast_lock);
+	rcu_read_lock();
+	br_multicast_lock_rcu(br);
 	if (!netif_running(br->dev) || hlist_unhashed(&pg->mglist) ||
 	    !br_opt_get(br, BROPT_MULTICAST_ENABLED) ||
 	    !br_opt_get(br, BROPT_MULTICAST_QUERIER))
@@ -1590,7 +1605,8 @@ static void br_multicast_port_group_rexmit(struct
timer_list *t)
 		mod_timer(&pg->rexmit_timer, jiffies +
 					     br->multicast_last_member_interval);
 out:
-	spin_unlock(&br->multicast_lock);
+	br_multicast_unlock(br);
+	rcu_read_unlock();
 }
 
 static void br_mc_disabled_update(struct net_device *dev, bool value)
@@ -1636,11 +1652,11 @@ void br_multicast_del_port(struct net_bridge_port *port)
 	struct hlist_node *n;
 
 	/* Take care of the remaining groups, only perm ones should be left */
-	spin_lock_bh(&br->multicast_lock);
+	br_multicast_lock_rtnl_bh(br);
 	hlist_for_each_entry_safe(pg, n, &port->mglist, mglist)
 		br_multicast_find_del_pg(br, pg);
 	hlist_move_list(&br->mcast_gc_list, &deleted_head);
-	spin_unlock_bh(&br->multicast_lock);
+	br_multicast_unlock_bh(br);
 	br_multicast_gc(&deleted_head);
 	del_timer_sync(&port->multicast_router_timer);
 	free_percpu(port->mcast_stats);
@@ -1675,9 +1691,11 @@ void br_multicast_enable_port(struct net_bridge_port
*port)
 {
 	struct net_bridge *br = port->br;
 
-	spin_lock(&br->multicast_lock);
+	rcu_read_lock();
+	br_multicast_lock_rcu(br);
 	__br_multicast_enable_port(port);
-	spin_unlock(&br->multicast_lock);
+	br_multicast_unlock(br);
+	rcu_read_unlock();
 }
 
 void br_multicast_disable_port(struct net_bridge_port *port)
@@ -1686,7 +1704,8 @@ void br_multicast_disable_port(struct net_bridge_port
*port)
 	struct net_bridge_port_group *pg;
 	struct hlist_node *n;
 
-	spin_lock(&br->multicast_lock);
+	rcu_read_lock();
+	br_multicast_lock_rcu(br);
 	hlist_for_each_entry_safe(pg, n, &port->mglist, mglist)
 		if (!(pg->flags & MDB_PG_FLAGS_PERMANENT))
 			br_multicast_find_del_pg(br, pg);
@@ -1698,7 +1717,8 @@ void br_multicast_disable_port(struct net_bridge_port
*port)
 #if IS_ENABLED(CONFIG_IPV6)
 	del_timer(&port->ip6_own_query.timer);
 #endif
-	spin_unlock(&br->multicast_lock);
+	br_multicast_unlock(br);
+	rcu_read_unlock();
 }
 
 static int __grp_src_delete_marked(struct net_bridge_port_group *pg)
@@ -2374,7 +2394,7 @@ static int br_ip4_multicast_igmp3_report(struct net_bridge
*br,
 		if (!port || igmpv2)
 			continue;
 
-		spin_lock_bh(&br->multicast_lock);
+		br_multicast_lock_rcu_bh(br);
 		mdst = br_mdb_ip4_get(br, group, vid);
 		if (!mdst)
 			goto unlock_continue;
@@ -2413,7 +2433,7 @@ static int br_ip4_multicast_igmp3_report(struct net_bridge
*br,
 		if (changed)
 			br_mdb_notify(br->dev, mdst, pg, RTM_NEWMDB);
 unlock_continue:
-		spin_unlock_bh(&br->multicast_lock);
+		br_multicast_unlock_bh(br);
 	}
 
 	return err;
@@ -2503,7 +2523,7 @@ static int br_ip6_multicast_mld2_report(struct net_bridge
*br,
 		if (!port || mldv1)
 			continue;
 
-		spin_lock_bh(&br->multicast_lock);
+		br_multicast_lock_rcu_bh(br);
 		mdst = br_mdb_ip6_get(br, &grec->grec_mca, vid);
 		if (!mdst)
 			goto unlock_continue;
@@ -2552,7 +2572,7 @@ static int br_ip6_multicast_mld2_report(struct net_bridge
*br,
 		if (changed)
 			br_mdb_notify(br->dev, mdst, pg, RTM_NEWMDB);
 unlock_continue:
-		spin_unlock_bh(&br->multicast_lock);
+		br_multicast_unlock_bh(br);
 	}
 
 	return err;
@@ -2731,7 +2751,7 @@ static void br_ip4_multicast_query(struct net_bridge *br,
 	unsigned long now = jiffies;
 	__be32 group;
 
-	spin_lock(&br->multicast_lock);
+	br_multicast_lock_rcu(br);
 	if (!netif_running(br->dev) ||
 	    (port && port->state == BR_STATE_DISABLED))
 		goto out;
@@ -2790,7 +2810,7 @@ static void br_ip4_multicast_query(struct net_bridge *br,
 	}
 
 out:
-	spin_unlock(&br->multicast_lock);
+	br_multicast_unlock(br);
 }
 
 #if IS_ENABLED(CONFIG_IPV6)
@@ -2813,7 +2833,7 @@ static int br_ip6_multicast_query(struct net_bridge *br,
 	bool is_general_query;
 	int err = 0;
 
-	spin_lock(&br->multicast_lock);
+	br_multicast_lock_rcu(br);
 	if (!netif_running(br->dev) ||
 	    (port && port->state == BR_STATE_DISABLED))
 		goto out;
@@ -2879,7 +2899,7 @@ static int br_ip6_multicast_query(struct net_bridge *br,
 	}
 
 out:
-	spin_unlock(&br->multicast_lock);
+	br_multicast_unlock(br);
 	return err;
 }
 #endif
@@ -2897,7 +2917,7 @@ br_multicast_leave_group(struct net_bridge *br,
 	unsigned long now;
 	unsigned long time;
 
-	spin_lock(&br->multicast_lock);
+	br_multicast_lock_rcu(br);
 	if (!netif_running(br->dev) ||
 	    (port && port->state == BR_STATE_DISABLED))
 		goto out;
@@ -2984,7 +3004,7 @@ br_multicast_leave_group(struct net_bridge *br,
 		break;
 	}
 out:
-	spin_unlock(&br->multicast_lock);
+	br_multicast_unlock(br);
 }
 
 static void br_ip4_multicast_leave_group(struct net_bridge *br,
@@ -3261,13 +3281,15 @@ static void br_multicast_query_expired(struct net_bridge
*br,
 				       struct bridge_mcast_own_query *query,
 				       struct bridge_mcast_querier *querier)
 {
-	spin_lock(&br->multicast_lock);
+	rcu_read_lock();
+	br_multicast_lock_rcu(br);
 	if (query->startup_sent < br->multicast_startup_query_count)
 		query->startup_sent++;
 
 	RCU_INIT_POINTER(querier->port, NULL);
 	br_multicast_send_query(br, NULL, query);
-	spin_unlock(&br->multicast_lock);
+	br_multicast_unlock(br);
+	rcu_read_unlock();
 }
 
 static void br_ip4_multicast_query_expired(struct timer_list *t)
@@ -3292,9 +3314,11 @@ static void br_multicast_gc_work(struct work_struct
*work)
 					     mcast_gc_work);
 	HLIST_HEAD(deleted_head);
 
-	spin_lock_bh(&br->multicast_lock);
+	rcu_read_lock();
+	br_multicast_lock_rcu_bh(br);
 	hlist_move_list(&br->mcast_gc_list, &deleted_head);
-	spin_unlock_bh(&br->multicast_lock);
+	br_multicast_unlock_bh(br);
+	rcu_read_unlock();
 
 	br_multicast_gc(&deleted_head);
 }
@@ -3441,11 +3465,11 @@ void br_multicast_dev_del(struct net_bridge *br)
 	HLIST_HEAD(deleted_head);
 	struct hlist_node *tmp;
 
-	spin_lock_bh(&br->multicast_lock);
+	br_multicast_lock_rtnl_bh(br);
 	hlist_for_each_entry_safe(mp, tmp, &br->mdb_list, mdb_node)
 		br_multicast_del_mdb_entry(mp);
 	hlist_move_list(&br->mcast_gc_list, &deleted_head);
-	spin_unlock_bh(&br->multicast_lock);
+	br_multicast_unlock_bh(br);
 
 	br_multicast_gc(&deleted_head);
 	cancel_work_sync(&br->mcast_gc_work);
@@ -3457,7 +3481,8 @@ int br_multicast_set_router(struct net_bridge *br,
unsigned long val)
 {
 	int err = -EINVAL;
 
-	spin_lock_bh(&br->multicast_lock);
+	rcu_read_lock();
+	br_multicast_lock_rcu_bh(br);
 
 	switch (val) {
 	case MDB_RTR_TYPE_DISABLED:
@@ -3475,7 +3500,8 @@ int br_multicast_set_router(struct net_bridge *br,
unsigned long val)
 		break;
 	}
 
-	spin_unlock_bh(&br->multicast_lock);
+	br_multicast_unlock_bh(br);
+	rcu_read_unlock();
 
 	return err;
 }
@@ -3499,7 +3525,8 @@ int br_multicast_set_port_router(struct net_bridge_port
*p, unsigned long val)
 	unsigned long now = jiffies;
 	int err = -EINVAL;
 
-	spin_lock(&br->multicast_lock);
+	rcu_read_lock();
+	br_multicast_lock_rcu(br);
 	if (p->multicast_router == val) {
 		/* Refresh the temp router port timer */
 		if (p->multicast_router == MDB_RTR_TYPE_TEMP)
@@ -3532,7 +3559,8 @@ int br_multicast_set_port_router(struct net_bridge_port
*p, unsigned long val)
 	}
 	err = 0;
 unlock:
-	spin_unlock(&br->multicast_lock);
+	br_multicast_unlock(br);
+	rcu_read_unlock();
 
 	return err;
 }
@@ -3565,7 +3593,8 @@ int br_multicast_toggle(struct net_bridge *br, unsigned
long val)
 	struct net_bridge_port *port;
 	bool change_snoopers = false;
 
-	spin_lock_bh(&br->multicast_lock);
+	rcu_read_lock();
+	br_multicast_lock_rcu_bh(br);
 	if (!!br_opt_get(br, BROPT_MULTICAST_ENABLED) == !!val)
 		goto unlock;
 
@@ -3586,7 +3615,8 @@ int br_multicast_toggle(struct net_bridge *br, unsigned
long val)
 	change_snoopers = true;
 
 unlock:
-	spin_unlock_bh(&br->multicast_lock);
+	br_multicast_unlock_bh(br);
+	rcu_read_unlock();
 
 	/* br_multicast_join_snoopers has the potential to cause
 	 * an MLD Report/Leave to be delivered to br_multicast_rcv,
@@ -3623,9 +3653,11 @@ bool br_multicast_router(const struct net_device *dev)
 	struct net_bridge *br = netdev_priv(dev);
 	bool is_router;
 
-	spin_lock_bh(&br->multicast_lock);
+	rcu_read_lock();
+	br_multicast_lock_rcu_bh(br);
 	is_router = br_multicast_is_router(br);
-	spin_unlock_bh(&br->multicast_lock);
+	br_multicast_unlock_bh(br);
+	rcu_read_unlock();
 	return is_router;
 }
 EXPORT_SYMBOL_GPL(br_multicast_router);
@@ -3636,7 +3668,8 @@ int br_multicast_set_querier(struct net_bridge *br,
unsigned long val)
 
 	val = !!val;
 
-	spin_lock_bh(&br->multicast_lock);
+	rcu_read_lock();
+	br_multicast_lock_rcu_bh(br);
 	if (br_opt_get(br, BROPT_MULTICAST_QUERIER) == val)
 		goto unlock;
 
@@ -3659,7 +3692,8 @@ int br_multicast_set_querier(struct net_bridge *br,
unsigned long val)
 #endif
 
 unlock:
-	spin_unlock_bh(&br->multicast_lock);
+	br_multicast_unlock_bh(br);
+	rcu_read_unlock();
 
 	return 0;
 }
@@ -3675,9 +3709,11 @@ int br_multicast_set_igmp_version(struct net_bridge *br,
unsigned long val)
 		return -EINVAL;
 	}
 
-	spin_lock_bh(&br->multicast_lock);
+	rcu_read_lock();
+	br_multicast_lock_rcu_bh(br);
 	br->multicast_igmp_version = val;
-	spin_unlock_bh(&br->multicast_lock);
+	br_multicast_unlock_bh(br);
+	rcu_read_unlock();
 
 	return 0;
 }
@@ -3694,9 +3730,11 @@ int br_multicast_set_mld_version(struct net_bridge *br,
unsigned long val)
 		return -EINVAL;
 	}
 
-	spin_lock_bh(&br->multicast_lock);
+	rcu_read_lock();
+	br_multicast_lock_rcu_bh(br);
 	br->multicast_mld_version = val;
-	spin_unlock_bh(&br->multicast_lock);
+	br_multicast_unlock_bh(br);
+	rcu_read_unlock();
 
 	return 0;
 }
diff --git a/net/bridge/br_multicast_eht.c b/net/bridge/br_multicast_eht.c
index fea38b9a7268..24732578115c 100644
--- a/net/bridge/br_multicast_eht.c
+++ b/net/bridge/br_multicast_eht.c
@@ -209,7 +209,8 @@ static void br_multicast_eht_set_entry_expired(struct
timer_list *t)
 	struct net_bridge_group_eht_set_entry *set_h = from_timer(set_h, t, timer);
 	struct net_bridge *br = set_h->br;
 
-	spin_lock(&br->multicast_lock);
+	rcu_read_lock();
+	br_multicast_lock_rcu(br);
 	if (RB_EMPTY_NODE(&set_h->rb_node) ||
timer_pending(&set_h->timer))
 		goto out;
 
@@ -217,7 +218,8 @@ static void br_multicast_eht_set_entry_expired(struct
timer_list *t)
 				       &set_h->eht_set->src_addr,
 				       &set_h->h_addr);
 out:
-	spin_unlock(&br->multicast_lock);
+	br_multicast_unlock(br);
+	rcu_read_unlock();
 }
 
 static void br_multicast_eht_set_expired(struct timer_list *t)
@@ -226,13 +228,15 @@ static void br_multicast_eht_set_expired(struct timer_list
*t)
 							      timer);
 	struct net_bridge *br = eht_set->br;
 
-	spin_lock(&br->multicast_lock);
+	rcu_read_lock();
+	br_multicast_lock_rcu(br);
 	if (RB_EMPTY_NODE(&eht_set->rb_node) ||
timer_pending(&eht_set->timer))
 		goto out;
 
 	br_multicast_del_eht_set(eht_set);
 out:
-	spin_unlock(&br->multicast_lock);
+	br_multicast_unlock(br);
+	rcu_read_unlock();
 }
 
 static struct net_bridge_group_eht_host *
@@ -870,9 +874,11 @@ int br_multicast_eht_set_hosts_limit(struct net_bridge_port
*p,
 	if (!eht_hosts_limit)
 		return -EINVAL;
 
-	spin_lock_bh(&br->multicast_lock);
+	rcu_read_lock();
+	br_multicast_lock_rcu_bh(br);
 	p->multicast_eht_hosts_limit = eht_hosts_limit;
-	spin_unlock_bh(&br->multicast_lock);
+	br_multicast_unlock_bh(br);
+	rcu_read_unlock();
 
 	return 0;
 }
diff --git a/net/bridge/br_private.h b/net/bridge/br_private.h
index d7d167e10b70..a4e35a9fbc45 100644
--- a/net/bridge/br_private.h
+++ b/net/bridge/br_private.h
@@ -863,6 +863,54 @@ static inline bool br_group_is_l2(const struct br_ip
*group)
 #define mlock_dereference(X, br) \
 	rcu_dereference_protected(X, lockdep_is_held(&br->multicast_lock))
 
+static inline void br_multicast_lock_rcu(struct net_bridge *br)
+{
+	unsigned char nest_level = netdev_get_nest_level_rcu(br->dev);
+
+	spin_lock_nested(&br->multicast_lock, nest_level);
+}
+
+static inline void br_multicast_lock_rcu_bh(struct net_bridge *br)
+{
+	unsigned char nest_level = netdev_get_nest_level_rcu(br->dev);
+
+	local_bh_disable();
+	spin_lock_nested(&br->multicast_lock, nest_level);
+}
+
+static inline void br_multicast_lock_rtnl(struct net_bridge *br)
+{
+	unsigned char nest_level = 0;
+
+#ifdef CONFIG_LOCKDEP
+	ASSERT_RTNL();
+	nest_level = br->dev->nested_level;
+#endif
+	spin_lock_nested(&br->multicast_lock, nest_level);
+}
+
+static inline void br_multicast_lock_rtnl_bh(struct net_bridge *br)
+{
+	unsigned char nest_level = 0;
+
+#ifdef CONFIG_LOCKDEP
+	ASSERT_RTNL();
+	nest_level = br->dev->nested_level;
+#endif
+	local_bh_disable();
+	spin_lock_nested(&br->multicast_lock, nest_level);
+}
+
+static inline void br_multicast_unlock(struct net_bridge *br)
+{
+	spin_unlock(&br->multicast_lock);
+}
+
+static inline void br_multicast_unlock_bh(struct net_bridge *br)
+{
+	spin_unlock_bh(&br->multicast_lock);
+}
+
 static inline bool br_multicast_is_router(struct net_bridge *br)
 {
 	return br->multicast_router == 2 ||
-- 
2.17.1

Nikolay Aleksandrov

2021-Apr-25 16:45 UTC

head link

[Bridge] [PATCH net 2/2] net: bridge: fix lockdep multicast_lock false positive splat

On 25/04/2021 18:57, Taehee Yoo wrote:> multicast_lock is a per-interface(bridge) lock.
> This lock can be used recursively because interfaces can be used
> recursively. So, it should use spin_lock_nested() but it doesn't.
> So lockdep false positive splat occurred.
> 
> Some inline helper functions are added.
> These functions internally get 'subclass' variable, which is used
as
> parameter of spin_lock_nested() and use spin_lock_nested() with a
> subclass parameter.
> 
> Test commands:
>     ip link add br0 type bridge
>     ip link add bond0 type bond
>     ip link add br1 type bridge
>     ip link set br0 master bond0
>     ip link set bond0 up
>     ip link set bond0 master br1
>     ip link set br0 up
>     ip link set br1 up
>     ip link set br0 type bridge mcast_router 1 mcast_querier 1
>     ip link set br1 type bridge mcast_querier 1 mcast_router 1
> 
> Splat looks like:
> ===========================================> WARNING: possible recursive
locking detected
> 5.12.0-rc7+ #855 Not tainted
> --------------------------------------------
> kworker/5:1/56 is trying to acquire lock:
> ffff88810f833000 (&br->multicast_lock){+.-.}-{2:2}, at:
> br_multicast_rcv+0x1484/0x5280 [bridge]
> 
[snip]> 
> Fixes: eb1d16414339 ("bridge: Add core IGMP snooping support")
> Signed-off-by: Taehee Yoo <ap420073 at gmail.com>
> ---
> 
> v2:
>  - No change
> 
>  net/bridge/br_mdb.c           |  12 +--
>  net/bridge/br_multicast.c     | 146 +++++++++++++++++++++-------------
>  net/bridge/br_multicast_eht.c |  18 +++--
>  net/bridge/br_private.h       |  48 +++++++++++
>  4 files changed, 158 insertions(+), 66 deletions(-)
> 
Hi Taehee,
Ugh.. that's just very ugly. :) The setup you've described above is by
all means invalid, but
possible unfortunately. The bridge already checks if it's being added as a
port to another
bridge, but not through multiple levels of indirection. These locks are
completely unrelated
as they're in very different contexts (different devices).

At the very least please push the rcu_read_lock() calls in
br_multicast_lock_rcu/_bh()
as they're needed only to get the nest level for
netdev_get_nest_level_rcu(), we don't need
them for the whole code paths (right ?), we could save a few lines in the
process and
avoid confusion about the locking rules for those code paths.

I wish there was a better solution.

Thanks,
 Nik

Heiner Kallweit

2021-Apr-25 18:03 UTC

head link

[Bridge] [PATCH net v2 1/2] net: core: make bond_get_lowest_level_rcu() generic

On 25.04.2021 17:57, Taehee Yoo wrote:> The purpose of bond_get_lowest_level_rcu() is to get nested_level under
> RCU. Because dev->nested_level is protected by RTNL, so it shouldn't
be
> used without RTNL. But bonding module needs this value under RCU without
> RTNL.
> So, this function was added.
> 
> But, there is another module, which needs this function.
> So, make this function generic.
> the new name is netdev_get_nest_level_rcu().
> 
> Signed-off-by: Taehee Yoo <ap420073 at gmail.com>
> ---
> 
> v2:
>  - No change
> 
>  drivers/net/bonding/bond_main.c | 45 +--------------------------------
>  include/linux/netdevice.h       |  1 +
>  net/core/dev.c                  | 44 ++++++++++++++++++++++++++++++++
>  3 files changed, 46 insertions(+), 44 deletions(-)
> 
> diff --git a/drivers/net/bonding/bond_main.c
b/drivers/net/bonding/bond_main.c
> index 83ef62db6285..a9feb039ffa6 100644
> --- a/drivers/net/bonding/bond_main.c
> +++ b/drivers/net/bonding/bond_main.c
> @@ -3754,47 +3754,6 @@ static void bond_fold_stats(struct rtnl_link_stats64
*_res,
>  	}
>  }
>  
> -#ifdef CONFIG_LOCKDEP
> -static int bond_get_lowest_level_rcu(struct net_device *dev)
> -{
> -	struct net_device *ldev, *next, *now, *dev_stack[MAX_NEST_DEV + 1];
> -	struct list_head *niter, *iter, *iter_stack[MAX_NEST_DEV + 1];
> -	int cur = 0, max = 0;
> -
> -	now = dev;
> -	iter = &dev->adj_list.lower;
> -
> -	while (1) {
> -		next = NULL;
> -		while (1) {
> -			ldev = netdev_next_lower_dev_rcu(now, &iter);
> -			if (!ldev)
> -				break;
> -
> -			next = ldev;
> -			niter = &ldev->adj_list.lower;
> -			dev_stack[cur] = now;
> -			iter_stack[cur++] = iter;
> -			if (max <= cur)
> -				max = cur;
> -			break;
> -		}
> -
> -		if (!next) {
> -			if (!cur)
> -				return max;
> -			next = dev_stack[--cur];
> -			niter = iter_stack[cur];
> -		}
> -
> -		now = next;
> -		iter = niter;
> -	}
> -
> -	return max;
> -}
> -#endif
> -
>  static void bond_get_stats(struct net_device *bond_dev,
>  			   struct rtnl_link_stats64 *stats)
>  {
> @@ -3806,9 +3765,7 @@ static void bond_get_stats(struct net_device
*bond_dev,
>  
>  
>  	rcu_read_lock();
> -#ifdef CONFIG_LOCKDEP
> -	nest_level = bond_get_lowest_level_rcu(bond_dev);
> -#endif
> +	nest_level = netdev_get_nest_level_rcu(bond_dev);
>  
>  	spin_lock_nested(&bond->stats_lock, nest_level);
>  	memcpy(stats, &bond->bond_stats, sizeof(*stats));
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 87a5d186faff..507c06bf5dba 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -4699,6 +4699,7 @@ int netdev_walk_all_lower_dev(struct net_device *dev,
>  			      int (*fn)(struct net_device *lower_dev,
>  					struct netdev_nested_priv *priv),
>  			      struct netdev_nested_priv *priv);
> +int netdev_get_nest_level_rcu(struct net_device *dev);
>  int netdev_walk_all_lower_dev_rcu(struct net_device *dev,
>  				  int (*fn)(struct net_device *lower_dev,
>  					    struct netdev_nested_priv *priv),
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 15fe36332fb8..efc2bf88eafd 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -7709,6 +7709,50 @@ static int __netdev_update_lower_level(struct
net_device *dev,
>  	return 0;
>  }
>  
> +int netdev_get_nest_level_rcu(struct net_device *dev)
> +{
> +#ifdef CONFIG_LOCKDEP
> +	struct net_device *ldev, *next, *now, *dev_stack[MAX_NEST_DEV + 1];
> +	struct list_head *niter, *iter, *iter_stack[MAX_NEST_DEV + 1];
> +	int cur = 0, max = 0;
> +
> +	now = dev;
> +	iter = &dev->adj_list.lower;
> +
> +	while (1) {
> +		next = NULL;
> +		while (1) {
> +			ldev = netdev_next_lower_dev_rcu(now, &iter);
> +			if (!ldev)
> +				break;
> +
> +			next = ldev;
> +			niter = &ldev->adj_list.lower;
> +			dev_stack[cur] = now;
> +			iter_stack[cur++] = iter;
> +			if (max <= cur)
> +				max = cur;
> +			break;
This looks odd. Why a while loop if it's left in the first iteration
anyway? The whole loop looks unnecessarily complex. The following
should do the same, just in a simpler way (untested!)

        while (1) {
                ldev = netdev_next_lower_dev_rcu(now, &iter);
                if (ldev) {
                        dev_stack[cur] = now;
                        iter_stack[cur++] = iter;
                        if (max <= cur)
                                max = cur;
                        now = ldev;
                        iter = &ldev->adj_list.lower;
                } else {
                        if (!cur)
                                break;
                        now = dev_stack[--cur];
                        iter = iter_stack[cur];
                }
        }

I know that you just copied the original function.
Simplifying the function should be something for a
follow-up patch.
> +		}
> +
> +		if (!next) {
> +			if (!cur)
> +				return max;
> +			next = dev_stack[--cur];
> +			niter = iter_stack[cur];
> +		}
> +
> +		now = next;
> +		iter = niter;
> +	}
> +
> +	return max;
> +#else
> +	return 0;
> +#endif
> +}
> +EXPORT_SYMBOL_GPL(netdev_get_nest_level_rcu);
> +
>  int netdev_walk_all_lower_dev_rcu(struct net_device *dev,
>  				  int (*fn)(struct net_device *dev,
>  					    struct netdev_nested_priv *priv),
>

Linux Ethernet Bridging - Apr 2021 - [Bridge] [PATCH net v2 0/2] net: fix lockdep false positive splat

[Bridge] [PATCH net v2 0/2] net: fix lockdep false positive splat

[Bridge] [PATCH net v2 1/2] net: core: make bond_get_lowest_level_rcu() generic

[Bridge] [PATCH net 2/2] net: bridge: fix lockdep multicast_lock false positive splat

[Bridge] [PATCH net 2/2] net: bridge: fix lockdep multicast_lock false positive splat

[Bridge] [PATCH net v2 1/2] net: core: make bond_get_lowest_level_rcu() generic