This rcu_barrier() came from a time call_rcu() calls were used in
net/bridge/br_multicast.c.

Now kfree_rcu() is there, we can remove this problematic rcu_barrier()
which causes extreme RTNL pressure in many syzbot reports.

INFO: task syz-executor:77945 is blocked on a mutex likely owned by task kworker/u1024:5:36537.
task:kworker/u1024:5 state:D stack:24616 pid:36537 tgid:36537 ppid:2      task_flags:0x4208060 flags:0x00080000 last_sleep:612797637337
Workqueue: netns cleanup_net
Call Trace:
 <TASK>
  [<ffffffff81914eaa>] context_switch+0xf2a/0x1730 kernel/sched/core.c:6483
  [<ffffffff81201143>] __schedule+0x1133/0x43a0 kernel/sched/core.c:8411
  [<ffffffff8120446b>] __schedule_loop kernel/sched/core.c:8514 [inline]
  [<ffffffff8120446b>] schedule+0xab/0x260 kernel/sched/core.c:8529
  [<ffffffff8121a093>] schedule_timeout+0xc3/0x2b0 kernel/time/sleep_timeout.c:75
  [<ffffffff81205347>] do_wait_for_common kernel/sched/completion.c:100 [inline]
  [<ffffffff81205347>] __wait_for_common kernel/sched/completion.c:121 [inline]
  [<ffffffff81205347>] wait_for_common kernel/sched/completion.c:132 [inline]
  [<ffffffff81205347>] wait_for_completion+0x2c7/0x5d0 kernel/sched/completion.c:153
  [<ffffffff81b8f27f>] rcu_barrier+0x49f/0x620 kernel/rcu/tree.c:3888
  [<ffffffff860091b3>] br_multicast_dev_del+0x303/0x350 net/bridge/br_multicast.c:4459
  [<ffffffff85fb5dbc>] br_dev_uninit+0x1c/0x40 net/bridge/br_device.c:157
  [<ffffffff8568058c>] unregister_netdevice_many_notify+0x1c1c/0x2300 net/core/dev.c:12599
  [<ffffffff8562be43>] ops_exit_rtnl_list net/core/net_namespace.c:187 [inline]
  [<ffffffff8562be43>] ops_undo_list+0x3d3/0x940 net/core/net_namespace.c:248

Signed-off-by: Eric Dumazet <edumazet@google.com>
---
Cc: Nikolay Aleksandrov <razor@blackwall.org>
Cc: Ido Schimmel <idosch@nvidia.com>
---
 net/bridge/br_multicast.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/net/bridge/br_multicast.c b/net/bridge/br_multicast.c
index 49ceea3ff9742d3043ac605d656b7fcf37dbcb96..8efa3add5d47508fb2b032ff6238ffbbabfd27ad 100644
--- a/net/bridge/br_multicast.c
+++ b/net/bridge/br_multicast.c
@@ -4453,8 +4453,6 @@ void br_multicast_dev_del(struct net_bridge *br)
 	br_multicast_ctx_deinit(&br->multicast_ctx);
 	br_multicast_gc(&deleted_head);
 	cancel_work_sync(&br->mcast_gc_work);
-
-	rcu_barrier();
 }
 
 int br_multicast_set_router(struct net_bridge_mcast *brmctx, unsigned long val)
-- 
2.54.0.563.g4f69b47b94-goog