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When dereferencing the port vlan group we should use the rcu helper
instead of the one relying on rtnl. In br_multicast_pg_to_port_ctx the
entry cannot disappear as we hold the multicast lock and rcu as explained
in the comment above it.
For the same reason we're ok in br_multicast_start_querier.

 =============================
 WARNING: suspicious RCU usage
 5.14.0-rc5+ #429 Tainted: G        W
 -----------------------------
 net/bridge/br_private.h:1478 suspicious rcu_dereference_protected() usage!

 other info that might help us debug this:

 rcu_scheduler_active = 2, debug_locks = 1
 3 locks held by swapper/2/0:
  #0: ffff88822be85eb0 ((&p->timer)){+.-.}-{0:0}, at: call_timer_fn+0x5/0x2da
  #1: ffff88810b32f260 (&br->multicast_lock){+.-.}-{3:3}, at: br_multicast_port_group_expired+0x28/0x13d [bridge]
  #2: ffffffff824f6c80 (rcu_read_lock){....}-{1:3}, at: rcu_lock_acquire.constprop.0+0x0/0x22 [bridge]

 stack backtrace:
 CPU: 2 PID: 0 Comm: swapper/2 Kdump: loaded Tainted: G        W         5.14.0-rc5+ #429
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-4.fc34 04/01/2014
 Call Trace:
  <IRQ>
  dump_stack_lvl+0x45/0x59
  nbp_vlan_group+0x3e/0x44 [bridge]
  br_multicast_pg_to_port_ctx+0xd6/0x10d [bridge]
  br_multicast_star_g_handle_mode+0xa1/0x2ce [bridge]
  ? netlink_broadcast+0xf/0x11
  ? nlmsg_notify+0x56/0x99
  ? br_mdb_notify+0x224/0x2e9 [bridge]
  ? br_multicast_del_pg+0x1dc/0x26d [bridge]
  br_multicast_del_pg+0x1dc/0x26d [bridge]
  br_multicast_port_group_expired+0xaa/0x13d [bridge]
  ? __grp_src_delete_marked.isra.0+0x35/0x35 [bridge]
  ? __grp_src_delete_marked.isra.0+0x35/0x35 [bridge]
  call_timer_fn+0x134/0x2da
  __run_timers+0x169/0x193
  run_timer_softirq+0x19/0x2d
  __do_softirq+0x1bc/0x42a
  __irq_exit_rcu+0x5c/0xb3
  irq_exit_rcu+0xa/0x12
  sysvec_apic_timer_interrupt+0x5e/0x75
  </IRQ>
  asm_sysvec_apic_timer_interrupt+0x12/0x20
 RIP: 0010:default_idle+0xc/0xd
 Code: e8 14 40 71 ff e8 10 b3 ff ff 4c 89 e2 48 89 ef 31 f6 5d 41 5c e9 a9 e8 c2 ff cc cc cc cc 0f 1f 44 00 00 e8 7f 55 65 ff fb f4 <c3> 0f 1f 44 00 00 55 65 48 8b 2c 25 40 6f 01 00 53 f0 80 4d 02 20
 RSP: 0018:ffff88810033bf00 EFLAGS: 00000206
 RAX: ffffffff819cf828 RBX: ffff888100328000 RCX: 0000000000000001
 RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffffffff819cfa2d
 RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000001
 R10: ffff8881008302c0 R11: 00000000000006db R12: 0000000000000000
 R13: 0000000000000002 R14: 0000000000000000 R15: 0000000000000000
  ? __sched_text_end+0x4/0x4
  ? default_idle_call+0x15/0x7b
  default_idle_call+0x4d/0x7b
  do_idle+0x124/0x2a2
  cpu_startup_entry+0x1d/0x1f
  secondary_startup_64_no_verify+0xb0/0xbb

Fixes: 74edfd48

 ("net: bridge: multicast: add helper to get port mcast context from port group")
Signed-off-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>