ipv4: Silence suspicious RCU usage warning

fib_info_notify_update() is always called with RTNL held, but not from
an RCU read-side critical section. This leads to the following warning
[1] when the FIB table list is traversed with
hlist_for_each_entry_rcu(), but without a proper lockdep expression.

Since modification of the list is protected by RTNL, silence the warning
by adding a lockdep expression which verifies RTNL is held.

[1]
 =============================
 WARNING: suspicious RCU usage
 5.9.0-rc1-custom-14233-g2f26e122d62f #129 Not tainted
 -----------------------------
 net/ipv4/fib_trie.c:2124 RCU-list traversed in non-reader section!!

 other info that might help us debug this:

 rcu_scheduler_active = 2, debug_locks = 1
 1 lock held by ip/834:
  #0: ffffffff85a3b6b0 (rtnl_mutex){+.+.}-{3:3}, at: rtnetlink_rcv_msg+0x49a/0xbd0

 stack backtrace:
 CPU: 0 PID: 834 Comm: ip Not tainted 5.9.0-rc1-custom-14233-g2f26e122d62f #129
 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-2.fc32 04/01/2014
 Call Trace:
  dump_stack+0x100/0x184
  lockdep_rcu_suspicious+0x143/0x14d
  fib_info_notify_update+0x8d1/0xa60
  __nexthop_replace_notify+0xd2/0x290
  rtm_new_nexthop+0x35e2/0x5946
  rtnetlink_rcv_msg+0x4f7/0xbd0
  netlink_rcv_skb+0x17a/0x480
  rtnetlink_rcv+0x22/0x30
  netlink_unicast+0x5ae/0x890
  netlink_sendmsg+0x98a/0xf40
  ____sys_sendmsg+0x879/0xa00
  ___sys_sendmsg+0x122/0x190
  __sys_sendmsg+0x103/0x1d0
  __x64_sys_sendmsg+0x7d/0xb0
  do_syscall_64+0x32/0x50
  entry_SYSCALL_64_after_hwframe+0x44/0xa9
 RIP: 0033:0x7fde28c3be57
 Code: 0c 00 f7 d8 64 89 02 48 c7 c0 ff ff ff ff eb b7 0f 1f 00 f3 0f 1e fa 64 8b 04 25 18 00 00 00 85 c0 75 10 b8 2e 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 51
c3 48 83 ec 28 89 54 24 1c 48 89 74 24 10
RSP: 002b:00007ffc09330028 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007fde28c3be57
RDX: 0000000000000000 RSI: 00007ffc09330090 RDI: 0000000000000003
RBP: 000000005f45f911 R08: 0000000000000001 R09: 00007ffc0933012c
R10: 0000000000000076 R11: 0000000000000246 R12: 0000000000000001
R13: 00007ffc09330290 R14: 00007ffc09330eee R15: 00005610e48ed020

Fixes: 1bff1a0c9b ("ipv4: Add function to send route updates")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_trie.c

@@ -2121,7 +2121,8 @@ void fib_info_notify_update(struct net *net, struct nl_info *info)
 		struct hlist_head *head = &net->ipv4.fib_table_hash[h];
 		struct fib_table *tb;
 
-		hlist_for_each_entry_rcu(tb, head, tb_hlist)
+		hlist_for_each_entry_rcu(tb, head, tb_hlist,
+					 lockdep_rtnl_is_held())
 			__fib_info_notify_update(net, tb, info);
 	}
 }