net: qualcomm: rmnet: Fix rcu splat in rmnet_is_real_dev_registered

Xiaolong reported a suspicious rcu_dereference_check in the device
unregister notifier callback. Since we do not dereference the
rx_handler_data, it's ok to just check for the value of the pointer.
Note that this section is already protected by rtnl_lock.

[  101.364846] WARNING: suspicious RCU usage
[  101.365654] 4.13.0-rc6-01701-gceed73a2 #1 Not tainted
[  101.370873] -----------------------------
[  101.372472] drivers/net/ethernet/qualcomm/rmnet/rmnet_config.c:57 suspicious rcu_dereference_check() usage!
[  101.374427]
[  101.374427] other info that might help us debug this:
[  101.374427]
[  101.387491]
[  101.387491] rcu_scheduler_active = 2, debug_locks = 1
[  101.389368] 1 lock held by trinity-main/2809:
[  101.390736]  #0:  (rtnl_mutex){+.+.+.}, at: [<8146085b>] rtnl_lock+0xf/0x11
[  101.395482]
[  101.395482] stack backtrace:
[  101.396948] CPU: 0 PID: 2809 Comm: trinity-main Not tainted 4.13.0-rc6-01701-gceed73a2 #1
[  101.398857] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.9.3-20161025_171302-gandalf 04/01/2014
[  101.401079] Call Trace:
[  101.401656]  dump_stack+0xa1/0xeb
[  101.402871]  lockdep_rcu_suspicious+0xc7/0xd0
[  101.403665]  rmnet_is_real_dev_registered+0x40/0x4e
[  101.405199]  rmnet_config_notify_cb+0x2c/0x142
[  101.406344]  ? wireless_nlevent_flush+0x47/0x71
[  101.407385]  notifier_call_chain+0x2d/0x47
[  101.408645]  raw_notifier_call_chain+0xc/0xe
[  101.409882]  call_netdevice_notifiers_info+0x41/0x49
[  101.411402]  call_netdevice_notifiers+0xc/0xe
[  101.412713]  rollback_registered_many+0x268/0x36e
[  101.413702]  rollback_registered+0x39/0x56
[  101.414965]  unregister_netdevice_queue+0x79/0x88
[  101.415908]  unregister_netdev+0x16/0x1d

Fixes: ceed73a2 ("drivers: net: ethernet: qualcomm: rmnet: Initial implementation")
Signed-off-by: Subash Abhinov Kasiviswanathan <subashab@codeaurora.org>
Reported-by: kernel test robot <xiaolong.ye@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/qualcomm/rmnet/rmnet_config.c

@@ -51,10 +51,7 @@ struct rmnet_walk_data {
 
 static int rmnet_is_real_dev_registered(const struct net_device *real_dev)
 {
-	rx_handler_func_t *rx_handler;
-
-	rx_handler = rcu_dereference(real_dev->rx_handler);
-	return (rx_handler == rmnet_rx_handler);
+	return rcu_access_pointer(real_dev->rx_handler) == rmnet_rx_handler;
 }
 
 /* Needs rtnl lock */