net: rmnet: remove rcu_read_lock in rmnet_force_unassociate_device()

The notifier_call() of the slave interface removes rmnet interface with
unregister_netdevice_queue().
But, before calling unregister_netdevice_queue(), it acquires
rcu readlock.
In the RCU critical section, sleeping isn't be allowed.
But, unregister_netdevice_queue() internally calls synchronize_net(),
which would sleep.
So, suspicious RCU usage warning occurs.

Test commands:
    modprobe rmnet
    ip link add dummy0 type dummy
    ip link add dummy1 type dummy
    ip link add rmnet0 link dummy0 type rmnet mux_id 1
    ip link set dummy1 master rmnet0
    ip link del dummy0

Splat looks like:
[   79.639245][ T1195] =============================
[   79.640134][ T1195] WARNING: suspicious RCU usage
[   79.640852][ T1195] 5.6.0-rc1+ #447 Not tainted
[   79.641657][ T1195] -----------------------------
[   79.642472][ T1195] ./include/linux/rcupdate.h:273 Illegal context switch in RCU read-side critical section!
[   79.644043][ T1195]
[   79.644043][ T1195] other info that might help us debug this:
[   79.644043][ T1195]
[   79.645682][ T1195]
[   79.645682][ T1195] rcu_scheduler_active = 2, debug_locks = 1
[   79.646980][ T1195] 2 locks held by ip/1195:
[   79.647629][ T1195]  #0: ffffffffa3cf64f0 (rtnl_mutex){+.+.}, at: rtnetlink_rcv_msg+0x457/0x890
[   79.649312][ T1195]  #1: ffffffffa39256c0 (rcu_read_lock){....}, at: rmnet_config_notify_cb+0xf0/0x590 [rmnet]
[   79.651717][ T1195]
[   79.651717][ T1195] stack backtrace:
[   79.652650][ T1195] CPU: 3 PID: 1195 Comm: ip Not tainted 5.6.0-rc1+ #447
[   79.653702][ T1195] Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS VirtualBox 12/01/2006
[   79.655037][ T1195] Call Trace:
[   79.655560][ T1195]  dump_stack+0x96/0xdb
[   79.656252][ T1195]  ___might_sleep+0x345/0x440
[   79.656994][ T1195]  synchronize_net+0x18/0x30
[   79.661132][ T1195]  netdev_rx_handler_unregister+0x40/0xb0
[   79.666266][ T1195]  rmnet_unregister_real_device+0x42/0xb0 [rmnet]
[   79.667211][ T1195]  rmnet_config_notify_cb+0x1f7/0x590 [rmnet]
[   79.668121][ T1195]  ? rmnet_unregister_bridge.isra.6+0xf0/0xf0 [rmnet]
[   79.669166][ T1195]  ? rmnet_unregister_bridge.isra.6+0xf0/0xf0 [rmnet]
[   79.670286][ T1195]  ? __module_text_address+0x13/0x140
[   79.671139][ T1195]  notifier_call_chain+0x90/0x160
[   79.671973][ T1195]  rollback_registered_many+0x660/0xcf0
[   79.672893][ T1195]  ? netif_set_real_num_tx_queues+0x780/0x780
[   79.675091][ T1195]  ? __lock_acquire+0xdfe/0x3de0
[   79.675825][ T1195]  ? memset+0x1f/0x40
[   79.676367][ T1195]  ? __nla_validate_parse+0x98/0x1ab0
[   79.677290][ T1195]  unregister_netdevice_many.part.133+0x13/0x1b0
[   79.678163][ T1195]  rtnl_delete_link+0xbc/0x100
[ ... ]

Fixes: ceed73a2 ("drivers: net: ethernet: qualcomm: rmnet: Initial implementation")
Signed-off-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -230,7 +230,6 @@ static void rmnet_force_unassociate_device(struct net_device *dev)
 
 	port = rmnet_get_port_rtnl(dev);
 
-	rcu_read_lock();
 	rmnet_unregister_bridge(dev, port);
 
 	hash_for_each_safe(port->muxed_ep, bkt_ep, tmp_ep, ep, hlnode) {
@@ -241,7 +240,6 @@ static void rmnet_force_unassociate_device(struct net_device *dev)
 		kfree(ep);
 	}
 
-	rcu_read_unlock();
 	unregister_netdevice_many(&list);
 
 	rmnet_unregister_real_device(real_dev, port);