net: fix a data race when get vlan device

We encountered a crash: in the packet receiving process, we got an
illegal VLAN device address, but the VLAN device address saved in vmcore
is correct. After checking the code, we found a possible data
competition:
CPU 0:                             CPU 1:
    (RCU read lock)                  (RTNL lock)
    vlan_do_receive()		       register_vlan_dev()
      vlan_find_dev()

        ->__vlan_group_get_device()	 ->vlan_group_prealloc_vid()

In vlan_group_prealloc_vid(), We need to make sure that memset()
in kzalloc() is executed before assigning  value to vlan devices array:
=================================
kzalloc()
    ->memset(object, 0, size)

smp_wmb()

vg->vlan_devices_arrays[pidx][vidx] = array;
==================================

Because __vlan_group_get_device() function depends on this order.
otherwise we may get a wrong address from the hardware cache on
another cpu.

So fix it by adding memory barrier instruction to ensure the order
of memory operations.
Signed-off-by: Di Zhu <zhudi21@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/8021q/vlan.c

@@ -71,6 +71,9 @@ static int vlan_group_prealloc_vid(struct vlan_group *vg,
 	if (array == NULL)
 		return -ENOBUFS;
 
+	/* paired with smp_rmb() in __vlan_group_get_device() */
+	smp_wmb();
+
 	vg->vlan_devices_arrays[pidx][vidx] = array;
 	return 0;
 }

net/8021q/vlan.h

@@ -57,6 +57,10 @@ static inline struct net_device *__vlan_group_get_device(struct vlan_group *vg,
 
 	array = vg->vlan_devices_arrays[pidx]
 				       [vlan_id / VLAN_GROUP_ARRAY_PART_LEN];
+
+	/* paired with smp_wmb() in vlan_group_prealloc_vid() */
+	smp_rmb();
+
 	return array ? array[vlan_id % VLAN_GROUP_ARRAY_PART_LEN] : NULL;
 }