Comment 3 for bug 1854968

net/core/dev.c netdev_wait_allrefs() states:

**
 * netdev_wait_allrefs - wait until all references are gone.
 * @dev: target net_device
 *
 * This is called when unregistering network devices.
 *
 * Any protocol or device that holds a reference should register
 * for netdevice notification, and cleanup and put back the
 * reference if they receive an UNREGISTER event.
 * We can get stuck here if buggy protocols don't correctly
 * call dev_put.
 */

...

                if (refcnt && time_after(jiffies, warning_time + 10 * HZ)) {
                        pr_emerg("unregister_netdevice: waiting for %s to become free. Usage count = %d\n",
                                 dev->name, refcnt);
                        warning_time = jiffies;
                }