Understanding Linux Network Internals

Notifying Events on a Chain

Notifications are generated with notifier_call_chain, defined in kernel/sys.c. This function simply invokes, in order of priority, all the callback routines registered against the chain. Note that callback routines are executed in the context of the process that calls notifier_call_chain. A callback routine could, however, be implemented so that it queues the notification somewhere and wakes up a process that will look at it.

int notifier_call_chain(struct notifier_block **n, unsigned long val, void *v)
{
    int ret = NOTIFY_DONE;
    struct notifier_block *nb = *n;
 
    while (nb)
    {
        ret = nb->notifier_call(nb, val, v);
        if (ret & NOTIFY_STOP_MASK)
        {
            return ret;
        }
        nb = nb->next;
    }
    return ret;
}

This is the meaning of its three input parameters:

n

Notification chain.

val

Event type. The chain itself identifies a class of events; val unequivocally identifies an event type (i.e., NETDEV_REGISTER).

v

Input parameter that can be used by the handlers registered by the various clients. This can be used in different ways under different circumstances. For instance, when a new network device is registered with the kernel, the associated notification uses v to identify the net_device data structure.

The callback routines called by notifier_call_chain can return any of the NOTIFY_ XXX values defined in include/linux/notifier.h:

NOTIFY_OK

Notification was processed correctly.

NOTIFY_DONE

Not interested in the notification.^[*]

NOTIFY_BAD

Something went wrong. Stop calling the callback ...