Understanding Linux Network Internals

Overview

A notification chain is simply a list of functions to execute when a given event occurs. Each function lets one other subsystem know about an event that occurred within, or was detected by, the subsystem calling the function.

Thus, for each notification chain there is a passive side (the notified) and an active side (the notifier), as in the so-called publish-and-subscribe model:

• The notified are the subsystems that ask to be notified about the event and that provide a callback function to invoke.

• The notifier is the subsystem that experiences an event and calls the callback function.

The functions executed are chosen by the notified subsystems. It is never up to the owner of the chain (the subsystem that generates the notifications) to decide what functions to execute. The owner simply defines the list; any kernel subsystem can register a callback function with that chain to receive the notification.

The use of notification chains makes the source code easier to write and maintain. Imagine how a generic routine might notify external subsystems about an event without using notification chains:

If (subsystem_X_enabled) {
    do_something_1
}
if (subsystem_Y_enabled) {
    do_something_2
}
If (subsystem_Z_enabled) {
    do_something_3
}
... ... ...

In other words, a conditional clause would have to be included for every possible subsystem that might be interested in an event, and the maintainer of this subsystem would have to add a new clause every time somebody else added a subsystem to the ...