Doc/cpu-hotplug: Specify race-free way to register CPU hotplug callbacks

Q: I don't see my action being called for all CPUs already up and running?

A: Yes, CPU notifiers are called only when new CPUs are on-lined or offlined.

   If you need to perform some action for each cpu already in the system, then

   do this:

	for_each_online_cpu(i) {

		foobar_cpu_callback(&foobar_cpu_notifier, CPU_UP_PREPARE, i);

		foobar_cpu_callback(&foobar_cpu_notifier, CPU_ONLINE, i);

	}

   However, if you want to register a hotplug callback, as well as perform

   some initialization for CPUs that are already online, then do this:

   Version 1: (Correct)

   ---------

   	cpu_notifier_register_begin();

		for_each_online_cpu(i) {

			foobar_cpu_callback(&foobar_cpu_notifier,

					    CPU_UP_PREPARE, i);

			foobar_cpu_callback(&foobar_cpu_notifier,

					    CPU_ONLINE, i);

		}

	/* Note the use of the double underscored version of the API */

	__register_cpu_notifier(&foobar_cpu_notifier);

	cpu_notifier_register_done();

   Note that the following code is *NOT* the right way to achieve this,

   because it is prone to an ABBA deadlock between the cpu_add_remove_lock

   and the cpu_hotplug.lock.

   Version 2: (Wrong!)

   ---------

	get_online_cpus();

		for_each_online_cpu(i) {

			foobar_cpu_callback(&foobar_cpu_notifier,

					    CPU_UP_PREPARE, i);

			foobar_cpu_callback(&foobar_cpu_notifier,

					    CPU_ONLINE, i);

		}

	register_cpu_notifier(&foobar_cpu_notifier);

	put_online_cpus();

    So always use the first version shown above when you want to register

    callbacks as well as initialize the already online CPUs.

Q: If i would like to develop cpu hotplug support for a new architecture,

   what do i need at a minimum?

A: The following are what is required for CPU hotplug infrastructure to work