cpumask: don't recommend set_cpus_allowed hack in Documentation/cpu-hotplug.txt

Q: I need to ensure that a particular cpu is not removed when there is some

   work specific to this cpu is in progress.

A: First switch the current thread context to preferred cpu

A: There are two ways.  If your code can be run in interrupt context, use

   smp_call_function_single(), otherwise use work_on_cpu().  Note that

   work_on_cpu() is slow, and can fail due to out of memory:

	int my_func_on_cpu(int cpu)

	{

		cpumask_t saved_mask, new_mask = CPU_MASK_NONE;

		int curr_cpu, err = 0;

		saved_mask = current->cpus_allowed;

		cpu_set(cpu, new_mask);

		err = set_cpus_allowed(current, new_mask);

		if (err)

			return err;

		/*

		 * If we got scheduled out just after the return from

		 * set_cpus_allowed() before running the work, this ensures

		 * we stay locked.

		 */

		curr_cpu = get_cpu();

		if (curr_cpu != cpu) {

			err = -EAGAIN;

			goto ret;

		} else {

			/*

			 * Do work : But cant sleep, since get_cpu() disables preempt

			 */

		}

		ret:

			put_cpu();

			set_cpus_allowed(current, saved_mask);

		int err;

		get_online_cpus();

		if (!cpu_online(cpu))

			err = -EINVAL;

		else

#if NEEDS_BLOCKING

			err = work_on_cpu(cpu, __my_func_on_cpu, NULL);

#else

			smp_call_function_single(cpu, __my_func_on_cpu, &err,

						 true);

#endif

		put_online_cpus();

		return err;

	}

Q: How do we determine how many CPUs are available for hotplug.

A: There is no clear spec defined way from ACPI that can give us that

   information today. Based on some input from Natalie of Unisys,