oprofile, nmi-timer: Fix CPU hotplug callback registration

Subsystems that want to register CPU hotplug callbacks, as well as perform
initialization for the CPUs that are already online, often do it as shown
below:

get_online_cpus();

for_each_online_cpu(cpu)
init_cpu(cpu);

register_cpu_notifier(&foobar_cpu_notifier);

put_online_cpus();

This is wrong, since it is prone to ABBA deadlocks involving the
cpu_add_remove_lock and the cpu_hotplug.lock (when running concurrently
with CPU hotplug operations).

Instead, the correct and race-free way of performing the callback
registration is:

cpu_notifier_register_begin();

for_each_online_cpu(cpu)
init_cpu(cpu);

/* Note the use of the double underscored version of the API */
__register_cpu_notifier(&foobar_cpu_notifier);

cpu_notifier_register_done();

Fix the nmi-timer code in oprofile by using this latter form of callback
registration.

Cc: Robert Richter <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Signed-off-by: Srivatsa S. Bhat <[e-mail]>
Signed-off-by: Rafael J. Wysocki <[e-mail]>

drivers/oprofile/nmi_timer_int.c

···
 	struct perf_event *event;
 	int cpu;
 
-	get_online_cpus();
-	unregister_cpu_notifier(&nmi_timer_cpu_nb);
+	cpu_notifier_register_begin();
+	__unregister_cpu_notifier(&nmi_timer_cpu_nb);
 	for_each_possible_cpu(cpu) {
 		event = per_cpu(nmi_timer_events, cpu);
 		if (!event)
···
 		perf_event_release_kernel(event);
 	}
 
-	put_online_cpus();
+	cpu_notifier_register_done();
 }
 
 static int nmi_timer_setup(void)
···
 	do_div(period, HZ);
 	nmi_timer_attr.sample_period = period;
 
-	get_online_cpus();
-	err = register_cpu_notifier(&nmi_timer_cpu_nb);
+	cpu_notifier_register_begin();
+	err = __register_cpu_notifier(&nmi_timer_cpu_nb);
 	if (err)
 		goto out;
+
 	/* can't attach events to offline cpus: */
 	for_each_online_cpu(cpu) {
 		err = nmi_timer_start_cpu(cpu);
-		if (err)
-			break;
+		if (err) {
+			cpu_notifier_register_done();
+			nmi_timer_shutdown();
+			return err;
+		}
 	}
-	if (err)
-		nmi_timer_shutdown();
+
 out:
-	put_online_cpus();
+	cpu_notifier_register_done();
 	return err;
 }