x86, msr: 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 msr code in x86 by using this latter form of callback registration.

Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

arch/x86/kernel/msr.c

@@ -259,14 +259,15 @@ static int __init msr_init(void)
 		goto out_chrdev;
 	}
 	msr_class->devnode = msr_devnode;
-	get_online_cpus();
+
+	cpu_notifier_register_begin();
 	for_each_online_cpu(i) {
 		err = msr_device_create(i);
 		if (err != 0)
 			goto out_class;
 	}
-	register_hotcpu_notifier(&msr_class_cpu_notifier);
-	put_online_cpus();
+	__register_hotcpu_notifier(&msr_class_cpu_notifier);
+	cpu_notifier_register_done();
 
 	err = 0;
 	goto out;
@@ -275,7 +276,7 @@ out_class:
 	i = 0;
 	for_each_online_cpu(i)
 		msr_device_destroy(i);
-	put_online_cpus();
+	cpu_notifier_register_done();
 	class_destroy(msr_class);
 out_chrdev:
 	__unregister_chrdev(MSR_MAJOR, 0, NR_CPUS, "cpu/msr");
@@ -286,13 +287,14 @@ out:
 static void __exit msr_exit(void)
 {
 	int cpu = 0;
-	get_online_cpus();
+
+	cpu_notifier_register_begin();
 	for_each_online_cpu(cpu)
 		msr_device_destroy(cpu);
 	class_destroy(msr_class);
 	__unregister_chrdev(MSR_MAJOR, 0, NR_CPUS, "cpu/msr");
-	unregister_hotcpu_notifier(&msr_class_cpu_notifier);
-	put_online_cpus();
+	__unregister_hotcpu_notifier(&msr_class_cpu_notifier);
+	cpu_notifier_register_done();
 }
 
 module_init(msr_init);