drivers/base/topology.c: 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 topology code by using this latter form of callback registration.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Git-commit: e12b7111
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Osvaldo Banuelos <osvaldob@codeaurora.org>

drivers/base/topology.c

@@ -181,16 +181,20 @@ static int __cpuinit topology_cpu_callback(struct notifier_block *nfb,
 static int __cpuinit topology_sysfs_init(void)
 {
 	int cpu;
-	int rc;
+	int rc = 0;
+
+	cpu_notifier_register_begin();
 
 	for_each_online_cpu(cpu) {
 		rc = topology_add_dev(cpu);
 		if (rc)
-			return rc;
+			goto out;
 	}
-	hotcpu_notifier(topology_cpu_callback, 0);
+	__hotcpu_notifier(topology_cpu_callback, 0);
 
-	return 0;
+out:
+	cpu_notifier_register_done();
+	return rc;
 }
 
 device_initcall(topology_sysfs_init);