[PATCH v3 32/52] powercap, intel-rapl: Fix CPU hotplug callback registration

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

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 intel-rapl code in the powercap driver by using this latter form
of callback registration. But retain the calls to get/put_online_cpus(),
since they also protect the function rapl_cleanup_data(). By nesting
get/put_online_cpus() *inside* cpu_notifier_register_begin/done(), we avoid
the ABBA deadlock possibility mentioned above.

Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 drivers/powercap/intel_rapl.c |   10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/powercap/intel_rapl.c b/drivers/powercap/intel_rapl.c
index 3c67683..d6c74c1 100644
--- a/drivers/powercap/intel_rapl.c
+++ b/drivers/powercap/intel_rapl.c
@@ -1369,6 +1369,9 @@ static int __init rapl_init(void)
 
 		return -ENODEV;
 	}
+
+	cpu_notifier_register_begin();
+
 	/* prevent CPU hotplug during detection */
 	get_online_cpus();
 	ret = rapl_detect_topology();
@@ -1380,20 +1383,23 @@ static int __init rapl_init(void)
 		ret = -ENODEV;
 		goto done;
 	}
-	register_hotcpu_notifier(&rapl_cpu_notifier);
+	__register_hotcpu_notifier(&rapl_cpu_notifier);
 done:
 	put_online_cpus();
+	cpu_notifier_register_done();
 
 	return ret;
 }
 
 static void __exit rapl_exit(void)
 {
+	cpu_notifier_register_begin();
 	get_online_cpus();
-	unregister_hotcpu_notifier(&rapl_cpu_notifier);
+	__unregister_hotcpu_notifier(&rapl_cpu_notifier);
 	rapl_unregister_powercap();
 	rapl_cleanup_data();
 	put_online_cpus();
+	cpu_notifier_register_done();
 }
 
 module_init(rapl_init);