[PATCH v3 49/52] mm, vmstat: 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 vmstat code in the MM subsystem by using this latter form of callback
registration.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Cody P Schafer <redacted>
Cc: Toshi Kani <redacted>
Cc: Dave Hansen <redacted>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: linux-mm@kvack.org
Acked-by: Christoph Lameter <redacted>
Acked-by: Rik van Riel <redacted>
Reviewed-by: Yasuaki Ishimatsu <redacted>
Signed-off-by: Srivatsa S. Bhat <redacted>
---

 mm/vmstat.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/vmstat.c b/mm/vmstat.c
index def5dd2..58c6f3d 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c

@@ -1292,14 +1292,14 @@ static int __init setup_vmstat(void)
 #ifdef CONFIG_SMP
 	int cpu;
 
-	register_cpu_notifier(&vmstat_notifier);
+	cpu_notifier_register_begin();
+	__register_cpu_notifier(&vmstat_notifier);
 
-	get_online_cpus();
 	for_each_online_cpu(cpu) {
 		start_cpu_timer(cpu);
 		node_set_state(cpu_to_node(cpu), N_CPU);
 	}
-	put_online_cpus();
+	cpu_notifier_register_done();
 #endif
 #ifdef CONFIG_PROC_FS
 	proc_create("buddyinfo", S_IRUGO, NULL, &fragmentation_file_operations);