[PATCH 48/51] mm, vmstat: Fix CPU hotplug callback registration

[Srivatsa S. Bhat]

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_maps_update_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_maps_update_done();


Fix the vmstat code in the MM subsystem by using this latter form of callback
registration.

Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Christoph Lameter <cl@xxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Yasuaki Ishimatsu <isimatu.yasuaki@xxxxxxxxxxxxxx>
Cc: Cody P Schafer <cody@xxxxxxxxxxxxxxxxxx>
Cc: Toshi Kani <toshi.kani@xxxxxx>
Cc: Dave Hansen <dave@xxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
---

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

diff --git a/mm/vmstat.c b/mm/vmstat.c
index 7249614..70668ba 100644
--- a/mm/vmstat.c
+++ b/mm/vmstat.c
@@ -1290,14 +1290,14 @@ static int __init setup_vmstat(void)
 #ifdef CONFIG_SMP
 	int cpu;
 
-	register_cpu_notifier(&vmstat_notifier);
+	cpu_maps_update_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_maps_update_done();
 #endif
 #ifdef CONFIG_PROC_FS
 	proc_create("buddyinfo", S_IRUGO, NULL, &fragmentation_file_operations);

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