[PATCH v2 06/52] ia64, palinfo: 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_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 palinfo code in ia64 by using this latter form of callback
registration.

Cc: Tony Luck <tony.luck@xxxxxxxxx>
Cc: Fenghua Yu <fenghua.yu@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: linux-ia64@xxxxxxxxxxxxxxx
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
---

 arch/ia64/kernel/palinfo.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/arch/ia64/kernel/palinfo.c b/arch/ia64/kernel/palinfo.c
index ab33328..c39c3cd 100644
--- a/arch/ia64/kernel/palinfo.c
+++ b/arch/ia64/kernel/palinfo.c
@@ -996,13 +996,17 @@ palinfo_init(void)
 	if (!palinfo_dir)
 		return -ENOMEM;
 
+	cpu_notifier_register_begin();
+
 	/* Create palinfo dirs in /proc for all online cpus */
 	for_each_online_cpu(i) {
 		create_palinfo_proc_entries(i);
 	}
 
 	/* Register for future delivery via notify registration */
-	register_hotcpu_notifier(&palinfo_cpu_notifier);
+	__register_hotcpu_notifier(&palinfo_cpu_notifier);
+
+	cpu_notifier_register_done();
 
 	return 0;
 }

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