[PATCH 12/51] sparc, sysfs: 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 sysfs code in sparc by using this latter form of callback
registration.

Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
Cc: sparclinux@xxxxxxxxxxxxxxx
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
---

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

diff --git a/arch/sparc/kernel/sysfs.c b/arch/sparc/kernel/sysfs.c
index c21c673..2177b9e 100644
--- a/arch/sparc/kernel/sysfs.c
+++ b/arch/sparc/kernel/sysfs.c
@@ -300,7 +300,7 @@ static int __init topology_init(void)
 
 	check_mmu_stats();
 
-	register_cpu_notifier(&sysfs_cpu_nb);
+	cpu_maps_update_begin();
 
 	for_each_possible_cpu(cpu) {
 		struct cpu *c = &per_cpu(cpu_devices, cpu);
@@ -310,6 +310,10 @@ static int __init topology_init(void)
 			register_cpu_online(cpu);
 	}
 
+	__register_cpu_notifier(&sysfs_cpu_nb);
+
+	cpu_maps_update_done();
+
 	return 0;
 }
 

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