[PATCH v2 05/45] CPU hotplug: Protect set_cpu_online() to avoid false-positives

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

When bringing a secondary CPU online, the task running on the CPU coming up
sets itself in the cpu_online_mask. This is safe even though this task is not
the hotplug writer task.

But it is kinda hard to teach this to the CPU hotplug debug infrastructure,
and if we get it wrong, we risk making the debug code too lenient, risking
false-negatives.

Luckily, all architectures use set_cpu_online() to manipulate the
cpu_online_mask. So, to avoid false-positive warnings from the CPU hotplug
debug code, encapsulate the body of set_cpu_online() within
get/put_online_cpus_atomic().

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

 kernel/cpu.c |    4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 5297ec1..35e7115 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -832,10 +832,14 @@ void set_cpu_present(unsigned int cpu, bool present)
 
 void set_cpu_online(unsigned int cpu, bool online)
 {
+	get_online_cpus_atomic();
+
 	if (online)
 		cpumask_set_cpu(cpu, to_cpumask(cpu_online_bits));
 	else
 		cpumask_clear_cpu(cpu, to_cpumask(cpu_online_bits));
+
+	put_online_cpus_atomic();
 }
 
 void set_cpu_active(unsigned int cpu, bool active)