[PATCH] smpboot: add missing get_online_cpus() when register

[Lai Jiangshan <laijs@cn.fujitsu.com>]

If the smpboot_register_percpu_thread() is called after smpboot_create_threads()
but before __cpu_up(), the smpboot thread of the online-ing CPU is not created,
and it results a bug.  So we use get_online_cpus() to prevent it.

smpboot_unregister_percpu_thread() travels all possible CPU, it doesn't need
get_online_cpus() which is removed in the patch.

CC: Thomas Gleixner <tglx@linutronix.de>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
CC: stable@kernel.org
Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---
 kernel/smpboot.c |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index eb89e18..8adab87 100644
--- a/kernel/smpboot.c
+++ b/kernel/smpboot.c
@@ -279,6 +279,7 @@ int smpboot_register_percpu_thread(struct smp_hotplug_thread *plug_thread)
 	unsigned int cpu;
 	int ret = 0;
 
+	get_online_cpus();
 	mutex_lock(&smpboot_threads_lock);
 	for_each_online_cpu(cpu) {
 		ret = __smpboot_create_thread(plug_thread, cpu);
@@ -291,6 +292,7 @@ int smpboot_register_percpu_thread(struct smp_hotplug_thread *plug_thread)
 	list_add(&plug_thread->list, &hotplug_threads);
 out:
 	mutex_unlock(&smpboot_threads_lock);
+	put_online_cpus();
 	return ret;
 }
 EXPORT_SYMBOL_GPL(smpboot_register_percpu_thread);
@@ -303,11 +305,9 @@ EXPORT_SYMBOL_GPL(smpboot_register_percpu_thread);
  */
 void smpboot_unregister_percpu_thread(struct smp_hotplug_thread *plug_thread)
 {
-	get_online_cpus();
 	mutex_lock(&smpboot_threads_lock);
 	list_del(&plug_thread->list);
 	smpboot_destroy_threads(plug_thread);
 	mutex_unlock(&smpboot_threads_lock);
-	put_online_cpus();
 }
 EXPORT_SYMBOL_GPL(smpboot_unregister_percpu_thread);
-- 
1.7.4.4