[PATCH 1/3] stop_machine: kill smp_hotplug_thread->pre_unpark, introduce stop_machine_unpark()

[Oleg Nesterov <oleg@redhat.com>]

1. Change smpboot_unpark_thread() to check ->selfparking, just
   like smpboot_park_thread() does.

2. Introduce stop_machine_unpark() which sets ->enabled and calls
   kthread_unpark().

3. Change smpboot_thread_call() and cpu_stop_init() to call
   stop_machine_unpark() by hand.

This way:

    - IMO the ->selfparking logic becomes more consistent.

    - We can kill the smp_hotplug_thread->pre_unpark() method.

    - We can easily unpark the stopper thread earlier. Say, we
      can move stop_machine_unpark() from smpboot_thread_call()
      to sched_cpu_active() as Peter suggests.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 include/linux/smpboot.h      |    4 ----
 include/linux/stop_machine.h |    1 +
 kernel/cpu.c                 |    1 +
 kernel/smpboot.c             |    5 ++---
 kernel/stop_machine.c        |   10 +++++++++-
 5 files changed, 13 insertions(+), 8 deletions(-)

diff --git a/include/linux/smpboot.h b/include/linux/smpboot.h
index da3c593..255661e 100644
--- a/include/linux/smpboot.h
+++ b/include/linux/smpboot.h
@@ -24,9 +24,6 @@ struct smpboot_thread_data;
  *			parked (cpu offline)
  * @unpark:		Optional unpark function, called when the thread is
  *			unparked (cpu online)
- * @pre_unpark:		Optional unpark function, called before the thread is
- *			unparked (cpu online). This is not guaranteed to be
- *			called on the target cpu of the thread. Careful!
  * @cpumask:		Internal state.  To update which threads are unparked,
  *			call smpboot_update_cpumask_percpu_thread().
  * @selfparking:	Thread is not parked by the park function.
@@ -42,7 +39,6 @@ struct smp_hotplug_thread {
 	void				(*cleanup)(unsigned int cpu, bool online);
 	void				(*park)(unsigned int cpu);
 	void				(*unpark)(unsigned int cpu);
-	void				(*pre_unpark)(unsigned int cpu);
 	cpumask_var_t			cpumask;
 	bool				selfparking;
 	const char			*thread_comm;
diff --git a/include/linux/stop_machine.h b/include/linux/stop_machine.h
index 7b76362..0adedca 100644
--- a/include/linux/stop_machine.h
+++ b/include/linux/stop_machine.h
@@ -34,6 +34,7 @@ void stop_one_cpu_nowait(unsigned int cpu, cpu_stop_fn_t fn, void *arg,
 int stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg);
 int try_stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg);
 void stop_machine_park(int cpu);
+void stop_machine_unpark(int cpu);
 
 #else	/* CONFIG_SMP */
 
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 8a7225a..ef52b3b 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -464,6 +464,7 @@ static int smpboot_thread_call(struct notifier_block *nfb,
 	switch (action & ~CPU_TASKS_FROZEN) {
 
 	case CPU_ONLINE:
+		stop_machine_unpark(cpu);
 		smpboot_unpark_threads(cpu);
 		break;
 
diff --git a/kernel/smpboot.c b/kernel/smpboot.c
index c697f73..cfd29cb 100644
--- a/kernel/smpboot.c
+++ b/kernel/smpboot.c
@@ -221,9 +221,8 @@ static void smpboot_unpark_thread(struct smp_hotplug_thread *ht, unsigned int cp
 {
 	struct task_struct *tsk = *per_cpu_ptr(ht->store, cpu);
 
-	if (ht->pre_unpark)
-		ht->pre_unpark(cpu);
-	kthread_unpark(tsk);
+	if (!ht->selfparking)
+		kthread_unpark(tsk);
 }
 
 void smpboot_unpark_threads(unsigned int cpu)
diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index 91fbb10..59096a5 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -513,6 +513,14 @@ static void cpu_stop_unpark(unsigned int cpu)
 	spin_unlock_irq(&stopper->lock);
 }
 
+void stop_machine_unpark(int cpu)
+{
+	struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
+
+	cpu_stop_unpark(cpu);
+	kthread_unpark(stopper->thread);
+}
+
 static struct smp_hotplug_thread cpu_stop_threads = {
 	.store			= &cpu_stopper.thread,
 	.thread_should_run	= cpu_stop_should_run,
@@ -521,7 +529,6 @@ static struct smp_hotplug_thread cpu_stop_threads = {
 	.create			= cpu_stop_create,
 	.setup			= cpu_stop_unpark,
 	.park			= cpu_stop_park,
-	.pre_unpark		= cpu_stop_unpark,
 	.selfparking		= true,
 };
 
@@ -537,6 +544,7 @@ static int __init cpu_stop_init(void)
 	}
 
 	BUG_ON(smpboot_register_percpu_thread(&cpu_stop_threads));
+	stop_machine_unpark(raw_smp_processor_id());
 	stop_machine_initialized = true;
 	return 0;
 }
-- 
1.5.5.1