stop_machine: introduce stop_machine_create/destroy.

Introduce stop_machine_create/destroy. With this interface subsystems
that need a non-failing stop_machine environment can create the
stop_machine machine threads before actually calling stop_machine.
When the threads aren't needed anymore they can be killed with
stop_machine_destroy again.

When stop_machine gets called and the threads aren't present they
will be created and destroyed automatically. This restores the old
behaviour of stop_machine.

This patch also converts cpu hotplug to the new interface since it
is special: cpu_down calls __stop_machine instead of stop_machine.
However the kstop threads will only be created when stop_machine
gets called.

Changing the code so that the threads would be created automatically
on __stop_machine is currently not possible: when __stop_machine gets
called we hold cpu_add_remove_lock, which is the same lock that
create_rt_workqueue would take. So the workqueue needs to be created
before the cpu hotplug code locks cpu_add_remove_lock.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

2 files changed, 50 insertions, 11 deletions

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 47fff3b..30e74dd 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -269,8 +269,11 @@ out_release:
 
 int __ref cpu_down(unsigned int cpu)
 {
-	int err = 0;
+	int err;
 
+	err = stop_machine_create();
+	if (err)
+		return err;
 	cpu_maps_update_begin();
 
 	if (cpu_hotplug_disabled) {
@@ -297,6 +300,7 @@ int __ref cpu_down(unsigned int cpu)
 
 out:
 	cpu_maps_update_done();
+	stop_machine_destroy();
 	return err;
 }
 EXPORT_SYMBOL(cpu_down);
diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index 286c417..0cd415e 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -38,7 +38,10 @@ struct stop_machine_data {
 static unsigned int num_threads;
 static atomic_t thread_ack;
 static DEFINE_MUTEX(lock);
-
+/* setup_lock protects refcount, stop_machine_wq and stop_machine_work. */
+static DEFINE_MUTEX(setup_lock);
+/* Users of stop_machine. */
+static int refcount;
 static struct workqueue_struct *stop_machine_wq;
 static struct stop_machine_data active, idle;
 static const cpumask_t *active_cpus;
@@ -109,6 +112,43 @@ static int chill(void *unused)
 	return 0;
 }
 
+int stop_machine_create(void)
+{
+	mutex_lock(&setup_lock);
+	if (refcount)
+		goto done;
+	stop_machine_wq = create_rt_workqueue("kstop");
+	if (!stop_machine_wq)
+		goto err_out;
+	stop_machine_work = alloc_percpu(struct work_struct);
+	if (!stop_machine_work)
+		goto err_out;
+done:
+	refcount++;
+	mutex_unlock(&setup_lock);
+	return 0;
+
+err_out:
+	if (stop_machine_wq)
+		destroy_workqueue(stop_machine_wq);
+	mutex_unlock(&setup_lock);
+	return -ENOMEM;
+}
+EXPORT_SYMBOL_GPL(stop_machine_create);
+
+void stop_machine_destroy(void)
+{
+	mutex_lock(&setup_lock);
+	refcount--;
+	if (refcount)
+		goto done;
+	destroy_workqueue(stop_machine_wq);
+	free_percpu(stop_machine_work);
+done:
+	mutex_unlock(&setup_lock);
+}
+EXPORT_SYMBOL_GPL(stop_machine_destroy);
+
 int __stop_machine(int (*fn)(void *), void *data, const struct cpumask *cpus)
 {
 	struct work_struct *sm_work;
@@ -146,19 +186,14 @@ int stop_machine(int (*fn)(void *), void *data, const struct cpumask *cpus)
 {
 	int ret;
 
+	ret = stop_machine_create();
+	if (ret)
+		return ret;
 	/* No CPUs can come up or down during this. */
 	get_online_cpus();
 	ret = __stop_machine(fn, data, cpus);
 	put_online_cpus();
-
+	stop_machine_destroy();
 	return ret;
 }
 EXPORT_SYMBOL_GPL(stop_machine);
-
-static int __init stop_machine_init(void)
-{
-	stop_machine_wq = create_rt_workqueue("kstop");
-	stop_machine_work = alloc_percpu(struct work_struct);
-	return 0;
-}
-core_initcall(stop_machine_init);