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>

3 files changed, 72 insertions, 11 deletions

diff --git a/include/linux/stop_machine.h b/include/linux/stop_machine.h
index 74d59a641362..baba3a23a814 100644
--- a/include/linux/stop_machine.h
+++ b/include/linux/stop_machine.h
@@ -35,6 +35,24 @@ int stop_machine(int (*fn)(void *), void *data, const struct cpumask *cpus);
  * won't come or go while it's being called.  Used by hotplug cpu.
  */
 int __stop_machine(int (*fn)(void *), void *data, const struct cpumask *cpus);
+
+/**
+ * stop_machine_create: create all stop_machine threads
+ *
+ * Description: This causes all stop_machine threads to be created before
+ * stop_machine actually gets called. This can be used by subsystems that
+ * need a non failing stop_machine infrastructure.
+ */
+int stop_machine_create(void);
+
+/**
+ * stop_machine_destroy: destroy all stop_machine threads
+ *
+ * Description: This causes all stop_machine threads which were created with
+ * stop_machine_create to be destroyed again.
+ */
+void stop_machine_destroy(void);
+
 #else
 
 static inline int stop_machine(int (*fn)(void *), void *data,
@@ -46,5 +64,9 @@ static inline int stop_machine(int (*fn)(void *), void *data,
         local_irq_enable();
         return ret;
 }
+
+static inline int stop_machine_create(void) { return 0; }
+static inline void stop_machine_destroy(void) { }
+
 #endif /* CONFIG_SMP */
 #endif /* _LINUX_STOP_MACHINE */
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 47fff3b63cbf..30e74dd6d01b 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 286c41722e8c..0cd415ee62a2 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);