[patch 1/2] stop_machine: introduce stop_machine_create/destroy.

Heiko Carstens
2008-12-22T11:45:55Z

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From: Heiko Carstens <heiko.carstens@de.ibm.com>

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>
---
include/linux/stop_machine.h | 22 +++++++++++++++++
kernel/cpu.c | 6 +++-
kernel/stop_machine.c | 55 +++++++++++++++++++++++++++++++++++--------
3 files changed, 72 insertions(+), 11 deletions(-)

Index: linux-2.6/kernel/stop_machine.c
===================================================================
--- linux-2.6.orig/kernel/stop_machine.c
+++ linux-2.6/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 cpumask_t *cpus)
{
struct work_struct *sm_work;
@@ -146,19 +186,14 @@ int stop_machine(int (*fn)(void *), void
{
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);
Index: linux-2.6/kernel/cpu.c
===================================================================
--- linux-2.6.orig/kernel/cpu.c
+++ linux-2.6/kernel/cpu.c
@@ -290,8 +290,11 @@ 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) {
@@ -318,6 +321,7 @@ int __ref cpu_down(unsigned int cpu)

out:
cpu_maps_update_done();
+ stop_machine_destroy();
return err;
}
EXPORT_SYMBOL(cpu_down);
Index: linux-2.6/include/linux/stop_machine.h
===================================================================
--- linux-2.6.orig/include/linux/stop_machine.h
+++ linux-2.6/include/linux/stop_machine.h
@@ -35,6 +35,24 @@ int stop_machine(int (*fn)(void *), void
* won't come or go while it's being called. Used by hotplug cpu.
*/
int __stop_machine(int (*fn)(void *), void *data, const cpumask_t *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)
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 */

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