Subject: kthread: call wake_up_process() whithout the lock being held Date: Wed, 20 Feb 2008 22:39:32 +0100 From: Dmitry Adamushko - from the POV of synchronization, there should be no need to call wake_up_process() with the 'kthread_create_lock' being held; - moreover, in order to support a lockless check for list_empty(&kthread_create_list) in kthreadd() : set_current_state(TASK_INTERRUPTIBLE); if (list_empty(&kthread_create_list)) schedule(); we must ensure that a modification of the list (i.e. list_add_tail()) has been completed by the moment a state of the task is checked in try_to_wake_up(). i.e. they must not be re-ordered. wake_up_process() (i.e. try_to_wake_up() effectively) doesn't provide a full mb. By moving wake_up_process() out of the locked section, we get an UNLOCK/LOCK pair (LOCK is in try_to_wake_up()) which is guaranteed to act as a full mb. Signed-off-by: Dmitry Adamushko Signed-off-by: Rusty Russell --- kernel/kthread.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/kernel/kthread.c b/kernel/kthread.c index d7a7897..ec68e0f 100644 --- a/kernel/kthread.c +++ b/kernel/kthread.c @@ -158,9 +158,9 @@ struct task_struct *kthread_create(int (*threadfn)(void *data), spin_lock(&kthread_create_lock); list_add_tail(&create.list, &kthread_create_list); - wake_up_process(kthreadd_task); spin_unlock(&kthread_create_lock); + wake_up_process(kthreadd_task); wait_for_completion(&create.done); if (!IS_ERR(create.result)) {