[PATCH 01/14] kthread: Fix invalid wakeup in kthreadd

[Libin]

If kthreadd is preempted at(or before) location a, and the other thread,
such as calling kthread_create_on_node(), adds a list item to
the kthread_create_list followed with wake_up_process(kthread). After that
when kthreadd is re-scheduled, calling set_current_state to set itself as
state TASK_INTERRUPTIBLE, if it is preempted again after that and before
__set_current_state(TASK_RUNNING), it triggers the invalid wakeup problem.
------------------------
kthreadd()
------------------------
...
for (;;) {
	//location a
	set_current_state(TASK_INTERRUPTIBLE);
	if (list_empty(&kthread_create_list)) {
		//location b
		schedule();
		//location c
	}
	__set_current_state(TASK_RUNNING);
	//location d
...
------------------------
kthread_create_on_node()
------------------------
...
spin_lock(&kthread_create_lock);
list_add_tail(&create.list, &kthread_create_list);
spin_unlock(&kthread_create_lock);
...
wake_up_process(kthreadd_task);
...

To solve this problem, using preempt_disable() to bound the operaion that
setting the task state and the conditions(set by the wake thread) validation.
------------------------
kthreadd()
------------------------
...
for (;;) {
	preempt_disable();
	set_current_state(TASK_INTERRUPTIBLE);
	if (list_empty(&kthread_create_list)) {
		preempt_enable();
		schedule();
		preempt_disable();
	}
...

Signed-off-by: Libin <huawei.libin@xxxxxxxxxx>
---
 kernel/kthread.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/kernel/kthread.c b/kernel/kthread.c
index 760e86d..25c3fed 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -456,10 +456,15 @@ int kthreadd(void *unused)
 	current->flags |= PF_NOFREEZE;
 
 	for (;;) {
+		preempt_disable();
 		set_current_state(TASK_INTERRUPTIBLE);
-		if (list_empty(&kthread_create_list))
+		if (list_empty(&kthread_create_list)) {
+			preempt_enable();
 			schedule();
+			preempt_disable();
+		}
 		__set_current_state(TASK_RUNNING);
+		preempt_enable();
 
 		spin_lock(&kthread_create_lock);
 		while (!list_empty(&kthread_create_list)) {
-- 
1.8.2.1


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