On Thu, Aug 29, 2013 at 09:57:36PM +0800, Libin wrote:
> 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.li...@huawei.com>
> ---
> 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))
And this one already has the list_empty() check after the call to
set_current_state(), so no change should be needed here.
On the other hand, if your testing shows that you are losing wakeups
with this exact code, please let us know!
Thanx, Paul
> + 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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