On Fri, 26 Feb 2021 17:02:17 -0800 Wei Wang wrote:
> static int napi_thread_wait(struct napi_struct *napi)
> {
> + bool woken = false;
> +
> set_current_state(TASK_INTERRUPTIBLE);
>
> while (!kthread_should_stop() && !napi_disable_pending(napi)) {
> - if (test_bit(NAPI_STATE_SCHED, &napi->state)) {
> + unsigned long state = READ_ONCE(napi->state);
> +
> + if ((state & NAPIF_STATE_SCHED) &&
> + ((state & NAPIF_STATE_SCHED_THREAD) || woken)) {
> WARN_ON(!list_empty(&napi->poll_list));
> __set_current_state(TASK_RUNNING);
> return 0;
> + } else {
> + WARN_ON(woken);
> }
>
> schedule();
> + woken = true;
> set_current_state(TASK_INTERRUPTIBLE);
> }
> __set_current_state(TASK_RUNNING);
>
> I don't think it is sufficient to only set SCHED_THREADED bit when the
> thread is in RUNNING state.
> In fact, the thread is most likely NOT in RUNNING mode before we call
> wake_up_process() in ____napi_schedule(), because it has finished the
> previous round of napi->poll() and SCHED bit was cleared, so
> napi_thread_wait() sets the state to INTERRUPTIBLE and schedule() call
> should already put it in sleep.
That's why the check says "|| woken":
((state & NAPIF_STATE_SCHED_THREAD) || woken))
thread knows it owns the NAPI if:
(a) the NAPI has the explicit flag set
or
(b) it was just worken up and !kthread_should_stop(), since only
someone who just claimed the normal SCHED on thread's behalf
will wake it up
