Re: [PATCH net v2] net: fix race between napi kthread mode and busy poll

[Jakub Kicinski <kuba@kernel.org>]

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