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

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