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

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Date:   Fri, 26 Feb 2021 18:08:33 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Wei Wang <weiwan@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Martin Zaharinov <micron10@...il.com>,
        Alexander Duyck <alexanderduyck@...com>,
        Eric Dumazet <edumazet@...gle.com>,
        Paolo Abeni <pabeni@...hat.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>
Subject: Re: [PATCH net v2] net: fix race between napi kthread mode and busy
 poll

On Fri, 26 Feb 2021 17:35:21 -0800 Wei Wang wrote:
> On Fri, Feb 26, 2021 at 5:22 PM Jakub Kicinski <kuba@...nel.org> wrote:
> >
> > 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  
> 
> The 'woken' is set after schedule(). If it is the first time
> napi_threaded_wait() is called, and SCHED_THREADED is not set, and
> woken is not set either, this thread will be put to sleep when it
> reaches schedule(), even though there is work waiting to be done on
> that napi. And I think this kthread will not be woken up again
> afterwards, since the SCHED bit is already grabbed.

Indeed, looks like the task will be in WAKING state until it runs?
We can switch the check in ____napi_schedule() from

	if (thread->state == TASK_RUNNING)

to

	if (!(thread->state & TASK_INTERRUPTIBLE))

?