From: Jakub Kicinski <kuba@kernel.org>
To: Wei Wang <weiwan@google.com>
Cc: "David S . Miller" <davem@davemloft.net>,
Linux Kernel Network Developers <netdev@vger.kernel.org>,
Martin Zaharinov <micron10@gmail.com>,
Alexander Duyck <alexanderduyck@fb.com>,
Eric Dumazet <edumazet@google.com>,
Paolo Abeni <pabeni@redhat.com>,
Hannes Frederic Sowa <hannes@stressinduktion.org>
Subject: Re: [PATCH net v2] net: fix race between napi kthread mode and busy poll
Date: Fri, 26 Feb 2021 18:08:33 -0800 [thread overview]
Message-ID: <20210226180833.09c98110@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com> (raw)
In-Reply-To: <CAEA6p_B6baYFZnEOMS=Nmvg0kA_qB=7ip4S96ys9ZoJWfOiOCA@mail.gmail.com>
On Fri, 26 Feb 2021 17:35:21 -0800 Wei Wang wrote:
> On Fri, Feb 26, 2021 at 5:22 PM Jakub Kicinski <kuba@kernel.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))
?
next prev parent reply other threads:[~2021-02-27 2:09 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-27 0:30 [PATCH net v2] net: fix race between napi kthread mode and busy poll Wei Wang
2021-02-27 0:48 ` Jakub Kicinski
2021-02-27 1:02 ` Wei Wang
2021-02-27 1:22 ` Jakub Kicinski
2021-02-27 1:35 ` Wei Wang
2021-02-27 2:08 ` Jakub Kicinski [this message]
2021-02-27 19:00 ` Wei Wang
2021-02-27 23:23 ` Wei Wang
2021-02-28 19:17 ` Jakub Kicinski
2021-03-01 18:16 ` Wei Wang
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