netdev - Re: [PATCH net] net: Use NAPI_* in test

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Message-ID: <CAAywjhQZ=4hYaCrO6Uue+cfB4xyyPDMbRTtucEQ4vvxozqxKEQ@mail.gmail.com>
Date: Thu, 11 Sep 2025 07:31:11 -0700
From: Samiullah Khawaja <skhawaja@...gle.com>
To: Jakub Kicinski <kuba@...nel.org>
Cc: "David S . Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Paolo Abeni <pabeni@...hat.com>, willemb@...gle.com, netdev@...r.kernel.org, 
	mkarsten@...terloo.ca
Subject: Re: [PATCH net] net: Use NAPI_* in test_bit when stopping napi kthread

On Thu, Sep 11, 2025 at 7:10 AM Samiullah Khawaja <skhawaja@...gle.com> wrote:
>
> On Thu, Sep 11, 2025 at 6:40 AM Jakub Kicinski <kuba@...nel.org> wrote:
> >
> > On Wed, 10 Sep 2025 20:37:16 +0000 Samiullah Khawaja wrote:
> > > napi_stop_kthread waits for the NAPI_STATE_SCHED_THREADED to be unset
> > > before stopping the kthread. But it uses test_bit with the
> > > NAPIF_STATE_SCHED_THREADED and that might stop the kthread early before
> > > the flag is unset.
> > >
> > > Use the NAPI_* variant of the NAPI state bits in test_bit instead.
> > >
> > > Tested:
> > >  ./tools/testing/selftests/net/nl_netdev.py
> > >  TAP version 13
> > >  1..7
> > >  ok 1 nl_netdev.empty_check
> > >  ok 2 nl_netdev.lo_check
> > >  ok 3 nl_netdev.page_pool_check
> > >  ok 4 nl_netdev.napi_list_check
> > >  ok 5 nl_netdev.dev_set_threaded
> > >  ok 6 nl_netdev.napi_set_threaded
> > >  ok 7 nl_netdev.nsim_rxq_reset_down
> > >  # Totals: pass:7 fail:0 xfail:0 xpass:0 skip:0 error:0
> > >
> > >  ./tools/testing/selftests/drivers/net/napi_threaded.py
> > >  TAP version 13
> > >  1..2
> > >  ok 1 napi_threaded.change_num_queues
> > >  ok 2 napi_threaded.enable_dev_threaded_disable_napi_threaded
> > >  # Totals: pass:2 fail:0 xfail:0 xpass:0 skip:0 error:0
> > >
> > > Fixes: 689883de94dd ("net: stop napi kthreads when THREADED napi is disabled")
> > > Signed-off-by: Samiullah Khawaja <skhawaja@...gle.com>
> >
> > Is this basically addressing the bug that Martin run into?
> Not really. That one was because the busy polling bit remained set
> during kthread stop. Basically in this function when we unset
> STATE_THREADED, we also needed to unset STATE_THREADED_BUSY_POLL.
>
> @@ -7000,7 +7002,8 @@ static void napi_stop_kthread(struct napi_struct *napi)
>                  */
>                 if ((val & NAPIF_STATE_SCHED_THREADED) ||
>                     !(val & NAPIF_STATE_SCHED)) {
> -                       new = val & (~NAPIF_STATE_THREADED);
> +                       new = val & (~(NAPIF_STATE_THREADED |
> +                                      NAPIF_STATE_THREADED_BUSY_POLL));
>
Just to add to my last email: I did find this test_bit issue while
working on the fix for that other problem.
> >
> > > diff --git a/net/core/dev.c b/net/core/dev.c
> > > index 93a25d87b86b..8d49b2198d07 100644
> > > --- a/net/core/dev.c
> > > +++ b/net/core/dev.c
> > > @@ -6965,7 +6965,7 @@ static void napi_stop_kthread(struct napi_struct *napi)
> > >        * the kthread.
> > >        */
> > >       while (true) {
> > > -             if (!test_bit(NAPIF_STATE_SCHED_THREADED, &napi->state))
> > > +             if (!test_bit(NAPI_STATE_SCHED_THREADED, &napi->state))
> > >                       break;
> > >
> > >               msleep(20);
> >