netdev - Re: [PATCH net-next v6 1/3] net: tcp/dcpp: prepare for tw

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Date: Mon, 3 Jun 2024 13:53:16 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Florian Westphal <fw@...len.de>
Cc: netdev@...r.kernel.org, Paolo Abeni <pabeni@...hat.com>, 
	"David S. Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, mleitner@...hat.com, 
	juri.lelli@...hat.com, vschneid@...hat.com, tglozar@...hat.com, 
	dsahern@...nel.org, bigeasy@...utronix.de, tglx@...utronix.de
Subject: Re: [PATCH net-next v6 1/3] net: tcp/dcpp: prepare for tw_timer un-pinning

On Mon, Jun 3, 2024 at 1:21 PM Florian Westphal <fw@...len.de> wrote:
>
> Eric Dumazet <edumazet@...gle.com> wrote:
> > On Mon, Jun 3, 2024 at 11:37 AM Florian Westphal <fw@...len.de> wrote:
> > > +       spin_lock(lock);
> > > +       if (timer_shutdown(&tw->tw_timer)) {
> > > +               /* releases @lock */
> > > +               __inet_twsk_kill(tw, lock);
> > > +       } else {
> >
> > If we do not have a sync variant here, I think that inet_twsk_purge()
> > could return while ongoing timers are alive.
>
> Yes.
>
> We can't use sync variant, it would deadlock on ehash spinlock.
>
> > tcp_sk_exit_batch() would then possibly hit :
> >
> > WARN_ON_ONCE(!refcount_dec_and_test(&net->ipv4.tcp_death_row.tw_refcount));
> >
> > The alive timer are releasing tw->tw_dr->tw_refcount at the end of
> > inet_twsk_kill()
>
> Theoretically the tw socket can be unlinked from the tw hash already
> (inet_twsk_purge won't encounter it), but timer is still running.
>
> Only solution I see is to schedule() in tcp_sk_exit_batch() until
> tw_refcount has dropped to the expected value, i.e. something like
>
> static void tcp_wait_for_tw_timers(struct net *n)
> {
>         while (refcount_read(&n->ipv4.tcp_death_row.tw_refcount) > 1))
>                 schedule();
> }
>
> Any better idea?


Maybe usleep_range(500, 1000)

>
> I started to sketch a patch that keeps PINNED as-is but schedules almost
> all of the actual work to a work item.
>
> Idea was that it would lower RT latencies to acceptable level but it got
> so ugly that I did not follow this path.
>
> I could resurrect this if you think its worth a try.

I would rather avoid a work queue.