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

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Date: Mon, 3 Jun 2024 15:50:35 +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 3:32 PM Florian Westphal <fw@...len.de> wrote:
>
> 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?
>
> Actually, I think we can solve this in a much simpler way.
>
> Instead of replacing:
>
> void inet_twsk_deschedule_put(struct inet_timewait_sock *tw)
> {
>  if (del_timer_sync(&tw->tw_timer))
>     inet_twsk_kill(tw);
>  inet_twsk_put(tw);
> }
>
> With:
>  spinlock_t *lock = inet_ehash_lockp(hashinfo, tw->tw_hash);
>  spin_lock(lock);
>  if (timer_shutdown(&tw->tw_timer)) {
>
> (Which gets us into the tcp_sk_exit_batch trouble Eric points out),
> we can simply add "empty" ehash lock unlock pair before calling
> del_timer_sync():
>
> void inet_twsk_deschedule_put(struct inet_timewait_sock *tw)
> {
> +       spinlock_t *lock = inet_ehash_lockp(hashinfo, tw->tw_hash);
> +       spin_lock(lock)
> +       spin_unlock(lock)
>
>         if (del_timer_sync(&tw->tw_timer))

Sounds good, maybe use timer_shutdown_sync() here ?

>                 inet_twsk_kill(tw);
>         inet_twsk_put(tw);
> }
>
> Rationale:
> inet_twsk_deschedule_put() cannot be called before hashdance_schedule
> calls refcount_set(&tw->tw_refcnt, 3).
>
> Before this any refcount_inc_not_zero fails so we never get into
> deschedule_put.
>
> Hashdance_schedule holds the ehash lock when it sets the tw refcount.
> The lock is released only after the timer is up and running.
>
> When inet_twsk_deschedule_put() is called, and hashdance_schedule
> is not yet done, the spinlock/unlock pair will guarantee that
> the timer is up after the spin_unlock.
>
> I think this is much better than the schedule loop waiting for tw_dr
> refcount to drop, it mainly needs a comment to explain what this is
> doing.
>
> Thoughts?