linux-kernel - Re: [PATCH v2 1/2] tcp/dcpp: Un-pin tw

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Date:   Thu, 23 Nov 2023 17:32:10 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     Valentin Schneider <vschneid@...hat.com>
Cc:     dccp@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        David Ahern <dsahern@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Tomas Glozar <tglozar@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 1/2] tcp/dcpp: Un-pin tw_timer

On Thu, Nov 23, 2023 at 3:34 PM Valentin Schneider <vschneid@...hat.com> wrote:
>
> Hi Eric,
>
> On Mon, 20 Nov 2023 at 18:56, Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > On Wed, Nov 15, 2023 at 10:05 PM Valentin Schneider <vschneid@...hat.com> wrote:
> > >
> > > @@ -53,16 +53,14 @@ void dccp_time_wait(struct sock *sk, int state, int timeo)
> > >                 if (state == DCCP_TIME_WAIT)
> > >                         timeo = DCCP_TIMEWAIT_LEN;
> > >
> > > -               /* tw_timer is pinned, so we need to make sure BH are disabled
> > > -                * in following section, otherwise timer handler could run before
> > > -                * we complete the initialization.
> > > -                */
> > > -               local_bh_disable();
> > > -               inet_twsk_schedule(tw, timeo);
> > > -               /* Linkage updates.
> > > -                * Note that access to tw after this point is illegal.
> > > -                */
> > > +              local_bh_disable();
> > > +
> > > +               // Linkage updates
> > >                 inet_twsk_hashdance(tw, sk, &dccp_hashinfo);
> > > +               inet_twsk_schedule(tw, timeo);
> >
> > We could arm a timer there, while another thread/cpu found the TW in
> > the ehash table.
> >
> >
> >
> > > +               // Access to tw after this point is illegal.
> > > +               inet_twsk_put(tw);
> >
> > This would eventually call inet_twsk_free() while the timer is armed.
> >
> > I think more work is needed.
> >
> > Perhaps make sure that a live timer owns a reference on tw->tw_refcnt
> > (This is not the case atm)
> >
>
> I thought that was already the case, per inet_twsk_hashdance():
>
> /* tw_refcnt is set to 3 because we have :
>  * - one reference for bhash chain.
>  * - one reference for ehash chain.
>  * - one reference for timer.
>
> and
>
> tw_timer_handler()
> `\
>   inet_twsk_kill()
>   `\
>     inet_twsk_put()
>
> So AFAICT, after we go through the hashdance, there's a reference on
> tw_refcnt held by the tw_timer.
> inet_twsk_deschedule_put() can race with arming the timer, but it only
> calls inet_twsk_kill() if the timer
> was already armed & has been deleted, so there's no risk of calling it
> twice... If I got it right :-)
>

Again, I think you missed some details.

I am OOO for a few days, I do not have time to elaborate.

You will need to properly track active timer by elevating
tw->tw_refcnt, or I guarantee something wrong will happen.