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From:   Valentin Schneider <vschneid@redhat.com>
Date:   Thu, 23 Nov 2023 15:34:23 +0100
Message-ID: <CAD235PRWd+zF1xpuXWabdgMU01XNpvtvGorBJbLn9ny2G_TSuw@mail.gmail.com>
Subject: Re: [PATCH v2 1/2] tcp/dcpp: Un-pin tw_timer
To:     Eric Dumazet <edumazet@google.com>
Cc:     dccp@vger.kernel.org, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        David Ahern <dsahern@kernel.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Tomas Glozar <tglozar@redhat.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Thomas Gleixner <tglx@linutronix.de>
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Hi Eric,

On Mon, 20 Nov 2023 at 18:56, Eric Dumazet <edumazet@google.com> wrote:
>
> On Wed, Nov 15, 2023 at 10:05=E2=80=AFPM Valentin Schneider <vschneid@red=
hat.com> wrote:
> >
> > @@ -53,16 +53,14 @@ void dccp_time_wait(struct sock *sk, int state, int=
 timeo)
> >                 if (state =3D=3D DCCP_TIME_WAIT)
> >                         timeo =3D DCCP_TIMEWAIT_LEN;
> >
> > -               /* tw_timer is pinned, so we need to make sure BH are d=
isabled
> > -                * 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 :-)