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

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Date:   Thu, 23 Nov 2023 17:45:29 +0100
From:   Valentin Schneider <vschneid@...hat.com>
To:     Eric Dumazet <edumazet@...gle.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, 23 Nov 2023 at 17:32, Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Thu, Nov 23, 2023 at 3:34 PM Valentin Schneider <vschneid@...hat.com> wrote:
> > 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.
>

Gotcha, let me dig into this then!