Hi Eric,
On Mon, 20 Nov 2023 at 18:56, Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>
> On Wed, Nov 15, 2023 at 10:05 PM Valentin Schneider <vschneid@xxxxxxxxxx> 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 :-)
