Re: [syzbot] [net?] WARNING: refcount bug in inet_twsk_kill

[Florian Westphal <fw@strlen.de>]

Jason Xing <kerneljasonxing@gmail.com> wrote:
> > I don't see how this helps, we need to wait until 'stolen' twsk
> > has gone through inet_twsk_kill() and decremented tw_refcount.
> > Obviously It would be a bit simpler if we had a reliable reproducer :-)
> 
> Allow me to say something irrelevant to this bug report.
> 
> Do you think that Kuniyuki's patch can solve the race between two
> 'killers' calling inet_twsk_deschedule_put()->inet_twsk_kill()
> concurrently at two cores, say, inet_twsk_purge() and tcp_abort()?

I don't think its possible, tcp_abort() calls inet_twsk_deschedule_put,
which does:

        if (timer_shutdown_sync(&tw->tw_timer))
                inet_twsk_kill(tw);

So I don't see how two concurrent callers, working on same tw address,
would both be able to shut down the timer.

One will shut it down and calls inet_twsk_kill(), other will wait until
the callback has completed, but it doesn't call inet_twsk_kill().

> It at least does help avoid decrementing tw_refcount twice in the
> above case if I understand correctly.

I don't think the refcount is decremented twice.

Problem is one thread is already at the 'final' decrement of
 WARN_ON_ONCE(!refcount_dec_and_test(&net->ipv4.tcp_death_row.tw_refcount));

in tcp_sk_exit_batch(), while other thread has not yet called
refcount_dec() on it (inet_twsk_kill still executing).

So we get two splats, refcount_dec_and_test() returns 1 not expected 0
and refcount_dec() coming right afterwards from other task observes the
transition to 0, while it should have dropped down to 1.