netdev - Re: [PATCH net] tcp: fix forever orphan socket caused by tcp

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Message-ID: <CAL+tcoADT8EnQdFctCHab0B6cf9WyhFoAzNMxOA+JraRuTfUug@mail.gmail.com>
Date: Mon, 5 Aug 2024 16:04:10 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: Xueming Feng <kuro@...oa.me>, Lorenzo Colitti <lorenzo@...gle.com>, 
	"David S . Miller" <davem@...emloft.net>, netdev@...r.kernel.org, 
	Neal Cardwell <ncardwell@...gle.com>, Yuchung Cheng <ycheng@...gle.com>, 
	Soheil Hassas Yeganeh <soheil@...gle.com>, David Ahern <dsahern@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net] tcp: fix forever orphan socket caused by tcp_abort

On Mon, Aug 5, 2024 at 3:23 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Mon, Aug 5, 2024 at 6:52 AM Jason Xing <kerneljasonxing@...il.com> wrote:
> >
> > On Sat, Aug 3, 2024 at 11:48 PM Jason Xing <kerneljasonxing@...il.com> wrote:
> > >
> > > Hello Eric,
> > >
> > > On Thu, Aug 1, 2024 at 9:17 PM Eric Dumazet <edumazet@...gle.com> wrote:
> > > >
> > > > On Thu, Aug 1, 2024 at 1:17 PM Xueming Feng <kuro@...oa.me> wrote:
> > > > >
> > > > > We have some problem closing zero-window fin-wait-1 tcp sockets in our
> > > > > environment. This patch come from the investigation.
> > > > >
> > > > > Previously tcp_abort only sends out reset and calls tcp_done when the
> > > > > socket is not SOCK_DEAD aka. orphan. For orphan socket, it will only
> > > > > purging the write queue, but not close the socket and left it to the
> > > > > timer.
> > > > >
> > > > > While purging the write queue, tp->packets_out and sk->sk_write_queue
> > > > > is cleared along the way. However tcp_retransmit_timer have early
> > > > > return based on !tp->packets_out and tcp_probe_timer have early
> > > > > return based on !sk->sk_write_queue.
> > > > >
> > > > > This caused ICSK_TIME_RETRANS and ICSK_TIME_PROBE0 not being resched
> > > > > and socket not being killed by the timers. Converting a zero-windowed
> > > > > orphan to a forever orphan.
> > > > >
> > > > > This patch removes the SOCK_DEAD check in tcp_abort, making it send
> > > > > reset to peer and close the socket accordingly. Preventing the
> > > > > timer-less orphan from happening.
> > > > >
> > > > > Fixes: e05836ac07c7 ("tcp: purge write queue upon aborting the connection")
> > > > > Fixes: bffd168c3fc5 ("tcp: clear tp->packets_out when purging write queue")
> > > > > Signed-off-by: Xueming Feng <kuro@...oa.me>
> > > >
> > > > This seems legit, but are you sure these two blamed commits added this bug ?
> > > >
> > > > Even before them, we should have called tcp_done() right away, instead
> > > > of waiting for a (possibly long) timer to complete the job.
> > > >
> > > > This might be important when killing millions of sockets on a busy server.
> > > >
> > > > CC Lorenzo
> > > >
> > > > Lorenzo, do you recall why your patch was testing the SOCK_DEAD flag ?
> > >
> > > I guess that one of possible reasons is to avoid double-free,
> > > something like this, happening in inet_csk_destroy_sock().
> > >
> > > Let me assume: if we call tcp_close() first under the memory pressure
> > > which means tcp_check_oom() returns true and then it will call
> > > inet_csk_destroy_sock() in __tcp_close(), later tcp_abort() will call
> > > tcp_done() to free the sk again in the inet_csk_destroy_sock() when
> > > not testing the SOCK_DEAD flag in tcp_abort.
> > >
> >
> > How about this one which can prevent double calling
> > inet_csk_destroy_sock() when we call destroy and close nearly at the
> > same time under that circumstance:
> >
> > diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> > index e03a342c9162..d5d3b21cc824 100644
> > --- a/net/ipv4/tcp.c
> > +++ b/net/ipv4/tcp.c
> > @@ -4646,7 +4646,7 @@ int tcp_abort(struct sock *sk, int err)
> >         local_bh_disable();
> >         bh_lock_sock(sk);
> >
> > -       if (!sock_flag(sk, SOCK_DEAD)) {
> > +       if (sk->sk_state != TCP_CLOSE) {
> >                 if (tcp_need_reset(sk->sk_state))
> >                         tcp_send_active_reset(sk, GFP_ATOMIC,
> >                                               SK_RST_REASON_NOT_SPECIFIED);
> >
> > Each time we call inet_csk_destroy_sock(), we must make sure we've
> > already set the state to TCP_CLOSE. Based on this, I think we can use
> > this as an indicator to avoid calling twice to destroy the socket.
>
> I do not think this will work.
>
> With this patch, a listener socket will not get an error notification.

Oh, you're right.

I think we can add this particular case in the if or if-else statement
to handle.

Thanks,
Jason