Date:   Tue, 17 Jan 2023 17:55:45 +0000
From:   "Iwashima, Kuniyuki" <kuniyu@...zon.co.jp>
To:     Jason Xing <kerneljasonxing@...il.com>
CC:     "davem@...emloft.net" <davem@...emloft.net>,
        "dsahern@...nel.org" <dsahern@...nel.org>,
        "edumazet@...gle.com" <edumazet@...gle.com>,
        "kernelxing@...cent.com" <kernelxing@...cent.com>,
        "kuba@...nel.org" <kuba@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "pabeni@...hat.com" <pabeni@...hat.com>,
        "yoshfuji@...ux-ipv6.org" <yoshfuji@...ux-ipv6.org>,
        "Iwashima, Kuniyuki" <kuniyu@...zon.co.jp>
Subject: Re: [PATCH v5 net] tcp: avoid the lookup process failing to get sk in
 ehash table

From: Jason Xing <kerneljasonxing@...il.com>
Date: Wed, 18 Jan 2023 01:43:32 +0800
> On Wed, Jan 18, 2023 at 12:36 AM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
> >
> > From:   Jason Xing <kerneljasonxing@...il.com>
> > Date:   Tue, 17 Jan 2023 10:15:45 +0800
> > > On Tue, Jan 17, 2023 at 12:54 AM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
> > > >
> > > > From:   Jason Xing <kerneljasonxing@...il.com>
> > > > Date:   Mon, 16 Jan 2023 18:33:41 +0800
> > > > > From: Jason Xing <kernelxing@...cent.com>
> > > > >
> > > > > While one cpu is working on looking up the right socket from ehash
> > > > > table, another cpu is done deleting the request socket and is about
> > > > > to add (or is adding) the big socket from the table. It means that
> > > > > we could miss both of them, even though it has little chance.
> > > > >
> > > > > Let me draw a call trace map of the server side.
> > > > >    CPU 0                           CPU 1
> > > > >    -----                           -----
> > > > > tcp_v4_rcv()                  syn_recv_sock()
> > > > >                             inet_ehash_insert()
> > > > >                             -> sk_nulls_del_node_init_rcu(osk)
> > > > > __inet_lookup_established()
> > > > >                             -> __sk_nulls_add_node_rcu(sk, list)
> > > > >
> > > > > Notice that the CPU 0 is receiving the data after the final ack
> > > > > during 3-way shakehands and CPU 1 is still handling the final ack.
> > > > >
> > > > > Why could this be a real problem?
> > > > > This case is happening only when the final ack and the first data
> > > > > receiving by different CPUs. Then the server receiving data with
> > > > > ACK flag tries to search one proper established socket from ehash
> > > > > table, but apparently it fails as my map shows above. After that,
> > > > > the server fetches a listener socket and then sends a RST because
> > > > > it finds a ACK flag in the skb (data), which obeys RST definition
> > > > > in RFC 793.
> > > > >
> > > > > Besides, Eric pointed out there's one more race condition where it
> > > > > handles tw socket hashdance. Only by adding to the tail of the list
> > > > > before deleting the old one can we avoid the race if the reader has
> > > > > already begun the bucket traversal and it would possibly miss the head.
> > > > >
> > > > > Many thanks to Eric for great help from beginning to end.
> > > > >
> > > > > Fixes: 5e0724d027f0 ("tcp/dccp: fix hashdance race for passive sessions")
> > > > > Suggested-by: Eric Dumazet <edumazet@...gle.com>
> > > > > Signed-off-by: Jason Xing <kernelxing@...cent.com>
> > > > > Link: https://lore.kernel.org/lkml/20230112065336.41034-1-kerneljasonxing@gmail.com/
> > > >
> > > > I guess there could be regression if a workload has many long-lived
> > >
> > > Sorry, I don't understand. This patch does not add two kinds of
> > > sockets into the ehash table all the time, but reverses the order of
> > > deleting and adding sockets only.
> >
> > Not really.  It also reverses the order of sockets in ehash.  We were
> > able to find newer sockets faster than older ones.  If a workload has
> > many long-lived sockets, they would add constant time on newer socket's
> > lookup.
> >
> >
> > > The final result is the same as the
> > > old time. I'm wondering why it could cause some regressions if there
> > > are loads of long-lived connections.
> > >
> > > > connections, but the change itself looks good.  I left a minor comment
> > > > below.
> > > >
> > > > Reviewed-by: Kuniyuki Iwashima <kuniyu@...zon.com>
> > > >
> > >
> > > Thanks for reviewing :)
> > >
> > > >
> > > > > ---
> > > > > v5:
> > > > > 1) adjust the style once more.
> > > > >
> > > > > v4:
> > > > > 1) adjust the code style and make it easier to read.
> > > > >
> > > > > v3:
> > > > > 1) get rid of else-if statement.
> > > > >
> > > > > v2:
> > > > > 1) adding the sk node into the tail of list to prevent the race.
> > > > > 2) fix the race condition when handling time-wait socket hashdance.
> > > > > ---
> > > > >  net/ipv4/inet_hashtables.c    | 17 +++++++++++++++--
> > > > >  net/ipv4/inet_timewait_sock.c |  6 +++---
> > > > >  2 files changed, 18 insertions(+), 5 deletions(-)
> > > > >
> > > > > diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
> > > > > index 24a38b56fab9..f58d73888638 100644
> > > > > --- a/net/ipv4/inet_hashtables.c
> > > > > +++ b/net/ipv4/inet_hashtables.c
> > > > > @@ -650,8 +650,20 @@ bool inet_ehash_insert(struct sock *sk, struct sock *osk, bool *found_dup_sk)
> > > > >       spin_lock(lock);
> > > > >       if (osk) {
> > > > >               WARN_ON_ONCE(sk->sk_hash != osk->sk_hash);
> > > > > -             ret = sk_nulls_del_node_init_rcu(osk);
> > > > > -     } else if (found_dup_sk) {
> > > > > +             ret = sk_hashed(osk);
> > > > > +             if (ret) {
> > > > > +                     /* Before deleting the node, we insert a new one to make
> > > > > +                      * sure that the look-up-sk process would not miss either
> > > > > +                      * of them and that at least one node would exist in ehash
> > > > > +                      * table all the time. Otherwise there's a tiny chance
> > > > > +                      * that lookup process could find nothing in ehash table.
> > > > > +                      */
> > > > > +                     __sk_nulls_add_node_tail_rcu(sk, list);
> > > > > +                     sk_nulls_del_node_init_rcu(osk);
> > > > > +             }
> > > > > +             goto unlock;
> > > > > +     }
> > > > > +     if (found_dup_sk) {
> > > > >               *found_dup_sk = inet_ehash_lookup_by_sk(sk, list);
> > > > >               if (*found_dup_sk)
> > > > >                       ret = false;
> > > > > @@ -660,6 +672,7 @@ bool inet_ehash_insert(struct sock *sk, struct sock *osk, bool *found_dup_sk)
> > > > >       if (ret)
> > > > >               __sk_nulls_add_node_rcu(sk, list);
> > > > >
> > > > > +unlock:
> > > > >       spin_unlock(lock);
> > > > >
> > > > >       return ret;
> > > > > diff --git a/net/ipv4/inet_timewait_sock.c b/net/ipv4/inet_timewait_sock.c
> > > > > index 1d77d992e6e7..6d681ef52bb2 100644
> > > > > --- a/net/ipv4/inet_timewait_sock.c
> > > > > +++ b/net/ipv4/inet_timewait_sock.c
> > > > > @@ -91,10 +91,10 @@ void inet_twsk_put(struct inet_timewait_sock *tw)
> > > > >  }
> > > > >  EXPORT_SYMBOL_GPL(inet_twsk_put);
> > > > >
> > > > > -static void inet_twsk_add_node_rcu(struct inet_timewait_sock *tw,
> > > > > +static void inet_twsk_add_node_tail_rcu(struct inet_timewait_sock *tw,
> > > > >                                  struct hlist_nulls_head *list)
> > > >
> > > > nit: Please indent here.
> > > >
> > >
> > > Before I submitted the patch, I did the check through
> > > ./script/checkpatch.py and it outputted some information (no warning,
> > > no error) as you said.
> > > The reason I didn't change that is I would like to leave this part
> > > untouch as it used to be. I have no clue about whether I should send a
> > > v7 patch to adjust the format if necessary.
> >
> > checkpatch.pl does not check everything.  You will find most functions
> > under net/ipv4/*.c have same indentation in arguments.  I would recommend
> > enforcing such styles on editor like
> >
> 
> Well, there are two other lines which have the same indent problem.
> I'm going to clean them both up as below.
> 1) inet_twsk_add_bind_node()
> 2) inet_twsk_add_bind2_node()

The two functions are properly indented.
And even if they have problems, unrelated cleanups should not be mixed
up with a fix and be posted separately to net-next.


> 
> Thanks,
> Jason
> 
> >   $ cat ~/.emacs.d/init.el
> >   (setq-default c-default-style "linux")
> >
> > Thanks,
> > Kuniyuki
> >
> > >
> > > Thanks,
> > > Jason
> > >
> > > >
> > > > >  {
> > > > > -     hlist_nulls_add_head_rcu(&tw->tw_node, list);
> > > > > +     hlist_nulls_add_tail_rcu(&tw->tw_node, list);
> > > > >  }
> > > > >
> > > > >  static void inet_twsk_add_bind_node(struct inet_timewait_sock *tw,
> > > > > @@ -147,7 +147,7 @@ void inet_twsk_hashdance(struct inet_timewait_sock *tw, struct sock *sk,
> > > > >
> > > > >       spin_lock(lock);
> > > > >
> > > > > -     inet_twsk_add_node_rcu(tw, &ehead->chain);
> > > > > +     inet_twsk_add_node_tail_rcu(tw, &ehead->chain);
> > > > >
> > > > >       /* Step 3: Remove SK from hash chain */
> > > > >       if (__sk_nulls_del_node_init_rcu(sk))
> > > > > --
> > > > > 2.37.3

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