Re: [PATCH v5 net] tcp: avoid the lookup process failing to get sk in ehash table

[Kuniyuki Iwashima]

From:   Jason Xing <kerneljasonxing@xxxxxxxxx>
Date:   Tue, 17 Jan 2023 10:15:45 +0800
> On Tue, Jan 17, 2023 at 12:54 AM Kuniyuki Iwashima <kuniyu@xxxxxxxxxx> wrote:
> >
> > From:   Jason Xing <kerneljasonxing@xxxxxxxxx>
> > Date:   Mon, 16 Jan 2023 18:33:41 +0800
> > > From: Jason Xing <kernelxing@xxxxxxxxxxx>
> > >
> > > 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@xxxxxxxxxx>
> > > Signed-off-by: Jason Xing <kernelxing@xxxxxxxxxxx>
> > > Link: https://lore.kernel.org/lkml/20230112065336.41034-1-kerneljasonxing@xxxxxxxxx/
> >
> > 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@xxxxxxxxxx>
> >
> 
> 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

  $ 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