Re: [PATCH v4 net] tcp: avoid the lookup process failing to get sk in ehash table

[Jason Xing]

On Mon, Jan 16, 2023 at 5:54 PM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>
> On Mon, Jan 16, 2023 at 8:38 AM Jason Xing <kerneljasonxing@xxxxxxxxx> wrote:
> >
> > 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.
> >
> >
> > 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/
> > ---
> > 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    | 18 ++++++++++++++++--
> >  net/ipv4/inet_timewait_sock.c |  6 +++---
> >  2 files changed, 19 insertions(+), 5 deletions(-)
> >
> > diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
> > index 24a38b56fab9..c64eec874b31 100644
> > --- a/net/ipv4/inet_hashtables.c
> > +++ b/net/ipv4/inet_hashtables.c
> > @@ -650,8 +650,21 @@ 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) {
> > +               if (sk_hashed(osk)) {
> > +                       /* 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);
> > +               } else {
> > +                       ret = false;
>
>
> Well, you added another 'else' statement...
>

Yeah, I want to make the code look more concise and easy to read. I
alway felt the previous series of commits are not human-readable
though it could work.

> What about the following ?
>
> diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
> index 24a38b56fab9e9d7d893e23b30d26e275359ec70..1bcf5ce8dd1317b2144bcb47a2ad238532b9accf
> 100644
> --- a/net/ipv4/inet_hashtables.c
> +++ b/net/ipv4/inet_hashtables.c
> @@ -650,8 +650,14 @@ 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) {
> +                       __sk_nulls_add_node_tail_rcu(sk, list);
> +                       sk_nulls_del_node_init_rcu(osk);
> +               }

Ah, I prefer this one :)

Thanks,
Jason

> +               goto unlock;
> +       }
> +       if (found_dup_sk) {
>                 *found_dup_sk = inet_ehash_lookup_by_sk(sk, list);
>                 if (*found_dup_sk)
>                         ret = false;
> @@ -659,7 +665,7 @@ 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;