netdev - Re: [PATCH net] inet: Avoid established lookup missing active sk

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Message-ID: <CANn89i+AMitXNYNo6h7GPBHsk2sVSbmH-8BQLRFDeihEd9-oQQ@mail.gmail.com>
Date: Wed, 3 Sep 2025 01:35:08 -0700
From: Eric Dumazet <edumazet@...gle.com>
To: Jason Xing <kerneljasonxing@...il.com>
Cc: Xuanqiang Luo <xuanqiang.luo@...ux.dev>, kuniyu@...gle.com, davem@...emloft.net, 
	kuba@...nel.org, kernelxing@...cent.com, netdev@...r.kernel.org, 
	Xuanqiang Luo <luoxuanqiang@...inos.cn>
Subject: Re: [PATCH net] inet: Avoid established lookup missing active sk

On Tue, Sep 2, 2025 at 11:53 PM Jason Xing <kerneljasonxing@...il.com> wrote:
>
> On Wed, Sep 3, 2025 at 2:40 PM Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > On Tue, Sep 2, 2025 at 7:46 PM Xuanqiang Luo <xuanqiang.luo@...ux.dev> wrote:
> > >
> > > From: Xuanqiang Luo <luoxuanqiang@...inos.cn>
> > >
> > > Since the lookup of sk in ehash is lockless, when one CPU is performing a
> > > lookup while another CPU is executing delete and insert operations
> > > (deleting reqsk and inserting sk), the lookup CPU may miss either of
> > > them, if sk cannot be found, an RST may be sent.
> > >
> > > The call trace map is drawn as follows:
> > >    CPU 0                           CPU 1
> > >    -----                           -----
> > >                                 spin_lock()
> > >                                 sk_nulls_del_node_init_rcu(osk)
> > > __inet_lookup_established()
> > >                                 __sk_nulls_add_node_rcu(sk, list)
> > >                                 spin_unlock()
> > >
> > > We can try using spin_lock()/spin_unlock() to wait for ehash updates
> > > (ensuring all deletions and insertions are completed) after a failed
> > > lookup in ehash, then lookup sk again after the update. Since the sk
> > > expected to be found is unlikely to encounter the aforementioned scenario
> > > multiple times consecutively, we only need one update.
> >
> > No need for a lock really...
> > - add the new node (with a temporary 'wrong' nulls value),
> > - delete the old node
> > - replace the nulls value by the expected one.
>
> Yes. The plan is simple enough to fix this particular issue and I
> verified in production long ago. Sadly the following patch got
> reverted...
> https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net.git/commit/?id=3f4ca5fafc08881d7a57daa20449d171f2887043

Please read again what I wrote, and compare it to your old patch.

- add the new node (with a temporary 'wrong' nulls value),
- delete the old node
- replace the nulls value by the expected one.

Can you see a difference ?