netdev - Re: [PATCH net-next v2 2/3] net: rps: protect filter locklessly

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Message-ID: <CAL+tcoDzUn=Qe9=Ar+VUPt1vgGvSAh+KBHZRO30yKPGwP87VDg@mail.gmail.com>
Date: Wed, 17 Apr 2024 20:20:47 +0800
From: Jason Xing <kerneljasonxing@...il.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: kuba@...nel.org, pabeni@...hat.com, davem@...emloft.net, horms@...nel.org, 
	netdev@...r.kernel.org, Jason Xing <kernelxing@...cent.com>
Subject: Re: [PATCH net-next v2 2/3] net: rps: protect filter locklessly

On Wed, Apr 17, 2024 at 8:14 PM Jason Xing <kerneljasonxing@...il.com> wrote:
>
> On Wed, Apr 17, 2024 at 7:58 PM Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > On Wed, Apr 17, 2024 at 1:52 PM Eric Dumazet <edumazet@...gle.com> wrote:
> > >
> > > @@ -4668,7 +4668,7 @@ bool rps_may_expire_flow(struct net_device *dev,
> > > u16 rxq_index,
> > >                 cpu = READ_ONCE(rflow->cpu);
> > >                 if (rflow->filter == filter_id && cpu < nr_cpu_ids &&
> > >                     ((int)(READ_ONCE(per_cpu(softnet_data,
> > > cpu).input_queue_head) -
> > > -                          READ_ONCE(rflow->last_qtail)) <
> > > +                          rflow->last_qtail) <
> > >                      (int)(10 * flow_table->mask)))
> > >                         expire = false;
> > >         }
> >
> > Oh well, rps_may_expire_flow() might be called from other contexts, so
> > only the  READ_ONCE()
> > from get_rps_cpu() is not really necessary.
>
> Thanks for telling me the access logic about qtail in the previous email.
>
> Yes, I'm writing exactly what you're saying now :) I can keep
> protecting rflow->cpu and rflow->filter locklessly.
>

I can keep these three patches just like now only without that
READ_ONCE(), I have to update my statement.

[...]
> I can remove the unneeded annotations around qtail as you suggested
> with those two patches if I can, or you can submit it first. It's up
> to you :)

The 'qtail' also needs protection. What I was saying is not true.

>
> Thanks,
> Jason