Subject: Re: [PATCH net-next 04/14] net_sched: sch_choke: implement lockless choke_dump()

From: Simon Horman <[email-masked]>
Date: Wed, 17 Apr 2024 15:44:21 +0100

On Wed, Apr 17, 2024 at 03:41:36PM +0200, Eric Dumazet wrote:
> On Wed, Apr 17, 2024 at 3:14 PM Simon Horman <[email-masked]> wrote:
> >
> > On Mon, Apr 15, 2024 at 01:20:44PM +0000, Eric Dumazet wrote:
> > > Instead of relying on RTNL, choke_dump() can use READ_ONCE()
> > > annotations, paired with WRITE_ONCE() ones in choke_change().
> > >
> > > Also use READ_ONCE(q->limit) in choke_enqueue() as the value
> > > could change from choke_change().
> >
> > Hi Eric,
> >
> > I'm wondering if you could expand on why q->limit needs this treatment
> > but not other fields, f.e. q->parms.qth_min (aka p->qth_min).
> 
> Other fields got their WRITE_ONCE() in red_set_parms()
> 
> +       WRITE_ONCE(p->qth_min, qth_min << Wlog);
> +       WRITE_ONCE(p->qth_max, qth_max << Wlog);
> +       WRITE_ONCE(p->Wlog, Wlog);
> +       WRITE_ONCE(p->Plog, Plog);

Thanks, understood.

I do wonder if other schedulers may need WRITE_ONCE() in
their enqueue() function. But I have not analysed this yet.

> > > Signed-off-by: Eric Dumazet <[email-masked]>
> > > ---
> > >  include/net/red.h     | 10 +++++-----
> > >  net/sched/sch_choke.c | 23 ++++++++++++-----------
> > >  2 files changed, 17 insertions(+), 16 deletions(-)
> > >
> > > diff --git a/include/net/red.h b/include/net/red.h
> >
> > ...
> >
> > > @@ -244,7 +244,7 @@ static inline void red_set_parms(struct red_parms *p,
> > >               max_P = red_maxp(Plog);
> > >               max_P *= delta; /* max_P = (qth_max - qth_min)/2^Plog */
> > >       }
> > > -     p->max_P = max_P;
> > > +     WRITE_ONCE(p->max_P, max_P);
> > >       max_p_delta = max_P / delta;
> > >       max_p_delta = max(max_p_delta, 1U);
> > >       p->max_P_reciprocal  = reciprocal_value(max_p_delta);
> >
> > A little further down in this function p->Scell_log is set.
> > I think it also needs the WRITE_ONCE() treatment as it
> > is read in choke_dump().
> >
> 
> I will squash in v2 the missing WRITE_ONCE(), thanks !

Likewise, thanks.
Your proposed update looks good to me.

> diff --git a/include/net/red.h b/include/net/red.h
> index 7daf7cf6130aeccf3d81a77600f4445759f174b7..802287d52c9e37e76ba9154539f511629e4b9780
> 100644
> --- a/include/net/red.h
> +++ b/include/net/red.h
> @@ -257,7 +257,7 @@ static inline void red_set_parms(struct red_parms *p,
>         p->target_min = qth_min + 2*delta;
>         p->target_max = qth_min + 3*delta;
> 
> -       p->Scell_log    = Scell_log;
> +       WRITE_ONCE(p->Scell_log, Scell_log);
>         p->Scell_max    = (255 << Scell_log);
> 
>         if (stab)
>