Subject: Re: [PATCH net-next 09/14] net_sched: sch_fq_codel: implement lockless fq_codel_dump()

From: Simon Horman <[email-masked]>
Date: Wed, 17 Apr 2024 18:22:44 +0100

On Wed, Apr 17, 2024 at 07:14:10PM +0200, Eric Dumazet wrote:
> On Wed, Apr 17, 2024 at 7:07 PM Simon Horman <[email-masked]> wrote:
> >
> > On Mon, Apr 15, 2024 at 01:20:49PM +0000, Eric Dumazet wrote:
> > > Instead of relying on RTNL, fq_codel_dump() can use READ_ONCE()
> > > annotations, paired with WRITE_ONCE() ones in fq_codel_change().
> > >
> > > Signed-off-by: Eric Dumazet <[email-masked]>
> > > ---
> > >  net/sched/sch_fq_codel.c | 57 ++++++++++++++++++++++++----------------
> > >  1 file changed, 35 insertions(+), 22 deletions(-)
> > >
> > > diff --git a/net/sched/sch_fq_codel.c b/net/sched/sch_fq_codel.c
> >
> > ...
> >
> > > @@ -529,30 +539,33 @@ static int fq_codel_dump(struct Qdisc *sch, struct sk_buff *skb)
> > >               goto nla_put_failure;
> > >
> > >       if (nla_put_u32(skb, TCA_FQ_CODEL_TARGET,
> > > -                     codel_time_to_us(q->cparams.target)) ||
> > > +                     codel_time_to_us(READ_ONCE(q->cparams.target))) ||
> > >           nla_put_u32(skb, TCA_FQ_CODEL_LIMIT,
> > > -                     sch->limit) ||
> > > +                     READ_ONCE(sch->limit)) ||
> > >           nla_put_u32(skb, TCA_FQ_CODEL_INTERVAL,
> > > -                     codel_time_to_us(q->cparams.interval)) ||
> > > +                     codel_time_to_us(READ_ONCE(q->cparams.interval))) ||
> > >           nla_put_u32(skb, TCA_FQ_CODEL_ECN,
> > > -                     q->cparams.ecn) ||
> > > +                     READ_ONCE(q->cparams.ecn)) ||
> > >           nla_put_u32(skb, TCA_FQ_CODEL_QUANTUM,
> > > -                     q->quantum) ||
> > > +                     READ_ONCE(q->quantum)) ||
> > >           nla_put_u32(skb, TCA_FQ_CODEL_DROP_BATCH_SIZE,
> > > -                     q->drop_batch_size) ||
> > > +                     READ_ONCE(q->drop_batch_size)) ||
> > >           nla_put_u32(skb, TCA_FQ_CODEL_MEMORY_LIMIT,
> > > -                     q->memory_limit) ||
> > > +                     READ_ONCE(q->memory_limit)) ||
> > >           nla_put_u32(skb, TCA_FQ_CODEL_FLOWS,
> > > -                     q->flows_cnt))
> > > +                     READ_ONCE(q->flows_cnt)))
> >
> > Hi Eric,
> >
> > I think you missed the corresponding update for q->flows_cnt
> > in fq_codel_change().
> 
> q->flows_cnt is set at init time only, it can not change yet.

Sorry, I missed that important detail.

> Blindly using READ_ONCE() in a dump seems good hygiene,
> it is not needed yet, but does no harm.

Thanks, understood.

Reviewed-by: Simon Horman <[email-masked]>