Subject: Re: [PATCH net-next 05/14] net_sched: sch_codel: implement lockless codel_dump()

From: Simon Horman <[email-masked]>
Date: Wed, 17 Apr 2024 17:21:48 +0100

On Wed, Apr 17, 2024 at 06:05:46PM +0200, Eric Dumazet wrote:
> On Wed, Apr 17, 2024 at 5:59 PM Simon Horman <[email-masked]> wrote:
> >
> > On Mon, Apr 15, 2024 at 01:20:45PM +0000, Eric Dumazet wrote:
> > > Instead of relying on RTNL, codel_dump() can use READ_ONCE()
> > > annotations, paired with WRITE_ONCE() ones in codel_change().
> > >
> > > Signed-off-by: Eric Dumazet <[email-masked]>
> > > ---
> > >  net/sched/sch_codel.c | 29 ++++++++++++++++++-----------
> > >  1 file changed, 18 insertions(+), 11 deletions(-)
> > >
> > > diff --git a/net/sched/sch_codel.c b/net/sched/sch_codel.c
> > > index ecb3f164bb25b33bd662c8ee07dc1b5945fd882d..3e8d4fe4d91e3ef2b7715640f6675aa5e8e2a326 100644
> > > --- a/net/sched/sch_codel.c
> > > +++ b/net/sched/sch_codel.c
> > > @@ -118,26 +118,31 @@ static int codel_change(struct Qdisc *sch, struct nlattr *opt,
> > >       if (tb[TCA_CODEL_TARGET]) {
> > >               u32 target = nla_get_u32(tb[TCA_CODEL_TARGET]);
> > >
> > > -             q->params.target = ((u64)target * NSEC_PER_USEC) >> CODEL_SHIFT;
> > > +             WRITE_ONCE(q->params.target,
> > > +                        ((u64)target * NSEC_PER_USEC) >> CODEL_SHIFT);
> > >       }
> > >
> > >       if (tb[TCA_CODEL_CE_THRESHOLD]) {
> > >               u64 val = nla_get_u32(tb[TCA_CODEL_CE_THRESHOLD]);
> > >
> > > -             q->params.ce_threshold = (val * NSEC_PER_USEC) >> CODEL_SHIFT;
> > > +             WRITE_ONCE(q->params.ce_threshold,
> > > +                        (val * NSEC_PER_USEC) >> CODEL_SHIFT);
> > >       }
> > >
> > >       if (tb[TCA_CODEL_INTERVAL]) {
> > >               u32 interval = nla_get_u32(tb[TCA_CODEL_INTERVAL]);
> > >
> > > -             q->params.interval = ((u64)interval * NSEC_PER_USEC) >> CODEL_SHIFT;
> > > +             WRITE_ONCE(q->params.interval,
> > > +                        ((u64)interval * NSEC_PER_USEC) >> CODEL_SHIFT);
> > >       }
> > >
> > >       if (tb[TCA_CODEL_LIMIT])
> > > -             sch->limit = nla_get_u32(tb[TCA_CODEL_LIMIT]);
> > > +             WRITE_ONCE(sch->limit,
> > > +                        nla_get_u32(tb[TCA_CODEL_LIMIT]));
> > >
> >
> > Hi Eric,
> >
> > Sorry to be so bothersome.
> >
> > As a follow-up to our discussion of patch 4/14 (net_choke),
> > I'm wondering if reading sch->limit in codel_qdisc_enqueue()
> > should be updated to use READ_ONCE().
> 
> No worries !
> 
> A READ_ONCE() in codel_qdisc_enqueue() is not needed
> because codel_change() writes  all the fields  under the protection of
> qdisc spinlock.
> 
> sch_tree_lock() / ... / sch_tree_unlock()
> 
> Note that I removed the READ_ONCE() in choke enqueue() in V2,
> for the same reason.

Perfect, now I understand.

With that cleared up, I'm happy with (my understanding of) this patch.

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