netdev - Re: [PATCH net-next 07/14] net_sched: sch_ets: implement lockless ets

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Message-ID: <20240417171728.GG2320920@kernel.org>
Date: Wed, 17 Apr 2024 18:17:28 +0100
From: Simon Horman <horms@...nel.org>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "David S. Miller" <davem@...emloft.net>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Jamal Hadi Salim <jhs@...atatu.com>,
	Cong Wang <xiyou.wangcong@...il.com>, Jiri Pirko <jiri@...nulli.us>,
	netdev@...r.kernel.org, eric.dumazet@...il.com
Subject: Re: [PATCH net-next 07/14] net_sched: sch_ets: implement lockless
 ets_dump()

On Wed, Apr 17, 2024 at 07:08:17PM +0200, Eric Dumazet wrote:
> On Wed, Apr 17, 2024 at 6:54 PM Simon Horman <horms@...nel.org> wrote:
> >
> > On Mon, Apr 15, 2024 at 01:20:47PM +0000, Eric Dumazet wrote:
> > > Instead of relying on RTNL, ets_dump() can use READ_ONCE()
> > > annotations, paired with WRITE_ONCE() ones in ets_change().
> > >
> > > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> > > ---
> > >  net/sched/sch_ets.c | 25 ++++++++++++++-----------
> > >  1 file changed, 14 insertions(+), 11 deletions(-)
> > >
> > > diff --git a/net/sched/sch_ets.c b/net/sched/sch_ets.c
> > > index 835b4460b44854a803d3054e744702022b7551f4..f80bc05d4c5a5050226e6cfd30fa951c0b61029f 100644
> > > --- a/net/sched/sch_ets.c
> > > +++ b/net/sched/sch_ets.c
> >
> > ...
> >
> > > @@ -658,11 +658,11 @@ static int ets_qdisc_change(struct Qdisc *sch, struct nlattr *opt,
> > >                       list_del(&q->classes[i].alist);
> > >               qdisc_tree_flush_backlog(q->classes[i].qdisc);
> > >       }
> > > -     q->nstrict = nstrict;
> > > +     WRITE_ONCE(q->nstrict, nstrict);
> > >       memcpy(q->prio2band, priomap, sizeof(priomap));
> >
> > Hi Eric,
> >
> > I think that writing elements of q->prio2band needs WRITE_ONCE() treatment too.
> 
> Not really, these are bytes, a cpu will not write over bytes one bit at a time.
> 
> I could add WRITE_ONCE(), but this is overkill IMO.

Thanks, armed with that understanding I'm now happy with this patch.

Reviewed-by: Simon Horman <horms@...nel.org>

> > >       for (i = 0; i < q->nbands; i++)
> > > -             q->classes[i].quantum = quanta[i];
> > > +             WRITE_ONCE(q->classes[i].quantum, quanta[i]);
> > >
> > >       for (i = oldbands; i < q->nbands; i++) {
> > >               q->classes[i].qdisc = queues[i];
> >
> > ...
> >
> > > @@ -733,6 +733,7 @@ static int ets_qdisc_dump(struct Qdisc *sch, struct sk_buff *skb)
> > >       struct ets_sched *q = qdisc_priv(sch);
> > >       struct nlattr *opts;
> > >       struct nlattr *nest;
> > > +     u8 nbands, nstrict;
> > >       int band;
> > >       int prio;
> > >       int err;
> >
> > The next few lines of this function are:
> >
> >         err = ets_offload_dump(sch);
> >         if (err)
> >                 return err;
> >
> > Where ets_offload_dump may indirectly call ndo_setup_tc().
> > And I am concerned that ndo_setup_tc() expects RTNL to be held,
> > although perhaps that assumption is out of date.
> 
> Thanks, we will add rtnl locking later only in the helper,
> or make sure it can run under RCU.
> 
> Note the patch series does not yet remove RTNL locking.

Yes, I understand that.
I was more flagging this as something that needs to be addressed.
Sorry for not being clearer.

> Clearly, masking and setting TCQ_F_OFFLOADED in sch->flags in a dump
> operation is not very nice IMO.

Yes, I noticed that too.

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