netdev - Re: [PATCH net-next 01/14] net_sched: sch_fq: implement lockless fq

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Message-ID: <CANn89i+X3zkk-RwRVuMursG-RY+R6P29AWK-pjjVuNKT91VsJw@mail.gmail.com>
Date: Tue, 16 Apr 2024 20:33:58 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Simon Horman <horms@...nel.org>
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 01/14] net_sched: sch_fq: implement lockless fq_dump()

On Tue, Apr 16, 2024 at 8:19 PM Simon Horman <horms@...nel.org> wrote:
>
> On Mon, Apr 15, 2024 at 01:20:41PM +0000, Eric Dumazet wrote:
> > Instead of relying on RTNL, fq_dump() can use READ_ONCE()
> > annotations, paired with WRITE_ONCE() in fq_change()
> >
> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> > ---
> >  net/sched/sch_fq.c | 96 +++++++++++++++++++++++++++++-----------------
> >  1 file changed, 60 insertions(+), 36 deletions(-)
> >
> > diff --git a/net/sched/sch_fq.c b/net/sched/sch_fq.c
> > index cdf23ff16f40bf244bb822e76016fde44e0c439b..934c220b3f4336dc2f70af74d7758218492b675d 100644
> > --- a/net/sched/sch_fq.c
> > +++ b/net/sched/sch_fq.c
> > @@ -888,7 +888,7 @@ static int fq_resize(struct Qdisc *sch, u32 log)
> >               fq_rehash(q, old_fq_root, q->fq_trees_log, array, log);
> >
> >       q->fq_root = array;
> > -     q->fq_trees_log = log;
> > +     WRITE_ONCE(q->fq_trees_log, log);
> >
> >       sch_tree_unlock(sch);
> >
> > @@ -931,7 +931,7 @@ static void fq_prio2band_compress_crumb(const u8 *in, u8 *out)
> >
> >       memset(out, 0, num_elems / 4);
> >       for (i = 0; i < num_elems; i++)
> > -             out[i / 4] |= in[i] << (2 * (i & 0x3));
> > +             out[i / 4] |= READ_ONCE(in[i]) << (2 * (i & 0x3));
> >  }
> >
>
> Hi Eric,
>
> I am a little unsure about the handling of q->prio2band in this patch.
>
> It seems to me that fq_prio2band_compress_crumb() is used to
> to store values in q->prio2band, and is called (indirectly)
> from fq_change() (and directly from fq_init()).
>
> While fq_prio2band_decompress_crumb() is used to read values
> from q->prio2band, and is called from fq_dump().
>
> So I am wondering if should use WRITE_ONCE() when storing elements
> of out. And fq_prio2band_decompress_crumb should use READ_ONCE when
> reading elements of in.

Yeah, you are probably right, I recall being a bit lazy on this part,
thanks !