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 16:59:09 +0100

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().

>  	if (tb[TCA_CODEL_ECN])
> -		q->params.ecn = !!nla_get_u32(tb[TCA_CODEL_ECN]);
> +		WRITE_ONCE(q->params.ecn,
> +			   !!nla_get_u32(tb[TCA_CODEL_ECN]));
>  
>  	qlen = sch->q.qlen;
>  	while (sch->q.qlen > sch->limit) {

...