netdev - Re: [PATCH net-next 05/14] net_sched: sch_codel: implement lockless codel

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Message-ID: <20240417155909.GZ2320920@kernel.org>
Date: Wed, 17 Apr 2024 16:59:09 +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 05/14] net_sched: sch_codel: implement lockless
 codel_dump()

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 <edumazet@...gle.com>
> ---
>  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) {

...