Subject: Re: [PATCH net-next 09/14] net_sched: sch_fq_codel: implement lockless fq_codel_dump()

From: Simon Horman <[email-masked]>
Date: Wed, 17 Apr 2024 18:07:39 +0100

On Mon, Apr 15, 2024 at 01:20:49PM +0000, Eric Dumazet wrote:
> Instead of relying on RTNL, fq_codel_dump() can use READ_ONCE()
> annotations, paired with WRITE_ONCE() ones in fq_codel_change().
> 
> Signed-off-by: Eric Dumazet <[email-masked]>
> ---
>  net/sched/sch_fq_codel.c | 57 ++++++++++++++++++++++++----------------
>  1 file changed, 35 insertions(+), 22 deletions(-)
> 
> diff --git a/net/sched/sch_fq_codel.c b/net/sched/sch_fq_codel.c

...

> @@ -529,30 +539,33 @@ static int fq_codel_dump(struct Qdisc *sch, struct sk_buff *skb)
>  		goto nla_put_failure;
>  
>  	if (nla_put_u32(skb, TCA_FQ_CODEL_TARGET,
> -			codel_time_to_us(q->cparams.target)) ||
> +			codel_time_to_us(READ_ONCE(q->cparams.target))) ||
>  	    nla_put_u32(skb, TCA_FQ_CODEL_LIMIT,
> -			sch->limit) ||
> +			READ_ONCE(sch->limit)) ||
>  	    nla_put_u32(skb, TCA_FQ_CODEL_INTERVAL,
> -			codel_time_to_us(q->cparams.interval)) ||
> +			codel_time_to_us(READ_ONCE(q->cparams.interval))) ||
>  	    nla_put_u32(skb, TCA_FQ_CODEL_ECN,
> -			q->cparams.ecn) ||
> +			READ_ONCE(q->cparams.ecn)) ||
>  	    nla_put_u32(skb, TCA_FQ_CODEL_QUANTUM,
> -			q->quantum) ||
> +			READ_ONCE(q->quantum)) ||
>  	    nla_put_u32(skb, TCA_FQ_CODEL_DROP_BATCH_SIZE,
> -			q->drop_batch_size) ||
> +			READ_ONCE(q->drop_batch_size)) ||
>  	    nla_put_u32(skb, TCA_FQ_CODEL_MEMORY_LIMIT,
> -			q->memory_limit) ||
> +			READ_ONCE(q->memory_limit)) ||
>  	    nla_put_u32(skb, TCA_FQ_CODEL_FLOWS,
> -			q->flows_cnt))
> +			READ_ONCE(q->flows_cnt)))

Hi Eric,

I think you missed the corresponding update for q->flows_cnt
in fq_codel_change().

>  		goto nla_put_failure;
>  
> -	if (q->cparams.ce_threshold != CODEL_DISABLED_THRESHOLD) {
> +	ce_threshold = READ_ONCE(q->cparams.ce_threshold);
> +	if (ce_threshold != CODEL_DISABLED_THRESHOLD) {
>  		if (nla_put_u32(skb, TCA_FQ_CODEL_CE_THRESHOLD,
> -				codel_time_to_us(q->cparams.ce_threshold)))
> +				codel_time_to_us(ce_threshold)))
>  			goto nla_put_failure;
> -		if (nla_put_u8(skb, TCA_FQ_CODEL_CE_THRESHOLD_SELECTOR, q->cparams.ce_threshold_selector))
> +		if (nla_put_u8(skb, TCA_FQ_CODEL_CE_THRESHOLD_SELECTOR,
> +			       READ_ONCE(q->cparams.ce_threshold_selector)))
>  			goto nla_put_failure;
> -		if (nla_put_u8(skb, TCA_FQ_CODEL_CE_THRESHOLD_MASK, q->cparams.ce_threshold_mask))
> +		if (nla_put_u8(skb, TCA_FQ_CODEL_CE_THRESHOLD_MASK,
> +			       READ_ONCE(q->cparams.ce_threshold_mask)))
>  			goto nla_put_failure;
>  	}
>  
> -- 
> 2.44.0.683.g7961c838ac-goog
> 
>