netdev - Re: [PATCH] virtio_net: Use u64_stats_fetch

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Date: Thu, 20 Jun 2024 07:09:08 -0700
From: Jakub Kicinski <kuba@...nel.org>
To: Li RongQing <lirongqing@...du.com>
Cc: mst@...hat.com, jasowang@...hat.com, xuanzhuo@...ux.alibaba.com,
 eperezma@...hat.com, hengqi@...ux.alibaba.com,
 virtualization@...ts.linux.dev, netdev@...r.kernel.org
Subject: Re: [PATCH] virtio_net: Use u64_stats_fetch_begin() for stats fetch

On Wed, 19 Jun 2024 10:55:29 +0800 Li RongQing wrote:
> This place is fetching the stats, so u64_stats_fetch_begin
> and u64_stats_fetch_retry should be used
> 
> Fixes: 6208799553a8 ("virtio-net: support rx netdim")
> Signed-off-by: Li RongQing <lirongqing@...du.com>
> ---
>  drivers/net/virtio_net.c | 14 ++++++++------
>  1 file changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 61a57d1..b669e73 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -2332,16 +2332,18 @@ static void virtnet_poll_cleantx(struct receive_queue *rq)
>  static void virtnet_rx_dim_update(struct virtnet_info *vi, struct receive_queue *rq)
>  {
>  	struct dim_sample cur_sample = {};
> +	unsigned int start;
>  
>  	if (!rq->packets_in_napi)
>  		return;
>  
> -	u64_stats_update_begin(&rq->stats.syncp);
> -	dim_update_sample(rq->calls,
> -			  u64_stats_read(&rq->stats.packets),
> -			  u64_stats_read(&rq->stats.bytes),
> -			  &cur_sample);
> -	u64_stats_update_end(&rq->stats.syncp);
> +	do {
> +		start = u64_stats_fetch_begin(&rq->stats.syncp);
> +		dim_update_sample(rq->calls,
> +				u64_stats_read(&rq->stats.packets),
> +				u64_stats_read(&rq->stats.bytes),
> +				&cur_sample);
> +	} while (u64_stats_fetch_retry(&rq->stats.syncp, start));

Did you by any chance use an automated tool of any sort to find this
issue or generate the fix?

I don't think this is actually necessary here, you're in the same
context as the updater of the stats, you don't need any protection.
You can remove u64_stats_update_begin() / end() (in net-next, there's
no bug).

I won't comment on implications of calling dim_update_sample() in 
a loop.

Please make sure you answer my "did you use a tool" question, I'm
really curious.
-- 
pw-bot: cr