Subject: Re: [PATCH] virtio_net: Use u64_stats_fetch_begin() for stats fetch

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 20 Jun 2024 11:52:36 -0400

On Thu, Jun 20, 2024 at 07:09:08AM -0700, Jakub Kicinski wrote:
> 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 <[email-masked]>
> > ---
> >  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.

I didn't realize there are any - it seems to be idempotent, no?

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