[Devel] [PATCH v2 rh7] vznetstat: fix statistics output when getting stats for same device in parallel

[Pavel Tikhomirov]

On 29.08.2023 18:01, Konstantin Khorenko wrote:
> Before this patch .get_stats() for venetdev was not defended from
> racing, Documentation/networking/netdevices.txt states ndo_get_stats()
> is to be syncronized by dev_base_lock rwlock, but we do READ stats, so
> can go in parallel.
> 
> At the same time the algorithm of gathering per-cpu stats into a single
> value used per-device structure, so in case .get_stats() is called in
> parallel, stats could be corrupted.
> 
> Example, 2 cpu node:
> 
> 	cpu A					cpu B
> .get_stats()
> memset(dev->...->stats, 0)
> stats.rx_bytes += cpuA.rx_bytes
> 					.get_stats()
> 					memset(dev->...->stats, 0)
> stats.rx_bytes += cpuB.rx_bytes
> return stats
> 					stats.rx_bytes += cpuA.rx_bytes
> 					stats.rx_bytes += cpuB.rx_bytes
> 					return stats
> 
> => process running on cpu A will print rx_bytes only taken from cpu B
> (so lower than expected), and the process running on cpu B will print
> too high numbers because cpu B values will be added twice.
> 
> Let's make venetdev provide ndo_get_stats64() callback which gets a
> storage as an argument, thus does not suffer from a race of filling
> a per-netdevice struct in parallel.
> 
> https://jira.vzint.dev/browse/PSBM-150027
> 

Reviewed-by: Pavel Tikhomirov <ptikhomirov at virtuozzo.com>

> Signed-off-by: Konstantin Khorenko <khorenko at virtuozzo.com>
> ---
> v2:
>    * get rid of extra function venet_stats_one()
>    * get rid of extra struct venet_device_stats and filling it
> 
>   drivers/net/venetdev.c | 20 ++++++++------------
>   include/linux/venet.h  |  1 -
>   2 files changed, 8 insertions(+), 13 deletions(-)
> 
> diff --git a/drivers/net/venetdev.c b/drivers/net/venetdev.c
> index 3ca4f8a49a5c..9785b9ba7247 100644
> --- a/drivers/net/venetdev.c
> +++ b/drivers/net/venetdev.c
> @@ -568,25 +568,21 @@ static int venet_xmit(struct sk_buff *skb, struct net_device *dev)
>   	return 0;
>   }
>   
> -static struct net_device_stats *get_stats(struct net_device *dev)
> +static void venet_get_stats64(struct net_device *dev,
> +			      struct rtnl_link_stats64 *total)
>   {
>   	int i;
> -	struct venet_stats *stats;
>   
> -	stats = (struct venet_stats *)dev->ml_priv;
> -	memset(&stats->stats, 0, sizeof(struct net_device_stats));
>   	for_each_possible_cpu(i) {
>   		struct net_device_stats *dev_stats;
>   
>   		dev_stats = venet_stats(dev, i);
> -		stats->stats.rx_bytes   += dev_stats->rx_bytes;
> -		stats->stats.tx_bytes   += dev_stats->tx_bytes;
> -		stats->stats.rx_packets += dev_stats->rx_packets;
> -		stats->stats.tx_packets += dev_stats->tx_packets;
> -		stats->stats.tx_dropped += dev_stats->tx_dropped;
> +		total->rx_bytes += dev_stats->rx_bytes;
> +		total->tx_bytes += dev_stats->tx_bytes;
> +		total->rx_packets += dev_stats->rx_packets;
> +		total->tx_packets += dev_stats->tx_packets;
> +		total->tx_dropped += dev_stats->tx_dropped;
>   	}
> -
> -	return &stats->stats;
>   }
>   
>   /* Initialize the rest of the LOOPBACK device. */
> @@ -712,7 +708,7 @@ static const struct ethtool_ops venet_ethtool_ops = {
>   
>   static const struct net_device_ops venet_netdev_ops = {
>   	.ndo_start_xmit = venet_xmit,
> -	.ndo_get_stats = get_stats,
> +	.ndo_get_stats64 = venet_get_stats64,
>   	.ndo_open = venet_open,
>   	.ndo_stop = venet_close,
>   	.ndo_init = venet_init_dev,
> diff --git a/include/linux/venet.h b/include/linux/venet.h
> index 51e0abeb03d7..f0a00832f6de 100644
> --- a/include/linux/venet.h
> +++ b/include/linux/venet.h
> @@ -21,7 +21,6 @@
>   struct ve_struct;
>   struct venet_stat;
>   struct venet_stats {
> -	struct net_device_stats	stats;
>   	struct net_device_stats	*real_stats;
>   };
>   

-- 
Best regards, Tikhomirov Pavel
Senior Software Developer, Virtuozzo.