Subject: Re: [PATCH] netdev: stats on multiqueue possible bug

From: Eric Dumazet <[email-masked]>
Date: Sun, 20 Sep 2009 09:24:08 +0200

Stephen Hemminger a écrit :
> Dave, I think you were trying to optimize something here that
> doesn't need optimization.
> 
> If transmit stats (all) wrap to zero, then the stats would not
> be set correctly. Move local variable into loop as well.
> 
> Signed-off-by: Stephen Hemminger <[email-masked]>
> 
> --- a/net/core/dev.c	2009-09-19 15:19:13.902495560 -0700
> +++ b/net/core/dev.c	2009-09-19 15:48:21.126458728 -0700
> @@ -5079,19 +5079,19 @@ const struct net_device_stats *dev_get_s
>  		unsigned long tx_bytes = 0, tx_packets = 0, tx_dropped = 0;
>  		struct net_device_stats *stats = &dev->stats;
>  		unsigned int i;
> -		struct netdev_queue *txq;
>  
>  		for (i = 0; i < dev->num_tx_queues; i++) {
> -			txq = netdev_get_tx_queue(dev, i);
> +			const struct netdev_queue *txq
> +				 = netdev_get_tx_queue(dev, i);
>  			tx_bytes   += txq->tx_bytes;
>  			tx_packets += txq->tx_packets;
>  			tx_dropped += txq->tx_dropped;
>  		}
> -		if (tx_bytes || tx_packets || tx_dropped) {
> -			stats->tx_bytes   = tx_bytes;
> -			stats->tx_packets = tx_packets;
> -			stats->tx_dropped = tx_dropped;
> -		}
> +
> +		stats->tx_bytes   = tx_bytes;
> +		stats->tx_packets = tx_packets;
> +		stats->tx_dropped = tx_dropped;
> +
>  		return stats;
>  	}
>  }

Most devices dont update txq->tx_bytes/tx_packets/tx_dropped yet, but
still update their device->stats

Your patch makes these devices clearing their stats.

In the case all stats wrap to zero, we'll give old values. If you think this is 
a bug, you should find another way to fix it :)