netdev - Re: [PATCH v2 4/5] drivers: net: xgene: fix statistics counters race condition

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Date:	Fri, 13 May 2016 18:41:44 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Iyappan Subramanian <isubramanian@....com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, patches@....com,
	toanle@....com
Subject: Re: [PATCH v2 4/5] drivers: net: xgene: fix statistics counters
 race condition

On Fri, 2016-05-13 at 16:53 -0700, Iyappan Subramanian wrote:
> This patch fixes the race condition on updating the statistics
> counters by moving the counters to the ring structure.
> 

...

> @@ -1127,12 +1127,31 @@ static struct rtnl_link_stats64 *xgene_enet_get_stats64(
>  {
>  	struct xgene_enet_pdata *pdata = netdev_priv(ndev);
>  	struct rtnl_link_stats64 *stats = &pdata->stats;
> +	struct xgene_enet_desc_ring *ring;
> +	int i;
> +
> +	memset(stats, 0, sizeof(struct rtnl_link_stats64));

The memset() should already be done in the caller.

> +	for (i = 0; i < pdata->txq_cnt; i++) {
> +		ring = pdata->tx_ring[i];
> +		if (ring) {
> +			stats->tx_packets += ring->tx_packets;
> +			stats->tx_bytes += ring->tx_bytes;
> +		}
> +	}
>  
> -	stats->rx_errors += stats->rx_length_errors +
> -			    stats->rx_crc_errors +
> -			    stats->rx_frame_errors +
> -			    stats->rx_fifo_errors;


Right, this piece was completely buggy.

Not a race, but rx_errors was growing every time stats were fetched on
this device.


stats->rx_errors =   sum(...)    was probably the intent.



> -	memcpy(storage, &pdata->stats, sizeof(struct rtnl_link_stats64));
> +	for (i = 0; i < pdata->rxq_cnt; i++) {
> +		ring = pdata->rx_ring[i];
> +		if (ring) {
> +			stats->rx_packets += ring->rx_packets;
> +			stats->rx_bytes += ring->rx_bytes;
> +			stats->rx_errors += ring->rx_length_errors +
> +				ring->rx_crc_errors +
> +				ring->rx_frame_errors +
> +				ring->rx_fifo_errors;
> +			stats->rx_dropped += ring->rx_dropped;
> +		}
> +	}
> +	memcpy(storage, stats, sizeof(struct rtnl_link_stats64));
>  
>  	return storage;
>  }
> diff --git a/drivers/net/ethernet/apm/xgene/xgene_enet_main.h b/drivers/net/ethernet/apm/xgene/xgene_enet_main.h
> index cc40c30..092fbec 100644
> --- a/drivers/net/ethernet/apm/xgene/xgene_enet_main.h
> +++ b/drivers/net/ethernet/apm/xgene/xgene_enet_main.h
> @@ -121,6 +121,16 @@ struct xgene_enet_desc_ring {
>  		struct xgene_enet_raw_desc16 *raw_desc16;
>  	};
>  	__le64 *exp_bufs;
> +	u64 tx_packets;
> +	u64 tx_bytes;
> +	u64 rx_packets;
> +	u64 rx_bytes;
> +	u64 rx_dropped;
> +	u64 rx_errors;
> +	u64 rx_length_errors;
> +	u64 rx_crc_errors;
> +	u64 rx_frame_errors;
> +	u64 rx_fifo_errors;

Note that there is still a race on 32bit arches.

It seems this driver should really use "unsigned long" counters and
provide standard ndo_get_stats()