Re: [PATCH net-next v3 1/7] net: ethtool: netlink: Allow per-netdevice DUMP operations

[Paolo Abeni <pabeni@redhat.com>]

On 3/13/25 7:26 PM, Maxime Chevallier wrote:
> We have a number of netlink commands in the ethnl family that may have
> multiple objects to dump even for a single net_device, including :
> 
>  - PLCA, PSE-PD, phy: one message per PHY device
>  - tsinfo: one message per timestamp source (netdev + phys)
>  - rss: One per RSS context
> 
> To get this behaviour, these netlink commands need to roll a custom
> ->dumpit().
> 
> To prepare making per-netdev DUMP more generic in ethnl, introduce a
> member in the ethnl ops to indicate if a given command may allow
> pernetdev DUMPs (also referred to as filtered DUMPs).
> 
> Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
> ---
>  net/ethtool/netlink.c | 45 +++++++++++++++++++++++++++++--------------
>  net/ethtool/netlink.h |  2 ++
>  2 files changed, 33 insertions(+), 14 deletions(-)
> 
> diff --git a/net/ethtool/netlink.c b/net/ethtool/netlink.c
> index a163d40c6431..7adede5e4ff1 100644
> --- a/net/ethtool/netlink.c
> +++ b/net/ethtool/netlink.c
> @@ -587,21 +587,38 @@ static int ethnl_default_dumpit(struct sk_buff *skb,
>  	int ret = 0;
>  
>  	rcu_read_lock();

Maintain the RCU read lock here is IMHO confusing...

> -	for_each_netdev_dump(net, dev, ctx->pos_ifindex) {
> -		dev_hold(dev);
> +	if (ctx->req_info->dev) {
> +		dev = ctx->req_info->dev;

.. as this is refcounted.

I suggest to move the rcu_read_lock inside the if.


>  		rcu_read_unlock();
> +		/* Filtered DUMP request targeted to a single netdev. We already
> +		 * hold a ref to the netdev from ->start()
> +		 */
> +		ret = ethnl_default_dump_one(skb, dev, ctx,
> +					     genl_info_dump(cb));
> +		rcu_read_lock();
> +		netdev_put(ctx->req_info->dev, &ctx->req_info->dev_tracker);
>  
> -		ret = ethnl_default_dump_one(skb, dev, ctx, genl_info_dump(cb));
> +		if (ret < 0 && ret != -EOPNOTSUPP && likely(skb->len))
> +			ret = skb->len;
>  
> -		rcu_read_lock();
> -		dev_put(dev);
> +	} else {
> +		for_each_netdev_dump(net, dev, ctx->pos_ifindex) {
> +			dev_hold(dev);
> +			rcu_read_unlock();
> +
> +			ret = ethnl_default_dump_one(skb, dev, ctx,
> +						     genl_info_dump(cb));
> +
> +			rcu_read_lock();
> +			dev_put(dev);
>  
> -		if (ret < 0 && ret != -EOPNOTSUPP) {
> -			if (likely(skb->len))
> -				ret = skb->len;
> -			break;
> +			if (ret < 0 && ret != -EOPNOTSUPP) {
> +				if (likely(skb->len))
> +					ret = skb->len;

IMHO a bit too many levels of indentation. It's possibly better to move
this code in a separate helper.

Thanks,

Paolo