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

[Maxime Chevallier <maxime.chevallier@bootlin.com>]

On Fri, 21 Mar 2025 17:31:17 +0100
Paolo Abeni <pabeni@redhat.com> wrote:

> 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.

Indeed, maybe not the best place indeed. I'll address that, thanks for
pointing this out

> 
> >  		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.

That's true, not the prettiest piece of that patch. I'll refactor this
better then.

Thanks for the review,

Maxime