Re: [PATCH net-next v2 1/8] ethtool: Add support for configuring frame preemption

On Mon, Jan 18, 2021 at 04:40:21PM -0800, Vinicius Costa Gomes wrote:

+int ethnl_set_preempt(struct sk_buff *skb, struct genl_info *info)
+{
+	struct ethnl_req_info req_info = {};
+	struct nlattr **tb = info->attrs;
+	struct ethtool_fp preempt = {};
+	struct net_device *dev;
+	bool mod = false;
+	int ret;
+
+	ret = ethnl_parse_header_dev_get(&req_info,
+					 tb[ETHTOOL_A_PREEMPT_HEADER],
+					 genl_info_net(info), info->extack,
+					 true);
+	if (ret < 0)
+		return ret;
+	dev = req_info.dev;
+	ret = -EOPNOTSUPP;
+	if (!dev->ethtool_ops->get_preempt ||
+	    !dev->ethtool_ops->set_preempt)
+		goto out_dev;
+
+	rtnl_lock();

I'm a bit of a noob when it comes to ethtool (netlink or otherwise).
Why do you take the rtnl_mutex when updating some purely hardware
values, what netdev state is there to protect? Can this get->modify->set
sequence be serialized using other locking mechanism than rtnl_mutex?

+	ret = ethnl_ops_begin(dev);
+	if (ret < 0)
+		goto out_rtnl;
+
+	ret = dev->ethtool_ops->get_preempt(dev, &preempt);
+	if (ret < 0) {
+		GENL_SET_ERR_MSG(info, "failed to retrieve frame preemption settings");
+		goto out_ops;
+	}
+
+	ethnl_update_u8(&preempt.enabled,
+			tb[ETHTOOL_A_PREEMPT_ENABLED], &mod);
+	ethnl_update_u32(&preempt.add_frag_size,
+			 tb[ETHTOOL_A_PREEMPT_ADD_FRAG_SIZE], &mod);
+	ret = 0;
+	if (!mod)
+		goto out_ops;
+
+	ret = dev->ethtool_ops->set_preempt(dev, &preempt, info->extack);
+	if (ret < 0) {
+		GENL_SET_ERR_MSG(info, "frame preemption settings update failed");
+		goto out_ops;
+	}
+
+	ethtool_notify(dev, ETHTOOL_MSG_PREEMPT_NTF, NULL);
+
+out_ops:
+	ethnl_ops_complete(dev);
+out_rtnl:
+	rtnl_unlock();
+out_dev:
+	dev_put(dev);
+	return ret;
+}