Subject: Re: [PATCHv4 net-next] bonding: add software tx timestamping support

From: Jakub Kicinski <[email-masked]>
Date: Fri, 14 Apr 2023 18:02:05 -0700

On Fri, 14 Apr 2023 16:35:26 +0800 Hangbin Liu wrote:
> v4: add ASSERT_RTNL to make sure bond_ethtool_get_ts_info() called via
>     RTNL. Only check _TX_SOFTWARE for the slaves.

> +	ASSERT_RTNL();
> +
>  	rcu_read_lock();
>  	real_dev = bond_option_active_slave_get_rcu(bond);
>  	dev_hold(real_dev);
> @@ -5707,10 +5713,36 @@ static int bond_ethtool_get_ts_info(struct net_device *bond_dev,
>  			ret = ops->get_ts_info(real_dev, info);
>  			goto out;
>  		}
> +	} else {
> +		/* Check if all slaves support software tx timestamping */
> +		rcu_read_lock();
> +		bond_for_each_slave_rcu(bond, slave, iter) {

> +			ret = -1;
> +			ops = slave->dev->ethtool_ops;
> +			phydev = slave->dev->phydev;
> +
> +			if (phy_has_tsinfo(phydev))
> +				ret = phy_ts_info(phydev, &ts_info);
> +			else if (ops->get_ts_info)
> +				ret = ops->get_ts_info(slave->dev, &ts_info);

My comment about this path being under rtnl was to point out that we
don't need the RCU protection to iterate over the slaves. This is 
a bit of a guess, I don't know bonding, but can we not use
bond_for_each_slave() ?

As a general rule we should let all driver callbacks sleep. Drivers 
may need to consult the FW or read something over a slow asynchronous
bus which requires process / non-atomic context. RCU lock puts us in 
an atomic context. And ->get_ts_info() is a driver callback.

It's not a deal breaker if we can't avoid RCU, but if we can - we should
let the drivers sleep. Sorry if I wasn't very clear previously.