netdev - Re: [PATCH V6 mlx5-next 11/16] RDMA/core: Add LAG functionality

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Message-ID: <20200428231525.GY13640@mellanox.com>
Date:   Tue, 28 Apr 2020 20:15:25 -0300
From:   Jason Gunthorpe <jgg@...lanox.com>
To:     Maor Gottlieb <maorg@...lanox.com>
Cc:     davem@...emloft.net, dledford@...hat.com, j.vosburgh@...il.com,
        vfalico@...il.com, andy@...yhouse.net, kuba@...nel.org,
        jiri@...lanox.com, dsahern@...nel.org, leonro@...lanox.com,
        saeedm@...lanox.com, linux-rdma@...r.kernel.org,
        netdev@...r.kernel.org, alexr@...lanox.com
Subject: Re: [PATCH V6 mlx5-next 11/16] RDMA/core: Add LAG functionality

On Sun, Apr 26, 2020 at 10:17:12AM +0300, Maor Gottlieb wrote:
> +int rdma_lag_get_ah_roce_slave(struct ib_device *device,
> +			       struct rdma_ah_attr *ah_attr,
> +			       struct net_device **xmit_slave)

Please do not use ** and also return int. The function should return
net_device directly and use ERR_PTR() 

> +{
> +	struct net_device *master;
> +	struct net_device *slave;
> +	int err = 0;
> +
> +	*xmit_slave = NULL;
> +	if (!(ah_attr->type == RDMA_AH_ATTR_TYPE_ROCE &&
> +	      ah_attr->grh.sgid_attr->gid_type == IB_GID_TYPE_ROCE_UDP_ENCAP))
> +		return 0;
> +
> +	rcu_read_lock();
> +	master = rdma_read_gid_attr_ndev_rcu(ah_attr->grh.sgid_attr);
> +	if (IS_ERR(master)) {
> +		err = PTR_ERR(master);
> +		goto unlock;
> +	}
> +	dev_hold(master);

What is the point of this dev_hold? This whole thing is under
rcu_read_lock()

> +
> +	if (!netif_is_bond_master(master))
> +		goto put;
> +
> +	slave = rdma_get_xmit_slave_udp(device, master, ah_attr);

IMHO it is probably better to keep with the dev_hold and drop the RCU
while doing rdma_build_skb so that the allocation in here doesn't have
to be atomic. This isn't performance sensitive so the extra atomic for
the dev_hold is better than the unnecessary GFP_ATOMIC allocation

> +	if (!slave) {
> +		ibdev_warn(device, "Failed to get lag xmit slave\n");
> +		err =  -EINVAL;
> +		goto put;
> +	}
> +
> +	dev_hold(slave);

And I think the dev_hold should be in the rdma_get_xmit_slave_udp() as
things called 'get' really ought to return with references.

Jason