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

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Date:   Tue, 28 Apr 2020 20:30:09 -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 Tue, Apr 28, 2020 at 08:15:25PM -0300, Jason Gunthorpe wrote:
> 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

Though if you do this be mindful that the create_ah call site is
conditionally non-sleeping, the best thing to do would be to make the
GFP_ATOMIC conditional on !RDMA_CREATE_AH_SLEEPABLE - ie pass in a gfp
flags argument.

Jason