RDMA/rxe: Use rdma_read_gid_attr_ndev_rcu to access netdev

Use rdma_read_gid_attr_ndev_rcu() to access netdevice attached to GID
entry under rcu lock.

This ensures that while working on the netdevice of the GID, it doesn't
get freed.
Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

drivers/infiniband/sw/rxe/rxe_net.c

@@ -458,7 +458,7 @@ struct sk_buff *rxe_init_packet(struct rxe_dev *rxe, struct rxe_av *av,
 				int paylen, struct rxe_pkt_info *pkt)
 {
 	unsigned int hdr_len;
-	struct sk_buff *skb;
+	struct sk_buff *skb = NULL;
 	struct net_device *ndev;
 	const struct ib_gid_attr *attr;
 	const int port_num = 1;
@@ -466,7 +466,6 @@ struct sk_buff *rxe_init_packet(struct rxe_dev *rxe, struct rxe_av *av,
 	attr = rdma_get_gid_attr(&rxe->ib_dev, port_num, av->grh.sgid_index);
 	if (IS_ERR(attr))
 		return NULL;
-	ndev = attr->ndev;
 
 	if (av->network_type == RDMA_NETWORK_IPV4)
 		hdr_len = ETH_HLEN + sizeof(struct udphdr) +
@@ -475,16 +474,26 @@ struct sk_buff *rxe_init_packet(struct rxe_dev *rxe, struct rxe_av *av,
 		hdr_len = ETH_HLEN + sizeof(struct udphdr) +
 			sizeof(struct ipv6hdr);
 
+	rcu_read_lock();
+	ndev = rdma_read_gid_attr_ndev_rcu(attr);
+	if (IS_ERR(ndev)) {
+		rcu_read_unlock();
+		goto out;
+	}
 	skb = alloc_skb(paylen + hdr_len + LL_RESERVED_SPACE(ndev),
 			GFP_ATOMIC);
 
-	if (unlikely(!skb))
+	if (unlikely(!skb)) {
+		rcu_read_unlock();
 		goto out;
+	}
 
 	skb_reserve(skb, hdr_len + LL_RESERVED_SPACE(ndev));
 
 	/* FIXME: hold reference to this netdev until life of this skb. */
 	skb->dev	= ndev;
+	rcu_read_unlock();
+
 	if (av->network_type == RDMA_NETWORK_IPV4)
 		skb->protocol = htons(ETH_P_IP);
 	else