net: Move flush list retrieval to where it is used.

The bpf_net_ctx_get_.*_flush_list() are used at the top of the function.
This means the variable is always assigned even if unused. By moving the
function to where it is used, it is possible to delay the initialisation
until it is unavoidable.
Not sure how much this gains in reality but by looking at bq_enqueue()
(in devmap.c) gcc pushes one register less to the stack. \o/.

 Move flush list retrieval to where it is used.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Jesper Dangaard Brouer <hawk@kernel.org>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

kernel/bpf/cpumap.c

@@ -707,7 +707,6 @@ static void bq_flush_to_queue(struct xdp_bulk_queue *bq)
  */
 static void bq_enqueue(struct bpf_cpu_map_entry *rcpu, struct xdp_frame *xdpf)
 {
-	struct list_head *flush_list = bpf_net_ctx_get_cpu_map_flush_list();
 	struct xdp_bulk_queue *bq = this_cpu_ptr(rcpu->bulkq);
 
 	if (unlikely(bq->count == CPU_MAP_BULK_SIZE))
@@ -724,8 +723,11 @@ static void bq_enqueue(struct bpf_cpu_map_entry *rcpu, struct xdp_frame *xdpf)
 	 */
 	bq->q[bq->count++] = xdpf;
 
-	if (!bq->flush_node.prev)
+	if (!bq->flush_node.prev) {
+		struct list_head *flush_list = bpf_net_ctx_get_cpu_map_flush_list();
+
 		list_add(&bq->flush_node, flush_list);
+	}
 }
 
 int cpu_map_enqueue(struct bpf_cpu_map_entry *rcpu, struct xdp_frame *xdpf,

kernel/bpf/devmap.c

@@ -448,7 +448,6 @@ static void *__dev_map_lookup_elem(struct bpf_map *map, u32 key)
 static void bq_enqueue(struct net_device *dev, struct xdp_frame *xdpf,
 		       struct net_device *dev_rx, struct bpf_prog *xdp_prog)
 {
-	struct list_head *flush_list = bpf_net_ctx_get_dev_flush_list();
 	struct xdp_dev_bulk_queue *bq = this_cpu_ptr(dev->xdp_bulkq);
 
 	if (unlikely(bq->count == DEV_MAP_BULK_SIZE))
@@ -462,6 +461,8 @@ static void bq_enqueue(struct net_device *dev, struct xdp_frame *xdpf,
 	 * are only ever modified together.
 	 */
 	if (!bq->dev_rx) {
+		struct list_head *flush_list = bpf_net_ctx_get_dev_flush_list();
+
 		bq->dev_rx = dev_rx;
 		bq->xdp_prog = xdp_prog;
 		list_add(&bq->flush_node, flush_list);

net/xdp/xsk.c

@@ -370,15 +370,17 @@ static int xsk_rcv(struct xdp_sock *xs, struct xdp_buff *xdp)
 
 int __xsk_map_redirect(struct xdp_sock *xs, struct xdp_buff *xdp)
 {
-	struct list_head *flush_list = bpf_net_ctx_get_xskmap_flush_list();
 	int err;
 
 	err = xsk_rcv(xs, xdp);
 	if (err)
 		return err;
 
-	if (!xs->flush_node.prev)
+	if (!xs->flush_node.prev) {
+		struct list_head *flush_list = bpf_net_ctx_get_xskmap_flush_list();
+
 		list_add(&xs->flush_node, flush_list);
+	}
 
 	return 0;
 }