net: sched: flower: handle concurrent filter insertion in fl_change

Check if user specified a handle and another filter with the same handle
was inserted concurrently. Return EAGAIN to retry filter processing (in
case it is an overwrite request).
Signed-off-by: Vlad Buslov <vladbu@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/cls_flower.c

@@ -1542,6 +1542,15 @@ static int fl_change(struct net *net, struct sk_buff *in_skb,
 			/* user specifies a handle and it doesn't exist */
 			err = idr_alloc_u32(&head->handle_idr, fnew, &handle,
 					    handle, GFP_ATOMIC);
+
+			/* Filter with specified handle was concurrently
+			 * inserted after initial check in cls_api. This is not
+			 * necessarily an error if NLM_F_EXCL is not set in
+			 * message flags. Returning EAGAIN will cause cls_api to
+			 * try to update concurrently inserted rule.
+			 */
+			if (err == -ENOSPC)
+				err = -EAGAIN;
 		} else {
 			handle = 1;
 			err = idr_alloc_u32(&head->handle_idr, fnew, &handle,