Revert "xsk: simplify AF_XDP socket teardown"

This reverts commit e2ce3674.

It turns out that the sock destructor xsk_destruct was needed after
all. The cleanup simplification broke the skb transmit cleanup path,
due to that the umem was prematurely destroyed.

The umem cannot be destroyed until all outstanding skbs are freed,
which means that we cannot remove the umem until the sk_destruct has
been called.
Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

net/xdp/xsk.c

@@ -366,7 +366,6 @@ static int xsk_release(struct socket *sock)
 
 	xskq_destroy(xs->rx);
 	xskq_destroy(xs->tx);
-	xdp_put_umem(xs->umem);
 
 	sock_orphan(sk);
 	sock->sk = NULL;
@@ -718,6 +717,18 @@ static const struct proto_ops xsk_proto_ops = {
 	.sendpage	= sock_no_sendpage,
 };
 
+static void xsk_destruct(struct sock *sk)
+{
+	struct xdp_sock *xs = xdp_sk(sk);
+
+	if (!sock_flag(sk, SOCK_DEAD))
+		return;
+
+	xdp_put_umem(xs->umem);
+
+	sk_refcnt_debug_dec(sk);
+}
+
 static int xsk_create(struct net *net, struct socket *sock, int protocol,
 		      int kern)
 {
@@ -744,6 +755,9 @@ static int xsk_create(struct net *net, struct socket *sock, int protocol,
 
 	sk->sk_family = PF_XDP;
 
+	sk->sk_destruct = xsk_destruct;
+	sk_refcnt_debug_inc(sk);
+
 	sock_set_flag(sk, SOCK_RCU_FREE);
 
 	xs = xdp_sk(sk);