[PATCH net] xsk: correct tx_ring_empty_descs count statistics

[Wang Liang]

The tx_ring_empty_descs count may be incorrect, when set the XDP_TX_RING
option but do not reserve tx ring. Because xsk_poll() try to wakeup the
driver by calling xsk_generic_xmit() for non-zero-copy mode. So the
tx_ring_empty_descs count increases once the xsk_poll()is called:

  xsk_poll
    xsk_generic_xmit
      __xsk_generic_xmit
        xskq_cons_peek_desc
          xskq_cons_read_desc
            q->queue_empty_descs++;

To avoid this count error, add check for tx descs before send msg in poll.

Fixes: df551058f7a3 ("xsk: Fix crash in poll when device does not support ndo_xsk_wakeup")
Signed-off-by: Wang Liang <wangliang74@xxxxxxxxxx>
---
 net/xdp/xsk.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
index 89d2bef96469..fb01e6736677 100644
--- a/net/xdp/xsk.c
+++ b/net/xdp/xsk.c
@@ -989,7 +989,7 @@ static __poll_t xsk_poll(struct file *file, struct socket *sock,
 	if (pool->cached_need_wakeup) {
 		if (xs->zc)
 			xsk_wakeup(xs, pool->cached_need_wakeup);
-		else if (xs->tx)
+		else if (xs->tx && xskq_has_descs(xs->tx))
 			/* Poll needs to drive Tx also in copy mode */
 			xsk_generic_xmit(sk);
 	}
-- 
2.34.1