rt2x00: do not pause queue unconditionally on error path

Pausing queue without checking threshold is racy with txdone path.
Moreover we do not need pause queue on any error, but only if queue
is full - in case when we send RTS frame ( other cases of almost full
queue are already handled in rt2x00queue_write_tx_frame() ).

Patch fixes of theoretically possible problem of pausing empty
queue.
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Tested-by: Enrico Mioso <mrkiko.rs@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>

drivers/net/wireless/ralink/rt2x00/rt2x00mac.c

@@ -142,22 +142,28 @@ void rt2x00mac_tx(struct ieee80211_hw *hw,
 	if (!rt2x00dev->ops->hw->set_rts_threshold &&
 	    (tx_info->control.rates[0].flags & (IEEE80211_TX_RC_USE_RTS_CTS |
 						IEEE80211_TX_RC_USE_CTS_PROTECT))) {
-		if (rt2x00queue_available(queue) <= 1)
-			goto exit_fail;
+		if (rt2x00queue_available(queue) <= 1) {
+			/*
+			 * Recheck for full queue under lock to avoid race
+			 * conditions with rt2x00lib_txdone().
+			 */
+			spin_lock(&queue->tx_lock);
+			if (rt2x00queue_threshold(queue))
+				rt2x00queue_pause_queue(queue);
+			spin_unlock(&queue->tx_lock);
+
+			goto exit_free_skb;
+		}
 
 		if (rt2x00mac_tx_rts_cts(rt2x00dev, queue, skb))
-			goto exit_fail;
+			goto exit_free_skb;
 	}
 
 	if (unlikely(rt2x00queue_write_tx_frame(queue, skb, control->sta, false)))
-		goto exit_fail;
+		goto exit_free_skb;
 
 	return;
 
- exit_fail:
-	spin_lock(&queue->tx_lock);
-	rt2x00queue_pause_queue(queue);
-	spin_unlock(&queue->tx_lock);
  exit_free_skb:
 	ieee80211_free_txskb(hw, skb);
 }