e1000e: Separate signaling for link check/link up

Lennart reported the following race condition:

\ e1000_watchdog_task
    \ e1000e_has_link
        \ hw->mac.ops.check_for_link() === e1000e_check_for_copper_link
            /* link is up */
            mac->get_link_status = false;

                            /* interrupt */
                            \ e1000_msix_other
                                hw->mac.get_link_status = true;

        link_active = !hw->mac.get_link_status
        /* link_active is false, wrongly */

This problem arises because the single flag get_link_status is used to
signal two different states: link status needs checking and link status is
down.

Avoid the problem by using the return value of .check_for_link to signal
the link status to e1000e_has_link().

Reported-by: Lennart Sorensen <lsorense@csclub.uwaterloo.ca>
Signed-off-by: Benjamin Poirier <bpoirier@suse.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/e1000e/mac.c

@@ -410,6 +410,9 @@ void e1000e_clear_hw_cntrs_base(struct e1000_hw *hw)
  *  Checks to see of the link status of the hardware has changed.  If a
  *  change in link status has been detected, then we read the PHY registers
  *  to get the current speed/duplex if link exists.
+ *
+ *  Returns a negative error code (-E1000_ERR_*) or 0 (link down) or 1 (link
+ *  up).
  **/
 s32 e1000e_check_for_copper_link(struct e1000_hw *hw)
 {
@@ -423,7 +426,7 @@ s32 e1000e_check_for_copper_link(struct e1000_hw *hw)
 	 * Change or Rx Sequence Error interrupt.
 	 */
 	if (!mac->get_link_status)
-		return 0;
+		return 1;
 
 	/* First we want to see if the MII Status Register reports
 	 * link.  If so, then we want to get the current speed/duplex
@@ -461,10 +464,12 @@ s32 e1000e_check_for_copper_link(struct e1000_hw *hw)
 	 * different link partner.
 	 */
 	ret_val = e1000e_config_fc_after_link_up(hw);
-	if (ret_val)
+	if (ret_val) {
 		e_dbg("Error configuring flow control\n");
+		return ret_val;
+	}
 
-	return ret_val;
+	return 1;
 }
 
 /**

drivers/net/ethernet/intel/e1000e/netdev.c

@@ -5081,7 +5081,7 @@ static bool e1000e_has_link(struct e1000_adapter *adapter)
 	case e1000_media_type_copper:
 		if (hw->mac.get_link_status) {
 			ret_val = hw->mac.ops.check_for_link(hw);
-			link_active = !hw->mac.get_link_status;
+			link_active = ret_val > 0;
 		} else {
 			link_active = true;
 		}