sfc: lock TX queues when calling netif_device_detach()

The assertion of netif_device_present() at the top of
efx_hard_start_xmit() may fail if we don't do this.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
diff --git a/drivers/net/ethernet/sfc/efx.c b/drivers/net/ethernet/sfc/efx.c
index 4f86d0c..6f073d0 100644
--- a/drivers/net/ethernet/sfc/efx.c
+++ b/drivers/net/ethernet/sfc/efx.c
@@ -2279,7 +2279,7 @@
 	netif_info(efx, drv, efx->net_dev, "resetting (%s)\n",
 		   RESET_TYPE(method));
 
-	netif_device_detach(efx->net_dev);
+	efx_device_detach_sync(efx);
 	efx_reset_down(efx, method);
 
 	rc = efx->type->reset(efx, method);
@@ -2758,7 +2758,7 @@
 	if (efx->state != STATE_DISABLED) {
 		efx->state = STATE_UNINIT;
 
-		netif_device_detach(efx->net_dev);
+		efx_device_detach_sync(efx);
 
 		efx_stop_all(efx);
 		efx_stop_interrupts(efx, false);
diff --git a/drivers/net/ethernet/sfc/efx.h b/drivers/net/ethernet/sfc/efx.h
index f11170b..50247df 100644
--- a/drivers/net/ethernet/sfc/efx.h
+++ b/drivers/net/ethernet/sfc/efx.h
@@ -163,4 +163,17 @@
 extern void efx_link_set_advertising(struct efx_nic *efx, u32);
 extern void efx_link_set_wanted_fc(struct efx_nic *efx, u8);
 
+static inline void efx_device_detach_sync(struct efx_nic *efx)
+{
+	struct net_device *dev = efx->net_dev;
+
+	/* Lock/freeze all TX queues so that we can be sure the
+	 * TX scheduler is stopped when we're done and before
+	 * netif_device_present() becomes false.
+	 */
+	netif_tx_lock(dev);
+	netif_device_detach(dev);
+	netif_tx_unlock(dev);
+}
+
 #endif /* EFX_EFX_H */
diff --git a/drivers/net/ethernet/sfc/selftest.c b/drivers/net/ethernet/sfc/selftest.c
index f6651d3..2069f51 100644
--- a/drivers/net/ethernet/sfc/selftest.c
+++ b/drivers/net/ethernet/sfc/selftest.c
@@ -722,7 +722,7 @@
 	/* Detach the device so the kernel doesn't transmit during the
 	 * loopback test and the watchdog timeout doesn't fire.
 	 */
-	netif_device_detach(efx->net_dev);
+	efx_device_detach_sync(efx);
 
 	if (efx->type->test_chip) {
 		rc_reset = efx->type->test_chip(efx, tests);