8139too: revisit napi_complete_done() usage

It seems we have to be more careful in napi_complete_done()
use. This patch is not a revert, as it seems we can
avoid bug that Ville reported by moving the napi_complete_done()
test in the spinlock section.

Many thanks to Ville for detective work and all tests.

Fixes: 617f0121 ("8139too: use napi_complete_done()")
Reported-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Tested-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/realtek/8139too.c

@@ -2135,11 +2135,12 @@ static int rtl8139_poll(struct napi_struct *napi, int budget)
 	if (likely(RTL_R16(IntrStatus) & RxAckBits))
 		work_done += rtl8139_rx(dev, tp, budget);
 
-	if (work_done < budget && napi_complete_done(napi, work_done)) {
+	if (work_done < budget) {
 		unsigned long flags;
 
 		spin_lock_irqsave(&tp->lock, flags);
-		RTL_W16_F(IntrMask, rtl8139_intr_mask);
+		if (napi_complete_done(napi, work_done))
+			RTL_W16_F(IntrMask, rtl8139_intr_mask);
 		spin_unlock_irqrestore(&tp->lock, flags);
 	}
 	spin_unlock(&tp->rx_lock);