ixgbevf: Prevent possible race condition by checking for message

The mailbox interrupt routine might cause a race condition sometimes
and cause a message to be missed.
Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c

@@ -917,31 +917,34 @@ static irqreturn_t ixgbevf_msix_mbx(int irq, void *data)
 	struct ixgbe_hw *hw = &adapter->hw;
 	u32 eicr;
 	u32 msg;
+	bool got_ack = false;
 
 	eicr = IXGBE_READ_REG(hw, IXGBE_VTEICS);
 	IXGBE_WRITE_REG(hw, IXGBE_VTEICR, eicr);
 
-	if (!hw->mbx.ops.check_for_ack(hw)) {
-		/*
-		 * checking for the ack clears the PFACK bit.  Place
-		 * it back in the v2p_mailbox cache so that anyone
-		 * polling for an ack will not miss it.  Also
-		 * avoid the read below because the code to read
-		 * the mailbox will also clear the ack bit.  This was
-		 * causing lost acks.  Just cache the bit and exit
-		 * the IRQ handler.
-		 */
-		hw->mbx.v2p_mailbox |= IXGBE_VFMAILBOX_PFACK;
-		goto out;
-	}
+	if (!hw->mbx.ops.check_for_ack(hw))
+		got_ack = true;
 
-	/* Not an ack interrupt, go ahead and read the message */
+	if (!hw->mbx.ops.check_for_msg(hw)) {
 		hw->mbx.ops.read(hw, &msg, 1);
 
 		if ((msg & IXGBE_MBVFICR_VFREQ_MASK) == IXGBE_PF_CONTROL_MSG)
 			mod_timer(&adapter->watchdog_timer,
 				  round_jiffies(jiffies + 1));
 
+		if (msg & IXGBE_VT_MSGTYPE_NACK)
+			pr_warn("Last Request of type %2.2x to PF Nacked\n",
+				msg & 0xFF);
+		goto out;
+	}
+
+	/*
+	 * checking for the ack clears the PFACK bit.  Place
+	 * it back in the v2p_mailbox cache so that anyone
+	 * polling for an ack will not miss it
+	 */
+	if (got_ack)
+		hw->mbx.v2p_mailbox |= IXGBE_VFMAILBOX_PFACK;
 out:
 	return IRQ_HANDLED;
 }