vfio: ccw: process ssch with interrupts disabled

When we call ssch, an interrupt might already be pending once we
return from the START SUBCHANNEL instruction. Therefore we need to
make sure interrupts are disabled while holding the subchannel lock
until after we're done with our processing.

Cc: stable@vger.kernel.org #v4.12+
Reviewed-by: Dong Jia Shi <bjsdjshi@linux.ibm.com>
Acked-by: Halil Pasic <pasic@linux.vnet.ibm.com>
Acked-by: Pierre Morel <pmorel@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/cio/vfio_ccw_fsm.c

@@ -20,12 +20,12 @@ static int fsm_io_helper(struct vfio_ccw_private *private)
 	int ccode;
 	__u8 lpm;
 	unsigned long flags;
+	int ret;
 
 	sch = private->sch;
 
 	spin_lock_irqsave(sch->lock, flags);
 	private->state = VFIO_CCW_STATE_BUSY;
-	spin_unlock_irqrestore(sch->lock, flags);
 
 	orb = cp_get_orb(&private->cp, (u32)(addr_t)sch, sch->lpm);
 
@@ -38,10 +38,12 @@ static int fsm_io_helper(struct vfio_ccw_private *private)
 		 * Initialize device status information
 		 */
 		sch->schib.scsw.cmd.actl |= SCSW_ACTL_START_PEND;
-		return 0;
+		ret = 0;
+		break;
 	case 1:		/* Status pending */
 	case 2:		/* Busy */
-		return -EBUSY;
+		ret = -EBUSY;
+		break;
 	case 3:		/* Device/path not operational */
 	{
 		lpm = orb->cmd.lpm;
@@ -51,13 +53,16 @@ static int fsm_io_helper(struct vfio_ccw_private *private)
 			sch->lpm = 0;
 
 		if (cio_update_schib(sch))
-			return -ENODEV;
-
-		return sch->lpm ? -EACCES : -ENODEV;
+			ret = -ENODEV;
+		else
+			ret = sch->lpm ? -EACCES : -ENODEV;
+		break;
 	}
 	default:
-		return ccode;
+		ret = ccode;
 	}
+	spin_unlock_irqrestore(sch->lock, flags);
+	return ret;
 }
 
 static void fsm_notoper(struct vfio_ccw_private *private,