s390/cio: fix race condition during online processing

A race condition exists in ccw_device_set_online() that can cause the
online process to fail, leaving the affected device in an inconsistent
state. As a result, subsequent attempts to set that device online fail
with return code ENODEV.

The problem occurs when a path verification request arrives after
a wait for final device state completed, but before the result state
is evaluated.

Fix this by ensuring that the CCW-device lock is held between
determining final state and checking result state.

Note that since:

commit 2297791c ("s390/cio: dont unregister subchannel from child-drivers")

path verification requests are much more likely to occur during boot,
resulting in an increased chance of this race condition occurring.

Fixes: 2297791c ("s390/cio: dont unregister subchannel from child-drivers")
Reviewed-by: Alexandra Winter <wintera@linux.ibm.com>
Reviewed-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Signed-off-by: Peter Oberparleiter <oberpar@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>

drivers/s390/cio/device.c

@@ -363,10 +363,8 @@ int ccw_device_set_online(struct ccw_device *cdev)
 
 	spin_lock_irq(cdev->ccwlock);
 	ret = ccw_device_online(cdev);
-	spin_unlock_irq(cdev->ccwlock);
-	if (ret == 0)
-		wait_event(cdev->private->wait_q, dev_fsm_final_state(cdev));
-	else {
+	if (ret) {
+		spin_unlock_irq(cdev->ccwlock);
 		CIO_MSG_EVENT(0, "ccw_device_online returned %d, "
 			      "device 0.%x.%04x\n",
 			      ret, cdev->private->dev_id.ssid,
@@ -375,7 +373,12 @@ int ccw_device_set_online(struct ccw_device *cdev)
 		put_device(&cdev->dev);
 		return ret;
 	}
-	spin_lock_irq(cdev->ccwlock);
+	/* Wait until a final state is reached */
+	while (!dev_fsm_final_state(cdev)) {
+		spin_unlock_irq(cdev->ccwlock);
+		wait_event(cdev->private->wait_q, dev_fsm_final_state(cdev));
+		spin_lock_irq(cdev->ccwlock);
+	}
 	/* Check if online processing was successful */
 	if ((cdev->private->state != DEV_STATE_ONLINE) &&
 	    (cdev->private->state != DEV_STATE_W4SENSE)) {