vfio-ccw: Prevent quiesce function going into an infinite loop

The quiesce function calls cio_cancel_halt_clear() and if we
get an -EBUSY we go into a loop where we:
- wait for any interrupts
- flush all I/O in the workqueue
- retry cio_cancel_halt_clear

During the period where we are waiting for interrupts or
flushing all I/O, the channel subsystem could have completed
a halt/clear action and turned off the corresponding activity
control bits in the subchannel status word. This means the next
time we call cio_cancel_halt_clear(), we will again start by
calling cancel subchannel and so we can be stuck between calling
cancel and halt forever.

Rather than calling cio_cancel_halt_clear() immediately after
waiting, let's try to disable the subchannel. If we succeed in
disabling the subchannel then we know nothing else can happen
with the device.

Suggested-by: Eric Farman <[e-mail]>
Signed-off-by: Farhan Ali <[e-mail]>
Message-Id: <[e-mail]>
Reviewed-by: Eric Farman <[e-mail]>
Acked-by: Halil Pasic <[e-mail]>
Signed-off-by: Cornelia Huck <[e-mail]>

drivers/s390/cio/vfio_ccw_drv.c

···
 	if (ret != -EBUSY)
 		goto out_unlock;
 
+	iretry = 255;
 	do {
-		iretry = 255;
 
 		ret = cio_cancel_halt_clear(sch, &iretry);
-		while (ret == -EBUSY) {
-			/*
-			 * Flush all I/O and wait for
-			 * cancel/halt/clear completion.
-			 */
-			private->completion = &completion;
-			spin_unlock_irq(sch->lock);
 
+		if (ret == -EIO) {
+			pr_err("vfio_ccw: could not quiesce subchannel 0.%x.%04x!\n",
+			       sch->schid.ssid, sch->schid.sch_no);
+			break;
+		}
+
+		/*
+		 * Flush all I/O and wait for
+		 * cancel/halt/clear completion.
+		 */
+		private->completion = &completion;
+		spin_unlock_irq(sch->lock);
+
+		if (ret == -EBUSY)
 			wait_for_completion_timeout(&completion, 3*HZ);
 
-			private->completion = NULL;
-			flush_workqueue(vfio_ccw_work_q);
-			spin_lock_irq(sch->lock);
-			ret = cio_cancel_halt_clear(sch, &iretry);
-		};
-
+		private->completion = NULL;
+		flush_workqueue(vfio_ccw_work_q);
+		spin_lock_irq(sch->lock);
 		ret = cio_disable_subchannel(sch);
 	} while (ret == -EBUSY);
 out_unlock: