vfio: Fix runaway interruptible timeout

Commit 13060b64 ("vfio: Add and use device request op for vfio
bus drivers") incorrectly makes use of an interruptible timeout.
When interrupted, the signal remains pending resulting in subsequent
timeouts occurring instantly.  This makes the loop spin at a much
higher rate than intended.

Instead of making this completely non-interruptible, we can change
this into a sort of interruptible-once behavior and use the "once"
to log debug information.  The driver API doesn't allow us to abort
and return an error code.
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
Fixes: 13060b64
Cc: stable@vger.kernel.org # v4.0

drivers/vfio/vfio.c

@@ -710,6 +710,8 @@ void *vfio_del_group_dev(struct device *dev)
 	void *device_data = device->device_data;
 	struct vfio_unbound_dev *unbound;
 	unsigned int i = 0;
+	long ret;
+	bool interrupted = false;
 
 	/*
 	 * The group exists so long as we have a device reference.  Get
@@ -755,9 +757,22 @@ void *vfio_del_group_dev(struct device *dev)
 
 		vfio_device_put(device);
 
-	} while (wait_event_interruptible_timeout(vfio.release_q,
-						  !vfio_dev_present(group, dev),
-						  HZ * 10) <= 0);
+		if (interrupted) {
+			ret = wait_event_timeout(vfio.release_q,
+					!vfio_dev_present(group, dev), HZ * 10);
+		} else {
+			ret = wait_event_interruptible_timeout(vfio.release_q,
+					!vfio_dev_present(group, dev), HZ * 10);
+			if (ret == -ERESTARTSYS) {
+				interrupted = true;
+				dev_warn(dev,
+					 "Device is currently in use, task"
+					 " \"%s\" (%d) "
+					 "blocked until device is released",
+					 current->comm, task_pid_nr(current));
+			}
+		}
+	} while (ret <= 0);
 
 	vfio_group_put(group);