virtio-blk: Call del_gendisk() before disable guest kick

del_gendisk() might not return due to failing to remove the
/sys/block/vda/serial sysfs entry when another thread (udev) is
trying to read it.

virtblk_remove()
  vdev->config->reset() : guest will not kick us through interrupt
    del_gendisk()
      device_del()
        kobject_del(): got stuck, sysfs entry ref count non zero

sysfs_open_file(): user space process read /sys/block/vda/serial
   sysfs_get_active() : got sysfs entry ref count
      dev_attr_show()
        virtblk_serial_show()
           blk_execute_rq() : got stuck, interrupt is disabled
                              request cannot be finished

This patch fixes it by calling del_gendisk() before we disable guest's
interrupt so that the request sent in virtblk_serial_show() will be
finished and del_gendisk() will success.

This fixes another race in hot-unplug process.

It is save to call del_gendisk(vblk->disk) before
flush_work(&vblk->config_work) which might access vblk->disk, because
vblk->disk is not freed until put_disk(vblk->disk).

Cc: virtualization@lists.linux-foundation.org
Cc: kvm@vger.kernel.org
Cc: stable@kernel.org
Signed-off-by: Asias He <asias@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/block/virtio_blk.c

@@ -584,13 +584,13 @@ static void __devexit virtblk_remove(struct virtio_device *vdev)
 	vblk->config_enable = false;
 	mutex_unlock(&vblk->config_lock);
 
+	del_gendisk(vblk->disk);
+
 	/* Stop all the virtqueues. */
 	vdev->config->reset(vdev);
 
 	flush_work(&vblk->config_work);
 
-	del_gendisk(vblk->disk);
-
 	/* Abort requests dispatched to driver. */
 	spin_lock_irqsave(&vblk->lock, flags);
 	while ((vbr = virtqueue_detach_unused_buf(vblk->vq))) {