block: avoid incorrect bdi_unregiter call

bdi_unregister() should be called after bdi_register() is called, so we should check whether WB_registered flag is set.

For example of the situation, error path in device driver may call blk_cleanup_queue() before the driver calls bdi_register().

Signed-off-by Masayoshi Mizuma <[email protected]>

---
  mm/backing-dev.c | 3 +++
  1 file changed, 3 insertions(+)

mm/backing-dev.c +3 -0

index 8fde443..f8b07d4 100644
@@ -853,6 +853,9 @@ static void bdi_remove_from_list(struct backing_dev_info *bdi)
 
  void bdi_unregister(struct backing_dev_info *bdi)
  {
+	if (!test_bit(WB_registered, &bdi->wb.state))
+		return;
+
  	/* make sure nobody finds us on the bdi_list anymore */
  	bdi_remove_from_list(bdi);
  	wb_shutdown(&bdi->wb);