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

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 8fde443..f8b07d4 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -853,6 +853,9 @@ static void bdi_remove_from_list(struct backing_dev_info 
 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 */
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