[PATCH 4.4 068/342] bcache: prevent crash on changing writeback_running

4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Stefan Bader <redacted>

commit 8d16ce540c94c9d366eb36fc91b7154d92d6397b upstream.

Added a safeguard in the shutdown case. At least while not being
attached it is also possible to trigger a kernel bug by writing into
writeback_running. This change  adds the same check before trying to
wake up the thread for that case.

Signed-off-by: Stefan Bader <redacted>
Cc: Kent Overstreet <redacted>
Signed-off-by: Jens Axboe <redacted>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 drivers/md/bcache/writeback.h |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

--- a/drivers/md/bcache/writeback.h
+++ b/drivers/md/bcache/writeback.h

@@ -63,7 +63,8 @@ static inline bool should_writeback(stru
 
 static inline void bch_writeback_queue(struct cached_dev *dc)
 {
-	wake_up_process(dc->writeback_thread);
+	if (!IS_ERR_OR_NULL(dc->writeback_thread))
+		wake_up_process(dc->writeback_thread);
 }
 
 static inline void bch_writeback_add(struct cached_dev *dc)