UBUNTU: SAUCE: bcache: prevent crash on changing writeback_running

BugLink: http://bugs.launchpad.net/bugs/1357295

commit a664d0f0
    bcache: fix crash on shutdown in passthrough mode

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.

BugLink: http://bugs.launchpad.net/bugs/1357295

(sent upstream and accepted into developer tree but not, yet Linus)
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

drivers/md/bcache/writeback.h

@@ -63,7 +63,8 @@ static inline bool should_writeback(struct cached_dev *dc, struct bio *bio,
 
 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)