bdi: wake up concurrent wb_shutdown() callers.

syzbot is reporting hung tasks at wait_on_bit(WB_shutting_down) in
wb_shutdown() [1]. This seems to be because commit 5318ce7d ("bdi:
Shutdown writeback on all cgwbs in cgwb_bdi_destroy()") forgot to call
wake_up_bit(WB_shutting_down) after clear_bit(WB_shutting_down).

Introduce a helper function clear_and_wake_up_bit() and use it, in order
to avoid similar errors in future.

[1] https://syzkaller.appspot.com/bug?id=b297474817af98d5796bc544e1bb806fc3da0e5eSigned-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reported-by: syzbot <syzbot+c0cf869505e03bdf1a24@syzkaller.appspotmail.com>
Fixes: 5318ce7d ("bdi: Shutdown writeback on all cgwbs in cgwb_bdi_destroy()")
Cc: Tejun Heo <tj@kernel.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/wait_bit.h

@@ -305,4 +305,21 @@ do {									\
 	__ret;								\
 })
 
+/**
+ * clear_and_wake_up_bit - clear a bit and wake up anyone waiting on that bit
+ *
+ * @bit: the bit of the word being waited on
+ * @word: the word being waited on, a kernel virtual address
+ *
+ * You can use this helper if bitflags are manipulated atomically rather than
+ * non-atomically under a lock.
+ */
+static inline void clear_and_wake_up_bit(int bit, void *word)
+{
+	clear_bit_unlock(bit, word);
+	/* See wake_up_bit() for which memory barrier you need to use. */
+	smp_mb__after_atomic();
+	wake_up_bit(word, bit);
+}
+
 #endif /* _LINUX_WAIT_BIT_H */

mm/backing-dev.c

@@ -383,7 +383,7 @@ static void wb_shutdown(struct bdi_writeback *wb)
 	 * the barrier provided by test_and_clear_bit() above.
 	 */
 	smp_wmb();
-	clear_bit(WB_shutting_down, &wb->state);
+	clear_and_wake_up_bit(WB_shutting_down, &wb->state);
 }
 
 static void wb_exit(struct bdi_writeback *wb)