[PATCH] bdi: wake up concurrent wb_shutdown() callers.

[Tetsuo Handa]

syzbot is reporting hung tasks at wait_on_bit(WB_shutting_down) in
wb_shutdown() [1]. This might be because commit 5318ce7d46866e1d ("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).

[1] https://syzkaller.appspot.com/bug?id=b297474817af98d5796bc544e1bb806fc3da0e5e

Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Reported-by: syzbot <syzbot+c0cf869505e03bdf1a24@xxxxxxxxxxxxxxxxxxxxxxxxx>
Fixes: 5318ce7d46866e1d ("bdi: Shutdown writeback on all cgwbs in cgwb_bdi_destroy()")
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Jens Axboe <axboe@xxxxxx>
---
 mm/backing-dev.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 023190c..dadac99 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -384,6 +384,8 @@ static void wb_shutdown(struct bdi_writeback *wb)
 	 */
 	smp_wmb();
 	clear_bit(WB_shutting_down, &wb->state);
+	smp_mb(); /* advised by wake_up_bit() */
+	wake_up_bit(&wb->state, WB_shutting_down);
 }
 
 static void wb_exit(struct bdi_writeback *wb)
-- 
1.8.3.1