Re: INFO: task hung in wb_shutdown (2)

[Tetsuo Handa]

Tejun, Jan, Jens,

Can you review this patch? syzbot has hit this bug for nearly 4000 times but
is still unable to find a reproducer. Therefore, the only way to test would be
to apply this patch upstream and test whether the problem is solved.

On 2018/04/24 21:19, Tetsuo Handa wrote:
>>From 39ed6be8a2c12dfe54feaa5abbc2ec46103022bf Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Date: Tue, 24 Apr 2018 11:59:08 +0900
> Subject: [PATCH] bdi: wake up concurrent wb_shutdown() callers.
> 
> 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)
>