linux-kernel - Re: [PATCH 4/9] writeback: track if we're sleeping on progress in balance_dirty

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Message-ID: <20160413130846.GA23550@quack2.suse.cz>
Date:	Wed, 13 Apr 2016 15:08:46 +0200
From:	Jan Kara <jack@...e.cz>
To:	Jens Axboe <axboe@...com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-block@...r.kernel.org
Subject: Re: [PATCH 4/9] writeback: track if we're sleeping on progress in
 balance_dirty_pages()

On Wed 30-03-16 09:07:52, Jens Axboe wrote:
> Note in the bdi_writeback structure if a task is currently being
> limited in balance_dirty_pages(), waiting for writeback to
> proceed.
...
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index 11ff8f758631..15e696bc5d14 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -1746,7 +1746,9 @@ pause:
>  					  pause,
>  					  start_time);
>  		__set_current_state(TASK_KILLABLE);
> +		wb->dirty_sleeping = 1;
>  		io_schedule_timeout(pause);
> +		wb->dirty_sleeping = 0;

Huh, but wb->dirty_sleeping is shared by all the processes in the system.
So this is seriously racy, isn't it? You rather need a counter for this to
work.

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR