linux-kernel - Re: [patch 4/5] fs: fix possible bdi writeback refcounting problem

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Message-ID: <20090915193055.GR23126@kernel.dk>
Date:	Tue, 15 Sep 2009 21:30:56 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	npiggin@...e.de
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 4/5] fs: fix possible bdi writeback refcounting problem

On Wed, Sep 16 2009, npiggin@...e.de wrote:
> wb_clear_pending AFAIKS should not be called after the item has been
> put on the list, except by the worker threads. It could lead to the
> situation where the refcount is decremented below 0 and cause lots of
> problems.

Good point!

> Presumably the !wb_has_dirty_io case is not a common one, so it can
> be discovered when the thread wakes up to check?

It's checked earlier as well, so I see no problem in killing the check
there.

> 
> Also add a comment in bdi_work_clear.
> 
> Signed-off-by: Nick Piggin <npiggin@...e.de>
> ---
>  fs/fs-writeback.c |   10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> Index: linux-2.6/fs/fs-writeback.c
> ===================================================================
> --- linux-2.6.orig/fs/fs-writeback.c
> +++ linux-2.6/fs/fs-writeback.c
> @@ -98,6 +98,11 @@ static void bdi_work_clear(struct bdi_wo
>  {
>  	clear_bit(WS_USED_B, &work->state);
>  	smp_mb__after_clear_bit();
> +	/*
> +	 * work can have disappeared at this point. bit waitq functions
> +	 * should be able to tolerate this, provided bdi_sched_wait does
> +	 * not dereference it's pointer argument.
> +	 */
>  	wake_up_bit(&work->state, WS_USED_B);
>  }
>  
> @@ -172,10 +177,7 @@ static void bdi_queue_work(struct backin
>  		 * thread always. As a safety precaution, it'll flush out
>  		 * everything
>  		 */
> -		if (!wb_has_dirty_io(wb)) {
> -			if (work)
> -				wb_clear_pending(wb, work);
> -		} else if (wb->task)
> +		if (wb->task)
>  			wake_up_process(wb->task);
>  	}
>  }
> 
> 

-- 
Jens Axboe

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