linux-kernel - Re: [RFC][PATCH 04/16] writeback: fix possible race when shutting down bdi

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Date:	Sun, 18 Jul 2010 02:47:20 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Artem Bityutskiy <dedekind1@...il.com>
Cc:	Jens Axboe <axboe@...nel.dk>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 04/16] writeback: fix possible race when shutting
 down bdi

On Fri, Jul 16, 2010 at 03:45:00PM +0300, Artem Bityutskiy wrote:
> From: Artem Bityutskiy <Artem.Bityutskiy@...ia.com>
> 
> Current bdi code has the following race between 'bdi_wb_shutdown()'
> and 'bdi_forker_thread()'.
> 
> Initial condition: BDI_pending is cleaned, bdi has no writeback thread,
> because it was inactive and exited, 'bdi_wb_shutdown()' and
> 'bdi_forker_thread()' are executed concurrently.

Wouldn't it be better to have a per-bdi mutex to serialize thread
creation and shutdown?  And please also kill the bit wait in favour
of a proper wait queue - the bit wait interface really is just a hack
for structures that are very size sensitive, which the backing device
is not.

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