linux-kernel - Re: [RFC] [PATCH] A clean aEvgeniy pproach to writeout throttling

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Message-ID: <20071210114123.GE5008@kernel.dk>
Date:	Mon, 10 Dec 2007 12:41:23 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Daniel Phillips <phillips@...nq.net>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [RFC] [PATCH] A clean aEvgeniy pproach to writeout throttling

On Mon, Dec 10 2007, Daniel Phillips wrote:
> On Monday 10 December 2007 02:47, Jens Axboe wrote:
> > Ehm, this patch is so broken it's not even funny - did you even
> > compile? You would have noticed the warning on request_queue_t,
> > surely. The big problem is the last hunk here though, how would that
> > work on stacked devices? Clue: ->bi_bdev is not const, it can change
> > after a call to ->make_request_fn().
> 
> Such paranoia.  Yes, the patch was compiled.   Yes, the warning was 
> slipped through.  No, it is not substantive, and in fact was removed 
> from another branch of our tree already.
> 
> Ignoring the rhetoric, apparently you missed the line:
> 
> +       if (q && q->metric && !bio->bi_queue) {
> 
> The prevents any reference ti bi_bdev after the intial call to 
> generic_make_request.  Thanks to Evgeniy for pointing out the need for 
> this measure on the last go round.

Which saves the initial target, for ease of accounting at end io time -
that's not the point. What happens when ->make_request_fn() changes
bio->bi_bdev and returns 1, causing another iteration of the
__generic_make_request() loop? 'q' is no longer the valid target,
bdev_get_queue(bio->bi_bdev) is.

> "So broken" is a gross exaggeration.  Substantive comments welcome.

Or you could try and make an effort to understand the comment instead of
just glancing over it.

-- 
Jens Axboe

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