netdev - Re: [1/1] Block device throttling [Re: Distributed storage.]

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Message-Id: <200708271457.38100.phillips@phunq.net>
Date:	Mon, 27 Aug 2007 14:57:37 -0700
From:	Daniel Phillips <phillips@...nq.net>
To:	Evgeniy Polyakov <johnpol@....mipt.ru>
Cc:	Jens Axboe <jens.axboe@...cle.com>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [1/1] Block device throttling [Re: Distributed storage.]

Say Evgeniy, something I was curious about but forgot to ask you 
earlier...

On Wednesday 08 August 2007 03:17, Evgeniy Polyakov wrote:
> ...All oerations are not atomic, since we do not care about precise
> number of bios, but a fact, that we are close or close enough to the
> limit. 
> ... in bio->endio
> +			q->bio_queued--;

In your proposed patch, what prevents the race:

			cpu1						cpu2

	read q->bio_queued
									q->bio_queued--
	write q->bio_queued - 1
	Whoops! We leaked a throttle count.

Regards,

Daniel
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