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

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Message-ID: <84144f020712101406i562ca243vd7b5fe6bfd1db05b@mail.gmail.com>
Date:	Tue, 11 Dec 2007 00:06:15 +0200
From:	"Pekka Enberg" <penberg@...helsinki.fi>
To:	"Jonathan Corbet" <corbet@....net>
Cc:	"Daniel Phillips" <phillips@...nq.net>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Peter Zijlstra" <peterz@...radead.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] [PATCH] A clean approach to writeout throttling

Hi,

On Dec 10, 2007 11:31 PM, Jonathan Corbet <corbet@....net> wrote:
> I'm just getting around to looking at this.  One thing jumped out at me:
>
> > +     if (bio->bi_throttle) {
> > +             struct request_queue *q = bio->bi_queue;
> > +             bio->bi_throttle = 0; /* or detect multiple endio and err? */
> > +             atomic_add(bio->bi_throttle, &q->available);
> > +             wake_up(&q->throttle_wait);
> > +     }
>
> I'm feeling like I must be really dumb, but...how can that possibly
> work?  You're zeroing >bi_throttle before adding it back into
> q->available, so the latter will never increase...

Heh, well, that's ok as long as bio->bi_vcnt is set to zero and I think we
have some md raid drivers do just that... ;-)

                                Pekka
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