linux-kernel - Re: [PATCH] device model: Do a quickcheck for driver binding before doing an expensive check

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Message-ID: <20080915133226.7e433379@gondolin.boeblingen.de.ibm.com>
Date:	Mon, 15 Sep 2008 13:32:26 +0200
From:	Cornelia Huck <cornelia.huck@...ibm.com>
To:	Arjan van de Ven <arjan@...radead.org>
Cc:	linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
	greg@...ah.com
Subject: Re: [PATCH] device model: Do a quickcheck for driver binding before
 doing an expensive check

On Sun, 14 Sep 2008 08:32:06 -0700,
Arjan van de Ven <arjan@...radead.org> wrote:

> This patch adds a quick check for the driver<->device match before taking
> the locks and doin gthe expensive checks. Taking the lock hurts in asynchronous
> boot context where the device lock gets hit; one of the init functions takes
> the lock and goes to do an expensive hardware init; the other init functions
> walk the same PCI list and get stuck on the lock as a result.

Hm, you call bus->match twice now; once without dev->sem held and once
with it. For the busses I'm familiar with that shouldn't be a problem,
but are you sure there aren't busses which want dev->sem held?
(Although I think not relying on dev->sem would be the sane thing...)
> 
> For the common case, we can know there's no chance whatsoever of a match
> if the device isn't in the drivers ID table... so this patch does that check
> as a best-effort-avoid-the-lock approach.

I've always thought of ->match being a quick check which just looks at
the IDs with ->probe doing the heavier stuff, so this should be
reasonable (if all busses play nicely). But driver_probe_device() still
calls ->match a second time, and device_attach() will thus always call
->match under the lock. Should it be moved out of the lock there as
well?
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