linux-kernel - Re: [PATCH 5/8] staging: vme: add functions for bridge module refcounting

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Message-ID: <20110805174728.GA1741@flamenco.cs.columbia.edu>
Date:	Fri, 5 Aug 2011 13:47:28 -0400
From:	"Emilio G. Cota" <cota@...ap.org>
To:	Martyn Welch <martyn.welch@...com>
Cc:	gregkh@...e.de, devel@...verdev.osuosl.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/8] staging: vme: add functions for bridge module
 refcounting

On Fri, Aug 05, 2011 at 10:24:55 +0100, Martyn Welch wrote:
> I think that by refcounting the resources being used we
> will know whether a bridge module is being used or not,
> thus whether it can be unloaded or not.

But the granularity is wrong; if you want to know whether the
bridge is being used, just keep track of the devices *which
want to make known* that they're under the bridge.

> By reference counting the use of resources we minimise the
> chance of poorly written drivers using resources, but not
> registering the fact that they are in fact using a VME bridge.

A driver leaking a resource will then leave a bogus refcount
on the bus driver--a clear case of self-inflicted pain.

The argument of "poorly written drivers" does not apply;
I would expect all the merged drivers to be "good quality"
only, that's why we want to be upstream and why we put effort
in reviewing. Your point is well-intentioned, but in
practice we'd be shooting ourselves in the foot, potentially
ending up with an unremovable vme bridge module--which is
worse than a driver leaking a resource.

Refcounting must be kept simple & stupid; doing it behind the
backs of the drivers we're trying to protect is a mistake.

		Emilio
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