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

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Date:	Mon, 08 Aug 2011 09:01:46 +0100
From:	Martyn Welch <martyn.welch@...com>
To:	"Emilio G. Cota" <cota@...ap.org>
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 05/08/11 18:47, Emilio G. Cota wrote:
> 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.
> 

I disagree, it shouldn't be up to the driver to determine whether it's use of
the bus should be known by the system. The system should know when the bus is
being used.

>> 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.
> 

As opposed to adding a function that *only* refcounts and therefore requires
every driver to make extra explicit function call just to keep the refcounts
up-to-date.

> 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.
> 

Which wouldn't happen because all the upstreamed drivers are "good quality". I
might add that failing to free a resource will mean that it won't be
re-allocated, so having visibility of the number of resources having been
allocated would be advantageous.

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

I just simply disagree. Forcing each driver to specifically maintain the
refcount is just stupid when an alternative is possible.

Martyn

-- 
Martyn Welch (Principal Software Engineer) | Registered in England and
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