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From: Robert Watson <rwatson@FreeBSD.org>
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To: Pawel Jakub Dawidek <pjd@FreeBSD.org>
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cc: Julian Elischer <julian@elischer.org>
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Subject: Re: Giant-free polling [PATCH]
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On Fri, 11 Mar 2005, Pawel Jakub Dawidek wrote:

> On Fri, Mar 11, 2005 at 01:14:38PM -0800, Julian Elischer wrote:
> +> >P> There is still an unresolved problem (in your and our patch as well) of
> +> >P> using ifnet structure fields without synchronization, as we don't have
> +> >P> access tointerface's internal mutex, which protects those fields.
> +> 
> +> you need to add an interface method that has access to it..
> 
> I was thinking more about moving interface mutex into ifnet structure,
> but Robert has some objections IIRC. 

My specific concern was to make sure that we don't lock device driver
writers into a locking model that forces them to protect the device driver
with a single mutex.  There are changes floating around to protect the
if_em receive and transmit components separately, since the're basically
independent hardware units and can be accessed from multiple CPUs in
parallel.  This is also, I believe, what several Linux device drivers do.

So I'm OK with a multi-layer mutex in ifnet protecting ifnet and device
driver data, but only if we don't preclude the parallelism benefits that
can be attained as suggested above.

Robert N M Watson