linux-kernel - Re: [patch 2.6.24-rc2 1/3] generic gpio -- gpio

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Date:	Wed, 14 Nov 2007 08:08:24 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	David Brownell <david-b@...bell.net>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	Florian Fainelli <florian.fainelli@...ecomint.eu>,
	Haavard Skinnemoen <hskinnemoen@...el.com>
Subject: Re: [patch 2.6.24-rc2 1/3] generic gpio -- gpio_chip support

On Wednesday 14 November 2007 19:37, David Brownell wrote:
> On Tuesday 13 November 2007, Nick Piggin wrote:

> > Upstream, all spinlocks prevent preemption.
>
> I chose my wording carefully though.  A preemption point is
> more than just a small region where preemption isn't allowed.
>
> It's one of those where preemption is *INVITED* ...

With CONFIG_PREEMPT upstream, that's exactly the same (unless
you're considering preempt breaking points, which you don't
seem t obe).

> Now, in the RT case, I believe the rationale for inviting
> preemption when dropping a lock is largely related to the
> way priority inversion is handled.  When lock contention can
> block higher priority activities, dropping the lock must
> be able to trigger the relevant activity switch.

There is no specific inviting of preemption. The locks are
preemptible -- they can be preempted even while being *held*

> ... and the raw spinlocks don't support that machinery,
> while "normal" spinlocks become inversion-aware mutexes.
>
> > But these ones
> > are raw locks rather than normal locks probably because that
> > they are trivially an innermost and correct lock.
>
> As in the $SUBJECT case, I'd say.
>
> Although another point is related to "trivial":  the data
> is being protected through an operation too trivial to be
> worth paying for any of that priority logic.

A driver shouldn't get to decide that, IMO. And if there is
some policy in the -rt tree allowing these decisions, then
it's exactly the kind of thing we don't want upsream.
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