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

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Message-Id: <200711150619.27571.nickpiggin@yahoo.com.au>
Date:	Thu, 15 Nov 2007 06:19:27 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	David Brownell <david-b@...bell.net>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	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 Thursday 15 November 2007 19:17, David Brownell wrote:
> On Wednesday 14 November 2007, Nick Piggin wrote:
> > > > > All this does is prevent constant and needless checking for
> > > > > "do you want to preempt me now?" "now?" "now?" in "now?" the
> > > > > middle "now?" of "now?" i/o "now?" loops.
> > > >
> > > > Actually that's wrong.
> > >
> > > Certainly it's right for the mainstream kernel.  Dropping a
> > > lock (other than a raw spinlock) does that checking; when a
> > > loop needs to acquire then drop such a lock, that's exactly
> > > what's going on.
> >
> > Obviously a raw spinlock is no different from a regular
> > spinlock upstream.
>
> Erm, no.  The raw ones don't have the extra logic when
> the lock gets dropped.

If you don't have preemption disabled already, then it is a
bug to use raw spinlocks. If you do have preemption disabled,
then a regular spinlock isn't going to check preemption after
the unlock either.
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