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From: Nick Piggin <nickpiggin@yahoo.com.au>
To: David Brownell <david-b@pacbell.net>
Subject: Re: [patch 2.6.24-rc2 1/3] generic gpio -- gpio_chip support
Date: Thu, 15 Nov 2007 05:51:53 +1100
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Cc: Ingo Molnar <mingo@elte.hu>, Andrew Morton <akpm@linux-foundation.org>,
       Linux Kernel list <linux-kernel@vger.kernel.org>,
       Florian Fainelli <florian.fainelli@telecomint.eu>,
       Haavard Skinnemoen <hskinnemoen@atmel.com>
References: <200711091136.20051.david-b@pacbell.net> <200711140818.59714.nickpiggin@yahoo.com.au> <200711142228.10934.david-b@pacbell.net>
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On Thursday 15 November 2007 17:28, David Brownell wrote:
> On Tuesday 13 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.


> And in the RT kernel, it's got to do the same thing ...
> because dropping that lock may mean that a higher priority
> task should immediately run and grab the lock.

No, it wakes up the highest priority waiter. If that wakeup
causes a preemption, then that's fine, but it does not have
to explicitly check preemption when dropping the lock.