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Date: Sat, 8 Aug 2009 02:31:33 +0200
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Jonathan Corbet <corbet@lwn.net>, Ingo Molnar <mingo@elte.hu>
Cc: LKML <linux-kernel@vger.kernel.org>, Mark Gross <mgross@linux.intel.com>
Subject: Re: [PATCH 1/2] pm_qos: remove BKL
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On Fri, Aug 07, 2009 at 09:08:18AM -0600, Jonathan Corbet wrote:
> On Fri, 7 Aug 2009 07:54:13 +0200
> Frederic Weisbecker <fweisbec@gmail.com> wrote:
> 
> > May be the last doubt could be the blocking_notifier_call_chain() call from
> > update_target(). Not sure if these notifier handlers can expect to be called
> > concurrently?
> 
> I will confess that I hadn't audited the notifiers.  One could easily
> argue that concurrent calls to update_target() are entirely possible
> with the current code (only one of the callers had BKL protection),
> but, then, I'm supposed to be trying to make things better.
> 
> The notifier call chain is already protected against concurrent
> modification, but, since an rwsem is used, concurrent calls to the
> notifiers themselves are possible.  A quick grep shows that, in 2.6.31-rc5,
> there is exactly one notifier registered.  It's in
> drivers/cpuidle/cpuidle.c; here's the whole thing:
> 
> static void smp_callback(void *v)
> {
> 	/* we already woke the CPU up, nothing more to do */
> }
> 
> After deep meditation on possible race condition scenarios, I am force to
> conclude that this particular notifier already has all of the protection it
> needs, and that any extra locking is likely to be superfluous.


Hehe :-)

So it would be nice to apply these patches.
Ingo?

 
> Thanks for looking at the patch,
> 
> jon