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Date: Fri, 26 Nov 2010 13:17:34 +0100
From: Heiko Carstens <heiko.carstens@de.ibm.com>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@elte.hu>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        linux-kernel@vger.kernel.org,
        Christof Schmitt <christof.schmitt@de.ibm.com>,
        Frank Blaschka <frank.blaschka@de.ibm.com>,
        Horst Hartmann <horsth@linux.vnet.ibm.com>, stable@kernel.org
Subject: Re: [patch 3/3] nohz/s390: fix arch_needs_cpu() return value on
 offline cpus
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On Fri, Nov 26, 2010 at 01:14:07PM +0100, Peter Zijlstra wrote:
> On Fri, 2010-11-26 at 13:01 +0100, Heiko Carstens wrote:
> > plain text document attachment (003_arch_needs_cpu.diff)
> > From: Heiko Carstens <heiko.carstens@de.ibm.com>
> > 
> > This fixes the same problem as described in the patch "nohz: fix
> > printk_needs_cpu() return value on offline cpus" for the arch_needs_cpu()
> > primitive.
> > This specific bug was indrocuded with 3c5d92a0 "nohz: Introduce arch_needs_cpu".
> > 
> > In this case a cpu hotplug notifier is used to fix the issue in order to keep
> > the normal/fast path small. All we need to do is to clear the condition that
> > makes arch_needs_cpu() return 1 since it is just a performance improvement which
> > is supposed to keep the local tick running for a short period if a cpu goes
> > idle. Nothing special needs to be done except for clearing the condition.
> > 
> > Cc: stable@kernel.org
> > Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
> 
> OK, and s390 seems to be the only architecture making use of that
> interface.
> 
> Did you audit all the other *_needs_cpu() interfaces for this same
> problem?

There's only rcu_needs_cpu() left and rcu indeed has a cpu hotplug notifier.
So there _shouldn't_ be any problems with that.

> Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> 
> Will you take this through the s390?

Yes. Thanks!