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Subject: Re: [PATCH 05/32] nohz: Move rcu dynticks idle mode handling to
 idle enter/exit APIs
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Anton Blanchard <anton@au1.ibm.com>, Avi Kivity <avi@redhat.com>,
        Ingo Molnar <mingo@elte.hu>, Lai Jiangshan <laijs@cn.fujitsu.com>,
        "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>,
        Stephen Hemminger <shemminger@vyatta.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Tim Pepper <lnxninja@linux.vnet.ibm.com>,
        Paul Menage <paul@paulmenage.org>
Date: Tue, 30 Aug 2011 17:42:32 +0200
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On Tue, 2011-08-30 at 17:33 +0200, Frederic Weisbecker wrote:
> > See all that is still kernelspace ;-) I think I know what you mean to
> > say though, but seeing as you note there is even now a known shortcoming
> > I'm not very confident its a solid construction. What will help us find
> > such holes?
> 
> This: https://lkml.org/lkml/2011/6/23/744
> 
> It's in one of Paul's branches and should make it for the next merge window.
> This should detect any of such holes. I made that on purpose for the nohz cpusets
> when I saw how much error prone that can be with rcu :)

OK, good ;-)

> > I would much rather we not rely on such fragile things too much.. this
> > RCU stuff wants way more thought, as it stands your patch-set doesn't do
> > anything useful IMO.
> 
> Not sure what you mean. Well that Rcu thing for sure is fragile but we have
> the tools ready to find the problems. 

Right that thing you linked above does catch abuse, still your current
proposal means that due to RCU it will basically never disable the tick.