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Subject: Re: [PATCH 2/2][RT] powerpc - Make the irq reverse mapping radix
	tree lockless
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Sebastien Dugue <sebastien.dugue@bull.net>
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Date: Fri, 25 Jul 2008 18:40:21 +1000
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Cc: Nick Piggin <nickpiggin@yahoo.com.au>, Tim Chavez <tinytim@us.ibm.com>,
	Linux-rt <linux-rt-users@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
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On Fri, 2008-07-25 at 10:36 +0200, Sebastien Dugue wrote:
> On Fri, 25 Jul 2008 18:27:20 +1000 Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:
> 
> > On Fri, 2008-07-25 at 09:49 +0200, Peter Zijlstra wrote:
> > > 
> > > 
> > > The only advantage of the concurrent radix tree over this model is that
> > > it can potentially do multiple modification operations at the same time.
> > 
> > Yup, we do not need that for the irq revmap... concurrent lookup is all we need.
> > 
> 
>   Shouldn't we care about concurrent insertion and deletion in the tree? I agree
> that concern might be a bit artificial but in theory that can happen.

Yes, we just need to protect it with a big hammer, like a spinlock, it's
not a performance critical code path.

Ben.