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From: Nick Piggin <nickpiggin@yahoo.com.au>
To: Sebastien Dugue <sebastien.dugue@bull.net>
Subject: Re: [PATCH 2/2][RT] powerpc - Make the irq reverse mapping radix tree lockless
Date: Thu, 24 Jul 2008 21:11:34 +1000
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Cc: Linux-rt <linux-rt-users@vger.kernel.org>,
       linux-ppc <linuxppc-dev@ozlabs.org>,
       linux-kernel <linux-kernel@vger.kernel.org>,
       Benjamin Herrenschmidt <benh@kernel.crashing.org>,
       Paul Mackerras <paulus@samba.org>,
       Michael Ellerman <michael@ellerman.id.au>,
       Tim Chavez <tinytim@us.ibm.com>,
       Jean Pierre Dion <jean-pierre.dion@bull.net>,
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On Thursday 24 July 2008 20:50, Sebastien Dugue wrote:
> From: Sebastien Dugue <sebastien.dugue@bull.net>
> Date: Tue, 22 Jul 2008 11:56:41 +0200
> Subject: [PATCH][RT] powerpc - Make the irq reverse mapping radix tree
> lockless
>
>   The radix tree used by interrupt controllers for their irq reverse
> mapping (currently only the XICS found on pSeries) have a complex locking
> scheme dating back to before the advent of the concurrent radix tree on
> preempt-rt.
>
>   Take advantage of this and of the fact that the items of the tree are
> pointers to a static array (irq_map) elements which can never go under us
> to simplify the locking.
>
>   Concurrency between readers and writers are handled by the intrinsic
> properties of the concurrent radix tree. Concurrency between the tree
> initialization which is done asynchronously with readers and writers access
> is handled via an atomic variable (revmap_trees_allocated) set when the
> tree has been initialized and checked before any reader or writer access
> just like we used to check for tree.gfp_mask != 0 before.

Hmm, RCU radix tree is in mainline too for quite a while. I thought
Ben had already converted this code over ages ago...

Nothing against the -rt patch, but mainline should probably be updated
to use RCU as well?