linux-kernel - Re: [PATCH 2/2][RT] powerpc - Make the irq reverse mapping radix tree lockless

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Message-Id: <200807242111.35338.nickpiggin@yahoo.com.au>
Date:	Thu, 24 Jul 2008 21:11:34 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Sebastien Dugue <sebastien.dugue@...l.net>
Cc:	Linux-rt <linux-rt-users@...r.kernel.org>,
	linux-ppc <linuxppc-dev@...abs.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <michael@...erman.id.au>,
	Tim Chavez <tinytim@...ibm.com>,
	Jean Pierre Dion <jean-pierre.dion@...l.net>,
	Gilles Carry <Gilles.Carry@....bull.net>
Subject: Re: [PATCH 2/2][RT] powerpc - Make the irq reverse mapping radix tree lockless

On Thursday 24 July 2008 20:50, Sebastien Dugue wrote:
> From: Sebastien Dugue <sebastien.dugue@...l.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?
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