linux-kernel - Re: [PATCH 0/2][RT] powerpc - fix bug in irq reverse mapping radix tree

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Date:	Thu, 24 Jul 2008 08:17:38 +1000
From:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
To:	Sebastien Dugue <sebastien.dugue@...l.net>
Cc:	Linux RT Users <linux-rt-users@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"linuxppc-dev@...abs.org" <linuxppc-dev@...abs.org>,
	Tim Chavez <tinytim@...ibm.com>,
	Jean Pierre Dion <jean-pierre.dion@...l.net>,
	Gilles Carry <Gilles.Carry@....bull.net>, paulus@...ba.org,
	michael@...erman.id.au
Subject: Re: [PATCH 0/2][RT] powerpc - fix bug in irq reverse mapping radix
	tree


>   The root cause of this bug lies in the fact that the XICS interrupt controller
> uses a radix tree for its reverse irq mapping and that we cannot allocate the tree
> nodes (even GFP_ATOMIC) with preemption disabled.

Is that yet another caes of -rt changing some basic kernel semantics ?

>   In fact, we have 2 nested preemption disabling when we want to allocate
> a new node:
> 
>   - setup_irq() does a spin_lock_irqsave() before calling xics_startup() which
>     then calls irq_radix_revmap() to insert a new node in the tree
> 
>   - irq_radix_revmap() also does a spin_lock_irqsave() (in irq_radix_wrlock())
>     before the radix_tree_insert()
> 
>   The first patch moves the call to irq_radix_revmap() from xics_startup() out to
> xics_host_map_direct() and xics_host_map_lpar() which are called with preemption
> enabled.

I suppose that would work.

>   The second patch is a little more involved in that it takes advantage of
> the concurrent radix tree to simplify the locking requirements and allows
> to allocate a new node outside a preemption disabled section.
> 
>   I just hope I've correctly understood the concurrent radix trees semantic
> and got the (absence of) locking right.

Hrm, that will need some scrutinity.

Thanks for looking at this.

Cheers,
Ben.


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