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SubjectRe: [PATCH 0/2][RT] powerpc - fix bug in irq reverse mapping radix tree
On Thu, 24 Jul 2008 08:17:38 +1000 Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:

>
> > 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 ?

Ahem, not really new: http://lkml.org/lkml/2007/11/12/211

>
> > 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.

It should indeed. Instead of inserting the new mapping at request_irq() time,
we do it a bit before at create_irq_mapping time.

>
> > 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.

Yep, that will need a few more pair of eyes along with brains behind those ;-)

Thanks,

Sebastien.

>
> Thanks for looking at this.
>
> Cheers,
> Ben.
>
>
>


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