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SubjectRe: [PATCH 0/2][RT] powerpc - fix bug in irq reverse mapping radix tree
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> 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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