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    SubjectRe: [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.


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