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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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