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    SubjectRe: [PATCH] ia64: prevent irq migration race in __cpu_disable path
    * Luck, Tony <tony.luck@intel.com>:
    > > This is wrong because fixup_irqs calls migrate_irqs, and in
    > > migrate_irqs, we use the cpu_online_map to:
    > >
    > > 1. look for interrupts on current CPU
    > > 2. if we find one, move it to the first available CPU in
    > > the cpu_online_map
    > >
    > > This means we can potentially migrate an interrupt off ourself
    > > back to... ourself. Uh oh.
    >
    > Should we make migrate_irqs smarter then ... does any caller really
    > expect that it would "migrate" the irq to the same cpu?

    The only thing migrate_irqs does is locate irqs that have their
    CPU affinity set to the current CPU, and if so, changes the
    affinity.

    We still have possible pending timer interrupts that we need to
    handle, so I'm not seeing how changing the migrate_irqs
    implementation (to avoid migrating to ourself) will handle that.

    On one hand, I think the only irq handler that can be called at
    this point is our timer_interrupt, which doesn't seem to be using
    any RCU APIs.

    On the other hand, if we really want to make sure that we're not
    calling interrupt handlers with our CPU marked as 'offline', then
    we need to fix ia64_process_pending_intr() so that we're not
    firing our timer_interrupt with a NULL pt_regs.

    Add in to the mix that x86 seems to have the same assumption that
    we do (that it's ok to call our timer interrupt handler even if
    we're already marked as 'offline').

    I'm leaning towards reverting Paul's previous commit entirely
    because

    - migrate_platform_irqs() doesn't cause any interrupt
    handlers to be fired by itself

    - it also uses cpu_online)map to find a new CPU to assign
    CPEI to

    Thoughts?

    /ac



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