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    Subject[PATCH] IRQ: prevent enabling of previously disabled interrupt
    Hi,
    This fix prevents re-disabling and enabling of a previously disabled
    interrupt in 2.6.16-rc5. On an SMP system with irq balancing enabled;
    If an interrupt is disabled from within its own interrupt context with
    disable_irq_nosync and is also earmarked for processor migration, the
    interrupt is blindly moved to the other processor and enabled without
    regard for its current "enabled" state. If there is an interrupt
    pending, it will unexpectedly invoke the irq handler on the new irq
    owning processor (even though the irq was previously disabled)

    The more intuitive fix would be to invoke disable_irq_nosync and
    enable_irq, but since we already have the desc->lock from __do_IRQ, we
    cannot call them directly. Instead we can use the same logic to
    disable and enable found in disable_irq_nosync and enable_irq, with
    regards to the desc->depth.

    This now prevents a disabled interrupt from being re-disabled, and
    more importantly prevents a disabled interrupt from being incorrectly
    enabled on a different processor.

    Signed-off-by: Bryan Holty <lgeek@frontiernet.net>

    --- 2.6.16-rc5/include/linux/irq.h
    +++ b/include/linux/irq.h
    @@ -155,9 +155,13 @@
    * Being paranoid i guess!
    */
    if (unlikely(!cpus_empty(tmp))) {
    - desc->handler->disable(irq);
    + if (likely(!desc->depth++))
    + desc->handler->disable(irq);
    +
    desc->handler->set_affinity(irq,tmp);
    - desc->handler->enable(irq);
    +
    + if (likely(!--desc->depth))
    + desc->handler->enable(irq);
    }
    cpus_clear(pending_irq_cpumask[irq]);
    }

    --
    - Bryan Holty

    -
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