i386 machine_restart unsafe in interrupt context

[Castor Fu]

I have a problem where systems fail to reboot on panic().  I've resolved
it by changing smp_send_stop() to use an NMI (like the KDB patch does to
manage communication).

The source of the problem is that the panic path has the following:

    panic()
        machine_restart()
            machine_real_restart()
                smp_send_stop()
                    smp_call_function()

and smp_call_function() is not safe in an interrupt context.

I imagine people might want to handle this differently, but I'd be
happy to diffs if there's interest.  It may be that there are enough
cases like this that smp_call_function might want a version that
uses an NMI. . .

    -Castor Fu
    [e-mail]