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    SubjectRe: The O(1) scheduler breaks UML
    On Sun, 13 Jan 2002, Jeff Dike wrote:

    > The new scheduler holds IRQs off across the call to context_switch. UML's
    > _switch_to expects them to be enabled when it is called, and things go
    > badly wrong when they are not.
    >
    > Because UML has a host process for each UML thread, SIGIO needs to be
    > forwarded from one process to the next during a context switch. A SIGIO
    > arriving during the window between the disabling of IRQs and forwarding of
    > IRQs to the next process will be trapped on the process going out of
    > context. This happens fairly regularly and causes hangs because some process
    > is waiting for disk IO which never arrives because the process that was notified
    > of the completion is switched out.
    >
    > So, is it possible to enable IRQs across the call to _switch_to?

    Yes, this should work :


    if (likely(prev != next)) {
    rq->nr_switches++;
    rq->curr = next;
    next->cpu = prev->cpu;
    spin_unlock_irq(&rq->lock);
    context_switch(prev, next);
    } else
    spin_unlock_irq(&rq->lock);

    and there's no need for barrier() and rq reload in this way.




    - Davide


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