Re: The O(1) scheduler breaks UML

[Davide Libenzi]

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