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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->curr = next;
next->cpu = prev->cpu;
context_switch(prev, next);
} else

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

- Davide

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