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Date: Mon, 14 Jan 2002 10:55:37 +0100 (CET)
From: Ingo Molnar <mingo@elte.hu>
Reply-To: <mingo@elte.hu>
To: Davide Libenzi <davidel@xmailserver.org>
Cc: Jeff Dike <jdike@karaya.com>, Linus Torvalds <torvalds@transmeta.com>,
        <linux-kernel@vger.kernel.org>
Subject: Re: The O(1) scheduler breaks UML
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On Sun, 13 Jan 2002, Davide Libenzi wrote:

> > 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);

this change is incredibly broken on SMP - eg. what protects 'prev' from
being executed on another CPU prematurely. It's even broken on UP:
interrupt context that changes current->need_resched needs to be aware of
nonatomic context switches. See my previous mail.

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

we can remove the barrier(), but for a different reason: the asm volatile
definition of the switch_to macro is a compilation barrier in itself
already. I've removed the barrier() from my tree, the change will be in
the -H8 patch. The rq = this_rq() reload is still necessery.

	Ingo

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