Re: A question about cpu_idle()

[yi li <liyi.dev@gmail.com>]

On Thu, Oct 22, 2009 at 8:34 PM, Gregory Haskins <ghaskins@novell.com> wrote:
> Yes, that is right.  irq_disable is a effective superset of preempt-disable() in this context because it also blocks those RESCHED_IPI events from being received.  Of course, disabling interrupts also has other side effects since it also disables *all* interrupts (like timers, etc) so it should be used sparingly.  In this case, we are simply bridging the preempt_enable_no_resched() and schedule() to make sure it is truly an atomic transition to a sleep state, so its use is justified.
>
> I hope this helps, and feel free to ask any more questions you wish.

Thanks. Yes it make sense to me.

Another question may be basic - but I just cannot figure out.

Why PREEMPT_RT patch disables local irq (with local_irq_disable())
before __schedule()? Is it to ensure atomic excution of __schedule()?
If so, does that mean, linux kernel without PREEMPT_RT patch suffers
from racy problem in schedule()?

asmlinkage void __sched schedule(void)
{
  need_resched:
         local_irq_disable();
         __schedule();
         local_irq_enable();

        if (need_resched())
                goto need_resched;
}

I saw similiar code in preempt_schedule(), __cond_schedule() and
preempt_schedule_irq().

Regards,
-Yi
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