On Thu, Oct 22, 2009 at 8:34 PM, Gregory Haskins <ghaskins@xxxxxxxxxx> 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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