On Mon, Aug 16, 2010 at 8:49 PM, Rik van Riel <riel@xxxxxxxxxxxx> wrote:
> On 08/16/2010 03:48 AM, Parmenides wrote:
>
>> if (!current_thread_info->preempt_count&& !irqs_disabled()) {
>> current_thread_info->preempt_count = PREEMPT_ACTIVE;
>> schedule();
>> current_thread_info->preempt_count = 0;
>> }
>>
>> The if statement checks whether irqs are enabled. If so, the
>> preemption will be carried out. I wonder why this check is necessary.
>> Why preemption is prohibited with irqs disabled?
>
> Because the thread you are scheduling to, won't know that
> it needs to reenable interrupts.
>
> If interrupts stay disabled "forever", the CPU will not
> get any more interrupts, like interrupts that tell it to
> reschedule, move the clock forward, etc...
Yes. And one more important point here is that the current thread may
have intentionally disabled the interrupts to disallow pre-emption
because it may be in some critical section and may want uninterrupted
access to some shared resource. Hence, no other thread should be given
control in this case.
>
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