Hi,
According to ULK3, the preempt_enable() macro decreases the preemption
counter, then checks whether the TIF_NEED_RESCHED flag is set. In this
case, a process switch is peding, so the macro invokes the
preempt_schedule() function, which essentially executes the following
code:
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?
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