Hi Steve,
On Fri, Dec 18, 2015 at 11:15:01AM -0800, Steve Muckle wrote:
> On 12/16/2015 11:17 PM, Leo Yan wrote:
> > Could you check if below corner case will introduce logic error?
> > The task still will be removed from rq if timer tick is triggered
> > between two time's set_current_state().
> >
> > set_current_state(TASK_INTERRUPTIBLE);
> > `-------> timer_tick and
> > schedule();
> > do_something...
> > set_current_state(TASK_RUNNING);
> >
> > It will be safe for combination for set_current_state()/schedule()
> > with waken_up_process():
> >
> > Thread_A: Thread_B:
> >
> > set_current_state(TASK_INTERRUPTIBLE);
> > `-------> timer_tick and
> > schedule();
> > ....
> > wake_up_process(Thread_A);
> > <---------------------/
> > schedule();
> >
> > The first time's schedule() will remove task from rq which is caused
> > by timer tick and call schedule(), and the second time schdule() will
> > be equal yeild().
>
> I was initially concerned about preemption while task state =
> TASK_INTERRUPTIBLE as well, but a task with state TASK_INTERRUPTIBLE is
> not dequeued if it is preempted. See core.c:__schedule():
>
> if (!preempt && prev->state) {
> if (unlikely(signal_pending_state(prev->state, prev))) {
> prev->state = TASK_RUNNING;
> } else {
> deactivate_task(rq, prev, DEQUEUE_SLEEP);
> prev->on_rq = 0;
>
> I knew this had to be the case, because this design pattern is used in
> many other places in the kernel, so many things would be very broken if
> this were a problem.
You are right, I went through the code again and sched tick irq will
call preempt_schedule_irq() and __schedule(true); so finally set the
parameter "preempt" = true. Sorry for noise :p
---8<---
arch/arm64/kernel/entry.S:
#ifdef CONFIG_PREEMPT
el1_preempt:
mov x24, lr
1: bl preempt_schedule_irq // irq en/disable is done inside
ldr x0, [tsk, #TI_FLAGS] // get new tasks TI_FLAGS
tbnz x0, #TIF_NEED_RESCHED, 1b // needs rescheduling?
ret x24
#endif
Thanks,
Leo Yan
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