Re: [PATCH] sched/rt.c: pick and check task if double_lock_balance() unlock the rq

[zhouchengming]

On 2017/9/26 3:40, Steven Rostedt wrote:
> On Mon, 11 Sep 2017 14:51:49 +0800
> Zhou Chengming<zhouchengming1@xxxxxxxxxx>  wrote:
>
> > push_rt_task() pick the first pushable task and find an eligible
> > lowest_rq, then double_lock_balance(rq, lowest_rq). So if
> > double_lock_balance() unlock the rq (when double_lock_balance() return 1),
> > we have to check if this task is still on the rq.
> >
> > The problem is that the check conditions are not sufficient:
> >
> > if (unlikely(task_rq(task) != rq ||
> > 	     !cpumask_test_cpu(lowest_rq->cpu,&task->cpus_allowed) ||
> > 	     task_running(rq, task) ||
> > 	     !rt_task(task) ||
> > 	     !task_on_rq_queued(task))) {
> >
> > cpu2				cpu1			cpu0
> > push_rt_task(rq1)
> >    pick task_A on rq1
> >    find rq0
> >      double_lock_balance(rq1, rq0)
> >        unlock(rq1)
> > 				rq1 __schedule
> > 				  pick task_A run
> > 				task_A sleep (dequeued)
> >        lock(rq0)
> >        lock(rq1)
> >      do_above_check(task_A)
> >        task_rq(task_A) == rq1
> >        cpus_allowed unchanged
> >        task_running == false
> >        rt_task(task_A) == true
> > 							try_to_wake_up(task_A)
> > 							  select_cpu = cpu3
> > 							  enqueue(rq3, task_A)
> How can this happen? The try_to_wake_up(task_A) needs to grab the rq
> that task A is on, and we have that rq lock.
>
> /me confused.
>
> -- Steve

Thanks for the reply!
After the task_A sleep on cpu1, the try_to_wake_up(task_A) on cpu0 select a different cpu3,
so it will grab the rq3 lock, not the rq1 lock.

Thanks.

> > 							task_A->on_rq = 1
> >        task_on_rq_queued(task_A)
> >      above_check passed, return rq0
> >      ...
> >      migrate task_A from rq1 to rq0
> >
> > So we can't rely on these checks of task_A to make sure the task_A is
> > still on the rq1, even though we hold the rq1->lock. This patch will
> > repick the first pushable task to be sure the task is still on the rq.
> >
> > Signed-off-by: Zhou Chengming<zhouchengming1@xxxxxxxxxx>
> .