Messages in this thread | | | Date | Sat, 7 Oct 2017 11:30:19 +0800 | From | zhouchengming <> | Subject | Re: [PATCH] sched/rt.c: pick and check task if double_lock_balance() unlock the rq |
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Hi Steven, Peter,
On 2017/9/26 11:18, Steven Rostedt wrote: > On Tue, 26 Sep 2017 09:23:20 +0800 > zhouchengming<zhouchengming1@huawei.com> wrote: > >> On 2017/9/26 3:40, Steven Rostedt wrote: >>> On Mon, 11 Sep 2017 14:51:49 +0800 >>> Zhou Chengming<zhouchengming1@huawei.com> 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. > Ah crap. This is caused by 7608dec2ce20 ("sched: Drop the rq argument > to sched_class::select_task_rq()"). Because this code depends on > try_to_wake_up() grabbing the task's rq lock. But it no longer does > that, and it causes this race. > > OK, I need to look at this deeper when I'm not so jetlagged and typing > this because I can't sleep at 5am. > > Thanks for pointing this out! > > It may be fixed by simply grabbing the run queue lock on migration, as > that would sync things up.
Is there any new solution? I don't think grabbing the rq lock without the task->pi_lock will fix this problem. And I think my patch is correct and the changes are small.
Thanks!
> Peter? > > > -- Steve > > > > . >
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