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SubjectRe: [PATCH] ubi: check kthread_should_stop() after the setting of task state
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在 2020/8/5 5:56, Richard Weinberger 写道:
> On Tue, Aug 4, 2020 at 4:58 AM Zhihao Cheng <chengzhihao1@huawei.com> wrote:
>> Oh, you're thinking about influence by schedule(), I get it. But I think
>> it still works. Because the ubi_thread is still on runqueue, it will be
>> scheduled to execute later anyway.
> It will not get woken. This is the problem.
>
>> op state of
>> ubi_thread on runqueue
>> set_current_state(TASK_INTERRUPTIBLE) TASK_INTERRUPTIBLE Yes
>> if (kthread_should_stop()) // not satisfy
>> TASK_INTERRUPTIBLE Yes
>> kthread_stop:
>> wake_up_process
>> ttwu_queue
>> ttwu_do_activate
>> ttwu_do_wakeup TASK_RUNNING Yes
>> schedule
>> __schedule(false)
>>
>> // prev->state is TASK_RUNNING, so we cannot move it from runqueue by
>> deactivate_task(). So just pick next task to execute, ubi_thread is
>> still on runqueue and will be scheduled to execute later.
> It will be in state TASK_RUNNING only if your check is reached.
>
> If kthread_stop() is called *before* your code:
> + if (kthread_should_stop()) {
> + set_current_state(TASK_RUNNING);
> + break;
> + }
>
> ...everything is fine.
> But there is still a race window between your if
> (kthread_should_stop()) and schedule() in the next line.
> So if kthread_stop() is called right *after* the if and *before*
> schedule(), the task state is still TASK_INTERRUPTIBLE
> --> schedule() will not return unless the task is explicitly woken,
> which does not happen.
Er, I can't get the point. I can list two possible situations, did I
miss other situations?

P1:ubi_thread
  set_current_state(TASK_INTERRUPTIBLE)
  if (kthread_should_stop()) {
    set_current_state(TASK_RUNNING)
    break
  }
  schedule()                            -> don't *remove* task from
runqueue if *TASK_RUNNING*, removing operation is protected by rq_lock

P2:kthread_stop
  set_bit(KTHREAD_SHOULD_STOP, &kthread->flags)
  wake_up_process(k)             -> enqueue task & set *TASK_RUNNING*,
these two operations are protected by rq_lock
  wait_for_completion(&kthread->exited)


Situation 1:
P1_set_current_state               on-rq, TASK_RUNNING -> TASK_INTERRUPTIBLE
P1_kthread_should_stop        on-rq, TASK_INTERRUPTIBLE
P2_set_bit                               on-rq, TASK_INTERRUPTIBLE ,
KTHREAD_SHOULD_STOP
P2_wake_up_process             on-rq, TASK_INTERRUPTIBLE -> TASK_RUNNING
, KTHREAD_SHOULD_STOP
P1_schedule                           on-rq, TASK_RUNNING ,
KTHREAD_SHOULD_STOP
P2_wait_for_completion        // wait for P1 exit

Situation 2:
P1_set_current_state             on-rq, TASK_RUNNING -> TASK_INTERRUPTIBLE
P1_kthread_should_stop       on-rq, TASK_INTERRUPTIBLE
P2_set_bit                             on-rq, TASK_INTERRUPTIBLE ,
KTHREAD_SHOULD_STOP
P1_schedule                          off-rq, TASK_INTERRUPTIBLE ,
KTHREAD_SHOULD_STOP
P2_wake_up_process             on-rq, TASK_INTERRUPTIBLE -> TASK_RUNNING
, KTHREAD_SHOULD_STOP
P2_wait_for_completion       // wait for P1 exit
> Before your patch, the race window was much larger, I fully agree, but
> your patch does not cure the problem
> it just makes it harder to hit.
>
> And using mdelay() to verify such a thing is also tricky because
> mdelay() will influence the task state.
>


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