Re: [BUG] TASK_DEAD task is able to be woken up in special condition

Yasunori Goto
2011-12-27T06:48:51Z

> On 12/26, Yasunori Goto wrote:
> >
> > >
> > > IIRC, this was already discussed a bit. Say, try_to_wake_up(TASK_INTERRUPTIBLE)
> > > can wakeup a TASK_UNINTERRUPTIBLE task if it temporary sets INTERRUPTIBLE but
> > > doesn't call schedule() in this state.
> >
> > Oleg-san,
> >
> > Could you point the discussion?
> > I don't understand yet how it occurred...
>
> Suppose that the task T does
>
> set_current_state(TASK_INTERRUPTIBLE);
>
> set_current_state(TASK_UNINTERRUPTIBLE);
> schedule();
>
> try_to_wake_up(TASK_UNINTERRUPTIBLE) in between can observe this task
> in TASK_INTERRUPTIBLE state. Then it can set RUNNING/WAKING after T
> sets ->state = TASK_UNINTERRUPTIBLE.
>
> For example, this is possibly if T simply does wait_event() twice when
> the the 1st wait_event() doesn't sleep.
>
> Basically this is the same race you described, but I think you found
> the case when we can't tolerate the spurious wakeup.
>
> Oleg.
>
> On 12/26, Yasunori Goto wrote:
> >
> > >
> > > IIRC, this was already discussed a bit. Say, try_to_wake_up(TASK_INTERRUPTIBLE)
> > > can wakeup a TASK_UNINTERRUPTIBLE task if it temporary sets INTERRUPTIBLE but
> > > doesn't call schedule() in this state.
> >
> > Oleg-san,
> >
> > Could you point the discussion?
> > I don't understand yet how it occurred...
>
> Suppose that the task T does
>
> set_current_state(TASK_INTERRUPTIBLE);
>
> set_current_state(TASK_UNINTERRUPTIBLE);
> schedule();
>
> try_to_wake_up(TASK_UNINTERRUPTIBLE) in between can observe this task
> in TASK_INTERRUPTIBLE state. Then it can set RUNNING/WAKING after T
> sets ->state = TASK_UNINTERRUPTIBLE.
>
> For example, this is possibly if T simply does wait_event() twice when
> the the 1st wait_event() doesn't sleep.
>
> Basically this is the same race you described, but I think you found
> the case when we can't tolerate the spurious wakeup.

Thanks.

I suppose your idea which uses cmpxchg(old_state, RUNNING) is best way.
I'll remake my patch.

>
> Oleg.
>

--
Yasunori Goto