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

Oleg Nesterov
2012-01-26T16:32:49Z

On 01/26, Peter Zijlstra wrote:
>
> So since we never call schedule() the p->on_rq thing will always be
> true. This means we don't need to consider all the icky ttwu after that,
> it also means the whole thing is inside ->pi_lock.
>
> So we only have to consider the exact case Yasunori-San illustrated, and
> waiting on ->pi_lock is sufficient.

Yes, and this is why I think Yasunori-san's patch should work. Because,
to remind, it adds unlock_wait(pi_lock).

> However I think your proposal:
>
> > for (;;) {
> > tsk->state = TASK_DEAD;
> > schedule();
> > }
>
> should equally work, if we hit the race and call schedule() with ->state
> = TASK_RUNNING,

Yes, in this case everything is fine, but we can shedule() with TASK_DEAD
state. preempt_disable() can't (and shouldn't) prevent deactivate_task().

To simplify, try_to_wake_up() does

spin_lock(pi_lock);

if (!(p->state & state))
goto out;

/* WINDOW */

if (p->on_rq) {
... everything is fine ...
}

p->state = TASK_WAKING;
ttwu_queue(p, cpu);

And the exiting task does

// but do not sleep ...
current->state = TASK_UNINTERRUPTIBLE;
// ttwu() checks ->state
...
tsk->state = TASK_DEAD;
schedule();
-> deactivate_task();
-> tsk->on_rq = 0;
-> finish_task_switch();

// ttwu() checks ->on_rq

In theory it can do this all in the WINDOW above. In this case we
can wake it up again, after finish_task_switch()-put_task_struct().

No?

Oleg.