On Sun, 5 Nov 2006, Steven Rostedt wrote:
>
> This whole situation is very theoretical, but I think this actually can
> happen *theoretically*.
>
> OK, the spin_lock doesn't do any serialization, but the unlock does. But
> the problem can happen before the unlock. Because of the loop.
>
> CPU 1 CPU 2
>
> task_rq_lock()
>
> p->state = TASK_RUNNING;
>
>
> (from bottom of for loop)
> set_current_state(TASK_INTERRUPTIBLE);
>
> for (;;) { (looping)
>
> if (timeout && !timeout->task)
>
>
> (now CPU implements)
> t->task = NULL
>
> task_rq_unlock();
>
> schedule() (with state == TASK_INTERRUPTIBLE)
Yeah, that seems a real bug. You _always_ need to actually do the thing
that you wait for _before_ you want it up. That's what all the scheduling
primitives depend on - you can't wake people up first, and then set the
condition variable.
So if a rt_mutex depeds on something that is set inside the rq-lock, it
needs to get the task rw-lock in order to check it.
Linus
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