Re: PATCH? hrtimer_wakeup: fix a theoretical race wrt rt_mutex_slowlock()

[Oleg Nesterov <oleg@tv-sign.ru>]

On 11/05, Steven Rostedt wrote:
> 
> On Sun, 5 Nov 2006, Linus Torvalds wrote:
> 
> >
> > That said, since "task->state" in only tested _inside_ the runqueue lock,
> > there is no race that I can see. Since we've gotten the runqueue lock in
> > order to even check task-state, the processor that _sets_ task state must
> > not only have done the "spin_lock()", it must also have done the
> > "spin_unlock()", and _that_ will not allow either the timeout or the task
> > state to haev leaked out from under it (because that would imply that the
> > critical region leaked out too).
> >
> > So I don't think the race exists anyway - the schedule() will return
> > immediately (because it will see TASK_RUNNING), and we'll just retry.
> >
> 
> 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.

Yes, yes, thanks.

( Actually, this was more "is my understanding correct?" than a patch )

Thanks!

> 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)
> 
> 
> Again, this is very theoretical, and I don't even think that this can
> happen if you tried to make it.  But I guess if hardware were to change in
> the future with the same rules that we have today with barriers, that this
> can be a race.
> 
> -- Steve
> 
>