Messages in this thread | | | Date | Sun, 5 Nov 2006 19:08:35 -0800 (PST) | From | Linus Torvalds <> | Subject | Re: PATCH? hrtimer_wakeup: fix a theoretical race wrt rt_mutex_slowlock() |
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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.
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