Messages in this thread | | | Date | Sun, 5 Nov 2006 17:53:32 -0500 (EST) | From | Steven Rostedt <> | Subject | Re: PATCH? hrtimer_wakeup: fix a theoretical race wrt rt_mutex_slowlock() |
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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.
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
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