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

    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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