On Mon, May 09, 2016 at 11:51:18AM -0700, Davidlohr Bueso wrote:
> On Mon, 09 May 2016, Peter Zijlstra wrote:
>
> >>>So I think you're wrong here; imagine this:
> >>>
> >>>
> >>> rwsem_down_read_failed() rwsem_wake()
> >>> get_task_struct();
> >>> raw_spin_lock_irq(&wait_lock);
> >>> list_add_tail(&waiter.list, &wait_list);
> >>> raw_spin_unlock_irq(&wait_lock);
> >>>
> >>> raw_spin_lock_irqsave(&wait_lock)
> >>> __rwsem_do_wake()
> >>> while (true) {
> >>> set_task_state(tsk, TASK_UNINTERRUPTIBLE);
> >>> waiter->task = NULL
> >>> if (!waiter.task) // true
> >>> break;
> >>>
> >>> __set_task_state(tsk, TASK_RUNNING);
> >>>
> >>> do_exit();
> >>>
> >>> wake_up_process(tsk); /* BOOM */
> >>
> >>I may be missing something, but rwsem_down_read_failed() will not return
> >>until
> >>after the wakeup is done by the rwsem_wake() thread.
> >
> >The above never gets to schedule(), and even if it did, a spurious
> >wakeup could've happened, no?
>
> Ah indeed, you are most certainly correct. For some reason I was always
> considering schedule() in the picture. Hmm I'll have to think about this
> some more, but given the small chance of a waiter actually seeing the nil
> task at the first iteration I'm wondering if we could just invert the code
> and call schedule() before the task check. Saving the refcounts will serve
> _all_ reader waiters otoh, but this would obviously need numbers...
So with where you're going -- using wake_q, it naturally goes away if
you do:
wake_q_add(&wake_q, tsk);
smp_store_release(&waiter->task, NULL);
Because the wake_q already takes a task ref, and we'll not actually
issue the wakeup until after the waiter->task store.
