On Thu, Aug 25, 2016 at 02:49:59PM +0300, Joonas Lahtinen wrote:
> > + * we move the task_list from this the next ready fence to the tail of
> > + * the original fence's task_list (and so added to the list to be
> > + * woken).
> > + */
> > + smp_mb__before_spinlock();
> > + if (!list_empty_careful(&x->task_list)) {
>
> if (list_empty_careful()
> return;
It's just broken. I added it recently after reading
void finish_wait(wait_queue_head_t *q, wait_queue_t *wait)
{
unsigned long flags;
__set_current_state(TASK_RUNNING);
/*
* We can check for list emptiness outside the lock
* IFF:
* - we use the "careful" check that verifies both
* the next and prev pointers, so that there cannot
* be any half-pending updates in progress on other
* CPU's that we haven't seen yet (and that might
* still change the stack area.
* and
* - all other users take the lock (ie we can only
* have _one_ other CPU that looks at or modifies
* the list).
*/
if (!list_empty_careful(&wait->task_list)) {
spin_lock_irqsave(&q->lock, flags);
list_del_init(&wait->task_list);
spin_unlock_irqrestore(&q->lock, flags);
}
}
and convinced myself that it was also safe to apply here.
Turns out that spinlock is very hard to avoid.
-Chris
--
Chris Wilson, Intel Open Source Technology Centre
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