On Mon, Aug 04, 2014 at 04:50:44AM -0700, Paul E. McKenney wrote:
> OK, I will bite...
> 
> What kinds of tasks are on a runqueue, but neither ->on_cpu nor
> PREEMPT_ACTIVE?

Userspace tasks, they don't necessarily get PREEMPT_ACTIVE when
preempted. Now obviously you're not _that_ interested in userspace tasks
for this, so that might be ok.

But the main point was, you cannot use ->on_cpu or PREEMPT_ACTIVE
without holding rq->lock.