Re: [PATCH v3 tip/core/rcu 1/9] rcu: Add call_rcu_tasks()

Paul E. McKenney
2014-08-04T12:37:36Z

On Mon, Aug 04, 2014 at 02:25:15PM +0200, Peter Zijlstra wrote:
> 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.

Right, I detect userspace tasks using RCU's scheduling-clock handler.
(And code to handle the NO_HZ_FULL case is under construction.)

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

OK, got it.

Thanx, Paul