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

Hmm, maybe you can, we have the context switch in between setting
->on_cpu and clearing PREEMPT_ACTIVE and vice-versa.

The context switch (obviously) provides a full barrier, so we might be
able to -- with careful consideration -- read these two separate values
and construct something usable from them.

Something like:

	task_preempt_count(tsk) & PREEMPT_ACTIVE
	smp_rmb();
	tsk->on_cpu

And because we set PREEMPT_ACTIVE before clearing on_cpu, this should
race the right way (err towards the inclusive side).

Obviously that wants a big fat comment...