On Thu, Aug 07, 2014 at 08:43:58AM -0700, Paul E. McKenney wrote:
> On Thu, Aug 07, 2014 at 10:49:21AM +0200, Peter Zijlstra wrote:
> > On Tue, Aug 05, 2014 at 02:55:10PM -0700, Paul E. McKenney wrote:
> > > +/* Check for nohz_full CPUs executing in userspace. */
> > > +static void check_no_hz_full_tasks(void)
> > > +{
> > > +#ifdef CONFIG_NO_HZ_FULL
> > > +	int cpu;
> > > +	struct task_struct *t;
> > > +
> > > +	for_each_online_cpu(cpu) {
> > > +		cond_resched();
> > > +		rcu_read_lock();
> > > +		t = rcu_dynticks_task_cur(cpu);
> > > +		if (t == NULL || is_idle_task(t)) {
> > > +			rcu_read_unlock();
> > > +			continue;
> > > +		}
> > > +		if (ACCESS_ONCE(t->rcu_tasks_holdout))
> > > +			ACCESS_ONCE(t->rcu_tasks_holdout) = 0;
> > > +		rcu_read_unlock();
> > > +	}
> > > +#endif /* #ifdef CONFIG_NO_HZ_FULL */
> > > +}
> > 
> > That's not hotplug safe afaict, and I've no idea if someone pointed that
> > out already because people refuse to trim email and I can't be arsed to
> > wade through pages and pages of quoting.
> 
> Hmmm...  That does look a bit suspicious, now that you mention it.
> After all, if a CPU is offline, its idle tasks cannot be on a
> trampoline.

But what about a CPU that in the process of getting on-line, it started
when there still were trampolines but hasn't quite made it to full
'online' status when you do this check.

Similar for going offline I suppose, we start to go offline when there
were still trampolines and got missed in that check because we already
cleared the 'online' bit but we're not quite dead yet.

I couldn't find any serialization against either on or off lining of
CPUs.