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

Paul E. McKenney
2014-08-07T15:44:07Z

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.

Thanx, Paul