On Sat, Apr 27, 2019 at 11:02:46AM -0700, Paul E. McKenney wrote:
> This actually passes rcutorture. But, as Andrea noted, not klitmus.
> After some investigation, it turned out that klitmus was creating kthreads
> with PF_NO_SETAFFINITY, hence the failures. But that prompted me to
> put checks into my code: After all, rcutorture can be fooled.
>
> void synchronize_rcu(void)
> {
> int cpu;
>
> for_each_online_cpu(cpu) {
> sched_setaffinity(current->pid, cpumask_of(cpu));
> WARN_ON_ONCE(raw_smp_processor_id() != cpu);
> }
> }
>
> This triggers fairly quickly, usually in less than a minute of rcutorture
> testing.
>
> And further investigation shows that sched_setaffinity()
> always returned 0.
> Is this expected behavior? Is there some configuration or setup that I
> might be missing?
ISTR there is hotplug involved in RCU torture? In that case, it can be
sched_setaffinity() succeeds to place us on a CPU, which CPU hotplug
then takes away. So when we run the WARN thingy, we'll be running on a
different CPU than expected.
If OTOH, your loop is written like (as it really should be):
void synchronize_rcu(void)
{
int cpu;
cpus_read_lock();
for_each_online_cpu(cpu) {
sched_setaffinity(current->pid, cpumask_of(cpu));
WARN_ON_ONCE(raw_smp_processor_id() != cpu);
}
cpus_read_unlock();
}
Then I'm not entirely sure how we can return 0 and not run on the
expected CPU. If we look at __set_cpus_allowed_ptr(), the only paths out
to 0 are:
- if the mask didn't change
- if we already run inside the new mask
- if we migrated ourself with the stop-task
- if we're not in fact running
That last case should never trigger in your circumstances, since @p ==
current and current is obviously running. But for completeness, the
wakeup of @p would do the task placement in that case.
