Hi, So Paul reported rcutorture hitting a NULL dereference, and patch #1 fixes it.
Now, patch #1 is obviously correct, but I can't explain how exactly it leads to the observed NULL pointer dereference. The NULL pointer deref happens in find_matching_se()'s last while() loop when is_same_group() fails even though both parents are NULL. The only explanation I have for that is that we just did an activate_task() while: 'task_cpu(p) != cpu_of(rq)', because then 'p->se.cfs_rq' doesn't match. However, I can't see how the lack of #1 would lead to that. Never-the-less, patch #2 adds assertions to warn us of this case. Patch #3 is a trivial rename that ought to eradicate some confusion. The last 3 patches is what I ended up with for cleaning up the whole smp_call_function/irq_work/ttwu thing more.
