cpu stopper threads and load balancing leads to deadlock

[Matt Fleming <matt@codeblueprint.co.uk>]

Hi guys,

We've seen a bug in one of our SLE kernels where the cpu stopper
thread ("migration/15") is entering idle balance. This then triggers
active load balance.

At the same time, a task on another CPU triggers a page fault and NUMA
balancing kicks in to try and migrate the task closer to the NUMA node
for that page (we're inside stop_two_cpus()). This faulting task is
spinning in try_to_wake_up() (inside smp_cond_load_acquire(&p->on_cpu,
!VAL)), waiting for "migration/15" to context switch.

Unfortunately, because "migration/15" is doing active load balance
it's spinning waiting for the NUMA-page-faulting CPU's stopper lock,
which is already held (since it's inside stop_two_cpus()).

Deadlock ensues.

This seems like a situation that should be prohibited, but I cannot
find any code to prevent it. Is it OK for stopper threads to load
balance? Is there something that should prevent this situation from
happening?