Re: [PATCH] mm/mmu_notifier: Remove non_block_start/end() from notifier invocation

> + * Atomic-flavor readers run with preemption disabled, so every
> + * read-side critical section is bounded and running (not blocked) on
> + * some CPU, which is what makes spinning here sane: the wait is
> + * bounded by the longest such section. No index flip and no
> + * grace-period sequence update occur, so concurrent call_srcu(),
> + * synchronize_srcu() and srcu_barrier() are entirely unaffected.

Making synchronize_srcu_atomic() spin in CONFIG_PREEMPT_RT is a
slightly tall request - it's basically putting it on the same level as
raw_spinlock. But if the MMU notifier guys really want to make OOM
notifiers atomic, this would be basically the only way to do so.

If instead it's okay to remove nonblock_start/end under
CONFIG_PREEMPT_RT, for RT kernels I'd rather avoid
preempt_disable/enable and only make synchronize_srcu_atomic() try to
skip the index flip:

   if (!try_synchronize_srcu(ssp))
      synchronize_srcu_expedited(ssp);

Paolo