callbacks are executed in batch periodically after a grace period.
Do _not_ use defer_rcu() within a read-side critical section, because
it may call synchronize_rcu() if the thread queue is full.
+ * Provides defer_rcu_ratelimit() primitive, which acts just like
+ defer_rcu(), but takes an additional rate limiter callback forcing
+ synchronized callback execution of the limiter returns non-zero.
Being careful with signals