[PATCH v2 tip/core/rcu 1/3] srcu: Implement more-efficient reader counts

Here is a more recent version of the patch. It has more accurate comments.

[PATCH v2 tip/core/rcu 1/3] srcu: Implement more-efficient reader counts

From: Lance Roy SRCU uses two per-cpu counters: a nesting counter to count the number of active critical sections, and a sequence counter to ensure that the nesting counters don't change while they are being added together in srcu_readers_active_idx_check(). This patch instead uses per-cpu lock