+/*
+ * rcu_gp_lock ensures mutual exclusion between threads calling
+ * synchronize_rcu().
+ */
+static pthread_mutex_t rcu_gp_lock = PTHREAD_MUTEX_INITIALIZER;
+/*
+ * rcu_registry_lock ensures mutual exclusion between threads
+ * registering and unregistering themselves to/from the registry, and
+ * with threads reading that registry from synchronize_rcu(). However,
+ * this lock is not held all the way through the completion of awaiting
+ * for the grace period. It is sporadically released between iterations
+ * on the registry.
+ * rcu_registry_lock may nest inside rcu_gp_lock.
+ */
+static pthread_mutex_t rcu_registry_lock = PTHREAD_MUTEX_INITIALIZER;
+
+int32_t gp_futex;
+
+/*
+ * Global grace period counter.
+ * Contains the current RCU_GP_CTR_PHASE.
+ * Also has a RCU_GP_COUNT of 1, to accelerate the reader fast path.
+ * Written to only by writer with mutex taken. Read by both writer and readers.
+ */
+unsigned long rcu_gp_ctr = RCU_GP_COUNT;
+/*
+ * Written to only by each individual reader. Read by both the reader and the
+ * writers.
+ */
+__DEFINE_URCU_TLS_GLOBAL(struct rcu_reader, rcu_reader);