rcu: Introduce rcu_cpu_online()

Export the RCU point of view as to when a CPU is considered offline
(ie: when does RCU consider that a CPU is sufficiently down in the
hotplug process to not feature any possible read side).

This will be used by RCU-tasks whose vision of an offline CPU should
reasonably match the one of RCU core.

Fixes: cff9b233 ("kernel/sched: Modify initial boot task idle setup")
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>

kernel/rcu/rcu.h

@@ -500,6 +500,7 @@ static inline void rcu_expedite_gp(void) { }
 static inline void rcu_unexpedite_gp(void) { }
 static inline void rcu_async_hurry(void) { }
 static inline void rcu_async_relax(void) { }
+static inline bool rcu_cpu_online(int cpu) { return true; }
 #else /* #ifdef CONFIG_TINY_RCU */
 bool rcu_gp_is_normal(void);     /* Internal RCU use. */
 bool rcu_gp_is_expedited(void);  /* Internal RCU use. */
@@ -509,6 +510,7 @@ void rcu_unexpedite_gp(void);
 void rcu_async_hurry(void);
 void rcu_async_relax(void);
 void rcupdate_announce_bootup_oddness(void);
+bool rcu_cpu_online(int cpu);
 #ifdef CONFIG_TASKS_RCU_GENERIC
 void show_rcu_tasks_gp_kthreads(void);
 #else /* #ifdef CONFIG_TASKS_RCU_GENERIC */

kernel/rcu/tree.c

@@ -4216,6 +4216,13 @@ static bool rcu_rdp_cpu_online(struct rcu_data *rdp)
 	return !!(rdp->grpmask & rcu_rnp_online_cpus(rdp->mynode));
 }
 
+bool rcu_cpu_online(int cpu)
+{
+	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
+
+	return rcu_rdp_cpu_online(rdp);
+}
+
 #if defined(CONFIG_PROVE_RCU) && defined(CONFIG_HOTPLUG_CPU)
 
 /*