rcu-tasks: Make RCU tasks trace also wait for idle tasks

This commit scans the CPUs, adding each CPU's idle task to the list of
tasks that need quiescent states.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/tasks.h

@@ -15,7 +15,7 @@ struct rcu_tasks;
 typedef void (*rcu_tasks_gp_func_t)(struct rcu_tasks *rtp);
 typedef void (*pregp_func_t)(void);
 typedef void (*pertask_func_t)(struct task_struct *t, struct list_head *hop);
-typedef void (*postscan_func_t)(void);
+typedef void (*postscan_func_t)(struct list_head *hop);
 typedef void (*holdouts_func_t)(struct list_head *hop, bool ndrpt, bool *frptp);
 typedef void (*postgp_func_t)(struct rcu_tasks *rtp);
 
@@ -331,7 +331,7 @@ static void rcu_tasks_wait_gp(struct rcu_tasks *rtp)
 	rcu_read_unlock();
 
 	set_tasks_gp_state(rtp, RTGS_POST_SCAN_TASKLIST);
-	rtp->postscan_func();
+	rtp->postscan_func(&holdouts);
 
 	/*
 	 * Each pass through the following loop scans the list of holdout
@@ -415,7 +415,7 @@ static void rcu_tasks_pertask(struct task_struct *t, struct list_head *hop)
 }
 
 /* Processing between scanning taskslist and draining the holdout list. */
-void rcu_tasks_postscan(void)
+void rcu_tasks_postscan(struct list_head *hop)
 {
 	/*
 	 * Wait for tasks that are in the process of exiting.  This
@@ -936,9 +936,17 @@ static void rcu_tasks_trace_pertask(struct task_struct *t,
 	trc_wait_for_one_reader(t, hop);
 }
 
-/* Do intermediate processing between task and holdout scans. */
-static void rcu_tasks_trace_postscan(void)
+/*
+ * Do intermediate processing between task and holdout scans and
+ * pick up the idle tasks.
+ */
+static void rcu_tasks_trace_postscan(struct list_head *hop)
 {
+	int cpu;
+
+	for_each_possible_cpu(cpu)
+		rcu_tasks_trace_pertask(idle_task(cpu), hop);
+
 	// Re-enable CPU hotplug now that the tasklist scan has completed.
 	cpus_read_unlock();