[PATCH v2] rcu-tasks: Make rude RCU-Tasks work well with CPU hotplug

[Zqiang]

Currently, invoke rcu_tasks_rude_wait_gp() to wait one rude
RCU-tasks grace period, if __num_online_cpus == 1, will return
directly, indicates the end of the rude RCU-task grace period.
suppose the system has two cpus, consider the following scenario:

	CPU0                                   CPU1 (going offline)
				          migration/1 task:
                                      cpu_stopper_thread
                                       -> take_cpu_down
                                          -> _cpu_disable
			                   (dec __num_online_cpus)
                                          ->cpuhp_invoke_callback
                                                preempt_disable
						access old_data0
           task1
 del old_data0                                  .....
 synchronize_rcu_tasks_rude()
 task1 schedule out
 ....
 task2 schedule in
 rcu_tasks_rude_wait_gp()
     ->__num_online_cpus == 1
       ->return
 ....
 task1 schedule in
 ->free old_data0
                                                preempt_enable

when CPU1 dec __num_online_cpus and __num_online_cpus is equal one,
the CPU1 has not finished offline, stop_machine task(migration/1)
still running on CPU1, maybe still accessing 'old_data0', but the
'old_data0' has freed on CPU0.

This commit add cpus_read_lock/unlock() protection before accessing
__num_online_cpus variables, to ensure that the CPU in the offline
process has been completed offline.

Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
---
 kernel/rcu/tasks.h | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 4a991311be9b..08e72c6462d8 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -1033,14 +1033,30 @@ static void rcu_tasks_be_rude(struct work_struct *work)
 {
 }
 
+static DEFINE_PER_CPU(struct work_struct, rude_work);
+
 // Wait for one rude RCU-tasks grace period.
 static void rcu_tasks_rude_wait_gp(struct rcu_tasks *rtp)
 {
+	int cpu;
+	struct work_struct *work;
+
+	cpus_read_lock();
 	if (num_online_cpus() <= 1)
-		return;	// Fastpath for only one CPU.
+		goto end;// Fastpath for only one CPU.
 
 	rtp->n_ipis += cpumask_weight(cpu_online_mask);
-	schedule_on_each_cpu(rcu_tasks_be_rude);
+	for_each_online_cpu(cpu) {
+		work = per_cpu_ptr(&rude_work, cpu);
+		INIT_WORK(work, rcu_tasks_be_rude);
+		schedule_work_on(cpu, work);
+	}
+
+	for_each_online_cpu(cpu)
+		flush_work(per_cpu_ptr(&rude_work, cpu));
+
+end:
+	cpus_read_unlock();
 }
 
 void call_rcu_tasks_rude(struct rcu_head *rhp, rcu_callback_t func);
-- 
2.25.1