[PATCH] rcu_barrier VS cpu_hotplug: Ensure callbacks in dead cpu are migrated to online cpu

[Lai Jiangshan <laijs@cn.fujitsu.com>]

[RFC]
I don't like this patch, but I thought for some days and I can't
thought out a better one.

I'm very hope rcu_barrier() can be called anywhere(any sleepable context).
But get_online_cpus() is a very large lock, it limits rcu_barrier().

We can avoid get_online_cpus() easily for rcupreempt by using a new rcu_barrier:
void rcu_barrier(void)
{
	for each rcu_data {
		lock rcu_data;
		if rcu_data is not empty, queue a callback for rcu_barrier;
		unlock rcu_data;
	}
}
But we cannot use this algorithm for rcuclassic and rcutree,
rcu_data in rcuclassic and rcutree have not a spinlock for queuing callback.

From: Lai Jiangshan <laijs@cn.fujitsu.com>

cpu hotplug may be happened asynchronously, some rcu callbacks are maybe
still in dead cpu, rcu_barrier() also needs to wait for these rcu callbacks
to complete, so we must ensure callbacks in dead cpu are migrated to
online cpu.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---
diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
index cae8a05..4665d18 100644
--- a/kernel/rcupdate.c
+++ b/kernel/rcupdate.c
@@ -129,6 +129,8 @@ static void rcu_barrier_func(void *type)
 static void _rcu_barrier(enum rcu_barrier type)
 {
 	BUG_ON(in_interrupt());
+	/* Ensure callbacks in dead cpu are migrated to online cpu */
+	get_online_cpus();
 	/* Take cpucontrol mutex to protect against CPU hotplug */
 	mutex_lock(&rcu_barrier_mutex);
 	init_completion(&rcu_barrier_completion);
@@ -147,6 +149,7 @@ static void _rcu_barrier(enum rcu_barrier type)
 		complete(&rcu_barrier_completion);
 	wait_for_completion(&rcu_barrier_completion);
 	mutex_unlock(&rcu_barrier_mutex);
+	put_online_cpus();
 }
 
 /**