[PATCH] rcupdate: fix 2 bugs of rcu_barrier*()

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

current rcu_barrier_bh() is like this:

void rcu_barrier_bh(void)
{
	BUG_ON(in_interrupt());
	/* Take cpucontrol mutex to protect against CPU hotplug */
	mutex_lock(&rcu_barrier_mutex);
	init_completion(&rcu_barrier_completion);
	atomic_set(&rcu_barrier_cpu_count, 0);
	/*
	 * The queueing of callbacks in all CPUs must be atomic with
	 * respect to RCU, otherwise one CPU may queue a callback,
	 * wait for a grace period, decrement barrier count and call
	 * complete(), while other CPUs have not yet queued anything.
	 * So, we need to make sure that grace periods cannot complete
	 * until all the callbacks are queued.
	 */
	rcu_read_lock();
	on_each_cpu(rcu_barrier_func, (void *)RCU_BARRIER_BH, 1);
	rcu_read_unlock();
	wait_for_completion(&rcu_barrier_completion);
	mutex_unlock(&rcu_barrier_mutex);
}

this is bug, rcu_read_lock() cannot make sure that "grace periods for RCU_BH
cannot complete until all the callbacks are queued".
it only make sure that race periods for RCU cannot complete
until all the callbacks are queued.

so we must use rcu_read_lock_bh() for rcu_barrier_bh().
like this:

void rcu_barrier_bh(void)
{
	......
	rcu_read_lock_bh();
	on_each_cpu(rcu_barrier_func, (void *)RCU_BARRIER_BH, 1);
	rcu_read_unlock_bh();
	......
}

and also rcu_barrier() rcu_barrier_sched() are implemented like this.
it will bring a lot of duplicate code. My patch uses another way to
fix this bug, please see the comment of my patch.

Bug 2:
on_each_cpu() do not imply wmb, so we need a explicit wmb.
I became a paranoid too.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
---
diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
index 467d594..a667e21 100644
--- a/kernel/rcupdate.c
+++ b/kernel/rcupdate.c
@@ -119,18 +119,23 @@ static void _rcu_barrier(enum rcu_barrier type)
 	/* Take cpucontrol mutex to protect against CPU hotplug */
 	mutex_lock(&rcu_barrier_mutex);
 	init_completion(&rcu_barrier_completion);
-	atomic_set(&rcu_barrier_cpu_count, 0);
 	/*
-	 * The queueing of callbacks in all CPUs must be atomic with
-	 * respect to RCU, otherwise one CPU may queue a callback,
-	 * wait for a grace period, decrement barrier count and call
-	 * complete(), while other CPUs have not yet queued anything.
-	 * So, we need to make sure that grace periods cannot complete
-	 * until all the callbacks are queued.
+	 * init and set rcu_barrier_cpu_count to 1, otherwise(set it to 0)
+	 * one CPU may queue a callback, wait for a grace period, decrement
+	 * barrier count and call complete(), while other CPUs have not yet
+	 * queued anything.
+	 * So, we need to make sure that rcu_barrier_cpu_count cannot become
+	 * 0 until all the callbacks are queued.
 	 */
-	rcu_read_lock();
+	atomic_set(&rcu_barrier_cpu_count, 1);
+	/*
+	 * rcu_barrier_cpu_count = 1 must be visible to cpus before
+	 * them call rcu_barrier_func().
+	 */
+	smp_wmb();
 	on_each_cpu(rcu_barrier_func, (void *)type, 1);
-	rcu_read_unlock();
+	if (atomic_dec_and_test(&rcu_barrier_cpu_count))
+		complete(&rcu_barrier_completion);
 	wait_for_completion(&rcu_barrier_completion);
 	mutex_unlock(&rcu_barrier_mutex);
 }