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Date: Fri, 14 Nov 2008 09:03:20 +0800
From: Lai Jiangshan <laijs@cn.fujitsu.com>
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To: paulmck@linux.vnet.ibm.com
CC: Ingo Molnar <mingo@elte.hu>, Peter Zijlstra <peterz@infradead.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       corbet@lwn.net
Subject: Re: [PATCH] rcupdate: move synchronize_sched() back to rcupdate.c
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Paul E. McKenney wrote:
> On Thu, Nov 13, 2008 at 10:48:33AM +0800, Lai Jiangshan wrote:
>> Hi, Paul,
>>
>> Could you add a RCU document about unloadable modules for kernel?
> 
> You thinking in terms of an ASCII version of
> http://lwn.net/Articles/217484/?
> 
> If so, please see attached patch and let me know what you think.
> Being too lazy to convert the cartoon to ASCII graphics, I simply
> left a URL to the .jpg on the LWN website.  Thus we need an ack/nack
> from Jon Corbet (CCed).

Hi, Paul

Thank you. it's a very good document.

I found several modules which need rcu_barrier(). So I'm going to
do some cleanup for them. A document for rcu_barrier() will help
these cleanup patches be accepted easily by maintainers.

Lai.

> 
> Of course, an alternative is to simply include the URL of the original
> LWN article in 00-INDEX.  Thoughts?
> 
> 							Thanx, Paul
> 
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---

> +The code for rcu_barrier() is as follows:
> +
> + 1 void rcu_barrier(void)
> + 2 {
> + 3   BUG_ON(in_interrupt());
> + 4   /* Take cpucontrol mutex to protect against CPU hotplug */
> + 5   mutex_lock(&rcu_barrier_mutex);
> + 6   init_completion(&rcu_barrier_completion);
> + 7   atomic_set(&rcu_barrier_cpu_count, 0);
> + 8   on_each_cpu(rcu_barrier_func, NULL, 0, 1);
> + 9   wait_for_completion(&rcu_barrier_completion);
> +10   mutex_unlock(&rcu_barrier_mutex);
> +11 }
> +

this is a little old.

> +
> +Quick Quiz #2: Why is there no rcu_barrier_bh()?
> +
> +Answer: Because no one has needed it yet. As soon as someone needs to
> +	use call_rcu_bh() from within an unloadable module, they will
> +	need an rcu_barrier_bh().
> +

add here.