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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Sat, Mar 07, 2009 at 06:54:38PM +0800, Lai Jiangshan wrote:
> [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.

Hmmm...  I thought that on_each_cpu() took care of interlocking with
CPU hotplug via smp_call_function().  During a CPU-hotplug operation,
the RCU callbacks do get migrated from the CPU going offline.  Are you
seeing a sequence of events that finds a hole in this approach?

Now, if a CPU were to go offline in the middle of smp_call_function()
there could be trouble, but I was under the impression that the
preempt_disable() in on_each_cpu() prevented this from happening.

So, please tell me more!

							Thanx, Paul

> 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();
>  }
> 
>  /**
>