linux-kernel - Re: [PATCH] rcupdate: fix bug of rcu

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Date:	Mon, 20 Oct 2008 15:41:09 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Lai Jiangshan <laijs@...fujitsu.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] rcupdate: fix bug of rcu_barrier*()


* Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:

> On Fri, Oct 17, 2008 at 02:40:30PM +0800, Lai Jiangshan wrote:
> > 
> > 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);
> > }
> > 
> > The inconsistency of the code and the comments show a bug here.
> > 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.
> > Thank Paul E. McKenney for he rewrote the comment.
> 
> Still looks good to me!  Thank you again, Jiangshan, for finding and
> fixing this one!!!
> 
> 							Thanx, Paul
> 
> > Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
> > Reviewed-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

applied to tip/core/urgent, thanks!

	Ingo
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