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SubjectRe: [PATCH] rcupdate: fix bug of rcu_barrier*()

* Paul E. McKenney <paulmck@linux.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@cn.fujitsu.com>
> > Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

applied to tip/core/urgent, thanks!

Ingo


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