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SubjectRe: [PATCH RFC tip/core/rcu 7/7] rcu: Quiet RCU-lockdep warnings involving interrupt disabling
On Sat, Dec 03, 2011 at 10:34:42AM -0800, Paul E. McKenney wrote:
> From: Yong Zhang <yong.zhang0@gmail.com>
>
> RCU-lockdep will issue warnings given the following use pattern:
>
> rcu_read_lock();
> local_irq_disable();
> rcu_read_unlock();
> local_irq_enable();
>
> However, this use pattern is legal except for the scheduler's runqueue
> and priority-inheritance locks (and any other locks that the scheduler
> might use during priority-inheritance operations).
>
> Signed-off-by: Yong Zhang <yong.zhang0@gmail.com>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
> kernel/rcutree_plugin.h | 8 ++++++--
> 1 files changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> index 8cd9efe..2020e8a 100644
> --- a/kernel/rcutree_plugin.h
> +++ b/kernel/rcutree_plugin.h
> @@ -401,8 +401,11 @@ static noinline void rcu_read_unlock_special(struct task_struct *t)
>
> #ifdef CONFIG_RCU_BOOST
> /* Unboost if we were boosted. */
> - if (rbmp)
> + if (rbmp) {
> + local_irq_save(flags);
> rt_mutex_unlock(rbmp);
> + local_irq_restore(flags);
> + }
> #endif /* #ifdef CONFIG_RCU_BOOST */
>
> /*
> @@ -1233,9 +1236,10 @@ static int rcu_boost(struct rcu_node *rnp)
> lockdep_set_class_and_name(&mtx.wait_lock, &rcu_boost_class,
> "rcu_boost_mutex");
> t->rcu_boost_mutex = &mtx;
> - raw_spin_unlock_irqrestore(&rnp->lock, flags);
> + raw_spin_unlock(&rnp->lock); /* rrupts remain disabled. */
> rt_mutex_lock(&mtx); /* Side effect: boosts task t's priority. */
> rt_mutex_unlock(&mtx); /* Keep lockdep happy. */

We permit rt_mutex_unlock() to be call with irq disabled,
but rt_mutex_lock() is still not allowed. So this usage
is not legal now.

Sounds we should hold this patch on until a more suitable
way is found.

Thanks,
Yong

> + local_irq_restore(flags);
>
> return rnp->exp_tasks != NULL || rnp->boost_tasks != NULL;
> }
> --
> 1.7.3.2
>
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