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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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