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SubjectRe: [PATCH RFC tip/core/rcu 7/7] rcu: Quiet RCU-lockdep warnings involving interrupt disabling
On Tue, Dec 06, 2011 at 09:26:35AM +0800, Yong Zhang wrote:
> On Mon, Dec 05, 2011 at 08:45:05AM -0800, Paul E. McKenney wrote:
> > On Mon, Dec 05, 2011 at 05:19:24PM +0800, Yong Zhang wrote:
> > > 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.
> >
> > Even after commit #5342e269b has been applied?
>
> Yeah, because we call might_sleep() in rt_mutex_lock() unconditionally.
> But in this case the 'BUG: sleeping function called from invalid context
> at *' is obviously false positive.
>
> Maybe we could teach might_sleep() about this special case?

That sounds very good to me!

Thanx, Paul



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