linux-kernel - Re: [PATCH RFC tip/core/rcu 7/7] rcu: Quiet RCU-lockdep warnings involving interrupt disabling

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Date:	Tue, 6 Dec 2011 09:26:35 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, peterz@...radead.org,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, patches@...aro.org
Subject: Re: [PATCH RFC tip/core/rcu 7/7] rcu: Quiet RCU-lockdep warnings
 involving interrupt disabling

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@...il.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@...il.com>
> > > Signed-off-by: Paul E. McKenney <paulmck@...ux.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?

Thanks,
Yong
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