linux-kernel - Re: [PATCH tip/core/urgent 4/7] rcu: protect __rcu_read_unlock() against scheduler-using irq handlers

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Date:	Wed, 20 Jul 2011 06:25:30 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
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, rostedt@...dmis.org,
	Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, patches@...aro.org,
	greearb@...delatech.com, edt@....ca,
	"Paul E. McKenney" <paul.mckenney@...aro.org>
Subject: Re: [PATCH tip/core/urgent 4/7] rcu: protect __rcu_read_unlock()
 against scheduler-using irq handlers

On Wed, Jul 20, 2011 at 02:54:27PM +0200, Peter Zijlstra wrote:
> On Tue, 2011-07-19 at 17:18 -0700, Paul E. McKenney wrote:
> > @@ -391,10 +400,15 @@ void __rcu_read_unlock(void)
> >         struct task_struct *t = current;
> >  
> >         barrier();  /* needed if we ever invoke rcu_read_unlock in rcutree.c */
> > -       if (--t->rcu_read_lock_nesting == 0) {
> > -               barrier();  /* decr before ->rcu_read_unlock_special load */
> > +       if (t->rcu_read_lock_nesting != 1)
> > +               --t->rcu_read_lock_nesting;
> > +       else {
> > +               t->rcu_read_lock_nesting = INT_MIN;
> > +               barrier();  /* assign before ->rcu_read_unlock_special load */
> >                 if (unlikely(ACCESS_ONCE(t->rcu_read_unlock_special)))
> >                         rcu_read_unlock_special(t);
> > +               barrier();  /* ->rcu_read_unlock_special load before assign */
> > +               t->rcu_read_lock_nesting = 0;
> >         }
> >  #ifdef CONFIG_PROVE_LOCKING
> >         WARN_ON_ONCE(ACCESS_ONCE(t->rcu_read_lock_nesting) < 0); 
> 
> But won't the above change make that WARN_ON_ONCE() invalid?

Yes, please see the patch I just sent.  So that warning was spurious,
and if that was the only problem...

							Thanx, Paul
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