Re: [PATCH -tip] rcu: local_irq_disable() also delimits RCU_SCHED read-site critical sections

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Mar 16, 2010 at 06:21:56AM -0700, Paul E. McKenney wrote:
> On Tue, Mar 16, 2010 at 07:09:21PM +0800, Lai Jiangshan wrote:
> > It is documented that local_irq_disable() also delimits
> > RCU_SCHED read-site critical sections.
> > See the document of synchronize_sched() or
> > Documentation/RCU/whatisRCU.txt.
> > 
> > So we have to test irqs_disabled() in rcu_read_lock_sched_held().
> > Otherwise rcu-lockdep brings incorrect complaint.
> 
> Interesting -- I was under the impression that preempt_count() covered
> this as well, due to the following in include/linux/hardirq.h:
> 
> 	#define PREEMPT_MASK    (__IRQ_MASK(PREEMPT_BITS) << PREEMPT_SHIFT)
> 	#define SOFTIRQ_MASK    (__IRQ_MASK(SOFTIRQ_BITS) << SOFTIRQ_SHIFT)
> 	#define HARDIRQ_MASK    (__IRQ_MASK(HARDIRQ_BITS) << HARDIRQ_SHIFT)
> 	#define NMI_MASK        (__IRQ_MASK(NMI_BITS)     << NMI_SHIFT)
> 
> But irqs_disabled() does look to sample the actual interrupt hardware.
> 
> So, if there are cases where RCU is used where the interrupt hardware
> is disabled, but preempt_count() has not been updated, this patch is
> the right thing to do.

In light of later emails in this thread, first a big "thank you!!!"
to Lai, and I will pull this one in.  Good stuff!!!

							Thanx, Paul

> > Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
> > ---
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index 3024050..2ce5674 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -160,7 +160,7 @@ static inline int rcu_read_lock_sched_held(void)
> >  		return 1;
> >  	if (debug_locks)
> >  		lockdep_opinion = lock_is_held(&rcu_sched_lock_map);
> > -	return lockdep_opinion || preempt_count() != 0;
> > +	return lockdep_opinion || preempt_count() != 0 || irqs_disabled();
> >  }
> >  #else /* #ifdef CONFIG_PREEMPT */
> >  static inline int rcu_read_lock_sched_held(void)
> > @@ -191,7 +191,7 @@ static inline int rcu_read_lock_bh_held(void)
> >  #ifdef CONFIG_PREEMPT
> >  static inline int rcu_read_lock_sched_held(void)
> >  {
> > -	return !rcu_scheduler_active || preempt_count() != 0;
> > +	return !rcu_scheduler_active || preempt_count() != 0 || irqs_disabled();
> >  }
> >  #else /* #ifdef CONFIG_PREEMPT */
> >  static inline int rcu_read_lock_sched_held(void)
> > 
> --
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