linux-kernel - Re: [PATCH RT v2 1/3] rcu: Acquire RCU lock when disabling BHs

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Date:   Thu, 22 Aug 2019 21:36:21 -0500
From:   Scott Wood <swood@...hat.com>
To:     paulmck@...ux.ibm.com
Cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        Joel Fernandes <joel@...lfernandes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Clark Williams <williams@...hat.com>
Subject: Re: [PATCH RT v2 1/3] rcu: Acquire RCU lock when disabling BHs

On Wed, 2019-08-21 at 16:33 -0700, Paul E. McKenney wrote:
> On Wed, Aug 21, 2019 at 06:19:04PM -0500, Scott Wood wrote:
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index 388ace315f32..d6e357378732 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -615,10 +615,12 @@ static inline void rcu_read_unlock(void)
> >  static inline void rcu_read_lock_bh(void)
> >  {
> >  	local_bh_disable();
> > +#ifndef CONFIG_PREEMPT_RT_FULL
> >  	__acquire(RCU_BH);
> >  	rcu_lock_acquire(&rcu_bh_lock_map);
> >  	RCU_LOCKDEP_WARN(!rcu_is_watching(),
> >  			 "rcu_read_lock_bh() used illegally while idle");
> > +#endif
> 
> Any chance of this using "if (!IS_ENABLED(CONFIG_PREEMPT_RT_FULL))"?
> We should be OK providing a do-nothing __maybe_unused rcu_bh_lock_map
> for lockdep-enabled -rt kernels, right?

OK.

> > @@ -185,8 +189,10 @@ void __local_bh_enable_ip(unsigned long ip,
> > > > unsigned int cnt)
> >  	WARN_ON_ONCE(count < 0);
> >  	local_irq_enable();
> >  
> > -	if (!in_atomic())
> > +	if (!in_atomic()) {
> > +		rcu_read_unlock();
> >  		local_unlock(bh_lock);
> > +	}
> 
> The return from in_atomic() is guaranteed to be the same at
> local_bh_enable() time as was at the call to the corresponding
> local_bh_disable()?

That's an existing requirement on RT (which rcutorture currently violates)
due to bh_lock.

> I could have sworn that I ran afoul of this last year.  Might these
> added rcu_read_lock() and rcu_read_unlock() calls need to check for
> CONFIG_PREEMPT_RT_FULL?

This code is already under a PREEMPT_RT_FULL ifdef.

-Scott