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

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Date:   Thu, 22 Aug 2019 19:54:18 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Scott Wood <swood@...hat.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 Thu, Aug 22, 2019 at 09:36:21PM -0500, Scott Wood wrote:
> 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.

Good enough, then!

							Thanx, Paul