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

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Message-ID: <20190823123038.GR28441@linux.ibm.com>
Date:   Fri, 23 Aug 2019 05:30:38 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Scott Wood <swood@...hat.com>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.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 10:23:23PM -0500, Scott Wood wrote:
> On Thu, 2019-08-22 at 09:39 -0400, Joel Fernandes wrote:
> > On Wed, Aug 21, 2019 at 04:33:58PM -0700, Paul E. McKenney wrote:
> > > On Wed, Aug 21, 2019 at 06:19:04PM -0500, Scott Wood wrote:

[ . . . ]

> > > >  include/linux/rcupdate.h |  4 ++++
> > > >  kernel/rcu/update.c      |  4 ++++
> > > >  kernel/softirq.c         | 12 +++++++++---
> > > >  3 files changed, 17 insertions(+), 3 deletions(-)
> > > > 
> > > > 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?
> > 
> > Since this function is small, I prefer if -rt defines their own
> > rcu_read_lock_bh() which just does the local_bh_disable(). That would be
> > way
> > cleaner IMO. IIRC, -rt does similar things for spinlocks, but it has been
> > sometime since I look at the -rt patchset.
> 
> I'll do it whichever way you all decide, though I'm not sure I agree about
> it being cleaner (especially while RT is still out-of-tree and a change to
> the non-RT version that fails to trigger a merge conflict is a concern).
> 
> What about moving everything but the local_bh_disable into a separate
> function called from rcu_read_lock_bh, and making that a no-op on RT?

That makes a lot of sense to me!

							Thanx, Paul