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

[Scott Wood <swood@redhat.com>]

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:
> > > Signed-off-by: Scott Wood <swood@redhat.com>
> > > ---
> > > Another question is whether non-raw spinlocks are intended to create
> > > an
> > > RCU read-side critical section due to implicit preempt disable.
> > 
> > Hmmm...  Did non-raw spinlocks act like rcu_read_lock_sched()
> > and rcu_read_unlock_sched() pairs in -rt prior to the RCU flavor
> > consolidation?  If not, I don't see why they should do so after that
> > consolidation in -rt.
> 
> May be I am missing something, but I didn't see the connection between
> consolidation and this patch. AFAICS, this patch is so that
> rcu_read_lock_bh_held() works at all on -rt. Did I badly miss something?

Before consolidation, RT mapped rcu_read_lock_bh_held() to
rcu_read_lock_bh() and called rcu_read_lock() from rcu_read_lock_bh().  This
somehow got lost when rebasing on top of 5.0.

> > >  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?

> > >  
> > > diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> > > index 016c66a98292..a9cdf3d562bc 100644
> > > --- a/kernel/rcu/update.c
> > > +++ b/kernel/rcu/update.c
> > > @@ -296,7 +296,11 @@ int rcu_read_lock_bh_held(void)
> > >  		return 0;
> > >  	if (!rcu_lockdep_current_cpu_online())
> > >  		return 0;
> > > +#ifdef CONFIG_PREEMPT_RT_FULL
> > > +	return lock_is_held(&rcu_lock_map) || irqs_disabled();
> > > +#else
> > >  	return in_softirq() || irqs_disabled();
> > > +#endif
> > 
> > And globally.
> 
> And could be untangled a bit as well:
> 
> if (irqs_disabled())
> 	return 1;
> 
> if (IS_ENABLED(CONFIG_PREEMPT_RT_FULL))
> 	return lock_is_held(&rcu_lock_map);
> 
> return in_softirq();

OK.

-Scott