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

[Paul E. McKenney]

On Thu, Aug 22, 2019 at 09:39:55AM -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:
> > > A plain local_bh_disable() is documented as creating an RCU critical
> > > section, and (at least) rcutorture expects this to be the case.  However,
> > > in_softirq() doesn't block a grace period on PREEMPT_RT, since RCU checks
> > > preempt_count() directly.  Even if RCU were changed to check
> > > in_softirq(), that wouldn't allow blocked BH disablers to be boosted.
> > > 
> > > Fix this by calling rcu_read_lock() from local_bh_disable(), and update
> > > rcu_read_lock_bh_held() accordingly.
> > 
> > Cool!  Some questions and comments below.
> > 
> > 							Thanx, Paul
> > 
> > > Signed-off-by: Scott Wood <swood@xxxxxxxxxx>
> > > ---
> > > 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?

I was interpreting Scott's question (which would be excluded from the
git commit log) as relating to a possible follow-on patch.

The question is "how special can non-raw spinlocks be in -rt?".  From what
I can see, they have been treated as sleeplocks from an RCU viewpoint,
so maybe that should continue to be the case.  It does deserve some
thought because in mainline a non-raw spinlock really would block a
post-consolidation RCU grace period, even in PREEMPT kernels.

But then again, you cannot preempt a non-raw spinlock in mainline but
you can in -rt, so extending that exception to RCU is not unreasonable.

Either way, we do need to make a definite decision and document it.
If I were forced to make a decision right now, I would follow the old
behavior, so that only raw spinlocks were guaranteed to block RCU grace
periods.  But I am not being forced, so let's actually discuss and make
a conscious decision.  ;-)

							Thanx, Paul