Re: [RFC PATCH RT 3/4] rcu: unlock special: Treat irq and preempt disabled the same

[Scott Wood <swood@redhat.com>]

On Thu, 2019-06-20 at 15:25 -0700, Paul E. McKenney wrote:
> On Thu, Jun 20, 2019 at 04:59:30PM -0500, Scott Wood wrote:
> > On Thu, 2019-06-20 at 14:10 -0700, Paul E. McKenney wrote:
> > > On Tue, Jun 18, 2019 at 08:19:07PM -0500, Scott Wood wrote:
> > > > [Note: Just before posting this I noticed that the invoke_rcu_core
> > > > stuff
> > > >  is part of the latest RCU pull request, and it has a patch that
> > > >  addresses this in a more complicated way that appears to deal with
> > > > the
> > > >  bare irq-disabled sequence as well.
> > > 
> > > Far easier to deal with it than to debug the lack of it.  ;-)
> > > 
> > > >  Assuming we need/want to support such sequences, is the
> > > >  invoke_rcu_core() call actually going to result in scheduling any
> > > >  sooner?  resched_curr() just does the same setting of need_resched
> > > >  when it's the same cpu.
> > > > ]
> > > 
> > > Yes, invoke_rcu_core() can in some cases invoke the scheduler sooner.
> > > Setting the CPU-local bits might not have effect until the next
> > > interrupt.
> > 
> > Maybe I'm missing something, but I don't see how (in the non-use_softirq
> > case).  It just calls wake_up_process(), which in resched_curr() will
> > set
> > need_resched but not do an IPI-to-self.
> 
> The common non-rt case will be use_softirq.  Or are you referring
> specifically to this block of code in current -rcu?
> 
> 		} else if (exp && irqs_were_disabled && !use_softirq &&
> 			   !t->rcu_read_unlock_special.b.deferred_qs) {
> 			// Safe to awaken and we get no help from enabling
> 			// irqs, unlike bh/preempt.
> 			invoke_rcu_core();

Yes, that one.  If that block is removed the else path should be sufficient,
now that an IPI-to-self has been added.

Also, shouldn't the IPI-to-self be conditioned on irqs_were_disabled? 
Besides that being the problem the IPI was meant to address, if irqs are
enabled the IPI is likely to happen before preempt is re-enabled and thus it
won't accomplish anything.

-Scott