Re: [PATCH v9 4/7] tracepoint: Make rcuidle tracepoint callers use SRCU

[Joel Fernandes]

On Wed, Jul 11, 2018 at 09:06:49AM -0400, Steven Rostedt wrote:
> On Wed, 11 Jul 2018 14:56:47 +0200
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> > On Thu, Jun 28, 2018 at 11:21:46AM -0700, Joel Fernandes wrote:
> > >  static inline void tracepoint_synchronize_unregister(void)
> > >  {
> > > +	synchronize_srcu(&tracepoint_srcu);
> > >  	synchronize_sched();
> > >  }  
> > 
> > Given you below do call_rcu_sched() and then call_srcu(), isn't the
> > above the wrong way around?
> 
> Good catch!
> 
> 	release_probes()
> 		call_rcu_sched()
> 			---> rcu_free_old_probes() queued
> 
> 	tracepoint_synchronize_unregister()
> 		synchronize_srcu(&tracepoint_srcu);
> 			< finishes right away >
> 		synchronize_sched()
> 			--> rcu_free_old_probes()
> 				--> srcu_free_old_probes() queued
> 	
> Here tracepoint_synchronize_unregister() returned before the srcu
> portion ran.

But isn't the point of synchronize_rcu to make sure that we're no longer in
an RCU read-side section, not that *all* queued callbacks already ran? So in that
case, I think it doesn't matter which order the 2 synchronize functions are
called in. Please let me know if if I missed something!

I believe what we're trying to guarantee here is that no tracepoints using
either flavor of RCU are active after tracepoint_synchronize_unregister
returns.

thanks!

- Joel