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

[Steven Rostedt]

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.


> 
> Also, does the above want to be barrier instead of synchronize, so as to
> guarantee completion of the callbacks.

Not sure what you mean here.

-- Steve

> 
> > +static void srcu_free_old_probes(struct rcu_head *head)
> >  {
> >  	kfree(container_of(head, struct tp_probes, rcu));
> >  }
> >  
> > +static void rcu_free_old_probes(struct rcu_head *head)
> > +{
> > +	call_srcu(&tracepoint_srcu, head, srcu_free_old_probes);
> > +}
> > +
> >  static inline void release_probes(struct tracepoint_func *old)
> >  {
> >  	if (old) {
> >  		struct tp_probes *tp_probes = container_of(old,
> >  			struct tp_probes, probes[0]);
> > +		/*
> > +		 * Tracepoint probes are protected by both sched RCU and SRCU,
> > +		 * by calling the SRCU callback in the sched RCU callback we
> > +		 * cover both cases. So let us chain the SRCU and sched RCU
> > +		 * callbacks to wait for both grace periods.
> > +		 */
> >  		call_rcu_sched(&tp_probes->rcu, rcu_free_old_probes);
> >  	}
> >  }