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

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Date:   Wed, 11 Jul 2018 17:31:00 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Boqun Feng <boqun.feng@...il.com>,
        Byungchul Park <byungchul.park@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Julia Cartwright <julia@...com>,
        linux-kselftest@...r.kernel.org,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Thomas Glexiner <tglx@...utronix.de>,
        Tom Zanussi <tom.zanussi@...ux.intel.com>
Subject: Re: [PATCH v9 4/7] tracepoint: Make rcuidle tracepoint callers use
 SRCU

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@...radead.org> 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