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

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Date:   Wed, 11 Jul 2018 12:40:19 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     rostedt <rostedt@...dmis.org>,
        "Joel Fernandes, Google" <joel@...lfernandes.org>,
        linux-kernel <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 <linux-kselftest@...r.kernel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tom Zanussi <tom.zanussi@...ux.intel.com>
Subject: Re: [PATCH v9 4/7] tracepoint: Make rcuidle tracepoint callers use
 SRCU

----- On Jul 11, 2018, at 11:17 AM, Peter Zijlstra peterz@...radead.org wrote:

> 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.
> 
> I just read the comment that goes with that function; the order doesn't
> matter. All we want to ensure is that the unregistration is visible to
> either sched or srcu tracepoint users.

Exactly, the order does not matter here.

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com