linux-kernel - Re: [RFC][PATCH 01/18 v2] ftrace: Add hash list to save RCU unsafe functions

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Date:	Wed, 4 Sep 2013 07:50:27 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	paulmck@...ux.vnet.ibm.com
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jiri Olsa <jolsa@...hat.com>
Subject: Re: [RFC][PATCH 01/18 v2] ftrace: Add hash list to save RCU unsafe
 functions

On Tue, 3 Sep 2013 21:18:28 -0700
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:

> On Tue, Sep 03, 2013 at 10:03:25PM -0400, Steven Rostedt wrote:
> > On Tue, 3 Sep 2013 22:01:15 -0400
> > Steven Rostedt <rostedt@...dmis.org> wrote:
> > 
> > > On Tue, 3 Sep 2013 18:24:04 -0700
> > > "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> > > 
> > > 
> > > > >  static DEFINE_PER_CPU(unsigned long, ftrace_rcu_func);
> > > > > @@ -588,15 +593,14 @@ static void
> > > > >  ftrace_unsafe_callback(unsigned long ip, unsigned long parent_ip,
> > > > >  		       struct ftrace_ops *op, struct pt_regs *pt_regs)
> > > > >  {
> > > > > -	int bit;
> > > > > -
> > > > > +	/* Make sure we see disabled or not first */
> > > > > +	smp_rmb();
> > > > 
> > > > 	smp_mb__before_atomic_inc()?
> > > > 
> > > 
> > > Ah, but this is before an atomic_read(), and not an atomic_inc(), thus
> > > the normal smp_rmb() is still required.
> > > 
> > 
> > Here's the changes against this one: 
> > 
> > diff --git a/kernel/trace/trace_functions.c b/kernel/trace/trace_functions.c
> > index cdcf187..9e6902a 100644
> > --- a/kernel/trace/trace_functions.c
> > +++ b/kernel/trace/trace_functions.c
> > @@ -569,14 +569,14 @@ void ftrace_unsafe_rcu_checker_disable(void)
> >  {
> >  	atomic_inc(&ftrace_unsafe_rcu_disabled);
> >  	/* Make sure the update is seen immediately */
> > -	smp_wmb();
> > +	smp_mb__after_atomic_inc();
> >  }
> > 
> >  void ftrace_unsafe_rcu_checker_enable(void)
> >  {
> >  	atomic_dec(&ftrace_unsafe_rcu_disabled);
> >  	/* Make sure the update is seen immediately */
> > -	smp_wmb();
> > +	smp_mb__after_atomic_dec();
> >  }
> > 
> >  static void
> > 
> > 
> > 
> > Which is nice, because the smp_mb() are now in the really slow path.
> 
> Looks good!
> 
> But now that I look at it more carefully, including the comments...
> The smp_mb__after_atomic_dec() isn't going to make the update be seen
> faster -- instead, it will guarantee that if some other CPU sees this
> CPU's later write, then that CPU will also see the results of the
> atomic_dec().

I don't need to have it seen "faster", just before anything that comes
next after the call to ftrace_unsafe_rcu_checker_disable(). That's
what I meant by the comment. In other words, don't delay this write, it
needs to go first.

-- Steve

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