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

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Date:	Tue, 3 Sep 2013 21:56:25 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	paulmck@...ux.vnet.ibm.com, 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 21:51:52 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:
 
> > >  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()?
> 
> Yep.
> 
> > 
> > >  }
> > > 
> > >  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()?
> 
> Yep.
> 
> > 
> > >  }
> > > 
> > >  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()?
> 
> Yep.
> 
> I'll update it and restart my tests.
> 

Although, this will punish archs that need the mb(), as IIUC, smp_mb()
is heavier weight than smp_wmb() or smp_rmb().

-- Steve
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