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

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Message-ID: <20130904041207.GO3871@linux.vnet.ibm.com>
Date:	Tue, 3 Sep 2013 21:12:07 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
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, Sep 03, 2013 at 10:01:15PM -0400, Steven Rostedt 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.

Good point, color me blind, dazed, and confused.

							Thanx, Paul

> -- Steve
> 
> > >  	if (atomic_read(&ftrace_unsafe_rcu_disabled))
> > >  		return;
> > > 
> > >  	preempt_disable_notrace();
> > > 
> 

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