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

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Message-ID: <20130903220325.273c2c7d@gandalf.local.home>
Date:	Tue, 3 Sep 2013 22:03:25 -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 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.

-- Steve
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