linux-kernel - Re: [PATCH] tracing/uprobes: Support ftrace_event

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Date:	Fri, 14 Jun 2013 10:25:16 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	"zhangwei(Jovi)" <jovi.zhangwei@...wei.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"yrl.pp-manager.tt@...achi.com" <yrl.pp-manager.tt@...achi.com>
Subject: Re: [PATCH] tracing/uprobes: Support ftrace_event_file base
 multibuffer

On Fri, Jun 14, 2013 at 12:33:27PM -0400, Steven Rostedt wrote:
> On Fri, 2013-06-14 at 09:21 -0700, Paul E. McKenney wrote:
> 
> > > > > @@ -548,15 +556,35 @@ static void uprobe_trace_print(struct trace_uprobe *tu,
> > > > >  /* uprobe handler */
> > > > >  static int uprobe_trace_func(struct trace_uprobe *tu, struct pt_regs *regs)
> > > > >  {
> > > > > -	if (!is_ret_probe(tu))
> > > > > -		uprobe_trace_print(tu, 0, regs);
> > > > > +	struct ftrace_event_file **file;
> > > > > +
> > > > > +	if (is_ret_probe(tu))
> > > > > +		return 0;
> > > > > +
> > > > > +	file = rcu_dereference_raw(tu->files);
> > > 
> > > Why are you using rcu_dereference_raw() and not rcu_dereference(). The
> > > _raw() is a bit special, and unless you know what you are doing with RCU
> > > here, don't use it.
> > > 
> > > As I see you using rcu_dereference_raw() all over this patch, along with
> > > mutexes, I believe that you are not using RCU correctly here.
> > 
> > If irqs and preempt are disabled, I suggest using rcu_dereference_sched().
> > That is what it is there for.  ;-)
> 
> I believe this just copied what kprobes did, where irqs and preemption
> is disabled. I don't believe that these probes have that same luxury.
> 
> But that begs the question. Should we convert the rcu_dereference_raw()
> in the kprobe code to rcu_dereference_sched()?

It makes a lot of sense to me, at least assuming no issues with the
interrupts being disabled, but the checks not spotting this.  Here
is the check:

	preempt_count() != 0 || irqs_disabled()

(With additional elaboration for if lockdep is enabled.)

							Thanx, Paul

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