linux-kernel - Re: [PATCH -tip v5 4/7] tracing: add kprobe-based event tracer

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Date:	Mon, 11 May 2009 23:26:04 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Masami Hiramatsu <mhiramat@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Steven Rostedt <rostedt@...dmis.org>,
	lkml <linux-kernel@...r.kernel.org>,
	systemtap <systemtap@...rces.redhat.com>,
	kvm <kvm@...r.kernel.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>
Subject: Re: [PATCH -tip v5 4/7] tracing: add kprobe-based event tracer

On Sat, May 09, 2009 at 01:33:53PM -0400, Masami Hiramatsu wrote:
> Frédéric Weisbecker wrote:
> > Hi,
> > 
> > 2009/5/9 Masami Hiramatsu <mhiramat@...hat.com>:
> [...]
> >> +
> >> +/* event recording functions */
> >> +static void kprobe_trace_record(unsigned long ip, struct trace_probe *tp,
> >> +                               struct pt_regs *regs)
> >> +{
> >> +       __trace_bprintk(ip, "%s%s%+ld\n",
> >> +                       probe_is_return(tp) ? "<-" : "@",
> >> +                       probe_symbol(tp), probe_offset(tp));
> >> +}
> > 
> > 
> > 
> > What happens here if you have:
> > 
> > kprobe_trace_record() {
> >       probe_symbol() {
> >             ....                         probes_open() {
> >                                               cleanup_all_probes() {
> >                                                          free_trace_probe();
> >      return tp->symbol ? ....; //crack!
> >
> > I wonder if you shouldn't use a per_cpu list of probes,
> > spinlocked/irqsaved  accessed
> > and also a kind of prevention against nmi.
> 
> Sure, cleanup_all_probes() invokes unregister_kprobe() via
> unregister_trace_probe(), which waits running probe-handlers by
> using synchronize_sched()(because kprobes disables preemption
> around its handlers), before free_trace_probe().
> 
> So you don't need any locks there :-)
> 
> Thank you,
> 
> 


Aah, ok :)
So this patch looks sane.

Thanks.

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