linux-kernel - Re: [RFC][PATCH 3/4] tracing/kprobes: Fail to unregister if probe event files are open

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Message-ID: <20130705172655.GA18576@redhat.com>
Date:	Fri, 5 Jul 2013 19:26:55 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Cc:	Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
	"zhangwei(Jovi)" <jovi.zhangwei@...wei.com>,
	Jiri Olsa <jolsa@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC][PATCH 3/4] tracing/kprobes: Fail to unregister if probe
	event files are open

On 07/05, Masami Hiramatsu wrote:
>
> (2013/07/05 3:48), Oleg Nesterov wrote:
> > On 07/04, Masami Hiramatsu wrote:
> >>
> >> Actually disable_kprobe() doesn't ensure to finish the current running
> >> kprobe handlers.
> >
> > Yes. in fact disable_trace_probe(file != NULL) does, but perf doesn't.
>
> Ah, right. we did that.

And thus we only need to synchronize kprobe_dispatcher()->kprobe_perf_func()
path. And afaics kprobe_perf_func() doesn't use anything which can be freed
by trace_remove_event_call?

> >> OTOH, unregister_kprobe() waits for that.
> >
> > Yes.
> >
> > So I think we only need to move kfree(tp->call.print_fmt).

OOPS. I was wrong. It seems that ->print_fmt is only for event/format ?
Then it is fine to kfree it right after trace_remove_event_call().

> > So the sequence should be:
> >
> > 	if (trace_remove_event_call(...))
> > 		return;
> >
> > 	/* does synchronize_sched */
> > 	unregister_kprobe();
> >
> > 	kfree(everything);
> >
> > Agreed?
>
> If we can free everything after all, I'd like to do so.
> Hmm, but AFAICS, trace_remove_event_call() supposes that
> all event is disabled completely.

Yes, but kprobe_trace_func() is really disabled?

> A safe way is to wait rcu always right after disable_*probe
> in disable_trace_probe. If we have an unused link, we can
> free it after that.

Aaaah... I am starting to understand... Even if kprobe_perf_func()
is fine, synchronize_sched() is calles _before_ disable_kprobe()
and thus it can't synchronize with the handlers which hit this probe
after we start synchronize_sched().

Did you mean this or I misssed something else?

Thanks!

Oleg.

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