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

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Message-ID: <20130708142533.GA23749@redhat.com>
Date:	Mon, 8 Jul 2013 16:25:33 +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 07/08, Masami Hiramatsu wrote:
>
> (2013/07/06 2:26), Oleg Nesterov wrote:
> (Perhaps, uprobes too.)

Sure, uprobes needs all the same fixes, lets ignore it for now.

> >> 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?
>
> Right, thus perf path also need to be synchronized.

kprobe_perf_func() looks fine. This path can't use anything which
can be freed by trace_remove_event_call(). We are going to free
tp / tp->call but only after unregister_kprobe().

Afaics. But even if I am right, I agree it would be safer to not
rely on this and simply do synchronize_sched() after disable as
you suggested.

Thanks,

Oleg.

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