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

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Date:	Thu, 01 Aug 2013 10:45:39 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	"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 Thu, 2013-08-01 at 16:33 +0200, Oleg Nesterov wrote:
> On 08/01, Steven Rostedt wrote:
> >
> > On Thu, 2013-08-01 at 15:34 +0200, Oleg Nesterov wrote:
> > > >
> > > >   __unregister_trace_probe(tp);
> > > >   list_del(&tp->list)
> > > >   unregister_probe_event(tp) <-- fails!
> > > >   free_trace_probe(tp)
> > >
> > > Yes. But again, this doesn't explain why unregister_probe_event()->
> > > __trace_remove_event_call() can't simply proceed and
> > > do ftrace_event_enable_disable() + remove_event_from_tracers().
> >
> > The problem is with the soft disable.
> 
> Exactly! This is another (also unlikely) race we need to prevent.
> 

Is there a race even with these patches? I don't see one. To link a
function to an event (set the soft disable mode), the event_mutex is
taken. If the event is gone, it wont be able to link. If the soft
disable is attached (after found and set under event_mutex), the event
can't be deleted.

Or are you just saying that these patches fix that case too?

-- Steve


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