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Message-ID: <1375368339.1152.28.camel@gandalf.local.home>
Subject: Re: [RFC][PATCH 3/4] tracing/kprobes: Fail to unregister if probe
 event files are open
From: Steven Rostedt <rostedt@goodmis.org>
To: Oleg Nesterov <oleg@redhat.com>
Cc: linux-kernel@vger.kernel.org,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        "zhangwei(Jovi)" <jovi.zhangwei@huawei.com>,
        Jiri Olsa <jolsa@redhat.com>, Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Arnaldo Carvalho de Melo <acme@ghostprotocols.net>,
        Srikar Dronamraju <srikar@linux.vnet.ibm.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Ingo Molnar <mingo@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>
Date: Thu, 01 Aug 2013 10:45:39 -0400
In-Reply-To: <20130801143307.GA12031@redhat.com>
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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