linux-kernel - Re: [PATCH] tracing/profile: Fix profile_disable vs module

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Date:	Thu, 27 Aug 2009 12:03:42 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
cc:	Li Zefan <lizf@...fujitsu.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tracing/profile: Fix profile_disable vs module_unload


On Thu, 27 Aug 2009, Mathieu Desnoyers wrote:
> > 
> > For this race to occur, the probe (code that hooks to the tracepoint) must 
> > be in module that does not contain the tracepoint. We don't even need more 
> > than one module, this could occur even with a core tracepoint. If a module 
> > registers it, if it unregisters before unloading, the tracepoint may be 
> > hit before the unregister and executing while the module is unloading.
> > 
> > I don't think we need to worry about this with the case of TRACE_EVENT and 
> > ftrace.h. The reason is that the trace point and probes are always in the 
> > same location. The MACROS set up the probe code with the modules. Thus, to 
> > remove the module, you must also remove the tracepoint itself along with 
> > the probe. If you can be executing in the probe, then you must have hit 
> > the trace point. If you hit the trace point, then you are executing code 
> > inside the module you are removing, which is a bug in the module code 
> > itself.
> > 
> > Using the ftrace.h MACROS limits the use of tracepoints and this race 
> > does not exist. I feel we are safe not needing to have the 
> > tracepoint_synchronize_unregister within the ftrace.h code.
> > 
> 
> Looks right. If you can guarantee that the probe is only called from
> tracepoints located within the same module as the probe, you should be
> safe without tracepoint_synchronize_unregister. It's worth adding a
> comment in ftrace.h explaining that though.

Yeah, I tried to trick the code to see if I can get a probe in another 
module than the trace point, and I get nothing but linking errors. I'm 
sure if I work hard enough, I may trick it to do so, but if anyone does 
that, I'll slap a big fat NAK on it ;-)

OK, I modify my fix to add a comment to this effect.

Thanks,

-- Steve
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