linux-kernel - Re: [PATCH 1/5] ftrace: use module notifier for function tracer

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Date:	Sun, 19 Apr 2009 20:55:02 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Tim Abbott <tabbott@....edu>
Subject: Re: [PATCH 1/5] ftrace: use module notifier for function tracer

On Thu, 16 Apr 2009 11:48:31 am Steven Rostedt wrote:
> From: Steven Rostedt <srostedt@...hat.com>
> 
> Impact: fix and clean up
> 
> The hooks in the module code for the function tracer must be called
> before any of that module code runs. The function tracer hooks
> modify the module (replacing calls to mcount to nops). If the code
> is executed while the change occurs, then the CPU can take a GPF.
> 
> To handle the above with a bit of paranoia, I originally implemented
> the hooks as calls directly from the module code.
> 
> After examining the notifier calls, it looks as though the start up
> notify is called before any of the module's code is executed. This makes
> the use of the notify safe with ftrace.

Hi Steven,

   Unfortunately not: we do parse_args, which can call into the module code.
(Though it shouldn't do anything "significant", as it won't get a chance to
clean up if module load fails later).

I think you need to do something else in general.  Share the module_mutex for
the ftrace code?  The ksplice guys have a similar issue, so maybe we should
generalize this into a "kernel_text" mutex?

Thanks,
Rusty.
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