From: Rusty Russell <rusty@rustcorp.com.au>
To: Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH 1/5] ftrace: use module notifier for function tracer
Date: Sun, 19 Apr 2009 20:55:02 +0930
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Cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@elte.hu>,
       Andrew Morton <akpm@linux-foundation.org>,
       Thomas Gleixner <tglx@linutronix.de>,
       Peter Zijlstra <peterz@infradead.org>,
       Frederic Weisbecker <fweisbec@gmail.com>, Tim Abbott <tabbott@mit.edu>
References: <20090416021830.556671772@goodmis.org> <20090416021928.040177084@goodmis.org>
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On Thu, 16 Apr 2009 11:48:31 am Steven Rostedt wrote:
> From: Steven Rostedt <srostedt@redhat.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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