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

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Date:	Tue, 21 Apr 2009 13:51:48 -0400 (EDT)
From:	Tim Abbott <tabbott@....EDU>
To:	Rusty Russell <rusty@...tcorp.com.au>
cc:	Steven Rostedt <rostedt@...dmis.org>, 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>,
	Anders Kaseorg <andersk@....edu>,
	Jeffrey B Arnold <jbarnold@....edu>
Subject: Re: [PATCH 1/5] ftrace: use module notifier for function tracer

On Sun, 19 Apr 2009, Rusty Russell wrote:

> 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?

Yes, a kernel_text mutex is on my list of things to propose once Ksplice 
gets merged.

There are at present several systems that modify the kernel text after a 
machine is booted (e.g. dynamic ftrace and toggling of smp_locks when 
hotplugging a cpu).  Currently, they avoid stepping on each other by only 
making changes inside stop_machine.

However, Ksplice does its run-pre matching checks outside stop_machine, 
and Ksplice needs a way to prevent e.g. dynamic ftrace from changing the 
code out from under it between those checks and actually applying the 
patches [1].  A kernel_text mutex would be a reasonable solution to this 
problem.

	-Tim Abbott

[1] At present this isn't a real problem because Ksplice and ftrace 
conflict with each other for unrelated reasons.
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