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

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Date:	Thu, 23 Apr 2009 18:31:25 -0400 (EDT)
From:	Tim Abbott <tabbott@....EDU>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Masami Hiramatsu <mhiramat@...hat.com>
cc:	Anders Kaseorg <andersk@....edu>, Ingo Molnar <mingo@...e.hu>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Jeffrey B Arnold <jbarnold@....edu>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>
Subject: Re: [PATCH 1/5] ftrace: use module notifier for function tracer

On Thu, 23 Apr 2009, Mathieu Desnoyers wrote:
> how about a kernel Ksplice API which lets your patch modules get
> their handlers executed under the right execution context ? E.g. :
[...]
> This way, the kernel code-base would contain the tricky locking bits.
> It seems much better than exporting the text_mutex to modules.

On Thu, 23 Apr 2009, Masami Hiramatsu wrote:
> Then, why can't you move that "matching" routine into the kernel too? :-)

I think both of you are assuming that Ksplice is grabbing text_mutex in 
the patch modules for each update, which isn't what's going on.  There are 
two types of modules associated with Ksplice, the per-update patch modules 
and the Ksplice core, which could be built as a module.

The patch modules for each update are already stub modules that just load 
some data into a data structure and call into the Ksplice core at 
module_init/cleanup (by Ksplice core, I mean kernel/ksplice.c).  All the 
tricky bits including the run-pre matching routine and the locking code 
are in the Ksplice core.

The reason that we want to export text_mutex is that it can be useful for 
the Ksplice core (still kernel/ksplice.c) to be built as a module.  This 
is desirable when a different version of the kernel/ksplice.c code is 
needed in order to apply a particular update (e.g. because there was a bug 
in Ksplice).

I agree that great care should be taken when writing code that patches the 
kernel's text.  However, I don't think failing to export text_mutex to GPL 
modules helps ensure that.  In particular, not exporting text_mutex 
doesn't prevent modules from patching the kernel text.  It just prevents 
any modules that do so from grabbing the appropriate lock.

	-Tim Abbott

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