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

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Date:	Thu, 23 Apr 2009 15:40:42 -0400 (EDT)
From:	Anders Kaseorg <andersk@....EDU>
To:	Masami Hiramatsu <mhiramat@...hat.com>
cc:	Tim Abbott <tabbott@....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>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>
Subject: Re: [PATCH 1/5] ftrace: use module notifier for function tracer

On Wed, 22 Apr 2009, Masami Hiramatsu wrote:
> Could you also add "use text_poke() on x86" to your plan? :-)

That should be possible now that it is usable inside stop_machine().  It 
would be nicer to have an API that isn’t x86-specific, though.

Another issue is that Ksplice supports patching rodata as well as text, 
and text_poke() does not support changes bigger than PAGE_SIZE.  Though 
perhaps text_poke() is not the right function for rodata patches anyway.

> Hmm, I can't agree that we allow module to modify kernel text...
> Is that possible to separate kernel-text swapping routine from Ksplice
> module? In that case, we don't need to export text_mutex.

No, it’s not enough for Ksplice to lock the kernel text only while 
actively swapping it.  We also need to prevent changes to the kernel text 
while Ksplice is doing run-pre matching to ensure safety.  This means that 
Ksplice wants to hold text_mutex for essentially the entire time it’s 
running.

Anders
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