linux-kernel - Re: [PATCH] ARM: ftrace: Ensure code modifications are synchronised across all cpus

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Date:	Fri, 07 Dec 2012 11:36:40 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	"Jon Medhurst (Tixy)" <tixy@...aro.org>,
	linux-arm-kernel@...ts.infradead.org,
	Ingo Molnar <mingo@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Rabin Vincent <rabin@....in>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ARM: ftrace: Ensure code modifications are synchronised
 across all cpus

On Fri, 2012-12-07 at 16:23 +0000, Russell King - ARM Linux wrote:

> That's fine if there are better ways.  If your view is that this would
> bring things "up to the future" consider this: what you suggest is possible
> with the standard ARM 32-bit instruction set.  With the more modern Thumb
> instruction set, because we now effectively have prefixes, where those
> prefixes control the execution of the following instructions, what you
> suggest becomes no longer possible.
> 
> So, it's not a question of bringing stuff up to the future at all... you
> can call it a design regression of you will, but you're really making
> demands about how CPUs work which are outside of your remit.
> 
> Think of this a bit like you changing the opcodes immediately following a
> 'LOCK' prefix on x86.  I suspect divorsing the following opcodes from its
> prefix would be very bad for the instructions atomicity.

But what about the limitations that the function tracer imposes on the
code that gets modified by stop_machine()?

1) the original code is simply a call to mcount

2) on boot up, that call gets converted into a nop

3) the code that gets changed will only be converting a nop to a call
into the function tracer, and back again.

IOW, it's a very limited subset of the ARM assembly that gets touched.
I'm not sure what the op codes are for the above, but I can imagine they
don't impose the prefixes as you described.

If that's the case, is it still possible to change to the breakpoint
method?

-- Steve


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