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

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Date:	Mon, 10 Dec 2012 13:57:48 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Will Deacon <will.deacon@....com>,
	"Jon Medhurst (Tixy)" <tixy@...aro.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Rabin Vincent <rabin@....in>, Ingo Molnar <mingo@...hat.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH] ARM: ftrace: Ensure code modifications are
	synchronised across all cpus

On Mon, Dec 10, 2012 at 08:02:17AM -0500, Steven Rostedt wrote:
> On Mon, 2012-12-10 at 10:04 +0000, Will Deacon wrote:
> > Yes, and I think if you do use two 16-bit nops, you can even get rid of all
> > the intermediate `sync' operations (I guess you might want one at the end if
> > you want the call to become visible at a particular point).
> 
> Wont work. We are replacing a 32bit call with a nop. That nop must also
> be 32bits, because we could eventually replace the nop(s) with a 32bit
> call.

... which, if it's misaligned to a 32-bit boundary, which can happen with
Thumb-2 code, will require the replacement to be done atomically; you will
need to use stop_machine() to ensure that other CPUs don't try to execute
the instruction mid-way through modification... as I have already
explained in my previous mails.
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