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Date: Mon, 10 Dec 2012 13:57:48 +0000
From: Russell King - ARM Linux <linux@arm.linux.org.uk>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Will Deacon <will.deacon@arm.com>, "Jon Medhurst (Tixy)" <tixy@linaro.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
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	<linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH] ARM: ftrace: Ensure code modifications are
	synchronised across all cpus
Message-ID: <20121210135747.GK14363@n2100.arm.linux.org.uk>
References: <1354888985.17101.41.camel@gandalf.local.home> <1354892111.13000.50.camel@linaro1.home> <1354894134.17101.44.camel@gandalf.local.home> <20121207162346.GW14363@n2100.arm.linux.org.uk> <1354898200.17101.50.camel@gandalf.local.home> <20121207164530.GX14363@n2100.arm.linux.org.uk> <1354900436.17101.58.camel@gandalf.local.home> <1354902347.8263.12.camel@linaro1.home> <20121210100433.GB6624@mudshark.cambridge.arm.com> <1355144537.17101.155.camel@gandalf.local.home>
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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.