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

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Message-ID: <1355148365.17101.168.camel@gandalf.local.home>
Date:	Mon, 10 Dec 2012 09:06:05 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
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, 2012-12-10 at 13:57 +0000, Russell King - ARM Linux wrote:
> 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.

If there's no way to modify a 32bit operation without stop_machine(),
ever with a breakpoint, than we can stop the discussion here. ARM will
forever require stop_machine() for use with tracepoints and ftrace. Too
bad, as ARM was the x86 competitor. Here's something that x86 has a one
up on ARM.

-- Steve


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