linux-kernel - Re: [PATCH 2/2] ftrace: add function-graph tracer support for ARM v2

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Message-ID: <20090520081150.GA22558@n2100.arm.linux.org.uk>
Date:	Wed, 20 May 2009 09:11:50 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Tim Bird <tim.bird@...sony.com>, Ingo Molnar <mingo@...e.hu>,
	linux kernel <linux-kernel@...r.kernel.org>,
	linux-arm-kernel <linux-arm-kernel@...ts.arm.linux.org.uk>,
	Steven Rostedt <rostedt@...dmis.org>,
	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
Subject: Re: [PATCH 2/2] ftrace: add function-graph tracer support for ARM
	v2

On Wed, May 20, 2009 at 04:34:35AM +0200, Frederic Weisbecker wrote:
> On Fri, May 15, 2009 at 10:06:36PM +0100, Russell King - ARM Linux wrote:
> > However, I'm not sure that this code is entirely right (and I'm not
> > sure what's going on with this patch - my editor is randomly changing
> > the placement of characters in it.  Are you submitting patches using
> > UTF-8 characters in the code?)
> > 
> > > >> @@ -139,8 +144,16 @@ ENTRY(mcount)
> > > >>       adr r0, ftrace_stub
> > > >>       cmp r0, r2
> > > >>       bne trace
> > 
> > If this is r0 != ftrace_stub, go to trace.  So the next block will
> > only be executed if r0 /was/ ftrace_stub, in which case it can't be
> > ftrace_graph_return.
> 
> Ah! This part concerns the function tracer.
> Let's see how it looks like in the original code: (added more comments
> inside)

You've completely missed the point I was trying to make.  Please go back
and re-read my email and think about precisely what's happening in the
assembly path, particularly which branches will be taken and which code
paths will be taken with various values of r2.

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