linux-kernel - Re: [PATCH v6 4/7] arm64: Add ftrace support

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Date:	Thu, 13 Mar 2014 18:37:05 +0000
From:	Will Deacon <will.deacon@....com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	AKASHI Takahiro <takahiro.akashi@...aro.org>,
	"fweisbec@...il.com" <fweisbec@...il.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"tim.bird@...ymobile.com" <tim.bird@...ymobile.com>,
	"gkulkarni@...iumnetworks.com" <gkulkarni@...iumnetworks.com>,
	"dsaxena@...aro.org" <dsaxena@...aro.org>,
	"arndb@...db.de" <arndb@...db.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 4/7] arm64: Add ftrace support

On Thu, Mar 13, 2014 at 06:27:39PM +0000, Steven Rostedt wrote:
> On Thu, 2014-03-13 at 17:08 +0000, Will Deacon wrote:
> 
> > > +       /*
> > > +        * Note:
> > > +        * No protection against faulting at *parent, which may be seen
> > > +        * on other archs. It's unlikely on AArch64.
> > > +        */
> > > +       old = *parent;
> > > +       *parent = return_hooker;
> > 
> > return_hook? People might take it personally otherwise ;)
> 
> No, return_hooker is consistent with all the other archs. Hey, it's a
> rugby position! Note, which I was when I played. ;-)

Hehe, in which case your children will be able to execute that line of
ftrace!

> > 
> > > +       trace.func = self_addr;
> > 
> > in kernel/ftrace/ftrace.c there's an smb_wmb() between setting up
> > tracing_graph_pause and setting the ret_stack with a comment saying:
> > 
> >     /* Make sure the tasks see the -1 first: */
> > 
> > Why don't we have a corresponding read-barrier here?
> 
> The corresponding rmb is in kernel/trace/trace_function_graph
> ftrace_push_return_trace().

Okey doke then, I guess we don't really care about tracing_graph_pause
getting out-of-sync with curr_ret_stack.

Will
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