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

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Date:	Thu, 13 Mar 2014 14:49:26 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Will Deacon <will.deacon@....com>
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, 2014-03-13 at 18:37 +0000, Will Deacon wrote:
>  
> > 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!
> 

My kids already know I was :-)

> > > 
> > > > +       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.

Yep, the tracing_graph_pause is more to prevent crazy stuff happening
when an interrupt comes in. So it is just local cpu context, and doesn't
have issues with other CPUS. If it takes a bit for the task to see it go
to zero, the worse thing is that you might miss a few tracing functions
until that gets synced. Which will probably happen by the time tracing
is fully activated anyway.

-- Steve


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