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From: "Rafael J. Wysocki" <rjw@rjwysocki.net>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>,
        Paul Menzel <pmenzel@molgen.mpg.de>, Len Brown <lenb@kernel.org>,
        linux-acpi@vger.kernel.org, linux-kernel@vger.kernel.org,
        Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH] acpi: fix incompatibility with mcount-based function graph tracing
Date: Sat, 25 Mar 2017 14:20:11 +0100
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On Friday, March 24, 2017 02:41:14 PM Steven Rostedt wrote:
> On Fri, 24 Mar 2017 13:12:54 -0500
> Josh Poimboeuf <jpoimboe@redhat.com> wrote:
> 
> 
> > Instead I was able to "fix" it by ignoring ftrace calls in real mode:
> > 
> > -----
> > index 8f3d9cf..5c0d0c6 100644
> > --- a/arch/x86/kernel/ftrace.c
> > +++ b/arch/x86/kernel/ftrace.c
> > @@ -983,6 +983,9 @@ void prepare_ftrace_return(unsigned long self_addr, unsigned long *parent,
> >  	unsigned long return_hooker = (unsigned long)
> >  				&return_to_handler;
> >  
> > +	if (__builtin_return_address(0) < TASK_SIZE_MAX)
> > +		return;
> > +
> >  	if (unlikely(ftrace_graph_is_dead()))
> >  		return;
> > ---------------
> > 
> > I'm not sure what the best fix should really be.  A few ideas off the
> > top of my head:
> > 
> > - A real mode check similar to the above (except it should probably be
> >   more precise)
> 
> The real mode check hack may be good enough for now. Make sure that
> it's commented well.

Agreed.

Thanks,
Rafael