linux-kernel - Re: [PATCH 3/4] x86/unwind: include _

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Date:   Tue, 10 Jan 2017 11:28:49 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Miroslav Benes <mbenes@...e.cz>
Cc:     x86@...nel.org, Dave Jones <davej@...emonkey.org.uk>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andy Lutomirski <luto@...capital.net>,
        Brian Gerst <brgerst@...il.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] x86/unwind: include __schedule() in stack traces

On Tue, Jan 10, 2017 at 11:14:51AM +0100, Miroslav Benes wrote:
> On Mon, 9 Jan 2017, Josh Poimboeuf wrote:
> > Signed-off-by: Josh Poimboeuf <jpoimboe@...hat.com>
> > ---
> >  arch/x86/include/asm/stacktrace.h |  5 +----
> >  arch/x86/include/asm/switch_to.h  | 10 +++++++++-
> >  2 files changed, 10 insertions(+), 5 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/stacktrace.h b/arch/x86/include/asm/stacktrace.h
> > index 20ce3db..2e41c50 100644
> > --- a/arch/x86/include/asm/stacktrace.h
> > +++ b/arch/x86/include/asm/stacktrace.h
> > @@ -52,16 +52,13 @@ static inline bool on_stack(struct stack_info *info, void *addr, size_t len)
> >  static inline unsigned long *
> >  get_frame_pointer(struct task_struct *task, struct pt_regs *regs)
> >  {
> > -	struct inactive_task_frame *frame;
> > -
> >  	if (regs)
> >  		return (unsigned long *)regs->bp;
> >  
> >  	if (task == current)
> >  		return __builtin_frame_address(0);
> >  
> > -	frame = (struct inactive_task_frame *)task->thread.sp;
> > -	return (unsigned long *)READ_ONCE_NOCHECK(frame->bp);
> > +	return &((struct inactive_task_frame *)task->thread.sp)->bp;
> 
> You effectively remove one of the changes from the previous patch - 
> READ_ONCE_NOCHECK. Is it intentional?

Yes, notice that it's no longer reading the value of bp on the stack.
It's instead getting a pointer to it.  Since there's no longer a stack
access, READ_ONCE_NOCHECK is no longer needed.

-- 
Josh