linux-kernel - Re: [RFC v2 1/4] ftrace: allow arch-specific check

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Date:	Tue, 18 Aug 2015 09:21:36 +0100
From:	Will Deacon <will.deacon@....com>
To:	AKASHI Takahiro <takahiro.akashi@...aro.org>
Cc:	Catalin Marinas <Catalin.Marinas@....com>,
	"rostedt@...dmis.org" <rostedt@...dmis.org>,
	"jungseoklee85@...il.com" <jungseoklee85@...il.com>,
	"olof@...om.net" <olof@...om.net>,
	"broonie@...nel.org" <broonie@...nel.org>,
	"david.griego@...aro.org" <david.griego@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC v2 1/4] ftrace: allow arch-specific check_stack()

On Mon, Aug 17, 2015 at 07:07:00AM +0100, AKASHI Takahiro wrote:
> On 08/12/2015 02:03 AM, Will Deacon wrote:
> > On Tue, Aug 04, 2015 at 08:44:06AM +0100, AKASHI Takahiro wrote:
> >> A stack frame pointer may be used in a different way depending on
> >> cpu architecture. Thus it is not always appropriate to slurp the stack
> >> contents, as currently done in check_stack(), in order to calcurate
> >> a stack index (height) at a given function call. At least not on arm64.
> >>
> >> This patch extract potentially arch-specific code from check_stack()
> >> and puts it into a new arch_check_stack(), which is declared as weak.
> >> So we will be able to add arch-specific and most efficient way of
> >> stack traversing Later.
> >>
> >> Signed-off-by: AKASHI Takahiro <takahiro.akashi@...aro.org>
> >
> > If arm64 is the only architecture behaving differently, then I'm happy
> > to reconsider the fix to unwind_frame that we merged in e306dfd06fcb
> > ("ARM64: unwind: Fix PC calculation"). I'd have thought any architecture
> > with a branch-and-link instruction would potentially have the same issue,
> > so we could just be fixing things in the wrong place if ftrace works
> > everywhere else.
> 
> I'm not the right person to answer for other architectures (and ftrace
> behavior on them.) The only thing I know is that current ftrace stack tracer
> works correctly only if the addresses stored and found on stack match to
> the ones returned by save_stack_trace().
> 
> Anyway, the fix above is not the only reason that I want to introduce arch-specific
> arch_check_stack(). Other issues to fix include
>    - combined case of stack tracer and function graph tracer (common across arch's)
>    - exception entries (as I'm trying to address in RFC 4/4)
>    - in-accurate stack size (for each function, my current fix is not perfect though.)

Ok, if you have other reasons for the callback, then fine. I just didn't
want you to think that you had to work around e306dfd06fcb at all costs.

Will
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