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

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Date:	Tue, 11 Aug 2015 18:03:22 +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 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.

Will
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