Re: [RFC v2 1/4] ftrace: allow arch-specific check_stack()

[Will Deacon <will.deacon@arm.com>]

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@linaro.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