linux-kernel - Re: [PATCH v6 6/7] arm64: ftrace: Add CALLER

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Message-ID: <20140314100914.GD13541@arm.com>
Date:	Fri, 14 Mar 2014 10:09:14 +0000
From:	Will Deacon <will.deacon@....com>
To:	AKASHI Takahiro <takahiro.akashi@...aro.org>
Cc:	"rostedt@...dmis.org" <rostedt@...dmis.org>,
	"fweisbec@...il.com" <fweisbec@...il.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	Catalin Marinas <Catalin.Marinas@....com>,
	"tim.bird@...ymobile.com" <tim.bird@...ymobile.com>,
	"gkulkarni@...iumnetworks.com" <gkulkarni@...iumnetworks.com>,
	"dsaxena@...aro.org" <dsaxena@...aro.org>,
	"arndb@...db.de" <arndb@...db.de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 6/7] arm64: ftrace: Add CALLER_ADDRx macros

On Fri, Mar 14, 2014 at 03:00:14AM +0000, AKASHI Takahiro wrote:
> On 03/14/2014 12:54 AM, Will Deacon wrote:
> > On Thu, Mar 13, 2014 at 10:13:49AM +0000, AKASHI Takahiro wrote:
> >> CALLER_ADDRx returns caller's address at specified level in call stacks.
> >> They are used for several tracers like irqsoff and preemptoff.
> >> Strange to say, however, they are refered even without FTRACE.
> >>
> >> Please note that this implementation assumes that we have frame pointers.
> >> (which means kernel should be compiled with -fno-omit-frame-pointer.)
> >
> > How do you ensure that -fno-omit-frame-pointer is passed?
> 
> arm64 selects ARCH_WANT_FRAME_POINTERS, then FRAME_POINTER is on (lib/Kconfig.debug)
> and so -fno-omit-frame-pointer is appended (${TOP}/Makefile).
> (stacktrace.c also assumes FRAME_POINTER.)
> 
> Do you think I should remove the comment above?

Yes please, it sounds like everything is taken care of automatically, so
there's no need to scare people in the commit message!

> >> diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
> >> index ed5c448..c44c4b1 100644
> >> --- a/arch/arm64/include/asm/ftrace.h
> >> +++ b/arch/arm64/include/asm/ftrace.h
> >> @@ -18,6 +18,7 @@
> >>
> >>   #ifndef __ASSEMBLY__
> >>   extern void _mcount(unsigned long);
> >> +extern void *return_address(unsigned int);
> >>
> >>   struct dyn_arch_ftrace {
> >>   	/* No extra data needed for arm64 */
> >> @@ -33,6 +34,16 @@ static inline unsigned long ftrace_call_adjust(unsigned long addr)
> >>   	 */
> >>   	return addr;
> >>   }
> >> -#endif /* __ASSEMBLY__ */
> >> +
> >> +#define HAVE_ARCH_CALLER_ADDR
> >> +
> >> +#define CALLER_ADDR0 ((unsigned long)__builtin_return_address(0))
> >> +#define CALLER_ADDR1 ((unsigned long)return_address(1))
> >> +#define CALLER_ADDR2 ((unsigned long)return_address(2))
> >> +#define CALLER_ADDR3 ((unsigned long)return_address(3))
> >> +#define CALLER_ADDR4 ((unsigned long)return_address(4))
> >> +#define CALLER_ADDR5 ((unsigned long)return_address(5))
> >> +#define CALLER_ADDR6 ((unsigned long)return_address(6))
> >
> > Could we change the core definitions of these macros (in linux/ftrace.h) to
> > use return_address, then provide an overridable version of return_address
> > that defaults to __builtin_return_address, instead of copy-pasting this
> > sequence?
> 
> I think I understand what you mean, and will try to post a separate RFC,
> but I also want to hold off this change on this patch since such a change
> may raise a small controversy from other archs' maintainers.

I don't see anything controversial here, but ok. Steve already posted
something you can get started with.

Will
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