linux-kernel - Re: [PATCH] MIPS: Makefile: workaround printk recursion bug

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Date:	Wed, 10 Apr 2013 09:37:42 -0700
From:	David Daney <ddaney.cavm@...il.com>
To:	Wladislav Wiebe <wladislav.kw@...il.com>
CC:	ralf@...ux-mips.org, linux-mips@...ux-mips.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] MIPS: Makefile: workaround printk recursion bug

On 04/10/2013 02:12 AM, Wladislav Wiebe wrote:
>
> From: Wladislav Wiebe <wladislav.kw@...il.com>
>
> Function tracing is broken due to removal of selecting FRAME_POINTER with
> FUNCTION_TRACER as result of commit: b732d439cb43336cd6d7e804ecb2c81193ef63b0
>
> Latest commit ad8c396936e328f5344e1881afde9e28d5f2045f "MIPS: Unbreak
> function tracer for 64-bit kernel." fixes just the early startup hang,
> but on MIPS64/CAVIUM_OCTEON2 are still random printk recursion bugs
> which cause also Kernel hangs, especially on late startup phase when
> network drivers get loaded. This patch enable for CAVIUM_OCTEON2/64 Bit
> architecture -fno-omit-frame-pointer cflag when FUNCTION_TRACER get
> enabled. This will fix random Kernel hangs with "BUG: recent printk
> recursion!" from linux/kernel/printk.c.
>
> Maybe there exist a other solution in mcount handling, since in the
> commit message from Al Cooper is mentioned that "MIPS frame pointers are
> generally considered to be useless because they cannot be used to unwind
> the stack. Unfortunately the MIPS function tracing code has bugs that
> are masked by the use of frame pointers. This commit fixes the bugs so
> that MIPS frame pointers don't need to be enabled."
>
> But this is just a solution for MIPS32 - on a symmetric multiprocessing
> @MIPS64/CAVIUM_OCTEON2 it doesn't work properly.

There are a couple of problems that I see with this patch:

1) It doesn't handle non-OCTEON2.  Surely other 64-bit targets are 
effected as well

2) You don't say how it is broken or how this fixes it.

3) Function graph tracing on 3.9.0-rc6 compiled with gcc-4.7.0 works 
fine for me without this.  So I see no need to clog up the make files 
with a rats nest of ifdef

Without more information about why this  is needed, I would have to say NAK.

David Daney

>
> Signed-off-by: Wladislav Wiebe <wladislav.kw@...il.com>
> ---
>   arch/mips/Makefile |    9 +++++++++
>   1 file changed, 9 insertions(+)
>
> diff --git a/arch/mips/Makefile b/arch/mips/Makefile
> index 6f7978f..8befe31 100644
> --- a/arch/mips/Makefile
> +++ b/arch/mips/Makefile
> @@ -119,6 +119,15 @@ cflags-$(CONFIG_SB1XXX_CORELIS)	+= $(call cc-option,-mno-sched-prolog) \
>   				   -fno-omit-frame-pointer
>
>   #
> +# FTrace depended compiler options, currently only needed by MIPS64/OCTEON2.
> +#
> +ifdef CONFIG_64BIT
> +ifdef CONFIG_CAVIUM_OCTEON2
> +cflags-$(CONFIG_FUNCTION_TRACER) += $(call cc-option,-fno-omit-frame-pointer)
> +endif
> +endif
> +
> +#
>   # CPU-dependent compiler/assembler options for optimization.
>   #
>   cflags-$(CONFIG_CPU_R3000)	+= -march=r3000
>

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