linux-kernel - Re: [PATCH 1/2] MIPS: ftrace.h: Fix the MCOUNT_INSN

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Date:	Mon, 22 Sep 2014 09:55:09 -0700
From:	David Daney <ddaney.cavm@...il.com>
To:	Markos Chandras <markos.chandras@...tec.com>
CC:	linux-mips@...ux-mips.org, Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] MIPS: ftrace.h: Fix the MCOUNT_INSN_SIZE definition

On 09/22/2014 06:32 AM, Markos Chandras wrote:
> The MCOUNT_INSN_SIZE is meant to be used to denote the overall
> size of the mcount() call. Since a jal instruction is used to
> call mcount() the delay slot should be taken into consideration
> as well.
> This also replaces the MCOUNT_INSN_SIZE usage with the real size
> of a single MIPS instruction since, as described above, the
> MCOUNT_INSN_SIZE is used to denote the total overhead of the
> mcount() call.

Are you seeing errors with the existing code?  If so please state what 
they are.

By changing this, we can no longer atomically replace the instruction. 
So I think shouldn't be changing this stuff unless there is a real bug 
we are fixing.

In conclusion: NAK unless the patch fixes a bug, in which case the 
change log *must* state what the bug is, and how the patch addresses the 
problem.

David Daney


>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: linux-kernel@...r.kernel.org
> Signed-off-by: Markos Chandras <markos.chandras@...tec.com>
> ---
>   arch/mips/include/asm/ftrace.h | 2 +-
>   arch/mips/kernel/ftrace.c      | 4 +++-
>   2 files changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/arch/mips/include/asm/ftrace.h b/arch/mips/include/asm/ftrace.h
> index 992aaba603b5..70d4a35fb560 100644
> --- a/arch/mips/include/asm/ftrace.h
> +++ b/arch/mips/include/asm/ftrace.h
> @@ -13,7 +13,7 @@
>   #ifdef CONFIG_FUNCTION_TRACER
>
>   #define MCOUNT_ADDR ((unsigned long)(_mcount))
> -#define MCOUNT_INSN_SIZE 4		/* sizeof mcount call */
> +#define MCOUNT_INSN_SIZE 8		/* sizeof mcount call + delay slot */
>
>   #ifndef __ASSEMBLY__
>   extern void _mcount(void);
> diff --git a/arch/mips/kernel/ftrace.c b/arch/mips/kernel/ftrace.c
> index 937c54bc8ccc..211460d4617d 100644
> --- a/arch/mips/kernel/ftrace.c
> +++ b/arch/mips/kernel/ftrace.c
> @@ -28,6 +28,8 @@
>   #define MCOUNT_OFFSET_INSNS 4
>   #endif
>
> +#define FTRACE_MIPS_INSN_SIZE 4 /* Size of single MIPS instruction */
> +
>   #ifdef CONFIG_DYNAMIC_FTRACE
>
>   /* Arch override because MIPS doesn't need to run this from stop_machine() */
> @@ -395,7 +397,7 @@ void prepare_ftrace_return(unsigned long *parent_ra_addr, unsigned long self_ra,
>   	 */
>
>   	insns = in_kernel_space(self_ra) ? 2 : MCOUNT_OFFSET_INSNS + 1;
> -	trace.func = self_ra - (MCOUNT_INSN_SIZE * insns);
> +	trace.func = self_ra - (FTRACE_MIPS_INSN_SIZE * insns);
>
>   	/* Only trace if the calling function expects to */
>   	if (!ftrace_graph_entry(&trace)) {
>

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