Date: Wed, 2 Dec 2015 12:36:55 +0000
From: Will Deacon <will.deacon@arm.com>
To: Li Bin <huawei.libin@huawei.com>
Cc: rostedt@goodmis.org, mingo@redhat.com, catalin.marinas@arm.com,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        guohanjun@huawei.com, dingtianhong@huawei.com
Subject: Re: [PATCH] arm64: ftrace: stop using kstop_machine to
 enable/disable tracing
Message-ID: <20151202123654.GC4523@arm.com>
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On Sat, Nov 28, 2015 at 03:50:09PM +0800, Li Bin wrote:
> On arm64, kstop_machine which is hugely disruptive to a running
> system is not needed to convert nops to ftrace calls or back,
> because that modifed code is a single 32bit instructions which
> is impossible to cross cache (or page) boundaries, and the used str
> instruction is single-copy atomic.

This commit message is misleading, since the single-copy atomicity
guarantees don't apply to the instruction-side. Instead, the architecture
calls out a handful of safe instructions in "Concurrent modification and
execution of instructions".

Now, those safe instructions *do* include NOP, B and BL, so that should
be sufficient for ftrace provided that we don't patch condition codes
(and I don't think we do).

> Cc: <stable@vger.kernel.org> # 3.18+

I don't think this is stable material.

Will

> Signed-off-by: Li Bin <huawei.libin@huawei.com>
> ---
>  arch/arm64/kernel/ftrace.c |    5 +++++
>  1 files changed, 5 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/arm64/kernel/ftrace.c b/arch/arm64/kernel/ftrace.c
> index c851be7..9669b33 100644
> --- a/arch/arm64/kernel/ftrace.c
> +++ b/arch/arm64/kernel/ftrace.c
> @@ -93,6 +93,11 @@ int ftrace_make_nop(struct module *mod, struct dyn_ftrace *rec,
>  	return ftrace_modify_code(pc, old, new, true);
>  }
>  
> +void arch_ftrace_update_code(int command)
> +{
> +	ftrace_modify_all_code(command);
> +}
> +
>  int __init ftrace_dyn_arch_init(void)
>  {
>  	return 0;
> -- 
> 1.7.1
> 
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