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Date:   Fri, 4 Jan 2019 13:06:48 -0500
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Mark Rutland <mark.rutland@arm.com>
Cc:     Torsten Duwe <duwe@lst.de>, Will Deacon <will.deacon@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
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        AKASHI Takahiro <takahiro.akashi@linaro.org>,
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        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        live-patching@vger.kernel.org
Subject: Re: [PATCH v6] arm64: implement ftrace with regs
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On Fri, 4 Jan 2019 17:50:18 +0000
Mark Rutland <mark.rutland@arm.com> wrote:

> At Linux Plumbers, I had a conversation with Steve Rostedt, and we came
> to the conclusion that (withut heavyweight synchronization) patching two
> NOPs at runtime isn't safe, since a CPU might have executed the first
> NOP as a NOP before another CPU patches both instructions. So a CPU
> might execute:
> 
> 	NOP
> 	BL	ftrace_regs_caller
> 
> ... rather than the expected:
> 
> 	MOV	X9, X30
> 	BL	ftrace_regs_caller
> 
> ... and therefore X9 contains some UNKNOWN value, rather than the
> original LR value.
> 
> I wonder if we could solve that by patching the kernel at build-time, to
> add the MOV X9, X30 in place of the first NOP. If we were to do that, we
> could also update the addresses to pooint at the second NOP, simplifying
> the changes to the runtime code.

You can also patch it at boot up when there's only one CPU running, and
interrupts are disabled.

-- Steve