Re: [PATCH v6] arm64: implement ftrace with regs

[Steven Rostedt]

On Fri, 4 Jan 2019 17:50:18 +0000
Mark Rutland <mark.rutland@xxxxxxx> 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