Re: [PATCH -next V6 1/7] riscv: ftrace: Fixup panic by disabling preemption

[Mark Rutland]

On Sat, Jan 07, 2023 at 08:35:43AM -0500, guoren@xxxxxxxxxx wrote:
> From: Andy Chiu <andy.chiu@xxxxxxxxxx>
> 
> In RISCV, we must use an AUIPC + JALR pair to encode an immediate,
> forming a jump that jumps to an address over 4K. This may cause errors
> if we want to enable kernel preemption and remove dependency from
> patching code with stop_machine(). For example, if a task was switched
> out on auipc. And, if we changed the ftrace function before it was
> switched back, then it would jump to an address that has updated 11:0
> bits mixing with previous XLEN:12 part.
> 
> p: patched area performed by dynamic ftrace
> ftrace_prologue:
> p|      REG_S   ra, -SZREG(sp)
> p|      auipc   ra, 0x? ------------> preempted
> 					...
> 				change ftrace function
> 					...
> p|      jalr    -?(ra) <------------- switched back
> p|      REG_L   ra, -SZREG(sp)
> func:
> 	xxx
> 	ret

What happens on SMP but not !PREEMPTION; can't a CPU be in the middle of this
while you're patching the sequence?

Do you have any guarantee as to the atomicity and ordering of instruction
fetches?

Thanks,
Mark.

> 
> Fixes: afc76b8b8011 ("riscv: Using PATCHABLE_FUNCTION_ENTRY instead of MCOUNT")
> Signed-off-by: Andy Chiu <andy.chiu@xxxxxxxxxx>
> Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
> ---
>  arch/riscv/Kconfig | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index e2b656043abf..ee0d39b26794 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -138,7 +138,7 @@ config RISCV
>  	select HAVE_DYNAMIC_FTRACE_WITH_REGS if HAVE_DYNAMIC_FTRACE
>  	select HAVE_FTRACE_MCOUNT_RECORD if !XIP_KERNEL
>  	select HAVE_FUNCTION_GRAPH_TRACER
> -	select HAVE_FUNCTION_TRACER if !XIP_KERNEL
> +	select HAVE_FUNCTION_TRACER if !XIP_KERNEL && !PREEMPTION
>  
>  config ARCH_MMAP_RND_BITS_MIN
>  	default 18 if 64BIT
> -- 
> 2.36.1
>