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SubjectRe: [PATCH -next V6 1/7] riscv: ftrace: Fixup panic by disabling preemption
On Sat, Jan 07, 2023 at 08:35:43AM -0500, guoren@kernel.org wrote:
> From: Andy Chiu <andy.chiu@sifive.com>
>
> 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@sifive.com>
> Signed-off-by: Guo Ren <guoren@kernel.org>
> ---
> 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
>

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