Re: [PATCH 4/7] ARM: Use hack from powerpc to get current cpu number

[Ard Biesheuvel <ardb@kernel.org>]

On Wed, 8 Sept 2021 at 00:00, Keith Packard <keithpac@amazon.com> wrote:
>
> When we enable THREAD_INFO_IN_TASK, the cpu number will disappear from
> thread_info and reappear in task_struct. As we cannot include
> linux/sched.h in asm/smp.h, there's no way to use that struct type in
> the raw_smp_processor_id macro. Instead, a hack from the powerpc code
> is used. This pulls the TI_CPU offset out of asm-offsets.h and uses
> that to find the cpu value.
>
> Signed-off-by: Keith Packard <keithpac@amazon.com>
> ---
>  arch/arm/Makefile          |  8 ++++++++
>  arch/arm/include/asm/smp.h | 18 +++++++++++++++++-
>  2 files changed, 25 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm/Makefile b/arch/arm/Makefile
> index 415c3514573a..6752995d2914 100644
> --- a/arch/arm/Makefile
> +++ b/arch/arm/Makefile
> @@ -284,6 +284,14 @@ stack_protector_prepare: prepare0
>         $(eval GCC_PLUGINS_CFLAGS += $(SSP_PLUGIN_CFLAGS))
>  endif
>
> +ifdef CONFIG_SMP
> +prepare: task_cpu_prepare
> +
> +PHONY += task_cpu_prepare
> +task_cpu_prepare: prepare0
> +       $(eval KBUILD_CFLAGS += -D_TI_CPU=$(shell awk '{if ($$2 == "TI_CPU") print $$3;}' include/generated/asm-offsets.h))
> +endif
> +
>  all:   $(notdir $(KBUILD_IMAGE))
>
>
> diff --git a/arch/arm/include/asm/smp.h b/arch/arm/include/asm/smp.h
> index d43b64635d77..f77ba3753bc4 100644
> --- a/arch/arm/include/asm/smp.h
> +++ b/arch/arm/include/asm/smp.h
> @@ -15,7 +15,23 @@
>  # error "<asm/smp.h> included in non-SMP build"
>  #endif
>
> -#define raw_smp_processor_id() (current_thread_info()->cpu)
> +/*
> + * This is particularly ugly: it appears we can't actually get the
> + * definition of task_struct here, but we need access to the CPU this
> + * task is running on, which is stored in task_struct when
> + * THREAD_INFO_IN_TASK is set.  Instead of using task_struct we're
> + * using TI_CPU which is extracted from asm-offsets.h by kbuild to get
> + * the current processor ID.
> + *
> + * This also needs to be safeguarded when building asm-offsets.s
> + * because at that time TI_CPU is not defined yet.
> + */
> +#ifndef _TI_CPU
> +#define raw_smp_processor_id()         (0)
> +#else
> +#define raw_smp_processor_id() \
> +       (*(unsigned int *)((void *)current_thread_info() + _TI_CPU))
> +#endif
>
>  struct seq_file;
>

As I stated before, I would really like to avoid using this hack - I
don't think we need to.