[RFC PATCH] arm64: move THREAD_* definitions to separated header

[Mark Rutland]

On Wed, Aug 30, 2017 at 12:22:49PM +0300, Yury Norov wrote:
> Hi Mark, all.

Hi Yury, 

> In patch 'dbc9344a68e506f19f8 ("arm64: clean up THREAD_* definitions")'
> you move THREAD_* definitions from arch/arm64/include/asm/thread_info.h
> to asm/memory.h. After that asm/thread_info.h starts to depend on
> asm/memory.h.
> 
> When I try to apply ilp32 series on top of it [1], it causes circular
> dependencies like this one:
> 
> In file included from ./arch/arm64/include/asm/memory.h:30:0,
>                  from ./arch/arm64/include/asm/thread_info.h:30,
>                  from ./include/linux/thread_bits.h:20,
>                  from ./include/linux/thread_info.h:13,
>                  from ./include/asm-generic/preempt.h:4,
>                  from ./arch/arm64/include/generated/asm/preempt.h:1,
>                  from ./include/linux/preempt.h:80,
>                  from ./include/linux/rcupdate.h:40,
>                  from ./include/linux/rculist.h:10,
>                  from ./include/linux/pid.h:4,
>                  from ./include/linux/sched.h:13,
>                  from arch/arm64/kernel/asm-offsets.c:21:
> ./arch/arm64/include/asm/is_compat.h: In function ‘is_a32_compat_task’:
> ./arch/arm64/include/asm/is_compat.h:25:9: error: implicit declaration of function ‘test_thread_flag’ [-Werror=implicit-function-declaration]
>   return test_thread_flag(TIF_32BIT);
>          ^~~~~~~~~~~~~~~~
> 
> The problem is that asm/memory.h depends on asm/is_compat.h to define
> TASK_SIZE, which in turn requires asm/thread_info.h.

... and <asm/thread_info.h> include <asm/memory.h>, giving a circular
dependency.

In other architectures, TASK_SIZE is defined in processor.h. Can we not
move TASK_SIZE instead of THREAD_SIZE, given that TASK_SIZE is what
causes the dependency?

We'd need a new __ASSEMBLY__ guard, but otherwise it looks like that
would solve the issue.

> The most obvious solution for it is to create is_compat.c file and make
> is_*_compat() real functions. The other option is to move THREAD_*
> definitions to separated macro. I would prefer 2nd one because of following
> reasons:
>  - TASK_SIZE macro is used many times in kernel, including hot paths;
>  - asm/memory.h is included widely, as well as asm/thread_info.h, and it's
>    better not to make them depend one from another;
>  - THREAD_SIZE etc are not memory-related definitions.

I disagree with this last point. THREAD_SIZE is in <asm/memory.h>
because it affects the kernel's memory layout, as with other definitions
at the top of <asm/memory.h>.

I would much prefer keeping those definitions together.

> In this patch THREAD_* definitions moved to separated asm/thread_size.h
> header. It's enough to resolve dependency above.
>
> If you find this approach useful, I can prepare other patch that moves
> TASK_* definitions from asm/memory.h to new header to remove the dependency
> from asm/is_compat.h.

I'd prefer that we moved the TASK_SIZE* definitions first, but if
necessary, I'm ok with seeing the THREAD_SIZE* definitions move.

[...]

> diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h

> -#include <asm/memory.h>
> +#include <asm/thread_size.h>
>  #include <asm/stack_pointer.h>
>  #include <asm/types.h>

Nit: please keep includes ordered alphabetically.

[...]

> diff --git a/arch/arm64/include/asm/thread_size.h b/arch/arm64/include/asm/thread_size.h

> +#ifdef __KERNEL__

This can go. We haven't needed __KERNEL__ ifdefs for a long time now...

[...]

> diff --git a/arch/arm64/kernel/hibernate-asm.S b/arch/arm64/kernel/hibernate-asm.S
> index e56d848b6466..46b91702ea26 100644
> --- a/arch/arm64/kernel/hibernate-asm.S
> +++ b/arch/arm64/kernel/hibernate-asm.S
> @@ -22,7 +22,6 @@
>  #include <asm/asm-offsets.h>
>  #include <asm/assembler.h>
>  #include <asm/cputype.h>
> -#include <asm/memory.h>
>  #include <asm/page.h>
>  #include <asm/virt.h>

AFAICT, we can also lose <linux/errno.h>, <asm/cputype.h>, and
<asm/virt.h>.

Changes to this file might be better as a preparatory cleanup.

Thanks,
Mark.