Re: [RFC 2/3] x86: Switch all C consumers of kernel_stack to this_cpu_sp0

[Denys Vlasenko <dvlasenk@redhat.com>]

On 02/27/2015 01:07 AM, Andy Lutomirski wrote:
> This will make modifying the semantics of kernel_stack easier.
> 
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
> Cc: Rusty Russell <rusty@rustcorp.com.au>
> Signed-off-by: Andy Lutomirski <luto@amacapital.net>
> ---
>  arch/x86/include/asm/thread_info.h | 3 +--
>  arch/x86/kernel/traps.c            | 2 +-
>  2 files changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
> index e82e95abc92b..92549053d86d 100644
> --- a/arch/x86/include/asm/thread_info.h
> +++ b/arch/x86/include/asm/thread_info.h
> @@ -163,8 +163,7 @@ DECLARE_PER_CPU(unsigned long, kernel_stack);
>  static inline struct thread_info *current_thread_info(void)
>  {
>  	struct thread_info *ti;
> -	ti = (void *)(this_cpu_read_stable(kernel_stack) +
> -		      KERNEL_STACK_OFFSET - THREAD_SIZE);
> +	ti = (void *)(this_cpu_sp0() - THREAD_SIZE);
>  	return ti;
>  }
>  
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index c74f2f5652da..d287ea779728 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -174,7 +174,7 @@ void ist_begin_non_atomic(struct pt_regs *regs)
>  	 * will catch asm bugs and any attempt to use ist_preempt_enable
>  	 * from double_fault.
>  	 */
> -	BUG_ON(((current_stack_pointer() ^ this_cpu_read_stable(kernel_stack))
> +	BUG_ON(((current_stack_pointer() ^ (this_cpu_sp0() - 1))
>  		& ~(THREAD_SIZE - 1)) != 0);

While we are at it, I propose a more readable version of this check:

BUG_ON(this_cpu_sp0() - current_stack_pointer() >= THREAD_SIZE);

Yes, I am aware that it is not equivalent to the existing condition
- it uses the fact that this_cpu_sp0(), previous check
wasn't making that assumption. But that assumption is true,
so shouldn't be a problem.