linux-kernel - Re: [PATCH] percpu: fix DEBUG_PREEMPT per

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Date:	Mon, 11 Feb 2008 11:37:05 -0800
From:	Mike Travis <travis@....com>
To:	Hugh Dickins <hugh@...itas.com>
CC:	Paul Mackerras <paulus@...ba.org>, Ingo Molnar <mingo@...e.hu>,
	David Miller <davem@...emloft.net>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] percpu: fix DEBUG_PREEMPT per_cpu checking

Thanks Hugh for catching this.  I've added it to my test code base
and it works fine for x86_64...

Reviewed-by: Mike Travis <travis@....com>

Hugh Dickins wrote:
> Recent percpu changes have broken CONFIG_DEBUG_PREEMPT's per_cpu checking
> on several architectures.  On s390, sparc64 and x86 it's been weakened to
> not checking at all; whereas on powerpc64 it's become too strict, issuing
> warnings from __raw_get_cpu_var in io_schedule and init_timer for example.
> 
> Fix this by weakening powerpc's __my_cpu_offset to use the non-checking
> local_paca instead of get_paca (which itself contains such a check);
> and strengthening the generic my_cpu_offset to go the old slow way via
> smp_processor_id when CONFIG_DEBUG_PREEMPT (debug_smp_processor_id is
> where all the knowledge of what's correct when lives).
> 
> Signed-off-by: Hugh Dickins <hugh@...itas.com>
> ---
> ia64 would be in the first group too, but does not support DEBUG_PREEMPT?
> 
>  include/asm-generic/percpu.h |    2 ++
>  include/asm-powerpc/percpu.h |    2 +-
>  2 files changed, 3 insertions(+), 1 deletion(-)
> 
> --- 2.6.24-git18/include/asm-generic/percpu.h	2008-02-08 11:31:30.000000000 +0000
> +++ linux/include/asm-generic/percpu.h	2008-02-08 12:27:08.000000000 +0000
> @@ -32,6 +32,8 @@ extern unsigned long __per_cpu_offset[NR
>   */
>  #ifndef __my_cpu_offset
>  #define __my_cpu_offset per_cpu_offset(raw_smp_processor_id())
> +#endif
> +#ifdef CONFIG_DEBUG_PREEMPT
>  #define my_cpu_offset per_cpu_offset(smp_processor_id())
>  #else
>  #define my_cpu_offset __my_cpu_offset
> --- 2.6.24-git18/include/asm-powerpc/percpu.h	2008-02-08 11:31:31.000000000 +0000
> +++ linux/include/asm-powerpc/percpu.h	2008-02-08 12:29:17.000000000 +0000
> @@ -13,7 +13,7 @@
>  #include <asm/paca.h>
>  
>  #define __per_cpu_offset(cpu) (paca[cpu].data_offset)
> -#define __my_cpu_offset get_paca()->data_offset
> +#define __my_cpu_offset local_paca->data_offset
>  #define per_cpu_offset(x) (__per_cpu_offset(x))
>  
>  #endif /* CONFIG_SMP */

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