Re: percpu: Implement this_cpu_add,sub,dec,inc_return

[Eric Dumazet <eric.dumazet@gmail.com>]

Le vendredi 19 novembre 2010 A  09:59 -0600, Christoph Lameter a A(C)crit :
> On Fri, 19 Nov 2010, Eric Dumazet wrote:
> 
> > > This isnt a use case for this_cpu_dec right? Seems that your message was
> > > cut off?
> > I wanted to show you the file were it was possible to use this_cpu_{dec|
> > inc}_return()
> >
> > My patch on kmap_atomic_idx() doesnt need your new functions ;)
> 
> Oh ok you mean this:
> 
> ---
>  include/linux/highmem.h |    8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> Index: linux-2.6/include/linux/highmem.h
> ===================================================================
> --- linux-2.6.orig/include/linux/highmem.h	2010-11-19 09:55:24.000000000 -0600
> +++ linux-2.6/include/linux/highmem.h	2010-11-19 09:57:54.000000000 -0600
> @@ -81,7 +81,9 @@ DECLARE_PER_CPU(int, __kmap_atomic_idx);
> 
>  static inline int kmap_atomic_idx_push(void)
>  {
> -	int idx = __get_cpu_var(__kmap_atomic_idx)++;
> +	int idx = __this_cpu_read(__kmap_atomic_idx);
> +
> +	__this_cpu_inc(__kmap_atomic_idx);
>  #ifdef CONFIG_DEBUG_HIGHMEM
>  	WARN_ON_ONCE(in_irq() && !irqs_disabled());
>  	BUG_ON(idx > KM_TYPE_NR);
> @@ -91,12 +93,12 @@ static inline int kmap_atomic_idx_push(v
> 
>  static inline int kmap_atomic_idx(void)
>  {
> -	return __get_cpu_var(__kmap_atomic_idx) - 1;
> +	return __this_cpu_read(__kmap_atomic_idx) - 1;
>  }
> 
>  static inline int kmap_atomic_idx_pop(void)
>  {
> -	int idx = --__get_cpu_var(__kmap_atomic_idx);
> +	int idx = __this_cpu_dec_return(__kmap_atomic_idx);
>  #ifdef CONFIG_DEBUG_HIGHMEM
>  	BUG_ON(idx < 0);
>  #endif


Yes, absolutely.

By the way, is your patch really ok ?

xadd %0,foo   returns in %0 the previous value of the memory, not the
value _after_ the operation.

This is why we do in arch/x86/include/asm/atomic.h :

static inline int atomic_add_return(int i, atomic_t *v)
...
       
        __i = i;
        asm volatile(LOCK_PREFIX "xaddl %0, %1"
                     : "+r" (i), "+m" (v->counter)
                     : : "memory");
        return i + __i;
...





--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Fight unfair telecom policy in Canada: sign http://dissolvethecrtc.ca/
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>