[PATCH] kref: prefer atomic_inc_not_zero to atomic_add_unless

[Daniel Vetter]

On Thu, Dec 15, 2016 at 06:01:10AM +0100, Jason A. Donenfeld wrote:
> On most platforms, there exists this ifdef:
> 
>  #define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
> 
> This makes this patch functionally useless. However, on PPC, there is
> actually an explicit definition of atomic_inc_not_zero with its own
> assembly that is slightly more optimized than atomic_add_unless. So,
> this patch changes kref to use atomic_inc_not_zero instead, for PPC and
> any future platforms that might provide an explicit implementation.
> 
> This also puts this usage of kref more in line with a verbatim reading
> of the examples in Paul McKenney's paper [1] in the section titled "2.4
> Atomic Counting With Check and Release Memory Barrier", which uses
> atomic_inc_not_zero.
> 
> [1] http://open-std.org/jtc1/sc22/wg21/docs/papers/2007/n2167.pdf
> 
> Signed-off-by: Jason A. Donenfeld <Jason at zx2c4.com>
> Reviewed-by: Thomas Hellstrom <thellstrom at vmware.com>

Applied to drm-misc, but for 4.11 since 4.10 is bugfixes only already.

Thanks, Daniel

> ---
> This was reviewed favorably 14 months ago but never picked up.
> I'm resubmitting it now in hopes that you can finally queue it
> up for 4.10.
> 
>  include/linux/kref.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/kref.h b/include/linux/kref.h
> index e15828fd71f1..62f0a84ae94e 100644
> --- a/include/linux/kref.h
> +++ b/include/linux/kref.h
> @@ -133,6 +133,6 @@ static inline int kref_put_mutex(struct kref *kref,
>   */
>  static inline int __must_check kref_get_unless_zero(struct kref *kref)
>  {
> -	return atomic_add_unless(&kref->refcount, 1, 0);
> +	return atomic_inc_not_zero(&kref->refcount);
>  }
>  #endif /* _KREF_H_ */
> -- 
> 2.11.0
> 
> _______________________________________________
> dri-devel mailing list
> dri-devel at lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/dri-devel

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch