linux-kernel - Re: [PATCH] list: Expand list_first_entry_or

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Date:	Sun, 24 Jul 2016 15:43:33 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Chris Wilson <chris@...is-wilson.co.uk>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dan Williams <dan.j.williams@...el.com>,
	Jan Kara <jack@...e.cz>, Josef Bacik <jbacik@...com>
Subject: Re: [PATCH] list: Expand list_first_entry_or_null()

On Sat, Jul 23, 2016 at 07:27:50PM +0100, Chris Wilson wrote:
> Due to the use of READ_ONCE() in list_empty() the compiler cannot
> optimise !list_empty() ? list_first_entry() : NULL very well. By
> manually expanding list_first_entry_or_null() we can take advantage of
> the READ_ONCE() to avoid the list element changing under the test while
> the compiler can generate smaller code.
> 
> Signed-off-by: Chris Wilson <chris@...is-wilson.co.uk>
> Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: Jan Kara <jack@...e.cz>
> Cc: Josef Bacik <jbacik@...com>
> Cc: linux-kernel@...r.kernel.org

Queued for review and testing, thank you!

							Thanx, Paul

> ---
>  include/linux/list.h | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/list.h b/include/linux/list.h
> index 5356f4d661a7..7f8b08492cb3 100644
> --- a/include/linux/list.h
> +++ b/include/linux/list.h
> @@ -381,8 +381,11 @@ static inline void list_splice_tail_init(struct list_head *list,
>   *
>   * Note that if the list is empty, it returns NULL.
>   */
> -#define list_first_entry_or_null(ptr, type, member) \
> -	(!list_empty(ptr) ? list_first_entry(ptr, type, member) : NULL)
> +#define list_first_entry_or_null(ptr, type, member) ({ \
> +	struct list_head *head__ = (ptr); \
> +	struct list_head *pos__ = READ_ONCE(head__->next); \
> +	pos__ != head__ ? list_entry(pos__, type, member) : NULL; \
> +})
> 
>  /**
>   * list_next_entry - get the next element in list
> -- 
> 2.8.1
>