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 <[email protected]> Cc: "Paul E. McKenney" <[email protected]> Cc: Andrew Morton <[email protected]> Cc: Dan Williams <[email protected]> Cc: Jan Kara <[email protected]> Cc: Josef Bacik <[email protected]> Cc: [email protected] --- 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
