linux-kernel - [PATCH] list: Expand list_first_entry_or

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Date:	Sat, 23 Jul 2016 19:27:50 +0100
From:	Chris Wilson <chris@...is-wilson.co.uk>
To:	linux-kernel@...r.kernel.org
Cc:	Chris Wilson <chris@...is-wilson.co.uk>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dan Williams <dan.j.williams@...el.com>,
	Jan Kara <jack@...e.cz>, Josef Bacik <jbacik@...com>
Subject: [PATCH] list: Expand list_first_entry_or_null()

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
---
 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