Commit-ID: 12adfd882c5f37548acaba4f043a158b3c54468b
Gitweb: http://git.kernel.org/tip/12adfd882c5f37548acaba4f043a158b3c54468b
Author: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
AuthorDate: Sat, 23 Jul 2016 19:27:50 +0100
Committer: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
CommitDate: Wed, 14 Sep 2016 12:57:43 -0700
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@xxxxxxxxxxxxxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: Jan Kara <jack@xxxxxxx>
Cc: Josef Bacik <jbacik@xxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---
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 5183138..5809e9a 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
--
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