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Subject[PATCH tip/core/rcu 1/7] list: Expand list_first_entry_or_null()
Date
From: Chris Wilson <chris@chris-wilson.co.uk>

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@chris-wilson.co.uk>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Josef Bacik <jbacik@fb.com>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
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 5183138aa932..5809e9a2de5b 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
--
2.5.2
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