llist: fix/simplify llist_add() and llist_add_batch()

1. This is mostly theoretical, but llist_add*() need ACCESS_ONCE().

   Otherwise it is not guaranteed that the first cmpxchg() uses the
   same value for old_entry and new_last->next.

2. These helpers cache the result of cmpxchg() and read the initial
   value of head->first before the main loop. I do not think this
   makes sense. In the likely case cmpxchg() succeeds, otherwise
   it doesn't hurt to reload head->first.

   I think it would be better to simplify the code and simply read
   ->first before cmpxchg().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrey Vagin <avagin@openvz.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

2 files changed, 12 insertions, 22 deletions

diff --git a/include/linux/llist.h b/include/linux/llist.h
index a5199f6d0e82..3e2b969d68f6 100644
--- a/include/linux/llist.h
+++ b/include/linux/llist.h
@@ -151,18 +151,13 @@ static inline struct llist_node *llist_next(struct llist_node *node)
  */
 static inline bool llist_add(struct llist_node *new, struct llist_head *head)
 {
-        struct llist_node *entry, *old_entry;
-
-        entry = head->first;
-        for (;;) {
-                old_entry = entry;
-                new->next = entry;
-                entry = cmpxchg(&head->first, old_entry, new);
-                if (entry == old_entry)
-                        break;
-        }
-
-        return old_entry == NULL;
+        struct llist_node *first;
+
+        do {
+                new->next = first = ACCESS_ONCE(head->first);
+        } while (cmpxchg(&head->first, first, new) != first);
+
+        return !first;
 }
 
 /**
diff --git a/lib/llist.c b/lib/llist.c
index 4a15115e90f8..4a70d120138c 100644
--- a/lib/llist.c
+++ b/lib/llist.c
@@ -39,18 +39,13 @@
 bool llist_add_batch(struct llist_node *new_first, struct llist_node *new_last,
                      struct llist_head *head)
 {
-        struct llist_node *entry, *old_entry;
+        struct llist_node *first;
 
-        entry = head->first;
-        for (;;) {
-                old_entry = entry;
-                new_last->next = entry;
-                entry = cmpxchg(&head->first, old_entry, new_first);
-                if (entry == old_entry)
-                        break;
-        }
+        do {
+                new_last->next = first = ACCESS_ONCE(head->first);
+        } while (cmpxchg(&head->first, first, new_first) != first);
 
-        return old_entry == NULL;
+        return !first;
 }
 EXPORT_SYMBOL_GPL(llist_add_batch);