llist: use try_cmpxchg in llist_add_batch and llist_del_first

Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in
llist_add_batch and llist_del_first.  x86 CMPXCHG instruction returns
success in ZF flag, so this change saves a compare after cmpxchg.

Also, try_cmpxchg implicitly assigns old *ptr value to "old" when cmpxchg
fails, enabling further code simplifications.

No functional change intended.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Uros Bizjak <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: ubizjak

lib/llist.c

@@ -30,7 +30,7 @@ bool llist_add_batch(struct llist_node *new_first, struct llist_node *new_last,
 
 	do {
 		new_last->next = first = READ_ONCE(head->first);
-	} while (cmpxchg(&head->first, first, new_first) != first);
+	} while (!try_cmpxchg(&head->first, &first, new_first));
 
 	return !first;
 }
@@ -52,18 +52,14 @@ EXPORT_SYMBOL_GPL(llist_add_batch);
  */
 struct llist_node *llist_del_first(struct llist_head *head)
 {
-	struct llist_node *entry, *old_entry, *next;
+	struct llist_node *entry, *next;
 
 	entry = smp_load_acquire(&head->first);
-	for (;;) {
+	do {
 		if (entry == NULL)
 			return NULL;
-		old_entry = entry;
 		next = READ_ONCE(entry->next);
-		entry = cmpxchg(&head->first, old_entry, next);
-		if (entry == old_entry)
-			break;
-	}
+	} while (!try_cmpxchg(&head->first, &entry, next));
 
 	return entry;
 }