llist: Move cpu_relax() to after the cmpxchg()

If in llist_add()/etc. functions the first cmpxchg() call succeeds, it is
not necessary to use cpu_relax() before the cmpxchg(). So cpu_relax() in
a busy loop involving cmpxchg() should go after cmpxchg() instead of before
that.

This patch fixes this for all involved llist functions.

Signed-off-by: Huang Ying <[email protected]>
Acked-by: Mathieu Desnoyers <[email protected]>
Signed-off-by: Peter Zijlstra <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

include/linux/llist.h

@@ -148,11 +148,14 @@ static inline void llist_add(struct llist_node *new, struct llist_head *head)
 	struct llist_node *entry, *old_entry;
 
 	entry = head->first;
-	do {
+	for (;;) {
 		old_entry = entry;
 		new->next = entry;
+		entry = cmpxchg(&head->first, old_entry, new);
+		if (entry == old_entry)
+			break;
 		cpu_relax();
-	} while ((entry = cmpxchg(&head->first, old_entry, new)) != old_entry);
+	}
 }
 
 /**

lib/llist.c

@@ -41,11 +41,14 @@ void llist_add_batch(struct llist_node *new_first, struct llist_node *new_last,
 	struct llist_node *entry, *old_entry;
 
 	entry = head->first;
-	do {
+	for (;;) {
 		old_entry = entry;
 		new_last->next = entry;
+		entry = cmpxchg(&head->first, old_entry, new_first);
+		if (entry == old_entry)
+			break;
 		cpu_relax();
-	} while ((entry = cmpxchg(&head->first, old_entry, new_first)) != old_entry);
+	}
 }
 EXPORT_SYMBOL_GPL(llist_add_batch);
 
@@ -68,13 +71,16 @@ struct llist_node *llist_del_first(struct llist_head *head)
 	struct llist_node *entry, *old_entry, *next;
 
 	entry = head->first;
-	do {
+	for (;;) {
 		if (entry == NULL)
 			return NULL;
 		old_entry = entry;
 		next = entry->next;
+		entry = cmpxchg(&head->first, old_entry, next);
+		if (entry == old_entry)
+			break;
 		cpu_relax();
-	} while ((entry = cmpxchg(&head->first, old_entry, next)) != old_entry);
+	}
 
 	return entry;
 }