list: Use WRITE_ONCE() when adding to lists and hlists

Code that does lockless emptiness testing of non-RCU lists is relying
on the list-addition code to write the list head's ->next pointer
atomically.  This commit therefore adds WRITE_ONCE() to list-addition
pointer stores that could affect the head's ->next pointer.
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

include/linux/list.h

@@ -42,7 +42,7 @@ static inline void __list_add(struct list_head *new,
 	next->prev = new;
 	new->next = next;
 	new->prev = prev;
-	prev->next = new;
+	WRITE_ONCE(prev->next, new);
 }
 #else
 extern void __list_add(struct list_head *new,
@@ -642,7 +642,7 @@ static inline void hlist_add_head(struct hlist_node *n, struct hlist_head *h)
 	n->next = first;
 	if (first)
 		first->pprev = &n->next;
-	h->first = n;
+	WRITE_ONCE(h->first, n);
 	n->pprev = &h->first;
 }
 
@@ -653,14 +653,14 @@ static inline void hlist_add_before(struct hlist_node *n,
 	n->pprev = next->pprev;
 	n->next = next;
 	next->pprev = &n->next;
-	*(n->pprev) = n;
+	WRITE_ONCE(*(n->pprev), n);
 }
 
 static inline void hlist_add_behind(struct hlist_node *n,
 				    struct hlist_node *prev)
 {
 	n->next = prev->next;
-	prev->next = n;
+	WRITE_ONCE(prev->next, n);
 	n->pprev = &prev->next;
 
 	if (n->next)

lib/list_debug.c

@@ -37,7 +37,7 @@ void __list_add(struct list_head *new,
 	next->prev = new;
 	new->next = next;
 	new->prev = prev;
-	prev->next = new;
+	WRITE_ONCE(prev->next, new);
 }
 EXPORT_SYMBOL(__list_add);