[PATCH] list: add missing rcu_dereference on first element

It seems that all the list_*_rcu primitives are missing a memory barrier
on the very first dereference.  For example,

#define list_for_each_rcu(pos, head) \
	for (pos = (head)->next; prefetch(pos->next), pos != (head); \
		pos = rcu_dereference(pos->next))

It will go something like:

	pos = (head)->next

	prefetch(pos->next)

	pos != (head)

	do stuff

We're missing a barrier here.

	pos = rcu_dereference(pos->next)

		fetch pos->next

		barrier given by rcu_dereference(pos->next)

		store pos

Without the missing barrier, the pos->next value may turn out to be stale.
In fact, if "do stuff" were also dereferencing pos and relying on
list_for_each_rcu to provide the barrier then it may also break.

So here is a patch to make sure that we have a barrier for the first
element in the list.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: "Paul E. McKenney" <paulmck@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/list.h

@@ -442,12 +442,14 @@ static inline void list_splice_init(struct list_head *list,
  * as long as the traversal is guarded by rcu_read_lock().
  */
 #define list_for_each_rcu(pos, head) \
-	for (pos = (head)->next; prefetch(pos->next), pos != (head); \
-        	pos = rcu_dereference(pos->next))
+	for (pos = (head)->next; \
+		prefetch(rcu_dereference(pos)->next), pos != (head); \
+        	pos = pos->next)
 
 #define __list_for_each_rcu(pos, head) \
-	for (pos = (head)->next; pos != (head); \
-        	pos = rcu_dereference(pos->next))
+	for (pos = (head)->next; \
+		rcu_dereference(pos) != (head); \
+        	pos = pos->next)
 
 /**
  * list_for_each_safe_rcu	-	iterate over an rcu-protected list safe
@@ -461,8 +463,9 @@ static inline void list_splice_init(struct list_head *list,
  * as long as the traversal is guarded by rcu_read_lock().
  */
 #define list_for_each_safe_rcu(pos, n, head) \
-	for (pos = (head)->next, n = pos->next; pos != (head); \
-		pos = rcu_dereference(n), n = pos->next)
+	for (pos = (head)->next; \
+		n = rcu_dereference(pos)->next, pos != (head); \
+		pos = n)
 
 /**
  * list_for_each_entry_rcu	-	iterate over rcu list of given type
@@ -476,9 +479,9 @@ static inline void list_splice_init(struct list_head *list,
  */
 #define list_for_each_entry_rcu(pos, head, member) \
 	for (pos = list_entry((head)->next, typeof(*pos), member); \
-	     prefetch(pos->member.next), &pos->member != (head); 	\
-	     pos = rcu_dereference(list_entry(pos->member.next, 	\
-					typeof(*pos), member)))
+		prefetch(rcu_dereference(pos)->member.next), \
+			&pos->member != (head); \
+		pos = list_entry(pos->member.next, typeof(*pos), member))
 
 
 /**
@@ -492,8 +495,9 @@ static inline void list_splice_init(struct list_head *list,
  * as long as the traversal is guarded by rcu_read_lock().
  */
 #define list_for_each_continue_rcu(pos, head) \
-	for ((pos) = (pos)->next; prefetch((pos)->next), (pos) != (head); \
-        	(pos) = rcu_dereference((pos)->next))
+	for ((pos) = (pos)->next; \
+		prefetch(rcu_dereference((pos))->next), (pos) != (head); \
+        	(pos) = (pos)->next)
 
 /*
  * Double linked lists with a single pointer list head.
@@ -696,8 +700,9 @@ static inline void hlist_add_after_rcu(struct hlist_node *prev,
 	     pos = n)
 
 #define hlist_for_each_rcu(pos, head) \
-	for ((pos) = (head)->first; pos && ({ prefetch((pos)->next); 1; }); \
-		(pos) = rcu_dereference((pos)->next))
+	for ((pos) = (head)->first; \
+		rcu_dereference((pos)) && ({ prefetch((pos)->next); 1; }); \
+		(pos) = (pos)->next)
 
 /**
  * hlist_for_each_entry	- iterate over list of given type
@@ -762,9 +767,9 @@ static inline void hlist_add_after_rcu(struct hlist_node *prev,
  */
 #define hlist_for_each_entry_rcu(tpos, pos, head, member)		 \
 	for (pos = (head)->first;					 \
-	     pos && ({ prefetch(pos->next); 1;}) &&			 \
+	     rcu_dereference(pos) && ({ prefetch(pos->next); 1;}) &&	 \
 		({ tpos = hlist_entry(pos, typeof(*tpos), member); 1;}); \
-	     pos = rcu_dereference(pos->next))
+	     pos = pos->next)
 
 #else
 #warning "don't include kernel headers in userspace"