[RFC PATCH 03/21] list: Annotate lockless list primitives with data_race()

[Will Deacon]

Some list predicates can be used locklessly even with the non-RCU list
implementations, since they effectively boil down to a test against
NULL. For example, checking whether or not a list is empty is safe even
in the presence of a concurrent, tearing write to the list head pointer.
Similarly, checking whether or not an hlist node has been hashed is safe
as well.

Annotate these lockless list predicates with data_race() and READ_ONCE()
so that KCSAN and the compiler are aware of what's going on. The writer
side can then avoid having to use WRITE_ONCE() in the non-RCU
implementation.

Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Marco Elver <elver@xxxxxxxxxx>
Signed-off-by: Will Deacon <will@xxxxxxxxxx>
---
 include/linux/list.h       | 10 +++++-----
 include/linux/list_bl.h    |  5 +++--
 include/linux/list_nulls.h |  6 +++---
 include/linux/llist.h      |  2 +-
 4 files changed, 12 insertions(+), 11 deletions(-)

diff --git a/include/linux/list.h b/include/linux/list.h
index 4fed5a0f9b77..4d9f5f9ed1a8 100644
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -279,7 +279,7 @@ static inline int list_is_last(const struct list_head *list,
  */
 static inline int list_empty(const struct list_head *head)
 {
-	return READ_ONCE(head->next) == head;
+	return data_race(READ_ONCE(head->next) == head);
 }
 
 /**
@@ -524,7 +524,7 @@ static inline void list_splice_tail_init(struct list_head *list,
  */
 #define list_first_entry_or_null(ptr, type, member) ({ \
 	struct list_head *head__ = (ptr); \
-	struct list_head *pos__ = READ_ONCE(head__->next); \
+	struct list_head *pos__ = data_race(READ_ONCE(head__->next)); \
 	pos__ != head__ ? list_entry(pos__, type, member) : NULL; \
 })
 
@@ -772,13 +772,13 @@ static inline void INIT_HLIST_NODE(struct hlist_node *h)
  * hlist_unhashed - Has node been removed from list and reinitialized?
  * @h: Node to be checked
  *
- * Not that not all removal functions will leave a node in unhashed
+ * Note that not all removal functions will leave a node in unhashed
  * state.  For example, hlist_nulls_del_init_rcu() does leave the
  * node in unhashed state, but hlist_nulls_del() does not.
  */
 static inline int hlist_unhashed(const struct hlist_node *h)
 {
-	return !READ_ONCE(h->pprev);
+	return data_race(!READ_ONCE(h->pprev));
 }
 
 /**
@@ -787,7 +787,7 @@ static inline int hlist_unhashed(const struct hlist_node *h)
  */
 static inline int hlist_empty(const struct hlist_head *h)
 {
-	return !READ_ONCE(h->first);
+	return data_race(!READ_ONCE(h->first));
 }
 
 static inline void __hlist_del(struct hlist_node *n)
diff --git a/include/linux/list_bl.h b/include/linux/list_bl.h
index ae1b541446c9..1804fdb84dda 100644
--- a/include/linux/list_bl.h
+++ b/include/linux/list_bl.h
@@ -51,7 +51,7 @@ static inline void INIT_HLIST_BL_NODE(struct hlist_bl_node *h)
 
 static inline bool  hlist_bl_unhashed(const struct hlist_bl_node *h)
 {
-	return !h->pprev;
+	return data_race(!READ_ONCE(h->pprev));
 }
 
 static inline struct hlist_bl_node *hlist_bl_first(struct hlist_bl_head *h)
@@ -71,7 +71,8 @@ static inline void hlist_bl_set_first(struct hlist_bl_head *h,
 
 static inline bool hlist_bl_empty(const struct hlist_bl_head *h)
 {
-	return !((unsigned long)READ_ONCE(h->first) & ~LIST_BL_LOCKMASK);
+	unsigned long first = data_race((unsigned long)READ_ONCE(h->first));
+	return !(first & ~LIST_BL_LOCKMASK);
 }
 
 static inline void hlist_bl_add_head(struct hlist_bl_node *n,
diff --git a/include/linux/list_nulls.h b/include/linux/list_nulls.h
index 3a9ff01e9a11..fa51a801bf32 100644
--- a/include/linux/list_nulls.h
+++ b/include/linux/list_nulls.h
@@ -60,18 +60,18 @@ static inline unsigned long get_nulls_value(const struct hlist_nulls_node *ptr)
  * hlist_nulls_unhashed - Has node been removed and reinitialized?
  * @h: Node to be checked
  *
- * Not that not all removal functions will leave a node in unhashed state.
+ * Note that not all removal functions will leave a node in unhashed state.
  * For example, hlist_del_init_rcu() leaves the node in unhashed state,
  * but hlist_nulls_del() does not.
  */
 static inline int hlist_nulls_unhashed(const struct hlist_nulls_node *h)
 {
-	return !READ_ONCE(h->pprev);
+	return data_race(!READ_ONCE(h->pprev));
 }
 
 static inline int hlist_nulls_empty(const struct hlist_nulls_head *h)
 {
-	return is_a_nulls(READ_ONCE(h->first));
+	return data_race(is_a_nulls(READ_ONCE(h->first)));
 }
 
 static inline void hlist_nulls_add_head(struct hlist_nulls_node *n,
diff --git a/include/linux/llist.h b/include/linux/llist.h
index 2e9c7215882b..c7f042b73899 100644
--- a/include/linux/llist.h
+++ b/include/linux/llist.h
@@ -186,7 +186,7 @@ static inline void init_llist_head(struct llist_head *list)
  */
 static inline bool llist_empty(const struct llist_head *head)
 {
-	return READ_ONCE(head->first) == NULL;
+	return data_race(READ_ONCE(head->first) == NULL);
 }
 
 static inline struct llist_node *llist_next(struct llist_node *node)
-- 
2.20.1