[PATCH 1/1] rculist: Replaced list_first_entry_rcu() with list_first_or_null_rcu()

[Michel Machado <michel@digirati.com.br>]

Replaced list_first_entry_rcu() with list_first_or_null_rcu() because
list_first_entry_rcu() is not safe as one can find in the comment that
this patch also adds.

This patch incorporated Paul's suggestions to the previous version of
this patch available here:

https://lkml.org/lkml/2012/4/2/536

This patch cannot break any upstream code because list_first_entry_rcu
is not being used anywhere in the kernel (tested with grep(1)), and
external code that uses it is probably broken already.

Signed-off-by: Michel Machado <michel@digirati.com.br>
CC: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
CC: Dipankar Sarma <dipankar@in.ibm.com>
---

diff --git a/include/linux/rculist.h b/include/linux/rculist.h
index d079290..ef5da30 100644
--- a/include/linux/rculist.h
+++ b/include/linux/rculist.h
@@ -228,18 +228,43 @@ static inline void list_splice_init_rcu(struct
list_head *list,
 	})
 
 /**
- * list_first_entry_rcu - get the first element from a list
+ * Where are list_empty_rcu() and list_first_entry_rcu()?
+ *
+ * Implementing those functions following their counterparts
list_empty() and
+ * list_first_entry() is not advisable because they lead to subtle race
+ * conditions as the following snippet shows:
+ * 
+ * if (!list_empty_rcu(mylist)) {
+ * 	struct foo *bar = list_first_entry_rcu(mylist, struct foo,
list_member);
+ * 	do_something(bar);
+ * }
+ *
+ * The list may not be empty when list_empty_rcu checks it, but it may
be when
+ * list_first_entry_rcu rereads the ->next pointer.
+ *
+ * Rereading the ->next pointer is not a problem for list_empty() and
+ * list_first_entry() because they would be protected by a lock that
blocks
+ * writers.
+ *
+ * See list_first_or_null_rcu for an alternative.
+ */
+
+/**
+ * list_first_or_null_rcu - get the first element from a list
  * @ptr:        the list head to take the element from.
  * @type:       the type of the struct this is embedded in.
  * @member:     the name of the list_struct within the struct.
  *
- * Note, that list is expected to be not empty.
+ * Note that if the list is empty, it returns NULL.
  *
  * This primitive may safely run concurrently with the _rcu
list-mutation
  * primitives such as list_add_rcu() as long as it's guarded by
rcu_read_lock().
  */
-#define list_first_entry_rcu(ptr, type, member) \
-	list_entry_rcu((ptr)->next, type, member)
+#define list_first_or_null_rcu(ptr, type, member) \
+	({struct list_head *__ptr = (ptr); \
+	  struct list_head __rcu *__next = list_next_rcu(__ptr); \
+	  likely(__ptr != __next) ? container_of(__next, type, member) : NULL;
\
+	})
 
 /**
  * list_for_each_entry_rcu	-	iterate over rcu list of given type